Electrochemical detection of simple alkanes by utilizing a solid-state zirconia-based gas sensor published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru LETTER 2023, vol. 10(1), No. 202310109 DOI: 10.15826/chimtech.2023.10.1.09 1 of 6 Electrochemical detection of simple alkanes by utilizing a solid-state zirconia-based gas sensor Anatoly S. Kalyakin , Alexander N. Volkov * Laboratory of Electrochemical Devices Based on Solid Oxide Proton Electrolytes, Institute of High Tem- perature Electrochemistry, Yekaterinburg 620066, Russia * Corresponding author: wolkov@ihte.uran.ru This paper belongs to a Regular Issue. Abstract Solid-state gas sensors composed of complex oxide electrolytes offer great potential for analyzing various atmospheres at high temperatures. While relatively simple gas mixtures (H2O+N2, O2+N2) have been suc- cessfully studied by means of ZrO2-based sensors, the precise detection of more complex compounds represents a challenging task. In this work, we present our findings regarding the analysis of lower alkanes (CH 4, C2H6, and C3H8) mixed with nitrogen as an inert gas, utilizing an am- perometric ZrO2-based sensor. This sensor, serving as an electrochemi- cal cell with a diffusion barrier, was tested at 500–600 °C to measure the limiting current, which depends on the gas composition and can be further used as a basis for calibration curves. In addition, the diffusion coefficients of the specified gas mixtures were successfully found and compared with references, confirming the applicability of the fabricated sensor for studying diffusion processes in wide concentration and tem- perature ranges. Keywords alkanes electrochemical approaches limiting current diffusion amperometric sensors diffusion barrier Received: 08.02.23 Revised: 13.02.23 Accepted: 13.02.23 Available online: 14.02.23 © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Complex oxide electrolytes based on zirconia are widely used in many fields of science and technology. One of the promising fields of ZrO2-containing materials application is analytical chemistry. For example, potentiometric solid- state sensors have been successfully used to monitor the exhaust gases of various heat units and vehicles [1–4]; moreover, they can be used to measure the oxygen activity in metal melts [5] and to detect some gases in the envi- ronments (NH3 [6], NO2 [7], H2S [8]). Based on the con- cept of potential reading, such types of sensors operate efficiently for detecting low oxygen contents in gas- es/melts compared to reference conditions (an electrode in air or oxygen atmospheres). To detect a relatively high concentration of the analyzed components, the so-called amperometric-type sensors can be employed. In detail, the ZrO2-based amperometric sensors have recently been used for measuring the contents of O2, H2, CO2 and H2O in inert gases [9–13]. The reading parameter of such sensors is a limiting current, representing an electrical equivalent of the concentration and diffusion parameters of the ana- lyzed gas media. The amperometric sensors allow the analysis of more complex gases, which may contain lower alkanes (CH4, C2H6, and C3H8), not only in terms of gas composi- tion, but also in terms of diffusion coefficients. At pre- sent, there are no reliable methods or appropriate tech- niques for measuring the diffusion coefficients of gases due to the complexity of the equipment and the meth- odological difficulties, although these coefficients are widely used in the calculation of combustion processes, in chemical kinetics; in addition, the diffusion coeffi- cients are included in many dimensionless criteria for heat and mass transfer. The knowledge of the binary diffusion coefficients for combustible gases is necessary when calculating the processes of transport and regasi- fication of natural gases. In the present work, we develop a new high- temperature YSZ-based sensor for the analysis of CH4+N2, C2H6+N2, and C3H8+N2 gas mixtures and the determination of their binary diffusion coefficients. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.1.09 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-8816-4303 https://orcid.org/0000-0002-5184-986X https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.1.09&domain=pdf&date_stamp=2023-02-14 Chimica Techno Acta 2023, vol. 10(1), No. 202310109 LETTER 2 of 6 DOI: 10.15826/chimtech.2023.10.1.09 2. Experimental part 2.1. Operation principles The working principle of the fabricated amperometric sensor is shown in Figure 1. This sensor consists of two plates made of YSZ (0.91ZrO2 + 0.09Y2O3) electrolyte glued together by a high-temperature glass sealant. Each plate has a cavity, in which a capillary was placed as a diffusion barrier. The capillary is a ceramic tube with an inner diameter of 265 µm and a length of 20 mm. The cavity has a depth of 0.5 mm and an area of approxi- mately 60 mm2. Platinum electrodes with wire current leads were organized on opposite surfaces of one solid electrolyte plate. During all the measurements, the electrochemical cell is in the flow of the analyzed gas, while the latter diffuses through the capillary into its internal cavity. There is no free oxygen in the analyzed gas, and oxygen is formed as a result of the dissociation of moisture present in the ana- lyzed gas: H2O + 2e ′ → H2 + O 2−. (1) The oxygen produced by the dissociation of water va- por must be pumped into the cavity of the cell. To provide such a pumping, it is necessary to apply a minus from a DC source to the outer electrode of the cell, and a plus to the inner electrode, respectively. Alkane oxidation then takes place at the inner electrode according to the follow- ing overall reactions: 4O2− + CH4 − 8e ′ → CO2 + 2H2O, (2) 7O2− + C2H6 − 14e ′ → 2CO2 + 3H2O, (3) 10O2− + C3H8 − 20e ′ → 3CO2 + 4H2O. (4) When working with mixtures of CH4+N2, C2H6+N2, and C3H8+N2, the oxygen pumping current into the sensor cav- ity ranged from several microamperes to tens of microam- peres. As the voltage applied to the cell increases, the cur- rent increases and then stabilizes, reaching a certain value corresponding to the as-called limiting current. Its values, as shown by the results of this study, are proportional to the concentration of methane, ethane, or propane in the analyzed gases. The appearance of the limiting current is due to the reaction between the hydrocarbons and oxygen ions on the inner surface of the zirconium electrolyte. It is important to note that the flow of the analyzed combus- tible gas entering the cavity of the electrochemical cell is limited by the diffusion barrier (capillary), which governs the limiting current value. Therefore, the measured limit- ing current is a function of the concentration of the meas- ured hydrocarbon in the analyzed gaseous medium, tem- perature and geometrical parameters of the diffusion bar- rier according to the following equation [13]: 𝐼lim = 𝑛 ∙ 𝐹 ∙ 𝑃 ∙ 𝐷 ∙ 𝑆 𝑅 ∙ 𝑇 ∙ 𝐿 ln(1 − 𝑐C𝑥H𝑦), (5) where n in the electron numbers required for full oxidation of 1 mole of alkane, F is the Faraday’s constant, P is the to- tal pressure of the analyzed gas (P = 1 atm), D is the binary diffusion coefficient of N2 and alkane mixture, S and L are the cross-section area and length of the capillary, R is the universal gas constant, T is the absolute temperature, 𝑐C𝑥H𝑦 is the volume concentration of alkane in nitrogen. At the relatively low concentration values, the ln(1 − 𝑐C𝑥H𝑦) term is equal to 𝑐C𝑥H𝑦. In turn, the D values depend on the standard conditions as follows [14]: 𝐷 = 𝐷0 ( 𝑇 𝑇0 ) 𝑛 ( 𝑃 𝑃0 ), (6) where To and Po are the standard parameters. Considering equations (5) and (6) together, one can see that the standard diffusion coefficients can be found, if the limiting current values are known. 2.2. Characterization of the sensor The fabricated sensor was placed in an oven and heated to the desired temperature (500, 550 or 600 °C). Then the analyzed gas mixtures (CH4+N2, C2H6+N2, or C3H8+N2) were fed to the sensor followed by equilibration of this system for 1 h. These gas mixtures were prepared by add- ing a portion of hydrocarbons into the nitrogen flow by F-201C-33-V gas flow meters. The DC voltage (GPS-18500 INSTEK) was applied to the electrodes to provide electro- chemical pumping of oxygen. 3. Results and Discussion The as-fabricated sensor was first tested with the CH4+N2 gas mixtures. Its typical volt-ampere dependences at dif- ferent gas compositions are presented in Figure 2a. Three clearly distinguished regions appear at these dependences with a gradual increase in the applied voltage: current growth (I region), a wide range of constant current values (II region) followed by further current growth (III region). In the I region, the applied voltage causes oxygen pumping from the external to inner electrode, where the methane oxidation reaction occurs (equation (1)). Figure 1 Principle scheme of operating the fabricated gas sensor: 1 is the electrolyte discs, 2 is the high-temperature glass sealant, 3 is the cavity, 4 is the platinum electrodes, and 5 is the capillary. https://doi.org/10.15826/chimtech.2023.10.1.09 Chimica Techno Acta 2023, vol. 10(1), No. 202310109 LETTER 3 of 6 DOI: 10.15826/chimtech.2023.10.1.09 Figure 2 Electrochemical response of the sensor operated in x vol.% CH4 + (1–x) vol.% N2 gas mixtures: (a) volt-ampere de- pendences at 550 °C and various methane concentration; (b) volt- ampere dependences at various temperature and x = 2.47; (c) calibration curve at 550 °C. An amount of methane is sufficiently high compared to the oxygen equivalent flow expressed in the current term; as a result, the current rises with increasing the applied voltage. When all the methane molecules in the sensor’s cavity (including those supplied through the capillary) are electrochemically oxidized, the current values stabilize and do not change with a further increase of the applied voltage, II region. This region corresponds to the realization of the limiting current. Finally, when the applied voltage exceeds a certain value, the current increases again (III region) due to the possible appearance of electronic conductivity [15]. Considering the effects of gas composition, it can be seen that the limiting current region increases with its range and in absolute current values, indicating that more oxygen ions should be pumped to oxidize more methane molecules. Figure 2b shows similar volt-ampere dependencies ob- tained for a given gas composition at various tempera- tures. The limiting current range is virtually unchanged with heating (~about 0.6 V), but the limiting current value increases at the same time. The latter can be explained by improved diffusion of methane molecules into the sensor’s cavity. Therefore, a higher oxygen-ion flux is required to oxidize these molecules. The concentration dependence of the limiting current is close to a linear form (Figure 2c), which allows these data to be used for the analysis of real CH4+N2 gas mixtures with unknown concentrations of the components. Similar characteristics were obtained for C2H6+N2 and C3H8+N2 mixtures in the temperature range of 500–600 °С (Figure 3). In general, the electrochemical characteristics have similar trends discussed for the CH4+N2 gas mixtures, with some distinctive points. First, the limiting current values considerably decrease in a se- ries of CH4–C2H6–C3H8 (at close concentrations and the same temperatures), indicating that heavy molecules dif- fuse more slowly through the capillary. Second, the con- centration range of the limiting current narrows as the molecular weight of the alkane increases, which limits the analytical capabilities of the sensor. Nevertheless, an important advantage of the fabricated sensor is the ability to determine the binary diffusion co- efficients from the measured limiting current values (even if they are not high) according to equations (5) and (6). Table 1 lists the experimentally obtained diffusion coeffi- cient values for methane in nitrogen for three tempera- tures and their comparison with reference data. As can be seen from Table 1, the experimentally ob- tained D values for methane in nitrogen have some dis- crepancies with the reference data obtained by other methods. This may be due to the fact that the reference data do not take the CH4 concentration into account, since in most cases, the D values were obtained for mixtures with a parity concentration of the components, usually 50% to 50%. In addition, available reference data are of- ten presented for several temperatures, typically 0 and 25 °C, and had to be recalculated for elevated tempera- tures according to equation (6). The experimental data show that the D values depend not only on the tempera- ture, but also on the component concentrations (especial- ly, methane), and this dependence is quite complex. Table 2 presents similar D data measured and com- pared for the C2H6+N2 gas mixtures. There is an accepta- ble agreement between the measured D values and refer- ence data. However, the reference D values are somewhat higher than the experimental data for the entire tempera- ture range and the considered ethane concentrations. This may be due to the different features of the experimental techniques used in the literature and in this work. Finally, Table 3 lists the experimental results for the propane – nitrogen gas mixtures. The comparison of the experimental and reference values shows that they are in the best agreement with each other. https://doi.org/10.15826/chimtech.2023.10.1.09 Chimica Techno Acta 2023, vol. 10(1), No. 202310109 LETTER 4 of 6 DOI: 10.15826/chimtech.2023.10.1.09 Figure 3 Electrochemical response of the sensor operated in x vol.% C2H6 + (1–x) vol.% N2 gas mixtures (left column) and x vol.% C3H8 + (1–x) vol.% N2 gas mixtures (right column): (a) volt-ampere dependences at 550 °C and various methane concentration; (b) volt-ampere dependences at various temperature and x = 2.47; (c) calibration curve at 550 °C. Table 1 Measured and reference binary diffusion coefficients (D) for the x vol.% CH4 + (1–x) vol.% N2 gas mixtures. Temperature (°С) D (cm2 s–1) x = 0.29 D (cm2 s–1) x = 0.61 D (cm2 s–1) x = 0.98 D (cm2 s–1) x = 1.41 D (cm2 s–1) x = 2.47 D (cm2 s–1) [16] D (cm2 s–1) [17] 500 1.8 1.8 1.78 1.77 1.6 1.22 1.32 550 1.3 1.4 1.8 1.9 1.7 1.35 1.48 600 2.2 2.1 2.05 1.7 1.8 1.50 1.66 Table 2 Measured and reference binary diffusion coefficients (D) for the x vol.% C2H6 + (1–x) vol.% N2 gas mixtures. Temperature (°С) D (cm2 s–1) x = 0.18 D (cm2 s–1) x = 0.36 D (cm2 s–1) x = 0.60 D (cm2 s–1) x = 1.28 D (cm2 s–1) x = 2.18 D (cm2 s–1) [17] 500 0.64 0.65 0.69 0.63 0.55 0.76 550 0.68 0.69 0.74 0.67 0.59 0.85 600 0.72 0.73 0.78 0.71 0.62 0.93 Table 3 Measured binary diffusion coefficients (D) for the x vol.% C3H8 + (1–x) vol.% N2 gas mixtures. Temperature (°С) D (cm2 s–1) x = 0.09 D (cm2 s–1) x = 0.13 D (cm2 s–1) x = 0.20 D (cm2 s–1) x = 0.51 D (cm2 s–1) [17] 500 0.52 0.49 0.59 0.50 0.55 550 0.62 0.61 0.67 0.60 0.61 600 0.72 0.72 0.73 0.71 0.67 https://doi.org/10.15826/chimtech.2023.10.1.09 Chimica Techno Acta 2023, vol. 10(1), No. 202310109 LETTER 5 of 6 DOI: 10.15826/chimtech.2023.10.1.09 4. Limitations As can be seen from the experimental data (Figures 2 and 3), the fabricated sensor shows good operability at low concentrations of alkanes in nitrogen. The higher hydro- carbon contents will provide the higher limiting current. However, a longer response time is required in this case. At the lower concentrations of hydrocarbons, a limiting current region is expected to be very weak, which does not allow the formation of the calibration curves. 5. Conclusions This work reports on the possibility of using an am- perometric sensor with a diffusion barrier to determine the concentration and binary diffusion coefficients of some light alkanes in nitrogen at elevated temperatures (500–600 °C). We have experimentally obtained the val- ues of the diffusion coefficients for CH4+N2, C2H6+N2, and C3H8+N2 gas mixtures. Our values are in good agreement with those reported in published data. ● Supplementary materials No supplementary materials are available. ● Funding This research had no external funding. ● Acknowledgments None. ● Author contributions Conceptualization: A.S.K., A.N.V. Data curation: A.N.V. Formal Analysis: A.S.K., A.N.V. Investigation: A.S.K., A.N.V. Methodology: A.S.K. Project administration: A.N.V. Resources: A.S.K., A.N.V. Software: A.S.K., A.N.V. Supervision: A.N.V. Validation: A.N.V. Visualization: A.S.K. Writing – original draft: A.N.V. Writing – review & editing: A.N.V. ● Conflict of interest The authors declare no conflict of interest. ● Additional information Author IDs: Anatoly S. Kalyakin, Scopus ID 8943432300 Alexander N. Volkov, Scopus ID 7402103148 Website: Institute of High Temperature Electrochemistry UB RAS, http://www.ihte.uran.ru. References 1. Bhardwaj A, Kim I-H, Mathur L, Park J-Y, Song S-J. Ultrahigh- sensitive mixed-potential ammonia sensor using dual- functional NiWO4 electrocatalyst for exhaust environment monitoring. J Hazard Mater. 2021;403:123797. doi:10.1016/j.jhazmat.2020.123797 2. Javed U, Ramaiyan KP, Kreller CR, Brosha EL, Mukundan R, Morozov AV. Using sensor arrays to decode NOx/NH3/C3H8 gas mixtures for automotive exhaust monitoring. Sensors Ac- tuators B Chem. 2018;264:110–118. doi:10.1016/j.snb.2018.02.069 3. Long Y, Li G, Zhang Z, Liang J. Application of reformed ex- haust gas recirculation on marine LNG engines for NO emis- sion control. 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