{Char of Tagetes erecta (African marigold) flower as a potential electrode material for supercapacitors:} http://dx.doi.org/10.5599/jese.1381 787 J. Electrochem. Sci. Eng. 12(4) (2022) 787-797; http://dx.doi.org/10.5599/jese.1381 Open Access : : ISSN 1847-9286 www.jESE-online.orghttp://www.jese-online.org/ Original scientific paper Char of Tagetes erecta (African marigold) flower as a potential electrode material for supercapacitors Venkata Naga Kanaka Suresh Kumar Nersu1,, Bhujanga Rao Annepu1, Subhakaran Singh Rajaputra2 and Satya Srinivasa Babu Patcha3 1Department of Instrument Technology, Andhra University College of Engineering (A), Visakhapatnam, Andhra Pradesh 530003, India 2Centre for Advanced Energy Studies, Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP, India 3Center for Flexible Electronics, Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Vaddeswaram, AP, India Corresponding author: sureshk834@gmail.com Received: May 16, 2022; Accepted: July 9, 2022; Published: July 25, 2022 Abstract A char of Tagetes erecta flowers (TFC) was derived through simple thermal decomposition of Tagetes erecta flowers (TF). Physico-chemical properties of as-prepared TFC were evaluated using XRD, FESEM, FTIR, TGA, N2 adsorption-desorption isotherm analysis and water contact angle measurements. The practicality and applicability of TFC as promising electrode material in supercapacitors (SCs) were evaluated in full-cell configuration by performing electrochemical characterizations like CV, GCD and EIS on a lab-scale TFC-based symmetric SC. TFC exhibited a remarkable specific capacitance of 118.4 F g-1 at a constant current density of 0.2 A g-1 and a specific energy of 4.1 Wh kg-1 at specific power of 0.1 kW kg-1. TFC showed excellent cyclic stability by retaining 92 % of its initial capacitance even after 6000 GCD cycles at 2 A g-1. The superior capacitive behaviour and cyclic stability of TFC could be attributed to its good wettability towards water. This excellent supercapacitive performance of TFC estab- lishes it as a potential floral waste-derived carbon-based electrode material for SCs. Keywords Biochar; flowers waste; gel polymer electrolyte; electric double-layer capacitor (EDLC); carbon cloth; hydrophilicity Introduction In India, nearly 300,000 hectares are under floriculture producing nearly 3 million tons of flowers annually (as per National Horticulture Board of India statistics of 2018-2019). Flowers like rose, marigold (Tagetes erecta), jasmine, hibiscus, etc., are most commonly used in the preparation of garlands, decoration of religious sites and statues during festivals, as well as offerings during http://dx.doi.org/10.5599/jese.1381 http://dx.doi.org/10.5599/jese.1381 http://www.jese-online.org/ http://www.jese-online.org/ mailto:sureshk834@gmail.com J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 CHAR OF Tagetes erecta FLOWER AS SUPERCAPACITORS 788 weddings and other ceremonies [1]. After serving their purpose, these flowers are left unused and are usually dumped in soil or disposed into water bodies [2]. Daily, around 40 % of flowers produced in India are left unsold, generating a huge amount of floral waste [3]. Disposing floral waste in open landfills would attract microorganisms that degrade it, thereby releasing harmful gases like methane, carbon dioxide and ammonia, developing a foul smell, and promoting greenhouse emissions [4]. In India, floral waste is dumped in rivers and other water bodies resulting in serious water pollution [5,6]. Disposal of flower waste is a major concern to the environment and there is a need for novel methods to convert floral waste into value-added products [3]. Currently, flower waste is being utilized for the preparation of compost, eco-friendly incense sticks, soaps, as well as conditioners in lawn dressing [7]. Eco-friendly energy storage devices are of utmost importance in the current scenario as alterna- tives to storing energy harvested from renewable energy sources [8]. Electric double-layer capacitors (EDLCs) are a type of supercapacitors (SCs) that store charge in the form of an electric double layer (EDL) at the electrode-electrolyte juncture and are familiar for their superior charge-discharge capability and long cyclic life [9]. Carbon-based materials are often used as electrode materials in EDLCs and commercially available EDLCs use activated carbon as electrode material [10]. Recently, the development of low-cost and eco-friendly electrode materials such as biological waste-derived carbon-based materials has gained a lot of attention from scientific communities worldwide [11]. Carbons obtained from bamboo [12], sunflower stems [13], lotus stems [14], rice straw [15], wheat straw [16], corn stalks [17], cotton stalks [18], and sugar cane bagasse [19] were already tested as electrode materials in SCs. Wood sawdust [20] and wood [21] were also used to derive carbons and tested as electrode materials in SCs. Carbons obtained from hemp fibers [22], bamboo fibers [23], jute fibers [24], lotus leaves [25], eucalyptus leaves [26], ficus religiosa leaves [27] and pine leaves [28] were tested for their potential as electrode materials in SCs. Rice husk [11] and shells of coconuts [29], oil palm kernel [30], and macadamia nuts [31] were used as precursors to derive carbon-based electrode materials for SCs. Biological waste of animal origin like scales of fish [32], hair of human beings [33], shells of shrimps [34] and chicken eggs [35] and chicken feathers [36] were used to derive carbon-based electrode materials for SCs. In the past, flowers like cherry blossom [37], rose [38] and catkins of willow [39-42] were used to derive carbon-based materials and applied as electrode materials in SCs. Flowers of Tagetes erecta (TF), also known as African or Mexican marigold, are commonly used for decoration, extraction of carotenoids and essential oils, adding colour to food items and medicinal purposes [43]. In India, around 3910 hectares of area is under marigold cultivation with an average yield of 22.4 tons per hectare [44], and producing nearly 87,000 tons of TF per year. Disposal of the resulting floral waste requires strategies like the production of bioethanol from TF waste, etc. [2]. Gupta et al. [45] reported the synthesis of graphene quantum dots from TF and tested its potential as electrode material in SCs in a three-electrode configuration. In the present work, char of Tagetes erecta flower (TFC) was obtained by facile thermal decomposition of TF excluding activation steps making the procedure eco-friendly. Physico-chemical properties of TFC were investigated. A TFC-based SC (full-cell) was constructed using TFC, hydrothermally reduced carbon cloth (CCHy), graphene incorporated sulfonated polyvinyl alcohol (SPVA-HRG-0.5) hydrogel, and Whatman® filter paper as electrode material, current collector, electrolyte and porous separator, respectively. Supercapacitive performance of the constructed cell was investigated using electrochemical techniques like cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). V. N. K. S. K. Nersu et al. J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 789 Experimental Materials TF waste was collected from nearby temples of Andhra Pradesh, India. Carbon cloth was procured from Avcarb, USA. Multiwalled carbon nanotubes (MWCNTs) were procured from Ad Nano technolo- gies, India. Sulfuric acid (H2SO4) (98 %), hydrogen peroxide (H2O2) (30 % w/v), graphite (<20 m), sodium nitrate (NaNO3), polyvinylidine difluoride (PVDF) (MW 320,000), ethanol (99.9 %), sodium hydroxide (NaOH), polyvinyl alcohol (PVA) (MW 1,25,000), nitric acid (HNO3), isopropanol (IPA), hydrochloric acid (HCl) (35-38 %), N-methyl-2-pyrrolidone (NMP) and potassium permanganate (KMnO4) were used as received. Whatman® qualitative filter paper was procured locally and used as a separator during the fabrication of TFC-based SC. Deionized (DI) water was utilized throughout the synthesis. Preparation of TFC Collected TF were washed with DI water and dried at room temperature (RT). Trimmed petals of TF were placed in a crucible and covered with aluminum foil in which tiny holes were made for the elimination of volatile compounds while thermal decomposition. TF petals were heated for 3 h at 350 °C to obtain TFC (Figure 1). Figure 1. Preparation of TFC through thermal decomposition of TF Preparation of CCHy CCHy was prepared from commercially acquired carbon cloth following the synthesis procedure reported elsewhere [46]. Carbon cloth was dropped into a 2:1 mixture (30 mL) of H2SO4 and HNO3, and KMnO4 (3 g) was added to the mixture while agitating at RT. Later, DI water (100 mL) was added to the mixture and stirred for 3 h continuously, followed by the addition of 30 % H2O2 (5-10 mL), leading to the formation of a transparent solution. The obtained oxidized carbon cloth was cleaned many times with DI water and later transferred into a DI water-filled polytetrafluoroethylene (PTFE) lined autoclave and heated for 14 h at 180 °C to obtain CCHy. Collected CCHy was dried and later used as a current collector in the fabrication of TFC-based SC. http://dx.doi.org/10.5599/jese.1381 J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 CHAR OF Tagetes erecta FLOWER AS SUPERCAPACITORS 790 Preparation of NGPE Hydrothermally reduced graphene oxide (HRG) was synthesized following the procedure reported elsewhere [47]. Graphite (2 g) was dispersed into a mixture of H2SO4 (98 mL) and NaNO3 (2 g) and agitated for 4 h continuously by preserving the suspension temperature under 5 °C using an ice bath. To this suspension, KMnO4 (12 g) was slowly added and stirred for 2 days continuously at RT. The temperature of the suspension was further decreased to <5 °C using an ice bath and then DI water (184 mL) was added to the suspension and stirred for another 2 h. Finally, 30 % H2O2 was slowly added to the suspension while stirring until the suspension color changed yellow, signifying graphene oxide (GO) formation. The yellow-coloured suspension was kept a side to allow sedimentation of GO. Later, the supernatant solution was decanted to obtain GO precipitate, which was washed and centrifuged multiple times using 1 M HCl and DI water until color of GO precipitate turned black. Finally, the GO precipitate was cleansed using ethanol and later centrifuged and vacuum dried for 12 h at 70 °C to obtain GO films, which were further powdered to obtain GO. Later, GO (0.2 g) was dispersed into DI water (200 mL) and transferred into a PTFE-lined autoclave while maintaining pH at 11 using NaOH pellets. The autoclave was heated at 180 °C for 14 h and the collected HRG was vacuum dried at 60 °C for 12 h to acquire HRG. NGPE was prepared by incorporating HRG into sulfonated PVA hydrogel to obtain HRG incorporated sulfonated PVA hydrogel (SPVA-HRG-0.5) following the procedure reported elsewhere [48]. To DI water (4 mL), H2SO4 (270 L) was added and heated up to 80 °C followed by the addition of PVA (0.5 g) and stirred until a clear sulfonated PVA (SPVA) hydrogel was obtained. Finely powdered HRG (25 mg) was dispersed in IPA (10 mL) and this dispersion was slowly added into SPVA hydrogel while stirring at 80 °C for 0.5 h to obtain SPVA-HRG-0.5, where 0.5 represents the wt.% of HRG in SPVA hydrogel. The as-prepared NGPE was used as an electrolyte in the fabrication of TFC- based SC. Characterization studies The structural properties of TF and TFC were analyzed utilizing X-ray diffraction (XRD) technique (Rigaku miniflex 600). The functional groups present in TF and TFC were determined using Fourier- transform infrared (FTIR) spectroscopy (Cary 630). Field emission scanning electron microscopy (FESEM) (FEI-Quanta FEG 200F) was used to investigate the surface morphology of TFC. The surface area and porosity profile of TFC was evaluated using a surface area and porosity analyser (Quantachrome Nova 2200e). Thermogravimetric analysis (TGA) (Simultaneous thermal analyser - STA 7200, Hitachi HTG) of TFC was carried out under N2 atmosphere up to 800 °C at a heating rate of 20 °C min-1. A pellet of TFC (diameter = 1.2 cm) was prepared to perform water contact angle measurement (Kyowa DM-501) through the sessile drop technique. A lab-scale TFC-based symmetric SC was fabricated by sandwiching TFC-coated CCHy on either side of a SPVA-HRG-0.5 electrolyte soaked Whatman® filter paper. The supercapacitive behaviour of TFC-based symmetric SC (full-cell) was tested using an electrochemical workstation (PARSTAT PMC 2000A) by performing CV, GCD and EIS techniques. TFC, MWCNTs and PVDF were dispersed in NMP in the ratio of 80:10:10 to obtain an electrode ink, which was later coated over two CCHy (area of each CCHy = 0.8 cm2) and heated at 120 °C for 15 min under vacuum, resulting in TFC coated CCHy (TFC-CCHy) with a TFC loading of 1 mg cm-2. A full cell was fabricated by sandwiching a pair of TFC-CCHy on either side of a SPVA-HRG-0.5 gel polymer electrolyte soaked Whatman® filter paper. V. N. K. S. K. Nersu et al. J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 791 Results and discussion Physico-chemical characterizations Figure 2 (a) illustrates the diffraction patterns of TF and TFC showing characteristic broad peaks, indicating the presence of amorphous carbon and the peaks in TFC are less sharp compared to TF, indicating an increase in its amorphous nature after thermal decomposition [49,50]. Figure 2. (a) X-ray diffractograms and (b) FTIR spectra of TF and TFC, respectively Figure 2(b) illustrates the FTIR spectra of TF and TFC. Both TF and TFC showed broad peaks around 3274 and 3271 cm-1 resulting from vibrational stretching O-H bonds of hydroxyl (-OH) groups [45]. The peaks observed around 2918 and 2857 cm-1 of TF and 2919 and 2854 cm-1 of TFC could result from vibrational stretching of C-H bonds of -CH groups [45]. The peaks observed around 1600 cm-1 in both TF and TFC, resulted from vibrational stretching of C=C bonds [45]. The peaks observed around 1017 and 1018 cm-1 in FTIR spectra of TF and TFC, respectively, could have resulted from vibrational stretching of C-N bonds [3,45]. Figure 3 (a and b) represents FESEM images of TFC at different magnifications showing layered morphology with voids in between, which could promote access for electrolyte ions into deeper regions of TFC, thereby improving EDL formation [51]. Figure 3. (a) and (b) FESEM images depicting surface morphology of TFC http://dx.doi.org/10.5599/jese.1381 J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 CHAR OF Tagetes erecta FLOWER AS SUPERCAPACITORS 792 Figure 4 (a) illustrates the TGA curve of TFC in which a weight loss of around 6 % was observed when heated up to 100 °C, which could be associated with water [52]. TFC lost 65 % of its weight when heated from 100 to 800 °C, which could be assigned to the degradation of lignin structures and remo- val of volatiles [53]. Figure 4 (b) illustrates the water contact angle of TFC with a contact angle of 52°, determining its hydrophilic property, which could improve the formation of EDL [54]. This hydrophilic nature of TFC could be attributed to the presence of -OH groups, as confirmed from FTIR data [45]. Figure 5(a) depicts the N2 adsorption-desorption isotherm of TFC and the Brunauer–Emmett–Teller (BET) surface area of TFC was calculated to be around 17 m2 g-1. Figure 5(b) represents pore size distribution in TFC, where dV₀ and D represent differential pore volume and pore diameter, respectively, while the average pore diameter was found to be around 2.8 nm. Figure 4. (a) TGA curve and (b) contact angle measurement of TFC Figure 5. (a) N2 adsorption-desorption isotherm and (b) pore size distribution in TFC Electrochemical characterizations Full cell studies The supercapacitive behaviour of the fabricated TFC-based full-cell was tested using techniques like CV, GCD and EIS. Figure 6(a) illustrates the CV curves of TFC at different scan rates confirming superior capacitive behaviour with good reversibility and rate probability [55]. Figure 6 (b) V. N. K. S. K. Nersu et al. J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 793 represents GCD curves of TFC at diverse constant current densities. The specific capacitance (Cs), specific energy (Es) and specific power (Ps) of TFC-based symmetric SC were calculated using equations (1-3) [56]: S 2 I t C m V  =  (1) 2 S S 8 C V E  = (2) S S E P t =  (3) where I, V, t and m represent constant discharge current, discharge voltage, discharge time and mass of TFC (0.8 mg) on each electrode, respectively. Figure 6 (c) illustrates the Ragone plot of TFC. At 0.2 A g-1 constant current density, TFC exhibited exceptional Cs of 118.4 F g-1. At a Ps of 0.1 kW kg-1, TFC exhibited an Es of 4.1 Wh kg-1. Table 1 represents the experimental data obtained from GCD studies of TFC, while Table 2 illustrates the comparison of TFC’s supercapacitive performance with carbon-based materials derived from some other flowers. Table 1. Experimental data obtained from GCD of TFC in full-cell studies Current density, A g-1 IR drop, V Discharge time, s Specific capacitance, F g-1 0.2 0.028 148 118.4 0.5 0.062 55 110 1 0.108 26 104 2 0.195 12 96 5 0.56 4.5 90 Table 2. Comparison of TFC supercapacitive performance with carbon-based materials derived from flowers Precursor Activating agent Measurement Specific capacitance, F g-1 Reference Cherry blossom petals ― 2-electrode 154 at 10 mV s-1 [37] Rose flower KOH 3-electrode 208 at 0.5 A g-1 [38] Willow catkin ― 3-electrode 251 at 0.5 A g-1 [39] Willow catkin KOH 3-electrode 292 at 1 A g-1 [40] Willow catkin KOH 3-electrode 298 at 0.5 A g-1 [41] Willow catkin KOH 3-electrode 340 at 0.1 A g-1 [42] Palmyra palm flowers KOH 3-electrode 155 at 1 A g-1 [57] TF NaOH 3-electrode 200 at 2 A g-1 [45] TF ― 2-electrode 118.4 at 0.2 A g-1 This work The cyclic stability of TFC was investigated by performing 6000 GCD cycles at a constant current density of 2 A g-1. Figure 7 (a and b) represents the CV and GCD curves of TFC before and after cycling for 6000 cycles. Figure 7 (c) illustrates the cyclic stability of TFC for 6000 GCD cycles, where TFC retained 92 % of its initial capacitance. EIS studies were conducted at 0 V, using an alternating signal of 5 mV amplitude and the frequency range from 100 kHz to 0.1 Hz. Figure 7 (d) illustrates Nyquist plots of TFC before and after cycling for 6000 GCD cycles, which indicates a slight increase in cell impedance after cycling. http://dx.doi.org/10.5599/jese.1381 J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 CHAR OF Tagetes erecta FLOWER AS SUPERCAPACITORS 794 Figure 7. (a) CV curves of TFC at 50 mV s-1 and (b) GCD curves of TFC at 1 A g-1, before and after cycling; (c) cyclic stability of TFC for 6000 GCD cycles at 2 A g-1; (d) Nyquist plots of TFC before and after cycling; inset image shows the magnified image of the high frequency region of Nyquist plots Conclusions TF waste was used as a precursor to obtain TFC, a flower waste-derived carbon-based material through thermal decomposition and its physico-chemical properties were investigated. The practicality and applicability of TFC as potential electrode material in EDLCs was determined by evaluating supercapacitive behaviour of fabricated TFC-based SC using techniques like CV, GCD and EIS. At 0.2 A g-1 constant current density, TFC exhibited an exceptional specific capacitance of 118.4 F g-1. At a Ps of 0.1 kW kg-1, TFC exhibited an Es of 4.1 Wh kg-1. TFC showed superior cyclic stability by preserving 92 % of its original capacitance in spite of 6000 GCD cycles. The layered morphology of TFC and presence of voids, as confirmed from FESEM analysis, promote efficient access of electrolyte ions into deeper regions of TFC, thereby enhancing EDL formation. Water contact angle measurements of TFC confirmed its hydrophilic nature, which could be attributed to the presence of -OH groups in TFC as confirmed from FTIR analysis. This hydrophilic nature of TFC could have contributed to the superior supercapacitive behaviour of TFC, establishing it as a potential flower- waste derived carbon-based electrode material for SCs. Acknowledgements: The authors are grateful to Er. Koneru Satyanarayana Garu, Hon’ble President, Koneru Lakshmaiah Education Foundation (Deemed to be University) for providing infrastructure to carry out this work. The authors thank Prof. Y. Anjaneyulu, Director, CAES, KLEF and V. N. K. S. K. Nersu et al. J. Electrochem. Sci. Eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 795 Dr. K. 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This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1381 https://doi.org/10.1016/j.ijhydene.2021.09.094 https://doi.org/10.1115/1.4051143 https://doi.org/10.1007/s11581-021-04144-4 https://doi.org/10.5599/jese.1031 https://doi.org/10.1590/S1517-70762007000300009 https://doi.org/10.3390/molecules25010096 https://doi.org/10.1016/j.jclepro.2020.120822 https://doi.org/10.1016/j.wasman.2018.04.041 https://doi.org/10.1016/j.ecoleng.2015.01.011 https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.134 https://doi.org/10.1039/C7CS00505A https://doi.org/10.1039/C3TA15046A https://doi.org/10.5599/jese.1314 https://creativecommons.org/licenses/by/4.0/) @Article{Nersu2022, author = {Nersu, Venkata Naga Kanaka Suresh Kumar and Annepu, Bhujanga Rao and Rajaputra, Subhakaran Singh and Patcha, Satya Srinivasa Babu}, journal = {Journal of Electrochemical Science and Engineering}, title = {{Char of Tagetes erecta (African marigold) flower as a potential electrode material for supercapacitors:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {787--797}, volume = {12}, abstract = {A char of Tagetes erecta flowers (TFC) was derived through simple thermal decompo­sition of Tagetes erecta flowers (TF). Physico-chemical properties of as-prepared TFC were evaluated using XRD, FESEM, FTIR, TGA, N2 adsorption-desorption isotherm analysis and water contact angle measurements. The practicality and applicability of TFC as promising electrode material in supercapacitors (SCs) were evaluated in full-cell configuration by performing electrochemical characterizations like CV, GCD and EIS on a lab-scale TFC-based symmetric SC. TFC exhibited a remarkable specific capacitance of 118.4 F g-1 at a constant current density of 0.2 A g-1 and a specific energy of 4.1 Wh kg-1 at specific power of 0.1 kW kg-1. TFC showed excellent cyclic stability by retaining 92 % of its initial capacitance even after 6000 GCD cycles at 2 A g-1. The superior capacitive behaviour and cyclic stability of TFC could be attributed to its good wettability towards water. This excellent supercapacitive performance of TFC estab­lishes it as a potential floral waste-derived carbon-based electrode material for SCs.}, doi = {10.5599/JESE.1381}, file = {:14_jESE_1381.pdf:PDF}, keywords = {Biochar, carbon cloth, electric double layer capacitor (EDLC), flower waste, gel polymer electrolyte, hydrophilicity}, publisher = {International Association of Physical Chemists (IAPC)}, url = {https://pub.iapchem.org/ojs/index.php/JESE/article/view/1381}, }