Hydrothermal Synthesis of CoWO4 as Active Material for Supercapacitor Electrode

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1 , pp Hydrothermal Synthesis of CoWO4 as Active Material for Supercapacitor Electrode Vaibhav Lokhande 1, Taeksoo Ji 1 * 1 Department of Electronics and Computer Engineering, Chonnam National University, Korea Abstract. Cobalt tungstate was synthesized by facile one step hydrothermal process and deposited on the carbon cloth to obtain a binder free electrode for supercapacitor. The as prepared electrode exhibited specific capacitance of 1372 F/g at 5 mv scan rate and stability over 95% after 4000 cycles. Keywords: Cobalt Tungstate, hydrothermal, supercapacitor, capacitance, stability. 1 Introduction Over the past decades, much of the research has been directed towards the development of energy storage system to satisfy the increasing energy demands. Owing to the fast chargingdischarging capability, high power density, higher life span than batteries and low cost, supercapacitors have received much attention in recent years. Supercapacitors can be categorized in two types namely EDLC (electrochemical double layer capacitors) and pseudocapacitors. EDLC devices are mainly carbon based devices having higher power density and higher cyclic life. Pseudocapacitors are redox reactions based devices comprising of metal oxides, conducting polymers and other such materials. They boast higher energy density but the cyclic life is considerably lower. Constant efforts are being made to increase the energy density while maintaining higher power density and cyclic life. Among the many transitional metal oxides studied as an electrode material for supercapacitor, Ruthenium oxide has shown best performance only to be marred by the high cost and limited availability of Ruthenium. Many other metals are being investigated for this purpose. Recent studies are directed towards binary metal oxides such as NiCo2O4, MnMoO4 etc. due to their low cost and abundant availability. The pseudocapacitive performance of these materials has been quiet encouraging. Cobalt tungstate is a similar binary metal oxide. A p-type semiconductor material having wide application in catalysis, magnetism and sensing. CoWO4 has good electrochemical performance but has been scarcely reported. Many reports on metal oxide electrodes have used slurry coating method to fabricate the electrode. The metal oxide powder is mixed with the binder and then pasted onto the current collector. This method has inherent problems due to the use of binder and non-contact of active material with the current collector. To improve this, binder free electrode has been fabricated by directly depositing the active material on the current collector. This minimizes the resistance due to the binder material and enables effective utilization of the active material. Chen et.al have reported specific capacitance of F/g at 1 A/g for cobalt tungstate [1]. Adib et.al have synthesized cobalt tungstate by chemical precipitation reaction. It exhibited specific capacitance of 378 F/g at 2mV scan rate [2]. ISSN: ASTL Copyright 2016 SERSC

2 In this report, Cobalt tungstate has been prepared and deposited on carbon cloth by facile one step hydrothermal process. The as prepared material showed better electrochemical performance than the previous reports. The material exhibited specific capacitance of 1372 F/g at 5mV scan rate and stability of 96% over 4000 cycles. On the basis of the findings, cobalt tungstate can be a good candidate as an active material for supercapacitor electrode. 2 Experimental Method The chemicals used in the experiment were obtained from Sigma Aldrich and were used as received without purification. 2.1 Synthesis and Deposition on Carbon cloth. Carbon cloth was cut into 2 cm* 2 cm sized piece. It was then dipped for several minutes in 6 M HCl to remove any impurities. In a typical synthesis, 5 mmol of K2WO4 and 5mmol of Co(NO3)2 were dissolved in 50 ml DI water. The solution was magnetically stirred for about 15 mins at room temperature. The solution was transferred into a 100ml Teflon-lined stainless steel autoclave. The piece of carbon cloth was dropped in the solution before sealing it and heating it in an oven for 18 h at 160 C. The autoclaves were allowed to cool down to room temperature naturally. Then carbon cloth substrate and the precipitate powder were collected and washed thoroughly several times ultrasonically with DI water. They were dried for 24 h at 60 C in an oven. 3 Characterization The X-ray diffraction analysis was done using Ultima3. The surface morphology of the thin films were characterized using field emission scanning electron microscopy (FE-SEM, Model: JSM-6700F, Japan). The electrochemical characterization was done using Wonatech WBCS3000S. The cyclic voltammetry characterization was done using three electrode system, Ag/AgCl as reference electrode, Pt as counter and 3 M KOH electrolyte was used. The potential was scanned between -0.2 V to 0.4 V at various scan rates. 4 Results and Discussion Cobalt tungstate (CoWO4) was synthesized through the reaction of Na2WO4, 2H2O and CoCl2, 6H2O. The two aqueous solutions were mixed together and subjected to hydrothermal processing to get the CoWO4 precipitate according to equation 3 Na2WO4, 2H2O 2Na + (aq) + Wo4 2- (aq) + 2H2O (1) CoCl2, 6H2O Co 2+ (aq) + 2Cl - (aq) + 6H2O (2) 410 Copyright 2016 SERSC

3 2Na + (aq) + Wo4 2- (aq) + 2H2O + Co 2+ (aq) + 2Cl - (aq) + 6H2O CoWO4 + 2Na + +2Cl - + 8H2O (3) The crystalline structure of the CoWO4 was characterized using the powder XRD patterns. Fig 1 shows the XRD pattern of the cobalt tungstate and reference XRD pattern of CoWO4 JCPDS # Fig 1. XRD pattern a b c Fig.2. SEM images a) carbon cloth coated with CoWO4.b, c) high magnification images of carbon cloth fiber. The micro structure and morphology were investigated by SEM. The figure 2 shows the SEM images of cobalt tungstate on carbon cloth. Carbon cloth was selected as the substrate because of high electrical conductivity and its ability to withstand harsh environment during hydrothermal process. The top view image of the carbon cloth figure 2a confirms that the active material has completely covered the cloth surface. Figures 2b,c show a single fiber of the cloth with higher magnification. Cobalt tungstate seems to form small spheres. Also the morphology seems to be very dense without any other significant structures being formed. Copyright 2016 SERSC 411

4 Current (A) Voltage (V) 50mV scan rate 100 mv scan rate 200 mv scan rate 20 mv scan rate 10 mv scan rate 5 mv scan rate Fig. 3. CV-graphs at different scan rates. The cyclic voltammetry studies were conducted over the potential range of -0.2 V to 0.4 V giving an effective potential window of 0.6 V. The CV graphs are shown in the fig 3. The weight of the active material was found out to be mg by recording the weight difference of the carbon cloth before and after the deposition process. The capacitance was calculated from the following formula: (4) Where, m is the mass of active material. v is the scan rate and P is the potential window. Capacitance (F/g) Capacitance (F/g) Scan Rate(mV) Fig.4. Rate stability of the sample at different scan rates. 412 Copyright 2016 SERSC

5 st cycle 4000 th cycle Current Voltage Fig.5. Cyclic stability after 4000 cycles. Fig 4 shows the rate capability of the electrode. There is a significant drop in the capacitance of the electrode at higher scan rates. The rate capability of 18% was observed at 200 mv scan rate. The cyclic stability of the electrode is quite high. Fig 5 shows the graph at cycle number 1 and The capacitance retention is about 96 % which is high for a pseudocapacitive type material. 5 Conclusion Cobalt tungstate seems to be a promising candidate for electrochemical storage devices. The binder free electrode prepared by facile one step hydrothermal process exhibited specific capacitance of 1372 F/g at 5 mv scan rate. The cyclic stability of the electrode is also above 95% after 4000 cycles. Herein we report highest capacitance for cobalt tungstate recorded till date to the best of our knowledge. References 1. Chen, S., Yang, G., Jia, Y., Zheng, H.: Facile Synthesis of CoWO4 Nanosheet Arrays Grown on Nickel Foam Substrates for Asymmetric Supercapacitors ChemElectroChem Volume 3, Issue 9 September 2016, pp Adib, K., Rahimi-Nasrabadi, M., Rezvani, Z., Pourmortazavi, S.M., Ahmadi, F., Naderi, H.R., Ganjali, M.R.: Facile chemical synthesis of cobalt tungstates nanoparticles as high performance supercapacitor, Journal of Materials Science Materials in Electronics, January Copyright 2016 SERSC 413