Pseudocapacitive Characteristics of Vanadyl hydroxide for energy storage

An adequate alternative for fossil fuels are batteries, fuel cells and supercapacitors, which is placed in the category of electrochemical energy storage (ESS), and powering the electrical grid by the linking of these technologies with renewable energy sources such as solar and wind [1, 2]. One of the difference between electrochemical energy systems is in power and energy density, so that fuel cells have higher energy density and supercapacitors have higher power density [2]. The combination of electrochemical energy systems like fuel cells/supercapacitors or batteries/supercapacitors can achieve high power and high energy together [2]. This property can be seen in pseudocapacitance with faradaic mechanism as like as batteries and deliver energy faster than batteries, will have potential to both high energy and power density in the same material [2]. With attention to pseudocapacitance mechanism, three mechanisms have been proposed: Underpotential deposition, Redox pseudocapacitance and Intercalation pseudocapacitance. In this work, we formed a thin film of VO(OH)2 on platinum working electrode by the cathodic deposition method which released hydroxides from the surface of working electrode, react with VO2+ cations contained in the vanadyl sulfate solution and produce brownish yellow vanadium hydroxide colloidal particles that stick to the surface of the electrode. Electrochemical tests of this work was carried out by using vanadyl hydroxide working electrode prepared by the above method, Ag/AgCl as reference electrode and platinum auxiliary electrode in 0.5 M sodium sulfate solution. 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 161 Cyclic voltammetry in different scan rates from 2 to 500 mV.s-1 and different pH (2 to 12) accomplished in range of -0.8 to 1.1 V to investigate pseudocapacitive mechanism. Then Galvanic charge-discharge test fulfilled in 5, 10, 20, 50 and 100 μA to compare with each other and estimate the specific capacitance as shown in figure below. According to figure, with decreasing the current density and increasing time of each charge or discharge cycle the specific capacitance increase. Also, in each charge or discharge cycle, two redox reactions are observed that related to the change of different oxidation numbers of vanadium species. Besides, the device exhibited a good cycling stability with capacitance retention of 87.8% after 1000 charging-discharging cycles.