Studies Of Supercapacitors Based On Vanadium Oxide Materials

Supercapacitors developed in1980s, which have been recognized as unique energy storage device. As the continually development of electronic device and electric vehicle, supercapacitors are referred to as green energy source. Many people, both at home and abroad devoted themselves to research. Studies on supercapacitors mainly focus on electrode materials, including carbon, metal oxide and conducting polymer. Based on the difference of energy storage mechanism, supercapacitors can be classified as electrochemical double–layer capacitors (EDLs) and faradaic capacitors. EDLs utilize the capacitance arising from charge separation at an electrode/electrolyte interface. Faradaic capacitors utilize the charge-transfer pseudocapacitance arising from reversible Faradaic reactions occurring at the electrode surface. Metal oxide materials are employed as electrodes for Faradaic capacitors, a typical representative of metal oxides is hydrated RuO2, which can yield a capacitance as high as 768 F·g-1 in H2SO4 electrolyte for a single electrode. However, it has a relatively high cost and this will prevent broader commercial applications; Besides, if RuO2 contacts with H2SO4 electrolyte for a long time, it may dissolve at a certain extent; at the same time, H2SO4 electrolyte can erode current collector, which cause entironment pollution. Hence, many efforts are made to replace ruthenium oxide with other low cost metal oxides or metal hydroxides as electrodes in supercapacitors or reduce the amount of ruthenium in the fabrication of supercapacitors.By integrating various electrochemical and material methods, the electric chemical performances of supercapacitors based on vanadium oxide were studied detailedly in our work. The thesis consists of five chapters. In the first chapter, the current survey of supercapacitors was summarized; the working principle, classification, status of electrode materials and eletrolyte, electrochemical testing principle were introduced . Meantime, we brought forward the mean of the thesis and some problems needed to be solved.In the second chapter, V2O5 sample was prepared by quenching method. The sample was characterized by FTIR, X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). The test results show that the sample is amorphous layer V2O5. The capacitive performances of V2O5 were measured by cyclic voltammetry (CV) and constant current charge/discharge method. Electrochemical reaction mechanism was also discussed. Electrochemical results indicated that the kinds of the aqueous electrolyte, potential limit, scan rate, current density had influence on V2O5 capacitive performance. In the range of -0.2~0.8V (vs. SCE), at 250 mA·g-1 constant current density, amorphous V2O5 exhibits a specific capacitance values of 185.1 F·g-1 with excellent cyclability in 1mol·L-1 NaNO3 electrolyte.In the third chapter, composite composed of V2O5 and PANI, which are designated in this paper as V2O5/PANI, was prepared by adding an appropriate amount of PANI doping with PTSA into the V2O5 sol diluted by acetone and water. The sample was characterized by X-ray diffraction (XRD), FTIR and Scanning Electron Microscope (SEM). The results show that PANI and V2O5 form equably amorphous composite. The capacitive performances of composite materials were measured by cyclic voltammetry (CV) and constant current charge/discharge method. Electrochemical results indicated that V2O5/PANI composite materials exhibit a specific capacitance values of 246.9 F·g-1, increasing 34% compared with V2O5 freed PANI (185.1F.g-1), with excellent cyclability in 1mol·L-1 NaNO3 electrolyte in the range of -0.2~0.8V (vs. SCE), at 250 mA·g-1 constant current density. In the fouth chapter, the sheet V6O13 sample for supercapacitors was prepared by a thermal decomposing and quenching method with NH4VO3 as starting material.The sample was characterized by means of TG-DSC, XRD, XPS and SEM. The results indicated that the prepared V6O13 product was being in the form of a sheet with average size of 2μm and thickness of 200 nm. Its electrochemical properties were determined by cyclic voltammetry and charge–discharge tests. The obtained results showed that potential range, scan rate and current density have great effects on capacitive properties of sheet V6O13. In the voltage range of -0.2~0.8V (vs. SCE) in 1 mol·L-1 NaNO3 electrolyte, V6O13 electrode exhibited obvious psudocapacitance performance. At a current density of 50 mA·g-1, the material delivered a specific capacitance value of 282F·g-1. At a current of 200 mA·g-1, the electrode exhibited an initial specific capacitance of 215 F·g-1 and 208 F·g-1 after 500 cycles. The electrochemical results for V6O13 showed that it could be considered as a kind of ideal electrode material for supercapacitor due to its good electrochemical cyclability.In the fifth chapter, conlusion and expectation for the whole content were summarized, and writer's some opinions for future research were also brought forward.