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Study On The Hybrid Supercapacitors With High Performance

Posted on:2011-12-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q T QuFull Text:PDF
GTID:1102360305997027Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
According to charge storage mechanisms, supercapacitors can be divided into three types, i.e., electrochemical double layer capacitors (EDLCs), faradic pseudo-capacitors, and hybrid supercapacitors. EDLCs store energy through the charge separation at the electrode/electrolyte interface, and they show very high power density and excellent cycling performance. Pseudocapacitors characterized by the presence of faradic current, store energy by using the fast chemical absorption/desorption or redox reactions on the electrode surface, and their theoretical specific capacitance and energy is 10-100 times higher than that of EDLC. The two electrodes used in hybrid supercapacitors store energy in different manners. One of them uses pseudocapacitors type or secondary battery type electrode materials, and the other electrode uses EDLCs type carbon materials.The power capability of aqueous electrolyte is much higher than that of organic electrolyte due to their higher ionic conductivity. In the case of energy density, it can be improved by increasing the working voltage (U) and specific capacitance (C). The working voltage of organic electrolyte can be up to 3V. The theoretical working voltage of aqueous electrolyte is 1.23V, which can be widened to 2V by increasing the overpotentials of H2 and O2 evolution. As for the specific capacitance, the capacitor-type electrode materials usually shows higher capacitance values in aqueous electrolyte than in organic electrolyte. Based on the above consideration, the energy density of aqueous electrolyte may exceed organic electrolyte. Moreover, aqueous electrolytes are cheaper, safer, and more environmentally friendly than organic electrolytes. Therefore, aqueous electrolyte seems to be more suitable as electrolyte for hybrid supercapacitors.Since the strong acidic and alkaline aqueous electrolytes pose environmental pollution, and the capacitors based on them still have the problems of high-price, precipitation of materials on the electrode, electrolyte consumption, poor electrode stability, low energy density, and poor cycling life, etc., here we focus on the investigation of neutral aqueous electrolytes based hybrid supercapacitors.The aim of this dissertation is to improve the power density, energy density and cycling performances of hybrid supercapacitors. On the one hand, nano-scaled faradic electrode materials with high specific surface area and structural stability were prepared to improve the power density, energy density and cycling performances of hybrid supercapacitors. On the other hand, the influence of various neutral aqueous electrolytes on the electrochemical performances of electrode materials was investigated, from which the optimal electrolyte was selected and the reaction mechanism of electrode material was analyzed.(1) The electrochemical performances of activated carbon in 0.5 mol/L Li2SO4, Na2SO4, and K2SO4 aqueous electrolytes were investigated. The equivalent series resistance (ESR) obtained from Nyquist plots decreases in the order of Li2SO4> Na2SO4>K2SO4, signifying that the migration speeds of the hydrated ions in the bulk electrolyte and within the inner pores of AC electrode increase in the order of Li+< Na+
Keywords/Search Tags:Hybrid supercapacitors, Electrochemical double layer capacitors, Faradic pseudo-capacitors, Aqueous electrolyte, MnO2 nanorods, Porous LiMn2O4, V2O5·6H2O, Activated carbon, Cyclic voltammetry, AC impedance, Charge-discharge, Energy density
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