Synthesis and Characterization of Oxide Supercapacitors
Date Issued
2006
Date
2006
Author(s)
Kuo, Shin-Liang
DOI
en-US
Abstract
In the first part of the thesis, RuO2-SnO2 composite supercapacitors were synthesized via both the impregnation and cyclic voltammetric deposition. The RuO2-impregnated SnO2 xerogel was optimized for its electrochemical capacitance in aqueous 1 M KOH electrolyte by adjusting the calcination temperature and the RuO2 loading. A specific RuO2 capacitance of 710 F/g-RuO2, is obtained with a RuO2 loading of 1.4 wt. % and by calcination at 200 oC. Higher loadings presumably result in a homogeneous nucleation, causing severe reduction in the total surface area of the RuO2 crystallites. On the other hand, after the optimization of crystallization protocol (150 oC), the electroplated RuO2-SnO2 composite exhibited a specific RuO2 capacitance of 930 F/g-RuO2 in 1 M H2SO4 electrolyte and an overall specific energy of ~ 0.5 Wh/Kg at a specific power>1.5 kW/kg. Comparative studies demonstrated that this composite electrode exhibited a far superior performance than the electrodes having RuO2 similarly plated onto either smooth Ti or porous conductive carbon black.
In addition, ferrites including MFe2O4 where M = Mn, Fe, Co, or Ni have been synthesized by coprecipitation methods and tested for their capacitive behaviors in aqueous NaCl solution. MnFe2O4 has been found to exhibit pseudocapacitance, while the other ferrites do not. The results indicated the pseudocapacitance was observed only for crystalline, rather than amorphous, MnFe2O4 phase. The MnFe2O4/CB composite showed pseudocapacitance in solutions of chloride, sulfate and sulfite salts of alkali and alkaline cations, with NaCl solution giving the highest capacitance. It has exhibited specific MnFe2O4 contributed capacitances of >100 F/g-MnFe2O4 and high-power delivering capabilities of >10 kW/kg. For the chloride electrolytes, the pseudocapacitance has been identified, by in-situ X-ray absorption near edge spectroscopy study, to involve charge transfer at both the Mn and Fe sites, predominantly at the Mn ions at the tetrahedral sites of the spinel, balanced by insertion of cations from the electrolyte and protonation process. The composite electrode exhibits an operating potential window of 1.0 V with a maximum leakage current of 10 mA/g, and it exhibits superior cycling stability and reduced self-discharge rate than amorphous MnO2. The specific capacitance of the composite is a strong function of the CB content and the optimum capacitance occurs with the ferrite:CB weight ratio of 7:3.
Besides, pseudocapacitive charge-storage reaction of MnO2·nH2O in several aqueous alkali and alkaline salts solutions, including LiCl, NaCl, KCl, CsCl and CaCl2, has been studied on fine-grained MnO2·nH2O thin-films and particles, which possess the e-MnO2-type crystal structure. In-situ synchrotron X-ray diffraction analysis shows that charge-transfer at Mn sites upon reduction/oxidation of MnO2·nH2O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charging/discharging cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that H3O+ plays the predominant (> 60%) role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as:
Mn(IV)O2-nH2O +δe- + δ(1-f)H3O+ δf M+-->(H3O)δ(1-f)Mδf[Mn(III)δMn(IV)1-δ]O2·nH2O,where M+ is alkali cation.
Subjects
偽電容
機制
MnFe2O4
MnO2
supercapacitor
pseudocapacitance
mechanism
Type
thesis
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