| Summary: | 博士 === 國立成功大學 === 化學工程學系 === 89 === This dissertation can be qualitatively divided into three parts, influence of physical surface area, surface oxides, and metal oxides in microporous carbons in the adsorption and capacitive behaviors at solid/liquid interfaces. Liquid-phase adsorption behaviors on microporous carbon with different activation levels were investigated in the first part. In the second part, carbon fabrics with different oxidation level were served as electrodes to explore the influence of carbon-oxygen complexes on the performance of capacitors. The electrochemical behaviors of nickel/carbon composite electrodes prepared for electrochemical capacitors were studied in the third part.
1. Liquid-phase adsorption on different carbons
The influence of the pore size distribution of activated carbon on the adsorption of phenol from aqueous solutions was explored. Activated carbons with different porous structures were prepared by gasifying a bituminous coal char to different extents of burn-off. The results of adsorption experiments show that the phenol capacity of these carbons does not proportionally increase with their BET surface area. This reflects the heterogeneity of the carbon surface for adsorption. The pore size distributions of these carbons, determined according to the Dubinin-Stoeckli equation, were found to vary with the burn-off level. By incorporating the distribution with the Dubinin-Radushkevich equation using an inverse proportionality between the micropore size and the adsorption energy, the isotherms for the adsorption of phenol onto these carbons can be well predicted. The present study has demonstrated that the heterogeneity of carbon surface for the phenol adsorption can be attributed to the different energies required for adsorption in different-size micropores.
2. Influence of surface oxides on electrochemical energy storage
Oxygen treatment at 250 °C on PAN-based activated carbon fabric was conducted to explore the influence of carbon-oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H2SO4, using potential sweep cyclic voltammetry and constant current charge-discharge cycling. It was found that the faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g. from 120 to 150 F g-1 at a current density of 0.5 mA cm-2). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition.
3. Influence of nickel oxides on electrochemical energy storage
Nickel oxides/carbon composite electrodes were prepared by precipitating nickel oxides in carbon and heating the electrodes in N2 at 350 °C, and the capacitors fabricated with the treated carbon electrodes was used to study the influence of NiOX deposited on the performance of capacitors. XPS analysis showed that type of nickel oxides deposited from chemical impregnation were NiO and Ni(OH)2. The performance of the capacitors was tested in 6 M KOH, using potential sweep cyclic voltammetry and constant current charge-discharge cycling. The capacitance enhancement of the capacitors showed an increase up to more than 200 % (e.g. from 116 to 237 F g-1 at a current density of 0.5 mA cm-2) due to pseudocapacitive effect. Comparing to the original electrode, a decrease inner resistance of the composite electrodes was suggested that the redox reaction of nickel oxides occur in outer surface of carbon electrodes. The increase faradaic leakage current of the electrode was found due to the electrolyte decomposition caused by electrocatalytic effect of nickel oxides. The capacitors prepared in the present study exhibit an excellent coulombic efficiency and stable capacitance over 400 cycles.
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