| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 92 === Composite electrodes which comprise a non-conductive activated carbon of large surface area (1436 m2/g) and a conductive carbon black (CB) of small surface area (220 m2/g) have been prepared and studied for their capacitive properties in aqueous KOH and Na2SO4 electrolytes. For either electrolyte, maximum capacitance exists at the composition believed to correspond to the percolation threshold for CB, the conductive phase. At a CB content less than the threshold, the capacitance is limited mainly by the electronic resistance on the electrode side. The interfacial surface area becomes the limiting factor as the threshold is exceeded. A maximum capacitance of 108 F/g at a voltage sweep rate of 20 mV/s is obtained in 1 M KOH aqueous electrolyte with a CB content of 25 wt.% (or ~ 14 vol.%).
Magnetite (Fe3O4) supercapacitor contained 10 wt.% CB as conductive additive (≧ percolation threshold), operating characteristics in aqueous electrolytes of Na2SO3, KOH and Na2SO4 were investigated. While the capacitance of the oxide was found to depend heavily on electrolyte composition, the self-discharge mechanism in these electrolytes appeared to be the same. Reduction in dissolved oxygen content (DOC) of electrolyte reduced leakage current and profoundly improved the cycling stability. In particular, Na2SO3(aq) gives the highest capacitance, nearly 30 F/g-Fe3O4 or 80 �媹/cm2 of actual surface area, with an operating range of 1.1 V and the electrode showed no deterioration after 104 cycles under a DOC < 0.1 ppm.
Since the surface Fe3O4 particles will easily raise the reversible redox below the low sweep rates (≦ 20 mV/s), the specific capacitances of coprecipitated electrode increased as magnetite loading content. The contributive specific capacitances of magnetite increased as decreasing magnetite content because of the dispersion of magnetite. The largest specific capacitances of coprecipitated electrode and magnetite were 42.4 F/g (66.4 wt.%) and 375 F/g-Fe3O4 (5.6 wt.%), respectively. We found that the capacitive behavior of magnetite with the optimal Ni-doped content would be larger than that without Ni-doped magnetite at the lower sweep rate.
The specific capacitance of thin film magnetite possess 130 F/g in 1 M Na2SO3(aq) which is due to higher conductivity by electroplating process. In order to explore the pseudocapacitive mechanism of magnetite in Na2SO3(aq), combined use of CV and EQCM affords a route to obtain the reversible oxdation and reduction reaction. We found that SO32- ions adsorbed on Fe atom of Fe3O4 surface structure, however, SO32- ions transformed into S2- ion (deoxidization) and adsorbed yet on Fe atom during the reduction process. During the oxidation process, S2- ions were returned to original SO32- ion on Fe atom site. On the other hand, the structure of Fe3O4 surface maybe has been transferred to another Fe2O3 phase during the oxidation or reduction process. The reaction of SO32- ions or phase transformations of Fe3O4 both take place on Fe3O4 surface structure and there was a consequent reversible pseudocapacitor.
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