| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === Effects of iron additions on the material and electrochemical properties of manganese oxides were investigated in this study. The Mn-Fe binary oxide electrodes were prepared by anodic deposition on graphite substrates. The deposition solution was a mixture of FeCl3 with manganese acetate aqueous electrolyte. It was found that Fe content in the binary oxide can be easily controlled by adjusting the composition of the plating solution. Crystal structure and surface morphology of the deposited oxides were examined by XRD and SEM, while their chemical state was analyzed by X-ray photoelectron spectroscopy and X-ray absorption near edge structure. Pseudo-capacitive performance of various oxide electrodes were determined by cyclic voltammetry (CV) in 2M KCl aqueous solution. SEM observation clearly showed that Fe dopings caused changes of surface morphology of the oxide electrodes. The data indicated that the doped Fe in proper concentration significantly increased specific capacitance of the oxide electrode. On the other hand, cyclic charge/discharge stability of the manganese oxide electrode was improved obviously by adding Fe.
Tailoring the material characteristics and thus the electrochemical performance of the oxide was attempted by annealing (up to 700?C in air). The 100?C-annealed oxide, evaluated by cyclic voltammetry at a potential sweep rate of 5 mVs-1, showed an optimum specific capacitance of 280 Fg-1. Cyclic stability of the oxide electrode can also be improved by post-heat treatment. However, the binary oxide loses its pseudocapacitive capability at the annealing temperature of 500?C, at which point the formation of crystalline (Mn–Fe)2O3 occurs.
In order to explore the electron storage mechanism, the deposited oxides were studied by in situ X-ray absorption spectroscopy (XAS) in 2 M KCl solution during the charging–discharging process. The experimental results clearly confirmed that the oxidation states of both Mn and Fe changed forth and back with adjusting the applied potential, contributing to the pseudocapacitive characteristics of the binary oxides. It was also found that, within a potential range of 1 V, Fe oxide addition would increase the variation in Mn oxidation state from 0.70 to 0.81, while Fe oxide itself demonstrated an oxidation state shift of only 0.55. Accordingly, an optimum pseudocapacitance of the binary Mn–Fe oxide could be only achieved as the amount of Fe oxide was properly controlled.
Developing Mn oxide supercapacitors incorporating protic and aprotic ionic liquid (IL) electrolytes was attempted. The experimental results indicate a possibility of achieving pseudocapacitive performance without involving protons and alkali cations, and thus open a new route of developing novel electrolytes for various metal oxide based pseudocapacitors. Significant enhancement in the electrochemical stability of Mn oxide was confirmed in EMI-DCA or BMP-DCA IL, when compared to aqueous electrolytes. Moreover, the analytical results indicate that Mn3+/Mn4+ redox transition during the charge-discharge process was charge compensated by the reversible insertion/desertion reaction of DCA- anion into/from the tunnels between the MnO6 octahedral units. The EMI+ or BMP+ cations were just adsorbed on the electrode surface and did not penetrate into the oxide.
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