Effects of amount of hydrates and valence states of manganese on charge transfer reaction coupled with cation diffusion in manganese oxide electrode

Abstract

The present work is concerned with charge transfer reaction coupled with diffusion reaction in the electrodeposited manganese oxide electrodes by using cyclic voltammetry, ac-impedance spectroscopy and current transient technique. The amorphous manganese oxide electrodes were potentiostatically electrodeposited on the Pt current collector at 1.1 V (vs. SCE) with the total passed charge of 0.5 C in a 0.25 M $MnSO_4ㆍ5H_2O$ solution. Here, in order to control the amount of water and the valence state of manganese in the electrode, the manganese oxide electrodes were heat-treated at various temperatures 25˚ to 400℃. The cyclic voltammogram experimentally measured on the as-deposited manganese oxide electrode exhibited a rectangular-shaped profile resulting from the overlap of several redox processes with the potential scanning. For the ac-impedance spectra experimentally measured on the manganese oxide electrodes heat-treated at temperatures 25˚ to 150℃, the Warburg impedance which is associated with a semi-infinite diffusion of cations or protons into the film electrode decreased in value with increasing heat-treatment temperature. In addition, from the analyses of the anodic current transient, it was found that the transition time ttr, i.e. the time to the transition of semi-infinite diffusion to finite diffusion decreased with decreasing amount of water in the electrodes. From the results, it was recognised that the diffusion depth of cations or protons into the electrode decreases with decreasing amount of water, i.e. with decreasing proton-oxygen sites. For the manganese oxide electrodes heat-treated above 200℃, the contribution of the valence state of manganese becomes more dominant rather than the contribution of the amount of water to the electrochemical performance of the manganese oxide electrodes. The ac-impedance spectra measured on the manganese oxide electrodes heat-treated at temperatures 200° to 400℃ did not exhibit the Warburg impedance at in...