Electrodeposition of manganese and molybdenum mixed oxide thin films and their charge storage properties

Manganese and molybdenum mixed oxides in a thin film form were deposited anodically on a platinum substrate by cycling the electrode potential between 0 and +1.0 V vs Ag/AgCl in aqueous manganese(II) solutions containing molybdate anion (MoO42-). A possible mechanism for the film formation is as follows. First, electrooxidation of Mn2+ ions with H2O Yields Mn oxide and protons. Then, the protons being accumulated near the electrode surface react with MoO42- to form polyoxomolybdate through a dehydrated condensation reaction (by protonation and dehydration). The condensed product coprecipitates with the Mn oxide. Cyclic voltammetry of the Mn/Mo oxide film-coated electrode in aqueous 0.5 M Na2SO4 solution exhibited a pseudocapacitive behavior with higher capacitance and better rate capability than that of the pure Mn oxide prepared similarly, most likely as a result of an increase in electrical conductivity of the film. Electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy clearly demonstrated that the observed pseudocapacitive behavior results from reversible extraction/insertion of hydrated protons to balance the charge upon oxidation/reduction of Mn3+/Mn4+ in the film.