Cathodic reduction of aqueous MnO 4− ions in the presence of various alkaline metals except for Na + ions led to the deposition of birnessite-type layered MnO 2 . The highest crystallinity was obtained when electrolyzed in a 2 mM KMnO 4 and 50 mM KCl at a constant potential of 0 V vs Ag/AgCl. The cathodic formation of MnO 2 was prevented by the presence of divalent cations, and Na + ions gave rise to an amorphous or low crystalline product. The birnessite film thus formed exhibited an excellent pseudocapacitive behavior in the as-deposited state, with a specific capacitance as high as 322 F g −1 at a scan rate of 2 mV s −1 , which is much larger than that (75 F g −1 ) of the birnessite film grown anodically. The resulting pseudocapacitive electrode functioned as an efficient catalyst toward the oxidation of L-cysteine, where the anodic overpotential was reduced by 0.15 to 0.3 V.

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We have fabricated composite electrode materials consisting of manganese-molybdenum mixed oxide and carbon nanotubes (CNTs) for supercapacitor application. The process involves electrophoretic deposition (EPD) of multi-walled CNTs onto an ITO electrode, followed by anodic deposition of Mn-Mo oxide from an aqueous Mn(II) solution containing MoO4 2− anions. Electrochemical properties of the composite electrodes with various combinations of Mn-Mo oxide and CNTs were evaluated systematically with the aim of maximizing the electrochemical utilization of the oxide. Optimization led to a high specific capacitance of 408 F g−1 at a scan rate of 2 mV s−1, with an area-normalized capacitance of 31 mF cm−2 (or a volume-normalized capacitance of 76 F cm−3).