Cathodic Synthesis of Birnessite-Type Layered Manganese Oxides for Electrocapacitive Catalysis

Abstract

Cathodic reduction of aqueous MnO4− ions in the presence of various alkaline metals except for Na+ ions led to the deposition of birnessite-type layered MnO2. The highest crystallinity was obtained when electrolyzed in a 2 mM KMnO4 and 50 mM KCl at a constant potential of 0 V vs Ag/AgCl. The cathodic formation of MnO2 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.