Cathodic Synthesis of Birnessite Films for Pseudocapacitor Application

Cathodic reduction of an aqueous solution of potassium permanganate in the presence of various alkaline metals except for Na+ ions led to a deposition of birnessite-type layered MnO2. The highest crystallinity was achieved in a bath containing 2 mM KMnO4 and 50 mM KCl by applying a constant potential of 0 V vs Ag/AgCl. The cathodic formation of birnessite was prevented by the presence of divalent cations, and Na+ ions yielded an amorphous or low crystalline product. The birnessite film thus formed showed excellent pseudocapacitive characteristics in its as-deposited state; i.e., a specific capacitance as high as 322 F g(-1) at a potential scan rate of 2 mV s(-1), which is much higher than that (75 F g(-1)) of the birnessite film prepared anodically from Mn2+ ions.