Bifunctional catalytic activity of manganese oxide toward O2 reduction:: novel insight into the mechanism of alkaline air electrode

The mechanism of electro-catalytic reduction of oxygen (O-2) with manganese oxide was studied by cyclic voltammetry and rotating ring-disc electrode voltammetry using MnOOH-modified glassy carbon (GC) electrodes in alkaline media. The O-2 reduction at the MnOOH-modified GC electrodes was found to undergo an electrochemical process followed by sequent disproportionations of the electrochemical reduction intermediates, i.e., superoxide anion (O-2(circle-)) and hydrogen peroxide (HO2-, in 0.10 M KOH solution). The manganese oxides, which are used as the electrocatalysts in the alkaline air electrodes, were first found to possess an excellent catalytic activity not only for the disproportionation of the produced O-2(circle-) into O-2 and HO2- but also for that of the produced HO2- into O-2 and OH-. Such bifunctional catalytic activities of MnOOH for the sequent disproportionations of O-2(circle-) and HO2-, combined with electrochemical reduction of O-2 mediated by surface functional groups at carbon electrode surface, substantially effect a quasi-four-electron reduction of O-2 at the MnOOH-based alkaline air electrodes. (C) 2004 Elsevier B.V. All rights reserved.