Electrochemical characterization of catalytic activities of manganese oxides to oxygen reduction in alkaline aqueous solution

The catalytic function and activity of manganese oxides (MnOx: Mn2O3,Mn3O4,Mn5O8, and MnOOH) to the electrochemical reduction of O-2 in 0.10 M KOH aqueous solution have been investigated by cyclic voltammetry at MnOx/Nafion-modified gold electrodes. Two successive reduction current peaks were observed at Nafion-modified electrodes in the cyclic voltammograms, i(p,1) for a two-electron reduction of O-2 to hydrogen peroxide (HO2-) and i(p,2) for a two-electron reduction of HO2- to OH-. The peak current heights of i(p,1) and i(p,2) changed greatly depending on the kind of MnOx species incorporated into the MnOx/Nafion-modified gold electrodes; i(p,1) increased and i(p,2) decreased. On the assumption that HO2- produced in the first reduction step is chemically decomposed into O-2 and OH- with a catalytic action of MnOx and that this regenerated O-2 is reduced again in the same first reduction step, we evaluated the catalytic activity of MnOx using the values of i(p,1) and i(p,2). MnOOH provided the highest catalytic activity to the electrochemical reduction of O-2. This result was supported by another experiment by using a chemical method where catalytic decomposition of HO2- with MnOx was estimated by measuring the O-2 concentration directly with a commercial oxygen sensor. (C) 2002 The Electrochemical Society.