Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Electrochemical reduction of oxygen (O-2) in air electrode with manganese oxides (MnOx) as electrocatalysts was studied with MnOx/Nafion-modified gold (Au) electrodes using cyclic voltammetry, potential-controlled amperometry and rotating ring-disk electrode (RRDE) voltammetry in alkaline aqueous solution. At Nafion-modified (MnOx free) An electrode, O-2 reduction undergoes two successive two-electron processes with HO2- as intermediate. The presence of MnOx, including Mn2O3, Mn3O4, Mn5O8 and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O-2 to hydrogen peroxide (HO2- in this case) and decreases the second one of HO2- to OH-, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO2- to O-2 and OH- and thus, the O-2 reduction in air electrode was considered to include an initial two-electron reduction of O-2 to HO2- followed by a disproportionation reaction of HO2- into O-2 and OH- catalyzed by MnOx. The excellent activity of MnOx for the follow-up disproportionation reaction substantially results in an overall four-electron reduction of O-2 at MnOx/Nafion-modified Au electrodes in the first reduction step, depending on potential scan rate and the kind of MnOx. The present work provides a scientific significance of the mechanism of O-2 reduction in air electrode using MnOx as electrocatalysts to effect a four-electron reduction of O-2 to OH-. (C) 2002 Elsevier Science Ltd. All rights reserved.