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

Abstract

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) Au electrode, O 2 reduction undergoes two successive two-electron processes with HO 2 − as intermediate. The presence of MnOx, including Mn 2O 3, Mn 3O 4, Mn 5O 8 and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O 2 to hydrogen peroxide (HO 2 − in this case) and decreases the second one of HO 2 − to OH −, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO 2 − 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 HO 2 − followed by a disproportionation reaction of HO 2 − 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 −.