Influence of Inner- and Outer-Sphere Electron Transfer Mechanisms during Electrocatalysis of Oxygen Reduction in Alkaline Media

Abstract

Oxygen reduction reaction (ORR) is generally considered to be more facile in alkaline media compared to its acidic counterparts. The fundamental reasoning for this statement has been quite elusive and not understood very well. A pertinent review of the literature in alkaline media on noble and non-noble metal electrocatalysts is presented here along with experimental results to investigate the rationale behind the so-called kinetic facility in alkaline media. Increasing the pH from 0 to 14 has several effects on the electrode–electrolyte interface in terms of the working electrode potential range, the strength of adsorption of the reaction intermediates, and spectator species. Besides these, the reasons for kinetic facility are investigated from the perspective of the changes in the double layer structure and electrochemical reaction mechanisms in transitioning from acidic to alkaline environment. In this context, specifically adsorbed hydroxyl species are found to promote an outer-sphere electron transfer ORR mechanism in alkaline media. A surface independent outer-sphere electron transfer component is proposed to be the reason for the so-called facile kinetics of ORR in alkaline media on a wide range of non-noble metal surfaces. However, this outer-sphere process predominantly leads only to a 2e– peroxide intermediate as the final product. The importance of promoting the electrocatalytic inner-sphere electron transfer mechanism by facilitation of direct adsorption of molecular oxygen and the stabilization of the peroxide intermediate on the active site are emphasized with the usage of chalcogen modified transition metals and pyrolyzed biomimetic metal porphyrins as electrocatalysts.