ORR on Simple Manganese Oxides: Molecular-Level Factors Determining Reaction Mechanisms and Electrocatalytic Activity

Abstract

In this study we combine experimental rotating ring disc electrode data, theory, molecular-level modeling and microkinetic simulations in order to gain a deeper insight into the oxygen reduction reaction (ORR) mechanism on simple manganese oxides in alkaline media. We demonstrate that “thermodynamic” approach based on periodical density functional theory calculations is unable to explain the experimentally observed differences in the ORR kinetics on the most (α-Mn 2 O 3 ) and the least (α-MnOOH) active oxide. We perform quantum mechanical cluster calculations and show that faster kinetics of the hydrogen peroxide reduction on the surface of Mn 2 O 3 oxide and the ensuing lower peroxide yield during the ORR are corroborated by the lower barrier for the dissociation of hydrogen peroxide adsorbed on the surface of Mn 2 O 3 arising from adsorbate-adsorbate interactions. We provide the arguments in favor of the outersphere nature of initial O 2 reduction steps and demonstrate that this hypothesis does not contradict the experimental trends observed for ORR and hydrogen peroxide reduction reactions on Mn 2 O 3 and MnOOH.