Oxygen evolution reaction on IrO2(110) is governed by Walden-type mechanisms

Oxygen evolution reaction (OER) is a key process for sustainable energy, although renewable sources require the use of proton exchange membrane electrolyzers, with IrO2-based materials being the gold standard under anodic polarization conditions. However, even for the (110) facet of a single-crystalline IrO2 model electrode, the reaction mechanism is not settled yet due to contradictory reports in literature. In the present manuscript, we disentangle the conflicting results of previous theoretical studies in the density functional theory approximation. We demonstrate that dissimilar reaction mechanisms and limiting steps for the OER over IrO2(110) are obtained for different active surface configurations present on the IrO2 electrode. In contrast to previous studies, we factor Walden-type mechanisms, in which the formation of the product O2 and adsorption of the reactant H2O occur simultaneously, into the analysis of the elementary steps. Combining free-energy diagrams along the reaction coordinate and Bader charge analysis of the active site, we elucidate why mononuclear- or bifunctional-Walden pathways excel the traditional OER mechanisms for the OER over IrO2(110). Our computational methodology to identify the reaction mechanism and limiting step of proton-coupled electron transfer steps is widely applicable to electrochemical processes in the field of energy conversion and storage.