Cations determine the mechanism and selectivity of alkaline ORR on Pt(111)

The proton-coupled electron transfer (PCET) mechanism of oxygen reduction reaction (ORR) is a long-standing issue in electrocatalysis. Despite decades of research, factors determining the ORR-PCET mechanism performance, even on the prototypical Pt(111) surface, as a function of the pH, electrolyte and electrode potential remain unknown. Herein, we integrate advanced experiments, simulations, and theory to study alkaline ORR on a well-defined Pt(111) in the presence of different cations. We unveil a dual-cation effect where the cations simultaneously determine the active electrode surface, i.e. Pt-O/OH formation, and the competition between inner- and outer-sphere PCET steps. This finding provides direct evidence that the electrolyte affects the pathways, activity, and selectivity which has important consequences for the practical design of electrochemical systems and computational catalyst screening studies, for example. Our work shows that all components of the electrochemical interface, not the surface alone, are integral in electrocatalysis.