Proton-Relaying Adsorbates Induce Non-Nernstian Behavior in Oxygen Reduction

Proton-coupled electron transfer (PCET) is central to energy conversion processes in fuel cells, electrolysis, and biological systems. According to the Nernst equation, the equilibrium potential of PCET shifts by around -60 mV/pH relative to the standard hydrogen electrode at room temperature. Here, we reveal significant deviations from this expected Nernstian behavior in the oxygen reduction reaction (ORR) at Pt(111) in H₂SO₄/M₂SO₄ (M = Li, Cs) solutions, with a pronounced dependence on the cation identity, whereas Nernstian behavior is retained in HClO₄/LiClO₄ solutions. To elucidate the origin of these pH effects, we employ a hierarchical theoretical framework that integrates density functional theory calculations, multistep microkinetic modeling, and the local reaction environment (LRE) model describingmass transport and electrical double layer effects. Our analysis uncovers a previously unrecognized mechanistic role of adsorbed sulfate anions in mediating proton transfer. Specifically, sulfate anions attract hydrated protons via electrostatic interactions, leading to the formation of adsorbed bisulfate species, which then act as proton donors in the ORR. This shift in the proton donor species explains the observed reduction in the proton reaction order from 1 in HClO₄/LiClO₄ to 0.5 in H₂SO₄/Li₂SO₄ and 0.75 in H₂SO₄/Cs₂SO₄ solutions. This work advances the understanding of anion- and cation-dependent pH effects in electrocatalysis by highlighting the role of LRE modulation. Furthermore, it demonstrates how a combined theoretical and computational approach can disentangle complex, multiscale interactions in electrochemical reactions.