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

ACS Catalysis, vol. 15, pp. 14191–14206

Abstract

High Resolution Image Download MS PowerPoint Slide 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 H 2 SO 4 /M 2 SO 4 (M = Li, Cs) solutions, with a pronounced dependence on the cation identity, whereas Nernstian behavior is retained in HClO 4 /LiClO 4 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 HClO 4 /LiClO 4 to 0.5 in H 2 SO 4 /Li 2 SO 4 and 0.75 in H 2 SO 4 /Cs 2 SO 4 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.

Authors 4

1. Lulu Zhang Aachen

   University of Science and Technology of China · Forschungszentrum Jülich · RWTH Aachen University

   Affiliation as printed

   Forschungszentrum Jülich GmbH

   Hefei National Research Center for Physical Sciences at Microscale, Department of Chemical Physics

   Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials

   RWTH Aachen University

   Theory of Electrocatalytic Interfaces, Faculty of Georesources and Materials Engineering

   University of Science and Technology of China

   Hefei National Research Center for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China

2. Weiqiang Tang

   Forschungszentrum Jülich

   Affiliation as printed

   Forschungszentrum Jülich GmbH

   Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials

3. Yanxia Chen corresponding

   University of Science and Technology of China

   Affiliation as printed

   Hefei National Research Center for Physical Sciences at Microscale, Department of Chemical Physics

   University of Science and Technology of China

   Hefei National Research Center for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China

4. Jun Zh. Huang corresponding Aachen

   Forschungszentrum Jülich · RWTH Aachen University

   Affiliation as printed

   Forschungszentrum Jülich GmbH

   Institute of Energy Technologies, IET-3: Theory and Computation of Energy Materials

   RWTH Aachen University

   Theory of Electrocatalytic Interfaces, Faculty of Georesources and Materials Engineering