Understanding the hydrated proton at the electrode–electrolyte interface

Abstract

Most aqueous electrocatalytic reactions proceed via proton-coupled electron transfer (PCET), in which protons are either consumed or generated. In water, hydrated protons exist as a variety of protonated water clusters, most notably Eigen and Zundel cations. Therefore, understanding the structural and chemical behavior at the electrochemical interface is essential for elucidating aqueous electrocatalysis. However, the highly dynamic nature of hydrated protons makes atomic-scale characterization particularly challenging, particularly within buried electrode–electrolyte interfaces. To overcome these limitations, we performed mean-field quantum mechanics/molecular mechanics (QM/MM) simulations, in which the electrode was described at the quantum-mechanical level and the surrounding electrolyte was treated using reactive molecular dynamics based on a reactive force field. Our simulations revealed that hydrated protons at electrified interfaces adopt three distinct structural motifs: the Eigen, perpendicular Zundel (Perp-Zundel), and parallel Zundel (Para-Zundel) cations. We further identified Perp-Zundel as the dominant pathway for proton migration from the bulk electrolyte to the electrode surface. Additionally, we showed that the solvating water molecules associated with the Eigen and Perp-Zundel species were likely proton donors during the PCET step. These findings provide molecular-level insight into the interfacial proton structure and its transport, and may facilitate the rational design of improved electrochemical systems via the modulation of hydrated proton behavior at electrified interfaces.