Understanding Surface Charge Effects in Electrocatalysis. Part I: Peroxodisulfate Reduction at Pt(111)

Electrocatalytic reactions invariably occur in the electrochemical double layer (EDL) whose properties are markedly dictated by the excess free charge on the electrode surface. The peroxodisulfate (PDS) reduction has long served as a model reaction to understand the surface charge effect on electrocatalytic reactions. Herein, we develop a mechanistic model for the PDS reduction at Pt(111) based on a mechanism consisting of a dissociative chemisorption step and two one-electron reduction steps. The electron-transfer step is described using the Marcus–Hush–Chidsey theory considering the double-layer structure and the metal electronic structure. The model reveals that considering the metal electronic structure in the kinetic rate expression is essential to explain the experimental observation that the reduction current first increases as the electrode potential decreases and is then suppressed when the electrode potential is lower than the potential of zero charge (pzc). In addition, a quantitative match with experimental data requires electrostatic interactions that are much stronger than that described by the mean-field Poisson–Boltzmann equation. Furthermore, hydroxyl adsorption forms surface dipoles at the metal surface and changes the surface-charging behavior, leading to a second pzc in high potential range that coincides with the onset potential of PDS reduction.