pH Effects on Hydrogen Evolution and Oxidation over Pt(111): Insights from First-Principles

A long-standing question in electrocatalysis is how the electrolyte pH affects the catalytic activity of proton electron transfer reactions. A prime example for this is the hydrogen evolution/oxidation reaction (HER/HOR) over metal catalysts. While it has long been established that alkaline conditions result in a more sluggish reaction kinetics than in acidic conditions, the underlying reason for this trend remains contentious. We apply density functional calculations to evaluate prevailing hypotheses for the origin of this effect: shifts in hydrogen binding, proton donor, and water reorganization energy. We present a microkinetic model, based on ab initio reaction energetics of all possible elementary steps. Our model shows a good agreement with experimental trends. We find that with increasing pH, the proton donor changes from hydronium to water. Our model suggests that the intrinsically larger barriers for the splitting of water with respect to hydronium are the cause of HER kinetics being slower in alkaline than in acidic media.