Modeling Hydrogen Evolution Reaction Kinetics through Explicit Water-Metal Interfaces

Despite the apparent simplicity of the hydrogen evolution reaction (HER) and the decades of research into it, controversy remains in the literature regarding the identity of the active site and the competition between the Heyrovsky and Tafel steps. In this work, we use charge-extrapolated ab initio simulations with explicit water in conjunction with mean-field microkinetic modeling to explore the mechanism for HER on both close-packed (111) and stepped (211) transition metals. First, we show that atop H*, beyond a monolayer of hollow H*, is unlikely to play a role in the HER mechanism, given its very positive adsorption energies. The energetics suggests the Volmer-Heyrovsky mechanism to predominate on fcc transition metals under typical operating conditions. We evaluate our theoretical results vs several experimental observations. We show that the Volmer-Heyrovsky mechanism predicts an activity volcano with its peak at a H* binding Delta G(H)* approximate to 0 eV, consistent with experiment. In contrast, the Volmer-Tafel volcano shows a broad rate plateau between Delta G(H)* approximate to 0 eV and Delta G(H)* approximate to - 0.4 eV. We find our theoretical Tafel slopes to be consistent with experimental ones on a range of transition metals. We show that, in line with experimental observations, the introduction of a CO(g) atmosphere shifts the strong binding metals toward the weak binding leg. Our study suggests that the simple thermodynamic approach to HER activity still holds, even when a detailed kinetic picture is considered.