The fundamental drivers of electrochemical barriers

Reaction barriers dictate the rates of elementary reactions, and therefore are crucial to understanding electrochemical kinetics. Since these reactions tend to occur at heterogeneous surfaces, principles from catalysis can be expected to apply. Here, we use electronically grand-canonical calculations on a model system of the proton-deposition reaction on a range of metal surfaces to explore the extent to which these principles hold. First, we show that while reaction barriers are functions of potential, unlike endstates they tend to exhibit a nonlinear dependence, and this nonlinearity can be traced to differences in the electron transfer at the reaction barrier. We show that along a reaction path, this same electron-transfer difference forces the barrier to move earlier for downhill reactions, enforcing and explaining the Hammond-Leffler postulate for electrochemical reactions, as well as explaining the curvature in the Marcus-like relations. We further examine trends in barrier energies, at equivalent driving forces, for this reaction across metals. We find that the barrier energy correlates weakly to the hydrogen binding energy and the d-band center, but instead correlates strongly with the charge presented at the metal surface -- which is a direct consequence of the native work function of the material. This suggests that the energetics of the barrier are driven more strongly by the electrostatic, rather than the covalent, nature of the metal-adsorbate interaction, and suggests electrochemical barriers may have an independent driving force from electrochemical adsorbates.