The role of the Helmholtz potential on electrocatalytic activity

The electrification of the chemical industry is required for a rapid reduction of its carbon footprint and necessitates sustainable and highly active electrocatalysts. New concepts, such as the entropy of the electrolyte at the interface, are emerging as critical descriptors of electrocatalytic activity. However, a theoretical understanding of these properties of the electrochemical interface is still missing. Here, we include the electronic equilibrium at the electrode-electrolyte interface in the Butler-Volmer formalism for metal and metal/semiconductor electrodes. We demonstrate, using experimental data on the hydrogen evolution reaction from the literature, that the electrochemical reaction kinetics are not only governed by the Sabatier principle, but also by the Helmholtz potential at the electrode surface. Based on this concept, we explain why adding a thin semiconductor layer (1 to 10 nm) on a metal electrode can enhance electrocatalytic activity, which may guide the discovery of thin-film catalysts. We also establish that the physical limit for the exchange current density reachable for the hydrogen evolution reaction is 10 A cm^-2 for an ideal material.