Hydrogen Deposition on Pt(111) during Electrochemical Hydrogen Evolution from a First-Principles Multiadsorption-Site Study

We study the simultaneous adsorption of H* on Pt(111) for multiple, interacting adsorption sites (i.e., fcc, atop, and hcp) and, over a wide range of electrode potential, examine the equilibrium site coverage during the hydrogen evolution reaction (HER) and oxidation reaction (HOR). We use a first-principles-based cluster expansion (CE) and Monte Carlo simulations. We predict the adsorption isotherm and cyclic voltammogram for −0.9 V < U < 0.5 V versus the standard hydrogen potential. Although strongly adsorbed H*fcc are the majority species for U > 0, we show that traces of weakly adsorbed H*atop and H*hcp are present, and they are expected to be active in the HER. For U < 0, we predict that H*atop takes over as the majority species ca. U = −0.4 V, where a simultaneous decrease in H*fcc occurs—contradicting the general assumption that H*fcc remains the majority species, even at very negative potential. We identify the favorable HER operating potentials by mapping out the coverage of the kinetically active species H*atop.