Potential-Dependent Hydricities and Bond Dissociation Free Energies of Metal Surface Hydrides.

Abstract

Recent experimental studies demonstrate that the thermodynamic hydricity of surface-bound intermediates is continuously tunable via electrical polarization. Canonical density functional theory relies on constant-charge models that fail to capture this potential-dependent polarization at the electrified solid-liquid interface. This work implements grand-canonical density functional theory (GC-DFT) to compute the surface hydricity, bond dissociation free energy (BDFE), and acidity of chemisorbed hydrogen across transition metal facets. Assuming an outer-sphere transfer isolates the intrinsic hydrogen-donating capacity of the active site. By explicitly controlling the system Fermi level, we demonstrate that surface hydricity exhibits a universal linear dependence on applied cathodic bias across all evaluated facets. In contrast, the potential dependence of the homolytic BDFE is strictly governed by material identity: platinum maintains a relatively static bond strength, while silver and copper exhibit potential-induced bond stabilization. The computed hydricity slopes for Pt facets quantitatively reproduce the experimentally measured polarization-dependent hydride-transfer sensitivity. Coordination geometry dictates the baseline donor capacity, with lower coordination atop sites on Ag and Cu functioning as significantly stronger hydride donors. Adlayer saturation perturbs the thermodynamic values in a metal-dependent manner, weakening hydride donation on silver but strengthening it on copper. However, at the two coverages examined, it does not substantially alter the voltage sensitivity of the thermodynamic properties. Evaluating voltage-dependent hydricity provides a thermodynamic framework for characterizing the intrinsic hydride- or hydrogen-donating ability of electrified solid-liquid interfaces.

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