PdH bulk and PdH(hkl) surfaces: Pourbaix stability and hydrogen adsorption revealed

Palladium (Pd) is a benchmark electrocatalyst for the hydrogen evolution reaction (HER), yet it readily absorbs hydrogen under cathodic polarization, forming palladium hydride (PdHx) phases that complicate the identification of the active surface state. Here, we employ ab initio thermodynamic calculations based on density functional theory to investigate hydrogen adsorption on low-index and stepped Pd(hkl) surfaces and their fully formed hydride counterparts, PdH(hkl). Surface energies and hydrogen adsorption energetics in the lowcoverage regime are evaluated and integrated into surface Pourbaix diagrams constructed within the computational hydrogen electrode framework. Hydride formation thermodynamically stabilizes all Pd surfaces, with a pronounced facet dependence, and systematically weakens the hydrogen adsorption strength through electronic and geometric effects. As a result, hydrogen binding on PdH surfaces shifts toward near thermoneutral values, enhancing predicted HER activity relative to metallic Pd, particularly on close-packed facets. Surface Pourbaix diagrams reveal that, across all facets and pH values considered, the thermodynamic equilibrium line for the HER falls within the hydride stability region, indicating that hydrogen absorption into Pd is thermodynamically favored under HER-relevant conditions. Furthermore, molecular dynamics simulations of bulk PdHx show that increasing the hydrogen concentration slows hydrogen diffusion while maintaining preferential octahedral site occupancy, consistent with the stability of the hydride phase under operating conditions. Our results demonstrate that the catalytically active state of Pd during electrochemical hydrogen evolution is best described as a PdHx phase rather than metallic Pd, and underscore the need to explicitly incorporate bulk phase transformations into theoretical descriptions of electrocatalytic reactions on hydrogen-absorbing metals.