Field-Dependent Redox Thermodynamics of MoO_mH_n Species on Cu(111) and Ni(111) Surfaces under Alkaline Hydrogen Evolution Conditions

Whether copper fundamentally alters Mo-centered redox thermodynamics or mainly tunes hydrogen adsorption in Ni–Mo electrocatalysts under alkaline hydrogen evolution reaction (HER) conditions remains unresolved. Density functional theory calculations combined with a field-corrected computational hydrogen electrode framework are used to evaluate the thermodynamic stability of H3Mo, H3MoOH, H2Mo(OH)2, and MoO(OH)3 on Cu(111) and Ni(111) and to construct surface Pourbaix diagrams under electrochemical conditions. The results show that substrate identity reorganizes the redox stabilization hierarchy of these Mo intermediates. Across the examined conditions, at least one of H3Mo, H3MoOH, or MoO(OH)3 is thermodynamically favored over H2Mo(OH)2 on both surfaces. However, only Cu(111) exhibits measurable pH-dependent free-energy shifts, reaching 0.28 eV on the reversible hydrogen electrode scale. The magnitude of this electrostatic modulation is comparable to the intrinsic substrate-dependent relative Gibbs free-energy differences, suggesting that Cu reshapes Mo redox thermodynamics rather than merely weakening hydrogen binding strength. Electronic structure and vibrational analyses further show that Cu(111) preferentially weakens Mo–O interactions, whereas Ni(111) more strongly perturbs Mo–H bonding in hydrogen-rich complexes. Overall, these results establish that substrate identity governs the electrostatic modulation of Mo redox thermodynamics under alkaline HER conditions and provide a mechanistic insight into substrate effects relevant to Cu-containing Ni–Mo systems.