Metal-dependent Volmer free energies track opposite cation trends on Pt(111) and Au(111) as catalysts for hydrogen evolution

The hydrogen evolution reaction in alkaline media exhibits opposite alkali-cation activity trends on Pt and Au, but the microscopic origin of this inversion remains unresolved. In this work, we combine electrochemical measurements and in situ Raman spectroscopy on polycrystalline Pt and Au with constrained ab initio molecular dynamics (AIMD) simulations of Pt(111)/water and Au(111)/water interfaces. The results show the following. (i) Larger alkali-metal cations increase the fitted high-wavenumber Raman contribution associated with weakly hydrogen-bonded water on both Pt and Au, with a more pronounced effect on Pt. In the simulations, increasing interfacial cation number broadens the local water-depleted region. (ii) Cation positioning follows a monotonic size ordering on Pt(111), where larger cations reside closer to the surface, whereas on Au(111) the cation-height sequence is nonmonotonic. (iii) Most importantly, single-cation constrained AIMD free-energy profiles for the Volmer step recover the qualitative inversion observed on polycrystalline Pt and Au. This agreement suggests that the Volmer barrier in the model may be an intrinsic contributing factor to the observed experimental trends.