Microscopic Origins of Metal-Dependent pH Effects in Hydrogen Evolution: Metal-Water Interactions Dictate Proton Transfer Pathways

The hydrogen evolution reaction (HER) is a prototypical proton-coupled electron transfer (PCET) reaction, yet the atomistic origin of its metal-dependent pH effect remains unclear. Here, combining constant-potential ab initio molecular dynamics with in situ infrared spectroscopy, we unravel the microscopic origins of metal-dependent pH effects in HER catalysis on Pt(111) and Au(111). We reveal that HER kinetics are governed by two competing spatial proton transfer topologies associated with the Volmer water-dissociation step: a low-barrier cross-layer proton transfer (CLPT) pathway and a high-barrier intra-layer proton transfer (ILPT) pathway. For Pt(111), strong metal-water interactions yield a pH-responsive water network. Under acidic conditions, a balanced intra- and cross-layer hydrogen-bond network promotes the CLPT in the Volmer step, thereby accelerating HER kinetics. By contrast, alkaline media induce a rigid H-down interfacial water layer that favors the sluggish ILPT pathway. Conversely, weak metal-water interactions on Au(111) maintain a pH-insensitive, predominantly parallel-flat water orientation that restricts the Volmer step to cooperate only with the high-barrier ILPT in both acidic and alkaline solutions. Our findings establish a fundamental causal link among metal-water interactions, interfacial hydrogen-bonding network, proton transfer pathways, and HER kinetics, providing atomistic insight into the design of pH-robust electrocatalysts.