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

ABSTRACT

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.