Facet Modulation of Interfacial Water Structure Tunes Electrocatalytic Hydrogen Reaction Kinetics

Interfacial water structure is theoretically envisioned as the master regulator of electrocatalytic reaction kinetics, but the direct experimental validation of this role has remained a formidable challenge. Probing kinetic isotope effects (KIEs) is paramount to deciphering the elusive proton-coupled electron transfer (PCET) mechanisms in alkaline media, a long-standing enigma in electrocatalysis. Such precision, however, demands the uncompromising rigor of atomic-level-precise single-crystal electrodes and ultrapure D2O to ensure definitive results. Here, we uncover a hydrogen evolution reaction KIE that varies sharply with the surface structure: ∼6 for Pt(100), ∼1 for Pt(111), and intermediate values for Pt(110). Through a combination of nuclear-electronic orbital density functional theory (NEO-DFT) and in situ laser-induced temperature-jump (LITJ) experiments, we propose that this disparity originates from the difference between a rigid, hexagonal H-bonded network on Pt(111) and a more disordered, flexible interfacial layer on Pt(100) that would be more sensitive to isotopic substitution, as well as from differences in H/D adsorption sites, which would influence the effective proton transfer distances. Furthermore, measurements with stepped surfaces show that step density acts as a critical dial to tune these water environments. Finally, to bridge the knowledge gap between bulk single-crystals and practical nanoparticles, {100}-oriented Pt nanocubes and {111}-oriented Pt nanotetrahedrons reveal a facet-dependent KIE trend comparable to their single-crystal counterparts. These insights provide a transformative framework for the rational design of high-performance interfaces for green hydrogen production.