Asymmetric Interfacial Water Induced by OH _ad -Cation Coupling Reorganizes the Electric Double Layer for Enhanced Alkaline Hydrogen Evolution

The origin of sluggish alkaline hydrogen evolution kinetics on Pt remains elusive. Here, we reveal an often-overlooked dynamic—the coupling between adsorbed hydroxyl (OH ad ) and alkali cations—that reshapes the electric double layer (EDL) in a spatial-dependent manner. This coupling draws outer water molecules into initially depleted gap-zone, restoring disrupted H-bond connectivity to an extent that varies with distance from the surface. Critically, this induces pronounced structural asymmetry specifically in the first water layer, creating a pre-activated state where elongated O–H bonds coexist with a compressed H-bond network. The extent to which this spatial activation lowers the proton transfer barrier correlates with cation identity, consistent with experimental observations. In situ Raman spectroscopy and theoretical calculations confirm the enhanced H-bond network, linking the red-shift of O–H stretching vibrations to the compressed H-bond geometry and reduced kinetic barriers. By resolving the spatial heterogeneity of interfacial water restructuring, we establish a molecular framework where OH ad -cation coupling drives EDL reorganization across multiple length scales, with the resulting water asymmetry serving as a crucial bridge between structural rearrangement and enhanced alkaline HER kinetics.