Cation-Driven Modulation of Interfacial Solvation Structures for Enhanced Alkaline Hydrogen Oxidation Kinetics

The role of interfacial water and hydrogen-bonding structures in an electric double layer (EDL) in alkaline hydrogen oxidation reaction (HOR) kinetics has garnered widespread attention. However, the dynamic evolution of alkali metal cations (AM⁺), as key components in EDL, and their impact on interfacial solvation structure and alkaline HOR kinetics remain poorly understood. Here, based on the Ni₃S₂-island-encapsulated Ni (Ni₃S₂/Ni) catalyst, we demonstrate that the AM⁺ arrangement in the EDL can be regulated by the potential of zero charge (PZC) of the electrode, which in turn controls the associated solvation environment (i.e., the ordering of interfacial water and hydrogen-bonding network). Ab initio molecular dynamics simulations, in situ surface-enhanced infrared absorption spectroscopy, and electrochemical experiments indicate that the introduction of Ni₃S₂ fosters a lower PZC for Ni₃S₂/Ni, which thus promotes a less crowded cation arrangement and more disordered interfacial water structures, ultimately contributing to the accelerated shuttling of H⁺/OH^- across the EDL through a more interconnected H-bonding network, thereby leading to a reduced energy barrier of proton-coupled electron transfer (PCET) and enhanced HOR kinetics under alkaline electrolytes. This molecular-level picture is further supported by the unconventional cation dependence of HOR activities on Ni₃S₂/Ni (i.e., KOH > NaOH > LiOH) by which we reveal a new mechanistic insight; that is, the coordination of the Ni₃S₂ island with a partially desolvated K⁺ cation (Ni₃S₂-K⁺) promotes dynamic evolution of cation-solvated water into strongly hydrogen-bonded water on adjacent Ni, significantly accelerating the proton transfer process. This work highlights the dominant role of AM⁺ in controlling the EDL structure and PCET kinetics in alkaline HOR.