Cation-Limited Hydroxide Anion Diffusion Drives Asymmetric Hydrogen Kinetics on Transition-Metal Decorated Platinum Surface

Electrocatalytic reactions occur at dynamic, ion-regulated electrochemical interfaces. In electrocatalytic processes such as alkaline hydrogen evolution/oxidation reactions (HER/HOR), which operate at potentials below the electrode’s potential of zero charge, electrolyte cations are not mere spectators but profoundly shape reaction behavior. It is generally believed that the alkaline HER and HOR are reversible reactions that share the sluggish Volmer step as their common rate-determining step and therefore exhibit symmetric behavior on Pt-based catalysts. Here, we show that the apparent kinetic symmetry between HER and HOR can be broken on certain transition-metal (TM)-decorated Pt surfaces. Using Ni-decorated Pt as a model system, we show that the formation of Ni–OH species enhances HER kinetics by promoting water dissociation yet paradoxically suppresses HOR kinetics, particularly in highly alkaline electrolytes at potentials above 0.05 V versus the reversible hydrogen electrode. Systematic analyses indicate that abundant TM–OH species drive strong cation accumulation at the outer Helmholtz plane, forming a compact cation layer that pairs with hydroxide anions (OH – ) and suppresses their inward diffusion during HOR. Extending this framework across TM-decorated Pt surfaces reveals a systematic trend depending on the TM oxidation potential. Low-oxidation-potential TMs enhance HER but lead to pronounced OH – -diffusion-limited HOR. Intermediate TMs promote both HER and HOR, whereas high-oxidation-potential TMs suppress both reactions due to insufficient TM–OH formation. Overall, this work establishes a unified mechanistic framework that links cation accumulation and interfacial OH – transport to distinct HER/HOR kinetics on TM-decorated Pt surfaces in alkaline media, providing design principles for alkaline electrocatalysts.