Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis

The sluggish kinetics of the alkaline hydrogen electrode have been attributed to the need to adsorb both H and OH optimally. In this work, single-crystal voltammetry and microkinetic modeling show that an OH-mediated mechanism is not viable on Pt(110). Only a direct Volmer step can explain observed kinetic trends with OH adsorption strength in KOH and LiOH electrolytes. Instead, OH behaves as a rapidly equilibrated spectator species that decreases available surface sites and slows hydrogen kinetics. These results identify kinetic barriers from interfacial water structure, not adsorption energies, as key to explaining changes in hydrogen kinetics with pH.