The successful deployment of advanced energy-conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl *OH) at electrified metal–aqueous electrolyte interfaces. The effect of alkali metal cations (Li+, Na+, K+, Cs+) on the non-Nernstian pH shift of the step-related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. The co-adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step-related peak on Pt(553). Density functional calculations explain the observations on the identity and concentration of alkali cations on the non-Nernstian pH shift, and demonstrate that cation–hydroxyl co-adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.
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