The role of cations in hydrogen evolution reaction on a platinum electrode in mildly acidic media

In this work, we study the influence of cation concentration and identity on the hydrogen evolution reaction (HER) on polycrystalline platinum (Pt) electrode in pH 3 electrolytes. Our observations indicate that cations in the electrolyte do not affect proton reduction at low potentials. However, an increase in cation concentration significantly enhances water reduction. Simultaneously, we identify a non-negligible migration current under mass transport limited conditions in electrolytes with low cation concentration. To separate migration effects from specific cation-promotion effects on HER, we carried out further experiments with electrolytes with mixtures of Li⁺ and K⁺ cations. Our results show that, adding strongly hydrated cations (Li⁺) to a K⁺-containing electrolyte leads to a less negative onset potential of water reduction. Interfacial pH measurements reveal a same interfacial pH at the platinum electrode in pH 3 in the presence of 80 mM LiClO₄ and KClO₄, respectively, at potentials where water reduction occurs. Based on these results, we suggest that under the current conditions, the strongly hydrated cations (Li⁺) promote water dissociation on the Pt electrode more favorably in comparison with the more weakly hydrated cations (K⁺), and that this promotion is not related to a local pH effect.