Even though hydrogen electrocatalysis has been studied for almost a century, it remains to be explained why higher pH slows the hydrogen evolution and oxidation reactions (HER/HOR) by a factor of 200, even on platinum [1]. This lack of understanding has serious implications not only for industrial electrolysis and next-generation energy storage, but for all electrocatalytic reactions in which pH may play a role. In this work, we investigate possible explanations for the effect of pH by combining microkinetic modeling with traditional electroanalytical methods. We specifically determine the viability of the proposed `bifunctional mechanism’, in which slow water dissociation is overcome by mediation of adsorbed hydroxide [2].

Our calculations [3] show that either a direct (hydroxide-as-spectator) or indirect (hydroxide-mediated) Volmer step can describe the reaction thermodynamics as revealed by slow-scan experimental cyclic voltammetry, but that only the direct Volmer step yields adsorption values consistent with literature. Modulating hydroxide adsorption strength via the electrolyte cation provides additional support for the direct mechanism. Experimentally, stronger hydroxide binding decreases kinetics, as observed by the dependence of peak-potential splitting on scan rate. Comparison with the model shows that this observation is consistent only with the direct mechanism.

Altogether, these results strongly suggest that adsorbed hydroxide serves as a competitive spectator in the alkaline Volmer step, and that the bifunctional HOR/HER mechanism plays only a minor role at best. This study contributes to resolving a long-standing paradox in electrocatalysis and surface science by determining that oxophilicity is not an accurate descriptor for alkaline hydrogen electrocatalysts. Other parameters, such as water orientation and non-covalent interactions, must play a greater role in overall activity. Efforts to identify and measure these parameters are ongoing.

[1] Durst et al. Energy Environ. Sci. 2014, 2255.
[2] Li et al. Angew. Chem. Int. Ed. 2017, in press.
[3] Intikhab et al, ACS Catal., 2017, in press.