(561e) Combining Experiment and Simulations to Determine the Role of Adsorbed Hydroxide in Alkaline Hydrogen Electrocatalysis (Invited)

Authors: 
Tang, M. H., Drexel University
Snyder, J., Drexel University
Intikhab, S., Drexel University
Rebollar, L., Tufts University
It has long been recognized that the reaction rates of the hydrogen oxidation and hydrogen evolution reactions (HOR and HER) are slower in basic than acidic electrolytes, even though the surface intermediate of adsorbed hydrogen is independent of solution pH [1]. Understanding the root of this observation is critical to designing catalysts for a multitude of electrochemical reactions with relevance to energy conversion and storage. 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 previous calculations [3] have shown 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. Furthermore, increasing hydroxide adsorption strength via the electrolyte cation 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. 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.

In this work, we extend our approach to consider the two-site model recently proposed in the literature [2], in which oxophilic ruthenium or nickel sites are responsible for facilitating water dissociation. We find that incorporating chemical dissociation steps results in anomalous trends with potential, and that oxophilicity does not affect hydrogen coverage. Comparison with experiment strongly indicates that the so-called ‘bifunctional mechanism’ is unrelated to hydroxide binding strength. Overall, our work resolves 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, 15594.

[3] Intikhab et al, ACS Catal., 2017, 8314.