The electrochemical interface between a catalytic electrode surface and electrolyte is complex and its structure and composition have a direct effect on the rates and mechanisms of electrocatalytic reactions. For example, it has been found that the rate of the hydrogen oxidation reaction on a platinum electrode is strongly dependent on electrolyte pH, and this effect had tentatively been correlated to a shift in hydrogen binding strength with pH (beyond the expected Nernstian shift) as measured by cyclic voltammetry [1]. We used density functional theory (DFT) to calculate the thermodynamics of adsorption of hydrogen, hydroxide, water, and alkali cations on the low index facets of platinum to support prior work which has shown that the low potential peaks measured by cyclic voltammetry on Pt(100), Pt(110), and higher order facets containing 100 and 110 steps, actually correspond to the competitive and co-adsorption of hydrogen and hydroxide [2]. We further show that increasing pH leads to an increasing amount of specifically adsorbed alkali metal cation which weakens the binding of hydroxide, leading to the experimentally observed shift in the low potential peaks measured by cyclic voltammetry, as opposed to a strengthening of hydrogen binding [2]. Now that we understand the mechanism for these experimentally measured effects of pH and alkali cations, we use experiment and DFT to examine their effect on the rate of the hydrogen oxidation reaction, which we find, on a platinum electrode, to depend on cation identity. We also discuss this effect of pH and alkali metal cation on cyclic voltammograms and the hydrogen oxidation reaction across transition metal surfaces (Ru, Ir, Pt, and Au) as measured by Strmcnik et al. [3] and Durst et al. [4].           

[1] Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating Hydrogen Oxidation and Evolution Activity on Platinum at Different pH with Measured Hydrogen Binding Energy. Nat. Commun. 2015, 6:5848.

[2] McCrum, I.T.; Janik, M. J. pH and alkali cation effects on the Pt cyclic voltammogram explained using density funtional theory. J. Phys. Chem. C. 2016, In Press. DOI: 10.1021/acs.jpcc.5b10979

[3] Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der, V.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Improving the Hydrogen Oxidation Reaction Rate by Promotion of Hydroxyl Adsorption. Nat. Chem. 2013, 5, 300-306.

 [4] Durst, J.; Siebel, A.; Simon, C., Hasché, F.; Herranz, J.; Gasteiger, H.A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 2014, 7, 2255-2260.