Understanding how the complex interface between a catalytic electrode and electrolyte affects the rate and mechanism of electrocatalytic reactions is crucial to designing more active and selective electrocatalysts. As an example of this effect, it is now well known that the rate of the hydrogen oxidation reaction on a platinum electrode is orders of magnitude faster in acid than in base [1, 2] and that the rate varies continuously as a function of pH [3]. Sheng et al. have shown that the dependence of the rate on pH is correlated with what is believed to be a shift in the binding energy of hydrogen with pH, as measured by cyclic voltammetry [3]. We have recently shown using density functional theory (DFT) simulations that the low potential peaks measured by cyclic voltammetry on Pt(100) and Pt(110) are actually the co-adsorption or competitive adsorption of hydrogen and hydroxide, and that the shift of this peak with pH may not be solely related to the binding energy of hydrogen. We further show, in conjunction with our prior work on alkali cation specific adsorption [4], that potassium can specifically adsorb to Pt(111), Pt(100), and Pt(110) electrode surfaces at high pH and weaken the adsorption of hydroxide [5]. We see similar results on stepped surfaces of platinum, in particular Pt(553) and Pt(533), where both the binding strength of hydrogen on the step in the gas phase and in an electrochemical environment match only when competitive adsorption of hydroxide is considered. We extended this analysis of hydrogen and hydroxide adsorption across transition metal surfaces and find a similar effect of adsorbed alkali cation on hydroxide adsorption. Hydrogen and hydroxide competitive adsorption occurs on most surfaces which bind both species more strongly than Pt(111). Recent work by Zheng et al. has shown that the pH dependence of the hydrogen oxidation reaction and of the adsorption peaks measured by cyclic voltammetry are not unique to platinum, and occur across transition metals [6]. This has significant implications in the understanding of the hydrogen oxidation mechanism, where it has been proposed that adsorbed hydroxide may play a role in dictating the rate [7]. We will discuss our work using DFT to model hydrogen oxidation reaction mechanisms as well as other important conclusions we can draw (such as more accurately calculating electrode surface area or the surface restructuring which occurs with platinum electrodes under electrochemical â??conditioning/cleaningâ?) with our improved understanding of hydrogen and hydroxide adsorption on transition metal surfaces.
[1] Sheng, W.; Gasteiger, H.A.; Shao-Horn, Y. Hydrogen oxidation and evolution reaction kinetics on platinum: Acid vs Alkaline Electrolytes. Journal of the Electrochemical Society. 2010, 157, B1529-B1536.
[2] 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.
[3] 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.
[4] Mills, J.N.; McCrum, I.T.; Janik, M.J.; Alkali cation specific adsorption onto fcc(111) transition metal electrodes. Phys. Chem. Chem. Phys. 2014, 16, 13699-13707.
[5] 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, 120, 457-471.
[6] Zheng, J.; Sheng, W.; Zhuang, Z.; Xu, B.; Yan, Y. Universal dependence of hydrogen oxidation and evolution reaction activity of platinum group metals on pH and hydrogen binding energy. Sci. Adv. 2016, 2, e1501602.
[7] 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.
