(145g) Experimental Determination of Water Orientation Effects on Kinetic Barriers to Alkaline Hydrogen Oxidation and Evolution
The pH-dependence of HOR/HER kinetics has been attributed to a variety of reasons. The research groups of Gasteiger and Yan have both suggested that in base, hydroxide ions stabilize the Pt-H bond for stronger binding and slower catalysis [1,2]. Central to this hypothesis is an experimentally measured shift with pH in the peak potential for hydrogen underpotential deposition on the (100) and (110) steps on the Pt surface. Although it is unclear why pH would stabilize adsorbed hydrogen, stronger binding would push platinum further to the right of the volcano peak and decrease the overall activity. Markovic et al proposed instead that in base, the water recombination/dissociation step of HOR/HER requires specific adsorption of hydroxide ions and therefore optimal binding to two adsorbates . One observation that has been less discussed than either hydrogen or hydroxide adsorption is the effect of pH not only on the apparent hydrogen adsorption energy, but the kinetic barrier to adsorption. Koper et al attributed this observation to a pH-dependent water orientation at the surface, and hypothesized that â??H-downâ? in acid vs â??H-upâ? in base resulted in slower transfer of hydrogen to and from the surface . The same pH-dependence of water orientation was recently supported computationally by Rossmeisl's group.
In this work, we examine specifically the hypothesis that water orientation governs the rate of hydrogen adsorption and thus the overall HER/HOR kinetics. The native oxide formed on chromium metal is known from colloidal literature to have a potential of zero charge (Epzc) of 1.22V vs. RHE. We hypothesize that if water orientation truly governs the alkaline HOR/HER rate, the negative oxide surface charge on chromium oxide will induce an H-down water orientation adjacent to platinum nanoparticles supported on the surface. This will result in faster hydrogen adsorption kinetics and thus, higher activity for alkaline HOR/HER than extended platinum surfaces.
To test this hypothesis, platinum nanoparticles were synthesized via pulsed electrodeposition on chromium oxide. The nanoparticle loading was controlled by the duration of deposition pulse, with longer times corresponding to higher coverage as well as larger particles. The hydrogen adsorption kinetics were measured by cyclic voltammetry in the hydrogen underpotential deposition region. Varying the sweep rate results in greater peak separation that can be used to extract rate constants for the adsorption reaction via traditional electroanalysis.
A typical voltammogram of Pt/CrOx in 0.1M KOH is shown in Fig. 1a. As the scan rate increases, kinetic barriers cause greater peak separation (dEp) for hydrogen adsorption on the 100 and 110 facets of platinum. For lower loadings with shorter pulse periods, both smaller currents and smaller peak separations are observed due to the lower Pt surface area. The effects of surface area can be accounted for by normalizing current to Pt surface area as measured by Hupd coulometry. The resulting surface-adsorption Tafel plot is shown in Fig 1b. The peak potential separation dEp is much greater for the larger nanoparticles with higher loading. This difference suggests that smaller particles have qualitatively more rapid adsorption kinetics due to the advantageous water orientation induced by negatively charged CrOx.
The results of this study contribute to resolving a long-standing paradox in electrocatalysis and surface science by highlighting the importance of kinetic barriers, as well as adsorption energies, and suggest the ability to modulate alkaline hydrogen activity through metal-support interactions. Future work will discuss the relation of hydrogen underpotential deposition kinetics to HER/HOR activity and the effects of different oxide supports.
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