Abstract
The hydrogen reaction rates on Pt and other catalysts are facile at low pH, however, the kinetics is found to be several orders of magnitude slower in base. The explanation for this unusual pH dependence has been controversial within the scientific community. Traditionally, hydrogen binding energy (HBE) has been used as the sole activity descriptor, independent of electrolyte pH [1]. This alone, however, is not enough to describe the pH dependence of the HER/HOR activity on metals other than Pt. Markovic et al. have proposed that hydroxide adsorption is necessary to facilitate water recombination/dissociation in base [2]. In our previous work, we have shown that adsorbed hydroxide is not an active participant in HER and HOR reactions but decreases the availability of sites for H adsorption [3][4]. According to a more recent explanation, the kinetic barriers associated with water mobility and reorganization at the electrode surface affects the hydrogen reaction rates in base. The magnitudes of these kinetic barriers are hypothesized to correlate with the rigidity of water. Koper et al. showed through laser-induced temperature-jump experiments that Ni(OH)x clusters on the surface of Pt (111) shift the potential of zero free charge (pzfc) at alkaline pH closer to 0 V vs. RHE by ~25 mV [5]. The shift of the pzfc closer to the equilibrium potential for HER/HOR increases the water mobility at the surface leading to a more efficient H/OH-X charge transfer (equivalent to the Volmer step) and consequently improves the HER kinetics. In this work, we discuss the role of Ru(OH)x on Pt in alkaline electrolytes. Surface decoration of Ru(OH)x on Pt leads to a dramatic increase in the HER/HOR activity. The improved activity is correlated to a negative shift in the potential of zero total charge (pztc) towards 0 V vs RHE, where the pztc is measured through CO displacement experiments. The negative shift in pztc indicates a higher OHad coverage near HER/HOR relevant potentials. While OHad is not an active participant as we have shown in our previous work [3], it decreases the activation energy of the hydrogen reactions by bringing water closer to the interface through the OHad-H2Ox-AM+ adducts, as also indicated by Liu et al. [6]. Alternatively, a shift in pztc may reflect a shift in pzfc, since both follow the same qualitative trends with surface, pH and electrolyte [7]. The behavior of Ru(OH)x is also found to display structural sensitivity to the atomic geometry of the underlying Pt substrate, with greater improvements on Pt(111) over Pt(110). Translating this insight to design a highly active nanocatalyst, we synthesize octahedral Pt nanoparticles with a high density of (111)-like domains. Decorating Octa-Pt/C nanocatalyst with Ru(OH)x clusters yields a higher HER/HOR activity than spherical Pt/C + Ru(OH)x, where spherical Pt/C has a higher density of steps, analogous to the Pt(110) surface. Future work involves probing the water mobility and solvent dynamics for a surface decorated with Ru(OH)x. through the kinetic isotope effect.
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