Investigating the Effect of Ru(OH)x Surface Decoration on HER/HOR Kinetics

Abstract

With an increase in energy demand and a rapidly changing climate, there has been an increasing effort within the scientific community to move towards a more sustainable and clean energy system. A clean, safe and sustainable hydrogen energy based system can be provided by electrochemical energy conversion devices such as fuel cells and electrolyzers. Electrolyzers produce hydrogen as a fuel through hydrogen evolution reaction (HER), while fuel cells consume hydrogen to generate energy by hydrogen oxidation reaction (HOR) [1]. It is observed that the more stable alkaline environment is not necessarily the most active for HOR and HER. On Platinum and other catalysts, HOR and HER are extremely fast in acidic electrolytes but several orders of magnitude slower in alkaline electrolytes. The explanation of this unusual pH dependence has been a matter of debate within the scientific community. Understanding the hydrogen reaction kinetics across the pH spectrum will allow the rational design of catalysts for the alkaline electrolyte. Traditionally, hydrogen binding energy (HBE) has been used as the sole activity descriptor, independent of electrolyte pH [2]. 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 [3]. 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 [4][5]. Therefore, any effects of OHad must serve to reduce kinetic barriers associated with the hydrogen reactions. Liu et al. very recently proposed that OHad electrostatically attracts water close to the surface, thereby reducing kinetic barriers [6]. Koper et al. also discussed the role of kinetic barriers in HER kinetics. Through laser-induced temperature-jump experiments, it was shown that Ni(OH)x clusters on the surface of Pt (111), which improves HER kinetics in base, shift the potential of zero free charge (pzfc) at alkaline pH closer to 0 V vs. RHE by ~25 mV [7]. It was hypothesized that the shift of the pzfc closer to the equilibrium potential for HER/HOR increases the water mobility at the surface that leads 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. Our results indicated that pztc is a valid descriptor for the hydrogen reactions along with pzfc. Two possible hypotheses can explain the effect of pztc. 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 [4], 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. A negative shift in the pzfc may weaken the electric field at the interface. The weaker electric field leads to a more fluent water structure at the surface and facilitates lower transition-state barriers. More work is required to determine which of these factors plays a greater role in HER/HOR kinetics. Decoration of Ru(OH)x on Pt(111) showed greater improvement of HER/HOR activity compared to when Pt(110) was decorated with the same Ru(OH)x coverage. Based on the structural sensitivity observed on the underlying Pt substrate, we synthesized 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. Hence, a highly active nanocatalyst was designed based on the insight from single crystal experiments.