(286b) pH, Cation, and Ad-Atom Effects on Hydrogen Oxidation and Evolution Reaction Kinetics

The kinetics of the hydrogen oxidation and evolution reactions (HOR/HER), important reactions in hydrogen fuel cells and water electrolyzers, depend strongly on electrolyte pH, where the rate on even the best catalyst for these reactions, platinum, is 2-3 orders of magnitude slower in an alkaline electrolyte than in an acid electrolyte [1, 2]. A similar pH dependence has been observed on other active transition metal catalysts [3], hindering the development of high-performance, low-cost alkaline fuel cells and electrolyzers. The pH dependence of the HOR/HER kinetics on polycrystalline platinum has been correlated with the pH dependence of the sharp, step-associated, low-potential peaks in current measured by cyclic voltammetry on polycrystalline platinum surfaces [2]; assumed to indicate an apparent pH dependence of the binding strength of hydrogen onto these step sites [2]. However, we have shown these step-associated peaks actually correspond to an exchange between adsorbed hydrogen and adsorbed hydroxide, and shift to more positive potentials with increasing pH due to the adsorption of an alkali metal cation, which subsequently weakens hydroxide and water co-adsorption along the step-edge [4-6]. This motivates considering properties beyond hydrogen binding strength in understanding HOR/HER kinetics. Using a combination of density functional theory (DFT) modeling and detailed experiments on single crystal electrodes, we examine how pH, alkali metal cations, and ad-atoms affect the binding strength of hydroxide and water on platinum, and the role these species (OH*/H₂O*) play in the mechanism of hydrogen oxidation and evolution. We additionally probe the ability of water to re-orient near the electrode surface, as this has been previously proposed to impact hydrogen evolution reaction kinetics [7, 8]. Adsorbed ad-atoms in particular affect the orientation of water near the electrode surface in a well-defined fashion [9]; this provides a controlled opportunity to explore the effect of water orientation on catalysis with both experiment and theory. Discussion will be given on how the role of hydroxide/water adsorption strength and orientation in HOR/HER kinetics can be used to design catalysts with improved activity for these reactions.

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