Understanding the Effects of pH and Alkali Metal Cations on H/OH Adsorption and the Hydrogen Oxidation/Evolution Reaction on Transition Metal Electrodes
Authors: McCrum Ian T. , Chen Xiaoting , Meduri Praveen , Hickner Michael A , Janik Michael J. , Koper Marc T. M. 
Published: February 2020

The structure and composition of the complex electrode/electrolyte interface have a direct effect on the rate and mechanisms of electrocatalytic reactions. Recent experimental evidence has shown that the rate of the hydrogen oxidation and evolution reactions (HOR/HER) are strongly dependent on electrolyte pH; the rate on the most active catalyst, platinum, is two orders of magnitude slower in an alkaline electrolyte than in an acid electrolyte [1]. This pH dependence has been correlated with an anomalous pH dependence of the sharp, low potential peaks in current measured by cyclic voltammetry on polycrystalline and nanoparticle platinum electrodes, believed to be due to hydrogen adsorption on 110 and 100 step sites [2]. In our prior work, we used density functional theory (DFT) to show that these sharp peaks measured by cyclic voltammetry correspond to competitive hydrogen and hydroxide adsorption onto 110 and 100 step sites and not solely hydrogen adsorption. We further showed that the apparent pH dependence of the location of these step-associated peaks is actually due to the co-adsorption of alkali metal cations, which weaken hydroxide adsorption along the step, shifting these peaks to more positive potentials [3]. We have recently extended this work using both single crystal experiments and DFT to find that the location of the sharp, step-associated peak is independent of pH in the absence of alkali metal cations and is dependent on alkali metal cation concentration and identity, supporting that this effect is mediated by the presence of alkali metal cations along the step [4]. We also find the rate of the hydrogen oxidation reaction on a polycrystalline platinum electrode to be dependent on the identity of the alkali metal cation present in an alkaline electrolyte, and using DFT find that the rate of the HOR is correlated with the alkali cation dependent hydroxide binding strength. Given that we find only a small interaction between the adsorbed cations and adsorbed hydrogen, we support that adsorbed hydroxide may play a role in the HOR mechanism, as has been previously proposed [5]. We show a volcano-type relationship between the experimentally measured hydrogen oxidation rate with the DFT calculated hydroxide adsorption potential across a variety of surfaces. A proposed mechanism will be discussed.
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