Effects of Alkali Metal and Organic Cations on the Hydrogen Oxidation Reaction in Alkaline Electrolytes

Abstract

Spectator ionic species in electrolyte solutions affect the rate and mechanism of electro-catalytic reactions. For example, it is well known that many anions, including (bi)sulfate and organic sulfonate can specifically adsorb to a platinum electrode at high potentials and affect the rate of the oxygen reduction reaction, important to fuel cell performance. If organic anions can specifically adsorb at high potentials and effect reaction rates, it should be determined if the organic cations proposed for use in new anion exchange membrane (AEM) fuel cells could specifically adsorb to platinum at low potentials and affect the rate of the hydrogen oxidation reaction. Recent evidence has shown that alkali metal cations can affect a variety of electro-catalytic reactions on platinum, including methanol oxidation, hydrogen oxidation, and oxygen reduction, but direct experimental evidence for their specific adsorption is lacking. Previously, we have used density functional theory (DFT), an ab initio quantum mechanics based technique to show that alkali metal cations can specifically adsorb to platinum and palladium electrodes at low potentials and compete with hydrogen adsorption in high pH electrolytes. We have found experimentally that the rate of the hydrogen oxidation reaction is dependent on the cation present in an alkaline electrolyte solution. We proposed and evaluated a mechanism for this effect using DFT to show alkali metal cations can specifically adsorb to the most active low index platinum facet for hydrogen oxidation, Pt(110), and can promote the adsorption of hydroxide, an important reaction intermediate. We have extended both our computational and experimental work to find that quaternary ammonium cations can also specifically adsorb to low index platinum electrode surfaces. The development of the mechanism for the effect of alkali metal cations on the rate of the hydrogen oxidation reaction and the DFT methods for simulating alkali metal cation and organic cation specific adsorption will be discussed.