Electrocatalysts development for hydrogen oxidation reaction in alkaline media: From mechanism understanding to materials design

Anion exchange membrane (AEM) fuel cells have gained great attention partially due to the advantage of using non-precious metal as catalysts. However, the reaction kinetics of hydrogen oxidation reaction (HOR) is two orders of magnitude slower in alkaline systems than in acid. To understand the slower kinetics of HOR in base, two major theories have been proposed, such as (1) pH dependent hydrogen binding energy as a major descriptor for HOR; and (2) bifunctional theory based on the contributions of both hydrogen and hydroxide adsorption for HOR in alkaline electrolyte. Here, we discuss the possible HOR mechanisms in alkaline electrolytes with the corresponding change in their Tafel behavior. Apart from the traditional Tafel-Volmer and Heyrovsky-Volmer HOR mechanisms, the recently proposed hydroxide adsorption step is also discussed to illustrate the difference in HOR mechanisms in acid and base. We further summarize the representative works of alkaline HOR catalyst design (e.g., precious metals, alloy, intermetallic materials, Ni-based alloys, carbides, nitrides, etc.), and briefly describe their fundamental HOR reaction mechanism to emphasize the difference in elementary reaction steps in alkaline medium. The strategy of strengthening local interaction that facilitates both H2 desorption and Hads + OHads recombination is finally proposed for future HOR catalyst design in alkaline environment.