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 H-2 desorption and H-ads + OHads recombination is finally proposed for future HOR catalyst design in alkaline environment. (c) 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Automated summary

Anion exchange membrane (AEM) fuel cells have attracted attention due to the use of non-precious metals as catalysts, but the reaction kinetics of hydrogen oxidation reaction (HOR) in alkaline systems is much slower than in acidic systems. Major theories for understanding this include pH dependent hydrogen binding energy and the bifunctional theory based on both hydrogen and hydroxide adsorption. Different HOR mechanisms in acid and base electrolytes are discussed, along with strategies for catalyst design in alkaline environments.