Electrocatalysts for hydrogen oxidation and evolution reactions

ABSTRACT The hydrogen economy is a clean, efficient, and sustainable energy system that delivers energy using hydrogen. Electrolyzers are used to generate hydrogen, and fuel cells consume hydrogen as a fuel. These devices rely on hydrogen evolution and oxidation reactions. Catalysts are required to accelerate the reactions. Pt is the best hydrogen oxidation/evolution reaction (HOR/HER) catalyst to date. However, the scarcity of Pt hinders its applications. Various processes have been developed to increase the catalyst activity in order to reduce the amount of Pt, or to develop non-precious metal catalysts to replace Pt. In this review, we focus on electrocatalysts for the hydrogen oxidation and evolution reactions. The reaction mechanism, factors influencing the catalyst activity, and the influence of the electrolyte (acid vs . base) are discussed. The recent advances in catalyst development, especially of non-precious metal catalysts, are also summarized. Due to the different reaction conditions and catalytic activities associated with the different electrolytes, the catalysts are classified into two categories:active in acid or in base.