Advances in the study of HOR reaction mechanisms under alkaline conditions

Hydrogen energy is an important energy carrier, which is an ideal choice to meet energy demand and reduce harmful gas emissions. The green recycling of hydrogen energy depends on water electrolysis and hydrogen fuel cells, which involves hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER). The activity of HER/HOR in alkaline electrolyte, however, exhibits a significantly lower magnitude (2–3 orders) compared to that observed in an acidic medium, which hinders the development of alkaline water electrolysis and alkaline membrane fuel cells. Therefore, comprehending the characteristics of HOR/HER activity in alkaline electrolytes and elucidating its underlying mechanism is a prerequisite for the design of advanced electrocatalysts. Based on this background, this review will briefly summarize the explanations and controversies about the basic HOR mechanism, including bifunctional mechanism and hydrogen binding energy theory. Moreover, the crucial affecting factors of the HOR kinetics, such as d-band center theory, interfacial water recombination, alkali metal cations and electronic effects, are discussed. Thus, based on the above theories, the design principle, catalytic performance, and latest progress of HOR electrocatalysts are summarized. An outlook and future research perspectives of advanced catalysts for hydrogen energy recycling are addressed. This review is helpful to understand the latest development of HOR mechanism and design cost-effective and high-performance HOR electrocatalysts towards the production of clean renewable energies.