Research Progress of Hydrogen Oxidation and Hydrogen Evolution Reaction Mechanism in Alkaline Media

Abstract

Hydrogen oxygen fuel cells and water electrolysis serves as two important systems for realizing the recycling of hydrogen energy, which involves two crucial electrochemical reactions, the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER). The kinetics of HOR/HER in alkaline media is 2 or 3 orders of magnitude slower than that in acidic media, which is the main bottleneck that hinders the development of alkaline membrane fuel cells and alkaline water electrolysis. Thus, clarifying the underlying difference of HOR/HER activity in alkaline and acid electrolytes, and exploring the alkaline HOR/HER mechanism are the significant challenges for widely commercial application of low temperature alkaline energy conversion devices. Here, this paper briefly reviews the related explanations and controversies about the alkaline HOR/HER mechanism in recent years, including bifunctional mechanism, hydrogen binding energy (HBE) theory and electronic effect. The bifunctional mechanism emphasizes the influence of water dissociation and OH adsorption on HER and HOR, respectively, which possesses guiding significance for designing and fabricating composite catalysts. The HBE theory stresses that Had is the key reaction intermediate of HOR/HER, and other external factors, such as electrode potential, pH, ions and so on, affect the HOR/HER mechanism and kinetics by disturbing HBE. HBE is widely considered to be the only activity descriptor of HOR/HER. The electronic effect emphasizes the role of catalysts’ composition and reaction intermediates in regulating electronic structure of active sites and changing HOR/HER mechanism. It provides an effective strategy to construct active sites and optimize catalytic activity. In addition, we summarize the theoretical simulation methods of electrochemical interface and their applications in exploring HOR/HER mechanism. In-depth theoretical simulation of HOR/HER mechanism requires the establishment of a more reasonable explicit solvation model on electrode/electrolyte interface and the combination of density functional theory (DFT), ab initio molecular dynamics (AIMD), and microkinetic model, to calculate the electronic structure and the dynamic processes of electrode/electrolyte interface, such as bond breaking and formation, solvent recombination, and proton migration in the electric double layer during reaction process, and then to analyze the HOR/HER mechanism and reaction kinetics under different electrode potentials and electrolytes. The present review is helpful for understanding the ongoing developments of HOR/HER mechanism. And the combination of experiment and theoretical calculation can be employed to explore the pH-dependence of HOR/HER deeply, and design novel HOR/HER catalysts with high activity and stability.