Some remaining puzzles in hydrogen electrocatalysis mechanisms on platinum surfaces

Hydrogen energy is of significant importance in tackling the global CO₂ challenge. The interconversion between renewable/surplus electricity and chemical energy contained in H₂ molecules can be achieved through H₂ evolution and oxidation reactions (HER and HOR) in electrolyzers and fuel cells, respectively. Despite the apparent simplicity of this pair of fundamental electrochemical reactions, a comprehensive picture of interfacial processes has not been obtained, which partially hinders the rational development of more advanced electrocatalysts/interfaces and the full realization of the H₂ economy. Herein, we summarize some of the most intensively debated puzzles in HER/HOR mechanisms on platinum surfaces and the evolution and current status of our understanding, including rate-determining steps, structures of active intermediates, electrolyte pH effects, the role of cations, and the origin of high activity on bimetallic interfaces. Perspectives on further research efforts that may help decipher these enigmas are also provided.