Competing hydrogen evolution reaction: a challenge in electrocatalytic nitrogen fixation

Ammonia (NH3) is an important chemical for human beings; however, the majority of NH3 is produced by the energy-intensive and environmentally unfriendly Haber-Bosch process. The electrocatalytic N-2 reduction reaction (NRR) under mild conditions is a promising candidate for replacing the conventional Haber-Bosch process. Nevertheless, competition between the H-2 evolution reaction (HER) and the NRR results in a low NRR performance (that is, a low faradaic efficiency and a low NH3 yield rate). In this review, several NRR mechanisms have been summarized: dissociative/associative, Mars-van Krevelen, and surface hydrogenation mechanisms. Moreover, some methods, such as optimization of reaction systems and other approaches for N-2 fixation, to improve the electrocatalytic NRR performance are discussed. Directly suppressing the competing HER seems to be a feasible approach to enhance the NRR performance; however, interestingly, protons are also involved as a reactant in the NRR, and directly suppressing the HER would definitely restrict the NRR. Furthermore, the reaction potentials of the HER and NRR are similar, and the potential of the HER is always lower than that of the NRR under both acidic and alkaline conditions; this indicates that the dominant NRR mechanism will involve vigorous H-2 evolution. Therefore, it is necessary to control the HER during the NRR via both catalyst design and optimization of experimental conditions rather than directly suppressing it.