Strategies to suppress hydrogen evolution for highly selective electrocatalytic nitrogen reduction: Challenges and perspectives

Ammonia, as a significant chemical for fertilizer production and also a promising energy carrier, is mainly produced through the traditional energy-intensive Haber-Bosch process. Recently, the electrocatalytic N2 reduction reaction (NRR) for ammonia synthesis has received tremendous attention with the merits of energy saving and environmental friendliness. To date, the development of the NRR process is primarily hindered by the competing hydrogen evolution reaction (HER), whereas the corresponding strategies for inhibiting this undesired side reaction to achieve high NRR selectivity are still quite limited. Furthermore, for such a complex reaction involving three gas-liquid-solid phases and proton/electron transfer, it is also rather meaningful to decouple and summarize the current strategies for suppressing H2 evolution in terms of NRR mechanisms, kinetics, thermodynamics, and electrocatalyst design in detail. Herein, on the basis of the NRR mechanisms, we systematically summarize the recent strategies to inhibit the HER for a highly selective electrocatalytic NRR, focusing on limiting the proton- and electron-transfer kinetics, shifting the chemical equilibrium, and designing the electrocatalysts. Additionally, insights into boosting the NRR selectivity and efficiency for practical applications are also presented in detail with regard to the determination of ammonia, the activation of the N2 molecule, the regulation of the gas-liquid-solid three-phase interface, the coupled NRR with value-added oxidation reactions, and the development of flow cell reactors.