Comprehensive understanding of the thriving electrocatalytic nitrate/nitrite reduction to ammonia under ambient conditions

Ammonia (NH3) is a multifunctional compound that is an important feedstock for the agricultural and pharmaceutical industries and attractive energy storage medium. At present, NH3 synthesis is highly dependent on the conventional Haber–Bosch process that operates under harsh conditions, which consumes large quantities of fossil fuels and releases a large amount of carbon dioxide. As an alternative, electrosynthesis is a prospective method for producing NH3 under normal temperature and pressure conditions. Although electrocatalytic nitrogen reduction to ammonia has attracted considerable attentions, the low solubility of N2 and high N≡N cracking energy render the achievements of high NH3 yield rate and Faradaic efficiency difficult. Nitrate and nitrite (NOx−) are common N-containing pollutants. Due to their high solubilities and low dissociation energy of N=O, NOx− are ideal raw materials for NH3 production. Therefore, electrocatalytic NOx− reduction to NH3 (eNOxRR) is a prospective strategy to simultaneously realise environmental protection and NH3 synthesis. This review offers a comprehensive understanding of the thriving eNOxRR under ambient conditions. At first, the popular theory and mechanism of eNOxRR and a summary of the measurement system and evaluation criteria are introduced. Thereafter, various strategies for developing NOx− reduction catalysts are systematically presented and discussed. Finally, the challenges and possible prospects of electrocatalytic NOx− reduction are outlined to facilitate energy-saving and environmentally friendly large-scale synthesis of NH3 in the future.