Recent progress in the electrochemical ammonia synthesis under ambient conditions

Ammonia not only serves as a building block for nitrogen-rich fertilizer, but offers a carbon-free energy carrier. At present, industrial ammonia synthesis is dominated by the Hablocker-Bosch process under high temperature and pressure, resulting in high energy consumption and serious environmental issues. Considerable recent at tention has focused on designing electrocatalysts for the N-2 reduction reaction (NRR). However, the ammonia synthesis through electrocatalytic N-2 reduction is still far from practical applications. In this review, recent theoretical and experiment investigations on various catalysts for the ammonia synthesis under mild conditions are highlighted. Firstly, the mechanisms for the electrochemical NRR are reviewed briefly. Then, based on these mechanisms, various catalysts, such as noble metal catalysts, non-noble metal catalysts, metal-free catalysts and even single atoms catalysts, for ammonia production are reviewed, with a particular focus on the improvement of the catalytic activity and selectivity toward ammonia production through optimizing the electrolyte, pH and the structure of the catalyst, etc. Lastly, the challenges and outlook for the ammonia synthesis are shown. This review systematically retrospects recent advances in the electrocatalytic NRR to show the readers a thorough understanding in this field. Most importantly, this review sheds some light toward the future development of electrocatalytic NRR.