Electrochemical synthesis of ammonia: Progress and challenges

Ammonia, as an essential chemical and a promising fuel, has been predominantly synthesized by the Haber-Bosch process since the beginning of the 20th century. However, the harsh conditions, high energy consumption, and large carbon footprint of this century-old process enforce human being to develop clean and sustainable ammonia synthesis technologies. The electrochemical synthesis of ammonia has advantages of mild operating conditions, zero emission of carbon dioxide, capability to store renewable electricity in chemical bonds, and possibilities for distributed ammonia production. Benefiting from these merits, the electrochemical synthesis of ammonia is expected as a promising alternative to the Haber-Bosch process and has been extensively explored. Research on the electrochemical synthesis of ammonia by nitrogen reduction reaction (NRR) can be principally divided into several aspects including the development of catalysts, innovations on the configurations of electrolytic cells, strategies to improve the selectivity, and verification of ammonia synthesis results. Moreover, the electrochemical reduction of nitrogenous pollutants has also been tried to produce ammonia. In this review, we summarize the recent progress on these points and conclude with perspectives on challenges and future directions of electrochemical ammonia synthesis.