A Review of Non-Precious Transition Metal Single-Atom Catalysts in NO ₃ RR: Recent Advances and Future Perspectives

Ammonia (NH 3 ) is an essential raw material for modern agriculture and industry, and is important for maintaining the sustainable development of modern societies. The conventional Haber–Bosch process has been highly successful as a commercially mature method of synthesizing ammonia, but it still suffers from the disadvantages of harsh reaction conditions, high energy consumption, and high CO 2 emissions. In recent years, the electrocatalytic nitrate reduction reaction (NO 3 RR) driven by renewable electrical energy has gradually become a multidisciplinary research hotspot, which can efficiently synthesize NH 3 under mild conditions. Single-atom catalysts show great potential for improving NO 3 RR yield and FE due to their maximum atom utilization, unique coordination environment, and electronic structure. This paper focuses on the experimental progress and recent theories of nonprecious transition metal single-atom electrocatalysts for NO 3 RR. First, we introduce the mechanism of nitrate reduction for ammonia synthesis and discuss the preparation of nonprecious transition metal single-atom electrocatalysts and related in situ characterization techniques. Second, we review the progress of nonprecious transition metal single-atom electrocatalysts for nitrate reduction and explore the relationship between the coordination environments of single-atom catalysts and their catalytic performance to guide the rational design of efficient catalysts. Finally, the paper summarizes the conclusions of the review and presents the challenges and prospects for the development of this promising field.