Homogeneous, Heterogeneous, and Biological Catalysts for Electrochemical N2 Reduction toward NH3 under Ambient Conditions

Ammonia (NH3) synthesis is an important industrial chemical process. Recently, electrochemically converting the earth-abundant dinitrogen (N-2) in the aqueous phase to NH3 at ambient conditions has been proposed as an alternative to the well-established Haber-Bosch process. Catalysts for the electrochemical N-2 reduction to NH3 play crucial roles in realizing this NH3 synthesis route. Electrochemical N-2 reduction has been studied for decades, and many studies have emerged in the past few years. Herein, we provide a comprehensive review to summarize various catalysts used for achieving electrochemical N-2 reduction to NH3, including homogeneous, heterogeneous and biological catalysts, as well as relevant computational studies to understand their reaction mechanisms. We compare the advantages and shortcomings of these catalytic systems. Future research directions for realizing catalysts with low overpotentials, high energy efficiency, good scalability, and stability modularity are also proposed. This review provides an overview of this fast-growing research field and encourages more studies toward the rational design of catalysts for electrochemical N-2 reduction to NH3 under ambient conditions.