Transition metal-based single-atom catalysts (TM-SACs); rising materials for electrochemical CO2 reduction

The continuous increase of global atmospheric CO2 concentrations brutally damages our environment. A series of methods have been developed to convert CO2 to valuable fuels and value-added chemicals to maintain the equilibrium of carbon cycles. The electrochemical CO2 reduction reaction (CO2RR) is one of the promising methods to produce fuels and chemicals, and it could offer sustainable paths to decrease carbon intensity and support renewable energy. Thus, significant research efforts and highly efficient catalysts are essential for converting CO2 into other valuable chemicals and fuels. Transition metal-based single atoms catalysts (TM-SACs) have recently received much attention and offer outstanding electrochemical applications with high activity and selectivity opportunities. By taking advantage of both heterogeneous and homogeneous catalysts, TM-SACs are the new rising star for electrochemical conversion of CO2 to the value-added product with high selectivity. In recent years, enormous research effort has been made to synthesize different TM-SACs with different M–Nx sites and study the electrochemical conversion of CO2 to CO. This review has discussed the development and characterization of different TM-SACs with various catalytic sites, fundamental understanding of the electrochemical process in CO2RR, intrinsic catalytic activity, and molecular strategics of SACs responsible for CO2RR. Furthermore, we extensively review previous studies on 1st-row transition metals TM-SACs (Ni, Co, Fe, Cu, Zn, Sn) and dual-atom catalysts (DACs) utilized for electrochemical CO2 conversions and highlight the opportunities and challenges.