Understanding the role of central metal and coordination environment of single atom catalysts embedded in graphene flakes on CO2RR performance

Single atom catalysts are promising for enhancing performance of electrocatalytic CO2 reduction reaction (CO2RR). Herein, first-principle calculations were conducted to investigate the mechanism of two electron/proton transfers during CO2RR on two types of structures of single metal atom with four coordinated nitrogen atoms (M-N-4, M = Fe, Co, Ni, and Cu), namely metal phthalocyanine (MPc) and metal anchored nitrogen-containing carbon (M-NC). Our results demonstrate that nitrogen-containing carbon plane effectively immobilizes and activates metal centers. The physical adsorption of CO2 suggests that the initial activation of CO2 proceeds through a concerted proton-electron transfer step. M-NC surfaces exhibit higher favorability toward reactions of CO2 -> *COOH and CO2 -> *HCOO compared to MPc with the same metal center. Noteworthily, MPc structures featuring pyrrolic N coordination environment exhibit greater stability, whereas M-NC with pyridinic N coordination environment demonstrate higher activity in the initial reduction of CO2. CoPc and Fe-NC are the optical catalysts for achieving high CO2RR performance in the formation of CO and HCOOH, respectively. The strong binding of *COOH/*HCOO on catalyst can effectively reduce the reaction energy required for its subsequent reduction. The results of this study provide profound insights into the intricate coordination environment and the distinct role by various metal centers in M-N-x materials.