The activity and selectivity in CO2 to HCOOH conversion on TM-G-N4 with axially coordinated nonmetal

Electrocatalytic CO₂ reduction reaction (CO₂RR) is an effective strategy to address the energy crisis and the greenhouse effect. However, the efficiency of CO₂RR is still insufficient for industrial applications. It is needed to develop suitable electrocatalysts to increase efficiency of CO₂RR. The transition metal coordinated with nitrogen doped graphene (TMN₄@G) combined with the unique advantages of axial coordination engineering are the promising methods for catalytic CO₂RR. In this study, the CO₂RR performance of TMN₄@G (TM = Co, Ni, and Cu) with various axial ligands (TMN₄-L@G, L = B, N, P, O, S, F, Cl, Br, I, and OH) is systematically evaluated by density functional theory (DFT) calculations. The results indicate that axial ligands have different degree of effect on the CO₂RR of TMN₄@G. CuN₄-N@G and CuN₄-P@G display excellent suppression ability of competitive HER via P2 pathway mechanism (reduction into HCOOH) among TMN₄-L@G, with the corresponding potential limiting (U_L) are −0.16 and − 0.11 V, respectively. CO₂ molecule is predicted physically adsorbed on CuN₄-N@G and CuN₄-P@G, while following hydrogenation intermediates is chemisorbed on these two catalysts surface, which indicates that axial coordination engineering can improve the adsorption stability of intermediates by regulating the electric charge distribution of the CuN₄-N@G and CuN₄-P@G. Moreover, density of states (DOS) and charge density difference analysis indicate that the introduced axial nonmetal atoms can enhance intermediates adsorption ability and improve the activity of CO₂ conversion into HCOOH. Meanwhile, the d-band center, work function, and ICOHP results reflect that the axial ligands can modulate electronic structures of CuN₄@G and improve its surface CO₂RR catalytic activity. The ab initio molecular dynamics (AIMD) simulations verified that CuN₄-N@G and CuN₄-P@G are dynamically stabilized at the reaction temperature. This study offers novel insight into the mechanism of CO₂ electrocatalytic reduction on axially coordinated graphene.