Role of atomic substitution in first coordination shell of Fe–N–C single atom catalyst towards oxygen reduction reaction: A theoretical study

Due to structural complexity and limited understanding, precise chemical modification in atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts requires theoretical calculations to optimize electronic, geometric, and catalytic performance and experimental implementation of learned lessons to improve their catalytic performance towards oxygen reduction reaction (ORR). Herein, we conducted a density functional theory (DFT) investigation to understand the effect of the replacement of O-atoms in Fe–O4–C by N-atoms in the first coordination shell of Fe-site (constructing five models: Fe–O4–C, Fe–O3N–C, Fe–N2O2–C, Fe–ON3–C and Fe–N4–C) towards ORR. The geometric and electronic calculations of these models suggested that replacing all the O-atoms with N-atoms around the Fe-site would make it more conducive to the adsorption of ORR intermediates. Moreover, the Gibbs free energy calculations demonstrated that the Fe-site becomes the active centre for 2e− ORR, leading to the dominant generation of H2O2 when Fe is coordinated with four O-atoms (Fe–O4–C). Whereas in compounds like Fe–N2O2–C, Fe–O3N–C, and Fe–N4–C, where the Fe-site displays 4e− ORR, form water is a preferred product if the O-atoms are replaced by N-atoms, as in Fe–N2O2–C, Fe–O3N–C, and Fe–N4–C. This study would provide a fundamental understanding of how atomically dispersed M-N-C catalysts' electrocatalytic activity is affected by changes in the coordination environment of metal sites.