The relationship between the local environment, N-type, spin state and catalytic functionality of carbon-hosted Fe II/III -N 4 for the conversion of CO 2 to CO.

Iron and nitrogen codoped carbon (Fe-N-C) materials are promising alternatives to precious metal catalysts for the carbon dioxide electrochemical reduction reaction (CO 2 RR); however, the influence of the oxidation state, spin state, N-type and local environment of Fe-N on its catalytic activity remains poorly understood. In this study, we employed density functional theory (DFT) calculations to evaluate the catalytic activity of the pyridine-type Fe III/II N 4 motifs at the armchair and zigzag edges, the activity of the pyrrole-type Fe III/II N 4 sites in the bulk plane of carbon-based materials for the two-electron CO 2 RR by analyzing the stability of initial reactants, free-energy evolutions and energy barriers for the possible elementary reactions in the different spin states. The Fe ions in the armchair-edge pyridine-type FeN 4 are mainly in the +2 oxidation state, and use the high spin state in the spin uncoupling manner to achieve the most efficient CO 2 -COOH-CO conversion. In contrast, the zigzag-edge pyridine-type Fe II N 4 employs the medium spin state in the spin uncoupling manner to achieve the highest catalytic activity in the two-electron CO 2 RR. However, the Fe ions in the pyrrole-type bulk-hosted FeN 4 mainly remain in the +3 valence state during the conversion process of CO 2 to CO and utilize the medium spin state with spin coupling to obtain the highest catalytic activity. The corresponding kinetic analyses show that the armchair-edge pyridine-type Fe II N 4 catalyst exhibited the best catalytic performance among the three cases. Consequently, these findings present significant insights into the design of Fe single-atom catalysts for enhancing CO 2 RR catalytic activity by producing more armchair-edge pyridine-type FeN 4 sites, which may be constructed by introducing micropores in the carbon materials.