The relationship between the local environment, N-type, spin state and catalytic functionality of carbon-hosted Fe-N for the conversion of CO 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 (CORR); 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 FeN motifs at the armchair and zigzag edges, the activity of the pyrrole-type FeN sites in the bulk plane of carbon-based materials for the two-electron CORR 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 are mainly in the +2 oxidation state, and use the high spin state in the spin uncoupling manner to achieve the most efficient CO-COOH-CO conversion. In contrast, the zigzag-edge pyridine-type FeN employs the medium spin state in the spin uncoupling manner to achieve the highest catalytic activity in the two-electron CORR. However, the Fe ions in the pyrrole-type bulk-hosted FeN mainly remain in the +3 valence state during the conversion process of CO 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 FeN 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 CORR catalytic activity by producing more armchair-edge pyridine-type FeN sites, which may be constructed by introducing micropores in the carbon materials.