The relationship between the local environment, N-type, spin state and catalytic functionality of carbon-hosted FeII/III–N4 for the conversion of CO2 to CO

Abstract

Iron and nitrogen codoped carbon (Fe–N–C) materials are promising alternatives to precious metal catalysts for the carbon dioxide electrochemical reduction reaction (CO₂RR); 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/IIN₄ motifs at the armchair and zigzag edges, the activity of the pyrrole-type Fe^III/IIN₄ sites in the bulk plane of carbon-based materials for the two-electron CO₂RR 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 FeN₄ are mainly in the +2 oxidation state, and use the high spin state in the spin uncoupling manner to achieve the most efficient CO₂–COOH–CO conversion. In contrast, the zigzag-edge pyridine-type Fe^IIN₄ employs the medium spin state in the spin uncoupling manner to achieve the highest catalytic activity in the two-electron CO₂RR. However, the Fe ions in the pyrrole-type bulk-hosted FeN₄ mainly remain in the +3 valence state during the conversion process of CO₂ 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^IIN₄ 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 CO₂RR catalytic activity by producing more armchair-edge pyridine-type FeN₄ sites, which may be constructed by introducing micropores in the carbon materials.