Axial chlorine coordinated iron-nitrogen-carbon single-atom catalysts for efficient electrochemical CO₂ reduction

Iron-nitrogen-carbon single-atom catalysts (Fe-N-C SACs) are promising low-cost catalysts for electrochemical CO2 reduction reaction (CO2RR) to achieve carbon neutrality. However, their relatively low selectivity and activity are related to the strong binding of reaction intermediates (e.g., CO*) on single Fe atoms. Here, combining experimental and theoretical studies, we show that introducing axial chlorine (Cl) atom can modulate the electronic structure of Fe atoms in catalytically active FeN4 sites, which facilitates the desorption of CO* and inhibits the adsorption of H*, resulting in improved activity and selectivity in CO2RR. The Cl modified Fe-N-C SAC embedded in nitrogen-doped carbon nanosheets (FeN4Cl/NC) was synthesized in two steps: pyrolyzing Fe-loaded two-dimensional zeolite imidazole framework nanosheets and low-temperature incubation in hydrochloric acid solution. X-ray absorption spectroscopy results reveal that most atomically dispersed Fe atoms are coordinated with one axial Cl atom at 2.26 angstrom and four N atoms at 2.02 angstrom. The optimized FeN4Cl/NC exhibits a CO Faradaic efficiency of 90.5%, a high current density of 10.8 mA cm(-2) at a low overpotential of 490 mV, and a high turnover frequency of 1566 h(-1), one of the best among recently reported Fe-based CO2RR catalysts. FeN4Cl/ NC was further applied as a bifunctional catalyst to construct rechargeable zinc-CO2 batteries, delivering a power density of 0.545 mW cm(-2) with excellent stability over 15 h. Tailoring the coordination environment of metal atoms in M-N-C SACs by introducing axial atoms may be further extended as an efficient general approach to design advanced catalysts for various electrochemical applications, such as fuel cells, nitrogen fixation, and lithium-sulfur batteries.