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

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• 2D ZIF nanosheets derived Fe-N-C single-atom catalysts for CO 2 reduction. • Axial Cl atoms modulate Fe atoms in catalytically active FeN 4 sites. • Axial Cl facilitates the formation, desorption, and adsorption of intermediates. • FeN 4 Cl/NC exhibits a high CO Faradaic efficiency of 90.5% at −0.6 V vs. RHE. Iron-nitrogen-carbon single-atom catalysts (Fe-N-C SACs) are promising low-cost catalysts for electrochemical CO 2 reduction reaction (CO 2 RR) 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 FeN 4 sites, which facilitates the desorption of CO* and inhibits the adsorption of H*, resulting in improved activity and selectivity in CO 2 RR. The Cl modified Fe-N-C SAC embedded in nitrogen-doped carbon nanosheets (FeN 4 Cl/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 Å and four N atoms at 2.02 Å. The optimized FeN 4 Cl/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 CO 2 RR catalysts. FeN 4 Cl/NC was further applied as a bifunctional catalyst to construct rechargeable zinc-CO 2 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.