Unveiling the In Situ Generation of a Monovalent Fe(I) Site in the Single-Fe-Atom Catalyst for Electrochemical CO₂ Reduction

Atomically dispersed single-atom catalysts are among the most attractive electrocatalysts for the CO₂ reduction reaction (CRR). To elucidate the origin of the exceptional activity of atomically dispersed Fe–N–C catalyst in CRR, we have performed operando ⁵⁷Fe Mössbauer spectroscopic studies on a model single-Fe-atom catalyst with a well-defined N coordination environment. Combining with operando X-ray absorption spectroscopy, the in situ-generated four pyrrolic nitrogen atom-coordinated low-spin Fe­(I) (LS Fe^IN₄) featuring monovalent iron is identified as the reactive center for the conversion of CO₂ to CO. Furthermore, density functional theory calculations reveal that the optimal binding strength of CO₂ to the LS Fe^IN₄ site, with strong orbital interactions between the singly occupied d_z² orbital of the Fe­(I) site and the singly occupied π* orbital of [COOH] fragment, is the key factor for the excellent CRR performance.