Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO₂ Reduction

Single-atom catalysts with a well-defined metal centeropen uniqueopportunities for exploring the catalytically active site and reactionmechanism of chemical reactions. However, understanding of the electronicand structural dynamics of single-atom catalytic centers under reactionconditions is still limited due to the challenge of combining operando techniques that are sensitive to such sites andmodel single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of thedynamic structural and electronic evolution during the electrochemicalCO(2) reduction reaction (CO2RR) of a model catalystcomprising iron only as a high-spin (HS) Fe(III)N-4 centerin its resting state. Operando Fe-57 Mo''ssbauerand X-ray absorption spectroscopies clearly evidence the change froma HS Fe(III)N-4 to a HS Fe(II)N-4 center withdecreasing potential, CO2- or Ar-saturation of the electrolyte,leading to different adsorbates and stability of the HS Fe(II)N-4 center. With operando Raman spectroscopyand cyclic voltammetry, we identify that the phthalocyanine (Pc) ligandcoordinating the iron cation center undergoes a redox process fromFe(II)Pc to Fe(II)Pc-. Altogether, the HS Fe(II)Pc- species is identified as the catalytic intermediatefor CO2RR. Furthermore, theoretical calculations revealthat the electroreduction of the Pc ligand modifies the d-band centerof the in situ generated HS Fe(II)Pc- species, resulting in an optimal binding strength to CO2 and thus boosting the catalytic performance of CO2RR.This work provides both experimental and theoretical evidence towardthe electronic structural and dynamics of reactive sites in single-Fe-atommaterials and shall guide the design of novel efficient catalystsfor CO2RR.