Unraveling the Dynamic Low-Spin State Evolution of Single-Fe-Atom Sites for Efficient CO 2 Electroreduction

Precisely tailoring the electronic structure of the single-atom center is significant to improve the intrinsic reactivity of single-atom catalysts and elucidate the underlying reaction mechanism, but this remains highly challenging. Herein, we construct covalently oxygen-bridged single-Fe-atom sites on carbon nanotubes, dominated by low-spin (LS) Fe(III) sites, as an efficient catalyst for boosting the intrinsic catalytic performance for the electrochemical CO₂ reduction reaction (CO₂RR). A maximal CO Faradaic efficiency of 99% with an extremely high turnover frequency of 5.3 × 10⁴ h^-1 at an applied cathodic potential of -0.7 V vs RHE is achieved, showing a more than 20-fold increase as compared to that of high-spin (HS) Fe(III) sites. Taking advantage of operando and rapid freeze-quenched ⁵⁷Fe Mössbauer spectroscopy, together with operando X-ray absorption spectroscopy, a spin-driven CO₂ electroreduction mechanism is identified, wherein the in-situ-generated HS Fe(II) and LS Fe(II) sites dominate the CO₂RR at the low and high overpotentials, respectively. Furthermore, results from operando Raman and attenuated total reflectance surface-enhanced infrared absorption spectroscopy reveal that the one-electron reduction of phthalocyanine (Pc) coordinated to the central Fe leads to a weaker bonding strength of *CO on the LS O-Fe(II)Pc^- sites. Density functional theory calculations further illustrate the increased Bader charge and d-band center of the in-situ-generated LS O-Fe(II)Pc^-, facilitating the delocalization of electrons from the Fe 3d orbital to the 2p_z orbital of CO₂, thus reducing the formation free energy of the *COOH intermediate and boosting the CO₂RR performance.