Unraveling the Dynamic Low-Spin State Evolution of Single-Fe-Atom Sites for Efficient CO <sub>2</sub> Electroreduction
Yaqiong Zeng, Jian Zhao, Shifu Wang, Weijue Wang, Ying‐Rui Lu, Shibo Xi, Wei Xu, Yoshitaka Yoda, Ryo Masuda, Xuning Li, Yanqiang Huang, Bin Liu, Tao Zhang
Abstract
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 CO 2 reduction reaction (CO 2 RR). A maximal CO Faradaic efficiency of 99% with an extremely high turnover frequency of 5.3 × 10 4 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 57 Fe Mössbauer spectroscopy, together with operando X-ray absorption spectroscopy, a spin-driven CO 2 electroreduction mechanism is identified, wherein the in-situ-generated HS Fe(II) and LS Fe(II) sites dominate the CO 2 RR 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 CO 2, thus reducing the formation free energy of the *COOH intermediate and boosting the CO 2 RR performance.