Vacancy Defects Inductive Effect of Asymmetrically Coordinated Single-Atom Fe─N₃ S₁ Active Sites for Robust Electrocatalytic Oxygen Reduction with High Turnover Frequency and Mass Activity

The development of facile, efficient synthesis method to construct low-cost and high-performance single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is extremely important, yet still challenging. Herein, an atomically dispersed N, S co-doped carbon with abundant vacancy defects (NSC-vd) anchored Fe single atoms (SAs) is reported and a vacancy defects inductive effect is proposed for promoting electrocatalytic ORR. The optimized catalyst featured of stable Fe─N₃ S₁ active sites exhibits excellent ORR activity with high turnover frequency and mass activity. In situ Raman, attenuated total reflectance surface enhanced infrared absorption spectroscopy reveal the Fe─N₃ S₁ active sites exhibit different kinetic mechanisms in acidic and alkaline solutions. Operando X-ray absorption spectra reveal the ORR activity of Fe SAs/NSC-vd catalyst in different electrolyte is closely related to the coordination structure. Theoretical calculation reveals the upshifted d band center of Fe─N₃ S₁ active sites facilitates the adsorption of O₂ and accelerates the kinetics process of *OH reduction. The abundant vacancy defects around the Fe─N₃ S₁ active sites balance the OOH* formation and *OH reduction, thus synergetically promoting the electrocatalytic ORR process.