Vacancy defects inductive effect of asymmetrically coordinated single-atom Fe-N 3 S 1 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, we report an atomically dispersed N, S co-doped carbon with abundant vacancy defects (NSC-vd) anchored Fe single atoms (SAs) derived from coal pitch by an encapsulation-pyrolysis-etching strategy and propose a vacancy defects inductive effect based on asymmetrically coordinated Fe SAs/NSC-vd catalyst for promoting electrocatalytic ORR. The optimized catalyst featured of stable Fe-N 3 S 1 active sites exhibits high ORR activity with turnover frequency (18.5 and 2.87 s -1 ) and mass activity (56.2 and 15.9 A mg -1 ), which are 132 and 11, 312 and 42 times higher than that of 20% Pt/C at 0.87 and 0.74 V in 0.1 M KOH and 0.5 M H 2 SO 4 , respectively. In-situ Raman and attenuated total reflectance surface enhanced infrared absorption spectroscopy reveal the Fe-N 3 S 1 active sites are beneficial to the generation of OH - species and exhibits 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. The Fe-N coordination is major contributor as active site for O 2 adsorption-activation in 0.1 M KOH, while the Fe-S coordination is main active center for ORR in 0.5 M H 2 SO 4 . Theoretical calculation reveals that the upshifted d band center of Fe-N 3 S 1 active sites facilitate the adsorption of O 2 and accelerate the kinetics process of *OH reduction. The abundant vacancy defects around the Fe-N 3 S 1 active sites balance the OOH* formation and *OH reduction, thus synergetic promoting the electrocatalytic ORR process. This work proposes a novel controllable synthesis method and structural regulation strategy of SACs for high-efficiency electrocatalytic ORR in energy conversion devices. This article is protected by copyright. All rights reserved.