Unraveling the Origin of Sulfur-Doped Fe-N-C Single-Atom Catalyst for Enhanced Oxygen Reduction Activity: Effect of Iron Spin-State Tuning

Heteroatom doped atomically dispersed Fe₁-NC catalysts have been found to show excellent activity toward oxygen reduction reaction (ORR). However, the origin of the enhanced activity is still controversial because the structure-function relationship governing the enhancement remains elusive. Herein, sulfur(S)-doped Fe₁-NC catalyst was obtained as a model, which displays a superior activity for ORR towards the traditional Fe-NC materials. ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance spectroscopy revealed that incorporation of S in the second coordination sphere of Fe₁-NC can induce the transition of spin polarization configuration. Operando ⁵⁷Fe Mössbauer spectra definitively identified the low spin single-Fe³⁺-atom of C-FeN₄-S moiety as the active site for ORR. Moreover, DFT calculations unveiled that lower spin state of the Fe center after the S doping promotes OH* desorption process. This work elucidates the underlying mechanisms towards S doping for enhancing ORR activity, and paves a way to investigate the function of broader heteroatom doped Fe₁-NC catalysts to offer a general guideline for spin-state-determined ORR.