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

2021 
Heteroatom doped atomically dispersed Fe 1 -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 Fe 1 -NC catalyst was obtained as a model, which displays a superior activity for ORR towards the traditional Fe-NC materials. 57 Fe Mossbauer spectroscopy and electron paramagnetic resonance spectroscopy revealed that incorporation of S in the second coordination sphere of Fe 1 -NC can induce the transition of spin polarization configuration. Operando 57 Fe Mossbauer spectra definitively identified the low spin  single-Fe 3+ -atom of C-FeN 4 -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 Fe 1 -NC catalysts to offer a general guideline for spin-state-determined ORR.
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