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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. [ABSTRACT FROM AUTHOR]

Dual-atomic-site catalysts (DASCs), as an extension of single-atom catalysts (SACs), have attracted increasing attention owing to the synergistic effect. However, the study of DASCs is still at the early stages and mainly based on metal atom pairs. More experimental and theoretical exploration needed for further guiding the reasonable design of diatomic active sites. Herein, remarkable activity for oxygen reduction reaction (ORR) of Se 1 -NC single-atom catalyst was discovered, and then Fe/Se dual-atom catalysts were constructed to demonstrate the dual-atom sites synergistic effect. Encouraging, the Fe 1 Se 1 -NC catalyst displays significant enhancement for ORR towards Fe 1 -NC and Se 1 -NC in both alkaline and acid electrolytes. Spectroscopic characterizations and theoretical calculations reveal that there are multiple effects for the introduction of Se, especially for supplying new active sites, and effectively tuning charge redistribution and the spin-state of Fe active sites, which presenting a new strategy to improve the electrochemical performance based on the metal-nonmetal dual-atomic-site catalysts. Atomically dispersed metal(Fe) and non-metal (Se) dual atoms anchored on nitrogen-doped carbon for the first time. The co-exist of Fe-N 5 moiety and SeC 2 dual active sites displays significant synergetic enhancement for ORR towards Fe 1 -NC and Se 1 -NC. This work provides a unique insights of atomical introducing of non-metal active sites for rational design of more effective catalysts. [Display omitted] • Metal(Fe) and nonmetal(Se) dual-atom sites are prepared for the first time, which displays significant enhancement for ORR. • DFT calculations indicate that the co-exist of Fe and Se dual-atomic-site is more beneficial to *OH desorption process. • Se atom has several functions: (1) adjust the electronic state of the Fe-N X. (2) Serve as ORR site. (3) Create rich pores. [ABSTRACT FROM AUTHOR]