While FeN 4 species are widely suggested as the active sites of noble-metal-free Fe−N−C oxygen reduction reaction (ORR) electrocatalysts, the ORR mechanism, particularly the rate-determining steps (RDSs) at the Fe centers, and the likely contribution of co-existed C−N active site remain disputed. Moreover, the dynamic structures of the FeN 4 active sites during ORR electrocatalysis also remain elusive. By in situ (isotope-labeled) Raman spectroscopy of molecular Fe phthalocyanine (FePc) model catalysts and pyrolyzed Fe−N−C catalysts, we achieve direct, simultaneous spectral identification of the ORR intermediates/RDSs at different active sites under different pH conditions, from which their intrinsic activities and ORR mechanisms can be quantitatively decoupled. Besides the single-atomic Fe−N x site, two kinds of C−N sites were pinpointed and clarified as separate active sites in pyrolyzed Fe−N−C catalysts, showing different ORR intermediates (*O 2 − and *OOH) and RDSs. Furthermore, from the FePc model catalyst, we reveal a pH-dependent structural switching of the FeN 4 between planar and non-planar structures during ORR electrocatalysis, which provides important insights into their pH-dependent ORR activity (RDS) and stability.

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The past decade has witnessed the great potential of Fe‐based single‐atom electrocatalysis in catalyzing oxygen reduction reaction (ORR). However, it remains a grand challenge to substantially improve their intrinsic activity and long‐term stability in acidic electrolytes. Herein, we report a facile chemical vapor deposition strategy, by which high‐density Fe atoms (3.97 wt%) are coordinated with square‐planar para‐positioned nitrogen and phosphorus atoms in a hierarchical carbon framework. The as‐crafted atomically dispersed Fe catalyst (denoted Fe‐SA/PNC) manifests an outstanding activity towards ORR over the entire pH range. Specifically, the half‐wave potential of 0.92 V, 0.83 V, and 0.86 V vs. reversible hydrogen electrode (RHE) are attained in alkaline, neutral, and acidic electrolytes, respectively, representing the high performance among reported catalysts to date. Furthermore, after 30,000 durability cycles, the Fe‐SA/PNC remains to be stable with no visible performance decay when tested in 0.1 M KOH and 0.5 M H2SO4, and only a minor negative shift of 40 mV detected in 0.1 M HClO4, significantly outperforming commercial Pt/C counterpart. The coordination motif of Fe‐SA/PNC is validated by density functional theory (DFT) calculations. This work provides atomic‐level insight into improving the activity and stability of non‐noble metal ORR catalysts, opening up an avenue to craft the desired single‐atom electrocatalysts.