Probing the Oxygen Reduction Reaction Intermediates and Dynamic Active Site Structures of Molecular and Pyrolyzed Fe–N–C Electrocatalysts by In Situ Raman Spectroscopy

While FeN4 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 FeN4 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–Nx 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 (*O2– and *OOH) and RDSs. Furthermore, from the FePc model catalyst, we reveal a pH-dependent structural switching of the FeN4 between planar and non-planar structures during ORR electrocatalysis, which provides important insights into their pH-dependent ORR activity (RDS) and stability.