It is unclear by far how the FeN₄ site, which is responsible for the high activity toward the oxygen reduction reaction (ORR) of Fe-N-C catalysts, forms during pyrolysis, which limits further advancement of Fe-N-C catalysts for the ORR. Herein by conducting operando in-temperature x-ray absorption spectroscopy on Fe compounds mixed with a variety of species during pyrolysis, we observed the formation of in-plane FeN₄ moieties starting around 600 ℃, accompanied with an ORR activity jump in acid in a rotating disk electrode. By varying the N and C precursors, as well as pyrolysis temperature, we showed that inorganic Fe species such as Fe nanoparticles, oxides, carbides et al. will form if Fe cannot be effectively incorporated into N-doped micropores during pyrolysis. The formation mechanism of in-plane FeN₄ moieties during pyrolysis will be discussed.

Acknowledgement

This work was supported by the US Department of Energy under award number DE-EE0008416. The authors declare no competing financial interests. MRCAT operations are supported by the Department of Energy and the MRCAT member institution. This research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.