Observation of Thermal Formation of FeN4 Sites during Pyrolysis

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.