(Invited) Revisiting the Nature of Active Sites in Pyrolyzed Fe-N-C Electrocatalysts: In Situ Monitoring the Structure Evolution of Active Sites in Fe-N-C Catalysts during Pyrolysis

Since it was found in 1980s that pyrolysis of the mixture of Fe, N, and C precursors leads to decent electrocatalysts for the oxygen reduction reaction (ORR), the nature of active sites high inherent ORR activity formed upon the heat treatment remains elusive. This mystery hasn’t been unraveled by recent applications of advanced spectroscopy and microscopy techniques.1-4 Herein, we conducted in situ x-ray absorption spectroscopy on carbon supported iron(II) phthalocyanine (FePc), as well as the mixture of ZIF-8, phenanthroline, Fe(II) acetate (namely MOF-derived catalyst) in a sealed inert gas environment as a function of pyrolysis temperature, in attempt to directly monitor the formation of new active sites. The FePc represents the Fe precursors with preexisting Fe-N4 moieties, and the Fe(II) acetate represents the precursors without. We observed the destruction of the in-plane Fe-N4 sites in FePc as a result of demetalization starting at a low temperature of ~300 ℃. However, we did not see the formation of new Fe-Nx active sites but metallic Fe clusters at elevated temperatures. The maximum of the ORR activity of the FePC/C pyrolyzed at 600 ℃ was attributed to the counterbalance between the loss of Fe-N4 sites and the increase of the carbon porosity caused by the demetalization, plus strengthening of the interactions between the FePc and carbon substrate, rather than creation of new active sites. On the other hand, during the pyrolysis of the MOF-derived catalyst, the XAS reveals the evolution from the Fe-O6 moiety to the Fe-N4 moiety with the local geometry resembling that of FePc but with longer Fe-N bond distance. The formation of metallic Fe clusters was not observed even at a high temperature of 1000 ℃. These results together suggest that the new active sites are likely formed upon the incorporation of Fe atoms into the N-doped carbon matrix during the pyrolysis. Compared to FePc/C, the carbon matrix in the MOF-derived catalyst appears to capture Fe atoms more efficiently forming Fe-N4 sites, owing probably to more abundant C-N cavities. 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. References (1) Li, J.; Alsudairi, A.; Ma, Z.-F.; Mukerjee, S.; Jia, Q. Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: In Situ Identification of the Site-Blocking Effect. J. Am. Chem. Soc. 2017, 139, 1384-1387. (2) Li, J.; Ghoshal, S.; Liang, W.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; McKinney, S.; McCool, G.; Ma, C.; Yuan, X.; Ma, Z.-F.; Mukerjee, S.; Jia, Q. Structural and mechanistic basis for the high activity of Fe-N-C catalysts toward oxygen reduction. Energ Environ Sci 2016, 9, 2418-2432. (3) Jia, Q.; Ramaswamy, N.; Tylus, U.; Strickland, K.; Li, J.; Serov, A.; Artyushkova, K.; Atanassov, P.; Anibal, J.; Gumeci, C.; Barton, S. C.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; Mukerjee, S. Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon electrocatalysts for oxygen reduction in acid. Nano Energy 2016, 29, 65-82. (4) Jia, Q.; Ramaswamy, N.; Hafiz, H.; Tylus, U.; Strickland, K.; Wu, G.; Barbiellini, B.; Bansil, A.; Holby, E. F.; Zelenay, P.; Mukerjee, S. Experimental Observation of Redox-Induced Fe–N Switching Behavior as a Determinant Role for Oxygen Reduction Activity. ACS Nano 2015, 9, 12496-12505. Figure 1