In Situ Observation of the Formation of Fe-N4 Sites Via Metalation during the Pyrolysis of Fe Precursors and N-Doped Carbon Matrix

A clear understanding of the formation mechanism of single atom M-N4 sites (M represents transition metals such as Mn, Fe, Co, etc) is required for rational synthesis of M-N-C catalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, multi-component characterizations including in-temperature x-ray absorption spectroscopy and temperature-programmed reaction experiments were combined to unravel the formation of Fe-N4 sites during pyrolysis. Two synthesis routes were used to form Fe-N4 sites: (1) pyrolyzing the mixture of FeCl2·4H2O with zeolitic imidazolate framework (ZIF8)-derived N-doped carbon matrix (N-C); (2) implementing the chemical vapor deposition (CVD) of FeCl3 vapor onto the N-C at 750 ℃ wherein the FeCl3 and N-C are separately placed without direct physical contact. We found in the synthesis route (1) the FeCl2·4H2O transforms to metal oxides, followed by the formation of single-atom tetrahedral Fe-O4 sites as the temperature increases to 600 ℃.1 The Fe-O4 then gradually transforms to Fe-N4 as the temperature gradually increases from 600 ℃ to 1000 ℃. We accordingly propose that the Fe-O4 reacts with the Zn-N4 in the N-C forming Fe-N4 via metalation: Fe-O4 + Zn-N4 ↔ Fe-N4 + Zn-O4, followed by the carbothermic reaction: ZnO + C ↔ Zn (vapor) + CO around 950 ℃. This is different from our previous hypothesis that the Fe-O4 releases a single Fe atom that is captured by a N4-C cavity forming Fe-N4.1 Higher temperature promotes the metalation due probably to the comparative thermostability between Fe-O4 and Fe-N4, plus driving the formation of Zn vapor that leaves the N-C. In addition, if the Fe-O4 is not in proximity with the Zn-N4 during pyrolysis, the Fe-O4 transforms to small nanoparticles or FeOx rather than Fe-N4. Therefore, high pyrolysis temperature and sufficient mix of the Fe precursor and N-C are two salient features of this synthesis route that is essentially the same as the traditional synthesis route. These two features severely limit the Fe-N4 site density in the synthesized Fe-N-C, because Fe-N4 decomposes at elevated potentials and Fe-N4 sites are formed throughout the N-C and only those on surface accessible by air are ORR-active. These limitations are alleviated in the CVD route wherein the Fe-N4 is formed via a different metalation process: FeCl3 (vapor) + Zn-N4 ↔ Fe-N4 + ZnCl2 (vapor). The boiling point of ZnCl2 (732 ℃) is more than 200 ℃ lower than that of Zn (950 ℃), which leads to an optimistic pyrolysis temperature of 750 ℃ rather than 1000±100 ℃ in traditional synthesis. In addition, Fe-N4 sites are formed exclusively on the surface sites of the N-C accessible by FeCl3 vapor, and they are thus accessible by air. Consequently, we demonstrated a highly active Fe-N-C ORR catalyst synthesized via CVD at 750 ℃ with a surface Fe-N4 site density of 1.8 wtFe% with full site utilization of Fe-N4 sites. We also showed that the CVD synthesis route is applicable for the synthesis of M-N-C with other elements such as Mn and Co. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. The authors acknowledge the support from the DOE Energy Efficiency and Renewable Energy Fuel Cell Technologies Office (DOE-EERE-FCTO) ElectroCat consortium. This research used beamline 6-BM and 8-ID (ISS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. (1) Li, J.; Jiao, L.; Wegener, E.; Richard, L. L.; Liu, E.; Zitolo, A.; Sougrati, M. T.; Mukerjee, S.; Zhao, Z.; Huang, Y.; Yang, F.; Zhong, S.; Xu, H.; Kropf, A. J.; Jaouen, F.; Myers, D. J.; Jia, Q. J. Am. Chem. Soc. 2020, 142, 1417-1423.