Optimization of Synthesis Variables Towards Improved Activity and Stability of Fe-N-C PGM-Free Catalysts

Materials in the Fe-N-C family are the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).1-3 Although significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable their use in applications such as propulsion power for light-duty vehicles.3 The most active ORR catalysts in the Fe-N-C family were synthesized by heat treating iron salts or other iron-containing compounds with zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline (as carbon and nitrogen sources), or by heat treating iron-substituted ZIFs. For this family of PGM-free materials, it has been shown that many synthesis variables, such as the metal and carbon-nitrogen macrocycle content, the heat treatment temperature, atmosphere, and temperature profile all affect the activity and durability of the resulting catalysts.4-7 Optimization of these variables and testing the resulting catalyst properties is not a trivial task, and only a limited portion of the composite composition and temperature space has been explored for this family of catalysts. To accelerate optimization of the synthesis variables to obtain improved ORR activity and stability for the Fe-N-C catalysts, high-throughput synthesis and characterization methods were developed and utilized. An automation platform, a multi-port ball-mill, and parallel fixed bed reactors in Argonne’s High-throughput Research Laboratory were used to rapidly synthesize the PGM-free catalysts with systematically-varied synthesis conditions. A multi-channel flow double electrode (m-CFDE) cell and other cells were designed and constructed for the simultaneous testing the ORR activity and stability of the multiple catalysts synthesized. The ORR activity and stability of the catalysts were correlated with their Fe speciation, as determined using Fe K-edge X-ray absorption spectroscopy (XAFS), electrochemically-determined surface areas, and other variables, which is beneficial for the further improved catalyst activity and stability. References B. Pivovar, Nature Catalysis, 2 (2019) 562. S. Thompson and D. Papageorgopoulos, Nature Catalysis, 2 (2019) 558. L. Osmieri, J. Park, D.A. Cullen, P. Zelenay, D.J. Myers, and K.C. Neyerlin, Curr. Opin. Electrochem., 25 (2021) 100627. X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu, and X. Li, Nano Energy, 25 (2016) 110. H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, and G. Wu, J. Am. Chem. Soc., 139 (2017) 14143-14149. E. Proietti, F. Jaouen, M. Lefevre, N. Larouche, J. Tian, J. Herranz, and J.-P. Dodelet, Nature Comm. 2 (2011) 1. A. Zitolo, V. Goellner, V. Armel, M.-T. Sougrati, T. Mineva, L. Stievano, E. Fonda, and F. Jaouen, Nature Materials, 14 (2015) 937. This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.