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). Significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts. However, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable use of these catalysts in practical applications, such as powering light and heavy-duty vehicles. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts that catalyze the ORR reaction, due to the complexity of the Fe-N-C catalysts composition and structure. The most active Fe-N-C catalysts have been synthesized by pyrolyzing 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. It has been shown that many synthesis variables, such as type of iron precursors, the metal and carbon-nitrogen macrocycle ratio, the heat treatment temperature, atmosphere, and temperature profile, all affect the properties and structure and thus the activity and stability/durability of the resulting catalysts. In order to further improve the Fe-N-C catalysts, it is critical to gain insights into the origins of ORR activity and the reaction mechanism. In this work, the redox behavior of different Fe-N-C catalysts, synthesized by a variety of techniques mentioned above, and correlation of this redox behavior with ORR catalytic activity in different electrolytes will be discussed and the effect of nitric oxide gas-phase adsorption on the catalyst redox feature and ORR activity will be presented. The implication of these results on the identity or identities of the ORR active sites of the Fe-N-C catalysts will be discussed.

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 2.0). 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.