A promising class of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts, with the highest ORR half-wave potentials in acidic aqueous media, are those derived from iron salts and zinc-based zeolitic imidazolate framework (ZIF) precursors.^1-6 These precursors are typically heat-treated in an inert atmosphere at temperatures ranging from 900 to 1100°C to form the ORR-active and acid-stable material. In addition to their high intrinsic activity, under certain preparation conditions these catalysts are free of crystalline iron species. Characterization by electron microscopy, X-ray absorption spectroscopy, and nuclear resonance vibrational spectroscopy indicates that the Fe species are atomically dispersed in a nitrogen-doped carbon matrix, that the iron is in an Fe-N₄-like coordination environment, and that the majority of iron is on the surface and thus accessible to bind oxygen or a probe molecule, such as nitric oxide.^6,7 However, the fuel cell performance of these materials is not commensurate with the high activities.⁷ X-ray tomography and transport modeling of polymer electrolyte fuel cell cathode catalyst layers based on this material indicate that the fuel cell performance is limited by reactant transport throughout the 70-95 mm electrode thickness.⁸ Such thick electrodes result from the high catalyst loadings necessary for high kinetic performance (e.g., 6 mg/cm²). Therefore, in addition to durability, one of the major challenges for PGM-free catalysts is increasing active site density which will enable a decrease in the catalyst layer thickness. However, attempts to increase the active site density of the Fe-ZIF catalysts by increasing the iron content in the precursors has led to formation of spectator Fe species, such as carbides, at the expense of the proposed Fe-N₄ active sites. Previous studies have shown that the standard heat treatment protocol with high temperatures and high precursor iron content favors the formation of iron carbides and, conversely, low temperatures and low iron contents favor the formation of Fe-N₄ species. Insight into the species formed as a function of temperature and time during pyrolysis can provide the necessary information to tailor the process to favor Fe-N₄ formation when using high iron content precursors. Toward this goal, we have developed an apparatus that allows the measurement of the Fe K-edge X-ray absorption (XAFS) spectra of multiple iron-containing precursor samples in rapid succession during heat-treatment to temperatures up to 1000°C (Fig. 1). The results from the XAFS measurements taken during pyrolysis of precursors containing varying amounts of iron show that Fe-N₄ species are formed at temperatures as low as ~715°C (Fig. 2) and that carbide species are formed during the temperature hold at 1000°C for the precursors containing higher iron contents. These data guide the way for modifying the heat treatment profile to favor the formation of the proposed Fe-N₄ active sites toward the goal of increasing the active site density in the Fe-ZIF-derived class of catalysts.

Acknowledgements

This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). This research used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.

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