The activity and stability of platinum group metal-free (PGM-free) oxygen reduction electrocatalysts and the performance of the electrodes based on these catalysts depends on the atomic species present, their stability in the fuel cell environment, and the nano- and micro-structure of the catalyst-ionomer composite electrode. As part of the U.S. Department of Energy Fuel Cell Technologies Office’s Electrocatalysis Consortium (ElectroCat),¹ a suite of in situ and ex situ synchrotron X-ray techniques is being used to characterize these properties of PGM-free electrocatalysts and electrodes. X-ray absorption spectroscopy (XAFS) is used to determine the coordination environment and oxidation state of the transition metals (e.g., Fe, Co, and Mn) in the as-prepared catalyst powders, precursors during the pyrolysis step, and catalyst-ionomer-solvent inks as a function of potential in an aqueous electrolyte and during adsorption and stripping of probe molecules, such as nitric oxide. Ultra-small angle and small angle X-ray scattering (USAXS and SAXS) are used to determine the aggregate and agglomerate size of the catalysts in the as-prepared catalyst, catalyst-ionomer-solvent inks during sonication, and electrodes. Nano-X-ray tomography (XCT) is used to determine the three-dimensional distribution of catalyst, pores, and ionomer.

This X-ray characterization methodology has thus far been applied to a series of catalysts, inks, and electrodes prepared through pyrolysis of iron salts and carbon-nitrogen precursors. The XAFS studies are being used to correlate the ORR activity of the catalysts synthesized by the core national laboratory, university, and industrial partners, including those prepared using high-throughput techniques, with the content of various iron species. The studies thus far have shown that the activity is correlated with the content of Fe-N₄-like species and declines with the formation of iron carbide at higher pyrolysis temperatures and higher iron contents (Fig. 1). The in situ XAFS experiments have shown that the Fe centers are highly oxidized at open circuit and gradually reduced as potential is decreased and as ORR current increases. The electrode structural information from XCT is being combined with characterization results obtained via other techniques, such as porosimetry and ultra-small angle X-ray scattering (USAXS), to build a structural and transport model of the electrode using a hybrid computational method. The XCT data, when correlated with fuel cell polarization curve data, show that electrode performance is limited by oxygen and proton transport through the thick electrode layers (~70-95 mm thick). The application of the full suite of X-ray techniques to an iron zeolitic imidazolate framework-derived catalyst² will be presented.

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.

References

1. T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics, 319 (2018) 68-76.
2. Zelenay, D.J. Myers, H. Dinh, and K.L. More, “ElectroCat (Electrocatalysis Consortium)”, 2017 Department of Energy Hydrogen and Fuel Cells Program 2017 Annual Merit Review and Peer Evaluation Meeting, Washington DC, June, 2017. (https://www.hydrogen.energy.gov/pdfs/review17/fc160_zelenay_2017_o.pdf)

Figure 1. Non-phase-corrected Fourier transform of the Fe K-edge XAFS spectra of
pyrolyzed iron sulfate, zeolitic imidazolates, an Fe-N₄ standard (Fe³⁺ phthalocyanine),
and iron carbide.