While high oxygen reduction reaction (ORR) activities for polymer electrolyte fuel cells (PEFCs) have been demonstrated for high surface area carbon-supported Pt alloy nanoparticle catalysts in aqueous cell rotating disk electrode tests and when operating on oxygen in membrane-electrode assemblies (MEAs), the performance improvement expected from the high intrinsic ORR activities is not realized at moderate to high current densities. ¹ One possible reason may arise from the complex requirements for full utilization of the electrocatalytic sites and for adequate reactant transport in the cathode catalyst layer. Knowledge about the structural features of the catalyst layer, understanding the distribution in this porous layer and understanding the proton mobility in ionomer between structural elements of the catalyst could help to design more efficient electrodes for PEFCs. Small angle X-ray scattering (SAXS), which probes objects with dimensions of 1 to 100 nm, is emerging as a powerful tool for structural investigations of PEFC electrodes to estimate the size, shape, and structure of the catalyst and ionomer formulations as well as of fuel cell electrodes. ² Ultra-small angle X-ray scattering (USAXS) is the scattering at ultra-low angles by structures up to several micrometers in size. USAXS combined with SAXS can probe the length scales relevant to the carbon structures in PEFC electrodes (10 nm to 6000 nm). ^{3, 4}

This presentation will highlight X-ray scattering microstructural characterization of state-of-the-art alloy cathode catalysts including catalyst powders, catalyst-ionomer-solvent dispersions, optimized electrodes and MEAs at different stages of testing and using different testing protocols. The microstructural results will be compared with the physico-chemical properties of the electrode components, the electrode fabrication processes, as well as the fuel cell performance and in-cell diagnaotics to establish a relationship between electrode microstructure and electrode performance.

Acknowledgments

This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the support of the Fuel Cell Performance and Durability Consortium (FC-PAD). The Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC-02-06CH11357 by UChicago Argonne, LLC.

References

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