(Invited) In Situ and Operando Synchrotron X-Ray Spectroscopy and Scattering Characterization of PEFC Cathode Catalysts

Abstract

With wavelengths on the order of atomic dimensions and the ability to penetrate through low atomic number materials (e.g., Nafion® and carbon), hard X-rays, particularly those arising from synchrotron sources with high brilliance, are ideal for the atomic-level characterization of polymer electrolyte fuel cell (PEFC) catalysts in situ, in aqueous environments that mimic PEFC conditions, and operando, in the membrane-electrode assembly (MEA) environment. Two particularly valuable X-ray techniques for this purpose are X-ray absorption spectroscopy (XAS) and small angle X-ray scattering (SAXS) or anomalous SAXS (ASAXS), which allows discrimination of scattering from the absorbing element. Analysis of the X-ray spectrum near the absorption edge of interest (i.e., the near edge or XANES region) provides information on the oxidation state and electronic structure (e.g., d-band vacancy) of the material, properties that have been associated with catalyst activity. The extended region of the XAS spectrum (EXAFS) yields information on the atomic structure and coordination environment of the absorbing element (i.e., number and identity of nearest neighbors and distances between atoms). Unlike X-ray diffraction, XAS is particularly useful for characterization of platinum nanoparticle catalysts and of single-site platinum-group metal-free catalysts, such as Fe-N-C catalysts. However, EXAFS can be limited in the information it can provide regarding the bonding geometries, but absorption in the pre-edge region, which is sensitive to bonding geometry, can provide this essential information. SAXS and ASAXS are well-established techniques for probing particle sizes and particle size distributions on the nanometer level. While not as widely-used in the fuel cell community as XAS, SAXS and ASAXS are well-suited to examining nanoparticle fuel cell catalysts, which typically have particle diameters of 2-5 nm.This presentation will discuss the information that can be obtained from X-ray spectroscopy and scattering techniques and provide examples of application of these techniques to the study of platinum and platinum alloy nanoparticle catalysts and atomically-dispersed Fe-N-C and Co-N-C PEFC cathode catalysts. The utility of using a combination of XAS and ASAXS to study platinum alloy catalyst degradation and of the combination of information from XANES, EXAFS, and XAS pre-edge data to an Fe-N-C catalyst will be presented.Acknowledgements:This research was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO), through the Electrocatalysis Consortium (ElectroCat). This research used 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 DE-AC02-06CH11357. The authors thank Hoon Chung of Los Alamos National Laboratory for providing the Fe-N-C catalyst and Johnson Matthey Fuel Cells for providing the Pt and PtCo catalysts. This work was conducted at Argonne National Laboratory, a U.S. DOE Office of Science Laboratory operated for the U.S. DOE by UChicago Argonne, LLC, under Contract DE-AC02-06CH11357.