X-Ray Scattering Characterization of the Structure of PEM Electrolyzer and Reversible Fuel Cell Catalysts and Electrodes

Abstract

The efficiency of hydrogen fuel generation from electrolysis is severely limited by the sluggish kinetics of the oxygen evolution reaction (OER).1 Iridium oxide (IrO2) is among the most active and stable OER catalysts reported to date. The combination of electrolyzer and fuel cell in one device is termed a unitized reversible or regenerative fuel cell (URFC). These devices are being developed to decrease material and system components versus systems with separate electrolyzers and fuel cells to reduce the cost of hydrogen generation and conversion for applications such as energy storage coupled with renewable sources. Recently, bifunctional hydrogen oxidation reaction and oxygen evolution reaction (HOR/OER) catalysts utilizing Ir and Pt have been explored for URFC anodes.3 Knowledge about the structural features of the catalyst layer, understanding the distribution in the porous layer and understanding the proton mobility in ionomer between structural elements of the catalyst could help to design more efficient electrodes for PEM water electrolyzers (PEMWEs) and URFCs. Small angle X-ray scattering (SAXS), which probes objects with dimensions of 1 to 100 nm, is a powerful tool for structural investigations to estimate the size, shape, and structure of the catalyst and ionomer formulations as well as of electrolyzer and URFC anodes. Ultra-small angle X-ray scattering (USAXS) combined with SAXS can probe the length scales relevant to aggregate and agglomerate structures in electrodes (100 nm to 6000 nm).This presentation will highlight X-ray scattering microstructural characterization of PEM-URFC and PEMWE anodes and associated materials, including catalyst powders, catalyst-ionomer-solvent dispersions, and membrane-electrode assemblies 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 performance to establish a relationship between electrode microstructure and electrode performance. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the H2NEW Consortium. This work was authored in part by Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357. This work was also funded in part under Contract Number DE-AC02-05CH11231. Use of the Advanced Photon Source (APS), an Office of Science user facility operated by Argonne National Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AS02-06CH11357. The authors would like to thank Jan Ilavsky and Ivan Kuzmenko of the X-ray Science Division, beamline 9-ID, of the APS.