Soft X-ray Spectromicroscopy of Radiation Damaged Perfluorosulfonic Acid

Abstract

Climate change has propelled the development of alternative power sources that minimize the emission of greenhouse effect gases. Widespread commercialization of polymer electrolyte membrane fuel cell (PEM-FC) technology for transportation and stationary applications requires cost-competitiveness with improved durability and performance. Advantages compared to battery electric vehicles include fast refueling and long distance range. One way to improve performance and minimize costs of PEM-FC involves the optimization of the nanostructure of the catalyst layer. The rate limiting oxygen reduction reaction occurs at a triple-phase interface in the cathode catalyst layer (CL) between the proton conductor perfluorosulfonic acid, PFSA, the Pt catalyst particles decorating the electron conductor carbon support and gaseous O2 available through the porous framework of the carbon support. Visualization and quantitation of the distribution of components in the CL requires microscopy techniques. Electron and X-ray microscopy have been used to characterize the distribution of the PFSA relative to the carbon support and porosity in CLs. Understanding and limiting the analytical impact of radiation damage, which occurs due to the ionizing nature of electrons and X-rays, is needed to improve quantitation, particularly of PFSA. This thesis developed scanning transmission X-ray microscopy (STXM) methods for quantitation of damage due to electron and soft X-ray irradiation in PFSA materials. Chemical damage to PFSA when irradiated by photons and electrons is dominated by fluorine loss and CF2-CF2 amorphization. The quantitative results are used to set maximum dose limits to help optimize characterization and quantitation of PFSA in fuel cell cathode catalyst layers using: analytical electron microscopy, X-ray microscopy, spectromicroscopy, spectrotomography, spectroptychography and spectro-ptycho-tomography.