(635b) CO2-Selective Membranes for H2 Purification for Fuel Cells

Polymer electrolyte materials are of great interest in the continuing development of fuel cell and battery technologies, which offer cleaner, more efficient alternatives to power generated through tradition technologies.  While substantial progress has been made in improving the performance of polymer electrolyte membrane (PEM) fuel cells, challenges in their development remain.  Due to the complex structure present in these PEM materials, including ionic clusters and crystallites, their performance is intimately linked to the specific morphology of the system.  As film thickness is decreased to improve performance and lower costs, the morphology of the system (i.e., size and connectivity ionic domains and crystallites) will become more strongly influenced by interactions at the material interfaces, impacting the performance of the membrane layer and the polymer electrolyte in the catalyst layer.  Nafion, the most prevalent PEM material, has been studied in detail in its bulk form; however, relatively little information has been published on the behavior of thin films of Nafion.  The focus of this work is to quantify how the morphology and material properties of PEM materials change under confinement at interfaces, determine how these interfacial effects are influenced by processing, and to establish a correlation between these effects and fuel cell performance.  

Using a variety of experimental techniques, including: quartz crystal microbalance (QCM), grazing incidence small angle x-ray scattering (GISAXS), ellipsometry, and x-ray reflectivity, we have investigated the influences of confinement and thermal annealing on the behavior of thin Nafion films.  Consistent with previous studies, the sorption of water in thin (40 nm) Nafion films is lower than in bulk membranes.  Additionally, the previous history of these films, i.e., thermal annealing, has a significant impact on water uptake behavior.  The nanostructure within the Nafion films, as studied using GISAXS, was also influenced by confinement and annealing.  In addition to changes in the ionic domain morphology in this material, which can be connected to the water uptake behavior, changes in the crystalline regions were also observed that relate to changes in the film strength and stability.  To complement the studies on structure and transport properties, we have also investigated the mechanical properties and stress change upon hydration of these films using a cantilever bending technique.  The observed deviations from bulk behavior in these thin Nafion films highlight the importance of understanding the influences of confinement in detailed modeling efforts, which typically use bulk-film parameters to describe transport in thin PEM layers and at catalyst interfaces.