(452b) Influence of Confinement and Interfacial Interactions On the Structure and Transport Properties of Polymer Electrolyte Materials

The drive for cleaner, more sustainable energy alternatives has motivated research worldwide, including extensive efforts to develop reliable and efficient polymer electrolyte materials.  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.  It is expected that 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.

The focus of this work is to quantify how the morphology and material properties of PEM materials change under confinement at interfaces, determine if these interfacial effects are influenced by processing conditions, and then establish a correlation between the interfacial effects and fuel cell performance.  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.  Results from our lab indicate strong deviations from expected bulk behavior of Nafion occur in films thinner than ~100 nm.  Film thicknesses measured by x-ray reflectivity as a function of relative humidity reveal that the swelling is suppressed in supported films <60 nm in thickness.  Furthermore, neutron reflectivity studies reveal alternating water-rich and Nafion-rich layers exist near hydrophilic supporting layers.  The development of these interfacial nanostructures, which typically span ~50-75 angstroms, has been shown to depend on the surface energy of the underlying support layer.  Water uptake and sorption kinetics have also been studied using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and using quartz crystal microbalance (QCM) technique.  Additionally, because these thin films are formed from complex dispersions, their thermal and processing history also strongly influence their behavior.  The observed deviations from bulk behavior in these thin Nafion films highlight shortcomings in current modeling efforts that typically use bulk-film parameters to describe transport in thin PEM layers at catalyst interfaces.

This work represents an initial step in uncovering the impact of interfacial interactions that influence material performance in more complex systems, such as the triple phase boundary present in fuel cell catalyst layers.  Understanding how confinement and interfacial interactions affect the behavior of these materials will enable more accurate predictions of model systems, and perhaps lead to strategies for improving device performance.