Optical Properties and Swelling of Thin Film Perfluorinated Sulfonic-Acid Ionomer

Abstract

Perfluorinated sulfonic-acid (PFSA) ionomers are members of a class of ion-conductive polymers that are commonly used as a proton-exchange membrane or catalyst layers in polymer-electrolyte fuel cells. Numerous studies have been undertaken to understand how the structure of PFSA ionomers with thickness of 1 to 200 µm and their water uptake affect the material properties and the performance of fuel cells. Thin film confinement affects the hydration behavior of thin film ionomers; the deviation from transport properties are still unclear. Advanced tools have been used to study water uptake of sub-micron PSFA ionomers at different humidity levels; however, their temporal resolution are of the order of 1 minute. Label free, high-speed Surface Plasmon Resonance imaging (SPRi) is proposed to provide a 3D map of a thin film (<300 nm) at high thickness resolution (~ 0.1 nm), high lateral resolution (~ 4 μm), and high temporal resolution (up to 3000 frames per second). Nafion films with 28-250 nm and 3M films with 7-150 nm are used for a systematic study of the swelling and diffusion transport phenomena of water in thin film ionomers deposited on a 50 nm gold film on a glass substrate. SPRi with angular modulation (AM) and intensity modulation (IM) is employed to determine the swelling ratios of the thin film PSFA’s at different humidity levels. SPRi with IM is used to study the kinetics of diffusion transport of water in the thin film ionomers. The preliminary results show that the water uptake in the thin film ionomers locally decreases the refractive index of the material, whose changes are tracked by SPRi. This can provide the diffusion map during water transports through the thin film ionomer.