X-ray Contrast Variation in Anion Exchange Membranes

Synopsis

Hydroxide-exchange fuel cells have the potential to bring hydrogen power into the commercial mainstream by providing the benefits of proton-exchange cells with lower production costs. Achieving this potential depends on optimizing the anion exchange membranes (AEMs) which facilitate hydroxide transfer. To guide their development, a more detailed understanding of AEM morphology is sought. Determining how water is distributed throughout the membrane upon absorption is particularly important to understanding how these membranes function as ion conductors. Small angle X-ray scattering (SAXS) is a proven technique for determining characteristic length scales within AEMs. However, SAXS measurements of some recently developed AEMs were unable to identify water-rich regions within hydrated membranes, and when these regions are detected they appear less distinct than corresponding regions within PEMs. It is hypothesized that there is insufficient contrast between the polymer backbones of AEMS and their water-rich regions, with the result that SAXS fails to fully resolve the structure of the hydrated membrane.

This thesis investigates the role of contrast in SAXS measurements of AEMs by attempting to vary the contrast between water-rich and backbone-rich regions within TMP-PMPI, a state-of-the-art AEM. Three methods of contrast variation are employed. One attempts to use organic solvents as contrast media, replacing water as the membrane’s liquid phase. Another employs anomalous scattering to vary the contrast of anions as a function of incident photon energy. The third method varies anion contrast by exchanging the membrane to different counter-ion forms. For each method, samples are prepared with different water contents to test whether the method clarifies the changes the membrane undergoes as it takes up water. These methods of contrast enhancement ultimately have minimal effect on the scattering features associated with phase separation, suggesting that the obscurity of these features has less to do with contrast than the intrinsic disorder of water regions in the membrane.