Characterization of Heterogeneous Solvent Diffusion Environments in Anion Exchange Membranes

1H high resolution magic angle spinning (HRMAS) NMR spectroscopy was used to characterize the solvent environments in a series of poly(phenylene)- and poly(phenylene alkylene)-based anion exchange membranes (AEMs). Multiple water and methanol environments were resolved in the membranes under HRMAS NMR. This allowed the self-diffusion rate constants to be evaluated for each different solvent environment as a function of the membrane identity, ion exchange capacity, water content, and sample temperature. These ionomers have been designed to function as binders within the catalyst layers of direct methanol fuel cells. In such applications, it is desirable to maximize the diffusion of the fuel (methanol) as well as the solvated ions to increase power output. To that end, the flexibilities of the backbone and the cationic side chains have been varied with the expectation that greater polymer mobility will lead to improved permeability. For the two types of AEMs investigated, it was observed that the methanol self-diffusion rates were preferentially reduced with respect to the water diffusion rates. It was also shown that the water diffusion rates within the AEMs were the largest at high water concentration, as observed in membranes containing the hexamethylene chain spacer in both the polymer backbone and the trimethylammonium (TMA+) cation-containing side chains.