Transport Properties of Perfluorosulfonate Membranes Ion Exchanged with Cations

In this work, the properties of univalent, that is, Li⁺, Na⁺, NH₄⁺, and TEA⁺ form perfluorosulfonate (PFSA) membranes are studied and compared to the properties of H⁺ form materials. Properties of these polymer membranes including water uptake, density and conductivity, were investigated for membranes exposed to various water activity levels. The water uptake by the membranes decreased in the order H⁺ > Li⁺ > Na⁺ > NH₄⁺ > TEA⁺, the same order as the hydration enthalpy (absolute values) of cations. Conductivity values did not strictly follow this order, indicating the importance of different factors besides the hydration level. The partial molar volume of water is derived from the density data as a function of water content for the various membrane forms. This provides further insight into the water, cation, and polymer interactions. Factors that contribute to the conductivity of these membranes include the size of cations, the electrostatic attraction between cations and sulfonate group, and the ion-dipole and hydrogen bonding interactions between cations and water. NH₄⁺ transport is surprisingly high given the low water uptake in NH₄⁺ form membranes. We attribute this to the ability of this ion to develop hydrogen bonded structures that helps to overcome electrostatic interactions with sulfonates. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was used to measure the diffusion coefficient of water in the membranes. FT-IR spectroscopy is employed to probe cation interactions with water and sulfonate sites in the polymer. Overall, the results reflect a competition between the strong electrostatic interaction between cation and sulfonate versus hydration and hydrogen bonding which vary with cation type.