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-dipol...