On the swelling properties of proton conducting membranes for direct methanol fuel cells

The swelling properties of different fluorinated (Nafion® 117, poly(ethylene-alt-tetrafluoroethylene) and poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid)) and hydrocarbon (sulfonated poly(phenylene sulfone)) proton conducting ionomers were investigated in alcohol (MeOH, EtOH, 2-propanol, tBuOH)–water and H2SO4–water mixtures. Solvent uptake and its water/alcohol selectivity were determined by thermal desorption using combined thermogravimetry (TGA) and mass spectrometry (MS). Clear trends are observed for the selectivity: preferential water uptake (alcohol rejection) correlates with high IEC (ion exchange capacity) and low solvent uptake (swelling) indicating a preferred interaction of the ionic groups (–SO3−, H+) and the groups on the polymerbackbone carrying partial charges (e.g. sulfone units (–SO2−) in the sulfonated poly(phenylene sulfone)) with water. The total swelling significantly decreases in the presence of H2SO4 indicating osmosis being a major driving force in the swelling process. This is particularly low for ETFE-SA, which is highlighting its supreme mechanical properties and its high selectivity for water uptake (selectivity coefficients up to 10 for λ = 7). Unlike Nafion®, which shows high swelling in alcohol–water mixtures and almost no selectivity, high selectivity coefficients (up to 8) are observed for sulfonated poly(phenylene sulfone) even at medium solvent uptake (λ = 12). The total solvent uptake in these highly polar ionomers even decreases with methanol mole fraction and is virtually independent of temperature (up to T = 120 °C). Together with the high proton conductivity and high hydrolytic stability reported in the literature, the swelling properties presented in this work are making sulfonated poly(phenylene sulfone) interesting membrane materials for DMFC applications.