The proton conductivity and water uptake were measured for a series of 3M ionomer membranes with various equivalent weights (EWs) and different acid groups. For the EW variation, the conductivity of PFSA samples with EW as low as 580 g/eq were studied. Results show that conductivity increases with decreasing EW. In particular, at lower EW conductivity dramatically increases at low RH. For 580 EW at 120 o C, the conductivity exceeds the target set by DOE for automotive fuel cells (100 mS/cm). The effect of water content is investigated through water uptake measurements that showed the clear dependence of conductivity on lambda ( ). Moreover, the nature and effects of changes in the morphology are studied with simulations and NMR relaxation and diffusion measurements probing the effect of water content on long vs. short range motions. For the different acid groups, conductivity and water uptake of polymers with sulfonic acid, sulfonyl amide and asymmetric sulfonyl imide end groups were compared.
Sulfonated poly(ether sulfone) (SPES) copolymers with a degree of sulfonation higher than 30% show high proton conductivity (> 60 mS/cm) as well as a high extent of swelling. Two methods were considered to decrease the swelling and improve dimensional stability. First, composite membranes were prepared by blending SPES and polyacrylonitrile (PAN) and annealing under nitrogen at 280 oC. In a second approach, SPES was blended with polysulfone (PSU) and heated at 260-280 oC to form sulfone crosslinks in the membrane. The membranes prepared by these methods showed significantly lower swelling in water and methanol with minor decreases in conductivity.
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