Proton Exchange Membranes for High Temperature Fuel Cells: Equivalent Weight and End Group Effects on Conductivity

Abstract

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 120oC, 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 (λ). 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.