Pore size distribution and water uptake in hydrocarbon and perfluorinated proton-exchange membranes as studied by NMR cryoporometry

Sulfonated polysulfone (sPSU) membranes were analysed by nuclear magnetic resonance (NMR) cryoporometry, conventional gravimetric water uptake measurements as well as by differential scanning calorimetry (DSC). NMR cryoporometry is based on the relation between the pore size and the melting point depression of the pore-filling liquid, i.e. water in fuel cell membranes; thus providing a relation between the amount of molten water and the temperature shift, i.e. the pore size, in hydrated membranes. An sPSU membrane with high ion-exchange capacity (IEC 1.45 mequiv. g(-1)) possessed a significant amount of large pores after hydrothermal pretreatment at 80 degrees C, which was related to its high hydrophilicity and low resistance towards swelling. An sPSU membrane with low IEC (0.95 mequiv. g(-1)) showed a significant fraction of small pores (r similar to 1 nm) after hydrothermal pre-treatment at 80 degrees C, implying a controlled water uptake of the membrane. NMR cryoporometry was also carried out on Nafion membranes, the results were found to be in agreement with the water uptake measurements and revealed a pore size distribution peak at r similar to 1 nm. The NMR cryoporometry, gravimetric water uptake and DSC results are also discussed in terms of the state of the water and methodological differences.