The relationship between proton conductivity and water permeability in composite carboxylate/sulfonate perfluorinated ionomer membranes

Composite carboxylate/sulfonate Nafion® films were prepared with different ratios of anion equivalents as a test of the relationship between proton conductivity and water permeability in perfluorinated ionomers. Substitution of carboxylate groups for sulfonate groups dramatically reduced water permeability and modestly reduced proton conductivity in the films. Small angle X-ray scattering (SAXS) was used to probe the polymer microstructure and showed that with up to 20% carboxylate equivalents, the size of hydrated ionic domains within the ionomer remains constant. Water sorption measurements showed little decrease in water uptake with increased carboxylate functionality, supporting the SAXS data. Methanol/water permeability experiments showed that the carboxylate moieties slow the diffusion of both water and methanol, with no permselectivity of water. Results indicate that the proton transport mechanism is altered by addition of carboxylic acid. As such, the composites may show interesting behavior in H2/O2 and direct methanol fuel cells. It is proposed that permeability and conductivity measurements may provide an inexpensive and simple screening tool for early evaluation of new experimental proton exchange membranes.