Mechanisms of ion and water transport in perfluorosulfonated ionomer membranes for fuel cells

To clarify transport mechanisms of ions and water molecules in perfluorosulfonated ionomer membranes, various membranes, such as one Nafion, two Aciplex, and four Flemion types, having different equivalent weight values (EW) were examined. H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, and their properties in the fully hydrated state were investigated systematically. The water content of the membranes showed the tendency that the size and/or the number of ionic cluster region increases with decreasing EW value and the Li-form membranes have the most largely expanded ionic cluster regions. The ionic conductivity of the H-form membranes was considerably higher than that of the Li- and Na-form membranes. It was suggested that the proton in the membranes transports by the hopping mechanism and the Li+ and Na+ ions by the vehicle mechanism. In addition, the ionic conductivity of all membranes increased with increasing water content within the same kinds of membranes. Although the cation concentration in the membranes is insensitive to the EW value, the cation mobility increased with decreasing EW value. This means that the increased mobility of the carrier cations is the major factor to enhance the ionic conductivities due to the expansion of the ionic cluster regions. The water transference coefficients for the Li- and Na-form membranes showed higher values than those of the H-form membranes, while the water permeabilities gave the inverse tendency. This means that the water molecules in the Li- and Na-form membranes interact with the Li+ and Na+ cations more strongly than the proton when the cations move in the membranes, and as a result the diffusion of the water molecules is reduced. All of the above results were in good agreement with the results of the self-diffusion coefficients of the water molecules and the Li+ cation in the membranes measured by pulsed field gradient NMR.