NMR diffusion studies of proton-exchange membranes in wide temperature range

The diffusion coefficients in proton-exchange perfluorosulfonated membranes fabricated by different techniques have been analyzed in the temperature range from 200 K to 300 K using NMR diffusometry in a static magnetic gradient field. The influence of the water content as well as the effects of the membrane thickness and the side-chain length on the diffusion coefficient was analyzed. It is found that the fabrication by the extrusion cast method leads to faster diffusion as compared to the solution cast fabrication method. Shorter side chains are also preferable for faster diffusion. Both the decrease of the temperature and the water content leads to a reduced diffusion coefficient, affecting the transport micromechanism in a similar way by increasing the role of the water on the pore walls with respect to proton transport. The model of confined water in Nafion pores has been applied to analyze the results. It is proposed that both conductivity and diffusion at low temperatures are determined by protonic transport of the surface water via bond defects in contrast to the bulk water or ice, where the ionic defects are responsible for the mass transport.