Ionic transport mechanism in cation-exchange membranes studied by NMR technique

The ionogenic groups, H+ and alkaline metal cations hydration and mobility in perfluorinated sulfocation membrane MF-4SC, carboxyl perfluorinated membrane F-4CF, aromatic mono- and bisulfocontaining polyamides films were investigated by 1H, 7Li, 23Na, and 133Cs high resolution NMR. Water and Li+, Na+, and Cs+ ion self-diffusion coefficients were measured by pulsed field gradient NMR. The ionic conductivities calculated from self-diffusion data were compared with experimental values measured by impedance spectroscopy. The cation hydration numbers were calculated from proton magnetic resonance data. In aromatic sulfocontaining polymers, the amide groups solvate the cations, taking part in additional hydrogen bond network formation. Water self-diffusion coefficients increase in the following cation row in the sulfonated membrane H+ > Cs+ > Na+ > Li+. The self-diffusion coefficients of the carboxyl membrane salt forms and in aromatic polyamide membranes are ordered in the same row Cs+ > Na+ > Li+. The self-diffusion behaviour in acid form of the carboxyl membrane is restricted; the restricted size is about 1 μm. The hindered self-diffusion coefficient is two orders of magnitude low compared with sulfonate membrane acid form. The conductivity data are the same order of magnitude with the values calculated from the self-diffusion results.