Ion mobility in Nafion-117 membranes

Transport phenomena in Nafion-117 perfluorosulphonic membranes have been studied by different techniques. Membrane electric conductivity has been measured under alternative current as a function of the equilibrating concentrations of MCl (M = Li, Na, K) and HCl solutions. Conductivities were found to increase for lithium and sodium membrane forms and decrease for the potassium form with an electrolyte concentration increase. The obtained data were compared with the results of diffusion permeability, ion-exchange kinetics, 7Li and 35Cl NMR measurements. In all cases permeability increased with electrolyte concentration growth. Diffusion coefficients and transport numbers for different counter- and co-ions were determined. The obtained data are discussed with the use of the two-phase membrane structure model. The selectivity of membrane sorption was found to increase with growth of cation radius.

Ion mobility and transport numbers in Nafion membranes depend strongly on the nature and composition of equilibrating solutions. Counterion and co-ion mobility in the membrane is highly affected by the presence of other electrolytes. A comparison of the data obtained by different techniques has given an estimate for diffusion coefficients for different ions in Nafion117 membranes. Transport numbers of anions in the salt forms of the Nafion-117 membrane increase with the growth of radius of corresponding cations and with an increase in the equilibrating electrolyte concentration.