DIFFUSION-COEFFICIENTS OF ALKALI-METAL CATIONS IN NAFION(R) FROM ION-EXCHANGE MEASUREMENTS - AN ADVANCED KINETIC-MODEL

An advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion-exchange membrane is developed considering the eff ect of the varying ionic composition on the membrane water content. Th e transport problem is solved numerically for a set of ratios of the d iffusion coefficients of the two counterions and 1:1 ion-exchange stoi chiometry. The model is used to evaluate the diffusion coefficients of alkali metal cations in the as-received and expanded H- and M-form Na fion(R) (M = Li, Na, K, Rb, Cs) from ion-exchange measurements. Owing to a compensating effect of the electro-osmotic pore fluid flow the in itial rates of ion exchange correspond to a fixed water content which, however, is different in H- and M-form membranes. A strong correlatio n is revealed between the ratio of the membrane to aqueous ion diffusi on coefficients and the polymer-phase volume fraction. It is concluded that the polymer phase presents mainly a steric effect without signif icantly changing the mechanism of transport of alkali metal cations or protons, which resembles that in bulk water. The different behaviors of the as-received and expanded Nafion(R) forms are probably associate d with the prevailing cluster- or pore-network morphology respectively .