Influence of the water content on the kinetics of counter-ion transport in perfluorosulphonic membranes

Abstract

The high frequency electrical resistance of a Nafion ® 117 perfluorosulphonic membrane has been measured by means of the ac impedance technique. Using a mercury cell, the variations of the membrane conductance as a function of the degree of swelling and of the nature of the counter-ion have been determined. The variations of the conductance with the water content have two different origins: the first is a change in the rate constant of the elementary ion transfer reaction due to interactions between the mobile ions and the fixed sites, and the second is related to a change in the microstructure of the membrane material. An analysis of the membrane conductance on the basis of the absolute rate theory is proposed. It enables one to explain the shape of the impedance diagrams and shows that the variations of the membrane conductance with its water content cannot be accounted for by geometric considerations alone. On the basis of a simple electrostatic interaction model, the energy barrier of the elementary ion transfer jump can be related to the hydration radius of the cation.