A study of the transport properties of perfluorosulfonate ionomers using microelectrodes

Abstract

Chronoamperometry at microelectrodes (10 μm to 0.5 mm in diameter) is a powerful and convenient technique for the determination of apparent diffusion coefficients and concentrations of electroactive cations ion-exchanged into Nafion and Dow perfluorosulfonate films. This technique offers tremendous advantages over the conventional electrochemical methods which employ film-coated macro-sized electrodes (0.5 mm in diameter and larger). These advantages stem from the elimination of iR distortion at the microelectrode and from the ability to determine both the concentration and the apparent diffusion coefficient of the electroactive cation from a single experiment. Further improvements over the conventional approach were achieved by eliminating coupled diffusion and migration contributions to the measured current response. Under these conditions, charge is transported via ionic diffusion rather than through electron hopping. Furthermore, the apparent diffusion coefficients decrease with increasing concentration of the electroactive cation in the film. This decreasing apparent diffusion coefficient with increasing concentration was attributed to a bottleneck effect caused by the narrow channels which interconnect the ionic clusters in these ionomers.