AC Impedance and Conductivity Study of Alkali Salt Form Perfluorosulfonate Ionomer Membranes

AC impedance studies were performed on Na+ and K+ alkali salt forms of the short sidechain perfluorosulfonate ionomer (PFSI) membrane films manufactured by Dow Chemical Company. With impressive performances of 4 A/cm2current density and power densities near 2.5 W/cm2, the acid forms of these short sidechain PFSI are very promising candidates for use in fuel cells for future electric vehicles.1 Since, at present, little is known about the exact transport mechanisms for the ionic species within PFSIs, an ac impedance study of the Na+ and K+ forms has been performed. It is hoped that this will provide some insight and understanding of the transport mechanisms in the PFSI and thus will aid in the development and optimization of fuel cells. Results suggest that there are marked differences with respect to host environments within the Dow membrane as compared to Nafion® long sidechain PFSI membrane films. Impedance spectra of the Dow salt form membranes displaying two distinct relaxation peaks while the spectra for all forms of Nafion reveal only a single peak. This second low temperature peak in the Dow membrane has been attributed to a much larger [OH−]/[SO3−] ratio, possibly greater than one, existing within the Dow membrane as compared to Nafion, which contains an [OH−]/[SO3−] ratio between 0.23 and 0.29. The temperature behavior of the dc conductivity was extracted from the temperature Cole‐Cole plots and was found to be non‐Arrhenius. Room temperature conductivities of the order of 10−2S/cm were achieved for the Na+ and K+ salt forms of the XUS membrane. It was observed that for all T<25K , σdc(Na+)<σdc(K+) . It has been proposed that this behavior arises from the hydration phenomena of the alkali ions. These results suggest that substantial differences exist between host environments of the Dow and Nafion PFSI.