The ion and water transport properties of K+ and Na+ form perfluorosulfonic acid polymer

Abstract

In this work, we explore the behavior of membranes under conditions that are relevant to ion-exchange membranes used in a variety of electrochemical engineering applications, extending beyond the heavily-studied fuel cell application and previous studies of fully hydrated systems. We report the water uptake, density and conductivity of membranes determined at various hydration states of K+, Na+ and K+/Na+ mixed form perfluorosulfonate membranes. Water uptake decreases with increasing K+ content in the membrane, consistent with increasing average cation size and decreasing hydration energy. However, for membranes at the same water content, the conductivity increased with increasing K+ fraction, in spite of the fact that the size of K+ is larger than that of Na+. Analysis of data on the basis of percentage conducting volume reveals that the membranes with higher K+ content show a much higher conductivity (with higher cation mobility) at percentage conducting volumes higher than 10%. One reason for this behavior may be due to a higher extent of dissociation for K+ from the fixed anion site. TEM results show a larger cluster size in K+ form membranes, another possible reason for faster K+ transport. Pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) shows that water diffusion coefficient in the membranes with higher K+ fraction is higher than for samples with lower K+ fraction. FT-IR bands shift with the change of cation content, supporting the suggestion of a higher degree of cation-sulfonate dissociation and weaker hydrogen bonding interactions between cation and water molecules for membranes with higher K+ content.