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

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.