Hydroxide, halide and water transport in a model anion exchange membrane

Abstract

Transport of water and anions (OH−, HCO3−, F−, Cl−, Br−, and I−) in a model poly(arylene ether) anion exchange membrane (AEM) with quaternary ammonium (QA) functional groups is studied for water volume fractions of Φwater=5–99.9%. At elevated water content, OH− conductivity recorded under CO2 free conditions occurs mainly through structural diffusion, approaching half of the H+ conductivities of Nafion. Severe conductivity decrease at low hydration suggests incomplete dissociation of NR4+ OH− and inferior percolation within the aqueous domain of the AEM compared to Nafion. Further conductivity decrease result from CO2 contamination, forming carbonates which are less mobile, less hydrated and also less dissociated from the QA compared to OH− at a given relative humidity (RH). For other anions, conductivity decreases in the order F−>Cl−>Br−>I− for a given RH and also for the same hydration number λ=[H2O]/[QA]. These trends correlate with both decreasing water uptake and degree of dissociation, which is incomplete for some anions even at very high levels of hydration. Consequently, the functional groups in their halide forms display weak electrolyte behavior in contrast to the corresponding salts. Varying degrees of dissociation of different anions are held responsible for distinct variations of the nano-morphology of hydrated membranes.