(777a) Ion Transport in Anion Exchange Membranes for Water Purification and Power Generation Applications
The global need for sustainable water and energy sources presents an opportunity for polymer science as many of the membrane-based technologies used to purify water [e.g., reverse osmosis (RO), forward osmosis (FO), and electrodialysis (ED)] and emerging membrane-based power generation technologies [e.g., pressure-retarded osmosis (PRO) and reverse electrodialysis (RED)] rely on polymeric membranes to control rates of water and ion transport. In ED and RED processes, ion exchange membranes are used to restrict the transport of either cations or anions across the membrane while simultaneously allowing passage of oppositely charged species. Currently, the performance and understanding of anion exchange membranes (AEMs), which contain positively charged functional groups such as quaternary ammonium moieties, lags behind that of cation exchange membranes (CEMs), which contain tethered negative charges such as sulfonate moieties. Fundamental structure-transport property relations for AEMs are therefore needed to optimize and extend technologies such as ED and RED. The electrical potential-driven ionic resistance and permselectivity of AEMs based on quaternary ammonium-functionalized poly(sulfone), poly(phenylene oxide), and poly(styrene) backbones were measured to study the influence of degree of functionalization, water sorption, polymer backbone polarity, and morphology on ion transport properties. The polymer films were characterized during immersion in two different aqueous solutions containing either sodium chloride or ammonium bicarbonate, as this thermolytic salt is of interest for RED, and these polymers were compared to commercially available membranes. Ion transport properties measured using each of these electrolytes were analyzed with regard to ion hydration and dilute solution mobilities of the charge carrying ion. For many polymers, the relative rates of ion transport for different electrolytes can be described in terms of the dilute solution mobilities of the charge-carrying ions. For some highly functionalized polymers, however, dilute solution mobilities do not provide a reliable estimate of the intrinsic resistance of a polymer for a given electrolyte based on the intrinsic resistance measured using a different electrolyte. This phenomenon is rationalized by considering the nature of the charge carrying ion, water sorption of the polymer, and polymer morphology.