Bicarbonate and chloride anion transport in anion exchange membranes

Quaternary ammonium poly(sulfone) based anion exchange membrane (AEM) in Cl^- and HCO₃^-forms were characterized chemically and morphologically. It was found that the surface of the membrane in both of the forms has highly connective island-like structure, where the diameters of the hydrophilic regions are approximately 5-20 nm. Thermal gravimetric analysis of the membrane in the HCO₃^- form presented lower decomposition temperatures for the backbone and the side chains, than the membrane in the Cl^- form. In addition, the AEM in its HCO₃^- form showed higher water uptake values than in its Cl^- form across the temperature range of 25-80 °C. Conductivity experiment measured at same temperatures in both AEM forms showed higher results for Cl^- form than for HCO₃^- form. A computational model was developed in order to understand the conductivity mechanism and the relevant parameters that limit ion transport in these materials. Together with the experimental results, it was found that only 40% of the ions are free for ionic conductivity, while 60% of the ions are bound to the cationic groups, therefore unavailable to participate in the conduction process.