The influence of anion-exchange membrane nanostructure onto ion transport: Adjusting membrane performance through fabrication conditions

We fabricated dense anion-exchange membranes from the same polymer with variations in the self-assembled hydrophilic and hydrophobic domains. We show how the nanostructure forms during membrane formation and how it can be tuned by rationally designing fabrication conditions. We found that the hydrophilic and hydrophobic domain structure evolves during the membrane solidification. This can be controlled by using casting solvents with different evaporation times or by the addition of an ionic liquid which alters the interactions in the polymer film. We could further tailor the structure of solidified membranes by heat or solvent annealing and developed conditions that selectively adjust the nanostructure in different parts of the membrane. With these methods we could significantly optimize the macroscopic properties and ion transport rates of membranes made from the same polymer. The ionic liquid additive led to an almost doubling of the water uptake without influencing the swelling degree and also to a 3-fold higher diffusion rate for HBr. By using a post-treatment with dioxane we also managed to fabricate a membrane with a 5-fold higher ionic conductivity. We further found that different transport mechanisms are involved in ionic conductance and acid diffusion and could relate them to different membrane nanostructures.