Preparation of acid block anion exchange membrane with quaternary ammonium groups by homogeneous amination for electrodialysis-based acid enrichment

•Strong base Anion-exchange membrane with outstanding acid block performance is developed.•Anion-exchange membranes prepared by homogeneous and heterogeneous amination are compared.•Reduction of water uptake resulting from phase separation microstructure deserves special attention. Proton leakage defect of anion exchange membrane (AEM) seriously restricts the applications of electrodialysis (ED) in acid enrichment. Different from conventional scheme in which weak base groups are usually introduced, strong base AEMs are endowed with acid block property by the specific design of membrane microstructure in this work. Firstly, as confirmed by 1H NMR and FTIR, a series of quaternary ammonium polysulfone containing long alkyl side chains pendant are synthesized for AEM preparation. Small Angle X-ray Scattering experiments show that ionic domains form a distinct and well-separated phase. The introduced hydrophobic side chains strengthen the hydrophobic matrix and then restrict AEM water uptake. Besides, some interesting confinement effects are observed, for examples, the reinforcement of Donnan exclusion for H+ and facilitated transport of counter-ions. Furthermore, influences of pendant carbon chain length and ion exchange capacity are investigated. Notably, ED measurements show as-prepared AEMs indeed display excellent acid block performance, a current efficiency of almost 100% under certain condition, which substantially exceeds that of conventional AEM fabricated by heterogeneous aminaiton. This observation demonstrates an unprecedented result that novel strong base AEM with both outstanding acid block performance and membrane conductivity can be achieved by constructing phase separation microstructure.