Cyclodextrin-alkyldiamine rotaxane cross-linked anion exchange membranes for electrodialysis

Anion exchange membranes (AEMs) with high selective conductivity and low surface resistance are important for the development of electrodialysis desalination industry. In this work, we design and synthesize a rotaxane of β-cyclodextrin (CD)-alkyldiamine inclusion complex (CDIC) via a host-guest molecular recognition mechanism; the resulting CDIC rotaxane is employed for cross-linking chloromethylated, hydroxy quaternized polysulfone (HQPSf) to make AEMs. β-CD is inherently hydrophilic and its -OH on the outer surface can hydrogen-bond with HQPSf to promote ionic cluster aggregation, leading to enhanced microphase-separated morphology and improved conductivity; the rotaxane-cross-linked structure guarantees good ductility and ion selectivity of the membrane. Therefore, the optimized HQPSf-CDIC membrane gives the most excellent current efficiency discharge current efficiency (92.04 %), the lowest energy consumption (4.68 kWh kg-1), and the maximum Cl- mobility (75.38 mg m-2 s-1) in the electrodialysis tests, significantly better than the un-crosslinked HQPSf membrane (86.41 %, 5.17 kWh kg-1 and 71.29 mg m-2 s-1). Our work demonstrates that introducing CDIC rotaxane cross-linked structure can help to enhance microphase separation and improve the performance of AEMs for electrodialysis.