Polybenzimidazolium bilayer membranes with high permselectivity and low ionic resistance

In an electrodialysis (ED) system, anion exchange membranes (AEMs) that increase the permselectivity of anions, while maintaining low ionic resistance and low fouling, are crucial to enhancing the separation efficiency of an ED process. This work reports the fabrication and study of bilayer membranes prepared from poly (benzimidazolium)-based polymers (HMT-PMBI) possessing varying degrees of methylation (dm). A single layer membrane of HMT-PMBI possessing 75% methylation (75dm) exhibits exceptionally large mono/divalent permselectivity but low ionic conductivity, while the single layer membrane of HMT-PMBI 89dm possesses increased ionic conductivity at the expense of permselectivity compared to the 75dm counterpart. Bilayer 75dm/ 89dm membranes exhibited high ion exchange capacity (IEC, 1.49 mmol g(-1)) and lower ionic resistance compared to single layer 75dm membranes without significantly impacting water uptake and permselectivity. In an electrodialysis system, bilayer membranes demonstrate internal permselectivity (4.7 for 23 mu m), with the interface between 89dm and 75dm layers posing a greater resistance to divalent ions (SO42- ) while minimally affecting the transport of monovalent ions (Cl- ). When the 89dm layer of the bilayer membrane is placed facing the cathode, the permselectivity-to-resistance ratio is enhanced 3x compared to a single layer 89dm membrane. Additionally, the bilayer membrane exhibits >10x greater fouling resistance than the single layer 75dm membrane. The bilayer configuration membrane effectively affords the advantage of the higher water uptake of the 89dm layer in order to enhance both ionic conductivity and fouling resistance while preserving the permselectivity afforded by the 75dm layer.