Ultrathin porous cation exchange membranes for high‐performance diffusion dialysis alkali recovery

Abstract Salt‐laden waste alkali streams pose significant environmental and economic challenges and demand efficient recovery. As an energy‐efficient route for alkali recovery, diffusion dialysis (DD) is limited by the poor hydroxide permeability () of conventional dense cation exchange membranes (CEMs). Herein, porous CEMs were fabricated via non‐solvent induced phase separation of a blend of chloromethylated polyethersulfone and sulfonated polyethersulfone, followed by crosslinking with polyethylenimine and post‐sulfonation with 1,3‐propane sultone. Through optimization of polymer composition, crosslinking degree, and sulfonation level, we engineered membranes with tailored microstructure and transport properties. The optimized membrane demonstrates a record‐high of 23.8 × 10 −3 m/h in NaOH/Na 2 WO 4 systems, while maintaining OH − /WO 4 2− selectivity () of 21.2; the permeability exceeds commercial benchmarks and reported values. This breakthrough establishes a practical and scalable approach for sustainable alkali recovery from high‐salinity industrial waste streams.