Sandwich-like COF membranes with enhanced chargeability and narrowed nanopores for nanofiltration-based desalination

Ionic covalent organic framework (iCOF) membranes are promising for nanofiltration, yet combining high charge density with confined pore size remains challenging because strong electrostatic repulsion disrupts ordered stacking and generates structural defects. Here, we propose a “moderately charged–highly charged–moderately charged” strategy to construct sandwich-like COF membranes (SCOFMs). By layer-by-layer assembly of TpPa-SO 3 H and TpPa-2SO 3 H nanosheets, SCOFMs with enhanced chargeability and narrowed nanopores are obtained. Increasing the outer-to-inner layer ratio from 10:0 to 7:3 raises the membrane surface charge density from −1.51 to −1.84 mC m −2 and reduces the pore diameter from 0.95 to 0.68 nm. This improvement originates from the higher chargeability and smaller pore size of TpPa-2SO 3 H, while the sandwich structure mitigates interlayer repulsion and promotes dense stacking. The synergistic Donnan exclusion and size-sieving endow SCOFM (7:3) with high rejection of 96.4 % for Na 2 SO 4 , 97.2 % for MgSO 4 , 84.4 % for NaCl, and >99 % for various antibiotics. Benefiting from this architecture, it also maintains Na 2 SO 4 rejection larger than 91 % and stable water permeance for over 50 days. This approach simultaneously narrows nanopores and enhances the membrane surface charge density, offering a promising route for high-performance nanofiltration membranes.