Electric-Field-Driven Assembly of Ultrathin and Crystalline iCOF Membranes for High-Performance Molecular Sieving

The fabrication of high-performance ionic covalent organic framework (iCOF) membranes is fundamentally constrained by the kinetic conflict between the slow crystallization required for structural order and the rapid film formation needed for defect-free continuity. Here, we introduce an electric-field-driven assembly (EFDA) strategy that actively overcomes this limitation. By applying a directional electric field across the liquid-liquid interface, we create a steep field gradient that electrophoretically pumps ionic monomers against diffusion barriers, achieving rapid and focused interfacial enrichment. This active supply enables the fast (∼4 h) growth of an ultrathin (∼45 nm), continuous, and highly crystalline iCOF layer-a combination unattainable by passive diffusion methods. The resultant membrane exhibits an exceptional water permeance of ∼70 L m^-2·h^-1·bar^-1coupled with precise charge-selective separation (>98% rejection of anionic dyes), outperforming most reported nanofiltration membranes. Demonstrated with various ionic monomers, the EFDA strategy presents a universal platform for the rational construction of advanced molecular sieving membranes.