Dynamic Electrophoretic Assembly Coupled With Water-Splitting-Induced pH Control Enables Precision Ion Sieving Membranes.

Discriminating among monovalent ions of similar charge and size, such as Li+, Na+, and K+, remains one of the most persistent challenges in separation science. Here we present a green, external-acid-free route to fabricate defect-free covalent organic framework (COF)-based thin-film composite (TFC) membranes for high precision monovalent-ion sieving. The method relies on a dynamic electric-field-assisted strategy in which an alternating electric field couples electrophoretic monomer assembly at the surface of an ion-exchange membrane (IEM) substrate with spatiotemporally resolved delivery of hydronium ions generated in situ via water splitting. This localized in situ-generated acid pulse initiates interfacial condensation into primary COF nuclei, which subsequently undergo continuous growth and fusion into a uniform ultrathin film through a self-healing mechanism. The proposed membrane formation strategy effectively suppresses non-selective transport pathways and enables rapid formation of highly selective ion-transport channels, resulting in a membrane with near-complete fractionation of Li+ from other monovalent ions (e.g., K+ and Na+), while preserving fast ion permeation that exceeds that of previously reported membranes. The governing ion separation mechanism is elucidated as hydration-shell restructuring under electrostatic confinement. The work establishes a scalable and energy-efficient platform for precision ion separation and opens new opportunities for membrane-based resource recovery and molecular purification.