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

ABSTRACT 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.