Covalent organic framework membranes for ion separation and ion-driven energy conversion

Abstract

Covalent organic framework (COF) membranes have moved from crystalline porous solids to processable platforms for ion separation and ion-transport-based energy conversion. Their ordered nanochannels and chemically addressable pore walls enable precise control over effective pore size and topology, channel alignment, and embedded charge or affinity sites, parameters that are often difficult to tune independently in conventional polymer membranes. This review summarizes major fabrication routes and the membrane attributes they shape, including interfacial growth, casting-based methods, layer assembly, and composite architectures that tune crystallinity, alignment, thickness, and interface quality. We then organize advances in aqueous ion separation around recurring design logics that transfer across targets. For monovalent to multivalent discrimination, case studies show how pathway continuity and alignment, intrapore charge localization, Mg²⁺ trapping, and Li⁺-favored coordination environments can sustain selectivity in mixed electrolytes and concentrated feeds. For monovalent cation separation, functional-group grafting, pore narrowing, orientation control, and host–guest motifs amplify subtle differences in hydration and binding and can enable pH-regulated selectivity switching. For proton and metal-ion separation and acid recovery, sub-nanometre sieving coupled with hydrogen-bond-assisted proton conduction enables high H⁺/Mⁿ⁺ selectivities under strongly acidic conditions. For Cl⁻/SO₄²⁻ separation, COF selective layers and COF-enabled polyamide architectures illustrate how charge density, pore uniformity, and interfacial control can raise monovalent to divalent anion selectivity while maintaining practical flux. We further connect these transport principles to salinity-gradient energy conversion and coupled thermal or photo fields, where permselectivity and internal resistance jointly set power output under polarization and stability constraints. Looking forward, translation will depend on scalable fabrication with reproducible defect control and orientation, mechanistic validation in complex electrolytes under operando conditions, and process-relevant benchmarking that links membrane descriptors to module-level performance and durability.