In Situ Growth of Imine‐Bridged Anion‐Selective COF/AAO Membrane for Ion Current Rectification and Nanofluidic Osmotic Energy Conversion

Abstract Facing the energy crisis, using the salinity gradient between seawater and freshwater for osmotic energy conversion is a direct way to obtain energy. So far, most nanofluidic membranes utilized for osmotic energy generation are cation‐selective. Given that both anion‐ and cation‐selective membranes have the identical importance for energy conversion devices, it is of great significance to develop anion‐selective membranes. Herein, an anion‐selective membrane is synthesized by in situ growth of imine‐bridged covalent organic framework (COF) on ordered anodic aluminum oxide (AAO) at room temperature. The imine groups and residual amino groups of COF can combine with protons in neutral solution, enabling the COF positively charged and efficiently transport of anions. Particularly, due to the asymmetry in the charge and structure of COF/AAO, the as‐prepared membrane exhibits excellent ionic current rectification property, which can inhibit ion concentration polarization effectively and possess high ion selectivity and permeability. Using the present COF/AAO membrane, salinity gradient energy can be successfully harvested from solutions with high salt content, and the output power density reached 17.95W m −2 under a 500‐fold salinity gradient. The study provides a new avenue for construction and application of anion‐selective membranes in the smart ion transport and efficient energy conversion.