Giant osmotic power density generation with an anion-selective AB-stacking covalent-organic framework bilayer

Atomically thin two-dimensional membranes are promising for osmotic energy generation. However, the trade-off between permeability and selectivity remains a long-standing bottleneck. Herein, we demonstrate that a cationic AB-stacking covalent-organic framework (COF) bilayer achieves high ion conductivity and selectivity. Through precise molecular design and the Langmuir-Blodgett (LB) technique, we fabricate an anion-selective COF bilayer (EB-COF) using tetradentate 4,4',4″,4'''-(porphyrin-5,10,15,20-tetrayl)tetrabenzaldehyde (TFPP) and bidentate ethidium bromide (EB) with a covalently tethered pyridinium moiety. The EB-COF bilayer shows a record-high output power density of 7,174 W m^-2 (0.5/0.01 M NaCl), significantly outperforming the AA-stacking PDA-COF bilayer constructed with TFPP and neutral p-phenylenediamine (PDA). This exceptional performance stems from highly ordered sub-2-nm nanopores, exceptional pore utilization efficiency, and a large surface charge density of 4.4 mC m^-2, which together synergistically enhance anion selectivity and ion permeability. This work not only provides fundamental insights into the structure-performance relationship of permselective membranes but also offers a promising strategy for high-efficiency osmotic energy harvesting.