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

Public summary

* A novel cationic AB-stacking COF (EB-COF) bilayer tackles the permeability-selectivity challenge.

* The EB-COF bilayer membrane achieves a record 7,174 W m−2 power output, outperforming older designs.

* Tiny sub-2 nm pores and high charge density enhance both ion flow and selectivity for better performance.

* This study offers fundamental insights into the structure-performance relationship of permselective membranes.

Abstract

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.