Ultrathin Azine Covalent Organic Framework Membrane for Highly‐Efficient Nanofluidic Osmotic Energy Generator

Charged covalent organic framework (COF) membranes have gained wide interest as the key component in the reverse electrodialysis technique to harness salinity energy. However, maintaining rapid ion transport and high selectivity in a Ca²⁺-rich environment remains a formidable challenge. Herein, a highly cation-conductive azine COF membrane is synthesized via a layer-by-layer chemical reaction between 2,4-dihydroxy-1,3,5-diphenyltrialdehyde (DHTA) and hydrazine hydrate (HZ). The osmotic energy generator based on this membrane delivers a high power density of 17.8 W m^-2 under 2.5 M/0.05 M CaCl₂, outperforming the TFP-HZ membrane (3.2 W m^-2), commercial benchmark (5 W m^-2), and other literature reported membranes owing to the simultaneous modulation of charges in angstrom scale channels and selective layer thickness. Moreover, this osmotic power density is comparable to that in a NaCl gradient (2.5 M/0.05 M, 16.9 W m^-2), which is rare. These results indicate that the DHTA-HZ membrane is highly suitable for application in hypersaline environments containing Ca²⁺, serving as an inspiration for the development of COF-based nanofluidic membranes with high power output efficiency in a practical high-salinity environment.