Axial alignment of covalent organic framework membranes for giant osmotic energy harvesting

The membrane-based osmotic power generation technology can both provide sustainable energy and address environmental pollution utilizing an eco-friendly energy conversion mechanism. Covalent organic framework (COF) membranes are an attractive option for this application due to their porosity, well-defined pores and tunable surface chemistry. However, precise engineering of the porous structure for rapid ion transport remains a challenge. Here we engineer the initially randomly oriented COF nanochannels into a highly axially aligned configuration, delivering a metal ion-coordinated COF framework, through interfacial polymerization followed by coordination to different ions, including Ca²⁺, Mg²⁺, Al³⁺, Fe³⁺, Zn²⁺, Co²⁺ and Cu²⁺. Notably, the representative Ca-COF demonstrates a superior cation selectivity of 0.93 and ionic conductivity of 0.06 S m⁻¹. When applied to osmotic energy harvesting, the Ca-COF membranes deliver a record output power density of 320.8 W m⁻² in the presence of a mixture of natural seawater and river water. By highlighting the importance of aligning metal ion-coordinated COF nanochannels in improving ion selectivity and permeability, our strategy suggests a pathway in unlocking the potential of osmotic energy harvesting technologies.