Enhancing Selective Ion Transport by Stacking Covalent Organic Framework Monolayers

Nanopore-based power generation represents an efficient way for harvesting salinity gradient energy. Due to its ultrahigh ion conductivity and moderate ion selectivity, the crystalline covalent organic framework (COF) monolayer demonstrates the record-high output power density by mixing river water and seawater. To further improve energy conversion performance, it is necessary to enhance ion selectivity while achieving high membrane permeability. Here, a layer-by-layer stacking approach is developed to notably enhance the selective ion transport of ultra-thin COF layers, offering advantageous in both conversion efficiency and scalability. Under a standard NaCl salinity gradient (0.5 M/0.1 M), the ratio of ionic mobility between Cl^- and Na⁺ increases from 1.4 to 2.9 with stacking the anion-selective COF monolayer from one to ten layers, leading to a more than seven-fold enhancement in osmotic energy conversion efficiency. By maximizing selectivity and permeability, the output power can reach 411 pW by stacking three layers in a single device. This strategy provides an effective approach for the integration of atomically thin membranes in selective mass transport applications.