Ultrafast Ion-transport at Hierarchically Porous Covalent-Organic Membrane Interface for Efficient Power Production

Abstract Highly ordered free-standing membranes are challenging to fabricate using conventional polymeric material. Additionally, several organic-inorganic materials have been studied to develop a stable free-standing membrane for energy applications. Still, fragile structural issues under realistic conditions remain one of the most significant barriers to making them commercially viable. Here, we have prepared a series of large-area 10 ×10 cm2 free-standing, proton-conducting covalent organic membrane (COM) for reverse electrodialysis. COM possesses a hierarchical nanoporous stable structure formed with a highly crystalline organic framework. The random arrangement of layered micropores with a pore size of 1.16 nm plus mesopores and macropores provides a hierarchical porous structure in COM. However, the crystalline arrangement is highly ordered with fixed pores in COF, a pore size of 1.34 nm. The determined surface porosity of cross-sectional COM is ~66%. The pore size distribution is ~1.2 nm, and the estimated surface area of COM is up to ~33 m2 g-1. The prepared free-standing structure is stable at elevated temperatures under 100% hydrated conditions. COM structure is also mechanically stable ~2 MPa with elongation up to 4% and maintains its free-standing structure under acidic conditions. Besides robust hierarchical porous structure and chemical stability under stress conditions, acid-treated COM offers better ion selectivity with enhanced ion transport at elevated temperatures. It is only possible because of low membrane swelling density while maintaining the high ion-exchange capacity, which are crucial factors for implementing it in an electrochemical application. The free-standing COM combined with FAA-3 membrane for assembling the reverse electrodialysis's stack for power production. The obtained power density of reverse electrodialysis is ~1.44 W m-2 at a fixed flow rate of 2 mL min-1. With negligible hydrodynamic loss and maintaining stable cell performance.