Sandwich-structured covalent organic framework membranes for selective sodium ion transport

Reverse electrodialysis (RED) is a promising renewable energy technology for salinity gradient power generation, while its key component (i.e., ion exchange membranes) faces the challenge of the lower energy density caused by the uphill transport of divalent ions (e.g., Mg2+). In this study, we carried out a series of molecular dynamics (MD) simulations to investigate the permselective transport of Na+ and Mg2+ through a novel sandwich-stacked hybrid covalent organic framework (COF) membrane. The alternate set of TpHz and TpPa-SO3H constructed reduced effective pore size ranging from 5.6 to 7.5 angstrom, producing an offset-block effect on the ion transport. Besides, the tunable interlayer spacing of the hybrid COF membrane could manipulate the rehydration/dehydration behavior of the confined Na+ ions. A suitable set of the interlayer spacing could achieve an adorable Na+/Mg2+ permselectivity and Na+ permeance by the synergistic effect of aperture sieving and the electrostatic affinity. This work provides useful insights into the Na+/Mg2+ permselective mechanism and further helps to design COF-based membranes for realizing a high energy efficiency for the application of RED.