Covalent organic framework-based membrane improved the performance of reverse electrodialysis under Na+/Mg2+ mixed solution

Reverse electrodialysis (RED) is a promising membrane-based technology for salinity gradient power while it faces the challenge of the energy loss caused by the insufficient monovalent/multivalent ion selectivity of the membranes (such as Na+/Mg2+). Here, covalent organic frameworks (COFs) with nanometre-scale pores (pore size of ~8 Å) are designed as composite membranes to selectively conduct monovalent ion (Na+ ~7.1 Å vs Mg2+ ~8.6 Å) by size sieving. Compared with the commercial Neosepta CMX membrane, the COF composite membrane effectively inhibits the penetration of Mg2+ without sacrificing Na+ penetration flux in RED, which to a large extent reduces the energy loss in RED caused by the penetration of Mg2+. Even for Mg2+ solution with high molar fraction of 30 %, the maximum power density (Pd, max) of CCM only shows a slight decrease of 1.7 %. In contrast, the Pd, max of the commercial Neosepta CMX membrane severely decreases by 35.8 %. This study highlights the prospects of COFs as the significant material for designing monovalent ion selective membranes which can improve energy efficiency in RED.