Self-supporting sulfonated covalent organic framework as a highly selective continuous membrane for vanadium flow battery

The application of sulfonated covalent organic frameworks (SCOFs) in vanadium redox flow battery (VRFB) is limited by the nano-scale non-sieving pores and low additive amount in mixed matrix membranes. Herein, a self-supporting continuous SCOF membrane interlaced with Nafion chains (SCOF/Nf) is proposed to possess high H+/Vn+ selectivity via the wedge-tenon like reinforce structure. The continuous self-supporting SCOF layer provides crystalline ordered dense sulfonic acid groups for proton hopping and contributes to the considerably high proton conductivity (143.9 mS cm−1, 25 °C) at a very low swelling ratio (3.1 %). The flexible Nafion polymer chains can be inserted into the pores of SCOF by hydrogen bonds and ionic interactions, reducing the pore size from 15.0 Å to 5–10 Å for efficiently sieving H+/Vn+ ions. With a SCOF weight proportion of 0.6, the SCOF/Nf-0.6 membrane achieves to a high energy efficiency and extremely low discharge capacity decay rate (85.5 % and 0.13 % per cycle at 100 mA cm−2, respectively), which are much superior to the most state-of-the-art Nafion and COFs based ion conductive membranes. With the interlaced Nafion layer, the SCOF/Nf-0.6 membrane keeps stable and intact in the 500-cycle test (>900 h) in VRFB. The application of SCOF in VRFB from mixed matrix membranes to self-supporting continuous membrane provides an efficient way to maximize the function of the crystalline ordered two-dimensional materials.