Self-supporting sulfonated covalent organic framework as a highly selective continuous membrane for vanadium flow battery
Bo Pang, Ruohan Du, Wanting Chen, Fujun Cui, Ning Wang, Huimin Zhao, Guihui Xie, Tiantian Li, Gaohong He, Xuemei Wu
Abstract
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 + /V n+ 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 + /V n+ 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.