Engineering amphoteric COFs nanofibers ion selective channel network for high-performance vanadium redox flow batteries

Covalent organic frameworks (COFs) possess highly ordered ionic conductive channels to compensate the challenges of polymer materials, are expected to address the critical issue of ion selective conduction in vanadium redox flow batteries (VRFBs). In this work, the amphoteric COFs nanofibers ion selective channel network is designed for the first time through the in-situ growth of the triazine COF and sulfonated COF from monomers contained electrospun blend nanofibers, and then densified by sulfonated polybenzimidazole (SPBI). The amphoteric COF nanofibers show notably higher diffusivity ratio of proton to vanadium ions compared to the individual nanofiber, demonstrating synergistical effect to construct high ion selective channel network, in which the triazine COF fiber repels vanadium ions via Donnan effect of the protonated nitrogen contained channels, and the sulfonated COF fiber facilitates rapid proton hopping along the acidic sulfonic acid groups contained fiber network. With amphoteric COFs nanofibers content of 55 vol%, the composite membrane exhibits low area resistance (0.17 Ω cm 2 ) and vanadium ions permeation (3.9 × 10 −9  cm 2  s −1 ) simultaneously, achieving an impressive battery energy efficiency of 85.1% at 200 mA cm −2 , outperforming that of the Nafion 212 (74.9%) and currently reported amphoteric ion conductive membranes for VRFBs. The amphoteric COFs composite membrane also shows stable operation over 1500 cycles (with periodic electrolyte refresh), exhibiting a discharge capacity decay rate of 0.25% per cycle (calculated over the first 100 cycles). This study provides an effective microstructure design for COFs materials to improve ion selectivity and flow batteries performance.