Effect of non-sulfonated diamine monomer on branched sulfonated polyimide membrane for vanadium redox flow battery application

Abstract

Branched sulfonated polyimide (bSPI) membranes based on three different non-sulfonated diamine monomers were fabricated for vanadium redox flow battery (VRFB) usage. Both FT-IR and 1H NMR spectra confirm that bSPIs were successfully synthesized. TGA results show that bSPI membranes have good thermal stability. FESEM pictures indicate that the surface of membrane in contact with catholyte keeps better integrity than that in contact with anolyte after 500-time cycling charge-discharge test. The water uptake, ion exchange capacity, mechanical strength of bSPI membranes are larger than or close to those of Nafion 117 membrane, and bSPI membranes exhibit remarkably lower vanadium ion permeability compared with Nafion 117 membrane. In addition, bSPI membranes have outstanding chemical stability in contrast with linear SPI membrane in ex-situ degradation test. Further, the bSPI (BAPP) membrane displays the best chemical stability and highest proton selectivity (1.23×105Smincm−3) among all bSPI membranes, thus it is seen as an optimum one for further single VRFB test. During 500-time cyclic charge-discharge test, the coulomb efficiency (CE: 97.1∼99.7%) of VRFB containing bSPI (BAPP) membrane is higher than that containing Nafion 117 (CE: 95.2∼98.6%) at 30∼120mAcm−2. The VRFB using bSPI (BAPP) membrane presents higher or comparable energy efficiency (EE) and capacity retention compared with that using Nafion 117 membrane at low or high current density. Based on the excellent physico-chemical properties and single cell performance together with much lower cost compared with widely used Nafion 117 membrane, the as-prepared bSPI (BAPP) membrane has great potential to be applied in VRFB.