Novel branched sulfonated polyimide/molybdenum disulfide nanosheets composite membrane for vanadium redox flow battery application

A novel branched sulfonated polyimide/molybdenum disulfide nanosheets (bSPI/MoS 2 -ns) composite membrane was fabricated for vanadium redox flow battery (VRFB) application. Scanning probe microscope (SPM) and field emission-scanning electron microscope (FE-SEM) images verify the successful exfoliation of monolayer and a few layers of MoS 2 nanosheets. X-ray diffraction (XRD), attenuated total reflection-fourier transform infrared spectrometer (ATR-FTIR) and FE-SEM results demonstrate that bSPI, bSPI/MoS 2 -ns and bSPI/MoS 2 nanopowders (bSPI/MoS 2 -np) membranes are successfully prepared. Both thermogravimetric analysis/differential thermogravimetry (TGA/DTG) and ex-situ chemical stability testing results confirm bSPI/MoS 2 -ns membrane has improved thermal and chemical stabilities in contrast with pristine bSPI membrane. The bSPI/MoS 2 -ns membrane owns high proton conductivity (5.04 × 10 −2  S cm −1 ) and extremely low vanadium ion permeability (0.65 × 10 −7  cm 2  min −1 ), leading to its highest proton selectivity (7.75 × 10 5  S min cm −3 ) among all the membranes tested. Besides, during 500 times cycle charge–discharge test, the coulombic efficiency, energy efficiency and discharge capacity retention of VRFB using bSPI/MoS 2 -ns membrane are higher than or comparable to those of VRFB using Nafion 117 membrane. And the surfacial morphological integrity of bSPI/MoS 2 -ns membrane maintains better than reported pristine bSPI membrane after long-time VRFB usage. Above results indicate that the as-prepared bSPI/MoS 2 -ns membrane has great potential for VRFB application.