Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10 −7  cm 2  min −1 ) and higher proton selectivity (1.06 × 10 5  S min cm −3 ) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies ( CE s: 96.0–98.2%) and energy efficiencies ( EE s: 79.7–69.5%) than that with Nafion 212 membrane ( CE s: 86.5–92.5% and EE s: 78.5–67.6%) at 80–160 mA cm −2 . The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm −2 , whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.