Abstract A series of branched sulfonated polyimide (bSPI)/sulfonated methylcellulose (s‐MC) composite membranes composed of a designed and synthesized bSPI polymer and functionalized s‐MC are prepared by using a facile solution casting method for vanadium redox flow batteries (VRFBs). Among all bSPI/s‐MC composite membranes, the optimized bSPI/s‐MC‐20 % composite membrane has the best proton selectivity of 2.45×105 S min cm−3, which is 14.4 times as high as the Nafion 115 membrane. The bSPI/s‐MC‐20 % composite membrane possesses superior proton conductivity compared to most reported SPI‐based composite membranes for VRFBs. The VRFB with a bSPI/s‐MC‐20 % composite membrane shows excellent battery efficiencies (CE=99.2–98.0 %, EE=66.3–77.6 %) and capacity retention (73.3–47.2 %). Moreover, the cost of the bSPI/s‐MC‐20 % composite membrane is only a quarter of that of a commercial Nafion 115 membrane. This work develops a new strategy to fabricate cheap bSPI‐based composite membranes by introducing a suitable functionalized biomass material.

Silicon carbide (SiC) was pretreated and functionalized by using α, ω-diaminopropyl polydimethylsiloxane (PDMS) to obtain a novel inorganic filler SiC (PDMS). Then, a series of branched sulfonated polyimide/SiC (PDMS) (bSPI/SiC (PDMS)) composite membranes with different contents of SiC (PDMS) were fabricated for vanadium redox flow battery (VRFB) application. Fourier transform infrared spectra, X-ray diffraction, and field emission scanning electron microscope demonstrate the successful preparation of SiC (PDMS) and bSPI/SiC (PDMS) membranes. The thermogravimetric analysis shows that bSPI/SiC (PDMS)-1.5% membrane has better thermal stability than pure bSPI, bSPI/SiC-1.5%, and Nafion 117 membranes. The ex situ chemical stability test results show that bSPI/SiC (PDMS)-0.5–2.5% composite membranes have better chemical stabilities than pure bSPI membrane. The physicochemical properties of bSPI/SiC (PDMS) membranes, including water uptake, swelling ratio, ion exchange capacity are investigated. Thereinto, bSPI/SiC (PDMS)-1.5% membrane has the highest proton selectivity (S: 2.99 × 105 S min cm−3) and was chosen as an optimum VRFB membrane. And the VRFB assembled with bSPI/SiC (PDMS)-1.5% membrane exhibits better battery performance than that assembled with Nafion 117 membrane during 500-time cyclic charge–discharge test at 20–60 mA cm−2. Above results indicate that as-optimized bSPI/SiC (PDMS)-1.5% membrane has great potential for VRFB application.