High-performance ether-free branched poly(isatin terphenyl fluorene) with sulfonated pendant side chains for proton exchange membrane fuel cells

Ether-free polyaromatic proton exchange membranes (PEMs) are regarded as an optimal and cost-effective substitute for perfluorosulfonic acid membranes due to their excellent oxidation resistance stability, low cost and inherent low permeability. The potential applications of the branched structure are significant in enhancing membrane processability and physical stability, but the ether-free branched sulfonated polyaryl proton exchange membrane remains to be further explored. Herein, we present the synthesis of branched sulfonated poly(isatin terphenyl fluorene) (b-SPITF-x), featuring a branched comb-like structure design incorporating sturdy backbones devoid of ether, branched monomers and flexible side chains containing sulfopropyl groups at specific locations. This branched comb-like structure endows the membrane with sufficient microphase-separated morphology and dense ion clusters, thus ensuring the optimized b-SPITF-10 PEM exhibits simultaneously high H + conductivity (>185 mS cm −1  at 80 °C) and desirable physical properties. Furthermore, the b-SPITF-10 PEMFCs obtained the maximum peak power density of 819 mW cm −2 operated at 80 °C. Moreover, during a 100-h durability test carried out at a temperature of 80 °C, this particular PEM exhibited an impressively low rate of voltage degradation, measuring only 0.743 mV h −1 . The ether-free structure and branched comb-like design offer valuable insights into the advancement of polyaromatic PEMs.