Development of a high-performance anion exchange membrane using poly(isatin biphenylene) with flexible heterocyclic quaternary ammonium cations for alkaline fuel cells

A series of quaternary ammonium-tethered poly(isatin biphenylene)s (PIBs) were prepared devoid of alkaline labile aryl ether bonds via superacid-catalyzed polymerization and explored as highly alkaline stable backbones of anion exchange membranes (AEMs). The long flexible alkyl chains with pendent trimethylammonium (QA), 1-methylpyrrolidinium (Pyr), and 1-methylpiperidinium (Pip) were grafted to PIB backbones to construct AEMs endowed with high alkaline stability and OH− ion conductivity. Results show that the AEM tethered long alkyl side chain exhibits a distinct microphase-separated morphology. In particular, the presence of the long alkyl side chain pendant QA possesses the highest OH− ion conductivity of 93.88 mS cm−1 at 80 °C among different AEMs, whereas the PIB tethered Pip shows the best alkaline stability and retains 71.7% of the original OH− ion conductivity after storage in 2.0 M aqueous NaOH at 80 °C for 1050 h. Besides, the oxidation stability of the as-prepared AEMs reaches 108 h, along with a weight decrease of 37.4% after immersion into 4 ppm Fenton's reagent at 80 °C. Furthermore, a platinum-catalyzed H2–O2 fuel cell using a QAPIB membrane shows a higher peak power density of 1.24 W cm−2 than that using a PipPIB membrane (1.13 W cm−2). Therefore, the present study demonstrates that PIB-based AEMs exhibit good alkaline stability, oxidation durability, hydroxide ion conductivity and superior fuel cell performance. Thus, we emphasized this type of PIB-based AEM as a promising prospect in anion exchange membrane fuel cells.