Synthesis and properties of highly durable poly(fluorene-alt-tetrafluorophenylene) anion-conducting membranes with cyclic ammonium groups

The alkaline durability of anion exchange membranes is essential for the long-term operation of anion exchange membrane water electrolyzers and fuel cells. In this study, we investigated the alkaline durability of anion exchange groups incorporated into the alkyl side-chain terminals of model low-molecular-weight fluorene compounds. As a result, we found that quaternary ammonium groups with a cyclic structure such as quinuclidinium and piperidinium have high durability. These cyclic ammonium groups were incorporated into high-molecular-weight poly-(fluorene-alt-tetrafluorophenylene) to form membranes with high OH⁻ conductivity (>100 mS cm⁻¹) in water at 80 °C and higher alkaline durability than similar polymers with linear alkyl trimethyl ammonium groups as the anion exchange group. A synthesized polymer with the piperidinium group showed optimal alkaline durability among all membranes examined in this study, with almost no decrease in ionic conductivity observed for 240 h under the accelerated test conditions at 80 °C in 8 M NaOH solution.