Anion exchange membranes and ionomer properties of a polyfluorene-based polymer with alkyl spacers for water electrolysis

Aryl ether-free polymers have attracted considerable attention in recent years for their application in anion exchange membrane (AEM) fuel cells (AEMFCs) and water electrolysis (AEMWE). Here, an aryl ether-free polyfluorene-based polymer bearing pendent ammonium groups in side chains and propyl spacers in its polymer backbone is synthesized through the Suzuki cross-coupling reaction. The propyl spacer introduced into the backbone improves the flexibility of the polymer and allows effective ionic cluster formation by the polymer. Compared to polyphenylene oxide (PPO), which has a similar molecular weight, quaternized poly[9,9-bis(6-bromohexyl)fluorene]–co-[4,4-bis((4-phenyl)propyl)biphenyl)] (PFPB-QA) polymers show significantly superior membrane-forming properties owing to the propyl spacers. The PFPB-QA membrane also exhibits high ionic conductivity (122 mS cm−1 at 80 °C) and excellent alkaline stability (1 M KOH at 80 °C for 720 h). Moreover, the AEMWE utilizing PFPB-QA exhibits a current density of 1.53 A cm−2 at 2.0 V in 1 M KOH at 70 °C. Therefore, the present study shows that an aryl ether-free polymer with an alkyl spacer has excellent mechanical properties, ionic conductivity, alkaline stability, and cell performance.