Ionic Triptycene Additive-Blended Poly(2,6-dimethyl-1,4-phenylene oxide)-Based Anion Exchange Membranes for Water Electrolyzers

Triptycene is a popular molecule with a bulky and rigid molecular structure and is three-dimensionally connected by three benzene rings. Owing to this, triptycene has been employed in complicated synthetic processes to form an inefficient polymer chain packing structure with high free volume. However, it is difficult to control the polymerization degree and increase its mechanical properties because of the three-dimensional contorted bulky structure of the triptycene moiety. In this study, we simply incorporated the triptycene molecule without a complex polymerization process. We fabricated brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and chloromethylated triptycene (triptycene-Cl) to form uniformly blended membranes with various small triptycene molecule additive weight loadings. Based on the BPPO matrix with appropriate bromination degree, the ionic triptycene additive provides an additional quaternary ammonium group and free volume for efficient hydroxide ion transport. Additionally, the dimensional stability of the dual-quaternized BPPO/triptycene-Cl blended membrane was effectively controlled because of the increased free volume, despite the high water uptake. As a result, the dual-quaternized BPPO/triptycene-Cl blended membrane with high triptycene-Cl weight loading shows a significantly enhanced hydroxide ion conductivity and reduced activation energy estimated within the water electrolyzer operation temperature range. The dual-quaternized BPPO/triptycene-Cl blended membrane with 20% triptycene-Cl exhibits a significantly lower and more stable area-specific resistance as well as higher water electrolyzer cell performance when compared to those of the commercial FAA-3-50 membrane.