Poly(dibenzofuran-p-terphenyl piperidinium)-based anion exchange membranes with enhanced phase separation for water electrolysis

Anion exchange membranes (AEMs) based on non-aryl ether–type poly(aryl piperidinium) have recently drawn significant attention because of their excellent ion exchange capacity (IEC), chemical stability, and ionic conductivity. However, the reactivity of poly(aryl piperidinium) varies significantly depending on the type of monomer used and is also accompanied by very rapid polymerization. These limitations have hindered the development of an AEM with controllable morphology. In the present study, AEMs with the desired morphology were obtained using hydrophilic and hydrophobic monomers; indeed, a well-controlled morphology was achieved even though they were random copolymers. Specifically, hydrophilic dibenzofuran and hydrophobic p-terphenyl were used as raw materials to develop the novel AEM material p(BF-TP)-Pip-x, i.e., a poly(dibenzofuran-p-terphenyl-piperidinium)-based copolymer. The dibenzofuran content relative to p-terphenyl was 5%, 10%, and 30%, resulting in three types of p(BF-TP)-Pip-x-based membranes. Their properties were compared with a poly(p-terphenyl-piperidinium)(pTP-Pip)-based AEM, a homopolymer composed solely of p-terphenyl. p(BF-TP)-Pip-10, a membrane with a dibenzofuran content of 10%, exhibited excellent mechanical properties with a tensile strength of 42 MPa and an elongation of break of 9.7%, along with high ionic conductivity at 144 mS cm−1 at 80 °C, owing to its well-developed, phase-separated morphology. Furthermore, its conductivity retention was 93% after treatment in a 2 M KOH solution at 80 °C for 960 h. For a given IEC, excellent alkaline stability was achieved compared to existing poly(aryl piperidinium)-based AEMs. p(BF-TP)-Pip-10 also exhibited excellent thermal and oxidative stability. In AEM water electrolysis tests, the membrane showed a very high current density of 1309 mA cm−2 at 1.8 V, a 120% improvement over pTP-Pip.