Olefin metathesis-crosslinked, bulky imidazolium-based anion exchange membranes with excellent base stability and mechanical properties

Abstract

To improve the mechanical properties of anion exchange membranes (AEMs) with bulky imidazolium cations, a series of anion-conductive poly(2,6-dimethyl-phenylene oxide)s (PPO)s with crosslinkable terminal double bonds were synthesized by the Menshutkin reaction. Following crosslinking via olefin metathesis at room temperature catalyzed by a Grubb's second generation catalyst, tough, transparent, and flexible PPO-based AEMs were obtained. The crosslinked AEMs exhibited good mechanical properties (tensile strength at maximum load of 20.8–49.9 MPa and values of elongation at break of 1.5–3.0%) though their bulky imidazolium cation groups which could destroy the film-forming ability of polymer. And the obtained mechanical properties were considerably better than those of the non-crosslinked AEMs with a similar architecture, which broke into small pieces during the process of membrane fabrication using solvent casting. Furthermore, the crosslinked AEMs exhibited an extremely low water uptake (up to 13.9 wt% at 80 °C) and minimal swelling (<7% at 80 °C), attributed to the high-density crosslinking network. A high bromide conductivity (22.9 mS/cm at 80 °C) was achieved despite the low water uptake for the crosslinked AEM. Moreover, long-term alkaline stability testing in 1 M NaOH at 80 °C, no obvious degradation of the imidazolium ring was observed, with the conductivity of aged crosslinked membranes remaining at ~100% after 960 h. It was assumed that the bulky substituents at the C2, C4, and C5 positions of imidazolium cations and the crosslinked architecture prevented H2O and/or OH− from attacking the cationic center. Moreover, the AEM fuel cell performance and durability was further explored for the membrane electrode assembly (MEA) with crosslinked and uncrosslinked bulky imidazolium-based ionomers in catalyst layers. An optimistic cell performance (a peak power density of 173 mW/cm2 at 410 mA/cm2) and improved short-term durability at a constant current density of 200 mA/cm2 were obtained for the crosslinked ionomers, and its lifetime was ca. 3 times longer than that of benzyltrimethylammonium functionalized PPO ionomer.