Enhancement of the mechanical properties of anion exchange membranes with bulky imidazolium by “thiol-ene” crosslinking

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To improve the poor membrane-forming ability of polymer containing sterically-protected organic cations with the excellent alkaline stability, a series of robust and crosslinked poly(2,6-dimethyl-1,4-phenylene oxide)s (PPO) anion exchange membranes (AEMs) with bulky and chemically stable imidazolium cations was prepared by quaternization with C1,C3,C4-substituted imidazole and crosslinking reaction via “thiol-ene” chemistry. Compared to the uncrosslinked counterpart, the crosslinked AEMs showed excellent film-forming properties with the highest of tensile strength of 53.2 MPa, probably owing to the increased entanglement interactions in polymer chains that may in turn result in the controlled the water uptake and dimensional stability of the AEMs. Moreover, by using two different crosslinkers, water uptake of the resulting AEMs could be effectively tuned. The crosslinked AEM had a water uptake of 12.8 wt% with the ion exchange capacity (IEC) of 1.27 meq./g when using a hydrophilic dithiol as the crosslinker, which was almost twice than that for the membrane with hydrophobic crosslinker AEM (water uptake was 6.6 wt%, IEC = 1.28 meq./g). The higher water uptake further contributed to high ionic transport with bromide conductivity of 11.2 mS/cm at 20 °C in water and 24.8 mS/cm at 80 °C. The excellent alkaline stability of these materials was demonstrated by the fact that, more than 90.4% of bromide conductivity and 60.5% of tensile strength was retained after 1000 h immersion in 1 M NaOH at 80 °C, due to the presence of crosslinked networks and alkali-resistant sterically-protected imidazolium. Furthermore, we compared the AEM fuel cell performance and durability of the membrane electrode assembly (MEA) using the crosslinked and uncrosslinked bulky imidazolium-functionalized PPO copolymers as ionomers in catalyst layers. A higher peak power density of 200 mW/cm2 at a current density of 475 mA/cm2 and improved short-term durability (20.5 h) under a constant current density of 200 mA/cm2 at 60 °C were obtained for the membrane electrode assembly with the crosslinked ionomer. These results suggest that crosslinking of anion conductive materials with stable sterically-protected organic cations is a facile and effective strategy to prepare the robust and alkaline stable AEMs for high-performance alkaline fuel cells.