UV-crosslinking of polystyrene anion exchange membranes by azidated macromolecular crosslinker for alkaline fuel cells

Abstract

Poly(2,6-dimethyl-phenyleneoxide)s (PPO)s with photo-crosslinkable azide groups were employed as macromolecular crosslinker for poly(vinylbenzyl chloride) (PVBC)-based anion exchange membranes (AEMs). The crosslinked AEMs were achieved by solution casting and UV- irradiation, subsequently quaternization with trimethylamine. Although the presence of azide groups in IR spectra in crosslinked AEMs suggested the incomplete reaction of azide groups using UV-irradiation, the gel fractions were more than 85% indicating the successful crosslinking between PVBC and PPO macromolecular crosslinker. The key properties of the crosslinked AEMs, such as mechanical property, ion exchange capacity, thermal stability, water uptake, ion conductivity, alkaline stability and initial H2/O2 fuel cell performance were investigated. Excellent mechanical properties were observed for all of the crosslinked AEMs. The as-obtained AEMs showed comparable λ-normalized hydroxide conductivity with previous reported crosslinked AEMs. The highest hydroxide conductivity of 14.8mS/cm at 20°C was achieved for the crosslinked AEMs with IEC value of 1.95meq./g in spite of its low water uptake (19.8wt%). Moreover, excellent alkaline stability was observed for crosslinked AEMs, 15% decrease in hydroxide conductivity after alkaline stability testing 1M NaOH at 80°C for 500h, while the conductivity of uncrosslinked membrane with similar IEC value decreased sharply with 58% of conductivity loss after only 120h. The improved alkaline stability of the crosslinked AEMs may be derived from its compact crosslinked architecture, protecting the cations from severe degradation. A peak power density of 11mW/cm2 was obtained at 16mA/cm2 when the crosslinked AEM was used as polymer electrolyte membrane in H2/O2 alkaline fuel cell at 60°C.