Crosslinked PPO-based anion exchange membranes: The effect of crystallinity versus hydrophilicity by oxygen-containing crosslinker chain length

Crosslinked poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) using hydrophilic crosslinkers that contain ethylene oxide (EO) were prepared as novel anion exchange membranes (AEMs), and the effect of crosslinker EO length was systematically investigated. The physicochemical, morphological and electrochemical properties of the corresponding AEMs were also compared against the membrane prepared from a typical hydrophobic alkyl-type crosslinker. The ethylene oxide crosslinker helped to form ion clusters and to enhance the water-holding capacity of the corresponding AEMs, which promoted high conductivity both in water and in 95% room humidity conditions, whilst maintaining high physicochemical stability. However, it was also found that the presence of a long ethylene oxide as a crosslinker may induce polymer crystallinity, which reduces both conductivity and the alkaline stability of the corresponding crosslinked membranes. The effect of ethylene oxide chain length on the morphology, electrochemical and physicochemical properties, were also investigated. The xBEO-PPO membrane, having bis(ethylene oxide) (BEO) as a crosslinker, showed the highest conductivity of 131.96 mS/cm in water at 80 °C, and 55.21 mS/cm in 95% RH at 80 °C; this was owing to its crystalline nature. The single-cell performance of 444 mW/cm2 peak power density, was also obtained from the xBEO-PPO membrane.