Quaternized poly(2,6-dimethyl-1,4-phenylene oxide) anion exchange membranes with pendant sterically-protected imidazoliums for alkaline fuel cells

Abstract

Quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) copolymers featuring pendant sterically-protected imidazolium groups are presented as new anion exchange membranes (AEMs) for alkaline fuel cell application. Four kinds of imidazoliums, in which the substitutions were located at different positions in imidazolium rings, were grafted on the PPO backbones for systematically assessing structure-property relationship in the resulting imidazolium-based AEMs. Grafting imidazoliums with less substitutions leads to high water uptake as well as sufficient ionic conductivity. The 1,2,4,5-tetramethylimidazolium-functionalized PPO AEM (PPO-TMIm) showed the higher water uptake (53.2 wt%) and hydroxide conductivity (31.7 mS/cm) at room temperature in comparison to the AEM (PPO-TPIm) having sterically-protected 2-(2,4,6-trimethyl)phenyl-4,5-diphenyl-1-methyl- imidazolium. With increasing steric hindrance in the positions of imidazolium rings, PPO-TPIm AEM exhibited superior alkaline stability. After 192 h of immersion in 1 M NaOH at 80 °C, PPO-TPIm membrane retained 86.7% of the ionic conductivity with no obvious structure change as evidenced by 1H NMR spectroscopy, while dealkylation degradation was observed for AEMs having 1,2,4,5-tetramethylimidazolium and 2-(2,6-dimethyl)phenyl-1-methyl-benzimidazoliums with 16.93% and 19.76% retention of conductivity. Furthermore, these imidazolium-based PPO copolymers were utilized as both polymer electrolyte membranes and ionomers in the membrane electrode assemble for alkaline fuel cell application. A single H2/O2 fuel cell testing showed that high peak power density of 128 mA/cm2 at 60 °C was obtained for PPO-TMIm copolymer as an AEM, probably due to its high ion conductivity and comparable alkaline stability. However, under the same conditions, PPO-TPIm copolymer with the highest alkaline stability failed to be a separator in cells, and only 22.1 mW/cm2 of peak power density was achieved as an ionomer in fuel cells. These results highlight that both ionic conductivity and alkaline stability of anion conductive polymers are important for fuel cell application as membranes and ionomers.