Alkaline polymer electrolytes containing pendant dimethylimidazolium groups for alkaline membrane fuel cells

Abstract

Novel anion exchange membranes (AEMs), based on poly(phenylene oxide) (PPO) chains linked to pendant 1,2-dimethylimidazolium (DIm) functional groups, have been prepared for evaluation in alkaline polymer electrolyte membrane fuel cells (APEFCs). Successful functionalisation of the PPO chains was confirmed using ¹H-NMR and FT-IR spectroscopies. The ionic conductivities of the resulting DIm–PPO AEMs at 30 °C are in the ranges of 10–40 mS cm⁻¹ and 18–75 mS cm⁻¹ at 60 °C. The high ionic conductivities are attributed to the highly developed microstructures of the membranes, which feature well-defined and interconnected ionic channels (confirmed by atomic force microscopy, AFM, measurements). Promisingly, the ion-exchange capacities (IECs) of the DIm–PPO AEM are maintained after immersion in an aqueous KOH solution (2 mol dm⁻³) for 219 h at 25 °C; a previously developed monomethyl imidazolium PPO analogue AEM (Im–PPO) showed a significant decline in IEC on similar treatment. This reduction in undesirable attack by the OH⁻ conducting anions is ascribed to an increase in steric interference and removal of the acidic C2 proton [in the monomethyl Im-groups] by the methyl group in the DIm cationic ring. Moreover, the maximum power densities produced in simple beginning-of-life single cell H₂/O₂ fuel cell tests increased from 30 mW cm⁻² to 56 mW cm⁻² when switching from the Im–PPO AEM (fuel cell temperature = 50 °C) to the DIm–PPO-0.54 AEM (fuel cell temperature = 35 °C) respectively (even with the use of lower temperatures).