Exploring Different Cationic Alkyl Side Chain Designs for Enhanced Alkaline Stability and Hydroxide Ion Conductivity of Anion-Exchange Membranes

Abstract

In order to systematically improve the performance of anion-exchange membranes (AEMs) for alkaline fuel cells, a series of poly(phenylene oxide)s (PPOs) was tethered with cationic alkyl side chains of different lengths and configurations. PPO was first functionalized with bromomethyl and longer bromoalkyl side chains, respectively, before introducing quaternary ammonium (QA) groups via Menshutkin reactions involving trimethylamine and dimethyloctylamine, respectively. This resulted in samples with QA groups attached to PPO either directly in benzylic positions, or via flexible pentyl and heptyl spacer units, respectively. In addition, the polymers were configured with or without octyl extender chains pendant to the QA groups. All the cationic PPOs had an excellent solubility in, e.g., methanol and dimethyl sulfoxide, and flexible and mechanically robust AEMs with an ion exchange capacity of ∼1.4 mequiv g –1 were cast from solution. Analysis by small-angle X-ray scattering showed that the flexible spacer units greatly facilitated efficient ionic phase separation, regardless of the presence of the extender chain. These AEMs reach very high OH – conductivities, exceeding 0.1 S cm –1 at 80 °C. A clear optimum conductivity was observed for the AEMs with pentyl spacers. Despite a markedly lower water uptake, AEMs configured with additional extender chains still reached a high conductivity, 0.07 S cm –1 at 80 °C. Importantly, the spacer units induced a high alkaline stability and no degradation of these AEMs was detected after storage in 1 M NaOH at 80 °C during 8 days. In comparison, the benchmark materials with QA groups placed in conventional benzylic positions severely degraded under the same conditions. The findings demonstrated that AEMs suitable for fuel cell applications can be achieved by tuning the configuration of flexible cationic alkyl side chains to reach an excellent combination of ionic phase separation, chemical stability, water uptake, and OH – conductivity.