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.

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Quaternary ammonium (QA) cations with high alkaline stability are crucial for the long term performance of anion-exchange membrane (AEM) fuel cells.

New durable and hydroxide ion conducting anion--exchange membranes (AEMs) are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In the present work we have attached quaternary piperidinium (QPi) groups to poly(2,6--dimethyl--1,4--phenylene oxide)s (PPOs) via flexible alkyl spacer chains with the aim to prepare AEMs. The bromine atoms of bromoalkylated PPOs were displaced in Menshutkin reactions to attach one or two QPi groups, respectively, via heptyl spacers. The cationic polymers have excellent solubility in, e.g., methanol, dimethylsulfoxide and N--methyl--2--pyrrolidone at room temperature, and form tough and transparent membranes. AEMs with bis--QPi side chains efficiently form ionic clusters and show very high hydroxide ion conductivities, up to 69 and 186 mS cm --1 at 20 and 80 °C, respectively. The AEMs further have excellent alkaline stability, and 1 H NMR analysis show no degradation of the AEMs after storage in 1 M NaOH at 90 °C during 8 days. Thermogravimetry indicate decomposition only above 225 °C. The AEM properties were further tuned by controlled formation of bis--QPi crosslinks through an efficient reaction between bromoalkylated PPO and 4,4′--trimethylenebis(1--methylpiperidine) during a reactive membrane casting process. In conclusion, alkali--stable and highly conductive AEMs for alkaline fuel cells can be prepared by placing cycloaliphatic quaternary ammonium cations on flexible side chains and crosslinks. 15This opens up for many synthetic possibilities to target new high--performance PPO--based AEMs.By employing standard procedures to introduce the cationic groups, e.g., chloromethylation and subsequent reaction with trimethylamine, benzyl trimethylammonium (BTA) groups are usually formed directly on aromatic polymer backbones. Unfortunately, quaternary ammonium (QA) groups in benzylic positions have in most cases proven to be quite sensitive towards nucleophilic attack by OH − , and also seem to activate the cleavage of adjacent ether links in aromatic polymer backbones. 16--18 QA groups with long alkyl chain segments are generally sensitive to Hofmann β--eliminations.Recently however, β--protons have been found to be far less

Poly(phenylene oxide)s carrying pendant polycationic alkyl side chains with a suitable configuration show high hydroxide ion conductivity and excellent alkaline stability.