High-Performing Hydroxide Exchange Membranes with Flexible Tetra-Piperidinium Side Chains Linked by Alkyl Spacers

The objective of the present work is to, in a single material, combine a number of molecular design strategies that have proven successful in the preparation of high-performance anion-exchange membranes (AEMs) for alkaline fuel cells. Hence, we here report on highly conductive and alkali-stable poly(phenylene oxide)s carrying flexible side chains attached via alkyl spacer units, where each side chain contains four quaternary piperidinium (QPip) cations interconnected with alkyl chain segments. These materials are completely soluble in, e.g., methanol and form mechanically tough transparent AEMs with efficiently segregated ions, as indicated by X-ray scattering. At 80 °C, the hydroxide ion conductivity reaches up to 170 and 221 mS cm–1 at ion-exchange capacities (IECs) of 2.1 and 2.6 mequiv g–1, respectively. Taking into account the IEC value and water uptake, the tetra-QPip side chain AEMs are found to be significantly more efficient hydroxide ion conductors than corresponding AEMs with mono-QPip side chains. Both the IEC value and hydroxide conductivity of the AEMs show a minor decrease after storage in 1 M aq NaOH at 90 °C during 240 h. However, this decline is not associated with any ionic loss or polymer structure degradation, as confirmed by 1H NMR spectroscopy and thermogravimetrical analysis, even after 380 h storage under the same conditions.