Elastic Long-Chain Multication Cross-Linked Anion Exchange Membranes

Anion exchange membranes (AEMs) are a promising class of materials that enable non-noble metals to be used as catalysts in fuel cells. Compared to their acidic counterparts, typically Nafion and other perfluorosulfonate-based membranes, the low OH^- conductivity in AEMs remains a concern as these materials are developed for practical applications. Cross-linked macromolecular structures are a popular way to optimize the trade-off between the ionic conductivity and the water swelling of AEMs with high ion exchange capacities (IECs). However, common cross-linked AEMs (e.g., x(QH)QPPO) that have high degrees of cross-linking with low molecular weight between cross-links are usually mechanically brittle. Moreover, the cross-links in AEMs can hinder the transport of OH^-, leading to unsatisfactory conductivities. Here we report a series of elastic and highly conductive poly(2,6-dimethylphenylene oxide) (PPO)-based AEMs (x(QH)₃QPPO) containing flexible, long-chain, multication cross-links. The strength and flexibility of the x(QH)₃QPPO samples are significantly improved as compared to the conventional x(QH)QPPO membranes and multication un-cross-linked materials reported previously. The high conductivities in these new materials (x(QH)₃QPPO-40, IEC = 3.59 mmol/g, σ_OH^- = 110.2 mS/cm at 80 °C) are attributed to the distinct microphase separation observed in the x(QH)₃QPPO membranes by SAXS and TEM analyses. Furthermore, the x(QH)₃QPPO samples exhibit good dimensional (swelling ratio of x(QH)₃QPPO-40 is 25.0% at 80 °C) and chemical (22% and 25% decrease in IEC and OH^- conductivity in 1 M NaOH at 80 °C for 30 days, respectively) stabilities, making this cross-linking motif suitable for potential membrane applications in fuel cells and other electrochemical devices.