Alkaline Stability of Pure Aliphatic-based Anion Exchange Membranes Containing Cycloaliphatic Quaternary Ammonium Cations

The alkaline stability of functional cations tethered to anion exchange membranes (AEMs) is essential for long-term operation in electrochemical devices. Here, we report the use of AEMs with pure aliphatic polymer backbones containing N-spirocyclic quaternary ammonium cation groups as highly conductive, crosslinked, and reinforced separators. Synthesizing these pure aliphatic-based AEMs via irradiation with ultraviolet (UV) light at room temperature avoids the use of carcinogenic solvents and the solution casting step generally used in AEM preparation. The resultant aliphatic-based AEMs have a chloride ion conductivity of 82 mS cm⁻¹ at 70 °C with an ion exchange capacity (IEC) of 3.0 ± 0.2 mmol g⁻¹. The aliphatic-based AEM retains 40% of its initial IEC after immersion in 1 M KOH at 80 °C for 30 d. A direct nucleophilic substitution degradation mechanism is proposed for such AEMs, based on FT-IR, solid state ¹³C-NMR spectroscopy, and XPS.