Structure–Stability Correlations on Quaternary Ammonium Cations as Model Monomers for Anion-Exchange Membranes and Ionomers

Understanding the alkaline stability of quaternary ammonium (QA) cations tethered to polymer backbones in anion-exchange membranes (AEMs) is crucial to advance the long-term performance of anion-exchange polyelectrolyte-based fuel cells and electrolyzers. A library of model QA cations with <i>N</i>-phenyl and <i>N</i>-benzyl tethers has been synthesized, and comparative alkaline degradation studies revealed that the former are much less stable toward hydroxide attack than their benzylic counterparts. Density functional theory (DFT) studies support the relative stability of the QA cations and demonstrate the critical effect of hydroxide solvation on alkaline stability as well as the degradation pathway. The 3-benzyl-3,6-diazaspiro[5.5]-undecane-6-ium (<i>N</i>-benzyl-ASU, <b>8</b>) cation was found to be the most stable QA group, with a half-life of 2,595 h at 80 °C and 14,363 h at 60 °C in 3 M NaOD at a hydration number of 4.8, despite its <i>N</i>-phenyl-ASU counterpart (<b>5</b>) having a higher energy lowest unoccupied molecular orbital (LUMO); this suggests that the LUMO energy alone may not be an accurate indicator of alkaline stability. This study highlights the importance of considering the method of tethering the QA group to the polymer backbone and controlling the level of hydroxide hydration when developing QA cations for use in AEM-based devices. The structure-stability correlations arising from this work will inform the design of heteroatom donor-containing QA-based head groups with improved stability profiles.