Multication Anion Exchange Membranes with Robust Chemical Stability and High Conductivity: Effect of the Equatorial Position on Membrane Alkaline Stability

The main issues with anion exchange membranes (AEMs) are their low ionic conductivity and their inability to retain ion conductivity. To address these problems, we have developed a series of multication AEMs with varying ratios of dimethylamine, pyrrolidine, or methyl cyclohexylamine (20, 30, 40, 50, and 60%). The most significant innovation presented in this study is the synthesis of a multication AEM modified with cyclohexyl ammonium (QPE-MCy/OH–). This modification involves placing the nitrogen atom in the equatorial position of the cyclohexane ring to prevent 1,3-diaxial repulsion. As a result, there are no hydrogens at the proper angle for Hoffman's elimination. Consequently, the QPE-MCy-40/OH- membrane exhibits stability due to the inability of the quaternary ammonium salt group to nucleophilically attack the cyclohexane ring (due to the large size of the ring). The OH– conductivity of QPE-MCy-40/OH– reaches 103 mS cm–1 at 80 °C in a 2 M NaOH solution. The alkaline chemical stability of the AEMs reveals that membranes containing cyclohexyl groups (QPE-MCy-40) exhibit exceptional alkaline stability with 95% retention of ionic conductivity in a 2 M NaOH solution at 80 °C after 480 h. When applied in a direct ethanol fuel cell (DEFC), QPE-MCy-40 membranes demonstrate superior single-cell performance, with a peak power density of 110 mW cm–2 at 80 °C.