To assess the chemical stability of tetrazoles as the hydrophilic functionality for use in alkaline ion-solvating membranes, 5-phenyl tetrazole was dissolved in 25 wt% aqueous KOH and kept at 85 °C. The 1 H NMR evolution over the test period of around 3700 h is shown in Figure , determined using an internal standard, and no significant change in concentration could be observed during the tests, as shown in Figure .