On the Radical‐Induced Degradation of Quaternary Ammonium Cations for Anion‐Exchange Membrane Fuel Cells and Electrolyzers

Abstract Four benzylic‐type quaternary ammonium (QA) compounds with different electron density at the phenyl group were evaluated for their susceptibility against degradation by radicals. Time‐resolved absorption spectroscopy indicated that radicals with oxidizing and reducing character were formed upon oxidation by HO⋅ and O⋅ − (conjugate base of HO⋅). It was estimated that, dependent on the QA, 18–41 % of the formed radicals were oxidizing with standard electrode potentials ( E 0 ) above 0.276 V and 13–23 % exceeded 0.68 V, while 13–48 % were reducing with E 0 <−0.448 V. The stability of these model compounds against oxidation and reductive dealkylation was evaluated at both neutral and strongly alkaline conditions, pH 14. Under both conditions, electron‐donating groups promoted radical degradation, while electron‐withdrawing ones increased stability. Therefore, durability against radical‐induced degradation shows an opposite trend to alkaline stability and needs to be considered during the rational design of novel anion‐exchange membranes for fuel cells and electrolyzers.