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

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 center dot and O center dot- (conjugate base of HO center dot). 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.