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 are evaluated for their susceptibility against degradation by radicals. Time-resolved absorption spectroscopy indicates that radicals with oxidizing and reducing character are formed upon oxidation by HO • and O •− (conjugate base of HO • ). We estimate that, dependent on the QA, 18-41% of the formed radicals are oxidizing with standard electrode potentials ( E 0 ) above 0.276 V and 13-23% exceed 0.68 V, while 13-48% are 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 promote radical degradation, while electron withdrawing ones increase 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.