Effect of Radical Scavenging Extender on the Chemical Stability of Anion Exchange Membrane Under Alkaline Conditions

Abstract

A radical-scavenging extender is investigated as a novel strategy for improving the chemical stability of anion exchange membranes (AEMs). Unlike typical alkyl chain extenders used in AEMs, the bulky piperidine derivative 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), which possesses a redox-active core structure, improved the chemical stability of AEMs by serving as both a radical scavenger and a chain extender. Reactive oxygen species (ROS), such as hydroxyl and superoxide radicals (∙OH and ∙O₂^–), which contribute to AEM degradation, were efficiently scavenged through the reversible TEMPO/TEMPO⁺ redox cycle. Simultaneously, the bulky TEMPO moiety also increased steric hindrance around the quaternary ammonium groups, thereby suppressing hydroxide-induced S_N2 degradation by hindering nucleophilic attack of hydroxide ions on the α-carbon adjacent to the ammonium group. As a result, the QPPO-35-TEMPO membrane exhibited much greater chemical stability than QPPO-25-TMA, QPPO-30-DMHA, and QPPO-35-DATMP, confirming the effectiveness of the radical-scavenging-extender concept. QPPO-35-TEMPO retained its ionic conductivity after 500 h in 1 M KOH at 80 °C and achieved a current density of 3.69 A cm^–2 at 2.0 V in alkaline water electrolysis and a peak power density of 0.49 W cm^–2 in fuel cells. When applied as an anode ionomer, QPPO-35-TEMPO further improved the electrode performance. This work establishes the radical-scavenging-extender concept as a promising molecular design strategy for developing chemically durable and high-performance AEMs for alkaline electrochemical energy technologies.