Alkaline‐Stable Anion‐Exchange Membranes with Barium [2.2.2]Cryptate Cations: The Importance of High Binding Constants

The stability of cationic functional groups is one of the key factors determining lifetime of alkaline anion-exchange membranes (AAEMs) and the AAEM-based electrochemical devices. Main-group metal and crown ether complexes are stable cations due to the absence of degradation pathways including nucleophilic substitution, Hofmann elimination, and cation redox. However, the binding strength, a key feature for AAEM applications, is overlooked in previous work. We herein propose the use of barium [2.2.2]cryptate ([Cryp-Ba]2+ ) as a new cationic functional group for AAEMs due to its extremely strong binding (109.5 M-1 in water at 25 °C). The [Cryp-Ba]2+ -AAEMs with polyolefin backbones remain stable after treatment in 15 M KOH at 60 °C for over 1500 hours. More importantly, these AAEMs are successfully applied in water electrolyzers, and an anolyte-feeding switch method is designed to further reveal the influence of binding constants.