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

Abstract 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 (10 9.5 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.