Anion-exchange membranes derived from main group and metal-based cations

The design and synthesis of anion-exchange membranes (AEMs) has attracted a great deal of attention given the potential utility of these materials for ion transport in electrochemical energy systems. AEMs are polymer electrolytes with positively charged groups covalently bound to a polymer backbone that can conduct either hydroxide or other anions. The vast majority of AEMs are based on ammonium groups (+NR4) but questions surrounding the long-term stability of these functionalities to hydroxide has led to examination of other cation alternatives. In this review, we highlight some of the recent efforts towards AEMs with cations derived from inorganic elements, with a specific emphasis on phosphonium, sulfonium, and metal-based cations. These studies demonstrate the potential of inorganic building blocks for construction of novel AEMs, with unique properties relative to their ammonium analogues.