Toward alkaline-stable anion exchange membranes in fuel cells: cycloaliphatic quaternary ammonium-based anion conductors

Anion exchange membrane (AEM) stability has been a long-standing challenge that limited the widespread development and adoption of AEM fuel cells (AEMFCs). The past five years have been a period of exceptional progress in the development of several alkaline-stable AEMs with remarkable both ex situ and in situ AEMFC stability. Certain cycloaliphatic quaternary ammonium (cQA) (mainly five- and six-membered) based AEMs appear to be among those having the most promising overall performance. In this review, we categorize cQAs as cage-like (such as quaternized 1,4-diazabicyclo[2.2.2]octane, (QDABCO) and quinuclidinium), non-cage-like (such as pyrrolidinium and piperidinium) and N-spirocyclic (such as 6-azonia-spiro[5.5]undecane (ASU)). The degradation mechanisms of categorized cQAs are first elucidated. Through an understanding of how the cations are attacked by strongly nucleophilic OH–, improved structural design of incorporating alkaline-stable cations into AEMs is facilitated. Before a detailed description and comparison of the alkaline stability of cQAs and their respective AEMs, current protocols for the assessment of alkaline stability are discussed in detail. Furthermore, the initial AEMFC performance and fuel cell performance stability based on cQA AEMs are also examined. The main focus and highlight of this review are recent advances (2015–2020) of cQA-based AEMs, which exhibit both excellent cation and membrane alkaline stability. We aim to shed light on the development of alkaline-stable cQA-type AEMs, which are trending in the AEM community, and to provide insights into possible solutions for designing long-lived AEM materials.