Alkali stability of anion exchange membrane

Abstract

Anion exchange membrane fuel cells (AEMFCs) are attractive alternatives to proton exchange membrane fuel cells due to using non-noble metal catalysts and faster cathode reaction kinetics. Anion exchange membranes (AEMs) are one of the key materials composed of AEMFCs. The hydroxide conductivity of AEMs has been able to meet operating requirements of fuel cells. Although the hydroxide conductivity is no longer a problem, the stability of AEMs is still a notable challenge. This work mainly introduces the development of alkaline stability of AEMs. AEMs mainly consist of functional groups, side chains and polymer backbones. The degradation of functional groups causes by attacking of hydroxide ion through Hofmann elimination, nucleophilic substitution and ylide-intermediate degradation pathways. Numerous functional groups have been developed in order to improve the alkaline stability of AEMs. The introductions of appropriate electron donors and steric shielding are effective ways to weaken the attack of hydroxide ion. Developing new functional groups is a very important way. In addition, the development of novel stable groups is an important way for long-lived AEMs. For example, hetero-cycloaliphatic quaternary ammonium cations (QAs) are proved to be especially stable in both model compound studies and even AEM studies. The most commonly studied AEMs were synthesized by chloromethylation/bromination of the aromatic backbones, followed by a Menshutkin reaction to introduce QAs. Benzylic groups, electron-withdrawing groups, are prone to degrade by nucleophilic substitution on the benzylic carbon/α-carbon, Hofmann elimination and N-ylide intermediate formation in the presence of hydroxide ion. Therefore, the tethering linkage between the polymer backbone and functional groups is quite important. Recent studies have mainly introduced long side chains between functional groups and polymer backbones, leaving functional groups away from the benzene ring of the polymer backbone. The all-alkyl, amine- and ether-containing branched-chain structures are proved to be stable. Poly(arylene ether)s have been extensively studied in AEMs due to the advantages of good thermal stability, high mechanical property and easy modification. Frequently employed polyaromatic electrolytes are quaternized poly(aryl ether sulfone)s, which are prepared via the nucleophilic polycondensation reaction. The introduction of functional groups leads to degrade of these main chains containing aryl ether bonds, which are unable to avoid in the polycondensation reaction. It is necessary that main chains are devoid of aryl ether bonds so as to develop AEMs with excellent alkaline stability. Diels-Alder reaction, coupling reaction and acid-catalyzed Friedel-Craft polycondensation are effective methods for synthesizing aromatic backbones devoid of aryl ether bonds. In addition, aliphatic backbones, such as poly(ethylene-co-tetrafluoroethylene), are proved to have good stability in alkaline conditions, so its modification for AEMs has also become an important research direction. At present, the alkaline stability of AEMs has made great progress. Three major aspects have been studied in an effort to develop stable AEMs: (1) screening stable functional groups such as five- or six-membered cyclic amines, substituted imidazoles; (2) developing effective linkages between functional groups and polymer backbones; (3) designing aryl ether-free aromatic backbones and stable aliphatic backbones. The designed AEMs can remain for hundreds of hours without degradation in alkaline solution, which is a significant advancement in increasing alkaline stability of AEMs.