Preparation and properties of anion exchange membranes based on spirocyclic quaternary ammonium salts

Abstract

<p indent=0mm>Among various types of fuel cells, alkaline anion exchange membrane fuel cells (AEMFCs) have attracted enormous attention as clean and highly efficient energy conversion devices for vehicles and portable electronic applications. As the key component of AEMFCs, anion exchange membranes (AEMs) act both as a barrier to separate the fuel and an electrolyte to transport hydroxide anion. To fulfill the application of AEMFCs, an AEM should possess good thermal stability, good mechanical properties, high conductivity and excellent alkaline stability. The lack of commercially available AEMs with excellent alkaline stability is limited the application of AEMs in the AEMFCs. In the recent years, a variety of AEMs based on poly(sulfone)s, poly(styrene)s, polypropylene, poly(arylene ether)s, poly(phenylene)s, poly(phenylene oxide)s and poly(olefin)s, have been synthesized for the application of AEMs. Most of these AEMs showed a similar chemical structure that a polymer backbone with with pendent quaternary ammonium cations. However, quaternary ammonium cations are unstable in alkaline environment due to the Hofmann degradation, the nucleophilic substitution and the ylide reaction, even in high temperature. Therefore, various types of AEMs based on some potential alternative cations, including imidazolium, guanidinium, phosphonium, metal-cation, pyridinium, tertiary sulfonium, and pyrrolidinium cations have been prepared and investigated in the last few years. Recently, different quaternary ammonium small molecules were synthesized and their alkaline stability was thoroughly investigated by Marino and Kreuer. Their study showed that spirocyclic quaternary ammonium have good stability in alkaline condition. To date, there is few work on spirocyclic quaternary ammonium-based AEMs, and to understand the properties of this type of AEMs, a systematic study is needed. In the present work, <italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide [DAPy][Br] was synthesized and used as hydrophilic phase in the polymeric membranes. The purity and chemical structure of [DAPy][Br] were confirmed by ¹H NMR measurements. However, poly(<italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide), the homopolymer of [DAPy][Br], shows poor film forming properties, and it is very soluble in water. Therefore, styrene and acrylonitrile were chosed to synthesize the copolymers with [DAPy][Br] due to the ease of processing, good mechanical properties and chemical resistance. A mixture of styrene/acrylonitrile, [DAPy][Br], divinylbenzene and benzoin ethyl ether was stirred to obtain a homogeneous solution. Then, the mixture was casted onto a glass mold irradiation with UV light. The prepared spirocyclic quaternary ammonium-based membranes were immersed in <sc>1 mol/L</sc> KOH solution at 60°C for <sc>24 h</sc> to convert the anion of membrane from Br^- to OH^-. Then the membranes was immersed in deionized water and washed with deionized water until the pH of residual water was neutral. In summary, a series of spirocyclic quaternary ammonium-based AEMs were successfully designed and prepared via a simple synthetic strategy. The resultant AEMs demonstrated great potential for alkaline anion exchange membrane fuel cell applications based on its good thermal stability, sufficient mechanical properties and high conductivity. The ion exchange capacity can be controlled by change the mixture ratio, and the swelling ratio, water uptake and ion exchange capacity of the spirocyclic quaternary ammonium-based membranes increase with the increasement of the content of [DAPy][Br] in the membranes. The transparent and mechanically robust spirocyclic quaternary ammonium-based AEMs show high conductivity up to <sc>7.29×10^-2 S/cm</sc> at 80°C, and all the membranes showed conductivity up to <sc>1×10^-2 S/cm</sc> at room temperature. In addition, all the AEMs synthesize in this work showed excellent long-term alkaline stability at elevated temperature. The spirocyclic quaternary ammonium-based AEMs with an aliphatic backbone exhibit high alkaline stability, which will open up new prospects for the preparation of AEMs with excellent alkaline stability and high conductivity.