Poly(phenylene oxide)s functionalized with cationic side chains: design, synthesis and properties as anion exchange membranes

Abstract: Anion exchange membrane fuel cells (AEMFCs) are considered as one of the most important next-generation energy conversion technologies. AEMFCs convert chemical energy into useful electrical energy and have great potential forachieving high conversion efficiencies. In addition, the faster oxygen reduction kinetics under alkaline conditions are advantageous for the use of non-noble metal catalysts (e.g., nickel). This is a great benefit for the large-scalecommercialization of AEMFCs.The anion exchange membrane (AEM), which is the core component of the AEMFC system, greatly affect the device’s performance, cost and durability. The most desired properties of an AEM include high hydroxide ion conductivity, excellent long-term alkaline stability, and low cost. In general, AEMs can be tailored by the appropriate selection of polymer backbone, polymer architecture and cation. Throughout the present work, a commercial poly(phenylene oxide) (PPO) backbone was selected for the preparation of AEMs because of its excellent performances and properties including high alkaline stability, and excellent mechanical and physical properties. PPO was extensively functionalized with tetraalkylammonium cations (R4N+, e.g., trimethylalkyl ammonium cations) due to their low toxicity, relatively low cost, abundance and straightforward synthesis. Based on a spacer concept, quaternary (QA) cations were attached to the PPO backbone via flexible alkyl spacer units using lithiation chemistry. This molecular design was expected to simultaneously enhance the alkaline stability (by avoiding the presence ofreactive benzyl trimethylammonium cations in the AEM) and the hydroxide ion conductivity (by improving the local mobility of QA cations and facilitating a distinct phase separation).To further enhance the properties of AEMs, side chains containing multiple QA cations were covalently attached to the PPO backbone via sequential Menshutkin reactions involving α,ω-diamines and α,ω-dibromoalkanes. This design was found to simultaneously enhance the hydroxide ion conductivity (by further facilitating the formation of ionic clusters and increasing the local concentration of QA cations) and the alkaline stability (by decreasing the degree of modification of the PPO backbone). In addition, an extensive study on the effects of various heterocycloaliphatic QA cations on AEM properties was performed in order to identify the most suitable cations.Furthermore, a cross-linking modification involving 4,4′-trimethylenebis(1-methylpiperidine) was utilized to enhance the mechanical properties and to reduce the water uptake of the AEMs. Using these different approaches, thesynthesized and studied AEMs were found to combine many crucial properties including high hydroxide ion conductivity, high alkaline stability and excellent thermal properties. The results suggest that optimized AEMs based on the spacer concept are promising candidates for the development of high-performance AEMFCs.