Preparation And Properties Of Anion Exchange Membranes Based On Quarternary Ammonium Poly(Arylene Ether Sulfone)s For Fuel Cell Applications

Recently, anion exchange membrane fuel cells (AEMFCs) have drawn the researchers’ attention. Theoretically, the oxygen reduction reaction is more dynamically favourable and the over-potential can be largely reduced in basic conditions than in acid conditions, and some non-noble metal catalysts, such as silver and nickel can be used as the electrode catalysts, resulting in substantial cost down and higher efficiency. As one of the key components of the AEMFC, the anion exchange membrane (AEM) serves as the ion carrier, fuel and oxidizer separator as well as catalyst supporter, and the membrane properties are closely related to the performance of the cell system. In this dissertation, several types of novel AEMs derived from quaternized poly(arylene ether sulfone)s were succefully prepared and their fundamental properties were investigated.Firstly, a series of ductile and transparent AEMs based on random type quaternary ammonium poly(arylene ether sulfone) were successfully synthesized from bulky monomer of9,9’-bis(4-hydroxyphenyl) fluorene (BHPF), flexible monomer of4,4’-(hexafluoroisopropylidene)(HFBPA) diphenol and4,4’-difluorodiphenyl sulfone (DFDPS) via random polymerization, Friedel-Crafts chloromethylation, quaternization, alkalization and solution casting. Trimethylamine and imidazolium were selected as the quaternary reagents, and their influence on the performance of the obtained AEMs were studied. The results indicated that at similar ion exchange capacity (IEC), the imidazolium quaternary ammonium salt type random membranes (MIM-rQPAES) showed much smaller size changes, water uptake and conductivity than the trimethylamine quaternary ammonium salt type random membranes (rQPAES). For instance, the R5membrane with IEC of1.23mmol/g and rMIM-1membrane with IEC of1.24mmol/g exhibited water uptakes of39.7%and16.7%at60℃, respectively. The R6membrane with IEC of1.38mmol/g and rMIM-1membrane with IEC of1.34mmol/g exhibited hydroxide ionic conductivity of32.8mS/cm and13.2mS/cm at60℃, respectively. After immersing in4M NaOH solution for72h at30℃, the ion conductivity of rMIM-3(1.49mmol/g) and rMIM-5(1.69mmol/g) decreased about21.1%and24.0%, respectively, while it decreased41.5%for R5, indicating the much better chemical stability of the MIM-rQPAES membranes.Secondly, hydroxide-terminated oligomers with certain block length from BHPF and DFDPS and fluoride-terminated oligomers with certain block length from HFBPA and DFDPS were synthesized separately, and subsequently used to prepare block poly(arylene ether sulfone) copolymers (bPAES) through high temperature polymerization. A series of block trimethylamine quaternary ammonium salt type AEMs (bQPAES) based on the block copolymers were prepared through Friedel-Crafts chloromethylation, quaternization, alkalization and solution casting. The obtained bQPAES membranes are ductile and transparent. Furthermore, the water uptakes, size changes and ion conductivities of the obtained bQPAES membranes increased with the temperature, IEC and the hydrophilic block length. Comparing with the rQPAES membranes with similar chemical structure and IEC, bQPAES membranes showed lower affinity to water molecules, but better mechanical properties, dimensional stability and chemical stability, and comparable ion conductivities. For example, the M4membrane with IEC of1.25mmol/g exhibited size changes in the in-plane and through-plane directions of0.05and0.05, respectively, ion conductivity of15.6mS/cm in water at60℃, while the R4membrane with IEC of1.23mmol/g exhibited size changes in the in-plane and through-plane directions of0.12and0.14, respectively, hydroxide ionic conductivity of26.8mS/cm. After immersing in4M NaOH solution for72h at30℃, the ion conductivity of M5(1.31mmol/g) decreased about34.5%, while it decreased41.5%for R4(1.23mmol/g), indicating the much better chemical stability of the bQPAES membranes.Finally, imidazolium salt type bQPAES membranes (MIM-bQPAES) were prepared from chloromethylated bPAES copolymers, and the influence of introduction of imidazolium quaternary ammonium groups on the membranes’properties was studied and compared with bQPAES membranes using the same chloromethylated bQPAES precursors, the IECs of the MIM-bQPAES membranes (0.67-1.10mmol/g) were much lower than the bQPAES ones (1.13-1.31mmol/g). At similar IEC levels, MIM-bQPAES membranes showed slightly lower water uptakes, but comparable ion conductivities with the bQPAES ones. For example, the bMIM-5membrane with IEC of1.1Ommol/g and the M2membrane with IEC of1.13mmol/g exhibited water uptakes of13.5%and15.1%, ion conductivities of14.6mS-cm-and14.8mS/cm, size changes in the in-plane and through-plane directions of0.05and0.07,0.05and0.06, respectively, at60℃. After immersing in4M NaOH solution for144h at30℃, all membranes decreased less than25%of their original ion conductivities, indicating the much better chemical stability of the MIM-bQPAES membranes.