Novel Sulfonated Poly (Arylene Ether Ketone) Proton Exchange Membrane Materials For Direct Methanol Fuel Cells: Synthesis And Properties Studies

Author: Zhang YangAdvisor: prof. Na HuiMajor: Polymer Chemistry and PhysicalDirect methanol fuel cells (DMFCs) have attracted much attention as alternative sources of electricity due to their low emission and high conversion efficiency. As one of the key components of DMFCs, proton exchange membrane (PEM) functions as an electrolyte for transferring protons as well as providing a barrier to the gases or fuel cross-leaks between the electrodes. Perfluorosulfonic acid membranes, such as Nafion, are the current state-of-the-art PEM materials, because of their excellent mechanical properties, chemical stability, and high proton conductivity. However, their high cost, low operating temperature (dehydration and loss of proton conductivity> 80℃) and high methanol permeability hinder their applications in fuel cells. Hence, great efforts have been devoted to the development of alternative PEMs.Sulfonated aromatic polymers have been studied as the promising candidates for PEMs. Among of them, sulfonated poly(arylene ether ketone)s (SPAEKs) have attracted much attention due to their good mechanical properties, excellent proton conductivity as well as high methanol resistance. Generally, conventional SPAEK polymers are unable to form defined hydrophilic domains, as the rigid polyaromatic backbone prevents continuous ionic clustering from occurring. As a result, in order to obtain high proton conductivity, these sulfonated polymers usually need high ion exchange capacity (IECs) values and water content. However, these condition will lead to high water uptake, poor dimensional stability and loss of mechanical properties of the membranes. There are several promising way to improve the properties of SPAEKs with high IEC values, such as acid-basic blend, cross-linking and improving nanophase separation of hydrophilic and hydrophobic domains. This work is aiming to synthesize novel SPAEK with excellent proton conductivity, good dimentional stability and low cost as proton exchange membrane materials for DMFCs.Firstly, we synthesized a series of novel sulfonated poly(arylene ether ketone)s bearing pendant carboxylic acid group (C-SPAEKs) via nucleophilic substitution reaction. The structures of all the C-SPAEKs had been characterized by FT-IR and'H NMR spectra. These C-SPAEKs showed good thermal stability and excellent mechanical properties. Meanwhile, the membrane properties for DMFCs such as water uptake, proton conductivity and methanol permeability had been investigated. The results exhibited the C-SPAEKs with high IEC values had excellent proton conductivity, but their poor dimentional stability and high methanol permeability prevented their applications as PEM materials on DMFCs.In chapter 4, in order to decrease the water uptake and improve the dimentional stability of the C-SPAEKs, we introduced benzimidazole groups into the system by the reaction between carboxylic acid group and 1,2-diaminobenzene. The structure of the sulfonated poly(arylene ether ketone)s bearing benzimidazole group (BI-SPAEKs) had been confirmed by'H NMR spectra. Compared to C-SPAEKs, BI-SPAEKs showed improved thermal stability and mechanical properties. Due to the interaction between sulfonic acid group and benzimidazole group, BI-SPAEKs exhibited much lower water uptake and methanol permeability than the C-SPAEKs with the same sulfonation degree (DS). However, the acid-basic interaction between sulfonic acid group and benzimidazole group decreased the amount of free sulfonic acid groups, resulting in the reduction of proton conductivity.In chapter 5, in order to not only improve the dimentional stability of the C-SPAEKs with high DS but also remain excellent proton conductivity, we modified the C-SPAEKs by self-cross-linking method. Cross-linked membranes (X-SPAEKs) were obtained by Friedel-Craft reaction at 160℃between carboxylic acid group and the nucleophilic phenyl rings. After cross-linking, the water uptake, swelling ratio and methanol permeability decreased substantially compared to the pristine membranes. For example, C-SPAEK-40 showed a water uptake of 169.3% and swelling ratio of 75.8% at 80℃, after cross-linking, its water uptake and swelling ratio decreased to 57.9% and 24.9%, respectively. These results indicated that cross-linking restricted the hydrophilic domains, thus decreased the water uptake and swelling ratio. On the other hand, the sulfonic acid groups had not been involved into the cross-linking reaction, meanwhile other cross-linkers are not introduced into the system, so that the cross-linked membrane can achieve reasonable proton conductivity without decreasing IEC value.In chapter 6, we synthesized hexafluoro-bisphenol-A novolac epoxy resin by a two-step reaction (HFANER). By the curing reaction between the carboxylic acid group in C-SPAEK-50 and epoxy group in HFANER, we prepared a series of cross-linked proton exchange membrane materials (Cr-SPAEKs). The cross-linked degree can be controlled by varing the content of cross-linker. The cross-linked network structure caused significant enhancement in the mechanical properties and oxidative stability. Meanwhile, water uptake, swelling ratio and methanol permeability substantially decreased with increasing the content of cross-linker. Although the proton conductivities of the cross-linked membranes were lower than that of the pristine membrane, the higher selectivity defined as the proton conductivity to methanol permeability was obtained for the cross-linked membranes.In chapter 7, we synthesized a series of novel SPAEK with sulfonic acid group on the flexible side chain. The bisphenol monomers (4-methoxy)phenylhydroquinone and (2,5-dimethoxy)phenylhydroquinone were prepared by a two-step reaction. The poly(arylene ether ketone)s containing different methoxy groups content were prepared by polycondensation. Then after demethylation reaction, poly(arylene ether ketone)s containing different hydroxyl groups content were obtained. Finally, the sulfobutyl groups were grafted onto the PAEKs by the nucleophilic ring opening reaction between the hydroxyl group and 1,4-butane sultone. The structure of all the polymers had been confirmed by'H NMR spectra. All the polymers were characterized by TGA and mechanical properties, meanwhile, the membrane properties for DMFCs such as water uptake, swelling ratio, proton conductivity as well as methanol permeability had been investigated. These side-chain-type SPAEK showed higher proton conductivity, lower water uptake and much more improved dimensional stability than the conventional main-chain-type SPAEK with similar IEC values.