Proton-conducting Sulfonated Poly(Aryl Ether Ketone) Membranes: Synthesis And Properties Studies

Proton exchange membrane fuel cells (PEMFCs) are receiving tremendous attention as transport, stationary, and portable future power sources because of their lower missions and high energy conversion efficiency. The proton exchange membrane (PEM), which serves as a separator of fuels between the anode and the cathode as well as allowing protons to pass through the membranes to complete the entire reaction, is the key component of the PEMFCs. Perfluorinated copolymers, such as Dupont Nafion(?), are the current state-of-the-art PEM materials with features of high conductivity and excellent chemical stability atmoderate temperatures (<90℃). However, high methanol crossover indirect methanol fuel cells (DMFCs) and environmental inadaptability limited their large-scale commercial applications. Therefore, it is significant to develop new PEM materials as alternatives with considerable performances.SPAEKs are considered as promising alternative materials because of their advantages of low cost, easy preparation, good mechanical strength and high chemical and thermal stabilities. Like other sulfonated aromatic copolymers, SPAEKs require a high sulfonation level to achieve sufficient proton conductivity. Unfortunately, too high loading of acidic groups may lead to undesirable large swelling and a dramatic loss of the mechanical strength. In order to solve these problems and improve selected proton exchange membranes properties, several methods were proposed in this dissertation.Firstly, we investigated novel sulfonated poly (arylene ether ketone) copolymers containing carboxylic acid groups (SPAEK-C). The carboxylic group in the SPAEK side chain is an active pendant group that could react with many fuctional groups for cross-linking, which is often otherwise achieved through the sulfonic acid group. Fourier transform infrared spectroscopy and 1H NMR measurements are used to characterize and confirm the structures of these copolymers. All the SPAEK-C were characterized by TGA and mechanical properties, meanwhile, the membrane properties for fuel cells application such as water uptake, proton conductivity and methanol transport had been studied. The result showed that the sulfonated poly (arylene ether ketone) copolymers containing carboxylic acid groups with proper sulfonated degree possessed excellent dimensional stability, high proton conductivity and low methanol permeability, and with the degree of sulfonated groups increasing, the proton conductivity is increasing while the dimensional stability is decreased. Then the carboxylic acid groups attached to the SPAEK-C are transformed to benzimidazole units through condensation reactions and we compared the proton conductivity, water uptake, dimensional stability and methanol permeability between the SPAEK-C and the poly (arylene ether ketone) copolymers containing bezimidazole groups.In chapter 4, we grafted a silane monomer, namely 3-glycidoxypropyltrimethoxysilane (KH-560), to the SPAEK-C, and hydrolysis-condensation is then performed on the grafted membranes to make them cross-link to prepare a cross-linked hybrid proton exchange membrane.1H NMR measurements and Fourier transform infrared spectroscopy are used to characterize and confirm the structures of SPAEK-Cs and hybrid polymer electrolyte membranes, respectively. The Si-O-Si cross-linking structure enhances the stability of the PEM greatly. The water uptake, water swelling and methanol permeability were drastically reduced as compared to the pristine SPAEK-C membranes. The water uptake and swelling, methanol permeability, mechanical properties, proton conductivity are investigated in detail.In chapter 5, since layer-by-layer (LBL) self-assembly is a simple and elegant method of constructing organic-inorganic composite thin films from environmentally benign aqueous solutions, we utilize this method to develop proton-exchange membranes for fuel cells. The methanol crossover current density can be significantly decreased with the introduction of deposited layers while the effect of the composite on the proton conductivity is not very distinct or even negative in some case due to the addition of non-conductive materials. However, the negative effect on the proton conductivity appears to depend on the nature of the polyelectrolyte bilayers and can be reduced to a minimum or eliminated by using a proper combination of polycation and polyanion. So, we choose variously polyelectrolytes coated on the SPAEK-C, including chitosan (CS)/phosphotungstic acid (PWA), polypyrrole (PPY)/PWA and multiwalled carbon nanotubes (MCNT-C)/CS.(ⅰ) The multilayer film is constructed onto the surface of SPAEK-C membrane by LBL self-assembly of CS and negatively charged inorganic particle PWA. The highly conductive inorganic nanoparticles ensure SPAEK-C-(CS/PWA)n membranes to maintain comparable proton conductivity values with the pure SPAEK-C, and these multilayer systems exhibit extremely low water swelling ratio and methanol permeability.(ⅱ) The multilayer film is constructed onto the surface of SPAEK-C membrane by LBL self-assembly of electrolyte-conductive PPY and negatively charged inorganic particle PWA. The methanol permeability are over 1 order of magnitude less than SPAEK-C while the proton conductivities and water swelling are decreased with the number of bilayers increasing.(ⅳ) The multilayer film is constructed onto the surface of SPAEK-C membrane by LBL self-assembly of electrolyte-conductive MCNT-C and negatively charged CS. The SPAEK-C-(MCNT-C/CS)n membranes maintain high proton conductivity values which are superior than the pure SPAEK-C especially in application at low humidity environment, while methanol permeability is effectively reduced.With different hydrophilicity of different polycations and polyanions, the four self-assembeled membranes show various changes in water uptake and water swelling, and the SPAEK-C-(CS/PWA)n showed the lowest water uptake and water swelling comparing with the others.In chapter 6, two methods were propsed to decrease the methanol permeability of Nafion membranes by introducing the SPAEK-C with high sulfonated degree in Nafion membranes.(ⅰ) Since the carboxylic acid groups in SPAEK-C can undergo Friedel-Craft acylation with the nucleophilic phenyl rings in the main chain of polymer, we blended Nafion and SPAEK-C and subsequently cross-linked by a Friedel-Craft reaction using the carboxylic acid groups in the SPAEK-C to achieve lower methanol permeability. The perfluoroalkyl sulfonic acid groups of Nafion act as a benign solid catalyst, which assist the cross-linking of SPAEK-C. The physical and chemical characterizations of the cross-linked composite membranes are performed by varying the contents of SPAEK-C. The methanol permeability of the crosslink membrane was effectively decreased while the high proton conductivity was maintained compared with the pure Nafion membranes.(ⅱ) Since the SPAEK-C with high sulfonated degree was water soluble, we alternated deposition of oppositely charged polyelectrolyte CS and SPAEK-C on the surface of Nafion(?)117 via the layer-by-layer (LBL) method. Then the crosslinking PEC-coated Nafion membranes are prepared from heat-induced cross-linking between CS and SPAEK-C via formation of amide bonds. Fourier transform infrared spectroscopy (FT-IR) verifies that cross-linking between ammonium groups of CS and carboxyl groups of SPAEK-C occurs by the formation of amide bonds. The morphology of the membranes is studied in detail by SEM. The c-PEC-coated Nafion(?) membranes exhibit lower methanol permeability and higher proton conducitivity than pure Nafion membranes while maintaining low water uptake and water swelling.