| Fuel cells have the potential to become an important energy conversion technology. Research efforts directed toward the widespread commercialization of fuel cells have accelerated in light of ongoing efforts to develop a hydrogen-based energy economy to reduce dependence on foreign oil and decrease pollution. Proton exchange membrane (also termed"polymer electrolyte membrane") (PEM) fuel cells employing a solid polymer electrolyte to separate the fuel from the oxidant were first deployed in the Gemini space program in the early 1960s using cells that were extremely expensive and had short lifetimes(<500 h) due to the oxidative degradation of their sulfonated polystyrene-divinylbenzene copolymer membranes. These cells were considered too costly and short-lived for real-world applications. The commercialization of Nafion by DuPont in the late 1960s helped to demonstrate the potential interest in terrestrial. PEM fuel cells are being developed for three main applications: automotive, stationary, and portable power. However, various types of application performance were very well, the nature of useful perfluorinated sulfonic acid membrane, due to high prices has been limited to widely use. Of course, there are some defects: for example, PEM fuel cells rely on absorbed water and its interaction with acid groups to produce protonic conductivity. Due to the large fraction of absorbed water in the membrane, both mechanical properties and water transport become key issues. All the above defects and high methanol permeability of Nafion have limited to usage. To develop a cost-effective superior proton exchange membrane materials have been becoming more and more important. High attention has been given to the new polymers by international corporations and the academic communityPoly (ether ether ketone) (PEEK), due to the excellent stability, the excellent mechanical properties and good thermal stability, which are recognized as a potential high-performance proton exchange membrane materials. Researchers through concentrated sulfuric acid and other acid-based reagents will be introduced the sulfonated groups to the PEEK chains to achieve proton conductivity? With the increase of sulfonation conditions of the study,there are two kinds of sulfonated methods: One approach is postsulfonation of existing polymers, and the other is direct copolymerization of sulfonated monomers. In the case of the postsulfonation approach, this method is attractive because of the available resource of commercial polymers, such as Victrex poly(ether ether ketone) (PEEK) as well as a simple reaction procedure, enabling the process to be readily scaled up. However, difficulties may be encountered in the precise control of the sulfonation sites and the DS. In addition, rigorous reaction conditions, such as high temperature and strongly acidic sulfonating agent, are usually used to sulfonate polymers such as PEEK, which in some cases may lead to the occurrence of side reactions and degradation of the polymer backbone. In the case of the sulfonated monomer approach, despite the limited number of available sulfonated monomers and the preparation difficulties of the some sulfonated monomers, the direct copolymerization of sulfonated monomer with other nonsulfonated monomers has the potential for synthesizing random copolymers with a better control of sulfonation content (SC) and more defined chain structures in comparison with the copolymers by the postsulfonation method. The obtained sulfonation PEEK polymer by which method, are still retained good thermal stability, chemical stability and good mechanical properties. And studies have shown that SPEEK in the fuel cell have a good application prospect. Of course, the sulfonated PEEK still have some shortcomings, such as in the case of a high sulfonation degree, although there are good proton conductivity, but the excessive swelling rate caused by water absorption of the membrane have been leaded to a decline of the mechanic properties and limit to its normal useage.In order to solve the shortcomings of the traditional SPEEK material, we have prepared new molecular structures, cross-linked structure and composite materials with other polymers, as well as the preparation methods such as nano-fiber membrane to further improve the properties of SPEEK in proton exchange membrane fuel cell usage.In chapter 3, a novel series of sulfonated PAEKs with stereocontrolled sites and high DS were prepared via postsulfonation method. The DS of the copolymers were controlled by polymer chain composition that contains two types of segments: those that are readily sulfonated and those that are not. Moreover, we controlled the reaction time to obtain the anticipant position of the sulfonated groups. PAEKs with phenyl and (3-trifluoromethyl) phenyl pendant groups were synthesized successfully and were found to have controlled sulfonation sites of the pendant rings via a postsulfonation approach. The properties of these sulfonated copolymers were all invested, such as thermal stabilities, mechanical stabilities, water uptake, and proton conductivity. It was found that this series of Ph-3F-SPAEK had good thermal stability, good mechanical property, high proton conductivity (up to 0.187 S cm-1 at 80oC), and low methanol permeability. Combined with all these advantages, these membranes may be a potential PEM material for PEMFC applications.In chapter 4, we have prepared a novel membrane based on the blend of sulfonated poly (ether ether ketone)s and PAA, and immersed in phosphoric acid phosphoric acid to obtain a acid doped composite film. This work aims at improving the proton conductivity, the mechanical properties and methanol resistance. The SPEEK/PAA blend membranes with content of PAA in the range of 2%-15% were confirmed as a good alternative for proton conductive membranes with potential application in PEMFC. All the polymers showed good thermal stability. SPEEK/PAA membranes (2 wt%, 5 wt%) showed better IEC and proton conductivity. As expected, proton conductivity as a key component of PEM was better than that of pure SPEEK, making series of SPEEK/PAA membranes were good alternative to use in PEMFC with high proton conductivity.In chapter 5, we prepared the SPAEK copolymer with carboxylic acid groups tethered to the flexible aliphatic chains. We proposed that the carboxylic group in the SPAEK copolymer side chain is an active pendant group which could react with a crosslinking agent containing a hydroxyl group to form a crosslinked membrane material. And we prepared crosslinked SPAEK/PEG membrane, in which PEG was used as cross-linker. A series of novel C-SPAEK/PEG membranes have been prepared and characterized. These membranes were formed as a result of physical (via H-bonding) and chemical (via PEG) crosslinking. The C-SPAEK/PEG membranes showed more flexible property and lower water uptake. The SR decrease from 25.3% to 14.5% with the increasing PEG content at room temperature. The result also suggested that one can control swellability of the membranes simply by changing the content of crosslinking PEG molecules. And the C-SPAEK/PEG membranes showed higher proton conductivity at higher temperature (80oC).In chapter 6, a nanofiber composite network ionexchange membrane was fabricated using a four-step procedure: (i) electrospinning a nanofiber mat of ion-exchange polymer, (ii) compacting the mat to increase the volume density of fibers in the final membrane, (iii) forming polymer welds between intersecting fibers to create a three-dimensional interconnecting network, and (iv) filling the void space between fibers with an inert polymer. The properties of the solution-casting membrane and electrospinning performance of Ph-SPAEK were investigated. The main focus was to understand how formation of fibers and the resulting fiber structure are affected. The measured properties of the nanofiber network membranes compare very well to those of solution-casting sample. All the results showed that these membranes may be a potential PEM material for PEMFC applications.
|
PDF Full Text Request