Synthesis And Study Of Properties Of Novel Proton Exchange Membrane For Fuel Celle

The polymer electrolyte membrane fuel cell (PEMFC) is the rapidly developingfifth-generation fuel cell. With the lowest working temperaturethe highest specificenergy, the fastest startup, the longest service life, and the widest applications,PEMFC has very important applications and good prospects in the fields of the sea,the land and the air. The proton-exchange membrane is the core component ofPEMFC. It differs from the membranes used in chemical cells in that it not only actsas an insulator between the fuel and the oxidant, but also works as an electrolyte.Most proton-exchange membranes consist of a polymer matrix and ion-exchangegroups. At present, the only available commercial products of proton-exchangemembrane are the Nafion series of perflurosulfonic acid membranes from Dupont. Although Nafion has good chemical and physical properties for use in fuel cellfields. It has three major draw backs, including low conductivity at low humidity orhigh temperature, as well as high methanol permeability. Unreacted methanol at theanode can diffuse through the membrane and react at the cathode, lowering thevoltage efficiency of the cell and reducing the system's fuel efficiency. Thedrawbacks limited application in the direct methanol polymer Electrolyte membranefuel cells. Besides, the perfluorinated membranes are very expensive, which is alsoregarded as a limiting factor. Thus new polymer electrolyte membranes for fuel cellare being explored. Synthesis and study of properties of novel proton exchangemembrane for fuel cells.Sulfonated polyimide is a promising candidate of Nafion for proton-exchangemembranes due to its outstanding thermal and chemical stability, high mechanicaland electrical property as well as good film-forming capability. Additionally, theorganic-inorganic composite proton exchange membranes have received a great dealof attention because they possess the excellence of organic and inorganicmembranes. The incorporation of inorganic components in polymer electrolytematerials improves the water uptake and water retention, which caused the highproton conductivity of membranes at high temperature. Meanwhile, the introductionof inorganic particles also reduces the methanol permeability.In this thesis, a series of sulfonated polyimides were synthesized by condensationof bis(p-hydroxylphenyl)-1,4,5,8-naphthalenetetracarboxylic di-imide with variousratios of 3,3'-disulfonate-4,4'-dichlorodiphenylsulfone to4,4'-dichlorodiphenyl-sulfone and were prepared for proton exchange membranes.At the same time, the SPEEK/TiO2 hybrid membranes with various contents of TiO2were prepared through sol-gel reactions. Both of them were evaluated for protonexchange membrane fuel cell application. The sodium5,5-carbonylbis(2-fluorobenzene-sulfonate) was synthesized by dissolving4,4-difluorobenzophenone in 30% fuming sulfuric acid throughelectron-withdrawing substitute reaction. The bisphenol monomer containing theimide ring was synthesized by NTDA and p-aminophenol. In addition, a series ofsulfonated polyimides containing ether ketone structure were synthesizedsuccessfully by typical nucleophilic substitution reactions. The results of charactersproved that sulfonated polyimides can be synthesized by nucleophilic substitutionreactions.The resulting polymers showed good film-forming capability, which indicatedhigh molecular weight of sulfonated polyimides. Besides, the polymers possessedhigh thermal decomposition temperature(T10%>400℃)and Tg(Tg>200℃). Thepolymers had good solubility and easily dissolved in m-cresol and NMP. The data oftensile modulus and tensile strength of SPEK-PI membranes indicated that theywere suitable as membrane materials for proton exchange membranes. SPEEK-PImembranes showed favorable proton conductivity and the proton conductivity ofmembrane with DS=1was close to that of Nafion. All the membranes showed lowermethanol permeabilities compared with Nafion117. Consequently, the protonexchange membranes were promising for PEMFC applications.In this thesis, the SPEEKs containing tetramethyl were selected as matrix owingto their good mechanical properties, low methanol permeability, highthermo-oxidative stability and proton conductivity. At the same time, nanosizedTiO2 was selected as additive due to its classical procedure of synthesize.A series of SPEEK/ TiO2 composite membranes with various DS and contentsof TiO2 were prepared and characterized by FTIR, SEM and TEM. It could beconclude that TiO2 particles were homogeneously distributed in the membrane.andthe diameters of TiO2 particles were below 100 nm. The composite membranesdisplayed excellent thermal stability and mechanical properties, which could suit theuse of proton exchange membrane.The water uptake of composite membranes improved with increasing TiO2 at thesame DS owing to large specific surface area and high surface energy. We noticedthat SPEEK/TiO2 composite membranes showed better water retention than SPEEKmembranes. It could be explained that abounding O-H restrained the movement ofwater in composite membranes, and further reduced water diffusion speed. It wasalso because that the introducing of nanosized TiO2 to proton exchange membranescan decrease radius of curvature of water diffuse interphase, further resulted in theenhancement of saturated vapor tension, which restricted the evaporation of water.We can see that the introduction of nanosized TiO2 increased the density ofmembrane and impeded the moving of methanol, which resulted in obvious decreaseof methanol permeability. First the proton conductivity of composite membranesincreased with enhancive content of nanosized TiO2, and then they decreaseddepending on same DS and temperature. It was because that the proton conductivitywas influenced by nanosized TiO2 in two aspects: one is to improve water uptakeand water retain;another is to block the channels of protons. The protonconductivity enhanced when the former play a dominant role, whereas, the protonconductivity decreased.