Preparation And Properties Of Phosphoric Acid Doped Cross-linked Quaternized Poly (Ether Ether Ketone) Membrane

Fuel cells are able to convert chemical energy of fuel into electric energy directly,which is not limited by Carnot cycle because the energy conversion process does notundergo combustion. Therefore, the rate of energy conversion is very high, up to60%-80%. Besides of high rate of energy conversion, fuel cell would not releasepollution to the environment. There are many advantages for operation of fuel cell athigh temperatures, such as, high rate of electrochemical reaction, simplified water-heattreatment, high CO tolerance. Proton exchange membrane fuel cell (PEMFC) is onekind of fuel cells, which is composed by anode, cathode and proton exchangemembrane (PEM). The PEM transports protons, isolates electron and fuel and is thecore of PEMFC.The studies of high temperature PEM (HTPEM) are mainly based on thefunctionalized sulfonated membranes and phosphoric acid (PA) doped membranes. Thepolybenzimidazole (PBI) is the most widely studied material. In recent years, theresearches about functionalized poly(aryl ether ketone or sulfone) as the HTPEM haveincreased gradually. Because these poly(aryl ether ketone or sulfone) also have highthermal stability and could be functionalized by different basic groups.First of all, four quaternized poly(ether ether ketone)(QPEEK) were prepared based on bromomethylated poly(ether ether ketone)(BrPEEK), with trimethylamine(TMeA), triethylamine (TEtA), tripropylamine (TPrA) and1-methylimidazole (MeIm)as the quaternary ammonium reagents, and marked as methyl-QPEEK, ethyl-QPEEK,propyl-QPEEK and im-QPEEK, respectively. The nuclear magnetic resonance wasused to confirmed the successful preparation. The thermal stability, mechanicalproperties, PA doping level and oxidative stability were studied. For these fourQPEEKs, the PA doping level was closely related to the structure of onium salt, andhad no business to the basicity of quaternary ammonium reagents. The large sterichindrance would hinder absorbing PA. The onium salt of im-QPEEK was a conjugatering, therefore, the PA doping level of im-QPEEK was highest, followed bymethyl-QPEEK. The results of dynamic mechanical analyses (DMA) and thermalgravimetric analyses (TGA) exhibited that these QPEEKs owned high glass transitiontemperature (Tg) and thermal stability. Once the QPEEKs membranes were doped withPA, the tensile strength decreased. The tensile strength decreased with the increase ofPA doping level and the high temperature also would lead the decrease of tensilestrength. The results of oxidative stability showed that the bigger volume swelling andhigher PA doping level would lead the worse oxidative stability. After thecomprehensive comparison, the im-QPEEK and methyl-QPEEK were potential to beused as HTPEM.The cross-linking method could hinder volume swelling and improve tensilestrength of materials, effectively. This method has been used in low and hightemperature proton exchange membrane. However, the dense cross-linking networkstructure usually leads to decrease of PA doping level. The cross-linker, which couldabsorb PA, is able to resolve this problem. In the next three chapters of this paper, thedifferent cross-linkers are used to prepare HTPEM. Both im-QPEEK and methyl-QPEEK have high PA doping level. Although methyl-QPEEK has a lower PAdoping level than im-QPEEK, the preparation of methyl-QPEEK is very agile. Themethyl-QPEEK membrane is able to be prepared by immersing in trimethylamineaqueous solution. Therefore, the trimethylamine and1-methylimidazole are used asquaternary ammonium reagents in the following chapters.In the chapter three, the PBI was used as a macromolecule cross-linker to preparea series of cross-linked membranes with BrPEEK by the reaction of-NH-and-CH2Br.The quaternized cross-linked membranes were obtain by immersing in trimethylamineaqueous solution. After being doped with PA, the PA doped cross-linked quaternizedmembranes with PBI as a cross-linker were obtained and were marked asPA-QPEEK-x%PBI. Because PBI had a high PA doping level, the PA doping level ofPA-QPEEK-x%PBI improved. However, when the content of PBI was high, the PAdoping level showed decrease because of the dense cross-linking network structure.The cross-linked membranes showed improved Tg, oxidative stability, mechanicalproperties and thermal stability because of the introduction of PBI. The PA dopinglevel in volume was calculated in this chapter, which was the amount of PA in permembrane volume. Compared PA doping in weight and PA doping level in volumewith proton conductivity, the PA doping level in volume had a closer relationship withproton conductivity.In the chapter four, the dual cross-linked HTPEMs were prepared by BrPEEK and3-triethoxysilylpropylamine (APTES). The cross-linked membranes were furtherquaternized, doped with PA to obtain PA doped HTPEM. The-NH2of APTES and-CH2Br of APTES reacted, forming the primary cross-linking network structure. TheSi-O-Si network structure produced by hydrolysis of APTES was the secondcross-linking network structure. The dual cross-linked membranes were marked as PA-QPEEK-x%APTES. The hydrolysis of APTES released many small molecules, H2Oand C2H5OH, leading to the appearance of small pores in QPEEK-x%APTESmembranes. These pores and the obtained Si-OH groups contributed to increasing PAdoping level and proton conductivity. These closed pores reduced the damages to themechanical properties in some degree. The PA gathered on the surfaces of membranebecause of the existence of Si-OH groups. When the content of APTES was low, the PAclusters were coherent, which was benefit to the proton conduction. The dualcross-linked organic-inorganic hybrid network had improved the PA doping level,mechanical properties and oxidative stability.There are many oxygen-containing groups on the surfaces of graphene oxide (GO),these oxygen-containing groups could form hydrogen-bond with PA and provide theconduction site for protons. Therefore, GO was introduced into im-QPEEK membraneto improve the proton conductivity, obtaining QPEEK-x%GO membranes. However,GO had a bad dispersion in organic solvent and a bad compatibility with im-QPEEK.The functionalized GO (FGO) by p-aminobenzene sulfonic acid was prepared. FGO hada good dispersion in DMSO and a good compatibility with im-QPEEK. When thecontent of GO or FGO was same, the PA doping level and proton conductivity ofQPEEK-x%GO were higher than those of QPEEK-x%FGO. The mechanical properties,oxidative stability and dimension stability of QPEEK-x%FGO were higher than those ofQPEEK-x%GO, especially, tensile strength.