Preparation And Proton Exchange Properties Of Sulfonated Polyimide/Silica Composite Membranes

Proton exchange membrane (PEM) is one of the key components for fuel cell. The service life and efficiency of fuel cell are significantly affected by the performance of PEM. In recent years, perflurosulfonic acid membranes are the only commercial available polymeric materials used as proton exchange membrane due to their excellent chemical stability, high proton conductivity and long service life. However, there are some drawbacks, including high cost, low dimensional stability, low performance at high temperature and low humidity, as well as high methanol crossover, have to be overcame, in order to meet the requirements of direct methanol fuel cell (DMFC) which was operated at elevated temperature. Therefore, it is very important to develop the novel proton exchange membranes with good thermal and dimensional stability, high proton conductivity and low methanol permeability. Sulfonated polyimides are promising candidates for DMFC application because of their outstanding thermal and chemical stability, high mechanical property as well as much lower methanol permeability.In this research, sulfonated polyimide/silica composite membranes with different silicon contents were prepared via sol-gel method. The inorganic SiO2 particles have good hydrophilicity, which can improve the water uptake of membranes and their water retention at high temperature. Meanwhile, the introduction of SiO2 particles can also improve the thermal and dimensional stability of membranes. The influence of the silica content on mechanical and thermal properties, dimensional stability oxidation and water stability, as well as proton conductivity and methanol permeability of composite membranes were investigated.This thesis consists of following contents:1. The 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (BAPBDS) was synthesized by 4,4'-bis(4-aminophenoxy)biphenyl sulfonated by sulfuric acid through electron-withdrawing substitute reaction, which was then copolymerized with NTDA and 6FBAB to get sulfonated polyimide. Then, sulfonated polyimide/silica composite membranes with different silicon contents were prepared via sol-gel method. The structure and morphology of membranes were characterised by FT-IR, solid 29Si-NMR, TEM and XRD. The result indicated that the silica distribute uniformly and have the average particle size of 2-10 nm. There is no obvious aggregation of partials in membranes with the silica content increasing.The influence of the silica content on mechanical and thermal properties, dimensional stability oxidation and water stability, as well as proton conductivity and methanol permeability of composite membranes were investigated. The results indicated that the thermal, oxidation and dimensional stabilities of membrane were significantly improved by the introduction of silica. Meanwhile, the composite membranes exhibited excellent methanol permeability with the values only 1/5-1/3 of that for Nation 115. Although the proton conductivity was reduced with the introducing of SiO2, the SPI/SiO2-2 membranes showed the higher proton conductivity at elevated temperature. Moreover, the low methanol permeation of membranes made them exhibit obvious advantage in selectivity, which is expected to application in direct methanol fuel cell.