Characterization And Synthesis Of Imidazolium-based Polymer Electrolyte Membranes

Polymer electrolyte membranes fuel cells have increasingly received worldwide attention because of their potential application in transportation and in stationary and portable electronics. Based on the polymer electrolyte membrane, the polymer electrolyte membrane fuel cells (PEMFCs) could be classified as proton exchange membrane fuel cells and alkaline anion exchange membrane fuel cells (AEMFCs).The operation of PEMFCs at elevated temperature is receiving much attention because it could enhance reaction kinetics at both electrodes, improve the carbonmonoxide tolerance of the platinum catalyst at the anode, and simplify heat and water managements of PEMFCs. However, the most commonly used proton exchange membranes (PEM), represented by Nafion, cannot be used at high temperature because of the evaporation of water, which results in a rapid loss of conductivity. The protic ionic liquid(PIL)-based proton exchange membrane show high conductivity at elevated temperature, but the progressive release of the PIL component is still a challenge.Compared with traditional PEMFCs, a advantage of AEMFCs is their potential to use non-precious-metal-based electro catalysts such as silver or nickel, Furthermore, AEMFCs also offer the fuel flexibility, reduced fuel (such as methanol) crossover, and enhancedreaction kinetics for both oxygen reduction and fuel oxidation. However, The traditional anion exchange membranes (AEMs) were typically prepared by attachment of chloromethyl groups to polymer backbones and followed by quaternization to form ammonium salts. Chloromethyl ether is carcinogenic and harmful to human, and the quaternary ammonium was not stable in alkaline condition at high temperature.To solve these problems, PIL-base PEMs and imidazolium-based AEMs were prepared in this dissertation. The main conclusion in this dissertation are as follows:(1) Silica nanoparticles with different size, mesoporous silica nanospheres and imidazolium surface functionalized SiO2(Im-SiO2) nanoparticle were prepared. Protic ionic liquid (PIL)/poly(styrene-co-acrylonitrile)/silica hybrid PEMs have been synthesized by incorporating silica nanoparticles, mesoporous silica nanospheres or Im-SiO2into the membrane. The resultant PIL-based hybrid membranes have quite a good thermal stability and mechanical properties. Under anhydrous conditions, the produced composite membranes have conductivity up to the order of1×10-2S/cm at160℃. Incorporation of proper amount of silica fillers significantly increased the proton conductivity of the membranes, probably due to the ion transport channel or network structures formed in the membranes. However, further addition of silica fillers might block the formed ion transport channels and decrease the conductivity of hybrid membranes.The mesoporous silica nanospheres and the Im-SiO2are effective in preventing the release of ionic liquid component from the composite membranes.(2) We have demonstrated a facile and effective synthetic procedure for the preparation of novel anion exchange membranes (AEMs) based on imidazolium salts. The AEMs were prepared via in situ cross-linking of1-vinyl-3-methylimidazolium iodide with styrene and acrylonitrile, and followed by anion-exchange with hydroxide ions. The synthetic procedures of AEMs are simple and without the use of chloromethyl ether which is harmful to human. The resulting cross-linked copolymer membranes exhibit high hydroxide ion conductivity (above10-2S/cm at room temperature) and good mechanical properties. The membranes display an excellent chemical stability up to400h in high pH solution without an obvious loss of ion conductivity and mechanical properties.(3) We developed a novel fluorene derivative (2,7-di(4’-phenol)-9,9-bis(6’-(1,2-dimethylimidazole))fluorene Bromine) which contains imidazolium groups. The imidazolium-based AEMs prepared via polycondensation polymerization, casting from DMF solution, and followed by anion-exchange with OH-anions. The membranes are soluble in polar aprotic solvents such as DMSO and DMF. The solubility properties of polyfluorene ionomers enable theuse for not only alkaline anion-exchange membrane but also ionomer electrode material.’H NMR and hydroxide ion conductivity measurements demonstrated an excellent chemical stability of the synthesized polyfluorene ionomers in high pH solution at elevated temperatures.(4) A series of imidazolium salts and imidazolium-based alkaline anion exchange membranes were synthesized and characterized. The methyl, isopropyl or phenyl groups were covalently attached to the C2-position of imidazole rings. The introduced of methyl, isopropyl or phenyl to C2-position of imidazolium ring enhanced the chemical stabiliy of imidazolium cations at elevated temperature under alkaline condition. The results of the study suggest that the δ-π hypercongugative effect, electronic effect and steric hindrance effect of the2-substituted groups are effective in stabilizing the imidazolium cation. These results of the study suggest a feasible approach for enhancing the chemical stability of imidazolium salts and the synthesis and practical applications of alkaline anion exchange membranes (AEMs).