Research On Doped Speek Proton Exchange Membrane Treated With Ultrastrong Magnetic Field

Direct methanol fuel cell (DMFC) has been attracting considerable attention as one of the ideal power generation devices, since they can offer numerous benefits, including high efficiency, high power density, low emissions, renewable fuel and more convenient for storage and transportation, in which the proton exchange membrane as a key component affecting the applied properties of DMFC can play an important role in fuel isolating and proton conducting. High performance sulfonated poly (ether ether ketone)(SPEEK) would replace the traditional perfluorinated soulfonic acid for a proton exchange membrane. However, the SPEEK membrane still has the weaknesses of the comprehensive performance. In this paper, based on the unary doping of one of two rare earth metal oxides such as cerium oxide (CeO3) and yttrium oxide (Y2O3) and four ionic liquids, the binary or ternary doping among Y2O3, or/and phosphotungstic acid (PWA) and ionic liquids and the6T or12T ultrastrong magnetic field treatment for the SPEEK membranes have been studied. The influence rules on membrane performance and modification mechanism with doping or ultrastrong magnetic field treatment would be discussed.Firstly, the PEEK was sulfonated by the concentrated sulfuric acid, and SPEEK resins with different sulfonation degree (DS) of36%to90%were obtained by controlling the reaction time and temperature. The proton conductivity of SPEEK membrane increased with increasing DS and reached a maximum value of1.20×10-2S·cm-1at90%DS. However, the methanol permeability, water uptake and swelling degree increased until invalid with raise of DS. Therefore, the SPEEK with medium DS of40%to70%would be used suitably to prepare the membranes. Comparing properties of the57%DS SPEEK membranes prepared with different solvents, such as N, N’-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO) and N, N’-dimethyl-formamide (DMF), the membrane prepared with DMAC as the solvent could possess the higher conductivity and medium methanol permeability coefficient. It was shown in the analysis results that the proton conduction mechanism in the SPEEK proton exchange membrane present the proton transition between the sulfonic acid groups in SPEEK, then the proton conductivities would directly relate to the number of the sulfonic acid groups.The influences of different rare earth oxide doping such as CeO2or Y2O3and the ultrastrong magnetic field treatment on structure and performance of the composite membrane with DS of48%were analyzed. It was found by FTIR that CeO2or Y2O3could coordinate with the sulfonic acid groups in SPEEK molecular chains, and especially the Y2O3doping caused observable structure change in the SPEEK membrane. The conductivity of the both doped membranes increased with increasing temperature and decreased with increasing the doping quantity, but the methanol permeability and water uptake also decreased with increase of the doping quantity. The conductivity of the doped membrane increased by4.2times through the ultrastrong magnetic field treatment, but its water uptake changed scarcely, and its methanol permeability decreased with ultrastrong magnetic field treatment at perpendicular, reached a minimum value of1.56×10-7cm2·s-1lower of70%than that of the pure SPEEK membrane. The8wt.%Y2O3doped membranes treated with12T ultrastrong magnetic field at perpendicular direction could prove weak paramagnetism, indicating that the ultrastrong magnetic field could produce orientation action on the polar groups in membrane and improve the proton conductivity.Through analysis of the65%DS SPEEK membranes doped with binary Y2O3and PWA, it was proved further that Y2O3could coordinate with the sulfonic acid groups in SPEEK molecular chains and caused the change of membrane structure. In a certain Y2O3content, the conductivity, methanol permeability, water uptake and swelling degree all increased with increasing PWA, while without changing PWA content and increasing the dope of Y2O3, these properties decreased, then the PWA loss rate decreased, and the storage capacity of the membrane increased. Through treating with12T ultrastrong magnetic field at120℃for10h, the conductivity of the membrane increased by about2.0times, and the methanol permeability decreased slightly, and the water uptake was unchangeable.A series of composite membranes of65%DS SPEEK was prepared by binary or ternary doping with one of four ionic liquids such as1-butyl-3-methylimidazole mesylate,1-ethide-3-methylimidazole tetrafluoroborate,1-butyl-3-methylimidazole hexafluorophosphate and1-carboxymethyl-3-methylimidazole hexafluorophosphate and Y2O3or/and PWA. Under different relative humidity condition, the proton conductivity of these doped membranes increased significantly than that of pure SPEEK membrane, in which1.98×10-1S·cm-1was reached at90℃and100%relative humidity. Their methanol permeabilities were increased slightly by the binary or ternary doping, in which that of the membranes doped with binary ionic liquid and Y2O3was relatively low. The membrane doped with binary1-ethide-3-methylim-idazole tetrafluorob orate and Y2O3have the best comprehensive performance and would apply for the DMFC...