Preparation And Properties Of High Temperature And Low Relative Humidity Proton Exchange Membranes

Direct methanol fuel cells (DMFCs) have attracted substantial research attention due to their high efficiency and non-toxicity to the environment. The conventional Nafion membrane can’t use at high temperature. So, develop high temperature and low relative humidity (RH) proton exchange membrane become one of key issues in high temperature DMFCs. Polybenzimidazole (PBI) is one of the most promising proton exchange membrane material for its excellent themo-chemical stability and mechanical properties.IronⅢ sulfophenyl phosphate (FeSPP) was successfully synthesized using simplified process as m-sulfophenyl phosphonic acid (msPPA) for precursor. The micro morphology of FeSPP was characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Energy-dispersive X-ray spectroscopy was used to analysis its elementary composition. Crystal structure was characterized by X-ray diffraction. Thermo stability is characterized by thermogravimetric-differential thermal analysis (TG-DTA), and proton conductivities were tested at different temperature and different relative humidity. SEM shows that FeSPP has a kind of sheeting structure. TEM micrograph indicates that the sheet is build up by multilayer sheets which thickness is about2nm. The TGA results reveal that FeSPP has good thermal stability at200℃. At180℃and100%relative humidity, the conductivity of FeSPP reaches0.115S/cm, at RH50%, the conductivity reaches5.41x10-2S/cm. In drying oven, the proton conductivity reaches2.15×10-3S/cm.PBI was synthesized by solution polymerization method. Poly[4,4’-(diphenyl ether)-5,5’-5,5’-bibenzimidazole](oPBI) is condensation products of4,4’-oxybis-(benzoic acid) and3,3’-Diaminobenzidine in polyphosphoric acid. A series of oPBI/FeSPP/FG composite membranes are prepared using hot-press method. The membranes are characterised by infrared spectroscopy (IR), X-ray diffraction spectroscopy, thermal gravimetric analysis, scanning electron microscopy (SEM). The properties of the membranes are evaluated by their water uptake, mechanical properties, proton conductivity and methanol permeability. SEM shows FeSPP is well dispersed in composite membranes, IR indicates that strong hydrogen bond is formed between FeSPP and oPBI. FeSPP enhanced mechanical properties, reduced the swelling ratios and enhanced the oxidation resistance of composite membranes of membranes. The mechanical strength of oPBI/FeSPP(10wt%) membrane is14MPa. The TG indicates composite membranes have excellent thermal stability; there is no weight loss below200℃. Composite membranes exhibit good proton conductivities. At170℃and100%RH, the conductivity of oPBI/FeSPP(30wt%) membrane is0.095S/cm,50%RH, the conductivity of oPBI/FeSPP(30wt%) membrane is0.052S/cm. FG further decreased the swelling ratio and methanol permeability, enhanced the mechanical strength. The mechanical strength of oPBI/FeSPP(30wt%)/FG(3wt%) membrane is16MPa, it is about two times of oPBI/FeSPP(30wt%) membrane (5.6MPa). At170℃and100%RH, the conductivity of oPBI/FeSPP(30wt%)/FG(3wt%) is0.078S/cm, and at50%RH, the conductivity of oPBI/FeSPP(30wt%)/FG(3wt%) is0.0418S/cm.Preparation process of poly[2,2’-(m-phenylene)-5,5’-bibenzimidazole](mPBI) membranes doped with FeSPP and thermally treated acrylic fibers (TAF) were studied. The-SEM indicates that the composite membranes have dense structure and FeSPP particles are homogenous dispersed into the composite membranes. FT-IR revealed that intense hydrogen bonds have formed between FeSPP and mPBI. Addition of FeSPP was in favor of increasing their proton conductivity at high temperature, improving mechanical properties. The mechanical strength of mPBI/FeSPP(20wt%) membrane is18.1MPa. At170℃and100%RH, the conductivity of the mPBI/FeSPP(30wt%) is0.118S/cm. The activation energy of proton transfer is19.8kJ·mol-1. The mechanical strength of mPBI/FeSPP(30wt%)/TAF(5wt%) membrane is21.7MPa, higher than mPBI/FeSPP(30wt%)(16.7MPa). At170℃and100%RH, the conductivity of mPBI/FeSPP(30wt%)/TAF(5wt%) membrane is (?)92S/cm. At50%RH, the conductivity is0.0494S/cm. Oxidative degradation of mPBI was studied under the Fenton test conditions by the weight loss, molecular weight, SEM, IR and XPS.