Preparation And Characterization Of Composite Proton Conductors Based On1-Buty-3-Methylimidazolium Dihydrogenphosphate And Heteropolyacids

Due to its good thermal stability and high proton conductivity, heteropolyacids (HPA) have been widely used as the inorganic proton conductors in the last few years. Among them,12-phosphotungstic acid is stable up to500℃, and its proton conductivity is about0.1S/cm at room temperature. However, HPA is soluble in water and the proton conductivity depends on the crystal water and the humidity. Therefore the application of HPA as the electrolyte in fuel cells is limited. Recently, ionic liquids (ILs) have been attracted many attentions on developing high temperature proton conductors due to their high thermal stability and high ionic conductivity.In this work, the protic ionic liquid1-buty-3-methylimidazolium dihydrogenphosphate ([BMIM]H2PO4), was synthesized from N-methylimizolium,1-chlorobutane and phosphate acid. Composite proton conductors of12-phosphotungstic or silicotungstic acid with [BMIM]H2PO4were prepared via mechanical grinding. The structure and thermal stability of the composite proton conductors were characterized by infrared spectra (IR), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), separately. The decomposition temperature of the composite conductors was about300℃, which was about100℃higher than that of the ionic liquid because of the strong interaction between HPA and IL. In addition, the content of ionic liquid as well as the temperature affects the conductivity of the composite proton conductors obviously. Under the anhydrous condition, the proton conductor, in which the mol ratio of PWA/[BMIM]H2PO4is1:5, exhibited the highest proton conductivity of6.7×10-3S/cm at180℃.The proton conductivity of the composite proton conductors could be enhanced by introducing of P2O5, significantly. With10wt%P2O5in PWA/[BMIM]H2PO4/P2O5, the conductivity reached3.0×10-2S/cm at180℃under the anhydrous condition. Comparing to the conductivity of PWA/[BMIM]H2PO4, i.e.,6.7×10-3S/cm, the increase in conductivity is close to one of magnitude. The presence of SiO2in the composite conductors did not benefit the conductivity of both PWA/[BMIM]H2PO4and SiWA/[BMIM]H2PO4.