A Study On Electrolyte Membranes Based On Poly(Phthalazinone Aromatic Ether)s

Direct methanol fuel cells (DMFC) have attracted considerable attentions as analternative to the present power sources, since they offer numerous benefits, includinghigh efficiency, high power density, low or zero emissions and easy fuel carriage.However, at present, two major technology problems in DMFC still restrict theirperformance and applications. One problem is methanol crossover from anode tocathode and the other is slow kinetics of methanol oxidation. If DMFC can beoperated at higher temperature, the activity of catalyst can be improved and heatmanagement simplified. This will require new membranes that are more heat-tolerantthan presently used Nafion membrane. In this study, development of methanol barrierconducting membranes with high thermal stability was attempted.Poly(phthalazinones) (PPs) including poly(phthazinone ether ketone) (PPEK),poly(phthalazinone ether sulfone) (PPES) and poly(phthalazinone ether solfoneketone) (PPESK) are polymers with high thermal stability and excellent mechanicalstrength. Moreover, those aromatic hydrocarbons are much cheaper compared withthe perfluorosulfonic acid (PFSA) polymer of present fuel cell membranes.SPPEK, SPPES and SPPESK membranes with various degree of sulfonation (DS)were prepared and the feasibility of applying these membranes in DMFC was studied.The proton conductivity of SPPEK (DS=50%), SPPES (DS=49%) membranes isabout 10-2S·cm-1 at temperatures above 100℃, and the conductivity of SPPESK(DS=131%) membrane is closed to that of Nafion membrane. Methanol permeabilityof the three membranes is lower than that of Nafion membrane. If the ratio ( σ /P) isused to characterize the performance of a membrane for DMFC, the ratio of thesethree membranes are better than Nafion115 membrane at the temperatures tested.Heteropoly acids (HPAs) are inorganic solid electrolytes with high protonconductivity and thermal stability. But HPAs cannot be made to a self-supportingmembrane. On the other hand, PPEK has good film-forming property and highthermal stability but cannot conduct proton. To combine the advantages of the two, aserial of HPAs doped PPEK membranes are prepared. The proton conductivity of thePWA/PPEK and SiWA/PPEK membrane is low due to poor dispersion of HPAs inPPEK. With the hydrophilic polymer of PVP added, the conductivity of the compositemembranes was improved. But phase separation of the two polymers was observedand the HPAs mainly disperse in PVP. PPEK becomes more hydrophilic whenmodified by sulfonation. A homogeneous membrane can be gotten with PWA dopedSPPEK. When immersed in water, less than 2% PWA was found lost in the PWAdoped SPPEK(DS=58%) membranes. But the dispersion of SiWA in SPPEKmembrane is poor and the lost of SiWA is much higher. So SiWA/SiO2 was derivedand SiWA was found well dispersed in the SiWA/SiO2 doped SPPEK membrane andthe lost of SiWA decrease when immersed in water.