The Research Of Heteropoly Acid Supported On Molecular Sieves-Organic Composite Proton Exchange Membrane

Direct methanol fuel cell (DMFC) has attracted considerable attention as an alternative to the present power sources, because its wide application as a portable or mobile power. Proton exchange membrane (PEM) is the key part of the DMFC and much efforts have been made to the development of the PEM. At present, two major problems in commerial membrane Nafion still restrict their performances and applications. One problem is the high cost and poor performance in higher temperture. The other is the crossover of methanol through the electrolyte membrane. In this study, heteropoly acid supported on molecular sieves-organic composite proton exchange membranes with high proton conductivity, good thermal tolerance, and low methanol permeability were investigated.The blanked Polyvinyl alcohol (PVA), chitosan (CS) and polyimide (PI) membranes are prepared at first. PVA, CS and PI composite proton exchange membranes are prepared with PVA, CS, PI, molecular sieves (13X, or MCM41, or Y) and phosphotungstic acid (PWA). The proton conductivity, water uptake, swelling degree and methanol permeability of the membranes are measured. The composite membranes with PWA supported on molecular sieves have both excellent proton conductivity and good barrier property for methanol.The blanked PVA membrane has high water uptake and swelling degree. The proton mobility in blanked PVA membrane may rely on Grotthuss mechanism. Work in our group has proven through the process that PWA supported on 13X, the molecular sieves can strengen the maintainability of membrane for PWA and improve the proton conductivity. PWA in the membrane makes a part of proton transfer rely on Grotthuss mechanism. PVA-PWA-13X composite membrane is high in water content, but also high in swelling degree, which may result in its bad mechanical performance.The blanked PI membrane has low water uptake and swelling degree. PI with PWA supported on molecular sieves(13X, MCM41, Y) composite membranes are prepared respectively. The conductivity of the polyimide composite membrane is increased with the increasing of the temperature. The influence of adding inorganic material was studied, and the Ea is lower than the blank membrane. The proton conductivity of the PI-PWA- MCM41 is higher than PI-PWA- Y, for the good specific surface of MCM41 molecular sieves. Additionally, the influence of adding ionic liquid into the composite membrane is also studied.The blanked CS membrane has high water uptake, low swelling degree and poor proton conductivity. The CS-PWA-13X composite membrane is prepared and the proton conductivity of the composite membranes is studied. At 90℃, the maximum value of proton conductivity is 0.123S·cm^-1.The methanol permeability of the composite membranes is lower than the Nafion 117(16.5×10^-7cm²Â·s^-1). The existence of the PWA supported on molecular sieves blocks the mobility of methanol and lead to the low methanol permeability.The methanol permeability of the three membranes is: PI< CS < PVA