Studies On Composite Membranes For Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells （PEMFCs） have high power density, zero emissionand quick start-up and have received intensive attentions. As one of the key components,proton exchange membrane separates anode and cathode to prevent direct reaction of fuel andoxidant, meanwhile, it acts as proton conductor in fuel cells. Currently, the most commonlyused perflurosulfonic acid membrane is not cheap at all and its performance is still notsatisfactory; this becomes one of the obstacles to hinder the commercialization of fuel cells.As a result, extensive focuses have been directed at the research and development ofinexpensive and highly performed proton exchange membranes.In this thesis, a solution casting method had been used to prepare composite membranesby introducing perfluorosulfonic acid resin （PFSR） into the strengthened porouspolytetrafluoroethylene （PTFE） substrate. This was aimed at the improvement of both tensilestrength and size stablility as well as reduced cost. Based on this type of strengthenedmembrane, hydrophilic and inorganic SiO₂and phosphorus tungsten acid （PWA） had beenfurther doped to fabricate self-humidified membrane with enhanced water uptake and protonconductivity.Different perfluorosulfonic acid resin （PFSR） and different porous PTFE substrate havebeen selected to prepare composite membrane PFSR/PTFE. The effects of membranethickness, porous PTFE thickness and kinds of perfluorosulfonic acid resin on the compositemembrane performance had been investigated. Scanning electron microscopy （SEM） imagesshowed that the PTFE was uniformly filled by perflurosulfonic acid resin. The thicknessuniformity, ion exchange capacity, water uptake, proton conductivity, mechanical strength andthe single cell performance had been studied. Experimental results showed that the compositemembrane with40μm thickness displayed the highest mechanical strength, and the sizestability was better than that of Nafion NRE212. The tensile strength of the compositemembrane using porous PTFE with35μm thickness was double than that of Nafion212.Theas prepared Nafion/PTFE and NR50/PTFE composite membranes performed the best in asingle cell test.SiO₂/PFSR/PTFE composite membrane was prepared by introducing aerosil into the resin solution. The effects of the aerosol amount on the proton conductivity and water uptakeof the resulted composite membranes had been investigated. It was found that the increase ofSiO₂content led to the enhancement of proton conductivity and water uptake for thecomposite membrane. Single cell performances showed that the incorporation of SiO₂resulted in superior performance for the SiO₂/PFSR/PTFE to that of the Nafion/PTFEmembrane under the conditions of both humidification and non-humidification.To improve the proton conductivity and water uptake of the membrane, compositemembranes were prepared by either introducing phosphorus tungsten acid（PWA/Nafion/PTFE） or simultaneously doping PWA/SiO₂of （PWA/SiO₂/Nafion/PTFE）.sandwiched PWA/Nafion/PTFE membrane was also prepared aiming for improvedperformances. It was found that the cell performance and proton conductivity ofPWA/Nafion/PTFE was superior to that of Nafion/PTFE under non-humidification condition.Because of the duel functions of phosphorus tungsten acid and SiO₂, the cellperformance/stability and non-humidification ability of PWA/SiO₂/Nafion/PTFE membranewere the best amongst all the membranes fabricated in this thesis. Although the sandwichedPWA/Nafion/PTFE can effectively prevent phosphorus tungsten acid from leaching in thecomposite membrane, the cell performance with this type of membrane was not satisfactoryand further improvements are definitely needed for desirable performances.