A Study On Nafion Composite Membranes For High Temperature Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell (PEMFC) is a new kind of efficient andenvironment-friendly power technology, which is expected to be widely used invarious fields in the future. When running under high temperatures (>100C),PEMFC possesses faster electrode reaction kinetics, higher levels of CO tolerance andeasier heat and water management, therefore resolving the existing problems ofPEMFC when running under low temperatures. At present, the most commonly usedproton exchange membranes in the fuel cells are Nafion membranes, which areproduced by DuPont Company. However, the proton conductivity of the Nafionmembrane depends heavily on the water content of the membrane, and decreasesquickly under high temperatures due to the water loss. It is inevitable and necessary todevelop high-performance proton exchange membranes which are applicable in hightemperature PEMFC (HT-PEMFC). The water uptake and microstructure of themembrane have an effect on the proton conductivity. In order to increase the protonconductivity of Nafion membranes, we incorporate phosphonic acid functionalizedtitanate nanotubes into Nafion membranes, expecting that the nanotubes could be ableto rearrange the microstructure of Nafion membranes, therefore enhancing theperformance of membranes in high temperatures.More charges are generated on the surfaces of functionalized titanate nanotubesafter treated with ammonial solution, therefore the nanotubes could be steadilydispersed in the casting solution. The nanotubes are uniformly distributed in theprepared composite membranes. The microphase separation structures of compositemembranes are tuned by the functionalized titanate nanotubes which have largerspecific surface area and length diameter ratio. Through interacting with thephosphonic acid groups, the sulfonic ion clusters of Nafion are distributed along thesurfaces of nanotubes. As a result, the sizes of ion clusters increase to more than5nmfrom3.6nm, and longer and more continuous proton transport channels are formed inthe membranes.The functionalized titanate nanotubes also enhance the water uptake of thecomposite membranes, accelerating the formation of hydrogen bond network forproton transport. The ion exchange capacity (IEC) of composite membrane is alsoincreased from0.923mmol·g-1of a pure Nafion membrane to0.956mmol·g-1at most.At100C and100%relative humidity (RH), the proton conductivity of the composite membrane with1wt.%phosphorylated TiO2nanotubes reaches0.154Scm-1from0.139Scm-1of a pure Nafion membrane. The composite membrane still holds higherconductivity than the recast Nafion membrane when the temperature is above100C.The conductivity under high temperature and low humidity is also increased slightly,the performance of the Nafion membrane is enhanced at high temperature.