A Study On The Structural Control Of Proton Exchange Membranes In PEM Fuel Cell

Proton exchange membrane fuel cells (PEMFC) have an expansive developingforeground,since they offer numerous benefits, including high efficiency,environmental protection and high power density. The performance of PEMFC isgreatly dependent on the proton conductivity of the membranes.However, the protonconductivity of membranes widely used in PEMFC is undesirable, especially inconditions of high temperature and low humidity. In this study, a new attempt is toimprove the water content and the proton conductivity of Nafion and solfonated poly（ether ether ketone）(SPEEK) membranes.For the first time, membrane casting solution of Nafion dispersed in the mixture ofsolvent dimethyl formamide (DMF) and nonsolvent tetrachloroethylene (TCE) is usedto prepare Nafion electrolyte membranes. The microstructure,the proton conductivity,the water uptake and the water permeability of the membranes are measured andcompared with those of membrane cast from Nafion/DMF solution.Both the size ofthe hydrophilic channels and aggregation are increased. The proton conductivity andwater permeability are increased by around20%together while these of the SPEEKmembranes prepared from SPEEK/DMF/TCE(or SPEEK/DMF/para-xylene(PX))solutions are increased by around40%and30%, respectively.The structure of the membranes is controlled via the application of analternating-current field (AC field) during solvent evaporation.The perfluorocarbonpolymers aggregate and entangle to form fibril-like structures which are aligned alongthe electric field. The ionic pedant side chains locate around the surface of fibril-likestructures.The field-induced strucure caused the membranes to display an enhancedwater uptake, proton conductivity and water permeability in comparison to solventcast membranes prepared without an applied field.A series of Nafion membranes areprepared under eletric field with frequencies of0.1Hz,1Hz,10Hz and100Hz.1Hzturns out to be the best choice. Membranes prepared under1Hz possess the highesttrans-plane conductivity. The real-time resistance measurement cell is used tomeasure the in-plane resistance of the casting solution in the membrane castingprocess under electric filed. However, the electric field has nothing to do with thehomogeneous solution.Light scattering technique and particle size test are adopted to characterize the critical point of phase seperation and the particles size in the casting solution,respectively. The effect of nonsolvent on the molecular conformation of Nafion in thesolution is studied through intrinsic viscosity analysis method.The phase-seperation ofNafion/DMF/TCE system happens when TCE content is up to55%. With the additionof TCE, the particle size first decreases and then increases while the intrinsic viscosityof Nafion in the mixed solvents decreased merely.