Developing alternative polymeric membranes for fuel cells

Fuel cells are electrochemical devices that convert chemical energy to electrical energy. They are attractive alternative power sources because they have a higher efficiency than diesel or gas engines, they can operate silently and they can eliminate pollution caused by burning fossil fuels. One of the most important components of polymer electrolyte fuel cells and of direct methanol fuel cells (DMFCs) that use hydrogen and methanol as a fuel respectively are the polymer electrolyte membranes (PEMs). The PEM provides proton conduction and functions as a barrier to avoid direct contact between fuel and oxygen. High proton conductivity, low water or fuel permeability, thermal and mechanical stability as well as a low cost are desirable properties for fuel cell membranes. For DMFCs, the proton-conducting membranes must also exhibit low methanol permeability to minimize fuel crossover. In this study, polymer electrolyte mebranes based on sulfonated poly(aryl ether sulfone)s and their cross-linked derivatives were prepared and characterized for DMFC applications. The partially sulfonated poly(aryl ether sulfone)s (PESS) were prepared via polycondensation of hydroquinone 2- potassium sulfonate, bisphenol A and 4- fluorophenyl sulfone. The resulting polymers were then methacrylated with glycidyl methacrylate (PESSGMA) and then cross-linked and copolymerized with comonomers, vinyl phosphonic acid (VPA) and styrene (STY) via radical polymerization to improve mechanical properties and decrease methanol permeability. Cross-linked membranes of PESSGMA and its copolymers were prepared via solution casting method through optimization steps both in the synthesis of the PESSGMA pre-polymer and curing cycles. The crosslinking of the PESS polymer significiantly reduced ion exchange capacity, proton conductivity, swelling in water and methanol permeability of the membranes while increasing the modulus and the glass transition temperature. However the introduction of the VPA co-monomer to the PESSGMA network increased the proton conductivity while maintaining excellent resistance to methanol cross-over which was significantly higher as compared to both PESS and the commercial Nafion membranes.