Polymer Composites for High-Temperature Proton-Exchange Membrane Fuel Cells
Recent advances in composite proton-exchange membranes for fuel cell applications at elevated temperature and low relative humidity are briefly reviewed in this chapter. Although a majority of research has focused on new sulfonated hydrocarbon and fluorocarbon polymers and their blends to directly enhance high temperature performance, we emphasize on polymer/inorganic composite membranes with the aim of improving the mechanical strength, thermal stability, and proton conductivity, which depend on water retention at elevated temperature and low relative humidity conditions. The polymer systems include perfluoronated polymers such as Nafion, sulfonated poly(arylene ether)s, polybenzimidazoles (PBI)s, and many others. The inorganic proton conductors are silica, heteropolyacids (HPA)s, layered zirconium phosphates, and liquid phosphoric acid. Direct use of sol-gel silica requires pressurization of fuel cells to maintain 100% relative humidity for high proton conductivity above 100°C. Direct incorporation of HPAs such as phosphotungstic acid (PTA) into polyelectrolyte membranes is capable of improving both proton conductivity and fuel cell performance above 100°C; however, they tend to leach out of the membrane whenever fuel cell flooding happens. To prevent HPA leaching, amine-functionalized mesoporous silica is used to immobilize PTA in Nafion membranes, whose proton conductivity and fuel cell performance are discussed. Compared with Nafion, sulfonated poly(arylene ether)s such as sulfonated poly(arylene ether sulfone)s are cost-effective materials with excellent thermal and electrochemical stability. Their composites with HPAs show increased proton conductivity at elevated temperatures when fully hydrated. Organic/inorganic hybrid membranes from acid-doped PBIs and other polymers are also discussed.
KeywordsFuel Cell Proton Conductivity Arylene Ether Zirconium Phosphate Fuel Cell Performance
The authors appreciate the financial support of this work by U.S. Army Phase II portable fuel cell program through Connecticut Global Fuel Cell Center at the University of Connecticut. We also appreciate the partial support of this work from National Science Foundation of China (grant no. 50373005). The authors are grateful to Dr. Ruichun Jiang (Chemical Engineering Department at University of Connecticut) for the assistance in the fuel cell performance tests and cyclic voltametry study. Helpful discussions with Prof. James M. Fenton (Central Florida University) and Prof. H. Russell Kunz (University of Connecticut) are also acknowledged.
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