Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.

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Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.

In this paper we use continuum modeling to analyze the mechanism of the oxygen reduction reaction at a porous mixed-conducting oxygen electrode. We show that for La0 5(Ca, Sr)04Fe08Co0203,, at 700°C, solid-state oxygen diffusion and 02 surface exchange dominate the electrochemical behavior, producing effective "chemical" resistances and capacitances. This behavior can be explained both qualitatively and quantitatively in terms of the known bulk and surface properties of the materials. This mechanism appears to be generally valid for mixed conductors with high rates of internal mass transfer, but breaks down for mixed conductors that have poor ionic transport. Our analysis also suggests that, for the best electrode materials, extension of the reaction zone beyond the three-phase boundary is limited to a few micrometers. We also show that gas phase diffusion resistance can contribute significantly to cell impedance at P0 0.1 atm. InfroductionElectrode reactions in solid-state electrochemical systems involve a complex interaction of mobile electronic, ionic, and molecular species. One example that has drawn

▪ Abstract  After a brief survey of fuel cell types, attention is focused on material requirements for SOFC and PEMFC stacks, with an introductory section on materials technology for reformers. Materials cost and processing, together with durability issues, are emphasized as these now dominate materials selection processes for prototype stack units. In addition to optimizing the cell components, increasing attention is being given to the composition and processing of the bipolar plate component as the weight and volume of the relevant material has a major influence on the overall power density and cost of the fuel cell stack. It is concluded that the introduction of alternative materials/processes that would enable PEMFC stacks to operate at 150–200°C, and IT-SOFC stacks to operate at 500–700°C, would have a major impact on the successful commercialization of fuel cell technology.

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