Electrolytes for Solid-Oxide Fuel Cells
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Electrolytes for SolidOxide Fuel Cells
Harumi Yokokawa, Natsuko Sakai, Teruhisa Horita, Katsuhiko Yamaji, and M.E. Brito Abstract Three solid-oxide fuel cell (SOFC) electrolytes, yttria-stabilized zirconia (YSZ), rareearth–doped ceria (REDC), and lanthanum strontium gallium magnesium oxide (LSGM), are reviewed on their electrical properties, materials compatibility, and mass transport properties in relation to their use in SOFCs. For the fluorite-type oxides (zirconia and ceria), electrical properties and thermodynamic stability are discussed in relation to their valence stability and the size of the host and dopant ions. Materials compatibility with electrodes is examined in terms of physicochemical features and their relationship to the electrochemical reactions. The application of secondary ion mass spectrometry (SIMS) to detect interface reactivity is demonstrated. The usefulness of doped ceria is discussed as an interlayer to prevent chemical reactions at the electrode–electrolyte interfaces and also as an oxide component in Ni–cermet anodes to avoid carbon deposition on nickel surfaces. Finally, the importance of cation diffusivity in LSGM is discussed, with an emphasis on the grain-boundary effects. Keywords: conductivity, diffusion, electrolytes, materials compatibility, solid-oxide fuel cells.
Introduction The most important breakthrough in solid-oxide fuel cells (SOFCs) was made by researchers at the Westinghouse Electric Company in the mid-1980s.1 Optimization of materials led to yttria-stabilized zirconia (YSZ) as the favored electrolyte. The most important property of a SOFC electrolyte is its oxide ionic conductivity. Although rare-earth–doped ceria (REDC) exhibits higher oxide ion conductivity than YSZ, doped ceria is less stable. In 1994, a new electrolyte, LaGaO3-based perovskite, was invented. Extensive investigation revealed that proper functioning of this electrolyte depends upon its interaction with other components in the fuel cell. Only a few materials have survived as practical SOFC electrolytes because many electrical, electrochemical, mechanical, and chemical properties are required of a successful electrolyte. In this overview, we make an attempt to clarify how those different properties are closely related to each other. Particular emphases will be placed on materials compatibility and
MRS BULLETIN • VOLUME 30 • AUGUST 2005
εelectrolyte
J extV term , JO V TH
(1)
2
where V term and V TH are the terminal voltage and the thermodynamically derived voltage, respectively, and Jext and JO 2– are the external current and the oxide ion flux through the electrolyte, respectively. Here, electrode effects such as the electrode internal resistivity and the electrode reaction resistivity are not included. A low value of (O 2–) lowers V term (corresponding to the Joule loss); a high value of el lowers the Jext/J(O2–) ratio and also the V term (this is called the loss due to electronic shorting or oxygen permeation). Both terms lead to lowering of ε electrolyte. Among three materials, YSZ [
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