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Mixed Oxygen and Electron Conduction in Transition Metal Perovskites

A serious concern with present designs of solid oxide fuel cells is the need for "triple-point junctions," points at which the cathode, electrolyte, and oxidizing gas are in simultaneous contact. Only at these junctions can the cathode catalyze the reduction of oxygen into O= ions and initiate their subsequent transport through the electrolyte. Extended operation at elevated temperatures can lead to sintering of the cathode and the loss of a large fraction of the triple-point junctions. One potential solution involves the use of cathodes with enhanced ionic conductivity, otherwise known as mixed conductors. Because of their ability to transport O= ions, such materials may effectively increase the surface area available for reduction to take place and thus relax the triple-point constraint. To this end, we are examining the electrical and structural properties of LaCo1-xMgxO3-a materials within the range 0.05 x 0.2 prepared by co-precipitated chemical methods. Magnesium has been selected as the dopant in order to enhance the concentration of oxygen vacancies. Our measurements to date (using electron blocking methods) confirm that oxygen conduction occurs via a vacancy mechanism, as observed in similar compounds. A "knee" in the conductivity data, combined with transmission electron micrographs that show twinned crystals, suggest a phase transition at approximately 525 C.

Oxygen Ion Conducting Defect Fluorites

Because of their role as the electrolyte in solid oxide fuel cells, rare-earth or yttrium doped zirconias are technologically important oxygen ion conductors. The dopant in these materials serves not only to stabilize the cubic, fluorite phase, but also to introduce anion defects that presumably increase the ionic conductivity. As the concentration of the dopant is increased, the structure may transform from a defect fluorite to an ordered pyrochlore, which, in turn reverts to the fluorite structure at elevated temperatures. This high-temperature order-disorder transition is of interest because unlike similar transitions in metals and other oxides, two species (cations and anions) are involved in the transition, and furthermore, in the case of Gd2Zr2O7 higher conductivity is observed in the ordered phase than in the disordered. In order to determine the nature of the transition and to address the fundamental questions of whether and how disorder on the cation and anion arrays is coupled during the transformation process and which of these has the greater influence on the oxygen ion conductivity, we have examined the transformation kinetics of Gd2Zr2O7 by in situ high-temperature X-ray diffraction and in situ impedance spectroscopy. To date, our results suggest that the transition occurs by a continuous process, rather than one involving nucleation and growth of the ordered pyrochlore phase.

 

Selected Publications

  • S.M. Haile and S. Meilicke, "The Kinetics of Ordering in Gd2Zr2O7: An Unusual Oxygen Ion Conductor," Mat. Res. Soc. Symp. Proc., 398 (1996) 599-604.
  • S. Meilicke and S.M. Haile, "Order-Disorder Transitions in Gadolinium Zirconate: A Potential Electrolyte in Solid Oxide Fuel Cells," Mat. Res. Soc. Symp. Proc., 393 (1995) 55-60.
  • A. Yu and S.M. Haile "Ionic Conductivity in LaCo1-xMgxO3-d: A Potential Cathode Material for Solid Oxide Fuel Cell Applications," Mat. Res. Soc. Symp. Proc., 393 (1995) 43-48.
  • S.M. Haile, E. Prince and B.J. Wuensch, "Neutron Rietveld Analysis of Anion and Cation Disorder in the Fast-Ion Conducting Pyrochlore System Y2(ZrxTi1-x)O7," Mat. Res. Soc. Symp. Proc. 166 (1989) 81-86.

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