Effect of Er, Gd, and Nd Co-Dopants on the Properties of Sm-Doped Ceria Electrolyte for IT-SOFC
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TRODUCTION
THE solid oxide fuel cell (SOFC) is one of the most attractive energy conversion devices because of its high efficiency, flexibility of fuel choice, and environmental friendliness. In general, conventional SOFC systems, which use yttria-stabilized zirconia (YSZ) as an electrolyte for their high conversion efficiency, are operated at approximately 1273 K (1000 C). However, at such high operating temperatures, there are some problems, such as interfacial reaction between the components, mechanical and thermal degradation, thermal expansion mismatch, and high cost of materials.[1–4] Therefore, it is necessary to lower [773 K to 1073 K (500 C to 800 C)] the operating temperature of the SOFCs. Ceria doped with a tri-valent cation, such as, Sm3+, Gd3+, and Nd3+, is a potential electrolyte for solid oxide fuel cell applications at temperatures below 1073 K (800 C) because of its appreciable oxygen ion conductivity above 873 K (600 C).[1,5–7] In particular, ceria doped with samarium oxide and gadolinium oxide was found to have the highest electrical conductivity at a fixed dopant level because the corresponding distortions of the ceria lattice are the smallest.[8] Kim[9] found that the expansion of the lattice deviation of the doped ceria from pure ceria would lead to an increase of the lattice strain of doped ceria, thereby resulting in an increase of the activation energy of
ALIYE ARABACI is with the Department of Metallurgical Engineering, Faculty of Engineering, Istanbul University, Avcilar 34320, Istanbul, Turkey. Contact e-mail: [email protected] Manuscript submitted December 15, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
conduction and a decrease of the ionic conductivity of the doped ceria. To further improve the ionic conductivity of ceria, some co-doped ceria-based electrolytes have been studied.[10–16] Yamamura and coworkers[10] found that co-doping might suppress the ordering of oxygen vacancies and therefore lower the activation energy of conduction and improve the ionic conductivity. Moreover, Wang and coworkers suggested that ceria co-doped with two elements has a higher electrical conductivity in comparison with single element-doped ceria.[12] Andersson et al.[13] reported that, among all rare earth-doped ceria materials, Nd3+ and Sm3+ co-doped ceria or Pr3+ and Gd3+ co-doped ceria should have the highest electrical conductivity. They applied quantum-mechanical first-principles methods to simulate the crystal structure and calculate the activation energy of oxygen vacancies within all types of rare earth-doped ceria materials. They reached a conclusion that the optimized dopants in ceria should have an effective atomic number between 61 (Pm) and 62 (Sm). Mori et al.[14] found that the ionic conductivity of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891 was higher than that of the single element-doped ceria. Similarly, Van Herle et al.[15] stated that co-doped ceria with three, five, or ten dopants showed a considerably higher conductivity in air than the best single element-doped material with the
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