Page 135 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

112 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

possible fast proton conductors [ 1081. Although detailed measurements of
proton conduction in these oxides have not been reported, proton conductivity
up to 8.7 x S/cm at 550°C has been estimated from the electrical resistance
in power density curves. But improvement of chemical stability is
the key requirement for application of all proton-conducting ceramics as
SOFC electrolytes.

4.9 Summary
During the past decades, many oxide formulations have been extensively
examined in the search for candidate SOFC electrolyte materials. Zirconia-based
compositions are still the best electrolytes at present owing to their good stability
under reducing atmospheres, low electronic conductivity, and acceptabIe oxide
ion conductivity above 800°C. The recent trend of SOFC development is to
operate at lower temperatures. The lowest operation temperature limit of the
cell, for thin YSZ electrolytes, is estimated to be about 700°C from YSZ
conductivity and mechanical property data. Scandia-doped zirconia, which
shows a higher conductivity than that of YSZ, could be preferred at temperatures
below 700°C, if the cost of scandia was acceptable.
Ceria-based electrolytes could be used at 5 50°C or less. To operate at higher
temperatures, a dual layer electrolyte, with a thin YSZ layer on CGO, has been
proposed to avoid the electronic current leakage. The interdiffusion issues at the
interface are important in this case for long-life electrolytes.
Another possibility is to use perovskite compositions. The most promising
candidate at this time is LaGa03 doped with Sr and Mg. Other possible
perovskites are Baz In205 doped with Ce or La. Other ionic conducting oxides
have also been found including Lal&,026 composition. Proton conductors such
as SrCe03, SrZr03 or BaCe03 doped with Y or Yb may also be effective
electrolytes, but reaction with C02 has to be resolved first.

References

[l] W. Nernst, Z. Elektrochem., 6 (1899) 41.
[2] E. Baur andH. Preis, Z. Electrochem., 43 (193 7) 72 7.
[3] T. H. Etsell and S. N. Flengas, Chem. Rev., 70 (1970) 339.
[4] N. Q. Minh and T. Takahashi, in Science and Technologg of Ceramic Fuel
CeZls, Elsevier, Amsterdam (1995).
[5] T. Takahashi, T. Esaka and H. Iwahara, I. AppI. Electrochem. 7 (1977)
299.
[6] B. C. H. Steele, in High Conductivitg Solid Ionic Conductors, ed. T.
Takahashi, World Scientific, Singapore (1 9 89).
[7] E.ICochandC. Wagner,Z. Phys. Chem.,B38 (1937)295.