Page 129 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

106 High Temperature SoZid Oxide Fuel Cells: Fundamentals, Design and Applications

monotonically with increasing content of Co. In particular, the OCV drop was
significant for Co concentrations above 10 mol%. This was caused by the onset of
hole conduction. But power density increased with Co concentration and
attained a maximum value at 8.5 mol% Co. This was explained by the improved
ionic conductivity of the doped electrolyte. However, current leakage became
dominant at higher Co levels. When the electrolyte thickness was reduced, the
power density further increased and at 180 pm thickness the maximum power
density was 1.58 W/cm2 at 800°C and 0.5 W/cm2 at 600°C. Larger cells of 150
mm diameter using La~.~Sro.2Gao.sMgo.15C00.0~03 electrolyte have also been
investigated recently 11921.

4.7 Oxides with Other Structures

4.7.1 Brownmillerites (e.g. BazlnzOd
Anther perovskite-related structure, which is interesting from the viewpoint of
oxide ion conduction, is brownmillerite with a general formula of A2B’B1‘05 or
A2B205. This structure can be viewed as a perovskite with oxygen vacancies
ordered along the [loll direction in alternate layers. Such vacancy ordering
results in an increased unit cell relative to the perovskite. In other words, Iattice
parameters of the a and c axis of the ideal brownmillerite oxide are larger than
those of the ideal perovskite oxide by 2 and the b axis of brownmillerite is the
same as that of perovskite. In some cases the oxygen vacancies do not order,
which results in a perovskite structure with a statistical distribution of oxygen
vacancies on the oxygen sites. Therefore, high oxide ion conductivity is also
expected in brownmillerites.
Goodenough et al. [93] have reported the high oxide ion conductivity in
several brownmillerite oxides. A listing [94] is given in Table 4.5. All these
brownmillerites exhibit oxide ion conductivity and the conductivities are rather

Table 4.5 Oxide ion conductivity for selected brownmillerite compounds [94]
Compound T(g) o(S/cm) Compound T(K) o(s/cm)
Ba21n205 973 5x10-3 Ba31n2Hf08 673 1.0~10-3
1223 1x10-I Sr31n2Hf08 973 1x10-4
BaZr03 973 1x10-6 Ba3SczZrOs 973 7x10-3
BaZr0.5In0.~02.7~ 973 1x10-2 Ba2GdIno.8Gao.205 873 5 x low3
Ba3h2ZrOg 973 5~10-~ Ba2GdIno.6Gao,405 873 5 x
Ba3In1.7Zrl ,308.15 973 5x10-2 Ca2Cr205 973 ~xIO-~
Ba2In1.33Zr0.6705.33 973 1x10-3 Sr2ScAI05 973 1x10-5
Ba21nl.7~Ceo.250j.125 973 9 x Sr~Sc1.3A10.705 973 1x10-3
1223 6~10-~ Sr2S~A10.8Mg0.204.9 973 5 x
BasIn2TiOs 973 7x10-4 Sr2SCA10.gZn0.204.9 973 2 x
Ba31n2ZrOs 673 6.8 x SrzSco.sYo.2A105 973 1x10-4
Ba&&Og 673 1.5 x Srl.8Bao,2ScA10j 973 1x10-4