Gallate based complex oxide electrolyte material

Details Gallate based complex oxide electrolyte material Gallate based complex oxide electrolyte material

2001-08-28

2004-10-12

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C429S006000, C501S152000, C252S062200, C252S062570, C252S521100

Reexamination Certificate

active

06803140

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gallate based complex oxide solid electrolyte material, a method of manufacturing the same and a solid oxide fuel cell.
2. Description of the Related Art
A solid oxide fuel cell (SOFC) has been constantly improved since Baur and Preis drove the SOFC at 1000° C. in 1937 after Nernst discovered a solid electrolyte (SE) in 1899. Currently, a zirconia ceramic cell with a capacity of several kilowatts shows a driving performance of several thousand hours. Because the SOFC is usually driven at high temperature of 1000° C. or higher, a hydrocarbon based fuel gas can be subjected to internal reforming in the cell, and that a high combustion efficiency of 60% or higher can be thereby obtained.
Typically, the SOFC is composed of a solid electrolyte and a pair of electrodes formed on both surfaces of the solid electrolyte. The electrodes are porous bodies. On the surface of one electrode, a gas containing oxygen is supplied, and on the surface of the other electrode, a gas containing hydrogen is supplied. The oxygen supplied to one electrode migrates via the solid electrolyte as oxide ions and reacts with a hydrogen component on the other electrode side to generate an electric charge and water.
Constituent materials of the SOFC must be stable in an oxidation/reduction atmosphere. In addition, since the SOFC is operated at high temperature, thermal expansion coefficients of the constituent components must be approximate from one to another, and the constituent components must be very strong and toughness. Moreover, high conductivity is required for the electrodes, and selectively high oxide-ion conductivity is required for the solid electrolyte.
Currently, as a solid electrolyte, stabilized zirconia (ZrO2) is mainly used. As a stabilizer for the zirconia, oxide of two-valence alkaline earth metal such as CaO, MgO and Sc
2
O
3
, rare earth oxide such as Y
2
O
3
and the like are used. Ion conductivity of ZrO
2
doped with CaO as alkaline earth metal is 0.01 (&OHgr;cm)
−1
at 800° C. In addition, ion conductivity of ZrO
2
doped with rare earth oxide, for example, Y
2
0
3
, Yb
2
O
3
, Gd
2
O
3
is about 1×10
−1
to 1×10
−2
(&OHgr;cm)
−1
at 800° C. However in this case, when the temperature is 650° C. or lower, the ion conductivity becomes 2×10
−2
(&OHgr;cm)
−1
or lower.
The stabilized zirconia added with single rare earth has been publicly known since 1970. The stabilized zirconia added with the rare earth and the alkaline earth is disclosed in Japanese Laid-Open Patent Publications Sho 57-50748 (published in 1982) and Sho 57-50749 (published in 1982).
Besides the above, as a solid electrolyte material, stabilized bismuth oxide is also used. A high temperature phase (&dgr; phase) of the bismuth oxide (Bi
2
O
3
) has a deficient fluorite structure, and exhibits a low activation energy for the migration of the oxide ions, but exhibits high oxide-ion conductivity. The high temperature phase of the bismuth can be stabilized to low temperature by dissolving the rare earth oxide thereinto, and exhibits high oxide-ion conductivity. In J. Appl. Electrochemistry, 5(3), pp. 187-195 (1975) by T. Takahashi, et al., described is that the oxide-ion conductivity of the rare metal stabilized bismuth, for example, (Bi
2
O
3
)
1-X
(Y
2
O
3
)
X
, is 0.1 (&OHgr;cm)
−1
at 700° C. and 0.01 (&OHgr;cm)
−
at 500° C., which is 10 to 100 times as high as that of the stabilized zirconia.
In Japanese Patent Publication Sho 62-45191(published in 1987), disclosed is that a mixture of the stabilized bismuth oxide and the stabilized zirconia has an oxideion conductivity of 0.1 (&OHgr;cm)
−1
or higher at 700° C. In this case, it can be expected that high ion conductivity is obtained in a temperature range lower than 1000° C. However, since the mixture is reduced and Bi metal is deposited in a reduction atmosphere. This Bi metal deposition exhibits electronic conductivity, thus making it difficult to use the mixture as a solid electrolyte.
As another solid electrolyte, there is a ceria based solid solution. Ceria (CeO
2
) has a fluorite cubic structure in a temperature range from room temperature to its melting point. When rare metal or CaO is added to the oxide, a solid solution is formed in a wide temperature range. This ceria based solid solution has been reported by Kudo, Obayashi, et al (J. Electrochem., Soc., 123[3] pp. 416-419, (1976)). With regard to CeO
2
-Gd
2
O
3
based solid solution, which is a topic compound in the recent research, a structure thereof is represented as Cel
1-X
Gd
X
O
2-X/2
, where oxide vacancies are formed. Since the valence of Ce is varied in this CeO
2
-Gd
2
O
3
based solid solution, the solid solution is reduced to Ce metal in a reduction atmosphere similarly to the bismuth base, and exhibits the electronic conductivity. Accordingly, it is difficult to use the solid solution as a solid electrolyte.
As still another solid electrolyte material usable at low temperature, there is a perovskite compound, on which research and development have been conducted. The perovskite compound is typically represented by a chemical formula ABO3, which includes, for example, Ba(Ce
0.9
Gd
0.1
)O
3
, (La
0.9
Sr
0.1
)(Ga
0.8
Mg
0.2
)O
3
, (Ca
0.9
Al
0.1
)TlO
3
, Sr(Zr
0.9
Sc
0.1
)O
3
and the like. Moreover, with regard to the (La
1-X
Sr
X
) (Ga
1-y
Mg
y
)O
3
based perovskite compounds have been reported in J. Am. Chem. soc., 116, pp. 3801-3803 (1994) by T. Ishihara, et al. and Eur. J. Solid State Inorg. Chem. t. 31, pp. 663-672 (1994) by M. Feng and J. B. Goodenough. Each of the compounds is expected to exhibit high oxide-ion conductivity in the oxidation-reduction atmosphere at low temperature.
SUMMARY OF THE INVENTION
Since an output of a single cell is just few volts, the conventional cell must be constructed in a laminated structure in order to obtain a high voltage. The laminated ceramic cell thus constructed becomes large in size, thus making a system designing difficult. Therefore it is desire to use a metal part such as ferrite based stainless steel for a vessel of a combustor body.
Accordingly, it is required to develop a solid oxide fuel cell operatable at low temperature of about 600 to 700° C. at which a stainless steel material can be used. Also for the solid electrolyte material, selective high oxide-ion conductivity at low temperature is desired.
For example, the zirconia based solid electrolyte material conventionally used as a main solid electrolyte exhibits low oxide-ion conductivity at low temperature. Meanwhile, the bismuth or ceria based solid electrolyte material is apt to be reduced, and the electronic conductivity thereof is increased by the reduction. Therefore, both of the materials are not suitable for the solid electrolyte for the fuel cell.
Meanwhile, the gallate based perovskite compound material exhibits superior oxide-ion conductivity at low temperature as compared with the other compounds. However at low temperature, the electronic conductivity is increased as well as the oxide-ion conductivity, leading to the exhibition of the mixed electric conductivity. Therefore, there occurs a problem that a ratio of relative oxide-ion conduction, that is, a transport number, is lowered.
Accordingly, an object of the present invention is to provide a solid electrolyte material in which oxide-ion conductivity is stable and high even at low temperature, particularly to provide a gallate based complex oxide material that has the stable and high oxide-ion conductivity at low temperature.
Another object of the present invention is to provide a method of manufacturing the gallate based complex oxide material.
Still another object of the present invention is to provide a solid oxide fuel cell operatable at low temperature.
A first aspect of the present invention provides a solid electrolyte material that comprises an A site-deficient complex oxide represented by a chemical formula A
1
.
&agr;
BO
3-&dgr;
, in which th

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