Solid electrolyte, method of producing same and fuel cell...

Details Solid electrolyte, method of producing same and fuel cell... Solid electrolyte, method of producing same and fuel cell...

2000-08-22

2002-09-10

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C429S304000, C252S062200

Reexamination Certificate

active

06447944

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to improvements in a solid electrolyte, in a method of producing the solid electrolyte and in a fuel cell using the solid electrolyte, and more particularly to the solid electrolyte which is active to maintain high ionic conductivity at low temperatures and stabilized in ionic conductivity, the method of producing the solid electrolyte and the fuel cell using the solid electrolyte.
Recently, researches and developments have been positively proceeded on solid electrolytes because the solid electrolytes are safe from the viewpoint of liquid leak and specified ions being conducted, so that they are very effective as electronic materials of a variety of devices such as cells and gas sensors. Particularly, developments have been proceeded on ceramic solid electrolyte fuel cells called SOFC (solid oxide fuel cell). A fuel cell having a zirconia-based ceramic solid electrolyte has made an operational achievement in which power generation of several kW is maintained for several thousands hours. It is supposed that the SOFC is operated at high temperatures higher than 1000° C., and therefore hydrocarbon fuels can be reformed inside the fuel cell (accomplishing so-called internal reforming) thereby obtaining a high combustion or conversion efficiency higher than 60%.
In general, the SOFC is composed of a solid electrolyte, an anode and a cathode. All such materials are required to be stable in oxidizing and reducing atmosphere, to have suitable ionic conductivity, and to have their thermal expansion coefficients close to each other. Additionally, the materials of the anode and the cathode are required to be so porous that gas is permeable. Further, the materials of the SOFC are desired to be high in strength and stiffness, to be inexpensive, to be operable at temperatures as low as possible (as basic requirements for electric conductive materials) from the viewpoint of safe during operation of the SOFC.
Presently, stabilized ZrO
2
is in the mainstream of the materials of the solid electrolytes, in which oxide of bivalent alkaline earth metal such as CaO, MgO, Sc
2
O
3
or rare earth oxide such as Y
2
O
3
are used as a stabilizer. ZrO
2
doped with CaO (oxide of alkaline earth metal) exhibits an ionic conduction characteristic value of 0.01 (&OHgr;cm)
−1
. Additionally, H. Tannenberger et al has reported in “Proc. Int'l Etude Piles Combust, 19-26 (1965)” that the ionic conductivity of ZrO
2
doped with one of Y
2
O
3
, Yb
2
O
3
, and Gd
2
O
3
is around a range of from 1×10
−1
to 1×10
−2
S/cm at 800° C., and decreases to a value lower than 2×10
−2
S/cm when temperature is below 650° C.
Concerning zirconia stabilized by rare earth and alkaline earth compounds, they are disclosed in Japanese Patent Publication No. 57-50748 and Patent Publication No. 57-50749.
Additionally, stabilized bismuth oxide is also used as solid electrolyte. A high temperature phase (&dgr; phase) of Bi
2
O
3
has a deficiency fluorite structure (Bi
4
O
6
□
2
where □ is vacancy) and low in activation energy for oxide ion movement thereby exhibiting a high oxide ion conductivity. The high temperature phase is stabilized also in a low temperature region by forming solid solution of rare earth oxide, thus exhibiting a high oxygen ion conductivity. T. Takahashi et al reports in “J. Appl. Electrochemistry, 5(3), 187-195(1975)” that bismuth oxide stabilized by oxide of rare earth element, for example, (Bi
2
O
3
)
1−x
(Y
2
O
3
)
x
exhibits ionic conductivity characteristics of 0.1 (&OHgr;cm)
−1
at 700° C., 0.01 (&OHgr;cm)
−1
at 500° C. which are higher 10 to 100 times than stabilized zirconia.
Japanese Patent Publication No. 62-45191 recites that a mixture of stabilized bismuth and stabilized zirconium oxide exhibits an ionic conductivity of 0.1 (&OHgr;cm)
−1
at 700° C. Accordingly, it may be expected that a high ionic conductivity can be obtained in a temperature region lower than 1000° C. However, bismuth oxide is reduced into bismuth in metal state under a reduction atmosphere thereby exhibiting electronic conductivity, and therefore it is difficult to directly use the mixture as solid electrolyte.
Additionally, ceria-based solid solution is also used as solid electrolyte. CeO
2
has a fluorite-type cubic structure in a temperature ranging from loom temperature to melting point. Kudo and H. Obayashi et al reports in “J. Electrochem., Soc., 123[3] 416-419, (1976)” that solid solution is formed in a wide temperature region by adding rare earth oxide or CaO to CeO
2
.
CeO
2
—Gd
2
O
3
-based solid electrolyte which is in the main stream compound of recent researches and developments is represented by Ce
1−x
Gd
x
O
2−x/2
in which vacancy of oxygen is formed. In such compound, the valency of Ce is changed and therefore cerium oxide is reduced into cerium in metal state under a reduction atmosphere similarly to bismuth oxide, thereby exhibiting electronic conductivity. Accordingly, it is difficult to directly use such compound as solid electrolyte.
As other materials usable in a low temperature region, attention has been paid on research and development of perovskite compound. This compound is composed of ABO
3
having two ions (A and B) and has such examples as BaCe
0.9
Gd
0.1
O
3
, La
0.8
Sr
0.1
Ga
0.8
Mg
0.2
O
3
, CaAl
0.7
TiO
3
and SrZr
0.9
Sc
0.1
O
3
. Additionally, La
1−x
Srx Ga
1−y
Mg
y
O
3
is reported by T. Ishihara et al reports in “J. Am. Chem. soc., 116, 3801-03 (1994) and by M. Feng and J. B. Goodenough reports in “Eur. J. Solid. State Inorg. Chem. t31, 663-672 (1994)”.
SUMMARY OF THE INVENTION
However, such zirconia is low in ionic conductivity in a low temperature region, and electronic conductivity of bismuth oxide and ceria is in the reduction atmosphere. Accordingly, they are not suitable for solid electrolyte of fuel cell in a low temperature region. Additionally, although perovskite compound is high in ionic conductivity in a low temperature region as compared with other compounds, it is lowered in oxygen ion conductivity in such a low temperature region under Hall effect.
In the above-discussed fuel cells, power output of a single cell is limited to about 1V, and therefore it is required to obtain a high power output that a fuel cell takes a laminated structure including a plurality of single cells. Such a ceramic fuel cell having the laminated structure becomes large-sized, which makes it difficult to select structures (for example, tube-type or plate-type) of parts and to produce a large-sized fuel cell. A container such as a combustor main body of such a large-sized ceramic fuel cell requires to effectively use metal parts formed of ferrite stainless steel or the like from the economical view points. In order to effectively use metal, the fuel cell requires stabilized solid electrolyte materials which are active throughout a wide temperature region, for example, in a low temperature region (600 to 800° C.) so as to have an ionic conductivity generally equal to that in a high temperature region higher than 1000° C.
Additionally, solid electrolyte has crystal which is liable to break at temperatures around 650 ° C. Accordingly, it has been required to establish a technique for stabilizing crystal phase of solid electrolyte in a wide temperature region and to prevent solid electrolyte from lowering in strength at high temperatures. In this regard, Japanese Patent Provisional Publication No. 5-225820 discloses that AlO
3
is added for the purpose of stabilizing crystal structure of the solid electrolyte.
It is, therefore, an object of the present invention to provide an improved solid electrolyte and an improved method of producing the solid electrolyte, which can overcome drawbacks encountered in conventional techniques in connection with solid electrolyte.
Another object of the present invention is to provide an improved solid electrolyte which is sufficiently active throughout a wide temperature region including a relatively

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