Perovskite-type oxide materials containing nickel and iron for air electrode and solid oxide fuel cell using the same

A solid oxide fuel cell, which converts a chemical reaction between a fuel and air or oxygen into electric energy, and which is constructed by a solid electrolyte, an air electrode mounted adjacent to the solid electrolyte, a fuel electrode, and an inter-connector for connecting with other cell units is provided. The air electrode of the above fuel cell is made of a perovskite-type oxide material with a composition expressed by LnNi.sub.1-x Fe.sub.x O.sub.3 and YNi.sub.1-x Fe.sub.x O.sub.3 and wherein Ln represents lanthanide elements and x is in a range of 0.30 to 0.60. The perovskite-type oxide materials containing nickel and iron satisfy requirements to provide higher electronic conductivity than the conventional material and closer thermal expansion coefficient to that of the solid electrolyte than that of the conventional air electrode material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to nickel-iron-type perovskite materials for an air 
electrode used for a solid oxide fuel cell, and particularly relates to 
the materials which are capable of improving the reliability of the solid 
oxide fuel cell and also capable of improving generation efficiency of 
electricity of this type of fuel cell. 
This application is based on Patent Application No. Hei 9-356061 filed in 
Japan, the contents of which are incorporated herein by reference. 
2. Background Art 
A fuel cell is a type of gas-electric cell capable of operating for long 
periods of time, by, on one hand, supplying oxygen or air to the cathode 
and supplying hydrogen or a hydrocarbon to the anode and, on the other 
hand, by continuously removing the reaction product (such as H.sub.2 O or 
CO.sub.2) from the fuel cell. In particular, from the point of view of 
effective utilization of energy, the fuel cell has a high conversion 
efficiency of energy, since the fuel cell is free from the thermodynamic 
restraints of the Carnot efficiency, so that the fuel cell is expected to 
be advantageous in environmental protection. 
Of the various types of fuel cells, recently, solid oxide type fuel cells 
have been investigated extensively and, in particular, the solid oxide 
cells using an ionic conductor of oxygen are attracting attentions. 
A tube-type cell unit as a representative example of the solid oxide fuel 
cell has the structure which is schematically shown in FIG. 1. This 
tubular cell unit, as shown in FIG. 1, is formed by a cylindrical porous 
substrate of the air electrode 1, a solid electrolyte 2 and a fuel 
electrode 3 disposed on the opposite sides of the air electrode 1, and an 
inter-connector for connecting cell units to each other in the fuel cell. 
This structure is advantageous in the ease of constructing a sturdy cell 
and attaining a gas-tight structure. However, a drawback is found that the 
length of the circuit of the electric current to flow is too long, causing 
energy loss. 
At present, YSZ (Yttrium Stabilized Zirconia) or SASZ (Scandium Aluminum 
Stabilized Zirconia) are the most promising materials as the solid 
electrolyte. Although many materials are examined for the air electrode, a 
manganese-type oxide with a perovskite type structure such as La.sub.0.8 
Sr.sub.0.2 MnO.sub.3 is now being investigated. However, due to its low 
electric conductivity, the resistance loss of energy of the above 
perovskite material degrades the power generation efficiency of the fuel 
cell. Thus, a material with high electronic conductivity is required. 
In general, it is necessary to operate a fuel cell at a temperature of 
1,000.degree. C. at present. This is because the fuel cell, composed of 
the air electrode, the fuel electrode, and the solid electrolyte, cannot 
generate sufficient power effectively at lower temperatures than 
1,000.degree. C. However, since high temperatures such as 1,000.degree. C. 
cause, for example, sintering of the fuel electrode, which degrades the 
power generation efficiency and which restricts the development of the 
fuel cell. Thus, it is desired to reduce the operation temperature of the 
fuel cell to 800.degree. C. 
In order to reduce the operation temperature of fuel cells, it is necessary 
to take many measures. An important point is to improve the electronic 
conductivity of the air electrode materials such as perovskite-type 
ceramics. 
Among perovskite-type ceramic materials, some example such as 
La(Sr)CoO.sub.3 is known as a conductive material with high electric 
conductivity. However, a problem arises that cracks may occur at an 
interface with the solid electrolyte, since this material has an higher 
expansion coefficient about two-times than that of the solid electrolyte 
of YSZ or SASZ. 
That is, the air electrode material is required to have an approximately 
equal thermal expansion coefficient to that of the solid electrolyte of 
YSZ or SASZ. This is required to avoid cracks in the electrolyte caused by 
a stress originated by mismatching of thermal expansion coefficients 
between the solid electrolyte and the air electrode in temperature cycles 
between room temperature and the operation temperature of 1,000.degree. C. 
As hereinabove described, there are two essential problems for the 
conventional tubular cell unit of the fuel cell, one of which is a problem 
concerning the reliability of the cell unit in term of cracking of the 
solid electrolyte, and another one of which is a problem concerning the 
low power generation efficiency caused by the low conductivity of the air 
electrode. 
Here, the thermal expansion coefficients of conventional materials used in 
a conventional solid oxide fuel cell are shown in Table 1 as a reference. 
TABLE 1 
______________________________________ 
Thermal expansion coefficient of materials used in 
a conventional solid oxide fuel cell 
Thermal expansion 
coefficients 
Materials (.times.10.sup.6) (1/K)* 
______________________________________ 
YSZ 10.0 
SASZ 10.0 
La.sub.0.8 Sr.sub.0.2 MnO.sub.3 
12.0 
YSZ-Ni Cermet (Ni: 60 mol %) 
13.0 
La.sub.0.8 Sr.sub.0.2 CrO.sub.3 
10.0 
______________________________________ 
*: average thermal expansion coefficients from 25 to 800.degree. C. 
As shown in Table 1, the thermal expansion coefficients of the dense solid 
electrolytes (YSZ and SASZ) and the inter-connector (La.sub.0.8 Sr.sub.0.2 
CrO.sub.3) are identical. In contrast, a conventional material for the 
fuel electrode of a cermet represented by Ni-YSZ cermet and the material 
for the air electrode expressed by La.sub.0.8 Sr.sub.0.2 MnO.sub.3 have 
higher thermal expansion coefficients by 20 to 30% than that of the 
electrolyte of YSZ or SASZ. A 20% to 30% differences in thermal expansion 
coefficients between the solid electrolyte and the materials for the fuel 
electrode and the material for the sir electrode might be allowable 
because the fuel electrode is formed in a porous layer which is capable of 
absorbing the difference of thermal expansion. However, in order to 
improve the reliability of the solid oxide fuel cell, a difference in 
thermal expansion coefficient between the solid electrolyte and the air 
electrode material is desired to be restricted within 10%, even though the 
air electrode is formed in a porous body. 
It is therefore an object of the present invention to provide a material 
which has a thermal expansion coefficient close to that of the solid 
electrolyte, and, at the same time, has a high level of electric 
conductivity so as not to degrade the energy generation efficiency by a 
high resistivity. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, the present invention 
provides a nickel iron perovskite-type material used for an air electrode 
of a solid oxide fuel cell, which converts the chemical reaction energy 
between a fuel and air or oxygen into electric energy, and which is 
constructed by a solid electrolyte, an air electrode mounted adjacent to 
the solid electrolyte, a fuel electrode, and an inter-connector for 
connecting cell units in the fuel cell to each other, wherein the 
perovskite material for the air electrode has a composition expressed by 
LnNi.sub.1-x Fe.sub.x O.sub.3 and YNi.sub.1-x Fe.sub.x O.sub.3, wherein Ln 
represents lanthanide elements and x is in a range of 0.30 to 0.60. 
The above mentioned perovskite-type material for the air electrode has 
higher electronic conductivity than the conventional material with a 
composition of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. At the same time, the air 
electrode materials with in the system expressed by LnNi.sub.1-x Fe.sub.x 
O.sub.3 (Ln: lanthanides, x=0.30 to 0.60) and YNi.sub.1-x Fe.sub.x O.sub.3 
(x=0.30 to 0.60) have approximately the same thermal expansion 
coefficients as the solid electrolyte. Therefore, this type of 
perovskite-type material satisfy the required properties in terms of 
higher electronic conductivity and closer thermal expansion coefficients 
to those of the solid electrolyte. 
According another aspect of the present invention, a nickel iron 
perovskite-type material for the air electrode of the solid fuel cell is 
provided with compositions expressed by LnNi.sub.1-x Fe.sub.x O.sub.3 and 
YNi.sub.1-x Fe.sub.x O.sub.3, wherein Ln represents lanthanide elements 
and x is within a range of 0.40 to 0.55. 
According to another aspect of the present invention, a nickel iron 
perovskite-type material for the air electrode wherein the lanthanide 
elements in the general formula of LnNi.sub.1-x Fe.sub.x O.sub.3 are at 
least one element selected from the group consisting of La, Pr, Nd, Sm, 
and Eu and more than two elements selected from the group consisting of 
La, Pr, Nd, Sm, Eu, and Ce. 
According to another aspect of the present invention, this invention 
provides a solid oxide fuel cell, which converts a chemical reaction 
between a fuel and air or oxygen into electric energy, and which is 
composed of a solid electrolyte, an air electrode mounted adjacent to the 
solid electrolyte, a fuel electrode, and an interconnector for connection 
with other cell units, wherein the solid oxide fuel cell is provided with 
an air electrode made of the perovskite material with a composition 
expressed by LnNi.sub.1-x Fe.sub.x O.sub.3 and YNi.sub.1-x Fe.sub.x 
O.sub.3 and wherein Ln represents lanthanide elements and x is in a range 
of 0.30 to 0.60. 
According to still another aspect of the present invention, this invention 
provides a solid oxide fuel cell comprising an air electrode made of a 
nickel iron perovskite-type material according to the preceding paragraph, 
wherein the nickel-iron perovskite material is provided with compositions 
expressed by LnNi.sub.1-x Fe.sub.x O.sub.3 and YNi.sub.1-x Fe.sub.x 
O.sub.3, wherein Ln represents lanthanide elements, and x is within a 
range of 0.40 to 0.55, and wherein lanthanide elements in said general 
formula of LnNi.sub.1-x Fe.sub.x O.sub.3 is at least one element selected 
from the group consisting of La, Pr, Nd, Sm, and Eu, and more than two 
elements selected from the group consisting of La, Pr, Nd, Sm, Eu, and Ce.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will be described below with reference to preferred 
embodiments and the attached drawings. It should be understood that the 
present invention is not limited to the described embodiments below. 
In general, a solid oxide fuel cell is constructed by a plurality of unit 
cells with cylindrical shapes, as depicted in FIG. 1. Air or oxygen (shown 
as O.sub.2 in FIG. 1), which is one of the reaction gases, is blown in the 
inside of an electrically conductive air electrode, and hydrogen or 
hydrocarbon (shown as H.sub.2 in FIG. 1), which is the other reaction gas, 
is blown around the outermost surface of a fuel electrode. As shown in 
FIG. 1, the solid oxide fuel cell is composed of a solid electrolyte, an 
air electrode, a fuel electrode, and an inter-connecter. The 
inter-connector 4 is used for connecting unit cells in the fuel cell to 
each other. 
The above unit cell of the fuel cell shown in FIG. 1 is not necessarily 
suitable for evaluating the characteristics of fuel cells by changing the 
air electrode materials. In the present invention, a simplified unit cell 
and the fuel cell simulation test equipment as illustrated in FIGS. 2 and 
3 are used for evaluation of the air electrode materials, and test results 
were confirmed by assembling practical unit cells as shown in FIG. 1. 
First Embodiment 
Various materials in the system of LaNi.sub.1-x Fe.sub.x O.sub.3 (x=0.3, 
0.4, 0.5, 0.55, 0.6) for the air electrode were tested using the 
simplified unit cell shown in FIGS. 2 and 3. FIG. 2 shows a model unit 
cell used in the tests, and FIG. 3 shows the fuel cell simulation test 
equipment for evaluating the materials for the air electrode of solid 
oxide fuel cells. The fuel cell simulation test equipment shown in FIG. 3 
was constructed by an air electrode 1, a solid electrolyte 2, a fuel 
electrode, a current collecting platinum mesh 5, a platinum electrode 6, 
and a gas seal 7. 
In the model unit cell, the air electrode and the fuel electrode were 
formed with a thickness of 0.2 mm, and the solid electrolyte was formed 
with a thickness of 0.2 mm and a diameter of 20 mm. SASZ (with a 
composition of 0.890ZrO.sub.2 -0.105ScO.sub.3 -0.005Al.sub.2 O.sub.3) was 
used for forming the solid electrolyte, and the fuel electrode was formed 
by Ni-YSZ (containing 60 mol % Ni). Various perovskite materials expressed 
by the general formula of La Ni.sub.1-x Fe.sub.x O.sub.3 were used for 
forming the air electrode. 
A method of forming model unit cells is described hereinafter. First, a 
green sheet of YSZ for the solid electrolyte 2 was formed by a 
doctor-blade method and the green sheet was sintered at 1400.degree. C. in 
air. On one surface of the plate of the sintered YSZ, the Ni-YSZ was 
coated and sintered at 1200.degree. C. to form the fuel electrode 3. On 
the opposite surface of the plate of the solid electrolyte, a material for 
the air electrode 1 was coated and sintered at 1,000.degree. C. That is, 
the air electrode 1 and the fuel electrode 3 were each formed on one of 
the two surfaces of the plate of the electrolyte 2 and both electrodes 
were then covered with platinum meshes 5 for collecting electric current. 
After connecting the platinum terminal 6 to each platinum mesh 5, terminal 
voltages were measured while flowing oxygen on the air electrode 1 and 
flowing hydrogen on the fuel electrode 3. 
The test results at 800.degree. C. obtained by applying various materials 
of the present embodiment to the air electrode 1 are shown in Table 2 
(Sample Nos.1 to 5). The terminal voltages shown in the Table 2 correspond 
to voltages measured at the time when the current density was 1.0 
A/cm.sup.2. Therefore, the higher the terminal voltage, the higher the 
power generation efficiency of the fuel cell. 
Thermal and electrical properties of the materials of this embodiment were 
measured using sintered blocks. These blocks were prepared by forming 
green bodies using a mixture of three types of powder of the present 
embodiment (LaO, NiO, and Fe.sub.2 O.sub.3), and the thus prepared green 
bodies are calcined and sintered at an temperature range from 1,250 to 
1,400.degree. C. to obtain the sintered blocks. 
The sintered blocks were then cut into sample bars to be used for 
measurement of the thermal expansion coefficient in a temperature range 
ranging from room temperature to 800.degree. C. The composition dependence 
of the thermal expansion coefficients of the materials in the system of 
LaNi.sub.1-x Fe.sub.x O.sub.3 are shown in Table 2. Here, the thermal 
expansion coefficients are the average thermal expansion in the 
temperature range of room temperature to 800.degree. C. 
The materials in the system of LaNi.sub.1-x Fe.sub.x O.sub.3 of the present 
embodiment had thermal expansion coefficients closer to the solid 
electrolyte than that of the conventional material of La.sub.0.8 
Sr.sub.0.2 MO.sub.3. That is, the thermal expansion coefficient of SASZ 
used for the solid electrolyte is 10.times.10.sup.-6, and that of the 
conventional material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3 was 
12.times.10.sup.-6, which is larger by 20% than that of the solid 
electrolyte. In contrast, differences in thermal expansion coefficients 
between the solid electrolyte and materials in the composition of 
LaNi.sub.1-x Fe.sub.x O.sub.3 (x=0.3 to 0.6) were within around 10%. When 
practical unit cells are assembled using these materials as the air 
electrode, damage caused by the difference in thermal expansion was not 
observed. From the point of view of the thermal expansion coefficient, 
more preferable materials are obtained in the composition of LaNi.sub.1-x 
Fe.sub.x O.sub.3, when x is in the range of 0.4 to 0.55. 
The measurement of the electric conductivity was conducted by the DC four 
terminal method for the same sample bars used for measurement of the 
thermal expansion coefficient, after attaching platinum terminals on both 
ends of the bar by baking. As an example, the temperature dependence of 
the electric conductivity is shown in FIG. 4 for two materials including 
the conventional material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3 and a 
material of LaNi.sub.0.6 Fe.sub.0.4 O.sub.3 of the present invention. FIG. 
4 clearly indicates the improved electrical conductivity of a material of 
the present embodiment, since the electronic conductivity of the material 
of this embodiment at an operating temperature of 800.degree. C. is 580 
Scm.sup.31 1, which is far higher than 150 Scm.sup.-1 of the conventional 
material. Due to this high electric conductivity, when the materials of 
the present embodiment are used for the air electrode of the solid oxide 
fuel cell, increased terminal voltages in a range of 0.24 to 0.28 volts 
are obtained as shown in Table 2, in contrast to the conventional terminal 
voltage of 0.20 volts. 
As described above, the materials for the air electrode in the system of 
LaNi.sub.1-x Fe.sub.x O.sub.3 show preferable in thermal expansion and the 
electric conductivity properties which lead to generation efficiency of 
the fuel cell, when x is in a range of 0.3 to 0.6. Furthermore, more 
desirable properties were obtained for materials in the system of 
LaNi.sub.1-x Fe.sub.x O.sub.3, when x is in the range of 0.4 to 0.55, as 
shown for Sample Nos. 2 to 4 in Table 2. 
Second Embodiment 
The same tests as for the first embodiment were conducted, using the same 
model unit cells, by replacing the air electrode material with five 
materials in the system of PrNi.sub.1-x Fe.sub.x O.sub.3 (where, x=0.3, 
0.4, 0.5, 0.55, and 0.6). The test results are shown in Table 2 (Sample 
Nos. 6 to 10). Every sample material of the present embodiment had a 
closer thermal expansion coefficient to that of the solid electrolyte than 
that of the conventional material of La.sub.0.8 Sr.sub.0.2 MO.sub.3. In 
addition, the terminal voltages (at the current density of 1.0 A/cm.sup.2) 
observed for samples 6 to 10 were higher than for the conventional 
material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3, as shown in Table 2. That is, 
all of sample materials Nos. 5 to 10 showed preferable properties for the 
air electrode. However, more preferable results were obtained for sample 
Nos. 7 to 9 in the system of PrNi.sub.1-x Fe.sub.x O.sub.3, when x is in 
the range of 0.4 to 0.55. 
Third Embodiment 
The same tests as for the first embodiment were conducted, using the same 
model unit cells, by replacing the air electrode material with five 
materials in the system of NdNi.sub.1-x Fe.sub.x O.sub.3 (where, x=0.3, 
0.4, 0.5, 0.55, and 0.6). The test results are shown in Table 2 (Sample 
Nos. 11 to 15). Every sample material of the present embodiment showed 
closer thermal expansion coefficient to the solid electrolyte than that of 
the conventional material of La.sub.0.8 Sr.sub.0.2 MO.sub.3. In addition, 
the terminal voltages (at the current density of 1.0 A/cm.sup.2) observed 
for samples from No. 11 to No. 15 were higher than for the conventional 
material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. That is, all of sample 
materials Nos. 11 to 15 showed preferable properties for the air 
electrode. However, more preferable results were obtained for sample Nos. 
12 to 14 in the system of NdNi.sub.1-x Fe.sub.x O.sub.3, when x is in the 
range of 0.4 to 0.6. 
TABLE 2 
______________________________________ 
Terminal voltages and thermal expansion coefficients 
of various materials according to respective embodiments 
*Thermal 
*Terminal expansion 
Sample Composition of voltage coefficient 
No. air-electrode materials 
(V) (.times. 10.sup.6, 1/K) 
______________________________________ 
Refer. La.sub.0.8 Sr.sub.0.2 MnO.sub.3 
0.20 12.0 
Sample 
1 LaNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.26 11.5 
2 LaNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.28 11.0 
3 LaNi.sub.0.5 Fe.sub.0.5 O.sub.3 
0.25 10.8 
4 LaNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.24 10.5 
5 LaNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.23 10.3 
6 PrNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.27 11.2 
7 PrNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.30 10.9 
8 PrNi.sub.0.5 Fe.sub.0.5 O.sub.3 
0.26 10.8 
9 PrNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.25 10.4 
10 PrNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.24 10.2 
11 NdNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.27 11.1 
12 NdNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.29 10.9 
13 NdNi.sub.0.5 Fe.sub.0.5 O.sub.3 
0.26 10.8 
14 NdNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.25 10.4 
15 NdNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.24 10.2 
16 SmNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.24 11.2 
17 SmNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.25 10.7 
18 SmNi.sub.0.5 Fe.sub.0.5 O.sub.3 
0.24 10.3 
19 SmNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.23 10.2 
20 SmNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.22 10.1 
21 EuNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.24 11.1 
22 EuNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.25 11.1 
23 EuNi.sub.0.5 Fe.sub.0.5 O.sub.3 
0.24 10.7 
24 EuNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.23 10.3 
25 EuNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.22 10.2 
26 YNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.24 11.2 
27 YNi.sub.0.6 Fe.sub.0.4 O.sub.3 
0.26 10.7 
28 YNi.sub.0.7 Fe.sub.0.3 O.sub.3 
0.24 10.3 
29 YNi.sub.0.45 Fe.sub.0.55 O.sub.3 
0.23 10.2 
30 YNi.sub.0.4 Fe.sub.0.6 O.sub.3 
0.22 10.1 
31 La.sub.0.4 Ce.sub.0.1 Pr.sub.0.3 Nd.sub.0.1 Sm.sub.0.1 -- 
0.28 11.2 
Ni.sub.0.6 Fe.sub.0.4 O.sub.3 
______________________________________ 
*: Terminal voltages are measured when the current density is 1.0 
A/cm.sup.2. 
*: The thermal expansion coefficients are average thermal expansion of 
room temperature to 800.degree. C. 
Fourth Embodiment 
The same tests as for the first embodiment were conducted, using the same 
model unit cells, by replacing the air electrode material with five 
materials in the system of SmNi.sub.1-x Fe.sub.x O.sub.3 (where, x=0.3, 
0.4, 0.5, 0.55, 0.6). The test results are shown in Table 2 (Sample Nos. 
16 to 20). Every sample materials of the present embodiments showed closer 
thermal expansion coefficient to that of the solid electrolyte than that 
of the conventional material of La.sub.0.8 Sr.sub.0.2 MO.sub.3. In 
addition, the terminal voltages (at the current density of 1.0 A/cm.sup.2) 
observed for samples 16 to 20 were higher than for the conventional 
material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. That is, all of sample 
materials Nos. 16 to 20 showed preferable properties for the air 
electrode. However, more preferable results were obtained for sample Nos. 
17 to 19 in the system of SmNi.sub.1-x Fe.sub.x O.sub.3, when x is in the 
range of 0.40 to 0.55. 
Fifth Embodiment 
The same tests as the first embodiment were conducted, using the same model 
unit cells, by replacing the air electrode material with five materials in 
the system of EuNi.sub.1-x Fe.sub.x O.sub.3 (where, x=0.3, 0.4, 0.5, 0.55, 
and 0.6). The test results are shown in Table 2 (Sample Nos. 21 to 25). 
Every sample material of the present embodiment showed closer thermal 
expansion coefficient to that of the solid electrolyte than that of the 
conventional material of La.sub.0.8 Sr.sub.0.2 MO.sub.3. In addition, the 
terminal voltages (at the current density of 1.0 A/cm.sup.2) observed for 
sample Nos. 21 to 25 were higher than for the conventional material of 
La.sub.0.8 Sr.sub.0.2 MnO.sub.3. That is, all of sample materials Nos. 21 
to 25 showed preferable properties for the air electrode. However, more 
preferable results were obtained for sample Nos. 22 to 24 in the 
compositional range of EuNi.sub.1-x Fe.sub.x O.sub.3, when x is in a range 
of 0.4 to 0.55. 
Sixth Embodiment 
The same tests as for the first embodiment were conducted, using the same 
model unit cells, by replacing the air electrode material with five 
materials in the system of YNi.sub.1-x Fe.sub.x O.sub.3 (where, x=0.3, 
0.4, 0.5, 0.55, and 0.6). The test results are shown in Table 2 (Sample 
Nos. 26 to 30). Every sample material of the present embodiment showed 
closer thermal expansion coefficient to that of the solid electrolyte than 
that of the conventional material of La.sub.0.8 Sr.sub.0.2 MO.sub.3. In 
addition, the terminal voltages (at the current density of 1.0 A/cm.sup.2) 
observed for samples 6 to 10 were higher than for the conventional 
material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. That is, all of sample 
materials Nos. 26 to 30 showed preferable properties for the air 
electrode. However, more preferable results were obtained for samples Nos. 
26 to 29 in the system of YNi.sub.1-x Fe.sub.x O.sub.3, when x is in the 
range of 0.4 to 0.55. 
Seventh Embodiment 
The same tests as for the first embodiment were conducted, using the same 
model unit cells, by replacing the air electrode material with a 
multi-component material with a composition of La.sub.0.1 Ce.sub.0.1 
Pr.sub.0.3 Nd.sub.0.1 Sm.sub.0.1 Ni.sub.0.6 Fe.sub.0.4 O.sub.3. The test 
result is shown in Table 2 (Sample No. 31). The sample material of the 
present embodiment had closer thermal expansion coefficient to that of the 
solid electrolyte than that of the conventional material of La.sub.0.8 
Sr.sub.0.2 MO.sub.3. In addition, the terminal voltage (at the current 
density of 1.0 A/cm.sup.2) observed for the sample 31 was higher than the 
conventional material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. That is, the 
material No. 31 showed preferable properties for the air electrode. 
In the above embodiments, although the properties of each material were 
evaluated by the fuel cell simulation test equipment, it should be 
understood that the performance of the solid oxide cell can be 
sufficiently evaluated by use of the simulation test equipment. It was 
confirmed that the same performances as those measured by the simulation 
test equipment are obtained by the practical unit cell of the solid oxide 
fuel cell by applying various nickel iron perovskite-type materials to the 
air electrode. 
As hereinabove described, the performance of the solid oxide fuel cell can 
be sufficiently improved by applying to the air electrode the nickel-iron 
perovskite-type materials in the systems of LnNi.sub.1-x Fe.sub.x O.sub.3 
and YNi.sub.1-x Fe.sub.x O.sub.3 (wherein, Ln represents lanthanide 
elements and x=0.3.about.0.6), when the performance is compared with that 
of the fuel cell using the conventional material in the composition of 
La.sub.0.8 Sr.sub.0.2 MnO.sub.3. This favorable effect is caused by the 
higher conductivity of the air electrode materials than the conventional 
air electrode material, and closer thermal expansion coefficient of the 
air electrode materials to that of the solid electrolyte than the 
conventional material of La.sub.0.8 Sr.sub.0.2 MnO.sub.3. Accordingly, the 
present invention contributes to the improvement of the reliability as 
well as to the high efficiency operation of the solid oxide fuel cell.