CA2467120A1

CA2467120A1 - Composite oxide for solid oxide fuel cell and process for its production

Info

Publication number: CA2467120A1
Application number: CA002467120A
Authority: CA
Inventors: Fumio Munakata; Kenji Furuya; Masaharu Hatano; Yoshinori Fujie; Manabu Suhara
Original assignee: Individual
Current assignee: Seimi Chemical Co Ltd
Priority date: 2001-11-15
Filing date: 2002-11-15
Publication date: 2003-05-22
Also published as: JP4393027B2; US7368095B2; CN1586020A; EP1450425A1; US20050031518A1; KR20040066118A; WO2003043111A1; JP2003151579A; CN1285138C; EP1450425A4

Abstract

A composite oxide for a solid oxide fuel cell which comprises a rare earth element, has a perovskite type crystal structure, contains a phase having a structure different from a perovskite type structure in an average area percentage of 0.3 % or less, has constituent elements being homogeneously dispersed therein, and has a melting point of 1470 ~C or higher; and a method for preparing the composite oxide which comprises reacting one or more metal carbonates, one or more metal oxides and/or one or more metal hydroxides with citric acid. The composite oxide can be fired at a relatively low temperature and is reduced in the content of an impurity phase having a structure different from the primary structure, and thus can be used for manufacturing a fuel cell having high performance capabilities and capable of being operated at a relatively low temperature.

Description

DESCRIPTION
COMPOSITE OXIDE FOR SOLID OXIDE FUEL CELL AND PROCESS FOR
ITS PRODUCTION
TECHNICAL FIELD
The present invention relates to a composite oxide to be used as an electrolyte or a cathode of a fuel cell employing a solid electrolyte, and a process for its production. More particularly, the present invention relates to a composite oxide for a solid oxide fuel cell, io having oxygen ionic conductivity and constituting a perovskite structure, and an industrially suitable process for its production.
BACKGROUND ART
At present, development of a solid oxide fuel cell (SOFC) is in progress. As an electrolyte for this fuel cell, stabilized zirconia is commonly used. However, the stabilized zirconia has low ionic conductivity at a lower temperature, whereby it is used at a higher temperature of at least 1000°C. Therefore, expensive ceramics had to 2o be used instead of metals as components of the fuel cell.
In order to solve this problem, in recent years, perovskite type electrolytes of LaGa03 which can be used at a lower temperature as compared with stabilized zirconia, were developed. Among them, it is reported that LaSrGaMg03 shows a good performance (KHuang, R.S.
Tichy, and J.B. Goodenough, J. Am. Ceram. Soc., 81,2565(1998), U.S.Patent 6,004,688, JP-A-11-335164 and JP-A-11-665165).
However, in the preparation of this LaGa03 type compound, Ga as a typical element is not likely to constitute a perovskite structure. Therefore, firing at a high temperature is required, and there is a problem that heterogeneous phases of impurities other than the desired composition are likely to remain. As the heterogeneous phases of impurities, LaSrGa04 having a melting point of about 1400°C which is lower than that of 1o perovskite and having low oxygen ionic conductivity, and LaSrGa30~ having a melting point of at least 1600°C and having low oxygen ionic conductivity, are typical ones.
In a solid state reaction method, oxides, carbonates or hydroxides of the respective metals are mixed, as they z5 are, with starting materials by pulverizing, followed by firing. Therefore, microscopic unevenness in the mixed state tends to occur, whereby heterogeneous phases of impurities tend to remain. In order to prepare perovskite having little heterogeneous phases of 2o impurities, firing at a high temperature of at least 1500°C has been required.
On the other hand, in a case where the prepared solid composite oxide powder is to be molded into an electrolyte or an electrode for a fuel cell, usually, the 25 solid composite oxide powder is press-molded, and then sintered by heating to a temperature of from 1300°C to 1600°C to obtain a sintered body structure.

Accordingly, when the oxide powder prepared by means of the solid state reaction method, is press-molded and sintered, the heterogeneous phases of impurities contained in the oxide powder are fused, and pores of the sintered body are covered with the heterogeneous phases of impurities having low oxygen ionic conductivity, whereby it has been difficult to form a homogeneous electrolyte body.
As described above, according to the solid state 1o reaction method which has been used as a common preparation method, the temperature becomes high during firing in a state where the desired composition, an intermediate and a starting material are mixed, whereby there may be a case where a part of the mixture is fused i5 to remain in the final product as heterogeneous phases of impurities.
As a common process for a production to constitute a composition at a lower temperature, a method using nitrates or acetates is known. However, in this method, 2o a large amount of a harmful gas such as nitric acid gas, nitrogen oxide or acetic acid will be generated during the firing, and it is not suitable for the industrial production. Further, a sol-gel process using an organic solvent such as ethylene glycol as a solvent is also 25 known, but a gel state substance is likely to deposit on a wall of a container and will burn intensely, and therefore this is not suitable for the industrial production either.
As a preparation method for other metal composite oxides, a citric acid method for an yttrium-alkaline earth metal-transition metal composite oxide, a bismuth-alkaline earth metal-transition metal composite oxide, a lanthanum-strontium-cobalt composite oxide or a lanthanum-cobalt-iron composite oxide, is disclosed in e.g. JP-B-7-96443, JP-3081212, JP-A-9-086928 or JP-A-08-130018, as proposed by the present inventors. However, 1o each obtained oxide has low electrical conductivity in a low temperature range of from 600°C to 800°C, and it is not suitable for a material for a solid oxide fuel cell of lower temperature operation type.
A solid oxide ceramics for a fuel cell is required z5 to be a solid oxide of lower temperature operation type having particularly little heterogeneous phases of impurities. Therefore, the object of the present invention is to provide a composite oxide for a high performance solid oxide fuel cell of lower temperature 20 operation type, which can be fired at a relatively low temperature, and which has little heterogeneous phases of impurities other than the desired composition, and an industrially advantageous process for its production.
DISCLOSURE OF THE INVENTION
25 The present inventors have found that the desired perovskite type composition (composite oxide) can be produced at a low temperature where an intermediate or a starting material will not be fused, by reacting a specific starting material compound by means of a citric acid method under a specific condition, and as a finally obtainable composition, it is possible to produce a 5 composition which has little heterogeneous phases different from a perovskite phase, and which is close to a single phase having constituent elements homogeneously dispersed therein. By using this composite oxide, the performance of a lower temperature operation type solid 1o fuel cell electrolyte can be improved.
Namely, the present invention is essentially directed to the following.
A composite oxide for a solid oxide fuel cell having a perovskite type crystal structure represented by the following formula:
Lnl_XAXGal_Y_ZBYCZ03_s ( 1 ) (provided that in the formula (1), Ln is at least one element selected from the group consisting of lanthanium, cerium, praseodymium, neodymium, samarium, gadolinium and 2o dysprosium, A is at least one element selected from the group consisting of strontium, calcium and barium, B is at least one element selected from the group consisting of magnesium, aluminum and indium, and C is at least one element selected from the group consisting of iron, cobalt, nickel and manganese, 0.05<x<0.4, 0.02<0.4, 0.1<y+z<0.45 and 0<~<1), characterized in that the abundance ratio of heterogeneous phases, which are different from the phase having a perovskite structure, is at most 0.3% by average area ratio.
Further, the present invention provides a process for producing a composite oxide for a solid oxide fuel cell having a perovskite type crystal structure represented by the above formula (1), characterized in that starting materials for metal elements constituting the composite oxide are metal compounds of any of hydroxides, oxides or carbonates, and a composite citrate 1o which is a reaction product obtained by reacting the metal compounds and citric acid, is subjected to thermal decomposition.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1(a): X-ray analysis pattern of the composite oxide of Example 1 according to the present invention.
Fig. 1(b): Scanning electron microscope image (magnifying power of 2000) of the sintered body of the same Example 1.
Fig. 1(c): An image obtained by cutting out about 8.8 um2 from a portion having no void in Fig. 1(b), followed by enlargement, and then by binarization by image processing.
Fig. 2: X-ray analysis pattern of Example 2 according to the present invention.
Fig. 3: X-ray analysis pattern of Example 3 according to the present invention.
Fig. 4: X-ray analysis pattern of Example 4 according to the present invention.
Fig. 5(a): X-ray analysis pattern of Comparative Example 1 according to a conventional method.
Fig. 5(b): Scanning electron microscope image (magnifying power of 2000) of the sintered body of Comparative Example 1.
Fig. 5(c): An image obtained by cutting out about 22 ~m2 from a portion having no void in Fig. 5(b), followed by enlargement, and then by binarization by image 1o processing.
Fig. 6: X-ray analysis pattern of Comparative Example 2 according to the conventional method.
BEST MODE FOR CARRYING OUT THE INVENTION
In the formula (1) which represents the composite s5 oxide of the present invention, the conditions of 0.05<x<0.40, 0.02<y<0.40 and 0.10<y+z<0.45 are required to constitute a perovskite structure. b satisfies 0<~cl.
If b is out of this range, the perovskite structure tends to be unstable such being not desirable. Among them, 20 0.10<x<0.25, 0.05<y<0.25 and 0.15<y+z<0.40 are particularly preferred, whereby the perovskite structure can more readily be formed, and the oxygen ionic conductivity can be made high.
Further, in the formula (1), it is especially 25 preferred that Ln (lanthanoid type rare earth metal) is La, A (alkaline earth metal) is Sr, and B (non-transition metal) is Mg, whereby the electrical conductivity at a low temperature can be made high. In addition, it is preferred that C (transition metal) is Co, whereby the electrical conductivity at a lower temperature can be made higher.
In a case where the composite oxide of the present invention is to be produced, citric acid is added to a starting material slurry, having carbonates, oxides or hydroxides of metal elements contained in the composite oxide, mixed in water, in an amount of from 25 to 100%, so preferably from 60 to 100%, of the chemical equivalent of citric acid which is required to have all the metal ions citrated, followed by reacting the mixture at a temperature of preferably from 25°C to 100°C, particularly preferably from 50°C to 70°C. Here, if e.g.
i5 nitrates, chlorides or sulfates, are used as the metal compounds, a harmful by-product gas will be generated during the preparation of the composite citrate or during its thermal decomposition, or it will be difficult to carry out the thermal decomposition, such being not 2o desirable.
In the present invention, it is particularly preferred to use hydroxides for lanthanoid type rare earth element starting materials of Ln to obtain a uniform composite citrate, wherein the constituent 25 elements are homogeneously dispersed. Further, as gallium, in order to reduce heterogeneous phases, it is particularly preferred to employ gallium hydroxide. Also, with respect to alkaline earth metals of constituent component A, it is preferred to use carbonates to reduce the heterogeneous phases. According to a combination of these specific compounds, a remarkably uniform composite citrate can be prepared. As a result, the homogeneity of perovskite after firing is improved.
After the above reaction with citric acid, drying and dehydration are carried out, and then thermal decomposition and calcination are carried out. In this 1o case, it is also possible to carry out the thermal decomposition and the calcination in one step of firing.
However, if the thermal decomposition and the calcination are carried out at the same time, it is difficult to take uniform heating of the reaction system. Therefore, it is i5 preferred to separately carry out the thermal decomposition and the calcination in two steps.
Then, the produced composite citrate is subjected to a thermal decomposition preferably at a temperature of from 350°C to 500°C, and then calcined preferably at a 2o temperature of from 900°C to 1470°C. This aspect is one of the characteristics of the present invention. The calcination may be carried out as it is in a powder form, or firing may be carried out after molding it by a press or the like. If the calcination temperature is less than ~5 900°C, sintering tends to be insufficient, whereby a dense powder can not be obtained, such being undesirable.
On the other hand, if the calcination temperature exceeds 1470°C, the furnace body material tends to deteriorate, and the heat consumption increases, such being undesirable.
The calcination temperature is more preferably from 5 1200°C to 1450°C, and particularly preferably from 1300°C
to 1420°C. The atmosphere for the thermal decomposition and the calcination may be either an oxidizing atmosphere such as air or an inert atmosphere. Pulverization may be carried out after the thermal decomposition. Further, 1o the product after the calcination may be pulverized by e.g. a jet mill or a ball mill. There is no particular limitation to the method of the pulverization.
According to the present invention, a composite oxide can be obtained, wherein the abundance ratio of i5 heterogeneous phases, which are different from the phase having a perovskite structure in the sintered body constitution, is at most 0.3o by average area ratio. If the abundance ratio of heterogeneous phases exceeds 0.3~
by average area ratio, the melting point decreases, the 2o toughness of the sintered body decreases, and further the electrical conductivity decreases, such being undesirable.
Particularly preferably, the abundance ratio of the heterogeneous phases is at most 0.15%, and more preferably at most 0.1% by an average area ratio.
25 The heterogeneous phases structure in the perovskite composite oxide can be detected by the x-ray diffraction spectrum when the fraction of the heterogeneous phases is high, but when the fraction of the heterogeneous phases is low, the heterogeneous phases structure can be quantified by a reflection electronic image by a scanning electron microscope. In the present invention, the quantitative determination of the heterogeneous phases in the perovskite composite oxide is carried out by an image analysis about the reflection electronic image.
The composite oxide obtained in the present invention has little heterogeneous phases of impurities 1o such as LaSrGa04, and constitutes a single crystal structure, and thus, it has a characteristic that the melting point is preferably at least 1470°C, particularly preferably at least 1500°C. If the melting point is less than 1470°C, the composite oxide tends to fuse during z5 molding, such being undesirable.
Further, the composite oxide according to the present invention has an advantage that a molded body having high toughness can be easily obtained as compared with the composite oxide prepared by the conventional 2o solid phase reaction method. Furthermore, it has a characteristic that fines will scarcely be formed at the time of pulverization of a calcined powder as compound with the composite oxide prepared by the conventional solid phase reaction method. Accordingly, the tap 25 density of the powder after the pulverization can be made high, whereby a dense molded body can be easily obtained.
Thus, the tap density of the composite oxide according to the present invention is preferably at least 1.0 g/cm3.
If the tap density is less than 1.0 g/cm3, it tends to be difficult to obtain a dense and high strength molded body, such being undesirable. Especially, in the present invention, one having a tap density of at least 1.2 g/cm3 can be obtained.
Further, the composite oxide according to the present invention has a weight-average particle diameter of preferably from 0.4 to 2 um. If the weight-average so particle diameter is less than 0.4 um, it tends to be difficult to obtain a dense electrode molded body, such being undesirable. On the other hand, if the weight-average particle diameter is more than 2.0 um, the strength of the molded body decreases, such being z5 undesirable. A particularly preferred range of the weight-average particle diameter is from 0.8 to 1.3 um.
Now, specific Examples 1 to 4 of the present invention and its Comparative Examples 1 and 2 will be described.
2o In each Example, identification of the crystal structure was carried out by X-ray diffraction (Cu-Ka ray, manufactured by Rigaku Corporation). Further, the melting point was obtained by a TG-DTA analysis (manufactured by Seiko Corporation) by raising the 25 temperature at a rate of 10°C/min. The particle size distribution was measured by a laser diffraction type analysis (manufactured by Microtrac Inc.), and the weight-average particle diameter was calculated. The tap density was obtained by a powder tester (manufactured by Hosokawa Micron Corporation) in accordance with JIS
89301-2-3.
Further, in each of Examples, as for the abundance ratio of the heterogeneous phases which are different from a phase having perovskite structure in the sintering body constitution, the composite oxide powder was molded at a pressure of 2 ton/cmz by a hydrostatic press, 1o followed by sintering at 1450°C for 6 hours to obtain a solid electrolyte sintered body. Then, an image of the same sintered body was obtained with a magnifying power of 2000 by scanning electron microscope. Using a high-speed image processor (VIDAS Plus, manufactured by Carl Zeiss Corporation), sampling was made from five visual fields (area: 2902 umz) per sample, by means of a high-speed image processing software (KS400, manufactured by Carl Zeiss Corporation), and an average value was calculated to obtain an average area ratio of the heterogeneous phases.

As starting material powders, lanthanum hydroxide, strontium carbonate, gallium hydroxide and magnesium carbonate, were mixed to be Lao.a~Sro,l3Gao.al''jgo.a~3-s, and 2s dispersed in water. The temperature was raised to 70°C, and citric acid was added in an amount required to have all metal ions converted to citrates, and then reacted.

After the reaction, the product was dried at 120°C
and pulverized. Then, pre-firing was carried out at 400°C for six hours, followed by thermal decomposition.
After that, further pulverization and mixing were carried out, followed by firing at 1450°C for 12 hours. The form after the firing was a white powder. After the firing, it was pulverized by a ball mill for six hours. The weight-average particle diameter of the obtained composite oxide powder was 0.56 Vim, and the tap density 1o was 1.27 g/cm3. The result of the crystal structure analysis is shown in Fig. 1(a), and further, the state after the firing, the melting point and the identification result of the X-ray diffraction are shown in Table 1.
A sintered body was prepared from this powder, and the image observed by a scanning electron microscope with a magnifying power of 2000 is shown in Fig. 1(b). From a portion having no voids (black holes) in the image of this Fig. 1(b), about 8.8 ~m2 was cut out, followed by 2o enlargement and then by binarization by image processing to obtain an image, which is shown in Fig. 1(c). Only a white part was measured based on this Fig. 1(c).
By this image analysis, with respect to the heterogeneous phases composed of fine particles which exist in a continuous phase of perovskite, the area ratios of the heterogeneous phases were obtained in five visual fields from the number of particles per unit area and the mean particle size and found to be 0.292%, 0.172%, 0.141%, 0.065% and 0.0$2%, and the average area ratio of the heterogeneous phases was 0.150%.

5 As starting material powders, lanthanum hydroxide, strontium carbonate, gallium hydroxide and magnesium carbonate, were mixed to be Lao.B~Sro.l3Gao.si''IJo.2~3-b. and dispersed in water. The temperature was raised to 70°C, and citric acid was added in an amount required to have 1o all metal ions converted to citrates, and then reacted.
After the reaction, the product was dried at 120°C
and pulverized. Then, pre-firing was carried out at 400°C for six hours, followed by thermal decomposition.
After that, further pulverization and mixing were carried i5 out, followed by firing at 1350°C for 12 hours. The form after the firing was a white powder. After the firing, it was pulverized by a ball mill for six hours. The weight-average particle diameter of the obtained composite oxide powder was 0.49 um, and the tap density 2o was 1.21 g/cm3. The result of the crystal structure analysis is shown in Fig. 2, and further, the state after the firing, the melting point and the identification result of the X-ray diffraction are shown in Table 1.
The average area ratio of the heterogeneous phases of the sintered body calculated in the same manner as in Example 1 was 0.159%.

As starting material powders, lanthanum hydroxide, strontium carbonate, gallium hydroxide, magnesium carbonate and cobalt carbonate were mixed to be Lao,$Sro_zGao,6Mgo.zCoo.a~3-s, and dispersed in water. The temperature was raised to 70°C, and citric acid was added in an amount required to have all metal ions converted to citrates, and then reacted.
After the reaction, the product was dried at 120°C
and pulverized. Then, pre-firing was carried out at so 400°C for six hours, followed by thermal decomposition.
After that, further pulverization and mixing were carried out, followed by firing at 1450°C for 12 hours. The form after the firing was a black powder. After the firing, it was pulverized by a ball mill for six hours. The i5 weight-average particle diameter of the obtained composite oxide powder was 0.86 ~zm, and the tap density was 1.34 g/cm3. The result of the crystal structure analysis is shown in Fig. 3, and further, the state after the firing, the melting point and the identification 2o result of the X-ray diffraction are shown in Table 1.
The average area ratio of the heterogeneous phases of the sintered body calculated in the same manner as in Example 1 was 0.107%.

25 As starting material powders, lanthanum hydroxide, strontium carbonate, gallium hydroxide and magnesium carbonate, were mixed to be Lao.B~Sro.l3Gao.el"Igo.z~3-s. and dispersed in water. The temperature was raised to 70°C, and citric acid was added in an amount corresponding to 70% of the amount required to have all metal ions converted to citrates, and then reacted.
After the reaction, the product was dried at 120°C
and pulverized. Then, pre-firing was carried out at 400°C for six hours, followed by thermal decomposition.
After that, further pulverization and mixing were carried out, followed by firing at 1350°C for 12 hours. The form 1o after the firing was a white powder. After the firing, it was pulverized by a ball mill for six hours. The weight-average particle diameter of the obtained composite oxide powder was 0.66 um, and the tap density was 1.22 g/cm3. The result of the crystal structure i5 analysis is shown in Fig. 4, and further, the state after the firing, the melting point and the identification result of the X-ray diffraction are shown in Table 1.
The average area ratio of the heterogeneous phases of the sintered body calculated in the same manner as in Example 20 1 was 0.168%.

As starting material powders, lanthanum oxide, strontium carbonate, gallium oxide and magnesium carbonate, were mixed to be Lao.a7Sro.i3Gao.aMgo.z03-s, and 25 pulverized and mixed by a ball mill. The mixture was pre-fired at 400°C for six hours, followed by thermal decomposition. After that, further pulverization and mixing were carried out, followed by firing at 1450°C for 12 hours.
The form after the firing was a brown block. After the firing, it was pulverized by the ball mill for six hours. The weight-average particle diameter of the obtained composite oxide powder was 2.21 Vim, and the tap density was 0.98 g/cm3. The result of the crystal structure analysis is shown in Fig. 5(a), and the state after the firing, the melting point and the Zo identification result of the X-ray diffraction are shown in Table 1.
Further, the image observed by a scanning electron microscope with a magnifying power of 2000 in the same manner as in Example 1, is shown in Fig. 5(b). From a z5 portion having no voids (black holes) in the image of the Fig. 5(b), about 22 ~m2 was cut out, followed by enlargement, and then by binarization by image processing to obtain an image, which is shown in Fig. 5(c). Only a white part was measured based on this Fig. 5(c).
2o By this image analysis, the area ratios of the heterogeneous phases were obtained in five visual fields in the same manner as in Example 1 and found to be 0.770%, 0.406%, 0.547%, 1.234% and 0.596%, and the average area ratio of the heterogeneous phases was 0.711%.

As starting material powders, lanthanum oxide, strontium carbonate, gallium oxide, magnesium carbonate ' CA 02467120 2004-05-13 and cobalt oxide, were mixed to be Lao_BSro,zGao.6Mgo.zCoo.2~3-b, and pulverized and mixed by a ball mill. The mixture was pre-fired at 400°C for six hours, followed by thermal decomposition. After that, further pulverization and mixing were carried out, followed by firing at 1450°C for 12 hours.
The form after the firing was a black block. After the firing, it was pulverized by the ball mill for six hours. The weight-average particle diameter of the obtained composite oxide powder was 2.11 um, and the tap density was 0.92 g/cm3. The result of the crystal structure analysis is shown in Fig. 6, and further, the state after the firing, the melting point and the identification result of the X-ray diffraction are shown i5 in Table 1. A perovskite phase was not identified.

Table 1 Identification State after Melting point result of the firing (C) X-ray diffraction Perovskite At least Ex. 1 White powder structure Single phase Perovskite At least Ex. 2 White powder structure Single phase Perovskite At least Ex. 3 Black powder structure Single phase Perovskite At least Ex. 4 White powder structure Single phase Perovskite structure Comp. Ex. 1 Brown block 1375C

Containing impurity phase Unknown Comp. Ex. 2 Black block 1420C

structure INDUSTRIAL APPLICABILITY
According to the present invention, it is possible to provide an extremely uniform composite oxide for a 5 high performance solid oxide fuel cell of low temperature operation type, which can be fired at a relatively low temperature and which has little heterogeneous phases of impurities other than the desired composition. Further, it is possible to provide a process whereby the composite 10 oxide can be industrially advantageously produced in large quantities without generation of a harmful gas at the time of firing, as compared with the conventional process.

Claims

1. A composite oxide for a solid oxide fuel cell having a perovskite type crystal structure represented by the following formula:

Ln1-x A x Ga1-y-z B y C2O3-.delta. (1) (provided that in the formula (1), Ln is at least one element selected from the group consisting of lanthanium, cerium, praseodymium, neodymium, samarium, gadolinium and dysprosium, A is at least one element selected from the group consisting of strontium, calcium and barium, B is at least one element selected from the group consisting of magnesium, aluminum and indium, and C is at least one element selected from the group consisting of iron, cobalt, nickel and manganese, 0.05~x~0.4, 0.02~y~0.4, 0.1~y+z~0.45 and 0~.delta.~1), characterized in that the abundance ratio of heterogeneous phases, which are different from the phase having a perovskite structure, is at most 0.3% by average area ratio.

2. The composite oxide according to Claim 1, wherein the melting point of the composite oxide is at least 1470°C.

3. The composite oxide according to Claim 1 or 2, wherein the weight-average particle diameter of the composite oxide is from 0.4 to 2 µm, and the tap density is at least 1.0 g/cm3.

4. The composite oxide according to Claim 1, 2 or 3, wherein Ln material is La, A material is Sr, and B
material is Mg.

5. A process for producing a composite oxide for a solid oxide fuel cell having a perovskite type crystal structure represented by the following formula:

Ln1-X A X Ga1-Y-Z B Y C Z O3-.delta. (1) (provided that in the formula (1), Ln is at least one kind of elements selected from the group consisting of lanthanium, cerium, praseodymium, neodymium, samarium, gadolinium and dysprosium, A is at least one kind of elements selected from the group consisting of strontium, calcium and barium, B is at least one kind of elements selected from the group consisting of magnesium, aluminum and indium, and C is at least one kind of elements selected from the group consisting of iron, cobalt, nickel and mangan, 0.05~x~0.4, 0.02~y~0.4, 0.1~y+z~0.45 and 0~.delta.~1), characterized in that starting materials for metal elements constituting the composite oxide are metal compounds of any of hydroxides, oxides or carbonates, and a composite citrate which is a reaction product obtained by reacting the metal compounds and citric acid, is subjected to thermal decomposition.

6. The process for producing a composite oxide according to Claim 5, wherein the above metal compounds are formed into a slurry by employing water as a medium, then the slurry is reacted with citric acid to produce the composite citrate, which is subjected to thermal decomposition at a temperature of from 350°C to 500°C, and then calcined at a temperature of from 900°C to 1470°C.

7. The process for producing a composite oxide according to Claim 5 or 6, wherein each of a metal compound containing Ln and a metal compound containing gallium, is a hydroxide, and a metal compound containing element A is a carbonate.