JP3359413B2 - Solid oxide fuel cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an air electrode made of conductive ceramics used for a solid oxide fuel cell.

[0002]

2. Description of the Related Art Research and development have been made on two types of solid oxide fuel cells, a cylindrical type and a flat type. The flat fuel cell has the feature that the power density per unit volume of power generation is high, but there are problems such as imperfect gas seal and non-uniformity of the temperature distribution in the cell in practical use. On the other hand, the cylindrical fuel cell has the features that the output density is low, but the mechanical strength of the cell is high and the temperature uniformity in the cell can be maintained. Both types of solid electrolyte fuel cells are being actively researched and developed utilizing their respective features.

As shown in FIG. 1, a single cell of a cylindrical fuel cell has a support tube 1 made of CaO-stabilized ZrO ₂ having an open porosity of about 40%, on which a porous air electrode is formed by a slurry-dip method. A LaMnO ₃ -based material 2 is applied, and the surface thereof is coated with a Y ₂ O ₃ -stabilized ZrO ₂ film serving as an electrolyte 3 by a vapor phase synthesis method (EVD) or a thermal spraying method. A zirconia anode 4 is formed; In the fuel cell module, each single cell is connected via a LaCrO ₃ -based interconnector 5. Power generation is performed at a high temperature of 1000 to 1050 ° C. by flowing air (oxygen) inside the support tube 1 and fuel (hydrogen) outside the cell.

In recent years, attempts have been made to use a LaMnO ₃ -based material, which is an air electrode material, directly as a porous support tube in order to simplify the process in the cell fabrication process. As a support tube material having the function as an air electrode, a LaMnO ₃ solid solution material in which La is substituted with 10 to 20 atomic% of Ca or Sr is generally used.

On the other hand, a single cell of a flat type fuel cell uses the same material system as that of a cylindrical type, and as shown in FIG. 8 are provided, and a connection between the single cells is provided by a dense LaCrO called a separator 9 to which a dense MgO or CaO is added.
₃ Solid solution material is used. For power generation, air (oxygen) is supplied to the air electrode side of the cell, and fuel (hydrogen) is supplied to the fuel electrode side.
It is performed at a high temperature of 0 to 1050 ° C.

[0006]

However, the above-mentioned CaO-stabilized ZrO ₂ is used as a support tube, and Ca, S
When long-term power generation is performed in a cell having a structure in which a LaMnO ₃ material in which r is dissolved is provided as an air electrode, and a cylindrical fuel cell and a flat fuel cell having a structure in which the air electrode is directly used as a support tube, There is a problem that the cell is deformed and the output gradually decreases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a long-life fuel cell that has a small cell deformation and a stable power generation output in a long-term operation of a fuel cell system in a cylindrical or flat fuel cell.

[0008]

Means for Solving the Problems The present inventors have repeatedly studied the above problems, and as a result, have found that a solid electrolyte type in which an air electrode is formed on one surface of a solid electrolyte and a fuel electrode is formed on the other surface. In the fuel cell, the air electrode is represented by the following formula 1,

[0009]

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Wherein A in Chemical Formula 1 is Y, Yb, Sc,
At least one element selected from the group consisting of rare earth elements of Er, Nd, Gd, Dy, Sm, Pr, and Ce, B is
At least one element selected from the group consisting of alkaline earth elements of Ca, Ba and Sr, C is Ni, Co, Fe, C
at least one element selected from the group consisting of r, Ce and Zr, wherein m, n and z in Chemical formula 1 are: 0.02 ≦ m ≦ 0.98 0 ≦ n ≦ 0.50 0.80 ≦ It has been found that the above object is achieved by using a conductive ceramic having a crystal phase satisfying z ≦ 1.10.

According to the present invention, the main crystal phase in the air electrode is

[0012]

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Where A is Y, Yb, Sc, E
at least one element selected from the group consisting of rare earth elements of r, Nd, Gd, Dy, Sm, Pr and Ce;
a, Ba, Sr, at least one element selected from alkaline earth elements, and C is at least one element selected from the group consisting of Ni, Co, Fe, Cr, Ce, and Zr. M, n and z of the composite oxide satisfying 0.02 ≦ m ≦ 0.980 0 ≦ n ≦ 0.50 0.80 ≦ z ≦ 1.10.
Oxides of at least one element selected from the group consisting of rare earth elements of Yb, Sc, Er, Nd, Gd, Dy, Sm, Pr and Ce are contained in a proportion of 0.01 to 10% by weight based on the total amount. It is characterized by being dispersedly contained.

The air electrode in the fuel cell unit of the present invention is
It has a composition as shown in the above chemical formula 1, and mainly has a so-called ABO ₃ type perovskite crystal structure. In the present invention, the reason why m, n, and z in Chemical Formula 1 are limited to the above ranges is that the substitution ratio m of the alkaline earth elements Ca, Sr, and Ba with respect to Y and various rare earth elements is smaller than 0.02. When the value of m is greater than 0.98, the sinterability increases, and the sintering of the air electrode progresses during long-term power generation at 1000 ° C, causing the air electrode to be deformed. And the power generation performance becomes worse. In addition, A site and B in the perovskite type crystal structure
A similar result is obtained in a nonstoichiometric system in which the atomic ratio z of the site is up to 0.80, because the lattice defect structure is similar to the stoichiometric system. When z is smaller than 0.8, precipitation of the second component such as Mn ₂ O ₃ is promoted, sintering is promoted, and deformation of the air electrode is caused. Further, Ni, Co, Fe, Cr, Ce with respect to Mn are produced. Similar characteristics can be obtained by substituting elements such as Zr and Zr.
If it exceeds 50, sinterability deteriorates, and sintering is difficult unless the temperature is 1650 ° C. or more, which is uneconomical.

In the present invention, the preferred ranges of m, n and z in Chemical Formula 1 are 0.10 ≦ m ≦ 0.40 and 0 ≦ n ≦ 0.2
0, 0.95 ≦ z ≦ 1.0.

In a fuel cell, the air electrode is maintained at a temperature higher than or equal to 1000 ° C., which is the operating temperature of the cell. However, in such a case, the air electrode is densified, the gas transmission coefficient is reduced, the electrode is deformed, and the electrode performance is reduced. As a result, the power generation output is reduced. . In order to solve such a problem, the present invention is characterized in that the composition represented by Chemical Formula 1 is used as a main crystal phase and further contains a second crystal phase. The effect of the precipitation of the second crystal phase tends to increase as the grain size of the second crystal phase is smaller and the amount of the second crystal phase is larger.

Based on these findings, the present inventors have proposed a second
When the type and amount of the crystal phase were examined, Y, Yb, Sc, Er, Nd, Gd, Dy, S
oxides of at least one element selected from the group consisting of rare earth elements of m, Pr and Ce are contained in an amount of 0.01 to
It has been found that it is best to disperse and contain 10% by weight. That is, if the amount of the second crystal phase is less than 0.01% by weight, the effect of suppressing densification is not significant, and if it exceeds 10% by weight, the conductivity of the air electrode is reduced. The amount of the second crystal phase is desirably 1 to 5% by weight.

The second crystal phase is located at the interface between the two grains by several tens to ten.
It is effective to have a size of 500 nm and an average particle size of 0.1 to 5 μm in a portion called a triple junction neck. In particular, a size of 0.1 to 3 μm is preferable. As the amount of the second crystal phase increases, the second crystal phase tends to precipitate at the neck of the triple point. Although the second crystal phase may slightly dissolve the components of the main crystal phase, there is no particular problem as long as the crystal structure does not change. A part of the second crystal phase is YMnO ₃ ,
It may be precipitated as a composite oxide such as YbMnO _3, but there is no particular problem as long as the main crystal phase and the second crystal phase satisfy the above ranges.

In the present invention, the air electrode is, for example, 10 to 1 in a cylindrical fuel cell as shown in FIG.
5 mol% CaO or 8 to 20 mol% Y ₂ O _3, Yb
It is also used as an air electrode formed on the surface of a porous, non-electron conductive support tube made of ZrO ₂ to which ₂ O ₃ is added, or as an air electrode support tube provided with a function as an air electrode. It can also be used as an air electrode and a gas diffuser in a flat fuel cell.

The air electrode itself must be porous and have gas permeability. To increase gas permeability, it is preferable that the open porosity is large, but if the open porosity is large, the strength of the air electrode itself decreases. In particular, when the air electrode is directly used as a support tube, if the strength is low, problems such as breakage of the support tube when forming an electrolyte on the surface and breakage due to handling in other manufacturing processes occur. According to the configuration of the present invention, the open porosity is desirably in the range of 20 to 45%, particularly 30 to 40%, in view of the relationship between the open porosity, strength, and gas permeability. Further, the gas permeation amount and the strength of the support tube also depend on the pore diameter in the air electrode. As the size of the pores, an average pore diameter in the range of 1.0 to 5.0 μm is excellent in gas permeability and strength. In particular, the average pore diameter is 1.0 μm
If it is smaller, the strength is excellent but the gas permeability is not sufficient. If the average pore diameter is more than 5.0 μm, the strength becomes small, and problems such as breakage due to handling in the production process occur. A particularly preferred range of the average pore size is 1.5
〜3.0 μm.

The air electrode material in the present invention is prepared by mixing oxides, carbonates, nitrates and the like of the constituent metal elements shown in the above formula 1 by a well-known mixing method. Calcination for 5 hours to produce a solid solution powder. When the rare earth element oxide is precipitated as the second crystal phase, it may be added in advance as an excess or may be added to the above-mentioned solid solution powder.

In order to produce a so-called self-supporting tube having a function as an air electrode, the above-mentioned solid solution powder is pulverized to a predetermined crystal particle diameter, and is extruded into a predetermined shape by extrusion molding or cold isostatic pressing (CIP). And fired at a temperature of 1400 to 1550 ° C. in the air or in an inert atmosphere such as Ar for 2 to 7 hours.

In the cylindrical cell, the air electrode of the present invention is applied to the surface of a porous self-supporting tube CaO-stabilized ZrO _{2 by} a slurry dipping method or a spraying method, and an electrolyte is formed on the surface by a vapor phase synthesis method or a spraying method. Y ₂ O ₃ or Yb ₂ O ₃ stabilized ZrO ₂
It is manufactured by coating a thin film and further providing a fuel electrode of Ni-zirconia (containing Y ₂ O ₃ ). In addition, Ca
Instead of using O-stabilized ZrO ₂ , a cell can also be produced by forming an electrolyte and a fuel electrode in a self-supporting tube made of the air electrode material of the present invention by the same method as described above.

On the other hand, a flat plate cell is prepared by preparing an electrolyte with Y ₂ O ₃ or Yb ₂ O ₃ stabilized ZrO ₂ by extrusion molding, press molding, doctor blade method or the like, and forming the air of the present invention on one surface. The electrode material is obtained by applying a fuel electrode to the other surface by screen printing or the like, drying the fuel electrode, and then baking.

[0025]

The conventionally used La is 10 to 20 atomic% S.
As a result of research focusing on the lattice defect structure of LaMnO ₃ substituted with r or Ca, oxygen was taken into the lattice in the air in a solid solution and at high temperature as shown in the following chemical formula, and the electrical It was found that vacancies of La and Mn ions were generated to maintain the properties.

[0026]

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Due to this lattice defect structure, the conventional sintering of an air electrode material has a high La and Mn ion vacancy concentration in a LaMnO ₃ solid solution, so that the diffusion rate of cations is increased, and as a result, sintering is promoted. It is thought to be. In a fuel cell unit, a reaction between fuels is partially released to electric power, and the others are released as heat. The deformation of the cell in question is due to the non-uniform temperature distribution of the fuel cell,
It is determined that the promotion of sintering of the air electrode occurs uniformly.

Based on these inferences, the inventors of the present invention believe that if the lattice defect structure in the crystal is changed to suppress the generation of La and Mn ion vacancies, the air electrode can be densified and the accompanying cell deformation can be suppressed. Thought. The present invention pays attention to this lattice defect structure, and as a result of examining the defect structure of the perovskite oxide-based material in more detail, it was found that YMnO ₃ , YbMnO ₃ , GdMnO _3, etc. in which Ca, Sr, Ba, etc. were dissolved in solid form were sintered. It has been found that the effect of promoting power is small and stable power generation performance is exhibited.

For example, YMnO ₃ containing Ca as a solid solution mainly generates lattice defects represented by the following formula ₃ at high temperatures in the air.

[0030]

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In the material having this lattice defect, C
a is substituted with Y to form a solid solution, a hole is formed to maintain the electrical neutrality of the system, and the formed hole contributes to electric conduction.
In this system, since the La and Mn ion vacancy concentrations are also small, the diffusion speed of the component ions is suppressed, and as a result, cell deformation is prevented. The present invention relates to the above (Y, Ca) Mn
In addition to O ₃ , Yb, Sc, Er, Nd, Gd, Dy, S
composite perovskite oxides composed of rare earth elements of m, Pr, Ce, alkaline earth elements of Ca, Sr, Ba and Mn, (Y, Ba) MnO ₃ , (Yb, Ca) Mn
O ₃ , (Y, Sr) MnO ₃ , (Gd, Sr) MnO ₃
Etc. also have a lattice defect structure similar to the above, and when used as an air electrode, cell deformation is prevented. Further, in the non-stoichiometric system in which the A site atoms in the structure of the present invention are deficient, the same effect as in the stoichiometric system can be obtained because the lattice defect structure is similar to the stoichiometric system. Also, a part of Mn is converted to C
Materials substituted with o, Ni, Fe, Cr, Ce, and Zr also have similar lattice defects and can be used as an air electrode with extremely small cell deformation.

In the present invention, in a cell in which the air electrode material is composed of small crystal particles and the performance of the fuel cell is improved, the air electrode material becomes denser and the electrode performance deteriorates, so that (Y, Ba) MnO ₃ , (Yb, Ca) MnO
_3. The densification can be suppressed by further depositing the second crystal phase in a composite perovskite-type oxide material such as (Y, Sr) MnO ₃ or (Gd, Sr) MnO ₃ . When precipitates are present at the grain boundaries, the limit size of particles that can grow into empirical movement, D is given by

[0033]

(Equation 1)

Is represented by This shows that the effect of the precipitate is larger as the precipitate is smaller and as the amount of the precipitate is larger. In the present invention, as a result of examining the types and amounts of precipitates based on the above empirical formula, Y, Yb, S
(Y, Ca) MnO ₃ , (Yb, S) containing rare earth oxides of c, Er, Nd, Gd, Dy, and Sm as the second crystal phase
r) It has been found that an oxide solid solution such as MnO ₃ has a large effect of suppressing densification.

In the fuel cell of the present invention, the air electrode is made of the above-mentioned specific manganite-based conductive ceramics so that the air electrode and the cell are deformed due to the sintering of the air electrode. A cell with excellent long-term stability can be provided by preventing poor connection. Further, even in a flat-plate type fuel cell, it is possible to provide a cell whose output is stable for a long time by preventing separation of the electrolyte during operation. In the flat fuel cell, the air electrode may be used as a gas diffuser.In this case, however, the gas diffuser has a small deformation, so that the connection with the electrolyte is excellent. Output reduction and the like can also be suppressed.

[0036]

Next, the present invention will be described based on embodiments. Example 1 Starting from a commercially available oxide powder of various metal elements and a carbonate of an alkaline earth element having a purity of 99.9% or more as shown in Tables 1 and 2, the composition was prepared as shown in Tables 1 and 2. The resulting mixture was mixed with a zirconia ball for 10 hours, and then subjected to a solid phase reaction at 1400 ° C. for 10 hours to obtain a solid solution powder. This powder was further ground for 10 hours using a zirconia ball. The powder is formed into a cylindrical shape, fired at 1470 to 1550 ° C. in the air, and
A cylindrical sintered body having a diameter of 6 mm, an inner diameter of 12 mm, and a length of 100 mm was obtained.

The open porosity of the obtained sintered body was measured by the Archimedes method. As a result, the open porosity was 30 to 35.
%Met. The average crystal grain size determined by a scanning electron micrograph (SEM) was 8 to 12 μm. afterwards,
The temperature of the cylindrical sintered body was increased from room temperature to 1000 ° C. at a rate of 200 ° C./h in the air using an electric furnace.
After it was kept at 2000C for 2000 hours, it was cooled to room temperature at a rate of 200C / h. Then, the outer diameter of the cylindrical sintered body after the heat treatment was measured, and compared with that before the heat treatment,

[0038]

(Equation 2)

The deformation rate was calculated according to the following equation.

In addition, the above-mentioned cylindrical sintered body is 50 mm long.
And a Pt electrode was attached so that the distance between the voltage terminals was 40 mm, and the resistance was measured at 1000 ° C. in the atmosphere.

[0041]

(Equation 3)

Thus, the sheet resistance Rs was measured. The measurement results are shown in Tables 1 and 2.

[0043]

[Table 1]

[0044]

[Table 2]

According to Tables 1 and 2, the periodic table 3a
In Samples Nos. 13, 28 and 39 in which the substitution amount m of Ca for the group element was larger than 0.98, the deformation was large and the sheet resistance was large. Sample No. where m is smaller than 0.02.
The sheet resistances of the samples Nos. 2 and 41 also increased. A site and B
In the sample No. 14 where the site ratio z is smaller than 0.8, M
n ₂ O ₃ was precipitated and deformed. In sample No. 19 where z was larger than 1.1, La ₂ O ₃ was precipitated and the sintered body was weathered in a short time. Further, in Samples Nos. 24 and 53 in which the amount n of substitution with Mn was larger than 0.5, the sinterability was reduced, and a sintered body having a predetermined porosity could not be obtained.

Example 2 Y ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ having a purity of 99.9% or more and an average particle diameter of about 1 μm were added to the solid solution powder prepared in Example 1.
O ₃ , Er ₂ O ₃ , Nd ₂ O ₃ , Gd ₂ O ₃ , Dy ₂ O
₃ and powders of Sm ₂ O ₃ were excessively added at the ratios shown in Table 3 and mixed and pulverized with a zirconia ball for 20 hours. This powder was formed into a cylindrical shape and fired at 1400 to 1500 ° C. for 3 to 5 hours to produce a cylindrical sintered body having an outer diameter of 16 mm, an inner diameter of 12 mm, a length of 100 mm and an open porosity of 30 to 34%. . The average crystal grain size determined by SEM is 3 to
It was 5 μm. Further, the deformation ratio was measured in the same manner as in Example 1. In addition, a part of the sample was prepared according to Example 1.
The sheet resistance at 00 ° C was measured. Table 3 shows the results.

[0047]

[Table 3]

From the results shown in Table 3, the deformation can be further suppressed when the amount of the second crystal phase (precipitate) is 0.01% by weight or more, but in Samples Nos. 62 and 74 where this amount exceeds 10% by weight. The sheet resistance increases and does not function as an air electrode. Also, according to the SEM observation, it was found that the air electrode 2
The size of the second crystal phase between the two crystals and at the triple junction neck was 0.1 to 5 μm. According to a transmission electron microscope (TEM), a precipitate of several tens to 500 nm was recognized at the grain boundary between the two crystal grains.

Example 3 With respect to the composition of Sample Nos. 1, 2, 7, 20, and 57 in Example 1, the open porosity was 30 to 34%, and the average pore diameter was 2.0.
3 to 2.35 μm, length 200 mm, outer shape 16 mm,
A cylindrical sintered body having an inner diameter of 12 mm and sealed at one end was prepared, and used as a support tube for a cell functioning as an air electrode.

Thereafter, a solid electrolyte (10 μm thick) having a thickness of about 50 μm was formed on the surface of the cylindrical sintered body at 1150 ° C. by a gas phase synthesis method.
mol% Y ₂ O ₃ -90 mol% ZrO ₂ )
Further, a ZrO ₂ (8 mol%) containing 70% by weight of Ni in a thickness of 30 μm was formed thereon by a slurry dipping method.
(Y ₂ O _{3 included} ) to form a single cell.

The cell was held in an electric furnace, and an acidity gas was supplied inside the cell and a hydrogen gas was supplied outside the cell, and power was generated at 1000 ° C. FIG. 3 shows changes in the power generation time and the output density.

As is apparent from FIG. 3, the sample N of the present invention
With respect to o, 7, 20, and 57, the power density hardly changes and the power density is high. In contrast, the conventional product No.
Outputs 1 and 2 decreased with time. This indicates that the product of the present invention has excellent long-term stability.

[0053]

As is clear from the above description, the fuel cell of the present invention prevents connection failure between cells due to deformation of the air electrode and the cell due to sintering of the air electrode, and the output is stable for a long time. It is possible to provide a highly reliable fuel cell unit.

[Brief description of the drawings]

FIG. 1 is a view for explaining the structure of a cylindrical fuel cell.

FIG. 2 is a diagram illustrating the structure of a flat fuel cell.

FIG. 3 is a diagram showing a change in output density with respect to a power generation time in Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Support pipe 2 Air electrode 3 Solid electrolyte 4 Fuel electrode 5 Interconnector

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/86 C04B 35/495 H01M 8/02 H01M 8/12

Claims (2)

(57) [Claims] 1. A solid electrolyte fuel cell comprising an air electrode formed on one surface of a solid electrolyte and a fuel electrode formed on the other surface, wherein the air electrode is represented by the following formula (A _{1 -MB) m} ) _z (Mn _1-n C _n ) O _{3 ± δ} , wherein A in Chemical Formula 1 is Y, Yb, Sc, Er, Nd,
B is at least one element selected from the group of rare earth elements of Gd, Dy, Sm, Pr and Ce; B is at least one element selected from the group of alkaline earth elements of Ca, Ba and Sr; Is at least one element selected from the group consisting of Ni, Co, Fe, Cr, Ce and Zr;
M, n and z in Chemical formula 1 are conductive ceramics having a crystal phase satisfying 0.02 ≦ m ≦ 0.980 0 ≦ n ≦ 0.50 0.80 ≦ z ≦ 1.10. A solid oxide fuel cell unit characterized by the following features. 2. A solid electrolyte fuel cell comprising an air electrode formed on one surface of a solid electrolyte and a fuel electrode formed on the other surface, wherein the main crystal phase in the air electrode is represented by the following chemical formula (A). _1-m B _m ) _z (Mn _1-n C _n ) O _{3 ± δ} , wherein A in Chemical Formula 1 is Y, Yb, Sc, Er, Nd,
B is at least one element selected from the group of rare earth elements of Gd, Dy, Sm, Pr and Ce; B is at least one element selected from the group of alkaline earth elements of Ca, Ba and Sr; Is at least one element selected from the group consisting of Ni, Co, Fe, Cr, Ce and Zr;
M, n and z in Chemical formula 1 are composed of a composite oxide satisfying 0.02 ≦ m ≦ 0.980 0 ≦ n ≦ 0.50 0.80 ≦ z ≦ 1.10. Y,
Oxides of at least one element selected from the group consisting of rare earth elements of Yb, Sc, Er, Nd, Gd, Dy, Sm, Pr and Ce are contained in a proportion of 0.01 to 10% by weight based on the total amount. A solid oxide fuel cell cell characterized by being dispersedly contained.