JP3342571B2 - 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 a solid oxide fuel cell, and more particularly, to an improvement in a conductive cathode material.

[0002]

2. Description of the Related Art In solid oxide fuel cells, research and development are being conducted on two types of fuel cells, cylindrical and flat. 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. In contrast, the cylindrical fuel cell has a low power density but a high mechanical strength of the cell,
Also, there is a feature that the uniformity of the temperature 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 as a solid electrolyte 3 by a vapor phase synthesis method (EVD) or a thermal spraying method,
Further, a porous Ni-zirconia fuel electrode 4 is formed on this surface.
Is provided. In the fuel cell module,
Each single cell is connected via a LaCrO ₃ -based interconnector 5. Power is generated by air (oxygen) inside the support tube 1.
And flowing fuel (hydrogen) to the outside at 1000 to 1050 ° C.
At a temperature of In recent years, in order to simplify the process in this cell manufacturing process, LaMn, which is an air electrode material, has been used.
Attempts have been made to use O ₃ -based materials directly as porous support tubes. As a support tube material having the function as an air electrode, La is 20 at% by Ca or 10 at Sr.
A LaMnO ₃ solid solution material substituted with １５15 atomic% is used.

[0004] Further, a single cell of a flat plate type fuel cell uses the same material system as that of the cylindrical type, and as shown in FIG. A pole 8 is provided. For connection between the single cells, a dense L called "separator 9" to which dense MgO or CaO is added.
An aCrO ₃ solid solution material is used. Power generation is performed at a temperature of 1000 to 1050 ° C. by supplying air (oxygen) to the air electrode side of the cell and fuel (hydrogen) to the fuel electrode side.

[0005]

However, a cell having a structure in which the above CaO-stabilized ZrO ₂ is used as a support tube and a LaMnO ₃ material in which SrO is dissolved as a solid solution is provided as an air electrode, and the air electrode is directly supported In producing a cylindrical fuel cell having a structure used as a tube, production of an air electrode is usually performed at 1200 to 1500 ° C. For this reason, the air electrode is maintained at a high temperature of 1300 ° C. in the above-described EVD method, and at a temperature close to that temperature in thermal spraying. For this reason, the deformation of the air electrode occurs during this time, which lowers the yield of cell fabrication. Further, even when power is generated at 1000 to 1050 ° C. for a long time, there is a problem that the air electrode is gradually deformed, the connection between cells is deteriorated, and the output gradually decreases. Similarly, in a flat fuel cell, the air electrode is deformed and peeled off, or the electrolyte is damaged.

The air electrode in the fuel cell needs to have gas permeability by itself, and when the air electrode functions as a support tube, the support tube needs to have high strength. To increase gas permeability, the open porosity should be large, but if the open porosity is large, the strength of the air electrode itself decreases, and when the air electrode is used directly as a support tube,
There are problems such as breakage of the support tube when forming an electrolyte on the surface and breakage due to handling in other manufacturing processes.
The present invention can improve gas permeability and strength of an air electrode in a cylindrical fuel cell and a flat-plate fuel cell, and can also cause cell damage or air electrode deformation due to deformation of the air electrode during cell production or long-term operation. An object of the present invention is to provide a stable and long-life solid oxide fuel cell having an output that does not cause separation.

[0007]

Means for Solving the Problems The present inventor has conducted various studies on the characteristics of the material constituting the air electrode in order to achieve the above object, and as a result, has found that the crystal phase is LaMnO ₃ (lanthanum manganite). When a crystal made of an oxide containing at least one element selected from rare earth elements other than Y and La in addition to the perovskite-type main crystal phase was present as a second crystal phase in the sintered body, Have been achieved, and the present invention has been achieved.

That is, a solid electrolyte fuel cell according to the present invention is a solid electrolyte fuel cell having an air electrode formed on one side and a fuel electrode formed on the other side of the solid electrolyte, wherein the air electrode contains at least La and Mn. It is composed of a main crystal phase composed of a perovskite-type composite oxide and a second crystal phase composed of an oxide containing at least one element selected from rare earth elements other than Y and La. 01-2
0% by weight, the average grain size of the main crystal phase is 3 to 20 μm, the average grain size of the second crystal phase at the triple junction is 0.1 to 7 μm, and the perovskite type The composite oxide contains, in addition to La and Mn, at least one selected from the group consisting of Y and rare earth elements and / or Ba, Ca and Sr.

Hereinafter, the present invention will be described in detail. The solid oxide fuel cell of the present invention has a cylindrical shape as shown in FIG.
Alternatively, the present invention can be applied to any form of a flat plate type as shown in FIG. Further, the air electrode is usually formed of a conductive porous body, and in some cases, the air electrode is also used as a support tube.

The air electrode in the fuel cell of the present invention is:
Although a LaMnO ₃ -based perovskite-type crystal is contained as a main crystal phase, a major feature of the present invention is that an oxide containing at least one element selected from rare earth elements other than Y and La in addition to the main crystal phase. Is included as the second crystal phase, and this configuration can suppress the deformation of the air electrode at a high temperature.

FIG. 3 shows the state of the second crystal phase. As is clear from FIG. 2, the second crystal phase 10
Among the grain boundaries of the main crystal grains 11, there is a so-called two-plane region 12 sandwiched between the two main crystal grains 11,
A so-called 3 surrounded by three or more main crystal grains 11
It exists in the focus area 13. When present in the triple point region 13, the second crystal phase has a thickness of 0.1 to 7 μm, particularly 0.1 μm.
Exists as crystal grains having an average particle size of 1 to 2 μm, and 10 nm to 1000 nm when present in the inter-plane region 12;
In particular, it is desirable to exist as crystal particles having an average particle size of 50 to 500 nm. If such a second crystal phase exists in at least one of the inter-plane region and the triple point region, it has a function of suppressing deformation.

The second crystal phase is contained in the total amount of 0.01 to 20.
%, Particularly preferably 0.01 to 10% by weight, more preferably 1 to 5% by weight. If the amount is less than 0.01% by weight, the effect of suppressing the deformation of the air electrode is insufficient. If the amount exceeds 20% by weight, the effect of suppressing the deformation is obtained, but the electric conductivity is reduced and the function as the air electrode is achieved. Disappears. In the present invention, the metal component of the main crystal phase may be very slightly dissolved in the second crystal phase. For example, diffusion of Y or Ca component into ZrO ₂ or CeO ₂ and diffusion of Ca into NiO are observed depending on firing conditions. However, no particular problem occurs when the main crystal phase satisfies the above formula 1. .

The main crystal phase in the present invention is LaM
It is an nO ₃ -based perovskite crystal, and its composition formula is shown below.

[0014]

Embedded image

Wherein A is at least one element selected from the group consisting of Y and rare earth elements, B is at least one element selected from the group consisting of Ca, Sr and Ba, and C is Ni, C
and at least one element selected from the group consisting of o, Fe, Cr, Ce and Zr, and 0 ≦ x ≦ 0.4
0, 0.10 ≦ y ≦ 0.60, 0.85 ≦ z ≦ 1.1
It is important that 0, 0 ≦ p ≦ 0.30 be satisfied.

[0016] This, Ca for La, substitution ratio y of such Sr is smaller than LaMnO ₃ specific 0.10 8
Since the volume change occurs without suppressing the phase transformation around 00 ° C., the air electrode is easily deformed. If y is larger than 0.60, the sinterability is reduced and the air electrode having a predetermined open porosity is reduced. It is necessary to bake at a temperature of 1650 ° C. or more for forming, which is uneconomical.

On the other hand, when the substitution ratio x of Y or the rare earth element with respect to La exceeds 0.40, the sintering suppressing effect is remarkably reduced. It is inferred that the lattice distortion in the crystal increases due to the difference in ion size between La and rare earth elements such as Y and Yb, the activation energy for diffusion of La and Mn decreases, and as a result, the sintering rate increases. You.

Further, the ratio between the A site and the B site is not necessarily required to be 1: 1.
Even in the range of 5-1.10, similar results are obtained because the lattice defect structure is similar to that of the stoichiometric system. However, when z is smaller than 0.85, Mn ₂ O ₃ precipitates and sintering is promoted. Conversely, when z exceeds 1.10, La ₂ O ₃ is precipitated and reacts with moisture or carbon dioxide in the air to decompose the material in a short time.

Further, Ni, Co, Fe, C with respect to Mn
When the substitution ratio p of r, Ce and Zr exceeds 0.3, M
Lattice strain increases due to the difference in ion size from n ions, and the activation energy for diffusion of La and Mn decreases. As a result, the sintering rate increases and the deformation of the air electrode is promoted.

The preferred composition range of the main crystal phase of the air electrode material in the present invention is 0 ≦ x ≦ 0.10, 0.10 ≦ y ≦
0.30, 0.95 ≦ z ≦ 1.00, 0 ≦ p ≦ 0.10
It is.

In the system of the present invention in which the A site is deficient, Ni, Co, Fe, Cr,
When Ce and Zr are slightly dissolved, or Ni, C
In a system to which o, Fe, Cr, Ce, Zr, Ni and Cr are added, an oxide containing the component, for example, NiO, Cr ₂
If O ₃ or the like is only a small amount deposited there is also no particular problems both shows a sufficient inhibitory effect on the firing.

The air electrode in the fuel cell of the present invention is
It is necessary to have gas permeability by itself, and when the air electrode functions as a support tube, the support tube needs to have high strength. The larger the open porosity is, the better the gas permeability is, but if the open porosity is large, the strength of the air electrode itself decreases, and when the air electrode is used directly as a support tube, an electrolyte is formed on the surface. In such a case, there is a problem that the support tube is damaged at the time, or damaged by handling in other manufacturing processes. Therefore, from the relationship between strength and gas permeability,
The main crystal phase is constituted by average crystal grains of 3 to 20 μm, particularly 5 to 15 μm. This is because if the particle diameter of the main crystal phase is smaller than 3 μm, the gas permeability is low although the strength is high, and if it exceeds 20 μm, the gas permeability is high but the strength is insufficient. The open porosity of the air electrode is suitably from 20 to 45%, particularly from 30 to 40%. The average pore diameter in the range of 1.0 to 5.0 μm is excellent in gas permeability.

In the present invention, as a method for producing the above-mentioned air electrode, specifically, an oxide of a metal constituting the composition of the LaMnO ₃ -based material as shown in the above-mentioned chemical formula 1 or heat treatment is carried out. Carbonates, nitrates, acetates and the like capable of forming oxides are mixed at a predetermined ratio, and
A calcination treatment is performed at a temperature of 00 ° C. to 1600 ° C. to perform a solid solution treatment. Thereafter, the solid solution is pulverized to obtain a solid solution powder. Next, a powder of a metal oxide constituting the second crystal phase is mixed with the solid solution powder at a predetermined ratio, and a desired molding means such as a mold press, a cold isostatic press is used by using the mixture. After forming into an arbitrary shape by extrusion molding or the like, baking is performed. The firing may be performed in an oxidizing atmosphere at 1100 to 1600 ° C. for about 2 to 15 hours. In addition, the precipitation of the second crystal phase can be prepared by adding an oxide to be a target precipitate in advance at the time of preparing the solid solution powder in advance, and then depositing the excess at the stage of firing using the solid solution powder. .

The solid oxide fuel cell of the present invention can be applied to any of a flat plate type as well as a cylindrical shape as shown in FIG. 1, and the air electrode is usually made of a conductive porous material. In some cases, the cathode is also used as a support tube.

[0025]

As a result of research focusing on the lattice defect structure of LaMnO ₃ in which La is replaced by Sr or Ca of 10 to 20 atomic%, the following results are obtained in solid solution in air and at high temperature.

[0026]

Embedded image

As shown in the above, it was found that oxygen was taken into the lattice and vacancies of La and Mn ions were generated to maintain the electrical neutrality of the system.

From these results, the sintering of the air electrode during power generation is as follows:
It is considered that the La, Mn ion vacancy concentration is high in the LaMnO ₃ solid solution, so that the diffusion rate of cations is high, and as a result, sintering is promoted. In a fuel cell unit, a reaction between fuels is partially released as electric power and partially as heat.

It is determined that the deformation of the cell is due to the non-uniformity of the temperature distribution of the fuel cell, which does not uniformly promote the sintering of the air electrode. From these inferences, the present inventor thought that if the lattice defect structure in the crystal is changed to suppress the generation of La and Mn ion vacancies, the sintering of the air electrode and the accompanying cell deformation can be suppressed.

In the present invention, attention was paid to the lattice defect structure of the perovskite-type oxide system, and as a result of further detailed investigation, it was found that La
It has been found that an effect of suppressing sintering occurs by dispersing another crystal phase different from the MnO ₃ -based crystal grain boundaries. When precipitates are present at the grain boundaries, the limit size D of particles that can grow for empirical movement is as follows:

[0031]

(Equation 1)

## EQU2 ##

From the results, it can be seen 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 the precipitates based on the above empirical formula, the oxide containing at least one element selected from rare earth elements other than Y and La effectively suppresses sintering. They have found what they can do.

Further, the present inventors have repeatedly studied the composition of the LaMnO ₃ -based main crystal phase with respect to the effect of suppressing the sintering of the air electrode. As a result, it was found that La was replaced with an alkaline earth element such as Ca, Sr, and Ba. And in some cases La
It has also been found that by substituting with Y or a rare earth element, the effect of promoting sintering is small, the cell deformation is small, and stable power generation performance is exhibited. The reason for this is, for example, that LaMnO ₃ in which La is simultaneously replaced by Y and Ca is mainly

[0035]

Embedded image

A lattice defect represented by the formula (1) is generated. In a material having this lattice defect, oxygen ion vacancies are predominantly generated instead of La and Mn ion vacancies. Therefore, the diffusion rates of La and Mn ions are suppressed,
It is believed that the sintering rate is reduced, thereby preventing cell deformation. Further, in the present invention, even in a non-stoichiometric system in which the atoms at the A site in the structure are deficient, the same effect as in the stoichiometric system can be obtained because the lattice defect structure is similar to the stoichiometric system. Further, a system in which a part of Mn is replaced with Co, Ni, Fe, CrCe, and Zr also has similar lattice defects, and can further prevent cell deformation. Further, by setting the average particle size of the main crystal phase to 3 to 20 μm, the gas permeability and strength of the air electrode can be improved.

[0037]

Next, the present invention will be described based on embodiments.

Example 1 Using various commercially available metal oxide powders having a purity of 99.9% or more as shown in Tables 1 to 3 as starting materials, they were blended so as to have predetermined compositions as shown in Tables 1 to 3, and zirconia bottles were prepared. 10 using
After mixing for 1 hour, a solid phase reaction was performed at 1500 ° C. for 10 hours to prepare an oxide powder composed of a perovskite phase. This powder and an oxide powder containing Y, a rare earth element and an alkaline earth element shown in Tables 1 to 3 are mixed so as to have a predetermined ratio,
This mixed powder was mixed and ground using a zirconia ball for 10 to 15 hours. After this, outer diameter 14mm, inner diameter 10m
m, length of 60 mm and formed into a cylindrical shape, 1400-15
It was fired at 00 ° C. to obtain a cylindrical sintered body having an open porosity of 28 to 35%. The precipitate in the sintered body was confirmed by X-ray diffraction. The particle diameter of the main crystal phase was measured by SEM, and the particle diameter of the second crystal phase was measured by SEM and TEM observation.

After maintaining the cylindrical sintered body in an electric furnace at 1200 ° C. for 400 hours in the atmosphere, the outer diameter of the cylindrical sintered body was measured.

[0040]

(Equation 2)

The deformation ratio was calculated according to the following.

The length of the cylindrical sintered body is 50 m.
m was cut out from the cylinder, and the resistance R was measured by the four-terminal method in the atmosphere at 1000 ° C.

[0043]

(Equation 3)

Thus, the sheet resistance Rs was measured. The results of each measurement are shown in Tables 1 to 3, respectively.

For comparison, a commercially available purity of 99.9% was used.
La _0.85 Sr _0.15 MnO ₃ and La _0.85 Ca _0.15 MnO
Using No. ₃ , a cylindrical sintered body was produced in the same manner as above, and the particle diameter, deformation rate, and sheet resistance were measured.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

According to Tables 1 to 3, the conventional samples Nos. 1 and 2 consisting of a single phase of perovskite type crystal have a large deformation rate, whereas the inclusion of the second crystal phase has a large deformation rate. Can be reduced. However, from Tables 1 and 2, it was found that the amount of the second crystal phase was less than 0.01 wt%,
It can be seen that the samples Nos. 11 and 58 show large deformation and the samples No. 17 and 62 exceeding 20% by weight have a large sheet resistance. From this result, the amount of the second crystal phase is 0.01 to
It can be seen that 20% by weight is excellent.

Further, from Table 1, it is found that Ca, Sr with respect to La.
Sample No. 3, 25 where y is less than 0.1
In this case, the phase transformation unique to LaMnO _{3 at} around 800 ° C. could not be suppressed, resulting in large deformation and high resistance. Also,
In Sample No. 10 in which y exceeded 0.6, the sinterability was poor and a sintered body having a predetermined open porosity could not be obtained.

From Table 2, it can be seen that the substitution ratio x for La such as Y is x.
The sample No. 43 having a value of more than 0.4 had a large deformation.
Sample No. 4 in which z is smaller than 0.88 for indeterminate z
In No. 8, Mn ₂ O ₃ was precipitated and the deformation was large. In the sample No. 44 in which z exceeds 1.10, La ₂ O ₃
Precipitated and the sample decomposed in a short time. Further, regarding the substitution of Cr, Ce, etc. for Mn, the deformation ratio was increased in Sample No. 66, in which the ratio p exceeded 0.3.

On the other hand, all of the products of the present invention exhibited excellent characteristics with a deformation rate of 1.0% or less and a sheet resistance of 0.08Ω or less. In addition, as a result of observing the precipitation of the second crystal phase, the second crystal phase is present at both the triple junction and the interplanar boundary of the main crystal phase. 7
10 to 10 at the grain boundary between two surfaces
It was confirmed that it was present as fine particles of 00 nm.

Example 2 The powder of No. 6, 63 composition prepared in Example 1
mm, an inner diameter of 11 mm and a length of 100 mm into a cylindrical tube, fired at 1400 ° C. to 1580 ° C., and an open porosity of 30 to
A 40% cylindrical sintered body was obtained. Thereafter, the cylindrical sintered body was cut into lengths of 10 mm and 50 mm, and the gas permeability coefficient of the former was measured at room temperature (20 to 22 ° C.) using N ₂ gas. The radial crushing strength was determined. The crystal particle diameter of the sintered body was determined by SEM observation, and the results are shown in Table 4.

[0054]

[Table 4]

According to Table 4, the larger the crystal particle diameter, the larger the gas permeability coefficient but the smaller the strength. Therefore, taking into account the handling in the manufacturing process, the crystal grain size is 3
The range of ２０20 μm is excellent.

In this range, the gas permeability coefficient is 0.05 (l · c).
m ² / g · min · (cmHg)) or more, radial crushing strength 8
(Kg / cm ² ) or more can be achieved.

Example 3 Using the compositions of Nos. 1, 2, 5, 46 and 67 in Example 1, the porosity was 34 to 38%, the length was 200 mm, and the outer diameter was 16
A cylindrical sintered body having an inner diameter of 12 mm and an inner diameter of 12 mm sealed at one end was prepared and used as a support tube for a cell having a function as an air electrode.
Thereafter, an electrolyte (10 mol% Y ₂ O _3-) having a thickness of about 50 μm was formed on the surface of the cylindrical sintered body at 1300 ° C. by a gas phase synthesis method.
90 mol% of ZrO ₂ ), and a 70-% by weight N layer having a thickness of 50 μm by slurry-dipping.
The fuel electrode of zirconia containing i (containing 8 mol% Y ₂ O ₃ ) was covered to form a single cell. The cell was held in an electric furnace, oxygen gas was supplied inside the cell, and hydrogen gas was supplied outside, and power was generated at 1000 ° C. for 3000 hours.

FIG. 3 shows the result. No. of the present invention.
With respect to 5, 46 and 67, the power density hardly changed. In contrast, the output density of the conventional product No. 1 decreased with time. Further, No. 2 was gradually deformed, and the cell was broken after about 1600 hours. From this, the excellent performance of the present invention was recognized.

[0059]

As described in detail above, according to the present invention,
When used as an air electrode of a cylindrical solid electrolyte fuel cell, gas permeability and strength can be improved, and cell growth due to grain growth and sintering deformation of the air electrode during cell fabrication or inter-cell damage during power generation due to this. Can be provided, and a cell having long-term stability can be provided. Further, even in a flat-plate type fuel cell, it is possible to provide a fuel cell whose output is stable for a long time by preventing separation due to deformation of the air electrode, solving problems such as a decrease in output, and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a cylindrical fuel cell.

FIG. 2 is a view showing the structure of a flat fuel cell.

FIG. 3 is a view for explaining a state of existence of a second crystal phase in an air electrode material according to the present invention.

FIG. 4 is a diagram showing the relationship between the power generation time and the output density of each sample of Example 3.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 support tube 2 7 air electrode 3, 6 solid electrolyte 4 8 fuel electrode 5 interconnector 9 separator 10 second crystal phase 11 main crystal particles

Claims (1)

(57) [Claims] 1. A solid electrolyte fuel cell in which an air electrode is formed on one side of a solid electrolyte and a fuel electrode is formed on the other side, wherein the air electrode has a main crystal phase comprising a perovskite-type composite oxide containing at least La and Mn. , Y, and a second crystal phase made of an oxide containing at least one element selected from rare earth elements other than La.
The main crystal phase has an average particle size of 3 to 20 μm, and the second crystal phase at the triple point boundary has an average particle size of 0.1 to 7 μm; at least the composition formula of the perovskite-type composite oxide represented by _{(La 1-xy a x B} y) z (Mn 1-p C p) O 3 ± δ, a is selected from rare earth elements other than Y and La 1 Seed elements, B is Ca, Sr and Ba
At least one element selected from the group of
i, Co, Fe, Cr, Ce and Zr, at least one element selected from the group consisting of
y, z and p are 0 ≦ x ≦ 0.40, 0.10 ≦ y ≦
0.60, 0.85 ≦ z ≦ 1.10, 0 ≦ p ≦ 0.30
A solid oxide fuel cell that satisfies the following.