JP2003331866A - Solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell stably realizing efficient power generation even at a low temperature. <P>SOLUTION: This solid electrolyte fuel cell is constituted by arranging an electrolyte layer 2 between an air electrode layer 3 and a fuel electrode layer 4, and the electrolyte layer 2 is made of lanthanum gallate base oxides, and the air electrode layer 3 is made of lanthanum cobaltite base oxides, and an intermediate layer comprising ceria base oxides and having a thickness of 20 μm or less is formed between the electrolyte layer 2 and the air electrode layer 3 by a spin coating method or a dip coating method. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solid oxide fuel cell, and more particularly to a solid oxide fuel cell (SOFC) in which an electrolyte layer is disposed between an air electrode layer and a fuel electrode layer.

[0002]

2. Description of the Related Art Solid oxide fuel cells (SOFC) are
Since the power generation capacity is high among fuel cells, research and development for practical use are being advanced for applications such as power plants.

In such a solid oxide fuel cell, an electrolyte layer is composed of a plurality of unit cells arranged between an air electrode layer and a fuel electrode layer, and an interconnector for electrically connecting the unit cells. It is configured to supply an oxidizing gas such as air to the air electrode layer, and to supply a fuel gas such as hydrogen to the fuel electrode layer, and to supply oxygen to the oxygen in the supplied oxidizing gas to the electron in the air electrode layer. Is given to ionize the oxygen ions (O ²⁻ ) to diffusely move toward the fuel electrode layer in the electrolyte layer. In the fuel electrode layer, the oxygen ions reaching the fuel electrode layer react with the fuel gas. Then, electric power is obtained by generating water and carbon dioxide gas and at the same time emitting electrons.

In addition, yttrium-doped stabilized zirconia (YSZ), which has high oxygen ion conductivity and is chemically stable, is widely used in the electrolyte layer of such a solid oxide fuel cell. There is. However, this stabilized zirconia usually needs to be operated at around 1000 ° C., and has a defect that the ionic conductivity is lowered when the operating temperature is lowered.

Therefore, in recent years, as an electrolyte layer, it has a higher oxygen ion conductivity than that of stabilized zirconia, and
Various attempts have been made to reduce the operating temperature of a solid oxide fuel cell by using a lanthanum gallate-based oxide having a perovskite structure, in which the decrease in oxygen ion conductivity is small even when the operating temperature is lowered.

[0006]

[Problems to be Solved by the Invention] However, in the electrolyte layer,
When a lanthanum gallate-based oxide is used, long-term operation causes a mutual diffusion of the metal elements contained in each layer between the electrolyte layer and the air electrode layer, and a chemical reaction between both layers causes the electrolyte layer and the air electrode to react. From a change in material composition with the layer, an impurity layer or an insulation resistance layer having a high resistance value is generated at the interface between the electrolyte layer and the air electrode layer, and the electrode overvoltage rises with time. As a result, during a long-term operation, the power generation voltage is lowered due to an increase in the electrode overvoltage, and the power generation performance is degraded.

The present invention has been made in view of the above problems, and an object thereof is a solid oxide type which can stably realize efficient power generation for a long time even at a low operating temperature. It is to provide a fuel cell.

[0008]

In order to achieve the above object, the present invention provides a solid oxide fuel cell in which an electrolyte layer is arranged between an air electrode layer and a fuel electrode layer, The layer is made of a lanthanum gallate-based oxide represented by the following general formula (1): La _1-x A _x Ga _{1- y By} O _3-α (1) (wherein A is Sr, Ca, Ba) , B is Co, Mg, F
e, Ni, Mn, Cu, x is 0 ≦ x ≦ 0.2, y
Indicates a numerical value of 0 ≦ y ≦ 0.25. ) The cathode layer is
It consists of a lanthanum cobaltite oxide represented by the following general formula (2): La _{1-m A'm} Co _{1-n B'n} O _3-β (2) (In the formula, A'is Sr, Ba, Ca, B'is Fe, M
n, Cu, Ni, m is 0 ≦ m <1.0, n is 0 ≦
A numerical value of n <1.0 is shown. ) Wherein between the electrolyte layer and the air electrode layer, ceria based oxide _{Ce 1-z A '' z} O 2-γ (3) (A ' represented by the following general formula (3)' is a rare earth element, At least one element selected from alkaline earth elements and alkaline elements is shown, and z is 0 <
A numerical value of z ≦ 0.6 is shown. ) Is provided as an intermediate layer.

Further, the solid oxide fuel cell of the present invention is
The thickness of the intermediate layer is preferably 20 μm or less, and the intermediate layer is preferably formed by a spin coating method or a dip coating method.

[0010]

1 is a schematic explanatory view showing an embodiment of a single cell of a solid oxide fuel cell of the present invention.
In FIG. 1, the single cell 1 includes an electrolyte layer 2 and an air electrode layer 3 and a fuel electrode layer 4 which are arranged on both sides of the electrolyte layer 2, respectively. An intermediate layer 5 is further provided between them.

The electrolyte layer 2 is formed of a lanthanum gallate oxide represented by the following general formula (1).

[0012] La in _{1-x A x Ga 1-} y B y O 3-α (1) ( wherein, A is Sr, Ca, and Ba, B is Co, Mg, F
e, Ni, Mn, Cu, x is 0 ≦ x ≦ 0.2, y
Indicates a numerical value of 0 ≦ y ≦ 0.25. That is, the electrolyte layer 2 has a perovskite structure (AB
O ₃ ), and the A site has La and A elements (Sr,
At least one element selected from Ca and Ba occupies the B site, and Ga and B elements (Co, Mg, F)
At least one element selected from e, Ni, Mn, and Cu) occupies, and oxygen vacancies generated due to the A element and the B element express oxygen ion conductivity. The number of oxygen atoms is reduced by the amount of oxygen vacancies.

The element A is preferably Sr, and the element B is preferably Mg.

X is the atomic ratio of the A element, and is 0 to 0.
2 range, that is, A element is not contained,
When it is contained, it is in the range of 0.2 or less, preferably in the range of 0.05 to 0.15. When the atomic ratio of the A element exceeds 0.2, impurities containing the A element as a main component are generated.

Further, y is the atomic ratio of the B element, and is 0
To 0.25, that is, B element is not contained, or, if contained, is 0.25 or less, preferably 0.1 to 0.2. . B
If the atomic ratio of the elements exceeds 0.25, impurities containing the B element as the main component are produced.

Actually, since it is necessary to generate oxygen vacancies, at least one of the A element and the B element is contained.

When the electrolyte layer 2 is formed from such a lanthanum gallate-based oxide, it is stable from a high temperature oxidizing atmosphere to a reducing atmosphere and has a higher oxygen ion conductivity at any temperature than that of stabilized zirconia. Therefore, for example, efficient power generation can be realized even at an operating temperature of 800 ° C. or lower.

For such an electrolyte layer 2, for example, a slurry in which a salt containing each of the above components is blended in the above composition ratio is prepared, a green sheet or the like is molded, and then fired. It is not particularly limited and can be formed by a known method. In addition, the electrolyte layer 2
Is formed to have a thickness of, for example, about 200 to 500 μm.

The air electrode layer 3 is made of a lanthanum cobaltite oxide represented by the following general formula (2).

La _{1-m A'm} Co _{1-n B'n} O _3-β (2) (In the formula, A'is Sr, Ba and Ca, and B'is Fe and M.
n, Cu, Ni, m is 0 ≦ m <1.0, n is 0 ≦
A numerical value of n <1.0 is shown. That is, the air electrode layer 3 has a perovskite structure (ABO ₃ ), similar to the electrolyte layer 2, and the A site is La.
And A'element (at least one element selected from Sr, Ca and Ba) occupies the B site, Co and B '
An element (at least one element selected from Fe, Mn, Cu, and Ni) is occupied, and oxygen vacancies generated due to the elements A and B express oxygen ion conductivity. The number of oxygen atoms is reduced by the amount of oxygen vacancies.

The A'element is preferably Sr, and the B'element is preferably Fe.

M is the atomic ratio of the element A ', and is 0 to
The range of 1.0 (less than), that is, the A'element is not contained or, if contained, is less than 1.0, and preferably in the range of 0.1 to 0.5. It is said that

N is the atomic ratio of the B'element, and is 0 to
The range of 1.0 (less than), that is, the B ′ element is not contained, or, if contained, is less than 1.0, preferably 0 to 0.3. It

Actually, since it is necessary to generate oxygen vacancies, at least one of A'element and B'element is contained.

If the air electrode layer 3 is formed of such a lanthanum cobaltite oxide, the air electrode layer 3 with a small overvoltage can be formed even at an operating temperature of 800 ° C. or lower.

The air electrode layer 3 as described above is prepared, for example, by preparing a slurry in which a salt containing each of the above-mentioned components is blended in the above-mentioned composition ratio, and preparing the slurry on the electrolyte layer 2 described later. After applying by a doctor blade method or a screen printing method on the intermediate layer 5 formed in
It can be formed by baking. The thickness of the air electrode layer 3 is, for example, 50 to 100 μm.
Formed as a degree.

The fuel electrode layer 4 is not particularly limited as long as it can be used as a fuel electrode layer of a solid oxide fuel cell. For example, Ni and ceria-based oxide similar to the intermediate layer 5 described later can be used. It is formed by a known method from a cermet material or the like, which is a composite material with a product (Ce _1-x A ″ _x O _2-x ). The fuel electrode layer 4 is formed to have a thickness of, for example, about 50 to 100 μm.

The intermediate layer 5 is provided between the electrolyte layer 2 and the air electrode layer 3, and is made of a ceria-based oxide represented by the following general formula (3).

Ce _1-z A ″ _z O _2-γ (3) (A ″ represents at least one element selected from rare earth elements, alkaline earth elements, and alkali elements, and z is 0 <
A numerical value of z ≦ 0.6 is shown. ) That is, the intermediate layer 5 has a fluorite structure (AO ₂ ), and the A site is replaced with Ce and A ″ elements (rare earth elements,
At least one element selected from alkaline earth elements and alkali elements) occupies the oxygen vacancy caused by the A ″ element, and thus oxygen ion conductivity is exhibited.
The number of oxygen atoms is reduced by the amount of oxygen vacancies.

The A ″ element is a rare earth element such as Sm, Y, La, Nd, Gd, Yb, Tb, Er,
Examples of the alkaline earth element include Ca and Sr, and examples of the alkaline element include Pr.
For example, Na, K, Cs, etc. may be mentioned. The A ″ element is preferably a rare earth element, more preferably Sm,
Examples thereof include Y and Gd.

Z is the atomic ratio of the A ″ element, and is 0
The range of (<) to 0.6, that is, the A ″ element is 0.
It is in the range of 6 or less, and preferably in the range of 0.2 to 0.4 in order to develop high oxygen ion conductivity.

If the atomic ratio of the A ″ element exceeds 0.6, the fluorite structure cannot be maintained.

When the intermediate layer 5 is formed of such a ceria-based oxide, it exhibits a high oxygen ion conductivity at a temperature of 800 ° C. or lower, and for the electrolyte layer 2 of a lanthanum gallate-based oxide, At 1250 ° C or higher, a solid solution was observed between some components, but no reaction product was produced.
Further, below 1200 ° C., no solid solution between the components was observed,
It becomes chemically stable without the formation of reaction products. Further, it is chemically stable with respect to the air electrode layer 3 made of a lanthanum cobaltite oxide.

Therefore, by providing such an intermediate layer 5, the electrochemical action of the electrolyte layer 2 is not hindered, and the intermediate layer 5 serves as the electrolyte layer 2 and the air electrode layer 3.
Since it is stable to both layers, the formation of impurity layers and insulation resistance layers due to mutual diffusion of metal elements and chemical reaction between the electrolyte layer 2 and the air electrode layer 3 is prevented, and long-term operation Stable operation can be realized.

If the intermediate layer 5 is made of a ceria-based oxide, the air electrode layer 3 may be peeled or cracked.
Since it is possible to prevent the electrochemical action from decreasing and to form the intermediate layer 5 having fine particles, it is possible to improve the initial performance due to the improvement of the three-phase interface length of the air electrode layer 3. .

For such an intermediate layer 5, for example, a slurry in which a salt containing each of the above components is blended in the above composition ratio is prepared, and the slurry is formed on the surface of the electrolyte layer 2, for example. Applying by liquid phase thin film forming method such as spin coating method or dip coating method, or forming into thin film by vapor phase thin film forming method such as sputtering method or CVD method, and then baking 20 μm or less, preferably 10
It can be formed as a thin film having a thickness of less than or equal to μm, preferably 3 μm or more.

When the thickness of the intermediate layer 5 exceeds 20 μm, the intermediate layer 5 serves as a resistance layer, which may rather lower the power generation performance. Further, in order to form the intermediate layer 5 having a thickness of 20 μm or less, the spin coating method or the dip coating method is preferably used. According to the spin coating method or the dip coating method, a thin film can be formed easily and reliably.

More specifically, the intermediate layer 5 can be formed as follows. That is, first, a nitrate aqueous solution of each metal component having the above composition is prepared (for example, a nitrate salt of each metal component is dissolved in a solution of 10 ml of nitric acid and 10 ml of pure water with respect to 0.02 mol of a target oxide). ), And with respect to the obtained nitrate aqueous solution,
Add ethylene glycol if necessary (for example, about 40 mol for 0.02 mol of the target oxide).
Stir to prepare a uniform aqueous nitrate solution. And 70
C. to 90.degree. C., the viscosity of the aqueous nitrate solution is 0.0
When the temperature reaches 5 to 0.2 Pa · s, stop heating and
A polymer precursor solution (slurry) is obtained. Then, as described above, a thin film of the polymer precursor is formed on the electrolyte layer 2 by the spin coating method or the dip coating method.

For example, in the case of forming by the spin coating method, the electrolyte layer 2 formed into a predetermined shape.
Is rotated for about 1 minute at a rotation speed of 1800 to 2500 min ⁻¹ , a polymer precursor solution is dropped on the surface of the rotating electrolyte layer 2, and then a thin film of the obtained polymer precursor is, for example, Heat at 320 ° C. to 340 ° C. for 1 minute or more. Then, a thin film having a required thickness can be formed on the electrolyte layer 2 by repeating such a process from dropping to heating several times.

Then, the thin film of the polymer precursor thus formed is heated at 1000 ° C. to 1150 ° C. for 2 to 1
The intermediate layer 5 is formed by baking for 0 hours.

In forming the intermediate layer 5 as described above, for example, in the case of spin coating, the viscosity of the polymer precursor solution is 0.05 to 0.2 P as described above.
It is preferably a · s. If it is less than 0.05 Pa · s, most of the particles may not be fixed as a film on the electrolyte layer 2 but may be scattered to the outside due to rotation. Also,
If it exceeds 0.2 Pa · s, it may not spread evenly over the entire surface of the electrolyte layer 2, and the intermediate layer 5 having a uniform film thickness may not be formed.

The baking temperature is 1 as described above.
It is preferably 000 ° C to 1150 ° C. 1150
If the temperature exceeds ° C, impurities may be generated due to the reaction with the electrolyte layer 2 during baking. Further, when the temperature is less than 1000 ° C., when the air electrode layer 3 is baked on the intermediate layer 5,
Impurities may be generated by reacting with the air electrode layer 3.

In the solid oxide fuel cell of the present invention, actually, a plurality of such single cells 1 are laminated by a known method, and in each single cell 1, the air electrode layer 3 is formed.
Is supplied with an oxidizing gas such as air, and a fuel gas such as hydrogen is supplied to the fuel electrode layer 4. In the air electrode layer 3, oxygen in the supplied oxidizing gas is ionized by the addition of electrons. . And the oxygen ion (O ²⁻ )
Are transferred to the electrolyte layer 2 through the intermediate layer 5 and diffused and moved in the electrolyte layer 2 toward the fuel electrode layer 4. After that, in the fuel electrode layer 4, oxygen ions that have reached the fuel electrode layer 4 react with the fuel gas to generate water and carbon dioxide gas, and at the same time, electrons are emitted, so that electric power can be obtained. .

In the solid oxide fuel cell of the present invention, an electrolyte layer 2 made of a lanthanum gallate-based oxide,
By combining with the air electrode layer 3 made of a lanthanum cobaltite oxide, for example, efficient power generation can be realized even at a temperature of 800 ° C. or lower, and moreover, the electrolyte layer 2 and the air electrode layer 3 can be combined. Since an intermediate layer 5 made of a ceria-based oxide that is stable with respect to both layers is provided between them, impurities due to mutual diffusion of metal elements or chemical reaction between the electrolyte layer 2 and the air electrode layer 3 are provided. Since it is possible to prevent the formation of a layer or an insulation resistance layer, it is possible to stabilize the electrochemical action between the electrolyte layer 2 and the air electrode layer 3. As a result, efficient power generation can be stably realized over a long period of time.

[0045]

EXAMPLES 1) Evaluation of oxygen ion conductivity La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _2.85
The polymer precursor solution of 0.16 Pa · s was repeatedly applied 10 times by spin coating on the surface of the electrolyte layer 2 having the composition of 0.1, and then heat treatment was performed at 1000 ° C. for 2 hours to obtain Ce _{0. 8} Sm _0.2 O
An intermediate layer 5 having a composition of _1.8 and a thickness of 10 μm was formed. When the oxygen ion conductivity of the intermediate layer 5 was measured by a DC 4-terminal method, it was 1.24 × 10 at 800 ° C.
It was ^-1 S / cm. Moreover, when the ionic conductivity of only the electrolyte layer 2 was measured, it was 1.24 × 10 ⁻¹ S / cm.
Met. From these, it was confirmed that the intermediate layer 5 did not affect the oxygen ion conductivity of the electrolyte layer 2.

2) Evaluation of durability Example 1 La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85
Electrolyte layer 2 having a composition of LaSr_0.6Co
_0.4O_3-βAir electrode layer 3 having a composition of Ni / C
e_0.8Sm_0.2O_1.8Electrode layer 4 having the composition
And Ce_0.8Sm _0.2O_1.8Intermediate with composition
A single cell solid oxide fuel cell comprising layer 5 and
It was made. The intermediate layer 5 is a spin coater on the electrolyte layer 2.
Coating 10 times by coating method and heat treatment at 1100 ℃
It was formed by doing. The thickness of the intermediate layer 5 is 1
It was 0 μm.

Example 2 La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85
Electrolyte layer 2 having a composition of LaSr_0.6Co
_0.4O_3-βAir electrode layer 3 having a composition of Ni / C
e_0.8Sm_0.2O_1.8Electrode layer 4 having the composition
And Ce_0.8Sm _0.2O_1.8Intermediate with composition
A single cell solid oxide fuel cell comprising layer 5 and
It was made. The intermediate layer 5 is a spin coater on the electrolyte layer 2.
Coating 5 times by coating method and heat treatment at 1000 ℃
It was formed by The thickness of the intermediate layer 5 is 5 μm.
It was m.

Example 3 La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85
Electrolyte layer 2 having a composition of LaSr_0.6Co
_0.4O_3-βAir electrode layer 3 having a composition of Ni / C
e_0.8Sm_0.2O_1.8Electrode layer 4 having the composition
And Ce_0.8Sm _0.2O_1.8Intermediate with composition
A single cell solid oxide fuel cell comprising layer 5 and
It was made. The intermediate layer 5 is a spin coater on the electrolyte layer 2.
Coating 3 times by coating method and heat treatment at 1000 ℃
It was formed by The thickness of the intermediate layer 5 is 3 μm.
It was m.

Comparative Example 1 La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _2.85
And an electrolyte layer 2 having a composition of LaSr _0.6 Co
Air electrode layer 3 having a composition of _0.4 O _3-β , and Ni / C
e Fuel electrode layer 4 having a composition of _0.8 Sm _0.2 O _1.8
From the above, a single cell solid oxide fuel cell in which the intermediate layer was not formed was produced.

Durability Test 800 for each cell of Examples 1-3 and Comparative Example 1.
At 0 ° C, air is supplied to the air electrode layer 3 and humidified hydrogen of 3 vol% H ₂ O + H ₂ is supplied to the fuel electrode layer 4, and a current of 0.3 A / cm ² is applied to generate power for 500 hours. The behavior of the cell voltage was observed, and the cell voltage was measured 100 hours and 500 hours after the cell was stabilized. The results are shown in Table 1 and FIG.

[0051]

[Table 1] For each cell of Example 1 and Comparative Example 1, 0.
After applying a current of 3 A / cm ² , the current is cut off, the voltage between the air electrode and the reference electrode that decays with time is measured, and the change over time of the air electrode reaction overvoltage excluding the internal resistance due to ohmic loss is measured. It was measured by the interruption method. The result is shown in FIG.

From Table 1, FIG. 2 and FIG. 3, Examples 1-3 are shown.
It was confirmed that, in comparison with Comparative Example 1 having no intermediate layer, the air electrode overvoltage was low, the initial performance was improved, and the cell voltage was stable.

[0053]

As described above, according to the solid oxide fuel cell of the present invention, between the electrolyte layer made of lanthanum gallate type oxide and the air electrode layer made of lanthanum cobaltite type oxide. Since an intermediate layer composed of a stable ceria-based oxide is provided for both of these layers, the impurity layer and the insulation resistance layer due to the mutual diffusion and chemical reaction of the metal element between the electrolyte layer and the air electrode layer are formed. Since the generation can be prevented, the electrochemical action between the electrolyte layer and the air electrode layer can be stabilized.

Further, by forming the intermediate layer from a ceria-based oxide, it is possible to prevent a decrease in electrochemical action due to the action of peeling or cracking of the air electrode layer, and to form an intermediate layer having fine particles. Therefore, it is possible to improve the initial performance due to the improvement of the three-phase interface length of the air electrode layer. As a result, efficient power generation can be stably realized over a long period of time.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing an embodiment of a single cell of a solid oxide fuel cell of the present invention.

FIG. 2 is a diagram showing changes with time in cell voltage of Examples 1 to 3 and Comparative Example 1 during 500 hours of operation.

FIG. 3 is a diagram showing a change with time of an air electrode reaction overvoltage in Example 1 and Comparative Example 1 during a 500-hour operation.

[Explanation of symbols]

1 single cell 2 Electrolyte layer 3 Air electrode layer 4 Fuel pole layer 5 Middle class

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toru Inagaki             3-3-22 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture             Kansai Electric Power Co., Inc. (72) Inventor Hiroyuki Yoshida             3-3-22 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture             Kansai Electric Power Co., Inc. F term (reference) 5H018 AA06 AS03 EE13 HH05                 5H026 AA06 BB03 BB04 EE13 HH03                       HH05

Claims (3)

[Claims] 1. A solid oxide fuel cell in which an electrolyte layer is disposed between an air electrode layer and a fuel electrode layer, wherein the electrolyte layer is a lanthanum gallate-based oxide represented by the following general formula (1). made things, _La in _{1-x a x Ga 1-} y B y O 3-α (1) ( wherein, a is Sr, Ca, and Ba, B is Co, Mg, F
e, Ni, Mn, Cu, x is 0 ≦ x ≦ 0.2, y
Indicates a numerical value of 0 ≦ y ≦ 0.25. ) The air cathode layer is made of a lanthanum cobaltite oxide represented by the following general formula (2): La _{1-m A'm} Co _{1-n B'n} O _3-β (2) (wherein A'is Sr, Ba, Ca, B'is Fe, M
n, Cu, Ni, m is 0 ≦ m <1.0, n is 0 ≦
A numerical value of n <1.0 is shown. ) Wherein between the electrolyte layer and the air electrode layer, ceria based oxide _{Ce 1-z A '' z} O 2-γ (3) (A ' represented by the following general formula (3)' is a rare earth element, At least one element selected from alkaline earth elements and alkaline elements is shown, and z is 0 <
A numerical value of z ≦ 0.6 is shown. ), A solid oxide fuel cell. 2. The solid oxide fuel cell according to claim 1, wherein the thickness of the intermediate layer is 20 μm or less. 3. The solid oxide fuel cell according to claim 1, wherein the intermediate layer is formed by a spin coating method or a dip coating method.