JP2003308846A - Perovskite oxide and air electrode for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a perovskite oxide whereby activated energy of an oxygen ion even at low temperatures from 400°C to 600°C is equivalent to the energy of a conventional example at a temperature of 800°C, and thereby electrode performance is enhanced, and to provide an air electrode for a fuel cell, which comprises the pervskite oxide. <P>SOLUTION: An rare earth element, alkaline earth metal and cobalt are contained in the A1 site, A2 site and B site of an ABO<SB>3</SB>type perovskite oxide, respectively. Meanwhile, the occupancy ratio of the rare earth element to the alkaline earth metal is determined as one from 0.05:0.5 to 0.1:0.9. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a perovskite type oxide and a fuel cell air electrode comprising the perovskite type oxide.

[0002]

2. Description of the Related Art A perovskite oxide is represented by the chemical formula ABO ₃ , as shown in FIG. A site is A
One of the perovskite type oxides consisting of 1 and A2,
There is one in which the A1 site is made of a rare earth element, the A2 site is made of an alkaline earth metal, and the B site is made of Co (cobalt). This type of perovskite oxide generally has a property of easily adsorbing oxygen molecules in oxygen gas.

A case where a perovskite type oxide of A (A1A2) BO ₃ type is used for an air electrode of a fuel cell will be described. A fuel cell is a cell that directly supplies power by externally supplying hydrogen and the like obtained from a fuel such as natural gas and oxygen in the atmosphere and electrochemically reacting these. It has been attracting attention as a power source for power generation and a power source for electric vehicles in recent years because it has good power generation efficiency because it is not restricted by the Carnot cycle, and it has a clean exhaust gas and little impact on the environment.

Fuel cells have different types of electrolytes.
It is classified into phosphoric acid type, molten carbonate type, solid electrolyte type (sometimes referred to as “solid oxide type”), and polymer type. Of these, the solid oxide fuel cell (SOFC) uses natural gas and has a high operating temperature (800 ° C to 1000 ° C)
Therefore, it has advantages such as good power generation efficiency.

As shown in FIG. 17, a solid oxide fuel cell 50 has a unit cell 5 including a solid electrolyte 51 and a pair of electrodes (air electrode 52 and fuel electrode 53) arranged on both sides of the solid electrolyte 51.
5 and the separator 57 interposed between the adjacent single cells
And consists of. As shown in FIG. 18, air (oxygen gas, nitrogen gas, etc.) is supplied to the air electrode 52, and fuel gas (hydrogen gas, etc.) is supplied to the fuel electrode 53. Oxygen molecule O ₂ in the supplied oxygen gas receives an electron e ⁻ at the surface of the air electrode 52 and becomes an oxygen ion O ²⁻ . The oxygen ions O ²⁻ diffusely move in the solid electrolyte 51 from the air electrode 52 toward the fuel electrode 53, and react with hydrogen in the vicinity of the interface with the fuel electrode 53. As a result, electrons e ⁻ are emitted to the fuel electrode 53, and a reaction product (water) is generated. The electron e ⁻ flows to the air electrode 52 through the external circuit 58, and the load 59 in the middle of the external circuit 58.
To work against.

Next, the materials of the solid electrolyte 51, the air electrode 52 and the fuel electrode 53 will be described. The solid electrolyte 51 is a conductor of oxygen ions O ²⁻ and separates hydrogen from oxygen. Therefore, it is required that the conductivity of oxygen ion O ²⁻ is high, that it is chemically stable in an oxidizing atmosphere or a reducing atmosphere, and that it is dense. In consideration of this, as the material, for example, yttria-stabilized zirconia (YSZ) is adopted.

An air electrode (also called "cathode") 5
2 is required to easily adsorb oxygen molecules. In consideration of this, the material is, for example, A (A1A2) BO ₃
LaSr which is one of the perovskite type oxides
MnO ₃ is adopted. Further, the fuel electrode (also referred to as “anode”) 52 is required to be thermodynamically stable in a high-temperature reducing atmosphere. In consideration of this, as the material, for example, Ni-YSZ is adopted.

The above A1 site is composed of a rare earth element,
2 sites consist of alkaline earth metal, B site is Co
As one of the perovskite type oxides composed of SmC
There is a case where oO3 is doped with Sr (strontium), which is represented by Sm _1-x Sr _x CoO ₃ . In the conventional example (see Non-Patent Document 1), La is used as the solid electrolyte 51.
(Sr) Ga (Mg) O ₃ -based perovskite type oxide is used, and Sm _1-x Sr _x CoO ₃ is used as the air electrode 52.
We are investigating the doping amount of Sr to the Sm site in the case of adopting.

Curve a in FIG. 19 shows Sm _0.7 Sr _0.3 CoO ₃ in which 30% (molar amount) of Sm is doped with Sr.
Fig. 3 shows the relationship between the maximum power density and operating temperature in an air electrode consisting of. The maximum power density depends on the performance of the electrolyte, anode and cathode. The maximum power density is increasing due to the increase in operating temperature.
It is considered that the conductivity of the electrolyte is improved and the reaction (catalyst) activity is improved at the fuel electrode and the air electrode.

Curves b to f in FIG. 20 are Sm _1-x Sr _x C
₃ shows an Arrhenius plot of conductivity of a cathode 52 made of oO _{3 in} a pure oxygen atmosphere. The cases where the Sr content (value of x) is 0, 0.3, 0.4, 0.5 and 1 are shown by curves b, c, d, e and f, respectively. x = 0
In the case of SmCoO ₃ (see curve b), the conductivity of the air electrode (conductivity of oxygen ions and conductivity of electrons) is S
As the doping amount of r is increased to x = 0.3 (see the curve c) and 0.4 (see the curve d), it becomes higher, and x = 0.5.
In the case of (see curve e), that is, the air electrode made of Sm _0.5 Sr _0.5 CoO ₃ has the highest value. This is because oxygen vacancies are formed in the air electrode due to an increase in the doping amount of Sr, and more oxygen ions and electrons move. On the other hand, x = 1 is considered to pores in the air electrode 52 made of the time i.e. SrCoO ₃ (see curve f) is increased, conductivity is smaller than that of the air electrode 52 made of SmCoO _3.

Further, FIG. 21 shows that when the air electrode 52 made of Sm _1-x Sr _x CoO ₃ is operated at a temperature of 1073 K (about 800 ° C.) and a current density of 0.3 A / cm ^-2 , the value of x changes. The IR loss (see the curve g) and the overpotential (see the curve h) at the air electrode are shown. Here, "I
“R loss” is an internal resistance loss due to the electric resistance of the substance forming the air electrode, and is represented by a voltage value. The smaller the voltage value, the better the air electrode.

On the other hand, the "overpotential" is the internal loss of the air electrode due to the electrode reaction. In other words, it is the activation energy required for the electrode reaction and is also called activation polarization, and is represented by a voltage value (polarization value). The voltage value is determined by the activity of the material itself, the structure of the electrode, the reaction area, and the like.

Generally, the IR loss and the overpotential depend on the composition of the air electrode 52, and are a function of the doping amount of Sr doped in SmCoO ₃ . Here, regarding IR loss and overpotential, the doping amount of Sr is x =
It gradually decreases as it increases from 0.1 to 0.2 and 0.3, and when x = 0.5, that is, Sm _0.5 Sr _0.5 CoO
It is minimized in the air electrode 52 consisting of _three . As described above, the maximum power density at this time is the maximum. Therefore, it can be seen that the IR loss and the decrease in overpotential of the air electrode 52 are the result of the increase in the conductivity thereof. After that, as the value of x increases from 0.6 to 1.0,
IR loss and overpotential are gradually increasing.

[0014]

[Non-Patent Document 1] J. Electrochem. Soc. Vol. 145, No. 9,
September 1998

[0015]

The operating temperature of the solid oxide fuel cell will be described below. A solid oxide fuel cell is generally operated at 800 to 1000 ° C., but in order to use it as a power source for an electric vehicle, the operating temperature is about 600 ° C., preferably 600 ° C., from the viewpoint of startability and reliability. It is desirable that the temperature is lower than ° C. Here, the "startability" is to heat the fuel cell with a heater to raise the temperature to an operating temperature so that power can be generated. It is difficult to arrange a large heater in an electric vehicle having a limited space, and it is desirable to reduce the operating temperature as much as possible and use a small heater.

On the other hand, "reliability" means the thermal stress generated in the electrolyte and the electrode, and the thermal and chemical stability of the electrolyte and the electrode. The lower the operating temperature is, the more the thermal stress is prevented from occurring.
Thermal stability is improved. From the above viewpoints, development of high-performance air electrodes, fuel electrodes, and solid electrolytes that operate well even at temperatures of 600 ° C. or less is expected.

However, in the above-mentioned conventional example, Sm _0.5 Sr _0.5 Co
It only shows the electrode performance (IR loss and overpotential) when a solid oxide fuel cell including an air electrode made of O ₃ is operated at an operating temperature of 800 ° C. In a fixed electrolyte fuel cell employing Sm _1-x Sr _x CoO ₃ as an air electrode, if the operating temperature is lowered to 600 ° C. or lower, problems such as a decrease in conductivity and a decrease in reaction rate occur. Therefore, a large activation energy is required to generate oxygen ions, which deteriorates the electrode performance.

The deterioration of the battery performance can be prevented to some extent by reducing the thickness of the solid electrolyte and increasing its conductivity, but the practical technique for thinning the film is still undeveloped. Further, if a large amount of expensive noble metal catalyst is used to compensate for the deterioration of the cell performance, the manufacturing cost of the air electrode and thus of the fuel cell increases.

The present invention has been made in view of the above circumstances, and is A (A1A2) BO ₃ type (A1: rare earth element, A2: alkaline earth) capable of adsorbing oxygen molecules in oxygen gas even at a low temperature of 400 ° C to 600 ° C. It is an object of the present invention to provide a perovskite type oxide of a (metal). Further, the present invention is 4
An object of the present invention is to provide a fuel cell air electrode made of a perovskite type oxide, which operates well even at a low temperature of 00 ° C to 600 ° C, and has an electrode performance equivalent to that obtained when the conventional example is operated at 800 ° C. To do.

[0020]

The inventor of the present application has found that the overpotential in the above-mentioned conventional example gradually decreases as the value of x increases from 0.1 to 0.5. The reason why the number gradually increased as it increased from 6 to 1.0 was considered. As a result, the operating temperature is 800 ℃
In this case, it is thought that this is because, as the value of x gradually increases from 0.6, a large number of vacancies are formed and the structural stability of the perovskite type oxide is lowered, and the conduction of oxygen ions is hindered.

However, the overpotential is determined by the conductivity of the air electrode and the solid electrolyte, the catalytic activity of the air electrode, etc., in addition to the structural stability. If the operating temperature is 600 ° C. or lower, The present invention has been completed on the assumption that satisfactory electrode performance can be obtained even when the value of x gradually increases from 0.5.

The first invention of the present application is, as described in claim 1, an A (A1A2) BO ₃ type perovskite type oxide, wherein one of Sm, Pr, Gd and Nd at the A1 site or Multiple kinds of rare earth elements are added to S
r, Ca, and Ba each contain one or more kinds of alkaline earth metals, and each B site contains cobalt, and the occupation ratio of the rare earth element to the alkaline earth metal is 0.5: 0.5 to 0.1: It is characterized by being 0.9. This perovskite type oxide is used in fields and applications where adsorption of oxygen molecules in oxygen gas is required, and it effectively adsorbs oxygen molecules.

In the perovskite type oxide according to the second aspect, in the first aspect, the occupation ratio of the rare earth element and the alkaline earth metal is preferably 0.4: 0.6 to 0.
2: 0.8. The perovskite type oxide according to claim 3 is the same as that according to claim 1, wherein the operating temperature is from 400 ° C to 6 ° C.
It operates at 00 ° C and is used to adsorb oxygen molecules.

Further, the second invention of the present application has the A (A1A2) BO ₃ format as described in claim 4,
It consists of a perovskite type oxide containing a rare earth element at one site, an alkaline earth metal at A2 site, and cobalt at B site, and one surface of which is laminated on one surface of an electrolyte of a fuel cell to adsorb oxygen in the air. The air electrode is characterized in that the element occupancy ratio between the rare earth element and the alkaline earth metal is 0.5: 0.5 to 0.1: 0.9. This air electrode adsorbs oxygen molecules in the supplied air and serves as a conductor when oxygen ions move to the electrolyte.

According to a fifth aspect of the present invention, in the fuel cell air electrode according to the fourth aspect, the occupancy ratio of the rare earth element and the alkaline earth metal is preferably 0.4: 0.6 to 0.2: 0.
8 The air electrode for a fuel cell according to a sixth aspect is the air electrode according to the fourth aspect, wherein the air electrode has a flat plate shape or a cylindrical shape. In the air electrode for a fuel cell according to claim 7, in the fuel cell according to claim 4, the fuel cell is a solid electrolyte type.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION <Perovskite type oxide> The rare earth element at the A1 site of the perovskite type oxide of the present invention contains one or more of Sm, Pr, Gd and Nd.
Plural kinds of Sm, Pr, Gd and Nd mean that two, three or four elements can be selected from four elements. Of these rare earth elements, Sm is particularly desirable. On the other hand, A2
The alkaline earth metal at the site contains one or more of Sr, Ca and Ba. With plural kinds of Sr, Ca and Ba,
This means that two or three elements can be selected from the three elements. Of these alkaline earth metals, Sr is particularly desirable.

The number of combinations of the rare earth element at the A1 site and the alkaline earth metal at the A2 site is the number obtained by multiplying the number of rare earth elements by the number of alkaline earth metals. Among them, the combination of Sm and Sr is particularly desirable.

The occupation ratio of the rare earth element at the A1 site to the alkaline earth metal at the A2 site is 0.5: 0.5 to 0.
It is 1: 0.9. Therefore, the occupation ratio is 0.5: 0.
5, 0.4: 0.6, 0.3: 0.7, 0.2: 0.
8, 0.1: 0.9, etc. are included. However, the occupancy ratio other than these (between these), for example, 0.45: 0.55,
It is also possible to adopt 0.35: 0.65 or the like. In addition,
Desirable occupation ratio is 0.4: 0.6, 0.3: 0.7,
It is 0.2: 0.8.

If the alkaline earth metal increases more than the occupancy rate of 0.5: 0.5, the above oxygen adsorption power is reduced or insufficient, and such an occupancy rate is not desirable. On the contrary, when the occupancy rate is less than 0.1: 0.9, the crystal structure of the perovskite type oxide is distorted and the conductivity decreases, so that the occupancy rate is not desirable.

The perovskite type oxide of the present invention has a property of adsorbing oxygen. The reason is that the rare earth element at the A1 site and the Co at the B site are +3 valent ions, whereas the alkaline earth metal at the A2 site is a +2 valent ion, so that the alkaline earth metal is uncharged by injection of the alkaline earth metal. Stable C
This is because o ⁴⁺ is generated and the oxidizing power (activity for oxygen) is increased.

Due to this oxygen adsorption property, the perovskite type oxide is used, in addition to the air electrode of the fuel cell, firstly as a CO or lower hydrocarbon oxidation catalyst, and secondly as a complete combustion catalyst for methanol, methane, etc. Thirdly, it can be used as a catalyst for removing NOx in exhaust gas, fourthly as an oxidative coupling catalyst for methane, fifthly as a hydrogenation catalyst for ethylene, and sixthly as a CO or oxygen gas sensor or the like. <Fuel Cell, Air Electrode> A case where the perovskite oxide of the present invention is used as an air electrode of a solid oxide fuel cell (hereinafter, abbreviated as “fuel cell” in the embodiments of the invention) will be described. . The fuel cell includes a solid electrolyte, an air electrode on one surface side, and a fuel electrode on the other surface side. The form may be a flat plate type or a cylindrical type.

As described above in part, the air electrode is easily adsorbed with oxygen, the oxygen ions are easily moved toward the solid electrolyte, it is thermodynamically stable in a high temperature oxygen atmosphere, and Are required to be porous so that they can easily move to the interface with the solid electrolyte. Taking this into consideration, a perovskite-type oxide that contains a rare earth element at the A1 site, an alkaline earth metal at the A2 site, and cobalt at the B site, is porous and rich in gas diffusion, and has electron conductivity is It is suitable as a material. The air electrode is stacked on one surface of the solid electrolyte on one surface thereof, and has a plurality of recessed grooves forming a flow passage for oxygen gas or the like on the other surface. The solid electrolyte is a conductor of oxygen ions O ²⁻ and physically separates hydrogen from oxygen. Therefore, as described above in part, the material has high conductivity of oxygen ions O ²⁻ , is chemically stable in both the oxidizing atmosphere on the air electrode side and the reducing atmosphere on the fuel electrode side, and is dense and dense. It is required to be. Yttria-stabilized zirconia (YSZ) is generally used as the material. However, since the air electrode of the present invention operates at a low temperature of about 400 ° C. to 600 ° C., the material of the solid electrolyte is La (Sr) Ga (Mg, Co).
O ₃ is suitable. In addition, it is preferable to use lanthanum gallate doped with strontium or magnesium, or ceria doped with samarium. As mentioned above, the fuel electrode is required to be thermodynamically stable in a high-temperature reducing atmosphere, have high electronic conductivity, have an affinity for hydrogen, and be porous. To be done. Cermet is generally suitable as the material, and for example, Ni-YSZ is used. However, since the air electrode of the present invention operates at a low temperature of 400 ° C. to 600 ° C., it is desirable to use Ni-SDC or the like as the material of the fuel electrode.

The fuel electrode is laminated on one surface of the solid electrolyte on the other surface side of the solid electrolyte, and on the other surface thereof, a plurality of concave grooves forming a flow passage for hydrogen gas or the like are formed. A cell (unit cell) is formed by the cell, the separator (interconnector) solid electrolyte, the fuel electrode and the air electrode. Since a single cell has a low voltage, a plurality of cells are connected in series to obtain a desired voltage.

As described above, the separator partially connects adjacent cells in series in the flat plate type fuel cell. In a cylindrical fuel cell, it is called an interconnector. In either case, it is necessary to firmly connect the air electrode in the oxidizing atmosphere at high temperature and the fuel electrode in the reducing atmosphere. In consideration of this, it is composed of a dense substance that is strong against oxidation and reduction, has good electron conductivity in any atmosphere, and has no ionic conductivity. <Manufacturing Method> In the first type, the air electrode is Sm _1-x Sr _x
CoO ₃ raw material powder preparation process, calcination process, crushing process,
It is formed by a paste making process, a screen printing process and a baking process. In the second type, Sm by coprecipitation method
_{It comprises a} step of synthesizing a raw material powder of _1-x Sr _x CoO _3, a step of forming the raw material powder into a paste, a step of screen-printing the paste on an electrolyte, and a step of baking the paste. In both the first type and the second type, the temperature of the baking process can be 1000 ° C or 900 ° C.

[0035]

Embodiments of the present invention will be described below with reference to the accompanying drawings. <Example> (Solid-electrolyte fuel cell) As shown in FIG. 14, the flat-plate solid-electrolyte fuel cell (hereinafter, abbreviated as "fuel cell" in the Examples section) is a solid electrolyte 11
And a single cell 15 including an air electrode 12 and a fuel electrode 13 arranged on both sides thereof, and a separator 17 interposed between adjacent single cells.

The plate-shaped solid electrolyte 11 is La (Sr) G.
It is made of a (Mg, Co) O ₃ and has a thickness of 0.5 mm. The plate-shaped air electrode 12 is made of Sm _0.2 Sr _0.8 CoO ₃ and has a thickness of about 20 μm, and a concave groove 12 a through which oxygen gas flows is formed on one surface thereof. The flat plate-shaped fuel electrode 13 is made of Ni-SDC, has a thickness of about 20 μm, and has a groove 13a through which hydrogen gas flows, formed on one surface thereof. (Manufacturing Method of Solid Electrolyte Fuel Cell) A manufacturing method of the fuel cell will be described. Fuel electrode 1 on one side of solid electrolyte 11
After forming 3, the air electrode 12 is formed on the other surface. For example, the solid electrolyte 11 is made of La (Sr) Ga (MgCo) O ₃ by a known method (for example, a doctor blade).
Fuel electrode 1 made of Ni-SDC on one surface of the solid electrolyte 11
3 is formed by a known method.

As shown in FIG. 1, the step of forming the air electrode 12 includes a step of preparing Sm _1-x Sr _x CoO ₃ raw material powder (step S 1), a step of calcination (step S 2) and a step of crushing (step S 3). , A paste production process (step S4), a screen printing process (step S5), and a baking process (step S6). Among these, in the preparation step of S1, six kinds of raw material powders, that is, Sm _0.6 Sr _0.4 Co
O ₃ , Sm _0.5 Sr _0.5 CoO ₃ , Sm _0.4 Sr _0.6 Co
O ₃ , Sm _0.3 Sr _0.7 CoO ₃ , Sm _0.2 Sr _0.8 Co
A raw material powder consisting of O ₃ and Sm _0.1 Sr _0.9 CoO ₃ is prepared.

In the calcination step of S2, the above six kinds of raw material powders are baked at 1000 ° C. for 24 hours. In the crushing step of S3, the calcined powder is crushed using a pestle and a pestle.
In the paste preparation step of S4, 0.2 g of polyethylene glycol is mixed with 0.5 g of the ground powder to prepare a paste.

In the screen printing step of S5, the solid electrolyte 11 is placed on a coating device (not shown), and a screen plate having the same shape as the air electrode 12 is placed above it. When the squeegee is moved along the screen plate, the paste (slurry) passes through the mold and the fixed electrolyte 11
Applied over. In the baking step of S6, the air electrode on which the paste is screen-printed is baked at 1000 ° C. for 4 hours.

Next, the preparation of the raw material powder will be described in detail with reference to FIG. As shown in FIG. 2, the raw material powder preparation step includes a raw material metal ion carbonate or hydroxide preparation step (step S11), a blending step (step S12), a synthesis step (step S13), and a drying step (step S14). ),
Decomposition firing step (step S15), decarbonization firing step (step S16), main firing step (step S17), crushing step (step S18) and product completion step (step S)
19).

Of these, the raw material metal ion carbonate or hydroxide prepared in S11 is mixed with water in S12. At this time, the composition can be controlled at a level of 1/1000 mol. In S13, citric acid is synthesized in the formulation. At this time, the chemical reaction represented by the formula (3) occurs.

When the compound is dried with M ₁ CO ₃ + M ₂ (OH) + CH ₃ OH (COOH) → M ₁ M ₂ {CH ₃ OH (COOH) ₃ } + CO ₂ + H ₂ O .. (3) S14 A uniform and fine complex oxide precursor citrate is formed. When the compound is decomposed and fired in S15, citric acid is decomposed. In the decarbonization firing of S16, firing is performed while removing carbon. At this time, only carbon dioxide and water are generated.

In the main calcination of S17, 300 times more than in the solid phase method.
Crystals are assembled at a temperature of about 1000 ° C lower than about 400 ° C. S1
By pulverizing No. 8, a powder having a large specific surface area and good reactivity can be obtained. The powder SmSrCoO ₃ as a product is completed in S19. Since this manufacturing method is a wet method, uniform and highly pure raw materials can be industrially produced, and the cost can be reduced.

In operation, the fuel cell is heated by the heater to 60
After being heated to 0 ° C., oxygen is supplied to the air electrode 12 and hydrogen is supplied to the fuel electrode 13. Then, oxygen becomes the air electrode 12
To reach the vicinity of the interface with the solid electrolyte 11 and receive electrons from the air electrode 12 to become oxygen ions. The oxygen ions diffusely move in the solid electrolyte 11 from the air electrode 12 toward the fuel electrode 13, and react with hydrogen in the vicinity of the interface with the fuel electrode 13. As a result, electrons are emitted to the fuel electrode 13 and water is generated. The electrons pass through the external circuit 18 and the air electrode 12
Current flows from the air electrode 12 to the fuel electrode 13. (Preparation of Test Fuel Cell) A test fuel cell (half cell) was prepared to examine the electrode performance (polarization value) of the air electrode 12. As shown in FIG. 3, the half cell 20 includes six kinds of air electrodes 22 having the same structure as the air electrode 12 on one surface and the other surface of the solid electrolyte 21 having the same structure as the solid electrolyte 11 of the fuel cell. It was formed. In this way, the reaction of only one of the air electrodes 22 can be taken out to evaluate the electrode performance, which is convenient. Below, the Sr content of the air electrode 22 is 0.4, 0.5, 0.6,
0.7, 0.8 and 0.9 half cells are respectively replaced by a first half cell, a second half cell, a third half cell and a fourth half cell.
They are called a half cell, a fifth half cell, and a sixth half cell.

The electrode performance of the air electrode 12 is evaluated by the polarization value (overpotential) for the following reason. Thermal energy for raising the air electrode 12 to an operating temperature (600 ° C.) and changing oxygen molecules into oxygen ions is externally supplied to the fuel cell. However, since the fuel cell 10 has internal resistance, not all of the added heat energy is used for power generation. The energy required for the oxygen molecules to change into oxygen ions is called activation polarization, and it can be said that the more easily the oxygen molecules are changed to oxygen ions, the smaller the polarization value and the better the performance of the air electrode. (Test of Electrode Performance) The electrode performance when oxygen was supplied to the first to sixth half cells to generate electricity was examined by the current interruption method. The test temperature is about 600 ° C., and the oxidant (atmosphere) is air.

FIG. 4 shows the electrode performance of the air electrodes in the first to sixth half cells. Current densit
y) was changed from 200 to 1200 mA / cm ^2, and the overpotential at that time was investigated. The "current density" is the amount of current per unit area of the air electrode, which is 10
When a current of 1 A is applied at cm ² , 1 A / cm ² = 0.1 A / c
It becomes m ² . The results of FIGS. 4 and 5 are shown in Table 1.

[0047]

[Table 1]

In each half cell, the overpotential also rises as the current density rises. The maximum output of the cell, that is, the fuel cell is obtained from the relationship between current and voltage. Therefore, when evaluating the electrode performance of the air electrode, it is meaningful to consider the relationship between the current density and the overpotential.

According to FIG. 4, for example, the current density is 600 m.
At A / cm ⁻² , as the amount of Sm decreases and the Sr content, that is, the content value of x increases from 0.4 to 0.5, 0.6, 0.7, the curves i, j, k And l, the overpotential is reduced. Then, as indicated by the curve m, when x = 0.8, that is, the air electrode is Sm _0.2 Sr.
The polarization value of the half cell 5 made of _0.8 CoO ₃ is the minimum. This result is the same for current densities other than 600 mA / cm ^-2 . In each of the first half cell to the fifth half cell, the amount of overpotential increases as the current density increases.

By the way, as shown by the curve n, x = 0.
9, that is, the sixth half cell in which the air electrode is made of Sm _0.1 Sr _0.9 CoO ₃ has deteriorated electrode characteristics. This is 600
It is considered that this is because the structure of the perovskite oxide became unstable even at 0 ° C.

The curve p in FIG. 5 shows that the air electrode is Sm _0.2 Sr _0.8.
When the fifth half cell made of CoO ₃ is operated at 600 ° C., it exhibits polarization characteristics with a current density of up to 2 A / cm ² = 2000 mA / cm ² . The polarization value increases as the cell density increases, and at 600 ° C, about 20 mV @ 300 mA /
cm ² , which is the overpotential of the conventional example (Sm _0.5 Sr _0.5 CoO ₃ ) 20 mV @ 300 mA / cm ² , 80
Comparable to (0 ° C operation).

Decrease in Sm content, ie increase in Sr content
The reason why the electrode performance of the air electrode 12 is improved by the addition of conductivity is
I looked it up from the side. In detail, it is the same as the above 6 types of air electrodes.
Six kinds of dense sintered bodies with the same composition, that is, Sm_0.6Sr
_0.4CoO₃, Sm_0.5Sr_0.5CoO₃, Sm_0.4Sr_0.6
CoO₃, Sm_0.3Sr_0.7CoO₃, Sm_0.2Sr_0.8Co
O₃, Sm_0.1Sr_0.9CoO₃Dense sintered body (1st
Dense sintered bodies 1 to 6) were produced. Dense firing
6 kinds of Sm_1-xSr_xCoO₃material
Powder preparation process, calcination process for 24 hours at 1000 ℃, 25
0 kgf / cm² Uniaxial forming process at 3000 kgf /
cm²CIP (Cold Isostatic Pressing) molding process, and 1250
It consists of a firing step at 5 ° C. for 5 hours. In addition, raising and lowering of the firing process
The temperature rate is 150 ° C / h from room temperature to 850 ° C.
It is 50 ° C./h from 850 ° C. to 1250 ° C.

From the first to the sixth dense sintered body, 500 to 80
The conductivity when operated at 0 ° C. was measured by the DC 4-terminal method. The results are shown in FIG. 6 as curves r, q, r,
Denote by s, t, u and v. Curve q is Sm _0.6 Sr _0.4 Co
O ₃ , the curve r is Sm _0.5 Sr _0.5 CoO ₃ , and the curve s is S
m _0.4 Sr _0.6 CoO ₃ and the curve t is Sm _0.3 Sr _0.7 Co
O ₃ , the curve u corresponds to Sm _0.2 Sr _0.8 CoO ₃ , and the curve v corresponds to Sm _0.1 Sr _0.9 CoO ₃ .

As is clear from this, the electrical conductivity of all the dense sintered bodies was almost the same regardless of the content of Sr. Therefore, it is considered that the increase of the ion conductivity in the total conductivity due to the increase of oxygen vacancies and the improvement of the oxygen adsorption property and the catalytic activity of the surface contribute to the improvement of the electrode performance. (Observation of Micro Structure) Next, in order to investigate the relationship between the micro structure of the air electrode 22 of the first to sixth Haar cells after the test and the electrode performance, the micro structure of each air electrode 22 was examined by a scanning electron microscope (SEM). ). The results are shown in FIGS. 7 to 12.

FIG. 7 shows the microstructure of the first half cell (Sr content is 0.4), FIG. 8 is the microstructure of the second half cell (Sr content is 0.5), and FIG. 9 is the third half cell ( Microstructure with Sr content of 0.6).
The microstructure of the half cell (Sr content is 0.7), FIG. 11 is the microstructure of the fifth half cell (Sr content is 0.8), and FIG. 12 is the sixth half cell (Sr content is 0.
The microstructure of 9) is shown respectively.

From these, the air electrodes 2 of all half cells
In 2 it can be seen that the porous structure is homogeneous. Moreover, as the Sm content decreased and the Sr content increased, the size of the particles forming the porous structure was slightly increased. From this, as mentioned above,
It is considered that the decrease in overpotential as the Sr content increases from 0.4 to 0.8 in the first half cell to the fifth half cell does not depend on the particle size.

However, in the sixth half cell in which the overpotential increased, the Sr content was 0.9, and the grain size suddenly increased, causing grain growth.
This grain growth is considered to be the reason why the electrode performance of the sixth half cell is lower than that of the fifth Haar cell.

Summarizing the above results, when operating at 600 ° C., the overpotential gradually decreased with increasing Sr content in the first half cell to the fifth half cell, and the polarization value of the fifth half cell became the minimum. The reason is considered as follows. Firstly, a large number of oxygen vacancies are formed in the air electrode 12, more oxygen ions can be moved, and the oxygen ion conductivity of the air electrode 12 is increased. Secondly, the structure of the perovskite-type oxide is homogeneous and the size of the particles is small, which makes the structure fine and stable. Thirdly, the oxygen adsorption property and the catalytic activity are improved by the addition of the alkaline earth metal.

The content of Sr, the electrode performance and the composition
The relationship with the structure is when the operating temperature is 600 ° C.
It was If the operating temperature is 500 ° C, the catalyst activity may decrease.
The only difference is that the activation polarization increases, but otherwise
This is almost the same as when the operating temperature is 600 ° C. Also, the product
When the kinetic temperature is 400 ° C, the activation polarization increases further
The only difference is that the operating temperature is 600 ° C otherwise.
It is almost the same as the case. Therefore, the operating temperature from 400 ℃ to 6
Electrode almost equivalent to 600 ° C in the range of 00 ° C
Performance and microstructure are obtained. <Modification> Next, the perovskite oxide of the present invention and
A modified example of the air electrode will be described. First modification The first modification is Sm._1-xSr_xCoO₃The raw powder of
Baking temperature for electrolyte in air electrode
However, it is different from the above embodiment. In detail, by the coprecipitation method
6 types of Sm_1-xSr_xCoO₃Raw material powder of Sm_0.6
Sr_0.4CoO₃, Sm_0.5Sr_0.5CoO₃, Sm_0.4Sr
_0.6CoO₃, Sm_0.3Sr_0.7CoO₃, Sm_0.2Sr_0.8
CoO₃, And Sm_0.1Sr_0.9CoO₃Of raw material powder
To achieve. Add polyethylene glycol to these raw material powders
In addition, it is made into a paste, and the paste is La (Sr) Ga (M
g, Co) O3 (LSGMCo) electrolyte (thickness
Screen printing on top of 5 mm). Then Sm
_0.6Sr_0.4CoO₃, Sm_0.5Sr_0.5CoO₃, Sm_0.4
Sr_0.6CoO₃, Sm_0.3Sr_0.7CoO₃, And Sm_{0. 1}
Sr_0.9CoO₃The paste consisting of the raw material powder of 1000 ℃
Then Sm_0.2Sr_0.8CoO₃Pace consisting of raw powder of
Bake at 1000 ° C and 900 ° C and use 7 air
Pole (electrode area: about 0.3 cm²) Was produced.

Then, a platinum electrode was prepared by screen printing and baking on the side opposite to the air electrode with the electrolyte in between, and eight half cells for electrode performance evaluation were completed. A platinum wire was fixed on the side surface of the electrolyte to serve as a reference electrode.

FIG. 13 shows the results of evaluating the electrode performance of the seven half cells at 600 ° C. by the current interruption method. In FIG. 13, the horizontal axis represents the current density (A / cm ² ) and the vertical axis represents the polarization value (overpotential) ηc (V). The curves a, b, c, d, e and f are Sm
_0.6 Sr _0.4 CoO ₃ , Sm _0.5 Sr _0.5 CoO ₃ , Sm _0.4
Sr _0.6 CoO ₃ , Sm _0.3 Sr _0.7 CoO ₃ , Sm _0.2 Sr
_This is a case where a paste containing raw material powders of _0.8 CoO ₃ and Sm _0.1 Sr _0.9 CoO ₃ was baked at 1000 ° C. On the other hand, the curve g is the case where the paste containing the raw material powder of Sm _0.2 Sr _0.8 CoO ₃ was baked at 900 ° C. That is, Sm
Paste containing raw material powder of _0.2 Sr _0.8 CoO ₃ is 1000 ℃
And baked at 900 ° C.

When an air electrode having a composition ratio of Sm _0.2 Sr _0.8 CoO ₃ baked at 1000 ° C. was operated at 600 ° C., the polarization value was 73 mV (current density: 1000 mV / c).
m ² ).

The air electrode having a paste baking temperature of 900 ° C. has a lower polarization value, and it was confirmed from the numerical data used to prepare FIG. 13 that it was 52 mV (current density: 1000 m
It was (V / cm ² ). This decrease in polarization value is due to Sm _0.2 Sr
_It is considered to be related to the refinement of the _0.8 CoO ₃ porous structure. 14 and 15 show the microstructures of the air electrode when the baking temperature was 1000 ° C. and 900 ° C., respectively. As is clear from this, due to the lowering of the baking temperature, Sm _0.2 Sr
It is considered that the porous structure of _0.8 CoO ₃ became finer and the number of reaction sites increased accordingly. In the second modification, the rare earth element at the A1 site of the perovskite type oxide was Sm, and the alkaline earth metal at the A2 site was Ca. Then, 70% of Sm is doped with Ca, and the composition formula is represented by Sm _0.3 Ca _0.7 CoO ₃ . Other configurations and test conditions are the same as those in the above embodiment.

As a result, the results shown in Table 2 were obtained. This polarization value is almost the same as that of the above-mentioned embodiment, and it is understood that it is excellent as an air electrode.

[0065]

[Table 2]

In the third modification, the rare earth element at the A1 site of the perovskite type oxide was Nd, and the alkaline earth metal at the A2 site was Sr. Then, 70% of Nd is S
Doping with r, the composition formula is represented by Nd _0.3 Sr _0.7 CoO ₃ . Other configurations and test conditions are the same as those in the above embodiment.

As a result, the results shown in Table 3 were obtained. This polarization value is almost the same as that of the above-mentioned embodiment, and it is understood that it is excellent as an air electrode.

[0068]

[Table 3]

[0069]

As described above, according to the perovskite type oxide of the first invention, oxygen is surely adsorbed in the atmosphere where the adsorption of oxygen molecules in oxygen gas is required.

The perovskite type oxide according to the second aspect more reliably adsorbs oxygen due to the desirable occupation ratio of the rare earth element and the alkaline earth metal. The perovskite type oxide according to claim 3 more reliably adsorbs oxygen at 400 to 600 ° C.

According to the fuel cell air electrode of the second aspect of the invention, oxygen molecules in the supplied air are reliably adsorbed by the air electrode and move to the electrolyte. As a result, electrode performance equivalent to that of a conventional fuel cell operating at around 800 ° C. can be obtained.

The air electrode for a fuel cell according to claim 5 has a desired electrode characteristic due to a desired occupation ratio of the rare earth element and the alkaline earth metal. The air electrode for a fuel cell according to claim 6 is suitable for incorporation in a fuel cell. The air electrode for a fuel cell according to claim 7 can be used as an air electrode in a solid oxide fuel cell.

[Brief description of drawings]

FIG. 1 is a process drawing showing a manufacturing process of an air electrode for a fuel cell which is an embodiment of the present invention.

FIG. 2 is a process drawing showing a manufacturing process of the raw material powder for the air electrode for a fuel cell.

FIG. 3 is a cross-sectional view of a half cell manufactured to investigate the electrode performance of the fuel air electrode.

FIG. 4 is a graph showing the relationship between the current density and the overpotential at the air electrodes of the first to sixth Haar cells.

FIG. 5 is a graph showing the relationship between the current density and the overpotential in the air electrode of the fifth Haar cell.

FIG. 6 is a graph showing the relationship between the operating temperature and the electrical conductivity of the air electrodes of the first to sixth Haar cells.

FIG. 7 is an electron microscopic observation view of the fine structure of the first half cell.

FIG. 8 is an electron microscopic observation view of a fine structure of a second half cell.

FIG. 9 is an electron microscopic observation view of the fine structure of the third half cell.

FIG. 10 is an observation view of a fine structure of a fourth half cell by an electron microscope.

FIG. 11 is an observation view of a fine structure of a fifth half cell by an electron microscope.

FIG. 12 is an observation view of a fine structure of a sixth half cell by an electron microscope.

FIG. 13 is a graph showing the electrode characteristics of a modified example of the above embodiment.

FIG. 14 is an electron microscopic observation view of the fine structure of the air electrode according to the modification.

FIG. 15 is an electron microscopic observation view of the fine structure of the air electrode according to the modification.

FIG. 16 is a perspective view showing the structure of a perovskite oxide.

FIG. 17 is an exploded perspective view of a solid oxide fuel cell.

FIG. 18 is an operation explanatory view of the fuel cell. Is.

FIG. 19 is a graph showing the relationship between the temperature and the maximum power density of a conventional solid oxide fuel cell.

FIG. 20 is a graph showing the electric conductivity of the air electrode in the conventional example.

FIG. 21 is a graph showing IR loss and overpotential of the air electrode in the above conventional example.

[Explanation of symbols]

10: Solid oxide fuel cell 11: Solid electrolyte 12: Air electrode 13: Fuel electrode 15: Cell 17: Separator

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[Procedure amendment]

[Submission date] November 19, 2002 (2002.11.
19)

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   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Ohara             2-4-1 Rokuno, Atsuta-ku, Nagoya-shi, Aichi               Fine Ceramics Center Foundation             Within (72) Inventor Takehisa Fukui             2-4-1 Rokuno, Atsuta-ku, Nagoya-shi, Aichi               Fine Ceramics Center Foundation             Within (72) Inventor Kenji Murata             2-4-1 Rokuno, Atsuta-ku, Nagoya-shi, Aichi               Fine Ceramics Center Foundation             Within F-term (reference) 4G030 AA08 AA09 AA10 AA11 AA28                       BA03 CA01 CA08 GA01 GA03                       GA04 GA08 GA14 GA20 GA27                 4G048 AA05 AB01 AB02 AC06 AD03                       AD08 AE05                 5G301 CA02 CD10                 5H018 AA06 AS03 CC03 CC06 EE13                       HH05 HH08                 5H026 AA06 CV02 EE13 HH05 HH08

Claims (7)

[Claims] 1. A (A1A2) BO ₃ type perovskite type oxide, wherein one or more rare earth elements of Sm, Pr, Gd and Nd are contained in the A1 site, and Sr, Ca and Ba are contained in the A2 site. One or more kinds of alkaline earth metals are contained at the B sites, respectively, and cobalt is contained, and the occupation ratio of the rare earth element and the alkaline earth metal is 0.5: 0.5 to 0.1: 0.9. A perovskite type oxide characterized by the above. 2. The occupation ratio of the rare earth element and the alkaline earth metal is preferably 0.4: 0.6 to 0.
The perovskite type oxide according to claim 1, wherein the ratio is 2: 0.8. 3. The perovskite-type oxide according to claim 1, which operates at an operating temperature of 400 ° C. to 600 ° C. and is used for adsorbing oxygen molecules. 4. A1 having an A (A1A2) BO ₃ format
It consists of a perovskite type oxide containing a rare earth element at the site, an alkaline earth metal at the A2 site, and cobalt at the B site. One surface of the perovskite oxide is laminated on one surface of the electrolyte of the fuel cell to adsorb oxygen molecules in the air. An air electrode for a fuel cell, wherein an element occupancy ratio between the rare earth element and the alkaline earth metal is 0.5: 0.5 to 0.1: 0.9. 5. The occupation ratio of the rare earth element and the alkaline earth metal is preferably 0.4: 0.6 to 0.
The air electrode for a fuel cell according to claim 4, wherein the air electrode is 2: 0.8. 6. The air electrode for a fuel cell according to claim 4, wherein the air electrode has a flat plate shape or a cylindrical shape. 7. The air electrode for a fuel cell according to claim 4, wherein the fuel cell is a solid electrolyte type.