JP2003229142A - Oxyfluoride, solid electrolyte and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxyfluoride in which good conductivity of the oxygen ion can be obtained even at the low temperature of 500-700°C and a solid electrolyte or the like made of that oxyfluoride. <P>SOLUTION: The oxyfluoride contains at least lanthanum La in the cationic part and contains florine F and oxygen O in the anionic part, and the ratio of the anion to the cation is 2.3 to 2.5. By making the ratio of the anion part to the cation part 2.3 to 2.5, the same crystalline structure as the fluorite structure can be maintained even at low temperatures of 500-700°C, and good conductivity of oxygen ions can be obtained. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an oxyfluoride,
The present invention relates to a solid electrolyte composed of oxyfluoride and a fuel cell containing the solid electrolyte.

[0002]

2. Description of the Related Art Oxyfluoride is an oxide in which a part of oxygen (O) is replaced with fluorine (F).
One of the oxides has a fluorite structure and has the chemical formula AB
₂ (A site is a cation part or a cation part, and B site is an anion part or an anion part). In FIG. 10 showing a fluorite structure, A represented by a black circle forms a face-centered cubic lattice, and there are lattice points at the center of each face other than the vertex. On the other hand, B represented by a white circle forms a simple cubic lattice, and has lattice points only at the vertices.

When one of O in the anion part of La ₂ O ₃ is replaced with F, it becomes an oxyfluoride (LaOF) containing La in the cation part. As shown in FIG. 11, in the crystal structure of LaOF, La indicated by a white circle forms a face-centered cubic lattice,
A part of O (here, four O's located in a diagonal direction) which are shown by black circles to form a simple cubic lattice are replaced with F by a circle with diagonal lines. As a result, the fluorite structure is asymmetric in the left-right direction, the front-back direction, and the height direction.

Oxyfluoride has a property of excellent conductivity of oxygen ions, and due to this characteristic, it has been attempted to use oxyfluoride as a solid electrolyte of a fuel cell. 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 are classified according to the type of electrolyte, one of which is a solid electrolyte type (also referred to as "solid oxide type"). Since natural gas is used as fuel and the operating temperature is high (around 1000 ° C), it has the advantage of good power generation efficiency.

As shown in FIG. 13, a solid oxide fuel cell 50 includes a solid electrolyte 51 and a unit cell 5 including 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. 14, 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 molecules O ₂ in the supplied oxygen gas pass through the air electrode 52 and reach the vicinity of the interface with the solid electrolyte 51, where the electrons e ⁻ are received from the air electrode 52 and become oxygen ions O ²⁻ . Oxygen ion O
^2- diffuses and moves in the solid electrolyte 51 from the air electrode 52 toward the fuel electrode 53, and reacts 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,
A reaction product (water) is produced. Electron e ^- is the external circuit 5
8 to the air electrode 52 to work on the load 59 in the middle of the external circuit 58.

The electrode reaction that takes place at the air electrode 52 is the ionization of oxygen molecules O ₂ into oxygen ions O ^2- , which is represented by the equation (1).

1 / 2O ₂ + 2e ⁻ → O ^{2 −} (1) On the other hand, the electrode reaction occurring at the fuel electrode 53 is the release of the electron e ⁻ due to the bond between the oxygen ion O ²⁻ and hydrogen. Yes,
It is expressed by equation (2).

H ₂ + O ₂ ⁻ → H ₂ O + 2e ⁻ (2) The solid electrolyte 51, the air electrode 52, and the fuel electrode 53 are made of materials suitable for the above electrochemical reaction. 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, for example, yttria-stabilized zirconia (YSZ) is adopted.

An air electrode (also called "cathode") 5
No. 2 is that oxygen molecules are easily adsorbed, oxygen ions are easily moved, electron conductivity is large, thermodynamically stable in a high temperature oxygen atmosphere, and porous. Required. Considering this, for example, La (Sr) MnO ₃ which is one of the perovskite type oxides is adopted. Furthermore, a fuel electrode (also called "anode")
52 is required to be thermodynamically stable in a high temperature reducing atmosphere, have high electron conductivity, have an affinity with hydrogen, be porous, and the like. In consideration of this, for example, Ni-YSZ is adopted.

The material of the solid electrolyte 51 will be described in detail. When YSZ is used as the material, a heat-resistant material that can withstand the high operating temperature of the fuel cell (around 1000 ° C.) must be adopted, and there is a problem of large internal resistance. The use of refractory materials limits the choice of materials. The internal resistance can theoretically be reduced to some extent by reducing the thickness of the solid electrolyte 51, but a practical technique for reducing the thickness has not been developed.

Under these circumstances, developments have been made to lower the operating temperature of solid oxide fuel cells. CeO ₂ and Bi ₂ O ₃ are also used as the material of the solid electrolyte 51 in order to operate the fuel cell at a lower intermediate temperature (around 800 ° C.). This lowers the operating temperature of the fuel cell, but has the drawback that the fixed electrolyte 51 is easily reduced in a hydrogen atmosphere and the conductivity of oxygen ions is easily lowered.

For example, when a fuel cell is used as a power source for an electric vehicle, it is required to further lower the operating temperature of the fuel cell. The operating temperature is preferably 500 to 700 ° C. (for example, 600 ° C.) from the viewpoint of startability and reliability. The "startability" is to heat the fuel cell with a heater and 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 that thermal stress is unlikely to occur in the electrolyte and the electrodes, and that the solid electrolyte 51 and the electrodes 52 and 53 are thermochemically stable. Is prevented and thermal and chemical stability is improved.

A fixed electrolyte 51 made of oxyfluoride having a LaOF type crystal structure is, for example, A, Pelloux, CR A.
cad.Sci., Ser.C, 276, 241 (1973) (hereinafter,
It is disclosed in "Conventional Example 1"). However, the composition of the solid electrolyte of Conventional Example 1 is close to the stoichiometric ratio and close to LaOF.

On the other hand, in JP-A-5-47404 (hereinafter referred to as "conventional example 2"), in a solid electrolyte made of LaOF, in order to improve the conductivity of oxygen ions at an operating temperature of 300 to 500.degree. A part of La in the cation part is replaced with strontium (Sr).
Specifically, a solid electrolyte powder in which LaF ₃ and SrF ₂ are mixed is fired in the air or an oxygen atmosphere to change to a LaOF type crystal structure. Next, the powder obtained by pulverizing this is formed into a fixed electrolyte membrane by plasma spraying.

[0016]

As described above, the composition of the oxyfluoride of Conventional Example 1 is close to LaOF. It has been confirmed that this oxyfluoride has a relatively small conductivity of oxygen ions at an operating temperature of 700 to 1000 ° C.
The conductivity of oxygen ions at 0 ° C or lower has not been confirmed.

On the other hand, in Conventional Example 2, relatively high conductivity of oxygen ions can be obtained. However, the composition of the solid electrolyte changes depending on the molding conditions and is unstable, and the desired composition is not always obtained. This is a problem because the quality of the solid electrolyte is not stable. In addition, in this solid electrolyte, F and O in the raw material react (LaF ₃ + 1 / 2O ₂ → L
aOF + F ₂ ), and the sintered density is poor because the gas-released pores remain. Therefore, the electromotive force of the fuel cell containing this solid electrolyte is about 1/2 of the theoretical electromotive force (theoretical value), and it cannot be said to be practical.

The present invention has been made in view of the above circumstances, and provides an oxyfluoride, a solid electrolyte composed of the oxyfluoride and the solid electrolyte which can obtain satisfactory oxygen ion conductivity even at an operating temperature of 500 to 700 ° C. An object is to provide a fuel cell including the same.

[0019]

Means for Solving the Problems
Attention was paid to the ratio of oxygen to fluorine in the anion part and the molar ratio of the anion part to the cation part in the oxyfluorides of Examples 1 and 2. As described above, the ratio of oxygen to fluorine in the anion part of the oxyfluoride of Conventional Example 1 (mol%
Was 1: 1 and the molar ratio of the anion part (F and O) to the cation part (La) was 2. Further, the ratio of oxygen to fluorine in the anion part of the conventional oxyfluoride 2 (ratio of mol%) is about 1: 1, and the molar ratio of the anion part (F and O) to the cation part (La and Sr) is 2: 1. Met.

Therefore, the present invention has been completed with the idea of increasing the ratio of fluorine at least in the anion part to change the ratio of oxygen to fluorine, that is, the molar ratio of the anion part to the cation part.

That is, the oxyfluoride according to the first invention of the present application contains at least lanthanum La in the cation part, contains fluorine F and oxygen O in the anion part, and has a molar ratio of the anion part to the cation part (hereinafter, "anion / cation"). "Ratio" is abbreviated as 2.3) to 2.5 (2.3≤).
Anion / cation ratio ≦ 2.5).

In this oxyfluoride, since the anion / cation ratio is set to 2.3 to 2.5, the symmetry of the crystal structure is improved as compared with LaOF (fluorine ion F ⁻ occupies the lattice point position, Oxygen ions O ^2- are randomly present between lattice points), and the conductivity of oxygen ions increases.

The solid electrolyte according to the second invention of the present application comprises an oxyfluoride containing at least La in the cation part, F and O in the anion part, and having an anion / cation ratio of 2.3 to 2.5. And has a flat plate shape or a cylindrical shape.

Further, the fuel cell according to the third invention of the present application,
A solid electrolyte comprising an oxyfluoride containing at least lanthanum La in the cation portion, F and O in the anion portion, and having an anion / cation ratio of 2.3 to 2.5, and having a flat plate shape or a cylindrical shape; And an air electrode arranged on one surface side of the solid electrolyte; and a fuel electrode arranged on the other surface side of the solid electrolyte.

Since the solid electrolyte and the fuel cell are used at a relatively low operating temperature (500 to 700 ° C.), a metal material can be used and damage due to thermal stress is prevented.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION <Oxyfluoride> The oxyfluoride of the present invention is a LaOF having a fluorite structure in which the composition of the anion portion is changed and an alkaline earth metal and / or an alkali metal is added to the cation portion. And has a fluorite structure. Therefore, in the following description, it is referred to as "fluorite oxyfluoride". 1st type The cation part of the 1st type fluorite type oxyfluoride is La
It has a LaOF type crystal structure. Anion /
Various compositions of the anion part having a cation ratio of 2.3 to 2.5 can be considered. However, La is +3 and oxygen is -2.
Considering that the valence and fluorine are −1, the composition formula LaO _1-x F _{1 + 2x} was considered in order to reduce the overall valence to zero. Then, the solid electrolyte 11 is 1 mm long, 5 mm wide, and 15 mm long.
A test piece cut out into a strip of mm was prepared, and the conductivity of oxygen ion at N ₂ (nitrogen) atmosphere and about 500 ° C. was examined while changing the value of x, that is, the structure of the anion portion and the anion / cation ratio. . The result is shown in FIG.

According to FIG. 1, when x is 0.4, that is, La
It can be seen that when O _0.6 F _1.8 , the conductivity of oxygen ions becomes maximum, and the conductivity decreases when x is larger or smaller than this. The conductivity of oxygen ions is Logσ = −2.6.
If it is larger than (σ = 0.0025 S / cm), it can be used as a fuel cell for an electric vehicle that generates power at an operating temperature of 500 to 700 ° C. From this, the value of x is about 0.
It can be seen that the selection can be made within the range of 3 to 0.5 (0.3 ≦ x ≦ 0.5).

When x = 0.3, the composition formula of oxyfluoride is LaO _0.70 F _1.6 , and the conductivity of oxygen ions is L.
ogσ = −2.55 (σ = 0.028 S / cm). Ratio of oxygen to fluorine in the anion part (ratio of mol%)
Is 0.70: 1.6≈1: 2.3, and the anion / cation ratio is 2.3. The ratio of oxygen to fluorine is 1:
If the ratio is smaller than 2.3 (the ratio of fluorine is small) or the anion / cation ratio is smaller than 2.3 (the anion portion is decreased), the conductivity of oxygen ions is sharply reduced.

On the other hand, when x = 0.5, the composition formula of oxyfluoride is LaO _0.5 F _2.0 , and the conductivity of oxygen ions is approximately Logσ = −2.3 (σ = 0.005S / cm). The ratio of oxygen to fluorine in the anion part is 0.5: 2.
0 or 1: 4, giving an anion / cation ratio of 2.5.

LaO _0.5 F _2.0 has the crystal structure shown in FIG. 12, white circles indicate La, black circles indicate O, and shaded circles indicate F. When the proportion of F is larger than this, the fluorite structure collapses and it approaches LaF ₃ rather than LaOF. From this, when the ratio of oxygen to fluorine exceeds 1: 4 and F increases, ionic conduction of fluorine ions (F ⁻ ) is exhibited, and the anion / cation ratio is larger than 2.5 (including fluorine). It is understood that it becomes difficult to maintain the fluorite structure when the anion portion increases.

From the above, the desirable range of the ratio of oxygen to fluorine in the anion portion is about 1: 2.3 to 1: 4 (1/2.
3 ≦ oxygen / fluorine ≦ 1/4), and the desirable range of the anion / cation ratio is 2.3 to 2.5. Highest conductivity about Log σ = −2.1 (σ = 0.008 S / c
m) was obtained at x = 0.4, ie LaO _0.6 F _1.8 , the ratio of oxygen to fluorine in the anion part was 0.6: 1.
8 = 1: 3 and the anion / cation ratio is (0.6+
1.8) /1.0=2.4.

When x = 0.35, that is, LaO _0.65 F
_{At 1.7} , the conductivity of oxygen ions is Logσ = −2.3.
(Σ = 0.005 S / cm). The ratio of oxygen to fluorine in the anion part is 0.65: 1.7≅1: 2.6, and the anion / cation ratio is 2.35.

When x = 0.45, that is, LaO _0.55
At F _1.9 , the conductivity of oxygen ions is about Logσ = -2.
2 (σ = 0.006 S / cm). The ratio of oxygen to fluorine in the anion portion is 0.55: 1.9≈1: 3.5, and the anion / cation ratio is 2.45.

The oxyfluoride of the present invention has a fluorite structure and is excellent in oxygen ion conductivity. Therefore, in addition to the solid electrolyte for fuel cells, it can be applied to, for example, an oxygen sensor, an oxygen pump, and the like. The case where it is applied to a solid electrolyte will be described later. The oxygen sensor is used, for example, to measure the concentration ratio of fuel and oxygen in the fuel atmosphere of an automobile engine in real time and control the air feed amount so that the optimum fuel is obtained. To form an oxygen sensor, electrodes made of platinum or the like are fixed on both surfaces of a thin film fluorite type oxyfluoride. 2nd type The cation part of the 2nd type fluorite type oxyfluoride is L
In addition to a, it contains one or more kinds of alkaline earth metal Ma and has a LaMaOF (Ma: alkaline earth metal) type crystal structure. Anion / cation ratio of 2.3 to 2.5
Various compositions of the cation part and the anion part are considered. However, La is +3 valence, alkaline earth metal is +2 valence,
Considering that oxygen is -2 and fluorine is -1,
Composition for the entire valence zero formula _{La 1-y Ma y O 1} -xy
F _{1 + 2x + y} (0.3 ≦ x ≦ 0.5, 0 <y ≦ 0.2) was considered. The anion / cation ratio of 2.4 is x =
0.4 and y = 0.1, which is the most desirable. The anion / cation ratio of 2.5 is x
= 0.5, y = 0.2 (La _0.8 Ma _0.2 0 _0.3 F
_2.2 ), and 2.3 becomes (La0 _0.7 F _1.6 ) when x = 0.3 and y = 0.

[0035] Since the value of y is from 0 to 0.2, (the ratio of the mol%) percentage of lanthanum to alkali earth metal in the cation portion La _1-y Ma _y is 1: 0 (provided that the alkaline earth metal Is 0% except 0.8: 0.2 = 8: 2 (0 <
Alkaline earth metal / lanthanum ≦ 1/4), preferably 0.9: 0.1 = 9: 1 to 8: 2 (1/9)
≦ Alkaline earth metal / lanthanum ≦ 1/4) can be selected. When this ratio exceeds 8: 2 and the value of alkaline earth metal, that is, y, increases, _{1 + 2x} in F _{1 + 2x + y}
Since the value of + y is 2.0 or more (the basic skeleton is LaF ₂ ), it becomes difficult for the oxyfluoride to maintain the fluorite structure.

Specific examples of the alkaline earth metal include magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba). Plural types mean two or more types. As described above, Sr or the like may be included in the cation portion in an amount of 10 to 20% by mol. Specific examples of the cation portion include La _0.9 S
r _0.1 and La _0.8 Sr _0.2 .

On the other hand, the anion portion O_1-xyF_{1 + 2x + y}Oke
The ratio of oxygen to fluorine in the range 1: 2.3 to 1: 4
You can choose. As mentioned above, x is between 0.3 and 0.5
In the range, y can be selected in the range of 0 to 0.2. This percent
If, for example, x = 0.4, y = 0.1 (O_0.5F_1.9)of
When it becomes 1: 3.8, x = 0.4, y = 0.2 (O
_0.4F_2.0), It becomes 1: 5. This ratio is 1: 2.3
Oxygen ion conductivity decreases as fluorine decreases below
However, when the amount of fluorine increases beyond 1: 5, the value of F is 2 or more.
Therefore, the fluorite structure collapses.

In the second type, the cation portion is La.
In addition to the inclusion of alkaline earth metal, the conductivity of oxygen ions is improved. It is known that the battery performance is improved by adding a small amount of alkaline earth metal to the cation portion in a LaOF-based solid electrolyte. However, in the case where the anion / cation ratio was 2.0 as in the conventional example, the battery performance was lowered by adding 10% or more of the alkaline earth metal. The reason is considered to be that the excess oxygen located between the lattice points decreases.

On the other hand, in the present invention, the anion / cation ratio is set to 2.3 to 2.5, and the alkaline earth metal is added in an amount of 0 to 20 mol%, preferably 10 to 20 mol%. Performance improved. It is considered that the reason is that the symmetry of the crystal structure of the oxyfluoride was improved by adding Sr or the like. Third type The cation part of the third type oxyfluoride contains one or more kinds of alkali metal Mb in addition to La, and LaMbO
It has an F (Mb: alkali metal) type crystal structure. Various compositions of the cation portion and the anion portion that make the anion / cation ratio 2.3 to 2.5 are possible. However, La is +
Considering that trivalent, alkali metal is +1 valent, oxygen is -2 valent, and fluorine is -1 valent, the composition formula La _1-z Mb _z O _{1-x- is set} to make the entire valence zero. _2z F _{1 + 2x + 2z} (0.3 ≦ x
≦ 0.5, 0 <z ≦ 0.1) were considered. (The anion / cation ratio becomes 2.3 when x = 0.3 and z = 0 (La0 _0.7 F _1.6 ), and the ratio becomes 2.4 when x = 0.
4, when z = 0.05 (La _0.95 Mb _0.05 0
_0.5 F _1.9 ) or when x = 0.4 and z = 0.1 (La
_0.9 Ma _{0.10 0.4} F _2.0 ).

Since the value of z is 0 to 0.1, the ratio of lanthanum to alkaline earth metal in the cation portion La _1-z Mb _z is 1: 0 (excluding 0% of alkaline earth). It can be selected within the range of 0.9: 0.1 = 9: 1. When the ratio of lanthanum to alkali metal exceeds 9: 1 and the amount of alkali metal increases, the oxyfluoride fluorite structure collapses (an impurity layer is formed).

Examples of alkali metals include lithium (Li) and sodium (Na). Plural types mean two or more types. Li or the like may be contained in the cation portion in an amount of 5 to 10% by mol. There are La _0.9 Na _{0 .1} Specific examples of the cation portion.

On the other hand, the ratio of oxygen to fluorine in the anion portion O _1-x-2z F _{1 + 2x + 2z} can be selected in the range of 1: 2.3 to 1: 5. As described above, x is 0.3 to 0.5
, Z can be selected in the range of 0 to 0.1. This ratio is 1: when x = 0.3 and z = 0 (O _0.7 F _1.6 ).
2.3, x is 0.4 and z is 0.1 (O _0.4 F _2.0 ).
Then it becomes 1: 5. If this ratio exceeds 1: 2.3 and the amount of fluorine decreases, the conductivity of oxygen ions decreases, and
When the amount of fluorine increases beyond the range, the fluorite structure collapses.

The conventional LaOF is not so good in sinterability and sometimes cracks when used in a fuel cell, or the electromotive force of an open circuit becomes lower than the theoretical value. The reason is that fuel gas and air are mixed and burned through the pores, resulting in local heat generation.
It is considered that energy loss occurs.

On the other hand, as in the case of the third type, by setting the anion / cation ratio to 2.3 to 2.5 and adding an alkali metal having a low melting point in addition to La to the cation portion, the calcining is performed. The bondability has improved. It is considered that the reason is that the alkali metal having a low melting point acts as a sintering aid. Owing to the improved sinterability, the conductive path of oxygen ions was increased, and the conductivity of oxygen ions was also improved. As a result, the open circuit electromotive force became almost the theoretical value. 4th type The cation part of the 4th type fluorite type oxyfluoride is La
In addition, one or more kinds of alkaline earth metal Ma and one or more kinds of alkali metal Mb are contained, and LaMaMb
OF (Ma: alkaline earth metal, Mb: alkali metal)
Has a crystal structure of type. Various compositions of the cation portion and the anion portion that make the anion / cation ratio 2.3 to 2.5 are possible. However, considering that La is +3 valent, alkaline earth metal is +2 valent, alkali metal is +1 valent, oxygen is -2 valent, and fluorine is -1 valent, the composition is set to make the entire valence zero. formula _{La 1-yz Ma y Mb z} O 1-xy-2z F 1 + 2x + y + 2z
Thought.

Fluorine F _{1 + 2x + y + 2z} in the anion part
When the ratio of exceeds a certain limit, that is, 1 + 2x + y + 2
When z exceeds 2, the fluorite structure of oxyfluoride is not maintained and fluorine ion conduction is exhibited. Therefore, 1 + 2x
+ Y + 2z must be less than 2 (1 + 2
x + y + 2z ≦ 2). Further, since the anion / cation ratio is 2.3 to 2.5, 2.3 ≦ (1-x-y-
2z) + (1 + 2x + y + 2z) / (1-yz) + y
From + z ≦ 2.5, 0.3 ≦ x ≦ 0.5 is obtained.

When x = 0.3, 1 + 0.6 + y + x≤
From 2.0, 0 <y ≦ 0.4 and 0 <z ≦ 0.2 are obtained.
Further, when x = 0.4, 0 <y ≦ 0.2 and 0 <
z ≦ 0.1. Furthermore, when x = 0.5, y + 2z
From ≦ 2.0, y = z = 0. As is clear from the above, the value of y and the value of z change depending on the value of x.

From the above, the cation part La _1-yz May _y Mb
The ratio of lanthanum La to alkaline earth metal Ma to alkali metal Mb at _z is 9: 0.5: 0.5 or 8.5:
It can be 1.0: 0.5. This ratio is 9:
It becomes 0.5: 0.5 when y = z = 0.05 (L =
a _0.9 Ma _0.05 Mb _0.05 ), and 8.5: 1.0:
It becomes 0.5 when y = 0.1 and z = 0.05 (L
a _0.85 Ma _0.1 Mb _0.05 ).

When the ratio of lanthanum to alkaline earth metal and alkali metal exceeds 8.5: 1.5 and the amount of alkaline earth metal and alkali metal increases, it becomes difficult for the oxyfluoride to maintain the fluorite structure.

As the combination of alkaline earth metal and alkali metal, one kind of alkaline earth metal and one kind of alkali metal, plural kinds of alkaline earth metal and one kind of alkali metal, plural kinds of alkaline earth metal and alkali There are one kind of metals and plural kinds of alkaline earth metals and plural kinds of alkali metals. When combining lanthanum and one kind of alkaline earth metal and one kind of alkali metal, as a specific example, La
_{There is 0.85} Ba _0.1 Na _0.05 .

On the other hand, the anion portion O _1-xy-2z F _{1 + 2x + y + 2z}
The ratio of oxygen to fluorine at 1 :: 2.3 to 1: 4
You can select in the range. This ratio becomes 1: 2.3 when x = 0.3 and y = z = 0 (O _0.7 F _1.6 ),
The ratio of 1: 4 is x = 0.4, y = 0.05, z = 0.
It is 025 (O _0.5 F _1.9 ).

By changing the anion / cation ratio from 2.3 to 2.5 as in the case of the fourth type, and by incorporating an alkaline earth metal and an alkali metal in the cation part,
The oxygen ion conductivity was further improved as compared with the case where only the alkaline earth metal was added. The reason is considered to be that the symmetry of the crystal structure of the oxyfluoride is improved and the sintering density is improved (the conductive path of oxygen ions is increased). <Solid Electrolyte and Fuel Cell> Solid Electrolyte The solid electrolyte of the present invention comprises fluorite-type oxyfluorides of the first to fourth types. As described above, each type of fluorite-type oxyfluoride has excellent conductivity of oxygen ions, and the solid electrolyte is a conductor of oxygen ions (O ²⁻ ), which can physically separate hydrogen and oxygen. Required. Considering this, it is desirable that the solid electrolyte of the present invention be formed of fluorite type oxyfluoride having excellent conductivity of oxygen ions. The fuel cell of the present invention includes a solid electrolyte composed of the above-mentioned first to fourth types of fluorite-type oxyfluoride, an air electrode on one side thereof, and a fuel electrode on the other side thereof. Here, the air electrode is required to easily adsorb oxygen and to easily move oxygen ions. Considering this, the air electrode is _preferably made of Sm _0.5 Sr _0.5 CoO ₃ or Pt. On the other hand, the fuel electrode is required to have high electronic conductivity and affinity with hydrogen. Considering this, it is desirable that the fuel electrode is made of Ni or Pt. It is particularly desirable that the air electrode be Pt and the fuel electrode be Pt.

The fuel cell may be a flat plate type or a cylindrical type and generates power at an operating temperature of 500 to 700 ° C. A cell (unit cell) is formed by the 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.

The fuel cell includes a separator in addition to the solid electrolyte, the fuel electrode and the air electrode. The separator connects adjacent cells in series in a flat plate fuel cell, and is called an interconnector in a cylindrical fuel cell. 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 desirable that the separator is made of a material which is strong against oxidation and reduction and has good electron conductivity and no ion conductivity in any atmosphere.

[0054]

Embodiments of the present invention will be described below with reference to the accompanying drawings. (Fluorite type oxyfluoride, solid electrolyte of Example 1) As shown in FIG. 13, the flat plate type fuel cell includes a solid electrolyte 11, an air electrode 12 and a fuel electrode 1 arranged on both sides thereof.
It is composed of a unit cell 15 including 3 and a separator 17 interposed between adjacent unit cells. The solid electrolyte 11 is LaO _0.6 which corresponds to the above-mentioned first type fluorite type oxyfluoride.
It consists of F _1.8 and has a flat plate shape. The oxygen to fluorine ratio in the anion part is 1: 3 and the anion / cation ratio is 2.4.

The fuel electrode 12 is made of Pt, and the solid electrolyte 1
The concave groove 12 which is laminated on one surface of the No. 1 and serves as a flow passage for hydrogen gas.
The air electrode 13 in which a is formed is made of Pt, is laminated on the other surface of the solid electrolyte 11, and has a concave groove 13a serving as a flow passage for oxygen gas. (Manufacturing Method of Solid Electrolyte 11) The solid electrolyte 11 was prepared as follows. As shown in FIG. 2, various oxides or La ₂ O ₃ or fluorides or LaF ₃ are weighed in S1 (for example, 1 g of La ₂ O ₃ and ₃ g of LaF ₃ ) and various oxides or fluorides are added in S2. Mix for 30 minutes in an alumina mortar.

Next, in S3, the mixture is calcined in an argon atmosphere at a temperature of 750 ° C. for 4 hours, and in S4, mixed and ground in a mortar. Next, in S5, hydrostatic pressure is applied to the pulverized product to form it into a disk shape, and in S6, it is sintered in an argon atmosphere at a temperature of 900 to 1100 ° C. for 3 hours.

As a result, the solid electrolyte 11 is manufactured.
Finally, in S7, the structure of the solid electrolyte 11 is analyzed by XRD. (Operation / Effect) A fuel cell including the solid electrolyte 11 generates power at an operating temperature of 600 ° C., for example. That is, oxygen is supplied to the air electrode 12 and hydrogen is supplied to the fuel electrode 13. Oxygen passes through the air electrode 12 and reaches the vicinity of the interface with the solid electrolyte 11, and receives 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 flow through the external circuit 18 to the air electrode 12, and a current flows from the air electrode 12 to the fuel electrode 13.

FIG. 3 shows the oxygen ion conductivity of the solid electrolytes of Example 1 and the solid electrolytes of Comparative Examples 1, 3 and 4. Example 1 is composed of LaO _0.6 F _1.8 , and Comparative Example 1 is L
aOF, Comparative Example 3 is YSZ, and Comparative Example 4
Is LSGM9182 (La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O
_2.85 ). The conductivity of oxygen ions of Example 1 is shown by curve c, and the conductivity of oxygen ions of Comparative Example 1, Comparative Example 3 and Comparative Example 4 are shown by curves d, e and f, respectively.

Comparing the conductivity of oxygen ions at 600 ° C., the conductivity of oxygen ions in Example 1 is about Logσ.
= -1.8 (σ = 0.016 S / cm), which is higher than the oxygen ion conductivity of the solid electrolytes of Comparative Examples 1, 3 and 4, and is said to be excellent in ionic conductivity. Even higher than. This tendency is the same in the entire range of 500 to 700 ° C.

FIG. 4 shows the X-ray diffraction pattern of each of the above solid electrolytes. This is a solid electrolyte with X at a predetermined angle (horizontal axis).
It is a result of examining the intensity (vertical axis) of the reflected light of the X-ray reflected by the atom of La, O or F by applying a line. When the X-ray hits the atom, the intensity of the reflected light increases. As shown by the curve h, in the solid electrolyte made of LaOF having a fluorite structure (Comparative Example 1), the intensity of the reflected light increases at a predetermined angle (about 27 degrees, about 32 degrees).

Also in Example 1 made of LaO _0.6 F _1.8 , the strength increases at almost the same angle as LaOF, as indicated by the curve g. This is because in the solid electrolyte of Example 1, the ratio of oxygen body fluorine in the anion part was 1: 3, the anion / cation ratio was 2.4, La was located at the cation site, and O was the lattice. It is considered that this is because F is located at the inter site and F is located at the anion site and has a crystal structure similar to that of LaOF.

[0062] On the other hand, La which is a kind of fluoride
F₃Comparative Example 2 consisting of Tissot with a slightly distorted fluorite structure
It has a knight structure and has a certain angle as shown by curve i.
There are a plurality of outputs in the vicinity, and the size is the same as in the first embodiment.
Smaller than solid electrolyte. (The fluorite type oxyfluorides of Examples 2, 3 and 4 and solid state electrodes)
Degradation) Corresponds to the second type of fluorite-type oxyfluoride
In Examples 2, 3 and 4, the cation moiety is La and al.
It contains Sr, which is one of the alkaline earth metals. In Figure 5,
Fluorite while maintaining an on / cation ratio of 2.3 to 2.4
The composition of the cation part and the anion part of the type oxyfluoride
The conductivity of oxygen ions when changed is shown. That is, implementation
The solid electrolyte of Example 2 is La_0.8Sr_0.2O_0.4F_2.0Consisting of
And the ratio of oxygen body fluorine in the anion part is 1: 5.
And the anion / cation ratio is 2.4. Also implemented
The solid electrolyte of Example 3 is La_0.9Sr_0.1O_0.5F_1.9Consisting of
And the ratio of oxygen body fluorine in the anion part is 5:19.
And the anion / cation ratio is 2.4. Furthermore, the real
The solid electrolyte of Example 4 is La_0.8Sr_0.2O_0.5F_1.8Consisting of
Therefore, the ratio of oxygen body fluorine in the anion part is 5:18 ≈
1: 3.6, anion / cation ratio of 2.3
It

Example 2 containing Sr in the cation part,
3 and 4 were manufactured by changing the mixing ratio of the raw materials and adding SrO or SrF ₂ as compared with the above-mentioned Example 1. SrO or SrF ₂ has a cubic fluorite structure,
The addition improved the symmetry of the oxyfluoride crystal structure. That is, F occupies the lattice point position of the fluorite structure, and O
Are presumed to exist randomly between the lattice points. It is considered that the O existing between the lattice points moves between the lattice points to improve the conductivity of oxygen ions.

At operating temperatures of 500 to 700 ° C., the oxygen ion conductivities of the solid electrolyte of Example 2 and the solid electrolyte of Example 3 of Example 1 are shown by curve 1 as shown by curves j and k. Greater than that of a solid electrolyte.

From this, it is desirable that Sr be contained in addition to La in the cation portion while maintaining the anion / cation ratio of 2.3 to 2.4, and the content thereof is 10%.
It turns out that 20% is better than. Further, it is understood that the ratio of oxygen to fluorine in the anion part is preferably 1: 5 rather than 0.5: 1.9≈1: 3.8.

The conductivity of oxygen ions of the solid electrolyte of Example 4 is lower than that of the solid electrolyte of Example 1. The anion / cation ratio of the solid electrolyte of Example 4 is 2.3. From this, it is found that the optimum anion / cation ratio is 2.4 when Sr is contained as the alkaline earth metal in the cation part. However, the conductivity of oxygen ions of the solid electrolyte of Example 4 is higher than the conductivity of the conventional example in which the anion / cation ratio is 2. (Fluorite type oxyfluoride of Example 5, solid electrolyte) Example 5 corresponding to the above-mentioned third type of fluorite type oxyfluoride
Then, the cation part is La and N is one of the alkali metals.
a is included. That is, 10% of La is replaced by Na, and the composition formula is represented by La _0.9 Na _0.1 O _0.4 F _2.0 .
The ratio of lanthanum to sodium in the cation part is 9:
1, the ratio of oxygen to fluorine in the anion part is 1: 5, and the anion / cation ratio is 2.4.

Example 5 was manufactured by changing the mixing ratio of the raw materials to be mixed and adding NaF, as compared with Example 1 described above.

The relationship between the operating temperature and the oxygen ion conductivity in the solid electrolyte of Example 5 is shown by the curve n in FIG. At the operating temperature of 500 to 700 ° C., the conductivity of oxygen ion of Example 5 is higher than that of Example 1 indicated by the curve p.

The relationship between the theoretical electromotive force and the actual electromotive force was investigated for Examples 3 and 5, and the results are shown in FIG. In FIG. 7, a curve q is the ratio of the actual electromotive force to the theoretical electromotive force of Example 5 (hereinafter abbreviated as “electromotive force ratio”), and a curve r is the electromotive force ratio of Example 3. From this, it is understood that in Example 5 in which Na was added to the cation portion, the actual electromotive force was closer to the theoretical electromotive force than in Example 3 in which Sr was added to the cation portion.

The sintered bodies were analyzed by SEM photographs in order to investigate the relationship between the conductivity of oxygen ions and the crystal structure of the sintered bodies in Examples 1 and 5. The result is shown in FIG. FIG. 8A is a photograph showing the crystal structure of Example 1, in which the crystal density is rough. (B) is a photograph showing the crystal structure of Example 5, in which the crystal density is high. By adding 10% Na to the cation portion, the sintered density of the sintered body became dense, and it is considered that the increase in this density is the cause of the increase in electromotive force. (Fluorite-Type Oxyfluoride of Example 6 and Solid Electrolyte) Example 6 corresponding to the above-mentioned fourth type of fluorite-type oxyfluoride.
In, the cation portion contains La, Sr which is one of the alkaline earth metals, and Na which is one of the alkali metals, and is represented by the composition formula La _0.85 Sr _0.1 Na _0.05 O _0.4 F _2.0 .

In FIG. 9, the relationship between the operating temperature and the oxygen ion conductivity in the solid electrolyte of Example 6 is shown by a curve s. The conductivity of oxygen ions in Example 3 (La _0.9 Sr _0.1 O _0.5 F _1.9 ) and Comparative Example 4 (LSGM9182) are shown by curves t and u. As is apparent from this, at 500 to 700 ° C., the conductivity of oxygen ion in Example 6 is higher than that in Example 3 and Comparative Example 4.

Thus, the anion / cation ratio is 2.
4, and by incorporating Sr and Na in the cation portion, the oxygen ion conductivity was further improved as compared with the case where only Sr was added. The reason is considered to be that the symmetry of the crystal structure of the oxyfluoride is improved and the sintered density is improved (conductive paths are increased).

[0073]

As described above, according to the oxyfluoride of the present invention, since the anion / cation ratio is 2.3 to 2.5, fluorine ion occupies the lattice point position and oxygen ion It exists randomly between the lattice points. As a result, the conductivity of oxygen ions is increased.

Further, according to the solid electrolyte and the fuel cell of the present invention, since they are used at a relatively low operating temperature (500 to 700 ° C.), it becomes possible to use a metal material, and the range of material selection is widened. . Also, damage due to thermal stress is prevented and the life is extended. Such a solid electrolyte and a fuel cell are suitable for use in, for example, a power source of an electric vehicle that requires startability and reliability.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the value of x and the conductivity of oxygen ions in the oxyfluoride of the present invention represented by the composition formula LaO _1-x F _{1 + 2x} (Example 1).

FIG. 2 is a flow chart illustrating a method for producing the oxyfluoride.

FIG. 3 is a graph showing the relationship between operating temperature and oxygen ion conductivity in Example 1 and Comparative Example.

FIG. 4 is a graph showing the relationship between the angle of X-rays and the intensity of reflected light in Example 1 and Comparative Examples 1 and 2 above.

FIG. 5 is a graph showing the relationship between operating temperature and oxygen ion conductivity in the cation portion of Example 1 and Examples 2, 3 and 4 to which an alkaline earth metal was added.

FIG. 6 is a graph showing the relationship between operating temperature and oxygen ion conductivity in the cation portion of Example 1 and Example 5 to which an alkali metal was added.

FIG. 7 is a graph showing the relationship between operating temperature and actual electromotive force / theoretical electromotive force in Examples 3 and 5.

8A is a micrograph showing a detailed structure of Example 3, and FIG. 8B is a micrograph showing a detailed structure of Example 5.

9 is a graph showing the relationship between operating temperature and oxygen ion conductivity in Examples 3 and 6 and Comparative Example 4. FIG.

FIG. 10 is a perspective explanatory view showing a general fluorite structure.

FIG. 11 is a perspective explanatory view showing a crystal structure of LaOF.

FIG. 12 is a perspective explanatory view showing a crystal structure of LaO _0.5 F _2.0 .

FIG. 13 is an exploded perspective view for explaining a conventional example and a fuel cell of the present invention.

14 is an operation explanatory view of the fuel cell of FIG.

[Explanation of symbols]

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

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 15, 2002 (2002.2.1
5)

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] Figure 8

[Correction method] Change

[Correction content]

[Figure 8]

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01M 4/92 H01M 4/92 8/12 8/12 F term (reference) 5G301 CA30 CD01 5H018 AA06 AS02 AS03 EE02 EE03 EE13 HH08 5H026 AA06 EE02 EE11 EE13 HH05 HH08

Claims (21)

[Claims] 1. An oxyfluoride characterized in that the cation part contains at least lanthanum La, the anion part contains fluorine F and oxygen O, and the molar ratio of the anion part to the cation part is 2.3 to 2.5. . 2. The oxyfluoride according to claim 1, wherein the ratio of oxygen to fluorine in the anion part is 1: 2.3 to 1: 4. 3. The oxyfluoride according to claim 2, which is represented by the composition formula LaO _1-x F _{1 + 2x} (x is 0.3 to 0.5). 4. The oxyfluoride according to claim 1, which operates at an operating temperature of 500 to 700 ° C. 5. The oxyfluoride according to claim 1, wherein the cation portion further contains one or more kinds of alkaline earth metals. 6. A composition formula La _1-y May _y O _1-xy F
The oxyfluoride according to claim 5, which is represented by _{1 + 2x + y} (x is 0.3 to 0.5 and y is 0 to 0.2 (excluding 0 for y)). 7. The oxyfluoride according to claim 5, wherein the ratio of lanthanum in the cation part to alkaline earth metal is 1: 0 to 8: 2 (excluding 0% of alkaline earth metal). 8. The oxyfluoride according to claim 5, wherein the ratio of oxygen to fluorine in the anion part is 1: 2.3 to 1: 5. 9. The oxyfluoride according to claim 1, wherein the cation portion further contains one or more kinds of alkali metals. 10. A composition formula La _1-z Mb _z O _1-x-2z F
_{1 + 2x + 2z} (x is 0.3 to 0.5, z is 0 to 0.1
(However, z excludes 0) The oxyfluoride according to claim 9. 11. The ratio of lanthanum to alkali metal in the cation portion is 1: 0 to 9: 1 (provided that alkali metal is 0
% Is excluded) The oxyfluoride according to claim 10. 12. The oxyfluoride according to claim 10, wherein the ratio of oxygen to fluorine in the anion portion is 1: 2.3 to 1: 5. 13. The oxyfluoride according to claim 1, wherein the cation portion further contains one or more kinds of alkaline earth metals and one or more kinds of alkali metals. 14. A composition formula La _{1 -yz} May _y Mb _z O
_1-xy-2z F _{1 + 2x + y + 2z} (x is 0.3 to 0.5, y is 0
To 0.2 (however, y is not 0), z is 0 to 0.1
The oxyfluoride according to claim 13, which is represented by (where z is not 0). 15. The ratio of lanthanum to one or more kinds of alkaline earth metal to one or more kinds of alkali metal in the cation part is 9: 0.5: 0.5 or 8.5: 1.
The oxyfluoride according to claim 13, which is 0: 0.5. 16. The oxyfluoride according to claim 13, wherein the molar ratio of oxygen to fluorine in the anion part is 1: 2.3 to 1: 5. 17. The cation portion contains at least lanthanum La, the anion portion contains fluorine F and oxygen O, and the molar ratio of the anion portion to the cation portion is 2.
A solid electrolyte comprising an oxyfluoride of 3 to 2.5 and having a flat plate shape or a cylindrical shape. 18. The solid electrolyte according to claim 17, which operates at an operating temperature of 500 to 700 ° C. 19. The cation portion contains at least lanthanum La, the anion portion contains fluorine F and oxygen O, and the molar ratio of the anion portion to the cation portion is 2.
A solid electrolyte composed of oxyfluoride having a size of 3 to 2.5 and having a flat plate shape or a cylindrical shape, an air electrode arranged on one surface side of the solid electrolyte, and a fuel electrode arranged on the other surface side of the solid electrolyte. And a fuel cell. 20. The air electrode is Sm _0.5 Sr _0.5 CoO ₃
20. The fuel cell according to claim 19, wherein the fuel electrode is made of Pt or Pt, and the fuel electrode is made of Ni or Pt. 21. The fuel cell according to claim 19, which operates at an operating temperature of 500 to 700.degree.