JPH1050329A - Solid electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To raise a releasing voltage of a seria-based electrolyte and the same the improve durability. SOLUTION: Fine films (protective films) 12, made of mixed conductivity material (conductive to electron and ion) are bonded with each other on a hydrogen pole side of a solid electrolyte 11 formed of a seria-based material. Platinum is used respectively for an oxygen pole 13 and a hydrogen pole 14, and oxygen and hydrogen are supplied to an oxygen supply chamber 15 and a fuel gas supply chamber 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid electrolyte fuel cell in which a solid electrolyte protective film is improved.

[0002]

2. Description of the Related Art FIG. 14 is a schematic configuration diagram showing the principle configuration of a solid oxide fuel cell. In FIG. 14, a porous oxygen electrode 52 (cathode) made of perovskite and Ni-Y are provided on both sides of a solid electrolyte 51 having oxygen ion conductivity.
Porous hydrogen electrode 53 (anode) made of SZ cermet
Attach. When oxygen gas O ₂ or air flows into the oxygen supply chamber 54 on the oxygen electrode 52 side, oxygen molecules are converted to oxygen ions O ²⁻ by the catalyst 55 of the oxygen electrode at the oxygen electrode 52. The following equation shows the reaction formula.

[0003] ^{_{1 / 2O 2 + 2e - →}} O 2- ...... (1) oxygen ions O ^2- oxygen ion conductivity of a solid electrolyte 5
1 reaches the hydrogen electrode 53 while diffusing. A fuel gas such as hydrogen gas or natural gas is poured into the fuel gas supply chamber 56 on the hydrogen electrode 53 side, and the oxygen ions O ²⁻ that have passed through the solid electrolyte 51 are assisted by the catalyst 57 of the hydrogen electrode. It reacts with the fuel gas to become water vapor or carbon dioxide and is removed from the fuel gas. The following equation is a reaction equation when oxygen ions react with hydrogen gas.

H ₂ + O ²⁻ → H ₂ O + 2e ⁻ (2) Although oxygen ions pass through the solid electrolyte 51,
A substance that is insulating is used for electrons. 58 is a load,
This load 58 is connected between the cathode and the anode.

The material of the solid electrolyte 51 used in the solid oxide fuel cell is a stabilized zirconia formed by dissolving an oxide such as yttrium in zirconia.
Many use (YSZ). Since the operation of the fuel cell using YSZ is at a high temperature of about 1000 ° C., a heat-resistant material such as ceramics is used as a constituent material of the solid oxide fuel cell. However, there is a problem that ceramics are expensive and difficult to process. Therefore,
Attempts have been made to lower the operating temperature of the battery to lower the heat resistance of the battery components. As a means for doing so, research has been conducted to use a material such as CeO ₂ or Bi ₂ O _{3 as} the material of the solid electrolyte and raise the operating temperature of the fuel cell to about 700 to 800 ° C. Although these solid electrolytes have the problem of being reduced in a reducing atmosphere such as a hydrogen electrode atmosphere, they have the advantage that the operating temperature can be lower than in the case of YSZ. For this reason, fuel cells using CeO ₂ -based electrolytes as alternative materials to YSZ have recently been used.

[0006]

In general, a fuel cell has an internal resistance in the fuel cell unit. Therefore, when a large amount of current is taken out, a voltage drop occurs due to the internal resistance and the output voltage that can be taken out outside is reduced. There's a problem.

[0007] There are two types of internal resistance, and the first problem with internal resistance is the resistance of the bulk of the electrodes and the electrolyte, which causes the voltage to drop in proportion to the current. Since the battery has an internal resistance R, if the current I to be taken out is increased, the output voltage is reduced by the product RI of the resistance R and the current I according to Ohm's law. The bulk resistance is Ce
In the O ₂ system, Sm-doped (CeO ₂ ) _X (Sm ₂ O ₃ )
_1-X (X; real number, 0 <X <1) (hereinafter referred to as SDC) is considered to have the lowest resistance of the electrolyte to oxygen ions.

[0008] The second problem of the internal resistance is called overvoltage, and this overvoltage is generated by the Tafel equation shown below, which is the resistance of the interface between the electrode and the solid electrolyte.
In the following equation, η indicates an overvoltage, A and B indicate constants, and I indicates a current.

Η = A + B * ln (I) (3) Usually, in a large current region used in a fuel cell, the value of the overvoltage does not change so much with a change in current. Therefore, resistance due to overvoltage is rarely displayed in units of resistance Ω divided by current, and is often displayed in units of V (volt) as loss of voltage.

As described above, the fuel cell has a problem of internal resistance, but the above-mentioned CeO ₂ -based electrolyte is characterized by being superior to the YSZ-based electrolyte in the following two points.

(1) The oxygen ion conductivity is much higher.
In general, oxygen ion conductivity decreases exponentially with decreasing temperature. Compared to the YSZ-based electrolyte used at 1000 ° C., the CeO ₂ -based electrolyte is Y
Approximately 0.1S /, almost equal to SZ-based electrolyte (1000 ° C)
cm oxygen ion conductivity.

(2) The overvoltage is smaller than the YSZ-based electrolyte. That is, at the same temperature and the same current, CeO ₂
The loss of the output voltage due to the overvoltage is much smaller in the system electrolyte than in the YSZ system electrolyte.

However, the CeO ₂ -based electrolyte has a problem that an open-circuit voltage drop is caused by being reduced by hydrogen to change into Ce ₂ O ₃ which is a semiconductor. For example, YS using normal pressure hydrogen (containing 3% water vapor) as fuel gas at 800 ° C. and normal pressure oxygen as cathode gas
This is because the open-circuit voltage of the Z-based electrolyte is 1.15 V as theoretically, whereas the open-circuit voltage of the CeO ₂ -based electrolyte is only about 0.8 V. The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a solid oxide fuel cell that increases the open circuit voltage of a CeO ₂ (ceria) electrolyte and improves durability. .

[0014]

According to a first aspect of the present invention, a hydrogen electrode or a natural gas is formed by forming a hydrogen electrode and an oxygen electrode on a solid electrolyte having oxygen ion conductivity. In a solid electrolyte fuel cell in which a fuel gas such as is supplied to the hydrogen electrode side, oxygen gas or air is supplied to the oxygen electrode side to generate electric power between the two electrodes, the hydrogen electrode side of the solid electrolyte has oxygen ion conductivity. An oxide film having an oxygen-deficient perovskite crystal structure containing CeO ₂ and ZrO ₂ is formed.

The second invention is characterized in that the oxide film is formed of a dense film. The third invention uses an electrolyte containing CeO ₂ as a main component for the solid electrolyte,
On the hydrogen electrode side of the electrolyte, oxygen ion conductive Ce
An oxide film having an oxygen-deficient perovskite crystal structure containing O ₂ and ZrO ₂ is formed.

A fourth invention is characterized in that the oxide film is a dense film. In the fifth invention, the C
An oxygen-deficient perovskite prepared by doping a perovskite containing CeO ₂ and ZrO ₂ having oxygen ion conductivity with an oxide containing a trivalent rare earth element on the hydrogen electrode side of an electrolyte containing eO ₂ as a main component. It comprises a protective film of an oxide having a type crystal structure.

According to a sixth aspect of the present invention, the protective film is formed of a dense film.

According to a seventh aspect of the present invention, the oxide containing a rare earth element is Y ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O _3.
It is characterized by including.

An eighth invention is characterized in that the oxide having a perovskite crystal structure contains Ba, Sr, Ca, and Mg.

According to a ninth invention, the oxygen-deficient oxide having a perovskite crystal structure has a molar ratio of CeO ₂ and ZrO ₂ of about 0.8 and a molar ratio of BaO, SrO, CaO and MgO of about 1. _{a, Y 2 O 3, Gd 2} O 3, Nd 2 O 3, Sc 2
The molar ratio of O ₃ to Sm ₂ O ₃ is set to about 0.1.

A tenth invention is characterized in that a powder of Ni with respect to hydrogen gas or a powder which is reduced in a hydrogen gas atmosphere to produce Ni is mixed into the oxide having a perovskite crystal structure. .

According to an eleventh aspect of the present invention, a hydrogen electrode and an oxygen electrode are formed on a solid electrolyte having oxygen ion conductivity, and a fuel gas such as hydrogen gas or natural gas is supplied to the hydrogen electrode side, and oxygen gas or air is supplied to the oxygen electrode side. In a solid oxide fuel cell that generates electric power between both electrodes by flowing, a ceria-based electrolyte is used as the solid electrolyte, and Ba, Sr,
It is characterized in that Ca is diffused to form an electrolyte having an oxygen-deficient perovskite crystal structure on the surface.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, in describing embodiments of the present invention, first, the principle that the open-circuit voltage of a conventional CeO ₂ -based electrolyte is reduced has been described using a transport model.

The transport model is an Israeli Ri
According to es, it is a model in which CeO ₂ (hereinafter referred to as ceria) is degraded, has electronic conductivity, and a short-circuit electron current is generated to lower the voltage. Here, assuming that the resistance of ceria to oxygen ions is R _i , the actual open circuit voltage (Voc) is expressed by the following equation.

Voc (actual open circuit voltage) = theoretical open circuit voltage−R _i * I _i (4) In equation (4), I _i is the oxygen ion (electron) current inside when the external circuit is open. Value. Figure 7 is a schematic diagram of a transport model, the ceria electrolyte 1 as shown in FIG. 7, inside has current even when the external circuit opened, is equal to current I _i of the electron current Ie and oxygen ions The current flows in the opposite direction.

Next, a falsification experiment of the transport model will be described with reference to FIG. FIG. 8 is a schematic configuration diagram of a transport model rebuttal experiment. In FIG. 8, first, a dense YSZ film 2 is formed on the cathode. Since YSZ is insulated from electrons, short-circuit electron current can be prevented. Therefore, according to the transport model, there is no current inside when the external circuit is open, and the open circuit voltage in the experiment is 800 degrees and hydrogen at normal pressure (water vapor 3%
) And oxygen, an open-circuit voltage of 1.15 V as in the same theory as that of YSZ must be obtained. From the open-circuit voltage characteristic diagram shown in FIG.
It produces only 6 mV, which disproves the transport model.

Next, a new model for explaining the reduction of the open circuit voltage of the ceria-based electrolyte without using the internal current will be described. The outline of the new model will be described using an electrostatic field. It is assumed that the voltage drops because the electrons cancel the electrostatic field created by oxygen ions. 10 (A), (B),
(C) is a characteristic diagram showing the state of the potential inside the electrolyte according to the new model.

Next, the method of obtaining the value of the voltage drop at the anode was estimated for the following reasons.

Reason 1: The open-circuit voltage drop occurs near the anode.

Reason 2: In the experimental results already conducted and published, the voltage drop from the theoretical voltage is a constant independent of the current except for the RI drop and the overvoltage.

Reason 3: The open-circuit voltage slightly changes depending on the conditions for preparing the ceria electrolyte.

From these reasons 1 to 3, the following assumption was made. It seems that the value of the open circuit voltage drop depends on the physical properties of ceria. Its physical property value is related to a voltage of about 0.34V. Now, since oxygen ions have a charge of 2e,
The energy of oxygen ions is 0.68 eV. This value is very close to the activation energy of oxygen ions. Is one of the activation energy _{(CeO 2) 0 · 9 (} Sm 2
O ₃₎ Figure 11 shows the Arrhenius plot of _{0, 1.} Therefore, the equation for obtaining the open circuit voltage was estimated as in the following equation (5).

V _OC (actual open-circuit voltage) = theoretical open-circuit voltage−Eα / 2e (5) where Eα is the activation energy of oxygen ions near the anode of the electrolyte, and e is the electronic charge. . The above equation (5) holds when there is no short circuit of electrons inside the electrolyte, in other words, when there is an insulating layer for electrons inside the electrolyte and there is sufficient electron conduction on the anode side inside the electrolyte.

Next, verification of the new model will be described.
In the new model, the actual open voltage V _OC depends on the activation energy of oxygen ions in the electrolyte. _{So, La 0 · 8 Sr 0 ·} 2
A demonstration experiment was performed using MnO ₃ (hereinafter, referred to as LSM). LSM is a substance having both oxygen ion conduction and electron conduction. The activation energy of oxygen ions is already known, and the measured value of the activation energy at that time is 1.60.
eV to 1.65 eV. In order to obtain the same conditions as the CeO ₂ -based electrolyte, a dense YSZ film was formed on the cathode, and the open-circuit voltage was measured. FIG. 12 shows an outline of the configuration of the cell used in the verification experiment at this time.

In FIG. 12, 2 is a YSZ film, 3 is an LSM, and the experimental conditions in the configuration shown in FIG.
At 00 ° C., normal pressure hydrogen (containing 3% water vapor) was used as a fuel gas on the anode side, and normal pressure oxygen was used as the cathode side gas. FIG. 13 shows the experimental results under these conditions. 600 ° C
Then, theoretical open circuit voltage is 1.18V, and V _OC =
Since the voltage is 0.38 V, the activation energy of ions in oxygen is (1.18−0.38) V * 2e = 1.6 from equation (5).
It was 0 eV, which proved that it could be demonstrated.

FIG. 1 is a schematic configuration diagram of a solid oxide fuel cell according to an embodiment of the present invention. In FIG. 1, reference numeral 11 denotes a solid electrolyte formed of a ceria-based material. On the (anode) side, a dense film (protective film) 12 made of a mixed conductive (conductive for electrons and ions) material having a sufficiently small activation energy of ions according to the formula (5) is bonded. Note that platinum is used for the oxygen electrode 13 and the hydrogen electrode 14, respectively. Reference numeral 15 denotes an oxygen supply chamber, 16 denotes a fuel gas supply chamber, and 17 denotes a load.
The load 17 is connected between the oxygen electrode 13 and the hydrogen electrode 14.

In the solid oxide fuel cell configured as described above, the protective film 12 made of the mixed conductive material is required to have a small oxygen ion activation energy and a high durability against hydrogen at high temperatures. In selecting the material, the material was developed with the following two points in mind.

First point: An oxygen ion conductive material whose resistance to reduction has already been proven is used as a main component. For example, it is already known that ZrO ₂ and CeO ₂ based materials have durability against reduction.

Second point: The activation energy of ions is
It depends strongly on the crystal structure. For example, ZrO ₂ -based, CeO ₂ -based, Bi ₂ , which are oxygen ion conductors having a fluorite-type crystal structure
The activation energy of the O ₃ system is about 0.7 eV to about 1 eV. On the other hand, YBCO (YBa ₂ Cu ₃ O ₇ ) having an oxygen ion deficient perovskite crystal structure is
It has a low activation energy of about 0.3 eV. The structure of this material is characterized in that the concentration of oxygen ion vacancies is large and the distance between two oxygen ion vacancies is short. An example of the crystal structure is shown in FIG.

From the above first and second points of consideration, based on the materials ZrO ₂ and CeO ₂ which are strong in reduction, it is sufficient to use a material in which the crystal structure is changed and the ion activation energy is reduced. There was found. Particularly, in order to change the crystal structure, it is sufficient to change the doping material, so that selection of the crystal structure of the material will be described below.

The perovskite structure belongs to the cubic system and is represented by the chemical formula of ABO ₃ . FIG. 3 shows the structure diagram. Examples of oxygen ion conductivity are CaZrO ₃ , S
rZrO ₃ , BaZrO ₂ , CaCeO ₃ , SrCeO ₃ ,
BaCeO ₃ and the like. Many have a slightly distorted structure.

Next, the fluorite type structure will be described. The fluorite type also belongs to the cubic system, and the chemical formula is AO ₂ . The structure is shown in FIG. A is a +4 ion, which is a divalent or trivalent cation (Mg2 +, Ca2
+. Ba2 +, Sr2 +, Y3 +, Gd3 +, Nd3 +, Sc3 +,
(Sm3 +). Examples of oxygen ion conductivity are ZrO ₂ and CeO ₂ .

Next, the reason why oxygen ion vacancies are generated will be described. The concentration of oxygen ion vacancies depends on the dopant. For example, in a CeO ₂ -based electrolyte, when a trivalent rare earth oxide Sm ₂ O ₃ is appropriately doped, Ce ⁴⁺ ions are replaced by Sm ³⁺ ions.
One oxygen ion vacancy is generated by two m ³⁺ ions. However, if the doping material is added too much, Sm ³⁺ ions will associate with each other and the crystal structure will be broken.

BaCe _0.8 Y _{0 2} O _3-X is an oxide having a stable oxygen-deficient perovskite crystal structure.
The activation energy is known to be 0.3 eV. This structure can be prepared by both doping the perovskite structure with +3 valence ions and the fluorite structure with +2 valence ions.

From the above, the following Table 1 shows the types in which the stable defective perovskite structure existing between the fluorite-type structure and the perovskite-type structure is combined with a material resistant to reduction such as ceria. The combinations of the materials are shown in Table 1, A,
There are three types, B and C, where A is a main material, B is a +2 valent material, and C is a +3 valent material.

[0046]

[Table 1]

As Example 1 of the protective film, powders of the materials A, B and C shown in Table 1 were mixed at a molar ratio of 0.8: 1: 0.1.
After calcination, the pellet (protective film) was press-molded under a pressure of 1 ton / cm ² and baked at 1300 ° C. for 10 hours to produce. The pellets were 25 mm in diameter and the thickness was reduced to 1 mm by polishing. A solid electrolyte (CeO ₂ containing Sm ₂ O ₃ ) pellet having a diameter of φ25 mm and a thickness of 1 mm was prepared and bonded to the pellet (protective film) prepared as described above, and the solid electrolyte was placed on the cathode side when the circuit was opened. The voltage between the anode and cathode was measured. Note that oxygen gas is supplied to the cathode, and hydrogen gas (water vapor 3%) is supplied to the anode, and the temperature is 600 ° C. The electrode uses porous platinum for the anode and the cathode, and the electrode area is 0.785 cm ^2.
(Φ: 10 mm). This measurement result will be described later.

Next, a method of forming a protective film having an oxygen-deficient perovskite crystal structure chemically bonded to a solid electrolyte will be described as a second embodiment. BaCO ₃ powder was applied on a solid electrolyte substrate having a diameter of φ20 mm and heat-treated at 1000 ° C. for 5 hours. Ba diffuses into the solid electrolyte, and near the surface of the solid electrolyte, Ba _X C of a defective perovskite is formed.
The formation of e _1-XY Sm _Y O _Z was confirmed by observation with an electron microscope and identification by XRD. The conditions of the characteristics and the like are the same as in the first embodiment.

Material number A2 (CeO ₂ ),
With respect to the blending of B4 (BaCO ₃ ) and C5 (Sm ₂ O ₃ ), a mixture doped with NiO (40% by weight) was applied as Example 3 on a solid electrolyte substrate having a diameter of 25 mm and a thickness of 1 mm. It was baked at 10 ° C for 10 hours. NiO
Is reduced in a hydrogen atmosphere to become Ni, which serves as a catalyst for hydrogen and becomes an electrode. FIG. 5 shows the structure diagram. In FIG. 5, 21 is a solid electrolyte, 22 is an anode also serving as a protective film doped with NiO, and 23 is a cathode.

Table 2 shows the measurement results of Example 1. The reading of the numbers in Table 2 is, for example, "134" means ZrO ₂ (No. 1 of material A shown in Table 1) and SrO (Similarly, material B No. 3)
And Sc ₂ O ₃ (similarly, material C No. 4).

[0051]

[Table 2]

Normally, the open voltage of only the solid electrolyte is 0.7.
Since the voltage was 6 V to 0.8 V, it was found that each of them was effective in increasing the open-circuit voltage. An increase in the open circuit voltage means an increase in the external output, so that the effect is clear.

The result of Example 2 was as effective as the open circuit voltage of 1.09 V. Similar experiments were performed with strontium acetate, CaO
However, the results were effective at 0.98 V and 0.95 V.

FIG. 6 shows the results of measurement of the current density-voltage and output density characteristics of Example 3, and from the measurement results, it was found that NiO functions as an electrode.

[0055]

As described above, according to the present invention,
There is an advantage that the open circuit voltage of the ceria-based electrolyte can be increased and durability can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a solid oxide fuel cell showing an embodiment of the present invention.

FIG. 2 shows an example of an oxygen-deficient perovskite, YBCO.
FIG.

FIG. 3 is a diagram showing a perovskite structure.

FIG. 4 is a fluorite type structure diagram.

FIG. 5 is a structural diagram of Example 3 in which an electrode serving also as a protective film is bonded to a solid electrolyte.

FIG. 6 is a graph showing current density-voltage and output density characteristics.

FIG. 7 is a schematic configuration diagram of a transport model.

FIG. 8 is a schematic configuration diagram of a falsification experiment of a transport model.

FIG. 9 is a characteristic diagram of an open-circuit voltage showing a result of a disproof experiment.

FIGS. 8A to 8C are characteristic diagrams showing a state of a potential inside an electrolyte according to a new model.

FIG. 11 is an Arrhenius plot characteristic diagram.

FIG. 12 is a schematic configuration diagram of a demonstration experiment of a new model.

FIG. 13 is a characteristic diagram showing the results of a demonstration experiment using a new model.

FIG. 14 is a schematic configuration diagram of a conventional solid oxide fuel cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Solid electrolyte 12 ... Mixed conductive protective film 13 ... Oxygen electrode 14 ... Hydrogen electrode 15 ... Oxygen supply chamber 16 ... Fuel gas supply chamber 17 ... Load

[Procedure amendment]

[Submission date] October 4, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a solid oxide fuel cell showing an embodiment of the present invention.

FIG. 2 shows an example of an oxygen-deficient perovskite, YBCO.
FIG.

FIG. 3 is a diagram showing a perovskite structure.

FIG. 4 is a fluorite type structure diagram.

FIG. 5 is a structural diagram of Example 3 in which an electrode serving also as a protective film is bonded to a solid electrolyte.

FIG. 6 is a graph showing current density-voltage and output density characteristics.

FIG. 7 is a schematic configuration diagram of a transport model.

FIG. 8 is a schematic configuration diagram of a falsification experiment of a transport model.

FIG. 9 is a characteristic diagram of an open-circuit voltage showing a result of a disproof experiment.

FIGS . 10A to 10C are characteristic diagrams showing a state of a potential inside an electrolyte according to a new model.

FIG. 11 is an Arrhenius plot characteristic diagram.

FIG. 12 is a schematic configuration diagram of a demonstration experiment of a new model.

FIG. 13 is a characteristic diagram showing the results of a demonstration experiment using a new model.

FIG. 14 is a schematic configuration diagram of a conventional solid oxide fuel cell.

[Description of Signs] 11 ... Solid electrolyte 12 ... Mixed conductive protective film 13 ... Oxygen electrode 14 ... Hydrogen electrode 15 ... Oxygen supply chamber 16 ... Fuel gas supply chamber 17 ... Load

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code Reference number in the agency FI Technical display location C01G 25/00 C01G 25/00

Claims (11)

[Claims] 1. A hydrogen electrode and an oxygen electrode are formed on a solid electrolyte having oxygen ion conductivity, and a fuel gas such as hydrogen gas or natural gas flows to the hydrogen electrode side, and oxygen gas or air flows to the oxygen electrode side. In a solid oxide fuel cell that generates electric power between them, an oxide film having an oxygen-deficient perovskite crystal structure containing CeO ₂ and ZrO ₂ having oxygen ion conductivity was formed on the hydrogen electrode side of the solid electrolyte. A solid oxide fuel cell, comprising: 2. The solid oxide fuel cell according to claim 1, wherein said oxide film is a dense film. 3. An oxygen-deficient perovskite crystal structure containing CeO ₂ and ZrO ₂ having oxygen ion conductivity on the hydrogen electrode side of the solid electrolyte using an electrolyte containing CeO ₂ as a main component. 2. The solid oxide fuel cell according to claim 1, wherein an oxide film is formed. 4. The solid oxide fuel cell according to claim 3, wherein said oxide film is a dense film. 5. An oxygen ion conductive CeO ₂ , Zr is provided on the hydrogen electrode side of the electrolyte containing CeO ₂ as a main component.
4. The solid electrolyte according to claim 3, comprising a protective film of an oxide having an oxygen-deficient perovskite crystal structure formed by doping a perovskite containing O ₂ with an oxide containing a +3 valent rare earth element. Type fuel cell. 6. The solid oxide fuel cell according to claim 5, wherein said protective film is formed of a dense film. 7. The oxide containing a rare earth element is Y
7. The solid oxide fuel cell according to claim 6, comprising ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , Sc ₂ O ₃ , and Sm ₂ O ₃ . 8. The solid oxide fuel cell according to claim 1, wherein the oxide having a perovskite crystal structure contains Ba, Sr, Ca, and Mg. 9. The oxide having an oxygen deficient perovskite crystal structure has a mole ratio of CeO ₂ and ZrO ₂ of about 0.8, a mole ratio of BaO, SrO, CaO and MgO of about 1 and a Y of about 1; ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , Sc ₂ O ₃ , Sm ₂
6. The solid oxide fuel cell according to claim 1, wherein the molar ratio of O ₃ is set to about 0.1. 10. The oxide having a perovskite crystal structure mixed with a powder of Ni with respect to hydrogen gas or a powder which is reduced in a hydrogen gas atmosphere to produce Ni. 6. The solid oxide fuel cell according to 5. 11. A hydrogen electrode and an oxygen electrode are formed on a solid electrolyte having oxygen ion conductivity, and a fuel gas such as hydrogen gas or natural gas is caused to flow to the hydrogen electrode side, and oxygen gas or air is caused to flow to the oxygen electrode side. A solid electrolyte fuel cell for generating electric power between the cells, wherein a ceria-based electrolyte is used as the solid electrolyte, Ba, Sr, and Ca are diffused from the surface of the electrolyte to form an electrolyte having an oxygen-deficient perovskite crystal structure on the surface. A solid oxide fuel cell comprising: