JP7659717B2 - Solid electrolyte, its manufacturing method and electrochemical cell - Google Patents

Description

The present invention relates to a solid electrolyte, its manufacturing method, and an electrochemical cell.

In recent years, solid oxide fuel cells (SOFCs) and proton conductive ceramic fuel cells (PCFCs), which have high energy conversion efficiency, have been attracting attention as next-generation energy supply systems.

SOFCs use a solid electrolyte with ion conductivity (oxide ion conductivity) as the electrolyte, and are constructed by joining a fuel electrode (anode) to one surface of the electrolyte layer and an air electrode (cathode) to the other surface. When fuel containing hydrogen is supplied to the anode and oxygen (air) to the cathode, electrical energy can be extracted through the following electrochemical reaction.
Anode reaction: 2H ₂ + 2O ²⁻ → 2H ₂ O + 4e ⁻ (Reaction 1)
Cathodic reaction: O ₂ + 4e ⁻ → 2O ²⁻ (reaction 2)
Total reaction: _2H2 + _O2 → _2H2O

On the other hand, PCFCs use a solid electrolyte with proton conductivity (hydrogen ion conductivity) as the electrolyte, and are constructed by joining a fuel electrode (anode) to one surface of the electrolyte layer and an air electrode (cathode) to the other surface. When a hydrogen-containing fuel (fuel gas) is supplied to the anode and oxygen (air) to the cathode, electrical energy can be extracted through the following electrochemical reaction:
Anode reaction: _2H2 → 4H ^{++ 4e-} (reaction 3)
Cathodic reaction: 4H ⁺ +O ₂ +4e ⁻ → 2H ₂ O (reaction 4)
Total reaction: _2H2 + _O2 → _2H2O

The solid electrolyte forming the electrolyte layer of PCFC is required to have proton conductivity (hydrogen ion conductivity). Known solid electrolytes with proton conductivity include BaCeO3 _-based and _BaZrO3- based compounds (oxides) with a perovskite structure. These compounds are conventionally made of sintered bodies obtained by sintering oxides.

To improve the performance of PCFCs, further research is being conducted into solid electrolyte materials. In particular, solid electrolytes for PCFCs are required to have improved proton conductivity, good sintering properties, and chemical stability.

Specifically, _BaCeO3 -based oxides with a perovskite structure are chemically unstable. When the oxides are chemically unstable, the long-term reliability of the PCFC decreases when the PCFC is formed. On the other hand, _BaZrO3 -based oxides are compounds that are difficult to sinter, and are difficult to densify. When densification becomes difficult, substances other than protons (e.g., atomic ions and gas molecules) become permeable through the solid electrolyte (electrolyte layer).

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a solid electrolyte that has high proton conductivity and excellent sinterability and chemical stability, a method for producing the same, and an electrochemical cell.

The solid electrolyte of the present invention, which solves the above problems, is characterized by being a compound represented by A ₂ B ¹ _1-x M ¹ _x B ² _1-y M ² _y O ₆ (A: alkaline earth metal, B ¹ : at least one selected from La, Nd, Gd, Ce, Pr and Yb, B ² : at least one selected from transition metals, M ¹ : at least one selected from Be, Mg, Ca and Sr, M ² : at least one selected from Ta, Sb, Zr, Sn, Ti and Mg, 0≦x≦0.30, 0≦y≦0.30).

The method for producing a solid electrolyte of the present invention, which solves the above problems, is characterized by having a step of mixing raw material powders weighed to have a predetermined atomic ratio, and heat-treating the mixture at a temperature below the sintering temperature to produce a precursor, and a step of heat-treating the precursor at a temperature equal to or higher than the sintering temperature.

The electrochemical cell of the present invention, which solves the above problems, is characterized by using the solid electrolyte described in any one of claims 1 and 2.

FIG. 1 is a diagram showing a double perovskite crystal structure. FIG. 1 is a diagram showing a perovskite crystal structure. 1 is a flowchart showing steps of a method for producing a solid electrolyte according to an embodiment. FIG. 2 is a diagram showing the results of XRD measurements in Examples 1 and 2. FIG. 1 is a diagram showing the results of XRD measurements in Examples 3 to 5. FIG. 13 is a diagram showing the results of XRD measurement in Example 6. FIG. 2 is a diagram showing the measurement results of proton conductivity in Examples and Comparative Examples.

The present invention will be specifically described below using the embodiments. Note that the present invention is not limited to the configurations shown in the embodiments, and includes any configuration that is modified without departing from the gist of the present invention.

[Solid electrolyte]
The solid electrolyte of this embodiment is a compound represented by A ₂ B ¹ _1-x M ¹ _x B ² 1 _-y M ² _y O ₆ (A: alkaline earth metal, B ¹ : at least one selected from La, Nd, Gd, Ce, Pr and Yb, B ² : at least one selected from transition metals, M ¹ : at least one selected from Be, Mg, Ca and Sr, M ² : at least one selected from Ta, Sb, Zr, Sn, Ti and Mg, 0≦x≦0.30, 0≦y≦0.30).

In the solid electrolyte of the present embodiment, the valences of ^B1 and ^B2 in the above composition are preferably smaller than the valences of ^M1 and ^M2 . When the valences satisfy this relationship, the solid electrolyte of the present embodiment has suitable proton conductivity. Here, the valences of ^B1 and ^B2 may be the same or different, but are preferably the same. The valences of ^M1 and ^M2 may also be the same or different. When the valences of ^M1 and ^M2 are different, it is preferable that the valences of ^B1 and ^B2 are both smaller than the smaller valence of ^M1 and ^M2 .

In the above composition, the solid electrolyte of the present embodiment is preferably Ba, where A is Ba. The elements ^B1 and ^B2 may be the same or different. The solid electrolyte of the present embodiment has a double perovskite crystal structure as described below when 0≦x≦0.30 and 0≦y≦0.30 in the above composition. When x and y exceed 0.30, two crystal phases are likely to occur, and the proton conductivity (proton conductivity value) is likely to decrease. It is more preferable that 0≦x≦0.25 and 0≦y≦0.25, and even more preferable that 0≦x≦0.20 and 0≦y≦0.20.

The compound of the solid electrolyte of this embodiment is an oxide having a crystal structure called a double perovskite type. The double perovskite type oxide is an oxide having suitable proton conductivity (ionic conductivity). Here, having suitable proton conductivity means that the proton conductivity is 10 ⁻⁵ S/cm or more, preferably 10 ⁻³ S/cm or more. That is, when an electrochemical cell (e.g., a fuel cell) is formed, the oxide exhibits high proton conductivity at the operating temperature to the extent that the desired battery characteristics can be exhibited.

The double perovskite crystal has the crystal structure shown in Fig. 1. Fig. 1 is a diagram showing the crystal structure of _a double perovskite compound represented by the general formula: _A2MM'O6 (A: Ba, M: Nd or Gd, M': Nb).
In contrast, perovskite-type crystals have the crystal structure shown in Fig. 2. Fig. 2 is a diagram showing the structure of a perovskite-type compound represented by the general formula: AMO ₃ (A: Ba, M: Zr or Ce).

As shown in Fig. 1, the double perovskite type compound has a structure in which an oxygen octahedron surrounding M' (pentavalent atomic ion, Nb5 ⁺ ) and an oxygen octahedron surrounding M (trivalent atomic ion, M3 ⁺ ) surround A (Ba2 ⁺ ). The space group of this structure is Fm-3m (No. 225).
In contrast, perovskite-type compounds have a structure in which an oxygen octahedron surrounding M (a tetravalent atomic ion, M ⁴⁺ ) surrounds A (Ba ²⁺ ), as shown in Figure 2. The space group of this structure is Pm-3m (No. 221).

As is clear from Figures 1 and 2, the symmetry of the crystal structure is clearly different between the double perovskite type compound and the perovskite type compound. Due to this difference in crystal structure, the double perovskite type compound has higher chemical stability than the perovskite type compound.
Double perovskite type compounds have multiple oxygen octahedra surrounding elements with different valences, and the distance between the oxygen octahedra surrounding the bulky elements is long. This makes it easier for atoms to move within the solid electrolyte. In other words, sintering reactions involving the movement of atoms are more likely to occur. In other words, they have high sinterability.

The compound of the solid electrolyte of this embodiment is preferably Ba ₂ B ¹ _1-x Ca _x NbO _6. This compound is a compound in the above composition formula where A: Ba, B ² : Nb, M ¹ : Ca, and y = 0. This compound is a compound with a crystal structure in which part of M (trivalent) is replaced with Ca (divalent) in the crystal structure shown in Figure 1.
The solid electrolyte of the present embodiment preferably has an oxygen-deficient crystal structure, which allows the solid electrolyte of the present embodiment to have higher proton conductivity.

(Effects of this embodiment)
The solid electrolyte of the present embodiment is a compound having the above-mentioned composition and is made of an oxide having suitable proton conductivity (ion conductivity). This oxide has excellent sinterability and chemical stability, and the solid oxide of the present embodiment also has excellent sinterability and chemical stability.

The solid electrolyte of the present embodiment is not limited in its manufacturing method, and can be manufactured, for example, by the following manufacturing method.
[Method of manufacturing solid electrolyte]
The method for producing a solid electrolyte according to the present embodiment includes the steps shown in FIG.
(Raw material weighing process: S1)
The raw material weighing step is a step of weighing the raw material powder of the solid electrolyte. Specifically, the raw material powder of the solid electrolyte is weighed in a ratio that allows the solid electrolyte to be produced. The raw material powder of the solid electrolyte can be a powder of a compound (powder of oxide or carbide) containing inorganic elements (each element of A, B ¹ , M ¹ , B ² , and M ² ) that constitute the solid electrolyte. It may also be a salt compound of an inorganic element such as a carbonate. Furthermore, the raw material of the solid electrolyte may be either a compound containing one type of inorganic element that constitutes the solid electrolyte or a compound containing two or more types of inorganic elements.

The raw powder of the solid electrolyte is weighed so that the inorganic elements have a predetermined atomic ratio (atomic ratio of the solid electrolyte after production). Note that some of the inorganic elements may evaporate during the sintering process, causing the atomic ratio to change. In this case, it is preferable to weigh the raw powder of the inorganic element that evaporates at an atomic ratio that includes the amount of evaporation. In other words, it is preferable to weigh out the raw powder of the inorganic element in an amount that is greater than the predetermined atomic ratio by the amount of evaporation.

(Mixing step: S2)
The mixing step is a step of mixing the weighed raw material powders. The mixing step is a step of mixing the raw material powders to obtain a uniformly mixed raw material powder. The method of mixing the raw material powders is not limited as long as each raw material powder can be mixed in a uniformly dispersed state. For example, a method of mixing using a mixing device such as a ball mill, a planetary ball mill, or a tromill can be mentioned.
The mixing step may be performed by either a dry mixing method in which the raw material powders are mixed in a dry state or a wet mixing method in which the raw material powders are mixed in a state where they are dispersed in a solvent or the like.

(Drying process: S3)
The drying process is a process for drying the mixed raw material powders. The drying process is not limited to a specific drying method as long as it can dry the mixed powders. The drying process removes moisture contained in the mixed powders (moisture adsorbed on the surfaces of the raw material powder particles). Furthermore, when wet mixing is performed in the mixing process, a dispersion medium such as a solvent is removed.
The method for drying the mixed powder is not limited. The drying step is, for example, a step of heating the mixed powder at a temperature higher than room temperature. Preferably, the drying step is a step of heating at a temperature at which the moisture (and dispersion medium) in the mixed powder evaporates. The heating temperature is not limited, and is preferably a temperature of 50° C. or higher. More preferably, it is 100° C. or higher. The pressure during heating is also not limited, and is preferably normal pressure or reduced pressure.

(Pre-firing step: S4)
The calcination process is a process of heat-treating the dried raw material powder (mixed powder) at a temperature lower than the sintering temperature. In the calcination process, the raw material powder (mixed powder) is heat-treated to promote the reaction (atomic diffusion reaction) between the inorganic elements in each raw material powder. When the reaction between the inorganic elements progresses, a precursor (a compound having a solid electrolyte composition as a whole. The crystal structure is a mixture of the raw material powders, and the crystal structure is different from that of the final product) is generated. That is, the calcination process is a process of heat-treating the precursor at a temperature lower than the sintering temperature. Here, the sintering temperature is the lowest temperature among the temperatures at which the reaction progresses in the sintering process described below.

The heat treatment in the calcination step may be performed at a temperature lower than the sintering temperature and capable of producing a precursor, for example, at a temperature 100 to 300° C. lower than the sintering temperature.
The pressure during the heat treatment in the calcination step is not limited, but is preferably normal pressure or reduced pressure.
The treatment time for the heat treatment in the calcination step is not limited either, but may be the time required for the reaction to produce the precursor to be completed.

(Crushing step: S5)
The pulverization step is a step of pulverizing the precursor. In the pulverization step, the precursor produced by the calcination is pulverized to produce a precursor powder. The particle size (average particle size: D50) of the precursor powder is not limited.
In the pulverization step, the specific method for pulverizing the precursor is not limited. For example, the precursor is physically pulverized using an apparatus such as a ball mill, a planetary ball mill, or a tromill to obtain a precursor powder.

(Molding process: S6)
The forming step is a step of forming the precursor powder obtained in the pulverization step into a predetermined shape. The predetermined shape is the shape when the solid electrolyte of this embodiment is used (i.e., the shape corresponding to the electrolyte layer in the electrochemical cell described later), and examples of such shapes include a plate-like or sheet-like thin plate shape. Note that, when a volume change (specifically, shrinkage) occurs during the sintering step, the predetermined shape is a shape that assumes the volume change.

The molding step is preferably a step of compression molding the precursor powder. When compression molding the precursor powder, the molding conditions such as pressure, pressure time, and pressure temperature are not limited. For example, molding methods such as die molding, which uses a mold having a cavity corresponding to a predetermined shape for pressure molding, and die molding, which uses an isostatic pressure method such as HIP molding or CIP molding, can be used.

(Sintering step: S7)
The sintering step is a step of heat-treating the precursor molded body at a temperature equal to or higher than the sintering temperature. In the sintering step, the precursor molded body is heat-treated to sinter the precursor. When the precursor is sintered, an exchange reaction of the precursor elements proceeds, and a reaction to generate a compound having a double perovskite crystal structure proceeds, and a compound of the solid electrolyte of this embodiment is generated. Here, the sintering temperature is the temperature at which the precursor is sintered, and is the lowest temperature among the temperatures at which the exchange reaction of the precursor elements proceeds, and the reaction to generate a compound having a double perovskite crystal structure proceeds to generate a compound of the solid electrolyte. Even if the heat treatment temperature is lower than the sintering temperature, an exchange reaction of the precursor elements may occur, but the reaction to generate a compound having a double perovskite crystal structure does not proceed.
In the sintering step, a sintering reaction between particles of the precursor powder progresses, and a solid electrolyte compound having a predetermined shape can be obtained. More specifically, a solid electrolyte made of a compound having the above composition (a compound having a double perovskite crystal structure) is produced. That is, a solid electrolyte having a thin plate shape such as a plate or sheet shape can be obtained by molding in the molding step.

In the sintering step, the precursor compact is heat-treated under treatment conditions that allow the sintering reaction to proceed. The conditions of the heat treatment are not limited. That is, the heat treatment conditions, such as the heat treatment temperature, heat treatment time, and atmosphere, are not limited. The heat treatment temperature may be a temperature at which the exchange reaction of the precursor elements proceeds and at which the reaction to generate a compound having a double perovskite crystal structure proceeds (i.e., a temperature equal to or higher than the sintering temperature). The heat treatment time may be a time at which the reaction to generate a solid electrolyte compound is completed. The atmosphere may be an atmosphere that allows the reaction to generate a solid electrolyte compound to proceed. It is preferable that the atmosphere is air to oxygen atmosphere (i.e., the heat treatment at a temperature equal to or higher than the sintering temperature is performed in an oxygen-containing atmosphere).
The solid electrolyte of the present embodiment is produced by carrying out these steps (S1 to S7). Regarding the conditions other than those described above, conventionally known methods (steps) can be appropriately used.
For example, after the sintering step, a step of polishing and shaping the surface may be carried out, which improves the dimensional accuracy and exposes a clean surface of the solid electrolyte compound.

(Effects of this embodiment)
According to the method for producing a solid electrolyte of the present embodiment, the above-mentioned solid electrolyte of the present embodiment, i.e., a solid electrolyte made of a compound having the above-mentioned composition, can be produced. In other words, the method for producing a solid electrolyte of the present embodiment exhibits the effect of being able to produce a solid electrolyte having suitable proton conductivity, as well as excellent sinterability and chemical stability.

[Electrochemical cell]
The solid electrolyte of the present embodiment has proton conductivity and can be used in components that require this property, for example, as an electrolyte (solid electrolyte) for electrochemical cells.
An electrochemical cell refers to a battery cell (or battery) that extracts electric power from the chemical energy of fuel through an electrochemical reaction. An example of an electrochemical cell is a battery cell of a fuel cell. The solid electrolyte of the present embodiment is preferably a solid electrolyte of a fuel cell.

[Fuel Cell]
The structure of the fuel cell (fuel cell) using the solid electrolyte of this embodiment is not limited. The fuel cell may have the same structure as a conventionally known proton conductive ceramic fuel cell (PCFC). The structure of the fuel cell not specifically mentioned may be a conventionally known structure. The fuel cell may have, for example, an electrolyte layer, a fuel electrode, and an air electrode.
The electrolyte layer is made of the solid electrolyte of the present embodiment. The electrolyte layer is made of the solid electrolyte formed in a plate shape.
The fuel electrode is formed on one surface of a plate-shaped electrolyte layer. The fuel electrode is an electrode to which a fuel (fuel gas) containing hydrogen is supplied. The fuel electrode has an electrode body integrally provided on one surface of the electrolyte layer. This electrode body is in the form of a foil (mesh) having a large number of holes that allow the passage of a gas (fuel gas) containing hydrogen. In the fuel electrode, the fuel gas passes through the large number of holes in the electrode body, and hydrogen comes into contact with one surface of the electrolyte layer, causing an electrode reaction. Electrons generated in the electrode reaction flow from the electrode body to an external element. That is, the above-mentioned reaction 3 occurs.
The air electrode is formed on the other surface of the plate-like electrolyte layer. The air electrode is an electrode to which oxygen-containing gas (air) is supplied. The air electrode has a catalyst and an electrode body integrally provided on the other surface of the electrolyte layer. This electrode body is in the form of a foil (mesh) having a large number of holes that allow the gas to pass through. In the air electrode, the oxygen-containing gas (air) passes through the large number of holes in the electrode body, and the oxygen comes into contact with the other surface of the electrolyte layer and reacts. The electrons consumed in the electrode reaction flow from the external element through the electrode body. In other words, the above reaction 4 occurs.

The fuel cell may further have a configuration in which the electrolyte layer is sandwiched between a fuel electrode separator and an air electrode separator. The fuel electrode separator has a plurality of fuel gas flow paths, and the air electrode separator has a plurality of oxygen-containing gas flow paths. The cross-sectional shapes of the flow paths (fuel gas flow paths, oxygen-containing gas flow paths) formed on the surfaces of the fuel electrode separator and the air electrode separator facing the electrolyte layer are not limited.
The fuel cell may have a configuration in which a plurality of fuel cells are stacked.

(Effects of this embodiment)
The electrochemical cell of the present embodiment is characterized in that it uses the solid electrolyte of the present embodiment in an electrolyte layer, that is, it uses the solid electrolyte having the above-mentioned effects, and exerts the above-mentioned effects (especially the effect of a long life).

The present invention will be described in detail below with reference to examples. As examples, solid electrolytes are produced.
Example 1
The solid electrolyte of this example is made of a compound represented by Ba ₂ Nd _0.8 Ca _0.2 NbO _6. This compound is a compound in which A: Ba, B ¹ : Nd, B ² : Nb, M ¹ : Ca, x=0.2, and y=0 in the above composition formula.

The solid electrolyte of this example is produced by the above-mentioned production method, specifically, by carrying out the following steps.
(Raw material weighing process: S1)
In the raw material weighing process, _BaCO3 is prepared as the Ba source, _Nd2O3 is _prepared as the Nd source, _CaCO3 is prepared as the Ca source, and _{Nb2O5 is} prepared as the Nb source.
The powders of the prepared compounds were weighed out so that the atomic ratio of Ba:Nd:Ca:Nb was 2:0.8:0.2:1.
(Mixing step: S2)
The powders of the prepared compounds are mixed uniformly in a ball mill at 100 rpm for 24 hours.
(Drying process: S3)
The mixed raw material powders are kept under the conditions of 50° C.×1 hour (at atmospheric pressure) and dried.
(Pre-firing step: S4)
The dried raw material powder is charged into the firing vessel, where it is maintained in an uncompressed state.
The firing container is then placed in a heating furnace and heat-treated at 1300° C. for 10 hours (at atmospheric pressure). This heat treatment produces a precursor oxide (a mixture of raw material powders with an overall composition of Ba ₂ Nd _0.8 Ca _0.2 NbO ₆ ).

(Crushing step: S5)
The precursor oxide produced in the calcination step (S4) is allowed to cool, and then pulverized in a ball mill at 100 rpm for 24 hours. The precursor oxide is pulverized to have a particle size of 5 μm or less.
(Molding process: S6)
The obtained precursor oxide powder is molded into a disk (predetermined shape) having a diameter of 18 mm and a thickness of 1.5 mm. The precursor oxide powder is molded by pressing at a molding pressure of 98.0665 MPa (1 ton weight/cm ² ).
(Sintering step: S7)
The compact is placed in a heating furnace and heat-treated at 1500°C for 10 hours (atmospheric atmosphere, normal pressure). The sintering temperature of the solid electrolyte of this example is 1500°C. This heat treatment produces a disk-shaped solid electrolyte of this example. The produced solid electrolyte of this example is a dense sintered body without defects such as cracks. In other words, it is a solid electrolyte that does not transmit gases such as _H2 and _O2 .

Example 2
The solid electrolyte of this example is made of a compound represented by Ba ₂ NdNbO _6. This compound is a compound in which A: Ba, B ¹ : Nd, B ² : Nb, x=0, and y=0 in the above composition formula.
The solid electrolyte of this example was produced in the same manner as in Example 1, except that the blending ratio of the powders of the respective compounds in the raw material weighing step (S1) was changed. The sintering temperature of the solid electrolyte of this example was 1500° C.

Example 3
The solid electrolyte of this example is made of a compound represented by Ba ₂ Gd _0.7 Ca _0.3 NbO _6. This compound is a compound in which A: Ba, B ¹ : Gd, B ² : Nb, M ¹ : Ca, x=0.3, and y=0 in the above composition formula.
The solid electrolyte of this example was produced in the same manner as in Example 1. The differences from Example 1 are as follows.
In the raw material weighing step (S1), Gd ₂ O ₃ was used as the Gd source.
In the calcination step (S4), heat treatment is performed under the conditions of 1350° C.×10 hours (at atmospheric pressure) to generate a precursor oxide (a mixture of raw material powders with an overall composition of Ba ₂ Gd _0.7 Ca _0.3 NbO ₆ ).
In the sintering step (S7), the solid electrolyte is heat-treated at 1600° C. for 10 hours (at atmospheric pressure). The sintering temperature of the solid electrolyte in this example is 1600° C.

Example 4
The solid electrolyte of this example is made of a compound represented by Ba ₂ Gd _0.8 Ca _0.2 NbO _6. This compound is a compound in which A: Ba, B ¹ : Gd, B ² : Nb, M ¹ : Ca, x=0.2, and y=0 in the above composition formula.
The solid electrolyte of this example was produced in the same manner as in Example 3, except that the blending ratio of the powders of the respective compounds in the raw material weighing step (S1) was changed. The sintering temperature of the solid electrolyte of this example was 1600° C.

Example 5
The solid electrolyte of this example is made of a compound represented by Ba ₂ GdNbO _6. This compound is a compound in which A: Ba, B ¹ : Gd, B ² : Nb, x=0, and y=0 in the above composition formula.
The solid electrolyte of this example was produced in the same manner as in Example 3, except that the blending ratio of the powders of the respective compounds in the raw material weighing step (S1) was changed. The sintering temperature of the solid electrolyte of this example was 1600° C.

Example 6
The solid electrolyte of this example is made of a compound represented _{by Ba2La0.9Ca0.1NbO6} . This compound is a compound in which A:Ba, ^B1 :La, ^B2 :Nb, ^M1 :Ca, x= _{0.1 ,} and y=0 in the above composition formula.
The solid electrolyte of this example was produced in the same manner as in Example 1. The differences from Example 1 are as follows.
In the raw material weighing step (S1), La ₂ O ₃ was used as the La source.
In the calcination step (S4), heat treatment is performed under the condition of 1350° C.×10 hours (at atmospheric pressure) to generate a precursor oxide (a mixture of raw material powders with an overall composition of Ba ₂ La _0.9 Ca _0.1 NbO ₆ ).
In the sintering step (S7), the solid electrolyte is heat-treated at 1600° C. for 10 hours (at atmospheric pressure). The sintering temperature of the solid electrolyte in this example is 1600° C.

(Comparative Example 1)
The solid electrolyte of this example is made of a _compound represented _{by BaZr0.95Y0.05O3} . The solid electrolyte of this example differs from those of the other examples in the composition and crystal structure of the compound. The compound of this example has a perovskite-type crystal structure.
The solid electrolyte of this example was produced in the same manner as in Example 1. The differences from Example 1 are as follows.
_In the raw material weighing step (S1), _BaCO3 was used as the Ba source, _ZrO2 was used as the Zr source, and _Y2O3 was used as the Y source. The powders of the prepared compounds were weighed so that the atomic ratio of Ba:Zr:Y was 1:0.95:0.05.
In the calcination step (S4), heat treatment is performed under conditions of 1400° C.×10 hours (at atmospheric pressure) to generate a precursor oxide (a mixture of raw material powders with an overall composition of BaZr _0.95 Y _0.05 O ₃ ).
In the sintering step (S7), heat treatment is performed under the conditions of 1600° C.×10 hours (in air, under normal pressure).

(Comparative Example 2)
The solid electrolyte of this example is made of a _compound represented _{by BaCe0.9Nd0.1O3} . The solid electrolyte of this example differs from those of the other examples in the composition and crystal structure of the compound. The compound of this example has a perovskite-type crystal structure.
The solid electrolyte of this example was produced in the same manner as in Example 1. The differences from Example 1 are as follows.
In the raw material weighing step (S1), _BaCO3 was used as the Ba source, _CeO2 was used as the Ce source, and _Nd2O3 was used as the _Nd source. The powders of the prepared compounds were weighed so that the atomic ratio of Ba:Ce:Nd was 1:0.9:0.1.
In the calcination step (S4), heat treatment is performed under the conditions of 1400° C.×10 hours (at atmospheric pressure) to generate a precursor oxide (a mixture of raw material powders with an overall composition of BaCe _0.9 Nd _0.1 O ₃ ).
In the sintering step (S7), heat treatment is performed under the conditions of 1600° C.×10 hours (in air, under normal pressure).

[evaluation]
The solid electrolytes of the above examples were evaluated by measuring the XRD diffraction pattern and the proton conductivity.

[XRD]
The XRD of the solid electrolyte of each Example was measured, and the measurement results of Examples 1 and 2 are shown in FIG. 4, and the measurement results of Examples 3 to 5 are shown in FIG.
4 and 5, when Example 1 is compared with Example 2, it can be seen that in the solid electrolyte of Example 1, a part of Nd in the solid electrolyte of Example 2 is partially substituted with Ca. When Examples 3 and 4 are compared with Example 5, it can be seen that in the solid electrolytes of Examples 3 and 4, a part of Gd in the solid electrolyte of Example 5 is partially substituted with Ca.

That is, in the solid electrolytes of Examples 1 to 5, which are Ba ₂ B ¹ _1-x Ca _x NbO ₆ (B ¹ : Nd or Gd, 0≦x≦0.30), B ¹ can be partially substituted with Ca, and this substitution causes oxygen deficiency (having an oxygen-deficient crystal structure), and it is found that the solid electrolytes have high proton conductivity.
Moreover, it can be confirmed that the solid electrolytes of Examples 1 to 5 have a double perovskite crystal structure from the XRD diffraction patterns of Figures 4 and 5. By having a double perovskite crystal structure, the solid electrolytes of Examples 1 to 5 have high chemical stability. Furthermore, since the solid electrolytes of Examples 1 to 5 have a double perovskite crystal structure, they also have excellent sinterability.
In addition, the XRD diffraction patterns of the solid electrolytes of Examples 1 to 5 show sharp peaks, as shown in Figures 4 and 5. In other words, the solid electrolytes of Examples 1 to 5 have high crystallinity. Moreover, the solid electrolytes of Examples 1 to 5 are made of sintered bodies without defects, and have excellent sinterability.

[XRD]
The solid electrolyte of Example 6 was subjected to XRD measurement, and the measurement results are shown in FIG.
The XRD diffraction pattern in Fig. 6 has peaks similar to those of the XRD diffraction patterns in Figs. 4 and 5. That is, it can be confirmed that the solid electrolyte of Example 6 has a double perovskite crystal structure, similar to the solid electrolytes of Examples 1 to 5. Therefore, the solid electrolyte of Example 6 has high chemical stability, similar to the solid electrolytes of Examples 1 to 5. Furthermore, since the solid electrolyte of Example 6 has a double perovskite crystal structure, it also has excellent sinterability.
The XRD diffraction pattern of the solid electrolyte of Example 6 also shows a sharp peak. In other words, the solid electrolyte of Example 6 also has high crystallinity like Examples 1 to 5, and is excellent in sinterability.

[Proton Conductivity]
To evaluate Examples 1 and 4, the proton conductivity was measured by an AC impedance method, and the measurement results are shown in Fig. 7. Fig. 7 also shows the proton conductivity (literature value) of Comparative Examples 1 and 2.
7, the solid electrolytes of Examples 1 and 4 have higher proton conductivity than the solid electrolytes of Comparative Examples 1 and 2. In other words, the compound having a double perovskite crystal structure has better proton conductivity than the compound having a perovskite crystal structure.
As described above, the solid electrolytes of the Examples have superior proton conductivity compared to the solid electrolytes of the Comparative Examples.

As described above in detail, the solid electrolytes of the Examples have superior proton conductivity compared to the Comparative Examples, and the solid electrolytes of the Examples have superior sinterability and chemical stability due to their crystal structures.
As a result, electrochemical cells (fuel cells) using these solid electrolytes have the effect of being excellent in performance and having a long life.
Furthermore, according to the manufacturing method of the present invention, a solid electrolyte exhibiting these effects can be manufactured.

Claims (7)

_{A solid electrolyte comprising} at _{least one compound} selected from _{Ba2Nd0.8Ca0.2NbO6 , Ba2NdNbO6 , Ba2Gd0.7Ca0.3NbO6 , Ba2Gd0.8Ca0.2NbO6 , Ba2GdNbO6 , and Ba2La0.9Ca0.1NbO6 .} The solid electrolyte according to claim 1, which has an oxygen-deficient crystal structure. _{A method for producing a solid electrolyte which is} at _least one compound selected from _{Ba2Nd0.8Ca0.2NbO6} , _Ba2NdNbO6 , _{Ba2Gd0.7Ca0.3NbO6 , Ba2Gd0.8Ca0.2NbO6} , _Ba2GdNbO6 , _{and Ba2La0.9Ca0.1NbO6 , comprising} :
A step of mixing raw material powders weighed to have a predetermined atomic ratio and heat treating the mixture at a temperature lower than the sintering temperature to generate a precursor;
heat treating the precursor at a temperature equal to or greater than the sintering temperature;
2. A method for producing a solid electrolyte comprising the steps of: The method for producing a solid electrolyte according to claim 3, wherein the heat treatment at a temperature equal to or higher than the sintering temperature is carried out in an oxygen-containing atmosphere. An electrochemical cell characterized by using the solid electrolyte according to any one of claims 1 to 2. The electrochemical cell of claim 5, which is a fuel cell. The electrochemical cell according to claim 6, comprising an electrolyte layer made of the solid electrolyte, a fuel electrode provided on one surface of the electrolyte layer, and an air electrode provided on the other surface of the electrolyte layer.

Images