JPH09139219A - Solid electrolyte type electrochemical cell, and manufacture of it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cell, having less aged deterioration despite of long-time operation. SOLUTION: In a solid electrolyte type electrochemical cell composed of an air pole, a solid electrolyte coat, and a fuel pole; a zirconia-yttria solid solution wherein; a yttria, of 9mol% or more and 10.5mol% or less, is solidly resolved into zirconia, the own crystal structure has fluorite-like structure or has the coexistent condition of the fluorite-like structure and a cubic crystal, and purity is 99.9wt.% or more; ia used as a solid electrolyte.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solid electrolyte type electrochemical cell such as a solid electrolyte type fuel cell or a high temperature steam electrolysis cell, and a method for producing the same.

[0002]

2. Description of the Related Art Hereinafter, a solid oxide electrolyte fuel cell (S
The description will be made by taking OFC for short) as an example. SOFC generates electromotive force by generating an electromotive force due to the difference in oxygen concentration generated at both ends of the electrolyte membrane. As an electrolyte, zirconia (ZrO ₂ ) -yttria (Y ₂ O ₃ ) cubic crystal having a Y ₂ O ₃ concentration of 8 mol% is used as an electrolyte. (Fluorite structure) Solid solutions have been used conventionally. This is because the cubic stabilized zirconia solid solution (YSZ) has a conductivity of Y ₂
This is because the maximum O ₃ concentration is 8 mol% composition.

[0003]

(100-m) ZrO
_When the solid solution of ₂ · mY ₂ O ₃ composition is abbreviated as ZmY, the conventional solid solution Z8Y is the operating temperature of SOFC.
When used at 000 ° C. for a long time, the conductivity is reduced to about half, and SOFC using this as an electrolyte has a problem that its performance deteriorates with time. In order to elucidate the cause of this deterioration over time, the phase diagram of the ZrO ₂ —Y ₂ O ₃ system is clarified, and the composition and the solid solution crystal necessary for that purpose show a small decrease in conductivity over long-term use. It is necessary to clarify the conditions such as the structure and to provide a stable SOFC with less output reduction.

As a result of various tests conducted by the present inventors, Z
Strictly speaking, the 8Y solid solution is not a solid solution having a stable cubic crystal structure, and it is found that when it is held at 1000 ° C. for a long time, tetragonal particles are precipitated, and the conductivity decreases with an increase in the amount of tetragonal precipitation. It was found to be the cause of deterioration. Furthermore, in order to reduce the manufacturing cost of SOFC, 9
If a raw material powder having a purity of about 9.8% is used, the strength of the membrane may be reduced over a long period of time, the solid electrolyte membrane may be cracked, and the electrical conductivity may be greatly reduced. all right.

[0005]

In order to solve the above problems, the present invention provides (i) a solid oxide fuel cell comprising an air electrode, a solid electrolyte membrane and a fuel electrode, wherein zirconia (ZrO ₂₎ is used as the solid electrolyte. ) To yttria (Y ₂
O ₃ ) is dissolved in a solid solution of 9 mol% or more and less than 10.5 mol%, and the crystal structure thereof is a pseudofluorite structure (as shown in FIG. 1, the oxygen atom is slightly different from the position of the oxygen atom of the fluorite structure. By using a zirconia-yttria (ZrO ₂ —Y ₂ O ₃ ) solid solution that has a displaced structure) or a pseudofluorite structure and a cubic crystal (fluorite structure) in a coexisting state and has a purity of 99.9% by weight or more. Provides a solid oxide fuel cell, which is characterized by little deterioration in performance even when used at high temperature for a long time.
i) Further, as a method for producing the same, Zr is obtained by uniformly dissolving Y ₂ O ₃ in ZrO ₂ in an amount of at least 9 mol% and less than 10.5 mol%.
A step of forming an air electrode or a fuel electrode by using O ₂ —Y ₂ O ₃ solid solution fine particles as a raw material powder of a solid electrolyte; and a step of forming the ZrO ₂ —Y ₂ O ₃ solid solution fine particle film thereon. And a step of forming a film of the remaining one of the fuel electrode and the air electrode on the film, and the method for producing a solid oxide fuel cell.

(Operation) (1) As a solid electrolyte, ZrO ₂ is solid-dissolved with Y ₂ O _{3 in an amount} of 9 mol% or more and less than 10.5 mol%, and its crystal structure is a pseudofluorite structure or a pseudofluorite structure. It is assumed that cubic crystals coexist: Y ₂ O _{3 as} shown in Examples below.
Since concentrations conductivity decreases with particles of tetragonal structure when held for a long time to precipitate at 1000 ° C. in the composition of less than 9 mol%, it is not suitable because a large deterioration with time of the output of the SOFC, Y ₂ O ₃ concentration Is 10.5 mol% or more, the crystal structure is cubic and stable, but the conductivity is small from the beginning, the output is small, the sinterability is poor, and it is necessary to raise the sintering temperature. This is because the cost is high and it is not suitable as a solid electrolyte material. Y ₂ O
_{When the} concentration of ₃ is between 9 mol% and less than 10.5 mol%, there is no precipitation of tetragonal structure particles even after long-term holding, and the crystal structure of the solid solution is pseudofluorite structure or pseudofluorite structure and cubic structure. This is because they are in a coexisting state, and in this state, they have high conductivity and little deterioration over time, and are therefore suitable as a solid electrolyte. This is because the oxygen atoms in the pseudofluorite structure are slightly displaced from the positions of the oxygen atoms in the fluorite structure and the lattice is also slightly expanded, so that the oxygen atoms are easier to move than in the cubic structure. Because there is.

(2) Z having a purity of 99.9% by weight or more
rO ₂ -Y ₂ O ₃ using a solid solution: impurity contained in the solid solution is _{SiO 2, Al 2 O 3,} Na 2 O or the like. If the purity is lower than 99.9% and a large amount of these impurities are contained, a grain boundary glass phase is formed at the grain boundaries of solid solution particles forming a solid electrolyte, and the grain boundary glass phase is crystallized when used for a long time. As a result, cracks are likely to occur, and the deterioration of conductivity over time increases. Therefore, in order to provide a stable SOFC with little deterioration in output over time, the purity is 99.
It is necessary to use 9% by weight or more of raw material powder.

(3) Using ZrO ₂ --Y ₂ O ₃ solid solution fine particles in which Y ₂ O ₃ is homogeneously dissolved in ZrO ₂ as a raw material powder: Even if the composition has a Y ₂ O ₃ concentration of 9 mol%, Z
When particles of rO ₂ and Y ₂ O ₃ were mixed and sintered at 1400 ° C. for 4 hours, it was revealed by X-ray diffraction that particles having a tetragonal structure were present. This is because the composition is non-uniform. In this case, it is necessary to perform heat treatment at a higher temperature to form a homogeneous solid solution. As described above, in order to provide a solid electrolyte having high conductivity without tetragonal particles being deposited and reducing the manufacturing cost of SOFC, solid solution fine particles in which Zr and Y atoms are homogeneously mixed in advance are used as raw material powders. It is essential to use. As a method for producing the raw material powder for this purpose, it is preferable to use a method for producing solid solution fine particles by decomposing a solution of a nitrate or an alkoxide uniformly containing Zr and Y atoms at a high temperature.

(4) The SOFC has a structure in which a solid electrolyte membrane is further formed on a fuel electrode, and an air electrode is formed thereon, a film is formed in the reverse order, and the fuel electrode and the air are formed depending on the stacking method. Some interconnectors are provided between the poles, but the present invention is applicable to any of these.

[0010]

EXAMPLES Reference examples for the present invention will be given below, and then examples will be described. Before producing an SOFC, the relationship between the properties of ZrO ₂ —Y ₂ O ₃ solid solution and its composition, crystal structure, and phase diagram was examined to find out that SOF with little deterioration over time.
The solid electrolyte material necessary for obtaining C was narrowed down.

Reference Example 1 ZrO ₂ obtained by decomposing an alkoxide uniformly containing Zr and Y atoms at high temperature was added to Y ₂ O.
The composition dependence of the electrical conductivity was measured while maintaining the sintered body obtained by heat-treating the solid solution fine powder in which ₃ was dissolved at 1500 ° C. for 5 hours at 30 ° C. at 1000 ° C. in FIG. 2.
Table 1 shows the results of crystal structure determination by X-ray diffraction after heat treatment for 00 hours. In Table 1, c phase is a cubic crystal phase of fluorite structure, t phase is a tetragonal structure phase, and t ″ phase is a pseudofluorite structure shown in FIG. 1 (metal ions form a cubic crystal lattice, oxygen ions 2 is a phase shifted from the lattice position of the cubic crystal), and ZmY represents a solid solution having a Y ₂ O ₃ concentration of mol% in FIG.
When the Y ₂ O ₃ concentration is less than 9 mol%, the conductivity decreases during the 1000 ° C. retention, and the deterioration over time is large. From the results of X-ray diffraction in Table 1, the crystal structures of these deteriorated samples are t + t ″.
It was found that the above two phases coexist with each other, and that the tetragonal crystal particles are deposited during the 1000 ° C. retention, which is the cause of the decrease in conductivity.

When the Y ₂ O ₃ concentration is 9 mol% or more and less than 10.5 mol%, the crystal structure is the t "or t" + c phase,
Tetragonal particles, which cause deterioration, are not deposited. t "
In the structure, it is clear that oxygen ions are slightly displaced from the cubic lattice position and the lattice is slightly expanded, which is considered to bring about the effect of improving the conductivity and suppressing deterioration. It is considered that these are the reasons why this composition region is suitable as a solid electrolyte material with little deterioration. When the Y ₂ O ₃ concentration is higher than 10.5 mol%, the crystal structure is cubic and stable, but the electrical conductivity is low, and therefore it is not suitable as a solid electrolyte material.

[0013]

[Table 1]

Reference Example 2 ZrO ₂ -9 mol% Y ₂ O ₃
Table 2 shows the effects of 1000 ° C. heat treatment and purity on the electrical conductivity of the solid solution (Z9Y). For solid solutions with a purity of 99.9% or higher, the decrease in conductivity by heat treatment at 1000 ° C. for 3000 hours is about 20%, but for solid solutions with a purity lower than 99.9%, the conductivity immediately after fabrication is also small and deteriorates. Is also big. Impurities contained in the sample are mainly SiO ₂ and A
It is 1 ₂ O ₃ , and when these impurities are present, a grain boundary glass phase is formed at the grain boundaries of the solid solution particles, and this is considered to be the cause of the low conductivity in the low purity product. So YSZ
It was revealed that it is necessary to use a high-purity raw material powder having a purity of 99.9% by weight or more in order to provide an SOFC with little deterioration over time.

[0015]

[Table 2]

(Embodiment 1) In order to show the structure of a cylindrical SOFC manufactured as an embodiment of the present invention, a sectional view of the structure is shown in FIG. The structure is such that a porous calcia-stabilized zirconia cylindrical tube with an outer diameter of about 21 mm is used as a base tube 1, and about 10 fuel electrodes 2 composed of composite particles of NiO and YSZ are provided thereon.
0 μm, on which the YSZ solid electrolyte 3 of the present invention is about 16
0 μm, an air electrode 4 made of lanthanum cobalt oxide of about 200 μm, and a current collecting electrode 5 made of composite particles of nickel alumina and alumina of about 150 μm were formed on each by a thermal spraying method to prepare an SOFC single cell. Is. As a result of operating the obtained SOFC at 1000 ° C., the change with time of the output is shown in FIG. The curve B in FIG. 4 is the output of the SOFC using the conventional Z8Y as the electrolyte.
Is the output of the SOFC with Z9.5Y as the electrolyte.
As a result, the output of the SOFC using the conventional ZrO ₂ -8 mol% Y ₂ O ₃ solid solution as an electrolyte is greatly deteriorated with time, whereas the ZrO ₂ -9.5 mol% Y ₂ O ₃ solid solution of the present invention is used as the electrolyte. The output of the SOFC is slightly smaller than that of the former in the initial stage, but its deterioration over time is small. It is understood that an SOFC with little deterioration over time can be obtained by the present invention. The reason for this is that the solid electrolyte of the present invention has a stable composition such that tetragonal particles do not precipitate even when used for a long period of time as described in the section of the above-mentioned action, and the phase of the pseudofluorite structure is This is because, since it is present and a high-purity raw material powder of 99.9% is used, the impurities that form the grain boundary glass phase are small, and therefore the conductivity has little deterioration over time.

[0017]

According to the present invention, it is possible to provide a solid electrolyte type electrochemical cell in which the output is less deteriorated with time even if it is operated for a long time, and it is highly useful in industry such as diversification of power sources.

[Brief description of the drawings]

FIG. 1 is a diagram showing a pseudo-fluorite structure of zirconia (arrows indicate displacement of oxygen ions from positions in the fluorite structure).

FIG. 2 is a chart showing the relationship between the electrical conductivity at 1000 ° C. and the holding time of a ZrO ₂ -mmol% Y ₂ O ₃ solid solution (ZmY) produced as a reference example of the present invention.

FIG. 3 is a cylindrical SOF manufactured as an example of the present invention.
Sectional drawing which shows the structure of C single cell.

FIG. 4 is a chart showing the relationship between the output of the SOFC manufactured as an example of the present invention and the operating time at 1000 ° C.

Claims (2)

[Claims] 1. A solid electrolyte type electrochemical cell comprising an air electrode, a solid electrolyte membrane and a fuel electrode, wherein the solid electrolyte is a solid solution of yttria in zirconia of 9 mol% or more and less than 10.5 mol% and a crystal thereof. The structure is a pseudofluorite structure or a coexisting state of a pseudofluorite structure and a cubic crystal, and the purity is 99.
A solid electrolyte type electrochemical cell comprising a zirconia-yttria solid solution in an amount of 9% by weight or more. 2. The electrochemical cell according to claim 1, wherein
Yttria in zirconia more than 9 mol% 10.5 mol%
Less than homogeneously dissolved zirconia-yttria solid solution fine particles as a raw material powder of the solid electrolyte, a step of forming an air electrode or a fuel electrode, a step of forming the zirconia-yttria solid solution fine particle film thereon, And a step of forming a film of the remaining one of the fuel electrode and the air electrode thereon.