JPH07130386A - Supporting tube for solid electrolyte type fuel cell - Google Patents

Abstract

PURPOSE:To improve the percentage of good products in a cell manufacturing process and ensure stability and reliability of a fuel cell over a long period. CONSTITUTION:A porous stabilizing ZrO2 sintered body which contains at least CaO, Y2O3 and/or Yb2O3 as stabilizer and has the content of Al and Si being 100-5000ppm in metal conversion is used for a fuel cell supporting tube. In this way, the supporting tube is excellent in its strength at the beginning and after used for a long period at a high temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a porous stabilized zirconia sintered body used as a support tube for a solid oxide fuel cell.

[0002]

2. Description of the Related Art A solid oxide fuel cell has an essentially high energy conversion efficiency because it directly converts chemical energy contained in a fuel such as hydrogen or methane into electric energy. Therefore, in recent years, solid oxide fuel cells have attracted attention as a power generation device, and much research and development has been advanced.

Therefore, FIG. 1 shows an example of a solid oxide fuel cell. This is called a cylindrical fuel cell, and a cylindrical support tube 1 of CaO-stabilized ZrO ₂ having an open porosity of about 35% is formed on which LaMnO ₃ based material is applied as a porous air electrode 2 by a slurry-dip method. Then, the surface thereof is coated with a Y ₂ O ₃ -stabilized ZrO ₂ film, which is the solid electrolyte 3, by a vapor phase synthesis method (EVD) or a thermal spraying method, and the surface is further covered with a porous Ni-zirconia (Y ₂ O ₂ A fuel electrode 4 (containing ₃ ) is provided. In the fuel cell module, each unit cell is connected via an LaCrO ₃ system interconnector 5. For power generation, air (oxygen) is flown inside the support tube, and fuel (hydrogen) is flown outside to generate 1000 to 105
It is carried out at a temperature of 0 ° C.

Further, a porous Ni-zirconia fuel electrode was prepared by a thermal spraying method on a cylindrical support tube of CaO-stabilized ZrO ₂ , and Y ₂ O ₃ -stabilized ZrO which is a solid electrolyte was prepared.
_{A structure in which two} films are covered and a porous air electrode such as LaCoO ₃ is provided thereon is also under study. C in this cell structure
The aO-stabilized ZrO ₂ is supplied with fuel (hydrogen) inside the support tube and air outside.

[0005]

Problems to be Solved by the Invention The CaO-stabilized Z
When the rO ₂ sintered body is used as a cylindrical support tube, the cylindrical support tube is destroyed by thermal shock in the process of forming the solid electrolyte membrane and the interconnector membrane by the vapor phase synthesis method or the thermal spraying method, which causes There is a problem of worsening the non-defective rate.

[0006] Further, CaO-stabilized ZrO ₂ undergoes a phase transformation when power is generated for a long time, so that the strength of the supporting tube itself is lowered, and as a result, the cell is broken by a small impact.

[0007]

The inventors of the present invention have repeatedly studied a porous sintered zirconia cylindrical sintered body in order to solve the above-mentioned problems, and as a result, to stabilize the zirconia crystallographically. CaO and Y ₂ O ₃ as stabilizers for
And / or Yb ₂ O ₃ is used, and
l and Si are 100 to 5000 ppm in terms of metal
It has been found that the inclusion of these elements improves the strength and suppresses the phase transformation even during long-term use at high temperature to ensure stable strength.

The present invention will be described in detail below. In the stabilized ZrO ₂ sintered body used in the present invention, the stabilizing material is CaO.
And Y ₂ O ₃ and / or Yb ₂ O ₃ are contained in combination. More specifically, these stabilizers are added in a total amount of 7 to 30 mol% in consideration of the matching of the coefficient of thermal expansion with the solid electrolyte, and particularly CaO: Y ₂ O ₃
/ Yb ₂ O ₃ = 1: 8 to 5:10 is desirable.

Further, according to the present invention, Al in the sintered body,
Si 100ppm-5000ppm, especially 500-2
It is also a great feature to add in the range of 000 ppm. These Al and Si are for increasing the strength by forming glass at the grain boundaries in the sintered body, and when this amount is less than 100 ppm, the generation of glass in the grain boundary phase is extremely small and the strength of the strength is high. No improvement effect, 5000pp
If it exceeds m, the thickness of the grain boundary phase itself becomes too thick and the strength of the sintered body decreases. It is presumed that this is probably due to the difference in the coefficient of thermal expansion between the glass phase and the crystalline phase.

Further, according to the present invention, the above-mentioned porous Zr
When the O ₂ sintered body is used as a cylindrical support tube, it is important to have gas permeability in addition to the above-mentioned strength. The gas permeability changes depending on the pore size in the sintered body. Normal,
In the support tube, the larger the average pore diameter, the higher the gas permeability and the better the power generation characteristics of the cell. On the contrary, the strength of the support tube decreases, the handling property deteriorates, and the yield rate of the cell decreases. According to the experiments by the present inventor, the average pore diameter of the sintered body is 0.8 in order to achieve both strength and power generation performance.
It has been found that the range is preferably to 5.0 μm, particularly 1 to 3 μm. The open porosity of the whole sintered body is 25
~ 45% is suitable.

To manufacture the support tube of the present invention, Z
rO ₂ powder, stabilizers such as CaO, Y ₂ O ₃ , Yb
₂ O ₃ is in the form of an oxide or carbonate, and each powder of Al ₂ O ₃ and SiO ₂ is weighed and mixed at a predetermined ratio as described above. In some cases, a raw material obtained by coprecipitating ZrO ₂ and the above-mentioned stabilizer can also be used.

The mixed powder thus obtained is molded into a desired shape by a desired molding means such as a die press, a cold isostatic press, and extrusion molding. When the support tube has a cylindrical shape, extrusion molding, cold isostatic molding, etc. are suitable.

Then, the molded body is fired at a temperature of 1400 to 1600 ° C. in an oxidizing atmosphere such as air, but in the firing at this time, since the sintered body has gas permeability, it is not completely densified. So it is necessary to bake. Therefore, it is necessary to perform the firing at a temperature slightly lower than the optimum firing temperature or to perform the firing time in a shorter firing time than when the firing time becomes dense.

The porous ZrO ₂ thus obtained
The sintered body is mainly composed of cubic ZrO _{2 and} has an average particle size of 5 to 5.
The stabilized ZrO ₂ is composed of fine crystals of about 15 μm, and CaO—SiO—Al ₂ O ₃ system glass is formed at the grain boundaries of the crystals.

In order to fabricate a fuel cell using the porous stabilized ZrO ₂ as a support tube for a fuel cell, (La, Ca) MnO ₃ is formed on the surface of this sintered body by a slurry dipping method or a thermal spraying method. A porous air electrode made of (La, Sr) MnO ₃ or the like is formed, and a Y ₂ O ₃ -stabilized ZrO ₂ film which is a solid electrolyte is formed on the surface thereof by a vapor phase synthesis method (EVD) or a thermal spraying method. Then, a porous anode of Ni-zirconia (containing Y ₂ O ₃ ) is formed on the surface. Further, it can be obtained by forming an interconnector made of a LaCrO ₃ system material on the surface of the air electrode in order to further connect the cells.

In addition, a porous Ni-zirconia fuel electrode was prepared on the surface of the porous stabilized ZrO ₂ support tube by a thermal spraying method and the solid electrolyte Y ₂ O ₃ stabilized Zr was prepared.
A cell can be prepared by coating an O ₂ film and providing a porous air electrode such as LaCoO ₃ on the O ₂ film.

[0017]

When Si and Al are added to the CaO-stabilized ZrO ₂ sintered body, CaO-SiO-Al ₂ O ₃ system glass is formed in the grain boundary phase, so that the strength and thermal shock resistance of the sintered body are increased. Become. However, CaO-stabilized ZrO ₂
When kept at a high temperature of 1000 ° C. for a long time, a phase transformation from cubic crystal to tetragonal crystal to monoclinic crystal occurs, and the strength of the sintered body decreases. On the other hand, a ZrO ₂ sintered body containing Y ₂ O ₃ and Yb ₂ O ₃ as a stabilizer does not easily undergo phase transformation at high temperature, and in view of long-term stability, CaO stabilized Z
Preferred over rO ₂ . However, Y ₂ O ₃ and Yb ₂
When Al and Si are added to O ₃ stabilized ZrO ₂ ,
SiO—Al ₂ O ₃ based glass forms a grain boundary phase, but in this case, it does not show sufficient strength. This is SiO-A
Write from l ₂ O ₃ based glass of CaO-SiO-Al ₂ O ₃ based glass, probably because the melting point and softening temperature is high.

For this reason, in the present invention, a crystal of fluorite structure is formed by adding Y ₂ O ₃ or Yb ₂ O ₃ as a stabilizer to a cylindrical sintered body of porous stabilized zirconia. Is stabilized for a long period of time and CaO is added, whereby CaO—SiO—Al ₂ O ₃ based glass can be generated at the grain boundaries to improve strength and thermal shock resistance.

Further, the support tube of the present invention is required to have gas permeability in addition to the above-mentioned strength. According to the invention, using the average pore size as a measure of gas permeability,
By setting this to 0.8 to 5.0 μm, excellent gas permeability can be imparted while maintaining high strength.

The support tube of the present invention has high strength and thermal shock resistance due to the above-mentioned constitution, and does not deteriorate in strength during long-term use at high temperature. Therefore, the yield rate of the fuel cell is improved and the long-term reliability is improved. It is possible to fabricate a high cell.

[0021]

【Example】

Example Commercially available Y ₂ O ₃ , Yb ₂ O ₃ , having a purity of 99.9%,
CaCO ₃ and ZrO ₂ were used as starting materials, and were prepared so as to have the compositions shown in Tables 1 and 2. This was mixed with a zirconia ball for 10 hours, and then calcined at 1500 ° C. for 10 hours to repeat solid phase reaction three times to obtain a stabilized zirconia powder. Furthermore, the average particle size is about 0.5 μm.
Al ₂ O ₃ powder and SiO ₂ powder were each added in a predetermined amount, and the powder was crushed and mixed for 20 to 30 hours using a zirconia ball in a vibration mill. After this,
Molded into a cylindrical shape by extrusion molding method, and then 1500
Baking at ~ 1560 ° C, outer diameter 15mm, inner diameter 10m
A cylindrical sintered body having a length of m and a length of 100 mm was produced.

The average pore diameter of the obtained sintered body was measured by the mercury porosimetry and found to be 1.7 to 2.3 μm. This cylindrical sintered body was cut into a length of 50 mm, and the breaking strength was measured with a radial crushing test device. Also. This part is 100
After annealing in the atmosphere at 0 ° C. for 500 hours, a radial crushing test was performed. The results are shown in Table 1.

[0023]

[Table 1]

According to Table 1, No. 1 in which only CaO was added to ZrO ₂ had a high initial strength but a large decrease in strength after annealing. Further, in No. 2 in which only Y ₂ O ₃ was added, the initial strength was low, although the decrease in strength after annealing was small. On the other hand, the amount of Al and Si is 100 pp
No. less than m. 3, 11, 16 have low initial strength.
In addition, No. 2 in which the amounts of Al and Si are more than 5000 ppm. 1
Regarding 0 and 19, both the initial strength and the strength after annealing are low.

In contrast to these comparative examples, the other products of the present invention all have an initial strength of 10 kg / mm ² or more,
The strength after annealing was as high as 5 kg / mm ² or more.

Example 2 The compositions of No. 5 and No. 17 in Table 1 of Example 1 were crushed and mixed for 15 to 40 hours by a vibration mill using a zirconia ball, and then extruded. Molded into a cylindrical shape 1
Fired at 480 to 1560 ° C, outer diameter 15 mm, inner diameter 10
A cylindrical sintered body having a length of 100 mm and a length of 100 mm was produced. A cylindrical sample having a length of 50 mm was cut out from this cylindrical sintered body, and the breaking strength of the cylindrical sample was measured by a radial crushing test device. On the other hand, a sample with a length of 10 mm was cut out from the above cylindrical sintered body, and the gas permeation coefficient was measured using N ₂ gas at room temperature (22 to 24 ° C.). The average pore size was also measured by the mercury porosimetry method. The results are shown in Table 2.

[0027]

[Table 2]

It can be seen from Table 2 that the larger the average pore diameter, the higher the gas permeability coefficient, but conversely the material strength decreases. When the average pore diameter is smaller than 0.8 μm, the fracture strength is sufficient, but the gas permeation coefficient becomes extremely small. On the other hand, when the average pore diameter exceeds 5.0 μm, the gas permeation coefficient increases, but the strength extremely decreases.

Example 3 Using the compositions of Nos. 1, 2, 3, 5, 8, and 13 in Example 1, the average pore diameter was 2.1 to 2.5 μm and the outer diameter was 16 m.
A hollow cylindrical sintered body having a diameter of m, an inner diameter of 12 mm and a length of 200 mm, which was sealed at one end, was prepared as a supporting tube. La _0.85 Sr
_A powder of about 3 μm having a composition of _0.15 MnO ₃ was coated by a slurry-dip method to a thickness of about 1.5 mm, and the powder was exposed to air 1
It was annealed at 400 ° C. for 3 hours, baked on a cylindrical sintered body and sintered the powder itself to obtain an air electrode. After that, the solid electrolyte membrane (1
0 mol% Y ₂ O ₃ -90 mol% ZrO ₂ ) about 30
It is coated to a thickness of μm, and further as a fuel electrode on this, by a slurry-dip method, a thickness of about 40 μm and 60 wt.
% Ni-40 wt% zirconia (8mol% Y ₂ O ₃ -
92 mol% ZrO ₂ ) was coated to prepare a single cell as shown in FIG.

This cell was held in an electric furnace and the temperature was raised from room temperature to 1000 ° C in 10 hours while flowing oxygen gas inside the cell and hydrogen gas outside, and the temperature was raised at 1000 ° C for 1 hour.
After power generation for 0 hours, the temperature was lowered from 1000 ° C. to room temperature in 20 hours. This thermal cycle is repeated 20 times, and the number of times the single cell is destroyed and the number of cycles of 5, 10 or 20 times is 100.
The power density by power generation at 0 ° C was measured. The results are shown in Table 3.

[0031]

[Table 3]

According to Table 3, Nos. 1, 2 other than the present invention,
The cells using the support tube of composition No. 3 were destroyed 6 to 11 times, whereas the cells using the support tubes of compositions No. 5, 8, and 13 of the present invention were all 20 times. It did not break even in the thermal cycle. Moreover, the power density also showed a high value. From this result, it was proved that the material of the present invention was sufficiently excellent.

[0033]

As described in detail above, the fuel cell support tube of the present invention has excellent initial strength and excellent strength even after long-term use at high temperature. Therefore, it is possible to improve the non-defective rate in the cell manufacturing process, and it is possible to secure the stability and reliability of the fuel cell for a long period of time.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining the structure of a cylindrical fuel cell unit.

[Explanation of symbols]

 1 Cylindrical support tube 2 Air electrode 3 Solid electrolyte 4 Fuel electrode 5 Interconnector

Claims (1)

[Claims] 1. A stabilizing material comprising at least CaO and Y ₂
For a solid oxide fuel cell, comprising a porous stabilized ZrO ₂ sintered body containing O ₃ and / or Yb ₂ O ₃ and containing 100 to 5000 ppm of Al and Si in terms of metal, respectively. Support tube.