JP2001102060A - Fuel pole for solid electrolyte fuel electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel electrode for a solid electrolyte fuel cell having better performance than that of prior art with keeping conventional property of electron conduction and coefficient of thermal expansion. SOLUTION: A fuel electrode comprises two layers of an electrode reacting layer adjacent to a solid electrolyte and an electron conductive layer adjacent to the electrode reacting layer, wherein the electron reacting layer being a mixture body with a conductive body of fine Ni and of fine oxygen ion and all of their average particle length being less than 0.5 micron, the electron conductive layer being mixture of fine Ni and fine oxide powder and all of their average particle length being not only between Ni: oxide=1:2 and 1:20, but also the average particle length of the oxide being more than 1 micron. It can obtain an electrode having an excellent electrode property and a coefficient of thermal expansion over prior art by separating the electrode reacting layer and the electron conductive layer. It has significantly contributed to a high efficiency operation of fuel cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel electrode for a solid oxide fuel cell.

[0002]

2. Description of the Related Art In recent years, interest has been growing in solid electrolyte fuel cells using oxygen ion conductors. In particular, from the viewpoint of effective use of energy, solid fuel cells have essentially high energy conversion efficiencies because they are not restricted by Carnot efficiency, and have excellent features such as better environmental protection. . Since the solid oxide fuel cell needs to operate at a high temperature of about 800 ° C. to 1000 ° C., the cell is made of a ceramic material. In the cell, an air electrode serving as an electron conductor and a fuel electrode are arranged with a solid electrolyte serving as an oxygen ion conductor interposed therebetween.
For these electrodes, a porous body is used to facilitate gas diffusion. To stack the cells, an interconnector material, which is an electronic conductor, is used between the cells.

[0003] With respect to these element materials, Table 1 shows materials which have been conventionally studied. From 20 ° C to 1000 ° C
The average values of the thermal expansion coefficients up to are also shown. Conventionally, YSZ (yttrium-stabilized zirconia) was used as the solid electrolyte, and La _0.8 Sr _0.2 MnO ₃ (LS
M), Ni-YSZ for the fuel electrode, and La _0.9 Sr _0.1 CrO ₃ for the interconnect material are most promising.

Since the electrodes are connected to a dense solid electrolyte, it is desirable that their thermal expansion coefficients be close to each other. This is because the operating temperature of the solid oxide fuel cell is much higher than room temperature, and materials having different coefficients of thermal expansion may be separated from each other at the joint due to stress due to the thermal cycle following the shutdown.

Here, since the fuel electrode uses a mixture of Ni and an oxygen ion conductor such as zirconia or ceria, the fuel electrode has an average thermal expansion coefficient of Ni and these ceramic materials. Since Ni has a much higher thermal expansion coefficient than electrolyte materials such as ceria and zirconia, Ni in the fuel electrode must be maintained in order to maintain consistency of the thermal expansion coefficient with the solid electrolyte.
Needs to be as small as possible. However, this conflicts with increasing the volume of Ni from the viewpoint of ensuring electron conduction.

For this reason, conventionally, the average particle diameter of Ni is made small and the average particle diameter of the oxygen ion conductor is made large, so that the Ni particles cover the surface of the large particles of the oxygen ion conductor. I have. Thereby, the electron conductivity can be secured while keeping the volume of Ni small. As described above, it is necessary to increase the average particle size ratio between Ni and the oxygen ion conductor from the viewpoint of the thermal expansion coefficient and the electron conductivity.

[0007] By the way, Ni is mixed with an oxygen ion conductor at the fuel electrode so that it is in close contact with the electrolyte, and near the interface with the electrolyte, a three-phase reaction gas, electrons and ions necessary for an electrochemical reaction coexist. Provides an interface. This electrochemical reaction is possible only in a limited area (three-phase interface) on the electrolyte-electrode interface. As described above, it is preferable to increase the average particle diameter of the oxygen ion conductor by reducing only the average particle diameter of Ni from the viewpoint of the thermal expansion coefficient and the electron conductivity.

However, fine Ni grains tend to cause grain growth, and such grain growth leads to a decrease in the three-phase interface, leading to deterioration of electrode performance. By mixing together a fine oxygen ion conductor of the same size as the fine Ni grains, grain growth is suppressed,
A fine structure can be maintained even at high temperatures. From the viewpoint of increasing the three-phase interface and improving the electrode performance in this manner, Ni
It is necessary that the average particle diameter ratio between the oxygen ion conductor and the oxygen ion conductor be substantially the same.

Heretofore, it has been difficult to simultaneously satisfy the above requirements for the average particle size ratio between Ni and the oxygen ion conductor in the fuel electrode.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a fuel electrode for a solid oxide fuel cell, and comprises an electrode reaction layer having a finer structure than a conventional fuel electrode at a portion in contact with the electrolyte, and an electron conductive layer adjacent thereto. By producing a fuel electrode having a two-layer structure, it is an object to provide a high-performance fuel electrode while maintaining the conventional characteristics of electron conductivity and thermal expansion coefficient.

[0011]

Means for Solving the Problems In order to solve the above problems, a fuel electrode for a solid oxide fuel cell according to the present invention comprises a solid electrolyte and a porous air electrode and a fuel electrode provided adjacent thereto. In the fuel electrode for a solid oxide fuel cell of a solid fuel cell having a cell, and an interconnector for electrically connecting the electrodes, and converting a chemical reaction between the fuel gas and air or oxygen gas into electric energy,
The structure is composed of two layers, an electrode reaction layer adjacent to the solid electrolyte and an electron conductive layer adjacent to this layer, and the electrode reaction layer is a mixture of fine Ni and fine oxygen ion conductor, and an average of these. Both have a particle size of 0.5 micron or less, and the electron conductive layer is a mixture of fine Ni and oxide powder, and the average particle size ratio is between Ni: oxide = 1: 2 and 1:20 and The average particle size of the oxide is 1 micron or more.

According to the present invention, a fuel electrode is bonded to an extremely thin electrode reaction layer composed of a composite of fine Ni and an oxygen ion conductor which is directly bonded to an electrolyte and has a close average particle size ratio. By using an electron conductive layer made of a mixture of Ni and an oxide having a significantly different average particle size ratio, it is possible to simultaneously satisfy the three requirements of electrode characteristics, thermal expansion coefficient, and electron conductivity, which were conventionally difficult. it can.

The present invention will be described below. In the present invention, the fuel electrode has two layers, an electrode reaction layer having a fine structure and an electron conductive layer having a relatively coarse structure. That is, the portion of the anode in contact with the electrolyte is an electrode reaction layer having a very small thickness of 0.01 to 10 μm, which is made of a mixture of Ni and oxygen ion conductors of very fine particles of 0.5 μm or less. I do. By mixing together a fine oxygen ion conductor of the same size as the fine Ni grains, the grain growth can be suppressed, and the fine structure can be maintained even at high temperatures, but the average grain size of the Ni and the oxygen ion conductor can be maintained. If the diameter exceeds 0.5 micron,
There is a possibility that the three-phase interface does not become large and the effect as an electrode reaction layer is reduced.

By adopting the above configuration, the effective surface area for the electrode reaction becomes extremely large, and the electrode characteristics are greatly improved. This electrode reaction layer is slightly inferior to the conventional fuel electrode in terms of matching of the conductivity and the coefficient of thermal expansion. However, since it is as thin as 10 μm or less, it is not a problem in practice. The thickness of the electrode reaction layer is preferably 0.05 to 5 microns, and most preferably 0.1 to 2 microns. If the thickness is too large, the gas will be difficult to permeate and it will be difficult to conduct electrons.If the thickness is too small compared to the particle size of the adjacent electron conductive layer, a current will flow in the lateral direction and the electrical resistance will increase, resulting in poor electrode characteristics. This is because there is a possibility that it will decrease.

Here, when the volume ratio between Ni and the oxygen ion conductor in the electrode reaction layer is from 4: 1 to 1: 4, a mixture having a close average particle size ratio is easily obtained in the firing process. . Thereby, the grain growth of both Ni and the oxygen ion conductor is suppressed. More preferably from 2: 1 to 1: 3, most preferably from 1.5: 1 to 1: 2. This is because the grain growth can be suppressed most in the vicinity of 1: 1.

The electron conductive layer is a mixture of fine Ni and oxide powder and has an average particle size ratio of Ni: oxide = 1: 1.
Between 2 and 1:20 and an average particle size of the oxide of 1 micron or more. The thickness of the electron conducting layer is preferably 2
0 microns or more. If the average particle diameter of the oxide powder exceeds 20 times the average particle diameter of fine Ni, Ni grain growth is likely to occur, and the oxide grains may become too large and mechanical strength may be reduced. Because. On the other hand, if the ratio is less than twice, the Ni particles covering the oxide will be cut off, making it difficult for current to flow, and possibly deteriorating the performance as an electrode.

If the thickness of the electron conductive layer is less than 20 microns, the current also flows in the direction of the screen, the resistance increases, and the performance as an electrode may be deteriorated.

Since the electron conductive layer does not contribute to the electrode reaction, it is not always necessary to use an oxygen ion conductor, and any material may be used as long as it does not adversely affect the material of the electrode reaction layer.
Alumina and mullite (alumina added with SiO ₂ ) that can coexist with zirconia and ceria even at high temperatures around 000 ° C
Etc. can also be used. Since an oxide having a low coefficient of thermal expansion can be used as described above, the coefficient of thermal expansion of the electron conductive layer can be reduced as compared with the conventional case, and the consistency with the solid electrolyte can be improved.

[0019]

Embodiments of the present invention will be described below. Note that, needless to say, the present invention is not limited to the following embodiments.

[0020]

EXAMPLE 1 In order to show the effect of the present invention, a test was conducted with a single cell having the structure shown in FIG. In the present invention, 1 is a fuel electrode, 2 is a solid electrolyte, 3 is an air electrode, 4 is a current collecting mesh, 5 is a platinum terminal, and 6 is a gas seal.

As a solid electrolyte, SASZ ((ZrO_Two)
_0.89(Sc_TwoO_Three)_0.105(Al_TwoO_Three) _0.005)
Lay the electrolyte sheet on the air electrode_0.8Sr_0.2MnO_ThreeFor
Was.

Cell used for comparison (cell # 0-1)
Prepared a slurry in which a mixture of SASZ having an average particle diameter of 2 μm and NiO having an average particle diameter of 0.5 μm was dispersed in an aqueous PVA solution at the fuel electrode, and applied in a circular shape having a diameter of 6 mm.
It was fired at 100 ° C. to obtain a fuel electrode having a thickness of 40 μm.
An LSM air electrode (6 mm diameter) was applied to the back surface, and a platinum reference electrode was applied to the end of the sheet, and baked at 900 ° C. to form a single cell. Here, platinum mesh was used for current collection of the fuel electrode and the air electrode.

As an example of the present invention, a fuel electrode was manufactured by the following method. First, the electrode reaction layer was r. It was produced by the f sputtering method. Here, in order to obtain a Ni-SASZ mixture, simultaneous sputtering was performed using a Ni metal target and a SASZ target. Here, Ni and SASZ
The ratio of the deposition rates was adjusted such that the volume ratio of the mixture became 1: 1. The same fuel electrode as that of the above-mentioned comparative example was coated thereon and heat-treated at 1100 ° C., and the average particle size was 0.2 μm, the layer thickness was 2.0 μm, the electrode reaction layer and the An electron conductive layer having a thickness of 40 μm and comprising a mixture of SASZ having an average particle diameter of 2 μm and NiO having an average particle diameter of 0.5 μm was obtained.

These cells (cell # 1, cell # 0-1)
Using a cell measurement system as shown in FIG.
A power generation test was performed. Table 2-1 summarizes the experimental conditions. Overvoltage of this single cell at 800 ° C. (current density 1
A / cm ² ) is shown in cell # 1 of Table 2-2. Here, hydrogen was supplied to the fuel electrode, and oxygen was supplied to the air electrode. Here, the reduction of NiO at the fuel electrode was performed by flowing hydrogen as a fuel at the measurement temperature. The current density is a value obtained based on the area of the fuel electrode. The overvoltage was determined by a current interruption method, that is, the current was interrupted by a relay, and the overvoltage was determined from this response.

For the sake of comparison, the characteristics of a cell in which only the fuel electrode of the above-mentioned single cell is a conventional one are also shown. Here, the overvoltage is a value when the current density is 1.0 A / cm ² . The cell using the fuel electrode of the present invention exhibited better overvoltage characteristics than the cell using the conventional fuel electrode (cell # 0-1).

[0026]

Example 2 In the cell # 1 of Example 1, the SA in the solid electrolyte sheet, the electrode reaction layer of the fuel electrode, and the electron conductive layer
A cell in which SZ is changed to YSZ (cell # 2), and SDC
(Cell # 3) was prepared. Then, as a comparative example, the SASZ solid electrolyte sheet of cell # 0-1 used in Example 1 was a YSZ sheet, the SASZ powder was changed to YSZ powder (cell # 0-2), and the SASZ solid electrolyte sheet was an SDC sheet. And a cell (Cell # 0-3) in which the SASZ powder was changed to the SDC powder. The same experiment as in Example 1 was performed on these cells. Table 2-2
As shown in the figure, the corresponding standard cell (cell #
0-2, cell # 0-3), respectively, the overvoltage characteristics were improved.

[0027]

Example 3 In the electron conductive layer of the fuel electrode described in Example 1, the average particle diameter of NiO was fixed at 0.5 μm, and
The average particle size ratio between iO and SASZ was 1: 2, 1: 8, 1:
A single cell was manufactured by changing it to 20. Here, the weight ratio of NiO to SASZ was adjusted to be 70:30, and the thickness of the electron conductive layer was adjusted to be 100 microns. Using this cell (cell # 4-1, cell # 4-2, cell # 4-3), an experiment similar to that of the first embodiment was performed. As shown in Table 2-2, the overvoltage characteristics were improved as compared with the standard cell (cell # 0-1) in Example 1.

[0028]

Example 4 In the electrode reaction layer of the fuel electrode described in Example 1, the thickness of the layer was 0.01 μm, 0.1 μm,
A single cell having a thickness of 1 micron or 10 microns was manufactured. Here, the weight ratio of Ni to SASZ is 70: 3.
0, the thickness of the electron conductive layer was adjusted to be 100 microns. Using the cells (cell # 5-1, cell # 5-2, cell # 5-3, cell # 5-4), an experiment similar to that of the first embodiment was performed. As shown in Table 2-2, the overvoltage characteristics were improved as compared with the standard cell (cell # 0-1) in Example 1.

[0029]

Embodiment 5 In the electrode reaction layer of the fuel electrode described in Embodiment 1, the volume ratio of Ni to SASZ was 4: 1, 1: 1 and
A single cell was prepared with the ratio changed to 1: 4. Here, the volume ratio of Ni and SASZ was changed by adjusting the ratio of the deposition rates in the simultaneous sputtering of Ni and SASZ. These cells (cell # 6-1, cell # 6-2, cell # 6-
The same experiment as in Example 1 was performed using 3). Table 2-
As shown in FIG. 2, the standard cell (cell # 0-
The overvoltage characteristic was improved compared to 1).

[0030]

Embodiment 6 In the electron conductive layer of the fuel electrode described in Embodiment 1, the kind of oxide was changed from SASZ powder to alumina (Al).
₂ O ₃ ) powder, mullite (0.6 Al ₂ O ₃ -0.4 SiO)
₂ ) A single cell was prepared in place of the powder. Here, the weight ratio of NiO to SASZ was adjusted to 70:30, and the thickness of the electron conductive layer was adjusted to 40 microns. Using this cell (cell # 7-1, cell # 7-2), the same experiment as in Example 1 was performed. As shown in Table 2-2, the overvoltage characteristics were improved as compared with the standard cell (cell # 0-1) in Example 1, and the mismatch between the fuel electrode and the electrolyte in the coefficient of thermal expansion was also improved.

Table 1

Table 2-1

Table 2-2

[0034]

As described above, by dividing the fuel electrode of the solid electrolyte fuel cell into the electrode reaction layer and the electron conductive layer shown in the present invention, the electrode characteristics are superior to those of the prior art and the thermal expansion characteristics are improved. Succeeded in obtaining a sufficiently excellent electrode. The present invention makes a great contribution to high efficiency operation of a solid fuel cell.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a fuel cell used in an example.

FIG. 2 is a plan view of a single cell used in an example.

[Explanation of symbols]

 1 fuel electrode 2 solid electrolyte 3 air electrode

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Yoji Sakurai 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation F-term (reference) 5H018 AA06 AS02 CC06 DD08 EE04 EE12 HH01 HH03 HH05 5H026 AA06 CC06 CV01 CX04 EE02 EE12 HH01 HH03 HH05

Claims (4)

[Claims] 1. A cell comprising a solid electrolyte, a porous air electrode and a fuel electrode provided adjacent to the solid electrolyte, and an interconnector for electrically connecting the electrodes.
In a fuel electrode for a solid oxide fuel cell of a solid fuel cell that converts a chemical reaction between a fuel gas and air or oxygen gas into electric energy, an electrode reaction layer having a structure adjacent to the solid electrolyte and an electron conductive layer adjacent to this layer Consists of two layers,
The electrode reaction layer is a mixture of fine Ni and a fine oxygen ion conductor, both of which have an average particle size of 0.5 μm or less, and the electron conductive layer is a mixture of fine Ni and oxide powder. A fuel electrode for a solid oxide fuel cell, characterized in that the average particle size ratio is between Ni: oxide = 1: 2 and 1:20 and the average particle size of the oxide is 1 micron or more. 2. The method according to claim 1, wherein the thickness of the electrode reaction layer is 0.01 to 10 μm and the thickness of the electron conductive layer is 20 μm.
A fuel electrode for a solid oxide fuel cell, having a diameter of at least one micron. 3. The electrode reaction layer according to claim 2, wherein the volume ratio of Ni to the oxygen ion conductor in the electrode reaction layer is from 4: 1 to 1: 4.
And a fuel electrode for a solid oxide fuel cell. 4. The fuel electrode for a solid oxide fuel cell according to claim 1, wherein the oxide of the electron conductive layer is alumina or mullite.