JPH1167226A - Fuel electrode of fuel cell and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel electrode with high electrode activity and less difference in coefficient of thermal expansion with YSZ (stabilized zirconia), used in an SOFC (solid electrolyte fuel cell), and its manufacturing method. SOLUTION: In the fuel electrode of a solid electrolyte fuel cell using a zirconia material, the concentration of a metal element on the surface of the electrode, coming in contact with an electrolyte is made higher than that on the onside of the electrode. The fuel electrode is manufactured by mixing, sintering zirconia and an oxide of metal element to prepare a substrate, arranging the powder of the metal element or the powder of the oxide of the metal element on one side of the substrate, then heating them. While the difference in coefficient of thermal expansion with the electrolyte is suppressed within the allowable value, the electrode having high activity to the power generating reaction can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel electrode for a fuel cell and a method for producing the same, and more particularly to a structure and a method for producing a sintered body used as a fuel electrode for a solid oxide fuel cell.

[0002]

2. Description of the Related Art Solid oxide fuel cells (hereinafter referred to as "solid electrolyte fuel cells").
In general, an SOFC is configured by arranging two electrodes (an air electrode and a fuel electrode) on both sides via a solid electrolyte made of a substance having a selective permeability for oxygen ions.
Then, by flowing oxygen and hydrogen through each electrode, a chemical reaction proceeds, and power generation is performed.

[0003] As an electrolyte material, stabilized zirconia (YSZ) having oxygen ion permeability, which is obtained by adding yttrium oxide to zirconium oxide to stabilize the crystal structure.
Is used. The air electrode has lanthanum manganite (La _1-x (M) _x ) _y MnO having a perovskite structure in which part of lanthanum is replaced with an alkaline earth metal.
₃ (M: alkaline earth metal)), and a nickel zirconia cermet obtained by sintering a mixture of YSZ and nickel metal is used as the fuel electrode.

There are several types of SOFC structures. For example, as shown in FIG. 5A, an electrolyte 1 is used as a support of a cell (one power generation unit) C, and an air electrode 2 and a fuel electrode 3 are formed on both sides thereof. )
As shown in the figure, one electrode (in this case, the fuel electrode substrate 6)
Is used as a support of the cell C, and an electrolyte 1 and another electrode (in this case, the air electrode 2) are formed on the surface thereof. In these figures, 4 is an interconnector, 5 is an air electrode substrate, 7 is a fuel passage, and 8 is an air passage.

[0005] Each member of the SOFC is a ceramic material as described above, and its electric conductivity is very small as compared with that of a metal. In particular, since the conductivity of the electrolyte is the lowest among the materials, it is necessary to reduce the cell resistance in order to obtain practical power generation characteristics, and it is essential to reduce the resistance in the electrolyte part. . YSZ which is the material of the electrolyte
Although the conductivity of the fuel cell increases as the temperature rises, the conductivity is small even at a temperature of about 1000 ° C., which is the operating temperature of this fuel cell. Therefore, reduction of the cell resistance by reducing the thickness of the electrolyte is necessary for realizing a cell having practical performance. That is, in order to realize an SOFC having good power generation characteristics, application of a thin-film electrolyte is an essential issue.

However, in a structure in which the electrolyte supports the entire cell as in the former method, an extremely thin film cannot be used because mechanical strength must be imparted to the electrolyte.
A thickness of at least about 200 to 500 μm is required,
It is difficult to reduce the resistance in the electrolyte part. On the other hand, in the latter method in which one of the electrodes serves as a support for the cell, the electrode material having a conductivity 1000 to 10,000 times larger than the electrolyte material serves as the support. It is also possible to use an electrode substrate having a thickness of mm. Then, several to several tens on such a support having high conductivity and strength.
An electrolyte having a thickness of 0 μm can be formed, and the realization of a cell having low resistance and high power generation characteristics is expected.

By the way, in a cell in which an electrode is used as a supporting body of the cell, two electrodes, i.e., an air electrode and a fuel electrode, are considered as electrodes to be a support. However, comparing the two, first, the strength of the fuel electrode is about three times greater than that of the air electrode, and the material price shows that the fuel electrode is 60 times stronger than the air electrode.
The cost is as low as ~ 70%. Therefore, from the viewpoints of cell fabrication and economy, the fuel electrode material is more advantageous as the cell support.

On the other hand, as described above, nickel zirconia cermet is currently used for the fuel electrode.
This cermet mainly consists of YSZ powder and nickel oxide (N
iO) It is produced by molding and sintering of a mixture of powders. By the way, the fuel electrode substrate used as a support for the cell of the SOFC is required to have electrode characteristics (porosity and conductivity) and mechanical strength, and an electrolyte (usually YSZ) formed on this surface. It is also required that the difference in thermal expansion coefficient from the above is small.

These physical properties of the cermet are mainly affected by the amount of NiO added. For example, the coefficient of thermal expansion shows a simple increase by the addition of NiO as shown in FIG. The expansion coefficient of YSZ is about 10 × 10 ^-6 (1 /
K), indicating that at least the cermet has a difference in thermal expansion coefficient between YSZ. Since the operating temperature of the SOFC is 1000 ° C., the thermal expansion coefficient of the cermet is 1
In the case of 4 × 10 ⁻⁶ (1 / K), for example, in a single cell having a size of 20 cm square, YSZ expands by 2 mm and cermet expands by 2.8 mm, which leads to a considerable separation between the two. Stress occurs.

On the other hand, as shown in FIG.
It changes greatly with an increase in the amount of O added.
At 70 wt%, it increases by 10%.
A value of not less than 00 S / cm is obtained (in the figure, ● indicates a sintering temperature of 1350 ° C, and ▲ indicates a value of 1300 ° C). Since the power generation reaction at the fuel electrode occurs at the nickel metal portion existing at the interface between the electrode and the electrolyte, a large amount of added NiO means an increase in the interface with YSZ involved in the power generation reaction, and the electrode characteristics are also improved.

However, as described above, an increase in NiO causes an increase in the coefficient of thermal expansion.
It is difficult to select the value of the addition amount.
The amount of NiO added is limited from the viewpoint of suppressing the difference in the coefficient of thermal expansion from the above, and it is not always possible to obtain a support having sufficiently high electrode activity. There was.

In the present invention, therefore, the present invention is used for SOFCs.
A method for producing a fuel electrode support having high electrode activity and a small difference in thermal expansion coefficient from YSZ will be clarified.

[0013]

In order to solve the above-mentioned problems, a fuel electrode of a fuel cell according to the present invention is a fuel electrode of a fixed electrolyte type fuel cell using a zirconia-based material. It is characterized in that the element concentration is distributed so as to be higher than inside the electrode.

Further, a method of manufacturing a fuel electrode of a fuel cell according to the present invention comprises the steps of mixing and sintering zirconia and an oxide of a metal element to produce a substrate; A step of arranging a powder of an oxide of a metal element and performing a heat treatment.

That is, a sintered body serving as a support is prepared from a mixture of zirconia and an oxide of a metal element, and then a metal element or a metal element is oxidized on a surface of the substrate on which an electrolyte is formed. By placing the material powder and heat treating,
It is characterized in that the concentration of the metal is increased on the surface of the electrode on which the electrolyte is formed and on the inside near the surface.

As described above, the present invention relates to a structure and a manufacturing method of an anode used in an SOFC, and in particular, to a YSZ
A sintered body to be a support is prepared from a mixture of NiO and NiO powder, and then nickel (Ni) or NiO is arranged on the surface of the substrate on which an electrolyte is formed, and heat treatment is performed in this state. From this surface to the inside
This is to cause diffusion of the metal component, thereby improving the concentration of Ni metal at the interface with the electrolyte in which the power generation reaction proceeds. At this time, the NiO concentration in the cermet sintered body to be prepared in advance is within the allowable value of the expansion coefficient, and
Since the concentration of Ni at the interface involved in the reaction can be increased, a cell support having high electrode activity can be manufactured.

Heretofore, no specific proposal as in the present invention has been made on the structure and composition of the fuel electrode used as a support for the SOFC.

As the zirconia-based material as described above,
For example, it can be yttrium stabilized zirconium (YSZ). Nickel can be given as an example of the metal element. However, in the present invention, the zirconia-based material and the metal element are not basically limited as long as the effects of the present invention can be achieved.

The mixing amount of the metal element oxide powder is 40
Preferably, it is 〜60% by weight. This is because a large voltage drop does not occur even when a current flows during power generation, and the difference in the coefficient of thermal expansion with the electrolyte is within an allowable range.

[0020]

EXAMPLES The fuel electrode of the present invention will be specifically described below with reference to examples.

[0021]

Embodiment 1 An electrode manufacturing process according to the present invention is as shown in FIG. 1. First, YSZ and NiO powder are prepared (step a), and they are mixed (step b). After molding (step c), sintering is performed to produce a sintered body (step d). Next, Ni or NiO powder is placed on the surface of the sintered body where the Ni concentration is to be increased (step e), and then heat-treated again (step f), so that the target surface and the inside thereof have a Ni metal component. Is to diffuse. Steps a to d are a sintered body preparation process, and steps e to f are a Ni diffusion process.

First, an example in which yttria-stabilized zirconia powder having a particle diameter of 10 to 40 μm and nickel oxide powder having an average particle diameter of 1 μm or less are used as raw materials will be described. Particle size 10
A 40 μm yttria-stabilized zirconia powder was prepared by heat-treating a commercially available powder, and this powder and nickel oxide powder were placed in a polyethylene pot, ethanol was added, and the mixture was mixed by a ball mill to prepare a cermet raw material powder. The amount of the nickel oxide powder added in the raw material powder prepared here was 45% by weight. Next, a polyvinyl alcohol-based binder was added to the raw material powder, and 2 t / cm
^It was press-formed (φ30 mm) at ² , and then sintered at 1400 ° C. The porosity of the produced sintered body was about 25%, and the coefficient of thermal expansion was 12.5 × 10 ⁻⁶ (1 / K). In addition,
This sintered body is referred to as Conventional Example 1.

Next, in the present invention, the φ30
The NiO powder was placed on the surface of the sintered body having a thickness of about 2 mm with a thickness of about 2 mm, and again heat-treated at 1200 ° C. in this state. As a result, only the Ni metal component was diffused from the surface of the sintered body toward the inside from NiO placed on the sintered body. The sintered body embodying the present invention (Example 1) and the N
The distribution of i metal was determined. FIG. 2a shows a cross section of the substrate of the sintered body to be measured, wherein the upper surface indicates the side in contact with the electrolyte, and the lower surface indicates the other side. As a result, as shown in FIG. 2B, the metal component in the cross section of the sintered body was almost constant in Conventional Example 1, but in the embodiment of the present invention, the distribution of Ni on the surface on which NiO was placed was high ( A distribution that gradually decreases toward the inside was confirmed (B 'direction: lower surface direction).

Next, a cell for a power generation test was prepared using the two types of sintered bodies thus prepared, and the power generation characteristics were determined. The electrolyte of this cell is made of yttria-stabilized zirconia as a material, and is plasma-sprayed to about 200 μm.
m. Next, the air electrode was adjusted to an average particle size of 1 μm.
Formed by applying and sintering a slurry prepared from the La _0.8 Sr _0.2 MnO ₃ powder of the above. With this cell, 100
The power generation characteristics at 0 ° C. were determined. As a result, the open-circuit voltage of both cells was 1.0 V, but the power generation characteristics differed. In Conventional Example 1, 250 mA / cm ² at 0.7 V.
However, in Example 1, it was 400 mA at 0.7 V.
/ Cm ² , confirming that the fuel electrode according to the present invention has more excellent power generation characteristics. From this, it can be seen that, according to the embodiment of the present invention, the concentration of Ni metal in the vicinity of the interface with the electrolyte is increased, and the effect is exhibited.

The porosity and the coefficient of thermal expansion are as follows:
No significant difference was observed between these two samples. The reason for this is that these characteristics are the first
The composition and state of the structure formed by sintering of the iO powder are uniquely determined, and the degree of the Ni attached by diffusion is due to the degree of attachment to the surface of the structure forming these skeletons. Conceivable.

The activity of the fuel electrode is generally determined by adding Ni
Since it is affected by the amount of O, it has been necessary to increase the amount of NiO to produce a highly reactive electrode. However, when the amount of NiO increases, the coefficient of thermal expansion increases, and the difference in expansion coefficient with the electrolyte increases. Therefore, there has been a problem that the cell is easily damaged by operation at a high temperature and the reliability is reduced. On the other hand, in the present invention, the amount (4
A base material to which NiO (0% to 60% by weight) is added is used, and the concentration of Ni is increased only on the surface on which the electrolyte is formed. Moreover, Ni added in the present invention does not affect the porosity and thermal expansion characteristics of the base material. Therefore, it is possible to produce an electrode having a high activity with respect to the power generation reaction in a state where the difference in thermal expansion coefficient with the electrolyte is kept within an allowable value.

[0027]

Example 2 In Example 1, a yttria-stabilized zirconia powder having a particle size of 10 to 40 μm produced by heat-treating a commercially available YSZ was used as a cermet starting powder. Commercial YS not applied
A cermet sintered body was prepared from Z powder and nickel oxide powder. Mixture of both powders as raw materials (NiO added amount: 45)
% By weight) was prepared by putting each powder together with ethanol in a polyethylene pot and mixing with a ball mill in the same manner as described above. Next, a molded body of cermet was prepared from the powder by an extrusion molding method. In this extrusion molding, a kneaded body was prepared by adding 10 and 20% by weight of a methylcellulose-based binder (Metroze) and water to the powder, respectively, and molded from the kneaded body. The produced molded body has a width of 50, a thickness of 5, and a length of 50 (mm). A compact obtained by sintering the compact at 1300 ° C. is referred to as Inventive Example 2. The porosity of Conventional Example 2 was 20%.

Next, in the present invention, one side of this sintered body is
NiO powder was placed, and heat treatment was again performed at 1200 ° C. in this state. With respect to Example 2 and Conventional Example 2, the distribution of Ni in the thickness direction of the electrode was obtained. As a result, similarly to Example 1, in the present invention, the NiO powder was placed on the surface and on the lower portion in the vicinity of the surface. It was confirmed that the distribution of Ni was large.

As described above, according to the present invention, it is possible to realize a fuel electrode substrate having a sufficient electrode activity in a state where the amount of NiO added is suppressed, so that a difference in thermal expansion coefficient from YSZ can be suppressed. In addition, it is possible to manufacture a highly reliable SOFC cell without breakage of the cell or peeling of YSZ even after long-time operation at high temperature.

[0030]

As described above, according to the present invention, a sintered body as a support is produced from a mixture of a zirconia-based material and a powder of an oxide of a metal element. The metal element or the oxide powder of the metal element is arranged on the surface on which is formed, and heat treatment is performed to increase the concentration of nickel metal on the electrode surface on the side where the electrolyte is formed and on the inside near the surface.

Since the activity of the fuel electrode is affected by the amount of the metal oxide added, it has been necessary to increase the amount of the metal oxide to produce a highly reactive electrode. However, when the amount of metal oxide increases, the coefficient of thermal expansion of the fuel electrode substrate increases, and the difference in expansion coefficient between the electrolyte and the electrolyte increases. There was a problem of lowering. However, in the present invention, the base material to which NiO is added is used to the extent that a large voltage drop does not occur even when a current flows during power generation, and the Ni concentration is increased only on the surface on which the electrolyte is formed. Therefore, it is possible to produce an electrode having a high activity with respect to the power generation reaction in a state where the difference in thermal expansion coefficient with the electrolyte is kept within an allowable value. Until now, a sintered body that satisfies these two contradictory physical properties has not been obtained. For this reason, if used for a long period of time, separation of the electrode and the electrolyte will occur, and a highly reliable SOFC The cell was not realized.

According to the present invention, a highly reliable SOFC can be realized, and an extremely large industrial advantage can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a fuel electrode manufacturing process of the present invention.

FIG. 2A is a diagram showing a cross section of a fuel electrode (substrate).

FIG. 2B is a diagram showing the distribution of Ni metal on the cross section of the fuel electrode together with Example 1 and Comparative Example 1.

FIG. 3 is a diagram showing the relationship between the thermal expansion coefficient of a fuel electrode and the amount of NiO added.

FIG. 4 is a graph showing the relationship between the conductivity of the fuel electrode and the amount of NiO added.

FIG. 5a is a perspective view showing the structure of a conventional solid oxide fuel cell.

FIG. 5B is a perspective view showing the structure of another conventional solid oxide fuel cell.

[Explanation of symbols]

 C single cell 1 solid electrolyte 2 air electrode 3 fuel electrode 4 interconnector 5 air electrode substrate 6 fuel electrode substrate 7 fuel passage 8 air passage

Claims (7)

[Claims] In a fuel electrode of a fixed electrolyte fuel cell using a zirconia-based material, the concentration of a metal element on the surface in contact with the electrolyte is distributed so as to be higher than inside the electrode. The fuel electrode of the fuel cell. 2. The fuel electrode for a fuel cell according to claim 1, wherein said metal element is nickel. 3. The fuel electrode for a fuel cell according to claim 1, wherein the zirconia is yttrium-stabilized zirconia. 4. A process for preparing a substrate by mixing and sintering zirconia and an oxide of a metal element, and arranging a powder of the metal element or a powder of an oxide of the metal element on one surface of the substrate. A method for producing a fuel electrode for a fuel cell, comprising the step of: 5. The method for manufacturing a fuel electrode for a fuel cell according to claim 4, wherein the mixing ratio of the oxide of the metal element is 40% by weight when the zirconia and the oxide of the metal element are mixed and sintered. % To 60% by weight of the fuel cell. 6. The method for manufacturing a fuel electrode for a fuel cell according to claim 4, wherein said metal element is nickel. 7. The method for manufacturing a fuel electrode for a fuel cell according to claim 4, wherein said zirconia is yttrium stabilized zirconia.