JPH08213029A - Fuel electrode of solid electrolyte fuel cell - Google Patents

Abstract

PURPOSE: To improve characteristics of a fuel electrode of a solid electrolyte fuel cell. CONSTITUTION: A fuel electrode 2 of a solid electrolyte fuel cell where the fuel electrode 2 and an air electrode are arranged by sandwiching solid electrolyte 1 mainly composed of zirconia, is formed as a film having a porous structure composed of a solid electrolyte material, a ceria material and nickel or a nickel oxide. The mixing ratio of solid electrolyte material is made large on the solid electrolyte 1 side, and the mixing ratio of nickel or nickel oxide is made large on the surface side of the fuel electrode 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid oxide fuel cell for producing an electromotive force by causing an oxidation / reduction reaction through a solid electrolyte having oxygen ion permeability, and more particularly to a fuel electrode thereof.

[0002]

2. Description of the Related Art In this type of fuel cell, as schematically shown in FIG. 4, a fuel electrode 2 and an air electrode 3 which are porous membranes are formed on both sides sandwiching a thin film solid electrolyte 1. Fuel gas (hydrogen gas, carbon monoxide gas, etc.) flowing on the fuel electrode 2 side and oxygen in a gas containing oxygen (for example, air) flowing on the air electrode 3 side are electrically connected via the solid electrolyte 1. An electromotive force can be obtained through each electrode 2 and 3 by reacting chemically.

That is, air diffuses inside the air electrode 3 to the surface of the solid electrolyte 1, and oxygen contained in the air is ionized, and the inside of the solid electrolyte 1 is caused by a difference in concentration of oxygen ions. Move to side 2. Also fuel electrode 2
On the side, hydrogen gas diffuses inside the fuel electrode 2 to the surface of the solid electrolyte 1 where it reacts with oxygen that has moved through the solid electrolyte 1. The electromotive force generated by such an electrochemical reaction between hydrogen and oxygen is extracted to the outside via the electrodes 2 and 3.

Since the above reaction is carried out at a high temperature of about 1000 ° C. at which the activity of the solid electrolyte 1 is excellent, the solid electrolyte 1 is, of course, excellent in oxygen ion permeability.
Properties such as excellent stability at high temperature and lack of conductivity are required. Therefore, conventionally, zirconia (YSZ or CSZ) stabilized with yttria or calcia is used as a solid electrolyte.

Further, since the air electrode 3 is placed in a strong oxidizing atmosphere, it has high electronic conductivity and oxygen ion conductivity and hardly causes polarization, or has a small difference in thermal expansion coefficient from the solid electrolyte 1. In addition to the above, it is required to have excellent oxidation resistance. Therefore, conventionally, the air electrode 3 is formed of a perovskite-type lanthanum-based composite oxide.

Further, since the fuel electrode 2 is an electrode for taking out an electromotive force to the outside, it has a high electron conductivity and hardly causes polarization, and is exposed to a high temperature reducing atmosphere. Stability is required, and in order to prevent thermal stress from the solid electrolyte 1 and peeling due to this, it is desirable that the coefficient of thermal expansion be close to that of the solid electrolyte 1. Therefore, in order to meet these requirements, at present, a cermet of nickel (Ni) or nickel oxide (NiO), or a weight ratio of Ni / YS of 4: 6 to 6: 4.
Z and Ni O / YSZ cermets are used as fuel electrodes. Here, NiO becomes conductive by being exposed to a high temperature reducing atmosphere to become Ni.

[0007]

However, when such a solid oxide fuel cell is used in a high temperature state for a long time, sintering (sintering) progresses on the fuel electrode 2 to cause Ni (melting point). (About 1450 ° C.) aggregates. As a result, the porous structure of the fuel electrode gradually collapses, the fuel gas permeability of the fuel electrode 2 decreases, the electron conductivity of the fuel electrode 2 also decreases, and the reaction area of the electrode decreases. There are various problems that the polarization resistance increases. Therefore, recently, there has been a demand for the development of a fuel electrode that can further satisfy the conditions required for a fuel electrode of a solid oxide fuel cell.

The present invention has been made to meet the above-mentioned demands, and an object thereof is to improve the characteristics of the fuel electrode of a solid oxide fuel cell.

[0009]

In order to achieve the above object, the present invention provides a ceria-based material for a fuel electrode of a solid oxide fuel cell. Specifically, according to the invention described in claim 1, in a fuel electrode of a solid oxide fuel cell in which a fuel electrode and an air electrode are provided with a solid electrolyte mainly composed of zirconia interposed therebetween, a nickel or nickel oxide material is used. CeO ₂ , (Ce O ₂ ) _1-X was added to the powder.
(Sm ₂ O ₃ ) _X , (Ce O ₂ ) _1-X (La
₂ O ₃ ) _X , (Ce O ₂ ) _1-X (Y ₂ O ₃ ) _{X and the} like are mixed with a mixed powder material to which one or more kinds of ceria-based materials are added to form a porous material on the surface of the solid electrolyte. It is characterized in that it is formed into a film structure.

At this time, in the invention described in claim 2, the mixing ratio of the mixed material is such that the mixing ratio of the nickel or nickel oxide material is small and the mixing ratio of the ceria-based material is large on the solid electrolyte side. It is characterized in that a film having a graded porous structure is formed by sequentially setting the mixing ratio of the nickel or nickel oxide material to be large and the mixing ratio of the ceria-based material to be small. is there.

According to a third aspect of the present invention, in a fuel electrode of a solid oxide fuel cell having a fuel electrode and an air electrode sandwiching a solid electrolyte mainly composed of zirconia, the solid electrolyte material and nickel or the material powder obtained by mixing the nickel _{oxide, Ce O 2, (Ce O} 2)
_1-X (Sm ₂ O ₃ ) _X , (Ce O ₂ ) _1-X (La
₂ O ₃ ) _X , (Ce O ₂ ) _1-X (Y ₂ O ₃ ) _X or the like is added to the surface of the solid electrolyte by a mixed powder material containing at least one of ceria-based materials. It is characterized by being formed into a film having a porous structure.

At this time, in the invention described in claim 4, the mixing ratio of the mixed material is set so that the mixing ratio of the solid electrolyte material on the solid electrolyte side is large and the mixing ratio of nickel or nickel oxide is small. It is characterized in that the film is formed into a film having a graded porous structure by setting the mixing ratio of the solid electrolyte material to be successively smaller and the mixing ratio of nickel or nickel oxide to be larger.

Further, in the invention described in claim 5, the mixing ratio of the mixed material is set such that the mixing ratio of the ceria-based material is reduced on the solid electrolyte side and the surface side of the fuel electrode, and The mixing ratio of the ceria-based material is set to be gradually increased in the central portion of the film, and the film is formed into a film having an inclined porous structure.

[0014]

Since the ceria-based material has excellent oxygen ion permeability, the oxidation / reduction reaction occurs even at the portion where the ceria-based material and nickel or nickel oxide are joined and contact with the fuel gas. Specifically, the electrode reaction according to the conventional structure was carried out only at the boundary portion between the solid electrolyte and nickel or nickel oxide.
The electrode reaction in the fuel electrode according to claim 5 is also performed in the boundary portion between the ceria-based material and nickel. Therefore, the portion that reacts as an electrode is enlarged, the polarization resistance is not increased, and the characteristics of the electrode are improved. Further, the ceria-based material has a melting point (for example, if CeO ₂ is 2
(Around 600 ° C.) is high, which prevents nickel agglomeration.

Particularly, in the fuel electrode according to the second aspect, the fourth aspect and the fifth aspect, the ceria-based material has a linear expansion coefficient (for example, 12.0 × 10 ^-6 / Ce ₂ for CeO _2).
K is the coefficient of linear expansion of the solid electrolyte material (for example, YSZ
If so, since it is located between 10.5 × 10 ⁻⁶ / K) and the linear expansion coefficient of nickel (16.0 × 10 ⁻⁶ / K),
The thermal matching between the solid electrolyte portion and the electrode portion can be easily adjusted, and the peeling phenomenon of the fuel electrode and the like can be prevented.

[0016]

First, an embodiment of the invention described in claim 1 will be described. This fuel electrode is made of a mixed material of nickel (Ni) and a ceria-based material, for example, cerium oxide (CeO ₂ ). At this time, nickel oxide (NiO) can also be used in place of Ni, which is reduced to Ni during operation of the fuel cell, and thus has conductivity and functions as an electrode.

The ceria-based material prevents the increase of polarization resistance and N
By preventing the aggregation of i or the phenomenon of electrode peeling,
It improves the characteristics of the electrode and is present in the electrode in a highly dispersed state. The fuel electrode containing the ceria-based material can be formed by various methods, examples of which are as follows.

Explaining the slurry method, a mixed material of Ni powder and CeO ₂ powder is prepared as a powder material, and terpineol (special grade) and polypinyl butyral as a solvent and an auxiliary agent are added thereto. The materials thus adjusted are stirred and mixed to form a slurry, which is screen-printed (applied) on the surface of the solid electrolyte to form a predetermined thickness. Then, it can be heated in an inert gas atmosphere for drying and degreasing, and further heated to a high temperature to form a porous fuel electrode.

Explaining the thermal spraying method, a mixed material of Ni powder and CeO ₂ powder, each of which has a particle size adjusted to about 40 μm, is prepared and sufficiently stirred and mixed, and then plasma sprayed, arc sprayed or flame sprayed. A porous thin film electrode can be formed by spraying the surface of the solid electrolyte by an appropriate method such as.

At this time, as described in claim 2, N
When the mixing ratio of the i powder and the CeO ₂ powder is made to incline in the thickness direction of the fuel electrode 2, stabilized zirconia (Y
The thermal compatibility with the solid electrolyte layer made of SZ) or the like can be improved, and peeling phenomenon and cracks can be prevented.

Next, an embodiment of the invention described in claim 3 will be described with reference to FIG. The fuel electrode 2 shown in FIG.
It is made of a mixed material of nickel (Ni), stabilized zirconia (YSZ), and a ceria-based material such as cerium oxide (CeO ₂ ). At this time, nickel oxide (NiO) may be used instead of Ni. The fuel electrode containing this ceria-based material can be easily obtained by mixing YSZ powder with the material powder used in the above-mentioned slurry method or thermal spraying method. It can also be formed by a method in combination with a chemical vapor deposition method (EVD method).

In this method, a mixed powder of Ni powder and CeO ₂ powder is stirred and mixed with terpineol (special grade) and polyvinyl butyral as a solvent and an auxiliary agent, and the obtained slurry is applied to the surface of the solid electrolyte 1. It is applied and calcined in an inert gas atmosphere, and YSZ is attached to its surface by the EVD method. This coating and calcination and EVD
The method is repeated to form the electrode 2 having a predetermined thickness.

An embodiment of the invention described in claims 4 and 5 will now be described with reference to FIGS. 2 and 3.
This fuel electrode 2 is composed of YSZ powder and N in the above-mentioned embodiment.
The mixing ratio of the i powder and the CeO ₂ powder is made to be inclined in the thickness direction of the fuel electrode 2. That is, the mixing ratio of YSZ is increased on the solid electrolyte 1 side, and the mixing ratio of Ni is increased on the surface side of the fuel electrode 2.

At this time, the ratio of the ceria-based material can be slightly small on the solid electrolyte 1 side and the front side of the fuel electrode 2, and can be slightly large on the intermediate portion. For example, as shown in FIG. 2, the fuel electrode 2 is changed from the solid electrolyte 1 side to the first
When it is composed of the layer electrode 2a, the second layer electrode 2b, the third layer electrode 2c, and the fourth layer electrode 2d, these four layers 2a, 2b, 2
The mixing ratios of the respective materials in c and 2d are configured as shown in FIG.

In the fuel electrode 2 with a gradient function formed as described above, due to the difference in the rate of thermal expansion between the solid electrolyte 1 and the first layer electrode 2a and between the layers 2a, 2b, 2c and 2d. Since the thermal stress generated is small, the peeling phenomenon and the occurrence of cracks are prevented. At this time, the linear expansion coefficient of CeO ₂ is located between the linear expansion coefficients of YSZ and Ni, so that the fuel electrode 2 can be easily formed.

Specifically, when the thermal stress in the fuel electrode 2 is made equal to the thermal stress between the respective layers in the case where the conventional fuel electrode made of the mixed material of Ni and YSZ is configured to be inclined, Since the fuel electrode 2 can be composed of fewer layers than the conventional fuel electrode, the productivity is improved. On the contrary, when the fuel electrode 2 is configured to have the same number of layers as that of the conventional gradient fuel electrode, the thermal matching property is further enhanced.

Further, in the fuel electrode 2 with the tilt function, the amount of Ni on the solid electrolyte 1 side is reduced, so N
The sintering of i is suppressed, and even if Ni is sintered, the oxygen ion permeability of the solid electrolyte 1 is not significantly deteriorated, so that the deterioration of the solid electrolyte of the solid oxide fuel cell with time is prevented, that is, the life characteristics. Is improved.

The fuel electrode 2 with the tilt function is YSZ.
The composition is such that the mixing ratio of Ni, Ce, and Ni and CeO ₂ is sequentially adjusted. It is also possible to determine the mixing ratio of CeO ₂ with respect to YSZ or the mixing ratio of CeO ₂ with respect to Ni by adjusting the ratio to improve the thermal matching of the fuel electrode 2.

In each of the above examples, calcia-stabilized zirconia (CSZ) can be used in place of YSZ as the solid electrolyte material, and other ceria-based materials, specifically CeO ₂ , can be used. Is (Ce O ₂ ) _1-X (S
m ₂ O ₃ ) _X , (Ce O ₂ ) _1-X (La ₂ O ₃ ) _X ,
Of course, (Ce O ₂ ) _1-X (Y ₂ O ₃ ) _{X and the} like can be used.

[0030]

As described above, according to the fuel electrode described in any one of claims 1 to 5, the electrode reaction occurs even at the joint between nickel or nickel oxide and the ceria-based material, and therefore the electrode The activity of is improved and the polarization can be reduced.

Further, since the ceria-based material has a high melting point, the sintering / aggregation of nickel is suppressed or prevented. As a result, the life characteristics of the fuel electrode are improved, that is, the power generation capacity of the solid oxide fuel cell can be favorably maintained for a long period of time.

In particular, according to the inventions described in claims 2 and 4 and 5, it is possible to easily obtain a fuel electrode in which peeling phenomenon and cracks are prevented, and a solid electrolyte having improved life characteristics. Type fuel cell can be provided at low cost.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an example of a fuel electrode according to the invention described in claim 3.

FIG. 2 is a cross-sectional view schematically showing an example in which a tilt function is added to the fuel electrode shown in FIG.

FIG. 3 is a table showing an example of mixing ratios of various materials in the example shown in FIG.

FIG. 4 is a schematic cross-sectional view showing the principle structure of a solid oxide fuel cell.

[Explanation of symbols]

1 ... Solid electrolyte, 2 ... Fuel electrode, 2a ... 1st layer,
2b ... 2nd layer, 2c ... 3rd layer, 2d ... 4th layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoru Yamaoka 1-5-1, Kiba, Koto-ku, Tokyo Inside Fujikura Ltd.

Claims (5)

[Claims] 1. A fuel electrode for a solid oxide fuel cell in which a fuel electrode and an air electrode are provided with a solid electrolyte mainly composed of zirconia sandwiched therebetween, and a nickel or nickel oxide material powder is added to one of the following ceria-based materials. A fuel electrode for a solid oxide fuel cell, characterized in that it is formed into a film having a porous structure on the surface of the solid electrolyte by using a mixed powder material containing one or more kinds thereof. Ceria-based material: Ce O ₂ , (Ce O ₂ ) _1-X (M
₂ O ₃ ) _X , where M is a rare earth element 2. The mixing ratio of the mixed material is such that the mixing ratio of the nickel or nickel oxide material is small and the mixing ratio of the ceria-based material is large on the solid electrolyte side, and the nickel or nickel oxide material is sequentially added. 2. The solid oxide fuel cell according to claim 1, wherein the mixed electrolyte of Ceria is set to be large and the mixing ratio of the ceria-based material is set to be small to form a film having a graded porous structure. Fuel electrode. 3. A fuel electrode of a solid oxide fuel cell in which a fuel electrode and an air electrode are provided with a solid electrolyte containing zirconia as a main component sandwiched between the solid electrolyte material and nickel or nickel oxide. A fuel electrode for a solid oxide fuel cell, characterized in that it is formed into a film having a porous structure on the surface of the solid electrolyte by a mixed powder material obtained by adding one or more of the following ceria-based materials. Ceria-based material: Ce O ₂ , (Ce O ₂ ) _1-X (M
₂ O ₃ ) _X , where M is a rare earth element 4. The mixing ratio of the mixed material is set so that the mixing ratio of the solid electrolyte material on the solid electrolyte side is large and the mixing ratio of nickel or nickel oxide is small.
The solid electrolyte according to claim 3, wherein the solid electrolyte material is formed into a film having a graded porous structure by setting the mixing ratio of the solid electrolyte material to be smaller and the mixing ratio of nickel or nickel oxide to be larger. Type fuel cell fuel electrode. 5. The mixing ratio of the mixed material is such that the mixing ratio of the ceria-based material is reduced on the solid electrolyte side and the surface side of the fuel electrode, and the mixing ratio of the ceria-based material is reduced in the central portion in the thickness direction of the fuel electrode. The fuel electrode for a solid oxide fuel cell according to claim 3, wherein the fuel electrode of the solid oxide fuel cell according to claim 3 is formed so that the mixing ratio is set to be gradually increased and the film is formed into a slanted porous structure.