JP2005190862A - Solid oxide fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell having improved gas sealing property and joint strength (thermal shock resistance, long-term heat resistance) at a joint between a metal frame and an electrolyte layer in the fuel cell, and a separator and the fuel cell. A fuel cell stack having good long-term durability by preventing gas damage due to a difference in thermal expansion during stacking and preventing deterioration of a brazing joint portion.
A solid comprising a power generation unit and a metal frame, an insulating barrier layer disposed in a gap between the metal frame and an electrolyte layer, and the insulating barrier layer and the metal frame joined by a brazing material. This is an oxide fuel cell.
This is a fuel cell stack formed by alternately stacking solid oxide fuel cells and separators.
[Selection figure] None

Description

  The present invention relates to a solid oxide fuel cell and a fuel cell stack, and more particularly to a solid oxide fuel cell and a fuel cell stack excellent in gas sealing performance.

  Conventionally, a solid oxide fuel cell (SOFC) is known as a device that converts chemical energy into electrical energy by an electrochemical reaction. In this SOFC, a fuel cell power generation unit is formed by laminating a fuel electrode layer, a solid electrolyte layer, and an air electrode layer, and a fuel gas such as hydrogen or hydrocarbon is supplied to the fuel electrode layer from the outside. Electricity is generated by supplying an oxidant gas such as air to the polar layer.

In a fuel cell stack in which a power generation unit and a separator are stacked, a type that operates at a high temperature generally has a configuration in which a separator plate and a power generation unit are sealed with a glass material and gas-sealed.
However, the glass-based sealing material has a problem in that the gas sealing property is deteriorated due to breakage due to heat cycle or the glass partially softened and melted out.

Furthermore, a fuel cell in which a gas seal portion is formed of a laminated body of glass and metal has been proposed. In this technique, the glass component is liquefied at the operating temperature of the fuel cell to improve the gas sealing property, and the seepage and breakage of the glass material can be prevented (for example, see Patent Document 1).
Japanese Patent Laid-Open No. 6-84530

  The fuel cell is premised on a stationary generator that operates at 1000 ° C. for a long time, and the temperature raising / lowering rate during the heat cycle is relatively small. On the other hand, in-vehicle fuel cells are frequent and frequent in the temperature change accompanying the start-stop of the fuel cell and the fluctuation of the operating state according to the required power generation output. When a gas seal material that liquefies and seals glass during power generation is used in such a fuel cell stack for use, there is a problem that durability is insufficient due to extremely frequent and rapid thermal shock.

Furthermore, a flat plate type fuel cell in which an alloy holding thin plate frame is attached around the flat plate type cell has been proposed. Specifically, the peripheral part where the electrode layer on the electrolyte layer of the cell is not formed and the alloy thin plate frame are joined with a glass-based sealing material or a brazing material. A fuel cell stack formed by laminating a cell plate using this cell and a separator and sealing a gap between each other by applying a predetermined load is obtained by attaching a holding thin plate frame around the cell. A gas seal can be obtained by sealing the upper and lower separators at a portion, and damage to the stack due to a difference in thermal expansion can be prevented (for example, see Patent Document 2).
JP 2000-331692 A

In order to improve the output density of the fuel cell and manufacture a small fuel cell, it is important to reduce the electrolyte layer to improve the ionic conductivity. In particular, in-vehicle fuel cells are limited in mounting space and weight, and therefore, it is an extremely important issue to manufacture fuel cells with high output density. Recently, it has become possible to manufacture cells in which an electrolyte layer having a thickness of 10 μm or less has been formed, and the output density has been improved.
When this thinned electrolyte layer and holding metal frame are joined by the brazing method, although the joint strength and thermal shock resistance are excellent, the temperature during the brazing process is high, so the thickness of the electrolyte layer is reduced. As a result, cracks may occur in the electrolyte layer due to thermal stress, or the electrolyte layer may peel from the lower electrode layer or the cell substrate. That is, there is a problem that a gas leak occurs and the power generation output decreases or the fuel cell is damaged.
Also, the molten brazing material during the brazing process may soak into the cracks in the electrolyte layer and become electrically connected to the lower electrode and the cell substrate. That is, there is a problem that the battery is short-circuited and cannot generate power.
Furthermore, since the electrolyte layer is ion conductive, ions move within the layer during power generation operations. A local electric field is also formed at the interface between the electrolyte layer and the brazing material, and ions are supplied. As a result, from the vicinity of the interface with the electrolyte, deterioration of the brazing filler metal component, oxidative degradation, and volatilization of the component are promoted, and gas sealing properties and bonding strength are degraded.

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide gas sealability and bonding strength (heat resistance) at the joint between the metal frame and the electrolyte layer in the fuel cell. The object is to provide a solid oxide fuel cell having improved impact resistance and long-term heat resistance.
In addition, the object of the present invention is that the gas sealability between the separator and the fuel cell is good, the damage due to the difference in thermal expansion at the time of stacking is prevented, and the deterioration of the brazing material joint is prevented for a long time. The object is to provide a fuel cell stack with improved durability.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by disposing an insulating barrier layer in the gap between the metal frame and the electrolyte layer in the fuel cell. The present invention has been completed.

  Since the insulating barrier layer is disposed in the gap between the metal frame and the electrolyte layer in the fuel battery cell, the gas sealability and bonding strength (thermal shock resistance, Solid oxide fuel cells with improved long-term heat resistance) and good gas sealability between the separator and the fuel cells, preventing damage due to thermal expansion differences during stacking, and A fuel cell stack in which deterioration is prevented and long-term durability is improved can be provided.

  Hereinafter, the solid oxide fuel cell of the present invention will be described in detail. In the present specification, “%” indicates a mass percentage unless otherwise specified.

The solid oxide fuel cell of the present invention is formed by arranging a metal frame in a power generation unit. This power generation unit is formed by sandwiching an electrolyte layer between a fuel electrode layer and an air electrode layer. Further, the metal frame is disposed at the outer edge of the power generation unit and in the gap between one or both of the air electrode layer and the fuel electrode layer and the electrolyte layer.
Further, an insulating barrier layer is provided in the gap between the metal frame and the electrolyte layer. The insulating barrier layer is bonded to the metal frame with a brazing material. The brazing material may be used at least between the insulating barrier layer and the metal frame.

Here, the insulating barrier layer has a function of preventing the ions conducting in the electrolyte layer from coming into contact with the brazing material component, and is further gas-impermeable. That is, the ion insulating and dense layer is formed. Furthermore, an electronic insulating and dense layer is preferable.
As a result, the insulating barrier layer prevents the brazing material from being oxidized or deteriorated, and the durability of the fuel cell is improved. In addition, the thermal stress applied to the electrolyte layer during brazing and the thermal stress caused by the difference in thermal expansion during power generation operation are alleviated, and a fuel battery cell with improved thermal shock resistance can be obtained.

  In addition, the insulating barrier layer has a function of relaxing thermal stress applied to the electrolyte layer during brazing, a function of relaxing thermal stress caused by a difference in thermal expansion during power generation, and a function of improving the wettability of the brazing material. It is preferable. At this time, it is possible to form a good joint that does not include a gas leak portion such as a pinhole. In addition, the metal frame and the electrolyte layer can be joined with a small amount of brazing material. Furthermore, the brazing material having a large thermal expansion coefficient can be made into a thin film to improve the thermal shock resistance.

  Furthermore, the insulating barrier layer can be composed of a plurality of layers. For example, the structure which laminated | stacked two layers from which a thermal expansion coefficient differs, and the structure which formed the active metal layer and metallization layer which improve the wettability with a brazing material on the surface of the insulating 1st layer are mentioned. At this time, since it is possible to prevent the brazing material from adhering to the electrolyte layer during the brazing process in addition to the region where the insulating barrier layer is formed, the region of the power generation function can be secured with good yield.

Examples of the insulating barrier layer include layers mainly composed of oxides such as alumina, zirconia, mica and magnesia, carbides such as silicon carbide and titanium carbide, and nitrides such as boron nitride, silicon nitride and aluminum nitride. Can be mentioned. Also includes siloxane (Si-O) bond, carbosilane (Si-C) bond, Al-O bond, Mg-O bond, Zr-O bond, BN bond, Al-N bond and Ti-C bond. Examples include layers using amorphous ceramics or glass. Furthermore, a layer in which the above material is the main component and the crystalline component and the amorphous component or the glass component are mixed can be employed.
The insulating barrier layer may be formed by, for example, applying and baking using a paste in which fine particles containing the above materials are dispersed in an organic binder, a sol-gel solution, a ceramic adhesive, or the like, or spraying and baking. The method can be adopted. Alternatively, the film may be formed by vapor deposition, sputtering, aerosol deposition, thermal spraying, or the like.

Specifically, as the insulating barrier layer, an insulating barrier layer formed by a thermal spraying method using a raw material containing alumina, zirconia, aluminum nitride, silicon oxide or magnesia, and any combination thereof. Is mentioned. For example, a dense insulating barrier layer having a good gas sealing property can be obtained by HVOF (high-speed flame spraying) method or plasma spraying method using fine particles having an average particle diameter of several tens of nanometers to 10 μm or raw material powder obtained by granulating fine particles. Can be formed.
As described above, since it can be formed with a thickness of several tens of μm, which is equivalent to that of the brazed bonding layer, it can be an insulating barrier layer having an excellent thermal stress relaxation effect. In addition, even if a crack occurs in the electrolyte layer during the formation of the insulating barrier layer, the insulating barrier layer can be bonded while ensuring gas leakage because the film can be formed while covering the damaged portion.

Furthermore, as another insulating barrier layer, the layer formed by the aerosol deposition method using the same raw material powder is also mentioned. The aerosol deposition method is a method in which fine particle powder is aerosolized and transported to a film forming chamber whose pressure is reduced to several Torr to several tens of Torr, and sprayed onto a substrate from a nozzle on a slit to form a film. Thus, a dense insulating barrier layer can be formed. Moreover, since it has excellent adhesion to the electrolyte layer, it is possible to form a dense insulating barrier layer with improved durability against mechanical vibration and excellent gas sealing properties. Furthermore, since it is easy to mix and form the additive for adjusting a thermal expansion coefficient, the insulating barrier layer excellent in the thermal stress relaxation function can be formed.
Furthermore, an insulating barrier layer can be formed by a known vapor deposition method or sputtering method using alumina, silica, magnesia, chromia, hafnia, silicon nitride, titanium nitride, or the like. Since the damage to the electrolyte layer can be reduced and the film can be formed, it is suitable when the strength of the electrolyte layer is relatively small. In addition, since the Ti active layer for the purpose of improving the brazing material wettability and the bondability can be continuously formed, the process can be simplified.

Examples of other insulating barrier layers include Si-Ti-CO-based amorphous ceramic layers formed by a spray method or a coating method, sol-gel solutions mixed with silica, magnesia, mica, and alumina fine particles. At this time, since it is dense and can be formed with a thickness of several tens of μm, which is equivalent to the brazing joint layer, it can be an insulating barrier layer having an excellent thermal stress relaxation effect.
For example, after spraying or applying a binder component to a known polytitano carbosilane polymer mixed with additive ceramic fine powders such as zirconia and boron nitride, it is fired and amorphous containing silicon, titanium, carbon and oxygen elements Ceramics can be obtained. Since this material has a low firing temperature of 200 to 500 ° C., a dense insulating barrier layer can be formed without damaging the electrolyte layer. In addition, the brazing property in the post-process is good and the insulation during the brazing process is difficult to break. Furthermore, since the electrical insulation does not decrease significantly up to about 800 ° C., long-term durability can be improved. If the firing temperature for densification or ceramics is very high compared to the operating temperature of the fuel cell, the electrolyte layer will be easily broken by thermal stress, the metal frame may be altered or hardened. There are things to do.

On the other hand, as the metal frame, for example, known Ni-based alloys, Fe-based alloys, and heat-resistant alloys such as stainless steel can be used. The metal frame is preferably a thin plate or foil having a thickness of 0.5 mm or less, and such a shape provides an effect of relaxing thermal stress and improving vibration resistance.
In addition, as the brazing material, for example, a known silver brazing, gold brazing, nickel brazing, cobalt brazing or titanium brazing, and any combination thereof can be used. In addition to the brazing material layer, the brazed joint includes an interface reaction layer in which the brazing material and the insulating barrier layer have reacted, and an interface reaction layer in which the brazing material layer and the metal frame member have reacted.

In the solid oxide fuel cell of the present invention, a protective substrate can be disposed on one or both of the front surface and the back surface of the power generation unit. In addition, one or both of the fuel electrode layer and the air electrode layer and the protective substrate are integrated, in other words, the electrode layer can have a function as a protective substrate.
As a result, electrolyte thin film cells with excellent power generation characteristics can be manufactured without causing electrical insulation breakdown due to diffusion of brazing filler metal components in the electrolyte layer during the brazing process, with high power output density and thermal shock resistance. Fuel cell excellent in heat resistance and heat resistance can be provided.
Examples of the protective substrate include the same material as the electrode material that is in contact with the protective substrate, and known Ni-based alloys, Fe-based alloys, and heat-resistant alloys such as stainless steel.

Furthermore, the fuel electrode layer or the air electrode layer can be electrically joined to the metal frame. Accordingly, since the air electrode layer and the metal frame are electrically joined, the potential difference in the electrolyte layer can be suppressed, the brazing material can be prevented from being deteriorated, and the long-term durability of the joint can be improved. . In addition, since the membrane potential is aligned in the thickness direction of the electrolyte layer, loss of ionic conductivity can be reduced, and the power generation output density can be improved.
For example, after joining a metal frame, an air electrode layer (fuel electrode layer) can be formed in a pattern in contact with the metal frame to be electrically joined, or a current collector wiring can be formed to form an air electrode layer ( The fuel electrode layer) and the metal frame can be electrically joined.
In addition, even if there is a gap between the air electrode layer and the metal frame, generally, for example, through a current collector installed on the air electrode layer and a separator or interconnector installed thereabove, It is also possible to establish conduction with the metal frame joined to these. However, when the adjacent metal frame and the electrode layer are electrically joined through many electrical contacts, a potential difference occurs according to the contact resistance, so a local battery is formed between the metal frame and the electrode layer. Then, the electrode layer and the electrolyte layer are deteriorated, which is not preferable. When operated at high temperature and in air for a long time, the contact resistance of the contact portion is further increased, and the deterioration is accelerated.

Next, the manufacturing method of the solid oxide fuel cell of the present invention will be described in detail.
The fuel cell described above has, for example, a structure as shown in FIG. 1, and a fuel electrode layer and an electrolyte layer are sequentially laminated on a porous substrate, and an insulating barrier layer and a brazing material are sequentially arranged on the outer periphery of the electrolyte layer. After disposing, it is obtained by coating an air electrode layer on the electrolyte layer.
In the present invention, the insulating barrier layer is formed by a thermal spraying method using fine particles or an aerosol deposition method. By such a method, a dense layer that does not leak gas can be formed at a relatively low temperature at the junction of the electrolyte layer without destroying or altering the electrolyte layer. In addition, it has excellent adhesion between the electrolyte layer and the insulating barrier layer, and suppresses the loss of power generation characteristics of the cell without causing gas leakage from the interface during operation of the fuel cell that repeatedly raises and lowers the temperature. Furthermore, since the mixed powder of a plurality of materials can be adjusted as a raw material, it becomes easy to adjust, for example, the difference in thermal expansion coefficient with the electrolyte layer and the wettability with the brazing material according to the purpose.
The insulating barrier layer melts, volatilizes, becomes porous, or the brazing filler metal component diffuses throughout the insulating barrier layer against the temperature of the brazing step, which is the subsequent step of the insulating barrier layer forming step. It is better to use materials that do not react violently so that insulation is lost.

Next, the fuel cell stack of the present invention will be described in detail.
Such a fuel cell stack is formed by alternately stacking the above-described solid fuel cell electrolyte fuel cells and separators.
Since the metal frame is provided on the outer edge of the fuel battery cell, it is possible to form a highly reliable joint with respect to gas sealability and thermal shock resistance by directly joining with a metal separator. Further, when both the metal frame and the separator plate are made of metal, it is easy to form a gas flow path and fine processing for adjusting the stacking interval, and the stacking interval can be reduced to form a stack. As a result, a highly integrated stack can be formed, and a small fuel cell stack with high output density can be formed.
As said separator, what is comprised from heat resistant alloys, such as a well-known Ni type alloy, Fe type alloy, and stainless steel, for example can be used.
When stacking, a cell plate in which a plurality of the fuel cells are connected in the horizontal direction can be employed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to these Examples.

(Example 1)
1) Alumina is formed as an insulating barrier layer on the electrolyte-supported power generation section by thermal spraying method. A Ni-ScSZ cermet fuel electrode layer is printed on a sintered plate of ScSZ electrolyte having a diameter of 40φ and a thickness of 0.2 mm and fired. An electrolyte thin film type power generation part with no layer formed was formed.
An alumina layer having a width of 5 mm and a thickness of 40 μm was formed as an insulating barrier layer on the outer edge of the surface of the electrolyte layer on the air electrode layer-unformed side. The alumina layer was formed by using a known HVOF method using alumina having an average particle diameter of 2 μm as a raw material powder.
A Ti layer having a width of 3 mm was formed on the alumina layer by a known sputtering method to have a thickness of 0.2 μm. The Ti layer improves the wettability of the brazing material on the alumina layer during the brazing process and improves the adhesion between the brazing material joining layer and the alumina layer. In addition, depending on the brazing conditions and brazing material, which has a function of preventing the molten brazing material during the brazing process from flowing on the alumina layer or the electrolyte layer where the Ti layer is not formed, the Ti element after brazing In some cases, a layer that is clearly observed as a layer containing a large amount of selenium may be formed, or in the optical microscope field of view, it may be in a state where it cannot be clearly distinguished from the brazing material bonding layer.
A 25 μm thick BNi-6 brazing material layer was formed on the Ti layer by a known plating method.
Next, as a metal frame, a metal member having a diameter of 60φ, a thickness of 0.1 mm, and a through hole having an inner diameter of 34φ formed in the central portion, a cell in which an insulating alumina layer and a brazing material layer are formed without an air electrode, It laminated | stacked so that it might become the same center, and it installed in the brazing jig so that the load of 100g might be applied to a brazing part.
According to a known brazing method, the metal frame and the insulating barrier layer were brazed at 930 ° C. in Ar flow.
A lanthanum-strontium-manganate air electrode layer was formed by sputtering with a thickness of 2 μm so as to cover the electrolyte layer on the upper surface of the power generation unit and the edge of the through hole of the metal frame, thereby obtaining a fuel cell. The schematic cross-sectional view of this fuel cell is the same as that shown in FIG.
Further, FIG. 2 shows a fuel cell stack configuration assembled by using a plurality of cells having the same configuration as the fuel cell obtained in Example 1, and the shape of the fuel cell portion in the stack of FIG. 2 is viewed from above. The figure is shown in FIG. By using the fuel battery cell of the present invention, such a stack is obtained and the object of the present application is achieved.

(Example 2)
2) An alumina layer is formed as an insulating barrier layer on the fuel electrode-supported power generation unit by the aerosol deposition method. Green sheet of Ni-YSZ fuel electrode layer and YSZ electrolyte layer on porous Inconel YSZ substrate with a diameter of 40φ and a thickness of 1 mm Were laminated and co-sintered to form an electrolyte thin film type power generation part in which the air electrode layer was not formed.
The insulating barrier layer uses alumina raw material powder having an average particle size of 0.1 μm, and is 5 mm wide and thick as an insulating barrier layer on the outer surface of the electrolyte layer on the air electrode non-formation side by a known aerosol deposition method. A 5 μm thick alumina layer was formed.
The same operation as in Example 1 was repeated, the metal frame was brazed, the air electrode layer was formed, and a fuel cell was obtained.
By making the inner diameter edge part of the metal frame into a stepped shape, the contact between the electrode layer and the brazing material can be avoided, and deterioration of the brazing material can be further suppressed. Further, deterioration due to exposure to an air atmosphere can be greatly suppressed. Further, since the construction thickness of the brazing material joining layer is defined, a fuel battery cell having stable characteristics such as joint strength can be formed.

(Example 3)
3) A Si—Ti—C—O-based layer is formed as an insulating barrier layer in the fuel electrode supported power generation unit by spray coating and baking. The same operation as in Example 2 is repeated except for the insulating barrier layer and the brazing material. A fuel cell was formed.
The insulating barrier layer is spray-coated with a coating containing a polytitano carbosilane polymer mixed with additive ceramic fine powder, dried at 80 ° C., and then baked at 370 ° C. to obtain Ti—Si—C—. An O-based amorphous ceramic layer was formed to 20 μm.
Subsequent to the formation of the Ti sputter layer, 1 μm of a brazing material BAg-8 was formed by sputtering. A metal frame was set by the same operation as in Example 1, and the metal layer and the insulating barrier layer were brazed at 820 ° C. in a vacuum to obtain a fuel cell.

Example 4
4) Applying and firing ceramic ultrafine particle-containing paste as insulating barrier layer on fuel electrode supported battery part By applying and firing, the same operation as in Example 3 was repeated to obtain a fuel cell. The insulating barrier layer was spray-coated with an alumina-based sol mixed with silica fine particles, dried at 80 ° C. and then fired at 370 ° C. to form an amorphous ceramic layer having a thickness of 2 μm.

(Evaluation methods)
Example 1
It installed in the well-known cell electric power generation output evaluation apparatus, air and hydrogen gas were each introduce | transduced at 750 degreeC, and the open end electromotive force (OCV) was measured.
An OCV of 1.08 V was obtained, and it was determined that there was no leakage of hydrogen gas. That is, there was no joint failure location in the brazing layer, the insulating barrier layer was sufficiently dense, and cracks in the electrolyte layer did not occur due to thermal stress during the brazing process.
Further, even after the 600 ° C. holding time reached 100 hours, no decrease in OCV was detected, and no discoloration or alteration of the brazed part was observed.
Example 2
The OCV was evaluated at 600 ° C. by the same measurement method as in Example 1. 1.09 V was obtained, and there was no change even after holding at 600 ° C. for 100 hours.
Example 3
OCV was evaluated by the same measurement method as in Example 2. 1.01 V was obtained at 600 ° C., and there was no change even after holding at 600 ° C. for 100 hours.
Example 4
OCV was evaluated by the same measurement method as in Example 2. 1.02 V was obtained at 600 ° C., and there was no change even after holding at 600 ° C. for 100 hours.

1 is a schematic view showing a fuel battery cell obtained in Example 1. FIG. FIG. 2 is a schematic cross-sectional view of a stack on which cells having the same configuration as the fuel cell obtained in Example 1 are mounted. FIG. 3 is a schematic view of a fuel cell constituting the stack of FIG. 2.

Claims (5)

A power generation unit formed by laminating a fuel electrode layer, an electrolyte layer, and an air electrode layer in this order, and a metal frame disposed at the outer edge of the power generation unit and at the gap between the air electrode layer and / or the fuel electrode layer and the electrolyte layer. A solid electrolyte fuel cell configured,
A solid oxide fuel cell comprising: an insulating barrier layer disposed in a gap between the metal frame and the electrolyte layer; and the insulating barrier layer and the metal frame joined with a brazing material.   The solid oxide fuel cell according to claim 1, wherein a protective substrate is disposed on the front surface and / or the back surface of the power generation section.   The solid oxide fuel cell according to claim 1 or 2, wherein the fuel electrode layer and / or the air electrode layer and a protective substrate are integrated.   The solid oxide fuel cell according to any one of claims 1 to 3, wherein the fuel electrode layer or the air electrode layer is electrically joined to the metal frame.   5. A fuel cell stack comprising the solid oxide fuel cells according to claim 1 and separators alternately stacked.

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