JP2005203257A - Solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell in which destruction of power generation cell or deterioration of battery characteristics or the like does not occur, while stable internal reforming generation is made possible. <P>SOLUTION: The fuel cell 5 is constituted of fuel electrode layers 3 and oxidizer electrode layers 4 arranged on both faces of solid electrolyte layers 2, then a fuel electrode current collector 6 and an oxidizer electrode current collector 7 respectively composed of foamed metals are arranged on the outside of the power generation cell 5, while separators 8 are installed outside the fuel electrode current collector 6 and the oxidizer electrode current collector 7. A hydrocarbon reforming catalyst 10 is filled inside the fuel electrode current collector 6. This is arranged so that the filled amount of hydrocarbon reforming catalyst 10 is made more as the position goes toward the peripheral part than from the center area of the current collector. The foamed metal body is formed in the three dimensional skeleton structure, and when the hydrocarbon reforming catalyst is carried on the surface of that skeleton structure, because a superior gas passage can be constantly secured, efficient and stabilized power generation can be carried out. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an internal reforming mechanism of a flat plate type solid oxide fuel cell.

  Solid oxide fuel cells are being developed as a third generation fuel cell for power generation. At present, three types of solid oxide fuel cells of this type have been proposed: a cylindrical type, a monolith type, and a flat plate type, each of which has a solid electrolyte composed of an oxide ion conductor as an air electrode (cathode) from both sides. ) And the fuel electrode (anode). A fuel cell stack is configured by stacking a plurality of power generation cells composed of this laminate alternately with separators with a fuel electrode current collector and an air electrode current collector interposed therebetween.

In the solid oxide fuel cell, an oxidant gas (oxygen) is supplied to the air electrode side and a fuel gas (H ₂ , CO, CH _4, etc.) is supplied to the fuel electrode side as a reaction gas. The air electrode and the fuel electrode are both porous layers so that the gas can reach the interface with the solid electrolyte.
Oxygen supplied to the air electrode side passes through the pores in the air electrode layer and reaches the vicinity of the interface with the solid electrolyte layer. At this part, electrons are received from the air electrode and ionized to oxide ions (O ²⁻ ). Is done. The oxide ions diffuse and move in the solid electrolyte layer toward the fuel electrode. Oxide ions that have reached the vicinity of the interface with the fuel electrode react with the fuel gas at this portion to generate reaction products (H ₂ O, CO _2, etc.), and emit electrons to the fuel electrode. This electron can be taken out as an electromotive force in an external circuit of another route.

  By the way, since the fuel gas used in such a solid oxide fuel cell is a hydrocarbon compound (this is called raw fuel) such as natural gas (methane gas), this raw fuel is actually mainly composed of hydrogen. It must be used after reforming to fuel gas. As a reforming method, when the raw fuel is a hydrocarbon-based gas fuel or liquid fuel, a steam reforming method is usually used.

For example, a reforming reaction using methane gas as a raw fuel is as follows.
The desulfurized methane gas is added with water vapor in the reformer to become hydrogen and carbon monoxide. This reforming reaction is an endothermic reaction, and the temperature is as high as 650 to 800 ° C.
CH ₄ + H ₂ O → 3H ₂ + CO
At this time, the generated carbon monoxide further reacts with water vapor and changes into hydrogen and carbon dioxide.
CO + H ₂ O → H ₂ + CO ₂

  Conventionally, as a fuel gas reforming of a solid oxide fuel cell, there are an external reforming method in which a reformer is installed outside, and an internal reforming method in which a reforming mechanism is directly incorporated in a high-temperature stack. It has been known.

  The external reforming method is a method in which a reformer having a hydrocarbon reforming catalyst is installed outside the fuel cell, and after reforming the raw fuel outside, the reformed gas is supplied into the cell. Since the reforming reaction is an endothermic reaction, it is necessary to supply heat for the reforming reaction into the external reformer, resulting in a problem that the efficiency of the power generation system is reduced.

  On the other hand, the internal reforming method generates a steam reforming reaction between raw fuel, water vapor, and a fuel electrode (for example, on a nickel electrode), and uses a part of Joule heat generated in the fuel cell as an endothermic reaction of the reforming reaction. This is an extremely rational method and has the potential to realize a highly efficient system. In addition, because of the cooling effect of absorbing high-temperature exhaust heat generated during power generation due to this endothermic reaction, this internal reforming method has recently attracted attention as a fuel reforming method for solid oxide fuel cells. .

  However, in this internal reforming method, a temperature distribution is generated inside the power generation cell due to an endothermic reaction due to the reforming reaction, and the power generation cell deteriorates or breaks due to the thermal stress at that time, or the battery performance decreases due to a local temperature decrease. Had the problem of doing.

In view of such a problem of the internal reforming method, Patent Documents 1 and 2 are disclosed as techniques for making the temperature distribution inside the power generation cell uniform.
JP-A-6-349504 JP-A-5-325996

  In Patent Document 1, a gas diffusion layer that restricts the arrival of the fuel gas to the fuel electrode is provided on the gas flow path side surface of the fuel electrode, and the reforming reaction at the fuel electrode occurs uniformly, so that the temperature distribution in the cell In addition, Patent Document 2 describes a reforming reaction by filling a catalyst with poor performance upstream of the catalyst packed bed inside the reformer and a catalyst with good performance downstream. This balances the endotherm and prevents the occurrence of temperature distribution in the cell.

  However, although various internal reforming methods have been proposed in this way, they are not always satisfactory in terms of the reformer mechanism or the durability of the fuel cell and the stability of the power generation performance, and are still being solved. Currently, it has problems to be solved and has not yet reached the practical level.

  In view of the above problems, the present invention is a simple reforming mechanism that prevents the occurrence of an in-cell temperature distribution caused by non-uniform reforming reactions, and does not cause destruction of power generation cells or deterioration of battery characteristics. It is an object of the present invention to provide a solid oxide fuel cell that enables stable and efficient internal reforming power generation.

  That is, according to the present invention, the fuel electrode layer and the oxidant electrode layer are disposed on both sides of the solid electrolyte layer, and the fuel electrode assembly made of a foam metal is provided outside the fuel electrode layer and the oxidant electrode layer, respectively. An electric current collector and an oxidant electrode current collector are disposed, a separator is disposed outside the fuel electrode current collector and the oxidant electrode current collector, and the fuel electrode current collector and the oxidant electrode current collector are disposed from the separator. In the solid oxide fuel cell for supplying fuel gas and oxidant gas to the fuel electrode layer and oxidant electrode layer via the fuel electrode current collector, the inside of the fuel electrode current collector is filled with a hydrocarbon reforming catalyst, and The amount of the reforming catalyst is increased from the upstream side to the downstream side of the fuel gas.

  The present invention described in claim 2 is the solid oxide fuel cell according to claim 1, wherein the fuel gas and the oxidant gas are fed from the central part of the separator to the fuel electrode current collector and the oxidant. The fuel electrode layer and the oxidant electrode layer are supplied to the fuel electrode layer and the oxidant electrode layer via an electrode current collector, and the filling amount of the hydrocarbon reforming catalyst is larger in the peripheral part than in the central part of the fuel electrode current collector. It is characterized by that.

  The present invention according to claim 3 is the solid oxide fuel cell according to claim 1 or 2, wherein the foam metal has a three-dimensional skeleton structure, and the skeleton surface thereof. The above-mentioned hydrocarbon reforming catalyst is supported.

Here, in the configuration according to claims 1 and 2, the temperature distribution inside the power generation cell is made uniform, thereby leveling the current density distribution and preventing the generation of thermal stress due to the temperature distribution. Deterioration and destruction of the power generation cell 5 can be prevented.
Further, in the configuration according to claim 3, since the hydrocarbon reforming catalyst is dispersed and supported on the surface of the foam metal body, the gas flow does not stagnate by the reforming catalyst, and the fuel electrode current collector A good gas flow path is always secured inside the body, enabling efficient and stable power generation.

  As described above, according to the first and second aspects of the present invention, the fuel electrode current collector is filled with the hydrocarbon reforming catalyst, and the filling amount of the reforming catalyst is reduced to that of the fuel gas. Since the temperature distribution inside the power generation cell is increased from the upstream side to the downstream side, the current distribution inside the power generation cell can be made uniform, and the current density distribution can be leveled. Destruction can be prevented.

  According to the third aspect of the present invention, since the foam metal body has a three-dimensional skeleton structure and the hydrocarbon reforming catalyst is supported on the skeleton surface, a good gas flow path is always provided in the anode current collector. Can be ensured, and sufficient reformed gas can always be supplied to the fuel electrode layer, so that efficient and stable power generation can be performed. Also, the reforming mechanism is simplified.

Hereinafter, a flat plate type solid oxide fuel cell according to the present embodiment will be described with reference to FIGS.
Here, FIG. 1 is an exploded perspective view showing the configuration of the fuel cell stack, FIG. 2 is a cross-sectional view showing the internal structure of a single cell, and FIG. 3 is a view showing the temperature distribution in the power generation cell.

  As shown in FIGS. 1 and 2, the fuel cell stack 1 includes a power generation cell 5 in which a fuel electrode layer 3 and an oxidant electrode layer 4 (air electrode layer 4) are disposed on both surfaces of a solid electrolyte layer 2, and a fuel electrode layer. 3, an anode current collector 6 disposed outside the air electrode layer 3, an oxidant electrode current collector 7 (air electrode current collector 7) disposed outside the air electrode layer 4, and outside the current collectors 6, 7. A single cell is formed by the separator 8 disposed, and a cylindrical body is formed by stacking a large number of the single cells.

The solid electrolyte layer 2 is composed of stabilized zirconia (YSZ) or the like to which yttria is added, and the fuel electrode layer 3 is composed of a metal such as Ni or Co or a cermet such as Ni—YSZ or Co—YSZ. Is made of LaMnO ₃ , LaCoO ₃ or the like, the fuel electrode current collector 6 is made of a sponge-like porous sintered metal plate (foamed metal plate) such as a Ni-based alloy, and the air electrode current collector 7 is made of an Ag base. It is made of a sponge-like porous sintered metal plate (foamed metal plate) such as an alloy, and the separator 8 is made of stainless steel or the like.

  The separator 8 has a function of electrically connecting the power generation cells 5 and supplying a reaction gas to the power generation cells 5. The fuel gas is introduced from the outer peripheral surface of the separator 8 to collect the fuel electrode of the separator 8. A fuel passage 11 that is discharged from substantially the center of the surface facing the electric body 6 and an oxidant that introduces an oxidant gas from the outer peripheral surface of the separator 8 and discharges it from the surface of the separator 8 facing the air electrode current collector 7. Each has a passage 12.

  Further, on the side of the fuel cell stack 1, a fuel manifold 15 that supplies fuel gas to the fuel passage 11 of each separator 8 through the connection pipe 13, and an oxidant gas through the connection pipe 14 to the oxidant passage 12 of each separator 8. An oxidant manifold 16 for supplying (air) is provided extending in the stacking direction of the power generation cells 5. Further, by interposing the joints 17 made of ceramics in the connection pipes 13 and 14, electrical insulation between the cells is ensured.

Here, the metal foam bodies 6 and 7 described above can be manufactured through the following steps. The order of the process is slurry preparation process → molding process → foaming process → drying process → degreasing process → sintering process.
First, in the slurry preparation step, a metal powder, an organic solvent (such as n-hexane), a surfactant (such as sodium dodecylbenzenesulfonate), a water-soluble resin binder (such as hydroxypropylmethylcellulose), a plasticizer (such as glycerin), A foam slurry is prepared by mixing water. This is formed into a thin plate shape on a carrier sheet by a doctor blade method in a forming step to obtain a green sheet. Next, in the foaming process, the green sheet is foamed in a sponge shape using the vapor pressure of the volatile organic solvent and the foaming property of the surfactant in a high-temperature and high-humidity environment, followed by a drying process, a degreasing process, A metal foam plate is obtained through a firing step. The thickness of the metal foam plate is about 1.6 mm.
In this case, in the foaming process, the bubbles generated inside the green sheet receive a substantially equivalent pressure from all directions and grow in a substantially spherical shape. When the bubbles diffuse from the inside and approach the interface with the atmosphere, the bubbles grow into a thin portion of the slurry between the bubbles and the atmosphere, eventually breaking the bubbles, and the gas inside the bubbles from the small holes created It diffuses into the atmosphere. Therefore, a metal foam plate having a three-dimensional network skeleton structure having continuous pores opened on the surface can be obtained.

In the present invention, as shown in FIG. 2, the internal reforming mechanism is configured by filling the fuel electrode current collector 6 with the hydrocarbon reforming catalyst 10 out of the foam metal body. As the hydrocarbon reforming catalyst 10, a composite material in which an Ni catalyst is supported on alumina powder is usually used, but Ru or the like can be used instead of Ni. As the alumina carrier, γ-alumina powder having a large surface area is preferable.
This Ni-supported alumina powder or Ru-supported alumina powder is used after being sized to a predetermined particle size with a mesh. The alumina powder is adhered in a state of being dispersed inside the pores 6a of the foam metal (that is, the skeleton surface).

  By the way, in the internal reforming type fuel cell, the temperature distribution is generated inside the power generation cell due to the endothermic reaction due to the reforming reaction. As described above, the destruction of the power generation cell and the deterioration of the battery performance due to the temperature distribution are problems. It has become.

FIG. 3B shows the temperature distribution of the power generation cell when the hydrocarbon reforming catalyst 10 is uniformly dispersed throughout the anode current collector 6, and FIG. 3B shows the hydrocarbon reforming. The amount of catalyst is shown, and (b) shows the temperature distribution.
The reforming reaction of the fuel gas is active at the beginning of the reaction and has a large endothermic amount, and the endothermic amount tends to decrease exponentially as the reforming reaction proceeds. Therefore, as shown in FIG. 2, when the fuel gas is supplied from the central portion of the separator 8, the amount of heat absorbed at the central portion of the anode current collector 6 on the fuel gas inlet side is large and approaches the fuel gas outlet side. As a result, the amount of heat absorbed decreases. As a result, as shown in the temperature distribution (b), the temperature of the central portion of the power generation cell is lower than that of the peripheral portion.

Therefore, in the present embodiment, as shown in FIG. 3A, the hydrocarbon reforming catalyst 10 is filled more from the central portion (upstream side) of the anode current collector 6 toward the peripheral portion (downstream side). I did it.
That is, since the steam reforming reaction that causes a temperature drop is likely to occur at the center of the anode current collector 6, the reforming reaction is suppressed by reducing the filling amount of the hydrocarbon reforming catalyst 10, and the anode current collecting is performed. The steam reforming reaction inside the current collector is uniformly performed by increasing the filling amount of the hydrocarbon reforming catalyst 10 toward the periphery of the body 6 and activating the reforming reaction. . Thereby, generation | occurrence | production of the temperature distribution in the electric power generation cell 5 can be suppressed, the temperature distribution inside an electric power generation cell can be equalize | homogenized, and current density distribution can be equalized.

  In the stack 1 configured by stacking a plurality of single cells having the above-described configuration, the oxidant gas (air) supplied from the outside passes through the oxidant manifold 16 and is connected to the oxidant passages 12 of the separators 8 from the plurality of connection pipes 14. In the process of moving through the current collector 7 while diffusing, the air in the power generation cell 5 is discharged from the discharge hole 12a of the oxidant gas at the end of the passage and supplied to the facing air electrode current collector 7. The extreme layer 4 is reached.

On the other hand, fuel gas (mixed gas of CH ₄ and high-temperature steam) supplied from the outside is introduced into the fuel passages 11 of the separators 8 from the plurality of connecting pipes 13 through the fuel manifold 15, and the fuel gas at the end of the passages is introduced. The fuel is discharged from the discharge hole 11a and supplied to the facing fuel electrode current collector 6.
Here, in the process in which the fuel gas diffuses and moves in the anode current collector 6, it comes into contact with the hydrocarbon reforming catalyst 10 carried in the holes 6 a of the current collector 6, and the reforming reaction is started. . In the present embodiment, as shown in FIG. 3A, the fuel gas inlet portion has a small filling amount of the hydrocarbon reforming catalyst 10, so that the reforming reaction is suppressed and the endothermic amount is also reduced. On the other hand, since the filling amount of the hydrocarbon reforming catalyst 10 increases from the central portion of the anode current collector 6 toward the peripheral portion, unreacted fuel gas that has not been subjected to the reforming reaction diffuses and moves to the peripheral portion. It is activated in the process of gradually increasing the endothermic amount.
As a result, as shown in (b) of FIG. 3A, the temperature distribution inside the power generation cell can be made uniform, and the current density distribution can be leveled. In addition, the thermal stress generated inside the cell due to the temperature distribution can be prevented, and the power generation cell 5 can be prevented from being deteriorated or broken.

  Note that the reforming reaction inside the anode current collector 6 is as described in the section of the prior art, and description thereof is omitted here, but the fuel gas is changed to hydrogen and carbon monoxide by this reforming reaction. Quality. The reformed gas reaches the fuel electrode layer 3 of the power generation cell 5 from the fuel electrode current collector 6, and the reforming reaction is performed again on the fuel electrode.

  As described above, in the present invention, the internal reforming mechanism is simplified as a structure in which the hydrocarbon reforming catalyst 10 is supported in the holes 6a of the anode current collector 6, thereby enabling efficient and stable internal reforming power generation. Yes.

The disassembled perspective view which shows the structure of the fuel cell stack to which this invention was applied. Sectional drawing which shows the internal structure of a single cell. The figure which shows the temperature distribution inside a power generation cell.

Explanation of symbols

2 Solid electrolyte layer 3 Fuel electrode layer 4 Oxidant electrode layer (air electrode layer)
6 Fuel electrode current collector 7 Oxidant electrode current collector (air electrode current collector)
8 Separator 10 Hydrocarbon reforming catalyst

Claims (3)

A fuel electrode layer and an oxidant electrode layer are arranged on both sides of the solid electrolyte layer, and a fuel electrode current collector and an oxidant electrode current collector made of foam metal are arranged outside the fuel electrode layer and the oxidant electrode layer, respectively. A separator is disposed outside the fuel electrode current collector and the oxidant electrode current collector, and the fuel electrode layer and the oxidant electrode layer are passed from the separator through the fuel electrode current collector and the oxidant electrode current collector. In a solid oxide fuel cell that supplies fuel gas and oxidant gas to
A solid oxide fuel cell, wherein the fuel electrode current collector is filled with a hydrocarbon reforming catalyst, and the amount of the reforming catalyst is increased from the upstream side to the downstream side of the fuel gas. The fuel gas and the oxidant gas are supplied from the central portion of the separator to the fuel electrode layer and the oxidant electrode layer via the fuel electrode current collector and the oxidant electrode current collector; and 2. The solid oxide fuel cell according to claim 1, wherein a filling amount of the hydrocarbon reforming catalyst is increased from a central portion to a peripheral portion of the fuel electrode current collector. 3. The solid oxide according to claim 1, wherein the foam metal has a three-dimensional skeleton structure, and the hydrocarbon reforming catalyst is supported on the skeleton surface. Fuel cell.

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