JPH06349504A - Solid electrolyte type fuel cell - Google Patents

Abstract

PURPOSE:To provide a solid electrolyte fuel cell which solves the generation of temperature distribution caused by the irregular vapor reforming reaction of fuel gas as endothermic reaction at a fuel electrode and prevents the deterioration of a cell and cell property due to temperature distribution. CONSTITUTION:All or part of the surface of a fuel cell 1 in contact wit fuel gas is covered with a gas diffused layer which is formed of material having no catalytic activity to the vapor reforming reaction of the fuel gas, the layer being thicher at the entrance of the fuel gas and gradually thinner toward the exit. The gas diffused layer 6 is preferably formed of ceramics identical with electrolyte or ceramics with the thermal expansion coefficient equal to that of the electrolyte, the fuel electrode 1 or an oxidant electrode 3.

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 capable of internally reforming a fuel gas and having a small temperature distribution due to a reforming reaction.

[0002]

2. Description of the Related Art Solid oxide fuel cells generally contain oxygen.
Using a solid substance having ion conductivity as an electrolyte,
It is used at high temperature around 00 ℃. Therefore, the raw materials used
Is limited, and not only the electrolyte but also most of each electrode, current collector, etc.
Solid materials are used for all of them, for example, electrolytes
Toria (Y₂O₃) Mixed zirconia (ZrO₂) (Hereafter, YSZ and
Abbreviated), the fuel electrode has nickel and YSZ cermet.
LaSrO for the oxidizer electrode ₃And LaCoO₃Oxygen ion transmission
A perovskite oxide having electrical conductivity and electronic conductivity is used.
Is used.

In the solid oxide fuel cell, oxygen in the oxidizer receives electrons and is ionized on the surface of the oxidizer electrode by the reaction represented by the formula (1). The generated oxygen ions move in the solid electrolyte and release electrons according to equation (2) on the surface of the fuel electrode to become oxygen, and the generated oxygen produces hydrogen or one of the hydrogen obtained by steam reforming the fuel gas. It reacts with carbon oxide according to the equations (3) and (4) to generate water vapor or carbon dioxide. The electrons move from the fuel electrode to the oxidant electrode through the external circuit and are used as electric energy. Since the conductivity of oxygen ions in the solid electrolyte is higher as the temperature is higher, the solid oxide fuel cell is usually operated at a high temperature of 1000 ° C.

1/2 O ₂ + e ~ → O ~ (1) O ~ → 1/2 O ₂ + e ~ (2) CO + 1/2 O ₂ → CO ₂ (3) H ₂ + 1/2 O ₂ → H ₂ O (4) In order to perform steam reforming of fuel gas, a reformer can be installed separately from the solid oxide fuel cell.
There is a problem that a space for installing the reforming device is required, which hinders downsizing of the device and increases the cost of the device. In order to solve this problem, in recent years, a direct internal reforming method in which the fuel electrode directly performs steam reforming of the fuel gas has been studied. The direct internal reforming system is a system in which the fuel electrode, for example, Ni-YSZ electrode is used not only as an electrode necessary for the cell reaction but also as a catalyst for the steam reforming reaction to perform steam reforming of the fuel gas, So far, it has been reported that fuel gas such as methane can be steam reformed on the Ni-YSZ electrode.

FIG. 3 shows a schematic structure of a conventional solid oxide fuel cell. In the direct internal reforming system, a mixed gas 6 of fuel and steam is supplied to the fuel electrode 1 to cause a steam reforming reaction on the fuel electrode 1, and hydrogen or carbon monoxide generated by the steam reforming reaction is generated. The fuel cell is caused to generate power by reacting with oxygen by the above-mentioned cell reaction.

[0006]

However, in this case, the steam reforming reaction of the fuel gas does not occur uniformly on the surface of the fuel electrode, and the reaction amount of the fuel gas is large near the fuel gas inlet of the fuel electrode. It decreases as it goes to the fuel gas outlet. On the other hand, it is known that the steam reforming reaction of fuel gas such as methane is an endothermic reaction. Therefore, when the direct internal reforming method is applied to a conventional solid oxide fuel cell, the temperature distribution as shown in FIG. 4 is generated inside the cell due to the progress of the steam reforming reaction which is an endothermic reaction. The cell internal temperature is 1000 near the rapidly advancing fuel gas inlet.
It will drop rapidly from ℃ to 500 ℃. When such a temperature distribution is generated inside the cell, the solid oxide fuel cell formed of ceramic is destroyed due to the difference in thermal expansion, or the temperature in the solid electrolyte is locally reduced. Since the conductivity of oxygen ions is lowered, there is a problem that the battery performance is also lowered.

An object of the present invention is to generate a temperature distribution caused by the nonuniform heat reforming reaction of the fuel gas, which is an endothermic reaction at the fuel electrode, in the conventional solid oxide fuel cell as described above. A solution is to provide a solid oxide fuel cell that can undergo internal reforming and that does not cause cell deterioration or cell characteristic deterioration due to temperature distribution.

[0008]

The above object is to provide a solid electrolyte fuel cell in which an oxidant electrode and a fuel electrode are arranged via an electrolyte, and a fuel gas and an oxidant gas are supplied to generate electric power. A solid oxide fuel cell characterized in that a part or all of the surface of the fuel electrode in contact with the fuel cell is covered with a gas diffusion layer composed of a substance having no catalytic activity for the steam reforming reaction of the fuel gas. Can be achieved by

Further, a more effective result can be obtained by making the gas diffusion layer on the surface of the fuel electrode thicker on the fuel gas inlet side and thinner on the fuel gas outlet side. The gas diffusion layer is preferably made of the same ceramic as the electrolyte, or ceramics having the same coefficient of thermal expansion as the electrolyte, the fuel electrode or the oxidizer electrode.

[0010]

The present invention, on the surface of the fuel electrode on the side of the fuel gas passage,
The main feature is that a gas diffusion layer for restricting the arrival of the fuel gas at the fuel electrode is provided.With such a configuration, the steam reforming reaction of the fuel gas at the fuel electrode is prevented. Since it occurs uniformly, the temperature distribution in the cell is less likely to occur, and the deterioration and deterioration of the characteristics of the fuel cell due to internal reforming will not occur.

[0011]

EXAMPLES The constitution of the solid oxide fuel cell of the present invention will be specifically described below with reference to examples.

FIG. 1 shows the cell structure of an embodiment of the solid oxide fuel cell of the present invention. 3 is different from the conventional solid oxide fuel cell shown in FIG. 3 in that a gas diffusion layer is provided on the fuel gas flow path side surface of the fuel electrode 1. The thickness of the fuel electrode 1 and the solid electrolyte 2 is 100 μm, and the thickness of the oxidizer electrode is 5 mm.
And The thickness of the gas diffusion layer 6 is thicker on the fuel gas inlet side and thinner toward the fuel gas outlet side.
It was set to m to 0 μm. The electrode area was 25 cm ² . Further, Ni-YSZ cermet containing 50% nickel was used for the fuel electrode 1, YSZ was used for the solid electrolyte 2 and the gas diffusion layer 6, and LaSrO ₃ was used for the oxidizer electrode 3. Methane was used as the fuel gas, and the supply rate was 400 cc / min. Further, the supply amount of water vapor was 1200 cc / min. On the other hand, air was used as the oxidant gas 9, and the supply rate was 2500 cc / min. The mixed gas 7 of fuel gas and water vapor and the oxidant gas 9 were preheated to 1000 ° C. in advance and then supplied to the cell. Power was generated at 0.4 W / cm ² . By providing the gas diffusion layer 6 on the surface of the fuel electrode 1 and controlling the amount of the fuel gas reaching the fuel electrode 1, the steam reforming reaction of the fuel gas can be uniformly performed in the fuel electrode 1. As shown in, it became possible to prevent the temperature distribution from occurring inside the cell. The cell internal temperature was constant at 1000 ° C.

As a material of the gas diffusion layer 6, a material having a thermal expansion coefficient close to that of other constituent members of the cell and having no catalytic activity for the steam reforming reaction of the fuel gas is suitable. As an example, YSZ is used. Can be mentioned. The amount of the fuel gas reaching the fuel electrode 1 was controlled by adjusting the thickness of the gas diffusion layer. In addition to the examples of the present invention, the amount of fuel gas reaching can be controlled by adjusting the pore size of the gas diffusion layer. At the fuel gas inlet of the fuel electrode 1, a steam reforming reaction that causes a temperature decrease is likely to occur. Therefore, the thickness of the gas diffusion layer 6 is increased to reduce the amount of fuel gas reaching the fuel electrode 1 and reduce the fuel gas outlet. Since the unreacted fuel gas decreases as the fuel electrode 1 approaches, it is necessary to make the gas diffusion layer thin so that the fuel gas can easily reach the fuel electrode 1 so that the steam reforming reaction is performed uniformly and It is effective in suppressing the occurrence of temperature distribution.

For the gas diffusion layer used in the solid oxide fuel cell of the present invention, the conventional manufacturing techniques of fuel electrode, oxidizer electrode, and solid electrolyte, such as sputtering, thermal spraying, and integral sintering with a green sheet, are used. By doing so, it can be easily formed on the fuel electrode surface.

[0015]

As described above, by using a solid oxide fuel cell as a fuel cell having the constitution of the present invention, it is possible to solve the problems of the prior art and to perform internal reforming. Moreover, it was possible to provide a solid oxide fuel cell having excellent characteristics without causing deterioration of cells and deterioration of cell characteristics due to generation of temperature distribution.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a cell structure of an embodiment of a solid oxide fuel cell of the present invention.

FIG. 2 is a diagram showing the temperature distribution inside the cell between the fuel gas inlet and outlet of the solid oxide fuel cell of the present invention.

FIG. 3 is a schematic cross-sectional view showing a cell configuration of a conventional solid oxide fuel cell.

FIG. 4 is a view showing a temperature distribution inside a cell between a fuel gas inlet and an outlet of a conventional solid oxide fuel cell.

[Explanation of symbols]

1 ... Fuel electrode, 2 ... Solid electrolyte, 3 ... Oxidizer electrode, 4 ...
Fuel gas channel, 5 ... Oxidant gas channel, 6 ... Gas diffusion layer,
7 ... mixed gas of fuel and water vapor, 8 ... fuel electrode exhaust gas, 9
… Oxidizer gas, 10… Oxidizer electrode exhaust gas.

Claims (3)

[Claims] 1. A solid oxide fuel cell in which an oxidant electrode and a fuel electrode are disposed with an electrolyte interposed between them, and a fuel gas and an oxidant gas are supplied to generate electric power. A solid oxide fuel cell, characterized in that a part or all thereof is covered with a gas diffusion layer composed of a substance having no catalytic activity for a steam reforming reaction of fuel gas. 2. The thickness of the gas diffusion layer on the surface of the fuel electrode is
2. The solid oxide fuel cell according to claim 1, wherein the solid electrolyte fuel cell is thicker on the fuel gas inlet side and thinner toward the fuel gas outlet side. 3. The solid electrolyte fuel according to claim 1, wherein the gas diffusion layer is made of the same ceramics as the electrolyte or a ceramic having the same coefficient of thermal expansion as the electrolyte, the fuel electrode or the oxidizer electrode. battery.