JP2003263997A - Solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve power generation efficiency of a solid oxide fuel cell. <P>SOLUTION: The solid oxide fuel cell includes a power generating cell 1 with a fuel electrode layer 4 disposed on one face of a solid electrolyte layer 3 and with an air electrode layer 2 arranged on the other face of the layer 3, wherein the solid electrolyte layer 3 has a first electrolyte layer 3a comprising a ceria based oxide material and a second electrolyte layer 3b comprising a lanthanum gallate based oxide material forming a two-layer structure. The second electrolyte layer 3b is formed at the side of the air electrode layer 2, by which contact state of the solid electrolyte layer 3 to each electrode layer 2, 4 is improved and internal resistance of each electrode layer at an interface can be reduced, so that the power generating cell 1 of high electromotive force with reduced IR loss can be achieved. <P>COPYRIGHT: (C)2003,JPO

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 having a power generating cell in which a fuel electrode layer is arranged on one surface of a solid electrolyte layer and an air electrode layer is arranged on the other surface. In particular, the present invention relates to the structure of a solid electrolyte layer that improves the power generation performance of a power generation cell.

[0002]

2. Description of the Related Art A solid oxide fuel cell having a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer (oxidant electrode layer) and a fuel electrode layer is a third generation type. Development is progressing as a fuel cell for power generation. In the solid oxide fuel cell, oxygen (air) is supplied to the air electrode side and fuel gas (H ₂ , CO, etc.) is supplied to the fuel electrode side. Both the air electrode and the fuel electrode are made porous so that the gas can reach the interface with the solid electrolyte.

Oxygen supplied to the air electrode reaches the vicinity of the interface with the solid electrolyte layer through the pores in the air electrode layer, and at this portion, electrons are received from the air electrode and an oxide ion (O ^{2 -} ) Is ionized. 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 and react with the reaction products (H ₂ O, CO _{2) in} this portion.
Etc.) and emit electrons to the fuel electrode.

The electrode reaction when hydrogen is used as the fuel is as follows. Air electrode: 1/2 O ₂ + 2e ⁻ → O ^{2 −} Fuel electrode: H ₂ + O ² → → H ₂ O + 2e ⁻ Overall: H ₂ + 1/2 O ₂ → H ₂ O

FIG. 2 shows the internal structure of a power generation cell 1 in a conventional solid oxide fuel cell. In the figure, reference numeral 2 is an air electrode (cathode) layer, reference numeral 3 is a solid electrolyte layer, and reference numeral 4 is a fuel electrode. The air electrode layer 2 and the fuel electrode layer 4 are disposed on both sides so as to sandwich the solid electrolyte layer 3 which is an (anode) layer.

Here, both the air electrode layer 2 and the fuel electrode layer 4 must be made of a material having a high electron conductivity. Since the air electrode material must be chemically stable in an oxidizing atmosphere at a high temperature of about 700 ° C., a metal is not suitable, and a perovskite type oxide material having electronic conductivity, specifically, LaMnO _{3 is used.} Or LaCoO ₃ , or
A solid solution obtained by substituting a part of La for Sr, Ca or the like is generally used. The fuel electrode material is Ni, C
Metals such as o or cermets such as Ni-YSZ and Co-YSZ are generally used.

On the other hand, the solid electrolyte layer 3 is a moving medium for oxide ions and at the same time functions as a partition wall for preventing the fuel gas and air from coming into direct contact with each other, so that a gas-impermeable and dense structure is formed. There is. The solid electrolyte layer 3 has a high oxide ion conductivity, and is made of a material that is chemically stable and resistant to thermal shock under the conditions from the oxidizing atmosphere on the air electrode layer side to the reducing atmosphere on the fuel electrode layer side. A yttria-stabilized zirconia (YSZ) that exhibits relatively high oxide ion conductivity at high temperatures is a material that needs to meet such requirements.
Is generally used, but with the recent trend toward lower operating temperatures of solid oxide fuel cells, an inexpensive ceria-based oxide that is slightly weak in high-temperature / reducing atmospheres but exhibits excellent electrical conductivity at low temperatures. Material materials (samarium-added ceria) are being used. Also, Japanese Patent Laid-Open No. 2001-52722
Discloses a solid oxide fuel cell using a lanthanum gallate oxide material having high oxide ion conductivity as the solid electrolyte layer. As described above, the material and the like of the solid electrolyte layer have reached the present while much research and improvement have been made.

[0008]

By the way, a power generation cell using yttria-stabilized zirconia or samarium-added ceria as a solid electrolyte layer has a drawback that the internal resistance on the air electrode side becomes large, and especially samarium-added ceria is used. In this case, as described above, excellent electrical characteristics are exhibited at low temperatures, but the fact that the electron-oxide mixed conductor is low in the proportion of oxide ion conductivity is a cause of increasing the internal resistance. In addition, in a power generation cell using a lanthanum gallate-based oxide material as the solid electrolyte layer, the internal resistance on the fuel electrode side tends to increase, contrary to the above case. There was a drawback that it was expensive. In any case, if the internal resistance is high, IR loss increases and efficient power generation cannot be expected.

The present invention has been made in view of the above-mentioned conventional problems.
It is an object of the present invention to provide a solid oxide fuel cell including an inexpensive solid electrolyte layer that reduces the contact resistance at the interface between the solid electrolyte layer and each electrode and improves power generation efficiency.

[0010]

That is, according to the present invention as set forth in claim 1, the fuel electrode layer (4) is arranged on one surface of the solid electrolyte layer (3), and the air electrode layer (4) is arranged on the other surface. In the solid oxide fuel cell provided with the power generation cell (1) in which 2) is arranged, the solid electrolyte layer (3) includes a first electrolyte layer (3a) made of a ceria-based oxide material and a lanthanum gallate. A two-layer structure of a second electrolyte layer (3b) made of a system oxide material,
The second electrolyte layer (3b) is formed on the side of the air electrode layer (2).

According to a second aspect of the present invention, in the solid oxide fuel cell according to the first aspect, the thickness of the first electrolyte layer (3a) is smaller than that of the second electrolyte layer (3b). It is characterized by forming.

In the present invention, the contact resistance at the interface with the fuel electrode (4) is reduced by using the ceria-based oxide material for the first electrolyte layer (3a), and the second electrolyte layer (3b) is made high. The contact resistance at the interface with the air electrode (2) is reduced by using a lanthanum gallate-based oxide material exhibiting oxide ion conductivity. As a result, the internal resistance of the power generation cell is reduced and the power generation characteristics are improved. Further, the cost of the power generation cell can be reduced by forming the second electrolyte layer (3b) thin and reducing the amount of expensive lanthanum gallate-based oxide material.

[0013]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a cross-sectional view showing the internal structure of a power generation cell 1 (single cell) in a solid oxide fuel cell.

As shown in FIG. 1, the power generation cell 1 of the present embodiment has a porous air electrode layer 2 which is in contact with air, a solid electrolyte layer 3 which is a medium for transporting oxide ions, and hydrogen gas and the like. The solid electrolyte layer 3 is sandwiched between the air electrode layer 2 and the fuel electrode layer 4 so as to sandwich the solid electrolyte layer 3 therebetween.

The air electrode layer 2 is a perovskite type oxide material having electron conductivity, specifically LaMnO ₃ or LaCoO ₃ , or a part of these La is S.
The fuel electrode layer 4 is made of a solid solution substituted with r, Ca or the like, and the fuel electrode layer 4 is made of a metal such as Ni or Co, or Ni-YSZ, C.
It is composed of a cermet or the like such as o-YSZ. Also,
Unlike the conventional structure, the solid electrolyte layer 3 has a two-layer electrolyte structure including a first electrolyte layer 3a and a second electrolyte layer 3b. The first electrolyte layer 3a in contact with the fuel electrode layer 4 is an inexpensive samarium-added ceria (CeSmO ₂ ) which is an electron-oxide ion mixed conductor exhibiting excellent electrical conductivity at low temperature.
And a second electrolyte layer 3 in contact with the cathode layer 2
For b, a lanthanum gallate-based oxide material having high oxide ion conductivity is used.

In the present embodiment, as shown in the figure, the inexpensive first electrolyte layer 3a is made thick and the relatively expensive second electrolyte layer 3b is made thin. Incidentally, it is preferable that the thickness of the second electrolyte layer 3b is about 30 to 100 μm while the thickness of the first electrolyte layer 3a is 100 to 400 μm. If the thickness of the second electrolyte layer 3b is 30 μm or less, it may not be possible to function as an electrolyte layer, and if it is 100 μm or more, the following IR loss problem and cost problem occur.

The so-called discharge reaction of the fuel cell proceeds irreversibly, and as the current drawn to the outside increases, the irreversibility increases and the cell voltage decreases. The maximum voltage obtained in an actual fuel cell decreases as the irreversibility increases with equilibrium electromotive force. Such a voltage drop is due to the diffusion transfer resistance of electrons in the fuel electrode layer and the air electrode layer, in other words, due to the ionization reaction rate of oxide ions,
The deviation of the electrode potentials of the air electrode layer and the fuel electrode layer from the equilibrium potential is called overvoltage (polarization). Also, electrons flow to each electrode, ions flow to the electrolyte, and current flows to the outside.
At this time, a voltage loss (IR loss) corresponding to the product of the current flowing in the battery, the contact resistance between the solid electrolyte layer and each electrode layer, the electrical resistance of the electrode material and the solid electrolyte layer itself, etc. occurs, and this IR loss is It increases in proportion to the current drawn to the outside. All the energy of the difference between the potential converted from the thermal energy obtained by burning the fuel and the potential that can be taken out from the power generation cell is wastefully released from the power generation cell of the fuel cell as thermal energy. Therefore, the above-mentioned overvoltage of each electrode and the magnitude of IR loss inside the power generation cell greatly influence the power generation efficiency of the fuel cell.

In the present invention, the first electrolyte layer 3a is made of a ceria-based oxide material to reduce the contact resistance at the interface with the fuel electrode 4, and the second electrolyte layer 3b has a high oxide ion conductivity. By using the lanthanum gallate-based oxide material shown, the contact resistance at the interface with the air electrode 2 was reduced. this is,
It is considered that the compatibility of the electrode layers 2 and 4 and the material of the solid electrolyte layer in contact with the electrode layers is good, and the respective contact properties are improved. With such an electrolyte structure, the internal resistance of the power generation cell 1 is reduced, and a decrease in electromotive force due to IR loss is suppressed by that amount, and a high electromotive force can be obtained. Therefore, improvement in power generation characteristics can be expected.

Further, the cost of the power generation cell 1 can be reduced by reducing the thickness of the second electrolyte layer 3b to reduce the amount of expensive lanthanum gallate-based oxide material. Therefore, it is more cost effective to form the second electrolyte layer 3b as thin as possible without affecting the performance. Accordingly, the inexpensive first electrolyte layer 3a is thickened to form two layers so that the solid electrolyte layer 3 maintains a predetermined thickness.

[0020]

As described above, according to the present invention as set forth in claim 1, the solid electrolyte layer 3 comprises the first electrolyte layer made of the ceria-based oxide material and the solid electrolyte layer 3 made of the lanthanum gallate-based oxide material. Since the two-electrolyte layer has a two-layer structure and the second electrolyte layer is formed on the air electrode layer side, the contact between the solid electrolyte layer and each electrode layer is improved, and the internal resistance of each electrode layer at the interface can be reduced. This makes it possible to realize a high-electromotive force power generation cell with reduced IR loss.

According to the present invention as defined in claim 2,
Since the thickness of the first electrolyte layer is formed thinner than that of the second electrolyte layer, the cost of the power generation cell can be reduced by reducing the expensive lanthanum gallate-based oxide material.

[Brief description of drawings]

FIG. 1 is a sectional view showing an internal structure of a power generation cell to which the present invention is applied.

FIG. 2 is a cross-sectional view showing the internal structure of a conventional power generation cell.

[Explanation of symbols]

1 power generation cell 2 Air electrode layer 3 Solid electrolyte layer 3a First electrolyte layer 3b Second electrolyte layer 4 Fuel pole layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kei Hosoi             1002-14 Mukoyama, Naka-machi, Naka-gun, Ibaraki Prefecture             Terari Co., Ltd.             Inside (72) Inventor Takashi Yamada             1002-14 Mukoyama, Naka-machi, Naka-gun, Ibaraki Prefecture             Terari Co., Ltd.             Inside F term (reference) 5H026 AA06 EE13 HH03

Claims (2)

[Claims] 1. A solid having a power generation cell (1) comprising a fuel electrode layer (4) on one surface of a solid electrolyte layer (3) and an air electrode layer (2) on the other surface. In the oxide fuel cell, the solid electrolyte layer (3) comprises two layers, a first electrolyte layer (3a) made of a ceria-based oxide material and a second electrolyte layer (3b) made of a lanthanum gallate-based oxide material. A solid oxide fuel cell having a structure, wherein the second electrolyte layer (3b) is formed on the air electrode layer (2) side. 2. The solid oxide fuel cell according to claim 1, wherein the first electrolyte layer (3a) is formed thinner than the second electrolyte layer (3b).