WO2010095728A1

WO2010095728A1 - Electrolyte electrode assembly and method for producing the same

Info

Publication number: WO2010095728A1
Application number: PCT/JP2010/052583
Authority: WO
Inventors: Jun Mizuno
Original assignee: Honda Motor Co., Ltd.
Priority date: 2009-02-19
Filing date: 2010-02-16
Publication date: 2010-08-26
Also published as: JP2010192288A

Abstract

The present invention relates to an electrolyte electrode assembly (18) containing an anode (12) of a first electrode and a cathode (14) of a second electrode with a solid electrolyte (16) interposed between the first electrode and the second electrode, as well as to a method for producing the same. For example, the thickness of an intermediate layer (20) is controlled depending on the average pore diameter of the anode (12). Thus, the intermediate layer (20) is formed such that the thickness of the intermediate layer (20) is equal to or greater than the average pore diameter of the anode (12), and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 µm.

Description

DESCRIPTION

Title of Invention

ELECTROLYTE ELECTRODE ASSEMBLY AND METHOD FOR PRODUCING THE SAME

Technical Field

The present invention relates to an electrolyte electrode assembly having first and second electrodes with a solid electrolyte interposed between the first and second electrodes, as well as to a method for producing the same. The first electrode acts as either an anode or a cathode, whereas the second electrode acts as the other.

Background Art

Fuel cells have an electrolyte electrode assembly produced by interposing an electrolyte between an anode and a cathode. The electrolyte may be a solid electrolyte such as a solid oxide. A fuel cell that uses such a solid electrolyte is referred to as a solid electrolyte fuel cell.

As is well known, fuel cells produce electric power when a fuel gas (such as hydrogen gas) is supplied to the anode and an oxidant gas (such as air) is supplied to the cathode. The fuel gas is diffused in the anode and reaches the electrolyte, whereas the oxidant gas is diffused in the cathode and reaches the electrolyte. Therefore, the anode and cathode each have a structure with pores for facilitating diffusion of the fuel or oxidant gas . For example, this type of structure may be a porous body. In the solid electrolyte fuel cell, on the anode. hydrogen molecules in the fuel gas are ionized to generate protons and electrons . Electrons are extracted from the cell and used as electric energy for energizing a load (such as a motor), which is connected electrically to the cell, and then the electrons reach the cathode .

Meanwhile, on the cathode, oxygen molecules in the oxidant gas are bonded to the electrons to generate oxide ions (O^2"). The oxide ions are transferred through the electrolyte to the anode, and become bonded to the protons on the anode, thereby generating water molecules (H2O) . Taken together, such reactions on the electrodes are generally referred to as electrode reactions.

More particularly, in the solid electrolyte fuel cell, for example, a component in the cathode may react with a component in the solid electrolyte to generate a high- resistance reaction phase during the electrode reactions . In this case, the internal resistance of the electrolyte electrode assembly is disadvantageously increased. To prevent this problem, Japanese Laid-Open Patent Publication No. 2003-173802 proposes a method, which comprises forming an intermediate layer as a reaction-preventing layer at least between the electrolyte and one of the electrodes .

Aside from this, solid electrolyte fuel cells are obviously disadvantageous in that the cell has a relatively high contact resistance between the electrolyte and the anode, and thus it is difficult to lower internal resistance. Thus, a method, which comprises disposing a mixed conductor having both ion conductivity and electron conductivity (such as a TiO₂- or CeO₂-based oxide) between the electrolyte and the anode, has been studied in the related art, as described in Japanese Laid-Open Patent Publication No. 11-073982. This patent publication describes that by forming a layer made up of such a mixed conductor, adhesion between the electrolyte and the anode is improved so as to lower contact resistance.

In addition, Japanese Laid-Open Patent Publication No. 06-295730 describes a technique of using an anode having a bilayer structure, which contains a mixture of nickel (Ni) and yttria- stabilized zirconia (YSZ) in an upper layer thereof facing the electrolyte, for increasing conductivity in order to lower internal resistance.

Summary of the Invention

In each of the above related arts , an intermediate layer is formed between the electrode and electrolyte.

However, compared to the electrodes, the intermediate layer has a higher resistance than the electrodes , whereby the formation of the intermediate layer itself increases the internal resistance of the electrolyte electrode assembly. As described above, gas is diffused through each electrode. Therefore, the intermediate layer should be formed in such a manner that gas diffusion is not inhibited by the intermediate layer.

Thus , the intermediate layer is required to have a low resistance, allow gas diffusion readily, and be firmly connected to both the electrolyte and electrode. Such requirements have not been comprehensively examined in the conventional known technologies .

A general object of the present invention is to provide a particular electrolyte electrode assembly having properties related to the physical property values of an intermediate layer.

A principal object of the present invention is to provide an electrolyte electrode assembly having low resistance.

Another object of the present invention is to provide an electrolyte electrode assembly, which can be made compact .

A further object of the present invention is to provide a method for producing the above electrolyte electrode assembly.

According to an aspect of the present invention, there is provided an electrolyte electrode assembly comprising a first electrode and a second electrode with a solid electrolyte interposed between the first electrode and the second electrode, the first electrode acting as one of an anode and a cathode, and the second electrode acting as the other of the anode and the cathode , wherein : the first electrode comprises a porous body with pores therein for facilitating passage of gas therethrough, and a concave portion and a convex portion on a surface facing the solid electrolyte, an intermediate layer is disposed between the first electrode and the solid electrolyte such that the concave portion is filled with the intermediate layer and the convex portion is embedded in the intermediate layer, a thickness of the intermediate layer is equal to or greater than the average pore diameter of the first electrode, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm. Thus, in the present invention, the thickness of the intermediate layer is equal to or slightly greater than the average pore diameter of the first electrode. Therefore, the intermediate layer does not have an excessively large thickness, so that the internal resistance (i.e., IR loss) of the electrolyte electrode assembly does not increase as a result of the intermediate layer.

Furthermore, since the intermediate layer has a small thickness, the electrolyte electrode assembly, and therefore the fuel cell, etc., can be prevented from increasing in size.

In the first electrode, the pore opening on the surface facing the solid electrolyte (i.e., the concave portion) is filled with the intermediate layer, and a protrusion on the surface (i.e., the convex portion) is embedded within the intermediate layer. Therefore, even when the solid electrolyte on the intermediate layer has a small thickness , depressions and protrusions of the first electrode do not become embossed on the solid electrolyte, and the number of stress concentration areas on the solid electrolyte is reduced significantly, whereby the solid electrolyte can be prevented from cracking. Thus, the solid electrolyte can have a small thickness, ions can readily be transferred through the solid electrolyte, and the solid electrolyte has a low internal resistance.

Furthermore, the contact area between the first electrode and the intermediate layer and the contact area between the intermediate layer and the solid electrolyte both are increased, whereby the interface contact resistance between the first electrode and the intermediate layer and the interface contact resistance between the intermediate layer and the solid electrolyte both are lowered. The internal resistance of the electrolyte electrode assembly also is lowered due to this increase in the contact area. Further, since the contact areas are increased, the bonding strength between the first electrode and the intermediate layer and the bonding strength between the intermediate layer and the solid electrolyte are improved.

In addition, since the intermediate layer is small in thickness, the gas supplied to the first electrode is readily diffused in the intermediate layer. Therefore, electrode reactions are not inhibited, and non-IR losses are reduced.

In fact, in the present invention, the electrolyte electrode assembly can have excellent durability as well as exhibiting excellent electrical properties with low IR loss and low non-IR loss.

The first electrode preferably has an average pore diameter of 3 to 20 μm. In this case, the intermediate layer may have a thickness of 3 to 20 μm in view of the average pore diameter .

The porosity of the intermediate layer preferably is lower than that of the first electrode. In this case, the concave portion (the pore opening on the surface) of the first electrode can reliably be filled with the intermediate layer .

For example, when the first electrode has a porosity of 10% to 40% by volume before reduction, the intermediate layer preferably has a thickness of 3 to 20 μm. In the above configuration, the first electrode and the intermediate layer preferably are made of the same material. In this case, the interface resistance between the first electrode and the intermediate layer is lowered, and the intermediate layer is prevented from becoming cracked or peeling away from the first electrode during production of the electrolyte electrode assembly, because the shrinkage ratio of the intermediate layer and the first electrode match each other. Furthermore, even when the first electrode and the intermediate layer expand during driving of the fuel cell, the intermediate layer is prevented from becoming cracked or peeling away from the first electrode, because the expansion rate also matches .

A fuel cell containing a solid oxide or the like as the electrolyte is driven at a high temperature, and thus is required to be excellent in durability and to exhibit good electrical properties even at high temperatures . As described above, the electrolyte electrode assembly of the present invention is excellent in durability and exhibits excellent electrical properties, and can exhibit sufficient durability and good electrical properties even at high temperatures. Thus, in the present invention, a solid oxide may be used as the solid electrolyte, which is suitable for driving at high temperatures .

According to another aspect of the present invention, there is provided a method for producing an electrolyte electrode assembly comprising a first electrode and a second electrode with a solid electrolyte interposed between the first electrode and the second electrode, the first electrode acting as one of an anode and a cathode, and the second electrode acting as the other of the anode and the cathode, wherein the method comprises the steps of: forming the first electrode by tape casting, forming an intermediate layer on the first electrode by tape casting and pressure-bonding the intermediate layer to the first electrode, forming the solid electrolyte on the intermediate layer by tape casting and pressure-bonding the solid electrolyte to the intermediate layer, and subjecting at least the first electrode, the intermediate layer, and the solid electrolyte to a firing treatment , wherein the first electrode comprises a porous body with pores for facilitating passage of gas therethrough, and a concave portion and a convex portion on a surface facing the solid electrolyte, the intermediate layer is disposed between the first electrode and the solid electrolyte such that the concave portion is filled with the intermediate layer and the convex portion is embedded in the intermediate layer, a thickness of the intermediate layer is equal to or greater than the average pore diameter of the first electrode after sintering, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm.

The first electrode, the intermediate layer, and the solid electrolyte may be formed by screen printing.

According to a further aspect of the present invention, there is provided a method for producing an electrolyte electrode assembly comprising a first electrode and a second electrode with a solid electrolyte interposed between the first electrode and the second electrode, the first electrode acting as one of an anode and a cathode , and the second electrode acting as the other of the anode and the cathode, wherein the method comprises the steps of: forming the first electrode by screen printing, forming an intermediate layer on the first electrode by screen printing, forming the solid electrolyte on the intermediate layer by screen printing, and subjecting at least the first electrode, the intermediate layer, and the solid electrolyte to a firing treatment , wherein the first electrode comprises a porous body with pores therein for facilitating passage of gas therethrough, and a concave portion and a convex portion on a surface facing the solid electrolyte, the intermediate layer is disposed between the first electrode and the solid electrolyte such that the concave portion is filled with the intermediate layer and the convex portion is embedded in the intermediate layer, a thickness of the intermediate layer is equal to or greater than the average pore diameter of the first electrode after sintering, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm.

As described above, when the thickness of the intermediate layer is controlled in this manner in view of the average pore diameter of the first electrode, both IR and non-IR losses of the electrolyte electrode assembly are lowered, and gas is readily diffused throughout the intermediate layer. The obtained electrolyte electrode assembly exhibits superior electrical properties, and the bonding strength between the first electrode and the intermediate layer is excellent.

The concave portion of the first electrode is filled with the intermediate layer, whereas the convex portion is embedded within the intermediate layer, and the solid electrolyte is formed on the intermediate layer . In this case, even when the solid electrolyte has a remarkably small thickness, the solid electrolyte can still be formed as a flat layer. Further, the obtained solid electrolyte has only a small number of stress concentration areas, and thus the solid electrolyte can be prevented from cracking during the sintering process. Consequently, the electrolyte electrode assembly has excellent durability against cracking . When the first electrode, the intermediate layer, and the solid electrolyte are formed by screen printing, a preliminary firing treatment may be carried out after at least one of them has been formed. Thus, the preliminary firing treatment may be carried out after at least one of the steps of forming the first electrode, forming the intermediate layer, and forming the solid electrolyte.

When a material for the first electrode contains 10% to 30% by weight of a pore-forming agent therein based on 100% by weight of the inorganic content , and the firing treatment is carried out within a range of 1200 ⁰C to 1500 ⁰C in order to sinter the material, the porosity of the first electrode can easily be controlled within a range of 10% to 40% by volume prior to reduction. When the first electrode is used as an anode, examples of the inorganic components include YSZ and NiO. For example, when the ratio of a pore-forming agent to an inorganic component in the material for the intermediate layer is lower than that in the material for the first electrode, the porosity of the intermediate layer can be made lower than that of the first electrode. The material for the intermediate layer may also be free of any pore- forming agent .

In the above production methods , when the first electrode and the intermediate layer are made of the same material, the intermediate layer is prevented from becoming cracked or peeling away from the first electrode.

Furthermore, when the solid electrolyte comprises a solid oxide, the resultant electrolyte electrode assembly can be driven at high temperatures . As described above, in the present invention, the thickness of the intermediate layer is controlled in view of the average pore diameter of the first electrode. Therefore, the intermediate layer is not excessively large in thickness, and the contact area between the first electrode and the intermediate layer is increased to improve adhesion therebetween. As a result of this improvement, the internal resistance of the electrolyte electrode assembly is lowered, so that an increase in IR loss is prevented.

Furthermore, since the intermediate layer is small in thickness, the gas supplied to the first electrode is readily diffused within the intermediate layer. Therefore, electrode reactions are not inhibited, and non-IR losses are reduced.

In the first electrode, the pore opening on the surface facing the solid electrolyte is filled with the intermediate layer, and the protrusion on the surface is embedded within the intermediate layer. Therefore, even though the solid electrolyte on the intermediate layer has a small thickness, the depressions and protrusions of the first electrode do not become embossed on the solid electrolyte, and the number of stress concentration areas on the solid electrolyte is reduced significantly, whereby the solid electrolyte can be prevented from cracking. Thus, the resultant electrolyte electrode assembly is excellent in durability. In addition, since the intermediate layer does not have an excessively large thickness, the electrolyte electrode assembly can be made smaller in size.

Brief Description of the Drawings FIG. 1 is a schematic longitudinal sectional view showing a power generating fuel cell containing an electrolyte electrode assembly according to an embodiment of the present invention;

FIG. 2 is an enlarged explanatory view showing a principal part of FIG. 1;

FIG. 3 is a schematic plan view for explaining the definition of pore diameter for calculating average pore diameter;

FIG. 4 is a graph showing the relation between the intermediate layer thickness and loss, including both IR loss and non-IR loss;

FIG. 5 is a graph showing the relation between the ratio of a pore-forming agent to inorganic components in an anode material and the anode porosity before reduction; FIG. 6 is a general flowchart of a production method for an electrolyte electrode assembly according to the embodiment ;

FIG. 7 is a general flowchart of a first production method using green sheets (tapes) for forming an anode side electrode as a first electrode, the intermediate layer, and the solid electrolyte; and

PIG. 8 is a general flowchart of a second production method using screen printing for forming an anode side electrode as a first electrode, the intermediate layer, and the solid electrolyte.

Description of the Embodiments

A preferred embodiment of an electrolyte electrode assembly, as well as a method for producing the electrolyte electrode assembly according to the present invention, will be described in detail below with reference to the accompanying drawings . In the following descriptions , all of the porosity values are obtained by an Archimedes method or by a mercury intrusion method, before reduction of an inorganic component in the anode, for example, before reduction of nickel oxide (NiO) to nickel (Ni) .

FIG. 1 is a schematic longitudinal sectional view showing a power generation fuel cell containing an electrolyte electrode assembly according to the present embodiment. The power generation cell 10 contains an electrolyte electrode assembly 18 formed by connecting an anode 12, a cathode 14, and a solid electrolyte 16 interposed between the anode 12 and the cathode 14. An intermediate layer 20 also is disposed between the anode 12 and the solid electrolyte 16. For example, the anode 12 comprises a cermet (a sintered body) of Ni and an oxide ion conductor such as YSZ, a scandia- stabilized zirconia (ScSZ), a samarium-doped ceria (SDC), or a gadolinium-doped ceria (GDC). The ratio of Ni to the oxide ion conductor preferably is 1:1 by weight . In the case that the electrolyte electrode assembly 18 is a so- called anode-supported-type assembly, which uses the anode 12 as a support, the thickness of the anode 12 may be approximately 300 μm to 1 mm, typically about 500 μm. As shown in FIG. 2, the anode 12 has both small pores 22 and large pores 24 therein. The small pores 22 are formed due to volume shrinkage, by reducing NiO particles to Ni in the material used for the anode 12. The large pores 24 are formed, for example, by removing a pore-forming agent, to be described later.

As shown in FIG. 3, the pore diameter of each of the small pores 22 and the large pores 24 is defined as a longest dimension a between both ends in the longitudinal direction of a two-dimensional section. The small pores 22 generally have a pore diameter of 0.5 to 1 μm, whereas the large pores 24 generally have a pore diameter of 3 to 20 μm. In the present embodiment , the average pore diameter of the pores is calculated without including the pore diameters of the small pores 22. Thus, in the present embodiment, pores having a longest dimension a (shown in FIG. 3) of more than 1 μm are extracted as the large pores 24, whereby a sum of the pore diameters of the large pores 24 is obtained. This sum is divided by the number of large pores 24 in order to obtain the average pore diameter. This calculation may practically be carried out using an arbitrary section observed by a scanning electron microscope (SEM) .

In the case of forming the large pores 24 by removing the pore-forming agent as described above, the average pore diameter of the large pores 24 can be controlled by changing the average particle diameter of the pore-forming agent. Furthermore, a narrow pore diameter distribution for the large pores 24 can be obtained by narrowing the particle diameter distribution of the pore-forming agent. For example, when a pore-forming agent having an average particle diameter of 8 μm or 20 μm is used, the large pores 24 of the anode 12 generally have an average pore diameter of 4 to 6 μm or 12 to 14 μm, respectively.

The porosity of the anode 12 can be controlled by changing the amount of the pore-forming agent. The porosity may be obtained by a mercury intrusion method, which involves filling all the pores including the small pores 22 and the large pores 24 with mercury.

As described below, when 10% to 30% by weight of the pore-forming agent is added to the inorganic components in the material for the anode 12, the anode 12 generally has a porosity of 10% to 40% by volume.

In the present embodiment, the intermediate layer 20 adjacent to the anode 12 (see FIG. 2) comprises the same material as the anode 12, such as a cermet containing Ni and YSZ at a weight ratio of 1:1. Further, in the present embodiment, the porosity of the intermediate layer 20 is lower than that of the anode 12. More specifically, the porosity of the intermediate layer 20 may be 0% to 35%, typically 0% to 30%, by volume. Thus, the anode 12 and the intermediate layer 20 comprise the same materials with the same composition ratio, however, the materials of the anode 12 and the intermediate layer 20 have different porosities. The thickness of the intermediate layer 20 is defined as a distance D (see FIG. 2) from the bottom of the large pores 24 opening on the surface of the anode 12 to the surface of the intermediate layer 20 facing the cathode 14. The thickness of the intermediate layer 20 is equal to or greater than the average pore diameter of the large pores 24. For example, when the large pores 24 have an average pore diameter of approximately 3 μm or 12 μm, the intermediate layer 20 has a thickness of at least 3 μm or 12 μm, respectively. In the case that the thickness of the intermediate layer 20 is equal to or greater than the average pore diameter of the large pores 24, the large pores 24 opening on the surface facing the solid electrolyte 16 (i.e., the concave portions of the anode 12) are reliably filled with the intermediate layer 20.

Convex portions 26, which protrude on the surface, are covered by and embedded within the intermediate layer 20. Thus, the concave portions are filled with the intermediate layer 20, whereas the convex portions 26 are embedded within the intermediate layer 20, as described above, whereby the surface of the anode 12 that faces the solid electrolyte 16 is made flat. The anode 12 is firmly attached to the solid electrolyte 16 by the intermediate layer 20 in the foregoing manner. Therefore, the interface contact resistance between the anode 12 and the intermediate layer 20, as well as the interface contact resistance between the intermediate layer 20 and the solid electrolyte 16, both are lowered. As a result , IR loss in a region from the anode 12 to the solid electrolyte 16 is reduced, whereby the internal resistance of the electrolyte electrode assembly is lowered. Furthermore, the contact area between the anode 12 and the intermediate layer 20, as well as the contact area between the intermediate layer 20 and the solid electrolyte 16, both are increased, whereby the bonding strength between the anode 12 and the intermediate layer 20 and the bonding strength between the intermediate layer 20 and the solid electrolyte 16 also are improved.

Since the intermediate layer 20 has a small thickness, fuel gas supplied to the anode 12 readily becomes diffused in the intermediate layer 20, so that electrode reactions proceed rapidly. Therefore, non-IR loss also is reduced. FIG. 4 is a graph showing the relationship between the thickness of the intermediate layer and loss, including both IR loss and non-IR loss. As is clear from FIG. 4, losses can be reduced to 0.33 V or less by optimizing the intermediate layer thickness. It is clear from the above that a satisfactory thickness of the intermediate layer 20 is such that the open large pores 24 (the concave portions) are filled with the intermediate layer 20, and the convex portions 26 are embedded in the intermediate layer 20. If the thickness of the intermediate layer 20 becomes excessively larger than the average pore diameter of the large pores 24, the total internal resistance of the electrolyte electrode assembly 18 may disadvantageously be increased. Thus, the thickness of the intermediate layer 20 preferably is controlled in view of the average pore diameter of the large pores 24 in the anode 12 .

More specifically, a difference calculated by subtracting the average pore diameter of the anode 12 from the thickness of the intermediate layer 20 is 0 to 17 μm. Thus, for example, when the large pores 24 in the anode 12 have an average pore diameter of approximately 3 μm, the intermediate layer 20 should preferably have a thickness of 3 to 20 μm.

As described above, when the thickness of the intermediate layer 20 is controlled in view of the average pore diameter of the large pores 24 in the anode 12, the electrolyte electrode assembly 18 is prevented from becoming excessively large in thickness. In other words, the internal resistance (IR loss) of the electrolyte electrode assembly 18 is not increased as a result of the intermediate layer 20, and the power generation cell 10 is prevented from becoming increased in size.

The thickness of the intermediate layer 20 may also be controlled in view of the porosity of the anode 12. Thus, in this case, the thickness of the intermediate layer 20 is selected depending on the porosity of the anode 12. As described above, when the thickness of the intermediate layer 20 is controlled in view of the porosity of the anode 12, the electrolyte electrode assembly 18 is prevented from becoming excessively large in thickness.

Therefore, also in this case, the internal resistance (IR loss) of the electrolyte electrode assembly 18 is not increased as a result of the intermediate layer 20, and the power generation cell 10 is prevented from becoming increased in size. Although not shown in FIG. 2, the intermediate layer 20 also has pores therein for diffusing the fuel gas supplied to the anode 12. In the present embodiment, the porosity of the intermediate layer 20 is lower than that of the anode 12, so that the large pores 24 opening on the anode 12 are reliably filled with the intermediate layer 20.

As described above, the intermediate layer 20 preferably has a porosity of 0% to 35% by volume. For example, the anode 12 and the intermediate layer 20, which have different porosities, may be obtained by adding a pore- forming agent to the material used for forming the anode 12, while not adding a pore-forming agent to the material used for forming the intermediate layer 20. Alternatively, the ratio of the pore-forming agent to the inorganic components in the material for the intermediate layer 20 may be controlled, so as to be lower than that in the material for the anode 12.

For example, the solid electrolyte 16 may comprise a stabilized zirconia (such as YSZ or ScSZ), a ceria-based oxide (such as SDC, GDC, or yttrium-doped ceria (YDC)), or a lanthanum gallate-based oxide (such as LaSrGaMgO₃ or LaSrGaMgCoO₃) . The thickness of the solid electrolyte 16 may be approximately 3 to 15 μm, and generally is about 5 μm. The solid electrolyte 16 has a flat surface facing the cathode 14. Although the solid electrolyte 16 has a remarkably small thickness of 3 to 15 μm, the number of cracks on the flat surface is significantly less than those found in conventional electrolyte-electrode joined assemblies, which do not have the intermediate layer 20. A reaction-preventing layer 28, composed of SDC, GDC, YDC, etc., is interposed between the solid electrolyte 16 and the cathode 14. The reaction-preventing layer 28 has a thickness of about 1 μm at most. Since the reaction- preventing layer 28 has a remarkably small thickness, the internal resistance of the electrolyte electrode assembly 18 is not increased as a result of the reaction-preventing layer 28.

For example, the cathode 14 may comprise an La-Sr-Co- Fe-based composite oxide (LSCF) , an La-Sr-Mn-based composite oxide (LSM) , an La-Sr-Co-based composite oxide (LSC) , a Ba- Sr-Co-Fe-based composite oxide (BSCF), a Ba-Sr-Co-based composite oxide (BSC) , or an Sm-Sr-Co-based composite oxide (SSC), etc. The thickness of the cathode 14 may be on the order of 10 μm or more, and preferably is about 30 μm. The electrolyte electrode assembly 18 having the aforementioned structure is interposed between a pair of separators 30a, 30b (see FIG. 1). Terminal electrodes 32a, 32b are disposed outside of the separators 30a, 30b, respectively, and end plates 34a, 34b are disposed outside of the terminal electrodes 32a, 32b, respectively. The end plates 34a, 34b are connected to each other by a tie rod or the like (not shown) , whereby the electrolyte electrode assembly 18, the separators 30a, 30b, and the terminal electrodes 32a, 32b are held by the end plates 34a, 34b, thereby forming the power generation cell 10. Gas passages 36a, 36b for supplying a fuel gas or an oxygen-containing gas to the anode 12 or the cathode 14 are formed in each of the separators 30a, 30b, respectively.

The electrolyte electrode assembly 18 according to the present embodiment has a basic structure as described above. Advantageous effects of the electrolyte electrode assembly 18 will be described below.

The power generation cell 10 having the above structure is driven after being heated to a temperature of approximately 500 ⁰C to 1000 ⁰C. After heating, the oxygen- containing oxidant gas is supplied through the gas passages 36b in the separator 30b, while the hydrogen-containing fuel gas is supplied through the gas passages 36a in the separator 30a. Oxygen molecules in the oxygen-containing gas become bonded to electrons on the cathode 14 in order to generate oxide ions (O^2"). The generated oxide ions are transferred from the cathode 14 to the solid electrolyte 16.

As described above, the solid electrolyte 16 has a flat surface facing the cathode 14. Therefore, the contact area between the cathode 14 and the solid electrolyte 16 is increased, and interface resistance between the cathode 14 and the solid electrolyte 16 is lowered. Thus, the voltage drop in the electrolyte electrode assembly 18 is reduced. The oxide ions are further transferred in the solid electrolyte 16 toward the surface facing the anode 12. As described above, the number of cracks, which might otherwise inhibit oxide ion transfer, is significantly small on the solid electrolyte 16. Thus, oxide ions can be readily transferred in the solid electrolyte 16. Since the solid electrolyte 16 can be small in thickness, oxide ions can readily be transferred in the solid electrolyte 16, and the solid electrolyte 16 can exhibit low internal resistance. In the present embodiment, the electrolyte electrode assembly 18 exhibits a low voltage drop, while the solid electrolyte 16 exhibits low internal resistance and low volume resistance. Thus, even when the power generation cell 10 is discharged at a high current density, a relatively high discharge voltage can be achieved. In the case that the intermediate layer 20 is not formed, the voltage is lowered in a relatively short time upon initiating supply of the fuel gas to the anode 12 (i.e., a relatively short reduction time is achieved). In contrast, when the intermediate layer 20 is provided, a constant voltage can be obtained over a long time period. As is clear from this fact, by forming the intermediate layer 20, a fuel cell with excellent power generation functionality can be obtained.

The oxide ions are further transferred through the YSZ particles contained in the anode 12 (the cermet).

The fuel gas supplied to the anode 12 is diffused, mostly through the large pores 24, and partially through the small pores 22. Hydrogen molecules in the fuel gas that is introduced into the pores react with the oxide ions , which are transferred through the YSZ particles in the anode 12, to thereby release water vapor and electrons .

The released electrons are introduced to an external circuit , which is connected electrically to the terminal electrodes 32a, 32b. The electrons are used as direct- current electric energy for energizing the external circuit, whereupon the electrodes are transferred to the cathode 14, and then become bonded to the oxygen molecules supplied to the cathode 14.

Water vapor is rapidly diffused through the large pores 24 and the small pores 22 of the anode 12 toward the separator 30a, and is discharged to the exterior of the system from the gas passages 36a of the separator 30a.

Production of the electrolyte electrode assembly 18 having the aforementioned structure will be described below. " As shown in FIG. 6, the electrolyte electrode assembly 18 of the present embodiment may be produced by carrying out a first step of forming a porous body of the anode 12, a second step of forming the intermediate layer 20 on the anode 12, a third step of forming the solid electrolyte 16 on the intermediate layer 20, and a fourth step of sintering at least the anode 12, the intermediate layer 20, and the solid electrolyte 16. The anode 12, the intermediate layer 20, and the solid electrolyte 16 may be formed either by tape casting or by screen printing. A first production method, in which the anode 12, the intermediate layer 20, and the solid electrolyte 16 are formed by tape casting, will be described below.

As shown in FIG. 7, in the first production method, the anode 12, the intermediate layer 20, and the solid electrolyte 16 are stacked, and the anode 12, the intermediate layer 20, and the solid electrolyte 16 are simultaneously subjected to a firing treatment.

According to this method, in the first step, for example, NiO particles and YSZ particles are mixed at a weight ratio of 1:1. Then, a binder (such as a polyvinyl alcohol binder or an acrylic binder) and a pore-forming agent (such as a polymethyl methacrylate resin or carbon) are added to the resultant mixed particles .

The average particle diameter of the pore-forming agent is selected in view of controlling the average pore diameter of the large pores 24 in the anode 12, so as to reside within a desired range. For example, a pore-forming agent having an average particle diameter of 8 μm or 20 μm may be selected in order to obtain large pores 24, which have an average pore diameter of approximately 4 to 6 μm or 12 to 14 μm, respectively.

The amount of the pore-forming agent to be added is selected in view of controlling the porosity of the anode 12 within a desired range. For example, the ratio of the pore- forming agent to the mixed particles (i.e., the inorganic components in the material for the anode 12) may be controlled at 10% to 30% by weight, so as to obtain a porosity of 10% to 40% by volume.

The above mixed particles are dispersed in a solvent to prepare a slurry. Using the slurry, the anode 12 is formed as a green sheet by a doctor blade method. The anode 12 is shrunk in the following fourth step. Thus, the thickness of the green sheet for the anode 12 may be controlled so that the anode 12 has a thickness of approximately 300 μm to 1 mm after shrinkage.

In the second step, the intermediate layer 20 is formed as a green sheet by preparing a slurry of NiO and YSZ, and by carrying out a doctor blade method using the slurry in the same manner as described above, except for not adding the pore-forming agent. The intermediate layer 20 is shrunk in the fourth step, and thus the thickness of the green sheet for the intermediate layer 20 may be controlled so that the intermediate layer 20 has a thickness of approximately 3 to 20 μm after shrinkage. The green sheet for the intermediate layer 20 is pressure-bonded onto a surface of the anode 12.

In the third step, the solid electrolyte 16 is formed as a green sheet by preparing a slurry containing particles of YSZ, ScSZ, SDC, GDC, YDC, lanthanum gallate-based oxide, etc., as described above, and by carrying out a doctor blade method using the slurry. The thickness of the green sheet for the solid electrolyte 16 may be controlled so that the solid electrolyte 16 has a thickness of approximately 3 to 15 μm after the firing treatment is performed in the next fourth step. The green sheet for the solid electrolyte 16 is pressure-bonded onto a surface of the intermediate layer 20.

A stacked structure of the three green sheets is prepared by the above steps. Then, in the fourth step, the stacked structure is subjected to a firing treatment, whereby the green sheets are sintered and shrunk in order to form the anode 12, the intermediate layer 20, and the solid electrolyte 16 having the above thicknesses.

Upon completion of. the firing treatment, the pore- forming agent that was added to the anode 12 is removed. Large pores 24 are formed by removal of the pore-forming agent, the large pores 24 having an average pore diameter corresponding to the average particle diameter of the pore- forming agent . In the material for the anode 12, the ratio of the pore-forming agent was changed based on 100% by weight of the total content of the mixed particles and the pore- forming agent (i.e., the inorganic components), wherein the relation of the porosity of the anode 12 with this change was examined prior to reduction. The results are shown in the graph of FIG. 5. As shown in FIG. 5, when the material for the anode 12 contains 10% to 30% by weight of the pore- forming agent, and the firing treatment is carried out at 1200 ⁰C to 1500 ⁰C, the porosity of the anode 12 can easily be controlled to within a range of 10% to 40% by volume.

Since the pore-forming agent is not added to the material for the intermediate layer 20, large pores 24 are not formed in the intermediate layer 20, and the porosity of the intermediate layer 20 is lower than that of the anode 12. Thus, the intermediate layer 20 comprises the same material and has the same composition ratio as the anode 12, however, the porosity of the intermediate layer 20 is lower than that of the anode 12.

When the anode 12 and the intermediate layer 20 comprise the same material and the same composition ratio, the anode 12 and the intermediate layer 20 have approximately the same shrinkage ratio. Therefore, peeling of the intermediate layer 20 away from the anode 12 can be reduced, because the shrinkage ratio of the anode 12 and the intermediate layer 20 match each other.

For example, a slurry of SDC, GDC, YDC, etc., is applied to the exposed surface of the solid electrolyte 16 by a screen printing method or the like. The slurry is sintered by a firing treatment in order to obtain the reaction-preventing layer 28.

Finally a slurry of LSCF, LSM, LSC, BSFC, BSC, SSC, etc., is applied to the reaction-preventing layer 28, and the slurry is sintered by a firing treatment, whereby the cathode 14 is formed to produce the electrolyte electrode assembly 18, containing the anode 12, the intermediate layer 20, the solid electrolyte 16, the reaction-preventing layer 28, and the cathode 14 stacked in this order.

A second production method, in which the anode 12, the intermediate layer 20, and the solid electrolyte 16 are formed by screen printing, will be described below with reference to FIG. 8.

As shown in FIG. 8, in a preferred example of the second production method, respective preliminary firing treatments are carried out after each of the steps of forming the anode 12, forming the intermediate layer 20, and forming the solid electrolyte 16. A principal firing treatment is carried out after the preliminary firing treatment of the solid electrolyte 16.

More specifically, first the above prepared slurry for the anode 12 is screen-printed in order to form the anode

12. Then, the anode 12 is shrunk by the preliminary firing treatment. During this treatment, large pores 24 may be formed utilizing the pore-forming agent.

The above prepared slurry for the intermediate layer 20 is screen-printed onto the anode 12 in order to form the intermediate layer 20. Then, the intermediate layer 20 is shrunk by the preliminary firing treatment .

The above prepared slurry for the solid electrolyte 16 is screen-printed onto the intermediate layer 20 in order to form the solid electrolyte 16. Then, the solid electrolyte 16 is shrunk by the preliminary firing treatment.

The anode 12, the intermediate layer 20, and the solid electrolyte 16 are sintered as a result of the principal firing treatment. In this example, the anode 12, the intermediate layer 20, and the solid electrolyte 16 exhibit small amounts of shrinkage in the principal firing treatment , because they have already been shrunk to a certain extent beforehand. Thus, the anode 12, the intermediate layer 20, and the solid electrolyte 16 can effectively be prevented from peeling away from each other, even if they have significantly different thermal expansion coefficients .

Then, the reaction-preventing layer 28 and the cathode 14 are formed in the same manner as above in order to produce the electrolyte electrode assembly 18.

All or a portion of the preliminary firing treatments may be included in the principal firing treatment . Thus , for example, at least one of the preliminary firing treatments for the anode 12, the intermediate layer 20, and the solid electrolyte 16 may be omitted, and the anode 12, the intermediate layer 20, and the solid electrolyte 16 may be fired simultaneously during the principal firing treatment .

The power generation cell 10 (see FIG. 1) may be produced by disposing the separators 30a, 30b, the terminal electrodes 32a, 32b, and the end plates 34a, 34b on respective surfaces of the anode 12 and cathode 14 in the electrolyte electrode assembly 18.

Although, in the above embodiment, the intermediate layer 20, the solid electrolyte 16, the reaction-preventing layer 28, and the cathode 14 are formed in this order on the anode 12, alternatively, the reaction-preventing layer 28, the solid electrolyte 16, the intermediate layer 20, and the anode 12 may be formed in this order on the cathode 14, which is prepared first. In either case, the reaction- preventing layer 28 may be omitted.

The anode 12 or the cathode 14, whichever is prepared first, may be formed by subjecting the material particles to press forming or the like, and by firing the green body. The intermediate layer is not particularly limited to the above described layer, which contains the same materials and the same composition ratio as the substrate of the anode or cathode. For example, when the anode is composed of a cermet containing YSZ and Ni at a weight ratio of 1:1, the intermediate layer may be composed of a cermet containing YSZ and Ni at a weight ratio of 3:7 or 7:3.

The average pore diameter of the large pores 24 is not limited to the above-described range, and may be changed by selecting the particle diameter and the particle diameter distribution of the pore-forming agent.

In addition, the pores may be formed without addition of the pore-forming agent. In this case, for example, the respective porosities of the anode 12 (the first electrode) and the intermediate layer 20 may be differentiated by controlling the temperature and/or time at which the preliminary sintering treatment of the anode 12 and the sintering treatment of the intermediate layer 20 are performed.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims .

Claims

Claim 1. An electrolyte electrode assembly (18) comprising a first electrode and a second electrode with a solid electrolyte (16) interposed between the first electrode and the second electrode, the first electrode acting as one of an anode (12) and a cathode (14), and the second electrode acting as the other of the anode (12) and the cathode (14), wherein: the first electrode comprises a porous body with pores (24) therein for facilitating passage of gas therethrough, and a concave portion and a convex portion (26) on a surface facing the solid electrolyte (16), an intermediate layer (20) is disposed between the first electrode and the solid electrolyte (16) such that the concave portion is filled with the intermediate layer (20) and the convex portion (26) is embedded in the intermediate layer (20), a thickness of the intermediate layer (20) is equal to or greater than the average pore diameter of the first electrode, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm.

Claim 2. An electrolyte electrode assembly (18) according to claim 1, wherein the first electrode has an average pore diameter of 3 to 20 μm, and the intermediate layer (20) has a thickness of 3 to 20 μm.

Claim 3. An electrolyte electrode assembly (18) according to claim 1 or 2 , wherein the porosity of the intermediate layer (20) is lower than that of the first electrode.

Claim 4. An electrolyte electrode assembly (18) according to claim 1, wherein the first electrode has a porosity of 10% to 40% by volume in an unreduced state, and the intermediate layer (20) has a thickness of 3 to 20 μm.

Claim 5. An electrolyte electrode assembly (18) according to any one of claims 1 to 4, wherein the first electrode and the intermediate layer (20) comprise the same material.

Claim 6. An electrolyte electrode assembly (18) according to any one of claims 1 to 5 , wherein the solid electrolyte (16) comprises a solid oxide.

Claim 7. An electrolyte electrode assembly (18) according to any one of claims 1 to 6 , wherein the first electrode is the anode (12), and the second electrode is the cathode (14).

Claim 8. A method for producing an electrolyte electrode assembly (18) comprising a first electrode and a second electrode with a solid electrolyte (16) interposed between the first electrode and the second electrode, the first electrode acting as one of an anode (12) and a cathode (14), and the second electrode acting as the other of the anode (12) and the cathode (14), wherein the method comprises the steps of: forming the first electrode by tape casting, forming an intermediate layer (20) on the first electrode by tape casting and pressure-bonding the intermediate layer (20) to the first electrode, forming the solid electrolyte (16) on the intermediate layer (20) by tape casting and pressure-bonding the solid electrolyte (16) to the intermediate layer (20), and subjecting at least the first electrode, the intermediate layer (20), and the solid electrolyte (16) to a firing treatment, wherein the first electrode comprises a porous body with pores (24) therein for facilitating passage of gas therethrough, and a concave portion and a convex portion (26) on a surface facing the solid electrolyte (16), the intermediate layer (20) is disposed between the first electrode and the solid electrolyte (16) such that the concave portion is filled with the intermediate layer (20) and the convex portion (26) is embedded in the intermediate layer (20), a thickness of the intermediate layer (20) is equal to or greater than the average pore diameter of the first electrode after sintering, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm.

Claim 9. A method according to claim 8, wherein a material for the first electrode contains 10% to 30% by weight of a pore-forming agent based on 100% by weight of the inorganic content, and the firing treatment is carried out at 1200 ⁰C to 1500 ⁰C to form the pores (24) utilizing the pore-forming agent.

Claim 10. A method according to claim 8, wherein the ratio of a pore-forming agent to an inorganic component in a material for the intermediate layer (20) is lower than that in a material for the first electrode.

Claim 11. A method according to claim 8, wherein the first electrode and the intermediate layer (20) comprise the same material.

Claim 12. A method according to claim 8, wherein the solid electrolyte (16) comprises a solid oxide.

Claim 13. A method according to claim 8, wherein the first electrode is the anode (12), and the second electrode is the cathode (14).

Claim 14. A method for producing an electrolyte electrode assembly (18) comprising a first electrode and a second electrode with a solid electrolyte (16) interposed between the first electrode and the second electrode, the first electrode acting as one of an anode (12) and a cathode (14), and the second electrode acting as the other of the anode (12) and the cathode (14), wherein the method comprises the steps of: forming the first electrode by screen printing, forming an intermediate layer (20) on the first electrode by screen printing, forming the solid electrolyte (16) on the intermediate layer (20) by screen printing, and subjecting at least the first electrode, the intermediate layer (20), and the solid electrolyte (16) to a firing treatment , wherein the first electrode comprises a porous body with pores (24) therein for facilitating passage of gas therethrough, and a concave portion and a convex portion (26) on a surface facing the solid electrolyte (16), the intermediate layer (20) is disposed between the first electrode and the solid electrolyte (16) such that the concave portion is filled with the intermediate layer (20) and the convex portion (26) is embedded in the intermediate layer (20), a thickness of the intermediate layer (20) is equal to or greater than the average pore diameter of the first electrode after sintering, and a difference calculated by subtracting the average pore diameter from the thickness is 0 to 17 μm.

Claim 15. A method according to claim 14, wherein a preliminary firing treatment is carried out after at least one of the steps of forming the first electrode, forming the intermediate layer (20), and forming the solid electrolyte (16).

Claim 16. A method according to claim 14, wherein a material for the first electrode contains 10% to 30% by weight of a pore-forming agent based on 100% by weight of the inorganic content , and the firing treatment is carried out at 1200 ⁰C to 1500 ⁰C to form the pores (24) utilizing the pore-forming agent.

Claim 17. A method according to claim 14, wherein the ratio of a pore-forming agent to an inorganic component in a material for the intermediate layer (20) is lower than that in a material for the first electrode.

Claim 18. A method according to claim 14, wherein the first electrode and the intermediate layer (20) comprise the same material.

Claim 19. A method according to claim 14, wherein the solid electrolyte (16) comprises a solid oxide.

Claim 20. A method according to claim 14, wherein the first electrode is the anode (12), and the second electrode is the cathode (14).