WO2008146927A1

WO2008146927A1 - Electrolyte-electrode joined assembly and method for producing the same

Info

Publication number: WO2008146927A1
Application number: PCT/JP2008/060093
Authority: WO
Inventors: Teruaki Komiya
Original assignee: Honda Motor Co., Ltd.
Priority date: 2007-05-25
Filing date: 2008-05-26
Publication date: 2008-12-04

Abstract

Secondary electrolytes (22a, 22b) are formed on surfaces of a primary electrolyte (20) to prepare a multi-layered electrolyte (16) having a thickness of 100 m or less. The materials and thicknesses of the primary electrolyte (20) and the secondary electrolytes (22a, 22b) are selected such that the total sheet resistivity of the primary electrolyte (20) and the secondary electrolytes (22a, 22b) is less than 0.40 cm2, and the multi-layered electrolyte (16) has a fracture energy of 0.3 mJ or more. For example, the multi-layered electrolyte (16) may be formed such that the primary electrolyte (20) is composed of a stabilized zirconia doped with 10 mol% of Sc2O3 and has a thickness of 70 m, and the secondary electrolytes (22a, 22b) are composed of a stabilized zirconia doped with 6 mol% of Sc2O3 and have a thickness of 10 m.

Description

DESCRIPTION

ELECTROLYTE-ELECTRODE JOINED ASSEMBLY AND METHOD FOR

PRODUCING THE SAME

Technical Field

The present invention relates to an electrolyte- electrode joined assembly for a unit cell of a solid electrolyte fuel cell, and a method for producing the same.

Background Art

In a solid electrolyte fuel cell (hereinafter referred to as the SOFC), a solid electrolyte capable of conducting an oxide ion (O^2") as the electrolyte is sandwiched between an anode and a cathode. It is preferred that the solid electrolyte has a high oxide ion conductivity, and a stabilized ZrO₂ doped with about 10 to 12 mol% of Sc₂O₃ has been attracting attention because of its high conductivity. An electrolyte-electrode joined assembly (hereinafter referred to as an MEA) may be produced such that the electrolyte is formed first, and then the anode and the cathode are formed on the surfaces thereof. In the MEA, the electrolyte is thicker than the anode and the cathode, and thereby is responsible for the strength of the entire assembly. Thus, it is referred to as an electrolyte- supported-type MEA.

A scandia-stabilized zirconia doped with a large amount (8 to 15 mol%) of Sc₂O₃ is described in Japanese Patent No. 3458863, etc. In the case of using the scandia-stabilized zirconia as the electrolyte, it has a thickness of about 200 μm or more to obtain a sufficient strength of the MEA. In general, as the thickness of the electrolyte is reduced, the oxide ion can be diffused more readily. In other words, by reducing the thickness of the electrolyte, the internal resistance of the MEA, and therefore of the SOFC, can be lowered. Thus, there is a demand for an MEA that has a sufficient strength and is not broken even under a high load while having a small electrolyte thickness.

An electrolyte disclose in WO 2006/050071 has a high resistance in power generation reaction at a temperature around 700⁰C, that is, the SOFC operation temperature. Thus, it is not possible for a fuel cell employing such an electrolyte to exhibit sufficient power generation property.

A method for increasing the strength of the MEA is proposed in Japanese Laid-Open Patent Publication No. 2003- 263996. In this method, intermediate layers of a samarium- doped ceria are formed on the surfaces of an electrolyte of an yttria-doped stabilized zirconia, so that a compressive stress is applied to the electrolyte, resulting in improvement of cracking resistance of the MEA.

Further, a unit cell for a low-temperature-operation- type SOFC, excellent in power generation function and reliability at a low temperature of 600⁰C to 900⁰C, is proposed in Japanese Laid-Open Patent Publication No. 2005- 322547. The unit cell contains a first electrolyte having an oxygen ion conductivity of 0.015 S/cm or more at 800⁰C and a bending strength of 600 MPa or more. The first electrolyte contains a scandia-stabilized zirconia doped with 3 to 6 mol% of Sc₂O₃.

As described in Japanese Patent No. 3458863, the scandia-stabilized zirconia doped with a large amount (8 to 15 mol%) of Sc₂O₃ is poor in mechanical strength, and a thin solid electrolyte thereof is easily broken. In other words, the thin solid electrolyte of the scandia-stabilized zirconia may cause a problem of reliability in a prolonged use of the SOFC.

The MEA described in WO 2006/050071 is relatively easily warped due to a power generation reaction during the operation of the SOFC. This is because the electrolyte layer stacked has a relatively small thickness, so that the stack is likely to be affected by a thermal stress, and a redox reaction is caused on the anode, causing deformation of the MEA. When the MEA is warped, a clearance may be formed between the MEA and a separator in the unit cell of the SOFC. Thus, the MEA and the separator may be separated into insufficient contact with each other, thereby causing reduction in current collection efficiency and uneven diffusion of a reaction gas. Further, a defect such as a crack may be generated in the anode or the cathode . Of course, the resultant SOFC exhibits a poor power generation property in each case.

In Japanese Laid-Open Patent Publication No. 2003- 263996, the thicknesses of the electrolyte and the intermediate layer are not disclosed at all. Thus, it is not clear that to what extent the total thickness of the MEA can be reduced practically.

The above strength of the first electrolyte described in Japanese Laid-Open Patent Publication No. 2005-322547 is a value measured by a three-point bending test using a sample having a thickness of 3 mm and a width of 4 mm. This thickness is greatly different from the thickness of the electrolyte used in practical MEAs . Thus , it is unclear whether an MEA using the first electrolyte can exhibit, in practical use, a strength equal to that of the sample.

Disclosure of Invention

A general object of the present invention is to provide an electrolyte-electrode joined assembly that has a remarkably small thickness but is not easily broken. A principal object of the present invention is to provide an electrolyte-electrode joined assembly containing an electrolyte excellent in conductivity.

Another object of the present invention is to provide an electrolyte-electrode joined assembly containing an electrolyte heretofore considered to be poor in mechanical strength, such as a lanthanum gallate-based compound or a scandia-stabilized zirconia doped with about 10 to 12 mol% of Sc₂O₃ .

A further object of the present invention is to provide a method for producing the above electrolyte-electrode joined assembly. According to an aspect of the present invention, there is provided an electrolyte-supported-type electrolyte- electrode joined assembly comprising an anode and a cathode with an electrolyte layer interposed therebetween, the electrolyte layer being thicker than the anode and the cathode, wherein the electrolyte layer comprises a multi-layered electrolyte containing a primary electrolyte and secondary electrolytes connected to surfaces of the primary electrolyte, the secondary electrolytes being thinner than the primary electrolyte, the total sheet resistivity of the primary electrolyte and the secondary electrolytes is less than 0.40 Ωcm², and when the multi-layered electrolyte has a thickness acceptable for a practical use of the electrolyte-electrode joined assembly in a fuel cell, the multi-layered electrolyte has a fracture energy of 0.3 mJ or more .

In the present invention, the multi-layered electrolyte having a fracture energy of 0.3 mJ or more can function to relax an applied load. The multi-layered electrolyte can be extended to a remarkably large extent before broken in the electrolyte-electrode joined assembly. Thus, the multi- layered electrolyte can be largely bent under the load, and thereby is not easily broken.

Further, since the total sheet resistivity of the primary electrolyte and the secondary electrolytes is less than 0.40 Ωcm², the multi-layered electrolyte is excellent in the oxide ion conductivity, and excessive resistance increase due to deteriorated oxide ion conductivity can be prevented.

As described above, by using the multi-layered electrolyte, the conductivity and the breakage resistance (toughness) can be well balanced.

The multi-layered electrolyte preferably has a thickness of less than 150 μm. Since the fracture energy is increased by using the composite of the primary electrolyte and the secondary electrolytes as described above, the multi-layered electrolyte is not easily broken in spite of such a small thickness. Further, as the electrolyte has a small thickness or is made thin, a stack of separators and an electrolyte-electrode joined assembly employing such an electrolyte also has a small thickness. Thus, the stack can be miniaturized.

It is preferred that, when the primary electrolyte and the secondary electrolytes have the same thickness, the resistance of the primary electrolyte is lower than those of the secondary electrolytes. In this case, the multi-layered electrolyte exhibits a sufficient oxide ion conductivity.

It is preferred that, when the primary electrolyte and the secondary electrolytes have the same thickness, the fracture energies of the secondary electrolytes are higher than that of the primary electrolyte. In this case, the electrolyte-electrode joined assembly exhibits a sufficient toughness. The primary electrolyte is preferably thicker than the secondary electrolytes. In this case, the conductivity and the toughness are balanced better.

In every case, the primary electrolyte preferably comprises a stabilized zirconia or a lanthanum gallate-based substance.

According to another aspect of the present invention, there is provided a method for producing an electrolyte- supported-type electrolyte-electrode joined assembly comprising an anode and a cathode with an electrolyte layer interposed therebetween, the electrolyte layer being thicker than the anode and the cathode , wherein the method comprises the steps of: forming secondary electrolytes on surfaces of a primary electrolyte to form a multi-layered electrolyte, the secondary electrolytes being thinner than the primary electrolyte; forming one of the anode and the cathode on one of the secondary electrolytes ; and forming the other of the anode and the cathode on the other of the secondary electrolytes , the total sheet resistivity of the primary electrolyte and the secondary electrolytes is less than 0.40 Ωcm², and when the multi-layered electrolyte has a thickness acceptable for a practical use of the electrolyte-electrode joined assembly in a fuel cell, the multi-layered electrolyte has a fracture energy of 0.3 mJ or more. An electrolyte-electrode joined assembly, which exhibits well-balanced conductivity and toughness in spite of a small thickness, can be easily produced by the method. It is particularly preferred that the total thickness of the primary electrolyte and the secondary electrolytes is less than 150 μm in the multi-layered electrolyte.

The primary electrolyte and the secondary electrolytes are preferably formed from sheets. The sheets can be easily prepared with controlled thicknesses. Thus, by using the sheets, the primary electrolyte and the secondary electrolytes, and therefore the multi-layered electrolyte, can be easily obtained with excellent dimensional accuracy. In the present invention, the sheet resistivity is a normalized resistance value obtained by converting the resistance of the electrolyte in terms of a size of 1 cm x 1 cm. The resistance can be obtained by forming a platinum electrode having a predetermined area on the electrolyte, and then measuring the impedance of the electrolyte. Thus,, the resistance of the electrolyte is practically equal to a value obtained by dividing the sheet resistivity of the electrolyte by the actual area of the electrolyte. The sheet resistivity may be calculated from IR loss due to power generation, if necessary.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

Brief Description of Drawings

FIG. 1 is a schematic, overall, explanatory cross- sectional view of an electrolyte-electrode joined assembly (MEA) according to an embodiment of the present invention;

FIG. 2 is a graph for explaining the concept of toughness and fracture energy according to the embodiment;

FIG. 3 is a table showing the material, thickness, fracture energy, and sheet resistivity of each electrolyte used in Examples 1 to 10 according to the embodiment and Comparative Examples 1 to 7 ; FIG. 4 is a flowchart showing a process of preparing a sheet for the primary electrolyte of a multi-layered electrolyte used in the electrolyte-electrode joined assembly shown in FIG. 1;

FIG. 5 is a flowchart showing a process of preparing the multi-layered electrolyte from the sheet of FIG. 4; and

FIG. 6 is a flowchart showing a process of preparing the MEA from the multi-layered electrolyte of FIG. 5.

Best Mode for Carrying Out the Invention A preferred embodiment of the electrolyte-electrode joined assembly and the producing method of the present invention will be described in detail below with reference to accompanying drawings .

FIG. 1 is a schematic, overall, explanatory cross- sectional view showing an electrolyte-electrode joined assembly (MEA) 10 according to this embodiment. The MEA 10 is such an electrolyte-supported-type assembly that a multi- layered electrolyte 16 is sandwiched between an anode 12 and a cathode 14, and the multi-layered electrolyte 16 is thicker than the anode 12 and the cathode 14.

The anode 12, which is disposed on one surface of a current collection layer 18 of Ni, is preferably composed of a cermet of Ni and a material for the electrolyte . Preferred examples of the materials for the electrolyte include yttria-stabilized zirconias (YSZ) doped with 8 to 10 mol% of Y₂O₃, scandia-stabilized zirconias (SSZ) doped with 9 to 12 mol% of Sc₂O₃, and samarium-doped cerias (SDC). In this embodiment, the anode 12 is composed of a cermet of Ni and an yttria-stabilized zirconia (YSZ), and has a thickness of 5 to 20 μm.

In the multi-layered electrolyte 16, a primary electrolyte 20 comprising a stabilized zirconia doped with 10 mol% of Sc₂O₃ (10SSZ) is sandwiched between secondary electrolytes 22a, 22b comprising a stabilized zirconia doped with 6 mol% of Sc₂O₃ (6SSZ). Thus, each of the secondary electrolytes 22a, 22b is connected to each of the surfaces of the primary electrolyte 20.

In this embodiment, the primary electrolyte 20 has a thickness of 70 μm, and each of the secondary electrolytes 22a, 22b has a thickness of 10 μm. Thus, the multi-layered electrolyte 16 has a thickness of 90 μm, which is less than half a common electrolyte thickness of about 200 μm in the electrolyte-supported-type MEA 10. In this embodiment, the primary electrolyte 20 composed of the 10SSZ has a thickness of 70 μm and a sheet resistivity of 0.15 Ωcm², and the secondary electrolytes 22a, 22b composed of the 6SSZ has a thickness of 10 μm and a sheet resistivity of 0.06 Ωcm². Thus, the total sheet resistivity of the primary electrolyte 20 and the secondary electrolytes 22a, 22b is 0.21 Ωcm². In this embodiment, the total sheet resistivity of the multi-layered electrolyte 16 is less than 0.40 Ωcm², preferably 0.20 to 0.35 Ωcm².

Since the primary electrolyte 20 of the 10SSZ and the secondary electrolytes 22a, 22b of the 6SSZ have the above thicknesses, the multi-layered electrolyte 16 exhibits a fracture energy of 1.0 mJ or more in a toughness evaluation test to be hereinafter described.

Thus, the multi-layered electrolyte 16 has an excellent toughness, and can be extended to a remarkably large extent before broken. The multi-layered electrolyte 16 can be largely bent under a load to relax the load, and thereby is not easily broken.

The relation between strength and toughness (breakage resistance) of a material will be described below. As described above, an electrolyte (a ceramic) for an SOFC is required to be excellent in strength. An absolute fracture strength obtained from a stress-strain curve has conventionally been emphasized in view of the requirement . The absolute fracture strength of the electrolyte is generally measured as a three-point bending strength in a three-point bending test according to JIS R 1601 as described in Japanese Laid-Open Patent Publication No. 2005- 322547, etc. When the electrolyte is poor in the three- point bending strength in a practical MEA, the breakage of the electrolyte is prevented, for example, by increasing the thickness of the electrolyte. However, as the electrolyte thickness is increased, the resistance of the electrolyte, and the internal resistance of a fuel cell are increased disadvantageousIy .

Further, as described above, a sample used in the three-point bending test according to JIS R 1601 has a size greatly different from that of an electrolyte used in a practical MEA. Thus, the strength of the electrolyte or the MEA cannot be accurately evaluated by the three-point bending test. It is clear from the above that the strength of the electrolyte cannot be sufficiently defined only by the evaluation of the three-point bending strength in the three- point bending test. Therefore, in the present invention, the electrolyte is evaluated in terms of a toughness to be described below. The following method for measuring the toughness has been established by the inventor.

Thus, a sample having an electrolyte thickness acceptable for a practical use of the MEA is subjected to the strength measurement according to JIS R 1601. An example of a load-strain line obtained by the measurement is represented by a line B in FIG. 2. In general, in view of only the strength of the MEA, an electrolyte, which begins to bend under a high load as represented by a line A in FIG. 2, is used to prevent the deformation of the MEA. Such a highly rigid electrolyte is suddenly broken when the load exceeds the capacity. Thus, the electrolyte has brittleness. On the other hand, the electrolyte represented by the line B in FIG. 2 is bent to relax the load.

When a load is applied to an electrolyte, the electrolyte is broken or not depending on the fracture energy accumulation capacity of the electrolyte. An electrolyte that can accumulate a large amount of fracture energy is not easily broken, while an electrolyte that cannot accumulate fracture energy is easily broken. This point will be described using FIG. 2 below. In FIG. 2, the area of a triangle formed by the line A, the horizontal axis, and a line Ll extending from the line A perpendicular to the horizontal axis is equal to the area of a triangle formed by the line B, the horizontal axis, and a line L2 extending from the line B perpendicular to the horizontal axis. Thus, in FIG. 2, the lines A, B are drawn such that the electrolytes have the same fracture energy. The lines A, B can be obtained by serially plotting the values of load and strain until the breakage.

The electrolyte of the line B is broken at a lower load as compared with the electrolyte of the line A. However, it is clear that the electrolyte of the line B is more highly strained at one load than the electrolyte of the line A.

The electrolyte of the line B can be deformed to accumulate the fracture energy. Thus, the electrolyte can be highly deformed under an applied load, whereby the load can be relaxed. As a result, the electrolyte is not easily broken and exhibits an excellent toughness in spite of the low fracture strength.

In this embodiment, the above area of the triangle is used as an index of the toughness (breakage resistance) , and is defined as the fracture energy. Further, the amount of the bending in the fracture energy measurement can be obtained from the area.

The electrolyte is evaluated not in terms of the conventional absolute mechanical strength according to JIS R 1601, but in terms of the fracture energy of the sample having a thickness acceptable for a practical use of the MEA as described above. Thus, a further improved structure of the electrolyte can be designed in the present invention. When the 10SSZ and the 6SSZ have the same thickness, the 10SSZ has a higher oxide ion conductivity, and the 6SSZ has a higher strength. Thus, in this embodiment, the primary electrolyte 20 of the 10SSZ having the higher oxide ion conductivity is reinforced by the secondary electrolytes 22a, 22b of the 6SSZ having the higher strength. In other words, the secondary electrolytes 22a, 22b act as reinforcing layers .

As shown in FIG. 3, the 10SSZ electrolyte of Comparative Example 1 having a thickness of 100 μm exhibits a fracture energy of about 0.1 mJ . The electrolyte having the low fracture energy is broken when extended by 1.0 mm (span 40 mm) in the three-point bending test. The 10SSZ electrolytes of Comparative Examples 2 and 3 having thicknesses of 200 μm and 250 μm exhibit only slightly increased fracture energies of 0.3 mJ and 0.4 mJ respectively. Further, the sheet resistivities of the electrolytes are disadvantageously increased with the thickness increase. It should be noted that all the sheet resistivity values shown in FIG. 3 are measured at 700⁰C.

The 6SSZ electrolyte of Comparative Example 4 having a thickness of 80 μm exhibits a fracture energy of about 0.4 mJ. Even though the 6SSZ electrolyte of Comparative Example 4 is thinner by 20 μm than the 10SSZ electrolyte of Comparative Example 1, its sheet resistivity is higher by about 0.20 Ωcm² than that of the 10SSZ electrolyte of Comparative Example 1. Thus the 6SSZ electrolyte is insufficient in the conductivity.

The electrolytes of Comparative Examples 5 to 7 have multi-layered electrolytes prepared according to WO 2006/050071. As shown in FIG. 3, the electrolyte thicknesses are equal to those of Comparative Example 1 to 4, i.e., the electrolytes are not thinned (downsized). Further, the electrolytes exhibit high sheet resistivities.

In contrast, in Example 1, the thicknesses of the 10SSZ primary electrolyte 20 and the 6SSZ secondary electrolytes 22a, 22b are controlled as described above, so that the multi-layered electrolyte 16 exhibits a remarkably higher fracture energy of 1.2 mJ in spite of the thickness smaller than that of Comparative Example 1 (10SSZ having a thickness of 100 μm) . Further, the electrolyte of Example 1 has a sheet resistivity of 0.21 Ωcm², equal to that of Comparative Example 1. Thus, by using the multi-layered electrolyte 16, the conductivity and the breakage resistance (the toughness) can be well balanced.

Also in the case of using the 4SSZ or 5SSZ as a material of the secondary electrolytes 22a, 22b in Examples 2 and 3 , each electrolyte has a fracture energy of 1.0 mJ or more and a sheet resistivity of less than 0.40 Ωcm², and thus the conductivity and the toughness can be well balanced. In Examples 4 to 10, though the material and thickness of the primary electrolyte, the thickness of the secondary electrolyte, and the total thickness are variously changed, the fracture energies are 1.0 mJ or more, and the sheet resistivities are less than 0.40 Ωcm². Thus, also in the examples, the conductivity and the toughness can be well balanced in the multi-layered electrolytes 16. In FIG. 3, the lOSclCeSZ of Example 7, the lOSclYSZ of Example 8, the lOScAlSZ of Example 9, and the lOSclOAlSZ of Example 10 represent a stabilized zirconia doped with 10 mol% of Sc₂O₃ and 1 mol% of CeO₃, a stabilized zirconia doped with 10 mol% of Sc₂O₃ and 1 mol% of Y₂O₃, a stabilized zirconia doped with 10 mol% of Sc₂O₃ and 1 mol% of Al₂O₃, and a stabilized zirconia doped with 10 mol% of Sc₂O₃ and 10 mol% of Al₂O₃, respectively.

It is clear from the above results that each secondary electrolyte 22a, 22b according to the embodiment has a high oxide ion conductivity sufficient for use as an oxide ion conductor.

A diffusion preventing layer 24 is interposed between the cathode 14 and the multi-layered electrolyte 16 that are formed as mentioned above, to prevent elemental diffusion from the secondary electrolyte 22b or the primary electrolyte 20 to the cathode 14, or from the cathode 14 to the secondary electrolyte 22b or the primary electrolyte 20. For example, the diffusion preventing layer 24 is composed of Sm₂O₃-doped CeO₂ (SDC), Y₂O₃-doped CeO₂ (YDC), Gd₂O₃-doped CeO₂ (GDC), La₂0₃-doped CeO₂ (LDC), etc., and has a thickness of 1 to 2 μm.

In this case, the cathode 14 layered on the diffusion preventing layer 24 has a thickness of 5 to 20 μm. Preferred examples of materials for the cathode 14 include La-Co-O perovskite oxides, La-Sr-Co-O (LSC) perovskite oxides, La-Sr-Co-Fe-O (LSCF) perovskite oxides, and mixtures of one of the perovskite oxides and oxide ion conductor. In this embodiment, the cathode 14 is composed of an LSCF perovskite oxide. Specific examples of the oxide ion conductor include SDC, YDC, GDC, and LDC.

Further, a current collection layer 26 of a lanthanum strontium cobaltite (LSC) is disposed on the cathode 14, to form the MEA 10.

The MEA 10 according to this embodiment has the above basic structure. The advantageous effects thereof will be described below. The MEA 10 may be sandwiched between separators to form a unit cell, and a plurality of the unit cells may be stacked to produce a fuel cell. The fuel cell is driven such that, after heating the fuel cell to a predetermined temperature, a fuel gas containing hydrogen is supplied to the anode 12 of each unit cell, and an oxidant gas containing oxygen is supplied to the cathode 14. The oxygen is ionized on the cathode 14, and the generated oxygen ions are diffused through the multi-layered electrolyte 16 to the anode 12. As described above, the multi-layered electrolyte 16 has a sheet resistivity of about 0.21 Ωcm², so that the oxide ions can be readily diffused in the multi-layered electrolyte 16. Thus, the SOFC is excellent in power generation properties because of the small internal resistance.

In the power generation process, a load or disturbance may be applied to the MEA 10. In this embodiment, the multi-layered electrolyte 16 can be extended by 5.0 mm (a span of 40 mm) and have a fracture energy of 0.3 mJ or more, and thereby can be bent to relax the load or disturbance.

The multi-layered electrolyte 16 can be greatly extended and cannot be broken without a high energy, so that the MEA 10 can effectively relax the load or disturbance to improve the durability of the SOFC.

Thus, in this embodiment, the MEA 10 can relax the load or disturbance even though the multi-layered electrolyte 16 has a significantly small thickness of 90 μm. The resultant MEA 10 is high in the conductivity and breakage resistance, and the SOFC is excellent in the power generation properties and durability.

As described above, since the multi-layered electrolyte 16 has a small thickness of less than 150 μm, when a stack is formed of the MEA 10 and separators, the stack is made thin in the stacking direction . Thus , the stack can be miniaturized.

Because the miniaturized stack has a small heat capacity, heat balance of such stack can advantageously easily managed while the operation condition of the stack greatly changes during rising or falling of temperature. In addition, since the miniaturized stack has a small volume, the power density per stack becomes large. The MEA 10 can be produced in the following manner.

First, the multi-layered electrolyte 16 is formed. As shown in FIG. 4, a powder of the 10SSZ and a binder are added to a solvent to prepare a slurry. The slurry is formed into a sheet shape to prepare a primary electrolyte sheet. The thickness of the primary electrolyte sheet is controlled such that the primary electrolyte has a thickness of 70 μm after a firing treatment to be hereinafter described. Two secondary electrolyte sheets of the 6SSZ are prepared in the same manner such that each secondary electrolyte has a thickness of 10 μm after the firing treatment . Such a method using the sheets is advantageous in that the thicknesses can be easily controlled, whereby the multi- layered electrolyte 16 can be easily produced with excellent dimensional accuracy.

Then, as shown in FIG. 5, the secondary electrolyte sheets (reinforcing layer sheets) are compressed under heating to the surfaces of the primary electrolyte sheet , and the sheets are connected by a firing treatment . For example, the firing temperature may be 1500⁰C. Thus obtained multi-layered electrolyte 16 contains the 10SSZ primary electrolyte 20 having a thickness of 70 μm and the 6SSZ secondary electrolytes 22a, 22b having a total thickness of 20 μm. Since the primary electrolyte 20 and the secondary electrolytes 22a, 22b have such thicknesses, the multi-layered electrolyte 16 has a sheet resistivity of 0.21 Ωcm², an extensibility of 5.0 mm (span 40 mm), and a fracture energy of 0.3 mJ or more .

In comparison between the multi-layered electrolyte 16 and an electrolyte composed of only the 6SSZ, the multi- layered electrolyte 16 can have a larger thickness within the range of less than 150 μm to obtain a desired resistance, assuming that both of them have the same resistance. By thickening the multi-layered electrolyte 16, the separators, which the MEA 10 is sandwiched between, are prevented from being short-circuited due to the contact with each other. Though the electromotive force of the fuel cell may be reduced due to electron conductivity increase depending on a temperature range, also the reduction can be prevented by thickening the multi-layered electrolyte 16.

As shown in FIG. 6, an intermediate SDC layer (the diffusion preventing layer 24) is printed on one surface of the multi-layered electrolyte 16. Further, the cathode 14 of the LSCF is printed on the diffusion preventing layer 24, and the anode 12 of the Ni-YSZ is printed on the other surface. Then, the current collection layers 18, 26 are formed on the outer surfaces of the anode 12 and the cathode 14 if necessary, to produce the MEA 10 of FIG. 1. Though the primary electrolyte 20 is composed of the 10SSZ and has a thickness of 70 μm, and the secondary electrolytes 22a, 22b are composed of the 6SSZ and have a thickness of 10 μm in this embodiment, the electrolytes are not particularly limited thereto . The electrolytes may have any materials and thicknesses as long as the total sheet resistivity of the primary electrolyte 20 and the secondary electrolytes 22a, 22b is less than 0.40 Ωcm², and the multi- layered electrolyte has a fracture energy of 0.3 mJ or more, while the multi-layered electrolyte has a thickness of less than 150 μm. For example, as Examples 2 and 3 in FIG. 3, the primary electrolyte 20 may be composed of the 10SSZ and may have a thickness of 60 μm, and the secondary electrolytes 22a, 22b may be composed of the 4SSZ or 5SSZ and may have a thickness of 10 μm. Further specific examples of the materials for the primary electrolyte 20 include lanthanum gallate-based substances. Also the materials of the anode 12 and the cathode 14 are not particularly limited, and may be selected from materials usable in SOFCs .

Though the sheets are prepared in the above producing method, the production is not limited thereto. For example, the primary electrolyte 20 may be formed by a known process, and the secondary electrolytes 22a, 22b may be formed thereon by a printing process, a CVD process, a PVD process, a coating process such as a spin coating process, or a dipping process . The invention has been particularly shown and described with reference to preferred embodiments , it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. An electrolyte-supported-type electrolyte-electrode joined assembly (10) comprising an anode (12) and a cathode (14) with an electrolyte layer interposed therebetween, said electrolyte layer being thicker than said anode (12) and said cathode (14), wherein said electrolyte layer comprises a multi-layered electrolyte (16) containing a primary electrolyte (20) and secondary electrolytes (22a, 22b) connected to surfaces of said primary electrolyte (20), said secondary electrolytes (22a, 22b) being thinner than said primary electrolyte (20), a total sheet resistivity of said primary electrolyte (20) and said secondary electrolytes (22a, 22b) is less than 0.40 Ωcm², and when said multi-layered electrolyte (16) has a thickness acceptable for practical use of said electrolyte- electrode joined assembly (10) in a fuel cell, said multi- layered electrolyte (16) has a fracture energy of 0.3 mJ or more .

2. An electrolyte-electrode joined assembly (10) according to claim 1, wherein said multi-layered electrolyte (16) has a thickness of less than 150 μm.

3. An electrolyte-electrode joined assembly (10) according to claim 1 or 2 , wherein when said primary electrolyte (20) and said secondary electrolytes (22a, 22t>) have the same thickness, a resistance of said primary electrolyte (20) is lower than those of said secondary electrolytes (22a, 22b).

4. An electrolyte-electrode joined assembly (10) according to claim 1 , wherein when said primary electrolyte (20) and said secondary electrolytes (22a, 22b) have the same thickness, fracture energies of said secondary electrolytes (22a, 22b) are higher than that of said primary electrolyte (20).

5. An electrolyte-electrode joined assembly (10) according to claim 4, wherein said primary electrolyte (20) is thicker than said secondary electrolytes (22a, 22b).

6. An electrolyte-electrode joined assembly (10) according to any one of claims 1 to 5 , wherein said primary electrolyte (20) comprises a stabilized zirconia or a lanthanum gallate-based substance.

7. A method for producing an electrolyte-supported-type electrolyte-electrode joined assembly (10) comprising an anode (12) and a cathode (14) with an electrolyte layer interposed therebetween, said electrolyte layer being thicker than said anode (12) and said cathode (14), wherein said method comprises the steps of: forming secondary electrolytes (22a, 22b) on surfaces of a primary electrolyte (20) to form a multi-layered electrolyte (16), said secondary electrolytes (22a, 22b) being thinner than said primary electrolyte (20); forming one of said anode (12) and said cathode (14) on one of said secondary electrolytes (22a, 22b); and forming the other of said anode (12) and said cathode (14) on the other of said secondary electrolytes (22a, 22b), a total sheet resistivity of said primary electrolyte (20) and said secondary electrolytes (22a, 22b) is less than 0.40 Ωcm², and when said multi-layered electrolyte (16) has a thickness acceptable for practical use of said electrolyte- electrode joined assembly (10) in a fuel cell, said multi- layered electrolyte (16) has a fracture energy of 0.3 mJ or more.

8. A method according to claim 7, wherein the total thickness of said primary electrolyte (20) and said secondary electrolytes (22a, 22b) is less than 150 μm in said multi-layered electrolyte (16).

9. A method according to claim ^"7 or 8, wherein said primary electrolyte (20) and said secondary electrolytes (22a, 22b) are formed from sheets.