JP2015173091A - fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell capable of reducing an occurrence frequency of peeling at an interface of an intermediate layer interposed in a boundary part between a support substrate and a fuel electrode, and also capable of operating at a high limitation fuel utilization ratio.SOLUTION: In a fuel cell, a support substrate 10 includes magnesium oxide MgO and first oxide ceramic. A fuel electrode collector part 21 includes nickel Ni and second oxide ceramic. An intermediate layer 15 is interposed in a boundary part between the support substrate 10 and the fuel electrode collector part 21. The intermediate layer 15 includes (Mg,Ni)O as a solid solution of magnesium oxide (MgO) and nickel oxide (NiO), the first oxide ceramic and the second oxide ceramic. In a state after being subjected to reduction treatment, the intermediate layer 15 has a lower porosity as compared with the support substrate 10 and the fuel electrode collector part 21. The support substrate 10 has a porosity of 25-55%, the fuel electrode collector part 21 has a porosity of 25-55%, and the intermediate layer 15 has a porosity of 10-43%.

Description

  The present invention relates to a fuel cell.

  Conventionally, “a support substrate having a gas flow path formed therein and including magnesium oxide (MgO) and first oxide ceramics” and “a nickel substrate (Ni) ) And a second oxide ceramics, a solid oxide fuel cell comprising a power generating element portion in which a fuel electrode, a solid electrolyte, and an air electrode are laminated in this order is known. (For example, refer to Patent Document 1).

  In general, each component of the fuel cell described above is formed by firing in an oxygen-containing atmosphere. By firing in this oxygen-containing atmosphere, the Ni component contained in the fuel electrode is NiO. NiO does not have electronic conductivity. Therefore, in order to acquire the electron conductivity of the fuel electrode, after the fuel electrode (fuel cell) is fired, the fuel electrode is “reduced”. The “reduction treatment” is a heat treatment performed in a reducing atmosphere. Specifically, for example, NiO in the fuel electrode is heated at a high temperature of about 800 to 1000 ° C. while flowing a reducing fuel gas from the support substrate side. It is the process which reduces by.

  In the fuel cell described above, cracks may occur at the boundary between the support substrate and the fuel electrode during the reduction treatment after firing. This is considered based on the following reasons. That is, during the reduction process, due to the fact that the fuel electrode contains a large amount of NiO, the NiO is reduced to Ni, so that the fuel electrode contracts (reduction shrinkage) while being reduced. The amount of shrinkage of the support substrate with a low content of the substance is small. Therefore, excessive strain (and hence thermal stress) due to the difference in the amount of contraction between the support substrate and the fuel electrode can occur. It is considered that cracks are generated at the boundary portion between the support substrate and the fuel electrode due to this excessive thermal stress.

Japanese Patent No. 4888733

  In order to cope with this problem, the present inventor reduces the occurrence frequency of the above-described cracks by inserting an intermediate layer having a higher strength than the support substrate and the fuel electrode at the boundary portion between the support substrate and the fuel electrode. I am thinking of that. Specifically, the intermediate layer includes (Mg, Ni) O, which is a solid solution of magnesium oxide (MgO) and nickel oxide (NiO), the first oxide ceramic, the second oxide ceramic, including. In addition, in order to increase the strength of the intermediate layer compared to the support substrate and the fuel electrode, the porosity of the intermediate layer is the support layer in a state where the fuel cell is “reduced body subjected to the reduction treatment”. It is smaller than the porosity of the substrate and the porosity of the fuel electrode.

  By the way, when the fuel cell having the intermediate layer inserted therein is operated under a severe thermal stress environment, the interface between the intermediate layer and the support substrate or the interface between the intermediate layer and the fuel electrode (hereinafter referred to as the fuel cell). In some cases, peeling occurs at the interface of the “intermediate layer”. In addition, when the intermediate layer is interposed in this manner, the limit fuel utilization rate of the fuel cell may be extremely reduced. It has been desired to reduce the frequency of occurrence of the above-described peeling of the interface of the intermediate layer and to suppress a decrease in the limit fuel utilization rate of the fuel cell.

  As described above, the present invention provides a fuel cell that can reduce the frequency of occurrence of delamination at the interface of the intermediate layer inserted in the boundary portion between the support substrate and the fuel electrode and can operate under a high fuel utilization rate. For the purpose.

  In the fuel cell according to the present invention, the support substrate including the same support substrate as described above and the same power generation element unit as described above may be flat or cylindrical.

  In the fuel cell according to the present invention, an intermediate layer is interposed at a boundary portion between the support substrate and the fuel electrode, and the intermediate layer is a solid solution of magnesium oxide (MgO) and nickel oxide (NiO) (Mg, Ni) O, the first oxide ceramic, and the second oxide ceramic. The first and second oxide ceramics may be the same or different.

  The fuel cell according to the present invention is characterized in that the porosity of the intermediate layer is more than the porosity of the support substrate and the porosity of the fuel electrode when the fuel cell is a reductant that has been heat-treated in a reducing atmosphere. The porosity of the support substrate is 25 to 55%, the porosity of the fuel electrode is 25 to 55%, and the porosity of the intermediate layer is 10 to 43%. In this case, it is preferable that the thickness of the intermediate layer is 3 to 100 μm.

  As described above, the present inventor has a lower porosity than the support substrate and the fuel electrode when the porosity of the support substrate is 25 to 55% and the porosity of the fuel electrode is 25 to 55%. It has been found that when the porosity of the intermediate layer is 10 to 43%, the frequency of occurrence of delamination at the interface of the intermediate layer is reduced, and the limit fuel utilization rate of the fuel cell is less likely to decrease (about this point) Will be detailed later).

1 is a perspective view showing a fuel cell according to the present invention. FIG. 2 is a cross-sectional view corresponding to line 2-2 of the fuel cell shown in FIG. It is the top view which showed the state of the fuel electrode and interconnector which were embed | buried under the recessed part of the support substrate shown in FIG. It is a figure for demonstrating the operation state of the fuel cell shown in FIG. It is a figure for demonstrating the flow of the electric current in the operating state of the fuel cell shown in FIG. It is a perspective view which shows the support substrate shown in FIG. FIG. 3 is a cross-sectional view corresponding to FIG. 2 in a first stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 3 is a cross-sectional view corresponding to FIG. 2 in a second stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 3 is a cross-sectional view corresponding to FIG. 2 in a third stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 4 is a cross-sectional view corresponding to FIG. 2 in a fourth stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 6 is a cross-sectional view corresponding to FIG. 2 in a fifth stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 7 is a cross-sectional view corresponding to FIG. 2 in a sixth stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 8 is a cross-sectional view corresponding to FIG. 2 in a seventh stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 7 is a cross-sectional view corresponding to FIG. 2 in an eighth stage in the manufacturing process of the fuel cell shown in FIG. 1. FIG. 10 is a cross-sectional view corresponding to FIG. 2 in a ninth stage in the manufacturing process of the fuel cell shown in FIG. 1. It is a schematic diagram for demonstrating the structure which the intermediate | middle layer intervened in the boundary part of a support substrate and a fuel electrode current collection part. It is a figure for demonstrating the specific structure of the intermediate | middle layer shown in FIG. It is a schematic diagram corresponding to FIG. 16 of the modification of the intermediate | middle layer shown in FIG. It is sectional drawing corresponding to FIG. 2 of the modification of the fuel cell which concerns on this invention.

(Constitution)
FIG. 1 shows a structure of a solid oxide fuel cell (SOFC) according to an embodiment of the present invention. This SOFC is electrically connected in series to each of the upper and lower surfaces (main surfaces (planes) on both sides parallel to each other) of the flat support substrate 10 having a longitudinal direction (x-axis direction) (this example). In this case, the four power generation element portions A having the same shape are arranged at predetermined intervals in the longitudinal direction, so-called “horizontal stripe type”.

  The shape of the entire SOFC as viewed from above is, for example, a rectangle whose length in the longitudinal direction is 50 to 500 mm and whose length in the width direction (y-axis direction) perpendicular to the longitudinal direction is 10 to 100 mm. The total thickness of this SOFC is 1-5 mm. The entire SOFC has a vertically symmetrical shape with respect to a plane passing through the center in the thickness direction and parallel to the main surface of the support substrate 10. Hereinafter, in addition to FIG. 1, the details of the SOFC will be described with reference to FIG. 2, which is a partial cross-sectional view of the SOFC corresponding to line 2-2 shown in FIG. FIG. 2 is a partial cross-sectional view showing a configuration (part of) each of a typical pair of adjacent power generation element portions A and A and a configuration between the power generation element portions A and A. The configuration between the other power generation element portions A and A in other sets is the same as the configuration shown in FIG.

  The support substrate 10 is a flat plate-like fired body made of a porous material having no electronic conductivity. As shown in FIG. 6 to be described later, a plurality of (six in this example) fuel gas passages 11 (through holes) extending in the longitudinal direction are provided in the support substrate 10 at predetermined intervals in the width direction. Is formed. In this example, the recesses 12 are respectively formed at positions corresponding to the plurality of power generation element portions A on the upper and lower surfaces of the support substrate 10. Each recess 12 has a bottom wall made of the material of the support substrate 10 and side walls closed in the circumferential direction made of the material of the support substrate 10 (two side walls along the longitudinal direction and two side walls along the width direction). ) And a rectangular parallelepiped depression defined by Each recess 12 has a length (dimension in the x-axis direction) of 5 to 50 mm, a width (dimension in the y-axis direction) of 2 to 95 mm, and a depth (dimension in the z-axis direction) of 0.03 to 1. .5 mm.

The support substrate 10 includes MgO (magnesium oxide) and a first oxide ceramic. The support substrate 10 contains the first oxide ceramic because the thermal expansion coefficient of MgO alone (about 14 ppm / K) is compared with the thermal expansion coefficient of normal electrode material (10-13 ppm / K). This is because the equivalent thermal expansion coefficient of the support substrate 10 is brought close to the thermal expansion coefficient of a normal electrode material due to the large size. Therefore, as the first oxide ceramic, those having a smaller thermal expansion coefficient than that of a normal electrode material (10 to 13 ppm / K) are preferable. Specifically, as the “first oxide ceramic”, Y ₂ O ₃ (yttria), YSZ (8YSZ) (yttria stabilized zirconia), CSZ (calcia stabilized zirconia) and the like are suitable. The support substrate 10 may contain “transition metal oxide or transition metal”. As the “transition metal oxide or transition metal”, NiO (nickel oxide) or Ni (nickel) is suitable. The transition metal can function as a catalyst for promoting a reforming reaction of the fuel gas (hydrocarbon-based gas reforming catalyst).

  As described above, since the support substrate 10 contains “transition metal oxide or transition metal”, the gas containing the residual gas component before the reforming is supplied to the fuel through the numerous pores inside the porous support substrate 10. In the process of being supplied from the gas flow path 11 to the fuel electrode, the catalytic action can promote the reforming of the residual gas component before the reforming. In addition, when the support substrate 10 contains insulating oxide ceramics, the insulation of the support substrate 10 can be ensured. As a result, insulation between adjacent fuel electrodes can be ensured.

  The thickness of the support substrate 10 is 1 to 5 mm. The porosity of the support substrate 10 is 25 to 55%. In addition, the value of porosity is a value after the reduction process described later (the same applies to other porosity values). The porosity was measured by polishing the cross section of the resin-embedded sample (after the reduction treatment) and analyzing the image (secondary electron image) of the cross section by SEM (scanning electron microscope). . Specifically, the ratio of the “total area of portions corresponding to the resin-filled region on the cross section” to the “total area of the cross section” was defined as the “porosity” of the cross section. The acceleration voltage of SEM was set to 5 kV, and the magnification of SEM was set to 5000 times or 10,000 times. The measurement of the porosity was performed on arbitrary 10 cross-sections of the sample, and the average value thereof was adopted as the porosity value.

  Hereinafter, only the configuration on the upper surface side of the support substrate 10 will be described in consideration of the fact that the shape of the structure is vertically symmetrical. The same applies to the configuration of the lower surface side of the support substrate 10.

  As shown in FIGS. 2 and 3, the entire fuel electrode current collector 21 is embedded (filled) in each recess 12 formed in the upper surface (upper main surface) of the support substrate 10. Therefore, each fuel electrode current collector 21 has a rectangular parallelepiped shape. As will be described later, an intermediate layer, which is a porous fired body, is provided at the boundary between the support substrate 10 and each fuel electrode current collector 21 (that is, the portion corresponding to the bottom wall and side wall of each recess 12). 15 is interposed.

  A recess 21 a is formed on the upper surface (outer surface) of each fuel electrode current collector 21. Each recess 21a includes a bottom wall made of the material of the fuel electrode current collector 21 and a side wall (two side walls along the longitudinal direction) closed in the circumferential direction made of the material of the fuel electrode current collector 21 over the entire circumference. And two side walls along the width direction).

  The entire anode active portion 22 is embedded (filled) in each recess 21a. Accordingly, each fuel electrode active portion 22 has a rectangular parallelepiped shape. A fuel electrode 20 is configured by the fuel electrode current collector 21 and the fuel electrode active unit 22. The fuel electrode 20 (fuel electrode current collector 21 + fuel electrode active part 22) is a fired body made of a porous material having electron conductivity. The four side surfaces and the bottom surface of each anode active portion 22 are in contact with the anode current collecting portion 21 in the recess 21a.

  A recess 21b is formed in a portion of the upper surface (outer surface) of each fuel electrode current collector 21 excluding the recess 21a. Each recess 21b includes a bottom wall made of a material of the fuel electrode current collector 21 and a circumferentially closed side wall made of the material of the fuel electrode current collector 21 (two side walls along the longitudinal direction). And two side walls along the width direction).

  An interconnector 30 is embedded (filled) in each recess 21b. Accordingly, each interconnector 30 has a rectangular parallelepiped shape. The interconnector 30 is a fired body made of a dense material having electronic conductivity. The four side surfaces and the bottom surface of each interconnector 30 are in contact with the fuel electrode current collector 21 in the recess 21b.

  The upper surface (outer surface) of the fuel electrode 20 (the fuel electrode current collector 21 and the fuel electrode active unit 22), the upper surface (outer surface) of the interconnector 30, and the main surface of the support substrate 10 form one plane (recessed portion). The same plane as the main surface of the support substrate 10 when 12 is not formed) is formed. That is, no step is formed between the upper surface of the fuel electrode 20, the upper surface of the interconnector 30, and the main surface of the support substrate 10.

The fuel electrode current collector 21 includes NiO (nickel oxide) and a second oxide ceramic. The anode current collector 21 contains the second oxide ceramic because the thermal expansion coefficient of NiO alone (about 14 ppm / K) is the thermal expansion coefficient of normal electrode materials (10-13 ppm / K). This is because the equivalent thermal expansion coefficient of the fuel electrode current collector 21 is brought close to the thermal expansion coefficient of a normal electrode material due to the fact that it is larger than the above. Therefore, as the second oxide ceramic, those having a smaller thermal expansion coefficient than that of a normal electrode material (10 to 13 ppm / K) are preferable. Specifically, as the “second oxide ceramic”, Y ₂ O ₃ (yttria), YSZ (8YSZ) (yttria stabilized zirconia), CSZ (calcia stabilized zirconia) and the like are suitable. The thickness of the fuel electrode current collector 21 (that is, the depth of the recess 12) is 50 to 500 μm. The porosity of the fuel electrode current collector 21 is 25 to 55%.

  The anode active part 22 includes a substance having electron conductivity and a substance having oxygen ion conductivity. As the “substance having electron conductivity”, NiO (nickel oxide) is suitable. As the “substance having oxygen ion conductivity”, YSZ (8YSZ) (yttria stabilized zirconia), GDC (gadolinium doped ceria) and the like are suitable. The thickness of the fuel electrode active part 22 is 5 to 30 μm. The porosity of the anode active portion 22 is 25 to 55%.

  Note that NiO in the fuel electrode current collector 21 and in the fuel electrode active part 22 is changed to Ni by a reduction process, which will be described later, to acquire electron conductivity. The “volume ratio of the substance having oxygen ion conductivity with respect to the entire volume excluding the pore portion” in the anode active portion 22 is “having oxygen ion conductivity with respect to the entire volume excluding the pore portion” in the anode current collecting portion 21. It is larger than the “volume ratio of the substance”.

The interconnector 30 can be composed of, for example, LaCrO ₃ (lanthanum chromite). Alternatively, it may be composed of (Sr, La) TiO ₃ (strontium titanate). The thickness of the interconnector 30 is 10 to 100 μm.

  The entire surface excluding the central portion in the longitudinal direction of each portion where the plurality of interconnectors 30 are formed on the outer peripheral surface extending in the longitudinal direction of the support substrate 10 in a state where the fuel electrode 20 and the interconnector 30 are embedded in the respective recesses 12. Is covered with a solid electrolyte membrane 40. The solid electrolyte membrane 40 is a fired body made of a dense material having ion conductivity. The solid electrolyte membrane 40 can be made of, for example, YSZ (8YSZ) (yttria stabilized zirconia). Or you may comprise from LSGM (lantern gallate). The thickness of the solid electrolyte membrane 40 is 3 to 50 μm.

  That is, the entire outer peripheral surface extending in the longitudinal direction of the support substrate 10 in a state where the fuel electrode 20 is embedded in each recess 12 is covered with a dense layer composed of the interconnector 30 and the solid electrolyte membrane 40. This dense layer exhibits a gas sealing function that prevents mixing of the fuel gas flowing in the space inside the dense layer and the air flowing in the space outside the dense layer. In the present application, “dense” means “high density so that gas does not pass”, and specifically means “porosity is 10% or less”.

  As shown in FIG. 2, in this example, the solid electrolyte membrane 40 covers the upper surface of the fuel electrode 20, both end portions in the longitudinal direction on the upper surface of the interconnector 30, and the main surface of the support substrate 10. Here, as described above, no step is formed between the upper surface of the fuel electrode 20, the upper surface of the interconnector 30, and the main surface of the support substrate 10. Therefore, the solid electrolyte membrane 40 is flattened. As a result, compared with the case where a step is formed in the solid electrolyte membrane 40, the generation of cracks in the solid electrolyte membrane 40 due to stress concentration can be suppressed, and the gas sealing function of the solid electrolyte membrane 40 is reduced. Can be suppressed.

  An air electrode 60 is formed on the upper surface of a portion in contact with each fuel electrode active part 22 in the solid electrolyte membrane 40 via a reaction preventing film 50. The reaction preventing film 50 is a fired body made of a dense material, and the air electrode 60 is a fired body made of a porous material having electron conductivity. The shape of the reaction preventing film 50 and the air electrode 60 viewed from above is substantially the same rectangle as the fuel electrode active part 22.

The reaction preventing film 50 can be made of, for example, GDC = (Ce, Gd) O ₂ (gadolinium-doped ceria). The thickness of the reaction preventing film 50 is 3 to 50 μm. The air electrode 60 can be made of, for example, LSCF = (La, Sr) (Co, Fe) O ₃ (lanthanum strontium cobalt ferrite). Alternatively, from LSF = (La, Sr) FeO ₃ (lanthanum strontium ferrite), LNF = La (Ni, Fe) O ₃ (lanthanum nickel ferrite), LSC = (La, Sr) CoO ₃ (lanthanum strontium cobaltite), etc. It may be configured. Further, the air electrode 60 may be configured by two layers of a first layer (inner layer) made of LSCF and a second layer (outer layer) made of LSC. The thickness of the air electrode 60 is 10 to 100 μm.

  The reaction preventing film 50 is interposed because the YSZ in the solid electrolyte film 40 and the Sr in the air electrode 60 react with each other in the SOFC during the production or operation of the SOFC, and the solid electrolyte film 40 and the air electrode. This is in order to suppress the occurrence of a phenomenon in which a reaction layer having a large electric resistance is formed at the boundary portion with 60.

  Here, the laminated body formed by laminating the fuel electrode 20, the solid electrolyte membrane 40, the reaction preventing membrane 50, and the air electrode 60 corresponds to the “power generation element portion A” (see FIG. 2). In other words, a plurality (four in this example) of power generating element portions A are arranged on the upper surface of the support substrate 10 at a predetermined interval in the longitudinal direction.

  For each pair of adjacent power generation element portions A and A, the air electrode 60 of one power generation element portion A (on the left side in FIG. 2) and the interconnector of the other power generation element portion A (on the right side in FIG. 2). The air electrode current collecting film 70 is formed on the upper surfaces of the air electrode 60, the solid electrolyte film 40, and the interconnector 30. The air electrode current collector film 70 is a fired body made of a porous material having electron conductivity. The shape of the air electrode current collector film 70 as viewed from above is a rectangle.

The air electrode current collector film 70 can be made of, for example, LSCF = (La, Sr) (Co, Fe) O ₃ (lanthanum strontium cobalt ferrite). Alternatively, LSC = (La, Sr) CoO ₃ (lanthanum strontium cobaltite) may be used. Or you may comprise from Ag (silver) and Ag-Pd (silver palladium alloy). The thickness of the air electrode current collector film 70 is 50 to 500 μm.

  By forming each air electrode current collecting film 70 in this way, for each pair of adjacent power generation element portions A and A, the air electrode 60 of one power generation element portion A (on the left side in FIG. 2), The other fuel electrode 20 (particularly, the fuel electrode current collector 21) of the power generating element part A (on the right side in FIG. 2) passes through the “air electrode current collector film 70 and interconnector 30” having electronic conductivity. Are electrically connected. As a result, a plurality (four in this example) of power generation element portions A arranged on the upper surface of the support substrate 10 are electrically connected in series. Here, the “air electrode current collector film 70 and the interconnector 30” having electronic conductivity correspond to “electrical connection portions”.

  The interconnector 30 corresponds to the “first portion made of a dense material” in the “electrical connection portion” and has a porosity of 10% or less. The air electrode current collecting film 70 corresponds to the “second portion made of a porous material” in the “electrical connection portion”, and has a porosity of 20 to 60%.

As described above, the reformed fuel gas (hydrogen gas or the like) flows through the fuel gas flow path 11 of the support substrate 10 as shown in FIG. By exposing the upper and lower surfaces (in particular, each air electrode current collecting film 70) to “gas containing oxygen” (air or the like) (or flowing a gas containing oxygen along the upper and lower surfaces of the support substrate 10), a solid is obtained. An electromotive force is generated by an oxygen partial pressure difference generated between both side surfaces of the electrolyte membrane 40. Furthermore, when this structure is connected to an external load, chemical reactions shown in the following formulas (1) and (2) occur, and current flows (power generation state).
(1/2) · O ₂ + 2e ⁻ → O ²⁻ (where: air electrode 60) (1)
H ₂ + O ²⁻ → H ₂ O + 2e ⁻ (in the fuel electrode 20) (2)

  In the power generation state, as shown in FIG. 5, a current flows as indicated by an arrow in each pair of adjacent power generation element portions A and A. As a result, as shown in FIG. 4, from this entire SOFC (specifically, the interconnector 30 of the power generating element part A on the frontmost side in FIG. 4 and the air electrode 60 of the power generating element part A on the innermost side in FIG. Power).

(Production method)
Next, an example of a method for manufacturing the “horizontal stripe type” SOFC shown in FIG. 1 will be briefly described with reference to FIGS. 6 to 15, “g” at the end of the reference numeral of each member indicates that the member is “before firing”.

First, a support substrate molded body 10g having the shape shown in FIG. 6 is produced. For example, this support substrate molded body 10g uses a slurry obtained by adding a binder or the like to the powder of the material of the support substrate 10 (for example, MgO and Y ₂ O ₃ ). It can be made using. Hereinafter, the description will be continued with reference to FIGS. 7 to 15 showing partial cross sections corresponding to the line 7-7 shown in FIG.

  As shown in FIG. 7, when the support substrate molded body 10g is manufactured, next, as shown in FIG. 8, the bottom and side surfaces of the recesses 12 formed on the upper and lower surfaces of the support substrate molded body 10g are formed. Then, an intermediate layer forming film 15g is formed. The formation of the intermediate layer forming film 15g will be described later.

Next, as shown in FIG. 9, the molded body 21g of the fuel electrode current collector is formed in the “recesses 12 formed with the molded film 15g of the intermediate layer” formed on the upper and lower surfaces of the molded body 10g of the support substrate. Each is buried and formed. Next, as shown in FIG. 10, a molded body 22g of the fuel electrode active portion is embedded and formed in each recess formed in the outer surface of the molded body 21g of each fuel electrode current collector. The molded body 21g of each fuel electrode current collector and each fuel electrode active part 22g are, for example, a slurry obtained by adding a binder or the like to the powder of the material of the fuel electrode 20 (for example, Ni and Y ₂ O ₃ ). It is embedded and formed using a printing method or the like.

Subsequently, as shown in FIG. 11, in each concave portion formed in “the portion excluding the portion where the molded body 22 g of the fuel electrode active portion is embedded” on the outer surface of the molded body 21 g of each fuel electrode current collector. The interconnector molded bodies 30g are respectively embedded and formed. The molded body 30g of each interconnector is embedded and formed by using a slurry obtained by adding a binder or the like to the material of the interconnector 30 (for example, LaCrO ₃ ), using a printing method or the like. .

  Next, as shown in FIG. 12, the outer periphery extending in the longitudinal direction of the molded body 10g of the support substrate in a state in which the molded body (21g + 22g) of the plurality of fuel electrodes and the molded body 30g of the plurality of interconnectors are respectively embedded and formed. A solid electrolyte membrane molded film 40g is formed on the entire surface excluding the central portion in the longitudinal direction of each portion where the plurality of interconnector molded bodies 30g are formed. The molded membrane 40g of the solid electrolyte membrane is formed using, for example, a printing method, a dipping method, etc., using a slurry obtained by adding a binder or the like to the powder of the material of the solid electrolyte membrane 40 (for example, YSZ). The

  Next, as shown in FIG. 13, a reaction preventing film forming film 50 g is formed on the outer surface of the solid electrolyte film forming body 40 g in contact with the fuel electrode forming body 22 g. The molded film 50g of each reaction preventing film is formed using a slurry obtained by adding a binder or the like to the powder of the material (for example, GDC) of the reaction preventing film 50, using a printing method or the like.

  Then, 10 g of the support substrate molded body in which various molded films are thus formed is fired in air at 1500 ° C. for 3 hours. As a result, a structure in which the air electrode 60 and the air electrode current collector film 70 are not formed in the SOFC shown in FIG. 1 is obtained.

  Next, as shown in FIG. 14, an air electrode forming film 60 g is formed on the outer surface of each reaction preventing film 50. The molded film 60g of each air electrode is formed using a slurry obtained by adding a binder or the like to the powder of the material of the air electrode 60 (for example, LSCF), using a printing method or the like.

  Next, as shown in FIG. 15, for each pair of adjacent power generation element portions, air is formed so as to straddle the air electrode molding film 60 g of one power generation element portion and the interconnector 30 of the other power generation element portion. On the outer surface of the electrode forming film 60 g, the solid electrolyte film 40, and the interconnector 30, the air electrode current collecting film forming film 70 g is formed. The forming film 70g of each air electrode current collector film is obtained by using a slurry obtained by adding a binder or the like to the powder of the material of the air electrode current collector film 70 (for example, LSCF), using a printing method or the like. It is formed.

  Then, the support substrate 10 in which the molded films 60g and 70g are thus formed is baked in air at 1050 ° C. for 3 hours. Thereby, the SOFC shown in FIG. 1 is obtained. At this point, the Ni component in the fuel electrode 20 (current collector 21 + active portion 22) is NiO due to firing in an oxygen-containing atmosphere. Therefore, in order to acquire the electron conductivity of the fuel electrode 20 (current collector 21 + active part 22), a reducing fuel gas is then flowed from the support substrate 10 side, and NiO is heated at 800 to 1000 ° C. for 1 to 10 hours. The reduction treatment is performed over a period of time. This reduction process may be performed during power generation. In the above, an example of the manufacturing method of SOFC shown in FIG. 1 was demonstrated.

(Intermediate layer)
In the above embodiment, as shown in FIG. 16, an intermediate layer is formed on the boundary portion between the support substrate 10 and each fuel electrode 20 (current collector 21) (that is, the portion corresponding to the bottom wall and the side wall of each recess 12). 15 is interposed. Even if the intermediate layer 15 is provided over the entire boundary portion between the support substrate 10 and each fuel electrode current collector 21 (that is, the entire region corresponding to the bottom wall and the side wall of each recess 12), It may be provided only in a part of the boundary portion. The thickness of the intermediate layer 15 (see T in FIG. 16 and T (= Ta + Tb) in FIG. 18 described later) is 3 to 100 μm. The definition of the boundary between the support substrate 10 and the intermediate layer 15 and the definition of the boundary between the intermediate layer 15 and the anode current collector 21 (therefore, the definition of the thickness of the intermediate layer 15) will be described later.

As shown in FIG. 17, after the reduction treatment, the intermediate layer 15 includes “MgO-containing particles” (first particles. Typically, solid solution (Mg, Ni) O particles) and “first particles”. Particles containing oxide ceramics and particles containing second oxide ceramics "(second particles, typically Y ₂ O ₃ particles) and" metal fine particles containing Ni "(third particles, typically Ni particles). In the intermediate layer 15, the third particles are fixed to the surface of the first particles. And adjacent 1st particle | grains and the adjacent 1st particle | grain and 2nd particle | grain are couple | bonded through 3rd particle | grains. The presence of the third particles (fixed Ni particles) can promote the fuel gas “reforming reaction of methane (CH ₄ ) → hydrogen (H ₂ )”. The porosity of the intermediate layer 15 is smaller than the porosity of the support substrate 10 and the porosity of the fuel electrode current collector 21. The appropriate range of the porosity of the intermediate layer 15 will be described later.

  In other words, the intermediate layer 15 includes (Mg, Ni) O, which is a solid solution of MgO and NiO, “first oxide ceramics”, “second oxide ceramics”, and Ni. In addition, in the intermediate layer 15 (in the thickness direction of the intermediate layer 15), the portion close to the anode current collector 21 (on the near side), the support layer 10 in the intermediate layer 15 (in the thickness direction of the intermediate layer 15). Compared with the close part (near side), “Ni molar ratio” and “second oxide ceramics molar ratio” are relatively large, and “Mg molar ratio” and “first oxide”. The “ceramic content molar ratio” is relatively small.

The intermediate layer 15 is formed as follows. First, the MgO powder and NiO powder as the raw material powder of the intermediate layer 15 were weighed so as to have a molar ratio of 1: 1. Subsequently, these mixtures were fired at 1400 ° C. in an air atmosphere for 5 hours. Thereby, the solid solution ((Mg, Ni) O) of MgO and NiO was produced. This solid solution has the property that it is very difficult to reduce in a reducing atmosphere. In addition, complete solid solution was confirmed by powder X-ray diffraction or the like. This solid solution was pulverized by a pot mill to obtain a powder having D50 = 0.52 μm. A slurry was prepared by adding a solvent and a binder to the obtained powder. This slurry was applied to each concave portion of the above-mentioned support substrate molded body 10g (see FIG. 6) by a spray coating method to form an intermediate layer molded film 15g (see FIG. 8). Thereafter, as described above, the intermediate layer molded film 15g is co-fired with the support substrate molded body 10g and the fuel electrode current collecting layer molded body 21g (see FIGS. 13 to 14). During the co-firing, “MgO and first oxide ceramics” in the support substrate 10 and “NiO and second oxide ceramics” in the fuel electrode current collector 21 are in the intermediate layer 15. It enters by diffusion. As a result, the “side of the intermediate layer 15 close to the fuel electrode current collector 21” and the “side of the intermediate layer 15 close to the support substrate 10” and “the molar ratio of Ni” and “second oxide ceramics” The “content molar ratio of” is relatively large, and the “content molar ratio of Mg” and “content molar ratio of the first oxide ceramics” are relatively small. Note that NiO and MgO that have entered the intermediate layer 15 by diffusion react with each other to form a new solid solution ((Mg, Ni) O). Thereafter, when the reduction of the solid solution (Mg, Ni) O proceeds by the above-described reduction treatment or the like, as shown in FIG. 17, Ni fine particles are deposited on the surface of the (Mg, Ni) O particles. come. Accompanying the precipitation of the Ni fine particles, the adjacent (Mg, Ni) O particles, and the adjacent (Mg, Ni) O particles and Y ₂ O ₃ particles are fixed and bonded through the Ni fine particles. As a result, the intermediate layer 15 having the structure shown in FIG. 17 is obtained.

In addition, in the state where the above-described intermediate layer molding film 15g is not present at the boundary portion between the support substrate molded body 10g and the fuel electrode current collector molded body 21g, Even when the molded body 21g is co-fired, MgO in the support substrate 10 and NiO in the fuel electrode current collector 21 diffuse into the intermediate layer 15, and thus the support substrate 10 and the fuel A solid solution (Mg, Ni) O can be naturally generated at the boundary with the pole current collector 21. Even in the case where the solid solution (Mg, Ni) O thus naturally generated is reduced by performing the above-described reduction treatment or the like, the surface of the (Mg, Ni) O particles is similar to the above. Fine particles are deposited. Accompanying the precipitation of the Ni fine particles, the adjacent (Mg, Ni) O particles, and the adjacent (Mg, Ni) O particles and Y ₂ O ₃ particles are fixed and bonded through the Ni fine particles. That is, also in this case, the intermediate layer 15 having the structure shown in FIG. 17 is obtained.

  As described above, when the intermediate layer 15 shown in FIG. 16 is interposed in the boundary portion between the support substrate 10 and the fuel electrode current collector 21, the following operations and effects are achieved. That is, during the reduction process, the fuel electrode 20 contains a large amount of NiO, and the NiO is reduced to Ni, so that the fuel electrode 20 contracts (reduction contraction). On the other hand, the shrinkage amount of the support substrate 10 with a small content of the substance to be reduced is small. Therefore, when the intermediate layer 15 is not interposed at the boundary portion between the support substrate 10 and the fuel electrode current collector 21, the boundary portion between the support substrate 10 and the fuel electrode current collector 21 is caused by the difference in contraction amount between the two. Excessive strain (and hence thermal stress) can occur. Due to this excessive thermal stress, a crack may occur at the boundary between the support substrate 10 and the fuel electrode current collector 21. On the other hand, in the SOFC according to the above embodiment, the intermediate layer 15 having a lower porosity than the support substrate 10 and the fuel electrode current collector 21 is interposed at the boundary portion between the support substrate 10 and the fuel electrode current collector 21. doing. In other words, the intermediate layer 15 having a higher strength than the support substrate 10 and the fuel electrode current collector 21 is interposed at the boundary between the support substrate 10 and the fuel electrode current collector 21. As a result, it is considered that the cracks described above are difficult to occur.

  In the intermediate layer 15 shown in FIG. 16, the intermediate layer 15 is composed of one layer, “a side near the fuel electrode current collector 21 in the intermediate layer 15” and “a side near the support substrate 10 in the intermediate layer 15”. There is no clear boundary between On the other hand, as shown in FIG. 18, the intermediate layer 15 is composed of two layers (or three or more layers). A clear boundary may exist between the layer 15b on the side close to the fuel electrode current collector 21. The layer 15a has a relatively high Mg content molar ratio and a relatively low Ni content molar ratio compared to the layer 15b. The configuration shown in FIG. 18 is created as follows, for example. That is, when the intermediate layer molding film 15g is formed in each concave portion of the above-described support substrate molding 10g (see FIG. 6), the molding film 15g becomes “a film closer to the support substrate 10” and “ It is composed of two layers, “a membrane close to the fuel electrode current collector 21”. The slurry (including (Mg, Ni) O) on the side closer to the support substrate 10 includes the slurry ((Mg, Ni) O on the side closer to the fuel electrode current collector 21). ”And“ Molar ratio of Mg ”and“ Molar ratio of the first oxide ceramics ”are relatively large, and“ Molar mole ratio of Ni ”and“ Molar ratio of the second oxide ceramics ” "Is relatively small. The two-layered “middle layer molded film 15g” is co-fired with the support substrate molded body 10g and the fuel electrode current collecting layer molded body 21g. Thereafter, these fired bodies are subjected to the reduction treatment described above. As a result, the intermediate layer 15 (= layer 15a + layer 15b) shown in FIG. 18 is obtained. Both the layer 15a and the layer 15b have the structure shown in FIG.

(Proper range of porosity of the intermediate layer)
In the SOFC according to the above-described embodiment, when operated in a normal environment, the interface between the intermediate layer 15 and the support substrate 10 or the interface between the intermediate layer 15 and the fuel electrode current collector 21 (hereinafter simply referred to as “intermediate layer 15”). No peeling occurs at the “interface of the intermediate layer 15”). However, when this SOFC is operated under a severe environment in terms of thermal stress, peeling may occur at the interface of the intermediate layer 15. In addition, when the intermediate layer 15 is inserted in this way, the SOFC limit fuel utilization rate may be extremely reduced. The inventor has found that the occurrence of delamination at the interface of the intermediate layer 15 and the decrease in the SOFC limit fuel utilization rate have a strong correlation with the porosity of the intermediate layer 15. Hereinafter, a test for confirming this will be described.

(test)
In this test, a plurality of samples having different combinations of the porosity of the support substrate 10, the porosity of the fuel electrode current collector 21 and the porosity of the intermediate layer 15 were produced for the SOFC shown in FIG. Specifically, as shown in Table 1, eight levels (combinations) were prepared. Ten samples (N = 10) were made for each level. For each sample, the boundary of the intermediate layer 15 was defined as follows. First, elemental quantitative analysis (line analysis) of Ni and Mg using EDS (energy dispersive X-ray analysis) in the cross section of the sample is continuously performed in the stacking direction from the support substrate side to the fuel electrode current collector side. To do. With respect to the cross section, the position where the value of “Ni / (Ni + Mg)” exceeds 0.30 in terms of molar ratio is defined as “the interface between the intermediate layer and the support substrate”, and “Ni / (Ni + Mg)” in molar ratio. The position where the value exceeds 0.90 is defined as “the interface between the intermediate layer and the fuel electrode current collector”. As the thickness of the intermediate layer 15, an average value of “distance between both interfaces” measured at any 10 points in the cross section is adopted.

In each sample (SOFC shown in FIG. 1), the intermediate layer 15 extends over the entire boundary portion between the support substrate 10 and the anode current collector 21 (that is, the entire bottom wall and side walls of each recess 12). Been formed. The intermediate layer 15 is obtained by co-firing the “middle layer molded film 15 g” with the support substrate molded body 10 g and the anode current collecting layer molded body 21 g, and then subjecting the fired body to a reduction treatment. Thus formed. For each sample, MgO—Y ₂ O ₃ , NiO / Ni—MgO—Y ₂ O ₃ , MgO—MgAl ₂ O ₄ , NiO / Ni—MgO—MgAl ₂ O _4, etc. are used as the material of the support substrate 10. It was. NiO / Ni—YSZ, NiO / Ni—CSZ, NiO / Ni—Y ₂ O ₃ or the like was used as the material for the fuel electrode current collector 21. As the material of the intermediate layer 15, NiO / Ni—MgO—YSZ—Y ₂ O ₃ , NiO / Ni—MgO—YSZ—MgAl ₂ O ₄ , NiO / Ni—MgO—CSZ—MgAl ₂ O ₄ or the like was used. . The thickness of the intermediate layer 15 was 3 to 100 μm. The adjustment of the porosity of each of the support substrate 10, the fuel electrode current collector 21, and the intermediate layer 15 is to adjust the amount and diameter of the pore former contained in the slurry used for producing the corresponding molded body. Made by etc. As shown in Table 1, the porosity of the support substrate was 25 to 55%, and the porosity of the fuel electrode was 25 to 55%.

In this test, first, the limit fuel utilization rate of SOFC was measured for each sample after the reduction treatment (SOFC shown in FIG. 1). This measurement was performed under the conditions of “temperature is 800 ° C. and current density is 0.2 A / cm ² ”. The results of this measurement are as shown in Table 1.

  Thereafter, for each sample, “a pattern in which the ambient temperature is raised from room temperature to 750 ° C. in 2 hours and then reduced from 750 ° C. to room temperature in 4 hours while reducing fuel gas is circulated through the fuel electrode 20” is repeated 10 times. A thermal cycle test was conducted. And about each sample, the presence or absence of peeling of the interface of the intermediate | middle layer 15 was confirmed. This confirmation was made by visual observation as well as observation using a microscope. The results are as shown in Table 1.

  As understood from Table 1, when the porosity of the support substrate 10 is 25 to 55% and the porosity of the fuel electrode current collector 21 is 25 to 55%, the porosity of the intermediate layer 15 is 10%. If it is less than that, the limit fuel utilization characteristic of the SOFC is extremely lowered as compared with the case where the porosity of the intermediate layer 15 is 10% or more. This is considered based on the following reasons. That is, the fuel gas flowing through the fuel gas flow path 11 of the support substrate 10 sequentially passes through the pores inside the support substrate 10, the pores inside the intermediate layer 15, and the pores inside the fuel electrode current collector 21. The fuel electrode active part 22 is reached. In this process, if the porosity of the intermediate layer 15 is too small, the fuel gas hardly passes through the pores in the intermediate layer 15 and the fuel gas hardly reaches the fuel electrode active portion 22. As a result, the amount of fuel gas that can react with oxygen at the anode active portion 22 (more precisely, near the interface between the anode active portion 22 and the solid electrolyte membrane 40) is extremely reduced, and the fuel limit of SOFC The utilization rate drops extremely.

  Further, as can be understood from Table 1, when the porosity of the support substrate 10 is 25 to 55% and the porosity of the fuel electrode current collector 21 is 25 to 55%, the above-mentioned severe thermal stress After the thermal cycle test is performed, if the porosity of the intermediate layer 15 is greater than 43%, the interface of the intermediate layer 15 is more likely to be peeled than when the porosity of the intermediate layer 15 is 43% or less. This is considered based on the following reasons. That is, when the SOFC is operated in a normal environment, the relative magnitude relationship of the porosity is established that “the porosity of the intermediate layer 15 is smaller than that of the support substrate 10 and the fuel electrode current collector 21”. By doing so, it is possible to ensure sufficient strength of the intermediate layer 15 to such an extent that peeling of the interface of the intermediate layer 15 does not occur. On the other hand, after the thermal cycle test that is severe in terms of thermal stress, even if the relative magnitude relationship of the porosity is established, if the porosity of the intermediate layer 15 is too large, Sufficient strength of the intermediate layer 15 to such an extent that no peeling of the interface occurs can not be ensured. As a result, peeling occurs at the interface of the intermediate layer 15.

  From the above, if the porosity of the intermediate layer 15 is in the range of 10 to 43%, the frequency of occurrence of delamination at the interface of the intermediate layer 15 is reduced, and the limit fuel utilization rate of the SOFC is difficult to decrease. Can do.

  In addition, when the above embodiment is used under normal conditions and environment (for example, a pattern in which the temperature is raised from room temperature to 750 ° C. in 4 hours and then lowered from 750 ° C. to room temperature in 12 hours), It has been separately confirmed that even if the porosity of the layer 15 is greater than 43%, no peeling of the interface of the intermediate layer 15 occurs. Further, the porosity of the intermediate layer 15 may not be uniform over the inside of the intermediate layer 15. Specifically, the porosity of the “side of the intermediate layer 15 close to the fuel electrode current collector 21” may be larger than the porosity of the “side of the intermediate layer 15 close to the support substrate 10”.

  In addition, this invention is not limited to the said embodiment, A various modification can be employ | adopted within the scope of the present invention. For example, in the above embodiment, as shown in FIG. 6 and the like, the planar shape of the recess 12 formed in the support substrate 10 (the shape when viewed from the direction perpendicular to the main surface of the support substrate 10) is a rectangle. However, it may be, for example, a square, a circle, an ellipse, or a long hole shape. Further, the support substrate 10 has a flat plate shape, but may have a cylindrical shape.

  Further, in the above embodiment, a plurality of recesses 12 are formed on each of the upper and lower surfaces of the flat support substrate 10 and a plurality of power generation element portions A are provided, but a plurality of recesses 12 are provided only on one side of the support substrate 10. , And a plurality of power generating element portions A may be provided. Further, in the above-described embodiment, a so-called “horizontal stripe type” configuration in which a plurality of power generation element portions A electrically connected in series is disposed on one main surface of the support substrate 10 is employed. However, a configuration in which one power generation element portion A is disposed on one main surface of the support substrate 10 (so-called “vertical stripe type”) may be employed.

  Further, in the above-described embodiment, the fuel electrode 20 is composed of two layers of the fuel electrode current collector 21 and the fuel electrode active unit 22, but the fuel electrode 20 is one layer corresponding to the fuel electrode active unit 22 ( (Ni + oxide ceramics). Further, in the above embodiment, a plurality of recesses 12 are formed on the main surface of the support substrate 10 and the fuel electrode 20 is embedded in each recess 12, but as shown in FIG. The fuel electrode 20 may be laminated on the surface. In this case, as shown in FIG. 22, the intermediate layer 15 is formed on a boundary portion between the support substrate 10 and the fuel electrode 20 on the main surface of the support substrate 10.

  In addition, in the above embodiment, each fuel electrode 20 (current collector 21 and active part 22) is embedded in the recess 12 formed in the main surface of the support substrate. However, as shown in FIG. The recesses may not be formed on the main surface of the substrate, and each fuel electrode 20 (current collector 21 and active portion 22) may be formed so as to protrude from the main surface of the support substrate. In this case, no step is formed in the intermediate layer 15 as shown in FIG.

  DESCRIPTION OF SYMBOLS 10 ... Support substrate, 11 ... Fuel gas flow path, 12 ... Recess, 15 ... Intermediate layer, 20 ... Fuel electrode, 21 ... Fuel electrode current collector, 21a, 21b ... Recess, 22 ... Fuel electrode active part, 30 ... Inter Connector: 40 ... Solid electrolyte membrane, 50 ... Reaction prevention membrane, 60 ... Air electrode, 70 ... Air electrode current collector membrane, A ... Power generation element part

Claims (2)

A support substrate configured to include a magnesium oxide (MgO) and a first oxide ceramic while a gas flow path is formed therein;
A power generating element unit that is provided on the support substrate and in which a fuel electrode including nickel (Ni) and a second oxide ceramic, a solid electrolyte, and an air electrode are stacked in this order;
A fuel cell comprising:
An intermediate layer is interposed at a boundary portion between the support substrate and the fuel electrode,
The intermediate layer is
(Mg, Ni) O that is a solid solution of magnesium oxide (MgO) and nickel oxide (NiO), the first oxide ceramics, and the second oxide ceramics,
In a state where the fuel cell is a reductant that has been heat-treated in a reducing atmosphere,
The porosity of the intermediate layer is smaller than the porosity of the support substrate and the porosity of the fuel electrode, the porosity of the support substrate is 25 to 55%, and the porosity of the fuel electrode is 25 to 55%. The fuel cell has a porosity of 10 to 43%. The fuel cell according to claim 1, wherein
The thickness of the said intermediate | middle layer is a fuel cell which is 3-100 micrometers.

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