JP6182286B1 - Fuel cell - Google Patents

Abstract

A fuel cell capable of suppressing the occurrence of cracks in an air electrode is provided. A fuel cell includes a fuel electrode, an air electrode, and a solid electrolyte layer disposed between the fuel electrode and the air electrode. The air electrode 50 is represented by the general formula ABO3, and contains a perovskite oxide containing at least Sr at the A site as a main component. The air electrode 50 has a surface region 51 within 5 μm from the surface opposite to the solid electrolyte layer 30 and an internal region 52 formed in the solid electrolyte layer 30 in the surface region 51. Each of the surface region 51 and the internal region 52 is represented by the general formula ABO3, and includes a main phase composed of a perovskite oxide containing at least Sr at the A site and a second phase composed of strontium sulfate. The area occupation ratio of the second phase in the cross section of the surface region 51 is larger than the area occupation ratio of the second phase in the cross section of the internal region. [Selection] Figure 1

Description

  The present invention relates to a fuel cell.

Conventionally, a fuel cell including a fuel electrode, an air electrode, and a solid electrolyte layer disposed between the fuel electrode and the air electrode is known. As a material for the air electrode, a perovskite oxide represented by the general formula ABO ₃ and containing at least one of La and Sr at the A site is suitable (see, for example, Patent Document 1).

JP 2006-32132 A

  However, if the temperature of the air electrode falls from high to low, such as when the air electrode is fired or when the operation is once stopped after starting the operation of the fuel cell, micro cracks may occur near the surface of the air electrode. . Although the minute crack does not immediately affect the characteristics of the air electrode, when the fuel cell is operated for a long time, there is a possibility that a crack that may affect the characteristics of the air electrode starts from the minute crack. Therefore, there is a desire to suppress the generation of microcracks near the surface of the air electrode. As a result of intensive studies by the present inventors in order to meet such demands, new knowledge has been obtained that the amount of strontium sulfate contained in the air electrode is related to the occurrence of microcracks.

  The present invention is based on such new knowledge, and an object thereof is to provide a fuel cell capable of suppressing the occurrence of cracks in the air electrode.

The fuel cell according to the present invention includes a fuel electrode, an air electrode, and a solid electrolyte layer disposed between the fuel electrode and the air electrode. The air electrode is represented by the general formula ABO ₃ and contains a perovskite oxide containing at least Sr at the A site as a main component. The air electrode has a surface region within 5 μm from the surface opposite to the solid electrolyte layer, and an internal region formed on the solid electrolyte layer side of the surface region. Each of the surface region and the internal region is represented by a general formula ABO ₃ and includes a main phase composed of a perovskite oxide containing at least Sr at the A site and a second phase composed of strontium sulfate. The area occupancy of the second phase in the cross section of the surface region is larger than the area occupancy of the second phase in the cross section of the internal region.

  According to the present invention, a fuel cell capable of suppressing the occurrence of cracks in an air electrode is provided.

Sectional drawing which shows the structure of the fuel cell which concerns on embodiment

  Next, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, the drawings are schematic, and the ratio of each dimension may be different from the actual one.

(Configuration of fuel cell 10)
The configuration of the fuel cell 10 will be described with reference to the drawings. The fuel cell 10 is a so-called solid oxide fuel cell (SOFC). The fuel cell 10 may take the form of a vertical stripe type, a horizontal stripe type, a fuel electrode support type, an electrolyte flat plate type, or a cylindrical type.

  FIG. 1 is a cross-sectional view showing the configuration of the fuel cell 10. The fuel cell 10 includes a fuel electrode 20, a solid electrolyte layer 30, a barrier layer 40, an air electrode 50, and a current collecting layer 60.

  The fuel electrode 20 functions as an anode of the fuel cell 10. As illustrated in FIG. 1, the fuel electrode 20 includes a fuel electrode current collecting layer 21 and a fuel electrode active layer 22.

The anode current collecting layer 21 is a porous body excellent in gas permeability. As a material constituting the anode current collecting layer 21, materials conventionally used for SOFC anode current collecting layers can be used. For example, NiO (nickel oxide) -8YSZ (8 mol% yttria is stabilized). Zirconia) and NiO—Y ₂ O ₃ (yttria). When the anode current collecting layer 21 contains NiO, at least a part of NiO may be reduced to Ni during operation of the fuel cell 10. The thickness of the anode current collecting layer 21 can be set to 0.1 mm to 5.0 mm, for example.

  The anode active layer 22 is disposed on the anode current collecting layer 21. The anode active layer 22 is a denser porous body than the anode current collecting layer 21. As a material constituting the anode active layer 22, materials conventionally used for the anode active layer of SOFC can be used, for example, NiO-8YSZ. When the anode active layer 22 contains NiO, at least a part of NiO may be reduced to Ni during operation of the fuel cell 10. The thickness of the anode active layer 22 can be set to, for example, 5.0 μm to 30 μm.

  The solid electrolyte layer 30 is disposed between the fuel electrode 20 and the air electrode 50. In the present embodiment, the solid electrolyte layer 30 is sandwiched between the fuel electrode 20 and the barrier layer 40. The solid electrolyte layer 30 has a function of transmitting oxygen ions generated at the air electrode 50. The solid electrolyte layer 30 is denser than the fuel electrode 20 and the air electrode 50.

The solid electrolyte layer 30 may contain ZrO ₂ (zirconia) as a main component. The solid electrolyte layer 30 may contain additives such as Y ₂ O ₃ (yttria) and / or Sc ₂ O ₃ (scandium oxide) in addition to zirconia. These additives function as stabilizers. In the solid electrolyte layer 30, the molar composition ratio of the stabilizer to zirconia (stabilizer: zirconia) can be about 3:97 to 20:80. Therefore, examples of the material of the solid electrolyte layer 30 include 3YSZ, 8YSZ, 10YSZ, or ScSZ (scandia-stabilized zirconia). The thickness of the solid electrolyte layer 30 can be 3 micrometers-30 micrometers, for example.

  In the present embodiment, the composition X containing the substance Y as the main component means that the substance Y accounts for 70% by weight or more, more preferably 90% by weight or more in the entire composition X. To do.

  The barrier layer 40 is disposed between the solid electrolyte layer 30 and the air electrode 50. The barrier layer 40 suppresses the formation of a high resistance layer between the solid electrolyte layer 30 and the air electrode 50. The barrier layer 40 is denser than the fuel electrode 20 and the air electrode 50. The barrier layer 40 can contain a ceria-based material such as GDC (gadolinium doped ceria) or SDC (samarium doped ceria) as a main component. The thickness of the barrier layer 40 can be set to 3 μm to 20 μm, for example.

The air electrode 50 is disposed on the barrier layer 40. The air electrode 50 functions as a cathode of the fuel cell 10. The air electrode 50 is a porous body. The air electrode 50 is represented by the general formula ABO ₃ and contains a perovskite oxide containing at least one of La and Sr at the A site as a main component. As such a perovskite oxide, (La, Sr) (Co, Fe) O ₃ (lanthanum strontium cobalt ferrite), (La, Sr) FeO ₃ (lanthanum strontium ferrite), (La, Sr) CoO ₃ ( Examples thereof include, but are not limited to, lanthanum strontium cobaltite, La (Ni, Fe) O ₃ (lanthanum nickel ferrite), and (La, Sr) MnO ₃ (lanthanum strontium manganate).

  The content of the perovskite oxide in the air electrode 50 is 70% by weight or more. The content of the perovskite oxide in the air electrode 50 is preferably 90% by weight or more.

  The air electrode 50 has a first surface 50S (an example of “surface”) and a second surface 50T. The first surface 50 </ b> S is a surface opposite to the solid electrolyte layer 30. In the present embodiment, since the fuel cell 10 includes the current collecting layer 60, the air electrode 50 is in contact with the current collecting layer 60 on the first surface 50S. That is, in the present embodiment, the first surface 50 </ b> S is an interface between the air electrode 50 and the current collecting layer 60. The second surface 50T is a surface on the solid electrolyte layer 30 side. In the present embodiment, since the fuel cell 10 includes the barrier layer 40, the air electrode 50 is in contact with the barrier layer 40 on the second surface 50T. That is, in the present embodiment, the second surface 50T is an interface between the air electrode 50 and the barrier layer 40.

  The current collecting layer 60 is disposed on the air electrode 50 (surface region 51). The thickness of the current collecting layer 60 is not particularly limited, but can be 30 μm to 500 μm. The current collecting layer 60 can be formed of a perovskite complex oxide represented by the following composition formula (1), but is not limited thereto. The material of the current collecting layer 60 is preferably smaller in electrical resistance than the material of the air electrode 50.

_{La m (Ni 1-x-} y Fe x Cu y) n O 3-δ ··· (1)
Substances other than La may be contained in the A site of the composition formula (1), and substances other than Ni, Fe and Cu may be contained in the B site. In the composition formula (1), m and n are 0.95 or more and 1.05 or less, x (Fe) is 0.03 or more and 0.3 or less, and y (Cu) is 0.05 or more and 0.5 or less. And δ is 0 or more and 0.8 or less.

(Configuration of air electrode 50)
The air electrode 50 has a surface region 51 and an internal region 52. The surface region 51 is disposed on the inner region 52. The surface region 51 is a region within 5 μm from the first surface 50S. The inner region 52 is disposed on the solid electrolyte layer 30 side of the surface region 51. In the present embodiment, the internal region 52 is disposed between the surface region 51 and the barrier layer 40. The thickness of the internal region 52 in the thickness direction can be 5 μm to 300 μm.

  The first surface 50 </ b> S can be defined as a line in which the concentration distribution of a predetermined component rapidly changes when the component concentration is mapped in a cross section parallel to the thickness direction of the air electrode 50 and the current collecting layer 60. Specifically, a line in which the concentration of an element substantially contained only in one of the air electrode 50 or the current collecting layer 60 is 10% of the maximum concentration in one of the elements is defined as the first surface 50S. The second surface 50T can be defined as a line in which the concentration distribution of a predetermined component changes abruptly when the component concentration is mapped in a cross section parallel to the thickness direction of the barrier layer 40 and the air electrode 50. Specifically, a line where the concentration of an element substantially contained only in one of the barrier layer 40 or the air electrode 50 is 10% of the maximum concentration inside the one is defined as the second surface 50T.

The surface region 51 is represented by the above-described general formula ABO ₃ and includes a perovskite oxide containing at least Sr at the A site as a main phase. In the cross section of the surface region 51, the area occupation ratio of the main phase composed of the perovskite oxide can be 91% or more and 99.5% or less. In the present embodiment, the “area occupation ratio of the substance Z in the cross section” refers to the ratio of the total area of the substance Z to the total area including the pores and the solid phase. Details of the method for calculating the area occupancy will be described later.

The surface region 51 contains strontium sulfate (SrSO ₄ ) as a second phase. In the cross section of the surface region 51, the area occupation ratio of the second phase composed of SrSO ₄ is larger than the area occupation ratio of the second phase in the cross section of the internal region 52 described later. Thereby, since the strength of the porous structure of the surface region 51 can be improved, the occurrence of microcracks in the surface region 51 can be suppressed.

  The area occupation ratio of the second phase in the cross section of the surface region 51 can be 0.2% or more and 10% or less, and preferably 0.25% or more and 8.5% or less. Accordingly, it is possible to suppress the occurrence of micro cracks in the surface region 51 and to reduce the number of inactive regions existing in the surface region 51, and thus it is possible to suppress the deterioration of the characteristics of the air electrode.

  The average equivalent circle diameter of the second phase in the cross section of the surface region 51 can be 0.03 μm or more and 3.2 μm or less, and preferably 0.05 μm or more and 2.0 μm or less. Accordingly, even when the temperature rise and fall of the air electrode are repeated, it is possible to suppress the micro cracks generated in the surface region 51 from progressing to the solid electrolyte layer 30 side.

  In the present embodiment, the equivalent circle diameter is the diameter of a circle having the same area as the second phase on an FE-SEM (Field Emission-Scanning Electron Microscope) image. The average equivalent circle diameter is a value obtained by arithmetically averaging the equivalent circle diameters of 20 second phases selected at random. Twenty second phases to be measured for the average equivalent circle diameter are selected at random from five or more reflected electron images.

In the second phase of the surface region 51, a constituent element of the main phase (for example, La or Co) may be dissolved. Further, the second phase of the surface region 51 may contain a small amount of impurities other than SrSO ₄ .

In addition to the main phase and the second phase, the surface region 51 is represented by a general formula ABO ₃ , a third phase composed of a perovskite oxide (for example, LaCoO ₃ etc.) different from the main phase, and the main phase It may contain at least one of third phases constituted by oxides of constituent elements. Examples of the constituent element oxide of the main phase include SrO, (Co, Fe) ₃ O ₄ , and Co ₃ O ₄ . (Co, Fe) ₃ O ₄ includes Co ₂ FeO ₄ , Co _1.5 Fe _1.5 O ₄ , CoFe ₂ O ₄ , and the like. The area occupation ratio of the third phase in the cross section of the surface region 51 can be 0.3% or less.

The internal region 52 is represented by the general formula ABO ₃ described above, and includes a main phase composed of a perovskite oxide containing at least Sr at the A site. In the cross section of the internal region 52, the area occupancy of the main phase can be 91% or more.

  The inner region 52 contains strontium sulfate as the second phase. Thereby, since the strength of the porous structure of the internal region 52 can be improved, even if a micro crack is generated in the surface region 51, it is possible to suppress the micro crack from progressing to the solid electrolyte layer 30 side. In addition, the area occupation ratio of the second phase in the cross section of the internal region 52 is smaller than the area occupation ratio of the second phase in the cross section of the surface region 51. Thereby, since the inactive area | region which exists in the internal area | region 52 can be decreased, it can suppress that the characteristic of an air electrode falls.

  The area occupation ratio of the second phase in the cross section of the internal region 52 can be 0.05% or more and 3% or less, and preferably 0.1% or more and 2.5% or less. Thereby, it is possible to further suppress the minute cracks generated in the surface region 51 from progressing to the solid electrolyte layer 30 side and to further suppress the deterioration of the characteristics of the air electrode.

  The average equivalent circle diameter of the second phase in the cross section of the internal region 52 can be 0.02 μm or more and 2.6 μm or less, and preferably 0.05 μm or more and 2.0 μm or less. Accordingly, even when the temperature rise and fall of the air electrode are repeated, it is possible to suppress the micro cracks generated in the surface region 51 from progressing to the solid electrolyte layer 30 side. The calculation method of the average equivalent circle diameter of the second phase in the cross section of the internal region 52 is the same as the calculation method of the average equivalent circle diameter of the second phase in the cross section of the surface region 51 described above.

In the second phase of the internal region 52, a constituent element of the main phase (for example, La or Co) may be dissolved. The second phase of the inner region 52 may contain a trace amount of impurities other than SrSO ₄ .

In addition to the main phase and the second phase, the internal region 52 is represented by the general formula ABO ₃ , and a third phase composed of a perovskite oxide (for example, LaCoO ₃ ) different from the main phase, It may contain at least one of third phases constituted by oxides of constituent elements.

(Calculation method of area occupancy)
Next, a method for calculating the area occupancy ratio of the second phase in the cross section of the surface region 51 will be described. In the following, a method for calculating the area occupancy of the second phase in the cross section of the surface region 51 will be described, but the area occupancy of the second phase in the cross section of the internal region 52 can be calculated in the same manner. Further, the area occupancy ratios of the main phase and the third phase in the cross sections of the surface region 51 and the internal region 52 can be calculated in the same manner.

(1) FE-SEM Image First, a reflected electron image showing a cross section of the surface region 51 magnified by a magnification of 10,000 is obtained by FE-SEM using a reflected electron detector. The backscattered electron image is obtained by FE-SEM (model: ULTRA55) manufactured by Zeiss (Germany) set to acceleration voltage: 1.5 kV and working distance: 2 mm. The cross section of the surface region 51 is preferably preliminarily subjected to ion milling processing by IM4000 of Hitachi High-Technologies Corporation after precision mechanical polishing.

In this backscattered electron image, the main phase (LSCF) and the second phase (SrSO ₄ ) are different in the brightness of the pores, the main phase is “gray white”, the second phase is “gray”, and the pores are “black”. Displayed. Such ternarization by light and dark difference can be realized by classifying the luminance of an image into 256 gradations.

  However, the method for discriminating the main phase, the second phase, and the pores is not limited to using the light / dark difference in the reflected electron image. For example, after acquiring an element mapping image of the same field of view by SEM-EDS (Scanning Electron Microscope Energy Dispersive X-ray Spectrometry), each particle in the image is identified with high accuracy by comparing with the reflected electron image. The second phase and pores can be discriminated.

(2) Analysis of reflected electron image Next, an image obtained by analyzing the reflected electron image by image analysis software HALCON manufactured by MVTec (Germany) is acquired.

(3) Calculation of area occupancy Then, in the acquired analysis image, the total area of the outlined second phase is calculated. Next, the ratio of the total area of the second phase to the area of the entire reflected electron image (including the pores and the solid phase) is calculated. The ratio of the total area of the second phase calculated in this way is the area occupation ratio of the second phase in the surface region 51.

(Material of surface region 51 and internal region 52)
As an air electrode material constituting the surface region 51 and the internal region 52, a mixed material containing a perovskite oxide as a main component and SrSO ₄ as a subcomponent can be used.

By adjusting the addition amount of the material powder containing SrSO ₄ , the area occupancy of the second phase in each of the surface region 51 and the internal region 52 can be adjusted separately.

Moreover, the average equivalent circular diameter of the second phase in each of the surface region 51 and the internal region 52 can be adjusted by adjusting the particle size of the material powder containing SrSO ₄ . By using an airflow classifier to adjust the particle size of the material powder containing SrSO ₄ , precise classification including adjustment of the upper limit value and the lower limit value is possible. By setting “rough particle size” or / and “large particle size distribution” of the material powder containing SrSO ₄ , the average equivalent circle diameter of the second phase can be increased, and conversely “fine particle size” or / and “ If the particle size distribution is set to “small”, the average equivalent circle diameter of the second phase can be reduced.

(Manufacturing method of fuel cell 10)
Next, an example of a method for manufacturing the fuel cell 10 will be described.

  First, a molded body of the anode current collecting layer 21 is formed by molding the anode current collecting layer material powder by a die press molding method.

  Next, a fuel electrode active layer slurry is prepared by adding PVA (polyvinyl alcohol) as a binder to a mixture of the fuel electrode active layer material powder and a pore former (for example, PMMA). And the molded object of the fuel electrode active layer 22 is formed by printing the slurry for fuel electrode active layers on the molded object of the fuel electrode current collecting layer 21 by a printing method or the like. Thus, a molded body of the fuel electrode 20 is formed.

  Next, terpineol and a binder are mixed with the solid electrolyte layer material powder to prepare a solid electrolyte layer slurry. Then, the solid electrolyte layer 30 molded body is formed by applying the solid electrolyte layer slurry on the molded body of the fuel electrode active layer 22 by a printing method or the like.

  Next, a barrier layer slurry is prepared by mixing terpineol and a binder with the barrier layer material powder. And the molded object of the barrier layer 40 is formed by apply | coating the slurry for barrier layers on the molded object of the intermediate | middle layer 40 by the printing method etc.

  Next, the fuel electrode 20, the solid electrolyte layer 30, and the barrier layer 40 are fired (1350 ° C. to 1450 ° C., 1 hour to 20 hours) to thereby form the fuel electrode 20, the solid electrolyte layer 30, and the barrier layer 40. Form.

Next, the slurry for the inner region is prepared by mixing the material of the inner region 52 (a mixed material containing a perovskite oxide as a main component and SrSO ₄ as a subcomponent), water, and a binder with a ball mill for 24 hours. Make it. At this time, by adjusting the amount of SrSO ₄ mixed in the mixed material, the area occupancy of the second phase in the inner region 52 after firing can be controlled. In the present embodiment, the mixing amount of SrSO ₄ is adjusted so that the area occupancy of the second phase in the internal region 52 is smaller than the area occupancy of the second phase in the surface region 51.

  Next, the molded body of the inner region 52 is formed by applying the slurry for the inner region onto the barrier layer 40 by a printing method or the like.

Next, a slurry for the surface region is obtained by mixing the material of the surface region 51 (a mixed material containing a perovskite oxide as a main component and SrSO ₄ as a subcomponent), water, and a binder with a ball mill for 24 hours. Make it. At this time, the area occupation ratio of the second phase in the surface region 51 after firing can be controlled by adjusting the amount of SrSO ₄ mixed in the mixed material. In the present embodiment, the mixing amount of SrSO ₄ is adjusted so that the area occupancy of the second phase in the surface region 51 is larger than the area occupancy of the second phase in the internal region 52.

  Next, the molded body of the surface region 51 is formed by applying the slurry for the surface region onto the molded body of the inner region 52.

  Next, a current collecting layer slurry is prepared by adding and mixing water and a binder to the material of the current collecting layer 60 described above.

  Next, the current collector layer slurry is applied onto the surface region 51 compact to form the current collector layer 60 compact.

  Next, the air electrode 50 and the current collecting layer 60 are formed by firing (1000 to 1100 ° C., 1 to 10 hours) the molded body of the inner region 52, the surface region 51, and the current collecting layer 60.

(Other embodiments)
The present invention is not limited to the embodiment described above, and various modifications or changes can be made without departing from the scope of the present invention.

  Although the fuel cell 10 includes the current collecting layer 60, the fuel cell 10 may not include the current collecting layer 60. In this case, since the first surface 50S of the air electrode 50 is the outer surface of the fuel cell 10, the surface region 51 of the air electrode 50 is a region within 5 μm from the outer surface.

  Although the fuel cell 10 includes the barrier layer 40, the fuel cell 10 may not include the barrier layer 40. In this case, since the second surface 50T of the air electrode 50 comes into contact with the solid electrolyte layer 30, the internal region 52 of the air electrode 50 is sandwiched between the surface region 51 and the solid electrolyte layer 30.

  The barrier layer 40 has a single layer structure, but may have a multilayer structure in which a dense barrier layer and a porous barrier layer are laminated (in no particular order).

  Examples of the fuel cell according to the present invention will be described below, but the present invention is not limited to the examples described below.

(Production of sample No. 1 to No. 14)
Sample no. 1-No. 14 was produced.

First, a mixed powder was prepared by drying, in a nitrogen atmosphere, a slurry in which NiO powder, Y ₂ O ₃ powder, a prepared powder of pore former (PMMA), and IPA were mixed.

  Next, the mixed powder is uniaxially pressed (molding pressure 50 MPa) to form a plate 30 mm long × 30 mm wide and 3 mm thick, and the plate is further consolidated with CIP (molding pressure: 100 MPa) to collect the fuel electrode. A layered compact was prepared.

  Next, a slurry obtained by mixing NiO-8YSZ and PMMA mixed powder and IPA was applied onto the molded body of the anode current collecting layer.

  Next, 8YSZ was mixed with terpineol and a binder to prepare a solid electrolyte layer slurry. Next, the solid electrolyte layer molded body was formed by applying the slurry for the solid electrolyte layer onto the molded body of the fuel electrode.

  Next, a GDC slurry was prepared, and the molded body of the barrier layer was prepared by applying the GDC slurry onto the molded body of the solid electrolyte layer.

  Next, the molded body of the fuel electrode, the solid electrolyte layer, and the barrier layer was fired (1450 ° C., 5 hours) to form the fuel electrode, the solid electrolyte layer, and the barrier layer.

Next, SrSO ₄ powder was added to the perovskite oxide material shown in Table 1 to prepare an internal region material. At this time, the amount of SrSO ₄ added was adjusted for each sample so that the area occupancy of the second phase (SrSO ₄ ) in the cross section of the internal region became the value shown in Table 1.

  Next, terpineol and a binder were mixed with the material for the inner region to prepare a slurry for the inner region. And the molded object of the internal area | region was produced by apply | coating the slurry for internal areas on a barrier layer.

Next, SrSO ₄ powder was added to the perovskite oxide material powder shown in Table 1 to prepare a surface region material. At this time, the amount of SrSO ₄ added was adjusted for each sample so that the area occupancy of the second phase in the cross section of the surface region became the value shown in Table 1.

  Next, terpineol and a binder were mixed with the surface region material to prepare a surface region slurry. And the surface area molded object was produced by apply | coating the slurry for surface areas on the molded object of an internal area | region. At this time, the coating amount was adjusted so that the thickness of the surface region after firing was 5 μm or less.

  Next, sample no. In Nos. 2, 5, 7, 8, and 10, current collecting layer slurry shown in Table 1 was mixed with water and a binder to prepare a current collecting layer slurry. Then, the current collector layer slurry was applied onto the surface region compact to form a current collector layer compact.

  Next, the air electrode and the current collecting layer were formed by firing (1100 ° C., 1 hour) the molded body of the inner region, the surface region, and the current collecting layer (sample Nos. 2, 5, 7, 8, 10 only). .

(Measurement of area occupancy)
After the cross section of the air electrode of each sample was subjected to precision mechanical polishing, ion milling was performed by IM4000 manufactured by Hitachi High-Technologies Corporation.

  Next, a backscattered electron image of the cross section of the surface region magnified by a magnification of 10,000 times was obtained by FE-SEM using a backscattered electron detector.

  Next, an analysis image was acquired by analyzing the reflected electron image of each sample with image analysis software HALCON manufactured by MVTec.

  Then, the ratio of the total area of the second phase to the total area of the reflected electron image (including the gas phase and the solid phase) was calculated as the area occupancy. The calculation results of the area occupancy ratio of the second phase are as shown in Table 1.

(Observation of microcracks after firing)
For each sample, after firing the air electrode, observe 20 portions of the cross section of the surface region with an electron microscope, and observe the presence or absence of microcracks that occur in the surface region when the temperature of the air electrode falls from high to low. did. The observation results are summarized in Table 1.

(Observation of cracks after thermal cycle test)
Sample No. 3-No. For No. 14, while maintaining the reducing atmosphere by supplying Ar gas and hydrogen gas (4% with respect to Ar) to the fuel electrode, the temperature is raised from room temperature to 800 ° C. in 2 hours and then lowered to room temperature in 4 hours. The cycle was repeated 10 times.

  Thereafter, cross sections of the surface region and the internal region were observed with an electron microscope, and the presence or absence of cracks that progressed from the surface region to the internal region was observed when the temperature of the air electrode was repeatedly raised and lowered. The observation results are summarized in Table 1.

  As shown in Table 1, in the sample in which the area occupancy of the second phase in the surface region is larger than the area occupancy of the second phase in the inner region, the surface region is obtained by firing the air electrode, that is, by cooling the air electrode. It was possible to suppress the occurrence of microcracks in This is because the strength of the porous structure in the surface region and the internal region could be improved.

  In particular, in this example, the area occupancy of the second phase in the surface region is 0.25% to 8.5%, and the area occupancy of the second phase in the internal region is 0.1% to 2.5%. It was confirmed that the generation of microcracks in the surface region can be suppressed in samples with a% or less.

  Further, as shown in Table 1, the average equivalent circle diameter of the second phase in the surface region is 0.05 μm or more and 2.0 μm or less, and the average equivalent circle diameter of the second phase in the inner region is 0.05 μm or more. In the sample having a thickness of 2.0 μm or less, it was possible to suppress the development of cracks from the surface region to the internal region after the thermal cycle test, that is, when the temperature increase and decrease of the air electrode were repeated.

DESCRIPTION OF SYMBOLS 10 Fuel cell 20 Fuel electrode 30 Solid electrolyte layer 40 Barrier layer 50 Air electrode 51 Surface area 52 Internal area 60 Current collection layer

Claims (3)

An anode,
An air electrode represented by the general formula ABO ₃ and containing as a main component a perovskite oxide containing at least Sr at the A site;
A solid electrolyte layer disposed between the fuel electrode and the air electrode;
With
The air electrode has a surface region within 5 μm from the surface opposite to the solid electrolyte layer, and an internal region formed on the solid electrolyte layer side of the surface region,
Each of the surface region and the internal region includes a main phase composed of the perovskite oxide and a second phase composed of strontium sulfate,
The area occupancy of the second phase in the cross section of the surface region is 0.25% or more and 8.5% or less, and is larger than the area occupancy of the second phase in the cross section of the internal region.
Fuel cell. The average equivalent circle diameter of the second phase in the cross section of the surface region is 0.05 μm or more and 2.0 μm or less,
The average equivalent circle diameter of the second phase in the cross section of the internal region is 0.05 μm or more and 2.0 μm or less.
The fuel cell according to claim 1. A current collecting layer disposed on the surface region of the air electrode;
The fuel cell according to claim 1 or 2.

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