Electrochemical cell

Abstract

To provide an electrochemical cell capable of suppressing mechanical deterioration of an air electrode layer and suppressing voltage degradation in the cell due to long-term operation.SOLUTION: An electrochemical cell 1 includes a fuel electrode layer 2, a solid electrolyte layer 3 having oxygen ion conductivity, and an air electrode layer 4, in this order. The air electrode layer 4 includes: a plurality of air electrode catalyst particles 41 comprising an air electrode catalyst material having electron conductivity and oxygen ion conductivity; a plurality of air electrode electrolyte particles 42 comprising a solid electrolyte material having oxygen ion conductivity; and a gap 43. In the air electrode layer 4, the air electrode catalyst material has a coefficient of linear thermal expansion at 700°C within a range of greater than 15×10-6 / K and less than 30×10-6 / K. In the air electrode lay 4, when the total surface area of the air electrode catalyst particles 41 is Scat, and the total surface area of an interface portion where the surface of the air electrode catalyst particles 41 is in contact with the surface of the air electrode electrolyte particles 42 is Scat-ele, Scat-ele / Scat is 0.6 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to electrochemical cells. [Background technology]

[0002] Electrochemical cells using a solid electrolyte layer with oxygen ion conductivity as an electrolyte layer have been known. Examples of this type of electrochemical cell include solid oxide fuel cells (SOFCs) and solid oxide electrochemical cells (SOECs).

[0003] For example, Patent Document 1 discloses a battery comprising an electrolyte layer containing a solid oxide, an air electrode disposed on one side of the electrolyte layer and containing an air electrode material, an anode disposed on the other side of the electrolyte layer, and an intermediate layer disposed between the electrolyte layer and the air electrode and containing a solid electrolyte material having oxide ion conductivity and an air electrode material, wherein the difference in thermal expansion coefficient between the air electrode material and the solid electrolyte material is 5×10 -6 (1 / K) or greater electrochemical cells are disclosed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-73653 Summary of the Invention [Problem to be solved by the invention]

[0005] The prior art has the following problems. According to Patent Document 1, an intermediate layer containing a solid electrolyte material and a cathode material is disposed between the electrolyte layer and the cathode, which is said to prevent the occurrence of air electrode peeling even when the cathode is constructed using an air electrode material with a high thermal expansion coefficient. However, even with the technology of Patent Document 1, in electrochemical cells having an air electrode layer containing an air electrode catalyst material with a relatively high linear thermal expansion coefficient, cracks occur due to the thermal expansion of the air electrode catalyst material, making it difficult to prevent mechanical deterioration of the air electrode layer itself. Furthermore, mechanical deterioration of the air electrode layer leads to changes in cell voltage over long-term durability.

[0006] The present invention has been made in view of the above problems, and aims to provide an electrochemical cell that can suppress mechanical deterioration of the air cathode layer itself and suppress changes in cell voltage due to long-term durability. [Means for solving the problem]

[0007] One aspect of the present invention is An electrochemical cell (1) comprising, in this order, a fuel electrode layer (2) which is an electrode to which fuel is supplied, a solid electrolyte layer (3) having oxygen ion conductivity, and an air electrode layer (4) which is an electrode paired with the fuel electrode layer, The air electrode layer is The battery includes a large number of air electrode-side catalyst particles (41) made of an air electrode catalyst material having electronic conductivity and oxygen ion conductivity, a large number of air electrode-side electrolyte particles (42) made of a solid electrolyte material having oxygen ion conductivity, and voids (43), The air electrode catalyst material has a linear thermal expansion coefficient of 15×10 at 700°C. -6 / K super 30×10 -6 / K or less, The total surface area of ​​the air electrode side catalyst particles is S cat The total surface area of ​​the interface between the surface of the air electrode side catalyst particle and the surface of the air electrode side electrolyte particle is S cat-ele When S cat-ele / S cat is 0.6 or more, In an electrochemical cell (1). [Effects of the Invention]

[0008] The electrochemical cell has the above-mentioned configuration. In the air electrode layer of the electrochemical cell, the surface of the air electrode-side catalyst particles made of the air electrode catalyst material having the above-mentioned high linear thermal expansion coefficient is in a range of 0.6≦S cat-ele / S cat The cathode-side catalyst particles are covered (wrapped) by the cathode-side electrolyte particles so as to satisfy the above condition. Therefore, in the electrochemical cell, even if the cathode-side catalyst particles attempt to thermally expand, the cathode-side electrolyte particles act as a skeleton to suppress the thermal expansion. Therefore, the electrochemical cell can suppress mechanical deterioration of the cathode layer itself and suppress changes in cell voltage due to long-term durability.

[0009] In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is an explanatory diagram that schematically shows the microstructure of an electrochemical cell according to an embodiment. [Figure 2] FIG. 2 is an explanatory view schematically showing an example of a stacked structure of an electrochemical cell according to an embodiment. [Figure 3] FIG. 3 is a diagram showing an example of a cross section (SEM image) taken along the thickness direction of the air electrode layer in the electrochemical cell according to the embodiment. [Figure 4] FIG. 4 is a diagram showing an example of continuous cross-sectional images perpendicular to the thickness direction of the air electrode layer, which are acquired when determining Scat-ele / Scat in the air electrode layer in an electrochemical cell according to an embodiment. [Figure 5] FIG. 5 is a diagram showing an example of an air cathode layer model (DT model) acquired when calculating Scat-ele / Scat in the air cathode layer in an electrochemical cell according to an embodiment. [Figure 6]FIG. 6 shows (a) an example of an 8-bit grayscale SEM image obtained when measuring the porosity of the air electrode layer by image analysis in an electrochemical cell according to an embodiment, and (b) an example of an SEM image after ternarization processing. [Figure 7] FIG. 7 is an explanatory diagram for explaining threshold setting in ternary processing when measuring the porosity of the air electrode layer by image analysis in an electrochemical cell according to an embodiment. [Figure 8] FIG. 8 is an explanatory diagram for explaining an analysis region where Raman analysis is performed when measuring the content of Co3O4 in the air cathode layer in the electrochemical cell according to the embodiment. [Figure 9] FIG. 9 is a diagram showing an example of a Co3O4 detection position map formed for the analysis region shown in FIG. [Figure 10] FIG. 10 shows an example of a Raman spectrum obtained by Raman analysis when measuring the content of Co3O4 in the air electrode layer in an electrochemical cell according to an embodiment, where (a) is an example of a Raman spectrum when Co3O4 is not detected, and (b) is an example of a Raman spectrum when Co3O4 is detected. [Figure 11] FIG. 11 is an explanatory diagram for explaining a method for forming a map showing the detection positions of Co3O4 from (a) the Raman spectrum when Co3O4 is not detected and (b) the Raman spectrum when Co3O4 is detected, obtained by Raman analysis when measuring the content of Co3O4 in the air electrode layer in an electrochemical cell according to an embodiment. [Figure 12] FIG. 12 is an explanatory diagram for explaining a method for measuring intercept lengths Lpore, Lcat, and Lele in the air electrode layer in an electrochemical cell according to an embodiment, in which (a) is a diagram showing an example of an SEM image after ternarization processing, and (b) is a diagram showing an enlarged view of the area surrounded by a square in the upper left corner shown in (a). [Figure 13] FIG. 13 is an explanatory diagram for explaining the relationship between intercept lengths Lcat, Lele, and Lpore in the air electrode layer in the electrochemical cell according to the embodiment. [Figure 14]FIG. 14 is a graph showing the relationship between the porosity (%) (horizontal axis) of the air electrode layer and the initial resistance (Ω) (vertical axis) of the electrochemical cell, obtained in an experimental example. DETAILED DESCRIPTION OF THE INVENTION

[0011] The electrochemical cell of this embodiment will be described with reference to FIGS.

[0012] As illustrated in FIGS. 1 and 2 , the electrochemical cell 1 of this embodiment includes an anode layer 2, a solid electrolyte layer 3, and an air cathode layer 4, in this order. In FIGS. 1 and 2 , the anode layer 2 is disposed on one side of the solid electrolyte layer 3, and the air cathode layer 4 is disposed on the other side of the solid electrolyte layer 3. Specifically, FIGS. 1 and 2 show an example in which the anode layer 2, the solid electrolyte layer 3, and the air cathode layer 4 are stacked in this order, the anode layer 2 and the solid electrolyte layer 3 are bonded to each other, and the solid electrolyte layer 3 and the air cathode layer 4 are bonded via an intermediate layer 6 (described later). The anode layer 2 is an electrode to which fuel is supplied. In other words, the anode layer 2 can be considered an electrode layer having electrode activity that allows it to function as an anode. The air cathode layer 4 is an electrode that forms a pair with the anode layer 2. In other words, the air cathode layer 4 can be considered an electrode layer having electrode activity that allows it to function as an air cathode. The electrochemical cell 1 may have a flat cell structure as exemplified in FIGS. 1 and 2, or may have a cylindrical cell structure, although not shown.

[0013] In the electrochemical cell 1, the solid electrolyte layer 3 has oxygen ion conductivity. The solid electrolyte layer 3 may be composed of multiple layers, as illustrated in FIGS. 1 and 2, or may be composed of a single layer (not shown). FIGS. 1 and 2 show an example in which the solid electrolyte layer 3 has two layers: an electrolyte main layer 31 and an electron blocking layer 32 formed on the surface of the electrolyte main layer 31 facing the air cathode layer 4. The electrolyte main layer 31 is an electrolyte layer that forms the main body of the solid electrolyte layer 3 and functions as the electrolyte of the electrochemical cell 1. When the solid electrolyte layer 3 is a single layer, the single layer serves as the electrolyte main layer 31. The electron blocking layer 32 is a layer that blocks the movement of electrons. When the electron blocking layer 32 is included, the electron blocking layer 32 can block the movement of electrons, so the electrolyte main layer 31 may exhibit electronic conductivity in addition to oxygen ion conductivity, for example, under a reducing atmosphere. As described above, the layer structure of the solid electrolyte layer 3 is not particularly limited as long as it is configured to function as an electrolyte of the electrochemical cell 1.

[0014] As illustrated in FIG. 2 , the electrochemical cell 1 can also have an anode support layer 5 on the side of the anode layer 2 opposite the solid electrolyte layer 3. The anode support layer 5 functions as a support for each layer. The presence of the anode support layer 5 ensures the strength of the electrochemical cell 1 and facilitates handling. The anode support layer 5 may be in contact with the anode layer 2 to support the anode layer 2, as illustrated in FIG. 2 . Alternatively, the anode support layer 5 may support the anode layer 2 via another layer (not shown) disposed between the anode support layer 5 and the anode layer 2. Examples of such another layer include an anode diffusion layer for diffusing fuel supplied to the anode layer 2. The anode support layer 5 described above may function not only as a support but also as a fuel diffusible layer. In this case, the anode diffusion layer described above may be unnecessary. The anode support layer 5 and anode diffusion layer described above can function as current collecting layers for the anode layer 2.

[0015] 1 and 2, the electrochemical cell 1 can also have an intermediate layer 6 between the solid electrolyte layer 3 and the air cathode layer 4. The intermediate layer 6 is a layer (reaction suppression layer) that mainly suppresses reaction between the material of the solid electrolyte layer 3 and the material of the air cathode layer 4. FIGS. 1 and 2 show an example in which the intermediate layer 6 is in contact with the solid electrolyte layer 3 and the air cathode layer 4 and is bonded to these layers.

[0016] 2, the electrochemical cell 1 can also have an air electrode current collecting layer 7 on the side of the air electrode layer 4 opposite to the solid electrolyte layer 3. The air electrode current collecting layer 7 functions as a current collector for the air electrode layer 4.

[0017] In the electrochemical cell 1, the thickness of the fuel electrode layer 2 can be, for example, 10 μm or more and 100 μm or less. The thickness of the solid electrolyte layer 3 can be, for example, 2 μm or more and 20 μm or less. The thickness of the electrolyte main layer 31 can be, for example, 1 μm or more and 15 μm or less. The thickness of the electron blocking layer 32 can be, for example, 1 μm or more and 15 μm or less. The thickness of the air electrode layer 4 can be, for example, 10 μm or more and 100 μm or less. The thickness of the fuel electrode support layer 5 can be, for example, 100 μm or more and 800 μm or less. The thickness of the intermediate layer 6 can be, for example, 1 μm or more and 20 μm or less. The thickness of the air electrode current collecting layer 7 can be, for example, 1 μm or more and 100 μm or less.

[0018] In the electrochemical cell 1 that can have the above-described stacked structure, the air electrode layer 4 includes a large number of air electrode-side catalyst particles 41, a large number of air electrode-side electrolyte particles 42, and voids 43, as illustrated in FIGS. 1 and 3.

[0019] In the air electrode layer 4, the air electrode side catalyst particles 41 are made of an air electrode catalyst material having electronic conductivity and oxygen ion conductivity. However, the air electrode catalyst material making up the air electrode side catalyst particles 41 has a coefficient of linear thermal expansion (CTE) of 15×10 at 700°C. -6 / K super 30×10 -6 / K. The linear thermal expansion coefficient at 700°C is 15×10-6 / K or less is a material with a low linear thermal expansion coefficient, and therefore is within the range of S defined in this disclosure. cat-ele / S cat Even if the requirement (described in detail later) is not satisfied, cracks due to thermal expansion of the air electrode catalyst material do not occur, and mechanical deterioration of the air electrode layer 4 itself is suppressed. Therefore, in the electrochemical cell 1, the air electrode catalyst material constituting the air electrode-side catalyst particles 41 is selected from the air electrode catalyst material having a linear thermal expansion coefficient of 15×10 at 700°C. -6 / K or more. On the other hand, the air electrode catalyst material constituting the air electrode side catalyst particles 41 is limited to a material having a relatively high linear thermal expansion coefficient of 30×10 -6 / K or more, S defined in this disclosure cat-ele / S cat Even if the requirement above is satisfied, the linear thermal expansion coefficient is too high, which causes cracks due to thermal expansion of the air electrode catalyst material, making it impossible to suppress mechanical deterioration of the air electrode layer 4 itself. Therefore, in the electrochemical cell 1, the air electrode catalyst material constituting the air electrode-side catalyst particles 41 is selected to have a linear thermal expansion coefficient of 30×10 at 700°C. -6 / K.

[0020] The air electrode catalyst material preferably has a linear thermal expansion coefficient of 15.5 × 10 at 700°C from the viewpoints of suppressing cracking due to thermal expansion of the air electrode catalyst material and ensuring the effects of the present disclosure. -6 / K or more, more preferably 16×10 -6 / K or more, more preferably 16.5 × 10 -6 / K or more, and even more preferably 17×10 -6 / K or more. From the viewpoints of suppressing cracking due to thermal expansion of the air electrode catalyst material and ensuring the effects of the present disclosure, the air electrode catalyst material preferably has a linear thermal expansion coefficient of 29×10 -6 / K or less, more preferably 28×10 -6 / K or less, more preferably 27×10 -6 / K or less, and even more preferably 26×10 -6 / K or less, and even more preferably 25×10 -6below / K, more preferably, further below 24×10 -6 / K or below. The upper and lower limit values can be arbitrarily combined.

[0021] The linear thermal expansion coefficient of the air electrode catalyst material is determined by specifying the crystal structure of the air electrode side catalyst particles 41 in the air electrode layer 4 by X-ray diffraction method (XRD) and specifying the contained elements by energy dispersive X-ray spectroscopy (EDS) to identify the air electrode catalyst material. For the identified air electrode catalyst material, it can be measured by thermomechanical analysis (TMA) in accordance with JIS R1618:2002. The conditions during thermomechanical analysis are: temperature range: room temperature to 900 °C, heating rate: 5 °C / min, sample shape: 20 mm × 5 mm × 1.5 mm, atmosphere: air, load control: constant load 5 gf.

[0022] Examples of the air electrode catalyst material include perovskite-type oxides containing La, Sr, and Co, perovskite-type oxides containing Pr, Ba, and Co, perovskite-type oxides containing Gd, Ba, and Co, perovskite-type oxides containing Nd, Ba, and Co, etc. These can be used alone or in combination of two or more. As the air electrode catalyst material, from the viewpoints such as easy assurance of the above-mentioned linear thermal expansion coefficient, which has excellent mixed conductivity with coexistence of both electronic conductivity and oxygen ion conductivity and has high air electrode catalyst activity, perovskite-type oxides containing La, Sr, and Co can be preferably used. Specific examples of the above-mentioned perovskite-type oxides containing La, Sr, and Co include La 0.6 Sr 0.4 CoO3, etc., and metal oxides represented by La 1-x Sr x CoO 3-δ (0 < x ≦ 1, preferably 0.1 ≦ x ≦ 0.5). Specific examples of the perovskite-type oxides containing Pr, Ba, and Co include Pr 2-x Ba x Co2O 5+δ(0.7≦x≦1.3, preferably 0.8≦x≦1). Specific examples of perovskite oxides containing Gd, Ba, and Co include Gd 2-x Ba x Co2O 5+δ (0.7≦x≦1.3, preferably 0.8≦x≦1). Specific examples of perovskite oxides containing Nd, Ba, and Co include Nd 2-x Ba x Co2O 5+δ (0.7≦x≦1.3, preferably 0.8≦x≦1). The oxides described above may or may not have oxygen non-stoichiometry. The crystal structure of the air electrode catalyst material can be measured by XRD, and the elements contained in the oxide can be measured by EDS.

[0023] In the air electrode layer 4, the air electrode side electrolyte particles 42 are composed of a solid electrolyte material having oxygen ion conductivity. Examples of the solid electrolyte material constituting the air electrode side electrolyte particles 42 include ceria (CeO2) (hereinafter, sometimes referred to as an oxide containing Ce and RE) doped with one or more elements selected from Gd, Sm, Y, Sc, La, Nd, Yb, Ca, and Ho (hereinafter, these elements may be collectively referred to as RE elements), and ceria. These materials may be used alone or in combination. From the viewpoint of excellent oxygen ion conductivity at a relatively low temperature of about 700°C, the solid electrolyte material constituting the air electrode side electrolyte particles 42 is preferably ceria doped with at least one of Gd and Sm, and more preferably ceria doped with Gd. Specific examples of the oxide containing Ce and RE described above include Ce, 1-x (RE) x O 2-x / 2(where 0.05≦x≦0.2, the RE element is one or more elements selected from Gd, Sm, Y, Sc, La, Nd, Yb, Ca, and Ho, preferably at least one of Gd and Sm, more preferably Gd). The oxides described above may or may not have oxygen non-stoichiometry.

[0024] The air electrode layer 4 has a total surface area of ​​the air electrode side catalyst particles 41 of S cat The total surface area of ​​the interface between the surface of the air electrode side catalyst particle 41 and the surface of the air electrode side electrolyte particle 42 is S cat-ele Then, S cat-ele / S cat is set to be 0.6 or more.

[0025] S cat-ele / S cat is an index that indicates the extent to which the surfaces of the air electrode-side catalyst particles 41 are covered (wrapped) by the air electrode-side electrolyte particles 42 in the air electrode layer 4. 0.6≦S cat-ele / S cat means that 60% or more of the total surface area of ​​the air electrode side catalyst particles 41 is covered (wrapped) by the air electrode side electrolyte particles 42. cat-ele / S cat If the coefficient of linear thermal expansion is <0.6, the amount of air electrode-side electrolyte particles 42 covering the surface of the air electrode-side catalyst particles 41 is too small, and the air electrode-side electrolyte particles 42 cannot suppress the thermal expansion of the air electrode-side catalyst particles 41, which are made of an air electrode catalyst material with a high coefficient of linear thermal expansion described above. As a result, mechanical deterioration of the air electrode layer 4 itself occurs, and it becomes impossible to suppress changes in cell voltage due to long-term durability. In contrast, if the air electrode layer 4 has a coefficient of linear thermal expansion of 0.6≦S cat-ele / S cat When the above condition is satisfied, even if the air electrode-side catalyst particles 41 attempt to thermally expand, the air electrode-side electrolyte particles 42 act as a skeleton to suppress the thermal expansion. As a result, an electrochemical cell 1 can be obtained that is capable of suppressing mechanical deterioration of the air electrode layer 4 itself and suppressing changes in cell voltage due to long-term durability.

[0026] In the air electrode layer 4, S cat-ele / S cat In order to ensure the above-mentioned effects, S is preferably 0.65 or more, and more preferably 0.7 or more. cat-ele / S cat From the viewpoint of the release of generated oxygen, it is preferable that the value of the upper and lower limits be 0.9 or less, and more preferably 0.85 or less. These upper and lower limit values ​​can be combined arbitrarily.

[0027] S cat-ele / S cat The S is calculated from an air electrode layer model (digital twin model, hereinafter sometimes referred to as DT model) which is a three-dimensional model of successive cross-sectional images of the air electrode layer 4 obtained by FIB-SEM (focused ion beam-scanning electron microscope). cat-ele , S cat Searching for S cat-ele S cat It can be calculated by dividing by S cat-ele / S cat Specifically, it can be measured as follows.

[0028] The imaging area in the FIB-SEM is determined appropriately so that the pore diameter and particle diameter are at least 1 / 10 or less of the model size to be simulated. Specifically, in the present disclosure, using the FIB-SEM, continuous cross-sectional images perpendicular to the thickness direction of the cathode layer 4 can be obtained under imaging conditions of X-axis direction: 7.6 μm, Y-axis direction: 9.2 μm, Z-axis direction: 10.2 μm, and slice pitch: 20 nm, as shown in FIG.

[0029] Next, the obtained continuous tomographic images are read using the importGeo-Vol function, which is one of the interfaces of GeoDict, a microstructure simulation software developed by Math2Market GmbH, with the pixel size of 1 pixel when photographed with an SEM being 1 voxel. In the present disclosure, the reading condition can be specifically set to 0.0118 μm / voxel.

[0030] Next, the loaded image is processed to remove parts unnecessary for calculation, such as electrolyte parts. Generally, in FIB-SEM, brightness varies in the Z-axis direction, so correction is performed within the Gray Value Adjustments tool to maintain a constant brightness. Next, if stripes due to the curtain effect are present, correction is performed within the FIB-SEM Filters tool. Specifically, in the present disclosure, this can be performed under the conditions of Tolerance: 1e-5 and Filter Strength: 0.1. Next, if the processed image contains noise or surface roughness, preventing particles from being recognized as the same material despite being within a single particle, appropriate noise reduction is performed using the Image Filters tool. From the 3D image captured in this manner, any number of FIB-SEM 2D images (preferably 10 or more) are extracted, and a volume ratio threshold is determined so that the volume ratio is the same as the average particle area ratio after ternarization, and ternarization is performed. In this manner, a DT model M, as illustrated in FIG. 5, is created.

[0031] Next, the total surface area S of the air electrode side catalyst particles 41 in the prepared DT model M cat , the total surface area S of the interface portion where the surface of the air electrode side catalyst particle 41 and the surface of the air electrode side electrolyte particle 42 come into contact cat-ele This is derived using the Estimate Surface Area function in the MatDict function, which is one of the modules of GeoDict.

[0032] Specifically, the air electrode side catalyst particles 41 and the air electrode side electrolyte particles 42 in the DT model M are selected as Material1 and Material2, respectively. Then, in the Chosen Material mode for calculating the surface area of ​​the selected material in the Estimate Surface Area, the total surface area S of Material1 (air electrode side catalyst particles 41) is calculated. catThis is achieved by integrating the surface area of ​​the surfaces of the selected Material 1 voxels that are shared with the space among the voxels that make up the DT model M. In addition, in the Between Materials mode that calculates the shared surface area between materials in the Estimate Surface Area, the total surface area S of the interface where Material 1 (air electrode side catalyst particle 41) and Material 2 (air electrode side electrolyte particle 42) come into contact is calculated. cat-ele This is achieved by adding up the surface area of ​​the shared faces of the two selected voxels, Material 1 and Material 2, from among the voxels that make up the DT model M. For details on how to calculate Estimate Surface Area in GeoDict, please refer to the GeoDict manual. As described above, S cat , S cat-ele After finding S cat-ele S cat By dividing by (S cat-ele ÷S cat (by calculating S cat-ele / S cat can be derived.

[0033] As described above, the air electrode layer 4 contains voids 43. In the electrochemical cell 1, the voids 43 are preferably reduced as much as possible within a range that ensures the necessary gas permeability, from the viewpoints of increasing the skeletal strength of the air electrode layer 4 and reducing the initial resistance of the electrochemical cell 1. Specifically, from the viewpoint of reducing the initial resistance of the electrochemical cell 1, the porosity of the air electrode layer 4 is preferably 25% or less, more preferably 23% or less, and even more preferably 20% or less. From the viewpoint of gas permeability, the porosity of the air electrode layer 4 is preferably 5% or more, more preferably 7% or more, and even more preferably 10% or more. These upper and lower limit values ​​can be combined as desired.

[0034] The porosity of the air electrode layer 4 is expressed as the area ratio of the voids 43 in a cross section along the thickness direction of the air electrode layer 4. Specifically, the porosity of the air electrode layer 4 can be measured by image analysis as follows.

[0035] The air electrode layer 4 of the electrochemical cell 1 is embedded in resin and cross-sectionally processed (specifically, processed with a cross-section polisher (CP)) to expose a cross section along the thickness direction of the air electrode layer 4. The cross section of the obtained air electrode layer 4 is observed with a field emission scanning electron microscope (FE-SEM) to obtain an SEM image based on a backscattered electron image. For the FE-SEM, for example, an "S-4800" manufactured by Hitachi High-Tech Corporation can be used.

[0036] The acquired SEM image was then imported into image analysis software (ImageJ, manufactured by the National Institutes of Health, USA) and converted into an 8-bit grayscale SEM image, as shown in Figure 6(a). This 8-bit grayscale SEM image was then subjected to ternarization processing using the "Threshold" setting in ImageJ. During ternarization processing, the detected intensity histogram of the SEM image, as shown in Figure 7, shows that brightness increases toward the right and decreases toward the left. In this detected intensity histogram, the midpoint between the peaks of the first high-brightness peak Y1 (corresponding to the cathode-side catalyst particle 41) and the second high-brightness peak Y2 (corresponding to the cathode-side electrolyte particle 42) was set as threshold 1 for ternarization processing. Next, the midpoint between the peaks of the second high-brightness peak Y2 and the third high-brightness peak Y3 (corresponding to the void 43) was set as threshold 2 for ternarization processing. Note that the relationship between brightness varies depending on the material, so thresholds 1 and 2 cannot be uniquely determined. Next, the area ratio of the voids 43 is calculated using the SEM image after ternarization processing as shown in Fig. 6(b). Specifically, the void ratio can be calculated by 100 x {(total area of ​​the voids 43) / (total area of ​​the SEM image after ternarization processing)}.

[0037] The content of the air electrode-side electrolyte particles 42 in the air electrode layer 4 can be preferably 50% or more, more preferably 52% or more, and even more preferably 55% or more, from the viewpoints of increasing the skeletal strength of the air electrode layer 4 by the air electrode-side electrolyte particles 42 and further suppressing mechanical deterioration. Furthermore, the content of the air electrode-side electrolyte particles 42 can be preferably 70% or less, more preferably 65% ​​or less, and even more preferably 60% or less, from the viewpoint of ensuring the availability of the air electrode-side catalyst particles 41. The content of the air electrode-side electrolyte particles 42 is expressed as the area ratio of the air electrode-side electrolyte particles 42 in a cross section taken along the thickness direction of the air electrode layer 4. Like the porosity of the air electrode layer 4, the content of the air electrode-side electrolyte particles 42 can be determined using a ternary-valued SEM image as shown in FIG. 6(b). Specifically, the content of the air electrode-side electrolyte particles 42 can be calculated by 100 × {(total area of ​​the air electrode-side electrolyte particles 42) / (total area of ​​the SEM image after ternary-valued)}.

[0038] In the air electrode layer 4, the content of the air electrode-side catalyst particles 41 can be preferably 35% or less, more preferably 33% or less, and even more preferably 30% or less, from the viewpoint of ensuring oxygen ion conductivity through the air electrode-side electrolyte particles 42. Furthermore, the content of the air electrode-side catalyst particles 41 can be preferably 15% or more, more preferably 18% or more, and even more preferably 20% or more, from the viewpoint of ensuring catalytic activity (electrode activity, hereinafter omitted) and electronic conductivity of the air electrode layer 4. These upper and lower limit values ​​can be arbitrarily combined. The content of the air electrode-side catalyst particles 41 is expressed as the area ratio of the air electrode-side catalyst particles 41 in a cross section taken along the thickness direction of the air electrode layer 4. Similar to the porosity of the air electrode layer 4, the content of the air electrode-side catalyst particles 41 can be determined using a ternary-valued SEM image as shown in FIG. 6(b). Specifically, the content of the air electrode side catalyst particles 41 can be calculated by 100×{(total area of ​​the air electrode side catalyst particles 41) / (total area of ​​the SEM image after ternarization processing)}.

[0039] In the air electrode layer 4, when the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co, if the content of the air electrode-side catalyst particles 41 is set to 35% or less, by containing the air electrode-side catalyst particles 41 made of an air electrode catalyst material that can exhibit high catalytic performance in the above range, the content of the air electrode-side electrolyte particles 42 can be ensured to be high, which has the advantage of making it easier to reduce the initial resistance of the electrochemical cell 1.

[0040] In the air electrode layer 4, when the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co, the Co3O4 content is preferably 10% or less. Perovskite oxides containing La, Sr, and Co can exhibit high catalytic activity. However, when perovskite oxides containing La, Sr, and Co are fired at high temperatures for a long time during the production of the air electrode layer 4 to reduce the voids 43 (to make the oxide denser), they chemically decompose to produce Co3O4. This decomposition reduces the catalytic activity of the air electrode layer 4. When the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co, the Co3O4 content is significant as an indicator of the degree of decomposition of the perovskite oxide containing La, Sr, and Co. If the Co3O4 content is 10% or less, decomposition of the perovskite oxide containing La, Sr, and Co is suppressed, which makes it easier to suppress an increase in cell resistance. The Co3O4 content can be preferably 9% or less, more preferably 8% or less, and even more preferably 7% or less. Note that, in order to reduce the Co3O4 content, firing under high-temperature, short-time firing conditions during production of the air electrode layer 4 is recommended. Furthermore, since a lower Co3O4 content is preferable, there is no particular lower limit for the Co3O4 content.

[0041] The above-mentioned Co3O4 content is expressed as the area ratio of Co3O4 detected in a cross section along the thickness direction of the air electrode layer 4. Specifically, the Co3O4 content can be measured by Raman analysis as follows.

[0042] As illustrated in FIG. 8, a cross section along the thickness direction of the cathode layer 4 is taken, and an area of ​​20 μm vertically (in the thickness direction) × 30 μm horizontally (perpendicular to the thickness direction) is defined as an analysis area 90 on the cross section. Next, Raman analysis is performed on this 20 μm × 30 μm analysis area 90, and the Co3O4 detection positions are mapped in 0.5 μm steps as illustrated in FIG. 9. In this case, one analysis point is depicted as one pixel. Therefore, as illustrated in FIG. 9, in the map 91, the total number of pixels in the vertical direction of the analysis area 90 is 40, and the total number of pixels in the horizontal direction of the analysis area 90 is 60. Specifically, the Raman analysis is performed by measuring a Raman shift of 30 cm for each point as illustrated in FIG. 10. -1 From 1300cm -1 For each Raman spectrum obtained, as illustrated in FIG. -1 The scattering intensity at 710cm -1 The scattering intensity at 660cm of the actual data is used as the end point. -1 From 710cm -1 The value is calculated by subtracting the area between the line segment connecting the start point and end point and the x-axis of the Raman spectrum from the integrated area up to the point. Note that the area between the line segment connecting the start point and end point and the x-axis of the Raman spectrum (the area surrounded by the line indicated by reference numeral 910) is the area derived from the background. If the above value is greater than 0, the calculated value is adopted; if the above value is 0 or less, it is uniformly set to 0 and this value is substituted for the part corresponding to the analysis position. Note that if there is a Co3O4 peak, the above value will be a number greater than 0. In this way, a map 91 showing the detection positions of Co3O4 can be created, as exemplified in Figure 9.

[0043] The map 91 is then imported into image analysis software (ImageJ, manufactured by the National Institutes of Health, USA) and binarized using the ImageJ "Threshold" function. The midpoint between the first and second peaks in the detection intensity histogram obtained during binarization is set as the threshold. The binarized map is then used to calculate the area ratio of the bright areas, i.e., the Co3O4 detection areas, relative to the total area of ​​the map. The Co3O4 content, expressed as the area ratio of Co3O4, can be calculated by 100 × {(total area of ​​Co3O4 detection areas) / (total area of ​​the map after binarization)}.

[0044] In the cumulative distribution of intercept lengths of the air electrode side catalyst particles 41, the air electrode side electrolyte particles 42, and the voids 43 obtained in a cross section along the thickness direction of the air electrode layer 4, the intercept length at which the cumulative frequency of the air electrode side catalyst particles 41 is 50% is defined as L cat , the intercept length at which the cumulative frequency of the electrolyte particles 42 on the air electrode side is 50% is L ele , the intercept length at the cumulative frequency of void 43 of 50% is L pore When L pore <L cat <L ele When each segment length satisfies this relationship, the size of the voids 43 that are the starting points for crack propagation is small, and the skeleton size of the air electrode-side electrolyte particles 42 that form the skeleton of the air electrode layer 4 is large, which further ensures an improvement in the mechanical strength of the air electrode layer 4.

[0045] L cat , L ele , L poreSpecifically, the porosity can be measured as follows. Similar to the measurement of the porosity of the air electrode layer 4, a ternarized SEM image (hereinafter simply referred to as a ternarized image) as shown in FIG. 12(a) is obtained. As shown in FIG. 12(b), the horizontal direction of this ternarized image is the x-axis direction, and the vertical direction is the y-axis direction. Next, from the left end to the right end of the ternarized image in the x-axis direction, the number of pixels of consecutive intercept lengths of the same material are counted in the x-axis direction for the air electrode-side catalyst particles 41, the air electrode-side electrolyte particles 42, and the voids 43. Then, one pixel down in the y-axis direction, the number of pixels of each intercept length in the x-axis direction is counted in the same way, and this process is repeated from the top to the bottom of the ternarized image. Similarly, from the top to the bottom of the ternary image in the y-axis direction, the number of pixels of consecutive intercept lengths of the same material are counted along the y-axis direction for the air electrode-side catalyst particles 41, the air electrode-side electrolyte particles 42, and the voids 43. Then, the pixel count for each intercept length along the y-axis direction is repeated from the left end to the right end of the ternary image. However, as illustrated in FIG. 12(b), the top, bottom, left, and right edges of the ternary image include the edge E when the SEM image was acquired, and the intercept lengths are truncated. Therefore, pixels starting from this edge E are not counted. For example, when calculating each intercept length along the x-axis direction in FIG. 12(b), the portion marked with symbol U0 is not used to calculate the intercept length, but the portions marked with symbols U1, U2, and U3 are used to calculate the intercept length. Furthermore, the specific value of each intercept length can be calculated by converting the side length per pixel of the ternary image from the SEM magnification and calculating the number of pixels of the intercept length x the side length per pixel. Next, as illustrated in Figure 13, the cumulative frequency distribution of the intercept length of the air electrode side catalyst particle 41 is obtained, and the intercept length at which the cumulative frequency counting from the shortest intercept length becomes 50% is defined as L. cat Similarly, the cumulative frequency distribution of the intercept lengths of the air electrode side electrolyte particles 42 is calculated, and the intercept length at which the cumulative frequency reaches 50% counting from the shortest intercept length is defined as L ele In addition, the cumulative frequency distribution of the intercept length of the void 43 is calculated, and the intercept length when the cumulative frequency reaches 50% counting from the shortest intercept length is defined as L pore It can be calculated as follows.cat , L ele、 L pore In the derivation of (a), the accumulation from the shorter intercept length accumulates all intercept lengths without distinction between the x-axis direction and the y-axis direction.

[0046] In the above-described electrochemical cell 1, in the air electrode layer 4, the surfaces of the air electrode-side catalyst particles 41 made of the air electrode catalyst material having the above-described high linear thermal expansion coefficient are in the range of 0.6≦S cat-ele / S cat The air electrode layer 4 is covered (enveloped) by the air electrode-side electrolyte particles 42 so as to satisfy the above condition. Therefore, in the electrochemical cell 1, even if the air electrode-side catalyst particles 41 attempt to thermally expand, the air electrode-side electrolyte particles 42 act as a skeleton to suppress the thermal expansion. Therefore, the electrochemical cell 1 can suppress mechanical deterioration of the air electrode layer 4 itself and suppress changes in cell voltage due to long-term durability.

[0047] As long as the electrochemical cell 1 is configured as a solid oxide cell using a material having oxygen ion conductivity as the electrolyte, there are no particular limitations on the materials and configurations of the above-mentioned fuel electrode layer 2, solid electrolyte layer 3 (electrolyte main layer 31, electron blocking layer 32, etc.), fuel electrode support layer 5, intermediate layer 6, and air electrode current collecting layer 7 other than the air electrode layer 4.

[0048] 1, the fuel electrode layer 2 can include a large number of fuel electrode-side catalyst particles 21, a large number of fuel electrode-side electrolyte particles 22, and voids 23. The fuel electrode-side catalyst particles 21 can be made of an electronically conductive fuel electrode catalyst material, etc. The fuel electrode-side electrolyte particles 22 can be made of an oxygen-ion conductive solid electrolyte material, etc. The oxygen-ion conductive solid electrolyte material used for the fuel electrode-side electrolyte particles 22 may or may not have electronic conductivity.

[0049] Examples of the anode catalyst material constituting the anode-side catalyst particles 21 include electron conductors (metals and alloys, hereinafter omitted) such as Ni, Ni alloys, Cu, Cu alloys, Co, and Co alloys, and oxides of electron conductors (metal and alloy oxides, hereinafter omitted) that become electron conductors upon reduction, such as Ni oxides (NiO, etc.), Cu oxides, and Co oxides. These materials can be used alone or in combination of two or more. Among these, Ni, Ni alloys, and Ni oxides (NiO, etc.) are preferred from the viewpoint of catalytic activity (electrode activity), and Ni and NiO are more preferred. Examples of the solid electrolyte material constituting the anode-side electrolyte particles 22 include oxides containing Ce and RE, the solid electrolyte material constituting the air electrode-side electrolyte particles 42, such as ceria, yttria-stabilized zirconia (YSZ), and scandia-stabilized zirconia (ScSZ). These materials can be used alone or in combination of two or more. Of these, ceria doped with at least one of Gd and Sm is preferred, and ceria doped with Gd is more preferred, from the viewpoint of excellent oxygen ion conductivity at a relatively low temperature of about 700°C.

[0050] When the solid electrolyte layer 3 is composed of an electrolyte main layer 31 and an electron blocking layer 32, for example, as illustrated in Fig. 1, the electrolyte main layer 31 can be composed of a solid electrolyte material having electronic conductivity and oxygen ion conductivity, and the electron blocking layer 32 can be composed of a solid electrolyte material having no electronic conductivity but oxygen ion conductivity. When the solid electrolyte layer 3 is composed of a single layer of the electrolyte main layer 31, the solid electrolyte layer 3 (electrolyte main layer 31) can be composed of a solid electrolyte material having no electronic conductivity but oxygen ion conductivity. The solid electrolyte layer 3 is usually formed to be dense so as to be gas impermeable.

[0051] Examples of solid electrolyte materials having electronic conductivity and oxygen ion conductivity in the solid electrolyte layer 3 include oxides containing Ce and RE, and solid electrolyte materials constituting the air electrode-side electrolyte particles 42, such as ceria. These materials can be used alone or in combination. Of these, ceria doped with at least one of Gd and Sm is preferable, and Gd-doped ceria is more preferable, from the viewpoint of excellent oxygen ion conductivity at relatively low temperatures of about 700°C. Examples of solid electrolyte materials that do not have electronic conductivity but have oxygen ion conductivity include yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ). These materials can be used alone or in combination. Of these, yttria-stabilized zirconia (YSZ) is preferable, from the viewpoint of stability in a reducing atmosphere and lacking electronic conductivity.

[0052] The anode support layer 5 can be porous, having voids or through-holes so as not to impede the supply of fuel to the anode layer 2. The anode support layer 5 can be made of, for example, a mixed material containing the above-mentioned anode catalyst material and an oxide material, or a metal (including an alloy, omitted below) material. In the electrochemical cell 1, when the anode support layer 5 is made of a metal material, a bonding layer (not shown) may be formed between the anode support layer 5 and the anode layer 2 to bond them together.

[0053] Examples of oxide materials for the anode support layer 5 include oxides containing Ce and RE, solid electrolyte materials such as ceria that constitute the air electrode-side electrolyte particles 42, yttria-stabilized zirconia (YSZ), and scandia-stabilized zirconia (ScSZ). These materials can be used alone or in combination. Of these, ceria doped with at least one of Gd and Sm is preferred, and Gd-doped ceria is more preferred, from the viewpoint of excellent oxygen ion conductivity at a relatively low temperature of about 700°C.

[0054] In the anode support layer 5, the oxide material does not necessarily have to be a solid electrolyte. In addition to the above, various oxides that are not solid electrolytes, such as CaO and MgO, can also be used. Furthermore, examples of the metal material include Fe-based alloys. Note that Fe-based alloys are formed by adding alloying elements to the base Fe. The alloying elements include at least one metal element and may also include at least one non-metal element. Furthermore, the alloying elements do not include Fe. Furthermore, the above metal elements may include metalloid elements. Examples of metal elements included in the alloying elements include Cr, Mn, Ti, Ni, Al, Cu, Mo, Nb, V, La, Ta, Hf, Zr, Si, and B. These may be used alone or in combination. Specifically, the metal element contained in the alloying element can include at least Cr, and more specifically, can include at least Cr and at least one selected from the group consisting of Mn, Ti, Ni, Al, Cu, Mo, Nb, V, La, Ta, Hf, Zr, Si, and B. The metal element with the highest content among the metal elements contained in the alloying element of the Fe-based alloy can be, for example, one selected from the group consisting of Cr, Mn, and Ti. Note that an Fe-based alloy containing Cr as the most added metal element can be referred to as an Fe-Cr-based alloy. Similarly, an Fe-based alloy containing Mn as the most added metal element can be referred to as an Fe-Mn-based alloy, and an Fe-based alloy containing Ti as the most added metal element can be referred to as an Fe-Ti-based alloy.

[0055] The intermediate layer 6 can be made of a mixed material containing a solid electrolyte material having oxygen ion conductivity and the air electrode material constituting the air electrode layer 4, or a solid electrolyte material having oxygen ion conductivity. The solid electrolyte material having oxygen ion conductivity used in the intermediate layer 6 may or may not have electronic conductivity.

[0056] Examples of solid electrolyte materials having oxygen ion conductivity used in the intermediate layer 6 include oxides containing Ce and RE, solid electrolyte materials constituting the air electrode side electrolyte particles 42, such as ceria, yttria-stabilized zirconia (YSZ), and scandia-stabilized zirconia (ScSZ). These materials can be used alone or in combination. Of these, ceria doped with at least one of Gd and Sm is preferred, and Gd-doped ceria is more preferred, from the viewpoints of excellent oxygen ion conductivity at relatively low temperatures of about 700°C and low reactivity with the air electrode catalyst material.

[0057] The air electrode current collecting layer 7 can be made of an air electrode current collecting material having electronic conductivity suitable for current collection on the air electrode side exposed to a high-temperature oxidizing atmosphere.

[0058] Examples of the air electrode current collecting material include metal materials such as Pt, Pt alloys, Ag, Ag alloys, and Au, and electronically conductive oxides such as perovskite oxides containing La, Sr, and Co, and perovskite oxides containing La, Ni, and Fe. These can be used alone or in combination. Of these, metal materials such as Pt and Pt alloys and electronically conductive oxides such as perovskite oxides containing La, Sr, and Co are preferred, from the viewpoints of being resistant to oxidation in high-temperature oxidizing atmospheres and having high electronic conductivity.

[0059] The electrochemical cell 1 can be used as at least one of a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). That is, the electrochemical cell 1 may be operated as an SOFC, or may be operated as an SOEC, or may be configured to be switchable between an SOFC mode in which it operates as an SOFC and an SOEC mode in which it operates as an SOEC, and may be operated as both an SOFC and an SOEC.

[0060] Specifically, when the electrochemical cell 1 is operated as an SOFC, a hydrogen-containing gas such as hydrogen gas can be supplied to the fuel electrode layer 2 as a fuel gas. In this case, an oxygen-containing gas such as air or oxygen gas can be supplied to the air electrode layer 4. On the other hand, when the electrochemical cell 1 is operated as an SOEC, the fuel electrode layer 2 functions as a hydrogen electrode. A water (H2O)-containing gas such as water vapor gas can be supplied to the fuel electrode layer 2 as a fuel gas. In this case, the air electrode layer 4 functions as an oxygen electrode. A gas such as air may or may not be supplied to the air electrode layer 4. Note that the hydrogen-containing gas may contain water vapor for humidification or the like, and the water-containing gas may contain a reducing gas such as hydrogen gas.

[0061] (Experimental example) <Material preparation> -Fuel electrode support layer- NiO powder (average particle size: 1.0 μm), yttria-stabilized zirconia (8YSZ) powder containing 8 mol% YO (average particle size: 0.5 μm), carbon (pore-forming agent), polyvinyl butyral, isoamyl acetate, and 1-butanol were mixed in a ball mill to prepare a slurry. The mass ratio of NiO powder to 8YSZ powder was 65:35. The slurry was applied in layers to a resin sheet using a doctor blade, dried, and then peeled off to prepare a sheet for forming an anode support layer. The average particle size is the particle size (diameter) d50 at which the volume-based cumulative frequency distribution measured by laser diffraction and scattering is 50% (the same applies hereinafter).

[0062] -Fuel electrode layer- NiO powder (average particle size: 1.0 μm), 8YSZ powder (average particle size: 0.5 μm), carbon (pore-forming agent), polyvinyl butyral, isoamyl acetate, and 1-butanol were mixed in a ball mill to prepare a slurry. The mass ratio of the NiO powder to the 8YSZ powder was 65:35. The amount of carbon in the fuel electrode layer-forming sheet was smaller than the amount of carbon in the fuel electrode support layer-forming sheet. Thereafter, a fuel electrode layer-forming sheet was prepared in the same manner as in the production of the fuel electrode support layer-forming sheet.

[0063] -Electrolyte body layer- A slurry was prepared by mixing Gd-doped CeO2 (hereinafter referred to as GDC) powder (average particle size: 0.3 μm), polyvinyl butyral, isoamyl acetate, 2-butanol, and ethanol in a ball mill. In this experimental example, CeO2 doped with 10 mol% Gd was used as the GDC. Thereafter, a sheet for forming the main electrolyte layer was prepared in the same manner as in the preparation of the sheet for forming the anode support layer.

[0064] -Electron Blocking Layer- A slurry was prepared by mixing 8YSZ powder (average particle size: 0.5 μm), polyvinyl butyral, isoamyl acetate, and 1-butanol in a ball mill. The subsequent steps were the same as in the preparation of the sheet for forming the anode support layer, and the sheet for forming the electron blocking layer was prepared.

[0065] -Middle class- A slurry was prepared by mixing GDC powder (average particle size: 0.3 μm), polyvinyl butyral, isoamyl acetate, 2-butanol, and ethanol in a ball mill. In this experimental example, CeO2 doped with 10 mol% Gd was used as the GDC. Subsequently, an intermediate layer-forming sheet was prepared in the same manner as in the preparation of the anode support layer-forming sheet.

[0066] -Air electrode layer- As shown in Table 1 below, La was used as the air electrode catalyst material powder. 0.6 Sr0.4 CoO3 powder or PrBaCo2O5 powder (average particle size: selected from the range of 1.0 to 2.0 μm) and GDC powder (average particle size: selected from the range of 0.15 μm to 1.0 μm) as a solid electrolyte material powder were mixed, and this was dispersed in terpineol as a solvent together with a dispersant. 0.6 Sr 0.4 The volume ratio of CoO3 powder to GDC powder was selected from the range of 33 to 66:34 to 66. Ethyl cellulose was added as a binder to the obtained dispersion and mixed to prepare a paste for forming an air cathode layer.

[0067] -Air electrode current collecting layer- La 0.6 Sr 0.4 CoO powder (average particle size: 1.6 μm) and a dispersant were dispersed in terpineol as a solvent, and ethyl cellulose as a binder was added to the resulting dispersion and mixed to prepare a paste for forming an air electrode current collecting layer.

[0068] <Preparation of electrochemical cell> A fuel electrode layer-forming sheet was laminated on a plurality of laminated fuel electrode support layer-forming sheets, and the sheets were pressed together using a hydrostatic press (WIP) molding method. Next, a sheet for forming an electrolyte main layer was laminated on the fuel electrode layer-forming sheet, and the sheets were pressed together in the same manner as above. Next, a sheet for forming an electron blocking layer was laminated on the sheet for forming an electrolyte main layer, and the sheets were pressed together in the same manner as above. Next, a sheet for forming an intermediate layer was laminated on the sheet for forming an electron blocking layer, and the sheets were pressed together in the same manner as above. The pressing conditions were a temperature of 85°C, a pressure of 50 MPa, and a pressing time of 10 minutes. Next, the obtained pressed body was cut to a predetermined size. Next, this pressed body was fired in an air atmosphere at 1350°C for 2 hours. This resulted in a ceramic substrate having a fuel electrode support layer (thickness 350 μm), a fuel electrode layer (thickness 30 μm), an electrolyte main layer (thickness 3 μm), an electron blocking layer (thickness 3 μm), and an intermediate layer (thickness 3 μm) laminated in this order.

[0069] Next, the paste for forming the air cathode layer was applied to the surface of the intermediate layer of the obtained ceramic substrate by screen printing, and fired (baked) in an air atmosphere at a predetermined temperature (selected from the range of 1100°C to 1200°C) for a predetermined time (selected from the range of 5 minutes to 1 hour) to form an air cathode layer (thickness 50 μm). At this time, the outer shape of the air cathode layer was formed smaller than the outer shape of the fuel electrode layer.

[0070] Next, the paste for forming the air electrode current collecting layer was applied to the surface of the air electrode layer by screen printing and fired (baked) at 900°C for 2 hours in an air atmosphere to form an air electrode current collecting layer (thickness 50 μm).

[0071] As described above, flat plate electrochemical cells (single cells) were prepared for Samples 1 to 15 shown in Table 1 below. Details of changes in the preparation of the electrochemical cells for Samples 1 to 15 are as follows. Sample 2: The material of the air electrode catalyst powder was changed from Sample 1. Sample 3: The average particle size of the solid electrolyte material powder is smaller than that of Sample 1. Sample 4: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 1, and the average particle size of the solid electrolyte material powder is smaller than that of Sample 2. Sample 5: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 4. The average particle size of the solid electrolyte material powder is smaller than that of Sample 4. Sample 6: The average particle size of the solid electrolyte material powder is smaller than that of Sample 1, and larger than that of Sample 5. Sample 7: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 5. The average particle size of the solid electrolyte material powder is smaller than that of Sample 5. Sample 8: Compared to sample 7, the ratio of air electrode catalyst material powder was increased and the ratio of solid electrolyte material powder was decreased. Sample 9: Compared to sample 8, the ratio of air electrode catalyst material powder was increased and the ratio of solid electrolyte material powder was decreased. Sample 10: Compared to sample 9, the ratio of air electrode catalyst material powder was increased and the ratio of solid electrolyte material powder was decreased. Sample 11: The material of the air electrode catalyst powder was changed from Sample 7. · Sample 12: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 7, the average particle size of the solid electrolyte material powder is smaller than that of Sample 7, and the firing temperature is lower than that of Sample 7. · Sample 13: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 12, the average particle size of the solid electrolyte material powder is smaller than that of Sample 12, and the firing temperature is lower than that of Sample 12. · Sample 14: The average particle size of the air electrode catalyst material powder is smaller than that of Sample 13, the average particle size of the solid electrolyte material powder is smaller than that of Sample 13, the firing temperature is lower than that of Sample 13, and the firing time is shorter than that of Sample 13. · Sample 15: The average particle size of the solid electrolyte material powder is smaller than that of Sample 14. In Table 1, when the particle size of the raw material was selected so that the average particle size of the solid electrolyte material powder < the average particle size of the air electrode catalyst material powder, it was denoted as "Ele<Cat". When the particle size of the raw material was selected so that the average particle size of the air electrode catalyst material powder < the average particle size of the solid electrolyte material powder, it was denoted as "Cat<Ele".

[0072] Also, for comparison, in the preparation of the electrochemical cell of Sample 1, an electrochemical cell of Sample 1C was prepared in the same manner except that the firing temperature of the paste for forming the air electrode layer was set to a low temperature within the above-mentioned predetermined temperature range.

[0073] In the preparation of the electrochemical cell of Sample 1, an electrochemical cell of Sample 2C was prepared in the same manner except that Ba 0.6 La 0.4 CoO3 powder (average particle size: 1.0 μm) was used as the air electrode catalyst material powder.

[0074] In the preparation of the electrochemical cell of Sample 1, in addition to the point that the firing temperature of the paste for forming the air electrode layer was set to a low temperature within the above-mentioned predetermined temperature range, and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O3 powder (average particle size: 1.0 μm) was used as the air electrode catalyst material powder, an electrochemical cell of Sample 3C was prepared in the same manner.

[0075] <Investigation of the composition of the air cathode layer> For each electrochemical cell, a cross section along the thickness direction of the air electrode layer was observed using an SEM. As a result, it was confirmed that each air electrode layer contained a large number of air electrode-side catalyst particles, a large number of air electrode-side electrolyte particles, and voids, as shown in Figure 3. In addition, using the measurement method described above, the linear thermal expansion coefficient, S, of the air electrode catalyst material at 700°C was measured for each air electrode layer. cat-ele / S cat , porosity, cathode side electrolyte particle content, cathode side catalyst particle content, Co3O4 content, L pore <L cat <L ele The relationship between these factors was measured.

[0076] <Evaluation> -Evaluation preparation- Platinum paste was applied to the surface of the anode support layer of the electrochemical cell by screen printing, and then baked in the air at 900°C for 1 hour to form an anode current collecting layer (thickness 20 μm). A platinum wire was fixed as a reference electrode to the portion of the single cell on the air cathode side where the air cathode layer was not formed, using platinum paste. Sealing glass was then applied to the end face of the electrochemical cell. The electrochemical cell was heated to 850°C in the air, and the fuel pipe was sealed with glass on the anode layer side to form a gas seal structure. The anode layer was then subjected to a reduction treatment. The reduction treatment conditions were a reduction temperature of 600°C, reduction treatment gas of hydrogen gas, and reduction time of 3 hours.

[0077] -Evaluation conditions- After the reduction treatment, the temperature was raised slightly to an operating temperature of 650°C, and a mixed gas of hydrogen and water vapor (volume ratio of hydrogen:water vapor = 50:50) was supplied to the fuel electrode layer of the electrochemical cell, and air was supplied to the air electrode layer. In this experimental example, the current density was 0.5 A / cm 2 After initial aging treatment was performed by passing a constant current at 400 kJ / min for 50 hours, the electrochemical cell was evaluated under the same current passing conditions.

[0078] -Machine deterioration (cracks)- Under the above evaluation conditions, the above mixed gas was supplied as fuel gas to the fuel electrode layer and air to the air electrode layer, and after 1000 hours of current application, the temperature was lowered. After 1000 hours of current application, a sample was taken from the electrochemical cell, embedded in resin, and cross-sectional processing was performed to obtain a cross-sectional SEM image along the thickness direction of the air electrode layer. The presence or absence of cracks in the air electrode layer was confirmed using the obtained cross-sectional SEM image.

[0079] - Cell voltage change rate - The cell was operated under the same test conditions as for the mechanical degradation test, and the cell voltage between the reference electrode and the air cathode layer at the beginning of the current application (after aging) and at the end of 1000 hours of current application were determined. The cell voltage change rate was then calculated using the formula: 100 × [{(cell voltage at the end of 1000 hours of current application) - (cell voltage at the beginning of current application)} / (cell voltage at the beginning of current application)]. Mechanical degradation increases resistance, which leads to a voltage drop during SOFC operation and a voltage increase during SOEC operation. Here, the single cell was operated as an SOFC.

[0080] -Initial resistance of the cell- After the initial aging, the AC impedance test was performed at 100mV amplitude for 10 6 The initial resistance of the entire cell was measured in the range from 10 Hz to 10 Hz.

[0081] Details of each of the air electrode layers and the evaluation results are summarized in Table 1. Also, Fig. 14 shows the relationship between the porosity (%) of the air electrode layer and the initial resistance (Ω) of the electrochemical cell.

[0082] [Table 1]

[0083] Table 1 and FIG. 14 reveal the following: Sample 1C has a S of the air electrode layer. cat-ele / S catTherefore, in sample 1C, cracks were observed due to thermal expansion of the catalyst particles on the air contact side, which are made of an air electrode catalyst material with a relatively high linear thermal expansion coefficient, and mechanical deterioration of the air electrode layer itself could not be suppressed, resulting in a large change in cell voltage (cell voltage drop) over long-term durability.

[0084] Sample 2C is S of the air electrode layer. cat-ele / S cat Although this is within the range specified in the present disclosure, the linear thermal expansion coefficient of the air electrode catalyst material constituting the air-contacting side catalyst particles is too high, which causes cracks due to thermal expansion of the air electrode catalyst material, makes it impossible to suppress mechanical deterioration of the air electrode layer itself, and results in a large change in cell voltage (cell voltage drop) over long-term durability.

[0085] Sample 3C has a low linear thermal expansion coefficient of the air electrode catalyst material that makes up the catalyst particles on the air contact side, so S cat-ele / S cat Even if the range specified in the present disclosure is not met, cracks due to thermal expansion of the air electrode catalyst material will not occur, and mechanical deterioration of the air electrode layer itself can be suppressed, so there is no need to take measures as specified in the present disclosure in the first place.

[0086] In contrast, samples 1 to 15 use air electrode catalyst materials with a relatively high linear thermal expansion coefficient as the air electrode catalyst material that constitutes the air electrode side catalyst particles. cat-ele / S cat satisfies the specific range specified in the present disclosure. Therefore, in Samples 1 to 15, even if the air electrode-side catalyst particles attempt to thermally expand, the surfaces of the air electrode-side catalyst particles are sufficiently covered (wrapped) by the air electrode-side electrolyte particles, and the air electrode-side electrolyte particles act as a skeleton to suppress the thermal expansion. As a result, it was confirmed that Samples 1 to 15 can suppress mechanical deterioration of the air electrode layer itself and suppress changes in cell voltage (cell voltage drop) due to long-term durability.

[0087] Furthermore, when comparing Samples 1 to 15, it can be seen that when the porosity of the air electrode layer is 25% or less, mechanical deterioration of the air electrode layer itself is suppressed, and changes in cell voltage (cell voltage drop) due to long-term durability are suppressed, and furthermore, the initial cell resistance can be reduced.

[0088] Furthermore, when comparing Samples 1 to 15, it can be seen that when the content of air electrode-side electrolyte particles in the air electrode layer is 50% or more, the air electrode-side electrolyte particles improve the skeletal strength of the air electrode layer, making it easier to suppress mechanical deterioration.

[0089] Furthermore, when comparing Samples 1 to 15, it can be seen that when the content of air electrode side catalyst particles in the air electrode layer is 35% or less, it is easier to ensure the content of air electrode side electrolyte particles, and it is easier to reduce the initial cell resistance.

[0090] Furthermore, when comparing Samples 1 to 15, it can be seen that when the content of Co3O4 in the air electrode layer is 10% or less, the decomposition of perovskite-type oxides containing La, Sr, and Co, which can exhibit high catalytic activity, is suppressed, making it easier to suppress an increase in cell resistance.

[0091] In addition, in Samples 1 to 15, the air electrode layer was L pore <L cat <L ele It can be seen that when the condition above is satisfied, the mechanical strength of the air electrode layer can be improved more reliably. This is because the size of the voids that serve as the starting points for crack propagation is small and the skeleton size of the air electrode-side electrolyte particles that form the skeleton of the air electrode layer is large.

[0092] The present invention is not limited to the above-described embodiments and experimental examples, and various modifications are possible without departing from the spirit and scope of the present invention. Furthermore, the configurations shown in the above-described embodiments and experimental examples can be combined in any manner.

[0093] The features of the present invention are as follows. [1] An electrochemical cell (1) comprising, in this order, a fuel electrode layer (2) which is an electrode to which fuel is supplied, a solid electrolyte layer (3) having oxygen ion conductivity, and an air electrode layer (4) which is an electrode paired with the fuel electrode layer, The air electrode layer is The battery includes a large number of air electrode-side catalyst particles (41) made of an air electrode catalyst material having electronic conductivity and oxygen ion conductivity, a large number of air electrode-side electrolyte particles (42) made of a solid electrolyte material having oxygen ion conductivity, and voids (43), The air electrode catalyst material has a linear thermal expansion coefficient of 15×10 at 700°C. -6 / K super 30×10 -6 / K or less, The total surface area of ​​the air electrode side catalyst particles is S cat The total surface area of ​​the interface between the surface of the air electrode side catalyst particle and the surface of the air electrode side electrolyte particle is S cat-ele When S cat-ele / S cat is 0.6 or more, Electrochemical cell (1). [2] The air electrode layer has a porosity of 25% or less, which is expressed as the area ratio of the voids in a cross section along the thickness direction. [1] The electrochemical cell according to [1]. [3] the air electrode layer has a content of the air electrode-side electrolyte particles, expressed as an area ratio of the air electrode-side electrolyte particles in a cross section along the thickness direction, of 50% or more; [1] The electrochemical cell according to [2]. [4] the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co; the content of the air electrode-side catalyst particles, expressed as an area ratio of the air electrode-side catalyst particles in a cross section along the thickness direction of the air electrode layer, is 35% or less; [1] An electrochemical cell according to any one of [1] to [3]. [5] the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co; The air electrode layer has a Co3O4 content of 10% or less, which is expressed as an area ratio of Co3O4 detected in a cross section along the thickness direction. [1] to [4]. An electrochemical cell according to any one of [1] to [4]. [6] The air electrode layer is In the cumulative distribution of intercept lengths of the air electrode side catalyst particles, the air electrode side electrolyte particles, and the voids obtained in a cross section along the thickness direction, the intercept length at a cumulative frequency of 50% of the air electrode side catalyst particles is defined as L cat , the intercept length at a cumulative frequency of 50% of the electrolyte particles on the air electrode side is L ele , the intercept length at a cumulative frequency of 50% of the voids is L pore When L pore <L cat <L ele fulfill, [1] to [5]. An electrochemical cell according to any one of [1] to [5]. [7] Used as at least one of a solid oxide fuel cell and a solid oxide electrolysis cell, [1] to [6], an electrochemical cell according to any one of [1] to [6]. [Explanation of symbols]

[0094] 1. Electrochemical cell 2 Fuel electrode layer 3 Solid electrolyte layer 4. Air electrode layer 41 Air electrode side catalyst particles 42 Air electrode side electrolyte particles 43 void

Claims

1. An electrochemical cell (1) comprising, in this order, a fuel electrode layer (2) which is an electrode to which fuel is supplied, a solid electrolyte layer (3) having oxygen ion conductivity, and an air electrode layer (4) which is an electrode paired with the fuel electrode layer, The air electrode layer is The battery includes a large number of air electrode side catalyst particles (41) made of an air electrode catalyst material having electronic conductivity and oxygen ion conductivity, a large number of air electrode side electrolyte particles (42) made of a solid electrolyte material having oxygen ion conductivity, and voids (43), The air electrode catalyst material has a linear thermal expansion coefficient of 15×10 at 700°C. －６ / K super 30×10 －６ / K, The total surface area of ​​the air electrode side catalyst particles is S ｃａｔ The total surface area of ​​the interface portion where the surface of the air electrode side catalyst particle and the surface of the air electrode side electrolyte particle come into contact is S ｃａｔ－ｅｌｅ When S ｃａｔ－ｅｌｅ / S ｃａｔ is 0.6 or more, Electrochemical cell (1).

2. the air electrode layer has a porosity of 25% or less, which is expressed as the area ratio of the voids in a cross section along the thickness direction; 10. The electrochemical cell of claim 1.

3. the air electrode layer has a content of the air electrode-side electrolyte particles, expressed as an area ratio of the air electrode-side electrolyte particles in a cross section along the thickness direction, of 50% or more; 3. The electrochemical cell of claim 1 or claim 2.

4. the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co; the content of the air electrode-side catalyst particles, expressed as an area ratio of the air electrode-side catalyst particles in a cross section along the thickness direction of the air electrode layer, is 35% or less; 3. The electrochemical cell of claim 1 or claim 2.

5. the air electrode catalyst material is a perovskite oxide containing La, Sr, and Co; The cathode layer has a Co ３ O ４ Co expressed as an area ratio of ３ O ４ The content of is 10% or less, 3. The electrochemical cell of claim 1 or claim 2.

6. The air electrode layer is In the cumulative distribution of intercept lengths of the air electrode side catalyst particles, the air electrode side electrolyte particles, and the voids obtained in a cross section along the thickness direction, the intercept length at a cumulative frequency of 50% of the air electrode side catalyst particles is defined as L ｃａｔ , the intercept length at a cumulative frequency of 50% of the air electrode side electrolyte particles is L ｅｌｅ , the intercept length at a cumulative frequency of 50% of the voids is L ｐｏｒｅ When L ｐｏｒｅ <L ｃａｔ <L ｅｌｅ fulfill, 3. The electrochemical cell of claim 1 or claim 2.

7. Used as at least one of a solid oxide fuel cell and a solid oxide electrolysis cell, 3. The electrochemical cell of claim 1 or claim 2.

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