JP7482718B2 - Electrochemical Cell - Google Patents

Description

The present invention relates to a solid oxide electrochemical cell.

Solid oxide electrochemical cells are required to have high durability. Patent Document 1 discloses a technology for controlling the structure of the fuel electrode in order to improve durability in an electrochemical cell that includes an electrolyte layer, a fuel electrode provided on one side of the electrolyte layer, and an air electrode provided on the other side of the electrolyte layer.

Japanese Patent No. 5624085

However, there is room for improvement in the above technology.

The present invention was made to meet this demand, and aims to provide an electrochemical cell with improved durability.

To achieve this object, the electrochemical cell of the present invention is an anode-supported electrochemical cell comprising an electrolyte layer, an anode provided on one side of the electrolyte layer, and an air electrode provided on the other side of the electrolyte layer, the anode including an active layer in contact with the electrolyte layer, the active layer including a first particle having ionic conductivity and a second particle having electronic conductivity, the first particle also having electronic conductivity in the anode atmosphere. In the first interface between the first particle and the second particle, the second interface between the first particle and the pore, and the third interface between the second particle and the pore, which appear in the cross section of the active layer, the ratio of the length of the second interface to the combined interface length of the first interface, the second interface, and the third interface is 40% or more.

According to the first aspect, the ratio of the length of the second interface to the combined interface length of the first interface (solid/solid interface), the second interface (solid/gas interface), and the third interface (solid/gas interface) is 40% or more, so the length of the third interface between the second particle and the pores is relatively short. When the length of the third interface is shortened, the surface diffusion of the atoms constituting the second particle is reduced, and the deterioration of the performance of the electrochemical cell can be reduced. Thus, durability can be improved.

According to the second aspect, when the ratio of the length of the first interface to the interface length is compared with the ratio of the length of the second interface to the interface length, and the ratio of the length of the third interface to the interface length, the ratio of the length of the third interface to the interface length is the lowest. In addition to the effect of the first aspect, durability can be further improved.

According to the third aspect, the value A/B obtained by dividing the average length A of the second particles in the first direction on the cross section by the average length B of the second direction on the cross section perpendicular to the first direction is greater than 0.9 and less than 1.1. Since the cross section of the second particles approaches a circle, the activity of the second particles can be reduced. Thus, in addition to the effect of the first or second aspect, durability can be further improved.

According to the fourth aspect, the value C/D obtained by dividing the average length C of the first particle in the first direction on the cross section by the average length D of the first particle in the second direction on the cross section perpendicular to the first direction is greater than 0.9 and less than 1.1. Since the cross section of the first particle approaches a circle, the activity of the first particle can be reduced. Thus, in addition to the effect of any one of the first to third aspects, durability can be further improved.

FIG. 1 is a schematic cross-sectional view of an electrochemical cell according to one embodiment. FIG. 2 is an enlarged cross-sectional view of the fuel electrode showing a portion indicated by II in FIG. FIG. 2 is a schematic diagram of the structure of an active layer. 2 is a schematic diagram of an active layer in a predetermined area on a cross section. FIG. 4 shows the particle size distribution of second particles in an example. 13 is a particle size distribution of second particles in a comparative example.

Below, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an electrochemical cell 10 in one embodiment. The electrochemical cell 10 comprises an electrolyte layer 11, a fuel electrode 12 provided on one side of the electrolyte layer 11, and an air electrode 13 provided on the other side of the electrolyte layer 11. The electrochemical cell 10 is used, for example, as a fuel cell (SOFC), a steam reforming cell, or a steam electrolysis cell (SOEC).

In this embodiment, the electrochemical cell 10 has an intermediate layer 14 between the electrolyte layer 11 and the air electrode 13. The electrochemical cell 10 in this embodiment is an anode-supported electrochemical cell in which the thickness of the anode 12 is thicker than the electrolyte layer 11 and the air electrode 13.

The electrolyte layer 11 is made of a solid electrolyte that exhibits oxide ion conductivity under the operating conditions of the electrochemical cell 10. Examples of the solid electrolyte include stabilized zirconia and ceria-based solid solutions. Examples of the solid electrolyte include a solid solution of one or more selected from stabilized zirconia and ceria-based solid solutions and alumina. Examples of stabilizers for stabilized zirconia include CaO, MgO, Y ₂ O ₃ , Sc ₂ O ₃ , and Yb ₂ O _3. Examples of elements that are solid-dissolved in ceria in the ceria-based solid solution include Gd, Sm, and Y. When the electrolyte layer 11 contains a ceria-based solid solution, the electrolyte layer 11 exhibits non-negligible electronic conductivity in the reducing atmosphere of the fuel electrode atmosphere, so it is preferable to provide a layer of stabilized zirconia or BaCe _1-X Sm _X O ₃ , which has low electronic conductivity in that atmosphere, in the electrolyte layer 11.

In a fuel cell, the air electrode 13 is a place where gaseous oxygen reacts with electrons to become oxide ions, and in steam electrolysis, it is a place where oxide ions release electrons to become oxygen. The air electrode 13 is required to be easy to adsorb oxidant gas (oxygen, air) and have high oxide ion conductivity, and also needs to have electronic conductivity. Examples of the material of the air electrode 13 include perovskite oxides such as La _1-X Sr _X MnO _3-δ , La _1-X Sr _X CoO _3-δ , La _1-X Sr _X Co _1-Y Fe _Y O _3-δ , and Pr _1-X Sr _X MnO _3-δ . Examples of the material of the air electrode 13 include a composite material of one or more oxides selected from these perovskite oxides and a solid electrolyte that can constitute the electrolyte layer 11.

The intermediate layer 14 reduces the reaction between the electrolyte layer 11 and the air electrode 13 at high temperatures during the manufacture (firing) and operation of the electrochemical cell 10. An example of the material for the intermediate layer 14 is a ceria-based solid solution. Examples of elements that dissolve in ceria in a ceria-based solid solution include Gd, Sm, La, and Y.

Figure 2 is a cross-sectional view of the fuel electrode 12, enlarging the portion indicated by II in Figure 1. The fuel electrode 12 is a porous body having gas permeability. The fuel electrode 12 includes a support layer 20 and an active layer 21 disposed between the support layer 20 and the electrolyte layer 11. The active layer 21 is in contact with the electrolyte layer 11. In this embodiment, the support layer 20 is in contact with the active layer 21.

The support layer 20 mainly functions to support the active layer 21. The thickness of the support layer 20 is, for example, 200 μm-1000 μm. In a fuel cell, the active layer 21 has the function of reacting oxide ions supplied from the electrolyte layer 11 with fuel (hydrogen, carbon monoxide, hydrocarbon, etc.) to generate electrons, and in steam electrolysis, it has the function of electrolyzing water vapor by passing electricity through it to convert it into hydrogen and oxide ions. The thickness of the active layer 21 is, for example, 10 μm-40 μm.

The material constituting the support layer 20 may be the same as the material constituting the active layer 21, or may be a material different from the material constituting the active layer 21. When the material constituting the support layer 20 is different from the material constituting the active layer 21, the material of the support layer 20 is exemplified by stabilized zirconia.

The fuel electrode 12 is formed into a shape such as a cylinder or a plate depending on the shape of the desired electrochemical cell 10. When the electrochemical cell 10 is cylindrical, the fuel electrode 12 serving as a support is formed into a hollow cylinder, the electrolyte layer 11 is provided on the outer peripheral surface of the fuel electrode 12, and the intermediate layer 14 and the air electrode 13 are provided on the outer peripheral surface of the electrolyte layer 11. When the electrochemical cell 10 is flat, the fuel electrode 12 serving as a support is formed into a plate shape, and the electrolyte layer 11, the intermediate layer 14, and the air electrode 13 are provided on the fuel electrode 12.

The electrochemical cell 10 is manufactured through the processes of preparing and mixing, molding, and firing the raw materials that make up the fuel electrode 12, electrolyte layer 11, intermediate layer 14, and air electrode 13. In the mixing process, binders, pore-forming materials, plasticizers, etc. are also mixed as necessary. Examples of the means for molding the fuel electrode 12 include extrusion molding, press molding, tape molding, and injection molding. Examples of the means for molding the electrolyte layer 11, intermediate layer 14, and air electrode 13 include thick film printing by applying a slurry in which the raw materials are dispersed in a liquid, and tape molding. The electrochemical cell 10 is obtained by firing the molded body.

For example, the molding and firing can be performed by stacking a molded body of the fuel electrode 12 and a molded body of the electrolyte layer 11 and firing them to obtain the fuel electrode 12 and the electrolyte layer 11, then providing a molded body of the intermediate layer 14 on the electrolyte layer 11 and firing them, and providing a molded body of the air electrode 13 on the intermediate layer 14 and firing them to obtain the electrochemical cell 10. Alternatively, the electrochemical cell 10 can be obtained by stacking the molded bodies of the fuel electrode 12, electrolyte layer 11, intermediate layer 14, and air electrode 13 and firing them.

Figure 3 is a schematic diagram of the structure of the active layer 21. The structure of the active layer 21 is identified by observing, with a scanning electron microscope (SEM), a surface (hereinafter referred to as the "observation surface") obtained by irradiating a cross section of the active layer 21, including the normal to the interface 21a (see Figure 2) between the electrolyte layer 11 and the active layer 21, with a focused ion beam (FIB) from an oblique direction. The observation surface is a smooth cross section with very little deformation of the metal contained in the active layer 21.

The active layer 21 includes first particles 22 having ion conductivity and second particles 23 having electronic conductivity. The first particles 22 also have electronic conductivity in the fuel electrode atmosphere. The oxide ion conductivity of the first particles 22 can be confirmed by the generation of electromotive force when an oxygen concentration cell is constructed using the material constituting the first particles 22 as an electrolyte. The electronic conductivity of the first particles 22 in the fuel electrode atmosphere can be confirmed by the presence of a slope in the oxygen concentration region corresponding to the fuel electrode atmosphere in a graph showing the oxygen concentration dependence when the oxygen concentration dependence of the electrical conductivity of the material constituting the first particles 22 is measured. In this embodiment, the fuel electrode atmosphere is defined as an atmosphere containing 1% or more hydrogen.

The first particles 22 are exemplified by a ceria-based solid solution and a perovskite-type oxide. Examples of elements dissolved in ceria in the ceria-based solid solution include Gd, Sm, La, and Y. An example of the perovskite-type oxide is Sr _1-x La _x TiO ₃ .

An example of the second particles 23 is Ni. Ni is contained in pure Ni, Ni-based alloys, cermets which are composites (sintered bodies) of NiO and solid electrolytes, and the like. NiO contained in cermets is converted to Ni by hydrogen reduction. NiO remaining after hydrogen reduction is not included in Ni. Examples of solid electrolytes contained in cermets include solid solutions of alumina and one or more types selected from ceria-based solid solutions, and ceria-based solid solutions.

The volume ratio of the second particles 23 in the active layer 21 is preferably 40 vol%-60 vol%. This is to ensure electronic conductivity by the second particles 23 and ionic conductivity by the first particles 22. The volume ratio of the second particles 23 is the volume ratio of the second particles 23 in the active layer 21 after reduction. The volume ratio of the second particles 23 is calculated from an SEM image, and is the value (%) obtained by dividing the area of the second particles 23 by the combined area of the first particles 22 and the area of the second particles 23.

The gas-permeable active layer 21 contains countless pores that are connected three-dimensionally. The pores that appear on the cross section (observation surface) of the active layer 21 are cross sections of the countless pores that are connected three-dimensionally, and include a first closed pore 24 contained in the first particle 22, a second closed pore 25 contained in the second particle 23, and a pore 26 that contacts the first particle 22 and the second particle 23. The closed pores 24 and 25 are shown to be isolated inside the first particle 22 and the second particle 23, respectively, on the observation surface, regardless of whether they are cross sections of pores isolated inside the active layer 21 or cross sections of open pores connected to the outside air. The closed pores 24 and 25 do not have to be observed.

The first interface 27 is the interface between the first particle 22 and the second particle 23. The pores 26 create a second interface 28 between the first particle 22 and the third pores 26, and a third interface 29 between the second particle 23 and the third pores 26. The first particle 22, the second particle 23, and the pores 26 create a three-phase interface 30 where the first interface 27, the second interface 28, and the third interface 29 intersect. The three-phase interface 30 is a reaction field where the fuel supplied through the pores 26, the first particle 22, and the second particle 23 come into contact.

A ceria-based solid solution is suitable for the first particles 22. It is presumed that H ⁺ dissolved in ceria moves to the surface of the ceria and reacts with ^O2- , and the reaction field expands not only to the three-phase interface 30 but also to the ceria surface near the three-phase interface 30. This is expected to reduce the reaction overvoltage.

Figure 4 is a schematic diagram of the active layer 21 in a specified range (field of view 31) on the observation surface (cross section). The structure of the active layer 21 is identified as follows. First, an SEM image of the field of view 31 of the observation surface of the active layer 21 is obtained. The size and shape of the field of view 31 is, for example, a rectangle with short sides of 10 μm and long sides of 20 μm. In this embodiment, the direction in which the long sides of the field of view 31 extend is the first direction, and the direction in which the short sides of the field of view 31 extend is the second direction.

The field of view 31 can be set at any position in the active layer 21 in any direction. When the active layer 21 is a flat plate, the field of view 31 can be set so that the long side of the field of view 31 is along the interface 21a (see FIG. 2). If the distance between the interface 21b and the interface 21a between the active layer 21 and the support layer 20 (the thickness of the active layer 21) is 10 μm or more, the long side (20 μm) of the field of view 31 is set along the interface 21a, and the short side (10 μm) of the field of view 31 is set in the thickness direction of the active layer 21. The area of the field of view 31 is at least 200 μm ² to ensure the accuracy of quality control. When the thickness of the active layer 21 is less than 10 μm, the short side of the field of view 31 is set to the maximum in the thickness direction of the active layer 21, and the long side of the field of view 31 is set along the interface 21a so that the area of the field of view 31 is 200 μm ² or more.

Based on the brightness of the acquired SEM image and the results of elemental analysis by an energy dispersive X-ray spectrometer (EDS), the entire field of view 31 is ternarized by image processing so that, for example, the first particles 22 are white, the second particles 23 are gray, and the closed pores 24, 25 and pores 26 are black. Next, by image analysis, the circle equivalent diameter of the second particles 23, which is the diameter of a circle having the same area as the second particles 23 (excluding the area of the closed pores 25), is calculated for each particle for all second particles 23 included in the field of view 31.

The length of the first interface 27, the length of the second interface 28, and the length of the third interface 29 are calculated for each interface by image analysis. The interface length is calculated by adding up the lengths of all interfaces 27, 28, and 29 included in the actual field of view 31, and the percentage (%) of the length of all first interfaces 27 included in the actual field of view 31 divided by the interface length, the percentage (%) of the length of all second interfaces 28 included in the actual field of view 31 divided by the interface length, and the percentage (%) of the length of all third interfaces 29 included in the actual field of view 31 divided by the interface length are calculated.

The average length of the particles 22 and 23 in the first direction and the average length of the particles 22 and 23 in the second direction are calculated by image analysis. These average values can be calculated according to the method (intercept method) described in "Ceramic Processing, by Mizutani Takayoshi, Ozaki Yoshiharu, Kimura Toshio, and Yamaguchi Takashi, published by Gihodo Publishing Co., Ltd. on March 25, 1985, pages 192 to 195". Specifically, straight lines (straight lines parallel to the long side) are drawn on the image in the first direction of the field of view 31 (for example, the direction in which the long side of the field of view 31 extends) at intervals finer than 1.0 μm, and the lengths of the line segments cut out by all the second particles 23 included in the field of view 31 are measured. Next, the total value of the lengths of the line segments is divided by the number of line segments for which the total value was calculated to obtain a numerical value. This numerical value is set as the average length A of the second particles 23 in the first direction.

By image analysis, straight lines (straight lines parallel to the short side) are drawn on the image in the second direction of the field of view 31 (for example, the direction in which the short side of the field of view 31 extends) at intervals finer than 1.0 μm, and the lengths of the line segments cut out by all second particles 23 included in the field of view 31 are measured. Next, the total length of each line segment is divided by the number of line segments for which this total was calculated to determine a numerical value. This numerical value is regarded as the average value B of the lengths of the second particles 23 in the second direction.

By image analysis, straight lines (straight lines parallel to the long side) are drawn on the image in the first direction of the field of view 31 (for example, the direction in which the long side of the field of view 31 extends) at intervals finer than 1.0 μm, and the lengths of the line segments cut out by all of the first particles 22 included in the field of view 31 are measured. Next, the total length of each line segment is divided by the number of line segments for which this total was found to determine a numerical value. This numerical value is regarded as the average value C of the lengths of the first particles 22 in the first direction.

By image analysis, straight lines (straight lines parallel to the short side) are drawn on the image in the second direction of the field of view 31 (for example, the direction in which the short side of the field of view 31 extends) at intervals finer than 1.0 μm, and the lengths of the line segments cut out by all of the first particles 22 included in the field of view 31 are measured. Next, the total length of each line segment is divided by the number of line segments for which this total was calculated to determine a numerical value. This numerical value is regarded as the average value D of the lengths of the first particles 22 in the second direction.

In these image analyses, the first particles 22, second particles 23, and pores 26 that are partially outside the field of view 31 (the first particles 22, second particles 23, and pores 26 that are cut off by the long and short sides of the field of view 31) are not used in the calculation of the values. This is to ensure the accuracy of the calculation of the values.

Figure 5 shows the particle size distribution of the second particles 23 appearing in the actual field of view 31 based on the equivalent circle diameter. The normalized particle diameter shown on the horizontal axis of Figure 5 is the value obtained by dividing the equivalent circle diameter of each particle of the second particles 23 appearing in the actual field of view 31 by the maximum value of the equivalent circle diameter. The horizontal axis of Figure 5 shows a classification divided into 20 intervals every 0.05, with the maximum value of the equivalent circle diameter being 1. The vertical axis of Figure 5 shows the frequency (%) of the second particles 23 having an equivalent circle diameter (normalized particle diameter) that exceeds the lower limit of each interval and is equal to or less than the upper limit of each interval (the same applies to Figure 6).

As shown in FIG. 5, in the particle size distribution of the second particles 23 appearing in the field of view 31 of the active layer 21, it is preferable that the frequency of second particles having a circular equivalent diameter of 1/20 or less of the maximum circular equivalent diameter of the second particles 23 is 10% or less. This can reduce the deterioration of the performance of the electrochemical cell 10 due to sintering of the minute second particles 23, thereby improving the durability of the electrochemical cell 10.

It is preferable that the area of the second particles 23 appearing in the actual field of view 31 of the active layer 21, which have a circular equivalent diameter of 1/20 or less of the maximum circular equivalent diameter, is 0.1% or less of the area of all the second particles 23 appearing in the actual field of view 31. This can reduce the deterioration of the performance of the electrochemical cell 10 due to sintering of the minute second particles 23, thereby improving the durability of the electrochemical cell 10.

The value A/B obtained by dividing the average value A by the average value B is preferably greater than 0.9 and less than 1.1. This makes the cross section of the second particles 23 closer to a circle, thereby reducing the activity of the second particles 23. Since the surface diffusion of the second particles 23 can be reduced, the durability of the electrochemical cell 10 can be further improved.

The value C/D obtained by dividing the average value C by the average value D is preferably greater than 0.9 and less than 1.1. This makes the cross section of the first particles 22 closer to a circle, thereby reducing the activity of the first particles 22. Since the surface diffusion of the first particles 22 can be reduced, the durability of the electrochemical cell 10 can be further improved.

It is preferable that the ratio of the length of the second interface 28 to the total interface length of the first interface 27, the second interface 28, and the third interface 29 is 40% or more. The diffusion of the atoms constituting the second particle 23 is more likely to occur as surface diffusion at the third interface 29 (solid/gas interface) than as grain boundary diffusion at the first interface 27 (solid/solid interface). When the ratio of the length of the second interface 28 is 40% or more, the length of the third interface 29 becomes relatively short, thereby reducing the surface diffusion of the atoms constituting the second particle 23. This can reduce the deterioration of the performance of the electrochemical cell 10. Therefore, durability can be improved.

The ratio of the length of the second interface 28 to the interface length is preferably 40% or more and 50% or less. More preferably, it is 40% or more and 45% or less. If the ratio of the length of the second interface 28 exceeds 45%, it becomes difficult for the second particles 23 to connect to each other in the active layer 21, and there is a tendency for electronic conductivity to decrease. In particular, if the ratio of the length of the second interface 28 exceeds 50%, this tendency becomes more pronounced.

When comparing the ratio of the length of the first interface 27 to the interface length, the ratio of the length of the second interface 28 to the interface length, and the ratio of the length of the third interface 29 to the interface length, it is preferable that the ratio of the length of the third interface 29 to the interface length is the lowest. If the ratio of the length of the third interface 29 is the lowest, the surface diffusion of the atoms constituting the second particle 23 can be further reduced, and therefore durability can be further improved.

The structure of the active layer 21 can be controlled by subjecting the electrochemical cell 10 to a reduction treatment and an electric current treatment before the operation of the electrochemical cell 10. The reduction treatment and the electric current treatment are performed by the manufacturer or user of the electrochemical cell 10.

In the reduction process, the fuel electrode 12 is reduced in a reducing gas atmosphere at a temperature close to the temperature of the fuel electrode 12 during operation of the electrochemical cell 10 (hereinafter referred to as the "operating temperature"). The reducing gas is a reducing gas containing hydrogen, and is preferably pure hydrogen. In the reduction process, the fuel electrode 12 is preferably heated to a temperature higher than the operating temperature. More preferably, the fuel electrode 12 is heated to a temperature at least 50°C higher than the operating temperature. The reduction process reduces the NiO contained in the second particles 23 to Ni. The reduction process reduces the electrical resistance of the fuel electrode 12, making it possible to perform the current flow process.

The hydrogen concentration of the reducing gas is set appropriately within the range of, for example, 4% to 100%. When using city gas as fuel, reducing gas (hydrogen concentration 60-70%) obtained by reforming city gas with water steam can be used. The reduction process time is set appropriately so that reduction of the catalyst can be achieved.

In the current application process, the electric potential of the air electrode 13 is alternately set between a state in which the electric potential of the air electrode 13 is in the range of -0.2 V to 0.45 V (hereinafter referred to as the "first range") relative to the fuel electrode 12 and a state in which the electric potential of the air electrode 13 is in the range of 0.9 V to 1.5 V (hereinafter referred to as the "second range") relative to the fuel electrode 12 so that the air electrode 13 has a positive electric potential relative to the fuel electrode 12. This stabilizes the polarization resistance of the fuel electrode 12 and improves the durability of the electrochemical cell 10. Note that the electric potential of the electrochemical cell 10 is usually treated as being positive relative to the fuel electrode 12, so hereafter, for convenience, the electric potential of the air electrode 13 will be expressed as being positive relative to the fuel electrode 12.

The times for which the potential is set to the first range and the second range are, for example, from 5 seconds to 30 minutes. The number of times for which these potential settings are repeated is, for example, from 1 cycle to 300 cycles, with the process of setting the potential to the first range and the second range once each being one cycle.

An example of a device for setting the potential of the air electrode 13 relative to the fuel electrode 12 is an electronic load device that can pass a load current of any magnitude in both directions. An electronic load device is connected between the fuel electrode 12 and the air electrode 13, and the load current value is set so that the potential of the air electrode 13 relative to the fuel electrode 12 becomes the desired value.

The electrochemical cell 10 is energized while supplying a fuel gas (hydrogen, carbon monoxide, hydrocarbon, etc.) to the fuel electrode 12 and an oxidant gas (oxygen, air) to the air electrode 13. The temperature of the fuel electrode 12 and the flow rate of the gas supplied during energization may be approximately equal to the temperature of the fuel electrode 12 and the flow rate of the gas supplied during operation of the electrochemical cell 10. The temperature of the fuel electrode 12 during energization is, for example, 600°C to 700°C.

The durability of the electrochemical cell 10 depends greatly on how well the chemical state and microstructure of the first particles 22 having oxide ion conductivity and the second particles 23 having electronic conductivity in the fuel electrode 12 (active layer 21) are maintained. It is presumed that the microstructure of the first particles 22 and second particles 23 and the chemical state of the first particles 22 are maintained by the oxidation and re-reduction of the surface of the second particles 23 due to the current application treatment.

The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

(Preparation of Electrochemical Cell in Example 1)
Yttria-stabilized zirconia (YSZ) powder and NiO powder were mixed, and butyral resin, polymethylmethacrylate beads as a pore former, plasticizer, dispersant, and solvent were added and mixed to obtain a slurry. The slurry was made into a green sheet with a thickness of 200 μm by a doctor blade method to obtain a molded body for the support layer.

Gd- _doped ceria ( _{Gd0.2Ce0.8O2 -δ} ) powder (GDC powder) and NiO powder were mixed, and butyral resin, polymethylmethacrylate beads as a pore former, plasticizer, dispersant, and solvent were added and mixed to obtain a slurry. The slurry was made into a green sheet with a thickness of 20 μm by the doctor blade method to obtain a green body for the active layer.

Butyral resin, plasticizer, dispersant, and solvent were added to the YSZ powder and mixed to obtain a slurry. The slurry was made into a green sheet of 10 μm thickness using the doctor blade method to obtain a green sheet for the electrolyte layer. The green sheets for the support layer, active layer, and electrolyte layer were stacked and pressed together, degreased, and then fired at 1350°C in air to obtain a laminate of the fuel electrode 12 and electrolyte layer 11.

Polyvinyl alcohol and a solvent were added to the GDC powder, mixed, and the viscosity was adjusted to obtain a paste. The paste was applied to the electrolyte layer 11 of the laminate by screen printing to form a film. After drying, it was fired at 1200°C in air to obtain a laminate provided with an intermediate layer 14.

The _{La0.6Sr0.4Co0.2Fe0.8O3 -δ} powder was mixed with polyvinyl alcohol and a solvent, and _the viscosity _was adjusted to obtain a paste. The paste was applied to the intermediate layer 14 of the laminate by screen printing to form a film. After drying _, it was fired at 1100°C in air to obtain an electrochemical cell 10 provided with an air electrode 13.

(Reduction Treatment)
The temperature of the fuel electrode 12 was raised to 700° C. under a nitrogen atmosphere, and after the temperature reached 700° C., the atmosphere was switched to a hydrogen atmosphere to reduce the fuel electrode 12. The hydrogen concentration was 100%, and the reduction time was 1 hour.

(Electrical supply processing)
While heating the electrochemical cell 10 so that the fuel electrode 12 of the electrochemical cell 10 that had undergone the reduction treatment was heated to 700° C., a fuel gas (hydrogen: 20 ml/min, steam: 100 ml/min) was supplied to the fuel electrode 12, and an oxidant gas (air: 100 ml/min) was supplied to the air electrode 13, and a current was passed between the fuel electrode 12 and the air electrode 13 of the electrochemical cell 10. The potential of the air electrode 13 with respect to the fuel electrode 12 was alternately set to a range of −0.2 V to 0.45 V (first range) and a range of 0.9 V to 1.5 V (second range) so that the air electrode 13 had a positive potential with respect to the fuel electrode 12 by the current passing.

The time for which the potential was set in the first and second ranges was not constant, but ranged from 5 seconds to 5 minutes. The process of setting the potential in the first and second ranges once each was counted as one cycle, and 200 cycles of current were passed. The total current passing time was 3 hours. In this way, the electrochemical cell in Example 1 was obtained.

Example 2
An electrochemical cell in Example 2 was obtained by producing an electrochemical cell in the same manner as in Example 1, except that the average particle size of the NiO powder contained in the active layer was changed.

(Comparative Example 1)
An electrochemical cell in Comparative Example 1 was obtained by producing an electrochemical cell in the same manner as in Example 1, except that no current application treatment was performed.

(Comparative Example 2)
An electrochemical cell was produced in the same manner as in Example 1, except that the active layer of the fuel electrode containing the GDC powder and the NiO powder was omitted and no current was applied, to obtain an electrochemical cell in Comparative Example 2 in which the composition of the fuel electrode was Ni-YSZ.

(Identification of the active layer structure)
A part of the electrochemical cell in Example 1, Example 2, and Comparative Example 1 was cut out to obtain a sample. Each sample was embedded in a room temperature curing resin, and a cross section of the active layer 21 perpendicular to the interface 21a between the active layer 21 and the electrolyte layer 11 was polished. A FIB was irradiated from an oblique direction to the cross section of the active layer 21 to prepare a smooth observation surface of the active layer 21. An image of the observation surface in a rectangular field of view 31 measuring 10 μm in length and 20 μm in width was obtained by SEM. The long side of the field of view 31 was set along the interface 21a.

After the entire field of view 31 was ternarized by image processing so that GDC (first particles 22) was white, Ni (second particles 23) was gray, and closed pores 24, 25 and pores 16 were black, the circle-equivalent diameters of all second particles 23 contained in the field of view 31 were calculated for each particle by image analysis. The maximum circle-equivalent diameter of the second particles 23 in the sample in Example 1 was 2.176 μm, the maximum circle-equivalent diameter of the second particles 23 in the sample in Example 2 was 1.968 μm, and the maximum circle-equivalent diameter of the second particles 23 in the sample in Comparative Example 1 was 1.788 μm.

For the second particles 23 appearing in the field of view 31, the normalized particle diameter was calculated by dividing the circle-equivalent diameter of each particle of the second particles 23 by the maximum circle-equivalent diameter. Next, the maximum circle-equivalent diameter was set to 1, and the frequency (%) of the second particles 23 having a circle-equivalent diameter (normalized particle diameter) included in the intervals divided by 0.05 was calculated. The particle size distribution of the second particles 23 in Example 1 is shown in Figure 5, and the particle size distribution of the second particles 23 in Comparative Example 1 is shown in Figure 6.

By image analysis, the length of the first interface 27, the length of the second interface 28, and the length of the third interface 29 were calculated for each interface, and the interface length was calculated by adding up the lengths of all interfaces 27, 28, and 29 included in the actual field of view 31. The interface length of the sample in Example 1 was 635 μm, the interface length of the sample in Example 2 was 643 μm, and the interface length of the sample in Comparative Example 1 was 641 μm. The percentage (%) of the length of all first interfaces 27 included in the actual field of view 31 divided by the interface length, the percentage (%) of the length of all second interfaces 28 included in the actual field of view 31 divided by the interface length, and the percentage (%) of the length of all third interfaces 29 included in the actual field of view 31 divided by the interface length were calculated.

Image analysis was performed by drawing 21 straight lines parallel to the long side of the field of view 31 at equal intervals on the image, and measuring the lengths of all the line segments cut by the second particles 23. Next, the average value A was calculated by dividing the total length of each line segment by the number of line segments for which the total value was calculated.

Similarly, 21 straight lines parallel to the short side of the field of view 31 were drawn at equal intervals on the image, and the lengths of all the line segments cut by the second particles 23 were measured. Next, the average value B was calculated by dividing the total length of each line segment by the number of line segments for which the total value was calculated, and A/B was calculated.

Image analysis was performed by drawing 21 straight lines parallel to the long side of the field of view 31 at equal intervals on the image, and measuring the lengths of all the line segments cut by the first particles 22. Next, the average value C was calculated by dividing the total length of each line segment by the number of line segments for which the total value was calculated.

Similarly, 21 straight lines parallel to the short side of the field of view 31 were drawn at equal intervals on the image, and the lengths of all the line segments cut by the first particles 22 were measured. Next, the average value D was calculated by dividing the total length of each line segment by the number of line segments for which the total value was calculated, and C/D was calculated.

(An endurance test)
While heating the electrochemical cell 10 (SOEC) to 800° C., hydrogen containing water vapor (H ₂ O: 90 ml/min, H ₂ : 10 ml/min) was supplied to the fuel electrode 12, and air (100 ml/min) was supplied to the air electrode 13, while a current of 1.5 A/cm ² was passed between the fuel electrode 12 and the air electrode 13. In this state, the change over time in the voltage between the fuel electrode 12 and the air electrode 13 was measured up to 1000 hours. From the voltage V ₀ at the start of the test and the voltage V _T after 1000 hours had elapsed, the deterioration rate (%)=(V _T -V ₀ )/V ₀ ×100, which represents the rate of increase in the electrical resistance between the fuel electrode 12 and the air electrode 13, was calculated.

Table 1 shows the composition of the active layer in the examples and comparative examples, the frequency (%) of second particles 23 having a circle equivalent diameter of 1/20 or less of the maximum diameter of the second particles 23 in the particle size distribution by the circle equivalent diameter of the second particles 23, the ratio (%) of the area of the second particles 23 having a circle equivalent diameter of 1/20 or less of the maximum diameter of the second particles 23 to the area of all the second particles 23, A/B, C/D, the ratio (%) of the first interface 27 to the interface length, the ratio (%) of the second interface 28 to the interface length, the ratio (%) of the third interface 29 to the interface length, and the deterioration rate (%) of the electrochemical cell in the durability test.

表１に示すように、第２の粒子２３の最大径の１／２０以下の円相当径をもつ第２の粒子２３の頻度が１０％以下である実施例１及び２は、その頻度が２１％である比較例１に比べて劣化率が低く、耐久性に優れることが明らかになった。

As shown in Table 1, Examples 1 and 2, in which the frequency of second particles 23 having a circular equivalent diameter that is 1/20 or less of the maximum diameter of second particles 23 is 10% or less, have a lower deterioration rate and are superior in durability compared to Comparative Example 1, in which the frequency is 21%.

It was revealed that Examples 1 and 2, in which the ratio of the area of second particles 23 having a circle equivalent diameter of 1/20 or less of the maximum diameter of the second particles 23 to the area of all second particles 23 is 0.1% or less, have a lower deterioration rate and are superior in durability compared to Comparative Example 1, in which the ratio is 0.54%.

It was revealed that Examples 1 and 2, in which A/B is greater than 0.9 and less than 1.1, have a lower deterioration rate and are more durable than Comparative Example 1, in which A/B is 1.11.

It was revealed that Examples 1 and 2, in which the C/D ratio was greater than 0.9 and less than 1.1, had a lower deterioration rate and superior durability compared to Comparative Example 1, in which the C/D ratio was 1.10.

It was revealed that Examples 1 and 2, in which the ratio of the second interface 28 to the interface length was 40% or more, had a lower deterioration rate and superior durability than Example 1, in which the ratio was 39.2%. In Examples 1 and 2, the ratio of the third interface 29 to the interface length was lower than the ratios of the other interfaces 27 and 28 to the interface length.

The present invention has been described above based on the embodiments, but the present invention is in no way limited to the above embodiments, and it can be easily imagined that various improvements and modifications are possible within the scope of the invention without departing from its spirit.

In the embodiment, the electrochemical cell 10 in which the intermediate layer 14 is disposed between the electrolyte layer 11 and the air electrode 13 has been described, but this is not necessarily limited to this. Of course, it is possible to omit the intermediate layer 14 depending on the materials of the electrolyte layer 11 and the air electrode 13.

In the embodiment, the electrolyte contained in the fuel electrode 12 is Gd _0.2 Ce _0.8 O _2-δ , but this is not necessarily limited to this. The electrolyte contained in the fuel electrode 12 can be set as appropriate.

REFERENCE SIGNS LIST 10 Electrochemical cell 11 Electrolyte layer 12 Anode 13 Cathode 21 Active layer 22 First particle 23 Second particle 26 Pore 27 First interface 28 Second interface 29 Third interface

Claims (4)

The fuel cell comprises an electrolyte layer, a fuel electrode provided on one surface of the electrolyte layer, and a cathode provided on the other surface of the electrolyte layer,
the anode includes an active layer in contact with the electrolyte layer,
the active layer includes first particles having ionic conductivity and second particles having electronic conductivity;
The first particles also have electronic conductivity in an anode atmosphere,
a first interface between the first particle and the second particle, a second interface between the first particle and a pore, and a third interface between the second particle and the pore, which appear in a cross section of the active layer,
An electrochemical cell, wherein a ratio of a length of the second interface to a total interface length of the first interface, the second interface, and the third interface is 40% or more. The electrochemical cell of claim 1, wherein the ratio of the length of the third interface to the interface length is the lowest when comparing the ratio of the length of the first interface to the interface length, the ratio of the length of the second interface to the interface length, and the ratio of the length of the third interface to the interface length. The electrochemical cell according to claim 1 or 2, wherein the value A/B obtained by dividing the average length A of the second particles in the first direction on the cross section by the average length B of the second particles in the second direction on the cross section perpendicular to the first direction is greater than 0.9 and less than 1.1. An electrochemical cell according to any one of claims 1 to 3, wherein the value C/D obtained by dividing the average length C of the first particles in a first direction on the cross section by the average length D of the first particles in a second direction on the cross section perpendicular to the first direction is greater than 0.9 and less than 1.1.

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