Electrochemical cells, electrochemical cell stacks, hot modules, and electrolytic reaction apparatuses

Abstract

This suppresses the migration and aggregation of Ni particles in the fuel polar layer of an electrochemical cell. [Solution] The electrolytic cell 21 comprises a solid electrolyte layer 211, an air electrode layer 212, and a fuel electrode layer 213 containing Ni particles. The cross-section of the fuel electrode layer has pores 102 composed of multiple particles. Of the pores in the cross-section, if we define the length in the thickness direction between the endpoints on both sides in the thickness direction as the pore length, and define the straight line passing through the midpoint and perpendicular to the thickness direction as the pore centerline, then the proportion of specific pores 102s in the region of interest that satisfy either the condition that the particles constituting the pore do not contain Ni particles or contain only one Ni particle, or the condition that the particles constituting the pore contain two or more Ni particles and all of those Ni particles are on the opposite side of the pore centerline from the solid electrolyte layer side is greater than 72.7%.

Description

Technical Field

[0001] The present invention relates to an electrochemical cell, an electrochemical cell stack, a hot module, and an electrolysis reaction device.

Background Art

[0002] Conventionally, a solid oxide type electrochemical cell using a solid oxide as an electrolyte has been known (see, for example, Patent Document 1). The solid oxide type electrochemical cell is characterized by performing an electrochemical reaction with high efficiency in a high temperature environment, and can be used as a solid oxide electrolysis cell (SOEC: Solid Oxide Electrolysis Cell) or a solid oxide fuel cell (SOFC: Solid Oxide Fuel Cell). The solid oxide electrolysis cell is an electrolysis device that decomposes water vapor into hydrogen and oxygen by electric energy. The solid oxide fuel cell is a power generation device that generates electric energy by the chemical reaction of hydrogen and oxygen.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] An electrochemical cell can be constructed comprising a solid electrolyte layer, an air electrode layer stacked on one side of the solid electrolyte layer, and a fuel electrode layer stacked on the other side of the solid electrolyte layer. Generally, the fuel electrode layer contains Ni (nickel) particles and ceramic particles as electrode catalysts. The ceramic particles are ion-conductive oxide particles, such as YSZ (yttria-stabilized zirconia). When an electrochemical cell with a fuel electrode layer containing Ni particles and ceramic particles is operated for a long time under harsh operating conditions (high temperature, high humidity, high current), migration of Ni particles within the fuel electrode layer and the resulting aggregation occur. When Ni particles migrate within the fuel electrode layer, the conductive paths within the fuel electrode layer decrease, increasing internal resistance. Furthermore, when Ni particles aggregate within the fuel electrode layer, the particle size of the Ni particles increases, which reduces the number of three-phase interfaces (boundaries between fuel gas, Ni particles, and ceramic particles) that serve as the reaction field, and also reduces the surface area of ​​the Ni particles, degrading catalytic performance and consequently increasing reaction resistance in the electrochemical reaction (in other words, reducing reaction efficiency). Such increases in internal and reaction resistance mean a decrease in the durability of the electrochemical cell.

[0005] This invention was made to address the problems described above. Specifically, one of the objectives of this invention is to provide a technique that can suppress the migration and aggregation of Ni particles in the fuel polar layer of an electrochemical cell.

[0006] The electrochemical cell (21) according to the present invention is Solid electrolyte layer (211), An air electrode layer (212) is stacked on one side of the solid electrolyte layer, A fuel electrode layer (213) containing Ni particles (100) is stacked on the other side of the solid electrolyte layer, It is equipped with. When the fuel electrode layer is cut along the thickness direction in any plane, the cross-section has multiple pores (102) composed of multiple particles of one or more types. Of the aforementioned cross-section, for each of the plurality of pores located in the region of interest (Ra) having a width of 5 μm in the thickness direction from the interface (S) between the solid electrolyte layer and the fuel electrode layer, if we define the length in the thickness direction between the endpoint (p1) located on the one side and the endpoint (p2) located on the other side as the pore length (Dy), and define the straight line passing through the midpoint of the pore length and perpendicular to the thickness direction as the pore centerline (Ly), then, Of the pores present in the region of interest, The proportion of specific pores (102s) that satisfy any of the following conditions is greater than 72.7%: 1) The particles constituting the pore do not contain the Ni particles; 2) The particles constituting the pore contain only one Ni particle; or 3) The particles constituting the pore contain two or more Ni particles, and all of these Ni particles are located on the side opposite to the solid electrolyte layer with respect to the pore centerline of the pore. Electrochemical cell.

[0007] In conventional configurations, when an electrochemical cell is operated under harsh operating conditions, Ni particles migrate and aggregate. This phenomenon occurs particularly frequently in a region of the fuel electrode layer with a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer (i.e., the region of interest). The inventors of this invention have found that the migration and aggregation of Ni particles tend to follow the following patterns. • When Ni particles are present in the vicinity of other Ni particles, they move toward those other Ni particles through the pores they comprise and aggregate. Ni particles tend to move easily in the thickness direction from the solid electrolyte layer toward the fuel electrode layer, and are less likely to move in the opposite direction (i.e., the thickness direction from the solid electrolyte layer toward the air electrode layer). Based on the above findings, the electrochemical cell according to the present invention is configured such that the proportion of specific pores in the region of interest that satisfy any of the first to third conditions is greater than 72.7%. For specific pores that satisfy the first condition, the particles constituting the specific pore do not contain Ni particles in the first place (in other words, the specific pore is composed of particles other than Ni particles), so the possibility of Ni particles moving and agglomerating through the specific pore is extremely low. Furthermore, for specific pores that satisfy the second condition, the particles constituting the specific pore contain only one Ni particle and do not contain another Ni particle that could trigger movement, so the possibility of that single Ni particle moving and agglomerating through the specific pore is extremely low. Moreover, for specific pores that satisfy the third condition, the particles constituting the specific pore contain two or more Ni particles, so at first glance it might seem that there is a trigger for Ni particles to move toward other Ni particles. However, all of these Ni particles are located on the opposite side from the solid electrolyte layer with respect to the pore centerline of that specific pore (a straight line passing through the midpoint of the pore length and perpendicular to the thickness direction). Therefore, it becomes less likely that these Ni particles will move toward the solid electrolyte layer (i.e., in the direction in which Ni particles are less likely to move), and as a result, the possibility of these Ni particles moving and agglomerating through specific pores becomes extremely low. According to the configuration of the present invention, since specific pores are present in greater than 72.7% of the region of interest, the movement and agglomeration of Ni particles in the fuel electrode layer of the electrochemical cell can be appropriately suppressed. As a result, the deterioration of the durability of the electrochemical cell can be appropriately suppressed.

[0008] Electrochemical cells can be broadly classified into two types: fuel electrode-supported type, where the fuel electrode layer supports the solid electrolyte layer and the air electrode layer, and electrolyte-supported type, where the solid electrolyte layer supports the air electrode layer and the fuel electrode layer. In fuel electrode-supported electrochemical cells, the fuel electrode layer is composed of a functional layer and a support layer. In fuel electrode-supported electrochemical cells, the "cross-section obtained when the fuel electrode layer is cut along the thickness direction by an arbitrary plane" in this invention refers to the cross-section in the thickness direction of the functional layer, not the cross-section in the thickness direction of the entire fuel electrode layer. Furthermore, in this specification, the "proportion of specific pores (among the pores present in the region of interest)" is defined as the "average proportion of specific pores in multiple ranges within the region of interest." The number of ranges is typically three, but it may be two or four or more. The more ranges there are, the more accurate the proportion of specific pores becomes.

[0009] In one aspect of the present invention, Of the pores (102) present in the region of interest (Ra), the proportion of pores with a pore length (Dy) of 0.05 μm or more and 0.6 μm or less is 85% or more.

[0010] If the pore length of the pores in the region of interest is 0.05 μm or more, the diffusivity of the gas can be adequately ensured. On the other hand, if the pore length is 0.6 μm or less, it becomes less likely that Ni particles constituting one pore will also constitute another pore, thus adequately suppressing the formation of a structure in the region of interest that is prone to Ni particle migration and aggregation. According to one aspect of the present invention, more than 85% of the pores in the region of interest have a pore length of 0.05 μm or more and 0.6 μm or less, thus adequately achieving both gas diffusivity and suppression of Ni particle migration and aggregation.

[0011] The electrochemical cell stack (20) according to the present invention is The electrochemical cell (21) according to the present invention is formed by stacking multiple cells.

[0012] This configuration provides an electrochemical cell stack in which the migration and aggregation of Ni particles in the fuel polar layer of the electrochemical cell are appropriately suppressed.

[0013] The hot module (10) according to the present invention includes the electrochemical cell stack (20) according to the present invention, a temperature raising device (40, 50) for raising the temperature of the gas supplied to the electrochemical cell stack, and a heat insulating material (60) in which the electrochemical cell stack and the temperature raising device are disposed. It is provided with.

[0014] According to the above configuration, it is possible to provide a hot module in which the movement and aggregation of Ni particles are appropriately suppressed in the fuel electrode layer of the electrochemical cell.

[0015] The electrolytic reaction device (1) according to the present invention includes the hot module (10) according to the present invention.

[0016] According to the above configuration, it is possible to provide an electrolytic reaction device in which the movement and aggregation of Ni particles are appropriately suppressed in the fuel electrode layer of the electrochemical cell. Note that this hot module means a hot module including an electrochemical cell stack formed by laminating solid oxide type electrolytic cells, and does not include a hot module including an electrochemical cell stack formed by laminating solid oxide type fuel cells.

[0017] In the above description, for the purpose of assisting the understanding of the invention, reference numerals used in the embodiments are added in parentheses to the constituent elements of the invention corresponding to the embodiments. However, each constituent element of the invention is not limited to the embodiments defined by the above reference numerals.

Brief Description of Drawings

[0018] [Figure 1] It is a block diagram of an electrolytic reaction device. [Figure 2] It is a perspective view of a cell stack. [Figure 3] It is a cross-sectional view taken along line III-III of FIG. 2. [Figure 4]It is a cross-sectional view when the electrolytic cell is cut by an arbitrary plane along the thickness direction. [Figure 5A] It is a diagram showing an example of pores on the cut surface of the functional layer. [Figure 5B] It is a diagram for explaining the pore length and the pore center line of the pores. [Figure 6] It is an enlarged view of the region R in FIG. 4. [Figure 7A] It is a diagram showing an example of a specific pore that satisfies the first condition. [Figure 7B] It is a diagram showing an example of a specific pore that satisfies the second condition. [Figure 7C] It is a diagram showing another example of a specific pore that satisfies the second condition. [Figure 7D] It is a diagram showing an example of a specific pore that satisfies the third condition. [Figure 8A] It is a diagram showing an example of a pore that does not correspond to the specific pore. [Figure 8B] It is a diagram showing another example of a pore that does not correspond to the specific pore. [Figure 9] It is a cross-sectional view when the electrolytic cell as a comparative example is cut by an arbitrary plane along the thickness direction. [Figure 10A] It is a partially enlarged view of the functional layer and its vicinity in the SEM image of the cut surface of the electrolytic cell in the initial state according to the embodiment. [Figure 10B] It is a partially enlarged view of the functional layer and its vicinity in the SEM image of the cut surface of the electrolytic cell after the durability deterioration test according to the embodiment. [Figure 11A] It is a partially enlarged view of the functional layer and its vicinity in the SEM image of the cut surface of the electrolytic cell in the initial state according to the comparative example. [Figure 11B] It is a partially enlarged view of the functional layer and its vicinity in the SEM image of the cut surface of the electrolytic cell after the durability deterioration test according to the comparative example. [Figure 12] It is a graph showing the pore length distribution in a predetermined range of the attention area of the electrolytic cell according to the embodiment and the pore length distribution in a predetermined range of the attention area of the electrolytic cell according to the comparative example.

Mode for Carrying Out the Invention

[0019] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a block diagram of the electrolytic reactor 1 according to this embodiment. The electrolytic reactor 1 according to this embodiment is a device that produces hydrogen by electrolyzing water vapor. As shown in Figure 1, the electrolytic reactor 1 comprises a hot module 10 and a condenser 90.

[0020] The hot module 10 is constructed by covering the main components that become hot among the elements constituting the electrolytic reaction apparatus 1 with an insulating material, and is a device in which the main components are concentrated within the insulating material so that the high temperature state of the main components is maintained. This hot module 10 comprises a cell stack 20, a vaporizer 30, a heat exchanger 40, a heater 50, and an insulating material 60.

[0021] As shown in Figure 1, water (H2O) is supplied to the vaporizer 30. The vaporizer 30 is heated to a temperature of 100°C or higher by a heating source (not shown). As a result, water vapor is generated when the water supplied to the vaporizer 30 evaporates within the vaporizer 30. The water vapor generated in the vaporizer 30 is introduced into the heat exchanger 40.

[0022] In addition to the steam mentioned above, air is introduced into the heat exchanger 40. High-temperature hydrogen (H2) and high-temperature oxygen (O2) generated in the cell stack 20, which will be described later, are also introduced into the heat exchanger 40. These high-temperature gases then exchange heat with the steam and air in the heat exchanger 40, thereby heating the steam and air introduced from the vaporizer 30.

[0023] The steam and air heated in the heat exchanger 40 are further heated by the heater 50 to the operating temperature of the cell stack 20 (i.e., the temperature required to operate the cell stack 20). After that, the steam and air are introduced into the cell stack 20. The heat exchanger 40 and heater 50 are heating devices that raise the gas (steam and air) supplied to the cell stack 20 to the operating temperature of the cell stack 20.

[0024] The cell stack 20 is formed by stacking solid oxide electrolytic cells. The cell stack 20 is heated to its operating temperature by a heating source (burner, etc.) not shown. A predetermined voltage is also applied to the cell stack 20. As a result, the water vapor introduced into the cell stack 20 is electrolyzed to produce hydrogen and oxygen. The hydrogen produced in the cell stack 20 is introduced into the heat exchanger 40 along with the unreacted water vapor, and after being used to heat the water vapor and air introduced into the heat exchanger 40 from the vaporizer 30, it is introduced into the condenser 90. In the condenser 90, the unreacted water vapor is condensed. The condensed water produced in the condenser 90 is introduced into the vaporizer 30. Meanwhile, the hydrogen separated by the condensation of water vapor in the condenser 90 is recovered. The oxygen produced in the cell stack 20 is introduced into the heat exchanger 40, and after being used to heat the water vapor and air, it is introduced into the vaporizer 30 to heat the water supplied to the vaporizer 30. The oxygen discharged from the vaporizer 30 is then recovered (or released into the atmosphere).

[0025] The cell stack 20, vaporizer 30, heat exchanger 40, and heater 50 are arranged inside the insulation material 60. This suppresses heat dissipation from each component 20, 30, 40, and 50. The insulation material 60 may be made of heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), biosoluble fiber (AES), and / or heat-resistant containers formed from these heat-resistant fibers. The heat-resistant fibers are arranged to fill the gaps between the cell stack 20, vaporizer 30, heat exchanger 40, and heater 50.

[0026] Figure 2 is a perspective view of the cell stack 20, and Figure 3 is a cross-sectional view taken along line III-III in Figure 2. As shown in Figures 2 and 3, the cell stack 20 comprises an electrolytic unit group consisting of a plurality of rectangular flat electrolytic units Ue stacked in the thickness direction (vertical direction), and a pair of end plates 27 and 28 positioned on the upper and lower surfaces of the electrolytic unit group, respectively. The end plates 27 and 28 are both rectangular flat members having the same outer shape as the electrolytic units Ue, and a rectangular opening is formed in the center of each. The electrolytic unit group and the end plates 27 and 28 are fastened to each other at their four corners by bolts B and nuts (not shown) inserted through them in the thickness direction. The end plates 27 and 28 are made of metal (for example, stainless steel) and function as the anode and cathode, respectively, when voltage is applied. For the sake of explanation, the proportions of each component in the drawings may differ from the actual proportions.

[0027] The electrolytic unit Ue will be described with reference to Figure 3. As shown in Figure 3, the electrolytic unit Ue comprises a solid oxide type electrolytic cell 21 (hereinafter simply referred to as "electrolytic cell 21"), an interconnector 22, a separator 23, an air electrode frame 24, a fuel electrode frame 25, and a current collector 26.

[0028] The electrolytic cell 21 is the smallest unit of SOEC and comprises a solid electrolyte layer 211, an air electrode layer 212, and a fuel electrode layer 213. The air electrode layer 212 is stacked on the upper side of the solid electrolyte layer 211 so as to be in contact with the upper surface of the solid electrolyte layer 211. The air electrode layer 212 has a smaller outer shape than the solid electrolyte layer 211 and the fuel electrode layer 213 and is located in the center of the upper surface of the solid electrolyte layer 211 in a plan view of the electrolytic cell 21. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 211 is exposed. The fuel electrode layer 213 is stacked on the lower side of the solid electrolyte layer 211 so as to be in contact with the lower surface of the solid electrolyte layer 211.

[0029] The interconnector 22 is a rectangular metal (e.g., stainless steel) component with a rectangular current collector 22a protruding downward from the center of its lower surface. A pair of interconnectors 22 are arranged on both sides of the electrolytic cell 21 in the thickness direction. Two adjacent electrolytic units Ue,Ue share one interconnector 22. The interconnector 22 also functions as a separator separating two adjacent electrolytic units Ue,Ue. The lower surface of the current collector 22a is in contact with the upper surface of the air electrode layer 212 of the electrolytic cell 21. The lowermost electrolytic unit Ue is provided with a pair of interconnectors 22,29 instead of the pair of interconnectors 22,22. The interconnector 29 is located at the very bottom of the cell stack 20 and differs from the interconnector 22 in that it does not have a current collector 22a.

[0030] The separator 23 is a rectangular plate-shaped metal (e.g., stainless steel) component with a rectangular opening in its center. The periphery of the opening of the separator 23 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 211 of the electrolytic cell 21 with a brazing material (e.g., Ag brazing) not shown. The separator 23 suppresses the mixing of oxygen generated in the air electrode layer 212 and hydrogen generated in the fuel electrode layer 213 by the electrolysis of water vapor.

[0031] The air electrode frame 24 is a rectangular plate-shaped insulating member, which may be formed from, for example, a mica sheet. A rectangular opening is formed in the center of the air electrode frame 24. The air electrode frame 24 is positioned to be interposed between the separator 23 and the interconnector 22 above it.

[0032] The fuel electrode frame 25 is a rectangular plate-shaped metal (for example, stainless steel) component with a rectangular opening formed in its center. The fuel electrode frame 25 is positioned between the separator 23 and the interconnector 22 below it.

[0033] The internal space of the electrolytic unit Ue is divided into an air chamber Sa and a fuel chamber Sf by a separator 23. The air chamber Sa is a space that allows the flow of oxygen generated in the air electrode layer 212 and is composed of a space enclosed by the separator 23, the interconnector 22 above the separator 23, the air electrode frame 24, and the electrolytic cell 21. The fuel chamber Sf is a space that allows the flow of hydrogen generated in the fuel electrode layer 213 and is composed of a space enclosed by the separator 23, the interconnector 22 (or interconnector 29) below the separator 23, the fuel electrode frame 25, and the electrolytic cell 21.

[0034] The current collector 26 is a rectangular, porous metal (e.g., nickel) member formed smaller than the fuel electrode layer 213 in plan view, allowing hydrogen to pass through. The current collector 26 is positioned in the fuel chamber Sf to contact the lower surface of the fuel electrode layer 213 and the upper surface of the lower interconnector 22. Multiple electrolytic cells 21 are electrically connected in series by stacking two adjacent electrolytic cells 21 in the thickness direction so that they share the interconnector 22 via the current collector 26.

[0035] As shown in Figures 2 and 3, four gas pathways Pfi, Pfo, Pai, and Pao are formed on the outer periphery of the cell stack 20. These pathways Pfi, Pfo, Pai, and Pao are all formed to penetrate the cell stack 20 in the thickness direction, excluding the "end plate 27" and the "interconnector 22 above the upper electrolytic unit Ue at the upper end".

[0036] Path Pfi is formed near one corner of side E1, which is one of the four sides that make up the outer periphery of the cell stack 20. Path Pfo is formed near the other corner of side E2, which is opposite side E1 (the corner located diagonally opposite to one corner of side E1). As shown in Figure 3, path Pfi communicates with the fuel chamber Sf through a lateral hole 25a formed in the fuel electrode frame 25 of each electrolytic unit Ue. Path Pfo communicates with the fuel chamber Sf through a lateral hole 25b formed in the fuel electrode frame 25 of each electrolytic unit Ue.

[0037] Pathway Pai is formed near one corner of side E2. Pathway Pao is formed near the other corner of side E1. Pathways Pai and Pao communicate with the air chamber Sa, respectively, through transverse holes (not shown) formed in the air electrode frame 24 of each electrolytic unit Ue.

[0038] Next, the configuration of the electrolytic cell 21 will be described in more detail with reference to Figure 4. The size and thickness of each layer of the electrolytic cell 21 shown below are examples and are not limited to these values. Figure 4 is a cross-sectional view when the electrolytic cell 21 is cut along an arbitrary plane in the thickness direction. In Figure 4, the xyz coordinate system is set up so that the thickness direction of the cut surface corresponds to the y-axis direction, and the direction perpendicular to the thickness direction corresponds to the x-axis direction. As described above, the electrolytic cell 21 comprises a solid electrolyte layer 211, an air electrode layer 212, and a fuel electrode layer 213. The solid electrolyte layer 211 is a rectangular plate-shaped layer measuring 150 mm on each side and 6 μm thick, and is configured to contain ceramic particles and formed by sintering. These ceramic particles are oxide particles having ion conductivity, and in this embodiment, YSZ is used. The solid electrolyte layer 211 has high oxide ion conductivity. The solid electrolyte layer 211 is a dense layer designed to prevent leakage between the atmosphere on the air electrode layer 212 side (atmospheric atmosphere) and the atmosphere on the fuel electrode layer 213 side (reducing atmosphere). The ceramic particles in the solid electrolyte layer 211 are not limited to YSZ, but may also be, for example, GDC (gadolinia-doped ceria).

[0039] The air electrode layer 212 is laminated on the upper side of the solid electrolyte layer 211 so as to be in contact with the upper surface of the solid electrolyte layer 211. The air electrode layer 212 is a rectangular, flat layer with a thickness of 108 μm, and is composed of a perovskite-type oxide such as LSCF (lanthanum strontium cobalt iron oxide), and is formed by sintering. The air electrode layer 212 has a functional layer and a current collector layer. The current collector layer is thicker than the functional layer and is located on the upper surface of the functional layer. The air electrode layer 212 has high electronic conductivity and collects electrons well in the current collector layer. The air electrode layer 212 is a porous layer and has pores inside.

[0040] The fuel electrode layer 213 is a rectangular, flat layer measuring 150 mm on each side, and its thickness is greater than that of the solid electrolyte layer 211 and the air electrode layer 212, for example, it is formed to be 410 μm. The solid electrolyte layer 211 and the air electrode layer 212 are supported by the fuel electrode layer 213. In other words, the electrolytic cell 21 is a fuel electrode-supported cell. The fuel electrode layer 213 has a functional layer 213a and a support layer 213b. The functional layer 213a is stacked on the underside of the solid electrolyte layer 211 so as to be in contact with the underside of the solid electrolyte layer 211. The support layer 213b is located below the functional layer 213a. In other words, the functional layer 213a and the support layer 213b are stacked in this order on the underside of the solid electrolyte layer 211. The thickness of the support layer 213b is formed to be significantly thicker than the thickness of the functional layer 213a. In this embodiment, the functional layer 213a has a thickness of 10 μm, and the support layer 213b has a thickness of 400 μm. However, the thicknesses of the functional layer 213a and the support layer 213b are not limited to these values, and the ratio of the thickness of the support layer 213b to the thickness of the functional layer 213a can be set to, for example, about 16 to 40 times.

[0041] The structure of the functional layer 213a will be described in more detail with reference to Figures 5A to 8B. The functional layer 213a mainly contains a cermet of Ni particles and ceramic particles. The Ni particles are particles made of Ni as a catalytic metal. The ceramic particles are ion-conductive oxide particles, and in this embodiment, YSZ is used. Pores are formed in the gaps between the Ni particles and / or ceramic particles. Figure 5A shows an example of pores 102 in a cross-section of the functional layer 213a. As shown in Figure 5A, the pores 102 are formed in the gaps between a plurality of Ni particles 100 and ceramic particles 101. In other words, the pores 102 are composed of a plurality of Ni particles 100 and ceramic particles 101. In the example shown in Figure 5A, the pores 102 are composed of multiple particles of two types (Ni particles 100 and ceramic particles 101), but the pores 102 may also be composed of multiple particles of one type (Ni particles 100 or ceramic particles 101). In addition, the functional layer 213a may contain small amounts of particles other than Ni particles 100 and ceramic particles 101 (other particles). In this case, the pores 102 may be composed of multiple particles of two or three or more types, including other particles. Thus, the functional layer 213a is a porous layer configured to be porous, containing multiple pores 102. The gas diffuses into the functional layer 213a via the pores 102.

[0042] Figure 5B is a diagram illustrating the pore length Dy and pore centerline Ly of pore 102, and shows the outline of pore 102 in Figure 5A. As shown in Figure 5B, the pore length Dy is the length in the thickness direction (y-axis direction) between points p1 and p2. Here, point p1 is the endpoint of pore 102 located furthest towards the +y-axis direction, and point p2 is the endpoint of pore 102 located furthest towards the -y-axis direction. That is, the pore length Dy is the length when pore 102 is projected onto the y-axis. The pore length Dy is an index that defines the size of pore 102 in the thickness direction on the cross-section. The pore centerline Ly is a straight line passing through the midpoint of the pore length Dy and perpendicular to the thickness direction.

[0043] In conventional electrolytic cells, during cell stack operation, Ni particles within the functional layer migrate towards other Ni particles via pores and aggregate. This phenomenon occurs particularly frequently in a region of the functional layer with a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the functional layer. Therefore, in this embodiment, the functional layer 213a is configured to make it difficult for Ni particle migration and aggregation to occur in this region. This will be explained in detail below.

[0044] Figure 6 is an enlarged view of region R in Figure 4. As shown in Figure 6, the functional layer 213a includes region Ra and region Rc. Region Ra is a region of the functional layer 213a with a width of 5 μm in the thickness direction from the interface S between the solid electrolyte layer 211 and the functional layer 213a. In this embodiment, since the thickness of the functional layer 213a is 10 μm, region Ra can also be said to be the upper half of the functional layer 213a. As described above, region Ra corresponds to a region where the migration and aggregation of Ni particles 100 are particularly likely to occur. Hereinafter, region Ra will be referred to as the "region of interest Ra". On the other hand, region Rc is a region with a width of 2.5 μm above and below the straight line Lm that divides the thickness of the functional layer 213a in half. Hereinafter, region Rc will be referred to as the "central region Rc". In this embodiment, the lower half of region of interest Ra and the upper half of central region Rc overlap. Whether the region of interest Ra and the central region Rc partially overlap depends on the thickness of the functional layer 213a. When the thickness of the functional layer 213a is 15 μm or more, the two do not overlap. The central region Rc is explained in Figures 10A to 11B.

[0045] The inventors of the present invention have found that the movement and aggregation of Ni particles 100 tend to follow the following patterns. If another Ni particle 100 is present nearby, the Ni particle 100 will move toward the other Ni particle 100 through the pores 102 that it comprises and aggregate. Ni particles 100 move easily downwards (-y axis direction) and are difficult to move upwards (+y axis direction).

[0046] According to the second trend described above, the Ni particles 100 move primarily downward. For this reason, this phenomenon will also be referred to as "the downward movement of Ni particles 100" below. The downward movement of Ni particles 100 is thought to be caused by factors such as the direction of the electric field and the difference in gas concentration. Based on the above findings, the functional layer 213a is configured such that specific pores 102s, which are pores 102 that satisfy any of the following first to third conditions, are predominantly present in the region of interest Ra. In other words, in the region of interest Ra, the ratio of the number of specific pores 102s to the number of pores 102 is high. Below, "the ratio of the number of specific pores 102s to the number of pores 102 in the region of interest Ra" will also be simply referred to as "the ratio of specific pores 102s". (Condition 1) The particles constituting the pores 102 do not contain Ni particles 100. (Second condition) The particles constituting the pore 102 contain only one Ni particle 100. (Third condition) The particles constituting the pore 102 contain two or more Ni particles 100, and all of these Ni particles 100 are located below (-y axis direction) with respect to the pore centerline Ly of the pore 102.

[0047] Figure 7A illustrates a specific pore 102s that satisfies the first condition, Figures 7B and 7C illustrate specific pores 102s that satisfy the second condition, and Figure 7D illustrates a specific pore 102s that satisfies the third condition. Figures 8A and 8B illustrate pores 102 that do not fall under the category of specific pores 102s. In Figures 7A to 8B, the outer shape of the pores 102 (including specific pores 102s) is simplified to an elliptical shape for easier viewing, and only the Ni particles 100 among the particles constituting the pores 102 are shown.

[0048] The pore 102 in Figure 7A is composed of multiple particles other than the Ni particle 100. That is, the particles constituting the pore 102 do not contain the Ni particle 100. Therefore, this pore 102 is a specific pore 102s that satisfies the first condition. With this configuration, since the particles constituting the specific pore 102s do not contain the Ni particle 100 in the first place, the possibility of the Ni particle 100 moving and agglomerating through this specific pore 102s becomes extremely low.

[0049] The pore 102 in Figure 7B is composed of one Ni particle 100 and multiple particles of other types (typically ceramic particles 101). That is, the particles constituting the pore 102 contain only one Ni particle 100. Therefore, this pore 102 is a specific pore 102s that satisfies the second condition. In Figure 7B, the Ni particle 100 constitutes the pore 102 at a relatively lower position, but as shown in Figure 7C, the second condition is also met when the Ni particle 100 constitutes the pore 102 at a relatively upper position. With these configurations, since the particles constituting the specific pore 102s contain only one Ni particle 100 and do not contain another Ni particle 100 that could trigger movement, the possibility of that single Ni particle 100 moving and agglomerating through this specific pore 102s becomes extremely low.

[0050] The pore 102 in Figure 7D is composed of two Ni particles 100 and several other types of particles (typically ceramic particles 101), and both Ni particles 100 constitute the pore 102 at a relatively lower position. That is, the particles constituting the pore 102 contain two or more Ni particles 100, and all of these Ni particles 100 are located below the pore centerline Ly. For this reason, this pore 102 is a specific pore 102s that satisfies the third condition. With this configuration, since the particles constituting the specific pore 102s contain two or more Ni particles 100, it might appear at first glance that there is an opportunity for Ni particles 100 to move toward other Ni particles 100. However, since all of these Ni particles 100 are located below the pore centerline Ly, it is unlikely that these Ni particles 100 will move upward (i.e., in a direction in which it is difficult for Ni particles 100 to move), and as a result, the possibility of these Ni particles 100 moving and agglomerating through this specific pore 102s becomes extremely low.

[0051] In contrast, the particles constituting pore 102 in Figure 8A contain two Ni particles 100, one of which is located above the pore centerline Ly. Therefore, this pore 102 does not satisfy the latter half of the third condition and does not qualify as a specific pore 102s. Similarly, the particles constituting pore 102 in Figure 8B contain three Ni particles 100, two of which are located below the pore centerline Ly, while the remaining Ni particle 100 is located above the pore centerline Ly. Therefore, this pore 102 also does not satisfy the latter half of the third condition and does not qualify as a specific pore 102s.

[0052] The dominant presence of specific pores 102s, as illustrated in Figures 7A to 7C, in the region of interest Ra significantly suppresses the downward movement of Ni particles 100.

[0053] Let's return to Figure 4 and continue the explanation. The support layer 213b, like the functional layer 213a, mainly contains Ni particles 100 and a cermet of ceramic particles 101 (YSZ). Pores 102 are formed in the gaps between the Ni particles 100 and / or ceramic particles 101. In other words, the support layer 213b is also a porous layer configured to be porous, containing multiple pores 102. Gas diffuses into the support layer 213b via the pores 102. The pores 102 in the support layer 213b may include specific pores 102s. However, it differs from the functional layer 213a in that the proportion of specific pores 102s is not particularly controlled. In addition, the functional layer 213a is more densely formed than the support layer 213b. In other words, the porosity of the functional layer 213a is smaller than that of the support layer 213b. The fuel electrode layer 213 is formed by sintering, similar to the solid electrolyte layer 211 and the air electrode layer 212.

[0054] The operation of the cell stack 20 will be explained with reference to Figures 2 to 4. First, a voltage is applied between the end plates 27 and 28 of the cell stack 20. Subsequently, high-temperature steam is supplied from path Pfi. The steam supplied to path Pfi flows into the fuel chamber Sf of each electrolytic unit Ue through the lateral hole 25a. High-temperature air is also supplied from path Pai. The air supplied to path Pai flows into the air chamber Sa of each electrolytic unit Ue through a lateral hole (not shown). The reason for supplying high-temperature air to the air chamber Sa is to control the temperature of the cell stack 20.

[0055] Water vapor flowing into the fuel chamber Sf travels through the support layer 213b of the fuel electrode layer 213 to the functional layer 213a. In the functional layer 213a, the water vapor reacts with electrons supplied from the end plate 28 via the current collector 26 to decompose into hydrogen and oxide ions (water vapor electrolysis). The hydrogen produced by the water vapor electrolysis diffuses within the fuel chamber Sf and is discharged from the path Pfo through the lateral hole 25b and recovered by a well-known method. At this time, unreacted water vapor may be discharged from the path Pfo along with the hydrogen. Meanwhile, the oxide ions travel via the solid electrolyte layer 211 to the air electrode layer 212 in the air chamber Sa, where they release electrons in the functional layer of the air electrode layer 212 to become oxygen. The oxygen diffuses within the air chamber Sa and, together with the air flowing into the air chamber Sa, is discharged from the path Pao through a lateral hole (not shown) and recovered (or released into the atmosphere) by a well-known method. Electrons emitted from the functional layer of the air electrode layer 212 are collected by the current collector section 22a of the interconnector 22 via the current collector layer and circulate from the end plate 27 to the end plate 28 via an external power supply.

[0056] As the cell stack 20 operates as described above, hydrogen is produced in the electrolytic reactor 1.

[0057] A method for manufacturing the electrolytic cell 21 will be described. First, a mixture of NiO powder and YSZ powder is prepared in a predetermined ratio. Butyral resin, DOP (dioctyl phthalate) as a plasticizer, a well-known dispersant, a mixed solvent of toluene and ethanol, and, if necessary, a pore-forming agent (typically organic beads) are added in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of support layer 213b having a predetermined thickness is prepared from the slurry using the doctor blade method. The mixing ratio of NiO powder and YSZ powder can be appropriately set according to the performance required of the support layer 213b.

[0058] Next, butyral resin, DOP as a plasticizer, a well-known dispersant, and a mixed solvent of toluene and ethanol are added to the YSZ powder in predetermined proportions and mixed in a ball mill to prepare a slurry. Then, a green sheet of solid electrolyte layer 211 having a predetermined thickness is prepared from the slurry using the doctor blade method.

[0059] Next, a mixture of NiO powder and YSZ powder in a predetermined ratio is mixed to produce a slurry. Butyral resin, DOP as a plasticizer, a well-known dispersant, and a mixed solvent of toluene and ethanol are added in predetermined ratios and mixed in a ball mill. In the manufacturing method according to this embodiment, the mixing ratio of NiO powder to YSZ powder is smaller compared to conventional manufacturing methods. For example, in conventional manufacturing methods, the mixing ratio (vol%) of NiO powder to YSZ powder is 2:3, while in the manufacturing method according to this embodiment it is 1:3. After preparing the slurry, a green sheet of functional layer 213a having a predetermined thickness is produced from the slurry using the doctor blade method. Specifically, the slurry is cast to the upper surface of a carrier film to a predetermined thickness, and the cast green sheet is dried for a predetermined time to produce a green sheet of functional layer 213a.

[0060] Next, the green sheet of the functional layer 213a and the green sheet of the support layer 213b are laminated in this order on the upper surface of the green sheet of the solid electrolyte layer 211 (the surface that will become the lower surface of the solid electrolyte layer 211 in Figure 4 after firing). At this time, the side of the green sheet of the functional layer 213a that was in contact with the carrier film is placed on top, and the green sheet of the support layer 213b is placed on that side. These laminated green sheets are then pressed together under high pressure for a predetermined time using warm isostatic pressing (WIP) while heating and vacuuming (for example, 35 seconds at a pressure of 12.9 MPa at 65°C) to produce a laminate containing the green sheet of the solid electrolyte layer 211, the green sheet of the functional layer 213a, and the green sheet of the support layer 213b.

[0061] Subsequently, the laminate is placed on a honeycomb setter with the green sheet of the solid electrolyte layer 211 facing upwards, and degreased at a predetermined temperature (e.g., 200 to 300°C). Next, the laminate is fired at a predetermined first temperature (e.g., 1300 to 1500°C) for a predetermined time (e.g., 1 to 5 hours) (primary firing). This forms a primary sintered body having the solid electrolyte layer 211 and the fuel electrode layer 213.

[0062] Next, a material containing LSCF is screen printed onto the upper surface of the solid electrolyte layer 211 of the molded primary sintered body (the side on which the fuel electrode layer 213 is not formed), and fired at a predetermined second temperature (e.g., 900 to 1000°C) for a predetermined time (e.g., 1 to 5 hours) (secondary firing). This forms a secondary sintered body having a solid electrolyte layer 211, a fuel electrode layer 213, and an air electrode layer 212. By performing a reduction treatment in which the molded secondary sintered body is reduced at a predetermined temperature (e.g., 700°C) in a hydrogen atmosphere, an electrolytic cell 21 is produced in which the NiO contained in the fuel electrode layer 213 is reduced to Ni. The above is a description of the method for manufacturing the electrolytic cell 21.

[0063] The pores 102 in the fuel electrode layer 213 are formed by the reduction treatment described above. That is, the areas where NiO is reduced and oxygen is removed become pores 102 (however, pores 102 may also be formed by other factors). Also, the reduced Ni becomes Ni particles 100. In this embodiment, the mixing ratio of NiO powder to YSZ powder in the slurry that forms the material of the functional layer 213a is smaller compared to the conventional manufacturing method. Therefore, the number of pores 102 and Ni particles 100 contained in the green sheet of the functional layer 213a after the reduction treatment is less than the number of pores and Ni particles contained in the green sheet of the functional layer according to the conventional manufacturing method. Consequently, the number of pores 102 and Ni particles 100 in the region Ra of the functional layer 213a is less than in the conventional method. In other words, the proportion of ceramic particles 101 in the region Ra of the functional layer becomes relatively higher.

[0064] This reduces the likelihood that the particles constituting the pores 102 contain Ni particles 100, thus increasing the number of specific pores 102s that satisfy the first condition in the region Ra of interest. Furthermore, as the number of pores 102 and Ni particles 100 in the functional layer 213a decreases compared to the conventional method, it becomes less likely that one Ni particle 100 will become a constituent particle of multiple pores 102. This is equivalent to the particles constituting the pores 102 containing multiple Ni particles 100. As this situation becomes less likely, the likelihood that the particles constituting the pores 102 contain only one Ni particle 100 increases, thus increasing the number of specific pores 102s that satisfy the second condition in the region Ra of interest. In addition, since the specific gravity of NiO is greater than that of YSZ, NiO settles relatively lower (towards the carrier film side) than YSZ during the drying of the green sheet of the functional layer 213a. The carrier film side of the green sheet of the functional layer 213a is the side that becomes the bottom surface of the functional layer 213a in Figure 4 after firing. Therefore, when the particles constituting the pore 102 include two or more Ni particles 100, there is a high probability that all of those Ni particles 100 will constitute the pore 102 at a relatively lower position, and the number of specific pores 102s that satisfy the third condition in the region of interest Ra increases. Thus, in this embodiment, by adjusting the mixing ratio of NiO powder to YSZ powder contained in the slurry of the functional layer 213a to be smaller than in the conventional method, specific pores 102s are predominantly present in the region of interest Ra (i.e., the proportion of specific pores 102s increases).

[0065] The methods for increasing the proportion of specific pores 102s in the region of interest Ra are not limited to those described above. For example, a method may be used that utilizes the difference in specific gravity between NiO and YSZ described above to cause NiO to settle lower than conventionally during the drying of the green sheet of the functional layer 213a. Specifically, the following three methods can be employed. The first is to increase the particle size of NiO (more precisely, NiO powder). The particle size of NiO can be appropriately selected from, for example, a range of 0.1 to 0.8 μm (conventionally, for example, 0.5 μm). The second is to reduce the viscosity of the slurry. With either the first or second method, the settling rate of NiO increases, so that NiO can settle further down during the drying of the green sheet. The third is to increase the drying time of the green sheet. The drying time can be appropriately selected from, for example, a range of 0.5 to 3 hours (conventionally, for example, 2 hours). In this case, it is desirable to set the drying temperature lower than conventionally (for example, 40°C or higher and less than 80°C in this method, compared to 80°C conventionally). The third method allows for a longer settling distance of NiO, enabling NiO to settle further down during the drying of the green sheet. All three methods reduce the number of pores 102 and Ni particles 100 in the region Ra compared to conventional methods, resulting in a higher proportion of specific pores 102s for the same reasons mentioned above. These three methods may be used individually or in combination of two or more. They may also be used in combination with methods for adjusting the mixing ratio of NiO powder and YSZ powder.

[0066] Alternatively, a method may be used in which a portion of the NiO powder in the slurry of the functional layer 213a is replaced with Ni powder. According to this method, the number of O atoms is reduced relative to the number of Ni atoms, so the number of pores 102 present in the functional layer 213a can be reduced relative to the number of Ni particles 100. Therefore, by adjusting the proportion to which Ni powder is replaced, the relative abundance of pores 102 in the region of interest Ra to Ni particles 100 can be controlled (specifically, reduced). Here, if the number of pores 102 is relatively large, one Ni particle 100 may become a constituent particle of multiple pores 102. In contrast, by using the above method, the possibility of one Ni particle 100 becoming a constituent particle of multiple pores 102 can be reduced, so the proportion of specific pores 102s in the region of interest Ra can be increased. In addition, since the number of Ni particles 100 is the same as before, the reduction in the number of conductive paths can be suppressed, and the electrical characteristics of the electrolytic cell 21 can be maintained well. Note that this method may be used in combination with the method described above.

[0067] Furthermore, a method may be used to change the size and number of pores 102 by adjusting the reduction time and temperature during the reduction treatment. By making the size of the pores 102 smaller than conventional methods, the possibility that the particles constituting the pores 102 contain multiple Ni particles 100 can be reduced, and as a result, the proportion of specific pores 102s in the region of interest Ra can be increased.

[0068] The inventors of the present invention conducted an experiment to verify the extent to which the downward movement of Ni particles 100 can be suppressed by increasing the proportion of specific pores 102s in the region of interest Ra compared to conventional methods. The experiment used an electrolytic cell 21 as an example manufactured by the manufacturing method according to the present embodiment, and an electrolytic cell 31 as a comparative example manufactured by a conventional manufacturing method. Figure 9 is a cross-sectional view of the electrolytic cell 31 as a comparative example when cut along an arbitrary plane in the thickness direction, and corresponds to Figure 4. As shown in Figure 9, the electrolytic cell 31 comprises a solid electrolyte layer 311, an air electrode layer 312, and a fuel electrode layer 313. The fuel electrode layer 313 consists of a functional layer 313a and a support layer 313b. The manufacturing method of the electrolytic cell 31 is the same as that of the electrolytic cell 21, except that the mixing ratio (vol%) of NiO powder and YSZ powder contained in the slurry that forms the material of the functional layer 313a is 2:3.

[0069] In this experiment, two samples (Samples A and B) were prepared for electrolytic cell 21, and two samples (Samples C and D) were prepared for electrolytic cell 31. Samples B and D underwent a 400-hour durability degradation test to simulate the long-term operation of electrolytic cells 21 and 31. In the durability degradation test, samples B and D were heated to 700°C, and a constant flow rate of steam was supplied to the fuel electrode side while simultaneously applying a voltage between the fuel electrode and air electrode to induce a steam electrolytic reaction, ensuring a constant current flow between them. In other words, samples B and D were operated as SOECs (Steam-Optical Emission Control). Subsequently, samples A through D were cut along their thickness in an arbitrary plane, and the changes in porosity composition of the region Ra of interest in electrolytic cells 21 and 31 due to the durability degradation test were observed. In this specification, porosity is defined as the ratio of the area of ​​all pores 102 present in the region Ra to the area of ​​the region Ra in the SEM image of the cross-section of electrolytic cell 21 or 31. In the following, the state of samples A and C, which have not undergone durability degradation testing, will be referred to as the "initial state."

[0070] Figure 10A is a magnified view of the functional layer 213a and its vicinity in an SEM image of a cross-section of sample A (i.e., electrolytic cell 21 in its initial state), and Figure 10B is a magnified view of the functional layer 213a and its vicinity in an SEM image of a cross-section of sample B (i.e., electrolytic cell 21 after the durability degradation test). Figure 11A is a magnified view of the functional layer 313a and its vicinity in an SEM image of a cross-section of sample C (i.e., electrolytic cell 31 in its initial state), and Figure 11B is a magnified view of the functional layer 313a and its vicinity in an SEM image of a cross-section of sample D (i.e., electrolytic cell 31 after the durability degradation test). In this experiment, the porosity composition of the region of interest Ra was defined as "the porosity Pa% of the region of interest Ra relative to the porosity Pc% of the central region Rc (see Figure 6) (Pa-Pc)". Therefore, below, the porosity composition of the region of interest Ra will also be referred to as "relative porosity".

[0071] Table 1 below shows the change in relative porosity in the area of ​​interest Ra for the electrolytic cell 21 in the example and the electrolytic cell 31 in the comparative example. [Table 1]

[0072] For sample A, the porosity Pc of the central region Rc was 17.4%, while the porosity Pa of the region Ra was 16.0%. For sample B, the porosity Pc of the central region Rc was 16.0%, while the porosity Pa of the region Ra was 14.4%. Therefore, as shown in Table 1, the relative porosity (Pa-Pc) of the region Ra in the initial state of the electrolytic cell 21 was -1.4%, and the relative porosity of the region Ra after the durability degradation test was -1.6%. That is, the change in the relative porosity of the region Ra of the electrolytic cell 21 due to the durability degradation test (change in porosity composition) was -0.2% (=-1.6-(-1.4)). In contrast, for sample C, the porosity Pc of the central region Rc was 14.7%, while the porosity Pa of the region Ra was 16.6%. For sample D, the porosity Pc of the central region Rc was 12.2%, while the porosity Pa of the region Ra was 18.3%. Therefore, as shown in Table 1, the relative porosity of the region Ra in the electrolytic cell 31 in its initial state was 1.9%, and the relative porosity of the region Ra after the durability degradation test was 6.1%. In other words, the change in the relative porosity of the region Ra of the electrolytic cell 31 due to the durability degradation test (change in porosity composition) was 4.2% (=6.1-1.9).

[0073] Here, when the Ni particles 100 move downward and aggregate, the location where the Ni particles 100 were located becomes a pore 102. Therefore, a large change in the relative porosity of the region Ra in the durability degradation test means that many instances of Ni particles 100 being pulled down occur in the region Ra due to long-term operation. In this experiment, different samples were used for the initial state and after the durability degradation test, so a 2% individual error was allowed. According to Table 1, the change in relative porosity (-0.2%) in electrolytic cell 21 was below the individual error, while the change in relative porosity (4.2%) in electrolytic cell 31 significantly exceeded the individual error. Therefore, it was confirmed that while many instances of Ni particles 100 being pulled down occur in the region Ra due to long-term operation in electrolytic cell 31 as a comparative example, the pulling down of Ni particles 100 in the region Ra can be significantly suppressed in electrolytic cell 21 as an example, even with long-term operation. In other words, it was confirmed that the manufacturing method according to this embodiment can appropriately suppress changes in the composition of porosity in the region of interest, Ra, over time, compared to conventional methods.

[0074] The inventors of this application calculated the proportion of specific pores 102s present in the area Ra of electrolytic cells 21 and 31 in order to investigate the correlation between "the degree of suppression of the downward movement of Ni particles 100" and "the proportion of specific pores 102s present in the area Ra of interest (the ratio of the number of specific pores 102s to the number of pores 102 in the area Ra of interest)." Specifically, a new sample E was prepared for electrolytic cell 21 and a new sample F was prepared for electrolytic cell 31, and SEM images of their cross-sections were acquired. Then, the proportion of specific pores 102s in predetermined ranges within the area Ra of interest of samples E and F was calculated. The predetermined range includes n locations. Each of the n locations includes the thickness of the area Ra of interest and is set so as not to overlap with each other. In this embodiment, the proportion of specific pores 102s in three locations (n=3) was calculated.

[0075] Table 2 below shows the percentage and average of specific pores 102s present in three areas of the Ra region of interest for the electrolytic cell 21 in the example and the electrolytic cell 31 in the comparative example. [Table 2]

[0076] As shown in Table 2, for sample E (electrolytic cell 21), the proportion of specific pores 102s in ranges 1 to 3 was 84%, 90%, and 95%, respectively, with an average of 89.7%. That is, the proportion of specific pores 102s in the region Ra of interest for sample E was 89.7%. In contrast, for sample F (electrolytic cell 31), the proportion of specific pores 102s in ranges 1 to 3 was 77%, 69%, and 72%, respectively, with an average of 72.7%. That is, the proportion of specific pores 102s in the region Ra of interest for sample F was 72.7%. From these results, it was confirmed that by increasing the proportion of specific pores 102s present in the region Ra of interest compared to conventional methods, the degree of Ni particle 100 displacement can be appropriately suppressed. Specifically, in the case of n=3, it is considered that the downward movement of Ni particles 100 can be appropriately suppressed by manufacturing the electrolytic cell 21 such that the proportion of specific pores 102s present in the region of interest Ra is greater than 72.7%. However, in the case of n=3, it is preferable that the proportion of specific pores 102s be 80.5% or more, which is the midpoint between the minimum value of sample E (84%) and the maximum value of sample F (77%), and more preferably 84% or more, which is the minimum value of sample E.

[0077] Note that the value of n is not limited to 3; n may be 2 or n ≥ 4. The larger the number of n, the more accurate the proportion of specific pores 102s becomes. When n=2, the lower limit of the proportion of specific pores 102s in the electrolytic cell 21 that satisfies the condition of adequately suppressing the downward movement of Ni particles 100 was 74.5%. Therefore, when n=2, it is considered that the downward movement of Ni particles 100 can be adequately suppressed by manufacturing the electrolytic cell 21 such that the proportion of specific pores 102s in the region of interest Ra is greater than 74.5%.

[0078] In addition, the inventors of this application also investigated the correlation between the degree of suppression of the downward movement of Ni particles 100 and the pore length distribution of pores 102 present in the region Ra of interest. Specifically, they measured the distribution of pore length Dy of all pores 102 present in ranges 1 to 3 of sample E (electrolytic cell 21) and sample F (electrolytic cell 31). Figure 12 shows the measurement results. Note that the number of pores 102 is not equal, as there were 107 pores 102 present in ranges 1 to 3 of sample E and 184 pores 102 present in ranges 1 to 3 of sample F. Comparing the two, the pore length Dy corresponding to the peak of the pore length distribution is smaller in electrolytic cell 21 than in electrolytic cell 31. Furthermore, while the pore length Dy of electrolytic cell 31 is distributed relatively broadly in the range of 0.1 μm to 1.45 μm, the pore length Dy of electrolytic cell 21 is distributed in a relatively narrow range of 0.05 μm to 0.6 μm. From these findings, it was found that the region Ra of interest in electrolytic cell 21 has more pores 102 with smaller pore lengths Dy compared to the region Ra of electrolytic cell 31, and that the variation range of the size of the pores 102 is also smaller. Therefore, it is presumed that by reducing the pore length Dy and the variation range of the size of the pores 102 present in the region Ra of interest compared to conventional methods, the downward movement of Ni particles 100 can be appropriately suppressed. In sample E, the pore lengths Dy of all pores 102 in the region of interest Ra are distributed in the range of 0.05 μm to 0.6 μm. It was confirmed that as long as 85% or more of the pores 102 with pore lengths Dy in the range of 0.05 μm to 0.6 μm are present in the region of interest Ra, the downward movement of Ni particles 100 can be appropriately suppressed (in other words, the change in the relative porosity of the region of interest Ra is 2% or less).

[0079] As described above, the electrolytic cell 21 of this embodiment is configured such that the proportion of specific pores 102s among the pores present in the region of interest Ra is 89.7% (>72.7%), thereby appropriately suppressing the movement and aggregation (downward pull) of Ni particles 100 in the region of interest Ra. As a result, the deterioration of the durability of the electrolytic cell 21 can be appropriately suppressed.

[0080] Furthermore, since the pore length Dy of the pores 102 in the region Ra of interest of the electrolytic cell 21 is 0.05 μm or more (i.e., the same as conventionally), gas diffusivity can be adequately ensured. On the other hand, since the pore length Dy is 0.6 μm or less, it becomes less likely that Ni particles 100 constituting one pore 102 will also constitute another pore 102, and the structure of the region Ra of interest can be adequately suppressed from becoming a structure where the movement and aggregation of Ni particles 100 are likely to occur. In other words, it is possible to adequately achieve both the assurance of gas diffusivity and the suppression of the movement and aggregation (downward pull) of Ni particles.

[0081] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications are possible as long as they do not depart from the purpose of the present invention.

[0082] For example, the electrolytic cell 21 is not limited to a fuel electrode-supported type, but may also be an electrolyte-supported type.

[0083] Furthermore, the configuration of this embodiment may be applied to a solid oxide fuel cell instead of the electrolytic cell 21.

[0084] Furthermore, the type of fuel gas is not limited to water vapor; for example, it could be carbon dioxide, or a mixture of water vapor and carbon dioxide.

[0085] Furthermore, the ceramic particles 101 in the fuel polar layer 213 are not limited to YSZ, but other materials may be used. In other words, the type of additive used to stabilize the zirconia is not limited. For example, CSZ (calcia-stabilized zirconia) or ScSZ (scandia-stabilized zirconia) may be used for the ceramic particles 101. CSZ is zirconia with added CaO (calcium oxide), and ScSZ is zirconia with added Sc2O3 (scandium oxide). In addition, ion-conducting oxides other than zirconia (e.g., GDC) may be used for the ceramic particles 101.

[0086] Furthermore, the present invention may include the following embodiments. [1] A solid electrolyte layer, An air electrode layer is stacked on one side of the solid electrolyte layer, A fuel electrode layer containing Ni particles is stacked on the other side of the solid electrolyte layer, An electrochemical cell comprising, When the fuel electrode layer is cut along the thickness direction in any plane, the cross-section has multiple pores composed of multiple particles of one or more types. With respect to each of the plurality of pores in the region of interest having a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer in the aforementioned cross-section, if we define the length in the thickness direction between the endpoint located on the one side and the endpoint located on the other side as the pore length, and define the straight line passing through the midpoint of the pore length and perpendicular to the thickness direction as the pore centerline, Of the pores present in the region of interest, The percentage of specific pores that satisfy any of the following conditions is greater than 72.7%: 1) The particles constituting the pore do not contain the Ni particles; 2) The particles constituting the pore contain only one Ni particle; or 3) The particles constituting the pore contain two or more Ni particles, and all of those Ni particles are located on the side opposite to the solid electrolyte layer with respect to the pore centerline of the pore. Electrochemical cell. [2] [1] The electrochemical cell described above, Of the pores present in the region of interest, the proportion of pores with a length of 0.05 μm or more and 0.6 μm or less is 85% or more. Electrochemical cell. [3] An electrochemical cell stack comprising the electrochemical cells described in [1] or [2] stacked on top of each other. [4] [3] The electrochemical cell stack described above, A heating device for heating the gas supplied to the electrochemical cell stack, The electrochemical cell stack and the heating device are insulated from an insulating material placed inside them. Equipped with, Hot module. [5] An electrolytic reactor equipped with the hot module described in [4]. [Explanation of symbols]

[0087] 1…Hydrogen production equipment, 10…Hot module, 20…Cell stack, 21…Solid oxide electrolytic cell (electrochemical cell), 22,29…Interconnector, 23…Separator, 24…Air electrode frame, 25…Fuel electrode frame, 26…Current collector, 27,28…End plate, 30…Vaporizer, 40…Heat exchanger, 50…Heater, 60…Insulation material, 90…Condenser, 100…Ni particles, 101…Ceramic particles, 102…Pores, 102s…Specific pores, 211…Solid electrolyte layer, 212…Air electrode layer, 213…Fuel electrode layer, 213a…Functional layer, 213b…Support layer

Claims

1. A solid electrolyte layer, An air electrode layer is stacked on one side of the solid electrolyte layer, A fuel electrode layer containing Ni particles is stacked on the other side of the solid electrolyte layer, An electrochemical cell comprising, When the fuel electrode layer is cut along the thickness direction in any plane, the cross-section has multiple pores composed of one or more types of particles. With respect to each of the plurality of pores in the region of interest having a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer in the aforementioned cross-section, if we define the length in the thickness direction between the endpoint located on the one side and the endpoint located on the other side as the pore length, and define the straight line passing through the midpoint of the pore length and perpendicular to the thickness direction as the pore centerline, Of the pores present in the region of interest, The percentage of specific pores that satisfy any of the following conditions is greater than 72.7%: 1) The particles constituting the pore do not contain the Ni particles; 2) The particles constituting the pore contain only one Ni particle; or 3) The particles constituting the pore contain two or more Ni particles, and all of these Ni particles are located on the side opposite to the solid electrolyte layer with respect to the pore centerline of the pore. Electrochemical cell.

2. An electrochemical cell according to claim 1, Of the pores present in the region of interest, the proportion of pores with a pore length of 0.05 μm or more and 0.6 μm or less is 85% or more. Electrochemical cell.

3. An electrochemical cell stack comprising an electrochemical cell stacked according to claim 1.

4. The electrochemical cell stack according to claim 3, A heating device for heating the gas supplied to the electrochemical cell stack, The electrochemical cell stack and the heating device are insulated from an insulating material placed inside them. Equipped with, Hot module.

5. An electrolytic reaction apparatus comprising the hot module described in claim 4.

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