Electrochemical cell, electrochemical cell stack, hot module, and electrolysis reactor

Abstract

To suppress the migration and aggregation of Ni particles in the fuel electrode layer of an electrochemical cell. [Solution] The electrolysis cell 21 includes a solid electrolyte layer 211, an air electrode layer 212, and an anode layer 213 containing Ni particles. A cut surface of the anode layer has pores 102 composed of a plurality of particles. For pores present in a region of interest in the cut surface that is 5 μm wide in the thickness direction from the interface between the solid electrolyte layer and the anode layer, the length in the thickness direction between both end points in the thickness direction is defined as the pore length, and the line passing through the midpoint of the pore length and perpendicular to the thickness direction is defined as the pore centerline. Among the pores present in the region of interest, the proportion of specific pores 102s that satisfy either the condition that the particles constituting the pores contain no Ni particles or contain only one Ni particle, or the condition that the particles constituting the pores contain two or more Ni particles and all of the Ni particles are present 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 reactor. [Background technology]

[0002] Solid oxide electrochemical cells using a solid oxide as an electrolyte have been known for some time (see, for example, Patent Document 1). Solid oxide electrochemical cells are characterized by performing electrochemical reactions with high efficiency in high-temperature environments, and can be used as solid oxide electrolysis cells (SOECs) or solid oxide fuel cells (SOFCs). A solid oxide electrolysis cell is an electrolysis device that uses electrical energy to decompose water vapor into hydrogen and oxygen. A solid oxide fuel cell is a power generation device that generates electrical energy through a chemical reaction between hydrogen and oxygen. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2019-8914 Summary of the Invention

[0004] An electrochemical cell can be configured with a solid electrolyte layer, an air electrode layer laminated on one side of the solid electrolyte layer, and a fuel electrode layer laminated on the other side of the solid electrolyte layer. The fuel electrode layer typically contains nickel (Ni) particles and ceramic particles as an electrode catalyst. The ceramic particles are ionically conductive oxide particles, such as yttria-stabilized zirconia (YSZ). When an electrochemical cell including a fuel electrode layer containing Ni particles and ceramic particles is operated for a long period of time under harsh operating conditions (high temperature, high humidity, and high current), the Ni particles in the fuel electrode layer migrate and aggregate. The migration of Ni particles in the fuel electrode layer reduces the conduction paths within the fuel electrode layer, increasing the internal resistance. Furthermore, aggregation of Ni particles in the fuel electrode layer increases the particle size of the Ni particles, which reduces the number of three-phase interfaces (boundaries between fuel gas, Ni particles, and ceramic particles) that serve as reaction fields. This reduces the surface area of ​​the Ni particles, reducing catalytic performance and resulting in increased reaction resistance in the electrochemical reaction (i.e., reduced reaction efficiency). Such an increase in internal resistance and reaction resistance means a decrease in the durability of the electrochemical cell.

[0005] The present invention has been made to address the above-mentioned problems, and one of the objects of the present invention is to provide a technique capable of suppressing the migration and aggregation of Ni particles in the fuel electrode layer of an electrochemical cell.

[0006] The electrochemical cell (21) according to the present invention comprises: a solid electrolyte layer (211); an air cathode layer (212) laminated on one side of the solid electrolyte layer; a fuel electrode layer (213) including Ni particles (100) and disposed on the other side of the solid electrolyte layer; Equipped with. a cut surface of the anode layer cut along a thickness direction along an arbitrary plane has a plurality of pores (102) constituted by a plurality of particles of one or a plurality of types; For each of the plurality of pores present in a region of interest (Ra) of the cut surface having a width of 5 μm in the thickness direction from the interface (S) between the solid electrolyte layer and the fuel electrode layer, the length in the thickness direction between the end point (p1) located on one side and the end point (p2) located on the other side is defined as a pore length (Dy), and a straight line passing through the midpoint of the pore length and perpendicular to the thickness direction is defined as a pore center line (Ly), Among the pores present in the region of interest, the proportion of specific pores (102s), which are pores that satisfy any one of the following conditions: a first condition that the particles that form the pores do not contain the Ni particles; a second condition that the particles that form the pores contain only one Ni particle; or a third condition that the particles that form the pores contain two or more Ni particles and all of the Ni particles are present on the side opposite to the solid electrolyte layer side with respect to the pore center line of the pore; is greater than 72.7%; Electrochemical cell.

[0007] In conventional electrochemical cells, Ni particles migrate and aggregate when operated in a harsh operating environment. This phenomenon occurs particularly frequently in a region of the fuel electrode layer that is 5 μm wide in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer (i.e., the target region). The present inventors have discovered that the migration and aggregation of Ni particles have the following tendency. When Ni particles are present in the vicinity of other Ni particles, they move toward the other Ni particles through the pores that they form and aggregate. Ni particles tend to move in the thickness direction from the solid electrolyte layer toward the fuel electrode layer, but do not move easily 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 so that the proportion of specific pores that satisfy any one of the first to third conditions among the pores present in the region of interest is greater than 72.7%. For specific pores that satisfy the first condition, the particles that make up the specific pore do not contain Ni particles (in other words, the specific pores are composed of particles other than Ni particles), making it extremely unlikely that Ni particles will migrate and aggregate through the specific pore. For specific pores that satisfy the second condition, the particles that make up the specific pore contain only one Ni particle and do not contain any other Ni particles that could serve as a migration trigger, making it extremely unlikely that the single Ni particle will migrate and aggregate through the specific pore. Furthermore, for specific pores that satisfy the third condition, the particles that make up the specific pore contain two or more Ni particles, which at first glance appears to provide an opportunity for a Ni particle to migrate toward another Ni particle. However, all of these Ni particles are located on the opposite side of the solid electrolyte layer from the pore centerline of the specific pore (a line passing through the midpoint of the pore length and perpendicular to the thickness direction). Therefore, the Ni particles are less likely to migrate toward the solid electrolyte layer (i.e., in a direction where Ni particles have difficulty moving), and as a result, the Ni particles are less likely to migrate and aggregate through the specific pores. According to the configuration of the present invention, the specific pores are present in the target region at a rate of more than 72.7%, so that the migration and aggregation of Ni particles can be appropriately suppressed in the fuel electrode layer of the electrochemical cell. As a result, the deterioration of the durability of the electrochemical cell can be appropriately suppressed.

[0008] Electrochemical cells can be broadly divided into two types: anode-layer-supported electrochemical cells, in which an anode layer supports a solid electrolyte layer and an air cathode layer, and electrolyte-supported electrochemical cells, in which a solid electrolyte layer supports an air cathode layer and an anode layer. In anode-supported electrochemical cells, the anode layer includes a functional layer and a support layer. In anode-supported electrochemical cells, the "cut surface when the anode layer is cut along an arbitrary plane in the thickness direction" in this specification does not refer to a cut surface in the thickness direction of the entire anode layer, but rather to a cut surface in the thickness direction of the functional layer. In addition, in this specification, the "proportion of specific pores (among pores present in a region of interest)" is defined as the "average proportion of specific pores in multiple ranges in the region of interest." The number of ranges is typically three, but may be two, four, or more. The greater the number of ranges, the more accurate the proportion of specific pores.

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

[0010] When the pore length of the pores present in the region of interest is 0.05 μm or more, gas diffusibility can be appropriately ensured. On the other hand, when the pore length is 0.6 μm or less, it is less likely that Ni particles constituting one pore will also constitute another pore, thereby appropriately preventing the region of interest from becoming a structure prone to Ni particle migration and aggregation. According to one aspect of the present invention, 85% or more of the pores present in the region of interest have pore lengths of 0.05 μm or more and 0.6 μm or less, thereby appropriately ensuring gas diffusibility and suppressing Ni particle migration and aggregation.

[0011] The electrochemical cell stack (20) according to the present invention comprises: The electrochemical cell (21) according to the present invention is stacked.

[0012] This configuration makes it possible to provide an electrochemical cell stack in which the migration and aggregation of Ni particles in the fuel electrode layer of the electrochemical cell is appropriately suppressed.

[0013] The hot module (10) according to the present invention comprises: an electrochemical cell stack (20) according to the present invention; a heating device (40, 50) for heating a gas supplied to the electrochemical cell stack; a thermal insulator (60) in which the electrochemical cell stack and the heating device are disposed; Equipped with.

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

[0015] The electrolytic reactor (1) according to the present invention comprises: The hot module (10) according to the present invention is provided.

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

[0017] In the above description, in order to facilitate understanding of the invention, the symbols used in the embodiments are added in parentheses to the constituent elements of the invention corresponding to the embodiments, but each constituent element of the invention is not limited to the embodiment defined by the said symbols. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram of an electrolytic reactor. [Figure 2] FIG. 2 is a perspective view of a cell stack. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. [Figure 4]FIG. 2 is a cross-sectional view of the electrolysis cell cut along an arbitrary plane in the thickness direction. [Figure 5A] FIG. 2 is a diagram showing an example of pores present in a cut surface of a functional layer. [Figure 5B] FIG. 2 is a diagram for explaining the pore length and pore center line of a pore. [Figure 6] FIG. 5 is an enlarged view of a region R in FIG. [Figure 7A] FIG. 3 is a diagram showing an example of a specific pore that satisfies a first condition. [Figure 7B] FIG. 10 is a diagram showing an example of a specific pore that satisfies the second condition. [Figure 7C] FIG. 10 is a diagram showing another example of a specific pore that satisfies the second condition. [Figure 7D] FIG. 10 is a diagram showing an example of a specific pore that satisfies the third condition. [Figure 8A] FIG. 10 is a diagram showing an example of a pore that does not fall under the category of a specific pore. [Figure 8B] FIG. 10 is a diagram showing another example of a pore that does not fall under the category of a specific pore. [Figure 9] FIG. 2 is a cross-sectional view of an electrolysis cell as a comparative example, cut along an arbitrary plane in the thickness direction. [Figure 10A] FIG. 1 is a partially enlarged view of a functional layer and its vicinity in an SEM image of a cross section of an electrolysis cell in an initial state according to an example. [Figure 10B] FIG. 1 is a partially enlarged view of the functional layer and its vicinity in an SEM image of a cross section of an electrolysis cell after a durability degradation test according to an example. [Figure 11A] FIG. 10 is a partially enlarged view of the functional layer and its vicinity in an SEM image of a cross section of an electrolysis cell in an initial state according to a comparative example. [Figure 11B] FIG. 10 is a partially enlarged view of the functional layer and its vicinity in an SEM image of a cross section of an electrolysis cell after a durability degradation test according to a comparative example. [Figure 12] 1 is a graph showing the pore length distribution in a predetermined range of the region of interest of an electrolytic cell according to an example, and the pore length distribution in a predetermined range of the region of interest of an electrolytic cell according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an electrolytic reactor 1 according to this embodiment. The electrolytic reactor 1 according to this embodiment is an apparatus for producing hydrogen by electrolyzing water vapor. As shown in FIG. 1, the electrolytic reactor 1 includes a hot module 10 and a condenser 90.

[0020] The hot module 10 is constructed by covering with insulating material the main components that become hot among the elements that make up the electrolytic reaction device 1, 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 includes a cell stack 20, a vaporizer 30, a heat exchanger 40, a heater 50, and an insulating material 60.

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

[0022] In addition to the water vapor described 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 exchange heat with the water vapor and air in the heat exchanger 40, whereby the water vapor and air introduced from the vaporizer 30 are heated in the heat exchanger 40.

[0023] The water vapor and air heated by 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). The water vapor and air are then introduced into the cell stack 20. The heat exchanger 40 and the heater 50 are temperature raising devices for raising the temperature of the gas (water vapor 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 electrolysis cells. The cell stack 20 is heated to an operating temperature by a heat source (such as a burner) not shown. A predetermined voltage is applied to the cell stack 20. As a result, 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 together with unreacted water vapor, where it is used to heat the water vapor and air introduced into the heat exchanger 40 from the vaporizer 30, and then introduced into the condenser 90. The unreacted water vapor is condensed in the condenser 90. The condensed water produced in the condenser 90 is introduced into the vaporizer 30. Meanwhile, hydrogen separated by condensation of the water vapor in the condenser 90 is recovered. The oxygen produced in the cell stack 20 is introduced into the heat exchanger 40, where it is used to heat the water vapor and air, and then introduced into the vaporizer 30 to heat the water supplied to the vaporizer 30. Then, the oxygen discharged from the vaporizer 30 is recovered (or released to the atmosphere).

[0025] The cell stack 20, vaporizer 30, heat exchanger 40, and heater 50 are disposed inside a thermal insulation material 60. This suppresses heat radiation from each of the components 20, 30, 40, and 50. Heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), and / or a heat-resistant container formed from these heat-resistant fibers, may be used for the thermal insulation material 60. The heat-resistant fibers are disposed so as to fill gaps between the cell stack 20, vaporizer 30, heat exchanger 40, and heater 50.

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

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

[0028] The electrolysis cell 21 is the smallest unit of the SOEC and includes a solid electrolyte layer 211, an air electrode layer 212, and an anode layer 213. The air electrode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact 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 anode layer 213, and is disposed in the center of the upper surface of the solid electrolyte layer 211 in a plan view of the electrolysis cell 21. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 211 is exposed. The anode layer 213 is laminated on the lower surface of the solid electrolyte layer 211 so as to contact the lower surface of the solid electrolyte layer 211.

[0029] The interconnector 22 is a rectangular metal (for example, stainless steel) member that has a rectangular current collecting part 22a that protrudes downward from the center of its lower surface. A pair of interconnectors 22 is arranged on both sides of the electrolysis cell 21 in the thickness direction. Two adjacent electrolysis units Ue, Ue share one interconnector 22. The interconnector 22 also functions as a separator that separates the two adjacent electrolysis units Ue, Ue. The lower surface of the current collecting part 22a is in contact with the upper surface of the air cathode layer 212 of the electrolysis cell 21. The lower electrolysis unit Ue includes a pair of interconnectors 22, 29 instead of the pair of interconnectors 22, 22. The interconnector 29 is arranged at the bottom end of the cell stack 20 and differs from the interconnector 22 in that it does not have a current collecting part 22a.

[0030] The separator 23 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in the 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 electrolysis cell 21 with a brazing material (e.g., Ag brazing) (not shown). The separator 23 prevents mixing of oxygen generated in the air electrode layer 212 by electrolysis of water vapor and hydrogen generated in the fuel electrode layer 213.

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

[0032] The fuel electrode frame 25 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in the center thereof. The fuel electrode frame 25 is disposed between the separator 23 and the interconnector 22 below it.

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

[0034] The current collector 26 is a rectangular porous member made of metal (for example, nickel) that is smaller than the fuel electrode layer 213 in plan view and allows hydrogen to pass through. The current collector 26 is arranged in the fuel chamber Sf so as to be in contact with the lower surface of the fuel electrode layer 213 and the upper surface of the lower interconnector 22. Two adjacent electrolysis cells 21 are stacked in the thickness direction so as to share the interconnector 22 via the current collector 26, thereby electrically connecting the multiple electrolysis cells 21 in series.

[0035] 2 and 3, four paths Pfi, Pfo, Pai, and Pao are formed as gas flow paths around the outer periphery of the cell stack 20. These paths Pfi, Pfo, Pai, and Pao are each formed to penetrate through members of the cell stack 20 in the thickness direction, excluding the "end plate 27" and the "upper interconnector 22 of the upper electrolysis unit Ue."

[0036] The 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. The path Pfo is formed near the other corner of side E2 that faces side E1 (the corner located diagonally from the one corner of side E1). As shown in FIG. 3 , the path Pfi communicates with the fuel chamber Sf via a horizontal hole 25a formed in the anode frame 25 of each electrolysis unit Ue. The path Pfo communicates with the fuel chamber Sf via a horizontal hole 25b formed in the anode frame 25 of each electrolysis unit Ue.

[0037] The path Pai is formed near one corner of the side E2. The path Pao is formed near the other corner of the side E1. The path Pai and the path Pao each communicate with the air chamber Sa via a horizontal hole (not shown) formed in the air electrode frame 24 of each electrolysis unit Ue.

[0038] Next, the configuration of the electrolysis cell 21 will be described in more detail with reference to FIG. 4. The following dimensions and thicknesses of each layer of the electrolysis cell 21 are merely examples and are not limited to these values. FIG. 4 is a cross-sectional view of the electrolysis cell 21 cut at an arbitrary plane along the thickness direction. In FIG. 4, an xyz coordinate system is set 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 electrolysis cell 21 includes a solid electrolyte layer 211, an air electrode layer 212, and an anode layer 213. The solid electrolyte layer 211 is a rectangular flat layer measuring 150 mm square and 6 μm thick. The solid electrolyte layer 211 is configured to contain ceramic particles and is formed by sintering. The ceramic particles are oxide particles with ion conductivity, and YSZ is used in this embodiment. The solid electrolyte layer 211 has high oxide ion conductivity. The solid electrolyte layer 211 is a dense layer and is designed so that the atmosphere on the air electrode layer 212 side (air atmosphere) and the atmosphere on the fuel electrode layer 213 side (reducing atmosphere) do not leak to each other. The ceramic particles in the solid electrolyte layer 211 are not limited to YSZ, and may be, for example, GDC (gadolinia-doped ceria).

[0039] The air electrode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact 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 configured to contain a perovskite 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 collecting layer. The current collecting layer is thicker than the functional layer and is disposed on the upper surface of the functional layer. The air electrode layer 212 has high electronic conductivity and effectively collects electrons from the current collecting layer. The air electrode layer 212 is a porous layer and has pores inside.

[0040] The anode layer 213 is a rectangular flat layer measuring 150 mm on each side, and is formed to have a thickness greater than that of the solid electrolyte layer 211 and the air cathode layer 212, for example, 410 μm. The anode layer 213 supports the solid electrolyte layer 211 and the air cathode layer 212. In other words, the electrolysis cell 21 is an anode-supported cell. The anode layer 213 includes a functional layer 213 a and a support layer 213 b. The functional layer 213 a is laminated on the lower surface of the solid electrolyte layer 211 so as to contact the lower surface of the solid electrolyte layer 211. The support layer 213 b is located below the functional layer 213 a. In other words, the functional layer 213 a and the support layer 213 b are laminated in this order on the lower surface of the solid electrolyte layer 211. The support layer 213 b is formed to be significantly thicker than the functional layer 213 a. 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, approximately 16 to 40 times.

[0041] The structure of the functional layer 213a will be described in more detail with reference to FIGS. 5A to 8B. The functional layer 213a contains a cermet of Ni particles and ceramic particles as a main component. The Ni particles are particles made of Ni as a catalytic metal. The ceramic particles are ion-conductive oxide particles, and YSZ is used in this embodiment. Pores are formed in the gaps between the Ni particles and / or ceramic particles. FIG. 5A is a diagram showing an example of pores 102 in a cut surface of the functional layer 213a. As shown in FIG. 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 formed by a plurality of Ni particles 100 and ceramic particles 101. In the example of FIG. 5A, the pores 102 are composed of a plurality of particles of two types (Ni particles 100 and ceramic particles 101), but the pores 102 may also be composed of a plurality of particles of one type (Ni particles 100 or ceramic particles 101). In addition, the functional layer 213a may contain small amounts of particles (other particles) other than the Ni particles 100 and ceramic particles 101. In this case, the pores 102 may be composed of a plurality of particles of two or more types, including other particles. In this way, the functional layer 213a is a porous layer configured to be porous and include a plurality of pores 102. Gas diffuses into the functional layer 213a via the pores 102.

[0042] FIG. 5B is a diagram for explaining the pore length Dy and pore center line Ly of the pore 102, and shows the outline of the pore 102 in FIG. 5A. As shown in FIG. 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 end point of the pore 102 located closest to the +y-axis direction, and point p2 is the end point of the pore 102 located closest to the -y-axis direction. In other words, the pore length Dy is the length of the pore 102 when projected onto the y-axis. The pore length Dy is an index that defines the size of the pore 102 in the thickness direction on the cross section. The pore center line Ly is a straight line that passes through the midpoint of the pore length Dy and is perpendicular to the thickness direction.

[0043] In conventional electrolysis cells, Ni particles in the functional layer tend to migrate toward other Ni particles through pores and aggregate during cell stack operation. This phenomenon occurs particularly frequently in a region of the functional layer that is 5 μm wide 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 prevent Ni particle migration and aggregation in this region. This will be described in detail below.

[0044] FIG. 6 is an enlarged view of region R in FIG. 4. As shown in FIG. 6, the functional layer 213a includes region Ra and region Rc. Region Ra is a region of the functional layer 213a having 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 referred to as the upper half of the functional layer 213a. As described above, region Ra corresponds to a region where migration and aggregation of Ni particles 100 are particularly likely to occur frequently. Hereinafter, region Ra will be referred to as the "attention region Ra." Meanwhile, region Rc is a region having a width of 2.5 μm above and below the 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 attention region Ra and the upper half of central region Rc overlap. Whether or not the attention region 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 will be described with reference to FIGS. 10A to 11B.

[0045] The present inventors have found that the movement and aggregation of the Ni particles 100 have the following tendency. When another Ni particle 100 is present in the vicinity, the Ni particle 100 moves toward the other Ni particle 100 through the pores 102 that the Ni particle 100 constitutes and aggregates. The Ni particles 100 tend to move downward (in the negative y-axis direction) and have difficulty moving upward (in the positive y-axis direction).

[0046] According to the second tendency, the Ni particles 100 move primarily downward. Therefore, hereinafter, this phenomenon is also referred to as "pulling down of the Ni particles 100." The pulling down of the Ni particles 100 is thought to be caused by the direction of the electric field, differences in gas concentration, etc. Based on the above findings, the functional layer 213a is configured so that specific pores 102s, which are pores 102 that satisfy any of the following first to third conditions, are predominantly present in the attention area Ra. In other words, the attention area Ra is configured so that the ratio of the number of specific pores 102s to the number of pores 102 is high. Hereinafter, the "ratio of the number of specific pores 102s to the number of pores 102 in the attention area Ra" will also be simply referred to as the "ratio of specific pores 102s." (First condition) The particles that form the pores 102 do not include Ni particles 100. (Second condition) The particles that form the pore 102 include only one Ni particle 100 . (Third condition) The particles constituting the pores 102 include two or more Ni particles 100, and all of these Ni particles 100 are present below the pore center line Ly of the pores 102 (in the -y-axis direction).

[0047] Fig. 7A is a diagram illustrating specific pores 102s that satisfy the first condition, Figs. 7B and 7C are diagrams illustrating specific pores 102s that satisfy the second condition, and Fig. 7D is a diagram illustrating specific pores 102s that satisfy the third condition. Figs. 8A and 8B are diagrams illustrating pores 102 that do not fall under the category of specific pores 102s. In Figs. 7A to 8B, for ease of viewing, the outer shapes of the pores 102 (including the specific pores 102s) are simplified to elliptical shapes, and only Ni particles 100 are shown among the particles that make up the pores 102.

[0048] 7A is composed of a plurality of particles other than Ni particles 100. In other words, the particles that compose the pore 102 do not include Ni particles 100. Therefore, this pore 102 is a specific pore 102s that satisfies the first condition. According to this configuration, since the particles that compose the specific pore 102s do not include Ni particles 100 in the first place, the possibility that the Ni particles 100 will move and aggregate through this specific pore 102s is extremely low.

[0049] The pore 102 in FIG. 7B is composed of one Ni particle 100 and multiple particles of another type (typically, ceramic particles 101). That is, the particles that compose the pore 102 include only one Ni particle 100. Therefore, this pore 102 is a specific pore 102s that satisfies the second condition. Note that in FIG. 7B, the Ni particle 100 composes the pore 102 at a relatively lower position. However, as shown in FIG. 7C, the second condition is also met when the Ni particle 100 composes the pore 102 at a relatively higher position. With these configurations, the particles that compose the specific pore 102s include only one Ni particle 100 and do not include other Ni particles 100 that could trigger migration. Therefore, the possibility of the single Ni particle 100 migrating and agglomerating through this specific pore 102s is extremely low.

[0050] The pore 102 in FIG. 7D is composed of two Ni particles 100 and a plurality of other types of particles (typically, ceramic particles 101), and the two Ni particles 100 are both located relatively lower in the pore 102. That is, the particles that make up the pore 102 contain two or more Ni particles 100, and all of these Ni particles 100 are located lower than the pore center line Ly. Therefore, this pore 102 is a specific pore 102s that satisfies the third condition. According to this configuration, the particles that make up the specific pore 102s contain two or more Ni particles 100, and at first glance, it appears that there is an opportunity for the Ni particles 100 to move toward other Ni particles 100. However, since these Ni particles 100 are all located below the pore center line Ly, it is unlikely that these Ni particles 100 will move upward (i.e., in a direction in which it is difficult for the Ni particles 100 to move), and as a result, it is extremely unlikely that these Ni particles 100 will move and agglomerate through this specific pore 102s.

[0051] In contrast, the particles constituting the pore 102 in FIG. 8A include two Ni particles 100, one of which is located above the pore center line Ly. Therefore, this pore 102 does not satisfy the second half of the third condition and does not qualify as a specific pore 102s. Furthermore, the particles constituting the pore 102 in FIG. 8B include three Ni particles 100, two of which are located below the pore center line Ly, but the remaining Ni particle 100 is located above the pore center line Ly. Therefore, this pore 102 also does not satisfy the second half of the third condition and does not qualify as a specific pore 102s.

[0052] When the specific pores 102s as exemplified in FIGS. 7A to 7C are predominantly present in the attention region Ra, the pulling down of the Ni particles 100 can be significantly suppressed.

[0053] Returning to FIG. 4, the explanation continues. Like the functional layer 213a, the support layer 213b also contains a cermet of Ni particles 100 and ceramic particles 101 (YSZ) as a main component. Pores 102 are formed in the gaps between the Ni particles 100 and / or ceramic particles 101. That is, the support layer 213b is also a porous layer configured to have a porous shape including a plurality of pores 102. Gas diffuses into the support layer 213b via the pores 102. The pores 102 of the support layer 213b may include specific pores 102s. However, the functional layer 213a differs from the functional layer 213a in that the proportion of the specific pores 102s is not specifically controlled. In addition, the functional layer 213a is formed more densely than the support layer 213b. In other words, the porosity of the functional layer 213a is smaller than that of the support layer 213b. Like the solid electrolyte layer 211 and the air electrode layer 212, the fuel electrode layer 213 is also formed by sintering.

[0054] The operation of the cell stack 20 will be described with reference to Figures 2 to 4. First, a voltage is applied between the end plates 27, 28 of the cell stack 20. Next, high-temperature steam is supplied from the path Pfi. The steam supplied to the path Pfi flows into the fuel chamber Sf of each electrolysis unit Ue via the horizontal holes 25a. In addition, high-temperature air is supplied from the path Pai. The air supplied to the path Pai flows into the air chamber Sa of each electrolysis unit Ue via a horizontal 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] The water vapor that flows into the fuel chamber Sf passes through the support layer 213b of the fuel electrode layer 213 and travels 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 and is decomposed into hydrogen and oxide ions (water vapor electrolysis reaction). The hydrogen generated by the water vapor electrolysis reaction diffuses within the fuel chamber Sf, is discharged through the horizontal hole 25b via path Pfo, and is recovered by a well-known method. At this time, unreacted water vapor can be discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 211 to the air cathode layer 212 in the air chamber Sa, release electrons in the functional layer of the air cathode layer 212, and become oxygen. The oxygen diffuses within the air chamber Sa, and is discharged through path Pao via a horizontal hole (not shown) together with the air that flowed into the air chamber Sa, and is recovered (or released to the atmosphere) by a well-known method. Electrons emitted from the functional layer of the air electrode layer 212 are collected by the current collecting portion 22a of the interconnector 22 via the current collecting layer, and circulate from the end plate 27 to the end plate 28 via the external power supply.

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

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

[0058] Next, butyral resin, DOP as a plasticizer, a 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 doctor blade method is used to produce a green sheet of the solid electrolyte layer 211 having a predetermined thickness from the slurry.

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

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

[0061] Thereafter, the laminate is placed on a honeycomb setter with the green sheet of the solid electrolyte layer 211 facing upward, and is 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 on the upper surface (the surface on which the anode layer 213 is not formed) of the solid electrolyte layer 211 of the formed primary sintered body, and the resulting product is 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 results in a secondary sintered body having the solid electrolyte layer 211, anode layer 213, and cathode layer 212. The formed secondary sintered body is subjected to a reduction treatment in a hydrogen atmosphere at a predetermined temperature (e.g., 700°C), thereby producing an electrolysis cell 21 in which the NiO contained in the anode layer 213 is reduced to Ni. This concludes the description of the method for producing the electrolysis cell 21.

[0063] The pores 102 in the fuel electrode layer 213 are formed by the reduction process described above. That is, the pores 102 are formed where NiO is reduced and O is removed (however, the pores 102 can also be formed by other factors). Furthermore, the reduced Ni becomes Ni particles 100. In this embodiment, the mixing ratio of NiO powder to YSZ powder contained in the slurry that is the material for the functional layer 213a is smaller than that in conventional manufacturing methods. Therefore, the number of pores 102 and Ni particles 100 contained in the green sheet of the functional layer 213a after the reduction process is smaller than the number of pores and Ni particles contained in the green sheet of the functional layer according to the conventional manufacturing method. Therefore, the number of pores 102 and Ni particles 100 in the target region Ra of the functional layer 213a is smaller than that in the conventional method. In other words, the presence ratio of ceramic particles 101 in the target region Ra is relatively high.

[0064] This reduces the likelihood that the particles constituting the pores 102 contain Ni particles 100, thereby increasing the number of specific pores 102s that satisfy the first condition in the target region Ra. Furthermore, because the number of pores 102 and Ni particles 100 in the functional layer 213a is reduced compared to conventional methods, it becomes less likely that a single Ni particle 100 will constitute multiple pores 102. This is equivalent to the particles constituting the pores 102 containing multiple Ni particles 100. Since this situation becomes less likely to occur, the particles constituting the pores 102 are more likely to contain only one Ni particle 100, thereby increasing the number of specific pores 102s that satisfy the second condition in the target region Ra. Furthermore, because NiO has a larger specific gravity than YSZ, NiO settles relatively lower (toward the carrier film) than YSZ during drying of the green sheet of the functional layer 213a. The surface of the green sheet of the functional layer 213a that faces the carrier film will become the lower surface of the functional layer 213a in FIG. 4 after firing. Therefore, when the particles constituting the pores 102 include two or more Ni particles 100, there is a high possibility that the Ni particles 100 will all form the pores 102 at relatively lower positions, and the number of specific pores 102s that satisfy the third condition in the target region Ra increases. In this way, in this embodiment, the mixing ratio of NiO powder to YSZ powder contained in the slurry of the functional layer 213a is adjusted to be smaller than in the past, so that the specific pores 102s are predominantly present in the target region Ra (i.e., the proportion of specific pores 102s is increased).

[0065] The method for increasing the proportion of the specific pores 102s in the target region Ra is not limited to the above. For example, a method utilizing the difference in specific gravity between NiO and YSZ to cause NiO to settle further downward than conventionally during drying of the green sheet of the functional layer 213a may be used. Specifically, the following three methods can be adopted. The first method is to increase the particle size of NiO (strictly speaking, NiO powder). The particle size of NiO can be appropriately selected from a range of, for example, 0.1 to 0.8 μm (the conventional particle size is, for example, 0.5 μm). The second method is to reduce the viscosity of the slurry. Both of the first and second methods increase the settling rate of NiO, allowing NiO to settle further downward during drying of the green sheet. The third method is to increase the drying time of the green sheet. The drying time can be appropriately selected from a range of, for example, 0.5 to 3 hours (the conventional method is, for example, 2 hours). In this case, it is desirable to set the drying temperature lower than conventionally (for example, 40°C or higher but lower than 80°C in this method, compared to 80°C in conventional methods). According to the third method, the settling distance of NiO is increased, allowing NiO to settle further downward during drying of the green sheet. All three of these methods can reduce the number of pores 102 and Ni particles 100 in the target region Ra compared to conventional methods, thereby increasing the proportion of specific pores 102s for the same reasons as described above. The above three methods may be used alone or in combination. They may also be used in combination with a method of adjusting the mixing ratio of NiO powder and YSZ powder.

[0066] Alternatively, a method of replacing a portion of the NiO powder in the slurry for the functional layer 213a with Ni powder may be used. This method reduces the number of O particles relative to the number of Ni particles, thereby reducing the number of pores 102 in the functional layer 213a relative to the number of Ni particles 100. Therefore, by adjusting the proportion of Ni powder used, the ratio of pores 102 to Ni particles 100 in the target region Ra 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, the above method reduces the possibility that one Ni particle 100 may become a constituent particle of multiple pores 102, thereby increasing the proportion of specific pores 102s in the target region Ra. Additionally, since the number of Ni particles 100 is the same as in the conventional method, the number of conductive paths is prevented from decreasing, thereby maintaining favorable electrical characteristics of the electrolysis cell 21. This method may be used in combination with the above-described method.

[0067] Furthermore, a method may be used in which the reduction time and temperature in the reduction treatment are adjusted to change the size and number of the pores 102. By making the size of the pores 102 smaller than conventionally, it is possible to reduce the possibility that the particles that make up the pores 102 contain multiple Ni particles 100, and as a result, it is possible to increase the proportion of specific pores 102s in the region of interest Ra.

[0068] The present inventors conducted an experiment to verify the extent to which the pulling down of Ni particles 100 can be suppressed by increasing the proportion of specific pores 102s in the target region Ra compared to conventional methods. The experiment used an electrolytic cell 21 as an example manufactured by the manufacturing method according to this embodiment and an electrolytic cell 31 as a comparative example manufactured by a conventional manufacturing method. FIG. 9 is a cross-sectional view of the electrolytic cell 31 as the comparative example cut along an arbitrary plane in the thickness direction, and corresponds to FIG. 4 . As shown in FIG. 9 , the electrolytic cell 31 includes a solid electrolyte layer 311, a cathode layer 312, and an anode layer 313. The anode layer 313 includes a functional layer 313a and a support layer 313b. The manufacturing method of the electrolytic cell 31 is similar to that of the electrolytic cell 21, except that the mixing ratio (vol%) of NiO powder and YSZ powder contained in the slurry used to form the functional layer 313a is 2:3.

[0069] In this experiment, two samples (Samples A and B) were prepared for the electrolytic cell 21, and two samples (Samples C and D) were prepared for the electrolytic cell 31. A 400-hour durability test was then conducted on Samples B and D to simulate the long-term operation of the electrolytic cells 21 and 31. In the durability test, Samples B and D were heated to 700°C, and a constant flow rate of steam was supplied to the anode layer side to ensure a constant current flowed between the anode layer and the cathode layer. A voltage was also applied between the anode layer and the cathode layer to perform a steam electrolysis reaction. In other words, Samples B and D were operated as SOECs. Samples A to D were then cut along arbitrary planes in the thickness direction, and the compositional changes in the porosity of the target region Ra of the electrolytic cells 21 and 31 due to the durability test were observed. In this specification, porosity is defined as the ratio of the area of ​​all pores 102 present in the target region Ra to the area of ​​the target region Ra in an SEM image of a cross section of the electrolytic cell 21 or 31. Hereinafter, the state of samples A and C before the durability degradation test is referred to as the "initial state."

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

[0071] Table 1 below shows the change in relative porosity in the noted region Ra of the electrolytic cell 21 according to the example and the electrolytic cell 31 according to 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 target 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 target region Ra was 14.4%. Therefore, as shown in Table 1, the relative porosity (Pa - Pc) of the target region Ra in the initial state of the electrolytic cell 21 was -1.4%, and the relative porosity of the target region Ra after the durability degradation test was -1.6%. In other words, the change in the relative porosity of the target region Ra of the electrolytic cell 21 due to the durability degradation test (the 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 target 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 target region Ra was 18.3%. Therefore, as shown in Table 1, the relative porosity of the target region Ra in the initial state of the electrolytic cell 31 was 1.9%, and the relative porosity of the target region Ra after the durability degradation test was 6.1%. In other words, the change in the relative porosity of the target region Ra of the electrolytic cell 31 due to the durability degradation test (the change in porosity composition) was 4.2% (=6.1-1.9).

[0073] Here, when the Ni particles 100 move downward and aggregate, the locations where the Ni particles 100 were located become pores 102. Therefore, a large change in the relative porosity of the attention area Ra due to the durability degradation test means that many Ni particles 100 will fall down in the attention area Ra after long-term operation. In this experiment, different samples were used for the initial state and after the durability degradation test, so an individual error of 2% was allowed. According to Table 1, the change in relative porosity (-0.2%) in the electrolytic cell 21 was less than the individual error, whereas the change in relative porosity (4.2%) in the electrolytic cell 31 greatly exceeded the individual error. Therefore, it was confirmed that while many Ni particles 100 fell down in the attention area Ra after long-term operation in the electrolytic cell 31 of the comparative example, the electrolytic cell 21 of the example was able to significantly suppress the fall of Ni particles 100 in the attention area Ra. In other words, it was confirmed that the manufacturing method according to this embodiment can appropriately suppress the change over time in the porosity composition in the attention area Ra, compared to the conventional method.

[0074] To investigate the correlation between the degree of suppression of Ni particle 100 descent and the proportion of specific pores 102s present in the region of interest Ra (the proportion of the number of specific pores 102s relative to the number of pores 102 in the region of interest Ra), the inventors calculated the proportion of specific pores 102s present in the region of interest Ra of electrolytic cells 21 and 31. 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 obtained. Then, the proportions of specific pores 102s in predetermined ranges in the region of interest Ra of samples E and F were calculated. The predetermined ranges included n ranges. Each of the n ranges was set to include the thickness of the region of interest Ra and not overlap each other. In this embodiment, the proportions of specific pores 102s in three ranges (n = 3) were calculated.

[0075] Table 2 below shows the proportions and averages of the specific pores 102s present in three ranges within the attention region Ra of the electrolytic cell 21 according to the example and the electrolytic cell 31 according to the comparative example. [Table 2]

[0076] As shown in Table 2, for sample E (electrolytic cell 21), the proportions of specific pores 102s in ranges 1 to 3 were 84%, 90%, and 95%, respectively, with an average of 89.7%. That is, the proportion of specific pores 102s in the noted region Ra of sample E was 89.7%. In contrast, for sample F (electrolytic cell 31), the proportions of specific pores 102s in ranges 1 to 3 were 77%, 69%, and 72%, respectively, with an average of 72.7%. That is, the proportion of specific pores 102s in the noted region Ra of sample F was 72.7%. From these results, it was confirmed that the degree of withdrawal of Ni particles 100 can be appropriately suppressed by increasing the proportion of specific pores 102s present in the noted region Ra compared to conventional methods. Specifically, when n = 3, it is believed that the pulling down of the Ni particles 100 can be appropriately suppressed by manufacturing the electrolytic cell 21 so that the proportion of the specific pores 102s present in the attention area Ra is greater than 72.7%. However, when n = 3, the proportion of the specific pores 102s is preferably 80.5% or more, which is intermediate 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] The value of n is not limited to 3, and may be n=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 that the pulling down of the Ni particles 100 can be appropriately suppressed was 74.5%. Therefore, when n=2, it is believed that the pulling down of the Ni particles 100 can be appropriately suppressed by manufacturing the electrolytic cell 21 so that the proportion of specific pores 102s present in the attention area Ra is greater than 74.5%.

[0078] In addition, the inventors also investigated the correlation between the "degree of suppression of the Ni particles 100 from descending" and the "pore length distribution of the pores 102 present in the region of interest Ra." Specifically, the distribution of pore lengths Dy of all pores 102 present in ranges 1 to 3 of sample E (electrolytic cell 21) and sample F (electrolytic cell 31) was measured. FIG. 12 shows the measurement results. Note that the number of pores 102 present in ranges 1 to 3 of sample E was 107, while the number of pores 102 present in ranges 1 to 3 of sample F was 184, so the numbers are not equal. 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, the pore length Dy of the electrolytic cell 31 is distributed over a relatively wide range from 0.1 μm to 1.45 μm, whereas the pore length Dy of the electrolytic cell 21 is distributed over a relatively narrow range from 0.05 μm to 0.6 μm. From these findings, it was found that the attention area Ra of the electrolytic cell 21 has more pores 102 with small pore lengths Dy than the attention area Ra of the electrolytic cell 31, and the range of variation in the size of the pores 102 also tends to be smaller. From the above, it is presumed that the pulling down of the Ni particles 100 can be appropriately suppressed by reducing the pore length Dy and the range of variation in size of the pores 102 present in the attention area Ra more than conventionally. In sample E, the pore lengths Dy of all pores 102 present in the target area Ra are distributed in the range of 0.05 μm to 0.6 μm. However, it was confirmed that as long as 85% or more of the pores 102 in the target area Ra have pore lengths Dy within the range of 0.05 μm to 0.6 μm, the pulling down of the Ni particles 100 can be appropriately suppressed (in other words, the change in the relative porosity of the target area Ra is 2% or less).

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

[0080] Furthermore, since the pore length Dy of the pores 102 present in the attention area Ra of the electrolytic cell 21 is 0.05 μm or more (i.e., the same as in the conventional case), gas diffusibility can be appropriately ensured. On the other hand, since the pore length Dy is 0.6 μm or less, it is unlikely that the Ni particles 100 constituting one pore 102 will also constitute another pore 102, and it is possible to appropriately prevent the attention area Ra from becoming a structure in which the Ni particles 100 are likely to move and aggregate. In other words, it is possible to appropriately achieve both ensuring gas diffusibility and suppressing the movement and aggregation (pull-down) of Ni particles.

[0081] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0082] For example, the electrolysis cell 21 is not limited to an anode-supported type, but may be an electrolyte-supported type.

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

[0084] Furthermore, the type of fuel gas is not limited to water vapor, but may be, for example, carbon dioxide or a mixed gas of water vapor and carbon dioxide.

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

[0086] Furthermore, the present invention may include the following aspects. [1] a solid electrolyte layer; an air electrode layer disposed on one surface of the solid electrolyte layer; a fuel electrode layer including Ni particles and disposed on the other surface of the solid electrolyte layer; An electrochemical cell comprising: a cut surface of the fuel electrode layer cut along a thickness direction along an arbitrary plane has a plurality of pores constituted by a plurality of particles of one or a plurality of types; For each of the plurality of pores present in a region of interest on the cut surface having a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer, the length in the thickness direction between the end point located on one side and the end point located on the other side in the thickness direction is defined as a pore length, and a straight line passing through the midpoint of the pore length and perpendicular to the thickness direction is defined as a pore center line. Among the pores present in the region of interest, the proportion of specific pores that satisfy any one of the following conditions: a first condition that the particles that form the pores do not contain the Ni particles; a second condition that the particles that form the pores contain only one Ni particle; or a third condition that the particles that form the pores contain two or more Ni particles and all of the Ni particles are present on the side opposite to the solid electrolyte layer side with respect to the pore center line of the pore; and Electrochemical cell. [2] [1] The electrochemical cell according to [1], Among the pores present in the region of interest, the proportion of pores having 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 formed by stacking the electrochemical cells according to [1] or [2]. [4] [3] The electrochemical cell stack according to [3], a heating device that heats the gas supplied to the electrochemical cell stack; a thermal insulator in which the electrochemical cell stack and the heating device are disposed; Equipped with Hot module. [5] [4] An electrolytic reactor comprising the hot module according to [4]. [Explanation of symbols]

[0087] 1...hydrogen production device, 10...hot module, 20...cell stack, 21...solid oxide electrolysis cell (electrochemical cell), 22, 29...interconnector, 23...separator, 24...air electrode frame, 25...fuel electrode frame, 26...current collector, 27, 28...end plate, 30...evaporator, 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 disposed on one surface of the solid electrolyte layer; a fuel electrode layer including Ni particles and disposed on the other surface of the solid electrolyte layer; An electrochemical cell comprising: a cut surface of the fuel electrode layer cut along a thickness direction along an arbitrary plane has a plurality of pores constituted by a plurality of particles of one or a plurality of types; For each of the plurality of pores present in a region of interest on the cut surface having a width of 5 μm in the thickness direction from the interface between the solid electrolyte layer and the fuel electrode layer, the length in the thickness direction between the end point located on one side and the end point located on the other side is defined as a pore length, and a straight line passing through the midpoint of the pore length and perpendicular to the thickness direction is defined as a pore center line. Among at least 100 pores present in the region of interest, the proportion of specific pores that satisfy any one of the following conditions: a first condition that the particles that form the pores do not contain the Ni particles; a second condition that the particles that form the pores contain only one Ni particle; or a third condition that the particles that form the pores contain two or more Ni particles, and all of the Ni particles are present on the side opposite to the solid electrolyte layer side with respect to the pore center line of the pore; and Electrochemical cell.

2. 10. The electrochemical cell of claim 1, Among the at least 100 pores present in the region of interest, the proportion of pores having a pore length of 0.05 μm or more and 0.6 μm or less is 85% or more. Electrochemical cell.

3. 10. The electrochemical cell of claim 1, Among the at least 100 pores present in the region of interest, the proportion of the specific pores is 84% ​​or more. Electrochemical cell.

4. An electrochemical cell stack formed by stacking the electrochemical cells according to any one of claims 1 to 3.

5. The electrochemical cell stack of claim 4; a heating device that heats the gas supplied to the electrochemical cell stack; a thermal insulator in which the electrochemical cell stack and the heating device are disposed; Equipped with Hot module.

6. An electrolytic reactor comprising the hot module according to claim 5.

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