Electrochemical cell, solid oxide type electrolytic cell, cell stack, hot module, and hydrogen production device

Abstract

To improve the durability of an electrochemical cell.SOLUTION: An electrolytic cell 21 as an electrochemical cell comprises a solid electrolyte layer 211, a fuel electrode layer 213 overlaid on a reverse surface 211A side of the solid electrolyte layer 211, and an air electrode layer 212 overlaid on an upper surface 211B side of the solid electrolyte layer 211. The fuel electrode layer 213 includes Ni and Fe. When the position of the fuel electrode layer 213 in the thickness direction, is represented by a ratio X of the distance from the boundary between the fuel electrode layer 213 and the solid electrolyte layer 211 relative to the thickness of the fuel electrode layer 213, the concentration of Fe present at a position where the ratio X is 0 or higher and 0.6 or lower, is (-14X / 60+0.14) wt.% or more and (-56X / 60+0.8) wt.% or less.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to an electrochemical cell, a solid oxide electrolysis cell, a cell stack, a hot module, and a hydrogen production device. [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). Solid oxide electrolysis cells are electrolysis devices that use electrical energy to decompose water vapor into hydrogen and oxygen. Solid oxide fuel cells are power generation devices that generate 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] The electrochemical cell can be configured to include a solid electrolyte layer, a fuel electrode layer laminated on one side of the solid electrolyte layer, and an air electrode layer laminated on the other side of the solid electrolyte layer. The fuel electrode layer can be configured to contain Ni (nickel) as an electrode catalyst and an oxide having ion conductivity (e.g., YSZ (yttria-stabilized zirconia)).

[0005] When an electrochemical cell containing Ni in the anode layer is operated for a long time under a harsh operating environment (high temperature, high humidity, high current), Ni migration and aggregation occurs within the anode layer. When Ni migrates or aggregates within the anode layer, the internal resistance of the anode layer increases and the three-phase interface (the boundary between the fuel gas, Ni, and the electrolyte (YSZ)) that serves as the reaction field decreases. This reduces the performance of the electrochemical cell, making it impossible to achieve the desired performance. Therefore, there is a need for an electrochemical cell with improved durability that can maintain the desired performance even when operated for a long time.

[0006] The present disclosure has an object to solve the above-mentioned problems, that is, one object of the present disclosure is to improve the durability of electrochemical cells.

[0007] The electrochemical cell (21) according to the present disclosure includes a solid electrolyte layer (211), a fuel electrode layer (213) laminated on one surface (211A) of the solid electrolyte layer (211), and a cathode layer (212) laminated on the other surface (211B) of the solid electrolyte layer (211). The fuel electrode layer (213) contains Ni and Fe. When a position in the thickness direction of the fuel electrode layer (213) is represented by a ratio X of the distance from the boundary between the fuel electrode layer (213) and the solid electrolyte layer (211) to the thickness of the fuel electrode layer (213), the concentration of Fe present at a position where the ratio X is between 0 and 0.6 is equal to or greater than (-14X / 60+0.14) wt% and equal to or less than (-56X / 60+0.8) wt%.

[0008] The fuel electrode layer of the electrochemical cell according to the present disclosure contains Ni and Fe (iron). The Fe contained in the fuel electrode layer inhibits the migration and aggregation of Ni. However, if the Fe concentration is too high, the interior of the fuel electrode layer is densified during molding (sintering), which reduces the diffusibility of gas passing through the fuel electrode layer and reduces the initial performance of the electrochemical cell. Furthermore, if the Fe concentration is too low, the effect of adding Fe in inhibiting the migration and aggregation of Ni is reduced, and durability is not sufficiently improved.

[0009] In the electrochemical cell according to the present disclosure, when the position in the thickness direction of the fuel electrode layer is normalized and expressed as a ratio X that ranges from 0 to 1 from the side closer to the solid electrolyte layer to the side farther from the solid electrolyte layer, the concentration of Fe at a position where the ratio X is 0 to 0.6 is (-14X / 60+0.14) wt% or more and (-56X / 60+0.8) wt% or less. By including Fe in the fuel electrode layer in this way, it is possible to improve durability while suppressing a decrease in the initial performance of the electrochemical cell.

[0010] In the present disclosure, the concentration range of Fe is defined in the region where the ratio X, which represents the position in the thickness direction of the fuel electrode layer, is equal to or greater than 0 and equal to or less than 0.6, but this does not deny the presence of Fe at positions where the ratio X is greater than 0.6.

[0011] The concentration of Ni contained in the fuel electrode layer is not particularly limited, but is preferably within the range of 30 to 70 wt %.

[0012] The boundary between the anode layer and the solid electrolyte layer can be determined by the difference in porosity between the two layers. The solid electrolyte layer is a very dense layer, while the anode layer is a porous layer containing pores. Therefore, the boundary can be determined by the presence or absence of pores.

[0013] Furthermore, although the Fe concentration is defined above when the ratio X is in the range of 0 to 0.6, it is not necessary that the Fe concentration be within the above range for all ratios X between 0 and 0.6. It is sufficient that the Fe concentration be within the above range for multiple ratios X within the range of 0 to 0.6. For example, it is sufficient that the Fe concentration be greater than or equal to (-14X / 60+0.14) wt% and less than or equal to (-56X / 60+0.8) wt% at positions X (=0, 0.6, 1.2, 1.8, . . . 5.4, 6.0) that divide the range of ratio X between 0 and 0.6 into 10 equal parts.

[0014] Furthermore, as described above, even if Fe is present at a position where the ratio X is greater than 0.6, the effect of improving the durability of the electrochemical cell is small. However, from the viewpoint of gas diffusibility and the like, it is more preferable that the concentration of Fe present at a position where the ratio X is greater than 0.6 is low.

[0015] In one embodiment of the electrochemical cell according to the present disclosure, The fuel electrode layer (213) includes a first layer (213a) and a second layer (213b) having different porosities, the porosity of the first layer (213a) being smaller than the porosity of the second layer (213b), the first layer (213a) and the second layer (213b) being stacked in this order on one surface (211A) of the solid electrolyte layer (211), and the Fe concentration in the first layer (213a) being higher than the Fe concentration in the second layer (213b).

[0016] The reaction in the anode layer is likely to occur in a region of the anode layer that is close to the solid electrolyte layer. Therefore, by suppressing the migration and aggregation of Ni present in the region of the anode layer that is close to the solid electrolyte layer, the durability of the electrochemical cell can be further improved. In this regard, according to the above configuration, the anode layer includes a first layer that is close to the solid electrolyte layer and a second layer that is far from the solid electrolyte layer, and the Fe concentration in the first layer is higher than the Fe concentration in the second layer. Therefore, the migration and aggregation of Ni in the first layer that is close to the solid electrolyte layer is sufficiently suppressed by the larger amount of Fe present in the first layer, thereby further improving the durability of the electrochemical cell.

[0017] The Fe concentration in the first layer may be the average value of Fe concentrations at multiple different positions in the thickness direction of the first layer, rather than the Fe concentration at any one point in the first layer. Similarly, the Fe concentration in the second layer may be the average value of Fe concentrations at multiple different positions in the thickness direction of the second layer, rather than the Fe concentration at any one point in the second layer.

[0018] The thickness of the first layer is smaller than 0.6 times the thickness of the entire fuel electrode layer, so the position in the thickness direction where the ratio X is 0.6 is located within the second layer.

[0019] In another embodiment of the electrochemical cell according to the present disclosure, The first layer (213a) has a first surface (S11) that forms an interface with the solid electrolyte layer (211), and the second layer (213b) has a second surface (S21) that forms an interface with the first layer (213a), and the concentration of Fe present on the second surface (S21) is 0.7 times or more and 1.0 times or less than the concentration of Fe present on the first surface (S11).

[0020] The Fe in the second layer has the effect of preventing the diffusion of Fe in the first layer into the second layer. In particular, when the Fe concentration in the second surface of the second layer, which forms an interface with the first layer, is 0.7 times or more the Fe concentration in the first surface of the first layer, which forms an interface with the solid electrolyte layer, the effect of preventing the diffusion of Fe in the first layer into the second layer is high. This prevents a decrease in Fe in the first layer and the resulting migration and aggregation of Ni in the first layer, which would reduce durability.

[0021] The first and second surfaces can be identified, for example, from an SEM image of a cross section of the electrochemical cell parallel to the thickness direction of the fuel electrode layer. Then, by determining the thickness direction positions (or ratio X) of the identified first and second surfaces and comparing the Fe concentrations at the determined thickness direction positions (or ratio X), it can be determined whether the Fe concentration present on the second surface is 0.7 to 1.0 times the Fe concentration present on the first surface.

[0022] In another embodiment of the electrochemical cell according to the present disclosure, The concentration of Fe present at a position where the ratio X is equal to or greater than 0 and equal to or less than 0.6 is equal to or greater than (-28X / 60+0.28) wt% and equal to or less than (-39X / 60+0.56) wt%.

[0023] According to the above configuration, it is possible to further improve the durability of the electrochemical cell while further suppressing the deterioration of the initial performance.

[0024] Furthermore, the solid oxide electrolysis cell (21) according to the present disclosure is comprised of the electrochemical cell according to the present disclosure.

[0025] According to the above configuration, it is possible to provide a solid oxide electrolysis cell with improved durability.

[0026] The cell stack (20) according to the present disclosure is formed by stacking a plurality of solid oxide electrolysis cells (21) according to the present disclosure.

[0027] According to the above configuration, a cell stack with improved durability can be provided.

[0028] The hot module (10) according to the present disclosure includes a cell stack (20) according to the present disclosure, a vaporizer (30) that generates steam to be supplied to the cell stack (20), a heat exchanger (40) that exchanges heat with gas supplied to the cell stack (20), a heater (50) that heats the cell stack (20), and a heat insulating material (60) in which the cell stack (20), the vaporizer (30), the heat exchanger (40), and the heater (50) are disposed.

[0029] According to the above configuration, it is possible to provide a hot module with improved durability.

[0030] The hydrogen production device (1) according to the present disclosure includes the hot module (10) according to the present disclosure.

[0031] According to the above configuration, it is possible to provide a hydrogen production device with improved durability. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a block diagram of a hydrogen production device according to an embodiment of the present invention. [Figure 2]FIG. 1 is a perspective view of a cell stack of a solid oxide electrolysis cell (SOEC). [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 an electrolysis cell in the thickness direction. [Figure 5] FIG. 10 is a diagram showing the appropriate concentration range of Fe in the fuel electrode layer. [Figure 6] FIG. 10 is a diagram showing a more appropriate concentration range of Fe in the fuel electrode layer. [Figure 7] 6 is a graph showing the results of measuring the Fe concentration along the thickness direction of the fuel electrode layer for each sample according to Example 1, Example 2, and Example 3, and a diagram showing the relationship with region A1 in FIG. 5. FIG. [Figure 8] 7 is a graph showing the results of measuring the Fe concentration along the thickness direction of the fuel electrode layer for each sample according to Example 1, Example 2, and Example 3, and a diagram showing the relationship with region A2 in FIG. 6. FIG. [Figure 9] FIG. 10 is a graph showing the results of measuring the Fe concentration along the thickness direction of the fuel electrode layer after a 400-hour durability degradation test was carried out on the sample according to Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. FIG. 1 is a block diagram of a hydrogen production device 1 according to this embodiment. The hydrogen production device 1 according to this embodiment is a device that produces hydrogen by electrolyzing water vapor. As shown in FIG. 1, the hydrogen production device 1 includes a hot module 10 and a condenser 90.

[0034] The hot module 10 is constructed by covering with insulating material the main components that become hot among the elements that make up the hydrogen production device 1, and is a device in which the main components are concentrated within the insulating material so that they can be kept at a high temperature. This hot module 10 includes a cell stack 20, a vaporizer 30, a heat exchanger 40, a heater 50, and insulating material 60.

[0035] 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.

[0036] 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, thereby heating the water vapor and air introduced from the vaporizer 30 in the heat exchanger 40.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] 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 rectangular flat-plate-shaped members having the same outer shape as the electrolysis units Ue, and each has a rectangular opening formed in its 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 end plates 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. For ease of explanation, the proportions of the components in the drawings may differ from the actual proportions.

[0041] 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 the electrolysis cell 21), an interconnector 22, a separator 23, a cathode frame 24, a fuel electrode frame 25, and a current collector 26.

[0042] The electrolysis cell 21 is the smallest unit of an SOEC and includes a solid electrolyte layer 211, an air electrode layer 212 laminated on the upper surface of the solid electrolyte layer 211, and an anode layer 213 laminated on the back 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.

[0043] 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 centre 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 share one interconnector 22. The interconnector 22 also functions as a separator that separates the two adjacent electrolysis units 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The internal space of the electrolysis unit Ue is partitioned by the separator 23 into an air chamber Sa and a fuel chamber Sf. 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 upper interconnector 22, separator 23, air electrode frame 24, and 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 lower interconnector 22, separator 23, fuel electrode frame 25, and electrolysis cell 21.

[0048] 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.

[0049] 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."

[0050] 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.

[0051] 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.

[0052] Next, the configuration of the electrolysis cell 21 will be described in more detail with reference to FIG. 4. FIG. 4 is a cross-sectional view of the electrolysis cell 21 in the thickness direction. As described above, the electrolysis cell 21 includes a solid electrolyte layer 211, an air cathode layer 212, and an anode layer 213. In this embodiment, the solid electrolyte layer 211 is a rectangular flat layer measuring 150 mm square and 6 μm thick, and is configured to contain YSZ (yttria-stabilized zirconia) and formed by sintering. The solid electrolyte layer 211 has high oxide ion conductivity. The solid electrolyte layer 211 is a dense layer and is designed to prevent leakage between the atmosphere on the air cathode layer 212 side (air atmosphere) and the atmosphere on the anode layer 213 side (reducing atmosphere).

[0053] The air electrode layer 212 is laminated on the upper surface 211B (other 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.

[0054] The anode layer 213 is a rectangular flat layer measuring 150 mm on each side, and is formed to have a thickness of, for example, approximately 400 μm, which is greater than the thicknesses of the solid electrolyte layer 211 and the air cathode layer 212. 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 has a functional layer 213a (first layer) and a support layer 213b (second layer). The support layer 213b is formed to be significantly thicker than the functional layer 213a, and the thickness ratio can be set to, for example, approximately 16 to 40 times.

[0055] The fuel electrode layer 213 is laminated on the back surface 211A (one side) of the solid electrolyte layer 211, which is the bottom surface in FIG. 4. Specifically, the functional layer 213a of the fuel electrode layer 213 is laminated on the back surface 211A of the solid electrolyte layer 211, and the support layer 213b is laminated on the back surface (the bottom surface in FIG. 4) of the functional layer 213a. That is, the functional layer 213a and the support layer 213b are laminated in this order on the back surface 211A (one side) of the solid electrolyte layer 211. Therefore, the functional layer 213a of the fuel electrode layer 213 has a first upper surface S11 (first surface) that is in contact with the back surface 211A of the solid electrolyte layer 211 and forms an interface with the back surface 211A, and a first back surface S12 on the opposite side. The support layer 213b of the fuel electrode layer 213 has a second upper surface S21 (second surface) that is in contact with the first back surface S12 of the functional layer 213a and forms an interface with the first back surface S12, and a second back surface S22 on the opposite side.

[0056] The support layer 213b is primarily composed of a cermet of Ni and YSZ. The support layer 213b is a porous layer configured to have a porous shape including a plurality of micropores (not shown). The diameter of the micropores is on the order of several μm, which ensures water vapor permeability (gas diffusibility). The functional layer 213a is also primarily composed of a cermet of Ni and YSZ. Like the support layer 213b, the functional layer 213a is also a porous layer configured to have a porous shape including a plurality of micropores (not shown). 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 the porosity of the support layer 213b. As will be described later, the fuel electrode layer 213 contains a trace amount of Fe. The fuel electrode layer 213, which includes the functional layer 213a and the support layer 213b, is also formed by sintering, similar to the solid electrolyte layer 211 and the air electrode layer 212.

[0057] The operation of the cell stack 20 will be described. 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 horizontal holes (not shown). The reason for supplying high-temperature air to the air chamber Sa is to control the temperature of the cell stack 20.

[0058] 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 29 and is decomposed into hydrogen and oxide ions (water electrolysis reaction). The hydrogen generated by the water electrolysis reaction diffuses within the fuel chamber Sf, is discharged through the horizontal hole 25b via path Pfo, and is collected 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 electrode layer 212 in the air chamber Sa, release electrons in the functional layer of the air electrode layer 212, and become oxygen. The oxygen diffuses within the air chamber Sa and, together with the air that flowed into the air chamber Sa, is discharged through path Pao via a horizontal hole (not shown) and is collected (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.

[0059] The cell stack 20 operates as described above, and hydrogen is produced in the hydrogen production device 1.

[0060] It has been confirmed that, when the cell stack 20 operates for a long time, Ni in the fuel electrode layer 213 migrates and aggregates within the fuel electrode layer 213. When Ni migrates and aggregates within the fuel electrode layer 213, the Ni concentration near the interface between the functional layer 213a and the solid electrolyte layer 211 decreases, and the three-phase interface (the boundary between water vapor, Ni, and YSZ) serving as a reaction field decreases. This reduces the reaction efficiency of the water electrolysis reaction and the performance of the electrolysis cell 21. Furthermore, when Ni migrates and aggregates within the fuel electrode layer 213, the conduction paths within the fuel electrode layer 213 decrease, and the internal resistance increases. This decrease in reaction efficiency and increase in internal resistance deteriorates the fuel electrode layer 213.

[0061] It is known that the progression of deterioration of the fuel electrode layer 213 can be suppressed by including an Fe (iron) component in the fuel electrode layer 213. However, because Fe is also a sintering aid, it promotes densification of the fuel electrode layer 213 when the fuel electrode layer 213 is formed by sintering, reducing the internal pores. This results in poor gas diffusivity and a decrease in initial performance. The fuel electrode layer 213 according to this embodiment is configured to contain an appropriate concentration of Fe in the fuel electrode layer 213 so as to improve durability while suppressing a decrease in initial performance. This is explained below.

[0062] FIG. 5 is a diagram showing an appropriate concentration range of Fe in the anode layer 213. In FIG. 5, the horizontal axis is the ratio X, which represents the position in the thickness direction of the anode layer 213 (hereinafter referred to as the thickness position). The ratio X is the ratio of the distance in the thickness direction from the boundary between the anode layer 213 and the solid electrolyte layer 211 to that position, relative to the thickness of the anode layer 213. In other words, the ratio X is the normalized thickness position of the anode layer 213. For example, if the thickness of the anode layer 213 is 400 μm, the thickness position where the ratio X is 0.1 is a position shifted 40 μm in the thickness direction from the surface of the anode layer 213 that is in contact with the solid electrolyte layer 211 (i.e., the first upper surface S11 of the functional layer 213a) toward the second back surface S22 of the support layer 213b of the anode layer 213. The thickness position where the ratio X is 0 is a position within the first upper surface S11 of the functional layer 213a. The thickness position where the ratio X is 1.0 is a position within the second back surface S22 of the support layer 213b of the fuel electrode layer 213. In addition, in Figure 5, the vertical axis represents the mass concentration of Fe relative to all elements present at the thickness position determined by the ratio X on the horizontal axis, and all the elements present are Ni, Fe, Zr, Y, and O.

[0063] In FIG. 5, the appropriate Fe concentration range in the fuel electrode layer 213 is the range indicated by region A1. This region A1 is the region where the ratio X is between 0 and 0.6. The Fe present in region D where the ratio X is greater than 0.6 has little effect on improving the durability of the electrolysis cell 21. Therefore, Fe may or may not be present in the position where the ratio X is greater than 0.6. As mentioned above, Fe also functions as a sintering aid. Therefore, the presence of a large amount of Fe in the position where the ratio X is greater than 0.6 deteriorates gas diffusibility and leads to a decrease in initial performance. Therefore, it is preferable that Fe is absent or, if present, has a low concentration in the position where the ratio X is greater than 0.6. For example, it is preferable that the Fe concentration be 0.10 wt% or less in the position where the ratio X is greater than 0.6.

[0064] The Fe concentration range indicated by region A1 represents the appropriate concentration range of Fe present at the thickness position represented by ratio X when ratio X is between 0 and 0.6. Here, the "thickness position" of the anode layer 213 is a position in a plane perpendicular to the thickness direction of the anode layer 213 that corresponds to that thickness position. Therefore, the Fe concentration present at the thickness position represented by ratio X is preferably the average value of the Fe concentrations at multiple positions in the plane corresponding to that thickness position. For example, the anode layer 213 can be cut along a plane parallel to its thickness direction, and the average value of the Fe concentrations at multiple points on the line corresponding to each thickness position represented on the cut surface can be used as the Fe concentration present at that thickness position.

[0065] The Fe concentration range indicated by region A1 narrows as the ratio X increases. Specifically, when the ratio X is 0, the Fe concentration range indicated by region A1 is 0.14 wt% or more and 0.80 wt% or less. As the ratio X increases, the width of the Fe concentration range indicated by region A1 decreases. When the ratio X is 0.6, the Fe concentration range indicated by region A1 is 0.00 wt% or more and 0.24 wt% or less.

[0066] The lower limit concentration of the Fe concentration range indicated by region A1 (hereinafter, sometimes referred to as the first Fe lower limit concentration) decreases as the ratio X increases. Specifically, when the ratio X is 0, the first Fe lower limit concentration is 0.14 wt%, and as the ratio X increases, the first Fe lower limit concentration decreases linearly. When the ratio X is 0.6, the first Fe lower limit concentration is 0.00 wt%. In this way, the first Fe lower limit concentration changes according to the ratio X. The first Fe lower limit concentration that changes according to the ratio X can be expressed by the following formula (1) using the ratio X. First Fe lower limit concentration [wt%]=(-14X / 60)+0.14 (1) The first Fe lower limit concentration shown in the above formula (1) is represented by a straight line Ld1 in Fig. 5. When the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component, the straight line Ld1 is a straight line that passes through points a (0.00, 0.14) and b (0.60, 0.00).

[0067] Furthermore, the upper limit concentration of the Fe concentration range indicated by region A1 (hereinafter, sometimes referred to as the first Fe upper limit concentration) also decreases as the ratio X increases. Specifically, when the ratio X is 0, the first Fe upper limit concentration is 0.80 wt%, and as the ratio X increases, the first Fe upper limit concentration decreases linearly. When the ratio X is 0.6, the first Fe upper limit concentration is 0.24 wt%. In this way, the first Fe upper limit concentration changes according to the ratio X. The first Fe upper limit concentration that changes according to the ratio X can be expressed by the following formula (2) using the ratio X. First Fe upper limit concentration [wt%]=(-56X / 60)+0.8 (2) The first upper limit of the Fe concentration shown in the above formula (2) is represented by a straight line Lu1 in Fig. 5. When the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component, the straight line Lu1 is a straight line that passes through points c (0, 0.80) and d (0.6, 0.24).

[0068] Therefore, when the ratio X is in the range of 0 to 0.6, the appropriate concentration [wt%] of Fe present at the thickness position represented by the ratio X is ((-14X / 60)+0.14) or more and ((-56X / 60)+0.8) or less.

[0069] When the concentration of Fe present at the thickness position represented by the ratio X is a concentration within region B below region A1 in FIG. 5, the amount of Fe contained in the fuel electrode layer 213 is too small to sufficiently suppress the migration and aggregation of Ni, and the durability of the fuel electrode layer 213 cannot be sufficiently improved.

[0070] Furthermore, because Fe is also a sintering aid, if the content is too high, it promotes sintering of the fuel electrode layer 213, leading to increased densification, making it impossible to obtain the desired porosity and causing a decrease in initial performance. For this reason, if the concentration of Fe present at the thickness position represented by the ratio X is within region C above region A1 in Figure 5, the initial performance will decrease.

[0071] Therefore, when the concentration of Fe present at the thickness position represented by the ratio X is within the concentration range represented by region A1 in FIG. 5, that is, when the Fe concentration distribution in the thickness direction of the fuel electrode layer 213 shifts within region A1 in FIG. 5, it is possible to improve durability while suppressing deterioration in the initial performance of the electrolysis cell 21.

[0072] 5, the thickness position represented by the ratio Xa is the boundary position between the functional layer 213a and the support layer 213b of the fuel electrode layer 213, i.e., a position within the second upper surface S21 of the support layer 213b. As described above, the thickness of the functional layer 213a is much smaller than the thickness of the support layer 213b. Therefore, the ratio X (Xa) representing the boundary position between the functional layer 213a and the support layer 213b is smaller than 0.6. The position where the ratio X is 0.6 is a position within the support layer 213b.

[0073] Furthermore, the region in the fuel electrode layer 213 where the water electrolysis reaction mainly occurs is the region close to the solid electrolyte layer 211, i.e., the region where the ratio X is small, i.e., the region in the functional layer 213a. To effectively suppress the migration and aggregation of Ni in the functional layer 213a, it is preferable that more Fe is present in the region where the ratio X is small. Therefore, the Fe concentration distribution should be such that it is high in the region where the ratio X is small and low in the region where the ratio X is large. In this case, it is preferable that the Fe concentration in the functional layer 213a is higher than the Fe concentration in the support layer 213b.

[0074] The Fe concentration in the functional layer 213a may be the average value of Fe concentrations present at multiple thickness positions in the functional layer 213a, rather than the Fe concentration at any one point in the functional layer 213a. Similarly, the Fe concentration in the support layer 213b may be the average value of Fe concentrations present at multiple thickness positions in the support layer 213b, rather than the Fe concentration at any one point in the support layer 213b.

[0075] Furthermore, if the Fe concentration in the support layer 213b is significantly lower than the Fe concentration in the functional layer 213a, the large difference between the Fe concentrations in the functional layer 213a and the support layer 213b may cause a large amount of Fe in the functional layer 213a to diffuse toward the support layer 213b. If a large amount of Fe in the functional layer 213a diffuses toward the support layer 213b, the Fe concentration in the functional layer 213a may decrease significantly, which may result in insufficient suppression of Ni migration and aggregation within the functional layer 213a. Therefore, it is preferable that the support layer 213b also contains a predetermined concentration or more of Fe. If the support layer 213b contains a predetermined concentration or more of Fe, the difference between the Fe concentration in the functional layer 213a and the Fe concentration in the support layer 213b becomes small, making it difficult for Fe in the functional layer 213a to diffuse into the support layer 213b. That is, the Fe in the support layer 213b effectively prevents the Fe in the functional layer 213a from flowing into the support layer 213b.

[0076] In this case, the concentration of Fe present on the second upper surface S21 (second surface) of the surface of the support layer 213b that forms an interface with the functional layer 213a, i.e., the concentration of Fe present at the thickness position represented by the ratio X = Xa, is defined as Na, and the concentration of Fe present on the first upper surface S11 (first surface) of the surface of the functional layer 213a that forms an interface with the solid electrolyte layer 211, i.e., the concentration of Fe present at the thickness position represented by the ratio X = 0, is defined as N0. The concentration Na should be 0.7 times or more and 1.0 times or less the concentration N0. If the concentration Na is within the above range, it is possible to sufficiently prevent a large amount of Fe from the functional layer 213a from diffusing into the support layer 213b.

[0077] FIG. 6 is a diagram showing a more appropriate concentration range of Fe in the fuel electrode layer 213. In FIG. 6, the horizontal axis represents the ratio X, which indicates the thickness position of the fuel electrode layer 213, and the vertical axis represents the Fe concentration. In FIG. 6, a more appropriate concentration range of Fe in the fuel electrode layer 213 is the range indicated by region A2. This region A2 is part of the region A1 shown in FIG. 5. Note that in FIG. 6, a straight line Ld1 defining the upper limit of region A1 and a straight line Lu1 defining the lower limit of region A1 are indicated by dashed lines.

[0078] The Fe concentration range indicated by region A2 represents a more appropriate concentration range of Fe present at the thickness position represented by ratio X when ratio X is in the range of 0 to 0.6. The Fe concentration range indicated by region A2 narrows as ratio X increases. Specifically, when ratio X is 0, the Fe concentration range indicated by region A2 is in the range of 0.28 wt% to 0.56 wt%. As ratio X increases, the width of the Fe concentration range indicated by region A2 decreases. When ratio X is 0.6, the Fe concentration range indicated by region A2 is in the range of 0.00 wt% to 0.17 wt%.

[0079] The lower limit concentration of the Fe concentration range indicated by region A2 (hereinafter sometimes referred to as the second Fe lower limit concentration) decreases as the ratio X increases. Specifically, when the ratio X is 0, the second Fe lower limit concentration is 0.28 wt%, and as the ratio X increases, the second Fe lower limit concentration decreases linearly. When the ratio X is 0.6, the second Fe lower limit concentration is 0.00 wt%. In this way, the second Fe lower limit concentration changes according to the ratio X. The second Fe lower limit concentration that changes according to the ratio X can be expressed using the ratio X as in the following equation (3). Fe lower limit concentration [wt%]=(-28X / 60)+0.28 (3) The second Fe lower limit concentration shown in the above formula (3) is represented by the straight line Ld2 in Fig. 6. When the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component, the straight line Ld2 is a straight line that passes through the points e (0, 0.28) and f (0.6, 0.00).

[0080] Furthermore, the upper limit concentration of the Fe concentration range indicated by region A2 (hereinafter, sometimes referred to as the second Fe upper limit concentration) also decreases as the ratio X increases. Specifically, when the ratio X is 0, the second Fe upper limit concentration is 0.56 wt%, and as the ratio X increases, the second Fe upper limit concentration decreases linearly. When the ratio X is 0.6, the second Fe upper limit concentration is 0.17 wt%. In this way, the second Fe upper limit concentration also changes according to the ratio X. The second Fe upper limit concentration that changes according to the ratio X can be expressed as the following equation (4) using the ratio X. Fe upper limit concentration [wt%]=(-39X / 60)+0.56 (4) The second upper limit of the Fe concentration shown in the above formula (4) is represented by the line Lu2 in Fig. 6. When the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component, the line Lu2 is a line that passes through the points g (0, 0.56) and h (0.6, 0.17).

[0081] Therefore, when the ratio X is in the range of 0 to 0.6, a more appropriate concentration [wt%] of Fe present at the thickness position represented by the ratio X is ((-28X / 60)+0.28) or more and ((-39X / 60)+0.56) or less.

[0082] When the Fe concentration at the thickness position represented by the ratio X is within the concentration range represented by region A2 in FIG. 6, that is, when the Fe concentration distribution in the thickness direction of the fuel electrode layer 213 shifts within region A2 in FIG. 6, it is possible to further suppress the deterioration of the initial performance of the electrolysis cell 21 while further improving its durability.

[0083] (Example) 1. Sample Preparation NiO powder and Fe2O3 powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. The mixture was then washed with alcohol, and the liquid mixture was placed in a bowl. The alcohol was evaporated in the bowl to dry the mixture and form it into a powder. The powder was calcined at approximately 800°C. This produced a composite oxide powder containing Ni and Fe. The produced composite oxide powder and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin (G-260, manufactured by Sekisui Chemical Co., Ltd.) as a plasticizer, a known dispersant, a mixed solvent of toluene and ethanol, and optionally a pore-forming agent (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. The slurry was then formed into a green sheet of the functional layer of the fuel electrode layer having a predetermined thickness using a doctor blade method.

[0084] NiO powder and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin as a plasticizer, a known dispersant, a mixed solvent of toluene and ethanol, and optionally a pore-forming agent (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of the support layer for the fuel electrode layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0085] In addition, butyral resin, polyvinyl acetal resin as a plasticizer, a known dispersant, and a mixed solvent of toluene and ethanol were added to the YSZ powder in predetermined proportions and mixed in a ball mill to prepare a slurry, which was then molded into a green sheet of a solid electrolyte layer having a predetermined thickness using a doctor blade method.

[0086] Next, a green sheet for the functional layer of the fuel electrode layer and a green sheet for the support layer of the fuel electrode layer were stacked in this order on one side of the green sheet for the solid electrolyte layer. These stacked green sheets were then pressed together under high pressure using a press while heating and evacuating. This resulted in the formation of a laminate including the green sheet for the solid electrolyte layer, the green sheet for the functional layer of the fuel electrode layer, and the green sheet for the support layer of the fuel electrode layer.

[0087] The laminate formed as described above was then degreased at a predetermined temperature (e.g., 200 to 300°C). The laminate was then fired (primary sintering) at a predetermined first temperature (e.g., 1300 to 1400°C) for a predetermined time (e.g., 1 to 5 hours). This resulted in the formation of a primary sintered body having a solid electrolyte layer and a fuel electrode layer stacked on one side of the solid electrolyte layer.

[0088] Next, a material containing LSCF was screen-printed onto the other side of the solid electrolyte layer of the formed primary sintered body, and the resulting product was fired at a predetermined second temperature (e.g., 900 to 1000°C) for a predetermined time (e.g., 1 to 5 hours) (secondary sintering). This produced a sample electrolysis cell comprising a solid electrolyte layer, an anode layer laminated on one side of the solid electrolyte layer, and an cathode layer laminated on the other side of the solid electrolyte layer.

[0089] Furthermore, when forming the green sheet for the functional layer of the fuel electrode layer, the ratio (compounding ratio) of Fe2O3 powder to the total volume of NiO powder and Fe2O3 powder was varied within the range of 0 to 30 vol% to produce multiple electrolysis cell samples with different Fe compounding ratios in the fuel electrode layer. Note that in the sample with a compounding ratio of Fe2O3 powder of 0 vol% (sample according to Comparative Example 1 described below), no Fe2O3 powder was used when forming the green sheet for the functional layer of the fuel electrode layer. Furthermore, the amount of pore-forming material, etc., was adjusted when forming these layers so that the porosity in the functional layer of the fuel electrode layer of each sample was smaller than the porosity in the support layer of the fuel electrode layer.

[0090] 2. Measurement of initial performance and durability deterioration rate The current density (current density) per unit area was measured when a constant voltage (1 to 1.3 V) was applied between the anode and cathode layers of each sample. The measured current density was used as an index for evaluating initial performance. A higher current density indicates better initial performance. A voltage was applied between the anode and cathode layers of each sample to ensure a constant current flow. A durability degradation test was then conducted, in which a constant current was continuously applied for a predetermined period of time. When a constant current is continuously applied during the durability degradation test, the applied voltage increases over time due to an increase in the internal resistance within the sample. The difference ΔV (= V1 - V0) between the initially applied voltage (V0) and the voltage (V1) applied after a predetermined time (e.g., 400 hours) was calculated, and this value was converted to the difference between the voltage expected to be applied after 1000 hours and the initially applied voltage. The converted value was then divided by the initially applied voltage to calculate the percentage as the durability degradation rate. A smaller durability degradation rate indicates higher durability.

[0091] 3.Measurement results Table 1 shows the initial performance (current density) measured for each sample, its evaluation, the durability deterioration rate, its evaluation, and an overall evaluation. [Table 1]

[0092] In Table 1, the Fe blending ratio [vol%] is the ratio of the volume of Fe2O3 powder to the sum of the volume of NiO powder and the volume of Fe2O3 powder used when forming the green sheet for the functional layer of the fuel electrode layer during fabrication of each sample. The blending ratio of the total amount of NiO powder and Fe2O3 powder (amount of metal powder) to the amount of YSZ powder (amount of ceramic powder) used when forming the green sheet for the functional layer of the fuel electrode layer for each sample was the same for all samples. The fabrication conditions for each sample, other than the Fe blending ratio, were all the same.

[0093] In addition, for each sample, the current density was 1.30 A / cm 2 If the current density is 1.30 A / cm or more, the initial performance evaluation result is judged to be good (〇). 2 If the durability degradation rate was less than 2.0% / kh, the evaluation result of the initial performance was judged to be poor (×). Furthermore, for each sample, if the durability degradation rate was less than 2.0% / kh, the evaluation result of the durability degradation rate was judged to be very good (◎); if the durability degradation rate was 2.0% / kh or more but less than 3.4% / kh, the evaluation result of the durability degradation rate was judged to be good (◯); and if the durability degradation rate was 3.4% / kh or more, the evaluation result of the durability degradation rate was judged to be poor (×). If the evaluation result of the initial performance was good (◯) and the evaluation result of the durability degradation rate was very good (◎), the overall evaluation was judged to be very good (◎); and if the evaluation result of the initial performance was good (◯) and the evaluation result of the durability degradation rate was good (◯), the overall evaluation was judged to be good (◯). Furthermore, if the evaluation result of the durability degradation rate was poor (×), the overall evaluation was judged to be poor (×) regardless of the evaluation result of the initial performance.

[0094] As can be seen from Table 1, the overall evaluations of the samples of Example 1, in which the Fe blending ratio is 5 vol%, Example 2, in which the Fe blending ratio is 10 vol%, and Example 3, in which the Fe blending ratio is 20 vol% are good (◯) or very good (◎). On the other hand, the overall evaluations of the samples of Comparative Example 1 and Comparative Example 2, in which the Fe blending ratio is less than 5 vol%, and the overall evaluation of the sample of Comparative Example 3, in which the Fe blending ratio is more than 20 vol%, are poor (×). This shows that the durability of the prepared samples is improved when the Fe blending ratio is 5 vol% or more and 20 vol% or less.

[0095] 4. Measurement of Fe concentration in the fuel electrode layer The samples of Examples 1, 2, and 3 were cut along a plane along the thickness direction of the fuel electrode layer (thickness direction of the cell), and the cut surface was subjected to area analysis using an EPMA device (manufactured by JEOL Ltd.). The Fe concentration distribution in the thickness direction of the fuel electrode layer was calculated from the element mapping image obtained by the area analysis. In this case, the Fe concentration at each thickness position was calculated at 1.0 μm intervals. To calculate the Fe concentration at each thickness position, the Fe concentrations detected at multiple points on the line corresponding to that thickness position were averaged, and the calculated average Fe concentration was used as the Fe concentration at that thickness position.

[0096] 7 is a graph showing the results of measuring the Fe concentration [wt%] along the thickness direction of the fuel electrode layer for each sample according to Example 1, Example 2, and Example 3, and a diagram showing the relationship between the graph and region A1 in FIG. 5. In FIG. 7, the horizontal axis represents the ratio X, and the vertical axis represents the Fe concentration. FIG. 7 also shows a line Ld1 defining the lower limit concentration of region A1 in FIG. 5 and a line Lu1 defining the upper limit concentration of region A1. In FIG. 7, graph G1 is a graph showing the measurement results of the Fe concentration for the sample according to Example 1, graph G2 is a graph showing the measurement results of the Fe concentration for the sample according to Example 2, and graph G3 is a graph showing the measurement results of the Fe concentration for the sample according to Example 3.

[0097] As shown in Fig. 7, the Fe concentration distributions represented by the graphs G1, G2, and G3, when the ratio X is in the range of 0 to 0.6, are within the region between the lines Ld1 and Lu1, i.e., within the region A1 in Fig. 5. Therefore, the Fe concentrations contained in the fuel electrode layers of the samples according to Examples 1, 2, and 3 are within the region A1 in Fig. 5.

[0098] Furthermore, the fuel electrode layer of the sample according to Comparative Example 1 does not contain Fe. Although the fuel electrode layer of the sample according to Comparative Example 2 contains Fe, as can be seen from Table 1, the Fe blending ratio is 2.5 vol%, which is smaller than the Fe blending ratio (5 vol%) in the sample according to Example 1. As can be seen from FIG. 7, the graph G1 for the sample according to Example 1 moves close to the line Ld1 that defines the lower limit concentration of region A1. Therefore, it is predicted that the concentration distribution of Fe contained in the sample according to Comparative Example 2 will move within region B below region A1.

[0099] As can be seen from Table 1, the Fe blending ratio for the sample according to Comparative Example 3 is 30 vol%, which is significantly higher than the Fe blending ratio (20 vol%) for the sample according to Example 3. As can be seen from FIG. 7, the graph G3 for the sample according to Example 3 fluctuates near the line Lu1 that defines the upper limit concentration for region A1, and the Fe concentration contained in the sample according to Example 3 coincides with line Lu1, particularly at the position where ratio X = 0. Therefore, the concentration distribution of Fe contained in the sample according to Comparative Example 3 is predicted to fluctuate within region C above region A1, particularly in the region where ratio X is small.

[0100] Based on the above considerations, when the concentration of Fe at the position where the ratio X, which represents the thickness direction position within the fuel electrode layer, is equal to or greater than 0 and equal to or less than 0.6, is within the range indicated by region A1 in FIGS. 5 and 7 , the durability of the electrolysis cell can be improved while suppressing deterioration in the initial performance.

[0101] Furthermore, in all of graphs G1, G2, and G3, it can be seen that the Fe concentration tends to decrease as the ratio X increases. Specifically, in the Fe concentration distribution shown in each graph, when the ratio X is between 0 and approximately 0.05, the Fe concentration decreases rapidly as the ratio X increases. Furthermore, from the point where the rapid decrease in the Fe concentration ends (the point where the ratio X is around 0.05), the Fe concentration decreases gradually as the ratio X increases, fluctuating between high and low within a predetermined range.

[0102] In each of graphs G1, G2, and G3, the position where the rapid decrease in Fe concentration ends is the boundary position (ratio Xa) between the functional layer and support layer of the fuel electrode layer. Furthermore, as can be seen from each of graphs G1, G2, and G3, the Fe contained in the fuel electrode layer of the samples according to Examples 1, 2, and 3 straddles the boundary position (ratio Xa), indicating that Fe is contained in both the functional layer and support layer of the fuel electrode layer. Here, during the fabrication of the samples according to each Example, Fe was contained only in the green sheet of the functional layer of the fuel electrode layer. However, in the molded samples, Fe was contained not only in the functional layer of the fuel electrode layer but also in the support layer. This is because Fe present on the functional layer side diffused to the support layer side during sample molding (sintering).

[0103] In each of graphs G1, G2, and G3, the position where the Fe concentration is highest is the thickness position where the ratio X is 0, i.e., the surface in contact with the solid electrolyte layer (i.e., the first upper surface S11 of the functional layer 213a). Also, at the position where the ratio X is Xa, i.e., the contact interface position between the functional layer and the support layer of the fuel electrode layer (i.e., the second upper surface S21 of the support layer 213b), the Fe concentration (concentration Na) is approximately 0.7 times the Fe concentration (concentration N0) present at the position where the ratio X is 0.

[0104] In this way, if the concentration of Fe (concentration Na) at the position where the ratio X = Xa, i.e., at the interface between the functional layer of the fuel electrode layer and the support layer (a position within the second upper surface S21 of the support layer 213b), is 0.7 times or more the concentration of Fe (concentration N0) at the position where the ratio X = 0, i.e., at the interface between the functional layer of the fuel electrode layer and the solid electrolyte layer (a position within the first upper surface S11 of the functional layer 213a), then since the difference between the concentration of Fe in the functional layer and the concentration of Fe in the support layer is small, it is considered that further diffusion (flow) of Fe in the functional layer into the support layer can be effectively suppressed.

[0105] FIG. 8 shows the relationship between graphs G1, G2, and G3 and region A2 shown in FIG. 6. In FIG. 8, the horizontal axis represents the ratio X, and the vertical axis represents the Fe concentration. FIG. 8 also shows a line Ld2 defining the lower limit concentration of region A2 in FIG. 6 and a line Lu2 defining the upper limit concentration of region A2. As shown in FIG. 8, graph G2 moves within region A2. In contrast, graphs G1 and G3 have portions that extend beyond region A2, and therefore do not move within region A2. As can be seen from Table 1, the overall evaluation of the sample according to Example 2 is very good (◎). Therefore, when the concentration of Fe contained in the fuel electrode layer is within the range indicated by region A2 shown in FIGS. 6 and 8, the deterioration of the initial performance of the electrolysis cell can be further suppressed and durability can be further improved.

[0106] Fig. 9 is a graph G4 showing the results of measuring the Fe concentration along the thickness direction of the fuel electrode layer after a 400-hour durability test was performed on the sample according to Example 2. Note that Fig. 9 also shows a graph (graph G2) by a dashed line showing the measurement results of the Fe concentration measured before the durability test was performed.

[0107] As shown in FIG. 9 , graph G4 shows that the concentration distribution (indicated by arrow P) in graph G2, in which the Fe concentration rapidly decreases as the ratio X ranges from 0 to Xa, disappears. This is thought to be because the Fe in the functional layer diffuses toward the support layer due to the long-term operation of the sample according to Example 2. However, even after the durability test, the Fe concentration at the position where the ratio X is 0 (the Fe concentration at the position where the ratio X is 0 in graph G4) is approximately 0.7 times the Fe concentration at the position where the ratio X is 0 before the durability test (the Fe concentration at the position where the ratio X is 0 in graph G2), and the Fe concentration distribution shown in graph G4 remains within region A2. This tendency is thought to be similar when durability tests are performed on the samples according to Examples 1 and 3. Therefore, it can be said that the samples according to Examples 1, 2, and 3 are able to sufficiently suppress the deterioration of initial performance and have high durability even after the durability test.

[0108] The region A1 (lines Ld1 and Lu1) shown in FIGS. 5 and 7 is determined as follows. First, line Ld1 is determined from graph G1, which represents the Fe concentration distribution measured for the sample according to Example 1. Specifically, the Fe concentration at ratio X=0 on graph G1 is approximately 0.2 wt%, and it is predicted that if this sample is used for a long period of time, the Fe concentration at ratio X=0 will become approximately 0.14 wt%, which is 0.7 times the Fe concentration at ratio X=0. Therefore, the lower limit of the Fe concentration at the position where ratio X=0 is estimated to be 0.14 wt%. This determines point a (0, 0.14) in FIG. 5. Furthermore, the lower limit of the Fe concentration at the position where ratio X=0.6 is 0.00 wt%. This determines point b (0.6, 0.00) in FIG. 5. Therefore, line Ld1, which defines the lower limit of the Fe concentration in region A1, is the line connecting point a (0, 0.14) and point b (0.6, 0). This line is expressed by the above equation (1) with X as a variable.

[0109] Furthermore, line Lu1 is determined by graph G3, which represents the Fe concentration distribution measured for the sample according to Example 3. The Fe concentration at ratio X = 0 on graph G3 is 0.80 wt%. Therefore, the upper limit of the Fe concentration at the position where ratio X = 0 is estimated to be 0.80 wt%. This determines point c(0, 0.80) in FIG. 5 . The Fe blending ratio used to prepare the sample according to Example 3 was 20 vol%, while the Fe blending ratio used to prepare the sample according to Example 1 was 5 vol%. That is, the sample according to graph G3 contains four times the amount of Fe used in the sample according to graph G1. Therefore, the Fe concentration gradient in the thickness direction of the sample according to graph G3 is predicted to be approximately four times the Fe concentration gradient in the thickness direction of the sample according to graph G1. Furthermore, since line Ld1 is determined by graph G1, the gradient of line Lu1 determined by graph G3 is considered to be approximately four times the gradient of line Ld1. Therefore, the straight line Lu1 that defines the upper limit concentration of region A1 passes through the point (point c) where the concentration is 0.8 wt% when ratio X = 0, and has a gradient with respect to ratio X that is four times -0.14 / 0.6, i.e., -0.56 / 0.6. This straight line is expressed by the above formula (2) with X as a variable. In this case, the straight line expressed by formula (2) passes through point d(0.6, 0.24).

[0110] Furthermore, region A2 (lines Ld2 and Lu2) shown in FIGS. 6 and 8 is determined so that graph G2 moves within the range between them, but graphs G1 and G3 do not fit within (move within) the range. In this case, first, the lower limit concentration of region A2 (point e in FIG. 6) when ratio X=0 is estimated. Here, the Fe concentration at ratio X=0 in graph G2 is approximately 0.48 wt%. However, the Fe blending ratio (10 vol%) used in the sample related to graph G2 is twice the Fe blending ratio (5 vol%) used in the sample related to graph G1. Furthermore, since the Fe concentration at ratio X=0 in the sample related to graph G1 is approximately 0.2 wt%, it is also possible that the Fe concentration at ratio X=0 is approximately 0.4 wt% when the Fe blending ratio is 10 vol%. If this sample is operated for a long time, the Fe concentration at ratio X = 0 may decrease to approximately 0.28 wt%, which is approximately 0.7 times 0.4 wt%. Therefore, the lower limit concentration of region A2 at ratio X = 0 is estimated to be 0.28 wt%. This determines point e(0,0.28) in Figure 6. Furthermore, the lower limit concentration of region A2 at ratio X = 0.6 is 0.00 wt%. This determines point f(0.6,0) in Figure 6. Therefore, line Ld2, which defines the lower limit concentration of region A2, is the line connecting points e(0,0.28) and f(0.6,0) in Figure 6. This line is expressed by the above equation (3) using X as a variable.

[0111] Furthermore, when ratio X = 0, the lower limit of the Fe concentration of the sample according to graph G3 is 0.7 times 0.8 wt%, which is 0.56 wt%. Therefore, when ratio X = 0, the upper limit of the concentration in the region that graph G3 does not enter is 0.56 wt%. This determines point g(0,0.56) in Figure 6. Furthermore, if the Fe concentration at the position where ratio X = 0 is 0.56 wt%, the Fe blend ratio at the time of manufacture is estimated to be approximately 14 vol%. This blend ratio is 2.8 times the Fe blend ratio (5 vol%) used in the sample according to graph G1. Furthermore, the gradient of line Ld1 is determined by graph G1. From these relationships, the line Lu2 that defines the upper limit concentration of region A2 can be considered to pass through point g (0, 0.56) and have a slope 2.8 times that of line Ld1, i.e., 2.8 times -14X / 60 (= -39X / 60). This line is expressed by the above equation (4) with X as a variable. In this case, the line expressed by equation (4) passes through point h (0.6, 0.17).

[0112] Although the embodiments of the present disclosure have been described above, the technology according to the present disclosure should not be limited to the above embodiments. For example, in the above embodiments, a cermet of Ni and YSZ was exemplified as the main component of the fuel electrode layer. However, a ceria-based oxide (e.g., gadolinia-doped ceria (GDC)) can be used instead of YSZ. As shown in FIG. 7 or FIG. 8, the Fe concentration distribution in the thickness direction of the fuel electrode layer decreases with increasing thickness position, fluctuating between high and low values ​​within a predetermined range. In this case, even if the Fe concentration falls outside region A1 or region A2 within the predetermined range, the Fe concentration distribution can be considered to be within region A1 or region A2 as long as the median value of the range does not fall outside region A1 or region A2. Furthermore, the cell stack configurations shown in the above embodiments are merely examples, and various configurations can be adopted. In this way, the technology according to the present disclosure can be modified without departing from the spirit and scope of the present disclosure.

[0113] Furthermore, the present disclosure may include the following aspects. [1] a solid electrolyte layer; a fuel electrode layer disposed on one side of the solid electrolyte layer; an air electrode layer disposed on the other side of the solid electrolyte layer; An electrochemical cell comprising: the anode layer contains Ni and Fe, when a position in the thickness direction of the anode layer is represented by a ratio X of a distance from a boundary between the anode layer and the solid electrolyte layer to the thickness of the anode layer, the concentration of Fe present at a position where the ratio X is 0 or more and 0.6 or less is (-14X / 60+0.14) wt% or more and (-56X / 60+0.8) wt% or less. Electrochemical cell. [2] [1] The electrochemical cell according to [1], the anode layer comprises a first layer and a second layer having different porosities; the porosity of the first layer is less than the porosity of the second layer; the first layer and the second layer are stacked in this order on the one surface side of the solid electrolyte layer, An electrochemical cell wherein the concentration of Fe present in the first layer is greater than the concentration of Fe present in the second layer. [3] [2] The electrochemical cell according to [2], the first layer has a first surface that forms an interface with the solid electrolyte layer; the second layer has a second surface that interfaces with the first layer; An electrochemical cell, wherein the concentration of Fe in the second surface is 0.7 times or more and 1.0 times or less than the concentration of Fe in the first surface. [4] [1] The electrochemical cell according to any one of [1] to [3], An electrochemical cell, wherein the concentration of iron present at a position where the ratio X is equal to or greater than 0 and equal to or less than 0.6 is equal to or greater than (-28X / 60+0.28) wt% and equal to or less than (-39X / 60+0.56) wt%. [5] A solid oxide electrolysis cell comprising the electrochemical cell according to any one of [1] to [4]. [6] A cell stack formed by stacking the solid oxide electrolysis cells according to [5]. [7] [6] The cell stack according to [6], a vaporizer that generates water vapor to be supplied to the cell stack; a heat exchanger that exchanges heat with the gas supplied to the cell stack; a heater for heating the cell stack; a heat insulating material in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed; Equipped with Hot module. [8] A hydrogen production device comprising the hot module according to [7]. [Explanation of symbols]

[0114] 1...hydrogen production device, 10...hot module, 20...cell stack, 21...solid oxide electrolysis cell (electrochemical cell), 211...solid electrolyte layer, 211A...rear surface (one side), 211B...upper surface (other side), 212...air electrode layer, 213...fuel electrode layer, 213a...functional layer (first layer), 213b...support layer (second layer), 30...evaporator, 40...heat exchanger, 50...heater, 60...insulating material, 90...condenser, S11...first upper surface (first surface), S12...first rear surface, S21...second upper surface (second surface), S22...second rear surface

Claims

1. a solid electrolyte layer; a fuel electrode layer disposed on one side of the solid electrolyte layer; an air electrode layer disposed on the other side of the solid electrolyte layer; An electrochemical cell comprising: the anode layer contains Ni and Fe, when a position in the thickness direction of the anode layer is represented by a ratio X of a distance from a boundary between the anode layer and the solid electrolyte layer to the thickness of the anode layer, the concentration of Fe present at a position where the ratio X is 0 or more and 0.6 or less is (-14X / 60+0.14) wt % or more and (-56X / 60+0.8) wt % or less; Electrochemical cell.

2. 10. The electrochemical cell of claim 1, the anode layer comprises a first layer and a second layer having different porosities; the porosity of the first layer is less than the porosity of the second layer; the first layer and the second layer are stacked in this order on the one surface side of the solid electrolyte layer, The concentration of Fe in the first layer is higher than the concentration of Fe in the second layer. Electrochemical cell.

3. 3. The electrochemical cell of claim 2, the first layer has a first surface that forms an interface with the solid electrolyte layer; the second layer has a second surface that interfaces with the first layer; the concentration of Fe present on the second surface is 0.7 times or more and 1.0 times or less than the concentration of Fe present on the first surface; Electrochemical cell.

4. 4. The electrochemical cell according to claim 1, wherein the concentration of Fe present at a position where the ratio X is 0 or more and 0.6 or less is (-28X / 60+0.28) wt% or more and (-39X / 60+0.56) wt% or less; Electrochemical cell.

5. A solid oxide electrolysis cell comprising the electrochemical cell according to claim 1.

6. A cell stack formed by stacking the solid oxide electrolysis cells according to claim 5 .

7. The cell stack according to claim 6; a vaporizer that generates water vapor to be supplied to the cell stack; a heat exchanger that exchanges heat with the gas supplied to the cell stack; a heater for heating the cell stack; a heat insulating material in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed; Equipped with Hot module.

8. A hydrogen production device comprising the hot module according to claim 7.

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