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

Abstract

To improve the durability of electrochemical cells. SOLUTION: An electrolysis cell 21 as an electrochemical cell includes a solid electrolyte layer 211, a fuel electrode layer 213 laminated on a back surface 211A side of the solid electrolyte layer 211, and an air electrode layer 212 laminated on an upper surface 211B side 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 a distance from a 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 0 or more and 0.6 or less is (-14X / 60+0.14) wt% or more and (-56X / 60+0.8) wt% or less.

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] Conventionally, solid oxide electrochemical cells using a solid oxide as an electrolyte have been known (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] JP 2019-8914 A 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 include 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 fuel electrode layer is operated for a long time under a severe operating environment (high temperature, high humidity, high current), Ni migration and aggregation occurs in the fuel electrode layer. When Ni migrates or aggregates in the fuel electrode layer, the internal resistance of the fuel electrode layer increases, and the three-phase interface (the boundary between the fuel gas, Ni, and electrolyte (YSZ)) as a reaction field decreases. This reduces the performance of the electrochemical cell, making it impossible to obtain the desired performance. Therefore, there is a demand 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 of the objects of the present disclosure is to improve the durability of an electrochemical cell.

[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 an air electrode 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 a distance from a 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 0 or more and 0.6 or less is (-14X / 60+0.14) wt% or more and (-56X / 60+0.8) wt% or less.

[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 suppresses the movement and aggregation of Ni. However, if the concentration of Fe is too high, the densification of the inside of the fuel electrode layer is promoted during molding (sintering) of the fuel electrode layer, which deteriorates the diffusibility of gas passing through the fuel electrode layer and reduces the initial performance of the electrochemical cell. Furthermore, if the concentration of Fe is too low, the effect of suppressing the movement and aggregation of Ni by adding Fe decreases, and durability does not improve sufficiently.

[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 ranging 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 deterioration in the initial performance of the electrochemical cell.

[0010] In the present disclosure, the Fe concentration range is defined in the region where the ratio X, which represents the thickness direction position 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 that contains pores. Therefore, the boundary can be determined by the presence or absence of pores.

[0013] In the above, the Fe concentration is defined when the ratio X is in the range of 0 to 0.6, but it is not necessary that the Fe concentration be within the above range for all ratios where the ratio X is in the range of 0 to 0.6. It is sufficient that the Fe concentration be within the above range for multiple ratios where the ratio X is in 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), which divide the range of the ratio X from 0 to 0.6 into 10 equal parts.

[0014] 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) comprises a first layer (213a) and a second layer (213b) having different porosities, the porosity of the first layer (213a) is smaller than the porosity of the second layer (213b), the first layer (213a) and the second layer (213b) are stacked in this order on one surface (211A) of the solid electrolyte layer (211), and the concentration of Fe in the first layer (213a) is higher than the concentration of Fe in the second layer (213b).

[0016] The reaction in the fuel electrode layer is likely to occur in a region of the fuel electrode layer close to the solid electrolyte layer. Therefore, by suppressing the movement and aggregation of Ni present in the region of the fuel electrode layer 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 fuel electrode layer includes a first layer close to the solid electrolyte layer and a second layer far from the solid electrolyte layer, and is configured so that the concentration of Fe in the first layer is higher than the concentration of Fe in the second layer. Therefore, the movement and aggregation of Ni in the first layer close to the solid electrolyte layer is sufficiently suppressed by the larger amount of Fe present in the first layer, so that the durability of the electrochemical cell can be further improved.

[0017] The Fe concentration in the first layer is preferably not the Fe concentration at any one point in the first layer but the average value of the Fe concentrations at multiple different positions in the thickness direction of the first layer. Similarly, the Fe concentration in the second layer is preferably not the Fe concentration at any one point in the second layer but the average value of the Fe concentrations at multiple different positions in the thickness direction of 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 or more and 1.0 or less times the concentration of Fe present on the first surface (S11).

[0020] The Fe in the second layer has the effect of blocking the diffusion of Fe in the first layer into the second layer. In particular, when the concentration of Fe present in the second surface of the second layer that forms an interface with the first layer is 0.7 times or more the concentration of Fe present in the first surface of the first layer that forms an interface with the solid electrolyte layer, the effect of blocking the diffusion of Fe in the first layer into the second layer is high. This makes it possible to prevent a decrease in Fe in the first layer and, as a result, a decrease in durability due to the movement and aggregation of Ni in the first layer.

[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, the thickness direction positions (or ratio X) of the identified first and second surfaces are obtained, and the Fe concentrations at the obtained thickness direction positions (or ratio X) are compared, thereby making it possible to determine whether the Fe concentration present on the second surface is 0.7 times or more and 1.0 times or less than 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] A solid oxide electrolysis cell (21) according to the present disclosure comprises an 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, it is possible to provide a cell stack with improved durability.

[0028] A hot module (10) according to the present disclosure includes a cell stack (20) according to the present disclosure, a vaporizer (30) that generates water vapor to be supplied to the cell stack (20), a heat exchanger (40) that exchanges heat with a gas supplied to the cell stack (20), a heater (50) that heats the cell stack (20), and a thermal insulation 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 description of the drawings]

[0032] [Figure 1] 1 is a block diagram of a hydrogen production device according to an embodiment of the present invention. [Diagram 2]FIG. 1 is a perspective view of a cell stack of a solid oxide electrolysis cell (SOEC). [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 2 is a cross-sectional view of an electrolysis cell in the thickness direction. [Diagram 5] FIG. 13 is a diagram showing an appropriate concentration range of Fe in a fuel electrode layer. [Figure 6] FIG. 13 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] FIG. 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. [Figure 9] FIG. 11 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 deterioration test was performed on the sample according to Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Hereinafter, an embodiment 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 constituting the hydrogen production 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 insulating material 60.

[0035] Water (H2O) is supplied to the vaporizer 30 as shown in Fig. 1. The vaporizer 30 heats the supplied water to a temperature of 100°C or higher by a heating source (not shown). Therefore, the water supplied to the vaporizer 30 evaporates within the vaporizer 30 to generate 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. Then, in the heat exchanger 40, these high-temperature gases exchange heat with the water vapor and air, whereby the water vapor and air introduced from the vaporizer 30 are heated in the heat exchanger 40.

[0037] The water vapor and air heated in the heat exchanger 40 are further heated by the heater 50 to the operating temperature of the cell stack 20 (i.e., the temperature required to operate the cell stack 20). Thereafter, the water vapor and air are 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 heating source (burner, etc.) not shown. A predetermined voltage is applied to the cell stack 20. As a result, the water vapor introduced into the cell stack 20 is electrolyzed to generate hydrogen and oxygen. The hydrogen generated in the cell stack 20 is introduced into the heat exchanger 40 together with unreacted water vapor, and 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 generated in the condenser 90 is introduced into the vaporizer 30. Meanwhile, the hydrogen separated by condensing the water vapor in the condenser 90 is collected. The oxygen generated in the cell stack 20 is introduced into the heat exchanger 40, and 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, the vaporizer 30, the heat exchanger 40, and the heater 50 are disposed inside the heat insulating 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 heat insulating material 60. The heat-resistant fibers are disposed so as to fill gaps between the cell stack 20, the vaporizer 30, the heat exchanger 40, and the 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 includes an electrolysis unit group including a plurality of rectangular flat-plate-shaped electrolysis units Ue stacked in the thickness direction (up-down direction), and a pair of end plates 27, 28 disposed on the upper and lower surfaces of the electrolysis unit group. The end plates 27, 28 are rectangular flat-plate-shaped members having the same outer shape as the electrolysis units Ue, and have rectangular openings formed in their centers. The electrolysis unit group and the end plates 27, 28 are fastened to each other at their four corners by bolts B that are 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 ratio of each member in the drawings may differ from the actual ratio.

[0041] The electrolysis unit Ue will be described with reference to Fig. 3. As shown in Fig. 3, the electrolysis unit Ue includes 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 an outer shape smaller 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 and has a rectangular current collecting part 22a that protrudes downward from the center of its lower surface. A pair of interconnectors 22 are arranged on both sides in the thickness direction of the electrolysis cell 21. 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. A lower surface of the current collecting part 22a is in contact with an 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 the oxygen generated in the air electrode layer 212 by electrolysis of water vapor and the hydrogen generated in the fuel electrode layer 213 from mixing.

[0045] The air electrode 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 air electrode frame 24. The air electrode frame 24 is disposed so as to be interposed 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 having a rectangular opening at its center. The fuel electrode frame 25 is disposed so as to be interposed between the separator 23 and the interconnector 22 below it.

[0047] 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 upper interconnector 22, 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 lower interconnector 22, the separator 23, the fuel electrode frame 25, and the 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 a plan view and allows hydrogen to pass through. The current collector 26 is arranged in the fuel chamber Sf so as to contact 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, whereby the multiple electrolysis cells 21 are electrically connected 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 in the thickness direction through members of the cell stack 20 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 a side E1, which is one of the four sides constituting the outer periphery of the cell stack 20. The path Pfo is formed near the other corner of a side E2 opposite to the side E1 (the corner diagonally opposite to the one corner of the side E1). As shown in Fig. 3, the path Pfi communicates with the fuel chamber Sf via a horizontal hole 25a formed in the fuel electrode frame 25 of each electrolysis unit Ue. The path Pfo communicates with the fuel chamber Sf via a horizontal hole 25b formed in the fuel electrode 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 electrode layer 212, and a fuel electrode layer 213. In this embodiment, the solid electrolyte layer 211 is a rectangular flat layer having a size of 150 mm square and a thickness of 6 μm, is configured to contain YSZ (yttria-stabilized zirconia), and is 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 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 from each other.

[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, 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 in the current collecting layer. The air electrode layer 212 is a porous layer and has pores inside.

[0054] The fuel electrode layer 213 is a rectangular flat layer measuring 150 mm square, and is formed to have a thickness greater than those of the solid electrolyte layer 211 and the air electrode layer 212, for example, about 400 μm. The solid electrolyte layer 211 and the air electrode layer 212 are supported by the fuel electrode layer 213. In other words, the electrolysis cell 21 is an anode-supported cell. The fuel electrode 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 ratio can be set to, for example, about 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 lower 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 lower surface in FIG. 4) of the functional layer 213a. That is, the functional layer 213a and the support layer 213b are laminated on the back surface 211A (one side) of the solid electrolyte layer 211 in the order of the functional layer 213a and the support layer 213b. Therefore, the functional layer 213a of the fuel electrode layer 213 has a first upper surface S11 (first surface) that contacts 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 main component of the support layer 213b is 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 the permeability (gas diffusibility) of water vapor. The main component of the functional layer 213a is also a cermet of Ni and YSZ. The functional layer 213a is also a porous layer configured to have a porous shape including a plurality of micropores (not shown) like the support layer 213b. The functional layer 213a is formed to be denser than the support layer 213b. That is, the porosity of the functional layer 213a is smaller than the porosity of the support layer 213b. Note that, as described later, the fuel electrode layer 213 contains a trace amount of Fe. The fuel electrode layer 213 having the functional layer 213a and the support layer 213b is also formed by sintering like 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 water vapor is supplied from the path Pfi. The water vapor supplied to the path Pfi flows into the fuel chamber Sf of each electrolysis unit Ue through the horizontal hole 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 through a horizontal hole (not shown). The high-temperature air is supplied to the air chamber Sa in order to control the temperature of the cell stack 20.

[0058] The water vapor that has flowed into the fuel chamber Sf passes through the support layer 213b of the fuel electrode layer 213 and moves 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 in the fuel chamber Sf, is discharged from the path Pfo through the horizontal hole 25b, and is collected by a well-known method. At this time, unreacted water vapor can be discharged from the path Pfo together with the hydrogen. Meanwhile, the oxide ions move to the air electrode layer 212 in the air chamber Sa via the solid electrolyte layer 211, and release electrons in the functional layer of the air electrode layer 212 to become oxygen. The oxygen diffuses in the air chamber Sa, and is discharged from the path Pao through a horizontal hole (not shown) together with the air that has flowed into the air chamber Sa, 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 source.

[0059] The cell stack 20 operates as described above, whereby 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 moves and aggregates within the fuel electrode layer 213. When Ni moves and aggregates within the fuel electrode layer 213, the concentration of Ni 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) as a reaction field decreases. This reduces the reaction efficiency of the water electrolysis reaction, and the performance of the electrolysis cell 21 decreases. Furthermore, when Ni moves and aggregates within the fuel electrode layer 213, the conductive path within the fuel electrode layer 213 decreases, and the internal resistance increases. Due to such a decrease in reaction efficiency and increase in internal resistance, the fuel electrode layer 213 deteriorates.

[0061] It is known that the progress of deterioration of the fuel electrode layer 213 can be suppressed by including an Fe (iron) component in the fuel electrode layer 213. However, since 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, and the internal pores are reduced. This leads to deterioration of gas diffusibility and a decrease in initial performance. The fuel electrode layer 213 according to this embodiment is configured to include an appropriate concentration of Fe in the fuel electrode layer 213 so as to improve durability while suppressing the decrease in initial performance. This will be described below.

[0062] FIG. 5 is a diagram showing an appropriate concentration range of Fe in the fuel electrode layer 213. In FIG. 5, the horizontal axis is the ratio X, which indicates the position in the thickness direction of the fuel electrode 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 fuel electrode layer 213 and the solid electrolyte layer 211 to that position, relative to the thickness of the fuel electrode layer 213. In other words, the ratio X is the normalized thickness position of the fuel electrode layer 213. For example, when the thickness of the fuel electrode layer 213 is 400 μm, the thickness position where the ratio X is 0.1 is a position that is moved 40 μm in the thickness direction from the surface of the fuel electrode 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 fuel electrode 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 FIG. 5, the vertical axis represents the mass concentration of Fe with respect 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 concentration range of Fe in the fuel electrode layer 213 is the range indicated by region A1. This region A1 is a region where the ratio X is 0 or more and 0.6 or less. 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 at the position where the ratio X is greater than 0.6. As described above, Fe is also a sintering aid, so if a large amount of Fe is present at the position where the ratio X is greater than 0.6, gas diffusibility deteriorates, leading to a decrease in initial performance. Therefore, it is better that Fe is not present at the position where the ratio X is greater than 0.6, or even if it is present, the concentration is low. For example, it is better that the concentration of Fe is 0.10 wt% or less at the position where the ratio X is greater than 0.6.

[0064] The Fe concentration range indicated by region A1 represents an 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. Here, the "thickness position" of the fuel electrode layer 213 is a position in a plane perpendicular to the thickness direction of the fuel electrode layer 213 corresponding to the 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 the thickness position. For example, the fuel electrode layer 213 can be cut on 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 taken as the Fe concentration present at that thickness position.

[0065] The Fe concentration range indicated by region A1 narrows as ratio X increases. Specifically, when ratio X is 0, the Fe concentration range indicated by region A1 is in the range of 0.14 wt% or more and 0.80 wt% or less. As ratio X increases, the width of the Fe concentration range indicated by region A1 decreases. And, when ratio X is 0.6, the Fe concentration range indicated by region A1 is in the range of 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. The straight line Ld1 is a straight line that passes through points a (0.00, 0.14) and b (0.60, 0.00) when the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component.

[0067] Furthermore, the upper limit concentration of the Fe concentration range indicated by region A1 (hereinafter, may be 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 manner, 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 Fe upper limit concentration shown in the above formula (2) is represented by a straight line Lu1 in Fig. 5. The straight line Lu1 is a straight line that passes through points c (0, 0.80) and d (0.6, 0.24) when the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component.

[0068] Therefore, when the ratio X is in the range of 0 to 0.6, the appropriate concentration [wt%] of Fe 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 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] In addition, since Fe is also a sintering aid, if the content is too high, it promotes sintering of the fuel electrode layer 213, which leads to densification, making it impossible to obtain the desired porosity and causing a decrease in initial performance. Therefore, when the concentration of Fe present at the thickness position represented by the ratio X is within region C above region A1 in Fig. 5, the initial performance decreases.

[0071] Therefore, when the Fe concentration at the thickness position represented by the ratio X is within the concentration range represented by region A1 in FIG. 5, i.e., when the Fe concentration distribution in the thickness direction of the fuel electrode layer 213 transitions 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 takes place is the region close to the solid electrolyte layer 211, i.e., the region where the ratio X is small, that is, the region in the functional layer 213a. In order to effectively suppress the movement 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, it is preferable that the Fe concentration distribution 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 is preferably not the Fe concentration at any one point in the functional layer 213a but the average Fe concentration at multiple thickness positions in the functional layer 213a. Similarly, the Fe concentration in the support layer 213b is preferably not the Fe concentration at any one point in the support layer 213b but the average Fe concentration at multiple thickness positions in the support layer 213b.

[0075] In addition, when the concentration of Fe contained in the support layer 213b is extremely low compared to the concentration of Fe contained in the functional layer 213a, the Fe in the functional layer 213a may diffuse in large amounts to the support layer 213b due to the large concentration difference between the Fe concentration in the functional layer 213a and the Fe concentration in the support layer 213b. When the Fe in the functional layer 213a diffuses in large amounts to the support layer 213b, the Fe concentration in the functional layer 213a may be significantly reduced, and the movement and aggregation of Ni in the functional layer 213a may not be sufficiently suppressed. Therefore, it is preferable that the support layer 213b also contains Fe at a predetermined concentration or more. When the support layer 213b contains Fe at a predetermined concentration or more, the concentration difference between the Fe concentration in the functional layer 213a and the Fe concentration in the support layer 213b becomes small, and the Fe in the functional layer 213a is less likely to diffuse to the support layer 213b. In other words, 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 is preferably 0.7 times or more and 1.0 times or less than the concentration N0. If the concentration Na is within the above range, it is possible to sufficiently suppress the diffusion of a large amount of Fe in the functional layer 213a 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 is the ratio X representing the thickness position of the fuel electrode layer 213, and the vertical axis is 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 a part of the region within 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 or more and 0.6 or less. 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% or more and 0.56 wt% or less. As ratio X increases, the width of the Fe concentration range indicated by region A2 decreases. And, when ratio X is 0.6, the Fe concentration range indicated by region A2 is in the range of 0.00 wt% or more and 0.17 wt% or less.

[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 as the following formula (3) using the ratio X. 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. The straight line Ld2 is a straight line that passes through points e (0, 0.28) and f (0.6, 0.00) when the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component.

[0080] Furthermore, the upper limit concentration of the Fe concentration range indicated by region A2 (hereinafter, may be 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 manner, 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 formula (4) using the ratio X. Fe upper limit concentration [wt%]=(-39X / 60)+0.56 (4) The second upper limit Fe concentration shown in the above formula (4) is represented by a straight line Lu2 in Fig. 6. The straight line Lu2 is a straight line that passes through points g (0, 0.56) and h (0.6, 0.17) when the ratio X is represented as the X-coordinate component and the Fe concentration is represented as the Y-coordinate component.

[0081] Therefore, when the ratio X is in the range of 0 to 0.6, a more appropriate concentration [wt%] of Fe 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, i.e., when the Fe concentration distribution in the thickness direction of the fuel electrode layer 213 transitions within region A2 in FIG. 6, it is possible to further improve durability while further suppressing the deterioration of the initial performance of the electrolysis cell 21.

[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 poured into a bowl. The mixture was dried by volatilizing the alcohol in the bowl and formed into a powder. The powder was calcined at about 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 a pore-forming agent (typically organic beads) as necessary were added to the mixed powder in a predetermined ratio, and mixed in a ball mill to prepare a slurry. Then, a green sheet of the functional layer of the fuel electrode layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0084] In addition, 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 a pore-forming agent (typically organic beads) as necessary were added to the mixed powder in a predetermined ratio and mixed in a ball mill to prepare a slurry. Then, a green sheet of a support layer for a 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 a predetermined ratio, and mixed in a ball mill to prepare a slurry. A doctor blade method was used to mold the slurry into a green sheet of a solid electrolyte layer having a predetermined thickness.

[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 laminated in this order on one side of the green sheet for the solid electrolyte layer. These laminated green sheets were then pressed together under high pressure while being heated and evacuated using a press. 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] Thereafter, the laminate formed as described above was degreased at a predetermined temperature (e.g., 200 to 300°C). Next, the laminate was sintered at a predetermined first temperature (e.g., 1300 to 1400°C) for a predetermined time (e.g., 1 to 5 hours) (primary sintering). This resulted in forming a primary sintered body having a solid electrolyte layer and a fuel electrode layer laminated on one side of the solid electrolyte layer.

[0088] Next, a material containing LSCF was screen-printed on the other side of the solid electrolyte layer of the molded primary sintered body, and then fired at a predetermined second temperature (e.g., 900 to 1000°C) for a predetermined time (e.g., 1 to 5 hours) (secondary sintering). In this way, a sample electrolysis cell was produced that included 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.

[0089] In addition, when forming the green sheet for the functional layer of the fuel electrode layer, the ratio (mixture ratio) of Fe2O3 powder to the total volume of NiO powder and Fe2O3 powder was changed in the range of 0 to 30 vol% to prepare a number of electrolysis cell samples with different Fe mixture ratios in the fuel electrode layer. In the sample with a mix ratio of Fe2O3 powder of 0 vol% (sample according to Comparative Example 1 described later), no Fe2O3 powder was used when forming the green sheet for the functional layer of the fuel electrode layer. In addition, the amount of pore former, 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 per unit area (current density) flowing when a constant voltage (1 to 1.3 V) was applied between the fuel electrode layer and the air electrode layer of each sample was measured. The measured current density was used as an evaluation index for initial performance. It can be said that the higher the current density, the better the initial performance. In addition, a voltage was applied between the fuel electrode layer and the air electrode layer of each sample so that a constant current flows through each sample. Then, a durability degradation test was performed in which a constant current was continued to flow for a predetermined time. When a constant current is continued to flow in the durability degradation test, the applied voltage increases over time due to an increase in internal resistance in the sample. The difference ΔV (= V1 - V0) between the voltage applied initially (V0) and the voltage applied after a predetermined time (e.g., 400 hours) was calculated, and the calculated value was converted into the difference between the voltage that would be applied after 1000 hours and the voltage applied initially. The converted value was divided by the voltage applied initially to calculate a percentage as the durability degradation rate. It can be said that the smaller the durability degradation rate, the higher the 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 judgment. [Table 1]

[0092] In Table 1, the Fe compounding 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 of the functional layer of the fuel electrode layer in the production of each sample. In addition, the compounding ratio of the total amount (amount of metal powder) of NiO powder and Fe2O3 powder used when forming the green sheet of the functional layer of the fuel electrode layer in each sample is the same for all samples. In addition, the production conditions for all samples are the same except for the Fe compounding ratio described above.

[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 is evaluated as good (〇). 2 If the durability degradation rate was less than 2.0%, 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 and 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 (×). From this, it can be seen that the durability of the samples produced 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 according to Examples 1, 2, and 3 were cut along a surface along the thickness direction of the fuel electrode layer (cell thickness direction), and the cut surface was subjected to surface analysis using an EPMA device (manufactured by JEOL). The Fe concentration distribution in the thickness direction of the fuel electrode layer was calculated from an element mapping image obtained by surface analysis. In this case, the Fe concentration at each thickness position was calculated at intervals of 1.0 μm. In calculating the Fe concentration at each thickness position, the average value of the Fe concentrations detected at multiple points on the line corresponding to that thickness position was calculated, and the calculated average value of the Fe concentrations was taken 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 with the region A1 in FIG. 5. In FIG. 7, the horizontal axis is the ratio X, and the vertical axis is the Fe concentration. FIG. 7 also shows a line Ld1 that defines the lower limit concentration of the region A1 in FIG. 5, and a line Lu1 that defines the upper limit concentration of the region A1. In FIG. 7, a graph G1 is a graph showing the measurement results of the Fe concentration for the sample according to Example 1, a graph G2 is a graph showing the measurement results of the Fe concentration for the sample according to Example 2, and a 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 shift in the region between the lines Ld1 and Lu1, that is, in the region A1 in Fig. 5, when the ratio X is in the range of 0 to 0.6. 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] Moreover, 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 blend ratio is 2.5 vol%, which is smaller than the Fe blend 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 a line Ld1 that defines the lower limit concentration of region A1. From this, it is predicted that the concentration distribution of Fe contained in the sample according to Comparative Example 2 moves 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 larger 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 moves in the vicinity of the line Lu1 that defines the upper limit concentration of region A1, and the concentration of Fe contained in the sample according to Example 3 coincides with the line Lu1, particularly at the position where the ratio X = 0. Therefore, the concentration distribution of Fe contained in the sample according to Comparative Example 3 is predicted to move within region C above region A1, particularly in the region where the ratio X is small.

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

[0101] Moreover, in all of graphs G1, G2, and G3, it can be seen that as ratio X increases, the Fe concentration tends to decrease. Specifically, in the Fe concentration distribution shown in each graph, when ratio X is between 0 and approximately 0.05, the Fe concentration drops sharply as ratio X increases. Moreover, from the position where the sharp drop in the Fe concentration ends (the position where ratio X is around 0.05), as ratio X increases, the Fe concentration gradually drops while repeatedly rising and falling within a predetermined range.

[0102] In each of the 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 the support layer of the fuel electrode layer. In addition, as can be seen from each of the graphs G1, G2, and G3, the Fe contained in the fuel electrode layer of the samples according to Example 1, Example 2, and Example 3 exists across the boundary position (ratio Xa), so it can be seen that Fe is contained in both the functional layer and the support layer of the fuel electrode layer. Here, when the samples according to each example are produced, Fe is contained only in the green sheet of the functional layer of the fuel electrode layer. However, in the molded sample, Fe is 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 diffuses to the support layer side when the sample is molded (sintered).

[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). In addition, at the position where the ratio X is Xa, i.e., the contact interface position between the functional layer of the fuel electrode layer and the support layer (i.e., the second upper surface S21 of the support layer 213b), the Fe concentration (concentration Na) is about 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 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 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 is a diagram showing the relationship between the graphs G1, G2, and G3 and the region A2 shown in FIG. 6. In FIG. 8, the horizontal axis is the ratio X, and the vertical axis is the concentration of Fe. FIG. 8 also shows a straight line Ld2 that defines the lower limit concentration of the region A2 in FIG. 6 and a straight line Lu2 that defines the upper limit concentration of the region A2. As shown in FIG. 8, the graph G2 moves within the region A2. In contrast, the graphs G1 and G3 have parts that extend beyond the region A2, so these graphs do not move within the region A2. And, as can be seen from Table 1, the overall evaluation of the sample according to Example 2 is very good (◎). From this, when the concentration of Fe contained in the fuel electrode layer is within the range shown by the region A2 shown in FIG. 6 and FIG. 8, the deterioration of the initial performance of the electrolysis cell can be further suppressed and the durability can be further improved.

[0106] Fig. 9 is a diagram showing a graph G4 representing 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 in Fig. 9, a graph (graph G2) representing the measurement results of the Fe concentration measured before the durability test was performed is shown by a dashed line.

[0107] As shown in FIG. 9, according to the graph G4, the concentration distribution (concentration distribution indicated by the arrow P) in which the concentration of Fe drops sharply when the ratio X is from 0 to Xa, which was shown in the graph G2, has disappeared. This is considered to be because Fe in the functional layer diffuses to the support layer side by operating the sample according to the embodiment 2 for a long time. However, even after the durability deterioration test, the concentration of Fe present at the position where the ratio X=0 (the concentration of Fe at the position where the ratio X=0 in the graph G4) is about 0.7 times the concentration of Fe present at the position where the ratio X=0 before the durability deterioration test (the concentration of Fe at the position where the ratio X=0 in the graph G2), and the concentration distribution of Fe shown in the graph G4 is shifted within the region A2. It is considered that such a tendency is similar even when the durability deterioration test is performed on the samples according to the embodiment 1 and the embodiment 3. Therefore, according to the samples according to the embodiment 1, the embodiment 2, and the embodiment 3, it can be said that the initial performance deterioration can be sufficiently suppressed even after the durability deterioration test, and the durability is also high.

[0108] The area A1 (lines Ld1 and Lu1) shown in FIG. 5 and FIG. 7 are determined as follows. First, the line Ld1 is determined by the graph G1 representing the Fe concentration distribution measured for the sample according to Example 1. Specifically, the Fe concentration at the ratio X=0 in the graph G1 is about 0.2 wt%, and when this sample is used for a long time, it is predicted that the Fe concentration at the ratio X=0 will be about 0.7 times, or about 0.14 wt%. Therefore, the lower limit of the Fe concentration at the position where the ratio X=0 is estimated to be 0.14 wt%. This determines the point a(0,0.14) in FIG. 5. Also, the lower limit of the Fe concentration at the position where the ratio X=0.6 is 0.00 wt%. This determines the point b(0.6,0.00) in FIG. 5. Therefore, the line Ld1 defining the lower limit concentration of the area A1 is the line connecting the point a(0,0.14) and the point b(0.6,0). This straight line is expressed by the above equation (1) with X as a variable.

[0109] Also, the line Lu1 is determined by the graph G3 representing the concentration distribution of Fe measured for the sample according to Example 3. The concentration of Fe at the ratio X=0 in the graph G3 is 0.80 wt%. Therefore, the upper limit of the concentration of Fe at the position where the ratio X=0 is estimated to be 0.80 wt%. This determines the point c(0,0.80) in FIG. 5. The compounding ratio of Fe used when preparing the sample according to Example 3 is 20 vol%, and the compounding ratio of Fe used when preparing the sample according to Example 1 is 5 vol%. That is, the sample according to the graph G3 uses four times the amount of Fe used in the sample according to the graph G1. Therefore, the concentration gradient of Fe in the thickness direction of the sample according to the graph G3 is predicted to be about four times the concentration gradient of Fe in the thickness direction of the sample according to the graph G1. And, since the line Ld1 is determined by the graph G1, the gradient of the line Lu1 determined by the graph G3 is considered to be about four times the gradient of the 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] In addition, the area A2 (lines Ld2 and Lu2) shown in FIG. 6 and FIG. 8 is determined so that the graph G2 moves within the range between them, and the graphs G1 and G3 do not fit (move) within the range. In this case, first, the lower limit concentration (point e in FIG. 6) of the area A2 when the ratio X=0 is estimated. Here, the Fe concentration at the ratio X=0 in the graph G2 is about 0.48 wt%. However, the Fe compounding ratio (10 vol%) used in the sample related to the graph G2 is twice the Fe compounding ratio (5 vol%) used in the sample related to the graph G1. And, since the Fe concentration at the ratio X=0 in the sample related to the graph G1 is about 0.2 wt%, it is also assumed that the Fe concentration at the ratio X=0 is about 0.4 wt% when the Fe compounding ratio is 10 vol%. If this sample is operated for a long time, the Fe concentration at ratio X=0 may decrease to about 0.28wt%, which is about 0.7 times 0.4wt%. Therefore, the lower limit concentration of region A2 at ratio X=0 is estimated to be 0.28wt%. This determines point e(0,0.28) in FIG. 6. Also, the lower limit concentration of region A2 at ratio X=0.6 is 0.00wt%. This determines point f(0.6,0) in FIG. 6. Therefore, the straight line Ld2 that defines the lower limit concentration of region A2 is the straight line that connects point e(0,0.28) and point f(0.6,0) in FIG. 6. This straight line is expressed by the above formula (3) with X as a variable.

[0111] Furthermore, when the ratio X=0, the lower limit of the Fe concentration of the sample according to the graph G3 is 0.7 times 0.8 wt%, so it is 0.56 wt%. Therefore, when the ratio X=0, the upper limit of the concentration of the area where the graph G3 does not enter is 0.56 wt%. This determines the point g (0, 0.56) in FIG. 6. Furthermore, when the Fe concentration at the position where the ratio X=0 is 0.56 wt%, the Fe blend ratio at the time of manufacture is estimated to be about 14 vol%. This blend ratio is 2.8 times the Fe blend ratio (5 vol%) used in the sample according to the graph G1. Furthermore, the gradient of the straight line Ld1 is determined by the graph G1. From these relationships, the straight 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 gradient 2.8 times that of straight line Ld1, i.e., 2.8 times -14X / 60 (=-39X / 60). This straight line is expressed by the above formula (4) with X as a variable. In this case, the straight line expressed by formula (4) passes through point h (0.6, 0.17).

[0112] Although the embodiment of the present disclosure has been described above, the technology according to the present disclosure should not be limited to the above embodiment. For example, in the above embodiment, the cermet of Ni and YSZ is exemplified as the main component of the fuel electrode layer, but instead of YSZ, a ceria-based oxide (for example, GDC (gadolinia-doped ceria) can be exemplified. Also, as shown in FIG. 7 or FIG. 8, the Fe concentration distribution in the thickness direction of the fuel electrode layer decreases while repeating highs and lows within a predetermined fluctuation range as the thickness position increases. In this case, even if the Fe concentration falls outside the region A1 or region A2 within the predetermined fluctuation range, as long as the median value of the fluctuation range does not fall outside the region A1 or region A2, the Fe concentration distribution can be considered to be moving within the region A1 or region A2. Also, the configuration of the cell stack shown in the above embodiment is one example, and various configurations can be adopted. In this way, the technology according to the present disclosure can be modified as long as it does not deviate from the gist of the technology.

[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 surface side of the solid electrolyte layer; An electrochemical cell comprising: the anode layer contains Ni and Fe, when a position in a thickness direction of the fuel electrode layer is represented by a ratio X of a distance from a boundary between the fuel electrode layer and the solid electrolyte layer to a thickness of the fuel electrode 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] The electrochemical cell according to [1], the anode layer comprises a first layer and a second layer having different porosities; the first layer has a smaller porosity than the second layer; the first layer and the second layer are laminated in this order on the one surface side of the solid electrolyte layer, An electrochemical cell wherein a concentration of Fe present in said first layer is greater than a concentration of Fe present in said second layer. [3] The electrochemical cell according to [2], the first layer has a first surface forming an interface with the solid electrolyte layer; the second layer has a second surface that interfaces with the first surface; 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] to [3], an electrochemical cell according to any one of the above, An electrochemical cell, wherein the concentration of iron 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. [5] A solid oxide electrolysis cell comprising the electrochemical cell according to any one of [1] to [4]. [6] A cell stack comprising the solid oxide electrolysis cells according to [5] stacked together. [7] [6] A cell stack according to the present invention; a vaporizer for generating water vapor to be supplied to the cell stack; a heat exchanger for exchanging heat with the gas supplied to the cell stack; a heater for heating the cell stack; a thermal insulator 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 described in [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...vaporizer, 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 surface side of the solid electrolyte layer; An electrochemical cell comprising: the anode layer contains Ni and Fe; when a position in a thickness direction of the fuel electrode layer is represented by a ratio X of a distance from a boundary between the fuel electrode layer and the solid electrolyte layer to a thickness of the fuel electrode 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. 2. The electrochemical cell of claim 1 , the anode layer comprises a first layer and a second layer having different porosities; the first layer has a smaller porosity than the second layer; the first layer and the second layer are laminated 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 forming an interface with the solid electrolyte layer; the second layer has a second surface that interfaces with the first surface; 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. An electrochemical cell according to any one of claims 1 to 3, 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 comprising the solid oxide electrolysis cells according to claim 5 stacked together.

7. The cell stack according to claim 6 ; a vaporizer for generating water vapor to be supplied to the cell stack; a heat exchanger for exchanging heat with the gas supplied to the cell stack; a heater for heating the cell stack; a thermal insulation 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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