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

Abstract

According to the present invention, an electrolysis cell 21 that serves as an electrochemical cell comprises: a solid electrolyte layer 211; a fuel electrode layer 213 which is superposed on the rear surface 211A side of the solid electrolyte layer 211 and contains Ni and Fe; and an air electrode layer 212 which is superposed on the upper surface 211B side of the solid electrolyte layer 211. The fuel electrode layer 213 is composed of a first layer 213F and a second layer 213S. The first layer 213F and the second layer 213S are constituted in the order of the first layer 213F and the second layer 213S from the side close to the rear surface 211A of the solid electrolyte layer 211. The concentration of Fe contained in the first layer 213F is 0.10 wt% or more and 0.80 wt% or less, and the concentration of Fe contained in the second layer 213S is less than 0.10 wt%.

Description

Electrochemical cells, solid oxide electrolysis cells, cell stacks, hot modules, and hydrogen production devices

[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.

[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.

[0003] Japanese Patent Application Laid-Open No. 2019-8914

[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 a cathode 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 fuel electrode layer is operated for a long time under a severe operating environment (high temperature, high humidity, high current), Ni migration and aggregation occur within the fuel electrode layer. When Ni migrates or aggregates within 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 the electrolyte (YSZ)) serving as a 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) containing Ni and Fe 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) is composed of a first layer (213F) and a second layer (213S). The first layer (213F) and the second layer (213S) are configured in this order from the side closest to the one surface (211A) of the solid electrolyte layer (211). The concentration of Fe contained in the first layer (213F) is 0.10 wt % or more and 0.80 wt % or less, and the concentration of Fe contained in the second layer (213S) is less than 0.10 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 suppresses the migration and aggregation of Ni. However, if the Fe concentration is too high, the densification of the interior 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 Fe concentration is too low, the effect of adding Fe in suppressing the migration and aggregation of Ni is reduced, and durability is not improved. When the Fe concentration is 0.10 wt% or more and 0.80 wt% or less, durability can be improved while suppressing the deterioration of the initial performance of the electrochemical cell.

[0009] Furthermore, since the electrochemical reaction occurring in the fuel electrode layer is likely to occur in a region close to the solid electrolyte layer, durability can be improved by preventing the migration and aggregation of Ni present in the region of the fuel electrode layer close to the solid electrolyte layer.

[0010] In the electrochemical cell according to the present disclosure, the fuel electrode layer is composed of a first layer and a second layer, and the Fe concentration in the first layer, which is the layer closest to the solid electrolyte layer, is 0.10 wt% or more and 0.80 wt% or less. Therefore, the high Fe content in the first layer effectively suppresses the migration and aggregation of Ni in the first layer, which is closer to the solid electrolyte layer. As a result, the durability of the electrochemical cell can be improved while suppressing a decrease in initial performance. Furthermore, in the electrochemical cell according to the present disclosure, the Fe concentration in the second layer, which is the layer farther from the solid electrolyte layer than the first and second layers constituting the fuel electrode layer, is low, at less than 0.10 wt%. Therefore, a high Fe concentration in the second layer can suppress densification inside the fuel electrode layer, which can lead to deterioration in gas diffusivity and consequent decrease in initial performance.

[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] The first and second layers constituting the anode layer according to the present disclosure may be composed of the same components except for the Fe concentration. Furthermore, by analyzing the components in the formed anode layer, it is possible to distinguish between a first layer having an Fe concentration of 0.10 wt% or more and 0.80 wt% or less and a second layer having an Fe concentration of less than 0.10 wt%. It is assumed that elements with concentrations lower than the detection limit (e.g., 0.001 wt%) of the device used for the component analysis in the anode layer are not contained. Therefore, if Fe is detected in the second layer, the Fe concentration in the second layer is equal to or greater than 0.001 wt% (detection limit) and less than 0.10 wt%.

[0014] The fuel electrode layer (213) may also include a functional layer (213a) laminated on one surface (211A) of the solid electrolyte layer (211) and a support layer (213b) laminated on the back surface (the surface opposite the solid electrolyte layer) of the functional layer (213a). The functional layer (213a) is a layer where the reaction that occurs mainly in the fuel electrode layer (213) takes place, and the support layer (213b) is a layer that mainly supports the electrochemical cell and forms a path for supplying gases necessary for the reaction that occurs in the functional layer (213a). In this case, the functional layer (213a) constitutes a part of the first layer (213F), and the support layer (213b) constitutes the other part of the first layer (213F) and the second layer (213S). Therefore, the boundary position between the functional layer (213a) and the support layer (213b) is located within the first layer (213F), and the boundary position between the first layer (213F) and the second layer (213S) is located within the support layer (213b).

[0015] Furthermore, the concentration of Fe contained in the first layer does not have to be uniform within the first layer. The concentration of Fe contained in the first layer may form a predetermined concentration distribution along the thickness direction of the fuel electrode layer, with the lower limit concentration of the concentration distribution being 0.10 wt% or more and the upper limit concentration being 0.80 wt% or less. Similarly, the concentration of Fe contained in the second layer does not have to be uniform within the second layer. The concentration of Fe contained in the second layer may form a predetermined concentration distribution along the thickness direction of the fuel electrode layer, with the upper limit concentration of the concentration distribution being less than 0.10 wt%.

[0016] In one embodiment of the electrochemical cell according to the present disclosure, the sum (S=T1+T2) of the thickness (T1) of the first layer (213F) and the thickness (T2) of the second layer (213S) is 489 μm or less.

[0017] According to the above configuration, by setting the thickness of the fuel electrode layer (the sum of the thickness of the first layer and the thickness of the second layer) to 489 μm or less, deterioration of gas diffusivity within the fuel electrode layer is suppressed, thereby further suppressing deterioration in the initial performance of the electrochemical cell.

[0018] In another aspect of the electrochemical cell according to the present disclosure, the ratio (R=T1 / T2) of the thickness (T1) of the first layer (213F) to the thickness (T2) of the second layer (213S) is 0.39 or more and 1.32 or less.

[0019] According to the above configuration, the first layer is not too thick relative to the second layer (i.e., the ratio R is 1.32 or less), which prevents sintering-induced densification due to the large number of regions with high Fe concentration. This further prevents a decrease in initial performance due to deterioration in gas diffusivity caused by densification of the fuel electrode layer, while further improving the durability of the electrochemical cell. Furthermore, the first layer is not too thin relative to the second layer (i.e., the ratio R is 0.39 or more), which prevents a decrease in Ni reactivity due to an overall decrease in Fe content. This further prevents a decrease in initial performance and suppresses Ni migration and aggregation, thereby further improving the durability of the electrochemical cell. Therefore, by keeping the ratio R within the above range, it is possible to further prevent a decrease in initial performance of the electrochemical cell and further improve durability.

[0020] Moreover, the solid oxide electrolysis cell (21) according to the present disclosure is composed of the electrochemical cell according to the present disclosure.

[0021] According to the above configuration, it is possible to provide a solid oxide electrolysis cell with improved durability.

[0022] 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.

[0023] According to the above configuration, a cell stack with improved durability can be provided.

[0024] The hot module (10) according to the present disclosure also 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) for heating the cell stack (20), and a thermal insulator (60) within which the cell stack (20), the vaporizer (30), the heat exchanger (40), and the heater (50) are disposed.

[0025] According to the above configuration, it is possible to provide a hot module with improved durability.

[0026] The hydrogen production device (1) according to the present disclosure includes the hot module (10) according to the present disclosure.

[0027] According to the above configuration, it is possible to provide a hydrogen production device with improved durability.

[0028] 1 is a block diagram of a hydrogen production device according to an embodiment. FIG. 2 is a perspective view of a cell stack of a solid oxide electrolysis cell (SOEC). FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. FIG. 4 is a cross-sectional view of an electrolysis cell in the thickness direction. FIG. 5 is a diagram showing an appropriate concentration range of Fe in a fuel electrode layer. FIG. 6 is a graph showing an example of the Fe concentration distribution in the thickness direction of the fuel electrode layer. FIG. 7 is a graph showing another example of the Fe concentration distribution in the thickness direction of the fuel electrode layer. FIG. 8 is a graph showing yet another example of the Fe concentration distribution in the thickness direction of the fuel electrode layer.

[0029] 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.

[0030] 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 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.

[0031] The vaporizer 30 contains water (H ２ 0) is supplied to the vaporizer 30. The vaporizer 30 heats 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.

[0032] In addition to the water vapor, air is introduced into the heat exchanger 40. The heat exchanger 40 also receives high-temperature hydrogen (H ) generated in the cell stack 20 (described later). ２ ) and high-temperature oxygen (O ２ ) is introduced into the heat exchanger 40. The high-temperature gas exchanges heat with the steam and air in the heat exchanger 40, thereby heating the steam and air introduced from the vaporizer 30.

[0033] 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.

[0034] 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 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, before being 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, hydrogen separated by condensing the water vapor in the condenser 90 is recovered. Note that a portion of the hydrogen separated in the condenser 90 may be introduced into the vaporizer 30 together with water as hydrogen for reduction. Furthermore, the oxygen produced in the cell stack 20 is introduced into the heat exchanger 40 to heat the water vapor and air, and then introduced into the vaporizer 30 to heat the water to be supplied to the vaporizer 30. The oxygen discharged from the vaporizer 30 is then recovered (or released to the atmosphere).

[0035] 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.

[0036] 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.

[0037] 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 an electrolysis cell 21), an interconnector 22, a separator 23, a cathode frame 24, an anode frame 25, and a current collector 26.

[0038] The electrolysis cell 21 is the smallest unit of the 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.

[0039] 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.

[0040] The separator 23 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in its center. The periphery of the opening of the separator 23 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 211 of the 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.

[0041] 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.

[0042] The fuel electrode frame 25 is a rectangular plate-shaped metal (e.g., stainless steel) member having a rectangular opening at its center, and is disposed between the separator 23 and the interconnector 22 below it.

[0043] 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.

[0044] The current collector 26 is a rectangular porous member made of metal (for example, nickel) that is smaller than the anode 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 be in contact with the lower surface of the anode 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.

[0045] 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."

[0046] The path Pfi is formed near one corner of the 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 the side E2 that faces the side E1 (the corner located diagonally from 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 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.

[0047] 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.

[0048] 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 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 electrode layer 212 side (air atmosphere) and the atmosphere on the anode layer 213 side (reducing atmosphere).

[0049] 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 with pores inside.

[0050] 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 213 a and a support layer 213 b. The support layer 213 b is formed to be significantly thicker than the functional layer 213 a, and the thickness ratio can be set to, for example, approximately 16 to 40 times.

[0051] The fuel electrode layer 213 is laminated on the back surface 211A (one side), which is the bottom surface in Fig. 4 of the solid electrolyte layer 211. 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 (bottom surface in Fig. 4) of the functional layer 213a. In other words, 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 this order.

[0052] 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, thereby ensuring water vapor permeability (gas diffusibility). The main component of the functional layer 213a is also 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 to be denser than the support layer 213b. In other words, the functional layer 213a and the support layer 213b are formed so that the porosity of the functional layer 213a is smaller than the porosity of the support layer 213b. Note that, as described below, 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, similar to the solid electrolyte layer 211 and the air electrode layer 212.

[0053] 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 a horizontal hole (not shown). The reason for supplying high-temperature air to the air chamber Sa is to control the temperature of the cell stack 20.

[0054] 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 recovered by a well-known method. At this time, unreacted water vapor can be discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 211 to the air 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 recovered (or released to the atmosphere) by a well-known method. Electrons emitted from the functional layer of the air electrode layer 212 are collected by the current collecting portion 22a of the interconnector 22 via the current collecting layer, and circulate from the end plate 27 to the end plate 28 via the external power source.

[0055] The cell stack 20 operates as described above, and hydrogen is produced in the hydrogen production device 1 .

[0056] 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 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) 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 deteriorate the fuel electrode layer 213.

[0057] 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 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 a decrease in initial performance. This will be explained below.

[0058] FIG. 5 is a diagram showing the appropriate concentration range of Fe in the anode layer 213. In FIG. 5, the horizontal axis represents the position in the thickness direction of the anode layer 213 (hereinafter referred to as the thickness position). This thickness position is 0 at the position in the thickness direction of the anode layer 213 where the anode layer 213 contacts the rear surface 211A (one side) of the solid electrolyte layer 211, i.e., the position within the contact interface between the anode layer 213 and the solid electrolyte layer 211, and the thickness position increases as the position moves away from the solid electrolyte layer 211 in the thickness direction. Therefore, the thickness position represents the width (thickness) from that position to the solid electrolyte layer 211. Also, in FIG. 5, the vertical axis represents the mass concentration of Fe relative to all elements present at the thickness position determined by the horizontal axis, and all present elements are Ni, Fe, Zr, Y, and O.

[0059] In Figure 5, the appropriate Fe concentration range in the anode layer 213 is the range indicated by region A1 and region A2. Region A1 indicates the appropriate Fe concentration range present at a thickness position between 0 and P, and region A2 indicates the appropriate Fe concentration range present at a thickness position greater than P. Here, a "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 each thickness position 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 a line corresponding to each thickness position represented on the cut surface can be used as the Fe concentration present at that thickness position.

[0060] The Fe concentration ranges indicated by regions A1 and A2 represent the appropriate Fe concentration ranges present at each thickness position. As can be seen from region A1, the appropriate Fe concentration range present at each thickness position from 0 to P is 0.10 wt% or more and 0.80 wt% or less. Also, as can be seen from region A2, the appropriate Fe concentration range present at each thickness position above P is less than 0.10 wt%. Therefore, the fuel electrode layer 213 is composed of a first layer 213F, which is a layer from 0 to P in thickness position and has an Fe concentration of 0.10 wt% or more and 0.80 wt% or less, and a second layer 213S, which is a layer from a thickness position above P in thickness position and has an Fe concentration less than 0.10 wt%.

[0061] Furthermore, first layer 213F is a layer adjacent to back surface 211A of solid electrolyte layer 211, and second layer 213S is a layer spaced apart from solid electrolyte layer 211 and adjacent to first layer 213F. Therefore, first layer 213F and second layer 213S are configured adjacent to each other in this order from the side closer to back surface 211A of solid electrolyte layer 211.

[0062] The thickness position X in Figure 5 is the boundary position between the functional layer 213a and the support layer 213b of the fuel electrode layer 213. The thickness position X is smaller than the thickness position P. Therefore, the functional layer 213a is included in the first layer 213F. The first layer 213F is composed of the entire functional layer 213a and the portion of the support layer 213b that is close to the functional layer 213a, and the second layer 213S is composed of the portion of the support layer 213b that is far from the functional layer 213a.

[0063] Because the first layer 213F is a layer including the functional layer 213a, the water electrolysis reaction mainly occurs within the first layer 213F. Therefore, by suppressing the migration and aggregation of Ni within the first layer 213F, the durability of the electrolysis cell 21 can be improved. In this regard, when the first layer 213F contains Fe at a concentration of 0.10 wt % or more, the migration and aggregation of Ni within the first layer is effectively suppressed.

[0064] Furthermore, because Fe also acts as a sintering aid, if the Fe concentration in the fuel electrode layer exceeds 0.80 wt %, the fuel electrode layer will become more dense during molding (sintering), which will deteriorate the gas diffusivity and thereby reduce the initial performance of the electrolysis cell.

[0065] From the above, when the concentration range of Fe in the first layer 213F is 0.10 wt % or more and 0.80 wt % or less, the durability of the electrolysis cell 21 can be improved while suppressing a decrease in the initial performance.

[0066] The inventors investigated the relationship between the Fe concentration in the fuel electrode layer and the durability and initial performance of the cell for the test piece cells, and obtained the findings shown in Table 1 below. As can be seen from Table 1 below, when the Fe concentration in the fuel electrode layer is greater than 0.05 wt% and less than 1.2 wt%, the durability and initial performance of the cell are high. This also shows that when the Fe concentration in the fuel electrode layer is 0.1 wt% or more and 0.8 wt% or less, the durability can be improved while suppressing a decrease in the initial performance of the cell.

[0067] The second layer 213S is composed of a portion of the support layer 213b that is far from the functional layer 213a. This second layer 213S must have good gas diffusivity because it serves as a path for supplying gas (water vapor) to the functional layer 213a in the first layer 213F. In this regard, if the Fe concentration in the second layer 213S is less than 0.10 wt%, there is almost no densification due to Fe during molding (sintering). This further suppresses deterioration of gas diffusivity.

[0068] In order for the concentration of Fe contained in the anode layer 213 to be within the ranges shown in regions A1 and A2 in Fig. 5, the concentration of Fe present at each thickness position in the anode layer 213 must be within the ranges shown in regions A1 and A2 in Fig. 5. Therefore, the concentration distribution of Fe along the thickness direction of the anode layer 213 must shift within the ranges of regions A1 and A2 in Fig. 5.

[0069] Furthermore, the boundary position P between the first layer 213F and the second layer 213S is not a predetermined fixed position, but varies depending on the concentration distribution of Fe. In other words, the boundary position P is a position determined by the concentration distribution of Fe.

[0070] Fig. 6 shows an example of a graph of the Fe concentration distribution in the thickness direction of the anode layer 213. The horizontal axis of Fig. 6 represents the thickness position of the anode layer 213, and the vertical axis represents the Fe concentration. In this example, the thickness of the anode layer 213 is 402 µm. Therefore, the maximum value at the thickness position is 402 µm.

[0071] As shown in graph G1 in Figure 6, the Fe concentration at the thickness position 0 of the anode layer 213 is highest, at 0.48 wt%. Furthermore, as the thickness position increases, i.e., as the distance from the solid electrolyte layer 211 increases, the Fe concentration gradually decreases, fluctuating between high and low values ​​within a predetermined range. At position P, where the thickness position is 165 μm, the Fe concentration decreases to 0.10 wt%, and at thickness positions greater than this, the Fe concentration is less than 0.10 wt%.

[0072] 6 , the Fe concentration distribution shown by graph G1 transitions between region A1 and region A2. Therefore, the fuel electrode layer 213 having the Fe concentration distribution shown by graph G1 is composed of a first layer 213F and a second layer 213S. The boundary position P between the first layer 213F and the second layer 213S is 165 μm, and the thickness of the first layer 213F is 165 μm. The thickness of the second layer 213S is the length obtained by subtracting the value of the boundary position P (165 μm) from the thickness of the fuel electrode layer 213 (402 μm), i.e., 237 μm.

[0073] 7 shows another example of a graph of the Fe concentration distribution in the thickness direction of the fuel electrode layer 213. In this example, the thickness of the fuel electrode layer 213 is 485 μm. Therefore, the maximum value at the thickness position is 485 μm.

[0074] As shown in graph G2 of Figure 7, the Fe concentration at the thickness position of 0 is the highest, at 0.80 wt%. Furthermore, as the thickness position increases, i.e., as the distance from the solid electrolyte layer 211 increases, the Fe concentration gradually decreases, fluctuating between high and low values ​​within a predetermined range. The Fe concentration then decreases to 0.10 wt% at position P, where the thickness position is 331 μm, and is less than 0.10 wt% at thickness positions greater than this.

[0075] 7 , the Fe concentration distribution shown by graph G2 shifts between region A1 and region A2. Therefore, the fuel electrode layer 213 having the Fe concentration distribution shown by graph G2 is composed of a first layer 213F and a second layer 213S. The boundary position P between the first layer 213F and the second layer 213S is 331 μm, and the thickness of the first layer 213F is 331 μm. The thickness of the second layer 213S is the length obtained by subtracting the value at boundary position P (331 μm) from the thickness of the fuel electrode layer 213 (485 μm), i.e., 154 μm.

[0076] 8 shows yet another example of a graph of the Fe concentration distribution in the thickness direction of the fuel electrode layer 213. In this example, the thickness of the fuel electrode layer 213 is 454 μm. Therefore, the maximum value at the thickness position is 454 μm.

[0077] As shown in graph G3 of Figure 8, the Fe concentration at the thickness position of 0 is the highest, at 0.23 wt%. Furthermore, as the thickness position increases, i.e., as the distance from the solid electrolyte layer 211 increases, the Fe concentration gradually decreases, fluctuating between high and low values ​​within a predetermined range. The Fe concentration then decreases to 0.10 wt% at position P, where the thickness position is 115 μm, and is less than 0.10 wt% at thickness positions greater than this.

[0078] 8 , the Fe concentration distribution shown by graph G3 shifts between region A1 and region A2. Therefore, the fuel electrode layer 213 having the Fe concentration distribution shown by graph G3 is composed of a first layer 213F and a second layer 213S. The boundary position P between the first layer 213F and the second layer 213S is 115 μm, and the thickness of the first layer 213F is 115 μm. The thickness of the second layer 213S is the length obtained by subtracting the value of the boundary position P (115 μm) from the thickness of the fuel electrode layer 213 (454 μm), i.e., 339 μm.

[0079] Thus, the anode layer 213 having the Fe concentration distributions shown by graphs G1, G2, and G3 all comprises a first layer 213F having an Fe concentration of 0.10 wt % or more and 0.80 wt % or less and a second layer 213S having an Fe concentration of less than 0.10 wt %, with the first layer 213F and second layer 213S arranged in this order from the side closest to the solid electrolyte layer 211. An electrolysis cell 21 having an anode layer 213 configured in this manner exhibits high durability while suppressing deterioration in initial performance.

[0080] 6 to 8 , the thickness of the fuel electrode layer 213 having the Fe concentration distribution shown in Fig. 6 to 8 , i.e., the sum S (= T1 + T2) of the thickness T1 of the first layer 213F and the thickness T2 of the second layer 213S, is 402 µm, 489 µm, and 454 µm, respectively. If the sum S exceeds 489 µm, the gas diffusibility within the fuel electrode layer 213 deteriorates, resulting in a decrease in the initial performance of the electrolysis cell. Therefore, it is preferable that the sum S be 489 µm or less.

[0081] When measuring the concentration of Fe present at each thickness position of the fuel electrode layer 213, a cross section of the fuel electrode layer 213 cut along its thickness direction can be analyzed by an EPMA (Electron Probe Micro Analyzer), and the average value of the Fe concentrations at multiple points on a line corresponding to each thickness position can be determined as the concentration of Fe present at that thickness position.

[0082] Furthermore, when determining the boundary position P, the Fe concentration is calculated based on the above-described EPMA analysis at predetermined small intervals (e.g., every 1.0 μm) from the thicker side to the thinner side of the anode layer 213. If the calculated Fe concentration exceeds 0.10 wt % twice consecutively, the first of the two consecutive exceeded positions can be determined as the boundary position P between the first layer 213F and the second layer 213S. Then, a region closer to the solid electrolyte layer 211 than the determined boundary position P can be determined as the first layer 213F, and a region farther from the solid electrolyte layer than the determined boundary position P can be determined as the second layer 213S.

[0083] Graphs G1, G2, and G3 in Figures 6 to 8 show a right-shoulder-sloping concentration distribution in which the Fe concentration decreases with increasing thickness position. When the Fe concentration distribution has such a right-shoulder-sloping concentration distribution, the concentration distribution may move through regions A1 and A2. However, even if the concentration distribution has a right-shoulder-sloping concentration distribution, if the maximum value of the Fe concentration is less than 0.10 wt% or the minimum value of the Fe concentration is greater than 0.10 wt%, the concentration distribution will not move through regions A1 and A2. Therefore, a fuel electrode layer having such a concentration distribution is not composed of the first layer 213F and the second layer 213S.

[0084] 6 to 8, the Fe concentration gradually decreases with increasing thickness position, fluctuating between high and low values ​​within a predetermined range. Therefore, within the predetermined range, the Fe concentration distribution may be slightly below the lower limit of region A1 or slightly above the upper limit of region A2. Even in such a case, if the line connecting the centers of the fluctuation range passes through regions A1 and A2, the fuel electrode layer having that concentration distribution can be recognized as consisting of first layer 213F and second layer 213S.

[0085] (Example) 1. Preparation of sample NiO powder and Fe ２ O ３The powder was mixed in a predetermined compounding 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 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 the mixture was 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.

[0086] 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.

[0087] A butyral resin, a 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.

[0088] 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.

[0089] The laminate formed as described above was then degreased at a predetermined temperature (e.g., 200 to 300°C). The laminate was then fired at a predetermined first temperature (e.g., 1300 to 1400°C) for a predetermined time (e.g., 1 to 5 hours) (primary firing). This resulted in a primary sintered body having a solid electrolyte layer and a fuel electrode layer stacked on one side of the solid electrolyte layer.

[0090] Subsequently, a material containing LSCF was screen-printed onto the other surface 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 firing). This produced a sample electrolysis cell including a solid electrolyte layer, an anode layer laminated on one surface of the solid electrolyte layer, and an cathode layer laminated on the other surface of the solid electrolyte layer.

[0091] In addition, when forming the green sheet of the functional layer of the fuel electrode layer, NiO powder and Fe ２ O ３ Fe relative to the total volume of the powder ２ O ３ By varying the powder ratio (mixing ratio) in the range of 0 to 30 vol%, a plurality of electrolytic cell samples with different Fe mixing ratios in the fuel electrode layer were produced. In addition, by varying the thickness of the green sheet when forming the green sheet for the support layer of the fuel electrode layer, a plurality of electrolytic cell samples with different fuel electrode layer thicknesses were produced. ２ O ３ In the sample with a powder blending ratio of 0 vol%, Fe was not added during the molding of the green sheet for the functional layer of the fuel electrode layer. ２ O ３The amount of pore-forming material was adjusted during molding of 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.

[0092] 2. Measurement of Thickness of Fuel Electrode Layer Each sample was cut along the thickness direction of the fuel electrode layer (thickness direction of the cell), and the cut surface was photographed using a SEM (Scanning Electron Microscope). The thickness of the fuel electrode layer was measured based on the obtained SEM image.

[0093] 3. Measurement of Fe Concentration in the Fuel Electrode Layer Each sample was cut along a plane along the thickness direction of the fuel electrode layer (cell thickness direction), 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 elemental mapping image obtained by the area analysis. In this case, the Fe concentration at each thickness position was calculated at 1.0 μm intervals from the maximum thickness position of the fuel electrode layer (the position farthest from the solid electrolyte layer) toward the direction of decreasing thickness position. To calculate the Fe concentration at each thickness position, the average of the Fe concentrations detected at multiple points on the line corresponding to that thickness position was calculated, and the calculated average Fe concentration was used as the Fe concentration at that thickness position.

[0094] 4. Calculation of the Thickness of Layer A and Layer B The Fe contained in the green sheet of the functional layer of the fuel electrode layer diffuses from the functional layer side to the support layer side during sintering. Therefore, Fe is present not only in the functional layer of the fuel electrode layer but also in the support layer. However, the Fe concentration in the fuel electrode layer tends to decrease from the thinner position (the functional layer side) to the thicker position (the support layer side). Taking advantage of this tendency, the fuel electrode layer was divided into Layer A, which has an Fe concentration of 0.10 wt% or more, and Layer B, which has an Fe concentration of less than 0.10 wt%. Layer B corresponds to the second layer shown in Figure 5. Layer A, which has an Fe concentration of 0.80 wt% or less, corresponds to the first layer shown in Figure 5.

[0095] To divide the fuel electrode layer into layer A and layer B, the Fe concentration was calculated from the maximum thickness position toward the decreasing thickness position. In this case, the Fe concentration gradually increases, fluctuating within a predetermined range, as the thickness position decreases. When the Fe concentration first exceeded 0.10 wt% continuously for 2 μm or more, the first position where it exceeded 0.10 wt% was determined as boundary position P. The region with a thickness position smaller than the determined boundary position P (i.e., the region closer to the solid electrolyte layer than boundary position P) was determined as layer A, and the region with a thickness position larger than boundary position P (i.e., the region farther from the solid electrolyte layer than boundary position P) was determined as layer B. The thicknesses of layer A and layer B thus determined were measured. The thickness of layer A is the thickness corresponding to boundary position P, and the thickness of layer B is the thickness of the fuel electrode layer minus the thickness corresponding to boundary position P.

[0096] 5. Measurement of Initial Performance and Durability Degradation Rate The current per unit area (current density) flowing when a constant voltage (1 to 1.3 V) was applied between the anode layer and the cathode 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. Furthermore, a voltage was applied between the anode layer and the cathode layer of each sample so that a constant current flowed through each sample. The applied voltage increased 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 the calculated value was converted to the difference between the voltage that would be applied after 1000 hours and the initially applied voltage. The converted value was then divided by the initially applied voltage to calculate the durability degradation rate as a percentage. It can be said that the smaller the durability degradation rate, the higher the durability.

[0097] 6. Measurement Results Table 2 shows the initial performance (current density) measured for each sample, its evaluation, the durability deterioration rate, its evaluation, and an overall evaluation.

[0098] In Table 2, T1 is the thickness of layer A [μm], T2 is the thickness of layer B [μm], T1+T2 is the thickness of the fuel electrode layer [μm], T1 / T2 is the ratio of the thickness of layer A to the thickness of layer B, and the maximum Fe concentration is the maximum value of the Fe concentration in layer A.

[0099] In Table 2, when the durability degradation rate was less than 2.4% / kh, the durability degradation rate was evaluated as very good (◎), when the durability degradation rate was 2.4% / kh or more and less than 3.4% / kh, the durability degradation rate was evaluated as good (◯), and when the durability degradation rate was 3.4% / kh or more, the durability degradation rate was evaluated as poor (×). ２ When the current density was 1.30 mA / cm or more, the initial performance was evaluated as very good (◎). ２ or more and 1.40 mA / cm ２ When the current density is less than 1.00 mA / cm, the initial performance is evaluated as good (◯). ２ or more and 1.30 mA / cm ２ When the current density is less than 1.00 mA / cm, the initial performance is evaluated as fair (Δ). ２ If the evaluation result of the durability degradation rate was less than 1 / 2, the initial performance was evaluated as poor (×). If the evaluation result of the durability degradation rate was very good (◎) and the evaluation result of the initial performance was also very good (◎), the overall evaluation was judged to be very good (◎). If the evaluation result of the durability degradation rate and the evaluation result of the initial performance were both good (◯), or if the evaluation result of the durability degradation rate was very good (◎) and the evaluation result of the initial performance was fair (△) or good (◯), the overall evaluation was judged to be good (◯). 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.

[0100] As can be seen from Table 2, the overall evaluation of the samples according to Examples 1 to 7 is very good (◎) or good (◯). On the other hand, the overall evaluation of the samples according to Comparative Examples 1 to 5 is all poor (×). This shows that the samples according to Examples 1 to 7 have improved durability while suppressing the deterioration of initial performance.

[0101] The samples according to Examples 1 to 7 consisted of layers A and B, with the maximum Fe concentration in layer A being 0.80 wt% or less. Therefore, layer A in the samples according to Examples 1 to 7 corresponds to the first layer shown in FIG. 5 . As described above, layer B corresponds to the second layer shown in FIG. 5 . Therefore, the fuel electrode layers of the samples according to Examples 1 to 7 consisted of a first layer having an Fe concentration of 0.10 wt% or more and 0.80 wt% or less, and a second layer having an Fe concentration of less than 0.10 wt%, with the first and second layers arranged in this order from the side closest to the solid electrolyte layer. The Fe concentration distribution shown in FIG. 6 corresponds to the Fe concentration distribution in the fuel electrode layer of the sample according to Example 2, the Fe concentration distribution shown in FIG. 7 corresponds to the Fe concentration distribution in the fuel electrode layer of the sample according to Example 6, and the Fe concentration distribution shown in FIG. 8 corresponds to the Fe concentration distribution in the fuel electrode layer of the sample according to Example 5.

[0102] Furthermore, the fuel electrode layers of the samples according to Comparative Examples 1, 2, and 3 do not contain any layer with an Fe concentration of 0.10 wt % or more, and only contain layer B with an Fe concentration of 0.10 wt % or less. Therefore, the fuel electrode layers of the samples according to Comparative Examples 1, 2, and 3 are composed only of the second layer, and do not contain the first layer.

[0103] The fuel electrode layer of the sample according to Comparative Example 4 does not include a B layer (second layer) with an Fe concentration of 0.10 wt% or less, but only an A layer with an Fe concentration of 0.10 wt% or more. Furthermore, in the sample according to Comparative Example 4, the maximum Fe concentration in the A layer is 0.25 wt%, making this A layer the first layer. Therefore, the fuel electrode layer of the sample according to Comparative Example 4 is composed only of the first layer, and does not include a second layer.

[0104] The fuel electrode layer of the sample according to Comparative Example 5 is composed of Layer A and Layer B. However, the maximum Fe concentration in Layer A is 1.10 wt%. Therefore, Layer A of Comparative Example 5 is not a layer in which the Fe concentration is 0.10 wt% or more and 0.80 wt% or less. Therefore, the fuel electrode layer of the sample according to Comparative Example 5 does not have a first layer.

[0105] From the above, as in the samples of Examples 1 to 7, when the fuel electrode layer is composed of a layer (first layer) having an Fe concentration of 0.10 wt % or more and 0.80 wt % or less and a layer (second layer) having an Fe concentration of less than 0.10 wt %, and the first layer and second layer are arranged from the side closest to the solid electrolyte, it is possible to improve durability while suppressing deterioration in the initial performance of the electrolytic cell.

[0106] The initial performance of the samples according to Examples 1 to 6 was good (◯) or very good (◎). On the other hand, the initial performance of the sample according to Example 7 was average (△). The thicknesses of the fuel electrode layers of the samples according to Examples 1 to 6 were 489 μm or less, whereas the thickness of the fuel electrode layer of the sample according to Example 7 was 560 μm, exceeding 489 μm. This suggests that when the thickness of the fuel electrode layer, i.e., the sum S (= T1 + T2) of the thickness (T1) of Layer A (first layer) and the thickness (T2) of Layer B (second layer), is 489 μm or less, the deterioration of initial performance can be further suppressed. The reason for this is that when the fuel electrode layer is not too thick, the deterioration of gas diffusivity within the fuel electrode layer can be further suppressed.

[0107] Furthermore, the durability and initial performance of the samples according to Examples 1 to 3 were both very good (◎). On the other hand, the durability and initial performance of the samples according to Examples 4 to 6 were both good (◯). Here, for the samples according to Examples 1 to 3, the ratio R (=T1 / T2) of the thickness T1 of the A layer (first layer) to the thickness T2 of the B layer (second layer) was 0.39 or more and 1.32 or less. In contrast, for the samples according to Examples 4 and 5, the ratio R was less than 0.39, and for the sample according to Example 6, the ratio R was greater than 1.32. Therefore, when the ratio R was 0.39 or more and 1.32 or less, the deterioration in the initial performance of the electrolysis cell could be further suppressed and durability could be further improved. This is because the thickness of the first layer, which has a higher Fe concentration than the second layer, was appropriate (neither too thick nor too thin), making the Fe concentration in the fuel electrode layer more appropriate for initial performance and durability, thereby enhancing both the effect of suppressing the deterioration in initial performance and the effect of improving durability. In the sample of Example 7, although the ratio R was 0.40, the thick fuel electrode layer (more than 489 μm) deteriorated gas diffusibility, which reduced the effect of suppressing the decline in initial performance. As a result, it is presumed that the initial performance was average (△).

[0108] 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 is exemplified as the main component of the fuel electrode layer, but a ceria-based oxide (e.g., GDC (gadolinia-doped ceria)) can be exemplified instead of YSZ. Furthermore, the cell stack configuration shown in the above embodiment is merely an 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 spirit thereof.

[0109] The present disclosure may further include the following aspects. [1] An electrochemical cell comprising: a solid electrolyte layer; an anode layer containing Ni and Fe laminated on one side of the solid electrolyte layer; and an air cathode layer laminated on the other side of the solid electrolyte layer, wherein the anode layer comprises a first layer and a second layer, the first layer and the second layer being configured in this order from the side closest to the one side of the solid electrolyte layer, the first layer having an Fe concentration of 0.10 wt % or more and 0.80 wt % or less, and the second layer having an Fe concentration of less than 0.10 wt %. [2] The electrochemical cell according to [1], wherein the sum of the thickness of the first layer and the thickness of the second layer is 489 μm or less. [3] The electrochemical cell according to [1] or [2], wherein a ratio of a thickness of the first layer to a thickness of the second layer is 0.39 or more and 1.32 or less. [4] A solid oxide electrolysis cell comprising the electrochemical cell according to any one of [1] to [3]. [5] A cell stack formed by stacking the solid oxide electrolysis cells according to [4]. [6] A hot module comprising: the cell stack according to [5] 5; a vaporizer that generates water vapor to be supplied to the cell stack; a heat exchanger that exchanges heat with a gas to be supplied to the cell stack; a heater for heating the cell stack; and a thermal insulator in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed. [7] A hydrogen production device comprising the hot module according to [6].

[0110] 1... Hydrogen production device, 10... Hot module, 20... Cell stack, 21... Solid oxide electrolysis cell (electrochemical cell), 211... Solid electrolyte layer, 211A... Back surface (one side), 211B... Top surface (other side), 212... Air electrode layer, 213... Fuel electrode layer, 213F... First layer, 213S... Second layer, 213a... Functional layer, 213b... Support layer, 30... Vaporizer, 40... Heat exchanger, 50... Heater, 60... Insulating material, 90... Condenser

Claims

1. An electrochemical cell comprising: a solid electrolyte layer; an anode layer containing Ni and Fe laminated on one side of the solid electrolyte layer; and an air cathode layer laminated on the other side of the solid electrolyte layer, wherein the anode layer comprises a first layer and a second layer, the first layer and the second layer being configured in this order from the side closest to the one side of the solid electrolyte layer; the concentration of Fe contained in the first layer is 0.10 wt% or more and 0.80 wt% or less; and the concentration of Fe contained in the second layer is less than 0.10 wt%.

2. An electrochemical cell according to claim 1, wherein the sum of the thickness of said first layer and the thickness of said second layer is 489 μm or less.

3. An electrochemical cell according to claim 1 or 2, wherein the ratio of the thickness of said first layer to the thickness of said second layer is 0.39 or more and 1.32 or less.

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

5. A cell stack comprising a stack of solid oxide electrolysis cells according to claim 4.

6. A hot module comprising: a cell stack according to claim 5; a vaporizer that generates steam to be supplied to the cell stack; a heat exchanger that exchanges heat with gas supplied to the cell stack; a heater that heats the cell stack; and a heat insulating material in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed.

7. A hydrogen production device comprising the hot module according to claim 6.

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