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

Abstract

An electrolysis cell 21 includes: a solid electrolyte layer 211; a fuel electrode layer 213 stacked and arranged on the rear surface 211A side of the solid electrolyte layer 211; and an air electrode layer 212 stacked and arranged on the front surface 211B side of the solid electrolyte layer 211. A mutual diffusion layer 214 in contact with both the solid electrolyte layer 211 and the fuel electrode layer 213 is formed between the solid electrolyte layer 211 and the fuel electrode layer 213. The mutual diffusion layer 214 includes: a first element which is one element constituting the solid electrolyte layer 211; and a second element which is one element constituting the fuel electrode layer 213 and is different from the first element. The thickness T1 of the mutual diffusion layer 214 falls within the range of 1.5 μm or more and 4.8 μm or less.

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 an air electrode layer laminated on the other side of the solid electrolyte layer. The solid electrolyte layer generally contains an ion-conductive oxide (also referred to as an oxide ion conductor). YSZ (yttria-stabilized zirconia) is typically used as the ion-conductive oxide. The fuel electrode layer can be configured to include Ni (nickel) as an electrode catalyst and YSZ as an ion-conductive oxide.

[0005] When an electrochemical cell with a fuel electrode layer containing Ni and YSZ is operated for a long time under a harsh operating environment (high temperature, high humidity, high current), migration and aggregation of Ni occurs in the fuel electrode layer. When Ni in the fuel electrode layer migrates or aggregates, 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.

[0006] When gadolinia-doped ceria (GDC) is used instead of YSZ as the ion-conductive oxide in the anode layer, the migration and aggregation of Ni in the anode layer are suppressed to some extent. However, when the ion-conductive oxide in the solid electrolyte layer is YSZ and the ion-conductive oxide in the anode layer is GDC, the ion-conductive oxides in the two layers are different. In this case, elements constituting the ion-conductive oxide in the solid electrolyte layer diffuse toward the anode layer, and elements constituting the ion-conductive oxide in the anode layer diffuse toward the solid electrolyte layer. This interdiffusion forms an interdiffusion layer between the solid electrolyte layer and the anode layer, which is in contact with both the solid electrolyte layer and the anode layer and contains elements constituting the ion-conductive oxide in the solid electrolyte layer and elements constituting the ion-conductive oxide in the anode layer. For example, if the ion-conductive oxide in the solid electrolyte layer is YSZ and the ion-conductive oxide in the anode layer is GDC, Zr (zirconium) in the YSZ in the solid electrolyte layer diffuses toward the anode layer, and Ce (cerium) in the GDC in the anode layer diffuses toward the solid electrolyte layer, forming an interdiffusion layer containing Zr and Ce between the solid electrolyte layer and the anode layer, which is in contact with both layers.

[0007] The durability of an electrochemical cell is significantly affected by the components in the region near the boundary between the solid electrolyte layer and the fuel electrode layer. The interdiffusion layer, formed between the solid electrolyte layer and the fuel electrode layer and in contact with both, significantly affects the durability of the electrochemical cell. Because the interdiffusion layer is highly resistive and electronically conductive, these properties adversely affect the durability of the electrochemical cell. Therefore, if the interdiffusion layer is too thick, the durability of the electrochemical cell deteriorates. On the other hand, since the interdiffusion layer ensures adhesion between the solid electrolyte layer and the fuel electrode layer, if the interdiffusion layer is too thin, the adhesion deteriorates. Therefore, in electrochemical cells having an interdiffusion layer formed between the solid electrolyte layer and the fuel electrode layer, it is desirable to suppress deterioration in durability while ensuring adhesion of the electrochemical cell.

[0008] The present disclosure aims to solve the above-mentioned problems. That is, one of the objectives of the present disclosure is to suppress deterioration of durability while ensuring adhesion of an electrochemical cell in which an interdiffusion layer is formed between a solid electrolyte layer and a fuel electrode layer.

[0009] The electrochemical cell (21) according to the present disclosure includes a solid electrolyte layer (211), a fuel electrode layer (213) laminated on one side (211A) of the solid electrolyte layer (211), and a cathode layer (212) laminated on the other side (211B) of the solid electrolyte layer (211). An interdiffusion layer (214) is formed between the solid electrolyte layer (211) and the anode layer (213) and is in contact with both the solid electrolyte layer (211) and the anode layer (213). The interdiffusion layer (214) contains a first element that is one of the elements constituting the solid electrolyte layer (211) and a second element that is one of the elements constituting the anode layer (213) and different from the first element. The thickness (T1) of the interdiffusion layer (214) is within a range of 1.5 μm to 4.8 μm. The thickness of the interdiffusion layer is preferably 3.9 μm or less, more preferably 2.9 μm or less, and even more preferably 2.2 μm or less.

[0010] If the thickness of the interdiffusion layer is less than 1.5 μm, adhesion is reduced, and there is a high possibility that the solid electrolyte layer and the fuel electrode layer will peel off. Furthermore, if the thickness of the interdiffusion layer exceeds 4.8 μm, durability deteriorates. In contrast, in the electrochemical cell according to the present disclosure, the thickness of the interdiffusion layer is within the range of 1.5 μm or more and 4.8 μm or less, so that deterioration of durability can be suppressed while ensuring adhesion of the electrochemical cell.

[0011] The first and second elements contained in the interdiffusion layer are different elements. The first and second elements may be elements constituting an ion-conductive oxide. The first element may be an element constituting an ion-conductive oxide in the solid electrolyte layer, and the second element may be an element constituting an ion-conductive oxide in the anode layer. The first element may be a metal element in the ion-conductive oxide in the solid electrolyte layer, and the second element may be a metal element in the ion-conductive oxide in the anode layer. Furthermore, when the anode layer includes an anode functional layer and an anode support layer, i.e., when the electrochemical cell is an anode-supported electrochemical cell, the interdiffusion layer is formed between the solid electrolyte layer and the anode functional layer. In this case, the first element may be a metal element in the ion-conductive oxide in the solid electrolyte layer, and the second element may be a metal element in the ion-conductive oxide in the anode functional layer. Furthermore, the concentration of the first element contained in the interdiffusion layer decreases from the solid electrolyte layer side toward the anode layer side, and the concentration of the second element contained in the interdiffusion layer decreases from the anode layer side toward the solid electrolyte layer side. Therefore, the interdiffusion layer can also be said to be a layer containing a first element and a second element, in which the concentration of the first element decreases from the solid electrolyte layer side toward the fuel electrode layer side, and the concentration of the second element decreases from the fuel electrode layer side toward the solid electrolyte layer side.

[0012] When measuring the thickness of the interdiffusion layer, it is difficult to accurately measure the extent to which the first element and the second element have diffused. Therefore, in the present disclosure, the thickness of the interdiffusion layer is measured as follows. The concentrations of the first element and the second element are measured along the thickness direction of the solid electrolyte layer and the anode layer using an EPMA device. Next, the point where the measured concentrations of the first element and the second element are equal is defined as the midpoint. The thickness of the interdiffusion layer is defined as the distance in the thickness direction between the thickness position (tb) where the concentration of the first element, measured from the midpoint toward the anode layer, first drops to 2.000 wt %, and the thickness position (ta) where the concentration of the second element, measured from the midpoint toward the solid electrolyte layer, first drops to 2.000 wt %.

[0013] In one aspect of the electrochemical cell according to the present disclosure, the inter-diffusion layer (214) has a solid electrolyte-side diffusion layer (214a) that is in contact with the solid electrolyte layer (211) and in which the concentration of a first element is equal to or greater than the concentration of a second element, and the sum of the thickness (T2) of the solid electrolyte layer (211) and the thickness (Ta) of the solid electrolyte-side diffusion layer (214a) is within a range of 2.3 μm or more and 151.0 μm or less.

[0014] If the sum of the thicknesses of the solid electrolyte layer and the solid electrolyte-side diffusion layer is less than 2.3 μm, the gas barrier performance is reduced, resulting in a decrease in the reaction efficiency of the electrochemical cell. Furthermore, if the sum of the thicknesses of the solid electrolyte layer and the solid electrolyte-side diffusion layer exceeds 151.0 μm, the electrical resistance increases, resulting in a decrease in the performance of the electrochemical cell. In contrast, with the above-described configuration, the sum of the thicknesses of the solid electrolyte layer and the solid electrolyte-side diffusion layer is within a range of 2.3 μm or more and 151.0 μm or less, thereby ensuring gas barrier performance while suppressing an increase in electrical resistance. This allows the electrochemical cell to maintain desired performance without reducing its reaction efficiency.

[0015] In another embodiment of the electrochemical cell according to the present disclosure, the ratio (R=T2 / T1) of the thickness (T2) of the solid electrolyte layer (211) to the thickness (T1) of the interdiffusion layer (214) is 0.9 or more.

[0016] Because the interdiffusion layer has electronic conductivity, some of the electrons supplied to the fuel electrode layer may enter the interdiffusion layer without contributing to the reaction in the fuel electrode layer. Furthermore, because a solid electrolyte layer is interposed between the interdiffusion layer and the air electrode layer, the solid electrolyte layer suppresses the movement of electrons that have entered the interdiffusion layer to the air electrode layer, i.e., electron leakage. However, if the ratio of the thickness of the solid electrolyte layer to the thickness of the interdiffusion layer is small, the thickness of the solid electrolyte layer relative to the amount of electrons that enter the interdiffusion layer is small, increasing the probability of electron leakage. In contrast, according to the above configuration, the ratio of the thickness of the solid electrolyte layer to the thickness of the interdiffusion layer is 0.9 or more. That is, the thickness of the solid electrolyte layer is 0.9 times or more the thickness of the interdiffusion layer. This allows the solid electrolyte layer to have a thickness sufficient to sufficiently suppress electron leakage due to the amount of electrons that have entered the interdiffusion layer. Therefore, electron leakage can be effectively suppressed, thereby further suppressing a decrease in the reaction efficiency of the electrolysis cell.

[0017] In yet another embodiment of the electrochemical cell according to the present disclosure, the first element is Zr and the second element is Ce.

[0018] When the combination of the first element and the second element contained in the interdiffusion layer is Zr and Ce, deterioration of the durability of the electrochemical cell can be suppressed compared to interdiffusion layers containing other combinations of the first element and the second element. In this case, the solid electrolyte layer can be configured to contain YSZ, and the anode layer can be configured to contain Ni and GDC. Furthermore, the anode layer can be configured to have an anode functional layer in contact with the interdiffusion layer and an anode support layer that supports the electrochemical cell, and the anode functional layer can be configured to contain Ni and GDC.

[0019] Furthermore, the solid oxide electrolysis cell (21) according to the present disclosure is comprised of the electrochemical cell according to the present disclosure.

[0020] According to the above configuration, it is possible to provide a solid oxide electrolysis cell in which adhesion is ensured and deterioration of durability is suppressed.

[0021] 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.

[0022] According to the above configuration, it is possible to provide a cell stack in which the adhesion of the solid oxide electrolysis cells is ensured while deterioration of durability is suppressed.

[0023] The hot module (10) according to the present disclosure includes a cell stack (20) according to the present disclosure, a vaporizer (30) that generates steam to be supplied to the cell stack (20), a heat exchanger (40) that exchanges heat with gas supplied to the cell stack (20), a heater (50) that heats the cell stack (20), and a heat insulating material (60) in which the cell stack (20), the vaporizer (30), the heat exchanger (40), and the heater (50) are disposed.

[0024] According to the above configuration, it is possible to provide a hot module in which the adhesion of the solid oxide electrolysis cell is ensured and deterioration of durability is suppressed.

[0025] The hydrogen production device (1) according to the present disclosure includes the hot module (10) according to the present disclosure.

[0026] According to the above configuration, it is possible to provide a hydrogen production device in which the adhesion of the solid oxide electrolysis cell is ensured and deterioration of durability is suppressed.

[0027] 3 is a block diagram of a hydrogen production device. FIG. 4 is a perspective view of a cell stack. FIG. 5 is a cross-sectional view taken along line III-III in FIG. 2. FIG. 6 is a cross-sectional view in the thickness direction of an electrolysis cell. FIG. 7 is a schematic diagram showing the formation of an interdiffusion layer. FIG. 8 is a diagram showing the Zr concentration distribution and the Ce concentration distribution with respect to the thickness position calculated for sample 2. FIG. 9 is a diagram showing the Zr concentration distribution and the Ce concentration distribution with respect to the thickness position calculated for sample 4. FIG. 10 is a diagram showing the Zr concentration distribution and the Ce concentration distribution with respect to the thickness position calculated for sample 5.

[0028] 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.

[0029] 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.

[0030] The vaporizer 30 contains water (H ２ 0) is supplied to the vaporizer 30. The vaporizer 30 is configured to heat the supplied water to a temperature of 100°C or higher by a heat source. 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.

[0031] In addition to the water vapor, air is introduced into the heat exchanger 40. The heat exchanger 40 also receives high-temperature hydrogen (H 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.

[0032] 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.

[0033] The cell stack 20 is formed by stacking solid oxide electrolysis cells. The cell stack 20 is heated to an operating temperature by a heat source (such as a burner) not shown. A predetermined voltage is applied to the cell stack 20. As a result, water vapor introduced into the cell stack 20 is electrolyzed to produce hydrogen and oxygen. The hydrogen produced in the cell stack 20 is introduced into the heat exchanger 40 together with unreacted water vapor, where it is used to heat the water vapor and air introduced into the heat exchanger 40 from the vaporizer 30, and then introduced into the condenser 90. The unreacted water vapor is condensed in the condenser 90. The condensed water produced in the condenser 90 is introduced into the vaporizer 30. Meanwhile, hydrogen separated by the condensation of the water vapor in the condenser 90 is recovered. The oxygen produced in the cell stack 20 is introduced into the heat exchanger 40, where it is used to heat the water vapor and air, and then introduced into the vaporizer 30 to heat the water supplied to the vaporizer 30. The oxygen discharged from the vaporizer 30 is then recovered (or released to the atmosphere).

[0034] 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.

[0035] 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 electrolysis units 27, 28 in the thickness direction and nuts (not shown). The end plates 27, 28 are made of metal (for example, stainless steel) and function as an anode and a cathode, respectively, when a voltage is applied. Note that for ease of explanation, the proportions of the components in the drawings may differ from the actual proportions.

[0036] 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.

[0037] The electrolysis cell 21 is the smallest unit of an SOEC and includes a solid electrolyte layer 211, a cathode layer 212, an anode layer 213, and an interdiffusion layer 214. The cathode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact the upper surface of the solid electrolyte layer 211. The cathode layer 212 has a smaller outer shape than the solid electrolyte layer 211 and the anode layer 213, and is disposed in the center of the upper surface of the solid electrolyte layer 211 in a plan view of the electrolysis cell 21. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 211 is exposed. The anode layer 213 is laminated on the back surface (lower surface) of the solid electrolyte layer 211, with the interdiffusion layer 214 sandwiched therebetween. Therefore, the interdiffusion layer 214 is formed between the solid electrolyte layer 211 and the anode layer 213.

[0038] 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, 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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 separator 23, the interconnector 22 above the separator 23, the air electrode frame 24, and the electrolysis cell 21. The fuel chamber Sf is a space that allows the flow of hydrogen generated in the fuel electrode layer 213, and is defined by a space surrounded by the separator 23, the interconnector 22 (or interconnector 29) below the separator 23, the fuel electrode frame 25, and the electrolysis cell 21.

[0043] 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.

[0044] 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."

[0045] 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.

[0046] 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.

[0047] 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, an anode layer 213, and an interdiffusion layer 214. In this embodiment, the solid electrolyte layer 211 is a rectangular flat layer approximately 150 mm square and approximately 6 μm thick. The solid electrolyte layer 211 is configured to contain YSZ (yttria-stabilized zirconia) as an ion-conductive oxide 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 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).

[0048] The air electrode layer 212 is laminated on the upper surface 211B side of the solid electrolyte layer 211 so as to contact the upper surface 211B of the solid electrolyte layer 211. The air electrode layer 212 is a rectangular flat layer with a thickness of approximately 100 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 from the current collecting layer. The air electrode layer 212 is a porous layer with pores inside.

[0049] The anode layer 213 is a rectangular flat layer measuring approximately 150 mm on each side, and is formed to have a thickness greater than the thicknesses of the solid electrolyte layer 211 and the air cathode layer 212, for example, approximately 400 μm. The solid electrolyte layer 211 and the air cathode layer 212 are supported by the anode layer 213. In other words, the electrolysis cell 21 is an anode-supported cell. The anode layer 213 has an anode functional layer 213 a and an anode support layer 213 b. The anode support layer 213 b is formed to be significantly thicker than the anode functional layer 213 a, and the thickness ratio can be set to, for example, approximately 16 to 40 times.

[0050] The anode layer 213 is laminated on the back surface 211A side of the solid electrolyte layer 211, which is the lower surface in Figure 4. As described above, the anode layer 213 has an anode functional layer 213a and an anode support layer 213b. The anode functional layer 213a and the anode support layer 213b are in contact with each other, and are laminated in this order on the back surface 211A side of the solid electrolyte layer 211, with the interdiffusion layer 214 sandwiched between them. Therefore, the interdiffusion layer 214, anode functional layer 213a, and anode support layer 213b are laminated in this order on the back surface 211A of the solid electrolyte layer 211.

[0051] The main component of the anode support layer 213b is a cermet of Ni as a catalytic metal and YSZ, an ion-conductive oxide. The anode 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 of water vapor (gas diffusibility).

[0052] The main component of the anode functional layer 213a is a cermet of Ni as a catalytic metal and GDC (gadolinia-doped ceria), an ion-conductive oxide. GDC is CeO containing Gd (gadolinium). ２The anode functional layer 213a is made of ceria (ceria). Like the anode support layer 213b, the anode functional layer 213a is also a porous layer configured to have a porous shape including a plurality of micropores (not shown). The anode functional layer 213a is formed to be denser than the anode support layer 213b. In other words, the porosity of the anode functional layer 213a is smaller than the porosity of the anode support layer 213b. Like the solid electrolyte layer 211 and the air cathode layer 212, the anode layer 213 including the anode functional layer 213a and the anode support layer 213b is also formed by sintering.

[0053] The interdiffusion layer 214 formed between the solid electrolyte layer 211 and the anode layer 213 (anode functional layer 213 a) is a layer containing Zr (first element) in YSZ, which is an ion-conductive oxide in the solid electrolyte layer 211, and Ce (second element) in GDC, which is an ion-conductive oxide in the anode functional layer 213 a.

[0054] The interdiffusion layer 214 is a layer that is generated when the electrolytic cell 21 is sintered. FIG. 5 is a schematic diagram illustrating the generation of the interdiffusion layer 214. As shown in FIG. 5( a), before the electrolytic cell 21 is sintered, a green sheet G1 of the solid electrolyte layer 211, which is primarily composed of YSZ, and a green sheet G2 of the anode functional layer 213a, which is primarily composed of Ni and GDC, are stacked so that they are in surface contact with each other. When the stacked green sheets G1 and G2 are sintered, the metal element (Zr) in the YSZ, which is an ion-conductive oxide in the green sheet G1, diffuses into the green sheet G2, and the metal element (Ce) in the GDC, which is an ion-conductive oxide in the green sheet G2, diffuses into the green sheet G1 (FIG. 5( b)). After sintering, the green sheet G1 becomes the solid electrolyte layer 211, and the green sheet G2 becomes the anode functional layer 213a, with an interdiffusion layer 214 containing Zr and Ce formed between them. The inter-diffusion layer 214 includes a solid electrolyte-side diffusion layer 214a and an anode-side diffusion layer 214b. The solid electrolyte-side diffusion layer 214a is in contact with the solid electrolyte layer 211 and contains Zr and Ce, with the Zr concentration [wt%] being equal to or greater than the Ce concentration [wt%] and the Ce concentration being 2.000 wt% or greater. The anode-side diffusion layer 214b is in contact with the anode functional layer 213a and contains Zr and Ce, with the Ce concentration [wt%] being equal to or greater than the Zr concentration [wt%] and the Zr concentration being 2.000 wt% or greater. The thickness of the inter-diffusion layer 214 is the sum of the thicknesses of the solid electrolyte-side diffusion layer 214a and the anode-side diffusion layer 214b.

[0055] As described above, the solid electrolyte-side diffusion layer 214a is a layer in which the Zr concentration is equal to or greater than the Ce concentration and the Ce concentration is 2.000 wt % or greater. The solid electrolyte-side diffusion layer 214a can be considered a layer in which the solid electrolyte layer 211 has been altered, but in this embodiment, the solid electrolyte-side diffusion layer 214a is not included in the solid electrolyte layer 211. The anode-side diffusion layer 214b is a layer in which the Ce concentration is equal to or greater than the Zr concentration and the Zr concentration is 2.000 wt % or greater. The anode-side diffusion layer 214b can be considered a layer in which the anode layer 213 has been altered, but in this embodiment, the anode-side diffusion layer 214b is not included in the anode layer 213. Furthermore, because the Zr contained in the interdiffusion layer is diffused from the solid electrolyte layer 211, the Zr concentration in the interdiffusion layer decreases from the solid electrolyte layer 211 side toward the anode layer 213 side. Furthermore, because the Ce contained in the inter-diffusion layer diffuses from the anode layer 213, the Ce concentration in the inter-diffusion layer decreases from the anode layer 213 side toward the solid electrolyte layer 211 side. Therefore, there is a point in the inter-diffusion layer where the Ce concentration and the Zr concentration match, and this point is the boundary between the solid electrolyte-side diffusion layer 214a and the anode-side diffusion layer 214b. The solid electrolyte-side diffusion layer 214a is formed from this boundary point (the midpoint in the embodiment described below) toward the solid electrolyte layer 211 side, and the anode-side diffusion layer 214b is formed from the boundary point toward the anode layer 213 side. The boundary point is both the solid electrolyte-side diffusion layer 214a and the anode-side diffusion layer 214b.

[0056] In this way, the interdiffusion layer 214 is formed between the solid electrolyte layer 211 and the anode layer 213 (anode functional layer 213 a), and is a layer containing Zr (first element), which is a metal element in the ion-conductive oxide (YSZ) in the solid electrolyte layer 211, and Ce (second element), which is a metal element in the ion-conductive oxide (GDC) in the anode functional layer 213 a.

[0057] Furthermore, the interdiffusion layer 214 is configured to contain Ni because it has an anode-side diffusion layer 214b, which can be considered a layer formed by alteration of the anode layer 213. As a result, the interdiffusion layer 214 has a three-phase interface consisting of Ni, an ion-conductive oxide such as YSZ or GDC, and pores, and therefore the interdiffusion layer 214 can also contribute to the electrolytic reaction.

[0058] 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.

[0059] The water vapor that flows into the fuel chamber Sf passes through the anode support layer 213b of the anode layer 213 and travels to the anode functional layer 213a. In the anode functional layer 213a, the water vapor reacts with electrons supplied from the end plate 28 via the current collector 26 to decompose into hydrogen and oxide ions (water vapor electrolysis reaction). The hydrogen generated by the water vapor electrolysis reaction diffuses within the fuel chamber Sf, is discharged through the horizontal hole 25b via path Pfo, and is recovered by a well-known method. At this time, unreacted water vapor can be discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 211 to the air cathode layer 212 in the air chamber Sa, release electrons in the functional layer of the air cathode layer 212, and become oxygen. The oxygen diffuses within the air chamber Sa and, 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.

[0060] The cell stack 20 operates as described above, and hydrogen is produced in the hydrogen production device 1 .

[0061] As described above, the ion-conductive oxide in the solid electrolyte layer 211 of the electrolysis cell 21 according to this embodiment is YSZ. Meanwhile, the ion-conductive oxide in the anode functional layer 213a of the electrolysis cell 21 according to this embodiment is GDC. It is known that when the ion-conductive oxide in the anode functional layer 213a is GDC, the migration and aggregation of Ni in the anode functional layer 213a is suppressed to some extent, thereby suppressing to some extent the degradation of the performance and durability of the electrolysis cell 21 due to the migration and aggregation of Ni.

[0062] However, as clearly shown in FIG. 5 , when the ion-conductive oxide (YSZ) in the solid electrolyte layer 211 and the ion-conductive oxide (GDC) in the anode functional layer 213 a are different, an interdiffusion layer 214 is formed between the solid electrolyte layer 211 and the anode functional layer 213 a, in contact with both layers ( 211 , 213 a ).

[0063] In the electrolytic cell 21 in which the interdiffusion layer 214 is formed, the interdiffusion layer 214 is also a layer that ensures adhesion between the solid electrolyte layer 211 and the fuel electrode layer 213, so if the interdiffusion layer 214 is too thin, the adhesion of the electrolytic cell 21 will decrease.

[0064] Furthermore, the durability of the electrolysis cell 21 is significantly affected by the components in the region near the boundary between the solid electrolyte layer 211 and the anode layer 213 (anode functional layer 213 a), and the interdiffusion layer 214 is formed between the solid electrolyte layer 211 and the anode layer 213 (anode functional layer 213 a) so as to be in contact with both. Therefore, the presence of this interdiffusion layer 214 significantly affects the durability of the electrolysis cell 21. However, the interdiffusion layer 214 is a highly resistive layer and has electronic conductivity, and these properties adversely affect the durability of the electrolysis cell 21. Therefore, if the interdiffusion layer 214 is too thick, the durability of the electrolysis cell 21 may be deteriorated.

[0065] In the electrolytic cell 21 according to this embodiment, the thickness (T1) of the interdiffusion layer 214 is adjusted to be 1.5 μm or more and 4.8 μm or less. The thickness (T1) of the interdiffusion layer 214 being 1.5 μm or more ensures the adhesion of the electrolytic cell 21. Furthermore, the thickness (T1) of the interdiffusion layer 214 being 4.8 μm or less prevents a decrease in durability. Therefore, the electrolytic cell 21 according to this embodiment can prevent a decrease in durability while ensuring adhesion.

[0066] The thickness (T1) of the interdiffusion layer 214 is preferably 3.9 μm or less, more preferably 2.9 μm or less, and even more preferably 2.2 μm or less.

[0067] The thickness (T1) of the interdiffusion layer 214 is affected by the firing temperature (first temperature t0 described below) when sintering the solid electrolyte layer 211 and the anode layer 213. Specifically, the lower the first temperature t0, the smaller the thickness (T1) of the interdiffusion layer 214. On the other hand, if the first temperature t0 is low, the electrolysis cell 21 may not be sufficiently sintered, which may result in a decrease in the strength and performance of the electrolysis cell 21. Therefore, in this embodiment, the particle size of the YSZ powder used as the raw material for the solid electrolyte layer 211 is controlled so that the electrolysis cell 21 can be sufficiently sintered even when the first temperature t0 is low. Specifically, by using YSZ powder obtained by pre-pulverizing commercially available YSZ powder to reduce its particle size (average particle size D50), the sinterability of the electrolysis cell 21 is sufficiently maintained even when the first temperature t0 is low. Hereinafter, the YSZ powder used to form the electrolysis cell 21 and whose particle size has been reduced by pre-pulverization will be referred to as pulverized YSZ powder. The average particle size (median size D50) of the pulverized YSZ powder is preferably 0.20 μm or less.

[0068] Furthermore, the solid electrolyte layer 211 and the solid electrolyte-side diffusion layer 214a contain a large amount of Zr, which enhances sinterability and makes them dense layers. Therefore, they are basically gas-tight. However, if they are too thin, particularly if the sum of the thicknesses of the two layers is less than 2.3 μm, their ability to block gas passage (gas barrier performance) decreases. If the gas barrier performance decreases, gas (water vapor) supplied to the anode layer 213 leaks to the cathode layer 212, and gas (air) supplied to the cathode layer 212 leaks to the anode layer 213. Such gas leakage (cross leakage) reduces the reaction efficiency of the electrolysis cell 21. Furthermore, if the sum of the thicknesses of the solid electrolyte layer 211 and the solid electrolyte-side diffusion layer 214a exceeds 151.0 μm, electrical resistance in the thickness direction increases, resulting in reduced performance (initial performance) of the electrolysis cell 21. In contrast, in the electrolytic cell 21 according to this embodiment, the sum of the thickness (T2) of the solid electrolyte layer 211 and the thickness (Ta) of the solid electrolyte-side diffusion layer 214a is 2.3 μm or more and 151.0 μm or less, thereby suppressing a decrease in reaction efficiency and performance.

[0069] Furthermore, in the electrolysis cell 21 according to this embodiment, the ratio R (=T2 / T1) of the thickness (T2) of the solid electrolyte layer 211 to the thickness (T1) of the interdiffusion layer 214 is 0.9 or greater. That is, the thickness (T2) of the solid electrolyte layer 211 is 0.9 times or greater than the thickness (T1) of the interdiffusion layer 214. When the ratio R is less than 0.9, the thickness of the solid electrolyte layer 211 is relatively small compared to the thickness of the interdiffusion layer 214. Here, because the interdiffusion layer 214 has electronic conductivity, electrons supplied to the anode layer 213 may enter the interdiffusion layer 214 without contributing to a reaction in the anode layer 213. Furthermore, because the solid electrolyte layer 211 does not essentially have electronic conductivity, it inhibits electrons that enter the interdiffusion layer 214 from migrating to the cathode layer 212. However, when the thickness (T2) of the solid electrolyte layer 211 is small relative to the thickness (T1) of the inter-diffusion layer 214 (when the ratio R is small), a relatively large number of electrons enter the inter-diffusion layer 214, but the thickness of the solid electrolyte layer 211 is insufficient to adequately prevent the electrons that have entered the inter-diffusion layer 214 from migrating toward the air cathode layer 212. As a result, electron leakage occurs, and the reaction efficiency decreases. In contrast, when the ratio R is 0.9 or greater, the thickness (T2) of the solid electrolyte layer 211 is not small relative to the thickness (T1) of the inter-diffusion layer 214, so the solid electrolyte layer 211 can effectively prevent the electrons that have entered the inter-diffusion layer 214 from migrating toward the air cathode layer 212. This further reduces the decrease in the reaction efficiency of the electrolysis cell 21.

[0070] (Examples) 1. Sample Preparation NiO powder and GDC 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 MEK (methyl ethyl ketone), 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 an anode functional layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0071] 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 MEK, 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 anode support layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0072] In addition, butyral resin, polyvinyl acetal resin as a plasticizer, a known dispersant, and a mixed solvent of toluene and MEK were added to the crushed 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.

[0073] The crushed YSZ powder used to form the green sheets for the solid electrolyte layers is a powder obtained by crushing commercially available YSZ powder in advance so as to have a predetermined average particle size or less.

[0074] Next, the green sheet for the anode functional layer and the green sheet for the anode support 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 using a press while heating and evacuating. This produced a laminate including the green sheet for the solid electrolyte layer, the green sheet for the anode functional layer, and the green sheet for the anode support layer.

[0075] 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 t0 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 an anode layer laminated on one side of the solid electrolyte layer. Because the ion-conductive oxide (YSZ) in the solid electrolyte layer and the ion-conductive oxide (GCD) in the anode functional layer were different, an interdiffusion layer containing Zr and Ce was formed between the solid electrolyte layer and the anode functional layer after sintering.

[0076] 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 stacked on one surface of the solid electrolyte layer with an interdiffusion layer sandwiched therebetween, and an cathode layer stacked on the other surface of the solid electrolyte layer.

[0077] Furthermore, by varying the firing temperature (first temperature t0) during the primary firing between 1000°C and 1400°C, for example, multiple electrolytic cell samples (Samples 1 to 9) with different solid electrolyte layer and interdiffusion layer thicknesses were produced. The first temperature t0 for each sample was specifically as follows: for Sample 2, the first temperature t0 was 1240°C; for Sample 3, the first temperature t0 was 1190°C; for Sample 4, the first temperature t0 was 1340°C; for Sample 5, the first temperature t0 was 1140°C; and for Sample 1 and Samples 6 to 9, the first temperature t0 was 1290°C.

[0078] As described above, the lower the firing temperature (first temperature t0) of the primary sintered body, the smaller the thickness of the interdiffusion layer. When the firing temperature (first temperature t0) of the primary sintered body is low, there is a concern that strength and performance may be reduced due to insufficient sintering. However, by controlling the particle size of the YSZ powder (pulverized YSZ powder) used to form the green sheet of the solid electrolyte layer, good sinterability can be maintained even when the firing temperature (first temperature t0) is low. Specifically, since the smaller the particle size of the pulverized YSZ powder, the better the sinterability. Therefore, by using a pulverized YSZ powder with a small average particle size for samples with a low firing temperature (first temperature t0) and a pulverized YSZ powder with a large average particle size for samples with a high firing temperature (first temperature t0), the sinterability of the primary sintered body of each sample can be maintained good. In this example, for samples (samples 2, 3, and 5) in which the firing temperature (first temperature t0) of the primary sintered body was 1240°C or less, the YSZ powder was pre-pulverized so that the average particle diameter (median diameter) PD of the pulverized YSZ powder was 0.20 μm or less (specifically, 0.16 μm), and for samples (samples 1, 4, and 6 to 9) in which the firing temperature (first temperature t0) was greater than 1240°C, the YSZ powder was pre-pulverized so that the average particle diameter (median diameter) PD of the pulverized YSZ powder was greater than 0.20 μm (specifically, 0.27 μm).

[0079] Furthermore, the amount of pore-forming material was adjusted during molding of these layers so that the porosity in the anode functional layer of each sample was smaller than the porosity in the anode support layer.

[0080] 2. Measurement of the Thickness of the Interdiffusion Layer (T1), the Thickness of the Solid Electrolyte Layer (T2), and the Thickness of the Solid Electrolyte-Side Diffusion Layer (Ta) Each prepared sample was cut along a plane along the thickness direction of the electrolytic cell, and the cut surface was subjected to area analysis using an EPMA device (manufactured by JEOL Ltd.). The Zr concentration distribution and the Ce concentration distribution in the thickness direction of the electrolytic cell were calculated from the elemental mapping image obtained by the area analysis. In this case, the Zr concentration and the Ce concentration at each thickness position were calculated at 0.1 μm intervals along the thickness direction of the electrolytic cell. To calculate the Zr (or Ce) concentration at each thickness position, the average value of the Zr (or Ce) concentrations detected at multiple points on the line corresponding to that thickness position was calculated, and the calculated average value of the Zr (or Ce) concentrations was used as the Zr (or Ce) concentration at that thickness position.

[0081] FIG. 6 shows the Zr concentration distribution and Ce concentration distribution with respect to thickness position calculated for Sample 2. In FIG. 6, the horizontal axis represents the thickness position (μm) from the midpoint position (described below). Here, the thickness position refers to the distance from the midpoint position. Note that thickness positions from the midpoint position toward the anode functional layer side are shown as positive thickness positions (distances), and thickness positions from the midpoint position toward the solid electrolyte layer side are shown as negative thickness positions (distances). The vertical axis represents the mass concentration of Zr (or Ce) relative to all elements present at the thickness position determined by the horizontal axis, and all present elements are Ni, Zr, Y, O, Gd, and Ce. Graph A1 shown by the solid line in FIG. 6 represents the Zr concentration distribution, and graph B1 shown by the dashed line represents the Ce concentration distribution. Note that FIG. 6 and FIGS. 7 and 8 (described below) show the Zr and Ce concentrations at thickness positions ±5 μm from the midpoint position.

[0082] As shown in graph A1 of Figure 6, the Zr concentration is approximately 80 wt% on the solid electrolyte layer side and less than approximately 2 wt% on the anode functional layer side. The Zr concentration distribution has a region where it rapidly decreases from the solid electrolyte layer side toward the anode functional layer side. As shown in graph B1 of Figure 6, the Ce concentration is approximately 40 wt% on the anode functional layer side and less than approximately 2 wt% on the solid electrolyte layer side. The Ce concentration distribution also has a region where it rapidly decreases from the anode functional layer side toward the solid electrolyte layer side.

[0083] 6, there is a thickness position between the solid electrolyte layer and the anode functional layer where the Zr concentration and the Ce concentration are equal. This thickness position is defined as the midpoint position, and the thickness position at the midpoint position is defined as 0.

[0084] The thickness position where the Zr concentration, calculated from the midpoint toward the anode functional layer, first decreased to 2.000 wt% was defined as position tb, and the thickness position where the Ce concentration, calculated from the midpoint toward the solid electrolyte layer, first decreased to 2.000 wt% was defined as position ta. The length in the thickness direction between positions ta and tb was calculated as the thickness (T1) of the interdiffusion layer. In the case shown in Figure 6, the thickness (T1) of the interdiffusion layer is 2.9 μm. Therefore, the thickness of the interdiffusion layer in Sample 2 is 2.9 μm.

[0085] The region from the thickness position ta toward the solid electrolyte layer side was defined as the solid electrolyte layer, and the region from the thickness position tb toward the anode functional layer side was defined as the anode layer. The distance from the thickness position ta to the thickness position on the opposite surface of the solid electrolyte layer was calculated as the thickness (T2) of the solid electrolyte layer. In the case shown in Figure 6, the thickness (T2) of the solid electrolyte layer was 4.5 μm. Therefore, the thickness of the solid electrolyte layer in Sample 2 was 4.5 μm.

[0086] Furthermore, the layer in the region between the midpoint and position ta was defined as the solid electrolyte-side diffusion layer, and the layer in the region between the midpoint and position tb was defined as the anode-side diffusion layer. The length in the thickness direction between the midpoint and position ta was calculated as the thickness (Ta) of the solid electrolyte-side diffusion layer. In the case shown in FIG. 6 , the thickness (Ta) of the solid electrolyte-side diffusion layer is 1.7 μm. Therefore, the thickness of the solid electrolyte-side diffusion layer in Sample 2 is 1.7 μm. Furthermore, in Sample 2, the sum of the thickness (T2) of the solid electrolyte layer and the thickness (Ta) of the solid electrolyte-side diffusion layer is 6.2 μm.

[0087] Figure 7 is a diagram showing the Zr concentration distribution and the Ce concentration distribution with respect to thickness position calculated for Sample 4. In Figure 7, the horizontal axis represents the thickness position (µm) from the midpoint position, with the thickness position from the midpoint position toward the anode functional layer side being a positive thickness position and the thickness position from the midpoint position toward the solid electrolyte layer side being a negative thickness position. The vertical axis, like the vertical axis in Figure 6, represents the mass concentration of Zr (or Ce). Graph A2 shown by the solid line in Figure 7 represents the Zr concentration distribution, and graph B2 shown by the dashed line represents the Ce concentration distribution.

[0088] As shown in graph A2 of Figure 7, the Zr concentration is approximately 80 wt% on the solid electrolyte layer side and less than approximately 2 wt% on the anode functional layer side. The Zr concentration distribution has a region where it rapidly decreases from the solid electrolyte layer side toward the anode functional layer side. As shown in graph B2 of Figure 7, the Ce concentration is approximately 40 wt% on the anode functional layer side and less than approximately 2 wt% on the solid electrolyte layer side. The Ce concentration distribution also has a region where it rapidly decreases from the anode functional layer side toward the solid electrolyte layer side.

[0089] In the example shown in FIG. 7 , there is also a thickness position (midpoint) between the solid electrolyte layer and the anode functional layer where the Zr concentration and the Ce concentration are equal. Then, thickness positions ta and tb were determined in the same manner as in FIG. 6 , and the distance between these positions was defined as the thickness (T1) of the interdiffusion layer. In the example shown in FIG. 7 , the thickness (T1) of the interdiffusion layer is 6.2 μm. Therefore, the thickness of the interdiffusion layer in Sample 4 is 6.2 μm. Furthermore, the thickness (T2) of the solid electrolyte layer is 3.6 μm. Therefore, the thickness of the solid electrolyte layer in Sample 4 is 3.6 μm. Furthermore, the thickness (Ta) of the solid electrolyte side diffusion layer is 2.7 μm. Therefore, the thickness of the solid electrolyte side diffusion layer in Sample 4 is 2.7 μm. Furthermore, in Sample 4, the sum of the thickness (T2) of the solid electrolyte layer and the thickness (Ta) of the solid electrolyte side diffusion layer is 6.3 μm.

[0090] Figure 8 is a diagram showing both the Zr concentration distribution and the Ce concentration distribution with respect to thickness position, calculated for Sample 5. In Figure 8, the horizontal axis represents the thickness position (µm) from the midpoint position, with the thickness position from the midpoint position toward the anode functional layer side being a positive thickness position and the thickness position from the midpoint position toward the solid electrolyte layer side being a negative thickness position. The vertical axis, like the vertical axis in Figure 6, represents the mass concentration of Zr (or Ce). Graph A3 shown by the solid line in Figure 8 represents the Zr concentration distribution, and graph B3 shown by the dashed line represents the Ce concentration distribution.

[0091] As shown in graph A3 of Figure 8, the Zr concentration is approximately 80 wt% on the solid electrolyte layer side and less than approximately 2 wt% on the anode functional layer side. The Zr concentration distribution has a region where it rapidly decreases from the solid electrolyte layer side toward the anode functional layer side. As shown in graph B3 of Figure 8, the Ce concentration is approximately 40 wt% on the anode functional layer side and less than approximately 2 wt% on the solid electrolyte layer side. The Ce concentration distribution also has a region where it rapidly decreases from the anode functional layer side toward the solid electrolyte layer side.

[0092] In the example shown in FIG. 8 , there is also a thickness position (midpoint) between the solid electrolyte layer and the anode functional layer where the Zr concentration and the Ce concentration are equal. Then, thickness positions ta and tb were determined in the same manner as in the case shown in FIG. 6 , and the distance between these positions was defined as the thickness (T1) of the interdiffusion layer. In the example shown in FIG. 8 , the thickness (T1) of the interdiffusion layer is 1.0 μm. Therefore, the thickness of the interdiffusion layer in Sample 5 is 1.0 μm. Furthermore, the thickness (T2) of the solid electrolyte layer is 5.9 μm. Therefore, the thickness of the solid electrolyte layer in Sample 5 is 5.9 μm. Furthermore, the thickness (Ta) of the solid electrolyte side diffusion layer is 0.6 μm. Therefore, the thickness of the solid electrolyte side diffusion layer in Sample 5 is 0.6 μm. Furthermore, in Sample 5, the sum of the thickness (T2) of the solid electrolyte layer and the thickness (Ta) of the solid electrolyte side diffusion layer is 6.5 μm.

[0093] As described above, the thickness of the interdiffusion layer (T1), the thickness of the solid electrolyte layer (T2), and the thickness of the solid electrolyte-side diffusion layer (Ta) were measured for each sample. Furthermore, the sum (T2 + Ta) of the thickness of the solid electrolyte layer (T2) and the thickness of the solid electrolyte-side diffusion layer (Ta) was calculated. As can be seen from Figures 6 to 8 , the interdiffusion layer is a layer containing Zr and Ce at a concentration of 2.000 wt % or more, in which the mass concentration of Zr decreases from the solid electrolyte layer side to the anode layer (anode functional layer) side, and the mass concentration of Ce decreases from the anode layer (anode functional layer) side to the solid electrolyte layer side. The solid electrolyte-side diffusion layer is a layer containing Zr and Ce, in which the Zr concentration is equal to or greater than the Ce concentration, and the Ce concentration is 2.000 wt % or more.

[0094] 3. Calculation of the ratio R of the thickness of the solid electrolyte layer to the thickness of the interdiffusion layer The ratio R was calculated using the thickness of the interdiffusion layer (T1) and the thickness of the solid electrolyte layer (T2) calculated for each sample. The ratio R was calculated by dividing the thickness of the solid electrolyte layer (T2) by the thickness of the interdiffusion layer (T1).

[0095] 4. Measurement of Durability Degradation Rate For Samples 1, 2, 3, 4, and 5, a durability degradation test was conducted in which the samples were heated to 700°C, a constant flow rate of water vapor was supplied to the anode layer side so that a constant current flowed between the anode layer and the cathode layer, and a voltage was applied between the anode layer and the cathode layer to continuously perform a steam electrolysis reaction. When a constant current was continuously flowed during the durability degradation test, the applied voltage increased over time due to an increase in 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.

[0096] 5. Evaluation of Peeling State After the durability degradation test was performed on Samples 1, 2, 3, 4, and 5, each sample was cut in a plane parallel to the thickness direction, and the cut surface was observed with an SEM to examine the peeling state between the fuel electrode layer and the solid electrolyte layer.

[0097] 6. Current Density Measurement For Samples 1, 6, 7, 8, and 9, the samples were heated to 700°C, and a constant voltage (1 to 1.3 V) was applied between the anode layer and the cathode layer while a constant flow rate of steam was supplied to the anode layer side to perform a steam electrolysis reaction. The current density per unit area (current density) was measured. The measured current density was used as an index for evaluating performance. It can be said that the higher the current density, the better the performance. Note that the current density of Sample 1 was measured before the durability degradation test was conducted.

[0098] 7. Measurement of Open Circuit Voltage (OCV) For Samples 1, 6, 7, 8, and 9, the samples were heated to 700°C, hydrogen was supplied to the fuel electrode layer at a flow rate of 8.2 cc / min and water vapor at a flow rate of 74 cc / min, and air was supplied to the air electrode layer at a flow rate of 100 cc / min. Under these conditions, the potential generated between the fuel electrode layer and the air electrode layer of each sample was measured as the OCV. It can be said that the higher the OCV, the higher the reaction efficiency. Note that the OCV of Sample 1 was measured before the durability degradation test was conducted.

[0099] 8. Evaluation Table 1 shows the thickness T1 of the interdiffusion layer, the thickness T2 of the solid electrolyte layer, the thickness Ta of the solid electrolyte-side diffusion layer, the sum of the thickness T2 of the solid electrolyte layer and the thickness Ta of the solid electrolyte-side diffusion layer (= T2 + Ta), the ratio R (= T2 / T1), the state of peeling, and the durability degradation rate obtained for Samples 1, 2, 3, 4, and 5.

[0100] In Table 1, t0 indicates the firing temperature (first temperature) of the primary sintered body, and PD indicates the average particle diameter (median diameter D50) of the pulverized YSZ powder used in producing the green sheet of the solid electrolyte layer. Regarding adhesion, in Table 1, if no peeling was observed by SEM observation, the adhesion evaluation result was judged as good (◯), and if peeling was observed, the adhesion evaluation result was judged as poor (×). Furthermore, if the durability degradation rate was less than 3.0%, the durability was rated as good (◯), and if the durability degradation rate was 3.0% or more, the durability was rated as poor (×).

[0101] As can be seen from Table 1, the adhesion and durability ratings for Samples 1, 2, and 3 are all good (◯). In contrast, the durability rating for Sample 4 is poor (×), and the adhesion rating for Sample 5 is poor (×).

[0102] In Sample 4, the thickness (T1) of the interdiffusion layer was 6.2 μm, which is thicker than the thicknesses of the interdiffusion layers in Samples 1, 2, and 3, and is therefore thought to have caused the deterioration in durability. In Sample 5, the thickness (T1) of the interdiffusion layer was 1.0 μm, which is thinner than the thicknesses of the interdiffusion layers in Samples 1, 2, and 3, and is therefore thought to have caused the deterioration in adhesion. Therefore, as long as the thickness (T1) of the interdiffusion layer is within the range of that in Samples 1, 2, and 3, it is possible to ensure adhesion and suppress deterioration in durability. In other words, when the thickness (T1) of the interdiffusion layer is 1.5 μm or more and 4.8 μm or less, it is possible to ensure adhesion of the electrolysis cell and suppress deterioration in durability.

[0103] Furthermore, the smaller the thickness (T1) of the interdiffusion layer, the lower the firing temperature (first temperature t0) of the primary sintered body can be, thereby reducing the thermal energy required to fabricate the electrolytic cell and, as a result, reducing the manufacturing cost of the electrolytic cell. Therefore, from the perspective of reducing the manufacturing cost of the electrolytic cell, it is desirable that the thickness (T1) of the interdiffusion layer be as small as possible. Specifically, the thickness (T1) of the interdiffusion layer is preferably 3.9 μm or less, more preferably 2.9 μm or less, and even more preferably 2.2 μm or less.

[0104] Table 2 shows the thickness of the interdiffusion layer (T1), the thickness of the solid electrolyte layer (T2), the thickness of the solid electrolyte-side diffusion layer (Ta), the sum of the thickness T1 of the solid electrolyte layer and the thickness Ta of the solid electrolyte-side diffusion layer (= T2 + Ta), the ratio R (= T2 / T1), the current density, and the OCV obtained for Samples 1, 6, 7, 8, and 9.

[0105] In Table 2, t0 indicates the firing temperature (first temperature) of the primary sintered body, and PD indicates the average particle diameter (median diameter D50) of the pulverized YSZ powder used in preparing the green sheet for the solid electrolyte layer. ２ When the current density was 0.7 A / cm or more, the performance was evaluated as very good (◎). ２ 0.9A / cm or more ２ The performance was evaluated as good (◯) when the current density was less than 0.5 A / cm ２ 0.7A / cm or more ２ When the current density is less than 0.5 A / cm, the performance is evaluated as fair (△). ２ When the OCV was less than 0.80 V, the performance was evaluated as poor (×). In Table 2, when the OCV was 0.87 V or more, the reaction efficiency was evaluated as very good (◎), when the OCV was 0.85 V or more but less than 0.87 V, the reaction efficiency was evaluated as good (◯), when the OCV was 0.80 V or more but less than 0.85 V, the reaction efficiency was evaluated as fair (△), and when the OCV was less than 0.80 V, the reaction efficiency was evaluated as poor (×). Here, the reaction efficiency is the amount of hydrogen generated per unit output (W), and the higher the OCV, the better (higher) the reaction efficiency.

[0106] In each sample shown in Table 2, the thickness (T1) of the interdiffusion layer is 1.5 μm or more and 4.8 μm or less, and therefore, it is considered that the adhesion and durability of these samples are good.

[0107] Furthermore, in Sample 9, in which the sum (T2 + Ta) of the solid electrolyte layer thickness (T2) and the solid electrolyte-side diffusion layer thickness (Ta) was less than 2.3 μm, the OCV was low at 0.78 V, indicating poor reaction efficiency (×). In contrast, in Samples 1, 6, 7, and 8, in which the sum (T2 + Ta) of the solid electrolyte layer thickness (T2) and the solid electrolyte-side diffusion layer thickness (Ta) was 2.3 μm or greater, the OCV was 0.84 V or greater, indicating fair or better reaction efficiency. Therefore, it can be seen that a decrease in reaction efficiency can be suppressed if the sum (T2 + Ta) of the solid electrolyte layer thickness (T2) and the solid electrolyte-side diffusion layer thickness (Ta) is 2.3 μm or greater. The reason for the low OCV in Sample 9 is thought to be that the sum (T2 + Ta) of the solid electrolyte layer thickness and the solid electrolyte-side diffusion layer thickness was thin (less than 2.3 μm), resulting in poor gas barrier performance and leakage of gas (water vapor and air) supplied to the sample through the solid electrolyte layer.

[0108] In addition, in sample 7, in which the sum of the thickness of the solid electrolyte layer (T2) and the thickness of the solid electrolyte-side diffusion layer (Ta) exceeded 151.0 μm, the current density was 0.29 A / cm ２ In contrast, in Samples 1, 6, 8, and 9, in which the sum of the thickness of the solid electrolyte layer (T2) and the thickness of the solid electrolyte-side diffusion layer (Ta) was 151.0 μm or less, the current density was 0.52 A / cm ２ The results are above average (Δ), and the performance is at least fair. Therefore, from the viewpoint of performance, it is preferable that the sum of the thickness of the solid electrolyte layer (T2) and the thickness of the solid electrolyte-side diffusion layer (Ta) be 151.0 μm or less. The reason for the low current density in Sample 7 is thought to be that the sum of the thickness of the solid electrolyte layer and the thickness of the solid electrolyte-side diffusion layer was greater than 151.0 μm, resulting in high electrical resistance in the thickness direction.

[0109] From the above, when the sum of the thickness of the solid electrolyte layer (T2) and the thickness of the solid electrolyte-side diffusion layer (Ta) is 2.3 μm or more and 151.0 μm or less, it is possible to maintain performance while suppressing a decrease in efficiency.

[0110] Furthermore, according to Table 2, in Samples 1, 6, and 7, where the ratio R is 0.9 or more, the OCV is 0.87 V or more, and the reaction efficiency is very good (◎). In contrast, in Samples 8 and 9, where the ratio R is less than 0.9, the OCV is 0.84 V or less. Therefore, when the ratio R is 0.9 or more, the reaction efficiency can be further improved. The reason for the low reaction efficiency in Samples 8 and 9, where the ratio R is less than 0.9, is thought to be that the thickness of the solid electrolyte layer is relatively small compared to the thickness of the interdiffusion layer, which increases the probability that electrons that enter the interdiffusion layer will leak to the air cathode layer through the solid electrolyte layer, causing an internal short circuit.

[0111] Thus, in Samples 1, 2, and 3, which are examples of the present disclosure, the thickness (T1) of the inter-diffusion layer is 1.5 μm or more and 4.8 μm or less, thereby ensuring adhesion of the electrolysis cell while suppressing deterioration in durability. Furthermore, in Samples 1, 6, and 8, which are examples of the present disclosure, the sum (T2 + Ta) of the thickness (T2) of the solid electrolyte layer and the thickness (Ta) of the solid electrolyte-side diffusion layer is 2.3 μm or more and 151.0 μm or less, thereby suppressing performance degradation while suppressing a decrease in reaction efficiency of the electrolysis cell. Furthermore, in Samples 1 and 6, which are examples of the present disclosure, the ratio R is 0.9 or more, thereby further increasing reaction efficiency.

[0112] Although the embodiments of the present disclosure have been described above, the technology according to the present disclosure should not be limited to the above embodiments. For example, in the above embodiments, an example was shown in which the interdiffusion layer contains Zr as a first element and Ce as a second element, but the first element and the second element may be other elements. Furthermore, in the above embodiments, an example was shown in which the ion-conductive oxide in the solid electrolyte layer is YSZ and the ion-conductive oxide in the anode functional layer is GDC. However, the technology according to the present disclosure can be applied, for example, to cases in which the metal elements in the ion-conductive oxides in both layers are different. In this way, the technology according to the present disclosure can be modified as long as it does not deviate from the spirit thereof.

[0113] The present invention may further include the following aspects: [1] An electrochemical cell comprising a solid electrolyte layer, an anode layer 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 an interdiffusion layer is formed between the solid electrolyte layer and the anode layer in contact with both the solid electrolyte layer and the anode layer, the interdiffusion layer containing a first element that is one of the elements constituting the solid electrolyte layer and a second element that is one of the elements constituting the anode layer and different from the first element, and the thickness of the interdiffusion layer is within the range of 1.5 μm to 4.8 μm. [2] The electrochemical cell according to [1], wherein the interdiffusion layer has a solid electrolyte-side diffusion layer that is in contact with the solid electrolyte layer and has a concentration of the first element equal to or higher than the concentration of the second element, and the sum of the thickness of the solid electrolyte layer and the thickness of the solid electrolyte-side diffusion layer is within the range of 2.3 μm to 151.0 μm. [3] The electrochemical cell according to [1] or [2], wherein the ratio of the thickness of the solid electrolyte layer to the thickness of the interdiffusion layer is 0.9 or more. [4] The electrochemical cell according to any of [1] to [3], wherein the first element is Zr and the second element is Ce. [5] A solid oxide electrolysis cell comprising the electrochemical cell according to any of [1] to [4]. [6] A cell stack formed by stacking the solid oxide electrolysis cells according to [5]. [7] A hot module comprising: the cell stack according to [6]; a vaporizer that generates water vapor to be supplied to the cell stack; a heat exchanger that exchanges heat with gas 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. [8] A hydrogen production device comprising the hot module according to [7].

[0114] DESCRIPTION OF SYMBOLS 1...hydrogen production device, 10...hot module, 20...cell stack, 21...solid oxide electrolysis cell (electrochemical cell), 22, 29...interconnector, 23...separator, 24...air electrode frame, 25...fuel electrode frame, 26...current collector, 27, 28...end plate, 30...vaporizer, 40...heat exchanger, 50...heater, 60...insulating material, 90...condenser, 211...solid electrolyte layer, 211A...rear surface (one side), 211B...upper surface (other side), 212...air electrode layer, 213...fuel electrode layer, 213a...fuel electrode functional layer, 213b...fuel electrode support layer, 214...interdiffusion layer, 214a...solid electrolyte side diffusion layer, 214b...fuel electrode side diffusion layer

Claims

1. An electrochemical cell comprising: 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, wherein an interdiffusion layer is formed between the solid electrolyte layer and the fuel electrode layer, the interdiffusion layer being in contact with both the solid electrolyte layer and the fuel electrode layer, the interdiffusion layer including a first element which is one of the elements constituting the solid electrolyte layer, and a second element which is one of the elements constituting the fuel electrode layer and different from the first element, and the thickness of the interdiffusion layer is within the range of 1.5 μm or more and 4.8 μm or less.

2. An electrochemical cell according to claim 1, wherein the interdiffusion layer has a solid electrolyte-side diffusion layer that is in contact with the solid electrolyte layer and in which the concentration of the first element is equal to or greater than the concentration of the second element, and the sum of the thickness of the solid electrolyte layer and the thickness of the solid electrolyte-side diffusion layer is within the range of 2.3 μm or more and 151.0 μm or less.

3. An electrochemical cell according to claim 2, wherein the ratio of the thickness of said solid electrolyte layer to the thickness of said interdiffusion layer is 0.9 or greater.

4. An electrochemical cell according to claim 1, wherein said first element is Zr and said second element is Ce.

5. A solid oxide electrolysis cell comprising the electrochemical cell according to any one of claims 1 to 4.

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

7. A hot module comprising: a cell stack according to claim 6; 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.

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

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