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

Abstract

An electrolysis cell 21 comprises a solid electrolyte layer 211, a fuel electrode layer 213 stacked and arranged on one surface side of the solid electrolyte layer 211, and an air electrode layer 212 stacked and arranged on the other surface side of the solid electrolyte layer 211. The fuel electrode layer 213 includes a functional layer 213a, a support layer 213b positioned on the side farther from the solid electrolyte layer 211 than from the functional layer 213a, and a mutual diffusion layer 213c positioned between the functional layer 213a and the support layer 213b so as to be in contact with both of the functional layer 213a and the support layer 213b. The mutual diffusion layer 213c includes: a first element which is one element constituting the functional layer 213a; and a second element which is one element constituting the support layer 213b and is different from the first element. The thickness of the mutual diffusion layer 213c is 1.1 μm or more and 9.7 μm or less.

Description

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

[0001] The present invention 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. 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 decompose water vapor into hydrogen and oxygen using electrical energy. Solid oxide fuel cells are power generation devices that generate electrical energy through a chemical reaction between hydrogen and oxygen.

[0003] An electrochemical cell can be configured with 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. Various studies have been conducted on the structure of each layer to improve the characteristics of electrochemical cells. For example, Patent Document 1 describes an SOFC in which a ceria-based first intermediate layer is provided between the solid electrolyte layer and the anode layer, and a ceria-based second intermediate layer is provided between the first intermediate layer and the anode layer. Patent Document 1 describes that by specifying the magnitude relationship of the Ni content in each of the anode layer, the first intermediate layer, and the second intermediate layer, and setting the mass percent concentration of NiO in the second intermediate layer within a predetermined range during manufacturing, an SOFC can be realized that is resistant to peeling even with repeated redox cycles and has good power generation performance.

[0004] JP 2012-156098 A

[0005] Generally, the solid electrolyte layer contains an ion-conductive oxide (also referred to as an oxide ion conductor). The fuel electrode layer contains Ni (nickel) as an electrode catalyst and an ion-conductive oxide. The fuel electrode layer can be configured to include a functional layer and a support layer located on the side farther from the solid electrolyte layer than the functional layer. Typically, the functional layer and the support layer contain the same type of ion-conductive oxide as the ion-conductive oxide contained in the solid electrolyte layer. However, in order to improve the performance of the electrochemical cell, the functional layer and the support layer may be configured to contain different types of ion-conductive oxide.

[0006] In this case, elements constituting the ion-conductive oxide in the functional layer diffuse toward the support layer, and elements constituting the ion-conductive oxide in the support layer diffuse toward the functional layer. This interdiffusion forms an interdiffusion layer between the functional layer and the support layer, which is in contact with both the functional layer and the support layer and contains elements constituting the ion-conductive oxide in the functional layer and elements constituting the ion-conductive oxide in the support layer. In this way, configuring the functional layer and the support layer to contain different types of ion-conductive oxides can alleviate problems associated with configurations in which both layers contain the same type of ion-conductive oxide, improving the performance of the electrochemical cell in some respects. However, this can also cause other problems associated with the interdiffusion layer, potentially degrading the performance of the electrochemical cell in other respects.

[0007] This example will be described using an example in which the ion-conductive oxide in the solid electrolyte layer is YSZ (yttria-stabilized zirconia), and the ion-conductive oxides in both the functional layer and the support layer are YSZ. When this electrochemical cell is operated for a long period of time under a harsh operating environment (high temperature, high humidity, high current), Ni migration and aggregation occurs in the anode layer (i.e., the functional layer and the support layer). When Ni migrates or aggregates in the anode layer, the internal resistance of the anode layer increases and the three-phase interface (the boundary between the fuel gas, Ni, and the electrolyte (YSZ)) serving as a reaction field decreases, resulting in a problem of reduced performance of the electrochemical cell. This problem is particularly pronounced in the functional layer.

[0008] Therefore, if GDC (gadolinia-doped ceria) is used instead of YSZ as the ion-conductive oxide in the functional layer, the degradation of electrochemical cell performance due to Ni migration and aggregation can be improved because GDC has the property of reducing the degree of Ni migration and aggregation compared to YSZ. However, in this configuration, Ce (cerium) in GDC in the functional layer diffuses to the support layer side, and Zr (zirconium) in YSZ in the support layer diffuses to the functional layer side, forming an interdiffusion layer between the functional layer and the support layer that is in contact with both and contains Zr and Ce.

[0009] This interdiffusion layer contains a composite oxide of Zr and Ce (so-called ceria-zirconia). Ceria-zirconia has a high water vapor adsorption capacity. Therefore, when the electrochemical cell is used as an SOEC, adsorption of water vapor in the interdiffusion layer hinders the supply of water vapor to the main reaction site of the electrochemical cell (i.e., the surface of the functional layer on the solid electrolyte layer side and its vicinity). Furthermore, when the electrochemical cell is used as an SOFC, adsorption of water vapor generated in the reaction site to the interdiffusion layer is a problem. These problems contribute to a decrease in the performance of the electrochemical cell. Additionally, ceria-zirconia has a high oxygen release / storage capacity. The oxygen release / storage capacity refers to the property of ceria-zirconia releasing oxide ions in a reducing atmosphere and absorbing surrounding oxide ions in an oxidizing atmosphere. The oxygen release / storage capacity of ceria-zirconia causes a decrease in the performance of the electrochemical cell, whether the electrochemical cell is used as an SOEC or an SOFC.

[0010] The water vapor adsorption capacity and oxygen release / storage capacity of ceria-zirconia described above are factors that reduce the durability of electrochemical cells. This is thought to be because, when an electrochemical cell is operated over a long period of time, the interdiffusion layer gradually changes, causing the water vapor adsorption capacity and oxygen release / storage capacity of ceria-zirconia to increase. The durability of an electrochemical cell decreases as the interdiffusion layer becomes thicker.

[0011] On the other hand, when an interdiffusion layer is formed, the interdiffusion layer ensures adhesion between the functional layer and the support layer, so if the interdiffusion layer is too thin, the adhesion will decrease.

[0012] In this way, when the functional layer contains Ni and GDC and the support layer contains Ni and YSZ, the problem caused by the configuration in which both layers contain Ni and YSZ (deterioration in performance due to Ni migration and aggregation) can be improved, but another problem caused by the interdiffusion layer formed between the functional layer and the support layer (deterioration in durability or adhesion) occurs. This is not limited to the configuration in which the ion-conductive oxide in the functional layer is GDC and the ion-conductive oxide in the support layer is YSZ, but is thought to occur when the types of ion-conductive oxides in both layers are different from each other (in other words, when an interdiffusion layer is formed between the functional layer and the support layer).

[0013] The present invention has been made to address the above-mentioned problems. That is, one of the objects of the present invention is to provide a technology that can suppress a decrease in durability while ensuring adhesion of an electrochemical cell in which an interdiffusion layer is formed between a functional layer and a support layer of a fuel electrode layer.

[0014] The electrochemical cell (21) according to the present invention comprises: a solid electrolyte layer (211), an anode layer (213) laminated on one side of the solid electrolyte layer, and an air cathode layer (212) laminated on the other side of the solid electrolyte layer. The anode layer includes a functional layer (213a), a support layer (213b) located farther from the solid electrolyte layer than the functional layer, and an interdiffusion layer (213c) located between the functional layer and the support layer so as to be in contact with both, the interdiffusion layer including a first element that is one of the elements constituting the functional layer and a second element that is one of the elements constituting the support layer and different from the first element, and the thickness (T1) of the interdiffusion layer is 1.1 μm or more and 9.7 μm or less.

[0015] If the thickness of the interdiffusion layer is less than 1.1 μm, adhesion is reduced, and there is a high possibility that the functional layer and the support layer will peel off. Furthermore, if the thickness of the interdiffusion layer exceeds 9.7 μm, durability is reduced. In contrast, in the electrochemical cell according to the present invention, the thickness of the interdiffusion layer is in the range of 1.1 μm or more and 9.7 μm or less, so that the adhesion of the electrochemical cell is ensured while reducing durability. In addition, deterioration in the initial performance of the electrochemical cell due to reduced adhesion can be suppressed. The initial performance refers to the performance of the electrochemical cell after an initial reduction treatment. The initial reduction treatment is a treatment for reducing NiO (nickel oxide) contained in the green sheets of each layer constituting the fuel electrode layer to Ni during the manufacturing process of the electrochemical cell.

[0016] 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 functional layer, and the second element may be an element constituting an ion-conductive oxide in the support layer. The first element may be a metal element in the ion-conductive oxide in the functional layer, and the second element may be a metal element in the ion-conductive oxide in the support layer. Furthermore, the concentration of the first element contained in the interdiffusion layer decreases from the functional layer side to the support layer side, and the concentration of the second element contained in the interdiffusion layer decreases from the support layer side to the functional layer side. Therefore, the interdiffusion layer can also be said to be a layer containing the first and second elements, in which the concentration of the first element decreases from the functional layer side to the support layer side and the concentration of the second element decreases from the support layer side to the functional layer side.

[0017] In this specification, the thickness of the interdiffusion layer is calculated as follows. Specifically, the electrochemical cell is first cut along a plane along the thickness direction, and the concentration distributions of the first element and the second element in the fuel electrode layer at the cut surface are calculated along the thickness direction. Next, the point where the calculated concentrations of the first element and the second element are equal is defined as the midpoint. The thickness of the interdiffusion layer is calculated as the length between the thickness position (i.e., the thickness position of the cut surface) where the concentration of the first element, measured from the midpoint toward the support layer, first drops to 2.000 wt %, and the thickness position where the concentration of the second element, measured from the midpoint toward the functional layer, first drops to 2.000 wt %. The concentration of the first element can be calculated by measuring the concentration of the first element at multiple locations on the cut surface at the same thickness position and averaging these measurements. Similarly, the concentration of the second element can be calculated by measuring the concentration of the second element at multiple locations on the cut surface at the same thickness position and averaging these measurements.

[0018] The fuel electrode layer may be configured to include a plurality of functional layers, in which case the fuel electrode layer includes an interdiffusion layer located between the functional layer that is located closest to the support layer and the support layer so as to be in contact with both of them.

[0019] In one aspect of the present invention, the first element is one of Ce or Zr, and the second element is the other of Ce or Zr.

[0020] When the combination of the first element and the second element contained in the interdiffusion layer is Ce and Zr, or Zr and Ce, the influence of the water vapor adsorption capacity and oxygen release / storage capacity of the complex oxide is stronger than in interdiffusion layers containing other combinations of the first element and the second element. However, in the present invention, because the thickness of the interdiffusion layer is within the above-mentioned numerical range, the influence of the water vapor adsorption capacity and oxygen release / storage capacity can be suppressed, and as a result, the durability of the electrochemical cell can be improved.

[0021] In one aspect of the present invention, the thickness (T1) of the interdiffusion layer (213c) is smaller than the thickness (T2) of the functional layer (213a).

[0022] This configuration ensures sufficient reaction sites within the functional layer (in other words, it prevents the presence of the interdiffusion layer from reducing the reaction sites within the functional layer), thereby improving the durability of the electrochemical cell compared to a configuration in which the interdiffusion layer is thicker than the functional layer.

[0023] In one aspect of the present invention, the functional layer (213a), the support layer (213b), and the interdiffusion layer (213c) each contain Ni, and the Ni concentration in each layer increases in the order of the support layer, the interdiffusion layer, and the functional layer.

[0024] With this configuration, the three-phase interface increases in the order of the support layer, the interdiffusion layer, and the functional layer. Therefore, the reaction activity is highest in the functional layer, second highest in the interdiffusion layer, and lowest in the support layer. By configuring each layer in this way so that the reaction activity is higher in the layer closer to the solid electrolyte layer, the performance of the electrochemical cell can be maintained at a good level.

[0025] The Ni concentration in the interdiffusion layer has a gradient along the thickness direction. For this reason, in this specification, the Ni concentration at the midpoint position in the interdiffusion layer is defined as the Ni concentration in the interdiffusion layer. The Ni concentration at the midpoint position can be calculated by measuring the Ni concentration at the midpoint position on the cut surface at multiple points and averaging these measured values.

[0026] The solid oxide electrolysis cell (21) according to the present invention comprises an electrochemical cell according to the present invention.

[0027] According to the above configuration, it is possible to provide a solid oxide electrolysis cell in which adhesion is ensured and a decrease in durability is suppressed.

[0028] The cell stack (20) according to the present invention is formed by stacking the solid oxide electrolysis cells (21) according to the present invention.

[0029] 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 and a decrease in durability is suppressed.

[0030] The hot module (10) of the present invention comprises a cell stack (20) of the present invention, a vaporizer (30) that generates steam to be supplied to the cell stack, a heat exchanger (40) that exchanges heat with gas supplied to the cell stack, a heater (50) that heats the cell stack, and a thermal insulator (60) in which the cell stack, the vaporizer, the heat exchanger, and the heater are arranged.

[0031] 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 a decrease in durability is suppressed.

[0032] The hydrogen production device (1) according to the present invention comprises the hot module (10) according to the present invention.

[0033] 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 a decrease in durability is suppressed.

[0034] Fig. 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 view showing how an interdiffusion layer is formed. Fig. 8 is a view for explaining a method for calculating the thickness of an interdiffusion layer.

[0035] Hereinafter, an embodiment of the present invention 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.

[0036] 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.

[0037] The vaporizer 30 contains water (H ２0) 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.

[0038] 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.

[0039] 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.

[0040] 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 condensing 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).

[0041] 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.

[0042] 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 (up-down 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.

[0043] 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.

[0044] The electrolysis cell 21 is the smallest unit of the SOEC and includes a solid electrolyte layer 211, a cathode layer 212, and an anode layer 213. 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 lower surface of the solid electrolyte layer 211.

[0045] The interconnector 22 is a rectangular metal (for example, stainless steel) member that has a rectangular current collecting part 22a that protrudes downward from the center of its lower surface. A pair of interconnectors 22 is arranged on both sides of the electrolysis cell 21 in the thickness direction. Two adjacent electrolysis units Ue, Ue share one interconnector 22. The interconnector 22 also functions as a separator that separates the two adjacent electrolysis units Ue, Ue. The lower surface of the current collecting part 22a is in contact with the upper surface of the air cathode layer 212 of the electrolysis cell 21. The lower electrolysis unit Ue includes a pair of interconnectors 22, 29 instead of the pair of interconnectors 22, 22. The interconnector 29 is arranged at the bottom end of the cell stack 20 and differs from the interconnector 22 in that it does not have a current collecting part 22a.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The internal space of the electrolysis unit Ue is partitioned into an air chamber Sa and a fuel chamber Sf by the separator 23. The air chamber Sa is a space that allows the flow of oxygen generated in the air electrode layer 212, and is defined by a space surrounded by the separator 23, the interconnector 22 above the separator 23, the air electrode frame 24, and the electrolysis cell 21. The fuel chamber Sf is a space that allows the flow of hydrogen generated in the fuel electrode layer 213, and is defined by a space surrounded by the separator 23, the interconnector 22 (or interconnector 29) below the separator 23, the fuel electrode frame 25, and the electrolysis cell 21.

[0050] 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.

[0051] 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 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."

[0052] 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.

[0053] 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.

[0054] Next, the configuration of the electrolysis cell 21 will be described in more detail with reference to FIG. 4 . The size and thickness of each layer of the electrolysis cell 21 shown below are merely examples and are not limited to these values. 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. The solid electrolyte layer 211 is a rectangular flat layer measuring 150 mm square and 6 μm thick, and is configured to contain YSZ, 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).

[0055] The air electrode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact the upper surface of the solid electrolyte layer 211. The air electrode layer 212 is a rectangular flat layer with a thickness of 108 μm, and is configured to contain a perovskite-type oxide such as LSCF (lanthanum strontium cobalt iron oxide), and is formed by sintering. The air electrode layer 212 has a functional layer and a current collecting layer. The current collecting layer is thicker than the functional layer and is disposed on the upper surface of the functional layer. The air electrode layer 212 has high electronic conductivity and effectively collects electrons from the current collecting layer. The air electrode layer 212 is a porous layer and has pores inside.

[0056] The anode layer 213 is a rectangular flat layer measuring 150 mm on each side, and is formed to have a thickness greater than that of the solid electrolyte layer 211 and that of the air cathode layer 212, for example, approximately 400 μm. The anode layer 213 supports the solid electrolyte layer 211 and the air cathode layer 212. In other words, the electrolysis cell 21 is an anode-supported cell. The anode layer 213 includes a functional layer 213 a, a support layer 213 b, and an interdiffusion layer 213 c. The functional layer 213 a is laminated on the lower surface side of the solid electrolyte layer 211. The support layer 213 b is located below the functional layer 213 a (i.e., on the side away from the solid electrolyte layer 211). The interdiffusion layer 213 c is located between the functional layer 213 a and the support layer 213 b so as to be in contact with both. That is, a functional layer 213a, an interdiffusion layer 213c, and a support layer 213b are stacked in this order on the lower surface of the solid electrolyte layer 211. The thickness of the support layer 213b is significantly thicker than the functional layer 213a and the interdiffusion layer 213c, and the thickness ratio can be set to, for example, about 16 to 40 times.

[0057] The main component of the support layer 213b is a cermet of Ni as a catalytic metal and YSZ, an ion-conductive oxide. The support layer 213b is a porous layer configured to have a porous shape including a plurality of micropores (not shown). The diameter of the micropores is on the order of several μm, which ensures the permeability of water vapor (gas diffusibility).

[0058] The main component of the functional layer 213a is a cermet of Ni as a catalytic metal and GDC, an ion-conductive oxide. Like the support layer 213b, the functional layer 213a is a porous layer configured to have a porous shape including a plurality of micropores (not shown). However, the functional layer 213a is formed more densely than the support layer 213b. That is, the porosity of the functional layer 213a is smaller than the porosity of the support layer 213b. Like the solid electrolyte layer 211 and the air electrode layer 212, the fuel electrode layer 213 is formed by sintering.

[0059] The interdiffusion layer 213c is a layer containing Ce in GDC, an ion-conductive oxide in the functional layer 213a, and Zr in YSZ, an ion-conductive oxide in the support layer 213b. Ce and Zr are examples of the "first element" and "second element," respectively.

[0060] The interdiffusion layer 213c is a layer generated when the electrolytic cell 21 is sintered. FIG. 5 is a schematic diagram showing the generation of the interdiffusion layer 213c. As shown in FIG. 5(a), before the electrolytic cell 21 is sintered, a green sheet G1 of the functional layer 213a mainly composed of Ni and GDC and a green sheet G2 of the support layer 213b mainly composed of Ni and YSZ 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 (Ce) in GDC, which is an ion-conductive oxide in the green sheet G1, diffuses into the green sheet G2, and the metal element (Zr) in YSZ, 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 functional layer 213a and the green sheet G2 becomes the support layer 213b, and an interdiffusion layer 213c containing Ce and Zr is formed between them. 5( c), the inter-diffusion layer 213c includes a functional layer-side diffusion layer 213c1 and a support layer-side diffusion layer 213c2. The functional layer-side diffusion layer 213c1 is in contact with the functional layer 213a and contains Ce and Zr, with the Ce concentration [wt%] equal to or greater than the Zr concentration [wt%] and the Zr concentration being 2.000 wt% or greater. The support layer-side diffusion layer 213c2 is in contact with the support layer 213b and contains Ce and Zr, with the Zr concentration [wt%] equal to or greater than the Ce concentration [wt%] and the Ce concentration being 2.000 wt% or greater. The thickness T1 of the inter-diffusion layer 213c is the sum of the thicknesses of the functional layer-side diffusion layer 213c1 and the support layer-side diffusion layer 213c2.

[0061] As described above, the functional layer-side diffusion layer 213c1 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 functional layer-side diffusion layer 213c1 can be considered a layer in which the functional layer 213a has been altered, but in this embodiment, the functional layer-side diffusion layer 213c1 is not included in the functional layer 213a. The support layer-side diffusion layer 213c2 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 support layer-side diffusion layer 213c2 can be considered a layer in which the support layer 213b has been altered, but in this embodiment, the support layer-side diffusion layer 213c2 is not included in the support layer 213b. The points in the thickness direction where the Ce concentration and the Zr concentration are the same are considered to be both the functional layer-side diffusion layer 213c1 and the support layer-side diffusion layer 213c2.

[0062] Thus, the interdiffusion layer 213c is formed between the functional layer 213a and the support layer 213b, and is a layer containing Ce (first element), which is a metal element in the ion-conductive oxide (GDC) in the functional layer 213a, and Zr (second element), which is a metal element in the ion-conductive oxide (YSZ) in the support layer 213b.

[0063] As is clear from the above description, the functional layer 213a, the support layer 213b, and the inter-diffusion layer 213c all contain Ni. In this embodiment, the electrolytic cell 21 is configured so that the Ni concentration in each layer increases in the order of the support layer 213b, the inter-diffusion layer 213c, and the functional layer 213a. The Ni concentration in the inter-diffusion layer 213c has a gradient along the thickness direction. For this reason, in this specification, the Ni concentration at the midpoint position (described below) in the inter-diffusion layer 213c is defined as the Ni concentration in the inter-diffusion layer 213c. The Ni concentration at the midpoint position can be calculated by measuring the Ni concentration at multiple points at the midpoint position on a cross section obtained by cutting the electrolytic cell 21 along the thickness direction and averaging these measurements. Meanwhile, the Ni concentrations in the functional layer 213a and the support layer 213b can be calculated using a well-known method.

[0064] The thickness T1 of the interdiffusion layer 213c can be controlled by adjusting the baking temperature and baking time of the fuel electrode layer 213. In this embodiment, the baking temperature and baking time of the fuel electrode layer 213 are adjusted so that the thickness T1 of the interdiffusion layer 213c is 1.1 μm or more and 9.7 μm or less.

[0065] A method for calculating the thickness T1 of the interdiffusion layer 213c will be described with reference to FIG. 6 . FIG. 6 shows the Ce concentration distribution and the Zr concentration distribution with respect to the thickness position (i.e., the position in the thickness direction of the cut surface) of the fuel electrode layer 213 on a cut surface obtained by cutting the electrolytic cell 21 along the thickness direction. These concentration distributions were calculated from elemental mapping images obtained by area analysis of the cut surface using an EPMA device (manufactured by JEOL Ltd.). In this embodiment, the Ce and Zr concentrations are calculated at intervals of 0.1 μm along the thickness direction, but the concentration calculation interval is not limited to this. The Ce concentration is calculated by measuring the Ce concentration at multiple locations at the same thickness position on the cut surface and averaging these measurements. Similarly, the Zr concentration is calculated by measuring the Zr concentration at multiple locations at the same thickness position on the cut surface and averaging these measurements.

[0066] The horizontal axis of the graph in Figure 6 indicates the thickness position (µm) from the midpoint position (described below) (in other words, the distance in the thickness direction from the midpoint position). In Figure 6, the thickness position from the midpoint position toward the functional layer 213a is indicated by a negative value, and the thickness position from the midpoint position toward the support layer 213b is indicated by a positive value. The vertical axis indicates the mass percent concentration of Ce or Zr relative to all elements present at the thickness position determined by the value on the horizontal axis, and all present elements are Ni, Ce, Y, O, Gd, and Zr. In the graph, the solid line A1 indicates the concentration distribution of Ce, and the dashed line B1 indicates the concentration distribution of Zr. Figure 6 shows the concentrations of Zr and Ce at thickness positions from the midpoint position to ±10 µm.

[0067] In the example of Figure 6, as shown by the solid line A1, the Ce concentration is approximately 40 wt% on the functional layer 213a side and less than approximately 2 wt% on the support layer 213b side. The Ce concentration distribution has a region where it rapidly decreases from the functional layer 213a side toward the support layer 213b side. Also, as shown by the dashed line B1, the Zr concentration is approximately 40 wt% on the support layer 213b side and less than approximately 2 wt% on the functional layer 213a side. The Zr concentration distribution has a region where it rapidly decreases from the support layer 213b side toward the functional layer 213a side.

[0068] As shown in FIG. 6 , there is a thickness position between the functional layer 213a and the support layer 213b where the Ce concentration and the Zr concentration are equal. In this specification, this thickness position is defined as the midpoint position, and the thickness position at the midpoint position is defined as 0 μm. Furthermore, the thickness position where the Ce concentration calculated from the midpoint position toward the support layer 213b first drops to 2.000 wt % is defined as position tb, and the thickness position where the Zr concentration calculated from the midpoint position toward the functional layer 213a first drops to 2.000 wt % is defined as position ta. In this case, the length in the thickness direction between positions ta and tb is calculated as the thickness T1 of the interdiffusion layer 213c. In the example of FIG. 6 , the thickness T1 is 6.0 μm, which is within the range of the thickness T1 described above.

[0069] As is clear from Figure 6, the interdiffusion layer 213c is a layer containing Ce and Zr at concentrations of 2.000 wt% or more, and in which the mass percentage concentration of Ce decreases from the functional layer 213a side toward the support layer 213b side, and the mass percentage concentration of Zr decreases from the support layer 213b side toward the functional layer 213a side.

[0070] Although not shown in FIG. 6 , the Ce concentration distribution includes a region where the concentration rapidly decreases from the functional layer 213a side toward the solid electrolyte layer 211 side. If the thickness position where the calculated Ce concentration toward the solid electrolyte layer 211 side in this region first drops to 2.000 wt % is defined as position tc (not shown), the length in the thickness direction between positions ta and tc is calculated as the thickness T2 of the functional layer 213a. That is, position tc is the thickness position of the interface of the functional layer 213a on the solid electrolyte layer 211 side. However, the method for defining the thickness position of the interface is not limited to this. For example, the thickness position of the interface may be defined by porosity. This is because the functional layer 213a is porous, while the solid electrolyte layer 211 is densely formed.

[0071] In this embodiment, the thickness T1 of the interdiffusion layer 213c and the thickness T2 of the functional layer 213a satisfy the relationship T1<T2. In other words, the green sheet G1 of the functional layer 213a is pre-prepared to have a thickness such that T1<T2 holds when the green sheets G1 and G2 are fired so that 1.1 μm≦T1≦9.7 μm is satisfied.

[0072] 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.

[0073] The water vapor that flows into the fuel chamber Sf passes through the support layer 213b and the interdiffusion layer 213c of the fuel electrode layer 213 and travels to the functional layer 213a. In the functional layer 213a, the water vapor reacts with electrons supplied from the end plate 28 via the current collector 26 and is decomposed into hydrogen and oxide ions (water vapor electrolysis reaction). The hydrogen generated by the water vapor electrolysis reaction diffuses within the fuel chamber Sf, is discharged through the horizontal hole 25b via path Pfo, and is recovered by a well-known method. At this time, unreacted water vapor can be discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 211 to the air 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.

[0074] The cell stack 20 operates as described above, and hydrogen is produced in the hydrogen production device 1 .

[0075] As described above, the ion-conductive oxide in the functional layer 213a of the electrolytic cell 21 according to this embodiment is GDC. Meanwhile, the ion-conductive oxide in the support layer 213b of the electrolytic cell 21 according to this embodiment is YSZ. Thus, when the functional layer 213a contains Ni and GDC and the support layer 213b contains Ni and YSZ, the performance degradation of the electrolytic cell 21 due to Ni migration and aggregation can be improved, but another problem occurs due to the interdiffusion layer 213c formed between the functional layer 213a and the support layer 213b. Specifically, when the thickness T1 of the interdiffusion layer 213c is small, adhesion decreases (and in some cases, initial performance also decreases). When the thickness T1 of the interdiffusion layer 213c is large, the interdiffusion layer 213c gradually changes during long-term operation of the electrolytic cell 21, and the water vapor adsorption capacity and oxygen release / occlusion capacity of the ceria-zirconia (a composite oxide of Ce and Zr present in the interdiffusion layer 213c) increase, resulting in reduced durability.

[0076] In the electrolytic cell 21 according to this embodiment, the thickness T1 of the inter-diffusion layer 213c is adjusted to be 1.1 μm or more and 9.7 μm or less. Having the thickness T1 of 1.1 μm or more ensures the adhesion of the electrolytic cell 21. Furthermore, having the thickness T1 of 9.7 μm or less suppresses the effect on the electrolytic cell 21 of an increase in the water vapor adsorption capacity and oxygen release / occlusion capacity of the ceria-zirconia due to alteration of the inter-diffusion layer 213c, thereby suppressing a decrease in durability. Thus, the electrolytic cell 21 according to this embodiment can suppress a decrease in durability while ensuring adhesion. Additionally, a decrease in the initial performance of the electrolytic cell 21 due to a decrease in adhesion can be suppressed.

[0077] Furthermore, in the electrolysis cell 21, the interdiffusion layer 213c contains one of the elements constituting the functional layer 213a and one of the elements constituting the support layer 213b, and in this embodiment, the former element is Ce and the latter element is Zr. This configuration can improve the durability of the electrolysis cell 21. Note that the former element may be Zr and the latter element may be Ce. That is, for example, the ion-conductive oxide in the functional layer 213a may be YSZ, and the ion-conductive oxide in the support layer 213b may be GDC. This configuration can also improve the durability of the electrolysis cell 21.

[0078] Furthermore, in the electrolysis cell 21, the thickness T1 of the inter-diffusion layer 213c is smaller than the thickness T2 of the functional layer 213a. This configuration ensures sufficient reaction sites within the functional layer 213a (in other words, it prevents the presence of the inter-diffusion layer 213c from reducing the reaction sites within the functional layer 213a). This improves the durability of the electrolysis cell 21 compared to a configuration in which the thickness T1 is larger than the thickness T2.

[0079] Furthermore, in the electrolytic cell 21, the functional layer 213a, the support layer 213b, and the interdiffusion layer 213c each contain Ni, and the Ni concentration in each layer increases in the order of the support layer 213b, the interdiffusion layer 213c, and the functional layer 213a. With this configuration, the three-phase interface increases in the order of the support layer 213b, the interdiffusion layer 213c, and the functional layer 213a. Therefore, the reaction activity is highest in the functional layer 213a, second highest in the interdiffusion layer 213c, and lowest in the support layer 213b. By configuring each layer in this way so that the reaction activity increases the closer to the solid electrolyte layer 211, the better the performance of the electrolytic cell 21 can be maintained.

[0080] (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 a functional layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0081] 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 a support layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0082] A butyral resin, a polyvinyl acetal resin as a plasticizer, a known dispersant, and a mixed solvent of toluene and MEK 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.

[0083] Next, a green sheet for the functional layer and a green sheet for the support 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 being heated and evacuated. This resulted in the formation of a laminate including the green sheet for the solid electrolyte layer, the green sheet for the functional layer, and the green sheet for the support layer.

[0084] 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 (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. Since the ion-conductive oxide (GDC) in the functional layer and the ion-conductive oxide (YSZ) in the support layer were different, an interdiffusion layer containing Ce and Zr was formed between the functional layer and the support layer after sintering.

[0085] 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.

[0086] In this example, eight electrolysis cell samples (Samples 1 to 8) with different interdiffusion layer thicknesses T1 and functional layer thicknesses T2 were prepared by adjusting the firing time while changing the firing temperature (first temperature t0) during primary firing, for example, between 1000°C and 1400°C. In Samples 1 to 8, the first temperature t0 increases and the firing time increases as the sample number (1 to 8) increases. The thickness of the interdiffusion layer tends to increase as the first temperature t0 increases and the firing time increases. The first temperatures t0 for each sample are specifically as follows: The first temperature t0 for sample 1 is 1090°C, the first temperature t0 for sample 2 is 1120°C, the first temperature t0 for sample 3 is 1140°C, the first temperature t0 for sample 4 is 1190°C, the first temperature t0 for sample 5 is 1240°C, the first temperature t0 for sample 6 is 1290°C, the first temperature t0 for sample 7 is 1340°C, and the first temperature t0 for sample 8 is 1390°C.

[0087] Furthermore, if the first temperature t0 is low, the electrolytic cell may not be sufficiently sintered. Therefore, in this embodiment, the particle size of the YSZ powder used to form the green sheet for the solid electrolyte layer is controlled so that the electrolytic cell can be sufficiently sintered even when the first temperature t0 is low. Specifically, a commercially available YSZ powder is pre-pulverized to reduce its particle size (average particle size D50), and the YSZ powder used to form the green sheet for the solid electrolyte layer is used. It is known that the smaller the particle size of the YSZ powder used, the better the sinterability. Therefore, even when the first temperature t0 is low, sufficient sinterability can be maintained by reducing the particle size of the YSZ powder used to form the green sheet for the solid electrolyte layer. Hereinafter, the pre-pulverized YSZ powder used to form the green sheet for the solid electrolyte layer is referred to as pulverized YSZ powder.

[0088] In this example, for samples (samples 1, 2, 3, 4, and 5) having a firing temperature (first temperature t0) of 1240°C or less, the YSZ powder was pre-ground 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 6, 7, and 8) having a firing temperature (first temperature t0) of more than 1240°C, the YSZ powder was pre-ground so that the average particle diameter (median diameter) PD of the pulverized YSZ powder was more than 0.20 μm (specifically, 0.27 μm).

[0089] 2. Measurement of Thickness T1 of the Interdiffusion Layer and Thickness T2 of the Functional Layer Samples 1 to 8 were cut along a plane along the thickness direction of the electrolysis cell, and the thickness T1 of the interdiffusion layer and the thickness T2 of the functional layer of Samples 1 to 8 at the cut plane were measured using the method described above.

[0090] 3. Measurement of Ni concentration in each layer constituting the fuel electrode layer The Ni concentration in the interdiffusion layer of Samples 1 to 8 on the cut surface was measured by the method described above, and the Ni concentration in the functional layer and support layer was measured by a known method.

[0091] 4. Measurement of Current Density For Samples 1 to 8, the samples were heated to 700°C, and a constant voltage (1.3 V in this example) 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 per unit area (current density) flowing during the steam electrolysis reaction was measured, and the measured current density was used as an index for evaluating performance. That is, Samples 1 to 8 were operated as SOECs. It can be said that the higher the current density, the better the initial performance.

[0092] 5. Measurement of Durability Degradation Rate For Samples 1 to 8, 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. That is, Samples 1 to 8 were operated as an SOEC. 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.

[0093] 6. Evaluation Table 1 shows the thickness T1 of the interdiffusion layer, the thickness T2 of the functional layer, the Ni concentration in each fuel electrode layer, the current density, and the durability deterioration rate obtained for Samples 1 to 8.

[0094] 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 to form the green sheet for the solid electrolyte layer. ２ When the current density was 0.7 A / cm or more, the initial performance was evaluated as very good (◎). ２ 0.9A / cm or more ２ When the current density is less than 0.5 A / cm, the initial performance is evaluated as good (◯). ２ 0.7A / cm or more ２ When the current density is less than 0.5 A / cm, the initial performance is evaluated as fair (△). ２When the current density was less than 3.0%, the initial performance was evaluated as poor (×). When the current density could not be measured, the initial performance was evaluated as ineligible for evaluation (-). Regarding the durability degradation rate, when the durability degradation rate was less than 3.0%, the durability was evaluated as good (◯), and when the durability degradation rate was 3.0% or more, the durability was evaluated as poor (×). When the durability degradation test could not be continued for the specified time, the durability was evaluated as ineligible for evaluation (-).

[0095] As can be seen from Table 1, for Samples 1 to 8, as the sample number (1 to 8) increases, the thickness T1 of the interdiffusion layer increases and the thickness T2 of the functional layer decreases. For Samples 3 to 6, the initial performance is very good (◎) and the durability is good (◯). In contrast, for Sample 1, both the initial performance and durability are unassessable (-). For Sample 2, the initial performance is good (◯), but the durability is unassessable (-) because the durability degradation test showed rapid deterioration in less than 400 hours, forcing the test to be discontinued. For Samples 7 and 8, the initial performance is very good (◎), but the durability is poor (×).

[0096] The initial performance and durability of Sample 1 were rated "unclassifiable" (-). This is likely due to the fact that the thickness T1 of the interdiffusion layer in Sample 1 was 0.3 μm, significantly smaller than the thicknesses of the interdiffusion layers in Samples 3 to 8, resulting in decreased adhesion. The initial performance of Sample 2 was improved because the thickness T1 of the interdiffusion layer was slightly larger than that of Sample 1, resulting in slightly improved adhesion. However, because the thickness T1 was still thin, adhesion decreased over the course of long-term operation, which is likely to be the reason for the durability rating of "unclassifiable" (-). The initial performance of Samples 7 and 8 was rated "very good" (◎). This is likely due to the sufficiently large thickness T1 of the interdiffusion layer, ensuring adhesion. On the other hand, the durability was rated "poor" (x). This is likely due to the excessive thickness T1, which gradually altered the interdiffusion layer over long-term operation, resulting in increased water vapor adsorption capacity and oxygen release / storage capacity of the ceria-zirconia.

[0097] Therefore, adhesion can be ensured and deterioration in durability can be suppressed within the range of the thickness T1 of the inter-diffusion layer in Samples 3 to 6. That is, when the thickness T1 of the inter-diffusion layer is 1.1 μm or more and 9.7 μm or less, adhesion of the electrolysis cell can be ensured and deterioration in durability can be suppressed.

[0098] Furthermore, as described above, the durability of Samples 3 to 6 is all good (◯), but among these, the durability of Samples 3 to 5 is particularly good compared to the durability of Sample 6. This is thought to be because the relationship between the thickness T1 of the interdiffusion layer and the thickness T2 of the functional layer is T1<T2 for Samples 3 to 5, which ensures sufficient reaction sites within the functional layer, whereas for Sample 6, T1>T2, which means that the presence of the interdiffusion layer slightly reduces the reaction sites in the functional layer.

[0099] Furthermore, the very good initial performance (◎) and good durability (◯) for Samples 3 to 6 are thought to be due to the fact that the thickness T1 of the inter-diffusion layer was within the above-mentioned range and the Ni concentration of each layer of the fuel electrode layer increased in the order of the support layer, the inter-diffusion layer, and the functional layer. That is, when the Ni concentration of each layer increases in this order, the functional layer had the highest reactivity, the inter-diffusion layer had the second highest reactivity, and the support layer had the lowest reactivity. Thus, by configuring each layer so that the layer closer to the solid electrolyte layer has the highest reactivity, it is thought that the electrolysis cell performance can be maintained at a good level. The Ni concentrations of each layer for Samples 1, 2, 7, and 8 also had the above-mentioned magnitude relationship. However, for these samples, the "influence of an excessively small or large thickness T1 of the inter-diffusion layer (particularly the influence on durability)" outweighs the "influence of configuring the Ni concentration of each layer as described above," and therefore a decrease in the performance of the electrolysis cell is thought to be unavoidable.

[0100] Furthermore, the smaller the thickness T1 of the inter-diffusion 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 inter-diffusion layer be as small as possible. Specifically, the thickness T1 of the inter-diffusion layer is preferably 7.9 μm or less, more preferably 6.0 μm or less, and even more preferably 4.6 μm or less.

[0101] In the above examples, the performance of the electrolytic cell was evaluated by operating it as an SOEC, but similar results were obtained when Samples 1 to 8 were fabricated as fuel cells (the cell configuration was the same) and operated as SOFCs. That is, for Samples 3 to 6, the initial performance was very good (◎) and the durability was good (◯).

[0102] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0103] For example, although the above embodiment shows an example in which the interdiffusion layer contains Ce as the first element and Zr as the second element, the first element and the second element may be other elements. Also, although the above embodiment shows an example in which the ion-conductive oxide in the functional layer is GDC and the ion-conductive oxide in the support layer is YSZ, the technology according to the present invention can be applied when the metal elements in the ion-conductive oxides in both layers are different.

[0104] Furthermore, as long as the thickness T1 of the interdiffusion layer 213c is 1.1 μm or more and 9.7 μm or less, the thickness T1 may be greater than the thickness T2 of the functional layer 213a (see Sample 6 of the example).

[0105] Furthermore, as long as the thickness T1 of the interdiffusion layer 213c is within the above numerical range, the Ni concentration in each layer of the fuel electrode layer 213 is not limited to the configuration in which the Ni concentration increases in the order of the support layer 213b, the interdiffusion layer 213c, and the functional layer 213a.

[0106] 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 the anode layer includes a functional layer, a support layer located farther from the solid electrolyte layer than the functional layer, and an interdiffusion layer located between the functional layer and the support layer so as to be in contact with both, wherein the interdiffusion layer includes a first element that is one of the elements constituting the functional layer, and a second element that is one of the elements constituting the support layer and different from the first element, and wherein the thickness of the interdiffusion layer is 1.1 μm or more and 9.7 μm or less. [2] The electrochemical cell according to [1], wherein the first element is one of Ce or Zr, and the second element is the other of Ce or Zr. [3] The electrochemical cell according to [1] or [2], wherein the thickness of the interdiffusion layer is smaller than the thickness of the functional layer. [4] The electrochemical cell according to any of [1] to [3], wherein the functional layer, the support layer, and the interdiffusion layer each contain Ni, and the Ni concentration in each layer increases in the order of the support layer, the interdiffusion layer, and the functional layer. [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 a gas supplied to the cell stack, a heater for heating the cell stack, and a thermal insulation material in which the cell stack, the vaporizer, the heat exchanger, and the heater are placed. [8] A hydrogen production device comprising the hot module according to [7].

[0107] 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, 212...air electrode layer, 213...fuel electrode layer, 213a...functional layer, 213b...support layer, 213c...interdiffusion layer, 213c1...functional layer-side diffusion layer, 213c2...support layer-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 the fuel electrode layer includes a functional layer, a support layer located farther from the solid electrolyte layer than the functional layer, and an interdiffusion layer located between the functional layer and the support layer so as to be in contact with both, wherein the interdiffusion layer includes a first element which is one of the elements constituting the functional layer, and a second element which is one of the elements constituting the support layer and different from the first element, and wherein the thickness of the interdiffusion layer is 1.1 μm or more and 9.7 μm or less.

2. An electrochemical cell according to claim 1, wherein the first element is one of Ce or Zr, and the second element is the other of Ce or Zr.

3. An electrochemical cell according to claim 1 or 2, wherein the thickness of the interdiffusion layer is smaller than the thickness of the functional layer.

4. An electrochemical cell according to claim 1 or 2, wherein the functional layer, the support layer and the interdiffusion layer each contain Ni, and the Ni concentration in each layer increases in the order of the support layer, the interdiffusion layer and the functional layer.

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

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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