Interconnector for solid oxide electrochemical cell stack and solid oxide electrochemical cell stack

Abstract

To improve the adhesion of a protective film to a metal substrate and also improve electrical conduction properties by reducing electrical resistance.SOLUTION: An interconnector for a solid oxide electrochemical cell stack includes: a metal substrate which includes a chromium-containing iron-based alloy; and a protective film which is provided on the metal substrate in a first direction. The protective film has: a metal layer which is provided on a surface of the metal substrate and contains a first metal element; an oxide layer which is provided above the metal layer and contains at least one oxide selected from the group consisting of a spinel oxide and a perovskite oxide containing a second metal element different from the first metal element; and a mixed layer which is provided between the metal layer and the oxide layer and has a first phase containing the first metal element and a second phase containing the at least one oxide.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to an interconnector for a solid oxide electrochemical cell stack and a solid oxide electrochemical cell stack. [Background technology]

[0002] Hydrogen is one of the new energy sources for a decarbonized society. Fuel cells, which convert chemical energy into electrical energy through an electrochemical reaction between hydrogen and oxygen, have attracted attention as a hydrogen-based energy source. Fuel cells have high energy utilization efficiency and are being developed for large-scale distributed power generation, residential power generation, and mobile power generation. Among fuel cells, solid oxide fuel cells (SOFCs), which generate electrical energy through an electrochemical reaction using a solid oxide electrolyte, have attracted attention due to their efficiency. Furthermore, research into solid oxide electrolysis cells (SOECs), which apply high-temperature steam electrolysis to electrolyze water in a high-temperature steam state, is also underway for hydrogen production. The operating principle of SOECs is the reverse reaction of SOFCs, and like SOFCs, they use an electrolyte made of a solid oxide. Furthermore, SOECs electrolyze carbon dioxide (CO2) to produce carbon monoxide (CO), which can then be synthesized with hydrogen (H2) to ultimately produce fuels such as methane (CH4). Therefore, they are attracting attention as a technology for realizing a decarbonized society.

[0003] Solid oxide electrochemical cells used in SOFCs and SOECs have a laminated structure consisting of an air electrode (oxygen electrode), a solid oxide electrolyte layer, and a hydrogen electrode (fuel electrode). Multiple electrochemical cells with such laminated structures are stacked together via interconnectors to form large-capacity electrochemical cell stacks. Interconnectors for solid oxide electrochemical cells require high-temperature resistance, so stainless steel alloys with a high chromium content are typically used as the interconnector substrate. However, at high temperatures, an oxide film consisting primarily of Cr2O3 forms on the surface of high-chromium stainless steel alloys. Because Cr2O3 has high electrical resistance, the surface oxide film reduces the electrical conductivity of the interconnect and increases the resistance of the solid oxide electrochemical cell stack. Furthermore, if the Cr component in the Cr2O3 film gasifies and adheres to the electrodes of the solid oxide electrochemical cell, it degrades the performance of the solid oxide electrochemical cell. To mitigate these issues, various materials for interconnectors in solid oxide electrochemical cell stacks have been extensively investigated.

[0004] Generally, Cr dispersion is suppressed by covering interconnectors for solid oxide electrochemical cell stacks with a dense protective film. The protective film is required to suppress Cr dispersion, provide electrical conductivity, adhesion, and workability. Cr dispersion suppression is a function to prevent Cr contained in interconnectors for solid oxide electrochemical cell stacks from vaporizing under high-temperature operating conditions, which reduces the performance of the solid oxide electrochemical cell. Electrical conductivity is a function to reduce electrical resistance during current flow and minimize energy loss, since the reaction in solid oxide electrochemical cells requires electricity. Because the operating temperature of solid oxide electrochemical cells is high (600°C or higher), repeated temperature increases and decreases between such high temperatures and room temperature can cause the protective film to peel off. Workability is a function to prevent this from happening, as interconnectors for solid oxide electrochemical cell stacks often have complex shapes.

[0005] For the protective film of the interconnector, an oxide that exhibits high electrical conductivity in the operating temperature range, such as an electrically conductive spinel-type oxide or a perovskite-type oxide, is used. However, from the viewpoint of adhesion with the interconnector for a solid oxide electrochemical cell, it is necessary to improve the material, structure, etc., even for a spinel-type oxide that has a smaller difference in thermal expansion coefficient from the interconnector for a solid oxide electrochemical cell.

[0006] In response to this, a technique is known in which the composition of a spinel-type oxide is adjusted to reduce the difference in thermal expansion with the interconnector for a solid oxide electrochemical cell, thereby improving adhesion. Also, a technique is known in which a protective film has an anchor structure that resembles roots growing on the interconnector for a solid oxide electrochemical cell, thereby improving adhesion and further improving electrical conductivity. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5770659 [Patent Document 2] Japanese Patent Publication No. 2020-066758 Summary of the Invention [Problem to be solved by the invention]

[0008] The problem to be solved by the present invention is to provide an interconnector for a solid oxide electrochemical cell stack that can improve the adhesion of a protective film to a metal substrate and reduce electrical resistance, thereby improving electrical conductivity, and a solid oxide electrochemical cell stack using such an interconnector. [Means for solving the problem]

[0009] An embodiment of the interconnector for a solid oxide electrochemical cell stack includes a metal substrate containing an iron-based alloy containing chromium, and a protective film provided on the metal substrate in a first direction. The protective film includes a metal layer provided on the surface of the metal substrate and containing a first metal element, an oxide layer provided above the metal layer and containing at least one oxide selected from the group consisting of spinel-type oxides and perovskite-type oxides containing a second metal element different from the first metal element, and a mixed layer provided between the metal layer and the oxide layer and having a first phase containing the first metal element and a second phase containing at least one oxide. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing an interconnector for a solid oxide electrochemical cell stack according to an embodiment. [Figure 2] 1 is a cross-sectional view showing a solid oxide electrochemical cell stack according to an embodiment. [Figure 3] FIG. 2 is a cross-sectional view of a protective film of an interconnector for a solid oxide electrochemical cell stack according to an embodiment. [Figure 4] 1 is a diagram showing, for comparison, backscattered electron images of cross sections of interconnectors for electrochemical cell stacks according to Comparative Example 1, Comparative Example 2, Example 1, and Example 2. FIG. [Figure 5] 1 is a diagram showing a comparison of the electrical resistance of the interconnectors for an electrochemical cell stack according to Comparative Example 1, Comparative Example 2, Example 1, and Example 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an interconnector for a solid oxide electrochemical cell stack and a solid oxide electrochemical cell stack according to an embodiment will be described with reference to the drawings. In each embodiment shown below, substantially identical components are denoted by the same reference numerals, and some of their descriptions may be omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each part, etc. may differ from the actual ones.

[0012] FIG. 1 shows a cross section of an interconnector for a solid oxide electrochemical cell stack. The interconnector 1 shown in FIG. 1 is an interconnector with an interconnector protective film for a solid oxide electrochemical cell. The interconnector 1 has a metal substrate 2 having a surface 2a and a surface 2b. When the interconnector 1 is used in a solid oxide electrochemical cell stack, the surface 2a of the metal substrate 2 is the surface that is placed on the hydrogen electrode (fuel electrode) side and is exposed to an atmosphere containing hydrogen. The surface 2b of the metal substrate 2 is the surface that is placed on the air electrode (oxygen electrode) side and is exposed to air. The surface 2b is provided, for example, on the opposite side of the surface 2a. Note that the flow of hydrogen is not limited to the surface 2a side; for example, in SOFCs, methanol (CHOH) or the like may be flowed, so that the surface 2a may be a surface that is exposed to an atmosphere containing a substance having hydrogen atoms. The flow of air is not limited to the surface 2b side; for example, in SOECs, nothing may be flowed, or oxygen may be flowed, so that the surface 2b may be a surface that is exposed to an atmosphere containing oxygen. A protective film 3 is provided on surfaces 2a and 2b of the metal base material 2. Although Fig. 1 shows the interconnector 1 in which the protective film 3 is provided on surfaces 2a and 2b of the metal base material 2, the protective film 3 may be provided on only one of surfaces 2a and 2b.

[0013] The interconnector 1 shown in FIG. 1 is used, for example, in an electrochemical cell stack 10 shown in FIG. 2. The electrochemical cell stack 10 shown in FIG. 2 is a solid oxide electrochemical cell stack, and has a structure in which an electrochemical cell 11 and an electrochemical cell 12 are stacked via the interconnector 1. Although FIG. 2 shows a structure in which the electrochemical cell 11 and the electrochemical cell 12 are stacked, the number of stacked electrochemical cells is not particularly limited, and the stacked structure may have three or more electrochemical cells. When three or more electrochemical cells are stacked, an interconnector is disposed between each two adjacent electrochemical cells, and the electrochemical cells are electrically connected by the interconnector.

[0014] The electrochemical cells 11 and 12 have the same configuration, each having an electrode 13 functioning as a hydrogen electrode (fuel electrode), an electrode 14 functioning as an oxygen electrode (air electrode), and a solid oxide electrolyte layer 15 disposed between the electrodes 13 and 14. The electrodes 13 and 14 are each formed of a porous electrical conductor. The solid oxide electrolyte layer 15 is made of a dense solid oxide electrolyte and is an ion conductor that does not conduct electricity. A porous current collecting member 16 may be disposed between the electrode 13 and the interconnector 1, as needed. Similarly, a porous current collecting member 17 may be disposed between the electrode 14 and the interconnector 1, as needed. The current collecting members 16 and 17 allow reactant gases to pass through while improving the electrical connection between the electrochemical cells 11 and 12 and the interconnector 1.

[0015] Although not shown in FIG. 2, gas flow paths are provided around the electrochemical cells 11 and 12. That is, supply gases according to the intended use of the electrochemical cell stack 10 are supplied to the electrodes 13 and 14 via parts of the gas flow paths, respectively. Exhaust gases generated and discharged from the electrodes 13 and 14 are discharged from the electrochemical cells 11 and 12 via other parts of the gas flow paths. The gases supplied to the electrodes 13 and 14 and the atmosphere surrounding the electrodes 13 and 14 are separated by a dense solid oxide electrolyte layer 15 and an interconnector 1. When the electrochemical cell stack 10 is used as a fuel cell such as an SOFC, a reducing gas such as hydrogen (H) or methanol (CHOH) gas is supplied to the electrode 13, which serves as the hydrogen electrode (fuel electrode), and an oxidizing gas such as air or oxygen (O) is supplied to the electrode 14, which serves as the oxygen electrode (air electrode). When the electrochemical cell stack 10 is used as an electrolysis cell such as an SOEC that employs high-temperature steam electrolysis, water vapor (H 2 O) is supplied to the electrode 13 that serves as a hydrogen electrode.

[0016] The interconnector 1 shown in Fig. 1 is used as an interconnector 1 disposed between an electrochemical cell 11 and an electrochemical cell 12 in an electrochemical cell stack 10 shown in Fig. 2. In the interconnector 1, a surface 2a of a metal substrate 2 is disposed on the side of an electrode 13 serving as a hydrogen electrode, and a surface 2b of the metal substrate 2 is disposed on the side of an electrode 14 serving as an air electrode. Therefore, the surface 2a of the metal substrate 2 is exposed to an atmosphere containing hydrogen, such as hydrogen supplied to the electrode 13 serving as a hydrogen electrode, or a mixed gas of hydrogen and water vapor, or a similar atmosphere containing hydrogen discharged from the electrode 13. The surface 2b of the metal substrate 2 is exposed to an atmosphere containing oxygen, such as air supplied to the electrode 14 serving as an air electrode.

[0017] In the interconnector 1 used in the electrochemical cell stack 10 shown in FIG. 2, the metal substrate 2 is an iron-based alloy containing chromium (Cr), i.e., stainless steel (SUS). In the electrochemical cell stack 10, the metal substrate 2 is made of ferritic stainless steel, such as SUS430, which has a thermal expansion coefficient close to that of the electrochemical cells 11 and 12. If stainless steel is used for the metal substrate 2, the Cr contained in the metal substrate 2 may react with oxygen or water vapor and vaporize at high temperatures of 600°C or higher and 1000°C or lower, which are the operating temperatures of SOFCs and SOECs, and may adhere to the electrodes 14, resulting in a decrease in performance. Therefore, to suppress the vaporization of chromium and the resulting diffusion, the interconnector 1 has a protective film 3 that covers at least the surface 2b of the metal substrate 2.

[0018] As shown in the cross-sectional schematic diagram of FIG. 3, the protective film 3 includes a metal layer 31, an oxide layer 32, and a mixed layer 33. FIG. 3 shows the X-axis, Y-axis, and Z-axis. The X-axis, Y-axis, and Z-axis are perpendicular to one another. FIG. 3 shows a portion of the XZ cross section. The thickness of the protective film 3 is preferably 1 μm or more and 50 μm or less. If the thickness is less than 1 μm, it becomes difficult to obtain the effects of the protective film 3. If the thickness exceeds 50 μm, the electrical resistance increases and the electrical conductivity may decrease significantly.

[0019] The metal layer 31 is provided in contact with the surfaces (surfaces 2a and 2b) of the metal substrate 2. The metal layer 31 has a metal phase 301 as a main phase. The main phase is the phase that has the largest volume ratio among the constituent phases of the layer. The metal phase 301 includes a first metal element. The first metal element is preferably a metal element that is resistant to oxidation in, for example, the operating environment of the solid oxide electrochemical cell. Examples of metal elements that are resistant to oxidation include gold (Au), silver (Ag), and platinum (Pt). Therefore, the first metal element is at least one of gold, silver, and platinum. The presence of the metal layer 31 between the metal substrate 2 and the oxide layer 32 can reduce the rate of cracking in the oxide layer 32.

[0020] Examples of methods for forming the metal layer 31 include electrolytic plating, electroless plating, spin coating, dip coating, and sol-gel. The metal layer 31 may be formed by depositing a metal film, or by depositing a metal oxide film and then reducing it. When reducing the metal oxide film, the interconnector with the protective film may be reduced alone, or may be reduced after being incorporated into a cell stack or a module.

[0021] The thickness of the metal layer 31 is preferably 0.5 μm or more and 10 μm or less. If it is less than 0.5 μm, it becomes difficult to obtain the effects of the metal layer 31. It is preferable that the metal layer 31 is thick because the first metal element may diffuse into the mixed layer 33 and decrease, but if it exceeds 10 μm, the thickness of the metal layer 31 in the planar direction of the interconnector 1 will vary greatly, which may have an adverse effect on the soundness of the interconnector 1.

[0022] The oxide layer 32 is provided above the metal layer 31. The oxide layer 32 is provided in contact with the outermost surface 3a of the protective film 3. The oxide layer 32 has an oxide phase 302 as a main phase. It is preferable that the oxide phase 302 is not contained in the metal layer 31. It is also preferable that the oxide layer 32 does not contain a metal phase 301.

[0023] The oxide phase 302 contains at least one oxide selected from spinel-type oxides and perovskite-type oxides that exhibit high electrical conductivity in the operating temperature range of SOFCs, SOECs, and the like. At least one oxide contains a second metal element different from the first metal element. The spinel-type oxide is an oxide expressed as AB2O4 (A and B are the same or different cationic elements such as metal elements). The perovskite-type oxide is an oxide expressed as ABO3 (A and B are the same or different cationic elements such as metal elements). Both spinel-type oxides and perovskite-type oxides exhibit electrical conductivity in the operating temperature range of SOFCs, SOECs, and the like, and are therefore suitable for the oxide layer 32, which requires electrical conductivity.

[0024] The second metal element is at least one element selected from the group consisting of cobalt (Co), nickel (Ni), manganese (Mn), copper (Cu), iron (Fe), chromium (Cr), zinc (Zn), aluminum (Al), titanium (Ti), lanthanum (La), and strontium (Sr). A spinel oxide containing Co is an effective material for the oxide layer 32. Co functions as both an A-site element and a B-site element in spinel oxides, and therefore can form a spinel oxide represented by Co3O4. Furthermore, materials containing such Co-containing spinel oxides and at least one element selected from Ni, Mn, Cu, Fe, Cr, Zn, Al, and Ti are also effective, and are expected to have effects such as improved electrical conductivity and reduced mismatch in thermal expansion coefficients. Alternatively, spinel oxides using Fe, Ni, Mn, or the like instead of Co can be used as the material for the oxide layer 32.

[0025] Examples of perovskite oxides include oxides containing Co and at least one selected from La and Sr, such as LaCoO3, SrCoO3, and (La,Sr)CoO3. Materials in which at least one selected from Ni, Mn, Cu, Fe, Cr, Zn, Al, and Ti is added to such perovskite oxides, such as La(Co,Fe)O3, Sr(Co,Fe)O3, and (La,Sr)(Co,Fe)O3, may also be used. Perovskite oxides in which Mn, Ni, or the like is added instead of Fe may also be used. Furthermore, perovskite oxides containing Mn, Fe, Ni, or the like instead of Co, such as SrMnO3, SrFeO3, and SrNiO3, or perovskite oxides in which the above-mentioned metal elements are added to these, may also be used as the constituent material of the oxide layer 32.

[0026] Examples of methods for forming the oxide layer 32 include electrolytic plating, electroless plating, spin coating, dip coating, and sol-gel methods. The oxide layer 32 may be formed by depositing an oxide film, or by depositing a metal film and then oxidizing it. When oxidizing a metal film, the interconnector with the protective film may be oxidized alone, or may be oxidized after being incorporated into a cell stack or a module.

[0027] The thickness of the oxide layer 32 is preferably 3 μm or more and 20 μm or less. If it is less than 3 μm, it is difficult to obtain the effect of the oxide layer 32. If it is more than 20 μm, peeling or cracking of the oxide layer 32 may occur.

[0028] The above-mentioned Co-containing spinel oxides and the like have better adhesion than perovskite oxides, but have inferior electrical conductivity compared to perovskite oxides. In response to this, the formation of the mixed layer 33 can increase electrical conductivity and further improve adhesion. The same applies to the use of perovskite oxides.

[0029] The mixed layer 33 is provided between the metal layer 31 and the oxide layer 32. The mixed layer 33 has a metal phase 301 containing a first metal element, similar to the metal layer 31, and an oxide phase 302 containing at least one oxide containing a second metal element, similar to the oxide layer 32. The mixed layer 33 has a mixture of the metal phase 301 and the oxide phase 302 in a direction intersecting the thickness direction of the cross section (for example, the X-axis direction). The metal phase 301 may extend to the metal layer 31.

[0030] The thickness of mixed layer 33 is preferably 10% to 70% of the thickness of protective film 3, and more preferably 30% to 50%. A thicker mixed layer 33 leads to a lower electrical resistance, but if the thickness exceeds 70% of the thickness of protective film 3, the metal contained in interconnector 1 may be more likely to diffuse to the surface of protective film 3 via metal phase 301 or the interface between metal phase 301 and oxide phase 302.

[0031] The mixed layer 33 has a complex shape having a metal phase 301 and an oxide phase 302, and the interface between the mixed layer 33 and the metal layer 31 and the interface between the mixed layer 33 and the oxide layer 32 are not flat. This can improve the adhesion between the metal layer 31 and the oxide layer 32. Furthermore, in the mixed layer 33, the metal phase 301, which has a lower electrical resistance, extends in a direction perpendicular to the plane of the interconnector 1, so that current flows preferentially through the metal phase 301 when current is applied, thereby significantly reducing the electrical resistance of the interconnector 1. In the mixed layer 33, it is assumed that the metal phase 301 extends in a direction perpendicular to the plane of the interconnector 1, but the metal phase 301 distributed in the perpendicular direction may be connected to each other, so that the metal phase 301 is distributed in the planar direction of the interconnector 1. In the mixed layer 33, the metal phase 301 functions as an electrical conduction path, so that, depending on the thickness of the protective film 3, the metal phase 301 in the mixed layer 33 preferably has a length of 1 μm or more and less than the thickness of the mixed layer 33, and even 5 μm or more and less than the thickness of the mixed layer 33, in a direction perpendicular to the plane of the interconnector 1 (thickness direction of the protective film 3, for example, the Z-axis direction). On the other hand, if the metal phase 301 reaches the outermost surface 3a of the protective film 3, there is a possibility that the Cr element contained in the interconnector 1 will diffuse through the metal phase 301. Therefore, it is preferable that the outermost surface 3a of the protective film 3 is an oxide phase 302. On the other hand, the metal component of the metal layer 31 may be present as the metal phase 301 partially in the region of the oxide layer 32.

[0032] The mixed layer 33 may be formed by, for example, using a method such as electrolytic plating, electroless plating, spin coating, dip coating, or sol-gel method to address defects such as pinholes that occur in the oxide layer 32, similar to the metal layer 31. The mixed layer 33 is not limited to this method, and may be formed by diffusing the metal component of the metal layer 31 so as to fill voids resulting from element diffusion that form at high temperatures such as the operating temperature of a solid oxide electrochemical cell. The mixed layer 33 may be formed by reducing an oxide film to form a metal phase 301, or conversely, the mixed layer 33 may be formed by oxidizing a metal film to form an oxide phase 302. These oxidation / reduction treatments may be performed on the interconnector with protective film alone, or may be performed after it is incorporated into a cell stack or a module.

[0033] For example, when electroplating is applied to form the protective film 3, electroplating is performed by immersing the metal substrate 2 in a first plating bath containing the constituent elements of the metal layer 31. Next, electroplating is performed by immersing the metal substrate 2 in a plating bath containing the constituent elements of the oxide layer 32. Next, heat treatment is performed at a temperature of 600°C or higher in an oxidizing atmosphere, such as air or an oxygen atmosphere, to oxidize the plating film. Because the metal elements constituting the oxide layer 32 are easily oxidized, high-temperature oxidation treatment produces oxides such as spinel-type oxides and perovskite-type oxides. The oxides produced can be controlled by the metal element ratio in the plating bath, the acid atmosphere, and the like. This makes it possible to form the protective film 3 having the mixed layer 33. Even when using other film formation methods, the interconnector 1 having the above-mentioned metal substrate 2 and the protective film 3 formed on its surface can be obtained by applying similar conditions.

[0034] The protective film 3 including the above-described mixed layer 33 contributes to improving the electrical conductivity and adhesion of the oxide layer 32 containing a spinel-type oxide or a perovskite-type oxide. Furthermore, it is possible to suppress the formation of pores at the interface between the metal substrate 2 and the protective film 3 and within the protective film 3. This increases the contact area between the metal substrate 2 and the protective film 3, thereby improving the adhesion of the protective film 3. Furthermore, the improved adhesion of the protective film 3 can reduce the electrical resistance of the protective film 3.

[0035] The metal layer 31, oxide layer 32, and mixed layer 33 can be analyzed using a cross-sectional backscattered electron image obtained by observing a cross section including the thickness direction (Z-axis direction) of the protective film 3 using a scanning electron microscope (SEM). The composition of each layer can be analyzed by elemental analysis using SEM-energy dispersive X-ray spectroscopy (SEM-EDX). The thickness of the protective film 3 is defined as the average thickness of the protective film 3 in the cross-sectional backscattered electron image.

[0036] In the cross-sectional backscattered electron image, the metal layer 31 is defined by a first region in which a first phase (metal phase 301) having a concentration of a first metal element of 90 atomic % or more and 100 atomic % or less continues from the surface 2b of the metal substrate 2, and when a first line perpendicular to a direction (e.g., the Z-axis direction) perpendicular to the thickness direction of the protective film 3 is drawn in the cross-sectional backscattered electron image, the total length of the first phase is 90% or more and 100% or less of the entire length of the first line. The thickness of the metal layer 31 is defined by the average thickness of the first region in the cross-sectional backscattered electron image.

[0037] The oxide layer 32 is defined by a second region in the cross-sectional backscattered electron image, where a second phase (oxide phase 302), which is different from the metal substrate 2 and the first phase and in which oxygen elements are detected, continues from the outermost surface 3a of the protective film 3, and when a second line perpendicular to the thickness direction of the protective film 3 (for example, the Z-axis direction) is drawn in the cross-sectional backscattered electron image, the total length of the second phase is 90% to 100% of the entire length of the second line. The thickness of the oxide layer 32 is defined by the average thickness of the second region in the cross-sectional backscattered electron image.

[0038] The mixed layer 33 is defined by a third region between the first region and the second region in the cross-sectional backscattered electron image. The third region includes a region in which, when a third line perpendicular to a direction perpendicular to the thickness direction of the protective film 3 (e.g., the Z-axis direction) is drawn in the cross-sectional backscattered electron image, the total length of the first phase is 20% to 80% and preferably 40% to 60% of the entire length of the third line, the total length of the second phase is 20% to 80% and preferably 40% to 60% of the entire length of the third line, and the sum of the total lengths of the first phase and the second phase is 95% to 100% of the entire length of the third line. The thickness of the mixed layer 33 is defined by the maximum thickness of the third region in the cross-sectional backscattered electron image.

[0039] In the above-described embodiment, the solid oxide electrochemical cells and the electrochemical cell stack 10 formed by stacking the cells have been mainly described as being applied to SOFCs and SOECs. However, the solid oxide electrochemical cells and the electrochemical cell stack 10 formed by stacking the cells of the embodiment can also be applied to CO2 electrolysis reaction devices, etc. [Example]

[0040] Next, specific examples of the interconnector according to the embodiment and the evaluation results thereof will be described.

[0041] (Comparative Example 1) A SUS430-based ferrite metal substrate was prepared as the metal substrate. This SUS substrate was immersed in a Co plating bath to perform electrolytic plating, forming a 2 μm-thick Co film. The SUS substrate with the Co plating film formed thereon was then exposed to the atmosphere at a temperature of 700°C to oxidize the Co plating film. The cross section of the SUS substrate with the Co oxide film thus obtained was observed using an SEM to obtain a cross-sectional backscattered electron image. The cross-sectional backscattered electron image of Comparative Example 1 is shown in FIG. 4. It was also confirmed that the oxidized Co film was a Co oxide film primarily composed of a spinel-type oxide represented by Co3O4.

[0042] The electrical resistance of the SUS substrate with the Co oxide film thus obtained was measured. Pt electrodes were formed on the surface of the Co oxide film by Pt vapor deposition, and the electrical resistance was measured by the four-terminal method at a temperature of 700°C. The results are shown in Figure 5.

[0043] (Comparative Example 2) A SUS430-based ferrite metal substrate was prepared as the metal substrate. This SUS substrate was immersed in a CoNi plating bath for electrolytic plating to form a 4 μm-thick CoNi film. The SUS substrate with the CoNi plating film formed thereon was then exposed to the atmosphere at a temperature of 700°C to oxidize the CoNi plating film. The cross section of the SUS substrate with the CoNi oxide film thus obtained was observed using an SEM in the same manner as in Comparative Example 1, and a cross-sectional backscattered electron image was obtained. The cross-sectional backscattered electron image of Comparative Example 2 is shown in FIG. 4. Furthermore, it was confirmed that the oxidized CoNi film was a CoNi oxide film primarily composed of a spinel-type oxide represented by (Co,Ni)3O4.

[0044] The electrical resistance of the SUS substrate with the CoNi oxide film thus obtained was measured. Similar to Comparative Example 1, Pt electrodes were formed on the CoNi oxide film by Pt vapor deposition, and the electrical resistance was measured by the four-terminal method at a temperature of 700°C. The results are shown in Figure 5.

[0045] Example 1 A SUS430-based ferrite metal substrate was prepared as the metal substrate. This SUS substrate was immersed in an Ag plating bath for electroplating, followed by a Co plating bath for electroplating, forming a 1 μm-thick Ag film and a 2 μm-thick Co film on top of it. The SUS substrate with the Ag and Co plating films was then exposed to the atmosphere at 700°C to oxidize the Co film. The oxidized Co film was confirmed to be a Co oxide film primarily composed of a spinel-type oxide represented by Co3O4. The cross section of the SUS substrate with the Ag and Co oxide films thus obtained was observed using an SEM, as in Comparative Example 1, to obtain a cross-sectional backscattered electron image. The cross-sectional backscattered electron image of Example 1 is shown in Figure 4.

[0046] The electrical resistance of the SUS substrate with the Ag film and Co oxide film thus obtained was measured. Similar to Comparative Example 1, Pt electrodes were formed on the Co oxide film by Pt vapor deposition, and the electrical resistance was measured by the four-terminal method at a temperature of 700°C. The results are shown in Figure 5.

[0047] Example 2 A SUS430-based ferrite metal substrate was prepared as the metal substrate. This SUS substrate was immersed in an Ag plating bath for electroplating, followed by a CoNi plating bath for electroplating, forming a 1 μm-thick Ag film and a 4 μm-thick CoNi film on top of it. The SUS substrate with the Ag and CoNi plating films was then exposed to the atmosphere at 700°C to oxidize the CoNi film. The oxidized CoNi film was confirmed to be a CoNi oxide film primarily composed of a spinel-type oxide represented by Co3O4. The cross section of the SUS substrate with the Ag and CoNi oxide films thus obtained was observed using an SEM, as in Comparative Example 1, to obtain a cross-sectional backscattered electron image. The cross-sectional backscattered electron image of Example 2 is shown in Figure 4.

[0048] The electrical resistance of the SUS substrate with the Ag film and CoNi oxide film thus obtained was measured. Similar to Comparative Example 1, Pt electrodes were formed on the CoNi oxide film by Pt vapor deposition, and the electrical resistance was measured by the four-terminal method at a temperature of 700°C. The results are shown in Figure 5.

[0049] As shown in Figure 4, in Examples 1 and 2, which have a metal layer (Ag metal layer) that is an Ag film, it can be seen that a mixed layer containing an Ag phase and a Co oxide phase or a CoNi oxide phase is formed between the metal layer and the oxide layer, with a structure having a complex interface between the metal layer and the oxide layer. The length of the Ag phase extending in the direction perpendicular to the plane of the metal substrate (thickness direction of the protective film) is approximately 4 µm in Example 1 and approximately 7 µm in Example 2. The thickness of the mixed layer is approximately 30% of the thickness of the protective film in Example 1 and approximately 50% of the thickness of the protective film in Example 2. Ag phases were also observed locally in some parts of the oxide layer.

[0050] Furthermore, as shown in Figure 5, it can be seen that Example 1 and Example 2, which have an Ag metal layer, have lower electrical resistance than Comparative Example 1 and Comparative Example 2, which do not have an Ag metal layer. Furthermore, it can be seen that Example 2 has lower electrical resistance than Example 1.

[0051] From the above, it can be seen that the formation of a mixed layer reduces electrical resistance. The protective film has a complex boundary between the oxide layer and the metal layer, which is expected to improve adhesion, and the protective film contains an Ag phase with low electrical resistance. Therefore, the formation of a mixed layer having an oxide phase and a metal phase is thought to be the reason for the low electrical resistance in Examples 1 and 2. From the above, it is possible to provide an interconnector with a protective film that is less likely to peel off and has low electrical resistance. A cell stack equipped with this has improved protective film integrity and can perform electrochemical reactions with higher energy efficiency.

[0052] The configurations of the above-described embodiments can be applied in combination with each other, and some of them can be replaced with other configurations. Although several embodiments of the present invention have been described herein, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. [Explanation of symbols]

[0053] 1...interconnector, 2...metal substrate, 2a...surface, 2b...surface, 3...protective film, 3a...outer surface, 4...protective film, 10...electrochemical cell stack, 11...electrochemical cell, 12...electrochemical cell, 13...electrode, 14...electrode, 15...solid oxide electrolyte layer, 31...metal layer, 32...oxide layer, 33...mixed layer, 301...metal phase, 302...oxide phase

Claims

1. a metal substrate comprising an iron-based alloy containing chromium; a protective film provided on the metal substrate in a first direction; An interconnector for a solid oxide electrochemical cell stack, comprising: The protective film is a metal layer provided on the surface of the metal substrate and containing a first metal element; an oxide layer provided above the metal layer and including at least one oxide selected from the group consisting of spinel-type oxides and perovskite-type oxides containing a second metal element different from the first metal element; a mixed layer provided between the metal layer and the oxide layer, the mixed layer having a first phase containing the first metal element and a second phase containing the at least one oxide; An interconnector for a solid oxide electrochemical cell stack, comprising:

2. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the first metal element is at least one element selected from the group consisting of gold, silver, and platinum.

3. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the second metal element is at least one element selected from the group consisting of cobalt, nickel, manganese, copper, iron, chromium, zinc, aluminum, titanium, lanthanum, and strontium.

4. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the second metal element is cobalt and nickel.

5. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the thickness of said mixed layer is 10% or more and 70% or less of the thickness of said protective film.

6. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the first phase has a length in the thickness direction of the protective film of not less than 1 μm and not more than the thickness of the mixed layer.

7. 2. The interconnector for a solid oxide electrochemical cell stack according to claim 1, wherein the protective film has a thickness of 1 μm or more and 50 μm or less.

8. a first electrochemical cell including a first electrode in contact with an atmosphere containing a substance having hydrogen atoms, a second electrode in contact with an atmosphere containing oxygen, and a solid oxide electrolyte layer interposed between the first electrode and the second electrode; a second electrochemical cell including a first electrode in contact with an atmosphere containing a substance having hydrogen atoms, a second electrode in contact with an atmosphere containing oxygen, and a solid oxide electrolyte layer interposed between the first electrode and the second electrode; a first electrode of the first electrochemical cell and a second electrode of the second electrochemical cell; and an interconnector disposed between the first electrode and the second electrode of the second electrochemical cell so as to be electrically connected to the first electrode and the second electrode of the second electrochemical cell, The interconnector includes a metal substrate including an iron-based alloy containing chromium, and a protective film provided on a surface of the metal substrate, The protective film is a metal layer provided on the surface of the metal substrate and containing a first metal element; an oxide layer provided above the metal layer and including at least one oxide selected from the group consisting of spinel-type oxides and perovskite-type oxides containing a second metal element different from the first metal element; a mixed layer provided between the metal layer and the oxide layer, the mixed layer having a first phase containing the first metal element and a second phase containing the at least one oxide; 1. A solid oxide electrochemical cell stack comprising:

9. 9. The solid oxide electrochemical cell stack according to claim 8, wherein the first metal element is at least one element selected from the group consisting of gold, silver, and platinum.

10. 9. The solid oxide electrochemical cell stack according to claim 8, wherein the second metal element is at least one element selected from the group consisting of cobalt, nickel, manganese, copper, iron, chromium, zinc, aluminum, titanium, lanthanum, and strontium.

11. 9. The solid oxide electrochemical cell stack of claim 8, wherein the second metallic element is cobalt and nickel.

12. 9. The solid oxide electrochemical cell stack according to claim 8, wherein the thickness of the mixed layer is 10% to 70% of the thickness of the protective film.

13. 9. The solid oxide electrochemical cell stack according to claim 8, wherein the first phase has a length in the thickness direction of the protective film of 1 μm or more and not more than the thickness of the mixed layer.

14. 9. The solid oxide electrochemical cell stack according to claim 8, wherein the protective film has a thickness of 1 μm or more and 50 μm or less.

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