Solid oxide electrolysis cell and use thereof

Abstract

Provided is a technique that makes it possible to suppress the occurrence of cracks in a solid electrolyte layer. This solid oxide electrolysis cell includes: an air electrode including a composite oxide having a perovskite structure; a fuel electrode; and a solid electrolyte layer disposed between the air electrode and the fuel electrode. The air electrode has cracks formed in both an interfacial region located within 10 μm from the interface on the solid electrolyte layer side, and a surface region located within 10 μm from the surface on the reverse side from the solid electrolyte layer side, wherein the number of the cracks is greater in the surface region than in the interfacial region.

Description

Solid oxide electrolysis cells and their uses

[0001] The present disclosure relates to solid oxide electrolysis cells and their uses.

[0002] Solid oxide fuel cells (hereinafter referred to as "SOFCs") are known that generate electricity by utilizing an electrochemical reaction between hydrogen and oxygen. For example, a single fuel cell, which is a constituent unit of an SOFC described in Patent Document 1, includes an electrolyte layer containing a solid oxide, an air electrode disposed on one side of the electrolyte layer, and an anode electrode disposed on the other side of the electrolyte layer. Oxygen ions dissociated from oxygen supplied to the air electrode migrate to the anode electrode based on the oxygen conductivity of the solid electrolyte, and react with hydrogen contained in the fuel gas supplied to the anode electrode to generate water vapor and electricity.

[0003] The above-mentioned SOFC can be used as a solid oxide electrolysis cell (hereinafter simply referred to as "SOEC") by passing current in the reverse direction, and is known to be used as an energy storage technology in the form of converting water vapor into hydrogen using surplus electricity, which is an issue in the process of introducing renewable energy. When water vapor is supplied to the anode and an electric current is passed between the cathode and the anode, the water vapor is electrolyzed to generate hydrogen from the anode, and oxygen ions generated at the anode migrate to the cathode due to the oxygen conductivity of the solid electrolyte, generating oxygen from the cathode.

[0004] JP 2023-080459 A

[0005] Generally, in SOFCs, oxygen is consumed at the anode during use, causing a drop in gas pressure, but in SOECs, oxygen is generated at the cathode during use, causing an increase in gas pressure, which creates stresses different from those in SOFCs and may cause cracks in the solid electrolyte layer. Furthermore, in general, there is a need for technology to prevent a drop in current density in SOECs.

[0006] The present disclosure can be realized in the following forms.

[0007] (1) According to one aspect of the present disclosure, there is provided a solid oxide electrolysis cell. The solid oxide electrolysis cell includes an air electrode containing a composite oxide having a perovskite structure, an anode, and a solid electrolyte layer disposed between the air electrode and the anode, wherein cracks are formed in the air electrode in an interface region within 10 μm from the interface on the solid electrolyte layer side and in a surface region within 10 μm from the surface opposite the solid electrolyte layer side, the number of cracks being greater in the surface region than in the interface region. The solid oxide electrolysis cell of this aspect can suppress the occurrence of cracks in the solid electrolyte layer.

[0008] (2) In the solid oxide electrolysis cell described in (1) above, a fuel gas containing water vapor is supplied to the fuel electrode at a rate of 100 to 130 liters / min cm ２ ) may be supplied at a flow rate of 0.1 to 1.0 times the flow rate of the solid oxide electrolysis cell of this embodiment. The solid oxide electrolysis cell of this embodiment can suppress the occurrence of cracks in the solid electrolyte layer.

[0009] (3) In the solid oxide electrolysis cell described in (1) or (2) above, the air electrode is supplied with an oxygen-containing gas at a rate of 30 to 50 liters / min cm ２ ) may be supplied at a flow rate of 0.1 to 1.0 times the flow rate of the solid oxide electrolysis cell of this embodiment. The solid oxide electrolysis cell of this embodiment can suppress the occurrence of cracks in the solid electrolyte layer.

[0010] (4) In the solid oxide electrolysis cell according to any one of (1) to (3) above, the volume of the air chamber facing the air electrode is 9 cm ３ ~11cm ３ According to the solid oxide electrolysis cell of this embodiment, it is possible to suppress the occurrence of cracks in the solid electrolyte layer.

[0011] (5) According to another aspect of the present disclosure, there is provided a separator-equipped cell including the solid oxide electrolysis cell according to any one of (1) to (4) above and a separator with a central opening that is disposed on the solid electrolyte layer. The separator-equipped cell of this aspect can suppress the occurrence of cracks in the solid electrolyte layer.

[0012] (6) According to another aspect of the present disclosure, there is provided an electrolysis stack including a plurality of stacked solid oxide electrolysis cells according to any one of (1) to (4). The electrolysis stack of this aspect can suppress the occurrence of cracks in the solid electrolyte layer.

[0013] (7) According to another aspect of the present disclosure, there is provided a hot module including the electrolytic stack according to (6), a vaporizer that generates water vapor to be supplied to the electrolytic stack, a heat exchanger that exchanges heat with a gas supplied to the electrolytic stack, a heater that heats the electrolytic stack, and a thermal insulator in which the electrolytic stack, the vaporizer, the heat exchanger, and the heater are disposed. The hot module of this aspect can suppress the occurrence of cracks in the solid electrolyte layer.

[0014] (8) According to another aspect of the present disclosure, there is provided a hydrogen production device including the hot module described in (7) above. The hydrogen production device of this aspect can suppress the occurrence of cracks in the solid electrolyte layer.

[0015] The present invention can be realized in various forms, for example, in the form of a method for manufacturing a solid oxide electrolysis cell.

[0016] 1 is a perspective view showing the external configuration of an electrolysis stack according to an embodiment of the present disclosure; 2 is an exploded schematic view of the electrolysis stack taken along line II-II in FIG. 1; 3 is a schematic top view of a cell with a separator; 4 is a block diagram of a hydrogen production device; 5 is an SEM image of a crack in an air electrode;

[0017] Fig. 1 is a perspective view showing the external configuration of an electrolysis stack 10 according to one embodiment of the present disclosure. The electrolysis stack 10 in this embodiment is a stack of solid oxide electrolysis cells (SOECs). Fig. 1 shows an example of the configuration of the electrolysis stack 10.

[0018] The electrolysis stack 10 includes a plurality of rectangular reaction units 11 stacked in the thickness direction, and approximately rectangular end plates 12 and 13 sandwiching the reaction units 11 in the thickness direction. Bolts 14 are arranged at the four corners of the periphery of the electrolysis stack 10, penetrating the end plates 12, the reaction units 11, and the end plates 13 in the thickness direction. The reaction units 11 and the end plates 12 and 13 are fastened together by the bolts 14.

[0019] The electrolysis stack 10 includes a terminal plate 52 disposed between the end plate 12 and the reaction unit 11, and a terminal plate 53 disposed between the end plate 13 and the reaction unit 11. The reaction units 11 are connected in series between the terminal plates 52 and 53. The protruding portions of the terminal plates 52 and 53 function as terminals. It is of course possible to omit the terminal plates 52 and 53 and electrically connect the reaction units 11 to the end plates 12 and 13, thereby using the end plates 12 and 13 as terminals of the electrolysis stack 10.

[0020] Four spaces penetrating the electrolysis stack 10 in the thickness direction are formed at the periphery of the electrolysis stack 10. These four spaces function as a passage 15a through which gas enters from outside the electrolysis stack 10 to a fuel chamber 33 (described later) of the reaction unit 11, a passage 15b through which gas exits from the fuel chamber 33 to the outside of the electrolysis stack 10, a passage 15c through which gas enters from outside the electrolysis stack 10 to an air chamber 35 (described later) of the reaction unit 11, and a passage 15d through which gas exits from the air chamber 35 to the outside of the electrolysis stack 10, respectively.

[0021] Figure 2 is an exploded schematic view of the electrolysis stack 10 taken along line II-II in Figure 1, which passes through the passages 15a and 15b. Figure 2 shows a schematic cross-sectional view taken along line II-II, in which components constituting one reaction unit 11 are separated in the thickness direction. The reaction unit 11 includes, in order in the thickness direction, an interconnector 16, an anode frame 17, a separator-equipped cell 47, and an cathode frame 19. Note that the thickness of each part is exaggerated in Figure 2.

[0022] 3 is a schematic top view of a separator-equipped cell 47. The separator-equipped cell 47 includes an electrolytic cell 20 and a separator 30 disposed in the electrolytic cell 20. Holes (passages 15a, 15b, 15c, 15d) penetrate through the interconnector 16, the anode frame 17, the separator 30, and the cathode frame 19. The electrolytic cell 20 will be described later.

[0023] The separator 30 is a substantially rectangular frame-shaped member provided with an opening 37 that is larger than the air electrode 29 (described later). The separator 30 can be made of stainless steel, for example. The separator 30 is airtightly joined to the surface 24a of the solid electrolyte layer 24 (described later) with a brazing material 31, avoiding the air electrode 29.

[0024] The interconnectors 16 are disposed at both ends of the electrolysis cell 20 in the thickness direction. The interconnectors 16 are formed of conductive, substantially rectangular plate-like members. The interconnectors 16 electrically connect the reaction units 11 adjacent to each other in the thickness direction. Stainless steel is an example of a material for the interconnectors 16.

[0025] The anode frame 17 is a substantially rectangular frame-shaped member disposed between the interconnector 16 and the separator 30. Stainless steel is an example of the material for the anode frame 17. The anode frame 17 surrounds the electrolysis cell 20 and a current collector 32 provided in the center of the interconnector 16.

[0026] The current collector 32 electrically connects the anode 21 and the interconnector 16. An example of a material for the current collector 32 is a gas-permeable porous body made of a metal such as Ni. A fuel chamber 33 surrounded by the interconnector 16, the anode frame 17, and the separator 30 is formed inside the anode frame 17.

[0027] The air electrode frame 19 is a substantially rectangular frame-shaped member disposed between the interconnector 16 and the separator 30. An example of the material of the air electrode frame 19 is an insulator such as mica. The air electrode frame 19 surrounds a current collector 34 provided in the center of the interconnector 16. The current collector 34 electrically connects the air electrode 29 and the interconnector 16. In this embodiment, the current collector 34 is formed integrally with the interconnector 16, but this is not limitative. It is of course possible for the current collector 34 to be a member separate from the interconnector 16.

[0028] An air chamber 35 is formed inside the air electrode frame 19 and is surrounded by the interconnector 16, the air electrode frame 19, and the separator 30. The separator 30 separates the fuel chamber 33 from the air chamber 35, preventing the fuel gas in the fuel chamber 33 and the oxidant gas (oxygen, air, etc.) in the air chamber 35 from mixing.

[0029] A hydrogen production device 60 including the electrolysis stack 10 and a hot module 61 will be described with reference to Fig. 4. Fig. 4 is a block diagram of the hydrogen production device 60. The hydrogen production device 60 is a device that produces hydrogen from water, and includes a hot module 61.

[0030] The hot module 61 includes the electrolysis stack 10, a vaporizer 62 that generates steam to be supplied to the electrolysis stack 10, a heat exchanger 63 that exchanges heat between the gas supplied to the electrolysis stack 10 and the gas generated by the electrolysis stack 10, and a heater 64 that heats the electrolysis stack 10. In the hot module 61, the electrolysis stack 10, the vaporizer 62, the heat exchanger 63, and the heater 64 are disposed inside a thermal insulator 65 to reduce heat radiation.

[0031] The vaporizer 62 includes a heat exchanger that exchanges heat with high-temperature gas containing oxygen produced by the electrolysis stack 10, and heats water to produce water vapor. The water vapor produced by the vaporizer 62 contains hydrogen that reduces oxidation of the catalyst contained in the anode 21. The hydrogen-containing water vapor exchanges heat with hydrogen and oxygen produced by the electrolysis stack 10 in a heat exchanger 63, is heated to the operating temperature of the electrolysis stack 10 by a heater 64, and is supplied to the fuel chamber 33 of the electrolysis stack 10. The air exchanges heat with hydrogen and oxygen produced by the electrolysis stack 10 in the heat exchanger 63, is heated to the operating temperature of the electrolysis stack 10 by the heater 64, and is supplied to the air chamber 35 of the electrolysis stack 10.

[0032] Examples of the heat insulating material 65 include heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), and a heat-resistant container made of heat-resistant fibers. The heat-resistant fibers fill gaps between the electrolysis stack 10, the vaporizer 62, the heat exchanger 63, and the heater 64. The condenser 66 is a device that cools the hydrogen gas, and liquefied water is supplied to the vaporizer 62 as raw water.

[0033] (Electrolysis Cell) As shown in FIG. 2 , the electrolysis cell 20 includes a cathode 29, an anode 21, and a solid electrolyte layer 24 disposed between the cathode 29 and the anode 21. The electrolysis cell 20 of this embodiment includes a reaction prevention layer 25 between the solid electrolyte layer 24 and the cathode 29, although the reaction prevention layer 25 may be omitted. Furthermore, in this embodiment, the cathode 29 includes, in order from the side closest to the solid electrolyte layer 24, a cathode functional layer 26 and an cathode current collecting layer 27. The thickness of the electrolysis cell 20 is not particularly limited, but may be, for example, 300 μm to 3 mm. The shape of the electrolysis cell 20 viewed from above is not particularly limited, but examples include a square with sides measuring 1 to 10 cm, a rectangle with long sides measuring 5 to 30 cm and short sides measuring 3 to 15 cm, or a circle with a diameter of 10 cm. A plurality of electrolysis cells 20 are connected in series by interconnectors 16 to form an electrolysis stack 10.

[0034] The fuel electrode 21 is formed using nickel oxide and oxygen ion conductive ceramic particles as materials. Nickel oxide (NiO) used as the material of the fuel electrode 21 is converted into nickel through a reduction process described below. The fuel electrode 21 of this embodiment is a porous thin-plate sintered body made of nickel and YSZ (yttria stabilized zirconia). Examples of ceramic materials having oxygen ion conductivity include YSZ (yttria stabilized zirconia), ScSZ (scandia stabilized zirconia), and (Gd, Ce)O. ２ That is, GDC (Gadolinium doped ceria), (Sm, Ce)O ２ SDC (Samarium doped ceria), and LaGaO ３ (lanthanum gallate). The ceramic material contained in the anode 21 may be one type or two or more types. The thickness of the anode 21 is not particularly limited, but is, for example, 0.3 to 3 mm. In this embodiment, the thickness of the anode 21 is the greatest among the thicknesses of the components of the electrolysis cell 20, and the anode 21 functions as a support (support substrate, the member with the highest rigidity) for the electrolysis cell 20.

[0035] The anode 21 functions as the cathode of the electrolysis cell 20. The anode 21 includes, in order from the solid electrolyte layer 24 side, an anode functional layer 23 and an anode substrate layer 22. In this embodiment, the anode 21 includes, in order from the solid electrolyte layer 24 side, the anode functional layer 23 and the anode substrate layer 22, which has a porosity greater than that of the anode functional layer 23. This ensures the strength of the anode functional layer 23. The thickness of the anode substrate layer 22 is preferably greater than that of the anode functional layer 23. The porosity of the anode functional layer 23 is not particularly limited, but is preferably, for example, 30% or less, more preferably 25% or less, and preferably 5% or more, and more preferably 10% or more. The porosity of the anode substrate layer 22 is not particularly limited, but is, for example, preferably 50% or less, more preferably 40% or less, and preferably 10% or more, and more preferably 20% or more. The anode 21 may be formed of a single layer instead of the anode functional layer 23 and the anode substrate layer 22 .

[0036] The anode substrate layer 22 is a porous, plate-shaped sintered body containing a transition metal and an oxygen ion conductive material. The anode substrate layer 22 may contain nickel (Ni) as the transition metal. The anode substrate layer 22 may contain, as the oxygen ion conductive material, a zirconia-based material such as yttria-stabilized zirconia (8YSZ, 10YSZ, etc.) or scandia-stabilized zirconia (ScSZ), or a gadolinium-doped ceria (GDC: (Ce, Gd)O ２ ) and samarium-doped ceria (SDC: (Ce, Sm)O ２ ) or yttria (Y ２ O ３ ) may be included.

[0037] The thickness of the anode substrate layer 22 can be, for example, 0.2 mm to 5.0 mm. When the anode substrate layer 22 functions as a substrate, the thickness of the anode substrate layer 22 may be the largest among the components of the electrolysis cell 20. In the anode substrate layer 22, the volume ratio of Ni can be, for example, 20 to 50 volume %, and the volume ratio of the oxygen ion conductive material can be, for example, 30 to 60 volume %.

[0038] The anode functional layer 23 is disposed between the anode substrate layer 22 and the solid electrolyte layer 24. The anode functional layer 23 is a porous, plate-shaped sintered body containing a transition metal and an oxygen ion conductive material. The anode functional layer 23 contains at least Ni as a transition metal. The anode functional layer 23 may further contain Fe or Cu as a transition metal. The anode functional layer 23 contains, as an oxygen ion conductive material, a zirconia-based material such as yttria-stabilized zirconia (8YSZ, 10YSZ, etc.) or scandia-stabilized zirconia (ScSZ), or gadolinium-doped ceria (GDC: (Ce, Gd)O ２ ) and samarium-doped ceria (SDC: (Ce, Sm)O ２ ) and other ceria-based materials.

[0039] The thickness of the anode functional layer 23 may be, for example, 1.0 μm to 30 μm. In the anode functional layer 23, the Ni volume ratio may be, for example, 30 to 60 volume %, and the oxygen ion conductive material volume ratio may be, for example, 40 to 70 volume %.

[0040] The solid electrolyte layer 24 is a dense, thin-plate sintered body. The solid electrolyte layer 24 is formed of a solid oxide such as YSZ (yttria-stabilized zirconia), ScSZ (scandia-stabilized zirconia), SDC (samarium-doped ceria), GDC (gadolinium-doped ceria), or a perovskite-type oxide. An example of a perovskite-type oxide is a lanthanum gallate-based oxide having a perovskite-type structure. The thickness of the solid electrolyte layer 24 is not particularly limited, but is, for example, 3 to 30 μm.

[0041] In this embodiment, the solid electrolyte layer 24 is disposed between the fuel electrode 21 and the reaction prevention layer 25. The solid electrolyte layer 24 has a function of allowing oxygen ions generated at the air electrode 29 to pass therethrough. The solid electrolyte layer 24 may contain zirconium (Zr), and the solid electrolyte layer 24 may contain Zr in the form of zirconia (ZrO ２ The solid electrolyte layer 24 may contain ZrO ２ The solid electrolyte layer 24 may contain ZrO as a main component. ２ In addition to that, Y ２O ３ and / or Sc ２ O ３ The solid electrolyte layer 24 may contain additives such as ZrO. These additives function as stabilizers. ２ Molar composition ratio (stabilizer: ZrO ２ ) is preferably about 3:97 to 20:80. That is, examples of the material for the solid electrolyte layer 24 include yttria-stabilized zirconia such as 3YSZ, 8YSZ, and 10YSZ, and zirconia-based materials such as ScSZ.

[0042] The air electrode 29 is disposed on the reaction prevention layer 25. The air electrode 29 functions as the anode of the electrolysis cell 20. The air electrode 29 may contain, for example, a lanthanum-containing perovskite-type complex oxide as a main component. Examples of lanthanum-containing perovskite-type complex oxides include LSCF (lanthanum strontium cobalt ferrite), lanthanum manganite, lanthanum cobaltite, and lanthanum ferrite. The lanthanum-containing perovskite-type complex oxide may also be doped with strontium, calcium, chromium, cobalt, iron, nickel, aluminum, or the like. The thickness of the air electrode 29 may be, for example, 5 μm to 150 μm.

[0043] Cracks are formed in the air electrode 29 in an interface region R1 within 10 μm of the interface Q on the solid electrolyte layer 24 side and in a surface region R2 within 10 μm of the surface opposite the solid electrolyte layer 24 side. The number of cracks is greater in the surface region R2 than in the interface region R1. The inventors discovered that by doing so, it is possible to suppress the occurrence of cracks in the solid electrolyte layer 24. Methods for controlling the occurrence and amount of cracks include, for example, adjusting the maximum temperature during firing when preparing the electrolysis cell 20, the high-temperature retention time, the temperature increase rate, and the temperature decrease rate, which will be described in detail later. Here, in this specification, the term "crack" refers to a crack with a width exceeding 100 nm and not exceeding 500 nm, and a length of 500 nm or more and not exceeding 2000 nm, which occurs along the grain boundaries between particles constituting the air electrode 29. The number of cracks can be measured by observing the cross sections of the layers in each of the interface region R1 and the surface region R2 using an SEM at 30,000x magnification. The number of cracks refers to the total number of cracks observed in 10 visual fields selected for each region. Since the vertical and horizontal dimensions of each visual field are 1.3 μm × 1.8 μm, the total number of cracks in the 10 visual fields is 23.4 μm. ２ is the total number of cracks present in the area.

[0044] Figure 5 is an SEM image of a crack in the air electrode 29. Figure 5 is an SEM image of the surface region R2 of the air electrode 29. The black area in the center of Figure 5 is a crack. This crack is elongated in the vertical direction of the page and bends to the right at the center of the page.

[0045] The air electrode 29 contains a complex oxide having a perovskite structure. The composition of the complex oxide is represented by the general formula ABO ３ However, the ratio of A, B, and O does not necessarily have to be 1:1:3. ３The ideal unit lattice of a complex oxide having a perovskite structure represented by the formula (1) is a cube, with element A located at a corner of the unit lattice, element B located at a body-center position of the unit lattice, and element oxygen located at a face-center position of the unit lattice. In the present disclosure, the corner position of the unit lattice where element A is located is referred to as the "A site," and the body-center position of the unit lattice where element B is located is referred to as the "B site."

[0046] The A site preferably contains at least one of La and Sr atoms. The B site preferably contains at least one of Co and Fe atoms. A specific example of such a composite oxide is LSCF, i.e., (La,Sr)(Co,Fe)O, which is the material for the air electrode 29. ３ , LSF, i.e. (La,Sr)FeO ３ , LSC i.e. (La,Sr)CoO ３ , LNF, that is, La(Ni,Fe)O ３ , SSC, i.e. (Sm,Sr)CoO ３ These composite oxides are substances that have both oxygen ion conductivity and electron conductivity, and are also called mixed conductive materials. In this embodiment, the air electrode 29 is made of lanthanum strontium cobalt ferrite LSCF (La ０．６ Sr ０．４ Co ０．２ Fe ０．８ O ３ It is a porous, thin-plate-shaped sintered body formed from a material containing

[0047] The air electrode 29 can contain a composite oxide as a "main component." The phrase "composition X contains substance Y as a "main component"" means that substance Y accounts for 60 mass % or more, preferably 70 mass % or more, and more preferably 90 mass % or more of the entire composition X. The air electrode 29 may contain components other than the composite oxide. The material of the air electrode 29 may be a powder (for example, with an average particle size of about 0.1 μm to 5 μm) or a crushed material (for example, with an average particle size of about 5 μm to 500 μm), or may be a lump larger than the crushed material.

[0048] The cathode current collecting layer 27 is made of a material having the general formula ABO ３The cathode functional layer 26 includes a complex oxide having a perovskite structure represented by the formula (I). Examples of such complex oxides include, but are not limited to, LSCF, LSF, LSC, LNF, and SSC. The cathode functional layer 26 may contain a component other than the complex oxide contained in the cathode current collecting layer 27. For example, the cathode functional layer 26 may be composited with a material of the solid electrolyte layer 24 (e.g., ceria or zirconia), which will be described later. The thickness of the cathode current collecting layer 27 is not particularly limited, but may be, for example, 5 to 100 μm. The thickness of the cathode functional layer 26 is not particularly limited, but may be, for example, 5 to 20 μm. The cathode functional layer 26 may not be provided, and the cathode functional layer 26 may not contain Ce.

[0049] Due to the difference in thermal expansion coefficient, there is a risk that the air electrode 29 may peel off from the solid electrolyte layer 24. To prevent this, there is a technique of providing a composite material in which an electrolyte is mixed into the air electrode 29 between the air electrode 29 and the solid electrolyte layer 24. However, when a Zr-based material is used as the electrolyte material, if the air electrode 29 contains Sr, the composite material may peel off from the solid electrolyte layer 24. ３ In this case, for example, gadolinium-doped ceria (GDC) or samarium-doped ceria (SDC) is used as the mixed electrolyte material. Preferably, an intermediate layer containing a mixture of an electrolyte and a composite oxide can be disposed between the air electrode 29 and the solid electrolyte layer 24, and this intermediate layer is the air electrode functional layer 26. However, the air electrode 29 does not necessarily have to have the air electrode functional layer 26.

[0050] Sr (strontium) contained in the air electrode 29 diffuses to the solid electrolyte layer 24 side, and when this diffused Sr reacts with Zr (zirconium) contained in the solid electrolyte layer 24, a highly resistive substance, SrZrO ３(hereinafter, also referred to as "SZO") is generated. If SZO is generated in a layer in the region between the air electrode 29 and the solid electrolyte layer 24, electrolysis performance deteriorates. To suppress the generation of SZO, it is preferable to dispose a reaction prevention layer 25 containing, for example, gadolinium-doped ceria (GDC) between the air electrode 29 and the solid electrolyte layer 24. This effectively suppresses the reaction of Sr diffused from the air electrode 29 with Zr contained in the solid electrolyte layer 24 to generate SZO. However, the electrolysis cell 20 does not necessarily have to include the reaction prevention layer 25. Examples of materials for the reaction prevention layer 25 include ceria-based materials containing cerium (Ce) and rare earth metal oxides solid-dissolved in Ce. The reaction prevention layer 25 is preferably, but is not particularly limited to, a dense, thin-plate-shaped sintered body made of ceria. Examples of ceria include, but are not limited to, gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC).

[0051] In this embodiment, the reaction prevention layer 25 is disposed between the solid electrolyte layer 24 and the air electrode 29. The reaction prevention layer 25 has the function of suppressing the formation of a high-resistance layer between the solid electrolyte layer 24 and the air electrode 29. The thickness of the reaction prevention layer 25 can be, for example, 3 μm to 20 μm.

[0052] In this SOEC electrolysis cell 20, water vapor is supplied to the fuel electrode 21, and a gas containing oxygen is supplied to the air electrode 29. At the same time, a current is passed between the fuel electrode 21 and the air electrode 29, and the chemical reactions shown in the following formulas (1) and (2) occur. As a result, hydrogen is generated from the fuel electrode 21, and oxygen is generated from the air electrode 29. H ２ O + 2e － →H ２ +O ２－ (Location: Fuel electrode 21) …(1) O ２－ → (1 / 2)・O ２ +2e － (At: air electrode 29) ... (2)

[0053] In this SOEC electrolysis cell 20, conductive connecting members (interconnectors 16) for collecting current are usually joined and fixed to each of the fuel electrode 21 and the air electrode 29 with a bonding agent, and a potential difference is applied via each interconnector 16 to allow a current to flow. Then, hydrogen gas generated from the fuel electrode 21 is collected.

[0054] The fuel electrode 21 is supplied with a fuel gas containing water vapor at a rate of, for example, 90 to 140 liters / min.cm. ２ ) and may be used at a flow rate of 100 to 130 liters / (min cm ２ In the case of co-electrolysis, water vapor and CO may be used as fuel gases. ２ In a stack in which cells are stacked in multiple stages, the amount of fuel gas to be introduced by a blower, pump, etc. naturally increases in proportion to the number of cells stacked.

[0055] The air electrode 29 is supplied with a gas containing oxygen at a flow rate of, for example, 20 to 60 liters / min.cm. ２ ) and may be used at a flow rate of 30 to 50 liters / (min cm ２ ) may be supplied and used. As the oxygen-containing gas, oxygen gas or air may be used. In a stack in which cells are stacked in multiple stages, the amount of gas introduced to the air electrode side by a blower, pump, etc. naturally increases in proportion to the number of cells stacked.

[0056] The volume of the air chamber 35 facing the air electrode 29 is not particularly limited, but is preferably 8 cm ３ ~13cm ３ Preferably, 9 cm ３ ~11cm ３ Here, the volume of the air chamber 35 facing the air electrode 29 refers to the volume per cell, excluding the space occupied by current collecting members and the like.

[0057] (Manufacturing Method) Next, an example of a manufacturing method for the electrolysis cell 20 will be described. Various conditions such as the material, particle size, temperature, and application method described below can be changed as appropriate. In the following description, the term "molded body" refers to the state before firing.

[0058] (Preparation of Green Sheet for Anode Substrate Layer) A mixture of NiO powder and YSZ powder is mixed with organic beads as a pore former, butyral resin, DOP as a plasticizer, Florene G-700 as a dispersant, and a mixed solvent of toluene and ethanol, and the mixture is mixed in a ball mill to prepare a slurry. The organic beads are, for example, spherical particles formed from a polymer such as polymethyl methacrylate or polystyrene. The resulting slurry is thinned using a doctor blade method to prepare a green sheet for an anode substrate layer with a predetermined thickness (e.g., 200 μm to 300 μm). The mixing ratio of NiO powder and YSZ powder when preparing the anode green sheet can be appropriately set as long as the performance is satisfied. The mixing ratio of NiO powder and YSZ powder may be, for example, 20 to 50 vol% NiO and 30 to 60 vol% YSZ.

[0059] (Preparation of Green Sheet for Anode Functional Layer) A butyral resin, DOP (plasticizer), Florene G-700 (dispersant), and a mixed solvent of toluene and ethanol are added to a mixed powder of NiO powder and YSZ powder, and the mixture is mixed in a ball mill to prepare a slurry. The resulting slurry is thinned by a doctor blade method to prepare a green sheet for an anode functional layer having a predetermined thickness (e.g., 5 μm to 50 μm). The mixing ratio of NiO powder and YSZ powder when preparing the green sheet for an anode functional layer can be appropriately set as long as the performance is satisfied. The mixing ratio of NiO powder and YSZ powder may be, for example, 30 to 60 vol% NiO and 40 to 70 vol% YSZ.

[0060] (Preparation of Green Sheet for Solid Electrolyte Layer) To the YSZ powder, butyral resin, DOP as a plasticizer, Florene G-700 as a dispersant, and a mixed solvent of toluene and ethanol are added, and the mixture is mixed in a ball mill to prepare a slurry. The obtained slurry is thinned by a doctor blade method to prepare a green sheet for the solid electrolyte layer of a predetermined thickness (for example, 10 μm).

[0061] (Fabrication of a laminate of the solid electrolyte layer 24, the anode functional layer 23, and the anode substrate layer 22) The anode substrate layer green sheet, the anode functional layer green sheet, and the solid electrolyte layer green sheet are attached together and degreased at a predetermined temperature (e.g., about 280°C). The degreased green sheet laminate is then fired at a predetermined temperature (e.g., about 1350°C). This yields a laminate of the solid electrolyte layer 24, the anode functional layer 23, and the anode substrate layer 22.

[0062] The cell manufacturing method is not limited to this method, and the following method may also be used. A slurry is prepared by adding polyvinyl alcohol (PVA) as a binder to a mixture of NiO powder and YSZ powder. This slurry is then dried and granulated using a spray dryer, and a green body for the anode 21 is formed by die press molding. Next, water and a binder are added to the YSZ powder, and the mixture is mixed in a ball mill for 24 hours to prepare a slurry. This slurry is then applied and molded onto the green body for the anode 21, forming a green body for the solid electrolyte layer 24. This green body stack is then co-sintered in air in an electric furnace (in an oxygen-containing atmosphere) at, for example, 1350°C to form a green body for the anode 21 and the solid electrolyte layer 24. A tape lamination method, printing method, or the like may also be used to form a film that will later become the solid electrolyte layer 24 on the anode 21.

[0063] (Formation of Reaction Preventive Layer 25) Next, the reaction preventive layer 25 is formed. Specifically, polyvinyl alcohol as an organic binder and butyl carbitol as an organic solvent are added to and mixed with GDC powder, and the viscosity is adjusted to prepare a paste for the reaction preventive layer. The obtained paste for the reaction preventive layer is applied by, for example, screen printing to the surface of the above-mentioned laminate of the solid electrolyte layer 24 and the anode 21 on the side of the solid electrolyte layer 24, and then fired at, for example, 1180°C. This forms the reaction preventive layer 25, and a laminate of the anode 21, the solid electrolyte layer 24, and the reaction preventive layer 25 (hereinafter referred to as the "intermediate laminate") is produced.

[0064] (Formation of Air Electrode Functional Layer 26) Next, the air electrode functional layer 26 is formed. First, LSCF powder and GDC powder are mixed in a mass ratio of 1:1, and polyvinyl alcohol as an organic binder and butyl carbitol as an organic solvent are mixed. The viscosity is adjusted to prepare a paste for the air electrode functional layer. Next, the prepared paste for the air electrode functional layer is applied to the surface of the intermediate laminate on the side of the reaction prevention layer 25 by, for example, screen printing, and then dried.

[0065] (Formation of Air Electrode Current Collecting Layer 27) Next, the air electrode current collecting layer 27 is formed. First, LSCF powder, polyvinyl alcohol as an organic binder, butyl carbitol as an organic solvent, and organic beads as a pore-forming material are mixed, and the viscosity is adjusted to prepare an air electrode current collecting layer paste. Next, the prepared air electrode current collecting layer paste is applied to the surface of the intermediate laminate facing the air electrode functional layer 26 by, for example, screen printing and dried. The intermediate laminate with the current collecting layer paste applied is fired at a predetermined firing temperature (for example, 1100°C). This firing process forms the air electrode current collecting layer 27, and produces a laminate of the anode 21, the solid electrolyte layer 24, the reaction prevention layer 25, the air electrode functional layer 26, and the air electrode current collecting layer 27, i.e., the single electrolytic cell 20. In this embodiment, cracks are formed in the interface region R1 and the surface region R2 of the air electrode 29, and the number of cracks is greater in the surface region R2 than in the interface region R1. As described above, methods for adjusting the occurrence and amount of cracks include, for example, adjusting the maximum temperature during firing, the holding time at high temperature, the temperature increase rate, and the temperature decrease rate. The maximum temperature during firing is, for example, preferably 1200°C or less, more preferably 1150°C or less, and even more preferably 1100°C or less. The maximum temperature during firing is, for example, preferably 900°C or more, more preferably 1000°C or more, and even more preferably 1050°C or more. The holding time at high temperature is, for example, preferably 180 minutes or less, more preferably 150 minutes or less, and even more preferably 130 minutes or less. The holding time at high temperature is, for example, preferably 30 minutes or more, more preferably 40 minutes or more, and even more preferably 50 minutes or more. For example, the temperature rise rate is preferably 3° C. / min or less, more preferably 2° C. / min or less, even more preferably 1.5° C. / min or less, and even more preferably 1.3° C. / min or less. The temperature rise rate is, for example, preferably 0.3° C. / min or more, more preferably 0.5° C. / min or more, even more preferably 0.8° C. / min or more, and even more preferably 1.0° C. / min or more.The temperature drop rate is, for example, preferably 2°C / min or less, more preferably 1.5°C / min or less, and even more preferably 1.3°C / min or less. The temperature drop rate is, for example, preferably 0.3°C / min or more, more preferably 0.5°C / min or more, and even more preferably 0.8°C / min or more.

[0066] Thereafter, a reduction step is performed to reduce the fuel electrode 21 (i.e., reduce NiO contained in the fuel electrode 21 to Ni) to enable the electrolysis cell 20 to operate for hydrogen production. The reduction step is achieved, for example, by exposing the fuel electrode 21 to a hydrogen atmosphere at a predetermined temperature for a predetermined time. The reducing gas used in the reduction step is not limited to hydrogen, but may be other gases such as methane gas, and the concentration of the reducing gas is also not limited. A reducing gas having a reducing agent concentration of less than 100% by volume may contain, for example, nitrogen gas in addition to hydrogen gas.

[0067] (Explanation of Effects) As described above, cracks are formed in the interface region R1 and the surface region R2 of the air electrode 29 having the above configuration, with the number of cracks being greater in the surface region R2 than in the interface region R1. The inventors have discovered that use of such an air electrode 29 can suppress the occurrence of cracks in the solid electrolyte layer 24. As a result, a decrease in current density at the thermal neutral point (1.3 V) can be suppressed. Furthermore, as a result, an electrolysis cell 20 capable of operating for a long period of time can be provided. The mechanism by which the air electrode 29 having the above configuration suppresses the occurrence of cracks in the solid electrolyte layer 24 is unclear, but the following probable mechanism is thought to be the case. That is, the formation of cracks in the interface region R1 and the surface region R2 is thought to alleviate stress caused by thermal cycles and the like. This stress is thought to be caused by the difference in thermal expansion coefficients between the solid electrolyte layer 24 and the air electrode 29. Furthermore, since the number of cracks is greater in the surface region R2 than in the interface region R1, it is thought that the occurrence of cracks in the solid electrolyte layer 24 due to cracks in the interface region R1 is suppressed compared to when the number of cracks is greater in the interface region R1 than in the surface region R2.

[0068] A test that confirmed that the occurrence of cracks in the solid electrolyte layer 24 was suppressed by the air electrode 29 having the above configuration will be described below.

[0069] (Test) In this test, multiple types of test specimens (sintered bodies) were used for the SOEC (electrolysis cell 20) according to the embodiment described above, with the thicknesses of the air electrode functional layer 26 and the air electrode current collecting layer 27 varied. Specifically, as shown in Table 1, 10 levels were prepared, and one test specimen was produced for each level. Table 1 shows the results of evaluating the number of cracks, the current density at the thermal neutral point after the thermal cycle test, and the presence or absence of cracks for each of Samples No. 1 to No. 10.

[0070] In these test samples, the thickness of the anode 21 was constant at 500 μm, the thickness of the solid electrolyte layer 24 (8YSZ) was constant at 10 μm, and the thicknesses of the cathode functional layer 26 and the cathode current collecting layer 27 are shown in Table 1. Two thicknesses of the cathode functional layer 26 and the cathode current collecting layer 27 were used. Furthermore, the shape of these test samples viewed from above was a 10 cm square. The occurrence and number of cracks were adjusted by adjusting the firing temperature, the high-temperature retention time, the temperature increase rate, and the temperature decrease rate during firing.

[0071] The following thermal cycle electrolysis test was performed on these prototypes. Specifically, the prototypes were heated from room temperature (25°C) to 700°C, and then an electrolysis test was performed in which hydrogen and water vapor were flowed toward the fuel electrode 21 side and air was flowed toward the air electrode 29 side while an electric current was passed. This series of steps was then repeated five times. The current density at the thermal neutral point of the prototypes that had undergone this thermal cycle electrolysis test was then measured. Furthermore, the presence or absence of cracks on the surface of the solid electrolyte layer 24 after the measurement was confirmed using a binocular microscope.

[0072] Furthermore, the number of cracks in each of the interface region R1 and the surface region R2 of the prototype after the current density measurement was observed using an SEM at a magnification of 30,000.

[0073] (Results) The evaluation results are shown in Table 1.

[0074]

[0075] As shown in Table 1, in samples in which cracks were formed in the interface region R1 and the surface region R2 and the number of cracks was greater in the surface region R2 than in the interface region R1, no cracks were observed in the solid electrolyte layer 24. Note that cracks with a width of 1 nm to 100 nm and a length of 100 nm to 1000 nm are also called microcracks. However, in the test results, microcracks were rarely observed in the interface region R1 and the surface region R2, and most cracks were longer than 1000 nm. Therefore, the average length of the cracks was 1200 nm or more. However, in sample No. 1, approximately two microcracks were observed in the interface region R1, which is thought to be due to the fast temperature increase and decrease rates.

[0076] The present invention is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted.

[0077] The electrolysis cell 20 only needs to have an anode 21, a solid electrolyte layer 24, and an air electrode 29, and the presence or absence of other components, as well as the shape, material, and dimensions of each component, can be modified. For example, the configuration of the electrolysis cell 20 can be modified as follows: (1) The shape of the electrolysis cell 20 can be, for example, an anode-supported type, a flat plate type, a cylindrical type, a flat type, a vertical stripe type, a horizontal stripe type, a type for a single-end-supported stack, or a type for a double-end-supported stack. The cross section of the cell can also be elliptical. (2) The configurations listed as different forms can be combined with each other.

[0078] 10...electrolysis stack, 12, 13...end plates, 14...bolt, 15a, 15b, 15c, 15d...passageway, 16...interconnector, 17...anode frame, 19...cathode frame, 20...electrolysis cell, 21...anode, 22...anode substrate layer, 23...anode functional layer, 24...solid electrolyte layer, 24a...surface, 25...reaction prevention layer, 26...cathode functional layer, 27...cathode current collecting layer , 29...air electrode, 30...separator, 31...brazing material, 32...current collector, 33...fuel chamber, 34...current collector, 35...air chamber, 37...opening, 47...cell with separator, 52, 53...terminal plate, 60...hydrogen production device, 61...hot module, 62...vaporizer, 63...heat exchanger, 64...heater, 65...insulation material, 66...condenser, Q...interface, R1...interface region, R2...surface region

Claims

1. A solid oxide electrolysis cell comprising: an air electrode containing a composite oxide having a perovskite structure; an anode; and a solid electrolyte layer disposed between the air electrode and the anode, wherein cracks are formed in the air electrode in an interface region within 10 μm from the interface on the solid electrolyte layer side and in a surface region within 10 μm from the surface opposite the solid electrolyte layer side, and the number of cracks is greater in the surface region than in the interface region.

2. In the solid oxide electrolysis cell according to claim 1, the fuel electrode is supplied with a fuel gas containing water vapor at a flow rate of 100 to 130 liters / min.cm ２ ) supplied at a flow rate of 1000 kJ / hour.

3. In the solid oxide electrolysis cell according to claim 1 or 2, the air electrode is supplied with an oxygen-containing gas at a flow rate of 30 to 50 liters / min.cm ２ ) supplied at a flow rate of 1000 kJ / hour.

4. In the solid oxide electrolysis cell according to claim 1 or 2, the volume of the air chamber facing the air electrode is 9 cm ３ ~11cm ３ A solid oxide electrolysis cell.

5. A cell with a separator, comprising: the solid oxide electrolysis cell according to claim 1 or 2; and a separator with a central opening disposed on the solid electrolyte layer.

6. An electrolysis stack comprising a plurality of solid oxide electrolysis cells according to claim 1 or 2 stacked together.

7. A hot module comprising: an electrolytic stack according to claim 6; a vaporizer that generates steam to be supplied to the electrolytic stack; a heat exchanger that exchanges heat with gas to be supplied to the electrolytic stack; a heater that heats the electrolytic stack; and a thermal insulator in which the electrolytic 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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