JP7799736B2 - Solid oxide electrolysis cells and their uses - Google Patents

Description

This disclosure relates to solid oxide electrolysis cells and their uses.

Solid oxide fuel cells (hereinafter referred to as "SOFCs") are known that generate electricity using the 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, comprises an electrolyte layer containing solid oxide, an air electrode located on one side of the electrolyte layer, and a fuel electrode located on the other side of the electrolyte layer. Oxygen ions dissociated from oxygen supplied to the air electrode migrate to the fuel electrode based on the oxygen conductivity of the solid electrolyte, and react with hydrogen contained in the fuel gas supplied to the fuel electrode to produce water vapor and generate electricity.

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 that converts 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 fuel electrode and an electric current is passed between the air electrode and the fuel electrode, the water vapor is electrolyzed, generating hydrogen from the fuel electrode. Oxygen ions generated at the fuel electrode migrate to the air electrode due to the oxygen conductivity of the solid electrolyte, and oxygen is generated from the air electrode.

JP 2023-080459 A

SOFCs generate heat during the reaction. For this reason, the temperature of SOFCs tends to rise. In contrast, there are times when the temperature of SOECs tends to fall between the start of operation and reaching the thermal neutral point. For this reason, with SOFCs, the air electrode may expand as the temperature rises, which can cause cracks to form in the air electrode, while with SOECs, the air electrode may shrink as the temperature drops, which can cause cracks to form in the air electrode.

This disclosure can be realized in the following forms:

(1) According to one embodiment of the present disclosure, a solid oxide electrolysis cell is provided. This solid oxide electrolysis cell includes an air electrode, an anode, and a solid electrolyte layer disposed between the air electrode and the anode. The air electrode has an uneven portion with alternating concave and convex portions along a portion or the entire periphery, and the top and bottom of the uneven portion are both arc-shaped, and the ratio (R1/R2) of the arc radius R1 of the top to the arc radius R2 of the bottom is 3.0 or greater. This embodiment of the solid oxide electrolysis cell can suppress the occurrence of cracks on the periphery of the air electrode.

(2) In the solid oxide electrolysis cell described in (1) above, the uneven portion may be present on at least one side of the air electrode. With this type of solid oxide electrolysis cell, the occurrence of cracks on the periphery of the air electrode can be efficiently suppressed.

(3) In the solid oxide electrolysis cell described in (2) above, the uneven portion may be present in two or more locations on one side. With a solid oxide electrolysis cell of this configuration, the occurrence of cracks on the periphery of the air electrode can be efficiently suppressed.

(4) In the solid oxide electrolysis cell described in any one of (1) to (3) above, the uneven portion may be present on at least one side on the upstream side with respect to the flow of oxygen-containing gas. With this type of solid oxide electrolysis cell, the occurrence of cracks on the periphery of the air electrode can be efficiently suppressed.

(5) In the solid oxide electrolysis cell according to any one of (1) to (4) above, a fuel gas containing water vapor may be supplied to the fuel electrode at a flow rate of 100 to 130 liters/min·cm ^2. This solid oxide electrolysis cell can efficiently prevent cracks from occurring on the periphery of the air electrode.

(6) In the solid oxide electrolysis cell according to any one of (1) to (5) above, an oxygen-containing gas may be supplied to the air electrode at a flow rate of 30 to 50 liters/min·cm ^2. This solid oxide electrolysis cell can efficiently prevent cracks from occurring on the periphery of the air electrode.

(7) In the solid oxide electrolysis cell according to any one of (1) to (6) above, the volume of the air chamber facing the air electrode may be 9 cm ³ to 11 cm ^3. The solid oxide electrolysis cell of this configuration can efficiently suppress the occurrence of cracks on the periphery of the air electrode.

(8) According to another aspect of the present disclosure, there is provided a cell with a separator, comprising the solid oxide electrolysis cell described in any one of (1) to (7) above and a separator with a central opening that is disposed on the solid electrolyte layer. This aspect of the cell with a separator can efficiently suppress the occurrence of cracks on the periphery of the air electrode.

(9) According to another aspect of the present disclosure, there is provided an electrolysis stack in which a plurality of solid oxide electrolysis cells described in any one of (1) to (7) above are stacked. With this form of electrolysis stack, it is possible to efficiently suppress the occurrence of cracks on the periphery of the air electrode. When a plurality of solid oxide electrolysis cells described in any one of (1) to (7) above are stacked, all of the solid oxide electrolysis cells may be used, or they may be used in areas where cracks are relatively likely to occur depending on the usage environment of the electrolysis stack, etc.

(10) According to another aspect of the present disclosure, there is provided a hot module comprising the electrolysis stack described in (9) above, a vaporizer that generates water vapor to be supplied to the electrolysis stack, a heat exchanger that exchanges heat with the gas supplied to the electrolysis stack, a heater for heating the electrolysis stack, and a thermal insulator in which the electrolysis stack, the vaporizer, the heat exchanger, and the heater are disposed. This aspect of the hot module can efficiently suppress the occurrence of cracks on the periphery of the air electrode.

(11) According to another aspect of the present disclosure, there is provided a hydrogen production device including the hot module described in (10) above. This aspect of the hydrogen production device can efficiently prevent cracks from occurring on the periphery of the air electrode.

The present invention can be realized in various forms, such as a method for manufacturing a solid oxide electrolysis cell.

1 is a perspective view showing an external configuration of an electrolysis stack according to an embodiment of the present disclosure; FIG. 2 is an exploded schematic view of the electrolysis stack taken along line II-II in FIG. 1 . Schematic top view of a cell with a separator. Block diagram of a hydrogen production device. FIG. 4 is a schematic diagram for explaining a concave-convex portion. FIG. 4 is a schematic diagram for explaining the shape of the concave and convex portions. FIG. 10 is a schematic diagram for explaining another example of the concave-convex portion. FIG. 10 is a schematic diagram for explaining another example of the concave-convex portion.

Figure 1 is a perspective view showing the external configuration of an electrolysis stack 10 according to one embodiment of the present disclosure. In this embodiment, the electrolysis stack 10 is a stack of solid oxide electrolysis cells (SOECs). Figure 1 shows an example of the configuration of the electrolysis stack 10.

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 that sandwich 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, reaction units 11, and end plates 13 in the thickness direction. The reaction units 11 and end plates 12 and 13 are fastened together by the bolts 14.

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

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

Figure 2 is an exploded schematic diagram of the electrolysis stack 10 cut along line II-II in Figure 1, which passes through the passages 15a and 15b. Figure 2 shows a schematic cross-sectional view along line II-II, with the components constituting one reaction unit 11 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 air electrode frame 19. Note that the thickness of each part is exaggerated in Figure 2. Holes (passages 15a, 15b, 15c, and 15d) penetrate the interconnector 16, anode frame 17, separator 30, and air electrode frame 19. The separator 30 is hermetically joined to the surface 24a of the solid electrolyte layer 24 (described later) with brazing filler metal 31, avoiding the air electrode 29.

Figure 3 is a schematic top view of a separator-equipped cell 47. The separator-equipped cell 47 includes an electrolysis cell 20 and a separator 30 disposed in the electrolysis cell 20. The electrolysis cell 20 will be described later.

The separator 30 is a roughly rectangular frame-shaped member with an opening 37 that is larger than the air electrode 29 described below. An example of a material for the separator 30 is stainless steel.

The interconnectors 16 are arranged at both ends of the electrolysis cell 20 in the thickness direction. The interconnectors 16 are formed from conductive, approximately rectangular plate-shaped members. The interconnectors 16 electrically connect adjacent reaction units 11 in the thickness direction. Stainless steel is an example of a material for the interconnectors 16.

The fuel electrode frame 17 is a roughly rectangular frame-shaped member positioned between the interconnector 16 and the separator 30. Stainless steel is an example of the material for the fuel electrode frame 17. The fuel electrode frame 17 surrounds the electrolysis cell 20 and the current collector 32 located in the center of the interconnector 16.

The current collector 32 electrically connects the fuel electrode 21 and the interconnector 16. Examples of materials for the current collector 32 include a gas-permeable porous body made of a metal such as Ni. Inside the fuel electrode frame 17, a fuel chamber 33 is formed, surrounded by the interconnector 16, the fuel electrode frame 17, and the separator 30.

The air electrode frame 19 is a substantially rectangular frame-shaped member disposed between the interconnector 16 and the separator 30. Examples of materials for the air electrode frame 19 include an insulator such as mica. The air electrode frame 19 surrounds the current collector 34 located 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 limited to this. It is of course possible for the current collector 34 to be a separate member from the interconnector 16.

An air chamber 35 is formed inside the air electrode frame 19, 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 from mixing with the oxidizer gas (oxygen, air, etc.) in the air chamber 35.

The hydrogen production device 60 equipped with the electrolysis stack 10 and the hot module 61 will be described with reference to Figure 4. Figure 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 is equipped with a hot module 61.

The hot module 61 includes the electrolysis stack 10, a vaporizer 62 that generates steam 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. To reduce heat radiation, the hot module 61 has the electrolysis stack 10, vaporizer 62, heat exchanger 63, and heater 64 arranged inside a thermal insulator 65.

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

Examples of the insulating material 65 include heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), as well as heat-resistant containers made from heat-resistant fibers. The heat-resistant fibers fill the gaps between the electrolysis stack 10, vaporizer 62, heat exchanger 63, and 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.

(Electrolytic 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 a 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.

The anode 21 is formed using nickel oxide and oxygen ion conductive ceramic particles. The nickel oxide (NiO) used as the material for the anode 21 is converted to nickel through a reduction process described below. The anode 21 of this embodiment is a porous, thin-plate-shaped sintered body composed of nickel and YSZ (yttria-stabilized zirconia). Examples of ceramic materials having oxygen ion conductivity include YSZ (yttria-stabilized zirconia), ScSZ (scandia-stabilized zirconia), (Gd, Ce) _O2 (i.e., GDC) (gadolinium-doped ceria), (Sm, Ce) _O2 (i.e., SDC) (samarium-doped ceria), and _LaGaO3 (lanthanum gallate). The anode 21 may contain one type of ceramic material or two or more types of ceramic materials. The thickness of the anode 21 is not particularly limited, but is, for example, 0.3 to 3 mm. In this embodiment, the anode 21 has the greatest thickness of all the components of the electrolysis cell 20, and functions as a support (support substrate, the member with the highest rigidity) for the electrolysis cell 20.

The anode 21 functions as the cathode of the electrolysis cell 20. The anode 21 comprises, 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 comprises, in order from the solid electrolyte layer 24 side, an anode functional layer 23 and an anode substrate layer 22 having a higher porosity than 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 thicker 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 preferably, for example, 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 from a single layer instead of the anode functional layer 23 and the anode substrate layer 22.

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), a ceria-based material such as gadolinium-doped ceria (GDC: (Ce, Gd)O 2 ) or samarium-doped ceria (SDC: (Ce, Sm) _{O 2} ), or yttria (Y ₂ O ₃ ).

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 %.

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, sintered plate 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 a ceria-based material such as gadolinium-doped ceria (GDC: (Ce, Gd)O ₂ ) or samarium-doped ceria (SDC: (Ce, Sm)O ₂ ).

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

The solid electrolyte layer 24 is a dense, thin-plate sintered body. The solid electrolyte layer 24 is formed from 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. Examples of perovskite-type oxides include lanthanum gallate-based oxides having a perovskite structure. The porosity of the solid electrolyte layer 24 is not particularly limited, but is, for example, 0 to 10%. The thickness of the solid electrolyte layer 24 is not particularly limited, but is, for example, 0.5 to 30 μm.

In this embodiment, the solid electrolyte layer 24 is disposed between the anode 21 and the reaction prevention layer 25. The solid electrolyte layer 24 has a function of allowing oxygen ions generated at the cathode 29 to pass through. The solid electrolyte layer 24 may contain zirconium (Zr), or may contain Zr as zirconia (ZrO ₂ ). The solid electrolyte layer 24 may contain ZrO ₂ as a main component. In addition to ZrO ₂ , the solid electrolyte layer 24 may also contain additives such as Y ₂ O ₃ and/or Sc ₂ O _3. These additives function as stabilizers. In the solid electrolyte layer 24, the molar composition ratio of the stabilizer to ZrO ₂ (stabilizer:ZrO ₂ ) is preferably approximately 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.

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 primary component. Examples of lanthanum-containing perovskite-type complex oxides include LSCF (lanthanum strontium cobalt ferrite), lanthanum manganite, lanthanum cobaltite, and lanthanum ferrite. Lanthanum-containing perovskite-type complex oxides 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 300 μm.

The air electrode 29 contains a complex oxide having a perovskite structure. The composition of the complex oxide is expressed by the general formula _ABO3 . However, the ratio of A, B, and O is not required to be strictly 1:1:3. The ideal unit lattice of a complex oxide having a perovskite structure expressed by the general formula _ABO3 is a cube, with the A element located at a corner of the unit lattice, the B element located at the body center of the unit lattice, and the oxygen element located at the face center of the unit lattice. In the present disclosure, the corner position of the unit lattice where the A element is located is referred to as the "A site," and the body center position of the unit lattice where the B element is located is referred to as the "B site."

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. Specific examples of such complex oxides, for example, for the material of the air electrode 29, include LSCF (i.e., (La,Sr)(Co,Fe)O _{3 )} , LSF (i.e., (La,Sr)FeO ₃ ), LSC (i.e., (La,Sr)CoO ₃ ), LNF (i.e., La(Ni,Fe)O _{3 )} , and SSC (i.e., (Sm,Sr)CoO ₃ ). These complex oxides are substances that have both oxygen ion conductivity and electronic conductivity and are also called mixed conductive materials. In this embodiment, the air electrode 29 is a porous, thin-plate-shaped sintered body formed from a material containing lanthanum strontium cobalt ferrite LSCF (La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ ).

The air electrode 29 can contain a complex oxide as its "main component." When composition X "contains substance Y as its main component," this means that substance Y preferably accounts for 60% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more of the entire composition X. The air electrode 29 may also contain components other than the complex oxide. Furthermore, the material for the air electrode 29 may be a powder (e.g., average particle size of approximately 0.1 μm to 5 μm) or a crushed material (e.g., average particle size of approximately 5 μm to 500 μm), or it may be a lump larger than the crushed material.

The cathode current collecting layer 27 contains a complex oxide having a perovskite structure represented by the general formula _ABO3 . 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 the 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 300 μm. The thickness of the cathode functional layer 26 is not particularly limited, but may be, for example, 5 to 30 μm. The cathode functional layer 26 may not be provided, and the cathode functional layer 26 may not contain Ce. The porosity of the cathode current collecting layer 27 is not particularly limited, but may be, for example, 35 to 45%. The porosity of the air electrode functional layer 26 is not particularly limited, but may be, for example, 35 to 45%. The electrical conductivity of the material constituting the air electrode current collecting layer 27 may be higher than the electrical conductivity of the material constituting the air electrode functional layer 26.

Due to the difference in thermal expansion coefficients, there is a risk that the air electrode 29 will peel off from the solid electrolyte layer 24. To address this issue, a technique exists in which a composite material containing an electrolyte mixed with the air electrode 29 is provided between the air electrode 29 and the solid electrolyte layer 24. However, when a Zr-based material is used as the electrolyte material, the presence of Sr in the air electrode 29 poses a problem of the generation of SrZrO ₃ (SZO). 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 may include the air electrode current collecting layer 27 but not the air electrode functional layer 26. Furthermore, depending on the actual usage environment, the air electrode 29 may include the air electrode functional layer 26 but not the air electrode current collecting layer 27.

When Sr (strontium) contained in the air electrode 29 diffuses toward the solid electrolyte layer 24 and 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 a layer of SZO is generated 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 not 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).

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 preventing 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.

In this embodiment, the periphery of the air electrode 29 has an uneven portion V with alternating concave and convex portions along part or the entire periphery.

Figure 5 is a schematic diagram illustrating the uneven portion V. In Figure 5, the periphery of the air electrode 29 has an uneven portion V in part. In this embodiment, the uneven portion V exists on at least one side of the air electrode 29, and there are sides on which the uneven portion V exists in two or more places, and sides on which the uneven portion V exists in one place.

In Figure 5, the oxygen-containing gas flows from the left side of the page to the right side. This flow is in a fixed direction. In this embodiment, the uneven portion V exists on at least one side upstream of the flow of the oxygen-containing gas. This effectively prevents cracks from occurring in the upstream air electrode 29, where the temperature is prone to change.

Figure 6 is a schematic diagram illustrating the shape of the uneven portion V. In this specification, the apex T of the uneven portion V refers to the portion away from the center of the air electrode 29. Furthermore, the bottom B of the uneven portion V refers to the portion close to the center of the air electrode 29. In the uneven portion V, both the apex T and the bottom B are arc-shaped. The ratio (R1/R2) of the arc radius R1 of the apex T to the arc radius R2 of the bottom B is 3.0 or greater. The inventors have found a strong correlation between the occurrence of cracks in the air electrode 29 and the ratio (R1/R2) of the arc radius R1 of the apex T to the arc radius R2 of the bottom B. The ratio (R1/R2) is not particularly limited as long as it is 3.0 or greater, but the upper limit is preferably 20 or less, more preferably 18 or less, and even more preferably 17 or less. Specifically, the ratio (R1/R2) increases as R1 increases (the convexity decreases), as R2 decreases (if there are extremely concave portions), or as a combination of these. The pitch P of the concave-convex portion V refers to the distance between adjacent bottom portions B. There are no particular limitations on the pitch P, but examples include 0.2 to 2 mm. The height difference H refers to the height difference from the point on the top portion T farthest from the center to the point on the bottom portion B closest to the center. There are no particular limitations on the height difference H, but it is preferably 0.1 to 4, more preferably 0.2 to 3, and even more preferably 0.25 to 2.5.

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, causing the chemical reactions shown in the following formulas (1) and (2). 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⁻ (at fuel electrode 21) …(1)
O ^2- → (1/2)・O ₂ +2e ^- (Location: Air electrode 29) ...(2)

In this SOEC electrolysis cell 20, a conductive connecting member (interconnector 16) for collecting current is typically joined and fixed to each of the fuel electrode 21 and air electrode 29 with a bonding agent, and a potential difference is applied via each interconnector 16 to allow current to flow. Hydrogen gas generated from the fuel electrode 21 is then collected.

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

An oxygen-containing gas may be supplied to the air electrode 29 at a flow rate of, for example, 20 to 60 liters/(min. cm ² ), or 30 to 50 liters/(min. cm ² ). The oxygen-containing gas may be oxygen gas or air. 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 layers.

The volume of the air chamber 35 facing the air electrode 29 is not particularly limited, but is preferably 8 cm ³ to 13 cm ³ , and more preferably 9 cm ³ to 11 cm ^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.

(Manufacturing method)
Next, an example of a method for manufacturing 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.

(Preparation of green sheet for fuel electrode substrate layer)
A mixture of NiO powder and YSZ powder is mixed with organic beads (pore former), butyral resin, DOP (plasticizer), a dispersant, and a mixed solvent of toluene and ethanol in a ball mill to prepare a slurry. The organic beads are spherical particles formed from a polymer such as polymethyl methacrylate or polystyrene. The resulting slurry is thinned using a doctor blade method to produce 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 producing 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 volume % NiO and 30 to 60 volume % YSZ.

(Preparation of green sheet for fuel electrode functional layer)
A butyral resin, a plasticizer (DOP), a 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 using a doctor blade method to produce a green sheet for an anode functional layer having a predetermined thickness (e.g., 5 μm to 50 μm). The mixing ratio of the NiO powder and the YSZ powder when producing the green sheet for an anode functional layer can be appropriately set as long as the performance is satisfied. The mixing ratio of the NiO powder and the YSZ powder may be, for example, 30 to 60 volume % NiO and 40 to 70 volume % YSZ.

(Preparation of Green Sheet for Solid Electrolyte Layer)
Butyral resin, DOP (a plasticizer), a dispersant, and a mixed solvent of toluene and ethanol are added to the YSZ powder and mixed in a ball mill to prepare a slurry. The resulting slurry is thinned by a doctor blade method to produce a green sheet for the solid electrolyte layer of a predetermined thickness (e.g., 10 μm).

(Fabrication of a Laminate of Solid Electrolyte Layer 24, Anode Functional Layer 23, and Anode Substrate Layer 22)
The green sheet for the anode substrate layer, the green sheet for the anode functional layer, and the green sheet for the solid electrolyte layer are attached together and degreased at a predetermined temperature (e.g., about 280°C). The laminate of the degreased green sheets is then fired at a predetermined temperature (e.g., about 1350°C). This results in a laminate of the solid electrolyte layer 24, the anode functional layer 23, and the anode substrate layer 22. The maximum temperature during firing is preferably, for example, 1500°C or less, more preferably 1450°C or less, and even more preferably 1400°C or less. The maximum temperature during firing is preferably, for example, 1100°C or more, more preferably 1200°C or more, and even more preferably 1300°C or more. The holding time at high temperature is, for example, preferably 420 minutes or less, more preferably 360 minutes or less, and even more preferably 300 minutes or less. Furthermore, 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 increase 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. For example, the temperature increase rate is 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. For example, the temperature decrease rate is preferably 2°C/min or less, more preferably 1.5°C/min or less, and even more preferably 1.3°C/min or less. For example, the temperature decrease rate is 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.

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 using a mold press molding method. 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 stack of green bodies is then co-sintered in air in an electric furnace (in an oxygen-containing atmosphere) at a temperature of, for example, 1350°C, to form a stack of the anode 21 and the solid electrolyte layer 24. Note that tape lamination, printing, and other methods may also be used to form the film that will later become the solid electrolyte layer 24 on the anode 21.

(Formation of reaction prevention layer 25)
Next, the reaction prevention 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 prevention layer. The obtained paste for the reaction prevention layer is applied by, for example, screen printing to the surface of the solid electrolyte layer 24 side of the laminate of the solid electrolyte layer 24 and the anode 21, and then fired at, for example, 1180°C. This forms the reaction prevention layer 25, and a laminate of the anode 21, solid electrolyte layer 24, and reaction prevention layer 25 (hereinafter referred to as the "intermediate laminate") is produced.

(Formation of the 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. At this time, a screen mask is used that forms an uneven portion V on the periphery of the air electrode 29. Examples of the screen mask include a mesh mask and a metal mask.

(Formation of Air Electrode Current Collecting Layer 27)
Next, the cathode 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 cathode current collecting layer paste. Next, the prepared cathode current collecting layer paste is applied to the surface of the intermediate laminate facing the cathode functional layer 26 by, for example, screen printing, and then dried. At this time, a screen mask is used that forms an uneven portion V on the periphery of the cathode 29. Examples of the screen mask include a mesh mask and a metal mask. Then, the intermediate laminate to which the current collecting layer paste is applied is fired at a predetermined firing temperature (for example, 1100°C). This firing process forms the cathode current collecting layer 27, and a laminate of the anode 21, the solid electrolyte layer 24, the reaction prevention layer 25, the cathode functional layer 26, and the cathode current collecting layer 27, i.e., the single electrolytic cell 20, is produced.

Then, a reduction process is carried out to reduce the fuel electrode 21 (i.e., reduce the NiO contained in the fuel electrode 21 to Ni) so that the electrolysis cell 20 is ready for hydrogen production operation. The reduction process is achieved, for example, by exposing the fuel electrode 21 to a hydrogen atmosphere at a predetermined temperature for a predetermined period of time. The reducing gas used in the reduction process is not limited to hydrogen, but may be other gases such as methane gas, and the concentration of the reducing gas is not limited. A reducing gas with a reducing agent concentration of less than 100% by volume may contain, for example, nitrogen gas in addition to hydrogen gas.

(Explanation of effects)
As described above, the periphery of the air electrode 29 in the above-described embodiment has an uneven portion V in which concave and convex portions alternate with each other. In the uneven portion V, both the top portion T and the bottom portion B are arc-shaped. The ratio (R1/R2) of the arc radius R1 of the top portion T to the arc radius R2 of the bottom portion B is 3.0 or greater. When an SOEC is operated in a typical environment after reaching thermal neutrality, cracks rarely occur on the periphery of the air electrode. However, before reaching thermal neutrality, such as during startup of the SOEC, the electrolysis cell 20 operates in a harsh environment, which may result in cracks on the periphery of the air electrode 29. In other words, shrinkage due to a temperature drop generally occurs on the periphery of the air electrode 29, generating stress in the air electrode 29 and potentially resulting in cracks on the air electrode 29. However, because the air electrode 29 of the above configuration has the uneven portion V as described above, stress on the air electrode 29 is dispersed, suppressing stress concentration and thereby suppressing the occurrence of cracks in the air electrode 29. As a result, peeling of the air electrode 29 due to the occurrence of cracks in the air electrode 29 can be suppressed. Furthermore, the hot module 61 including the air electrode 29 of the above configuration has a heat source such as the heater 64 or the heat exchanger 63, and is therefore able to increase the temperature of the gas supplied to the electrolysis stack 10 and the electrolysis cell 20, thereby effectively suppressing the occurrence of cracks in the air electrode 29 when the temperature drops.

Below, we will explain the tests that confirmed that the above-described configuration of the air electrode 29 suppresses the occurrence of cracks in the air electrode 29.

(test)
In this test, for the SOEC (electrolysis cell 20) according to the above embodiment, multiple types of test specimens (sintered bodies) were used, each of which had different combinations of the material of the cathode current collecting layer 27, the height difference H of the uneven portion V formed on the periphery of the cathode current collecting layer 27, the arc radius R1 of the top portion T, and the arc radius R2 of the bottom portion B. Specifically, as shown in Table 1, six levels were prepared, and 10 test specimens were produced for each level. The values of H, R1, and R2 in Table 1 each indicate an average value. In the parentheses within the evaluation results, the denominator indicates the total number of test specimens, and the numerator indicates the number of test specimens that had cracks.

In these test samples, the thickness of the air electrode functional layer 26 was 5 to 30 μm, and the thickness of the air electrode current collecting layer 27 was 80 to 300 μm. The pitch P of the irregularities was 0.2 to 2 mm. The height difference H was 0.05 to 1 mm. The porosity of the air electrode functional layer 26 was 35 to 45%. The porosity of each layer was adjusted by the particle size of the powder in the slurry and the amount of pore-forming material added. The co-firing temperature of the air electrode 29 (air electrode functional layer 26 and air electrode current collecting layer 27) was adjusted within the range of 900 to 1100°C. The co-firing time was adjusted within the range of 1 to 10 hours. The reduction treatment temperature was adjusted within the range of 800 to 1000°C. The reduction treatment time was adjusted within the range of 1 to 10 hours.

The following thermal cycle electrolysis test was then performed on these test specimens. Specifically, for each test specimen, with a reducing fuel gas flowing through the anode 21, the ambient temperature was increased from room temperature (25°C) to 750°C over 8 hours, and then decreased from 750°C to room temperature (25°C) over 2 hours. This series of steps was repeated 10 times. For test specimens that underwent this thermal cycle electrolysis test, the presence or absence of cracks was confirmed around the periphery of the cathode current collecting layer 27 (i.e., the portion of the cathode current collecting layer 27 where irregularities are formed). This confirmation was made by visual inspection and observation with a microscope.

(result)
The evaluation results are shown in Table 1.

This thermal cycle electrolysis test is a severe test in terms of thermal stress, but as shown in Table 1, when R1/R2 is 3.0 or greater, no cracks were observed, although the reason is unclear. On the other hand, when R1/R2 is less than 3.0, cracks were observed. The inventors have separately confirmed that when the electrolytic cell 20 of this embodiment is used under typical usage conditions and environments, when R1/R2 is 3.0 or greater, no cracks occur around the periphery of the air electrode current collecting layer 27.

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 replaced or combined as appropriate 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 deleted as appropriate.

The electrolysis cell 20 only needs to have the anode 21, the solid electrolyte layer 24, and the cathode 29, and the presence or absence of other components, as well as the shape, material, dimensions, etc. of each component, can be modified. For example, the configuration of the electrolysis cell 20 may be modified as follows.
(1) The shape of the electrolysis cell 20 may be, for example, anode-supported, flat, cylindrical, oblate, vertically striped, horizontally striped, for a single-end-supported stack, or for a double-end-supported stack. The cross section of the cell may also be elliptical.
(2) The configurations listed as different embodiments can be combined with each other.
(3) In the above embodiment, the periphery of the air electrode 29 has a concave-convex portion V in part. In the above embodiment, the concave-convex portion V is present on at least one side of the air electrode 29, and the concave-convex portion V is present in one or more locations on one side. However, this is not limited to this. For example, the periphery of the air electrode 29 may have the concave-convex portion V around the entire periphery ( FIG. 7 ), may have the concave-convex portion V on two opposing sides ( FIG. 8 ), may have the concave-convex portion V in three or more locations on one side, may have the concave-convex portion V on a side downstream of the flow of the oxygen-containing gas, or may have the concave-convex portion V on a side parallel to the flow of the oxygen-containing gas.

In addition, in the above embodiment, the air electrode 29 has the uneven portion V on the periphery of both the air electrode functional layer 26 and the air electrode current collecting layer 27. However, the uneven portion V may also be on the periphery of one of the air electrode functional layer 26 or the air electrode current collecting layer 27. If the air electrode current collecting layer 27 is larger than the air electrode functional layer 26 in a planar view, the uneven portion V of the air electrode functional layer 26 may be omitted, and the uneven portion V may be on the periphery of the air electrode current collecting layer 27 only. On the other hand, if the air electrode functional layer 26 is larger than the air electrode current collecting layer 27 in a planar view, the uneven portion V of the air electrode current collecting layer 27 may be omitted, and the uneven portion V may be on the periphery of the air electrode functional layer 26 only. In the above embodiment, the air electrode 29 has the air electrode functional layer 26 and the air electrode current collecting layer 27, but this is not limited thereto, and the air electrode 29 may be formed of a single layer. Furthermore, in the above embodiment, the air electrode current collecting layer 27 is in contact only with the upper surface of the air electrode functional layer 26, but this is not limited thereto, and the air electrode current collecting layer 27 may cover the upper and side surfaces of the air electrode functional layer 26. In this case, the uneven portion V of the air electrode functional layer 26 may be omitted, and the uneven portion V may be present only on the periphery of the air electrode current collecting layer 27.

10...Electrolytic stack, 11...Reaction unit, 12, 13...End plates, 14...Bolt, 15a, 15b, 15c, 15d...Passageway, 16...Interconnector, 17...Anode frame, 19...Cathode frame, 20...Electrolytic 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...Cathode 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...Insulating material, 66...Condenser, B...Bottom, H...Height difference, P...Pitch, R1...Arc radius, R2...Arc radius, T...Top, V...Uneven portion

Claims (11)

an air electrode;
a fuel electrode;
a solid electrolyte layer disposed between the air electrode and the fuel electrode;
Equipped with
The periphery of the air electrode has an uneven portion in which unevenness alternates over a part or the entire periphery,
In the concave-convex portion, both the top and bottom are arc-shaped,
a ratio (R1/R2) of the arc radius R1 of the top portion to the arc radius R2 of the bottom portion is 3.0 or more;
A fuel gas containing water vapor is supplied to the fuel electrode,
The temperature drops from the start of operation until it reaches the thermal neutral point .
Solid oxide electrolysis cell. 2. The solid oxide electrolysis cell according to claim 1,
The uneven portion is present on at least one side of the air electrode.
Solid oxide electrolysis cell. 3. The solid oxide electrolysis cell according to claim 2,
The uneven portion is present at two or more locations on the one side.
Solid oxide electrolysis cell. The solid oxide electrolysis cell according to claim 1 or 2,
The uneven portion is present on at least one side on the upstream side with respect to the flow of the oxygen-containing gas.
Solid oxide electrolysis cell. The solid oxide electrolysis cell according to claim 1 or 2,
A fuel gas containing water vapor is supplied to the fuel electrode at a flow rate of 100 to 130 liters/(min·cm ² ).
Solid oxide electrolysis cell. The solid oxide electrolysis cell according to claim 1 or 2,
An oxygen-containing gas is supplied to the air electrode at a flow rate of 30 to 50 liters/(min·cm ² ).
Solid oxide electrolysis cell. 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 to 11 cm ³ .
Solid oxide electrolysis cell. The solid oxide electrolysis cell according to claim 1 or 2; and
a separator having an opening in the center and disposed on the solid electrolyte layer;
Cell with separator. An electrolysis stack comprising multiple stacked solid oxide electrolysis cells according to claim 1 or claim 2. 10. The electrolytic stack of claim 9;
a vaporizer that generates water vapor that is supplied to the electrolysis stack;
a heat exchanger that exchanges heat with the gas supplied to the electrolysis stack;
a heater for heating the electrolysis stack;
a thermal insulator in which the electrolysis stack, the vaporizer, the heat exchanger, and the heater are disposed,
Hot module. A hydrogen production device comprising the hot module according to claim 10.