Electrochemical single cells, cell stacks, hot modules, and gas production equipment

Abstract

To provide an electrochemical unit cell capable of maintaining stable performance over a long period of time. [SOLUTION] The electrochemical unit cell 10 comprises a solid oxide type solid electrolyte layer 12, an air electrode 14 laminated on the front side of the solid electrolyte layer 12, an anode 16 laminated on the back side of the solid electrolyte layer 12, and an intermediate layer 20 laminated between the solid electrolyte layer 12 and the anode 16. The anode 16 is made of a cermet, and the intermediate layer 20 is porous and does not contain a cermet.

Description

[Technical field]

[0001] The present invention relates to a solid oxide electrochemical unit cell, a cell stack, a hot module, and a gas production device. [Background technology]

[0002] Conventionally, solid oxide electrochemical cells using solid oxide as an electrolyte have been known. Solid oxide electrochemical cells are characterized by performing electrochemical reactions with high efficiency in a high-temperature environment, and are broadly classified into solid oxide electrolysis cells (SOEC: Solid Oxide Electrolysis Cell) and solid oxide fuel cells (SOFC: Solid Oxide Fuel Cell). Solid oxide electrolysis cells are devices that decompose fuel gas using electrical energy. Solid oxide fuel cells are devices that generate electrical energy (i.e., generate electricity) by combining multiple different gases. In the following, the former may be simply referred to as "SOEC" and the latter may be simply referred to as "SOFC". In addition, the "multiple different gases" are referred to as "combination target gases".

[0003] An electrochemical cell has a structure in which a plurality of electrochemical single cells (hereinafter also referred to as "single cells") are stacked. Usually, a single cell includes a solid electrolyte layer, an air electrode laminated on the front side of the solid electrolyte layer, and a fuel electrode laminated on the back side of the solid electrolyte layer. Various research and development efforts have been conducted in order to suppress the performance degradation of a single cell. For example, Patent Document 1 describes a single cell in which an intermediate layer is provided between the solid electrolyte layer and the fuel electrode. Patent Document 1 describes that the performance degradation of a single cell can be suppressed by devising the material of this intermediate layer and the bonding state of the material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-8914 A Summary of the Invention

[0005] However, there is a possibility that the single cell of Patent Document 1 cannot maintain stable performance over a long period of time. This will be explained in detail below.

[0006] Since the single cell is exposed to a large temperature change during manufacture and at the start / stop of operation, stress is generated at the interface of each layer due to the difference in the thermal expansion coefficient of each layer constituting the single cell, and the single cell may be warped. Patent Document 1 describes that peeling occurs between the solid electrolyte layer and the fuel electrode due to the difference in the expansion coefficient of the two in a reducing atmosphere, but does not consider any problems caused by the difference in the thermal expansion coefficient in an environment with large temperature changes. Therefore, with the technology of Patent Document 1, the single cell is exposed to a large temperature change during manufacture and at the start / stop of operation, which may cause the single cell to warp and reduce its durability.

[0007] Incidentally, the electrochemical reaction of the electrochemical cell is particularly active on the surface of the fuel electrode of each unit cell on the solid electrolyte layer side and in the region nearby the surface (hereinafter, this region is referred to as the "reaction field"). When the electrochemical cell is operated as an SOEC, the first product gas (e.g., hydrogen) generated by electrolysis of the fuel gas (e.g., water vapor) increases in the reaction field, and the reaction field tends to become a reducing atmosphere. Here, depending on the usage mode of the electrochemical cell, there is a demand to produce a large amount of the first product gas in a short period of time. In this case, if the electrochemical cell is operated at high output in order to increase the production efficiency of the first product gas, the discharge pace of the first product gas may not keep up with the production pace, and the concentration of the first product gas in the reaction field may exceed its reference concentration, resulting in a highly reducing atmosphere in the reaction field.

[0008] On the other hand, when the electrochemical cell is operated as a SOFC, the second product gas (e.g., water vapor) generated by the combination of the gases to be combined (e.g., hydrogen and oxygen) increases in the reaction field, and the reaction field tends to become an oxidizing atmosphere. Here, depending on the manner in which the electrochemical cell is used, there is a demand for generating a large amount of electricity in a short period of time. In this case, if the electrochemical cell is operated at high output in order to increase the power generation efficiency, the discharge rate of the second product gas cannot keep up with the generation rate, and the concentration of the second product gas in the reaction field may exceed its reference concentration, causing the reaction field to become a highly oxidizing atmosphere.

[0009] In this way, when the reaction field becomes a highly reducing or oxidizing atmosphere by operating the electrochemical cell at high power output, the electrodes of the single cell may deteriorate or the gas flow may be impeded, which may reduce the reaction efficiency of the single cell and cause the output to deteriorate (reduced or unstable output). The deterioration of the output is a factor in reducing the durability of the single cell.

[0010] In addition, the electrochemical cell can be operated by switching between SOEC and SOFC (i.e., reversibly). In this case, steam is used as the fuel gas, and hydrogen and oxygen are used as the gas to be combined. If the electrochemical cell is operated at high power and the switching between SOEC and SOFC is performed frequently, the electrodes may deteriorate. In addition, immediately after switching from SOEC to SOFC, the hydrogen remaining in the reaction field may be consumed by the combination reaction before the hydrogen supplied from the outside reaches the reaction field, which may result in depletion of hydrogen. On the other hand, immediately after switching from SOFC to SOEC, the water vapor remaining in the reaction field may be consumed by the electrolysis reaction before the water vapor supplied from the outside reaches the reaction field, which may result in depletion of water vapor. In this way, when the gas used in the reaction is depleted, the resistance of the electrochemical cell increases, and the output becomes unstable. For this reason, when the electrochemical cell is operated reversibly at high power, the problem of reduced durability of the single cell becomes more prominent.

[0011] These problems are not specifically mentioned in Patent Document 1. For this reason, in the technology of Patent Document 1, the reaction field becomes a highly reducing atmosphere or a highly oxidizing atmosphere, which may deteriorate the output of the unit cell, resulting in a decrease in the durability of the unit cell.

[0012] For the above reasons, the unit cell of Patent Document 1 may not be able to maintain stable performance over a long period of time because durability decreases due to the occurrence of warping and deterioration of output.

[0013] The present invention has been made to address the above-mentioned problems. That is, one of the objects of the present invention is to provide a technique capable of stably maintaining the performance of an electrochemical single cell for a long period of time.

[0014] The solid oxide electrochemical cell according to the first aspect of the present invention is A solid electrolyte layer (12); an air electrode (14) disposed on a surface side of the solid electrolyte layer; a fuel electrode (16) laminated on the back surface of the solid electrolyte layer; an intermediate layer (20) laminated between the solid electrolyte layer and the fuel electrode; Equipped with. The fuel electrode is made of a cermet, The intermediate layer is porous and does not include a cermet.

[0015] This electrochemical single cell includes an intermediate layer laminated between the solid electrolyte layer and the fuel electrode, and this intermediate layer is porous. Therefore, when the single cell is exposed to a large temperature change during manufacturing and at the start / end of operation, and the solid electrolyte layer and the fuel electrode expand or contract at their own thermal expansion coefficients, the intermediate layer elastically deforms in response to the movement. Specifically, the intermediate layer in the portion in contact with the solid electrolyte layer elastically deforms in response to the amount of expansion or contraction of the solid electrolyte layer, and the intermediate layer in the portion in contact with the fuel electrode elastically deforms in response to the amount of expansion or contraction of the fuel electrode. This makes it possible to significantly reduce the stress generated at the interface of each layer, even if the materials of the ceramic particles contained in the solid electrolyte layer and the fuel electrode are different from each other (i.e., even if the thermal expansion coefficients of the two are different from each other). As a result, it is possible to suppress the occurrence of warping in the single cell, and to suppress the deterioration of the durability of the single cell.

[0016] In addition, in this single cell, the fuel electrode is made of cermet, whereas the intermediate layer does not contain cermet. That is, no electrochemical reaction takes place in the intermediate layer. Therefore, the reaction field is formed on the surface of the fuel electrode on the intermediate layer side and in the area nearby it. When an electrochemical cell made of this single cell is operated as an SOEC, a part of the first product gas (gas generated by electrolysis of fuel gas) generated in the reaction field diffuses into the intermediate layer and is temporarily stored in a large number of holes. The stored first product gas is discharged from the fuel electrode in stages. This configuration can improve the dischargeability of the first product gas. Therefore, even when the electrochemical cell is operated at high output, the first product gas can be discharged at an appropriate pace, and the concentration of the first product gas in the reaction field can be prevented from exceeding the reference concentration. As a result, the reaction field can be prevented from becoming a highly reducing atmosphere. Similarly, when this electrochemical cell is operated as an SOFC, a part of the second product gas (gas generated by the combination of the gases to be combined) generated in the reaction field diffuses into the intermediate layer and is temporarily stored in a large number of holes. The stored second product gas is discharged from the fuel electrode in stages. This configuration can improve the dischargeability of the second product gas. Therefore, even when the electrochemical cell is operated at high power, the second product gas can be discharged at an appropriate pace, and the concentration of the second product gas in the reaction field can be prevented from exceeding its reference concentration. As a result, the reaction field can be prevented from becoming a highly oxidizing atmosphere. That is, the intermediate layer functions as a buffer that temporarily stores the first product gas or the second product gas. In this way, by providing the intermediate layer, which has a large number of holes and is a layer in which an electrochemical reaction does not occur, in contact with the reaction field, the reaction field can be prevented from becoming a highly reducing atmosphere or a highly oxidizing atmosphere, and the possibility that the electrodes of the single cell will deteriorate or the flow of gas will be stagnated can be significantly reduced. Therefore, it is possible to suppress a decrease in reaction efficiency and maintain a stable output, and as a result, it is possible to suppress a decrease in durability of the single cell.

[0017] Furthermore, as described above, according to the configuration of the present invention, the reaction field can be prevented from becoming a highly reducing atmosphere or a highly oxidizing atmosphere, so that the electrodes are less likely to deteriorate even if the electrochemical cell is operated at high power and the SOEC and SOFC are switched frequently. In addition, immediately after switching from SOEC to SOFC, hydrogen stored in the intermediate layer (in a large number of holes) during operation as SOEC is gradually discharged to the reaction field, so that the possibility of hydrogen depletion can be significantly reduced. On the other hand, immediately after switching from SOFC to SOEC, water vapor stored in the intermediate layer (in a large number of holes) during operation as SOFC is gradually discharged to the reaction field, so that the possibility of water vapor depletion can be significantly reduced. Therefore, it is possible to prevent the resistance of the electrochemical cell from increasing due to gas depletion. As a result, it is possible to maintain a stable output even when the electrochemical cell is operated reversibly at high power, and it is possible to prevent the durability of the single cell from decreasing.

[0018] As described above, immediately after switching from SOFC to SOEC, the water vapor remaining in the reaction field is consumed in the electrolysis reaction. The consumption rate is higher as the distance between the electrodes is shorter and / or the insulation resistance between the electrodes is higher. In the present invention, an intermediate layer is interposed between the reaction field and the solid electrolyte layer. Therefore, the electrodes can be separated from each other by the thickness of the intermediate layer. In addition, since the intermediate layer has many holes, the insulation resistance between the electrodes can be reduced. Therefore, the consumption rate of the water vapor remaining in the reaction field is slower than in a configuration without an intermediate layer. This also reduces the possibility that the water vapor will run out immediately after switching.

[0019] As described above, according to the present invention, the performance of the electrochemical unit cell can be stably maintained for a long period of time.

[0020] In one aspect of the invention, The intermediate layer (20) comprises ceria or ceria containing a rare earth element.

[0021] In this electrochemical single cell, the intermediate layer contains ceria (CeO2) or ceria containing a rare earth element. In general, CeO2 has a characteristic that in a reducing atmosphere, the crystal lattice releases oxide ions and the valence changes from tetravalent to trivalent, and in an oxidizing atmosphere, the crystal lattice absorbs (takes in) the surrounding oxide ions and the valence changes from trivalent to tetravalent. Therefore, when the concentration of the first product gas in the reaction field increases (i.e., the atmosphere changes to a reducing atmosphere) by operating the electrochemical cell composed of this single cell at high output as an SOEC, oxide ions are released from the CeO2 in the intermediate layer, and the reduction tendency is alleviated, so that it is possible to suppress the atmosphere from becoming highly reducing.

[0022] Here, the single cell is characterized in that the solid electrolyte layer has oxide ion conductivity, so that one of the gases to be combined is always oxygen. When the electrochemical cell is operated as a SOFC, oxygen receives electrons at the air electrode and becomes oxide ions, and these oxide ions move through the solid electrolyte layer and intermediate layer to flow into the reaction field of the fuel electrode. Therefore, when the electrochemical cell is operated at high power as a SOFC and the concentration of the second product gas in the reaction field increases (i.e., the atmosphere changes to an oxidizing atmosphere), the oxide ions (more specifically, the oxide ions moving from the air electrode to the reaction field) are absorbed in CeO2 in the intermediate layer, so that the concentration of the oxide ions in the reaction field decreases. When the concentration of the oxide ions decreases, the increase in the concentration of the second product gas is suppressed and the tendency for oxidation is alleviated, so that the atmosphere can be prevented from becoming highly oxidizing.

[0023] In particular, in the conventional configuration (i.e., a configuration not including an intermediate layer containing ceria or ceria containing a rare earth element), when the electrochemical cell is operated reversibly at high power, the electrochemical reaction proceeds rapidly immediately after switching, causing a problem that the reaction field changes rapidly to an oxidizing atmosphere or a reducing atmosphere. Specifically, immediately after switching from SOEC to SOFC, a combination reaction of hydrogen and oxide ions remaining in the reaction field proceeds rapidly, causing the reaction field to change rapidly to an oxidizing atmosphere, and immediately after switching from SOFC to SOEC, an electrolysis reaction of water vapor remaining in the reaction field proceeds rapidly, causing the reaction field to change rapidly to a reducing atmosphere. In contrast, in the present invention, immediately after switching from SOEC to SOFC, the intermediate layer occludes oxide ions in the process of the reaction field changing to an oxidizing atmosphere, so that the combination reaction proceeds slowly and the rapid change to an oxidizing atmosphere can be suppressed. Immediately after switching from SOFC to SOEC, oxide ions are released from the intermediate layer in the process of changing the reaction field to a reducing atmosphere, so the electrolysis reaction proceeds slowly and a sudden change to a reducing atmosphere can be suppressed. Therefore, even when the electrochemical cell is operated reversibly at high output, a sudden change to an oxidizing atmosphere or a reducing atmosphere can be suppressed immediately after switching.

[0024] As described above, according to the present invention, the performance of the electrochemical single cell can be more stably maintained for a long period of time. Note that the "ceria containing a rare earth element" is typically obtained by adding an oxide of a rare earth element to ceria.

[0025] In one aspect of the invention, The ceria containing rare earth elements is gadolinia-doped ceria. A be.

[0026] According to this configuration, the crystal lattice of ceria contained in the intermediate layer can appropriately release or store oxide ions, that is, the intermediate layer can be appropriately endowed with oxygen releasing / storing ability.

[0027] In one aspect of the invention, The solid electrolyte layer (12) is Yttria Stabilized Zirconia The ceramic particles are The fuel electrode (16) and the intermediate layer (20) each include Gadolinia-doped ceria The ceramic particles comprise:

[0028] In this electrochemical single cell, the metal element (first metal element) constituting the ceramic particles contained in the solid electrolyte layer is different from the metal element (second metal element) constituting the ceramic particles contained in the intermediate layer. Therefore, a layer containing a solid solution containing the first metal element and the second metal element (so-called element diffusion layer) is formed at the interface between the solid electrolyte layer and the intermediate layer and in the region near the interface. On the other hand, the metal element (second metal element) constituting the ceramic particles contained in the fuel electrode is the same as the metal element constituting the ceramic particles contained in the intermediate layer. Therefore, the element diffusion layer is not formed at the interface between the intermediate layer and the fuel electrode. In the conventional configuration (for example, a configuration in which the single cell does not have an intermediate layer), there was a problem that the activity of the reaction field significantly decreased due to the reaction field overlapping with the element diffusion layer. However, according to the configuration of the present invention, the reaction field can be kept away from the element diffusion layer. Therefore, the decrease in activity of the reaction field can be suppressed, and the performance of the single cell can be maintained more stably for a long period of time.

[0029] In one aspect of the invention, The porosity of the intermediate layer (20) is greater than the porosity of the fuel electrode (16).

[0030] With this configuration, compared to a configuration in which the porosity of the intermediate layer is equal to or lower than that of the fuel electrode, the gas generated in the reaction field can be more easily diffused into the intermediate layer, and the gas discharge performance can be further improved, so that the reaction field can be more appropriately prevented from becoming a highly reducing atmosphere or a highly oxidizing atmosphere, and therefore the performance of the unit cell can be maintained more stably over a long period of time.

[0031] The second aspect of the present invention relates to a solid oxide electrochemical cell, A solid electrolyte layer (12); an air electrode (14) disposed on a surface side of the solid electrolyte layer; a fuel electrode (16) laminated on the back surface of the solid electrolyte layer; an intermediate layer (20) laminated between the solid electrolyte layer and the fuel electrode; Equipped with. the anode includes metal particles and ceramic particles, The intermediate layer is porous and contains ceramic particles but does not contain metal particles.

[0032] According to this configuration, it is possible to suppress deterioration in durability due to the occurrence of warping of the unit cell and deterioration in output, and it is possible to maintain stable performance of the unit cell for a long period of time.

[0033] The cell stack according to the present invention comprises: The electrochemical cell is formed by stacking a plurality of electrochemical unit cells according to the first invention or a plurality of electrochemical unit cells (10) according to the second invention.

[0034] The electrochemical unit cells used in this cell stack are less likely to warp, so that a plurality of unit cells can be appropriately stacked.

[0035] The hot module according to the present invention comprises: A cell stack (1); a heat exchanger (33) for exchanging heat with the gas supplied to the cell stack; a heater (34) for heating the cell stack; a thermal insulation material (35) in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with.

[0036] The gas production apparatus according to the present invention comprises: The apparatus includes a hot module (31) for use in an operation mode in which each of the plurality of electrochemical single cells (10) is used as an electrolysis single cell.

[0037] In the above description, in order to facilitate understanding of the invention, the symbols used in the embodiments are added in parentheses to the constituent elements of the invention corresponding to the embodiments, but each constituent element of the invention is not limited to the embodiment defined by the said symbols. [Brief description of the drawings]

[0038] [Figure 1] FIG. 1 is a perspective view of a cell stack of a solid oxide electrochemical cell according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] 2 is a cross-sectional view in the thickness direction of a single cell included in the electrochemical unit of FIG. 1. [Figure 4] FIG. 4 is a diagram showing an SEM image of area R1 of the single cell in FIG. 3. [Diagram 5] FIG. 4 is a partially enlarged view of a range R1 of the unit cell in FIG. [Figure 6] FIG. 1 is a block diagram of a gas production apparatus equipped with a hot module. [Figure 7] FIG. 1 is a cross-sectional view in the thickness direction of a conventional single cell. [Figure 8] FIG. 8 is a partial enlarged view of a range R2 of the unit cell in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Hereinafter, a cell stack, an electrochemical single cell, a gas production device, and a hot module according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a cell stack 1, and FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1. As shown in FIGS. 1 and 2, the cell stack 1 includes an electrochemical unit group formed by stacking a plurality of rectangular flat electrochemical units U in the thickness direction (vertical direction), and a pair of end plates 2, 3 arranged on the upper and lower surfaces of the electrochemical unit group. In this embodiment, the upper side corresponds to an example of the "front side", and the lower side corresponds to an example of the "rear side". The electrochemical unit U is the smallest unit of the electrochemical device. The electrochemical device includes an electrolyzer and a power generator. The end plates 2, 3 are rectangular flat members having the same outer shape as the electrochemical unit U, and have a rectangular opening in the center. The electrochemical unit group and the end plates 2, 3 are fastened to each other at their four corners by bolts Bo inserted through them in the thickness direction and nuts (not shown). The end plates 2 and 3 are made of metal (e.g., stainless steel). When each electrochemical unit U functions as an electrolyzer, the end plates 2 and 3 act as an anode and a cathode, respectively, and when each electrochemical unit U functions as a fuel cell, the end plates 2 and 3 act as a positive electrode and a negative electrode, respectively. For convenience of explanation, the ratio of each component in the drawings may differ from the actual ratio.

[0040] The electrochemical unit U will be described in detail with reference to Fig. 2. As shown in Fig. 2, the electrochemical unit U includes a single cell 10, an interconnector 4, a separator 5, an air electrode frame 6, an anode frame 7, and a current collector 8.

[0041] The unit cell 10 is the minimum unit of a solid oxide electrochemical cell (i.e., an electrochemical unit cell) and includes a solid electrolyte layer 12, an air electrode 14 laminated on the upper surface side of the solid electrolyte layer 12, an anode 16 laminated on the lower surface side of the solid electrolyte layer 12, and an intermediate layer 20 laminated between the solid electrolyte layer 12 and the anode 16. The air electrode 14 has an outer shape smaller than the solid electrolyte layer 12, the anode 16, and the intermediate layer 20, and is disposed in the center of the upper surface of the solid electrolyte layer 12 in a plan view of the unit cell 10. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 12 is exposed to the outside.

[0042] The interconnector 4 is a rectangular metal (e.g., stainless steel) member, and has a rectangular current collecting portion 4a protruding downward from the center of its lower surface. A pair of interconnectors 4 are arranged on both sides of the single cell 10 in the thickness direction. Two adjacent electrochemical units U share one interconnector 4. In other words, the interconnector 4 also functions as a separator that separates the two adjacent electrochemical units U. The lower surface of the current collecting portion 4a is in contact with the upper surface of the air electrode 14 of the single cell 10. The electrochemical unit U at the lower end has a pair of interconnectors 4,9 instead of the pair of interconnectors 4,4. The interconnector 9 is arranged at the lowermost end of the cell stack 1, and differs from the interconnector 4 in that it does not have a current collecting portion 4a.

[0043] The separator 5 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in the center. The periphery of the opening of the separator 5 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 12 of the unit cell 10 with a brazing material (e.g., Ag brazing) (not shown).

[0044] The air electrode frame 6 is a rectangular plate-shaped insulating member and may be formed of, for example, a mica sheet. A rectangular opening is formed in the center of the air electrode frame 6. The air electrode frame 6 is disposed so as to be interposed between the interconnector 4 (strictly speaking, the upper interconnector in the electrochemical unit U) and the separator 5.

[0045] The fuel electrode frame 7 is a rectangular plate-shaped metal (e.g., stainless steel) member having a rectangular opening at its center. The fuel electrode frame 7 is disposed so as to be interposed between the separator 5 and the interconnector 4 (strictly speaking, the lower interconnector in the electrochemical unit U).

[0046] The internal space of the electrochemical unit U is divided into an air chamber Sa and a fuel chamber Sf. The air chamber Sa is a space on the air electrode 14 side, and is composed of an upper interconnector 4, a separator 5, an air electrode frame 6, and a single cell 10. The fuel chamber Sf is a space on the fuel electrode 16 side, and is composed of a separator 5, a lower interconnector 4, an fuel electrode frame 7, and a single cell 10.

[0047] The current collector 8 is a rectangular porous member made of metal (for example, nickel) that is smaller than the fuel electrode 16 in a plan view. The current collector 8 is arranged in the fuel chamber Sf so as to contact the lower surface of the fuel electrode 16 and the upper surface of the lower interconnector 4. Two adjacent unit cells 10 are stacked in the thickness direction so as to share the interconnector 4 via the current collector 8, whereby the unit cells 10 are electrically connected in series.

[0048] 1 and 2, four paths Pfi, Pfo, Pai, and Pao are formed as gas flow paths on the outer periphery of the cell stack 1. All of these paths Pfi, Pfo, Pai, and Pao are formed to penetrate the members of the cell stack 1 except for the "end plate 2" and the "upper interconnector 4 of the upper electrochemical unit U" in the thickness direction.

[0049] The path Pfi is formed along a side E1, which is one of the four sides constituting the outer periphery of the cell stack 1, in the vicinity of one corner of the side E1. The path Pfo is formed along a side E2 opposite to the side E1, in the vicinity of the other corner of the side E2 (the corner located diagonally opposite to the one corner of the side E1). As shown in FIG. 2, the path Pfi communicates with the fuel chamber Sf via a horizontal hole 7a formed in the fuel electrode frame 7 of each electrochemical unit U. The path Pfo communicates with the fuel chamber Sf via a horizontal hole 7b formed in the fuel electrode frame 7 of each electrochemical unit U.

[0050] The path Pai is formed along the side E2 in the vicinity of one corner of the side E2. The path Pao is formed along the side E1 in the vicinity of the other corner of the side E1. The paths Pai and Pao are each connected to the air chamber Sa via a horizontal hole (not shown) formed in the air electrode frame 6 of each electrochemical unit U.

[0051] Next, the configuration of the unit cell 10 will be described in more detail with reference to Fig. 3 to Fig. 5. Fig. 3 is a cross-sectional view in the thickness direction of the unit cell 10. Fig. 4 is a diagram showing an SEM image of a region R1 of the unit cell in Fig. 3. Fig. 5 is a partial enlarged view of the region R1 of the unit cell 10 in Fig. 3.

[0052] As shown in FIG. 3, the solid electrolyte layer 12 is a rectangular flat layer and is configured to contain YSZ (yttria-stabilized zirconia) particles. Here, YSZ is ZrO2 (zirconia) to which Y2O3 (yttria) is added as a stabilizer. ZrO2 is an oxide of Zr (zirconium). That is, the solid electrolyte layer 12 contains ceramic particles made of an oxide of Zr. The solid electrolyte layer 12 has high oxide ion conductivity. The solid electrolyte layer 12 can be formed to have, for example, a size of 150 mm square and a thickness of 3 μm to 30 μm. Zr corresponds to an example of a "first metal element."

[0053] The air electrode 14 is a rectangular flat layer and is configured to contain perovskite oxide particles such as LSCF (lanthanum strontium cobalt iron oxide). The air electrode 14 has a functional layer and a current collecting layer. The current collecting layer is thicker than the functional layer and is disposed on the upper surface of the functional layer. The air electrode 14 has high electronic conductivity and effectively collects electrons from the current collecting layer. The air electrode 14 can be formed to have a thickness of, for example, 5 to 200 μm.

[0054] The anode 16 is a rectangular flat layer and can be formed to have a thickness of, for example, 200 μm to 1000 μm. The thickness of the anode 16 is greater than the thicknesses of the solid electrolyte layer 12, the air electrode 14, and the intermediate layer 20. In other words, the unit cell 10 is an anode-supported cell in which the anode 16 supports the solid electrolyte layer 12, the air electrode 14, and the intermediate layer 20. The anode 16 has a support layer 18a and a functional layer 18b. The support layer 18a is significantly thicker than the functional layer 18b. The functional layer 18b is laminated between the upper surface of the support layer 18a and the lower surface of the intermediate layer 20.

[0055] The support layer 18a is a cermet of Ni and YSZ. In other words, the support layer 18a is configured to include Ni particles and YSZ particles. The support layer 18a is configured to be porous including a plurality of micropores. The support layer 18a has high electronic conductivity.

[0056] The functional layer 18b is a cermet of Ni and GDC (gadolinia-doped ceria). In other words, the functional layer 18b is configured to include Ni particles and GDC particles. Here, GDC is CeO2 (ceria) doped with Gd2O3 (gadolinia). CeO2 is an oxide of Ce (cerium). That is, the functional layer 18b includes ceramic particles made of an oxide of Ce. The functional layer 18b is configured to be porous including a plurality of micropores h1 (described later). The diameter of the micropores h1 is smaller than the diameter of the micropores of the support layer 18a. The functional layer 18b has high electronic conductivity. Ce corresponds to an example of a "second metal element", and the Ni particles correspond to an example of a "metal particle".

[0057] The intermediate layer 20 is a rectangular flat layer and is configured to include GDC particles. Therefore, the intermediate layer 20 includes ceramic particles made of an oxide of Ce, similar to the functional layer 18b. On the other hand, the intermediate layer 20 does not include Ni particles. That is, the intermediate layer 20 does not include cermet. Whether a layer includes cermet or not can be determined by identifying the type of element that constitutes the layer using EDS or EDX (energy dispersive X-ray spectroscopy). The intermediate layer 20 is configured to be porous including a plurality of micropores h2 (described later). The intermediate layer 20 has high oxide ion conductivity. The intermediate layer 20 can be formed to have a thickness of, for example, 4.21 μm to 6.53 μm.

[0058] The micropores h1 of the functional layer 18b and the micropores h2 of the intermediate layer 20 will be specifically described with reference to FIG. 4. As shown in FIG. 4, the functional layer 18b can be formed so that the diameter of the micropores h1 is 0.317 μm to 0.353 μm and the porosity is 20.95% to 21.32%. The intermediate layer 20 can be formed so that the diameter of the micropores h2 is 0.508 μm to 0.539 μm and the porosity is 24.10% to 25.81%. That is, the diameter of the micropores h2 is larger than the diameter of the micropores h1, and the porosity of the intermediate layer 20 is larger than the porosity of the functional layer 18b. According to this configuration, the gas generated by the electrochemical reaction in the functional layer 18b can be temporarily stored in the many micropores h2 of the intermediate layer 20. This can suppress the occurrence of a highly reducing atmosphere or a highly oxidizing atmosphere. In addition, the possibility that the gas necessary for the electrochemical reaction will be depleted immediately after switching of the operation mode described later can be significantly reduced (described later). The diameter and porosity of these micropores h1 and h2 can be controlled to be within a desired numerical range by adjusting the size and amount of the pore-forming agent (typically, organic beads) used in forming the functional layer 18b and the intermediate layer 20.

[0059] The internal structure of the unit cell 10 will be described in more detail with reference to Fig. 5. As is clear from the above description, in the unit cell 10, the metal element (Zr) constituting the ceramic particles (ZrO2) contained in the solid electrolyte layer 12 is different from the metal element (Ce) constituting the ceramic particles (CeO2) contained in the intermediate layer 20. Therefore, an element diffusion layer Ld, which is a layer including a solid solution containing these metal elements (Zr and Ce), is formed at the interface between the solid electrolyte layer 12 and the intermediate layer 20 and in the region nearby the interface.

[0060] On the other hand, the element diffusion layer Ld is not formed at the interface between the intermediate layer 20 and the functional layer 18b. This is because the metal elements constituting the ceramic particles contained in the intermediate layer 20 and the functional layer 18b are the same. In addition, in the single cell 10, the fuel electrode 16 (specifically, the functional layer 18b) is made of cermet, whereas the intermediate layer 20 does not contain cermet. That is, no electrochemical reaction takes place in the intermediate layer 20. For this reason, the reaction field rf, which is a region where the electrolysis reaction takes place particularly actively, is formed on the surface of the functional layer 18b on the intermediate layer 20 side and in a region in the vicinity thereof. Therefore, in the single cell 10, the reaction field rf is located at a position separated from the element diffusion layer Ld. With this configuration, it is possible to suppress a decrease in the activity of the reaction field rf (described later).

[0061] In addition, in this embodiment, the intermediate layer 20 is configured to contain GDC particles. Generally, CeO2 has a characteristic that in a reducing atmosphere, the crystal lattice releases oxide ions and changes its valence from tetravalent to trivalent, and in an oxidizing atmosphere, the crystal lattice absorbs (takes in) surrounding oxide ions and changes its valence from trivalent to tetravalent (hereinafter, the former is referred to as "oxygen release ability" and the latter is referred to as "oxygen storage ability"). Therefore, in a reducing atmosphere, oxide ions are released from the CeO2 of the intermediate layer 20, and in an oxidizing atmosphere, surrounding oxide ions are absorbed into the CeO2 of the intermediate layer 20. With this configuration, it is possible to suppress a highly reducing atmosphere or a highly oxidizing atmosphere (described later). The oxygen release / storage ability of CeO2 is not affected by the type of additive. Therefore, instead of GDC, the ceramic material of the intermediate layer 20 may be CeO2 (i.e., ceramics with no additives added), YDC (yttria-doped ceria), SDC (samaria-doped ceria), LDC (lanthania-doped ceria), or the like.

[0062] The cell stack 1 is configured to be operable in two operation modes, namely, "SOEC mode" and "SOFC mode". The SOEC mode is a mode in which the cell stack 1 is operated as an SOEC (in other words, a mode in which each unit cell 10 is operated as an electrolytic unit cell). Here, the electrolytic unit cell is the smallest unit of an SOEC. The electrolytic unit cell produces (generates) gas by electrolyzing a fuel gas (gas to be electrolyzed) supplied to the fuel chamber Sf. On the other hand, the SOFC mode is a mode in which the cell stack 1 is operated as an SOFC (in other words, a mode in which each unit cell 10 is operated as a fuel cell). Here, the fuel cell unit cell is the smallest unit of an SOFC. The fuel cell unit cell generates electricity by combining gases to be combined (gases to be combined) supplied to the fuel chamber Sf and the air chamber Sa, respectively.

[0063] In this embodiment, water vapor is used as the fuel gas in the SOEC mode, and hydrogen and oxygen are used as the gases to be combined in the SOFC mode. In this case, in the SOEC mode, hydrogen is produced by electrolyzing water vapor into hydrogen and oxygen, and in the SOFC mode, hydrogen and oxygen are combined to generate electricity and water vapor. That is, electrochemical reactions proceed in reverse in the SOEC mode and the SOFC mode. In this embodiment, the cell stack 1 is configured to be able to switch between these two types of operation modes at a desired timing (i.e., to be able to operate reversibly).

[0064] The operation of the cell stack 1 when it is operated in the SOEC mode will be described. First, a voltage is applied to the end plates 2 and 3 of the cell stack 1 by an external power source (not shown). Next, when a high-temperature mixed gas of water vapor and hydrogen is supplied from the path Pfi, the mixed gas flows into the fuel chamber Sf of each electrochemical unit U through the horizontal hole 7a. Here, the water vapor is a fuel gas, and the hydrogen is a reducing gas for reducing the oxidation of the catalyst contained in the fuel electrode 16. In addition, when high-temperature air is supplied from the path Pai, the air flows into the air chamber Sa of each electrochemical unit U through a horizontal hole (not shown). The high-temperature air is supplied in order to control the temperature of the cell stack 1.

[0065] The water vapor that has flowed into the fuel chamber Sf passes through the support layer 18a of the fuel electrode 16 and moves to the functional layer 18b. In the functional layer 18b, the water vapor reacts with electrons (electrons supplied from the end plate 3 via the current collector 8) and is electrolyzed into hydrogen and oxide ions. This reaction is particularly active in the reaction field rf of the functional layer 18b. Hydrogen (specifically, hydrogen generated by electrolysis and hydrogen as a reducing gas) diffuses in the fuel chamber Sf, is discharged from the path Pfo through the horizontal hole 7b, and is collected by a well-known method. At this time, unreacted water vapor is discharged from the path Pfo together with hydrogen. Meanwhile, the oxide ions move to the air electrode 14 in the air chamber Sa through the intermediate layer 20 and the solid electrolyte layer 12, release electrons in the functional layer of the air electrode 14, and become oxygen. The oxygen diffuses in the air chamber Sa, is discharged from the path Pao through the horizontal hole (not shown) together with the air that has flowed into the air chamber Sa, and is collected by a well-known method. The electrons emitted from the functional layer are collected by the current collecting portion 4a of the interconnector 4 via the current collecting layer, and circulate from the end plate 2 to the end plate 3 via the external power supply. As a result, a current corresponding to the applied voltage flows through the cell stack 1. Note that carbon monoxide may be used as the reducing gas instead of hydrogen.

[0066] Next, the operation of the cell stack 1 when it is operated in the SOFC mode will be described. First, the end plates 2 and 3 are connected to an external circuit. Next, when high-temperature hydrogen is supplied from the path Pfi, the hydrogen flows into the fuel chamber Sf through the horizontal hole 7a. Also, when high-temperature air is supplied from the path Pai, the air flows into the air chamber Sa through a horizontal hole (not shown). The oxygen in the air that flows into the air chamber Sa receives electrons (electrons supplied from the end plate 2 through the current collector 4a) at the air electrode 14 and becomes oxide ions. "Gases other than oxygen in the air" and "unreacted oxygen" diffuse in the air chamber Sa and are discharged from the path Pao through a horizontal hole (not shown) and collected. The oxide ions move to the fuel electrode 16 in the fuel chamber Sf through the solid electrolyte layer 12 and the intermediate layer 20, release electrons in the functional layer 18b, and combine with hydrogen to become water vapor. This reaction is particularly active in the reaction field rf of the functional layer 18b. The water vapor diffuses in the fuel chamber Sf and is discharged from the path Pfo via the horizontal hole 7b together with the unreacted hydrogen and is collected. The emitted electrons are collected by the current collector 8 via the support layer 18a and circulate from the end plate 3 to the end plate 2 via the external circuit. Electricity is taken from the external circuit by a predetermined method.

[0067] 6 is a block diagram of the gas production apparatus 30. In the SOEC mode, hydrogen is produced by operating the gas production apparatus 30. A specific description will be given below.

[0068] The gas production apparatus 30 includes a hot module 31 and a condenser 36. The cell stack 1 is a component of the hot module 31. That is, the hot module 31 includes the cell stack 1, a vaporizer 32, a heat exchanger 33, a heater 34, and a thermal insulator 35.

[0069] The vaporizer 32 generates water vapor to be supplied to the cell stack 1. Specifically, the vaporizer 32 has a heat exchanger (not shown), and uses this heat exchanger to exchange heat with high-temperature exhaust gas containing oxygen generated by the cell stack 1, and generates water vapor by heating water with the heat obtained by the heat exchange. The heat exchanger 33 exchanges heat between the supply gas to be supplied to the cell stack 1 and the generated gas generated by the cell stack 1. Specifically, the heat exchanger 33 exchanges heat between "water vapor generated by the vaporizer 32 (strictly speaking, water vapor containing hydrogen as a reducing gas)" and "hydrogen and oxygen generated by the cell stack 1". The heat exchanger 33 also exchanges heat between "air" and "hydrogen and oxygen generated by the cell stack 1". The heater 34 heats the water vapor and air that have passed through the heat exchanger 33 to the operating temperature of the cell stack 1. The water vapor that has passed through the heater 34 is supplied from a path Pfi of the cell stack 1 to the fuel chamber Sf, and the air that has passed through the heater 34 is supplied from a path Pai of the cell stack 1 to the air chamber Sa.

[0070] Heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), and / or a heat-resistant container formed from these heat-resistant fibers may be used for the heat insulating material 35. The cell stack 1, the vaporizer 32, the heat exchanger 33, and the heater 34 are disposed inside the heat insulating material 35. The heat-resistant fibers are disposed so as to fill the gaps between the cell stack 1, the vaporizer 32, the heat exchanger 33, and the heater 34. This suppresses heat radiation from the cell stacks 1 and 32 to 34.

[0071] The condenser 36 is a device for cooling hydrogen containing unreacted water vapor. The water liquefied by the condenser 36 is supplied again to the vaporizer 32 as raw water.

[0072] The inventors of the present application carried out three measurement tests to examine the extent to which the performance of the unit cell 10 is improved compared to the conventional unit cell 50 shown in Figures 7 and 8. Below, the unit cell 50 will be described, and then the measurement results will be described.

[0073] FIG. 7 is a cross-sectional view of the unit cell 50 in the thickness direction. FIG. 8 is a partial enlarged view of the range R2 of the unit cell 50 in FIG. 7. As shown in FIG. 7, the unit cell 50 is different from the unit cell 10 in that it does not include the intermediate layer 20, and otherwise has the same structure as the unit cell 10. As shown in FIG. 8, in the unit cell 50, the metal element (Zr) constituting the ceramic particles contained in the solid electrolyte layer 12 is different from the metal element (Ce) constituting the ceramic particles contained in the functional layer 18b. Therefore, an element diffusion layer Ld, which is a layer containing a solid solution containing these metal elements (Zr and Ce), is formed at the interface between the solid electrolyte layer 12 and the functional layer 18b and in the area nearby the interface. Meanwhile, the reaction field rf is formed on the surface of the functional layer 18b on the solid electrolyte layer 12 side and in the area nearby the interface. As a result, the reaction field rf partially overlaps with the element diffusion layer Ld.

[0074] In the first measurement test, the initial characteristics of the unit cells 10 and 50 were compared. The initial characteristics are characteristics when the unit cells 10 and 50 are operated in SOEC mode after the initial reduction treatment. Here, the initial reduction treatment is a treatment carried out at the final stage of the manufacturing process of the unit cells 10 and 50, and specifically, a treatment to reduce NiO2 (nickel oxide) contained in the green sheet of the fuel electrode 16 (i.e., the support layer 18a and the functional layer 18b) to Ni. The initial characteristics are evaluated based on the voltage value when the same current is passed while supplying the same concentration of water vapor to the unit cells 10 and 50. Note that, strictly speaking, the "current" here means the current per unit area of ​​the unit cell. The same applies to other measurement tests.

[0075] When the voltage values ​​were measured while changing the current flowing through the single cells 10 and 50 in the range of 0.0 A to 1.5 A, the result was that the voltage value of the single cell 10 was lower than that of the single cell 50 in the entire current range. It was also confirmed that the difference in the voltage values ​​between the two became larger as the current value became higher. This measurement result is considered to be due to the positional relationship between the reaction field rf and the element diffusion layer Ld of the single cells 10 and 50. That is, in the single cell 50, the reaction field rf partially overlaps with the element diffusion layer Ld (see FIG. 8), whereas in the single cell 10, the reaction field rf is located at a position separated from the element diffusion layer Ld (see FIG. 5). Therefore, in the single cell 50, the electrochemical reaction in the reaction field rf is inhibited by the element diffusion layer Ld, and the activity of the reaction field rf is significantly reduced, whereas in the single cell 10, the reduction in the activity of the reaction field rf is suppressed. It is considered that the above result was obtained as a result.

[0076] In the second measurement test, the durability of the unit cells 10 and 50 was compared when they were continuously operated in SOEC mode. Specifically, the voltage required to obtain a specified current value was measured before and after the unit cells 10 and 50 were operated in SOEC mode for 365 hours, and the ratio was calculated as the deterioration rate. The deterioration rate is an index for evaluating durability and is calculated by dividing the "voltage after operation" by the "voltage before operation." The lower the deterioration rate, the higher the durability. Note that for the unit cell 10, a sample with a porosity of the intermediate layer 20 of 35% was used. The water vapor concentration was set to 80%.

[0077] As a result, for unit cell 10, the voltage before operation was 1.293 V and the voltage after operation was 1.387 V. For unit cell 50, the voltage before operation was 1.355 V and the voltage after operation was 1.508 V. Therefore, the deterioration rate of unit cell 10 was 1.073 and the deterioration rate of unit cell 50 was 1.113, confirming that unit cell 10 has higher durability than unit cell 50.

[0078] This measurement result is believed to be due to the fact that the single cell 10 has a porous intermediate layer 20 containing CeO2 particles, while the single cell 50 does not have the intermediate layer 20. That is, in the single cell 10, hydrogen generated in the reaction field rf by operating in the SOEC mode is diffused into a large number of micropores h2 in the intermediate layer 20 and temporarily stored. In addition, when the atmosphere changes to a reducing atmosphere due to the generation of hydrogen, oxide ions are released from the CeO2 in the intermediate layer 20, and the reduction tendency is alleviated. For this reason, it is possible to suppress a sudden increase in hydrogen concentration, and as a result, a highly reducing atmosphere is suppressed. Therefore, it is believed that the electrodes are less likely to deteriorate and gas flows appropriately (described later), and as a result, the deterioration of the durability of the single cell 10 is suppressed.

[0079] On the other hand, since the single cell 50 does not have the intermediate layer 20, it continues to operate under a highly reducing atmosphere. As a result, the electrodes deteriorate and the flow of gas stagnates (described later), which is considered to result in a decrease in the durability of the single cell 50. Here, "deterioration of the electrodes" in the SOEC mode specifically means deterioration of the functional layer 18b. That is, CeO2 has a property that, in a reducing atmosphere, it releases oxide ions, weakening the cohesive force of the crystal lattice and expanding the volume. For this reason, in a highly reducing atmosphere, the volume of the functional layer 18b expands significantly, which results in the distortion of the crystal structure and deterioration of the functional layer 18b. In addition, "stagnation of the flow of gas" in the SOEC mode specifically means "the hydrogen concentration in the reaction field rf increases due to operation at high power, and hydrogen is not properly discharged and stagnates," and "water vapor supplied from the outside does not properly flow into the reaction field rf due to the increase in hydrogen concentration."

[0080] In the third measurement test, the redox resistance (durability when oxidizing and reducing atmospheres are alternately repeated) of the single cells 10 and 50 was compared. Specifically, 200 cycles were performed in which the SOEC mode and the SOFC mode were alternately switched at 10-second intervals, with each cycle consisting of one set of operating both modes once. The current value was measured against the applied voltage or output voltage, and the rate of decrease in the current value was calculated as the degradation rate. The lower the degradation rate, the higher the redox resistance. The applied voltage was 1V in the SOEC mode, and the output voltage was -0.1V in the SOFC mode. The operating temperature was 700 degrees, and in both the SOEC mode and the SOFC mode, 100cc of steam per minute and 20cc of hydrogen per minute were supplied to the fuel chamber Sf, and 100cc of air per minute was supplied to the air chamber Sa.

[0081] As a result, for single cell 50, the current value in the first cycle in SOEC mode was 0.393 A / cm 2 The current value at the 200th cycle was 0.234 A / cm 2 Therefore, the degradation rate was 40.5%. On the other hand, the current value in the first cycle in SOFC mode was 1.163 A / cm 2 The current value at the 200th cycle was 0.904 A / cm 2 As a result, the degradation rate was 22.3%. On the other hand, the degradation rate of the unit cell 10 was 0% in both operation modes. This confirmed that the unit cell 10 has significantly higher redox resistance than the unit cell 50.

[0082] This measurement result is believed to be due to the fact that the unit cell 10 has a porous intermediate layer 20 containing CeO2 particles, whereas the unit cell 50 does not have the intermediate layer 20. Here, the considerations in the case of operation in the SOEC mode are as described in the explanation of the second measurement test. Therefore, the following will explain the considerations in the case of operation in the SOFC mode and the considerations in the case of reversible high-frequency operation.

[0083] In the single cell 10, water vapor generated in the reaction field rf by operating in the SOFC mode is diffused into many micropores h2 in the intermediate layer 20 and temporarily stored. In addition, when the atmosphere changes to an oxidizing atmosphere due to the generation of water vapor, oxide ions are absorbed in the CeO2 of the intermediate layer 20, and the tendency to oxidize is alleviated. This makes it possible to suppress a sudden increase in water vapor concentration, and as a result, a highly oxidizing atmosphere is suppressed. Therefore, the electrodes are less likely to deteriorate and gas flows appropriately (described later), which is thought to result in suppressing a decrease in durability of the single cell 10.

[0084] On the other hand, since the single cell 50 does not have the intermediate layer 20, it continues to operate in a highly oxidizing atmosphere. As a result, the electrodes deteriorate and the flow of gas stagnates (described later), which is considered to result in a decrease in the durability of the single cell 50. Here, "deterioration of the electrodes" in the SOFC mode specifically means deterioration of the functional layer 18b. That is, Ni has a property that when oxidized, the movement speed increases and multiple Ni particles aggregate. Therefore, in a highly oxidizing atmosphere, multiple Ni particles aggregate in the functional layer 18b, which significantly reduces the number of three-phase interfaces. As a result, the reaction efficiency is significantly reduced and the functional layer 18b is deteriorated. In addition, "stagnation of the flow of gas" in the SOFC mode specifically means "operation at high power increases the water vapor concentration in the reaction field rf, and water vapor is not properly discharged and stagnates," and "hydrogen supplied from the outside does not properly flow into the reaction field rf due to the increase in water vapor concentration."

[0085] Furthermore, in the single cell 10, even if the operation mode is switched frequently, the reaction field rf is prevented from becoming a highly reducing atmosphere or a highly oxidizing atmosphere, so that the electrodes are less likely to deteriorate. In addition, when operating in the SOEC mode, hydrogen is temporarily stored in the micropores h2 of the intermediate layer 20. On the other hand, when operating in the SOFC mode, water vapor is temporarily stored in the micropores h2, and oxide ions are absorbed in the CeO2 of the intermediate layer 20. Therefore, immediately after the operation mode is switched to the SOFC mode, the hydrogen stored in the intermediate layer 20 during operation in the SOEC mode is gradually discharged to the reaction field rf, so that the possibility that hydrogen supplied from the outside will be depleted before it reaches the reaction field rf is greatly reduced. On the other hand, immediately after switching to the SOEC mode, the water vapor stored in the intermediate layer 20 during operation in the SOFC mode is gradually discharged to the reaction field rf, so that the possibility that water vapor supplied from the outside will be depleted before it reaches the reaction field rf is greatly reduced. In addition, the oxide ions absorbed in the CeO2 of the intermediate layer 20 during operation in the SOFC mode are released, slowing down the pace of water vapor electrolysis, which also significantly reduces the possibility of water vapor depletion. As a result, it is believed that the deterioration of the durability of the unit cell 10 is suppressed.

[0086] In contrast, the unit cell 50 does not include the intermediate layer 20, and therefore continues to operate in an environment in which a highly reducing atmosphere and a highly oxidizing atmosphere are frequently switched between. As a result, the electrodes deteriorate and the gas is easily depleted immediately after the operation mode is switched, which is thought to result in a decrease in the durability of the unit cell 50.

[0087] In addition, the inventors of the present application also conducted a measurement test to examine the extent to which the warpage of the unit cell 10 could be improved. In this measurement test, a unit cell (not shown) having a different configuration from the unit cell 50 was used as a comparison object. Hereinafter, this unit cell will be referred to as "unit cell B." Unit cell B is common to unit cell 10 in that it has an intermediate layer containing GDC particles, but differs from unit cell 10 in that the intermediate layer does not have micropores (i.e., it is not porous). In the test, six samples of unit cell 10 and ten samples of unit cell B were prepared, and the amount of warpage of unit cell 10 and B was measured when each sample was fired.

[0088] As a result, the maximum amount of warpage of unit cell B was 2.161 mm, the minimum was 1.934 mm, and the average was 2.078 mm, whereas the maximum amount of warpage of unit cell 10 was 1.862 mm, the minimum was 1.801 mm, and the average was 1.836 mm. This confirmed that unit cell 10 had a significantly smaller amount of warpage than unit cell B, and that the variation in the amount of warpage was also smaller.

[0089] This measurement result is believed to be due to the fact that the intermediate layer 20 in the unit cell 10 is porous, whereas the intermediate layer in the unit cell B is not porous. That is, when the unit cell 10 is exposed to a large temperature change during firing, the intermediate layer 20 elastically deforms in accordance with the amount of expansion or contraction of the solid electrolyte layer 12 and the functional layer 18b, and it is believed that this significantly alleviates the generation of stress at the interfaces of the layers, thereby reducing the amount of warping of the unit cell 10. On the other hand, in the unit cell B, the intermediate layer does not elastically deform even when exposed to a large temperature change, and therefore stress is generated at the interfaces of the layers, and it is believed that a relatively large warping occurred in the unit cell B.

[0090] These measurement tests confirmed that the configuration of the unit cell 10 can suppress deterioration in durability due to the occurrence of warping and deterioration of output, and can maintain stable performance over a long period of time.

[0091] The electrochemical single cell, cell stack, hot module, and gas production apparatus according to the embodiments have been described above. However, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the purpose of the present invention.

[0092] For example, a ceramic material other than CeO2 (e.g., YSZ) may be used for the intermediate layer 20. In other words, the intermediate layer 20 does not need to have oxygen releasing / storing ability. When YSZ is used for the intermediate layer 20, it is preferable that the functional layer 18b is formed so as to be composed of a cermet of Ni and YSZ.

[0093] The porosity of the intermediate layer 20 may be equal to or smaller than the porosity of the functional layer 18b. As long as the pores h1 and h2 are large enough to allow water vapor and hydrogen to pass through, the diameter of the pores h2 of the intermediate layer 20 may be equal to or smaller than the diameter of the pores h1 of the functional layer 18b.

[0094] Furthermore, a reaction prevention layer may be disposed between the solid electrolyte layer 12 and the cathode 14. The reaction prevention layer may be configured to contain, for example, CeO2 or CeO2 containing a rare earth element. The reaction prevention layer serves to prevent elements contained in the cathode 14 from reacting with the solid electrolyte layer 12.

[0095] Furthermore, when the single cell 10 is operated in the SOEC mode, carbon dioxide may be used as the fuel gas, or a mixed gas of carbon dioxide and water vapor may be used. In the former case, carbon monoxide and oxygen are generated by electrolysis of carbon dioxide. In the latter case, carbon monoxide, hydrogen, and oxygen are generated by electrolysis of the mixed gas. In the former case, the vaporizer 32 is not required in the gas production device 30. This is because carbon dioxide undergoes heat exchange in the heat exchanger 33 in a gaseous state, and the carbon dioxide is supplied directly to the heat exchanger 33 without passing through the vaporizer 32.

[0096] Furthermore, the unit cell 10 is not limited to being an anode-supported type, but may be configured as, for example, an electrolyte-supported type or a metal-supported type, in which case the anode does not include a support layer and is configured only by a functional layer.

[0097] Furthermore, the present invention may include the following aspects. [1] A solid electrolyte layer; an air electrode disposed on a surface side of the solid electrolyte layer; a fuel electrode disposed on a back surface of the solid electrolyte layer; an intermediate layer disposed between the solid electrolyte layer and the fuel electrode; Equipped with The fuel electrode is made of a cermet, The intermediate layer is porous and does not contain a cermet. Solid oxide electrochemical single cell. [2] [1] An electrochemical single cell according to the present invention, The intermediate layer comprises ceria or ceria containing a rare earth element. Electrochemical single cell. [3] [2] The electrochemical single cell according to the present invention, The ceria containing a rare earth element is any one of gadolinia-doped ceria, yttria-doped ceria, samaria-doped ceria, and lanthania-doped ceria. Electrochemical single cell. [4] The electrochemical single cell according to any one of [1] to [3], the solid electrolyte layer includes ceramic particles made of an oxide of a first metal element, the anode and the intermediate layer each contain ceramic particles made of an oxide of a second metal element; Electrochemical single cell. [5] The electrochemical single cell according to any one of [1] to [4], The porosity of the intermediate layer is greater than the porosity of the anode. Electrochemical single cell. [6] A solid electrolyte layer; an air electrode disposed on a surface side of the solid electrolyte layer; a fuel electrode disposed on a back surface of the solid electrolyte layer; an intermediate layer disposed between the solid electrolyte layer and the fuel electrode; Equipped with the anode includes metal particles and ceramic particles, The intermediate layer is porous and contains ceramic particles but does not contain metal particles. Solid oxide electrochemical single cell. [7] A cell stack comprising a plurality of electrochemical single cells according to any one of [1] to [6] stacked together. [8] [7] A cell stack according to the present invention; a heat exchanger for exchanging heat with the gas supplied to the cell stack; a heater for heating the cell stack; a thermal insulation material in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with Hot module. [9] The hot module according to [8] is used in an operation mode in which each of the plurality of electrochemical single cells is used as an electrolysis single cell; Gas production equipment. [Explanation of symbols]

[0098] 1: cell stack, 2: end plate, 3: end plate, 4: interconnector, 5: separator, 6: air electrode frame, 7: fuel electrode frame, 8: current collector, 9: (lowest) interconnector, 10: solid oxide electrochemical unit cell, 12: solid electrolyte layer, 14: air electrode, 16: fuel electrode, 18a: support layer, 18b: functional layer, 20: intermediate layer, 30: gas production device, 31: hot module, 32: vaporizer, 33: heat exchanger, 34: heater, 35: insulation material, 36: condenser

Claims

1. A solid electrolyte layer; an air electrode disposed on a surface side of the solid electrolyte layer; a fuel electrode disposed on a back surface of the solid electrolyte layer; an intermediate layer disposed between the solid electrolyte layer and the fuel electrode; Equipped with The fuel electrode is made of a cermet, The intermediate layer is porous and does not contain a cermet.

1. A solid oxide electrochemical cell comprising: The fuel electrode has a functional layer and a support layer, The functional layer and the support layer are laminated on the intermediate layer in this order, the functional layer and the support layer; the solid electrolyte layer comprises yttria-stabilized zirconia; the intermediate layer comprises gadolinia-doped ceria; the functional layer comprises nickel and gadolinia doped ceria; an element diffusion layer containing zirconium and cerium is formed at the interface between the solid electrolyte layer and the intermediate layer and in a region in the vicinity of the interface; a reaction field formed on the surface of the functional layer on the side of the intermediate layer and in a region adjacent thereto is located away from the element diffusion layer; Electrochemical single cell.

2. 2. The electrochemical unit cell of claim 1, The porosity of the intermediate layer is greater than the porosity of the anode. Electrochemical single cell.

3. A cell stack comprising a plurality of electrochemical unit cells according to claim 1 or 2 stacked together.

4. The cell stack according to claim 3 ; a heat exchanger for exchanging heat with the gas supplied to the cell stack; a heater for heating the cell stack; a thermal insulation material in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with Hot module.

5. The hot module according to claim 4 is used in an operation mode in which each of the plurality of electrochemical single cells is used as an electrolysis single cell. Gas production equipment.

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