Electrochemical unit cell, cell stack, hot module, and gas production apparatus

Abstract

To provide an electrochemical unit cell capable of maintaining a stable performance for a long period of time.SOLUTION: An electrochemical unit cell 10 comprises a solid oxide type solid electrolyte layer 12, an air electrode 14 overlaid on the surface side of the solid electrolyte layer 12, a fuel electrode 16 overlaid on the back surface side of the solid electrolyte layer 12, and an intermediate layer 20 laminated between the solid electrolyte layer 12 and the fuel electrode 16. The fuel electrode 16 is formed of cermet, and the intermediate layer 20 is porous and does not contain cermet.SELECTED DRAWING: Figure 4

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] Solid oxide electrochemical cells, which use solid oxide as an electrolyte, have been known for some time. Solid oxide electrochemical cells are characterized by their ability to perform electrochemical reactions with high efficiency in high-temperature environments, and are broadly classified into solid oxide electrolysis cells (SOECs) and solid oxide fuel cells (SOFCs). 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. Hereinafter, the former may be simply referred to as "SOECs," and the latter may be simply referred to as "SOFCs." Furthermore, the "multiple different gases" will be referred to as "gases to be combined."

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

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-8914 Summary of the Invention

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

[0006] Because a single cell is exposed to significant temperature changes during manufacturing and at the start and end of operation, differences in the thermal expansion coefficients of the layers that make up the single cell can cause stress at the interfaces of the layers, potentially resulting in warping of the single cell. Patent Document 1 describes the occurrence of peeling between the solid electrolyte layer and the anode due to differences in the expansion coefficients of the two in a reducing atmosphere, but does not address any issues caused by differences in the thermal expansion coefficients in environments with large temperature changes. Therefore, with the technology described in Patent Document 1, the single cell is exposed to significant temperature changes during manufacturing and at the start and end of operation, which can cause warping of the single cell and reduce its durability.

[0007] Incidentally, the electrochemical reaction in an electrochemical cell occurs particularly actively on the surface of the fuel electrode of each unit cell on the solid electrolyte layer side and in a region nearby (hereinafter, this region will be referred to as the "reaction field"). When the electrochemical cell is operated as an SOEC, the first product gas (e.g., hydrogen) generated by the electrolysis of a 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 for producing 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 power to improve the production efficiency of the first product gas, the discharge rate of the first product gas may not keep up with the production rate, 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 an 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, which tends to create an oxidizing atmosphere in the reaction field. Depending on the mode of use of the electrochemical cell, there is a demand for generating large amounts of electricity in a short period of time. In this case, if the electrochemical cell is operated at high power output to improve power generation efficiency, the discharge rate of the second product gas may not keep up with the production rate, causing the concentration of the second product gas in the reaction field to exceed its reference concentration, which may create a highly oxidizing atmosphere in the reaction field.

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

[0010] Furthermore, electrochemical cells can be operated reversibly by switching between SOEC and SOFC. In this case, water vapor is used as the fuel gas, and hydrogen and oxygen are used as the compound gases. Frequent switching between SOEC and SOFC while operating an electrochemical cell at high power output can lead to electrode degradation. Furthermore, immediately after switching from SOEC to SOFC, the remaining hydrogen in the reaction field may be consumed in the compound reaction before the externally supplied hydrogen reaches the reaction field, resulting in hydrogen depletion. On the other hand, immediately after switching from SOFC to SOEC, the remaining water vapor in the reaction field may be consumed in the electrolysis reaction before the externally supplied water vapor reaches the reaction field, resulting in water vapor depletion. As such, depletion of the gases used in the reaction increases the resistance of the electrochemical cell, leading to unstable output. Therefore, when an electrochemical cell is operated reversibly at high power output, the problem of reduced durability of the single cell becomes more pronounced.

[0011] These problems are not particularly mentioned in Patent Document 1. Therefore, with the technology of Patent Document 1, the reaction field becomes a highly reducing atmosphere or a highly oxidizing atmosphere, which may cause the output of the single cell to deteriorate, and as a result, the durability of the single cell may decrease.

[0012] As described above, the single cell of Patent Document 1 may not be able to maintain stable performance over a long period of time because durability is reduced due to the occurrence of warping and deterioration of output.

[0013] The present invention has been made to address the above-mentioned problems, and one of the objects of the present invention is to provide a technology that can stably maintain the performance of an electrochemical single cell over a long period of time.

[0014] The solid oxide electrochemical unit cell according to the first aspect of the present invention comprises: a solid electrolyte layer (12); an air electrode (14) laminated on the 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 and disposed between the solid electrolyte layer and the fuel electrode; Equipped with. the anode is made of cermet, The intermediate layer is porous and does not contain cermet.

[0015] This electrochemical single cell includes an intermediate layer laminated between the solid electrolyte layer and the anode, and the intermediate layer is porous. Therefore, when the single cell is exposed to significant temperature changes during manufacturing and during start-up and shutdown of operation, causing the solid electrolyte layer and the anode to expand or contract at their respective thermal expansion coefficients, the intermediate layer elastically deforms in response to the changes. Specifically, the portion of the intermediate layer in contact with the solid electrolyte layer elastically deforms in response to the expansion or contraction of the solid electrolyte layer, while the portion of the intermediate layer in contact with the anode elastically deforms in response to the expansion or contraction of the anode. This significantly reduces stress at the interface between the layers, even when the ceramic particles contained in the solid electrolyte layer and the anode are made of different materials (i.e., even when the thermal expansion coefficients of the two are different). As a result, warping of the single cell can be suppressed, thereby preventing a decrease in the durability of the single cell.

[0016] Furthermore, in this single cell, the anode is made of cermet, while the intermediate layer does not contain cermet. In other words, no electrochemical reaction occurs in the intermediate layer. Therefore, the reaction field is formed on the surface of the anode facing the intermediate layer and in the region adjacent to it. When an electrochemical cell composed of this single cell is operated as an SOEC, a portion of the first product gas (gas produced by electrolysis of the fuel gas) generated in the reaction field diffuses into the intermediate layer and is temporarily stored in the numerous pores. The stored first product gas is gradually discharged from the anode. This configuration improves the dischargeability of the first product gas. Therefore, even when the electrochemical cell is operated at high power, 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 its 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 portion of the second product gas (gas produced by the combination of the target gases) generated in the reaction field diffuses into the intermediate layer and is temporarily stored in the numerous pores. The stored second product gas is gradually discharged from the fuel electrode. This configuration improves 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 an intermediate layer that has many pores and where electrochemical reactions do not occur in contact with the reaction field, the reaction field can be prevented from becoming a highly reducing or oxidizing atmosphere, thereby significantly reducing the possibility of deterioration of the electrodes of the single cell or stagnation of gas flow. Therefore, a decrease in reaction efficiency can be prevented, allowing stable power to be maintained, and as a result, a decrease in the durability of the single cell can be prevented.

[0017] Furthermore, as described above, the configuration of the present invention prevents the reaction field from becoming a highly reducing or oxidizing atmosphere. Therefore, even if the electrochemical cell is operated at high power and the switching between SOEC and SOFC is performed frequently, the electrode is less likely to deteriorate. In addition, immediately after switching from SOEC to SOFC, hydrogen stored in the (numerous pores in) the intermediate layer during operation as an SOEC is gradually released to the reaction field, significantly reducing the possibility of hydrogen depletion. On the other hand, immediately after switching from SOFC to SOEC, water vapor stored in the (numerous pores in) the intermediate layer during operation as an SOFC is gradually released to the reaction field, significantly reducing the possibility of water vapor depletion. Therefore, an increase in the resistance of the electrochemical cell due to gas depletion can be suppressed. As a result, stable output can be maintained even when the electrochemical cell is operated reversibly at high power, and a decrease in the durability of the single cell can be suppressed.

[0018] As mentioned 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 increases as the distance between the electrodes decreases and / or the insulation resistance between the electrodes increases. In the present invention, an intermediate layer is interposed between the reaction field and the solid electrolyte layer. This allows the electrodes to be spaced apart by the thickness of the intermediate layer. Furthermore, the intermediate layer has many pores, which reduces the insulation resistance between the electrodes. 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 of water vapor depletion 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 (CeO) or ceria containing a rare earth element. Generally, CeO has the property that in a reducing atmosphere, the crystal lattice releases oxide ions, changing its valence from tetravalent to trivalent, and in an oxidizing atmosphere, the crystal lattice absorbs (takes in) surrounding oxide ions, changing its valence from trivalent to tetravalent. Therefore, when an electrochemical cell composed of this single cell is operated as an SOEC at high power, and the concentration of the first product gas in the reaction field increases (i.e., the atmosphere changes to a reducing atmosphere), oxide ions are released from the CeO in the intermediate layer, mitigating the reduction tendency, and thus preventing a highly reducing atmosphere from occurring.

[0022] Here, because the single cell is characterized by the oxide-ion conductivity of the solid electrolyte layer, one of the gases to be combined is always oxygen. When the electrochemical cell is operated as an SOFC, oxygen receives electrons at the air electrode and becomes oxide ions. These oxide ions then migrate through the solid electrolyte layer and intermediate layer and flow into the reaction field at the anode. Therefore, when the electrochemical cell is operated as an SOFC at high power output, the concentration of the second product gas in the reaction field increases (i.e., the atmosphere changes to an oxidizing atmosphere). As a result, oxide ions (more specifically, oxide ions migrating from the air electrode to the reaction field) are absorbed by CeO2 in the intermediate layer, resulting in a decrease in the concentration of oxide ions in the reaction field. The decrease in the oxide ion concentration suppresses the increase in the concentration of the second product gas, mitigating the tendency for oxidation, thereby preventing the atmosphere from becoming highly oxidizing.

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

[0024] As described above, according to the present invention, the performance of the electrochemical single cell can be maintained more stably over 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 a rare earth element is any one of gadolinia-doped ceria, yttria-doped ceria, samaria-doped ceria, and lanthania-doped ceria.

[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 release / storage capabilities.

[0027] In one aspect of the invention, the solid electrolyte layer (12) contains ceramic particles made of an oxide of a first metal element, The fuel electrode (16) and the intermediate layer (20) each contain ceramic particles made of an oxide of a second metal element.

[0028] In this electrochemical single cell, the metal element (first metal element) constituting the ceramic particles in the solid electrolyte layer is different from the metal element (second metal element) constituting the ceramic particles in the intermediate layer. Therefore, a layer containing a solid solution containing the first metal element and the second metal element (a so-called element diffusion layer) is formed at the interface between the solid electrolyte layer and the intermediate layer and in the vicinity thereof. Meanwhile, the metal element (second metal element) constituting the ceramic particles in the anode is the same as the metal element constituting the ceramic particles in the intermediate layer. Therefore, no element diffusion layer is formed at the interface between the intermediate layer and the anode. In conventional configurations (e.g., single cells without an intermediate layer), there is a problem in that the reaction field overlaps with the element diffusion layer, significantly reducing the activity of the reaction field. However, the configuration of the present invention allows the reaction field to be separated from the element diffusion layer. This suppresses the reduction in activity of the reaction field, thereby enabling the performance of the single cell to be maintained more stably over 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 less than that of the anode, the gas generated in the reaction field can more easily diffuse into the intermediate layer, further improving the gas discharge performance, thereby more appropriately preventing the reaction field from becoming a highly reducing or highly oxidizing atmosphere, thereby enabling the performance of the unit cell to be maintained more stably over a long period of time.

[0031] The solid oxide electrochemical unit cell according to the second aspect of the present invention is a solid electrolyte layer (12); an air electrode (14) laminated on the 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 and 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.

[0032] This configuration can suppress deterioration in durability due to warping of the unit cell and deterioration in output, and can 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 (10) 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 multiple unit cells can be stacked appropriately.

[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 heat insulating material (35) in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with.

[0036] The gas production device according to the present invention comprises: The electrochemical unit cells (10) each include a hot module (31) used in an operation mode in which they are used as electrolysis unit cells.

[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 embodiments defined by the symbols. [Brief explanation of the drawings]

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

[0039] A cell stack, electrochemical unit cell, gas production apparatus, and hot module according to embodiments of the present invention will be described below 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-plate-shaped electrochemical units U in the thickness direction (vertical direction), and a pair of end plates 2, 3 disposed 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 an electrochemical device. The electrochemical device includes an electrolyzer and a power generator. The end plates 2, 3 are rectangular flat-plate-shaped members having the same outer shape as the electrochemical unit U and each have a rectangular opening formed in its 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 the end plates 2, 3 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 serve as an anode and a cathode, respectively, and when each electrochemical unit U functions as a fuel cell, the end plates 2 and 3 serve as a positive electrode and a negative electrode, respectively. For ease of explanation, the proportions of the components in the drawings may differ from the actual proportions.

[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 smallest 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 thereof, an anode 16 laminated on the lower surface thereof, and an intermediate layer 20 laminated between the solid electrolyte layer 12 and the anode 16. The air electrode 14 has a smaller outer shape 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 having a rectangular current collecting portion 4a that protrudes downward from the center of its lower surface. A pair of interconnectors 4 is arranged on both sides of the unit 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 unit cell 10. The lower electrochemical unit U has a pair of interconnectors 4, 9 instead of a pair of interconnectors 4, 4. The interconnector 9 is arranged at the bottom 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 thereof. 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 with a square opening formed in the center. The fuel electrode frame 7 is disposed 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 the space on the air electrode 14 side and is composed of the upper interconnector 4, separator 5, air electrode frame 6, and unit cell 10. The fuel chamber Sf is the space on the fuel electrode 16 side and is composed of the separator 5, lower interconnector 4, fuel electrode frame 7, and unit 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, thereby electrically connecting the unit cells 10 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. These paths Pfi, Pfo, Pai, and Pao are all formed to penetrate through the members of the cell stack 1 in the thickness direction, excluding the "end plate 2" and the "interconnector 4 above the upper electrochemical unit U at the upper end."

[0049] The path Pfi is formed along side E1, which is one of the four sides that make up the outer periphery of the cell stack 1, near one corner of side E1. The path Pfo is formed along side E2, which faces side E1, near the other corner of side E2 (the corner located diagonally from one corner of 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 near one corner of the side E2. The path Pao is formed along the side E1 near 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 of the unit cell 10 in the thickness direction. 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 plate-shaped layer and is configured to contain YSZ (yttria-stabilized zirconia) particles. Here, YSZ is ZrO (zirconia) to which Y2O3 (yttria) is added as a stabilizer. ZrO is an oxide of Zr (zirconium). That is, the solid electrolyte layer 12 contains ceramic particles made of Zr oxide. The solid electrolyte layer 12 has high oxide ion conductivity. The solid electrolyte layer 12 can be formed, for example, to have 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 cathode 14 is a rectangular, flat layer containing perovskite oxide particles such as lanthanum strontium cobalt iron oxide (LSCF). The cathode 14 includes a functional layer and a current collecting layer. The current collecting layer is thicker than the functional layer and is disposed on top of the functional layer. The cathode 14 has high electronic conductivity and effectively collects electrons from the current collecting layer. The cathode 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 solid electrolyte layer 12, the air electrode 14, and the intermediate layer 20 are supported by the anode 16. 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 contain 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 CeO (ceria) doped with GdO (gadolinia). CeO 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 below). The diameter of the micropores h1 is smaller than the diameter of the micropores in the support layer 18a. The functional layer 18b has high electronic conductivity. Ce is an example of a "second metal element," and the Ni particles are an example of a "metal particle."

[0057] The intermediate layer 20 is a rectangular plate-shaped layer and is configured to contain GDC particles. Therefore, like the functional layer 18b, the intermediate layer 20 contains ceramic particles composed of Ce oxide. On the other hand, the intermediate layer 20 does not contain Ni particles. In other words, the intermediate layer 20 does not contain cermet. Whether a layer contains cermet can be determined by identifying the type of elements constituting 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 below). 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 described in detail with reference to FIG. 4. As shown in FIG. 4, the functional layer 18b may 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 may 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. This configuration allows gas generated by the electrochemical reaction in the functional layer 18b to be temporarily stored in the numerous micropores h2 of the intermediate layer 20. This prevents a highly reducing or highly oxidizing atmosphere from occurring. Furthermore, the possibility of the gas required for the electrochemical reaction being depleted immediately after switching the operation mode (described later) can be significantly reduced. 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 when 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 Figure 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 containing 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.

[0060] On the other hand, no element diffusion layer Ld is 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. Furthermore, in the unit cell 10, the fuel electrode 16 (specifically, the functional layer 18b) is made of cermet, while the intermediate layer 20 does not contain cermet. That is, no electrochemical reaction occurs in the intermediate layer 20. Therefore, the reaction field rf, which is the region where the electrolysis reaction is particularly active, is formed on the surface of the functional layer 18b on the intermediate layer 20 side and in a region nearby. Therefore, in the unit cell 10, the reaction field rf is located at a position separated from the element diffusion layer Ld. This configuration can suppress a decrease in the activity of the reaction field rf (described below).

[0061] Additionally, in this embodiment, the intermediate layer 20 is configured to contain GDC particles. Generally, CeO2 has the property that in a reducing atmosphere, the crystal lattice releases oxide ions, changing its valence from tetravalent to trivalent, and in an oxidizing atmosphere, the crystal lattice absorbs (takes in) surrounding oxide ions, changing its valence from trivalent to tetravalent (hereinafter, the former is referred to as "oxygen release capacity" and the latter is referred to as "oxygen storage capacity"). 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 by the CeO2 of the intermediate layer 20. This configuration can prevent the atmosphere from becoming highly reducing or highly oxidizing (described below). The oxygen release / storage capacity 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), or LDC (lanthania-doped ceria), etc.

[0062] The cell stack 1 is configured to be operable in two operating modes: "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 electrolysis unit cell). Here, an electrolysis unit cell is the smallest unit of an SOEC. An electrolysis 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 unit cell). Here, a fuel cell unit cell is the smallest unit of an SOFC. A 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, hydrogen is produced by electrolyzing water vapor into hydrogen and oxygen in the SOEC mode, and hydrogen and oxygen are combined in the SOFC mode to generate electricity and water vapor. That is, electrochemical reactions proceed in opposite directions 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 operating modes at the 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 from an external power supply (not shown). Next, a high-temperature mixed gas of water vapor and hydrogen is supplied from the path Pfi, and the mixed gas flows into the fuel chamber Sf of each electrochemical unit U through the horizontal holes 7a. Here, the water vapor is the fuel gas, and the hydrogen is a reducing gas for reducing the oxidation of the catalyst contained in the fuel electrode 16. Furthermore, when high-temperature air is supplied from the path Pai, the air flows into the air chamber Sa of each electrochemical unit U through the horizontal holes (not shown). The high-temperature air is supplied in order to control the temperature of the cell stack 1.

[0065] Water vapor that flows into the fuel chamber Sf passes through the support layer 18a of the fuel electrode 16 and travels 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 occurs particularly actively in the reaction field rf of the functional layer 18b. Hydrogen (specifically, hydrogen generated by electrolysis and hydrogen as a reducing gas) diffuses within the fuel chamber Sf and is discharged through the horizontal holes 7b via path Pfo and collected by a well-known method. At this time, unreacted water vapor is discharged along with the hydrogen via path Pfo. Meanwhile, oxide ions travel through the intermediate layer 20 and solid electrolyte layer 12 to the air electrode 14 in the air chamber Sa, where they release electrons in the functional layer of the air electrode 14 and become oxygen. The oxygen diffuses within the air chamber Sa and is discharged through path Pao via horizontal holes (not shown) together with the air that flowed into the air chamber Sa and 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, we will explain the operation of the cell stack 1 when it is operated in SOFC mode. First, the end plates 2 and 3 are connected to an external circuit. Next, when high-temperature hydrogen is supplied through the path Pfi, the hydrogen flows into the fuel chamber Sf through the horizontal hole 7a. When high-temperature air is supplied through 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 via the current collector 4a) at the air electrode 14 and becomes oxide ions. "Gases other than oxygen in the air" and "unreacted oxygen" diffuse within the air chamber Sa and are discharged and collected through the horizontal hole (not shown) via the path Pao. The oxide ions travel through the solid electrolyte layer 12 and intermediate layer 20 to the fuel electrode 16 in the fuel chamber Sf, where they release electrons and combine with hydrogen to form water vapor in the functional layer 18b. This reaction occurs particularly actively in the reaction field rf of the functional layer 18b. The water vapor diffuses within the fuel chamber Sf and is discharged along with unreacted hydrogen through the horizontal holes 7b and the path Pfo for recovery. 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 an external circuit. Electric power is extracted from the external circuit in a predetermined manner.

[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. This will be described in detail 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 heat 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) that exchanges heat with high-temperature exhaust gas containing oxygen generated by the cell stack 1, and generates water vapor by heating the water with the heat obtained through the heat exchange. The heat exchanger 33 exchanges heat between the supply gas supplied to the cell stack 1 and the product 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. After passing through the heater 34, the water vapor is supplied to the fuel chamber Sf through a path Pfi of the cell stack 1, and the air after passing through the heater 34 is supplied to the air chamber Sa through a path Pai of the cell stack 1.

[0070] Heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), and / or a heat-resistant container made of these heat-resistant fibers, can be used for the heat insulating material 35. The cell stack 1, vaporizer 32, heat exchanger 33, and heater 34 are arranged inside the heat insulating material 35. The heat-resistant fibers are arranged so as to fill gaps between the cell stack 1, vaporizer 32, heat exchanger 33, and 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 to the vaporizer 32 again as raw water.

[0072] The inventors of the present application conducted 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, followed by a description of each measurement result.

[0073] FIG. 7 is a cross-sectional view of the unit cell 50 in the thickness direction. FIG. 8 is a partially enlarged view of the unit cell 50 in a region R2 in FIG. 7. As shown in FIG. 7, the unit cell 50 differs from the unit cell 10 in that it does not include an intermediate layer 20. Otherwise, the unit cell 50 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 vicinity thereof. Meanwhile, a reaction field rf is formed on the surface of the functional layer 18b on the solid electrolyte layer 12 side and in the vicinity thereof. 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 refer to the characteristics when the unit cells 10 and 50 are operated in SOEC mode after undergoing an initial reduction treatment. Here, the initial reduction treatment is a treatment performed in the final stage of the manufacturing process of the unit cells 10 and 50, specifically, a treatment to reduce NiO2 (nickel oxide) contained in the green sheet of the anode 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 through the unit cells 10 and 50 while supplying water vapor of the same concentration. Note that, strictly speaking, the "current" here refers to the current per unit area of ​​the unit cell. The same applies to other measurement tests.

[0075] The voltage values ​​were measured while varying the current flowing through the unit cells 10 and 50 between 0.0 A and 1.5 A. The results showed that the unit cell 10 had a lower voltage value than the unit cell 50 across the entire current range. Furthermore, it was confirmed that the difference in voltage values ​​between the two cells increased as the current increased. This measurement result is believed to be due to the positional relationship between the reaction field rf and the element diffusion layer Ld in the unit cells 10 and 50. Specifically, in the unit cell 50, the reaction field rf partially overlaps with the element diffusion layer Ld (see FIG. 8 ), whereas in the unit cell 10, the reaction field rf is located at a distance from the element diffusion layer Ld (see FIG. 5 ). Therefore, in the unit cell 50, the electrochemical reaction in the reaction field rf is inhibited by the element diffusion layer Ld, significantly reducing the activity of the reaction field rf, whereas in the unit cell 10, the reduction in the activity of the reaction field rf is suppressed. This is believed to be the reason for the above results.

[0076] In the second measurement test, the durability of 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 operating unit cells 10 and 50 in SOEC mode for 365 hours, and the ratio was calculated as the degradation rate. The degradation rate is an index for evaluating durability and is calculated by dividing the "voltage after operation" by the "voltage before operation." The lower the degradation rate, the higher the durability. Note that for unit cell 10, a sample with a porosity of 35% for the intermediate layer 20 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 includes a porous intermediate layer 20 containing CeO2 particles, while the single cell 50 does not include such an intermediate layer 20. Specifically, in the single cell 10, hydrogen generated in the reaction field rf by operating in SOEC mode diffuses into the numerous micropores h2 in the intermediate layer 20 and is temporarily stored. Furthermore, as the hydrogen generation changes the atmosphere to a reducing one, oxide ions are released from the CeO2 in the intermediate layer 20, mitigating the reduction tendency. This suppresses a rapid increase in hydrogen concentration, thereby preventing a highly reducing atmosphere. Therefore, the electrodes are less susceptible to deterioration and gas flows appropriately (described below), which is believed to have resulted in suppressing a decrease in the durability of the single cell 10.

[0079] On the other hand, since the unit cell 50 does not include the intermediate layer 20, it continues to operate in a highly reducing atmosphere. As a result, the electrodes deteriorate and gas flow stagnates (described below), which is thought to result in a decrease in the durability of the unit cell 50. Here, "electrode deterioration" in the SOEC mode specifically refers to deterioration of the functional layer 18b. That is, CeO2 has the property of releasing oxide ions in a reducing atmosphere, weakening the cohesion of the crystal lattice and causing volume expansion. Therefore, in a highly reducing atmosphere, the volume of the functional layer 18b expands significantly, resulting in distortion of the crystal structure and deterioration of the functional layer 18b. Furthermore, "gas flow stagnation" in the SOEC mode specifically refers to "operation at high power increases the hydrogen concentration in the reaction field rf, causing hydrogen to stagnate without being properly discharged" and "the increase in hydrogen concentration prevents water vapor supplied from the outside from properly flowing into the reaction field rf."

[0080] The third measurement test compared the redox resistance (durability when alternating between oxidizing and reducing atmospheres) of single cells 10 and 50. Specifically, 200 cycles were performed, with SOEC and SOFC modes alternately switched every 10 seconds. Each cycle consisted of one cycle in both modes. The current versus applied voltage or output voltage was measured, and the rate of decrease in current was calculated as the degradation rate. The lower the degradation rate, the higher the redox resistance. The applied voltage was 1 V in SOEC mode, and the output voltage was -0.1 V in SOFC mode. The operating temperature was 700°C. In both SOEC and SOFC modes, 100 cc of steam per minute and 20 cc of hydrogen per minute were supplied to the fuel chamber Sf, and 100 cc of air per minute was supplied to the air chamber Sa.

[0081] As a result, for the single cell 50, the current value in the first cycle in the 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 Therefore, 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 thought to be due to the fact that the unit cell 10 has a porous intermediate layer 20 containing CeO particles, while the unit cell 50 does not have such an intermediate layer 20. Here, the considerations for operation in the SOEC mode are as described in the explanation of the second measurement test. Therefore, below, the considerations for operation in the SOFC mode and for operation at high reversible frequencies will be described.

[0083] In the single cell 10, water vapor generated in the reaction field rf by operating in SOFC mode diffuses into the numerous micropores h2 in the intermediate layer 20 and is temporarily stored there. Furthermore, as the atmosphere changes to an oxidizing atmosphere due to the generation of water vapor, oxide ions are absorbed into the CeO2 of the intermediate layer 20, mitigating the tendency toward oxidation. This prevents a sudden increase in water vapor concentration, thereby preventing a highly oxidizing atmosphere. Therefore, the electrodes are less susceptible to deterioration and gas flows appropriately (described below), which is thought to have resulted in reduced deterioration in the durability of the single cell 10.

[0084] On the other hand, since the unit cell 50 does not include the intermediate layer 20, it continues to operate in a highly oxidizing atmosphere. As a result, the electrodes deteriorate and gas flow stagnates (described below), which is thought to have resulted in a decrease in the durability of the unit cell 50. Here, "electrode deterioration" in the SOFC mode specifically refers to deterioration of the functional layer 18b. That is, Ni has the property that, when oxidized, its migration speed increases and multiple Ni particles aggregate. Therefore, in a highly oxidizing atmosphere, multiple Ni particles aggregate within the functional layer 18b, significantly reducing the number of three-phase interfaces. As a result, the reaction efficiency significantly decreases and the functional layer 18b deteriorates. Furthermore, "gas flow stagnation" in the SOFC mode specifically refers to "operation at high power increases the water vapor concentration in the reaction field rf, causing water vapor to stagnate without being properly discharged" and "externally supplied hydrogen does not properly flow into the reaction field rf due to the increased water vapor concentration."

[0085] Furthermore, in the single cell 10, even when the operation mode is switched frequently, the reaction field (rf) is prevented from becoming a highly reducing or highly oxidizing atmosphere, thereby making the electrodes less susceptible to deterioration. Furthermore, when operating in the SOEC mode, hydrogen is temporarily stored in the micropores h2 of the intermediate layer 20. Meanwhile, when operating in the SOFC mode, water vapor is temporarily stored in the micropores h2, and oxide ions are occluded 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 released to the reaction field (rf), significantly reducing the possibility that hydrogen supplied from the outside will be depleted before reaching the reaction field (rf). Meanwhile, immediately after switching to the SOEC mode, the water vapor stored in the intermediate layer 20 during operation in the SOFC mode is gradually released to the reaction field (rf), significantly reducing the possibility that water vapor supplied from the outside will be depleted before reaching the reaction field (rf). In addition, the oxide ions stored in the CeO2 of the intermediate layer 20 during operation in the SOFC mode are released, slowing down the rate at which water vapor is electrolyzed, 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. Hereinafter, this unit cell will be referred to as "unit cell B." Unit cell B is similar to unit cell 10 in that it includes 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 unit cell B was measured after firing each sample.

[0088] As a result, the maximum warpage of unit cell B was 2.161 mm, the minimum was 1.934 mm, and the average was 2.078 mm, while the maximum 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 warpage than unit cell B, and that the warpage variation was also smaller.

[0089] This measurement result is thought to be due to the fact that the intermediate layer 20 in unit cell 10 is porous, while the intermediate layer in unit cell B is not porous. That is, when unit cell 10 is exposed to large temperature changes during firing, the intermediate layer 20 elastically deforms in accordance with the amount of expansion or contraction of the solid electrolyte layer 12 and functional layer 18b, which is thought to have significantly alleviated the stress generated at the interface of each layer and reduced the amount of warping of unit cell 10. On the other hand, in unit cell B, the intermediate layer does not elastically deform even when exposed to large temperature changes, which is thought to have caused stress to be generated at the interface of each layer, resulting in a relatively large warping of 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 in 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, but 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 CeO (e.g., YSZ) may be used for the intermediate layer 20. That is, the intermediate layer 20 does not need to have oxygen releasing / storing ability. When YSZ is used for the intermediate layer 20, it is desirable that the functional layer 18b be 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 less than the porosity of the functional layer 18b. As long as the micropores h1 and h2 are large enough to allow water vapor and hydrogen to pass through, the diameter of the micropores h2 of the intermediate layer 20 may be equal to or less than the diameter of the micropores 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, CeO or CeO 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 unit 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 produced by electrolysis of carbon dioxide. In the latter case, carbon monoxide, hydrogen, and oxygen are produced by electrolysis of the mixed gas. In the former case, the gas production device 30 does not require the vaporizer 32. This is because the 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 the 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 with 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 the back surface of the solid electrolyte layer; an intermediate layer disposed between the solid electrolyte layer and the fuel electrode; Equipped with the anode is made of cermet, the intermediate layer is porous and does not contain a cermet; Solid oxide electrochemical single cell. [2] [1] The electrochemical unit cell according to [1], The intermediate layer contains ceria or ceria containing a rare earth element. Electrochemical single cell. [3] [2] The electrochemical unit cell according to [2], 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 unit cell according to any one of [1] to [3], the solid electrolyte layer contains 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 unit 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 the 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 formed by stacking a plurality of electrochemical unit cells according to any one of [1] to [6]. [8] [7] The cell stack according to [7], a heat exchanger that exchanges heat with the gas supplied to the cell stack; a heater for heating the cell stack; a heat insulating material in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with Hot module. [9] [8] The hot module 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 the back surface of the solid electrolyte layer; an intermediate layer disposed between the solid electrolyte layer and the fuel electrode; Equipped with the anode is made of cermet, the intermediate layer is porous and does not contain a cermet; Solid oxide electrochemical single cell.

2. 10. The electrochemical unit cell of claim 1, The intermediate layer contains ceria or ceria containing a rare earth element. Electrochemical single cell.

3. 3. The electrochemical unit cell according to claim 2, 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. 4. The electrochemical unit cell according to claim 1, the solid electrolyte layer contains 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. 4. The electrochemical unit cell according to claim 1, 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 the 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 formed by stacking a plurality of electrochemical unit cells according to claim 1 or 6.

8. The cell stack according to claim 7; a heat exchanger that exchanges heat with the gas supplied to the cell stack; a heater for heating the cell stack; a heat insulating material in which the cell stack, the heat exchanger, and the heater are disposed; Equipped with Hot module.

9. The hot module according to claim 8 is used in an operation mode in which each of the plurality of electrochemical unit cells is used as an electrolysis unit cell. Gas production equipment.

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