Electrochemical cells, electrochemical cell stacks, hot modules, and electrolytic reactors

Abstract

To improve the durability of an electrochemical cell by reducing the η ratio. [Solution] The electrolytic cell 311 comprises a solid electrolyte layer 311a, a fuel electrode layer 311c in which pores are formed, and an air electrode layer 311b. The fuel electrode layer 311c has a functional layer L1 that contacts the solid electrolyte layer 311a, and a substrate layer L2 that contacts the side of the functional layer L1 opposite to the side that contacts the solid electrolyte layer 311a. The porosity φ2 of the substrate layer-side interface vicinity region NA2, which is the region near the contact interface S between the substrate layer L2 and the functional layer L1, is greater than the porosity φ3 of the region of the substrate layer L2 other than the substrate layer-side interface vicinity region NA2, and less than 48%. The average diameter (D) of the pores formed in the functional layer L1 is less than 0.672 μm.

Description

Technical Field

[0001] The present disclosure relates to an electrochemical cell, an electrochemical cell stack, a hot module, and an electrolysis reaction device.

Background Art

[0002] Conventionally, as a solid electrolyte, a solid oxide type electrochemical cell using an ion conductive oxide (for example, YSZ (yttria stabilized zirconia)) has been known. The solid oxide type electrochemical cell is characterized by performing an electrochemical reaction with high efficiency in a high temperature environment, and can be used as a solid oxide electrolysis cell (SOEC: Solid Oxide Electrolysis Cell) or a solid oxide fuel cell (SOFC: Solid Oxide Fuel Cell). Further, the solid oxide type electrochemical cell can also be used as an r-SOC (reversible solid oxide type electrochemical cell) that can switch between an SOEC mode in which it operates as an SOEC and an SOFC mode in which it operates as an SOFC.

[0003] The solid oxide type electrochemical cell includes a solid electrolyte, a fuel electrode, and an air electrode. Patent Document 1 discloses a fuel electrode supported type electrochemical cell in which the fuel electrode includes a functional layer and a substrate layer. Patent Document 1 describes limiting the Ni (nickel) content in the functional layer of the fuel electrode or the Ni content in the substrate layer to a predetermined range.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] Electrochemical cells require high durability to maintain a certain level of performance even after prolonged operation. To improve durability, it is effective to suppress the degradation of electrodes (e.g., fuel electrodes), and to suppress electrode degradation, it is considered effective to reduce the activation overvoltage ηact and concentration overvoltage ηcon applied to the electrochemical cell. If the resistance due to the activation overvoltage ηact and concentration overvoltage ηcon is expressed as the reaction resistance Rη, then by reducing the ratio of the reaction resistance Rη to the impedance Z of the electrochemical cell (hereinafter referred to as the η ratio), it is possible to reduce the overvoltage due to electrode reactions (activation overvoltage ηact and concentration overvoltage ηcon) from the required voltage (e.g., 1.3V), thereby improving durability. This disclosure aims to solve the above-mentioned problems. Specifically, one of the objectives of this disclosure is to improve the durability of an electrochemical cell by reducing the η ratio.

[0006] The electrochemical cell (311) according to this disclosure comprises a solid electrolyte (311a), a fuel electrode (311c) in which pores are formed, and an air electrode (311b). The fuel electrode (311c) has a functional layer (L1) in contact with the solid electrolyte (311a) and a substrate layer (L2) in contact with the side of the functional layer (L1) opposite to the side in contact with the solid electrolyte (311a). The porosity (φ2) of the region near the substrate layer interface (NA2), which is the region within the substrate layer (L2) that is close to the contact interface (S) between the substrate layer (L2) and the functional layer (L1), is greater than the porosity (φ3) of the region within the substrate layer (L2) other than the region near the substrate layer interface (NA2), and is less than 48%. The average diameter (D) of the pores formed in the functional layer (L1) is less than 0.672 μm.

[0007] According to the electrochemical cell described herein, the η ratio can be reduced, thereby improving durability.

[0008] In one embodiment of the electrochemical cell according to this disclosure, the ratio (H) of the porosity (φ1) of the region near the interface (NA2) on the substrate layer side to the porosity (φ2) of the region near the interface (NA1) on the functional layer side, which is a region within the functional layer (L1) near the contact interface (S), is less than 1.89. This allows the η ratio to be further reduced, thereby further improving durability.

[0009] The electrochemical cell stack (1) relating to this disclosure is made up of stacked electrochemical cells (311) as described above. This makes it possible to provide an electrochemical cell stack equipped with electrochemical cells with improved durability.

[0010] The hot module (6) according to this disclosure comprises the electrochemical cell stack (1) described above, a heating device (3) for heating the gas supplied to the electrochemical cell stack (1), and an insulating material (4) in which the electrochemical cell stack (1) and the heating device (3) are arranged. This makes it possible to provide a hot module equipped with an electrochemical cell stack with improved durability.

[0011] The electrolytic reactor (100) according to this disclosure includes the hot module (6) described above. This makes it possible to provide an electrolytic reactor equipped with an electrochemical cell with improved durability. [Brief explanation of the drawing]

[0012] [Figure 1] This is a block diagram of a hydrogen production system. [Figure 2] This is a perspective view of an electrochemical cell stack. [Figure 3] This is a cross-sectional view along line III-III in Figure 2. [Figure 4] This is a cross-sectional view of the electrolytic cell in the thickness direction. [Figure 5] Figure 4 is a cross-sectional view along the VV line. [Figure 6] This is a schematic diagram showing a portion of a cross-section of an electrolytic cell cut in the direction of its thickness. [Figure 7]This is an example of an image of a cross-section of a sample. [Modes for carrying out the invention]

[0013] Embodiments of this disclosure will be described below with reference to the drawings. Figure 1 is a block diagram of a hydrogen production apparatus 100 as an electrolytic reaction apparatus according to this embodiment. The hydrogen production apparatus 100 according to this embodiment is an apparatus that produces hydrogen by electrolyzing water vapor. As shown in Figure 1, the hydrogen production apparatus 100 comprises a hot module 6 and a condenser 7.

[0014] The hot module 6 is constructed by covering the main components of the hydrogen production apparatus 100 that become hot with an insulating material. In other words, the hot module 6 is a device that concentrates the main components of the hydrogen production apparatus 100 within an insulating material so that the high temperature state of the main components of the hydrogen production apparatus 100 is maintained. This hot module 6 comprises an electrochemical cell stack 1, a vaporizer 2, a heating device 3, and an insulating material 4.

[0015] As shown in Figure 1, water (H2O) and reducing hydrogen (H2) are supplied to vaporizer 2. Vaporizer 2 is designed to heat the supplied water to a temperature of 100°C or higher. Therefore, water vapor is generated as the water supplied to vaporizer 2 evaporates within vaporizer 2. The water vapor generated in vaporizer 2 and the reducing hydrogen heated in vaporizer 2 are introduced into the heating device 3.

[0016] The heating device 3 is a device for heating the gas introduced into the electrochemical cell stack 1. The heating device 3 is equipped with a heat exchanger 3a and a heater 3b. Water vapor, hydrogen for reduction, and air produced in the vaporizer 2 are introduced into the heat exchanger 3a. High-temperature gas produced in the electrochemical cell stack 1, which will be described later, is also introduced into the heat exchanger 3a. In the heat exchanger 3a, the high-temperature gas introduced from the electrochemical cell stack 1 exchanges heat with the water vapor, hydrogen for reduction, and air, thereby heating these gases.

[0017] The gas heated in the heat exchanger 3a is further heated by the heater 3b to the operating temperature of the electrochemical cell stack 1 (i.e., the temperature required to operate the electrochemical cell stack 1). The heated gas is then introduced into the electrochemical cell stack 1.

[0018] The electrochemical cell stack 1 is formed by stacking solid oxide electrolytic cells (hereinafter referred to as electrolytic cells). The electrochemical cell stack 1 is heated to its operating temperature by a heating source (burner, etc.) not shown. A predetermined voltage is also applied to the electrochemical cell stack 1. As a result, the water vapor introduced into the electrochemical cell stack 1 is electrolyzed to produce hydrogen and oxygen. The produced hydrogen, along with unreacted water vapor and hydrogen for reduction, is discharged from the electrochemical cell stack 1, and these high-temperature gases are then introduced into the heat exchanger 3a. These high-temperature gases are then used to heat the gas and air introduced from the vaporizer 2 to the heat exchanger 3a, and then introduced into the condenser 7. In the condenser 7, unreacted water vapor is condensed. The condensed water produced in the condenser 7 is introduced into the vaporizer 2. Meanwhile, the hydrogen separated by the condensation of water vapor in the condenser 7 is recovered. A portion of the hydrogen separated in the condenser 7 may be supplied to the vaporizer 2 as hydrogen for reduction. The oxygen generated in the electrochemical cell stack 1 is discharged from the electrochemical cell stack 1 along with the air introduced into the electrochemical cell stack 1, and these high-temperature gases are then introduced into the heat exchanger 3a. These high-temperature gases are then used to heat the gas and air introduced from the vaporizer 2 into the heat exchanger 3a, and are then introduced into the vaporizer 2 to heat the water and hydrogen for reduction supplied to the vaporizer 2, after which they are recovered (or released into the atmosphere).

[0019] The electrochemical cell stack 1, the vaporizer 2, and the temperature raising device 3 are arranged inside the heat insulating material 4. Thereby, heat radiation from each of the members 1, 2, 3 is suppressed. As the heat insulating material 4, heat resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), bio-soluble fiber (AES), and / or a heat resistant container formed of these heat resistant fibers can be used. The heat resistant fibers are arranged so as to fill the gaps between the electrochemical cell stack 1, the vaporizer 2, and the temperature raising device 3 (the heat exchanger 3a and the heater 3b).

[0020] FIG. 2 is a perspective view of the electrochemical cell stack 1. Here, when directions are used to explain the electrochemical cell stack 1 and its components, the three directions shown in FIG. 2, namely the vertical direction, the width direction, and the depth direction, are used. The vertical direction, the width direction, and the depth direction are perpendicular to each other. The planes extending in the width direction and the depth direction are horizontal planes perpendicular to the vertical direction. Also, one of the depth directions is defined as the front and the other as the rear.

[0021] As shown in FIG. 2, the electrochemical cell stack 1 includes a cell cassette group formed by laminating a plurality of rectangular flat plate-shaped cell cassettes 30 in the thickness direction, an upper insulating plate 20 laminated on the upper surface of the cell cassette group, an upper end plate 10 laminated on the upper surface of the upper insulating plate 20, a terminal plate 4 laminated on the lower surface of the cell cassette group, a lower insulating plate 50 laminated on the lower surface of the terminal plate 40, and a lower end plate 60 laminated on the lower surface of the lower insulating plate 50. These plate-like members are a laminate laminated in the thickness direction. The thickness direction of each member coincides with the vertical direction in FIG. 2. Therefore, the above-described members are laminated in the vertical direction.

[0022] Each of the above-described members is formed in a rectangular plate shape having sides along the width direction and the depth direction. Further, each of the above-described members is fastened to each other by bolts B inserted through the four corners thereof in the stacking direction and nuts not shown in the drawings. The upper end plate 10, the upper insulating plate 20, the terminal plate 40, the lower insulating plate 50, and the lower end plate 60 have the same outer shape as the cell cassette 30 when viewed from the vertical direction. The upper end plate 10, the terminal plate 40, and the lower end plate 60 are all made of metal (for example, stainless steel). The upper insulating plate 20 and the lower insulating plate 50 are formed in a plate shape by insulating members, for example, mica or resin. Further, a rectangular opening is formed in the central portions of the upper end plate 10 and the upper insulating plate 20. For the sake of convenience of explanation, the ratios of the members in the drawings may be different from the actual ratios.

[0023] Two gas supply passages Pfi, Pai and two gas discharge passages Pfo, Pao penetrating in the stacking direction thereof are formed in the plurality of cell cassettes 30, the terminal plate 40, the lower insulating plate 50, and the lower end plate 60 constituting the cell cassette group. The gas supply passage Pfi is formed in the vicinity of one corner of a side E1 which is one of the four sides constituting the outer peripheral portion of the electrochemical cell stack 1. The gas discharge passage Pfo is formed in the vicinity of the other corner of the side E2 facing the side E1 (the corner located on the diagonal line of one corner of the side E1). The gas supply passage Pai is formed in the vicinity of one corner of the side E2, and the gas discharge passage Pao is formed in the vicinity of the other corner of the side E1. The gas supply passage Pfi forms a passage through which water vapor and hydrogen for reduction supplied to the electrochemical cell stack 1 pass, and the gas supply passage Pai forms a passage through which air supplied to the electrochemical cell stack 1 passes. The gas discharge passage Pfo forms a passage through which hydrogen, hydrogen for reduction, and water vapor discharged from the electrochemical cell stack 1 pass, and the gas discharge passage Pao forms a passage through which oxygen and air discharged from the electrochemical cell stack 1 pass.

[0024] Figure 3 is a cross-sectional view taken along line III-III in Figure 2. Figure 3 is a cross-sectional view of the electrochemical cell stack 1 cut in the depth direction so that the cross-sections of the gas supply passage Pfi and the gas discharge passage Pfo are visible. As shown in Figure 3, the multiple cell cassettes 30 constituting the cell cassette group are stacked between the upper end plate 10 and the lower end plate 60. An upper insulating plate 20 is interposed between the multiple cell cassettes and the upper end plate 10. A terminal plate 40 and a lower insulating plate 50 are interposed between the multiple cell cassettes and the lower end plate 60.

[0025] Figure 4 is a cross-sectional view of the cell cassette 30 shown in Figure 3. Note that the gas supply passage Pfi and gas discharge passage Pfo shown in Figure 3 are omitted in Figure 4. As shown in Figure 4, the cell cassette 30 comprises an electrochemical cell unit 31, a separator section 32, and a frame section 33. The electrochemical cell unit 31 is a reaction section in which an electrochemical reaction takes place, and includes a solid oxide type electrolytic cell (hereinafter referred to as an electrolytic cell) as an electrochemical cell. Specifically, the electrochemical cell unit 31 includes an electrolytic cell 311, a fuel electrode current collector 312, and an interconnector 313. In Figure 4, the fuel electrode current collector 312 is located below the electrolytic cell 311, and the interconnector 313 is located below the fuel electrode current collector 312.

[0026] In this embodiment, the electrolytic cell 311 is a substantially rectangular plate-like member having a predetermined thickness. Figure 6 is a schematic diagram showing a part of the cross-section of the electrolytic cell 311 cut in the thickness direction. As shown in Figures 4 and 6, the electrolytic cell 311 comprises a solid electrolyte layer 311a (solid electrolyte), an air electrode layer 311b (air electrode) laminated on the upper surface (one side) of the solid electrolyte layer 311a, and a fuel electrode layer 311c (fuel electrode) laminated on the lower surface (the other side) of the solid electrolyte layer 311a, and is formed by laminating these in the thickness direction. The thickness direction of each layer (solid electrolyte layer 311a, air electrode layer 311b, fuel electrode layer 311c) coincides with the thickness direction of the electrolytic cell 311. Also, as shown in Figure 6, a reaction prevention layer 311d is formed between the solid electrolyte layer 311a and the air electrode layer 311b.

[0027] The solid electrolyte layer 311a is a layer made of a solid oxide type electrolyte. The solid electrolyte layer 311a is a rectangular, flat layer. The solid electrolyte layer 311a contains YSZ (yttria-stabilized zirconia) as an ion-conducting oxide and is formed by sintering. The solid electrolyte layer 311a has good oxide ion conductivity. The solid electrolyte layer 311a is a dense layer and is designed so that the atmosphere on the air electrode layer 311b side and the atmosphere on the fuel electrode layer 311c side do not leak to each other through the solid electrolyte layer 311a.

[0028] The air electrode layer 311b is a rectangular, flat layer, composed of perovskite-type oxides such as LSCF (lanthanum strontium cobalt iron oxide), and is formed by sintering. The air electrode layer 311b has high electron conductivity and collects electrons well. This air electrode layer 311b is a porous layer and has pores inside. Also, as shown in Figure 4, the air electrode layer 311b has a smaller outer shape than the solid electrolyte layer 311a and the fuel electrode layer 311c, and is located in the center of the upper surface of the solid electrolyte layer 311a in a plan view of the electrolytic cell 311. For this reason, the upper surface of the outer periphery of the solid electrolyte layer 311a is exposed and not covered by the air electrode layer 311b.

[0029] The reaction prevention layer 311d is provided to prevent the reaction between the components in the solid electrolyte layer 311a and the components in the air electrode layer 311b. The reaction prevention layer 311d can be formed, for example, by GDC (gadolinium-doped ceria).

[0030] The fuel electrode layer 311c is a rectangular, flat layer. The fuel electrode layer 311c is molded to be thicker than the solid electrolyte layer 311a and the air electrode layer 311b. The fuel electrode layer 311c supports the solid electrolyte layer 311a and the air electrode layer 311b. In other words, the electrolytic cell 311 is a fuel electrode-supported electrochemical cell. In plan view, the external shape of the fuel electrode layer 311c matches the external shape of the solid electrolyte layer 311a.

[0031] As shown in Figure 6, the fuel electrode layer 311c has a functional layer L1 and a substrate layer L2. The functional layer L1 is a layer in which the electrolytic reaction of water vapor mainly takes place. The substrate layer L2 is a layer that mainly supplies water vapor to the functional layer L1 and supports the electrolytic cell 311. The electrolytic reaction of water vapor may also take place in the substrate layer L2. The thickness of the substrate layer L2 is formed to be significantly thicker than that of the functional layer L1, and the ratio can be set to, for example, about 16 to 40 times. The functional layer L1 and the substrate layer L2 are stacked on the lower surface of the solid electrolyte layer 311a in the order of functional layer L1, then substrate layer L2. Therefore, the functional layer L1 is in contact with the solid electrolyte layer 311a. The substrate layer L2 is in contact with the side of the functional layer L1 opposite to the side that is in contact with the solid electrolyte layer 311a.

[0032] The main component of the substrate layer L2 is a cermet made of Ni (nickel) and YSZ (yttria-stabilized zirconia) as an ion-conducting oxide. The substrate layer L2 is a porous layer configured to be porous, containing multiple pores. The multiple pores ensure permeability (gas diffusion) of water vapor. In this embodiment, the main component of the functional layer L1 is the same cermet as the substrate layer L2, namely, a cermet made of Ni and YSZ. The functional layer L1 may also be configured to contain Fe. Fe has the effect of suppressing the movement and aggregation of Ni within the functional layer L1. The functional layer L1 is also a porous layer configured to be porous, containing multiple pores, similar to the substrate layer L2. Therefore, the fuel electrode layer 311c is a layer in which pores are formed internally. However, the functional layer L1 is formed more densely than the substrate layer L2. That is, the functional layer L1 and the substrate layer L2 are formed such that the porosity of the functional layer L1 is smaller than that of the substrate layer L2. The fuel electrode layer 311c has both ionic and electronic conductivity. Like the solid electrolyte layer 311a and the air electrode layer 311b, the fuel electrode layer 311c with this configuration is formed by sintering.

[0033] Figure 5 is a cross-sectional view of the VV line in Figure 4. As shown in Figure 5, the fuel electrode current collector 312 is disposed between the electrolytic cell 311 and the interconnector 313. In this embodiment, the fuel electrode current collector 312 is composed of a plurality of elastic insulators 312a and a plurality of conductors 312b. Each of the plurality of elastic insulators 312a is formed in an elongated shape extending in the depth direction and is arranged in parallel with a predetermined interval in the width direction. The elastic insulators 312a may be made of mica, for example. The plurality of conductors 312b are provided so as to cover the outer circumference of each elastic insulator 312a and may be made of metal foil, for example. Each conductor 312b contacts the interconnector 313 with the portion covering the lower surface of the elastic insulator 312a and contacts the fuel electrode layer 311c of the electrolytic cell 311 with the portion covering the upper surface of the elastic insulator 312a.

[0034] The interconnector 313 is a metal (for example, stainless steel) component and, as shown in Figure 4, has a rectangular flat body portion 313a and a projection portion 313b that protrudes downward from the lower surface of the body portion 313a. The projection portion 313b is formed by a plurality of protrusions that extend parallel to each other along the direction perpendicular to the plane of the paper (width direction) in Figures 3 and 4. The interconnector 313 is positioned so that the body portion 313a contacts the conductor 312b of the fuel electrode current collector 312 disposed above it, and the projection portion 313b connects to the air electrode layer 311b of the adjacent electrolytic cell 311 disposed below it.

[0035] The frame section 33 includes a fuel electrode frame 331 and an air electrode frame 332. The fuel electrode frame 331 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in its center. The air electrode frame 332 is positioned below the fuel electrode frame 331. The air electrode frame 332 is a rectangular plate-shaped insulating member, which may be formed from, for example, a mica sheet. The air electrode frame 332 also has a rectangular opening in its center, similar to that of the fuel electrode frame 331.

[0036] Furthermore, as can be seen from Figures 4 and 5, the electrochemical cell unit 31 is disposed inside an opening formed in the center of the frame portion 33. In other words, the frame portion 33 is disposed around the electrochemical cell unit 31 so as to surround its outer periphery. A predetermined gap is formed between the frame portion 33 and the electrochemical cell unit 31, and the separator portion 32 is disposed to close this gap. The separator portion 32 includes a cell-side separator 321 and an interconnector-side separator 322. The cell-side separator 321 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in its center. The periphery (i.e., inner periphery) of the opening of the cell-side separator 321 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 311a of the electrolytic cell 311 with a brazing material (e.g., Ag brazing material) not shown. On the other hand, the outer periphery of the cell-side separator 321 is positioned on the upper surface of the fuel electrode frame 331 and is joined to the fuel electrode frame 331 by means of welding, for example.

[0037] The interconnector-side separator 322, like the cell-side separator 321, is a rectangular plate-shaped metal (for example, stainless steel) component with a rectangular opening formed in its center. The periphery (i.e., inner periphery) of the opening of the interconnector-side separator 322 is joined to the upper surface of the main body portion 313a of the interconnector 313, for example, by welding. The outer periphery of the interconnector-side separator 322 is sandwiched between the lower surface of the fuel electrode frame 331 and the upper surface of the air electrode frame 332, and is joined to the fuel electrode frame 331 above it, for example, by welding. In this way, the separator portion 32 (cell-side separator 321 and interconnector-side separator 322) is configured to connect the electrochemical cell unit 31 and the frame portion 33.

[0038] When the cell cassettes 30 configured as described above are stacked on the terminal plate 40, an electrochemical cell stack 1 is formed in which multiple electrochemical cell units 31, including an electrolytic cell 311, are stacked vertically. In addition, the separator section 32 (cell-side separator 321 and interconnector-side separator 322) partitions the space between adjacent separators. As a result, multiple fuel chambers Sf and air chambers Sa are formed inside the electrochemical cell stack 1, with gas flow being blocked from each other. Specifically, the fuel chamber Sf is formed by the space surrounded by the fuel electrode frame 331, the cell-side separator 321 joined to the upper surface of the fuel electrode frame 331, the electrolytic cell 311 connected to the cell-side separator 321, the interconnector-side separator 322 joined to the lower surface of the fuel electrode frame 331, and the interconnector 313 connected to the interconnector-side separator 322. This fuel chamber Sf is formed between the electrolytic cell 311 and the interconnector 313. Furthermore, the fuel electrode layer 311c of the electrolytic cell 311 is exposed in the fuel chamber Sf. In addition, an air chamber Sa is formed by the space surrounded by the air electrode frame 332, the interconnector-side separator 322 in contact with the upper surface of the air electrode frame 332, the interconnector 313 connected to the interconnector-side separator 322, the cell-side separator 321 of the cell cassette 30 adjacent to the air electrode frame 332 below, and the electrolytic cell 311 connected to the cell-side separator 321. The air electrode layer 311b of the electrolytic cell 311 is exposed in the air chamber Sa. Water vapor as fuel gas and hydrogen for reduction are supplied to the fuel chamber Sf. Air is supplied to the air chamber Sa.

[0039] Furthermore, as shown in Figure 3, the gas supply passage Pfi and the gas discharge passage Pfo are formed to penetrate the lower end plate 60, the lower insulating plate 50, the terminal plate 40, and the frame portion 33 of each cell cassette 30 constituting the cell cassette group in the stacking direction. The gas supply passage Pfi communicates with the fuel chamber Sf via a lateral hole 331a formed in the fuel electrode frame 331 of each cell cassette 30. The gas discharge passage Pfo communicates with the fuel chamber Sf via a lateral hole 331b formed in the fuel electrode frame 331 of each cell cassette 30.

[0040] Furthermore, the gas supply passage Pai and the gas discharge passage Pao are formed to penetrate the lower end plate 60, the lower insulating plate 50, the terminal plate 40, and the frame portion 33 of each cell cassette 30 constituting the cell cassette group in the stacking direction. The gas supply passage Pai and the gas discharge passage Pao are in communication with the air chamber Sa, respectively, through lateral holes (not shown) formed in the air electrode frame 332 of each cell cassette 30.

[0041] Furthermore, cell cassette 30 (30A), located at the top of the cell cassette group, is a dummy cell cassette in which a metal plate PL is provided in place of the electrolytic cell 311. The fuel electrode frame 331 of this dummy cell cassette 30A functions as a terminal plate.

[0042] The operation of the electrochemical cell stack 1 will now be described. First, a voltage is applied to the electrochemical cell stack 1. At this time, for example, the negative electrode of the power supply is connected to the terminal plate 40, and the positive electrode is connected to the fuel electrode frame 331 of the dummy cell cassette 30A. Subsequently, high-temperature steam and hydrogen for reduction are supplied from the gas supply passage Pfi. The steam and hydrogen for reduction supplied to the gas supply passage Pfi flow into the fuel chamber Sf of each cell cassette 30 through the side hole 331a. High-temperature air is also supplied from the gas supply passage Pai. The air supplied to the gas supply passage Pai flows into the air chamber Sa of each cell cassette 30 through a side hole (not shown). The reason for supplying high-temperature air to the air chamber Sa is to control the temperature of the electrochemical cell stack 1.

[0043] The water vapor and hydrogen for reduction that flow into the fuel chamber Sf flow from the front side (one end) to the rear side (the other end) in the depth direction, as shown in Figure 3. Therefore, the front side in the depth direction is the inlet side of the fuel chamber Sf, and the rear side in the depth direction is the outlet side of the fuel chamber Sf. The water vapor in the fuel chamber Sf comes into contact with the fuel electrode layer 311c exposed in the fuel chamber Sf, and reacts with electrons supplied via the interconnector 313 and the fuel electrode current collector 312, etc., in the fuel electrode layer 311c (functional layer L1). This decomposes the water vapor into hydrogen and oxide ions (water vapor electrolysis reaction). The hydrogen generated by the water vapor electrolysis reaction flows further to the rear in the depth direction within the fuel chamber Sf together with the unreacted water vapor and hydrogen for reduction, and is discharged from the gas discharge passage Pfo through the lateral hole 331b. Meanwhile, oxide ions move through the solid electrolyte layer 311a to the air electrode layer 311b exposed to the air chamber Sa, where they release electrons to become oxygen. The oxygen diffuses within the air chamber Sa and, together with the air flowing into the air chamber Sa, is discharged through a lateral hole (not shown) from the gas discharge passage Pao.

[0044] Electrons emitted from the functional layer of the air electrode layer 311b are collected by the interconnector 313 via the current collector layer and then returned to the positive electrode of the power supply unit via the dummy cell cassette 30A.

[0045] Incidentally, the impedance of an electrolytic cell can be measured by the AC impedance measurement method. The AC impedance measurement method involves inputting an AC potential (or current) signal into the electrolytic cell, measuring the response current (or potential) at that time, and comparing these inputs and responses to determine the impedance of the electrochemical cell.

[0046] The impedance Z (total impedance) of an electrolytic cell can be expressed as shown in equation (1) below. (1) Z = Rohm + Rη In equation (1) above, Rohm is the resistance component due to the resistive overpotential IR (so-called ohmic loss), and includes electrical resistance due to ion and electron conduction inside the electrolytic cell, as well as contact resistance between materials. The reaction resistance Rη is the resistance component due to the activation overpotential ηact and the concentration overpotential ηcon, and can be expressed as in equation (2). (2)Rη=Rηact.a+Rηact.c+Rηcon In equation (2) above, Rηact.c is the resistance due to the activation overpotential ηact.c applied to the fuel electrode side, and includes the resistance resulting from the delay in the charge transfer process in the electrochemical reaction in the fuel electrode layer 311c. Rηact.a is the resistance due to the activation overpotential ηact.a applied to the air electrode layer 311b, and includes the resistance resulting from the delay in the charge transfer process in the electrochemical reaction in the air electrode layer 311b. Rηcon is the resistance due to the concentration overpotential ηcon, and includes the resistance resulting from the supply of reactants to the reaction field and the dissipation of products (i.e., gas diffusivity). Strictly speaking, Rηcon can be divided into Rηcon.c on the fuel electrode layer 311c side and Rηcon.a on the air electrode layer 311b side, but here they are collectively referred to as Rηcon.

[0047] When Rηact.c and Rηcon (more precisely, Rηcon.c) are large, the voltage applied to the fuel electrode layer increases to advance the electrolytic reaction. This leads to deterioration of the fuel electrode layer, such as the movement of Ni within the fuel electrode layer, and consequently, a decrease in the durability of the electrolytic cell. Therefore, by reducing Rηact.c and Rηcon, it is possible to suppress the deterioration of the fuel electrode layer and thereby improve the durability of the electrolytic cell. Furthermore, when Rηact.c and Rηcon are reduced, the ratio of Rη to impedance Z (η ratio) also decreases. Consequently, by reducing the η ratio, the durability of the electrolytic cell can be improved.

[0048] Based on the above, the fuel electrode layer 311c according to this embodiment will now be described. As described above, the fuel electrode layer 311c comprises a functional layer L1 and a substrate layer L2. A contact interface S exists between the functional layer L1 and the substrate layer L2, as shown in Figure 6. Here, the region within the functional layer L1 that is near the contact interface S is defined as the functional layer-side interface vicinity region NA1, the region within the substrate layer L2 that is near the contact interface S is defined as the substrate layer-side interface vicinity region NA2, and the region formed by combining the functional layer-side interface vicinity region NA1 and the substrate layer-side interface vicinity region NA2 is defined as the interface vicinity region NA.

[0049] The functional layer interface vicinity region NA1 and the substrate layer interface vicinity region NA2 can be predetermined by various methods. The functional layer interface vicinity region NA1 and the substrate layer interface vicinity region NA2 may be layered regions having thickness in a direction perpendicular to the contact interface S. In this case, the thickness of the functional layer interface vicinity region NA1 and the thickness of the substrate layer interface vicinity region NA2 may be different or the same. Generally, the substrate layer L2 is thicker than the functional layer L1, so the thickness of the substrate layer interface vicinity region NA2 may be thicker than the thickness of the functional layer interface vicinity region NA1. In this embodiment, the functional layer interface vicinity region NA1 and the substrate layer interface vicinity region NA2 are determined based on the average diameter r of the pores in the substrate layer L2. Specifically, the substrate layer interface vicinity region NA2 is defined as a layered region between the contact interface S and a line S2 that is separated from the contact interface S by a distance of 5 times the average diameter r in the thickness direction and parallel to the contact interface S. The functional layer interface vicinity region NA1 is defined as a layered region between the contact interface S and a line S1 that is separated from the contact interface S toward the functional layer L1 by a distance of twice the average diameter r in the thickness direction and is parallel to the contact interface S.

[0050] The fuel electrode layer (especially the functional layer) undergoes a reduction treatment after the electrolytic cell is manufactured to reduce the internal Ni. During the reduction treatment of the fuel electrode layer and when the electrolytic cell is operated, Ni in the region of the functional layer near the contact interface B (see Figure 6) with the solid electrolyte layer may move into the interior of the functional layer (towards the substrate layer). The Ni that moves into the interior of the functional layer enters and clogs the pores formed within the functional layer, thus worsening the gas diffusivity within the functional layer. When the gas diffusivity within the functional layer worsens, the concentration overpotential ηcon increases. Furthermore, when the gas diffusivity within the functional layer worsens, the H2 generated in the reaction field within the functional layer during the operation of the electrolytic cell does not diffuse sufficiently, resulting in a locally highly reducing atmosphere in the functional layer. The Ni in the functional layer becomes more mobile when exposed to a highly reducing atmosphere, leading to deterioration of the fuel electrode layer, which in turn reduces the durability of the electrolytic cell.

[0051] In this regard, in order to improve the gas diffusion within the fuel electrode layer L1, in the electrolytic cell 311 according to this embodiment, the porosity φ2 of the region NA2 near the substrate layer interface within the substrate layer L2 is made larger than the porosity φ3 of the region within the substrate layer L2 other than the region NA2 near the substrate layer interface (hereinafter sometimes referred to as the region OA outside the substrate layer interface (see Figure 6)). That is, the porosity within the substrate layer L2 is large in the region close to the contact interface S with the functional layer L1 and small in the region far from the contact interface S. As a result, the gas in the functional layer L1 can easily escape to the substrate layer L2 side through the contact interface S, improving the gas diffusion within the functional layer L1, and thereby reducing the concentration overvoltage ηcon and the resistance component Rηcon due to the concentration overvoltage ηcon. As a result, the η ratio also decreases, improving the durability of the electrolytic cell 311. Furthermore, if the gas diffusivity within the functional layer L1 is improved, the H2 generated by the reaction will be quickly discharged from the reaction field. This will suppress the localization of the atmosphere within the functional layer L1 into a highly reducing atmosphere due to the retention of H2, and thus the durability of the electrolytic cell 311 is expected to improve.

[0052] The porosity φ2 should be greater than the porosity φ3 and at least 20%. If the porosity φ2 is 20% or more, the diffusivity of the gas within the fuel electrode layer L2 can be sufficiently increased. Furthermore, the strength of the substrate layer L2 can be maintained by having a porosity φ3 that is smaller than the porosity φ2. In this case, the porosity φ3 should be smaller than the porosity φ2 and at least 47%.

[0053] Furthermore, when Ni in the substrate layer interface region NA2 moves to the outside, the amount of Ni in the substrate layer interface region NA2 decreases. This reduces the number of conductive paths (electron conduction paths) in the substrate layer interface region NA2, making electrolytic reactions less likely to occur. In other words, the activation overpotential ηact.c in the fuel electrode layer 311c increases.

[0054] In this regard, in the electrolytic cell 311 according to this embodiment, the porosity φ2 of the substrate layer interface vicinity region NA2 is less than 48%. When the porosity φ2 is less than 48%, the remaining half or more of the region will contain cermets of Ni and YSZ. If 50% or more of cermets of Ni and YSZ are present in the substrate layer interface vicinity region NA2, the necessary conductive paths will be secured even if Ni moves to the outside from the substrate layer interface vicinity region NA2 during reduction or sintering. Therefore, the increase in activation overvoltage ηact.c can be suppressed. Thus, the increase in resistance Rηact.c due to activation overvoltage can be suppressed, and as a result, the increase in the η ratio is suppressed, and the deterioration of the durability of the electrolytic cell 311 is suppressed.

[0055] Furthermore, in the electrolytic cell 311 according to this embodiment, the average diameter D of the pores in the functional layer L1 is less than 0.672 μm. When the average diameter D is small, less than 0.672 μm, there are many reaction fields (three-phase interfaces) in the functional layer L1, making electrolytic reactions more likely to occur and reducing the activation overpotential ηact.c. Therefore, the resistance component Rηact.c due to the activation overpotential ηact.c can be reduced, and as a result, the η ratio also becomes smaller, improving the durability of the electrolytic cell 311. The average diameter D is preferably 0.580 μm or less. The average diameter D is more preferably 0.488 μm or less.

[0056] Thus, according to the electrolytic cell 311 of this embodiment, the porosity φ2 of the region NA2 near the substrate layer interface is greater than the porosity φ3 of the region outside the substrate layer interface and is less than 48%, and the average diameter D of the pores formed in the functional layer L1 is less than 0.672 μm. By using an electrolytic cell 311 equipped with a fuel electrode layer 311c with such a structure, the η ratio can be reduced, and thus the durability of the electrolytic cell 311 can be improved.

[0057] Furthermore, in the electrolytic cell 311 according to this embodiment, the ratio H (=φ2 / φ1) of the porosity φ2 of the substrate layer interface vicinity region NA2 to the porosity φ1 of the functional layer interface vicinity region NA1 is less than 1.89. When the ratio H is less than 1.89, the difference in porosity across the contact interface S is small, which can improve the diffusivity of the gas passing through the contact interface S. Therefore, the concentration overvoltage ηcon and the resistance component Rηcon due to the concentration overvoltage ηcon can be further reduced. As a result, the η ratio can be further reduced, and the durability of the electrolytic cell 311 can be further improved.

[0058] (Examples) [Sample preparation] 1. Molding of the green sheet for the functional layer NiO (nickel oxide) powder and Fe2O3 (iron oxide) powder were mixed in a predetermined ratio. Then, a well-known dispersant and ethanol were added in predetermined proportions, and the mixture was stirred for a predetermined time using a ball mill or the like. After that, the liquid mixture was poured into a bowl. The mixture was dried by evaporating the solvent in the bowl and formed into a powder. The powder was calcined at a predetermined calcination temperature. This produced Ni oxide powder containing Fe (iron). It is known that when Fe is contained in the functional layer of the fuel electrode, the movement and aggregation of Ni within the functional layer are suppressed.

[0059] The prepared Ni oxide powder and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin (G-260, manufactured by Sekisui Chemical Co., Ltd.) as a plasticizer, a well-known dispersant, a mixed solvent of toluene and ethanol, and, if necessary, a pore-forming material (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of the functional layer of the fuel electrode layer having a predetermined thickness was formed from the slurry using the doctor blade method.

[0060] 2. Forming of the green sheet for the substrate layer NiO powder (commercially available) and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin as a plasticizer, a well-known dispersant, a mixed solvent of toluene and ethanol, and, if necessary, a pore-forming material (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of the substrate layer for the fuel electrode layer having a predetermined thickness was formed from the slurry using the doctor blade method.

[0061] 3. Molding of the green sheet of the solid electrolyte layer YSZ powder was mixed with butyral resin, polyvinyl acetal resin as a plasticizer, a well-known dispersant, and a mixed solvent of toluene and ethanol in predetermined proportions, and mixed in a ball mill to prepare a slurry. Then, a green sheet of solid electrolyte layer having a predetermined thickness was formed from the slurry using the doctor blade method.

[0062] 4. Molding of the laminate Next, a green sheet of the functional layer was laminated onto one side of the green sheet of the solid electrolyte layer, and then a green sheet of the substrate layer was laminated on top of the green sheet of the functional layer. These laminated green sheets were then pressed together under high pressure using a press machine while heating and vacuuming. This formed a laminate in which the green sheet of the solid electrolyte layer, the green sheet of the functional layer of the fuel electrode layer, and the green sheet of the substrate layer of the fuel electrode layer were laminated in that order.

[0063] 5. Forming of the primary sintered body Subsequently, the laminate formed as described above was degreased at a predetermined temperature (e.g., 200 to 300°C). Then, the laminate was fired at a predetermined temperature (e.g., 1100 to 1350°C) for a predetermined time (e.g., 1 to 5 hours) (primary sintering). This formed a primary sintered body having a solid electrolyte layer and a fuel electrode layer laminated on one side of the solid electrolyte layer.

[0064] 6. Molding of the secondary sintered body Next, a paste containing GDC was screen printed onto the other side of the solid electrolyte layer of the molded primary sintered body, and it was fired at a predetermined temperature (e.g., 1100°C to 1350°C) (secondary sintering). This formed a secondary sintered body in which a reaction prevention layer was formed on the other side of the solid electrolyte layer.

[0065] 7. Preparation of electrolytic cell samples Next, a material containing LSCF was screen printed onto the surface of the reaction prevention layer of the secondary sintered body, and it was fired at a predetermined temperature (e.g., 900 to 1000°C) for a predetermined time (e.g., 1 to 5 hours) (tertiary sintering). This formed a tertiary sintered body. Then, the tertiary sintered body was placed in an electric furnace, and while heated to approximately 700°C in the electric furnace, hydrogen gas was supplied to the fuel electrode layer side to perform a reduction treatment on the fuel electrode layer. The reduction treatment reduced the nickel oxide in the fuel electrode layer to metallic nickel. In this way, a sample of an electrolytic cell was prepared, comprising a solid electrolyte layer, a fuel electrode layer laminated on the solid electrolyte layer so as to be in contact with one side of the solid electrolyte layer, and an air electrode layer laminated on the other side of the solid electrolyte layer via a reaction prevention layer. Furthermore, by changing the calcination temperature of the Ni oxide powder used when forming the green sheet of the substrate layer, the thickness of the functional layer, etc., multiple samples with different porosity φ2 of NA2 in the region near the substrate layer interface were prepared.

[0066] Furthermore, if the calcination temperature of the Ni oxide powder used during the molding of the functional layer's green sheet is low, the particle size of the Ni oxide powder produced will be smaller, resulting in smaller particle sizes of Ni oxide particles within the functional layer. Additionally, when the Ni oxide within the functional layer is reduced to metallic Ni by the reduction treatment, pores form where oxygen was previously present. Therefore, smaller particle sizes of Ni oxide result in smaller pore diameters within the functional layer formed by the reduction treatment. Moreover, pores within the substrate layer interface region NA2 are formed in the locations where Ni that migrated from the substrate layer interface region NA2 existed before the migration. It is thought that Ni that migrated from the substrate layer interface region NA2 to the functional layer side enters the pores within the functional layer. However, if the pore diameter within the functional layer is small, there is insufficient space for Ni to enter, thus restricting its movement. When the movement of Ni within the substrate layer interface region NA2 is restricted in this way, sufficient pores are not formed within the substrate layer interface region NA2. For this reason, the porosity φ2 within the substrate layer interface region NA2 is considered to be small. In other words, when the calcination temperature of the Ni oxide powder used during the molding of the functional layer green sheet is low, the porosity φ2 of NA2 in the region near the substrate layer interface tends to decrease.

[0067] Furthermore, when the functional layer is thin, heat is more easily transferred to the Ni within the functional layer during sintering, thus promoting the sintering and aggregation of Ni. As a result, the particle size of Ni oxide within the functional layer increases. Consequently, the diameter of the pores formed within the functional layer during the reduction process increases, making it easier for Ni to migrate from the substrate layer interface region NA2 to the functional layer side. Therefore, it is thought that the migration of Ni within the substrate layer interface region NA2 is promoted, and the porosity φ2 of the substrate layer interface region NA2 increases. In other words, when the functional layer is thin, the porosity φ2 of the substrate layer interface region NA2 tends to increase.

[0068] [Calculation of the η ratio] Each prepared sample was placed in an electric furnace, and the fuel polar layer and the air polar layer were electrically connected via a frequency response analyzer (FRA). Next, the electric furnace temperature was set to a predetermined temperature (e.g., 700°C), water vapor was supplied to the fuel polar layer side, and air was supplied to the air polar layer side. The impedance spectrum (CCP data) for each sample was measured by acquiring the response signal of each sample while sweeping the frequency and amplitude of the AC using the frequency response analyzer. A Cole-Cole plot was created from the measured CCP data, and the impedances of various components were separated by curve fitting of the created Cole-Cole plot. The change in the real part of the impedance of the separated components was obtained as various resistive components. The various resistive components are the resistive component Rohm due to the resistive overvoltage IR, the resistive component Rηact due to the activation overvoltage ηact, and the resistive component ηcon due to the concentration overvoltage ηcon. Furthermore, Rηact is separated into the resistive component Rηact.c on the fuel polar layer side and the resistive component Rηact.a on the air polar layer side.

[0069] The sum of the various resistance components obtained (Rohm + Rηact + Rηcon) was calculated as the total impedance Z. Furthermore, the sum of the resistance component Rηact due to the obtained activation overvoltage ηact and the resistance component Rηcon due to the concentration overvoltage (Rηact + Rηcon) was calculated as Rη. Finally, the ratio of Rη to the total impedance Z was calculated as the η ratio.

[0070] [Calculation of the average diameter r of pores in the substrate layer] Each sample was cut along its thickness, and the cut surface was imaged using a scanning electron microscope (SEM). Ten pores were selected from the substrate layer shown in the image, and the pore diameter was calculated for each of the ten selected pores using the intercept method. The average diameter r of the pores in the substrate layer of the sample corresponding to the image was calculated by taking the arithmetic mean of the pore diameters calculated for each pore. The average diameter r was calculated for each sample in the same manner as described above.

[0071] [Calculation of porosity φ1 of NA1 in the region near the functional layer interface] Figure 7 shows a portion of the images of the cross-section of the sample. Figure 7 shows the cross-sections of the substrate layer and the functional layer of the fuel electrode. As shown in Figure 7, a reference line SL is drawn on the contact interface between the functional layer and the substrate layer of the fuel electrode, as shown in the images taken for each sample. The contact interface between the functional layer and the substrate layer can be determined from the difference in porosity of the two layers. In Figure 7, the functional layer is above the reference line SL and the substrate layer is below it. Next, a first line SL1 is drawn parallel to the reference line SL, at a distance of twice the average diameter r of the pores in the substrate layer (2r) from the reference line SL in the thickness direction (perpendicular to the reference line SL) toward the functional layer. The region between the reference line SL and the first line SL1 is the functional layer side interface vicinity region NA1.

[0072] Next, two lines A1 and A2 are drawn within the functional layer interface vicinity region NA1, parallel to the reference line SL and the first line SL1, at an average diameter r interval. Specifically, line A1 is drawn parallel to the reference line SL at a distance of half the average diameter r toward the functional layer from the reference line SL, and line A2 is drawn parallel to line A1 at a distance of the average diameter r away from line A1 relative to line A1.

[0073] For each of lines A1 and A2, the porosity of each line was calculated as the ratio (percentage) of the total length of the porous portion (the black portion in Figure 7) to the total length of the non-porous portion (the portion other than the black portion in Figure 7) on each line. The average of the calculated porosity of each line was then calculated as the porosity φ1 of the functional layer interface vicinity region NA1.

[0074] [Calculation of porosity φ2 of NA2 in the vicinity of the substrate layer interface] On the image of the sample's cross-section, a second line SL2 is drawn parallel to the reference line SL, at a distance of 5 times the average diameter r (5r) toward the substrate layer in the thickness direction (perpendicular to the reference line SL) from the reference line SL drawn as described above. The region between the reference line SL and the second line SL2 is the substrate layer side interface vicinity region NA2.

[0075] Next, five lines B1, B2, B3, B4, and B5 are drawn within the substrate layer interface region NA2, parallel to the reference line SL and the second line SL2, at an average diameter interval of r. Specifically, line B1 is drawn parallel to the reference line SL at a distance of half the average diameter r from the reference line SL towards the substrate layer. Then, lines B2, B3, B4, and B5 are drawn in order parallel to line B1, at an average diameter interval of r, in a direction away from the reference line SL relative to line B1.

[0076] For each of lines B1 to B5, the porosity of each line was calculated as the ratio (percentage) of the total length of the porous portion to the total length of the non-porous portion on that line. The average of the calculated porosity of each line was then used to determine the porosity φ2 within the NA2 region near the substrate layer interface.

[0077] [Calculation of porosity φ3 in the region outside the substrate layer interface] As shown in Figure 7, line C1 is drawn parallel to the reference line SL in the region of the substrate layer other than the region NA2 near the substrate layer interface (region OA outside the substrate layer interface). In this example, the thickness of the substrate layer is greater than 10 times the average diameter r, so the midpoint of the substrate layer thickness is in the region outside the substrate layer interface. Therefore, in this example, line C1 is drawn parallel to the reference line SL at the midpoint in the thickness direction of the substrate layer as shown in the cross-sectional image of each sample. Next, line C2 is drawn parallel to line C1 at a position away from the reference line SL by the average diameter r from line C1, and then line C3 is drawn parallel to line C2 at a position away from the reference line SL by the average diameter r from line C2.

[0078] For each of lines C1 to C3, the porosity of each line was calculated as the ratio (percentage) of the total length of the porous portion to the total length of the non-porous portion on each line. The average of the calculated porosity for each line was then calculated as the porosity φ3 of the region outside the substrate layer interface.

[0079] [Calculation of ratio H] For each sample, the ratio H of porosity φ2 to porosity φ1 was calculated by dividing porosity φ2 by porosity φ1.

[0080] [Calculation of the average diameter D of pores within the functional layer] Each sample was cut along its thickness, and the cut surface was imaged using a scanning electron microscope (SEM). Ten pores were selected from the functional layer shown in the image, and the pore diameter was calculated for each of the ten selected pores using the intercept method. The average pore diameter D of the pores in the functional layer of the sample corresponding to the image was calculated by taking the arithmetic mean of the pore diameters calculated for each pore. The average diameter D was calculated for each sample in the same manner as described above.

[0081] [Measurement of durability degradation rate] A voltage was applied between the fuel electrode and air electrode of each sample so that a constant current flowed through each sample. Then, a durability degradation test was conducted in which a constant flow rate of water vapor was supplied to the fuel electrode side while a constant current was continuously applied for a predetermined time, thereby continuously performing a water vapor electrolysis reaction. In the durability degradation test, when a constant current is continuously applied, the applied voltage increases over time due to the increase in resistance within the sample. The difference ΔV (=V1-V0) between the initially applied voltage (V0) and the voltage applied after a predetermined time (e.g., 400 hours) (V1) was calculated, and this calculated value was converted into the difference between the voltage that would be applied after 1000 hours and the initially applied voltage. The percentage obtained by dividing this converted value by the initially applied voltage was calculated as the durability degradation rate. A smaller durability degradation rate indicates less voltage change and higher durability.

[0082] Table 1 shows the calculated or measured values ​​for each sample (porosity φ1, porosity φ2, porosity φ3, ratio H, average diameter D, Rohm, Rη, η ratio, and durability degradation rate). [Table 1] In Table 1, an η ratio of less than 70% was evaluated as a small η ratio (○), and an η ratio of 70% or more was evaluated as a large η ratio (×). Furthermore, a durability degradation rate of less than 5% was evaluated as good durability (○), a durability degradation rate of 5% or more but less than 10% was evaluated as average durability (△), and a durability degradation rate of 10% or more was evaluated as poor durability (×).

[0083] As shown in Table 1, sample P5 exhibits a high η ratio and poor durability. This is thought to be because the porosity φ2 of the region NA2 near the substrate layer interface is smaller than the porosity φ3 of the region OA outside the substrate layer interface. In the preparation of sample P5, the calcination temperature of the Ni oxide powder used when forming the green sheet of the functional layer was low. Therefore, it is thought that Ni could not move sufficiently from the region NA2 near the substrate layer interface to the functional layer side during the reduction treatment, and thus the porosity φ2 of the region NA near the substrate layer interface became small. Consequently, the porosity φ2 of sample P5 became smaller than the porosity φ3, which worsened gas diffusivity and increased the η ratio, resulting in poor durability.

[0084] Furthermore, sample P4 also exhibits a high η ratio and poor durability. This is thought to be because, although the porosity φ2 is greater than the porosity φ3, the porosity φ2 is high at 48%. In addition, in sample P4, the calcination temperature of the Ni oxide powder used during the molding of the green sheet of the functional layer was low. Therefore, the Ni oxide particles in the functional layer should be small, but the thickness of the functional layer in sample P4 is about half that of the other samples. As a result, heat is easily transferred to the Ni in the functional layer during sintering, which promotes sintering and leads to Ni aggregation, resulting in a larger particle size of Ni oxide. Therefore, it is thought that the amount of Ni moving from the substrate layer interface region NA2 to the functional layer side increased during the reduction treatment, and as a result, the porosity φ2 of the substrate layer interface region NA2 increased.

[0085] Furthermore, sample P3 also exhibits a high η ratio and poor durability. This is thought to be because the porosity φ2 is greater than the porosity φ3, and although the porosity φ2 is less than 48%, the average pore diameter D within the functional layer is large at 0.672 μm. In addition, in sample P3, the calcination temperature of the Ni oxide powder used during the molding of the green sheet of the functional layer was high. Therefore, it is thought that the particle size of the Ni oxide particles within the functional layer increased, and the average pore diameter D within the functional layer formed during the reduction treatment increased.

[0086] In contrast, samples P1 and P2 exhibited a low η ratio and above-average durability. This is likely because the porosity φ2 was greater than that of φ3, the porosity φ2 was less than 48%, and the average pore diameter D within the functional layer was less than 0.672 μm. Furthermore, in samples P1 and P2, the calcination temperature of the Ni oxide powder used during the molding of the green sheet of the functional layer was appropriate. Therefore, the average pore diameter D within the functional layer was less than 0.672 μm. Additionally, the porosity φ2 was slightly larger than that of φ3 because Ni in the substrate layer interface region NA2 moved appropriately towards the functional layer. Finally, the porosity φ2 was less than 48% because Ni did not move excessively from the substrate layer interface region NA2 towards the functional layer.

[0087] Furthermore, sample P1 exhibits a low η ratio and good durability. This is thought to be because the porosity φ2 is greater than the porosity φ3, the porosity φ2 is less than 48%, the average pore diameter D in the functional layer is less than 0.672 μm, and the ratio H (=φ2 / φ1) is small, less than 1.89 (1.67). On the other hand, the ratio H of sample P2 is large at 2.17, which worsens the diffusivity of gas passing through the contact interface between the functional layer and the substrate layer. For this reason, the η ratio of sample P2 is considered to be larger than that of sample P1. Note that the porosity φ1 of sample P2 is 20%, while the porosity φ1 of sample P1 is 26%. In other words, the functional layer of sample P2 was formed more densely than the functional layer of sample P1. Meanwhile, the porosity φ2 (=43%) of sample P1 and the porosity φ2 (=44%) of sample P2 are not significantly different. In other words, it is thought that the functional layer of sample P2 was formed more densely than the functional layer of sample P1, resulting in a smaller ratio H for sample P1 than for sample P2.

[0088] While embodiments of this disclosure have been described above, this disclosure is not limited to the above embodiments. For example, in the above embodiments, an example was described in which the ion-conducting oxide in the functional layer is YSZ, but the ion-conducting oxide in the functional layer may be something other than YSZ, for example, GDC (gadolinium-doped ceria). Also, the electrochemical cell according to this disclosure is applicable to electrolytic cells (SOEC), fuel cell cells (SOFC), and r-SOC. Furthermore, in the above embodiments, a hydrogen production apparatus as an electrolytic reactor was disclosed, but the electrolytic reactor may be an apparatus other than a hydrogen production apparatus, for example, a co-electrolytic apparatus for water vapor and carbon dioxide. Also, in the above embodiments, an example was shown in which a vaporizer 2 is included in the hot module 6, but a hot module without a vaporizer 2 is also possible. In addition, in the electrochemical cell stack 1 shown in Figures 2 and 3, the number of stacked cell cassettes 30 is 4, including the dummy cell cassette, but a larger number of cell cassettes, for example 20 to 30 cell cassettes, may be stacked. Thus, this disclosure is modifiable as long as it does not deviate from its spirit.

[0089] Furthermore, this disclosure may include the following aspects: [1] An electrochemical cell comprising a solid electrolyte, a fuel electrode in which pores are formed internally, and an air electrode, The fuel electrode has a functional layer that contacts the solid electrolyte and a substrate layer that contacts the side of the functional layer opposite to the side that contacts the solid electrolyte. The porosity of the region near the substrate layer interface, which is the region within the substrate layer that is close to the contact interface between the substrate layer and the functional layer, is greater than the porosity of the region within the substrate layer other than the region near the substrate layer interface, and is less than 48%. The average diameter of the pores formed within the functional layer is less than 0.672 μm. Electrochemical cell. [2] [1] The electrochemical cell described above, The ratio of the porosity of the region near the interface on the substrate side to the porosity of the region near the interface on the functional side, which is the region near the contact interface within the functional layer, is less than 1.89. Electrochemical cell. [3] An electrochemical cell stack comprising the electrochemical cells described in [1] or [2] stacked on top of each other. [4] [3] The electrochemical cell stack described above, A heating device for heating the gas supplied to the electrochemical cell stack, The electrochemical cell stack and the heating device are insulated from an insulating material placed inside them. Equipped with, Hot module. [5] An electrolytic reactor equipped with the hot module described in [4]. [Explanation of symbols]

[0090] 1... Electrochemical cell stack, 2... Vaporizer, 3... Heating device, 4... Insulation material, 6... Hot module, 7... Condenser, 10... Upper end plate, 20... Upper insulating plate, 30... Cell cassette, 31... Electrochemical cell unit, 311... Electrolytic cell (electrochemical cell), 311a... Solid electrolyte layer (solid electrolyte), 311b... Air electrode layer (air electrode), 311c... Fuel electrode layer (fuel electrode), 311d... Reaction prevention layer, 312... Fuel electrode current collector, 313... In Terminal connector, 32...Separator section, 321...Cell-side separator, 322...Interconnector-side separator, 33...Frame section, 331...Fuel electrode frame, 332...Air electrode frame, 40...Terminal plate, 50...Lower insulating plate, 60...Lower end plate, 100...Hydrogen production equipment, NA...Near-interface region, NA1...Near-interface region on the functional layer side, NA2...Near-interface region on the substrate layer side, OA...Outer region near the interface on the substrate layer side, S...Contact interface

Claims

1. An electrochemical cell comprising a solid electrolyte, a fuel electrode in which pores are formed internally, and an air electrode, The fuel electrode has a functional layer that contacts the solid electrolyte, and a substrate layer that contacts the side of the functional layer opposite to the side that contacts the solid electrolyte, and has a higher porosity than the functional layer. The porosity of the region near the substrate layer interface, which is a layered region within the substrate layer between the contact interface between the substrate layer and the functional layer and a line parallel to the contact interface and located in the thickness direction at a distance five times the average diameter of the pores in the substrate layer, is greater than the porosity of the region within the substrate layer other than the region near the substrate layer interface, and is 44% or less. The average diameter of the pores formed within the functional layer is 0.488 μm or less. Electrochemical cell.

2. An electrochemical cell according to claim 1, The ratio of the porosity of the region near the substrate layer interface to the porosity of the region near the functional layer interface, which is a layered region within the functional layer between the contact interface and a line parallel to the contact interface and separated in the thickness direction by a distance of twice the average diameter of the pores in the substrate layer from the contact interface toward the functional layer, is greater than 1 and less than 1.

89. Electrochemical cell.

3. An electrochemical cell stack comprising an electrochemical cell unit, each unit comprising an electrochemical cell according to claim 1 or 2, and an interconnector disposed on the opposite side of the substrate layer from the functional layer, stacked together.

4. The electrochemical cell stack according to claim 3, A heating device for heating the gas supplied to the electrochemical cell stack, The electrochemical cell stack and the heating device are insulated from an insulating material placed inside them. Equipped with, Hot module.

5. An electrolytic reaction apparatus comprising the hot module described in claim 4.

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