Electrochemical reaction unit and electrochemical reaction cell stack

Abstract

To suppress Cr diffusion from a surface of a current collector in an electrochemical reaction unit.SOLUTION: An electrochemical reaction unit includes a single cell, a conductive current collector, a conductive coat, and a conductive bonding layer. The single cell includes an air electrode, an electrolyte layer including a solid oxide and a fuel electrode which are layered in this order. The current collector contains Cr, and is disposed on the electrode side of the single cell. The coat covers a surface of the current collector. The bonding layer bonds the air electrode and the coat. A porosity near the interface between the bonding layer and the air electrode is higher than an internal porosity in the bonding layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology disclosed herein relates to an electrochemical reaction unit and an electrochemical reaction cell stack. [Background technology]

[0002] Solid oxide fuel cells (hereinafter referred to as "SOFCs") are known as one type of fuel cell that generates electricity using the electrochemical reaction between hydrogen and oxygen. A fuel cell power generation unit (hereinafter simply referred to as "power generation unit"), which is a constituent element of an SOFC, comprises a single cell and a conductive current collector. The single cell is composed of an air electrode, an electrolyte layer containing solid oxide, and a fuel electrode stacked in this order. The current collector is located on the air electrode side of the single cell.

[0003] The current collector contains Cr. When such a current collector is exposed to a high-temperature atmosphere during operation of the SOFC, Cr may diffuse from the surface of the current collector. If Cr diffused from the current collector adheres to the surface of the air electrode, a phenomenon known as "Cr poisoning of the air electrode" may occur, in which the electrode reaction rate at the air electrode decreases. Therefore, the power generation unit may further include a conductive coating covering the surface of the current collector and a conductive bonding layer bonding the coating to the air electrode, thereby suppressing Cr poisoning (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6268209 Summary of the Invention [Problem to be solved by the invention]

[0005] In fuel cell power generation units, cracks in the bonding layer can also cause cracks in the coating, and these cracks can cause Cr to diffuse from the surface of the current collector exposed through the coating.

[0006] These issues are also common to electrolysis cell units, which are constituent units of electrolysis cells (hereinafter referred to as "SOECs") that generate hydrogen by electrolysis of water. In this specification, a fuel cell unit and an electrolysis cell are collectively referred to as an electrochemical reaction unit, a fuel cell power generation unit and an electrolysis cell unit are collectively referred to as an electrochemical reaction unit, and a fuel cell stack and an electrolysis cell stack are collectively referred to as an electrochemical reaction cell stack.

[0007] This specification discloses a technique that can solve the above-mentioned problems. [Means for solving the problem]

[0008] The technology disclosed in this specification can be realized, for example, in the following forms.

[0009] (1) The electrochemical reaction unit disclosed in this specification comprises a single cell, a conductive current collector, a conductive coating, and a conductive bonding layer. The single cell comprises an air electrode, an electrolyte layer containing a solid oxide, and a fuel electrode stacked in this order. The current collector contains Cr and is disposed on the air electrode side of the single cell. The coating covers the surface of the current collector. The bonding layer bonds the air electrode and the coating. The porosity of the bonding layer near the interface with the air electrode is greater than the porosity of the interior of the bonding layer.

[0010] In this electrochemical reaction unit, the porosity of the bonding layer near the interface with the air electrode is greater than the porosity inside the bonding layer, so that stress applied to the bonding layer selectively peels the interface between the bonding layer and the air electrode, releasing the stress. This prevents cracks in the bonding layer from propagating into the coating and inhibits Cr diffusion from the surface of the current collector.

[0011] (2) In the electrochemical reaction unit described in (1) above, the difference in porosity of the bonding layer near the interface with the air electrode and the porosity of the interior of the bonding layer may be 5% or more and 60% or less. With this configuration, the difference in porosity of the bonding layer near the interface with the air electrode and the porosity of the interior of the bonding layer is 5% or more and 60% or less, which allows for more effective stress relief at the interface between the bonding layer and the air electrode and maintains good electrical connection between the air electrode and the bonding layer.

[0012] (3) In the electrochemical reaction unit described in (1) above, when the electrochemical reaction unit is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the porosity of the bonding layer near the interface with the air electrode after the heat treatment may be greater than the porosity of the bonding layer near the interface with the air electrode before the heat treatment. With this configuration, even if stress on the bonding layer increases due to sintering of the bonding layer during operation of the electrochemical reaction unit, stress is more effectively relieved at the interface between the bonding layer and the air electrode. This prevents cracks in the bonding layer from progressing into the coating, and more effectively suppresses Cr diffusion from the surface of the current collector.

[0013] (4) In the electrochemical reaction unit described in (1) above, when the electrochemical reaction unit is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the rate of increase in porosity of the bonding layer near the interface with the air electrode due to the heat treatment may be greater than the rate of increase in porosity of the bonding layer inside the bonding layer due to the heat treatment. According to this configuration, even if stress applied to the bonding layer increases due to sintering of the bonding layer during operation of the electrochemical reaction unit, stress is more effectively relieved at the interface between the bonding layer and the air electrode. This prevents cracks in the bonding layer from progressing into the coating, and more effectively suppresses Cr diffusion from the surface of the current collector.

[0014] (5) The electrochemical reaction cell stack disclosed in this specification includes a plurality of electrochemical reaction units, at least one of which is the electrochemical reaction unit described in any one of (1) to (4) above. According to this electrochemical reaction cell stack, the porosity of the bonding layer of the electrochemical reaction unit near the interface with the air electrode is greater than the porosity of the interior of the bonding layer. Therefore, stress applied to the bonding layer selectively peels the interface between the bonding layer and the air electrode, thereby releasing the stress. This prevents cracks in the bonding layer from progressing into the coating, and inhibits Cr diffusion from the surface of the current collector.

[0015] The technology disclosed in this specification can be realized in various forms, such as an electrochemical reaction unit, an electrochemical reaction cell stack including an electrochemical reaction unit, and a manufacturing method thereof. [Brief explanation of the drawings]

[0016] [Figure 1] A perspective view showing the appearance of a fuel cell stack. [Figure 2] FIG. 2 is an explanatory diagram showing an XZ cross section of the fuel cell stack taken along the line II-II in FIG. 1; [Figure 3] FIG. 2 is an explanatory diagram showing an XZ cross section of the fuel cell stack taken along the line III-III in FIG. 1; [Figure 4] FIG. 3 is an explanatory diagram showing an XZ cross section of two adjacent power generating units at the same position as the cross section shown in FIG. 2. [Figure 5] FIG. 4 is an explanatory diagram showing an XZ cross section of two adjacent power generating units at the same position as the cross section shown in FIG. 3. [Figure 6] An explanatory diagram showing an enlarged view of the X1 portion of FIG. 5. DETAILED DESCRIPTION OF THE INVENTION

[0017] A. Implementation: (Configuration of fuel cell stack 10) FIG. 1 is a perspective view showing the exterior of a fuel cell stack 10. FIG. 2 is an explanatory diagram showing an XZ cross section of the fuel cell stack 10 taken along line II-II in FIG. 1 . FIG. 3 is an explanatory diagram showing an XZ cross section of the fuel cell stack 10 taken along line III-III in FIG. 1 . Each figure shows mutually orthogonal X, Y, and Z axes for identifying directions. For convenience, the Z-axis direction is referred to as the up-down direction, the positive Z-axis direction as the up-down direction, and the negative Z-axis direction as the down-down direction in this specification. However, the fuel cell stack 10 may actually be installed in a different orientation. Furthermore, the "thickness" of each component in this specification refers to the length of each component in the up-down direction unless otherwise specified. Furthermore, the up-down direction may also be referred to as the "thickness direction." The fuel cell stack 10 is an example of an electrochemical reaction cell stack.

[0018] 1 to 3, the fuel cell stack 10 includes a power generation block 100, an end separator 230, a first plate 232, a second plate 260, a first terminal plate 240, a second terminal plate 250, an insulating section 220, a first end plate 210, a second end plate 270, and four gas passage members 280. The first end plate 210, the insulating section 220, the end separator 230, the first terminal plate 240, the power generation block 100, the second terminal plate 250, the second plate 260, and the second end plate 270 have rectangular outer shapes of approximately the same size and are arranged in a stacked manner in this order in a predetermined arrangement direction (vertical direction).

[0019] As shown in FIG. 1, the fuel cell stack 10 has bolt holes BH near each of the four corners, penetrating from the first end plate 210 to the second end plate 270. A bolt B is inserted into each bolt hole BH. A nut N is screwed onto both ends of each bolt B. These bolts B and nuts N fasten the members from the first end plate 210 to the second end plate 270 together. As shown in FIGS. 2 and 3, the first plate 232 is supported by the terminal separator 230. Four gas passage members 280 are connected to the second end plate 270.

[0020] As shown in FIGS. 2 and 3, the power generation block 100 is made up of a plurality of (seven in this embodiment) power generation units 100U arranged side by side in a predetermined arrangement direction (vertical direction).

[0021] The first end plate 210 is a member formed by pressing (bending) a single plate-like member, and is formed of a conductive material such as stainless steel. As shown in FIGS. 1 to 3 , the first end plate 210 includes a rectangular, frame-like flat portion 211 having a through-hole 212 near the center, and an outer convex portion 213 and an inner convex portion 214 that protrude from the flat portion 211 in the direction opposite the insulating portion 220 (upward in FIG. 2 ). The flat portion 211 has holes that form the bolt holes BH described above. The outer convex portion 213 protrudes from the outer periphery of the flat portion 211. The outer convex portion 213 is formed around the entire outer periphery of the flat portion 211. The inner convex portion 214 protrudes from the inner periphery of the flat portion 211. The inner convex portion 214 is formed around the entire inner periphery of the flat portion 211.

[0022] Insulating section 220 is a rectangular frame-shaped member with a through-hole near the center, and is made of, for example, an insulating material. As shown in Figures 2 and 3, insulating section 220 is sandwiched between first end plate 210 and end separator 230, thereby ensuring insulation between first end plate 210 and end separator 230.

[0023] As shown in FIGS. 2 and 3, the terminal separator 230 is a rectangular frame-shaped member having a through-hole 231 near the center, and is made of, for example, metal.

[0024] The first plate 232 is a rectangular, flat member made of a conductive material such as stainless steel. As shown in Figures 2 and 3, the first plate 232 is joined by welding, for example, to the periphery of the through-hole 231 in the terminal separator 230. The terminal separator 230 and the first plate 232 separate the power generating block 100 from the external space of the fuel cell stack 10.

[0025] The first plate 232 is connected to an interconnector 190 (described later) provided in a power generation unit 100U arranged at one end (the upper end in Figure 2) of the multiple power generation units 100U that make up the power generation block 100, via a connecting member having the same structure as the anode current collecting member 144 (described later), thereby electrically connecting the power generation unit 100U and the first plate 232.

[0026] The first terminal plate 240 is a rectangular frame-shaped member having a through-hole 241 near the center, and is made of a conductive material such as ferritic stainless steel that forms an alumina oxide coating on its surface. The first terminal plate 240 is electrically connected to the power generating unit 100U located at one end (the upper end in FIG. 2) of the multiple power generating units 100U that make up the power generating block 100, via a first plate 232 and a terminal separator 230. One end (the right end in FIG. 2) of the first terminal plate 240 protrudes laterally from the power generating block 100, and this protruding portion functions as the positive output terminal of the fuel cell stack 10.

[0027] The second terminal plate 250 is a rectangular plate-shaped member made of a conductive material such as ferritic stainless steel that has an alumina oxide coating on its surface. The second terminal plate 250 is electrically connected to the power generating unit 100U that is located at the other end (the lower end in FIG. 2) of the multiple power generating units 100U that make up the power generating block 100. One end (the right end in FIG. 2) of the second terminal plate 250 protrudes laterally from the power generating block 100, and this protruding portion functions as the negative output terminal of the fuel cell stack 10.

[0028] Second plate 260 is a rectangular, flat member made of, for example, an insulating material. The peripheral edge of second plate 260 is sandwiched between second terminal plate 250 and second end plate 270, thereby ensuring insulation between second terminal plate 250 and second end plate 270.

[0029] The second end plate 270 is a member formed by pressing (bending) a single plate-like member, and is formed of a conductive material such as stainless steel. The second end plate 270 has a rectangular, frame-like flat portion 271 having a through-hole 272 near the center, and an outer convex portion 273 and an inner convex portion 274 that protrude from the flat portion 271 in the direction opposite to the second terminal plate 250 (downward in FIG. 2). The flat portion 271 has holes that form the bolt holes BH described above. The outer convex portion 273 protrudes from the outer periphery of the flat portion 271. The outer convex portion 273 is formed around the entire outer periphery of the flat portion 271. The inner convex portion 274 protrudes from the inner periphery of the flat portion 271. The inner convex portion 274 is formed around the entire inner periphery of the flat portion 271.

[0030] 1 to 3, the fuel cell stack 10 has four holes that penetrate from the power generation block 100 to the second end plate 270. The four holes are an oxidant gas supply manifold 311, an oxidant gas discharge manifold 312, a fuel gas supply manifold 321, and a fuel gas discharge manifold 322, respectively.

[0031] 2, the oxidant gas supply manifold 311 is a gas flow path that supplies the oxidant gas OG introduced from outside the fuel cell stack 10 to an air chamber 313 (described later) of each power generating unit 100U. The oxidant gas discharge manifold 312 is a gas flow path that discharges the oxidant off-gas OOG discharged from the air chamber 313 of each power generating unit 100U to the outside of the fuel cell stack 10. For example, air is used as the oxidant gas OG. The oxidant gas supply manifold 311 and the oxidant gas discharge manifold 312 are arranged on opposite sides of the air chamber 313.

[0032] As shown in Fig. 3, the fuel gas supply manifold 321 is a gas flow path that supplies fuel gas FG introduced from outside the fuel cell stack 10 to a fuel chamber 323 (described later) of each power generating unit 100U. The fuel gas discharge manifold 322 is a gas flow path that discharges fuel off-gas FOG discharged from the fuel chamber 323 of each power generating unit 100U to the outside of the fuel cell stack 10. For example, hydrogen-rich gas obtained by reforming city gas is used as the fuel gas FG. The fuel gas supply manifold 321 and the fuel gas discharge manifold 322 are arranged on opposite sides of the fuel chamber 323.

[0033] As shown in FIGS. 1 to 3 , each of the four gas passage members 280 includes a main body portion 281 and a flange portion 282. The main body portion 281 has a gas through hole 283 formed therethrough in the vertical direction. The flange portion 282 is provided so as to protrude outward from the other end (the lower end in FIGS. 2 and 3 ) of the main body portion 281. The flange portion 282 has a plurality of bolt holes 284. A bolt (not shown) for connecting the fuel cell stack 10 to an external device is inserted into each bolt hole 284. One end (the upper end in FIGS. 2 and 3 ) of the main body portion 281 included in each of the four gas passage members 280 is joined to the second end plate 270 by, for example, welding, and the gas through hole 283 is connected to the manifolds 311, 312, 321, and 322, respectively. A gas pipe (not shown) for supplying or discharging gas is connected to each of the main body portions 281.

[0034] FIG. 4 is an explanatory diagram showing an XZ cross section of two adjacent power generating units 100U at the same position as the cross section shown in FIG. 2. FIG. 5 is an explanatory diagram showing an XZ cross section of two adjacent power generating units 100U at the same position as the cross section shown in FIG. 3. As shown in FIGS. 4 and 5, the power generating unit 100U includes a single cell 110, a single cell separator 120, an air electrode frame 130, an anode frame 140, an anode current collecting member 144, two interconnectors 190, and two IC separators 180. One IC separator 180, the air electrode frame 130, the single cell separator 120, the anode frame 140, and the other IC separator 180 are stacked in this order. The power generating unit 100U is an example of an electrochemical reaction unit.

[0035] The unit cell 110 includes an electrolyte layer 112, an air electrode 114, an anode 116, and a reaction prevention layer 118. As shown in Figures 4 and 5, the air electrode 114, the reaction prevention layer 118, the electrolyte layer 112, and the anode 116 are arranged in this order. The unit cell 110 of this embodiment is an anode-supported unit cell in which the anode 116 supports the other layers (electrolyte layer 112, air electrode 114, and reaction prevention layer 118) that make up the unit cell 110. The unit cell 110 is supported by a unit cell separator 120.

[0036] The electrolyte layer 112 is a rectangular, flat member having one surface (the upper surface in FIGS. 4 and 5 ) on which the air electrode 114 is disposed and another surface (the lower surface in FIGS. 4 and 5 ) parallel to the first surface on which the anode 116 is disposed. The electrolyte layer 112 is a layer containing a solid oxide (e.g., YSZ (yttria-stabilized zirconia)). The cathode 114 is a layer having a rectangular shape smaller than that of the electrolyte layer 112 and containing, for example, a perovskite-type oxide (e.g., LSCF (lanthanum strontium cobalt iron oxide)). The anode 116 is a layer having a rectangular shape and approximately the same size as the electrolyte layer 112 and containing, for example, Ni (nickel), a cermet made of Ni and ceramic particles, a Ni-based alloy, or the like. The reaction prevention layer 118 is a layer having a rectangular shape and approximately the same size as the air electrode 114 and containing, for example, GDC (gadolinium-doped ceria). The reaction prevention layer 118 has the function of preventing elements (e.g., Sr) diffused from the air electrode 114 from reacting with elements (e.g., Zr) contained in the electrolyte layer 112 to produce a highly resistive substance (e.g., SrZrO3).

[0037] 4 and 5, the single cell separator 120 is a rectangular frame-like member having a substantially rectangular through-hole 121 near the center, and is made of, for example, metal. The peripheral edge of the through-hole 121 in the single cell separator 120 is joined to the peripheral edge of one surface of the electrolyte layer 112 (the surface on which the air electrode 114 is disposed: the upper surface in FIGS. 4 and 5) by a joint 124. The joint 124 is made of, for example, a brazing material (Ag brazing).

[0038] 4 and 5, the air electrode frame 130 is a rectangular frame-like member having a substantially rectangular through-hole 131 near the center, and is made of, for example, mica. As shown in Fig. 4, the air electrode frame 130 has an oxidant gas supply communicating channel 132 that connects the oxidant gas supply manifold 311 and the air chamber 313, and an oxidant gas discharge communicating channel 133 that connects the air chamber 313 and the oxidant gas discharge manifold 312.

[0039] 4 and 5, the anode frame 140 is a rectangular frame-like member having a substantially rectangular through-hole 141 near the center, and is made of, for example, metal. As shown in Fig. 5, the anode frame 140 has a fuel gas supply communication channel 142 that connects the fuel gas supply manifold 321 and the fuel chamber 323, and a fuel gas discharge communication channel 143 that connects the fuel chamber 323 and the fuel gas discharge manifold 322.

[0040] As shown in FIGS. 4 and 5, IC separator 180 is a rectangular frame-shaped member having a through-hole 181 near the center, and is made of, for example, metal.

[0041] As shown in FIGS. 4 and 5 , the interconnector 190 includes a rectangular flat plate portion 191, a plurality of plate-shaped air electrode current collectors 192 protruding from one surface of the flat plate portion 191 toward the air electrode 114, and a coating layer 193. The flat plate portion 191 and the air electrode current collector 192 are formed of an alloy containing Fe and Cr (e.g., ferritic stainless steel) and are electrically conductive. The flat plate portion 191 and the air electrode current collector 192 are disposed on the air electrode 114 side of the single cell 110. In this embodiment, an oxide coating 192OM is formed on the surface of the air electrode current collector 192 (see FIG. 6 ). The oxide coating 192OM is a coating containing Cr oxide. The coating layer 193 is electrically conductive and is disposed so as to cover the surface of the air electrode current collector 192 and the surface of the flat plate portion 191 on which the air electrode current collector 192 is disposed. The flat plate portion 191 is joined to the periphery of the through hole 181 in the IC separator 180 by, for example, welding. The interconnector 190 is supported by the IC separator 180. The air electrode current collecting portion 192 is an example of a current collecting portion. The coating layer 193 is an example of a coat.

[0042] The anode current collecting member 144 is a member that connects the interconnector 190 and the anode 116. The anode current collecting member 144 is formed of a conductive material such as nickel, a nickel alloy, or stainless steel. As shown in FIGS. 4 and 5 , the anode current collecting member 144 has an interconnector facing portion 146, an electrode facing portion 145 that is parallel to the interconnector facing portion 146, and a connecting portion 147 that connects the electrode facing portion 145 and the interconnector facing portion 146, and has an overall U-shape. The electrode facing portion 145 is in contact with the anode 116, and the interconnector facing portion 146 is in contact with a flat portion 191 of the interconnector 190.

[0043] As shown in FIGS. 4 and 5, the interconnector 190 is shared by two adjacent power generating units 100U. More specifically, as shown in FIGS. 4 and 5, the air electrode current collecting portion 192 is joined to the air electrode 114 of the unit cell 110 included in one of the two adjacent power generating units 100U via a conductive bonding material 196 formed, for example, of a spinel-type oxide, which bonds the air electrode 114 and the coating layer 193. This electrically connects the air electrode current collecting portion 192 to the air electrode 114. The flat plate portion 191 is electrically connected to the anode 116 of the unit cell 110 included in the other of the two adjacent power generating units 100U via an anode current collecting member 144. This ensures electrical continuity between the two adjacent power generating units 100U. The conductive bonding material 196 is an example of a bonding layer.

[0044] However, as shown in Figure 2, the power generating unit 100U located at the other end (the lower end in Figure 2) of the multiple power generating units 100U does not have an interconnector 190 on the fuel electrode 116 side. The fuel electrode 116 included in this power generating unit 100U is connected to the second terminal plate 250 via the fuel electrode current collecting member 144.

[0045] A spacer 149 made of, for example, mica is disposed between the electrode facing portion 145 and the interconnector facing portion 146. This allows the anode current collecting member 144 to follow deformation of the power generating unit 100U due to temperature cycles and reactant gas pressure fluctuations, and good electrical connection is maintained between the anode 116 and the interconnector 190 (or second terminal plate 250) via the anode current collecting member 144.

[0046] 4 and 5, the space partitioned by the single cell separator 120, single cell 110, air electrode frame 130, IC separator 180, and interconnector 190 faces the air electrode 114 and serves as an air chamber 313 through which oxidant gas OG flows. The air electrode frame 130 partitions the entire periphery of the air chamber 313 from the external space and seals the gap between the single cell separator 120 and the IC separator 180, preventing gas from leaking from the air chamber 313 to the external space.

[0047] 4 and 5, the space partitioned by the single cell separator 120, the single cell 110, the fuel electrode frame 140, the IC separator 180, and the interconnector 190 faces the fuel electrode 116 and forms a fuel chamber 323 through which fuel gas FG flows. The fuel electrode frame 140 partitions the entire periphery of the fuel chamber 323 from the external space, and seals the gap between the single cell separator 120 and the IC separator 180, thereby preventing gas from leaking from the fuel chamber 323 to the external space.

[0048] The single cell separator 120 separates the air chamber 313 and the fuel chamber 323, thereby preventing gas leakage (cross leakage) from the air electrode 114 side to the fuel electrode 116 side or from the fuel electrode 116 side to the air electrode 114 side around the single cell 110. In addition, the IC separator 180 and the interconnector 190 prevent gas leakage between adjacent power generating units 100U.

[0049] (Operation of fuel cell stack 10) As shown in Figures 2 and 4, the oxidizing gas OG is supplied to the oxidizing gas supply manifold 311 via a gas pipe (not shown) and a gas passage member 280, and is supplied to the air chamber 313 via the oxidizing gas supply communicating passage 132.

[0050] As shown in FIGS. 3 and 5, the fuel gas FG is supplied to the fuel gas supply manifold 321 via a gas pipe (not shown) and a gas passage member 280, and is supplied to the fuel chamber 323 via the fuel gas supply communication passage 142.

[0051] When an oxidant gas OG is supplied to the air chamber 313 of each power generating unit 100U and a fuel gas FG is supplied to the fuel chamber 323, power is generated in the unit cell 110 by an electrochemical reaction between the oxidant gas OG and the fuel gas FG. This power generation reaction is exothermic. As described above, the interconnector 190 is shared by two adjacent power generating units 100U, and the interconnector 190 ensures electrical continuity between the two adjacent power generating units 100U. In other words, the multiple power generating units 100U included in the fuel cell stack 10 are electrically connected in series. Furthermore, a second terminal plate 250 is electrically connected to the power generating unit 100U located at the other end (the lower end in FIG. 2 ) of the multiple power generating units 100U, and a first terminal plate 240 is electrically connected to the power generating unit 100U located at one end (the upper end in FIG. 2 ). As a result, electrical energy generated in each power generating unit 100U is extracted from the terminal plates 240, 250, which function as output terminals of the fuel cell stack 10. Since SOFCs generate electricity at relatively high temperatures (e.g., 700°C to 1000°C), after startup, the fuel cell stack 10 may be heated by a heater (not shown) until the high temperature can be maintained using the heat generated by power generation.

[0052] 2 and 4, the oxidant off-gas OOG discharged from the air chamber 313 of each power generating unit 100U to the oxidant gas discharge manifold 312 via the oxidant gas discharge communicating passage 133 is discharged to the outside of the fuel cell stack 10 through the internal space of the main body 281. Also, as shown in FIGS. 3 and 5, the fuel off-gas FOG discharged from the fuel chamber 323 of each power generating unit 100U to the fuel gas discharge manifold 322 via the fuel gas discharge communicating passage 143 is discharged to the outside of the fuel cell stack 10 through the internal space of the main body 281.

[0053] (Detailed structure around conductive adhesive material 196) Fig. 6 is an explanatory diagram showing an enlarged view of part X1 in Fig. 5. Fig. 6 shows in detail the periphery of the conductive bonding material 196. Fig. 6 shows an interface IF1 between the conductive bonding material 196 and the air electrode 114, and an interface IF2 between the conductive bonding material 196 and the coating layer 193.

[0054] Interface IF1 can be identified as follows. First, an electron probe microanalyzer (EPMA) is used to map the entire thickness of the conductive bonding material 196 and the air electrode 114 in a cross section perpendicular to the surface of the conductive bonding material 196 that is bonded to the air electrode 114. In the EPMA mapping, element concentrations are obtained for each measurement range of 1 μm in the thickness direction and 100 μm in the direction intersecting the thickness direction from the coating layer 193 toward the air electrode 114. Regarding element A, which is the most abundant element in the conductive bonding material 196 excluding carbon, oxygen, and the elements contained in the air electrode 114, when the average concentration of element A throughout the conductive bonding material 196 is taken as 100, the measurement range where element A is 10 can be identified as interface IF1. In other words, near interface IF1 in the conductive bonding material 196, the concentration of element A gradually decreases toward the air electrode 114, and the measurement range in which the concentration of element A is one-tenth of the average concentration of element A in the entire conductive bonding material 196 can be identified as interface IF1.

[0055] The interface IF2 can be identified as follows. First, the entire thickness direction of the conductive bonding material 196 and the coating layer 193 is mapped using an EPMA in a cross section perpendicular to the surface of the conductive bonding material 196 that is bonded to the coating layer 193. In the EPMA mapping, element concentrations are obtained for each measurement range of 1 μm in the thickness direction and 100 μm in the direction intersecting the thickness direction from the air electrode 114 toward the coating layer 193. Regarding element B, which is the most abundant element in the conductive bonding material 196 excluding carbon, oxygen, and the elements in the coating layer 193, when the average concentration of element B throughout the conductive bonding material 196 is taken as 100, the measurement range where element B is 10 can be identified as interface IF2. In other words, near interface IF2 in the conductive bonding material 196, the concentration of element B gradually decreases toward the coating layer 193, and the measurement range where the concentration of element B is one-tenth of the average concentration of element B throughout the conductive bonding material 196 can be identified as interface IF2.

[0056] As shown in FIG. 6 , the conductive bonding material 196 has a thickness L. The conductive bonding material 196 has a surface layer portion 196S and an inner layer portion 196IN. The surface layer portion 196S is a portion of the conductive bonding material 196 sandwiched between an interface IF1 and a position 3 μm away from the interface IF1 in the thickness direction of the conductive bonding material 196 toward the interior of the conductive bonding material 196. The inner layer portion 196IN is a portion of the conductive bonding material 196 sandwiched between a position L / 4 away from the interface IF1 in the thickness direction of the conductive bonding material 196 toward the interior of the conductive bonding material 196 and a position L / 4 away from the interface IF2 in the thickness direction of the conductive bonding material 196 toward the interior of the conductive bonding material 196.

[0057] The porosity of the conductive bonding material 196 near its interface with the air electrode 114 is greater than the porosity inside the conductive bonding material 196. The difference between the porosity of the conductive bonding material 196 near its interface with the air electrode 114 and the porosity inside the conductive bonding material 196 is 5% or more and 60% or less. "The porosity of the conductive bonding material 196 near its interface with the air electrode 114" more specifically refers to the porosity of a surface layer portion 196S of the conductive bonding material 196. "The porosity inside the conductive bonding material 196" more specifically refers to the porosity of an inner layer portion 196IN of the conductive bonding material 196.

[0058] The porosity of the surface portion 196S of the conductive bonding material 196 can be determined as follows. First, for the power generating unit 100U, an SEM image of a cross section perpendicular to the surface of the conductive bonding material 196 that is bonded to the air electrode 114 is obtained. Next, an image analysis of the surface portion 196S is performed using image analysis software (e.g., ImageJ) with an analysis target of a range of 60 μm in the direction perpendicular to the thickness direction (i.e., 3 μm) of the surface portion 196S. In the image analysis, the analysis target is binarized. When the contrast of the pixel with the maximum contrast is set to 100 and the contrast of the pixel with the minimum contrast is set to 0, regions with a contrast of 50 or less are defined as pores. The porosity can be determined by dividing the area of ​​the pores in the analysis target by the total area of ​​the analysis target.

[0059] The porosity of the inner layer portion 196IN of the conductive bonding material 196 can be determined as follows. First, for the power generating unit 100U, an SEM image of a cross section perpendicular to the surface of the conductive bonding material 196 that is bonded to the air electrode 114 is obtained. Next, an image analysis of the inner layer portion 196IN is performed using image analysis software (e.g., ImageJ) with an analysis target of a range of 60 μm in the thickness direction (i.e., L / 2) of the inner layer portion 196IN in the thickness direction x the direction intersecting the thickness direction. In the image analysis, a binarization process is performed on the analysis target. When the contrast of the pixel with the maximum contrast is set to 100 and the contrast of the pixel with the minimum contrast is set to 0, areas with a contrast of 50 or less are defined as pores. The porosity can be determined by dividing the area of ​​the pores in the analysis target by the total area of ​​the analysis target.

[0060] Furthermore, when the power generating unit 100U is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the porosity of the conductive bonding material 196 near the interface with the air electrode 114 after the heat treatment is greater than the porosity of the conductive bonding material 196 near the interface with the air electrode 114 before the heat treatment. In other words, the porosity of the conductive bonding material 196 near the interface with the air electrode 114 increases as a result of the heat treatment.

[0061] Furthermore, when the power generating unit 100U is subjected to a heat treatment at 900°C for 200 hours in an atmospheric environment, the rate of increase in the porosity of the conductive bonding material 196 near the interface with the air electrode 114 due to the heat treatment is greater than the rate of increase in the porosity inside the conductive bonding material 196 due to the heat treatment.

[0062] (Effects of this embodiment) As described above, the power generating unit 100U of this embodiment includes a single cell 110, a conductive air electrode current collecting part 192, a conductive coating layer 193, and a conductive adhesive 196. The single cell 110 includes a cathode 114, an electrolyte layer 112 containing a solid oxide, and a fuel electrode 116 stacked in this order. The cathode current collecting part 192 contains Cr and is disposed on the cathode 114 side of the single cell 110. The coating layer 193 covers the surface of the air electrode current collecting part 192. The conductive adhesive 196 bonds the air electrode 114 and the coating layer 193. The porosity of the conductive adhesive 196 near the interface with the cathode 114 is greater than the porosity of the conductive adhesive 196 inside.

[0063] In the power generating unit 100U of this embodiment, the porosity of the conductive bonding material 196 near the interface with the air electrode 114 is greater than the porosity inside the conductive bonding material 196. Therefore, stress applied to the conductive bonding material 196 selectively peels off the interface between the conductive bonding material 196 and the air electrode 114, thereby releasing the stress. This prevents cracks in the conductive bonding material 196 from progressing to the coating layer 193 and prevents Cr from diffusing from the surface of the air electrode current collecting part 192.

[0064] In the power generating unit 100U of this embodiment, the difference in porosity between the conductive bonding material 196 near its interface with the air electrode 114 and the conductive bonding material 196's interior is 5% or more and 60% or less. According to the power generating unit 100U of this embodiment, the difference in porosity between the conductive bonding material 196 near its interface with the air electrode 114 and the conductive bonding material 196's interior is 5% or more and 60% or less. This allows for more effective stress relief at the interface between the conductive bonding material 196 and the air electrode 114, and maintains good electrical connection between the air electrode 114 and the conductive bonding material 196.

[0065] When the power generating unit 100U of this embodiment is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the porosity of the conductive bonding material 196 near the interface with the air electrode 114 after the heat treatment is greater than the porosity of the conductive bonding material 196 near the interface with the air electrode 114 before the heat treatment. According to the power generating unit 100U of this embodiment, even if stress applied to the conductive bonding material 196 increases due to sintering of the conductive bonding material 196 during operation of the power generating unit 100U, stress is more effectively released at the interface between the conductive bonding material 196 and the air electrode 114. This prevents cracks in the conductive bonding material 196 from progressing to the coating layer 193, and more effectively prevents Cr diffusion from the surface of the air electrode current collecting part 192.

[0066] In the power generating unit 100U of this embodiment, when the power generating unit 100U is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the rate of increase in porosity in the conductive bonding material 196 near the interface with the air electrode 114 due to the heat treatment is greater than the rate of increase in porosity inside the conductive bonding material 196 due to the heat treatment. According to the power generating unit 100U of this embodiment, even if stress applied to the conductive bonding material 196 increases due to sintering of the conductive bonding material 196 during operation of the power generating unit 100U, stress is more effectively released at the interface between the conductive bonding material 196 and the air electrode 114. This prevents cracks in the conductive bonding material 196 from progressing to the coating layer 193, and more effectively prevents Cr diffusion from the surface of the air electrode current collecting part 192.

[0067] The fuel cell stack 10 of this embodiment includes a plurality of power generating units, at least one of which is the power generating unit 100U. According to the fuel cell stack 10 of this embodiment, the porosity of the conductive bonding material 196 of the power generating unit 100U near the interface with the air electrode 114 is greater than the porosity of the interior of the conductive bonding material 196. Therefore, stress applied to the conductive bonding material 196 selectively peels the interface between the conductive bonding material 196 and the air electrode 114, thereby releasing the stress. This prevents cracks in the conductive bonding material 196 from progressing to the coating layer 193 and prevents Cr from diffusing from the surface of the air electrode current collecting portion 192.

[0068] (Performance evaluation) Next, a performance evaluation of this embodiment will be described. A plurality of fuel cell stack samples (SA1 to SA5) each including conductive bonding materials with various different physical properties were fabricated, and performance evaluation was carried out using the plurality of samples.

[0069] First, multiple fuel cell stack samples were fabricated. The fuel cell stacks were fabricated by a known method. The conductive bonding material was formed by forming a coating layer on the surface of the air electrode current collector, applying a precursor of the conductive bonding material to the surface of the coating layer, and baking the precursor. The precursor of the conductive bonding material is a paste containing a material (more specifically, a spinel-type oxide) that forms the conductive bonding material. For example, the physical properties of the conductive bonding material can be changed by changing conditions such as the solid concentration of the precursor and the powder particle size of the spinel-type oxide contained in the precursor.

[0070] Next, the stack deterioration rate was evaluated. First, the fuel cell stack manufactured by the above method was started to operate at 700°C, and the potential V I After that, the temperature is kept at 700°C and the voltage V is measured after 10,000 hours of continuous operation. E The stack deterioration rate due to continuous operation of each sample of the fuel cell stack was calculated using the following formula. Stack deterioration rate (%) = 100 × (V I -V E ) / V I

[0071] Next, the porosity was measured before and after the heat treatment. First, a portion including the cathode current collector, coating layer, conductive bonding material, and cathode was cut out from the fuel cell stack sample prepared by the above method, embedded in resin, and the cross section was mirror-polished. In this state, the porosity of the surface layer of the conductive bonding material and the porosity of the inner layer of the conductive bonding material were measured to determine the porosity near the interface with the cathode in the conductive bonding material before the heat treatment and the internal porosity of the conductive bonding material before the heat treatment. Next, another sample manufactured by the same manufacturing method as the sample for which the porosity near the interface with the cathode in the conductive bonding material before the heat treatment and the internal porosity of the conductive bonding material before the heat treatment were measured was subjected to a heat treatment at 900°C for 200 hours in an air atmosphere. Next, a portion including the cathode current collector, coating layer, conductive bonding material, and cathode was cut out from the sample after the heat treatment, embedded in resin, and the cross section was mirror-polished. In this state, the porosity of the surface layer of the conductive bonding material and the porosity of the inner layer of the conductive bonding material were measured to determine the porosity of the conductive bonding material near the interface with the air electrode after the heat treatment and the porosity of the interior of the conductive bonding material after the heat treatment. Note that in this performance evaluation, "before the heat treatment" is synonymous with the time when the product manufacturing is completed.

[0072] The results of the performance evaluation are explained below. Table 1 shows the results of the performance evaluation. [Table 1]

[0073] The "Solid content concentration (wt%)" column in Table 1 indicates the solid content concentration of the conductive bonding material precursor when preparing each sample. The "Powder particle size D50 (μm)" column in Table 1 indicates the median diameter of the spinel-type oxide powder contained in the precursor. The "Porosity near the interface (%)" column in Table 1 indicates the porosity near the interface with the air electrode in the conductive bonding material. The "Internal porosity (%)" column in Table 1 indicates the internal porosity in the conductive bonding material. The "Difference (%)" column in Table 1 indicates the difference in the internal porosity of the conductive bonding material relative to the porosity near the interface with the air electrode in the conductive bonding material. The "Increase in porosity near the interface (%)" column in Table 1 is the porosity near the interface with the air electrode in the conductive bonding material after heat treatment minus the porosity near the interface with the air electrode in the conductive bonding material before heat treatment. The column "Increase in internal porosity (%)" in Table 1 is the value obtained by subtracting the internal porosity in the conductive bonding material before the heat treatment from the internal porosity in the conductive bonding material after the heat treatment.

[0074] For samples SA2 to SA5, the porosity of the conductive bonding material near the interface with the air electrode after completion of manufacturing was greater than the internal porosity of the conductive bonding material after completion of manufacturing, while for sample SA1, the porosity of the conductive bonding material near the interface with the air electrode after completion of manufacturing was equal to the internal porosity of the conductive bonding material after completion of manufacturing. Furthermore, the stack deterioration rate of samples SA2 to SA5 was lower than that of sample SA1. This confirms that the porosity of the conductive bonding material near the interface with the air electrode is greater than the internal porosity of the conductive bonding material, thereby reducing the deterioration rate of the fuel cell stack (i.e., increasing durability).

[0075] For samples SA2 to SA5, the difference in the porosity of the conductive bonding material near its interface with the air electrode at the time of completion of manufacturing was 5% or more and 60% or less, and for sample SA1, the difference in the porosity of the conductive bonding material near its interface with the air electrode at the time of completion of manufacturing was less than 5%. Furthermore, the stack deterioration rate of samples SA2 to SA5 was lower than that of sample SA1. This confirms that the deterioration rate of the fuel cell stack is lower (i.e., durability is increased) when the difference in the porosity of the conductive bonding material near its interface with the air electrode at the time of completion of manufacturing is 5% or more and 60% or less.

[0076] Furthermore, for sample SA3, the porosity of the conductive bonding material near the interface with the air electrode after the heat treatment was greater than the porosity of the conductive bonding material near the interface with the air electrode before the heat treatment. For samples SA1, SA2, SA4, and SA5, the porosity of the conductive bonding material near the interface with the air electrode after the heat treatment was less than the porosity of the conductive bonding material near the interface with the air electrode before the heat treatment. Furthermore, the stack degradation rate of sample SA3 was lower than that of samples SA1, SA2, SA4, and SA5. This confirms that the greater porosity of the conductive bonding material near the interface with the air electrode after the heat treatment compared to the greater porosity of the conductive bonding material near the interface with the air electrode before the heat treatment reduces the degradation rate of the fuel cell stack (i.e., increases durability).

[0077] Furthermore, for samples SA2 and SA3, the rate of increase in porosity near the interface with the air electrode in the conductive bonding material due to heat treatment was greater than the rate of increase in internal porosity in the conductive bonding material due to heat treatment. For samples SA1, SA4, and SA5, the rate of increase in porosity near the interface with the air electrode in the conductive bonding material due to heat treatment was less than or equal to the rate of increase in internal porosity in the conductive bonding material due to heat treatment. Furthermore, the stack degradation rates of samples SA2 and SA3 were lower than those of samples SA1, SA4, and SA5. This confirms that the rate of increase in porosity near the interface with the air electrode in the conductive bonding material due to heat treatment is greater than the rate of increase in internal porosity in the conductive bonding material due to heat treatment, thereby reducing the degradation rate of the fuel cell stack (i.e., increasing durability).

[0078] B. Variations: The technology disclosed in this specification is not limited to the above-described embodiments, and can be modified in various forms without departing from the spirit thereof, for example, the following modifications are also possible.

[0079] The configurations of the fuel cell stack 10 and the power generation unit 100U in the above embodiment are merely examples and can be modified in various ways. For example, the number of unit cells (number of power generation units) included in the fuel cell stack is merely an example, and the number of unit cells is determined appropriately depending on the output voltage required for the fuel cell stack.

[0080] In the above embodiment, the oxide film 192OM is formed on the surface of the air electrode current collecting portion 192, but the oxide film does not necessarily have to be formed.

[0081] In the above embodiment, the difference in the porosity of the conductive bonding material 196 near the interface with the air electrode 114 and the porosity inside the conductive bonding material 196 is 5% or more and 60% or less, but the difference in the porosity of the bonding layer near the interface with the air electrode and the porosity inside the bonding layer does not necessarily have to be 5% or more and 60% or less.

[0082] In the above embodiment, when the power generation unit 100U is subjected to a heat treatment at 900°C for 200 hours in an atmospheric air, the porosity of the conductive bonding material 196 near the interface with the air electrode 114 after the heat treatment is greater than the porosity of the conductive bonding material 196 near the interface with the air electrode 114 before the heat treatment, but the power generation unit does not necessarily have to be configured in this way.

[0083] In the above embodiment, when the power generation unit 100U is subjected to a heat treatment at 900°C for 200 hours in an atmospheric air, the rate of increase in the porosity of the conductive bonding material 196 near the interface with the air electrode 114 due to the heat treatment is greater than the rate of increase in the porosity inside the conductive bonding material 196 due to the heat treatment, but the power generation unit does not necessarily have to be configured in this way.

[0084] The fuel cell stack may include at least one power generation unit such as that in the above embodiment among a plurality of power generation units.

[0085] The fuel cell stack 10 of the above embodiment is a co-flow type SOFC, but the technology disclosed in this specification is also applicable to counter-flow type SOFCs and cross-flow type SOFCs.

[0086] In the above embodiment, the unit cell 110 is an anode-supported unit cell, but it may be another type of unit cell such as an electrolyte-supported type or a metal-supported type.

[0087] In the above embodiment, the fuel cell stack 10 is configured to have a plurality of flat-type unit cells 110, but the technology disclosed in this specification is equally applicable to fuel cell stacks having a plurality of unit cells of other types (e.g., cylindrical, flat cylindrical, etc.).

[0088] In the above embodiment, the electrochemical reaction unit is a fuel cell power generation unit, which is a constituent unit of a solid oxide fuel cell (SOFC), but the present invention can also be applied to an electrolysis cell unit, which is a constituent unit of a solid oxide electrolysis cell (SOEC). [Explanation of symbols]

[0089] 10: fuel cell stack 100: power generation block 100U: power generation unit 110: single cell 112: electrolyte layer 114: air electrode 116: fuel electrode 118: reaction prevention layer 120: single cell separator 124: joint 130: air electrode frame 132: oxidant gas supply communicating channel 133: oxidant gas discharge communicating channel 140: fuel electrode frame 142: fuel gas supply communicating channel 143: fuel gas discharge communicating channel 144: fuel electrode current collecting member 149: spacer 180: IC separator 190: interconnector 191: flat plate portion 192: air electrode current collecting portion 192OM: oxide coating 193: coating layer 196: conductive bonding material 196IN: inner layer portion 196S: surface layer portion 210: first end plate 220: Insulation part 230: End separator 232: First plate 240: First terminal plate 250: Second terminal plate 260: Second plate 270: Second end plate 280: Gas passage member 311: Oxidizer gas supply manifold 312: Oxidizer gas discharge manifold 313: Air chamber 321: Fuel gas supply manifold 322: Fuel gas discharge manifold 323: Fuel chamber B: Bolt BH: Bolt hole N: Nut FG: Fuel gas FOG: Fuel off-gas OG: Oxidizer gas OOG: Oxidizer off-gas IF1: Interface IF2: Interface

Claims

1. a single cell in which an air electrode, an electrolyte layer containing a solid oxide, and a fuel electrode are stacked in this order; a conductive current collector containing Cr and disposed on the air electrode side of the single cell; a conductive coating covering the surface of the current collecting portion; a conductive bonding layer that bonds the air electrode and the coating; In an electrochemical reaction unit comprising: the porosity of the bonding layer near the interface with the air electrode is greater than the porosity of the interior of the bonding layer; An electrochemical reaction unit characterized by:

2. The electrochemical reaction unit according to claim 1, a difference between the porosity of the bonding layer near the interface with the air electrode and the porosity of the bonding layer inside the bonding layer is 5% or more and 60% or less; An electrochemical reaction unit characterized by:

3. The electrochemical reaction unit according to claim 1, when the electrochemical reaction unit is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, the porosity of the bonding layer near the interface with the air electrode after the heat treatment is greater than the porosity of the bonding layer near the interface with the air electrode before the heat treatment. An electrochemical reaction unit characterized by:

4. The electrochemical reaction unit according to claim 1, when the electrochemical reaction unit is subjected to a heat treatment at 900°C for 200 hours in an air atmosphere, an increase rate of porosity in the bonding layer near the interface with the air electrode due to the heat treatment is greater than an increase rate of porosity in the interior of the bonding layer due to the heat treatment; An electrochemical reaction unit characterized by:

5. In an electrochemical reaction cell stack having a plurality of electrochemical reaction units, At least one of the plurality of electrochemical reaction units is the electrochemical reaction unit according to any one of claims 1 to 4. An electrochemical reaction cell stack comprising:

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