Electrochemical reaction cell, and electrochemical reaction cell stack

Abstract

To improve durability of an electrochemical reaction cell.SOLUTION: An electrochemical reaction cell comprises: an electrolyte layer including a first face and a second face on the opposite side of the first face; a fuel electrode containing nickel and disposed on the first face; and an air electrode disposed on the second face. The electrolyte layer contains a stabilized metal in which a second metal oxide containing second metal elements to be bivalent or trivalent cations is solid-dissolved to a first metal oxide containing first metal elements to be tetravalent cations. The first metal element is zirconium or cerium. When a concentration of the first metal elements contained in the electrolyte layer is defined as M1 and a concentration of the second metal elements contained in the electrolyte layer is defined as M2, the electrolyte layer includes a change region which is disposed on the first face and in which a value of M2 / M1 changes gradually in a direction from the first face toward the second face.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The technology disclosed herein relates to electrochemical reaction cells and electrochemical reaction cell stacks. [Background technology]

[0002] Solid oxide fuel cells (hereinafter referred to as "SOFCs"), which have an electrolyte layer containing solid oxide, are known as one type of fuel cell that generates electricity using the electrochemical reaction between hydrogen and oxygen. SOFCs are generally used in the form of a fuel cell stack, in which multiple structural units (electrochemical reaction units) are arranged in a predetermined direction. Each electrochemical reaction unit has a single cell including an electrolyte layer containing solid oxide, a cathode active layer (air electrode) disposed on one side of the electrolyte layer, and an anode active layer (fuel electrode) disposed on the other side of the electrolyte layer.

[0003] When the fuel electrode contains nickel oxide, the fuel electrode may peel off from the electrolyte layer, resulting in a decrease in the power generation efficiency of the unit cell. To prevent this, a technology has been proposed in which the anode-side electrolyte layer is disposed between the cathode-side electrolyte layer and the anode active layer so as to be in contact with the anode active layer (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-207215 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above configuration, there are two electrolyte layers and an anode active layer (fuel electrode layer), and there is a concern that the difference in thermal expansion coefficient between adjacent layers may cause interfacial peeling, reducing the durability of the single cell.

[0006] These issues are also common to electrolysis cell stacks that include multiple electrolysis cell units, which are constituent units of solid oxide electrolysis cells (hereinafter referred to as "SOECs") that generate hydrogen using the electrolysis reaction of water, and are not limited to SOFCs and SOECs, but are also common to other types of electrochemical reaction cell stacks.

[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. (1) The present specification discloses an electrochemical reaction cell comprising: an electrolyte layer having a first surface and a second surface opposite to the first surface; an anode containing nickel and disposed on the first surface; and an cathode disposed on the second surface, wherein the electrolyte layer comprises a stabilized metal oxide in which a first metal oxide containing a first metal element that becomes a tetravalent cation is solid-dissolved with a second metal oxide containing a second metal element that becomes a divalent or trivalent cation, the first metal element being zirconium or cerium; and wherein, when the concentration of the first metal element contained in the electrolyte layer is M1 and the concentration of the second metal element contained in the electrolyte layer is M2, the electrolyte layer is disposed on the first surface and has a change region in which the value of M2 / M1 changes stepwise in a direction from the first surface to the second surface.

[0009] According to the above configuration, peeling of the fuel electrode from the electrolyte layer is suppressed, and durability of the unit cell is improved.

[0010] (2) In the electrochemical reaction cell described in (1) above, the value of M2 / M1 at the boundary between the change region and the fuel electrode and the value of M2 / M1 at the boundary between the change region and other regions of the electrolyte layer excluding the change region are both 0.10 to 0.13 when the first metal element is zirconium, and 0.08 to 0.25 when the first metal element is cerium. With this configuration, an electrochemical reaction cell with excellent initial characteristics can be obtained.

[0011] (3) In the electrochemical reaction cell described in (1) or (2) above, the larger of the M2 / M1 value at the boundary between the change region and the fuel electrode and the smaller of the M2 / M1 value at the boundary between the change region and other regions of the electrolyte layer excluding the change region may be designated as ML and MS, respectively, such that ML / MS≧1.01. This configuration further enhances the durability of the electrochemical reaction cell.

[0012] (4) The electrochemical reaction cell stack disclosed in this specification includes the electrochemical reaction cell according to any one of (1) to (3) above.

[0013] The technology disclosed in this specification can be realized in various forms, for example, in the form of an electrochemical reaction cell stack and a manufacturing method thereof. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view showing the external configuration of a fuel cell stack according to an embodiment; [Figure 2] FIG. 2 is a cross-sectional view of the fuel cell stack according to the embodiment taken along line II-II in FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view of the fuel cell stack according to the embodiment taken along line III-III in FIG. 1. [Figure 4] 2 is a cross-sectional view showing two adjacent electrochemical reaction units in the fuel cell stack of the embodiment, cut at the same position as line II-II in FIG. 1. [Figure 5] 3 is a cross-sectional view showing two adjacent electrochemical reaction units in the fuel cell stack of the embodiment, cut at the same position as line III-III in FIG. 1. [Figure 6] Partially enlarged cross-sectional view of a single cell according to an embodiment. [Figure 7] Partially enlarged cross-sectional view of the electrolyte layer of the single cell of the test example DETAILED DESCRIPTION OF THE INVENTION

[0015] A. Implementation: A-1. Configuration of fuel cell stack 10: The embodiment will be described with reference to Figures 1 to 6. A fuel cell stack 10 (an example of an electrochemical reaction cell stack) of the present embodiment is used in a solid oxide fuel cell having an electrolyte layer 112 containing a solid oxide.

[0016] (Overall configuration of fuel cell stack 10) 1 to 3, the fuel cell stack 10 includes a power generation block 100, a first end plate 210, a second end plate 230, an end separator 220, and four gas passage members 280A, 280B. The power generation block 100 is composed of a plurality of (seven in this embodiment) electrochemical reaction units 100U (hereinafter, sometimes abbreviated as "reaction units 100U"). Each electrochemical reaction unit 100U includes a single cell 110 (an example of an electrochemical reaction cell), 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.

[0017] The first end plate 210, power generation block 100, terminal separator 220, and second end plate 230 have rectangular outer shapes of approximately the same size, and are stacked in this order in a predetermined arrangement direction (the vertical direction in Figure 2).

[0018] 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 terminal separator 220. A bolt B is inserted into each bolt hole BH. The second end plate 230 has four screw holes (not shown) at positions corresponding to the four bolt holes BH. One end of each bolt B is screwed into the corresponding screw hole, and a nut N is screwed onto the other end of each bolt B. These bolts B and nuts N fasten the members from the first end plate 210 to the second end plate 230 together. As shown in FIGS. 2 and 3, the four gas passage members 280A, 280B are connected to the second end plate 230.

[0019] The multiple reaction units 100U that make up the power generation block 100 are arranged side by side in a predetermined arrangement direction (the vertical direction in FIG. 2). As shown in FIGS. 4 and 5, each reaction unit 100U is composed of one IC separator 180, an air electrode frame 130, a single cell separator 120, an anode frame 140, and another IC separator 180 stacked in this order. The single cell 110 is supported by the single cell separator 120, and two interconnectors 190 are supported by the two IC separators 180, respectively. The anode current collecting member 144 is disposed between the single cell 110 and the interconnector 190.

[0020] 4 and 5, the IC separator 180 and the interconnector 190 are shared by two adjacent reaction units 100U. However, as shown in Fig. 2, the reaction unit 100U located at one end (the lower end in Fig. 2) of the multiple reaction units 100U does not have the IC separator 180 and the interconnector 190 adjacent to the fuel electrode frame 140, and the terminal separator 220 overlaps the fuel electrode frame 140.

[0021] (single cell 110) The unit cell 110 includes an electrolyte layer 112, an air electrode 114, and an anode 116. As shown in Figures 3 and 4, the air electrode 114, the electrolyte layer 112, and the anode 116 are stacked in this order, with a reaction prevention layer 118 interposed between the electrolyte layer 112 and the air electrode 114. The unit cell 110 of this embodiment is an anode-supported unit cell in which the other layers (electrolyte layer 112, air electrode 114, and reaction prevention layer 118) that constitute the unit cell 110 are supported by the anode 116.

[0022] The electrolyte layer 112 is a rectangular, flat-plate member and contains a solid oxide. The air electrode 114 is a layer having a rectangular shape smaller than the electrolyte layer 112 and contains, for example, a perovskite oxide (e.g., lanthanum strontium cobalt iron oxide (LSCF)). The fuel electrode 116 is a layer having a rectangular shape and approximately the same size as the electrolyte layer 112 and contains Ni (nickel). The reaction prevention layer 118 is a layer having a rectangular shape and approximately the same size as the air electrode 114 and contains, for example, GDC (gadolinium-doped ceria). The reaction prevention layer 118 has the function of suppressing the reaction of an element (e.g., Sr) diffused from the air electrode 114 with an element (e.g., Zr) contained in the electrolyte layer 112 to produce a highly resistive substance (e.g., SrZrO).

[0023] (Single cell separator 120) As shown in Figures 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 plate thickness of the single cell separator 120 is relatively thin, for example, not less than 0.05 mm and not more than 0.2 mm. The peripheral portion of the through-hole 121 in the single cell separator 120 is joined to the electrolyte layer 112 by a joining portion 124. The joining portion 124 is made of, for example, brazing material.

[0024] (Air electrode frame 130) As shown in FIGS. 4 and 5, the cathode frame 130 is a rectangular frame-like member having a substantially rectangular through-hole 131 near the center, and is made of, for example, insulating ceramics (mica, etc.).

[0025] (fuel electrode frame 140) As shown in FIGS. 4 and 5, the fuel electrode 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.

[0026] (IC separator 180) As shown in FIGS. 4 and 5, IC separator 180 is a frame-like member having a substantially rectangular through-hole 181 near the center, and is made of, for example, metal.

[0027] (Interconnector 190 and anode current collecting member 144) As shown in Figures 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 the flat plate portion 191 toward the air electrode 114, a coating layer 193, and an oxide coating 194. The flat plate portion 191 and the air electrode current collector 192 are electrically conductive and formed of a metal (e.g., ferritic stainless steel). 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 side of the flat plate portion 191 on which the air electrode current collector 192 is disposed. The oxide coating 194 is disposed so as to cover the side of the flat plate portion 191 opposite to the side on which the air electrode current collector 192 is disposed.

[0028] 4 and 5, the interconnector 190 has a rectangular outer shape that is slightly larger than the edge of the through-hole 181, and is placed on the IC separator 180 so as to close the through-hole 181. The flat plate portion 191 is placed on the peripheral portion of the through-hole 181 in the IC separator 180 via an oxide film 194, and is joined by welding.

[0029] The anode current collecting member 144 is a member that connects the interconnector 190 and the anode 116, and is formed of a conductive material such as nickel, a nickel alloy, or stainless steel. As shown in Figures 4 and 5, the anode current collecting member 144 includes an interconnector facing portion 146, an electrode facing portion 145 that is parallel to the interconnector facing portion 146, and a connecting portion (not shown) that connects the electrode facing portion 145 and the interconnector facing portion 146. 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.

[0030] As described above, the interconnector 190 is shared by two adjacent reaction units 100U. More specifically, as shown in Figures 4 and 5, the air electrode current collecting part 192 is joined to the air electrode 114 of the unit cell 110 provided in one of the two adjacent reaction units 100U via a conductive bonding material 196 made of, for example, a spinel-type oxide, and is thereby electrically connected to the air electrode 114. The flat plate part 191 is electrically connected to the anode 116 of the unit cell 110 provided in the other of the two adjacent reaction units 100U via an anode current collecting member 144. This ensures electrical continuity between the two adjacent reaction units 100U.

[0031] However, as described above, the reaction unit 100U located at one end (the lower end in FIG. 2) of the multiple reaction units 100U does not have an interconnector 190 on the fuel electrode 116 side. The fuel electrode 116 included in this reaction unit 100U is connected to the terminal separator 220 via the fuel electrode current collecting member 144.

[0032] 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 reaction unit 100U due to temperature cycles and fluctuations in reactant gas pressure, and good electrical connection between the anode 116 and the interconnector 190 (or terminal separator 220) via the anode current collecting member 144 is maintained.

[0033] (Air chamber 313 and fuel chamber 323) 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.

[0034] 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.

[0035] 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 reaction units 100U.

[0036] A glass seal portion 125 containing glass is disposed near the edge of the through-hole 121 in the unit cell separator 120. The glass seal portion 125 covers the edge of the through-hole 121 and is disposed so as to be in contact with the surface of the unit cell 110 (electrolyte layer 112 in this embodiment), thereby sealing the gap between the edge of the through-hole 121 and the unit cell 110. The glass seal portion 125 effectively prevents gas 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.

[0037] (First end plate 210) The first end plate 210 is a rectangular frame-shaped member having a through-hole 211 near the center, and is made of a conductive material such as stainless steel. The first end plate 210 is electrically connected to the reaction unit 100U arranged at the other end (the upper end in FIG. 2) of the multiple reaction units 100U that make up the power generation block 100. The first end plate 210 functions as a positive output terminal of the fuel cell stack 10.

[0038] (Terminal separator 220) The terminal separator 220 is a rectangular flat plate member made of a conductive material such as metal.

[0039] (Second end plate 230) The second end plate 230 is a rectangular frame-shaped member having a through-hole 231 near the center and is made of a conductive material such as stainless steel. The peripheral edge of the terminal separator 220 is sandwiched between the power generation block 100 and the second end plate 230 and joined to the second end plate 230 by welding, for example, and is electrically connected to the second end plate 230. The second end plate 230 is connected to the anode 116 of the reaction unit 100U located at one end (the lower end in FIG. 2 ) of the multiple reaction units 100U via the anode current collecting member 144 and the terminal separator 220, thereby electrically connecting the reaction unit 100U to the second end plate 230. The second end plate 230 functions as the negative output terminal of the fuel cell stack 10.

[0040] (Manifolds 311, 312, 321, 322) 1, 2, and 3, the fuel cell stack 10 has four holes that penetrate from the power generation block 100 to the second end plate 230. 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.

[0041] 2, the oxidant gas supply manifold 311 is a gas flow path for supplying the oxidant gas OG introduced from outside the fuel cell stack 10 to the air chamber 313 of each reaction unit 100U. The oxidant gas discharge manifold 312 is a gas flow path for discharging the oxidant off-gas OOG discharged from the air chamber 313 of each reaction unit 100U to the outside of the fuel cell stack 10. As the oxidant gas OG, for example, air is used.

[0042] 3, the fuel gas supply manifold 321 is a gas flow path for supplying fuel gas FG introduced from outside the fuel cell stack 10 to the fuel chamber 323 of each reaction unit 100U. The fuel gas discharge manifold 322 is a gas flow path for discharging fuel off-gas FOG discharged from the fuel chamber 323 of each reaction unit 100U to the outside of the fuel cell stack 10. As the fuel gas FG, for example, a hydrogen-rich gas obtained by reforming city gas is used.

[0043] (Gas passage members 280A, 280B) Two of the four gas passage members 280A, 280B are gas passage members 280A connected to the oxidant gas supply manifold 311 and the oxidant gas discharge manifold 312, respectively. As shown in FIG. 2, each gas passage member 280A has a substantially rectangular tubular main body 281A and a cylindrical branch portion 282A branching off from the side surface of the main body 281A. The internal space of the branch portion 282A is in communication with the internal space of the main body 281A. A gas pipe (not shown) is connected to the branch portion 282A. The two gas passage members 280A are each connected to the second end plate 230 via an insulating sheet S. The internal spaces of the two gas passage members 280A are in communication with the oxidant gas supply manifold 311 and the oxidant gas discharge manifold 312, respectively.

[0044] The other two of the four gas passage members 280A, 280B are gas passage members 280B connected to the fuel gas supply manifold 321 and the fuel gas discharge manifold 322, respectively. As shown in FIG. 3, each gas passage member 280B has a cylindrical main body 281B and a cylindrical branch portion 282B branching off from the side surface of the main body 281B. The internal space of the branch portion 282B is in communication with the internal space of the main body 281B. A gas pipe (not shown) is connected to the branch portion 282B. The two gas passage members 280B are each connected to the second end plate 230 via an insulating sheet S. The internal spaces of the two gas passage members 280B are in communication with the fuel gas supply manifold 321 and the fuel gas discharge manifold 322, respectively.

[0045] A-2.Detailed configuration of single cell 110: 6, the electrolyte layer 112 has a first surface 112F1 (the lower surface in FIG. 6) and a second surface 112F2 (the upper surface in FIG. 6) opposite the first surface 112F1. The fuel electrode 116 is disposed on the first surface 112F1, and the air electrode 114 is disposed on the second surface 112F2 with a reaction prevention layer 118 interposed therebetween.

[0046] The electrolyte layer 112 is an ion-conductive layer and includes a stabilizing metal oxide. The stabilizing metal oxide is a solid solution in which a first metal oxide containing a first metal element is solid-dissolved with a second metal oxide containing a second metal element. The first metal element is Zr (zirconium) or Ce (cerium), which form tetravalent cations. The second metal element is a metal element that forms divalent or trivalent cations. Preferred examples of the second metal element include Y (yttrium) and Sc (scandium) when the first metal element is Zr, and Gd (gadolinium) when the first metal element is Ce. That is, preferred examples of the stabilizing metal oxide include YSZ (yttria-stabilized zirconia), ScSZ (scandia-stabilized zirconia), and GDC (gadolinium-doped ceria). The solid solution of the second metal oxide in the first metal oxide stabilizes the crystal structure.

[0047] Preferred examples of the material of the fuel electrode 116 include Ni (nickel), cermet made of Ni and ceramics (for example, YSZ, GDC), and Ni-based alloys.

[0048] The electrolyte layer 112 has a change region R1 where the concentration of the second metal element changes. The change region R1 is located on the first surface 112F1. In other words, the change region R1 is a region of the electrolyte layer 112 adjacent to the anode 116, i.e., a region extending from the first surface 112F1 to a predetermined depth. When the concentration of the first metal element contained in the electrolyte layer 112 is M1 and the concentration of the second metal element contained in the electrolyte layer 112 is M2, the value of M2 / M1 changes stepwise within the change region R1 in a direction from the first surface 112F1 to the second surface 112F2 (in FIG. 6, from the bottom surface of the electrolyte layer 112 to the top). In this specification, the term "the value of M2 / M1 changes stepwise" refers to cases where the value of M2 / M1 changes continuously, discontinuously, linearly, or nonlinearly. Furthermore, the values ​​of the concentrations M1 and M2 are values ​​(unit: mass %) obtained by a concentration mapping method using an EPMA (electron probe microanalyzer).

[0049] The electrolyte layer 112 having the change region R1 suppresses peeling of the fuel electrode 116 from the electrolyte layer 112, improving the durability of the unit cell 110. The reason for this is believed to be as follows.

[0050] First, we will explain the case where the value of M2 / M1 within the change region R1 decreases as the distance from the first surface 112F1 increases, that is, the case where the content ratio of the second metal element within the change region R1 becomes relatively higher in the area closer to the first surface 112F1.

[0051] Generally, in metal oxides, the lower the ionic valence of the contained metal element, the weaker the Coulomb force bonding and the lower the melting point. Since the first metal element is an element that forms tetravalent cations and the second metal element is an element that forms divalent or trivalent cations, the larger the value of M2 / M1, i.e., the higher the content ratio of the second metal element, the lower the melting point and the easier it is to sinter. By making the value of M2 / M1 relatively large in the portion of the change region R1 close to the first surface 112F1, the sinterability of that portion, i.e., the portion in contact with the anode 116, is improved. This is thought to suppress peeling of the anode 116 and improve the durability of the unit cell 110.

[0052] Next, we will explain the case where the value of M2 / M1 increases as the distance from the first surface 112F1 increases within the change region R1, that is, the case where the content ratio of the second metal element becomes relatively smaller in the part close to the first surface 112F1 within the change region R1.

[0053] The lattice size (unit cell size) of a metal oxide crystal depends on the size of the metal atoms contained. More specifically, when comparing metal oxides containing elements belonging to different periods, the lattice size tends to be larger as the atomic number of the contained element increases. Furthermore, when comparing metal oxides containing elements belonging to the same period, the lattice size tends to be larger as the valence of the contained metal element decreases. Therefore, the relationship between the lattice sizes of the crystals of oxides containing nickel, a first metal element, and a second metal element is thought to be as follows:

[0054] Nickel < First metal element < Second metal element

[0055] If the difference in lattice size between the anode 116 and the electrolyte layer 112 is large, the lattice strain generated at the interface between them increases, making the interface more susceptible to peeling. By making the content ratio of the second metal element relatively small in a portion of the change region R1 close to the first surface 112F1, it is possible to reduce the difference in lattice size between this portion, i.e., the portion in contact with the anode 116, and the anode 116. This is thought to suppress peeling of the anode 116 and improve the durability of the unit cell 110.

[0056] From the viewpoint of improving the initial characteristics of the unit cell 110, the value of M2 / M1 at the boundary position of the changed region R1 with the anode 116 and the value of M2 / M1 at the boundary position with the non-changed region R2 (an example of another region) of the electrolyte layer 112 excluding the changed region R1 may both be 0.08 to 0.15 or 0.10 to 0.13 when the first metal element is Zr, and may be 0.001 to 0.32 or 0.08 to 0.25 when the first metal element is Ce. In this specification, the "boundary position with the anode" of the changed region refers to a region in the changed region from the interface with the anode (first surface 112F1) to a certain depth. Furthermore, the "boundary position with another region" of the changed region refers to a region in the changed region from the boundary with the other region (non-changed region R2) (the position indicated by the dashed line in FIG. 7) to a certain depth.

[0057] It is preferable to make the concentration gradient of the second metal element in the changed region R1 sufficiently large from the viewpoint of effectively suppressing peeling of the anode 116. More specifically, when the larger value of M2 / M1 at the boundary position of the changed region R1 with the anode 116 and the smaller value of M2 / M1 at the boundary position of the electrolyte layer 112 with the non-changed region R2 excluding the changed region R1 are designated as ML and MS, respectively, it is preferable that ML / MS≧1.01 be satisfied, and it is even more preferable that ML / MS≧1.05 be satisfied.

[0058] A-3. Manufacturing method of unit cell 110 An example of a method for manufacturing the unit cell 110 having the above configuration will be described below.

[0059] A solvent is added to the powder of the stabilized metal oxide and the mixture is wet mixed in a ball mill to obtain a slurry, which is then processed into a sheet shape using, for example, a doctor blade method to produce a green sheet for the electrolyte.

[0060] A stabilized metal oxide powder, the same as that used as the material for the electrolyte green sheet but with a different content of the second metal element, and NiO powder are weighed out to achieve the desired composition. A solvent is added to these raw material powders, and they are wet-mixed in a ball mill to obtain a slurry. The obtained slurry is processed into a sheet shape using, for example, a doctor blade method, to produce a green sheet for the anode.

[0061] The concentrations of the second metal element in the stabilizing metal oxides contained in the electrolyte green sheet and the anode green sheet may be set so that M2 / M1 is a target value. More specifically, the concentrations of the second metal element may increase or decrease in the order of the electrolyte green sheet and the anode green sheet.

[0062] Next, the resulting electrolyte green sheet and anode green sheet are stacked and pressed together to obtain a laminate. This laminate is degreased at a predetermined temperature and then subjected to primary firing to obtain a primary fired body having an electrolyte layer and anode. At this time, diffusion of the second metal element occurs between the electrolyte green sheet and the anode green sheet. As a result, the region of the electrolyte layer in contact with the anode becomes a transition region with a concentration gradient of the second metal element. By adjusting the firing conditions, a primary fired body having the target M2 / M1 value can be obtained. The longer the holding time at high temperature during firing, the more the diffusion of the second metal element between the electrolyte green sheet and the anode green sheet is promoted.

[0063] Next, a solvent is added to the metal oxide powder that will be the material for the reaction prevention layer, and the mixture is wet-mixed in a ball mill to prepare a reaction prevention layer paste. A solvent is also added to the metal oxide powder that will be the material for the air electrode, and the mixture is wet-mixed in a ball mill to prepare an air electrode paste. The reaction prevention layer paste is printed on the surface of the electrolyte layer of the primary fired body, and then dried to form a reaction prevention layer paste layer. The air electrode paste is printed on the surface of this reaction prevention layer paste layer, and then dried to form an air electrode paste layer. The primary fired body after the reaction prevention layer paste layer and air electrode paste layer are formed is degreased at a predetermined temperature and then subjected to secondary firing to obtain a single cell comprising an air electrode, electrolyte layer, reaction prevention layer, and fuel electrode. The firing temperature for the secondary firing is preferably low enough (e.g., 1000°C) to avoid densifying the reaction prevention layer and reducing gas diffusivity.

[0064] A-4. Operation of fuel cell stack 10: 2 and 4, the oxidizing gas OG is supplied to the air chamber 313 through the gas passage member 280A and the oxidizing gas supply manifold 311. Also, as shown in FIGS. 3 and 5, the fuel gas FG is supplied to the fuel chamber 323 through the gas passage member 280B and the fuel gas supply manifold 321.

[0065] When an oxidant gas OG is supplied to the air chamber 313 of each reaction unit 100U and a fuel gas FG is supplied to the fuel chamber 323, power is generated in the single 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 reaction units 100U, and the interconnector 190 ensures electrical continuity between the two adjacent reaction units 100U. In other words, the multiple reaction units 100U included in the fuel cell stack 10 are electrically connected in series. Furthermore, the reaction unit 100U located at one end (the lower end in FIG. 2 ) of the multiple reaction units 100U is electrically connected to a second end plate 230, and the reaction unit 100U located at the other end (the upper end in FIG. 2 ) is electrically connected to a first end plate 210. As a result, electrical energy generated in each reaction unit 100U is extracted from the second end plate 230, which functions as an output terminal 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.

[0066] 2, the oxidant off-gas OOG discharged from the air chamber 313 of each reaction unit 100U to the oxidant gas discharge manifold 312 passes through the inside of the gas passage member 280A and is discharged to the outside of the fuel cell stack 10. Also, as shown in FIG. 3, the fuel off-gas FOG discharged from the fuel chamber 323 of each reaction unit 100U to the fuel gas discharge manifold 322 passes through the inside of the gas passage member 280B and is discharged to the outside of the fuel cell stack 10.

[0067] A-5. Advantages of this embodiment: As described above, the fuel cell stack 10 of this embodiment includes a unit cell 110. The unit cell 110 includes an electrolyte layer 112 having a first surface 112F1 and a second surface 112F2 opposite the first surface 112F1, an anode 116 containing nickel and disposed on the first surface 112F1, and an cathode 114 disposed on the second surface 112F2. The electrolyte layer 112 includes a stabilized metal oxide in which a first metal oxide containing a first metal element that forms tetravalent cations is solid-dissolved with a second metal oxide containing a second metal element that forms divalent or trivalent cations, and the first metal element is zirconium or cerium. When the concentration of the first metal element contained in the electrolyte layer 112 is M1 and the concentration of the second metal element contained in the electrolyte layer 112 is M2, the electrolyte layer 112 is arranged on a first surface 112F1 and has a change region R1 in which the value of M2 / M1 changes gradually in the direction from the first surface 112F1 to the second surface 112F2.

[0068] According to the above configuration, separation of the fuel electrode 116 from the electrolyte layer 112 can be suppressed, and the durability of the unit cell 110 is improved.

[0069] Furthermore, it is preferable that the value of M2 / M1 at the boundary between the changed region R1 and the anode 116 and the value of M2 / M1 at the boundary between the changed region R1 and the non-changed region R2 of the electrolyte layer 112 excluding the changed region R1 are both 0.10 to 0.13 when the first metal element is Zr, and 0.08 to 0.25 when the first metal element is Ce. With this configuration, it is possible to obtain a unit cell 110 with excellent initial characteristics.

[0070] Furthermore, it is preferable that the unit cell 110 satisfies ML / MS≧1.01, where ML is the larger value of M2 / M1 at the boundary between the changed region R1 and the anode 116 and MS is the smaller value of M2 / M1 at the boundary between the changed region R1 and the non-changed region R2 of the electrolyte layer 112. Such a configuration further enhances the durability of the unit cell 110.

[0071] A-6. Performance evaluation: A-6-1. Sample creation (Test Example 1) A solvent was added to YSZ powder with an yttria concentration of 8 mol%, and the mixture was wet mixed in a ball mill to obtain a slurry, which was then processed into a sheet shape using a doctor blade method to produce a green sheet for the electrolyte.

[0072] NiO powder and YSZ powder with a 10 mol% yttria concentration were weighed to achieve the desired composition. A solvent was added to these raw material powders, and they were wet-mixed in a ball mill to obtain a slurry. The resulting slurry was processed into a sheet shape using a doctor blade method to produce a green sheet for the anode.

[0073] The electrolyte green sheet and the fuel electrode green sheet were stacked and pressed together to obtain a laminate. After degreasing the laminate at a predetermined temperature, it was placed in an electric furnace and fired at 1400°C for 3 hours (primary firing) to obtain a primary fired body having an electrolyte layer and a fuel electrode.

[0074] A solvent was added to GDC powder and wet-mixed in a ball mill to prepare a paste for the reaction prevention layer. A solvent was added to LSCF powder and wet-mixed in a ball mill to prepare a paste for the air electrode. The paste for the reaction prevention layer was printed on the surface of the electrolyte layer of the primary fired body and then dried to form a paste layer for the reaction prevention layer. The paste for the air electrode was printed on the surface of this paste layer for the reaction prevention layer and then dried to form a paste layer for the air electrode. The primary fired body after the paste layer for the reaction prevention layer and the paste layer for the air electrode were degreased at a predetermined temperature and then placed in an electric furnace and fired at 1000°C for a certain period of time (secondary firing) to obtain a single cell sample equipped with an air electrode, electrolyte layer, reaction prevention layer, and fuel electrode.

[0075] (Test Examples 2-6, 12, and 13) The concentration of yttria in YSZ and the conditions for primary firing were as shown in Table 1, and a single cell sample was obtained in the same manner as in Test Example 1. In Table 1, the firing condition "1300°C, 0 h" means that after the temperature inside the electric furnace was raised to 1300°C, the temperature was immediately lowered without being maintained at 1300°C.

[0076] (Test Examples 7-11, 14, and 15) A single cell sample was obtained in the same manner as in Test Example 1, except that GDC powder was used instead of YSZ powder, the concentration of gadolinia in GDC, and the firing conditions for the primary firing were as shown in Table 1.

[0077] [Table 1]

[0078] A-6-2. Analysis by EPMA For each sample obtained, an EPMA was used to obtain an image of a cross section perpendicular to the surface (first face 112F1) of the electrolyte layer 112 (see FIG. 7). The obtained images were subjected to mapping analysis of the first metal element (zirconium or cerium) and the second metal element (yttrium or gadolinium).

[0079] Multiple measurement lines were set perpendicular to the first surface 112F1 of the electrolyte layer 112, and measurement points were set every 0.1 μm on each measurement line to calculate the concentrations of the first metal element and the second metal element at each measurement point. The average value of the concentrations of the first metal element obtained at multiple measurement points at the same depth from the first surface 112F1 was calculated and defined as the concentration M1 of the first metal element at that depth. The concentration M2 of the second metal element was calculated in the same manner at each depth. The value of M2 / M1 at each depth was calculated from the obtained value. The region where the value of M2 / M1 changes stepwise from the first surface 112F1 was defined as the change region R1.

[0080] Within the changed region R1, a region extending from the first surface 112F1 to a depth one-fifth the thickness of the changed region R1 was defined as a first changed region R11 (corresponding to the "boundary position with the anode"). Furthermore, a region extending from the boundary between the changed region R1 and the non-changed region R2 (the position indicated by the dashed line in FIG. 7) to a depth one-fifth the thickness of the changed region R1 was defined as a second changed region R12 (corresponding to the "boundary position with another region"). The thickness of the changed region R1 is the distance between the first surface 112F1 and the boundary between the changed region R1 and the other non-changed region R2. The average value of the M2 / M1 values ​​calculated for multiple depth positions included in the first changed region R11 was defined as the M2 / M1 value for the first changed region R11. Furthermore, the average value of the M2 / M1 values ​​calculated for multiple depth positions included in the second changed region R12 was defined as the M2 / M1 value for the second changed region R12. The larger of the values ​​of M2 / M1 for the first change region R11 and M2 / M1 for the second change region R12 was defined as ML, and the smaller of the values ​​of M2 / M1 for the second change region R12 was defined as MS, and the value of ML / MS was calculated.

[0081] A-6-3. Initial Characterization: For a fuel cell stack using each sample as a single cell, oxidant gas was supplied to the air electrode at approximately 700°C, and fuel gas was supplied to the fuel electrode, and the current density was 0.55 (A / cm 2 The output voltage of the single cell at this time was measured, and this measurement value was designated as the initial voltage (output voltage before rated power generation operation). Each sample was then evaluated as follows: an initial voltage of 0.8 V or higher was rated as ◎, and an initial voltage of less than 0.8 V was rated as ○.

[0082] A-6-4. Thermal cycle test: Each sample prepared as a button cell was placed in an electric furnace and heated from room temperature to 700°C over 1.5 hours, held at 700°C for 3 hours, and cooled from 700°C to room temperature over 13 hours. This cycle was repeated 10 times. Before and after the thermal cycle test, oxidant gas was supplied to the air electrode at approximately 700°C, and fuel gas was supplied to the anode, and the current density was 0.55 (A / cm 2The output voltage of the button cell was measured. For each sample, the voltage drop rate (unit: %) calculated using the following formula (1) was evaluated as follows: ◎ if it was 1% or less, ◯ if it was more than 1% and 1.5% or less, and × if it was more than 1.5%. Voltage drop rate (%) = {(voltage before thermal cycle test - voltage after thermal cycle test) / voltage before thermal cycle test} × 100 (1)

[0083] A-6-5. Results: In Test Examples 12-15, no change region was formed in the electrolyte layer. Furthermore, in all of these test examples, the voltage drop rate exceeded 1.5%. It is believed that in these test examples, interfacial peeling occurred between the fuel electrode and the electrolyte layer due to the thermal cycle, reducing the durability of the single cell.

[0084] In Test Examples 1-11, a change region was formed in the electrolyte layer. Furthermore, in all of these test examples, the voltage drop rate was 1.5% or less. It is believed that the presence of a change region in the electrolyte layer in these test example samples suppressed interfacial delamination between the fuel electrode and the electrolyte layer, maintaining the durability of the single cell and resulting in a low voltage drop rate. In Test Examples 1-3, 7, and 8, the M2 / M1 value gradually decreased from the first surface to the second surface, while in Test Examples 4-6 and 9-11, the M2 / M1 value gradually increased from the first surface to the second surface. However, in all cases, it was confirmed that the durability of the single cell was maintained.

[0085] Comparing Test Examples 3 and 6, in which the M L / MS value was 1.04, with Test Examples 1, 2, 4, 5, and 7-11, in which the M L / MS value was greater than 1.04, the voltage drop rate was smaller in the latter than in the former. It is believed that the latter, in which the M L / MS value was greater, had a sufficiently large concentration gradient of the second metal element in the change region R1, which effectively suppressed peeling of the fuel electrode 116, resulting in a lower voltage drop rate.

[0086] Furthermore, in Test Examples 1-11, all of the initial voltages were values ​​that could withstand practical use. In particular, among Test Examples 1-6 in which the first metal element was Zr, Test Examples 1, 3, 4, and 6, in which the ML / MS values ​​in the first and second change regions were 0.10 or more and 0.13 or less, had initial voltages of 0.8 V or more, confirming that they had excellent initial characteristics. Furthermore, among Test Examples 7-11 in which the first metal element was Ce, Test Examples 7, 9, and 11, in which the ML / MS values ​​in the first and second change regions were 0.08 or more and 0.25 or less, had initial voltages of 0.8 V or more, confirming that they had excellent initial characteristics. In these Test Examples, the ratio of the content of the second metal element to the content of the first metal element was neither too high nor too low, and the two metal elements were contained in a balanced manner, which is thought to maintain good ionic conductivity and obtain excellent initial characteristics.

[0087] 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. (1) In the above embodiment, the reaction prevention layer 118 is interposed between the electrolyte layer 112 and the air electrode 114, but it is not necessary for a reaction prevention layer to be interposed between the electrolyte layer and the air electrode. (2) The above configuration can also be applied to cell stacks used in other types of fuel cells, such as polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), and molten carbonate fuel cells (MCFCs), or to electrolysis cell stacks that include electrolysis cell units, which are the constituent elements of solid oxide electrolysis cells (SOECs), as single cells. [Explanation of symbols]

[0088] 10: Fuel cell stack (electrochemical reaction cell stack) 100: Power generation block 100U: Electrochemical reaction unit 110: Single cell (electrochemical reaction cell) 112: Electrolyte layer 112F1: First surface 112F2: Second surface 114: Air electrode 116: Anode 118: Reaction prevention layer 120: Single cell separator 121: Through hole 124: Joint portion 125: Glass seal portion 130: Air electrode frame 131: Through hole 140: Anode frame 141: Through hole 144: Anode current collecting member 145: Electrode opposing portion 146: Interconnector opposing portion 149: Spacer 180: IC separator 181: Through hole 190: Interconnector 191: Flat plate portion 192: Air electrode current collecting portion 193: Covering layer 194: Oxide coating 196: Conductive bonding material 210: First end plate 211: Through hole 220: End separator 230: Second end plate 231: Through hole 280A, 280B: Gas passage member 281A, 281B: Main body 282A, 282B: Branching section 311: Oxidant gas supply manifold 312: Oxidant gas discharge manifold 313: Air chamber 321: Fuel gas supply manifold 322: Fuel gas discharge manifold 323: Fuel chamber B: Bolt BH: Bolt hole FG: Fuel gas FOG: Fuel off-gas N: Nut OG: Oxidant gas OOG: Oxidant off-gas R1: Change region R11: First change region (boundary position with fuel electrode) R12: Second change region (boundary position with other region) R2: Non-change region (other region) S: Insulation sheet

Claims

1. an electrolyte layer having a first surface and a second surface opposite the first surface; an anode including nickel and disposed on the first surface; a cathode disposed on the second surface; Equipped with the electrolyte layer includes a stabilized metal oxide in which a first metal oxide containing a first metal element that becomes a tetravalent cation is dissolved in a second metal oxide containing a second metal element that becomes a divalent or trivalent cation, the first metal element is zirconium or cerium; When the concentration of the first metal element contained in the electrolyte layer is M1 and the concentration of the second metal element contained in the electrolyte layer is M2, The electrolyte layer is a change region disposed on the first surface, in which the value of M2 / M1 changes stepwise in a direction from the first surface toward the second surface; Electrochemical reaction cell.

2. The value of M2 / M1 at the boundary position of the change region with the anode and the value of M2 / M1 at the boundary position of the electrolyte layer with other regions excluding the change region are both When the first metal element is zirconium, the ratio is 0.10 or more and 0.13 or less; When the first metal element is cerium, the ratio is 0.08 or more and 0.25 or less. The electrochemical reaction cell according to claim 1 .

3. When the larger value of M2 / M1 at the boundary position of the change region with the anode and the smaller value of M2 / M1 at the boundary position of the electrolyte layer with other regions excluding the change region are designated as ML and MS, ML / MS≧1.01 is satisfied; The electrochemical reaction cell according to claim 1 or 2.

4. An electrochemical reaction cell stack comprising the electrochemical reaction cell according to claim 1 or 2.

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