JP7465849B2 - Electrochemical reaction single cells and electrochemical reaction cell stacks - Google Patents

Description

The technology disclosed in this specification relates to a single electrochemical reaction cell.

One type of fuel cell that generates electricity using the electrochemical reaction between hydrogen and oxygen is the solid oxide fuel cell (hereinafter referred to as "SOFC"), which has an electrolyte layer containing solid oxide. The smallest structural unit of an SOFC, a single fuel cell (hereinafter simply referred to as "single cell"), includes an electrolyte layer, and an air electrode and a fuel electrode that face each other in a specific direction (hereinafter referred to as the "first direction") across the electrolyte layer.

The electrolyte layer has a plurality of electrolyte particles and grain boundaries that exist between the plurality of electrolyte particles. Conventionally, a technology has been known that attempts to achieve both high bending strength and high ionic conductivity in an electrolyte layer based on the number of grain boundaries in a cross section of the electrolyte layer along a first direction (see, for example, Patent Document 1).

JP 2017-199601 A

In a single cell, stress is generated in the electrolyte layer due to the difference in thermal expansion between the electrolyte layer and the adjacent layer on the air electrode side (e.g., the air electrode or intermediate layer), which can result in, for example, peeling between the electrolyte layer and the adjacent layer or damage to the electrolyte layer (e.g., the occurrence and growth of cracks). Until now, the relationship between the suppression of stress concentration in the electrolyte layer and the grain boundaries in the electrolyte layer has not been studied.

These issues are also common to single electrolytic cells, which are the building blocks of solid oxide electrolysis cells (hereinafter referred to as "SOECs") that generate hydrogen using the electrolysis reaction of water. In this specification, single fuel cell cells and single electrolytic cells are collectively referred to as single electrochemical reaction cells.

The technology disclosed in this specification can be realized, for example, in the following forms:

(1) The electrochemical reaction single cell disclosed in this specification is an electrochemical reaction single cell including an electrolyte layer, and an air electrode and a fuel electrode that face each other in a first direction with the electrolyte layer sandwiched therebetween, and when viewed in a specific cross section that is at least one cross section along the first direction, the surfaces of the particles located between two adjacent grain boundary points at the interface on the air electrode side of the electrolyte layer are located within a range of ±0.2 μm or less in a direction perpendicular to a virtual line connecting the two adjacent grain boundary points. According to this electrochemical reaction single cell, compared to a configuration in which the unevenness of the surface of the particles located between the two grain boundary points exceeds a predetermined range, it is possible to suppress the concentration of stress at a specific location at the interface between the electrolyte layer and the layer adjacent thereto (e.g., the air electrode or intermediate layer).

(2) In the electrochemical reaction single cell, when viewed in the specific cross section, the surface of a particle located between two adjacent grain boundary points in at least a part of the interface on the fuel electrode side of the electrolyte layer may be located outside a range of ±0.2 μm or less in a direction perpendicular to a virtual line connecting the two adjacent grain boundary points. According to this electrochemical reaction single cell, stress concentration is suppressed at the interface on the air electrode side of the electrolyte layer, while the contact area between the electrolyte layer and the fuel electrode is increased at the interface on the fuel electrode side, thereby improving the diffusibility of the fuel gas from the fuel electrode to the electrolyte layer. In particular, when the electrolyte layer and the fuel electrode contain a common component (e.g., a common oxygen ion conductive oxide), the thermal expansion difference between the electrolyte layer and the fuel electrode can be reduced, so that even if the unevenness of the particles at the interface on the fuel electrode side of the electrolyte layer is increased, stress concentration at a specific point at the interface between the electrolyte layer and the fuel electrode can be suppressed.

(3) In an electrochemical reaction cell stack having a plurality of electrochemical reaction single cells, at least one of the plurality of electrochemical reaction single cells may be configured to be an electrochemical reaction single cell of (1) or (2). This electrochemical reaction cell stack can suppress stress concentration at a specific location at the interface between the electrolyte layer and the layer adjacent thereto.

The technology disclosed in this specification can be realized in various forms, such as an electrochemical reaction unit cell (a fuel cell unit cell or an electrolysis unit cell), an electrochemical reaction cell stack (a fuel cell stack or an electrolysis cell stack) having a plurality of electrochemical reaction units each having an electrochemical reaction unit cell, or a manufacturing method thereof.

FIG. 1 is a perspective view showing an external configuration of a fuel cell stack 100 according to an embodiment. FIG. 2 is an explanatory diagram showing the XZ cross-sectional configuration of the fuel cell stack 100 taken along the line II-II in FIG. FIG. 2 is an explanatory diagram showing a YZ cross-sectional configuration of the fuel cell stack 100 taken along the line III-III in FIG. 1. FIG. 3 is an explanatory diagram showing the XZ cross-sectional configuration of two adjacent power generating units 102 at the same position as the cross-section shown in FIG. 2 . FIG. 4 is an explanatory diagram showing the YZ cross-sectional configuration of two adjacent power generating units 102 at the same position as the cross-section shown in FIG. 3 . FIG. 5 is an explanatory diagram showing a cross-sectional configuration of a portion X1 of a unit cell 110 in FIG. 4 . FIG. 7 is an explanatory diagram showing a cross-sectional configuration of a portion X2 of a unit cell 110 in FIG. 6. FIG. 7 is an explanatory diagram showing a cross-sectional configuration of a portion X3 of a unit cell 110 in FIG. 6 . Diagram showing the performance evaluation results

A. Embodiments:
A-1. composition:
(Configuration of fuel cell stack 100)
FIG. 1 is a perspective view showing the external configuration of a fuel cell stack 100 in this embodiment, FIG. 2 is an explanatory diagram showing the XZ cross-sectional configuration of the fuel cell stack 100 at the position II-II in FIG. 1, and FIG. 3 is an explanatory diagram showing the YZ cross-sectional configuration of the fuel cell stack 100 at the position III-III in FIG. 1. Each figure shows mutually orthogonal XYZ axes for specifying directions. In this specification, for convenience, the positive direction of the Z axis is called the upward direction, and the negative direction of the Z axis is called the downward direction, but the fuel cell stack 100 may actually be installed in a direction different from such directions. The same applies to FIG. 4 and subsequent figures. The up-down direction is an example of the first direction in the claims.

The fuel cell stack 100 comprises a plurality of (seven in this embodiment) power generating units 102 and a pair of end plates 104, 106. The seven power generating units 102 are arranged in a predetermined arrangement direction (vertical direction in this embodiment). The pair of end plates 104, 106 are arranged to sandwich the assembly consisting of the seven power generating units 102 from above and below.

A plurality of holes (eight in this embodiment) that penetrate in the vertical direction are formed in the peripheral portion around the Z-axis direction of each layer (power generation unit 102, end plates 104, 106) that constitutes the fuel cell stack 100, and corresponding holes formed in each layer communicate with each other in the vertical direction to form communication holes 108 that extend in the vertical direction from one end plate 104 to the other end plate 106. In the following description, the holes formed in each layer of the fuel cell stack 100 to form the communication holes 108 may also be referred to as communication holes 108.

A bolt 22 extending in the vertical direction is inserted into each communication hole 108, and the fuel cell stack 100 is fastened by the bolt 22 and a nut 24 fitted on both sides of the bolt 22. As shown in FIG. 2 and FIG. 3, an insulating sheet 26 is interposed between the nut 24 fitted on one side (upper side) of the bolt 22 and the upper surface of the end plate 104 constituting the upper end of the fuel cell stack 100, and between the nut 24 fitted on the other side (lower side) of the bolt 22 and the lower surface of the end plate 106 constituting the lower end of the fuel cell stack 100. However, at the location where the gas passage member 27 described later is provided, the gas passage member 27 and the insulating sheets 26 arranged on the upper and lower sides of the gas passage member 27 are interposed between the nut 24 and the surface of the end plate 106. The insulating sheet 26 is composed of, for example, a mica sheet, a ceramic fiber sheet, a ceramic powder sheet, a glass sheet, a glass ceramic composite, etc.

The outer diameter of the shaft of each bolt 22 is smaller than the inner diameter of each communication hole 108. Therefore, a space is provided between the outer peripheral surface of the shaft of each bolt 22 and the inner peripheral surface of each communication hole 108. As shown in FIG. 1 and FIG. 2, the space formed by the bolt 22 (bolt 22A) located near the midpoint of one side (the side on the positive side of the X-axis of the two sides parallel to the Y-axis) on the outer periphery around the Z-axis of the fuel cell stack 100 and the communication hole 108 through which the bolt 22A is inserted functions as an oxidizing gas inlet manifold 161, which is a gas flow path through which the oxidizing gas OG is introduced from the outside of the fuel cell stack 100 and the oxidizing gas OG is supplied to each power generation unit 102, and the space formed by the bolt 22 (bolt 22B) located near the midpoint of the opposite side (the side on the negative side of the X-axis of the two sides parallel to the Y-axis) and the communication hole 108 through which the bolt 22B is inserted functions as an oxidizing gas exhaust manifold 162 that exhausts the oxidizing off-gas OOG, which is a gas exhausted from the air chamber 166 of each power generation unit 102, to the outside of the fuel cell stack 100. In this embodiment, for example, air is used as the oxidizing gas OG.

As shown in Figures 1 and 3, the space formed by the bolt 22 (bolt 22D) located near the midpoint of one side (the side on the positive Y-axis side of the two sides parallel to the X-axis) on the outer periphery of the fuel cell stack 100 around the Z-axis direction and the communication hole 108 through which the bolt 22D is inserted functions as a fuel gas introduction manifold 171 through which fuel gas FG is introduced from outside the fuel cell stack 100 and supplied to each power generation unit 102, and the space formed by the bolt 22 (bolt 22E) located near the midpoint of the opposite side (the side on the negative Y-axis side of the two sides parallel to the X-axis) and the communication hole 108 through which the bolt 22E is inserted functions as a fuel gas exhaust manifold 172 that exhausts fuel off-gas FOG, which is gas exhausted from the fuel chamber 176 of each power generation unit 102, to the outside of the fuel cell stack 100. In this embodiment, the fuel gas FG is, for example, hydrogen-rich gas obtained by reforming city gas.

The fuel cell stack 100 is provided with four gas passage members 27. Each gas passage member 27 has a hollow cylindrical main body 28 and a hollow cylindrical branch portion 29 branched from the side of the main body 28. The hole of the branch portion 29 is connected to the hole of the main body 28. A gas pipe (not shown) is connected to the branch portion 29 of each gas passage member 27. As shown in FIG. 2, the hole of the main body 28 of the gas passage member 27 arranged at the position of the bolt 22A forming the oxidizing gas inlet manifold 161 is connected to the oxidizing gas inlet manifold 161, and the hole of the main body 28 of the gas passage member 27 arranged at the position of the bolt 22B forming the oxidizing gas exhaust manifold 162 is connected to the oxidizing gas exhaust manifold 162. Also, as shown in FIG. 3, the hole in the body 28 of the gas passage member 27 located at the position of the bolt 22D that forms the fuel gas inlet manifold 171 is connected to the fuel gas inlet manifold 171, and the hole in the body 28 of the gas passage member 27 located at the position of the bolt 22E that forms the fuel gas exhaust manifold 172 is connected to the fuel gas exhaust manifold 172.

(Configuration of end plates 104, 106)
The pair of end plates 104, 106 are conductive members having a substantially rectangular flat plate shape when viewed in the Z-axis direction, and are made of, for example, stainless steel. One end plate 104 is disposed above the uppermost power generating unit 102, and the other end plate 106 is disposed below the lowermost power generating unit 102. The plurality of power generating units 102 are sandwiched in a pressed state between the pair of end plates 104, 106. The upper end plate 104 functions as a positive output terminal of the fuel cell stack 100, and the lower end plate 106 functions as a negative output terminal of the fuel cell stack 100.

(Configuration of power generation unit 102)
FIG. 4 is an explanatory diagram showing the XZ cross-sectional configuration of two adjacent power generating units 102 at the same position as the cross-section shown in FIG. 2, and FIG. 5 is an explanatory diagram showing the YZ cross-sectional configuration of two adjacent power generating units 102 at the same position as the cross-section shown in FIG. 3.

As shown in Figures 4 and 5, the power generation unit 102 includes a single cell 110, a separator 120, an air electrode side frame 130, an air electrode side current collector 134, an anode side frame 140, an anode side current collector 144, and a pair of interconnectors 150 constituting the top and bottom layers of the power generation unit 102. Holes corresponding to the communication holes 108 through which the bolts 22 described above are inserted are formed on the periphery of the separator 120, the air electrode side frame 130, the anode side frame 140, and the interconnector 150 around the Z-axis direction.

The interconnector 150 is a conductive member having a substantially rectangular flat plate shape when viewed in the Z-axis direction, and is formed of a material containing Cr, such as ferritic stainless steel. The interconnector 150 ensures electrical conduction between the power generating units 102 and prevents the reaction gases from mixing between the power generating units 102. In this embodiment, when two power generating units 102 are arranged adjacent to each other, one interconnector 150 is shared by the two adjacent power generating units 102. That is, the upper interconnector 150 in one power generating unit 102 is made of the same material as the lower interconnector 150 in the other power generating unit 102 adjacent to the upper side of the power generating unit 102. In addition, since the fuel cell stack 100 has a pair of end plates 104, 106, the power generating unit 102 located at the top of the fuel cell stack 100 does not have an upper interconnector 150, and the power generating unit 102 located at the bottom does not have a lower interconnector 150 (see Figures 2 and 3).

The single cell 110 includes an electrolyte layer 112, an air electrode (cathode) 114 arranged above the electrolyte layer 112, a fuel electrode (anode) 116 arranged below the electrolyte layer 112, and an intermediate layer 180 arranged between the electrolyte layer 112 and the air electrode 114. Note that the single cell 110 of this embodiment is an anode-supported single cell in which the fuel electrode 116 supports the other layers (electrolyte layer 112, air electrode 114, intermediate layer 180) that make up the single cell 110.

The electrolyte layer 112 is a flat plate-shaped member that is approximately rectangular when viewed in the Z-axis direction, and is a dense layer (with low porosity). The electrolyte layer 112 contains a solid oxide (e.g., YSZ (yttria-stabilized zirconia), ScSZ (scandia-stabilized zirconia), and CSZ (calcia-stabilized zirconia)). Thus, the unit cell 110 of this embodiment is a solid oxide fuel cell (SOFC) that uses a solid oxide as an electrolyte.

The air electrode 114 is a substantially rectangular flat plate-shaped member smaller than the electrolyte layer 112 when viewed in the Z-axis direction, and is a porous layer having a porosity higher than that of the electrolyte layer 112. The air electrode 114 contains a perovskite-type oxide represented by _ABO3 (e.g., lanthanum strontium cobalt iron oxide (LSCF), lanthanum strontium iron oxide (LSF), lanthanum strontium cobalt oxide (LSC), lanthanum strontium manganese oxide (LSM), lanthanum nickel iron oxide (LNF), etc.).

The fuel electrode 116 is a substantially rectangular flat plate-shaped member having substantially the same size as the electrolyte layer 112 when viewed in the Z-axis direction, and is a porous layer having a higher porosity than the electrolyte layer 112. Although not shown, in this embodiment, the fuel electrode 116 includes a substrate layer constituting the lower surface of the fuel electrode 116, and a functional layer located between the substrate layer and the electrolyte layer 112. The functional layer of the fuel electrode 116 is a layer that mainly functions to generate electrons and water vapor by reacting oxygen ions supplied from the electrolyte layer 112 with hydrogen and the like contained in the fuel gas FG, and contains Ni, which is an electronic conductive material, and an oxygen ion conductive oxide (e.g., YSZ). The substrate layer of the fuel electrode 116 is a layer that mainly functions to support the functional layer, the electrolyte layer 112, and the air electrode 114, and contains Ni, which is an electronic conductive material, and an oxygen ion conductive oxide (e.g., YSZ).

The intermediate layer (reaction prevention layer) 180 is a flat plate-shaped member that is approximately rectangular when viewed in the Z-axis direction. The intermediate layer 180 is formed of a solid oxide having ion conductivity, such as SDC (samarium-doped ceria), GDC (gadolinium-doped ceria), LDC (lanthanum-doped ceria), or YDC (yttrium-doped ceria).

The separator 120 is a frame-like member with a substantially rectangular hole 121 formed near the center that penetrates vertically, and is made of, for example, metal. The periphery of the hole 121 in the separator 120 faces the peripheral portion of the surface of the electrolyte layer 112 on the side of the air electrode 114. The separator 120 is joined to the electrolyte layer 112 (single cell 110) by a joint 124 formed of a brazing material (e.g., Ag brazing) placed in the opposing portion. The separator 120 separates an air chamber 166 facing the air electrode 114 and a fuel chamber 176 facing the fuel electrode 116, thereby preventing gas leakage from one electrode side to the other electrode side at the peripheral portion of the single cell 110.

The air electrode side frame 130 is a frame-shaped member with a substantially rectangular hole 131 formed in the center that penetrates vertically, and is formed of an insulator such as mica. The hole 131 of the air electrode side frame 130 constitutes an air chamber 166 facing the air electrode 114. The air electrode side frame 130 is in contact with the peripheral portion of the surface of the separator 120 opposite to the side facing the electrolyte layer 112, and the peripheral portion of the surface of the interconnector 150 facing the air electrode 114. The air electrode side frame 130 also electrically insulates the pair of interconnectors 150 included in the power generation unit 102. The air electrode side frame 130 also has an oxidizer gas supply communication hole 132 that communicates between the oxidizer gas introduction manifold 161 and the air chamber 166, and an oxidizer gas discharge communication hole 133 that communicates between the air chamber 166 and the oxidizer gas discharge manifold 162.

The fuel electrode side frame 140 is a frame-shaped member with a substantially rectangular hole 141 formed near the center that penetrates in the vertical direction, and is made of, for example, metal. The hole 141 of the fuel electrode side frame 140 constitutes a fuel chamber 176 facing the fuel electrode 116. The fuel electrode side frame 140 is in contact with the peripheral portion of the surface of the separator 120 facing the electrolyte layer 112 and the peripheral portion of the surface of the interconnector 150 facing the fuel electrode 116. The fuel electrode side frame 140 also has a fuel gas supply communication hole 142 that connects the fuel gas introduction manifold 171 and the fuel chamber 176, and a fuel gas discharge communication hole 143 that connects the fuel chamber 176 and the fuel gas discharge manifold 172.

The fuel electrode side current collector 144 is disposed in the fuel chamber 176. The fuel electrode side current collector 144 includes an interconnector facing portion 146, an electrode facing portion 145, and a connecting portion 147 connecting the electrode facing portion 145 and the interconnector facing portion 146, and is formed of, for example, nickel, a nickel alloy, stainless steel, or the like. The electrode facing portion 145 is in contact with the surface of the fuel electrode 116 opposite to the side facing the electrolyte layer 112, and the interconnector facing portion 146 is in contact with the surface of the interconnector 150 facing the fuel electrode 116. However, as described above, since the power generation unit 102 located at the bottom of the fuel cell stack 100 does not include a lower interconnector 150, the interconnector facing portion 146 of the power generation unit 102 is in contact with the lower end plate 106. Since the fuel electrode side current collector 144 is configured in this way, it electrically connects the fuel electrode 116 and the interconnector 150 (or the end plate 106). In addition, a spacer 149 made of, for example, mica is disposed between the electrode facing portion 145 and the interconnector facing portion 146. Therefore, the fuel electrode side current collector 144 follows the deformation of the power generation unit 102 due to temperature cycles and reactant gas pressure fluctuations, and the electrical connection between the fuel electrode 116 and the interconnector 150 (or end plate 106) via the fuel electrode side current collector 144 is well maintained.

The air electrode side current collector 134 is disposed in the air chamber 166. The air electrode side current collector 134 is composed of a plurality of approximately rectangular prism-shaped current collector elements 135, and is formed of a material containing Cr, for example, ferritic stainless steel. The air electrode side current collector 134 is in contact with the surface of the air electrode 114 opposite to the electrolyte layer 112 and the surface of the interconnector 150 facing the air electrode 114. However, as described above, since the power generation unit 102 located at the top of the fuel cell stack 100 does not have an upper interconnector 150, the air electrode side current collector 134 of the power generation unit 102 is in contact with the upper end plate 104. Since the air electrode side current collector 134 has such a configuration, it electrically connects the air electrode 114 and the interconnector 150 (or the end plate 104). A conductive bonding layer that bonds the air electrode 114 and the air electrode side current collector 134 may be interposed between the air electrode 114 and the air electrode side current collector 134. The air electrode side current collector 134 and the interconnector 150 may be configured as an integrated member.

A-2. Operation of the fuel cell stack 100:
2 and 4, when the oxidant gas OG is supplied through a gas pipe (not shown) connected to the branch portion 29 of the gas passage member 27 provided at the position of the oxidant gas introduction manifold 161, the oxidant gas OG is supplied to the oxidant gas introduction manifold 161 through the branch portion 29 of the gas passage member 27 and a hole in the main body 28, and is supplied from the oxidant gas introduction manifold 161 to the air chamber 166 through the oxidant gas supply passage 132 of each power generating unit 102. Also, as shown in Figs. 3 and 5, when the fuel gas FG is supplied through a gas pipe (not shown) connected to the branch portion 29 of the gas passage member 27 provided at the position of the fuel gas introduction manifold 171, the fuel gas FG is supplied to the fuel gas introduction manifold 171 through the branch portion 29 of the gas passage member 27 and a hole in the main body 28, and is supplied to the fuel chamber 176 from the fuel gas introduction manifold 171 through the fuel gas supply passage 142 of each power generating unit 102.

When oxidant gas OG is supplied to the air chamber 166 of each power generation unit 102 and fuel gas FG is supplied to the fuel chamber 176, power is generated in the single cell 110 by an electrochemical reaction of the oxidant gas OG and the fuel gas FG. This power generation reaction is an exothermic reaction. In each power generation unit 102, the air electrode 114 of the single cell 110 is electrically connected to one interconnector 150 via the air electrode side current collector 134, and the fuel electrode 116 is electrically connected to the other interconnector 150 via the fuel electrode side current collector 144. In addition, the multiple power generation units 102 included in the fuel cell stack 100 are electrically connected in series. Therefore, the electrical energy generated in each power generation unit 102 is extracted from the end plates 104, 106 that function as the output terminals of the fuel cell stack 100. In addition, since SOFCs generate electricity at relatively high temperatures (e.g., 700°C to 1000°C), after startup, the fuel cell stack 100 may be heated by a heater (not shown) until the heat generated by power generation can maintain the high temperature.

As shown in Figs. 2 and 4, the oxidant off-gas OOG discharged from the air chamber 166 of each power generation unit 102 is discharged to the oxidant gas discharge manifold 162 through the oxidant gas discharge communication hole 133, and then through the holes in the main body 28 and the branching part 29 of the gas passage member 27 provided at the position of the oxidant gas discharge manifold 162, and is discharged to the outside of the fuel cell stack 100 through the gas pipe (not shown) connected to the branching part 29. Also, as shown in Figs. 3 and 5, the fuel off-gas FOG discharged from the fuel chamber 176 of each power generation unit 102 is discharged to the fuel gas discharge manifold 172 through the fuel gas discharge communication hole 143, and then through the holes in the main body 28 and the branching part 29 of the gas passage member 27 provided at the position of the fuel gas discharge manifold 172, and is discharged to the outside of the fuel cell stack 100 through the gas pipe (not shown) connected to the branching part 29.

A-3. Detailed configuration of the interface of the electrolyte layer 112:
Next, a detailed configuration of the interface of the electrolyte layer 112 in the unit cell 110 constituting the fuel cell stack 100 of this embodiment will be described. Fig. 6 is an explanatory diagram showing a cross-sectional configuration of the X1 portion of the unit cell 110 in Fig. 4. Fig. 6 shows an XZ cross-sectional configuration along the vertical direction of a portion of the unit cell 110 including the intermediate layer 180, the electrolyte layer 112, and the fuel electrode 116 arranged adjacent to each other in the vertical direction. Fig. 6 is a schematic diagram showing an SEM image (e.g., 5000x) of the X1 portion taken by FIB-SEM (accelerating voltage 1.0 kV) showing a cross section along the vertical direction.

Figure 6 shows the configuration of the electrolyte layer 112 near the first interface M1 (interface between the electrolyte layer 112 and the intermediate layer 180) on the intermediate layer 180 (air electrode 114) side, and the configuration of the electrolyte layer 112 near the second interface M2 (interface between the electrolyte layer 112 and the fuel electrode 116) on the fuel electrode 116 side. Note that the XZ cross section of the single cell 110 shown in Figure 6 is an example of a specific cross section within the scope of the claims. Below, the configuration of the electrolyte layer 112 near the first interface M1 and the configuration near the second interface M2 will be explained separately.

(Configuration of the First Interface M1 and Its Vicinity)
Fig. 7 is an explanatory diagram showing the cross-sectional configuration of the X2 portion of the unit cell 110 in Fig. 6. Fig. 7 shows the configuration of the electrolyte layer 112 near the first interface M1. Note that in Fig. 7, cross-sectional hatching of each layer is omitted in order to make it easier to see the virtual straight line L, which will be described later.

As shown in FIG. 7 , when viewed in the XZ cross section of the single cell 110, among the particles R constituting the electrolyte layer 112, the particle R constituting the first interface M1 (hereinafter referred to as the “first particle R1”) satisfies the following first surface condition.
First surface condition: The surface of a first grain R1 located between two adjacent grain boundary points P is located within a range of ± a predetermined distance or less in the vertical direction with respect to a virtual line L connecting the two adjacent grain boundary points P.

Specifically, the contour lines of the surfaces of all the first particles R1 (contour lines constituting the first interface M1) are located within a range of ± a predetermined distance or less in the vertical direction with respect to the virtual line L. Here, the grain boundary point P is the intersection point of the grain boundary existing between the adjacent first particles R1 and the first interface M1. The virtual line L is a line connecting two adjacent grain boundary points P. The upper limit virtual line LU is a line that is parallel to the virtual line L and is spaced a predetermined distance upward (towards the positive side of the Z axis) from the virtual line L. The lower limit virtual line LD is a line that is parallel to the virtual line L and is spaced a predetermined distance downward (towards the negative side of the Z axis) from the virtual line L. The predetermined distance may be 0.2 μm or less, 0.15 μm or less, or 0.1 μm or less.

In the example of FIG. 7, the first surface condition is satisfied for all the first particles R1 constituting the first interface M1. For example, the contour line of the surface of the first particle R1 located between the two grain boundary points P1 and P2 is located within a range of ± a predetermined distance or less from the virtual line L1. The contour line of the surface of the first particle R1 located between the two grain boundary points P2 and P3 is located within a range of ± a predetermined distance or less from the virtual line L2. The contour line of the surface of the first particle R1 located between the two grain boundary points P3 and P4 is located within a range of ± a predetermined distance or less from the virtual line L3. The contour line of the surface of the first particle R1 located between the two grain boundary points P4 and P5 is located within a range of ± a predetermined distance or less from the virtual line L4.

(Configuration of the vicinity of the second interface M2)
Fig. 8 is an explanatory diagram showing the cross-sectional configuration of the X3 portion of the unit cell 110 in Fig. 6. Fig. 8 shows the configuration of the electrolyte layer 112 near the second interface M2. Note that in Fig. 8, cross-sectional hatching of each layer is omitted in order to make it easier to see the virtual line N, which will be described later.

As shown in FIG. 8, when viewed in the XZ cross section of the single cell 110, among the particles R constituting the electrolyte layer 112, the particles R constituting the second interface M2 (hereinafter referred to as the “second particles R2”) satisfy the following second surface condition.
Second surface condition: The surface of the second grain R2 located between two adjacent grain boundary points Q is located outside the range of ± a predetermined distance in the vertical direction with respect to the virtual line N connecting the two adjacent grain boundary points Q. The predetermined distance is the same as the predetermined distance in the first surface condition.

Specifically, the contour line of the surface of at least a part of the second particles R2 (the contour line constituting the second interface M2) is located outside the range of ± a predetermined distance or less in the vertical direction with respect to the virtual line N. Here, the grain boundary point Q is the intersection point of the grain boundary existing between the adjacent second particles R2 and the second interface M2. The virtual line N is a line connecting two adjacent grain boundary points Q. The upper limit virtual line NU is a line that is parallel to the virtual line N and is spaced a predetermined distance upward (towards the positive side of the Z axis) from the virtual line N. The lower limit virtual line ND is a line that is parallel to the virtual line N and is spaced a predetermined distance downward (towards the negative side of the Z axis) from the virtual line N.

In the example of FIG. 8, the second surface condition is satisfied for some of the second particles R2 that constitute the second interface M2. For example, the contour line of the surface of the second particle R2 located between the two grain boundary points Q1 and Q2 is located outside the range of ± a predetermined distance or less from the virtual line N1. The contour line of the surface of the second particle R2 located between the two grain boundary points Q2 and Q3 is located outside the range of ± a predetermined distance or less from the virtual line N2. The contour line of the surface of the second particle R2 located between the two grain boundary points Q3 and Q4 is located within the range of ± a predetermined distance or less from the virtual line N3. The contour line of the surface of the second particle R2 located between the two grain boundary points Q4 and Q5 is located outside the range of ± a predetermined distance or less from the virtual line L4.

The thickness of the electrolyte layer 112 is preferably 3 μm or more and 20 μm or less. By making the thickness of the electrolyte layer 112 3 μm or more, sufficient strength of the electrolyte layer 112 can be ensured, and by making the thickness of the electrolyte layer 112 20 μm or less, excellent initial performance of the single cell 110 can be obtained.

A-4. Manufacturing method of the fuel cell stack 100:
The fuel cell stack 100 of this embodiment can be manufactured, for example, as follows.

(Formation of a laminate of the electrolyte layer 112 and the fuel electrode 116)
Butyral resin, dioctyl phthalate (DOP) as a plasticizer, a dispersant, and a mixed solvent of toluene and ethanol are added to the YSZ powder, and mixed in a ball mill to prepare a slurry. The obtained slurry is thinned by a doctor blade method to obtain a green sheet for an electrolyte layer of a predetermined thickness (for example, about 6 μm). In addition, organic beads as a pore former, butyral resin, DOP as a plasticizer, a dispersant, and a mixed solvent of toluene and ethanol are added to a mixed powder of NiO powder and YSZ powder, and mixed in a ball mill to prepare a slurry. The obtained slurry is thinned by a doctor blade method to obtain a green sheet for an anode substrate layer of a predetermined thickness (for example, about 400 μm). In addition, butyral resin, DOP as a plasticizer, a dispersant, and a mixed solvent of toluene and ethanol are added to a mixed powder of NiO powder and YSZ powder, and mixed in a ball mill to prepare a slurry. The resulting slurry is thinned by a doctor blade method to obtain a green sheet for the anode functional layer having a predetermined thickness (for example, about 11 μm). The green sheets are attached and pressed together to obtain a molded body in which the green sheet for the anode substrate layer, the green sheet for the anode functional layer, and the green sheet for the electrolyte layer are laminated in this order.

Here, as described below, by adjusting at least one of the pressure (hereinafter referred to as "bonding pressure"), temperature (hereinafter referred to as "bonding temperature"), and time for which pressure is applied (hereinafter referred to as "bonding time") when the green sheet for the anode substrate layer, the green sheet for the anode functional layer, and the green sheet for the electrolyte layer are bonded together, it is possible to manufacture a single cell 110 that satisfies the above-mentioned first and second surface conditions. In addition, when the green sheet for the anode substrate layer, the green sheet for the anode functional layer, and the green sheet for the electrolyte layer are bonded together, the flat surface of a metal body (not shown) is pressed against the green sheet for the electrolyte layer. As a result, as described above, the surface of the electrolyte layer 112 on the first interface M1 side becomes flat and satisfies the first surface condition, and the surface on the second interface M2 side satisfies the second surface condition.

Next, the molded body is degreased at a predetermined temperature (e.g., about 280°C) and then sintered at a predetermined temperature (e.g., about 1350°C) for a predetermined time (e.g., about 1 hour). This results in a laminate of the electrolyte layer 112 and the fuel electrode 116.

(Formation of intermediate layer 180)
Polyvinyl alcohol as an organic binder and butyl carbitol as an organic solvent are added to the GDC powder, mixed, and the viscosity is adjusted to prepare a paste for the intermediate layer. The obtained paste for the intermediate layer is applied to the surface of the electrolyte layer 112 in the above-mentioned laminate by, for example, screen printing, and fired at a predetermined temperature (for example, 1200° C.). As a result, the intermediate layer 180 is formed, and a laminate of the intermediate layer 180, the electrolyte layer 112, and the fuel electrode 116 is obtained.

(Formation of the air electrode 114)
A mixture of a perovskite oxide (e.g., LSCF) powder and a sulfate (e.g., SrSO ₄ ) powder is prepared, and polyvinyl alcohol as an organic binder and butyl carbitol as an organic solvent are added to the mixture, and the viscosity is adjusted to prepare a cathode paste. The obtained cathode paste is applied to the surface of the intermediate layer 180 in the laminate of the intermediate layer 180, electrolyte layer 112, and anode 116 described above, for example, by screen printing, and dried, and the laminate to which the cathode paste is applied is fired at a predetermined temperature (e.g., about 1100° C.). This forms the cathode 114, and the single cell 110 including the anode 116, electrolyte layer 112, intermediate layer 180, and cathode 114 is fabricated.

After producing multiple unit cells 110 according to the above-mentioned method, assembly processes (e.g., a process of attaching other components such as separators 120 to each unit cell 110, a process of stacking multiple unit cells 110, and a process of fastening them with bolts 22) are performed. This completes the manufacture of the fuel cell stack 100.

A-5. Advantages of this embodiment:
As described above, the single cell 110 constituting the fuel cell stack 100 of this embodiment includes the electrolyte layer 112, and the air electrode 114 and the fuel electrode 116 which face each other with the electrolyte layer 112 in between. Furthermore, an intermediate layer 180 is formed between the air electrode 114 and the fuel electrode 116.

Here, the inventor has newly found through intensive research that in the single cell 110, when the first particles R1 constituting the first interface M1 on the air electrode 114 side of the electrolyte layer 112 satisfy a first surface condition, stress concentration at a specific location at the interface between the electrolyte layer 112 and the intermediate layer 180 can be suppressed. This first surface condition is that the surface of the first particles R1 located between two adjacent grain boundary points P is located within a range of ± a predetermined distance in the vertical direction with respect to the virtual line L connecting the two adjacent grain boundary points P. In other words, the fact that the first particles R1 constituting the first interface M1 satisfy the first surface condition means that the surface shape of the first particles R1 constituting the first interface M1 constitutes a relatively gentle unevenness. As a result, it is considered that stress concentration at a specific location at the first interface M1 is suppressed.

Furthermore, as described below, in this embodiment, the first particles R1 constituting the first interface M1 satisfy the first surface condition, thereby improving the electrochemical reaction characteristics of the single cell 110 (e.g., the initial voltage of the single cell). This is because, according to this embodiment, the bonding strength between the electrolyte layer 112 and the intermediate layer 180 is improved, improving the ionic conductivity between the electrolyte layer 112 and the intermediate layer 180, which is believed to result in improved electrochemical reaction characteristics of the single cell 110.

In this embodiment, the second particle R2 constituting the second interface M2 on the fuel electrode 116 side of the electrolyte layer 112 satisfies the second surface condition. This second surface condition is that the surface of the second particle R2 located between two adjacent grain boundary points Q is located outside the range of ± a predetermined distance in the vertical direction with respect to the virtual line N connecting the two adjacent grain boundary points Q. In other words, at least a part of the second particle R2 constituting the second interface M2 does not satisfy the first surface condition. As a result, while stress concentration is suppressed at the first interface M1 on the air electrode 114 side, the contact area between the electrolyte layer 112 and the fuel electrode 116 is increased at the second interface M2 on the fuel electrode side, thereby improving the diffusibility of the fuel gas FG from the fuel electrode 116 to the electrolyte layer 112.

A-6. Performance evaluation:
The performance evaluation performed using a plurality of samples of the unit cell 110 will be described below. Fig. 9 is an explanatory diagram showing the performance evaluation results. As shown in Fig. 9, the samples S1 to S4 are different from each other in at least one of the compression pressure, compression temperature, and compression time among the compression conditions when the anode substrate layer green sheet, the anode functional layer green sheet, and the electrolyte layer green sheet are compressed together in the manufacturing process of the unit cell 110.

Specifically, in sample S1, the bonding pressure is 46 MPa, the bonding temperature is 65°C, and the bonding time is 15 seconds. Sample S2 differs from sample S1 in that the bonding pressure is 20 MPa, but the bonding temperature and bonding time are the same. Sample S3 differs from sample S1 in that the bonding temperature is 30°C, but the bonding pressure and bonding time are the same. Sample S4 differs from sample S1 in that the bonding time is 30 seconds, but the bonding pressure and bonding temperature are the same. The thickness of the electrolyte layer 112 in samples S1 to S4 is 10 μm.

The cross sections of each of the samples S1 to S4, which have different bonding conditions, are observed with an FIB-SEM (acceleration voltage 1.0 kV) at a magnification of, for example, 5000 times to check whether the first particles R1 constituting the electrolyte layer 112 satisfy the first surface condition and the second surface condition. As a result of the cross-sectional observation, as shown in FIG. 6, the first surface condition was satisfied in samples S1 and S4, but the first surface condition was not satisfied in samples S2 and S3. In addition, the second surface condition was satisfied in samples S1 to S3, but the second surface condition was not satisfied in sample S4.

Next, a thermal cycle test was performed on each of the samples S1 to S4. In the thermal cycle test, the samples S1 to S4, which were fabricated as button cells, were heated (from room temperature to 700°C) for 1.5 hours, then maintained at 700°C for 3 hours, and cooled (from 700°C to room temperature) for 13 hours. Each cycle consisted of 10 cycles. Before and after the thermal cycle test, an oxidant gas OG was supplied to the air electrode 114 at about 700°C, a fuel gas FG was supplied to the fuel electrode 116, and the output voltage of the button cell was measured when the current density was 0.55 (A/cm ² ). For each sample, if the decrease in output voltage before and after the thermal cycle test was equal to or less than that of sample S1, it was marked as "○", and if it was greater than that of sample S1, it was marked as "×".

As a result, samples S1 and S4, which satisfied the first surface condition, were evaluated as passing (◯). On the other hand, samples S2 and S3, which did not satisfy the first surface condition, were evaluated as failing (×). It is presumed that the reason for the large drop in output voltage before and after the thermal cycle test in samples S2 and S3 was that cracks and breaks occurred in the electrolyte layer 112, etc. due to stress concentration at specific points at the interface between the electrolyte layer 112 and the intermediate layer 180.

Also, an initial characteristic evaluation was performed for each of the samples S1 to S4. In the initial characteristic evaluation, for the fuel cell stack 100 using each sample, an oxidant gas OG was supplied to the air electrode 114 at about 700 (°C), a fuel gas FG was supplied to the fuel electrode 116, and the output voltage of the single cell 110 was measured when the current density was 0.55 (A/cm ² ), and the measured value was taken as the initial voltage (output voltage before rated power generation operation). Then, for each sample, if the initial voltage was greater than the judgment voltage (for example, 940 (mV)), it was marked as "◯", and if it was equal to or less than the judgment voltage, it was marked as "X".

As a result, in samples S1 to S3 that satisfied the second surface condition, the initial voltage was greater than the judgment voltage and was evaluated as pass (◯). On the other hand, in sample S4 that did not satisfy the second surface condition, the initial voltage was smaller than the judgment voltage and was evaluated as fail (×). In sample S4, the contact area between the electrolyte layer 112 and the fuel electrode 116 was smaller than in the other samples, so it is presumed that the reaction field at the interface of the electrolyte layer 112 on the fuel electrode 116 side was reduced, resulting in a low initial voltage evaluation. In the overall evaluation, sample S1, which satisfied both the first and second surface conditions, was evaluated as "◯", while samples S2 to S4, which did not satisfy either the first or second surface conditions, were evaluated as "×".

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 of the invention. For example, the following modifications are also possible.

The configuration of the single cell 110 or fuel cell stack 100 in the above embodiment is merely an example, and various modifications are possible. For example, in the above embodiment, the single cell 110 includes the intermediate layer 180, but the single cell 110 does not have to include the intermediate layer 180. In such a configuration that does not include the intermediate layer 180, by applying the present invention, it is possible to suppress the concentration of stress at a specific location at the interface between the electrolyte layer 112 and the air electrode 114.

In the above embodiment, it is not necessary for all of the second particles R2 constituting the second interface M2 on the fuel electrode 116 side of the electrolyte layer 112 to satisfy the second surface condition.

In the above embodiment, the fuel electrode 116 has a two-layer structure of a substrate layer and a functional layer, but the fuel electrode 116 may have a single layer structure or a structure of three or more layers. An intermediate layer may be formed between the fuel electrode 116 and the electrolyte layer 112. In the above embodiment, the number of unit cells 110 included in the fuel cell stack 100 is merely an example, and the number of unit cells 110 is appropriately determined depending on the output voltage required for the fuel cell stack 100, etc.

In addition, in the above embodiment, the space between the outer peripheral surface of the shaft of each bolt 22 and the inner peripheral surface of each communication hole 108 is used as each manifold, but instead, an axial hole may be formed in the shaft of each bolt 22 and the hole may be used as each manifold. Also, each manifold may be provided separately from each communication hole 108 into which each bolt 22 is inserted.

The materials constituting each component in the above embodiment are merely examples, and each component may be made of other materials.

In the above embodiment, the first particles R1 constituting the first interface M1 do not necessarily need to satisfy the first surface condition for all unit cells 110 included in the fuel cell stack 100. As long as the first particles R1 satisfy the first surface condition for at least one unit cell 110 included in the fuel cell stack 100, it is possible to suppress stress concentration at a specific location at the interface between the electrolyte layer 112 and the air electrode 114. In addition, in the above embodiment, the first particles R1 satisfy the first surface condition in any cross section along the vertical direction of the unit cell 110, but this is not limited thereto, and the effect of the present invention can be obtained as long as the first particles R1 satisfy the first surface condition in at least one cross section along the vertical direction of the unit cell 110.

In addition, while the above embodiment is directed to a flat-type single cell 110, the technology disclosed in this specification can be similarly applied to other single cells other than flat-type cells.

In addition, the above embodiment is directed to an SOFC that generates electricity by utilizing an electrochemical reaction between hydrogen contained in a fuel gas and oxygen contained in an oxidant gas, but the technology disclosed in this specification can be similarly applied to an electrolytic cell, which is a constituent unit of a solid oxide electrolytic cell (SOEC) that generates hydrogen by utilizing an electrolytic reaction of water, and an electrolytic cell stack including a plurality of electrolytic cell units. The configuration of the electrolytic cell stack is publicly known, for example, as described in JP 2016-81813 A, and will not be described in detail here, but it is generally similar to the configuration of the fuel cell stack 100 in the above embodiment. That is, the fuel cell stack 100 in the above embodiment may be read as an electrolytic cell stack, the power generation unit 102 as an electrolytic cell unit, and the single cell 110 as an electrolytic single cell. However, when the electrolytic cell stack is operated, a voltage is applied between the electrodes so that the air electrode 114 is positive (anode) and the fuel electrode 116 is negative (cathode), and water vapor is supplied as a raw material gas through the communication hole 108. As a result, a water electrolysis reaction occurs in each electrolysis cell unit, hydrogen gas is generated in the fuel chamber 176, and hydrogen is extracted to the outside of the electrolysis cell stack through the communication hole 108. In the electrolysis unit cell and electrolysis cell stack of this configuration, if a configuration that satisfies the first surface condition is adopted as in the above embodiment, the same effect can be achieved.

The manufacturing method of the fuel cell stack 100 in the above embodiment is merely an example and can be modified in various ways.

22: Bolt 24: Nut 26: Insulating sheet 27: Gas passage member 28: Main body 29: Branching section 100: Fuel cell stack 102: Power generation unit 104, 106: End plate 108: Communication hole 110: Single cell 112: Electrolyte layer 114: Air electrode 116: Fuel electrode 120: Separator 121, 131, 141: Hole 124: Joint 130: Air electrode side frame 132: Oxidant gas supply communication hole 133: Oxidant gas exhaust communication hole 134: Air electrode side current collector 135: Current collector element 140: Fuel electrode side frame 142: Fuel gas supply communication hole 143: Fuel gas exhaust communication hole 144: Fuel electrode side current collector 145: Electrode facing portion 146: Interconnector facing portion 147: Connection portion 149: Spacer 150: Interconnector 161: Oxidant gas inlet manifold 162: Oxidant gas outlet manifold 166: Air chamber 171: Fuel gas inlet manifold 172: Fuel gas outlet manifold 176: Fuel chamber 180: Intermediate layer

Claims (2)

An electrochemical reaction unit cell including an electrolyte layer, and an air electrode and a fuel electrode facing each other in a first direction with the electrolyte layer therebetween,
the electrolyte layer having a plurality of particles in the first direction;
when viewed in a specific cross section that is at least one cross section along the first direction, a surface of a particle located between two adjacent grain boundary points at an interface on the air electrode side of the electrolyte layer is located within a range of ±0.2 μm or less in a direction perpendicular to a virtual line connecting the two adjacent grain boundary points ,
When viewed from the specific cross section, in at least a part of the interface of the electrolyte layer on the fuel electrode side, a part of a surface of a particle located between two adjacent grain boundary points is located outside a range of ±0.2 μm or less in a direction perpendicular to a virtual line connecting the two adjacent grain boundary points.
Electrochemical reaction unit cell. In an electrochemical reaction cell stack including a plurality of electrochemical reaction single cells,
At least one of the plurality of electrochemical reaction single cells is the electrochemical reaction single cell according to claim 1 .
Electrochemical reaction cell stack comprising:

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