Patent Application
20160156058
The present invention relates to a composite material for a fuel cell, a method for producing a composite material for a fuel cell, and a fuel cell. Specifically, the present invention relates to, in a solid-oxide fuel cell, a composite material for a fuel cell and so, the composite material being capable of enhancing the power generation performance of an electrolyte layer.
A solid-oxide fuel cell (hereinafter, referred to as an “SOFC”) includes an electrolyte-electrode laminate in which an anode layer and a cathode layer are arranged on the respective sides of a solid electrolyte layer. To reduce resistance to ionic conduction in the solid electrolyte layer, the solid electrolyte layer is preferably formed so as to have a minimum thickness. The formation of a thinner solid electrolyte layer reduces the strength of the solid electrolyte layer, thereby causing problems in the production process and when the fuel cell is used. Thus, a structure (anode support structure) in which the anode layer stacked on the solid electrolyte layer has a large thickness to ensure the strength of the laminate is often used.
As a method for producing the electrolyte-electrode laminate, a method has been studied in which an electrolyte powder is applied to an anode layer powder compact in a thin layer and the resulting electrolyte-anode laminate is co-fired.
PTL 1: Japanese Unexamined Patent Application Publication No. 2001-307546
Using the foregoing structure ensures high strength of the electrolyte-anode laminate while the solid electrolyte layer is set to have a small thickness. However, in the case where Ni is used as a catalyst, the performance of the solid electrolyte layer is disadvantageously decreased at the time of firing.
For example, in the case where a BaZrO3—Y2O3 (hereinafter, referred to as “BZY”) powder is used as an electrolyte material and where an anode powder material in which nickel (Ni) or nickel oxide (NiO) serving as a catalyst is added to the BZY powder is used as an anode material, the ionic conductivity of the solid electrolyte layer is disadvantageously liable to decrease. Hitherto, the electrolyte-anode laminate has been produced by applying the BZY powder to a surface of a formed article, the formed article being produced by compacting the anode powder material to a predetermined thickness, and performing co-firing at 1400° C. to 1600° C. In this case, the ionic conductivity inherent to the solid electrolyte layer composed of BZY is decreased. In the case where the solid electrolyte layer is used for a fuel cell, the power generation performance is often decreased, compared with a theoretical power generation performance.
Although details of the cause of the decrease in power generation performance are not clear, nickel added to the anode layer is presumed to affect the solid electrolyte layer to inhibit the ionic conductivity.
The present invention has been accomplished in order to solve the foregoing problems. It is an object of the present invention to provide a composite material for a fuel cell, in which in the case where an electrolyte-anode laminate is co-fired, the composite material is capable of inhibiting a decrease in the ion conduction performance of a solid electrolyte layer to enhance the power generation performance of the fuel cell.
An aspect of the present invention provides a composite material for a fuel cell, the composite material including a solid electrolyte layer and an anode layer stacked on the solid electrolyte layer, in which the solid electrolyte layer is composed of an ionic conductor in which the A-site of a perovskite structure is occupied by at least one of barium (Ba) and strontium (Sr) and tetravalent cations in the B-sites are partially replaced with a trivalent rare-earth element, and the anode layer contains an electrolyte component having the same composition as the solid electrolyte layer, a nickel (Ni) catalyst, and an additive containing a rare-earth element, the additive being located at least at an interfacial portion with the solid electrolyte layer.
The incorporation of the additive containing the rare-earth element into the anode layer does not result in a decrease in the ion conduction performance of the solid electrolyte layer even in the case of co-firing a laminate composed of a solid electrolyte material and an anode material, and enhances the power generation performance of a fuel cell including the laminate.
Even in the case where nickel, which is inexpensive compared with noble metals, such as platinum (Pt), is used as a catalyst and where the anode layer and the solid electrolyte layer are co-fired, the ion conduction performance is not decreased.
The inventors of the present invention have conducted intensive studies on a conventional electrolyte-anode laminate and have found the following cause of a reduction in ion conduction performance.
For example, in a conventional electrolyte-anode laminate including a solid electrolyte layer composed of BZY, which is defined as above, and an anode layer composed of a material in which Ni is added as a catalyst to the BZY in the form of, usually, NiO, the inventors have conducted detailed studies on the composition of the solid electrolyte layer after firing and have found that the Ni component is present in the entire region of the solid electrolyte layer in high concentration. The Ni component was clearly the catalytic component added to the anode layer. However, it was unclear how the Ni component moved to the electrolyte layer and whether the Ni component inhibited the ionic conductivity of the solid electrolyte layer or not.
Thus, the inventors have made an experimental electrolyte-anode laminate in which the migration of the Ni component to the solid electrolyte layer is inhibited and the concentration of the Ni component in the solid electrolyte layer is reduced, and have compared a fuel cell including the electrolyte-anode laminate with a fuel cell including a conventional electrolyte-anode laminate. As a result, the inventors have found that a reduction in the amount of the Ni component in the solid electrolyte layer increases the power generation performance.
An embodiment of the present invention provides a composite material for a fuel cell, the composite material including a solid electrolyte layer and an anode layer stacked on the solid electrolyte layer, in which the solid electrolyte layer is composed of an ionic conductor in which the A-site of a perovskite structure is occupied by at least one of barium (Ba) and strontium (Sr) and tetravalent cations in the B-sites are partially replaced with a trivalent rare-earth element, and the anode layer contains an electrolyte component having the same composition as the solid electrolyte layer, a nickel (Ni) catalyst, and an additive containing a rare-earth element, the additive being located at least at an interfacial portion with the solid electrolyte layer.
Preferably, the amount of the additive containing the rare-earth element is, in an atomic ratio of the rare-earth element, 0.001 to 2 times the amount of the rare-earth element in the solid electrolyte component contained in the anode layer.
When the amount of the additive containing the rare-earth element is, in an atomic ratio of the rare-earth element, less than 0.001 times the amount of the rare-earth element in the solid electrolyte component contained in the anode layer, the effect of inhibiting a reduction in ionic conductivity is negligibly provided, thus failing to enhance the power generation performance of a fuel cell. When the amount of the additive containing the rare-earth element is, in an atomic ratio of the rare-earth element, more than 2 times the amount of the rare-earth element in the solid electrolyte component contained in the anode layer, an affinity for the solid electrolyte layer can be reduced to reduce interlayer adhesion, and the composition of the solid electrolyte layer can be changed to reduce the ionic conductivity. More preferably, the amount of the additive containing the rare-earth element is, in an atomic ratio of the rare-earth element, 0.01 to 1.5 times the amount of the rare-earth element in the solid electrolyte component contained in the anode layer. When the amount of the additive containing the rare-earth element is 0.01 or more times, a reaction inhibition effect is markedly provided. When the amount of the additive containing the rare-earth element is 1.5 or less times, the reduction in interlayer adhesion and the effect on the composition of the solid electrolyte layer are significantly small.
Preferably, the anode layer is such that the ratio (B/A) of the number (B) of atoms of the Ni catalyst to the number (A) of atoms of cationic elements other than the Ni catalyst is in the range of 0.5 to 10. When the ratio of the number of atoms of the Ni catalyst to the number of atoms of the cationic elements other than the Ni catalyst is less than 0.5, a sufficient catalytic effect is not provided, and the electron conductivity of the anode layer is not ensured. When the ratio of the number of atoms of the Ni catalyst to the number of atoms of the cationic elements other than the Ni catalyst is more than 10, a volume change during reduction from NiO to Ni can be increased. Furthermore, the thermal expansion coefficient between the solid electrolyte layer and the anode layer can be increased to increase the thermal stress, thereby possibly causing a break of the electrolyte layer and an increase in the amount of Ni diffused into the electrolyte layer.
Yttrium-doped barium zirconate may be used as a solid electrolyte contained in the solid electrolyte layer. As the additive, for example, an yttrium-containing additive may be used. As the yttrium-containing additive, for example, yttrium oxide (Y2O3) may be used. The additive may be added to the entire anode layer. The addition of the additive to at least an interfacial portion with the electrolyte layer should be effective. For example, an anode layer containing the additive may be arranged between the solid electrolyte layer and a conventional anode layer.
The composite material for a fuel cell according to the present invention may be produced by a method including a laminate formation step of integrally laminating a powder material to be formed into the solid electrolyte layer and a powder material to be formed into the anode layer, and a firing step of thermally sintering the resulting laminate. In the laminate formation step, the anode layer may have a structure including two layers: a layer which is located adjacent to the solid electrolyte layer and which contains the additive; and a layer which is located remote from the solid electrolyte layer the other side and which does not contain the additive.
Embodiments of the present invention will be described below with reference to the drawings.
The solid electrolyte layer 3 is produced by firing a powder composed of yttrium-doped barium zirconate (hereinafter, referred to as “BZY”) which is a solid solution of barium zirconate (BaZrO3) and yttrium oxide (Y2O3). The ratio of Zr to Yin the BZY is 8:2. The solid-solution powder seemingly has the chemical formula Ba10(Zr8.Y2)O29.
As powder materials used to form the anode layer 2 according to the embodiment, the BZY powder used for the solid electrolyte layer 3, a nickel oxide powder (hereinafter, referred to as “NiO”) serving as a catalyst, and an Y2O3 powder serving as an additive containing a rare-earth element were prepared in such a manner that the mixing ratio (cation·at %) listed in A of
Polyvinyl alcohol (PVA) serving as a molding aid was added to each of the powder mixtures in an amount of 20% by volume. The resulting powder mixtures were formed into compacts by uniaxial pressing so as to have a diameter of 20 mm and a thickness of 2 mm, thereby producing anode compact A according to the embodiment and anode compact B according to the comparative example.
To the BZY powder, 50% by weight of EC vehicle (experimental EC vehicle 3-097, manufactured by Nisshin Kasei Co., Ltd.) serving as a binder was added with respect to the amount of the BZY powder. A BZY powder slurry was prepared using 2-(2-butoxyethoxy)ethyl acetate and α-terpineol as a solvent. The BZY powder slurry was applied to a side of each of anode compact A and anode compact B by screen printing to form films to be formed into solid electrolyte layers, the films each having a thickness of about 20 μm, thereby forming multilayer laminates according to the embodiment A and the comparative example B respectively.
These multilayer laminates were heated at 700° C. for 24 hours in air to remove the resin components and then fired at 1500° C. for 10 hours in an oxygen atmosphere, thereby providing electrolyte-anode laminates. The rate of shrinkage due to the firing was about 20%.
To evaluate the state of the reaction of Ni with the solid electrolyte layer after the firing, the amount of Ni on a surface of each solid electrolyte layer opposite the surface adjacent to a corresponding one of the anode layers was quantitatively determined by energy dispersive X-ray spectroscopy (EDX).
The electrolyte-anode laminates were heated at 700° C. for 1 hour in a H2 atmosphere to reduce the anode layers and to deposit metallic Ni, thereby providing the composite material 1 for fuel cells. A La—Sr—Co—Fe—O (LSCF) powder slurry to be formed into cathode layers was applied to a surface of each solid electrolyte layer 3 opposite the surface adjacent to a corresponding one of the anode layers 2 to form the cathode layers each having a thickness of about 10 μm, thereby forming electrolyte-electrode laminates 11. Fuel cells 10 illustrated in
Each of the fuel cells 10 includes the electrolyte-electrode laminate 11 supported in the middle of a cylindrical case 12, channels 13 and 14 configured to allow a fuel gas to act on one side of the cylindrical case, and channels 15 and 16 configured to allow air to act on the other side. Platinum meshes 19 and 20 serving as collectors are arranged on a surface of the anode electrode and a surface of the cathode electrode, respectively, of each electrolyte-electrode laminate 11. Lead wires 17 and 18 extending to the outside are connected to the platinum meshes 19 and 20, respectively.
The power generation performance of the fuel cells 10 were measured when the fuel cells 10 were operated at 600° C. while hydrogen serving as a fuel gas was allowed to flow at a flow rate of 20 to 100 cc/mn to act on the anodes and air was allowed to flow at a flow rate of 20 to 100 cc/min to act on the cathodes.
As listed in
The cause of the diffusion of a considerable amount of the Ni component in the solid electrolyte layer in the conventional electrolyte-anode laminate and the mechanism of action in the present invention were considered from a kinetic point of view and a thermodynamic point of view.
From the kinetic point of view, the inventors made the hypothesis that the Ni component changed to a. liquid phase and migrated to the entire region of the solid electrolyte layer by capillarity and so forth in the firing step. This is presumably because the migration distance and the migration speed in the form of a liquid phase are markedly increased, compared with migration by solid-state diffusion.
The conventional anode layer is composed of the powder mixture of the BZY powder and the NiO powder. In the firing step, the following reaction seemingly occurs.
Ba10(Zr8Y2)O29+2NiO→Ba8Zr8O24+Y2BaNiO5+BaNiO2 (Reaction formula 1)
Based on the foregoing findings, the inventors have speculated that it is possible to inhibit the migration of the Ni component to the solid electrolyte layer by blocking the formation of the liquid phase of the Ni-containing compound. The inventors have conducted many experiments and have conceived the present invention.
In the embodiment, the powder materials to be formed into the anode layer, the powder materials containing the additive that contains the rare-earth element, is fired in order to block the formation of BaNiO2 in reaction formula 1.
Let us consider the case where regarding the anode layer, NiO is added as a catalyst component to the powder composed of the BZY, Y2O3 is added as the additive thereto, and the resulting mixture is fired.
If we assume that Y2BaNiO5 is formed in place of BaNiO2, the amount of Y2O3 added is, at the maximum, equal to the amount of Y2O3 contained in BZY in the anode layer. For example, in the case where 20 at % of Zr in BaZrO3 is replaced with Y, the addition of Y2O3 presumably leads to a reaction with NiO as represented by a reaction formula described below.
Ba10(Zr8Y2)O29+2NiO+Y2O3→Ba8Zr8O24+2Y2BaNiO5 (Reaction formula 2)
In the case where the reaction represented by the foregoing reaction formula occurs by the addition of Y2O3 to the anode layer, BaNiO2, which is formed according to reaction formula 1 described above, is not formed. In the case where this amount of Y2O3 is added, even if the total amount of Y in BZY reacts with NiO together with Ba, BaNiO2 is not formed.
A region, where Y2O3 is not added, denoted by A2 in the ternary phase diagram illustrated in
In the embodiment, Y2O3 is added; hence, a compound corresponding to a region denoted by D2 in the ternary phase diagram, i.e., BaY2NiO5, is presumed to be formed. BaY2NiO5 has a high melting point and is presumed to be in a solid-phase state even at 1500° C.
It is thus possible to block the formation of the liquid-phase state of the Ni-containing compound formed in the firing step and block the migration of the Ni component from the anode layer to the solid electrolyte layer. From a thermodynamic point of view, the addition of Y2O3 to the anode layer increases the chemical potential of Y in the anode layer. This is presumed to inhibit the migration of Y from BYZ in the anode layer. The migration of Ba is less likely to occur if Ba does not migrate together with cations in the B-sites. This is presumed to inhibit the migration of Y and Ba to the outside of the BZY grains, i.e., the reaction of Y, Ba, and NiO.
A larger amount of Y2O3 added is preferred from the viewpoint of inhibiting the formation of the liquid phase. However, from the viewpoint of maintaining an affinity for BZY in the solid electrolyte layer and inhibiting the effect on the anode layer, a smaller amount of Y2O3 added is preferred. When the amount of Y2O3 added is, in an atomic ratio of the rare-earth element, less than 0.001 times the amount of the rare-earth element in the electrolyte component contained in the anode layer, the effect of inhibiting the formation of the liquid phase is small. In the case of more than 2 times, an affinity for the solid electrolyte layer can be reduced to reduce interlayer adhesion, and the ratio of Zr to Y in the electrolyte can be changed to reduce the ionic conductivity. More preferably, the amount of Y2O3 added is, in an atomic ratio of the rare-earth element, 0.01 to 1.5 times the amount of the rare-earth element in the solid electrolyte component contained in the anode layer.
When the amount of Y2O3 added is 0.01 or more times, a reaction inhibition effect is markedly provided. When the amount of the additive containing the rare-earth element is 1.5 or less times, the reduction in interlayer adhesion and the effect on the composition of the solid electrolyte layer are significantly small.
Also in the case where the composite material (the embodiment) including A listed in
In the embodiment, the composite material including the solid electrolyte layer composed of the ionic conductor in which the A-site of the perovskite structure was occupied by barium (Ba) and the tetravalent cations in the B-sites were partially replaced with yttrium was used. However, according to the present invention, a composite material including a solid electrolyte layer composed of an ionic conductor in which the A-site is occupied by strontium (Sr), or barium (Ba) and strontium (Sr) may be used. In the embodiment, Y2O3 was added to the entire anode layer. However, the additive containing the rare-earth element may be added to at least an interfacial portion with the solid electrolyte layer. For example, a layer to which Y2O3 is added may be separately formed at the interfacial portion.
The scope of the present invention is not limited to the foregoing embodiments. The embodiments disclosed herein are to be considered in all respects as illustrative and not limiting. The scope of the invention is defined not by the foregoing description but by the following claims, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims.
The electrolyte-anode laminate for a fuel cell having high power generation performance is provided at low cost.
1 electrolyte-anode laminate (composite material for fuel cell)
2 anode layer
3 solid electrolyte layer
10 fuel cell
11 electrolyte-electrode laminate
12 cylindrical case
13 channel (fuel gas)
14 channel (fuel gas)
15 channel (air)
16 channel (air)
17 lead wire
18 lead wire
19 platinum mesh
20 platinum mesh
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-149336 | Jul 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/068285 | 7/9/2014 | WO | 00 |
