JP6779745B2 - Solid oxide fuel cell and material for forming the cathode of the fuel cell - Google Patents

Description

The present invention relates to a material used for forming a cathode (air electrode) of a solid oxide fuel cell, and a solid oxide fuel cell including a cathode formed by using the material.

Solid oxide fuel cells (SOFC: Solid Oxide Fuel Cell; hereinafter, also referred to simply as "SOFC") have high power generation efficiency and can use various fuels among various types of fuel cells. Therefore, it is being developed as a next-generation power generation device with less environmental load.
A typical structure of SOFC is a cathode (air electrode) having a porous structure on one surface of a dense solid electrolyte (for example, a dense film layer) composed of an oxygen ion conductor (typically an oxygen ion conductive ceramic body). ) Is formed, and an anode (fuel electrode) having a porous structure is formed (for example, laminated) on the other surface.

Conventionally, SOFCs have been operated in a high temperature range of 700 ° C. or higher, typically 800 ° C. to 1000 ° C., but from the viewpoint of improving durability and reducing costs, the operating temperature has been lowered in recent years. It is desired to do. For example, it is being studied to reduce the operating temperature of SOFC to about 600 ° C.
Cathode materials are being studied as an aid to effectively achieving low temperature operation. For example, by constructing the cathode using a so-called electron-oxygen ion mixed conductor material exhibiting electron conductivity and oxygen ion conductivity, not only at the three-phase interface where the electrode / electrolyte / gas phase is in contact, but also at the cathode surface. It is expected that the electrode reaction will proceed and the electrode activity will be enhanced. Examples of such a cathode material include a lanthanum cobaltate _- based material (for example, LaSrCoO _3-δ- based material) and a lanternstrontium cobalt ferrite (LSCF) -based material, which are perovskite-type oxide materials. Patent Documents 1 and 2 are examples of conventional techniques relating to these.

Japanese Unexamined Patent Publication No. 2013-84529 Japanese Unexamined Patent Publication No. 2008-234915

Patent Document 1 describes that a mixture of La _x Sr _1-x MnO ₃ and CeO ₂ is used as an air electrode material for SOFC. The publication describes that the air electrode and the solid electrolyte can be laminated and integrated by co-firing while suppressing the formation of a secondary reaction phase by such a configuration. However, cerium oxides such as CeO ₂ generally have low heat resistance. For example, in the temperature range of 600 ° C to 1000 ° C, which is the operating temperature of SOFC, air electrode particles are sintered due to insufficient heat resistance, resulting in dense electrodes. The conversion progresses. As a result, the porous state of the electrode could not be maintained, the three-phase interface was not sufficiently formed, and there was a risk that the desired electrode activity could not be obtained.

The present invention has been made in view of the above problems, and an object of the present invention is to increase the heat resistance of cerium-containing particles to prevent densification of the cathode during power generation, and to obtain SOFC capable of exhibiting desired electrode activity for a long period of time. It is to provide a material for forming a cathode. Further, as another aspect, the present invention provides an SOFC in which a cathode is formed by using the above-mentioned material.

In order to realize the above object, the present invention provides a material for forming a cathode of a solid oxide fuel cell (SOFC). This cathode forming material contains granular cerium-containing oxide particles. In the cerium-containing oxide particles, the surface portion of the particles has a higher concentration of Ce than the central portion. By using the cerium-containing oxide particles having an appropriate difference in Ce concentration between the surface portion and the central portion in this way, the heat resistance of the cerium-containing particles is enhanced and the densification of the cathode during power generation is prevented, which is desired. Electrode activity can be maintained for a long period of time.

In a preferred embodiment of the cathode forming material disclosed herein, the central portion of the cerium-containing oxide particles is one or more selected from the group consisting of Ce, La, Nd and Zr. Contains oxides as constituent metal elements. By including the oxide having the constituent metal elements as described above in the central portion of the cerium-containing oxide particles, the heat resistance of the cerium-containing oxide particles can be further improved. For this reason, for example, in the temperature range of 600 ° C to 1000 ° C, which is the general operating temperature of SOFC, the phenomenon that the densification of the cathode progresses due to insufficient heat resistance is eliminated or alleviated, and the durability with little performance deterioration during power generation is achieved. An excellent SOFC can be produced.

A preferred embodiment of the cathode forming material disclosed herein further comprises perovskite-type oxide particles exhibiting oxygen ion conductivity. By incorporating the perovskite-type oxide in the cathode, not only the three-phase interface but also the electrode surface can be an excellent reaction field. Therefore, it can exhibit high performance as an electrode even in a relatively low temperature range (for example, 600 ° C. to 800 ° C.). On the other hand, the perovskite-type oxide particles have a large difference from the coefficient of thermal expansion of the solid electrolyte material, and when repeatedly used at the general operating temperature of SOFC, cracks occur in the joint and the cathode tends to peel off. is there. On the other hand, by using the perovskite type oxide particles and the cerium-containing oxide particles in combination, the coefficient of thermal expansion can be reduced, and the heat can be reduced as compared with the case where the perovskite type oxide particles are used alone. Cycle durability can be improved.

In a preferred embodiment of the cathode forming material disclosed herein, the amount of the perovskite-type oxide particles is 10% by mass to 90% by mass with respect to the total mass of the perovskite-type oxide particles and the cerium-containing oxide particles. It is an amount corresponding to mass%. More preferably, the amount of the perovskite-type oxide particles is an amount corresponding to 20% by mass to 80% by mass with respect to the total mass of the perovskite-type oxide particles and the cerium-containing oxide particles. When the amount is within the range of such perovskite-type oxide particles, the above-mentioned performance improving effect (for example, low temperature operation effect and heat cycle durability improving effect) can be more effectively exhibited.

A preferred embodiment of the cathode forming material disclosed herein is that it contains at least one dispersion solvent and is prepared in a paste form (including a slurry form and an ink form). The materials disclosed herein are used to form the cathode of SOFCs. In such an application, a homogeneous cathode layer can be stably produced by using a material prepared as a paste by dispersing (or dissolving) the constituent material of the cathode in one or more kinds of dispersion solvents.

Further, according to the present invention, a solid oxide fuel cell including an anode, a solid electrolyte and a cathode, wherein the cathode is one of the cathode forming materials disclosed herein (typically, the above-mentioned cerium-containing material). A solid oxide fuel cell (SOFC) comprising a calcined product of a material containing oxide particles) is provided.
With the above cathode, it is difficult for the electrodes to become densified during power generation. Therefore, the SOFC provided with such a cathode can exhibit high durability.

It is sectional drawing which shows typically the anode support type SOFC which concerns on one Embodiment of this invention. It is a figure which shows typically the cerium-containing oxide particles disclosed here. It is sectional drawing which shows typically the manufacturing process of SOFC which concerns on one Embodiment of this invention. It is sectional drawing which shows typically the manufacturing process of SOFC which concerns on one Embodiment of this invention. It is sectional drawing which shows typically the manufacturing process of SOFC which concerns on one Embodiment of this invention.

Hereinafter, preferred embodiments of the present invention will be described. Matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art.

The cathode-forming material disclosed herein is characterized by the fact that the surface portion contains granular cerium-containing oxide particles having a higher concentration of Ce than the central portion. Therefore, the content, composition, and the like of the other constituents can be determined in light of various criteria as long as the object of the present invention is not deviated.

According to this configuration, a solid oxide fuel cell provided with an anode, a solid electrolyte, and a cathode, wherein the cathode is a solid oxide fuel cell (SOFC) provided with a cathode forming material disclosed herein. Provided. Cathodes using the materials disclosed herein are SOFCs of various structures, for example, conventionally known flat plate type (Planar), MOLB type, vertical stripe type (Tubular), or the peripheral side surface of the cylinder is vertically crushed. It can be suitably used for SOFCs such as a flat cylindrical type and an integrally laminated type, and the shape and size are not particularly limited. Further, the support (base material) is not particularly limited, and may be, for example, an anode, a cathode, a solid electrolyte, or the like.

As an example, it will be described with reference to FIG. FIG. 1 is a cross-sectional configuration diagram schematically showing a part of a power generation system including the SOFC 50 disclosed here. The SOFC 50 having the configuration shown here is an anode-supported SOFC, and is formed on a thin plate-shaped or sheet-shaped anode (fuel electrode) 10 as a support and at least a part of the surface of the anode 10 (film-like). The solid electrolyte 20 and the thin plate-shaped or sheet-shaped cathode (air electrode) 30 formed on the surface of the solid electrolyte 20 are laminated. Then, an end portion 12 of the anode 10 and a gas pipe for supplying fuel gas (typically hydrogen (H ₂ ), but for example, hydrocarbon (methane (CH ₄ )) or the like as long as it burns). The joint surface of 60 is joined by the connecting member 40, and is sealed so that gas (fuel gas or air) does not flow out (inflow). Further, the cathode 30 has a structure exposed to a gas containing oxygen (O ₂ ) (typically, a structure exposed to the outside air).
When an electric current is applied to such an SOFC, oxygen in an oxygen-containing gas (typically air) becomes oxygen ions ( ^O2- ) at the cathode 30. The oxygen ions are supplied from the cathode 30 to the anode 10 via the solid electrolyte 20. Then, at the anode 10, water (H ₂ O) is generated by reacting with the fuel gas, and electrons are emitted to generate electricity.

The anode (fuel electrode) 10 constituting the SOFC 50 disclosed here has a porous structure. The shape of the anode may be configured so as to be in contact with the fuel gas supplied to the SOFC, and can be appropriately selected depending on the shape of the SOFC described above. Since the SOFC 50 having the configuration shown in FIG. 1 is a so-called anode support type, a relatively thick thin plate-shaped or sheet-shaped anode 10 is formed as a support for the SOFC 50. The thickness of the anode 10 as the support is preferably set in consideration of handleability, durability, coefficient of thermal expansion, and the like. It is typically about 0.1 mm to 10 mm, preferably about 0.3 mm to 1 mm, but is not limited to such a thickness.

As the material constituting such an anode, one or more of the materials conventionally used for SOFC can be used without particular limitation. For example, nickel (Ni), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru) and other platinum group elements, cobalt (Co), lanthanum (La), strontium (Sr). ), Titanium (Ti) and the like, and / or a metal oxide composed of one or more of metal elements. Specific examples include metals and metal oxides composed of platinum group elements such as Ni, Co, and Ru. For example, Ni is a particularly suitable metal species because it is cheaper than other metals and has sufficiently high reactivity with a fuel gas such as hydrogen. Further, a composite in which these metals and metal oxides are mixed can also be used. Further, for example, a composite of the anode constituent material (metal or metal oxide) and the solid electrolyte constituent material described later can also be used. Specifically, for example, nickel (Ni) or ruthenium (Ru) and stabilized zirconia (for example, yttria-stabilized zirconia (YSZ), calcia-stabilized zirconia (CSZ), scandia-stabilized zirconia (ScSZ), etc.) Cermet is a good example. Although not particularly limited, for example, the mixing ratio (mass ratio) of the anode constituent material and the solid electrolyte constituent material described later is about 90: 10 to 40:60 (more preferably, about 80:20 to 45: It is preferably in the range of 55).

The solid electrolyte 20 constituting the SOFC 50 disclosed here has a dense structure. The solid electrolyte 20 is laminated on the anode 10, and its shape can be appropriately changed according to the shape of the anode 10. Further, the thickness of the solid electrolyte 20 is thickened to the extent that the denseness of the solid electrolyte layer is maintained, while being thinned to the extent that oxygen ion conductivity preferable for SOFC can be provided. It is preferable to set. It is typically about 0.1 μm to 50 μm, preferably about 1 μm to 40 μm, more preferably about 10 μm to 30 μm, but is not limited to such a film thickness.

As the material constituting such a solid electrolyte, one or more of the materials conventionally used for SOFC can be used without particular limitation, and for example, a compound having high oxygen ion conductivity is preferably used. Be done. For example, cerium (Ce), zirconium (Zr), magnesium (Mg), scandium (Sc), titanium (Ti), aluminum (Al), yttrium (Y), calcium (Ca), gadolinium (Gd), samarium (Sm). ), Barium (Ba), Lantern (La), Strontium (Sr), Gallium (Ga), Bismus (Bi), Niob (Nb), Tungsten (W), etc. It is preferable to have. Specifically, stabilizers (for example, Itria (Y ₂ O ₃ ), Calcia (CaO), Scandia (Sc ₂ O ₃ ), Magnesia (Mg O), Ittervia (Yb ₂ O ₃ ), Elvia (Er ₂ O ₃ ). Zirconia (ZrO ₂ ) whose crystal structure is stabilized by ₃ )), dope (for example, gadlinia (Gd ₂ O ₃ ), lanthania (La ₂ O ₃ ), sammaria (Sm ₂ O ₃ ), ytria (Y) ₂ O ₃₎₎ doped with cerium oxide (CeO ₂₎ may be mentioned as preferable examples. For example, yttria-stabilized zirconia (YSZ) doped with an oxide of yttrium (Y) (for example, yttria (Y ₂ O ₃ )) or an oxide of scandium (Sc) (for example, scandia (Sc ₂ O ₃ )). Dopeed Scandia-stabilized zirconia (ScSZ) and the like can be preferably used.

The cathode (air electrode) 30 constituting the SOFC disclosed here has a porous structure like the anode 10. The cathode 30 is laminated on the solid electrolyte 20, and its shape can be appropriately changed according to the shape of the solid electrolyte 20. The thickness of the cathode 30 is typically about 1 μm to 200 μm, preferably about 5 μm to 100 μm, more preferably 10 μm to 100 μm, still more preferably 20 μm to 50 μm, but is not limited to such thickness. ..

<Cerium-containing oxide particles>
Examples of the material constituting the cathode 30 include granular cerium-containing oxide particles. As shown in FIG. 2, such a cathode forming material contains cerium-containing oxide particles 1. In the cerium-containing oxide particles 1, the surface portion of the particles 1 has a higher concentration of Ce than the central portion. In this embodiment, the cerium-containing oxide particles 1 have a core-shell structure including a core 2 having a relatively low concentration of Ce and a shell 3 having a relatively high concentration of Ce. By using the cerium-containing oxide particles 1 having a difference in Ce concentration between the surface portion and the central portion in this way, the heat resistance of the cerium-containing particles 1 is enhanced to prevent the cathode 30 from becoming densified during power generation, which is desired. Electrode activity can be maintained for a long period of time. Further, the cerium-containing oxide particles 1 having the core-shell structure have many irregularities on the surface portion and a large specific surface area. Therefore, the cathode constructed by using the cerium-containing oxide particles has more three-phase interfaces in which the electrode / electrolyte / gas phase is in contact with each other, and the electrode activity is further improved. SOFCs using such cathodes can exhibit high power generation performance for a long period of time.

The concentration of Ce on the surface of the cerium-containing oxide particles 1 (hereinafter, may be simply referred to as “C _shell ”) may be referred to as the concentration of Ce on the central portion (hereinafter, simply referred to as “C _core ”). It may be higher than (that is, C _shell > C _core ), and is not particularly limited. The C _shell is preferably 10 mol% or more, for example, preferably 15 mol% or more, and more preferably 20 mol% or more from the viewpoint of improving heat resistance. The C _shell may be, for example, 30 mol% or more, typically 40 mol% or more (for example, 50 mol% or more). The upper limit of the C _shell is not particularly limited, but is preferably 90 mol% or less, more preferably 85 mol% or less, and particularly preferably 80 mol% or less. The C _{shell on the} surface may be 70 mol% or less of cerium-containing oxide particles, and the C _shell may be 60 mol% or less.

On the other hand, the central portion of the cerium-containing oxide particles 1 is not particularly limited as long as its Ce concentration C _core is lower than that of the C _shell . From the viewpoint of improving heat resistance and the like, cerium-containing oxide particles 1 having a C _core of 50 mol% or less at the center can be preferably used. The C _core may be, for example, 40 mol% or less, typically 20 mol% or less (for example, 10 mol% or less). For example, it is particularly preferable that the C _core is 0 mol%, that is, the cerium-containing oxide particles 1 that do not substantially contain Ce in the central portion.

From the viewpoint of better exerting the effect of providing a difference in Ce concentration between the central portion and the surface portion of the cerium-containing oxide particles 1, the relationship between C _core and C _shell is 0 ≦ (C _core / C _shell ) < It is preferable to satisfy 1. By using the cerium-containing oxide particles 1 in which Ce is unevenly distributed so that the central portion and the surface portion have a specific Ce concentration ratio, the heat resistance of the cerium-containing oxide particles can be further improved. In the technique disclosed here, the relationship between the C _core and the C _shell is 0 ≦ (C _core / C _shell ) ≦ 0.5, more preferably 0 ≦ (C _core / C _shell ) ≦ 0.3, and further preferably. _Can be preferably carried out in an embodiment of 0 ≦ (C _core / C _shell ) ≦ 0.2, particularly preferably 0 ≦ (C _core / C _shell ) ≦ 0.1.

From the viewpoint of realizing cerium-containing oxide particles having higher heat resistance, the C _shell is preferably 10 mol% or more higher than the C _{core, and} more preferably 20 mol% or more higher. The value obtained by subtracting the C _core from the C _shell (that is, the C _shell −C _core ) is preferably 90 mol% or less, more preferably 80 mol% or less, and further preferably 70 mol% or less. For example, the C _shell −C _core may be 50 mol% or less.

Regarding the Ce concentration of the cerium-containing oxide particles 1, for the surface portion, for example, using an analytical instrument based on the TEM-EDS method, the outermost surface portion of the cerium-containing oxide particles (about several nm from the surface, for example, the surface). The average ratio (mol%) of the Ce element to the total elements constituting (from about 5 nm) can be measured and grasped. As for the central portion, for example, using an analytical instrument based on the TEM-EDS method, the average ratio (mol%) of the Ce element to the total elements constituting the portion of the cerium-containing oxide particles excluding the outermost surface portion. Can be measured and grasped. It should be noted that the fact that Ce is unevenly distributed on the surface of the cerium-containing oxide particles from the central portion can be grasped by mapping Ce by, for example, SEM (scanning electron microscope) -EDX (energy dispersive X-ray spectroscopy). You may.

The material used in the core 2 of the cerium-containing oxide particles 1 is not particularly limited Ce concentration C _core in the center portion is as long as satisfying the above relationship (C _shell> C _core) with the C _shell. For example, rare earth elements such as lantern (La), cerium (Ce), neodymium (Nd), samarium (Sm), and gadolinium (Gd); group 3 such as zirconium (Zr), titanium (Ti), and ittrium (Y). A metal compound material containing a metal element of Group 4; or the like can be used. Of these, oxides containing one or more of Ce, La, Nd and Zr as constituent metal elements are preferable. Preferable examples of the material used for the core 2 of the cerium-containing oxide particles 1 include lanthanum oxide, neodymium oxide, zirconium oxide (for example, zirconia (ZrO ₂ )), zirconium oxide doped with rare earth elements, and rare earth elements. Examples thereof include cerium oxide doped with. Examples of rare earth element-doped zirconium oxides include lanthanum-doped zirconia, cerium-doped zirconia, neodymium-doped zirconia, and lanthanum-neodymium-doped zirconia. Examples of cerium oxides doped with rare earth elements include ceria doped with lanthanum. The suitable doping amount of the rare earth element is about 20 mol% to 80 mol% (preferably 30 mol% to 70 mol%). By forming the cathode using the cerium-containing oxide particles 1 containing the core 2 composed of such an oxide component, it is possible to prevent the cathode from becoming densified during power generation and maintain the desired power generation performance for a long period of time. ..

As a material used for the shell 3 of the cerium-containing oxide particles 1 is not particularly limited as long as it satisfies the relationship between the Ce concentration C _shell surface portion and _{C core (C shell> C core} ). For example, cerium oxide (for example, ceria (CeO ₂ )), ceria-zirconia composite oxide (CeO _2- ZrO ₂ composite oxide) and the like can be mentioned. On the surface of the cerium-containing oxide particles 1, the material constituting the shell 3 (for example, cerium oxide) may be mixed with the material constituting the core 2 (for example, zirconia doped with a rare earth element). Alternatively, the material constituting the shell 3 may react with the material constituting the core 2 by firing, and a part thereof may form a solid solution.

The properties of the cerium-containing oxide particles 1 are not particularly limited, but for example, the average particle diameter (D50) is usually preferably 5 μm or less, and particularly preferably 2 μm or less. From the viewpoints of handleability, sinterability of the cathode, enhancement of interfacial adhesion between the cathode and the solid electrolyte layer, etc., particles having an average particle diameter of 1 μm or less are preferably adopted as the cerium-containing oxide particles 1. it can. For example, cerium-containing oxide particles having an average particle diameter of 0.005 μm or more and 3 μm or less are preferable, those having an average particle diameter of 0.01 μm or more and 2 μm or less are more preferable, and those having an average particle diameter of 0.05 μm or more and 0.8 μm or less are particularly preferable. In the present specification, the "average particle size" is the particle size at an integrated value of 50% in the particle size distribution measured by a particle size distribution measuring device based on the laser diffraction / scattering method (50% cumulative volume average particle size (Median diameter). ); D50).

The method for producing such particulate cerium-containing oxide particles is not particularly limited. For example, a hydroxide of a metal element other than Ce is precipitated by a wet method, and this hydroxide is tentatively fired to produce a fired product (a calcined product (). Obtaining a precursor) (precursor formation step), adding an appropriate cerium source (acid solution) to the obtained precursor, and precipitating a hydroxide of cerium on the surface of the precursor (cerium precipitation step). , The precursor in which cerium is precipitated can be produced by firing at a predetermined temperature (baking step). Incidentally, if the average composition in the following ceria in the core 2 which is represented by La _x Zr _y O ₂ is as an example a supported cerium-containing oxide particles 1 as a shell 3, a method for manufacturing a cerium-containing oxide particles However, it is not intended to limit the present invention to such specific embodiments.

In the process of producing the precursor, an aqueous solution containing a lanthanum (La) source and a zirconium (Zr) source as starting materials is typically prepared, and these are prepared under alkaline conditions (pH> 7 conditions). By mixing, the hydroxide of the above metal element is precipitated (crystallized). The aqueous solution can be prepared by dissolving a predetermined amount of each of a metal element source (La source, Zr source, typically a water-soluble ionic compound) in an aqueous solvent. The order in which these metal element sources are added to the aqueous solvent is not particularly limited. Alternatively, an aqueous solution containing each metal element source may be prepared separately and prepared by mixing such an aqueous solution. The anion of the metal element source can be appropriately selected so that the metal element source becomes water-soluble, and may be, for example, sulfate ion, nitrate ion, carbonate ion, hydroxide ion, chloride ion or the like. The anions of these metal element sources may be all or part of the same or different from each other. Further, each of these metal element sources may be in a solvated state such as hydrate.

The aqueous solvent used when preparing the aqueous solution is typically water, but a mixed solvent mainly composed of water can also be used. As the solvent other than water constituting the mixed solvent, one or more kinds of organic solvents (for example, lower alcohols, lower ketones, etc.) that can be uniformly mixed with water can be appropriately selected and used. When the raw material compound to be used has low water solubility, a compound (acid, base, etc.) capable of improving the solubility can be appropriately added to the aqueous solvent.

As the compound capable of making the aqueous solution alkaline, a compound containing a strong base (a hydroxide of an alkali metal or the like) and / or a weak base (ammonia or the like) and which does not inhibit the formation (precipitation) of the hydroxide is preferably used. Can be. For example, sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide _(NH 4 OH), lithium hydroxide (LiOH), sodium carbonate (NaCO _3), potassium carbonate _(KCO 3), lithium carbonate (LiCO ₃ ), ammonium carbonate ((NH ₄ ) ₂ CO ₃ ), ammonium nitrate (NH ₄ NO ₃ ), ammonia (NH ₃ ) and the like can be selected from one or more. Of these, the use of ammonia is preferable.

In the process of producing the precursor, a compound (for example, ammonium) that can make the aqueous solution alkaline is added to the aqueous solution, and the mixture and stirring are mixed and stirred at an appropriate rate to form water of a metal element (here, La and Zr). The oxide is precipitated (crystallized). Next, the obtained hydroxide is calcined to form a precursor.

In the cerium precipitation step, an appropriate cerium source (acid solution) is added to the obtained precursor to precipitate a hydroxide of cerium on the surface of the precursor. The anion of the cerium source can be appropriately selected so that the metal element source becomes water-soluble, and may be, for example, an acidic ion such as a sulfate ion, a nitrate ion, or a carbonate ion. In the cerium precipitation step, first, the obtained precursor is mixed with a solution (acid solution) in which the cerium source is dissolved in an aqueous solvent. Then, a compound (for example, ammonia) capable of making the solution alkaline is added to the solution and stirred to make the solution alkaline (pH> 7), whereby the hydroxide of cerium is used as a precursor. Precipitate (crystallize) on the surface of. As the water-soluble solvent and the compound that can be made alkaline used in the cerium precipitation step, the same compounds as those used in the precursor production step described above can be used.

In the firing step, the cerium-containing oxide particles 1 disclosed herein can be produced by firing the precursor in which the cerium is precipitated at a predetermined temperature. The firing temperature is not particularly limited, but can be, for example, 700 ° C. or higher (typically 800 ° C. or higher) and 1300 ° C. or lower (typically 1200 ° C. or lower). Then, the obtained calcined product is typically pulverized and then sieved to a desired particle size as needed and used as the cerium-containing oxide particles 1. In this way, the average composition (ie has a surface portion concentration of Ce higher than the center portion) La _x Zr _y O ₂ ceria in the core 2 which is represented by is supported as a shell 3 cerium-containing oxide particles 1 can be manufactured.

The material for forming a cathode disclosed herein is the above-mentioned cerium-containing oxide particles, that is, a surface Ce-enriched cerium-containing oxide having a higher Ce concentration on the surface than in the center, as long as the effects of the present invention are not impaired. Cerium-containing oxide particles other than the particles (that is, non-surface Ce-enriched cerium-containing oxide particles) may be contained. Examples of such non-surface Ce-enriched cerium-containing oxide particles are substantially from any of gadolinium-doped ceria particles (GDC), lanthanum-doped ceria particles, samarium-doped ceria particles, and the like. Examples include particles that are composed.

The content of the non-surface Ce-enriched cerium-containing oxide particles is preferably, for example, 50% by mass or less of the total mass of the cerium-containing oxide particles contained in the cathode forming material, and is preferable. It is 30% by mass or less, more preferably 10% by mass or less.
The technique disclosed herein is preferably carried out in an embodiment in which the total ratio of the surface Ce-enriched cerium-containing oxide particles to the total mass of the cerium-containing oxide particles contained in the cathode forming material is larger than 90% by mass. obtain. The proportion of the surface Ce-enriched cerium-containing oxide particles is more preferably 95% by mass or more, further preferably 98% by mass or more, and particularly preferably 99% by mass or more. Among them, a cathode forming material in which 100% by mass (that is, all) of the cerium-containing oxide particles contained in the cathode forming material is the surface Ce-enriched cerium-containing oxide particles is preferable.

<Perovskite type oxide particles>
The cathode forming material disclosed herein preferably contains perovskite-type oxide particles exhibiting oxygen ion conductivity. Examples of the perovskite-type oxide particles include lanthanum covalent (for example, LaSrCoO _3-δ material), lanthanum strontium cobalt ferrite (LSCF) material, lanthanum manganate (for example, LaSrMnO _3-δ ) material, and lanthanum ferrite (for example, LaSrFeO _{3). −δ} ) -based materials, lanthanum nikelate (for example, LaSrNiO _3-δ ) -based materials, and the like can be mentioned.

As a preferable composition of the oxide material constituting the perovskite-type oxide particles disclosed herein, the following general formula (1):
Ln _a Ae _b M _c O _3-δ (1)
An example is a perovskite-type oxide represented by. Here, "Ln" in the general formula (1) is one kind or two or more kinds of elements selected from lanthanoids, and elements having atomic numbers 57 to 71 (for example, lanthanum (La), samarium (Ce)). ), Praseodymium (Pr), neodymium (Nd), samarium (Sm), etc.), or one or more of them can be selected. Among these, La can be used particularly preferably, and when two or more kinds of elements are contained as "Ln", it is preferable that the content of La is high.
Further, "Ae" in the above general formula (1) indicates an alkaline earth metal. Typically, it is one or more elements selected from the group consisting of strontium (Sr), calcium (Ca) and barium (Ba). Among them, those containing Sr are preferable. Further, a composition having a high content of Sr is preferable, for example, it is composed of only Sr or contains Sr at a high content (for example, 40 mol% of Sr in "Ae"). (Included above) is particularly preferable.
Further, "M" in the general formula (1) is cobalt (Co), manganese (Mn), nickel (Ni), iron (Fe), titanium (Ti), aluminum (Al), zirconium (Zr), Gallium (Ga), magnesium (Mg), copper (Cu), indium (In), tin (Sn), vanadium (V), chromium (Cr), zinc (Zn), germanium (Ge), scandium (Sc) and One or more elements selected from the group consisting of ittium (Y). Of these, any one of Co, Mn, Ni, and Fe, or a combination of any two of these is preferable. When the above element is contained, it has higher reactivity and oxygen ion conductivity than other perovskite-type oxides, so that it can exhibit excellent performance as an electrode even in a relatively low temperature range.

Further, a, b, and c in the general formula (1) are real numbers satisfying 0.3 ≦ a ≦ 0.7, 0.3 ≦ b ≦ 0.7, 1.7 ≦ a + b + c ≦ 2.0. is there. That is, any real number may be taken as long as it is within the above range as long as the structure can be maintained without breaking the perovskite type structure. More preferably, 0.5 ≦ a ≦ 0.7. More preferably, 0.3 ≦ b ≦ 0.6 (for example, 0.4 ≦ b ≦ 0.5). More preferably 0.8 ≦ c ≦ 1.2 (typically c = 1). As a specific example of the perovskite-type oxide, for example, "La _a Sr _b Co _c O _3-δ " in which La is selected as "Ln", Sr is selected as "Ae", and Co is selected as "M". Or, "La _a Sr _b (CoFe) _c O _3-δ " in which La was selected as "Ln", Sr was selected as "Ae", and Co and Fe were selected as "M", and La was selected as "Ln". Select Sr as "Ae", select Fe as "M", select "La _a Sr _b Fe _c O _3-δ ", select La as "Ln", and select Sr as "Ae". Then, “La _a Sr _b Mn _c O _3-δ ” in which Mn is selected as “M” can be mentioned.

Since the perovskite type oxide particles are mixed conductors (oxygen ion-electron mixed conductors) having both oxygen ion conductivity and electron conductivity, they are so-called three-phase interfaces (cathode electrodes (electron conductors) and electrolytes (. Not only the interface between the ionic conductor) and the gas phase), but also the electrode surface can be an excellent reaction field. Therefore, it can exhibit high performance as an electrode even in a relatively low temperature range (for example, 600 ° C. to 800 ° C.). On the other hand, the perovskite-type oxide particles have a large difference from the coefficient of thermal expansion of the solid electrolyte material, and when repeatedly used in the temperature range of 600 ° C. to 1000 ° C., which is the general operating temperature of SOFC, the joints are formed. Cracks tend to occur and the cathode tends to peel off. On the other hand, by using the perovskite type oxide particles and the cerium-containing oxide particles in combination, the coefficient of thermal expansion can be reduced, and the heat can be reduced as compared with the case where the perovskite type oxide particles are used alone. Cycle durability can be improved.

Although not particularly limited, the amount of the perovskite-type oxide particles is an amount corresponding to 10% by mass to 90% by mass with respect to the total mass of the perovskite-type oxide particles and the cerium-containing oxide particles. It is preferably an amount corresponding to 20% by mass to 80% by mass. When the amount is within the range of such perovskite-type oxide particles, the above-mentioned performance improving effect (for example, low temperature operation effect and heat cycle durability improving effect) can be more effectively exhibited.

The perovskite-type oxide particles and the cerium-containing oxide particles are not adhered to each other and may exist as independent particles. Alternatively, the perovskite-type oxide particles and the cerium-containing oxide particles may be bonded to each other and composited. That is, the cathode forming material may be a composite material in a composite state (typically a mixed state) containing the perovskite-type oxide particles and the cerium-containing oxide particles. When the material for forming a cathode is used as a composite material, the perovskite-type oxide particles and the cerium-containing oxide particles are weighed so as to have a predetermined blending ratio, and are charged into an appropriate mixer (for example, a ball mill) to be mixed. .. The mixed powder is fired under suitable high temperature conditions. The firing is preferably carried out at a temperature of 900 ° C. to 1600 ° C. (for example, 1000 ° C. to 1200 ° C.) for about 2 hours to 48 hours. Then, by appropriately pulverizing and / or classifying the calcined mixed powder, a composite material composed of a perovskite-type oxide and a cerium-containing oxide, which are the target substances, can be obtained.

The form of the cathode forming material disclosed here is, for example, a paste (including a slurry and an ink) containing the powder and an arbitrary dispersion solvent even if it is in the powder form described above. Also, the slurry may be in the form of a dried (or fired) solid.
As the dispersion solvent, one or more of those capable of preferably dispersing the constituent material of the cathode forming material can be used without particular limitation. As the solvent, either an organic solvent or an inorganic solvent may be used. Examples of the organic dispersion solvent include an ether solvent, an ester solvent, a ketone solvent, and other organic solvents. For example, terpineol, butyl diglycol acetate, isobutyl alcohol and the like are preferably used. The inorganic dispersion solvent is preferably water or a mixed solvent mainly composed of water. As the solvent other than water constituting the mixed solvent, for example, one kind or two or more kinds of organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. The content of the solvent in such a paste is not particularly limited, but is preferably about 1% by mass to 50% by mass (typically 5% by mass to 35% by mass) of the entire slurry.

If necessary, a binder or other components that can be optionally added (for example, additives such as a thickener and a dispersant) may be added to the paste-like cathode forming material disclosed herein. it can. The binder, additives, etc. used in the paste are not particularly limited, and can be appropriately selected from conventionally known ones in paste production. Examples of the binder include cellulose or a derivative thereof. Specific examples thereof include hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, carboxyethyl methyl cellulose, cellulose, ethyl cellulose, methyl cellulose, ethyl hydroxyethyl cellulose, butyral and salts thereof. The content of the binder is not particularly limited, but is preferably contained in the range of 2% by mass to 20% by mass of the entire slurry.

The technique disclosed herein provides a method for suitably producing the SOFC described above. That is, this manufacturing method is a method for manufacturing a solid oxide fuel cell having a laminated structure including an anode having a porous structure, a solid electrolyte composed of an oxygen ion conductor, and a cathode having a porous structure.

A specific manufacturing method will be described in detail with reference to FIGS. 3A to 3C. Such a manufacturing method is disclosed here by preparing an anode-solid electrolyte laminate 110 composed of an anode 10 and a solid electrolyte 20 formed on the anode 10 on the surface of the laminate 110 on the solid electrolyte side. By applying any of the cathode-forming materials (typically, the above-mentioned cerium-containing oxide particles and the perovskite-type oxide particles), and by firing the above-mentioned laminate to which the above-mentioned material is applied, the above-mentioned cathode Includes forming 30. With the manufacturing method disclosed here, SOFC50 having excellent power generation performance and durability can be manufactured more easily.

<Preparation of anode-solid electrolyte laminate 110>
Here, the "anode-solid electrolyte laminate" indicates a state in which the anode and the solid electrolyte are laminated, and there is no particular limitation on the method for preparing the anode-solid electrolyte laminate. Therefore, the "method for producing an anode" and the "method for producing a solid electrolyte" in the conventionally known methods for producing SOFC can be used.
For example, first, as shown in FIG. 3A, an anode 10 having a porous structure is formed as a supporting base material (support). Specifically, for example, the above-mentioned group 8 to 10 metal element materials (for example, nickel materials) and stabilized zirconia (for example, YSZ) are mixed to obtain a cermet material. Next, the cermet material is dispersed in an appropriate solvent (for example, xylene) together with a binder (for example, a methacrylic acid ester polymer) and a dispersant (for example, sorbitan trioleate) to prepare a paste-like material for an anode. Then, the anode material can be molded by an appropriate molding method (for example, sheet molding), and the molded body is fired to form a thin plate-shaped or sheet-shaped anode 10. The firing treatment of the molded product is preferably performed at a temperature of 1200 ° C. to 1400 ° C. for about 1 hour to 5 hours. The firing treatment of the anode material may be performed at the same time as the firing treatment of the solid electrolyte material described later.

Next, as shown in FIG. 3B, the solid electrolyte 20 is formed on the anode 10. Here, the raw material (for example, YSZ) of the above-mentioned solid electrolyte 20 is dispersed in an appropriate solvent (for example, xylene) together with a binder (for example, a methacrylic acid ester polymer) and a dispersant (for example, sorbitan trioleate) to form a paste-like solid. Prepare materials for electrolytes. Next, the prepared solid electrolyte material is applied onto the anode 10 (or a molded body of the anode material) by an appropriate method (for example, printing molding) to form a molded body of the solid electrolyte material. Then, after the molded product to which the material for the solid electrolyte is applied is dried, it is fired in an air atmosphere. The firing temperature at this time is preferably in the range of, for example, 1000 ° C. to 1500 ° C. (preferably 1200 ° C. to 1400 ° C.), and the firing time is preferably in the range of, for example, 1 hour to 5 hours. By this firing treatment, the solid electrolyte 20 is formed on the anode 10, and the anode-solid electrolyte laminate 110 composed of the anode 10 and the solid electrolyte 20 formed on the anode 10 can be obtained.

<Adding material for cathode formation>
Next, as shown in FIG. 3C, the above-mentioned cathode forming material is applied to the surface of the anode-solid electrolyte laminate 110 on the solid electrolyte 20 side. At this time, in order to stably produce a homogeneous electrode, a material prepared by dispersing (or dissolving) the constituent material in one or more kinds of dispersion solvents can be preferably used. For example, first, the cerium-containing oxide particles and perovskite-type oxide particles disclosed here, a dispersion solvent (for example, terpineol), and an additive such as a thickener (for example, ethyl cellulose) are kneaded by an arbitrary kneading method (for example). , Roll mill, mixer, etc.) and knead to form a paste. For example, in such a kneading process, the powdery composite material or the like is kneaded at a stirring speed of 50 rpm to 300 rpm for 0.5 hours to 1 hour to form a paste in which the powdery cathode forming material is suitably dispersed. A material for forming a cathode is obtained.
Then, an arbitrary method (for example, print molding) is used to apply a paste-like cathode forming material onto the solid electrolyte 20. Before applying the cathode forming material, a buffer layer (also referred to as a reaction suppression layer, an intermediate layer, etc.) composed of, for example, a solid electrolyte or a component of an electrode is formed on the surface of the solid electrolyte 20 as needed. .) Can also be provided.

<Cathode 30 firing>
Next, the laminate to which the cathode forming material is applied is fired. The firing method and the like are not particularly limited, but for example, the laminate 110 to which the cathode forming material is applied can be fired at 700 ° C. to 900 ° C. (preferably 750 ° C. to 850 ° C.). The cathode 30 is formed by the above firing, and the SOFC 50 in which the anode 10, the solid electrolyte 20, and the cathode 30 are laminated can be obtained.
The SOFC 50 includes a cathode obtained by firing a cathode forming material containing surface Ce-enriched cerium-containing oxide particles having a Ce concentration on the surface portion higher than that in the central portion. Therefore, the obtained SOFC 50 can have high power generation performance and excellent durability as compared with the conventional one.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not intended to be limited to those shown in such specific examples.
In this example, solid oxide fuel cells (SOFCs) having different types and ratios of cathode constituent materials were produced and their performances were evaluated. In the following description, "%" is a mass reference (mass%) unless otherwise specified.

<Preparation of surface Ce-enriched cerium-containing oxide particles>
An aqueous solution (salt solution) containing a metal source (any one or more metal elements of Zr, La, Nd and Ce) constituting the core of the cerium-containing oxide particles as a starting material was prepared. Ammonia (NH ₃ ) was added to this salt solution, and the mixture was mixed and stirred at an appropriate speed to precipitate the hydroxide of the above metal element. The obtained hydroxide was washed, dried and calcined to form a precursor. Next, the precursor obtained above is mixed with a solution (acid solution) in which a predetermined amount of Ce source is dissolved in water, ammonia (NH ₃ ) is added, and the mixture and stirring are performed at an appropriate rate. A hydroxide of Ce was deposited on the surface of the precursor. Next, the precursor in which the hydroxide of Ce was precipitated was washed, dried, and calcined, so that the surface portion had a higher concentration of Ce than the central portion, and the surface Ce-enriched cerium-containing oxide of Examples 1 to 7 was obtained. Particles (core-shell particles) were prepared. On the surface of the cerium-containing oxide particles of each example, the cerium oxide (typically ceria) constituting the shell and the metal oxide constituting the core are mixed or solid-solved. Here, the amount of ceria (and thus the Ce concentration) on the surface of the cerium-containing oxide particles was adjusted by changing the amount of the Ce source added to the precursor. That is, as the amount of the Ce source added to the precursor increases, the amount of ceria (and thus the Ce concentration) on the surface of the cerium-containing oxide particles tends to increase. Here, the Ce concentration on the surface of the cerium-containing oxide particles was 80 mol%.

Further, as non-surface Ce-enriched cerium-containing oxide particles, 10 mol% gadolinium-doped ceria particles (Examples 8 and 9) and 10 mol% lanthanum-doped ceria particles (Example 10) were prepared. ..

<Preparation of composite material powder>
Each of the cerium-containing oxide particles of Examples 1 to 10 and the perovskite-type oxide powder were weighed in the proportions shown in Table 1 and mixed using a ball mill. The mixed powder was calcined at 1300 ° C. for 24 hours in an air atmosphere, pulverized and classified to obtain a powdery composite material (Examples 1 to 10). Table 1 summarizes the constituent elements of the cerium-containing oxide used, the composition of the perovskite-type oxide, the content of the perovskite-type oxide, and the specific surface area of the composite material powder of each example. In Example 8, a mixture of core-shell particles and Gd-doped ceria particles at a mass ratio of 50:50 was used. Further, in Example 11, the perovskite type oxide was used alone without using the cerium-containing oxide particles. The specific surface area of the composite material powder was measured using the nitrogen adsorption method.

Using the composite material powder of each example, a circular fuel electrode support type SOFC having an anode (fuel electrode) of φ20 mm and a cathode (air electrode) of about φ10 mm was produced by the following procedure.

As a fuel electrode support material, the average nickel oxide particle size 0.5 [mu] m (NiO) powder and the average particle diameter of 0.5 [mu] m 8 mol% yttria-stabilized zirconia _{(8mol% Y 2 O 3 -ZrO} 2; hereinafter, 8YSZ) powder Was mixed at a mass ratio of NiO: 8YSZ = 60:40 to obtain a mixed powder. This mixed powder, binder (polyvinyl butyral; PVB), pore-forming material (carbon particles, average particle size: about 5 μm), plasticizer (octyl phthalate), solvent (toluene: ethanol = 1: 3) Was blended in a mass ratio of 58: 8.5: 5: 4.5: 24 and mixed to prepare a paste-like composition for forming a fuel electrode support. Next, this composition for forming a fuel electrode support was applied in a sheet shape by a doctor blade method and dried to form a fuel electrode support green sheet having a thickness of 0.5 mm to 1 mm.

Next, as fuel electrode materials, nickel oxide (NiO) powder having an average particle diameter of 0.4 μm, 8YSZ powder having an average particle diameter of 0.3 μm, binder (ethyl cellulose; EC), and a solvent (α-terpineol; TE). And were kneaded at a mass ratio of 48:32: 2:18 to prepare a paste-like composition for forming a fuel electrode. A fuel electrode green sheet was formed by supplying this in the form of a sheet on the fuel electrode support green sheet by a screen printing method.

Next, as a solid electrolyte material, 8YSZ powder having an average particle diameter of 0.5 μm, a binder (ethyl cellulose; EC), and a solvent (terpine type) are kneaded at a mass ratio of 65: 4: 31 to make a paste. A composition for forming a solid electrolyte layer in the form of a solid electrolyte layer was prepared. By supplying this in the form of a sheet on the fuel electrode green sheet by a screen printing method, a solid electrolyte layer green sheet having a thickness of about 10 μm was formed. The laminated green sheet prepared in this manner was co-baked at 1350 ° C. to obtain SOFC half cells.

Next, as a material for forming a cathode, each of the composite material powders of Examples 1 to 11, a binder (ethyl cellulose), and a solvent (alcohol-based) are kneaded at a mass ratio of 80: 3: 17 to form a paste. A composition for forming a cathode was prepared. By supplying this in the form of a sheet on the solid electrolyte layer of the half cell by a screen printing method, an air electrode green sheet was formed. Next, this was fired at 1100 ° C. to fire the air electrode to obtain SOFCs according to Examples 1 to 11.

<Initial power generation performance>
The output densities when the SOFCs of Examples 1 to 11 produced above were operated under the following conditions were measured, and the output (W / cm ² ) at a voltage of 0.8 V is shown in Table 1 as the initial power generation performance.
Operating temperature: 700 ° C
Fuel electrode supply gas: H ₂ gas (50 ml / min)
Air electrode supply gas: Air (100 ml / min)

<Durability test>
In addition, a durability test was conducted in which the SOFCs of Examples 1 to 11 were continuously operated for 1000 hours under the above conditions. Then, the output density after the durability test was measured at a voltage of 0.8 V, and the deterioration rate (%) was calculated by [(initial power generation performance-power generation performance after the durability test) / initial power generation performance] × 100. The results are shown in the corresponding columns of Table 1.

As shown in Table 1, the SOFCs of Examples 1 to 8 using core-shell particles whose surface portion has a higher concentration of Ce than the central portion have an initial power generation performance of 0.38 W / cm ² or more, and the power generation performance. Was good. The deterioration rate after the durability test is also 0.32% or less (for example, 0.2% or less), which is compared with Examples 9 and 10 using rare earth element-doped ceria particles and Example 11 using perovskite-type oxide alone. It was excellent in durability. From this result, it was confirmed that SOFC capable of maintaining high power generation performance for a long period of time can be realized by forming a cathode using core-shell particles whose surface portion has a higher concentration of Ce than that of the central portion. In the case of the SOFC tested here, by setting the content of the perovskite-type oxide to 20% by mass to 80% by mass, an extremely high power generation performance of 0.42 W / cm ² or more could be achieved. From the viewpoint of power generation performance, the content of the perovskite type oxide is preferably 20% by mass to 80% by mass.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples illustrated above.

1 Cerium-containing oxide particles 2 Core 3 Shell 10 Anode 20 Solid electrolyte 30 Cathode

Claims (6)

A material for forming the cathode of solid oxide fuel cells.
The surface portion contains granular cerium-containing oxide particles having a higher concentration of Ce than the central portion.
The cerium-containing oxide particles are core-shell particles composed of a core having a relatively low concentration of Ce and a shell having a relatively high concentration of Ce.
A material for forming a cathode , wherein the core is a core-shell particle composed of a lanthanum, cerium or neodymium- doped zirconium oxide or a lanthanum- doped cerium oxide. The cathode forming material according to claim 1, further comprising perovskite-type oxide particles exhibiting oxygen ion conductivity. The amount according to claim 2, wherein the amount of the perovskite-type oxide particles is an amount corresponding to 10% by mass to 90% by mass with respect to the total mass of the perovskite-type oxide particles and the cerium-containing oxide particles. Material for forming the cathode. The amount of the perovskite type oxide particles is an amount corresponding to 20% by mass to 80% by mass with respect to the total mass of the perovskite type oxide particles and the cerium-containing oxide particles, according to claim 2 or 3. The material for forming a cathode according to the above. The cathode forming material according to any one of claims 1 to 4, which contains at least one dispersion solvent and is prepared in the form of a paste. A solid oxide fuel cell having an anode, a solid electrolyte, and a cathode.
The cathode is a solid oxide fuel cell made of a fired body of the material according to any one of claims 1 to 5.

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