JP2013051043A - Fuel electrode for fuel battery, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel electrode for a fuel battery which allows the improvement of the structural reliability against thermal stress, and a manufacturing method and a material which are suitable for the production of the fuel electrode.SOLUTION: The fuel electrode 3 comprises at least a structure 31, and an electron conductor 32. The structure 31 has a core part 311, and a surface layer 312 covering the outer periphery of the core part 311. The core part 311 includes an oxygen ion conductor as its primary component, and the surface layer 312 includes a proton conductor as its primary component. The fuel electrode 3 can be manufactured by baking a mixture of at least a kind of powder formed from the structure 31, and a kind of powder formed from the electron conductor 32, provided that the structure has: the core part 311 including an oxygen ion conductor as its primary component; and the surface layer 312 covering the outer periphery of the core part 311 and including a proton conductor as its primary component.

Description

  The present invention relates to a fuel electrode for a fuel cell and a method for manufacturing the same, and more particularly to a fuel electrode used in a fuel cell using a solid electrolyte as an electrolyte and a method for manufacturing the same.

  A fuel cell is a converter that generates electric energy by supplying a reactant (fuel) and an oxidant (air or oxygen) into the cell. The basic elements of a fuel cell are a fuel electrode, an electrolyte, and an air electrode. A joined body of these basic elements is called a single cell. Conventionally, a solid electrolyte type fuel cell using a solid electrolyte as an electrolyte is known. An example of this type of fuel cell is a solid oxide fuel cell (hereinafter sometimes referred to as SOFC) using solid oxide ceramics such as stabilized zirconia as a solid electrolyte.

  In the above fuel cell, in order to increase the output voltage, it is necessary to reduce the ohmic resistance of the solid electrolyte and the overvoltage associated with the reaction occurring at the fuel electrode and the air electrode. In recent years, for example, attempts have been made to use proton conductors in order to reduce the overvoltage of the fuel electrode and improve the output voltage.

For example, in Patent Document 1, a perovskite oxide such as SrZrO ₃ as a proton conductor is added to a mixture of Ni as an electron conductor and yttria-stabilized zirconia as an oxygen ion conductor. A fuel electrode produced by firing is disclosed.

In Patent Document 2, a perovskite-type crystal of BaCe _(1-x) Sm _(x) O _(3-α) is obtained by forming a BaO coating film on the surface of a plate-like ceria base material and performing heat treatment. A ceria-based solid electrolyte grown on the surface is disclosed.

JP 2007-66813 A JP 2005-243473 A

  However, the conventional fuel electrode has the following problems. That is, the thermal expansion coefficient of the proton conductor is greatly different from the thermal expansion coefficient of the electronic conductor or the oxygen ion conductor. Therefore, the conventional fuel electrode including the proton conductor itself may be cracked due to thermal stress generated during use of the fuel cell, and the structural reliability is low.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a fuel electrode for a fuel cell capable of improving the structural reliability against thermal stress. Another object of the present invention is to provide a manufacturing method and materials suitable for manufacturing the fuel cell fuel electrode.

  One embodiment of the present invention includes at least a structure including a core portion and a surface layer covering the outer periphery of the core portion, and an electronic conductor, and the main component of the core portion is an oxygen ion conductor, The main component of the surface layer is a proton conductor, in the fuel electrode for a fuel cell (Claim 1).

  Another aspect of the present invention is a mixture forming step for obtaining a mixture including at least a powder composed of a structure including a core portion and a surface layer covering the outer periphery of the core portion, and a powder composed of an electronic conductor. And a firing step of firing the resulting mixture, wherein the main component of the core part is an oxygen ion conductor, and the main component of the surface layer is a proton conductor. It exists in the manufacturing method of a pole (Claim 7).

  Still another embodiment of the present invention includes a core portion and a surface layer covering the outer periphery of the core portion, the main component of the core portion is an oxygen ion conductor, and the main component of the surface layer is a proton conductor. It is in the powder characterized by being comprised from a structure (Claim 12).

  First, the effect of the fuel cell fuel electrode will be described. In the prior art, the proton conductor itself is added to the fuel electrode as it is. Therefore, the conventional fuel electrode has a large difference in thermal expansion coefficient between the proton conductor and other materials such as an electronic conductor and oxygen ion conductor contained therein, and is inferior in structural reliability against thermal stress.

  In contrast, the fuel electrode for a fuel cell (hereinafter sometimes simply referred to as “fuel electrode”) includes a structure having a core portion and a surface layer covering the outer periphery of the core portion, and an electronic conductor. doing. In the structure, the main component of the core part is an oxygen ion conductor, and the main component of the surface layer is a proton conductor. Therefore, the thermal expansion coefficient of the structure is close to the thermal expansion coefficient of the oxygen ion conductor in the core portion, and is less susceptible to the thermal expansion coefficient of the proton conductor in the surface layer. Therefore, the difference in thermal expansion coefficient between the structure and another material such as an electronic conductor or oxygen ion conductor that can be included in the fuel electrode can be reduced. As a result, the fuel electrode can relieve thermal stress caused by the difference in thermal expansion coefficient, and is excellent in structural reliability against thermal stress.

  Further, adsorption / dissociation of hydrogen (gas phase) contained in the fuel to the fuel electrode occurs on the surface layer of the structure whose main component is a proton conductor. Therefore, as compared with the conventional fuel electrode containing the proton conductor itself, the effect of promoting adsorption / dissociation can be obtained even with a small amount of proton conductor. Therefore, the fuel electrode can reduce an overvoltage (hereinafter, also referred to as “reaction overvoltage”) associated with a reaction, and can contribute to an improvement in output voltage. Moreover, since the amount of the proton conductor can be reduced, it is advantageous from the viewpoint of cost reduction.

  Next, the effect of the method for producing the fuel electrode for a fuel cell will be described. In the manufacturing method described above, as a material of the mixture, a structure having a core part and a surface layer covering the outer periphery of the core part, the main component of the core part is an oxygen ion conductor, and the main component of the surface layer is a proton conductor The powder comprised from is used. And the fuel electrode as a sintered compact is obtained by baking the unbaked mixture containing this powder and an electronic conductor. Therefore, the manufacturing method does not require the mixture of the proton conductor powder itself as in the prior art. Therefore, the above-described fuel electrode excellent in structural reliability against thermal stress can be suitably obtained.

  Next, the effect of the powder will be described. The powder includes a core portion and a surface layer that covers the outer periphery of the core portion, and is composed of a structure in which the main component of the core portion is an oxygen ion conductor and the main component of the surface layer is a proton conductor. Therefore, instead of using the proton conductor powder itself as a raw material during the production of the fuel electrode, by using the powder composed of the above structure, an electronic conductor, an oxygen ion conductor, etc. that can be contained in the fuel electrode The thermal stress resulting from the difference in thermal expansion coefficient with other materials can be relieved. Therefore, it is suitable for manufacturing the above-described fuel electrode excellent in structural reliability against thermal stress.

  As mentioned above, according to this invention, the fuel electrode for fuel cells which can improve the structural reliability with respect to a thermal stress can be provided. Moreover, the manufacturing method suitable for manufacture of the said fuel electrode fuel electrode can be provided. Moreover, the material suitable for manufacture of the said fuel electrode fuel electrode can be provided.

It is explanatory drawing which showed typically the fuel cell single cell which concerns on an Example. It is explanatory drawing which expanded and showed typically the site | part A of the fuel electrode in FIG. It is the figure which showed the electrode reaction resistance by the side of a fuel electrode in the sample of the produced fuel cell single cell. It is the figure which showed the electrode reaction resistance by the side of a fuel electrode in the sample of the produced fuel cell single cell. It is a figure for comparing the thermal expansion coefficient of a structure with the thermal expansion coefficient of barium zirconate and yttria stabilized zirconia.

  First, the fuel electrode will be described. The fuel electrode can be suitably applied to a fuel cell using a solid electrolyte as an electrolyte. Examples of the solid electrolyte include stabilized zirconia in which oxides such as yttria and scandia are solid-dissolved (including partially stabilized zirconia, hereinafter omitted) and solid oxides having oxygen ion conductivity such as ceria-based solid solutions. It can be illustrated. Hereinafter, a solid oxide fuel cell using a solid oxide as a solid electrolyte may be referred to as SOFC.

  The fuel electrode includes a structure having a core part and a surface layer covering the outer periphery of the core part, and an electronic conductor. In addition to the above, the fuel electrode can further contain an oxygen ion conductor (claim 2). In this case, the reaction field in the fuel electrode can be increased. For this reason, the effect of promoting the adsorption / dissociation of hydrogen by the structure and the effect of increasing the reaction field are combined, thereby making it easier to reduce the reaction overvoltage and further contribute to the improvement of the output voltage.

  In the structure, the main component of the core part is an oxygen ion conductor, and the main component of the surface layer is a proton conductor. “The main component of the core part is an oxygen ion conductor” means that 50% by mass or more of the core part is an oxygen ion conductor having oxygen ion conductivity. The oxygen ion conductor can include not only a substance having oxygen ion conductivity but also a substance having oxygen ion conductivity and slight electronic conductivity. Examples of the component other than the oxygen ion conductor in the core part include a proton conductor which is a component of the surface layer, impurities inevitable in production, and the like. Thus, the core portion may be mixed with a proton conductor that is a component of the surface layer. Further, “the main component of the surface layer is a proton conductor” means that 50% by mass or more of the surface layer is a proton conductor having proton conductivity. Examples of components other than the proton conductor in the surface layer include oxygen ion conductors that are components of the core part, impurities that are unavoidable in production, and the like. Thus, the surface layer may be mixed with an oxygen ion conductor that is a component of the core. Note that the proton conductor included in the structure may be present so as to be inclined so that the proportion of the proton conductor existing from the surface of the structure toward the inside decreases. The ratio of the oxygen ion conductor in the core and the proton conductor in the surface layer can be measured by element mapping using an electron probe microanalyzer (hereinafter referred to as EPMA).

  The surface layer of the structure may exist so as to cover almost the entire outer periphery of the core part, or may exist so as to cover a part of the outer periphery of the core part. From the viewpoint of ensuring proton conductivity, the coverage of the outer periphery of the core portion by the surface layer is preferably 60% or more, more preferably 80% or more, and even more preferably 90% or more.

  In the fuel electrode, the oxygen ion conductor that is the main component of the core in the structure is one or more selected from stabilized zirconia and ceria solid solution, and the proton conductor that is the main component of the surface layer is A perovskite oxide exhibiting proton conductivity can be obtained.

  In this case, the fuel electrode is suitable for SOFC. That is, since the SOFC can be operated in a temperature range of 600 ° C. to 800 ° C., for example, there is a large temperature difference between battery operation and battery stop. Therefore, if the difference in thermal expansion coefficient between the materials contained in the fuel electrode is large, thermal stress is likely to occur. However, in the case of the fuel electrode having the above-described configuration, even when used in the operating temperature range of SOFC, the generation of thermal stress due to the difference in thermal expansion coefficient can be mitigated. Excellent structural reliability.

As the stabilized zirconia in the structure, for example, one kind of rare earth oxide such as Y ₂ O ₃ , Sc ₂ O ₃ , Gd ₂ O ₃ , Sm ₂ O ₃ , Yb ₂ O ₃ , Nd ₂ O ₃ or the like is used. Stabilized zirconia containing 2 or more types can be exemplified. Examples of the ceria-based solid solution in the structure include a ceria-based solid solution containing one or more rare earth oxides such as Gd ₂ O ₃ , Sm ₂ O ₃ , Y ₂ O ₃ , and La ₂ O _3. can do. Examples of the perovskite oxide in the structure include rare earths such as Y ₂ O ₃ , Sc ₂ O ₃ , Gd ₂ O ₃ , Sm ₂ O ₃ , Yb ₂ O ₃ , Nd ₂ O ₃ , and La ₂ O _3. Examples thereof include barium zirconate, strontium zirconate, barium serate, and strontium serate which can contain one or more oxides. One or more of these may be contained.

  Examples of the oxygen ion conductor that can be contained in the fuel electrode include stabilized zirconia, ceria-based solid solution, and the like. In addition, the said stabilized zirconia and ceria type solid solution illustrated by description of the core part of a structure can be applied to the concrete stabilized zirconia and ceria type solid solution in this case.

  In the fuel electrode, the ratio of the thickness of the surface layer to the distance from the center of the core portion to the surface of the surface layer in the structure may be substantially constant, or there may be different portions. The ratio of the thickness of the surface layer to the distance from the center of the core portion to the surface of the surface layer in the structure is preferably 5 to 50%, more preferably 10 to 40% (claims). 4).

  In this case, the balance between the relaxation effect of thermal stress caused by the difference in thermal expansion coefficient between materials and the promotion effect of hydrogen adsorption / dissociation on the fuel electrode is excellent. Therefore, the balance between the improvement effect of the structural reliability against the thermal stress and the improvement effect of the output voltage due to the reduction of the reaction overvoltage is excellent. The thickness of the surface layer can be measured by element mapping using an electron probe microanalyzer (hereinafter referred to as EPMA).

  In the fuel electrode, the mass ratio of the surface layer to the structure is preferably 50 to 90%, more preferably 70 to 90%, and still more preferably 80 to 90%.

  Also in this case, the balance between the relaxation effect of thermal stress due to the difference in thermal expansion coefficient between materials and the promotion effect of hydrogen adsorption / dissociation on the fuel electrode is excellent. Therefore, the balance between the improvement effect of the structural reliability against the thermal stress and the improvement effect of the output voltage due to the reduction of the reaction overvoltage is excellent. The mass ratio of the surface layer can be measured by elemental analysis using EPMA.

  In the fuel electrode, the content of the structure is preferably 3 to 20% by mass, more preferably 5 to 10% by mass.

  Also in this case, the balance between the relaxation effect of thermal stress due to the difference in thermal expansion coefficient between materials and the promotion effect of hydrogen adsorption / dissociation on the fuel electrode is excellent. Therefore, the balance between the improvement effect of the structural reliability against the thermal stress and the improvement effect of the output voltage due to the reduction of the reaction overvoltage is excellent. The content of the structure can be measured by elemental analysis using EPMA.

  In the above fuel electrode, any material can be used as the electronic conductor as long as it can exhibit electronic conductivity at the operating temperature of the fuel cell. Examples of the electronic conductor include nickel, iron, titanium, and the like. These can be used alone or in combination of two or more.

  The thickness of the fuel electrode can be appropriately selected from the range of, for example, 30 to 1000 μm in consideration of the shape of the fuel cell and the like.

  Next, a single fuel cell and a fuel cell will be described. The single fuel cell has the fuel electrode on at least one surface of the solid electrolyte layer. More specifically, the fuel cell single cell includes a solid electrolyte layer, the fuel electrode provided on one surface of the solid electrolyte layer, and an air electrode provided on the other surface of the solid electrolyte layer. be able to. An intermediate layer can be interposed between the solid electrolyte layer and the air electrode.

  Further, the fuel cell has the single fuel cell (claim 6). More specifically, the fuel cell can be configured as a fuel cell stack in which a plurality of fuel cell single cells are stacked via a separator. A current collector may be provided between the single fuel cell and the separator.

  Since these fuel cell single cells and fuel cells use the above-described fuel electrode excellent in structural reliability against thermal stress, they become highly reliable fuel cell single cells and fuel cells. In addition, since the fuel electrode capable of reducing the reaction overvoltage is used, the fuel cell unit cell and the fuel cell are advantageous in improving the output voltage.

  In addition, as a solid electrolyte, what was mentioned above can be illustrated. Examples of the material of the air electrode include perovskite oxides having conductivity such as lanthanum-manganese oxide, lanthanum-cobalt oxide, and lanthanum-iron oxide.

  The shape of the fuel cell unit cell and the fuel cell may be a flat shape such as a flat plate shape or a flat shape, or a cylindrical shape. Preferably, it is a flat type excellent in cell productivity. In the fuel cell unit cell and the fuel cell, any power generation main element of the solid electrolyte layer, the fuel electrode layer, and the air electrode layer may function as a support. A thing other than the power generation main element can also be provided as a separate support.

  Next, the manufacturing method of the fuel electrode and the powder will be described. In the fuel electrode manufacturing method, in the mixture forming step, a mixture including at least a powder composed of a structure including a core part and a surface layer covering an outer periphery of the core part, and a powder composed of an electronic conductor. Form. In addition, about the structure of a structure and an electronic conductor, since it applies to the content illustrated by description of the fuel electrode, description is abbreviate | omitted.

  In the fuel electrode manufacturing method, the mixture may further include a powder composed of an oxygen ion conductor (claim 8). In addition, about the structure of an oxygen ion conductor, since it applies to the content illustrated by description of the fuel electrode, description is abbreviate | omitted.

  In this case, the reaction field in the obtained fuel electrode can be increased. For this reason, the effect of promoting the adsorption / dissociation of hydrogen by the structure and the effect of increasing the reaction field are combined, so that the reaction overvoltage can be easily reduced, and a fuel electrode that can further contribute to the improvement of the output voltage can be obtained.

  From the viewpoints of catalytic activity and manufacturability, the powder composed of the structure preferably has an average particle size of 0.1 to 10 μm, more preferably 0.1 to 3 μm. Similarly, the powder composed of the electronic conductor preferably has an average particle size in the range of 0.1 to 10 μm, more preferably 0.1 to 3 μm. The powder composed of the oxygen ion conductor preferably has an average particle diameter of 0.1 to 10 μm, more preferably 0.1 to 3 μm. In addition, the said average particle diameter is a 50% diameter in the integrated fraction of a volume reference | standard measured based on JISR1629.

  In the fuel electrode manufacturing method, the means for mixing each powder is not particularly limited, and various mixing means such as a ball mill and a planetary ball mill can be used. In addition, as a mixture, specifically, for example, after preparing a slurry by mixing each powder and a suitable solvent such as ethanol, the slurry is applied using various coating methods, sheet-like, Examples thereof include an unfired product formed into a film or the like, and an unfired molded body formed by mixing each powder and press-molding the mixed powder.

  Moreover, when there are three or more kinds of powders for forming the mixture, the mixture can be formed by going through a procedure of mixing each powder at once. Moreover, it is also possible to obtain a mixture by mixing two or more kinds of powders first and then adding and mixing one or more kinds of powders to the mixed powder.

  Specifically, for example, when mixing a powder composed of a structure, a powder composed of an electronic conductor, and a powder composed of an oxygen ion conductor, the powder composed of an electronic conductor and oxygen A mixture can be formed through a procedure of mixing a mixed powder obtained by mixing a powder composed of an ionic conductor and a powder composed of a structure. In this case, it becomes easy to obtain a fuel electrode in which the structure is relatively uniformly dispersed in the skeleton composed of the electron conductor and the oxygen ion conductor.

  In the fuel electrode manufacturing method, the powder composed of the structural body reacts with the oxygen ion conductor by heat treatment on the particle surface of the powder composed of the oxygen ion conductor, which is the raw material of the core portion, and conducts proton conduction. It can be suitably prepared through an attachment step of attaching a reaction raw material capable of generating a body and a heat treatment step of heat-treating a powder composed of an oxygen ion conductor to which the reaction raw material is attached (claim) Item 9).

  In this case, during the heat treatment in the heat treatment step, the powder particle surface composed of the oxygen ion conductor that is the raw material of the core part reacts with the reaction raw material attached to the particle surface, and the surface layer of the oxygen ion conductor Proton conductors are formed at the same time. Moreover, the oxygen ion conductor inside the surface layer remains as it is without substantially reacting with the reaction raw material, and the core portion is formed. Thus, a powder comprising a structure having a core portion and a surface layer covering the outer periphery of the core portion, the main component of the core portion being an oxygen ion conductor, and the main component of the surface layer being a proton conductor. Can be prepared relatively easily.

  That is, according to the fuel electrode manufacturing method including the preparation step, it is possible to prepare the powder composed of the structure relatively easily. Therefore, according to the manufacturing method having the preparation step, the fuel electrode having excellent structural reliability against thermal stress can be obtained relatively easily.

  Examples of the oxygen ion conductor used as the raw material for the core part include stabilized zirconia and ceria-based solid solution exemplified in the description of the fuel electrode. Examples of the reaction raw material that can generate a proton conductor by reacting with an oxygen ion conductor by heat treatment include, for example, oxides such as Ba and Sr, and the above elements such as nitrates, carbonates, and hydroxides of the above elements. Examples thereof include compounds. These can be used alone or in combination of two or more.

  Moreover, the method of attaching the said reaction raw material to the particle | grain surface which comprises the powder comprised from an oxygen ion conductor is not specifically limited. For example, a solution containing a reaction raw material and a solvent such as water, a slurry containing a reaction raw material and an appropriate binder or solvent, and a powder of an oxygen ion conductor used as a raw material for the core are employed. can do. In these cases, there is an advantage that the particle surface constituting the powder of the oxygen ion conductor is relatively uniformly covered with the reaction raw material. In addition, when using the said solution and slurry, the viewpoint of the reactivity with the raw material of a core part, etc. from the viewpoint of the reactivity with the raw material of a core part, etc., Preferably, it is 0.1 mol / L or more, More preferably, it is 0.00. It is good to be prepared at 3 mol / L or more.

  The heat treatment temperature when heat-treating the powder composed of the oxygen ion conductor to which the reaction raw material is attached is preferably from the viewpoints of reactivity, balance between the amount of the core and the surface layer, evaporation of the reaction elements, etc. 900 ° C to 1600 ° C, more preferably 1200 ° C to 1500 ° C. The heat treatment time is preferably in the range of 1 to 6 hours, more preferably in the range of 1 to 3 hours, from the viewpoints of reactivity, balance between the amount of the core part and the surface layer, evaporation of the reactive elements, and the like. It can be.

  In the fuel electrode manufacturing method, the firing temperature in the firing step may be equal to or lower than the heat treatment temperature in the heat treatment step. In this case, the reaction between the electronic conductor, oxygen ion conductor, and the like that can be included in the mixture and the surface layer of the structure can be effectively suppressed. Therefore, it becomes easy to obtain a fuel electrode with little composition deviation. The firing temperature in the firing step can be more preferably lower than the heat treatment temperature in the heat treatment step from the viewpoint of ensuring the above effect. More preferably, it can be in the range of the heat treatment temperature in the heat treatment step -500 ° C to the heat treatment temperature in the heat treatment step -50 ° C.

  In the fuel electrode manufacturing method, the oxygen ion conductor that is the main component of the core is one or more selected from stabilized zirconia and ceria-based solid solution, and the proton conductor that is the main component of the surface layer Can be a perovskite oxide exhibiting proton conductivity (claim 11).

  In this case, a fuel electrode suitable for SOFC and excellent in structural reliability against thermal stress can be obtained. In this case, the specific stabilized zirconia, ceria-based solid solution, and perovskite oxide exemplified in the description of the fuel electrode can be applied to the specific stabilized zirconia, ceria-based solid solution, and perovskite-type oxide.

  First, an outline of a fuel cell unit cell, a fuel cell fuel electrode, a method for manufacturing the same, and a powder composed of a structure will be described with reference to the drawings as appropriate.

  As shown in FIG. 1, the fuel cell single cell 1 according to the embodiment has the fuel electrode 3 for the fuel cell according to the embodiment on one surface of the solid electrolyte layer 2. Specifically, the fuel cell single cell 1 according to the embodiment includes a solid electrolyte layer 2, a fuel cell fuel electrode 3 according to the embodiment provided on one surface of the solid electrolyte layer 2, and a solid electrolyte layer 2. And an air electrode 4 provided on the other surface.

  The fuel electrode 3 for a fuel cell according to the example includes a structure 31 and an electronic conductor 32 as schematically shown in FIG. In this example, the fuel electrode 3 for the fuel cell according to the embodiment further includes an oxygen ion conductor 33 in addition to the structure 31 and the electronic conductor 32. The structure 31 includes a core portion 311 and a surface layer 312 that covers the outer periphery of the core portion 311. The main part of the core part 311 is an oxygen ion conductor. The main component of the surface layer 312 is a proton conductor.

  A method for producing a fuel electrode for a fuel cell according to an embodiment includes a mixture forming step for obtaining a mixture including at least the powder according to the embodiment and a powder composed of an electronic conductor, and a firing step for firing the obtained mixture. And have. The powder according to the example includes a core part and a surface layer covering the outer periphery of the core part, the core part is composed of an oxygen ion conductor, and the surface layer is composed of a proton conductor. (Hereinafter, this powder is simply referred to as “structure powder”). Hereinafter, it demonstrates in detail using an experiment example.

(Experimental example)
<Production of structure powder>
5 g of yttria-stabilized zirconia powder (average particle diameter = 0.7 μm) in which 8 mol% of yttria was dissolved and 0.1 L of Ba (NO ₃ ) ₂ aqueous solution (concentration 0.3 mol / L) were placed in a ball mill. Mixed for hours. Next, this yttria-stabilized zirconia powder was heat treated at 1500 ° C. for 2 hours to obtain a powder of Sample P1 (average particle size = 0.7 μm). About elemental mapping about the powder of obtained sample P1 using SEM-EDS (scanning electron microscope-energy dispersive X ray analyzer), Ba, Zr, and Y are detected from the surface layer of particles. At the same time, Zr and Y were detected from the inner side of the surface layer of the particles. Further, when the crystal structure of the surface layer of the particles was confirmed using XRD (X-ray diffractometer), it was confirmed that the crystal structure was barium zirconate which is a perovskite proton conductor. From these results, the powder of sample P1 has a core part and a surface layer covering the outer periphery of the core part, and the main component of the core part is yttria-stabilized zirconia, and the main component of the surface layer is yttria-doped barium zirconate. It was found to be composed of a group of particles having a certain core-shell structure. Further, the ratio of the thickness of the surface layer to the distance from the center of the core portion to the surface of the surface layer, measured by EPMA elemental mapping, was 55%.

  In the production of the structure powder of Sample P1, the structure powder of Sample P2 was obtained in the same manner except that the heat treatment condition was 1600 ° C. × 2 hours.

  In producing the structure powder of sample P1, the structure powder of sample P3 was obtained in the same manner except that the heat treatment condition was 1400 ° C. × 2 hours.

  In the production of the structure powder of sample P1, the same procedure was performed except that scandia-stabilized zirconia powder (average particle size = 0.8 μm) in which 6 mol% of scandia was dissolved was used instead of yttria-stabilized zirconia powder. A structural powder of Sample P4 was obtained.

  In the production of the structure powder of the sample P1, a gadria-doped ceria powder (average particle size = 0.5 μm) doped with 5 mol% gadria was used instead of the yttria-stabilized zirconia powder. A structure powder of Sample P5 was obtained.

  In the production of the structure powder of the sample P1, in place of the yttria-stabilized zirconia powder, except that a samaritan-doped ceria powder (average particle size = 0.5 μm) doped with 10 mol% samaria was used, A structure powder of Sample P6 was obtained.

  In the production of the structure powder of the sample P1, in place of the yttria-stabilized zirconia powder, except that yttria-doped ceria powder doped with 10 mol% yttria (average particle size = 0.5 μm) was used, A structural powder of Sample P7 was obtained. Table 1 shows the detailed configuration and the like of each prepared structure powder.

<Fabrication of fuel cell single cell>
Yttria-stabilized zirconia (8YSZ) powder (average) in which the structure powder (heat treatment temperature 1500 ° C.), NiO powder (average particle diameter: 0.8 μm), and 8 mol% of yttria prepared in the above were prepared. Each powder was weighed so that the mass ratio with respect to (particle diameter: 0.7 μm) was 10:50:40. The weighed NiO powder and 8YSZ powder and 1.7 times the mass of terpineol with respect to these powders were placed in a ball mill and mixed at 180 rpm for 1 hour. Next, the weighed structural powder of the sample P1 was added to the ball mill and mixed for 2 hours at 180 rpm. Thus, a fuel electrode forming slurry was prepared.

Also, La _0.6 Sr _0.4 Co _0.2 Fe _0.8 (LSCF) powder (average particle size: 1.0 μm) and gadria-doped ceria doped with 5 mol% of gadria (GDC powder) ( Each powder was weighed so that the mass ratio with respect to the average particle size = 0.6 μm was 80:20. The weighed LSCF powder and GDC powder and terpineol 1.5 times the mass of these powders were placed in a ball mill and mixed at 180 rpm for 2 hours. Thus, an air electrode forming slurry was prepared.

  In addition, gadria-doped ceria doped with 5 mol% of gadria (average particle size = 0.6 μm) and terpineol twice the mass of these powders were put in a ball mill and rotated at 180 rpm for 2 hours. Mixed. Thus, an intermediate layer forming slurry on the air electrode side was prepared.

  For comparison, a fuel electrode forming slurry C1 that does not contain the prepared structure powder at all, and a barium zirconate powder itself, which is a proton conductor powder, was contained in place of the prepared structure powder. A fuel electrode forming slurry C2 was also prepared.

  The fuel electrode forming slurry C1 was prepared as follows. The mass ratio of NiO powder (average particle size: 0.8 μm) to yttria-stabilized zirconia (8YSZ) powder (average particle size: 0.7 μm) in which 8 mol% of yttria is dissolved is 55:45. Each powder was weighed. The weighed NiO powder and 8YSZ powder and terpineol 1.5 times the mass of these powders were put into a ball mill and mixed at 180 rpm for 2 hours. Thus, a fuel electrode forming slurry C1 was prepared.

  The fuel electrode forming slurry C2 was prepared as follows. Barium zirconate powder (average particle size: 0.7 μm), NiO powder (average particle size: 0.8 μm), yttria-stabilized zirconia (8YSZ) powder (average particle size: 0) in which 8 mol% of yttria was dissolved. Each powder was weighed so that the mass ratio thereof was 10:50:40. The weighed NiO powder and 8YSZ powder and 1.7 times the mass of terpineol with respect to these powders were placed in a ball mill and mixed at 180 rpm for 1 hour. Next, a further weighed barium zirconate powder was added to the ball mill and mixed for 2 hours at a rotation speed of 180 rpm. Thus, a fuel electrode forming slurry C2 was prepared.

  Next, a fuel electrode forming slurry comprising yttria-stabilized zirconia (8YSZ) in which 8 mol% of yttria is dissolved and containing the structure powder of the sample P1 on one surface of a solid electrolyte plate having a thickness of 0.3 mm and a diameter of 20 mm. After screen printing, this was fired at 1400 ° C. The firing temperature is a temperature equal to or lower than the heat treatment temperature when the structure powder contained in the fuel electrode forming slurry is produced. Thereby, a fuel electrode (thickness 50 μm, diameter 8 mm) was formed on one surface of the solid electrolyte plate.

  Next, the intermediate layer forming slurry was screen-printed on the other surface of the solid electrolyte plate, and then fired at 1300 ° C. Next, the air electrode forming slurry was screen-printed and then fired at 1050 ° C. Thus, an air electrode (thickness 50 μm, diameter 8 mm) was formed on the other surface of the solid electrolyte plate via an intermediate layer (thickness 8 μm, diameter 16 mm). Thus, a fuel cell single cell of Sample F1 was produced.

  In the production of the fuel cell single cell of sample F1, a fuel cell single cell of sample F2 was obtained in the same manner except that the structure powder (heat treatment temperature 1600 ° C.) of sample P2 was used.

  A fuel cell single cell of sample F3 was obtained in the same manner except that the structure powder of sample P3 (heat treatment temperature 1400 ° C.) was used in the production of the fuel cell single cell of sample F1.

  In the production of the fuel cell single cell of sample F1, the structure powder of sample P2 is used, and each powder is weighed so that the mass ratio of the structure powder of sample P2, NiO powder, and 8YSZ powder is 5:53:42. A fuel cell single cell of Sample F4 was obtained in the same manner except that.

  In the production of the fuel cell single cell of sample F1, the structure powder of sample P2 was used, and each powder was weighed so that the mass ratio of the structure powder of sample P2, NiO powder, and 8YSZ powder was 20:44:36. A fuel cell single cell of Sample F5 was obtained in the same manner except for the above.

  A fuel cell single cell of Sample F6 was obtained in the same manner except that the fuel electrode forming slurry C1 prepared above was used in the production of the fuel cell single cell of Sample F1.

  In the production of the fuel cell single cell of sample F1, a fuel cell single cell of sample F7 was obtained in the same manner except that the fuel electrode forming slurry C2 prepared above was used. Table 2 summarizes the detailed configuration of each manufactured fuel cell single cell, each fuel electrode, and the like.

<Evaluation of fuel electrode>
1. Evaluation of Reaction Overvoltage The produced fuel cell single cell was operated at 800 ° C., AC impedance was measured by the four-terminal method, and the electrode reaction resistance on the fuel electrode side was calculated from the waveform. At this time, 97 vol% H ₂ -3 vol% H ₂ O fuel gas was supplied to the fuel electrode at a flow rate of 0.1 L / min. In addition, air was supplied to the air electrode as an oxidant gas at a flow rate of 0.1 L / min. The results are shown in FIGS.

2. Thermal expansion evaluation Each of the pellets for evaluation was prepared by molding the powders of the structures of the samples P1 to P7 prepared above into pellets and firing them at 1400 ° C. Next, using a thermomechanical analyzer (TMA), the thermal expansion amount of each pellet is measured from the difference in thermal expansion amount between each pellet and the standard sample (alumina) when the temperature is increased at a constant rate. The thermal expansion coefficient of each pellet was calculated. At this time, for comparison, pellets were similarly prepared for the above-described yttria-doped barium zirconate (BZY) powder and 8YSZ powder, and the thermal expansion coefficient was calculated. FIG. 5 shows the result.

<Discussion>
Comparing the results of FIG. 3 to FIG. 5 reveals the following. The fuel cell single cell of Sample F6 has a fuel electrode that does not contain a specific structure. Therefore, the electrode reaction resistance of the fuel electrode is extremely high, which is extremely disadvantageous for improving the output voltage. In addition, the fuel cell single cell of Sample F7 has a fuel electrode to which barium zirconate itself, which is a perovskite oxide showing proton conductivity, is added. Therefore, since the thermal expansion coefficient is greatly different from yttria-stabilized zirconia and barium zirconate, which are oxygen ion conductors constituting a part of the fuel electrode, structural reliability against thermal stress is lacking.

On the other hand, the fuel cell single cells of Samples F1 to F5 have a fuel electrode containing a specific structure. In the fuel electrode, the thermal expansion coefficient of the specific structure is close to the thermal expansion coefficient of the oxygen ion conductor in the core and is hardly affected by the thermal expansion coefficient of the proton conductor in the surface layer. Therefore, the fuel electrode can reduce the difference in thermal expansion coefficient between the other material constituting the fuel electrode and the structure. Although not shown in FIG. 5, the thermal expansion coefficient of Ni is about 13 × 10 ⁻⁶ K ⁻¹ . Therefore, it can be said that the fuel electrode and the single fuel cell can relieve the thermal stress caused by the difference in thermal expansion coefficient, and are excellent in structural reliability against the thermal stress.

  Further, adsorption / dissociation of hydrogen (gas phase) contained in the fuel to the fuel electrode occurs on the surface layer of the structure whose main component is a proton conductor. Therefore, compared with the fuel electrode of the sample F7 containing the proton conductor itself, an effect of promoting adsorption / dissociation can be obtained even with a small amount of proton conductor. Therefore, it can be said that the fuel electrode can reduce the overvoltage associated with the reaction and contribute to the improvement of the output voltage. Further, since the amount of proton conductor can be reduced, it can be said that it is advantageous from the viewpoint of cost reduction.

  Further, when the content of the structure in the fuel electrode is in the range of 3 to 20% by mass, the effect of improving the structural reliability against thermal stress and the effect of improving the output voltage by reducing the reaction overvoltage It can be seen that the balance is excellent.

  Although the embodiments have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 Fuel cell single cell 2 Solid electrolyte layer 3 Fuel electrode for fuel cells 31 Structure 311 Core part 312 Surface layer 32 Electronic conductor 33 Oxygen ion conductor 4 Air electrode

Claims (12)

A structure including a core portion and a surface layer covering the outer periphery of the core portion, and at least an electronic conductor,
A fuel electrode for a fuel cell, wherein a main component of the core portion is an oxygen ion conductor, and a main component of the surface layer is a proton conductor. The fuel electrode for a fuel cell according to claim 1,
Furthermore, the fuel electrode for fuel cells characterized by containing an oxygen ion conductor. The fuel electrode for a fuel cell according to claim 1 or 2,
The oxygen ion conductor as the main component of the core part is one or more selected from stabilized zirconia and ceria-based solid solution,
A fuel electrode for a fuel cell, wherein the proton conductor as a main component of the surface layer is a perovskite oxide exhibiting proton conductivity. The fuel electrode for a fuel cell according to any one of claims 1 to 3,
The fuel electrode for a fuel cell, wherein the structure has a ratio of the thickness of the surface layer to the distance from the center of the core portion to the surface of the surface layer in a range of 5 to 50%.   A fuel cell single cell comprising the fuel electrode for a fuel cell according to any one of claims 1 to 4 on at least one surface of the solid electrolyte layer.   A fuel cell comprising the single fuel cell according to claim 5. A mixture forming step of obtaining a mixture comprising at least a powder comprising a core body and a structure comprising a surface layer covering the outer periphery of the core part, and a powder comprising an electronic conductor;
A firing step of firing the resulting mixture,
A method for producing a fuel electrode for a fuel cell, wherein a main component of the core portion is an oxygen ion conductor, and a main component of the surface layer is a proton conductor. In the manufacturing method of the fuel electrode for fuel cells according to claim 7,
The said mixture contains the powder comprised from an oxygen ion conductor further, The manufacturing method of the fuel electrode for fuel cells characterized by the above-mentioned. In the manufacturing method of the fuel electrode for fuel cells according to claim 7 or 8,
The powder composed of the above structure is
An attachment step of attaching a reaction raw material capable of generating a proton conductor by reacting with the oxygen ion conductor by heat treatment on the particle surface of the powder composed of the oxygen ion conductor as a raw material of the core part;
A method for producing a fuel electrode for a fuel cell, comprising: a heat treatment step of heat treating a powder composed of an oxygen ion conductor to which the reaction raw material is adhered. In the manufacturing method of the fuel electrode for fuel cells according to claim 9,
A method for producing a fuel electrode for a fuel cell, wherein a firing temperature in the firing step is equal to or lower than a heat treatment temperature in the heat treatment step. In the manufacturing method of the fuel electrode for fuel cells of any one of Claims 7-10,
The oxygen ion conductor as the main component of the core part is one or more selected from stabilized zirconia and ceria-based solid solution,
The method for producing a fuel electrode for a fuel cell, wherein the proton conductor as a main component of the surface layer is a perovskite oxide exhibiting proton conductivity.   A core part and a surface layer covering the outer periphery of the core part, wherein the main part of the core part is an oxygen ion conductor, and the main part of the surface layer is a proton conductor. And powder.

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