JP5090575B1 - Solid oxide fuel cell - Google Patents

Abstract

A solid oxide fuel cell (SOFC) having a low reaction resistance of a fuel electrode is provided.
The SOFC includes a "porous fuel electrode for reacting a fuel gas and a fuel electrode comprising Ni and YSZ" and a "porous air electrode for reacting a gas containing oxygen". And “a solid electrolyte membrane provided between the fuel electrode and the air electrode and having an interface with the fuel electrode”. Regarding the “near interface region” located within 3 μm from the interface in the fuel electrode, the average diameter of Ni particles existing in the “near interface region” is 0.28 to 0.80 μm. The average value of the diameters of the YSZ particles existing in the “inside” is 0.28 to 0.80 μm, and the average value of the diameters of the pores existing in the “region near the interface” is 0.10 to 0.87 μm.
[Selection] Figure 1

Description

  The present invention relates to a solid oxide fuel cell (SOFC).

The SOFC includes a porous fuel electrode for reacting a fuel gas, a porous air electrode for reacting a gas containing oxygen, and a dense solid electrolyte membrane provided between the fuel electrode and the air electrode. (For example, refer patent document 1). By supplying a fuel gas (such as hydrogen gas) to the fuel electrode and a gas containing oxygen (such as air) to the air electrode for a high-temperature (for example, 500 to 1000 ° C.) SOFC, the following (1) , (2) chemical reaction occurs. Thereby, a potential difference is generated between the fuel electrode and the air electrode. This potential difference is based on the oxygen ion conductivity of the solid electrolyte.
(1/2) · O ₂ + 2e ⁻ → O ²⁻ (where: air electrode) (1)
H ₂ + O ²⁻ → H ₂ O + 2e ⁻ (in the fuel electrode) (2)

JP 2008-226789 A

Hereinafter, attention is paid to the reaction of the fuel electrode (the above formula (2)). The porous fuel electrode is composed of, for example, Ni (nickel) and YSZ (yttria stabilized zirconia). In this case, at the fuel electrode, Ni functions as “a catalyst for dissociating hydrogen molecules (H ₂ ) into hydrogen ions (2H ⁺ )” and “a substance that conducts the generated electrons (2e ⁻ )”. YSZ functions as a “substance that conducts oxygen ions (O ²⁻ )”. The pores function as “pathways for hydrogen molecules (H ₂ ) and generated water (H ₂ O)”. That is, the reaction of the fuel electrode strongly depends on the presence state of Ni particles, YSZ particles, and pores in the fuel electrode (particularly, in the vicinity of the interface with the solid electrolyte membrane in the fuel electrode active layer described later). Therefore, it is considered that the reaction resistance of the fuel electrode can be reduced by adjusting the existence state (particle size, etc.) of Ni particles, YSZ particles, and pores in the fuel electrode.

  By the way, when performing observation using a scanning electron microscope (SEM observation) on a cross section obtained by cutting a sample of a SOFC cell in the thickness direction (stacking direction), pores and “Ni particles or YSZ particles” While it was easy to distinguish between the Ni particles and the YSZ particles, it was very difficult to distinguish them. This is due to the fact that the difference in brightness between the Ni particles and the YSZ particles is large (more specifically, the difference in brightness in the SEM reflected electron image), while the difference in brightness between the Ni particles and the YSZ particles is small. It was.

  On the other hand, in recent years, an SEM observation method has been reported that can distinguish between Ni particles and YSZ particles, and therefore can individually distinguish between Ni particles, YSZ particles, and pores. This “new technique of SEM observation” is described in detail in Solid State lonics 178 (2008) 1984.

  The inventor has observed cross sections of various fuel electrodes using this “new technique for SEM observation”. At that time, in particular, attention was paid to the “region in the vicinity of the interface between the fuel electrode and the solid electrolyte membrane in the fuel electrode” that is considered to greatly contribute to the reaction of the fuel electrode. As a result, the present inventor has discovered that the combination of the three diameters of Ni particles, YSZ particles, and pores in the above region greatly affects the reaction resistance of the fuel electrode. In addition, the present inventor has found an appropriate range of combinations of the three diameters of Ni particles, YSZ particles, and pores in the above region, which are necessary for reducing the reaction resistance of the fuel electrode.

  In view of the above, an object of the present invention is to provide an SOFC with a small reaction resistance of the fuel electrode.

  The SOFC according to the present invention is “a porous fuel electrode that reacts with a fuel gas, and is configured to include Ni and an oxygen ion conductive material (oxide ceramic) having oxygen ion conductivity”. , “A porous air electrode for reacting a gas containing oxygen” and “a solid electrolyte membrane provided between the fuel electrode and the air electrode and having an interface with the fuel electrode” Electrolyte membrane ".

  Examples of the oxygen ion conductive material include yttria stabilized zirconia (YSZ), gadolinium doped ceria (GDC), and the like. The fuel electrode and the solid electrolyte membrane are preferably formed by simultaneous firing.

  The feature of the SOFC according to the present invention is that, for a region located within 3 μm from the interface in the fuel electrode, the average particle diameter of the Ni present in the region is 0.28 to 0.80 μm, The average particle diameter of the oxygen ion conducting material existing in the region is 0.28 to 0.80 μm, and the average pore diameter of the fuel electrode existing in the region is 0.10 to 0.87 μm. It is to be.

  As a result of observing the cross sections of various fuel electrodes using the above-described “new method of SEM observation”, the present inventor has found that the reaction resistance of the fuel electrode is reduced by adopting the above configuration ( Details will be described later).

  In the SOFC, the average diameter of the Ni particles existing in the region facing the solid electrolyte membrane is 0.31 to 0.70 μm, and in the region exists facing the solid electrolyte membrane. The average particle diameter of the oxygen ion conducting material is 0.32 to 0.78 μm, and the average pore diameter of the fuel electrode existing in the region facing the solid electrolyte membrane is 0.15. It is preferable that it is -0.80 micrometer. The inventor has also found that the reaction resistance of the fuel electrode becomes smaller by adopting this configuration (details will be described later). Note that “particles (pores) present facing the solid electrolyte membrane” means that when the fuel electrode is observed from the solid electrolyte membrane side when the solid electrolyte membrane is assumed to be absent (using SEM or the like). ) Refers to visible (visible) particles (pores). In other words, it refers to particles (pores) distributed at the “end near the interface in the thickness direction” in the fuel electrode (along the interface).

  If the above-mentioned “new method of SEM observation” is adopted, not only Ni particles, YSZ particles and pores can be individually distinguished, but also the Ni particles are “single without being connected to other adjacent Ni particles”. Ni particles that exist in non-percolating Ni (hereinafter referred to as “isolated Ni particles”) and “Ni particles that are connected to other adjacent Ni particles (continuously and continuously)” (Percolating Ni, hereinafter referred to as “continuous Ni particles”). This point is also described in detail in the above “Solid State lonics 178 (2008) 1984”.

  As a result of observing the cross sections of various fuel electrodes using the above-described “new method of SEM observation”, the present inventor found that “all the Ni particles existing in the region (ie, isolated Ni particles + continuous When the ratio of the “total volume of only isolated Ni particles existing in the region” to the “total volume of Ni particles” is (2% or more and 40% or less), the reaction resistance of the fuel electrode is further increased. It has also been found that it becomes smaller (details will be described later).

It is the schematic diagram which showed the structure of SOFC which concerns on embodiment of this invention. It is a SEM image which shows an example of the cross section of SOFC (especially fuel electrode and electrolyte membrane) which concerns on embodiment of this invention. It is a graph which shows an example of the relationship between the average value of the diameter of Ni particle | grains which exist in the interface vicinity area | region, and SOFC cell output. It is a graph which shows an example of the relationship between the average value of the diameter of the YSZ particle | grains which exist in the interface vicinity area | region, and SOFC cell output. It is a graph which shows an example of the relationship between the average value of the diameter of the pore which exists in the interface vicinity region, and SOFC cell output. It is a graph which shows an example of the relationship between the average value of the diameter of Ni particle | grains which face the electrolyte membrane in the interface vicinity area | region, and SOFC cell output. It is a graph which shows an example of the relationship between the average value of the diameter of the YSZ particle | grains which exist in the interface vicinity area | region, and faces an electrolyte membrane, and a SOFC cell output. It is a graph which shows an example of the relationship between the average value of the diameter of the pore which “exists in the vicinity of the interface and faces the electrolyte membrane”, and the SOFC cell output. It is a graph which shows an example of the relationship between "the ratio of isolated Ni particles in the interface vicinity region" and the SOFC cell output. It is a graph which shows an example of the relationship between the "isolated Ni particle ratio in the interface vicinity area" and the "output density deterioration rate" of SOFC.

(Constitution)
FIG. 1 schematically shows the configuration of a SOFC cell according to an embodiment of the present invention. This SOFC cell includes a fuel electrode 110, an electrolyte membrane 120 laminated on the fuel electrode 110, a reaction preventing membrane 130 laminated on the electrolyte membrane 120, and an air laminated on the reaction preventing membrane 130. A laminated body including the electrode 140. The shape of this cell viewed from above is, for example, a square having a side of 1 to 30 cm, a long side of 5 to 30 cm and a short side of 3 to 15 cm, or a circle having a diameter of 1 to 30 cm. The thickness of the cell is 0.1 to 3 mm.

  The fuel electrode 110 (anode electrode) is, for example, a porous thin plate-like fired body including nickel oxide NiO and yttria-stabilized zirconia YSZ. The thickness of the fuel electrode 110 is 0.1 to 3 mm. The thickness of the fuel electrode 110 is the largest among the thicknesses of the constituent members of the cell, and the fuel electrode 110 functions as a cell support substrate. Note that the member having the largest thickness among the constituent members of the cell may be a member other than the fuel electrode 110. The fuel electrode 110 obtains conductivity by receiving a known reduction process (for example, a process of reducing NiO to Ni). In this manner, the SOFC operates in a state where the fuel electrode 110 has obtained conductivity.

The fuel electrode 110 is composed of two layers: a fuel electrode active part (active layer) in contact with an electrolyte membrane 120 (to be described later) and a fuel electrode current collector part (current collection layer) that is the remaining part other than the fuel electrode active part. To obtain (see FIG. 1). The anode active portion may be composed of, for example, nickel oxide NiO and yttria stabilized zirconia YSZ (8YSZ). Alternatively, it may be composed of nickel oxide NiO and gadolinium-doped ceria GDC. The fuel electrode current collector may be composed of, for example, nickel oxide NiO and yttria stabilized zirconia YSZ (8YSZ). Alternatively, it may be composed of nickel oxide NiO and yttria Y ₂ O ₃ , or may be composed of nickel oxide NiO and calcia stabilized zirconia CSZ. The thickness of the anode active portion (active layer) is 5 to 30 μm, and the thickness of the anode current collecting portion (current collecting layer) is 50 to 500 μm.

  As described above, the fuel electrode current collector includes a substance having electronic conductivity. The fuel electrode active part includes a substance having electron conductivity and a substance having oxygen ion conductivity. “Volume ratio of the substance having oxygen ion conductivity with respect to the entire volume excluding the pore portion” in the anode active portion is “the volume ratio of the substance having oxygen ion conductivity with respect to the entire volume excluding the pore portion” in the anode current collecting portion. It is larger than the “volume ratio”.

  The electrolyte membrane 120 is a dense thin plate-like fired body including a material containing zirconium, for example, YSZ. The thickness of the electrolyte membrane 120 is 3 to 30 μm.

  The reaction preventing film 130 is a thin plate-like fired body including ceria. Specific examples of ceria include GDC (gadolinium doped ceria) and SDC (samarium doped ceria). The reaction preventing film 130 is a high resistance layer between the electrolyte film 120 and the air electrode 140 due to the reaction between the zirconia Zr in the electrolyte film 120 and the strontium Sr in the air electrode 140 at the time of cell fabrication or SOFC operation. Is interposed between the electrolyte membrane 120 and the air electrode 140. The thickness of the reaction preventing film 130 is 3 to 20 μm.

The air electrode 140 (cathode electrode) is, for example, a porous thin plate-like fired body including a perovskite oxide. Perovskite oxides include lanthanum strontium cobalt ferrite LSCF ((La, Sr) (Co, Fe) O ₃ ), LSF = (La, Sr) FeO ₃ (lanthanum strontium ferrite), LNF = La (Ni, Fe) O ₃ (lanthanum nickel ferrite), LSC = (La, Sr) CoO ₃ (lanthanum strontium cobaltite) or the like may be employed. The thickness of the air electrode 140 is 10 to 100 μm.

  The outline of the configuration of the SOFC cell shown in FIG. 1 has been described above. A plurality of SOFC cells can be used by being electrically connected in series via a current collecting member (interconnector). Thereby, high output can be obtained. As a mode in which a plurality of SOFC cells are electrically connected in series, a form in which a “power generation unit that is a stack of a fuel electrode, an electrolyte membrane, and an air electrode” is stacked in the stacking direction (so-called “vertical stripe type”) Alternatively, a form in which the power generation units are arranged at different positions on the surface of the plate-like support (so-called “horizontal stripe type”) may be employed.

(Production method)
Next, an example of a method for manufacturing the SOFC cell shown in FIG. 1 will be described. Hereinafter, the “molded body” refers to a state before firing. First, a molded body of the fuel electrode 110 was formed as follows. That is, NiO powder and YSZ powder were mixed, and polyvinyl alcohol (PVA) was added as a binder to this mixture to prepare a slurry. This slurry was dried and granulated with a spray dryer to obtain a fuel electrode powder. This powder was molded by a die press molding method to form a molded body of the fuel electrode 110.

  When the fuel electrode 110 is composed of the above-described “two layers of the fuel electrode active part and the fuel electrode current collector part”, first, a molded body of the fuel electrode current collector part is formed. The molded article of the fuel electrode current collector is formed by molding a powder for the fuel electrode current collector obtained by the same technique as described above by a die press molding method or the like. Next, the molded body of the anode active portion is laminated and formed on the upper surface of the molded body of the anode current collector as follows. That is, water and a binder were added to NiO powder and YSZ powder, and this mixture was mixed with a ball mill for 24 hours to prepare a slurry. This slurry is applied to the upper surface of the molded body of the fuel electrode current collector and dried to form a molded body (film) of the fuel electrode active section. In forming the fuel electrode active portion molded body on the upper surface of the fuel electrode current collector molded body, a tape lamination method, a printing method, or the like may be used.

  As will be described in detail later, in particular, the adjustment of the Ni particle diameter, the YSZ particle diameter, and the pore diameter in the fuel electrode active portion, and the adjustment of the “isolated Ni particle ratio” (described later) are carried out by adjusting the powder characteristics (particle It can be achieved by adjusting the diameter, specific surface area), the degree of mixing of the pore former, slurry characteristics (solid-liquid ratio, composition of organic material such as binder), conditions for the above-described reduction treatment, and the like.

  Next, the molded body of the electrolyte membrane 120 was laminated and formed on the upper surface of the molded body of the fuel electrode 110 as follows. That is, water and a binder were added to the YSZ powder, and this mixture was mixed with a ball mill for 24 hours to prepare a slurry. This slurry was applied and dried on the upper surface of the molded body of the fuel electrode 110 to form a molded body (film) of the electrolyte membrane 120. When forming the molded body of the electrolyte membrane 120 on the upper surface of the molded body of the fuel electrode 110, a tape lamination method, a printing method, or the like may be used.

  Next, a molded body of the reaction preventing film 130 was formed on the upper surface of the molded body of the electrolyte membrane 120 as follows. That is, water and a binder were added to the GDC powder, and this mixture was mixed with a ball mill for 24 hours to prepare a slurry. This slurry was applied and dried on the upper surface of the molded body of the electrolyte membrane 120, and a molded body (film) of the reaction preventing film 130 was formed. When forming the molded body of the reaction preventing film 130 on the upper surface of the molded body of the electrolyte membrane 120, a tape lamination method, a printing method, or the like may be used.

  From the above, a laminate (before firing) of the molded body of the fuel electrode 110, the molded body of the electrolyte membrane 120, and the molded body of the reaction preventing film 130 was formed. This laminate (before firing) is co-sintered at 1300 to 1600 ° C. for 2 to 20 hours to obtain a laminate (sintered) of the porous fuel electrode 110, the dense electrolyte film 120, and the dense reaction preventing film 130. After) was obtained.

  Next, an air electrode 140 was formed on the upper surface of the reaction preventing film 130 of the laminate as follows. That is, water and a binder were added to the LSCF powder, and this mixture was mixed with a ball mill for 24 hours to prepare a slurry. This slurry is applied and dried on the upper surface of the reaction preventing film 130, and baked at 1000 ° C. for 1 hour in the air in an electric furnace (in an oxygen-containing atmosphere), so that a porous air electrode is formed on the upper surface of the reaction preventing film 130. 140 was formed.

Thereafter, the above-described reduction process is performed on the fuel electrode 110. As a result, the fuel electrode 110 obtains conductivity and is in a state where the SOFC can operate. The specific conditions for the reduction treatment are as follows.
Temperature increase rate: 50 ° C / hr to 400 ° C / hr
Reducing gas: hydrogen concentration 4% or more Reducing gas introduction temperature: 400 ° C. or more, preferably 600 ° C. or more Reduction temperature: 700 to 900 ° C.
Reduction time: 1 to 50 hours The example of the method for producing the SOFC cell shown in FIG. 1 has been described above.

(Appropriate combination of Ni particle size, YSZ particle size, and pore size in the vicinity of the interface)
As stated in the “Summary of Invention” section, “Solid State lonics”
178 (2008) 1984 ”described above, the Ni particles existing in the fuel electrode in the cross section in the thickness direction (stacking direction) of the sample of the SOFC cell YSZ particles and pores can be distinguished individually.

Further, as described in the “Summary of the Invention” section, as the Ni particles existing in the fuel electrode, the “isolated Ni particles” (non-percolating Ni) and the “continuous Ni particles” (percolating Ni).
Ni) exists. By adopting the above-mentioned “new method of SEM observation”, it is possible to individually distinguish “isolated Ni particles” and “continuous Ni particles” present in the fuel electrode.

  2 shows a cross section (particularly, a cross section of the active portion of the fuel electrode 110 and the electrolyte membrane 120) obtained by cutting the sample of the SOFC cell shown in FIG. 1 along the thickness direction (stacking direction). An example of the SEM image obtained by observing using the "new technique of SEM observation" is shown. FIG. 2 shows an electrolyte membrane and a fuel electrode active layer enlarged by a magnification of 10,000 times by an FE-SEM (Field Emission Scanning Electron Microscope) using an in-lens secondary electron detector. It is a SEM image of a section including. This cross section is subjected to precision mechanical polishing and then subjected to ion milling processing by IM4000 of Hitachi High-Technologies Corporation. FIG. 2 is an SEM image obtained using an FE-SEM (model: ULTRA55) manufactured by Zeiss (Germany) set at an acceleration voltage of 1 kV and a working distance of 2 mm.

  In FIG. 2, light and dark are shown in gray scale. As can be understood from FIG. 2, according to the above “new technique for SEM observation”, “continuous Ni particles”, “isolated Ni particles”, and “YSZ particles” existing in the fuel electrode using the difference in brightness and darkness. Can be distinguished from "stomach" individually.

  Specifically, by using image analysis or the like, the SEM image can be classified into four types of regions according to the brightness. The brightest region corresponds to “continuous Ni particles”, the second brightest region corresponds to “isolated Ni particles”, the third brightest region corresponds to “YSZ particles”, and the darkest region corresponds to “pores” Correspond. This image analysis can be performed, for example, using image analysis software HALCON manufactured by MVTec (Germany).

  Note that the method of discriminating Ni particles, YSZ particles, and pores based on image analysis is not limited to the method using the light / dark difference in the SEM image. For example, after obtaining element mapping by SEM-EDS in the same field of view, this acquisition result and an FE-SEM image (including an in-lens image and an out-lens image) using a previously obtained in-lens secondary electron detector Each particle in the SEM image can be identified. As a result, Ni particles, YSZ particles, and pores are ternarized, so that Ni particles, YSZ particles, and pores can be discriminated. At this time, identification of each particle can be easily performed as follows. That is, first, the low luminance region on the out-lens image can be determined as a pore. Thereafter, the high luminance region in the region other than the pores on the in-lens image can be determined as Ni particles, and the low luminance region can be determined as YSZ particles.

  In addition to the image analysis, composition analysis using energy dispersive X-ray analysis (so-called EDS analysis) can be performed on the cross section corresponding to the SEM image. Also by this, Ni particles and YSZ particles can be discriminated, and therefore Ni particles can be specified. Of the identified Ni particles, bright particles can be identified as continuous Ni particles, and dark particles can be identified as isolated Ni particles.

  Continuous Ni particles and isolated Ni particles can also be discriminated by a conduction test using an atomic force microscope (AFM). Specifically, a conductive member is provided so that the sample of the SOFC cell in which the cross section corresponding to the SEM image is exposed is electrically connected to a portion different from the cross section of the fuel electrode. In a state where a predetermined voltage is applied between the conductive member and the AFM cantilever (needle), the cantilever is scanned over the cross section. Based on the relationship between the magnitude of the current flowing at that time and the position on the cross section, the area identified as Ni particles in the cross section is classified into two areas, a “conducting area” and a “non-conducting area”. obtain. The “region with continuity” can be determined as continuous Ni particles, and the “region without continuity” can be determined as isolated Ni particles.

  Regarding the discrimination between continuous Ni particles and isolated Ni particles, the various methods described above use a cross section obtained by actually cutting a SOFC sample by machining along the thickness direction (stacking direction). The Accordingly, it is considered that among the Ni particles distributed in the vicinity of the cross-section, there may exist those that were continuous Ni particles before cutting but changed to isolated Ni particles after cutting (or vice versa). .

  However, the proportion of Ni particles distributed in the vicinity of the cross-section changed from continuous Ni particles to isolated Ni particles (or vice versa) before and after cutting is considered to be very small. Therefore, when discussing the “isolated Ni particle ratio” described later, the “isolated Ni particle ratio” calculated based on the cross section obtained by actually cutting the SOFC sample by machining cuts the SOFC sample. This is considered to be almost coincident with the previous true “isolated Ni particle ratio”.

  The inventor has observed cross sections of various fuel electrodes 110 after the reduction treatment using the “new technique for SEM observation”. At that time, “the interface between the fuel electrode 110 and the electrolyte membrane 120 in the fuel electrode 110”, which is considered to greatly contribute to “the reaction of the fuel electrode” (the reaction represented by the formula (2) described in the “Background Art” column). In particular, attention was paid to the “region located within 3 μm” (see FIG. 2, hereinafter referred to as “region near the interface”).

  The following is a supplementary explanation. In this example, the fuel electrode 110 and the electrolyte membrane 120 are formed (sintered) by simultaneous firing (co-firing). For this reason, during the co-firing, the region close to the interface with the electrolyte membrane 120 in the fuel electrode 110 (the membrane thereof) (= the “region near the interface”) is easily affected from the electrolyte membrane 120 side. Specifically, the fuel electrode 110 and the electrolyte membrane 120 are affected by differences in firing start temperature, differences in firing shrinkage, mutual diffusion of constituent elements, and the like. Due to this, in the fuel electrode 110, the microstructure (therefore, the particle size (the particle size (air)) in the “region near the interface” and the other region (that is, the region far from the interface with the electrolyte membrane 120). It is very likely that the distribution of the pore diameter) is different. From the above, the present inventor has focused only on the “microstructure in the vicinity of the interface” that greatly contributes to the reaction of the fuel electrode 110. On the other hand, the present inventor does not pay attention to “a microstructure in a region far from the interface and does not contribute much to the reaction of the fuel electrode”, which is likely to be different from “a microstructure in the region near the interface”.

  When the fuel electrode is composed of two layers of the “fuel electrode current collector” and the “fuel electrode active part” described above, the “region near the interface” corresponds to a part of the fuel electrode active part. Hereinafter, when the fuel electrode is composed of the two layers of the “fuel electrode current collector” and the “fuel electrode active part” described above (that is, the “region near the interface” corresponds to a part of the fuel electrode active part) )

  The definition of “interface between the fuel electrode active portion and the electrolyte membrane 120” will be added. The “interface between the fuel electrode active part and the electrolyte membrane 120” is defined as follows, for example. That is, first, an SEM image (including an interface between the fuel electrode active portion and the electrolyte membrane) as shown in FIG. 2 obtained using the above-described “new technique for SEM observation” is acquired. In this SEM image (field of view), “continuous Ni particles”, “isolated Ni particles”, “YSZ particles”, and “pores” constituting the fuel electrode active portion (hereinafter collectively referred to as “constituents”) ), A plurality of components existing facing the electrolyte membrane are identified. For each of the plurality of identified structures, a point closest to the electrolyte membrane (upper side in FIG. 2) is identified in a region corresponding to the structure. Using the identified points and one of the well-known statistical methods (for example, the least square method), a straight line passing through the neighborhood of each of the identified points is determined. This line (in the example shown in FIG. 2, the line segment L) becomes “the interface between the fuel electrode active portion and the electrolyte membrane 120”.

  As a result of the various observations described above, the present inventor has found that the combination of the three diameters of Ni particles, YSZ particles, and pores in the “region near the interface” after the reduction treatment greatly affects the reaction resistance of the fuel electrode active portion. I discovered that. In addition, the present inventor found that the average value of the three diameters of Ni particles, YSZ particles, and pores in the “region near the interface” after the reduction treatment necessary to reduce the reaction resistance of the fuel electrode active portion. We found an appropriate range of combinations. Hereinafter, a test for confirming this will be described.

  In addition, the diameter of a certain Ni particle is an SEM image (including an interface between the fuel electrode active part and the electrolyte membrane) as shown in FIG. 2 obtained by using the above-mentioned “new technique for SEM observation”. It can be defined as the diameter of a circle having an area equal to the area of the region corresponding to the Ni particles (isolated Ni particles or continuous Ni particles). The same applies to the diameter of the YSZ particles and the diameter of the pores. As described above, the method for estimating the three-dimensional structure (particle diameter, pore diameter) from the two-dimensional structure (cross-sectional image) is described by Satoshi Mizutani, Yoshiharu Ozaki, Toshio Kimura, Satoshi Yamaguchi, “Ceramic Processing”, Gihodo Publishing Co., Ltd., published on March 25, 1985, pages 190-201.

  Further, the average value of the particle diameter (pore diameter) needs to be an average value calculated from data obtained by observing and analyzing each cross section (SEM image) of a plurality of different parts (fields of view). . This is based on the fact that in order to reduce the reaction resistance of the fuel electrode active portion, the particle size (pore size) needs to be distributed within an appropriate range over a wide region of “the vicinity of the interface”. Specifically, for example, cross-sectional observation is performed at a magnification of 5000 to 20000, and an average value calculated based on the analysis result of each cross section of 20 or more different parts (fields of view) is adopted. Is preferred.

<Test>
In this test, various samples having different combinations of the three diameters of Ni particles, YSZ particles, and pores in the fuel electrode active part after the reduction treatment were produced for the SOFC according to the above embodiment. Adjustment of the Ni particle diameter was achieved, for example, by adjusting the powder characteristics (particle diameter, specific surface area) of the NiO powder mixed in the slurry for the fuel electrode active part. The adjustment of the YSZ particle diameter was also achieved by adjusting the powder characteristics (particle diameter, specific surface area) of the YSZ powder mixed in the slurry for the fuel electrode active part, as in the case of the NiO particle diameter. The adjustment of the pore diameter was achieved, for example, by adjusting the particle diameter and mixing amount of the pore former mixed in the slurry for the fuel electrode active part. The adjustment of the NiO particle diameter, the adjustment of the YSZ particle diameter, and the adjustment of the pore diameter can also be achieved by adjusting the firing time and the firing temperature.

  In each sample described above, the thickness of the fuel electrode current collector (Ni-YSZ) is 1000 μm, the thickness of the fuel electrode active portion is 20 μm, the thickness of the electrolyte membrane 120 (YSZ) is 5 μm, and the reaction preventing membrane 130 (GDC). ) Was 20 μm, and the thickness of the air electrode 140 (LSCF) was 50 μm. In addition, the shape of these samples viewed from above was a circle with a diameter of 30 mm.

  While supplying nitrogen gas to the fuel electrode 110 side and air to the air electrode 140 side for each sample, the temperature was raised to 800 ° C., and when the temperature reached 800 ° C., hydrogen gas was supplied to the fuel electrode 110 and reduced. Processing was carried out for 3 hours. After this reduction treatment, the SOFC power density was measured for each sample. As the output density, a value at a temperature of 800 ° C. and a rated voltage of 0.8 V was used.

  In general, the electrical resistance of SOFC includes a resistance defined by “(voltage) / (current)” (so-called IR resistance) and a resistance based on the rate of chemical reaction (reaction resistance). As a result of measuring the IR resistance of the various samples described above, it has been separately confirmed that the difference in IR resistance is small. Therefore, it is considered that the power density depends strongly on the reaction resistance. That is, it can be said that a large output density means that the reaction resistance of the fuel electrode active part is small.

  FIG. 3 shows the relationship between “average value of Ni particle diameter” and “SOFC output density” obtained by changing the diameter of Ni particles existing in the “region near the interface” after the reduction treatment. For each sample, the “average value of Ni particle diameter” is the average value of the diameters of Ni particles (isolated Ni particles + continuous Ni particles) present in the “region near the interface” after the reduction treatment. In the relationship shown in FIG. 3, the diameter of the “YSZ particles” present in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm and is present in the “region near the interface” after the reduction treatment. The diameter of the “pores” is maintained at 0.3 to 0.5 μm.

  FIG. 4 shows the relationship between “average value of YSZ particle diameter” and “SOFC output density” obtained by changing the diameter of YSZ particles present in the “region near the interface” after the reduction treatment. For each sample, the “average value of YSZ particle diameter” is the average value of the diameters of YSZ particles present in the “region near the interface” after the reduction treatment. In the relationship shown in FIG. 4, the diameter of the Ni particles (isolated Ni particles + continuous Ni particles) existing in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm. The diameter of the “pores existing in the“ region close to the interface ”” is maintained at 0.3 to 0.5 μm.

  FIG. 5 shows the relationship between the “average pore diameter” and the “SOFC output density” obtained by changing the diameter of the pores existing in the “region near the interface” after the reduction treatment. For each sample, the “average value of pore diameter” is the average value of the diameters of pores existing in the “region near the interface” after the reduction treatment. In the relationship shown in FIG. 5, the diameter of the Ni particles (isolated Ni particles + continuous Ni particles) existing in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm. The diameter of the “YSZ particles” present in the “region near the interface” is maintained at 0.4 to 0.6 μm.

In this test, a region where the power density is 0.7 W / cm ² or more is determined as a high power state. As can be understood from FIGS. 3, 4, and 5, the average value of the Ni particle diameter is 0.28 to 0.80 μm in the “region near the interface” after the reduction treatment, and the YSZ particle diameter is When the average value is 0.28 to 0.80 μm and the average pore diameter is 0.10 to 0.87 μm, a high output state is obtained. That is, the reaction resistance of the fuel electrode active part is small when the combination of the average values of the three diameters of Ni particles, YSZ particles, and pores in the “region near the interface” after the reduction treatment is within the above range. Can do. The appropriate combination of the Ni particle diameter, the YSZ particle diameter, and the pore diameter in the “region near the interface” after the reduction process necessary for reducing the reaction resistance of the fuel electrode active portion has been described above.

  In the “region near the interface” after the reduction treatment, the standard deviation of the Ni particle diameter when the combination of the average values of the three diameters is within the above range is 0.10 to 0.60, and YSZ The standard deviation of the particle diameter was 0.15 to 0.60, and the standard deviation of the pore diameter was 0.05 to 0.70.

(Appropriate combination of Ni particle size, YSZ particle size, and pore size facing the electrolyte membrane)
The present inventor believes that “Ni particles existing in the vicinity of the electrolyte membrane” are considered to make the largest contribution to “reaction of the active portion of the fuel electrode” among “Ni particles, YSZ particles, and pores existing in the vicinity of the interface”. Further combinations of the respective diameters for “, YSZ particles, and pores” were also investigated.

  FIG. 6 shows the relationship between the “average value of the Ni particle diameter” obtained by changing the diameter of the “Ni particles existing facing the electrolyte membrane 120” after the reduction treatment and the “SOFC output density”. Show. For each sample, the “average value of Ni particle diameter” is the “average value of the diameters of Ni particles (isolated Ni particles + continuous Ni particles) existing facing the electrolyte membrane 120. The relationship shown in FIG. , The diameter of the “YSZ particles present facing the electrolyte membrane 120” after the reduction treatment is maintained at 0.4 to 0.6 μm, and the “pores present facing the electrolyte membrane 120” after the reduction treatment The diameter is maintained at 0.3 to 0.5 μm.

  FIG. 7 shows the relationship between the “average value of the YSZ particle diameter” obtained by changing the diameter of the “YSZ particles existing facing the electrolyte membrane 120” after the reduction treatment and the “SOFC output density”. Show. For each sample, “the average value of the YSZ particle diameter” is “the average value of the diameters of the YSZ particles present facing the electrolyte membrane 120. In the relationship shown in FIG. The diameter of “NI particles (isolated Ni particles + continuous Ni particles) present facing 120” is maintained at 0.4 to 0.6 μm, and “pores present facing electrolyte membrane 120” after reduction treatment are maintained. The diameter is maintained at 0.3 to 0.5 μm.

  FIG. 8 shows the relationship between the “average pore diameter” obtained by changing the diameter of the “pores facing the electrolyte membrane 120” after the reduction treatment and the “SOFC output density”. For each sample, “the average value of pore diameter” is “the average value of the diameter of pores existing facing the electrolyte membrane 120. In the relationship shown in FIG. The diameter of the Ni particles (isolated Ni particles + continuous Ni particles) present facing each other is maintained at 0.4 to 0.6 μm, and the diameter of the “YSZ existing facing the electrolyte membrane 120” after the reduction treatment is It is maintained at 0.4 to 0.6 μm.

  As can be understood from FIGS. 6, 7, and 8, the average value of the Ni particle diameter is 0.31 to 0.70 μm in the “construction body” after the reduction treatment that faces the electrolyte membrane. A high output state is obtained when the average value of the YSZ particle diameter is 0.32 to 0.78 μm and the average value of the pore diameter is 0.15 to 0.80 μm. That is, the reaction resistance of the fuel electrode active part is smaller when the combination of the average values of the three diameters of Ni particles after reduction treatment, YSZ particles, and pores present facing the electrolyte membrane is within the above range. It can be said.

  In addition, in the “construction body” after the reduction treatment that faces the electrolyte membrane, the standard deviation of the Ni particle diameter is 0.14 to 0 when the combination of the average values of the three diameters is within the above range. The standard deviation of the YSZ particle diameter was 0.13 to 0.60, and the standard deviation of the pore diameter was 0.10 to 0.65.

  In the present embodiment, in the calculation of the above-described particle size (Ni particle size, YSZ particle size), the average value of the pore size, and the standard deviation, particles of 0.1 μm or less recognized by the image analysis software described above. No diameter data is used. According to the observation result at a higher magnification, the existence of particles and pores of 0.1 μm or less recognized by the above-described image analysis software are uncertain, and they are taken into consideration as a factor governing the output performance. Is based on what is considered inappropriate.

(Appropriate range of isolated Ni particle ratio)
“Ratio of the sum of the volumes of only isolated Ni particles existing in the interface region” to “the total volume of all Ni particles (isolated Ni particles + continuous Ni particles) existing in the interface region” This is called “isolated Ni particle ratio”.

  The present inventor considered that the ratio of isolated Ni particles may contribute to the reduction of the reaction resistance of the fuel electrode active part, and further investigated the “isolated Ni particle ratio”. The “isolated Ni particle ratio” is determined as follows, for example. That is, first, an SEM image (including an interface between the fuel electrode active portion and the electrolyte membrane) as shown in FIG. 2 obtained using the above-described “new technique for SEM observation” is acquired. In this SEM image (field of view), the “interface between the fuel electrode active portion and the electrolyte membrane 120” is determined by the above-described method. Accordingly, the “region near the interface” is determined in this SEM image. Next, “isolated Ni particles” and “continuous Ni particles” existing in the “region near the interface” in the SEM image are specified. Next, the total area (total area 1) of the region corresponding to “isolated Ni particles” in this SEM image and the total area (total area 2) of the region corresponding to “continuous Ni particles” in this SEM image are acquired. The Then, “the ratio of isolated Ni particles” is determined by dividing “total area 1” by “sum of total area 1 and total area 2”.

  The adjustment of the “isolated Ni particle ratio” (that is, adjustment of the bonding state between adjacent Ni particles) is, for example, the powder characteristics (particle diameter, specific surface area) of NiO powder mixed in the slurry for the fuel electrode active part. ) Was achieved by adjusting. The adjustment of the “isolated Ni particle ratio” can also be achieved by adjusting the firing time and the firing temperature, or by adjusting the particle size and the amount of the pore former. Furthermore, the “isolated Ni particle ratio” can also be adjusted by a combination of the various adjustment elements described above.

  FIG. 9 shows the relationship between the “isolated Ni particle ratio” and the “SOFC output density” obtained by changing the “isolated Ni particle ratio” after the reduction treatment. In the relationship shown in FIG. 9, the diameter of the Ni particles (isolated Ni particles + continuous Ni particles) existing in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm. The diameter of YSZ particles existing in the “region near the interface” of the particle is maintained at 0.4 to 0.6 μm, and the diameter of pores existing in the “region near the interface” after the reduction treatment is 0.3 to 0.5 μm. Is maintained. It was impossible to make the ratio of isolated Ni particles less than 2%.

In this test, a region where the power density is 0.8 W / cm ² or more is determined as a high power state. As can be understood from FIG. 9, a larger output density can be obtained when the “isolated Ni particle ratio” is as small as 2 to 40%. That is, it can be said that when the “ratio of isolated Ni particles” is within the above range, a high output state is obtained, that is, the reaction resistance of the fuel electrode active part is smaller. This result shows that the smaller the ratio of isolated Ni particles (that is, the denser the continuous Ni particles are), the denser the paths (paths) that conduct electrons generated by the reaction of the fuel electrode active part. It is thought that it is based on that.

Further, assuming that the region where the power density is 1.2 W / cm ² or more is determined as a high power state, as can be understood from FIG. 9, when the “isolated Ni particle ratio” is as small as 2 to 25%. A larger power density can be obtained.

Further, the inventor considered that the ratio of isolated Ni particles may also correlate with the “degradation rate of output density” at the rated current density (0.3 A / cm ² ) of SOFC, and the ratio of isolated Ni particles and SOFC “ We also investigated the relationship with the power density deterioration rate. “Degradation rate of output density” means that the output density before the execution of the “endurance test that operates SOFC continuously for 1000 hours” is A, and the output density after the execution of the endurance test is B. Is defined as (AB) / A (%). A small "output density deterioration rate" means that the "rate of increase in reaction resistance of the fuel electrode active part" resulting from the execution of the durability test is small, that is, the fuel electrode active part even if the SOFC is operated for a long time. This means that the reaction resistance is difficult to increase.

  FIG. 10 shows the relationship between the “isolated Ni particle ratio” obtained by changing the “isolated Ni particle ratio” after the reduction treatment and the “output density deterioration rate” of the SOFC. Also in the relationship shown in FIG. 10, as in the case of FIG. 9, the diameter of the Ni particles (isolated Ni particles + continuous Ni particles) existing in the “region near the interface” after the reduction treatment is 0.4 to 0.00. The diameter of the YSZ particles existing in the “region near the interface” after the reduction treatment is maintained at 6 μm, and the diameter of the pores existing in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm. Is maintained at 0.3 to 0.5 μm. In this test as well, it was impossible to make the ratio of isolated Ni particles less than 2%.

  As can be understood from FIG. 10, when the “isolated Ni particle ratio” is as small as 2 to 40%, the “output density deterioration rate” is extremely higher than when the “isolated Ni particle ratio” is larger than 40%. (Boundary: Degradation rate = 2%). That is, when the “isolated Ni particle ratio” is within the above range, it can be said that the degree to which the reaction resistance of the fuel electrode active portion increases after a long operation is extremely small. Furthermore, when the “isolated Ni particle ratio” is further small, such as 2 to 25%, the “output density deterioration rate” is extremely smaller than when the “isolated Ni particle ratio” is larger than 25% ( Boundary: degradation rate = 0.5%). As a result, the smaller the ratio of isolated Ni particles (that is, the more dense the continuous Ni particles are), the smaller the ratio of the “electron conduction path (path)” that is destroyed due to long-time operation. It is thought to be based on

  As described above, according to the test results obtained by using the above-mentioned “new technique for SEM observation”, the average value of the particle diameters of Ni existing in the “region near the interface” after the reduction treatment is 0.28 to 0.8. The average value of the YSZ particle diameter existing in the “region near the interface” after the reduction treatment is 0.28 to 0.80 μm, and the average value of the pore diameter existing in the “region near the interface” after the reduction treatment It has been found that when the SOFC is configured to have a 0.10 to 0.87 μm, the reaction resistance of the fuel electrode active portion of the SOFC becomes small.

  In addition, the average value of the Ni particle diameter after reduction treatment existing facing the electrolyte membrane is 0.31 to 0.70 μm, and the average value of the YSZ particle diameter after reduction treatment existing facing the electrolyte membrane is When the SOFC is configured such that the average pore diameter after reduction treatment existing facing the electrolyte membrane is 0.15 to 0.80 μm, the active electrode active portion of the SOFC It was found that the reaction resistance of was smaller.

  Furthermore, if the SOFC is configured so that the “isolated Ni particle ratio” after the reduction treatment is 2 to 40%, the reaction resistance of the SOFC active portion of the fuel electrode becomes smaller, and further, after the operation for a long time. It has also been found that the degree of increase in the reaction resistance of the SOFC anode is extremely small.

Hereinafter, a method for realizing the microstructure of the fuel electrode active part according to the present embodiment will be additionally described.
1. The average particle diameter of the raw material powder (NiO and 8YSZ) was set to 0.3 μm or more and 1.3 μm or less. Fine particles having a particle size of 0.1 μm or less and coarse particles having a particle size of 20 μm or more were removed by classification treatment. For example, an airflow classifier can be applied to the classification process.
2. The average particle size of the pore former was 0.8 μm or more and 10 μm or less. Fine particles having a particle size of 0.2 μm or less and coarse particles having a particle size of 30 μm or more were removed by classification treatment. As the pore former, for example, cellulose or PMMA (polymethyl methacrylate resin) can be applied.
3. The amount of pore former added was 30% by volume or less with respect to the ceramic raw material (NiO + YSZ).
4). The co-firing temperature of the fuel electrode active part and the electrolyte membrane (including the reaction preventing film) was 1350 ° C. or higher and 1500 ° C. or lower, and the co-firing time was 1 hour or longer and 20 hours or shorter.
5). The material for forming the fuel electrode active layer (print paste when formed by a printing method) was uniformly mixed.
6). When printing paste is produced as a material for forming the anode active layer, a slurry with good dispersibility to which an appropriate dispersant (for example, a wet dispersant “DISPERBYKR-180” manufactured by Big Chemie Japan Co., Ltd.) is added After that, the slurry was uniformly mixed by sufficient pot mill mixing and triroll mill mixing.

Furthermore, in order to realize the microstructure of the fuel electrode (fuel electrode active part) according to the present embodiment, a substance having electron conductivity (Ni or the like) and a substance having oxygen ion conductivity (YSZ or the like) are the main components. It is necessary to stabilize the porous structure during firing and during reduction by controlling the mixing of the mixture into the fuel electrode (fuel electrode active part). Specifically, it has been found that it is important to manage the concentration of the mixture in the fuel electrode (fuel electrode active part) as described below. These mixtures may be mixed in the raw material in advance, or may be mixed in the manufacturing process.
Si: 200 ppm or less, desirably 100 ppm or less P: 50 ppm or less, desirably 30 ppm or less Cr: 100 ppm or less, desirably 50 ppm or less B: 100 ppm or less, desirably 50 ppm or less S: 100 ppm or less, desirably 30 ppm or less

  By adding a small amount of these elements, the sinterability can be improved and the skeleton of the porous fuel electrode can be strengthened. On the other hand, excessive addition of these elements causes a decrease in the power density. It is not preferable. This is considered to be based on the fact that the reaction field decreases (reaction resistance increases) due to the progress of sintering of Ni in the fuel electrode active portion during long-term operation. Hereinafter, the test which confirmed these things is demonstrated.

  In this test, the relationship between the concentration of the mixture in the above-described fuel electrode (fuel electrode active part) and the above-mentioned “degradation rate of power density” was investigated. Table 1 shows the results of this investigation. Also in this test, the diameter of the Ni particles (isolated Ni particles + continuous Ni particles) existing in the “region near the interface” after the reduction treatment is maintained at 0.4 to 0.6 μm. The diameter of YSZ particles existing in the “near region” is maintained at 0.4 to 0.6 μm, and the diameter of pores existing in the “near interface region” after the reduction treatment is maintained at 0.3 to 0.5 μm. ing.

  As understood from Table 1, the concentration of Si, P, Cr, B, and S in the fuel electrode (fuel electrode active part) is 200 ppm or less, 50 ppm or less, 100 ppm or less, 100 ppm or less, and 100 ppm, respectively. In the following cases (levels 2, 3, 4, and 5), the “output density deterioration rate” is extremely small as compared to the case (level 1) that is not. Furthermore, when the mixed concentrations of Si, P, Cr, B, and S are 100 ppm or less, 30 ppm or less, 50 ppm or less, 50 ppm or less, and 30 ppm or less, respectively (levels 4 and 5), Compared with levels 2 and 3), the “degradation rate of output density” is extremely reduced.

  110 ... Fuel electrode, 120 ... Electrolyte membrane, 130 ... Reaction prevention membrane, 140 ... Air electrode

Claims (6)

A porous fuel electrode for reacting a fuel gas, the fuel electrode comprising Ni and an oxygen ion conductive material having oxygen ion conductivity; and
A porous air electrode for reacting a gas containing oxygen;
A dense solid electrolyte membrane provided between the fuel electrode and the air electrode, the solid electrolyte membrane having an interface with the fuel electrode;
In a solid oxide fuel cell comprising:
Regarding the region located within 3 μm from the interface in the fuel electrode after the reduction treatment is performed, the average diameter of the Ni particles existing in the region is 0.28 to 0.80 μm, and the region The average particle diameter of the oxygen ion conductive material existing in the region is 0.28 to 0.80 μm, and the average pore diameter of the fuel electrode existing in the region is 0.10 to 0.87 μm. Oh it is,
After the reduction treatment is performed on the fuel electrode, the other of the Ni particles existing in the region is adjacent to the total volume of all the Ni particles present in the region. A solid oxide fuel cell, wherein the ratio of the sum of the volume of Ni particles present alone without being connected to the Ni particles is 40% or less . The solid oxide fuel cell according to claim 1, wherein
After the reduction treatment is performed on the fuel electrode, the average diameter of the Ni particles existing facing the solid electrolyte membrane in the region is 0.31 to 0.70 μm, The average particle diameter of the oxygen ion conductive material existing in the region facing the solid electrolyte membrane is 0.32 to 0.78 μm, and the region existing in the region facing the solid electrolyte membrane. A solid oxide fuel cell having an average pore diameter of a fuel electrode of 0.15 to 0.80 μm. The solid oxide fuel cell according to claim 1 or 2 ,
The fuel electrode and the solid electrolyte membrane are solid oxide fuel cells formed by simultaneous firing. The solid oxide fuel cell according to any one of claims 1 to 3 ,
The solid oxide fuel cell, wherein the oxygen ion conductive material is yttria stabilized zirconia (YSZ). The solid oxide fuel cell according to any one of claims 1 to 4 , wherein
The fuel electrode includes a fuel electrode active part which is a layer in contact with the solid electrolyte membrane, and a content ratio of a substance having oxygen ion conductivity with respect to the fuel electrode active part which is a remaining part other than the fuel electrode active part. Is composed of two layers, a fuel electrode current collector that is a small layer,
The solid oxide fuel cell, wherein the region is a part of the fuel electrode active part. The solid oxide fuel cell according to any one of claims 1 to 5 , wherein
As a mixture mixed in the fuel electrode, silicon (Si), phosphorus (P), chromium (Cr), boron (B), and sulfur (S) are included,
In the fuel electrode, mixing concentrations of silicon (Si), phosphorus (P), chromium (Cr), boron (B), and sulfur (S) are 200 ppm or less, 50 ppm or less, 100 ppm or less, 100 ppm or less, And a solid oxide fuel cell having a concentration of 100 ppm or less.