Patent Publication Number: US-11380902-B2

Title: Electrochemical cell

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of PCT/JP2017/027099, filed Jul. 26, 2017, which claims priority to Japanese Application No. 2016-147862, filed Jul. 27, 2016, the entire contents all of which are incorporated hereby by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an electrochemical cell. 
     BACKGROUND ART 
     In recent years, fuel cells that are a type of electrochemical cell have attracted attention in light of environmental problems and their effective use as an energy source. 
     A fuel cell generally includes an anode, a cathode, and a solid electrolyte layer disposed between the anode and the cathode. 
     The cathode for example is configured by a perovskite oxide such as (La, Sr)(Co, Fe)O 3 : (lanthanum strontium cobalt ferrite) or the like (for example, reference is made to Japanese Patent Application Laid-Open No. 2006-32132). 
     SUMMARY OF INVENTION 
     However, fuel cell output may be reduced by repetitive power generation. The present inventors have gained the new insight that one cause of a reduction in output results from deterioration of the cathode, and that this deterioration of the cathode is related to the total proportion of Co 3 O 4  and (Co, Fe) 3 O 4  that is introduced into a region of the cathode on the solid electrolyte layer side. 
     The present invention is proposed based on the new insight above and has the object of providing an electrochemical cell that is configured to inhibit a reduction in the fuel cell output. 
     The electrochemical cell according to the present invention has an anode, a cathode, and a solid electrolyte layer. The cathode contains a perovskite oxide expressed by the general formula ABO 3  and includes at least one of Sr and La at the A site as a main component. The solid electrolyte layer is disposed between the anode and the cathode. The cathode includes a solid electrolyte layer-side region within 3 μm from a surface of the solid electrolyte layer side. The solid electrolyte layer-side region includes a main phase which is configured by the perovskite oxide and a second phase which is configured by Co 3 O 4  and (Co, Fe) 3 O 4 . An occupied surface area ratio of the second phase in a cross section of the solid electrolyte layer-side region is less than or equal to 10.5%. 
     The present invention provides an electrochemical cell that is configured to inhibit a reduction in the fuel cell output. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration of a fuel cell. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Configuration of Fuel Cell  10   
     The configuration of the fuel cell  10  will be described making reference to the drawing. The fuel cell  10  is configured as a so-called solid oxide fuel cell (SOFC). The possible configurations of the fuel cell  10  include a flat-tubular type, a segmented-in-series type, an anode-supporting type, an electrolyte flat-plate type, a cylindrical type, or the like. 
       FIG. 1  is a cross-sectional view illustrating a configuration of a fuel cell  10 . The fuel cell  10  includes an anode  20 , a solid electrolyte layer  30 , a barrier layer  40 , and a cathode  50 . 
     The anode  20  functions as an anode for the fuel cell  10 . As illustrated in  FIG. 1 , the anode  20  includes an anode current-collecting layer  21  and an anode-active layer  22 . 
     The anode current-collecting layer  21  is configured as a porous body that exhibits superior gas permeability. The material that constitutes the anode current-collecting layer  21  includes a material that is used in the anode current-collecting layer of a conventional SOFC, and for example, includes NiO (nickel oxide)-8YSZ (8 mol % of yttria-stabilized zirconia), or NiO—Y 2 O 3  (yttria). However, when NiO is included in the anode current-collecting layer  21 , at least a portion of the NiO may be reduced to Ni during the operation of the fuel cell  10 . The thickness of the anode current-collecting layer  21  may be configured for example as 0.1 mm to 5.0 mm. 
     The anode-active layer  22  is disposed on the anode current-collecting layer  21 . The anode-active layer  22  is configured as a porous body that is denser than the anode current-collecting layer  21 . The material that configures the anode-active layer  22  includes use of a material used in an anode-active layer of a conventional SOFC, and for example, includes NiO-8YSZ. However, when NiO is included in the anode-active layer  22 , at least a portion of the NiO may be reduced to Ni during operation of the fuel cell  10 . The thickness of the anode-active layer  22  may be configured for example as 5.0 μm to 30 μm. 
     The solid electrolyte layer  30  is disposed between the anode  20  and the cathode  50 . The solid electrolyte layer  30  in the present embodiment is sandwiched between the anode  20  and the barrier layer  40 . The solid electrolyte layer  30  functions to enable the permeation of oxide ions that are produced by the cathode  50 . The solid electrolyte layer  30  is configured by a material that is denser than the anode  20  or the cathode  50 . 
     The solid electrolyte layer  30  may contain ZrO 2  (zirconia) as a main component. In addition to zirconia, the solid electrolyte layer  30  may contain an additive such as Y 2 O 3  (yttria) and/or Sc 2 O 3  (scandium oxide). These additives function as a stabilizing agent. The mol composition ratio (stabilizing agent:zirconia) of the stabilizing agent to zirconia in the solid electrolyte layer  30  may be configured to be approximately 3:97˜20:80. Therefore, the material used in the solid electrolyte layer  30  includes 3YSZ, 8YSZ, 10YSZ, or ScSZ (zirconia stabilized with scandia), or the like. The thickness of the solid electrolyte layer  30  for example may be configured as 3 μm to 30 μm. 
     In the present embodiment, the term “composition X contains as a main component composition Y” means that composition Y preferably occupies at least 70 wt % of the total of composition X, and more preferably occupies at least 90 wt %. 
     The barrier layer  40  is disposed between the solid electrolyte layer  30  and the cathode  50 . The barrier layer  40  inhibits the formation of a high resistivity layer between the solid electrolyte layer  30  and the cathode  50 . The barrier layer  40  is configured by a material that is denser than the anode  20  or the cathode  50 . The barrier layer  40  may include a main component of a ceria-based material such as GDC (gadolinium-doped ceria), SDC (samarium-doped ceria), or the like. The thickness of the barrier layer  40  may be configured, for example, as 3 μm to 20 μm. 
     The cathode  50  is disposed on the barrier layer  40 . The cathode  50  functions as a cathode for the fuel cell  10 . The cathode  50  is configured as a porous body. There is no particular limitation on the porosity of the cathode  50 , and it may be configured to be 20% to 60%. There is no particular limitation on the thickness of the cathode  50  and it may be configured to be 2 μm to 100 μm 
     The cathode  50  contains a main phase configured by a perovskite oxide expressed by the general formula ABO 3  and includes at least one of La or Sr at the A site. The perovskite oxide for example may suitably employ a composite perovskite oxide that contains lanthanum, or SSC (samarium strontium cobaltite (Sm, Sr)CoO 3 ) or the like that does not contain lanthanum. However, there is no limitation in this regard. The lanthanum-containing composite perovskite oxide includes LSCF (lanthanum strontium cobalt ferrite): (La, Sr) (Co, Fe)O 3 ), LSF: (lanthanum strontium ferrite: (La, Sr) FeO 3 ), LSC: (lanthanum strontium cobaltite: (La, Sr)CoO 3 ), and LNF (lanthanum nickel ferrite: (La(Ni, Fe)O 3 ), or the like. The density of the main phase that is configured by a perovskite oxide may be configured as 5.5 g/cm 3  to 8.5 g/cm 3 . 
     The cathode  50  includes a first region  51  and a second region  52  as shown in  FIG. 1 . 
     The first region  51  is an example of “a solid electrolyte layer-side region” according to the present embodiment. The first region  51  is a region within 3 μm of a solid electrolyte layer-side surface  50 S of the cathode  50 . In the present embodiment, since the fuel cell  10  includes a barrier layer  40 , the first region  51  makes contact with the barrier layer  40  on the solid electrolyte layer-side surface  50 S. The solid electrolyte layer-side surface  50 S is the interface between the barrier layer  40  and the cathode  50 . However, when the fuel cell  10  does not include a barrier layer  40 , the first region  51  will then make contact with the solid electrolyte layer  30  on the solid electrolyte layer-side surface  50 S. 
     When component densities in a cross section of the fuel cell  10  are mapped, the solid electrolyte layer-side surface  50 S can be defined with reference to a rapid change in the element densities included in the cathode. 
     The first region  51  contains a main phase that is configured by a perovskite oxide expressed by the general formula ABO 3  and includes at least one of Sr and La at the A site. The occupied surface area ratio of the main phase in a cross section of the first region  51  may be configured as greater than or equal to 89.5% and less than or equal to 99.5%. In the present embodiment, the term “occupied surface area ratio of the substance Z in the cross section” denotes the ratio of the sum total surface area of a substance Z phase relative to the total surface area of a solid phase in a cross section. The method of calculating the occupied surface area ratio will be described in detail below. 
     The first region  51  includes a second phase that is configured by (Co, Fe) 3 O 4  and Co 3 O 4  that have a spinel crystalline structure. (Co, Fe) 3 O 4  includes Co 2 FeO 4 , Co 1.5 Fe 1.5 O 4 , and CoFe 2 O 4 , or the like. In the second phase, Co 3 O 4  and (Co, Fe) 3 O 4  are in a mixed configuration and not in solid solution. More specifically, a feature such as “Co 3 O 4  and (Co, Fe) 3 O 4  are mixed” means a configuration in which an EDX spectrum detecting Co and O and an EDX spectrum detecting Co, Fe and O are separately acquired at different sites, and Co 3 O 4  and (Co, Fe) 3 O 4  are literally mixed (=present in a combined state) (Kojien Second Edition, Corrected Version, 15 Oct. 1979, Fourth Publication)). The density of the second phase may be configured as 5.2 g/cm 3  to 6.2 g/cm 3 . The density of the second phase is less than the density of the main phase. 
     The occupied surface area ratio of the second phase in a cross section of the first region  51  is less than or equal to 10.5%. The occupied surface area ratio of the second phase is the sum of the occupied surface area ratio of Co 3 O 4  and the occupied surface area ratio of (Co, Fe) 3 O 4 . More specifically, the occupied surface area ratio of the second phase includes the occupied surface area ratio of the particles that are configured by Co 3 O 4 , the occupied surface area ratio of the particles that are configured by (Co, Fe) 3 O 4 , the occupied surface area ratio of the particles that are configured by a mixture of Co 3 O 4  and (Co, Fe) 3 O 4  and the occupied surface area ratio of Co 3 O 4  and/or (Co, Fe) 3 O 4  that are mixed into the particles of the main phase. 
     Since the inactive part of the inner portion of the first region  51  is reduced by a configuration in which the occupied surface area ratio of the second phase is less than or equal to 10.5%, it is possible to suppress a reduction in the output of the fuel cell  10  during power supply. 
     It is more preferred that the occupied surface area ratio of the second phase in a cross section of the first region  51  is greater than or equal to 0.5%. In this manner, since the sintering characteristics of the first region  51  are improved by suitably introducing the second phase, the porous framework structure can be strengthened. As a result, since changes in the microstructure of the first region  51  can be inhibited, it is possible to suppress peeling of the first region  51  during power supply. 
     Although there is no particular limitation in relation to the ratio of the occupied surface area ratio of Co 3 O 4  and the occupied surface area ratio of (Co, Fe) 3 O 4  in the occupied surface area ratio of the second phase, a configuration in which (Co, Fe) 3 O 4  is added in a greater amount than Co 3 O 4  is effective for strengthening the framework structure of the first region  51 . For that purpose, the occupied surface area ratio of (Co, Fe) 3 O 4  is preferably greater than the occupied surface area ratio of Co 3 O 4 . The occupied surface area ratio of Co 3 O 4  in a cross section of the first region  51  may be configured as less than or equal to 2.5%, and the occupied surface area ratio of (Co, Fe) 3 O 4  in a cross section of the first region  51  may be configured as less than or equal to 9.5%. 
     Although there is no particular limitation in relation to the average equivalent circle diameter of the second phase in the cross section of the first region  51 , it is preferably greater than or equal to 0.05 μm and less than or equal to 0.5 μm. In this manner, it is possible to suppress a reduction in the output of the fuel cell  10  during power supply. The average equivalent circle diameter is the value of the arithmetic average of 50 randomly selected circle diameters that have the same surface area as the second phase. The 50 second phases that are the object of the equivalent circle diameter measurement are preferably selected in a random manner from 5 or more positions on an FE-SEM image (magnification 10000 times) on a cross section of the cathode  50 . 
     The first region  51  may include a third phase distinct from the main phase and second phase described above. The components that constitute the third phase may include CoO (cobalt oxide), SrO (strontium oxide), SrSO 4  (strontium sulfate), and an oxide of an element that constitutes the main phase. However, there is no limitation in this regard. The sum total occupied surface area ratio of the third phase in the cross section of the cathode  50  is preferably less than 10%. 
     The second region  52  contains a main phase that is configured by a perovskite oxide expressed by the general formula ABO 3  and includes at least one of Sr and La at the A site. Although there is no particular limitation in relation to the occupied surface area ratio of the main phase in a cross section of the second region  52 , it may be configured as greater than or equal to 89.5% and less than or equal to 100%. 
     The second region  52  may not include a second phase that is configured by (Co, Fe) 3 O 4  and Co 3 O 4 , or may include a second phase. Experimental confirmation has been carried out in relation to the feature that it is effective to limit the occupied surface area ratio of the second phase in a cross section of the first region  51  as described above to a predetermined range irrespective of the presence or absence of a second phase in a cross section of the second region  52  or the dimension of an occupied surface area ratio of a second phase in a cross section of the second region  52 . 
     Method of Calculation of Occupied Surface Area of Second Phase 
     Now, the method of calculation of the occupied surface area ratio of the second phase in a cross section of the first region  51  will be described. 
     Firstly, a cross section of the cathode  50  is polished with precision machinery followed by an ion milling process performed using an IM4000 manufactured by Hitachi High-Technologies Corporation. 
     Then, an SEM image of a cross section of the first region  51  that is enlarged with a magnification of 10,000 times is obtained by use of a field emission scanning electron microscope (FE-SEM) that uses an in-lens secondary electron detector. 
     Next, 3 values corresponding to the contrast of the main phase, second phase and pores are assigned by categorizing the luminosity of the SEM image into 256 gradations. For example, the main phase is displayed as faint gray, the second phase as gray and the pores as black. However, there is no limitation in this regard. 
     Next, an analysis image highlighting Co 3 O 4  and (Co, Fe) 3 O 4  is obtained by image analysis of an SEM image using HALCON image analysis software produced by MVTec GmbH (Germany). The total surface area of Co 3 O 4  and (Co, Fe) 3 O 4  in the analysis image is taken to be the total surface area of the second phase, and the occupied surface area ratio of the second phase is calculated by dividing the total surface area of the second phase by the total surface area of the total solid phase in the analysis image. The analysis described above is performed at 5 positions on the same cross section of the first region  51 , and a value that is the arithmetic average of the ratio of the total surface area of the second phase calculated respectively at 5 positions is the occupied surface area ratio of the second phase in the first region  51 . 
     The respective occupied surface area ratio of Co 3 O 4  and (Co, Fe) 3 O 4  can be confirmed by component analysis as described below. 
     Firstly, the position of the second phase is confirmed with reference to the SEM image used in the calculation of the occupied surface area ratio of the second phase. Next, an EDX spectrum at the position of the second phase is obtained using energy dispersive X-ray spectroscopy (EDX). The elements that are present at the position of the second phase are identified by semi-quantitative analysis of the EDX spectrum. In this manner, it can be confirmed that Co 3 O 4  and (Co, Fe) 3 O 4  are in a mixed configuration in the second phase and not present in a solid solution, and that the occupied surface area ratio of Co 3 O 4  and the occupied surface area ratio (Co, Fe) 3 O 4  in the second phase can be separately obtained. 
     Whether (Co, Fe) 3 O 4  is configured as any of CoFe 2 O 4 , Co 1.5 Fe 1.5 O 4  or Co 2 FeO 4  can be confirmed by analysis of the crystalline structure of the second phase (lattice constant, lattice type, crystal orientation) using selected area electron diffraction (SAED) with a transmission electron microscope (TEM). 
     The calculation method of the occupied surface area ratio for the second phase has been described above, and the occupied surface area ratio for the main phase may be calculated in the same manner. 
     Cathode Material 
     The material used to configure the second region  52  in the cathode  50  is a perovskite oxide raw powder that is expressed by the general formula ABO 3 . 
     The material used to configure the first region  51  in the cathode  50  is a mixture in which a Co 3 O 4  raw powder and a (Co, Fe) 3 O 4  raw powder are added to a perovskite oxide raw powder that is expressed by the general formula ABO 3 . 
     The composite perovskite oxide raw powder includes a raw powder such as LSCF, LSF, LSC, LNF, SSC, or the like. (Co, Fe) 3 O 4  includes a raw powder of Co 2 FeO 4 , Co 1.5 Fe 1.5 O 4  or CoFe 2 O 4 , or the like. 
     The total added amount of the Co 3 O 4  raw powder and (Co, Fe) 3 O 4  raw powder that is added to the material for the first region  51  is less than or equal to 9.5 wt %. In this manner, it is possible to inhibit the occupied surface area ratio of the second phase in a cross section of the first region  51  to less than or equal to 10.5%. 
     The total added amount of Co 3 O 4  and (Co, Fe) 3 O 4  in the material for the first region  51  is preferably greater than or equal to 0.46 wt %. In this manner, it is possible to control the occupied surface area ratio of the second phase in a cross section of the first region  51  to greater than or equal to 0.5%. 
     The occupied surface area ratio of the second phase in a cross section of the first region  51  can be minutely adjusted by adjusting the granularity of each raw powder or the configuration of the Co 3 O 4  raw powder and (Co, Fe) 3 O 4  raw powder (whether in an oxide or chloride configuration, or the like). 
     Adjusting the granularity of the Co 3 O 4  raw powder and the (Co, Fe) 3 O 4  raw powder enables the adjustment of the average equivalent circle diameter of the second phase in a cross section of the first region  51 . Adjusting the granularity of the Co 3 O 4  raw powder and the (Co, Fe) 3 O 4  raw powder is preferably performed by use of an air classifier. In this manner, accurate classification such as an upper limiting value and a lower limiting value for the grain diameter is possible. 
     Method of Manufacturing Fuel Cell  10   
     Next, an example will be described of a manufacture method for the fuel cell  10 . In the following description, the term “green body” denotes a member prior to firing. 
     Firstly, a slurry for the anode current-collecting layer is prepared by adding a binder (for example polyvinyl alcohol) to a mixture of an anode current-collecting layer powder (for example, an NiO powder and a YSZ powder) and a pore-forming agent (for example, PMMA (polymethylmethacrylate resin)). Next, an anode current-collecting layer powder is obtained by drying and granulating the slurry for the anode current-collecting layer in a spray drier. Then, a green body for the anode current-collecting layer  21  is formed by molding the anode powder using a die press molding method. At that time, a tape lamination method may be used in substitution for the die press molding method. 
     Next, a slurry for the anode-active layer is prepared by adding a binder (for example polyvinyl alcohol) to a mixture of an anode-active layer powder (for example, an NiO powder and a YSZ powder) and a pore-forming agent (for example, PMMA). Then, a green body for the anode-active layer  22  is formed by printing the slurry for the anode-active layer onto the green body for the anode current-collecting layer  21  using a printing method. In that manner, a green body for the anode  20  is formed. At that time, a tape lamination method or coating method or the like may be used in substitution for the printing method. 
     Next, a slurry for the solid electrolyte layer is prepared by mixing a mixture of water and a binder into a solid electrolyte layer powder (for example, a YSZ powder) in a ball mill. Then, a green body for the solid electrolyte layer  30  is formed by coating and drying the slurry for the solid electrolyte layer onto the green body for the anode  20 . At that time, a tape lamination method or printing method or the like may be used in substitution for the coating method. 
     Next, a slurry for the barrier layer is prepared by mixing a mixture of water and a binder into a barrier layer powder (for example, a GDC powder) in a ball mill. Then, a green body for the barrier layer  40  is formed by coating and drying the slurry for the barrier layer onto the green body for the solid electrolyte layer  30 . At that time, a tape lamination method or printing method or the like may be used in substitution for the coating method. 
     Next, a laminated body using the green bodies respectively for the anode  20 , the solid electrolyte layer  30  and the barrier layer  40  are cofired (1300 to 1600 degrees C. for 2 to 20 hours) to form a cofired body of the anode  20 , the solid electrolyte layer  30  and the barrier layer  40 . 
     Next, a first region slurry is prepared by mixing water and a binder with the first region material described above in a ball mill. Next, a green body for the first region  51  is formed by coating and drying the first region slurry onto the barrier layer  40  of the cofired body. 
     Next, a second region slurry is prepared by mixing water and a binder with the second region material described above in a ball mill. Next, the second region slurry is coated and dried onto the green body for the first region  51 . 
     Then, the green bodies for the first region  51  and the second region  52  are fired (1000 to 1100 degrees C. for 1 to 10 hours) in an electrical furnace (oxygen-containing atmosphere) to thereby form the cathode  50  on the barrier layer  40 . 
     Other Embodiments 
     Although an embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various changes or modifications may be added within a scope that does not depart from the spirit of the invention. 
     Although a configuration has been described in which the cathode  50  according to the present invention is applied to the fuel cell  10 , in addition to a fuel cell, the cathode of the present invention may be applied to an electrochemical cell having a solid oxide configuration that includes a solid oxide-type electrolytic cell. 
     In the present embodiment, although the fuel cell  10  comprises the anode  20 , the solid electrolyte layer  30 , the barrier layer  40  and the cathode  50 , there is no limitation in this regard. The fuel cell  10  may comprise the anode  20 , the solid electrolyte layer  30 , and the cathode  50 , or another layer may be interposed between the anode  20  and the solid electrolyte layer  30 , or between the solid electrolyte layer  30  and the cathode  50 . 
     Examples 
     Although the examples of a cell according to the present invention will be described below, the present invention is not thereby limited to the following examples. 
     Preparation of Samples No. 1 to No. 21 
     A fuel cell according to Samples No. 1 to No. 21 was prepared as described below. 
     Firstly, an anode current-collecting layer (NiO:8YSZ=50:50 (Ni volume % conversion)) was formed with a thickness of 500 μm using a die press molding method, and on that layer, an anode-active layer (NiO:8YSZ=45:55 (Ni volume % conversion)) was formed using a printing method with a thickness of 20 μm. 
     Then a green body for a GDC layer and an 8YSZ layer were sequentially formed using a coating method on the anode-active layer and co-fired (1400 degrees C., 2 hours). 
     Next, as shown in Table 1, a material to configure the first region on the solid electrolyte layer side of the cathode was prepared as a first region material that includes a main phase (LSCF, LSF or SSC) and a second phase (Co 3 O 4  and (Co, Fe) 3 O 4 ). As shown in Table 1, the occupied surface area ratio of the second phase in the first region was varied by varying the addition amount of the second phase in each sample. Furthermore, the granularity of the Co 3 O 4  raw powder and the (Co, Fe) 3 O 4  raw powder was adjusted so that the average equivalent circle diameter of the second phase took the values shown in Table 1. In Sample No. 1, No. 2, No. 11, and No. 12, Co 2 FeO 4  was used as (Co, Fe) 3 O 4 , in Sample No. 3 to No. 5, No. 13, No. 14, and No. 17, Co 1.5 Fe 1.5 O 4  was used as (Co, Fe) 3 O 4 , and in Sample No. 6 to No. 10, No. 15, No. 16 and No. 18 to No. 21, CoFe 2 O 4  was used as (Co, Fe) 3 O 4 . 
     Next, a first region slurry was prepared by mixing the first region material, water and PVA in a ball mill for 24 hours. 
     Next, a green body for the first region was formed by coating and drying the first region slurry onto the GDC layer of the cofired body. At that time, the thickness after firing was configured to 3 μm by adjusting the coating amount of the first region slurry. 
     Next, a second region slurry was prepared by mixing the LSCF, water and a binder in a ball mill for 24 hours. 
     Next, a green body for the second region was formed by coating and drying the second region slurry onto the green body for the first region. 
     A cathode was formed by firing the green bodies for the first region and the second region for one hour in an electrical furnace (oxygen-containing atmosphere, 1000 degrees C.). 
     Measurement of Occupied Surface Area Ratio of Second Phase 
     Firstly, after polishing a cross section of the cathode in each sample with precision machinery, ion milling processing was performed using an IM4000 manufactured by Hitachi High-Technologies Corporation. 
     An SEM image of five positions in a cross section of the first region enlarged with a magnification of 10,000 times was obtained by use of an FE-SEM that uses an in-lens secondary electron detector. The SEM image was obtained using an FE-SEM (model: ULTRA55 manufactured by Zeiss AG) with a working distance setting of 3 mm and an acceleration voltage of 1 kV. In the SEM image, 3 values were assigned in relation to the contrast of the main phase, the second phase and the pores by categorizing the luminosity of the image into 256 gradations. 
     Then, the SEM image was configured as an analysis image by use of HALCON image analysis software produced by MVTec GmbH (Germany) in order to obtain an analysis image that highlights Co 3 O 4  and (Co, Fe) 3 O 4 . 
     Then, the occupied surface area ratio of the second phase was calculated respectively at five positions by dividing the total surface area of Co 3 O 4  and (Co, Fe) 3 O 4  in the analysis image by the total surface area of the solid phase in the analysis image and then calculating the arithmetic average of those values as the occupied surface area ratio for the second phase. Furthermore, an occupied surface area ratio of Co 3 O 4  and an occupied surface area ratio of (Co, Fe) 3 O 4  in the second phase were separately obtained by acquiring an EDX spectrum at the position of the second phase with reference to the SEM image. The calculation results for the occupied surface area ratio of the second phase in a cross section of the first region are shown in Table 1. 
     Average Equivalent Circle Diameter of Second Phase 
     The average equivalent circle diameter of the second phase at  50  arbitrarily selected positions was calculated with reference to the 5 analysis images used in the calculation of the occupied surface area ratio. The calculation results for the average equivalent circle diameter of the second phase are shown in Table 1. 
     Measurement of Fuel Cell Output 
     While supplying nitrogen gas to the anode side and air to the cathode side of each sample, the temperature was increased to 750 degrees C. When reaching a temperature of 750 degrees C., hydrogen gas was supplied for 3 hours the anode to perform reduction process. 
     Next, a rated current density value of 0.2 A/cm 2  was set, and power generation for 1000 hours was performed while measuring the cell voltage. The voltage drop ratio per 1000 hours was calculated as a deterioration rate. 
     After 1000 hours of power generation, a cross section of the first region of the cathode was observed using an electron microscope to observe peeling at the interface of the first region and the barrier layer. Those samples that were observed to have peeling of less than or equal to 5 μm that has a slight effect on the characteristics of the fuel cell are designated in Table 1 as being “present (slightly).” 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                   
                   
                 Occupied Surface 
                 Occupied Surface 
                 Occupied Surface 
                 Average Equivalent 
               
               
                   
                   
                   
                 Area Ratio 
                 Area Ratio 
                 Area Ratio 
                 Circle Diameter 
               
               
                   
                   
                   
                 of Second Phase 
                 of Co 3 0 4   
                 of (Co,Fe) 3 0 4   
                 of Second Phase 
               
               
                   
                   
                   
                 in Cross Section 
                 in Cross Section 
                 in Cross Section 
                 in Cross Section 
               
               
                   
                 Main Phase of 
                 Second Phase of 
                 of First Region 
                 of First Region 
                 of First Region 
                 of First Region 
               
               
                 Sample 
                 First Region 
                 First Region 
                 (%) 
                 (%) 
                 (%) 
                 (μm) 
               
               
                   
               
               
                 No. 1 
                 LSCF 
                 Co 2 FeO 4  + Co 3 O 4   
                 0.2 
                 0.05 
                 0.15 
                 0.35 
               
               
                 No. 2 
                 LSCF 
                 Co 2 FeO 4  + Co 3 O 4   
                 0.6 
                 0.08 
                 0.52 
                 0.25 
               
               
                 No. 3 
                 LSCF 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 0.5 
                 0.1 
                 0.4 
                 0.05 
               
               
                 No. 4 
                 LSCF 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 1.5 
                 0.3 
                 1.2 
                 0.22 
               
               
                 No. 5 
                 LSCF 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 2.4 
                 0.3 
                 2.1 
                 0.26 
               
               
                 No. 6 
                 LSCF 
                 CoFe 2 O 4  + Co 3 O 4   
                 5.3 
                 0.6 
                 4.7 
                 0.28 
               
               
                 No. 7 
                 LSCF 
                 CoFe 2 O 4  + Co 3 O 4   
                 7.5 
                 1.6 
                 5.9 
                 0.42 
               
               
                 No. 8 
                 LSCF 
                 CoFe 2 O 4  + Co 3 O 4   
                 9.7 
                 1.8 
                 7.9 
                 0.50 
               
               
                 No. 9 
                 LSCF 
                 CoFe 2 O 4  + Co 3 O 4   
                 10.5 
                 1.7 
                 8.8 
                 0.52 
               
               
                 No. 10 
                 LSCF 
                 CoFe 2 O 4  + Co 3 O 4   
                 11.3 
                 1.8 
                 9.5 
                 0.93 
               
               
                 No. 11 
                 LSF 
                 Co 2 FeO 4  + Co 3 O 4   
                 0.25 
                 0.05 
                 0.2 
                 0.37 
               
               
                 No. 12 
                 LSF 
                 Co 2 FeO 4  + Co 3 O 4   
                 1.6 
                 0.3 
                 1.3 
                 0.42 
               
               
                 No. 13 
                 LSF 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 3.5 
                 0.6 
                 2.9 
                 0.48 
               
               
                 No. 14 
                 LSF 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 9.5 
                 1.6 
                 7.9 
                 0.50 
               
               
                 No. 15 
                 LSF 
                 CoFe 2 O 4  + Co 3 O 4   
                 10.2 
                 1.5 
                 8.7 
                 0.55 
               
               
                 No. 16 
                 LSF 
                 CoFe 2 O 4  + Co 3 O 4   
                 11.5 
                 0.6 
                 10.9 
                 0.97 
               
               
                 No. 17 
                 SSC 
                 Co 1.5 Fe 1.5 O 4  + Co 3 O 4   
                 0.4 
                 0.1 
                 0.3 
                 0.17 
               
               
                 No. 18 
                 SSC 
                 CoFe 2 O 4  + Co 3 O 4   
                 0.6 
                 0.1 
                 0.5 
                 0.08 
               
               
                 No. 19 
                 SSC 
                 CoFe 2 O 4  + Co 3 O 4   
                 2.9 
                 0.5 
                 2.4 
                 0.30 
               
               
                 No. 20 
                 SSC 
                 CoFe 2 O 4  + Co 3 O 4   
                 5.6 
                 0.5 
                 5.1 
                 0.40 
               
               
                 No. 21 
                 SSC 
                 CoFe 2 O 4  + Co 3 O 4   
                 10.9 
                 0.6 
                 10.3 
                 0.58 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Occupied Surface 
                   
                   
                   
               
               
                   
                   
                   
                 Area Ratio 
               
               
                   
                   
                   
                 of Second Phase 
               
               
                   
                   
                   
                 in Cross Section 
                 Deterioration 
                 Presence/Absence 
               
               
                   
                   
                 Second Phase of 
                 of Second Region 
                 Rate 
                 of 
               
               
                   
                 Sample 
                 Second Region 
                 (%) 
                 (%) 
                 Interface Peeling 
                 Evaluation 
               
               
                   
                   
               
               
                   
                 No. 1 
                 None 
                 0.00 
                 0.62 
                 Present (Slightly) 
                 ◯ 
               
               
                   
                 No. 2 
                 None 
                 0.00 
                 0.35 
                 No 
                 ⊚ 
               
               
                   
                 No. 3 
                 None 
                 0.00 
                 0.4 
                 No 
                 ⊚ 
               
               
                   
                 No. 4 
                 None 
                 0.00 
                 0.47 
                 No 
                 ⊚ 
               
               
                   
                 No. 5 
                 None 
                 0.00 
                 0.58 
                 No 
                 ⊚ 
               
               
                   
                 No. 6 
                 None 
                 0.00 
                 0.76 
                 No 
                 ⊚ 
               
               
                   
                 No. 7 
                 None 
                 0.00 
                 0.95 
                 No 
                 ⊚ 
               
               
                   
                 No. 8 
                 None 
                 0.00 
                 1.1 
                 No 
                 ⊚ 
               
               
                   
                 No. 9 
                 None 
                 0.00 
                 1.5 
                 No 
                 ⊚ 
               
               
                   
                 No. 10 
                 None 
                 0.00 
                 2.6 
                 Yes 
                 X 
               
               
                   
                 No. 11 
                 None 
                 0.00 
                 0.39 
                 Present (Slightly) 
                 ◯ 
               
               
                   
                 No. 12 
                 None 
                 0.00 
                 0.85 
                 No 
                 ⊚ 
               
               
                   
                 No. 13 
                 None 
                 0.00 
                 1.4 
                 No 
                 ⊚ 
               
               
                   
                 No. 14 
                 None 
                 0.00 
                 1.05 
                 No 
                 ⊚ 
               
               
                   
                 No. 15 
                 None 
                 0.00 
                 1.5 
                 No 
                 ⊚ 
               
               
                   
                 No. 16 
                 None 
                 0.00 
                 2.8 
                 Yes 
                 X 
               
               
                   
                 No. 17 
                 None 
                 0.00 
                 0.45 
                 Present (Slightly) 
                 ◯ 
               
               
                   
                 No. 18 
                 None 
                 0.00 
                 0.68 
                 No 
                 ⊚ 
               
               
                   
                 No. 19 
                 None 
                 0.00 
                 0.31 
                 No 
                 ⊚ 
               
               
                   
                 No. 20 
                 None 
                 0.00 
                 0.78 
                 No 
                 ⊚ 
               
               
                   
                 No. 21 
                 None 
                 0.00 
                 2.3 
                 Yes 
                 X 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, a reduction in the output of the fuel cell was suppressed in Sample No. 1 to No. 9, No. 11 to No. 15, and No. 17 to No. 20 in which the occupied surface area ratio of the second phase in a cross section of the first region of the cathode was suppressed to less than or equal to 10.5%. This feature is due to the reduction in the inactive part in an inner portion of the first region. 
     In the present embodiment, the effect of the present invention was simply confirmed by preparing a cathode into which a second phase had been introduced into a first region and measuring a deterioration rate after 1000 hours power generation immediately after manufacture of the fuel cell. However, the results of the present embodiment demonstrate that it is possible to suppress an output reduction of a fuel cell if the occupied surface area ratio of a second phase in a cross section of the first region is suppressed to less than or equal to 10.5% irrespective of the manner of manufacturing or the degree to which the fuel cell is used. 
     Furthermore, the peeling of the first region was suppressed in Sample No. 2 to No. 9, No. 12 to No. 15, and No. 18 to No. 20 in which the occupied surface area ratio of the second phase in a cross section of the first region was configured to be greater than or equal to 0.5%. This feature is due to the strengthening of the porous framework structure by improving the sintering characteristics of the first region with the second phase. 
     The occupied surface area ratio of (Co, Fe) 3 O 4  in those samples is generally greater than the occupied surface area ratio of Co 3 O 4 , and therefore such a feature exhibits a more useful effect in relation to the strengthening of the framework structure of the cathode. However, even when the occupied surface area ratio of (Co, Fe) 3 O 4  is smaller than the occupied surface area ratio of Co 3 O 4 , or even when it is the same, the framework structure of the cathode can be strengthened by a configuration in which the occupied surface area ratio of the second phase is greater than or equal to 0.5%. 
     In the present embodiment, the effect of the present invention was simply confirmed by preparing a cathode into which a second phase is introduced and observing the peeling of the first region after 1000 hours power generation immediately after manufacture of the fuel cell. However, the results of the present embodiment demonstrate that it is possible to suppress the production of cracks in a cathode if the occupied surface area ratio of the second phase in a cross section of the first region is configured to be greater than or equal to 0.5%, irrespective of the manner of manufacturing or the degree of to which the fuel cell is used. 
     In addition, a further suppression in a reduction of fuel cell output was enabled in Sample No. 1 to No. 8, No. 11 to No. 14, and No. 17 to No. 20 in which the average equivalent circle diameter of the second phase in a cross section of the first region was configured to be greater than or equal to 0.05 μm and less than or equal to 0.5 μm. 
     In the present embodiment, although those samples in which the second region does not contain the second phase were evaluated, even when the second region contains the second phase, the above results have been experimentally confirmed to be available irrespective of the dimension of the occupied surface area ratio of the second phase in a cross section of the second region.