U.S. patent application number 16/233477 was filed with the patent office on 2020-02-06 for cathode material and fuel cell.
The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Ayano KOBAYASHI, Makoto OHMORI.
Application Number | 20200044260 16/233477 |
Document ID | / |
Family ID | 51175662 |
Filed Date | 2020-02-06 |
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United States Patent
Application |
20200044260 |
Kind Code |
A9 |
OHMORI; Makoto ; et
al. |
February 6, 2020 |
CATHODE MATERIAL AND FUEL CELL
Abstract
A cathode material used in an anode and a cathode contains
(Co,Fe).sub.3O.sub.4 and a perovskite type oxide that is expressed
by the general formula ABO.sub.3 and includes at least one of La
and Sr at the A site. A content ratio of (Co,Fe).sub.3O.sub.4 in
the cathode material is at least 0.23 wt % and no more than 8.6 wt
%.
Inventors: |
OHMORI; Makoto; (Nagoya-shi,
JP) ; KOBAYASHI; Ayano; (Nagoya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-shi |
|
JP |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20190157687 A1 |
May 23, 2019 |
|
|
Family ID: |
51175662 |
Appl. No.: |
16/233477 |
Filed: |
December 27, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14819572 |
Aug 6, 2015 |
10312525 |
|
|
16233477 |
|
|
|
|
PCT/JP14/59861 |
Apr 3, 2014 |
|
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14819572 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 35/2641 20130101;
C04B 2235/83 20130101; H01M 2008/1293 20130101; C04B 2235/3279
20130101; H01M 4/8652 20130101; C04B 35/01 20130101; C04B 2235/3213
20130101; C04B 2235/77 20130101; C04B 2235/3272 20130101; H01M
8/1246 20130101; H01M 4/8657 20130101; H01M 8/1213 20130101; H01M
2300/0071 20130101; C04B 2235/3274 20130101; C04B 2235/763
20130101; C04B 2235/768 20130101; C04B 2235/80 20130101; H01M
4/9033 20130101; C04B 2235/3227 20130101; C04B 2235/3275 20130101;
C04B 2235/3224 20130101; C04B 2235/3277 20130101 |
International
Class: |
H01M 4/90 20060101
H01M004/90; H01M 4/86 20060101 H01M004/86; H01M 8/1246 20060101
H01M008/1246; C04B 35/01 20060101 C04B035/01; C04B 35/26 20060101
C04B035/26; H01M 8/1213 20060101 H01M008/1213 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2013 |
JP |
2013-084154 |
Claims
1. A cathode material containing (Co,Fe).sub.3O.sub.4 and a
perovskite type oxide, the perovskite type oxide being expressed by
the general formula ABO.sub.3 and including at least one of La and
Sr at the A site, wherein a content ratio of (Co,Fe).sub.3O.sub.4
is at least 0.23 wt % and no more than 8.6 wt %.
2. The cathode material according to claim 1, wherein the
perovskite type oxide is LSCF.
3. The cathode material according to claim 1, wherein a content
ratio of the perovskite type oxide is 91.4 wt % or more.
4. The cathode material according to claim 1, wherein the
(Co,Fe).sub.3O.sub.4 is at least one selected from the group
consisting of CoFe.sub.2O.sub.4, Co.sub.1.5Fe.sub.1.5O.sub.4 and
Co.sub.2FeO.sub.4.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 14/819,572 filed on Aug. 6, 2015, which is a
continuation application of International Application No.
PCT/JP2014/059861, filed Apr. 3, 2014, which claims priority to
Japanese Application No. 2013-084154, filed in Japan on Apr. 12,
2013, the contents of each of which is hereby incorporated herein
by reference.
BACKGROUND
Field of the Invention
[0002] The present invention relates to a cathode material and a
fuel cell.
Background Information
[0003] In recent years, fuel cell batteries have attracted
attention in light of effective use of energy resources and
environmental problems. A fuel cell includes a fuel battery cell
and an interconnector. A fuel cell generally includes an anode, a
cathode and a solid electrolyte layer that is disposed between the
anode and the cathode.
[0004] A widely known configuration for the raw material of the
cathode is a perovskite type oxide such as LSCF. (For example,
reference is made to Japanese Patent Application Laid-Open No.
2006-32132).
SUMMARY
[0005] However, repetitive use of the fuel cell for power
generation may lead to power reduction. The present inventors have
gained new insight that one of the causes of power reduction
resides in the deterioration of the cathode, and that the
deterioration of the cathode is related to the proportion of (Co,
Fe).sub.3O.sub.4 that is found in the cathode.
[0006] The present invention is proposed based on the above new
insight and has the purpose of providing a fuel cell and a cathode
material that enhances durability.
[0007] A fuel cell according to the present invention includes an
anode, a cathode that includes a main phase configured with a
perovskite type oxide and a secondary phase configured with (Co,
Fe).sub.3O.sub.4, and a solid electrolyte layer that is disposed
between the anode and the cathode. The occupied area ratio of the
secondary phase in a cross section of the cathode is no more than
9.5%.
[0008] The present invention provides a fuel cell and a cathode
material that enhances durability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Referring now to the attached drawings which form a part of
this original disclosure.
[0010] FIG. 1 is a cross sectional view illustrating the
configuration of a fuel cell.
[0011] FIG. 2 illustrates an example of a SEM image of a cross
section of a cathode.
[0012] FIG. 3 illustrates a histogram that divides the luminosity
distribution in the SEM image into 256 gradations.
[0013] FIG. 4 illustrates an image analysis results for the SEM
image illustrated in FIG. 2.
[0014] FIG. 5 illustrates a cross sectional view illustrating
another configuration of a fuel cell.
[0015] FIG. 6 illustrates an example of a TEM image of a cathode
cross section.
[0016] FIG. 7 illustrates a graph showing an example of an EDX
spectrum of a secondary phase.
[0017] FIG. 8 illustrates an example of an SAED image of the
secondary phase.
DETAILED DESCRIPTION OF EMBODIMENTS
[0018] Next, the embodiments of the present invention will be
described making reference to the figures. In the description of
the figures below, the same or similar portions are denoted by the
same or similar reference numerals. However, the figures are merely
illustrative and the ratio of respective dimensions or the like may
differ from the actual dimensions. Therefore, the actual dimensions
or the like should be determined by reference to the following
description. Furthermore, it goes without saying that the ratios or
the relations of dimensions used in respective figures may be
different.
[0019] In the following embodiments, a solid oxide fuel cell (SOFC)
will be described as an example of a fuel cell. Although the
following description relates to a flat-tubular type fuel cell, the
invention is not limited in this regard, and may also find
application in relation to a segmented-in-series fuel cell.
1. First Embodiment
Configuration of Fuel Cell 10
[0020] The configuration of a fuel cell (abbreviated below to
"cell") 10 will be described making reference to the figures. FIG.
1 is a cross sectional view of the configuration of the cell
10.
[0021] The cell 10 is a thin plate body configured using a ceramic
material. The thickness of the cell 10 is for example 300
micrometers to 3mm, and the diameter of the cell 10 is 5 mm to 50
mm. A plurality of cells 10 is connected in series by
interconnectors to form a fuel cell.
[0022] The cell 10 includes an anode 11, a solid electrolyte layer
12, a barrier layer 13 and a cathode 14.
[0023] The anode 11 has the function of an anode of the cell 10.
The anode 11 as illustrated in FIG. 1 is configured with an anode
current collecting layer 111 and an anode active layer 112.
[0024] The anode current collecting layer 111 may be configured as
a porous tabular fired body including a transition metal and an
oxygen ion conductive material. The anode current collecting layer
111 for example may include nickel oxide (NiO) and/or nickel (Ni)
and yttria-stabilized zirconia (3YSZ, 8YSZ, 10YSZ, or the like).
The thickness of the anode current collecting layer 111 may be 0.2
mm to 5.0 mm. The thickness of the anode current collecting layer
111 may be the largest of each component member of the cell 10 when
the anode current collecting layer 111 functions as a support
substrate. The volume ratio of Ni and/or NiO in the anode current
collecting layer 111 may be 35 volume % to 65 volume %, and the
volume ratio of YSZ may be 35 volume % to 65 volume %. The anode
current collecting layer 111 may include yttria (Y.sub.2O.sub.3) in
substitution for YSZ.
[0025] The anode active layer 112 is disposed between the anode
current collecting layer 111 and the solid electrolyte layer 12.
The anode active layer 112 is configured as a porous tabular fired
body including a transition metal and an oxygen ion conductive
material. The anode active layer 112 for example may include NiO
and/or Ni and yttria-stabilized zirconia in the same manner as the
anode current collecting layer 111. The thickness of the anode
active layer 112 may be 5.0 micrometers to 30 micrometers. The
volume ratio of Ni and/or NiO in the anode active layer 112 may be
25 volume % to 50 volume % using an Ni conversion, and the volume
ratio of YSZ may be 50 volume % to 75 volume %. In this manner, the
content ratio of YSZ in the anode active layer 112 may be greater
than the anode current collecting layer 111. The anode active layer
112 may include a zirconia based material such as
scandia-stabilized zirconia (ScSZ) in substitution for YSZ.
[0026] The solid electrolyte layer 12 is disposed between the anode
11 and the barrier layer 13. The solid electrolyte layer 12 has the
function of enabling transmission of oxygen ions produced by the
cathode 14. The solid electrolyte layer 12 includes zirconium (Zr).
The solid electrolyte layer 12 may include Zr as zirconia
(ZrO.sub.2). The solid electrolyte layer 12 may include ZrO.sub.2
as a main component. In addition to ZrO2, the solid electrolyte
layer 12 may include an additive such as Y.sub.2O.sub.3 and/or
Sc.sub.2O.sub.3. These additives may function as a stabilizer. In
the solid electrolyte layer 12, the stabilizer may have a mol
composition ratio with respect to the stabilizer ZrO.sub.2
(stabilizer: ZrO.sub.2) of 3:97.about.20:80. In other words, the
material used in the solid electrolyte layer 12 may include
zirconia-based materials such as ScSZ and yttria-stabilized
zirconia such as 3YSZ, 8YSZ, and 10YSZ, or the like. The thickness
of the solid electrolyte layer 12 may be 3 micrometers to 30
micrometers.
[0027] The barrier layer 13 is disposed between the solid
electrolyte layer 12 and the cathode 14. The barrier layer 13 has
the function of suppressing formation of a high resistive layer
between the solid electrolyte layer 12 and the cathode 14. The
material used in the barrier layer 13 includes cerium (Ce) and a
ceria-based material including Ce in which rare earth metal oxide
is entered into solid solution. More specifically, the ceria-based
material includes GDC ((Ce, Gd)O.sub.2: gadolinium doped ceria),
SDC ((Ce, Sm)O.sub.2: samarium doped ceria), or the like. The
thickness of the barrier layer 13 may be 3 micrometers to 20
micrometers.
[0028] The cathode 14 is disposed on the barrier layer 13. The
cathode 14 functions as the cathode of the cell 10. The thickness
of the cathode 14 may be 2 micrometers to 100 micrometers.
[0029] The cathode 14 has a main component being a perovskite type
oxide expressed by the general formula ABO.sub.3 and including at
least one of La and Sr at the A site. This type of perovskite type
oxide preferably includes a configuration of a perovskite type
complex oxide that contains lanthanum and SSC (samarium strontium
cobaltite: (Sm, Sr) CoO.sub.3) that does not contain lanthanum.
However, there is no limitation in this regard. A perovskite type
complex oxide that contains lanthanum includes LSCF (lanthanum
strontium cobalt ferrite: (La, Sr)(Co, Fe)O.sub.3), LSF (lanthanum
strontium ferrite: (La, Sr) FeO.sub.3), LSC (lanthanum strontium
cobaltite: (La, Sr)CoO3) and LNF (lanthanum nickel ferrite: (La
(Ni, Fe)O.sub.3). The cathode 14 may not contain Co (cobalt). The
density of the main phase configured by a perovskite type oxide may
be 5.5 g/cm.sup.3.about.8.5 g/cm.sup.3.
[0030] The occupied area ratio of the main phase in a cross section
of the cathode 14 may be at least 87.5% to no more than 99.75%. The
method of calculating the occupied area ratio will be described
below.
[0031] The cathode 14 includes a secondary phase that is configured
with (Co, Fe).sub.3O.sub.4 in a spinel crystal structure. (Co,
Fe).sub.3O.sub.4 includes Co.sub.2FeO.sub.4,
Co.sub.1.5Fe.sub.1.5O.sub.4 and CoFe.sub.2O.sub.4. The density of
the secondary phase may be configured as 5.2 g/cm.sup.3.about.6.2
g/cm.sup.3. The density of the secondary phase may be smaller than
the density of the main phase.
[0032] The occupied area ratio of the secondary phase in the cross
section of the cathode 14 is no more than 9.5%. In this manner,
since the inactive region in the interior of the cathode is
reduced, a decrease in the initial output can be suppressed and
progressive deterioration of the cathode resulting from a reaction
between the secondary phase and the main phase during current flow
can be suppressed. As a result, the durability of the cathode 14
can be enhanced.
[0033] The occupied area ratio of the secondary phase is preferably
at least 0.25%. In this manner, the porous framework structure can
be strengthened as a result of the improvement to the sintering
characteristics of the cathode 14 that result from suitable
introduction of the secondary phase. Consequently, microstructural
changes in the cathode 14 during current flow can be suppressed and
the durability of the cathode 14 can be further enhanced.
[0034] The average value of the equivalent circle diameter of the
particles comprising the secondary phase is preferably at least
0.05 micrometers to no more than 0.5 micrometers. The deterioration
rate of the cathode 14 can be further reduced as a result of
controlling the average value of the equivalent circle diameter of
the secondary phase to this range. The average value of the
equivalent circle diameter is the arithmetic mean value of the
diameter of a circle having the same surface area as the particles
comprising the secondary phase.
[0035] The cathode 14 may include a third phase that is configured
by Co.sub.3O.sub.4 (tricobalt tetroxide) or CoO (cobalt oxide). The
occupied area ratio of the third phase in the cross section of the
cathode 14 is preferably less than 3.0%. Furthermore, in addition
to the secondary phase and the third phase, the cathode 14 may
include an oxide of component element of the main phase.
Method of Calculation of Occupied Area Ratio
[0036] Next, the method of calculating the occupied area ratio of
the secondary phase will be described making reference to FIG. 2 to
FIG. 4.
1. SEM Image
[0037] FIG. 2 is an SEM image illustrating the cross section of the
cathode 14 enlarged with a magnification of 10,000 times by a field
emission scanning electron microscope (FE-SEM) using an in-lens
secondary electron detector. FIG. 2 illustrates the cross section
of the cathode 14 that contains a main component of LSCF
(La.sub.0.6Sr.sub.0.4)(Co.sub.0.2Fe.sub.0.8)O.sub.3). The cross
section of the cathode 14 is preprocessed by polishing with
precision machinery, and then ion milling processing is performed
using an IM4000 manufactured by Hitachi High-Technologies
Corporation. The SEM image in FIG. 2 is obtained by an FE-SEM
(model: ULTRA55) manufactured by Zeiss AG (Germany) with a working
distance setting of 3 mm, and an acceleration voltage of 1 kV.
[0038] In the SEM image in FIG. 2, the contrast of the pores, the
main phase (LSCF) and the secondary phase ((Co, Fe).sub.3O.sub.4)
differs from each other. The main phase is displayed as "faint
gray", the secondary phase as "dark gray" and the pores as "black".
In this manner, three values assigned by the contrast can be
realized by categorizing the luminosity of the image into 256
gradations. FIG. 3 is a histogram that divides the luminosity
distribution in an SEM image illustrated in FIG. 2 into 256
gradations. As illustrated in FIG. 3, the luminosity of the
secondary phase is detected at a low frequency from the low
luminosity side of the main phase to the high luminosity side of
the pores. Consequently, in FIG. 2, the secondary phase exhibits a
darker contrast than the main phase and a brighter contrast than
the pores.
[0039] The method of discriminating the main phase, the secondary
phase and the pores is not limited to the use of a contrast based
on the SEM image. For example, after acquiring an element mapping
in the same field by use of SEM-EDS, the respective particles in
the image are identified by illuminating and aligning the FE-SEM
image that is acquired in advance by use of an in-lens secondary
electron detector to thereby arrive at three accurate values.
2. Analysis of SEM Image
[0040] FIG. 4 illustrates the image analysis results for the SEM
image illustrated in FIG. 2 using HALCON image analysis software
produced by MVTec GmbH (Germany). In FIG. 4, the secondary phase is
enclosed by the white solid line. It is possible to calculate the
occupied area ratio of the main phase and the secondary phase based
on the analysis image illustrated in FIG. 4.
[0041] Firstly, the sum total surface area of the secondary phase
enclosed by the white solid line is calculated with reference to
the analysis image. Next, the proportion of the sum total surface
area of the secondary phase to the gross surface area of the
analysis image is calculated. The proportion of the sum total
surface area of the secondary phase calculated in this manner is
taken to be the occupied area ratio of the secondary phase.
Cathode Material
[0042] The cathode material that configures the cathode 14 is
preferably raw material mixture of (Co, Fe).sub.3O.sub.4 and a
perovskite type oxide that is expressed by the general formula
ABO.sub.3.
[0043] The perovskite type oxide includes LSCF, LSF, LSC, LNF, SSC,
or the like. (Co, Fe).sub.3O.sub.4 includes Co.sub.2FeO.sub.4,
Co.sub.1.5Fe.sub.1.5O.sub.4 and CoFe.sub.2O.sub.4.
[0044] The added amount of (Co, Fe).sub.3O.sub.4 in the cathode
material is no more than 8.6 wt %. In this manner, the occupied
area ratio of the secondary phase ((Co, Fe).sub.3O.sub.4) in the
cross section of the cathode 14 can be controlled to no more than
9.5%. Furthermore, the added amount of (Co, Fe).sub.3O.sub.4 in the
cathode material is preferably at least 0.23 wt %. In this manner,
the occupied area ratio of the secondary phase in the cross section
of the cathode 14 can be controlled to be at least 0.25%.
[0045] It is further possible to perform micro adjustment to the
occupied area ratio of the secondary phase by adjusting the grain
size or the status of the raw material of (Co, Fe).sub.3O.sub.4
(whether it is in the form of an hydroxide or a salt).
[0046] Furthermore, since the density of the raw material of (Co,
Fe).sub.3O.sub.4 is smaller than the density of the perovskite type
oxide, the density of the secondary phase in the cathode 14 can be
configured to be smaller than the density of the main phase.
[0047] In addition, the equivalent circle diameter of the particles
comprising the secondary phase in the cathode 14 can be adjusted by
adjusting the grain size of the raw material of (Co,
Fe).sub.3O.sub.4. An accurate classification that includes an upper
limiting value and a lower limiting value is possible by adjusting
the grain size of the raw material of (Co, Fe).sub.3O.sub.4 by use
of an air classifier.
Method of Manufacturing Cell 10
[0048] Next, an example will be described of a manufacture method
for the cell 10. However, respective conditions such as the
material, the particle diameter, the temperature and the method of
coating as described below may be varied as required. "Green body"
below denotes a state prior to firing.
[0049] Firstly, a slurry is formed by adding polyvinyl alcohol
(PVA) as a binder to a mixture of NiO powder, YSZ powder, and a
pore forming agent (for example, Polymethyl methacrylate (PMMA)).
Next, the slurry is dried and granulated by use of a spray drier to
form a powder for the anode current collecting layer. Then, the
powder for the anode current collecting layer is pressed using a
die press molding method to form a green body for the anode current
collecting layer 111.
[0050] Then, a slurry is formed by adding polyvinyl alcohol as a
binder to a mixture of NiO powder, YSZ powder, and a pore forming
agent (for example, PMMA). The slurry is printed onto the green
body of the anode current collecting layer 111 using a printing
method to thereby form a green body for the anode active layer 112.
In this manner, the green body for the anode 11 is formed.
[0051] Next, a mixture of YSZ powder, water and a binder is mixed
in a ball mill for 24 hours to prepare a slurry. Then the slurry is
coated on the green body for the anode active layer 112, and dried
to form a green body for the solid electrolyte layer 12. In
substitution for a method of coating, a method such as a tape
lamination method or a printing method may be used.
[0052] Then, a mixture of GDC powder, water and a binder is mixed
in a ball mill for 24 hours to prepare a slurry. Then the slurry is
coated on the green body for the electrolyte layer 12, and dried to
form a green body for the barrier layer 13. In substitution for a
method of coating, a method such as a tape lamination method or a
printing method may be used.
[0053] As described above, a laminated body configured with the
green body for the anode 11, the green body for the solid
electrolyte layer 12 and the green body for the barrier layer 13
can be formed.
[0054] Then the laminated body formed from the green bodies is
co-sintered for 2 to 20 hours at 1300 to 1600 degrees C. to form a
co-fired body formed from a dense barrier layer 13 and solid
electrolyte layer 12 and an anode 11 that is configured with the
anode current collecting layer 111 and the anode active layer
112.
[0055] Then, the above cathode material, water and a binder are
mixed in a ball mill for 24 hours to prepare a slurry.
[0056] The slurry is coated on the barrier layer 13 of the co-fired
body, and dried, and then fired for one hour in an electric furnace
(O.sub.2 containing atmosphere, 1000 degrees C.) to form the porous
cathode 14 on the barrier layer 13. In the above manner, the cell
10 is completed.
2. Second Embodiment
Configuration of Fuel Cell 20
[0057] The configuration of a fuel cell 20 according to a second
embodiment will be described making reference to the figures. FIG.
5 is a cross sectional view of the configuration of the cell 20.
The point of difference between the cell 10 according to the first
embodiment and the cell 20 according to the second embodiment
resides in the feature that the occupied area ratio of the
secondary phase is controlled only in relation to the region near
the solid electrolyte layer of a cathode 14. The following
description will focus mainly on the point of difference.
[0058] As illustrated in FIG. 5, the cathode 14' includes a first
region 141 and a second region 142.
[0059] The first region 141 is a region within 3 micrometers from
the surface 14S near to the solid electrolyte layer 12. The first
region 141 is an example of the "solid electrolyte layer-side
region". The cell 20 according to the present embodiment is
configured by interposing the barrier layer 13 between the solid
electrolyte layer 12 and the cathode 14' so that the first region
141 is in contact with the barrier layer 13. However, when the cell
20 is configured without provision of the barrier layer 13, the
first region 141 is in contact with the solid electrolyte layer
12.
[0060] The first region 141 includes a main phase configured with a
perovskite type oxide that is expressed by the general formula
ABO.sub.3 and includes at least one of Sr and La at the A site. The
occupied area ratio of the main phase in the cross section of the
first region 141 may be at least 87.5% to no more than 99.75%.
[0061] The first region 141 includes a secondary phase that is
configured with (Co, Fe).sub.3O.sub.4 in a spinel crystal
structure. The occupied area ratio of the secondary phase in a
cross section of the first region 141 is no more than 9.5%. In this
manner, the film strength of the first region 141 can be enhanced
by addition of (Co, Fe).sub.3O.sub.4, and it is possible to
suppress a large thermal expansion difference between the first
region 141 and the barrier layer 13 or the solid electrolyte layer
12 as a result of addition of (Co, Fe).sub.3O.sub.4. Consequently,
it is possible to prevent delamination of the cathode 14'. The
occupied area ratio of the secondary phase in the cross section of
the first region 141 is preferably at least 0.25%. In this manner,
since a sufficient enhancement to the film strength of the first
region 141 is enabled by addition of (Co, Fe).sub.3O.sub.4,
enhanced suppression of delamination of the cathode 14' is enabled.
The method of calculating the surface area occupied ratio of the
secondary phase is the same as that described with reference to
the. first embodiment.
[0062] The average value of the equivalent circle diameter of
particles comprising the secondary phase is preferably at least
0.05 micrometers to no more than 0.5 micrometers in the cross
section of the first region 141. The density of the secondary phase
may be smaller than the density of the main phase. The first region
141 may include a third phase that is configured by Co3O.sub.4
(tricobalt tetroxide) or CoO (cobalt oxide). The occupied area
ratio of the third phase in a cross section of the first region 141
is preferably less than 3.0%. Furthermore, in addition to the
secondary phase and the third phase, the first region 141 may
include an oxide of component element of the main phase.
[0063] The cathode material described with reference to the first
embodiment may be suitably used as the material of the first region
141 as described above.
[0064] The second region 142 is a region separated by at least 3
micrometers from the surface 14S near to the solid electrolyte
layer 12. The second region 142 is disposed on the first region
141. The second region 142 includes a perovskite type oxide
expressed by the general formula ABO.sub.3 and including at least
one of La and Sr at the A site. The second region 142 need not
include (Co, Fe).sub.3O.sub.4 in a spinel crystal structure.
[0065] The surface 14S of the cathode 14' near to the solid
electrolyte layer 12 can be defined by a line of rapid change in
the concentration distribution when the component density in the
cross sectional surface of the cathode 14' and the solid
electrolyte layer 12 is mapped. The surface 14S of the cathode 14'
near to the solid electrolyte layer 12 may also be defined by a
line of rapid change in the porosity in the cross sectional surface
of the cathode 14' and the solid electrolyte layer 12.
Method of Manufacturing Cell 20
[0066] Next, a manufacture method for the cell 20 according to the
second embodiment will be described. Since the method of
manufacturing the anode 11, the solid electrolyte layer 12 and the
barrier layer 13 has been described above in the first embodiment,
the following description will focus mainly on the manufacturing
method for the cathode 14'.
[0067] Firstly, a co-fired body is prepared from the anode 11, the
solid electrolyte layer 12 and the barrier layer 13.
[0068] Then, a slurry for the first region 141 is prepared by
mixing the cathode material for the first region 141 (including the
main phase and the secondary phase described above), water, and a
binder in a ball mill for 24 hours.
[0069] Next, the slurry for the first region is coated on the
barrier layer 13 of the cofired body and dried to thereby form a
green body for the first region 141.
[0070] Then, a known cathode material (for example, LSCF, or the
like), water and a binder are mixed in a ball mill for 24 hours to
thereby prepare the slurry for the second region.
[0071] Next, the slurry for the second region is coated on the
green body for the first region 141 and dried to thereby form a
green body for the second region 142.
[0072] Next, the green body for the first region 141 and the green
body for the second region 142 are fired for one hour in an
electric furnace (O.sub.2 containing atmosphere, 1000 degrees C.)
to form the cathode 14' on the barrier layer 13.
Other Embodiments
[0073] The present invention is not limited to the above
embodiments and various modifications or changes are possible
within a scope that does not depart from the spirit of the
invention.
[0074] (A) In the above embodiment, although FIG. 2 to FIG. 4
illustrate a cross sectional view in which the cathode 14 contains
the main phase of LSCF, the cathode 14 just have to contain a main
phase of a perovskite type oxide such as (LSC or SSC, or the
like).
[0075] (B) In the above embodiment, although the cell 10 includes
the anode 11, the solid electrolytic layer 12, the barrier layer 13
and the cathode 14, the invention is not thereby limited. The cell
10 may include the anode 11, the solid electrolytic layer 12 and
the cathode 14, and another layer may be interposed between the
anode 11, the solid electrolytic layer 12 and the cathode 14. For
example, in addition to the barrier layer 13, the cell 10 may
include a porous barrier layer that is interposed between the
barrier layer 13 and the cathode 14.
[0076] (C) Although such a feature has not been specifically
disclosed in the above embodiments, the configuration of the cell
10 may have an anode-support configuration, or a tabular,
cylindrical, a flat-tubular type, segmented-in-series
configuration, or the like. Furthermore, the cross section of the
cell 10 may be oval, or the like.
[0077] (D) The second embodiment includes the configuration of the
region within 3 micrometers from the surface 14S near to the solid
electrolyte layer 12 of the cathode 14' has been denoted as "a
solid electrolyte layer-side region". However, when the thickness
of the cathode 14' is less than or equal to 3 micrometers, the
whole of the cathode 14' may be configured as "a solid electrolyte
layer-side region".
EXAMPLES
[0078] Although the examples of the fuel 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. 22
[0079] As described below, Samples No. 1 to No. 22 of an anode
support cell were prepared in which the anode current collection
layer is configured as a support substrate.
[0080] Firstly, a green body for an anode current collection layer
(NiO:8YSZ=50:50 (Ni volume % conversion)) having a thickness of 500
micrometers was formed using a die press molding method. Then, a
green body for an anode active layer (NiO:8YSZ=45:55 (Ni volume %
conversion)) having a thickness of 20 micrometers was formed on the
green body for the anode current collection layer using a printing
method.
[0081] Then, a green body for an 8YSZ electrolyte having a
thickness of 5 micrometers and a green body for a GDC barrier film
having a thickness of 5 micrometers were formed in series on the
green body for the anode active layer to thereby form a laminated
body.
[0082] The laminated body was then co-sintered for two hours at
1400 degrees C. to obtain a co-fired body. Thereafter, a cathode
having a thickness of 30 micrometers was sintered with the
laminated body for 2 hours at 1000 degrees C., and thereby samples
No. 1 to No. 22 were prepared in an anode-support type coin cell
(.PHI.=15 mm).
[0083] As described in Table 1, the main component of the cathode
material was LSCF, LSF, and SSC. In samples No. 2, 7 and 13,
CoFe.sub.2O.sub.4 was added to the cathode material as (Co,
Fe).sub.3O.sub.4. In samples 4, 6, 14 and 20,
Co.sub.1.5Fe.sub.1.5O.sub.4 was added to the cathode material as
(Co, Fe).sub.3O.sub.4. In other samples, Co.sub.2FeO.sub.4 was
added to the cathode material as (Co, Fe).sub.3O.sub.4.
Measurement of Occupied Area Ratio
[0084] Firstly, the cathode of each of samples No. 1 to No. 22 was
polished with precision machinery, and then ion milling processing
was performed using an IM4000 manufactured by Hitachi
High-Technologies Corporation.
[0085] An SEM image of the cross section of the cathode enlarged
with a magnification of 10,000 times by the FE-SEM using the
in-lens secondary electron detector was acquired (reference is made
to FIG. 2).
[0086] Then, an analysis image was acquired by analyzing the SEM
image for each sample using HALCON image analysis software produced
by MVTec GmbH (Germany) (reference is made to FIG. 3).
[0087] Then, the occupied area ratio of the secondary phase
configured with (Co, Fe).sub.3O.sub.4 was calculated with reference
to the analysis image. The calculation results for the occupied
area ratio of the secondary phase are shown in Table 1.
Component Analysis of Secondary Phase
[0088] Next, component analysis of the secondary phase in samples
No. 1 to No. 22 was performed to identify the component material of
the secondary phase in each sample.
[0089] Firstly, a TEM image of the cathode cross section was
acquired by use of a transmission electron microscope (TEM). FIG. 6
illustrates an example of a TEM image of a cathode cross section,
and illustrates a secondary phase configured with
Co.sub.2FeO.sub.4. The position of the secondary phase was
confirmed with reference to the TEM image.
[0090] Next, energy dispersive x-ray spectroscopy (EDX) was used to
analyze the elements of the particles comprising the secondary
phase. FIG. 7 is a graph illustrating an example of an EDX spectrum
of the secondary phase configured with Co.sub.2FeO.sub.4.
Semi-quantitative analysis of the EDX spectrum enables an inference
regarding the component materials of the secondary phase. In FIG.
7, although Cu is detected, this is due to the component for the
sample holder in the analytic device and not a component material
of the secondary phase.
[0091] Next, the crystalline structure (lattice constant, lattice
configuration, crystal orientation) of the component particles of
the secondary phase was analyzed by selected area electron
diffraction (SAED). FIG. 8 illustrates an example of a SAED image
of a secondary phase configured with Co.sub.2FeO.sub.4. The
component materials of the secondary phase can be inferred by
analyzing the lattice constant, the lattice configuration, and the
crystal orientation based on the SAED image.
[0092] As a result of the analysis of the secondary phase of each
sample using the above methods, samples No. 2, 7 and 13 were
identified as CoFe.sub.2O.sub.4, samples No. 4, 6, 14 and 20 were
identified as Co.sub.1.5Fe.sub.1.5O.sub.4, and other samples were
identified as Co.sub.2FeO.sub.4.
Durability Testing
[0093] Samples No. 1 to No. 20 were heated to 750 degrees C. while
supplying nitrogen gas to the anode side and air to the cathode
side. When 750 degrees C. is reached, a reduction process was
performed for three hours while supplying hydrogen gas to the
anode.
[0094] Thereafter, the voltage depression rate per 1000 hours was
measured in relation to samples NO. 1 to No. 22 as a deterioration
rate. The output density at a rated current density of 0.2
A/cm.sup.2 and at a temperature of 750 degrees C. was used. The
measurement results are summarized in Table 1. A state of low
deterioration is evaluated for those samples in Table 1 in which
the deterioration rate was less than or equal to 1.5%.
[0095] After the durability testing above, the presence or absence
of cracks in the interior of the cathode was observed by
observation with an electron microscope of the cross section of the
cathode. The measurement results are summarized in Table 1.
TABLE-US-00001 TABLE 1 Equivalent Circle Diameter Main Type of
Content Ratio Occupied (micrometers) Component (Co,Fe).sub.3O.sub.4
(wt %) of Area Ratio of Particles of comprising
(Co,Fe).sub.3O.sub.4 (%) of comprising Presence or Cathode
Secondary in Cathode (Co,Fe).sub.3O.sub.4 Secondary Deterioration
Absence of Sample Material Phase Material in Cathode Layer Rate (%)
Microcracks Evaluation No. 1 LSCF Co.sub.2FeO.sub.4 0.14 0.16 0.33
0.51 Yes (minute) .largecircle. No. 2 LSCF Co.sub.2FeO.sub.4 0.17
0.19 0.08 0.26 Yes (minute) .largecircle. No. 3 LSCF
Co.sub.2FeO.sub.4 0.23 0.25 0.05 0.32 No .circleincircle. No. 4
LSCF Co.sub.1.5Fe.sub.1.5O.sub.4 0.47 0.52 0.34 0.44 No
.circleincircle. No. 5 LSCF Co.sub.2FeO.sub.4 0.79 0.88 0.18 0.38
No .circleincircle. No. 6 LSCF Co.sub.1.5Fe.sub.1.5O.sub.4 2.3 2.6
0.25 0.66 No .circleincircle. No. 7 LSCF CoFe.sub.2O.sub.4 3.8 4.2
0.15 0.83 No .circleincircle. No. 8 LSCF Co.sub.2FeO.sub.4 6.4 7.1
0.50 1.2 No .circleincircle. No. 9 LSCF Co.sub.2FeO.sub.4 8.6 9.5
0.62 1.5 No .circleincircle. No. 10 LSCF Co.sub.2FeO.sub.4 9.3 10.3
0.88 2.8 No X No. 11 LSF Co.sub.2FeO.sub.4 0.14 0.15 0.31 0.35 Yes
(minute) .largecircle. No. 12 LSF Co.sub.2FeO.sub.4 0.32 0.35 0.14
0.52 No .circleincircle. No. 13 LSF Co.sub.2FeO.sub.4 0.59 0.66
0.08 0.45 No .circleincircle. No. 14 LSF
Co.sub.1.5Fe.sub.1.5O.sub.4 1.4 1.5 0.39 0.76 No .circleincircle.
No. 15 LSF Co.sub.2FeO.sub.4 3.2 3.6 0.50 1.2 No .circleincircle.
No. 16 LSF Co.sub.2FeO.sub.4 5.1 5.6 0.42 0.91 No .circleincircle.
No. 17 LSF Co.sub.2FeO.sub.4 10 11.1 0.93 2.5 No X No. 18 SSC
Co.sub.2FeO.sub.4 0.19 0.21 0.28 0.45 Yes (minute) .largecircle.
No. 19 SSC Co.sub.2FeO.sub.4 0.30 0.33 0.35 0.65 No
.circleincircle. No. 20 SSC Co.sub.1.5Fe.sub.1.5O.sub.4 2.3 2.5
0.27 0.33 No .circleincircle. No. 21 SSC Co.sub.2FeO.sub.4 4.1 4.5
0.48 0.85 No .circleincircle. No. 22 SSC Co.sub.2FeO.sub.4 9.5 10.6
0.85 2.2 No X
[0096] As illustrated in Table 1, the deterioration rate of the
cathode was reduced to no more than 1.5% in those samples in which
the added amount of (Co, Fe).sub.3O.sub.4 to the cathode material
is less than or equal to 8.6 wt %. This is due to the fact that the
occupied area ratio of the secondary phase was suppressed to no
more than 9.5% by limiting the added amount of (Co,
Fe).sub.3O.sub.4 to the cathode material. More specifically, the
inactive region in the interior of the cathode is reduced by
suitably reducing the secondary phase. Therefore the progressive
deterioration of the cathode was suppressed as a result of
suppressing a reaction of the main phase with the secondary phase
during current flow.
[0097] As illustrated in Table 1, those samples in which the added
amount of (Co, Fe).sub.3O.sub.4 to the cathode material was at
least 0.25 wt % exhibited suppression of the production of cracks
in the interior of the cathode. This feature is due to the fact
that the occupied area ratio of the secondary phase was maintained
at more than or equal to 0.25% by maintaining the added amount of
(Co, Fe).sub.3O.sub.4 to the cathode material. More specifically,
the porous framework structure was strengthened as a result of an
improvement to sintering characteristics of the cathode by suitable
inclusion of the secondary phase.
[0098] As illustrated in Table 1, the deterioration rate is
suppressed to no more than 1.2% in samples exhibiting an average
value of the equivalent circle diameter in the secondary phase of
at least 0.05 micrometers to no more than 0.5 micrometers.
Preparation of Samples No. 23 to No. 44
[0099] Samples No. 23 to No. 44 were prepared as described
below.
[0100] Firstly, a green body for an anode current collection layer
(NiO:8YSZ=50:50 (Ni volume % conversion)) having a thickness of 500
micrometers was formed using a die press molding method. Then, a
green body for an anode active layer (NiO:8YSZ=45:55 (Ni volume %
conversion)) having a thickness of 20 micrometers was formed on the
green body for the anode current collection layer using a printing
method.
[0101] Then, a green body for an 8YSZ electrolyte having a
thickness of 5 micrometers and a green body for GDC barrier film
having a thickness of 5 micrometers were formed in series on the
green body for the anode active layer to thereby form a laminated
body.
[0102] The laminated body was then co-sintered for two hours at
1400 degrees C. to obtain a co-fired body.
[0103] Thereafter, a slurry was prepared by mixing cathode
materials including a secondary phase and a main phase as described
in Table 2, water and a binder in a ball mill for 24 hours.
[0104] The slurry for a first region was coated onto the
co-sintered body to thereby form a green body for the first region
of a cathode.
[0105] Thereafter, a slurry for the second region was prepared by
mixing LSCF, water and a binder in a mill.
[0106] The slurry for the second region was coated onto the green
body for the first region to thereby prepare a green body for the
second region of the cathode.
[0107] Next, the green body for the first region and the green body
for the second region were fired for two hours at 1000 degrees C.,
and thereby samples No. 23 to No. 44 were prepared in an
anode-support type coin cell (.PHI.=15 mm).
Measurement of Occupied Area Ratio
[0108] Firstly, the first region of each of samples No. 23 to No.
44 was polished with precision machinery, and then ion milling
processing was performed using an IM4000 manufactured by Hitachi
High-Technologies Corporation.
[0109] An SEM image of the cross section of the first region
enlarged with a magnification of 10,000 times by FE-SEM using an
in-lens secondary electron detector was acquired.
[0110] Then, an analysis image was acquired by analyzing SEM image
for each sample using HALCON image analysis software produced by
MVTec GmbH (Germany).
[0111] Next, the occupied area ratio of the secondary phase
configured with (Co, Fe).sub.3O.sub.4 was calculated with reference
to the analysis image. The calculation results for the occupied
area ratio of the secondary phase are shown in Table 2.
Component Analysis of Secondary Phase
[0112] Next, component analysis of the secondary phase in samples
No. 23 to No. 44 was performed in relation to the first region to
identify the component material of the secondary phase in each
sample.
[0113] Firstly, a TEM image of the cathode cross section was
acquired by use of the TEM.
[0114] Then, EDX was used to analyze the elements of the particles
comprising the secondary phase.
[0115] Next, the crystalline structure (lattice constant, lattice
configuration and crystal orientation) of the component particles
of the secondary phase was analyzed by SAED.
[0116] As a result of the analysis of the secondary phase of each
sample using the above method, sample No. 29 was identified as
CoFe.sub.2O.sub.4, samples No. 26, 28, 36 and 42 were identified as
Co.sub.1.5Fe.sub.1.5O.sub.4, and other samples were identified as
Co.sub.2FeO.sub.4.
Heat Cycle Testing
[0117] The process cycle in which heated from ambient temperature
to 750 degrees C. and fell to ambient temperature in four hours
while supplying nitrogen gas to the anode side and air to the
cathode side was repeated 10 times.
[0118] Thereafter, the presence or absence of delamination at the
interface between the cathode and the barrier layer was confirmed
by electron microscope observation of the cathode cross section for
each sample. The results are summarized in Table 2.
[0119] In Table 2, the samples that are confirmed to exhibit
delamination associated with a risk of an effect on the
characteristic properties of the cathode are evaluated by "X", the
samples that are confirmed to exhibit micro delamination not
associated with a risk of an effect on the characteristic
properties of the cathode are evaluated by "O ", and samples not
exhibiting delamination are evaluated by ".circle-w/dot.".
TABLE-US-00002 TABLE 2 Equivalent Circle Type of Diameter
(Co,Fe).sub.3O.sub.4 Content Ratio Occupied (micrometers)
comprising (wt %) of Area Ratio of Particles Presence or Main
Secondary (Co,Fe).sub.3O.sub.4 in (%) of comprising Absence of
Component of Phase of First Region (Co,Fe).sub.3O.sub.4 in
Secondary Interface Sample First Region First Region Material First
Region Layer Delamination Evaluation No. 23 LSCF Co.sub.2FeO.sub.4
0.14 0.16 0.33 Yes (minute) .largecircle. No. 24 LSCF
Co.sub.2FeO.sub.4 0.17 0.19 0.08 Yes (minute) .largecircle. No. 25
LSCF Co.sub.2FeO.sub.4 0.23 0.25 0.05 No .circleincircle. No. 26
LSCF Co.sub.1.5Fe.sub.1.5O.sub.4 0.47 0.52 0.34 No .circleincircle.
No. 27 LSCF Co.sub.2FeO.sub.4 0.79 0.88 0.18 No .circleincircle.
No. 28 LSCF Co.sub.1.5Fe.sub.1.5O.sub.4 2.3 2.6 0.25 No
.circleincircle. No. 29 LSCF CoFe.sub.2O.sub.4 3.8 4.2 0.15 No
.circleincircle. No. 30 LSCF Co.sub.2FeO.sub.4 6.4 7.1 0.50 No
.circleincircle. No. 31 LSCF Co.sub.2FeO.sub.4 8.6 9.5 0.62 No
.circleincircle. No. 32 LSCF Co.sub.2FeO.sub.4 9.3 10.3 0.88 Yes X
No. 33 LSF Co.sub.2FeO.sub.4 0.14 0.15 0.31 Yes (minute)
.largecircle. No. 34 LSF Co.sub.2FeO.sub.4 0.32 0.35 0.14 No
.circleincircle. No. 35 LSF Co.sub.2FeO.sub.4 0.59 0.66 0.08 No
.circleincircle. No. 36 LSF Co.sub.1.5Fe.sub.1.5O.sub.4 1.4 1.5
0.39 No .circleincircle. No. 37 LSF Co.sub.2FeO.sub.4 3.2 3.6 0.50
No .circleincircle. No. 38 LSF Co.sub.2FeO.sub.4 5.1 5.6 0.42 No
.circleincircle. No. 39 LSF Co.sub.2FeO.sub.4 10 11.1 0.93 Yes X
No. 40 SSC Co.sub.2FeO.sub.4 0.19 0.21 0.28 Yes (minute)
.largecircle. No. 41 SSC Co.sub.2FeO.sub.4 0.30 0.33 0.35 No
.circleincircle. No. 42 SSC Co.sub.1.5Fe.sub.1.5O.sub.4 2.3 2.5
0.27 No .circleincircle. No. 43 SSC Co.sub.2FeO.sub.4 4.1 4.5 0.48
No .circleincircle. No. 44 SSC Co.sub.2FeO.sub.4 9.5 10.6 0.85 Yes
X
[0120] As illustrated in Table 2, interface delamination was
suppressed in those samples in which the occupied area ratio of the
secondary phase in the first region was no more than 9.5% as a
result of the fact that the content ratio of (Co,Fe).sub.3O.sub.4
in the material of the first region was no more than 8.6 wt %. This
feature is due to the fact that the film strength of the first
region was enhanced by addition of (Co,Fe).sub.3O.sub.4, and
thereby an increase in the thermal expansion difference with the
barrier layer resulting from addition of (Co,Fe).sub.3O.sub.4 can
be suppressed.
[0121] As illustrated in Table 2, micro interface delamination was
also suppressed in those samples in which the occupied area ratio
of the secondary phase of the first region was no more than 0.25%
as a result of the fact that the content ratio of
(Co,Fe).sub.3O.sub.4 in the material of the first region was at
least 0.23 wt %.. This is due to the fact that the film strength of
the first region was sufficiently enhanced by maintaining the
addition amount of (Co,Fe).sub.3O.sub.4.
[0122] The cathode material of the present invention finds useful
application in the field of fuel cells for the purpose of enhancing
the durability of the cathode.
* * * * *