U.S. patent application number 13/756726 was filed with the patent office on 2013-06-13 for fuel electrodes for solid oxide electrochemical cell, processes for producing the same, and solid oxide electrochemical cells.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. The applicant listed for this patent is KABUSHIKI KAISHA TOSHIBA. Invention is credited to Takayuki Fukasawa, Yoshio Hanakata, Yoshiyasu Itoh, Tsuneji Kameda, Kentaro Matsunaga, Keizo Shimamura, Masato Yoshino.
Application Number | 20130149438 13/756726 |
Document ID | / |
Family ID | 40432193 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130149438 |
Kind Code |
A1 |
Fukasawa; Takayuki ; et
al. |
June 13, 2013 |
FUEL ELECTRODES FOR SOLID OXIDE ELECTROCHEMICAL CELL, PROCESSES FOR
PRODUCING THE SAME, AND SOLID OXIDE ELECTROCHEMICAL CELLS
Abstract
A fuel electrode for a solid oxide electrochemical cell
includes: an electrode layer constituted of a mixed phase including
an oxide having mixed conductivity and another oxide selected from
the group including an aluminum-based oxide and a magnesium-based
composite oxide, said another oxide having, supported on a surface
part thereof, particles of at least one member selected from
nickel, cobalt, and nickel-cobalt alloys; a meshy wiring formed on
a surface layer part of the electrode layer and made of a material
having higher electronic conductivity than the electrode layer; and
a current collector which overlies the electrode layer and is in
contact with at least the wiring.
Inventors: |
Fukasawa; Takayuki; (Tokyo,
JP) ; Shimamura; Keizo; (Tokyo, JP) ;
Hanakata; Yoshio; (Tokyo, JP) ; Yoshino; Masato;
(Tokyo, JP) ; Matsunaga; Kentaro; (Tokyo, JP)
; Kameda; Tsuneji; (Tokyo, JP) ; Itoh;
Yoshiyasu; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA; |
Tokyo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
40432193 |
Appl. No.: |
13/756726 |
Filed: |
February 1, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12204982 |
Sep 5, 2008 |
|
|
|
13756726 |
|
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Current U.S.
Class: |
427/115 |
Current CPC
Class: |
H01M 4/9025 20130101;
H01M 4/8885 20130101; H01M 4/9075 20130101; H01M 4/8621 20130101;
Y02P 70/50 20151101; H01M 8/0232 20130101; H01M 2008/1293 20130101;
H01M 8/0245 20130101; H01M 8/1016 20130101; H01M 8/1226 20130101;
H01M 8/1004 20130101; H01M 4/881 20130101; Y02E 60/50 20130101;
H01M 4/9041 20130101; H01M 8/0202 20130101 |
Class at
Publication: |
427/115 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2007 |
JP |
2007-230747 |
Claims
1. A process for producing a fuel electrode for a solid oxide
electrochemical cell comprising the steps of: producing a mixture
of oxide particles having mixed conductivity and nickel-aluminum
composite oxide particles, cobalt-aluminum composite oxide
particles, or composite oxide particles composed of a
nickel-aluminum composite oxide and a cobalt-aluminum composite
oxide; superposing a layer of the mixture on a surface of a solid
electrolyte and sintering the mixture layer; and reducing the
resultant sinter at a temperature of from 800.degree. C. to
1,000.degree. C.
2. The process of claim 1, wherein the oxide particles having mixed
conductivity are mixed with the nickel-aluminum composite oxide
particles, cobalt-aluminum composite oxide particles, or composite
oxide particles composed of a nickel-aluminum composite oxide and a
cobalt-aluminum composite oxide, in a ratio of from 50:50 to 95:5
by weight.
3. A process for producing a fuel electrode for a solid oxide
electrochemical cell comprising the steps of: producing a mixture
of oxide particles having mixed conductivity and nickel-magnesium
composite oxide particles, cobalt-magnesium composite oxide
particles, or composite oxide particles composed of a
nickel-magnesium composite oxide and a cobalt-magnesium composite
oxide; superposing a layer of the mixture on a surface of a solid
electrolyte and sintering the mixture layer; and reducing the
resultant sinter at a temperature of from 800.degree. C. to
1,000.degree. C.
4. The process of claim 3, wherein the oxide particles having mixed
conductivity are mixed with the nickel-magnesium composite oxide
particles, cobalt-magnesium composite oxide particles, or composite
oxide particles composed of a nickel-magnesium composite oxide and
a cobalt-magnesium composite oxide, in a ratio of from 50:50 to
95:5 by weight.
5. The process of claim 3, wherein the nickel-magnesium composite
oxide particles and the cobalt-magnesium composite oxide particles
contain at least one of Sc, Al, and Cr, the content of these
elements being from 0.01% by mole to 1.0% by mole based on the
magnesium-based composite oxides.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Division of U.S. patent application
Ser. No. 12/204,982 filed Sep. 5, 2008, the entire contents of
which are incorporated herein by reference.
[0002] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2007-230747, filed on Sep. 5, 2007, the entire contents of which
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field
[0004] The present invention relates to solid oxide electrochemical
cells such as, e.g., a solid oxide fuel cell (SOFC) and a solid
electrolyte high-temperature steam electrolysis cell (SOEC), fuel
electrodes for use in the cells, and processes for producing the
electrodes. 2. Description of the Related Art
[0005] A solid oxide fuel cell (SOFC) has an operating temperature
as high as about 700.degree. C.-1,000.degree. C. and hence a high
power generation efficiency, and is reduced in CO.sub.2 generation.
This fuel cell is hopeful as a next-generation clean power
generation system.
[0006] With respect to fuel electrode materials for solid
electrolyte fuel cells, a technique has been disclosed in which SDC
(CeO.sub.2 doped with Sm.sub.2O.sub.3) particles having
electron/oxygen ion mixed conductivity are used to highly
dispersedly deposit fine particles of nickel on the surface of the
SDC particles (see J. Electrochem. Soc., 141, [2], 342-346,
1994).
[0007] In this technique, nickel particles are formed in a porous
material constituted of an SDC network by the impregnation method
using, e.g., an aqueous metal salt solution. This technique has
succeeded in reducing the size of nickel particles by at least one
order of magnitude and in obtaining high catalytic activity with a
smaller nickel addition amount. In addition, since SDC further has
electronic conductivity, the boundary between each of all the fine
nickel particles and the SDC theoretically functions as a
three-phase interface. There is a description in that document to
the effect that bonding between the nickel and the SDC is
relatively satisfactory. However, because nickel particles are
formed by impregnation with the solution, burning, and reduction,
the particles change in size with time or the particles
unite/aggregate with one another during the burning step, resulting
in an uneven structure.
[0008] Furthermore, techniques concerning the use of
NiAl.sub.2O.sub.4 in a fuel electrode have been disclosed, which
include the use of a system including NiAl.sub.2O.sub.4 and NiO
(see Japanese Patent No. 3327789) and the use of a system including
nickel as a main component and further including NiAl.sub.2O.sub.4
(see JP-A 2003-242985). Moreover, a technique in which a solid
solution between NiO and MgAl.sub.2O.sub.4 is mixed with YSZ to
form an electrode and this electrode is reduced to precipitate
nickel has been disclosed (see JP-A 7-105956). There also is a
technique in which an NiO--MgO solid solution is used in a fuel
electrode (see JP-A 6-111829).
[0009] For reducing overvoltage and increasing catalytic activity
in a fuel electrode, it is necessary to use finer metal particles
as a catalyst and thereby increase the number of active sites.
However, in a high-temperature reducing atmosphere, the metal
particles are apt to readily move, grow, and aggregate.
Furthermore, it is difficult to incorporate nickel particles in an
unnecessarily large amount partly because of a difference in the
coefficient of thermal expansion. In addition, in case where abrupt
oxidation has occurred, the formation of an oxide results in volume
expansion and leads to the fear of causing cell breakage.
BRIEF SUMMARY OF THE INVENTION
[0010] According to the embodiments of the invention, examples of
oxides having, supported on a surface part thereof, particles of at
least one member selected from nickel, cobalt, and nickel-cobalt
alloys include an aluminum-based oxide (a first aspect) and a
magnesium-based composite oxide (a second aspect).
[0011] The invention may provide, according to a first aspect
thereof, a fuel electrode for a solid oxide electrochemical cell,
the fuel electrode comprising: an electrode layer comprising a
mixed phase constituted of an oxide having mixed conductivity and
an aluminum-based oxide having, supported on a surface part
thereof, particles of at least one member selected from nickel,
cobalt, and nickel-cobalt alloys; a meshy wiring formed on a
surface layer part of the electrode layer and comprising a material
having higher electronic conductivity than the electrode layer; and
a current collector which is in contact with the wiring.
[0012] The invention may provide, according to a second aspect
thereof, a fuel electrode for a solid oxide electrochemical cell,
the fuel electrode comprising: an electrode layer comprising a
mixed phase constituted of an oxide having mixed conductivity and a
magnesium-based composite oxide having, supported on a surface part
thereof, particles of at least one member selected from nickel,
cobalt, and nickel-cobalt alloys; a meshy wiring formed on a
surface layer part of the electrode layer and comprising a material
having higher electronic conductivity than the electrode layer; and
a current collector which is in contact with the wiring.
[0013] The invention may provide, according to a third aspect
thereof, a solid oxide electrochemical cell including: a solid
electrolyte plate having oxygen ion conductivity; the fuel
electrode according to the first or second aspect of the invention
formed on one side of the solid electrolyte plate; and an air
electrode formed on the other side of the solid electrolyte plate,
the air electrode comprising a composite oxide represented by
Ln.sub.1-xA.sub.xBO.sub.3-.delta. (wherein Ln is a rare-earth
element; A is Sr, Ca, or Ba; and B is at least one of Cr, Mn, Fe,
Co, and Ni) or comprising a composite phase constituted of that
composite oxide and any one of CeO.sub.2 doped with
Sm.sub.2O.sub.3, CeO.sub.2 doped with Gd.sub.2O.sub.3, and
CeO.sub.2 doped with Y.sub.2O.sub.3.
[0014] The invention may provide, according to a fourth aspect
thereof, a process for producing a fuel electrode for a solid oxide
electrochemical cell, the process comprising the steps of:
producing a mixture of oxide particles having mixed conductivity
and nickel-aluminum composite oxide particles, cobalt-aluminum
composite oxide particles, or composite oxide particles composed of
a nickel-aluminum composite oxide and a cobalt-aluminum composite
oxide; superposing a layer of the mixture on a surface of a solid
electrolyte and sintering the mixture layer; and reducing the
resultant sinter at a temperature of from 800.degree. C. to
1,000.degree. C.
[0015] The invention may provide, according to a fifth aspect
thereof, a process for producing a fuel electrode for a solid oxide
electrochemical cell, the process comprising the steps of:
producing a mixture of oxide particles having mixed conductivity
and nickel-magnesium composite oxide particles, cobalt-magnesium
composite oxide particles, or composite oxide particles composed of
a nickel-magnesium composite oxide and a cobalt-magnesium composite
oxide; superposing a layer of the mixture on a surface of a solid
electrolyte and sintering the mixture layer; and reducing the
resultant sinter at a temperature of from 800.degree. C. to
1,000.degree. C.
[0016] According to the embodiments of the invention, fuel
electrodes having high catalytic activity and high durability can
be provided, and solid oxide electrochemical cells which are stable
and have excellent output performance can be realized. Furthermore,
inexpensive production techniques such as screen printing and spray
coating are applicable, whereby the fuel electrodes can be produced
at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagrammatic sectional view illustrating the
structure of a solid oxide fuel cell (SOFC) according to a first
embodiment of the invention.
[0018] FIG. 2 is a diagrammatic sectional view illustrating a
three-phase interface in a fuel electrode according to the first
embodiment.
[0019] FIG. 3 is a flowchart showing steps for producing the fuel
electrode according to the first embodiment.
[0020] FIG. 4 is diagrammatic sectional views and a diagrammatic
plan view which illustrate structures obtained in steps for
producing the fuel electrode according to the first embodiment.
[0021] FIG. 5 is a presentation showing a change in weight of
NiAl.sub.2O.sub.4 during hydrogen reduction in an Example according
to the first embodiment.
[0022] FIGS. 6A, 6B and 6C are SEM photographs of reduced
NiAl.sub.2O.sub.4 in an Example according to the first embodiment
and of a Comparative Example.
[0023] FIG. 7 is a flowchart showing steps for producing the fuel
electrode according to a second embodiment of the invention.
[0024] FIG. 8 is a presentation showing changes in weight of
nickel-magnesium composite oxide solid solutions during hydrogen
reduction in an Example according to the second embodiment.
[0025] FIG. 9 is an SEM photograph of a reduced nickel-magnesium
composite oxide in an Example according to the second
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The solid oxide fuel cells, fuel electrodes for use therein,
and processes for fuel electrode production according to the
embodiments of the invention are explained below. However, the
invention should not be construed as being limited to the following
embodiments and Examples. The diagrammatic views which are referred
to in the following explanations are for illustrating the
positional relationship between constituents, and the size of the
particles, the thickness ratio among the layers, etc. do not always
coincide with actual ones.
First Embodiment
[0027] A first embodiment of the invention relates to the fuel
electrode according to the first aspect of the invention, the
process for producing this fuel electrode, and the solid oxide fuel
cell employing this fuel electrode.
[0028] First, the solid oxide fuel cell according to this
embodiment is explained by reference to the diagrammatic sectional
view of FIG. 1. This fuel cell has a multilayer structure composed
of a solid electrolyte plate 11, a fuel electrode material 12
disposed on one side of the plate 11, and an air electrode material
13 disposed on the other side thereof.
[0029] The air electrode 13 includes a composite oxide 18 which is
an oxide showing mixed conductivity and represented by the general
formula Ln.sub.1-xA.sub.xBO.sub.3-.delta. (wherein Ln is a
rare-earth element; A is Sr, Ca, or Ba; and B is at least one of
Cr, Mn, Fe, Co, and Ni). The composite oxide 18 efficiently
dissociates oxygen and has electronic conductivity. The composite
oxide is slightly deficient in ionic conductivity. This deficiency
may be compensated for by further adding an ionically conductive
oxide 16. As this ionically conductive oxide, use is made of any
one of CeO.sub.2 doped with Sm.sub.2O.sub.3, CeO.sub.2 doped with
Gd.sub.2O.sub.3, and CeO.sub.2 doped with Y.sub.2O.sub.3. Such
oxides are materials which show mixed conductivity in a reducing
atmosphere but show high ionic conductivity in an oxygen-containing
atmosphere and which do not react with that oxide showing mixed
conductivity. The term "mixed conductivity" herein means a property
simultaneously showing electronic conductivity and oxygen ion
conductivity.
[0030] It is likewise thought that a reaction can take place also
at the interface between the air electrode 13, which contains the
composite oxide 18 having mixed conductivity, and the solid
electrolyte plate 11. It is therefore preferred to form beforehand
a thin and dense reaction-preventive layer at the interface between
the solid electrolyte plate 11 and the air electrode 13 in order to
inhibit the reaction. As a material for forming the
reaction-preventive layer also, a ceria-based ion conductor is
effective. A thickness of about 1 .mu.m suffices for this
layer.
[0031] Oxygen ions (O.sup.2-) formed by dissociation in the air
electrode 13 pass through the solid electrolyte 11 and move to the
fuel electrode side, where the oxygen ions react with hydrogen to
yield water. Electrons generate upon this reaction and are taken
out through an external circuit to conduct power generation.
[0032] The dissociation of oxygen on the air electrode 13 side and
the reaction of hydrogen with oxygen ions on the fuel electrode 12
side each occur at those three-phase interfaces in the electrode
where all of the catalyst, oxygen ion conductor, and feed gas, are
present. Because of this, how to form such three-phase interfaces
in a larger amount is an important subject.
[0033] In general fuel electrodes 12 heretofore in use, a cermet
obtained by mixing nickel particles functioning as a catalyst and
having electronic conductivity with stabilized zirconia having
oxygen ion conductivity (such as YSZ) and sintering the mixture has
been employed. The nickel particles to be used for constituting
this cermet are produced by reducing NiO and, hence, generally have
a size of from several hundred nanometers to several micrometers,
which is close to the size of the NiO particles used as a raw
material. In the conventional technique, the nickel particles
having such a size are connected to one another to constitute a
network for electron conduction.
[0034] A measure which is thought to be effective in improving
catalytic activity is to reduce the size of nickel particles to
thereby increase the specific surface area. It is actually known
that a fuel electrode produced using nickel particles having a
reduced particle diameter obtained by a method in which nickel
particles are precipitated from a solution of, e.g., a nitric acid
salt or a method in which particles are mechanically pulverized as
in the production of mechanical alloys shows high catalytic
activity.
[0035] However, to reduce metal particles into finer particles
results in a higher tendency to particle growth due to nickel
particle sintering although it heightens activity. In addition, in
a high-temperature reducing atmosphere, metal particles readily
move. When the temperature is high, there has been a problem that
metal particles move to a surface layer part and an inner part of
the electrode increases in resistance. For overcoming such a
problem, it is necessary to take a measure in reducing the particle
diameter of the catalyst and simultaneously inhibiting nickel
movement/aggregation and thereby preventing internal resistance
from increasing. Moreover, in case where nickel is added in an
increased amount in order to heighten catalytic activity, this
results in an increase in the coefficient of thermal expansion to
form a cause of breakage of the cell itself. Because of this, the
amount of nickel is generally regarded as up to about 60% by weight
at the most.
[0036] The present inventors diligently made investigations in
order to eliminate those problems. As a result, it has been found
that a technique in which a composite oxide solid solution
(catalyst precursor) containing a metal functioning as an electrode
catalyst is used to constitute an electrode beforehand and then
reduced to form metal particles as a precipitate from the solid
solution is effective in reducing the size of particles and fixing
the particles to a base.
[0037] On the other hand, the nickel particles precipitated by
reduction are fine but are discontinuously present on the
insulator. It was thought that because of such state of the nickel
particles, the resistance of contact with a current collector
member is high. It has been found that to form beforehand a meshy
wiring having electronic conductivity in a surface layer part of
the electrode in order to compensate for a deficiency in electronic
conductivity is effective in inhibiting the contact resistance of
the fuel electrode from increasing due to current collection.
[0038] Namely, as shown in FIG. 1, the fuel electrode 12 according
to the first embodiment includes a mixed phase (electrode layer)
constituted of an oxide 16 having mixed conductivity and an
aluminum-based oxide 17 having, supported on a surface part
thereof, particles of at least one member selected from nickel,
cobalt, and nickel-cobalt alloys. This fuel electrode 12 further
includes: a wiring 21 formed on a surface layer part of the
electrode layer and made of a material having higher electronic
conductivity than the electrode layer; and a current collector 14
electrically connected to the wiring 21.
[0039] FIG. 2 is a diagrammatic view of a three-phase interface in
the fuel electrode 12. This figure diagrammatically illustrates a
reaction occurring at an interface where the catalyst Ni, material
having mixed conductivity (e.g., YSZ constituting the electrolyte
plate 11 or SDC as the oxides having mixed conductivity in the
oxygen-ion-conductor particles), and feed gas H.sub.2 exist. A
process for producing this fuel electrode 12 is explained
below.
[0040] An example of production steps in this embodiment is shown
in FIG. 3, and diagrammatic views of electrode structures are shown
in FIG. 4.
[0041] First, an NiO powder is mixed with an Al.sub.2O.sub.3
powder, and the mixture is burned to produce a nickel-aluminum
composite oxide solid solution represented by NiAl.sub.2O.sub.4.
This solid solution is pulverized and used as particles 19. The
powder thus obtained by pulverization has a particle diameter of
preferably about 0.5-20 .mu.m. For producing such a powder, use may
be made of a method in which an aqueous solution of a metal salt
such as, e.g., a nitrate, is used to mix the ingredients and the
mixture is pyrolyzed and burned to produce the target powder.
[0042] Subsequently, the composite oxide solid-solution particles
19 thus produced are mixed with particles 16 having electron/ion
mixed conductivity, and water is added thereto to obtain a paste.
Examples of the particles 16 having mixed conductivity to be used
include CeO.sub.2 doped with Sm.sub.2O.sub.3, CeO.sub.2 doped with
Gd.sub.2O.sub.3, and CeO.sub.2 doped with Y.sub.2O.sub.3. However,
the particles 16 should not be construed as being limited to these,
and any particles having high oxygen ion conductivity and high
electronic conductivity at temperatures of from 700.degree. C. to
1,000.degree. C. may be used.
[0043] The powder mixture in a paste form is then applied to a
surface of a solid electrolyte plate 11 by screen printing, and the
coating is burned by heating to a temperature which results in an
elevated strength of bonding between the plate 11 and the coating.
In general, it is preferred to burn the coating at a temperature in
the range of from 1,200.degree. C. to 1,400.degree. C. Methods for
disposing particles 16 having mixed conductivity and composite
oxide solid-solution particles 19 on a solid electrolyte plate 11
should not be construed as being limited to the method described
above. Use may be made of a method in which the powder mixture is
slurried and this slurry is applied by roller coating, dipping, or
spray coating to produce the target structure. Alternatively, the
slurry may be formed into a sheet and then superposed.
[0044] In FIG. 4, the particles 16 and 19 are exaggeratingly
illustrated for easy understanding. However, the actual particles
have been bonded together and united because of sintering and
constitute a network. From the standpoint of gas diffusibility in
the electrode, it is preferred that the electrode layer should be
porous. A pore-forming material which disappears upon burning to
form pores may be incorporated beforehand. Examples of the
pore-forming material include organic ones such as, e.g., acrylic
spherical particles.
[0045] A processing for heightening the efficiency of current
collection, which is a feature of this embodiment, is further
conducted. In a final material constitution, metal particles
functioning as a catalyst and having electronic conductivity are
fine particles which are isolated and dispersed. It is therefore
necessary to take a measure for securing sufficient contact with a
current collector 14. In ordinary techniques, a material capable of
serving as a current collector, such as a metal mesh, is pushed
against the electrode to establish contact. In this embodiment,
however, a meshy wiring 21 is formed on the electrode layer by
printing with a material having higher electronic conductivity than
the electrode layer, and this meshy wiring is brought into contact
with a current collector 14 to thereby secure current collection
(FIGS. 4(b-1) and (b-2)).
[0046] A line width of about 30 .mu.m and a line-to-line distance
of about 500 .mu.m suffice for the wiring 21. This wiring 21 exerts
almost no influence on fuel diffusion. Although the line width,
line-to-line distance, and wiring pattern are not limited to these,
it is preferred that the area occupied by the wiring part should
account for up to 40% of the whole electrode surface. As a material
for forming the wiring through printing, a mixture of a metal such
as Pt, Au, Ni, Co, or Fe and a material having mixed conductivity,
such as SDC or GDC, to be simultaneously used in the electrode
layer is used in the form of a paste. The two ingredients are mixed
preferably in such a ratio that the proportion of the metal is
40-90 vol % based on the whole. This is because not only such a
mixing ratio improves tight contact and adhesion to the electrode
layer but also the wiring itself can be expected to function as a
catalyst.
[0047] The printed wiring part is burned. Thereafter, the fuel
electrode 12 is subjected to a reduction treatment in a reducing
atmosphere having a temperature of from 800.degree. C. to
1,000.degree. C. Ordinary NiO reduction treatments are conducted at
a temperature of about 900.degree. C. without using an
unnecessarily high temperature. However, in this embodiment, in
which NiAl.sub.2O.sub.4 is used as a main component, it is more
preferred to conduct the reduction at 950.degree. C. or higher for
the purpose of sufficiently precipitating nickel. Although the
period of reduction is not particularly limited, about 10 minutes
may suffice.
[0048] Through the reduction, the nickel ingredient present in the
solid solution state in the NiAl.sub.2O.sub.4 part 19 precipitates
on the surface to give a base made of an aluminum oxide 17 (mainly
Al.sub.2O.sub.3). Namely, nickel-particle-supporting
Al.sub.2O.sub.3 17 is formed. The fine metal particles thus formed
have a size of generally tens of nanometers. From the standpoint of
enabling the fine metal particles to have high catalytic activity,
the size thereof is preferably about from 5 nm to 200 nm. Metal
particles having a size smaller than 5 nm are difficult to produce
actually. Metal particles larger than 200 nm are apt to bond to
adjoining ones and this may pose the same problem as in the
conventional technique in which NiO is used after having been
reduced. A more preferred range of the size of the fine metal
particles as a catalyst is about from 20 nm to 100 nm. This size is
smaller than the conventional electrode catalyst sizes by one to
two orders of magnitude. An improvement in catalytic activity is
hence expected. Consequently, the amount of the NiAl.sub.2O.sub.4
to be added is preferably in the range of from 5% by weight to 50%
by weight based on all materials constituting the electrode layer.
More preferably, the amount thereof is from 10% by weight to 30% by
weight.
[0049] According to this embodiment, a reduction in catalyst amount
can be attained and, hence, the proportion of the material having
mixed conductivity can be increased. Consequently, the difference
in thermal expansion between the electrode layer and the solid
electrolyte and the difference due to conformation mismatch can be
further reduced.
[0050] The metal particles precipitated are present on a surface
part of the Al.sub.2O.sub.3 as a base so as to form only one layer.
These particles have satisfactory conformability to the base and
are tenaciously bonded thereto. Consequently, the metal particles
are characterized in that even when exposed to a high-temperature
reducing atmosphere, the particles do not readily move.
[0051] In addition, since the metal particles are fine and
isolated, this fuel electrode has an advantage that even when the
electrode layer undergoes abrupt oxidation, the resultant volume
expansion is only local and is less apt to lead to breakage.
[0052] In each of Japanese Patent No. 3327789 and JP-A 2003-242985,
NiAl.sub.2O.sub.4 is added for the purposes of imparting the same
coefficient of thermal expansion as the YSZ used as a solid
electrolyte and of inhibiting the nickel particles from growing or
aggregating. These documents include no statement at all concerning
the technique of precipitating fine nickel particles from the
NiAl.sub.2O.sub.4. On the other hand, in JP-A 7-105956, YSZ is
added in an amount of about 10 vol % and there is a possibility
that such a YSZ amount might be too small to sufficiently secure
oxygen ion conductivity.
[0053] As described above, in producing a fuel electrode according
to this embodiment, fine nickel particles can be fixed to a base.
In addition, according to this fuel electrode, high activity and
long-term stability can be provided with a small nickel addition
amount. When this fuel electrode is used in combination with an air
electrode employing a preferable electrode catalyst, inexpensive
high-output cells which may be not only a flat cell but also a
cylindrical or electrode-supported cell or the like can be
realized.
[0054] The nickel-particle-supporting Al.sub.2O.sub.3 produced by
reducing NiAl.sub.2O.sub.4 is usable also as a catalyst for the
reforming of hydrocarbon fuels including methane. Namely, it is
applicable to a variety of fuels.
[0055] This embodiment is explained below in more detail by
reference to the following Examples. Electrodes employing
Sm.sub.2O.sub.3-doped CeO.sub.2 as particles having mixed
conductivity, ones employing Gd.sub.2O.sub.3-doped CeO.sub.2 as
particles having mixed conductivity, and ones employing
Y.sub.2O.sub.3-doped CeO.sub.2 as particles having mixed
conductivity all showed the same tendency.
[0056] Consequently, in the following Examples, cells employing
Sm.sub.2O.sub.3-doped CeO.sub.2 as the only example are explained.
The particle diameter and other properties of each of the powders
used should not be construed as being limited to those shown
below.
[0057] <Solid Electrolyte>
[0058] In all Examples and Comparative Examples, YSZ (ZrO.sub.2
stabilized with 8 mol % Y.sub.2O.sub.3) processed so as to have a
diameter of 15 mm and a thickness of 500 .mu.m was used as a solid
electrolyte.
[0059] In all Examples and Comparative Examples, that solid
electrolyte plate was used, and a porous platinum electrode was
used as an air electrode.
COMPARATIVE EXAMPLE 1
[0060] An NiO powder having an average particle diameter of about 1
.mu.m and an SDC powder (Sm.sub.0.2Ce.sub.0.8O.sub.2) having an
average particle diameter (D.sub.50) of about 0.3 .mu.m were
weighed out in such amounts as to result in a weight ratio of
50:50. Pure water was added to the powders in an amount of about
50% based on the total weight thereof. The resultant mixture was
treated with a high-speed rotary mixer to obtain a paste. Using a
screen printer, the paste was printed in a size with a diameter of
6 mm on a central part of a YSZ solid electrolyte plate. After the
printing, the solid electrolyte plate was placed in an atmospheric
furnace and the paste was burned at 1,300.degree. C. for 2 hours.
Thereafter, a platinum electrode likewise having a diameter of 6 mm
was deposited on the opposite side by screen printing and burned at
950.degree. C. for 1 hour to obtain an air electrode.
COMPARATIVE EXAMPLE 2
[0061] A nickel paste containing SDC particles (in such an amount
as to result in an SDC/Ni ratio of 50:50 by weight) was produced.
This nickel paste was applied to the fuel electrode of the sample
produced in Comparative Example 1, by screen printing through a
screen mesh produced so as to give a wiring having a line width of
about 30 .mu.m and a line-to-line distance of about 500 .mu.m.
COMPARATIVE EXAMPLE 3
[0062] A sample was produced under the same conditions as in
Comparative Example 1, except that the NiO powder and the SDC
powder were mixed in a ratio of 30:70 by weight. The same meshy
wiring as that shown in Comparative Example 2 was formed by
printing on a surface layer part of the fuel electrode
produced.
EXAMPLES 1 TO 3
[0063] An NiO powder having an average particle diameter of about 1
.mu.m and an Al.sub.2O.sub.3 powder having an average particle
diameter of about 0.4 .mu.m were weighed out in such amounts as to
result in a molar ratio of 1:1. The powders were mixed together by
means of a mortar. The resultant powder mixture was press-molded,
and the molding was sintered at 1,300.degree. C. of 5 hours in the
air. The constituent phases of the sinter obtained were examined by
X-ray diffractometry.
[0064] Subsequently, the sinter was pulverized and passed through a
40-.mu.m mesh sieve to obtain a starting powder (nickel-aluminum
composite oxide). The pulverized composite oxide particles were
mixed with SDC (Sm.sub.0.2Ce.sub.0.8O.sub.2) particles having an
average particle diameter of 0.3 .mu.m in each of such ratios as to
result in pulverized-particle proportions of 8, 20, and 50% by
weight. Thus, respective powder mixtures were prepared (Examples 1
to 3).
[0065] To each of the powder mixtures was added about 40% by weight
pure water. The resultant mixtures each were treated with a
high-speed rotary mixer to obtain pastes. Using a screen printer,
each paste was printed in a size with a diameter of 6 mm on a
central part of a YSZ solid electrolyte plate. After the printing,
the solid electrolyte plates were placed in an atmospheric furnace
and the pastes were burned at 1,300.degree. C. for 2 hours.
Subsequently, a platinum electrode was likewise deposited by
printing on the opposite side of each solid electrolyte plate and
burned at 950.degree. C. for 1 hour to obtain an air electrode. The
same meshy wiring as that shown in Comparative Example 2 was formed
by printing on a surface layer part of each fuel electrode
produced.
COMPARATIVE EXAMPLE 4
[0066] The same structure as that produced in Example 2 was
produced, except that the meshy wiring on the surface layer part
was omitted. Thus, the structure of Comparative Example 4 was
prepared.
EXAMPLES 4 AND 5
[0067] Samples were produced in the same manner as in Examples 1 to
3, except that the proportions of the nickel-aluminum composite
oxide particles were changed to 3 or 55% by weight (Examples 4 and
5, respectively). The same meshy wiring as that shown in
Comparative Example 2 was formed by printing on a surface layer
part of each fuel electrode produced.
EXAMPLE 6
[0068] A CoO powder having an average particle diameter of about 1
.mu.m and an Al.sub.2O.sub.3 powder having an average particle
diameter of about 0.4 .mu.m were weighed out in such amounts as to
result in a molar ratio of 1:1. The powders were mixed together by
means of a mortar. The resultant powder mixture was press-molded,
and the molding was sintered at 1,300.degree. C. of 5 hours in the
air. The constituent phases of the sinter obtained were examined by
X-ray diffractometry.
[0069] Subsequently, the sinter was pulverized and passed through a
40-.mu.m mesh sieve to obtain a starting powder (cobalt-aluminum
composite oxide). The pulverized composite oxide particles were
mixed with SDC particles having an average particle diameter of 0.3
.mu.m in such a ratio as to result in a pulverized-particle
proportion of 20% by weight. Thus, a powder mixture was prepared.
To the powder mixture was added about 40% by weight pure water. The
resultant mixture was treated with a high-speed rotary mixer to
obtain a paste. Using a screen printer, the paste was printed in a
size with a diameter of 6 mm on a central part of a YSZ solid
electrolyte plate.
[0070] After the printing, the solid electrolyte plate was placed
in an atmospheric furnace and the paste was burned at 1,300.degree.
C. for 2 hours. Subsequently, a platinum electrode was likewise
deposited by printing on the opposite side of the solid electrolyte
plate and burned at 950.degree. C. for 1 hour to obtain an air
electrode. The same meshy wiring as that shown in Comparative
Example 2 was formed by printing on a surface layer part of the
fuel electrode produced.
[0071] <Cell Characteristics Evaluation Test>
[0072] Each flat cell produced was set on an SOFC output
characteristics evaluation apparatus, and the fuel electrode side
and the air electrode side each were sealed with a Pyrex (R) glass
material. A platinum wire having a diameter of 0.1 mm was attached
as a reference electrode to a side of the electrolyte plate. The
cell was heated in an argon atmosphere. Thereafter, hydrogen was
introduced into the fuel electrode to conduct a reduction
treatment. In Comparative Examples 1 to 3, the treatment was
conducted at 900.degree. C. for 1 hour. In Examples 1 to 6 and
Comparative Example 4, the treatment was conducted at 1,000.degree.
C. for 10 minutes.
[0073] Subsequently, H.sub.2+H.sub.2O were introduced into the fuel
electrode at 50 cc/min, and dry air+argon were introduced into the
air electrode at 50 cc/min (dry air, 10 cc/min; argon, 40 cc/min).
Thus, the cell was evaluated for output characteristics.
Furthermore, an impedance measurement was made by the current
interruption method.
[0074] An explanation is made on the results. The thickness of each
electrode layer formed through one screen printing operation was
about 20 .mu.m. The results of the X-ray diffraction test revealed
that in Comparative Examples 1 to 3, the components of the fuel
electrode after the reduction were nickel and SDC. In contrast, in
Examples 1 to 6 and Comparative Example 4, peaks attributable to
SDC and NiAl.sub.2O.sub.4 were detected in the composition before
the reduction, and peaks attributable to SDC, Ni/Co, and
Al.sub.2O.sub.3 were detected in the composition after the
reduction.
[0075] The NiAl.sub.2O.sub.4 was examined with a thermo-gravimetric
analyzer (TG) for weight change with reduction in hydrogen. The
results obtained are shown in FIG. 5. Weight decrease, i.e., nickel
precipitation, began at around 800.degree. C., and a weight loss of
about 6% was observed at 1,000.degree. C. Reduction at
1,000.degree. C. means that the fuel electrodes produced can be
used more stably because the electrodes include metal particles
precipitated at high temperatures and are used at lower
temperatures.
[0076] An examination was further made with an SEM. A comparison
between the results for an SDC--NiO material (Comparative Example
1) and those for an SDC--NiAl.sub.2O.sub.4 material (Example 2) is
shown in FIGS. 6A, 6B and 6C. It was ascertained that in
Comparative Example 1, for which NiO had been used, nickel
particles having a size of about 1 .mu.m were present among SDC
particles. In contrast, in the system for which NiAl.sub.2O.sub.4
had been used, the presence of SDC particles and of Al.sub.2O.sub.3
particles having nickel particles with a diameter of from tens of
nanometers to 100 nm precipitated on the surface thereof was
ascertained. The nickel particles precipitated were highly
dispersed on the Al.sub.2O.sub.3 without overlapping.
[0077] In Table 1 are shown the results concerning maximum output
density obtained by I-V characteristics evaluation. In the table,
the Catalyst precursor amount means the proportion of NiO or
NiAl.sub.2O.sub.4.
TABLE-US-00001 TABLE 1 Constituent Constituent Catalyst Output
phases before phases after precursor amount density reduction
reduction (wt %) Meshy wiring (mW/cm.sup.2) Example 1
NiAl.sub.2O.sub.4-SDC Ni/Al.sub.2O.sub.3-SDC 8 present 122 Example
2 NiAl.sub.2O.sub.4-SDC Ni/Al.sub.2O.sub.3-SDC 20 present 120
Example 3 NiAl.sub.2O.sub.4-SDC Ni/Al.sub.2O.sub.3-SDC 50 present
101 Comparative NiO-SDC Ni-SDC 50 absent 120 Example 1 Comparative
NiO-SDC Ni-SDC 50 present 121 Example 2 Comparative NiO-SDC Ni-SDC
30 present 90 Example 3 Comparative NiAl.sub.2O.sub.4-SDC
Ni/Al.sub.2O.sub.3-SDC 20 absent 34 Example 4 Example 4
NiAl.sub.2O.sub.4-SDC Ni/Al.sub.2O.sub.3-SDC 3 present 90 Example 5
NiAl.sub.2O.sub.4-SDC Ni/Al.sub.2O.sub.3-SDC 55 present 85 Example
6 CoAl.sub.2O.sub.4-SDC Co/Al.sub.2O.sub.3-SDC 20 present 105
Example 7 (Ni,Mg)O-SDC Ni/(Ni,Mg)O-SDC 8 present 124 Example 8
(Ni,Mg)O-SDC Ni/(Ni,Mg)O-SDC 20 present 123 Example 9 (Ni,Mg)O-SDC
Ni/(Ni,Mg)O-SDC 50 present 99 Comparative (Ni,Mg)O-SDC
Ni/(Ni,Mg)O-SDC 20 absent 35 Example 5 Comparative (Ni,Mg)O-SDC
Ni/(Ni,Mg)O-SDC 3 present 98 Example 6 Comparative (Ni,Mg)O-SDC
Ni/(Ni,Mg)O-SDC 55 present 85 Example 7 Example 10 (Co,Mg)O-SDC
Co/(Co,Mg)O-SDC 20 present 108
[0078] As apparent from Table 1, in the case of the NiO--SDC
systems, which are conventional materials, output characteristics
tended to decrease with decreasing nickel amount. This may be
because a decrease in nickel amount results in difficulties in
forming electron-conducting paths. In contrast, in the Examples,
output characteristics tended to improve even with small nickel
amounts. This is thought to be mainly because the amount of
Al.sub.2O.sub.3, which is an insulator, decreased and this resulted
in reduced internal resistance of the electrode and also in an
enlargement of SDC paths, which conduct oxygen ions. Conversely
speaking, the electrodes have sufficiently high catalytic activity
even with small nickel amounts.
[0079] It was further found that the formation of a meshy wiring on
a surface layer part by printing was highly effective in the
SDC--NiAl.sub.2O.sub.4 systems although it exerted almost no
influence in the conventional SDC--NiO systems. Namely, in the
techniques according to the Examples, in which NiAl.sub.2O.sub.4 is
used, contact with the current collector for current collection is
important although catalytic activity improves with reducing
catalyst size. It has become obvious that the formation of a meshy
wiring on a surface layer part by printing is highly effective in
reducing the loss to be caused by current collection contact
resistance.
[0080] It can be seen from those results that output
characteristics equal to or higher than those of conventional
materials can be imparted with smaller catalyst amounts according
to the Examples. This is thought to be mainly caused by an
increased amount of three-phase interfaces due to the catalyst
particle size reduction and the resultant increase in catalytic
activity.
[0081] In the SDC--NiO materials shown in Comparative Examples 1 to
3, the nickel particles in the materials before a power generation
test had a size of about 1 .mu.m. However, after a 300-hour power
generation test at 900.degree. C., a decrease in output density of
about 25% was observed. In the structure of each of these materials
which had undergone the test, it was observed that the nickel
particles had grown (sintered) into particles larger than a
two-fold size.
[0082] In contrast, the electrode catalysts of the Examples had not
undergone the uniting of nickel particles although slight particle
growth was observed. No decrease in output density was also
observed. Namely, the results obtained show that the catalysts are
more effective also from the standpoint of durability. This is
attributable to the fact that these catalysts were synthesized at a
higher temperature than the conventional NiO-based materials, and
means that the catalysts according to the first embodiment have
excellent thermal stability.
[0083] Also in the case of using a cobalt-aluminum composite oxide
as a precursor, cobalt particle precipitation occurred and the same
electrode catalyst effects were ascertained.
[0084] Furthermore, cells employing the fuel electrodes according
to the Examples in combination with various air electrodes were
found to show satisfactory output performance. As air-electrode
materials, use can be made of, for example, one represented by the
general formula Ln.sub.1-xA.sub.xBO.sub.3-5 (wherein Ln is a
rare-earth element; A is Sr, Ca, or Ba; and B is at least one of
Cr, Mn, Fe, Co, and Ni), such as (La, Sr)(Co, Fe)O.sub.3, besides
platinum electrodes. Ionically conductive particles such as
CeO.sub.2 doped with Sm.sub.2O.sub.3, CeO.sub.2 doped with
Gd.sub.2O.sub.3, or CeO.sub.2 doped with Y.sub.2O.sub.3 may be
further added to those air-electrode materials. A fuel cell system
produced from one or more cells employing such a combination was
ascertained to have high durability and show high-performance
power-generating properties. Such cells can be extensively applied
not only to the flat type but also to the cylindrical type and
fuel-electrode-supported type.
Second Embodiment
[0085] A second embodiment of the invention relates to the fuel
electrode according to the second aspect of the invention and the
process for producing this fuel electrode. The fuel electrode which
will be explained below can be applied to solid oxide
electrochemical cells including the solid oxide fuel cell explained
by reference to FIG. 1.
[0086] An example of production steps for use in this embodiment is
explained by reference to FIG. 7. In this example, nickel is used
as a catalyst.
[0087] First, an NiO powder, an MgO powder, and a slight amount of
an Sc.sub.2O.sub.3 powder are mixed together, and the mixture is
burned to produce an MgO--NiO solid solution. Because the
Sc.sub.2O.sub.3 amount is slight, the Sc.sup.3+ ions are finally
incorporated in the solid solution. When this nickel-magnesium
composite oxide solid solution is reduced at 800-1,000.degree. C.,
more preferably at a temperature of from 900.degree. C. to
1,000.degree. C., nickel particles having a size of tens of
nanometers precipitate on the surface. Thus, the solid solution
becomes a composite including a magnesium-based oxide having fine
nickel particles supported thereon.
[0088] Other examples of such systems which form a solid solution
with magnesium oxide and from which fine metal particles
precipitate upon reduction include a CoO--MgO solid solution
system. In this system also, the precipitation of metal particles
can be accelerated with a slight amount of an additive element.
Examples of such substances having the effect of accelerating the
precipitation include Al.sub.2O.sub.3 and Cr.sub.2O.sub.3 besides
Sc.sub.2O.sub.3.
[0089] An amount of about 0.01-1.0% by mole based on the
magnesium-based composite oxide solid solution as a base suffices
for those additive ingredients. By adding such an additive
ingredient in such a slight amount, the amount of particles to be
precipitated and the specific surface area of the metal can be
improved by about one order of magnitude as compared with the case
in which the minor ingredient is not added at all.
[0090] Subsequently, this sinter is pulverized into particles. The
powder obtained by pulverization has a particle diameter of
preferably from 0.5 .mu.m to 20 .mu.m. For producing such a powder,
use may be made of a method in which an aqueous solution of a metal
salt such as, e.g., a nitrate, is used to mix the ingredients and
the mixture is pyrolyzed and burned to produce the target powder.
The magnesium-based composite oxide solid-solution particles 19 are
then mixed with particles 16 having mixed conductivity, and water
is added thereto to obtain a paste.
[0091] As the material having mixed conductivity, use is made of
SDC (Sm.sub.2O.sub.3-doped CeO.sub.2) or GDC (Gd.sub.2O.sub.3-doped
CeO.sub.2). The paste is applied to a solid electrolyte 11 by
screen printing, and the coating is burned by heating to a
temperature which results in an elevated strength of bonding
between the electrolyte 11 and the coating (FIG. 4(a)). In general,
it is preferred to burn the coating at a temperature in the range
of from 1,200.degree. C. to 1,400.degree. C. Methods for electrode
formation should not be construed at being limited to the printing
method. Use may be made of a method in which the powder mixture is
slurried and this slurry is applied by roller coating or spray
coating to produce the electrode. Alternatively, the slurry may be
formed into a sheet and then superposed. From the standpoint of gas
diffusibility in the electrode, it is preferred that the electrode
layer should be porous. A pore-forming material which disappears
upon burning to form pores may be incorporated beforehand. Examples
of the pore-forming material include organic ones such as, e.g.,
acrylic spherical particles.
[0092] Generally used as the solid electrolyte 11 is ZrO.sub.2
stabilized with Y.sub.2O.sub.3, Yb.sub.2O.sub.3, Sc.sub.2O.sub.3,
or the like. However, the solid electrolyte 11 should not be
construed as being limited to that, and any solid electrolyte
having high oxygen ion conductivity at temperatures of from
700.degree. C. to 1,000.degree. C. may be used.
[0093] A processing for heightening the efficiency of current
collection, which is a feature of this embodiment, is further
conducted. In a final material constitution to be obtained by the
process according to this embodiment, metal particles functioning
as a catalyst and having electronic conductivity are fine particles
which are isolated and dispersed. It is therefore necessary to take
a measure for securing sufficient contact with a current
collector.
[0094] In ordinary techniques, a material capable of serving as a
current collector, such as a metal mesh, is pushed against the
electrode to establish contact. In this embodiment, however, a
meshy wiring 21 is formed on the electrode layer by printing with a
material having higher electronic conductivity than the electrode
part, and this meshy wiring 21 is brought into contact with a
current collector to thereby secure current collection (FIGS.
4(b-1) and (b-2)). A line width of about 30 .mu.m and a
line-to-line distance of about 500 .mu.m suffice for the wiring
formed by printing. This wiring 21 exerts almost no influence on
fuel diffusion. Although the line width, line-to-line distance, and
wiring pattern are not limited to those, it is preferred that the
area occupied by the wiring part should account for up to 40% of
the whole electrode surface. As a material for forming the wiring
through printing, a mixture of Pt, Au, Ni, Co, Fe, or the like and
the material having mixed conductivity to be simultaneously used in
the electrode layer, i.e., an SDC, GDC, or YDC material, is used.
The two ingredients are mixed preferably in such a ratio that the
proportion of the metal material is 40-90 vol % based on the whole.
This is because not only such a mixing ratio improves tight contact
and adhesion to the electrode layer but also the wiring itself can
be expected to function as a catalyst.
[0095] The printed wiring part is burned. Thereafter, the resultant
structure is subjected to a reduction treatment at a temperature of
800-1,000.degree. C. in, e.g., a hydrogen atmosphere. Ordinary
reduction treatments for fuel electrode production using NiO as a
starting material are conducted at about 900.degree. C. without
using an unnecessarily high temperature because of the possibility
of aggregation, etc. However, in the case of using, e.g., the
nickel-magnesium composite oxide solid solution according to this
embodiment, it is preferred to conduct the reduction at 900.degree.
C. or higher in order to sufficiently precipitate nickel. More
preferably, the reduction is conducted at a temperature of from
950.degree. C. to 1,000.degree. C. Although the period of reduction
is not particularly limited, about 10 minutes may suffice. Through
the reduction, the metal ingredient present in the solid solution
state in the magnesium composite oxide solid solution part
precipitates on the surface to give a base made of a magnesium-rich
composite oxide. Namely, a magnesium-based composite oxide 17
having fine metal particles supported thereon is formed.
[0096] The metal particles formed by the method described above
have a size of about tens of nanometers. From the standpoint of
enabling the metal particles to have high catalytic activity, the
size of the precipitated metal particles is preferably regulated to
about from 5 nm to 200 nm. The reasons for this are as follows.
Metal particles having a size smaller than 5 nm are difficult to
produce actually. Metal particles larger than 200 nm cannot be
expected to produce high catalytic activity as compared with ones
obtained by, e.g., the conventional reduction of NiO particles. A
more preferred range of the size of the metal particles as an
actual catalyst is about from 20 nm to 100 nm. This size is smaller
than the conventional electrode catalyst sizes by one to two orders
of magnitude.
[0097] The amount of the magnesium-based composite oxide solid
solution to be added in constituting the electrode is preferably in
the range of from 5% by weight to 50% by weight. More preferably,
the amount thereof is from 10% by weight to 30% by weight. In the
process according to this embodiment, magnesium-based particles
having insulating properties remain even after the reduction
treatment and, hence, it is undesirable to add this part in a large
amount. Because activity is improved by reducing catalyst size, the
amount of the composite oxide solid solution to be added can be
reduced accordingly. As a result, the proportion of the material
having mixed conductivity, which is more akin in properties (e.g.,
the coefficient of thermal expansion) to the solid electrolyte as a
base, increases and the difference due to mismatch can be further
reduced.
[0098] The fine metal particles precipitated are present on a
surface part of the magnesium-based composite oxide as a base so as
to form only one layer. These particles have satisfactory
conformability to the base and are tenaciously bonded thereto.
Consequently, the fine metal particles are characterized in that
even when exposed to a high-temperature reducing atmosphere, the
particles do not readily move.
[0099] In addition, since the metal particles present are fine and
isolated, this fuel electrode has an advantage that even when the
electrode layer undergoes abrupt oxidation, the resultant volume
expansion is only local and is less apt to lead to breakage.
[0100] In JP-A 6-111829, a solid solution having a magnesium oxide
content regulated to 5-25% by mole is used, i.e., a nickel-rich
region is employed. This is intended to take account of electronic
conductivity. Because of such conditions, the amount of nickel to
be precipitated changes considerably with changing reduction
temperature and the nickel particles are apt to aggregate/unite
with one another. In this respect, in the process according to this
embodiment, nickel particles retaining a small particle diameter
can be precipitated on the surface of the NiO--MgO solid solution
without fail, although the amount of the nickel is small. The fuel
electrode thus produced is free from the fear of decreasing in
catalytic performance even when used over long.
[0101] As described above, in producing a fuel electrode according
to this embodiment, fine nickel particles can be fixed to a base.
In addition, according to this fuel electrode, high activity and
long-term stability can be provided with a small nickel addition
amount. When this fuel electrode is used in combination with an air
electrode employing a preferable electrode catalyst, inexpensive
high-output cells which may be not only a flat cell but also a
cylindrical or electrode-supported cell or the like can be
realized.
[0102] The nickel-particle-supporting composite oxide (Ni, Mg)O
produced by reducing NiO--MgO is usable also as a catalyst for the
reforming of hydrocarbon fuels including methane. Namely, it is
applicable to a variety of fuels.
[0103] The second embodiment is explained below in more detail by
reference to Examples. Electrodes employing SDC as oxide particles
having mixed conductivity, ones employing GDC as such oxide
particles, and ones employing YDC as such oxide particles all
showed the same tendency. Consequently, in the following Examples,
cells employing SDC as an example are explained. Likewise, the
particle diameter and other properties of each of the powders used
should not be construed as being limited to those shown below.
[0104] <Solid Electrolyte>
[0105] In all Examples and Comparative Examples, YSZ (ZrO.sub.2
stabilized with 8 mol % Y.sub.2O.sub.3) processed so as to have a
diameter of 15 mm and a thickness of 500 .mu.m was used as a solid
electrolyte.
EXAMPLES 7 TO 9
[0106] An NiO powder having an average particle diameter of about 1
.mu.m, an MgO powder having an average particle diameter of about 1
.mu.m, and an Sc.sub.2O.sub.3 powder were weighed out in such
amounts as to result in a molar ratio of 1:2:0.2. The powders were
mixed together by means of a mortar. The resultant powder mixture
was press-molded, and the molding was sintered at 1,300.degree. C.
of 5 hours in the air. The constituent phases of the sinter
obtained were examined by X-ray diffractometry. Subsequently, the
sinter was pulverized and passed through a 40-.mu.m mesh sieve to
obtain a starting powder (nickel-magnesium composite oxide solid
solution). The pulverized composite oxide solid-solution particles
were mixed with SDC (Sm.sub.0.2Ce.sub.0.8O.sub.2) particles having
an average particle diameter of 3 .mu.m in each of such ratios as
to result in pulverized-particle proportions of 8, 20, and 50% by
weight. Thus, respective powder mixtures were prepared (Examples 7
to 9).
[0107] To each of the powder mixtures was added about 40% by weight
pure water. The resultant mixtures each were treated with a
high-speed rotary mixer to obtain pastes. Using a screen printer,
each paste was printed in a size with a diameter of 6 mm on a
central part of a YSZ solid electrolyte plate. After the printing,
the solid electrolyte plates were placed in an atmospheric furnace
and the pastes were burned at 1,300.degree. C. for 2 hours.
Subsequently, a platinum electrode was likewise deposited by
printing on the opposite side of each solid electrolyte plate and
burned at 960.degree. C. for 1 hour to obtain an air electrode. The
same meshy wiring as that shown in Comparative Example 2 was formed
by printing on a surface layer part of each fuel electrode
produced.
COMPARATIVE EXAMPLE 5
[0108] The same structure as that produced in Example 8 was
produced, except that the meshy wiring on the surface layer part
was omitted. Thus, the structure of Comparative Example 5 was
prepared.
COMPARATIVE EXAMPLES 6 AND 7
[0109] Samples were produced in the same manner as in Examples 7 to
9, except that the proportions of the nickel-magnesium composite
oxide solid-solution particles were changed to 3 or 55% by weight
(Comparative Examples 6 and 7, respectively). The same meshy wiring
as that shown in Comparative Example 2 was formed by printing on a
surface layer part of each fuel electrode produced.
EXAMPLE 10
[0110] MgO was mixed with CoO and with Sc.sub.2O.sub.3 as an
additive for accelerating, cobalt precipitation, in a molar ratio
of 1:2:0.2. The resultant mixture was burned at 1,300.degree. C. to
obtain a cobalt-magnesium composite oxide solid solution. This
solid solution was pulverized to produce a fuel electrode in the
same manner as in Example 7, and a wiring was formed by printing on
a surface layer part of the electrode in the same manner.
[0111] <Cell Characteristics Evaluation Test>
[0112] Each flat cell produced was set on an SOFC output
characteristics evaluation apparatus, and the fuel electrode side
and the air electrode side each were sealed with a Pyrex (R) glass
material. A platinum wire having a diameter of 0.1 mm was attached
as a reference electrode to a side of the electrolyte plate. The
cell was heated in an argon atmosphere. Thereafter, hydrogen was
introduced into the fuel electrode to conduct a reduction
treatment. With respect to Examples 7 to 10 and Comparative
Examples 5 to 7, the reduction was conducted at 1,000.degree. C.
for 10 minutes.
[0113] Subsequently, H.sub.2+H.sub.2O were introduced into the fuel
electrode at 50 cc/min, and dry air+argon were introduced into the
air electrode at 50 cc/min (air electrode: dry air, 10 cc/min;
argon, 40 cc/min). Thus, the cell was evaluated for output
characteristics. Furthermore, an impedance measurement was made by
the current interruption method.
[0114] An explanation is made on the results below. The thickness
of each electrode layer formed through one screen printing
operation was about 20 .mu.m. The results of the X-ray diffraction
test are as follows. In Examples 7 to 10 and Comparative Examples 5
to 7, peaks attributable to SDC and an (Ni, Mg)O-based or (Co,
Mg)O-based solid solution were detected in the composition before
the reduction, while peaks attributable to SDC, nickel, and an (Ni,
Mg)O-based solid solution or peaks attributable to SDC, cobalt, and
a (Co, Mg)O-based solid solution were detected in the composition
after the reduction.
[0115] The nickel-magnesium composite oxide solid solutions were
examined with a thermo-gravimetric analyzer (TG) for weight change
with reduction in hydrogen. The results obtained are shown in FIG.
8. The addition of each of Sc.sub.2O.sub.3, Cr.sub.2O.sub.3, and
Al.sub.2O.sub.3 in a slight amount brought about a large weight
loss. The MgO--NiO composite oxide solid solution to which none of
these additives had been added had a reduction loss at
1,000.degree. C. of about 0.5%. It can hence be seen that the
addition of the additives accelerated the reduction (nickel
precipitation). Reduction at 1,000.degree. C. means that the fuel
electrodes produced can be used more stably because the electrodes
include metal particles precipitated at high temperatures and are
used at lower temperatures.
[0116] A structure examination was further made with an SEM. As a
result, it was ascertained that in Comparative Example 1, for which
NiO had been used, nickel particles having a size of about 1 .mu.m
were present among SDC particles. In contrast, in each of the
systems for which NiO--MgO had been used, nickel particles of a
size of from tens of nanometers to about 100 nm precipitated on the
surface of the magnesium-based composite oxide particles were
ascertained (FIG. 9). Although this figure shows an example for
which Sc.sub.2O.sub.3 had been used, the case of adding
Cr.sub.2O.sub.3 or Al.sub.2O.sub.3 gave the same structure. A
measurement of specific surface area revealed that the nickel
particles had been precipitated in an amount about 10 times the
nickel particle amount in the material obtained by reducing an
NiO--MgO composite oxide solid solution containing no additive. In
addition, the nickel particles precipitated were highly dispersed
on the magnesia solid-solution particles without overlapping and
were mostly in contact with SDC particles as a base.
[0117] In Table 1 are shown the results concerning output density
obtained by I-V characteristics evaluation. In the table, the
catalyst precursor amount means the proportion of NiO or of
MgO--NiO solid-solution particles.
[0118] In the processes according to the Examples, output
characteristics tended to improve even with small nickel amounts.
This is thought to be attributable to the following. The
nickel-magnesium composite oxide solid solution serving as a
nickel-supporting base had almost no electronic conductivity and,
hence, the internal resistance of the electrodes could be reduced
by adding the solid solution in a reduced amount. In addition, the
reduced solid-solution amount resulted in an increased amount of
SDC paths, which conduct oxygen ions. Conversely speaking, the
electrodes have sufficiently high catalytic activity even with
small nickel amounts.
[0119] It was further found that the formation of a meshy wiring on
a surface layer part by printing was highly effective. Namely, in
the techniques according to the Examples, in which (Ni, Mg)O was
used as a catalyst precursor, contact with the current collector
for current collection was important although catalytic activity
improved with decreasing catalyst size. It has become obvious that
the formation of a meshy wiring on a surface layer part by printing
is highly effective in reducing the loss to be caused by current
collection contact resistance.
[0120] It can be seen from those results that output
characteristics equal to or higher than those of conventional
materials can be imparted with smaller catalyst amounts according
to the Examples. This is thought to be mainly caused by an
increased amount of three-phase interfaces due to the catalyst
particle size reduction and the resultant increase in catalytic
activity. Furthermore, it was ascertained that because nickel
particles had been precipitated by conducting a reduction treatment
at 1,000.degree. C., the fuel electrodes underwent especially no
structural change (uniting or growth of nickel particles) with the
lapse of time even when continuously used at a temperature of
900.degree. C. or higher.
[0121] The electrode catalysts of the Examples had not undergone
the uniting of nickel particles although slight particle growth was
observed. No decrease in output density was also observed. Namely,
the results obtained show that the catalysts are more effective
also from the standpoint of durability. This is attributable to the
fact that these catalysts were synthesized at a higher temperature
than the conventional NiO-based materials, and means that the
catalysts according to the second embodiment have excellent thermal
stability.
[0122] Also in the case of using a cobalt-magnesium composite oxide
solid solution as a precursor, almost the same effects as in the
case of using the nickel system were ascertained to be obtained
when Sc.sub.2O.sub.3, Al.sub.2O.sub.3, or Cr.sub.2O.sub.3 was used
as a minor additive.
[0123] Furthermore, cells employing the fuel electrodes according
to the Examples in combination with various air electrodes were
found to show satisfactory output performance. As an air-electrode
material, use can be made of, for example, one represented by the
general formula Ln.sub.1-xA.sub.xBO.sub.3-6 (wherein Ln is a
rare-earth element; A is Sr, Ca, or Ba; and B is at least one of
Cr, Mn, Fe, Co, and Ni), such as (La, Sr)(Co, Fe)O.sub.3; this
material may be disposed over the electrolyte plate through an SDC
layer having a thickness of 1 .mu.m as a reaction-inhibitive layer
for inhibiting reaction with the electrolyte. Ionically conductive
particles such as CeO.sub.2 doped with Sm.sub.2O.sub.3, CeO.sub.2
doped with Gd.sub.2O.sub.3, or CeO.sub.2 doped with Y.sub.2O.sub.3
may be further added to those air-electrode materials. A fuel cell
system produced from one or more cells employing such a combination
was ascertained to have high durability and show high-performance
power-generating properties. Such cells can be extensively applied
not only to the flat type but also to the cylindrical type and
fuel-electrode-supported type.
[0124] The aluminum-based composite oxide solid solution or
magnesium-based composite oxide solid solution which comes to
support fine metal particles, which was used in the embodiments
described above, is useful also as a catalyst for the reforming of
hydrocarbon fuels including methane. This solid solution is hence
expected to be applied to techniques concerning SOFCs of the
internal reforming type.
[0125] The aluminum-based composite oxide solid solution and/or
magnesium-based composite oxide solid solution which comes to
support fine metal particles according to the embodiments of the
invention was applied to the fuel electrode (steam electrode) of an
electrolytic cell (SOEC) for electrolyzing steam to take out
hydrogen. As a result, it was ascertained that the cell efficiently
electrolyzed steam to yield hydrogen and that the cell showed
characteristics with little metal particle growth and excellent
durability.
[0126] The nickel-aluminum-based composite oxide sinter particles
(19) or nickel-magnesium-based composite oxide particles (19),
which are catalyst precursors, take a remarkable effect for making
the output density improve, when the specific surface area thereof
is set within the range of 5 m.sup.2/g or larger and preferably
from 10 to 30 m.sup.2/g. The larger the specific surface area of
the particles, that is, the smaller the particle diameter thereof,
the larger the amount of the metal fine particles exposed on the
surface thereof after conducting the reduction. Accordingly,
because the amount of the effective metal fine particles which can
be used as reactive sites is increased, the output density is
improved. However, when the specific surface area exceeds 30
m.sup.2/g, because the sinter density of a material constituting
the fuel electrode is increased and there is a possibility that the
dispersion path for water vapor to be formed by a reaction may be
curbed, it is necessary to device a countermeasure such as an
addition of pore-forming material to the constitution material.
[0127] Further, it is likewise preferable that the ionically
conductive particles ion to be combined with the above-described
composite oxide sinter particles have the specific surface area of
5 m.sup.2/g or larger. This is effective in forming a large number
of contact sites (reactive sites) with the metal fine particles
deposited by conducting the reduction. Further, the above-described
ionically conductive particles may be prepared by mixing a
plurality of particles having particle sizes different in the
specific surface areas with each other and used in order to
effectively form an ion conduction path. As described above, the
improvement of the catalytic activity by increasing the reactive
sites results in the decrease in the overvoltage and as a results
the output density is improved.
EXAMPLE 11
[0128] The nickel-aluminum-based composite oxide sinter particles
which were obtained by mixing the NiO powder and the
Al.sub.2O.sub.3 powder in a molar ratio of 1:1 and sintering the
resulting mixture in Example 1 were further pulverized by means of
a planetary ball mill to obtain powders having the specific surface
area of 20.0 m.sup.2/g. The thus pulverized composite oxide sinter
particles and SDC (Sm.sub.0.2Ce.sub.0.3O.sub.2) particles having an
average particle diameter of 0.3 .mu.m and the specific surface
area of 6.2 m.sup.2/g were weighed out in such amounts as to result
in a weight ratio of 20:80. The powders were mixed together.
[0129] Pure water was added to the mixed powders in an amount of
about 40% based on the total weight thereof. The resultant mixture
was treated with a high-speed rotary mixer to obtain a paste. Using
a screen printer, the paste was printed in a size with a diameter
of 6 mm on a central part of a YSZ solid electrolyte plate. After
the printing, the solid electrolyte plate was placed in an
atmospheric furnace and the paste was burned at 1,300.degree. C.
for 2 hours. Thereafter, a platinum electrode likewise having a
diameter of 6 mm was deposited on the opposite side by screen
printing and burned at 950.degree. C. for 1 hour to obtain an air
electrode. The same meshy wiring as that shown in Example 1 was
formed by printing on a surface layer part of the fuel electrode
produced.
EXAMPLE 12
[0130] The nickel-aluminum-based composite oxide sinter particles
used in Example 11 which were pulverized so as to have the specific
surface area of 20.0 m.sup.2/g and the SDC
(Sm.sub.0.2Ce.sub.0.8O.sub.2) particles which were likewise
pulverized so as to have the specific surface area of 26.7
m.sup.2/g were weighed out in such amounts as to result in a weight
ratio of 20:80. The powders were mixed together.
[0131] Pure water was added to the mixed powders in an amount of
about 40% based on the total weight thereof. The resultant mixture
was treated with a high-speed rotary mixer to obtain a paste. Using
a screen printer, the paste was printed in a size with a diameter
of 6 mm on a central part of a YSZ solid electrolyte plate. After
the printing, the solid electrolyte plate was placed in an
atmospheric furnace and the paste was burned at 1,300.degree. C.
for 2 hours. Thereafter, a platinum electrode likewise having a
diameter of 6 mm was deposited on the opposite side by screen
printing and burned at 950.degree. C. for 1 hour to obtain an air
electrode. The same meshy wiring as that shown in Example 1 was
formed by printing on a surface layer part of the fuel electrode
produced.
[0132] An explanation is made on the results. In Example 11, the
output density in the fuel electrode half cell constituted of the
mixed phase of the nickel-aluminum-based composite oxide sinter
particles (NiAl.sub.2O.sub.4 particles) the specific surface area
of which was increased to 20.0 m.sup.2/g by the pulverization and
the SDC powders having the specific surface area of 6.2 m.sup.2/g
was 140 mW/cm.sup.2. Even when the particle size of the
nickel-aluminum-based composite oxide sinter particles is
decreased, the size of nickel particles deposited on the surface of
the composite oxide sinter particles after conducting the reduction
hardly be changed. Accordingly, when the particle size of the
nickel-aluminum-based composite oxide sinter particles which are
catalyst precursors is reduced, because the contact sites of the
metal particles exposed on the surface thereof with the SDC
particles which are the ionically conductive particles are
increased, it is possible to make the catalytic activity
improve.
[0133] In Example 12, the output density in the fuel electrode half
cell constituted of the mixed phase of the nickel-aluminum-based
composite oxide sinter particles (NiAl.sub.2O.sub.4 particles) the
specific surface area of which was increased to 20.0 m.sup.2/g by
the pulverization and the SDC powders having the specific surface
area of 26.7 m.sup.2/g was 145 mW/cm.sup.2. When the overvoltage
due to only the fuel electrode except the overvoltage due to the
electrolyte resistance was investigated, the effect of remarkably
reducing the overvoltage at the low to medium electrical current
density region was taken. On the other hand, the overvoltage was
slightly increased at the high electrical current density region.
In this region, the overvoltage due to the dispersion condition of
the reaction gas was shown. Because the particle sizes of both
particles were reduced, the density of the fuel electrode was
increased. That is, because the pores are decreased, there is a
possibility that the dispersion of gas might be insufficient. Under
the circumstances, the increase in the density of the fuel
electrode can be controlled by introducing the pore-forming
material and the like.
* * * * *