U.S. patent application number 12/081124 was filed with the patent office on 2008-10-23 for heterogeneous ceramic composite sofc electrolyte.
This patent application is currently assigned to BLOOM ENERGY CORPORATION. Invention is credited to Tad Armstrong, Emad El Batawi, Dien Nguyen, Ravi Oswal.
Application Number | 20080261099 12/081124 |
Document ID | / |
Family ID | 39864237 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080261099 |
Kind Code |
A1 |
Nguyen; Dien ; et
al. |
October 23, 2008 |
Heterogeneous ceramic composite SOFC electrolyte
Abstract
A solid oxide fuel cell (SOFC) includes a cathode electrode, a
solid oxide electrolyte, and an anode electrode. The electrolyte
includes yttria stabilized zirconia and scandia stabilized
zirconia, such as scandia ceria stabilized zirconia.
Inventors: |
Nguyen; Dien; (San Jose,
CA) ; Oswal; Ravi; (Fremont, CA) ; Armstrong;
Tad; (Burlingame, CA) ; Batawi; Emad El;
(Sunnyvale, CA) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
BLOOM ENERGY CORPORATION
|
Family ID: |
39864237 |
Appl. No.: |
12/081124 |
Filed: |
April 10, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60907706 |
Apr 13, 2007 |
|
|
|
Current U.S.
Class: |
429/418 ;
429/465; 75/343 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01M 2008/1293 20130101; H01M 8/126 20130101; Y02P 70/56 20151101;
H01M 4/9066 20130101; C04B 2235/3224 20130101; C04B 35/64 20130101;
H01M 2300/0091 20130101; Y02E 60/525 20130101; H01M 4/8657
20130101; H01M 4/9033 20130101; C04B 2235/602 20130101; C04B 35/48
20130101; H01M 8/1253 20130101; C04B 2235/96 20130101; C04B
2235/3225 20130101; Y02E 60/50 20130101; H01M 4/8621 20130101; H01M
2300/0077 20130101; C04B 2235/3246 20130101 |
Class at
Publication: |
429/33 ;
75/343 |
International
Class: |
H01M 8/10 20060101
H01M008/10; B22F 1/00 20060101 B22F001/00 |
Claims
1. A solid oxide fuel cell (SOFC), comprising: a cathode electrode;
a solid oxide electrolyte; and an anode electrode; wherein the
electrolyte comprises a yttria stabilized zirconia and a scandia
stabilized zirconia.
2. The electrolyte of claim 1, wherein the electrolyte comprises a
mixture of the yttria stabilized zirconia and the scandia
stabilized zirconia.
3. The SOFC of claim 2, wherein the scandia stabilized zirconia
comprises up to 1 molar percent ceria, alumina or yttria, about 6
to about 11 molar percent scandia and a balance comprising
zirconia.
4. The SOFC of claim 3, wherein the electrolyte comprises a mixture
of the yttria stabilized zirconia and scandia ceria stabilized
zirconia.
5. The SOFC of claim 1, wherein the electrolyte comprises a mixture
of yttria stabilized zirconia and
[(ZrO.sub.2).sub.1-y(CeO.sub.2).sub.y].sub.1-x(Sc.sub.2O.sub.3).sub.x,
where 0.06.ltoreq.x.ltoreq.0.11 and 0.ltoreq.y.ltoreq.0.01.
6. The SOFC of claim 5, wherein the yttria stabilized zirconia
comprises 3 to 10 molar percent yttria.
7. The SOFC of claim 1, wherein a weight ratio of the yttria
stabilized zirconia to the scandia stabilized zirconia in the
electrolyte ranges from 1:4 to 1:1.
8. The SOFC of claim 7, wherein the weight ratio of the yttria
stabilized zirconia to the scandia stabilized zirconia in the
electrolyte ranges from 1:2 to 1:3.
9. The SOFC of claim 1, wherein the electrolyte is about 150 to
about 300 microns thick.
10. A solid oxide fuel cell (SOFC), comprising: a cathode
electrode; a solid oxide electrolyte comprising a mixture of about
25 weight percent yttria stabilized zirconia which comprises 3
molar percent yttria, and about 75 weight percent scandia ceria
stabilized zirconia which comprises 1 molar percent ceria and 10
molar percent scandia; and an anode electrode.
11. The SOFC of claim 10, wherein the anode electrode comprises: a
first sublayer comprising samaria doped ceria; and a second
sublayer comprising a scandia ceria stabilized zirconia and
gadolinia doped ceria ceramic phase and a nickel containing
phase.
12. A method of making a solid oxide fuel cell, comprising: mixing
yttria stabilized zirconia powder with scandia stabilized zirconia
powder to form a powder mixture; shaping the powder mixture;
sintering the shaped powder mixture to form an electrolyte; forming
an anode electrode on a first side of the electrolyte; and forming
a cathode electrode on a second side of the electrolyte.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] The present application claims benefit of U.S. provisional
application 60/907,706, filed Apr. 13, 2007, which is incorporated
herein by reference in its entirety.
BACKGROUND
[0002] The present invention is generally directed to fuel cell
components, and to solid oxide fuel cell electrolyte materials in
particular.
[0003] Fuel cells are electrochemical devices which can convert
energy stored in fuels to electrical energy with high efficiencies.
Electrolyzer cells are electrochemical devices which can use
electrical energy to reduce a given material, such as water, to
generate a fuel, such as hydrogen. The fuel and electrolyzer cells
may comprise reversible cells which operate in both fuel cell and
electrolysis mode.
[0004] In a high temperature fuel cell system, such as a solid
oxide fuel cell (SOFC) system, an oxidizing flow is passed through
the cathode side of the fuel cell while a fuel flow is passed
through the anode side of the fuel cell. The oxidizing flow is
typically air, while the fuel flow can be a hydrocarbon fuel, such
as methane, natural gas, propane, ethanol, or methanol. The fuel
cell, operating at a typical temperature between 750.degree. C. and
950.degree. C., enables the transport of negatively charged oxygen
ions from the cathode flow stream to the anode flow stream, where
the ion combines with either free hydrogen or hydrogen in a
hydrocarbon molecule to form water vapor and/or with carbon
monoxide to form carbon dioxide. The excess electrons from the
negatively charged ion are routed back to the cathode side of the
fuel cell through an electrical circuit completed between anode and
cathode, resulting in an electrical current flow through the
circuit. A solid oxide reversible fuel cell (SORFC) system
generates electrical energy and reactant product (i.e., oxidized
fuel) from fuel and oxidizer in a fuel cell or discharge mode and
generates the fuel and oxidant using electrical energy in an
electrolysis or charge mode.
SUMMARY
[0005] A solid oxide fuel cell (SOFC) includes a cathode electrode,
a solid oxide electrolyte, and an anode electrode. The electrolyte
includes yttria stabilized zirconia and a scandia stabilized
zirconia, such as a scandia ceria stabilized zirconia.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 illustrates a side cross-sectional view of a SOFC of
the embodiments of the invention.
[0007] FIG. 2 illustrates a side cross sectional view of a SOFC
stack of an embodiment of the invention.
[0008] FIG. 3 illustrates a plot of conductivity versus temperature
for the electrolyte of the embodiment of the invention and for
electrolytes of the comparative examples.
[0009] FIG. 4 illustrates a bar graph comparing the CTE of the
electrolyte of the embodiment of the invention and of electrolytes
of the comparative examples
[0010] FIG. 5 illustrates a plot of cell voltage versus time for a
SOFC cell containing the electrolyte of the embodiment of the
invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0011] The embodiments of the invention provide a higher strength
electrolyte material to enable a thinner electrolyte and/or larger
footprint electrolyte, while lowering the cost for electrolyte
production. The composite electrolyte material comprises a
composite yttria and scandia stabilized zirconias. The mixture of
yttria and scandia stabilized zirconia exhibits a good flexural
strength increase, and reasonable conductivity decrease compared to
scandia stabilized zirconia. The electrolyte composition provides a
coefficient of thermal expansion (CTE) which is closely matched to
that of a chromium-iron alloy interconnect component of a SOFC
stack. SOFC cells comprising the composite electrolyte can operate
for a long time with a low degradation rate. By mixing a lower cost
yttria stabilized zirconia powder with a higher cost, higher
performance scandia stabilized zirconia powder, the overall cost of
the electrolyte is reduced without significantly impacting the
electrolyte performance compared to a scandia stabilized zirconia
electrolyte.
[0012] FIG. 1 illustrates a solid oxide fuel cell (SOFC) 1
according to an embodiment of the invention. The cell 1 includes an
anode electrode 3, a solid oxide electrolyte 5 and a cathode
electrode 7. The electrolyte 5 may comprise a sintered mixture of
scandia stabilized zirconia ("SSZ") (including scandia ceria
stabilized zirconia ("SCSZ"), which can also be referred to as
scandium and cerium doped zirconia), and yttria stabilized zirconia
("YSZ"). The electrolyte may also contain unavoidable impurities.
For example, the electrolyte 5 may comprise a mixture of YSZ and
one of SSZ with no ceria or SCSZ, such as a YSZ/SCSZ mixture in an
about 1:1 to about 1:4 weight ratio, such as an about 1:2 to 1:3
weight ratio. Thus, YSZ may comprise up to 50% by weight of the
electrolyte 5. In alternative embodiments, the SCSZ may be
substituted by SSZ.
[0013] Preferably, 3 molar percent yttria YSZ is used. However, YSZ
compositions having more than 3 molar percent yttria, such as 3 to
10 molar percent yttria, for example 5 to 10 molar percent yttria
(i.e., (ZrO.sub.2).sub.1-z(Y.sub.2O.sub.3).sub.x, where
0.03.ltoreq.z.ltoreq.0.1) may be used.
[0014] Preferably, the scandia stabilized zirconia has the
following formula:
[(ZrO.sub.2).sub.1-y(CeO.sub.2).sub.y].sub.1-x(Sc.sub.2O.sub.3).-
sub.x, where 0.06.ltoreq.x.ltoreq.0.11 and 0.ltoreq.y.ltoreq.0.01.
While a stoichiometric stabilized zirconia is described by the
formula, a non-stoichiometric stabilized zirconia having more or
less than two oxygen atoms for each metal atom may be used. For
example, the electrolyte may comprise SCSZ having 1 molar percent
ceria and 10 molar percent scandia (i.e.,
[(ZrO.sub.2).sub.1-y(CeO.sub.2).sub.y].sub.1-x(Sc.sub.2O.sub.3).sub.x
where x=0.1 and y=0.01). The ceria in SCSZ may be substituted with
other ceramic oxides. Thus, alternative scandia stabilized
zirconias can be used, such as scandia yttria stabilized zirconia
("SYSZ"), which can also be referred to as scandium and yttrium
doped zirconia, and scandia alumina stabilized zirconia ("SAlSZ"),
which can also be referred to as scandium and aluminum doped
zirconia. The yttria or alumina may comprise 1 molar percent or
less in the scandia stabilized zirconia.
[0015] The cathode electrode 7 may comprise an electrically
conductive material, such as an electrically conductive perovskite
material, such as lanthanum strontium manganite (LSM). Other
conductive perovskites, such as La.sub.1-xSr.sub.xCoO.sub.3,
La.sub.1-xSr.sub.xFe.sub.1-yCO.sub.yO.sub.3 or
La.sub.1-xSr.sub.xMn.sub.1-yCO.sub.yO.sub.3 where
0.1.ltoreq.x.ltoreq.0.4 and 0.02.ltoreq.y.ltoreq.0.4, respectively,
may also be used. The cathode electrode 7 can also be composed of
two sublayers (a SCSZ/LSM functional layer adjacent to the
electrolyte and a LSM current collection layer over the functional
layer).
[0016] The anode electrode 3 may comprise one or more sublayers.
For example, the anode electrode may comprise a single layer Ni-YSZ
and/or a Ni-SSZ cermet. In a preferred embodiment, the anode
electrode comprises two sublayers, where the first sublayer closest
to the electrolyte is composed of samaria doped ceria ("SDC") and
the second sublayer distal from the electrolyte comprises nickel,
gadolinia doped ceria ("GDC") and a scandia stabilized zirconia
("SSZ"), such as a scandia ceria stabilized zirconia ("SCSZ").
[0017] The samaria doped ceria preferably comprises 15 to 25 molar
percent, such as for example 20 molar percent samaria and a balance
comprising ceria. The SDC may have the following formula:
Sm.sub.zCe.sub.1-zO.sub.2-.delta., where 0.15.ltoreq.z.ltoreq.0.25.
While a non-stoichiometric SDC is described by the formula where
there is slightly less than two oxygen atoms for each metal atom,
an SDC having two or more oxygen atoms for each metal atom may also
be used. Preferably, the first sublayer contains no other
materials, such as nickel, besides the SDC and unavoidable
impurities. However, if desired, other materials may be added to
the first sublayer, such as a small amount of nickel in an amount
less than the amount of nickel in the second sublayer.
[0018] The second sublayer comprises a cermet including a nickel
containing phase and a ceramic phase. The nickel containing phase
of the second sublayer preferably consists entirely of nickel in a
reduced state. This phase forms nickel oxide when it is in an
oxidized state. Thus, when the anode is fabricated, the nickel
containing phase comprises nickel oxide. The anode electrode is
preferably annealed in a reducing atmosphere prior to operation to
reduce the nickel oxide to nickel. The nickel containing phase may
include other metals and/or nickel alloys in addition to pure
nickel, such as nickel-copper or nickel-cobalt alloys (in a reduced
state) and their oxides (in an oxidized state), for example
Ni.sub.1-xCu.sub.xO or Ni.sub.1-xCo.sub.xO where
0.05.ltoreq.x.ltoreq.0.3. However, the nickel containing phase
preferably contains only nickel or nickel oxide and no other
metals. The nickel is preferably finely distributed in the ceramic
phase, with an average grain size less than 500 nanometers, such as
200 to 400 nanometers, to reduce the stresses induced when nickel
converts to nickel oxide.
[0019] The ceramic phase of the second sublayer preferably
comprises gadolinia doped ceria and scandia stabilized zirconia.
The ceramic phase may comprise a sintered mixture of GDC and SSZ
(containing some or no cerium) ceramic particles. The scandia
stabilized zirconia may have the same composition as the scandia
stabilized zirconia of the electrolyte 5. Preferably, the scandia
stabilized zirconia of sublayer 23 has the following formula:
[(ZrO.sub.2).sub.1-y(CeO.sub.2).sub.y].sub.1-x(Sc.sub.2O.sub.3).sub.x,
where 0.06.ltoreq.x.ltoreq.0.11 and 0.ltoreq.y.ltoreq.0.01. While a
stoichiometric stabilized zirconia is described by the formula, a
non-stoichiometric stabilized zirconia having more or less than two
oxygen atoms for each metal atom may be used. For example, the
electrolyte may comprise SCSZ having up to 1 molar percent ceria,
about 6 to about 11 molar percent scandia and a balance comprising
zirconia, such as SCSZ having 1 molar percent ceria and 10 molar
percent scandia (i.e., Sc.sub.xCe.sub.yZr.sub.1-x-yO.sub.2 where
x=0.1 and y=0.01).
[0020] Any suitable GDC may be used in the second sublayer. For
example, 10 to 40 molar percent gadolinia containing GDC may be
used. GDC is preferably slightly non-stoichiometric with less than
two oxygen atoms for each metal atom:
Ce.sub.1-mGd.sub.mO.sub.2-.delta. where 0.1.ltoreq.m.ltoreq.0.4.
However, GDC containing two or more oxygen atoms for each metal
atom may also be used. The weight ratio of GDC to SSZ or SCSZ in
the sublayer ranges from about 2:1 to about 5:1. For example, the
weight ratio may be 5:1. If the ceramic phase contains no other
components besides GDC and the stabilized zirconia, then the
ceramic phase in the second sublayer may range from about 70 (such
as for example 66.66) weight percent GDC and about 30 (such as for
example 33.33) weight percent stabilized zirconia to about 85 (such
as for example 83.33) weight percent GDC and about 15 (such as for
example 16.66) weight percent stabilized zirconia. The ceramic
phase preferably contains no other ceramic materials besides GDC,
one of SSZ or SCSZ and unavoidable impurities.
[0021] The second sublayer preferably comprises 60 to 80 weight
percent of the nickel containing phase and 40 to 20 weight percent
of the ceramic phase, such as for example 75 weight percent of the
nickel containing phase and 25 weight percent of the ceramic
phase.
[0022] Any suitable layer thicknesses may be used. For example, the
anode electrode 3 may be 20 to 40 microns thick, where the first
sublayer is about 5 to about 10 microns thick and the second
sublayer is about 15 to about 30 microns thick. The fuel cell is
preferably a planar electrolyte supported cell in which the
electrolyte is at least one order of magnitude thicker than the
anode electrode. For example, the electrolyte 5 may be about 150 to
about 300 microns thick. The cathode 7 may also be between 10 and
50 microns thick.
[0023] Fuel cell stacks are frequently built from a multiplicity of
SOFC's 1 in the form of planar elements, tubes, or other
geometries. Fuel and air has to be provided to the
electrochemically active surface, which can be large. As shown in
FIG. 2, one component of a fuel cell stack is the so called gas
flow separator (referred to as a gas flow separator plate in a
planar stack) 9 that separates the individual cells in the stack.
The gas flow separator plate separates fuel flowing to the fuel
electrode (i.e. anode 3) of one cell in the stack from oxidant,
such as air, flowing to the air electrode (i.e. cathode 7) of an
adjacent cell in the stack. The fuel may be a hydrocarbon fuel,
such as natural gas for internally reforming cells, or a reformed
hydrocarbon fuel comprising hydrogen, water vapor, carbon monoxide
and unreformed hydrocarbon fuel for externally reforming cells. The
separator 9 contains gas flow passages or channels 8 between the
ribs 10. Frequently, the gas flow separator plate 9 is also used as
an interconnect which electrically connects the fuel electrode 3 of
one cell to the air electrode 7 of the adjacent cell. In this case,
the gas flow separator plate which functions as an interconnect is
made of or contains electrically conductive material, such as a
Cr--Fe alloy. An electrically conductive contact layer, such as a
nickel contact layer, may be provided between the anode electrode
and the interconnect. FIG. 2 shows that the lower SOFC 1 is located
between two gas separator plates 9.
[0024] Furthermore, while FIG. 2 shows that the stack comprises a
plurality of planar or plate shaped fuel cells, the fuel cells may
have other configurations, such as tubular. Still further, while
vertically oriented stacks are shown in FIG. 2, the fuel cells may
be stacked horizontally or in any other suitable direction between
vertical and horizontal.
[0025] The term "fuel cell stack," as used herein, means a
plurality of stacked fuel cells which share a common fuel inlet and
exhaust passages or risers. The "fuel cell stack," as used herein,
includes a distinct electrical entity which contains two end plates
which are connected to power conditioning equipment and the power
(i.e., electricity) output of the stack. Thus, in some
configurations, the electrical power output from such a distinct
electrical entity may be separately controlled from other stacks.
The term "fuel cell stack" as used herein, also includes a part of
the distinct electrical entity. For example, the stacks may share
the same end plates. In this case, the stacks jointly comprise a
distinct electrical entity. In this case, the electrical power
output from both stacks cannot be separately controlled.
[0026] A method of forming a planar, electrolyte supported SOFC 1
shown in FIG. 1 includes forming the planar solid oxide electrolyte
5 followed by forming the cathode electrode 7 on a first side of a
planar solid oxide electrolyte 5 and forming the anode electrode 3
on a second side of electrolyte 5. The anode and the cathode may be
formed in any order on the opposite sides of the electrolyte.
[0027] For example, the electrolyte may be formed by mixing the YSZ
powder with SSZ or SCSZ powder followed by shaping (such as tape
casting, roll pressing or other suitable ceramic shaping
techniques) and sintering the powders at any suitable temperature
to form the electrolyte. The anode electrode containing a plurality
of sublayers shown in FIG. 1 may be formed by a screen printing
method or by other suitable methods. The first anode 3 sublayer can
be screen printed on the electrolyte 5, followed by screen printing
the second anode sublayer on the first sublayer using any suitable
ceramic powder screen printing techniques. The screen printed cell
is then sintered or fired at any suitable temperature, such as a
temperature between 1150 and 1400.degree. C. in air. The cell may
be separately fired or sintered after the anode deposition and
after the cathode deposition at the same or different temperature.
The completed cell is preferably further annealed in a reducing
atmosphere, such as a hydrogen or forming gas atmosphere, to covert
nickel oxide to nickel in the anode prior to using fuel cell to
generate electricity as part of a fuel cell system.
[0028] A performance of various electrolytes were tested.
Specifically, the performance of the YSZ and SCSZ composite
electrolyte of the embodiments of the invention having the
following composition (25% by weight of 3 molar percent yttria YSZ
and 75% by weight of
[(ZrO.sub.2).sub.1-y(CeO.sub.2).sub.y].sub.1-x(Sc.sub.2O.sub.3).sub.x
where x=0.1 and y=0.01) ("YSZ+SCSZ") was compared to the following
comparative example electrolyte compositions: (a) 3 molar percent
yttria YSZ ("3YSZ"); (b) 8 molar percent yttria YSZ ("8YSZ"); and
(c) Sc.sub.xCe.sub.yZr.sub.1-x-yO.sub.2 where x=0.1 and y=0.01
("SCSZ").
[0029] FIG. 3 illustrates a plot of conductivity versus temperature
for the four electrolytes. The conductivity of the YSZ+SCSZ
electrolyte is higher than that of the 8YSZ and 3YSZ electrolytes,
but slightly lower than that of the SCSZ electrolyte.
[0030] FIG. 4 illustrates a bar graph comparing the CTE of the four
electrolytes. The CTE of the YSZ+SCSZ electrolyte is about the same
as that of the electrolytes of the comparative examples.
[0031] FIG. 5 illustrates a plot of cell voltage versus time for a
SOFC cell containing the YSZ+SCSZ electrolyte. This endurance test
indicates that the cell voltage degrades about 3-4% for the first
1000 hours and about 1-2% for the second thousand hours of
operation.
[0032] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and modifications and variations are possible in
light of the above teachings or may be acquired from practice of
the invention. The description was chosen in order to explain the
principles of the invention and its practical application. It is
intended that the scope of the invention be defined by the claims
appended hereto, and their equivalents.
* * * * *