U.S. patent application number 11/312275 was filed with the patent office on 2006-06-29 for preconditioning treatment to enhance redox tolerance of solid oxide fuel cells.
This patent application is currently assigned to VERSA POWER SYSTEMS, LTD.. Invention is credited to David Waldbillig, Anthony Wood.
Application Number | 20060141300 11/312275 |
Document ID | / |
Family ID | 36611994 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060141300 |
Kind Code |
A1 |
Wood; Anthony ; et
al. |
June 29, 2006 |
Preconditioning treatment to enhance redox tolerance of solid oxide
fuel cells
Abstract
A high temperature, redox tolerant fuel cell anode electrode and
method of fabrication in which the anode electrode is
pre-conditioned by application of an initial controlled redox cycle
to the electrode whereby an initial re-oxidation of the anode
electrode is carried out at temperatures less than or equal to
about 650.degree. C.
Inventors: |
Wood; Anthony; (Calgary,
CA) ; Waldbillig; David; (Vancouver, CA) |
Correspondence
Address: |
MARK E. FEJER;GAS TECHNOLOGY INSTITUTE
1700 SOUTH MOUNT PROSPECT ROAD
DES PLAINES
IL
60018
US
|
Assignee: |
VERSA POWER SYSTEMS, LTD.
|
Family ID: |
36611994 |
Appl. No.: |
11/312275 |
Filed: |
December 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60639131 |
Dec 27, 2004 |
|
|
|
Current U.S.
Class: |
429/429 ;
429/442; 429/489; 429/496; 502/101 |
Current CPC
Class: |
H01M 8/04223 20130101;
H01M 4/8885 20130101; Y02E 60/50 20130101; H01M 8/2425 20130101;
H01M 4/9066 20130101; H01M 4/8621 20130101; H01M 8/04225
20160201 |
Class at
Publication: |
429/013 ;
429/045; 502/101; 429/030 |
International
Class: |
H01M 8/12 20060101
H01M008/12; H01M 4/88 20060101 H01M004/88; H01M 4/90 20060101
H01M004/90 |
Claims
1. A method for preconditioning a solid oxide fuel cell anode
electrode comprising the steps of: subjecting a sintered said anode
electrode to an initial redox cycle in which re-oxidation is
carried out at a temperature less than or equal to about
650.degree. C.
2. A method in accordance with claim 1, wherein said sintered anode
electrode comprises a metal oxide and zirconia.
3. A method in accordance with claim 2, wherein said zirconia is
stabilized with yttria.
4. A method in accordance with claim 2, wherein said metal oxide is
NiO.
5. A method in accordance with claim 1, wherein said redox cycle is
carried out to at least a 100% redox depth.
6. A method in accordance with claim 1, wherein said temperature is
in a range of about 400.degree. C. to about 650.degree. C.
7. A method of fabricating a solid oxide fuel cell anode electrode
comprising the steps of: forming a mixture of metal oxide particles
and YSZ particles into a green anode structure; sintering said
green anode structure, forming a sintered anode structure;
contacting said sintered anode structure with a reducing agent at a
reducing temperature in a range of about 600.degree. C. to about
1000.degree. C., forming a reduced anode structure having a first
microstructure; contacting said reduced anode structure with an
oxidizing agent at an oxidizing temperature in a range of
about400.degree. C. to about 650.degree. C., forming an oxidized
anode structure; and contacting said oxidized anode structure with
said reducing agent at said reducing temperature, forming said
reduced anode structure with a second microstructure.
8. A method in accordance with claim 7, wherein said metal oxide is
NiO.
9. A method in accordance with claim 7, wherein said second
microstructure is redox tolerant.
10. In a solid oxide fuel cell having a metal-cermet anode
electrode, a method for enhancing redox tolerance of said solid
oxide fuel cell comprising the steps of: contacting said
metal-cermet anode electrode with an oxidizing agent at a
temperature in a range of about 400.degree. C. to about 650.degree.
C., forming an oxidized anode electrode; and contacting said
oxidized anode electrode with a reducing agent at a reducing
temperature in a range of about 600.degree. C. to about
1000.degree. C., forming a preconditioned metal-cermet anode
electrode.
11. A method in accordance with claim 10, wherein said oxidizing
agent is provided in an amount sufficient to provide at least a
100% redox depth.
12. A method in accordance with claim 10, wherein said metal-cermet
comprises Ni and YSZ.
13. In a solid oxide fuel cell having an anode electrode, a cathode
electrode and a solid electrolyte disposed between said anode
electrode and said cathode electrode, the improvement comprising:
said anode electrode preconditioned by an initial re-oxidation at a
temperature less than or equal to about 650.degree. C.
14. A solid oxide fuel cell in accordance with claim 13, wherein
said anode electrode comprises Ni and YSZ.
15. A solid oxide fuel cell in accordance with claim 13, wherein
said anode electrode is preconditioned to at least a 100% redox
depth.
16. A solid oxide fuel cell in accordance with claim 13, wherein
said temperature of said initial re-oxidation is in a range of
about 400.degree. C. to about 650.degree. C.
17. A solid oxide fuel cell in accordance with claim 13, wherein
said anode electrode is preconditioned in-situ.
18. A solid oxide fuel cell in accordance with claim 13, wherein
said anode electrode is preconditioned prior to assembly into said
solid oxide fuel cell.
19. A solid oxide fuel cell anode electrode comprising: a porous
metal-YSZ structure having a microstructure produced by initially
re-oxidizing said structure at a temperature less than or equal to
about 650.degree. C.
20. A solid oxide fuel cell anode electrode in accordance with
claim 19, wherein said metal is Ni.
21. A solid oxide fuel cell anode electrode in accordance with
claim 19, wherein said temperature of said initial re-oxidizing is
in a range of about 400.degree. C. to about 650.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to high temperature fuel cells having
metal-containing anode electrodes, in particular, solid oxide fuel
cells and, more particularly, to solid oxide fuel cell anode
electrodes. More particularly yet, this invention relates to solid
oxide fuel cell anode electrodes that are redox tolerant, solid
oxide fuel cells comprising such electrodes, and a method for
enhancing the redox tolerance of such electrodes.
[0003] 2. Description of Related Art
[0004] Fuel cells are electrochemical devices that convert the
chemical energy of a fuel into electrical energy with high
efficiency. The basic physical structure of a fuel cell consists of
an electrolyte layer with a porous anode electrode and porous
cathode electrode on opposed sides of the electrolyte. In a typical
fuel cell, gaseous fuels, typically hydrogen, are continuously fed
to the anode electrode and an oxidant, typically oxygen from air,
is continuously fed to the cathode electrode. The electrochemical
reactions take place at the electrodes to produce an electric
current.
[0005] In a solid oxide fuel cell, the electrolyte is a solid,
nonporous metal oxide, normally Y.sub.2O.sub.3-stabilized ZrO.sub.2
(YSZ), the anode electrode is a metal/YSZ cermet and the cathode
electrode is typically Sr-doped LaMnO.sub.3. The solid oxide fuel
cell operating temperature is typically in the range of about
650.degree. C. to about 1000.degree. C., at which temperature ionic
conduction by oxygen ions occurs.
[0006] The most commonly used solid oxide fuel cell anode material
is a porous two phase nickel and yttria stabilized zirconia
(Ni/YSZ) cermet. During normal fuel cell operation, this anode
material remains a cermet. However, there are potentially several
occurrences, such as air leakage into the anode side of the fuel
cell due to seal leakage, fuel supply interruption, and emergency
stops, which may cause the anode electrode to re-oxidize, forming
an NiO/YSZ structure. Upon restarting of the fuel cell, the NiO/YSZ
structure is chemically reduced to re-form the Ni/YSZ anode
electrode. However, this reduction and oxidation process (referred
to as redox cycling) results in substantial bulk volume changes of
the anode electrode. The bulk volume of a fully dense NiO sample
would be expected to contract by about 40.9% upon reduction and
would be expected to expand by about 69.2% upon oxidation.
Although, due to expansion into the pores, a NiO/YSZ solid oxide
fuel cell anode electrode is unlikely to experience such a drastic
volume change, any volume change that does occur can have a
significant effect on the integrity of other cell components (e.g.
electrolyte cracking) and cell component interfaces, which can, in
turn, result in a significant degradation in the performance of the
fuel cell.
SUMMARY OF THE INVENTION
[0007] It is, thus, one object of this invention to provide a solid
oxide fuel cell having enhanced tolerance to the effects of redox
cycling.
[0008] It is one object of this invention to provide a solid oxide
fuel cell anode electrode having enhanced tolerance to the effects
of redox cycling.
[0009] It is another object of this invention to provide a method
for enhancing the redox tolerance of solid oxide fuel cell anode
electrodes.
[0010] It is another object of this invention to provide a method
for fabrication of a redox tolerant solid oxide fuel cell anode
electrode.
[0011] These and other objects of this invention are addressed by a
solid oxide fuel cell anode electrode comprising a porous metal-YSZ
structure having a microstructure produced by applying an initial
redox cycle to the structure where the re-oxidation step of the
cycle is carried out at a temperature less than or equal to about
650.degree. C.
[0012] These and other objects of this invention are also addressed
by a method of fabricating a solid oxide fuel cell anode electrode
comprising the steps of forming a mixture of metal oxide particles
and YSZ particles into a "green" or uncured anode structure
typically with binders and plasticizers, heating the green anode
structure in air to a suitable sintering temperature, forming a
sintered anode structure comprising the metal oxide and YSZ,
contacting the sintered anode structure with a reducing agent at a
temperature in the range of about 600.degree. C. to about
1000.degree. C., forming a reduced anode structure having a first
microstructure, contacting the reduced anode structure with an
oxidizing agent at a temperature in the range of about 400.degree.
C. to about 650.degree. C., forming an oxidized anode structure,
and contacting the oxidized anode structure with the reducing agent
at a temperature in the range of about 600.degree. C. to about
1000.degree. C., forming said reduced anode structure with a
second, redox tolerant, microstructure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other objects and features of this invention will
be better understood from the following detailed description taken
in conjunction with the drawings wherein:
[0014] FIGS. 1a, 1b and 1c show BSE (backscattered electron) SEM
(scanning electron microscope) images of a sintered (a), reduced
(b) and re-oxidized (c) solid oxide fuel cell anode electrode;
[0015] FIGS. 2a, 2b, 2c and 2d show TEM (transmission electron
microscope) images of a sintered (a), reduced (b), re-oxidized (c)
and rereduced (d) solid oxide fuel cell anode electrode;
[0016] FIG. 3 is a diagram showing the results of a
thermomechanical analysis (TMA) of solid oxide fuel cell anode
electrode samples during oxidation at 600.degree. C. and
750.degree. C.;
[0017] FIG. 4 is a diagram showing voltage degradation after redox
cycling for a baseline solid oxide fuel cell;
[0018] FIG. 5 is a diagram showing the cumulative degradation of a
solid oxide fuel cell after redox cycling at various
temperatures;
[0019] FIG. 6 is a diagram showing a comparison of the redox
tolerance of a solid oxide fuel cell having a preconditioned anode
electrode in accordance with one embodiment of this invention and a
baseline solid oxide fuel cell; and
[0020] FIG. 7 is a diagram showing a cumulative % degradation
comparison of solid oxide fuel cells having a baseline anode
electrode and solid oxide fuel cells having preconditioned anode
electrodes in accordance with this invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0021] Although the invention is described herein in the context of
solid oxide fuel cells and anode electrodes therefor, it will be
appreciated by those skilled in the art that the basic principles
of this invention may be advantageously applied to other high
temperature fuel cells employing metal-containing anode electrodes
that may be subject to redox cycling, and such other fuel cells and
fuel cell components are deemed to be within the scope of this
invention.
[0022] The invention disclosed herein is an anode electrode for a
solid oxide fuel cell and a method for fabrication and
preconditioning of the anode electrode which addresses the problems
associated with redox cycling of the solid oxide fuel cell. FIGS.
1a, 1b and 1c show BSE SEM images of solid oxide fuel cell anodes
in an as prepared, or sintered, state, after undergoing reduction
by contact with a reducing gas, and after re-oxidation,
respectively. FIGS. 2a, 2b, 2c and 2d show TEM images of solid
oxide fuel cell anodes in an as prepared state, after undergoing a
first reduction, after re-oxidation, and after a second reduction,
respectively. As can be seen from these figures, redox cycling of a
solid oxide fuel cell anode electrode irreversibly alters the anode
electrode microstructure after the first redox cycle. As used
herein, the first, or initial, redox cycle comprises the initial
reduction of the sintered metal oxide/YSZ anode structure to
produce the reduced anode structure followed by the initial
re-oxidation of the reduced anode structure.
[0023] Electron microscopy of the anode electrode shows that, in
the as prepared, i.e. sintered, condition, the anode electrode
comprises NiO particles several microns in size, YSZ grains about
one micron in size and intergranular porosity. After the first
reduction, the overall Ni grain size remains about the same as the
consumed NiO and epitaxial growth of Ni crystals on NiO grains is
observed. The amount of intergranular porosity increases and very
fine, 50 nanometer (nm), intergranular pores are formed throughout
the Ni grains. This increase in the amount of porosity is due to
the large volume change that occurs when NiO is reduced to Ni. When
the anode electrode samples are re-oxidized, the NiO particles in
the SEM images appear spongy with much smaller intergranular pores
than the as prepared anode electrode samples. The re-oxidized anode
electrode comprises smaller (less than about 100 nm), randomly
oriented grains of NiO. The grain refinement that occurs upon
re-oxidation is likely due to the large number of intragranular
pores that occur upon reduction which serve as nucleation sites.
Anode electrode samples reduced for a second time were also very
fine grained (less than about 200 nm) and contained significant
amounts of small intergranular porosity. The YSZ grains were
unaffected by the redox cycles. The grain refinement and
microstructural changes that occur after the first re-oxidation
cycle significantly alter the anode microstructure. These changes
also occur after every subsequent redox cycle, but the resulting
microstructure is similar to the microstructure resulting from the
initial re-oxidation. Thus, a new "redox cycled" equilibrium
microstructure is formed after the first redox cycle.
[0024] Notwithstanding, we have found that subsequent redox cycling
of solid oxide fuel cells in which the anode electrode has been
preconditioned in accordance with this invention does not
significantly affect the integrity of the other fuel cell
components and fuel cell component interfaces and, thus, does not
result in a significant degradation in the performance of the fuel
cell.
[0025] FIG. 3 shows the results of a thermomechanical analysis
(TMA) of solid oxide fuel cell anode electrode samples subjected to
oxidation at 600.degree. C. and 750.degree. C. As can be seen, the
rate and amount of oxidation-induced expansion of the anode
electrode sample was substantially reduced at 600.degree. C.
compared to expansion of the anode electrode at 750.degree. C. By
reducing the oxidation-induced expansion of the anode electrode,
the undesirable impact of the expansion on the integrity of other
fuel cell components and of the fuel cell component interfaces and,
thus, on the cell performance degradation is significantly
reduced.
[0026] The amount of electrochemical performance degradation of a
solid oxide fuel cell after redox cycling was characterized using a
single cell testing facility. The initial performance of the fuel
cell was characterized, after which air was blown over the anode
electrode for various amounts of time in order to re-oxidize the
anode electrode. The anode electrode was then reduced and the
electrochemical performance of the fuel cell was measured again.
The results are shown in FIG. 4. Baseline testing of fuel cell
redox tolerance showed that significant electrochemical performance
degradation occurs at redox times greater than about 60 minutes,
corresponding to a redox depth of about 30%, and that the greatest
amount of redox-inducing degradation occurs after the first redox
cycle to a 100% redox depth, occurring after 3.5 or more hours.
100% redox depth corresponds to all of the nickel in the reduced
anode electrode being re-oxidized.
[0027] FIG. 5 shows the results of single cell tests with redox
cycles performed at temperatures less than about 750.degree. C. As
can be seen, lowering the anode electrode oxidation temperature
significantly lowered the amount of electrochemical performance
degradation after redox cycling.
[0028] Thus, the combination of the TMA, SEM/TEM, baseline redox
single cell tests and lower temperature single cell tests clearly
suggests that the redox tolerance of the cell may be enhanced by a
low temperature oxidation treatment, which acts to condition the
microstructure of the anode electrode.
[0029] FIG. 6 shows a comparison of cumulative percent
electrochemical performance degradation versus redox time for a
baseline redox cell test and a preconditioned cell test, that is, a
test in which pre-oxidation of the electrode is carried out at
temperatures less than about 600.degree. C. As can be seen from the
figure, preconditioning the anode electrode microstructure
significantly enhances the cell redox tolerance compared to
baseline test cells. The results represent an average of three
tests for the baseline test cells and two tests for the
pre-oxidized test cells. All tests were carried out at 750.degree.
C. and 0.74 A/cm.sup.2.
[0030] Whereas FIG. 6 shows cumulative percentage degradation,
Table 1 shows the data in numerical form as a percentage of voltage
degradation per redox cycle. Comparison of the data easily shows
that the first full redox cycle (greater than 100% oxidation depth,
which means that more air is supplied to the electrode than is
needed to oxidize all of the nickel in the electrode) causes the
most degradation for baseline cells at -4.1% (Table 1). This is
still the case for the pre-oxidized cell, but the value is only
-0.9% degradation, thereby clearly showing that the pre-oxidation
of the electrode conditions the electrode microstructure to lower
further degradation. TABLE-US-00001 TABLE 1 Comparison of
Degradation from Thermal Cycling for Baseline and Pre-oxidized
Cells Redox Time Redox Depth Degradation (%) Degradation (%) (min)
(%) Baseline Cells Pre-oxidized Cells Initial 0 0 0.0 0.0 Redox 1
20 10 -0.1 -0.3 Redox 2 40 20 -0.2 -0.2 Redox 3 60 30 -0.7 -0.2
Redox 4 120 60 -1.6 -0.3 Redox 5 240 120 -4.1 -0.9 Redox 6 360 180
-1.7 -0.7
[0031] FIG. 7 shows the results of individual cell tests at the
same condition for a range of pre-oxidized cell tests and a
baseline test comparison. Several of these tests incorporate other
redox enhancements, but it still can be seen that pre-oxidation
alone produces the lowest redox degradation. This is likely due to
interference with the pre-oxidation process from the other redox
enhancements.
[0032] A key element for fabrication of a solid oxide fuel cell
anode electrode in accordance with one embodiment of the method of
this invention is subjecting the anode electrode metal oxide to a
controlled initial redox cycle where the initial re-oxidation step
takes place at temperatures of less than about 650.degree. C. There
are several stages in the fabrication of the anode electrode and/or
fuel cell at which the controlled cycle may be applied. In
accordance with one particularly preferred embodiment of this
invention, the initial controlled redox cycling is carried out
in-situ, that is, with the anode electrode as a component of an
assembled fuel cell. Alternatively, in accordance with one
embodiment of this invention, the initial redox cycling is carried
out on the anode electrode structure prior to assembly of the fuel
cell. In accordance with yet another embodiment of this invention,
the redox cycling is applied to the mixture of metal oxide/YSZ
particles prior to formation of the green anode structure or to the
metal oxide powders alone prior to mixing with YSZ.
[0033] While in the foregoing specification this invention has been
described in relation to certain preferred embodiments thereof, and
many details have been set forth for the purpose of illustration,
it will be apparent to those skilled in the art that the invention
is susceptible to additional embodiments and that certain of the
details described herein can be varied considerably without
departing from the basic principles of this invention.
* * * * *