U.S. patent application number 14/156251 was filed with the patent office on 2014-09-18 for ba-sr-co-fe-o based perovskite mixed conducting materials as cathode materials for intermediate temperature solid oxide fuel cells both in dual chamber and single chamber configuration.
The applicant listed for this patent is California Institute of Technology. Invention is credited to Sossina M. Haile, Zongping Shao.
Application Number | 20140272667 14/156251 |
Document ID | / |
Family ID | 33555414 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140272667 |
Kind Code |
A1 |
Haile; Sossina M. ; et
al. |
September 18, 2014 |
BA-SR-CO-FE-O BASED PEROVSKITE MIXED CONDUCTING MATERIALS AS
CATHODE MATERIALS FOR INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL
CELLS BOTH IN DUAL CHAMBER AND SINGLE CHAMBER CONFIGURATION
Abstract
Improved cathode active materials for reduced temperature
operation in single and dual chamber solid oxide fuel cells are
provided. The cathode active materials comprise perovskites of the
general form ABO.sub.3, where A is a cation with approximately a +2
charge, and B is a cation with approximately a +4 charge. These
perovskite cathode materials exhibit substantially enhanced power
generation at operation temperatures less than or equal to
600.degree. C.
Inventors: |
Haile; Sossina M.;
(Altadena, CA) ; Shao; Zongping; (Pasadena,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
California Institute of Technology |
Pasadena |
CA |
US |
|
|
Family ID: |
33555414 |
Appl. No.: |
14/156251 |
Filed: |
January 15, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12485816 |
Jun 16, 2009 |
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14156251 |
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10861828 |
Jun 4, 2004 |
7563533 |
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12485816 |
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60476413 |
Jun 5, 2003 |
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Current U.S.
Class: |
429/489 |
Current CPC
Class: |
H01M 2004/8689 20130101;
Y02E 60/50 20130101; H01M 4/92 20130101; H01M 8/1246 20130101; H01M
4/90 20130101; Y02E 60/525 20130101; H01M 4/9033 20130101; Y02P
70/50 20151101; H01M 4/8621 20130101; H01M 8/1253 20130101; H01M
8/126 20130101; Y02P 70/56 20151101 |
Class at
Publication: |
429/489 |
International
Class: |
H01M 4/90 20060101
H01M004/90 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The U.S. government has certain rights in this invention
pursuant to Grant No. N66 001-01-1-8966, awarded by the Defense
Advanced Research Projects Agency.
Claims
1.-63. (canceled)
64. A solid oxide fuel cell comprising: a cathode comprising a
compound of the general form
Ba.sub.1-xSr.sub.xCo.sub.1-yFe.sub.yO.sub.3-.delta. having a
perovskite crystal structure, wherein y is less than 1 and greater
than 0, x is greater than 0 and less than 1, and .delta. is less
than or equal to 1; an anode; and an electrolyte.
65. A solid oxide fuel cell according to claim 64, wherein the
solid oxide fuel cell is a single chamber solid oxide fuel
cell.
66. A solid oxide fuel cell according to claim 64, wherein the
solid oxide fuel cell is a dual chamber solid oxide fuel cell.
67. A solid oxide fuel cell according to claim 64, wherein x is
0.5.
68. A solid oxide fuel cell according to claim 64, wherein y is
0.2.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims the benefit of Provisional
Application. Ser. No. 60/476,413, filed Jun. 5, 2003, entitled
Ba--Sr--Co--Fe--O BASED PEROVSKITE MIXED CONDUCTING MATERIALS AS
CATHODE MATERIALS FOR INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL
CELLS BOTH IN DUAL CHAMBER AND SINGLE CHAMBER CONFIGURATION, the
entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to perovskite cathode
materials for use in reduced temperature solid oxide fuel
cells.
BACKGROUND OF THE INVENTION
[0004] A traditional solid oxide fuel cell comprises a cathode, an
anode and an electrolyte. Often, perovskites of the general form
ABO.sub.3-.delta. are used as the cathode active material. In such
a configuration, A and B both represent cations, and these cations
have historically both had charges of approximately +3.
[0005] The primary function of the cathode in the solid oxide fuel
cell is to facilitate the electrochemical reduction of oxygen,
which requires the diffusion of oxygen through the cathode. To that
end, the use of +3 charged cations in the perovskite cathode
material has long been thought to impart the fastest rate of oxygen
diffusion. However, even with this configuration, oxygen diffusion
remains the rate limiting step in the electroreduction process.
[0006] Notwithstanding the rate limiting nature of oxygen diffusion
in cathodes utilizing this perovskite configuration, solid oxide
fuel cells employing these cathode materials have exhibited
satisfactory power generation at very high temperatures, i.e.
800-1000.degree. C. Nonetheless, such high operating temperatures
lead to high costs and limit material compatibility. For example,
conventional solid oxide fuel cells use yttria-stabilized zirconia
(YSZ) as an electrolyte. In these fuel cells, the transition metal
perovskite (La.sub.1-xSr.sub.x)MnO.sub.3-.delta. (LSM) has
traditionally served as the cathode. However, the electrochemical
reduction of oxygen over LSM creates a high activation energy,
rendering the LSM cathode material inappropriate for reduced
temperature operation.
[0007] Efforts have been made to develop a cathode material
suitable for reduced temperature operation. However, these efforts
have focused on mixed electron and oxygen ion conducting
perovskites such as doped LaCoO.sub.3, doped LaFeO.sub.3 and doped
SmCoO.sub.3. For example, the perovskites
La.sub.1-xSr.sub.xCo.sub.yFe.sub.1-yO.sub.3-.delta. (LSCF) and
Sm.sub.0.5Sr.sub.0.5CoO.sub.3-.delta. (SSC) have shown particularly
high activities in the 600 to 800.degree. C. temperature range.
Although these cathode materials exhibit substantially improved
performance compared to LSM, no cathodes suitable for operation at
temperatures less than 600.degree. C. have yet been developed.
Furthermore, these perovskite cathode materials are far too active
for propane catalytic oxidation in high efficiency single chamber
fuel cells. Accordingly, a need arises for a perovskite cathode
material that exhibits accelerated oxygen diffusion, and that is
suitable for reduced temperature operation in both single and dual
chamber fuel cells.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to a perovskite cathode
material with accelerated oxygen diffusion for reduced temperature
operation in both single and dual chamber solid oxide fuel cells.
In one embodiment, the perovskite takes the general form
ABO.sub.3-.delta., where A is any cation with approximately a +2
charge, and B is any cation with approximately a +4 charge. In
another embodiment, the A site cation is selected from the group
consisting of alkaline earth metal cations, and the B site cation
is selected from the group consisting of transition metal
cations.
[0009] In a particularly preferred embodiment the A site cation
comprises a mixture of cations, the average charge of the mixed
cations being approximately +2. Alternatively, the B site cation
can comprise a mixture of cations, the average charge of the mixed
cations being approximately +4. Preferably, both the A and B site
cations comprise mixtures of cations, the average charge of the
mixed A site cations being approximately +2, and the average charge
of the mixed B site cations being approximately +4.
[0010] In another preferred embodiment, the A site cation is
selected from the group consisting of alkaline earth metal cations.
More preferably, the A site cation is a mixture of alkaline earth
metal cations. Even more preferably, the A site cation is a mixture
of Ba and another alkaline earth metal cation.
[0011] In yet another preferred embodiment, the B site cation is
selected from the group consisting of transition metal cations.
More preferably, the B site cation is a mixture of transition metal
cations. Even more preferably the B site cation is a mixture of Co
and another transition metal cation. In a particularly preferred
embodiment, the A site cation comprises a mixture of Ba and Sr, and
the B site cation comprises a mixture of Co and Fe.
[0012] Historically, perovskite cathode materials have been stable
only at high temperatures and high oxygen partial pressures.
However, the perovskite cathode materials of the present invention
are stable at substantially reduced temperatures. Furthermore, the
perovskite configuration with a +2 charged A site cation and a +4
charged B site cation exhibits substantially accelerated oxygen
diffusion through the cathode, eliminating oxygen diffusion as the
rate limiting step in the electrochemical reduction of oxygen. In
addition, this accelerated oxygen diffusion through the cathode
enables the fuel cell to operate at substantially reduced
temperatures, thereby reducing costs and eliminating limitations of
material compatibility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other features and advantages of the present
invention will be better understood by reference to the following
detailed description when considered in conjunction with the
accompanying drawings wherein:
[0014] FIG. 1 is a graphical representation of the cell voltages
and power densities of the BSCF cathode as functions of the current
density, obtained through operation in a dual chamber solid oxide
fuel cell;
[0015] FIG. 2 is a graphical representation of the cell voltages
and power densities of the BSCF+SDC cathode as functions of the
current density, obtained through operation in a single chamber
solid oxide fuel cell;
[0016] FIG. 3 is a schematic depicting a dual chamber fuel cell;
and
[0017] FIG. 4 is a schematic depicting a single chamber fuel
cell.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The present invention is directed to a perovskite cathode
active material for reduced temperature operation of single and
dual chamber solid oxide fuel cells. In one embodiment, the
perovskite takes the general form ABO.sub.3-.delta., wherein A is
any cation with approximately a +2 charge, and B is any cation with
approximately a +4 charge. As is known in the art, .delta.
represents the oxygen vacancy concentration, and is less than or
equal to 1. Preferably, the A site cation is selected from the
group consisting of alkaline earth metal cations having
approximately a +2 charge, and the B site cation is selected from
the group consisting of transition metal cations having
approximately a +4 charge.
[0019] As used herein, the term "+2 charge" refers to a charge of
approximately +2, and may vary slightly as necessary to maintain
the neutrality of the perovskite based on the oxygen vacancy
concentration, as determined by .delta.. Similarly, the term "+4
charge" refers to a charge of approximately +4 and may vary
slightly as necessary to maintain the neutrality of the perovskite
based on the oxygen vacancy concentration as determined by
.delta..
[0020] In a preferred embodiment, the perovskite takes the general
form (A'.sub.1-xA''.sub.x)BO.sub.3-.delta., where A' and A'' are
any cations, the average charge of A' and A'' is approximately +2,
B is any cation having approximately a +4 charge, and x is less
than or equal to 1. Preferably, both A' and A'' are selected from
the group consisting of alkaline earth metal cations, and B is
selected from the group consisting of transition metal cations.
[0021] Alternatively, the perovskite can take the general form
A(B'.sub.1-yB''.sub.y)O.sub.3-.delta., where A is any cation having
approximately a +2 charge, B' and B'' are any cations, the average
charge of B' and B'' is approximately +4, and y is less than or
equal to 1. Preferably, A is selected from the group consisting of
alkaline earth metal cations, and B' and B'' are both selected from
the group consisting of transition metal cations.
[0022] In a particularly preferred embodiment, the perovskite can
take the general form
(A'.sub.1-xA''.sub.x)(B'.sub.1-yB''.sub.y)O.sub.3-.delta., where A'
and A'' are any cations, the average charge of A' and A'' is
approximately +2, B' and B'' are any cations, the average charge of
B' and B'' is approximately +4, and x and y are both less than or
equal to 1. Preferably, both A' and A'' are selected from the group
consisting of alkaline earth metal cations, and both B' and B'' are
selected from the group consisting of transition metal cations.
[0023] In another preferred embodiment, the perovskite can take the
form BaBO.sub.3-.delta., where B is any cation having approximately
at +4 charge. Preferably, B is selected from the group consisting
of transition metal cations.
[0024] The perovskite of the invention can also take the form
Ba(B'.sub.1-yB''.sub.y)O.sub.3-.delta., where B' and B'' are any
cations, the average charge of B' and B'' is approximately +4, and
y is less than or equal to 1. Preferably, B' and B'' are selected
from the group consisting of transition metal cations.
[0025] In another preferred embodiment, the perovskite can take the
form (Ba.sub.1-xA.sub.x)BO.sub.3-.delta., where A is any cation
other than Ba, the average charge of Ba and A is approximately +2,
B is any cation having approximately a +4 charge, and x is less
than or equal to 1. Preferably, A is any alkaline earth metal
cation other than Ba, and B is selected from the group consisting
of transition metal cations.
[0026] In an even more preferred embodiment, the perovskite can
take the form
(Ba.sub.1-xA.sub.x)(B'.sub.1-yB''.sub.y)O.sub.3-.delta., where A is
any cation other than Ba, the average charge of Ba and A is
approximately +2, B' and B'' are any cations, the average charge of
B' and B'' is approximately +4, and both x and y are less than or
equal to 1. Preferably, A is any alkaline earth metal cation other
than Ba, and B' and B'' are both selected from the group consisting
of transition metal cations.
[0027] In another preferred embodiment, the perovskite can take the
general form
(Ba.sub.1-xSr.sub.x)(B'.sub.1-yB''.sub.y)O.sub.3-.delta., wherein
B' and B'' are any cations, the average charge of B' and B'' is
approximately +4, and both x and y are less than or equal to one.
Preferably, B' and B'' are selected from the group consisting of
transition metal cations.
[0028] In yet another preferred embodiment, the perovskite can take
the general form
(Ba.sub.1-xSr.sub.x)(Co.sub.1-yB.sub.y)O.sub.3-.delta., where B is
any cation other than Co, the average charge of Co and B is
approximately +4, and x and y are both less than or equal to 1.
Preferably, B is selected from the group consisting of transition
metal cations.
[0029] In a more preferred embodiment, the perovskite can take the
general form
(Ba.sub.1-xSr.sub.x)(Co.sub.1-yFe.sub.y)O.sub.3-.delta., where x
and y are both less than or equal to 1. Preferably, x is 0.5 and y
is 0.2.
[0030] In an alternative embodiment, the perovskite can be combined
with a compatible electrolyte material. In this embodiment, the
cathode material for use in the fuel cell comprises not only the
perovskite, but also a porous interlayer of the electrolyte
material. Preferably, the electrolyte material is present in the
cathode active material in an amount ranging from about 0 to about
40% by weight of the total weight of the cathode active material.
Nonlimiting examples of compatible electrolyte materials include
SDC, gadallinium doped ceria, Sc doped zirconia, yttria doped
zirconia and La--Sr--Ga--Mg--O perovskites.
[0031] In another alternative embodiment, the perovskite can be
combined with a precious metal. In this embodiment, the cathode for
use in the fuel cell comprises not only the perovskite, but also
the precious metal. Preferably, the precious metal is present in
the cathode active material in an amount ranging from about 0 to
about 60% by weight of the total weight of the cathode active
material. Nonlimiting examples of suitable precious metals include
Ag, Au, Pt, Pd and mixtures thereof. In yet another alternative
embodiment, the perovskite may be combined with both a porous
interlayer of electrolyte material and a precious metal.
[0032] The accelerated diffusion of oxygen through perovskites
employing a +2 charged A site cation and a +4 charged B site cation
was first noticed during the development of a cubic perovskite in
the BaCoO.sub.3--SrCoO.sub.3 system as a high temperature oxygen
permeation membrane material, and reported in Shao, Z. P., Yang, W.
S., Cong, T., Dong, H., Tong, J. H. & Xiong, G. X.,
"Investigation of the Permeation Behavior and Stability of a
Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3-.delta. Oxygen
Membrane," J. Membr. Sci. 172, 177-188 (2000). However, perovskites
of this configuration have not been used successfully as cathode
active materials for reduced temperature solid oxide fuel cells.
Related materials, such as SrCo.sub.0.8Fe.sub.0.2O.sub.3 and Sr- or
Mg-doped LaGaO.sub.3 have been used as cathodes, but only under
high operating temperatures, and they have exhibited only slight
improvements in power generation over SSC.
[0033] In contrast, the perovskites of the present invention
exhibit substantial improvement over SSC and are remarkably
efficient at substantially reduced temperatures. In addition, as
demonstrated by the following examples, these perovskites are
effective at these lower temperatures in both single chamber and
dual chamber fuel cells, and are compatible with known anode and
electrolyte materials.
[0034] A fuel cell 10 utilizing a cathode according to this
invention is illustrated in FIGS. 3 and 4 and includes an anode 14,
a cathode 16 and an electrolyte 12. A fuel cell may be operated in
a conventional dual chamber configuration, as shown in FIG. 3, or
in a single chamber configuration, as shown in FIG. 4. In a single
chamber configuration, the anode 14 and cathode 16 of the fuel cell
10 are located in the same chamber and are exposed to the same
oxidant-fuel mixture. The anode 14 is active and selective for fuel
partial oxidation and for electrooxidation of the resulting H.sub.2
and CO. The cathode 16, in contrast, is active and selective for
oxygen-electroreduction.
[0035] A dual chamber fuel cell, as shown in FIG. 3, operates in
much the same manner as the single chamber fuel cell. However, the
anode 14 and cathode 16 of the fuel cell 10, in a dual chamber
configuration, are located in separate chambers. Accordingly, the
fuel and the oxidant do not combine in a dual chamber fuel cell.
Rather, the fuel is introduced into the anode 14 chamber, and the
oxidant, usually air, is introduced into the cathode 16 chamber.
However, the functions of the anode 14 and cathode 16 are the same
as in a single chamber fuel cell.
[0036] When used in dual chamber fuel cells, the perovskites of
this invention are useful in the 350 to 1000.degree. C. temperature
range. However, the perovskites of the invention are particularly
useful in the 350 to 800.degree. C. range. For example, power
densities ranging from about 100 to about 1000 mW/cm.sup.2 were
obtained at a temperature of approximately 600.degree. C.
Nonlimiting examples of compatible fuels for this dual chamber
configuration include hydrogen, methane, propane and other
hydrocarbons, and mixtures of fuel and water.
[0037] The perovskites of the invention are also useful in single
chamber fuel cells utilizing fuel-oxidant mixtures. Nonlimiting
examples of compatible fuels for this single chamber fuel cell
include methane, ethane, propane and other hydrocarbons, such as
alcohols. The oxidant is primarily air. Under these conditions,
power densities ranging from about 100 to about 500 mW/cm.sup.2
were obtained at temperatures ranging from 450 to 600.degree.
C.
EXAMPLE 1
Dual Chamber Fuel Cell
[0038] A conventional, trilayer fuel cell was constructed using
samaria doped ceria (SDC) as the electrolyte. A 20 .mu.m, thin
electrolyte layer was supported on a 700 .mu.m thick Ni+SDC anode
having a porosity of approximately 46%. A 10 to 20 .mu.m thick
Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3 (BSCF) cathode
layer was deposited on the opposing side, after first depositing an
additional porous interlayer of SDC (less than 5 .mu.m in
thickness).The cathode contained approximately 70% by weight BSCF
and approximately 30% by weight SDC. The cathode was deposited on
the electrolyte by spraying a colloidal solution, and then calcined
at 950.degree. C. for five hours. The cathode had an area of 0.71
cm.sup.2 and a thickness of approximately 10 .mu.m.
[0039] Air was supplied to the cathode chamber and 3%
H.sub.2O-humidified H.sub.2 to the anode chamber. Peak power
densities of approximately 1010 mW/cm.sup.2 and 402 mW/cm.sup.2
were obtained at 600 and 500.degree. C. respectively. These values
are more than twice those measured for a similar cell using a
SSC+SDC cathode. In addition, the cell resistances under open
circuit conditions were measured at various temperatures by
impedance spectroscopy. The electrode polarization resistance, i.e.
the sum of anode and cathode area specific resistances, is only
approximately 0.021.OMEGA./cm.sup.2 at 600.degree. C., and
0.135.OMEGA./cm.sup.2 at 500.degree. C., amounting to just 14 and
26% of the resistance of the electrolyte at these respective
temperatures. Although still very active for oxygen
electroreduction, composite SDC+BSCF cathodes yielded lower power
densities than simple BSCF cathodes. FIG. 1 shows the cell voltage
and power densities of the BSCF cathode as functions of the current
density.
EXAMPLE 2
Single Chamber Fuel Cell
[0040] The same trilayer fuel cell as in Example 1 was constructed
and operated in a single chamber configuration with a
propane+O.sub.2+He mixture in a 4:9:36 volumetric ratio as the feed
gas at a total flow rate of 490 ml/min. The gas composition was
kept constant with propane flowing at a rate of 40 ml/min, O.sub.2
flowing at a rate of 90 ml/min, and He flowing at a rate of 360
ml/min. However, at 600.degree. C., the O.sub.2 flow rate was
increased to 100 ml/min and the He flow rate was increased to 400
ml/min. The linear gas flow velocity was about 10-15 cm/s. A peak
power density of approximately 391 mW/cm.sup.2 was observed at a
furnace set temperature of 575.degree. C., with a current density
at short circuit of approximately 1.9 A/cm.sup.2. A power density
of approximately 350 mW/cm.sup.2 was observed at a temperature of
525.degree. C. with a current density of 1.7 A/cm.sup.2. An
analogous fuel cell fabricated using SSC as the cathode yielded
near zero power density at 575.degree. C. and a power density of
approximately 175 mW/cm.sup.2 at 525.degree. C., with a current
density at short circuit of approximately 1.3 A/cm.sup.2. An open
cell voltage of 0.75 V was reached at 450.degree. C., and decreased
slightly with an increase of the furnace set temperature.
[0041] Upon modifying the BSCF cathode to incorporate 30 wt % SDC,
significant improvements over simple BSCF were observed. A peak
power density of approximately 440 mW/cm.sup.2 was achieved at a
furnace set temperature of 500.degree. C. A comparably high power
density of 403 mW/cm.sup.2 at 500.degree. C. has been reported for
an electrolyte-supported fuel cell using SSC+SDC as the cathode and
ethane as the fuel. However, this cathode was incompatible with
propane at temperatures higher than 450.degree. C. Because of the
heat release during partial oxidation at the anode, the real
temperature of the single chamber fuel cell is about 150 to
245.degree. C. higher than the furnace set temperature, depending
on the operation conditions. This self-heating phenomenon in single
chamber fuel cell configurations accounts for the higher power
densities achieved in single chamber configurations as compared to
dual chamber configurations at nominally low temperatures. FIG. 2
shows the cell voltage and power densities of the BSCF+SDC cathode
as functions of the current density.
Testing Methods
[0042] The mechanisms responsible for the excellent performance of
these cathode active materials were identified by oxygen
permeability measurements and extensive impedance spectroscopy
studies of symmetric cells using a BSCF perovskite configuration.
The oxygen permeation measurements, combined with thermal
gravimetric analysis to determine the oxygen vacancy concentration
as a function of oxygen partial pressure, revealed that the oxygen
vacancy diffusion rate is 1.3.times.10.sup.-4 cm.sup.2/s at
900.degree. C. and 7.3.times.10.sup.-5 cm.sup.2/s at 775.degree. C.
In comparison, a Sm.sub.0.5Sr.sub.0.5CoO.sub.3 (SSC) perovskite
configuration has a reported value of 8.6.times.10.sup.-7
cm.sup.2/s at 915.degree. C., a
La.sub.0.6Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3 (LSCF) perovskite
configuration has a value of 8.4.times.10.sup.-6 cm.sup.2/s at
900.degree. C., and a SrCo.sub.0.8Fe.sub.0.2O.sub.3 (SCF)
perovskite configuration has a value of 5.1.times.10.sup.-5
cm.sup.2/s at 900.degree. C. In addition, the activation energy for
oxygen diffusion in BSCF was found to be less than half that for
oxygen surface exchange, i.e. 46.+-.2 kJ/mol versus 113.+-.11
kJ/mol, suggesting that oxygen surface exchange is the rate
limiting step at low temperatures and that the exceptionally high
oxygen diffusivity through BSCF gives it its overall high rate of
oxygen electro-oxidation. The oxygen ion conductivity is, in fact,
higher than that of SDC, an electrolyte used in these solid oxide
fuel cells.
[0043] Impedance spectroscopy of the symmetric cells also
demonstrated that oxygen diffusion is rapid and surface exchange
kinetics are rate limiting. Specifically, good linearity of the
cathode area specific resistance versus reciprocal temperature was
observed over the temperatures investigated, i.e. 400 to
725.degree. C., and the derived activation energy (approximately
116 kJ/mol) was almost identical to that determined for the oxygen
surface exchange step (113.+-.114 kJ/mol). Also, at low
temperatures, the cathode area specific resistance was sensitive to
the presence of CO.sub.2 and H.sub.2O in the atmosphere, gases
which could only affect surface and not bulk properties.
Additionally, an increase in the cathode thickness decreased the
area specific resistance without changing the activation energy, a
result presumably due to the increase in area over which surface
exchange could occur. Finally, the possibility that interfacial
charge transfer could be the rate limiting step was eliminated by
the fact that no arc associated with this step appeared in the
impedance data.
[0044] When used in single chamber fuel cells, cathode active
materials must exhibit a low activity toward fuel oxidation under
the oxidant and fuel environment. The perovskite cathode materials
of this invention not only exhibit high activity of oxygen
electroreduction, but also exhibit low activity toward fuel
oxidation needed for use in single chamber fuel cells. For example,
under stoichiometric conditions, i.e. O.sub.2 to propane ratio of
5:1 with 95 vol % helium, and at 500.degree. C., the propane
conversion rates over BSCF, LSCF and SSC are 5.3%, 35.5% and 16.1%
respectively.
[0045] The preceding description has been presented with reference
to presently preferred embodiments of the invention. Workers in the
art and technology to which this invention pertains will appreciate
that alterations and changes may be made to the described
embodiments without meaningfully departing from the principal,
spirit and scope of this invention. Accordingly, the foregoing
description should not be read as pertaining only to the precise
embodiments described, but rather should be read consistent with
and as support for the following claims, which are to have their
fullest and fairest scope.
* * * * *