U.S. patent application number 15/493546 was filed with the patent office on 2017-08-03 for sulfur tolerant anode for solid oxide fuel cell.
The applicant listed for this patent is Ceramatec, Inc.. Invention is credited to Singaravelu Elangovan, Joseph J Hartvigsen.
Application Number | 20170222245 15/493546 |
Document ID | / |
Family ID | 40512076 |
Filed Date | 2017-08-03 |
United States Patent
Application |
20170222245 |
Kind Code |
A1 |
Elangovan; Singaravelu ; et
al. |
August 3, 2017 |
SULFUR TOLERANT ANODE FOR SOLID OXIDE FUEL CELL
Abstract
A solid oxide fuel cell (SOFC) (100) for use in generating
electricity while tolerating sulfur content in a fuel input stream.
The solid oxide fuel cell (100) includes an electrolyte (106), a
cathode (102), and a sulfur tolerant anode (104). The cathode (102)
is disposed on a first side of the electrolyte (106). The sulfur
tolerant anode (104) is disposed on a second side of the
electrolyte (106) opposite the cathode (102). The sulfur tolerant
anode (104) includes a composition of nickel, copper, and ceria to
exhibit a substantially stable operating voltage at a constant
current density in the presence of the sulfur content within the
fuel input stream. The solid oxide fuel cell (100) is useful within
a SOFC stack to generate electricity from reformate which includes
synthesis gas (syngas) and sulfur content. The solid oxide fuel
cell (100) is also useful within a SOFC stack to generate
electricity from unreformed hydrocarbon fuel.
Inventors: |
Elangovan; Singaravelu;
(South Jordan, UT) ; Hartvigsen; Joseph J;
(Kaysville, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ceramatec, Inc. |
Salt Lake City |
UT |
US |
|
|
Family ID: |
40512076 |
Appl. No.: |
15/493546 |
Filed: |
April 21, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12240725 |
Sep 29, 2008 |
9666871 |
|
|
15493546 |
|
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|
|
60975761 |
Sep 27, 2007 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/9066 20130101;
H01M 4/8652 20130101; H01M 4/8621 20130101; H01M 2008/1293
20130101; H01M 8/2404 20160201; H01M 8/2425 20130101; H01M 4/905
20130101; H01M 4/9058 20130101; H01M 4/8846 20130101; H01M 4/8642
20130101; H01M 8/1213 20130101; Y02E 60/50 20130101 |
International
Class: |
H01M 8/1213 20060101
H01M008/1213; H01M 4/90 20060101 H01M004/90; H01M 4/88 20060101
H01M004/88; H01M 4/86 20060101 H01M004/86 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The U.S. Government has certain rights in this invention as
provided for by the terms of Small Business Innovation Research
(SBIR) Contract No. FA8650-07-M-2704 awarded by the U.S. Air Force.
Claims
1. A method for making a solid oxide fuel cell (SOFC), the method
comprising: disposing a cathode on a first side of an electrolyte;
and disposing a sulfur tolerant anode on a second side of the
electrolyte, wherein the sulfur tolerant anode comprises a mixture
of nickel (Ni), copper (Cu), and ceria (CeO.sub.2) to operate at a
substantially stable operating voltage at a constant current
density in the presence of a fuel with a measurable sulfur
content.
2. The method of claim 1, wherein disposing the sulfur tolerant
anode on the second side of the electrolyte comprises: mixing
nitrates of the nickel, copper, and ceria; and using a Pechini
process to convert the nitrates to mixture of oxides to dispose on
the electrolyte.
3. The method of claim 2, further comprising mixing a nitrate of
magnesium (Mg) or another electrochemically inert ceramic oxide
precursor with the nitrates of the nickel, copper, and ceria.
4. The method of claim 2, further comprising mixing a nitrate of
cobalt (Co) or a nitrate of praseodymium (Pr) with the nitrates of
the nickel, copper, and ceria.
5. The method of claim 2, further comprising mixing the nitrate of
ceria with a. dopant.
6. The method of claim 1, wherein disposing the sulfur tolerant
anode on the second side of the electrolyte comprises: mixing
nitrates of the nickel, copper, and ceria; and using a glycine
nitrate process to convert the nitrates to mixture of oxides to
dispose the on the electrolyte.
7. The method of claim 1, wherein disposing the sulfur tolerant
anode on the second side of the electrolyte comprises: making a
mixture of: oxides of the nickel, copper, and ceria; an
electrochemically inert ceramic oxide; and a catalyst; and using a
solid state process to convert the oxides to mixture of oxides to
dispose the mixture on the electrolyte.
8. The method of claim 1, wherein disposing the sulfur tolerant
anode on the second side of the electrolyte comprises: making a
mixture of components for the sulfur tolerant anode, wherein the
components comprise the nickel, copper, and ceria; and.
infiltrating the mixture into a porous material.
9. The method of claim 8, wherein the mixture comprises nitrates of
the nickel, copper, and ceria.
10. The method of claim 8, wherein the mixture comprises fine
particles of the nickel, copper, and ceria.
11. The method of claim 8, wherein the porous material comprises an
electrolyte material.
12. The method of claim 8, wherein the porous material comprises an
inert material disposed on the electrolyte.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of and claims priority to
U.S. patent application Ser. No. 12/240,725, filed on Sep. 29, 2008
(the '725 Application). The '725 Application claims the benefit of
U.S. Provisional Patent Application No. 60/975,761, filed on Sep.
27, 2007, entitled "Sulfur Tolerant Anode." These applications are
expressly incorporated herein by reference.
BACKGROUND
[0003] Embodiments of this invention relate generally to the field
of solid oxide fuel cells and, more specifically, to anode
structures for solid oxide fuel cells.
[0004] A solid oxide fuel cell (SOFC) electrochemically converts
fuel into electricity. The solid oxide fuel cell has a solid oxide,
or ceramic, electrolyte between a cathode and an anode. A
conventional solid oxide fuel cell utilizes an yttria-stabilized
zirconia (YSZ) electrolyte between the cathode and the anode. In
general, the cathode reduces oxygen from the air into oxygen ions
and passes the oxygen ions through the electrolyte to the anode. A
conventional cathode material is lanthanum strontium manganite
(LSM), or a similar material. The anode uses the oxygen ions to
oxidize the fuel, which results in free electrons at the anode. The
anode is typically a ceramic/metallic (cermet) material that
includes YSZ as the ceramic and nickel (Ni) as the metal. By
connecting an electrical load between the anode and the cathode
(outside of the fuel cell), the electrons can return to the
cathode, and the electrical generation cycle can repeat.
[0005] FIG. 1 depicts a schematic block diagram of a conventional
SOFC system 10. The conventional SOFC system 10 includes a reformer
12, a sulfur trap 14, and a conventional solid oxide fuel cell 16.
Sulfur can rapidly poison and deactivate the Ni-YSZ cermet anode of
the solid oxide fuel cell 16. Since many fuels contain total sulfur
levels that far exceed the levels that can damage the typical anode
of the solid oxide fuel cell 16, the reformer 12 and the sulfur
trap 14 are used to remove sulfur content from the fuel. Typical
fuels which may be reformed by the reformer 12 include military
fuels such as JP-8, JP-5, and NATO F-76. While military fuel
sources are energy dense, these fuels are extremely complex in
composition and contain a number of impurities and additives that
present challenges for compact electrochemical power generation. JP
fuels can contain as much as 3,000-4,000 ppm by weight sulfur,
while Navy fuels (NATO F-76, etc.) can include as much as 10,000
ppm by weight.
[0006] The reformer 12 implements a reformation process to break
down hydrocarbons (C.sub.xH.sub.y) from the fuel into reformate
which includes synthesis gas (syngas) and hydrogen sulfide
(H.sub.25). The syngas includes hydrogen (H.sub.2) and carbon
monoxide (CO), and also may include other components such as carbon
dioxide (CO.sub.2) and steam (H.sub.2O). Although reformation by
the reformer 12 reduces the sulfur content, typical sulfur levels
for reformate from JP fuels is about 500-600 ppmv from an
endothermic steam reformer and 300-400 ppmv from a partial
oxidation (POx) reformer.
[0007] Hydrogen sulfide (H.sub.2S) content in the reformate of 2
ppmv (at 1000.degree. C.) is known to poison the anode of a
conventional solid oxide fuel cell 16. Additionally, sulfur
poisoning increases the polarization resistance and over-voltage of
the anode at as low as 0.5 ppmv (at 900.degree. C.). This
concentration of H.sub.2S is close to the equilibrium values
measured at that temperature for the chemisorption of H.sub.2S to
achieve full coverage on nickel steam-reforming catalysts. Because
of this coverage, conventional Ni-YSZ cermet anodes are not sulfur
tolerant.
[0008] Since the sulfur content of the reformate (i.e., syngas and
sulfur) from the reformer 12 is significant enough to poison the
anode, the reformate is directed through the sulfur trap 14 to
remove the remaining sulfur content, leaving only the syngas. Many
of the sulfur compounds present in these fuel streams are mildly
reactive and, therefore, are relatively easy to remove. However,
there are also considerable quantities of more complex sulfur
compounds, including substituted thiophenes, which can be
particularly difficult to remove via conventional liquid phase
adsorption processes. In addition, even with the best possible
liquid phase sulfur removal technology, the conventional solid
oxide fuel cell is not capable of accommodating sulfur in the fuel
stream due to intermittent malfunctions of the sulfur trap 14, or
another sulfur removal system. In other words, occasional sulfur
slip is anticipated. Additionally, the adsorption of liquid phase
sulfur removal materials utilizes relatively large amounts of
material, which increases the size and resources of the
conventional SOFC system 10.
[0009] In contrast to the conventional Ni-YSZ cermet anodes, other
conventional SOFC devices use a doped ceria (cerium oxide
(CeO.sub.2)) anode or a copper-ceria (Cu--CeO.sub.2) anode. While
ceria provides sulfur tolerance and some avoidance of coking if
there is hydrocarbon slip, ceria is a mixed conductor in a fuel
atmosphere and has low electronic conductivity. Thus, ceria alone
does not provide a low polarization loss for a high performance
anode. Infiltration of cerium nitrate (Ce(NO.sub.3).sub.3) into a
conventional Ni-YSZ cermet anode may show some level of tolerance
to the presence of H.sub.2S.
SUMMARY
[0010] Embodiments of an apparatus are described. In one
embodiment, the apparatus is a solid oxide fuel cell (SOFC). The
solid oxide fuel cell may be implemented to generate electricity
while tolerating sulfur content in a fuel input stream. The solid
oxide fuel cell includes an electrolyte, a cathode, and a sulfur
tolerant anode. The cathode is disposed on a first side of the
electrolyte. The sulfur tolerant anode is disposed on a second side
of the electrolyte opposite the cathode. The sulfur tolerant anode
includes a composition of nickel, copper, and ceria to exhibit a
substantially stable operating voltage at a constant current
density in the presence of the sulfur content within the fuel input
stream. Other embodiments of the solid oxide fuel cell are also
described.
[0011] The solid oxide fuel cell is useful within a SOFC stack,
which includes a plurality of solid oxide fuel cells coupled
together, for example, in a serial configuration. Embodiments of
the SOFC stack are useful to generate electricity from reformate
which includes synthesis gas (syngas) and sulfur content.
Embodiments of the SOFC stack are also useful to generate
electricity from unreformed hydrocarbon fuel. Other embodiments of
a SOFC system which uses a SOFC stack with sulfur tolerant anodes
are also described.
[0012] Embodiments of a method are also described. In one
embodiment, the method is a method of making a solid oxide fuel
cell. The method includes disposing a cathode on a first side of an
electrolyte. The method also includes disposing a sulfur tolerant
anode on a second side of the electrolyte. The sulfur tolerant
anode operates at a substantially stable operating voltage at a
constant current density in the presence of a fuel with a
measurable sulfur content. In some embodiments, nitrates or oxides
may be mixed and calcined using a Pechini or glycine nitrate
process and the resulting anode disposed on the electrolyte. In
some embodiments, a mixture of components may be disposed on the
electrolyte using a solid state process. In some embodiments, the
mixture of components may be a mixture or nitrates or fine
particles (e.g., nanoparticles) which are infiltrated into a porous
material, such as the electrolyte or an inert material disposed on
the electrolyte.
[0013] Some embodiments may combine two or more of the various
structures and/or functions described herein. Other aspects and
advantages of embodiments of the present invention will become
apparent from the following detailed description, taken in
conjunction with the accompanying drawings, illustrated by way of
example of the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 depicts a schematic block diagram of a conventional
solid oxide fuel cell (SOFC) system.
[0015] FIG. 2 depicts a schematic block diagram of one embodiment
of a sulfur tolerant solid oxide fuel cell.
[0016] FIG. 3 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell of FIG. 2,
including a catalyst disposed on the anode.
[0017] FIG. 4 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell of FIG. 2
with the anode infiltrated into a portion of the electrolyte.
[0018] FIG. 5 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell of FIG. 4
with a graded ceria anode.
[0019] FIG. 6 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell of FIG. 4
with a graded copper anode.
[0020] FIG. 7 depicts a schematic block diagram of one embodiment
of a SOFC system which includes the sulfur tolerant solid oxide
fuel cell of FIG. 2.
[0021] FIG. 8 depicts a schematic block diagram of another
embodiment of a SOFC system which includes the sulfur tolerant
solid oxide fuel cell of FIG. 2.
[0022] Throughout the description, similar reference numbers may be
used to identify similar elements.
DETAILED DESCRIPTION
[0023] It will be readily understood that the components of the
embodiments as generally described herein and illustrated in the
appended Figures could be arranged and designed in a wide variety
of different configurations. Thus, the following more detailed
description of various embodiments, as represented in the Figures,
is not intended to limit the scope of the present disclosure, but
is merely representative of various embodiments. While the various
aspects of the embodiments are presented in drawings, the drawings
are not necessarily drawn to scale unless specifically
indicated.
[0024] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by this detailed description. All changes which come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
[0025] Reference throughout this specification to features,
advantages, or similar language does not imply that all of the
features and advantages that may be realized with the present
invention should be or are in any single embodiment of the
invention. Rather, language referring to the features and
advantages is understood to mean that a specific feature,
advantage, or characteristic described in connection with an
embodiment is included in at least one embodiment of the present
invention. Thus, discussion of the features and advantages, and
similar language, throughout this specification may, but do not
necessarily, refer to the same embodiment.
[0026] Furthermore, the described features, advantages, and
characteristics of the invention may be combined in any suitable
manner in one or more embodiments. One skilled in the relevant art
will recognize, in light of the description herein, that the
invention can be practiced without one or more of the specific
features or advantages of a particular embodiment. In other
instances, additional features and advantages may be recognized in
certain embodiments that may not be present in all embodiments of
the invention.
[0027] Reference throughout this specification to "one embodiment,"
"an embodiment," or similar language means that a particular
feature, structure, or characteristic described in connection with
the indicated embodiment is included in at least one embodiment of
the present invention. Thus, the phrases "in one embodiment," "in
an embodiment," and similar language throughout this specification
may, but do not necessarily, all refer to the same embodiment.
[0028] In the following description, specific details of various
embodiments are provided. However, some embodiments may be
practiced without at least some of these specific details. In other
instances, certain methods, procedures, components, and circuits
are not described in detail for the sake of brevity and clarity,
but are nevertheless understood from the context of the description
herein.
[0029] In general, the described embodiments are directed to a
sulfur tolerant anode for a solid oxide fuel cell (SOFC). Since
typical hydrocarbon fuels have sulfur content, and sulfur can
irreparably damage the anode of a conventional solid oxide fuel
cell, embodiments described herein relate to a solid oxide fuel
cell with an anode that is tolerant of sulfur. The sulfur tolerance
of the anode may be manifested by the ability of the solid oxide
fuel cell to generate electricity at a substantially stable
operating voltage at a constant current density, even though the
operating voltage may be relatively lower when sulfur is present in
the input field stream.
[0030] Various improvements can be made over an anode of a
conventional solid oxide fuel cell in order to improve the sulfur
tolerance of the solid oxide fuel cell. In one embodiment, some or
all of the YSZ, in a conventional Ni-YSZ, cermet anode can be
replaced with doped ceria. The doped ceria contributes to sulfur
tolerance of the anode. The doped ceria also contributes to coke
suppression at the anode.
[0031] In another embodiment, some of the nickel in a conventional
Ni-YSZ cermet anode can be replaced with an electrochemically inert
oxide material. Some examples of electrochemically inert oxide
materials include thorium oxide (ThO.sub.2), zirconium oxide
(ZrO.sub.2), magnesium oxide (MgO), a titanium oxide (TiO.sub.2),
potassium oxide (K.sub.2O), and a. tungsten oxide (WO.sub.2),
although other embodiments may use other electrochemically inert
oxide materials. The addition of the electrochemically inert oxide
material contributes to sulfur tolerance and coke suppression of
the anode.
[0032] In another embodiment, copper is added to the anode
structure. The copper contributes to oxidation of some
hydrocarbons. Thus, the copper within the anode facilitates direct
oxidation of certain hydrocarbon fuels. When combined with the
sulfur tolerance properties achieved by the addition of ceria and
the electrochemically inert oxide material (e.g., MgO), the direct
oxidation by the copper allows a hydrocarbon fuel source with
sulfur content to be processed by the solid oxide fuel cell without
significant degradation of the performance of the solid oxide fuel
cell.
[0033] By implementing a sulfur tolerant anode in a solid oxide
fuel cell, the solid oxide fuel cell may be used in a system with a
reformer and a sulfur trap, even if the sulfur trap malfunctions
and allows sulfur to reach the solid oxide fuel cell. Also, the
solid oxide fuel cell with a sulfur tolerant anode may be used in a
system which omits the sulfur trap, so that the solid oxide fuel
cell processes reformate which includes syngas and sulfur content.
Additionally, the solid oxide fuel cell with the sulfur tolerant
anode may be used in a system which omits both the reformer and the
sulfur trap, so that the solid oxide fuel cell with the sulfur
tolerant anode processes hydrocarbon fuel using direct oxidation.
The omission of the sulfur trap and/or the reformer from a SOFC
power generation system allows embodiments of the SOFC power
generation system to be made lighter, simpler, and more
reliable.
[0034] FIG. 2 depicts a schematic block diagram of one embodiment
of a sulfur tolerant solid oxide fuel cell 100. The illustrated
sulfur tolerant solid oxide fuel cell 100 includes a cathode 102, a
sulfur tolerant anode 104, and an electrolyte 106. In general, the
cathode 102 extracts oxygen (O.sub.2) from an input oxidant (e.g.,
ambient air) and reduces the oxygen into oxygen ions. The remaining
gases are exhausted from the solid oxide fuel cell 100. The
electrolyte 106 diffuses the oxygen ions from the cathode 102 to
the anode 104. The anode 104 uses the oxygen ions to oxidize
hydrogen (H.sub.2) from the input fuel (i.e., combine the hydrogen
and the oxygen ions). The oxidation of the hydrogen forms water
(H.sub.2O) and free electrons (e.sup.-). The water exits the anode
104 with any excess fuel and sulfur. The free electrons can travel
through a circuit (shown dashed with a load 108) between the anode
104 and the cathode 102. When combined with other solid oxide fuel
cells 100 within a SOFC stack, the power generation capabilities of
all of the solid oxide fuel cells 100 can be combined to output
more power.
[0035] The anode 104 is sulfur tolerant and, in some embodiments,
receives fuel with sulfur content. The fuel may be reformate which
includes syngas and sulfur. Alternatively the fuel may be a
hydrocarbon fuel which includes sulfur containing compounds.
Although embodiments of the solid oxide fuel cell are described
herein as being sulfur tolerant and capable of processing fuel in
the presence of sulfur, without significant degradation,
embodiments of the solid oxide fuel cell are also capable of
processing fuel in the absence of sulfur.
[0036] One example of a sulfur tolerant anode 104 is an anode
formed with nickel, copper, magnesium oxide, and ceria (e.g.,
Ni--Cu--MgO--ceria). The nickel in the anode 104 contributes to
oxidation of hydrogen and carbon monoxide. The magnesium oxide
decreases the amount of nickel and increases the sulfur tolerance
of the anode 104. The magnesium oxide also suppresses coke
formation at the anode 104. Although magnesium oxide is
specifically referenced in this example, other embodiments may use
other types of electrochemically inert oxide materials. The ceria
in the anode 104 also increases sulfur tolerance of the anode 104
and suppresses coke formation at the anode 104. The copper allows
direct oxidation of certain hydrocarbons. The copper also maintains
a relatively high conductivity of the anode 104. Thus, the
combination of nickel, copper, magnesium oxide, and ceria results
in an anode structure 104 which is substantially sulfur tolerant
and coke resistant, while maintaining a relatively high
conductivity. The sulfur tolerance and direct oxidation
capabilities of the anode 104 allow embodiments of the solid oxide
fuel cell 100 to directly process hydrocarbon fuels which might
have sulfur content associated with the fuel.
[0037] The sulfur tolerant anode 104 may be made and disposed on
the electrolyte 106 using any suitable technique. In one
embodiment, the anode materials are applied to a porous surface of
the electrolyte 106 so that the anode materials at least partially
infiltrate the electrolyte 106. An example of infiltrating the
electrolyte 106 with anode materials is shown in FIG. 4 and
described in more detail below. In contrast to infiltration
techniques, some embodiments of the solid oxide fuel cell 100
implement the anode materials disposed substantially on a surface
of the electrolyte 106, rather than infiltrating the structure of
the electrolyte 106.
[0038] In one embodiment, the anode materials are synthesized using
a Pechini, or glycine nitrate, process. In general, the Pechini
process involves mixing nitrates of the anode materials (e.g., Ni,
Cu, Co, Mg, ceria, and a dopant for the ceria) with ethylene glycol
and citric acid, charring the mixture at about 150.degree. C.,
calcining the char at about 1000.degree. C., and milling the
resulting material. The resulting anode is made into a paste using
a binder and solvent system, and the paste is screen printed onto
the electrolyte 106 and fired. The Pechini process is described in
greater detail in U.S. Pat. No. 3,330,697, entitled "Method of
preparing lead and alkaline earth titanates and niobates and
coating method using the same to form a capacitor."
[0039] In another embodiment, the anode materials are synthesized
using a solid state process. In one embodiment, oxides and
carbonates of the anode components are admixed and calcined to
obtain a two phase mixture. A resulting anode powder is made into a
paste using a binder, and the paste is screen printed onto the
electrolyte 106, which is then fired.
[0040] Thus, in various embodiments, raw materials in the form of
nitrates, oxides, and carbonates of copper, nickel, and one or more
inert oxides, as appropriate for Pechini, glycine nitrate, solid
state or any known process for ceramic and cermet powder
manufacture, are used as the starting materials. When the anode
material is prepared, the anode material contains a mixture of the
oxides of nickel, copper, and the inert oxide. The oxides of
nickel, copper, and the inert oxide may be in the form of a solid
solution or as separate oxide phases. When exposed to the fuel gas
such as hydrogen or syngas, only the NiO and CuO in the solid
solution is reduced to a metallic phase as distinct nickel and
copper, or more commonly as an alloy of Ni and Cu, leaving an
extremely fine dispersion of the inert oxide. One example of the
inert oxide is MgO, although other inert oxides are described
herein. In particular, other inert oxides that have at least 1 to
20 mole percent solubility in the (NiCu)O solid solution can also
be employed. With a solid solution of NiO, CuO, and MgO, there is a
substantial region of the phase diagram (not shown) where the solid
solution of the three oxides is present as (Mg, Ni, Cu)O. In all
cases, only the oxides of Ni and Cu reduce to the metallic phase,
leaving a fine dispersion of inert oxide in the metal grains.
[0041] FIG. 3 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell 100 of FIG.
2, including a catalyst 110 disposed on the anode 104. The catalyst
110 also contributes to the sulfur tolerance of the anode 104.
Additionally, the catalyst 110 may improve other aspects of the
electrochemical performance of the anode 104 and the solid oxide
fuel cell 100. The catalyst material being present as very fine,
high surface area particles on the interior porous surface of the
anode structure enhances the adsorption and dissociation of the
fuel molecules and enables diffusion of the adsorbed and
dissociated species to the electrochemical reaction sites. This
improves the reaction kinetics and results in improved performance.
Some examples of catalyst materials include praseodymium (Pr) and
cobalt (Co), although other types of rare earth metals may be used.
In an embodiment which uses cobalt as the catalyst 110, the anode
structure can be described, generally, by the chemical formula
Ni--Co--Cu--MgO-ceria. In some embodiments, ceria may be used as
the catalyst 110.
[0042] The catalyst 110 is shown as a separate layer from the anode
104, but in some embodiments the catalyst 110 may be disposed
within a surface portion of the anode 104. For example, the
catalyst 110 may be infiltrated into a portion of the anode 104.
More specifically, infiltrant cations of the catalyst 110 may be
introduced during anode synthesis. In one embodiment, a liquid form
of nitrates of cobalt and/or praseodymium is impregnated into a
porous anode material so that porous spaces within the anode
material receive a coating of the liquid catalyst material. Upon
heating, the cobalt and praseodymium salts are converted to cobalt
and praseodymium oxides which are subsequently reduced to cobalt
and praseodymium metals prior to operation which coat the anode
104. In some embodiments, the resulting content of the catalyst 110
disposed on the anode 104 is generally less than about 1% by
weight, although other embodiments may result in more than 1% by
weight.
[0043] FIG. 4 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell 100 of FIG.
2 with the anode 104 infiltrated into a portion of the electrolyte
106. In the depicted solid oxide fuel cell 100, the anode 104 is
not formed as a separate material which is applied to a surface of
the electrolyte 106. Rather, the anode 104 is formed by
infiltration of materials which are applied to a porous surface and
allowed to fill in porous spaces of the electrolyte 106.
[0044] In one embodiment, the materials are combined within a
liquid mixture (e.g., a nitrate mixture) which is infiltrated into
a porous portion of the electrolyte 106. In some embodiments, the
materials are combined as fine particles (e.g., nanoparticles)
which are then infiltrated into a porous portion of the electrolyte
106. The porous portion of the electrolyte 106 may be formed of an
electrolyte material such as zirconia (e.g., YSZ) or ceria.
Alternatively, the mixture of anode materials may be infiltrated
into a porous inert material such as alumina (Al.sub.2O.sub.3)
disposed on the electrolyte 106. In this embodiment, the anode
materials are not infiltrated directly into the electrolyte 106,
but are infiltrated into the inert material disposed on a surface
of the electrolyte 106.
[0045] FIG. 5 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell 100 of FIG.
4 with a graded ceria anode 104. In general, the arrows within the
anode 104 portion of the solid oxide fuel cell 100 indicate an
increasing concentration of ceria in the direction of the arrows
and, correspondingly, a decreasing concentration of other
components such as zirconia electrolyte material in the opposite
direction. As mentioned above, the addition of ceria in the anode
104 can enhance the sulfur tolerance of the anode 104. The ceria
also suppresses coke formation at the anode 104 so that carbon
particles (e.g., from cracked hydrocarbons) do not form a film on
the anode 104 and prevent the anode 104 from facilitating the
oxidation of fuel and generation of free electrons. However, it may
be difficult to dispose a ceria-based anode on the electrolyte
106.
[0046] In order to dispose a ceria-based anode on the electrolyte
106, it may be useful to mix the ceria with at least some of the
electrolyte material. For example, the anode 104 may include a
mixture of ceria and YSZ. Additionally, the mixture of ceria and
YSZ may be graded within the anode 104 so that there is more
electrolyte material (e.g zirconia) near the electrolyte 106 and
more ceria near the exposed surface of the anode 104. In other
words, the graded ceria anode 104 has a proportional concentration
of the ceria and the YSZ, which varies approximately relative to a
distance from the electrolyte 106. In this way, the higher
concentration of electrolyte material near the electrolyte 106
helps the anode 104 to be disposed on the electrolyte 106, while
the higher concentration of ceria near the exposed surface of the
anode 104 facilitates sulfur tolerance and coke suppression.
[0047] FIG. 6 depicts a schematic block diagram of another
embodiment of the sulfur tolerant solid oxide fuel cell 100 of FIG.
4 with a graded copper anode 104. Like the graded ceria anode 104
shown in FIG. 5 and described above, the graded copper anode 104 of
FIG. 6 has a varying concentration of copper within the anode 104
of the solid electrolyte fuel cell 100. In the depicted embodiment,
the concentration of copper within the anode 104 is relatively
higher at a fuel input end of the anode 104, as indicated by the
arrow, and lower at a fuel output end of the anode 104. Thus, the
copper concentration within the sulfur tolerant anode 104 varies
along a distance between a fuel input end of the sulfur tolerant
anode 104 and a fuel output end of the sulfur tolerant anode
104.
[0048] While copper within the anode 104 provides direct oxidation
of certain hydrocarbons, as well as suppression of coke formation,
copper does not oxidize some lower hydrocarbons such as methane.
Therefore, fully distributed copper within the anode 104 (along the
length of the fuel path) may limit the type of fuel that may be
processed and utilized in the solid oxide fuel cell 100. Hence, the
graded copper anode 104 includes decreasing concentrations of
copper along the fuel flow direction of the anode 104. In this way,
higher hydrocarbons are oxidized near the fuel entrance, or intake,
and lower hydrocarbons are oxidized closer to the fuel output, or
outlet. Additionally, some embodiments of the graded copper anode
104 facilitate internal reformation, in addition to direct
oxidation. Thus, for these reasons, the graded copper anode 104 can
facilitate higher fuel utilization compared with an anode that does
not include copper or an anode that only includes evenly
distributed copper.
[0049] FIG. 7 depicts a schematic block diagram of one embodiment
of a SOFC system 120 which includes the sulfur tolerant solid oxide
fuel cell 100 of FIG. 2. The illustrated SOFC system 120 includes
the reformer 12 operably connected to the solid oxide fuel cell
100. The reformer 12 at least partially oxidizes the fuel to
produce reformate. The reformate includes syngas and sulfur,
although the sulfur concentration of the reformate may be
significantly lower than the sulfur concentration of the unreformed
fuel.
[0050] The reformate, including the syngas and sulfur, is directed
from the reformer 12 to the solid oxide fuel cell 100. In contrast
to the conventional SOFC system 10 illustrated in FIG. 1, the SOFC
system 10 of FIG. 7 omits the sulfur trap 14. However, the
illustrated SOFC system of FIG. 7 is also representative of a SOFC
system which includes a malfunctioning, or otherwise inoperable,
sulfur trap 14. In the absence of the sulfur trap 14, or in the
conditions of an inoperable sulfur trap 14, the solid oxide fuel
cell 100 with the sulfur tolerant anode 104 is nevertheless capable
of operating to generate electricity even though the reformate
includes sulfur. Moreover, embodiments of the solid oxide fuel cell
100 may exhibit a substantially stable operating voltage, despite
the presence of sulfur in the reformate which is used by the solid
oxide fuel cell 100.
[0051] FIG. 8 depicts a schematic block diagram of another
embodiment of a SOFC system 120 which includes the sulfur tolerant
solid oxide fuel cell 100 of FIG. 2. In the illustrated SOFC system
130, both the reformer 12 and the sulfur trap 14 are omitted. Thus,
the fuel is fed directly into the solid oxide fuel cell 100 to
generate electricity. Depending on the sulfur content of the fuel,
and the composition of the solid oxide fuel cell 100, the solid
oxide fuel cell 100 may be capable of directly oxidizing the fuel
despite relatively high sulfur content.
[0052] It should be noted that references herein to the solid oxide
fuel cell 100 may also be used to refer to a stack of solid oxide
fuel cells 100. The stack of solid oxide fuel cells 100 may be
referred to as a SOFC stack. Thus, depictions and descriptions of
an individual solid oxide fuel cell 100 are also representative of
a SOFC stack. In particular, the SOFC systems 120 and 130 of FIGS.
7 and 8 may include SOFC stacks instead of individual solid oxide
fuel cells 100.
[0053] With various sulfur concentrations of different fuels, for
the direct oxidation the sufur concentration can be up to 15 ppm by
weight and for the syngas case the sulfur concentration could be as
high as 1,000 ppm by volume. The effect of sulfur on the anode
performance is strongly temperature dependent. While sulfur
concentrations as high as 200 ppmv, typical of most reformed fuel,
is tolerated with only a minor loss in operating voltage at a
constant current density at 700 to 800.degree. C. operating
temperature, a much higher sulfur concentration as much as 1,000
ppm can be tolerated when the cell is operated at temperatures of
900 to 1000.degree. C. Some embodiments may operate at different
operating voltages, current densities, and/or temperatures.
[0054] At a typical constant current density operation, the anode
performance will drop only about 20 to 100 mV when sulfur
containing fuel is introduced, relative to the operating voltage of
the anode when operated with sulfur free fuel. The exact magnitude
of change is dependent on the sulfur concentration, operating
current density, and temperature. Higher sulfur concentration and
higher current density will cause higher voltage drop. In contrast
a higher operating temperature will cause lower voltage drop.
[0055] More generally, the relationships among sulfur
concentration, current density, operating temperature, and
operating voltage allows one or more of the operating
characteristics to be adjusted or optimized based on one or more of
the remaining operating characteristics. Additionally, the
relationship among voltage, current, and resistance can also affect
the operation of the sulfur tolerant anode. For example, the load
108 could be a constant resistance load, in which case the voltage
and current of the solid oxide fuel cell 100 effectively go down in
response to degradation of the sulfur tolerant anode 104.
Alternatively, the load 108 could be a constant current load, in
which case only the voltage goes down in response to degradation of
the sulfur tolerant anode 104. In another embodiment, the load 108
could be a constant voltage load, in which case only the current
goes down in response to degradation of the sulfur tolerant anode
104. Thus, embodiments of the solid oxide fuel cell 100 can achieve
a stable output power, in which the voltage and current are tied to
the internal cell resistance.
[0056] Although the operations of the method(s) herein are shown
and described in a particular order, the order of the operations of
each method may be altered so that certain operations may be
performed in an inverse order or so that certain operations may be
performed, at least in part, concurrently with other operations. In
another embodiment, instructions or sub-operations of distinct
operations may be implemented in an intermittent and/or alternating
manner.
[0057] Although specific embodiments of the invention have been
described and illustrated, the invention is not to be limited to
the specific forms or arrangements of parts so described and
illustrated. The scope of the invention is to be defined by the
claims appended hereto and their equivalents.
* * * * *