U.S. patent application number 09/835905 was filed with the patent office on 2002-02-07 for fuel cell anode structures for voltage reversal tolerance.
Invention is credited to Gascoyne, John M., Knights, Shanna D., Ralph, Thomas R., Taylor, Jared L., Wilkinson, David P..
Application Number | 20020015879 09/835905 |
Document ID | / |
Family ID | 26847469 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015879 |
Kind Code |
A1 |
Gascoyne, John M. ; et
al. |
February 7, 2002 |
Fuel cell anode structures for voltage reversal tolerance
Abstract
An improved fuel cell anode structure comprises a substrate and
a first carbon-based component. The first carbon-based component
exhibits little or no resistance to corrosion. When said anode
structure is incorporated into a membrane electrode assembly, the
membrane electrode assembly is tolerant to incidences of cell
voltage reversal.
Inventors: |
Gascoyne, John M.; (Bucks,
GB) ; Knights, Shanna D.; (Burnaby, CA) ;
Ralph, Thomas R.; (Reading, GB) ; Taylor, Jared
L.; (Davis, CA) ; Wilkinson, David P.; (North
Vancouver, CA) |
Correspondence
Address: |
McAndrews, Held & Malloy, Ltd.
500 West Madison Street, 34th Floor
Chicago
IL
60661
US
|
Family ID: |
26847469 |
Appl. No.: |
09/835905 |
Filed: |
April 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09835905 |
Apr 16, 2001 |
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09585696 |
Jun 1, 2000 |
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60150253 |
Aug 23, 1999 |
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Current U.S.
Class: |
429/432 ;
429/483; 429/492; 429/532; 429/534 |
Current CPC
Class: |
H01M 8/04582 20130101;
H01M 2300/0082 20130101; H01M 10/4235 20130101; H01M 4/926
20130101; H01M 8/1007 20160201; H01M 4/90 20130101; H01M 8/04119
20130101; H01M 8/04902 20130101; H01M 8/04671 20130101; Y02E 60/10
20130101; Y02E 60/50 20130101; H01M 2300/0005 20130101; H01M
8/04291 20130101; H01M 2004/8684 20130101; H01M 8/1004 20130101;
H01M 8/04559 20130101; H01M 4/8605 20130101; H01M 4/9083 20130101;
H01M 4/925 20130101; H01M 4/9075 20130101 |
Class at
Publication: |
429/44 ;
429/42 |
International
Class: |
H01M 004/96 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2001 |
GB |
PCT/GB01/00458 |
Claims
What is claimed is:
1. In a fuel cell anode structure comprising a substrate and a
first carbon-based component comprising a first carbon material,
the improvement comprising: said first carbon-based component
having substantially no resistance to corrosion.
2. The improved anode structure of claim 1 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
3. The improved anode structure of claim 1 wherein said substrate
is a gas diffusion layer.
4. The improved anode structure of claim 3 wherein said first
carbon-based component is disposed on said gas diffusion layer.
5. The improved anode structure of claim 3 wherein said first
carbon-based component is disposed within said gas diffusion
layer.
6. The improved anode structure of claim 3 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
7. The improved anode structure of claim 6 wherein said first
carbon-based component and said second carbon component are mixed
and disposed on said gas diffusion layer.
8. The improved anode structure of claim 6 wherein said first
carbon-based component and said second carbon component are mixed
and disposed within said gas diffusion layer.
9. The improved anode structure of claim 6 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers on said gas diffusion layer.
10. The improved anode structure of claim 6 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers within said gas diffusion layer.
11. The improved anode structure of claim 1 wherein said substrate
is a solid polymer electrolyte.
12. The improved anode structure of claim 11 wherein said first
carbon-based component is disposed on said solid polymer
electrolyte.
13. The improved anode structure of claim 11 wherein said first
carbon-based component is disposed within said solid polymer
electrolyte.
14. The improved anode structure of claim 11 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
15. The improved anode structure of claim 11 wherein said first
carbon-based component and said second carbon component are mixed
and disposed on said solid polymer electrolyte.
16. The improved anode structure of claim 11 wherein said first
carbon-based component and said second carbon component are mixed
and disposed within said solid polymer electrolyte.
17. The improved anode structure of claim 11 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers on said solid polymer electrolyte.
18. The improved anode structure of claim 11 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers within said solid polymer
electrolyte.
19. The improved anode structure of claim 1 wherein said first
carbon material has a BET surface area of at least 350
m.sup.2g.sup.-1.
20. The improved anode structure of claim 19 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
21. The improved anode structure of claim 19 wherein said substrate
is a gas diffusion layer.
22. The improved anode structure of claim 21 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
23. The improved anode structure of claim 22 wherein said first
carbon-based component and said second carbon component are mixed
and disposed on said gas diffusion layer.
24. The improved anode structure of claim 22 wherein said first
carbon-based component and said second carbon component are mixed
and disposed within said gas diffusion layer.
25. The improved anode structure of claim 22 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers on said gas diffusion layer.
26. The improved anode structure of claim 22 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers within said gas diffusion layer.
27. The improved anode structure of claim 20 wherein said substrate
is a solid polymer electrolyte.
28. The improved anode structure of claim 27 further comprising a
second carbon component, said second carbon component being
substantially more resistant to corrosion than said first
carbon-based component.
29. The improved anode structure of claim 28 wherein said first
carbon-based component and said second carbon component are mixed
and disposed on said solid polymer electrolyte.
30. The improved anode structure of claim 28 wherein said first
carbon-based component and said second carbon component are mixed
and disposed within said solid polymer electrolyte.
31. The improved anode structure of claim 28 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers on said solid polymer electrolyte.
32. The improved anode structure of claim 28 wherein said first
carbon-based component and said second carbon component are
disposed in separate layers within said solid polymer
electrolyte.
33. The improved anode structure of any one of claims 2, 6-10,
14-18, 20, 22-26 and 28-32 wherein the second carbon component acts
as a support for an electrocatalyst material.
34. The improved anode structure of any one of claims 6-10 and
22-26 wherein said second carbon component is a carbon fill for
said gas diffusion layer.
35. A membrane electrode assembly comprising the improved anode
structure of any one of claims 1-32, wherein said membrane
electrode assembly is voltage reversal tolerant.
36. A fuel cell comprising a membrane electrode assembly comprising
the improved anode structure of any one of claims 1-32.
37. A fuel cell comprising the improved anode structure of any one
of claims 1-32.
38. A method of improving tolerance of a fuel cell to voltage
reversal, the method comprising incorporating in said fuel cell the
improved anode structure of any one of claims 1-32.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/585,696 filed Jun. 1, 2000, entitled "Fuel
Cell Anode Structures For Voltage Reversal Tolerance". The '696
application is, in turn, related to and claims priority benefits
from U.S. Provisional Pat. App. Ser. No. 60/150,253 filed Aug. 23,
1999. This application is also related to and claims priority
benefits from PCT/International Application No. PCT/GB01/00458
filed Feb. 6, 2001. The '696, '253 and '458 applications are each
incorporated by reference herein in their entireties.
FIELD OF THE INVENTION
[0002] The present invention relates to an anode structure
comprising a substrate and a first carbon-based component that
shows little or no resistance to corrosion, such that when the
anode structure is incorporated into a membrane electrode assembly,
the membrane electrode assembly is substantially tolerant to
incidences of cell voltage reversal.
Background Of The Invention
[0003] A fuel cell is an energy conversion device that efficiently
converts chemical energy into electrical energy by
electrochemically combining either hydrogen, normally stored as a
gas, or methanol, normally stored as a liquid or gas, with oxygen,
normally in the form of air, to generate electrical power. At their
fundamental level, fuel cells comprise electrochemical cells formed
from a solid or liquid electrolyte and two electrodes, the anode
side and cathode side, at which the desired electrochemical
reactions take place. In the fuel cell, the hydrogen or methanol is
oxidized at the anode side and the oxygen is reduced at the cathode
side to generate the electrical power.
[0004] Normally in fuel cells the reactants are in gaseous form and
are diffused into the anode and cathode structures. The electrode
structures are therefore specifically designed to be porous to gas
diffusion in order to facilitate contact between the reactants and
the reaction sites in the electrode to promote the reaction rate.
Efficient removal of the reaction products from the electrode
structures is also important. In cases where liquid reactants and
products are present the electrode structures are often tailored to
efficiently feed reactants to and remove products from the reaction
sites. The electrolyte is also in contact with both electrodes and
in fuel cell devices may be acidic or alkaline, as well as liquid
or solid in nature.
[0005] The proton exchange membrane fuel cell (PEMFC) is the likely
type of fuel cell to find wide application as an efficient and low
emission power generation technology for a range of markets, such
as in a range of stationary, residential and portable power
generation devices and as an alternative to the internal combustion
engine for transportation. In the PEMFC, whether hydrogen or
methanol fuelled, the electrolyte is a solid proton-conducting
polymer membrane, commonly based on perfluorosulfonic acid
materials.
[0006] In the PEMFC, the combined laminate structure formed from
the membrane and the two electrode structures is known as a
membrane electrode assembly (MEA) . The MEA typically comprises
several layers, but in general can be considered to comprise five
layers that are characterized by their function. On either side of
the membrane an anode electrocatalyst or a cathode electrocatalyst
is incorporated to increase the rates of the desired electrode
reactions. In contact with the electrocatalyst containing layers,
on the opposite face to that in contact with the membrane, are the
anode and cathode gas diffusion layers. The anode gas diffusion
layer is typically porous to allow the reactant hydrogen or
methanol to enter from the face of the layer exposed to the
reactant fuel supply. The reactant then diffuses through the
thickness of the gas diffusion layer to the layer containing the
electrocatalyst, which is usually platinum metal based, to
facilitate the electrochemical oxidation of hydrogen or methanol.
The anode electrocatalyst layer also typically comprises some level
of proton-conducting electrolyte in contact with the same
electrocatalyst reaction sites. With acidic electrolyte types, the
product of the anode reaction is protons, and the protons are then
transported from the anode reaction sites through the electrolyte
to the cathode layers. The cathode gas diffusion layer is also
typically porous to allow oxygen or air to enter the layer and
diffuse through to the electrocatalyst layer reaction sites. The
cathode electrocatalyst facilitates the chemical combination of the
protons with oxygen to produce water, and also typically comprises
some level of the proton-conducting electrolyte in contact with the
same electrocatalyst reaction sites. Product water then diffuses
out of the cathode structure. The structure of the cathode is
normally designed to enable efficient removal of product water. If
water builds up at or in the cathode, it becomes more difficult for
the reactant oxygen to diffuse to the reactant sites, and thus the
performance of the fuel cell deteriorates. In the case of
methanol-fuelled PEMFCs, additional water is present due to the
water contained in the methanol, which can be transported through
the membrane from the anode to the cathode side. The increased
quantity of water at the cathode requires additional water removal
capabilities. However, it is also the case with proton-conducting
membrane electrolytes, that if too much water is removed from the
cathode structure, the membrane can dry out, thereby resulting in a
significant decrease in the performance of the fuel cell.
[0007] The complete MEA can be constructed by several methods. The
electrocatalyst layers can be bonded to one surface of the gas
diffusion layer to form what is known as a catalyzed gas diffusion
layer or gas diffusion electrode. Two gas diffusion electrodes can
be combined with the solid proton-conducting membrane to form the
MEA. Alternatively, two porous uncatalyzed gas diffusion layers can
be combined with a solid proton-conducting polymer membrane that is
catalyzed on both sides to form the MEA. Further, one gas diffusion
electrode can be combined with one uncatalyzed gas diffusion layer
and a solid proton-conducting polymer membrane that is catalyzed on
the side facing the gas diffusion layer to form the MEA.
[0008] The materials typically employed in the fabrication of the
uncatalyzed gas diffusion layers of the MEA comprise high density
materials such as rigid carbon fiber paper (such as, for example,
Toray TGP-H-60 or TGP-H-90 from Toray Industries, Japan) or woven
carbon cloths (such as Zoltek PWB-3 from Zoltek Corporation, 3101
McKelvey Road, St. Louis, Mo., USA 63044). Layers such as these are
usually modified with a particulate material either embedded within
the fiber network or coated on to the large planar surfaces, or a
combination of both. Typically, ,these particulate materials
comprise a carbon black and polymer mix. The particulate carbon
black material is, for example, an oil furnace black (such as
Vulcan XC72R from Cabot Chemicals, Billerica, Mass., USA) or an
acetylene black (such as Shawinigan from Chevron Chemicals,
Houston, Tex., USA). The polymer most frequently employed is
polytetrafluoroethylene (PTFE). The coating, or embedding, is
carried out in order to improve the water management properties,
improve gas diffusion characteristics, to provide a continuous
surface on which to apply the catalyst layer and to improve the
electrical conductivity. More recently, electrode structures based
on gas diffusion layers comprising a non-woven network of carbon
fibers (carbon fiber structures such as Optimat 203 from Technical
Fiber Products, Kendal, Cumbria, UK) with a particulate material
embedded within the fiber network, as disclosed in European Patent
Publication No. 0791974, have shown comparable performances to
structures based on carbon fiber paper or cloth.
[0009] The electrocatalyst materials for the anode and cathode
structures typically comprise precious metals, in particular
platinum, as these have been found to be the most efficient and
stable electrocatalysts for low-temperature fuel cells such as the
PEMFC. Platinum is employed either on its own as the only
electrocatalytic metal or in combination with other precious metals
or base metals. The platinum-based electrocatalyst is provided as
very small particles (approximately 20-50.ANG. in diameter) of high
surface area, which are usually distributed on and supported by
larger macroscopic conducting carbon particles to provide a desired
catalyst loading. Conducting carbons are the preferred materials to
support the catalyst. Particulate carbon black materials typically
employed include Vulcan XC72R and Shawinigan. It is also possible
to employ a platinum-based electrocatalyst that does not
incorporate a support, and in this case it is referred to as an
unsupported Pt electrocatalyst.
[0010] Each MEA in the PEMFC is sandwiched between electrically
conducting flow field plates that are conventionally based upon
carbon and contain channels that feed the MEA with the reactants
and through which the products are removed. Since each MEA
typically delivers 0.6-0.7 V, usually between 10 to 100 such MEAs
are each interposed between flow field plates to form stacks. These
stacks are combined electrically in series or parallel to produce
the desired power output for a given application.
[0011] Recently, it has been observed that during prolonged
operation some cells in large stacks can go into an undesired
condition known as cell voltage reversal or, simply, cell reversal.
This is shown by the cell potential becoming negative rather than
the positive value associated with normal PEMFC operation. Such
cell reversals can be due to depletion in the concentration of the
reactants at the cathode or anode sides, which can be caused by a
number of factors such as restricted gas flow due to blocked flow
fields or poor water distribution in the MEA. In combination with
this, especially in situations in which a fast dynamic response is
required, such as in transportation applications, it is possible
that the gas flow cannot respond quickly enough to sustain the
current demand. Further, if one cell in a stack shows cell
reversal, adjacent cells in the stack may also overheat, resulting
in cell reversal.
[0012] If the cell reversal is due to a restricted oxygen
concentration at the electrocatalyst sites in the cathode then, to
sustain the flow of current, hydrogen is produced at the
cathode,
2H.sup.++2e.sup.-.fwdarw.H.sub.2
[0013] Since hydrogen production at the cathode is very facile at
the platinum-based electrocatalysts typically employed, the
electrode potential is usually only slightly more negative than
that for hydrogen oxidation at the anode. The result is that at
normal operating current densities the cell voltage is normally
slightly negative, for example, -0.1 V. This type of cell reversal
raises safety and durability concerns, since hydrogen is being
produced in the oxidant side of the cell, a significant quantity of
heat is generated, and water is no longer being produced at the
cathode. Such product water helps to sustain membrane hydration,
especially at the membrane-anode interface, since it promotes the
back-diffusion of water.
[0014] A major problem occurs, however, if the hydrogen
concentration is restricted at the anode side. In this case to
sustain the flow of current water electrolysis and carbon corrosion
can occur, as follows:
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.-
C+2H.sub.2O.fwdarw.CO.sub.2+4H.sup.++4e.sup.-
[0015] Since both electrode reactions occur at more positive
electrode potentials than oxygen reduction at the cathode, again,
the cell voltage is negative, but in this case the cell voltage may
be as high as -0.8 V at typical operating current densities. While
carbon corrosion is favored over water electrolysis
thermodynamically, the electrochemical kinetics of water
electrolysis are sufficiently facile at the platinum-based
electrocatalysts typically employed in the PEMFC that initially
water electrolysis principally sustains the current. There is only
a small contribution from corrosion of the carbon components in the
anode to the cell current. If, however, the anode catalyst becomes
deactivated for water electrolysis or if the water concentration at
the electrocatalyst sites in the anode becomes significantly
depleted, the water electrolysis current is gradually replaced by
increased rates of carbon corrosion. In the case of carbon
corrosion, water need only be present in the vicinity of the
relevant, abundant carbon surfaces. During this period the cell
voltage becomes more negative (that is, the anode potential becomes
more positive) to provide the necessary driving force for carbon
corrosion. This in turn increases the driving force for the water
electrolysis reaction. The result, if such cell reversal is
prolonged, may be irreversible damage to the membrane and catalyst
layers due to excessive dehydration and localized heating. Further,
the catalyst carbon support in the anode structure corrodes, with
eventual dissolution of the platinum-based catalyst from the
support, and the anode gas diffusion layer may become degraded due
to corrosion of the carbon present in the gas diffusion layer
structure. In cases where the bipolar flow field plates are based
upon carbon the anode flow field plate may also be subjected to
significant carbon corrosion, thereby resulting in surface pitting
and damage to the flow field pattern.
[0016] It would therefore be a significant advantage to protect the
MEA from the effects of cell reversal should a cell go into cell
reversal.
SUMMARY OF THE INVENTION
[0017] An anode structure for a proton exchange membrane fuel cell
(PEMFC) comprises a substrate and a first carbon-based component
comprising a first carbon material. The first carbon-based
component exhibits little or no resistance to corrosion. When the
present anode structure is incorporated into a membrane electrode
assembly, the MEA is substantially tolerant to incidences of cell
reversal.
[0018] The term "anode structure" in the context of the present
specification means any of the functional components and structures
associated with the anode side of the MEA through which a fuel is
either transported or reacted, that is, within the gas diffusion
and electrocatalyst containing layers on the anode side of the
membrane. The practical embodiments of the present anode structure
as herein defined include:
[0019] (a) a gas diffusion layer;
[0020] (b) an electrocatalyst containing layer bonded to a gas
diffusion layer (also referred to as a gas diffusion electrode or a
catalyst-coated gas diffusion layer);
[0021] (c) an electrocatalyst containing layer bonded to the
proton-conducting membrane (also referred to as a catalyst-coated
membrane).
[0022] In the context of the present specification, the term
"substrate" refers to a gas diffusion layer or a polymer membrane
electrolyte.
[0023] The first carbon-based of the present anode structure
component may consist entirely of a first carbon material or may
comprise a first carbon material and one or more other materials
that may for example be present to promote the corrosion rate of
the first carbon material or to act as a binder. The one or more
other materials that may be present in the first carbon-based
component include polymeric materials such as, for example, a
proton-conducting polymer electrolyte, such as Nafion.RTM., or a
non-proton-conducting polymer such as, for example,
polytetrafluoro-ethylene (PTFE). The first carbon-based component
present in the anode structure (whether solely of first carbon
material or of first carbon material plus other material(s)) shows
little or no resistance to corrosion, and therefore when used in an
electrochemical cell that has entered a period of cell reversal,
the first carbon-based component will be corroded in preference to
other carbon also present in the anode structure, for example a
carbon support for the electrocatalyst. In other words, the first
carbon-based component is acting as a sacrificial carbon component.
This will protect further carbon present in the anode from
corrosion and thus maintain its desired function when the cell
returns to normal operation. For instance, this will inhibit the
carbon black in the electrocatalyst carbon support and the carbon
in the gas diffusion layer from corroding. Consequently, the anode
electrocatalyst and the anode gas diffusion layer will be protected
from the effects of cell reversal, thereby allowing the cell to
function without having suffered significant irreversible
performance decay when the cell reverts to normal fuel cell
operation after the cell reversal incident. To promote the
corrosion rate of the first carbon material used in the first
carbon-based component, the first carbon material may be
pre-treated with a suitable form of the proton-conducting membrane
electrolyte prior to incorporation into the anode structure.
Impregnating the first carbon material with proton-conducting
membrane electrolyte will promote the corrosion rate of the first
carbon-based component by providing an efficient conduction pathway
for the protons formed in the carbon corrosion reaction to the
membrane of the MEA.
[0024] Further, the first carbon-based component allows the
membrane and catalyst layer in the MEA to function without having
suffered significant irreversible performance decay when the cell
reverts to normal fuel cell operation after the cell reversal
incident. This is because corrosion of the first carbon-based
component helps sustain the current density at a less negative cell
voltage, corresponding to a less positive anode potential. At less
positive anode potentials the driving force for irreversible damage
to the membrane and catalyst layers is reduced.
[0025] As a general rule, the corrosion resistance of carbons is
related to the degree of the graphitic nature within the structure.
The more graphitic the structure of the carbon the more resistant
the carbon is to corrosion. The typical carbon blacks employed in
fuel cells, either as the electrocatalyst support or in the gas
diffusion layer, therefore tend to be those that are more highly
graphitic in nature as the environment particularly at the cathode
is very oxidising. It is contemplated that the first carbon
material will be chosen from the group of carbons that are much
less graphitic, that is, more amorphous than the typical carbon
materials employed in the fuel cell.
[0026] In a further embodiment of the present anode structure, the
anode structure further comprises a second carbon component that is
substantially more resistant to corrosion than the first
carbon-based component. For example, the second carbon component
may be a carbon support for an electrocatalyst or a carbon fill for
a gas diffusion substrate.
[0027] In embodiments of the present anode structure, a gas
diffusion layer may comprise a first carbon-based component. The
first carbon-based component may either be embedded within the gas
diffusion layer or applied as a coating to one or both surfaces, or
a mixture of both. To prepare a gas diffusion layer according to
the present technique, the first carbon-based component may be
mixed with a carbon black filler material typically employed to
coat or fill the carbon paper, cloth or non-woven fiber web
substrates employed in the PEMFC to produce the anode structure of
the invention in the form of a gas diffusion layer. To promote the
corrosion rate of the first carbon material used in the first
carbon-based component, the first carbon material may be
pre-treated or the resultant anode gas diffusion layer subsequently
treated with a suitable form of the proton-conducting membrane
electrolyte prior to incorporation in the MEA. The carbon black
filler material usually comprises a particulate carbon and a
polymer, the carbon suitably being in the form of a powder. The
carbon powder may be any of the materials generally designated as
carbon black, such as acetylene blacks, furnace blacks, pitch coke
based powders and graphitised versions of such materials. Suitably,
both natural and synthetic graphites may also be employed in this
application. Such materials may be employed either alone or in
combination. The particulate carbon, or carbons, in the fill are
held together by one or more polymers. The polymeric materials
employed contribute to the electrode structural properties, such as
pore size distribution, hydrophobic/hydrophilic balance and
physical strength of the gas diffusion layer. Examples of such
polymers include PTFE, fluorinated ethylenepropylene (FEP),
polyvinylidene difluoride (PVDF), Viton A, polyethylene,
polypropylene, ethylenepropylene. The preferred polymer is PTFE or
FEP.
[0028] In addition other modifier materials and catalyst materials,
which are not electro-catalysts, may be added to the carbon black
filler such as disclosed in PCT/International Publication No. WO
00/55933 (Johnson Matthey).
[0029] Furthermore, the first carbon-based component may be applied
to an anode gas diffusion layer that has previously been coated or
filled with typical carbon filler materials. To promote the
corrosion rate of the first carbon material employed in the first
carbon-based component, it may be pre-treated with the suitable
form of the proton-conducting membrane electrolyte. It is
contemplated that in the MEA formed using the resultant anode gas
diffusion layer, the first carbon-based component within the anode
layer may face either the electrocatalyst layer or the anode flow
field plate. In the present anode structure, the anode gas
diffusion layer should have sufficient electrical conductivity such
that on removal of the first carbon-based component during cell
reversal, the remaining anode layer does not have a significantly
lower electrical conductivity. Typical substrates that could be
employed include those based upon Toray carbon fiber paper and
Zoltek PWB-3 carbon cloth, which without a carbon coating or fill
have through plane specific electrical resistivities of below 0.15
.OMEGA.cm.
[0030] In another embodiment of the anode structure, a gas
diffusion electrode comprises a first carbon-based component. The
first carbon-based component may be admixed with an
electro-catalyst component and a polymeric material and the two
applied to a gas diffusion layer as a single admixed layer, or the
first carbon-based component and the electrocatalyst component may
be applied as separate layers, each separate layer also
incorporating a polymeric material. Alternatively, there could be a
combination of separate and mixed layers. The polymeric material
may be a soluble form of the proton-conducting membrane
electrolyte, or may be any of a wide range of polymeric materials
used to contribute to the structural and diffusional properties.
Examples of such polymers include PTFE, FEP, PVDF, Viton A,
polyethylene, polypropylene, ethylene-propylene. The preferred
polymer is PTFE or FEP. To promote the corrosion rate of the first
carbon material used in the first carbon-based component the first
carbon material may be pre-treated with a suitable form of the
proton-conducting membrane electrolyte prior to incorporation into
the anode electro-catalyst mixture.
[0031] The mixture of first carbon-based component and anode
electrocatalyst can be deposited onto the typical range of gas
diffusion layers employed in PEMFCs to produce the anode structure
of the invention in the form of a gas diffusion electrode.
[0032] Typical anode electrocatalysts employed in the PEMFC may be,
for example, a precious metal or a transition metal as the metal or
metal oxide, either unsupported or supported in a dispersed form on
a carbon support; an organic complex, in the form of a high surface
area finely divided powder or fiber, or a combination of these
options. An example of a suitable electrocatalyst material is
described in European Patent Publication No. 0731520. Particularly
suitable electrocatalysts are unsupported platinum (Pt) or alloys
or mixtures of platinum/ruthenium (PtRu) and carbon supported Pt or
PtRu. The PtRu electrocatalyst exhibits a higher level of tolerance
to CO and CO.sub.2 when they are present in the fuel stream than Pt
electrocatalysts.
[0033] Specific examples of this embodiment may be prepared
according to the procedure described in more detail
hereinbelow.
[0034] In another embodiment of the anode structure, a
catalyst-coated membrane comprises a first carbon-based component.
The first carbon-based component may be admixed with an
electrocatalyst component and a polymeric material and the two
applied to a membrane electrolyte as an admixed single layer, or
the first carbon-based component and the electrocatalyst component
may be applied as separate layers, each separate layer also
incorporating a polymeric material. Alternatively, there could be a
combination of separate and mixed layers. The polymeric material
may be a soluble form of the proton-conducting membrane
electrolyte, or may be any of a wide range of polymeric materials
used to contribute to the structural and diffusional properties.
Examples of such polymers include PTFE, FEP, PVDF, Viton A,
polyethylene, polypropylene, ethylene-propylene. The preferred
polymer is PTFE or FEP. To promote the corrosion rate of the carbon
in the first carbon component, the carbon may be pre-treated with a
suitable form of the proton-conducting membrane electrolyte prior
to incorporation into the anode electrocatalyst mixture.
[0035] The mixture of first carbon-based component and anode
electrocatalyst can be deposited onto the solid membrane
electrolyte to produce the anode structure of the invention in the
form of a catalyst-coated membrane. Subsequent compression of the
anode and cathode catalyst-coated membrane to the typical gas
diffusion layers employed in PEMFCs, or hot pressing of anode and
cathode catalyst-coated gas diffusion layers to the solid
proton-conducting membrane electrolyte forms the complete MEA.
[0036] Typical anode electrocatalysts employed in the PEMFC are as
previously described.
[0037] The proton-conducting polymers suitable for use in the
present anode structure may include, but are not limited to:
[0038] (a) Polymers which have structures with a substantially
fluorinated carbon chain optionally having attached to it side
chains that are substantially fluorinated. These polymers contain
sulfonic acid groups or derivatives of sulfonic acid groups,
carboxylic acid groups or derivatives of carboxylic acid groups,
phosphonic acid groups or derivatives of phosphonic acid groups,
phosphoric acid groups or derivatives of phosphoric acid groups
and/or mixtures of these groups. Perfluorinated polymers include
Nafion.RTM., Flemion.RTM. and Aciplex.RTM. commercially available
from E. I. DuPont de Nemours (U.S. Pat. Nos. 3,282,875; 4,329,435;
4,330,654; 4,358,545; 4,417,969; 4,610,762; 4,433,082 and
5,094,995), Asahi Glass KK and Asahi Chemical Industry
respectively. Other polymers include those disclosed in U.S. Pat.
Nos. 5,595,676 (Imperial Chemical Industries plc) and U.S. Pat. No.
4,940,525 (Dow Chemical Co.)
[0039] (b) Perfluorinated or partially fluorinated polymers
containing aromatic rings such as those described in
PCT/International Publication Nos. WO 95/08581, WO 95/08581 and WO
97/25369 (Ballard Power Systems Inc.), which have been
functionalized with SO.sub.3H, PO.sub.2H.sub.2, PO.sub.3H.sub.2,
CH.sub.2PO.sub.3H.sub.2, COOH, OSO.sub.3H, OPO.sub.2H.sub.2,
OPO.sub.3H.sub.2. Also included are radiation or chemically grafted
perfluorinated polymers, in which a perfluorinated carbon chain,
for example, PTFE, FEP, tetrafluoroethylene-ethylene (ETFE)
copolymers, tetrafluoroethylene-perfluoroalkoxy (PFA) copolymers,
poly (vinyl fluoride) (PVF) and poly (vinylidene fluoride) (PVDF)
is activated by radiation or chemical initiation in the presence of
a monomer, such as styrene, which can be functionalized to contain
an ion exchange group.
[0040] (c) Fluorinated polymers such as those disclosed in European
Patent Publication Nos. 0331321 and 0345964 (Imperial Chemical
Industries plc) containing a polymeric chain with pendant saturated
cyclic groups and at least one ion exchange group which is linked
to the polymeric chain through the cyclic group.
[0041] (d) Aromatic polymers such as those disclosed in European
Patent Publication No. 0574791 and U.S. Pat. No. 5,438,082 (Hoechst
AG), for example sulfonated polyaryletherketone. In addition,
aromatic polymers such as polyether sulfones, which can be
chemically grafted with a polymer with ion exchange functionality
such as those disclosed in PCT/International Publication No. WO
94/16002 (Allied Signal Inc.).
[0042] (e) Nonfluorinated polymers include those disclosed in U.S.
Pat. No. 5,468,574 (Dais Corporation), for example, hydrocarbons
such as styrene-(ethylene-butylene)-styrene,
styrene-(ethylene-propylene)-styrene and
acrylonitrile-butadiene-styrene copolymers and terpolymers, in
which the styrene components are functionalized with sulfonate,
phosphoric and/or phosphonic groups.
[0043] (f) Nitrogen containing polymers including those disclosed
in U.S. Pat. No. 5,599,639 (Hoechst Celanese Corporation), for
example, polybenzimidazole alkyl sulfonic acid and
polybenzimidazole alkyl, or aryl phosphonate.
[0044] (g) Any of the above polymers which have the ion exchange
group replaced with a sulfonyl chloride (SO.sub.2Cl) or sulfonyl
fluoride (SO.sub.2F) group, thereby rendering the polymers melt
processable. The sulfonyl fluoride polymers may form part of the
precursors to the ion exchange membrane or may be arrived at by
subsequent modification of the ion exchange membrane. The sulfonyl
halide moieties can be converted to a sulfonic acid using
conventional techniques such as, for example, hydrolysis.
[0045] In direct methanol fuel cells (DMFC), it is methanol that is
oxidized at the anode during normal fuel cell operation, as
follows:
CH.sub.3OH+H.sub.2OCO.sub.2+6H.sup.++6e.sup.-
[0046] Fuel starvation can also be a particular problem in the
methanol-fuelled DMFC. The methanol can be blocked from the
electrocatalyst sites by the significant quantities of water that
are present in the aqueous methanol fuel mixture and by the carbon
dioxide gas that is generated by the electro-oxidation of the
methanol. Consequently, the problems of cell reversal due to fuel
starvation in the anode structure, which are substantially
identical to those outlined for the H.sub.2-fuelled PEMFC, can be a
problem in the DMFC. The use of a first carbon-based component in
the anode structure of the DMFC offers a significant benefit. Just
as in the H.sub.2-fuelled PEMFC, the use of a first carbon-based
component protects the vital carbon components in the anode from
corrosion, by undergoing preferential corrosion, and also protects
the membrane and catalyst layers from excessive dehydration and
irreversible damage by helping to sustain the current density at
less positive anode potentials. The use of a first carbon-based
component in the anode structure of the DMFC allows the MEA to
provide a performance that is not significantly reduced after a
cell reversal incident. However, the problem of carbon corrosion in
the direct methanol fuelled PEM fuel cell is not likely to be as
great a problem as in the H.sub.2-fuel cell due to the increased
amount of water at the anode, and thus cell reversal current should
be consumed in electrolysis reactions.
[0047] In a further aspect, an MEA comprises the present anode
structure.
[0048] In a still further aspect, a fuel cell comprises an MEA
comprising the present anode structure. In a yet further aspect, a
fuel cell comprises the present anode structure.
[0049] While the present anode structures have been described for
use in solid polymer fuel cells, such as the proton exchange
membrane and direct methanol fuel cells, it is anticipated that
they would be useful in other fuel cells, as well. In this regard,
"fuel cell" generally refers to a fuel cell having an operating
temperature below about 250.degree. C. The present anode structures
are preferred for acid electrolyte fuel cells, which are fuel cells
comprising a liquid or solid acid electrolyte, such as phosphoric
acid, solid polymer electrolyte, and direct methanol fuel cells.
The present anode structures are particularly preferred for solid
polymer electrolyte fuel cells.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0050] Specific examples of a gas diffusion layer for use in
conjunction with the present anode structure may be prepared in the
following manner.
EXAMPLES
[0051] A particulate catalyst component, containing a first
carbon-based component is provided by dispersing 30 weight parts of
a high surface area carbon black (the first carbon-based component,
which may be Black pearls 2000 or PICACTIF CSO-D, both available
from Cabot Carbon Ltd., Stanlow, South Wirral, UK, or Norit A
Supra, from Norit Americas Inc., Atlanta, USA) and 100 weight parts
of a 20 wt% platinum, 10 wt% ruthenium catalyst, supported on Cabot
Vulcan XC72R (from Johnson Matthey Inc, New Jersey, USA) in 1200
parts of demineralized water. To this is added 10 weight parts of
polytetrafluoroethylene (PTFE) as a dispersion in water (ICI Fluon
GP1, 64 wt% solids suspension) and the mixture heated and stirred
to entrain the PTFE particles within the carbon catalyst materials.
The slurry is filtered to remove excess water and re-dispersed in a
2% methyl cellulose solution, using a high shear mixer, to produce
a smooth mixture.
[0052] The anode electrode may be prepared by applying a layer of
the carbon/catalyst slurry described above to a pre-teflonated (18%
by weight ICI Fluon GP1), rigid conducting carbon fiber paper
substrate (Toray TGP-H-090, available from Toray Industries Inc,
Tokyo, Japan) at an electrode platinum loading of 0.4 mg/cm.sup.2
of electrode geometric area. The dried electrode is heated to
375.degree. C. in air to sinter the PTFE A solution of
perfluorosulfonic acid in the aqueous form as described in European
Patent Publication No. 0731520 is applied to the surface of the
catalyst layer to provide a proton conductive interface with the
electrocatalyst and to act as a water reservoir for the carbon
corrosion process.
[0053] An electrode so prepared may form the anode of an MEA. The
cathode may be of the more conventional type, currently widely
employed in the PEMFC. The foregoing comprise a conventional
pre-teflonated rigid conducting carbon fiber paper substrate (Toray
TGP-H-090, available from Toray Industries Inc, Tokyo, Japan) to
which is applied a layer of a 40 wt% platinum, catalyst, supported
on Cabot Vulcan XC72R (from Johnson Matthey Inc, New Jersey, USA),
at an electrode platinum loading of 0.6 mg/cm.sup.2 of electrode
geometric area. The catalyst layer material is provided by
dispersing 100 weight parts of a 40 wt % platinum catalyst,
supported on carbon black (Johnson Matthey High-Spec 4000) in 30
parts of a 9.5% dispersion of Nafion EW1100 (E. I. DuPont de
Nemours & Co.) in water, prepared according to methods
described in EPA 731,520. The particulate catalyst is dispersed
using a high shear mixer to produce a smooth mixture and is then
applied to the cathode substrate. The complete MEA is fabricated by
bonding the anode and the cathode electrodes (with the face of the
electrode comprising the platinum catalyst component adjacent to
the membrane) to a Nafion 112 membrane (supplied by E. I. DuPont de
Nemours, Fayetteville, N.C., USA)
[0054] The MEA thus formed may be tested in a cell reversal
situation according to the following procedure. The MEA is
conditioned prior to voltage reversal by operating it normally at a
current density of about 0.5 A/cm.sup.2 and a temperature of
approximately 75.degree. C. Humidified hydrogen may be used as fuel
and humidified air as oxidant, both at 200 kPa pressure. The
stoichiometry of the reactants (that is, the ratio of reactant
supplied to reactant consumed in the generation of electricity) is
1.5 and 2.0 for the hydrogen and oxygen-containing air reactants,
respectively. The output cell voltage as a function of current
density (polarization data) is determined. After that, each cell is
subjected to a voltage reversal test by flowing humidified nitrogen
over the anode (instead of fuel) while forcing 10 A current through
the cell for a period of time long enough to cause some damage to a
conventional anode without causing the extensive damage associated
with large increases in the anode potential (23 minutes has been
found to be an appropriate length of time) using a constant current
power supply connected across the fuel cell. During the voltage
reversal, the cell voltage versus time is recorded. Polarization
data for each cell is obtained once the cell has returned to normal
stabilized operating conditions to determine the effect of a single
reversal episode on cell performance.
[0055] Each cell is then subjected to a second voltage reversal
test at a 10A current. This time, however, the reversal current is
interrupted five times during the test period to observe the effect
of repeated reversals on the cells. After 5 minutes of operation in
reversal, the current is cycled on and off five times (20 seconds
off and 10 seconds on) after which the current is left on until a
total "on" time of 23 minutes has been reached. Following the
second reversal test, polarization measurements of each cell are
obtained.
[0056] The above procedure for cell testing can be used not only
for the three specific examples described, but also for examples
falling within the scope of the present teachings. Furthermore,
although the examples described above relate to a gas diffusion
layer according to the present teachings, it is within the ability
of those skilled in the art to modify the procedure to produce a
gas diffusion substrate and/or a catalyst-coated membrane according
to the present teachings.
[0057] In the foregoing examples and embodiments, the rate of
carbon corrosion may be determined by appropriate adaptation of the
following procedure which is suitable for a liquid acid electrolyte
fuel cell such as, for example, a phosphoric acid fuel cell. A
complete cell is assembled by inserting an anode structure
(previously weighed) of the invention and a reference electrode
(for example, a dynamic hydrogen reference electrode) into a liquid
electrolyte. The cell was left until the test temperature is
reached (for example, 180.degree. C.) and for the open circuit
voltage (OCV) of the anode structure to stabilize. The cell was
activated and as soon as the potential of the working electrode
reached 1 volt, current readings were taken over a given time
period. The cell was dismantled and the anode structure reweighed.
The log (corrosion current) was plotted against log(time) and
extrapolated to 100 minutes. The corrosion rate is expressed as
current per unit weight of carbon (.mu.Amg.sup.-1C) after 100
minutes at 1 volt. Data for the corrosion rates of a number of
carbons in phosphoric acid fuel cells may be found in Catalysis
Today, 7 (1990) 113-137, which is incorporated herein by reference
in its entirety. Although the actual carbon corrosion rates will be
dependent on the particular environment in which the anode
structure is placed, the relative rates of the various carbons will
remain substantially similar.
[0058] One measure that can be taken as an indication of the
corrosion resistance of carbon is provided by the BET surface area
measured using nitrogen, as this detects the microporosity and
mesoporosity typically found in amorphous carbon structures. For
example, Vulcan XC72R, Shawinigan and graphitised Vulcan XC72R are
typical semi-graphitic/graphitic carbon blacks employed in fuel
cells. Vulcan XC72R has a surface area of 228 m.sup.2g.sup.-1. This
contrasts with a surface area of 86 m.sup.2g.sup.-1 for graphitised
Vulcan XC72R. The much lower surface area as a result of the
graphitization process reflects a loss in the more amorphous
microporosity in Vulcan XC72R. The microporosity is commonly
defined as the surface area contained in the pores of diameter less
than 2 nm. Shawinigan has a surface area of 55 m.sup.2g.sup.-1, and
BET analysis indicates a low level of carbon microporosity
available in this support for corrosion. This contrasts with the
much higher BET surface area of, for example, Black Pearls 2000
(1536 m.sup.2g.sup.-1) reflecting in this case a high degree of
microporosity in this carbon black that can corrode. Carbon blacks
with BET surface areas in excess of 350 m.sup.2g.sup.-1, such as
BP2000, could be employed as the first carbon material in the first
carbon-based component in the anode structure of the PEMFC.
[0059] There are other carbons that also have high BET surface
areas in excess of 350 m.sup.2g.sup.-1, such as those classified as
activated carbons. Such carbons are usually derived from the
carbonization of vegetable matter (typically wood, peat or coconut
husks), in which the carbon is generally amorphous in character and
there is a range of possible pore sizes from micropores to larger
mesopores and macropores. Typical examples of these activated
carbons are those produced under the general trade name Norit
(Norit Americas Inc., Atlanta, Ga., USA) and Pica (Pica, 92300
Levallois, France). Such carbons could also be employed as the
first carbon material in the first carbon-based component in the
anode structure of the PEMFC.
[0060] Another indication of the corrosion resistance may be
demonstrated by the carbon inter-layer separation d.sub.002
measured from the x-ray diffractograms. Synthetic graphite
(substantially pure graphite) has a spacing of 3.36 .ANG. compared
with 3.45 .ANG. for Vulcan XC72R (graphitised), 3.50.ANG. for
Shawinigan, and 3.64 .ANG. for Vulcan XC72R, with the higher
inter-layer separations reflecting the decreasing graphitic nature
of the carbon and the decreasing order of corrosion resistance.
Thus, a first carbon material with an inter-layer separation of
greater that 3.65 .ANG. may be suitable for use in the first
carbon-based component of the present invention. However, many
carbons that show poor resistance to corrosion (and therefore may
be of use in the first carbon-based component of the present
invention) are amorphous in nature and therefore no inter-layer
separation measurement can be obtained.
[0061] It is also possible that the first carbon-based component
may comprise a first carbon material which intrinsically
demonstrates a reasonably high resistance to corrosion but which is
treated in such a manner, for example, by coating with a
proton-conducting electrolyte, that the formed first carbon-based
component as a whole shows little or no resistance to
corrosion.
[0062] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the scope of the present disclosure, particularly in
light of the foregoing teachings.
* * * * *