U.S. patent application number 11/081765 was filed with the patent office on 2005-10-06 for modified carbon products, their use in proton exchange membranes and similar devices and methods relating to the same.
Invention is credited to Atanassova, Paolina, Bhatia, Rimple, Caruso, James, Gurau, Bogdan, Hampden-Smith, Mark J., Napolitano, Paul, Rice, Gordon L..
Application Number | 20050221141 11/081765 |
Document ID | / |
Family ID | 34962823 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050221141 |
Kind Code |
A1 |
Hampden-Smith, Mark J. ; et
al. |
October 6, 2005 |
Modified carbon products, their use in proton exchange membranes
and similar devices and methods relating to the same
Abstract
Proton exchange membranes incorporating modified carbon
products. The modified carbon products advantageously enhance the
properties of proton exchange membranes, leading to more efficiency
within a fuel cell or similar device.
Inventors: |
Hampden-Smith, Mark J.;
(Albuquerque, NM) ; Atanassova, Paolina;
(Albuquerque, NM) ; Napolitano, Paul;
(Albuquerque, NM) ; Bhatia, Rimple; (Albuquerque,
NM) ; Rice, Gordon L.; (Albuquerque, NM) ;
Caruso, James; (Albuquerque, NM) ; Gurau, Bogdan;
(Albuquerque, NM) |
Correspondence
Address: |
MARSH, FISCHMANN & BREYFOGLE LLP
3151 SOUTH VAUGHN WAY
SUITE 411
AURORA
CO
80014
US
|
Family ID: |
34962823 |
Appl. No.: |
11/081765 |
Filed: |
March 15, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60553612 |
Mar 15, 2004 |
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60553413 |
Mar 15, 2004 |
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60553672 |
Mar 15, 2004 |
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60553611 |
Mar 15, 2004 |
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Current U.S.
Class: |
524/544 ;
429/492; 429/494; 429/535; 521/27 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01M 4/8817 20130101; B01J 23/42 20130101; H01M 4/926 20130101;
H01M 8/0213 20130101; H01M 8/1023 20130101; B01J 23/8926 20130101;
H01M 4/921 20130101; H01M 8/1081 20130101; H01M 8/0245 20130101;
H01M 8/1027 20130101; Y02E 60/50 20130101; H01M 8/103 20130101;
H01M 8/1032 20130101; H01M 4/8636 20130101; Y02E 60/523 20130101;
H01M 4/86 20130101; H01M 4/90 20130101; H01M 8/1034 20130101; H01M
4/8652 20130101; H01M 4/886 20130101; H01M 8/1011 20130101; H01M
8/0234 20130101; H01M 8/1025 20130101; H01M 8/1048 20130101; H01M
8/0289 20130101; Y02P 70/56 20151101; H01M 4/9083 20130101; H01M
8/1004 20130101; H01M 2300/0094 20130101; B01J 37/0207 20130101;
H01M 8/1039 20130101; B01J 23/462 20130101; B01J 37/084 20130101;
B01J 21/18 20130101; C08J 5/22 20130101; H01B 1/122 20130101 |
Class at
Publication: |
429/033 ;
521/027 |
International
Class: |
H01M 008/10; C08J
005/22 |
Claims
What is claimed is:
1. A proton conducting membrane, said membrane comprising a
modified carbon product and a polymer.
2. A proton conducting membrane as recited in claim 1, wherein said
modified carbon product comprises modified carbon black.
3. A proton conducting membrane as recited in claim 1, wherein said
modified carbon product comprises a proton conducting functional
group.
4. A proton conducting membrane as recited in any of claim 1,
wherein said polymer is selected from the group consisting of
sulfonated PTFE and perfluorosulfonated PTFE.
5. A proton conducting membrane as recited in claim 1, wherein said
polymer is selected from the group consisting of polyvinylidene
fluoride (PVDF), acid-doped or derivatized hydrocarbon polymers,
such as polybenzimidizole (PBI), polyarylenes, polyetherketones,
polysulfones, phosphazenes and polyimides.
6. A proton conducting membrane as recited in claim 1, wherein said
modified carbon product is coated on a surface of said polymer.
7. A proton conducting membrane as recited in claim 1, wherein said
modified carbon product is dispersed within said polymer.
8. A proton conducting membrane as recited in claim 1, wherein said
proton-conducting membrane has a proton conducting group
concentration of at least about 5.0 mmol/mL.
9. A proton conducting membrane as recited in claim 1, wherein said
proton-conducting membrane has a proton conducting group
concentration of at least about 5.4 mmol/mL.
10. A proton conducting membrane as recited in claim 1, wherein
said modified carbon product is adapted to conduct protons in the
absence of water.
11. A proton conducting membrane as recited in claim 1, wherein
said modified carbon product is adapted to selectively conduct
protons in the presence of other hydrogen-comprising liquid
fuels.
12. A proton conducting membrane as recited in claim 1, wherein
said modified carbon product is adapted to selectively conduct
protons in the presence of methanol or ethanol.
13. A proton conducting membrane as recited in claim 1, wherein
said modified carbon product is adapted to yield an increased
mechanical strength without a substantial decrease in proton
conductivity.
14. A proton conducting membrane as recited in claim 1, wherein
said modified carbon product comprises at least one proton
conducting functional group selected from the group consisting of
SO.sub.3H, CO.sub.2H, PO.sub.3H.sub.2 and PO.sub.3 MH, where M is a
monovalent cation.
15. A proton conducting membrane, wherein said proton conducting
membrane consists essentially of a modified carbon product.
16. A proton conducting membrane as recited in claim 15, wherein
said modified carbon product comprises proton-conducting functional
groups.
17. A proton conducting membrane as recited in claim 15, wherein
said modified carbon product comprises proton-conducting functional
groups selected from the group consisting of carboxylic acids,
sulfonic acids, phosphonic acids and phosphonic acid salts.
18. A proton conducting membrane as recited in claim 15, wherein
said modified carbon product comprises modified carbon black.
19. A proton conducting membrane as recited in claim 15, wherein
said modified carbon product comprises modified carbon fibers.
20. A method for the fabrication of a proton conducting membrane,
comprising the steps of: a) mixing a polymer with a modified carbon
product to form a composite mixture; and b) forming said composite
mixture into a proton conducting membrane.
21. A method as recited in claim 20, wherein said forming step
comprises extruding said composite mixture.
22. A method as recited in claim 20, wherein said forming step
comprises casting said composite mixture.
23. A method as recited in claim 20, wherein said proton conducting
membrane has a volume density of proton conducting groups of at
least about 4.8 mmol/mL.
24. A method as recited in claim 20, wherein said composite mixture
comprises at least about 20 wt. % modified carbon product.
25. A method as recited in claim 20, wherein said proton conducting
membrane comprises at least about 40 wt. % modified carbon
product.
26. A method as recited in claim 20, wherein said carbon product
comprises carbon black.
27. A method as recited in claim 20, wherein said carbon product
comprises carbon fibers.
28. A method for the fabrication of a proton conducting membrane,
comprising the steps of: a) providing a modified carbon black
product; and b) forming said modified carbon black product into a
thin membrane.
29. A method as recited in claim 28, wherein said forming step
comprises analog deposition.
30. A method as recited in claim 28, wherein said forming step
comprises digital deposition.
31. A method as recited in claim 28, wherein said forming step
comprises dispersing said modified carbon product in a liquid
vehicle and ink-jet printing said modified carbon product.
32. A method as recited in claim 31, wherein said modified carbon
product comprises hydrophilic functional groups.
Description
CLAIM OF PRIORITY BENEFIT
[0001] Pursuant to 35 U.S.C. .sctn. 119(e), this patent application
claims a priority benefit to: (a) U.S. Provisional Patent
Application No. 60/553,612 entitled "MODIFIED CARBON PRODUCTS AND
THEIR USE IN GAS DIFFUSION LAYERS" filed Mar. 15, 2004; (b) U.S.
Provisional Patent Application No. 60/553,413 entitled "MODIFIED
CARBON PRODUCTS AND THEIR USE IN ELECTROCATALYSTS AND ELECTRODE
LAYERS" filed Mar. 15, 2004; (c) U.S. Provisional Patent
Application No. 60/553,672 entitled "MODIFIED CARBON PRODUCTS AND
THEIR USE IN PROTON EXCHANGE MEMBRANES" filed Mar. 15, 2004; and
(d) U.S. Provisional Patent Application No. 60/553,611 entitled
"MODIFIED CARBON PRODUCTS AND THEIR USE IN BIPOLAR PLATES" filed
Mar. 15, 2004. This application is also related to U.S. patent
application Ser. No. ______, entitled "MODIFIED CARBON PRODUCTS,
THEIR USE IN ELECTROCATALYSTS AND ELECTRODE LAYERS AND SIMILAR
DEVICES AND METHODS RELATING TO THE SAME", filed on Mar. 15, 2005,
and further identified by Attorney File No. 41890-01745, and U.S.
patent application Ser. No. ______, entitled "MODIFIED CARBON
PRODUCTS, THEIR USE IN FLUID/GAS DIFFUSION LAYERS AND SIMILAR
DEVICES AND METHODS RELATING TO THE SAME", filed on Mar. 15, 2005,
and further identified by Attorney File No. 41890-01744, and U.S.
patent application Ser. No. ______, entitled "MODIFIED CARBON
PRODUCTS, THEIR USE IN BIPOLAR PLATES AND SIMILAR DEVICES AND
METHODS RELATING TO THE SAME", filed on Mar. 15, 2005, and further
identified by Attorney File No. 41890-01747. Each of the above
referenced patent applications is hereby incorporated herein by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the production and use of
modified carbon products in fuel cell components and similar
devices. Specifically, the present invention relates to proton
exchange membranes incorporating modified carbon products and
methods for making proton exchange membranes including modified
carbon products. The modified carbon products can be used to
enhance and tailor the properties of the proton exchange
membrane.
[0004] 2. Description of Related Art
[0005] Fuel cells are electrochemical devices that are capable of
converting the energy of a chemical reaction into electrical energy
without combustion and with virtually no pollution. Fuel cells are
unlike batteries in that fuel cells convert chemical energy to
electrical energy as the chemical reactants are continuously
delivered to the fuel cell. As a result, fuel cells are used to
produce a continuous source of electrical energy, and compete with
other forms of continuous energy production such as the combustion
engine, nuclear power and coal-fired power stations. Different
types of fuel cells are categorized by the electrolyte used in the
fuel cell. The five main types of fuel cells are alkaline, molten
carbonate, phosphoric acid, solid oxide and proton exchange
membrane (PEM), also known as polymer electrolyte fuel cells
(PEFCs). One particularly useful fuel cell is the proton exchange
membrane fuel cell (PEMFC).
[0006] A PEMFC typically includes tens to hundreds of MEAs each of
which includes a cathode layer and an anode layer. One embodiment
of a MEA is illustrated in FIGS. 1(a) and 1(b). One embodiment of a
cathode side of an MEA is also depicted in FIG. 2. With references
to FIGS. 1(a), 1(b) and 2, the anode electrocatalyst layer 104 and
cathode electrocatalyst layer 106 sandwich a proton exchange
membrane 102. In some instances, the combined membrane and
electrode layer is referred to as a catalyst coated membrane 103.
Power is generated when a fuel (e.g., hydrogen gas) is fed into the
anode 104 and oxygen (air) 106 is fed into the cathode. In a
reaction typically catalyzed by a platinum-based catalyst in the
catalyst layer of the anode 104, the hydrogen ionizes to form
protons and electrons. The protons are transported through the
proton exchange membrane 102 to a catalyst layer on the opposite
side of the membrane (the cathode), where another catalyst,
typically platinum or a platinum alloy, catalyzes an
oxygen-reduction reaction to form water. The reactions can be
written as follows:
Anode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.- (1)
Cathode: 4H.sup.++4e.sup.-+O.sub.2.fwdarw.2H.sub.2O (2)
Overall: 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O (3)
[0007] Electrons formed at the anode and cathode are routed through
bipolar plates 114 connected to an electrical circuit. On either
side of the anode 104 and cathode 106 are porous gas diffusion
layers 108, which generally comprise a carbon support layer 107 and
a microporous layer 109, that help enable the transport of
reactants (H.sub.2 and O.sub.2 when hydrogen gas is the fuel) to
the anode and the cathode. On the anode side, fuel flow channels
110 may be provided for the transport of fuel, while on the cathode
side, oxidizer flow channels 112 may be provided for the transport
of an oxidant. These channels may be located in the bipolar plates
114. Finally, cooling water passages 116 can be provided adjacent
to or integral with the bipolar plates for cooling the MEA/fuel
cell.
[0008] A particularly preferred fuel cell for portable
applications, due to its compact construction, power density,
efficiency and operating temperature, is a PEMFC that can utilize
methanol (CH.sub.3OH) directly without the use of a fuel reformer
to convert the methanol to H.sub.2. This type of fuel cell is
typically referred to as a direct methanol fuel cell (DMFC). DMFCs
are attractive for applications that require relatively low power,
because the anode reforms the methanol directly into hydrogen ions
that can be delivered to the cathode through the PEM. Other liquid
fuels that may also be used in a fuel cell include formic acid,
formaldehyde, ethanol and ethylene glycol.
[0009] Like a PEMFC, a DMFC also is made of a plurality of membrane
electrode assemblies (MEAs). A cross-sectional view of a typical
MEA is illustrated in FIG. 3 (not to scale). The MEA 300 comprises
a PEM 302, an anode electrocatalyst layer 304, cathode
electrocatalyst layer 306, fluid distribution layers 308, and
bipolar plates 314. The electrocatalyst layers 304, 306 sandwich
the PEM 302 and catalyze the reactions that generate the protons
and electrons to power the fuel cell, as shown below. The fluid
diffusion layer 308 distributes the reactants and products to and
from the electrocatalyst layers 304, 306. The bipolar plates 314
are disposed between the anode and cathode of sequential MEA
stacks, and comprise current collectors 317 and fuel and oxidizer
flow channels, 310, 312, respectively, for directing the flow of
incoming reactant fluid to the appropriate electrode. Two end
plates (not shown), similar to the bipolar plates, are used to
complete the fuel cell stack.
[0010] Operation of the DMFC is similar to a hydrogen-gas based
PEMFC, except that methanol is supplied to the anode instead of
hydrogen gas. Methanol flows through the fuel flow channels 310 of
bipolar plate 314, through the fluid distribution layer 308 and to
the anode electrocatalyst layer 304, where it decomposes into
carbon dioxide gas, protons and electrons. Oxygen flows through the
oxidizer flow channels 312 of the bipolar plate 314, through the
fluid distribution layer 308, and to the cathode electrocatalyst
layer, where ionized oxygen is produced. Protons from the anode
pass through the PEM 302, and recombine with the electrons and
ionized oxygen to form water. Carbon dioxide is produced at the
anode 304 and is removed through the exhaust of the cell. The
foregoing reactions can be written as follows:
Anode: CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.-
(4)
Cathode: 6H.sup.++6e.sup.-+{fraction (3/2)}O.sub.2.fwdarw.3H.sub.2O
(5)
Overall: 2CH.sub.3OH+3O.sub.2.fwdarw.2CO.sub.2+6H.sub.2O+energy
(6)
[0011] There are a number of properties that are required for
efficient fuel cell operation. For example, the PEM should have a
high proton conductivity to enable efficient transport of protons
from the anode to the cathode, be electrically insulative to
prevent transport of electrons to the cathode, and act as a robust
physical separator. The membrane should also be impermeable to
gases. A common proton exchange material currently used is NAFION,
a perfluorosulfonic acid (PFSA) polymer available from E.I. duPont
deNemours, Wilmington, Del., from which different membrane
thicknesses can be formed.
[0012] One drawback to current PEMs is that they require a humid
fuel to operate efficiently due to the intermittent spacing of the
proton exchange materials within the membrane. Thus, water must be
present to act as a bridge between such sites for proton transport.
Water introduced into and/or produced at the electrodes must be
removed from the fuel cell to prevent clogging of the electrode
pores, a phenomenon known as "flooding".
[0013] Carbon is a material that has previously been used for some
components of the fuel cell structure. For example, U.S. Pat. No.
6,280,871 by Tosco et al. discloses gas diffusion electrodes
containing carbon products. The carbon product can be used for at
least one component of the electrodes, such as the active layer
and/or the blocking layer. Methods to extend the service life of
the electrodes, as well as methods to reduce the amount of
fluorine-containing compounds are also disclosed. Similar products
and methods are described in U.S. Pat. No. 6,399,202 by Yu et al.
Each of the foregoing patents is incorporated herein by reference
in its entirety.
[0014] U.S. Patent Application Publication No. 2003/0017379 by
Menashi, which is incorporated herein by reference in its entirety,
discloses fuel cells including a gas diffusion electrode, gas
diffusion counter-electrode, and an electrolyte membrane located
between the electrode and counter-electrode. The electrode,
counter-electrode, or both, contain at least one carbon product.
The electrolyte membranes can also contain carbon products. Similar
products and methods are described in U.S. Patent Application
Publication No. 2003/0022055 by Menashi, which is also incorporated
herein by reference in its entirety.
[0015] U.S. Patent Application Publication No. 2003/0124414 by
Hertel et al., which is incorporated herein by reference in its
entirety, discloses a porous carbon body for a fuel cell having an
electronically conductive hydrophilic agent and discloses a method
for the manufacture of the carbon body. The porous carbon body
comprises an electronically conductive graphite powder in an amount
of between 60 and 80 weight percent of the body, carbon fiber in an
amount of between 5 and 15 weight percent of the body, a thermoset
binder in an amount between 6 and 18 weight percent of the body and
an electronically created modified carbon black. Hertel et al.
disclose that the carbon body provides increased wettability
without any decrease in electrical conductivity, and can be
manufactured without high temperature steps to add graphite to the
body or to incorporate post molding hydrophilic agents into pores
of the body.
[0016] Composite membranes with increased strength have been
studied for use in industrial chloro-alkali electrolysis processes.
Similar fibril reinforced membranes have been produced for use in
PEMFCs. These membranes target the following properties: thin,
flat, good mechanical strength, chemical robustness and high water
permeability and proton conductivity. Such membranes were developed
by companies such as W.L. Gore and Associates and Asahi Glass Co.,
Ltd., all utilizing polytetrafluoroethylene (PTFE) as the
reinforcement agent, and an ion exchange material, such as a
sulfonated polymer, to act as the proton conduction sites. Others
have utilized inorganic fillers, such as silica, titania or
tungstosilicic acid, with solids loadings that range from 5 to 70
weight percent. Fillers such as these have been incorporated into
proton conducting and non-proton conducting polymers.
SUMMARY OF THE INVENTION
[0017] According to one aspect of the present invention, a proton
conducting membrane including a modified carbon product and a
polymer is provided. In one embodiment of the present aspect, the
modified carbon product is a modified carbon black. In another
embodiment of the present aspect, the modified carbon product is a
modified carbon fiber. In yet another embodiment, the modified
carbon product includes a proton conducting functional group. In
one embodiment, the proton conducting functional group is selected
from the group of SO.sub.3H, CO.sub.2H, PO.sub.3H.sub.2 and
PO.sub.3 MH, where M is a monovalent cation. In another embodiment,
the proton conducting functional group is selected from the group
of carboxylic acids, sulfonic acids, phosphonic acids and
phosphonic acid salts. In one embodiment, the polymer is selected
from the group of a sulfonated PTFE and a perfluorosulfonated PTFE.
In another embodiment, the polymer is selected from the group of
polyvinylidene fluoride (PVDF), acid-doped or derivatized
hydrocarbon polymers, such as polybenzimidizole (PBI),
polyarylenes, polyetherketones, polysulfones, phosphazenes and
polyimides. In one embodiment, the modified carbon product is
coated on a surface of the polymer. In another embodiment, the
modified carbon product is dispersed within the polymer. In one
embodiment, the modified carbon product is adapted to conduct
protons in the absence of water. In another embodiment, the
modified carbon product is adapted to selectively conduct protons
in the presence of other hydrogen-comprising liquid fuels. In yet
another embodiment, the modified carbon product is adapted to
selectively conduct protons in the presence of methanol or ethanol.
In one embodiment, the modified carbon product is adapted to yield
an increased mechanical strength without a substantial decrease in
proton conductivity. In another embodiment, the proton conducting
membrane has a proton conducting group concentration of at least
about 5.0 millimoles per milliliter, and, in some instances, a
proton conducting group concentration of at least about 5.4
millimoles per milliliter. In yet another embodiment, the proton
conducting membrane consists essentially of a modified carbon
product.
[0018] According to another aspect of the present invention, a
method for the fabrication of a proton conducting membrane is
provided, the method including the steps of mixing a polymer with a
modified carbon product to form a composite mixture and forming the
composite mixture into a proton conducting membrane. In one
embodiment of the present aspect, the forming step includes
extruding the composite mixture. In another embodiment, the forming
step comprises casting the composite mixture. In yet another
embodiment, the proton conducting membrane has a volume density of
proton conducting groups of at least about 5.0 millimoles per
milliliter. In one embodiment, the composite mixture includes at
least about 20 weight percent modified carbon product, and, in some
instances, includes at least about 40 weight percent modified
carbon product. In one embodiment, the modified carbon product
includes carbon black. In another embodiment, the modified carbon
product includes carbon fibers.
[0019] In yet another aspect of the present invention, a method for
the fabrication of a proton conducting membrane is provided, the
method including the steps of providing a modified carbon black
product and forming the modified carbon black product into a thin
membrane. In one embodiment of the present aspect, the forming step
includes analog deposition. In another embodiment of the present
aspect, the forming step includes digital deposition. In yet
another embodiment, the forming step includes dispersing the
modified carbon product in a liquid vehicle and ink-jet printing
the modified carbon product. In one embodiment, the modified carbon
product includes hydrophilic functional groups.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIGS. 1(a) and 1(b) illustrate a schematic cross-section of
a PEMFC MEA and bipolar plate assembly according to the prior
art.
[0021] FIG. 2 illustrates a cross-section of the cathode side of an
MEA showing the membrane and bipolar plate and O.sub.2, H+ and
H.sub.2O transport according to the prior art.
[0022] FIG. 3 illustrates a schematic cross-section of a direct
methanol fuel cell (DMFC) according to the prior art.
[0023] FIG. 4 illustrates a method for modifying a carbon product
to form modified carbon according to U.S. Pat. No. 5,900,029 by
Belmont et al.
[0024] FIGS. 5(a) and 5(b) illustrate functional groups attached to
a carbon surface according to one via a diazonium salt in
accordance with the present invention.
[0025] FIG. 6 illustrates the increase in active species phase size
as processing temperature increases.
[0026] FIG. 7 illustrates a method for formation of platinum/metal
oxide active sites using a modified carbon product according to the
present invention.
[0027] FIG. 8 illustrates the proton conduction mechanism in a PEM
according to the prior art.
[0028] FIG. 9 illustrates the proton conduction mechanism in a PEM
according an embodiment of the present invention.
[0029] FIG. 10 illustrates phosphonic groups incorporated into a
PBI membrane according to an embodiment of the present
invention.
[0030] FIG. 11 illustrates the use of sulphonic acid as a proton
conducting functional group according to an embodiment of the
present invention.
[0031] FIG. 12 illustrates the use of modified carbon products to
decrease cracking during drying as compared to the prior art and
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention relates to fuel cell components that
incorporate modified carbon products. Specifically, the present
invention relates to proton exchange membranes incorporating
modified carbon products. The use of such modified carbon products
enables the production of proton exchange membrane (PEMs) having
enhanced properties. For example, modified carbon products can be
utilized to enhance proton conductivity and electrical insulation
properties, as well as physical robustness.
[0033] As used herein, a modified carbon product refers to a
carbon-containing material having an organic group attached to the
carbon surface. In a preferred embodiment, the modified carbon
product is a carbon particle having an organic group covalently
attached to the carbon surface.
[0034] A native (unmodified) carbon surface is relatively inert to
most organic reactions, and the attachment of specific organic
groups at high coverage levels has been difficult. However, U.S.
Pat. No. 5,900,029 by Belmont et al., which is incorporated herein
by reference in its entirety, discloses a process (referred to
herein as the Belmont process) that significantly improves the
ability to modify carbon surfaces with organic groups. Utilizing
the Belmont process, organic groups can be covalently bonded to the
carbon surface such that the groups are highly stable and do not
readily desorb from the carbon surface.
[0035] Generally, the Belmont process includes reacting at least
one diazonium salt with a carbon material to reduce the diazonium
salt, such as by reacting at least one diazonium salt with a carbon
black in a protic reaction medium . . . The diazonium salt can
include the organic group to be attached to the carbon. The organic
group can be selected from an aliphatic group, a cyclic organic
group or an organic compound having an aliphatic portion and a
cyclic portion. The organic group can be substituted or
unsubstituted and can be branched or unbranched. Accordingly,
carbon can be modified to alter its properties such as its surface
energy, dispersability in a medium, aggregate size and size
distribution, dispersion viscosity and/or chemical reactivity.
[0036] The modified carbon product can be formed using an
electrically conductive crystalline form of carbon, such as
graphite, or can be an amorphous carbon. The carbon, whether
crystalline or amorphous, can be in the form of any solid carbon,
including carbon black, activated carbon, carbon fiber, bulk
carbon, carbon cloth, carbon nanotubes, carbon paper, carbon flakes
and the like.
[0037] It will be appreciated that the carbon material utilized to
form the modified carbon product can be selected to suit the
specific application of the modified carbon product in which the
carbon material will be utilized. For example, graphite has an
anisotropic plate-like structure and a well-defined crystal
structure, resulting in a high electrical conductivity. In one
embodiment, a modified carbon product including graphite is
utilized in a fuel cell component to increase or enhance its
electrical conductivity.
[0038] Carbon fibers are long, thin, rod-shaped structures which
are advantageous for physically reinforcing membranes and
increasing in-plane electrical conductivity. In one embodiment,
modified carbon fibers are utilized in a fuel cell component to
increase or maintain its structural integrity.
[0039] Carbon blacks are homologous to graphite, but typically have
a relatively low conductivity and form soft, loose agglomerates of
primarily nano-sized particles that are isotropic in shape. Carbon
black particles generally have an average size in the range of 9 to
150 nanometers and a surface area of from about 20 to 1500
m.sup.2/g. In one embodiment, a modified carbon product including
carbon black is utilized in the fuel cell component to decrease its
electrical conductivity. In another embodiment, modified carbon
product including carbon black is dispersed in a liquid to form a
modified carbon ink that can be utilized in the production of a
fuel cell component due to its shape and small particle size.
[0040] Generally, a carbon material is modified utilizing the
Belmont process via a functionalizing agent of the form:
X--R--Y
[0041] where:
[0042] X reacts with the carbon surface;
[0043] R is a linking group; and
[0044] Y is a functional group.
[0045] The functional group (Y) can vary widely, as can the linking
group (R), by selection of the appropriate diazonium salt
precursor. The diazonium precursor has the formula:
XN.ident.NRY
[0046] where:
[0047] N is nitrogen;
[0048] X is an anion such as Cl.sup.-, Br.sup.- or F.sup.-; R is
the linking group; and
[0049] Y is the functional group.
[0050] FIG. 4 schematically illustrates one method of surface
modifying a carbon material according to the Belmont process. The
carbon material 420 is contacted with a diazonium salt 422 to
produce a modified carbon product 424. The resulting modified
carbon product 424 includes surface groups that include the linking
group (R) and the functional group (Y), as discussed below in
relation to FIGS. 5(a) and 5(b).
[0051] FIGS. 5(a) and 5(b) illustrate different embodiments of a
modified carbon product 524a, 524b having a surface group,
including a functional group (Y) and linking group (R) attached to
the carbon material. In FIG. 5(a), sulfonic acid is attached to the
carbon material 520 to produce a modified carbon product 524a. In
FIG. 5(b) polyamines are attached to the carbon material 520 to
produce a modified carbon product 524b.
[0052] Examples of functional groups (Y) that can be used to modify
the carbon material according to the present invention include
those that are charged (electrostatic), such as sulfonate,
carboxylate and tertiary amine salts. Preferred functional groups
for fuel cell components according to one aspect of the present
invention include those that alter the hydrophobic/hydrophilic
nature of the carbon material, such as polar organic groups and
groups containing salts, such as tertiary amine salts. Particularly
preferred hydrophilic functional groups are listed in Table 1, and
particularly preferred hydrophobic functional groups are listed in
Table II.
1TABLE I Hydrophilic Functional Groups (Y) Examples Carboxylic
acids and salts (C.sub.6H.sub.4)CO.sub.2.sup.-K- .sup.+,
(C.sub.6H.sub.4)CO.sub.2H Sulfonic acids and salts
(C.sub.6H.sub.4)CH.sub.2SO.sub.3H Phosphonic acids and salts
(C.sub.10H.sub.6)PO.sub.3H.sub.2 Amines and amine salts
(C.sub.6H.sub.4)NH.sub.3.sup.+Cl.sup.- Alcohols
(C.sub.6H.sub.4)OH
[0053]
2TABLE II Hydrophobic Functional Groups (Y) Examples Saturated and
unsaturated cyclics and (CH.sub.2).sub.3CH.sub.3,
(C.sub.6H.sub.4)CH.sub.3 aliphatics Halogenated saturated and
unsaturated (C.sub.6H.sub.4)CF.sub.3,
(C.sub.6H.sub.4)(CF.sub.2).sub.7CF.sub.3 cyclics and aliphatics
Polymerics Polystyrene [CH.sub.2CH(C.sub.6H.sub.5)].sub.n
[0054] According to another aspect, preferred functional groups for
fuel cell components are those that increase proton conductivity,
such as SO.sub.3H, PO.sub.3H.sub.2 and others known to have good
proton conductivity. Particularly preferred proton conductive
functional groups according to the present invention are listed in
Table III.
3 TABLE III Proton Conducting Groups (Y) Examples Carboxylic acid
and salt (C.sub.6H.sub.4)COOH, (C.sub.6H.sub.4)COONa Sulfonic acid
and salt (C.sub.6H.sub.4)SO.sub.3H, (C.sub.6H.sub.4)SO.sub.3Na
Phosphonic acid and salt (C.sub.6H.sub.4)PO.sub.3H.sub.2,
(C.sub.6H.sub.4)PO.sub.3HN- a
[0055] According to another aspect of the present invention,
preferred functional groups for fuel cell components include those
that increase steric hindrance and/or physical interaction with
other material surfaces, such as branched and unbranched polymeric
groups. Particularly preferred polymeric groups according to this
aspect are listed in Table IV.
4 TABLE IV Polymeric Groups (Y) Examples Polyacrylate Polymethyl
methacrylate (C.sub.6H.sub.4)[CH.sub.2C(CH.sub.3)COOCH.sub.2].sub.n
Polystyrene (C.sub.6H.sub.4)[CH(C.sub.6H.sub.5)CH.sub.2].sub.n
Polyethylene oxide (PEO)
(C.sub.6H.sub.4)[OCH.sub.2CH.sub.2OCH.sub.2CH.su- b.2].sub.n
Polyethylene glycol (PEG) (C.sub.6H.sub.4)[CH.sub.2CH.s- ub.2O]n
Polypropylene oxide (PPO) (C.sub.6H.sub.4)[OCH(CH.sub.3)CH-
.sub.2].sub.n
[0056] The linking group (R) of the modified carbon product can
also vary. For example, the linking group can be selected to
increase the "reach" of the functional group by adding flexibility
and degrees of freedom to further enhance proton conduction, steric
hindrance and/or physical interaction with other materials. The
linking group can be branched or unbranched. Particularly preferred
linking groups according to the present invention are listed in
Table V.
5 TABLE V Linking Group (R) Examples Alkyls CH.sub.2,
C.sub.2H.sub.4 Aryls C.sub.6H.sub.4, C.sub.6H.sub.4CH.sub.2 Cyclics
C.sub.6H.sub.10, C.sub.5H.sub.4 Unsaturated aliphatics
CH.sub.2CH.dbd.CHCH.sub.2 Halogenated alkyl, aryl, cyclics and
C.sub.2F.sub.4, C.sub.6H.sub.4CF.sub.2, C.sub.8F.sub.10 unsaturated
aliphatics CF.sub.2CH.dbd.CHCF.sub.2
[0057] Generally, any functional group (Y) can be utilized in
conjunction with any linking group (R) to create a modified carbon
product for use according to the present invention. It will be also
appreciated that any other organic groups listed in U.S. Pat. No.
5,900,029 by Belmont et al. can be utilized in accordance with the
present invention.
[0058] It will further be appreciated that the modified carbon
product can include varying amounts of surface groups. The amount
of surface groups in the modified carbon product is generally
expressed either on a mass basis (e.g., mmol of surface groups/gram
of carbon) or on a surface area basis (e.g., .mu.mol of surface
groups per square meter of carbon material surface area). In the
latter case, the BET surface area of the carbon support material is
used to normalize the surface concentration per specific type of
carbon. In one embodiment, the modified carbon product has a
surface group concentration of from about 0.1 .mu.mol/m.sup.2 to
about 6.0 .mu.mol/m.sup.2. In a preferred embodiment, the modified
carbon product has a surface group concentration of from about 1.0
.mu.mol/m.sup.2 to about 4.5 .mu.mol/m.sup.2, and more preferably
of from about 1.5 .mu.mol/m.sup.2 to about 3.0 .mu.mol/m.sup.2.
[0059] The modified carbon product can also have more than one
functional group and/or linking group attached to the carbon
surface. In one such aspect of the present invention, the modified
carbon product includes a second functional group (Y') attached to
the carbon surface. In one embodiment, the second functional group
(Y') is attached to the carbon surface via a first linking group
(R), which also has a first functional group (Y) attached thereto.
In another embodiment, the second functional group (Y') is attached
to the carbon surface via a separate second linking group (R'). In
this regard, any of the above referenced organic groups can be
attached as the first and/or second organic surface groups, and in
any combination.
[0060] In one embodiment of the present invention, the modified
carbon products are modified carbon product particles having a
well-controlled particle size. Preferably, the volume average
particle size is not greater than about 100 .mu.m, more preferably
is not greater than about 20 .mu.m and even more preferably is not
greater than about 10 .mu.m. Further, it is preferred that the
volume average particle size is at least about 0.1 .mu.m, more
preferably 0.3 .mu.m, even more preferably is at least about 0.5
.mu.m and even more preferably is at least about 1 .mu.m. As used
herein, the average particle size is the median particle size
(d.sub.50). Powder batches having an average particle size within
the preferred parameters disclosed herein enable the formation of
thin layers which are advantageous for producing energy devices
such as fuel cells according to the present invention.
[0061] In a particular embodiment, the modified carbon product
particles have a narrow particle size distribution. For example, it
is preferred that at least about 50 volume percent of the particles
have a size of not greater than about two times the volume average
particle size and it is more preferred that at least about 75
volume percent of the particles have a size of not greater than
about two times the volume average particle size. The particle size
distribution can be bimodal or trimodal which can advantageously
provide improved packing density.
[0062] In another embodiment, the modified carbon product particles
are substantially spherical in shape. That is, the particles are
preferably not jagged or irregular in shape. Spherical particles
can advantageously be deposited using a variety of techniques,
including direct write deposition, and can form layers that are
thin and have a high packing density, as discussed in further
detail below.
[0063] Manufacture Of Modified Carbon Products Particles
[0064] Modified carbon products useful in accordance with the
present invention can be manufactured using any known methodology,
including, inter alia, the Belmont process, physical adsorption,
surface oxidation, sulfonation, grafting, using an alkylating agent
in the presence of a Friedel-Crafts type reaction catalyst, mixing
benzene and carbon black with a Lewis Acid catalyst under anhydrous
conditions followed by polymerization, coupling of a diazotized
amine, coupling of one molecular proportion of a tetrazotized
benzidine with two molecular proportions of an arylmethyl
pyrazolone in the presence of carbon black, use of an
electrochemical reduction of a diazonium salt, and those disclosed
in and by: Tsubakowa in Polym. Sci., Vol. 17, pp 417470, 1992, U.S.
Pat. No. 4,014,844 to Vidal et al., U.S. Pat. No. 3,479,300 to
Riven et al., U.S. Pat. No. 3,043,708 to Watson et al., U.S. Pat.
No. 3,025,259 Watson et al., U.S. Pat. No. 3,335,020 to Borger et
al., U.S. Pat. No. 2,502,254 to Glassman, U.S. Pat. No. 2,514,236
to Glassman, U.S. Pat. No. 2,514,236 to Glassman, PCT Patent
Application No. WO 92/13983 to Centre National De La Recherch
Scientifique, and Delmar et al., J. Am. Chem. Soc. 1992, 114,
5883-5884, each of which is incorporated herein by reference in its
entirety.
[0065] A particularly preferred process for manufacturing modified
carbon product particles according to the present invention
involves implementing the Belmont process by spray processing,
spray conversion and/or spray pyrolysis, the methods being
collectively referred to herein as spray processing. A spray
process of this nature is disclosed in commonly-owned U.S. Pat. No.
6,660,680 by Hampden-Smith et al., which is incorporated herein by
reference in its entirety.
[0066] Spray processing according to the present invention
generally includes the steps of: providing a liquid precursor
suspension, which includes a carbon material and a diazonium salt
or a precursor to a diazonium salt; atomizing the precursor to form
dispersed liquid precursor droplets; and removing liquid from the
dispersed liquid precursor droplets to form the modified carbon
product particles.
[0067] Preferably, the spray processing method combines the drying
of the diazonium salt and carbon-containing droplets and the
conversion of the diazonium precursor salt to a linking group and
functional group covalently bound to a carbon surface in one step,
where both the removal of the solvent and the conversion of the
precursor occur essentially simultaneously. Combined with a short
reaction time, this method enables control over the properties of
the linking group and functional group bound to the carbon surface.
In another embodiment, the spray processing method achieves the
drying of the droplets in a first step, and the conversion of the
diazonium salt to a linking group and functional group in a
distinct second step. By varying reaction time, temperature, type
of carbon material and type of precursors, spray processing can
produce modified carbon product particles having tailored
morphologies and structures that yield improved performance.
[0068] Spray processing advantageously enables the modified carbon
product particles to be formed while the diazonium salt phase is in
intimate contact with the carbon surface, where the diazonium salt
is rapidly reacted on the carbon surface. Preferably, the diazonium
salt is exposed to an elevated reaction temperature for not more
than about 600 seconds, more preferably not more than about 100
seconds and even more preferably not more than about 10
seconds.
[0069] Spray processing is also capable of forming an aggregate
modified carbon product particle structure. The aggregate modified
carbon product particles form as a result of the formation and
drying of the droplets during spray processing, and the properties
of the structure are influenced by the characteristics of the
carbon particles, such as the particle size, particle size
distribution and surface area of the carbon particles.
[0070] Spray processing methods for modified carbon product
particle manufacture according to the present invention can be
grouped by reference to several different attributes of the
apparatus used to carry out the method. These attributes include:
the main gas flow direction (vertical or horizontal); the type of
atomizer (submerged ultrasonic, ultrasonic nozzle, two-fluid
nozzle, single nozzle pressurized fluid); the type of gas flow
(e.g., laminar with no mixing, turbulent with no mixing, co-current
of droplets and hot gas, countercurrent of droplets and gas or
mixed flow); the type of heating (e.g., hot wall system, hot gas
introduction, combined hot gas and hot wall, plasma or flame); and
the type of collection system (e.g., cyclone, bag house,
electrostatic or settling).
[0071] For example, modified carbon product particles can be
prepared by starting with a precursor liquid including a protic
reaction medium (e.g., an aqueous-based liquid), colloidal carbon
and a diazonium salt. The processing temperature of the precursor
droplets can be controlled so the diazonium salt reacts, leaving
the carbon intact but surface functionalized. The precursor liquid
may also or alternatively include an aprotic reaction medium such
as acetone, dimethyl formamide, dioxane and the like.
[0072] The atomization technique has a significant influence over
the characteristics of the modified carbon product particles, such
as the spread of the particle size distribution (PSD), as well as
the production rate of the particles. In extreme cases, some
techniques cannot atomize precursor compositions having only
moderate carbon particle loading or high viscosities. Several
methods exist for the atomization of precursor compositions
containing suspended carbon particulates. These methods include,
but are not limited to: ultrasonic transducers (usually at a
frequency of 1-3 MHz); ultrasonic nozzles (usually at a frequency
of 10-150 KHz); rotary atomizers; two-fluid nozzles; and pressure
atomizers.
[0073] Ultrasonic transducers are generally submerged in a liquid,
and the ultrasonic energy produces atomized droplets on the surface
of the liquid. Two basic ultrasonic transducer disc configurations,
planar and point source, can be used. Deeper fluid levels can be
atomized using a point source configuration since the energy is
focused at a point that is some distance above the surface of the
transducer. The scale-up of submerged ultrasonic transducers can be
accomplished by placing a large number of ultrasonic transducers in
an array. Such a system is illustrated in U.S. Pat. No. 6,103,393
by Kodas et al. and U.S. Pat. No. 6,338,809 by Hampden-Smith et
al., each of which is incorporated herein by reference in its
entirety.
[0074] Spray nozzles can also be used, and the scale-up of nozzle
systems can be accomplished by either selecting a nozzle with a
larger capacity, or by increasing the number of nozzles used in
parallel. Typically, the droplets produced by nozzles are larger
than those produced by ultrasonic transducers. Particle size is
also dependent on the gas flow rate. For a fixed liquid flow rate,
an increased airflow decreases the average droplet size and a
decreased airflow increases the average droplet size. It is
difficult to change droplet size without varying the liquid or
airflow rates. However, two-fluid nozzles have the ability to
process larger volumes of liquid per unit time than ultrasonic
transducers.
[0075] Ultrasonic spray nozzles use high frequency energy to
atomize a fluid and have some advantages over single or two-fluid
nozzles, such as the low velocity of the spray leaving the nozzle
and lack of associated gas flow. The nozzles are available with
various orifice sizes and orifice diameters that allow the system
to be scaled for the desired production capacity. In general,
higher frequency nozzles are physically smaller, produce smaller
droplets, and have a lower flow capacity than nozzles that operate
at lower frequencies. A drawback of ultrasonic nozzle systems is
that scaling up the process by increasing the nozzle size increases
the average particle size. If a particular modified carbon product
particle size is required, then the maximum production rate per
nozzle is set. If the desired production rate exceeds the maximum
production rate of the nozzle, additional nozzles or additional
production units will be required to achieve the desired production
rate.
[0076] The shape of the atomizing surface determines the shape and
spread of the spray pattern. Conical, microspray and flat atomizing
surface shapes are available. The conical atomizing surface
provides the greatest atomizing capability and has a large spray
envelope. The flat atomizing surface provides almost as much flow
as the conical, but limits the overall diameter of the spray. The
microspray atomizing surface is for very low flow rates where
narrow spray patterns are needed. These nozzles are preferred for
configurations where minimal gas flow is required in association
with the droplets.
[0077] Particulate suspensions present several problems with
respect to atomization. For example, submerged ultrasonic atomizers
re-circulate the suspension through the generation chamber and the
suspension concentrates over time. Further, some fraction of the
liquid atomizes without carrying the suspended carbon particulates.
When using submerged ultrasonic transducers, the transducer discs
can become coated with the particles over time. Further, the
generation rate of particulate suspensions is very low using
submerged ultrasonic transducer discs, due in part to energy being
absorbed or reflected by the suspended particles.
[0078] For spray drying, an aerosol can be generated using three
basic methods. These methods differ in the type of energy used to
break the liquid masses into small droplets. Rotary atomizers
(utilization of centrifugal energy) make use of spinning liquid
droplets off of a rotating wheel or disc. Rotary atomizers are
useful for co-current production of droplets in the range of 20 to
150 .mu.m in diameter. Pressure nozzles (utilization of pressure
energy) generate droplets by passing a fluid under high pressure
through an orifice. These can be used for both co-current and
mixed-flow reactor configurations, and typically produce droplets
in the size range of 50 to 300 .mu.m. Multiple fluid nozzles, such
as a two fluid nozzle, produce droplets by passing a relatively
slow moving fluid through an orifice while shearing the fluid
stream with a relatively fast moving gas stream. As with pressure
nozzles, multiple fluid nozzles can be used with both co-current
and mixed-flow spray dryer configurations. This type of nozzle can
typically produce droplets in the range of 5 to 200 .mu.m.
[0079] For example, two-fluid nozzles are used to produce aerosol
sprays in many commercial applications, typically in conjunction
with spray drying processes. In a two-fluid nozzle, a low-velocity
liquid stream encounters a high-velocity gas stream that generates
high shear forces to accomplish atomization of the liquid. A direct
result of this interaction is that the droplet size characteristics
of the aerosol are dependent on the relative mass flow rates of the
liquid precursor and nozzle gas stream. The velocity of the
droplets as they leave the generation zone can be quite large which
may lead to unacceptable losses due to impaction. The aerosol also
leaves the nozzle in a characteristic pattern, typically a flat
fan, and this may require that the dimensions of the reactor be
sufficiently large to prevent unwanted losses on the walls of the
system.
[0080] The next step in the process includes the evaporation of the
solvent (typically water) as the droplet is heated, resulting in a
carbon particle of dried solids and salts. A number of methods to
deliver heat to the particle are possible: horizontal hot-wall
tubular reactors, spray drier and vertical tubular reactors can be
used, as well as plasma, flame and laser reactors. As the carbon
particles experience either higher temperature or longer time at a
specific temperature, the diazonium salt reacts. Preferably, the
temperature and amount of time that the droplets/particles
experience can be controlled, and, therefore, the properties of the
linking group and functional group formed on the carbon surface can
also be controlled.
[0081] For example, a horizontal, tubular hot-wall reactor can be
used to heat a gas stream to a desired temperature. Energy is
delivered to the system by maintaining a fixed boundary temperature
at the wall of the reactor and the maximum temperature of the gas
is the wall temperature. Heat transfer within a hot wall reactor
occurs through the bulk of the gas and buoyant forces that occur
naturally in horizontal hot wall reactors aid this transfer. The
mixing also helps to improve the radial homogeneity of the gas
stream. Passive or active mixing of the gas can also increase the
heat transfer rate. The maximum temperature and the heating rate
can be controlled independent of the inlet stream with small
changes in residence time. The heating rate of the inlet stream can
also be controlled using a multi-zone furnace.
[0082] The use of a horizontal hot-wall reactor according to the
present invention is preferred to produce modified carbon product
particles with a size of not greater than about 5 .mu.m. One
disadvantage of such reactors is the poor ability to atomize carbon
particles when using submerged ultrasonics for atomization.
[0083] Alternatively, a horizontal hot-wall reactor can be used
with a two-fluid nozzle. This method is preferred for precursor
feed streams containing relatively high levels of carbon. A
horizontal hot-wall reactor can also be used with ultrasonic
nozzles, which allows atomization of precursors containing
particulate carbons. However, large droplet size can lead to
material loss on reactor walls and other surfaces, making this an
expensive method for production of modified carbon product
particles.
[0084] While horizontal hot-wall reactors are useful according to
the present invention, spray processing systems in the
configuration of a spray dryer are the generally preferred
production method for large quantities of modified carbon product
particles. Spray drying is a process where particles are produced
by atomizing a precursor to produce droplets and evaporating the
liquid to produce a dry aerosol, where thermal decomposition of one
or more precursors (e.g., a carbon and/or diazonium salt) may take
place to produce the particle. The residence time in the spray
dryer is the average time the process gas spends in the drying
vessel as calculated by the vessel volume divided by the process
gas flow using the outlet gas conditions. The peak excursion
temperature (i.e., the reaction temperature) in the spray dryer is
the maximum temperature of a particle, averaged throughout its
diameter, while the particle is being processed and/or dried. The
droplets are heated by supplying a pre-heated carrier gas.
[0085] Three types of spray dryer systems are useful for spray
drying to form modified carbon product particles according to the
present invention. An open system is useful for general spray
drying to form modified carbon product particles using air as an
aerosol carrier gas and an aqueous feed solution as a precursor. A
closed system is useful for spray drying to form modified carbon
product particles using an aerosol carrier gas other than air. A
closed system is also useful when using a non-aqueous or a
semi-non-aqueous solution as a precursor. A semi-closed system,
including a self-inertizing system, is useful for spray drying to
form modified carbon product particles that require an inert
atmosphere and/or precursors that are potentially flammable.
[0086] Two spray dryer designs are particularly useful for the
production of modified carbon product particles according to the
present invention. A co-current spray dryer is useful for
production of modified carbon product particles that are sensitive
to high temperature excursions (e.g., greater than about
350.degree. C.), or that require a rotary atomizer to generate the
aerosol. Mixed-flow spray dryers are useful for producing modified
carbon product particles that require relatively high temperature
excursions (e.g., greater than about 350.degree. C.), or require
turbulent mixing forces. According one embodiment of the present
invention, co-current spray-drying is preferred for the manufacture
of modified carbon product particles, including modified carbon
black.
[0087] In a co-current spray dryer, the hot gas is introduced at
the top of the unit, where the droplets are generated with any of
the above-described atomization techniques. Generally, the maximum
temperature that a droplet/particle is exposed to in a co-current
spray dryer is the temperature at the outlet of the dryer.
Typically, this outlet temperature is limited to about 200.degree.
C., although some designs allow for higher temperatures. In
addition, since the particles experience the lowest temperature in
the beginning of the time-temperature curve and the highest
temperature at the end, the possibility of precursor surface
diffusion and agglomeration is high.
[0088] A mixed-flow spray dryer introduces the hot gas at the top
of the unit while precursor droplets are generated near the bottom
and directed upwardly. The droplets/particles are forced towards
the top of the unit, and then fall and flow back down with the gas,
increasing the residence time in the spray dryer. The temperature
experienced by the droplets/particles is higher compared to a
co-current spray dryer.
[0089] These conditions are advantageous for the production of
modified carbon product particles having a wide range of surface
group concentrations including surface concentrations up to 6
.mu.mol/m.sup.2 organic groups on carbon. For co-current spray
dryers the reaction temperatures can be high enough to enable
reaction of the diazonium salt (e.g., between 25.degree. C. and
100.degree. C.). The highest temperature in co-current spray dryers
is the inlet temperature (e.g., 180.degree. C.), and the outlet
temperature can be as low as 50.degree. C. Therefore, the carbon
particles and surface groups reach the highest temperature for a
relatively short time, which advantageously reduces migration or
surface diffusion of the surface groups. This spike of high
temperature can also quickly convert the diazonium salt to the
bonded surface group, and is followed by a mild quench since the
spray dryer temperature quickly decreases after the maximum
temperature is achieved. Thus, the spike-like temperature profile
can be advantageous for the generation of highly dispersed surface
groups on the surface of the carbon.
[0090] The range of useful residence times for producing modified
carbon product particles depends on the spray dryer design type,
atmosphere used, nozzle configuration, feed liquid inlet
temperature and the residual moisture content. In general,
residence times for the production of modified carbon product
particles can range from less than 3 seconds up to 5 minutes.
[0091] For a co-current spray-drying configuration, the range of
useful inlet temperatures for producing modified carbon product
particles depends on a number of factors, including solids loading
and droplet size, atmosphere used and energy required to perform
drying and/or reaction of the diazonium salt. Useful inlet
temperatures should be sufficiently high to accomplish the drying
and/or reaction of the diazonium salt without promoting significant
surface diffusion of the surface groups.
[0092] In general, the outlet temperature of the spray dryer
determines the residual moisture content of the modified carbon
product particles. For example, a useful outlet temperature for
co-current spray drying according to one embodiment of the present
invention is from about 50.degree. C. to about 80.degree. C. Useful
inlet temperatures according to the present invention are from
about 130.degree. C. to 180.degree. C. The carbon solids (e.g.,
particulate) loading can be up to about 50 wt. %.
[0093] Other equipment that is desirable for producing modified
carbon product particles using a spray dryer includes a heater for
heating the gas, directly or indirectly, including by thermal,
electrical conductive, convective and/or radiant heating.
Collection apparatus, such as cyclones, bag/cartridge filters,
electrostatic precipitators, and/or various wet collection
apparatus, may also be utilized to collect the modified carbon
product particles.
[0094] In one embodiment of the present invention, spray drying is
used to form aggregate modified carbon product particles, wherein
the aggregates include more than one modified carbon product
particle. In this regard, the individual modified carbon product
particles can all have essentially the same surface groups or
varying types of modified carbon product particles can be utilized
to provide an aggregate with a mixture of surface groups. For
example, a first modified carbon product particle within the
aggregate can have a hydrophilic surface group and a second
modified carbon product particle can have a hydrophobic surface
group.
[0095] In one aspect, first modified carbon product particles
(e.g., modified carbon black particles having a hydrophilic surface
group) and second modified carbon product particles (e.g., modified
carbon black particles having a hydrophobic surface group) are
dispersed in a aqueous precursor solution and spray dried to obtain
an aggregate modified carbon product particle having both
hydrophilic and hydrophobic properties. The aggregate may include
various particle sizes, from nano-sized particles to large,
sub-micron size particles.
[0096] Moreover, as described below with respect to electrocatalyst
materials, the aggregate structure can include smaller primary
carbon particles and two or more types of primary particles can be
mixed. For example, two or more types of particulate carbon (e.g.,
amorphous and graphitic carbon) can be combined within the
aggregate to tailor the aggregate to the desired electrical and/or
oxidation resistant properties.
[0097] In this regard, spray drying techniques can be used simply
to form the aggregate modified carbon product particles, or to
additionally effect a change in the structure of the individual
modified carbon product particles. For example, spray processing
techniques can be conducted at higher temperatures to effect at
least a partial decomposition of the previously attached surface
groups, such as those surface groups that are utilized to help the
spray processing, but are subsequently not desired in the
end-product. The specific temperature for the spray drying process
may be chosen depending on the desired outcome, which is a function
of the type and stability of the surface groups, the targeted final
composition, and the treatment distribution.
[0098] Electrocatalyst Materials
[0099] Electrocatalysts are used in the fuel cell to facilitate the
desired reactions. Particularly preferred electrocatalyst materials
useful in accordance with the present invention include those
having an active species phase, such as a metal, dispersed on a
support phase, such as a carbon material. Such electrocatalyst
materials are described in U.S. Pat. No. 6,660,680 by Hampden-Smith
et al. As used herein, the terms "electrocatalyst materials",
"electrocatalyst particles" and/or "electrocatalyst powders" and
the like refer to such electrocatalyst materials in a non-modified
native state.
[0100] With respect to electrocatalyst materials, the larger
structures formed from the association of discrete carbon particles
supporting the dispersed active species phase are referred to as
aggregates or aggregate particles, and typically have a size in the
range from 0.3 to 100 mm. In addition, the aggregates can further
associate into larger "agglomerates". The aggregate morphology,
aggregate size, size distribution and surface area of the
electrocatalyst powders are all characteristics that impact the
catalyst performance. The aggregate morphology, aggregate size and
size distribution determines the packing density, and the surface
area determines the type and number of surface adsorption centers
where the active species form during synthesis of the
electrocatalyst.
[0101] The aggregate structure can include smaller primary carbon
particles, constituting the support phase. Two or more types of
primary particles can be mixed to form the support phase. For
example, two or more types of particulate carbon (e.g., amorphous
and graphitic carbon) can be combined to form the support phase.
The two types of particulate carbon can have different performance
characteristics and the combination of the two types in the
aggregate structure can enhance the performance of the
catalyst.
[0102] The carbon support is a major component of the
electrocatalysts. To achieve adequate dispersion of the active
sites, the carbon support should have a high surface area, a large
accessible porous surface area (pore sizes from about 2 nm to about
50 nm preferred), low levels of contaminants that are poisons for
either the membrane or the active sites during long term operation
of the fuel cell, and good stability with respect to oxidation
during the operation of the fuel cell.
[0103] Among the forms of carbon available for the support phase,
graphitic carbon is preferred for long-term operational stability
of fuel cells due to its ability to resist oxidation. Amorphous
carbon (e.g., carbon black) is preferred when a smaller crystallite
size is desired for the supported active species phase. The carbon
support particles typically have sizes in the range of from about
10 nanometers to 5 .mu.m, depending on the nature of the carbon
material. However, carbon particulates having sizes up to 25 .mu.m
can also be used.
[0104] The compositions and ratios of the aggregate particle
components can be independently varied, and various combinations of
carbons, metals, metal alloys, metal oxides, mixed metal oxides,
organometallic compounds and their partial pyrolysis products can
be used. The electrocatalyst particles can include two or more
different materials as the dispersed active species. As an example,
combinations of Ag and MnO.sub.x dispersed on carbon can be useful
for some electrocatalytic applications. Other examples of multiple
active species are mixtures of metal porphyrins, partially
decomposed metal porphyrins, Co and CoO.
[0105] The supported electrocatalyst particles preferably include a
carbon support phase with at least about 1 weight percent active
species phase, more preferably at least about 5 weight percent
active species phase and even more preferably at least about 10
weight percent active species phase. In one embodiment, the
particles include from about 20 to about 80 weight percent of the
active species phase dispersed on the support phase. It has been
found that such compositional levels give rise to the most
advantageous electrocatalyst properties for many applications.
However, the preferred level of the active species supported on the
carbon support will depend upon the total surface area of the
carbon, the type of active species phase and the application of the
electrocatalyst. A carbon support having a low surface area will
require a lower percentage of active species on its surface to
achieve a similar surface concentration of the active species
compared to a support with higher surface area and higher active
species loading.
[0106] Metal-carbon electrocatalyst particles include a
catalytically active species of at least a first metal phase
dispersed on a carbon support phase. The metal active species phase
can include any metal and the particularly preferred metal will
depend upon the application of the powder. The metal phase can be a
metal alloy wherein a first metal is alloyed with one or more
alloying elements. As used herein, the term metal alloy also
includes intermetallic compounds between two or more metals. For
example, the term platinum metal phase refers to a platinum alloy
or platinum-containing intermetallic compound, as well as pure
platinum metal. The metal-carbon electrocatalyst powders can also
include two or more metals dispersed on the support phase as
separate active species phases.
[0107] Preferred metals for the active species include the platinum
group metals and noble metals, particularly Pt, Ag, Pd, Ru, Os and
their alloys. The metal phase can also include a metal selected
from the group consisting of Ni, Rh, Ir, Co, Cr, Mo, W, V, Nb, Al,
Ta, Ti, Zr, Hf, Zn, Fe, Cu, Ga, In, Si, Ge, Sn, Y, La, lanthanide
metals and combinations or alloys of these metals. Preferred metal
alloys include alloys of Pt with other metals, such as Ru, Os, Cr,
Ni, Mn and Co. Particularly preferred among these is Pt or PtRu for
use in the anode and Pt, PtCrCo or PtNiCo for use in the
cathode.
[0108] Alternatively, metal oxide-carbon electrocatalyst particles
that include a metal oxide active species dispersed on a carbon
support phase can be used. The metal oxide can be selected from the
oxides of the transition metals, preferably those existing in
oxides of variable oxidation states, and most preferably from those
having an oxygen deficiency in their crystalline structure. For
example, the metal oxide active species can be an oxide of a metal
selected from the group consisting of Au, Ag, Pt, Pd, Ni, Co, Rh,
Ru, Fe, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti or Al. A
particularly preferred metal oxide active species is manganese
oxide (MnO.sub.x, where x is 1 to 2). The active species can
include a mixture of different oxides, solid solutions of two or
more different metal oxides or double oxides. The metal oxides can
be stoichiometric or non-stoichiometric and can be mixtures of
oxides of one metal having different oxidation states. The metal
oxides can also be amorphous.
[0109] It is preferred that the average size of the active species
is such that the electrocatalyst particles include small single
crystals or crystallite clusters, collectively referred to herein
as clusters, of the active species dispersed on the support phase.
Preferably, the average active species cluster size (diameter) is
not greater than about 10 nanometers, more preferably is not
greater than about 5 nanometers and even more preferably is not
greater than about 3 nanometers. Preferably, the average cluster
size of the active species is from about 0.5 to 5 nanometers.
Preferably, at least about 50 percent by number, more preferably at
least about 60 percent by number and even more preferably at least
about 70 percent by number of the active species phase clusters
have a size of not greater than about 3 nanometers. Electrocatalyst
powders having a dispersed active species phase with such small
crystallite clusters advantageously have enhanced catalytic
properties as compared to powders including an active species phase
having larger clusters.
[0110] It should be recognized that the preferred electrocatalyst
powders are not mere physical admixtures of different particles,
but are comprised of support phase particles that include a
dispersed phase of an active species. Preferably, the composition
of the aggregate electrocatalyst particles is homogeneous. That is,
the different phases of the electrocatalyst are well dispersed
within a single aggregate particle. It is also possible to
intentionally provide compositional gradients within the individual
electrocatalyst aggregate particles. For example, the concentration
of the dispersed active species phase in a composite particle can
be higher or lower at the surface of the secondary support phase
than near the center and gradients corresponding to compositional
changes of 10 to 100 weight percent can be obtained. When the
aggregate particles are deposited using a direct-write tool, the
aggregate particles preferably retain their structural morphology
and therefore the functionality of the compositional gradient can
be exploited in the device.
[0111] In addition, the electrocatalyst powders preferably have a
surface area of at least about 25 m.sup.2/g, more preferably at
least about 90 m.sup.2/g and even more preferably at least about
600 m.sup.2/g. Surface area is typically measured using the BET
nitrogen adsorption method which is indicative of the surface area
of the powder, including the surface area of accessible pores on
the surface of the particles.
[0112] Moreover, many of the desired attributes of modified carbon
products may be desired attributes of electrocatalyst, and any of
the above-described attributes of the modified carbon products can
be acknowledged as being useful in the production, use and
application of electrocatalyst materials. For example, particle
size, size distribution and spherical nature can be an important
factor when utilizing such electrocatalyst materials in an
electrocatalyst ink, as described in further detail below.
[0113] Manufacture of Electrocatalyst Materials
[0114] Electrocatalyst materials may be produced in a variety of
ways including impregnation and co-precipitation. One preferred
method for preparing particulate electrocatalyst materials is by
spray processing, one approach of which is disclosed in U.S. Pat.
No. 6,660,680 to Hampden-Smith et al.
[0115] Production of electrocatalyst material by spray processing
generally involves the steps of: providing a precursor composition
which includes a support phase or a precursor to the support phase
(e.g., a carbon-containing material) and a precursor to the active
species; atomizing the precursor to form a suspension of liquid
precursor droplets; and removing liquid from liquid precursor
droplets to form the powder. At least one component of the liquid
precursor is chemically converted into a desired component of the
powder. The drying of the precursors and the conversion to a
catalytically active species can be combined in one step, where
both the removal of the solvent and the conversion of a precursor
to the active species occur essentially simultaneously. Combined
with a short reaction time, this enables control over the
distribution of the active species on the support, the oxidation
state of the active species and the crystallinity of the active
species. By varying reaction time, temperature, type of support
material and type of precursors, electrocatalyst materials having
well-controlled catalyst morphologies and active species structures
can be produced, which yield improved catalytic performance.
[0116] The precursor composition can include low temperature
precursors, such as a molecular metal precursor that has a
relatively low decomposition temperature. As used herein, the term
molecular metal precursor refers to a molecular compound that
includes a metal atom. Examples include organometallics (molecules
with carbon-metal bonds), metal organics (molecules containing
organic ligands with metal bonds to other types of elements such as
oxygen, nitrogen or sulfur) and inorganic compounds such as metal
nitrates, metal halides and other metal salts. The molecular metal
precursors can be either soluble or insoluble in the precursor
composition.
[0117] In general, molecular metal precursor compounds that
eliminate ligands by a radical mechanism upon conversion to metal
are preferred, especially if the species formed are stable
radicals, and, therefore, lower the decomposition temperature of
that precursor compound.
[0118] Furthermore, molecular metal precursors containing ligands
that eliminate cleanly upon precursor conversion are preferred
because they are not susceptible to carbon contamination or
contamination by anionic species such as nitrates. Therefore,
preferred precursors for metals include carboxylates, alkoxides or
combinations thereof that convert to metals, metal oxides or mixed
metal oxides by eliminating small molecules such as carboxylic acid
anhydrides, ethers or esters.
[0119] Particularly preferred molecular metal precursor compounds
are metal precursor compounds containing silver, nickel, platinum,
gold, palladium, copper, ruthenium, cobalt and chromium. In one
preferred embodiment of the present invention, the molecular metal
precursor compound comprises platinum.
[0120] Various molecular metal precursors can be used for platinum
metal. Preferred molecular precursors for platinum include
nitrates, carboxylates, beta-diketonates, and compounds containing
metal-carbon bonds. Divalent platinum (II) complexes are
particularly preferred. Preferred molecular precursors also include
ammonium salts of platinates such as ammonium hexachloro platinate
(NH.sub.4).sub.2PtCl.sub.6, and ammonium tetrachloro platinate
(NH.sub.4).sub.2PtCl.sub.4; sodium and potassium salts of halogeno,
pseudohalogeno or nitrito platinates such as potassium hexachloro
platinate K.sub.2PtCl.sub.6, sodium tetrachloro platinate
Na.sub.2PtCl.sub.4, potassium hexabromo platinate
K.sub.2PtBr.sub.6, potassium tetranitrito platinate
K.sub.2Pt(NO.sub.2).sub.4; dihydrogen salts of hydroxo or halogeno
platinates such as hexachloro platinic acid H.sub.2PtCl.sub.6,
hexabromo platinic acid H.sub.2PtBr.sub.6, dihydrogen hexahydroxo
platinate H.sub.2Pt(OH).sub.6; diammine and tetraammine platinum
compounds such as diammine platinum chloride
Pt(NH.sub.3).sub.2Cl.sub.2, tetraammine platinum chloride
[Pt(NH.sub.3).sub.4]Cl.sub.2, tetraammine platinum hydroxide
[Pt(NH.sub.3).sub.4](OH).sub.2, tetraammine platinum nitrite
[Pt(NH.sub.3).sub.4](NO.sub.2).sub.2, tetrammine platinum nitrate
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2, tetrammine platinum bicarbonate
[Pt(NH.sub.3).sub.4](HCO.sub.3).sub.2, tetraammine platinum
tetrachloroplatinate [Pt(NH.sub.3).sub.4]PtCl.sub.4; platinum
diketonates such as platinum (II) 2,4-pentanedionate
Pt(C.sub.5H.sub.7O.sub.2).sub.2; platinum nitrates such as
dihydrogen hexahydroxo platinate H.sub.2Pt(OH).sub.6 acidified with
nitric acid; other platinum salts such as Pt-sulfite and
Pt-oxalate; and platinum salts comprising other N-donor ligands
such as [Pt(CN).sub.6].sub.4.sup.+.
[0121] Modified Electrocatalyst Products
[0122] According to one embodiment of the present invention, the
modified electrocatalyst products are a subclass of the
above-described modified carbon products, and as used herein,
modified electrocatalyst products generally refers to an
electrocatalyst material having an organic group attached
thereto.
[0123] In one embodiment of the present invention, a modified
electrocatalyst product is provided having an active species phase,
a carbon support phase, and an organic surface group covalently
bonded to the carbon support phase.
[0124] In one preferred embodiment, the active species phase
includes a first metal, such as platinum. The active species phase
can also include a second metal, such as ruthenium, cobalt,
chromium or nickel. The first and second metals can be in metallic,
metal oxide or alloy form, as described in further detail below. In
yet another embodiment, the active species phase includes at least
three metals (e.g., Pt, Ni and Co). The active species phase may be
any of the above-mentioned metals or metal oxides utilized in the
above-described electrocatalyst materials.
[0125] The carbon support material can be any of the
above-described materials utilized in a modified carbon product or
electrocatalyst material. In one preferred embodiment, the carbon
support material is carbon black.
[0126] The organic group may include aliphatic groups, cyclic
organic groups and organic compounds having an aliphatic portion
and a cyclic portion. The organic group can be substituted or
unsubstituted and can be branched or unbranched. Generally, as
described above, the organic groups include a linking group (R) and
a functional group (Y), more generally known as surface groups.
[0127] Any of the above-described functional groups (Y) utilized to
form a modified carbon product can also be used in the production
of a modified electrocatalyst product, including those that are
charged (electrostatic), such as sulfonate, carboxylate and
tertiary amine salts. Preferred functional groups include those
that alter the hydrophobic or hydrophilic nature of the carbon
material, such as polar organic groups and groups containing salts,
such as tertiary amine salts, including those listed in Tables I
and II. Another particularly preferred class of functional groups
include those that increase proton conductivity, such as SO.sub.3H,
PO.sub.3H.sub.2 and others known to be part of the backbone of a
proton conducting membrane, including those listed in Table III.
Yet another particularly preferred class of functional groups
includes compounds that increase steric hindrance and/or physical
interaction with other material surfaces, such as such as those
listed in Table IV.
[0128] Any of the linking groups (R) utilized in the creation of a
modified carbon product can also be used in the production of a
modified electrocatalyst products, including those that increase
the "reach" of the functional group by adding flexibility and
degrees of freedom to further increase, for example, proton
conduction, steric hindrance and/or physical and/or interaction
with other materials, including branched and unbranched materials.
Particularly preferred linking groups are listed in Table V,
above.
[0129] It will be appreciated that, generally, any functional group
(Y) can be utilized in conjunction with any linking group (R) to
create a modified electrocatalyst product according to the present
invention to produce the desired effect within the fuel cell
component. It will be also appreciated that any other organic
groups disclosed in U.S. Pat. No. 5,900,029 by Belmont et al. can
be utilized.
[0130] As noted above, modified electrocatalyst products are a
subclass of modified carbon products. Thus, many of the desired
attributes of modified carbon products are also desired attributes
of modified electrocatalyst products, and any of the
above-described attributes of modified carbon products can be
acknowledged as being useful in the production, use and application
of modified electrocatalyst products. For example, particle size,
size distribution and spherical nature can be an important factor
when utilizing such modified electrocatalyst products in a modified
carbon ink, as described in further detail below. Moreover, many of
the attributes of electrocatalyst materials are also desired
attributes of modified electrocatalyst products and any of the
above-described attributes of electrocatalyst materials can be
acknowledged as being useful in the production, use and application
of modified electrocatalyst products. For example, surface area,
average active species cluster size and size distribution, mass
ratio of active species phase to carbon support phase, and particle
aggregation are important factors in catalytic activity. Other
attributes are described below.
[0131] The modified electrocatalyst product may include varying
concentrations of functional groups, such as from about 0.1
.mu.mol/m.sup.2 to about 6.0 .mu.mol/m.sup.2. In a preferred
embodiment, the modified electrocatalyst product has a surface
group concentration of from about 1.0 .mu.mol/m.sup.2 to about 4.5
.mu.mol/m.sup.2, and more preferably of from about 1.5
.mu.mol/m.sup.2 to about 3.0 .mu.mol/m.sup.2.
[0132] The modified electrocatalyst product may also have more than
one functional group and/or linking group attached to the carbon
material. In one aspect of the present invention, the modified
electrocatalyst product includes a second organic surface group
having a second functional group (Y') attached to the carbon
support. In one embodiment, the second functional group (Y') can be
attached to the carbon support via a first linking group (R), which
also has the first functional group (Y) attached thereto.
Alternatively, the second functional group (Y') can be attached to
the carbon support phase via a separate second linking group (R').
In this regard, any of the above referenced organic groups can be
attached as the first and/or second organic surface groups, and in
any combination.
[0133] In a particular embodiment, the first organic surface group
includes a first proton conductive functional group, such as a
sulfuric and/or carboxylic group, and the second organic surface
group includes a second proton conductive functional group, such as
a phosphoric group. The use of two different proton conducting
functional groups on the same carbon material is useful in
circumstances where a wide range of operating conditions may be
utilized so one of the proton conducting groups is always
functional. This enables a relatively flat rate of proton
conduction over a wide range of operating conditions. For example,
sulfuric groups are known to fail at temperatures of about
100.degree. C. However, phosphoric groups are capable of conducting
protons at temperatures above 100.degree. C. Thus, utilizing a
modified carbon product having two different proton conducting
functional groups can enable proton conduction over a wide range of
temperatures without requiring the incorporation of numerous
conventional proton conducting materials in the fuel cell
component. Such materials are especially useful in fuel cells
utilized in automobiles and other transportation devices where
temperatures can widely vary during start-up conditions.
[0134] Methods of producing modified electrocatalyst products are
described in further detail below. It will be appreciated that many
of such methods can be utilized to produce a modified
electrocatalyst product having first and second organic surface
groups attached thereto. Preferred methods for producing modified
electrocatalyst products having two different types of organic
surface group attached thereto (a multiply-modified electrocatalyst
product) include spray processing and surface contacting
techniques, such as immersion and spraying.
[0135] In one embodiment, a multiply-modified electrocatalyst
product having first and second organic surface groups is produced
by spray processing, where a diazonium salt and modified
electrocatalyst product having a first organic surface group
attached thereto are included in a precursor composition. The
precursor composition is subsequently spray processed to attach a
second organic group to the carbon support to produce the
multiply-modified electrocatalyst product. The multiply-modified
electrocatalyst product may then be utilized in the production of a
fuel cell component.
[0136] In another embodiment, a multiply-modified electrocatalyst
product having first and second organic surface groups is produced
by placing a modified electrocatalyst product having a first
organic surface group attached thereto in a solution comprising a
diazonium salt having a second organic group. The second organic
group from the diazonium salt will attach to the carbon support to
create the multiply-modified electrocatalyst product. The
multiply-modified electrocatalyst product may then be utilized in
the production of a fuel cell component.
[0137] It will be appreciated, that the multiply-modified
electrocatalyst product can be formed by modifying with the second
organic surface group before or after the multiply-modified
electrocatalyst product is incorporated into a component of the
fuel cell. For example, a modified electrocatalyst product can be
utilized in the production of a fuel cell component. Subsequently,
the modified electrocatalyst product can be contacted by a
diazonium salt to attach the second organic surface group.
[0138] In a particular embodiment, a modified electrocatalyst
product can be utilized in the production of an electrode.
Subsequently, the electrode can be contacted with a second
diazonium salt, such as by immersion and/or spraying, to attach the
second organic surface group.
[0139] Manufacture of Modified Electrocatalyst Products
[0140] Modified electrocatalyst products according to the present
invention can be manufactured by any appropriate method, including
Impregnation, co-precipitation and other methods utilized by those
skilled in the art to make supported electrocatalysts. One
preferred method for manufacturing modified electrocatalyst
products is spray processing, as described above in reference to
modified carbon particles.
[0141] When a non-modified carbonaceous material is utilized in a
spray processing precursor composition, a dispersant, such as a
surfactant, is typically required to enable dispersion and
increased loading of the carbonaceous material. Such dispersants
typically require high temperature processing to facilitate their
removal from the resultant products. Moreover, in the production of
electrocatalyst materials, any unremoved dispersants typically
poison the active sites.
[0142] However, according to the present invention, modified carbon
products having surface groups that match the polar or non-polar
nature of the precursor liquid composition can be used. Such
modified carbon products decrease or eliminate the need for such
dispersants as the modified carbon products may be more readily
dispersed in the precursor composition. Utilizing modified carbon
products may also lower spray processing manufacturing
temperatures. Processing at a lower temperature also enables
reduction of the active species crystallite size in the
electrocatalyst.
[0143] As schematically depicted in FIG. 6, and with specific
reference to platinum as the active species phase and carbon black
as the carbon material, as processing temperature increases, as
depicted left to right in the figure, crystallite size increases.
Conversely, as temperature decreases, crystallite size also
decreases. Reduced crystallite sizes at lower temperatures are also
evidenced due to the decreased ability for the active species phase
(e.g., platinum) to migrate during the production temperature.
[0144] An increased dispersability of the carbon material in the
precursor composition also enables an expanded range of carbon
products (e.g., graphite) and metal precursors that can be used.
Other materials that may be added to the precursor composition
include those that do not decompose during processing, such as
ionomers (e.g., PTFE) and molecular species (e.g., metal
porphryns).
[0145] Thus, one approach of the present invention is directed to
the production of modified electrocatalyst particles by spray
processing utilizing a modified carbon product in the precursor
composition. According to one particular aspect, modified
electrocatalyst products are produced utilizing spray processing,
where the precursor composition includes modified carbon product
particles as the support phase and a precursor to the active
species.
[0146] Preferred modified carbon products useful in accordance with
this aspect include those that are miscible in an aqueous precursor
composition, including those having polar surface groups, such as
those terminating in hydrophilic and/or proton conducting
functional groups as listed in Tables I and II, above. Preferred
modified carbon products useful in accordance with the present
aspect also include those that are miscible in a non-aqueous
precursor composition, including those having non-polar surface
groups, such those terminating in hydrophobic functional groups as
those listed in Table II above. Preferred modified carbon products
useful in accordance with this aspect also include those that are
readily atomizable to produce an aerosol comprising the modified
carbon product.
[0147] In a particularly preferred embodiment, the modified carbon
product used in the precursor composition is a low-conductivity
carbon material (e.g., a carbon black) having a hydrophilic surface
group (e.g., a sulfuric terminating functional group). In a
particular embodiment, the precursor solution includes from about 5
weight percent to about 15 weight percent of the modified carbon
product.
[0148] Preferred active species precursors include those listed
above for the production of non-modified electrocatalyst materials,
including molecular metal precursor compounds, such as
organometallics (molecules with carbon-metal bonds), metal organics
(molecules containing organic ligands with metal bonds to other
types of elements such as oxygen, nitrogen or sulfur) and inorganic
compounds such as metal nitrates, metal halides and other metal
salts. The molecular metal precursors can be either soluble or
insoluble in the precursor composition.
[0149] Particularly preferred molecular metal precursor compounds
are metal precursor compounds containing nickel, platinum,
ruthenium, cobalt and chromium. In one preferred embodiment of the
present invention, the molecular metal precursor compound comprises
platinum.
[0150] Various molecular metal precursors can be used for platinum
metal, such as those described above with reference to the
molecular metal precursors utilized in the production of
electrocatalyst materials. Any known molecular metal precursors can
also be utilized for other metals, including molecular metal
precursors of ruthenium, nickel, cobalt and chromium. Preferred
precursors for ruthenium include ruthenium (III) nitrosyl nitrate
[Ru(NO)NO.sub.3)] and ruthenium chloride hydrate. One preferred
precursor for nickel is nickel nitrate [(Ni(NO.sub.3).sub.2]. One
preferred precursor for cobalt is cobalt nitrate
[Co(NO.sub.3).sub.2]. One preferred precursor for chromium is
chromium nitrate [Cr(NO.sub.3).sub.3].
[0151] In accordance with this aspect, low temperature spray
processing conditions can be utilized to produce a modified
electrocatalyst product. The processing temperature within the
spray processor is preferably less than about 500.degree. C., more
preferably les than 400.degree. C., and even more preferably less
than 300.degree. C., The residence time within the spray processor
is preferably less than about 10 seconds, more preferably less than
5 seconds, and even more preferably less than 3 seconds.
[0152] In another embodiment, the precursor composition comprises
previously manufactured electrocatalyst materials, such as any of
those described above, and a diazonium salt or a diazonium salt
precursor. The above-described spray processing methods can be
utilized to produce a modified electrocatalyst product based on
this precursor composition.
[0153] In another aspect of the present invention, spray generation
methods are utilized in conjunction with a precursor composition
including a modified carbon product to generate an aerosol for use
in the spray processing methods. As noted above, dispersants, such
as surfactants, have previously been utilized to enable spray
processing of non-modified carbon material. Surfactants increase
the viscosity and change the surface tension, disallowing the use
of certain generation methods like ultrasonic nebulization. In one
embodiment, a precursor composition including modified carbon
products and/or modified electrocatalyst products is utilized in a
spray processing method to produce a modified carbon product,
wherein the precursor composition is atomized utilizing ultrasonic
generation, a vibrating orifice or spray nozzles.
[0154] Formation of Alloyed, Mixed Metal and/or Metal Oxide
Modified Electrocatalyst Products
[0155] Another aspect of the present invention is directed to the
use of modified carbon products to produce modified electrocatalyst
particles including alloys or mixed metal/metal oxides as the
active species. Traditionally, platinum is alloyed with various
elements such as ruthenium, nickel, cobalt or chromium.
[0156] Typically, alloys are produced by the deposition of two or
more metals or metal oxides on the surface of the carbon.
Subsequently, the metals/metal oxides are subjected to a high
temperature post-processing step in a reducing atmosphere to alloy
the active species. This post-processing step reduces any metal
oxides and allows the different species to migrate over the surface
and coalesce to form an alloy.
[0157] According to one embodiment of the present invention, spray
processing techniques are used to produce modified electrocatalyst
products having a platinum alloy having a small metal and/or metal
oxide crystallite size. Small crystallite sizes are possible
because the metal can be bound to a surface group (e.g., such as an
electron donating carboxylic and/or amine functional group) and
steric hindrance of the surface group prevents large crystallite
growth during consecutive steps of metal deposition and/or
post-processing or reduction steps. Other electron donating surface
groups useful in accordance with the present embodiment include
alcohols, ethers, polyalcohols, unsaturated alkyls or aryls, thiols
and amines.
[0158] By way of illustration, a carbon material, such as carbon
black, can be co-modified with carboxylic and amine groups (e.g.,
(C.sub.6H.sub.4)CO.sub.2H and (C.sub.6H.sub.4)CH.sub.2NH.sub.2).
When this carbon material is treated with a metal salt (e.g.,
RuCl.sub.3), the metal center will bind to the amine functionality
on the carbon surface. When this material is subsequently exposed
to a second metal salt (e.g. Pt(NH.sub.3).sub.4(OH).sub.2 or
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2), it will weakly bind to the
carboxylic group of the carbon surface. When this resulting
multiply-modified carbon product is heated under mild reducing
conditions, a finely dispersed alloy (e.g., a platinum plus
ruthenium) electrocatalyst is produced.
[0159] Mixing of the alloy constituents (e.g., Pt and Ru) at the
atomic level reduces the severity of, or eliminates, the
post-processing conditions that are required for alloy formation,
which in turn reduces crystallite sintering and improves alloy
crystallite homogeneity. In one embodiment, the modified
electrocatalyst products including a metal alloy are produced at
temperatures not greater than 400.degree. C. During processing, it
is preferable to have chemically inert surface groups, such as
C.sub.5H.sub.4N, (C.sub.6H.sub.4)NH.sub.2, C.sub.6H.sub.5,
C.sub.10H.sub.7 or (C.sub.6H.sub.4)CF.sub.3, due to pH effects with
the precursor salts. Additionally, if both metals have the same
affinity for the attached surface group(s), the addition of a
homogeneous distribution of a different metal species leads to an
even distribution over the carbon surface. Again, such an approach
will result in an atomic distribution of the metal species, for
example, Pt and Ru, anchored and distributed evenly over the carbon
surface. This even distribution enables alloy formation at lower
processing temperatures, and, hence, smaller alloy crystallite
sizes and better catalytic performance.
[0160] According to another embodiment of the present invention,
one or more metal or metal oxide precursors can be deposited on the
modified carbon product where a post-processing step occurs between
the two deposition steps. For example, a metal or metal oxide
precursor can be added to a modified carbon support where a surface
of the carbon that is not modified preferentially adsorbs the
precursor, and after a post-processing decomposition step, first
metal or metal oxide clusters are formed. This is followed by
deposition of a second precursor, which preferentially adsorbs on
the surface of previously formed first metal or metal oxide
clusters. After the second decomposition step, a fine composite
cluster is formed (e.g., where the second element deposits either
as a monolayer or as clusters onto the surface of the first metal
clusters).
[0161] By way of example, and as depicted in FIG. 7, a modified
carbon product having surface groups relatively inert to an active
species phase (e.g., SiMe.sub.3) can be utilized to selectively
produce an alloyed modified electrocatalyst product. A first
metal/metal oxide (e.g., RuO.sub.2) can be deposited on the carbon
material surface in locations where the surface groups are not
located. After a post-processing step, a second metal/metal oxide
(e.g., Pt) can then be selectively deposited, where the second
metal/metal oxide preferentially is adsorbed onto the surface of
the first metal/metal oxide clusters to form a composite active
species phase cluster and/or a thin surface active layer, which may
be used as a support for other deposited materials.
[0162] It will be appreciated that one or more of the metal or
metal oxide species described above can be deposited by any known
method, including liquid phase adsorption (as discussed above),
spray-based processing, chemical vapor deposition, under potential
deposition, electroless deposition or liquid-phase precipitation.
In such cases, the carbon can be modified such that the metal/metal
oxide precursors in the second deposition process cannot absorb
onto anything other than the first metal/metal oxide initially
deposited, resulting in improved crystallite dispersion, uniformity
of alloy formation and elimination of segregation phenomena.
[0163] As described above, alloy electrocatalysts typically require
post-processing to ensure a proper degree of alloying to provide
long-term stability of the active sites. During this
post-processing step, the alloy crystallites have a tendency to
sinter, resulting in crystallite growth. Modified carbon products
can be used to minimize this effect. With the integration of
modified carbons as the support structure, the metal/metal oxide
dispersion is significantly increased since the surface
modification blocks part of the surface and inhibits migration.
This prevents surface diffusion and agglomeration of the alloy
clusters during the reduction/alloying step.
[0164] For example, modifying the surface of the carbon material
with thermally stable steric groups, such as phenyl
(C.sub.6H.sub.5) or napthyl (C.sub.10H.sub.7) groups acts to
physically block migration of metal and metal oxide species across
the surface of the carbon. When an electrocatalyst is produced with
a greater metal/metal oxide dispersion (i.e., smaller crystallite
size), and the species to be alloyed (e.g. mixed metal/metal
oxides) requires a lower temperature for alloy formation, a reduced
grain growth and better dispersion of the alloy clusters results.
Reduced grain growth leads to smaller alloy crystallite size,
increased catalytic activity and increased precious metal
utilization. Additionally, as described previously, where surface
modification groups are intact after the post-processing procedure,
they can sterically prevent the metal crystallites from growing by
blocking diffusion paths.
[0165] Modified Carbon Products and Proton Exchange Membranes
[0166] As noted above, modified carbon products can be utilized in
proton exchange membranes. The use of modified carbon products
and/or diazonium salts in conjunction with a proton exchange
membrane are discussed below. It will be appreciated that the
various aspects, approaches, and/or embodiments of the present
invention described below, are primarily in reference to modified
carbon products in proton exchange membranes. However, it will be
appreciated that electrocatalyst materials can be utilized in
conjunction with modified carbon products in many of such aspects,
approaches and/or embodiments, where appropriate, although not
specifically mentioned, and the use of such electrocatalyst
materials in such aspects, approaches and/or embodiments is
expressly within the scope and spirit of the present invention.
[0167] Proton Conductivity
[0168] In a traditional PEM, protons conduct through a membrane,
such as a sulfonated polytetrafluoroethylene (sulfonated PTFE)
membrane, by means of protonated water (H.sub.3O.sup.+) that is
hydrogen bonded to the sulfonic acid groups, which in turn are
attached to the polymer backbone, as illustrated in FIG. 8. These
PEMs generally require the presence of water for efficient proton
conductivity.
[0169] According to one embodiment of the present invention, a
modified carbon product is utilized to increase the amount of
proton conducting sites within the PEM. Increasing the
concentration of proton conducting sites is beneficial for several
reasons, including increased transport of protons across the
membrane and a decrease in the amount of water needed to be
supplied with the anode reactant. In some situations, elimination
of the water supplied with the anode reactant can be achieved,
which greatly reduces the complexity and design of the fuel cell.
An added benefit is that surface groups having proton conducting
functional groups are also generally hydrophilic. Thus, increasing
the amount of proton conducting material also increases the water
retention capability of the fuel cell, thereby enabling rapid "dry"
starts.
[0170] Similarly, the functional group can be tailored to increase
K.sub.a (proton donating strength, like K.sub.a:
--SO.sub.3H>--CO.sub.2H), to form a better proton conductor. In
addition, a tailored functional group may also be utilized to
introduce different chemical reactivity to the membrane, such as by
increasing hydrogen bond strength, which may affect the bonding
strength of a modified carbon product to the membrane surface.
[0171] The linking group can also vary. Particularly preferred
branched and unbranched linking groups are listed in Table V. It
will be appreciated that any proton conducting functional group (Y)
can be utilized in conjunction with any linking group (R) to create
a modified carbon product in accordance with the present invention.
In a particular approach, the linking group is tailored to increase
the "reach" of the proton-conducting functional group by adding
flexibility and degrees of freedom and further increase proton
conduction. According to one preferred embodiment, the linking
group is selected such that the functional group extends to, but
not substantially beyond, the adjacent functional group.
[0172] One example of increased proton conductivity utilizing
tailored linking groups is the use of a modified carbon black
having (C.sub.6H.sub.4)(CH.sub.2).sub.5SO.sub.3H functional groups.
This modified carbon product can be incorporated into a membrane to
provide a longer effective "reach" and increased flexibility of the
proton conducting group. In the case of a
(C.sub.6H.sub.4)(CH.sub.2).sub.5SO.sub- .3H modified carbon
product, the saturated (CH.sub.2).sub.5 chain attached to the
phenyl ring adds significant length and an increased degree of
rotational flexibility about several C--C single bonds, resulting
in an increased cone angle. In this situation, the aliphatic chain
allows for increased proton conductivity, especially in dry,
reduced humidity or non-humidified conditions.
[0173] Modified carbon products can be utilized in conjunction with
any of a variety of types of PEMs. These include fluorinated PEMs
such as sulfonated PTFE, perfluorosulfonated PTFE, polyvinylidene
fluoride (PVDF), as well as acid-doped or derivatized hydrocarbon
polymers, such as polybenzimidizole (PBI), polyarylenes,
polyetherketones, polysulfones, phosphazenes and polyimides and
other similar membranes. A schematic illustration of a resulting
structure is illustrated in FIG. 9. As illustrated in FIG. 9, a
RSO.sub.3H(R=linking group) modified carbon product is added to a
sulfonated PTFE membrane, which results in an increased
concentration of proton conductive groups.
[0174] PBI is a proton conducting membrane that is often used in
high-temperature PEMFCs, and generally includes hydrogen-bonded
H.sub.3PO.sub.4 species in high concentrations. H.sub.3PO.sub.4 is
typically lost during fuel cell operation as it migrates toward the
membrane surface due to electro-osmotic drag, where it subsequently
evaporates. In one embodiment of the present invention, the proton
conducting modified carbon products, such as those having proton
conducting surface groups (e.g., those containing sulfuric and/or
phosphonic functional groups), can be incorporated into a PBI
membrane to increase the concentration of the proton conducting
sites, and hence proton conductivity of the PBI membrane.
[0175] In a particularly preferred embodiment, modified carbon
products having phosphonic groups are incorporated into a PBI
membrane, as is illustrated in FIG. 10. Adding modified carbon
products having phosphonic functional groups to PBI-based membranes
increases the number of proton conducting sites and increases the
hydrogen bonding to the H.sub.3PO.sub.4 acid bound within the pores
of the membrane. This prevents the PBI membrane from losing
H.sub.3PO.sub.4 acid over time.
[0176] One example of increased proton conductivity due to an
increased proton conducting material density in a proton conducting
membrane is the use of modified carbon black having
(C.sub.6H.sub.4)SO.sub.3H surface groups. Preferably, the
concentration of surface groups on the modified carbon black is at
least about 3.0 .mu.mol/m.sup.2, and more preferably at least about
5.0 .mu.mol/m.sup.2. Preferably, the volume density of proton
conducting groups on the modified carbon black is at least about
5.0 mmol/mL, more preferably at least about 5.4 mmol/mL. This
modified carbon black, when mixed into a perfluorintated sulfonic
acid (PFSA)(e.g., NAFION available from E.I. duPont de Nemours,
Wilmington, Del.) solution (e.g., at a 50:50 volume ratio), can be
utilized to produce a modified proton exchange membrane. In one
embodiment, the volume density of proton conducting groups on the
resulting membrane is at least about 5 mmol/mL (about 2.7
mmol/g).
[0177] Proton conducting modified carbon products can also be
advantageously incorporated into non-proton conducting membranes,
such as non acid-doped or derivatized PBI, polyimides,
polysulfones, polyphosphazenes, polyetherketones (PEEKs) and
polysiloxanes. Despite lacking proton conductivity, such membranes
can provide chemical robustness, mechanical strength and good
electrical insulation. By modification of such membranes with
proton conducting modified carbon products, a modified membrane can
be formed having proton conductivity. This is advantageous in cases
where a certain set of physical and/or chemical properties are
desired that can only be achieved through the use of a non-proton
conducting "base" membrane. Furthermore, incorporating modified
carbon products into a non-proton conducting membrane enables the
disconnection of the proton conductivity of the final membrane from
the membrane material utilized during fabrication.
[0178] Mechanical Strength Enhancement
[0179] Modified carbon products can also be utilized according to
the present invention to increase the mechanical strength of the
PEM. In one embodiment of the present invention, a modified carbon
product having a polymeric functional group is utilized to create
physical and/or chemical interactions with the membrane. For
example, a modified carbon having a surface group with aliphatic
surface groups can physically interact (e.g., intertwine) with the
polymer membrane backbone to increase the strength of the membrane.
Surface groups useful in accordance with the present invention
include acrylics, polypropyleneoxide (PPO), polyethyleneoxide (PEO)
polyethylene glycol (PEG) and polystyrene. In a particularly
preferred embodiment, the concentration of surface groups on the
carbon material is from about 0.1 to about 5 .mu.moles/m.sup.2.
[0180] According to another embodiment of the present invention,
the modified carbon product has at least one surface group that
forms a chemical bond to the sulfonate proton exchange membrane.
For example, a modified carbon product having an amine-containing
(NH.sub.2) surface group can be incorporated into a proton exchange
membrane, such as a PTFE membrane. The NH.sub.2 groups hydrogen
bond to the oxygen-containing sulfonate groups, resulting in a
membrane having increased strength. Other surface groups useful in
accordance with the present embodiment include SO.sub.3H,
PO.sub.3H.sub.2 and CO.sub.2H groups.
[0181] In another embodiment, modified carbon products having a
surface functional group can be incorporated in the raw polymer
material prior to forming the membrane to increase mechanical
strength of the membrane. For example, a modified carbon product
having an amine surface group can be added to a carboxylate-based
PTFE membrane and processed to create a mechanically enhanced
proton exchange membrane. In this regard, the NH.sub.2 groups react
with the carboxylate groups on the polymer during polymer
formation, forming an amide of the form RC(O)N(H)R and water.
[0182] In another embodiment, the surface groups on the carbon
materials can be selected to covalently bond to the polymer
backbone through a number of means, such as cross-polymerization or
condensation. For example, an ethylene-terminated modified carbon
product can be added to a batch of perfluorosulfonated PTFE
comonomer prior to final membrane formation. Polymerization is
initiated and the modified carbon product (through the ethylene
surface functional group) co-polymerizes with the co-monomers
resulting in a modified composite membrane including carbon
covalently bound to the polymer.
[0183] It will be appreciated that the foregoing embodiments (e.g.,
increased proton conductivity and increased mechanical strength)
are not mutually exclusive. The modified carbon product can
advantageously increase the mechanical strength while the resulting
proton conductivity remains unchanged. Furthermore, the addition of
proton conducting modified carbon products to the membrane can
increase both the mechanical strength and the proton conductivity
of the membrane. The modified carbon products can be co-modified
with a long chain functional group and a proton-conducting group.
That is, the carbon particles can include more than one type of
surface modification. For example, carbon black (e.g., VULCAN
XC-72, Cabot Corp., Boston Mass.) can be modified with 1
.mu.mol/m.sup.2 of PEG and also with 3 .mu.mol/m.sup.2 of
(C.sub.6H.sub.4)SO.sub.3H to produce a multiply-modified carbon
product. When this multiply-modified carbon product is introduced
into a substantially non-proton conducting membrane, such as
polyimide, the resultant membrane will evidence enhanced proton
conductivity and increased mechanical strength. It will be
appreciated that two different modified carbons can be utilized to
derive the benefit of two different functional groups. For example,
a first modified carbon product having a polymeric functional group
(e.g., PEG) and a second modified carbon product having a proton
conducting functional group (e.g., (C.sub.6H.sub.4)SO.sub.3H) can
be utilized in the production of a proton exchange membrane to
produce a membrane with enhanced proton conductivity and mechanical
strength.
[0184] The thermal stability of the membrane can also be enhanced
according to the present invention. Carbon is very thermally stable
compared to organic polymers, and increasing the loading of carbon
within the PEM will increase the overall thermal stability of the
membrane. A thermally stable PEM can be produced by the addition of
a thermally stable modified carbon to a standard PEM. Moreover, the
PEM can be reinforced with the addition of acicular, rod or whisker
like modified carbon products. Alternatively, thermally stable PEM
can be produced by the addition of a proton conducting modified
carbon product to a non-proton conducting but thermally stable
membrane such as, for example, polyimide. In either case, the
blending of two or more constituents, such as a membrane and a
modified carbon product or a membrane and two different modified
carbon products or a carbon material modified with more than one
surface group, can result in a unique combination of
properties.
[0185] Water and Fuel Transport Control, Humidification and
Porosity
[0186] Water transport is a key issue for PEMs. One feature that
the modified PEMs according to the present invention can impart to
the MEA structure is the ability to operate at decreased
humidification levels due to an increased concentration of proton
conducting groups per unit volume. In this regard, one aspect of
the present invention relates to modified carbon products including
proton conductive functional groups. This leads to a significant
overall increase in the concentration of proton conducting sites
per unit volume, and as well as increased mobility (reach), which
allows protons to migrate through the membrane without water acting
as a bridge. Moreover, the proton exchange membrane may also
operate at a significantly lower humidification level. The ability
to operate a proton exchange membrane in near humid and/or dry
conditions greatly simplifies the fuel cell design, especially at
high operating temperatures, where the difficulty associated with
adding water to the membrane is heightened by the accelerated
evaporation rate of the water.
[0187] The integration of modified carbon products can
significantly affect the humidification requirements and water
transport for fuel cell systems utilizing PEMs. In a particular
embodiment, a composite modified carbon product/polymer membrane is
utilized to increase proton conduction at reduced humidification
levels. Moreover, even where the modified membranes do not have a
high enough concentration of proton conduction groups to allow
fully dry conduction, the addition of strongly hydrophilic groups,
such as SO.sub.3H, advantageously decreases the requisite water
vapor pressure, allowing for water-based proton transfer at
elevated temperatures and/or reduced humidification levels of the
reactant gases.
[0188] According to another embodiment of the present invention,
the average pore size within the polymer membranes is reduced
through the use of modified carbon products having polymeric
surface groups, as described above, to decrease permeability and,
therefore, likelihood of water or fuel transport through the
membrane. For example, the use of a modified carbon product having
hydrophilic and long chained functional groups attached thereto
increases proton conductivity while also forming a denser membrane
with a smaller average internal pore size.
[0189] The porosity of the PEM can be tailored by selecting
appropriate carbon materials, linking groups and functional groups.
Porosity can be tailored to prevent and/or inhibit gas, water
and/or fuel transport across the membrane. In one embodiment,
porosity is tailored by utilizing a high surface area carbon black
having a high density of proton conducting groups co-modified with
large polymeric groups, such as PEG or PPO having a relatively low
electrical conductivity. One such carbon black is KETJEN BLACK
(available from Akzo Nobel), which has a surface area of about 1600
m.sup.2/g and an electrical resistivity of about 500 to 1500
milliOhm-cm. In a particular embodiment, the carbon black can be
modified with a proton conducting group, such as a sulfonate, and
long-chained polymeric group, such as PEG, and formulated into a
membrane having a small pore size. In one preferred embodiment, the
membrane has an average pore size of no greater than 3
angstroms.
[0190] As will be appreciated, the small pore size is achieved due
to the polymeric groups (e.g., PEG) having significant steric bulk
dangling from the carbon surfaces. To enhance/tailor porosity, it
is important to utilize such large molecular groups, like PEG or
PPO, to act as a binder and "filler" between the carbon materials.
Without the addition of such steric inhibitors, it is difficult to
achieve a dense membrane suitable for use in a fuel cell. The
length of the linking group regarding the proton conducting
functionality should be such that it extends to, but not
substantially beyond, the next proton-conducting functional group.
In one extreme, the resulting membrane is essentially based upon
modified carbon products and is proton conducting.
[0191] One embodiment of such an approach utilizing sulfonic acid
as the proton conducting functional group is depicted schematically
illustrated in FIG. 11. Modified carbon products 1124 are
protonically connected via the sulfonic acid functional groups 1150
extended by the linking groups (R), resulting in a membrane with
increased proton conductivity and decreased porosity.
[0192] Direct methanol fuel cells (DMFCs), as well as fuel cells
that utilize other hydrogen-containing liquid fuels, like ethanol,
also utilize a proton conductive membrane. In a DMFC, methanol
migration through the membrane (i.e., methanol cross-over) should
be avoided to maintain fuel cell performance. Methanol cross-over
results in both low fuel utilization and poisoning of active
catalyst sites at the cathode, which significantly decreases the
efficiency and performance of the fuel cell.
[0193] According to the present invention, methanol cross-over
through the membrane can be reduced by the addition of modified
carbon products to the membrane. As with PEMFCs, increasing the
density of proton conduction sites will allow protons to be
transported without the need for water. In addition, decreasing the
porosity utilizing modified carbon products having a polymeric
surface group, as described above, can also decrease methanol
cross-over.
[0194] For purposes of illustration, a carbon material, such as
carbon black, can be modified with a proton conducting functional
group and a functional and/or linking group having substantial
steric bulk (e.g., PEG). This modified carbon product can be mixed
with another modified carbon product having proton conducting
functional groups (e.g., (C.sub.6H.sub.4)SO.sub.3H). This mixture
can be added to a perflurosulfonated PTFE polymer co-monomer, and
cast to form a modified membrane having an increased proton
conductivity and reduced porosity, resulting in lower methanol
cross-over levels. Another embodiment of the present invention
utilizes modified carbon products to tailor the
hydrophilicity/hydrophobicity of the internal pores of the membrane
to selectively transport water and inhibit the transport of other
molecules, such as methanol.
[0195] Fabrication of PEMs Utilizing Modified Carbon Products
[0196] There are a variety of approaches to producing PEMs
including modified carbon products, which are discussed in further
detail below. It will be appreciated that any of the
above-described modified carbon products, including modified
electrocatalyst products, and/or electrocatalyst materials may be
utilized in any of the below-described approaches, aspects and/or
embodiments to produce and/or modify a proton exchange membrane. It
will also be appreciated that the thickness of the membrane should
be suitable for the specific fuel cell application, irrespective of
its manufacturing process. Preferably, the thickness of the
resultant proton exchange membrane is from about 25 to about 250
microns, more preferably from about 30 microns to about 150
microns, and even more preferably from about 35 microns to about 75
microns.
[0197] In one approach, a pre-existing PEM may be contacted with a
diazonium salt to attach an organic group to carbon materials
contained therein to create a PEM including a modified carbon
product. It will be appreciated that any of the below referenced
deposition methods can be utilized to directly deposit a diazonium
salt onto a carbonaceous surface for the purpose of directly
modifying such carbonaceous surface. Additionally, such deposition
techniques can be used to modify any carbonaceous materials
contained in the PEM, including non-modified carbon materials,
electrocatalyst materials, modified carbon products and/or modified
electrocatalyst products. It will also be appreciated that such
deposition techniques can be utilized to create a uniform modified
carbon layer across the entire surface of the proton exchange
membrane, or can be deposited in discrete patterns to produce
patterned modified carbon layers.
[0198] In another approach, modified carbon products may be
utilized in the extrusion and casting process to create a modified
carbon PEM. In one aspect, a modified carbon product is mixed into
a monomer prior to polymerization and casting/extrusion to form the
modified carbon PEM. In another aspect, the modified carbon product
is mixed into the polymer prior to casting/extrusion to form the
PEM. In one embodiment, a modified carbon product is included in
the fluid (e.g., monomer or polymer-containing fluid) utilized in
the casting and extrusion process to create a modified carbon
proton exchange membrane. In a particular embodiment, the modified
carbon product is tailored to increases its solubility and/or
dispersability in the fluid, such as by utilization of specific
functional groups. Such increased solubility and/or dispersability
within the fluid enables formation of a composite PEM with uniform
distribution of the modified carbon product. In a particular
embodiment, the surface group of the modified carbon product is
both hydrophilic and proton conducting.
[0199] By way of illustration, a modified carbon product including
a proton conducting functional group (e.g.,
(C.sub.6H.sub.4)SO.sub.3H attached to carbon black) can be added to
a formulation including a PFSA. This formulation can be fabricated
into a membrane by casting or extrusion. In one embodiment, the
formulation includes various amounts of modified carbon products,
such as at least about 20 wt. %, more preferably at least about 40
wt. % and up to about 70 wt. %. After fabrication, the PEM may
include from 5 wt. % to 30 wt. % of a modified carbon product. The
resulting membrane, after casting or extrusion, evidences an
increased proton conductivity at, or even below, typical PTFE-type
humidification levels. In addition, such membranes may also
evidence an increased mechanical stability, depending on the amount
and type of modified carbon product utilized.
[0200] In another example, modified carbon products can be mixed
with a non-proton conducting membrane material to form a composite
mixture. Preferably, the modified carbon product constitutes at
least about 10 vol. % of the dry mixture, up to about 70 vol. % of
the dry mixture. The dry mixture can be added to a solvent and
blended, if necessary, such as by using a ball mill. Solvent
systems can include aqueous solvents, non-aqueous solvents and
mixtures thereof. The resulting slurry can be cast and dried to
form a membrane. The dried and cast membrane can be pressed (to
further reduce thickness) and/or can be heated to an elevated
temperature, such as near or slightly above the glass transition
temperature (T.sub.g) of the polymer.
[0201] In another approach, modified carbon products may be
deposited onto or impregnated into a pre-existing PEM. In one
aspect, the modified carbon products may be deposited or
impregnated using a modified carbon ink, as discussed in further
detail below. In one embodiment, the proton exchange membrane may
be contacted by a modified carbon ink, such as by immersion or
spraying, to impregnate and/or deposit the modified carbon product
into/onto the membrane. In a particular embodiment, the modified
carbon ink is deposited utilizing a direct-write tool. In another
embodiment, the modified carbon ink is sprayed onto a membrane
(e.g., a polybenzimidazole membrane) to form a coated membrane. In
particular embodiment, the coated membrane can be further contacted
with a solution containing additional materials, such as a proton
conducting material or polymer backbone material. In a particular
embodiment a coated polybenzimidazole membrane is immersed in a
phosphoric acid (H.sub.3PO.sub.4) solution to increase the amount
of proton conducting groups in the membrane.
[0202] In yet another approach, proton-conducting membranes may be
fabricated directly from one or more modified carbon products. This
approach alleviates the need for conventional polymers, membrane
materials and/or binder systems, which eliminates the negative
effects of such materials on the PEM. In this regard, it will be
appreciated that large chain, polymeric insulating groups can be
attached to the carbon materials, essentially coating them and
making them electrically insulating, which aids in the production
of PEMs without the above described conventional polymers, membrane
materials and/or binder systems. Moreover, by selecting the proper
combination of proton conducting functional group, linking group
and carbon, a PEM having the desired properties can be obtained. In
one embodiment, the PEM is fabricated by deposition of a modified
carbon ink, as is discussed in further detail below.
[0203] As noted above, various methods may be utilized to
incorporate a modified carbon product in a proton exchange
membrane. One particular method includes the steps of contacting a
carbon material with a diazonium salt to form a modified carbon
product and incorporating the modified carbon product into the
proton exchange membrane. It will be appreciated that more than one
type of carbon material and/or diazonium salt may be utilized in
this approach to form a plurality of modified carbon products
and/or multiply-modified carbon products.
[0204] One specific embodiment utilizes spray processing
techniques, and includes the steps of providing a precursor
composition including a carbon material and a diazonium salt, spray
processing the precursor composition to form a modified carbon
product, and incorporating the modified carbon product into a
proton exchange membrane.
[0205] Another specific embodiment includes the steps of providing
a precursor composition including an electrocatalyst material and a
diazonium salt, spray processing the precursor composition to form
a modified electrocatalyst product, and incorporating the modified
electrocatalyst product into a proton exchange membrane.
[0206] Yet another specific embodiment includes the steps of
providing a precursor composition including an existing modified
carbon product and an active species precursor, spray processing
the precursor composition to form a modified electrocatalyst
product, and incorporating the modified electrocatalyst product
into a proton exchange membrane.
[0207] Another method for incorporating a modified carbon product
in a proton exchange membrane, includes the steps of mixing a
modified carbon product with another material (e.g., a second
modified carbon product, a conventional polymer used in the
production of a PEM, an electrocatalyst material, a conventional
carbon material, a resin and/or other materials utilized in the
production of a proton exchange membrane) to form a modified
carbon-containing mixture and incorporating the mixture into the
proton exchange membrane. It will be appreciated that more than one
type of modified carbon product and other material may be utilized
in this approach to form the mixture.
[0208] In one specific embodiment, modified carbon products are
dispersed in an ink to create a modified carbon ink that may be
utilized in the production of a proton exchange membrane, such as
by analog or digital printing, as discussed in further detail
below.
[0209] Yet another method includes the steps of incorporating a
carbonaceous material, such as a modified carbon product and/or a
conventional carbon material into a proton exchange membrane and
contacting the carbonaceous material with a diazonium salt to form
a modified carbon product in the proton exchange membrane. It will
be appreciated that more than one type of carbonaceous material
(e.g., modified carbon product) and/or diazonium salt may be
utilized in this approach to form a plurality of modified carbon
products and/or multiply-modified carbon products. In a specific
embodiment, a diazonium salt is deposited using a direct-write
tool, as discussed in further detail below, to form a modified
carbon product in the proton exchange membrane.
[0210] Interface with Adjacent Components/Adhesion
[0211] Another embodiment of the present invention is directed to
the incorporation of modified carbon products to improve the
interface contact between the proton exchange membrane and the
electrode layer. In this regard, the surface group can form a
physical or chemical bond between the materials on either the
electrode or the proton exchange membrane due to improved
dispersability of the modified carbon product, forming a smoother
surface and increasing contact area.
[0212] By way of illustration, modification of an electrocatalyst
material with a hydrophilic functional group enables preparation of
a uniform, low viscosity aqueous ink, which can be formed into a
smooth, crack-free electrode layer. Such a layer increases contact
because the hydrophilic functional groups of the modified
electrocatalyst product can hydrogen bond to any adjacent polymer
groups of the proton exchange membrane. This improves bonding
between the electrode and the proton exchange membrane, thereby
decreasing contact resistance and minimizing ohmic losses. Another
benefit is reduced delamination and structural deterioration during
long term operation of the fuel cell.
[0213] Another embodiment of the present invention is directed to
the use of modified carbon products to increase the adhesion of
between an the proton exchange membrane and adjacent fuel cell
component (e.g., the electrode). A proton exchange membrane
incorporating modified carbon products may form a physical
interlocking bond or a chemical bond to the adjacent component. In
this regard, polymeric functional groups and/or linking groups can
be utilized to increase adhesion of the gas/fluid diffusion. Such
polymeric functional groups can be physically intertwined with the
surface of the adjacent component, which increases adhesion. In
another embodiment, hydrophilic functional groups can be utilized
to increase hydrogen bonding between the proton exchange membrane
and the adjacent component. It will be appreciated that both
hydrophilic functional groups and/or polymeric functional and/or
linking groups can be utilized on a modified carbon product(s) to
achieve such properties.
[0214] Deposition of Modified Carbon Products and/or Diazonium
Salts
[0215] In one aspect, modified carbon products may be utilized in a
modified carbon ink to produce and/or modified the PEM, such as
analog printing (screen, lithographic, roll-coat, slot die and
flexographic) and digital printing (e.g., direct-write, spraying,
electrostatic, xerographic, laser transfer) methods as is discussed
in further detail below. As used herein a "modified carbon ink"
refers to any liquid phase solution, such as an ink, resin or
paste, that contains one or more of the above-described modified
carbon products and/or modified electrocatalyst products. As used
herein "electrocatalyst inks" refers to a liquid phase solution,
such as an ink, resin or paste, that contains one or more of the
above-described electrocatalyst materials.
[0216] Various aspects, approaches, and/or embodiments of the
present invention are described below, primarily in reference to
modified carbon inks. However, it will be appreciated that
electrocatalyst inks can be utilized in conjunction with a modified
carbon ink in some of such aspects, approaches and/or embodiments,
where appropriate, although not specifically mentioned, and the use
of such electrocatalyst inks in such aspects, approaches and/or
embodiments is expressly within the scope and spirit of the present
invention.
[0217] The incorporation of modified carbon products in a modified
carbon ink significantly improves ink uniformity, homogeneity, ease
of manufacture and ease of use. Various methods and mixing
techniques are currently utilized to improve the properties of inks
that include electrocatalysts, carbons and polymer solutions (e.g.,
PFSA or PTFE) and combinations thereof, such as ball milling and
sonication. The incorporation of modified carbon products having
surface groups that match the solubility requirements of the ink
there are dispersed in significantly simplifies ink preparation As
a result, the homogeneity and uniformity of the inks, and hence the
homogeneity of the deposited layer/feature are increased.
Homogenous deposition enables control over the concentration and
drying rate of the materials being deposited. For example, a
modified carbon product having hydrophilic surface groups
simplifies dispersion of carbon-based materials in aqueous-based
inks due to increased wetting and dispersability of the modified
carbon material. Other surface modifications can be chosen to
improve the wettability and dispersability of modified carbon
products when organic solutions are used.
[0218] In one embodiment of the present invention, modified carbon
products are utilized in a PFSA solution and/or a PTFE suspension
to create a modified carbon ink, where the aggregate size of the
modified carbon particles is not larger than the size of the
largest particle within the ink.
[0219] In a particular embodiment, a modified carbon product having
two different surface groups (e.g. a hydrophilic and a hydrophobic
group) is utilized in a PFSA solution and/or PTFE suspension to
create a modified carbon ink, where the aggregate size of the
modified carbon products is not larger than the size of the largest
particle within the ink.
[0220] Deposition of modified carbon inks preferably produce and/or
modify the PEM to enable proton conductivity, physical separation,
and/or electrical insulation, after deposition and/or
post-processing. In this regard, it should be noted that any
combination of surface groups described in U.S. Pat. No. 5,900,029
by Belmont et al. can be utilized in conjunction with any modified
carbon product in and modified carbon ink to create and/or modify
the proton exchange membrane. Preferably, the modified carbon ink
is formulated for deposition (e.g., via analog or digital printing)
to maintain a low manufacturing cost while retaining the above
noted properties.
[0221] The modified carbon inks according to the present invention
can be deposited to form patterned or unpatterned layers using a
variety of tools and methods. In one embodiment, a modified carbon
ink is deposited using a direct-write deposition tool. As used
herein, a direct-write deposition tool is a device that can deposit
a modified carbon ink onto a surface by ejecting the composition
through an orifice toward the surface without the tool being in
direct contact with the surface. The direct-write deposition tool
is preferably controllable over an x-y grid. One preferred
direct-write deposition tool according to the present invention is
an ink-jet device. Other examples of direct-write deposition tools
include aerosol jets and automated syringes, such as the MICROPEN
tool, available from Ohmcraft, Inc., of Honeoye Falls, N.Y.
[0222] An ink-jet device operates by generating droplets of a
liquid suspension and directing the droplets toward a surface. The
position of the ink-jet head is carefully controlled and can be
highly automated so that discrete patterns of the modified carbon
ink/electrocatalyst ink can be applied to the surface. Ink-jet
printers are capable of printing at a rate of 1000 drops per second
per jet, or higher, and can print linear features with good
resolution at a rate of 10 cm/sec or more, such as up to about 1000
cm/sec. Each drop generated by the ink-jet head includes
approximately 25 to 100 picoliters of the suspension/ink that is
delivered to the surface. For these and other reasons, ink-jet
devices are a highly desirable means for depositing materials onto
a surface.
[0223] Typically, an ink-jet device includes an ink-jet head with
one or more orifices having a diameter of not greater than about
100 .mu.m, such as from about 50 .mu.m to 75 .mu.m. Droplets are
generated and are directed through the orifice toward the surface
being printed. Ink-jet printers typically utilize a piezoelectric
driven system to generate the droplets, although other variations
are also used. Ink-jet devices are described in more detail in, for
example, U.S. Pat. No. 4,627,875 by Kobayashi et al. and U.S. Pat.
No. 5,329,293 by Liker, each of which is incorporated herein by
reference in its entirety. Functionalized carbon particles have
been demonstrated to be stable in inks at relatively high carbon
loadings by Belmont et al. in U.S. Pat. No. 5,554,739, which is
incorporated herein by reference in its entirety. Ink-jet printing
for the manufacture of DMFCs is disclosed by Hampden-Smith et al.
in commonly-owned U.S. patent application Ser. No. 10/417,417
(Publication No. 20040038808) which is also incorporated herein by
reference in its entirety.
[0224] It is important to simultaneously control the surface
tension and the viscosity of the modified carbon ink to enable the
use of industrial ink-jet devices. Preferably, the surface tension
of the ink is from about 10 to 50 dynes/cm, such as from about 20
to 40 dynes/cm. For use in an ink-jet, the viscosity of the
modified carbon ink is preferably not greater than about 50
centipoise (cp), such as in the range of from about 10 cp to about
40 cp. Automated syringes can use compositions having a higher
viscosity, such as up to about 5000 cp.
[0225] According to one embodiment, the solids loading of modified
carbon products in the modified carbon ink is preferably as high as
possible without adversely affecting the viscosity or other
necessary properties of the composition. For example, a modified
carbon ink can have a solids loading of up to about 20 wt. %. In
one embodiment, the solids loading is from about 2 wt. % to about
10 wt. %. In another particular embodiment, the solids loading is
from about 2 wt. % to about 8 wt %. As is discussed below, the
surface modification of a carbon product can advantageously enhance
the dispersion of the carbon product, and lead to higher obtainable
solids loadings.
[0226] The modified carbon inks used in an ink-jet device can also
include water and/or an alcohol. Surfactants can also be used to
maintain the modified carbon products in the ink. Co-solvents, also
known as humectants, can be used to prevent the modified carbon
inks from crusting and clogging the orifice of the ink-jet head.
Biocides can also be added to prevent bacterial growth over time.
Examples of such liquid vehicle compositions for use in an ink-jet
are disclosed in U.S. Pat. No. 5,853,470 by Martin et al.; U.S.
Pat. No. 5,679,724 by Sacripante et al.; U.S. Pat. No. 5,725,647 by
Carlson et al.; U.S. Pat. No. 4,877,451 by Winnik et al.; U.S. Pat.
No. 5,837,045 by Johnson et al.; and U.S. Pat. No. 5,837,041 by
Bean et al. Each of the foregoing U.S. patents is hereby
incorporated herein by reference in its entirety. The selection of
such additives is based upon the desired properties of the
composition. If necessary, modified carbon products can be mixed
with the liquid vehicle using a mill or, for example, an ultrasonic
processor. In this regard, it should be noted that modified carbon
products that are dispersible in their corresponding solvent (e.g.
a modified carbon product having a hydrophilic surface groups in an
aqueous solution) may require minimal or no mixing due to their
improved dispersability in their corresponding solvents.
[0227] The modified carbon inks according to the present invention
can also be deposited by aerosol jet deposition. Aerosol jet
deposition can enable the formation of features having a feature
width of not greater than about 200 .mu.m, such as not greater than
100 .mu.m, not greater than 75 .mu.m and even not greater than 50
.mu.m. In aerosol jet deposition, the modified carbon ink is
aerosolized into droplets and the droplets are transported to a
substrate in a flow gas through a flow channel. Typically, the flow
channel is straight and relatively short. For use in an aerosol jet
deposition, the viscosity of the ink is preferably not greater than
about 20 cp.
[0228] The aerosol in the aerosol jet can be created using a number
of atomization techniques, such as by ultrasonic atomization,
two-fluid spray head, pressure atomizing nozzles and the like.
Ultrasonic atomization is preferred for compositions with low
viscosities and low surface tension. Two-fluid and pressure
atomizers are preferred for higher viscosity inks.
[0229] The size of the aerosol droplets can vary depending on the
atomization technique. In one embodiment, the average droplet size
is not greater than about 10 .mu.m, and more preferably is not
greater than about 5 .mu.m. Large droplets can be optionally
removed from the aerosol, such as by the use of an impactor.
[0230] Low aerosol concentrations require large volumes of flow gas
and can be detrimental to the deposition of fine features. The
concentration of the aerosol can optionally be increased, such as
by using a virtual impactor. The concentration of the aerosol can
be greater than about 10.sup.6 droplets/cm.sup.3, such as greater
than about 10.sup.7 droplets/cm.sup.3. The concentration of the
aerosol can be monitored and the information can be used to
maintain the mist concentration within, for example, 10% of the
desired mist concentration over a period of time.
[0231] Examples of tools and methods for the deposition of fluids
using aerosol jet deposition include U.S. Pat. No. 6,251,488 by
Miller et al., U.S. Pat. No. 5,725,672 by Schmitt et al. and U.S.
Pat. No. 4,019,188 by Hochberg et al. Each of these patents is
hereby incorporated herein by reference in its entirety.
[0232] The modified carbon inks of the present invention can also
be deposited by a variety of other techniques including intaglio,
roll printer, spraying, dip coating, spin coating and other
techniques that direct discrete units, continuous jets or
continuous sheets of fluid to a surface. Other printing methods
include lithographic and gravure printing.
[0233] For example, gravure printing can be used with modified
carbon inks having a viscosity of up to about 5000 centipoise. The
gravure method can deposit features having an average thickness of
from about 1 .mu.m to about 25 .mu.m and can deposit such features
at a high rate of speed, such as up to about 700 meters per minute.
The gravure process also enables the direct formation of patterns
onto the surface.
[0234] Lithographic printing methods can also be utilized. In the
lithographic process, the inked printing plate contacts and
transfers a pattern to a rubber blanket and the rubber blanket
contacts and transfers the pattern to the surface being printed. A
plate cylinder first comes into contact with dampening rollers that
transfer an aqueous solution to the hydrophilic non-image areas of
the plate. A dampened plate then contacts an inking roller and
accepts the ink only in the oleophillic image areas.
[0235] The aforementioned deposition/printing techniques may
require one or more subsequent drying and/or curing (e.g., heating)
steps, such as by thermal, ultraviolet and/or infrared radiation,
to induce a chemical or physical bond formation. For example, if a
long chain fluoric substituted aryl is used, the resulting
deposited layer can be dried (e.g., at 100.degree. C.) and heated
(e.g., 350.degree. C.) to induce mobility and physical bond
formation between adjacent modified carbon products through a
surface substituted aryl group.
[0236] By way of illustration, a low viscosity modified carbon ink
including a modified carbon product having a hydrophobic surface
group can be deposited using a direct-write deposition tool (e.g.,
an ink jet printer) to form a hydrophobic layer. After the
deposited layer is dried (e.g., at about 100.degree. C.), it can be
heated (e.g., at about 350.degree. C.) for a certain period of time
(e.g., 30 minutes) to enable the hydrophobic groups to become
mobile and intertwine with adjacent surface groups on the same and
different carbon particles, thereby resulting in a hydrophobic
layer with a greater level of structural integrity.
[0237] Using one or more of the foregoing deposition techniques, it
is possible to deposit modified carbon inks on one side or both
sides of a surface to form and/or modify a proton exchange
membrane. According to one embodiment, such deposition techniques
are utilized to modify an existing proton exchange membrane. In
another embodiment, such deposition techniques are used to directly
create the proton exchange membrane, such as by depositing a
modified carbon ink onto another component of the MEA.
[0238] It will be appreciated that any of the above-noted processes
can be utilized in parallel or serial to deposit multiple layers of
the same or different modified carbon inks onto a surface, and can
be printed in one or more dimensions and in single or multiple
deposition steps. In this regard, one embodiment of the present
invention is directed to printing multiple layers of modified
carbon inks to generate gradients in the proton exchange
membrane.
[0239] In one particular embodiment, gradient structures can be
prepared that have material properties that transition from very
hydrophilic to very hydrophobic, such as by utilizing a plurality
of layers including modified carbon products or modified carbon
products having varying concentrations of surface groups. In this
regard, a first layer may include a modified carbon product that is
very hydrophilic, such as a modified carbon product having a
hydrophilic terminated surface group attached to the surface (e.g.,
a sulfuric group). On this first layer, a second, slightly less
hydrophilic layer can be formed, such as by using a modified carbon
product that has slightly hydrophilic surface groups (e.g., a
carboxylic group). A third, hydrophobic layer can be formed on the
second layer utilizing a hydrophobic modified carbon product having
a hydrophobic surface group. It will be appreciated that in any of
these layers, more than one type of surface group can be utilized
with the various modified carbon products.
[0240] The incorporation of modified carbon products in a modified
carbon ink can also affect the drying characteristics of ink. For
example, rapid drying can result in crack formation after
deposition. Drying can be slowed by utilizing modified carbon
products miscible with the solvent to reduce the vapor pressure of
the solvent after deposition. This can be achieved by increased the
solids loading of the modified carbon products in the modified
carbon ink. In one preferred embodiment, the modified carbon ink
has a solids loading of up to about 70 wt. %. Increased solids
loading of modified carbon produced results in more uniform drying
and less volume fraction of solvents being removed during drying
process. In addition, modified carbon products can include a long
chain surface group (e.g., polymeric) that can form physical and/or
chemical bonds to the solvent species (e.g., water, isopropanol or
TEFLON) or adjacent surface groups, resulting in more uniform
drying as depicted in FIG. 12, where 1271 is a deposition process
using a conventional ink and 1272 is a deposition processing using
a modified carbon ink.
EXAMPLES
Proton Exchange Membrane Examples
[0241] A. Production of PEG-VULCAN XC-72-Reinforced PFSA PEM
[0242] 90 mL of deionized water, 26.5 g of a treating agent
(aminophenylated polyethylene glycol ether, MW.about.2119 and
having the formula
(H.sub.2N--C.sub.6H.sub.4--CO--[O--(C.sub.2H.sub.4O).sub.n--CH.su-
b.3])) and 2.25 g of a 70% aqueous solution of nitric acid are
added to a beaker and slowly mixed. The temperature is slowly
raised to 40.degree. C. using a hot plate. When the temperature
reaches 40.degree. C., 10 g of VULCAN XC-72 carbon black is added
and the mixture is stirred and heated to 50.degree. C. When the
temperature reaches 50.degree. C., 4.3 g of a 20 wt. % aqueous
sodium nitrite solution is added slowly drop wise. The mixture is
then allowed to react at 50.degree. C. for 2 hours. When the
reaction is substantially complete, the sample is diafiltered using
10 volumes of fresh deionized water to remove any reaction
by-products. The resulting PEG-modified VULCAN XC-72 carbon is
added to a PFSA water/isopropanol solution to give a modified
carbon solids loading of 55 wt. %. The slurry is cast and dried at
80.degree. C. overnight to form a membrane.
[0243] B. Increased Proton Conductivity Composite Modified
Membrane
Example B-1
To Cast a 70 wt. % SO.sub.3H Modified Carbon Black/PFSA
Membrane
[0244] To 70 g of (C.sub.6H.sub.4)SO.sub.3H modified VULCAN XC-72
(3 .mu.mols/m.sup.2) is added 2000 g of a 5 wt. % PFSA
isopropanol/water solution to give a 70 wt % modified carbon
black/PFSA suspension. The resulting viscous suspension is mixed
for several minutes and poured onto a Teflonized glass plate. The
resulting film is doctor bladed to a thickness (wet) of 5 mils. The
resulting film is dried in air at room temperature for 24
hours.
Example B-2
To Cast a 15 wt. % SO.sub.3H Modified Carbon Black/PFSA
Membrane
[0245] To 15 g of (C.sub.6H.sub.4)SO.sub.3H modified VULCAN XC-72
(3 .mu.mols/m.sup.2) is added 2000 g of a 5 wt. % PFSA
isopropanol/water solution to give a 15 wt % modified carbon
black/PFSA suspension. The resulting suspension is mixed for
several minutes and poured onto a Teflonized glass plate. The
resulting film is doctor bladed to a thickness (wet) of 20 mils.
The resulting film is dried in air at room temperature for 24
hours. The resulting membrane has increased mechanical stability
and higher proton conductivity at lower humidification levels.
[0246] C. Direct Printed Modified Carbon Product/PFSA PEM
[0247] A modified carbon suspension containing modified carbon
black includes from about 2 to 10 wt. % solids loading of the
carbon black, a humectant, viscosity, surface tension modifier
and/or a biocide. CAB-O-JET 200 (Cabot Corporation, Boston, Mass.)
is a dispersion of 20 wt. % BLACKPEARLS 700, modified with
(C.sub.6H.sub.4)SO.sub.3H, in water. The dispersion has a viscosity
of 3.8 cP with a surface tension of 75.5 dynes/cm. The acceptable
viscosity for ink-jetting with the Spectra ink-jet heads is about
12 cP with a surface tension of about 30 dynes/cm.
[0248] Table VII illustrates an example of a low viscosity,
ink-jettable modified carbon black/PFSA ink composition.
6 TABLE VII Component Wt. % Modified carbon 5.9 black PFSA 52.4
Isopropyl 12.6 alcohol Water 29.1 Total 100.0
[0249] The formulation is ink jet printed on the electrode side of
a fluid diffusion electrode with a Spectra ink-jet head. The
ink-jet head temperature was set at 30.degree. C., fire pulse width
of 8 .mu.s, pulse rise and fall time of 3 .mu.s, and firing voltage
of 120 V. An ink-jet print is obtained at a speed of 10 inches/sec.
The printed film is dried at 80.degree. C. for 3 hours and the
printing and drying process is repeated two additional times.
[0250] D. Modified Polyimide-Based PEM
[0251] A mixture of C.sub.6H.sub.4SO.sub.3H surface functionalized
VULCAN XC-72 is mixed with finely divided polyimide to give a final
composition 70 wt. % modified carbon black. The mixture is blended
in a ball mill for 2 hours and is then suspended in a water/ethanol
solvent to form a slurry. The slurry is cast and dried overnight at
80.degree. C. The resulting film is pressed to a thickness of 0.12
mm and heated to a temperature of 250.degree. C. for 2 hours. The
resulting modified carbon black polyimide composite membrane has
structural integrity and strength and is proton conducting with and
without humidification.
[0252] E. Printed Modified Carbon Product-Based PEM
[0253] A powder batch of VULCAN XC-72 modified with
(C.sub.6H.sub.4)SO.sub.3H is suspended in water at a concentration
of 5 wt. %. To this suspension is added VULCAN XC-72 that has been
modified with polyethylene glycol (PEG), where the ratio of PEG
modified carbon to (C.sub.6H.sub.4)SO.sub.3H modified carbon is 5:1
and the total ink solids loading is 5 wt. %. The resulting ink is
ink jet printed directly onto an electrode layer, which is
supported by a fluid diffusion layer (e.g., a gas diffusion
electrode). The printed layer is then dried and heated at
130.degree. C. for 2 hours to remove the solvent and bind the
carbon black particles together via the PEG surface molecules. The
printing, drying and heating process is repeated two more
times.
[0254] F. Ink Jet Printed Modified Carbon Product-Based PEM
[0255] VULCAN XC-72 that has been co-modified with PEG and
(C.sub.6H.sub.4)SO.sub.3H in a 5:1 wt. ratio is dispersed into an
isopropanol/water solvent with a total solids content of 5 wt. %.
The resulting ink is printed by means of a piezoelectric,
drop-on-demand GALAXY ink-jet head (Spectra Corporation) in three
passes. The resulting layer is heated at 120.degree. C. for 30
minutes to allow the PEG surface groups to intertwine and to remove
the solvent.
[0256] G. Ink Jet Printed Modified Carbon Product/TEFLON PEM
[0257] To VULCAN XC-72 modified with (C.sub.6H.sub.4)SO.sub.3H
groups (3 .mu.mol/m.sup.2) is added a NAFION/isopropanol-water
solution to give an ink that consists of 5 wt. % carbon where the
carbon:NAFION weight ratio is 1:5. The resulting carbon
black/NAFION ink is ink jet printed utilizing a piezoelectric drop
on demand Galaxy ink jet head (Spectra Corporation) onto the
electrode side of a catalyst coated fluid diffusion layer and is
subsequently dried at 25.degree. C. for 12 hours. The printing and
drying process is repeated two more times.
[0258] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations to those embodiments will occur to those skilled in the
art. It is to be expressly understood, however, that such
modifications and adaptations are within the scope and spirit of
the present invention, as set forth in the claims below. Further,
it should be recognized that any feature of any embodiment
disclosed herein can be combined with any other feature of any
other embodiment in any combination.
* * * * *