US20050233203A1

US20050233203A1 - Modified carbon products, their use in fluid/gas diffusion layers and similar devices and methods relating to the same

Info

Publication number: US20050233203A1
Application number: US11/081,768
Authority: US
Inventors: Mark Hampden-Smith; Paolina Atanassova; Gordon Rice; James Caruso; James Brewster; Rimpla Bhatia; Paul Napolitano; Bogdan Gurau
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 2004-03-15
Filing date: 2005-03-15
Publication date: 2005-10-20
Also published as: US20050233183A1; CA2560069C; KR20060125918A; WO2005091416A3; EP1726018A2; US8227117B2; KR100891337B1; US20050221141A1; CA2560069A1; WO2005091416A2; JP2007535787A; US20050221139A1

Abstract

Gas/Fluid diffusion layers incorporating modified carbon products. The modified carbon products advantageously enhance the properties of gas/fluid diffusion layers, leading to more efficiency within a fuel cell or similar device incorporating the gas/fluid diffusion layer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY BENEFIT

Pursuant to 35 U.S.C. §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 Serial 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, and U.S. Patent Application Serial 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 Serial No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIR USE IN PROTON EXCHANGE MEMBRANES AND SIMILAR DEVICES AND METHODS RELATING TO THE SAME”, filed on Mar. 15, 2005, and further identified by Attorney File No. 41890-01746. Each of the above referenced patent applications hereby is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract (Award) No. 70NAN2B3021 awarded by the National Institute of Standards and Technology (NIST).

BACKGROUND OF THE INVENTION

1. Field of the Invention
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 gas and fluid diffusion layers incorporating modified carbon products and methods for making gas and fluid diffusion layers including modified carbon products. The modified carbon products can be used to enhance and tailor the properties of the gas and fluid diffusion layers.
2. Description of Related Art
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).
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: 2H₂→4H++4e ⁻ (1)
Cathode: 4H⁺+4e ⁻O₂→2H₂O (2)
Overall: 2H₂+O₂→2H₂O (3)
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 (H₂and O₂when 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.
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 (CH₃OH) directly without the use of a fuel reformer to convert the methanol to H₂. 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.
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.
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: CH₃OH+H₂O→CO₂+6H⁺+6e ⁻ (4)
Cathode: 6H⁺+6e ⁻+ 3/2O₂→3H₂O (5)
Overall: 2CH₃OH+3O₂→2CO₂+6H₂O+energy (6)
There are a number of properties that are required for efficient fuel cell operation. The gas/fluid diffusion layers, commonly referred to as gas diffusion layers (GDLs) in hydrogen gas based PEMFCs, and fluid diffusion layers in methanol fuel-based DMFCs, are used to control fluid distribution and water permeability in the fuel cell. Gas/fluid diffusion layers and similar structures are referred to in the literature by many different names. As used herein, the term gas/fluid diffusion layer includes all such structures including gas diffusion layers, gas diffusion media, gas diffusion electrodes and the like.
Ideally, gas/fluid distribution layers should be capable of transporting reactants to the electrodes, while removing by-products away from the electrodes. For example, in a PEMFC, the gas/fluid diffusion layer should enable efficient transport and distribution of hydrogen gas to the catalysts within the anode. In a DMFC, the gas/fluid diffusion layer should enable efficient transport and distribution of liquid methanol to the catalysts, while enabling removal of product carbon dioxide gas from the anode. In both PEMFCs and DMFCs, the gas/fluid distribution layer should enable efficient transport and distribution of oxygen-containing gas to the catalysts within the cathode and removal of water from the cathode.
Current techniques employed to control gas/fluid distribution include the incorporation of conventional hydrophilic and/or hydrophobic materials within the gas/fluid distribution layer. However, the use of such materials often results in decreased electrical performance and decreased porosity in the gas/fluid diffusion layer.
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.
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.
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.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a fuel cell is provided comprising an anode, a cathode and a proton exchange membrane dispose between the anode and the cathode, where the fuel cell further includes a first gas/fluid diffusion layer adjacent to the anode and opposite the proton exchange membrane, and a second gas/fluid diffusion layer adjacent to the cathode and opposite the proton exchange membrane, where at least one of the first and second gas/fluid diffusion layers includes a modified carbon product. In one embodiment of the present aspect, the modified carbon products include carbon black. In another embodiment of the present aspect, the modified carbon product includes carbon fibers. In yet another embodiment, the modified carbon product comprises graphite fibers. In one embodiment, the modified carbon product includes graphite fibers modified with a hydrophilic functional group. In yet another embodiment, at least one of the gas/fluid diffusion layers includes a carbon support and a microporous layer disposed on the carbon support, where the microporous layer includes the modified carbon product. In one embodiment, at least one of the gas/fluid diffusion layers has an average pore size of from about 100 nm to about 500 nm. In another embodiment, the microporous layer has an average thickness of from about 2 microns to about 20 microns. In yet another embodiment, at least one of the gas/fluid diffusion layers includes a carbon support, where the modified carbon product particles are dispersed within the carbon support. In one embodiment, the modified carbon product particles have an average particle size of from about 1 micron to about 30 microns. In yet another embodiment, at least one of the gas/fluid diffusion layers includes from about 5 volume percent to about 70 volume percent of the modified carbon product particles. In another embodiment, at least one of the gas/fluid diffusion layers includes a carbon support that is a modified carbon product. In one embodiment, both the first and second gas/fluid diffusion layers comprise a modified carbon product. In one embodiment, the modified carbon product includes a hydrophilic functional group. In another embodiment, the modified carbon product comprises a hydrophilic functional group selected from the group of carboxylic acids, carboxylic salts, sulfonic acids, sulfonic salts, phosphonic acids, phosphonic salts, amines, amine salts and alcohols. In another embodiment, the modified carbon product comprises a hydrophobic functional group. In one embodiment, the modified carbon product includes a hydrophobic functional group selected from the group of saturated cyclics, unsaturated cyclics, saturated aliphatics, unsaturated aliphatics and polymerics. In yet another embodiment, the modified carbon product includes both a hydrophilic functional group and a hydrophobic functional group. In another embodiment, the modified carbon product includes a steric inhibiting functional group. In one embodiment, at least one of the gas/fluid diffusion layers includes predominately hydrophilic modified carbon products on a first side and predominantly hydrophobic modified carbon products on a second side opposite the first side. In another embodiment, at least one of the gas/fluid diffusion layers includes a major planar surface and comprises a chemical gradient lateral to the planar surface. In another embodiment, at least one of the gas/fluid diffusion layers includes a major planar surface and comprises a chemical gradient perpendicular to the planar surface.
According to another aspect of the present invention, a method for the fabrication of the gas/fluid diffusion layer in a fuel cell provided, the method comprising providing a carbon support, depositing a first ink composition comprising a first modified carbon product on the carbon support to form a first sublayer and depositing a second ink composition comprising a second modified carbon product over the first sublayer, where the first modified carbon product and the second modified carbon product have different hydrophilic properties. In one embodiment of the present aspect, the depositing step includes depositing with an ink-jet device. In another embodiment of the present aspect, the depositing step includes depositing using a method selected from the group of the spraying, analog deposition, lithography, flexographic printing, slot-die, role-coating, xerography and electrostatic printing. In one embodiment, at least one of the first and second modified carbon products includes graphite. In another embodiment, at least one of the first and second modified carbon products includes carbon black.
According to yet another aspect of the present invention, a method for the fabrication of gas/fluid diffusion layer is provided, the method comprising providing a carbon support, contacting the carbon sport with the diazonium salt and treating the diazonium salt to covalently bond a functional group to the carbon support and form a modified carbon support product. According to one embodiment of the present aspect, the carbon support includes carbon cloth. In another embodiment of the present aspect, the carbon support includes carbon paper. In one embodiment, the functional group is a hydrophilic group. In another embodiment, the functional group is a hydrophobic group.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) and 1(b) illustrate a schematic cross-section of a PEMFC MEA and bipolar plate assembly according to the prior art.
FIG. 2 illustrates a cross-section of the cathode side of an MEA showing the membrane and bipolar plate and O₂, H⁺ and H₂O transport according to the prior art.
FIG. 3 illustrates a schematic cross-section of a direct methanol fuel cell (DMFC) according to the prior art.
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.
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.
FIG. 6 illustrates the increase in active species phase size as processing temperature increases.
FIG. 7 illustrates a method for formation of platinum/metal oxide active sites using a modified carbon product according to the present invention.
FIG. 8 illustrates modified carbon products used to form a microporous layer onto a gas/fluid diffusion layer in accordance with an embodiment of the present invention.
FIG. 9 illustrates impregnation of modified carbon products into a porous gas/fluid diffusion layer according to an embodiment of the present invention.
FIG. 10 illustrates a modified gas/fluid diffusion layer in accordance with an embodiment of the present invention.
FIG. 11 illustrates a method for forming a gas/fluid diffusion layer in accordance with an embodiment of the present invention.
FIG. 12 illustrates formation of through-plane gradient structures in accordance with an embodiment of the present invention.
FIG. 13 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.
FIGS. 14(a) and 14(b) illustrate SEM photomicrographs of a printed ink formulation comprising a modified carbon black product according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to fuel cell components that incorporate modified carbon products. Specifically, the present invention relates to gas/fluid diffusion layers that incorporate and utilize modified carbon products. The use of such modified carbon products enables the production of gas/fluid diffusion layers having enhanced properties. For example, modified carbon products can be utilized in gas/fluid diffusion layers to enhance the mass transport properties, electrical conductivity and physical robustness.
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.
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.
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.
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.
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.
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.
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 m²/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.
Generally, a carbon material is modified utilizing the Belmont process via a functionalizing agent of the form: X—R—Y

- where: X reacts with the carbon surface;
  - R is a linking group; and
  - Y is a functional group.

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≡NRY

- where: N is nitrogen;
  - X is an anion such as Cl⁻, Br⁻ or F⁻; R is the linking group; and
  - Y is the functional group.

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).
FIGS. 5(a) and 5(b) illustrate different embodiments of a modified carbon product 524 a, 524 b 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 524 a. In FIG. 5(b) polyamines are attached to the carbon material 520 to produce a modified carbon product 524 b.

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 I, and particularly preferred hydrophobic functional groups are listed in Table II.

TABLE 1


Hydrophilic Functional Groups (Y)	Examples

Carboxylic acids and salts	(C₆H₄)CO₂ ⁻K⁺, (C₆H₄)CO₂H
Sulfonic acids and salts	(C₆H₄)CH₂SO₃H
Phosphonic acids and salts	(C₁₀H₆)PO₃H₂
Amines and amine salts	(C₆H₄)NH₃ ⁺Cl⁻
Alcohols	(C₆H₄)OH

TABLE II


Hydrophobic Functional Groups (Y)	Examples

Saturated and unsaturated cyclics and	(CH₂)₃CH₃, (C₆H₄)CH₃
aliphatics
Halogenated saturated and unsaturated	(C₆H₄)CF₃, (C₆H₄)(CF₂)₇CF₃
cyclics and aliphatics
Polymerics	Polystyrene [CH₂CH(C₆H₅)]_n

According to another aspect, preferred functional groups for fuel cell components are those that increase proton conductivity, such as SO₃H, PO₃H₂and others known to have good proton conductivity. Particularly preferred proton conductive functional groups according to the present invention are listed in Table Ill.

TABLE III


Proton Conducting Groups (Y)	Examples

Carboxylic acid and salt	(C₆H₄)COOH, (C₆H₄)COONa
Sulfonic acid and salt	(C₆H₄)SO₃H, (C₆H₄)SO₃Na
Phosphonic acid and salt	(C₆H₄)PO₃H₂, (C₆H₄)PO₃HNa

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.

	TABLE IV


	Polymeric Groups (Y)	Examples

	Polyacrylate	Polymethyl methacrylate
		(C₆H₄)[CH₂C(CH₃)COOCH₂]_n
	Polystyrene	(C₆H₄)[CH(C₆H₅)CH₂]_n
	Polyethylene oxide (PEO)	(C₆H₄[OCH₂CH₂OCH₂CH₂]_n
	Polyethylene glycol (PEG)	(C₆H₄)[CH₂CH₂O]_n
	Polypropylene oxide (PPO)	(C₆H₄)[OCH(CH₃)CH₂]_n

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.

	TABLE V


	Linking Group (R)	Examples

	Alkyls	CH₂, C₂H₄
	Aryls	C₆H₄, C₆H₄CH₂
	Cyclics	C₆H₁₀, C₅H₄
	Unsaturated aliphatics	CH₂CH═CHCH₂
	Halogenated alkyl, aryl, cyclics and	C₂F₄, C₆H₄CF₂, C₈F₁₀
	unsaturated aliphatics	CF₂CH═CHCF₂

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.
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., μmol 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 μmol/m²to about 6.0 μmol/m². In a preferred embodiment, the modified carbon product has a surface group concentration of from about 1.0 μmol/m²to about 4.5 μmol/m², and more preferably of from about 1.5 μmol/m²to about 3.0 μmol/m².
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.
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 μm, more preferably is not greater than about 20 μm and even more preferably is not greater than about 10 μum. Further, it is preferred that the volume average particle size is at least about 0.1 μm, more preferably 0.3 μm, even more preferably is at least about 0.5 μm and even more preferably is at least about 1 μm. As used herein, the average particle size is the median particle size (d₅₀). 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.
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.
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.
Manufacture Of Modified Carbon Products Particles
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 417-470, 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. Nos. 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 Recherché Scientifique, and Delmar et al., J. Am. Chem. Soc. 1992, 114, 5883-5884, each of which is incorporated herein by reference in its entirety.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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 μm 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 μm. 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 μm.
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.
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.
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.
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 μm. One disadvantage of such reactors is the poor ability to atomize carbon particles when using submerged ultrasonics for atomization.
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.
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.
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.
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° 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° 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.
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° 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.
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.
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 μmol/m²organic 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° C. and 100° C.). The highest temperature in co-current spray dryers is the inlet temperature (e.g., 180° C.), and the outlet temperature can be as low as 50° 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.
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.
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.
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° C. to about 80° C. Useful inlet temperatures according to the present invention are from about 130° C. to 180° C. The carbon solids (e.g., particulate) loading can be up to about 50 wt. %.
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.
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.
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.
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.
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.
Electrocatalyst Materials
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.
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.
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.
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.
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 μm, depending on the nature of the carbon material. However, carbon particulates having sizes up to 25 μm can also be used.
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_xdispersed 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.
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.
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.
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.
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_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.
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.
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.
In addition, the electrocatalyst powders preferably have a surface area of at least about 25 m²/g, more preferably at least about 90 m²/g and even more preferably at least about 600 m²/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.
Moreover, many of the desired attributes of modified carbon products may be desired attributes of electrocatalyst, and any of the above-described attirbutes 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.
Manufacture of Electrocatalyst Materials
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.
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.
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.
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.
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.
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.
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 (NH₄)₂PtCl₆, and ammonium tetrachloro platinate (NH₄)₂PtCl₄; sodium and potassium salts of halogeno, pseudohalogeno or nitrito platinates such as potassium hexachloro platinate K₂PtCl₆, sodium tetrachloro platinate Na₂PtCl₄, potassium hexabromo platinate K₂PtBr₆, potassium tetranitrito platinate K₂Pt(NO₂)₄; dihydrogen salts of hydroxo or halogeno platinates such as hexachloro platinic acid H₂PtCl₆, hexabromo platinic acid H2PtBr6, dihydrogen hexahydroxo platinate H₂Pt(OH)₆; diammine and tetraammine platinum compounds such as diammine platinum chloride Pt(NH₃)₂Cl₂, tetraammine platinum chloride [Pt(NH₃)₄]Cl₂, tetraammine platinum hydroxide [Pt(NH₃)₄](OH)₂, tetraammine platinum nitrite [Pt(NH₃)₄](NO₂)₂, tetrammine platinum nitrate Pt(NH₃)₄(NO₃)₂, tetrammine platinum bicarbonate [Pt(NH₃)₄](HCO₃)₂, tetraammine platinum tetrachloroplatinate [Pt(NH₃)₄]PtCl₄; platinum diketonates such as platinum (II) 2,4-pentanedionate Pt(C₅H₇O₂)₂; platinum nitrates such as dihydrogen hexahydroxo platinate H₂Pt(OH)₆acidified 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)₆]₄ ⁺.
Modified Electrocatalyst Products
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.
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.
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.,
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.
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.
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 SO₃H, PO₃H₂and 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.
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.
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.
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.
The modified electrocatalyst product may include varying amounts of surface groups. In one embodiment, the modified electrocatalyst product has a surface group concentration of from about 0.1 μmol/m²to about 6.0 μmol/m². In a preferred embodiment, the modified electrocatalyst product has a surface group concentration of from about 1.0 μmol/m²to about 4.5 μmol/m², and more preferably of from about 1.5 μmol/m²to about 3.0 mol/m².
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.
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°C. However, phosphoric groups are capable of conducting protons at temperatures above 100° 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.
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.
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.
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.
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.
Manufacture of Modified Electrocatalyst Products
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.
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.
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.
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.
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).
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.
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 III, 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,.
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.
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.
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.
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)NO₃)] and ruthenium chloride hydrate. One preferred precursor for nickel is nickel nitrate [(Ni(NO₃)₂]. One preferred precursor for cobalt is cobalt nitrate [Co(No₃)₂]. One preferred precursor for chromium is chromium nitrate [Cr(NO₃)₃].
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° C., more preferably less than 400° C., and even more preferably less than 300° 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.
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.
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.
Formation of Alloyed, Mixed Metal and/or Metal Oxide Modified Electrocatalyst Products
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.
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.
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.
By way of illustration, a carbon material, such as carbon black, can be co-modified with carboxylic and amine groups (e.g.,(C₆H₄)CO₂H and (C₆H₄)CH₂NH₂). When this carbon material is treated with a metal salt (e.g., RuCl₃), 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(NH₃)₄(OH)₂or Pt(NH₃)₄(NO₃)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.
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° C. During processing, it is preferable to have chemically inert surface groups, such as C₅H₄N, (C₆H₄)NH₂, C₆H5, C₁₀H₇or (C₆H₄)CF₃, 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.
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).
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., SiMe₃) can be utilized to selectively produce an alloyed modified electrocatalyst product. A first metal/metal oxide (e.g., RuO₂) 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.
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.
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.
For example, modifying the surface of the carbon material with thermally stable steric groups, such as phenyl (C₆H₅) or napthyl (C₁₀H₇) 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.
Modified Carbon Products and Fluid Diffusion Layers
The present invention also relates to the use of modified carbon products in gas/fluid diffusion layers. Gas/fluid diffusion layers are utilized to distribute the gaseous and/or liquid reactants to the electrode layers, to enable removal of gaseous products and to conduct electrons from the electrode layer to the bipolar plate. Even distribution of the reactants is desirable to maximize the efficiency of the fuel cell by maximizing the utilization of the fuel (e.g., hydrogen gas or methanol)and the utilization of the electrocatalyst active sites in the electrode layer. Efficient removal of reaction products, such as water, is important to prevent flooding of the fuel cell and prevent clogging. Efficient conduction of electrons is important to increase the electrical output efficiency of the fuel cell. Other important properties of the gas/fluid diffusion layer include heat conductivity and mechanical strength. Heat generated during operation of the fuel cell should be removed to manage the humidification levels and the system efficiency of the fuel cell.
A typical gas/fluid diffusion layer generally comprises a porous carbonaceous support material, such as carbon fibers woven into a paper or cloth, for conducting electrons to the bipolar plate. The carbon based fibers, typically graphite, are generally conductive, especially in the “in-plane” direction due to their anisotropic shape. A typical gas/fluid diffusion layer carbonaceous support material has a porosity of about 70% due to the spacing of the carbon fibers.
In some instances, the gas/fluid diffusion layer also includes a microporous layer for distributing reactants to the electrode. A traditional microporous layer typically includes of a mixture of binder, such as a PTFE polymer, and carbon-based particles. These materials change the degree of hydrophilicity and the pore size of the gas/fluid diffusion layer. When the mixture is added to a gas/fluid diffusion layer, the conventional polymer material may aid in water distribution and removal within the cell, but also detrimentally reduces porosity of the layer.
It is an object of the present invention to utilize modified carbon products to improve the efficiency of the gas/fluid diffusion layer. There are several approaches in which modified carbon products can be incorporated into a gas/fluid diffusion layer. In one approach, the modified carbon products can be layered onto and/or impregnated into an existing gas/fluid diffusion layer. In another approach, an existing gas/fluid diffusion layer can be chemically modified with a diazonium salt to add surface groups to the gas/fluid diffusion layer. In another approach, a combination of modified carbon products and conventional materials (e.g., polymeric) are used to modify and/or produce a gas/fluid diffusion layer. Any of these approaches can be utilized to tailor the properties of the gas/fluid diffusion layer, such as the porosity, electron conductivity and/or mass transport properties. It will be appreciated that any of these approaches can be utilized to achieve the desired functionality, either alone, or in combination with the other approaches, and either in series or in parallel. It will be also appreciated that more than one type of modified carbon product can be utilized. In one embodiment, modified carbon products have two or more different functional groups on the same carbon particle, as described above. In another embodiment, modified carbon products having varying concentrations of surface groups are utilized to produce the desired functionality, such as gradients within the gas/fluid diffusion layer.
Various methods may be utilized to incorporate a modified carbon product in a gas/fluid diffusion layer. One 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 gas/fluid diffusion layer. 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.
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 gas/fluid diffusion layer.
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 gas/fluid diffusion layer.
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 gas/fluid diffusion layer.
Another method for incorporating a modified carbon product in a gas/fluid diffusion layer, includes the steps of mixing a modified carbon product with another material (e.g., a second modified carbon product, an electrocatalyst material, a conventional carbon material, and/or other materials utilized in the production of a gas/fluid diffusion layer) to form a modified carbon-containing mixture and incorporating the mixture into the gas/fluid diffusion layer. 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.
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 gas/fluid diffusion layer, such as by analog or digital printing, as discussed in further detail below.
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 gas/fluid diffusion layer and contacting the carbonaceous material with a diazonium salt to form a modified carbon product in the gas/fluid diffusion layer. 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 gas/fluid diffusion layer.
Various aspects, approaches, and/or embodiments of the present invention are described below, primarily in reference to modified carbon products in gas/fluid diffusion layers. 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.
Layering of Modified Carbon Products
According to one embodiment of the present invention, modified carbon products are applied to the surface of the gas/fluid diffusion layer to produce a microporous layer. While the below described aspects and embodiments are described as being applied to only a single side of the gas/fluid diffusion layer, it will be appreciated that modified carbon products can be added to both sides of the gas/fluid diffusion layer to achieve desired effects.
In a particular aspect, modified carbon products are added to the electrode side of the gas/fluid diffusion layer to produce a microporous layer having tailored hydrophobic and/or hydrophilic properties and porosity. In this aspect, modified carbon products utilizing hydrophobic and/or hydrophilic surface groups, such as those including hydrophilic and/or hydrophobic functional groups, can be utilized to manufacture a microporous layer by any of a variety of techniques, as described in a further detail below. In a preferred embodiment, the microporous layer comprises hydrophobic modified carbon products to increase the mass transport of water out of the fuel cell. In another embodiment, hydrophilic and/or a mixture of and/or a gradient of hydrophobic and hydrophilic modified carbon products are used to complete the microporous layer. Preferred hydrophobic functional groups according to this aspect are provided in Table I, and preferred hydrophilic functional groups according to this aspect are provided in Table II.
In one embodiment, the modified carbon products include a hydrophobic functional group. Such modified carbon products can be incorporated into a non-polar solution, such as a solution comprising PTFE. The resultant solution can be utilized to manufacture a microporous layer by any of a variety of techniques, as described in further detail below, having tailored hydrophobic and/or hydrophilic properties. The manufactured microporous layer will generally comprise a significant less amounts of conventional hydrophobic ionomer, resulting in increased porosity of the gas/fluid diffusion layer.
In another embodiment, the modified carbon products include a hydrophilic functional group. Such modified carbon products can be incorporated into a polar solution, such as a solution comprising PFSA. The resultant solution can be utilized to manufacture a microporous layer by any of a variety of techniques, having tailored hydrophilic and/or hydrophobic properties. The manufactured microporous layer will comprise less conventional hydrophilic ionomer, resulting in an increased porosity of the gas/fluid diffusion layer.
In yet another embodiment, as illustrated in FIG. 8, modified carbon products 824 are utilized to produce a microporous layer 809 without the addition of any conventional polymeric materials. Such modified carbon products 824 may include a surface group exhibiting hydrophilic and/or hydrophobic properties, such as one including a hydrophilic and/or hydrophobic functional group (Y), as applicable, to tailor the properties of the microporous layer for transport of reactants and/or products. The microporous layer 809 is substantially more porous than traditional microporous layers using conventional polymeric additives.
The porosity of the gas/fluid diffusion layer may also be controlled by utilizing well-controlled modified carbon products having a small average particle size and/or a narrow particle size distribution. Small particle size enables the formation of dense and thin microporous layers with small pore sizes. In one embodiment of the present invention, microporous layers having a pore size in a range of 100 nm to 500 nm can be produced utilizing modified carbon products. In one embodiment, the microporous layers have a thickness of from about 2 μm to 20 μm, more preferably from about 4 μm to about 12 μm. In one embodiment, modified carbon products are utilized in conjunction with conventional polymeric materials to create a gas/fluid diffusion layer having tailor porosity and/or hydrophobic/hydrophilic properties. In a particular embodiment, modified carbon products are mixed in with hydrophobic monomer ionomer solution to create a modified carbon ink, which may then be deposited to produce a microporous layer having tailored porosity and hydrophobic properties.
In any of the foregoing embodiments, the modified carbon products may include more than one surface group attached to the surface thereof to produce the modified microporous layer. For example, a modified carbon product including both hydrophilic and hydrophobic functional groups can be dispersed in an aqueous solution. The solution may then be deposited on the surface of the gas/fluid diffusion layer to produce a microporous layer having reduced pore size and an intimate mixture of hydrophilic and hydrophobic domains.
Impregnating The Gas/Fluid Diffusion Layer With Modified Carbon Products
As noted above, gas/fluid diffusion layers are often made of porous carbon papers or cloths. According to another aspect of the present invention, modified carbon products are utilized to change the hydrophilic, hydrophobic and/or permeability properties of the gas/fluid diffusion layer by impregnation of the modified carbon products into the internal pores of the gas/fluid diffusion layer. Modifying the properties of the gas/fluid diffusion layer by impregnation may eliminate the need for a microporous layer, which has many advantages, including reduced costs, fabrication time, and the elimination of handling and manufacture procedures associated with the microporous layer.
Gas/fluid diffusion layers generally have pore sizes of from 10 to 30 μm. In one embodiment of the present invention, modified carbons product particles having an average particle size of not greater than about 30 μm, such as from about 1 μm to 30 μm are utilized to impregnate a gas/fluid diffusion layer. In another embodiment, the solids loading of the modified carbon products in the gas/fluid diffusion layer is preferably from about 5 vol. % to 70 vol. %.
FIG. 9 illustrates an embodiment of the present invention where modified carbon products 924 are impregnated into a porous gas/fluid diffusion layer 908. The gas/fluid diffusion layer 1908 may be impregnated by a variety of techniques. In one embodiment, the gas/fluid diffusion layer is impregnated by depositing modified carbon products 924 on the surface of the gas/fluid diffusion layer 908, such as by ink-jet deposition, as described below, followed by filtering the deposited modified carbon products through the gas/fluid diffusion layer, such as by vacuum filtration.
In another embodiment, modified carbon products are impregnated into the gas/fluid diffusion layer during the manufacturing process of the gas/fluid diffusion layer. With respect to the manufacture of carbon papers, typical manufacturing processes entail extruding and pressing the papers through a series of rollers, which expose the paper to various aqueous and/or non-aqueous solutions. In this embodiment, a modified carbon product can be dispersed in any of the various aqueous and/or non-aqueous solutions. As the paper is manufactured, the modified carbon product is impregnated into the porous paper structure.
In yet another embodiment, the incorporated (e.g., layered or impregnated) modified carbon products comprise surface groups that undergo further reactions, such as polymerization or thermal decomposition. For example, a gas/fluid diffusion layer may incorporate a modified carbon product having a carboxylate functional group. As the gas/fluid diffusion layer is heated, such as in a subsequent step, the hydrophilic CO₂H will decompose into a hydrophobic phenyl group.
Modification Of Carbonaceous Materials In The Gas/Fluid Diffusion Layer
Another aspect of the present invention is directed to the modification of the gas/fluid diffusion layer with surface groups, such as modification of the carbonaceous support structure (e.g., the carbon paper and/or carbon cloth), or other carbonaceous materials contained therein, such as previously deposited and/or impregnated carbon materials and/or modified carbon products.
As described above, the gas/fluid diffusion layer generally comprises carbonaceous materials, such as carbon paper and/or carbon cloth. In one approach, these carbonaceous materials are modified by means of a diazonium salt to covalently attach an organic group thereto, such as any of the above described organic surface groups. In another approach, non-modified carbon materials and/or modified carbon products may be layered and/or impregnated onto to a diffusion layer, as described above. Such non-modified materials and/or modified carbon products can be subsequently modified with a diazonium salt to create modified carbon products and/or multiply-modified carbon products. A modified gas/fluid diffusion layer resulting from such a treatment is schematically illustrated in FIG. 10, where (Y) is a functional group attached to a carbon material 1020.
In one embodiment, the gas/fluid diffusion layer is immersed into a solution containing a diazonium salt. In another embodiment, the gas/fluid diffusion layer is contacted with a diazonium salt by means of an analog and/or digital printing technique. As will be appreciated, use of a digital printing enables the production of discrete patterns of surface groups on the gas/fluid diffusion layer, as described in further detail below.
By way of illustration, a solution including a carbon material, such as carbon black, can be deposited onto the surface of the gas/fluid diffusion layer, such as by screen printing. The gas/fluid diffusion layer can then be dried and treated with a diazonium salt, such as by spraying a diazonium salt-containing solution onto the gas/fluid diffusion layer. The resultant gas/fluid diffusion layer will comprise a microporous layer having surface groups attached thereto, and may evidence hydrophobic or hydrophilic properties.
As noted above, the modified carbon products can be used to control the hydrophilic/hydrophobic characteristics of the gas/fluid diffusion layer. Modified carbon products can also be used to tailor the pore size of the gas/fluid distribution layer to facilitate selective mass transport therein, such as by enabling a uniform gas/fluid distribution to the electrode. In one embodiment, the modified carbon products comprise steric inhibiting functional groups, such as polymeric groups. The characteristics of the steric inhibiting groups, such as length and branch quality can be selected to tailor the mass transport characteristics of the gas/fluid diffusion layer.
It will be appreciated, that both hydrophilic/hydrophobic functional groups and steric surface groups can be utilized to tailor the gas/fluid diffusion layer. In one embodiment, both the hydrophilic/hydrophobic and steric groups are added in a single step, such as by immersing the gas/fluid diffusion layer in a solution comprising diazonium salts corresponding to two or more functional groups. Thus, it is possible in one chemical treatment to tailor both the hydrophilic and/or hydrophilic nature and porosity of the gas/fluid diffusion layer.
In another embodiment, two different surface groups can be added to separate portions of the gas/fluid diffusion layer. For example, the gas/fluid diffusion layer can be placed in contact with a diazonium salt solution having a low concentration of a first surface group, such as one having a hydrophilic functional group. In a subsequent step, a solution comprising a modified carbon product or a diazonium salt can be deposited onto one side or a portion of the gas/fluid diffusion layer. This embodiment enables a gradient to form within the gas/fluid diffusion layer, where hydrophilic pores exist in the interior and the surface is predominately hydrophobic. It will be appreciated that this order can be reversed, where the interior is hydrophobic and the surface is hydrophilic.
In one embodiment, a solution comprising a modified carbon product or a diazonium salt is deposited in a discrete pattern on the gas/fluid diffusion layer to produce one or more discrete patterns of surface groups on the gas/fluid diffusion layer. In this regard, such patterns may be hydrophilic and/or hydrophobic to influence the movement of reactants through the gas/fluid diffusion layer, as schematically shown in FIG. 11. As depicted, a first diazonium salt contacts the gas/fluid diffusion layer, 1108, and then a second diazonium salt contacts the gas/fluid diffusion layer. In one preferred embodiment, the deposited patterns definable a path for water removal through the gas/fluid diffusion layer.
Gradients
According to another aspect of the present invention, modified carbon products are patterned on the gas/fluid diffusion layer to produce lateral gradients or in-plane gradients to vary the hydrophilic/hydrophobic properties and/or fluid permeability of the gas/fluid diffusion layer. In one approach, modified carbon products are deposited utilizing an analog and/or digital printing means, as discussed in further detail below, to produce the gradients. This aspect is useful, for example, to form defined water wicking and fluid distribution paths, which reduces the likelihood of water clogging in the gas/fluid diffusion layer and enables reactants to quickly reach the electrode, thereby increasing fuel cell performance, especially at high currents.
In one embodiment, multiple layers of modified carbon products are deposited to produce through-plane gradients. These gradient structures can transition from very hydrophilic to hydrophobic, or vice versa, moving from one side of the gas/fluid diffusion layer to the other.
According to another embodiment, through-plane gradient structures are fabricated by surface treatment of the native gas/fluid diffusion layers with a diazonium salt, or surface treatment of a deposited carbonaceous particulate layer by a diazonium salt, as illustrated in FIG. 12. By way of illustration, the gas/fluid diffusion layer can be chemically modified with a sulfonic diazonium salt to attach hydrophilic surface groups (X). Subsequently, a modified carbon product including a slightly less hydrophilic functional group (Y), such as a carboxylic group, can be deposited on the hydrophilic gas/fluid diffusion layer to form a less hydrophilic first layer. A final modified carbon product including a hydrophobic functional group (Z), can be deposited on the first layer to produce a hydrophobic layer. The resultant gas/fluid diffusion layer will have a through-plane gradient with respect to hydrophobic/hydrophilic properties.
According to another embodiment, the gradient structure, either in-plane or through-plane, includes different layers of modified carbon products where the modified carbon products include the same surface group, but in varying concentrations.
In yet another embodiment, various layers are deposited on the gas/fluid diffusion layer, where the various layers each utilize modified carbon products having one or more surface groups, where the ratios of the surface groups change from layer to layer to tailor the nature of gas/fluid diffusion layer.
By way of example, the first layer of the gas/fluid diffusion layer can be the carbonaceous support material, such as a carbon paper or a carbon cloth. A second layer can be deposited on the carbon support, where the second layer includes a modified carbon product containing two surface groups, such as a hydrophobic surface group and a hydrophilic surface group, where the ratio of the hydrophobic surface group to the hydrophilic surface group is relatively high, such as about 4:1. A third layer can be produced on the second layer, where the third layer comprises modified carbon products having hydrophobic surface groups and hydrophilic surface groups, where the ratio of hydrophobic surface groups to hydrophilic surface groups is less than that of the second layer, such as about 3:2. A fourth layer can be produced on the third layer, where the fourth layer comprises modified carbon products comprising hydrophobic surface groups and hydrophilic surface groups, where the ratio of hydrophobic surface groups to hydrophilic surface groups is even less than that of the third layer, such as 2:3. A top layer can be produced on the fourth layer, where the top layer comprises modified carbon products comprising only hydrophilic surface groups or hydrophobic surface groups and hydrophilic surface groups, where the ratio of hydrophobic surface groups to hydrophilic surface groups is even less than that of the third layer, such as 1:4, or even higher.
According to another embodiment of the present invention, through-plane gradient structures can be produced by utilizing different types of modified carbon products (e.g., one modified with a hydrophilic group and one modified with a hydrophobic group). The ratios of the different modified carbon products can be varied from layer to layer to achieve a gradient structure.
It will be appreciated, that the layers can be produced such that the most hydrophilic layer is on the electrode side and the most hydrophobic layer is near the bipolar plates of the gas/fluid diffusion layer, or vice versa. In one particular embodiment, a hydrophilic layer is produced on the electrode side of the gas/fluid diffusion layer in the cathode.
Pore Size
According to another aspect of the present invention, modified carbon products are utilized to tailor the porosity of the gas/fluid diffusion layer. In this regard, the steric bulk of the surface groups may utilized to control (decrease) the average pore size and pore size distribution within the gas/fluid diffusion layer, resulting in more uniform fluid distribution during operation of the cell. Long chain polymeric surface groups clog internal pores, and optionally can provide hydrophilic or hydrophobic and binding functionality within the layer. In one embodiment, modified carbon products having polymeric surface groups are utilized to enhance the porosity of the gas/fluid diffusion layer. Particularly preferred long-chained groups, according to the present aspect include the polymeric functional groups listed in Table IV, above. It will be appreciated that the modified carbon products having polymeric surface groups can be incorporated into a gas/fluid diffusion layer by any useful means, including layering and/or impregnating.
By way of example, modified carbon products having a polymeric surface group attached thereto, such as PEG, can be suspended in a solvent. A second modified carbon product having a hydrophilic surface group, such as one including a sulfuric functional group, can also be suspended in the solvent. The resultant modified carbon solvent solution can be deposited on the gas/fluid diffusion layer and dried to produce enhanced porosity and hydrophilicity within the gas/fluid diffusion layer.
Electrical Conductivity
The gas/fluid diffusion layer should also be relatively electrically conductive to conduct electrons to the bipolar plate. In one embodiment of the present invention, modified carbon products having an electrically conductive carbon are utilized in the production of the gas/fluid diffusion layer. In a particular embodiment, a microporous layer is manufactured utilizing a modified carbon product based on graphite to increase its conductivity. Since graphite materials are typically hydrophobic, a graphite flake can be made hydrophilic by adding a hydrophilic group, such as a sulfuric group, to the surface of the graphite. The graphite-containing modified carbon product can then be deposited on to a gas/fluid diffusion layer to make an electrically conductive and/or hydrophilic microporous layer.
Other Features of Modified Carbon Products in Gas/fluid Diffusion Layers
Carbon oxidation is a significant issue in fuel cells. Carbon within the gas/fluid diffusion layer can oxidize, which decreases the conductivity of the gas/fluid diffusion layer and its corresponding hydrophobic nature. Traditionally, control over oxidation has been difficult since many highly crystalline, oxidation resistant carbon materials, such as graphite, have been difficult to utilize in fuel cell components as there was no easy-method for incorporating such products into the fuel cell. As noted, conventional graphite materials are very hydrophobic and difficult to disperse in aqueous-based solutions. Thus, it was difficult to efficiently and effectively utilize such materials in spraying, immersion and other deposition techniques known in the production of fuel cells.
According to one embodiment of the present invention, carbon oxidation within the fuel cell is reduced/minimized by utilizing modified carbon products having a high crystallinity carbon, such as graphite, and a hydrophilic surface group. Utilizing a modified carbon product based on graphite and hydrophilic surface groups enables the graphite to be more, readily dispersed in an aqueous solution. Thus, aqueous solutions, such as a modified carbon ink, as discussed below, can be utilized to produce fuel cell components, such as gas/fluid diffusion layers, that include graphite. Such a modified carbon product can be utilized in the production of a gas/fluid diffusion layer to increase the oxidation resistance of the gas/fluid diffusion layer. In another aspect, carbon oxidation is controlled by increasing the amount of surface groups attached to the surface of the carbon material.
Another issue associated with the gas/fluid diffusion layer, and more specifically gas/fluid diffusion layers including a microporous layer, is cracking. Cracking is an issue because it inhibits even fluid distribution through the gas/fluid diffusion layer. If cracks are present in the gas/fluid diffusion layer, the incoming fluids may flow through the cracks (following the path of least resistance), which may prohibit/eliminate their transport to the active catalytic sites in the electrode layer. Typically, cracks occur when drying rates are too fast and/or are not uniform.
Thus, according to one aspect of the present invention, modified carbon products are utilized in the gas/fluid diffusion layer to reduce or substantially eliminate cracking. In one approach, cracking is controlled/eliminated by adjusting the hydrophilic or hydrophobic nature of the modified carbon product to slow the drying process during layer formation, such as by selecting a modified carbon product having miscibility characteristics that coincide with the solvent utilized to form the microporous layer. Such an approach is depicted in FIG. 13, as compared to a normal drying process. A normal drying process, such as depicted in FIG. 13(a), using standard products may result in cracks and/or a disconnected surface. Conversely, utilizing modified carbon products to create a layer, as shown in FIG., 13(b), for example, such as those including a long chain polymeric groups and or surface groups that have miscibility characteristics that coincide with the solvent used to produce the layer, results in a greater retention period of the liquid solvent, and, thus, a more uniform surface with minimal surface cracks. Another way to increase solvent retention time is by increasing 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.
For example, a modified carbon product including a hydrophilic surface group, such one having a sulfuric functional group, can be used in an aqueous-based solvent system. Use of a hydrophilic surface group in an aqueous-based solvent system slows water evaporation due to the hydrogen bonding between the water molecules and the hydrophilic group. Similarly, using a modified carbon product having a non-polar surface group in an organic solvent will also slow evaporation of the organic solvent.
In another approach, modified carbon products including a long chain polymeric surface groups (e.g., a PEG functional group) are utilized to decrease cracking. Such polymeric surface groups will entangle during concentration/drying. Cracking is thus prevented by the cross-linking between the surface groups, and/or the gas/fluid diffusion layer materials. Other surface groups useful in accordance with this embodiment include long chain branched and unbranched alkyls, aryl and polymerics such as PPO, polystyrene, and polyalcohols.
In another embodiment, multipass printing (e.g., printing the microporous layer more than once but over the same area and in series) is used to minimize crack formation. With multipass printing, subsequent prints fill cracks formed from previous drying steps, resulting in a substantially crack-free layer. In one particular embodiment, the last deposited layer of the multiple deposited layers is dried more slowly than the other layers to eliminate non-uniform drying.
In yet another embodiment, relatively thin layers of modified carbon products are deposited to eliminate cracking. Thinner layers generally are more likely to dry without cracks because of the small volume change involved as compared to thicker layers. In a particular embodiment, the deposited layers have a thickness of between 8 and 12 microns, and preferably between 2 and 8 microns.
Another embodiment of the present invention is directed to the use of dry coating techniques, such as dry electrostatic printing, to add modified carbon products to the gas/fluid diffusion layer. This embodiment advantageously eliminates a drying step, which in turn eliminates any mechanism of crack formation. In this embodiment, the surface groups are selected to act as the charge carrier and binder system typically utilized in electrostatic (or xerographic) printing processes. In a particular embodiment, a modified carbon product including two surface groups is used, such as one having a hydrophilic functional group and another having a polymeric functional group, where the hydrophilic group is utilized for dispersability purposes and the polymeric groups act as both the binder and the charge carrier. It will be appreciated that these specific surface groups can be added to the same or different carbons materials.
Another embodiment of the present invention is directed to the use of modified carbon products as a sorbentitrap for impurities in the reactant steam or corrosion byproducts, such as sulfur, chloride ions or sodium ions. Surface groups, such as those including sulfuric functional groups and carboxylic functional groups, can be utilized to bind chemical species such as sodium ions. In this embodiment, the removal for such chemical species helps minimize poisoning and degradation of the catalyst and proton exchange membrane, which leads to increased MEA lifetime and fuel cell reliability.
Interface with Adjacent Components/Adhesion
Another embodiment of the present invention is directed to the incorporation of modified carbon products to improve the interface contact between the electrode layer and the gas/fluid diffusion layer. In this regard, the surface group can form a physical or chemical bond between the materials on either the electrode or the gas/fluid diffusion layer due to improved dispersability of the modified carbon product, forming a smoother surface and increasing contact area.
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 gas/fluid diffusion layer. This improves bonding between the electrode and the gas/fluid diffusion layer, thereby decreasing contact resistance and minimizing ohmic losses. Another benefit is reduced delamination and structural deterioration during long term operation of the fuel cell.
Another embodiment of the present invention is directed to the use of modified carbon products to increase the adhesion of between an the gas/fluid diffusion layer and adjacent fuel cell component (e.g., the bipolar plate and/or the electrode). An gas/fluid diffusion layer 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 gas/fluid diffusion layer 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.
Deposition of Modified Carbon Products and/or Diazonium Salts
As noted above a modified carbon ink can be utilized to produce and/or modify the fluid diffusion layer. 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 includes the subclass of 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.
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.
In a particular embodiment, deposition of a modified carbon ink can be utilized to produce and/or modify a fluid diffusion layer. Deposition of a modified carbon ink preferably produces and/or modify the fluid diffusion layer to tailor the layer to one or more attributes. For example, a modified carbon ink utilized in the production of and/or modification of fluid diffusion layers may enable electrical conductivity, proton conductivity and physical separation 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 ink to create and/or modify the fluid diffusion layer. 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.
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 hydrophilic surface groups significantly simplifies ink preparation in aqueous-based inks due to increased wetting and dispersability of the modified carbon material. 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. Other surface modifications can be chosen to improve the wettability and dispersability of modified carbon products when organic solutions are used. 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.
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.
The modified carbon ink according to the present invention can be deposited to form patterned or unpatterned layers using a variety of tools and methods. In one embodiment, the 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 inkjet 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.
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 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.
Typically, an ink-jet device includes an ink-jet head with one or more orifices having a diameter of not greater than about 100 μm, such as from about 50 μm to 75 μm. 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. Pat. application Ser. No. 10/417,417 (Publication No. 20040038808) which is also incorporated herein by reference in its entirety.
It is important to simultaneously control the surface tension and the viscosity of the modified carbon ink. 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.
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 above, the surface modification of a carbon product can advantageously enhance the dispersion of the carbon product, and lead to higher obtainable solids loadings.
The modified carbon ink 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.
The modified carbon ink 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 μm, such as not greater than 100 μm, not greater than 75 μm and even not greater than 50 μm. 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.
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.
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 μm, and more preferably is not greater than about 5 μm. Large droplets can be optionally removed from the aerosol, such as by the use of an impactor.
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 10⁶droplets/cm³, such as greater than about 10⁷droplets/cm³. 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.
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.
The modified carbon ink 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.
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 μm to about 25 μm 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.
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.
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° C.) and heated (e.g., 350° C.) to induce mobility and physical bond formation between adjacent modified carbon products through a surface substituted aryl group.
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 tool (e.g., an ink jet printer) onto the fluid diffusion layer to form a hydrophobic layer on the electrode side of the fluid diffusion layer. After the deposited layer is dried (e.g., at about 100° C.), it can be heated (at about 350° 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.
Using one or more of the foregoing deposition techniques, it is possible to deposit a modified carbon ink on one side or both sides of a surface (e.g., a membrane surface) to form and/or modify a fluid diffusion layer. According to one embodiment, such deposition techniques are utilized to modify a fluid diffusion layer. In another embodiment, such deposition techniques are used to directly create the fluid diffusion layer(s), such as by depositing a modified carbon ink onto another component of the MEA, such as a bipolar plate and/or electrode layer.
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 one or more components of the fuel cell.
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.
It will be appreciated that any of the above referenced deposition methods can be utilized to directly deposit a diazonium salt onto carbonaceous surface for the purpose of directly modifying such carbonaceous surface, such as the surfaces of the fluid diffusion layer. Additionally, such depositions can be used to modify any previously deposited carbonaceous materials, including non-modified carbon materials, electrocatalyst materials, modified carbon products and/or modified electrocatalyst products, such as any of those contained in the fluid diffusion layers. 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 fuel cell component, or can be deposited in discrete patterns to produce patterned modified carbon layers.

EXAMPLES

1. Printed SO₃H and CF₃Terminated VULCAN XC-72 Microporous Layer
A solution containing ArNH₂, NaNO₂and HCL (where Ar is —C₆H₆—SO₃H), is added to a solution containing unmodified VULCAN XC-72 to form the diazonium salt, ArN≡N:⁺Cl⁻ which in turn reacts with the surface of the VULCAN XC-72 to form the modified carbon black (MCB) product. The quantity of diazonium salt formed is sufficient to only give a partial surface coverage, 0.4 μmol/m². The resulting MCB is isolated and redispersed in deionized water. To this solution additional ArNH₂, NaNO₂and HCI are added. However, in this case Ar is —C₆H₆—CF₃. The resulting reactions yield a MCB product containing 0.4 μmol/m²SO₃groups and 0.2 umol/m²CF₃groups, approximate total loading of MCBs is 0.6 μmol/m². The resulting MCB is then dispersed in a low viscosity ink, i.e., dispersed in deionized water and is sprayed onto the surface of a carbon cloth GDL to form the microporous layer. The resulting mix of hydrophobic/hydrophilic surface groups provides a percolation path for both O₂to the catalyst and H₂O away from the catalyst.
2. Printed PEG-VULCAN XC-72 Microporous Layer
90 ml of deionized water, 26.5 g of a treating agent (aminophenylated polyethylene glycol ether having a MW˜2119 and a formula of H₂N—C₆H₄—CO—[O—(C₂H₄O)_n—CH₃) 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° C. using a hot plate. When the temperature reaches 40° C., 10 g of VULCAN XC-72 carbon black is added and the mixture is stirred and heated to 50° C. When the temperature reaches 50° C., 4.3 g of a 20 wt. % aqueous sodium nitrite solution is added slowly dropwise. The mixture is then allowed to react at 50° C. for 2 hours. When the reaction is complete, the sample is diafiltered using 10 volumes of fresh deionized water to remove any reaction byproducts. The PEG-modified VULCAN XC-72 is formulated into an ink by the addition of ethylene glycol with a final solids loading of 40 vol. %. The ink is printed onto a carbon paper fluid diffusion layer to a wet thickness of about 5 mil (125 μm) by roll coating on a web drive and is subsequently heated at 130° C. for 30 minutes.
3. Ink-jettable Microporous Layers Including Carbon Black
A typical formulation for ink-jetted Modified Carbon Black (MCB) contains 2-10 wt % solids loading. The rest of the ink contains a humectant, viscosity, surface tension modifier and/or a biocide. CAB-O-JET 200 (Cabot Corporation, Boston, Mass.) is a dispersion of 20 wt. % (C₆H₄)SO₃H terminated Black Pearls 700 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 ˜12 cP with a surface tension of ˜30 dynes/cm.
Below in Table VI to VIII are examples of low viscosity ink-jettable modified carbon black inks.

TABLE VI

Formulation

1 Weight % Properties at 21 .4° C.

MCB 5.9 Viscosity 14.8 cP

DI H₂O 52.4 Shear Stress 19.6 dynes/cm²

IPA 12.6 Shear Rate 132 s⁻¹

Ethylene Glycol 29.1 Torque 19.6%

Total 100.0 Surface 35.1 dynes/cm

Tension

TABLE VII


Formulation 2	Weight %	Properties at 20.6° C.

MCB	5.5	Viscosity	14.4 cP
DI H₂O	40.0	Shear Stress	15.5 dynes/cm²
IPA	15.0	Shear Rate	132 s⁻¹
Ethylene Glycol	40.0	Torque	22.0%
Total	100.0	Surface Tension	30.8 dynes/cm

TABLE VIII


Formulation 3	Weight %	Properties at 19.6° C.

MCB	5.5	Viscosity	12.5 cP
DI H
₂0	57.8	Shear Stress	16.7 dynes/cm²
IPA	12.7	Shear Rate	132 s⁻¹
Ethylene Glycol	19.0	Torque	42.1%
CARBITOL	5.0	Surface Tension	34.3 dynes/cm
Total	100.0

Formulation 3 is prepared with 5.5 wt. % loading of modified carbon black. Jetting is achieved with a Spectra ink-jet head. The ink-jet head temperature was set at 30° C., fire pulse width of 8 μs, pulse rise and fall time of 3 μs, and firing voltage of 120 V. An ink-jet print was obtained at a speed of 10 inches/sec. FIGS. 14(a) and 14(b) shows the dried ink-jet printed formulation (Formulation 3). Thickness of the coating in a single pass is approximately 2 μm.
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.

Claims

1. A fuel cell, comprising an anode, a cathode and a proton exchange membrane disposed between said anode and said cathode, and further comprising a first gas/fluid diffusion layer adjacent to said anode and opposite said proton exchange membrane and a second gas/fluid diffusion layer adjacent to said cathode and opposite said proton exchange membrane, wherein at least one of said first and second gas diffusion layers comprises a modified carbon product.

2. A fuel cell as recited in claim 1, wherein said modified carbon product comprises carbon black.

3. A fuel cell as recited in claim 1, wherein said modified carbon product comprises carbon fibers.

4. A fuel cell as recited in claim 1, wherein said modified carbon product comprises graphite fibers.

5. A fuel cell as recited in claim 1, wherein said modified carbon product comprises graphite fibers modified with a hydrophilic functional group.

6. A fuel cell as recited in claim 1, wherein said at least one gas diffusion layer comprises a carbon support and a microporous layer disposed on said carbon support, wherein said microporous layer comprises said modified carbon product.

7. A fuel cell as recited in claim 6, wherein said at least one gas diffusion layer has an average pore size of from about 100 nanometers to about 500 nanometers.

8. A fuel cell as recited in claim 6, wherein said microporous layer has an average thickness of from about 2 μm to about 20 μm.

9. A fuel cell as recited in claim 1, wherein said at least one gas diffusion layer comprises a carbon support and wherein modified carbon product particles are disposed within said carbon support.

10. A fuel cell as recited in claim 9, wherein said modified carbon product particles have an average particle size of from about 1 μm to about 30 μm.

11. A fuel cell as recited in claim 9, wherein said at least one gas diffusion layer comprises from about 5 vol. % to about 70 vol. % of said modified carbon product particles.

12. A fuel cell as recited in claim 1, wherein said at least one gas diffusion layer comprises a carbon support that is a modified carbon product.

13. A fuel cell as recited in claim 1, wherein both of said first and said second gas diffusion layers comprises a modified carbon product.

14. A fuel cell as recited in claim 1, wherein said modified carbon product comprises a hydrophilic functional group.

15. A fuel cell as recited in claim 1, wherein said modified carbon product comprises a hydrophilic functional group selected from the group consisting of carboxylic acids, carboxylic salts, sulfonic acids, sulfonic salts, phosphonic acids, phosphonic salts, amines, amine salts and alcohols.

16. A fuel cell as recited in claim 1, wherein said modified carbon product comprises a hydrophobic functional group.

17. A fuel cell as recited in claim 1, wherein said modified carbon product comprises a hydrophobic functional group selected from the group consisting of saturated cyclics, unsaturated cyclics, saturated aliphatics, unsaturated aliphatics and polymerics.

18. A fuel cell as recited in claim 1, wherein said modified carbon product comprises a hydrophilic functional group and a hydrophobic functional group.

19. A fuel cell as recited in claim 1, wherein said modified carbon products comprise a steric inhibiting functional group.

20. A fuel cell as recited in claim 1, wherein said at least one gas diffusion layer comprises predominately hydrophilic modified carbon products on a first side and predominately hydrophobic modified carbon products on a second side opposite said first side.

21. A fuel cell as recited in claim 1, wherein said gas diffusion layer comprises a major planar surface and comprises a chemical gradient lateral to said planar surface.

22. A fuel cell as recited in claim 1, wherein said gas diffusion layer comprises a major planar surface and comprises a chemical gradient prependicular to said planar surface.

23. A method for the fabrication of a gas diffusion layer in fuel cell, comprising the steps of:

a) providing a carbon support;

b) depositing a first ink composition comprising a first modified carbon product on said carbon support to form a first sublayer; and

c) depositing a second ink composition comprising a second modified carbon product over said first sublayer, wherein said first modified carbon product and said second modified carbon product have different hydrophilic properties.

24. A method as recited in claim 23, wherein said depositing steps comprise depositing with an ink-jet device.

25. A method as recited in claim 23, wherein said depositing steps comprise depositing using a method selected from the group consisting of spraying, analog deposition, lithography, flexographic printing, slot die, roll-coating, xerography and electrostatic printing.

26. A method as recited in claim 23, wherein at least one of said first and second modified carbon products comprises graphite.

27. A method as recited in claim 23, wherein at least one of said first and second modified carbon products comprises carbon black.

28. A method for the fabrication of a gas diffusion layer, comprising the steps of:

a) providing a carbon support; and

b) contacting said carbon support with a diazonium salt; and

c) treating said diazonium salt to covalently bond a functional group to said carbon support and form a modified carbon support product.

29. A method as recited in claim 28, wherein said carbon support comprises carbon cloth.

30. A method as recited in claim 28, wherein said carbon support comprises carbon paper.

31. A method as recited in claim 28, wherein said functional group is a hydrophilic group.

32. A method are recited in claim 28, wherein said functional group is a hydrophobic group.