US20050221139A1

US20050221139A1 - Modified carbon products, their use in bipolar plates and similar devices and methods relating to same

Info

Publication number: US20050221139A1
Application number: US11/081,754
Authority: US
Inventors: Mark Hampden-Smith; Paolina Atanassova; James Caruso; Gordon Rice; Rimple 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: 2005-10-06
Also published as: WO2005091416A3; US20050221141A1; WO2005091416A2; US20050233183A1; KR100891337B1; JP2007535787A; KR20060125918A; CA2560069C; CA2560069A1; US8227117B2; US20050233203A1; EP1726018A2

Abstract

Bipolar plates incorporating modified carbon products. The modified carbon products advantageously enhance the properties of the bipolar plates, leading to more efficiency within a fuel cell or a similar device.

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 Ser. No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIR USE IN ELECTROCATALYSTS AND ELECTRODE LAYERS AND SIMILAR DEVICES AND METHODS RELATING TO THE SAME”, filed on Mar. 15, 2005, and further identified by Attorney File No. 41890-01745, and U.S. patent application Ser. No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIR USE IN FLUID/GAS DIFFUSION LAYERS AND SIMILAR DEVICES AND METHODS RELATING TO THE SAME”, filed on Mar. 15, 2005, and further identified by Attorney File No. 41890-01744, and U.S. patent application Ser. No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIR USE IN 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 is hereby incorporated herein by reference in its entirety.

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 bipolar plates incorporating modified carbon products and methods for making bipolar plates including modified carbon products. The modified carbon products can be used to enhance and tailor the properties of the bipolar plates.
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)
Bipolar plates are utilized to provide reactants, remove products and transport electrons to the circuit of the fuel cell. Bipolar plates are also utilized to prevent fluid communication between adjacent MEAs. There are several different types of bipolar plates utilized in fuel cells, differentiated by their material of construction and the manufacturing method. These include: 1) Graphite-filled Polymer—a molded graphite-loaded polymer/resin; 2) Machined Graphite—a machined single piece of graphite; 3) Expanded Foam—typically expanded porous metal foams; and 4) Stamped metal—a stamped metallic plate.
Bipolar plates are described in more detail in U.S. Pat. No. 5,776,624 by Neutzler and U.S. Pat. No. 6,255,012 by Wilson et al., each of which is incorporated herein by reference in its entirety.
Bipolar plates and similar structures are referred to in the literature by many different names, such as flow field plates, gas distribution manifolds, gas inlet manifolds, monopolar plates, collector plates, fuel manifolds and the like. As used herein, the term bipolar plate includes all such structures.
Some bipolar plates are manufactured from a resin compound, which is molded to form the bipolar plate. Most resin compounds are not very conductive and tend to decrease the electrical conductivity of the bipolar plates, thereby resulting in decreased performance of the fuel cell. Attempts have been made to add carbon materials and other additives to such resin compounds to increase electrical conductivity, but low solids loading levels are necessitated to maintain the viscosity of the resin compound within a useful range. Incorporation of such carbon materials can also increase the brittleness of the bipolar plate.
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 stack including a plurality of membrane electrode assemblies and at least one bipolar plate separating the membrane electrode assemblies is provided, where the bipolar plate comprises at least a first modified carbon product. In one embodiment of the present aspect, the first modified carbon product comprises a hydrophilic functional group. In another embodiment of the present aspect, the first modified carbon product comprises a hydrophobic functional group. In one embodiment, the bipolar plate includes a gradient hydrophilic structure. In another embodiment, the bipolar plate includes a gradient hydrophilic structure that is perpendicular to a major planar surface of the bipolar plate. In yet another embodiment, the bipolar plate includes a gradient hydrophilic structure that is lateral to a major planar surface of the bipolar plate. In one embodiment, the modified carbon product has been surface modified by the direct reaction of a diazonium salt having a general formula of YRN≡N⁺X⁻, where X is an anion, R is a linking group and Y is a functional group. In one embodiment, Y is a hydrophilic functional group. In another embodiment, Y is a hydrophobic functional group. In one embodiment, the bipolar plate includes a printed hydrophilic layer comprising the modified carbon product. In yet another embodiment, the hydrophobic layer is printed with a direct write tool. In one embodiment, the hydrophobic layer is digitally printed. In another embodiment, the modified carbon product includes a surface-modifying group selected from the group of saturated and unsaturated cyclics and aliphatics.
According to another aspect of the present invention, a method for treating a current collector having at least a first carbonaceous surface to increase the hydrophobicity of the first carbonaceous surface is provided, the method including the step of covalently bonding a surface-modifying group to the first carbonaceous surface.
According to another aspect of the present invention, a method for treating a current collector having at least a first carbonaceous surface to decrease the hydrophobicity of the first carbonaceous surface is provided, the method including the step of covalently bonding a surface-modifying group to the first carbonaceous surface.
According to get another aspect of the present invention, a method for treating a gas distribution system having at least a first carbonaceous surface to increase the hydrophobicity of the first carbonaceous surface is provided, the method including the step of covalently bonding a surface-modifying group to the first carbonaceous surface.
According to get another aspect of the present invention, a method for treating a gas distribution system having at least a first carbonaceous surface to decrease the hydrophobicity of the first carbonaceous surface is provided, the method including the step of covalently bonding a surface-modifying group to the first carbonaceous surface.

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 mixing a modified carbon product with a resin compound to form a bipolar plate according to an embodiment of the present invention.
FIG. 7 illustrates modified carbon product fibers and a bipolar plate according to an embodiment of the present invention.
FIG. 8 illustrates the surface modification of a bipolar plate in accordance with an embodiment of the present invention.
FIG. 9 illustrates the surface modification of a bipolar plate in accordance with an embodiment of the present invention.
FIG. 10 illustrates the surface modification of a bipolar plate in accordance with an embodiment of the present invention.
FIG. 11 illustrates the use of modified carbon products to decrease cracking during drying as compared to the prior art and according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to fuel cell components that incorporate modified carbon products. Specifically, the present invention relates to bipolar plates that incorporate and utilize modified carbon products. The use of such modified carbon products enables the production of bipolar plates having enhanced properties. For example, modified carbon products can be utilized in bipolar plates to enhance mass transport properties and electrical conductivity.
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 I


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 III.

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 μm. 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 arylmethylpyrazolone 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. No. 2,502,254 to Glassman, U.S. Pat. No. 2,514,236 to Glassman, U.S. Pat. No. 2,514,236 to Glassman, PCT Patent Application No. WO 92/13983 to Centre National De La 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.
Modified Carbon Products and Bipolar Plates
As noted above, the bipolar plates are generally utilized to provide reactants (e.g., fuel and oxidant), remove products (e.g., water) and conduct electrons. In addition, bipolar plates also provide mechanical support for the MEAs, and conduct heat out of the cell. According to the present invention, modified carbon products can be used to tailor the channel structure, hydrophilic/hydrophobic properties, thermal conductivity, electrical conductivity, physical and chemical integrity, structural rigidity and manufacturability of the bipolar plate.
The channel structure of the bipolar plates is typically serpentine in shape and enables the introduction of reactants into the fuel cell stack, directing them to the gas/fluid diffusion layer. In one aspect, modified carbon products are utilized to tailor the properties of the channel structure for efficient transport of the reactants to the gas/fluid diffusion layer, as described in further detail below.
The hydrophilic/hydrophobic properties of the bipolar plate impact water removal, both on the anode and cathode side of the MEA. Water is typically introduced into the fuel cell by humidification of the fuel gas. On the cathode side, the generated water must be removed via the bipolar plate. In one aspect of the present invention, modified carbon products are utilized to tailor the properties of the bipolar plate to efficiently permit introduction of humid and/or polar fuels at the anode side, while enabling efficient removal of water from the cathode side.
High thermal conductivity is important for removing excess heat generated by the fuel cell stack, which affects fuel cell performance. Accordingly, in one aspect of the present invention, modified carbon products are utilized to enhance the thermal conductivity of the bipolar plate.
Good electrical conductivity is desirable and impacts the ability to supply generated electrons to the external load. In one aspect of the present invention, modified carbon products are utilized to increase and/or maintain the electrical conductivity of the bipolar plate.
Physical and chemical integrity of the bipolar plate is important to control and/or maintain the properties of the bipolar plate over time, such as the hydrophobicity/hydrophilicity, electrical conductivity and structural rigidity of the bipolar plate. In one aspect of the present invention, modified carbon products are utilized to maintain the physical and chemical integrity of the bipolar plate over time.
Structural integrity is important to support adjacent MEAs and the fuel cell stack. In one aspect of the present invention, modified carbon products are used to increase the structural integrity of the bipolar plate. A bipolar plate must be also be reproducible, reliable and relatively low cost to manufacture. Use of modified carbon products in the manufacture of bipolar plates can enable reproducible results and reliability while reducing manufacturing cost.
In accordance with the foregoing, modified carbon products can be utilized in a bipolar plate by: (a) incorporating modified carbon products in compounds used to manufacture the bipolar plate, like a polymer/resin compound; or (b) direct surface modification of a carbonaceous bipolar plate. Such modified carbon products can enhance the properties of the bipolar plates as necessary to achieve the desired effect.
It is common to manufacture bipolar plates by molding or extruding a carbon-filled polymer/resin composite. In one embodiment of the present invention, modified carbon products are used as fillers in the polymer/resin based compound utilized to produce the bipolar plates, as is schematically illustrated in FIG. 6, where a modified carbon product is mixed with a resin compound and then molded into bipolar plate. This approach enables the impartation of a number of unique attributes to the bipolar plate. It will be appreciated that the term “resin compound” is used for illustrative purposes only, and that any compound utilized in the production of bipolar plates can be utilized in accordance with the present invention
According to the present approach, any carbon material that is capable of having a surface group attached to it can be used. In one embodiment, electrically conductive crystalline carbon materials are utilized. In another embodiment, amorphous carbon materials are utilized.
In one aspect of the present invention, surface groups that increase the dispersability of the modified carbon product in the resin compound are utilized. Polymers that can be utilized in the fabrication of carbon polymer/resin filled bipolar plates according to the present invention are given in Table VI.

TABLE VI

Examples of Resins Compounds

Polyvinylidenfluoride (PVDF)

Polyphenylene

Polypropylene

Polyphenylene Sulfide

Phenolic Resins

Vinyl Esters
Surface groups useful in accordance with this embodiment include those having polymeric groups miscible in the above resin compounds, such as a polymeric functional group and/or a polymeric linking group as listed in Tables III and IV, above. Particularly, preferred surface groups include those having PEG (polyethylene glycol), PEO (polyethylene oxide), PPO (polypropylene oxide) and polystyrene groups.
Generally, a resin compound becomes very viscous when the solids loading is too high. This complicates downstream processing (e.g., molding or extrusion) of the bipolar plates, as the viscous composite will have difficulty flowing to completely fill the interior of the mold, resulting in a poor “fill-factor”. Typically, carbon materials are immiscible in such resin compounds, and, therefore, are unable to be effectively utilized in the production of bipolar plates.
Incorporating modified carbon products in a resin compound according to the present invention can reduce viscosity changes in the resin compound, even at high solids loading. Maintaining viscosity at high solids loading increases the fill-factor in the molding process and simplifies the manufacture of the bipolar plate. Increasing the fill-factor also permits better control over the dimensional tolerances of the bipolar plate, which can enable production of narrower, more closely spaced channels and grooves. According to one aspect of the present invention, solids loading of modified carbon products in a resin compound can be as high as 70 wt. % carbon.
By way of illustration, native carbon blacks have a non-polar surface. A modified carbon product including carbon black and a hydrophilic surface group (e.g., one having a sulfuric functional group) can be dispersed at high loading levels in a polar phenolic resin. The resulting resin containing the modified carbon products will have a low viscosity as compared to a similar resin incorporating non-modified carbon black.
In another aspect of the present invention, the viscosity of the resin compound can be controlled by utilizing well-controlled modified carbon product particles and size distributions, such as controlled modified carbon product particle shapes (e.g., spherical) and size distributions. It is often important to tailor the particle size of a filler to maximize solids loading and reduce viscosity. In one embodiment, spray-processing methods are utilized to produce spherical, non-agglomerated carbon-based particulates, which may be subsequently utilized in native (non-modified) form or as modified carbon product particles in a resin compound to produce a bipolar plate. One particular embodiment in this regard is illustrated in FIG. 6, where a modified carbon product is mixed with a resin compound, which is subsequently molded to create a surface modified bipolar plate.
By way of illustration, a modified carbon product (e.g., a carbon black including a hydrophilic surface group) can be processed by spray processing to make spherical aggregate modified carbon product particles having a narrow and monomodal particle size distribution (e.g., 2 μm). This modified carbon black product particles can be added to a phenolic resin, and the resulting resin can be molded and cured to form a bipolar plate.
Native carbon fibers are also generally hydrophobic. In another example, a modified carbon product including a hydrophilic surface group attached to the carbon fiber can be dispersed at high loading levels into a phenolic resin. In this regard, because the fibers are anisotropic, they will tend to overlap one another in the bipolar plate, leading to a well-defined electron conduction path as illustrated in FIG. 7.
It will be appreciated that modified carbon products can be utilized in a resin compound to increase solids loading, by themselves or in the presence of other, non-modified carbon material, to produce resin compound filler blends having controlled particle size distribution, and mono-, bi- and/or tri-modal particle size distributions.
By way of illustration, a modified carbon product (e.g., a carbon black including a hydrophilic surface group) can be processed through a spray-pyrolysis reactor to make spherical aggregate modified carbon product particles having a narrow and monomodal particle size distribution (2 μm). This modified carbon black product can be added to a graphite powder in a phenolic resin, and the resulting mixture can be molded and cured to form a bipolar plate.
In another embodiment, modified carbon products are utilized in a resin compound to maintain and/or increase the structural integrity of the bipolar plates. Traditionally, high solids loading in a resin compound not only increases viscosity, as described above, but the resultant bipolar plate is generally more brittle and fragile. The use of modified carbon products in conjunction with a resin compound according to the present invention can advantageously enhance the physical and chemical properties of the bipolar plates to decrease their brittleness and fragileness.
According to a preferred embodiment of the present invention, modified carbon products having surface groups that can chemically or physically bond to the resin itself, such as hydrophilic and/or long-chained polymeric functional and/or linking groups, are utilized to increase the mechanical strength of the bipolar plates.
By way of illustration, a modified carbon product, such as modified carbon black including a polymeric surface group, such as PEG, can be mixed into a polar resin compound, such as a phenolic resin. A bipolar plate manufactured using the modified carbon product-containing resin compound will evidence an increased mechanical strength due to chemical effects (e.g., hydrogen bonding) and/or physical interactions of the surface group with the resin (e.g., physically intertwinement of the surface group with the resin material).
In yet another approach, modified carbon products including hydrophilic and/or hydrophobic surface groups are utilized to tailor mass transport properties of the bipolar plate. Generally, the bipolar plate is a porous carbonaceous product exhibiting hydrophobic properties. Utilizing modified carbon products according to the present invention, bipolar plates can be manufactured having selectively tailored properties to facilitate transport of reactants and products to and from the electrodes of the fuel cell.
In one embodiment, modified carbon products including a hydrophilic group are utilized in a bipolar plate to increase its hydrophilic properties. Water can be attracted into the pores of the bipolar plate through use of a hydrophilic modified carbon product as a means of extracting the water from the MEAs and the fuel cell stack on the cathode side. Advantageously, the retention of water in the porous bipolar plate also forms a wet seal, thereby substantially preventing reactant and/or product gases from traveling through the bipolar plate.
According to another embodiment, modified carbon products are utilized to change the surface properties of the bipolar plate. When a bipolar plate is manufactured with a resin compound including relatively low amounts of solids, the nature of the bipolar plate surface will be dominated by the properties of the polymer/resin. Generally, as the amount of solids in the resin compound increases a larger percentage of solids will reside at the surface of the bipolar plate. Hence, the hydrophilic and/or hydrophobic properties of the bipolar plate surface can be controlled by increasing or decreasing the amount of modified carbon product in the resin compound.
In one particular embodiment, the modified carbon product includes a hydrophilic surface group and this modified carbon product is incorporated into a resin compound. In a particularly preferred embodiment, the solids loading of the modified carbon product in the resin compound is from about 50 weight percent to about 85 weight percent. In such a case, the hydrophilic properties of the surface of the bipolar plate will be increased. In a particularly preferred embodiment, the modified carbon product includes a graphite material having a hydrophilic surface group.
In yet another embodiment, the modified carbon products include surface groups that facilitate deposition and adhesion to the bipolar plate. For example, a modified carbon product may include both hydrophilic surface groups and a surface group having a polymeric functional and/or linking group. Utilizing the hydrophilic surface group enables the modified carbon product to be more easily dispersed in an aqueous solvent, and, in some instances, tailors the properties of the bipolar plate after deposition. Utilizing the polymeric groups promotes adhesion of the modified carbon product to the bipolar plate by promoting physical and/or chemical interaction with other materials of the bipolar plate. Adhesion to the bipolar plate surface can be further promoted by heating the deposited modified carbon products to intertwine the polymeric groups with the resin utilized in the production of the bipolar plate. Methods for depositing modified carbon products on bipolar plates include analog and digital printing methods, as discussed in further detail below.
In yet another embodiment, a modified carbon product including both a hydrophilic surface group (e.g., one including a sulfuric, carboxylic, or amine functional group) and a long-chained surface group (e.g., one including a polymeric group such as PEG, PPO, and/or PEO group) is utilized in the production of a bipolar plate. In this regard, the hydrophilic properties of the plate can be tailored utilizing the hydrophilic surface groups, as described above, while also increasing the mechanical strength of the bipolar plate.
In another embodiment of the present invention, modified carbon products utilized in a resin compound can be post-processed (e.g., subjected to heat and/or pressure treatment) to make a composite modified carbon-resin composite product. This composite can subsequently be used to produced a bipolar plate, such as by casting and molding.
As noted above, various methods may be utilized to incorporate a modified carbon product in a bipolar plate. One particular method includes the steps of contacting a carbon material with a diazonium salt to form a modified carbon product and incorporating the modified carbon product into the bipolar plate. 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 bipolar plate.
Another method for incorporating a modified carbon product in a bipolar plate, includes the steps of mixing a modified carbon product with another material (e.g., a second modified carbon product, a conventional carbon material, a resin and/or other materials utilized in the production of a bipolar plate) to form a modified carbon-containing mixture and incorporating the mixture into the bipolar plate. 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 bipolar plate, 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 bipolar plate and contacting the carbonaceous material with a diazonium salt to form a modified carbon product in the bipolar plate. 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 bipolar plate.
Gradients
In another approach, modified carbon products are utilized to form gradient structures in the bipolar plates, such as through-plane and/or in-plane gradients. These gradient structures can be produced by layering or molding the parts in a distinct order using varying concentrations of the modified carbon products and/or by utilizing different modified carbon products in the resin compound.
In one embodiment, modified carbon products are utilized to form a hydrophilic and/or hydrophobic gradient in the bipolar plate structure, either in the through-plane or the in-plane. For example, a modified carbon product having a first surface group can be utilized in a first resin compound to create a first portion of the bipolar plate. A modified carbon product having a second surface group can be utilized in the first resin compound and/or a separate second resin compound to create a second portion of the bipolar plate. In a particularly preferred embodiment, the first surface group is a hydrophobic surface group, and the second surface group is a hydrophilic surface group. In another particularly preferred embodiment, the modified carbon product includes graphite. Graphite is preferred due to its conductive nature and its ability to fuse to adjacent layers.
In another embodiment, modified carbon products are deposited on pre-existing bipolar plates to form gradient structures. In this regard, modified carbon products varying in concentration and/or attached surface groups can be utilized in the deposition to create gradient structures in the through-plane. For example, a modified carbon product having a first surface group can be deposited on a first portion of a bipolar plate. A modified carbon product having a second surface group can be deposited on a second portion of the bipolar plate and/or the first deposited portion. In a particularly preferred embodiment, the first surface group is a hydrophobic surface group, and the second surface group is a hydrophilic surface group. In another particularly preferred embodiment, the modified carbon product includes graphite.
In another embodiment, in-plane gradients can be produced utilizing modified carbon products. For example, modified carbon products can be deposited on the channels and/or ribs of the bipolar plate to create gradients thereon. In a particular embodiment, modified carbon products including hydrophilic surface groups are deposited into channels of the bipolar plate. Modified carbon products including hydrophobic surface groups may also be deposited into other channels of the bipolar plate. Thus, the channels of the bipolar pate can be selectively tailored to enhance the transport of reactants/product by deposition of modified carbon products having properties suited for the transport of the desired reactants/product. Advantageously, a selective wet seal can be formed, where the hydrophilic areas are substantially impermeable to gases due to the presence of water, and the hydrophobic areas are more permeable to gases due to the absence of water.
Direct Surface Modification of the Bipolar Plate
Aside from utilizing modified carbon products to tailor the properties of bipolar plates, a diazonium salt can also be used to directly surface modify a carbonaceous material within the bipolar plate. As noted, bipolar plates generally include a non-trivial amount of carbonaceous material to increase the electron conductivity of its structure. In many instances, the bipolar plate includes graphite.
According to one aspect of the present invention, the carbonaceous materials within the bipolar plate are directly surface modified utilizing a diazonium salt. Direct surface modification of the bipolar plate is beneficial as it is an inexpensive and simple means of modifying the properties of the bipolar plate, such as its hydrophilic properties and permeability.
One embodiment of this aspect is illustrated in FIG. 8, where the surface of the bipolar plate is contacted by a diazonium salt, such as by immersion. Another embodiment of this aspect is illustrated in FIGS. 9 and 10, where only a portion of the bipolar plate surface is modified with a surface group, such as by utilizing a mask (FIG. 9), discussed in further detail below, or a direct-write tool (FIG. 10), discussed in further detail below.
In one embodiment, a mask/barrier is utilized on the surface of the bipolar plate to prevent attachment of surface groups. The mask/barrier can be applied by, for example, deposition of a film (e.g., a UV curable polymer) that is inert to chemical attachment. Subsequently, a diazonium salt can be contacted with a surface of the bipolar plate, such that surface groups attach to the exposed carbon surfaces. Subsequently, the mask/barrier can be physically and/or chemically removed (e.g., stripped by acids) resulting in a patterned bipolar plate. This method is useful when analog printing methods, such as dip coating, doctor blading or brushing/painting are used.
In another embodiment, a solution comprising a diazonium salt can be selectively deposited onto discrete portions of the bipolar plate to produce a patterned bipolar plate surface and/or to tailor specific components of the bipolar polar plate. For example, a digital printing method, such as direct-write deposition, can be utilized to deposit an ink solution comprising a diazonium salt onto select portions of the bipolar plate surface, such as the channels and/or ribs of the bipolar plate.
In a particularly preferred embodiment, one portion of the bipolar plate is contacted by a first diazonium salt, and a second portion of the bipolar plate is contacted by a second diazonium salt, thereby resulting in different surface groups attached to discrete surfaces of the bipolar plate. For example, a hydrophilic diazonium salt can be deposited into the channels of the bipolar plate to make them hydrophilic. A hydrophobic diazonium salt can be deposited onto the ribs of the bipolar plate to make them hydrophobic. The resultant bipolar plate thus includes a gradient water pathway to enable rapid water removal.
As noted above, analog methods for depositing diazonium salts generally require the use of the mask to produce a discrete pattern on the surface of the bipolar plate. Digital printing methods generally do not require such a mask, and, therefore, are preferred in many applications requiring the creation of discrete patterns on the bipolar plate surface.
In another embodiment, the bipolar plate surface is modified to enable water retention over long periods of time, in some instances even in relatively dry conditions. Long-term retention of water, such as on the anode side of the fuel cell, enables the fuel cell to begin operating from a “dry” start, thereby obviating and/or minimizing the need for external water at the anode. In this regard, the surface of the bipolar plate can be modified with very hydrophilic surface groups. Additionally, the concentration of hydrophilic surface groups on the surface of the bipolar plate can be increased. Deposition of a diazonium salt onto the surface of the bipolar plate by any of the above-mentioned methods can be utilized in this regard.
It will be appreciated, with respect to the above approaches, aspects and embodiments, that the diazonium salt can be any salt that will enable the formation of appropriate surface group(s) to tailor the properties of bipolar plate. Surface groups including hydrophobic and/or hydrophilic functional groups and/or including a polymeric functional and/or linking group are often preferred. Particularly preferred hydrophilic, hydrophobic and polymeric functional and linking groups are provided above in Tables II, III, IV and V. For example, the bipolar plate can be contacted by a hydrophobic diazonium salt, such as a fluoric salt, to produce hydrophobic surface groups on the bipolar plate surface. The bipolar plate may also or alternatively be contacted by a hydrophilic diazonium salt, such as a sulfuric salt, to produce hydrophilic surface groups on the bipolar plate surface.
The bipolar plate may also or alternatively be contacted by an ionic diazonium salt, such as a sodic and/or potassic salt to produce ionic surface groups on the bipolar plate surface. The bipolar plate also can be contacted by a diazonium salt that will produce an inert surface on the bipolar plate. Inert surface groups useful in this regard include C₅H₄N, (C₆H₄)NH₂, C₆H₅, C₁₀H₇and (C₆H₄)CF₃. It will be appreciated, that the bipolar plate can be contacted with one or more diazonium salts, and for various times to partially or fully modify the bipolar plate surface with one or more surface groups.
Other Features
Metal foams are often utilized in bipolar plates and generally a have a hydrophobic nature. According to one embodiment of the present invention, modified carbon products are impregnated into the pores of the metal foams to increase the hydrophilic properties of the bipolar plates. For example, a modified carbon product including a hydrophilic surface group can be mixed into a resin compound, such as an acrylic polymer, and the modified carbon-containing resin compound can be contacted with a metallic foam. Upon extraction and curing, the resin compound will adhere to the metallic structure, and the modified carbon products contained therein will modify the hydrophilic properties of the metal foam. In this regard, it should be noted that the above described dispersability properties of the modified carbon products are useful in this embodiment, as higher solids loading results in a higher degree of hydrophilic properties being imparted to the metal foam.
In yet another embodiment of the present invention, the surface groups of a modified carbon product and/or the surface groups attached to the bipolar plate undergo subsequent reactions, such as during a curing step. For example, a surface group can be selected that will decompose or partially decompose at selected temperatures, such as temperatures used to cure the resin compound. By way of illustration, a carboxylic functional group is slightly hydrophilic. However, at sufficient temperatures the carboxyl group will decompose to a phenyl group, which is slightly hydrophobic.
Deposition of Modified Carbon Products and/or Diazonium Salts
Use of modified carbon products and/or diazonium salts to produce and/or modify the properties of the various fuel cell components has been described above. Specific techniques for manufacturing such fuel cell components utilizing inks including such modified carbon products and/or diazonium salts are now discussed.
In one aspect, deposition of a modified carbon ink can be utilized to modify the a bipolar plate. 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
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 consisting of electrocatalysts, carbons and polymer solutions (e.g., PFSA or PTFE) and combinations thereof, such as ball milling and sonication. The incorporation of modified carbon products having surface groups that match the solubility requirements of the ink there are dispersed in significantly simplifies ink preparation As a result, the homogeneity and uniformity of the inks, and hence the homogeneity of the deposited layer/feature are increased. Homogenous deposition enables control over the concentration and drying rate of the materials being deposited. For example, a modified carbon product having hydrophilic surface groups simplifies dispersion of carbon-based materials in aqueous-based inks due to increased wetting and dispersability of the modified carbon material. Other surface modifications can be chosen to improve the wettability and dispersability of modified carbon products when organic solutions are used.
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.
Deposition of modified carbon ink preferably modifies the bipolar plate to help tailor the one or more attributes of the bipolar plates. For example, modified carbon inks utilized in the modification of bipolar plates may enable liquid and/or gas transport, such as by tailoring the hydrophilic/hydrophobic properties of the bipolar plate. 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 modify the bipolar plate. 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 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 ink-jet device. Other examples of direct-write deposition tools include aerosol jets and automated syringes, such as the MICROPEN tool, available from Ohmcraft, Inc., of Honeoye Falls, N.Y.
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. patent 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 to enable the use of industrial ink-jet devices. Preferably, the surface tension of the ink is from about 10 to 50 dynes/cm, such as from about 20 to 40 dynes/cm. For use in an ink-jet, the viscosity of the modified carbon ink is preferably not greater than about 50 centipoise (cp), such as in the range of from about 10 cp to about 40 cp. Automated syringes can use compositions having a higher viscosity, such as up to about 5000 cp.
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 inks used in an ink-jet device can also include water and/or an alcohol. Surfactants can also be used to maintain the modified carbon products in the ink. Co-solvents, also known as humectants, can be used to prevent the modified carbon inks from crusting and clogging the orifice of the ink-jet head. Biocides can also be added to prevent bacterial growth over time. Examples of such liquid vehicle compositions for use in an ink-jet are disclosed in U.S. Pat. No. 5,853,470 by Martin et al.; U.S. Pat. No. 5,679,724 by Sacripante et al.; U.S. Pat. No. 5,725,647 by Carlson et al.; U.S. Pat. No. 4,877,451 by Winnik et al.; U.S. Pat. No. 5,837,045 by Johnson et al.; and U.S. Pat. No. 5,837,041 by Bean et al. Each of the foregoing U.S. patents is hereby incorporated herein by reference in its entirety. The selection of such additives is based upon the desired properties of the composition. If necessary, modified carbon products can be mixed with the liquid vehicle using a mill or, for example, an ultrasonic processor. In this regard, it should be noted that modified carbon products that are dispersible in their corresponding solvent (e.g. a modified carbon product having a hydrophilic surface groups in an aqueous solution) may require minimal or no mixing due to their improved dispersability in their corresponding solvents.
The modified carbon inks according to the present invention can also be deposited by aerosol jet deposition. Aerosol jet deposition can enable the formation of features having a feature width of not greater than about 200 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 inks of the present invention can also be deposited by a variety of other techniques including intaglio, roll printer, spraying, dip coating, spin coating and other techniques that direct discrete units, continuous jets or continuous sheets of fluid to a surface. Other printing methods include lithographic and gravure printing.
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 bipolar plate to form a hydrophobic 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 the bipolar plate to modify a the properties of the bipolar plate). According to one embodiment, such deposition techniques are utilized to form a modified bipolar plate.
It will be appreciated that any of the above-noted processes can be utilized in parallel or serial to deposit multiple layers of the same or different modified carbon inks onto a surface, and can be printed in one or more dimensions and in single or multiple deposition steps. In this regard, one embodiment of the present invention is directed to printing multiple layers of modified carbon inks to generate gradients in the bipolar plates.
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/modified electrocatalyst 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 any of the surfaces of the bipolar plates and/or 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 any of the proton exchange membrane, the electrode, the fluid diffusion layers and/or the bipolar plates. 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 bipolar plate, or can be deposited in discrete patterns to produce patterned modified carbon layers.
The incorporation of modified carbon products in a modified carbon ink can also affect the drying characteristics of ink. For example, rapid drying can result in crack formation after deposition. Drying can be slowed by utilizing modified carbon products miscible with the solvent to reduce the vapor pressure of the solvent after deposition. This can be achieved by increased the solids loading of the modified carbon products in the modified carbon ink. In one preferred embodiment, the modified carbon ink has a solids loading of up to about 70 wt. %. Increased solids loading of modified carbon produced results in more uniform drying and less volume fraction of solvents being removed during drying process. In addition, modified carbon products can include a long chain surface group (e.g., polymeric) that can form physical and/or chemical bonds to the solvent species (e.g., water, isopropanol or TEFLON) or adjacent surface groups, resulting in more uniform drying as depicted in FIG. 11, where 1171 is a deposition process using a conventional ink and 1172 is a deposition processing using a modified carbon ink.

EXAMPLES

1. Surface Modification of a Machined Graphite Bipolar Plate with a Hydrophilic Modifying Group
270 ml of deionized water, 6.48 g of a functionalizing (treating) agent (H₂NC₆H₄SO₃H) and 6.75 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., a machined graphite bipolar plate is immersed into the solution and the solution is continually stirred and heated to 50° C. When the temperature reaches 50° C., 12.9 g of a 20 wt. % aqueous sodium nitrite solution is added slowly dropwise. The mixture is then allowed to react at 50° C. for 24 hours. The graphite plate is removed and washed with deionized water three times and dried at 50° C. overnight.
2. A Polymer/Resin Type Bipolar Plate Incorporating a Peg Modified Carbon
90 ml of deionized water, 26.5 g treating agent (aminophenylated polyethylene glycol ether (MW 2119) (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 a mixture of graphite powder (75% from TIMCAL COMPANY, KS75), 22% phenolic thermoset resin (VARCUM 29302 from DUREZ Company) and 3% of the above-described PEG-modified VULCAN XC-72. The mixture is place in a mold and pressed at 500 psi for 30 minutes at a temperature of 400° C.
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 stack comprising a plurality of membrane electrode assemblies and at least one bipolar plate separating said membrane electrode assemblies, wherein said bipolar plate comprises at least a first modified carbon product.

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

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

4. A fuel cell stack as recited in claim 1, wherein said bipolar plate comprises a gradient hydrophilic structure.

5. A fuel cell stack as recited in claim 1, wherein said bipolar plate comprises a gradient hydrophilic structure that is perpendicular to a major planar surface of said bipolar plate.

6. A fuel cell stack as recited in claim 1, wherein said bipolar plate comprises a gradient hydrophilic structure that is lateral to a major planar surface of said bipolar plate.

7. A fuel cell stack as recited in claim 1, wherein said modified carbon product has been surface modified by the direct reaction of a diazonium salt of the general formula YRN≡N⁺X⁻, where X is an anion, R is a linking group and Y is a functional group.

8. A fuel cell stack as recited in claim 7, wherein Y is a hydrophilic functional group.

9. A fuel cell stack as recited in claim 7, wherein Y is a hydrophobic functional group.

10. A fuel cell stack as recited in claim 1, wherein said bipolar plate comprises a printed hydrophilic layer comprising said modified carbon product.

11. A fuel cell stack as recited in claim 1, wherein said bipolar plate comprises a printed hydrophobic layer comprising said modified carbon product.

12. A fuel cell stack as recited in claim 1, wherein said hydrophobic layer is printed with a direct-write tool.

13. A fuel cell stack as recited in claim 1, wherein said hydrophobic layer is digitally printed.

14. A fuel cell stack as recited in claim 1, wherein said modified carbon product comprises a surface-modifying group selected from the group consisting of saturated and unsaturated cyclics and aliphatics.

15. A fuel cell stack as recited in claim 1, wherein said modified carbon product comprises a surface-modifying group selected from the group consisting of halogenated saturated and unsaturated cyclics and aliphatics.

16. A fuel cell stack as recited in claim 1, wherein said modified carbon product comprises a surface-modifying group selected from the group consisting of sulfonic, carboxylic and phosphonic acids and salts.

17. A method for treating a current collector having at least a first carbonaceous surface to increase the hydrophobicity of said first carbonaceous surface, comprising the step of covalently bonding a surface-modifying group to said first carbonaceous surface.

18. A method for treating a current collector having at least a first carbonaceous surface to decrease the hydrophobicity of said first carbonaceous surface, comprising the step of covalently bonding a surface-modifying group to said first carbonaceous surface.

19. A method for treating a gas distribution system comprising at least a first carbonaceous surface to increase the hydrophobicity of said first carbonaceous surface, comprising the step of covalently bonding a surface-modifying group to said first carbonaceous surface.

20. A method for treating a gas distribution system comprising at least a first carbonaceous surface to decrease the hydrophobicity of said first carbonaceous surface, comprising the step of covalently bonding a surface-modifying group to said first carbonaceous surface.