US20050221141A1

US20050221141A1 - Modified carbon products, their use in proton exchange membranes and similar devices and methods relating to the same

Info

Publication number: US20050221141A1
Application number: US11/081,765
Authority: US
Inventors: Mark Hampden-Smith; Paolina Atanassova; Paul Napolitano; Rimple Bhatia; Gordon Rice; James Caruso; 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: CA2560069C; JP2007535787A; EP1726018A2; CA2560069A1; US20050233183A1; KR20060125918A; US20050233203A1; WO2005091416A2; US8227117B2; US20050221139A1; WO2005091416A3; KR100891337B1

Abstract

Proton exchange membranes incorporating modified carbon products. The modified carbon products advantageously enhance the properties of proton exchange membranes, leading to more efficiency within a fuel cell or similar device.

Description

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 BIPOLAR PLATES AND SIMILAR DEVICES AND METHODS RELATING TO THE SAME”, filed on Mar. 15, 2005, and further identified by Attorney File No. 41890-01747. Each of the above referenced patent applications is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

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 proton exchange membranes incorporating modified carbon products and methods for making proton exchange membranes including modified carbon products. The modified carbon products can be used to enhance and tailor the properties of the proton exchange membrane.
2. Description of Related Art
Fuel cells are electrochemical devices that are capable of converting the energy of a chemical reaction into electrical energy without combustion and with virtually no pollution. Fuel cells are unlike batteries in that fuel cells convert chemical energy to electrical energy as the chemical reactants are continuously delivered to the fuel cell. As a result, fuel cells are used to produce a continuous source of electrical energy, and compete with other forms of continuous energy production such as the combustion engine, nuclear power and coal-fired power stations. Different types of fuel cells are categorized by the electrolyte used in the fuel cell. The five main types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid oxide and proton exchange membrane (PEM), also known as polymer electrolyte fuel cells (PEFCs). One particularly useful fuel cell is the proton exchange membrane fuel cell (PEMFC).
A PEMFC typically includes tens to hundreds of MEAs each of which includes a cathode layer and an anode layer. One embodiment of a MEA is illustrated in FIGS. 1(a) and 1(b). One embodiment of a cathode side of an MEA is also depicted in FIG. 2. With references to FIGS. 1(a), 1(b) and 2, the anode electrocatalyst layer 104 and cathode electrocatalyst layer 106 sandwich a proton exchange membrane 102. In some instances, the combined membrane and electrode layer is referred to as a catalyst coated membrane 103. Power is generated when a fuel (e.g., hydrogen gas) is fed into the anode 104 and oxygen (air) 106 is fed into the cathode. In a reaction typically catalyzed by a platinum-based catalyst in the catalyst layer of the anode 104, the hydrogen ionizes to form protons and electrons. The protons are transported through the proton exchange membrane 102 to a catalyst layer on the opposite side of the membrane (the cathode), where another catalyst, typically platinum or a platinum alloy, catalyzes an oxygen-reduction reaction to form water. The reactions can be written as follows:
Anode: 2H₂→4H⁺+4e ⁻ (1)
Cathode: 4H⁺+4e ⁻+O₂→2H₂O (2)
Overall: 2H₂+O₂→2H₂O (3)
Electrons formed at the anode and cathode are routed through bipolar plates 114 connected to an electrical circuit. On either side of the anode 104 and cathode 106 are porous gas diffusion layers 108, which generally comprise a carbon support layer 107 and a microporous layer 109, that help enable the transport of reactants (H₂and O₂when hydrogen gas is the fuel) to the anode and the cathode. On the anode side, fuel flow channels 110 may be provided for the transport of fuel, while on the cathode side, oxidizer flow channels 112 may be provided for the transport of an oxidant. These channels may be located in the bipolar plates 114. Finally, cooling water passages 116 can be provided adjacent to or integral with the bipolar plates for cooling the MEA/fuel cell.
A particularly preferred fuel cell for portable applications, due to its compact construction, power density, efficiency and operating temperature, is a PEMFC that can utilize methanol (CH₃OH) directly without the use of a fuel reformer to convert the methanol to H₂. This type of fuel cell is typically referred to as a direct methanol fuel cell (DMFC). DMFCs are attractive for applications that require relatively low power, because the anode reforms the methanol directly into hydrogen ions that can be delivered to the cathode through the PEM. Other liquid fuels that may also be used in a fuel cell include formic acid, formaldehyde, ethanol and ethylene glycol.
Like a PEMFC, a DMFC also is made of a plurality of membrane electrode assemblies (MEAs). A cross-sectional view of a typical MEA is illustrated in FIG. 3 (not to scale). The MEA 300 comprises a PEM 302, an anode electrocatalyst layer 304, cathode electrocatalyst layer 306, fluid distribution layers 308, and bipolar plates 314. The electrocatalyst layers 304, 306 sandwich the PEM 302 and catalyze the reactions that generate the protons and electrons to power the fuel cell, as shown below. The fluid diffusion layer 308 distributes the reactants and products to and from the electrocatalyst layers 304, 306. The bipolar plates 314 are disposed between the anode and cathode of sequential MEA stacks, and comprise current collectors 317 and fuel and oxidizer flow channels, 310, 312, respectively, for directing the flow of incoming reactant fluid to the appropriate electrode. Two end plates (not shown), similar to the bipolar plates, are used to complete the fuel cell stack.
Operation of the DMFC is similar to a hydrogen-gas based PEMFC, except that methanol is supplied to the anode instead of hydrogen gas. Methanol flows through the fuel flow channels 310 of bipolar plate 314, through the fluid distribution layer 308 and to the anode electrocatalyst layer 304, where it decomposes into carbon dioxide gas, protons and electrons. Oxygen flows through the oxidizer flow channels 312 of the bipolar plate 314, through the fluid distribution layer 308, and to the cathode electrocatalyst layer, where ionized oxygen is produced. Protons from the anode pass through the PEM 302, and recombine with the electrons and ionized oxygen to form water. Carbon dioxide is produced at the anode 304 and is removed through the exhaust of the cell. The foregoing reactions can be written as follows:
Anode: CH₃OH+H₂O→CO₂+6H⁺+6e ⁻ (4)
Cathode: 6H⁺+6e ⁻+ 3/2O₂→3H₂O (5)
Overall: 2CH₃OH+3O₂→2CO₂+6H₂O+energy (6)
There are a number of properties that are required for efficient fuel cell operation. For example, the PEM should have a high proton conductivity to enable efficient transport of protons from the anode to the cathode, be electrically insulative to prevent transport of electrons to the cathode, and act as a robust physical separator. The membrane should also be impermeable to gases. A common proton exchange material currently used is NAFION, a perfluorosulfonic acid (PFSA) polymer available from E.I. duPont deNemours, Wilmington, Del., from which different membrane thicknesses can be formed.
One drawback to current PEMs is that they require a humid fuel to operate efficiently due to the intermittent spacing of the proton exchange materials within the membrane. Thus, water must be present to act as a bridge between such sites for proton transport. Water introduced into and/or produced at the electrodes must be removed from the fuel cell to prevent clogging of the electrode pores, a phenomenon known as “flooding”.
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.
Composite membranes with increased strength have been studied for use in industrial chloro-alkali electrolysis processes. Similar fibril reinforced membranes have been produced for use in PEMFCs. These membranes target the following properties: thin, flat, good mechanical strength, chemical robustness and high water permeability and proton conductivity. Such membranes were developed by companies such as W.L. Gore and Associates and Asahi Glass Co., Ltd., all utilizing polytetrafluoroethylene (PTFE) as the reinforcement agent, and an ion exchange material, such as a sulfonated polymer, to act as the proton conduction sites. Others have utilized inorganic fillers, such as silica, titania or tungstosilicic acid, with solids loadings that range from 5 to 70 weight percent. Fillers such as these have been incorporated into proton conducting and non-proton conducting polymers.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a proton conducting membrane including a modified carbon product and a polymer is provided. In one embodiment of the present aspect, the modified carbon product is a modified carbon black. In another embodiment of the present aspect, the modified carbon product is a modified carbon fiber. In yet another embodiment, the modified carbon product includes a proton conducting functional group. In one embodiment, the proton conducting functional group is selected from the group of SO₃H, CO₂H, PO₃H₂and PO₃MH, where M is a monovalent cation. In another embodiment, the proton conducting functional group is selected from the group of carboxylic acids, sulfonic acids, phosphonic acids and phosphonic acid salts. In one embodiment, the polymer is selected from the group of a sulfonated PTFE and a perfluorosulfonated PTFE. In another embodiment, the polymer is selected from the group of polyvinylidene fluoride (PVDF), acid-doped or derivatized hydrocarbon polymers, such as polybenzimidizole (PBI), polyarylenes, polyetherketones, polysulfones, phosphazenes and polyimides. In one embodiment, the modified carbon product is coated on a surface of the polymer. In another embodiment, the modified carbon product is dispersed within the polymer. In one embodiment, the modified carbon product is adapted to conduct protons in the absence of water. In another embodiment, the modified carbon product is adapted to selectively conduct protons in the presence of other hydrogen-comprising liquid fuels. In yet another embodiment, the modified carbon product is adapted to selectively conduct protons in the presence of methanol or ethanol. In one embodiment, the modified carbon product is adapted to yield an increased mechanical strength without a substantial decrease in proton conductivity. In another embodiment, the proton conducting membrane has a proton conducting group concentration of at least about 5.0 millimoles per milliliter, and, in some instances, a proton conducting group concentration of at least about 5.4 millimoles per milliliter. In yet another embodiment, the proton conducting membrane consists essentially of a modified carbon product.
According to another aspect of the present invention, a method for the fabrication of a proton conducting membrane is provided, the method including the steps of mixing a polymer with a modified carbon product to form a composite mixture and forming the composite mixture into a proton conducting membrane. In one embodiment of the present aspect, the forming step includes extruding the composite mixture. In another embodiment, the forming step comprises casting the composite mixture. In yet another embodiment, the proton conducting membrane has a volume density of proton conducting groups of at least about 5.0 millimoles per milliliter. In one embodiment, the composite mixture includes at least about 20 weight percent modified carbon product, and, in some instances, includes at least about 40 weight percent modified carbon product. In one embodiment, the modified carbon product includes carbon black. In another embodiment, the modified carbon product includes carbon fibers.
In yet another aspect of the present invention, a method for the fabrication of a proton conducting membrane is provided, the method including the steps of providing a modified carbon black product and forming the modified carbon black product into a thin membrane. In one embodiment of the present aspect, the forming step includes analog deposition. In another embodiment of the present aspect, the forming step includes digital deposition. In yet another embodiment, the forming step includes dispersing the modified carbon product in a liquid vehicle and ink-jet printing the modified carbon product. In one embodiment, the modified carbon product includes hydrophilic functional groups.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) and 1(b) illustrate a schematic cross-section of a PEMFC MEA and bipolar plate assembly according to the prior art.
FIG. 2 illustrates a cross-section of the cathode side of an MEA showing the membrane and bipolar plate and O₂, H+ and H₂O transport according to the prior art.
FIG. 3 illustrates a schematic cross-section of a direct methanol fuel cell (DMFC) according to the prior art.
FIG. 4 illustrates a method for modifying a carbon product to form modified carbon according to U.S. Pat. No. 5,900,029 by Belmont et al.
FIGS. 5(a) and 5(b) illustrate functional groups attached to a carbon surface according to one via a diazonium salt in accordance with the present invention.
FIG. 6 illustrates the increase in active species phase size as processing temperature increases.
FIG. 7 illustrates a method for formation of platinum/metal oxide active sites using a modified carbon product according to the present invention.
FIG. 8 illustrates the proton conduction mechanism in a PEM according to the prior art.
FIG. 9 illustrates the proton conduction mechanism in a PEM according an embodiment of the present invention.
FIG. 10 illustrates phosphonic groups incorporated into a PBI membrane according to an embodiment of the present invention.
FIG. 11 illustrates the use of sulphonic acid as a proton conducting functional group according to an embodiment of the present invention.
FIG. 12 illustrates the use of modified carbon products to decrease cracking during drying as compared to the prior art and according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to fuel cell components that incorporate modified carbon products. Specifically, the present invention relates to proton exchange membranes incorporating modified carbon products. The use of such modified carbon products enables the production of proton exchange membrane (PEMs) having enhanced properties. For example, modified carbon products can be utilized to enhance proton conductivity and electrical insulation properties, as well as physical robustness.
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 1, 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 arylmethyl pyrazolone in the presence of carbon black, use of an electrochemical reduction of a diazonium salt, and those disclosed in and by: Tsubakowa in Polym. Sci., Vol. 17, pp 417470, 1992, U.S. Pat. No. 4,014,844 to Vidal et al., U.S. Pat. No. 3,479,300 to Riven et al., U.S. Pat. No. 3,043,708 to Watson et al., U.S. Pat. No. 3,025,259 Watson et al., U.S. Pat. No. 3,335,020 to Borger et al., U.S. Pat. No. 2,502,254 to Glassman, U.S. Pat. No. 2,514,236 to Glassman, U.S. Pat. No. 2,514,236 to Glassman, PCT Patent Application No. WO 92/13983 to Centre National De La Recherché Scientifique, and Delmar et al., J. Am. Chem. Soc. 1992, 114, 5883-5884, each of which is incorporated herein by reference in its entirety.
A particularly preferred process for manufacturing modified carbon product particles according to the present invention involves implementing the Belmont process by spray processing, spray conversion and/or spray pyrolysis, the methods being collectively referred to herein as spray processing. A spray process of this nature is disclosed in commonly-owned U.S. Pat. No. 6,660,680 by Hampden-Smith et al., which is incorporated herein by reference in its entirety.
Spray processing according to the present invention generally includes the steps of: providing a liquid precursor suspension, which includes a carbon material and a diazonium salt or a precursor to a diazonium salt; atomizing the precursor to form dispersed liquid precursor droplets; and removing liquid from the dispersed liquid precursor droplets to form the modified carbon product particles.
Preferably, the spray processing method combines the drying of the diazonium salt and carbon-containing droplets and the conversion of the diazonium precursor salt to a linking group and functional group covalently bound to a carbon surface in one step, where both the removal of the solvent and the conversion of the precursor occur essentially simultaneously. Combined with a short reaction time, this method enables control over the properties of the linking group and functional group bound to the carbon surface. In another embodiment, the spray processing method achieves the drying of the droplets in a first step, and the conversion of the diazonium salt to a linking group and functional group in a distinct second step. By varying reaction time, temperature, type of carbon material and type of precursors, spray processing can produce modified carbon product particles having tailored morphologies and structures that yield improved performance.
Spray processing advantageously enables the modified carbon product particles to be formed while the diazonium salt phase is in intimate contact with the carbon surface, where the diazonium salt is rapidly reacted on the carbon surface. Preferably, the diazonium salt is exposed to an elevated reaction temperature for not more than about 600 seconds, more preferably not more than about 100 seconds and even more preferably not more than about 10 seconds.
Spray processing is also capable of forming an aggregate modified carbon product particle structure. The aggregate modified carbon product particles form as a result of the formation and drying of the droplets during spray processing, and the properties of the structure are influenced by the characteristics of the carbon particles, such as the particle size, particle size distribution and surface area of the carbon particles.
Spray processing methods for modified carbon product particle manufacture according to the present invention can be grouped by reference to several different attributes of the apparatus used to carry out the method. These attributes include: the main gas flow direction (vertical or horizontal); the type of atomizer (submerged ultrasonic, ultrasonic nozzle, two-fluid nozzle, single nozzle pressurized fluid); the type of gas flow (e.g., laminar with no mixing, turbulent with no mixing, co-current of droplets and hot gas, countercurrent of droplets and gas or mixed flow); the type of heating (e.g., hot wall system, hot gas introduction, combined hot gas and hot wall, plasma or flame); and the type of collection system (e.g., cyclone, bag house, electrostatic or settling).
For example, modified carbon product particles can be prepared by starting with a precursor liquid including a protic reaction medium (e.g., an aqueous-based liquid), colloidal carbon and a diazonium salt. The processing temperature of the precursor droplets can be controlled so the diazonium salt reacts, leaving the carbon intact but surface functionalized. The precursor liquid may also or alternatively include an aprotic reaction medium such as acetone, dimethyl formamide, dioxane and the like.
The atomization technique has a significant influence over the characteristics of the modified carbon product particles, such as the spread of the particle size distribution (PSD), as well as the production rate of the particles. In extreme cases, some techniques cannot atomize precursor compositions having only moderate carbon particle loading or high viscosities. Several methods exist for the atomization of precursor compositions containing suspended carbon particulates. These methods include, but are not limited to: ultrasonic transducers (usually at a frequency of 1-3 MHz); ultrasonic nozzles (usually at a frequency of 10-150 KHz); rotary atomizers; two-fluid nozzles; and pressure atomizers.
Ultrasonic transducers are generally submerged in a liquid, and the ultrasonic energy produces atomized droplets on the surface of the liquid. Two basic ultrasonic transducer disc configurations, planar and point source, can be used. Deeper fluid levels can be atomized using a point source configuration since the energy is focused at a point that is some distance above the surface of the transducer. The scale-up of submerged ultrasonic transducers can be accomplished by placing a large number of ultrasonic transducers in an array. Such a system is illustrated in U.S. Pat. No. 6,103,393 by Kodas et al. and U.S. Pat. No. 6,338,809 by Hampden-Smith et al., each of which is incorporated herein by reference in its entirety.
Spray nozzles can also be used, and the scale-up of nozzle systems can be accomplished by either selecting a nozzle with a larger capacity, or by increasing the number of nozzles used in parallel. Typically, the droplets produced by nozzles are larger than those produced by ultrasonic transducers. Particle size is also dependent on the gas flow rate. For a fixed liquid flow rate, an increased airflow decreases the average droplet size and a decreased airflow increases the average droplet size. It is difficult to change droplet size without varying the liquid or airflow rates. However, two-fluid nozzles have the ability to process larger volumes of liquid per unit time than ultrasonic transducers.
Ultrasonic spray nozzles use high frequency energy to atomize a fluid and have some advantages over single or two-fluid nozzles, such as the low velocity of the spray leaving the nozzle and lack of associated gas flow. The nozzles are available with various orifice sizes and orifice diameters that allow the system to be scaled for the desired production capacity. In general, higher frequency nozzles are physically smaller, produce smaller droplets, and have a lower flow capacity than nozzles that operate at lower frequencies. A drawback of ultrasonic nozzle systems is that scaling up the process by increasing the nozzle size increases the average particle size. If a particular modified carbon product particle size is required, then the maximum production rate per nozzle is set. If the desired production rate exceeds the maximum production rate of the nozzle, additional nozzles or additional production units will be required to achieve the desired production rate.
The shape of the atomizing surface determines the shape and spread of the spray pattern. Conical, microspray and flat atomizing surface shapes are available. The conical atomizing surface provides the greatest atomizing capability and has a large spray envelope. The flat atomizing surface provides almost as much flow as the conical, but limits the overall diameter of the spray. The microspray atomizing surface is for very low flow rates where narrow spray patterns are needed. These nozzles are preferred for configurations where minimal gas flow is required in association with the droplets.
Particulate suspensions present several problems with respect to atomization. For example, submerged ultrasonic atomizers re-circulate the suspension through the generation chamber and the suspension concentrates over time. Further, some fraction of the liquid atomizes without carrying the suspended carbon particulates. When using submerged ultrasonic transducers, the transducer discs can become coated with the particles over time. Further, the generation rate of particulate suspensions is very low using submerged ultrasonic transducer discs, due in part to energy being absorbed or reflected by the suspended particles.
For spray drying, an aerosol can be generated using three basic methods. These methods differ in the type of energy used to break the liquid masses into small droplets. Rotary atomizers (utilization of centrifugal energy) make use of spinning liquid droplets off of a rotating wheel or disc. Rotary atomizers are useful for co-current production of droplets in the range of 20 to 150 μm in diameter. Pressure nozzles (utilization of pressure energy) generate droplets by passing a fluid under high pressure through an orifice. These can be used for both co-current and mixed-flow reactor configurations, and typically produce droplets in the size range of 50 to 300 μm. Multiple fluid nozzles, such as a two fluid nozzle, produce droplets by passing a relatively slow moving fluid through an orifice while shearing the fluid stream with a relatively fast moving gas stream. As with pressure nozzles, multiple fluid nozzles can be used with both co-current and mixed-flow spray dryer configurations. This type of nozzle can typically produce droplets in the range of 5 to 200 μm.
For example, two-fluid nozzles are used to produce aerosol sprays in many commercial applications, typically in conjunction with spray drying processes. In a two-fluid nozzle, a low-velocity liquid stream encounters a high-velocity gas stream that generates high shear forces to accomplish atomization of the liquid. A direct result of this interaction is that the droplet size characteristics of the aerosol are dependent on the relative mass flow rates of the liquid precursor and nozzle gas stream. The velocity of the droplets as they leave the generation zone can be quite large which may lead to unacceptable losses due to impaction. The aerosol also leaves the nozzle in a characteristic pattern, typically a flat fan, and this may require that the dimensions of the reactor be sufficiently large to prevent unwanted losses on the walls of the system.
The next step in the process includes the evaporation of the solvent (typically water) as the droplet is heated, resulting in a carbon particle of dried solids and salts. A number of methods to deliver heat to the particle are possible: horizontal hot-wall tubular reactors, spray drier and vertical tubular reactors can be used, as well as plasma, flame and laser reactors. As the carbon particles experience either higher temperature or longer time at a specific temperature, the diazonium salt reacts. Preferably, the temperature and amount of time that the droplets/particles experience can be controlled, and, therefore, the properties of the linking group and functional group formed on the carbon surface can also be controlled.
For example, a horizontal, tubular hot-wall reactor can be used to heat a gas stream to a desired temperature. Energy is delivered to the system by maintaining a fixed boundary temperature at the wall of the reactor and the maximum temperature of the gas is the wall temperature. Heat transfer within a hot wall reactor occurs through the bulk of the gas and buoyant forces that occur naturally in horizontal hot wall reactors aid this transfer. The mixing also helps to improve the radial homogeneity of the gas stream. Passive or active mixing of the gas can also increase the heat transfer rate. The maximum temperature and the heating rate can be controlled independent of the inlet stream with small changes in residence time. The heating rate of the inlet stream can also be controlled using a multi-zone furnace.
The use of a horizontal hot-wall reactor according to the present invention is preferred to produce modified carbon product particles with a size of not greater than about 5 μm. One disadvantage of such reactors is the poor ability to atomize carbon particles when using submerged ultrasonics for atomization.
Alternatively, a horizontal hot-wall reactor can be used with a two-fluid nozzle. This method is preferred for precursor feed streams containing relatively high levels of carbon. A horizontal hot-wall reactor can also be used with ultrasonic nozzles, which allows atomization of precursors containing particulate carbons. However, large droplet size can lead to material loss on reactor walls and other surfaces, making this an expensive method for production of modified carbon product particles.
While horizontal hot-wall reactors are useful according to the present invention, spray processing systems in the configuration of a spray dryer are the generally preferred production method for large quantities of modified carbon product particles. Spray drying is a process where particles are produced by atomizing a precursor to produce droplets and evaporating the liquid to produce a dry aerosol, where thermal decomposition of one or more precursors (e.g., a carbon and/or diazonium salt) may take place to produce the particle. The residence time in the spray dryer is the average time the process gas spends in the drying vessel as calculated by the vessel volume divided by the process gas flow using the outlet gas conditions. The peak excursion temperature (i.e., the reaction temperature) in the spray dryer is the maximum temperature of a particle, averaged throughout its diameter, while the particle is being processed and/or dried. The droplets are heated by supplying a pre-heated carrier gas.
Three types of spray dryer systems are useful for spray drying to form modified carbon product particles according to the present invention. An open system is useful for general spray drying to form modified carbon product particles using air as an aerosol carrier gas and an aqueous feed solution as a precursor. A closed system is useful for spray drying to form modified carbon product particles using an aerosol carrier gas other than air. A closed system is also useful when using a non-aqueous or a semi-non-aqueous solution as a precursor. A semi-closed system, including a self-inertizing system, is useful for spray drying to form modified carbon product particles that require an inert atmosphere and/or precursors that are potentially flammable.
Two spray dryer designs are particularly useful for the production of modified carbon product particles according to the present invention. A co-current spray dryer is useful for production of modified carbon product particles that are sensitive to high temperature excursions (e.g., greater than about 350° C.), or that require a rotary atomizer to generate the aerosol. Mixed-flow spray dryers are useful for producing modified carbon product particles that require relatively high temperature excursions (e.g., greater than about 350° C.), or require turbulent mixing forces. According one embodiment of the present invention, co-current spray-drying is preferred for the manufacture of modified carbon product particles, including modified carbon black.
In a co-current spray dryer, the hot gas is introduced at the top of the unit, where the droplets are generated with any of the above-described atomization techniques. Generally, the maximum temperature that a droplet/particle is exposed to in a co-current spray dryer is the temperature at the outlet of the dryer. Typically, this outlet temperature is limited to about 200° C., although some designs allow for higher temperatures. In addition, since the particles experience the lowest temperature in the beginning of the time-temperature curve and the highest temperature at the end, the possibility of precursor surface diffusion and agglomeration is high.
A mixed-flow spray dryer introduces the hot gas at the top of the unit while precursor droplets are generated near the bottom and directed upwardly. The droplets/particles are forced towards the top of the unit, and then fall and flow back down with the gas, increasing the residence time in the spray dryer. The temperature experienced by the droplets/particles is higher compared to a co-current spray dryer.
These conditions are advantageous for the production of modified carbon product particles having a wide range of surface group concentrations including surface concentrations up to 6 μmol/m²organic groups on carbon. For co-current spray dryers the reaction temperatures can be high enough to enable reaction of the diazonium salt (e.g., between 25° C. and 100° C.). The highest temperature in co-current spray dryers is the inlet temperature (e.g., 180° C.), and the outlet temperature can be as low as 50° C. Therefore, the carbon particles and surface groups reach the highest temperature for a relatively short time, which advantageously reduces migration or surface diffusion of the surface groups. This spike of high temperature can also quickly convert the diazonium salt to the bonded surface group, and is followed by a mild quench since the spray dryer temperature quickly decreases after the maximum temperature is achieved. Thus, the spike-like temperature profile can be advantageous for the generation of highly dispersed surface groups on the surface of the carbon.
The range of useful residence times for producing modified carbon product particles depends on the spray dryer design type, atmosphere used, nozzle configuration, feed liquid inlet temperature and the residual moisture content. In general, residence times for the production of modified carbon product particles can range from less than 3 seconds up to 5 minutes.
For a co-current spray-drying configuration, the range of useful inlet temperatures for producing modified carbon product particles depends on a number of factors, including solids loading and droplet size, atmosphere used and energy required to perform drying and/or reaction of the diazonium salt. Useful inlet temperatures should be sufficiently high to accomplish the drying and/or reaction of the diazonium salt without promoting significant surface diffusion of the surface groups.
In general, the outlet temperature of the spray dryer determines the residual moisture content of the modified carbon product particles. For example, a useful outlet temperature for co-current spray drying according to one embodiment of the present invention is from about 50° C. to about 80° C. Useful inlet temperatures according to the present invention are from about 130° C. to 180° C. The carbon solids (e.g., particulate) loading can be up to about 50 wt. %.
Other equipment that is desirable for producing modified carbon product particles using a spray dryer includes a heater for heating the gas, directly or indirectly, including by thermal, electrical conductive, convective and/or radiant heating. Collection apparatus, such as cyclones, bag/cartridge filters, electrostatic precipitators, and/or various wet collection apparatus, may also be utilized to collect the modified carbon product particles.
In one embodiment of the present invention, spray drying is used to form aggregate modified carbon product particles, wherein the aggregates include more than one modified carbon product particle. In this regard, the individual modified carbon product particles can all have essentially the same surface groups or varying types of modified carbon product particles can be utilized to provide an aggregate with a mixture of surface groups. For example, a first modified carbon product particle within the aggregate can have a hydrophilic surface group and a second modified carbon product particle can have a hydrophobic surface group.
In one aspect, first modified carbon product particles (e.g., modified carbon black particles having a hydrophilic surface group) and second modified carbon product particles (e.g., modified carbon black particles having a hydrophobic surface group) are dispersed in a aqueous precursor solution and spray dried to obtain an aggregate modified carbon product particle having both hydrophilic and hydrophobic properties. The aggregate may include various particle sizes, from nano-sized particles to large, sub-micron size particles.
Moreover, as described below with respect to electrocatalyst materials, the aggregate structure can include smaller primary carbon particles and two or more types of primary particles can be mixed. For example, two or more types of particulate carbon (e.g., amorphous and graphitic carbon) can be combined within the aggregate to tailor the aggregate to the desired electrical and/or oxidation resistant properties.
In this regard, spray drying techniques can be used simply to form the aggregate modified carbon product particles, or to additionally effect a change in the structure of the individual modified carbon product particles. For example, spray processing techniques can be conducted at higher temperatures to effect at least a partial decomposition of the previously attached surface groups, such as those surface groups that are utilized to help the spray processing, but are subsequently not desired in the end-product. The specific temperature for the spray drying process may be chosen depending on the desired outcome, which is a function of the type and stability of the surface groups, the targeted final composition, and the treatment distribution.
Electrocatalyst Materials
Electrocatalysts are used in the fuel cell to facilitate the desired reactions. Particularly preferred electrocatalyst materials useful in accordance with the present invention include those having an active species phase, such as a metal, dispersed on a support phase, such as a carbon material. Such electrocatalyst materials are described in U.S. Pat. No. 6,660,680 by Hampden-Smith et al. As used herein, the terms “electrocatalyst materials”, “electrocatalyst particles” and/or “electrocatalyst powders” and the like refer to such electrocatalyst materials in a non-modified native state.
With respect to electrocatalyst materials, the larger structures formed from the association of discrete carbon particles supporting the dispersed active species phase are referred to as aggregates or aggregate particles, and typically have a size in the range from 0.3 to 100 mm. In addition, the aggregates can further associate into larger “agglomerates”. The aggregate morphology, aggregate size, size distribution and surface area of the electrocatalyst powders are all characteristics that impact the catalyst performance. The aggregate morphology, aggregate size and size distribution determines the packing density, and the surface area determines the type and number of surface adsorption centers where the active species form during synthesis of the electrocatalyst.
The aggregate structure can include smaller primary carbon particles, constituting the support phase. Two or more types of primary particles can be mixed to form the support phase. For example, two or more types of particulate carbon (e.g., amorphous and graphitic carbon) can be combined to form the support phase. The two types of particulate carbon can have different performance characteristics and the combination of the two types in the aggregate structure can enhance the performance of the catalyst.
The carbon support is a major component of the electrocatalysts. To achieve adequate dispersion of the active sites, the carbon support should have a high surface area, a large accessible porous surface area (pore sizes from about 2 nm to about 50 nm preferred), low levels of contaminants that are poisons for either the membrane or the active sites during long term operation of the fuel cell, and good stability with respect to oxidation during the operation of the fuel cell.
Among the forms of carbon available for the support phase, graphitic carbon is preferred for long-term operational stability of fuel cells due to its ability to resist oxidation. Amorphous carbon (e.g., carbon black) is preferred when a smaller crystallite size is desired for the supported active species phase. The carbon support particles typically have sizes in the range of from about 10 nanometers to 5 μm, depending on the nature of the carbon material. However, carbon particulates having sizes up to 25 μm can also be used.
The compositions and ratios of the aggregate particle components can be independently varied, and various combinations of carbons, metals, metal alloys, metal oxides, mixed metal oxides, organometallic compounds and their partial pyrolysis products can be used. The electrocatalyst particles can include two or more different materials as the dispersed active species. As an example, combinations of Ag and MnO_xdispersed on carbon can be useful for some electrocatalytic applications. Other examples of multiple active species are mixtures of metal porphyrins, partially decomposed metal porphyrins, Co and CoO.
The supported electrocatalyst particles preferably include a carbon support phase with at least about 1 weight percent active species phase, more preferably at least about 5 weight percent active species phase and even more preferably at least about 10 weight percent active species phase. In one embodiment, the particles include from about 20 to about 80 weight percent of the active species phase dispersed on the support phase. It has been found that such compositional levels give rise to the most advantageous electrocatalyst properties for many applications. However, the preferred level of the active species supported on the carbon support will depend upon the total surface area of the carbon, the type of active species phase and the application of the electrocatalyst. A carbon support having a low surface area will require a lower percentage of active species on its surface to achieve a similar surface concentration of the active species compared to a support with higher surface area and higher active species loading.
Metal-carbon electrocatalyst particles include a catalytically active species of at least a first metal phase dispersed on a carbon support phase. The metal active species phase can include any metal and the particularly preferred metal will depend upon the application of the powder. The metal phase can be a metal alloy wherein a first metal is alloyed with one or more alloying elements. As used herein, the term metal alloy also includes intermetallic compounds between two or more metals. For example, the term platinum metal phase refers to a platinum alloy or platinum-containing intermetallic compound, as well as pure platinum metal. The metal-carbon electrocatalyst powders can also include two or more metals dispersed on the support phase as separate active species phases.
Preferred metals for the active species include the platinum group metals and noble metals, particularly Pt, Ag, Pd, Ru, Os and their alloys. The metal phase can also include a metal selected from the group consisting of Ni, Rh, Ir, Co, Cr, Mo, W, V, Nb, Al, Ta, Ti, Zr, Hf, Zn, Fe, Cu, Ga, In, Si, Ge, Sn, Y, La, lanthanide metals and combinations or alloys of these metals. Preferred metal alloys include alloys of Pt with other metals, such as Ru, Os, Cr, Ni, Mn and Co. Particularly preferred among these is Pt or PtRu for use in the anode and Pt, PtCrCo or PtNiCo for use in the cathode.
Alternatively, metal oxide-carbon electrocatalyst particles that include a metal oxide active species dispersed on a carbon support phase can be used. The metal oxide can be selected from the oxides of the transition metals, preferably those existing in oxides of variable oxidation states, and most preferably from those having an oxygen deficiency in their crystalline structure. For example, the metal oxide active species can be an oxide of a metal selected from the group consisting of Au, Ag, Pt, Pd, Ni, Co, Rh, Ru, Fe, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti or Al. A particularly preferred metal oxide active species is manganese oxide (MnO_x, where x is 1 to 2). The active species can include a mixture of different oxides, solid solutions of two or more different metal oxides or double oxides. The metal oxides can be stoichiometric or non-stoichiometric and can be mixtures of oxides of one metal having different oxidation states. The metal oxides can also be amorphous.
It is preferred that the average size of the active species is such that the electrocatalyst particles include small single crystals or crystallite clusters, collectively referred to herein as clusters, of the active species dispersed on the support phase. Preferably, the average active species cluster size (diameter) is not greater than about 10 nanometers, more preferably is not greater than about 5 nanometers and even more preferably is not greater than about 3 nanometers. Preferably, the average cluster size of the active species is from about 0.5 to 5 nanometers. Preferably, at least about 50 percent by number, more preferably at least about 60 percent by number and even more preferably at least about 70 percent by number of the active species phase clusters have a size of not greater than about 3 nanometers. Electrocatalyst powders having a dispersed active species phase with such small crystallite clusters advantageously have enhanced catalytic properties as compared to powders including an active species phase having larger clusters.
It should be recognized that the preferred electrocatalyst powders are not mere physical admixtures of different particles, but are comprised of support phase particles that include a dispersed phase of an active species. Preferably, the composition of the aggregate electrocatalyst particles is homogeneous. That is, the different phases of the electrocatalyst are well dispersed within a single aggregate particle. It is also possible to intentionally provide compositional gradients within the individual electrocatalyst aggregate particles. For example, the concentration of the dispersed active species phase in a composite particle can be higher or lower at the surface of the secondary support phase than near the center and gradients corresponding to compositional changes of 10 to 100 weight percent can be obtained. When the aggregate particles are deposited using a direct-write tool, the aggregate particles preferably retain their structural morphology and therefore the functionality of the compositional gradient can be exploited in the device.
In addition, the electrocatalyst powders preferably have a surface area of at least about 25 m²/g, more preferably at least about 90 m²/g and even more preferably at least about 600 m²/g. Surface area is typically measured using the BET nitrogen adsorption method which is indicative of the surface area of the powder, including the surface area of accessible pores on the surface of the particles.
Moreover, many of the desired attributes of modified carbon products may be desired attributes of electrocatalyst, and any of the above-described attributes of the modified carbon products can be acknowledged as being useful in the production, use and application of electrocatalyst materials. For example, particle size, size distribution and spherical nature can be an important factor when utilizing such electrocatalyst materials in an electrocatalyst ink, as described in further detail below.
Manufacture of Electrocatalyst Materials
Electrocatalyst materials may be produced in a variety of ways including impregnation and co-precipitation. One preferred method for preparing particulate electrocatalyst materials is by spray processing, one approach of which is disclosed in U.S. Pat. No. 6,660,680 to Hampden-Smith et al.
Production of electrocatalyst material by spray processing generally involves the steps of: providing a precursor composition which includes a support phase or a precursor to the support phase (e.g., a carbon-containing material) and a precursor to the active species; atomizing the precursor to form a suspension of liquid precursor droplets; and removing liquid from liquid precursor droplets to form the powder. At least one component of the liquid precursor is chemically converted into a desired component of the powder. The drying of the precursors and the conversion to a catalytically active species can be combined in one step, where both the removal of the solvent and the conversion of a precursor to the active species occur essentially simultaneously. Combined with a short reaction time, this enables control over the distribution of the active species on the support, the oxidation state of the active species and the crystallinity of the active species. By varying reaction time, temperature, type of support material and type of precursors, electrocatalyst materials having well-controlled catalyst morphologies and active species structures can be produced, which yield improved catalytic performance.
The precursor composition can include low temperature precursors, such as a molecular metal precursor that has a relatively low decomposition temperature. As used herein, the term molecular metal precursor refers to a molecular compound that includes a metal atom. Examples include organometallics (molecules with carbon-metal bonds), metal organics (molecules containing organic ligands with metal bonds to other types of elements such as oxygen, nitrogen or sulfur) and inorganic compounds such as metal nitrates, metal halides and other metal salts. The molecular metal precursors can be either soluble or insoluble in the precursor composition.
In general, molecular metal precursor compounds that eliminate ligands by a radical mechanism upon conversion to metal are preferred, especially if the species formed are stable radicals, and, therefore, lower the decomposition temperature of that precursor compound.
Furthermore, molecular metal precursors containing ligands that eliminate cleanly upon precursor conversion are preferred because they are not susceptible to carbon contamination or contamination by anionic species such as nitrates. Therefore, preferred precursors for metals include carboxylates, alkoxides or combinations thereof that convert to metals, metal oxides or mixed metal oxides by eliminating small molecules such as carboxylic acid anhydrides, ethers or esters.
Particularly preferred molecular metal precursor compounds are metal precursor compounds containing silver, nickel, platinum, gold, palladium, copper, ruthenium, cobalt and chromium. In one preferred embodiment of the present invention, the molecular metal precursor compound comprises platinum.
Various molecular metal precursors can be used for platinum metal. Preferred molecular precursors for platinum include nitrates, carboxylates, beta-diketonates, and compounds containing metal-carbon bonds. Divalent platinum (II) complexes are particularly preferred. Preferred molecular precursors also include ammonium salts of platinates such as ammonium hexachloro platinate (NH₄)₂PtCl₆, and ammonium tetrachloro platinate (NH₄)₂PtCl₄; sodium and potassium salts of halogeno, pseudohalogeno or nitrito platinates such as potassium hexachloro platinate K₂PtCl₆, sodium tetrachloro platinate Na₂PtCl₄, potassium hexabromo platinate K₂PtBr₆, potassium tetranitrito platinate K₂Pt(NO₂)₄; dihydrogen salts of hydroxo or halogeno platinates such as hexachloro platinic acid H₂PtCl₆, hexabromo platinic acid H₂PtBr₆, dihydrogen hexahydroxo platinate H₂Pt(OH)₆; diammine and tetraammine platinum compounds such as diammine platinum chloride Pt(NH₃)₂Cl₂, tetraammine platinum chloride [Pt(NH₃)₄]Cl₂, tetraammine platinum hydroxide [Pt(NH₃)₄](OH)₂, tetraammine platinum nitrite [Pt(NH₃)₄](NO₂)₂, tetrammine platinum nitrate Pt(NH₃)₄(NO₃)₂, tetrammine platinum bicarbonate [Pt(NH₃)₄](HCO₃)₂, tetraammine platinum tetrachloroplatinate [Pt(NH₃)₄]PtCl₄; platinum diketonates such as platinum (II) 2,4-pentanedionate Pt(C₅H₇O₂)₂; platinum nitrates such as dihydrogen hexahydroxo platinate H₂Pt(OH)₆acidified with nitric acid; other platinum salts such as Pt-sulfite and Pt-oxalate; and platinum salts comprising other N-donor ligands such as [Pt(CN)₆]₄ ⁺.
Modified Electrocatalyst Products
According to one embodiment of the present invention, the modified electrocatalyst products are a subclass of the above-described modified carbon products, and as used herein, modified electrocatalyst products generally refers to an electrocatalyst material having an organic group attached thereto.
In one embodiment of the present invention, a modified electrocatalyst product is provided having an active species phase, a carbon support phase, and an organic surface group covalently bonded to the carbon support phase.
In one preferred embodiment, the active species phase includes a first metal, such as platinum. The active species phase can also include a second metal, such as ruthenium, cobalt, chromium or nickel. The first and second metals can be in metallic, metal oxide or alloy form, as described in further detail below. In yet another embodiment, the active species phase includes at least three metals (e.g., Pt, Ni and Co). The active species phase may be any of the above-mentioned metals or metal oxides utilized in the above-described electrocatalyst materials.
The carbon support material can be any of the above-described materials utilized in a modified carbon product or electrocatalyst material. In one preferred embodiment, the carbon support material is carbon black.
The organic group may include aliphatic groups, cyclic organic groups and organic compounds having an aliphatic portion and a cyclic portion. The organic group can be substituted or unsubstituted and can be branched or unbranched. Generally, as described above, the organic groups include a linking group (R) and a functional group (Y), more generally known as surface groups.
Any of the above-described functional groups (Y) utilized to form a modified carbon product can also be used in the production of a modified electrocatalyst product, including those that are charged (electrostatic), such as sulfonate, carboxylate and tertiary amine salts. Preferred functional groups include those that alter the hydrophobic or hydrophilic nature of the carbon material, such as polar organic groups and groups containing salts, such as tertiary amine salts, including those listed in Tables I and II. Another particularly preferred class of functional groups include those that increase proton conductivity, such as SO₃H, PO₃H₂and others known to be part of the backbone of a proton conducting membrane, including those listed in Table III. Yet another particularly preferred class of functional groups includes compounds that increase steric hindrance and/or physical interaction with other material surfaces, such as such as those listed in Table IV.
Any of the linking groups (R) utilized in the creation of a modified carbon product can also be used in the production of a modified electrocatalyst products, including those that increase the “reach” of the functional group by adding flexibility and degrees of freedom to further increase, for example, proton conduction, steric hindrance and/or physical and/or interaction with other materials, including branched and unbranched materials. Particularly preferred linking groups are listed in Table V, above.
It will be appreciated that, generally, any functional group (Y) can be utilized in conjunction with any linking group (R) to create a modified electrocatalyst product according to the present invention to produce the desired effect within the fuel cell component. It will be also appreciated that any other organic groups disclosed in U.S. Pat. No. 5,900,029 by Belmont et al. can be utilized.
As noted above, modified electrocatalyst products are a subclass of modified carbon products. Thus, many of the desired attributes of modified carbon products are also desired attributes of modified electrocatalyst products, and any of the above-described attributes of modified carbon products can be acknowledged as being useful in the production, use and application of modified electrocatalyst products. For example, particle size, size distribution and spherical nature can be an important factor when utilizing such modified electrocatalyst products in a modified carbon ink, as described in further detail below. Moreover, many of the attributes of electrocatalyst materials are also desired attributes of modified electrocatalyst products and any of the above-described attributes of electrocatalyst materials can be acknowledged as being useful in the production, use and application of modified electrocatalyst products. For example, surface area, average active species cluster size and size distribution, mass ratio of active species phase to carbon support phase, and particle aggregation are important factors in catalytic activity. Other attributes are described below.
The modified electrocatalyst product may include varying concentrations of functional groups, such as from about 0.1 μmol/m²to about 6.0 μmol/m². In a preferred embodiment, the modified electrocatalyst product has a surface group concentration of from about 1.0 μmol/m²to about 4.5 μmol/m², and more preferably of from about 1.5 μmol/m²to about 3.0 μmol/m².
The modified electrocatalyst product may also have more than one functional group and/or linking group attached to the carbon material. In one aspect of the present invention, the modified electrocatalyst product includes a second organic surface group having a second functional group (Y′) attached to the carbon support. In one embodiment, the second functional group (Y′) can be attached to the carbon support via a first linking group (R), which also has the first functional group (Y) attached thereto. Alternatively, the second functional group (Y′) can be attached to the carbon support phase via a separate second linking group (R′). In this regard, any of the above referenced organic groups can be attached as the first and/or second organic surface groups, and in any combination.
In a particular embodiment, the first organic surface group includes a first proton conductive functional group, such as a sulfuric and/or carboxylic group, and the second organic surface group includes a second proton conductive functional group, such as a phosphoric group. The use of two different proton conducting functional groups on the same carbon material is useful in circumstances where a wide range of operating conditions may be utilized so one of the proton conducting groups is always functional. This enables a relatively flat rate of proton conduction over a wide range of operating conditions. For example, sulfuric groups are known to fail at temperatures of about 100° C. However, phosphoric groups are capable of conducting protons at temperatures above 100° C. Thus, utilizing a modified carbon product having two different proton conducting functional groups can enable proton conduction over a wide range of temperatures without requiring the incorporation of numerous conventional proton conducting materials in the fuel cell component. Such materials are especially useful in fuel cells utilized in automobiles and other transportation devices where temperatures can widely vary during start-up conditions.
Methods of producing modified electrocatalyst products are described in further detail below. It will be appreciated that many of such methods can be utilized to produce a modified electrocatalyst product having first and second organic surface groups attached thereto. Preferred methods for producing modified electrocatalyst products having two different types of organic surface group attached thereto (a multiply-modified electrocatalyst product) include spray processing and surface contacting techniques, such as immersion and spraying.
In one embodiment, a multiply-modified electrocatalyst product having first and second organic surface groups is produced by spray processing, where a diazonium salt and modified electrocatalyst product having a first organic surface group attached thereto are included in a precursor composition. The precursor composition is subsequently spray processed to attach a second organic group to the carbon support to produce the multiply-modified electrocatalyst product. The multiply-modified electrocatalyst product may then be utilized in the production of a fuel cell component.
In another embodiment, a multiply-modified electrocatalyst product having first and second organic surface groups is produced by placing a modified electrocatalyst product having a first organic surface group attached thereto in a solution comprising a diazonium salt having a second organic group. The second organic group from the diazonium salt will attach to the carbon support to create the multiply-modified electrocatalyst product. The multiply-modified electrocatalyst product may then be utilized in the production of a fuel cell component.
It will be appreciated, that the multiply-modified electrocatalyst product can be formed by modifying with the second organic surface group before or after the multiply-modified electrocatalyst product is incorporated into a component of the fuel cell. For example, a modified electrocatalyst product can be utilized in the production of a fuel cell component. Subsequently, the modified electrocatalyst product can be contacted by a diazonium salt to attach the second organic surface group.
In a particular embodiment, a modified electrocatalyst product can be utilized in the production of an electrode. Subsequently, the electrode can be contacted with a second diazonium salt, such as by immersion and/or spraying, to attach the second organic surface group.
Manufacture of Modified Electrocatalyst Products
Modified electrocatalyst products according to the present invention can be manufactured by any appropriate method, including Impregnation, co-precipitation and other methods utilized by those skilled in the art to make supported electrocatalysts. One preferred method for manufacturing modified electrocatalyst products is spray processing, as described above in reference to modified carbon particles.
When a non-modified carbonaceous material is utilized in a spray processing precursor composition, a dispersant, such as a surfactant, is typically required to enable dispersion and increased loading of the carbonaceous material. Such dispersants typically require high temperature processing to facilitate their removal from the resultant products. Moreover, in the production of electrocatalyst materials, any unremoved dispersants typically poison the active sites.
However, according to the present invention, modified carbon products having surface groups that match the polar or non-polar nature of the precursor liquid composition can be used. Such modified carbon products decrease or eliminate the need for such dispersants as the modified carbon products may be more readily dispersed in the precursor composition. Utilizing modified carbon products may also lower spray processing manufacturing temperatures. Processing at a lower temperature also enables reduction of the active species crystallite size in the electrocatalyst.
As schematically depicted in FIG. 6, and with specific reference to platinum as the active species phase and carbon black as the carbon material, as processing temperature increases, as depicted left to right in the figure, crystallite size increases. Conversely, as temperature decreases, crystallite size also decreases. Reduced crystallite sizes at lower temperatures are also evidenced due to the decreased ability for the active species phase (e.g., platinum) to migrate during the production temperature.
An increased dispersability of the carbon material in the precursor composition also enables an expanded range of carbon products (e.g., graphite) and metal precursors that can be used. Other materials that may be added to the precursor composition include those that do not decompose during processing, such as ionomers (e.g., PTFE) and molecular species (e.g., metal porphryns).
Thus, one approach of the present invention is directed to the production of modified electrocatalyst particles by spray processing utilizing a modified carbon product in the precursor composition. According to one particular aspect, modified electrocatalyst products are produced utilizing spray processing, where the precursor composition includes modified carbon product particles as the support phase and a precursor to the active species.
Preferred modified carbon products useful in accordance with this aspect include those that are miscible in an aqueous precursor composition, including those having polar surface groups, such as those terminating in hydrophilic and/or proton conducting functional groups as listed in Tables I and II, above. Preferred modified carbon products useful in accordance with the present aspect also include those that are miscible in a non-aqueous precursor composition, including those having non-polar surface groups, such those terminating in hydrophobic functional groups as those listed in Table II above. Preferred modified carbon products useful in accordance with this aspect also include those that are readily atomizable to produce an aerosol comprising the modified carbon product.
In a particularly preferred embodiment, the modified carbon product used in the precursor composition is a low-conductivity carbon material (e.g., a carbon black) having a hydrophilic surface group (e.g., a sulfuric terminating functional group). In a particular embodiment, the precursor solution includes from about 5 weight percent to about 15 weight percent of the modified carbon product.
Preferred active species precursors include those listed above for the production of non-modified electrocatalyst materials, including molecular metal precursor compounds, such as organometallics (molecules with carbon-metal bonds), metal organics (molecules containing organic ligands with metal bonds to other types of elements such as oxygen, nitrogen or sulfur) and inorganic compounds such as metal nitrates, metal halides and other metal salts. The molecular metal precursors can be either soluble or insoluble in the precursor composition.
Particularly preferred molecular metal precursor compounds are metal precursor compounds containing nickel, platinum, ruthenium, cobalt and chromium. In one preferred embodiment of the present invention, the molecular metal precursor compound comprises platinum.
Various molecular metal precursors can be used for platinum metal, such as those described above with reference to the molecular metal precursors utilized in the production of electrocatalyst materials. Any known molecular metal precursors can also be utilized for other metals, including molecular metal precursors of ruthenium, nickel, cobalt and chromium. Preferred precursors for ruthenium include ruthenium (III) nitrosyl nitrate [Ru(NO)NO₃)] and ruthenium chloride hydrate. One preferred precursor for nickel is nickel nitrate [(Ni(NO₃)₂]. One preferred precursor for cobalt is cobalt nitrate [Co(NO₃)₂]. One preferred precursor for chromium is chromium nitrate [Cr(NO₃)₃].
In accordance with this aspect, low temperature spray processing conditions can be utilized to produce a modified electrocatalyst product. The processing temperature within the spray processor is preferably less than about 500° C., more preferably les than 400° C., and even more preferably less than 300° C., The residence time within the spray processor is preferably less than about 10 seconds, more preferably less than 5 seconds, and even more preferably less than 3 seconds.
In another embodiment, the precursor composition comprises previously manufactured electrocatalyst materials, such as any of those described above, and a diazonium salt or a diazonium salt precursor. The above-described spray processing methods can be utilized to produce a modified electrocatalyst product based on this precursor composition.
In another aspect of the present invention, spray generation methods are utilized in conjunction with a precursor composition including a modified carbon product to generate an aerosol for use in the spray processing methods. As noted above, dispersants, such as surfactants, have previously been utilized to enable spray processing of non-modified carbon material. Surfactants increase the viscosity and change the surface tension, disallowing the use of certain generation methods like ultrasonic nebulization. In one embodiment, a precursor composition including modified carbon products and/or modified electrocatalyst products is utilized in a spray processing method to produce a modified carbon product, wherein the precursor composition is atomized utilizing ultrasonic generation, a vibrating orifice or spray nozzles.
Formation of Alloyed, Mixed Metal and/or Metal Oxide Modified Electrocatalyst Products
Another aspect of the present invention is directed to the use of modified carbon products to produce modified electrocatalyst particles including alloys or mixed metal/metal oxides as the active species. Traditionally, platinum is alloyed with various elements such as ruthenium, nickel, cobalt or chromium.
Typically, alloys are produced by the deposition of two or more metals or metal oxides on the surface of the carbon. Subsequently, the metals/metal oxides are subjected to a high temperature post-processing step in a reducing atmosphere to alloy the active species. This post-processing step reduces any metal oxides and allows the different species to migrate over the surface and coalesce to form an alloy.
According to one embodiment of the present invention, spray processing techniques are used to produce modified electrocatalyst products having a platinum alloy having a small metal and/or metal oxide crystallite size. Small crystallite sizes are possible because the metal can be bound to a surface group (e.g., such as an electron donating carboxylic and/or amine functional group) and steric hindrance of the surface group prevents large crystallite growth during consecutive steps of metal deposition and/or post-processing or reduction steps. Other electron donating surface groups useful in accordance with the present embodiment include alcohols, ethers, polyalcohols, unsaturated alkyls or aryls, thiols and amines.
By way of illustration, a carbon material, such as carbon black, can be co-modified with carboxylic and amine groups (e.g., (C₆H₄)CO₂H and (C₆H₄)CH₂NH₂). When this carbon material is treated with a metal salt (e.g., RuCl₃), the metal center will bind to the amine functionality on the carbon surface. When this material is subsequently exposed to a second metal salt (e.g. Pt(NH₃)₄(OH)₂or Pt(NH₃)₄(NO₃)₂), it will weakly bind to the carboxylic group of the carbon surface. When this resulting multiply-modified carbon product is heated under mild reducing conditions, a finely dispersed alloy (e.g., a platinum plus ruthenium) electrocatalyst is produced.
Mixing of the alloy constituents (e.g., Pt and Ru) at the atomic level reduces the severity of, or eliminates, the post-processing conditions that are required for alloy formation, which in turn reduces crystallite sintering and improves alloy crystallite homogeneity. In one embodiment, the modified electrocatalyst products including a metal alloy are produced at temperatures not greater than 400° C. During processing, it is preferable to have chemically inert surface groups, such as C₅H₄N, (C₆H₄)NH₂, C₆H₅, C₁₀H₇or (C₆H₄)CF₃, due to pH effects with the precursor salts. Additionally, if both metals have the same affinity for the attached surface group(s), the addition of a homogeneous distribution of a different metal species leads to an even distribution over the carbon surface. Again, such an approach will result in an atomic distribution of the metal species, for example, Pt and Ru, anchored and distributed evenly over the carbon surface. This even distribution enables alloy formation at lower processing temperatures, and, hence, smaller alloy crystallite sizes and better catalytic performance.
According to another embodiment of the present invention, one or more metal or metal oxide precursors can be deposited on the modified carbon product where a post-processing step occurs between the two deposition steps. For example, a metal or metal oxide precursor can be added to a modified carbon support where a surface of the carbon that is not modified preferentially adsorbs the precursor, and after a post-processing decomposition step, first metal or metal oxide clusters are formed. This is followed by deposition of a second precursor, which preferentially adsorbs on the surface of previously formed first metal or metal oxide clusters. After the second decomposition step, a fine composite cluster is formed (e.g., where the second element deposits either as a monolayer or as clusters onto the surface of the first metal clusters).
By way of example, and as depicted in FIG. 7, a modified carbon product having surface groups relatively inert to an active species phase (e.g., SiMe₃) can be utilized to selectively produce an alloyed modified electrocatalyst product. A first metal/metal oxide (e.g., RuO₂) can be deposited on the carbon material surface in locations where the surface groups are not located. After a post-processing step, a second metal/metal oxide (e.g., Pt) can then be selectively deposited, where the second metal/metal oxide preferentially is adsorbed onto the surface of the first metal/metal oxide clusters to form a composite active species phase cluster and/or a thin surface active layer, which may be used as a support for other deposited materials.
It will be appreciated that one or more of the metal or metal oxide species described above can be deposited by any known method, including liquid phase adsorption (as discussed above), spray-based processing, chemical vapor deposition, under potential deposition, electroless deposition or liquid-phase precipitation. In such cases, the carbon can be modified such that the metal/metal oxide precursors in the second deposition process cannot absorb onto anything other than the first metal/metal oxide initially deposited, resulting in improved crystallite dispersion, uniformity of alloy formation and elimination of segregation phenomena.
As described above, alloy electrocatalysts typically require post-processing to ensure a proper degree of alloying to provide long-term stability of the active sites. During this post-processing step, the alloy crystallites have a tendency to sinter, resulting in crystallite growth. Modified carbon products can be used to minimize this effect. With the integration of modified carbons as the support structure, the metal/metal oxide dispersion is significantly increased since the surface modification blocks part of the surface and inhibits migration. This prevents surface diffusion and agglomeration of the alloy clusters during the reduction/alloying step.
For example, modifying the surface of the carbon material with thermally stable steric groups, such as phenyl (C₆H₅) or napthyl (C₁₀H₇) groups acts to physically block migration of metal and metal oxide species across the surface of the carbon. When an electrocatalyst is produced with a greater metal/metal oxide dispersion (i.e., smaller crystallite size), and the species to be alloyed (e.g. mixed metal/metal oxides) requires a lower temperature for alloy formation, a reduced grain growth and better dispersion of the alloy clusters results. Reduced grain growth leads to smaller alloy crystallite size, increased catalytic activity and increased precious metal utilization. Additionally, as described previously, where surface modification groups are intact after the post-processing procedure, they can sterically prevent the metal crystallites from growing by blocking diffusion paths.
Modified Carbon Products and Proton Exchange Membranes
As noted above, modified carbon products can be utilized in proton exchange membranes. The use of modified carbon products and/or diazonium salts in conjunction with a proton exchange membrane are discussed below. It will be appreciated that the various aspects, approaches, and/or embodiments of the present invention described below, are primarily in reference to modified carbon products in proton exchange membranes. However, it will be appreciated that electrocatalyst materials can be utilized in conjunction with modified carbon products in many of such aspects, approaches and/or embodiments, where appropriate, although not specifically mentioned, and the use of such electrocatalyst materials in such aspects, approaches and/or embodiments is expressly within the scope and spirit of the present invention.
Proton Conductivity
In a traditional PEM, protons conduct through a membrane, such as a sulfonated polytetrafluoroethylene (sulfonated PTFE) membrane, by means of protonated water (H₃O⁺) that is hydrogen bonded to the sulfonic acid groups, which in turn are attached to the polymer backbone, as illustrated in FIG. 8. These PEMs generally require the presence of water for efficient proton conductivity.
According to one embodiment of the present invention, a modified carbon product is utilized to increase the amount of proton conducting sites within the PEM. Increasing the concentration of proton conducting sites is beneficial for several reasons, including increased transport of protons across the membrane and a decrease in the amount of water needed to be supplied with the anode reactant. In some situations, elimination of the water supplied with the anode reactant can be achieved, which greatly reduces the complexity and design of the fuel cell. An added benefit is that surface groups having proton conducting functional groups are also generally hydrophilic. Thus, increasing the amount of proton conducting material also increases the water retention capability of the fuel cell, thereby enabling rapid “dry” starts.
Similarly, the functional group can be tailored to increase K_a(proton donating strength, like K_a: —SO₃H>—CO₂H), to form a better proton conductor. In addition, a tailored functional group may also be utilized to introduce different chemical reactivity to the membrane, such as by increasing hydrogen bond strength, which may affect the bonding strength of a modified carbon product to the membrane surface.
The linking group can also vary. Particularly preferred branched and unbranched linking groups are listed in Table V. It will be appreciated that any proton conducting functional group (Y) can be utilized in conjunction with any linking group (R) to create a modified carbon product in accordance with the present invention. In a particular approach, the linking group is tailored to increase the “reach” of the proton-conducting functional group by adding flexibility and degrees of freedom and further increase proton conduction. According to one preferred embodiment, the linking group is selected such that the functional group extends to, but not substantially beyond, the adjacent functional group.
One example of increased proton conductivity utilizing tailored linking groups is the use of a modified carbon black having (C₆H₄)(CH₂)₅SO₃H functional groups. This modified carbon product can be incorporated into a membrane to provide a longer effective “reach” and increased flexibility of the proton conducting group. In the case of a (C₆H₄)(CH₂)₅SO₃H modified carbon product, the saturated (CH₂)₅chain attached to the phenyl ring adds significant length and an increased degree of rotational flexibility about several C—C single bonds, resulting in an increased cone angle. In this situation, the aliphatic chain allows for increased proton conductivity, especially in dry, reduced humidity or non-humidified conditions.
Modified carbon products can be utilized in conjunction with any of a variety of types of PEMs. These include fluorinated PEMs such as sulfonated PTFE, perfluorosulfonated PTFE, polyvinylidene fluoride (PVDF), as well as acid-doped or derivatized hydrocarbon polymers, such as polybenzimidizole (PBI), polyarylenes, polyetherketones, polysulfones, phosphazenes and polyimides and other similar membranes. A schematic illustration of a resulting structure is illustrated in FIG. 9. As illustrated in FIG. 9, a RSO₃H(R=linking group) modified carbon product is added to a sulfonated PTFE membrane, which results in an increased concentration of proton conductive groups.
PBI is a proton conducting membrane that is often used in high-temperature PEMFCs, and generally includes hydrogen-bonded H₃PO₄species in high concentrations. H₃PO₄is typically lost during fuel cell operation as it migrates toward the membrane surface due to electro-osmotic drag, where it subsequently evaporates. In one embodiment of the present invention, the proton conducting modified carbon products, such as those having proton conducting surface groups (e.g., those containing sulfuric and/or phosphonic functional groups), can be incorporated into a PBI membrane to increase the concentration of the proton conducting sites, and hence proton conductivity of the PBI membrane.
In a particularly preferred embodiment, modified carbon products having phosphonic groups are incorporated into a PBI membrane, as is illustrated in FIG. 10. Adding modified carbon products having phosphonic functional groups to PBI-based membranes increases the number of proton conducting sites and increases the hydrogen bonding to the H₃PO₄acid bound within the pores of the membrane. This prevents the PBI membrane from losing H₃PO₄acid over time.
One example of increased proton conductivity due to an increased proton conducting material density in a proton conducting membrane is the use of modified carbon black having (C₆H₄)SO₃H surface groups. Preferably, the concentration of surface groups on the modified carbon black is at least about 3.0 μmol/m², and more preferably at least about 5.0 μmol/m². Preferably, the volume density of proton conducting groups on the modified carbon black is at least about 5.0 mmol/mL, more preferably at least about 5.4 mmol/mL. This modified carbon black, when mixed into a perfluorintated sulfonic acid (PFSA)(e.g., NAFION available from E.I. duPont de Nemours, Wilmington, Del.) solution (e.g., at a 50:50 volume ratio), can be utilized to produce a modified proton exchange membrane. In one embodiment, the volume density of proton conducting groups on the resulting membrane is at least about 5 mmol/mL (about 2.7 mmol/g).
Proton conducting modified carbon products can also be advantageously incorporated into non-proton conducting membranes, such as non acid-doped or derivatized PBI, polyimides, polysulfones, polyphosphazenes, polyetherketones (PEEKs) and polysiloxanes. Despite lacking proton conductivity, such membranes can provide chemical robustness, mechanical strength and good electrical insulation. By modification of such membranes with proton conducting modified carbon products, a modified membrane can be formed having proton conductivity. This is advantageous in cases where a certain set of physical and/or chemical properties are desired that can only be achieved through the use of a non-proton conducting “base” membrane. Furthermore, incorporating modified carbon products into a non-proton conducting membrane enables the disconnection of the proton conductivity of the final membrane from the membrane material utilized during fabrication.
Mechanical Strength Enhancement
Modified carbon products can also be utilized according to the present invention to increase the mechanical strength of the PEM. In one embodiment of the present invention, a modified carbon product having a polymeric functional group is utilized to create physical and/or chemical interactions with the membrane. For example, a modified carbon having a surface group with aliphatic surface groups can physically interact (e.g., intertwine) with the polymer membrane backbone to increase the strength of the membrane. Surface groups useful in accordance with the present invention include acrylics, polypropyleneoxide (PPO), polyethyleneoxide (PEO) polyethylene glycol (PEG) and polystyrene. In a particularly preferred embodiment, the concentration of surface groups on the carbon material is from about 0.1 to about 5 μmoles/m².
According to another embodiment of the present invention, the modified carbon product has at least one surface group that forms a chemical bond to the sulfonate proton exchange membrane. For example, a modified carbon product having an amine-containing (NH₂) surface group can be incorporated into a proton exchange membrane, such as a PTFE membrane. The NH₂groups hydrogen bond to the oxygen-containing sulfonate groups, resulting in a membrane having increased strength. Other surface groups useful in accordance with the present embodiment include SO₃H, PO₃H₂and CO₂H groups.
In another embodiment, modified carbon products having a surface functional group can be incorporated in the raw polymer material prior to forming the membrane to increase mechanical strength of the membrane. For example, a modified carbon product having an amine surface group can be added to a carboxylate-based PTFE membrane and processed to create a mechanically enhanced proton exchange membrane. In this regard, the NH₂groups react with the carboxylate groups on the polymer during polymer formation, forming an amide of the form RC(O)N(H)R and water.
In another embodiment, the surface groups on the carbon materials can be selected to covalently bond to the polymer backbone through a number of means, such as cross-polymerization or condensation. For example, an ethylene-terminated modified carbon product can be added to a batch of perfluorosulfonated PTFE comonomer prior to final membrane formation. Polymerization is initiated and the modified carbon product (through the ethylene surface functional group) co-polymerizes with the co-monomers resulting in a modified composite membrane including carbon covalently bound to the polymer.
It will be appreciated that the foregoing embodiments (e.g., increased proton conductivity and increased mechanical strength) are not mutually exclusive. The modified carbon product can advantageously increase the mechanical strength while the resulting proton conductivity remains unchanged. Furthermore, the addition of proton conducting modified carbon products to the membrane can increase both the mechanical strength and the proton conductivity of the membrane. The modified carbon products can be co-modified with a long chain functional group and a proton-conducting group. That is, the carbon particles can include more than one type of surface modification. For example, carbon black (e.g., VULCAN XC-72, Cabot Corp., Boston Mass.) can be modified with 1 μmol/m²of PEG and also with 3 μmol/m²of (C₆H₄)SO₃H to produce a multiply-modified carbon product. When this multiply-modified carbon product is introduced into a substantially non-proton conducting membrane, such as polyimide, the resultant membrane will evidence enhanced proton conductivity and increased mechanical strength. It will be appreciated that two different modified carbons can be utilized to derive the benefit of two different functional groups. For example, a first modified carbon product having a polymeric functional group (e.g., PEG) and a second modified carbon product having a proton conducting functional group (e.g., (C₆H₄)SO₃H) can be utilized in the production of a proton exchange membrane to produce a membrane with enhanced proton conductivity and mechanical strength.
The thermal stability of the membrane can also be enhanced according to the present invention. Carbon is very thermally stable compared to organic polymers, and increasing the loading of carbon within the PEM will increase the overall thermal stability of the membrane. A thermally stable PEM can be produced by the addition of a thermally stable modified carbon to a standard PEM. Moreover, the PEM can be reinforced with the addition of acicular, rod or whisker like modified carbon products. Alternatively, thermally stable PEM can be produced by the addition of a proton conducting modified carbon product to a non-proton conducting but thermally stable membrane such as, for example, polyimide. In either case, the blending of two or more constituents, such as a membrane and a modified carbon product or a membrane and two different modified carbon products or a carbon material modified with more than one surface group, can result in a unique combination of properties.
Water and Fuel Transport Control, Humidification and Porosity
Water transport is a key issue for PEMs. One feature that the modified PEMs according to the present invention can impart to the MEA structure is the ability to operate at decreased humidification levels due to an increased concentration of proton conducting groups per unit volume. In this regard, one aspect of the present invention relates to modified carbon products including proton conductive functional groups. This leads to a significant overall increase in the concentration of proton conducting sites per unit volume, and as well as increased mobility (reach), which allows protons to migrate through the membrane without water acting as a bridge. Moreover, the proton exchange membrane may also operate at a significantly lower humidification level. The ability to operate a proton exchange membrane in near humid and/or dry conditions greatly simplifies the fuel cell design, especially at high operating temperatures, where the difficulty associated with adding water to the membrane is heightened by the accelerated evaporation rate of the water.
The integration of modified carbon products can significantly affect the humidification requirements and water transport for fuel cell systems utilizing PEMs. In a particular embodiment, a composite modified carbon product/polymer membrane is utilized to increase proton conduction at reduced humidification levels. Moreover, even where the modified membranes do not have a high enough concentration of proton conduction groups to allow fully dry conduction, the addition of strongly hydrophilic groups, such as SO₃H, advantageously decreases the requisite water vapor pressure, allowing for water-based proton transfer at elevated temperatures and/or reduced humidification levels of the reactant gases.
According to another embodiment of the present invention, the average pore size within the polymer membranes is reduced through the use of modified carbon products having polymeric surface groups, as described above, to decrease permeability and, therefore, likelihood of water or fuel transport through the membrane. For example, the use of a modified carbon product having hydrophilic and long chained functional groups attached thereto increases proton conductivity while also forming a denser membrane with a smaller average internal pore size.
The porosity of the PEM can be tailored by selecting appropriate carbon materials, linking groups and functional groups. Porosity can be tailored to prevent and/or inhibit gas, water and/or fuel transport across the membrane. In one embodiment, porosity is tailored by utilizing a high surface area carbon black having a high density of proton conducting groups co-modified with large polymeric groups, such as PEG or PPO having a relatively low electrical conductivity. One such carbon black is KETJEN BLACK (available from Akzo Nobel), which has a surface area of about 1600 m²/g and an electrical resistivity of about 500 to 1500 milliOhm-cm. In a particular embodiment, the carbon black can be modified with a proton conducting group, such as a sulfonate, and long-chained polymeric group, such as PEG, and formulated into a membrane having a small pore size. In one preferred embodiment, the membrane has an average pore size of no greater than 3 angstroms.
As will be appreciated, the small pore size is achieved due to the polymeric groups (e.g., PEG) having significant steric bulk dangling from the carbon surfaces. To enhance/tailor porosity, it is important to utilize such large molecular groups, like PEG or PPO, to act as a binder and “filler” between the carbon materials. Without the addition of such steric inhibitors, it is difficult to achieve a dense membrane suitable for use in a fuel cell. The length of the linking group regarding the proton conducting functionality should be such that it extends to, but not substantially beyond, the next proton-conducting functional group. In one extreme, the resulting membrane is essentially based upon modified carbon products and is proton conducting.
One embodiment of such an approach utilizing sulfonic acid as the proton conducting functional group is depicted schematically illustrated in FIG. 11. Modified carbon products 1124 are protonically connected via the sulfonic acid functional groups 1150 extended by the linking groups (R), resulting in a membrane with increased proton conductivity and decreased porosity.
Direct methanol fuel cells (DMFCs), as well as fuel cells that utilize other hydrogen-containing liquid fuels, like ethanol, also utilize a proton conductive membrane. In a DMFC, methanol migration through the membrane (i.e., methanol cross-over) should be avoided to maintain fuel cell performance. Methanol cross-over results in both low fuel utilization and poisoning of active catalyst sites at the cathode, which significantly decreases the efficiency and performance of the fuel cell.
According to the present invention, methanol cross-over through the membrane can be reduced by the addition of modified carbon products to the membrane. As with PEMFCs, increasing the density of proton conduction sites will allow protons to be transported without the need for water. In addition, decreasing the porosity utilizing modified carbon products having a polymeric surface group, as described above, can also decrease methanol cross-over.
For purposes of illustration, a carbon material, such as carbon black, can be modified with a proton conducting functional group and a functional and/or linking group having substantial steric bulk (e.g., PEG). This modified carbon product can be mixed with another modified carbon product having proton conducting functional groups (e.g., (C₆H₄)SO₃H). This mixture can be added to a perflurosulfonated PTFE polymer co-monomer, and cast to form a modified membrane having an increased proton conductivity and reduced porosity, resulting in lower methanol cross-over levels. Another embodiment of the present invention utilizes modified carbon products to tailor the hydrophilicity/hydrophobicity of the internal pores of the membrane to selectively transport water and inhibit the transport of other molecules, such as methanol.
Fabrication of PEMs Utilizing Modified Carbon Products
There are a variety of approaches to producing PEMs including modified carbon products, which are discussed in further detail below. It will be appreciated that any of the above-described modified carbon products, including modified electrocatalyst products, and/or electrocatalyst materials may be utilized in any of the below-described approaches, aspects and/or embodiments to produce and/or modify a proton exchange membrane. It will also be appreciated that the thickness of the membrane should be suitable for the specific fuel cell application, irrespective of its manufacturing process. Preferably, the thickness of the resultant proton exchange membrane is from about 25 to about 250 microns, more preferably from about 30 microns to about 150 microns, and even more preferably from about 35 microns to about 75 microns.
In one approach, a pre-existing PEM may be contacted with a diazonium salt to attach an organic group to carbon materials contained therein to create a PEM including a modified carbon product. It will be appreciated that any of the below referenced deposition methods can be utilized to directly deposit a diazonium salt onto a carbonaceous surface for the purpose of directly modifying such carbonaceous surface. Additionally, such deposition techniques can be used to modify any carbonaceous materials contained in the PEM, including non-modified carbon materials, electrocatalyst materials, modified carbon products and/or modified electrocatalyst products. It will also be appreciated that such deposition techniques can be utilized to create a uniform modified carbon layer across the entire surface of the proton exchange membrane, or can be deposited in discrete patterns to produce patterned modified carbon layers.
In another approach, modified carbon products may be utilized in the extrusion and casting process to create a modified carbon PEM. In one aspect, a modified carbon product is mixed into a monomer prior to polymerization and casting/extrusion to form the modified carbon PEM. In another aspect, the modified carbon product is mixed into the polymer prior to casting/extrusion to form the PEM. In one embodiment, a modified carbon product is included in the fluid (e.g., monomer or polymer-containing fluid) utilized in the casting and extrusion process to create a modified carbon proton exchange membrane. In a particular embodiment, the modified carbon product is tailored to increases its solubility and/or dispersability in the fluid, such as by utilization of specific functional groups. Such increased solubility and/or dispersability within the fluid enables formation of a composite PEM with uniform distribution of the modified carbon product. In a particular embodiment, the surface group of the modified carbon product is both hydrophilic and proton conducting.
By way of illustration, a modified carbon product including a proton conducting functional group (e.g., (C₆H₄)SO₃H attached to carbon black) can be added to a formulation including a PFSA. This formulation can be fabricated into a membrane by casting or extrusion. In one embodiment, the formulation includes various amounts of modified carbon products, such as at least about 20 wt. %, more preferably at least about 40 wt. % and up to about 70 wt. %. After fabrication, the PEM may include from 5 wt. % to 30 wt. % of a modified carbon product. The resulting membrane, after casting or extrusion, evidences an increased proton conductivity at, or even below, typical PTFE-type humidification levels. In addition, such membranes may also evidence an increased mechanical stability, depending on the amount and type of modified carbon product utilized.
In another example, modified carbon products can be mixed with a non-proton conducting membrane material to form a composite mixture. Preferably, the modified carbon product constitutes at least about 10 vol. % of the dry mixture, up to about 70 vol. % of the dry mixture. The dry mixture can be added to a solvent and blended, if necessary, such as by using a ball mill. Solvent systems can include aqueous solvents, non-aqueous solvents and mixtures thereof. The resulting slurry can be cast and dried to form a membrane. The dried and cast membrane can be pressed (to further reduce thickness) and/or can be heated to an elevated temperature, such as near or slightly above the glass transition temperature (T_g) of the polymer.
In another approach, modified carbon products may be deposited onto or impregnated into a pre-existing PEM. In one aspect, the modified carbon products may be deposited or impregnated using a modified carbon ink, as discussed in further detail below. In one embodiment, the proton exchange membrane may be contacted by a modified carbon ink, such as by immersion or spraying, to impregnate and/or deposit the modified carbon product into/onto the membrane. In a particular embodiment, the modified carbon ink is deposited utilizing a direct-write tool. In another embodiment, the modified carbon ink is sprayed onto a membrane (e.g., a polybenzimidazole membrane) to form a coated membrane. In particular embodiment, the coated membrane can be further contacted with a solution containing additional materials, such as a proton conducting material or polymer backbone material. In a particular embodiment a coated polybenzimidazole membrane is immersed in a phosphoric acid (H₃PO₄) solution to increase the amount of proton conducting groups in the membrane.
In yet another approach, proton-conducting membranes may be fabricated directly from one or more modified carbon products. This approach alleviates the need for conventional polymers, membrane materials and/or binder systems, which eliminates the negative effects of such materials on the PEM. In this regard, it will be appreciated that large chain, polymeric insulating groups can be attached to the carbon materials, essentially coating them and making them electrically insulating, which aids in the production of PEMs without the above described conventional polymers, membrane materials and/or binder systems. Moreover, by selecting the proper combination of proton conducting functional group, linking group and carbon, a PEM having the desired properties can be obtained. In one embodiment, the PEM is fabricated by deposition of a modified carbon ink, as is discussed in further detail below.
As noted above, various methods may be utilized to incorporate a modified carbon product in a proton exchange membrane. One particular method includes the steps of contacting a carbon material with a diazonium salt to form a modified carbon product and incorporating the modified carbon product into the proton exchange membrane. It will be appreciated that more than one type of carbon material and/or diazonium salt may be utilized in this approach to form a plurality of modified carbon products and/or multiply-modified carbon products.
One specific embodiment utilizes spray processing techniques, and includes the steps of providing a precursor composition including a carbon material and a diazonium salt, spray processing the precursor composition to form a modified carbon product, and incorporating the modified carbon product into a proton exchange membrane.
Another specific embodiment includes the steps of providing a precursor composition including an electrocatalyst material and a diazonium salt, spray processing the precursor composition to form a modified electrocatalyst product, and incorporating the modified electrocatalyst product into a proton exchange membrane.
Yet another specific embodiment includes the steps of providing a precursor composition including an existing modified carbon product and an active species precursor, spray processing the precursor composition to form a modified electrocatalyst product, and incorporating the modified electrocatalyst product into a proton exchange membrane.
Another method for incorporating a modified carbon product in a proton exchange membrane, includes the steps of mixing a modified carbon product with another material (e.g., a second modified carbon product, a conventional polymer used in the production of a PEM, an electrocatalyst material, a conventional carbon material, a resin and/or other materials utilized in the production of a proton exchange membrane) to form a modified carbon-containing mixture and incorporating the mixture into the proton exchange membrane. It will be appreciated that more than one type of modified carbon product and other material may be utilized in this approach to form the mixture.
In one specific embodiment, modified carbon products are dispersed in an ink to create a modified carbon ink that may be utilized in the production of a proton exchange membrane, such as by analog or digital printing, as discussed in further detail below.
Yet another method includes the steps of incorporating a carbonaceous material, such as a modified carbon product and/or a conventional carbon material into a proton exchange membrane and contacting the carbonaceous material with a diazonium salt to form a modified carbon product in the proton exchange membrane. It will be appreciated that more than one type of carbonaceous material (e.g., modified carbon product) and/or diazonium salt may be utilized in this approach to form a plurality of modified carbon products and/or multiply-modified carbon products. In a specific embodiment, a diazonium salt is deposited using a direct-write tool, as discussed in further detail below, to form a modified carbon product in the proton exchange membrane.
Interface with Adjacent Components/Adhesion
Another embodiment of the present invention is directed to the incorporation of modified carbon products to improve the interface contact between the proton exchange membrane and the electrode layer. In this regard, the surface group can form a physical or chemical bond between the materials on either the electrode or the proton exchange membrane due to improved dispersability of the modified carbon product, forming a smoother surface and increasing contact area.
By way of illustration, modification of an electrocatalyst material with a hydrophilic functional group enables preparation of a uniform, low viscosity aqueous ink, which can be formed into a smooth, crack-free electrode layer. Such a layer increases contact because the hydrophilic functional groups of the modified electrocatalyst product can hydrogen bond to any adjacent polymer groups of the proton exchange membrane. This improves bonding between the electrode and the proton exchange membrane, thereby decreasing contact resistance and minimizing ohmic losses. Another benefit is reduced delamination and structural deterioration during long term operation of the fuel cell.
Another embodiment of the present invention is directed to the use of modified carbon products to increase the adhesion of between an the proton exchange membrane and adjacent fuel cell component (e.g., the electrode). A proton exchange membrane incorporating modified carbon products may form a physical interlocking bond or a chemical bond to the adjacent component. In this regard, polymeric functional groups and/or linking groups can be utilized to increase adhesion of the gas/fluid diffusion. Such polymeric functional groups can be physically intertwined with the surface of the adjacent component, which increases adhesion. In another embodiment, hydrophilic functional groups can be utilized to increase hydrogen bonding between the proton exchange membrane and the adjacent component. It will be appreciated that both hydrophilic functional groups and/or polymeric functional and/or linking groups can be utilized on a modified carbon product(s) to achieve such properties.
Deposition of Modified Carbon Products and/or Diazonium Salts
In one aspect, modified carbon products may be utilized in a modified carbon ink to produce and/or modified the PEM, such as analog printing (screen, lithographic, roll-coat, slot die and flexographic) and digital printing (e.g., direct-write, spraying, electrostatic, xerographic, laser transfer) methods as is discussed in further detail below. As used herein a “modified carbon ink” refers to any liquid phase solution, such as an ink, resin or paste, that contains one or more of the above-described modified carbon products and/or modified electrocatalyst products. As used herein “electrocatalyst inks” refers to a liquid phase solution, such as an ink, resin or paste, that contains one or more of the above-described electrocatalyst materials.
Various aspects, approaches, and/or embodiments of the present invention are described below, primarily in reference to modified carbon inks. However, it will be appreciated that electrocatalyst inks can be utilized in conjunction with a modified carbon ink in some of such aspects, approaches and/or embodiments, where appropriate, although not specifically mentioned, and the use of such electrocatalyst inks in such aspects, approaches and/or embodiments is expressly within the scope and spirit of the present invention.
The incorporation of modified carbon products in a modified carbon ink significantly improves ink uniformity, homogeneity, ease of manufacture and ease of use. Various methods and mixing techniques are currently utilized to improve the properties of inks that include electrocatalysts, carbons and polymer solutions (e.g., PFSA or PTFE) and combinations thereof, such as ball milling and sonication. The incorporation of modified carbon products having surface groups that match the solubility requirements of the ink there are dispersed in significantly simplifies ink preparation As a result, the homogeneity and uniformity of the inks, and hence the homogeneity of the deposited layer/feature are increased. Homogenous deposition enables control over the concentration and drying rate of the materials being deposited. For example, a modified carbon product having hydrophilic surface groups simplifies dispersion of carbon-based materials in aqueous-based inks due to increased wetting and dispersability of the modified carbon material. Other surface modifications can be chosen to improve the wettability and dispersability of modified carbon products when organic solutions are used.
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 inks preferably produce and/or modify the PEM to enable proton conductivity, physical separation, and/or electrical insulation, after deposition and/or post-processing. In this regard, it should be noted that any combination of surface groups described in U.S. Pat. No. 5,900,029 by Belmont et al. can be utilized in conjunction with any modified carbon product in and modified carbon ink to create and/or modify the proton exchange membrane. Preferably, the modified carbon ink is formulated for deposition (e.g., via analog or digital printing) to maintain a low manufacturing cost while retaining the above noted properties.
The modified carbon inks according to the present invention can be deposited to form patterned or unpatterned layers using a variety of tools and methods. In one embodiment, a modified carbon ink is deposited using a direct-write deposition tool. As used herein, a direct-write deposition tool is a device that can deposit a modified carbon ink onto a surface by ejecting the composition through an orifice toward the surface without the tool being in direct contact with the surface. The direct-write deposition tool is preferably controllable over an x-y grid. One preferred direct-write deposition tool according to the present invention is an ink-jet device. Other examples of direct-write deposition tools include aerosol jets and automated syringes, such as the MICROPEN tool, available from Ohmcraft, Inc., of Honeoye Falls, N.Y.
An ink-jet device operates by generating droplets of a liquid suspension and directing the droplets toward a surface. The position of the ink-jet head is carefully controlled and can be highly automated so that discrete patterns of the modified carbon ink/electrocatalyst ink can be applied to the surface. Ink-jet printers are capable of printing at a rate of 1000 drops per second per jet, or higher, and can print linear features with good resolution at a rate of 10 cm/sec or more, such as up to about 1000 cm/sec. Each drop generated by the ink-jet head includes approximately 25 to 100 picoliters of the suspension/ink that is delivered to the surface. For these and other reasons, ink-jet devices are a highly desirable means for depositing materials onto a surface.
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 below, 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 deposition tool (e.g., an ink jet printer) to form a hydrophobic layer. After the deposited layer is dried (e.g., at about 100° C.), it can be heated (e.g., 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 modified carbon inks on one side or both sides of a surface to form and/or modify a proton exchange membrane. According to one embodiment, such deposition techniques are utilized to modify an existing proton exchange membrane. In another embodiment, such deposition techniques are used to directly create the proton exchange membrane, such as by depositing a modified carbon ink onto another component of the MEA.
It will be appreciated that any of the above-noted processes can be utilized in parallel or serial to deposit multiple layers of the same or different modified carbon inks onto a surface, and can be printed in one or more dimensions and in single or multiple deposition steps. In this regard, one embodiment of the present invention is directed to printing multiple layers of modified carbon inks to generate gradients in the proton exchange membrane.
In one particular embodiment, gradient structures can be prepared that have material properties that transition from very hydrophilic to very hydrophobic, such as by utilizing a plurality of layers including modified carbon products or modified carbon products having varying concentrations of surface groups. In this regard, a first layer may include a modified carbon product that is very hydrophilic, such as a modified carbon product having a hydrophilic terminated surface group attached to the surface (e.g., a sulfuric group). On this first layer, a second, slightly less hydrophilic layer can be formed, such as by using a modified carbon product that has slightly hydrophilic surface groups (e.g., a carboxylic group). A third, hydrophobic layer can be formed on the second layer utilizing a hydrophobic modified carbon product having a hydrophobic surface group. It will be appreciated that in any of these layers, more than one type of surface group can be utilized with the various modified carbon products.
The incorporation of modified carbon products in a modified carbon ink can also affect the drying characteristics of ink. For example, rapid drying can result in crack formation after deposition. Drying can be slowed by utilizing modified carbon products miscible with the solvent to reduce the vapor pressure of the solvent after deposition. This can be achieved by increased the solids loading of the modified carbon products in the modified carbon ink. In one preferred embodiment, the modified carbon ink has a solids loading of up to about 70 wt. %. Increased solids loading of modified carbon produced results in more uniform drying and less volume fraction of solvents being removed during drying process. In addition, modified carbon products can include a long chain surface group (e.g., polymeric) that can form physical and/or chemical bonds to the solvent species (e.g., water, isopropanol or TEFLON) or adjacent surface groups, resulting in more uniform drying as depicted in FIG. 12, where 1271 is a deposition process using a conventional ink and 1272 is a deposition processing using a modified carbon ink.

EXAMPLES

Proton Exchange Membrane Examples

A. Production of PEG-VULCAN XC-72-Reinforced PFSA PEM
90 mL of deionized water, 26.5 g of a treating agent (aminophenylated polyethylene glycol ether, MW˜2119 and having the formula (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 drop wise. The mixture is then allowed to react at 50° C. for 2 hours. When the reaction is substantially complete, the sample is diafiltered using 10 volumes of fresh deionized water to remove any reaction by-products. The resulting PEG-modified VULCAN XC-72 carbon is added to a PFSA water/isopropanol solution to give a modified carbon solids loading of 55 wt. %. The slurry is cast and dried at 80° C. overnight to form a membrane.
B. Increased Proton Conductivity Composite Modified Membrane

Example B-1

To Cast a 70 wt. % SO₃H Modified Carbon Black/PFSA Membrane

To 70 g of (C₆H₄)SO₃H modified VULCAN XC-72 (3 μmols/m²) is added 2000 g of a 5 wt. % PFSA isopropanol/water solution to give a 70 wt % modified carbon black/PFSA suspension. The resulting viscous suspension is mixed for several minutes and poured onto a Teflonized glass plate. The resulting film is doctor bladed to a thickness (wet) of 5 mils. The resulting film is dried in air at room temperature for 24 hours.

Example B-2

To Cast a 15 wt. % SO₃H Modified Carbon Black/PFSA Membrane

To 15 g of (C₆H₄)SO₃H modified VULCAN XC-72 (3 μmols/m²) is added 2000 g of a 5 wt. % PFSA isopropanol/water solution to give a 15 wt % modified carbon black/PFSA suspension. The resulting suspension is mixed for several minutes and poured onto a Teflonized glass plate. The resulting film is doctor bladed to a thickness (wet) of 20 mils. The resulting film is dried in air at room temperature for 24 hours. The resulting membrane has increased mechanical stability and higher proton conductivity at lower humidification levels.
C. Direct Printed Modified Carbon Product/PFSA PEM
A modified carbon suspension containing modified carbon black includes from about 2 to 10 wt. % solids loading of the carbon black, a humectant, viscosity, surface tension modifier and/or a biocide. CAB-O-JET 200 (Cabot Corporation, Boston, Mass.) is a dispersion of 20 wt. % BLACKPEARLS 700, modified with (C₆H₄)SO₃H, in water. The dispersion has a viscosity of 3.8 cP with a surface tension of 75.5 dynes/cm. The acceptable viscosity for ink-jetting with the Spectra ink-jet heads is about 12 cP with a surface tension of about 30 dynes/cm.
Table VII illustrates an example of a low viscosity, ink-jettable modified carbon black/PFSA ink composition.

TABLE VII

Component Wt. %

Modified carbon 5.9

black

PFSA 52.4

Isopropyl 12.6

alcohol

Water 29.1

Total 100.0
The formulation is ink jet printed on the electrode side of a fluid diffusion electrode with a Spectra ink-jet head. The ink-jet head temperature was set at 30° C., fire pulse width of 8 μs, pulse rise and fall time of 3 μs, and firing voltage of 120 V. An ink-jet print is obtained at a speed of 10 inches/sec. The printed film is dried at 80° C. for 3 hours and the printing and drying process is repeated two additional times.
D. Modified Polyimide-Based PEM
A mixture of C₆H₄SO₃H surface functionalized VULCAN XC-72 is mixed with finely divided polyimide to give a final composition 70 wt. % modified carbon black. The mixture is blended in a ball mill for 2 hours and is then suspended in a water/ethanol solvent to form a slurry. The slurry is cast and dried overnight at 80° C. The resulting film is pressed to a thickness of 0.12 mm and heated to a temperature of 250° C. for 2 hours. The resulting modified carbon black polyimide composite membrane has structural integrity and strength and is proton conducting with and without humidification.
E. Printed Modified Carbon Product-Based PEM
A powder batch of VULCAN XC-72 modified with (C₆H₄)SO₃H is suspended in water at a concentration of 5 wt. %. To this suspension is added VULCAN XC-72 that has been modified with polyethylene glycol (PEG), where the ratio of PEG modified carbon to (C₆H₄)SO₃H modified carbon is 5:1 and the total ink solids loading is 5 wt. %. The resulting ink is ink jet printed directly onto an electrode layer, which is supported by a fluid diffusion layer (e.g., a gas diffusion electrode). The printed layer is then dried and heated at 130° C. for 2 hours to remove the solvent and bind the carbon black particles together via the PEG surface molecules. The printing, drying and heating process is repeated two more times.
F. Ink Jet Printed Modified Carbon Product-Based PEM
VULCAN XC-72 that has been co-modified with PEG and (C₆H₄)SO₃H in a 5:1 wt. ratio is dispersed into an isopropanol/water solvent with a total solids content of 5 wt. %. The resulting ink is printed by means of a piezoelectric, drop-on-demand GALAXY ink-jet head (Spectra Corporation) in three passes. The resulting layer is heated at 120° C. for 30 minutes to allow the PEG surface groups to intertwine and to remove the solvent.
G. Ink Jet Printed Modified Carbon Product/TEFLON PEM
To VULCAN XC-72 modified with (C₆H₄)SO₃H groups (3 μmol/m²) is added a NAFION/isopropanol-water solution to give an ink that consists of 5 wt. % carbon where the carbon:NAFION weight ratio is 1:5. The resulting carbon black/NAFION ink is ink jet printed utilizing a piezoelectric drop on demand Galaxy ink jet head (Spectra Corporation) onto the electrode side of a catalyst coated fluid diffusion layer and is subsequently dried at 25° C. for 12 hours. The printing and drying process is repeated two more times.
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 proton conducting membrane, said membrane comprising a modified carbon product and a polymer.

2. A proton conducting membrane as recited in claim 1, wherein said modified carbon product comprises modified carbon black.

3. A proton conducting membrane as recited in claim 1, wherein said modified carbon product comprises a proton conducting functional group.

4. A proton conducting membrane as recited in any of claim 1, wherein said polymer is selected from the group consisting of sulfonated PTFE and perfluorosulfonated PTFE.

5. A proton conducting membrane as recited in claim 1, wherein said polymer is selected from the group consisting of polyvinylidene fluoride (PVDF), acid-doped or derivatized hydrocarbon polymers, such as polybenzimidizole (PBI), polyarylenes, polyetherketones, polysulfones, phosphazenes and polyimides.

6. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is coated on a surface of said polymer.

7. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is dispersed within said polymer.

8. A proton conducting membrane as recited in claim 1, wherein said proton-conducting membrane has a proton conducting group concentration of at least about 5.0 mmol/mL.

9. A proton conducting membrane as recited in claim 1, wherein said proton-conducting membrane has a proton conducting group concentration of at least about 5.4 mmol/mL.

10. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is adapted to conduct protons in the absence of water.

11. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is adapted to selectively conduct protons in the presence of other hydrogen-comprising liquid fuels.

12. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is adapted to selectively conduct protons in the presence of methanol or ethanol.

13. A proton conducting membrane as recited in claim 1, wherein said modified carbon product is adapted to yield an increased mechanical strength without a substantial decrease in proton conductivity.

14. A proton conducting membrane as recited in claim 1, wherein said modified carbon product comprises at least one proton conducting functional group selected from the group consisting of SO₃H, CO₂H, PO₃H₂and PO₃MH, where M is a monovalent cation.

15. A proton conducting membrane, wherein said proton conducting membrane consists essentially of a modified carbon product.

16. A proton conducting membrane as recited in claim 15, wherein said modified carbon product comprises proton-conducting functional groups.

17. A proton conducting membrane as recited in claim 15, wherein said modified carbon product comprises proton-conducting functional groups selected from the group consisting of carboxylic acids, sulfonic acids, phosphonic acids and phosphonic acid salts.

18. A proton conducting membrane as recited in claim 15, wherein said modified carbon product comprises modified carbon black.

19. A proton conducting membrane as recited in claim 15, wherein said modified carbon product comprises modified carbon fibers.

20. A method for the fabrication of a proton conducting membrane, comprising the steps of:

a) mixing a polymer with a modified carbon product to form a composite mixture; and

b) forming said composite mixture into a proton conducting membrane.

21. A method as recited in claim 20, wherein said forming step comprises extruding said composite mixture.

22. A method as recited in claim 20, wherein said forming step comprises casting said composite mixture.

23. A method as recited in claim 20, wherein said proton conducting membrane has a volume density of proton conducting groups of at least about 4.8 mmol/mL.

24. A method as recited in claim 20, wherein said composite mixture comprises at least about 20 wt. % modified carbon product.

25. A method as recited in claim 20, wherein said proton conducting membrane comprises at least about 40 wt. % modified carbon product.

26. A method as recited in claim 20, wherein said carbon product comprises carbon black.

27. A method as recited in claim 20, wherein said carbon product comprises carbon fibers.

28. A method for the fabrication of a proton conducting membrane, comprising the steps of:

a) providing a modified carbon black product; and

b) forming said modified carbon black product into a thin membrane.

29. A method as recited in claim 28, wherein said forming step comprises analog deposition.

30. A method as recited in claim 28, wherein said forming step comprises digital deposition.

31. A method as recited in claim 28, wherein said forming step comprises dispersing said modified carbon product in a liquid vehicle and ink-jet printing said modified carbon product.

32. A method as recited in claim 31, wherein said modified carbon product comprises hydrophilic functional groups.