MXPA01005063A - Fuel cell collector plate and method of fabrication - Google Patents

Abstract

An improved molding composition is provided for compression molding or injection molding a current collector plate for a polymer electrolyte membrane fuel cell. The molding composition is comprised of a polymer resin combined with a low surface area, highly-conductive carbon and/or graphite powder filler. The low viscosity of the thermoplastic resin combined with the reduced filler particle surface area provide a moldable composition which can be fabricated into a current collector plate having improved current collecting capacity vis-a-vis comparable fluoropolymer molding compositions.

Description

FUEL CELLS COLLECTOR PLATE AND MANUFACTURING METHOD FIELD OF THE INVENTION This invention relates in general to compositions and methods for manufacturing structures and coatings of electrically conductive polymer composite, and more particularly to a highly conductive, highly conductive graphite compound suitable for net forming a current collector plate for a fuel cell. BACKGROUND OF THE INVENTION Electrochemical solid electrolyte membrane (MPE) type fuel cells are well known. Generally, the MPE fuel cells comprise an electrode membrane assembly (EME) and a diffusion reinforcing structure interposed between the electrically conductive graphite current collector plates. In operation, multiple individual cells are accommodated to form a stack of fuel cells. When the individual cells are arranged in series to form a stack of fuel cells, the current collector plates are called bipolar collector plates. The collector plates perform multiple functions, including: (1) providing structural support; (2) provide electrical connection between the cells; (3) direct the fuel and oxidizing reagents and / or coolers to the individual cells; (4) distributing reagent streams and / or coolers within individual cells; (5) remove secondary products from the individual cells; and (6) separating fuel gas and oxidant streams between the electrically connected cells. In addition to being electrically conductive, the collector plates must have good mechanical strength, high thermal stability, high resistance to degradation caused by chemical attack and / or hydrolysis, and low permeability to hydrogen gas. Typically, collector plates have intricate patterns formed on their larger surfaces. For example, integral channels can be provided to drive the fuel, oxidant and / or secondary product through the fuel cell. Historically, graphite structures with a desired configuration have been machined from graphite composite materials. Due in part to the expense and the delayed nature of the machining, recent efforts have been focused on the fuel cell manufacturing industry in the development of compositions and methods for producing net molded fuel cell structures, such as bipolar collector plates. , using compression molding and injection molding techniques. These efforts, which have had limited success, have concentrated mainly on molding compositions incorporating fluoropolymer binding materials. For example, bipolar collector plates molded from thermoplastic fluoropolymers, such as vinylidene fluoride, are described in U.S. Patent Nos. 3,801,374; 4,214,969 and 4,988,583. In comparison with other polymeric materials, fluoropolymers have relatively high viscosities. Significantly, the relatively high viscosity associated with fluoropolymers limits their effectiveness as binder materials in molding and coating compositions. In an effort to maximize the electrical conductivity of the current collector plates for fuel cells, it is desirable to maximize the load levels of the electrically conductive filler. Generally, as the percentage of filler particles in a given polymer composition increases, there is a corresponding increase in the viscosity of the composition. Consequently, irrespective of the chosen polymer binder material, the addition of electrically conductive filler must be limited to ensure a minimum degree of flow during processing. These viscosity limitations are particularly pronounced in injection molding applications, wherein the viscosity of the polymer composition must be maintained at a sufficiently low level to allow the composition to move through intricate characteristics of the mold such as channels and ports. In the case of fluoropolymer compositions, the initial high viscosity level associated with the fluoropolymer binder restricts the amount of filler that can be loaded into the binder before processing. Accordingly, the electrical conductivity of the fuel cell collector plates manufactured using fluoropolymer binders is correspondingly limited. For these and other reasons, there is a well-established need for improved compositions and methods for processing highly conductive composite structures for electronic, thermoelectric and electrochemical device applications. SUMMARY OF THE INVENTION It is an object of this invention to provide a composition for manufacturing structures and coatings of electrically and thermally conductive polymer composites for use in very corrosive environments, wherein the electrical conductivity of the resulting structure or coating is improved as a result of an increased filler loading capacity of the composition. It is another object of this invention to provide a composition, and a method for processing said composition, to form a structure or coating composed of thermally and electrically conductive polymer B for use in electronic, thermoelectric and electrochemical devices. It is another object of this invention to provide a non-fluorinated composition for rapidly net casting a current collector plate for a polymer electrolyte membrane fuel cell, wherein the improved filler charge results in a common faith collector plate having greater electrical conductivity and volume than conventional current collector plates made from compositions based on fluoropolymers. These and other objects of the invention are achieved with the novel compositions and methods of the present invention. [0002] Novel polymer compositions are provided to produce highly conductive net-shaped molded coatings and structures for a variety of applications, including: corrosion-resistant electrical and thermal conductors and contacts; electrodes in battery and 20 capacitors; electrodes for electrochemical coating and synthesis of materials; and components of electrochemical devices, such as current collector plates for fuel cells of polymeric electrolyte membrane. Briefly, according to the invention, a high filler polymer composition is provided to make a structure or coating generally ^ B Suitable for use in electronic, thermoelectric and electrochemical devices. In the preferred embodiment of the invention, the composition is particularly convenient for the compression molding and / or injection molding of a current collector plate for a polymer electrolyte membrane fuel cell. The composition is composed of a low viscosity polymer loaded with a ^ Thermal and electrically conductive filler, chemically inert. The polymer is selected from the group of polymers having a melt viscosity of less than 1,000 Newton-seconds per square meter (N * s / m2) over a range of shear rate of 1,000 to 10,000 sec. "1. that he The polymer has properties and characteristics of material as summarized in Table 2 (below). Suitable families of polymers include: polyphenylene sulfide (SPF); modified polyphenylene oxide (OPF); liquid crystal polymer (PCL); polyamide; poly-imide; polyester; phenolic; resin that contains epoxides and vinyl ester. The polymer composition is loaded with a highly conductive filler. In the preferred embodiment of the invention, the filler comprises carbon and / or graphite particles having an average particle size varying from About 0.1 to 200 microns, and preferably in the range of about 23 to 26 microns. The particles of w? filler have a surface area ranging from about 1 to 100 m2 / gram, and preferably in the range of 7 to 10 m2 / gram (as measured by test standards) BET). The composition may include additional components, including: carbon nanofibers and / or graphite; carbon fibers and / or graphite; metal fibers such as stainless steel or nickel; and carbon fiber concentrates and / or ^ fc graphite metal covers that have thermoplastic finishes or thermostable materials chosen from the aforementioned group of potential polymers. The composition is subsequently shaped into a desired shape by compression molding, injection molding, or a combination thereof. Alternatively, the The composition can be used in plating or coating operations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A novel composition is provided to make a surface-resistant composite or surface coating. corrosion that has improved electrical conductivity. In the best mode of the invention, the composition is used to mold a unitary current collector plate for a polymer electrolyte membrane fuel cell. However, the composition can also be used to manufacture alternative collector plate structures. For example, the composition can coat the surface of a substrate w? convenient to form a multi-layer manifold plate structure. In accordance with the foregoing, the term"structure" as used herein is intended to refer to to a unitary part to a coated part. Preferably, the composition comprises a low viscosity thermoplastic resin combined with a highly conductive carbon or graphite filler material. ft The composition is chosen to produce a plate current collector capable of withstanding the rough environment of a polymer electrolyte membrane fuel cell. Preferably, the composition is used to make current collector plates that meet particular criteria listed in Table 1 (below). In addition to having the properties and characteristics identified below, it is preferred that the collector plate is resistant to chemical and electrochemical degradation and hydrolysis, and has an electrical resistance in volume of less than 50 mO-centimeter (or a conductivity in volume greater than 20 S / cm).

• • Table 1 10 Convenient binder resins are defined as thermoplastic or non-fluorinated thermosetting polymers that preferably have melt viscosities of less than 1,000 Newton-seconds per square meter (N * s / m2) over a shear index range of 1,000 to 10,000 sec "1, and • 15 properties and additional material characteristics defined in Table 2 (below). As used herein, the term "non-fluorinated" is intended to describe polymers other than fluoropolymers. In accordance with the above, nominal amounts of fluorine-containing components can be added to the present combination without changing the designation of the binder resin as a non-fluorinated polymer. For example, nominal amounts of Teflon® can be added to the binder resin to improve the mold release characteristics of the final composition. • 10 • 15 20 Table 2 fft Particular examples of polymer resins that meet these requirements include, but are not limited to, polyphenylene sulfide, low weight polyphenylene sulfide. molecular, liquid crystal polymer, and modified polyphenylene oxide. Suitable polyphenylene sulfides are commercially available from Phillips Chemical Company of Bartlesville, Oklahoma, under the trade name Ryton®, and in (ft. Ticona Corporation of Summit, New Jersey, under the name commercial of Fortron®. Liquid crystal polymers having the desired properties are commercially available from Ticona under the trade name of Vectra®, and from Amoco Performance Products, Inc. of Alpharetta, Georgia, under the trade name Xydar®. A polyphenylene oxide The modified one having the desired properties is commercially available from General Electric Company of Pittsfield, Massachusetts, under the trade name Noryl®. Combinations of the polymer resins identified above have the desired properties listed in the Table 2. Before molding, the polymer resin is combined with highly conductive filler particles. Preferably, the filler particles comprise carbon and / or graphite and have the properties and characteristics that are defined in Table 3 below. • • 10 Table 3 The filler can be provided in various forms, including powder, fibers and flakes. However, it is preferred that the filler material comprises a graphite powder of high purity having a carbon content greater than 98 percent. The use of graphite is preferred because graphite is electrochemically stable over a wide range of environments. Use in powder form is preferred because the powders are less able to prevent the flow of the composition during molding. Preferably, the graphite powder has an average particle size of about 23-26 microns, and a surface area measured by BET of approximately 7-10 m2 / gram. The incorporation of small surface area conductive particles, small in the novel composition of the present invention is a significant departure from the conventional conductive compounds used to make structures for electronic, thermoelectric and electrochemical devices. The Conventional conductive compounds, such as those used to make fuel cell collector plates, typically contain conductive particles having a very high surface area combined with small particle size. For example, carbon black particles that have an area superficial greater than 500 m / gram and particle size less than one miera are typical. Commonly, conventional conductive compounds also contain large fibers that have little surface area. For example, fibers having a surface area of less than 10 m2 / gram coupled with a fiber length greater than 250 micras are typical. The combination of reduced filler particle size and reduced filler particle surface area provides a means to maintain material flow while increasing the particle load. filling machines. Significantly, the relatively low particle size and low surface area allow densities Jft packing of highly filled filler particles compared to the known compositions for molding current collector plates. A corresponding increase in load solids results in a fabricated plate having increased electrical conductivity, while minimizing gas permeable voids. Graphite powders that have the properties identified above are available from UCAR Coal Company, Inc. of Lawrenceburg, Tennessee, as well as at Asbury Carbons, Inc. of Asbury, New Jersey. The carbon nanofibers can be added to the composition to improve the electrical conductivity and mechanical strength of the molded manifold plate. The carbon nanofibers typically have diameters ranging from a few nanometers to several hundred nanometers, and relationships between dimensions ranging from 50 to 1,500. Other additives may include carbon fibers, metal fibers such as stainless steel or nickel, and / or fiber concentrates. carbon-coated metal having polymer finishes chosen from the aforementioned group of potential polymers (ie, polyphenylene sulfides, modified polyphenylene oxides, liquid crystal polymers, polyamides, polyimides, polyesters, phenolics, resins containing epoxides, epoxy novolacs and vinyl esters).

The preferred composition contains 45-95 percent in fl | weight of graphite powder, 5-50 weight percent of polymer resin, and 0-20 weight percent of metallic fiber, carbon fiber and / or carbon nanofiber. When metal fibers are added, it is preferred that at least 50 percent of the fibers have diameters ranging from a few nanometers to about 50 microns, and dimensional relationships ranging from 10 to 5,000. The composition is formed in a compound that has a desired geometry by various methods including compression molding, injection molding, or a combination thereof. In the case of compression molding, graphite and polymer powders, and / or metal-coated carbon particles or fibers, are initially mixed together to obtain a uniform distribution and composition. A preform of the stirred mixture is created by compressing the mixture using a pressure of 5-100 x 106 N / m2 at a temperature below the melting temperature of the polymer constituent, and preferably at room temperature. The preform is heats at a temperature higher than the melting temperature of the polymer for a period of about 1-45 minutes. Subsequently, the preform is placed between mold plates heated at a temperature in the range of 180-350 ° C. The mold plates are brought together at a pressure of clamping of approximately 1-15 (10) d N / m2 and the gas trapped inside the mold is removed by a degassing step (ft in which vacuum is applied) The degassing step takes about one minute. , the clamping pressure of the mold increases to approximately 5-75 x 5 106 N / m 2, Subsequently, the mold is cooled to a temperature in the range of approximately 80-250 ° C, and the part is removed from the mold. case of injection molding, polymer and filler powders, and / or carbon particles or fibers coated with metal, they are initially mixed together to obtain a uniform distribution and composition, composed of granules, and then plasticized before injection into the mold. When injection molding is used, the composition must be maintained at a suitable temperature (ie say, well above the melting temperature of the polymer resin) to prevent the resin from freezing, or solidifying, as it flows to and through the cooling mold. To further assist in preventing the resin from freezing during injection, the same mold preferably is heated to a temperature of about 80-350 ° C. The mixture is rapidly injected into the mold to minimize heat loss due to the high thermal conductivity of the composition. The rapid injection also produces an improved flow of material to and through the mold generating higher shear forces. By varying the injection pressure the injection speed can be affected. The injection pressure may vary depending on several factors, such as the viscosity of the composition, the temperature of the mold, and so on. However, it is preferred that The injection pressure is set at the maximum level that can be achieved without creating excessive mold spillage. Mold spillage occurs when the mold material is squeezed out of the mold cavity. Injection pressures may vary from approximately 13-500 (106 N / m2). The injection step it takes approximately 1-15 seconds. After the injection, the part can be retained in the mold before being expelled. In some cases, it may be desirable to employ a combination of the injection / compression molding process in Wherein the injection molded structure is subjected to a compression step after molding. This final compression step can, for example, be used to further increase the conductivity of the molded structure by increasing the packing density of the conductive filler. In an alternative embodiment of the invention, the novel composition is melted and applied to a metal surface to provide a highly conductive protective layer, hardened after cooling. The composition provides a means to protect against corrosion underlying metal structure, while avoiding a significant increase in electrical resistance. Convenient structures for fuel cell applications (ie, having the properties listed in Table 1) can be formed using numerous different coating methods. For example, a coated structure can be formed by coating thin, stamped, etched metal substrates with the novel composition. The coating methods include plating or hot-rolling a sheet of metal, and then hot-stamping the coated surface to form a desired surface geometry. Although the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited thereby. Numerous modifications, changes, variations, substitutions and equivalents will be presented to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.

Claims (26)

1. CLAIMS ft 1. A composition for forming an electrically conductive polymer composite, comprising: a non-fluorinated polymer binder, having a viscosity of 5 melting less than 1000 Newton-seconds per square meter (N * s / m2) over a range of shear rate of 1,000 to 10,000 sec "1, and a plurality of electrically conductive particles fixed in the polymer binder, the compound has a volume conductivity of at least 10 approximately 10 S / cm.
2. 2. A composition according to claim 1, characterized in that the non-fluorinated polymer binder has a melt viscosity of less than 200 Newton-seconds per square meter (N * s / m2) on a 15 range of shear rate from 1,000 to 10,000 sec. " 1
3. 3. A composition according to claim 1, characterized in that the non-fluorinated polymer binder comprises a thermoplastic having a 20 thermal index of at least 45 ° C.
4. 4. A composition according to claim 1, characterized in that the non-fluorinated polymer binder comprises a thermoplastic having a melting temperature of at least 90 ° C.
5. 5. A composition according to claim 1, characterized in that the non-fluorinated polymer binder is selected from the group of thermoplastic polymers consisting of polyphenylene sulphides, modified polyphenylene oxides, liquid crystal polymers, polyamides. , poly-imides, polyesters, phenolics, resins containing epoxy and vinyl esters.
6. 6. A composition according to claim 1, characterized in that the plurality of electrically conductive particles comprise particles of 10 coal.
7. 7. A composition according to claim 1, characterized in that the plurality of electrically conductive particles comprises graphite particles.
8. 8. A composition according to claim 1, characterized in that the plurality of electrically conductive particles have an average particle size in the range of about 0.1 to 200 microns, and preferably in the range of about 23 microns. 20 to 26 microns.
9. 9. A composition as claimed in claim 8, characterized in that the plurality of electrically conductive particles have an average surface area measured by BET in the range of about 1 to 25 100 m2 / g, and preferably in the range of about 7 to 10 m2 / g. (ft
10. 10. A composition according to claim 1, characterized in that the plurality of electrically conductive particles comprises at least 45 5 weight percent of the composition
11. 11. A composition according to claim 1 , characterized in that the polymer composite comprises a current collector plate for a polymer electrolyte membrane fuel cell 10 (MPE).
12. 12. In a polymer electrolyte membrane (MPE) fuel cell having an ion exchange membrane and electrodes interposed between a pair of current collector plates, the improvement comprising 15 forming a current collector plate having a composition comprising a non-fluorinated polymer having a melt viscosity of less than 1,000 N * s / m 2 over a shear interval of 1,000 to 10,000 sec. "1. 20 electrolyte polymeric membrane fuel according to claim 12, characterized in that the current collector plate comprises a unitary structure molded from the composition 14. The electrolyte polymeric membrane fuel cell according to the invention. as claimed in claim 12, characterized in that the current collector plate comprises a metal substrate coated with the composition 15. The electrolyte polymer membrane fuel cell according to claim 12, characterized in that the non-fluorinated polymer binder has a melt viscosity of less than 200 Newton-seconds per square meter (N * s / m2) over a range of shear rate of 1,000 to 10,000 sec "1. The cell of polymer electrolyte membrane fuel according to claim 12, characterized in that the non-fluorinated polymer binder comprises a thermoplastic having a thermal index of at least 45 ° C. 17. The polymeric membrane fuel cell of electrolyte according to claim 12, characterized in that the non-fluorinated polymer binder comprises a thermoplastic having a melting temperature of at least 90 ° C, and preferably in the range of about 250 to 350 ° C. The electrolyte polymer membrane fuel cell according to claim 12, characterized in that the agglutinant The non-fluorinated polymer is selected from the group of thermoplastic polymers consisting of polyphenylene sulfides, modified polyphenylene oxides, liquid crystal polymers, polyamides, polyimides, polyesters, phenolics, epoxy-containing resins and vinyl esters. 19. The electrolyte polymeric membrane fuel cell according to claim 12, characterized in that the composition further comprises a plurality of electrically conductive filler particles having a particle size. 10 average in the range of about 0.1 to 200 microns, and preferably in the range of about 23 to 26 microns. 20. The polymer electrolyte membrane fuel cell in accordance with the claim in the 15 claim 19, characterized in that the plurality of electrically conductive filler particles has a ^ average surface area measured by BET in the range of about 1 to 100 m2 / g, and preferably in the range of about 7 to 10 m2 / g. 21. The polymer electrolyte membrane fuel cell according to claim 19, characterized in that the plurality of electrically conductive particles comprises at least 45 percent by weight of the composition. 22. The polymer electrolyte membrane fuel cell according to claim 12, characterized in that the collector plates comprise compression molded collector plates. 23. The polymer electrolyte membrane fuel cell according to claim 12, characterized in that the collector plates comprise collector plates molded by injection. 24. A method to make a collector plate of ^ fc current for a polymeric membrane fuel cell 10 electrolyte (MPE), comprising the steps of: combining a conductive filler powder and a non-fluorinated polymer powder to form a mixture; introducing the mixture into the cavity of a mold, heating the mold to a temperature higher than the melting temperature of the non-fluorinated polymer powder; 15 compress the mixture to form a current collector plate; cooling the mold to a temperature below the melting temperature of the non-fluorinated polymer powder; and remove the current collector plate from the mold. 25. A method for manufacturing a current collector plate for a polymer electrolyte membrane (MPE) fuel cell, comprising the steps of: combining a conductive filler powder and a non-fluorinated polymer powder to form a mixture; form the mixture in solid granules; heating the granules to a temperature higher than the melting temperature of the non-fluorinated polymer powder; injecting the mixture into a mold cavity; allowing the mixture to cool to a temperature below the melting temperature of the non-fluorinated polymer powder to form a unitary collector plate; and remove the unit manifold plate 5 the mold cavity. 26. The method according to claim 25, characterized in that it also comprises the step of compressing the unitary collector plate.