Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2002-04-29
2004-06-22
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S512000, C252S513000, C264S105000, C429S006000, C429S006000, C429S006000, C204S254000, C204S295000
Reexamination Certificate
active
06752937
ABSTRACT:
FIELD OF INVENTION
The field of invention is highly conductive compositions that are particularly suitable for intricately molded conductive products including fuel cell plates. These compounds can be used in molding processes such as injection, compression, transfer, and injection/compression molding.
Products molded from the composition of this invention desirably have a bulk conductivity of at least 70, and up to 170 or more S/cm. They also have desirable surface characteristics; heat, temperature, chemical and shrink resistance; strength; and cost. The compositions include a thermoset resin matrix such as a polyester or vinyl ester with a high loading of conductive inorganic filler, typically graphite having a particular particle size distribution wherein a significant portion of the particles exceed 75 microns, 100, 150, 200, 300 and even exceeding 500 or 600 microns. These conductivity values can be increased even more to better achieve through plane conductivity by the inclusion of conductive silver metal-coated ceramic fibers or other conductive additives.
The molding compositions in accordance with the invention can be formed into high definition complex configurations, including configurations, which are particularly suitable for a variety of molding techniques. Further labyrinthine plates can be made substantially exclusively by molding, meaning that the need for complex and expensive machining processes is virtually eliminated. Thin plate-like specimens (e.g. 60 to 200 thousandths of an inch) having an intricately patterned network of very narrow, relatively smooth, flow passages can be made by molding these compositions. Such specimens are used as electrochemical cell bipolar plates.
BACKGROUND OF THE INVENTION
Conductive polymeric compositions have applications in providing alternatives to traditional conductive materials, which often involve greater labor expense to manufacture into complex parts. In particular, in instances where the demand justifies significant volumes of a product, polymer-molding expenses may prove far more cost effective than comparable machining expenses for other materials. However in the past, it has proved difficult to achieve both a high level of conductivity and desirable molding characteristics. Generally, high-level weight percentages of an appropriate filler in a polymeric matrix are necessary to achieve satisfactory levels of conductivity. However, these high load levels lead to problems with the strength, durability, and moldability of the resulting composition
One area in particular where it would be beneficial to solve the previously mentioned strength, durability, and molding issues is for application in fuel cells. Electrochemical fuel cells have great appeal as a potentially limitless energy source that is clean and environmentally friendly. These fuel cells can, in addition, be constructed at an appropriate scale for small-scale energy consumption, such as household use, or for industrial scale use, and even for commercial power generation. They have portable applications to power small appliances (such as computers or camping equipment), or automobiles and other forms of transportation. Although these different applications involve differences in size, the fundamental construction remains the same for generation of power varying from less than one to a few thousand kilowatts.
Basically, a fuel cell is a galvanic cell in which the chemical energy of a fuel is converted directly into electrical energy by means of an electrochemical process. The fundamental components of the fuel cell are an electrode comprising an anode and a cathode, eletrocatalysts, and an electrolyte. Work has been done in perfecting both liquid and solid electrolyte fuel cells and the present invention may find use in both types of fuel cells.
Solid electrolytes include polymeric membranes, which act as proton exchange membranes typically fueled by hydrogen. These membranes usually comprise a perfluorinated sulphonic acid polymer membrane sandwiched between two catalyzed electrodes that may utilize platinum supported on carbon as an electrocatalyst. Hydrogen fuel cells form a reaction chamber, which consumes hydrogen at the anode. At the cathode, oxygen reacts with protons and electrons at the electrocatalytic sites yielding water as the reaction product. A three-phase interface is formed in the region of the electrode and a delicate balance must be maintained between the electrode, the electrolyte, and the gaseous phases.
Systems involving the use of other electrolytes have been also been studied. These would include alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. However, the principles are similar, as are some of the issues in perfecting these products.
A fuel cell reactor may comprise a single-cell or a multi-cell stack. In any case, the cell includes at least two highly conductive flow field plates that serve multiple functions. These plates may function as current collectors that provide electrical continuity between the fuel cell voltage terminals and electrodes. They also provide mechanical support (for example for the membrane/electrode assembly). In addition, these plates act to transport reactants to the electrodes and are essential to establishing the previously mentioned delicate phase balance.
Typically, the fuel cell plates are thin relatively flat plate members that include a highly complex network of interconnecting channels that form the flow field area of the plate. The configuration of these channels is highly developed in order to maintain the proper flow of reactants and to avoid channeling or the formation of stagnant areas, which results in poor fuel cell performance. It is critical that the flow of the reactants is properly managed, and that the electrocatalysts are continuously supplied with precisely the appropriate balance of reactants. Thus, it is essential for the plates to define and maintain clear passages within the highly engineered flow labyrinth. Moreover, in order to assure a satisfactory life, the plates must be able to resist surface corrosion under a variety of conditions. For example, fuel cells may be placed outside and subject to ambient weather. Thus, the cells must be resistant to stress cracking and corrosion at temperature ranging from −40 to 200 degrees Fahrenheit. Further, since the conditions within the cell are corrosive, the cells must also be resistant to chemical attack at these temperatures from various corrosive substances. For example, the plates may be subjected to de-ionized water, methanol, formic acid, formaldehyde, heavy naptha, hydrofluoric acid, tertafluoroethylene, and hexafluoropropylene depending on the fuel cell type. Moreover, the conditions within the fuel cell may lead to elevated temperatures, i.e. from 150 to 200 degrees Fahrenheit, as well as elevated pressures, i.e. from ambient to 30 psi. Corrosive decomposition needs to be avoided since it almost certainly would cause a system failure by changing the flow patterns within the fuel cell.
Past attempts at solving the various requirements for fuel cell plates have included the use of metal and machined graphite plates. The use of metal plates result in higher weight per cell, higher machining costs and possibly corrosion problems. Machined graphite plates solve the weight and corrosion problems but involve high machining cost and result in fragile products, especially when prepared as very thin plates. Some use of graphite/poly(vinylidene fluoride) plates has been made but these have been characterized as being expensive and brittle and having long cycle times. U.S. Pat. No. 4,197,178 is incorporated herein for its teaching of the working and compositions of electrochemical cells. U.S. Pat. No. 4,301,222 is incorporated herein for its teachings on graphite-based separators for electrochemical cells.
SUMMARY OF THE INVENTION
In the past, known conventional bulk molding compounds have been modified to be conductive by the addition of large amounts of conductive filler, s
Hudak, Shunk & Farine Co. LPA
Kopec Mark
Quantum Composites Inc.
Shunk Laura F.
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