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Diffusion media, fuel cells, and fuel cell powered systemsRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Fuel Cell, Subcombination Thereof Or Methods Of Operating, Catalytic Electrode Structure Or Composition, Having Organic Constituent As Part Of The ElectrodeDiffusion media, fuel cells, and fuel cell powered systems description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060240312, Diffusion media, fuel cells, and fuel cell powered systems. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD Of THE INVENTION [0001] The present invention relates generally to diffusion media, fuel cells employing diffusion media according to the present invention, and fuel cell powered systems utilizing such fuel cells. More specifically, the present invention is related to the use of diffusion media in addressing water transport difficulties in fuel cells and other types of devices. Background Art [0002] Fuel cells have been used as a power source in many applications and have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton conductive, non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is typically referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation. [0003] In PEM fuel cells hydrogen (H.sub.2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O.sub.2), or air (a mixture of O.sub.2 and N.sub.2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluorinated sulfonic acid ionomers. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies, which comprise the catalyzed electrodes, require certain controlled conditions in order to maintain certain hydration for optimized proton conductivity and avoid flooding. [0004] Efficient operation of a fuel cell depends, at least in part, on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of product water is desired so as to not inhibit flow of fresh reactants to the catalytic sites. Therefore, it is desirable to improve the mobility of reactant and product water to and from the MEA where reaction occurs. [0005] To improve the mobility of reactant and product species to and from the MEA where reactions occur, a diffusion structure which enhances mass transport to and from an electrode in a MEA of a fuel cell is used. The diffusion structure cooperates and interacts with an electrode at a major surface of the electrode opposite the membrane electrolyte of the cell, therefore, electrical and heat conductivity are required. The diffusion structure facilitates the supply of reactant gas to the electrode. The diffusion structure is hereinafter referred to as a diffusion media. See for example U.S. Pat. No. 6,350,539 issued to the assignee of the present application. The diffusion media is positioned between the MEA and the cathode or anode flow channels of an individual fuel cell. One example of a relatively typical diffusion media comprises an electrically conductive porous media such as carbon paper. [0006] In an operating PEM fuel cell, water is generated at the cathode side due to the electrochemical reaction between hydrogen and oxygen occurring within the MEA. Water is also typically introduced through reactant gas streams into fuel cells to humidify the membrane to ensure good proton conductivity. PEM fuel cells can experience a relative excess of water, which, if not removed from the system, could block the transportation path between oxidant gas and cathode electrode. In addition to possible oxidant starvation on the cathode side, water slugs in the gas flow channel may also be formed on the anode side which can cause hydrogen starvation. Water on the anode side can result from external humidification of the hydrogen gas and from back diffusion through the membrane (cathode to anode). If these occur, the fuel cell efficiency can decrease and may eventually lead to system shutdown, a phenomenon called "flooding." Managing water is therefore a relatively important aspect for the efficient operation of a fuel cell. [0007] The diffusion media plays a relatively important role in PEM fuel cells water management. The diffusion media can facilitate movement of water to ensure good transportation paths between reactant gases and catalyst electrodes in the MEA. One conventional practice to accomplish this is to coat the diffusion media (such as carbon paper) with a hydrophobic material such as polytetrafluoroethylene (PTEE). This PTEE coating makes the media more hydrophobic and thus helps to prevent water from blocking the flow channels in diffusion media. Even still, other water management properties are sought to provide more efficient water management. It would be desirable for the gas diffusion media to provide a flow path for increased water management in a fuel cell. SUMMARY OF THE INVENTION [0008] In at least one embodiment, the present invention comprises a porous matrix having an outer surface comprising a first major face and a second major face and a hydrophilic polymeric coating on at least a portion of the porous matrix wherein the hydrophilic coating comprises the cured product of a formulation comprising a hydrophilic monomer. [0009] In at least another embodiment, the present invention comprises a fuel cell comprising an anode, a cathode, a PEM disposed between the anode and the cathode, and a diffusion media disposed between at least one of the anode and the cathode and the PEM. The diffusion media comprises a porous matrix having an outer surface comprising a first major face and a second major face and a hydrophilic polymeric coating on at least a portion of the outer surface. The hydrophilic coating comprises the cured product of a formulation comprising a hydrophilic monomer. [0010] In at least yet another embodiment, the present invention comprises a method for making a diffusion media. The method comprises providing a porous matrix and coating at least a portion of the porous matrix with a hydrophilic polymeric coating. In at least one embodiment, the coating step comprises exposing the porous matrix to a hydrophilic formulation comprising a hydrophilic monomer to form a coating precursor on the porous matrix, masking a first portion of the precursor coated porous matrix while leaving a second portion of the matrix unmasked, and exposing the precursor coated porous matrix to UV light to UV cure the coating precursor on the second portion of the matrix to form the hydrophilic polymeric coating on the second portion. [0011] Further areas of applicability of the present invention will become apparent from the detailed description provided herein. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS [0012] The present invention will become more fully understood from the detailed description and the accompanying drawings where like structure is indicated with like reference numerals and in which: [0013] FIG. 1 is an exploded schematic illustration of a fuel cell incorporating a porous diffusion media according to the present invention; [0014] FIG. 2 is an illustration of a diffusion media according to embodiments of the present invention; [0015] FIG. 3 is an illustration of a diffusion media according to embodiments of the present invention positioned against a catalyst layer; [0016] FIG. 4 is a schematic representation of a diffusion media according to other embodiments of the present invention; and [0017] FIG. 5 is an illustration of a vehicle incorporating a fuel cell employing a porous diffusion media according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) [0018] The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative bases for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the present invention. [0019] Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight; the term "polymer" includes "oligomer", "copolymer", "terpolymer", and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 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