1. Field of the Invention
The present invention is related to bipolar plates with improved hydrophilicity for fuel cell applications.
2. Background
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
The electrically conductive plates currently used in fuel cells provide a number of opportunities for improving fuel cell performance. For example, it is desirable to minimize the agglomeration of water droplets within flow channels in the plates. To this end, fuel cells are typically coated with a hydrophilic coating. Currently, hydrophylic layers are applied to a conductive plate via a multilayer adsorption (MLA) process. Typically, such processes require 4 dip cycles (i.e., 4 bilayers, 1 bilayer consists of a layer of silica on top of a layer of a cationic polymer) in a hydrophilic coating such as silica-based NanoX. Although such processes work reasonably well, MLA methods are undesirably labor intensive often taking up to 40 minutes to complete.
Although recent stack data indicate that a superhydrophilic coating is not necessary in the active area of Au-coated stainless steel bipolar plates to pass low power stability (LPS), future plate designs and system operating conditions may require such a coating for water management. Presently, silica-based hydrophilic coatings (e.g., EMS, NanoX) applied using a multilayer adsorption (MLA) process (includes use of a cationic polymer) are not sufficiently water stable. In stacks S0340 (3500 hrs) and S0949 (5100 hrs), the silica-coated plates became grossly non-wicking (less hydrophilic) after fuel cell exposure due to silica and cationic polymer dissolution. A more hydrolytically stable material is needed to replace the water-soluble cationic polymer.
Accordingly, there is a need for improved methodology for applying hydrophilic coatings at the surfaces of bipolar plates used in fuel cell applications.
The present invention solves one or more problems of the prior art by providing in at least one embodiment a method for coating a hydrolytically-stable hydrophilic coating on a fuel cell component. The method comprises contacting a fuel cell component with a titanium oxide-containing liquid to form a titanium oxide-containing layer adhered to the fuel cell component. The titanium oxide-containing layer is optionally dried. In a subsequent step, the fuel cell component is contacted with a silicon oxide-containing liquid to form a silicon oxide coating adhered to the titanium oxide layer. The silicon oxide-containing layer is then optionally dried. The steps of the present embodiment are optionally repeated one or more additional times to produce a plurality of bilayers on the fuel cell component. The fuel cell component coated in accordance to the method of the present embodiment is found to have good hydrophilicity and a low water contact angle. Moreover, these coated articles are found to retain these properties after continued exposure to water.
In another embodiment of the present invention, a method for forming a fuel cell with a coated flow field plate is provided. The method includes a step of placing a membrane electrode assembly between a first flow field plate and a cathode flow field plate. At least one of the first flow field plate and the second flow field plate are coated by contacting a fuel cell component with a titanium oxide-containing liquid to form a titanium oxide-containing layer adhered to the fuel cell component. The titanium oxide-containing layer is then optionally dried. In a subsequent step, the fuel cell component is contacted with a silicon oxide-containing liquid to form a silicon oxide coating adhered to the titanium oxide layer. The silicon oxide-containing layer is then optionally dried. The steps of the present embodiment are optionally repeated one or more additional times to produce a plurality of bilayers on the fuel cell component.
In yet another embodiment of the present invention, a fuel cell including a coated fuel cell component is provided. The fuel cell component, which is usually a flow field plate is formed by the methods set forth above.
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 basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
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 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 applies mutatis mutandis 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.
It is also to be understood that this invention is not limited to the specific embodiments and methods described below as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
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In a variation of the present embodiment, the method of
The fuel cell component coated in the manner set forth above is found to have superior hydrophilicity. In particular, the coated fuel cell component is found to have a water contact angle less than about 40 degrees. In another refinement, the coated fuel cell component is found to have a contact angle less than about 30 degrees. In still another refinement, the coated fuel cell component is found to have a contact angle less than about 20 degrees. In yet another embodiment, the coated fuel cell component is found to have a contact angle from about 3 to 20 degrees. Coated fuel cell components are found to retain their properties upon prolonged water exposure.
With reference to
The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.
Materials: Hombikat® XXS 100 TiO2 (Sachtleben Corporation) and EMS silica (Electronic Microscopy Services, Inc.) sols. The commercial TiO2 sol consists of photocatalytic anatase nanoparticles suspended in water (<7 nm particle size, 18.6% TiO2 weight percent), and is diluted with deionized (DI) water and ethanol to obtain a final sol composition of 6 weight percent TiO2, 89 weight percent water, and 5 weight percent ethanol. The positively-charged TiO2 particles (pH=2.4, TiO2 isoelectric point: 5.5 to 6.0 pH) are stabilized electrostatically with nitric acid. The silica sol consists of 60 nm SiO2 particles suspended in water, diluted 10-fold with water to 0.5 weight percent SiO2 and pH-adjusted (pH=3.9, SiO2 isoelectric point: 2.0 to 2.6 pH) with H2SO4 to enable the negatively charged SiO2 particles to adsorb well in the MLA process to the pre-adsorbed, positively charged TiO2 layer.
Process Details: EMS silica applied to Au-coated stainless steel coupons using an MLA process in which the alkaline-cleaned coupons are: (1) dipped into the TiO2 sol for 3 minutes at room temperature to acquire a positively charged surface, (2) rinsed in deionized (DI) water with vigorous agitation for 2 minutes to remove unadhered TiO2, (3) dipped into a negatively charged EMS sol for 3 minutes, and then (4) rinsed in DI water with vigorous agitation for 2 minutes to remove unbonded silica particles and to form a monolayer of silica. This generates a single SiO2/TiO2 bilayer. Table 1 provides water soaking properties of gold coated stainless steel coupons coated with four bilayers of SiO2/Kemira C-442. and SiO2/TiO2. Kemira C-442 is an acrylamide/β-methacryl-oxyethyl-trimethyl-ammonium copolymer. The amount of silicon loss after 792 hours of water soaking is found to be much less for the SiO2/TiO2 coupons as compared to the SiO2/C-442 coated coupons. Also, the former show a slight increase in contact angle. In Table 1, the amounts of silicon oxide and titanium oxide are determined by electron probe microanalysis (EPMA). Similarly,
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.