Hydrophilic surface modification of bipolar plate

A bipolar plate having hydrophilic surfaces is disclosed. The bipolar plate includes multiple surfaces including channels having channel surfaces. A hydrophilic coating is provided on the surfaces to enhance the water management capabilties of a fuel cell.

FIELD OF THE INVENTION

The present invention relates to fuel cells which generate electricity to power vehicles or other machinery. More particularly, the present invention relates to bipolar plates having hydrophilic coatings and methods for rendering the surfaces of bipolar plates in a fuel cell hydrophilic to increase the wettability of the plates and enhance water management capabilities of the fuel cell.

BACKGROUND OF THE INVENTION

In recent years, much interest regarding fuel cell technology has developed due in large measures to fuel cell efficiency. Fuel cells have exhibited efficiencies as high as 55%. Furthermore, fuel cell power plants are environmentally-friendly, emitting only heat and water as by-products.

A PEM (polymer electrolyte membrane) fuel cell stack typically includes a central membrane electrode assembly (MEA) which is sandwiched between gas diffusion media. The MEA and gas diffusion media are sandwiched between a pair of bipolar plates. The bipolar plates are provided with flow field channels which conduct reactant gases to and product gases from the MEA through the gas diffusion media, as well as coolant channels which conduct coolant. The regions of the bipolar plate surface between the channels are known as lands and abut against the corresponding gas diffusion medium. It is desired that the surfaces of the bipolar plate, particularly the bipolar plate on the cathode side of the stack, be hydrophilic to facilitate optimum water management inside PEM fuel cell stacks.

Accordingly, bipolar plates having hydrophilic coatings and methods of rendering the surfaces of bipolar plates hydrophilic are needed to enhance water management in a PEM fuel cell stack.

SUMMARY OF THE INVENTION

The present invention is generally directed to bipolar plates having hydrophilic coatings and methods of providing hydrophilic coatings on bipolar plates, particularly on the cathode bipolar plate of a fuel cell stack. In one embodiment, the hydrophilic coating is a silicon dioxide. In another embodiment, the hydrophilic coating is a titanium oxide. In still another embodiment, the hydrophilic coating is silicon dioxide and titanium oxide. The hydrophilic coating enhances the wettability of the bipolar plate channel surfaces, thereby enhancing water management, performance, durability and efficiency of a fuel cell stack. During application of the coating to the bipolar plate using any of a variety of methods, a mask can be used to cover the lands of the bipolar plate to facilitate selective coating of the channel surfaces of the plate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates bipolar plates having hydrophilic coatings. In one embodiment, the hydrophilic coating is silicon dioxide. In another embodiment, the hydrophilic coating is titanium oxide. In still another embodiment, the hydrophilic coating is silicon dioxide and titanium oxide. The hydrophilic coating enhances the wettability of the bipolar plate channel surfaces, thereby enhancing the water management, performance, durability and efficiency of a fuel cell.

The invention further contemplates methods of providing hydrophilic coatings on bipolar plates, particularly on the cathode bipolar plate of a fuel cell stack. The methods include applying a silicon dioxide hydrophilic coating to surfaces, particularly the channel surfaces, of the bipolar plate using any of a variety of methods including but not limited to chemical vapor deposition, physical vapor deposition or plasma polymerization. The methods further include applying a titanium oxide hydrophilic coating to a bipolar plate using any of a variety of methods including but not limited to electrochemical methods, sputter deposition, chemical vapor deposition or reactive electron beam evaporation. The methods may further include applying both a silicon dioxide hydrophilic coating and a titanium oxide hydrophilic coating to the bipolar plate. During application of the coating to the bipolar plate, a mask can be used to cover the lands of the bipolar plate and facilitate selective coating of the channel surfaces of the plate; Subsequently, the lands can be coated with a thin layer of gold or a polymeric conductive carbon coating.

Silicon dioxide and titanium oxide have been shown to possess hydrophilic properties which could optimize the performance of bipolar plates. The spreading pressures of silicon dioxide and titanium oxide at 25 degrees C. are 336 and 300 dyne/cm2, respectively. These high values for the spreading pressure indicate that silicon dioxide and titanium oxide have considerably high surface energy that make them promising candidates for hydrophilic surfaces on bipolar plates.

Referring toFIG. 1, a bipolar plate32having hydrophilic surfaces according to the present invention is shown. The bipolar plate32may be metal, such as stainless steel; a carbon composite; or any other material which is suitable for use as a bipolar plate in a fuel cell. The bipolar plate32is typically a cathode bipolar plate which is provided on the cathode side of a fuel cell stack, as will be hereinafter further described. The cathode bipolar plate32includes multiple channels34which distribute oxygen to and exhaust streams from the fuel cell stack. Coolant channels are provided on the back of this plate (not shown). Lands42having land surfaces43separate the channels34from each other. Each channel34has channel surfaces35.

According to the present invention, a hydrophilic coating48is formed on the channel surfaces35of the channels34. The hydrophilic coating48may be silicon dioxide, titanium oxide or both silicon dioxide and titanium oxide. Prior to application of the hydrophilic coating48, which will be hereinafter described, a mask44, having mask openings45, is typically provided on the land surfaces43of the lands42. The channel surfaces35of the channels34are exposed through the mask openings45, whereas the land surfaces43are covered by the mask44. This prevents the land surfaces43from being coated with the non-conductive oxide.

Referring next toFIG. 1A, after formation of the hydrophilic coating48, which may be silicon dioxide, titanium oxide, or both, on the channel surfaces35, the mask44is removed from the land surfaces43. A conductive coating50can be formed on the land surfaces43to enhance electrical conductivity of the lands42. In the case of a composite carbon bipolar plate32, the conductive coating may not be required and the plates may be used as such. In the case of a stainless steel bipolar plate32, the conductive coating50is typically a thin layer of Au or conductive polymeric coating. Therefore, the hydrophilic coating48increases the hydrophilicity of the channel surfaces35without impacting the coating conductivity of the lands42.

In one embodiment, the silicon dioxide hydrophilic coating48is formed on the channel surfaces35using a conventional chemical vapor deposition (CVD) process or atomic layer deposition (ALD) process. The deposition temperature for the carbon composite bipolar plate32is typically about 200 degrees C. and for the stainless steel bipolar plate32is typically about 350 degrees C. Prior to the deposition process, the bipolar plate32is cleaned typically by exposure to far-UV radiation, which generates ozone and removes any organic contamination from the bipolar plate32by oxidation. Each cycle of the ALD process includes a dose of trimethylaluminum (TMA), followed by a dose of tris (tert-pentoxy) silanol. The thickness of the silicon dioxide hydrophilic coating48is typically about 10˜50 nm, and the contact angle of the coating48is typically about 10˜14 degrees.

In another embodiment, the silicon dioxide hydrophilic coating48is formed on the channel surfaces35by physical vapor deposition (PVD). In this method, magnetron sputtering is used to deposit the coating48at a BIAS potential of typically about 200 V in a reactive environment of O2/Ar mixture plasma and a chamber pressure of typically about 2.5×10−4Torr. The target used in the magnetron sputtering process is 99% pure Si. Witness coupons may be run with the bipolar plate substrate to obtain the composition and thickness of the SiO2hydrophilic coating48. Hydrophilic coatings48having a thickness of typically about 100 nm may be obtained using this method.

In still another embodiment, the silicon dioxide hydrophilic coating48is formed by plasma polymerization using open air plasma technology with air as a feeder gas. Samples obtained using this process are hydrophilic with contact angles of typically about 10˜15 degrees.

The titanium oxide hydrophilic coating48may be formed on the channel surfaces35using an electrochemical plating (ECP) technique. This method involves the use of a 0.5 M sulfuric acid solution, with a stainless steel bipolar plate32as the cathode and titanium coupons as the anode. The titanium coupons are anodized for typically about 10 minutes at an applied potential of typically about 4, 6, 8, 10, 12, 14 and 16 volts, respectively. Contact angle values for the hydrophilic coating48are typically about 35˜43. Alternative methods which may be used to form the titanium oxide hydrophilic coating48on the channel surfaces35include sputter deposition, chemical vapor deposition and reactive electron beam evaporation methods.

Referring next toFIG. 2, a fuel cell stack22is shown which includes the bipolar plate32having the hydrophilic coating48formed according to the present invention. The fuel cell stack22includes a membrane electrode assembly (MEA)24having a polymer electrolyte membrane (PEM)30which is sandwiched between a cathode26and an anode28. A gas diffusion medium10is attached to or abuts against the cathode26, and a gas diffusion medium10ais attached to or abuts against the anode28. The lands42of the bipolar plate32abut against the gas diffusion medium10, whereas lands42aof a bipolar plate32ahaving multiple channels34aabut against the gas diffusion medium10a. Although not shown, a hydrophilic coating48may be formed on the surfaces of the channels34aof the bipolar plate32ain the same manner as was heretofore described with respect to the bipolar plate32.

During operation of the fuel cell22, hydrogen gas36flows through the channels34aof the bipolar plate32aand diffuses through the substrate10ato the anode28. In like manner, oxygen38flows through the channels34of the bipolar plate32and diffuses through the substrate10to the cathode26. At the anode28, the hydrogen36is split into electrons and protons. The electrons are distributed as electric current from the anode28, through a drive motor (not shown) and then to the cathode26. The protons migrate from the anode28, through the PEM30to the cathode26. At the cathode26, the protons are combined with electrons returning from the drive motor and oxygen38to form water40. The water40diffuses from the cathode26, through the substrate10into the channels34of the bipolar plate32and is discharged from the fuel cell stack22.

In the fuel cell stack22, the polymer electrode membrane30requires a certain level of humidity. Irreversible damage to the fuel cell22will occur if the membrane30dries out. Therefore, maintenance of humidity in the membrane30, through humidity/water management, is very important for proper functioning of the fuel cell22. Accordingly, the hydrophilic coating48enhances the wettability of the bipolar plate channel surfaces35of the bipolar plate32, thereby enhancing water management, performance, durability and efficiency of the fuel cell stack22.

For proper functioning of fuel cell, it is required that the water generated does not create any flooding problems. Accumulation of water in the channels34can create mass transport limitation because of the limited solubility of oxygen in water. Such accumulation can cause the cell to perform poorly because of the reactant starve to eventually effect the performance of fuel cell.