Patent Grant
7700212
The present invention relates to fuel cells, which generate electricity to power vehicles or other machinery. More particularly, the present invention relates to a bipolar plate which is coated with a corrosion-resistant coating of a high-grade stainless steel using thermal spraying techniques to impart fluoride resistance to the bipolar plate when used in a polyelectrolyte membrane (PEM) fuel cell.
In recent years, much research has been devoted to the development of fuel cell technology, particularly for automotive applications. Fuel cell power plants have shown efficiencies as high as 55%. Furthermore, fuel cell power plants are environmentally-friendly since they emit only heat and water as by-products.
Fuel cells produce energy by combining hydrogen and oxygen to produce water and an end product. In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. In the PEM fuel cell, multiple fuel cells are frequently stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate.
Bipolar plates for PEM fuel cells must be electrochemically stable, electrically conductive and inexpensive. The corrosion of metallic bipolar plates in the fuel cell environment accelerates the corrosion process through degradation of the membrane. The degradation products of the membrane include hydrogen fluoride (HF), which accelerates the corrosion process, causing the corrosion process to become autocatalytic in nature. 316L stainless steel has been used as an inexpensive bipolar plate material.
While 316L stainless steel exhibits a fair corrosion resistance to fluoride ions, the corrosion rate increases with the increase in the fluoride ion leach out rate. This problem can be mitigated somewhat by removing the hydrogen fluoride ions from the fuel cell environment or by using higher grades of stainless steel which are more resistant to corrosion by fluoride ions than 316L stainless steel. However, the use of higher grades of stainless steel for the bipolar plate tends to increase the cost of the bipolar plate.
Various methods are known for increasing the corrosion resistance of a corrosion-susceptible substrate. For example, US20030228512 A1 discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate. US20040091768 A1 discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate. U.S. Pat. No. 6,372,376 B1 discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
It has been found that coating the surface of a lower grade stainless steel bipolar plate, such as a 316L stainless steel bipolar plate, for example, with a thin layer of high-grade stainless steel or alloy using thermal spraying imparts a high degree of fluoride ion corrosion resistance to the bipolar plate while maintaining the cost of the bipolar plate within acceptable levels. Only a small amount of the more expensive (more corrosion resistant) alloy is required.
The present invention is generally directed to a novel bipolar plate which is characterized by enhanced stability and resistance to fluoride corrosion in a fuel cell environment. The bipolar plate of the present invention includes a stainless steel bipolar plate substrate which is typically a low-grade stainless steel, such as 304L or 316L, for example, and a corrosion-resistant coating, which is a higher-grade stainless steel or alloy, coated on the bipolar plate substrate using thermal spraying techniques. The corrosion-resistant coating may be a high-grade stainless steel such as C-276, for example. Other alloys such as 904L, 254SMO and Carp-20, for example, can also be used as coating materials. Accordingly, the corrosion-resistant coating renders the lower-grade stainless steel bipolar plate substrate substantially resistant to fluoride ions in the fuel cell environment. This substantially prolongs the lifetime of the bipolar plate. A cover layer, which may be gold or an organic coating, for example, may be provided on the corrosion-resistant coating to reduce the contact resistance of the corrosion-resistant coating.
The present invention is further directed to a novel method for enhancing the corrosion resistance of a bipolar plate. The method includes providing a bipolar plate substrate, which is typically a lower-grade stainless steel such as 304L or 316L stainless steel, for example; and providing a corrosion-resistant coating on the bipolar plate substrate using thermal spraying techniques. The corrosion-resistant coating may be a thin film of C-276, for example. Alternatively, the corrosion-resistant coating may be an alloy such as 904L, 254SMO or Carp-20, for example. The method may further include deposition of a cover layer on the corrosion-resistant layer to reduce the contact resistance of the corrosion-resistant coating.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which:
According to the present invention, a bipolar plate substrate, which is a low-grade stainless steel such as 304L or 316L stainless steel, for example, is coated with a higher grade of stainless steel using a thermal spray process to fabricate a bipolar plate having an improved corrosion resistance to hydrogen fluoride. Preferably, the cathode side of the bipolar plate substrate is coated with a corrosion-resistant coating to increase the resistance of the cathode side of the bipolar plate to corrosion by chloride or fluoride during operation of a fuel cell. Coating the surface of a low-grade stainless steel bipolar plate substrate with a thin (0.1-2 μm) corrosion-resistant coating of the higher-grade C-276, for example, significantly lowers the corrosion rate of the bipolar plate in a fluoride ion environment while not adding significant cost to the bipolar plate. Alloys which may be coated on the bipolar plate substrate as the corrosion-resistant coating include 904-L, 254SMO or Carp-20 stainless steel, for example.
The contact resistance of the thermally-sprayed stainless steel coating can be minimized by providing a cover layer on the stainless steel coating. The cover layer may be a thin layer (<10 nm) of gold (Au) or an organic coating, for example. Other suitable materials for the cover layer include platinum and its alloys, rhodium, ruthenium and its alloys, and palladium and its alloys. Coating a lower-grade stainless steel or material with a thin, higher-grade stainless steel or material using a thermal spray process according to the invention is cost-effective, since fabricating a thick bipolar plate using a higher-grade stainless steel or material is cost-prohibitive.
A cover layer 16, which may be gold (Au) or an organic material, for example, is typically provided on the corrosion-resistant coating 14 to keep the contact resistance of the corrosion-resistant coating 14 low. Other suitable materials for the cover layer 16 include platinum and its alloys, rhodium, ruthenium and its alloys, and palladium and its alloys. The thickness of the cover layer 16 is preferably less than typically about 10 nm in the case of gold coatings and typically about 10˜28 micrometers in the case of polymeric coatings.
In step 2, a corrosion-resistant coating is formed on the outer surface of the bipolar plate substrate. The thickness of the corrosion-resistant coating is preferably in the range of typically about 0.1-30 μm. The corrosion-resistant coating may be a higher-grade stainless steel including, for example, C-276 stainless steel. Alternatively, the corrosion-resistant coating may be a corrosion-resistant alloy such as 904L, 254SMO or Carp-20, for example. The corrosion-resistant coating is coated onto the exterior surface of the bipolar plate substrate using a thermal spray process.
The process of thermal spraying proceeds with the formation of molten particles or particles which are able to deform plastically. The particles are transported at high speeds within a heat source towards a surface or substrate upon which deposition occurs. The particles undergo spreading and may create a chemical bond with the underlying surface or substrate. In the case of substrate materials that are not able to form a chemical bond with the particles, the substrate surface is pre-roughened to create a mechanical bond. Each droplet or particle impacts the roughened surface and mechanically interlocks with the asperities on the surface.
In step 3, a cover layer is deposited on the corrosion-resistant coating to minimize the contact resistance of the corrosion-resistant layer. The cover layer preferably has a thickness of less than typically about 10 nm. The cover layer may be, for example, gold (Au) or an organic material, for example.
Table (I) below presents the actual corrosion rates and the normalized corrosion rates for various grades of stainless steel, alloys and titanium. The corrosion rate and normalized corrosion rate of each sample was obtained by soaking the sample in an etching solution (1M H2SO4+0.1M HF) for an hour at 80 degrees C.
Table (I) above shows that the higher the grade of stainless steel/alloy, the better the corrosion resistance of the stainless steel/alloy in a sulfuric acid/hydrogen fluoride mixture. The corrosion resistance of titanium in the same solution is comparable to that of 304L stainless steel. The bar graph of
Referring next to
Referring next to
Next, the corrosion-resistant coating 14a, which is the leading end portion of the coating film 15, is stamped between the stamping die 38 and the bipolar plate substrate 12a. This stamps the corrosion-resistant coating 14a into the surface of the bipolar plate substrate 12a to complete fabrication of the enhanced stability bipolar plate 10a, shown in
While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.
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