2. Description of the Related Art Fuel cells are used to convert chemical fuels, particularly hydrogen, into electricity. The structure and function of one variety of fuel cell, using proton exchange membranes, is described in detail in EP 1 009 051 A2. A key element in this structure is the bipolar plate, which serves to separate mechanically the adjacent fuel cells in a series- connected stack of such cells, while providing low-resistance electrical connection between a series of the cells. The voltages of the series-connected cells add up to form a desired higher voltage output from the battery of cells. The bipolar plate also provides channels that carry the fuel gas (typically hydrogen) and the oxidant (oxygen or air) into the cell. The bipolar plate also may carry channels for the circulation of coolant to remove heat from the fuel cells. The bipolar plate may be formed from a single sheet of material, or it may be laminated from two or more thinner sheets.

During operation of the fuel cell, each side of the bipolar plate contacts the liquid electrolyte, which is normally an aqueous sulfuric acid solution, entrained within the pores of an electrically conductive carbon paper. For the current to pass through the series-connected cells in a stack, the bipolar plate must make low-resistance electrical contact with the carbon paper. The bipolar plate is exposed to highly corrosive conditions, including the acid electrolyte and both oxidizing and reducing conditions on opposite sides of the plate. Only a few materials are not destroyed by these harsh conditions. If the bipolar plate is made from a noble metal such as gold or platinum, then it remains unaffected by the acid solution or the electrical potentials developed within the fuel cell.

However, gold and platinum are too expensive for most applications of fuel cells. Most other metals are attacked by the highly corrosive conditions within the cell. Stainless

steel and nickel alloys function for a limited time as bipolar plates, but then develop an electrically insulating oxide surface layer. This oxide surface layer increases the electrical resistance and decreases the efficiency of the fuel cell stack as it is operated.

Graphite is stable as a bipolar plate, but it is expensive and difficult to machine with the intricate channels required for the gas distribution. Graphite is also too fragile unless it is used in such thick pieces that the volume and weight of the fuel cell stack are too large.

No known single material has all the properties desired for a practical bipolar plate: long-term durability in a stack of fuel cells, combined with low electrical resistance, low cost, easy fabrication, light weight and low volume.

Summary of the Invention A principal feature of the present invention includes the use of electrically conductive protective coatings on inexpensive metal bipolar plates in a proton exchange membrane fuel cell stack.

An advantage of the coating is that it protects the underlying metal of the bipolar plate from the acid electrolyte in the presence of both oxidizing and reducing conditions found in a fuel cell.

A related feature of the coating is that it is electrically conductive in order to provide low electrical contact resistance to adjacent components of a fuel cell.

Another advantage of the coating for bipolar plates is that it can be deposited rapidly and inexpensively.

Other features and advantages of the invention will be apparent to those skilled in the art on reading the instant invention.

The above features and advantages have been substantially achieved by the use of a protective layer coating on the metal bipolar separator plates used in fuel cell stacks.

The protective coating is an electrically conductive ceramic oxide that is not readily susceptible to chemical attack under the corrosive conditions of the fuel cell.

In at least some embodiments, a two-layer protective coating for metal bipolar separator plates in fuel cell stacks is provided. In at least some embodiments, the fuel cell includes one or more metal bipolar plates coated with a corrosion-resistant metal and then with electrically conductive, polycrystalline tin oxide. The outer layer can include tin

oxide that is made electrically conductive by addition of dopant elements such as fluorine and/or antimony. Deposition of the electrically conductive tin oxide preferably takes place at a high enough temperature so that the coating has excellent chemical resistance to acids under both oxidizing and reducing conditions found in a fuel cell. The second, inner protective layer lies between the outer tin oxide layer and the base metal of the bipolar separator plate. This inner protective layer comprises a ductile metal or metal alloy that is highly resistant to etching or dissolution in hot sulfuric acid solutions under both oxidizing and reducing conditions. Materials particularly suitable for the inner protective layer include tantalum metal, niobium metal, zirconium metal, hafnium metal, and nickel- based alloys such as Hastelloy C-276, Hastelloy C-22, Incoloy 625 and Inconel 625.

In at least some embodiments, the surface of the inner protective layer is treated to provide surface roughness. Some roughness of the surface of the inner protective metal is desirable in order to promote good adhesion between the tin oxide and the inner metal.

Roughening the surface of the base metal, and depositing the inner metal conformally over this roughened surface may achieve this necessary roughness.

Brief Description of the Drawing The foregoing and various other aspects, features, and advantages of the present invention, as well as the invention itself, may be more fully appreciated with reference to the following detailed description of the invention when considered in connection with the following drawing. The drawing is presented for the purpose of illustration only and is not intended to be limiting of the invention, in which: The Figure is a cross-sectional illustration of bipolar plate with a double coating used in the practice of at least one embodiment of the invention.

Detailed Description of the Invention Tin oxide is a suitable material for the outer protection layer in at least some embodiments of the invention. The tin oxide is doped, e. g. fluorine-and/or antimony- doped, to improve the electrical conductivity of the oxide. Electrically conductive fluorine-doped tin oxide films were deposited onto glass plates using chemical vapor

deposition (CVD). The tin source was dibutyltin diacetate, the fluorine source was trifluoroacetic acid anhydride, and the oxygen source was oxygen gas. 25 weight % trifluoroacetic acid anhydride was mixed with 75 weight % dibutyltin diacetate. 0.3 ml of this liquid mixture was pumped at a rate of 6 ml/hr into a tube through which there was an 8 liter/minute flow of nitrogen gas preheated to 220 °C into which the liquid mixture vaporized. This gas mixture was then mixed with a 2 liter/minute flow of oxygen gas that had been preheated to 220 °C. The combined gases flowed into a rectangular reactor at atmospheric pressure with a cross section 12 cm wide by 1 cm high. The 10 cm by 10 cm glass substrates rested on the bottom of this rectangular space, and they were heated from below to various temperatures from 400 °C to 550 °C. Electrically conductive tin oxide films about 0.5 micron thick were deposited on the glass plates.

The coated glass plates were then tested for corrosion resistance using an accelerated lifetime corrosion test to establish suitability of the material for use in bipolar plates. The material was partly immersed in 10% sulfuric acid solution in water at 80 °C for 3 days, with the material was held at +1 volt relative to a platinum electrode. The test was then repeated with the material held at-1 volt relative to a platinum electrode. The materials were then inspected visually and microscopically for changes. The sulfuric acid concentration used in the test is much higher (about a thousand times) than is normally used in fuel cells, so that any damage to the coating may be observed in a shorter time.

Likewise, the voltage used is higher than those normally encountered within a fuel cell.

The results are summarized in the Table 1.

Table 1 DEPOSITION CONDITIONS STRUCTURE OBSERVATIONS 400 °C, low growth rate (0.5 ml/hr) amorphous Film partly dissolved 450 °C, high growth rate (15 ml/hr) amorphous Film partly dissolved 450 °C, low growth rate (0.5 ml/hr) crystalline Film unchanged 500 °C, high growth rate (15 ml/hr) crystalline Film unchanged 550 °C, high growth rate (15 ml/hr) crystalline Film unchanged The observations in Table 1 show that the polycrystalline tin oxide films are suitable for

use in protecting bipolar plates. Amorphous tin oxide films deposited at lower temperatures and/or higher growth rates are dissolved by sulfuric acid more readily than the polycrystalline tin oxide.

In the next experiment, polycrystalline fluorine-doped tin oxide films were deposited on various metal plates held at substrate temperatures of 500 °C. Various surface finishes were given to the metals before the deposition of the tin oxide.

Table 2 SURFACE PREPARATION APPEARANCE As received Several defects #240 sandpaper Few defects # 600 sandpaper Tiny defects # 600 then # 2400 sandpaper No defects visible # 600 then # 2400 sandpaper + 0. 5 silica Tin oxide peeled off polished surface

The conclusion from these tests is that the most uniform tin oxide films are grown on smoother surfaces, but if the surface is too highly polished, then the adhesion of the film to the metal is not strong enough. Of the surfaces tested, a fine-grained matte finish (produced by #600 then #2400 sandpaper) provided the best preparation of the metal surface for coating with tin oxide.

These coated plates were then subjected to the accelerated lifetime tests in order to evaluate the suitability of the plate as a possible material for the inner protective layer.

The results of these tests are shown in Table 3.

Table 3 METAL SUBSTRATE OBSERVATIONS Aluminum coating flaked off and metal dissolved Carbon steel coating flaked off and metal dissolved Stainless Steel 304 coating flaked off and metal etched Stainless Steel 316 damaged in some areas Titanium damaged in a few small spots Inconel625 unchanged Incoloy 825 unchanged Hastelloy C-276 unchanged Hastelloy C-22'"'unchanged Niobium unchanged Tantalum unchanged Zirconium unchanged Hafnium unchanged

The observations in Table 2 show that aluminum and steels are attacked by hot sulfuric acid even when they are covered with tin oxide. Any small pinhole or crack in the tin oxide coating allows the acid to dissolve the substrate, eventually undercutting the coating and causing the remainder of the coating to flake off. In the cases of titanium and stainless steel 316, there is also some attack of the metal through defects in the coating, but the damage remains more localized. In the cases of niobium, tantalum, zirconium, hafnium and the nickel alloys (Inconel, Incoloy and Hastelloy) there was no apparent attack on these highly corrosion-resistant metals through defects in the tin oxide coatings. In the areas of pinholes or cracks in the tin oxide coating, the corrosion-resistant metal develops a protective oxide that prevents further attack on the metal. Even though this oxide is electrically insulating, its presence does not significantly degrade the overall performance of the bipolar plate, because the insulting oxide forms only on the very small portion of total area corresponding to the area of the pinholes and cracks in the tin oxide coating.

The novel bipolar plates of the invention are now described. As shown in the Figure, the body 10 of a plate is formed from an inexpensive base metal such as steel or aluminum, which would be rapidly attacked by acid in a fuel cell. Thus the base metal 10 is protected by a layer 20 of a corrosion-resistant metal such as tantalum or a nickel alloy, which in turn is overcoated with a layer 30 of electrically conductive tin oxide. The double layer of corrosion-resistant metal and tin oxide protects the base metal from attack by the acid electrolyte 40. The roughened surface of the base metal provides better adhesion between the tin oxide 30 and the corrosion-resistant metal 20.

The invention may be understood with reference to the following examples which are for the purpose of illustration only and which are not limiting of the invention, the full scope of which is set forth in the claims which follow.

Example 1. Bipolar plates made of carbon steel are roughened with # 600 sandpaper and then # 2400 sandpaper, cleaned and dried. The bipolar plates are supported inside a CVD chamber by touching only the outer parts of the plate that will have no contact with the acid electrolyte. Then they are coated with tantalum by the CVD process disclosed by Glanski (US Patent 3,784, 403), following which process they are cooled down to 500 °C in an atmosphere of pure argon. Then, in the same deposition chamber, they are coated with 0.5 micron of fluorine-doped tin oxide by the CVD process disclosed before Table 1. The doubly coated plates show excellent durability when used in a fuel cell.

Example 2. Example 1 is repeated, except that a niobium coating is formed in place of the tantalum coating. The results are similar to those of Ex. 1.

Example 3. Example 1 is repeated, except that a zirconium coating is formed in place of the tantalum coating. The results are similar to those of Ex. 1.

Example 4. Example 1 is repeated, except that a hafnium coating is formed in place of the tantalum coating. The results are similar to those of Ex. 1.

Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.