Corrosion resistant PEM fuel cell

Abstract

A pem fuel cell having electrical contact elements comprising a corrosion-susceptible substrate metal coated with an electrically conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles. The substrate may have an oxidizable metal first layer (e.g., stainless steel) underlying the polymer coating.

Description

The Government of the United States of America has rights in this invention pursuant to contract No. DE-AC02-90CH10435 awarded by the United States Department of Energy.

TECHNICAL FIELD

This invention relates to PEM fuel cells, and more particularly to corrosion-resistant electrical contact elements therefor.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, and may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H₂ & O₂/air) over the surfaces of the respective anode and cathode.

Bipolar PEM fuel cells comprise a plurality of the membrane-electrode-assemblies stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or septum. The septum or bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.

In an H₂-O₂/air PEM fuel cell environment, the bipolar plates and other contact elements (e.g., end plates) are in constant contact with highly acidic solutions (pH 3-5) containing F^-, oxidation resistant oxidation-resistant, and acid-resistant protective coating 94 having a resistivity less than about 50 ohm-cm, and comprising a plurality of oxidation-resistant, acid-insoluble, conductive particles (i.e. less than about 50 microns) dispersed throughout an acid-resistant, oxidation-resistant polymer matrix. Preferably, the conductive filler particles are selected from the group consisting of gold, platinum, graphite, carbon, nickel, conductive metal borides, nitrides and carbides (e.g. titanium nitride, titanium carbide, titanium diboride), titanium alloyed with chromium and/or nickel, palladium, niobium, rhodium, rare earth metals, and other nobel metals. Most preferably, the particles will comprise carbon or graphite (i.e. hexagonally crystallized carbon). The particles comprise varying weight percentages of the coating depending on the density and conductivity of the particles (i.e., particles having a high conductivity and low density can be used in lower weight percentages). Carbon/graphite containing coatings will typically contain 25 percent by weight carbon/graphite particles. The polymer matrix comprises any water-insoluble polymer that can be formed into a thin adherent film and that can withstand the hostile oxidative and acidic environment of the fuel cell. Hence, such polymers, as epoxies, silicones, polyamide-imides, polyether-imides, polyphenols, fluro-elastomers (e.g., polyvinylidene flouride), polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics, and urethanes, inter alia inter alia are seen to be useful with the present invention. Cross-linked polymers are preferred for producing impermeable coatings.

The substrate metal forming the contact element comprises a corrosion-susceptible metal such as (1) aluminum which is dissolvable by the acids formed in the cell, or (2) titanium or stainless steel which are oxidized/passivated by the formation of oxide layers on their surfaces. In accordance with one embodiment of the invention, the conductive polymer coating is applied directly to the substrate metal and allowed to dry/cure thereon. According to another embodiment of the invention, the substrate metal comprises an acid soluble metal (e.g., Al) that is covered with an oxidizable metal (e.g., stainless steel) before the electrically conductive polymer topcoat is applied.

The coating may be applied in a variety of ways, e.g., (1) electrophoretic deposition, (2) brushing, spraying or spreading, or (3) laminating. Electrophoretically deposited coatings are particularly advantageous because they can be quickly deposited in an automated process with little waste, and can be deposited substantially uniformly onto substrates having complex and recessed surfaces like those used to form the reactant flow fields on the working face(s) of the contact elements. Electrophoretic deposition is a well-known process useful to coat a variety of conductive substrates such as automobile and truck bodies. Electrophoretic deposition technology is discussed in a variety of publications including “Cathodic Electrodeposition”, Journal of Coatings Technology, Volume 54, No. 688, pages 35-44 (May 1982). Briefly, in electrophoretic deposition processes, a direct current is passed through a suspension of the conductive particles in an aqueous solution of a charged acid-soluble polymer. Under the influence of the applied current, the polymer migrates to, and precipitates upon, a conductive substrate of opposing charge, and carries with it the conductive particles. When cross-linkable polymers are used, the suspension also includes a catalyst for promoting the cross-linking. Cathodic and anodic electrophoretic processes are both known. Cathodically deposited coatings are preferred for fuel cell applications, and are deposited by a process wherein positively charged polymer is deposited onto a negatively charged substrate. Anodically deposited coatings are less desirable since they tend to dissolve some of the substrate metal and contaminate the coating therewith. In cathodic electrophoretic coating, the passage of electrical current causes the water to electrolyze forming hydroxyl ions at the cathode and establishing an alkaline diffusion layer contiguous therewith. The alkalinity of the diffusion layer is proportional to the cathode current density. Under the influence of the applied voltage, the positively charged polymer migrates to the cathode and into the alkaline diffusion layer where the hydroxyl ions react with the acid-solubilized polymer and cause the polymer to precipitate onto the cathodic substrate. The conductive filler particles become trapped in the precipitate and co-deposit onto the cathodic substrate. Cathodic epoxies, acrylics, urethanes and polyesters are useful with this method of depositing the coating as well as other polymers such as those disclosed in the “Cathodic Electrodeposition” publication (supra), and in Reuter et al. U.S. Pat. No. 5,728,283 and the references cited therein. Subsequent baking of the coated contact element cures and densities the coating.

According to another embodiment of the invention, the coating is first formed as a discrete film (e.g. by solvent casting, extrusion etc.), and then laminated onto the working surface of the contact element, e.g., by hot rolling. This technique will preferably be used to make laminated sheet stock from which the contact elements are subsequently formed, e.g. as by stamping. In this embodiment, the discrete film will preferably contain a plasticizer to improve handling of the film and to provide a coating layer atop the substrate that is supple enough so that it can be readily shaped, (e.g. stamped) without tearing or disrupting the film when the contact element is formed as by stamping. To insure adherence of the coating to the substrate, the surface of the substrate to which the film is applied is (1) cleaned of all undesirable surface films (e.g., oil), (2) oxides are removed by acid etching, and (3), most preferably, roughened or abraded to roughen the surface for anchoring the film thereto. Fluroelastomers such as polyvinyladiene diflouride or the like are useful with this embodiment, and may be used with conventional plasticizers such as dibutyl phthalate.

According to another embodiment of the invention, the electrically conductive polymer film is applied to the working face of the substrate by spraying, brushing or spreading (e.g. with a doctor blade). In this embodiment, a precursor of the coating is formed by dissolving the polymer in a suitable solvent, mixing the conductive filler particles with the dissolved polymer and applying it as a wet slurry atop the substrate. The wet coating is then dried (i.e. the solvent removed) and cured as needed (e.g., for thermosets). The conductive particles adhere to the substrate by means of the solvent-free polymer. A preferred polymer useful with this embodiment comprises a polyamide-imide thermosetting polymer. The polyamide-imide is dissolved in a solvent comprising a mixture of N-methylpyrrolidone, propylene glycol and methyl ether acetate. To this solution is added about 21% to about 23% by weight of a mixture of graphite and carbon black particles wherein the graphite particles range in size from about 5 microns to about 20 microns and the carbon black particles range in size from about 0.5 micron to about 1.5 microns with the smaller carbon black particles serving to fill the voids between the larger graphite particles and thereby increase the conductivity of the coating compared to all-graphite coatings. The mix is applied to the substrate, dried and cured to provide 15-30 micron thick coatings (preferably about 17 microns) having a carbon-graphite content of about 38% by weight. It may be cured slowly at low temperatures (i.e. <400° F.), or more quickly in a two step process wherein the solvent is first removed by heating for ten minutes at about 300° F.-350° F. (i.e., dried) followed by higher temperature heating (500° F.-750° F.) for various times ranging from about ½ min to about 15 min (depending on the temperature used) to cure the polymer.

Some coatings may be pervious to the cell's hostile environment. Previous Pervious coatings are used directly only on oxidizable metals (e.g., titanium or stainless steel) and not directly on metals that are susceptible to dissolution in the fuel cell environment (e.g., aluminum). Pervious coatings could however be used on dissolvable metal substrates (e.g., Al) which have first been coated or clad with an oxidizable/passivating metal layer (e.g., titanium or stainless steel). When pervious coatings are used on an oxidizable/passivating substrate or coating, oxides will form at the sites (i.e., micropores) where the coating is pervious, but not at sites where the polymer engages the substrate metal. As a result, only a small portion of the surface is oxidized/passivated (i.e. i.e., at the micropores in the coating) resulting in very little increase in electrical resistance attributable to the oxide formation.

According to one embodiment of the invention, the electrically conductive polymer coating is applied to an acid-dissolvable substrate metal (e.g., Al) which had previously been coated with a layer of oxidizable/passivating metal such as stainless steel. In this regard, a barrier/protective layer 96 of a metal that forms a low resistance, passivating oxide film is deposited onto the substrate 98, and is covered with a topcoat of conductive polymer 54 in accordance with the present invention. Stainless steels rich in chromium (i.e., at least 16% by weight), nickel (i.e., at least 20% by weight), and molybdenum (i.e., at least 3% by weight) are seen to be excellent such barrier/protective layers 96 as they form a dense oxide layer at the sites of the micropores in the polymer coating which inhibits further corrosion, but which does not significantly increase the fuel cell's internal resistance. One such stainless steel for this purpose is commercially available from the Rolled Alloy Company as alloy Al-6XN, and contains 23±2% by weight chromium, 21±2% by weight nickel, and 6±2% by weight molybdenum. The barrier/protective stainless steel layer is preferably deposited onto the metal substrate using conventional physical vapor deposition (PVD) techniques (e.g., sputtering), or chemical vapor deposition (CVD) techniques known to those skilled in these the art. Alternatively, electrolessly deposited nickel-phosphorous alloys appear to have good potential as a substitute for the stainless steel in that they readily form a passivating film when exposed to the fuel cell environment which provides a barrier to further oxidation/corrosion of the underlying coating.

While the invention has been described in terms of specific embodiments thereof it is not intended to be limited thereto but rather only to the extent set forth hereafter in the claims which follow.

Claims

1. In a pem fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte intedacent interjacent said electrodes for conducting ions therebetween, and an electrically conductive contact element having a working face confronting at least one of said electrodessfor electrodes for conducting electrical current from said one electrode, the improvement comprising: said contact element comprising a corrosion-susceptible metal substrate and an electrically conductive, corrosion-resistant protective coating on said face to protect said substrate from the corrosive environment of said fuel cell, said protective coating comprising a mixture of electrically conductive particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix and having a resistivity no greater than about 50 ohm-cm, said mixture comprising graphite particles having a first particle size and other electrically conductive particles selected from the group consisting of gold, platinum, nickel, palladium, rhodium, niobium, titanium carbide, titanium nitride, titanium diboride, chromium-alloyed titanium, nickel-alloyed titanium, rare earth metals and carbon, said other particles having a second particle size less than said first particle size to enhance the packing density of said particles.

2. A fuel cell according to claim 1 wherein said carbon comprises carbon black.

3. A fuel cell according to claim 1 wherein said coating is electrophoretically deposited onto said substrate from a suspension of said particles in an aqueous solution of acid-solubilized polymer.

4. A fuel cell according to claim 1 wherein a discrete film of said coating is laminated onto said substrate to form said electrically conductive contact element.

5. A fuel cell according to claim 1 wherein a precursor of said coating is deposited onto said substrate from a solution thereof, dried and cured to form said coating.

6. A fuel cell according to claim 1 wherein said substrate comprises a first acid-soluble metal underlying a second acid-insoluble, passivating metal layer susceptible to oxidation in said environment.

7. A fuel cell according to claim 1 wherein said polymer matrix is selected from the group consisting of epoxies, silicones, polyamide-imides, polyether-imides, polyphenols, fluro-elastomers, polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics and urethanes.

8. In a pem fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte intedjacent interjacent said electrodes for conducting ions therebetween, and an electrically conductive contact element having a working face confronting at least one of said electrodes for conducting electrical current from said one electrode, the improvement comprising: said contact element comprising a corrosion-susceptible metal substrate and an electrically conductive, corrosion-resistant protective coating on said face to protect said substrate from the corrosive environment of said fuel cell, said protective coating comprising a plurality of electrically conductive particles dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix and having a resistivity no greater than about 50 ohm-cm, said substrate comprising a first acid-soluble metal underlying a second acid-insoluble, passivating layer susceptible to oxidation in said environment.