Conductive coatings for PEM fuel cell electrodes

[0001] The present invention relates to fuel cells; more particularly, to fuel cells having bipolar plate electrodes which separate the individual modules of the fuel cell stack; and most particularly, to means for increasing the reliability and durability of the electrical contact surface of a bipolar plate electrode.

[0002] Fuel cell assemblies employing a plurality of individual fuel cell modules are well known. Each module has an anode and a cathode. In a proton-exchange fuel cell, the anode and cathode are separated by a catalytic proton exchange membrane (PEM) in which the modules in the stack typically are connected in series electrically through bipolar plates to provide a desired total output voltage. Fuel in the form of hydrogen and water vapor, or hydrogen-containing mixtures such as “reformed” hydrocarbons, is flowed through a first set of reaction channels formed in a first surface of the bipolar plate adjacent the anode. Oxygen, typically in the form of air, is flowed through a second set of reaction channels formed in a second surface of the bipolar plate adjacent the cathode.

[0003] In a PEM fuel cell, hydrogen is catalytically oxidized at the anode-membrane interface. The resulting proton, H⁺, migrates through the membrane to the cathode-membrane interface where it combines with ionic oxygen to form water. Electrons flow from the anode through a load to the cathode, doing electrical work in the load.

[0004] In fuel cells, a long-term electrical continuity problem is well known in the art. Metals typically used to form bipolar plates, for example, aluminum a stainless steel, either corrode or form high-resistance oxide passivation layers on the surface of the bipolar plates because of electrochemical activity at these surfaces. These high resistant oxide layers limit the current-collecting ability of the bipolar plates, significantly lower the efficiency and output of a fuel cell. In the prior art, bipolar plates are known to be coated with noble metals such as gold and platinum to prevent corrosion and the formation of high resistant passivation layers on the electrical contact surfaces, but such coatings are so expensive as to impact the widespread use of cost-effective fuel cells.

[0005] What is needed is a simple and cost-effective means for maintaining high electrical conductivity of the electrical-contact surfaces of a bipolar plate.

[0006] It is a principal object of the present invention to provide an improved bipolar plate which is simple and inexpensive to manufacture and which maintains high electrical conductivity of the surface during use in a fuel cell.

[0007] It is a further object of the invention to increase the durability and reliability of a fuel cell.

[0008] Briefly described, electrical contact surfaces of a bipolar plate, as disclosed in this invention, are formed of inexpensive metals which when oxidized form highly conductive surface oxide passivation layers, thus maintaining high electrical conductivity and continuity through the bipolar plates of a fuel cell. Alloy composition systems such as, but not limited to, Ti—Nb, Ti—Ta, La—Sr—Cr, and La—Sr—Co are known to form oxide passivation layers which are highly conductive. The passivation layers may be formed in situ after assembly of a fuel cell or may be provided in an oxidative step during manufacture prior to assembly. The bipolar plate may be formed entirely of one or more of such alloys or may be formed of an inexpensive substrate metal having the alloys coated thereupon.

[0012] Referring to FIG. 1, a portion 10 of a multiple-cell PEM fuel cell stack includes a first fuel cell unit 12a and a second fuel cell unit 12b, separated and mechanically/electrically connected by a bipolar plate 14. Each fuel cell 12a, 12b comprises a proton exchange membrane 16 separating an anode 18 and a cathode 20 in known fashion. A complete fuel cell stack (not shown) comprises a plurality of fuel cells 12 similarly separated and connected by a plurality of bipolar plates 14.

[0013] Each bipolar plate 14 is provided with a first set of channels 22 for providing oxygen, typically in the form of air, to the cathode surface 23. Channels 22 are separated by first lands 24 for making mechanical and electrical contact with cathode 20. Optionally, a conductive cathode diffuser 26 may be provided between cathode 20 and bipolar plate 14 to permit air to diffuse laterally and thereby reach those portions 25 of the cathode surface 23 covered by first lands 24.

[0014] Each bipolar plate 14 is provided with a second set of channels 28 for providing hydrogen fuel to the anode surface 29. Channels 28 are separated by second lands 30 for making mechanical and electrical contact with anode 18. Optionally, a conductive anode diffuser 32 may be provided between anode 18 and bipolar plate 14 to permit fuel to diffuse laterally and thereby reach those portions 31 of the anode surface 29 covered by second lands 30.

[0015] It should be understood that in an actual bipolar plate, air and fuel channels 22 and 28 may be oriented orthogonally of each other.

[0016] Referring to FIG. 2, surface portions of lands 24,30 of improved bipolar plate 14′ include pure metals, metal alloys, or metallic compounds which may be oxidized to form surface passivation oxide layers having high electrical conductivity, defined by a surface resistivity less than about 0.007 ohms-cm² and preferably less than about 0.0035 ohms-cm² or less. By contrast, the comparable surface resistivity of surface-oxidized pure titanium is about 0.035 ohm-cm². Thus, oxide passivation layers formed in accordance with the invention provide a conductivity improvement on the order of about ten-fold or greater. Examples of suitable metals and alloy systems include, but are not limited to, Ti—Nb, Ti—Ta, La—Sr—Cr, and La—Sr—Co. Those skilled in the art of metal chemistry may recognize other suitable metals or metal combinations which may form conductive oxide passivation layers within the scope of the invention.

[0017] In a currently preferred embodiment, contact surfaces of lands 24,30 of a bipolar plate substrate 14′ (formed of preferably aluminum or stainless steel) comprise a thin outer layer 36 of an alloy comprising between about 0 and about 3 atomic percent Nb or Ta and between about 97 and about 100 atomic percent Ti; most preferably, 1.5 atomic percent Nb or Ta and 98.5 atomic percent Ti. Such a layer is readily formed by any of several known methods, for example, by magnetron sputtering of Ti, Nb and Ta targets onto substrate 34, and can form an oxide layer having a surface resistivity of about 0.0035 ohms-cm².

[0018] Preferably, layer 36 is subsequently treated thermally, chemically, and/or electrochemically in known fashion during manufacture of bipolar plate 14 to cause a portion of the metallic constituents of layer 36 to be converted to metal-like electrically-conductive oxides, for example, Ti⁺³_xTi⁺⁴_(1-x)Nb⁺⁵_xO₂ or Ti⁺³_xTi⁺⁴_(1-x)Ta⁺⁵_xO₂ where x represents the atomic fraction of Nb or Ta metal based, typically but not limited to the range of 0.0≦x≦0.03. Alternatively, bipolar plate 14′ containing layer 36 may be assembled into a fuel cell stack and layer 36 may be allowed to oxidize spontaneously by exposure to atmospheric oxygen and electrical current during operation of the fuel cell.

[0019] In a second embodiment in accordance with the invention, most or all of bipolar plate 14′ is formed of metal material as recited hereinabove, the surfaces thereof then being oxidized to form layers 36. This embodiment is currently not preferred because it is wasteful of those materials, only a thin surface coating thereof being required for satisfactory performance of a bipolar plate.

[0020] While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.