Fuel cells are being developed as a power source for many applications including vehicular applications. One such fuel cell is the proton exchange membrane or PEM fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a membrane electrode assembly or MEA. The MEA is a thin, proton-conductive, polymeric, membrane-electrolyte having an anode electrode face formed on one side thereof and a cathode electrode face formed on the opposite side thereof. One example of a membrane-electrolyte is the type made from ion exchange resins. An exemplary ion exchange resin comprises a perfluoronated sulfonic acid polymer such as NAFION™ available from the E.I. DuPont de Nemeours & Co. The anode and cathode faces, on the other hand, 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 particles such as NAFION™ intermingled with the catalytic and carbon particles; or catalytic particles, without carbon, dispersed throughout a polytetrafluorethylene (PTFE) binder.
Multi-cell PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series and separated one from the next by a gas-impermeable, electrically-conductive fluid distribution plate known as a separator plate or a bipolar plate. Such multi-cell fuel cells are known as fuel cell stacks. The 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. Electrically conductive fluid distribution plates at the ends of the stack contact only the end cells and are known as end plates. The bipolar plates contain a flow field that distributes the gaseous reactants (e.g. H2 and O2/air) over the surfaces of the anode and the cathode. These flow fields generally include a plurality of lands which define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header and an exhaust header located at opposite ends of the flow channels.
A highly porous (i.e. ca. 60% to 80%), electrically-conductive material (e.g. cloth, screen, paper, foam, etc.) known as “diffusion media” is generally interposed between electrically conductive fluid distribution plates and the MEA and serves (1) to distribute gaseous reactant over the entire face of the electrode, between and under the lands of the electrically conductive fluid distribution plate, and (2) collects current from the face of the electrode confronting a groove, and conveys it to the adjacent lands that define that groove. One known such diffusion media comprises a graphite paper having a porosity of about 70% by volume, an uncompressed thickness of about 0.17 mm, and is commercially available from the Toray Company under the name Toray 060. Such diffusion media can also comprise fine mesh, noble metal screen and the like as is known in the art.
In an H2—O2/air PEM fuel cell environment, the electrically conductive fluid distribution plates can typically be in constant contact with mildly acidic solutions (pH 3-5) containing F−, SO4−−, SO3−, HSO4−, CO3−−, and HCO3−, etc. Moreover, the cathode typically operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is typically constantly exposed to hydrogen. Hence, the electrically conductive fluid distribution plates should be resistant to a hostile environment in the fuel cell.
One of the more common types of suitable electrically conductive fluid distribution plates include those molded from polymer composite materials which typically comprise 50% to 90% by volume electrically-conductive filler (e.g. graphite particles or filaments) dispersed throughout a polymeric matrix (thermoplastic or thermoset). Recent efforts in the development of composite electrically conductive fluid plates have been directed to materials having adequate electrical and thermal conductivity. Material suppliers have developed high carbon loading composite plates comprising graphite powder in the range of 50% to 90% by volume in a polymer matrix to achieve the requisite conductivity targets. Plates of this type will typically be able to withstand the corrosive fuel cell environment and, for the most part, meet cost and conductivity targets. One such currently available bipolar plate is available as the BMC plate from Bulk Molding Compound, Inc. of West Chicago, Ill.
Alternatively, discrete conductive fibers have been used in composite plates in an attempt to reduce the carbon loading and to increase plate toughness. See copending U.S. Pat. No. 6,607,857 to Blunk, et. al., issued Aug. 19, 2003 which is assigned to the assignee of this invention, and is incorporated herein by reference. Fibrous materials are typically ten to one thousand times more conductive in the axial direction as compared to conductive powders. See U.S. Pat. No. 6,827,747 to Lisi, et. al., issued Dec. 7, 2004 which is assigned to the assignee of the present invention and is incorporated herein by reference.
Another one of the more common types of suitable electrically conductive fluid distribution plates include those made of metal coated with polymer composite materials containing about 30% to about 40% by volume conductive particles. In this regard, see U.S. Pat. No. 6,372,376 to Fronk et al., issued Apr. 16, 2002, which (1) is assigned to the assignee of this invention, (2) is incorporated herein by reference, and (3) discloses electrically conductive fluid distribution plates made from metal sheets coated with a corrosion-resistant, electrically-conductive layer comprising a plurality of electrically conductive, corrosion-proof (i.e. oxidation-resistant and-acid resistant) filler particles dispersed throughout a matrix of an acid-resistant, water insoluble, oxidation-resistant polymer that binds the particles together and to the surface of the metal sheet. Fronk et al-type composite coatings will preferably have a resistivity no greater than about 50 ohm-cm and a thickness between about 5 microns and 75 microns depending on the composition, resistivity and integrity of the coating. The thinner coatings are preferred to achieve lower IR drop through the fuel cell stack.
As discussed above, a great percentage of the electrically conductive fluid distribution plates comprise either a conductive polymeric composite material or a metallic base layer coated with a conductive polymer composite material. While these types of plates currently have acceptable water management properties, there is a desire to provide an electrically conductive fluid distribution plate having increased water management properties.