Low contact resistance PEM fuel cell

Details Low contact resistance PEM fuel cell Low contact resistance PEM fuel cell

2001-11-20

2004-11-02

Yuan, Dah-Wei (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S010000, C429S010000, C429S006000

Reexamination Certificate

active

06811918

ABSTRACT:

TECHNICAL FIELD
This invention relates to PEM fuel cells, and more particularly to electrical current collectors (e.g. bipolar plates) therefor that are made from polymer composite materials, and have low contact resistance with adjacent diffusion media.
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” (MEA) 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 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 current collectors for the anode and cathode, which current collectors typically contain a number of lands that define a plurality of channels or grooves for supplying the fuel cell's gaseous reactants (i.e., H
2
& O
2
/air) to the surfaces of the respective anode and cathode.
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 current collector known as 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. Current collectors at the ends of the stack contact only the end cells and are known as end plates.
A highly porous (i.e. ca. 60%-80%), electrically-conductive material (e.g. cloth, screen, paper, foam, etc.) known as “diffusion media” is interposed between the current collectors and the MEA and serves (1) to distribute gaseous reactant over the entire face of the electrode, between and under the lands of the current collector, 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.
In an H
2
—O
2
/air PEM fuel cell environment, the current collectors are in constant contact with highly acidic solutions (pH 3-5) containing F
−
, SO
4
−−
, SO
3
−
, HSO
4
−
, CO
3
−−
, and HCO
3
−
, etc. Moreover, the cathode 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 constantly exposed to hydrogen. Hence, the current collectors must be resistant to a hostile environment in the fuel cell. Accordingly, current collectors have heretofore been either (1) machined from pieces of graphite, (2) molded from polymer composite materials comprising about 70% to about 90% % by volume electrically-conductive filler (e.g. graphite particles or filaments) dispersed throughout a polymeric matrix (thermoplastic or thermoset), or (3) fabricated from metals coated with polymer composite materials containing about 30% to about 40% by volume conductive particles. In this later regard, see co pending U.S. patent application, Fronk et al Ser. No. 09/456,478, filed Dec. 7, 1999 which (1) is assigned to the assignee of this invention, (2) is incorporated herein by reference, and (3) discloses current collectors 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.
Lightweight metals such as aluminum and their alloys have also been proposed for use in making fuel cell current collectors. Unfortunately, such metals are susceptible to dissolution in the hostile PEM fuel cell environment. Accordingly, it has been proposed to coat lightweight metal current collectors with a layer of metal or metal compound, which is both electrically conductive and corrosion resistant to thereby protect the underlying metal. See for example, Li et al RE 37,284E, issued Jul. 17, 2001, which is assigned to the assignee of the present invention, and discloses a lightweight metal core, a stainless steel passivating layer atop the core, and a layer of titanium nitride (TiN) atop the stainless steel layer.
SUMMARY OF THE INVENTION
The present invention relates to PEM fuel cells using current collectors made either entirely, or at least in part, from polymer composite materials such as described above. A high contact resistance exists between the diffusion media and current collectors made from such composite materials owing to the fact that the diffusion media is porous (ca. 60%-80% porosity) and the surface of the composite material engaging the diffusion media contains less than 70% (typically about 30% to about 40%) by volume of conductive filler (with the remainder being non-conductive polymer matrix material). Hence, current flow between the diffusion media and the composite material occurs only at sites where a conductive strand of the diffusion media contacts the conductive filler in the composite material. No current flows at sites (1) where a diffusion media strand meets the polymer matrix material of the composite, or (2) where a composite's conductive filler meets a pore in the diffusion media.
The present invention reduces the contact resistance between the diffusion media and composite-containing current collectors in PEM fuel cells. More specifically, the present invention is directed to a PEM fuel cell having at least one cell comprising (1) a pair of opposite polarity electrodes each having a first face exposed to a fuel cell reactant and a second face engaging a membrane-electrolyte interjacent said electrodes, (2) a porous, electrically-conductive diffusion media engaging the first face for distributing reactant over, and conducting electrical current from, the first face, and (3) a current collector (e.g. a bi-polar plate) engaging the diffusion media for conducting electrical current from the media. The current collector comprises a polymer composite that has a first electrical conductivity and comprises corrosion-proof electrically conductive filler dispersed throughout an oxidation-resistant and acid-resistant, water-insoluble polymeric matrix. The polymeric matrix may be either a thermoplastic or a thermoset material, and will preferably be selected from the group consisting of epoxies, polyamideimides, polyether-imides, polyphenols, fluro-elastomers, polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics, and urethanes. The electrically conductive filler in the composite (1) are preferably selected from the group consisting of gold, platinum, graphite, conductive carbon, palladium, rhodium, ruthenium, and the rare earth metals, and (2) may take many physical forms (e.g. elongated filaments, or spheroidal, flake, fibrillose particles or aggregates of such particle

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