Fuel cell current collector

Details Fuel cell current collector Fuel cell current collector

1999-10-07

2002-05-07

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S010000, C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06383677

ABSTRACT:

CROSS-REFERANCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERANCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention relate generally to fuel cells which utilize a current collector and/or a separator for the purpose of providing an electronic flow path for current generated by the fuel cell, to support the electrodes and electrolyte holding member, and to form the flow field for gas access to electrodes.
2. Description of Related Art
Conventional planar fuel cell stacks typically are comprised of a plurality of fuel cell subassemblies arranged in an electrical series relationship. Each fuel cell sub-assembly may be comprised of an anode electrode, a separator plate, and a cathode electrode. Each electrolyte holding member is located between adjacent fuel cell sub-assemblies so as to be in contact with the anode and the cathode of adjoining fuel cell sub-assemblies. Another approach is to provide a plurality of membrane-electrode assemblies, or MEA's, with the separators located between adjacent MEA's. At assembly, the fuel cell stack is compressed axially to afford good intimate contact at each interface of the fuel cell stack to establish the electronic flow path for the electrons liberated by the electrochemical fuel cell reaction.
The separator plate, being disposed between adjacent anodes and cathodes, is required to be constructed from a conductive material. Typically, the basis for selection of material to construct the separator is a function of the operating characteristics of the fuel cell type. Each of the various fuel cell types has its particular electrolyte and operating temperature and provides various degrees of operating efficiencies. Typically, fuel cells which operate at low temperatures ( ~<400 C.) may utilize a polymer separated carbon graphite for the separator material. Fuel cells which operate at temperatures greater than ~400 C. utilize stainless steels and ceramics as the separator material.
The separator plate of a conventional high temperature fuel cell stack serves multiple purposes.
The separator acts as a housing for the reactant gasses to avoid leakage to atmosphere and cross-contamination of the reactants.
The separator acts as a flow field for the reactant gasses to allow access to the reaction sites at the electrode/electrolyte interfaces.
The separator further acts as a current collector for the electronic flow path of the series connected fuel cells.
In many cases the separator is comprised of multiple components to achieve these purposes. Typically, three to four separate components, or sheets of material, are needed depending upon the flow configuration of the fuel cell stack. It is frequently seen that one sheet of material is used to provide the separation of anode/cathode gasses while two additional sheets are used to provide the flow field and current collection duties for the anode and the cathode sides of the separator. Another example of prior art is to use one sheet of ribbed or dimpled material to create the anode/cathode separation as well as the flow fields. Additional sheets of perforated material are used for current collection and, in some instances, to enhance the flow field cross sectional area.
There are three fundamental flow patterns of the reactant gasses which may be applied to the separator to achieve varying objectives. The three patterns consist of co-flow, counter-flow, and cross-flow. Each of the three flow patterns introduces varying degrees of complexity to the design and construction of the separator and current collectors. Low temperature fuel cells, employing carbon graphite as the separator material, often will utilize a combination of the three fundamental flow patterns resulting in sinusoidal flow paths or “Z” patterns.
U.S. Pat. No. 4,548,876 teaches the application of a “corrugated metallic electron collector” which “includes a plurality of corrugations therein”. A preferred embodiment described in this patent utilizes particles within the metallic electron collector to “provide support for a respective catalyst (i.e. electrode) immediately adjacent to and in contact with the metallic electron collector”. These collectors are adjacent to flat “separators”.
This approach has been further advanced through the application of an additional sheet metal component comprised of a perforated sheet positioned between the “corrugated metallic electron collector” and the respective electrode. These approaches have proved to be technically feasible, however, the material content of such structures is economically prohibitive, consisting of three to five sheet metal components of rather significant complexity.
U.S. Pat. Nos. 4,654,195 and 5,531,956 among others teaches the application of “ribs” to the anode electrode of the fuel cell. This approach is intended to apply the flow field directly to the electrode. While technically effective, typically the material cost of the anode is greater than that of sheet metal used to otherwise form an anode flow field with the current collector and separator. Additionally, depth of the ribs formed on the anodes is insufficient for large area fuel cells requiring large cross-sectional flow area. Furthermore, excessive mechanical creep of ribbed anodes can result in poor performance of the fuel cell.
U.S. Pat. No. 4,983,472 teaches the application of a “plurality of arches” to the current collector in a somewhat similar fashion as the above mentioned U.S. Pat. No. 4,548,876. However, the plurality of arches are distributed much more densely and create a finer degree of support to the electrodes thus eliminating the requirement for supporting particles or an additional perforated sheet metal component. This approach has proven to be technically successful but yet again has not reduced the component count of the separator plate below three sheets of material, two current collectors and one separator sheet.
U.S. Pat. No. 5,503,945 teaches the application of corrugations to the “main plate” of the separator and the use of perforated current collector for both the anode and cathode. This patent further teaches the integration of the current collector with its respective electrode. Additionally, this patent teaches the integration of the current collector of either the anode or the cathode with the peripheral sealing structure of the separator and claims a two piece separator with reduced material content and component count. However, the requirement for a current collector for the anode and for the cathode have not been eliminated. The active central area of the fuel cell typically constitutes the far greater portion of the area of the fuel cell relative to the peripheral sealing area. Therefore, while component count of the separator assembly has been reduced through integration of one of the current collectors with either the anode or with the cathode, the material content and component count of the separator as a whole has not appreciably been altered when viewing the total assembly. Furthermore, current collector/separator designs which utilize a ribbed separator and a nominally flat perforated current collector suffer from diminished cross-communication of the reactant gas from one rib to adjoining ribs.
U.S. Pat. No. 5,795,665 teaches the application of “a plurality of rows of dimples” to the separator plate and to the “current collector/active component subassembly”. Though resulting in modest reduction of material content the separator/current collector component count remains at three. This invention provides for cross-flow, co-flow, or counter-flow of reactant gasses utilizing three sheets of material.
U.S. Pat. No. 5,811,202 teaches the application of ribs to an “anode field plate” and to a “cathode field plate” separated by a “flat middle plate”. A perforated current collector is disposed between the cathode field plate and the cathode electrode throughout the central active area of the fuel cell. Again,

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