Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-11-16
2004-08-17
Cantelmo, Gregg (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06777126
ABSTRACT:
INTRODUCTION
The present invention is directed to a fuel cell bipolar separator plate and current collector assembly, and, more particularly, to an improved fuel cell bipolar separator plate and collector assembly having improved manufacturability and cooling, and a corresponding method of manufacture.
BACKGROUND OF THE INVENTION
A fuel cell stack consists of multiple planar cells stacked upon one another, to provide an electrical series relationship. Each cell is comprised of an anode electrode, a cathode electrode, and an electrolyte member. A device known in the art as a bipolar separator plate, an interconnect, a separator, or a flow field plate, separate the adjacent cells of a stack of cells in a fuel cell stack. The bipolar separator plate may serve several additional purposes, such as mechanical support to withstand the compressive forces applied to hold the fuel cell stack together, providing fluid communication of reactants and coolants to respective flow chambers, and to provide a path for current flow generated by the fuel cell. The plate also may provide a means to remove excess heat generated by the exothermic fuel cell reactions occurring in the fuel cells.
Each individual cell produces about 0.5-1.0 volts DC. Electrical current output capacity is based upon the area of the fuel cell electrodes times the current density capacity of the cell. Maximum achievable current density, measured in amps/cm
2
, varies from fuel cell type to fuel cell type. Therefore, the quantity of cells (voltage), the area of the cells (current), and the current density of the cells determine the kW output capacity of a fuel cell stack. In order to achieve output capacities suitable for distributed generation, the fuel cell stack must output a minimum of about 2-10 kW for residential applications, to about 50-100 kW for light commercial/industrial applications. In these scenarios, the fuel cell stack may consist of from about fifty to in excess of one-hundred-and-fifty cells having an area of about 200 cm
2
to about 3000 cm
2
.
Another important measure of a fuel cell stack, in addition to current density, is its volumetric power density. High volumetric power density is desirable for both stationary and transportation applications of fuel cells. Volumetric power density is measured as the watt density per cm
2
of an individual cell times the quantity of cells per linear centimeter of stack height. Therefore, it is desirable to design thin cells to achieve high volumetric power density.
While current density is more a function of the individual fuel cell type, volumetric power density is mostly a function of the physical design of the fuel cell components and the design of the bipolar separator plate.
The design of bipolar separator plates in the prior art has been driven by many wide ranging factors, such as cell chemistry, reactant flow configurations, material selection, system pressurization, operating temperature, system cooling requirements, and intellectual property considerations.
However, there are several common characteristics of bipolar separator plate design. Prior art bipolar separator plates have typically been produced in a discontinuous mode utilizing highly complex tooling that produces a plate with a finite cell area. Alternatively, prior art plates having a finite area may be produced from a collection of a mixture of discontinuously and continuously manufactured sheet-like components that are assembled to produce a single plate possessing a finite cell area. U.S. Pat. No. 6,040,076 to Reeder teaches an example of a Molten Carbonate Fuel Cell (MCFC) bipolar separator plate produced in this fashion, where plates are die formed with a specific finite area of up to eight square feet. U.S. Pat. No. 5,527,363 to Wilkinson et. al. teaches an example of a Proton Exchange Membrane Fuel Cell (PEMFC) “embossed fluid flow field plate,” also die formed with a discrete finite area. U.S. Pat. No. 5,460,897 to Gibson et. al. teaches an example of a Solid Oxide Fuel Cell (SOFC) interconnect, also produced having a finite area. Bipolar separator plates produced with a discontinuous finite area do not enjoy the advantages of continuous production methods such as are commonly used to produce the electrodes and electrolyte members of the fuel cell. Continuous production methods provide cost and speed advantages and minimize part handling. Continuous production using what is known as progressive tooling allows the use of small tools that are able to produce large plates from sheet material. The plate described in Reeder is able to be produced in a semi-continuous fashion, but requires tooling possessing an area equivalent to that of the finished bipolar plate area. The plate described in Reeder requires separately produced current collectors for both the anode and cathode. These current collectors may be produced in a continuous fashion. However, the resultant assembly is material intensive, comprised of three sheets of material. The area of the plate created by the design is fixed and unalterable unless retooled.
Production methods that utilize molds to produce plates from non-sheet material, such as injection molding with polymers, are wholly unable to stream the production process in a continuous mode. As a result, discontinuous production methods require complex tooling and are speed limited. Complex tooling further inhibits design evolution due to the costs associated with replacing or modifying the tools.
Another commonality among the bipolar separator plate designs of the various fuel cell types is the material of construction. Although carbon graphite, polymers, and ceramics are common examples of the materials of choice for the bipolar separator plate of the various fuel cell types, sheet metal can also be found as an example of the material of choice for each of the fuel cell types in the prior art literature. For example, Reeder teaches a metallic MCFC bipolar separator plate. U.S. Pat. No. 5,776,624 to Neutzler teaches a metallic PEMFC bipolar separator plate. Gibson teaches a metallic SOFC bipolar separator plate. U.S. Pat. No. 6,080,502 to Nolscher et. al. teaches a metallic bipolar separator for fuel cells and denotes fuel cells as including Phosphoric Acid Fuel Cell (PAFC) and Alkaline Fuel Cell (AFC). Sheet metal, or metal foil, permits the application of high-speed manufacturing methods such as continuous progressive tooling. Metallic bipolar separator plates for fuel cells further provide for high strength and compact design.
A third commonality of bipolar separator design can be found in the various methods to provide a means to cool the fuel cell. Although some fuel cell stack designs elect to disperse this critical function via dedicated cooling plates at intervals of several cells, or within a wholly separate cooling section, examples of a bipolar separator plate from each fuel cell type can be found to include an integral coolant chamber. These chambers may be designed for gaseous coolant, liquid coolant, or endothermic fuel reforming. Providing a coolant chamber to each individual bipolar separator plate presents engineering and design challenges. Specifically, plate thickness and reactant/coolant manifolding are impacted by the addition of a coolant chamber. The impact on plate thickness can be minimized by using a liquid coolant that possesses a greater heat carrying capacity than do gaseous coolants such as air. Neutzler teaches a “coolant flow passage” centrally located between two outer metallic sheets. Nolscher teaches a “cooling medium distribution duct” also located between two metallic sheets. In both cases, the design utilizes two opposing sheets of material die-formed with a plurality of grooves, or ribs. The cooling chamber is formed when the maximum elevation of one sheet rests on the maximum depression of the subsequent sheet. Both sheets are structural members of the bipolar plate and therefore must be of sufficient strength and robustness to withstand the compressive sealing force applied to the assembled fuel cell stack. U.S. Pat. No. 5,795,665
Banner & Witcoff , Ltd.
Cantelmo Gregg
GenCell Corporation
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