Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-12-26
2003-04-08
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S047000
Reexamination Certificate
active
06544681
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fuel cells and particularly to solid polymer electrolyte fuel cells incorporating a proton exchange membrane. More particularly, the present invention relates to a corrugated flow field plate assembly for a fuel cell and a means for interconnecting fluid flow channels therein.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Conventional proton exchange membrane (“PEM”) fuel cells generally employ a planar, layered structure known as a membrane, electrode assembly (“MEA”), comprising a solid polymer electrolyte or ion exchange membrane, which is neither electrically conductive nor porous, disposed between an anode electrode layer and a cathode electrode layer. The electrode layers are typically comprised of porous, electrically conductive sheets with electrocatalyst particles at each membrane-electrode interface to promote the desired electrochemical reaction.
In conventional fuel cells, the MEA is interposed between two rigid, planar, substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plate in contact with the anode electrode layer is referred to as the anode plate and the plate in contact with the cathode electrode layer is referred to as the cathode plate. The separator plates (1) serve as current collectors, (2) provide structural support for the MEA, and (3) typically provide reactant channels for directing the fuel and oxidant to the anode and cathode electrode layers, respectively, and for removing products, such as water, formed during operation of the fuel cell. Fuel channels and oxidant channels are typically formed in the separator plates, and such plates are then normally referred to as fluid flow field plates. Herein, “fluid” shall include both gases and liquids; although the reactants are frequently gaseous and the products may be liquids or liquid droplets as well as gases.
During operation of the fuel cell, hydrogen from a fuel gas stream moves from fuel channels through the porous anode electrode material and is oxidized at the anode electrocatalyst to yield electrons to the anode plate and hydrogen ions which migrate through the electrolyte membrane. At the same time, oxygen from an oxygen-containing gas stream moves from oxidant channels through the porous electrode material to combine with the hydrogen ions that have migrated through the electrolyte membrane and electrons from the cathode plate to form water. A useful current of electrons travels from the anode plate through an external circuit to the cathode plate to provide electrons for the reaction occurring at the cathode electrocatalyst.
Multiple unitary fuel cells can be stacked together to form a conventional fuel cell stack to increase the overall power output. Stacking is typically accomplished by the use of electrically conductive bipolar plates, which act both as the anode separator plate of one fuel cell and as the cathode separator plate of the next fuel cell in the stack. One side of the bipolar plate acts as an anode separator plate for one fuel cell, while the other side of the bipolar plate acts as a cathode separator plate for the next fuel cell in the stack. The bipolar plates combine the functions of anode and cathode plates referred to above and are provided with fuel channels and oxidant channels.
Fluid reactant streams are typically supplied to channels in the flow field plates via external inlet manifolds connected to the sides of the stack or by internal inlet manifolds formed by aligning openings formed in the bipolar plates and each MEA in the stack dimension. Similarly, fluid stream exhaust manifolds may be external or internal exhaust manifolds. Typically the stack also has coolant passageways extending through the bipolar plates and each MEA for circulating a coolant fluid to absorb heat generated by the fuel cell reaction.
A typical conventional bipolar flow field plate has a plurality of parallel open-faced oxidant channels on one side and a plurality of parallel open-faced fuel channels on the other side. The oxidant channels extend between an oxidant inlet manifold opening and an oxidant outlet manifold opening in the bipolar plate and typically traverse the central area of one plate surface in a plurality of passes, that is, in a serpentine manner, between the inlet manifold opening and the outlet manifold opening. Similarly, the fuel channels extend between a fuel inlet manifold opening and a fuel outlet manifold opening in the bipolar flow field plate and traverse the central area of the other plate surface in a similar plurality of passes between the fuel inlet manifold opening and the fuel outlet manifold opening.
The design of the fluid flow channel patterns is important for correct and efficient fuel cell operation. The flow channels should be sized correctly to provide the reactant species to the MEA layer, and the flow path should provide sufficient pressure drop for the flow velocities to be maintained in operation. It is desirable to be able to create flow fields with substantially different geometries on either side of the bipolar flow field plate. While serpentine channel patterns are a preferred design, channel patterns other than serpentine may be used.
The MEA is physically supported in a conventional fuel cell stack, as it is exposed to the compression forces to prevent fluid leaks between adjacent fluid flow channels in the fuel cell stack. In conventional fuel cell designs such leakage is undesirable, particularly if the channels are serpentine, as some of the fluid may move directly from the inlet manifold opening across the channels to the outlet manifold opening without passing through the passes of the channels and so missing most of the MEA. To prevent such fluid movement, flow field plates with extremely flat surfaces are required in conventional fuel cell stacks, necessitating either the grinding of the surfaces of the flow field plates or the use of molds to form the flow field plates to exacting specific tolerances.
Watkins et al. U.S. Pat. Nos. 4,988,583 and 5,108,849, issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates in which continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner, between an inlet manifold opening and an outlet manifold opening formed in the plate.
A conventional bipolar plate is fabricated from bulk materials such as graphite and formed by creating a substantially flat plate of a given thickness and then removing materials to form flow channels in the plate. Alternately, the plate can be molded or built-up by depositing layers of materials onto a planar substrate until the desired flow channels are created. Generally, however, the manufacture of the bipolar plate remains a difficult and somewhat expensive process.
Bipolar plates manufactured according to conventional means should be made thick enough to withstand the rigors of manufacturing and be rigid enough to support the large clamping forces exercised during stack operation. This typically requires the plate to be thick enough to accommodate the full depth of the flow channels on both sides of the plate, as well as to provide a sufficiently thick solid portion in between the flow channels. This in turn means that bipolar plate design usually involves a trade-off between preferred dimensions for fuel cell operation and feasible dimensions for structural strength and cost-effective manufacturing.
It would be desirable to fabricate bipolar flow field plates for use in fuel cell stacks using low cost materials and simple manufacturing techniques that do not rely on precise machining of components. Furthermore, it would be desirable to reduce the volume of bulk material used in the formation of a bipolar flow field p
Djilali Nedjib
Donis Walter Roberto Mérida
Lindstrom Jeremy
McLean Gerard F.
Reid Christopher E. J.
Ballard Power Systems Inc.
McAndrews Held & Malloy Ltd.
Scaltrito Donald V.
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