COMPOSITE BIPOLAR SEPARATOR PLATE FOR AIR COOLED FUEL CELL

BACKGROUND

Technical Field

The present invention relates to improved bipolar separator plates for use in an air cooled fuel cell and, in particular, for use in a solid polymer electrolyte fuel cell.

Description of the Related Art

Fuel cells are devices in which fuel and oxidant fluids electrochemically react to generate electricity. A type of fuel cell being developed for various commercial applications is the solid polymer electrolyte fuel cell, which employs a membrane electrode assembly (MEA) comprising a solid polymer electrolyte made of a suitable ionomer material (e.g., Nafion ^® ) disposed between two electrodes. Each electrode comprises an appropriate catalyst located next to the solid polymer electrolyte. The catalyst may be, for example, a metal black, an alloy, or a supported metal catalyst such as platinum on carbon. The catalyst may be disposed in a catalyst layer, and the catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte. A fluid diffusion layer (a porous, electrically conductive sheet material) is typically employed adjacent to the electrode for purposes of mechanical support and/or reactant distribution. In the case of gaseous reactants, such a fluid diffusion layer is referred to as a gas diffusion layer. If a catalyst layer is incorporated onto a gas diffusion layer, the unit is referred to as a gas diffusion electrode.

For commercial applications, a plurality of fuel cells are generally stacked in series in order to deliver a greater output voltage. Separator plates are typically employed adjacent the gas diffusion electrode layers in solid polymer electrolyte fuel cells to separate one cell from another in a stack. In a fuel cell stack, if a separator plate is adjacent one cell's anode on one side and another cell's cathode on the other side, it is referred to as a bipolar plate. Fluid distribution features, including

inlet and outlet ports, fluid distribution plenums and numerous fluid channels, are typically formed in the surface of the separator plates adjacent the electrodes in order to distribute reactant fluids to, and remove reaction by-products from, the electrodes. Such separator plates are also referred to as flow field plates. These bipolar separator/flow field plates also provide a path for electrical and thermal conduction, as well as mechanical support and dimensional stability to the MEA.

Certain fuel cell types are liquid cooled and, along with anode and cathode flow fields, also employ coolant flow fields. The coolant flow field is frequently located between an anode flow field and a cathode flow field in a composite (two or more piece) separator plate.

In air cooled fuel cells, the cathode flow field can serve both for oxidant distribution and for cooling purposes. The cathode flow field is thus sized appropriately to serve both purposes and a separate coolant flow field is generally not employed. In some constructions, additional cooling (if desired) may be achieved by incorporating cooling fins or other similar features. Published PCT application WO98/39809 and published U.S. application US2007/0148509 exemplify some options for air cooled fuel cell constructions. In particular, published U.S. application US2007/0148509 discloses a preferred bipolar plate construction for a commercial air cooled fuel cell stack.

While advances have been made in the field of air cooled fuel cells, there remains a need for improved fuel cell construction. The present invention fulfills this need and provides other advantages.

BRIEF SUMMARY

A composite bipolar separator plate is disclosed for use in an air cooled fuel cell, comprising an anode flow field on one major surface and a cathode flow field on the opposite major surface. The bipolar separator plate further comprises two component plates that are attached together. One component plate is a base plate comprising the anode flow field on one major surface. The other component plate is a corrugated plate adjacent the opposite major surface of the base plate. The major surface of the corrugated plate opposite the base plate comprises the cathode flow field.

In the middle of the composite separator plate, the adjacent major surfaces of the corrugated plate and the base plate together define air cooling channels for the fuel cell. Composite construction provides greater air cooling capacity for a given thickness of bipolar plate, whereas such air cooling channels would not generally be present if the plate were made in a single piece. Further, a thinner composite plate having the same air cooling capacity may be used in place of a thicker bipolar plate made from a single piece of material.

The corrugated plate may comprise corrugations that are linear and parallel and, in one embodiment, can be formed as a corrugated metal sheet. The corrugated plate may be attached to the base plate in various ways, such as with epoxy.

The base plate may be a molded carbon plate and may be of a high aspect ratio, and can have a rectangular shape. In such an embodiment, the corrugations on the corrugated plate may preferably be parallel to the short side of the rectangular base plate.

The composite bipolar separator plate is suitable for use in solid polymer electrolyte fuel cells making up an air cooled fuel cell series stack.

These and other aspects of the invention will be evident in view of the attached figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows exploded views of a bipolar plate design for use in a prior art air cooled solid polymer electrolyte fuel cell stack.

Figure 2 shows a side view of the bipolar plate of Figure 1.

Figures 3 a and 3b show an exploded side view and an assembled side view, respectively, of a composite bipolar separator plate of the invention.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced

without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.

Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as "comprises" and "comprising" are to be construed in an open, inclusive sense, that is, as "including but not limited to".

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Also herein, the word "corrugated" refers to a surface that has been formed into a set of alternating grooves and ridges. A typical corrugated surface is frequently, but not necessarily, comprised of a set of linear and parallel alternating grooves and ridges.

Figure 1 has been reproduced from the aforementioned published U.S. application US2007/149509 and shows two views of a bipolar separator plate assembly suitable for use in a commercial solid polymer electrolyte fuel cell stack. Bipolar separator plate 2 has a relatively high aspect ratio rectangular shape. The isometric exploded bottom view is of the cathode/air side and the isometric exploded top view is of the anode/fuel side. The anode/fuel side of bipolar separator plate 2 has serpentine fuel flow field channels 6 and is sealed against the bridge seal of an adjacent membrane electrode assembly (not shown) with perimeter seal 3 which rests in seal groove 1. The cathode/air side of bipolar separator plate 2 has parallel, linear air flow field channels 7 that are perpendicular to the fuel flow field channels 6 and are open to air flow on both ends of the plate. Air can be directed through channels 6 using a fan (not shown). The ports 10 of the plate on each end can be sealed with port seals 8 which rest within port

seal grooves 9. (In a further embodiment, port seals 8 can be replaced with either port plugs 4 or port plugs with tabs 5 to advantageously adapt the plate assemblies to the ends of the fuel cell stack where ports are not required such that two different types of plates are not needed.)

Figure 2 shows a side view of the same bipolar plate 2 of Figure 1 with the cathode side of plate 2 facing downwards. As depicted, air flow field channels 7 are seen to have a cross section that is roughly triangular. Bipolar plate 2 is typically made of a carbonaceous material as this can afford the required electrical, thermal, mechanical, and corrosion resistance properties needed in a solid polymer electrolyte fuel cell. For ease of manufacture, plate 2 may be made by forming a suitable porous carbonaceous blank (e.g., by molding or stamping), then impregnating with a suitable impregnant to fill any remaining pores, and carbonizing the impregnant if required/desired.

However, in many fuel cell applications, air flow field channels 7 are sized to meet the cooling requirements of the fuel cell stack and may be oversized with respect to the air reactant requirements of the fuel cell stack. In such a situation, the bipolar plate of Figure 2 may be replaced with a thinner composite bipolar plate as depicted in Figures 3 a and 3b. These Figures show an exploded side view and an assembled side view, respectively, of such a thinner composite bipolar separator plate.

Composite plate 12 in Figures 3a and 3b is comprised of base plate 12a and corrugated plate 12b. As depicted here, base plate 12a comprises fuel flow field 6 (not visible in these side view Figures) on its upper major surface and a flat location on its opposite lower major surface in which to nest corrugated plate 12b. Corrugated plate 12b may be attached to base plate 12a with epoxy (or like adhesive) at suitable locations near its ends, and in a manner to provide appropriate electrical contact.

Base plate 12a can be made of a similar material and in a similar manner as separator plate 2 in Figure 2. However, the forming process may be substantially simpler because the air flow fields now are not formed thereon. Also, as explained below, base plate 12a may be made significantly thinner than separator plate 2 of Figures 1 and 2.

Corrugated plate 12b is generally made of a suitable thin metal sheet for use in such fuel cell applications. The choice of materials and methods of corrugating such sheets is well known to those of skill in the art. As shown in Figures 3a and 3b, corrugated sheet 12b is formed into roughly a square wave shape. However, a variety of corrugation shapes may be employed instead. Channels 17 (created on the lower major surface of corrugated plate 12b) serve as the air flow field channels in this improved design. Channels 18 (created on the upper major surface of corrugated plate 12b), together with base plate 12a, define air cooling channels 19 through composite bipolar separator plate 12.

The total air cooling capacity in a fuel cell comprising the improved composite bipolar separator plate depends on the total cross sectional area provided by both channels 17 and air cooling channels 19. Thus, with regard to air cooling requirements, composite plate 12 can substitute for separator plate 2 in Figure 2 if, roughly speaking, the combined cross sectional area of channels 17 and 19 is equal to or greater than that of air flow field channels 7 in Figure 2. And, with regard to air reactant requirements, composite plate 12 can substitute for separator plate 2 if the cross sectional area of channels 17 meets the minimum requirements for the fuel cell application.

Thus, depending on the situation, the cross sectional area of air flow field channels 17 may be made significantly smaller than channels 7, without sacrificing overall cooling capacity. In particular, the height of channels 17, and hence the thickness of composite plate 12, can be made significantly smaller than the thickness of separator plate 2. The composite bipolar separator plate thus offers the advantages of a reduction in plate thickness as well as enhancing ease of manufacturing of the base plate component.

The following example is provided to illustrate certain aspects and embodiments of the invention but should not be construed as limiting in any way.

EXAMPLE

A prior art, single piece, impregnated carbon, bipolar separator plate such as depicted in Figure 2 can be about 5.0 mm thick overall with air channels that are roughly triangularly shaped with a cross sectional area of 4.4 mm ² . The openings of the air channels can be 2.2 mm wide with the adjacent peaks of the plate 1.3 mm wide.

The prior art plate can be replaced with a composite plate comprising a 3.5 mm thick (as opposed to 5.0 mm thick), impregnated carbon base plate to which is attached an inset corrugated metal plate as depicted in Figures 3 a and 3b. The composite plate can be made of an appropriate 0.15 mm thick metal sheet (although thinner sheets could be used) and is formed roughly into a square wave shape of 1.5 mm amplitude (the outside dimensions). The openings of the air channels and the widths of the adjacent square wave peaks are taken to be similar in dimension to the prior art plate. Air cooling is now provided through both the oxidant and the cooling channels defined by the corrugated plate. The cross sectional area available for the flow of cooling air in a combined oxidant and adjacent cooling channel is now 4.6 mm ² (i.e., greater than that in the prior art plate above).

This example illustrates that a composite bipolar plate can be made significantly thinner than the prior art plate, using components with appropriate dimensions, and while providing equal or better cooling capacity.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.

While particular elements, embodiments, and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.