FUEL CELL FLOW FIELD PLATE ASSEMBLY

BACKGROUND

Technical Field

The present invention relates generally to a flow field plate assembly for a fuel cell, and, more particularly, to a flow field plate assembly with reduced effects of the residual deformation of the plates on the reactant channel alignment.

Description of the Related Art

Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.

During normal operation of a PEM fuel cell, fuel is electrochemically oxidized on the anode side, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the membrane, to electrochemically react with the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant on the cathode side to generate water reaction product.

A broad range of reactants can be used in PEM fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.

One type of electrochemical fuel cell is the polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two

electrodes. Each electrode typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The membrane is ion conductive (typically proton conductive), and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION ^® . The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).

In a fuel cell, a MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates typically act as current collectors and provide support for the MEA. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the adjacent fuel cell assemblies.

In a fuel cell, these plates on either side of the MEA may incorporate flow fields for the purpose of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. The flow fields comprise fluid distribution channels separated by landings. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The landings act as mechanical supports for the fluid distribution layers in the MEA and provide electrical contact thereto. Because it is interposed between two adjacent MEAs a plate has to be provided with oxidant distribution channels on one side and with fuel distribution channels on the other. Alternatively, such plate may be replaced by a bipolar plate assembly comprising a first plate provided with fuel distribution channels on its outer surface and a second plate provided with oxidant distribution channels on its outer surface, the two plates being assembled together with their inner surfaces facing each other. Such a

construction allows an easier manufacturing process of each plate since channels need to be provided only on one side of each plate.

Since during operation the temperature of the fuel cell may increase considerably and needs to be controlled within admissible limits, flow field plates may also include channels for directing coolant fluids along specific portions of the fuel cell. Such coolant distribution channels are formed as channels that traverse through the plate or, in the case of a flow field plate assembly, as partial coolant channels incorporated on the inner surface of each plate to form complete coolant distribution channels when the two plates are assembled together. There are known advantages to having partial coolant channels formed on both the fuel and oxidant plates.

Plates are typically made of expanded graphite or metal. Expanded graphite (e.g., Grafoil®) plates are typically embossed side-by-side on one sheet of material, surrounded by a frame containing various additional features to aid in the manufacturing of the parts (e.g., alignment features, sacrificial edges to protect the part from handling damage). Anode and cathode plates can be embossed side by side on a single sheet of Grafoil. After embossing, the plates may be impregnated with a resin and cured. Bipolar plate assemblies are then made by applying glue to the sheet material containing the plates and bonding this sheet to another identical sheet that is flipped over to form two bipolar plate assemblies side by side. These bipolar plate assemblies are subsequently cut from the glued sheet assembly. Alternatively, especially in the case of metal plates, the fuel and oxidant plates may be each made on separate dies and they are afterwards aligned to one another to ensure proper alignment of the channels so that when the fuel cell stack is assembled, the MEA in each fuel cell is properly supported by opposing anode and cathode plate landings.

A problem that may arise during this manufacturing process is that, after the plates are embossed on the graphite sheet and impregnated, there is a certain amount of dimensional change due to the elastic recovery of the plate material and/or due to the material growth or shrinkage on curing the impregnating resin which can be different in X, Y and Z directions (where X is the flow field direction, Y is perpendicular to the flow field direction in the plane of the plate and Z is perpendicular to the plane of the plate). The dimensional change of the material is influenced by the geometry of the flow channels, such that very

small differences in the cross-section of the channels can cause significant differences in how the part if finally dimensioned. If the plates are made of metal the issues are similar, such that the differential dimensional changes of the fuel and oxidant plates may result in a finished plate assembly with misaligned channels and a distorted profile.

For example, residual deformation of the plate in the X direction tends to cause differences in the length of the flow field channels on the fuel and oxidant plates and, since the plates are surrounded by a frame and confined to a mold during the manufacturing process, this causes a bending in-plane of the plates. When the plates are assembled together, one over the other, the bends of the plates oppose each other resulting in poor alignment of the channels in the middle of the plate, while the ends of the plates are reasonably aligned.

When the plates are each made on separate dies, as is generally the case of metal plates, no residual forces are induced from a second plate embossed on the same sheet, as was the case in the previous example, but deformations still appear (even if at a smaller scale) due to the constraints imposed by the frame around the plate.

In the case of low aspect ratio plates (plate width being approximately the same as the plate length) the variations in the Y direction cause similar channel misalignments.

The effects of such residual stresses have generally been addressed in the past by manufacturing the oxidant and fuel plates from graphite or metal sheets having the same thickness and same total cross-sectional area.

More recently, various different systems and methods have been developed for reducing the effects of plate deformation. For example, published Japanese Patent Application No. 2006228533 discloses two metallic separator plates with superimposed landings, the plates being pressed together during assembly such that at least a part of the contact area between the landings making the plates stick together in spite of the plate warpage caused by its residual deformation after manufacturing. Another example is shown in published Japanese Patent Application No. 2006173090, where two metallic separator plates encompassing a membrane electrode assembly have their ends fixed together by caulking to reduce the effects of the residual deformation of the plates. Further, published U.S. Patent Application No. 2002/0168561 describes a fuel cell unit comprising two plates

having a similar channel region, offset from the geometric center of the plate, and a membrane electrode assembly (MEA) interposed between the plates, whereby the plates are arranged such that the forces acting on the stack are transmitted to the MEA without any flexural moment. Any damage to the MEA is thereby prevented. However, the stack operates without any coolant circulation and it does not address the problem of reactant channels misalignment.

Accordingly, although there have been advances in the field, there remains a need for improved and/or simplified flow field plates design and methods of assembly to reduce the effects of the stresses introduced in the plates during manufacturing. The present invention addresses these needs and provides further related advantages.

BRIEF SUMMARY

A flow field plate for a fuel cell is disclosed comprising: a first planar flow field plate having a surface with open channels, having a first channel cross-section and an opposite surface with open channels having a second channel cross-section with a larger area than the first channel cross-section; and a second flow field plate being essentially the same as the first flow field plate, wherein the surface of the first flow field plate with open channels with a larger cross-sectional area engages the surface of the second flow field plate with open channels of a smaller cross-sectional area to define a closed coolant flow field.

The cross-section of the plates is essentially identical and therefore they deform essentially in the same way during molding. By engaging the plates as described above the residual deformation of one plate is compensated for by the residual deformation of the other plate thereby eliminating the channel misalignments presented by the solutions in the prior art.

The first cross-section and the second cross-section of the open channels may be curvilinear or polygonal.

The first flow field plate and the second flow field plate may be made of a moldable material. In one embodiment the flow field plates are made of graphite or expanded graphite that can also be impregnated with a resin material. The resin material may

be, for example, a methacrylate, an epoxy or a phenolic resin. In a further embodiment the flow field plates are made of metal.

In both embodiments, the first flow field plate may be the anode plate such that the open channels of a smaller cross-section are used for fuel circulation and the open channels of a larger cross-section are used for coolant circulation. In this case, the second flow field plate is the cathode plate such that the open channels of a larger cross-section are used for the oxidant circulation and the open channels of a smaller cross-section are used for the coolant circulation.

In both embodiments, the surface of the first flow field may slidably engage the surface of the second flow field plate.

Herein, the term "slidably" is defined to mean that the engagement of the two plates allows some relative movement between them during the fuel cell stack operation. Such movement may be caused by the stack expansion during manufacturing or by the vibrations transferred to the stack from the fuel cell system.

A method of engaging the surfaces of the two plates making the flow field plate assembly is also presented.

These and other aspects of the invention will be evident upon reference to the following detailed description and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility.

Figure 1 is an exploded view of a conventional fuel cell unit, showing one membrane electrode assembly and the corresponding flow field plates in conformity to the prior art.

Figure 2 is a sectional view of a conventional flow field plate assembly showing the possible reactant channel misalignments caused by the residual deformation of the plates after manufacturing.

Figure 3 is a sectional view of one embodiment of the flow field assembly with plates made of graphite.

Figure 4 is a sectional view of another embodiment of the flow field assembly with plates made of metal.

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.

Figure 1 illustrates a conventional (prior art) fuel cell unit. For simplicity, a single cell, from the fuel cell stack is represented. It is to be understood that this represents a repeating unit of the fuel cell stack.

This repeating unit 1 includes a membrane electrode assembly (MEA) 2 interposed between a flow field plate 3 and a flow field plate in an adjacent unit (not shown), similar to plate 4. The MEA comprises a solid polymer ion exchange membrane 5 sandwiched between an anode 6 and a cathode 7. The anode 6 and cathode 7 each contain a fluid diffusion layer 8 and 9, respectively, and a catalyst layer 10, 11, respectively, on the respective sides facing membrane 5. Fluid distribution layers 8 and 9 serve as electrically conductive backings and mechanical supports for catalyst layers 10, 11, and also serve to distribute the reactants from the flow field plates to the catalyst layer. The reactants, typically hydrogen and oxygen or oxygen-containing air, are supplied to flow field plates 3 and 4 and then delivered through the fluid distribution channels 13 and 12 of the flow fields to the surfaces of the fluid distribution layers 8 and 9. As shown in Figure 1, flow field plate 4 contains channels 14 on the side facing flat surface 15 of flow field plate 3. The cooperating sides of channel 14 and flat surface 15 form a closed inner flow field for carrying a coolant fluid, typically water.

It should be understood, however, that the surface of flow field plate 3 shown in Figure 1 that forms a part of the closed inner coolant flow field does not have to be flat, as shown in another example of a conventional (prior art) flow field plate illustrated in Figure 2(a). The flow field plate assembly 16 shown in Figure 2(a) includes a first flow field plate 17 provided with open channels on both sides, and a second flow field plate 18 also provided with open channels on both sides, such that the cooperating sides of the plates form an inner flow field 19 for carrying a coolant fluid. Figure 2(a) further illustrates the possible effect of residual deformation of the plates in the X direction (the direction of the flow field as illustrated in Figure 1) on the channel alignment, which is more visible at the middle of the plate, by comparison with the fairly good alignment of the channels near the plate ends. As explained above, this is due to the deformation effect introduced by the residual stress in the plates after molding.

The fuel cell unit 20, according to one embodiment shown in Figure 3, includes a flow field plate assembly 21 and a MEA 22. The flow field plate assembly 21 includes a first flow field plate 23 having a surface 24 with open channels 25 having a first channel cross-section, and an opposite surface 26 with open channels 27 having a second

channel cross-section of a larger area than the first channel cross-section. The flow field plate assembly 21 further includes a second flow field plate 28, essentially the same as the first flow field plate. Surface 29 of the second flow field plate having open channels 30 of a smaller cross-sectional area engages surface 26 of the first flow field plate with open channels of a larger cross-sectional area and thereby defines a closed coolant flow field 31.

The other surface 32 of the second flow field plate has open channels 33 of a larger cross section and engages the MEA 22. Fuel cell unit 20 represents a repeating unit of the fuel cell stack such that surface 24 of the first flow field plate also engages the MEA of the neighboring fuel cell (not illustrated).

The first flow field plate may be the anode plate and, consequently, the second flow field plate will be the cathode plate, as illustrated in Figure 3, such that the fuel circulates through the open channels 25 of a smaller cross-section, and the oxidant circulates through the open channels 33 of a larger cross-section.

The fuel cell stack is made of a succession of fuel cells similar to the fuel cell unit illustrated in Figure 3 assembled together using tie rods that go through the plates and ensure their alignment. Tie rods or straps are also used to ensure the compression of the fuel cell for a good electrical contact between the plates and the electrodes. During stack assembly and operation some relative movement between adjacent flow field plate assemblies 21 may take place due to the stack expansion during operation and due to the vibrations induced on the stack from the fuel cell system or from the vehicle on board of which the stack is mounted. Seals or glue 34 may be placed between plates 23 and 28, and/or seals or glue 35 between the flow field plate assembly 21 and the MEA 22, to ensure a good sealing between the fuel cell components.

In the illustrated embodiment, the cross-section of the open channels of plates 23 and 28 is curvilinear. As one of ordinary skill in the art will appreciate, the shape of this cross-section may vary such that in alternate embodiments, the cross-section may be polygonal or of any other shape that would allow a smooth fluid movement along the channels and is compatible with the manufacturing methods appropriate to the plate material.

In the embodiment illustrated in Figure 3, plates 23 and 28 are made of graphite. Impregnated graphite may be used as the material for manufacturing the plates in

this case. Graphite plates may be impregnated with a methacrylate, an epoxy or a phenolic resin or any other suitable resin material. The resin is cured before plate assembly. In other embodiments the plates may be manufactured from any moldable material.

For example, in a second embodiment illustrated in Figure 4 plates 35 and 36 of flow field plate assembly 21 may be made of metal. Plate 35 has a surface 37 defining open channels 38 having a first channel cross-section, and an opposite surface 39 defining open channels 40 having a second channel cross-section of a larger area than the first channel cross-section. The second flow field plate 36 is essentially the same as the first flow field plate 35. Surface 41 of the second flow field plate with open channels 42 of a smaller cross- sectional area engages surface 39 of the first flow field plate with open channels 40 of a larger cross-sectional area, and thereby defines a closed coolant flow field 43.

The other surface 44 of the flow field plate 36 engages the MEA 22. Fuel cell unit 20 represents a repeating unit of the fuel cell stack, such that surface 37 of the flow field plate 35 engages the MEA of the neighboring fuel cell (not illustrated). It is to be understood that seals between the plates and between the flow field plate assembly and the MEA may be used in this embodiment, even if not illustrated in Figure 4. As a person skilled in the art may appreciate, they may be different in shape and composition from the seals used in the first embodiment in order to be compatible with the plate material.

In another embodiment, a method of making a flow field plate assembly with reduced residual deformation is disclosed. This method comprises the step of engaging a first flow field plate having open channels of a first cross-section on one surface and open channels of a second cross-section of a larger area than the first cross-section on the opposite surface of the plate with a second flow field plate that is essentially the same as the first flow field plate, such that the surface of the first flow field plate with channels of a larger cross- sectional area engages the surface of the second flow field plate with channels of a smaller cross-sectional area to define a closed coolant flow field.

The flow field plate assembly shown in Figure 3 and Figure 4 comprises two plates that are essentially the same, having essentially the same flow field, that are engaged with one another on the surfaces that have open channels of different cross-sectional area. The cross-section of the plates is essentially identical and therefore they deform essentially in

the same way during molding. By engaging the plates as described above the residual deformation of one plate is compensated for by the residual deformation of the other plate, which eliminates the channel misalignments presented by the prior art.

As one of ordinary skill in the art will appreciate, various modifications may be made to the embodiments illustrated in Figures 3 and 4 without deviating from the spirit and scope of the invention. For example, the first flow field plate could be the anode or the cathode plate, allowing the fuel or the oxidant flow through the channels of a smaller cross- section depending on the required pressure drop across the fuel or oxidant flow field, the preferred flow field design (straight, serpentine or any other layout) and the fuel cell system conditions (dead-ended or partially dead-ended operation or hydrogen or oxidant recirculation).

While particular steps, elements, embodiments and applications of the present invention have been shown and described herein for purposes of illustration, it will be understood, of course, that the invention is not limited thereto since modifications may be made by persons skilled in the art, particularly in light of the foregoing teachings, without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.