CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S. C. 119(e) of U.S. Provisional Patent Application No. 61/151,585 filed February 11, 2009 which provisional application is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present invention relates to solid polymer electrolyte fuel cell stacks employing framed membrane electrode assemblies and internal reactant manifolds.

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 which has been 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.

For commercial applications, a plurality of fuel cells is generally stacked in series in order to deliver a greater output voltage. Separator plates are typically employed adjacent the gas diffusion 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. These bipolar separators provide a path for electrical and thermal conduction, as well as mechanical support and dimensional stability to the MEA. 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 then also referred to as flow field plates. In certain fuel cell stack constructions, when the ports in the separator plates are stacked together in alignment, they create internal reactant manifolds that are used to distribute reactants to and from the individual fuel cells in the stack.

For sealing and other manufacturing purposes, frames may be incorporated around the periphery of the MEAs. A typical framed MEA may, for instance, have the membrane electrolyte extending out past the edges of the gas diffusion layers so as to provide a non-porous plastic surface that can be sealed to. The frame itself can comprise two pieces; namely, an anode frame piece and a cathode frame piece that sandwich the MEA and that are attached at their inner peripheries to the anode gas diffusion layer and the cathode gas diffusion layer, respectively. The frame pieces also attach and seal to the extended membrane electrolyte, and to each other at their outer peripheries. Seals may then be made between the outer edges of the frame and the adjacent separator plates. In fuel cell stacks with internal reactant manifolds, the frames typically also form part of the internal manifolds. These frames have appropriate ports formed in their outer edges and are aligned with the corresponding separator ports that make up the internal manifolds.

The repeating cells in such stacks can thus comprise ports for internal manifolds in both the separator plates and the frames of the MEAs. Reactants can be supplied and by-products removed from the stack by making appropriate fluid connections to the internal manifolds at one of the end cells in the stack. However, to seal the internal manifolds, at least one of the end cells generally does not have all the open ports in its outer separator plate as the repeating cells do. This end cell typically either employs a special separator plate without all the repeating cell ports or, alternatively, sealing plugs are inserted into the port openings to seal them off from ambient.

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.

However, 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. Further, the cathode flow field is generally open to the atmosphere and an internal oxidant manifold is not required/employed. Such simple stacks therefore may only require an internal fuel manifold.

BRIEF SUMMARY

A solid polymer electrolyte fuel cell stack can comprise a series stack of framed membrane electrode assemblies separated by a plurality of separator plates comprising reactant ports that, when stacked together, form an internal reactant manifold. In order to seal the internal reactant manifold at an end separator plate in the stack, a special framed membrane electrode assembly may be employed at this end of the stack. This can be a simple, cost effective alternative to employing a special separator plate without reactant ports or to employing sealing plugs in the separator plate ports.

The solid polymer electrolyte fuel cell stack comprises a plurality of solid polymer electrolyte fuel cells stacked in series. Each fuel cell therein comprises a framed membrane electrode assembly, and the stack comprises a plurality of separator plates that separate the framed membrane electrode assemblies. Further, each separator plate comprises a reactant port that forms an internal reactant manifold when stacked together in the fuel cell stack. At one end of the stack, the end of the reactant manifold is sealed by the frame of a special framed membrane electrode assembly which has no opening adjacent the reactant port in the neighboring separator plate. The frames in the other repeating framed membrane electrode assemblies in the stack do comprise openings adjacent the reactant ports in the neighboring separator plates and thus allow for reactant flow through the reactant manifold.

The fuel cell stack can comprise both fuel and oxidant internal manifolds. In addition, either or both of these may include both inlet and outlet internal manifolds and thus may include appropriate ports for these in the separator plates and MEA frames. The invention is particularly suitable for use in low fuel pressure (e.g., supplied fuel is less than about 7 psi), air cooled fuel cell stacks in which only an internal fuel manifold is used.

An exemplary embodiment employs thermoplastic polyimide MEA frames, carbon separator plates, and the internal manifold is sealed at the positive or cathode end of the fuel cell 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 DRAWING

Figure 1 shows a schematic side view of one end of a solid polymer electrolyte fuel cell stack comprising framed membrane electrode assemblies, separator plates with fuel ports, and openings for fuel flow through all the frames except in that MEA at the end of the stack.

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.

Herein, "framed membrane electrode assembly" or "framed MEA" refers to an assembly comprising a membrane solid polymer electrolyte, a cathode electrode, and an anode electrode in which the electrodes are located on either side of the membrane electrolyte. The assembly may also optionally comprise one or more porous gas diffusion or gas barrier layers adjacent either electrode. The assembly also comprises a frame that is sealed around the periphery of the membrane electrode assembly.

Referring to Figure 1, a representative example of an air-cooled solid polymer fuel cell stack 1 with an internal fuel manifold is shown. Here, a schematic side view of the cathode or positive end of stack 1 is shown. Figure 1 shows the last three cells 2 in the stack. Cells 2 comprise framed membrane electrode assemblies 3, and end cell 2a comprises framed membrane electrode assembly 3a. Each of these framed membrane electrode assemblies 3, 3 a comprise a conventional membrane electrode assembly 4 that includes electrolyte, electrodes, and gas diffusion layers and a gas diffusion barrier (details not shown in Figure 1). Frames 5 and 5a are attached to and seal to their respective membrane electrode assemblies 4. Frame 5 a is similar to frames 5 except that frames 5 comprise fuel inlet ports 6 and fuel outlet ports 7 in their outer periphery, while frame 5a does not have any such ports or openings (e.g., at dashed area 8).

Separator plates 9 separate the framed membrane electrode assemblies 3, 3 a and also comprise flow fields for distributing reactants to and by-products from the electrodes. In the representative example of Figure 1, ambient air may be used as the oxidant supply and also for cooling purposes. The flow of air occurs through oxidant flow fields 10 and is perpendicular to the plane of the paper. Fuel flow fields 11 are also formed in separator plates 9 and are illustrated by dashed lines 11. The flow of fuel in flow fields 11 is from left to right in Figure 1 (as indicated by the arrows).

Separator plates 9 also comprise fuel inlet ports 12 and fuel outlet ports 13. When stacked together, the ports in separator plates 9 and in frames 5 form inlet and outlet internal manifolds in the stack. These manifolds are fluidly connected to the inlets and outlets of the fuel flow fields 11.

Finally, Figure 1 shows gasket seals 14 (which are employed to seal frames 5, 5a to separator plates 9) and conventional end plate hardware 15. For illustrative purposes, sealing plugs 16 are shown (hatched lines) in the end separator plate. Such sealing plugs have been used to seal the ends of internal manifolds; however, in the inventive stack, these seals are instead made by solid frame 5 a.

As illustrated in Figure 1, the separator plates 9 may be identical, including that at the cathode end of the stack. The separator plates can typically be made of a resin impregnated carbon material and can require an expensive forming die for use in compression molding these parts. However, an advantage of the inventive stack is that there is no need for expensive tooling for a low volume part, i.e., the end separator plate (only one of which may be needed per upwards of a hundred plates in a stack). Also, with regards to the conventional alternative (when using similar separator plates throughout the stack), there is no need to use sealing plugs which represent undesirable extra parts, extra assembly steps (including deflashing and installation), and associated extra process checkpoints (to ensure proper installation).

The preceding construction does, of course, require that different framed MEAs (absent ports in the frame) be manufactured for the end cells in the stacks. The tooling requirement is, however, simpler than that for the separator plates. For instance, the manufacturing process for the framed MEAs may involve the use of rule die tooling to make the final cuts. In such an instance, separate rule die tooling may be employed to cut the special end cell framed MEAs that have no ports.

Except for the absence of ports, the framed MEA for the end cell in the stack may otherwise be similar to the other framed MEAs in the stack. It may be desirable, however, to incorporate an additional different feature (other than the absence of ports) to distinguish the end cell framed MEA from the others in order to prevent it from being mixed up with the others for manufacturing purposes. As is illustrated in the following examples, the fuel cell stacks of the invention seal at least as well under vacuum and pressure as existing embodiments.

While Figure 1 exemplifies a preferred air-cooled fuel cell stack, the invention may be practiced in stacks comprising a dead-ended fuel supply (thus not having a continuous outlet of fuel, but instead a purge capability), or in stacks comprising an internal oxidant manifold as well as or instead of an internal fuel manifold.

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

EXAMPLES

Two 10 cell, solid polymer electrolyte, air-cooled Markl020 ACS™ stacks were made according to the design generally shown in Figure 1. Of these, a comparative stack used sealing plugs to seal off the internal fuel manifold. An inventive stack used the frame of the end cell's framed MEA to make this seal.

In both cases, the MEAs, with the exception of the frames, were of conventional construction. The MEAs included a catalyst coated membrane electrolyte and carbon fiber-based gas diffusion layers bonded to each electrode. The catalyst coated membrane extended beyond the edges of the gas diffusion layers so that the frames could be attached to them.

The MEA frames sandwiched the MEAs at their periphery, and were impregnated into a small portion of the edges of the gas diffusion layers and thermally bonded to the membrane electrolytes at their outermost edges where there was no catalyst coating. The frames comprised both an inner pair and an outer pair of sheets to make up the sandwich. The inner sheets were made of a polyethylene and ethylene- vinyl-acetate -based thermoplastic adhesive layer approximately 80 micrometers thick. The outer sheets were made of a polyethylene terephthalate based thermoplastic layer approximately 165 microns thick with a higher melting temperature.

The comparative stack used plugs made from a low viscosity, two- component silicone rubber to seal the fuel ports in the end separator plate (e.g., plugs 16 in Figure 1). The inventive stack was sealed off by the frame in the framed MEA of the end cell (e.g., frame 5a in Figure 1). Both stacks were then subjected to accelerated stress testing that involved cyclic exposure of the internal fuel manifold to rough pump vacuum at elevated temperature. Starting from ambient, the cycle involved connecting the manifold to a vacuum pump for five minutes and then venting back to ambient again. No degradation was seen in the sealing function of either stack after 4000 vacuum cycles at 73 ⁰ C.

Another inventive 7 cell stack was made as above and subjected to overpressure testing. Here, air was used instead of fuel to pressurize the internal fuel manifold. The stack seals were considered to have failed if leaks > 91 seem total (or 13 sccm/cell) were observed. Initially, the pressure was increased incrementally from 7-30 psi at room temperature. Then, this ramping pressure test was repeated at 75° C for an hour, and then finally at 78° C for 67 hours, at which point the stack seals were considered to have failed. Interestingly, however, this failure occurred in cells other than the end cell and did not occur at the seals at the ends of the internal fuel manifold. Thus, the end cell, with the special framed MEA that sealed the internal fuel manifold, sealed at least as well as the other repeating cells in the stack.

The above examples illustrate that a satisfactory seal can be made for the internal stack manifolds using a framed MEA.

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.