Seals for fuel cells and fuel cell stacks

Details Seals for fuel cells and fuel cell stacks Seals for fuel cells and fuel cell stacks

2001-10-16

2004-07-13

Kalafut, Stephen J. (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000

Reexamination Certificate

active

06761991

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not applicable.
FIELD OF THE INVENTION
This invention relates to fuel cells, and more particularly to the design of a groove profile for use in forming seals between different elements of a conventional fuel cell or fuel stack assembly, to prevent leakage of gases and liquids required for operation of the individual fuel cells. The invention also relates to the formation of such seals with a novel sealing material.
BACKGROUND OF THE INVENTION
There are various types of known fuel cells. One type of fuel cell currently considered practical for use in many applications is a fuel cell employing a proton exchange membrane (PEM). PEM fuel cells enables simple, compact fuel cells to be designed, which are robust, can be operated at temperatures not too different from ambient temperature, and which do not have complex requirements with respect to fuel, oxidant, and coolant supplies.
A single conventional fuel cell generates a relatively low voltage. In order to provide a useable amount of power, therefore, fuel cells are commonly configured into fuel cell stacks typically containing 10, 20, 30, and even 100′s or more fuel cells in a single stack. While this provides a single unit capable of generating useful amounts of power at usable voltages, the design can be complex, and can include numerous elements all of which must be carefully assembled.
For example, a conventional PEM fuel cell requires two flow field plates, an anode flow field plate, and a cathode flow field plate. A membrane electrode assembly (MEA) including the actual proton exchange membrane is provided between the two flow field plates. Additionally, a gas diffusion media or layer (GDM/GDL) is sandwiched between each flow field plate and the proton exchange membrane. The gas diffusion media enables diffusion of an appropriate gas, either the fuel or the oxidant, to the surface of the proton exchange membrane, while at the same time providing conduction of electricity between the associated flow field plate and the PEM.
This type of a basic cell structure itself requires two seals, with each seal being provided between one of the flow field plates and the PEM. Moreover, the seals have to be of relatively complex configuration. In particular, and as detailed below, the flow field plates for use in the fuel cell stack have to provide a number of functions, and a complex sealing arrangement is therefore required.
For a fuel cell stack, the flow field plates typically provide apertures or openings at either end so that a stack of flow field plates defines elongate channels extending perpendicularly to the flow field plates. As fuel cells require flows of a fuel, an oxidant, and a coolant, this typically requires at least three pairs of ports or six ports in total. This is because it is necessary for the fuel and the oxidant to flow through each fuel cell. A continuous flow is required as it ensures that although most of the fuel or oxidant may be consumed, any contaminants are continually flushed through the fuel cell.
The foregoing assumes that the fuel cell is a compact type of configuration provided with water or the like as a coolant. There are also known stack configurations which use air as a coolant, either relying on natural convection or forced convection. Such fuel cell stacks typically provide open channels through the stacks for the coolant, and therefore the sealing requirements are diminished. Commonly, it is then only necessary to provide sealed supply channels for the oxidant and the fuel.
Consequently, each flow field plate typically has three apertures at each end, and each aperture represents either an inlet or outlet for one of the fuel, oxidant, or coolant. In a completed fuel cell stack, these apertures align to form distribution channels extending through the entire fuel cell stack. It should therefore be appreciated that the sealing requirements are complex and difficult to meet. However, it is possible to have multiple inlets and outlets to the fuel cell for each fluid, depending on the stack/cell design. For example, some fuel cells have two inlet ports for each of the anode, cathode, and coolant, two outlet ports for the coolant, and a single outlet port for each of the cathode and anode. However, other combinations are also possible.
The coolant most commonly flows across the back of each fuel cell so as to flow between adjacent individual fuel cells. This is not essential, however, and as a result, many fuel cell stack designs have cooling channels only at every second, third, or fourth plate. This allows for a more compact stack with thinner plates but such an arrangement may provide less than satisfactory cooling. It also requires another seal between each adjacent pair of individual fuel cells. Thus, in a completed fuel cell stack, each individual fuel cell requires two seals just to seal the membrane exchange assembly to the two flow field plates. A fuel cell stack with 30 individual fuel cells will require 60 seals for that purpose. Additionally, as noted, a seal is required between each adjacent pair of fuel cells and end seals for the current collectors. For a 30 cell stack, therefore, this requires an additional 31 seals, Thus, a 30 cell stack requires a total of 91 seals, excluding the seals for bus bars, current collectors, and endplates, and each of these would be of a complex and more elaborate construction. With additional gaskets required for bus bars, insulator plates, and endplates, the number easily reaches 100 seals of varying configurations in a single 30 cell stack.
These seals can be formed by providing channels or grooves in the flow field plates, and then providing prefabricated gaskets in the channels or grooves to effect a seal. In known configurations, the gaskets and/or the sealing material are specifically polymerized and formulated to resist degradation from contact with various of the materials of construction in the fuel cell, and the various gases and coolants which are aqueous, organic, and inorganic fluids used for heat transfer. This means that assembly technique for a fuel cell stack will be complex, time consuming, and offers many opportunities for error.
Accordingly, in a first technique, a resilient seal can be provided as a floppy gasket seal molded separately from individual elements of the fuel cells by known methods such as injection, transfer or compression molding of an elastomer.
A second technique for providing such resilient seals involves application of an uncured sealing material to the fuel cell plates by dispensing the uncured sealing material, silk screening the uncured sealing material, or spraying uncured sealing material onto the fuel cell plate to a predetermined thickness, and then curing the sealing material to achieve desired elastomeric properties.
A third technique for providing resilient seals involves insert injection molding in which a resilient seal is fabricated on a plate and assembly of the unit is simplified. According to this technique, the gasket is adhered to the fuel cell plate sufficiently to allow its handling and assembly in the fuel cell stack. Such insert injection molded gaskets can be designed with improved groove and seal profiles to optimize the various sealing forces occurring within a fuel cell stack. The basic process for insert injection molding is known in the art, and reference may be had, for example, to U.S. Pat. No. 4,865,793 (September 1989). As noted hereinafter, this is the most preferred technique according to the present invention.
As an additional consideration, formation or manufacture of such seals or gaskets is complex, and there are generally only two known methods of manufacture. For the first technique, the individual floppy gasket seal can be formed by molding it in a suitable mold. This can be relatively complex and expensive, and for each fuel cell configuration, it would require the design and manufacture of a mol

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