WO2000057500A1

WO2000057500A1 - Thin graphite bipolar plate with associated gaskets and carbon cloth flow-field for use in a fuel cell

Info

Publication number: WO2000057500A1
Application number: PCT/US2000/007644
Authority: WO
Inventors: George A. Marchetti
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-03-25
Filing date: 2000-03-23
Publication date: 2000-09-28
Anticipated expiration: 2001-09-25
Also published as: AU4175000A

Abstract

The present invention comprises a thin graphite plate with associated gaskets (11, 21, 41, 51) and a carbon cloth flow-field (7). The plate, gaskets (11, 21, 41, 51) and flow field (7) comprise a'plate and gasket assembly' for use in an ionomer membrane fuel cell, fuel cell stack or battery.

Description

THIN GRAPHITE BIPOLAR PLATE WITH ASSOCIATED GASKETS AND CARBON CLOTH FLOW-FIELD FOR USE IN A FUEL CELL

Field of the Invention:

The present invention relates to electrochemical fuel cells, and more particularly, to ionomer membrane fuel cells. This invention was made with government support under Grant No. DE-FG01-97EE15679 from the United States Department of Energy/Energy Related Inventions Program. The government has certain rights in the invention.

Background Art:

A bipolar plate is the backbone of an ionomer membrane fuel cell stack or battery. An ionomer membrane is virtually any ion-conducting membrane. The most technically advanced type of ion-conducting membrane currently available for fuel cell applications is the proton-exchange membrane, such as the Nafion series of membranes, the Dow membrane, etc. The fuel cell electrodes are hot-pressed or otherwise affixed to the membrane to form a unitized assembly. Bipolar plates, and associated gas seals, enclose the membrane and electrode assembly ("MEA") in a fuel cell.

Typical state-of-the-art bipolar plates are made of graphite that is compressed into a single block. Gas flow channels (the "flow-field" channels) are generally machined into the graphite block and permit the flow of the reactant gases from the manifolds and through the flow-field to the electrodes of the fuel cell. Bipolar plates serve three primary functions in overall fuel cell operation. First, they conduct electricity from the fuel side of the electrochemical reaction to the oxidant side of the reaction, where water is produced. Second, they separate the fuel and oxidant gases and prevent cross-mixing of the reactant gases in the cell. Third, they allow gases from the manifolds to reach the appropriate fuel cell electrode. The gas seals or gaskets (the "gaskets") serve to contain the gases within the fuel cell and also prevent cross-mixing of the reactant gases.

Graphite is an excellent material for use in fuel cell applications because it is relatively inert in the corrosive electrochemical environment of the cell. Although the material cost of graphite is not high, the manufacturing methods currently employed result in very costly bipolar plates. Also, because state-of-the-art bipolar graphite plates are compressed into a block, they tend to be relatively thick. A relatively thick plate is also required in order to accommodate the channels of the flow-field. Separate cooling plates are often included in fuel cell designs, which may further add thickness to the fuel cell stack. As the thickness of the graphite bipolar plates increase, the number of cells that can be placed in a given spatial volume decreases For example, some state-of-the-art ionomer membrane fuel cells, utilizing a standard machined graphite bipolar plate, may be approximately 100 mils (ca 2 5 mm) or more thick Up to ten cells can therefore be stacked per lineal inch of fuel cell stack using these types of cells If the thickness of the bipolar plate could be reduced, however, much thinner fuel cells could be produced and the cell "stacking density" (1 e , the number of cells in a given volumetπc space) could be correspondingly increased An increase m stacking density would be particularly beneficial in portable and transportation-related applications where more compact and light-weight fuel cell stacks and fuel cell batteπes are desirable

While some work has recently been done in reducing the thickness of the bipolar plate and increasing stacking density, these efforts have focused pπmaπly on the substitution of a metal plate, such as a plate made of titanium, for the traditional graphite bipolar plate For example, Lynntech, Inc of College Station, Texas, has reported that its titanium/foamed metal bipolar plate allows stacking densities of up to 5 5 cells per centimeter, or nearly 14 cells per inch H Power of Belleville, New Jersey has developed a bipolar "platelet", also made of titanium, which permits about 12 cells per inch to be stacked Dr Mahlon Wilson of Los Alamos National Laboratoπes has developed a stainless steel screen bipolar plate that allows stacking of about 10 cells per inch

Summary of the Invention

One embodiment of the present invention includes a graphite plate because of its proven performance in ionomer membrane fuel cell stacks and its relatively low cost The term ' graphite" as used herein refers to any mateπal which is pπmaπly composed of graphite, including mateπals composed of graphite, graphite flakes or graphite powders Unlike state- of-the-art graphite bipolar plates, however, the invention is a thm graphite bipolar plate with associated gaskets for use as a component m an ionomer membrane fuel cell, fuel cell stack or batten, The graphite bipolar plate and gaskets of this invention in certain embodiments are onh about 40 mils thick in total The invention further includes a carbon or graphite cloth ("carbon cloth") flow-field, as hereafter described Unlike state-of-the-art graphite bipolar plates, the invention does not have flow-field channels machined into the graphite Rather, the reactant gases enter the anode and cathode of the fuel cell from the manifolds via ' port channels" which are located in the gasket, not in the plate itself From the port channels, gas then flows into the carbon cloth flow -field, which lies on the first surface of the graphite plate The second surface of the graphite plate mav be smooth, may have a stamped flow-field or may have a carbon cloth flow-field This structure permits the fabrication of very thm bipolar plate and gasket assemblies The umtized plate, carbon cloth flow-field and gaskets of the invention are hereinafter referred to as a plate and gasket assembly ("PGA") When the MEA is inserted into adjacent PGAs, a fuel cell is fabricated By assembling multiple ionomer membrane fuel cells in a bipolar arrangement with endplates, a fuel cell stack or battery is fabπcated

Advantages and novel features of the invention will be set forth in part m the descπption which follows or may be learned by practice of the invention The advantages of the invention may be realized and attained by mechanisms of the instrumentalities and combinations particularly pointed out in the appended claims

Descπption of the Drawings

Figure 1 illustrates a top plan view of a graphite sheet with manifold slots and the carbon cloth flow-field

Figures 2, 3, 4 and 5 illustrate a top plan view of the port channel side of a four slot gasket

Figure 6 illustrates a top plan view of the membrane side of a four slot gasket Figure 7 illustrates a top plan view of a membrane and electrode assembly wherein the electrodes fit into the electrode seating area of the PGA

Figure 8 illustrates a graphical view of a N/I curve for a single cell

Figure 9 illustrates a graphical view of a N/I curve for a fuel cell with 2 MEAs and a bipolar plate

Detailed Description of the Preferred Embodiments The present invention comprises a thm graphite bipolar plate with associated gaskets and a carbon cloth flow-field for use as a component m an ionomer membrane fuel cell, fuel cell stack or batten This invention was made with government support under Grant No DE- FG01-97EE 15679 from the United States Depaπment of Energ /Energy Related Inventions Program The government has certain πghts m the invention In one embodiment of the present invention, a graphite sheet 2, such as Alfa

Aesar/Johnson Matthev Compam of Ward Hill Massachusetts Product No 10832. is cut to the size of the fuel cell as illustrated in Figure 1 The graphite sheet used in the preferred embodiment is 10 mils thick and has a densitx of about ca 1 13 grams per cubic centimeter The graphite is first compressed in a rotary press or by other means Manifolds 3, 4, 5 and 6 are then cut or stamped out of the graphite Normally, there will be four manifold slots The slots are for fuel in 3, fuel out 4, oxidant m 5, and oxidant out 6 The fuel may be hydrogen, a hydrogen-πch gas, methanol, etc The oxidant may be oxygen, air, etc For thermal control, the graphite may be extended to form a thermal control fin, if desired, as shown m Figure 1

For the mam body of the gaskets, a πgid mateπal is utilized In the preferred embodiment, the rigid material used is polycarbonate and all such rigid mateπals that may be used as gaskets in the present invention are hereafter geneπcally referred to, without limitation, as "polycarbonate" Manifolds 12, 13, 14 and 15 are cut or stamped out of the polvcarbonate gasket 1 1 as illustrated in Figure 2 and the electrode seating area 20 is likewise cut or stamped out Alternatively, the polycarbonate may be molded to compπse the gasket mam body The polycarbonate is slightly roughened on both surfaces Port channels 16, 17, 18 and 19 are sawed, scored, molded or otherwise impressed into two of the inteπor legs of each gasket as illustrated in Figure 2 The port channels can be located in vaπous numbers and at vaπous positions and intervals along the inteπor legs, as illustrated in Figure 5

Figure 3 illustrates the gasket 21 which is placed on the opposite side of the graphite plate from the first gasket 1 1 and which has port channels 26, 27. 28 and 29 that are rotated 90^" with respect to the port channels of the first gasket 11 The manifolds 22, 23, 24 and 25 of the gasket 21 are also illustrated

To enhance the gas sealing capability of the gasket, a compressible gasket mateπal, such as certain commercially-available automotive sihcone gasket mateπals, is applied to each surface of the gasket mam body The gasket mateπal is applied to the entire surface of the "membrane side" of the gasket, as illustrated in Figure 6 The "membrane side" of the gasket 55 is that surface of the gasket which is adjacent to the ionomer membrane, as hereinafter described

The gasket mateπal is applied onfy to a poπion of the surface of the "plate side" of the gasket 41 and 51. as illustrated m Figures 4 and 5 No gasket material is applied in the "port areas" defined by the dotted lines m Figures 4 and 5 Consequently, reactant gases from the manifolds enter the electrode seating area of the gasket by means of the port channels in the gasket The gasket material, however, forms a gas-tight seal with the remainder of the plate and also w ith the membrane of the MEA, when the MEA is inserted into the PGA A piece of carbon cloth 7 is then cut approximately to the size of the electrode seating area as illustrated in Figure 1 . The non-port edges of the carbon cloth and the MEA are sealed and attached to the graphite with gasket material. A slight gap between the carbon cloth and the interior legs of the gaskets where the port channels are located allows for distribution of the reactant gases along the length of the carbon cloth flow-field.

The reactant gases flow through the manifolds and into the port channels of the gasket. See, e.g., Figure 4. The compressible gasket material on the rigid gasket main body prevents the gases from cross-mixing in the cell. The gases are thereby distributed to the appropriate side of the graphite plate, either the fuel or the oxidant side. The gases then flow into the gap on the first surface of the plate and along the length of the carbon cloth flow field. See, Figure 1. The gases flow through the carbon cloth and into the appropriate fuel cell electrode. The gases, and the product water formed on the oxidant side of the electrochemical reaction, exit the cell through the carbon cloth, the opposite gap, the opposite port channel and the opposite manifold. Similarly, the other reactant gas is directed through the gasket ports to the second surface of the plate and into the second surface flow-field, which flow-field may be smooth graphite, an impressed flow-field or carbon cloth, depending on the operating condition parameters.

It is understood that the foregoing manifold, port channel and flow-field configuration is illustrative only and that other configurations may be fabricated by those skilled in the art without departing from the spirit and scope of the present invention.

Other embodiments of the present invention may include a thermal control fin 8, as illustrated in Figure 1. The graphite may be extended beyond the edge of the gasket to form the fin. Adjacent graphite fins may then be separated by an electronic insulating material to prevent short circuits between the fins. The thermal control fin permits air or liquid cooling of the fuel cell stack. It should also be noted that the thermal control function also allows the fuel cell stack to be heated in cold weather. By heating the fluid with, for example, a high resistance coil and a chemical battery, heat is transferred into the fuel cell stack via the fins of the graphite sheets.

Once the PGA has been assembled, the MEA61 can be inserted in the electrode seating area and sealed along the non-port edge as shown in Figure 7. The membrane portion 62 of the MEA is substantially the same width and length as the gasket. Slots 64, 65, 66 and 67 are cut in the membrane, which match the slots in the PGA. The fuel cell electrode 63 is also illustrated. The PGAs may be fabricated to ha\ e one of many types of symmetπes such as squares o\ als. circles octagons and so on Tie rod holes mav be drilled m the PGAs or the corners may be clipped to allow tie rod access To continue the fuel cell stack, the next PGA is stacked, the next MEA, etc The four-slot PGA illustrated herein is designed for operation on pressuπzed fuel and oxidant gases The PGA may be adapted for operation with atmospheric pressure air or in a convection mode by eliminating one or both of internal oxidant manifolds

The present invention meets the cπteπa, discussed above, for a thin graphite bipolar plate that is compatible with an ionomer membrane-type MEA in a fuel cell The graphite plate and gaskets prevent cross-mixing of the reactant gases in the cell The gases are distπbuted to the appropπate fuel cell electrode (either fuel or oxidant) by mechanisms of the manifolds, port channels in the gaskets, the first surface carbon cloth flow-field and the second surface flow-field The graphite sheet and carbon cloth compπse a low-resistance, electronic pathway for the flow of electrons generated by the electrochemical reaction in a bipolar configuration The carbon cloth serves not only as a flow-field but also as a soft, spπng-type electronic contact within each cell Thermal control may be achieved by a mechanism of the thermal conductivity of the graphite fin

Moreover, one embodiment of the present invention is compπsed of relatively inexpensive precursor mateπals graphite sheet, a rigid mateπal such as polycarbonate, gasket mateπal, and carbon cloth No machining is employed All of the component parts of the bipolar plate and associated gaskets can be stamped or cut. thereby enabling the potential reduction of manufacturing costs

The present invention further increases the cell stacking density of ionomer membrane fuel cells beyond that currently possible with state-of-the-art bipolar plates About twenty cells per lmeal inch can be stacked using the present invention The component or precursor mateπals are relatively inexpensive and light-weight m order to minimize the cost and weight of the invention

Figure 8 illustrates the representative performance of a single-cell fuel cell unit, using an MEA manufactured bv BCS Technolog} of Bryan, Texas Figure 9 illustrates the representative performance of a two-cell unit which includes one of the embodiments of the PGA of the present invention. 1 e , a non-fin embodiment with a smooth graphite surface on the fuel side of the PGA The heat produced bv the electrochemical fuel cell reaction is used in this particular non-fin tw o-cell embodiment to increase internal cell temperature which, m turn, increases the power generated by each of the cells. A comparison of Figures 8 and 9 indicates that both the voltage and amperage of the two-cell unit are approximately twice that of the one-cell unit, with an incremental increase in the two-cell unit being attributable to higher operating temperature. This comparison indicates that the PGA thus provides a relatively low-resistance electronic connection between the cells, adequately supplies reactant gases to the MEAs and allows for removal of depleted oxidant and product water produced by the cells.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

What is claimed is:

1. An assembly for use in a fuel cell, fuel stack, or battery comprising: a bipolar plate, comprising: a graphite portion that defines a periphery of the bipolar plate; a cloth portion; and a gasket portion that aids in attaching the cloth portion to the graphite portion.

2. The bipolar plate of claim 1 wherein the cloth portion is enclosed by the graphite portion.

3. The bipolar plate of claim 1 wherein the cloth portion comprises carbon cloth.

4. The bipolar plate of claim 1 wherein the gasket portion comprises a rigid, polymeric material portion and a compressible portion.

5. The bipolar plate of claim 4 wherein the rigid polymeric material is polycarbonate.

6. The bipolar plate of claim 1 wherein the graphite portion and the gasket portion define one or more manifolds.

7. The assembly of claim 1 wherein the graphite portion defies a fin.

8. The assembly of claim 1 wherein the gasket portion overlays the graphite portion.

9. The assembly of claim 1 wherein the carbon cloth provides channels for reactant gases.

10. The assembly of claim 1 wherein the manifolds provide a channel for reactant gases.

1 1. The assembly of claim 1 and further comprising a plurality of bipolar plates stacked on each other. 12 The assembly of claim 1 and further compπsing a fuel cell membrane and fuel cell electrode positioned on opposite sides of the bmolar plate

13 A gasket, comprising a πgid gasket mam body, port channels defines by the gasket mam body, and a compressible material positioned on the gasket main body

14 The gasket of claim 13 wherein the πgid gasket mam body is polycarbonate

15 A fuel cell comprising the assembly of claim 12

16 A fuel stack compπsing the assembly of claim 12

17 A battery compπsing the assembly of claim 12

18 A method for preventing gas cross-mixing m a fuel cell, compπsing providing a bipolar plate that defines one or more manifolds for gas flow, and overlaying a gasket on the bipolar plate, the gasket defining manifolds for gas flow wherein the manifolds defined by the rigid portion of the gasket are o\ er the plate and wherein the gasket further comprises a compressible portion that seals the gasket to the plate

19 The method of claim 18 and further compπsing passing reactant gases through the manifolds