CA2493554A1

CA2493554A1 - Bipolar plate for a fuel cell

Info

Publication number: CA2493554A1
Application number: CA002493554A
Authority: CA
Inventors: Hendrik Dohle; Thomas Bewer; Juergen Mergel; Detlef Stolten; Thorsten Wuester
Original assignee: Individual
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2002-07-25
Filing date: 2003-06-28
Publication date: 2004-02-26
Also published as: ATE467240T1; WO2004017448A3; DE10233982A1; DE50312695D1; EP1525634B1; EP1525634A2; DE10233982B4; WO2004017448A2

Abstract

The invention relates to a bipolar plate for a fuel cell, especially a low temperature fuel cell comprising a centre plate, at least one intermediate plate comprising a recess for the flow distribution of an operating medium a nd at least one cover plate which is adjacent thereto and which has at least on e recess. The centre plate and the cover plates can be made of metal, plastic or graphite. Additional network-type distributor structures arranged in the intermediate plate between the centre plate and the cover plate and the flow channels arranged in the cover plate advantageously improve the flow within the bipolar plate. The cover plate advantageously prevents the membrane- electrolyte-unit (MEA) from pressing into the bipolar plate. The inventive modular bipolar plate enables an extremely compact structure to be obtained. Space-saving fuel cell stacks can be obtained, wherein the layer thickness o f an individual bipolar plate (centre plate, two intermediate plates, two cove r plates) is as a rule significantly less than 1.4 mm, advantageously less tha n 1.0 mm.

Description

~WO 2004/017448 PCT/DE2003/002155 Description Bipolar Plate for a Fuel Cell Technical Domain The present invention relates to a bipolar plate for a fuel cell, in particular to a very compact bipolar plate for a low-temperature fuel cell.
Prior Art Fuel cells convert chemical energy into electrical energy without generating any emissions worth considering in so doing. Various types of fuel cells are known; for example, the high-temperature fuel cell (solid-oxide fuel cell - SOFC) or the so-called low temperature fuel cell (polymer-electrolyte-membrane - PEM).
As a rule, an individual fuel cell comprises a cathode, an anode, as well as an electrolyte;
for example, that is situated between them and is in the form in the form of an ion-conductive membrane. An oxidizing medium, such as oxygen or air, is supplied to the cathode and a fuel such as hydrogen is supplied to the anode. Although direct contact between the oxidizing agent and the fuel is prevented within the fuel cell, movement of ions are permitted to move through the membrane. The hydrogen is oxidized at the anode with the production of protons. These protons then migrate through the membrane to the cathode where they react with the oxidizing agent to form water. The whole of the electrochemical reaction is spontaneous, so that energy is produced in the form of voltage. As a rule, a plurality of fuel cells are connected to one another electrically and mechanically by means of connecting elements-so-called bipolar plates-in order to produce greater electrical power. Bipolar plates are used to stack fuel cells that are connected electrically in series so as to form so-called fuel-cell stacks.
The two principal elements in a fuel cell are first, the membrane-electrolyte assembly (MEA), and second, the operating medium distribution assemblies (bipolar plates). As a rule, these operating-medium distribution assemblies, which simultaneously ensure contact between the fuel cells, are of graphite or metal. They always incorporate a plurality of channels that are intended-ideally-to distribute the fuels evenly over the MEA and permit removal of the production water that is formed.
Up to now, the most frequently used material for a bipolar plate has been graphite. It is conductive, corrosion resistant, and extremely durable. It is, however, very costly and results in very large installed size.
Titanium is also suitable as material for a bipolar plate. It is extremely tough and can also exhibit the required conductivity and resistance to corrosion if appropriately treated.
However, it is very costly and is very difficult to machine.
Ferrous metals, for example stainless steels, have also been used as materials for a bipolar plate. Metals are very good conductors of electricity and can be machined quite easily.
One disadvantage of metals is that the individual components are heavy; in addition, they are not so durable because they are vulnerable to corrosion.
Up to now, costly structures that necessitate a certain, minimum installed size and are complicated to manufacture, have always been needed for the cell frames of the MEA

and for the bipolar plates. The bipolar plates are installed between two MEAs and thus form the connection between two fuel cells.
US S, 482,792 describes an operating-medium distribution assembly that is a porous, electron-conducting collector that distributes the operating medium across the MEA, removes the production water, and conducts electrical current away from the electrodes.
In one embodiment, a metallic wire fabric or mesh is set in a rubber seal as a collector/distributor; this has a plurality of channels for supply and removal. The rubber seal is approximately 2 mm thick. A bipolar plate for use between two fuel cells would thus be at least 2 - S mm thick.

3 891 describes a bipolar plate for a fuel cell that essentially comprises a number of planar components. Each of two mesh-like metal foils borders two mesh-like structures on each side. This assembly forms the so-called bipolar plate. One disadvantage of such an arrangement is that it requires costly seals between the individual components. They form a layered system that is approximately 3 mm thick.
Objective and Solution It is the objective of the present invention to create a bipolar plate for use in a fuel-cell stack, allows a very compact structure with a layer thickness of less that 1.4 mm overall, and does not have the disadvantages discussed heretofore, particularly vulnerability to corrosion. An additional objective of the present invention is to produce a compact and effective fuel-cell stack.

The present invention achieves this objective by a bipolar plate for a fuel-cell stack, as is defined in the principal claim, and by a fuel-cell stack as defined in the secondary claims.
Advantageous embodiments of the bipolar plate or the fuel-cell stack are set out in each of the related claims.
Description of the Present Invention The bipolar plate as defined in Claim 1 is a modular structure and comprises a thin centre plate, at least one intermediate plate and at least one cover plate. The plates are arranged as layers, with the intermediate plate disposed between the centre plate and the cover plate. Both the centre plate and the cover plate can be of metal, plastic, or graphite.
Within a fuel-cell stack, the bipolar plate advantageously has two intermediate plates and two cover plates, which are arranged on both sides of the centre plate.
The centre plate is so configured as to be gas-tight since, as a rule, within a fuel-cell stack it separates an anode space from a cathode space. As an edge plate, the centre plate can also simultaneously close off an electrode space of a cell in a fuel-cell stack to the outside in such a way as to render it gas-tight.
The centre plate can, in particular, be of metal or of graphite. Any metal that is gas-tight during the proposed operation of the fuel cell and which provides sufficient chemical resistance to the operating medium is suitable as material for the centre plate so that, in principle, plastic is also suitable as material for the centre plate. In such a case, the necessary conductivity can be provided by conductive contacts, e.g., in the form of metal inclusions.

According to the present invention, the centre plate has one raised area, but mostly a plurality of raised areas that are, as a rule, disposed in a central area and can be produced by stamping or embossing, for example. These raised areas serve as electrical contacts and/or provide for the mechanical spacing of the cover plate.
Between the centre plate (edge plate) and a cover plate there is an intermediate plate that serves as a flow distributor. To this end, the intermediate plate frequently incorporates a central cutout. Thus it has a frame-like appearance. The intermediate plate can advantageously be configured as a flat seal. It is advantageous that there be additional cutouts in the edge area to supply and remove operating medium. The raised areas of the centre plate are disposed in the area in which the intermediate plate has its central cutout, so that a gas-tight contact between the intermediate plate and the centre plate is made possible, despite the raised areas.
The central cutout in the intermediate plate corresponds for the most part to the area of an adjoining membrane-electrolyte unit (MEA); as a rule, it is in the form of a rectangle or square, there being .a connection to one of the supply channels. The predominantly square or rectangular part of the cutout can advantageously be filled with a web or mesh.
A web or a mesh is particularly good for distributing the operating medium evenly across the surface. The remaining supply part of the cutout, which is mostly triangular, and is left between the mesh and the supply or removal channel forms the so-called manifold.
In order to prevent an MEA from being pressed into the mesh, the intermediate plate is limited by a cover plate.

The cover plate, too, has cutouts in the edge area, and these serve to supply and remove operating medium. The cover plate also incorporates some cutouts within the area of the central cutout of the intermediate layer. These are so small or narrow that the MEA
cannot be pressed into them.. On the other hand, these channels can bring about additional even distribution of the operating medium from the cutout of the intermediate plate, across the cover plate and onto the surface of the adjacent MEA. In this connection, a channel in the cover plate that is configured so as to be parallel to the manifold of the intermediate plate has been found to be particularly advantageous.
The individual plates can advantageously be cemented, which provides for a broad selection of the materials that can be used.
Special descriptive section The present invention will be described in greater detail below on the basis of the drawings appended hereto and exemplary embodiments shown in the drawings, without the object of the present invention being restricted thereby. These drawings show the following:
Figure 1: A diagrammatic representation of the structure of a bipolar plate known from the prior art, with a flat centre or edge plate, two cover plates, two intermediate plates disposed between these, with mesh-like flow distributors and a membrane-electrode assembly (MEA).
Figure 2: A diagrammatic representation of the structure of an embodiment of the bipolar plate according to the present invention, with a centre plate, two cover plates and two intermediate plates disposed between these, with flow distributors, the centre plate having raised areas on at least one side;
Figure 3: Different embodiments of a cover plate for a bipolar plate according to the present invention.
Figure 1 shows a bipolar plate for a fuel cell according to the prior art. An intermediate plate (B) is adj acent to a centre or edge plate (A) that incorporates openings ( 1 ) for supplying the operating medium. It also incorporates a centre cutout (2). The square or rectangular centre part of the centre cutout is advantageously filled with a mesh-like web (3). The cover plate (C) is adjacent to the centre plate; on the one hand, this incorporates openings (1) for supplying operating medium and, on the other hand, it incorporates horizontal (4) and vertical (6) channels (5). These serve to distribute the operating medium even more evenly across the adjacent membrane-electrode assembly (MEA).
Figure 2 shows one embodiment of the bipolar plate according to the present invention.
A flow distributor (intermediate plate B') is adjacent to each side of a metal centre plate (A') that has raised areas (e.g., embossed areas) on both sides. Each of these adjacent intermediate plates B') incorporates a plurality of cutouts, the centre cutout being filled at least in part by a mesh. The area of the central cutout that is not filled with the mesh forms the so-called manifold. The intermediate plates (B') are each closed off by a cover plate (C), each such cover plate incorporating a plurality of cutouts. On the right-hand side, the cover plate has vertical and horizontal channels; the horizontal channels run parallel to the manifolds, and advantageously increase the size of these. The cover plate prevents the adjoining membrane-electrode assembly (MEA) from being pressed into the mesh-like distributor structures.
Different embodiments of the cover plate are shown in Figure 3. Embodiment C' is a cover plate with a cutout that is provided for an MEA. The configuration of the cover plate or the size of the MEA is so calculated that it cannot be pressed into an adjacent manifold, since the manifold is closed of by a solid area of the cover plate.
C" is a cover plate with channels for further improving the flow of an operating medium across the flow distributors that are, for example, mesh-like.
It is advantageous that the cover plate C"' be manufactured with the seal already applied to it so that there is no need far a seal between the bipolar plate and the MEA to be installed when a fuel cell is being assembled. The additional horizontal channels C"" run parallel to the manifolds in the adjacent flow distributors, enlarging them thereby.
The foregoing results in the following advantages for the present invention:
- it is a possible to arrive at a very flat structure, < 1.5 mm per bipolar plate;
- electrical contact between the anode and the cathode is ensured;
adequate application pressure on the electrodes is ensured, and delamination is prevented;
- even distribution of the operating medium is ensured;
- the bipolar plate is particularly stable and chemical resistance over the long term.

Key to drawings:
Figure 1 (Prior Art) A Centre or end plate B Intermediate plate with mesh insert and manifold C Cover plate with channels MEA Membrane-electrode assembly 1 Supply and removal channels for the operating medium, perpendicular to the plane of the fuel cell or to the bipolar plate 2 Central cutout in the intermediate plate 3 Mesh as flow distributor 4 Horizontal channels in the cover plate that run parallel to the adjacent manifold Vertical channels in the cover plate for improved distribution of the operating medium Figure 2: An embodiment of the present invention A' Centre plate according to the present invention, with raised areas B' intermediate plate matched to A' C', MEA as known from the prior art Figure 3: An embodiment of the cover plate C' Cover plate with a large cutout for a membrane-electrode assembly (MEA) C" Cover plate with vertical channels C"' Cover plate with seal applied C"" Cover plate with vertical channels and horizontal channels parallel to a manifold

Claims

1. Bipolar plate for a fuel cell, said bipolar plate comprising a centre or cover plate A, at least one cover plate C, and at least one intermediate plate B that is disposed between a centre and cover plate, said intermediate plate having at least one central cutout 2 for distributing an operating medium, characterized by at least one raised area on the side of the centre or end plate A' that is proximate to the intermediate plate B', said raised area extending into the central cutout 2 in the intermediate plate B'

2. Bipolar plate as defined in the preceding Claim 1, with a centre plate A' that incorporates a plurality of raised areas.

3. Bipolar plate as defined in one of the preceding Claims 1 and 2, with a gas-tight centre plate A' that has raised areas on both sides, two adjoining intermediate plates B', as well as two cover plates C that delimit the intermediate plates.

4. Bipolar plate as defined in one of the preceding Claims 1 to 3, with a centre plate A' that is of metal or graphite.

5. Bipolar plate as defined in one of the preceding Claims 1 to 4, in which the thickness of the centre plate A' without raised areas is less than 0.2 mm and preferably less than 0.1 mm.

6. Bipolar plate as defined in one of the preceding Claims 1 to 5, in which the centre plate is configured as a flat seal.

7. Bipolar plate as defined in one of the preceding Claims 1 to 6, in which the intermediate plate B' is less than 0.4 mm thick, and in particular less than 0.3 mm thick.

8. Bipolar plate as defined in one of the preceding Claims 1 to 7, in which the intermediate plate B' has an additional flow distributor.

9. Bipolar plate as defined in the preceding Claim 8, in which the flow distributor is in the form of a mesh, and the portion of the cutout of the intermediate plate that is not filled with mesh forms a manifold.

10. Bipolar plate as defined in the preceding Claim 9, with a mesh that is of metal or plastic.

11. Bipolar plate as defined in one of the preceding Claims 9 or 10, in which the flow distributor that is in the form of a mesh incorporates cutouts for the raised areas of the centre plate A'.

12. Bipolar plate as defined in one of the preceding Claims 1 to 11, with a cover plate C that is of metal, plastic, or graphite.

13. Bipolar plate as defined in one of the preceding Claims 1 to 12, in which a cover plate C is less than 0.2 mm thick, and in particular less than 0.1 mm thick.

14. Bipolar plate as defined in one of the preceding Claims 1 to 13, in which the cover plate C incorporates channels as a flow-distribution structure.

15. Bipolar plate as defined in one of the preceding Claims 1 to 14, in which the cover plate C has at least one channel parallel to a manifold.

16. Bipolar plate as defined in one of the preceding Claims 1 to 15, in which the raised area of the centre plate A' makes contact with the cover plate C.

17. Bipolar plate as defined in one of the preceding Claims 1 to 16, in which the raised area of the centre plate A' forms an electrically conductive contact to the cover plate C.

18. Bipolar plate as defined in one of the preceding Claims 1 to 17, in which the areas that are raised in the direction of the particular cover plate are produced by impressing or embossing.

19. Bipolar plate as defined in one of the preceding Claims 1 to 18, in which at least one cover plate C has a seal that is applied to it.

20. Bipolar plate as defined in one of the preceding Claims 1 to 19, comprising a centre plate A', an intermediate plate B', and a cover plate C, which are of a total thickness that is less than 0.8 mm, and in particular of less than 0.6 mm.

21. Bipolar plate as defined in one of the preceding Claims 1 to 19, comprising a centre plate A', two intermediate plates B', and two cover plates C, that are in total of a thickness that is less than 1.4 mm, and in particular of less than 1.0 mm.

22. Bipolar plate as defined in one of the preceding Claims 1 to 21, comprising a metal centre plate A', at least one metal mesh as a flow distributor, and at least one cover plate C that is of plastic.

23. Bipolar plate as defined in one of the preceding Claims 1 to 21, comprising a metal centre plate A' that is of plastic, at least one metal mesh as a flow distributor in the intermediate plate B', and at least one cover plate C that is of plastic.

24. Bipolar plate as defined in one of the preceding Claims 1 to 22, comprising a centre plate A' and at least one cover plate C that is of graphite.

25. Fuel cell stack comprising at least one fuel cell with a bipolar plate as defined in one of the preceding Claims 1 to 24.