WO2006125775A1 - Bipolar plate for fuel cells - Google Patents

Abstract

The invention relates to bipolar plates for fuel cell stacks, comprising an electrically non-conducting substrate plate with opposing surfaces, each comprising a partial surface. Said partial surface comprises channels for gas transport and at least one electrically-conducting film, fixed to the non-conducting substrate such that the partial surfaces of the opposing surfaces of the non-conducting substrate plate are covered by the at least one conducting film and the film has perforations in the region of the partial surfaces. The invention further relates to a method for production of such a bipolar plate, fuel cell stacks comprising said plates and use thereof in mobile and stationary devices.

Description

Bipolar plate for fuel cells

description

The present invention relates to a bipolar plate and its production and fuel cell stacks containing these bipolar plates and their use.

So far, in particular in motor vehicles predominantly internal combustion engines are used for propulsion, which require petroleum products as fuel. As the resources of oil are limited and the products of combustion may have an adverse environmental impact, research into alternative propulsion concepts has been intensified in recent years.

The use of electrochemical fuel cells for mobile and stationary energy supplies is growing in interest. Fuel cells are energy converters that convert chemical energy into electrical energy. In the fuel cell, the principle of electrolysis is reversed.

Currently, there are different types of fuel cells, whose principle of operation is generally based on the electrochemical recombination of hydrogen and oxygen to the final product water. They can be classified according to the type of conductive electrolyte used, the operating temperature level and realizable power ranges. For automotive applications, polymer electrolyte membrane (PEM) fuel cells are particularly suitable. They are usually operated at a temperature in the range of 50 ° C to 90 ° C. Since the electrical potential of a single cell is too low for practical applications, multiple such cells must be connected in series to a fuel cell stack or stack. At the moment, PEM fuel cells in the complete stack usually deliver electrical power in the range of 1 to 75 kW (cars) and up to 250 kW (commercial vehicles, buses).

In a PEM fuel cell, the electrochemical reaction of hydrogen with oxygen to water is divided by the insertion of a proton-conducting membrane between the anode and the cathode electrode in the two substeps reduction and oxidation. This is a charge separation, which can be used as a voltage source.

A single PEM fuel cell has a symmetrical construction. On a

Polymer membrane followed on both sides each have a catalyst layer and gas distribution layer, which is followed by a bipolar plate. Current collectors are used to pick up the electrical voltage, while end plates ensure the metered addition of the reaction gases and removal of the reaction products.

In a fuel cell stack, a plurality of cells are stacked in electrical series with each other separated by an impermeable, electrically conductive bipolar plate, referred to as a bipolar plate. The bipolar plate connects two cells mechanically and electrically. Since the voltage of a single cell is in the range of 1 volt, it is necessary for practical applications to switch numerous cells in series. Often, up to 400 cells are stacked separately by bipolar plates. The cells are stacked in such a way that the oxygen side of one cell is connected to the hydrogen side of the next cell via the bipolar plate. The bipolar plate fulfills several functions. It serves for the electrical connection of the cells, for the supply and distribution of reactants (reaction gases) and coolants and for the separation of the gas spaces. A bipolar plate should fulfill the following characteristics:

- chemical resistance to moist oxidizing and reducing conditions;

Gas leaks; - high conductivity;

- low contact resistance;

- Dimensional accuracy;

Low costs in terms of materials and production;

- freedom of design; - high mechanical load capacity;

Corrosion resistance;

- low weight.

At present, various types of bipolar plates are used.

Graphite bipolar plates can be shaped by pressing or milling. They are characterized by chemical resistance and low contact resistance, but have an insufficient mechanical behavior.

Composite materials are made of special plastics containing conductive fillers, such as those based on carbon.

Such a composite material is described for example in FR-A2 7765 723. Bipolar plates are most commonly composites, but bipolar plates of metal, such as stainless steels, which may optionally be coated, are also in use. These metallic bipolar plates are characterized by high gas-tightness, dimensional stability and high electrical conductivity. Due to the high requirements in terms of corrosion resistance, such bipolar plates, which include a metal plate, must be protected by appropriate measures. A disadvantage of such plates is in particular their high weight.

For example, GB-A 2359186 describes a metallic bipolar plate having a layer of electrically conductive polymer on its surface. This polymer should ensure sufficient resistance to the electrochemical reaction conditions.

US-A 2004/0081879 describes a fuel cell bipolar plate consisting of a metallic substrate or a composite having a conductive contact layer of noble metal with a thickness in the sub-micron range.

US-A 5,798,188 describes a bipolar plate including an aluminum plate on which projections of a polymer are applied and coated with a layer of metal, metal nitride or metal carbide.

US 2003/0228512 discloses a stainless steel bipolar plate in which the substrate is made of stainless steel coated with a noble metal conductive layer less than 100 nanometers thick.

WO-A 97/50139 describes a bipolar plate consisting of a polymer frame in which a carbon plate or other corrosion-resistant conductive plate is cast.

Furthermore, WO-A 01/28020 describes a bipolar plate which has a metal plate which is protected against corrosion by a graphite emulsion layer and a graphite foil.

US 2004/0038104 proposes a bipolar plate consisting of a chromium-nickel alloy having the required corrosion resistance.

Finally, DE-A10039674 describes a bipolar plate with a metal layer which is enclosed by non-conductive plastic layers, wherein the metal layer has with its surfaces one or more electrically conductive connections.

Furthermore, bipolar plates are proposed in the prior art, which do not include metal or graphite base plate. Here in particular plastics are used, however, they must be modified to ensure the required electrical conductivity of the bipolar plate.

The advantage of using such polymeric base plates or substrates in bipolar plates is their resistance and their low weight.

DE-A 19823880 discloses a bipolar plate made of thermoplastic material which is protected by electrically conductive fillers, e.g. Carbon powder, which are added to the thermoplastic, as well as by a thin film of a conductive material, which is applied, for example by thin-film vacuum deposition on its surface, is made electrically conductive.

DE-A 10261483 discloses a bipolar plate, made of a thermoplastic or thermosetting polymer, in which a metallic fabric is incorporated in such a way that the channels of the so-called "flow fields" present on both sides of the surface are covered by this tissue.

A disadvantage of the latter alternatives is the sometimes expensive modification of the plastic substrate in order to generate sufficient electrical conductivity.

The object of the present invention is thus to provide a bipolar plate which is based on a non-conductive substrate, which is made electrically conductive in a simple manner.

The object is achieved by a bipolar plate for fuel cell stacks containing

- An electrically non-conductive substrate plate having opposite surfaces, each containing a partial surface and these partial surfaces have channels for gas transport and

at least one electrically conductive foil which is fixed on the non-conductive substrate plate such that the partial surfaces of the opposite surfaces of the non-conductive substrate plate are covered by the at least one conductive foil and wherein the foil has perforations in the region of the partial surfaces.

It has been found that produced by simply coating the channels for gas transport having surfaces (flow fields) of the substrate plate with an electrically conductive foil a simple and inexpensive producible bipolar plate can be, wherein the electrical conduction is effected from one side of the bipolar plates to the other by the two sides spanning foil.

Preferably, the non-conductive substrate plate is made of a thermoplastic or thermosetting polymer.

In the context of the present invention, the term "electrically nonconductive" means that no or only negligibly low electrical conductivity is present.

In this case, all reinforced and unreinforced thermoplastic or thermosetting plastics which are chemically stable to moisture-oxidizing and reducing conditions, as prevalent in, for example, PEM fuel cells, can be used as plastic material. In addition, they should be gas-tight and dimensionally stable. Examples of suitable materials are PA, PBT, PPO, PP, PES, EP, UP, PF and other technically used plastics.

According to the invention, the flow fields of the substrate plate can be covered by an electrically conductive foil. This is most easily achieved by guiding a film over an outer edge of the substrate plate and covering the substrate plate at least as far as the flow fields are covered by the foil. Likewise, a plurality of films may be used, which may overlap, in order to be connected to each other gas and liquid-tight.

The film has perforations in the area of the covered flow fields. These serve to allow the gas components, which are conducted to the flow fields, to flow through the perforations to the electrolyte, preferably a polymer electrolyte membrane (PEM), distributed in a finely distributed manner.

However, the bipolar plate can also be designed such that the conductive foil is received by a gas-tight closed recess in the substrate plate outside the flow fields. In this case, the recess is expediently dimensioned in such a way that a film width results that allows complete coverage of the flow fields. Such a configuration corresponds to that proposed in DE-A 10261483, but here, instead of the fabric, the film occurs. This proposed variant can also be combined with that in which the film is guided over an edge of the substrate plate.

The flow field can be covered by bending the foil at the edge of the recess in the direction of the flow field on both sides of the bipolar plates. Furthermore, a film can also be guided over a plurality of edges or through a plurality of recesses in the substrate plate. This increases the conductivity.

There is also the possibility that the film is guided over three or four edges of the substrate plate and / or recesses in the substrate plate.

Alternatively, it is also possible to use a plurality of films which, if appropriate, are connected correspondingly at their abutting edge.

The electrically conductive foil preferably contains carbon, a metal or a metal alloy. Preference is given to nickel (pure), nickel alloys and high-alloy steel. Here, the most important alloying elements for the steel are Al, B, Bi, Co, Cr, Cu, La, Mn, Mo, Ni, Pb, Se, Si, Te, V, W, and Zr.

In order to prevent breakage or tearing of the electrically conductive film during bending over, for example, the edges of the substrate plate, an adhesive tape or several adhesive tapes may be applied to the film.

It is also the combination of two or more different film materials possible. When using a graphite foil, for example, in order to improve the electrical conductivity from one side of the bipolar plate to the other metal sheets may additionally be guided in a U-shape around one or more edges or through the recess or recesses provided for this purpose.

The conductive film according to the invention preferably has a thickness in the range from 50 μm to 700 μm, more preferably in the range from 100 μm to 500 μm and in particular in the range from 200 μm to 400 μm. As already stated above, the film is partially provided with a perforation, which is located after application of the film over the flow fields. The dimensioning of the perforation openings advantageously corresponds to the channel width. To ensure gas transport, at least part of the perforation openings must be located above the channels of the flow fields.

The film can be fixed on the substrate plate by various ways known to those skilled in the art. This fixation is particularly advantageous by gluing. This allows a particularly simple way of producing a bipolar plate according to the invention.

For gluing common adhesives can be used. However, it should be taken into account that the adhesive has sufficient resistance to the operating fluids used and to the operating temperature. Another object of the present invention is a method for producing a bipolar plate according to the invention comprising the steps:

- Providing an electrically non-conductive substrate plate with opposite surfaces, each containing a partial surface, and these partial surfaces have channels for gas transport;

Fixing at least one electrically conductive film on the non-conductive substrate plate such that the partial surfaces of the opposite surface of the non-conductive substrate plate are covered by the at least one conductive film, and wherein the film has perforations in the region of the partial surfaces; and

- Optionally gas-tight closing of recesses in the substrate plate for receiving the conductive film.

The substrate plate can be produced by injection molding. This allows a particularly flexible spatial design, so that complex geometric structures can be generated despite the simple production.

The at least one electrically conductive film can - as mentioned above - are preferably fixed by gluing. In this case, the part of the surfaces of the bipolar plate to be bonded is provided with a suitable adhesive. Alternatively, the film or foil and bipolar plate may be provided with adhesive. Subsequently, the foil is bent around the outer edge of the bipolar plate and / or the inner edge (s) of the recess (s) and pressed against the surface of the bipolar plate. If recesses are present, they must be sealed gas-tight, which can be done by suitable adhesives.

Alternatively, the film may be gas-tightly encapsulated during the production of the substrate plate when the substrate plate contains a thermoplastic polymer in order to achieve the gas-tight reception of the conductive film in a recess.

Another object of the present invention is a fuel cell stack of several fuel cells containing bipolar plates according to the invention.

The bipolar plates according to the invention are generally used in fuel cell stacks of several individual cells. Such fuel cell stacks are made by repeatedly stacking the bipolar plate, gas diffusion layer, catalyst layer, polymer membrane, catalyst layer, and gas diffusion layer. Between each two bipolar plates is a single cell. moreover terminal current collectors and end plates are inserted. The stacked elements of the fuel cell stack are connected and sealed.

The fuel cell stacks according to the invention can be used, for example, for power supply in mobile and stationary devices. In addition to a home supply in particular the power supply of vehicles, such as land, water and air vehicles as well as self-sufficient systems such as satellites into consideration.

The fuel cell stacks according to the invention are preferably stable in a temperature range from -40 to 120 ^° C. The working temperature range is in particular in the range around 80 ^° C. The temperature control can be achieved by suitable cooling media, which are at least in communication with a part of the stack.

The bipolar plates according to the invention combine an advantageous combination of low weight, good electrical conductivity, gas tightness and design of gas channels, and resistance to corrosion.

With reference to the figures, the present invention will be explained in more detail below.

Show it:

Figure 1 shows an embodiment of a bipolar plate according to the invention prior to fixation of the electrically conductive foil by corresponding bending, in which it is guided through a recess and

Figure 2 shows another embodiment of a bipolar plate according to the invention prior to fixation of the electrically conductive foil by corresponding bending, in which it is guided around an outer edge of the bipolar plate.

According to FIG. 1, the bipolar plate 1 comprises an electrically nonconductive substrate plate 2 in which opposite surfaces each have partial surfaces 3 (flow fields) which have a multiplicity of channels 5. Furthermore, the electrically non-conductive substrate plate 2 channels for supplying and discharging liquids and gases. For example, H _{2 can be} supplied via a first input 4. The hydrogen is then distributed in fuel cell operation on the anode side via the channels 5 on the respective surface of the partial surfaces. The hydrogen not consumed for the fuel cell reaction is removed again via the first outlet 6. Likewise, there exist a second input 7 and a second output 8 for the other gas involved in the fuel cell reaction (eg O ₂ ), which passes through the channels (not visible in FIG. 1) on the cathode side along the surface of the not shown th partial surface on the back of the bipolar plate is passed. Furthermore, the electrically non-conductive substrate plate 2 (not shown) further channels for a coolant having, which may be present in the region of the partial surfaces 3 in the interior of the bipolar plate 1. Furthermore, in the electrically non-conductive substrate plate 2 openings 9 are included, which are provided for mounting the bipolar plate 1 in the fuel cell stack. In the embodiment of the present invention shown in FIG. 1, the electrically conductive foil 10 is guided through a recess 11, so that the foil 10 can be bent over in the direction of the partial surfaces 3 on the respective opposite sides of the bipolar plate 1. Preferably, a bonding for fixing the film 10 takes place in areas of the surface of the bipolar plate 1, which lie outside the partial surfaces 3 on each side of the bipolar plate 1. Furthermore, the film 10 perforations 12, which are preferably in the region of the film surface, which come after bending the film 10 on the faces 3 of the opposite sides of the bipolar plate 1 to lie. The electrically conductive film 10 is dimensioned such that the opposite partial surfaces 3 are completely covered after bending and fixing of the film 10.

FIG. 2 shows a further embodiment of a bipolar plate 1 according to the invention, which, in contrast to FIG. 1, however, has no recess for receiving a lens Ne. FIG. 2 shows an electrically conductive foil 10, which is arranged on an outer edge 13 of the bipolar plate. In this case, after bending the film 10 in the direction of the opposite partial surfaces 3 and subsequent fixing also a bipolar plate 1 according to the invention is obtained. Here, the film 10 is also dimensioned such that the opposite partial surfaces 3 of the bipolar plate 1 after bending and fixing the film 10 are completely covered by this and the film 10 in the area of the partial surfaces 3 perforations 12 has. Further, if necessary, the film 10 may have openings (not shown) which ensure that after bending and fixing the film 10, the openings 4, 6, 7, 8, 9 of the bipolar plate 1 are not covered by the film 10.

As already stated above, both embodiments according to FIGS. 1 and 2 can be combined with one another. It is also possible to use further recesses and / or further outer edges of the bipolar plate 1 for guiding the film or the films.

Further conceivable embodiments are disclosed, for example, in FIGS. 4 and 5 of DE-A 10261483, wherein the electrically conductive film 10 according to the present invention must be used instead of the fabric. LIST OF REFERENCE NUMBERS

1 bipolar plate

2 electrically non-conductive substrate plate

3 part surface

4 first input (H ₂ )

5 channels

6 first output (H ₂ )

7 second input (O ₂ )

8 second output (O ₂ )

9 openings

10 electrically conductive foil

11 recess

12 perforations

13 outer edge

Claims

claims 1. Bipolar plate for fuel cell stacks comprising: - An electrically non-conductive substrate plate having opposite surfaces, each containing a partial surface and these partial surfaces have channels for gas transport and at least one electrically conductive foil, which is fixed on the non-conductive substrate plate such that the partial surfaces of the opposite Surfaces of the non-conductive substrate plate are covered by the at least one conductive film and wherein the film has perforations in the region of the partial surfaces. 2. bipolar plate according to claim 1, characterized in that the non-conductive substrate plate is a thermoplastic or thermosetting polymer. 3. bipolar plate according to claim 1 or 2, characterized in that the at least one conductive foil is guided over at least one outer edge of the substrate plate. 4. bipolar plate according to one of claims 1 to 3, characterized in that the at least one conductive foil is received by a gas-tight closed recess in the substrate plate outside the faces. 5. bipolar plate according to one of claims 1 to 4, characterized in that the at least one conductive foil contains carbon, metal or a metal alloy. 6. bipolar plate according to one of claims 1 to 5, characterized in that the at least one conductive foil has a thickness in the range of 50 to 700 microns. 7. bipolar plate according to one of claims 1 to 6, characterized in that the fixing is carried out by at least partially bonding the film with the opposite surfaces of the non-conductive substrate plate. 8. Bipolar plate according to one of claims 1 to 7, characterized in that in addition metal sheets are guided in a U-shape around an edge or a plurality of edges or through the recess or recesses provided for this purpose. 9. A method for producing a bipolar plate according to any one of claims 1 to 8, comprising the steps - Providing an electrically non-conductive substrate plate with opposite surfaces, each containing a partial surface and these partial surfaces have channels for gas transport; Fixing at least one electrically conductive film on the non-conductive substrate plate such that the partial surfaces of the opposite surfaces of the non-conductive substrate plate are covered by the at least one conductive film and wherein the film has perforations in the region of the partial surfaces; and - Optionally gas-tight closing of recesses in the substrate plate for receiving the conductive film. 10. A method for producing a bipolar plate according to any one of claims 4 to 8, comprising the steps - Providing at least one electrically conductive film having perforations; - Gas-tight encapsulation of the film to produce a thermoplastic electrically non-conductive substrate plate with opposite surfaces, each containing a partial surface and these partial surfaces have channels for gas transport; and Fixing the at least one electrically conductive film on the non-conductive substrate plate such that the partial surfaces of the opposite surfaces of the non-conductive substrate plate are covered by the at least one conductive film. 11. Fuel cell stack of a plurality of fuel cells, the bipolar plates according to one of claims 1 to 8 contains. 12. Use of fuel cell stacks according to claim 11 for power supply in mobile and stationary facilities.