DE102020204303A1 - Method of manufacturing bipolar plates for fuel cells and device - Google Patents

Abstract

The invention relates to a method for producing bipolar plates (7) for fuel cells (1), comprising the following steps: a) extruding a molding compound (60), b) rolling the molding compound (60) by means of at least one first pair of rollers (62), so that a first film (64) is formed, c) rolling and embossing of the first film (64) by means of at least one second pair of rollers (66), so that a second film (68) is formed, the at least one second pair of rollers (66) being a The roller surface (70), the roller surface (70) has a microstructure (72) and the microstructure (72) is heated to a structure temperature TS and d) possibly cutting the second film (68) so that the bipolar plates (7) are produced The invention also relates to a device for producing bipolar plates.

Description

The invention relates to a method for producing bipolar plates for fuel cells comprising the steps of extruding a molding compound, rolling the molding compound by means of at least one first pair of rollers so that a first film is formed, rolling the first film by means of at least one second pair of rollers, so that a second film is formed, and optionally cutting the second film, so that the bipolar plates are formed. The invention is also directed to an apparatus for producing bipolar plates for fuel cells.

State of the art

A fuel cell is an electrochemical energy converter. In known fuel cells, hydrogen (H ₂ ) and oxygen (O ₂ ) in particular are converted into water (H ₂ O), electrical energy and heat.

Among other things, proton exchange membrane (PEM) fuel cells are known. PEM fuel cells have a centrally arranged membrane that is permeable to protons, i.e. hydrogen ions. The oxidizing agent, in particular atmospheric oxygen, is thereby spatially separated from the fuel, in particular hydrogen.

Solid oxide fuel cells, which are also referred to as solid oxide fuel cells (SOFC), are also known. SOFC fuel cells have a higher operating temperature and exhaust gas temperature than PEM fuel cells and are used in particular in stationary operation.

Fuel cells have an anode and a cathode. The fuel is fed to the anode of the fuel cell and is catalytically oxidized to protons, releasing electrons, which then reach the cathode. The released electrons are diverted from the fuel cell and flow to the cathode via an external circuit.

The oxidizing agent, in particular atmospheric oxygen, is supplied to the cathode of the fuel cell and reacts by absorbing electrons from the external circuit and protons to form water. The resulting water is drained from the fuel cell. The gross response is: O ₂ + 4H ⁺ + 4e ^- → 2H ₂ O

The membrane and the electrodes, in particular the anode and the cathode, can be structurally combined by a membrane electrode unit, which is also referred to as a membrane electrode assembly (MEA).

A voltage is applied between the anode and the cathode of the fuel cell. To increase the voltage, several fuel cells can be arranged mechanically one behind the other to form a fuel cell stack, which is also referred to as a stack, and electrically connected in series. The fuel cell stack can be surrounded by a housing.

Fuel cell stacks generally have bipolar plates, which can also be referred to as gas distributor plates. They serve to distribute the fuel evenly to the anode and to distribute the oxidizing agent evenly to the cathode. Bipolar plates usually have a surface structure, for example channel-like structures, for distributing the fuel and the oxidizing agent to the electrodes. The channel-like structures serve to drain off the water produced during the reaction. In addition, bipolar plates can have structures for conducting a cooling liquid through the fuel cell in order to dissipate heat.

In addition to the media flow with regard to oxygen, hydrogen and water, bipolar plates ensure a flat electrical contact with the membrane.

In order to operate the electrochemical reaction in the fuel cell stack, bipolar plates must enable the electrons formed on the anode side to be transported to the cathode side of an adjacent fuel cell with the lowest possible electrical resistance. Correspondingly, both good bulk conductivity and, in particular, low contact resistance are required. The contact resistance depends on the surface quality of the bipolar plates, an adjacent contact medium and an applied contact pressure. In addition, the bipolar plates must ensure high media tightness and corrosion resistance, good heat dissipation and sufficient mechanical stability. The mechanical stability is particularly important for the contact pressure between the fuel cells in the fuel cell stack.

For the production of bipolar plates, in addition to thin, structured metal foils, which usually require corrosion protection, pure graphite plates are used which, although they offer high chemical resistance and good contact resistance, are complex to process. There are also sheets made of highly filled graphite-based thermoplastic or curable polymers, which are also available as composites are designated, for use. These combine the advantageous functional properties of graphite with a simpler and more cost-effective shaping by the polymers.

Polymer-based bipolar plates are usually manufactured using injection molding and pressing. However, due to a high proportion of fillers, the polymers used have a high viscosity and high flow limits, which makes processing difficult. Only relatively small and thick bipolar plates can be produced by injection molding, since the proportions of flow paths and lengths of the bipolar plates do not allow the production of other geometries from correspondingly highly filled polymers.

DE 10 243 592 A1 describes a bipolar plate for PEM fuel cells made from a polymer blend that contains a filler to increase conductivity.

WO 03/094270 A2 is also directed to a bipolar plate for fuel cells, which has a fiber reinforcement structure, a plastic matrix and an additive to create electrical conductivity. A press mold is used for production.

DE 10 2004 039 115 A1 deals with a conical bipolar plate that is manufactured by turning, milling, deep drawing, pressing or injection molding.

EP 1 753 605 B1 relates to roller arrangements for the embossing of web-shaped materials.

DE 10 2009 051 434 A1 has a molded body made of highly conductive molding compound, which is suitable for use as a bipolar plate for electrochemical cells.

Disclosure of the invention

1. a) Extrudieren einer Formmasse,
2. b) Walzen der Formmasse mittels mindestens eines ersten Walzenpaares, so dass eine erste Folie gebildet wird,
3. c) Walzen und Prägen der ersten Folie mittels mindestens eines zweiten Walzenpaares, so dass eine zweite Folie gebildet wird, wobei das mindestens eine zweite Walzenpaar eine Walzenoberfläche umfasst, die Walzenoberfläche eine Mikrostruktur aufweist und die Mikrostruktur auf eine Strukturtemperatur T_S erwärmt wird und
4. d) ggf. Schneiden der zweiten Folie, so dass die Bipolarplatten entstehen.

A method for producing bipolar plates for fuel cells is proposed, comprising the following steps:

1. a) extruding a molding compound,
2. b) rolling the molding compound by means of at least one first pair of rollers, so that a first film is formed,
3. c) rolling and embossing the first film by means of at least one second pair of rollers, so that a second film is formed, wherein the at least one second pair of rollers comprises a roller surface, the roller surface has a microstructure and the microstructure is heated _{to a structure temperature T S and}
4. d) if necessary, cutting the second film so that the bipolar plates are created.

Process steps a) to d) are preferably carried out in the order given. The at least one first pair of rollers and the at least one second pair of rollers can also be referred to as a roller frame or calender.

Furthermore, a device for producing bipolar plates for fuel cells is proposed, comprising an extruder, at least one first pair of rollers, at least one second pair of rollers, an external heat source and optionally a cutting tool, the at least one second pair of rollers comprising a roller surface, the roller surface having a microstructure and the external heat source for heating the microstructure is arranged on the at least one second pair of rollers.

The at least one first pair of rollers is used to produce the first film, which preferably has a thickness of no more than 1 mm. Then the thickness of the film is further reduced by means of the at least one second pair of rollers and the microstructure is embossed, so that the second film is produced. The microstructure preferably has structures which have a depth in a range from 50 μm to 450 μm and a width in a range from 50 μm to 800 μm. The first film preferably has a first thickness in a range from 100 μm to 800 μm, more preferably the second film has a second thickness in a range from 600 nm to 200 μm.

As a result of the heating of the microstructure in step c), material of the first film can flow into the microstructure of the at least one second pair of rollers, so that a high level of molding accuracy is established.

An arrangement of the external heat source on the at least second roller pair is to be understood in particular to mean that the external heat source is arranged at a smaller distance from the at least one second roller pair than from the at least one first roller pair. The external heat source is preferably arranged between the at least one first pair of rollers and the at least one second pair of rollers. The external heat source can alternatively be arranged after the second pair of rollers in the conveying direction.

The at least one second pair of rollers preferably comprises a first roller and a second roller. The roller surface which has the microstructure is preferably on the first roller and / or arranged on the second roller of the at least one second roller pair. The microstructure is more preferably arranged on the second roller and the external heat source is arranged at a smaller distance from the second roller than from the first roller. The roller surface which has the microstructure is preferably arranged on the second roller, more preferably exclusively on the second roller.

The first roller is preferably an upper roller and the second roller is preferably a lower roller. A center point of the upper roller is preferably arranged in a vertical direction above a center point of the lower roller. The external heat source is preferably arranged below the upper roller.

The external heat source is preferably arranged below the first film and in front of the at least one second pair of rollers in the conveying direction. In particular, the external heat source is directed to the microstructure of the at least one second pair of rollers.

The fuel cells are in particular PEM fuel cells. The device, in particular the extruder, preferably comprises a slot die. The extruder can be, for example, a single screw extruder, a twin screw extruder or a planetary roller extruder. The cutting tool is preferably a cutter. Furthermore, the device can comprise a pull-off device.

The molding compound, the first film and / or the second film are preferably biaxially stretched.

The microstructure is preferably heated in step c) by means of the external heat source or by means of induction. The external heat source can also be referred to as an additional heating station. The external heat source is in particular a component that is different from the first pair of rollers and / or the second pair of rollers and is arranged separately.

The external heat source preferably comprises an electrical heating element, a variothermal heater, a laser and / or an infrared source.

The microstructure is heated in a targeted manner, in particular structurally independently of the second pair of rollers. The structure temperature Ts is preferably higher than a surface temperature T _{O of} the roller surface of the at least one second roller pair. In a radial cross section of the first roller and / or the second roller, the structure temperature Ts is preferably measured at a first point of the microstructure, which is arranged further from the center point of the first roller and / or the second roller than a second point of the microstructure which the surface temperature T _{O is} measured.

The structure temperature Ts is more preferably at least as high as a minimum temperature T _min , the minimum temperature T _{min being} 10 ° C. less than a melting temperature T _{SM of} the molding compound, in particular of the first film. The minimum temperature T _min is particularly preferably equal to the melting temperature T _{SM of} the molding compound.

The molding compound is preferably a thermoplastic molding compound. The molding compound further preferably comprises polypropylene (PP), polyphenylene sulfide (PPS), thermoplastic styrene block copolymers (TPS), polyphthalamides (PPA) or mixtures thereof. In particular, the molding compound consists of PP, PPS, TPS, PPA or mixtures thereof.

The melting temperature T _SM is, for example, in a range from 160 ° C. to 185 ° C., in particular if the molding compound contains PP, in a range from 270 ° C. to 290 ° C., in particular in a range when the molding compound contains PPS from 150 ° C. to 220 ° C., in particular when the molding compound contains TPS or in a range from 285 ° C. to 315 ° C., in particular when the molding compound contains PPA.

_{A temperature T F of} the first film is preferably measured prior to rolling by means of the at least one second pair of rollers in step c), and more preferably the structure temperature Ts is regulated or set as a _{function of the temperature T F of the first film.}

The temperature T _{F of} the first film after step b) _{is preferably lower than an extrusion temperature T E of} the molding compound after step a). The heating of the microstructure in step c) counteracts the cooling of the molding compound and / or the first film by the rolling in step b), so that the microstructure can be molded.

The molding compound, the first film and the second film also preferably contain an electrically conductive filler, in particular graphite. The electrically conductive filler is in particular in the form of particles, fibers, fabrics and / or as a fleece. The molding compound preferably contains 60% by weight to 90% by weight of the electrically conductive filler, based on the total molding compound.

Advantages of the invention

With the method according to the invention, large and thin fuel cells can also be produced in a continuous process and on the basis of plastics.

In addition, by heating the microstructure, in particular by the external heat source, the molding accuracy of the microstructure on the bipolar plate can be increased. The targeted and locally directed heating of the microstructure improves the flow of the material of the molding compound or the films into the microstructure. In contrast, a uniformly high temperature of the complete pair of rollers would lead to high heat losses and an inhomogeneous temperature distribution due to the large surface area.

These heat losses are avoided by the method according to the invention and by the device according to the invention, and a homogeneous temperature distribution can be ensured at the contact surface between the film to be embossed and the microstructure.

Since a high pressure or a great force has to be used for embossing, large rollers are required, which are basically difficult to control, since high losses occur over the large surfaces of the rollers. The local application of the external heat source provides an accurate desired temperature.

Figure list

Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

• 1 eine schematische Darstellung eines Brennstoffzellenstapels und
• 2 eine schematische Darstellung einer Vorrichtung zur Herstellung von Bipolarplatten.

Show it:

• 1 a schematic representation of a fuel cell stack and
• 2 a schematic representation of an apparatus for the production of bipolar plates.

Embodiments of the invention

In the following description of the embodiments of the invention, the same or similar elements are denoted by the same reference numerals, a repeated description of these elements being dispensed with in individual cases. The figures represent the subject matter of the invention only schematically.

1 shows a schematic representation of a fuel cell stack 23 with several fuel cells 1 . Every fuel cell 1 has a membrane 33 , two gas diffusion layers 35 , an anode 37 and a cathode 39 on. The individual fuel cells 1 are by bipolar plates 7th who have favourited a cold plate 41 may include, delimited from one another. The fuel cell stack 23 , the hydrogen 47 and oxygen 45 in the form of air as well as a coolant 43 is fed through two end plates 49 completed and instructs current collectors 51 on.

2 shows a schematic representation of a device 90 for the production of bipolar plates 7th for fuel cells 1 . The device 90 includes an extruder 80 , a first pair of rollers 62 , a second pair of rollers 66 , an external heat source 74 and a cutting tool 82 .

With the extruder 80 , the one slot nozzle 88 has, is first a molding compound 60 extruded, which then to the first pair of rollers 62 is promoted. By rolling the molding compound 60 with the first pair of rollers 62 a first slide is created 64 from the molding compound 60 . The first slide 64 has a first thickness 76 on.

The first slide 64 then becomes the second pair of rollers 66 fed. The second pair of rollers 66 includes a roller surface 70 that have a microstructure 72 having. By means of the second pair of rollers 66 becomes the first slide 64 rolled and the microstructure 72 is imprinted. An embossed second foil 68 leaves the second pair of rollers 66 . The second slide 68 has a second thickness 78 that is smaller than the first thickness 76 .

To improve the molding accuracy with regard to the microstructure 72 becomes the microstructure 72 heated to a structure temperature Ts. For heating the microstructure 72 is the external heat source 74 near the microstructure 72 arranged and ensures local heating of the microstructure 72 before or during the microstructure 72 with the first slide 64 is in contact.

The second slide 68 then becomes the cutting tool 82 led where bipolar plates 7th from the second slide 68 be tailored.

The invention is not restricted to the exemplary embodiments described here and the aspects emphasized therein. Rather, a large number of modifications are possible within the range specified by the claims, which are within the scope of expert action.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 10243592 A1 [0014]
• WO 03/094270 A2 [0015]
• DE 102004039115 A1 [0016]
• EP 1753605 B1 [0017]
• DE 102009051434 A1 [0018]

Claims (10)

A method for producing bipolar plates (7) for fuel cells (1) comprising the following steps: a) extruding a molding compound (60), b) rolling the molding compound (60) by means of at least one first pair of rollers (62), so that a first film (64 ) is formed, c) rolling and embossing the first film (64) by means of at least one second pair of rollers (66), so that a second film (68) is formed, the at least one second pair of rollers (66) comprising a roller surface (70) , the roller surface (70) has a microstructure (72) and the microstructure (72) is _{heated to a structure temperature T S} and d) if necessary, cutting the second film (68) so that the bipolar plates (7) are created. Procedure according to Claim 1 , characterized in that the microstructure (72) is heated in step c) by means of an external heat source (74) or by means of induction. Method according to one of the preceding claims, characterized in that the external heat source (74) comprises an electrical heating element, a variothermal heater, a laser and / or an infrared source. Method according to one of the preceding claims, characterized in that the structure temperature Ts is at least as high as a minimum temperature T _min , the minimum temperature T _{min being} 10 ° C less than a melting temperature T _{SM of} the molding compound (60). Method according to one of the preceding claims, characterized in that before rolling by means of the at least one second pair of rollers (66) in step c) a temperature T _{F of} the first film (64) is measured and the structure temperature Ts as a function of the temperature T _F the first film (64) is regulated. Method according to one of the preceding claims, characterized in that the first film (64) has a first thickness (76) in a range from 100 µm to 800 µm. Method according to one of the preceding claims, characterized in that the second film (68) has a second thickness (78) in a range from 600 nm to 200 µm. Method according to one of the preceding claims, characterized in that the molding compound (60) is a thermoplastic molding compound and / or contains an electrically conductive filler, in particular graphite. Device (90) for producing bipolar plates (7) for fuel cells (1), comprising an extruder (80), at least one first pair of rollers (62), at least one second pair of rollers (66), an external heat source (74) and optionally a cutting tool (82), wherein the at least one second pair of rollers (66) comprises a roller surface (70), the roller surface (70) has a microstructure (72) and the external heat source (74) for heating the microstructure (72) on the at least one second Roller pair (66) is arranged. Device (90) after Claim 9 , characterized in that the at least one second pair of rollers (66) comprises a first roller (84) and a second roller (86), the microstructure (72) is arranged on the second roller (86) and the external heat source (74) in a smaller distance to the second roller (86) than to the first roller (84) is arranged.

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