DE102023105976A1 - Polymer-graphitic bipolar plate and method for producing a bipolar plate - Google Patents

Abstract

A polymer-graphitic bipolar plate (1) for a stack of electrochemical cells comprises at least one three-dimensionally structured polymer-graphite plate (3, 4) with an average wall thickness (dM), which has first flat areas (8) which lie in a first plane (E1), and second flat areas (9), which touch a second level (E2) which is spaced parallel from the first level (E1) by a multiple of the average wall thickness (dM), the different flat areas (8, 9) being connected to one another by flanks (11). , and wherein at least one of the three areas mentioned (8, 9, 11), i.e. flat areas (8, 9) and flanks (11), is an accumulation of material (14), compared to an undeformed section of the polymer graphite plate (3, 4), based on a projection onto the first plane (E1), and at the same time an impression (15), that is to say a section of reduced wall thickness (d15), is formed in at least one of the two flat areas (8, 9).

Description

The invention relates to a polymer-graphitic bipolar plate suitable for use in a stack of electrochemical cells, in particular fuel cells. The invention further relates to a method for producing a bipolar plate.

The DE 102 16 306 B4 discloses a method for producing a contact plate for an electrochemical cell. A composite material made of polypropylene and synthetic graphite with a graphite mass fraction of at least 80% is used. The entire contact plate is manufactured by injection molding, with a media distribution structure being formed in a plate surface.

The DE 11 2005 001 954 B4 The subject is a bipolar plate intended for use in a fuel cell system, which is a composite plate with a polymer and carbon fibers. It is proposed to chemically treat a surface of the bipolar plate to create a hydrophilic surface. The chemical treatment should be done by immersing the bipolar plate in an acid bath.

The DE 10 2020 204 303 A1 discloses a method for producing bipolar plates for fuel cells. As part of this process, a molding compound is extruded. The molding compound is then rolled and embossed using several pairs of rollers to obtain a film. One of the pairs of rollers has a microstructured roller surface, which is heated to carry out the manufacturing process.

Possible geometric features of a bipolar plate for a fuel cell are in the DE 10 2020 216 095 A1 described. In this case, the bipolar plate has an insert for a connection to a membrane-electrode arrangement.

The invention is based on the object of further developing the production of bipolar plates made of polymer-graphitic materials compared to the prior art, with the aim of achieving a particularly favorable relationship between material consumption and robustness of the end product, i.e. the bipolar plate.

This object is achieved according to the invention by a polymer-graphitic bipolar plate with the features of claim 1. The object is also achieved by a method for producing a bipolar plate according to claim 8. The embodiments and advantages of the invention explained below in connection with the manufacturing method also apply mutatis mutandis to the Devices, that is, the individual bipolar plate and a cell stack comprising a large number of such bipolar plates, and vice versa.

The polymer-graphitic bipolar plate according to the application, intended for use in a stack of electrochemical cells, in particular fuel cells, comprises at least one three-dimensionally structured polymer-graphite plate, which has first flat areas that lie in a first plane and second flat areas that are a multiple of the first level the average wall thickness of the polymer graphite plate affects the second plane spaced parallel to each other, so that the entire polymer graphite plate is arranged between the two planes mentioned. Here, the various flat areas are connected to one another by flanks of the polymer graphite plate, with at least one of the three areas mentioned, i.e. flat areas and flanks, having an accumulation of material compared to an undeformed section of the polymer graphite plate, based on a projection onto the first plane, and at the same time In at least one of the two flat areas, an impression, that is, a section of reduced wall thickness compared to the average wall thickness of the polymer graphite plate, is formed.

The polymer graphite plate can be made with a polyolefin binder. The graphite content of the polymer graphite plate can be 80 percent by weight or more.

• - Bereitstellung mindestens einer Polymergraphitfolie als Ausgangsprodukt des Herstellungsverfahrens,
• - Einlegen der Polymergraphitfolie in ein Werkzeug und Erzeugen einer Prägestruktur in einem kombinierten Präge- und Fließpressvorgang unter Temperatureinwirkung derart, dass Material der Polymergraphitfolie innerhalb eines in unveränderter Winkellage verbleibenden Abschnitts der Polymergraphitfolie verschoben und zumindest teilweise zum Wandstärkeaufbau in einem anderen Abschnitt, nämlich einer Flanke, der Polymergraphitfolie genutzt wird.

The process for producing the bipolar plate made up of a single polymer graphite plate or a plurality of such plates, in particular two pieces, comprises the following steps:

• - Provision of at least one polymer graphite film as the starting product of the manufacturing process,
• - Inserting the polymer graphite foil into a tool and producing an embossed structure in a combined embossing and extrusion process under the influence of temperature in such a way that material of the polymer graphite foil is displaced within a section of the polymer graphite foil remaining in an unchanged angular position and at least partially to build up the wall thickness in another section, namely a flank, the polymer graphite film is used.

The manufacturing process thus combines process features of embossing with features of extrusion. This means that there are very pronounced three-dimensional structures with high Degrees of forming can be achieved with extensive options for adjusting the wall thickness.

The invention is based on the idea that in classic deep drawing the wall thickness of the originally flat, flat workpiece inevitably decreases, with this effect becoming more pronounced the higher the degree of deformation. In the case of a product manufactured by deep drawing and having an embossed structure, depending on the process, stress peaks can occur in flank areas, in transition areas between flanks and flat areas or even within a flat area of the formed workpiece. In order to control all of these stress conditions, the wall thickness of the flat starting product is usually selected in such a way that no overstressing of the later, completely formed workpiece is to be expected. Accordingly, the wall thickness of the workpiece is typically oversized in large areas.

According to the application, such a partial oversizing of the wall thickness is effectively counteracted by moving material within a plane in which the workpiece lies during the forming process and shifting it into a section of the workpiece that is inclined in relation to this plane.

This material shift is made possible by tempering the workpiece to a level of 120 °C to 200 °C, in particular to a temperature of 150 °C ± 10 K, with moderate pressing forces.

According to a possible procedure, during the simultaneous embossing and extrusion, material of the polymer graphite film from a flat area, that is to say a section remaining in an unchanged angular position in one of the two mentioned mutually parallel planes, is used partly to build up a thickening in the same area and partly in a flank the polymer graphite plate being manufactured. The result is an inconsistent wall thickness in at least one of the flat areas, while at the same time material is piled up in the flank area. Such an accumulation of material in the flank does not necessarily mean that there is a local maximum in wall thickness in the relevant area. Rather, the accumulated material can also be evenly distributed over the entire area reinforced with it, so that its wall thickness is uniform. Particularly in a flat area, depending on the type of forming, a maximum and several minima of the wall thickness can also be generated, with the minima being located, for example, at the transition to a flank of the polymer graphite plate.

• - Die Flankenwandstärke der Polymergraphitplatte ist größer als die Quadratwurzel aus dem Produkt aus der mittleren Wandstärke und dem Cosinus des Schrägstellungswinkels α.
• - Die Flankenwandstärke ist kleiner als das Produkt aus der mittleren Wandstärke und dem Cosinus des halben Schrägstellungswinkels α.

According to various possible embodiments, the flanks of the polymer graphite plate having a medium wall thickness are inclined by an inclination angle α of at least 30° and a maximum of 84° relative to the two mutually parallel planes in which the flat areas lie, the flank wall thickness being inclined downwards and upwards as follows is limited:

• - The flank wall thickness of the polymer graphite plate is greater than the square root of the product of the average wall thickness and the cosine of the inclination angle α.
• - The flank wall thickness is smaller than the product of the average wall thickness and the cosine of half the inclination angle α.

The average wall thickness is understood to mean the original wall thickness of the not yet deformed polymer graphite plate. The limits mentioned can be valid both in embodiments in which the wall thickness of a flank is uniform and in variants with non-uniform flank wall thickness.

Regardless of the exact geometric design of the polymer graphite plates, in many cases it is possible to assemble two similar or different polymer graphite plates to form a bipolar plate. Any flow-conducting structures, for example in the form of straight or corrugated channels in the form of an alternating droplet geometry, can be formed on the two outer surfaces of the bipolar plate, particularly in the area of an active field. In this context we take the example of: DE 10 2021 105 373 A1 pointed out. Cavities between the interconnected polymer graphite plates can be used in particular to pass a cooling medium through.

• 1 eine aus zwei Polymergraphitplatten zusammengesetzte Bipolarplatte für ein elektrochemisches System,
• 2 und 3 weitere Gestaltungsmöglichkeiten von Polymergraphitplatten für elektrochemische Systeme,
• 4 in schematisierter Ansicht eine Anlage zur Fertigung der Bipolarplatte nach 1.

Several exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Herein show:

• 1 a bipolar plate composed of two polymer graphite plates for an electrochemical system,
• 2 and 3 further design options for polymer graphite plates for electrochemical systems,
• 4 A schematic view of a system for manufacturing the bipolar plate 1 .

Unless otherwise stated, the following explanations refer to all exemplary embodiments. Parts that correspond to one another or have the same effect in principle are marked with the same reference numerals in all figures.

A bipolar plate, marked overall with the reference number 1, which can be constructed in one or more layers, is intended for use in a stack of electrochemical cells, in particular fuel cells, not shown. The bipolar plate 1 is formed by a single polymer graphite plate 3 or by an arrangement of two polymer graphite plates 3, 4 lying one on top of the other and permanently connected to one another. In both cases, the polymer graphite plates 3, 4 have a graphite content of at least 80% by weight and a polyolefinic binder. The starting product for producing the polymer graphite plates 3, 4 is a film 19, i.e. polymer graphite film, with a wall thickness dM. In the finished polymer graphite plate 3, 4, dM represents the average wall thickness.

Each polymer graphite plate 3, 4 touches a first plane E1 and a plane E2 spaced parallel therefrom. The distance between the planes E1, E2 represents the thickness DH of the polymer graphite plate 3, 4. If two polymer graphite plates 3, 4 are assembled to form a bipolar plate 1, each polymer graphite plate 3, 4 is also referred to as a half plate.

Areas of the polymer graphite plate 3 which rest on the first level E1 are referred to as first flat areas 8. In a corresponding manner, areas of the polymer graphite plate 3, 4 which touch the second plane E2 are referred to as second flat areas 9. If two polymer graphite plates 3, 4 are assembled to form a single bipolar plate 1, as in 1 sketched, the first plane E1 of the first half plate 3 coincides with the first plane E1 of the second half plate 4 and forms a central plane ME of the bipolar plate 1. The total thickness of the bipolar plate 1, designated DG, corresponds to the sum of the thickness DH of the first half plate 3 and the thickness DH of the second half plate 4, the half plates 3, 4 not necessarily having the same thickness.

Each polymer graphite plate 3, 4 has an embossed structure 5 with elevations 6. At least some of the elevations 6 lie on the second level E2. In the case of assembled half-plates 3, 4, at least one cavity 7 is formed by the embossed structures 5, which can be used in particular for the passage of a coolant.

The different flat areas 8, 9 of one and the same polymer graphite plate 3, 4 lying on the first level E1 or on the second level E2 are connected to one another by flanks 11. The inclination angle of a flank 11 relative to the planes E1, E2 is designated α. Between the flank 11 and the flat areas 8, 9 there are rounded areas 12, 13 of the polymer graphite plate 3, 4, as from each of the 1 until 3 emerges.

In comparison to deep-drawn films, the flanks 11 in the exemplary embodiments are not or only slightly weakened or even strengthened in their wall thickness relative to the flat areas 8, 9. This becomes particularly clear when considering a projected wall thickness dP in the area of the flanks 11, as in each of the 1 until 3 drawn. If the flat areas 8, 9 were to be moved against one another exclusively by deep drawing, the projected wall thickness in every area of the originally flat plate, including in the flank area, would correspond to the initial wall thickness. The actual wall thickness in the flank area would then correspond at least approximately to the product of the original wall thickness with the cosine of the flank angle.

In comparison with such a hypothetical product formed purely by deep drawing, the flank wall thickness designated d11 is relatively large in the exemplary embodiments. This is made possible by the material of at least one flat area 8, 9 being displaced into the flank 11 during the manufacturing process.

To explain the manufacturing process, see the diagram below 4 referred. A production system 10 is sketched here, which is the bipolar plate 1 as the starting product 1 delivers. Likewise, the manufacturing plant 10 could 4 be set up to deliver a bipolar plate 1, which consists of two polymer graphite plates 3 2 , made from two polymer graphite plates 3 after 3 , or from a half plate 3 after 2 and a half plate 3 after 3 is composed.

In the in 4 In the case outlined, the first half plate 3 is formed in a continuous process and the second half plate 4 is formed in a discontinuous process. Deviating from this, manufacturing processes can also be implemented in which both half-plates 3, 4 are formed in a continuous process or both half-plates 3, 4 are formed in a discontinuous process.

In the process according to 4 It is assumed that the foils 19 are initially present as a coil 18. To shape the half-plates 3, 4, various embossing and extrusion devices 20 are used, the shaping in each case being carried out by mechanical force and the simultaneous effect of temperature. In the case of the first half plate 3, the embossed structure is produced by rollers 21, 22, with the help of heating elements 23 setting a temperature level in the range between 150° Celsius and 200° Celsius. After the continuous embossing and extrusion process the half plates 3 are separated by a cutting device 24.

As part of the production of the half plates 4, in 4 below, the cutting device 24 is located between the coil 18 and the embossing and extrusion device 20, which works discontinuously here. The embossing and extrusion device 20 in this case comprises an upper tool part 25 and a lower tool part 26, whereby here too the embossing and extrusion process is carried out under the influence of temperature he follows. The result of this process is the fully formed half plate 4, which is prepared for joining with the first half plate 3. In both in 4 With the embossing and extrusion processes illustrated, a precise geometry of the end products, that is to say the polymer graphite plates 3, 4, is achieved despite the comparatively widely tolerated geometric properties of the films 19 used as starting products.

The half plate 3 after 2 shows, unlike the half plates 3, 4 1 , inconsistent wall thicknesses of the flat areas 8, 9. Imprints 15, i.e. sections of reduced wall thickness, are clearly visible in the flat areas 8, 9. The wall thickness in the area of the imprint 15 is designated d15 and is significantly smaller than the average wall thickness dM. During the embossing and extrusion process, which is effected by tools 21, 22, 25, 26, material is displaced from the flat area 8, 9 into the flank 11, so that an accumulation of material 14 forms there. At the same time, material is displaced within the flat areas 8, 9 during the combined embossing and flowing. This leads to the formation of local thickenings 16, 17, i.e. wall thickness maxima, in the flat areas 8, 9.

A particularly strong manifestation of the material accumulation 14 is in the case 3 given. In this case, the wall thickness d11 in the area of the flank 11 is significantly larger than the wall thickness d8 in the first flat area 8 and the wall thickness d9 in the second flat area 9. The wall thickness d15 in the area of the impression 15 corresponds to the wall thickness d9 in this case.

Reference symbol list

1

Bipolar plate

2

Embossed structure

3

Half plate, polymer graphite plate

4

Half plate, polymer graphite plate

5

Embossed structure

6

survey

7

cavity

8th

first flat area

9

second flat area

10

Manufacturing facility

11

Flank

12

rounded area

13

rounded area

14

Material accumulation in the flank

15

Imprint, section of reduced wall thickness

16

local thickening in the elevation

17

local thickening in the first flat area

18

Coil

19

foil

20

Embossing and extrusion device

21

roller

22

roller

23

Heating element

24

cutting device

25

tool part

26

tool part

α

angle

d8

Wall thickness in the first flat area

d9

Wall thickness in the second flat area

d11

flank wall thickness

d15

Wall thickness in the area of the impression

dm

medium wall thickness

dp

projected wall thickness

DG

Total thickness

DH

Thickness of a half plate

E1

first floor

E2

second level

M.E

Middle level

QUOTES INCLUDED IN THE DESCRIPTION

This list of 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.

Cited patent literature

• DE 10216306 B4 [0002]
• DE 112005001954 B4 [0003]
• DE 102020204303 A1 [0004]
• DE 102020216095 A1 [0005]
• DE 102021105373 A1 [0018]

Claims (10)

Polymer-graphitic bipolar plate (1) for a stack of electrochemical cells, comprising at least one three-dimensionally structured polymer-graphite plate (3, 4) with an average wall thickness (dM), which has first flat areas (8) which lie in a first plane (E1), and second flat areas (9), which touch a second level (E2) which is spaced parallel from the first level (E1) by a multiple of the average wall thickness (dM), the different flat areas (8, 9) being connected to one another by flanks (11). , and wherein at least one of the three areas mentioned (8, 9, 11), i.e. flat areas (8, 9) and flanks (11), is an accumulation of material (14), compared to an undeformed section of the polymer graphite plate (3, 4), based on a projection onto the first plane (E1), and at the same time an impression (15), that is to say a section of reduced wall thickness (d15), is formed in at least one of the two flat areas (8, 9). Bipolar plate (1). Claim 1 , characterized in that the polymer graphite plate (3, 4) comprises a polyolefinic binder. Bipolar plate (1). Claim 2 , characterized in that the graphite content of the polymer graphite plate (3, 4) is at least 80% by weight. Bipolar plate (1) according to one of the Claims 1 until 3 , characterized in that the wall thickness of the polymer graphite plate (3, 4) is non-uniform in at least one flat area (8, 9). Bipolar plate (1). Claim 4 , characterized in that there is at least one flat area (8, 9) with a maximum and several minima of the wall thickness. Bipolar plate (1) according to one of the Claims 1 until 5 , characterized in that the flanks (11) of the polymer graphite plate (3, 4) are inclined by an angle α of at least 30° and a maximum of 84° relative to the two mentioned, mutually parallel planes (E1, E2), the following relationship between the average wall thickness (dM) of the polymer graphite plate (3, 4) and the flank wall thickness (d11) is given: $$(\text{SQRT}(\text{cos}\left(\mathrm{\alpha }\right)))\times \text{dm}<\text{d}11<(\text{cos}\left(0.5\mathrm{\alpha }\right))\times \text{dm}$$

Bipolar plate (1) according to one of the Claims 1 until 6 , characterized in that it is composed of two polymer graphite plates (3, 4) tangent to one another in a central plane (ME). Method for producing a bipolar plate (1), with the following steps: - Provision of a polymer graphite film (19), - Inserting the polymer graphite film (19) into a tool (21, 22, 25, 26) and producing an embossed structure (2, 5) in a combined embossing and extrusion process under the influence of temperature in such a way that the material of the polymer graphite film (19) remains unchanged within one Angular position of the remaining section (8, 9) of the polymer graphite film (19) is shifted and used to build up the wall thickness in another section, namely a flank (11), of the polymer graphite film (19). Procedure according to Claim 8 , characterized in that the simultaneous embossing and extrusion takes place at a temperature of at least 120 °C and a maximum of 200 °C. Procedure according to Claim 8 or 9 , characterized in that during the simultaneous embossing and extrusion, material of the polymer graphite film (19) is used from a flat area (8, 9), that is to say a section remaining in an unchanged angular position, partly to build up a thickening (16) in the same area and partly in an edge (11) is displaced.

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