Bipolar plate production method, bipolar plate and electrochemical cell

Abstract

The invention relates to a bipolar plate production method for producing a bipolar plate (4), in particular for an electrochemical cell, which method comprises the following steps: -providing two foil sections (2, 3) made of a polymer graphite material comprising at least one polymer and at least 75 wt.% of an electrically conductive filler comprising predominantly graphite and also carbon black, -inserting the two foil sections (2, 3) into an embossing and moulding tool (6), -closing the tool (6), wherein the foil sections (2, 3) are embossed and tightly connected to one another at their edges, -forming a hollow structure (40) between the foil sections (2, 3) by means of gas pressure differences at the foil surfaces, wherein the foil sections (2, 3) rest against surface structures (11, 12) of tool surfaces, which face one another, of the embossing and moulding tool (6), -removing the bipolar plate (4), formed from the foil sections (2, 3), from the embossing and moulding tool (6) after the foil sections (6) have solidified.

Description

[0001] Bipolar plate manufacturing process, bipolar plate and electrochemical cell

[0002] The invention relates to a bipolar plate manufacturing method for producing a bipolar plate, in particular for an electrochemical cell, in particular a fuel cell. Furthermore, the invention relates to a bipolar plate and an electrochemical cell.

[0003] DE 10 2008 028 549 A1 discloses a method for manufacturing a fuel cell bipolar plate using thermoformed plates. The individual plates are made from a resin mixture comprising an electrically conductive thermoplastic polymer composition and a solvent. After the individual plates are formed and cut to size, they are assembled into a bipolar plate.

[0004] Another method for producing an electrically conductive bipolar plastic plate intended for use as an electrode in a fuel cell battery is described in EP 1 506 585 B1. In this case, the production of a structure with electrically conductive, carbonized, or graphitized reinforcing fibers is proposed, wherein mechanical orientation of the reinforcing fibers by needling in a first direction corresponding to the preferred electrical conduction path is intended to lead to higher conductivity in said first direction. EP 1 506 585 B1 proposes graphitized PAN fibers and graphitized pitch fibers as reinforcing fibers. In the finished product, the fibers are in the form of a matrix, which may additionally contain filler fibers.Possible processes for obtaining the matrix are mentioned in EP 1 506 585 B1 as thermoforming, membrane forming, pressure die casting, resin transfer forming, forming under pressure and vacuum, lamination and compression molding.

[0005] DE 10 2011 116 993 A1 relates to a device for producing a metallic foil component intended for use as a fuel cell component. At the beginning of the production process, two foils are arranged one above the other and connected to each other in a fluid-tight manner, at least in some areas. By introducing a pressurized fluid into a space formed between the foils, with the foils located in a forming tool, the shape of the foils is intended to adapt to the surface structures of the forming tools. DE 10 2011 116 993 A1 stipulates that the tool mold parts of the forming tool are moved toward each other during the forming process. During this process, fluid is drained from the cavity formed between the foils in a controlled manner.

[0006] Another method for producing a metallic bipolar plate for a fuel cell stack is described, for example, in DE 10 2010 020 178 A1. In particular, DE 10 2010 020 178 A1 deals with the production of gas distribution structures, with shear cutting being recommended as the manufacturing technology.

[0007] DE 10 2009 044 112 A1 describes a method for producing a microstructured composite component. For this purpose, a first and a second film made of thermoplastic polymer material are arranged between molded components, which have microstructured cavities to be filled by the film material and are heated in the contact area with the films. Overpressure is generated between the two films, forcing the films into the cavities. Finally, the films are pressed together and cooled. After demolding, the microstructured composite component is formed with microstructures that provide channels for fluids.

[0008] US 6 217 699 B1 discloses a device and a method for joining pre-thermoformed plastic films by welding.

[0009] DE 12 50 627 A describes a method for producing a double-walled hollow body from thermoplastic films. For this purpose, two heated, plasticized plastic films are placed into die-like molds, where they are at least partially welded together, and formed by a pressure difference within the weld edges.

[0010] US 3 982 877 A discloses a laminated, rib-reinforced hollow body and a method and device for producing such a body. At least two films are used, at least one of which consists of heated thermoplastic material and at least one further film has grooves or projections on the surface which form fluid channels. The films are heated and arranged in opposing compression molds, one of the compression molds forming rib-shaped cavities. The thermoplastic film is arranged in contact with the rib-shaped cavities. After the compression molds have been closed, a fluid is fed between the films and the thermoplastic film is formed into the cavities and the films are bonded to one another. The thermoplastic material for forming a film can contain a filler in an amount of 1 to 70 wt.-%, with possible fillers being asbestos, carbon, glass fibers, calcium phosphate, calcium carbonate, kaolinitic clay, silicon dioxide, titanium dioxide, bentonite, talc and mica.

[0011] The invention is based on the object of further developing the production of bipolar plates for electrochemical cells compared to the cited prior art, striving for a particularly favorable balance between equipment complexity, geometric precision of the product to be manufactured, and process reliability. Furthermore, a bipolar plate and an electrochemical cell are to be provided.

[0012] This object is achieved according to the invention by a bipolar plate manufacturing method according to claim 1, a bipolar plate manufactured thereafter according to claim 9, and an electrochemical cell according to claim 10. The embodiments and advantages of the invention explained below in connection with the bipolar plate manufacturing method also apply mutatis mutandis to the bipolar plate manufactured therewith, and vice versa. The bipolar plate manufacturing method generally comprises the following steps:

[0013] - Providing two foil sections made of a polymer-graphitic material comprising at least one polymer and at least 75% by weight of an electrically conductive filler comprising predominantly graphite and furthermore carbon black,

[0014] - Inserting the two foil sections into an embossing and hollow forming tool,

[0015] - Closing the tool, whereby the film sections are embossed and tightly joined at their edges,

[0016] - forming a hollow structure between the film sections by means of gas pressure differences, in particular pressure differences of air, on the film surfaces, whereby the films adhere to the mutually facing tool surfaces,

[0017] - Removal of the bipolar plate formed from the film sections after the film sections have solidified.

[0018] It is thus a combined process that combines mechanical forming in the form of embossing with shaping using pressure differences in a gaseous medium. The first change in the geometric parameters of the foil sections occurs during the embossing process, immediately after the foil sections are inserted into the embossing and hollow forming tool, also referred to as the tool for short. Changes in geometric parameters include, among other things, changes in wall thickness and the formation of three-dimensional embossed structures.

[0019] The majority of the deformation of the film sections is achieved by the subsequent application of gas pressure, which can be negative pressure and / or positive pressure. The initial embossing of the film sections ensures the gas-tightness between the film sections. In this case, the term "film sections" refers to any flat polymer-graphite starting products. This also applies to cases where the starting products are in the form of sheets or plates. Typical wall thicknesses or film thicknesses range from 0.1 mm to 0.5 mm, particularly from 100 to 300 μm.

[0020] In any case, both foil sections are inserted into the tool together. Individual forming of foil sections is not intended. Depending on the composition and thickness of the foil sections, preheating of the foil sections before insertion into the tool may be considered. Likewise, the degree of preheating of the tool parts of the embossing and hollow forming tool depends, in particular, on the material properties of the foil sections.

[0021] A “polymer-graphitic material” is understood here to mean a material which contains a proportion of polymer and a total proportion of at least 75% by weight of electrically conductive fillers in the form of predominantly graphite and also carbon black.

[0022] In principle, the polymer can be selected from a thermoplastic or thermosetting material, although a uniform material structure is not necessarily required. Fiber reinforcement of the films is particularly suitable. In the case of thermoplastic materials, the process generally referred to as solidification occurs as solidification. In the case of thermosetting materials, solidification is a process of curing. Polypropylene (PP) or polyphenylene sulfide (PPS) have proven particularly suitable as thermoplastic materials. Polyester resins or epoxy resins have proven particularly suitable as thermosetting materials.

[0023] The maximum filler content or minimum polymer content in the film is reached when film formation is no longer possible and the polymer content is no longer sufficient to bind the filler particles into a film. This can be easily determined experimentally.

[0024] According to one possible process variant, the integral bond between the film sections at their edges is created directly by closing the embossing and hollow forming tool. Alternatively, the film sections can be permanently bonded, particularly by a material bond, to one another at a later stage of the manufacturing process, in any case still within the embossing and hollow forming tool. For this purpose, heating devices can be provided for the tool parts, which heat the tool parts and thus also the film sections in defined areas, usually in the edge areas of the film sections, beyond the otherwise given level.

[0025] A suitable manufacturing system with which the bipolar plate intended for use in a stack of electrochemical cells is produced generally comprises a two-part embossing and hollow forming tool which is designed both for embossing a two-layer foil arrangement and has fluid connections, in particular vacuum and / or compressed air connections, for forming at least one cavity between the foils by gas pressure.

[0026] The foil from which the foil sections are cut contains at least 75 wt.% electrically conductive fillers, predominantly graphite, especially ground graphite, and also carbon black, to provide sufficient electrical conductivity of at least 20 S / cm, especially at least 100 S / cm, for the intended use in a stack of electrochemical cells. This minimum achievable electrical conductivity value is required for a maximum foil thickness of 0.5 mm at a room temperature of 20 to 24°C for the application of the foil in a bipolar plate.

[0027] A so-called “through-plane measurement method” (TPV) is used to first determine the area-specific electrical volume resistance of the film, whereby a contact pressure of the measuring electrodes (= gold-plated contact pins with 39° tip) of 40 N / cm 2applied to the film. Surface-specific electrical resistances of < 10 mΩ*cm 2 at a room temperature in the range of 20 to 24°C. The determined electrical resistance value is then converted into electrical conductivity.

[0028] The filler used in the film with a total proportion of at least 75 wt.% preferably comprises a proportion of 5 to 10 wt.% carbon black and 65 to 70 wt.% ground graphite (calculated based on the composition of the film).

[0029] The filler particles of the electrically conductive filler preferably have a grain distribution with a dgo value of at most 200 pm, preferably 75 pm.

[0030] To process the foil in the embossing and hollow forming tool, the foil sections are advantageously heated to a temperature above the heat distortion temperature and below the melting temperature. This not only achieves good formability but also virtually completely prevents any separation of components, particularly filler and plastic, in the foil. Metallic components of the foil are not provided for in typical designs, but are not categorically excluded. Metal is by no means the main component of the foil. Thus, a proportion of up to 20 wt.% metal particles, for example, made from at least one of the metals from the group consisting of titanium, titanium alloys, aluminum, aluminum alloys, vanadium, or vanadium alloys, such as Ti6Al4V, can be admixed to the foil.

[0031] Depending on the materials used and the geometric structures to be created, it may be sufficient to use negative pressure to apply the film sections to the three-dimensionally structured surfaces of the tool parts, which correspond to the intended shape of the end product. According to a further developed process variant, compressed air is also introduced between the film sections, i.e. into the cavity to be formed. To support the heating of the film sections in the tool, this can be tempered, i.e. compressed air brought to an elevated temperature level. To support the subsequent solidification of the bipolar plate formed from the film sections once it has assumed its final shape, cooling air can be introduced between the film sections at an appropriate time instead of the heated compressed air.The foil sections are thus initially exposed to hot air in the mold and, at a later stage of the process, to cooled air. In all cases, the same connections that are initially used to apply a vacuum can be used to eject the finished bipolar plate formed from the foil sections from the mold.

[0032] A significant advantage of the bipolar plate manufacturing process according to the application compared to processes that involve the individual forming of plate-shaped elements is that the simultaneous processing of both foil sections in the embossing and hollow forming tool eliminates any alignment of the foil sections after their formation. Optionally, a leak test follows the production of the bipolar plate from the foil sections. The same openings formed at specific locations between the foil sections that were already used to introduce compressed air during the formation of the bipolar plate can be used as connections for the leak test.Furthermore, the same openings can be used to pass coolant, in particular cooling water, through the bipolar plates within the subsequent stack of electrochemical cells, in particular fuel cell stack, which comprises a plurality of bipolar plates of the type described.

[0033] A bipolar plate produced according to the method according to the invention has at least one hollow structure, in particular a channel-shaped one, for fluid passage through the bipolar plate.

[0034] An electrochemical cell, in particular a fuel cell, electrolysis cell, or redox flow cell, comprises at least one such bipolar plate according to the invention. The bipolar plate formed by the process according to the invention is therefore suitable for use in electrochemical cells, in particular fuel cells with a polymer electrolyte membrane, electrolysis cells for the electrolysis of water with a polymer electrolyte membrane, or redox flow cells with a polymer ion exchange membrane.

[0035] Several exemplary embodiments of the invention are explained below with reference to the drawings. These show, partly schematically:

[0036] Fig. 1 shows a production plant to illustrate a bipolar plate manufacturing process,

[0037] Fig. 2 shows the arrangement according to Figure 1 in partly more detailed representation,

[0038] Fig. 3 shows a modified system for illustrating a bipolar plate manufacturing process in a representation analogous to Figure 2,

[0039] Fig. 4 a bipolar plate in three-dimensional view, and

[0040] Fig. 5 is a schematic representation of an electrochemical cell and a Ze II stack.

[0041] Unless otherwise stated, the following explanations refer to both embodiments. Corresponding or essentially equivalent parts are identified by the same reference numerals in all figures.

[0042] A production plant 1 uses foil sections 2, 3 made of an electrically conductive polymer-graphitic material to produce bipolar plates 4 for electrochemical cells, in particular PEM fuel cells. Within a finished stack of electrochemical cells, each bipolar plate 4 separates a half-cell of a first electrochemical cell from a half-cell of another, similarly constructed electrochemical cell. Regarding the basic structure and function of stacked electrochemical cells, in particular fuel cells, reference is made to the prior art cited above.

[0043] The film sections 2, 3 are conveyed in a manner not shown in detail and heated to the temperature required for further processing in a preheating device 5. Arranged one above the other, the film sections 2, 3 are inserted into an embossing and hollow forming tool 6, shown here and below in section, which comprises a lower tool part 7 and an upper tool part 8.

[0044] Subsequently, the embossing and hollow forming tool 6 is closed, which constitutes an embossing process in which the two film sections 2, 3 are firmly bonded to one another at the contact points, and a partial geometry transfer from the tool parts 7, 8 to the film sections 2, 3, which are in the process of being further processed into the bipolar plate 4, already takes place. In this case, the film sections 2, 3 contact, in particular, sealing areas 9, 10 of the tool parts 7, 8.

[0045] In the further manufacturing process, the shape of the film sections 2, 3 is adapted to the shape of surface structures 11, 12 of the tool parts 7, 8 by the action of negative pressure and / or positive pressure, as explained in more detail below. With reference to the symbolic representation in Figure 1, this forming step takes place in the top row of the figure illustrating the manufacturing process. Subsequently, in the bottom left row in Figure 1, the finished bipolar plate 4 formed from the film sections 2, 3 is cooled within the still closed embossing and hollow forming tool 6. The last step is the release and demolding of the bipolar plate 4. The demolded bipolar plate 4 is shown in cross-section in Figures 1, 2 and 3, so that the hollow structure 40, here channel-shaped, formed between the connected film sections 3, 2 can be seen. This hollow structure 40 serves to conduct fluid through the bipolar plate 4.Typically, a coolant, such as cooling water, is passed through the hollow structure 40. Channels 13, 14 are formed in the tool parts 7, 8, which can be used for heating or cooling as needed. Alternatively, separate heating channels and cooling channels can be provided. The integration of electrical heating elements into the tool parts 7, 8 is also possible. Such heating elements can, in particular, effect or support the material-to-material bonding of the film sections 2, 3. Furthermore, compressed air channels 15, 16 are formed in the tool parts 7, 8, each extending from a collecting line 17, 18 to the tool surface, which has the surface structure 11, 12. The term "compressed air channel" is used in the present case regardless of the absolute pressure of the gas in the respective channel. In particular, the absolute pressure can be lower than the ambient air pressure.

[0046] As can be seen from Figures 2 and 3, a vacuum pump 19 is connected to each collecting line 17, 18. With the help of the vacuum pumps 19, a negative pressure is generated which sucks the film sections 2, 3 onto the surface structures 11, 12 of the tool parts 7, 8. In the variant according to Figure 3, an internal pressure pi acting between the film sections 2, 3 is additionally generated with the help of a compressor 20, which can be monitored by means of a manometer 21. In both the variant according to Figures 1 and 2 and in the variant according to Figure 3, the demolding of the bipolar plate 4 is assisted by compressed air being applied to the outer surfaces of the bipolar plate 4 via the compressed air channels 15, 16.

[0047] Compressed air, which is to be introduced between the film sections 2, 3, is temperature-controlled depending on the process stage. Heated compressed air can be used to quickly heat not only the film sections 2, 3, but also the insides of the tool parts 7, 8. The same applies to the demolding of the bipolar plate 4. When using thermoplastic materials, the temperature levels of the compressed air are adapted to the forming temperature or the demolding temperature of the material. In the case of thermosetting materials, the viscosity for forming is initially reduced by setting suitable temperature levels. The further temperature control depends on the activation temperature of the hardener contained in the material of the film sections 2, 3. By accelerating the temperature-dependent crosslinking reactions, the solidification of the bipolar plate 4 after forming is supported and thus the cycle time is reduced.

[0048] After removing any excess material, the bipolar plate 4 can be used for assembly in an electrochemical cell or a cell stack formed therewith, regardless of the materials used, without further processing.

[0049] Figure 4 shows a three-dimensional view of a bipolar plate 4. On each of its sides facing away from the hollow structure 40 (not visible here, see Figures 1 to 3), the plate has an active field 41, in the region of which electrochemical reactions take place in an electrochemical cell 70 (see Figure 5). The rectangular bipolar plate 4 has three fluid passages on each of its short sides. The middle fluid passages serve as coolant supply openings 50 and coolant discharge openings 51. These are fluidically connected to the hollow structure 40 within the bipolar plate 4 and enable the supply of coolant to the hollow structure 40, which flows through the hollow structure 40, and the discharge of the coolant after leaving the hollow structure 40.

[0050] Fig. 5 shows a schematic representation of an electrochemical cell 70 in a cell stack 100 comprising several such electrochemical cells 70. The electrochemical cell 70 comprises two bipolar plates 4 and a polymer electrolyte membrane 60 arranged therebetween, wherein adjacent electrochemical cells 70 share one bipolar plate 4. List of reference symbols

[0051] 1 production facility

[0052] 2 slide section

[0053] 3 slide section

[0054] 4 bipolar plate

[0055] 5 Preheating device

[0056] 6 Embossing and hollow forming tool

[0057] 7 lower tool part

[0058] 8 upper tool part

[0059] 9 Sealing area of ​​the upper tool part

[0060] 10 Sealing area of ​​the lower tool part

[0061] 11 Surface structure of the upper tool part

[0062] 12 Surface structure of the lower tool part

[0063] 13 Channel for tempering fluid in the upper part of the tool

[0064] 14 Channel for tempering fluid in the lower part of the tool

[0065] 15 Compressed air channel in the upper part of the tool

[0066] 16 Compressed air channel in the lower part of the tool

[0067] 17 Collecting line in the upper part of the tool

[0068] 18 Collecting line in the lower part of the tool

[0069] 19 Vacuum pump

[0070] 20 compressor

[0071] 21 pressure gauges

[0072] 40 Hollow structure

[0073] 41 Active field

[0074] 50 Coolant supply opening

[0075] 51 Cooling I middle discharge opening

[0076] 60 polymer electrolyte membranes

[0077] 70 electrochemical cell

[0078] 100 cell stacks

[0079] Pi internal pressure

Claims

Patent claims 1. Bipolar plate manufacturing method for producing a bipolar plate (4), in particular for an electrochemical cell, comprising the following steps: - providing two film sections (2, 3) made of a polymer-graphitic material comprising at least one polymer and at least 75% by weight of an electrically conductive filler comprising predominantly graphite and furthermore carbon black, - Inserting the two foil sections (2, 3) into an embossing and hollow forming tool (6), - closing the tool (6), whereby the film sections (2, 3) are embossed and tightly connected to one another at their edges, - forming a hollow structure (40) between the film sections (2, 3) by means of gas pressure differences on the film surfaces, wherein the film sections (2, 3) are applied to surface structures (11, 12) of mutually facing tool surfaces of the embossing and hollow forming tool (6), - Removing the bipolar plate (4) formed from the film sections (2, 3) from the embossing and hollow forming tool (6) after the film sections (6) have solidified.

2. Method according to claim 1, characterized in that film sections (2, 3) with a film thickness of maximum 0.5 mm and an electrical conductivity of at least 20 S / cm (at 20 to 24°C) are used.

3. Method according to claim 1 or 2, characterized in that the film sections (2, 3) are heated before being inserted into the tool (6).

4. Method according to one of claims 1 to 3, characterized in that the film sections (2, 3) are pressed against the surface structures (11, 12) of the tool surfaces are sucked in.

5. Method according to claim 4, characterized in that the suction of the film sections (2, 3) is supported by tempered compressed air introduced between the film sections (2, 3).

6. Method according to claim 4 or 5, characterized in that the solidification of the bipolar plate (4) is at least partly effected by cooling air which is introduced between the film sections (2, 3).

7. Method according to one of claims 1 to 6, characterized in that the film sections (2, 3) are materially joined to one another immediately upon closing of the embossing and hollow forming tool (6).

8. Method according to one of claims 1 to 6, characterized in that the film sections (2, 3) are only joined together in a material-to-material manner after their final shaping by means of gas pressure differences acting in the tool (6) by partial heating of the tool (6).

9. Bipolar plate (4), produced by a method according to one of claims 1 to 8, comprising at least one, in particular channel-shaped, hollow structure (40) for fluid passage through the bipolar plate (4).

10. Electrochemical cell (70), in particular fuel cell, electrolysis cell or redox flow cell, comprising at least one bipolar plate (4) according to claim 9.

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