Bipolar plate manufacturing method, bipolar plate, and electrochemical cell

Abstract

The present invention relates to a bipolar plate manufacturing method, in particular for manufacturing a bipolar plate (4) for an electrochemical cell, comprising the steps of: - providing two foil sections (2, 3) made of a polymer-graphite material comprising at least one polymer and at least 75 wt. % of a conductive filler, mainly graphite and also carbon black, - inserting the two foil sections (2, 3) into an embossing tool (6), - closing the tool (6) in which the foil sections (2, 3) are embossed and tightly connected to each other at their edges, - forming hollow structures (40) between the foil sections (2, 3) by a gas pressure difference at the foil surfaces, the foil sections (2, 3) abutting the surface structures (11, 12) of the facing tool surfaces of the embossing tool (6), - removing the bipolar plate (4) formed from the foil sections (2, 3) from the embossing tool (6) after the foil section (6) has solidified, The present invention relates to a method for manufacturing a bipolar plate, including:

Description

[Technical Field]

[0001] The present invention relates to a bipolar plate manufacturing method, in particular for manufacturing bipolar plates for electrochemical cells, in particular fuel cells.The present invention also relates to bipolar plates and electrochemical cells. [Background technology]

[0002] German Patent Application No. 102008028549 discloses a method for manufacturing fuel cell bipolar plates with thermoplastic plates. The individual plates are made from a resin mixture containing an electrically conductive thermoplastic polymer composition and a solvent. After forming and cutting the individual panels, they are assembled to form the bipolar plates.

[0003] Another method for producing conductive plastic bipolar plates intended for use as fuel cell electrodes is described in EP 1506585. In this case, the production of a structure with conductive, carbonized, or graphitized reinforcing fibers is proposed, and mechanical orientation of the reinforcing fibers by needling in a first direction corresponding to a preferred electrical conduction path is intended to result in higher electrical conductivity in said first direction. EP 1506585 proposes graphitized PAN fibers and graphitized pitch fibers as reinforcing fibers. In the final product, the fibers may be in the form of a matrix and may also contain filler fibers. Possible processes by which the matrix can be obtained are mentioned in EP 1506585, including thermoforming, film molding, pressure casting, resin transfer molding, molding under pressure and vacuum, lamination, and embossing presses.

[0004] German Patent Application Publication No. 102011116993 relates to an apparatus for manufacturing metal foil components intended to be usable as fuel cell components. At the start of the manufacturing process, two foils are placed one on top of the other and connected to each other in a liquid-tight manner in at least some areas. The foils are placed on a forming tool, and a fluid is introduced under pressure into the space formed between the foils, thereby adapting the shape of the foil to the surface structure of the forming tool. German Patent Application Publication No. 102011116993 specifies that the tool parts of the forming tool are moved toward each other during the forming process. During this process, the fluid is released in a controlled manner from the cavity formed between the foils.

[0005] Another method for producing metallic bipolar plates for fuel cell stacks is described, for example, in DE 10 2010 020178. In particular, DE 10 2010 020178 deals with the production of gas distribution structures and recommends shear cutting as a manufacturing technique.

[0006] German Patent Application No. 102009044112 describes a method for producing a microstructured composite component. For this purpose, first and second foils made of a thermoplastic polymer material are placed between a mold component that has microstructured hollow shapes that are filled with the foil material and is heated in the contact area with the foils. An overpressure is generated between the two foils, forcing the foils into the hollow mold. Finally, the foils are pressed together and cooled. After demolding, the microstructured composite component has microstructures that provide channels for the liquid.

[0007] US Pat. No. 6,217,699 discloses an apparatus and method for joining pre-thermoformed plastic foils by welding.

[0008] DE 1250627 A1 describes a method for producing double-walled hollow bodies from thermoplastic foils: for this purpose, two heated plastic foils in a plasticized state are introduced into a die-like mold, where they are at least partially welded together, forming an internal weld edge due to a pressure difference.

[0009] U.S. Patent No. 3,982,877 discloses a laminated, rib-reinforced hollow body, as well as a method and apparatus for manufacturing such a body. At least two foils are used, at least one of which is made of a heated thermoplastic material, and at least one other foil has grooves or protrusions on its surface that form fluid channels. The foils are heated and placed in opposing molds, one of which forms a ribbed cavity. A thermoplastic foil is placed in contact with the ribbed cavity. After the molds are closed, a fluid is supplied between the foils, molding the thermoplastic foil into the cavity and bonding the foils together. The thermoplastic material for forming the foils can contain a filler in an amount of 1 to 70 wt %. Possible fillers include asbestos, carbon, glass fiber, calcium phosphate, calcium carbonate, kaolinite clay, silicon dioxide, titanium dioxide, bentonite, talc, and mica. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention is based on the object of further developing the manufacture of bipolar plates for electrochemical cells compared to the aforementioned prior art, whereby a particularly favorable ratio between the cost of the equipment, the geometrical accuracy of the manufactured product, and the process reliability is sought. Furthermore, a bipolar plate and an electrochemical cell are provided. [Means for solving the problem]

[0011] This object is achieved according to the present invention by a method for manufacturing a bipolar plate as set forth in claim 1, a subsequently manufactured bipolar plate as set forth in claim 9, and an electrochemical cell as set forth in claim 10. The embodiments and advantages of the present invention described below in relation to the method for manufacturing a bipolar plate equally apply to the bipolar plate manufactured thereby, and vice versa.

[0012] The bipolar plate manufacturing method generally includes the following steps. - providing two foil sections made of a polymer-graphite material comprising at least one polymer and at least 75 wt. % of a conductive filler that also comprises mainly graphite and carbon black, - inserting the two foil sections into an embossing tool; - closing the tool, in which the foil sections are embossed and tightly connected to each other at their edges, - forming hollow structures between the foil sections by means of a gas pressure difference, in particular an air pressure difference, at the foil surface, whereby the foils abut against tool surfaces facing each other; - Removing the formed bipolar plate from the foil section after the foil section has solidified.

[0013] This is therefore a hybrid method that combines mechanical deformation of the embossed form with formation using pressure differences in a gaseous medium. The first change in the geometric parameters of the foil section occurs during embossing as soon as the foil section is inserted into the embossing tool, also known as the tool for short. The change in geometric parameters includes, among other things, a change in wall thickness and the formation of a three-dimensional embossed structure.

[0014] The change in shape of the foil sections is mostly achieved by the subsequent gas pressure, which can be negative and / or positive. The initial embossing of the foil sections already ensures the airtightness between the foil sections.

[0015] The term "foil section" is used in this case to refer to any flat polymer-graphite starting product. This also applies if the starting product is in the form of a sheet or plate. Typical wall thicknesses of foils or foil thicknesses are in the range of 0.1 mm to 0.5 mm, especially in the range of 100 μm to 300 μm.

[0016] In either case, both foil sections are inserted into the tool together. No individual foil section formation is provided. Depending on the composition and thickness of the foil sections, it may be considered to preheat the foil sections before inserting them into the tool. Likewise, the degree of preheating of the tool parts of the embossing tool depends in particular on the material properties of the foil sections.

[0017] "Polymer-graphite material" is understood here to mean a material comprising a proportion of polymer and an overall proportion of at least 75 wt. % of conductive fillers, mainly in the form of graphite and carbon black.

[0018] In principle, the polymer can be chosen from thermoplastic or thermosetting materials, but it does not necessarily have to be made of the same material. Particular consideration is given to the fiber reinforcement of the foil. In the case of thermoplastic materials, a process commonly called solidification occurs during setting. In the case of thermosetting materials, solidification is the process of hardening. 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.

[0019] When foil formation is no longer possible and the polymer content is no longer sufficient to bind the filler particles to the foil, the maximum filler or minimum polymer proportion in the foil is reached, which can be determined experimentally in a simple manner.

[0020] According to one possible method variant, the material bond between the foil sections at their edges is already formed directly by closing the embossing tool. Alternatively, the foil sections can be provided so that they are only permanently connected to one another, in particular substantially bonded, at a later stage of the manufacturing process, in any case inside the embossing and hollow-forming tool; for this purpose, a heating device can be provided for the tool part, which heats the tool part and thus also the foil sections above other given levels, usually in a defined area in the edge region of the foil section.

[0021] Suitable manufacturing plants in which bipolar plates intended for use in stacks of electrochemical cells are manufactured generally comprise a two-part embossing tool, both designed for embossing a two-layer foil arrangement and having fluid connections, in particular vacuum and / or compressed air connections, for forming at least one cavity between the foils by gas pressure.

[0022] The foil from which the foil sections are cut has a proportion of conductive filler of at least 75 wt. %, here mainly graphite, in particular crushed graphite, and carbon black, so as to provide a conductivity of at least 20 S / cm, in particular at least 100 S / cm, sufficient for the intended use in stacks of electrochemical cells. This minimum value to be achieved for the conductivity is required for a maximum foil thickness of 0.5 mm at room temperature of 20 to 24° C. for the application of the foil in bipolar plates.

[0023] The area specific electrical resistivity of the foil was first determined using the "Through-Plane Measurement" (TPV) method, and was 40 N / cm 2 A contact pressure of 10 mΩ*cm is applied to the foil by a measuring electrode (= a gold-plated contact pin with a 39° tip). At room temperature in the range of 20 to 24°C, the contact pressure is ≦10 mΩ*cm. 2 The specific surface electrical contact resistance is then converted to electrical conductivity.

[0024] The fillers are used in the foil in an overall proportion of at least 75 wt %, preferably comprising carbon black in a proportion of 5 to 10 wt % and crushed graphite in a proportion of 65 to 70 wt % (calculated based on the composition of the foil).

[0025] The filler particles of the conductive filler preferably have a diameter of 200 μm or less, preferably 75 μm. 90 It has a particle distribution with a value.

[0026] To process the foil in the embossing tool, the foil section is advantageously brought to a temperature above the heat distortion temperature and below the melting temperature. This not only ensures good formability but also substantially completely prevents separation of the foil's components, especially the filler and plastic. Metal components of the foil are not typically included in the design, but this is not entirely excluded. Metal is by no means the primary component of the foil. For example, up to 20 wt% of metal particles, such as at least one metal from the group consisting of titanium, titanium alloys, aluminum, aluminum alloys, vanadium, and vanadium alloys, e.g., Ti6Al4V, can be mixed into the foil.

[0027] Depending on the material used and the geometric structure to be created, it may be sufficient to use negative pressure to apply the foil sections to the three-dimensional structured surface of the tool part according to the intended shape of the final product. According to a further developed method variant, compressed air is also introduced between the foil sections, i.e., into the cavities being formed. To assist in heating the foil sections of the tool, this can be adjusted to compressed air, i.e., compressed air at an elevated temperature level. To aid in the subsequent solidification of the bipolar plate formed from the foil sections after they have taken their final shape, cooled air can be introduced between the foil sections at some point instead of heated compressed air. Thus, the foil sections are initially exposed to hot air in the tool and then exposed to cooled air at a later stage of the method. In all cases, the finished bipolar plate formed from the foil sections can be ejected from the tool using the same connection that was originally used to apply the negative pressure.

[0028] An important advantage of the bipolar plate manufacturing method according to the present application compared to processes providing for a single formation of plate-shaped elements is that the simultaneous processing of both foil sections in the embossing tool eliminates any need for alignment of the foil sections after their formation. Optionally, a leak test is performed after the bipolar plate has been manufactured from the foil sections. The same openings formed at specific points between the foil sections 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 a coolant, in particular cooling water, through the bipolar plates inside a subsequent stack of electrochemical cells, in particular a fuel cell stack containing multiple bipolar plates of the type described.

[0029] The bipolar plate manufactured according to the method according to the invention has at least one hollow structure, in particular in the form of a channel, for the passage of fluid through the bipolar plate.

[0030] An electrochemical cell, in particular a fuel cell, an electrolysis cell or a redox flow battery, comprises at least one such bipolar plate according to the invention.

[0031] The bipolar plates formed by the process according to the invention are 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 batteries with a polymer ion exchange membrane.

[0032] In the following, some exemplary embodiments of the invention are described by way of example with the aid of drawings which show, in a partly schematic and simplified manner: [Brief explanation of the drawings]

[0033] [Figure 1] 1 shows a manufacturing plant for illustrating a method for manufacturing bipolar plates. [Figure 2] 2 shows the arrangement according to FIG. 1 in more detail. [Figure 3] 2 shows a modified plant for illustrating the bipolar plate manufacturing method in the same representation as FIG. [Figure 4] 1 shows a bipolar plate in a three-dimensional view. [Figure 5] 1 shows a schematic representation of an electrochemical cell and cell stack. DETAILED DESCRIPTION OF THE INVENTION

[0034] Unless otherwise stated, the following description relates to both exemplary embodiments. In all figures, parts that correspond to each other or have essentially the same effect are provided with the same reference signs.

[0035] The manufacturing plant 1 uses foil sections 2, 3 made of conductive polymer-graphite material to manufacture bipolar plates 4 for electrochemical cells, in particular PEM fuel cells. Within a completed 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 first cited prior art.

[0036] The foil sections 2, 3 are conveyed in a manner not shown in detail and brought to the temperature required for further processing in a preheating device 5. In a superimposed arrangement, the foil sections 2, 3 are inserted into an embossing tool 6, shown in the following section, which comprises a lower tool part 7 and an upper tool part 8.

[0037] The embossing and hollow-forming tool 6 is then closed, which means that the two foil sections 2, 3 are substantially joined to each other at the contact points and the shape has already been partially transferred from the tool parts 7, 8 to the foil sections 2, 3, an embossing process which is further processed to form the bipolar plate 4. The foil sections 2, 3 contact, in particular, the sealing areas 9, 10 of the tool parts 7, 8.

[0038] In a further manufacturing method, the shape of the foil sections 2, 3 is adapted to the shape of the surface structures 11, 12 of the tool parts 7, 8 by the action of negative and / or positive pressure, as will be explained in more detail later. Referring to the symbolic representation in FIG. 1 , this forming step still takes place in the upper line of the manufacturing method diagram. Subsequently, in the lower left part of FIG. 1 , the completed bipolar plate 4 formed from the foil sections 2, 3 is cooled inside the still-closed embossing tool 6. The final step is the removal and demolding of the bipolar plate 4. The demolded bipolar plate 4 is shown in cross section in FIGS. 1, 2, and 3, where hollow structures 40 in the form of channels formed between the connected foil sections 3, 2 can be seen, which guide a fluid through the bipolar plate 4. Typically, a coolant, such as cooling water, passes through the hollow structures 40.

[0039] Channels 13, 14 are formed in each of the tool parts 7, 8, which can be used for heating or cooling as needed. Alternatively, separate heating and cooling channels can be provided. Integration of electric heating elements into the tool parts 7, 8 is also possible. Such heating elements can be used, in particular, to achieve or assist in bonding the materials of the foil sections 2, 3. Additionally, compressed air channels 15, 16 are formed in the tool parts 7, 8, each extending from the collection lines 17, 18 to the tool surface bearing the surface structures 11, 12. In this case, the term "compressed air channel" is used regardless of the absolute pressure of the gas in the channel in question. In particular, the absolute pressure can be lower than ambient air pressure.

[0040] As can be seen from Figures 2 and 3, a vacuum pump 19 is connected to each collection line 17, 18. Using the vacuum pump 19, a negative pressure is generated which sucks the foil sections 2, 3 onto the surface structures 11, 12 of the tool parts 7, 8. In the variant according to Figure 3, a compressor 20 is also used to create an internal pressure p i, which can be monitored by a pressure gauge 21. In both the variant according to Figures 1 and 2 and the variant according to Figure 3, the demoulding of the bipolar plate 4 is assisted by applying compressed air to the outer surface of the bipolar plate 4 via compressed air channels 15, 16.

[0041] The amount of compressed air introduced between the foil sections 2 and 3 is adjusted depending on the process stage. Heated compressed air allows for rapid heating of the foil sections 2 and 3, as well as the inside of the tool sections 7 and 8. The same applies to the demolding of the bipolar plate 4. When using thermoplastic materials, the temperature level of the compressed air is adapted to the forming or demolding temperature of the material. For thermosetting materials, the viscosity for forming is first reduced by setting the appropriate temperature level. Further temperature control depends on the activation temperature of the hardener contained in the material of the foil sections 2 and 3. By promoting a temperature-dependent crosslinking reaction, solidification of the bipolar plate 4 after forming is aided, thereby shortening the cycle time.

[0042] After removing any excess material, the bipolar plate 4, regardless of the material used, can be used for assembly in an electrochemical cell or cell stack formed therewith without further processing.

[0043] FIG. 4 shows a three-dimensional view of the bipolar plate 4. It has an active field 41 on each of its sides facing outward from the hollow structure 40 (see FIGS. 1 to 3), which is not visible here and in which areas electrochemical reactions do not occur in the electrochemical cells 70 (see FIG. 5). The rectangular bipolar plate 4 has three fluid passage openings on its short sides. The central fluid passage opening functions as a coolant supply opening 50 and a coolant discharge opening 51. These are fluidly connected to the hollow structure 40 inside the bipolar plate 4 and allow the supply of coolant to the hollow structure 40 through the hollow structure 40 and the discharge of the coolant after it has left the hollow structure 40.

[0044] 5 shows a schematic representation of an electrochemical cell 70 in a cell stack 100 containing a plurality of such electrochemical cells 70. The electrochemical cell 70 comprises two bipolar plates 4 with a polymer electrolyte membrane 60 disposed therebetween, with adjacent electrochemical cells 70 sharing a bipolar plate 4. [Explanation of symbols]

[0045] 1. Manufacturing Plant 2 Foil Sections 3 Foil Section 4 Bipolar Plates 5 Preheating device 6 embossing tools 7 Lower tool part 8 Upper tool section 9. Upper tool part sealing area 10 Lower tool part sealing area 11 Surface structure of upper tool part 12 Surface structure of the lower tool part 13 Channel for adjusting fluid in upper tool section 14 Channel for adjusting fluid in lower tool section 15 Compressed air channel in upper tool section 16 Lower tool section compressed air channel 17 Upper tool section collection line 18 Lower tool section collection line 19 Vacuum Pump 20 Compressor 21 Pressure gauge 40 Hollow structure 41 Active Place 50 Refrigerant supply opening 51 Refrigerant discharge opening 60 Polymer electrolyte membrane 70 Electrochemical Cells 100 cell stacks p i internal pressure

Claims

1. A bipolar plate manufacturing method, in particular for manufacturing a bipolar plate (4) for an electrochemical cell, comprising the steps of: - providing two foil sections (2, 3) made of a polymer-graphite material comprising at least one polymer and at least 75 wt % of conductive fillers, mainly graphite and also carbon black, - inserting said two foil sections (2, 3) into an embossing tool (6), - closing the tool (6) in which the foil sections (2, 3) are embossed and tightly connected to each other at their edges, - forming hollow structures (40) between the foil sections (2, 3) by a gas pressure difference at the foil surfaces, the foil sections (2, 3) abutting the surface structures (11, 12) of the facing tool surfaces of the embossing tool (6), - removing the bipolar plate (4) formed from the foil sections (2, 3) from the embossing tool (6) after the foil sections (6) have solidified; A method for manufacturing a bipolar plate, comprising:

2. 2. The method according to claim 1, characterized in that foil sections (2, 3) are used having a maximum foil thickness of 0.5 mm and a conductivity of at least 20 S / cm (at 20 to 24° C.).

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

4. 4. A method according to any one of claims 1 to 3, characterized in that the foil sections (2, 3) are sucked onto the surface structures (11, 12) of the tool surface by negative pressure.

5. 5. A method according to claim 4, characterized in that the suction of the foil sections (2, 3) is assisted by conditioned compressed air introduced between the foil sections (2, 3).

6. 6. A method according to claim 4 or 5, characterized in that the solidification of the bipolar plate (4) is at least partly caused by cooling air introduced between the foil sections (2, 3).

7. 7. A method according to any one of claims 1 to 6, characterized in that the foil sections (2, 3) are substantially joined to each other immediately after closing the embossing tool (6).

8. 7. A method according to any one of claims 1 to 6, characterized in that the foil sections (2, 3) are substantially joined together only after final shaping by local heating of the tool (6) with the aid of a gas pressure difference acting in the tool (6).

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

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

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