Method for manufacturing bipolar plates, bipolar plates, and electrochemical cells

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 particularly relates to a bipolar plate manufacturing method for manufacturing bipolar plates for electrochemical cells, especially for fuel cells. The present invention also relates to a bipolar plate and an electrochemical cell.

Background Art

[0002] German Patent Application Publication No. 102008028549 discloses a method for manufacturing a fuel cell bipolar plate having thermoplastic plates. Individual plates are produced from a resin mixture comprising a conductive thermoplastic polymer composition and a solvent. After individual panels are formed and cut, they are assembled to form the bipolar plate.

[0003] Another method for producing a conductive plastic bipolar plate intended for use as an electrode of a fuel cell is described in European Patent No. 1506585. In this case, production of a structure having conductive, carbonized, or graphitized reinforced fibers is proposed, and it is intended that mechanical orientation of the reinforced fibers by needling in a first direction corresponding to a preferred electrical conduction path results in higher conductivity in said first direction. European Patent No. 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 for obtaining the matrix are mentioned in European Patent No. 1506585 as thermoforming, membrane molding, pressure casting, resin transfer molding, molding under pressure and under vacuum, lamination and embossing pressing.

[0004] German Patent Application Publication No. 102011116993 relates to an apparatus for manufacturing metal foil components intended for use as components of a fuel cell. At the start of the manufacturing process, two foils are overlapped and connected to each other liquid-tightly in at least some areas. The shape of the foils is intended to conform to the surface structure of the forming tool by introducing a fluid under pressure into the space formed between the foils where the foils are placed on the forming tool. German Patent Application Publication No. 102011116993 specifies that the tool portions of the forming tool are moved toward each other during the forming process. In this process, the fluid is released in a controlled manner from the cavity formed between the foils.

[0005] Another method for manufacturing metal bipolar plates for fuel cell stacks is described, for example, in German Patent Application Publication No. 102010020178. In particular, German Patent Application Publication No. 102010020178 deals with the manufacture of gas distribution structures, and shear cutting is recommended as the manufacturing technique.

[0006] German Patent Application Publication No. 102009044112 describes a method for manufacturing a microstructured composite component. For this purpose, first and second foils made of a thermoplastic polymer material are placed between mold components that have a microstructured hollow shape filled with foil material and are heated in the contact area with the foils. Overpressure is generated between the two foils, pushing the foils into the hollow mold. Finally, the foils are pressed together and cooled. After demolding, the microstructured composite component has a microstructure that provides channels for liquids.

[0007] U.S. Patent No. 6,217,699 discloses an apparatus and method for joining pre-molded plastic foils by welding.

[0008] West German Patent Application Publication No. 1250627 describes a method for producing a double-walled hollow body from thermoplastic foil. For this purpose, two heated plastic foils in a plasticized state are introduced into a die-shaped mold, where they are welded together at least partially, with the difference in pressure forming the interior of the welded edges.

[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 rib-like cavity. The thermoplastic foils are placed in contact with the rib-like cavity. After the molds are closed, a fluid is supplied between the foils, and the thermoplastic foils are formed within the cavity, joining the foils together. The thermoplastic material for forming the foils may contain 1 to 70 wt% of fillers, and possible fillers are asbestos, carbon, glass fiber, calcium phosphate, calcium carbonate, kaolinite clay, silicon dioxide, titanium dioxide, bentonite, talc, and mica. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The present invention is based on the objective of further developing the manufacture of bipolar plates for electrochemical cells compared to the aforementioned prior art, seeking a particularly favorable ratio between equipment costs, the geometric accuracy of the manufactured product, and process reliability. Furthermore, bipolar plates and electrochemical cells are provided. [Means for solving the problem]

[0011] This objective is achieved, according to the present invention, by the bipolar plate manufacturing method described in claim 1, the bipolar plate subsequently manufactured according to claim 9, and the electrochemical cell described in claim 10. The embodiments and advantages of the present invention described below in relation to the bipolar plate manufacturing method also apply to the bipolar plate manufactured thereby, and vice versa.

[0012] The bipolar plate manufacturing method generally includes the following steps: - A step of providing two foil sections made of a polymer graphite material comprising at least one polymer and at least 75 wt% of a conductive filler, which also mainly includes graphite and carbon black, - The step of inserting two foil sections into the embossing tool, - The foil sections are embossed and tightly connected to each other at their edges, step of closing the tool, - A step of forming a hollow structure between foil sections on the foil surface by a difference in gas pressure, particularly a difference in air pressure, wherein the foils form a hollow structure in contact with the tool surfaces facing each other. - A step of removing the bipolar plate formed from the foil section after the foil section has solidified.

[0013] Therefore, this is a combined method that combines the mechanical deformation of the embossing form with formation using the pressure difference of a gaseous medium. The first change in the geometric parameters of the foil section occurs by embossing immediately after the foil section is inserted into the embossing tool, also abbreviated as the tool. Changes in the geometric parameters include, among other things, changes in wall thickness and the formation of a three-dimensional embossed structure.

[0014] Most of the changes in the shape of the foil sections are achieved by the subsequent gas pressure, which can exist as negative and / or positive pressure. The initial embossing of the foil sections already ensures airtightness between them.

[0015] The term "foil section" is used in this context for any flat polymer-graphite starting product. This also applies when the starting product is in the form of a sheet or plate. Typical wall thicknesses or foil thicknesses range from 0.1 mm to 0.5 mm, particularly from 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, preheating of the foil sections before inserting them into the tool may be considered. Similarly, the degree of preheating of the tool portion of the embossing tool depends particularly on the material properties of the foil sections.

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

[0018] In principle, polymers can be selected from thermoplastic or thermosetting materials, but they do not necessarily have to be made from the same material. Particular consideration should be given to fiber reinforcement of foils. In the case of thermoplastic materials, a process commonly called solidification occurs during fixation. In the case of thermosetting materials, solidification is the curing process. Polypropylene (PP) or polyphenylene sulfide (PPS) has been found to be particularly suitable as thermoplastic materials. Polyester resins or epoxy resins have been found to be 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 filler in the foil reaches its maximum proportion, or the polymer reaches its minimum proportion. This can be determined in a simple experimental manner.

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

[0021] A suitable manufacturing plant for manufacturing bipolar plates intended for use in a stack of electrochemical cells is generally both designed for embossing a two-layer foil arrangement, and comprises a two-part embossing tool having a fluid connection, in particular a vacuum and / or compressed air connection, for forming at least one cavity between the foils by means of gas pressure.

[0022] The foil from which the foil sections are cut has a proportion of at least 75 wt% of conductive fillers, which herein are mainly graphite, in particular ground graphite, and carbon black, and is configured to provide a conductivity of at least 20 S / cm, in particular at least 100 S / cm, sufficient for the intended use in a stack 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 the bipolar plate.

[0023] Using the "through-plane measurement method" (TPV), the area-specific electrical resistance of the foil is first determined, at 40 N / cm 2 contact pressure of the measuring electrode (= gold-plated contact pin with a 39° tip) is applied to the foil. At room temperature in the range of 20 to 24°C, a surface-specific electrical contact resistance of ≦10 mΩ*cm 2 is required. The determined electrical resistance value is then converted into conductivity.

[0024] The filler is used in the foil in a total proportion of at least 75 wt%, and preferably comprises carbon black in a proportion of 5 to 10 wt% and ground 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 d of 200 μm or less, more preferably 75 μm 90 value in the particle size distribution.

[0026] For processing the foil in an embossing tool, the foil section is advantageously brought to a temperature higher than the heat distortion temperature and lower than the melting temperature. This not only ensures good formability, but also substantially completely prevents separation of the constituent components of the foil, particularly the filler and the plastic. A metallic constituent of the foil is not provided in a typical design, but is not unequivocally excluded. Metal is by no means a main constituent of the foil. For example, up to 20 wt% of metal particles, for example at least one metal from the group consisting of titanium, titanium alloys, aluminum, aluminum alloys, vanadium, vanadium alloys, for example Ti6Al4V, can be mixed into the foil.

[0027] Depending on the materials used and the geometric structure to be created, it may be sufficient to use negative pressure to apply foil sections to the three-dimensional structured surface of the tool part according to the intended shape of the final product. According to a more developed variation of the method, compressed air is further introduced between the foil sections, i.e., into the cavity being formed. To assist in heating the foil sections of the tool, this can be adjusted to compressed air, i.e., compressed air with an increased temperature level. To aid in the subsequent solidification of the bipolar plate formed from the foil sections after the foil sections have taken their final shape, cooling 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 to cooling air in later stages of the method. In all cases, the completed bipolar plate formed from the foil sections can be discharged from the tool using the same connection initially used to apply negative pressure.

[0028] A key advantage of the bipolar plate manufacturing method according to this application, compared to a process that provides for the single formation of plate-like elements, is that the simultaneous processing of both foil sections in the embossing tool eliminates the need for any alignment of the foil sections after they have been formed. Optionally, a leak test is performed after the bipolar plate has been manufactured from the foil sections. The same openings formed at specific locations 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, particularly cooling water, through the bipolar plate inside a fuel cell stack, including a subsequent stack of electrochemical cells, particularly a plurality of bipolar plates of the type described.

[0029] A bipolar plate manufactured according to the method of the present invention has at least one hollow structure, particularly in the shape of a channel, for a fluid passage through the bipolar plate.

[0030] Electrochemical cells, particularly fuel cells, electrolytic cells, or redox flow batteries, include at least one such bipolar plate according to the present invention.

[0031] Therefore, the bipolar plates formed by the process according to the present invention are suitable for use in electrochemical cells, particularly fuel cells equipped with polymer electrolyte membranes, electrolytic cells for the electrolysis of water equipped with polymer electrolyte membranes, or redox flow batteries equipped with polymer ion exchange membranes.

[0032] Hereinafter, several exemplary embodiments of the present invention will be described illustratively with reference to the drawings. The drawings show the following in a partially schematic and simplified manner. [Brief explanation of the drawing]

[0033] [Figure 1] This shows a manufacturing plant to explain the bipolar plate manufacturing method. [Figure 2] Figure 1 shows a more detailed representation of the configuration. [Figure 3] A modified plant is shown to illustrate the bipolar plate manufacturing method using the same representation as in Figure 2. [Figure 4] This shows a bipolar plate in three dimensions. [Figure 5] A schematic representation of an electrochemical cell and cell stack is shown. [Modes for carrying out the invention]

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

[0035] Manufacturing plant 1 uses foil sections 2 and 3 made of conductive polymer-graphite material to produce bipolar plates 4 for electrochemical cells, particularly PEM fuel cells. Within the completed stack of electrochemical cells, each bipolar plate 4 separates the half-cells of a first electrochemical cell from the half-cells of another similarly configured electrochemical cell. The prior art cited earlier is referenced for the basic structure and function of the stacked electrochemical cells, particularly fuel cells.

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

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

[0038] In a further manufacturing process, the shapes of foil sections 2 and 3 are conformed to the shapes of the surface structures 11 and 12 of tool sections 7 and 8 by the action of negative and / or positive pressure, as will be described in more detail later. Referring to the symbolic representation in Figure 1, this forming step is still carried out along the upper line of the diagram showing the manufacturing process. Subsequently, in the lower left of Figure 1, the completed bipolar plate 4 formed from foil sections 2 and 3 is cooled inside the embossing tool 6, which remains closed. The final step is the removal and demolding of the bipolar plate 4. The demolded bipolar plate 4 is shown in cross-sectional views in Figures 1, 2, and 3, where a hollow structure 40 formed between the connected foil sections 3 and 2 in the form of a channel can be seen, which guides fluid through the bipolar plate 4. Typically, a coolant such as cooling water passes through the hollow structure 40.

[0039] Channels 13 and 14 are formed in each of the tool sections 7 and 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 sections 7 and 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 and 3. Furthermore, compressed air channels 15 and 16 are formed in the tool sections 7 and 8, each extending from collection lines 17 and 18 to the tool surface having surface structures 11 and 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 the ambient air pressure.

[0040] As can be seen in Figures 2 and 3, a vacuum pump 19 is connected to each collection line 17, 18. The vacuum pump 19 is used to generate negative pressure that sucks the foil sections 2, 3 onto the surface structures 11, 12 of the tool sections 7, 8. In the modified configuration shown in Figure 3, a compressor 20 is also used to create internal pressure p between the foil sections 2, 3. iThis generates a pressure which can be monitored by the pressure gauge 21. In both the modified forms shown in Figures 1 and 2 and the modified form shown in Figure 3, the demolding of the bipolar plate 4 is assisted by applying compressed air to the outer surface of the bipolar plate 4 through compressed air channels 15 and 16.

[0041] The compressed air introduced between foil sections 2 and 3 is adjusted according to the stage of the method. The heated compressed air allows for rapid heating not only of foil sections 2 and 3 but also of the inside of 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 matched to the formation temperature or demolding temperature of the material. In the case of thermosetting materials, the viscosity for formation is reduced by first setting an appropriate temperature level. Further temperature control depends on the activation temperature of the curing agent contained in the material of foil sections 2 and 3. By promoting a temperature-dependent crosslinking reaction, the solidification of the bipolar plate 4 after formation is aided, thus shortening the cycle time.

[0042] 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 material used, without further processing.

[0043] Figure 4 shows the bipolar plate 4 in three dimensions. It has active fields 41 on each of its sides facing outward from the hollow structure 40 (see Figures 1 to 3), which are not visible here, and in that region, electrochemical reactions do not occur in the electrochemical cell 70 (see Figure 5). The rectangular bipolar plate 4 has three fluid passage openings on its short side. The central fluid passage opening functions as a refrigerant supply opening 50 and a refrigerant discharge opening 51. These are fluidly connected to the hollow structure 40 inside the bipolar plate 4, allowing for the supply of coolant flowing through the hollow structure 40 to the hollow structure 40 and the discharge of coolant after it has left the hollow structure 40.

[0044] Figure 5 shows a schematic representation of an electrochemical cell 70 in a cell stack 100 containing multiple such electrochemical cells 70. Each electrochemical cell 70 comprises two bipolar plates 4 and a polymer electrolyte membrane 60 positioned between them, with adjacent electrochemical cells 70 sharing the bipolar plates 4. [Explanation of Symbols]

[0045] 1. Manufacturing plant 2 foil sections 3 Foil Sections 4 Bipolar Plates 5. Preheating device 6 Embossing Tools 7 Lower tool section 8 Upper tool section 9. Sealing area of ​​the upper tool section 10 Sealing area of ​​the lower tool section 11 Surface structure of the upper tool section 12 Surface structure of the lower tool section 13. Channel for adjusting the fluid in the upper tool section 14 Channels for adjusting the fluid in the lower tool section 15 Compressed air channel in the upper tool section 16 Compressed air channel in the lower tool section 17 Collection line of the upper tool section 18 Collection line of the lower tool section 19 Vacuum pump 20 Compressors 21 Pressure gauge 40 Hollow structure 41 Active space 50 Refrigerant supply opening 51 Refrigerant discharge opening 60 Polymer electrolyte membrane 70 Electrochemical Cells 100-cell stack p i internal pressure

Claims

1. A method for manufacturing a bipolar plate (4) for an electrochemical cell, comprising the following: - A step 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, also including graphite and carbon black, - The step of inserting the two foil sections (2, 3) into the embossing tool (6), - The step of closing the embossing tool (6) such that the foil sections (2, 3) are embossed and their edges are tightly connected to one another, - A step of forming a hollow structure (40) between foil sections (2, 3) by the difference in gas pressure on the surfaces of the foil sections (2, 3), wherein the foil sections (2, 3) abut against the surface structures (11, 12) of the mutually facing tool surfaces of the embossing tool (6) to form the hollow structure (40). - After the foil sections (2, 3) have solidified, the bipolar plate (4) formed from the foil sections (2, 3) is removed from the embossing tool (6). It has, The foil sections (2, 3) are attracted onto the surface structures (11, 12) of the tool surface by negative pressure. A method for manufacturing a bipolar plate, characterized in that the suction of the foil sections (2, 3) is assisted by controlled compressed air introduced between the foil sections (2, 3).

2. A method for manufacturing a bipolar plate (4) for an electrochemical cell, comprising the following: - A step 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, also including graphite and carbon black, - The step of inserting the two foil sections (2, 3) into the embossing tool (6), - The step of closing the embossing tool (6) such that the foil sections (2, 3) are embossed and their edges are tightly connected to one another, - A step of forming a hollow structure (40) between foil sections (2, 3) by the difference in gas pressure on the surfaces of the foil sections (2, 3), wherein the foil sections (2, 3) abut against the surface structures (11, 12) of the mutually facing tool surfaces of the embossing tool (6) to form the hollow structure (40). - After the foil sections (2, 3) have solidified, the bipolar plate (4) formed from the foil sections (2, 3) is removed from the embossing tool (6). It has, The foil sections (2, 3) are attracted onto the surface structures (11, 12) of the tool surface by negative pressure. A method for manufacturing a bipolar plate, characterized in that the solidification of the bipolar plate (4) is at least partially caused by cooling air introduced between the foil sections (2, 3).

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

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

5. The method according to claim 1 or 2, characterized in that the foil sections (2, 3) are substantially joined to each other immediately after the embossing tool (6) is closed.

6. The method according to claim 1 or 2, characterized in that the foil sections (2, 3) are substantially joined together only after they have been formed by partially heating the embossing tool (6) with the help of the pressure difference of the gas acting on the embossing tool (6).

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