Bipolar plate and method for manufacturing a bipolar plate

Abstract

Method for manufacturing a bipolar plate (2), comprising the following steps: - Provision of two steel half-sheets (3, 4), wherein at least one of the two half-sheets (3, 4) has an embossed structure (5), - Construction of a three-dimensional metal structure (7) from a first powder (12) of a metal different from the steel of the half-sheets (3, 4) or from at least two powders (12) of different metals, wherein at least one first powder of the at least two powders (12) is selected to be different from the steel of the half-sheets (3, 4), wherein the metal is selected from the group comprising titanium, titanium alloys, nickel, nickel alloys, chromium, chromium alloys, ferrochrome alloys, copper, copper alloys, gold, gold alloys, platinum, platinum alloys, silver, silver alloys, iridium, iridium alloys, rhodium, rhodium alloys, ruthenium, ruthenium alloys, palladium, palladium alloys, on a first half-sheet (3) of the two half-sheets (3, 4) by means of a laser (10) with green laser light and consequently a wavelength in the range of 495 nm to 550 nm, wherein the first Powder made from the metal copper is used.wherein, when using at least one further different second powder, a further laser in the form of an infrared laser is used to build up the three-dimensional metal structure (7), and , - Forming a materially bonded connection of the 3D metal structure (7) with a second half-sheet (4) of the two half-sheets (3, 4) using the same laser (10) with green laser light, wherein the half-sheets (3, 4) are joined to form a bipolar plate (2).

Description

[0001] The invention relates to a bipolar plate constructed from two half-sheets, which is intended for use in an electrochemical cell. The invention further relates to a method for manufacturing such a bipolar plate.

[0002] A method for manufacturing a bipolar plate is known, for example, from DE 10 2021 104 821 A1. The known manufacturing method comprises cutting and forming a workpiece, wherein cutting tools are operated prior to forming tools.

[0003] Forming processes are fundamentally distinct from primary forming processes, which include 3D printing. EP 3 851 227 A1 discloses a method for the additive manufacturing of a three-dimensional body from a precious metal-containing powder. This method uses a laser whose laser beam has a wavelength between 490 nm and 570 nm.

[0004] The use of laser pulses with green laser light, specifically with a wavelength in the range of 500 nm to 540 nm, is also recommended in a method for spot welding workpieces described in DE 10 2014 201 715 A1. The workpieces can be made of copper, copper alloys, or gold. A weld penetration depth of 0.8 mm to 1.3 mm is specified.

[0005] Additive manufacturing processes using wire- or fiber-shaped starting materials are described, for example, in documents CN 110548961 A and WO 2018 / 094276 A1. WO 2021 / 150691 A1 shows possibilities for depositing a metallic powder onto a workpiece using a laser. A possible composition of a steel material for forming components by additive manufacturing is disclosed in detail in WO 2021 / 245158 A1.

[0006] Documents EP 4 043 126 A1 and CN 108362147 A deal with various methods for not only building up material in additive manufacturing, but also for removing or dissolving it in defined areas.

[0007] US Patent 5,272,309 A proposes laser processing using multiple lasers emitting laser radiation of different wavelengths. This method is intended to enable the production of objects, in particular, that incorporate metal parts made of different materials, such as copper and gold.

[0008] EP 3 656 500 A1 relates to laser welding and laser additive processing of pure copper. The pure copper is contained in a mixture comprising at least 99.0 wt% copper and a boride compound as an absorber for electromagnetic radiation. The boride compound can be in the form of lanthanum hexaboride.

[0009] According to WO 2019 / 091827 A1, an aqueous composition is suitable for the additive manufacturing of a metallic component. The aqueous composition contains a powder of a metal, which may be copper, gold, silver, or aluminum, or an alloy or intermetallic phase of one of these metals. The additive manufacturing of the component is to be carried out by laser beam melting or sintering.

[0010] German patent DE 10 2014 226 567 A1 describes a method for manufacturing a bipolar plate, wherein at least a portion of the bipolar plate is produced from metal powder using an additive manufacturing process. The metal powder used comprises titanium, a titanium alloy, nickel, a nickel alloy, chromium, a chromium alloy, a ferrochrome alloy, and / or precious metals. A base plate with a topology for creating a flow field can be used, with powder being selectively applied to projections on the base plate.

[0011] DE 10 2021 113 597 A1 discloses a method and a laser processing device for manufacturing a bipolar plate. In this process, two plate elements are directly welded together using a laser processing beam.

[0012] The invention is based on the objective of achieving advances in the manufacture of bipolar plates suitable for use in electrochemical cells, particularly in fuel cells or electrolyzers, compared to the prior art, whereby a particularly favorable ratio between manufacturing effort, geometric precision, and reproducibility of the result should be maintained even under mass production conditions. At the same time, flexibility of the manufacturing process is sought.

[0013] This problem is solved according to the invention by a method for producing a bipolar plate according to claim 1. In particular, a bipolar plate can be produced using the method which has the features of claim 7.

[0014] The manufacturing process according to the invention comprises the following steps: - Provision of two steel sheet half-sheets, wherein at least one of the two half-sheets has an embossed structure, - Construction of a three-dimensional metal structure from a first powder of a metal different from the steel of the half-sheets or from at least two powders of different metals, wherein at least one of the at least two powders is selected to be different from the steel of the half-sheets, the metal being selected from the group comprising titanium, titanium alloys, nickel, nickel alloys, chromium, chromium alloys, ferrochrome alloys, copper, copper alloys, gold, gold alloys, platinum, platinum alloys, silver, silver alloys, iridium, iridium alloys, rhodium, rhodium alloys, ruthenium, ruthenium alloys, palladium, palladium alloys on a first half-sheet of the two half-sheets by means of a laser with green laser light and consequently a wavelength in the range of 495 nm to 550 nm, in particular in the range of 500 nm to 540 nm, wherein the first powder is made from the metal copper,and wherein, when using at least one further different second powder, a further laser in the form of an infrared laser is used to build up the three-dimensional metal structure, and, - Forming a material-bonded connection of the three-dimensional metal structure with a second half-sheet of the two half-sheets using the same laser with green laser light, whereby the half-sheets are joined to form a bipolar plate.

[0015] The process always combines forming and additive manufacturing steps. The forming manufacturing of half-sheets, from which bipolar plates for fuel cells or other electrochemical cells, such as electrolysis cells or redox flow cells, are assembled, is fundamentally known.

[0016] The combination with the three-dimensional metal structure enables the use of half-sheets, which, compared to conventional products formed solely from sheet metal, have a lower embossing depth and can therefore be manufactured with lower degrees of forming.

[0017] Copper is known for its exceptionally good thermal conductivity. This means that heat is dissipated particularly quickly from the laser-heated processing zone. Combined with copper's significantly lower melting point compared to iron, this results in relatively little heat input into the steel sheets during the manufacturing process according to the invention, compared to processing steel powder. This minimizes undesirable changes in the shape and / or properties of the half-sheets already formed during the construction of the 3D structure. Using a green laser has the advantage that, compared to a conventional infrared laser, a relatively high proportion of the laser radiation is absorbed by the copper and therefore reflected less.In addition to the three-dimensionally structured half-sheet, on which the three-dimensional metal structure is built up using green laser radiation, the second half-sheet, with which the bipolar plate is completed, can also have an embossed structure.

[0018] Regardless of whether a three-dimensional metal structure, for example made of copper, is built up on only one or both of the two half-sheets, the manufacturing process according to the invention is characterized by a precisely defined heat input and, consequently, high precision of the final product, i.e., the bipolar plate. Compared to products manufactured purely by forming, drastically expanded design possibilities exist. For example, very small internal radii can be achieved. Compared to laser processing with an infrared laser, the feed rate during laser processing can be approximately three times higher due to the use of a green laser light, resulting in higher component quality.

[0019] The three-dimensional metal structure, viewed perpendicular to the plane of a half-sheet, is preferably formed by parallel straight, zigzag, or wavy lines. The lines can also be arranged concentrically. They can be continuous or interrupted, for example, as dashed or dotted lines. Furthermore, the metal structure, viewed perpendicular to the plane of a half-sheet, can consist of local protrusions or include features with, for example, circular, annular, rectangular, or triangular perimeters.

[0020] The three-dimensional, additively produced metal structure can be built up in multiple layers, in particular at least three layers, on at least one of the half-sheets. The height of the metal structure(s) can be 15% or more of the total thickness of the bipolar plate composed of the two half-sheets.

[0021] Laser processing can be carried out in such a way that, with each new layer of the three-dimensional metal structure being built up, at most 10% of the laser power emitted is absorbed by underlying, already produced layers of the metal structure under construction.

[0022] In particular, the three-dimensional metal structure can be built upon raised areas of the embossed pattern of the half-sheet. This allows the finished bipolar plate to have a large overall thickness compared to the embossing depth of the half-sheets, which enables the retention of generously dimensioned flow cross-sections, for example in the form of coolant channels, within the bipolar plate.

[0023] The two half-sheets are joined using the same green laser light that was already used to build the metal structure. For example, a maximum of 60% of the laser power is absorbed by the steel half-sheet exposed to the laser radiation, while at least 20% of the laser radiation is absorbed by the previously formed three-dimensional metal structure.

[0024] The three-dimensional metal structure is preferably constructed from at least two powders of different metals, wherein at least one powder is selected to be different from the steel of the half-sheets.

[0025] Preferably, the at least two powders, which are different from each other, are also selected to be different from the steel of the half-sheets.

[0026] When using two or more different metals in powder form to create the three-dimensional metal structure, at least one additional laser, in the form of an infrared laser, is used to build the three-dimensional metal structure. This is particularly advantageous when processing steel powder.

[0027] The bipolar plate intended for use in an electrochemical cell, in particular in a fuel cell or in an electrolyzer, and produced according to the inventive method, generally comprises the two sheet steel half-sheets, which are structured three-dimensionally by embossing, wherein the half-sheets are joined together by the three-dimensional metal structure.

[0028] The height DH of the three-dimensional metal structure is in particular at least 15% of the total thickness D of the bipolar plate.

[0029] An exemplary embodiment of the invention is explained below with reference to the drawings. These show, in part schematically: Fig. 1. A production plant for the manufacture of three-dimensional metal structures on half-sheets of bipolar plates for electrochemical cells, Fig. 2 a detail of a production plant after Fig. 1 manufactured bipolar plate in cross-section, Fig. 3 a half sheet in three-dimensional view, and Fig. 4 a cell stack comprising several electrochemical cells.

[0030] A production plant 1 according to Fig. 1 is used in the production of bipolar plates 2 (compare Fig. 2 and Fig. 4) used, which are intended for use in an electrochemical cell 20. In particular, the bipolar plates 2 are usable in a cell stack 100, especially a fuel cell stack, wherein in such a cell stack 100, i.e., stack, each bipolar plate 2 separates a half-cell of a first electrochemical cell 20 from a half-cell of another electrochemical cell 20, i.e., fuel cell.

[0031] The bipolar plate 2 comprises two essentially parallel steel sheet half-sheets 3, 4, each of which has an embossed structure 5. Fig. Figure 2 merely indicates, by way of example, the embossed structure 5 of the lower half-sheet 4, which is visible on the reverse side of the half-sheet 4 as embossed structure 5'. Where the present text refers to "lower" and "upper" components, this refers only to the arrangement according to the Fig. 1 and Fig. 2 and does not include any statement about the actual installation position in the final product. In particular, the bipolar plates 2 within a completed cell stack 100 can be vertically oriented.

[0032] The total thickness of the bipolar plate 2 is denoted by D. d denotes the wall thickness of a half-sheet 3, 4, based on its undeformed area lying in a plane. PT denotes the embossing depth of the embossed structure 5. A cavity formed between the half-sheets 3, 4 is denoted by 6 and can be used, in particular, as a flow channel for a coolant. Operating fluids of the electrochemical cells are located on the outer surfaces of the bipolar plate 2.

[0033] In addition to the half-sheets 3 and 4, the bipolar plate 2 comprises a three-dimensional metal structure 7, which, in conjunction with the embossed structure 5 of the bipolar plate 2, gives it a spatial structure. The height DH of the metal structure 7, measured orthogonally to the planes in which the half-sheets 3 and 4 lie, together with the embossing depth PT, yields the channel height KH of the cavity 6. As can be seen from Fig. As can be seen from Figure 2, the three-dimensional metal structure 7 visible in this sectional view is located on a raised area of ​​the embossed structure 5 designated 15.

[0034] Regarding the production of metal structure 7, it is noted that Fig. As indicated in Figure 1, in the outlined manufacturing stage, a first 3D-printed layer 8, as part of the emerging metal structure 7, has already been built up on the half-sheet 4. A second 3D-printed layer 9 is currently being formed. A laser 10, which emits green laser radiation LS in the range of approximately 500 nm, is used to generate the 3D-printed layers 8 and 9 layer by layer. The feed direction during laser processing is designated VR. The reference numeral 11 denotes a power supply unit for the laser 10. The material to be processed by the laser 10 is initially in the form of a metal powder 12, here copper. A melt zone created by the laser 10 is designated 13. After solidification, the 3D printing layer 8, 9 is formed. The short wavelength of the laser radiation LS compared to infrared lasers makes the laser 10 particularly suitable for processing copper powder 12 without additives.

[0035] In Fig. In addition to the 3D-printed layers 8 and 9, a third 3D-printed layer 14 is visible in Figure 2, which was also generated from pure copper using green laser radiation (LS). In the exemplary embodiment, the third 3D-printed layer 14 is materially bonded to the upper half-sheet 3. This materially bonded connection, or rather the composite of the lower half-sheet 4 with the metal structure 7 built upon it and the upper half-sheet 3, i.e., the bipolar plate 2 to be manufactured, is located in the production plant 1 in the Fig. 1. Green laser radiation LS is applied to the upper half-sheet 3 in the manner outlined, striking it from the outside. The resulting heat input is sufficient for the desired metallurgical bond between the metal structure 7 and the half-sheet 3.

[0036] Fig. Figure 3 shows a three-dimensional view of a half-sheet 4. The embossed structure 5 with a linearly applied metal structure 7 is visible in the central area of ​​the half-sheet 4, where the electrochemically active area of ​​an electrochemical cell 20 is typically located. The half-sheet 4 has fluid passage openings 16 on its narrow sides in the edge region.

[0037] Additionally, it is possible to attach further 3D-printed structures 70 laterally to one of the half-sheets 3, 4 or to both half-sheets 3, 4, thereby increasing the surface area of ​​the bipolar plate 2 at its edges or in the area of ​​recesses, such as the through-openings 16, whereby channel structures can be integrated into the further 3D-printed structure 70 both in edge areas and in the area of ​​recesses of the bipolar plate 2.

[0038] Fig.Figure 4 shows several bipolar plates 2 in a cell stack 100 of electrochemical cells 20. Each electrochemical cell 20 comprises a so-called membrane electrode unit 30 comprising a polymer electrolyte membrane (shown schematically here), which is adjacent to a bipolar plate 2 on both sides. Reference symbol list 1 production plant 2 Bipolar plate 3 half-sheets 4 half-sheets 5 Embossing structure 5' Reverse side of the embossed structure 5 6 Cavity 7 Metal structure 8 first 3D printed layer 9 second 3D printing layer 10 lasers 11 Energy supply facility 12 metal powders 13 Melting zone 14 third 3D printing layer 15 Survey 16 Fluid passage opening 20 electrochemical cells 30 Membrane Electrode Unit 70 3D printed structure 100 cell stacks, wall thickness of the half sheet Total thickness DH 3D printing height KH Canal height LS laser beam PT embossing depth VR feed direction

Claims

[1] Method for manufacturing a bipolar plate (2), comprising the following steps: - Provision of two steel half-sheets (3, 4), wherein at least one of the two half-sheets (3, 4) has an embossed structure (5), - Construction of a three-dimensional metal structure (7) from a first powder (12) of a metal different from the steel of the half-sheets (3, 4) or from at least two powders (12) of different metals, wherein at least one first powder of the at least two powders (12) is selected to be different from the steel of the half-sheets (3, 4), wherein the metal is selected from the group comprising titanium, titanium alloys, nickel, nickel alloys, chromium, chromium alloys, ferrochrome alloys, copper, copper alloys, gold, gold alloys, platinum, platinum alloys, silver, silver alloys, iridium, iridium alloys, rhodium, rhodium alloys, ruthenium, ruthenium alloys, palladium, palladium alloys, on a first half-sheet (3) of the two half-sheets (3, 4) by means of a laser (10) with green laser light and consequently a wavelength in the range of 495 nm to 550 nm, wherein the first Powder made from the metal copper is used.wherein, when using at least one further different second powder, a further laser in the form of an infrared laser is used to build up the three-dimensional metal structure (7), and, - Forming a materially bonded connection of the 3D metal structure (7) with a second half-sheet (4) of the two half-sheets (3, 4) using the same laser (10) with green laser light, wherein the half-sheets (3, 4) are joined to form a bipolar plate (2). [2] Method according to claim 1, characterized by , that the three-dimensional metal structure (7) is built up in at least three layers on the half sheet (3, 4). [3] Method according to claim 2, characterized by , that during each build-up of a new layer (9, 14) of the three-dimensional metal structure (7) at most 10% of the incident laser power is absorbed by underlying, already produced layers (7, 8) of the metal structure (7) under construction. [4] Method according to claim 2 or 3, characterized by , that the three-dimensional metal structure (7) is built upon elevations (15) of the embossed structure (5) of the first half sheet (3). [5] Method according to any one of claims 1 to 4, characterized by , that when joining the two half-sheets (3, 4) by means of the laser (10) no more than 60% of the incident laser power is absorbed by the half-sheet (3, 4) exposed to the laser radiation, while at least 20% of the laser radiation is absorbed by the three-dimensional metal structure (7). [6] Method according to any one of claims 1 to 5, characterized by , that the at least two powders (12) are chosen differently from the steel of the half sheets (3, 4). [7] Bipolar plate (2) for an electrochemical cell, manufactured according to a method according to one of claims 1 to 6, comprising the two steel-made, three-dimensionally structured half-sheets (3, 4), wherein the half-sheets (3, 4) are metallurgically connected to each other by the three-dimensional metal structure (7). [8] Bipolar plate according to claim 7, characterized by , that the height (DH) of the three-dimensional metal structure (7) is at least 15% of the total thickness (D) of the bipolar plate (2).

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