METHOD FOR PRODUCING A BIPOLAR PLATE

Description

Technical field

[0001] The invention relates to a method for manufacturing a bipolar plate comprising an anode plate and a cathode plate of a fuel cell. Furthermore, the invention relates to the use of the method for manufacturing a bipolar plate. State of the art

[0002] DE 20 2021 104 930 U1 relates to a resistance welding system, which is used in particular for the production of bipolar plates for fuel cells. During welding, an additional wire material is provided as an auxiliary joining partner, independent of any foil material. The described device or method is based on the concept of forming a weld projection or the like using the described auxiliary joining partner. However, it is also possible to use the auxiliary joining partner in the form of the additional wire together with existing weld projections, for example, if spot welds are also required.The device and method described aim to minimize rejects and defective production, achieve high process quality, and at least partial automation, particularly with regard to high production volumes and short cycle times.

[0003] DE 10 2019 210 633 A1 relates to a method for manufacturing a distributor structure, in particular a bipolar plate for a fuel cell, from several components arranged in a stack. The method allows the components of a distributor structure, in particular a bipolar plate—namely a cathode structure, a separator, and an anode structure—to be joined together in a single operation by means of resistance welding. Furthermore, multiple welds or weld points can be produced in parallel to achieve a short cycle time, particularly in series production.

[0004] DE 10 2010 007 705 A1 relates to a method for manufacturing a bipolar plate for a fuel cell. In this process, the individual plates are joined together by soldering to create a material bond, thus avoiding the disadvantages of other methods, such as laser or resistance welding, such as longer manufacturing times and costs or local temperature stresses that can lead to damage to the bipolar plate.

[0005] A key component of fuel cells is the fuel cell stack, where the chemical reaction between oxygen and hydrogen, and thus the generation of electricity, takes place. The stack can be composed of several hundred individual cells, which in turn consist of an anode plate, a membrane electrode assembly, and a cathode plate. The connection between the anode and cathode plates forms the bipolar plate. The main function of the bipolar plate is to guide and separate hydrogen, air, and the cooling medium, for example, a water / glycol mixture. Minimal electrical resistance within the bipolar plate is desirable for the fuel cell's performance.Electrical contact is achieved by joining the anode and cathode plates of a fuel cell in the active area using short laser welds to minimize contact resistance between the anode and cathode plates. The channel structure of the active area is typically created through a forming process, such as embossing. Due to the nature of laser welding, the upper plate is welded through to either the underlying plate or to create the electrical contact. This process carries the risk of leakage due to defects at the top or bottom. Sealing between the different media in the ports of the anode and cathode plates, both externally and internally, is typically achieved using laser beam welding.This requires, firstly, complex clamping tools to ensure gap-free contact between the anode and cathode plates, as otherwise leaks are likely to occur. Instead of laser beam welding, resistance welding can be used, which does not have the aforementioned disadvantages, namely the welding through the upper plate and the complex clamping system inherent in laser beam welding.

[0006] Resistance welding creates a joint between the metal sheets without molten metal reaching the top or bottom surface. This is because the ohmic resistance at the contact surface between the anode and cathode sheets is higher than in the component through which the welding current flows. Consequently, the majority of the energy is converted into heat at this point, generating the molten metal. This inherently prevents leakage. Furthermore, the electrodes apply the necessary welding force to the components, closing any gaps that may be present. Should a gap remain, which is highly unlikely due to the low stiffness of the thin sheets, no current flows, and therefore no joint is formed. Description of the invention

[0007] According to the invention, a method for manufacturing a bipolar plate with an anode plate and a cathode plate of a fuel cell is proposed, comprising at least the following process steps: a) Production of a first embossing step in the anode plate or the cathode plate to form channels with the largest possible radii to create a first channel depth, b) in a subsequent embossing step, reduced radii and a required channel depth as well as flat areas are produced in the anode plate and / or the cathode plate, c) by suitable design of the embossing tool, protrusions remain in the first channel depth in the anode plate and / or the cathode plate when process step b) is carried out, d) production of tack welds by resistance welding on the protrusions between the anode plate or the cathode plate and e) production of a joining seam to ensure a sealing function between the anode plate and the cathode plate.

[0008] The solution proposed according to the invention allows weld projections to be created in the very thin anode and cathode sheets, which have a thickness of < 300 µm, in particular < 80 µm. These projections enable contact by resistance welding with a large electrode and thus prevent leakage inherent in the process. If further weld projections in the form of tack welds are created in the area of ​​the sealing weld to achieve a zero gap between the overlapping anode and cathode sheets, sealing welding can be carried out, for example, using a laser beam without complex clamping techniques, which is extremely advantageous for mass production.

[0009] In an advantageous further development of the method proposed according to the invention, anode or cathode sheets are used whose sheet thickness is below 300 µm, in particular below 100 µm.

[0010] In a further, advantageous embodiment of the method proposed according to the invention, capacitor discharge welding is used as the preferred resistance welding method. This advantageously allows relatively high, pulsed currents to be generated. A further advantage of capacitor discharge welding lies in the possibility of achieving short cycle times.

[0011] In the method proposed according to the invention, the joining seam can be produced after the tack welds have been made by a laser welding process, which can be automated and enables the production of high quantities, as shown in d).

[0012] Advantageously, in the method proposed according to the invention, channels with channel radii are produced in the first carrier step according to process step a) that allow for the formation of a first channel depth exceeding the required channel depth. This ensures that material is already provided in the first embossing step, which can advantageously be further deformed during the subsequent embossing step.

[0013] Advantageously, the method proposed according to the invention makes it possible, within the framework of the post-embossing step according to process step b), to post-emboss the channel radii designed in the first embossing step, to form an angular channel shape and to form flat contact surfaces.

[0014] In an advantageous further development of the method proposed according to the invention, in carrying out the post-embossing step according to process steps b) and c), an embossing tool comprising an upper tool and a lower tool is used, with whose upper tool the angular channel shape on the one hand and with whose lower tool having recesses areas formed between protrusions remaining in the first channel depth by back-pressing in the required channel depth on the other hand.

[0015] Advantageously, the method proposed according to the invention allows the formation of protrusions which can be designed as simple weld bumps, double bumps, or reduced-size weld bumps.

[0016] In an advantageous further development of the method proposed according to the invention, the tack points produced according to process step d), which are produced by means of resistance welding, serve to represent a technical zero gap between the superimposed anode sheet and the cathode sheet of the bipolar plate to be produced.

[0017] In an advantageous further development of the method proposed according to the invention, the joint seam produced according to process step e) can be manufactured by laser beam welding. Alternatively, it is possible to produce the sealing seam by fusion welding processes, such as electron beam welding, laser beam welding, or plasma welding.

[0018] Advantageously, in the method proposed according to the invention, the tack welds are arranged inside or outside the area to be sealed. The tack welds can be arranged either to the left or to the right of the continuous weld seam. Alternatively, the sealing seam is executed such that it extends in a serpentine pattern, alternately to the left and right of the tack welds.

[0019] Furthermore, the invention relates to the use of the method for manufacturing a bipolar plate comprising at least one anode plate and at least one cathode plate. Advantages of the invention

[0020] The solution proposed according to the invention is characterized in that resistance welding, in particular capacitor discharge welding, can be used in the mass production of bipolar plates both for creating the electrical contacts and, by means of tack welding, for sealing. This allows for a number of advantages compared to solutions known from the prior art. Due to the nature of the process, the joint formed in resistance welding is created in the contact surface in the form of a weld bead. This prevents leakage upwards or downwards without molten material, thus reducing the probability of rejection due to leakage in the flow field of a bipolar plate being manufactured.Pre-tacking the anode and cathode plates ensures a zero-gap gap near the weld seam for subsequent laser welding, so that only holding the entire plate down is required to ensure working in the focused position for the weld seam to be produced. This results in a reduced effort in the design of the hold-down device for welding the sealing contours.

[0021] Through targeted current flow, using the embossing structures formed in the first embossing step and the subsequent post-embossing step, the current flow is concentrated so that the energy available from resistance welding is sufficient to weld all connections with just a few current pulses. This makes it possible to create electrical contacts by welding as well as to perform tack welding, both of which can be achieved in a single process step.

[0022] Integrating the resistance welding system into a stamping system, where the embossing processes are carried out, is conceivable, so that subsequent processes can benefit from the easier handling of a more robust and stiffer manufactured bipolar plate.

[0023] The thermal expansion and contraction during the welding process primarily takes place in the embossed structures, so that component distortion can be minimized accordingly.

[0024] The targeted current flow mentioned above allows for shorter welding times, resulting in less discoloration and material transformations, which in turn has a positive effect on corrosion resistance. Brief description of the drawings

[0025] Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

[0026] They show: Figure 1 shows a cross-section through a fuel cell and a bipolar plate. Figure 2 shows a channel structure made of anode and cathode plates with welds to reduce the contact resistance. Figure 3 shows a linear bearing of cathode and anode plates. Figure 4 shows a schematic representation of a resistance weld joint. Figure 5 shows a channel shape after a first embossing step. Figure 6 shows the channel shape after a second embossing step with a partially cut protrusion serving as a weld bump. Figure 7 shows the formation of the channel structure by means of an embossing tool comprising an upper tool and a lower tool. Figure 8 shows a sketch of a protrusion designed as a double bump. Figure 9 shows a sketch of a reduced protrusion serving as a weld cap. Figure 10 shows tack welds to represent a technical zero gap and a continuous joining seam. Embodiments of the invention

[0027] In the following description of embodiments of the invention, identical or similar elements are designated by the same reference numerals, and repeated descriptions of these elements are omitted in individual cases. The figures represent the subject matter of the invention only schematically.

[0028] Figure 1 Figure 1 shows a cross-section through a fuel cell 10 and a bipolar plate 14. In addition to the bipolar plate 14, the fuel cell 10 comprises a membrane electrode assembly (MEA) 12. A gas diffusion layer 20 runs through each membrane electrode assembly 12. A cathode is designated with reference numeral 18, and an anode bears reference numeral 16. From the illustration according to Figure 1It is evident that the bipolar plate 14 extends below the membrane electrode assembly 12 and essentially comprises an anode plate 22 and a cathode plate 24. Channels 32 for a cooling medium flow 30, generally a water-glycol mixture, run through both the cathode plate 24 and the anode plate 22. Furthermore, channels 32 in the anode plate 22 are traversed by a hydrogen flow 28, and channels 32 in the cathode plate 24 are traversed by an air flow 26.

[0029] According to the representation Figure 2 A channel structure made of anode and cathode plates 22, 24 with welds to reduce the contact resistance can be seen. From the perspective top view according to Figure 2It is evident that channels 32 are incorporated into both the anode plate 22 and the cathode plate 24. At the base of the channels 32, individual short joining sections 34 are formed, which can, for example, be laser welds, in order to achieve the lowest possible contact resistance between the anode plate 22 and the cathode plate 24. Flat areas 36, which are undeformed and are also referred to as "lands," extend on both sides of the channels 32 in the anode plate 22 and the cathode plate 24.

[0030] According to the representation Figure 3 A channel structure can be seen in which the anode plate 22 and the cathode plate 24 are joined together at the bottom of the respective channels 32 by linear joining sections 38, which can be continuous.

[0031] According to the representation Figure 4 A resistance weld joint can be seen schematically. From the illustration according to Figure 4It is evident that a first sheet 44 and a second sheet 46 are joined together by a lens 42, in particular a weld lens, in a materially bonded manner. The lens 42, arranged between the first sheet 44 and the second sheet 46, can seal a medium 40 to be sealed against flowing through the gap between the first sheet 44 and the second sheet 46.

[0032] From the representation according to Figure 5 It is evident that a channel 32 is introduced into an anode plate 22 and / or a cathode plate 24 by means of a first embossing step 52. The first embossing step 52 is carried out using an embossing tool (see illustration according to Figure 7), wherein at least one channel 32 is embossed into the material of the anode plate 22 and / or the cathode plate 24 at a channel depth 50, forming relatively large channel radii 54. The anode plate 22 and / or the cathode plate 24 have a sheet thickness 88 of ≤ 300 µm, in particular ≤ 100 µm, in particular 80 µm.

[0033] According to Figure 5 The anode sheet 22 or cathode sheet 24 treated by the first embossing step 52 is according to Figure 6 The channel 32 is subjected to a post-embossing step 56, in which the channel radii 54 are reshaped to achieve a more angular channel shape 60 in the anode plate 22 or in the cathode plate 24. The post-embossing step 56 is carried out using a two-part embossing tool, of which the illustration shows... Figure 7 An upper tool 68 and a lower tool 72 are shown. In the post-embossing step 56, which is described in Figure 6As outlined, in addition to forming the more angular channel shape 60, flat contact surfaces 58 are formed. This results in a reduction of the first channel depth 50, which was formed in the first embossing step 52, such that a reduced required channel depth 64 is achieved.

[0034] During the execution of the re-embossing step 56, as described in Figure 6Although schematically depicted, the provision of recesses in the lower tool 72 results in the retention of protrusions 62, which are designed as weld bumps, in the area of ​​these recesses. These protrusions, resembling weld bumps, remain in the channels 32 of the anode plate 22 and / or the cathode plate 24, respectively, and are formed at the first channel depth 50. In contrast, the sections of the channels 32 extending between these protrusions have the reduced required channel depth 64. In the subsequent stamping step 56, the bottom of the channels 32 is thus pressed back from the channel depth 50 achieved in the first stamping step 52 to the reduced required channel depth 64.

[0035] In comparison to the representations in the Figures 5 and 6The longer channels 32 in the anode plate 22 and / or in the cathode plate 24 thus result in channels 32, along whose longitudinal extent a number of bump-like elevations 62, which later serve as weld bumps, are produced spaced apart from each other at the bottom of the channel 32.

[0036] According to the representation Figure 7 It can be seen that the upper tool 68 takes up the final contour of the channel 32 and forms it in the channel 32 by advancing the upper tool 68 against the material of the anode plate 22 and / or the cathode plate 24. The counter tool, i.e., the lower tool 72 of the embossing tool, is designed as a flat plate which has the recesses mentioned above in the area of ​​the protrusions 62 for forming the protrusions 62.

[0037] From the representations according to the Figures 8 and 9 Individual geometries of the elevations emerge.

[0038] Figure 8Figure 1 shows, for example, a channel 32 which is designed in an angular channel shape 60 and in whose channel wall contour 70, particularly at the bottom, a projection in the form of a double bump 76 is embossed. The two adjacent double bumps 76 are separated from each other by a flat section 78. By means of the double bump 76, as shown in Figure 1, Figure 8 As indicated, an additional centering function can be achieved when joining a canal-land structure. From the illustration according to Figure 8 It is further evident that in addition to the channel 32, the undeformed flat areas 36, i.e. the "Lands", extend.

[0039] From the representation according to Figure 9One embodiment of the channel 32 is characterized by the fact that at the bottom of the channel 32, which is formed in a more angular channel shape 60, i.e., in the channel wall contour 70, a reduced elevation, i.e., a reduced weld bump 80, is formed. This allows for a higher current concentration.

[0040] Out of Figure 10 This results in a combination of resistance-welded tack welds 82 and a continuous laser weld seam, designed here, for example, as a continuous joining seam 84. According to the illustration Figure 10It can be seen that the individual tack welds 82 are arranged at a distance from each other between the first sheet 44 and the second sheet 46, or between the anode sheet 22 and the cathode sheet 24. The function of the tack welds 82, which are created during resistance welding, for example by capacitor discharge welding, is to fix the sheets 44, 46, or between the anode sheet 22 and the cathode sheet 24, to each other. After the continuous weld seam 84 has been created, these sheets form the bipolar plate 14. The tack welds 82 created by resistance welding ensure a zero gap 86 between the sheets 44, 46, or between the anode sheet 22 and the cathode sheet 24, for the subsequent creation of the continuous weld seam 84.

[0041] From the representation according to Figure 10It is evident that the continuous joining seam 84, for example, is executed as a laser weld. This provides the technically required sealing function.

[0042] The position of the tack points 82 between the first sheet 44 and the second sheet 46, or between the anode sheet 22 and the cathode sheet 24, can be located either outside or inside the area to be sealed. Assuming that the medium to be sealed is located to the right of the joining seam 84 (sealing seam) formed between the two sheets 44 and 46, then the inner area would be on the right side of the joining seam 84, and the outer area would extend to the left of the joining seam 84. Alternatively, however, the joining seam 84 can also separate two media from each other, resulting in a seal between these two media. Although in the illustration according to Figure 10If the joining seam 84 has an essentially straight course, it would alternatively also be possible for the joining seam 84 to move in a serpentine or meandering shape around the tack points 82.

[0043] The continuous joining seam 84 according to the illustration in Figure 10 It can be manufactured not only by laser beam welding but also by alternative joining methods. Depending on the electrical contacting, the embossings required to create a multitude of joints with a single current pulse are made in the anode plate 22 and / or cathode plate 24, as shown in [reference to relevant section]. Figure 8 and 9 for example, illustrated or executed.

[0044] The invention is not limited to the embodiments described here and the aspects highlighted therein. Rather, within the scope specified by the claims, a multitude of modifications are possible that fall within the bounds of what is considered skilled in the art.

Claims

1. Method for producing a bipolar plate (14) with an anode sheet (22) and a cathode sheet (24) of a fuel cell (10) comprising at least the following method steps: a) carrying out a first embossing step (52) in the anode sheet (22) and / or the cathode sheet (24) for forming channels (32) with greatest possible roundings to create a first channel depth (50), b) in a re-embossing step (56), reduced radii and a required channel depth (64) as well as planar regions are created in the anode sheet (22) and / or in the cathode sheet (24) in such a way that, c) by suitable formation of the embossing tool (68, 72), elevations (62, 76, 80) in the first channel depth (50) remain in the anode sheet (22) and / or the cathode sheet (24) when method step b) is carried out, d) creating bonding points (82) between the anode sheet (22) and the cathode sheet (24) by resistance welding at the elevations (62, 76, 80) and e) creating a joining seam (84) to ensure a tightness between the anode sheet (22) and the cathode sheet (24).

2. Method according to Claim 1, characterized in that a sheet thickness (88) of the anode sheet (22) and of the cathode sheet (24) lies below 300 µm, in particular below 100 µm.

3. Method according to Claims 1 and 2, characterized in that capacitor discharge welding is used as the resistance welding method according to method steps d) and / or e).

4. Method according to Claims 1 and 2, characterized in that in method step e) the joining seam (84) is produced by a laser welding method.

5. Method according to Claims 1 to 4, characterized in that, in the first embossing step (52) according to method step a), channels (32) are created with channel radii (54) that allow formation of a first channel depth (50) exceeding the required channel depth (64).

6. Method according to Claims 1 to 5, characterized in that, in the re-embossing step (56) according to method step b), the channel radii (54) are re-embossed, an angular channel shape (60) is formed and planar contact areas (36, 78) are fashioned.

7. Method according to Claim 6, characterized in that, when carrying out the re-embossing step (56) according to b) and c), an embossing tool comprising an upper tool (68) and a lower tool (72) is used, with the upper tool (68) of which the angular channel shape (60) is created on the one hand and with the lower tool (72) of which, having cutouts, regions formed in the required channel depth (64) are created between elevations (62, 76, 78) remaining in the first channel depth by pressing back.

8. Method according to Claims 1 to 7, characterized in that the elevations (62, 76, 80) are made as a single weld bulge (62) or as double bulges (76) or as a reduced weld bulge (80).

9. Method according to Claims 1 to 8, characterized in that, according to method step d), the bonding points (82) created by resistance welding serve for creating a technical zero gap (86) between the anode sheet (22) and the cathode sheet (24) of the bipolar plate (14).

10. Method according to Claims 1 to 3, characterized in that, according to method step e), the joining seam (84) is created by laser beam welding or fusion welding methods, such as electron beam welding or plasma welding.

11. Method according to Claims 1 to 9, characterized in that the bonding points (82) are arranged inside or outside a region to be sealed off by the joining seam (84), wherein the joining seam (84) either runs in a straight line or assumes a serpentine or meandering course between the bonding points (82).

12. Use of the method according to one of Claims 1 to 11 for producing a bipolar plate (14), comprising at least one anode sheet (22) and at least one cathode sheet (24).