Method for manufacturing bipolar plates and fuel cell

Abstract

The invention relates to a method for producing a bipolar plate (5), comprising the following steps: a. providing two planar components (7), which are present, in particular, in a stack, b. materially bonding the two planar components (7) in a joining plane (34), in particular by means of welding, wherein, prior to the materially bonding, an intrinsic stress (9) is introduced into at least one of the two planar components (7). Furthermore, the invention relates to a fuel cell (1) comprising a bipolar plate (5) produced according to the method.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a bipolar plate, the method comprising the steps of: providing two planar members and locking the two planar members together with materials. Furthermore, this invention also relates to a fuel cell including a bipolar plate. Background Technology

[0002] A fuel cell is an electrochemical cell that converts the energy from the chemical reaction of a continuously supplied fuel and oxidant into electrical energy. Therefore, a fuel cell is an electrochemical energy converter. Among known fuel cells, hydrogen (H2) and oxygen (O2) are specifically converted into water (H2O), electrical energy, and heat.

[0003] Among them, proton exchange membrane (PEM) fuel cells are known. A PEM fuel cell consists of a centrally located membrane that is permeable to protons, i.e., hydrogen ions. The oxidant, particularly air oxygen, is thus spatially separated from the fuel, particularly hydrogen.

[0004] In addition, solid oxide fuel cells, also known as "solid oxide fuel cells" (SOFCs), are known. SOFC fuel cells have higher operating and exhaust temperatures than PEM fuel cells and are particularly useful in stable operation.

[0005] A fuel cell includes an anode and a cathode. Fuel is supplied at the anode of the fuel cell and is catalytically oxidized into protons in the release of electrons, which then reach the cathode. The released electrons are extracted from the fuel cell and flow to the cathode through an external circuit.

[0006] An oxidant, primarily oxygen from the air, is supplied at the cathode of the fuel cell and reacts to form water by accepting electrons and protons from an external circuit. The resulting water is then discharged from the fuel cell. The global reaction is as follows:

[0007] O2+4H + +4e - →2H2O

[0008] Here, a voltage exists between the anode and cathode of the fuel cell. To increase the voltage, multiple fuel cells can be mechanically arranged one after another in a fuel cell stack (also known as a pile) and connected in series electrically.

[0009] Fuel cell stacks typically have end plates that press the individual fuel cells together and provide stability to the stack. The end plates also serve as the positive or negative electrodes for discharging current.

[0010] Electrodes, namely anodes and cathodes, and membranes can be structurally combined to form membrane electrode assemblies (MEAs), also known as “Membrane Electrode Assemblies”.

[0011] Fuel cell stacks also include bipolar plates, also known as gas distribution plates. Bipolar plates are used to uniformly distribute fuel onto the anode and oxidant onto the cathode. Furthermore, bipolar plates typically have surface structures, such as channel-like structures, for distributing fuel and oxidant onto the electrodes. These channel-like structures also serve to drain water produced during the reaction. Additionally, bipolar plates may include structures for conveying cooling media through the fuel cell for heat removal.

[0012] In addition to guiding the medium for oxygen, hydrogen, and water, the bipolar plate ensures planar electrical contact with the diaphragm.

[0013] A fuel cell stack typically comprises up to several hundred individual fuel cells, which are stacked on top of each other in a layered structure known as a sandwich structure. A single fuel cell includes an MEA (Medium-Oxide Assembly) and a bipolar plate half on both the anode and cathode sides. In particular, a fuel cell includes an anode monopolar plate and a cathode monopolar plate, which are combined together to form a bio-bipolar plate.

[0014] To manufacture bipolar plates that separate hydrogen, air, and, if necessary, a cooling medium (e.g., water), steel plates are typically joined together in a locking manner, for example, by laser beam welding. To minimize component deformation during laser beam welding, process parameters are selected such that the smallest possible energy input is achieved, resulting in a narrow weld with a small molten pool volume.

[0015] Due to the small molten pool volume and the high process speed required, the gap bridging during laser beam welding is very small. This results in excessive gaps between the anode and cathode plates, leading to defects in the weld and thus to unsealing in the bipolar plates and, consequently, the fuel cell.

[0016] To manufacture bipolar plates, thin plates with low stiffness are typically used and formed into plates to be joined, depending on the heat input of the welding process and the resulting localized deformation of the components, i.e., the gap before the actual welding process. This gap cannot be kept sufficiently narrow by clamping the plates to be joined parallel to the weld seam to avoid defects.

[0017] DE 10 2016 200 387 A1 describes an apparatus and method for manufacturing bipolar plates, wherein two separator plates are connected to each other. The separator plates are stacked on top of each other and are tightly welded, for example, in an lap joint, by means of a laser. Energy for locking the materials of the two separator plates together is supplied through the two outer sides of the two separator plates respectively. Summary of the Invention

[0018] This invention proposes a method for manufacturing bipolar plates, the method comprising the following steps:

[0019] a. Provide two surface components, which are in particular stacked.

[0020] b. In particular, by means of welding, two faceted member materials are locked together in the joint plane, wherein, prior to the material locking, inherent stress is introduced into at least one of the two faceted members.

[0021] Two faceted members are prepared for a material-locking connection by introducing inherent stresses into at least one, preferably both, of the faceted members before the stresses induced by the actual material-locking connection occur. The introduced inherent stresses are targeted, time-stable, and do not cause material movement and / or deformation in the absence of external influences. In particular, the location and magnitude of the inherent stresses are adjustable. The inherent stresses are introduced, especially in the immediate surrounding environment of the joint to be manufactured. The stresses induced by the actual material-locking connection can be at least partially compensated directly by the reacting, introduced inherent stresses.

[0022] Welding is performed, in particular, by means of laser beam welding. A joint, also referred to as a seam, is preferably formed by material locking, and this seam preferably has a width of no more than 0.1 mm.

[0023] Stress induced by true material-locking joints, particularly by lasers used for material-locking joints, includes welding deformation, such as due to thermal expansion, plastic expansion, and material transport during the material-locking process. Stress induced by true material-locking joints is only temporary, especially during the heating of the two faceted components, and particularly localized, and is methodically unavoidable. It is mostly undesirable and arises from thermal expansion, the resulting material compression of the two faceted components, material movement or melt flow, and / or shrinkage that begins after solidification.

[0024] The stresses arising from the actual material-locking connection are compressive stresses, particularly when the two faceted members are heated, occurring before the weld, and tensile stresses, occurring after the actual material-locking connection process, when the two faceted members are cooled. The direction of strain and the resulting deformation depends on the heat source, joint geometry, time point, and location on the members.

[0025] According to the invention, the inherent stress introduced prior to the material locking connection points in the opposite direction to the stress caused by the actual material locking connection. The inherent stress introduced prior to the material locking connection preferably, and particularly locally, exists in the gap between the two face members, where a joint is to be formed. More preferably, the inherent stress is arranged laterally in the region of the joint and vertically along the thickness of the two face members.

[0026] Preferably, inherent stress is mechanically introduced, which is thus already present in at least one of the two faceted members before the materials are locked together. More preferably, the inherent stress is introduced by embossing, rolling, roll forming, and / or hot pressing.

[0027] Preferably, during the material locking connection, at least one of the two planar members deforms toward the joint plane. More preferably, at least one of the two planar members deforms toward the joint plane by releasing previously introduced inherent stresses. In the case of metallic materials, the rheological limit, or the maximum stress that may exist in the two planar members, decreases with increasing temperature. Therefore, the energy introduced by the material locking connection temporarily reduces the rheological limit, which is understood as the stress that can be tolerated until plastic deformation. In the molten state of the materials of the two planar members, the rheological limit decreases, for example, to near zero. Thus, the balance between the previously introduced inherent stresses is eliminated, and the two planar members deform.

[0028] Tensile and / or compressive stresses can be introduced as inherent stresses. The tensile and compressive stresses are preferably arranged such that the localized reduction in stiffness of the two planar members caused by temperature introduction from the material-locking connection reduces the previously introduced inherent stresses, and the residual stresses unaffected by temperature introduction cause deformation of the members towards the joint plane. Preferably, the tensile and / or compressive stresses are balanced to achieve a stable state.

[0029] Preferably, tensile stress is introduced into at least one of the two face members prior to the material locking connection. The introduced tensile stress at least partially compensates for the compressive stress in front caused by the actual material locking connection.

[0030] Preferably, especially additionally or auxiliaryly, in order to mechanically introduce inherent stresses, at least one temperature field is introduced into at least one of the two surface members before the materials are locked together. Introducing at least one temperature field particularly includes heating at least one of the two surface members. In particular, one or more temperature zones are used in the joint area to further compensate for the expansion caused by welding. At least one temperature field can be achieved, for example, by beamforming of a welding laser or by using an additional laser, particularly by a laser spot.

[0031] Preferably, during the material-locking connection, a portion of at least one of the two face members moves toward the joint plane. This effect is particularly caused by the geometry and thermal expansion of the two face members, as the heated material expands in all spatial directions. To produce a guiding effect, the material share is increased, especially in the direction of the desired deformation. In particular, a further preferred arrangement of at least one of the two face members at the joint is such that the compressive stress in front, caused by the actual material-locking connection, or the expansion of the two face members, causes movement toward the joint plane.

[0032] Preferably, at least one of the two face members has a geometrical element with a directional component perpendicular to the surface of the respective face member, which can also be formed during the material locking connection by the aforementioned movement. Perpendicularity should be understood herein as the geometrical element having a directional component, particularly a surface and / or a longitudinal axis, which forms an angle with the surface of the respective face member in the range of 60° to 120°, preferably 70° to 110°, and more preferably 80° to 100°, for example, an angle of 90°.

[0033] Movement toward the joint plane can also be achieved by reducing the stiffness of the individual faceted members along the deformation direction of the members due to the geometry. The individual faceted members, in particular, have reduced stiffness perpendicular to the member plane, i.e., the surface perpendicular to the member. By shaping the faceted members, for example, into a groove shape, the stiffness of the faceted members near the joint plane can be reduced. The faceted members expand due to the temperature introduced into the member plane, thereby generating a component of movement toward the joint plane through the lever action of the groove sidewalls, which may be unaffected by heat.

[0034] The two surface components preferably comprise metallic materials. More preferably, the two surface components are sheets, more preferably steel sheets, especially anode sheets or cathode sheets respectively. Furthermore, the two surface components preferably each have a thickness of no more than 0.1 mm.

[0035] The present invention also relates to a fuel cell comprising bipolar plates manufactured according to the method of the present invention.

[0036] By means of the method according to the invention, during material locking connections, the deformation of the components to be connected is oriented and restricted, thereby preventing or reducing the generation of process-determined increases in gaps between the components to be connected. Furthermore, it is also possible to prevent, reduce, or overcome increased gaps in the mating planes caused by component tolerances or contamination.

[0037] Accordingly, the material locking connection process can stabilize and reduce or avoid functionally relevant defects in the established connection that lead to unsealing in the fuel cell.

[0038] In addition, it can reduce the inherent welding stress and final deformation generated by the material locking connection of bipolar plates.

[0039] Furthermore, it enables higher process speeds or reduces cycle time, and offers greater freedom in the configuration of the clamping device used in the process, thereby increasing the service life of the clamping device and / or requiring less cleaning work.

[0040] Due to the process conditions of material locking connections, stress caused by actual material locking connections in components is unavoidable; however, these stresses can be directed or resisted in a targeted manner by the method according to the invention. Attached Figure Description

[0041] Embodiments of the invention are further illustrated with reference to the accompanying drawings and the following description.

[0042] The attached diagram shows:

[0043] Figure 1 fuel cell stack,

[0044] Figure 2 The cross-section of a fuel cell,

[0045] Figure 3 First connecting seam,

[0046] Figure 4 Second connecting seam,

[0047] Figure 5 Top view of the seam.

[0048] Figure 6 A schematic cross-sectional view of the joint during heating.

[0049] Figure 7 A schematic cross-sectional view of the joint during cooling.

[0050] Figure 8 A schematic diagram of a material locking connection between two planar components with pre-introduced inherent stress.

[0051] Figure 9 A schematic diagram of a material locking connection with an additional introduced temperature field, and

[0052] Figure 10 A schematic diagram of a material locking connection with geometrically matched structure.

[0053] In the following description of embodiments of the invention, the same or similar elements are designated by the same reference numerals, and in some cases these elements are not described repeatedly. The drawings are for illustrative purposes only, showing the subject matter of the invention. Detailed Implementation

[0054] Figure 1 A schematic diagram of a fuel cell stack 3 having multiple fuel cells 1 is shown. Each fuel cell 1 includes a membrane 35, two gas diffusion layers 37, an anode 39, and a cathode 41. The individual fuel cells 1 are separated from each other by bipolar plates 5, which may include cooling plates 43. The fuel cell stack 3, supplied with hydrogen and oxygen and a cooling medium, is enclosed by two end plates 45 and has current collectors 47.

[0055] Figure 2 A cross-section of fuel cell 1 is shown. Fuel cell 1 includes a bipolar plate 5 on which a membrane electrode unit 27 is arranged, located between two gas diffusion layers 37. Hydrogen gas 29 and water 31 for cooling are guided independently of each other in the bipolar plate 5.

[0056] Figure 3 A cross-sectional view of the first joint 33 in the form of a weld is shown. Two face members 7 are connected by the joint 33 in the mating plane 34. The medium 51 to be sealed flows between the two face members 7. The joint 33 shown here is implemented without defects, so that no medium 51 flows out.

[0057] Figure 4 The second connecting seam 33 is shown. In this illustration, the connecting seam 33 has a defective portion 55 through which the medium 51 can flow out. There is a gap 53 between the surface members 7, which is not adequately bridged by the connecting seam 33. The defective portion 55 may appear as seam intrusion, splashes, seam breaks or cracks in the bipolar plates 5, or as a pore or attachment interruption between the bipolar plates 5.

[0058] Figure 5 A top view of the connecting seam 33 implemented in the feed direction 57 is shown. For this purpose, a laser beam 59 moves in the feed direction 57, wherein the surface member 7 is heated near the connecting seam 33, thereby inducing stress and deformation in the surface member 7.

[0059] Heating occurs at laser beam 59, resulting in compressive stress 13. After laser beam 59 passes, surface member 7 cools down, resulting in tensile stress 11 pointing in the direction of connecting seam 33.

[0060] Figure 6 A cross-sectional view of the joint 33 is shown during heating. Compressive stress 13 is present, from which the deformation direction 15 is locally determined.

[0061] Figure 7 Showing according to Figure 6Another cross-sectional view of the joint 33. However, the illustration shown here shows the joint 33 during cooling, where tensile stress 11 exists, from which a relationship is derived with... Figure 6 Compared to the opposite deformation direction 15.

[0062] Figure 8 A schematic diagram of a material-locking connection is shown, in which two surface members 7 are connected by a joint 33 via a laser beam 59. Prior to the material-locking connection, inherent stresses 9, including tensile stresses 11, are introduced in the shaded areas of the surface members 7. This compensates for compressive stresses 13 in the joint 33, particularly in front of the laser beam 59.

[0063] Figure 9 Another schematic diagram of the material locking connection is shown, wherein a temperature field 17 is additionally introduced into the surface member 7 prior to the material locking connection.

[0064] Figure 10 Another schematic diagram of the material-locking connection is shown, wherein the deformation direction 15 exhibits directional welding deformation through geometric optimization in the seam region of the joint 33. The region surrounding the joint 33 is configured such that compressive stress 13 and thermal expansion in front of the joint 33 cause a portion 19 of the surface member 7 to move perpendicular to the surface 21 of the surface member 7, and this portion 19 of the surface member 7 deforms toward the joining plane (not shown here). This is achieved in the illustrated embodiment by a geometric element 23 having a directional component perpendicular to the surface 21 of the surface member 7.

[0065] This invention is not limited to the embodiments described herein and the aspects highlighted therein. Rather, various modifications that are within the scope of those skilled in the art can be implemented within the scope defined by the claims.

Claims

1. A method for manufacturing a bipolar plate (5), the method comprising the following steps: a. Provide two surface components (7), which exist in a stacked manner. b. The two face members (7) are materially locked together in the joint plane (34) by means of welding, wherein, prior to the material locking connection, an inherent stress (9) is introduced into at least one of the two face members (7), wherein the inherent stress (9) is directed opposite to the stress caused by the material locking connection, wherein the inherent stress exists in the gap between the two face members, on which a joint is to be made, wherein the inherent stress is arranged laterally in the region of the joint and vertically on the thickness of the two face members.

2. The method according to claim 1, characterized in that, The inherent stress (9) is mechanically introduced.

3. The method according to claim 1 or 2, characterized in that, During the material locking connection, at least one of the two surface members (7) deforms toward the joint plane (34).

4. The method according to claim 1 or 2, characterized in that, Tensile stress (11) is introduced into at least one of the two surface members (7) before the material locking connection.

5. The method according to claim 1 or 2, characterized in that, At least one temperature field (17) is introduced into at least one of the two surface members (7) before the material locking connection.

6. The method according to claim 1 or 2, characterized in that, During the material locking connection, a portion (19) of at least one of the two surface members (7) moves toward the joint plane (34).

7. The method according to claim 1 or 2, characterized in that, At least one of the two surface members (7) has a geometric element (23) having a directional component perpendicular to the surface (21) of the corresponding surface member (7).

8. The method according to claim 1 or 2, characterized in that, The two surface components (7) are sheets.

9. The method according to claim 1 or 2, characterized in that, The two surface components (7) each have a thickness (25) of no more than 0.1 mm.

10. The method according to claim 8, characterized in that, The two surface components (7) are either an anode plate or a cathode plate.

11. A fuel cell (1) comprising a bipolar plate (5) manufactured according to any one of claims 1 to 10.

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