METHOD FOR JOINING AT LEAST TWO COMPONENTS OF A FUEL CELL AND DEVICE FOR CARRYING OUT THE METHOD

Description

[0001] The invention relates to a method for joining at least two components of a fuel cell, in particular for joining two individual plates of a fuel cell to form a bipolar plate.

[0002] Bipolar plates are used in fuel cells, particularly in fuel cell stacks. Bipolar plates are used to guide and distribute the fuel to a neighboring anode of a first fuel cell and the cathode gas to a neighboring cathode of a second fuel cell. The bipolar plate also provides lines for conducting a cooling medium. A bipolar plate is usually made of two individual plates formed as half-shells, which are glued together in the case of graphite bipolar plates. Metallic bipolar plates typically comprise two individual plates that are welded together at least in sections.

[0003] DE 10 2018 219 056 A1 describes a device for the continuous production of bipolar plates using a laser welding device. WO 2018 / 237 049 A1 describes the additive manufacturing of bipolar plates using a laser sintering process. US 2020 / 0 206 843 A1 describes a device and a welding method for welding two individual plates to form a bipolar plate. US 8 354 203 B2 describes a device and a method for producing bipolar plates, wherein two individual plates are welded together to form a bipolar plate. US 2016 / 0 096 237A1 also describes a device for laser welding two individual plates, wherein a polygon wheel is used to deflect the laser beam.

[0004] It has been found that laser welding of fuel cell components, particularly bipolar plates, poses a risk of pore formation due to excessive local heat input into the individual plates. This excessive heating can lead to leaks at weld seams, which are particularly evident when the bipolar plates are designed with a very long weld seam. Due to the increased heat input into the joined material, undesirable distortion of the component can occur, which has negative effects on the subsequent process chain for fuel cell production, particularly during assembly.

[0005] It is therefore the object of the present invention to provide a method for joining at least two components of a fuel cell, in which there is a lower heat input into the material, thus reducing the disadvantages known from the prior art.

[0006] This object is achieved by a method having the features of claim 1. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims.

[0007] The method according to the invention is characterized by the following steps: Providing a first component of the fuel cell and providing at least one second component of the fuel cell, emitting a pulsed laser beam from a laser device onto a rotating, mirrored polygon wheel, whereby the laser beam forms a joining line from a plurality of overlapping, point-shaped and / or line-shaped joining points on the components.

[0008] This process not only makes it possible, for example, to join two (metallic) individual plates to form a bipolar plate, but also to permanently bond other components of a fuel cell. For example, the process makes it possible to simultaneously join the membrane electrode assembly to a bipolar plate or to an individual plate. The laser beam melts, for example, sealing material to such an extent that it develops adhesive properties and, upon cooling, bonds the individual components together. "Joining" preferably refers to welding the two components. Using the process according to the invention, a unit cell can also be permanently bonded to form a composite.

[0009] The joining process according to the invention is carried out using a new laser technology that utilizes the polygon wheel. The polygon wheel can serve as a high-speed scanner (preferably a high-speed galvano scanner) that rotates at a speed of between 1,000 and 12,000 revolutions per minute. If individual snapshots of the laser irradiation of the polygon wheel are captured in slow motion, it can be seen that, due to the use of a pulsed laser source and the temporally changing deflection of the beam from the outer surface of the polygon wheel, many individual points on the surface of the components are irradiated with the laser, creating the point-like and / or linear joining points that lead to the joining of the components. This method is particularly suitable for line-oriented, flat applications because it can significantly reduce process time.In addition, the use of the rotating polygon wheel allows for an overall reduction in heat input into the raw material. In other words, this method also allows for quasi-stationary machining of the components with reduced heat input.

[0010] It is not mandatory that rotationally symmetrical or completely circular points be created when creating the joints; thus, non-circular elliptical joints or joints consisting of short lines can also be created. To provide other spot geometries, such as rectangular or square spot geometries, for the point-shaped and / or linear joints, at least one beam-shaping element can be positioned in the laser beam path.

[0011] The mirrored polygon wheel preferably has a base surface formed as a regular polygon (e.g., polygon), a top surface corresponding to the base surface, and a lateral surface formed from mirrored rectangles connecting the base surface to the top surface. The polygon wheel is rotatably mounted about a rotation axis, wherein an electric drive unit is preferably present and configured to drive the polygon wheel in rotation about the rotation axis oriented perpendicular to the direction of incidence of the laser beams. When the laser beam strikes the mirrored rectangles of the lateral surface of the polygon wheel, it is reflected. Due to the rotation of the polygon wheel, a time-varying deflection occurs, so that a series of overlapping individual points and / or lines, which become the joining points, strike the components.

[0012] It is possible for the two components to be completely "scanned" in one dimension by deflecting the laser beam from the polygon wheel, thus joining them, in particular, welding them, completely in one dimension. Whether a single polygon wheel is sufficient to join the components, or whether a cascade of several polygon wheels with one or more laser devices is required, depends on factors such as the dimensions of the polygon wheel or its mirrored outer surfaces, the size of the components to be joined, and the distance between the polygon wheel and the components to be joined. Whether points or lines are created during the joining process depends, for example, on the width of the mirrored rectangles on the outer surface of the polygon wheel onto which the laser beam is directed and from which the laser beam is deflected.

[0013] The irradiation of at least one polygon wheel is carried out using the laser device, which is designed in particular as a pulsed laser. A laser that generates ultrashort light pulses with a maximum pulse duration of 10 -9< seconds can be used for this purpose. A picosecond laser with a pulse duration between 10 -9< and 10 -12< seconds can also be used. The use of a femtosecond laser with a pulse duration between 10 -12< and 10 -15< seconds is also possible. The length of the pulses can also be used to control the heat input into the material, with the heat input increasing with increasing pulse duration. If the pulse duration is particularly long, the laser device approximates a cw laser, where "cw" stands for "continuous wave" and means "a wave emitted at a constant time."If the pulse is sufficiently long, it is possible for the components to be completely scanned during a single pulse. The high-speed rotation of the polygon wheel allows multiple scans to occur within a single pulse. This also results in very little heat being introduced into the material due to the rapid process. The use of multiple laser devices is possible. The laser source of the laser device can be, for example, a gas laser, a semiconductor laser, or a solid-state laser.

[0014] In most cases, it is necessary for the components to be permanently bonded together along a desired two-dimensional contour. To create such a two-dimensional joining contour, it is advantageous if the components and the laser beam are moved relative to each other, whereby the relative movement creates a two-dimensional joining contour.

[0015] The relative movement between the two components and the laser beam impinging on them is generated, for example, by the advance of a transport device for transporting the components. This ensures that a series of individual point-like joining points is created to form the joining line. For example, several rows of overlapping point-like joining points can also be created, resulting in a particularly dense joining line.

[0016] In the present invention, the laser beam is directed onto the components essentially perpendicular to the plane in which the components extend. This creates weld seams, which can be continuous and also provide a reliable sealing function.

[0017] The use of an adjustable optical deflection device has proven advantageous, as it allows for the creation of joining lines that include a portion oriented perpendicular to the feed direction of the transport device. If a joining line is to be created that is oriented entirely perpendicular to the feed direction of the transport device, it is advantageous if the optical deflection device is adjustable or is adjusted at a speed that compensates for the feed speed of the transport device. This makes it possible to create straight joining lines even when the raw material or components are continuously transported by the transport device along the feed direction.

[0018] Analogous to an inkjet printer, it is also possible to generate the relative movement by adjusting a laser head of the laser device, which can be moved vertically relative to the feed direction, while driving the polygon wheel. This also makes it possible to "move" to various points on the components in order to irradiate them with the laser beam and to create a desired contour of a joining line from a plurality of overlapping point-like and / or linear joining points.

[0019] It is possible to fold and / or split the laser beam once or multiple times and thus to irradiate a plurality of polygon wheels with the laser light emitted by a single laser source, whereby a plurality of the plurality of point-shaped and / or linear joints on the components can be produced simultaneously.

[0020] In order to additionally ensure that the joining points are only point-like or with lines that are not too long on the components, it is advantageous if the joining points are created by a high-frequency clocked switching on and off of the laser device, in particular the associated laser source.

[0021] The present invention is not limited to the joining of components with prefabricated dimensions, but can also be used for manufacturing facilities using a continuous process, i.e., for strip-shaped raw material. A cutting device can be provided here, which is designed to separate the joined, strip-shaped raw material into individual components or to trim it to the final size. "Roll-to-roll" processing is also possible.

[0022] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 shows a sectional detailed view of a section of a fuel cell stack with a bipolar plate formed from two individual plates, Fig. 2 shows a schematic side view of a device for joining at least two components of a fuel cell, in particular for joining individual plates to form a bipolar plate, Fig. 3 shows a schematic detailed view of the surface of an already partially joined bipolar plate, and Fig. 4 shows a schematic view of the device from Figure 3 , in which the lines of the joining line are illustrated, whereby for the sake of clarity the bipolar plate is sketched in a top view and the device in a side view.

[0023] In Figure 1a section of a fuel cell stack can be seen, which is formed from several fuel cells 220. Each fuel cell 220 is formed with a membrane electrode assembly 222, which comprises a proton-conductive membrane, to which an electrode is assigned on each side. The membrane electrode assembly 222 is designed to carry out the electrochemical reaction of the fuel cell. A fuel (e.g. hydrogen) is fed to the electrode forming the anode, where it is catalytically oxidized to protons with the release of electrons. These protons are transported through the proton-conductive membrane (or ion exchange membrane) to the cathode. The electrons derived from the fuel cell flow via an electrical consumer, preferably via an electric motor for driving a vehicle, or to a battery. The electrons are then conducted to the cathode or electrons are made available at it.At the cathode, the oxidation medium (e.g. oxygen or oxygen-containing air) is reduced to anions by absorbing electrons, which react directly with the protons to form water.

[0024] With the help of bipolar plates 216, the fuel or the cathode gas is guided to gas diffusion layers 224, which diffusely distribute the respective gases to the electrodes of the membrane electrode assembly 222. The fuel, the oxidation medium, and optionally a cooling medium are guided through channels 208 of the bipolar plate 216, which are delimited on both sides by webs 206 of the bipolar plates 216 having web backs. As can be seen from the Figure 1 For this purpose, one set of the web backs rests on a gas diffusion layer 224 so that a reactant flowing in the channels 208 can be delivered to the gas diffusion layer 224 and thus to the electrode of the membrane electrode arrangement 222.

[0025] In this case, the bipolar plate 216 comprises two individual plates 200, 202 placed one on top of the other, which are selectively joined, in particular welded, to one another at their facing webs 206, in particular at their respective web backs. The facing webs 206 of the individual plates 200, 202 typically form conduits for a cooling medium with the channels 208 located between the webs 206.

[0026] Out of Figure 1 It is also apparent that the webs 206 or their web backs of the individual plates 200, 202 do not necessarily have to have the same width, so that different widths and / or depths can also be present for the channels 208. However, for a permanent connection of two individual plates 200, 202, it should be ensured that at least two of the opposing webs 206 rest on one another, which can be permanently connected to one another, namely joined, in particular welded.

[0027] In Figure 2A device 100 for joining at least two components of a fuel cell 218 is presented, which in the present case is designed in particular for joining two individual plates 200, 202 to form a bipolar plate 216.

[0028] This device 100 comprises a laser device 108, which has a laser source 106 for emitting a pulsed laser beam 110. The laser source 108 can be a gas laser, a semiconductor laser, or a solid-state laser, with particular use being made of a CO2 laser, an Nd:YAG laser, a semiconductor laser / diode laser, or a Yb:YAG laser. The individual components of the laser device 108 are controlled by a laser controller 104.

[0029] The laser source 106 directs the laser beam 110 onto a mirrored polygon wheel 114 driven by an electric motor to rotate. This mirrored polygon wheel 114 is configured to deflect the laser beam 110 very quickly and at a variety of angles depending on the angular position of the polygon wheel 114. The laser controller 104 can be configured to switch the laser device 108, in particular the laser source 106, on and off at high frequencies to additionally ensure that no overly long linear laser beams 110 are formed, which would lead to excessive heat input into the raw material, thus into the individual plates 200, 202 of the bipolar plate 216.

[0030] Furthermore, the laser device 108 has an adjustable optical deflection device 112 arranged downstream of the polygon wheel 114 in the beam path of the laser beam 110, which is configured to deflect the laser beam 110 depending on the feed rate of a transport device 116. The transport device 116 transports the individual panels 200, 202 in a feed direction 118. Two-dimensional joining contours can be realized by the feed of the transport device 116. In order to be able to provide any desired two-dimensional joining contours for the joining process, the laser device 108 can also be equipped with a laser head 102, which, as indicated by the diagonal double arrow shown next to the laser device 108, is electromotorically adjustable or "movable" perpendicular to the feed direction 118 of the transport device 116.

[0031] The operation of the device 100 is explained by Figure 3, which illustrates a top view of a surface 218 of the individual plates 200, 202. As soon as the transport device 116 has arranged or positioned the components of the fuel cell 218 relative to the laser device 108, the laser beam 110 is emitted onto the rotating, mirrored polygon wheel 114. Due to the rotation of the wheel, the laser beam 110 is deflected at a multitude of angles and forms a joining line 122 on the individual plates 200, 202 consisting of a multitude of overlapping, point-shaped and / or line-shaped joining points 120, whereby the two plates are permanently bonded to one another. The laser beam 110 can and should be focused onto the components so that a targeted melting of the material and joining of the components can be effectively brought about.In order to produce any desired joining contours, the plates are adjusted relative to the laser beam 110; either by the feed of the transport device 116 and / or by the adjustable laser head 102.

[0032] This sequential, single-point or single-line design allows only a small amount of heat to be introduced into the material when creating a continuous joining line 122, thus reducing heat-induced distortion in the component. If necessary, the point-shaped joining points 120 are traversed along a given line several times to create a closed or stronger joining line 122 from the overlapping joining points 120.

[0033] The Figure 3It can also be seen that the point-shaped and / or line-shaped joints 120 do not necessarily have to be circular. Therefore, the use of other advantageous spot geometries can also be considered, which can be generated, for example, by means of suitable beam-shaping elements. For example, the Figure 3 The joining line 122 shown at the very bottom is an overlapping series of rectangular, in particular square, point-shaped joints 120, which must have a lower degree of overlap than circular joints 120 in order to achieve the desired tightness of the joint.

[0034] In order to further accelerate the process for producing the joined composite, the optical deflection device 112 is used, which will be described with reference to Figure 4 will be discussed below. For the sake of clarity, the Figure 4the side view of the device 100, which is shown only in a simplified manner, and the top view of the surface 218 of the individual plates 200, 202, which are to be welded to form a bipolar plate 216.

[0035] The bipolar plate 216 is transported by the transport device 116 along the feed direction 118. If the optical deflection device 112 were not used, this would result in the dashed line of the joining line 122 formed from several point-shaped and / or line-shaped joining points 120. The optical deflection device 112, which is designed in particular as a mirror or a prism, is adjustable or pivotable like a scanner. The optical deflection device 112 can be adjusted depending on the feed speed of the transport device 116. In this way, the use of the optical deflection device 112 makes it possible to create a joining line 122 oriented perpendicular to the feed direction 118, as can be seen in the figure.For this purpose, the adjustment or deflection of the laser beam 110 preferably occurs at a speed that just compensates for the feed speed of the transport device 116. In this way, a joining line 122 can be created even when the bipolar plate 216 is transported by the transport device 116 in the feed direction 118.

[0036] The present invention therefore provides a device 100 and a method for joining at least two components of a fuel cell 218, which are distinguished from methods known from the prior art by a shortened cycle time. The device 100 and the method according to the invention are therefore suitable for series production and, compared to known methods and devices, reduce the proportion of rejects during production, in particular during the production of bipolar plates 216, due to the lower heat input into the (raw) material of the components. The joining lines 122 formed according to the invention ensure the necessary tightness and the required electrical contact, wherein, due to the lower heat input into the raw materials, no or reduced thermally induced distortion occurs, which could have a disadvantageous effect on subsequent production steps. LIST OF REFERENCE SYMBOLS:

[0037] 100Device 102Laser head 104Laser control 106Laser source 108Laser device 110Laser beam 112Deflection device (mirror / prism or similar) 114Polygon wheel (mirrored) 116Transport device 118Feed direction 120Joining point 122Joining line 200First single plate 202Second single plate 206Webs 208Channel 216Bipolar plate 218Outer surface 220Fuel cell 222Membrane electrode arrangement 224Gas diffusion layer

Claims

1. A method for joining at least two components of a fuel cell (218), in particular for joining two individual plates (200, 202) of a fuel cell (218) to form a bipolar plate (216), comprising the steps of: - providing a first component of the fuel cell (218) and providing at least a second component of the fuel cell (218), and characterized by the step of: - emitting a pulsed laser beam (110) from a laser device (108) onto a rotating, polygon mirrored wheel (114), whereby the laser beam (110) forms a joint line (122) from a plurality of overlapping, dot-shaped and / or line-shaped joints (120) on the components.

2. The method according to claim 1, characterized in that the components and the laser beam (110) are moved relative to one another, and in that a two-dimensional joining contour is formed by the relative movement.

3. The method according to claim 2, characterized in that the relative movement is generated by a feed of a transportation unit (116) for transporting the components.

4. The method according to claim 3, characterized in that in order to form the joint line (122), which comprises a portion oriented perpendicularly to the feed direction (118) of the transportation unit (116), the laser beam (110) partially expanded by the polygon wheel (114) is deflected by an adjustable optical deflecting device (112) as a function of the feed.

5. The method according to claim 4, characterized in that the optical deflecting device (112) for forming a joint line (122) oriented perpendicularly with respect to the feed direction (118) is adjusted at the components at a speed which compensates for the feed speed.

6. The method according to one of claims 1 to 5, characterized in that the relative movement is generated by adjusting a movable laser head (102) of the laser device (108) while carrying the polygon wheel (114).