Bipolar plate for electrochemical system and corresponding electrochemical system comprising such bipolar plate

Abstract

The invention relates to a bipolar plate for an electrochemical system, comprising a first separator and a second separator arranged one on top of the other, the first separator having a first electrochemically active surface for conducting a first fluid on a side facing away from the second separator; the second separator has a second electrochemically active surface for conducting a second fluid on a side facing away from the first separator; wherein the first electrochemically active surface and the second electrochemically active surface overlap each other and form an active region of the bipolar plate in the overlapping region, and wherein the separators are bonded together in the active region by means of a weld pattern having non-uniformly distributed welds.

Description

Technical Field

[0001] The invention relates to a separator for an electrochemical system and to an electrochemical system comprising at least one such bipolar plate. Background Art

[0002] Known electrochemical systems, such as fuel cell cell systems or electrochemical compressor systems, redox flow batteries and electrolyzers, usually include a large number of bipolar plates, which are arranged in a stack so that each two adjacent bipolar plates enclose an electrochemical cell. The bipolar plates usually include two separators, which are connected to each other along their rear sides facing away from the electrochemical cell. The bipolar plates can be used, for example, for the electrical contacting of electrodes of individual electrochemical cells (e.g. fuel cell cells) and / or for the electrical connection of adjacent cells (series connection of cells).

[0003] The separator of the bipolar plate may have a channel structure for supplying one or more mediums to the battery cell and / or for removing the medium. In the case of a fuel cell cell, the medium may be, for example, a fuel (e.g., hydrogen or methanol), a reaction gas (e.g., air or oxygen), or a coolant as a supplied medium, and a reaction product and a heated coolant as a removed medium. In addition, the bipolar plate can be used to conduct waste heat generated in the electrochemical battery cell, such as during the conversion of electrical or chemical energy in the fuel cell cell, and to seal various media or cooling channels from each other and / or from the outside. In the case of a fuel cell cell, the reaction medium is the fuel and the reaction gas, and the reaction medium is usually conducted on the mutually opposing surfaces of the individual plates, while the coolant is conducted between the individual plates. The bipolar plate generally has at least one or more through openings. Through the through openings, the medium and / or the reaction product can be fed to the electrochemical battery cell defined by the stacked adjacent bipolar plates, or to the cavity formed by the separator of the bipolar plate, or can be discharged from the battery cell or from the cavity. Electrochemical cells and in particular electrochemical cells of fuel cell cells, for example, can each include a membrane electrode assembly (or MEA) with a corresponding polymer electrolyte membrane (PEM) and electrodes. The MEA may also include one or more gas diffusion layers (GDL), which are generally oriented toward the separator and in particular toward the bipolar plate of the fuel cell system, and are, for example, in the form of carbon felt. In an example fuel cell cell, the hydrogen carried on the anode side and the oxygen carried on the cathode side are generally converted into water. The protons produced during the oxidation of the hydrogen pass from the anode side through the MEA to the cathode, where they react with oxygen and electrons to form water. The membrane of the fuel cell cell is electrically insulating and prevents electrical short circuits between the anode and the cathode. Electrons are directed from the anode to the cathode alone so that the current can be utilized.

[0004] Today, most fuel cell monomer systems are operated with an excess of oxidant. It has been found that in the electrochemically active area, the reaction occurs to a greater extent at points where the concentration of the oxidizing medium is particularly high than at other points. This may be the case, for example, near the point of entry of the oxidant into the electrochemically active area. In areas with a greater excess of oxidant, a more intense conversion of the reactants causes particularly high current densities. This leads to an uneven distribution of the current density over the active area. High reactant conversions cause high loads on the membrane, while the membrane is only subjected to low loads in areas of low material conversion. Overloaded areas have a negative impact on the service life of the MEA or its membrane. The disadvantage of low-load areas is the high area-related costs compared to the output, especially since the MEA is a relatively expensive component.

[0005] In other electrochemical systems, particularly other polymer membrane-based electrochemical systems, other factors may cause uneven loading on the polymer membrane surface. Summary of the invention

[0006] The object of the present invention is therefore to propose an improved bipolar plate and / or an improved electrochemical system, in particular to increase the service life of the bipolar plate and / or the service life of the MEA or polymer membrane arranged between the bipolar plates in a bipolar plate stack. Additionally or alternatively, the object may be to reduce the performance-dependent costs of the electrochemical system.

[0007] According to the invention, this object is solved by a bipolar plate according to claim 1 and an arrangement for an electrochemical system according to the independent claim. Further developments form the subject matter of the following description and of the dependent claims.

[0008] The proposed bipolar plate for an electrochemical system has a first separator and a second separator, which are arranged one above the other. The first separator has a first electrochemically active surface for conducting a first fluid on a side facing away from the second separator. The second separator has a second electrochemically active surface for conducting a second fluid on a side facing away from the first separator. The first electrochemically active surface and the second electrochemically active surface overlap each other and form an active area of ​​the bipolar plate in the overlapping area. The separators are bonded together in the active area by a weld pattern with unevenly distributed welds.

[0009] In this example, the term "first electrochemically active surface" is used to refer to the surface of the first separator opposite to the electrochemically active surface of the MEA when the separator is arranged in the electrochemical system. Similarly, the term "second electrochemically active surface" is used to refer to the surface of the second separator opposite to the electrochemically active surface of the MEA when the separator is arranged in the electrochemical system. In addition, it should be noted that when the present disclosure refers to the "overlapping" or "overlapping" of the first electrochemically active surface and the second electrochemically active surface, it means the overlap of the surfaces in the orthogonal projection. In the electrochemical system, the first electrochemically active surface can correspond in particular to the cathode side. The cathode side can, for example, have an oxygen conduction channel, i.e., in particular for conducting air or oxygen. The second electrochemically active surface can correspond in particular to the anode side. In particular, the anode side can have a hydrogen conduction channel, i.e., in particular for conducting hydrogen, methanol and / or another proton donor.

[0010] Surprisingly, a defined, non-uniform distribution of the weld seams enables a uniform current density or at least improves the uniformity of the current density over the electrochemically active area compared to a uniform distribution of the weld seams over the electrochemically active area. The weld seams can be distributed over the active area such that there are fewer weld seams in areas that would be overloaded areas if the weld seams were uniformly distributed. Additionally or alternatively, the weld seams can be distributed over the active area such that the surface density of the welds is increased in areas that would be underloaded areas if the weld seams were uniformly distributed.

[0011] The weld is preferably arranged in the region of the channel for conducting the first fluid and / or in the region of the channel for conducting the second fluid. A preferred region for arranging the weld is given in the contact region, which is the region in which the back side of the channel for conducting the first fluid rests, in particular in a flat manner, on the back side of the channel for conducting the second fluid. This results in a particularly durable and stable welded connection.

[0012] The distance between adjacent welds can be greater in the first region of the weld pattern than in the second region of the weld pattern. For example, the distance can be considered not only along the fluid channel, but also in all surface directions, ie, perpendicular to the stacking direction of the electrochemical system.

[0013] This makes it easy to achieve an uneven distribution of welds. The bipolar plate may have more than two regions, in particular whereby the welds within each region may have a constant distance and the distances between welds in different regions may be different.

[0014] In one embodiment, the first electrochemically active surface may have a first region and a second region. The welds may be arranged so that the surface density of the welds in the first region F1 is less than the surface density of the welds in the second region F2. This means that, due to the natural gradient of the reaction gas, the region that would otherwise react with a high current density can be turned into a less loaded region by local electrical throttling, more precisely, by local throttling of the electron flow between the two separators of the bipolar plate. Additionally or alternatively, the current density in the region that would otherwise react with a lower current density due to the natural gradient of the reaction gas can be increased by locally reducing the resistance.

[0015] The surface density of the weld can increase continuously or suddenly, for example, in regular or irregular jumps, downstream of the fluid flow. The second region can be arranged downstream of the first region, for example, downstream of the fluid flowing on the first surface of the cathode plate. For example, the surface density can be defined as the ratio of the product of the length of the weld seam and the number of weld seams to the product of the channel length, the channel spacing ("channel pitch") and the number of webs that span between the weld seams closest to each other and at right angles to the direction in which the channel extends. In a narrower sense, this is a ratio of length to area, because the width of the weld seam has a smaller effect on the electron flow in practice than its length and is generally constant for bipolar plates.

[0016] When the term "downstream fluid flow" is used, it particularly refers to the general flow direction pointing from the inlet port to the outlet port. One direction, particularly a local direction, of the fluid flow in a channel of the flow field may deviate from the general flow direction.

[0017] The ratio F1 / F2 between the area density of welds in the first region F1 and the area density of welds in the second region F2 may be less than 0.9, preferably less than 0.5. This enables a particularly advantageous, particularly uniform current density distribution across the electrochemically active area of ​​the bipolar plate.

[0018] In one embodiment, the surface proportion of the first region and / or the second region in the total area of ​​the active surface can be at least 5%, preferably at least 10%, preferably at least 15%, particularly preferably at least 25%. Additionally or alternatively, the first region and the second region can have the same area.

[0019] In one embodiment, the first electrochemically active surface may have a first region, a second region and a third region. As described above, the first region, the second region and the third region may be arranged one after another, for example in the flow direction of the first fluid. The weld pattern may be designed so that the surface density of the welds in the above-mentioned regions of the first electrochemically active surface deviates from each other by more than 5%, preferably more than 10%, and particularly preferably more than 20%. This may allow an advantageous, in particular more uniform current density distribution.

[0020] In one embodiment, only the second region, or the second region and the first region, or the second region and the third region may have a weld. Specifically, if the welds are evenly distributed, the region where the reaction gas will react with a high current density can be electrically throttled by providing fewer welds or no welds, so that the region is less loaded.

[0021] Within a channel or group of channels, the surface density of welds may first increase and then decrease again in the flow direction relative to the fluid flowing in the channel or relative to the fluid flowing on another partition in the channel. The gradient may be arranged at different points in different channels relative to the longitudinal extent of the flow field. Alternatively or additionally, the weld pattern formed by the total number of welds may vary in different areas of the flow field depending on the distance from the side edge.

[0022] The bipolar plate can be arranged to operate at a specific operating point of the fuel cell cell, wherein the specific operating point can include a specific concentration gradient of the first fluid along the first active surface. The weld pattern can be adapted to the concentration gradient of the first fluid, in particular such that the area density of welds in areas of the active surface with a relatively low concentration of the first fluid is relatively large, and the area density of welds in areas of the active surface with a relatively high concentration of the first fluid is relatively small.

[0023] Alternatively, the material flow on both gas sides can also be optimized. In areas where there is an excess of gas from both reactants, the lowest density welds can be provided. In areas where there is a shortage of at least one gas, a higher density weld can be provided.

[0024] The first separator may have a first inlet port for admitting a first fluid and a first outlet port for discharging the first fluid. The first inlet port may be fluidically connected to the first electrochemically active region via the first inlet region. The first outlet port may be fluidically connected to the first electrochemically active region via the first outlet region, in particular such that the first fluid may be conducted sequentially through the first inlet region, the first electrochemically active region, and the first outlet region.

[0025] The first dividing plate and the second dividing plate can be connected to each other by the other welding portion arranged in the first inlet region and / or the first outlet region.In addition, they can be closely welded together along their edge and at a distance from their edge.

[0026] In one embodiment, the bipolar plate can be designed so that the flow direction of the first fluid is opposite to the flow direction of the second fluid. This embodiment can be referred to as a countercurrent embodiment. The density of the welds in the second electrochemically active surface can decrease with the flow direction of the second fluid, while the density of the welds in the second electrochemically active surface can increase with the flow direction of the first fluid.

[0027] In one embodiment, the bipolar plate can be designed so that the flow direction of the first fluid is parallel to the flow direction of the second fluid. This embodiment can be called a direct current embodiment. The density of welds in the second electrochemically active surface can increase with the flow direction of the first fluid and the second fluid.

[0028] In most cases, an attempt is made to achieve the most uniform possible fluid distribution transversely to the flow direction by designing the distribution or collection areas. However, this is not always optimal due to the interdependence of the fluid channels of all three fluid systems, especially in the case of fuel cell monolithic bipolar plates. In this case, the most uniform possible current density and therefore the load on the membrane can be advantageously ensured by means of an uneven distribution of welds transversely to the flow direction, in particular perpendicular to the flow direction.

[0029] In order to protect structural weak points at the edges of the membrane, the areas with high current density can be moved from these edges to areas of the active area that are oriented slightly toward the center. For this purpose, fewer welds can be provided at the cathode inlet region and / or in the edge regions of the electrochemically active surface. This can lead to throttling of the electron flow in these regions. Many welds can be provided in the region of the cathode outlet to promote the electron flow.

[0030] The welds of the weld pattern may be spaced apart from the outer edge of the active area. The distance between each weld and the outer edge may be greater than 1 mm and / or greater than 2 mm and / or greater than 5 mm and / or greater than 8 mm and / or greater than 10 mm, in particular greater than 15 mm. Additionally or alternatively, at least 1 channel and / or at least 2 channels and / or at least 4 channels and / or at least 8 channels and / or at least 10 channels may be arranged between each weld and the outer edge.

[0031] The first electrochemically active surface may be at least partially surrounded by a first sealing element of the first separator. The weld may be spaced apart from the first sealing element. The minimum distance between each weld and the sealing element may be greater than 5 mm, preferably greater than 8 mm, particularly preferably greater than 10 mm. The sealing element may be designed as an elastomeric seal. Additionally or alternatively, the sealing element may be particularly designed as a chime seal. The minimum distance between each weld and the sealing element may be greater than 9 mm, preferably greater than 12 mm, particularly preferably greater than 14 mm, especially in the case of a chime seal.

[0032] Having fewer or no weld seams in the edge regions near the flange may be advantageous because separator springback at these points during production may cause misalignment between the anode and cathode, which may have a negative impact on weldability. Thus, in addition to or as an alternative to improved current density uniformity, fewer welds in the edge regions may facilitate production and / or improve the quality of the bipolar plates.

[0033] Another aspect of the present disclosure relates to an arrangement for an electrochemical system, comprising a bipolar plate as described above. The arrangement further comprises at least one membrane electrode assembly (MEA) having a frame-like reinforcement edge, wherein the MEA and the bipolar plate are stacked one on top of the other in a stacking direction, and an active area of ​​the first bipolar plate is enclosed by the reinforcement edge of the MEA in an orthogonal projection in a common plane.

[0034] In one embodiment, the weld pattern of the bipolar plate can be spaced at least 2 mm, at least 5 mm, at least 8 mm, at least 10 mm, or at least 15 mm from the reinforced edge in a direction perpendicular to the stacking direction. During operation of the arrangement as a fuel cell cell, the current density distribution of the bipolar plate can be uniform, or at least more uniform than the current density distribution of a similar bipolar plate having a uniform distribution of welds over the active area.

[0035] In the area of ​​the active area of ​​the bipolar plate where the density of welds is highest, thus in particular the second or third area of ​​the weld pattern, the length of the welds in the channel and / or the distance between the closest welds is advantageously shown to be between 0.05 mm and 3 mm, preferably between 0.1 mm and 1 mm, in particular between 0.25 mm and 0.55 mm. In this respect, it is particularly preferred that the length of the welds and their corresponding distances differ by at most 50%, in particular by at most 20%. In the area of ​​the highest density of welds, for example, up to four, preferably up to three, in particular up to two channels can be located between the two welds that are closest to each other on a line that extends orthogonally or transversely to the main extension direction of the channels, and therefore, at an angle greater than 0° and less than 90° to the main extension direction of the channels. Even in the second or third area, at least one channel without a weld can be arranged between two channels that include a weld. The closest welds do not necessarily need to be located on a line that extends orthogonally to the main extension direction of the channel, they can also be displaced in this direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Exemplary embodiments of separators and electrochemical systems are shown in the accompanying drawings and will be explained in more detail according to the following description. In the drawings:

[0037] Figure 1 schematically shows a perspective view of an electrochemical system including a plurality of separators or bipolar plates arranged in a stack;

[0038] Figure 2 Schematically shows the Figure 1 A perspective view of two bipolar plates of a system, wherein a membrane electrode assembly (MEA) is arranged between the bipolar plates;

[0039] Figure 3 Schematically shows the Figure 1 The system's approach to the cross-section of the system's plate stacking;

[0040] Figure 4 Schematically shows a top view of a bipolar plate with uniformly distributed welds according to the prior art;

[0041] Figure 5 schematically illustrates a top view of an exemplary bipolar plate having unevenly distributed welds;

[0042] Figure 6 schematically illustrates a top view of another exemplary bipolar plate having unevenly distributed welds;

[0043] Figure 7 schematically illustrates a top view of another exemplary bipolar plate having unevenly distributed welds;

[0044] Figure 8 schematically illustrates a top view of another exemplary bipolar plate having unevenly distributed welds;

[0045] Fig. 9 schematically illustrates a top view of another exemplary bipolar plate having unevenly distributed welds;

[0046] Fig.10 schematically illustrates a top view of another exemplary bipolar plate having unevenly distributed welds;

[0047] Fig.11 schematically illustrates a top view of a portion of an active area of ​​another exemplary bipolar plate having unevenly distributed welds; and

[0048] Fig.12 Fig.12 A schematic top view of a section of a second or third region of the active area of ​​another exemplary bipolar plate is shown.

[0049] Here and hereinafter, features which occur repeatedly in the different figures are in each case denoted by the same or similar reference numerals. In particular, the representation of the welds is greatly simplified and not drawn to scale. DETAILED DESCRIPTION

[0050] Figure 1 An electrochemical system 1 is shown, which comprises a plurality of structurally identical metal bipolar plates 2, which are arranged in a stack 6 and stacked in the z-direction 7. The bipolar plates 2 of the stack 6 are sandwiched between two end plates 3, 4. The z-direction 7 is also referred to as the stacking direction. In the present example, the system 1 is a stack of fuel cell cells. Two adjacent bipolar plates 2 of the stack in each case enclose an electrochemical cell cell between them, which is used, for example, to convert chemical energy into electrical energy. In order to form an electrochemical cell cell of the system 1, a corresponding membrane electrode assembly (MEA) is arranged between adjacent bipolar plates 2 of the stack (see, for example, Figure 2 ). Each MEA generally includes at least one membrane, such as an electrolyte membrane. A gas diffusion layer (GDL) may also be arranged on one or both surfaces of the MEA ( Figure 1 and Figure 2 (not shown in the figure).

[0051] In alternative embodiments, the system 1 can also be designed as an electrolyzer, an electrochemical compressor or a redox flow battery. The bipolar plates can also be used in these electrochemical systems. The structure of these bipolar plates can then correspond to the structure of the bipolar plates 2 described in detail herein, although in the electrolyzer, electrochemical compressor or redox flow battery, the medium fed onto and / or passing through the bipolar plates can in each case differ from the medium used for the fuel cell monomer system.

[0052] The z-axis 7 together with the x-axis 8 and the y-axis 9 constitute a right-handed Cartesian coordinate system. The bipolar plates 2 each define a plate plane, whereby the plate planes of the separators 2a, 2b are each aligned parallel to the xy plane and thus perpendicular to the stacking direction or z-axis 7. The end plate 4 comprises a plurality of media connections 5 via which media can be fed to the system 1 and via which media can be discharged from the system 1. These media that can be supplied to the system 1 and carried out of the system 1 can include fuels such as molecular hydrogen or methanol, reaction gases such as air or oxygen, reaction products such as water vapor, or spent fuel or coolant such as water and / or ethylene glycol.

[0053] Figure 2 The stereogram shows Figure 1 Two adjacent bipolar plates 2 of an electrochemical system of the type in the system 1, and a membrane electrode assembly (MEA) 10 known in the prior art arranged between these adjacent bipolar plates 2, Figure 2 The MEA 10 in the embodiment is largely obscured by the bipolar plate 2 facing the viewer. The bipolar plate 2 is formed by two separators 2a, 2b joined together by a material connection (see for example Figure 3),exist Figure 2 Only the first partition 2a facing the observer is visible, and the first partition hides the second partition 2b. The partitions 2a, 2b can each be made of metal, for example, from a stainless steel sheet. For example, the partitions 2a, 2b can be welded to each other, for example, by laser welding. On the one hand, the partitions 2a, 2b can be tightly welded along their outer edges and at a certain position away from their outer edges, or at least along the inner edges of a part of the through opening of the sealing seam 60, in particular outside and / or inside the peripheral flange 12d. Other welded seams are also possible, which will be discussed below.

[0054] The separators 2a, 2b have through openings that form through openings 11a-11c of the bipolar plate 2 and are aligned with each other. When a plurality of bipolar plates of the same type as the bipolar plate 2 are stacked, the through openings 11a-11c form pipelines extending through the stack 6 in the stacking direction 7 (see Figure 1 ). Typically, each of the lines formed by the through-openings 11a-11c is fluidically connected to one of the media ports 5 in the end plate 4 of the system 1. By means of the lines formed by the through-openings 11a, for example, a coolant can be introduced into the stack or discharged from the stack. Conversely, the lines formed by the through-openings 11b, 11c can be configured to supply fuel and reaction gases to the electrochemical cells of the fuel cell cell stack 6 of the system 1 and to discharge reaction products from the stack. The through-openings 11a-11c for medium conduction are substantially parallel to the plate plane.

[0055] In order to seal the through-openings 11a-11c from the interior of the stack 6 and from the surroundings, the first partitions 2a each have a sealing arrangement in the form of a sealing bead 12a-12c, which is each arranged around the through-opening 11a-11c and each completely surrounds the through-opening 11a-11c. Figure 2 On the rear side of the viewing device, there are corresponding sealing beads (not shown) for sealing the through-openings 11a-11c. Alternatively, an elastomeric seal may be used.

[0056] In the electrochemically active region 18, the first separators 2a are disposed on their facing sides. Figure 2 On the outside of the observer, there is a flow field 17 with a structure for guiding the reaction medium along the outside of the partition 2a. Figure 2 In the embodiment, these structures are provided by a plurality of webs and channels running between and defined by these webs. Figure 2On the outer side of the observer, the first partition 2a also each has a distribution and collection area 20. The distribution or collection area 20 in each case includes structures that are configured to distribute the medium introduced into the distribution area 20 from the first of the two through-openings 11b over the entire active area 18, or to collect or combine the medium flowing from the active area 18 toward the second through-opening 11b. The fluid guiding structures 29 of both the distribution or collection areas 20 are also located in Figure 2 The distribution area 20 is provided by webs and channels running between the webs and bounded by the webs. In the following, for simplicity, only the distribution area 20 will be discussed; a corresponding description also applies to the collection area 20.

[0057] The sealing ridges 12a-12c have passages 13a-13c, wherein the passage 13a is formed on both the lower side of the upper partition 2a and the upper side of the lower partition 2b, while the passage 13b is formed in the upper partition 2a, and the passage 13c is formed in the lower partition 2b. For example, the passage 13a is designed as a local bulge of the ridge, allowing the coolant to pass between the through-opening 12a and the distribution area 20, allowing the coolant to enter the distribution area between the partitions, or to be led out from the collection area 20. In addition, the passage 13b allows hydrogen to pass between the through-opening 12b and the distribution area on the upper side of the upper partition 2a; these passages 13b are characterized by perforations facing the distribution area and running at an angle to the plate plane. For example, hydrogen flows through the passage 13b from the through-opening 12b to the distribution area on the top of the partition 2a, or flows from the collection area in the opposite direction. The passage 13c allows, for example, air to pass between the through-opening 12c and the distribution area, allowing the air to enter the distribution area on the underside of the partition 2b located below or to be led out of the collection area. The relevant perforations are not visible here.

[0058] The first partitions 2a also each have a further sealing arrangement in the form of a peripheral lip 12d, which surrounds the flow field 17 of the active area 18, the distribution and collecting area 20 and the through-openings 11b, 11c and seals them to the through-opening 11a, i.e. to the coolant circuit, and to the environment of the system 1. The second partitions 2b each include a corresponding peripheral lip. The structure of the active area 18, the distribution structure of the distribution and collecting area 20 and the sealing lip 12a-12d are each formed in one piece with the partition 2a and are molded into the partition 2a in a embossing process or deep drawing process or by means of hydraulic forming. The same applies to the corresponding structure of the second partition 2b.

[0059] The two through-openings 11b or the lines formed by the through-openings 11b through the plate stack of the system 1 each lead via a passage 13b in the sealing collar 12b, via the distribution structure of the distribution or collecting area 20 and via the first partition 2a towards Figure 2In a similar manner, the two through-openings 11c or the lines formed by the through-openings 11c through the plate stack of the system 1 in each case lead away from the plate stack via corresponding flange passages, via corresponding distribution and collection structures and via the second partition plate 2b. Figure 2 The through-openings 11a or the lines formed by the through-openings 11a through the plate stack of the system 1 are fluidically connected to each other via cavities 19 enclosed or surrounded by the separators 2a, 2b. Each cavity 19 is used to guide a coolant through the bipolar plate 2, in particular to cool the electrochemically active area 18 of the bipolar plate 2.

[0060] Figure 3 Schematically shows the Figure 1 The section plane is oriented in the z-direction and is therefore perpendicular to the plate plane of the bipolar plate 2. Figure 3 In the example, the section plane runs along the curved section, i.e. along Figure 2 The section line AA in .

[0061] The stacked bipolar plates 2 of identical construction each comprise a first metal separator 2a described previously and a second metal separator 2b described previously. Structures for guiding the medium along the outer surface of the separator 2 are apparent, these structures being here in particular in each case in the form of webs and channels defined by the webs. In particular, channels on the surfaces of adjacent separators 2a, 2b pointing away from each other, as well as cooling channels in the cavity 19 between adjacent separators 2a, 2b are shown. Adjacent to the sealing ridge 12d and between the cooling channels in both the distribution or collection area 20 and the active area 18, the two separators 2a, 2b are superimposed on each other in the contact area 24 and are connected to each other there, in this case by means of laser welded seams 60, 70, 50.

[0062] A corresponding membrane electrode assembly (MEA) 10, such as is known from the prior art, is arranged between adjacent separators 2 of the stack. The MEA 10 generally comprises a membrane, such as an electrolyte membrane, and an edge portion 15 connected to the membrane 14. For example, the edge portion 15 may be bonded to the membrane 14, such as by an adhesive connection or by lamination.

[0063] In the present case, the term "first electrochemically active surface" is used to denote the surface of the first separator 2a, 2b which is opposite to the electrochemically active surface of the MEA 10 when the separator 2a is arranged in the electrochemical system. The membrane of the MEA 10 extends in each case at least over the active area 18 of the adjacent separator 2 and enables the transfer of protons across or through the membrane 14. The actual active area 18 is limited by the edge section 15 of the MEA. This means that the membrane does not extend into the distribution or collection area 20. The edge section 15 of the MEA 10 serves in each case to position, fasten and seal the membrane between the adjacent separators 2.

[0064] The edge portion 15 in each case covers the distribution or collection area 20 of the adjacent bipolar plate 2. The edge portion 15 can also extend outwards beyond the peripheral lip 12d and adjoin or protrude beyond the outer edge area of ​​the separators 2a and 2b (see Figure 2 ).

[0065] Furthermore, a gas diffusion layer 16 can be additionally arranged in the active region 18. The gas diffusion layer 16 can guide the flow to the membrane over as large an area of ​​the surface of the membrane as possible and can thus improve the proton transfer through the membrane. The gas diffusion layer 16 can, for example, be arranged on both sides of the membrane in the active region 18 at least between adjacent bipolar plates 2. The gas diffusion layer 16 can, for example, be formed by a fiber mat or include a fiber mat.

[0066] Although in Figure 2 In FIG. 1 , the outer side of the bipolar plate 2 carrying hydrogen, ie the anode plate, faces the viewer during operation, but the following illustrations show that the outer side of the bipolar plate 2 carrying oxygen, ie the cathode plate, faces the viewer.

[0067] Figure 4 Shows Figure 1 1 is a plan view of a bipolar plate 2 of an electrochemical system of the type of system 1 in FIG. The bipolar plate has a plurality of welds 50, of which only some are marked with reference numerals for the sake of clarity. The plurality of welds 50 results in a weld pattern. In the prior art, the welds 50 are uniform, i.e., uniformly distributed over the Figure 4 The active area 18 of the bipolar plate 2 of FIG. Such a uniform distribution of the weld seams may have the disadvantage that the current density during operation of the bipolar plate is not uniformly distributed over the active area due to the natural gradient of the reaction gases. This may lead to overloaded areas and thus to a shortened service life of the MEA or its membrane and / or to underloaded areas with reduced performance.

[0068] In contrast, the present invention provides for a non-uniform distribution of welds 50 in the active area 18, such as Figures 5 to 11 as shown in the example.

[0069] Figure 5 Shows Figure 1A plan view of a bipolar plate 2 of an electrochemical system of the type system 1 is shown in FIG. Figure 5 In the embodiment of the bipolar plate shown, the bipolar plate is designed so that the flow direction of the first fluid is parallel to the flow direction of the second fluid. In the image plane shown, the first fluid flows from left to right. The surface density of the weld in the first electrochemically active surface increases with the flow direction of the first fluid. The active area 18 is defined by its outer edge 18'. Specifically, no weld 50 is located in the first region I. In the second region II, the surface density F1 of the weld 50 is less than the surface density F2 of the weld in the third region III. The ratio F1 / F2 between the surface density F1 of the weld in the second region II and the surface density F2 of the weld in the third region III is less than 0.9, preferably less than 0.5. This allows a particularly advantageous and particularly uniform current density distribution across the electrochemically active area 18 of the bipolar plate. In the second region II, two rows of welds running along the y direction are provided in this example. The welds each extend in the x direction. The welds 50 of the first row are located in the same channel as the welds 50 of the second row. For all welds 50 in the first row, the distance between the weld 50 in the first row and the nearest weld 50 in the second row is the same. Between the channels in which the welds 50 are provided, there is at least one channel in which no weld 50 is provided. In the third region III, in this example, four rows of welds 50 are provided in the y direction. The welds 50 each extend in the x direction. The welds 50 in the first row are located in the same channel as the welds in the third row. The welds 50 in the second row are located in the same channel as the welds in the fourth row. For all welds 50 in the corresponding rows, the distance between the welds 50 in one row and the nearest weld 50 in the next row is the same. At least two welds 50 are arranged in each channel of the parallel channels of the active area 18. Since the welds 50 are arranged in this way, when in accordance with Figure 1-3 When used in an electrochemical system, Figure 5 The current density ratio of the bipolar plate shown in Figure 4 In the first region I, the Figure 4 Compared with the current density of the bipolar plate in the third region, the current density can be reduced. Figure 4 Compared with the current density of the bipolar plates in the reactor, the current density can be increased.

[0070] Figure 6 Shows Figure 1 A plan view of a bipolar plate 2 of an electrochemical system of the type system 1 is shown in FIG. Figure 6In the embodiment of the bipolar plate shown, the bipolar plate is designed so that the flow direction of the first fluid is opposite to the flow direction of the second fluid. In the image plane shown, the first fluid flows from left to right. The second fluid flows from right to left. In each case, the reverse fluid direction is also possible. The density of the weld in the first electrochemically active surface first increases with the flow direction of the first fluid and then decreases again. Specifically, no weld 50 is located in the two first regions I. In the two second regions II located between the two first regions, the surface density F1 of the weld 50 is less than the surface density F2 of the weld in the third region III. The third region III is located between the two second regions II. Regions II and III are arranged centrally between regions I. The ratio F1 / F2 between the surface density F1 of the weld in the second region II and the surface density F2 of the weld in the third region III is less than 0.9, preferably less than 0.5. This makes the current density distribution across the electrochemically active region 18 of the bipolar plate 2 particularly advantageous and uniform when the bipolar plate 2 is operated in reverse flow. In this example, in each second area II, two rows of welds 50 are arranged running along the y direction, and each weld 50 extends in the x direction. The welds 50 of the first row are located in the same channel as the welds 50 of the second row of the corresponding second area. For all welds 50 of the first row, the distance between the welds 50 of the first row and the nearest welds 50 of the second row is the same. Between the channels in which the welds 50 are arranged, there are three channels in which no welds 50 are arranged. In the third area, three rows of welds 50 are arranged in the y direction in this example. Each weld 50 extends in the x direction. All three rows of welds 50 are located in the same channel. The distance between the first row of welds 50 of the third area and the second row of welds of the second area II is the same as the distance between the first row of welds 50 of the second area II and the second row of welds of the second area II. Since the welds 50 are arranged in this way, when according to Figure 1-3 When used in an electrochemical system, Figure 6 The current density ratio of the bipolar plate shown in Figure 4 In the first region I, the Figure 4 Compared with the current density of the bipolar plate in the third region, the current density can be reduced. Figure 4 The current density may be increased compared to the current density of the bipolar plate in FIG. 1 . In the second region, the weld 50 is spaced apart from the outer edge 18 ′ of the active area 18 .

[0071] Figures 7 to 10 With Figures 4 to 6 The top view of the bipolar plate 2 is shown in a greatly simplified form. In particular, the weld 50 is not drawn to scale. It should also be noted that the arrangement of the through openings in Figures 4 to 5 is not shown in Figures 4 to 5. Figure 6 and Figure Group 7 to Fig.10 There are some differences between them.

[0072] Figure 7 Shows Figure 1 FIG. 1 is a top view of another example of a schematic depiction of a bipolar plate 2 of an electrochemical system of the type system 1 . Figure 7 In the embodiment of the bipolar plate shown, the bipolar plate is designed so that the flow direction of the first fluid is parallel to the flow direction of the second fluid. Although no weld 50 is provided in the first region I, the surface density of the weld 50 gradually increases in the flow direction in the second region. No weld 50 is provided in the edge region of the active region 18 to improve productivity and reduce stress on the outer region of the active surface of the membrane. At least 2 channels are arranged between each weld 50 and the outer edge 18' of the active region 18. The distance between each weld and the outer edge of the bipolar plate 2 is greater than 15 mm. The distance between each weld and the peripheral flange 12d is greater than 10 mm.

[0073] Figure 8 Shows Figure 1 A top view of another example of a schematic bipolar plate 2 of an electrochemical system of the type of system 1 in FIG. Figure 8 In the embodiment of the bipolar plate 2 shown, the bipolar plate is designed so that the flow direction of the first fluid is parallel to the flow direction of the second fluid. Although only two welds 50 arranged up and down along the y-direction are provided in the first region I, four welds 50 arranged centrally up and down are provided in the second region II. In the third region III, two rows are provided, each row having six welds 50 arranged centrally up and down. The fourth region IV corresponds to the third region III, wherein the two middle welds of each row are omitted. The fifth region V corresponds to the fourth region IV, wherein the two middle welds 50 of each row are again omitted. This produces a weld pattern similar to the "<" symbol in the top view shown. The distance between each row of weld seams 50 running along the y-direction increases from the center toward the upper outer edge and the lower outer edge, "upper" and "lower" refer to Figure 8 The distance between the weld seams 50 of the row running in the x-direction decreases from left to right, “left” and “right” see Figure 8 At least four channels are arranged between each welding portion 50 and the outer edge 18 ′ of the active area 18 .

[0074] Fig. 9 Shows Figure 1 FIG. 1 is a top view of another example of a schematic bipolar plate 2 of an electrochemical system of the type of system 1 in FIG. Arrow A marks the concentration gradient of the fluid flowing from the through hole 11c (located at Fig. 9 The upper left part of the active area 18 is guided into the distribution area to the through hole 11b (located at Fig. 9). An exemplary weld pattern has four rows of welds 50 running in the x-direction. The first three rows are offset from each other and arranged parallel to the x-axis. The fourth row corresponds to the third row and is arranged above the third row. Within the first and second rows, the distance between welds decreases unevenly. The number and spacing of welds 50 in the first and second rows are different from each other. Within the third and fourth rows, the distance between welds initially decreases, then increases again, and then decreases again. Fig. 9 The weld pattern in FIG. 5 has welds 50 only in the upper 2 / 3 of the active area 18. If, on the one hand, the lowest area of ​​the active area is undersupplied with dielectric, while the upper area is well supplied everywhere, and on the other hand, the middle area (relative to the height) has a lower than average current density at least in the downstream part of the fluid flow, without countermeasures, then Fig. 9 The arrangement of the welds shown may be particularly advantageous. In areas without weld seams, the current density during operation of the bipolar plate 2 can be lower than Figure 4 The current density during operation of the bipolar plate is shown. The current density can be increased in the areas with welds 50. The distance between each weld 50 and the outer edge 18' of the active area 18 is at least 10 mm.

[0075] Fig.10 Shows Figure 1 FIG. 1 is a top view of another example of a schematic bipolar plate 2 of an electrochemical system of the type of system 1 in FIG. Arrow A marks the flow direction of the fluid, which flows from the through hole 11c (located at Fig.10 The upper left part of the active area 18 flows into the distribution area and reaches the through hole 11c (located at Fig.10 First, an embodiment of a bipolar plate 2 is manufactured, which is provided with a Figure 4 Similar evenly spaced weld joints. Based on this, a current density distribution is determined, which shows that the current density in the upper left area of ​​the active area 18 is higher than in other areas. In addition, it is found that the current density distribution in the left area is higher than that in the right area. In addition, two main horizontal areas at a certain distance from each other in the lower third show too high current density. Based on this result, for the current panel 2, the original weld pattern is only reused in the rectangular area in the upper right part. The remaining distribution or surface density of the weld seam comes from the previously obtained measurements and compensates for the excessive current density through its uneven distribution. The distance between each weld 50 and the outer edge 18' of the active area 18 is at least 5 mm.

[0076] Fig.11 A cross section of the active area 18 of the bipolar plate 2 is shown. In order to ensure sufficient contact surface for welding, in contrast to the previous embodiment examples, the tube base or channel base is widened in the region of the weld 50 (shown here in white) compared to the adjacent tube section. Fig.11 Also shown are parameters from which the surface density can be derived. For example, this can be defined as the ratio of the product of the weld seam length LS and the number of weld seams to the channel length, the channel spacing ("channel pitch") KP and the number of webs 80 spanning between the two closest weld seams, the number of webs 80 being four in this example on a line extending at right angles to the channel extension direction, and two in this example on a line extending transversely to the channel extension direction, and therefore at an angle greater than 0° but less than 90° to the direction in which the channel extends. The surface density determined in this way is in a narrower sense the ratio of length to area, because the width of the weld seam has a smaller effect on the electron flow in practice than its length, and is generally constant for bipolar plates. The pipe length can be measured as a whole, but it can also be determined by the distance DP between the weld seams or the sum of the weld length LS in the main direction of the pipe extension and the distance DP, expressed as LP, and the number of welds. As an alternative to determining the channel pitch, the distance between the welds transverse to the main direction of the channel extension, such as AS, can also be directly measured.

[0077] Fig.11 It is also shown that the surface density of the weld can be changed by the distance of the weld in the direction of the channel extension, as shown in the comparison of regions I and II. On the other hand, it can be changed via the length of the weld seam, as shown in the comparison of the first two regions I, II and region III. In the region IV framed by the dotted line, the surface density of the weld is not uniform, but shows the ratio of regions I, II and III described above on the horizontal route. However, since the weld joint 50 is not present only in every two channels, but in every channel, it is achieved here that the surface density of the weld on the vertical route is twice that outside the region IV framed by the dotted line.

[0078] Fig.12 A top view of a cross section of the active area of ​​another exemplary bipolar plate is schematically shown. The figure shows a second or third area of ​​the active area, in which a relatively high density of weld seams is provided. The weld 50 is shown here with a length LS, for example 0.25 mm, which substantially corresponds to the distance between the end of the weld and the start of the nearest weld in the same channel 95. As in the previous example, the channel 90 without a weld is arranged between the two welds 50 closest to each other in a direction transverse to the main extension direction of the channel 90. When considering the welds 50 closest to each other on a line extending at right angles to the main extension direction of the channel, three channels and four webs are arranged between these welds. Among these channels, there are two channels 90 without welds and one channel 95 with welds.

Claims

1. A bipolar plate (2) for an electrochemical system (1), comprising a first separator (2a) and a second separator (2b) arranged in a stacked manner, wherein The first separator (2a) has, on a side facing away from the second separator (2b), a first electrochemically active surface for conducting a first fluid; The second separator (2b) has a second electrochemically active surface for conducting a second fluid on a side facing away from the first separator; in, The first electrochemically active surface and the second electrochemically active surface overlap each other and form an active area (18) of the bipolar plate (2) in the overlapping area, The separators (2a, 2b) are bonded together in the active area (18) by a weld pattern having unevenly distributed welds (50).

2. The bipolar plate according to the preceding claim, characterized in that The distance between adjacent welds (50) in the first region of the weld pattern is greater than the distance between adjacent welds (50) in the second region of the weld pattern.

3. The bipolar plate according to any one of the preceding claims, characterized in that The first electrochemically active surface comprises a first region and a second region, The distribution of the welding parts (50) is such that the surface density F1 of the welding parts (50) in the first region is smaller than the surface density F2 of the welding parts (50) in the second region.

4. The bipolar plate according to the preceding claim, characterized in that A ratio F1 / F2 between the surface density F1 of the welding portion (50) in the first region and the surface density F2 of the welding portion (50) in the second region is less than 0.9, preferably less than 0.

5.

5. The bipolar plate according to any one of the two preceding claims, characterized in that The surface proportion of the first area and / or the second area in each case is at least 5%, preferably at least 10%, preferably at least 15%, particularly preferably at least 25% of the total area of ​​the active surface, and / or the first area and the second area have the same area.

6. The bipolar plate according to any one of the preceding claims, characterized in that The first electrochemically active surface comprises a first region, a second region and a third region, wherein the second region is arranged between the first region and the third region relative to the flow direction of the fluid, The weld pattern is configured such that the surface densities of the welds (50) in the respective regions of the first electrochemically active surface deviate from each other by more than 5%, preferably more than 10%, and particularly preferably more than 20%.

7. The bipolar plate according to the preceding claim, characterized in that Only the second region, or the second region and the first region, or the second region and the third region have the welded portion (50).

8. A bipolar plate according to any one of the preceding claims, characterized in that The weld (50) is arranged in the region of a channel for conducting a first fluid and / or in the region of a channel for conducting a second fluid.

9. A bipolar plate according to any one of the preceding claims, characterized in that The welds (50) of the weld pattern are spaced apart from the outer edge of the active area (18), wherein the distance between each weld and the outer edge is greater than 2 mm, preferably greater than 5 mm, more preferably greater than 8 mm, more preferably greater than 10 mm, and particularly greater than 15 mm.

10. A bipolar plate according to any one of the preceding claims, characterised in that The welding part (50) of the welding part pattern is spaced apart from the outer edge of the active area, wherein at least 1 channel and / or at least 2 channels and / or at least 4 channels and / or at least 8 channels and / or at least 10 channels are arranged between each welding part (50) and the outer edge.

11. A bipolar plate according to any one of the preceding claims, characterized in that The first electrochemically active surface is at least partially surrounded by a first sealing element of the first separator, and the weld (50) is spaced apart from the first sealing element, wherein the minimum distance between each weld and the sealing element is greater than 5 mm, preferably greater than 8 mm, and particularly preferably greater than 10 mm.

12. A bipolar plate according to any one of the preceding claims, characterised in that The bipolar plate is adapted to operate at a specific operating point of a fuel cell cell, wherein the specific operating point includes a specific concentration gradient of the first fluid along the first active surface, Wherein, the welding portion pattern adapts to the concentration gradient of the first fluid, so that the surface density of the welding portion (50) in the area of ​​the active area with a relatively low concentration of the first fluid is relatively large, and the surface density of the welding portion (50) in the area of ​​the active area with a relatively high concentration of the first fluid is relatively small.

13. A bipolar plate according to any one of the preceding claims, characterised in that The first partition includes a first inlet port for letting the first fluid in and a first outlet port for discharging the first fluid, wherein the first inlet port is fluidly connected to the first electrochemically active region via a first inlet region, and the first outlet port is fluidly connected to the first electrochemically active region via a first outlet region, so that the first fluid can sequentially pass through the first inlet region, the first electrochemically active region, and the first outlet region, The first separator and the second separator are connected to each other via a further weld (50) arranged in the first inlet region and / or the first outlet region.

14. The bipolar plate according to any one of the preceding claims, wherein the bipolar plate is designed such that a flow direction of the first fluid is opposite to a flow direction of the second fluid, wherein: The surface density of the welds (50) in the second electrochemically active surface decreases along the flow direction of the second fluid, and the surface density of the welds (50) in the second electrochemically active surface increases along the flow direction of the first fluid.

15. A bipolar plate according to any one of the preceding claims, characterised in that The structure is such that the flow direction of the first fluid is parallel to the flow direction of the second fluid, wherein the surface density of the welding parts (50) in the second electrochemically active surface increases along the flow directions of the first fluid and the second fluid.

16. A bipolar plate according to any one of the preceding claims, characterised in that The first separator is a cathode plate and the second separator is an anode plate, wherein the first fluid is preferably oxygen or air, and / or the second fluid is preferably hydrogen, methanol or another proton donor.

17. A bipolar plate according to any one of the preceding claims, characterised in that In the channel, at least in the section, in particular in the second area or the third area of ​​the weld pattern, the length of the weld and / or the distance between the welds closest to each other is between 0.05 mm and 3 mm, preferably between 0.1 mm and 1 mm, in particular between 0.25 mm and 0.55 mm.

18. A bipolar plate according to any one of the preceding claims, characterised in that In a section, in particular in the second or third area of ​​the weld pattern, a maximum of four channels, preferably a maximum of three channels, in particular a maximum of two channel areas are located between two welds that are closest to each other on a line extending orthogonally or transversely to the main extension direction of the channel.

19. An arrangement for an electrochemical system, comprising a bipolar plate according to any one of the preceding claims and at least one membrane electrode assembly (MEA) with a frame-like reinforcing edge, wherein the membrane electrode assembly and the bipolar plate are arranged superimposed on each other in a stacking direction, and the active area of ​​the first bipolar plate is enclosed by the reinforcing edge of the membrane electrode assembly in an orthogonal projection in a common plane.

20. Arrangement according to the preceding claim, characterised in that The weld pattern of the bipolar plate is spaced apart from the reinforcement edge in a direction perpendicular to the stacking direction by at least 2 mm, at least 5 mm, at least 8 mm, at least 10 mm, or at least 15 mm.

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