Bipolar plate for an electrochemical system

Abstract

Bipolar plate (2) for an electrochemical system (1), comprising a first separator plate (2a) and a second separator plate (2a) arranged one above the other, wherein - the first separator plate (2a) has embossed ribs (41a) and channels (42a) on one side facing away from the second separator plate (2b) for guiding a first fluid; - the second separator plate (2b) has embossed ribs (41b) and channels (42b) on the side facing away from the first separator plate (2a) for guiding a second fluid; - the first and second separator plates (2a, 2b) each have webs (43a, 43b) and channels (44a, 44b) on their facing sides for guiding a cooling medium along an inner side of the bipolar plate (2); wherein at least one of the separator plates (2a, 2b) has structured web areas (80) on at least one side of the separator plate, wherein the structured web areas (80) have at least periodic surface structures (8) with a mean spatial period (Px, Py) of less than 10 µm, where a proportion of structured web areas is inhomogeneously distributed across the entire web surface.

Description

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

[0002] Known electrochemical systems, such as fuel cell systems or electrochemical compressor systems, redox flow batteries, and electrolyzers, typically comprise a plurality of bipolar plates arranged in a stack, so that each two adjacent bipolar plates enclose an electrochemical cell. The bipolar plates typically each comprise two separator plates that are connected to one another along their rear sides facing away from the electrochemical cells. The bipolar plates can serve, for example, to electrically contact the electrodes of the individual electrochemical cells (e.g., fuel cells) and / or to electrically connect adjacent cells (series connection of the cells). Particularly in electrolyzers, however, bipolar plates can also comprise only one separator plate; in this case, separate cooling is often not necessary.

[0003] The separator plates of the bipolar plates can have channel structures for supplying the cells with one or more media and / or for transporting media away. In the case of fuel cells, the media can be, for example, fuels (e.g., hydrogen or methanol), reaction gases (e.g., air or oxygen), or a coolant as supplied media, and reaction products and heated coolant as removed media. Furthermore, the bipolar plates can be used to conduct the waste heat generated in the electrochemical cell, such as that generated during the conversion of electrical or chemical energy in a fuel cell, and can be designed to seal the various media or cooling channels from one another and / or from the outside. In fuel cells, the reaction media, i.e.,Fuel and reaction gases are guided, while the coolant is guided between the individual plates. The bipolar plates usually each have at least one or more through-openings. The media and / or the reaction products can be guided through the through-openings to the electrochemical cells defined by adjacent bipolar plates of the stack or into the cavity formed by the separator plates of the bipolar plate, or can be discharged from the cells or from the cavity. The electrochemical cells, in particular of a fuel cell, can each comprise, for example, a membrane electrode assembly (MEA), each with a polymer electrolyte membrane (PEM) and electrodes. The MEA or the MEA system can also have one or more gas diffusion layers (GDL), which are usually oriented towards the separator plates, in particular towards bipolar plates of fuel cell systems, and can, for example,are formed as a carbon fleece. In an exemplary fuel cell, hydrogen supplied on the anode side is typically converted to water with oxygen supplied on the cathode side. Protons produced during hydrogen oxidation pass from the anode side through the MEA to the cathode, where they react with the oxygen and electrons to form water. The fuel cell membrane is electrically insulating and prevents an electrical short circuit between the anode and cathode. The electrons are separately supplied from the anode to the cathode, thus harnessing the electrical current.

[0004] Nowadays, most fuel cell systems are operated with an excess of oxidant. It has been found that the reaction takes place to a greater extent in places where there is a particularly high concentration of oxidizing medium than in other places in the electrochemically active region. This can be the case, for example, in the immediate vicinity of the points where the oxidant enters the electrochemically active region. The greater conversion of reactants leads to a particularly high current density in the areas where there is a larger excess of oxidant. This results in an uneven distribution of the current density over the active area. A high conversion of reactants leads to a high load on the membrane, whereas the membrane is only subjected to a low load in areas of low mass conversion. Overload areas have a negative impact on the service life of the MEA.of their membrane. Low-load ranges have the disadvantage of high area-related costs relative to the power, particularly since the MEA is a comparatively expensive component. Similar phenomena can occur in PEM electrolyzers, since water is the only medium supplied, and as it passes through the bipolar plate, an increasingly large proportion of oxygen is carried along in the water, while the actual water content decreases. In particular, low-load ranges can occur towards the end of the water path on a bipolar plate.

[0005] In other electrochemical systems, especially other polymer membrane-based electrochemical systems, other factors can lead to uneven loading of the polymer membrane surface.

[0006] The object of the 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 an MEA or polymer membrane arranged between bipolar plates in a bipolar plate stack. Additionally or alternatively, it may be an object to reduce the costs of an electrochemical system depending on its performance.

[0007] These objects are achieved according to the invention by the bipolar plate according to claim 1 and claim 3. Further developments are the subject of the subclaims and the following description.

[0008] The proposed bipolar plate for an electrochemical system comprises a first separator plate and a second separator plate arranged one on top of the other.

[0009] The first separator plate has embossed webs and channels for conducting a first fluid on a side facing away from the second separator plate. The second separator plate has embossed webs and channels for conducting a second fluid on a side facing away from the first separator plate. The first and second separator plates each have webs and channels on mutually facing sides for conducting a cooling medium along an inner side of the bipolar plate.

[0010] At least one of the separator plates has structured web regions on at least one side of the separator plate, wherein the structured web regions have at least periodic surface structures with an average spatial period (Px, Py) of less than 10 µm, wherein a proportion of structured web regions on the entire web surface is distributed inhomogeneously over the entire web surface, in particular that only individual sections of the web surface are structured.

[0011] Such a bipolar plate allows, in particular, the current density to be homogenized without having to intervene in the MEA system. In particular, this allows for homogenization of the current density.

[0012] The "entire web area" can refer in particular to the entire web area lying on one side of the separator plate. Alternatively, the "entire web area" can refer to only a region of one side of the separator plate, for example, to a region that overlaps with an electrochemically active area of ​​the MEA in orthogonal projection in the stacking direction. Here, the term "first electrochemically active area" is used to refer to the area of ​​the first separator plate that overlaps with an electrochemically active area of ​​the MEA, in particular the area of ​​the MEA surrounded by a reinforcing edge, when the separator plate is arranged in an electrochemical system.Analogously, the term "second electrochemically active region" is used to refer to the region of the second separator plate that overlaps with an electrochemically active surface of the MEA when the separator plate is arranged in an electrochemical system. Furthermore, it should be noted that when the present disclosure refers to "overlapping" or an "overlap" of the first electrochemically active region with the second electrochemically active region, this refers to an overlap of the regions in an orthogonal projection. A side of a separator plate that faces away from the other separator plate of the bipolar plate may be referred to herein as the gas side. A side of a separator plate that faces the other separator plate of the bipolar plate may be referred to herein as the coolant side.The separator plates have channels and webs on both their respective gas sides, the side facing the MEA, and the coolant side, the side facing away from the MEA. Webs on the gas side correspond, in an orthogonal projection, to channels on the coolant side, and channels on the gas side correspond, in an orthogonal projection, to webs on the coolant side.

[0013] A web region can be understood in particular as the region which lies centrally between two adjacent channels. A structured web region can be understood in particular as one in which at least 5% and / or at least 10% and / or at least 20% and / or at least 30% and / or at least 40% and / or at least 50% are structured. The structuring of the web region can be arranged symmetrically to the virtual center line, but can also be arranged asymmetrically. It can be advantageous for the webs, when viewed in cross section, to be completely structured between the inflection points, which can represent the boundary to the adjacent channel, and / or for a maximum of 90% and / or a maximum of 80% and / or a maximum of 70% and / or a maximum of 60% between the inflection points or the ends facing the web of the flanks which extend essentially rectilinearly in cross-section and contain the inflection point, to be structured as described above.

[0014] Typically, the surface structures are arranged periodically relative to one another in at least one spatial direction. The surface structures can also be arranged periodically relative to one another in two spatial directions. According to some embodiments, surface structures can result that run parallel to one another at least in sections. The alignment can be present over larger or smaller regions. Preferably, the surface structures run parallel to one another at least within a region enclosed by a grain boundary. Different regions with parallel surface structures, in which, however, a different orientation exists compared to another region, can also border one another, for example at grain boundaries.

[0015] The shape of the structures on the surface is therefore repeated in at least one spatial direction. The spatial period typically refers to the maximum distance between two adjacent surface structures of the same or similar shape. Due to manufacturing reasons, the surface structures are generally not completely identical to one another. Rather, the period can fluctuate along the surface. Therefore, an average spatial period of less than 10 µm is specified. It can also happen that the spatial period of the surface structures is always less than 10 µm. For example, the period lengths of the surface structures can be less than 5 µm and / or less than 3 µm. The average spatial period is preferably a maximum of 2 µm, in particular a maximum of 1.5 µm.

[0016] Such periodic surface structures are typically generated using ultrashort pulse laser radiation (see below) and are also known in the literature as "Laser-Induced Periodic Surface Structures" (LIPSS). For detailed explanations, details, and definitions on the topic of LIPSS, please refer to the following publication: "Dynamics of the Generation and Mechanisms of Formation of Periodic Surface Structures in the Nanometer Range (LIPSS) by Irradiation of Solids with Femtosecond Laser Pulses," dissertation by Sandra Höhm, Berlin, 2014 (hereinafter: Höhm 2014), which is incorporated by reference into this disclosure in its entirety. The separator plate is thus typically laser-treated in the area of ​​the periodic surface structures. The spatial period of the surface structures depends directly on the wavelength of the laser light used and is usually of the order of magnitude of the wavelength of the laser light used.

[0017] The surface structures can extend along their longitudinal direction, for example, in a wavy or linear manner. In one embodiment, the surface structures can have depressions and / or elevations. The depressions can run between the elevations and are usually delimited and / or formed by them. The depressions or elevations can run essentially parallel to one another, at least in sections (e.g., parallel next to one another). Often, the surface structures form, at least locally, a trench structure with a multiplicity of elongated depressions that are aligned essentially parallel to one another. The number of surface structures, depressions, or elevations can be varied as required. For example, the number of depressions can depend on the size of the area that is to have the surface structures.Typically, at least 10 or at least 20 trench structures, such as depressions, are present in an area with similar or identical surface structures, which run parallel to each other at least in sections. It is also possible to provide a different number of periods over a specific length in different areas in at least one spatial direction on the surface.

[0018] The dimensions of the depressions, especially the period, usually depend at least on the wavelength of the laser radiation used.

[0019] For example, the depressions have a depth of at least 8 nm, preferably at least 20 nm, for example at least 50 nm and / or at most 3 µm, preferably at most 1 µm, in particular at most 500 nm, preferably at most 300 nm, generally at most 250 nm. The depth is usually measured normal to the area formed by the elevations or normal to the surface of the separator plate that is free of the periodic surface structures. Furthermore, the depressions can have a width of at least 0.1 µm and / or at most 2 µm. The width is typically measured at half height and perpendicular to the local longitudinal direction of the depressions. The depressions can also have a period in a spatial direction of at least 100 nm, usually at least 0.3 µm and / or at most 3 µm, preferably at most 1.5 µm, in particular at most 1.2 µm, typically at most 1000 nm.The periodic surface structures thus often comprise nanostructures with a depth, width and / or period of less than one micrometer or, in particular with regard to the period, slightly above one micrometer.

[0020] In one embodiment, the portion of structured web regions can be formed on the side of the first separator plate that faces the second separator plate. Additionally or alternatively, the portion of structured web regions can be formed on the side of the second separator plate that faces the first separator plate. Additionally or alternatively, the portion of structured web regions can be formed on the side of the first separator plate that faces away from the second separator plate and / or the portion of structured web regions can be formed on the side of the second separator plate that faces away from the first separator plate. Structured web regions can therefore be formed on a coolant side of the first or second separator plate. Structured web regions can be formed on the coolant sides of the first and second separator plates.Structured land areas can be provided on only one anode side, only one cathode side, or both gas sides. The proportion of structured land areas and unstructured land areas on one side of a separator plate, particularly in an electrochemically active area of ​​the separator plate, can form a pattern. Unstructured land areas can be those land areas that do not have the periodic surface structure described above.

[0021] The invention further comprises a bipolar plate for an electrochemical system, comprising a first separator plate, wherein this first separator plate has embossed webs and channels on a first side for conducting a first fluid and embossed webs and channels on a second side for conducting a second fluid. Such an arrangement can be advantageous, for example, in electrolyzers, which are often operated without a separate cooling medium, since the supplied water itself cools the system.

[0022] In this arrangement, the first separator plate has structured ridge regions on at least one of its sides. These structured ridge regions have, at least in sections, periodic surface structures with an average spatial period (Px, Py) of less than 10 µm. A portion of the structured ridge regions is distributed inhomogeneously across the entire ridge surface.

[0023] The first separator plate can have a first electrochemically active region, and the second separator plate can have a second electrochemically active region, wherein the first electrochemically active region and the second electrochemically active region overlap one another and form an active region of the bipolar plate in an overlap region. Likewise, the second side of the first separator plate can have a second electrochemically active region. In particular, the structured land regions in the active region can be located on at least one side of at least one of the at least one separator plate(s). In particular, if the structured land regions are located only in the active region of the bipolar plate, the "entire land area" typically refers only to a region of one side of the separator plate and the land areas contained in this region.For example, if the structured land areas are arranged only on the coolant side of the first separator plate and only in an area that overlaps with the first active area of ​​the MEA, the total land area refers only to the first active area of ​​the coolant side. The proportion of structured land areas in the total land area of ​​this area is then, for example, distributed inhomogeneously across the entire land area of ​​this area.

[0024] The inhomogeneous distribution can be arranged in such a way that the current density is increased in the area of ​​the structured surfaces, so that a current density distribution across the bipolar plate is more homogeneous than in a corresponding bipolar plate which does not have structured surfaces arranged in such a way.

[0025] In one embodiment, the first separator plate and / or the second separator plate can form at least one contact surface with each other and / or with another component, with the surface structures being formed particularly in the region of the contact surfaces. The other component can be, for example, the MEA or the GDL. The surface structures can enable improved electrical conductivity, particularly in the region of the contact surfaces.

[0026] The surface structures can be distributed inhomogeneously over the contact surfaces.

[0027] The first separator plate can have a first region, a second region, and optionally a third region in the electrochemically active region of at least one of its sides. The surface structures can be distributed such that a proportion A11 of structured web regions in the first region, relative to the total web area of ​​the first region, is smaller than a proportion A12 of structured web regions in the second region, relative to the total web area of ​​the second region, and / or such that a proportion A13 of structured web regions in the third region, relative to the total web area of ​​the third region, is smaller than a proportion A12 and / or A11.

[0028] Additionally or alternatively, the surface structures can be distributed such that a proportion A11' of structured contact surfaces in the first region in relation to the total contact surface of the first region is smaller than a proportion A12' of structured contact surfaces in the second region in relation to the total contact surface of the second region and / or that a proportion A13' of structured contact surfaces in the third region in relation to the total contact surface of the third region is smaller than a proportion A12' and / or A11'.

[0029] The first, second, and third regions can all be located on the gas side and / or all three on the coolant side of the first separator plate.

[0030] Additionally or alternatively, the second separator plate can have a first region, a second region, and a third region in the electrochemically active region of at least one of its sides. The surface structures can be distributed such that a proportion A21 of structured web regions in the first region, relative to the total web area of ​​the first region, is smaller than a proportion A22 of structured web regions in the second region, relative to the total web area of ​​the second region, and / or such that a proportion A23 of structured web regions in the third region, relative to the total web area of ​​the third region, is smaller than a proportion A22 and / or A21.Additionally or alternatively, the surface structures can be distributed such that a proportion A21' of structured contact surfaces in the first region, relative to the total contact surface of the first region, is smaller than a proportion A22' of structured contact surfaces in the second region relative to the total contact surface of the second region and / or that a proportion A23' of structured contact surfaces in the third region relative to the total contact surface of the third region is smaller than a proportion A22' and / or A21'. All three of the first, second, and third regions can be located on the gas side and / or all three on the coolant side of the second separator plate.

[0031] A ratio A11 / A12 of the first separator plate and / or a ratio A21 / A22 of the second separator plate between the proportion A11 or A21 of structured web regions in the first region and the proportion A12 or A22 of structured web regions in the second region may be less than 0.9, preferably less than 0.7, preferably less than 0.5.

[0032] Additionally or alternatively, a ratio A11' / A12' of the first separator plate and / or a ratio A21' / A22' of the second separator plate between the proportion A11' or A21' of structured contact surfaces in the first region and the proportion A12' or A22' of structured contact surfaces in the second region can be less than 0.9, preferably less than 0.7, preferably less than 0.5.

[0033] A ratio A13 / A12 of the first separator plate and / or a ratio A23 / A22 of the second separator plate between the proportion A13 or A23 of structured web regions in the third region and the proportion A12 or A22 of structured web regions in the second region can be less than 0.9, preferably less than 0.7, preferably less than 0.5. A ratio A13' / A12' of the first separator plate and / or a ratio A23' / A22' of the second separator plate between the proportion A13' or A23' of structured contact surfaces in the third region and the proportion A12' or A22' of structured contact surfaces in the second region can be less than 0.9, preferably less than 0.7, preferably less than 0.5.

[0034] The surface area of ​​the first region and / or the second region and / or the third region can be at least 10%, preferably at least 15%, particularly preferably at least 25%, of the total surface area of ​​the active region of the bipolar plate. The first region, the second region, and the third region can have the same surface area. The distribution of the surface areas can be different for the two separator plates.

[0035] The first separator plate can be configured on at least one of its sides such that a proportion of structured web regions in the total web area and / or a proportion of structured contact surfaces in the total contact surfaces of the first and / or second and / or third region of the first electrochemically active region differ from one another by more than 5%, preferably by more than 10%, particularly preferably by more than 20%. The first and / or second and / or third region can be located on the coolant side or gas side.

[0036] The second separator plate can be configured such that a proportion of structured web regions in the total web area and / or a proportion of structured contact surfaces in the total contact surfaces of the first and / or second and / or third region of the second electrochemically active region differ from one another by more than 5%, preferably by more than 10%, particularly preferably by more than 20%. The first and / or second and / or third region can be located on the coolant side or gas side.

[0037] Different proportions of the areas on the coolant and gas sides of at least one of the separator plates are also possible.

[0038] In one embodiment, it can be provided that only one of the regions, in particular the second region, has the surface structures.

[0039] The first separator plate can have a first inlet opening for the inlet of the first fluid and a first outlet opening for the outlet of the first fluid. The first inlet opening can be fluidically connected to the first electrochemically active region via a first inlet region. The first outlet opening can be fluidically connected to the first electrochemically active region via a first outlet region. The first fluid can be passed sequentially through the first inlet region, the first electrochemically active region, and the first outlet region.

[0040] The second separator plate can have a first inlet opening for the inlet of the second fluid and a first outlet opening for the outlet of the second fluid. The first inlet opening can be fluidically connected to the first electrochemically active region via a first inlet region. The first outlet opening can be fluidically connected to the first electrochemically active region via a first outlet region. The first fluid can be passed sequentially through the first inlet region, the first electrochemically active region, and the first outlet region.

[0041] A flow direction of the first fluid can be opposite to a flow direction of the second fluid, wherein a proportion of structured web regions and / or structured contact surfaces of at least one side of a separator plate increases at least in sections with the flow direction of the first fluid.

[0042] The first, second, and third regions can be arranged in one flow direction. The flow direction can be a flow direction of a first gas on the first gas side, a second gas on the second gas side, or of the coolant. The flow direction of the first gas, the second gas, and / or the coolant can be different. The flow direction of the coolant can also be different from at least one gas flow direction. Within a web or a group of webs, a proportion of structured web regions and / or contact surfaces can initially increase in the flow direction and then decrease again. The gradients on different webs can be arranged at different locations relative to a longitudinal extent of the flow field. The increase can also originate from a section without any structure.

[0043] The first separator plate can be a cathode plate, and the second separator plate can be an anode plate. The cathode side can, for example, have oxygen-carrying channels, ie, in particular, carry air or oxygen. The anode side can, in particular, have hydrogen-carrying channels, ie, in particular, carry hydrogen, methanol, and / or another proton donor.

[0044] Alternatively or additionally, the area fractions may be different in different regions of the flow field depending on the distance from the side edges.

[0045] The surface structures can be spaced apart from an outer edge of the electrochemically active region, wherein a distance between the surface structures and the outer edge can be greater than 2 mm, preferably greater than 5 mm, preferably greater than 8 mm, particularly preferably greater than 10 mm, in particular greater than 15 mm. The surface structures can be spaced apart from an outer edge of the electrochemically active region, 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 can be arranged between the surface structures and the outer edge.The first electrochemically active surface can be surrounded at least in sections by a first sealing element of the first separator plate, and the surface structures can be spaced from the first sealing element, wherein a minimum distance between each surface structure and the sealing element can be greater than 5 mm, preferably greater than 8 mm, particularly preferably greater than 10 mm. The sealing element can be designed as an elastomer seal. Additionally or alternatively, the sealing element can be designed, in particular, as a bead seal. A minimum distance between each surface structure and the sealing element can be greater than 9 mm, preferably greater than 12 mm, particularly preferably greater than 14 mm, in particular in the case of a bead seal.

[0046] In peripheral areas of the electrochemically active region, surface structures can be avoided, or fewer surface areas with such structures can be provided. This can lead to a throttling of the electron flow in these areas. This can serve to protect structural weak points at the membrane edges, as areas with high current density can be shifted away from these edges toward a region of the active region oriented somewhat toward the center.

[0047] The present disclosure further relates to an arrangement for an electrochemical system, comprising a bipolar plate according to one of the preceding claims and at least one membrane electrode unit (MEA) with a frame-shaped reinforcing edge, wherein the MEA and the bipolar plate are arranged one on top of the other in a stacking direction and the electrochemically active region of the bipolar plate is enclosed by the reinforcing edge of the MEA in orthogonal projection into a common plane.

[0048] A surface structure pattern of the bipolar plate may be spaced 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.

[0049] The bipolar plate can be configured to operate at a specific operating point of an electrochemical system, in particular a fuel cell or an electrolyzer, wherein the specific operating point comprises a specific concentration gradient of the first fluid along the first electrochemically active region. The surface portions of the surface structure can be adapted to the concentration gradient of the first fluid, such that a proportion of structured web regions and / or structured contact surfaces is comparatively large in sections of the active region with a comparatively low concentration of the first fluid, and a proportion of structured web regions and / or structured contact surfaces is comparatively small in sections of the active region with a comparatively high concentration of the first fluid.

[0050] The proportion of structured web regions and / or structured contact surfaces can be adapted accordingly to the concentration gradient of the first fluid, wherein the adapted proportion of structured web regions and / or structured contact surfaces is formed on the gas side or coolant side.

[0051] Alternatively, the material flows on both gas sides can also be optimized. The proportion of structured web regions and / or structured contact surfaces can then be adapted accordingly to the concentration gradient of the first and second fluids, wherein the adapted proportion of structured web regions and / or structured contact surfaces is formed on the gas side or coolant side of the first and / or second separator plate, in particular in the active region. The smallest proportion of structured web regions and / or structured contact surfaces can be provided in the regions in which there is an excess of gas from both reactants. In the regions in which there is a shortage of at least one gas, at least close to the surface on or in the MEA, a higher proportion of structured web regions and / or structured contact surfaces can be provided.

[0052] It can be advantageous to achieve the most homogeneous fluid distribution possible across the flow direction. However, this is not always optimally possible. In such cases, it may be advantageous to ensure the most uniform current density and thus the loading of the membrane by adjusting the arrangement of the surface structures.

[0053] The areas provided with the surface structures can be provided at least in sections with a coating, in particular with a coating that increases the electrical conductivity. In one embodiment, the bipolar plate may have regions in which, in orthogonal projection, coating surfaces of one separator plate overlap with structured surfaces of the other separator plate. The structured surfaces may be referred to as UKP surfaces. The bipolar plate may have regions in which at least one separator plate has the periodic surface structure and coating described above.

[0054] The coating may contain one or more of the following substances or may consist of one or more of these substances or their alloys: electrically conductive oxides, carbon, electrically conductive carbon layers, precious metals, in particular Au, Ag or Pt, metals, in particular Ti, Zr, Nb, Ta or Cr, metal nitrides, in particular TiN, TiON (with varying oxygen / nitrogen ratio), CrN, Cr2N, metal carbides, metal borides, metal silicides and / or silicon carbide. The coating may be single-layer or multi-layer. The coating may be applied unevenly over at least one side of one or both separator plates. The coating may be applied directly to the structured web regions or partially to structured web regions and partially to unstructured web regions. The coating may additionally or alternatively be applied in channel structures.

[0055] The present disclosure further relates to a method for producing a bipolar plate as described above. The method comprises the steps: - Providing a first and a second separator plate, - irradiating the first and / or second separator plate by means of a pulsed laser, wherein a pulse duration of the laser pulses is less than 1 ns, preferably less than 100 ps, - Generation of periodic surface structures on the separator plate by means of laser radiation, whereby a proportion of structured web areas on the entire web surface is distributed inhomogeneously over the entire web surface.

[0056] Additionally, a coating can be applied.

[0057] The features described above with respect to the bipolar plate can be transferred to the process.

[0058] The aforementioned plurality of periodic surface structures can typically be generated within a spatially coherent projection of the respective laser pulse onto the separator plate. A plurality of periodic surface structures can be generated per laser pulse. The generation of each periodic surface structure by the respective laser pulse can be completed before the impact of the next laser pulse. At least 5, at least 10, or at least 20 periodic surface structures, in particular trench structures, can be generated per laser pulse.The periodic surface structures are thus generated with each laser pulse within the coherent surface irradiated by the respective laser pulse and not by scanning the separator plate in a spatially periodic manner or by irradiating it with a spatially periodic, non-coherent light pattern such as a diffraction pattern or interference pattern.

[0059] It is important that the laser pulses have a pulse duration of less than 1 ns, preferably less than 100 ps, ​​in particular less than 10 ps or, for example, less than 1 ps. The laser pulses preferably have a pulse frequency of 1 MHz or less. In particular, it is advantageous if the ratio of pulse frequency to pulse duration is at least 1000. This short pulse duration and the comparatively low pulse frequency enable very high intensities to be achieved, which are required for ablating the surface and / or rearranging the surface material. On the other hand, the short pulse duration, in interaction with the long dead times, allows largely heat diffusion-free processing of the surface material and thus the creation of periodic surface structures.

[0060] In preferred embodiments, the pulse duration is less than 100 ps, ​​less than 50 ps, ​​less than 20 ps, ​​less than 10 ps, ​​or even less than 1 ps. In some embodiments, pulse durations in the fs range are used, e.g., greater than 30 fs and / or less than 1000 fs and / or less than 500 fs, preferably greater than 50 fs and / or greater than 100 fs. In particular, picosecond or femtosecond lasers, which are collectively referred to as ultrashort pulse lasers, can be used for the method.

[0061] Furthermore, the invention comprises an electrochemical system with a plurality of bipolar plates as described above.

[0062] Examples of bipolar plates are shown in the figures and are explained in more detail in the following description. They show: Fig. 1 schematically shows in a perspective view an electrochemical system with a plurality of separator plates or bipolar plates arranged in a stack; Fig. 2 schematically shows a cross-section through an electrochemical system with a plurality of separator plates or bipolar plates arranged in a stack; Fig. 3A schematically shows in a perspective view two bipolar plates of the system according to Fig. 1 with a membrane electrode assembly (MEA) arranged between the bipolar plates, using the example of a fuel cell system; Fig. 3B schematically shows a section through a plate stack of a system according to the type of system according to Fig. 1; Fig. 4 schematically shows a plan view of a bipolar plate with evenly distributed welds according to the prior art; Fig. 5 schematically shows a plan view of an exemplary bipolar plate with structured web regions, wherein a portion of structured web regions on the entire web surface is distributed inhomogeneously over the entire web surface; Fig. 6 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 7 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 8 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 9 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 10 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 11 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 12 schematically shows a plan view of another exemplary bipolar plate with an unevenly distributed proportion of structured web areas over the entire web surface; Fig. 13 schematically shows a section of a separator plate in perspective view; Fig. 14 a microscopic image of periodic surface structures in plan view; Fig. 15 Detail A of the Fig. 13; Fig. 16 Detail B of the Fig. 13; Fig. 17 a plan view of the section of the separator plate according to Fig. 13; Fig. 18 another microscopic image of periodic surface structures in plan view; Fig. 19 an exemplary bipolar plate with an MEA arranged thereon; Fig. 20 a detailed view of the Fig. 19 and Fig. 21 shows another example bipolar plate with an MEA arranged thereon.

[0063] Here and in the following, features that recur in various figures are designated by the same or similar reference symbols.

[0064] Fig. Figure 1 shows an electrochemical system 1 with a plurality of identical metallic bipolar plates 2, which are arranged in a stack 6 and stacked along a z-direction 7. The bipolar plates 2 of the stack 6 are clamped between two end plates 3, 4. The z-direction 7 is also called the stacking direction. In the present example, the system 1 is a fuel cell stack. Two adjacent bipolar plates 2 of the stack enclose an electrochemical cell between them, which serves, for example, to convert chemical energy into electrical energy. To form the electrochemical cells of the system 1, a membrane electrode assembly (MEA) is arranged between each adjacent bipolar plate 2 of the stack (see, for example, Fig. 3A). The MEAs typically each contain at least one membrane, e.g., an electrolyte membrane. Furthermore, a gas diffusion layer (GDL) may be arranged on one or both surfaces of the MEA, in which Fig. 1 and 3A not shown, but in Fig. 2.

[0065] In alternative embodiments, system 1 can also be designed as an electrolyzer, an electrochemical compressor, or a redox flow battery. 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 explained in more detail here, even if the media flowing onto or through the bipolar plate in an electrolyzer, an electrochemical compressor, or a redox flow battery can differ from the media used in a fuel cell system.

[0066] The z-axis 7, together with an x-axis 8 and a y-axis 9, spans a right-handed Cartesian coordinate system. The bipolar plates 2 each define a plate plane, wherein the plate planes of the separator plates 2a, 2b are each aligned parallel to the xy-plane and thus perpendicular to the stacking direction or to the z-axis 7. The end plate 4 has a plurality of media connections 5 through which media can be supplied to the system 1 and through which media can be removed from the system 1. These media that can be supplied to the system 1 and removed from the system 1 can include, for example, fuels such as molecular hydrogen or methanol, reaction gases such as air or oxygen, reaction products such as water vapor or depleted fuels, or coolants such as water and / or glycol.

[0067] Fig. Figure 2 shows a cross-section through an electrochemical system 1 with a plurality of separator plates 2a arranged in a stack, which in the illustrated example from a PEM electrolyzer simultaneously each form the only layer of the bipolar plates 2. The structure of the membrane electrode assembly (MEA) 10 differs from that in fuel cells in that instead of two essentially identical or mirror-inverted gas diffusion layers (GDL) 16 on either side of the membrane 14, a GDL 16a, usually made of graphite fleece, is combined on the one hand with a so-called porous transport layer (PTL) 16b, usually made of sintered titanium material, on the opposite side of the membrane 14. For the sake of simplicity, a PTL is also subsumed under the term GDL in the context of this document. The media connections 5 are also designed somewhat differently, namely without protruding elements, but in their basic function, the media connections 5 made of Fig. 1. However, a PEM electrolyzer typically does not require a separate coolant, so that not only can the cavity for guiding the coolant within a bipolar plate be omitted, but also the number of media connections 5 and through-openings 11 is reduced.

[0068] Fig. Figure 3A shows in perspective two adjacent bipolar plates 2 of an electrochemical system of the type of system 1 from Fig. 1 and a membrane electrode assembly (MEA) 10 arranged between these adjacent bipolar plates 2, which is known from the prior art, wherein the MEA 10 is Fig. 3A is largely obscured by the bipolar plate 2 facing the viewer. The bipolar plate 2 is formed from two materially joined separator plates 2a, 2b (see e.g. Fig. 3B), of which Fig. 3A, only the first separator plate 2a facing the viewer is visible, concealing the second separator plate 2b. The separator plates 2a, 2b can each be made of a metal sheet, e.g., a stainless steel sheet. The separator plates 2a, 2b can be welded to one another, e.g., by laser welding. On the one hand, the separator plates 2a, 2b can be tightly welded along sealing seams 60, in particular outside and / or inside the perimeter bead 12d, at a distance from their outer edges or at least along the inner edge of a portion of the through-openings. Further weld seams are possible.

[0069] The separator plates 2a, 2b have through-openings aligned with one another, which form through-openings 11a-c of the bipolar plate 2. When stacking a plurality of bipolar plates of the type of bipolar plate 2, the through-openings 11a-c form lines that extend in the stacking direction 7 through the stack 6 (see Fig. 1). Typically, each of the lines formed by the through-openings 11a-c is in fluid communication with one of the ports or media connections 5 in the end plate 4 of the system 1. For example, coolant can be introduced into or discharged from the stack via the lines formed by the through-openings 11a. The lines formed by the through-openings 11b, 11c, on the other hand, can be designed to supply the electrochemical cells of the fuel cell stack 6 of the system 1 with fuel and reaction gas, as well as to discharge the reaction products from the stack. The media-carrying through-openings 11a-11c are designed essentially parallel to the plate plane.

[0070] To seal the through-openings 11a-c from the interior of the stack 6 and from the environment, the first separator plates 2a each have sealing arrangements in the form of sealing beads 12a-c, which are each arranged around the through-openings 11a-c and which completely enclose the through-openings 11a-c. The second separator plates 2b have, on the side visible from the viewer of the Fig. 3A, corresponding sealing beads for sealing the through openings 11a-c (not shown). Alternatively, elastomer seals can also be used.

[0071] In an electrochemically active area 18, the first separator plates 2a have, at their side facing the observer, Fig. 3A, a flow field 17 with structures for guiding a reaction medium along the outside of the separator plate 2a. These structures are shown in Fig. 3A by a plurality of webs 41 and channels 42 extending between the webs and defined by the webs. At the viewer of the Fig. 3A, the first separator plates 2a each have a distribution and collection area 20. Distribution or collection areas 20 each comprise structures that are configured to distribute a medium introduced from a first of the two through-openings 11b into the distribution area 20 via the active area 18 or to collect or bundle a medium flowing from the active area 18 to the second of the through-openings 11b. The fluid conducting structures 29 of both distribution or collection areas 20 are in Fig. 3A, also defined by webs and channels extending between the webs and delimited by the webs. For the sake of simplicity, only the distribution area 20 will be discussed below; the corresponding statements can equally apply to a collection area 20. Likewise, the distribution area is sometimes referred to as the inlet area 20, and the collection area is sometimes referred to as the outlet area 20.

[0072] The sealing beads 12a-12c have passages 13a-13c, of which the passages 13a are formed on both the underside of the upper separator plate 2a and the upper side of the lower separator plate 2b, while the passages 13b are formed in the upper separator plate 2a and the passages 13c are formed in the lower separator plate 2b. For example, the passages 13a, which are designed as local elevations of the bead, allow the passage of coolant between the through-opening 12a and the distribution area 20, so that the coolant reaches the distribution area between the bipolar plates or is guided out of the collection area 20.Furthermore, the passages 13b allow hydrogen to pass between the through-opening 12b and the distribution area on the upper side of the upper separator plate 2a. These passages 13b are characterized by perforations facing the distribution area and running obliquely to the plate plane. Thus, for example, hydrogen flows through the passages 13b from the through-opening 12b to the distribution area on the upper side of the upper separator plate 2a or in the opposite direction from the collection area. The passages 13c allow air to pass between the through-opening 12c and the distribution area, so that air enters the distribution area on the underside of the lower separator plate 2b or is led out of the collection area. The corresponding perforations are not visible here.

[0073] The first separator plates 2a each further comprise a further sealing arrangement in the form of a perimeter bead 12d, which surrounds the flow field 17 of the active region 18, the distribution and collection regions 20, and the through-openings 11b, 11c and seals them from the through-opening 11a, i.e., from the coolant circuit, and from the environment of the system 1. The second separator plates 2b each comprise corresponding perimeter beads. The structures of the active region 18, the distribution structures of the distribution and collection regions 20, and the sealing beads 12a-d are each formed integrally with the separator plates 2a and molded into the separator plates 2a, e.g., in a stamping or deep-drawing process or by hydroforming. The same applies to the corresponding structures of the second separator plates 2b.

[0074] The two through-openings 11b or the lines formed by the through-openings 11b through the plate stack of the system 1 are each connected via passages 13b in the sealing beads 12b, via the distribution structures of the distribution or collection area 20 and via the flow field 17 in the active area 18 of the viewer of the Fig. 3A facing first separator plates 2a are in fluid communication with each other. In an analogous manner, the two through-openings 11c or the lines formed by the through-openings 11c are connected through the plate stack of the system 1 via corresponding bead feedthroughs, via corresponding distribution and collection structures and via a corresponding flow field on an outer side of the Fig. 3A, the second separator plates 2b are in fluid communication with each other. The through-openings 11a, or the lines formed by the through-openings 11a through the plate stack of the system 1, are each in fluid communication with each other via a cavity 19 enclosed or surrounded by the separator plates 2a, 2b. This cavity 19 serves to guide a coolant through the bipolar plate 2, in particular to cool the electrochemically active region 18 of the bipolar plate 2.

[0075] The bipolar plate 2 is formed from two materially joined individual plates, namely separator plates 2a, 2b (see e.g. Fig. 3B), of which Fig. 3A, only the first individual plate facing the viewer is visible, which conceals the second individual plate. The individual plates can each be formed from a formed metal sheet, e.g., from a stamped or deep-drawn stainless steel sheet. This metal sheet can, for example, have a thickness of at most 150 µm, preferably of at most 100 µm, preferably of at most 90 µm, particularly preferably of at most 80 µm. The individual plates can be welded together, for example, by laser welding.

[0076] The Fig. 3B shows schematically a section through a portion of the plate stack 6 of the system 1 from Fig. 1, where the cutting plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. The cutting plane runs in the Fig. 3B along a bent section, along the section line CC in Fig. 3A.

[0077] The identical bipolar plates 2 of the stack each comprise the previously described first metallic separator plate 2a and the previously described second metallic separator plate 2b. Structures for media conduction along the outer surfaces of the separator plates 2 can be seen, here in particular in the form of webs and channels defined by the webs. In particular, channels are shown on the mutually facing surfaces of adjacent separator plates 2a, 2b, as well as cooling channels in the cavity 19 between adjacent separator plates 2a, 2b. Adjacent to the sealing bead 12d and between the cooling channels in both the distribution or collection area 20 and the active area 18, the two separator plates 2a, 2b rest on one another in a contact area 24 and are connected to one another there, in the present example by means of laser weld seams 60, 70, 50.

[0078] A membrane electrode assembly (MEA) 10, known from the prior art, is arranged between adjacent separator plates 2 of the stack. The MEA 10 typically comprises a membrane, e.g., an electrolyte membrane, and an edge portion 15 connected to the membrane 14. For example, the edge portion 15 can be integrally bonded to the membrane 14, e.g., by an adhesive bond or by lamination.

[0079] Here, the term "electrochemically active region" is used to refer to the surface of the separator plate 2a, 2b that is opposite an electrochemically active surface of the MEA 10 when the separator plate 2a, 2b is arranged in an electrochemical system. The membrane of the MEA 10 extends at least over the active region 18 of the adjacent separator plates 2 and enables proton transfer there over or through the membrane 14. The actual active region 18 is delimited by the edge section 15 of the MEA. Thus, the membrane does not extend into the distribution or collection region 20. The edge section 15 of the MEA 10 serves to position, secure, and seal the membrane between the adjacent separator plates 2.

[0080] The edge section 15 covers the distribution or collection area 20 of the adjacent bipolar plates 2. Towards the outside, the edge section 15 can also extend beyond the perimeter bead 12d and there border on the outer edge area of ​​the separator plates 2a, 2b or protrude beyond it (cf. Fig. 3A).

[0081] Furthermore, additional gas diffusion layers 16 can be arranged in the active region 18. The gas diffusion layers 16 enable the flow to the membrane over the largest possible area of ​​the membrane surface and can thus improve the proton transfer across the membrane. The gas diffusion layers 16 can, for example, be arranged on both sides of the membrane, at least in the active region 18, between the adjacent bipolar plates 2. The gas diffusion layers 16 can, for example, be formed from a fiber fleece or comprise a fiber fleece.

[0082] Fig. Figure 4 shows a plan view of a bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1. The bipolar plate has a plurality of welds 50, of which only a few are provided with reference symbols for the sake of clarity. The plurality of welds 50 results in a weld pattern. In the prior art, the welds 50 are homogeneous, ie, evenly distributed over the active area 18 of the bipolar plate 2 of the Fig. 4. Such a bipolar plate can have the disadvantage that the current density is inhomogeneously distributed across the active region during operation due to the natural gradients of the reaction gases. This can lead to overload areas and thus a reduced service life of the MEA or its membrane and / or low-load areas with reduced performance.

[0083] In contrast, the present invention provides that at least one of the separator plates 2a, 2b has structured web regions 80 on at least one side of the separator plate 2a, 2b, wherein the structured web regions 80 have at least periodic surface structures 8 with an average spatial period (Px, Py) of less than 10 µm. A proportion of structured web regions 80 on the entire web surface is distributed inhomogeneously over the entire web surface, as in the examples of Fig. 5 to 12. The examples of Fig. Numbers 5 to 12 are merely examples.

[0084] Fig. 5 to 12 each show plan views of a bipolar plate 2 of an electrochemical system. The illustration can correspond to a representation of a gas side of a first separator plate 2a as well as to a representation of a gas side of a second separator plate 2b. It is possible for only one of the separator plates 2a, 2b to have one of the illustrated designs or both; examples from different figures can also be combined. The description refers to the structured web regions 80; the same applies to the structured contact surfaces 80'; the difference between the two will be explained below, particularly in the context of Fig. 13. In the figures, the reference numeral 80 is only partially indicated together with the surface-structured sections 81-84, even though a surface structuring 8 is always present in these sections.

[0085] The Fig. Figure 5 shows a plan view of a bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. In the image plane shown, the first fluid flows from left to right. The active region 18 is delimited by its outer edge 18'. In the active region, there are sections 70 in which the web regions and thus also the contact surfaces are unstructured. The sections in which structured web regions 80 with the above-mentioned periodic surface structures 8 are present are highlighted here by hatching and are distributed differently across the active region 18. In a first region I, viewed in the direction of flow, there is only a short section 81 in which structured web regions 80 with the above-mentioned periodic surface structures 8 are present. In a second region II, the surface structures 8 are arranged evenly on the webs in two sections 82, 83, again shown by hatching.In a third region III, the surface structures 8 are arranged evenly on the webs in a hatched region 84. The first region I and the second region II have the same area as the third region III. A proportion of section 81 in the total web area of ​​the first region I is smaller than a proportion of the region A proportion of sections 82 and 83 in the total web area of ​​the second region II is smaller than a proportion of region 84 in the total web area of ​​the third region III. A proportion of structured web regions in the total web area is inhomogeneously distributed over the entire web area. The structured web regions 80 are spaced from an outer edge 18' of the active region 18; here, a circumferential section 71 is formed in which the web regions are formed without the surface structuring 8.

[0086] The Fig. Figure 6 shows a plan view of a bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. In the image plane shown, the first fluid flows from left to right. The active region 18 is delimited by its outer edge 18', which is adjoined by a circumferential section 71 in which the web regions are formed without the surface structuring 8. 70 denotes further sections in which the web regions are formed without the surface structuring 8. In a hatched section 81, the surface structures 8 are arranged evenly on the webs in the manner described above. The section 81 extends from left to right, i.e. in the x-direction, in a <-shape. In regions in which the surface structures 8 are provided, electrical conductivity is improved. Again, three regions I, II and III are shown.The surface structures are distributed here in such a way that a proportion of structured web regions 80 in the first region I in relation to the total web area of ​​the first region I is smaller than a proportion of structured web regions 80 in the second region II in relation to the total web area of ​​the second region II. Furthermore, a proportion of structured web regions 80 in the third region III in relation to the total web area of ​​the third region III is also less than 50%, which is significantly smaller than a proportion in the second region II and slightly smaller than a proportion in the first region I. Due to the inhomogeneous distribution of the proportion of structured web regions 80 in relation to the total web area over the entire web area of ​​the active region 18, a more homogeneous current density can be achieved.

[0087] The embodiments of the Fig. 5 and Fig. 6 are particularly suitable when one medium strongly dominates the current density distribution and its distribution is essentially mirror-symmetrical relative to the center line of the plate in the flow direction of this fluid.

[0088] The Fig. Figure 7 shows a plan view of another example of a schematically illustrated bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. The arrow A marks a concentration gradient of the fluid, which is drawn from the through hole 11c (in Fig. 7 located bottom left) via the distribution area into the active area 18 and further to the through hole 11c (in Fig. 7 (top right). The active area 18 is limited by its outer edge 18'. In a hatched section 81, the surface structures 8 of the type described above are arranged evenly on the webs. The section 81 extends in Fig. 7 from the top left of the active region 18 on both sides in a step-like manner to the bottom right of the active region 18. The surface structures 8 are spaced from the edge of the active region 18'; both in the corresponding sections 71 and in the sections 70, the web regions are formed without the surface structuring 8.

[0089] The Fig. Figure 8 shows a plan view of another example of a schematically illustrated bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. The arrow A marks a concentration gradient of the fluid, which is drawn from the through hole 11c (in Fig. 8 located bottom left) via the distribution area into the active area 18 and further to the through hole 11c (in Fig. 8 top right). The active area 18 is limited by its outer edge 18'. In a hatched section 81, the surface structures 8 of the type described above are evenly arranged on the webs. The section 81 extends in Fig. 8 from the top left of the active region 18 in a stepped manner almost to the right outer edge of the active region 18. The surface structures 8 are spaced from the edge of the active region 18'.

[0090] The embodiments of the Fig. 8 is particularly suitable when a medium strongly dominates the current density distribution and the medium is supplied and discharged off-center, ie a large part of the medium has an approximately diagonal course or a course parallel to the diagonal. The embodiment of the Fig. 12 also shows advantages in such a course of the medium.

[0091] The Fig. Figure 9 shows a plan view of another example of a schematically illustrated bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. The active region 18 is delimited by its outer edge 18'. In sections 81, 82, 83 shown in black, the surface structures 8 of the type described above are evenly arranged on the webs. The webs of the intermediate sections 70 shown in white do not have such surface structures 8. Section 81 in Fig. 9 has the shape of an elongated semicircle and is in Fig. 9 is located on the left in the active area 18. Section 82 in Fig. 9 has the shape of a compressed semicircle and is in Fig. 9 is arranged on the right in the active region 18. Strip-shaped sections 83, of which only a few are provided with the reference number 83 for the sake of clarity, extend essentially parallel to the y-axis and are arranged between the sections 81 and 82. The surface structures 8 are spaced from the upper and lower edge 18' of the active region. Regions I, II and III are again marked, but the sequence from left to right is I, III and II, in the third region III the smallest proportion of web regions with the surface structuring 8 is formed. Here, the second region II is arranged closer to the outlet opening 11 than the first region I and the third region III.

[0092] The Fig. 10 shows in plan view another example of a schematically illustrated bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. The active region 18 is delimited by its outer edge 18'. In sections 81, 82 shown in black, the surface structures 8 of the type described above are evenly arranged on the webs. The webs of the intermediate sections 70 shown in white do not have such surface structures 8. Section 81 in Fig. 10 has the shape of a trapezoid that extends centrally from right to left over approximately three-quarters of the active region 18. Strip-shaped sections 82, of which only some are provided with the reference numeral 82 for the sake of clarity, extend substantially parallel to the y-axis and are arranged substantially over the entire active region, being spaced from the lateral edges 18' of the active region 18. The surface structures 8 are further spaced from the upper and lower edges 18' of the active region 18. The left-hand region, in which only the strip-shaped sections 82 are provided with surface structures 8, can be regarded here as the first region I; the right-hand region, in which both the strip-shaped sections 82 and the trapezoidal section 81 are provided with surface structures 8, can be regarded here as the second region II.Therefore, there do not always have to be three areas I, II, III, and the area of ​​these areas can also vary.

[0093] The design of the Fig. 10 is suitable like the Fig. 5 and Fig. 6 especially when one medium strongly dominates the current density distribution and its distribution is essentially mirror-symmetric relative to the centerline of the plate in the flow direction of this fluid.

[0094] The Fig. 11 and Fig. 12 show in a plan view each a further example of a schematically illustrated bipolar plate 2 of an electrochemical system of the type of system 1 from Fig. 1 or Fig. 2. The active region 18 is delimited by its outer edge 18'. In sections 81, 82, 83, 84, the surface structures 8 are arranged on the webs in the manner described above; section 70 is again free of surface structures 8. The surface structures 8 are arranged evenly in each of the sections 81, 82, 83, 84. A proportion of structured web surfaces in section 81 to the total web surfaces of section 81 is greater than a proportion of structured web surfaces in section 82 to the total web surfaces of region 82. A proportion of structured web surfaces in section 82 to the total web surfaces of section 82 is greater than a proportion of structured web surfaces in section 83 to the total web surfaces of section 83. A proportion of structured web surfaces in section 83 to the total web surfaces of section 83 is greater than a proportion of structured web surfaces in section 84 to the total web surfaces of section 84. In Fig. 11, section 81 has a horizontal oval shape and is arranged centrally in the active area 18. Section 82 encloses section 81, section 83 encloses section 82, and section 84 encloses section 83. Section 70 is arranged at the right and left edges of the active area 18. In Fig. 12, section 81 is located at the top right corner of the active area 18. Section 82 extends adjacent to it, followed by sections 83, 84, and 70, resulting in a gradual progression from top left to bottom right. The electrical conductivity in the first section 81 is greater than in the second section 82, the electrical conductivity in section 82 is greater than in section 83, the electrical conductivity in section 83 is greater than in section 84, and the electrical conductivity in section 84 is greater than in section 70. The uneven arrangement of the portions of structured land surfaces can thus homogenize the current density.

[0095] Fig. 13 shows a schematic representation of a section of an exemplary separator plate 2a or 2b of the system of Fig. 1 or Fig. 2. The webs 41 and channels 42 are each formed integrally with the separator plates 2a, 2b and molded into the separator plates 2a, 2b, e.g., in a stamping, hydroforming, or deep-drawing process. The same usually applies to the corresponding conductive structures of the second separator plates 2b. The conductive structures of the flow field can comprise a plurality of webs 41 and channels 42 formed between the webs 41 in the electrochemically active region 18, as well as a plurality of webs and, if appropriate, channels formed between the webs in the distribution and collection region 20. The webs 41 on the gas side form channels on the coolant side, and channels 42 on the gas side form webs on the coolant side. In the case of bipolar plates formed from only one separator plate, the webs 41 on the first side form channels on the second side, and channels 42 on the first side form webs on the second side. The surface structures 8 are arranged on some of the webs 41.The orientation of the surface structures 8 relative to the direction of extension of the webs or channels is merely exemplary. The surface structure 8 of the web area is arranged symmetrically to the virtual center line in this case, but can also be arranged asymmetrically in other examples. As shown in the . Fig. 13, the two left-hand webs 41, when viewed in cross-section, are structured only in a central region between the turning points, which represent the boundary to the adjacent channel. In an installed bipolar plate 2, these regions correspond to the structured contact surfaces 80'. In contrast, the right-hand web 41 is provided with the surface structure 8 over much wider regions; in this case, the structured web region 80 essentially corresponds to the area of ​​the entire web 41. In the installed state, structured web regions 80 can extend further than the regions that form the contact or contact surfaces explained below. Those sections of the structured web regions 80 that form the contact or contact surfaces simultaneously represent structured contact surfaces 80'.

[0096] Fig. 14 shows a microscopic image of exemplary periodic surface structures 8 in plan view. Fig. 18 shows a further microscopic image of exemplary periodic surface structures 8 in plan view. The periodic surface structures 8 comprise a plurality of depressions and elevations. The depressions run between the elevations and are delimited and / or formed by them. The surface structures 8 are arranged periodically relative to one another in at least one spatial direction x, y. Thus, the surface structures 8 can be aligned relative to one another along their longitudinal direction. For example, the surface structures 8, i.e., the depressions and the elevations, run substantially parallel to one another. The surface structures 8 are, in particular, arranged parallel to one another. Fig. 15 and Fig. 16 show details A and B of the Fig. 13. Fig. 17 shows a top view of the section of the Fig. 13. Differently formed surface structures are shown as examples. The same or different surface structures 8 can be formed on a separator plate. For example, in Fig. 15 and Fig. 16 that the surface structures 8 are arranged parallel to one another, i.e. perpendicular to the direction of extension of the surface structures. Fig. 15 indicates that surface structures 8 are also arranged parallel to one another in groups one behind the other (one after the other in the direction of web extension). The surface structures 8 can extend along their longitudinal direction, for example, in a wave-like or linear manner. An example of a wave-like extension of the surface structures 8 is shown in Fig. 16 shown.

[0097] Fig. 19 shows a section of an exemplary bipolar plate 2 of the system of Fig. 1 with a first separator plate 2a and a second separator plate 2b arranged thereon. The first separator plate 2a and the second separator plate 2b touch each other at contact surfaces 91, 92, 93. The contact surfaces 91, 92, 93 are located centrally on webs of the respective coolant side of the respective separator plate 2a or 2b. An MEA is arranged on each of the gas sides of the separator plates 2a and 2b, so that the webs 41 of the gas sides form contact surfaces 101, 102, 103, 104 with the MEA 10. The contact surfaces on the gas side of the separator plate 2a are marked with the index a and the contact surfaces on the gas side of the separator plate 2b are marked with the index b.

[0098] Surface structures 8 are arranged on both sides of the contact surfaces 91, 92, 93, i.e., on the separator plate 2a and on the separator plate 2b. Additionally or alternatively, surface structures 8 can be applied to webs on the gas side, in particular to some of the contact surfaces 101a, 102a, 103a, 104a, 101b, 102b, 103b, and / or 104b.

[0099] In the Fig. 20 is a detail of the Fig. 19. A depth t, a width b and a period Px of the surface structures 8, in particular of the depressions, are indicated. The surface structures 8 can have a depth t of at least 8 nm, preferably at least 50 nm and / or at most 3 µm, preferably at most 1 µm, in particular at most 500 nm and / or at most 300 nm and / or at most 250 nm. In the present example it is, for example, t = 0.4 µm or t = 100 nm. In one exemplary embodiment, the surface structures 8 have a width b of at least 0.1 µm and / or at most 2 µm. In the present example it is b = 0.45 µm. In addition, the surface structures 8 can have a period Px in a spatial direction x of at least 0.3 µm and / or at most 3 µm. In the present example it is 1 µm. The period Px denotes in the Fig. 20 the lateral distance between two adjacent elevations.

[0100] Due to the surface structures 8, the surface of the separator plate 2a, 2b has chemical, electrical, and / or mechanical properties that differ from areas of the separator plate 2a, 2b without surface structures 8, in particular sections 70, 71. Due to the surface structures 8, for example, the oxygen content of the surface material of the separator plate 2a, 2b can be greater in the area of ​​the periodic surface structures 8 than outside the periodic surface structures 8.

[0101] Fig. 21 shows a schematic representation of a section of another exemplary bipolar plate 2 of the system of Fig.1 with a first separator plate 2a and a second separator plate 2b arranged thereon. The first separator plate 2a and the second separator plate 2b touch each other at contact surfaces 91, 92, 93. The contact surfaces 91, 92, 93 are located centrally on webs of the respective coolant side of the respective separator plate 2a or 2b. An MEA is arranged on each of the gas sides of the separator plates 2a and 2b, so that the webs 41 of the gas sides form contact surfaces 101, 102, 103, 104 with the MEA 10. The contact surfaces on the gas side of the separator plate 2a are marked with the index a and the contact surfaces on the gas side of the separator plate 2b are marked with the index b.

[0102] Surface structures 8 are arranged on the contact surfaces 91, 92, 93, thus forming structured contact surfaces 80'. At the contact surface 91, a surface structure 8 is applied to both the web of the first separator plate 2a and the web of the second separator plate 2b. In addition, a coating 90 is applied to at least one of the two surface structures 8. The coating 90 is described in more detail below. At the contact surface 92, a coating 90 is applied to the web of the first separator plate 2a, and a surface structure 8 is applied to the web of the second separator plate 2b. At the contact surface 93, neither a coating 90 nor a surface structure 8 is applied to the web of the first separator plate 2a. A surface structure 8 is applied to the web of the second separator plate 2b.

[0103] Additionally or alternatively, surface structures 8 and / or coatings may be applied to webs of the gas side, in particular to some of the contact surfaces 101a, 102a, 103a, 104a, 101b, 102b, 103b and / or 104b.

[0104] The coating 90 can be provided to increase the electrical conductivity and / or corrosion resistance of the separator plate 2a, 2b. The coating 90 can be applied over the entire surface or, as is preferred, in specific regions. Preferably, at least some of the regions provided with the periodic surface structures 8 are provided with the coating, in particular with a coating that increases electrical conductivity. In one embodiment, the coating 90 can be provided only in the region of the webs and can be left out in the region of the channels.The coating 90 can, for example, contain one or more of the following substances or consist of one or more of these substances or their alloys: electrically conductive oxides, carbon, preferably electrically conductive carbon layers such as graphite, precious metals such as Au, Ag, or Pt, metals such as Ti or Cr, metal nitrides, in particular TiN, TiON (with varying oxygen / nitrogen ratios), CrN, Cr2N, metal carbides, metal borides, metal silicides, and / or silicon carbide. Surface structures 8 and coatings 90 are known, for example, from DE 10 2021 202 214 A1, the content of which is hereby incorporated in its entirety into the present document by reference. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2021 202 214 A1

[0104]

Claims

[1] Bipolar plate (2) for an electrochemical system (1), with a first separator plate (2a) and a second separator plate (2a) which are arranged one on top of the other, wherein - the first separator plate (2a) has embossed webs (41a) and channels (42a) for guiding a first fluid on a side facing away from the second separator plate (2b); - the second separator plate (2b) has, on a side facing away from the first separator plate (2a), embossed webs (41b) and channels (42b) for guiding a second fluid; - the first and second separator plates (2a, 2b) each have webs (43a, 43b) and channels (44a, 44b) on mutually facing sides for guiding a cooling medium along an inner side of the bipolar plate (2); wherein at least one of the separator plates (2a, 2b) has structured web regions (80) on at least one side of the separator plate, wherein the structured web regions (80) have at least periodic surface structures (8) with an average spatial period (Px, Py) of less than 10 µm, whereby a proportion of structured web areas on the entire web surface is distributed inhomogeneously over the entire web surface. [2] Bipolar plate according to the preceding claim, wherein the portion of structured web regions (80) is formed on the side of the first separator plate (2a) facing the second separator plate (2b) and / or wherein the portion of structured web regions (80) is formed on the side of the second separator plate (2b) facing the first separator plate (2a) and / or wherein the portion of structured web regions (80) is formed on the side of the first separator plate (2a) facing away from the second separator plate (2b) and / or wherein the portion of structured web regions (80) is formed on the side of the second separator plate (2b) facing away from the first separator plate (2a). [3] Bipolar plate for an electrochemical system, comprising a first separator plate (2a), wherein - the first separator plate (2a) has embossed webs (41a) and channels (42a) on a first side for guiding a first fluid; - the first separator plate (2a) has embossed webs (41b) and channels (42b) on a second side for guiding a second fluid; wherein the first separator plate (2a) has structured web regions (80) on at least one side of the first separator plate (2a), wherein the structured web regions (80) have, at least in sections, periodic surface structures (8) with an average spatial period (Px, Py) of less than 10 µm, wherein a portion of structured web areas (80) on the entire web surface is distributed inhomogeneously over the entire web surface. [4] Bipolar plate according to claim 1 or 3, wherein the first separator plate (2a) has a first electrochemically active region (18a) and the second separator plate (2b) has a second electrochemically active region (18b) or the second side of the first separator plate (2a) has a second electrochemically active region (18b), wherein the first electrochemically active region (18a) and the second electrochemically active region (18b) overlap one another and form an active region (18) of the bipolar plate in an overlap region, wherein the structured web regions (80) in the active region (18) are located on at least one side of at least one of the at least one separator plate(s) (2a, 2b). [5] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) and / or the second separator plate (2b) form at least one contact or touch surface (91, 92, 93, 101, 102, 103, 104, 101a, 102a, 103a, 104a, 101b, 102b, 103b, 104b) with each other and / or with another component (10, 16, 16a, 16b), wherein the surface structures (8) in particular in the region of the contact or touch surfaces (91, 92, 93, 101, 102, 103, 104, 101a, 102a, 103a, 104a, 101b, 102b, 103b, 104b). [6] Bipolar plate according to the preceding claim, wherein the surface structures (8) are distributed inhomogeneously over the contact or touch surfaces (91, 92, 93, 101, 102, 103, 104, 101a, 102a, 103a, 104a, 101b, 102b, 103b, 104b). [7] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) has in the electrochemically active region (18a) of at least one of its sides a first region (I), a second region (II) and optionally a third region (III), wherein the surface structures (8) are distributed such that a proportion A11 of structured web regions (80) in the first region (I) of the total web area of ​​the first region (I) is smaller than a proportion A12 of structured web regions (80) in the second region (II) of the total web area of ​​the second region (II) and / or that a proportion A13 of structured web regions (80) in the third region (III) of the total web area of ​​the third region (III) is smaller than a proportion A12 and / or A11 and / or wherein the surface structures (8) are distributed such that a proportion A11' of structured contact surfaces (80') in the first region (I) of the total contact surface of the first region (I) is smaller than a proportion A12' of structured contact surfaces (80') in the second region (II) of the total contact surface of the second region (II) and / or that a proportion A13' of structured contact surfaces (80') in the third region (III) of the total contact surface of the third region (III) is smaller than a proportion A12' and / or A11'. [8] Bipolar plate according to one of claims 1 to 2 and 4 to 7, wherein the second separator plate (2b) has in the electrochemically active region (18b) of at least one of its sides a first region (I), a second region (II) and a third region (III), wherein the surface structures (8) are distributed such that a proportion A21 of structured web regions (80) in the first region (I) of the total web area of ​​the first region (I) is smaller than a proportion A22 of structured web regions (80) in the second region (II) of the total web area of ​​the second region (II) and / or that a proportion A23 of structured web regions (80) in the third region (III) of the total web area of ​​the third region (III) is smaller than a proportion A22 and / or A21 and / or wherein the surface structures (8) are distributed such that a proportion A21' of structured contact surfaces (80') in the first region (I) of the total contact surface of the first region (I) is smaller than a proportion A22' of structured contact surfaces (80') in the second region (II) of the total contact surface of the second region (II) and / or that a proportion A23' of structured contact surfaces (80') in the third region (III) of the total contact surface of the third region (III) is smaller than a proportion A22' and / or A21'. [9] Bipolar plate according to one of the two preceding claims, wherein a ratio, A11 / A12 of the first and / or a ratio A21 / A22 of the second separator plate (2a, 2b), between the proportion A11 or A21 of structured web regions (80) in the first region (I) and the proportion A12 or A22 of structured web regions (80) in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.5 and / or wherein a ratio, A11' / A12' of the first and / or a ratio A21' / A22' of the second separator plate (2a, 2b), between the proportion A11' or A21' of structured contact surfaces (80') in the first region (I) and the proportion A12' or A22' of structured contact surfaces (80') in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.5, and / or wherein a ratio A13 / A12 of the first and / or a ratio A23 / A22 of the second separator plate (2a, 2b) between the proportion A13 or A23 of structured web regions (80) in the third region (III) and the proportion A12 or A22 of structured web regions (80) in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.5, and / or wherein a ratio A13' / A12' of the first and / or a ratio A23' / A22' of the second separator plate (2a, 2b) between the proportion A13' or A23' of structured contact surfaces (80') in the third region (III) and the proportion A12' or A22' of structured contact surfaces (80') in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.

5. [10] Bipolar plate according to one of the preceding claims, wherein at least one side of the first separator plate (2a) is designed such that a proportion of structured web regions (80) in the total web area and / or a proportion of the structured contact surfaces (80') in the total contact surfaces of the first and / or second and / or third region (I, II, III) of the first electrochemically active region (18a) differ from one another by more than 5%, preferably differ from one another by more than 10%, particularly preferably differ from one another by more than 20%. [11] Bipolar plate according to one of claims 1 to 2 and 4 to 10, wherein the second separator plate (2b) is designed such that a proportion of structured web regions (80) in the total web area and / or a proportion of the structured contact surfaces (80') in the total contact surfaces of the first and / or second and / or third region (I, II, III) of the second electrochemically active region (18b) differ from one another by more than 5%, preferably differ from one another by more than 10%, particularly preferably differ from one another by more than 20%. [12] Bipolar plate according to one of the two preceding claims, wherein only one of the regions, in particular the second region (II), has the surface structures (8). [13] Bipolar plate according to one of the preceding claims, wherein the bipolar plate is configured to be operated at a specific operating point of an electrochemical system (1), wherein the specific operating point comprises a specific concentration gradient of the first fluid along the first electrochemically active region (18), wherein the surface structure pattern (8) is adapted to the concentration gradient of the first fluid, so that a proportion of structured web regions (80) and / or structured contact surfaces (80') in sections of the active region (18) with a comparatively low concentration of the first fluid is comparatively large and a proportion of structured web regions (80) and / or structured contact surfaces (80') in sections of the active region (18) with a comparatively high concentration of the first fluid is comparatively small. [14] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) has a first inlet opening (11) for the inlet of the first fluid and a first outlet opening (11) for the outlet of the first fluid, wherein the first inlet opening (11) is fluidically connected to the first electrochemically active region (18a) via a first inlet region (20) and the first outlet opening (11) is fluidically connected to the first electrochemically active region (18a) via a first outlet region (20), so that the first fluid can be conducted successively through the first inlet region (20), the first electrochemically active region (18a) and the first outlet region (20). [15] Bipolar plate according to claim 14 as dependent on claim 9, wherein the second region (II) is arranged closer to the outlet opening (11) than the first region (I) and the optional third region (III). [16] Bipolar plate according to one of the preceding claims, designed such that a flow direction of the first fluid is opposite to a flow direction of the second fluid, wherein a proportion of structured web regions (80) and / or structured contact surfaces (80') of at least one side of a separator plate (2a, 2b) increases at least in sections with the flow direction of the first fluid. [17] Bipolar plate according to one of the preceding claims, wherein at least one of the separator plates (2a, 2b) of the bipolar plate (2) consists of or contains a steel, in particular a stainless steel, or a titanium alloy, in particular a group 1 titanium alloy. [18] Bipolar plate according to one of the preceding claims, wherein at least the areas (80) provided with the surface structures (8) are provided at least in sections with a coating (90), in particular with a coating (90) increasing the electrical conductivity. [19] Bipolar plate according to the preceding claim, wherein the coating (90) contains one or more of the following substances or consists of one or more of these substances or their alloys: electrically conductive oxides, carbon, electrically conductive carbon layers, precious metals, in particular Au, Ag or Pt, metals, in particular Ti, Zr, Nb, Ta or Cr, metal nitrides, in particular TiN, TiON (with varying oxygen / nitrogen ratio), CrN, Cr2N, metal carbides, metal borides, metal silicides and / or silicon carbide, wherein the coating is single-layered or multi-layered. [20] Electrochemical system (1) comprising a plurality of bipolar plates (2) according to one of the preceding claims.

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