Bipolar plate with an inner coating and an outer coating, and methods for its manufacture

Abstract

The invention relates to a bipolar plate for an electrochemical system and an electrochemical system with a plurality of stacked such bipolar plates, as well as a method for manufacturing such bipolar plates. The electrochemical system can, in particular, be a fuel cell system, an electrochemical compressor, an electrolyzer, or a redox flow battery.

Description

[0001] The invention relates to a bipolar plate for an electrochemical system and an electrochemical system with a plurality of stacked such bipolar plates, as well as a method for manufacturing such bipolar plates. The electrochemical system can, in particular, be a fuel cell system, an electrochemical compressor, an electrolyzer, or a redox flow battery.

[0002] Depending on the application, bipolar plates can have various functions. On the one hand, they ensure an electrically conductive connection to an adjacent layer, which might be, for example, a gas diffusion layer. On the other hand, separator plates typically serve to transport reactants and / or reaction products to and / or from the substrate, with a fluid guidance structure in the form of a channel structure usually being provided for this transport. Furthermore, bipolar plates can be used to dissipate heat from the reaction, for example, by means of a coolant. This is often achieved by designing the bipolar plate as a two-layer plate, with the two layers defining an interior space through which a cooling fluid flows. Due to their separation function between media, the layers are often referred to as separator plates.

[0003] The reaction conditions during operation of the electrochemical system often have a negative impact on the lifespan of the bipolar plates. For example, aggressive reaction conditions (e.g., oxidation of H₂ and generation of H₂) can lead to significant damage. + and e - This often leads to corrosion of the bipolar plate, particularly on the outer surface of a two-layer bipolar plate. Conversely, there is significantly less corrosion on the inner surface of a two-layer bipolar plate because the electrochemical reactions take place on the outer surface.

[0004] To counteract corrosion of the bipolar plate, an anti-corrosion coating can be applied, for example, in the electrochemically active area. To ensure long-term corrosion resistance of the bipolar plate, this coating should not be damaged during manufacturing or transport.

[0005] However, applying such anti-corrosion coatings can be expensive and complex due to the materials used in the coating and / or the additional process steps. When selecting a suitable anti-corrosion coating, the electrical contact resistance of the bipolar plate should also be considered, as the bipolar plate should be electrically conductive in the electrochemically active area.

[0006] There is therefore a constant need to improve bipolar plates in terms of their corrosion resistance, electrical contact resistance, manufacturing costs and / or service life.

[0007] According to one aspect of this document, a bipolar plate is proposed. The bipolar plate comprises a first separator plate and a second separator plate, which are connected to each other and define an interior space of the bipolar plate. Each separator plate has an inner surface with an inner coating and an outer surface with an outer coating, wherein fluid guiding structures are formed into each separator plate within an electrochemically active region of the separator plate by reshaping the separator plate. The inner coating extends along the electrochemically active region and exhibits defects, at least in some areas. The outer coating extends along the electrochemically active region and has a substantially uniform surface finish.

[0008] During the forming of the fluid guide structures, sections of the separator plate's surface are typically stretched. Other sections may be compressed. This stretching or compression of the separator plate's surface affects the separator plate's coating if the coating was already present on the separator plate's substrate before forming. The coating is typically thinner and less elastic than the separator plate's substrate and therefore tears more easily. The stretching or compression thins, damages, or cracks the coating in certain areas. Thus, forming the separator plate leads to localized damage to the coating, subsequently referred to as defects.

[0009] For the reasons mentioned above, such damage to the coating should be avoided on the outer surfaces. Therefore, the outer coating was applied to the separator plate after it had been formed. This sequence of process steps is evident from the fact that the outer coating is largely intact and exhibits a substantially uniform surface finish, while the inner surface shows defects.

[0010] As described above, aggressive reaction conditions prevail, particularly on the outer surface of the bipolar plate, leading to corrosion. Conversely, the inner surface of the bipolar plate is shielded from these aggressive electrochemical reactions, and only coolant flows through its interior. Therefore, defects in the inner coating are less critical than defects in the outer coating. This is especially true when using a suitable substrate, such as AISI 316L stainless steel.

[0011] Applying the internal coating to the separator plates before forming can be advantageous, for example, if applying the coating to a smooth, unformed separator plate is technically simpler than applying it to a formed surface with fluid guidance structures. This can be the case, for instance, with printing processes such as screen printing, rolling, or painting the coating material onto the separator plate substrate.

[0012] In particular, it can be advantageous for a manufacturer of bipolar plates to procure pre-coated strip material for the production of the separator plates from a supplier, because in this case the manufacturer does not need to own or provide additional equipment for further coating lines for the internal coating. In addition to the aforementioned application methods, PVD, CVD, and electroplating processes are also suitable options.

[0013] It is also possible to use coating materials for the interior coatings that are somewhat less corrosion-resistant compared to the exterior coatings described in more detail below.

[0014] The internal coating may have a different surface finish in a formed area of ​​the respective separator plate than in a non-formed area. The internal coating may exhibit defects such as cracks, particularly microcracks, scratches, and other damage. It is also possible that the internal coating is partially or completely missing in the formed area of ​​the separator plate. The defects may also be characterized by a reduced or uneven coating thickness, caused by the expansion of the separator plate during forming. The formed area may generally include the electrochemically active area, and sometimes it may only include this electrochemically active area, i.e., no other areas. Areas of the separator plate or...Bipolar plates that are not designed to carry fluids may include flat areas, which can be described as unshaped areas.

[0015] It can be provided that in the area of ​​defects, i.e. between the intact sections, the substrate of the separator plate forms its surface; thus, in the areas where the inner coating is missing or does not cover the surface of the substrate, i.e., where the inner coating has defects, sections of the substrate, i.e., in particular the metallic plate material, can lie on the surface.

[0016] The two separator plates may be joined by at least one metallurgical bond, such as a weld. At least one or both outer coatings may be applied to this metallurgical bond, for example, a weld. The outer coating can thus be applied to the respective separator plate after the plates have been joined. The outer coating is therefore not damaged or burned through by the weld. Often, the at least one weld is located in flat areas of the two separator plates. Sometimes, the at least one weld is located outside the electrochemically active area, i.e., outside the flow field and / or within a non-fluid-carrying area of ​​the bipolar plate. However, welds may also be present within the electrochemically active area.

[0017] In one embodiment, at least one of the outer coatings, or the outer coating itself, is applied in an opaque manner. In other words, the substrate of the separator plate is covered by the outer coating wherever it is applied. A coating can be described as opaque if the smallest measured layer thickness of the coating is at least 50%, preferably at least 75%, of the average layer thickness. Likewise, a coating can be described as opaque if the mass fraction of the coating materials on the surface is higher than the mass fraction of the substrate in any given area increment of the electrochemically active region. In particular, the mass fraction of all coating materials on the surface can be at least 70%, and more specifically at least 75%, in any given area increment of the electrochemically active region.Advantageously, the mass fractions are determined by SEM-EDX. For the preferred materials and coating thicknesses, advantageously from 20 to 500 nm, the measurements are preferably taken at a voltage of 10 kV and with a working distance (WD) of 12 mm. The outer coating can have a uniform layer thickness. Over a distance of 10 µm on the outer surface, the variation in layer thickness is a maximum of ± 20%. This can apply in any direction or to any area, but preferably in continuous web crest or channel bottom regions. The uniformity of the outer coating is also demonstrated by the low variation in the mass fraction of the coating materials, whose mass fraction varies by a maximum of ± 20% over the aforementioned 10 µm distance within the electrochemically active range.

[0018] At least one, several, or all of the coatings can be applied to the entire surface of the respective side of the separator plates. "Full surface" in this context means that the respective area has been completely covered with the coating. On the inside, the coating may have been applied completely, but due to subsequent forming processes, it may no longer be opaque.

[0019] According to some embodiments, the inner and outer coatings differ in terms of their materials. This allows the inner and outer coatings to be selected to suit the operating conditions of the electrochemical system. Alternatively, the inner and outer coatings can be made of the same material and thus be materially identical. It is also possible for the orthogonal projections of the inner and outer coatings to overlap.

[0020] The outer coating can be designed as an anti-corrosion layer and / or to reduce the contact resistance of the bipolar plate. Exemplary layers include elements from transition metal groups IV, V, and IX to XI, in particular nitrides, carbonitrides, oxynitrides, carbides, and carbooxynitrides, as well as carbon. The carbon can be formed as a separate layer or as multiple layers; in particular, it can form the top layer of the coating.

[0021] The outer coating can be a PVD coating (PVD = physical vapor deposition). A PVD coating is generally characterized by its stability and durability due to the covalent bonds it forms with the substrate. Depending on the material chosen, a PVD coating is particularly well-suited as an anti-corrosion layer or for reducing the contact resistance of the bipolar plate. The outer coating, and optionally the inner coating, can, for example, include or consist of titanium and / or carbon.

[0022] The outer coating can have at least one layer, for example two or more layers. In the case of a PVD coating, for example, a first layer can be applied to the substrate and then a second layer applied to the first layer.

[0023] Applying PVD coatings can be time-consuming and expensive. Time can be saved if the outer coatings of the separator plates are applied to the bipolar plate substrate after the separator plates have been joined. In this case, it is not necessary to coat two separator plates individually; instead, the bipolar plate can be coated in a single step.

[0024] The inner coating is often designed to increase the electrical conductivity of the bipolar plate and / or to reduce its contact resistance. Typically, the contact resistance of an inner coating is a maximum of 6 mΩ·cm. 2The following materials, for example, are suitable for the internal coating: elements of transition metals IV, V, and IX to XI, in particular as nitrides, carbonitrides, oxynitrides, carbides, and carbooxynitrides, as well as carbon. The carbon can be formed as a single layer or as multiple layers; in particular, it can form the top layer of the coating.

[0025] The fluid guidance structures on the inside typically form complementary fluid guidance structures on the outside of the respective separator plate. The respective coatings are generally located at least in the area of ​​the fluid guidance structures and / or in an electrochemically active area of ​​the respective plate. The fluid guidance structures are typically formed into the respective separator plate by deep drawing, embossing, or hydroforming.

[0026] The following describes the steps of a process for manufacturing an object as described above, namely a bipolar plate.

[0027] Such a process for manufacturing the item described above may, for example, include at least the following steps: - Providing a first separator plate with an inside, an outside and an inner coating arranged on the inside, - Providing a second separator plate with an inside, an outside and an inner coating arranged on the inside, - Forming the first separator plate to create fluid guiding structures at least in an electrochemically active area of ​​the first separator plate, wherein the inner coating is arranged at least in the area of ​​the fluid guiding structures, - Forming the second separator plate to create fluid guiding structures at least in an electrochemically active area of ​​the second separator plate, wherein the inner coating is arranged at least in the area of ​​the fluid guiding structures, - Connecting, preferably by material bonding, the first separator plate to the second separator plate, and - Applying an external coating to the outside of the first separator plate, at least in the area of ​​the fluid guidance structures, and / or an external coating to the outside of the second separator plate, at least in the area of ​​the fluid guidance structures.

[0028] The internal coating develops an uneven surface texture with defects due to the forming process. The internal coating can exhibit a different surface texture in a formed area of ​​the respective separator plate compared to an unformed area. This unformed area does not necessarily have to be directly adjacent to the formed area. Unformed areas adjacent to the formed area can still exhibit a locally altered internal coating due to the expansion of the separator plates during forming. For example, the unformed area might be located in a flat edge region of the respective separator plate.

[0029] The following methods are suitable for applying the internal coating: PVD, other sputtering processes, CVD, screen printing, rollers, spraying, rolling and electroplating processes.

[0030] Such a process for manufacturing the item described above may additionally include at least the following step: It may be intended that the first separator plate and the second separator plate are welded together during the joining process.

[0031] Such a process for manufacturing the object described above may additionally include at least the following step: application of the outer coating by means of PVD, in particular after forming and / or joining the separator plates together. Alternatively, the outer coating may be applied by means of other sputtering processes, by means of CVD, printing processes and in particular by means of spraying processes.

[0032] Such a process for manufacturing the item described above may additionally include at least the following step: Applying the outer coating to the outside of the first separator plate and applying the outer coating to the outside of the second separator plate simultaneously or at least in the same process step.

[0033] In the same process step, this can mean that the layers can be applied in parallel or at different times in the same tool, without having to carry out other process steps in between that require a tool change.

[0034] The process is specifically designed for producing the bipolar plate of the type described above. The process can therefore be combined with features of the bipolar plate and vice versa.

[0035] Furthermore, an electrochemical system is proposed which comprises a large number of stacked bipolar plates of the previously described design. This electrochemical system could, for example, be a fuel cell system, an electrochemical compressor, an electrolyzer, or a redox flow battery.

[0036] Exemplary embodiments of the electrochemical cell and the electrochemical system are shown in the accompanying figures and are explained in more detail below. They show: Fig. 1. Schematic representation in a perspective view of an electrochemical system with a plurality of bipolar plates arranged in a stack; Fig. 2 schematically in a perspective view two bipolar plates of the system according to the prior art consisting of two separator plates with a membrane electrode arrangement (MEA) arranged between the bipolar plates; Fig. 3 schematically shows a section through a stack of a system according to the type of system according to Fig. 1 along a line comparable to that in the Fig. 2 shown section line AA; Fig. 4 a scanning electron microscope (SEM) image of a coated surface which was reshaped after coating; Fig. 5 a scanning electron microscope (SEM) image of a coated surface which was reshaped before coating; Fig. 6. A representation of SEM (top) and energy-dispersive X-ray spectroscopy (EDX) results (bottom) of a coated surface which was reshaped after coating; Fig. 7 a representation of SEM (top) and energy-dispersive X-ray spectroscopy (EDX) results (bottom) of a coated surface which was reshaped before coating; Fig. 8 schematically a cross-section through a bipolar plate in the area of ​​a weld joint according to one embodiment; and Fig. 9 a flowchart for a process for manufacturing a bipolar plate.

[0037] Here and in the following, recurring features in various figures are designated with the same or similar reference symbols. For the sake of clarity, the repeated use of reference symbols in subsequent figures is sometimes omitted.

[0038] Fig. Figure 1 shows an electrochemical system 1 with a plurality of identical metallic bipolar plates 2, which consist of separator plates 2a, 2b and together with membrane electrode units 10 and gas diffusion layers 14 form electrochemical cells 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 this example, the system 1 is a fuel cell stack. Each pair of adjacent separator plates 2a, 2b of two neighboring bipolar plates 2 of the stack defines an electrochemical cell, which serves, for example, to convert chemical energy into electrical energy.To form the electrochemical cells of system 1, a membrane electrode assembly (MEA) 10, sometimes also called a membrane electrode array, is arranged between adjacent bipolar plates 2 of the stack. The MEAs typically each comprise at least one membrane, e.g., an electrolyte membrane, and a frame-shaped membrane reinforcement layer that surrounds and reinforces the membrane. Furthermore, a gas diffusion layer (GDL) 14 can be arranged on one or both surfaces of the MEA. Fig. 1 and 2 are not shown.

[0039] In alternative embodiments, system 1 can also be configured as an electrolyzer. Separator plates can also be used in this case. The design of these separator plates can then correspond to the design of separator plates 2a and 2b described in more detail here, even though the media guided on or through the separator plates in an electrolyzer may differ from the media used in a fuel cell system.

[0040] The z-axis 7, together with an x-axis 8 and a y-axis 9, defines a right-handed Cartesian coordinate system. The separator plates 2a and 2b define a plate plane at their point of contact, with each plate plane being parallel to the xy-plane and thus perpendicular to the stacking direction or to the z-axis 7. The end plate 4 has a multitude of media connections 5 through which media can be supplied to and discharged from system 1. These media 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. Gases are often supplied by means of blowers and / or compressors, while the supply of coolants is usually carried out with the aid of at least one pump.

[0041] Fig. Figure 2 shows, in perspective, two adjacent bipolar plates 2 of an electrochemical system of the type of system 1. Fig. 1 and a membrane electrode assembly (MEA) 10 arranged between these adjacent bipolar plates 2, known from the prior art, wherein the MEA 10 is in Fig. 2 is largely obscured by the separator plate 2 facing the viewer. The bipolar plate 2 is formed from two separator plates 2a, 2b joined together, of which in Fig. In each case, only the first separator plate 2a, facing the viewer, is visible, concealing the second separator plate 2b. The separator plates 2a and 2b can each be made of a single sheet of metal, e.g., stainless steel. The separator plates 2a and 2b can be welded together, e.g., by laser welding. Two adjacent separator plates 2a and 2b, together with the MEA 10 and any GDLs present (not shown here), form an electrochemical cell.

[0042] The separator plates 2a, 2b have aligned through-holes that form through-holes 11a-c of the bipolar plate 2. When a plurality of plates of the type of bipolar plate 2 are stacked, the through-holes 11a-c form conductors that extend through the stack 6 in the stacking direction 7 (see Fig. 1) Typically, each of the conduits formed by the through-openings 11a-c is in fluid connection with one of the ports or media connections 5 in the end plate 4 of system 1. For example, coolant can be introduced into or discharged from the stack via the conduits formed by the through-openings 11a. The conduits formed by the through-openings 11b and 11c, on the other hand, can be configured to supply the electrochemical cells of the fuel cell stack 6 of 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 essentially parallel to the plane of the individual bipolar plates 2.

[0043] 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 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 to the observer, Fig. 2 corresponding sealing beads for sealing the through openings 11a-c on the opposite back side of the bipolar plates (not shown).

[0044] In an electrochemically active area 18, the first separator plates 2a exhibit, to the observer, the Fig. On the front side facing 2, a flow field 17 with structures for guiding a reaction medium along the front side of the separator plate 2a is located. These structures are in Fig. 2 is given by a multitude of footbridges and channels 16 running between and bounded by the footbridges. At the viewer's Fig. On the front faces of the bipolar plates 2, the first separator plates 2a each have a distribution and a collection area 20. Distribution or collection areas 20 each comprise structures designed to distribute a medium introduced into the distribution area 20 from one of the first of the two through-openings 11b across the active area 18, or to collect or concentrate a medium flowing from the active area 18 towards the second of the through-openings 11b. The fluid guide structures of both distribution or collection areas 20 are arranged in Fig. 2 channels also run through footbridges and between the footbridges and are bordered by the footbridges.

[0045] The sealing beads 12a-12c have feedthroughs 13a-13c which allow the passage of medium through the sealing beads 12a-12c.

[0046] The first separator plates 2a each have a further sealing arrangement in the form of a perimeter bead 12d, which surrounds the flow field 17 of the active area 18, the distribution and collection areas 20, and the through-openings 11b, 11c, and seals them against the through-opening 11a, i.e., against the coolant circuit, and against the environment of the system 1. The second separator plates 2b each include corresponding perimeter beads. The structures of the active area 18, the distribution structures of the distribution and collection areas 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 an embossing or deep-drawing process or by hydroforming. The same applies to the corresponding structures of the second separator plates 2b.

[0047] The two through-openings 11b and the conduits formed by the through-openings 11b through the stack of plates of system 1 are each connected via feedthroughs 13b in the sealing grooves 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. The first separator plates 2a facing each other are in fluid contact. Similarly, the two through-openings 11c and the conduits formed by the through-openings 11c through the plate stack of system 1 are each connected via corresponding corrugated penetrations, distribution and collection structures, and a corresponding flow field on an outer surface visible from the observer's perspective. Fig. The two separator plates 2b facing away from each other are in fluid communication. The through-openings 11a, or the channels formed by the through-openings 11a through the plate stack of system 1, are each in fluid communication 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 area 18 of the separator plates 2a, 2b.

[0048] In the following, the sealing beads 12a, 12b, 12c, 12d are collectively described as the sealing arrangement 12. The sealing arrangement 12 thus comprises only one, at least one, or all of the sealing beads 12a-d. Overall, the sealing arrangement 12 defines the fluid-carrying area 17 of the respective plate, within which the media (cooling fluid, reactants, product media) flow or are guided.

[0049] The present invention is based, among other things, on the following considerations. Inside the bipolar plate 2, i.e., in the cooling chamber 19, different operating conditions prevail than on the outer surfaces of the bipolar plate 2. Thus, the oxidation of the fuels and the resulting electrochemical potentials can lead to oxidation or corrosion of the outer surfaces of the bipolar plates 2, while the inner surface (coolant side) in the electrochemically active area 18 exhibits hardly any corrosion problems.

[0050] For the production of the bipolar plate 2, the individual separator plates 2a, 2b are formed to create the channels 16, 16' and other structures such as sealing beads. During the forming of the separator plates 2a, 2b to create the channels 16, 16', sections of the separator plate 2a, 2b, including the surface, are typically stretched or compressed. This has a particularly significant impact on pre-coated sheet metal, as the coating is also affected by the forming process and can potentially even be damaged. This is especially problematic if, as is usually the case, the coating has a lower ductility than the sheet metal substrate. However, due to the reduced corrosion risk, this is acceptable on the inside of the bipolar plate 2. Such damage to the coating should be avoided on the outside of the bipolar plate 2.Therefore, according to the present document, it is proposed not to use pre-coated material on the outside, but to coat the sheet metal only after forming.

[0051] The invention will be explained in more detail below with reference to the Fig. 3-8 explained. Here, the characteristics of the Fig. 1 and Fig. 2 with the characteristics of Fig. 3-8 can be combined, provided they do not contradict each other.

[0052] According to the present document, a bipolar plate 2 is proposed, which has a first separator plate 2a and a second separator plate 2b. As previously referred to the Fig. As discussed in section 2, the separator plates 2a, 2b are connected to each other and define an interior space 19 of the bipolar plate 2. Each separator plate 2a, 2b has an inner surface 24 with an inner coating 25 and an outer surface 26 with an outer coating 27.

[0053] Channel-shaped fluid guidance structures 16 are formed into each separator plate 2a, 2b by reshaping the separator plate 2a, 2b. The fluid guidance structures 16' on the inner surface 24 typically form complementary fluid guidance structures 16 on the outer surface 26 of the respective separator plate 2a, 2b. The inner coating 25 is located in the area of ​​the fluid guidance structures 16' and typically extends along the electrochemically active area 18. Optionally, the inner coating 25 can extend over the entire inner surface 24 and thus be applied to the entire inner surface 24 of the separator plates 2a, 2b. The inner coating 25 was applied to the separator plates 2a, 2b before reshaping. The inner coating 25 can be designed to increase the electrical conductivity of the bipolar plate 2 and / or to reduce the contact resistance of the bipolar plate 2.The contact resistance of the inner coating is typically a maximum of 6 mΩ · cm. 2 Because pre-coated plates or plates made from fully pre-coated strip material were used for the formation of the fluid guide structures 16, 16', the internal coating 25 exhibits defects in some areas due to the subsequent forming process. As a result, the internal coating 25 is often no longer completely opaque on the inside 24 of the respective separator plate 2a, 2b after forming.

[0054] The outer coating 27 is located in the area of ​​the surfaces of the fluid guidance structures 16 and typically extends at least along the electrochemically active area 18. Optionally, the outer coating 27 can extend over the entire outer surface 26 of the respective separator plate 2a, 2b and thus be applied to the entire outer surface 26 of the separator plates 2a, 2b. The outer coating 27 was applied to the substrate of the separator plates 2a, 2b after the separator plates 2a, 2b had been formed, resulting in the outer coating 27 having a substantially uniform surface finish. It can therefore be provided that the respective outer coating 27 is applied to the entire surface, covering the entire area and with a substantially constant layer thickness. The outer coating 27 can, for example, comprise or be a PVD (physical vapor deposition) coating.According to one embodiment, the outer coating 27 is designed as an anti-corrosion layer and / or to reduce the contact resistance of the bipolar plate 2. The outer coating 27 can, for example, comprise or consist of titanium and / or carbon. The outer coating 27 can be a single layer or have at least two layers. For example, titanium can be applied to the substrate of the separator plate 2a, 2b as a first layer. Subsequently, a carbon layer can be applied to the titanium layer. Alternative layers, particularly layers close to the substrate, comprise, for example, metal nitrides such as ZrN, TiN, CrN, or TiAlN, or other metal compounds such as TiCN and TiON. The first and second layers can, for example, have thicknesses of at least 20 nm, at least 25 nm, or at least 50 nm, and / or at most 500 nm, at most 300 nm, or at most 200 nm.

[0055] In the Fig. 3. On the outer coating 27, a further layer is present in sections, cf. the polymeric sealant 30, which was applied using screen carriers or rollers. The polymeric sealant 30 is located outside the electrochemically active area 18 on a corrugated roof of the corrugated arrangement 12 and is intended to provide local micro-sealing, while the sealing corrugation 12 itself is intended for macro-sealing. This means that a bipolar plate for micro-sealing in the area of ​​the sealing corrugations may have coatings other than those described here; for example, a coating for micro-sealing may also be applied to an outer or inner coating and cover it in sections.

[0056] The two separator plates 2a, 2b are connected to each other by means of at least one welded joint 21, wherein at least one of the outer coatings 27 can be arranged on the welded joint 21, cf. Fig. 8. In this case, the separator plates 2a, 2b are therefore provided with the outer coatings 27 after welding. Often, at least one weld joint 21 is provided in a flat area 22 of the separator plates 2a, 2b, such as in a non-fluid-carrying edge area, cf. area 22 in Fig. 2. A corresponding weld joint 21a, in particular for tight welding of the separator plates 2a, 2b, is adjacent to the perimeter bead 12d in Fig. 3. In this flat area, the separator plates 2a, 2b generally touch over their entire surface, which simplifies welding the plates 2a, 2b. Optionally, short weld seams 21b may also be present in the electrochemically active area, as is shown in Fig. Figure 3 is also shown. In all welds 21a, 21b, it is clear that the outer coating 27 covers the welds.

[0057] Due to the different requirements and conditions on the inside and outside of the bipolar plate 2, the inner coating 25 and the outer coating 27 can differ in their materials and layer thicknesses. Alternatively, the inner coating 25 and the outer coating 27 can also be made of the same material and / or have the same layer thicknesses.

[0058] In the Fig. Figures 4-7 compare coatings 25 applied before forming with coatings 27 applied after forming. For better comparability, both coatings 25 and 27 include the Fig. 4-7 the same materials, namely titanium and carbon.

[0059] In the Fig. 4 and Fig. In the SEM images, microcracks are clearly visible, which were caused by the subsequent reshaping of the separator plates 2a, 2b in the coating 25. The reshaping process stretches, thins, cracks, or damages the coating 25, resulting in the aforementioned microcracks.

[0060] In the REM images, which are in the Fig. 6 and Fig. Figure 7, shown in the upper part, depicts lines 23 along which energy-dispersive X-ray spectroscopy (EDX) measurements were taken. The SEM images were acquired at a voltage of 10 kV and a working distance (WD) of 12 mm. The microcracks manifest in the EDX images as fluctuating proportions of Fe, Ti, and C. At each crack, the proportion of the coating (C, Ti) decreases significantly, while the proportion of the plate material (Fe) increases to a similar extent. This suggests that the cracks extend to the plate material and are not limited to the coating.

[0061] On the other hand, in the Fig. 5 and Fig. 7. It is evident that the coating 27, which was applied to the sheet material after forming, does not exhibit these microcracks and has a significantly smoother, more uniform surface structure. In the Fig. Figure 7 below clearly shows that both Ti and C layers have a constant layer thickness, indicating that both layers are intact.

[0062] The following describes a method for manufacturing bipolar plate 2; see also Fig. 9. The procedure includes the following steps S10, S20, S12, S22, S30 and S40: S10 Providing a first separator plate 2a with an inner surface 24, an outer surface 26 and an inner coating 25 arranged on the inner surface 24. S12 Forming the first separator plate 2a to form fluid guiding structures 16, 16' in the electrochemically active area 18, wherein the inner coating 25 is arranged in the area of ​​the fluid guiding structures 16'.

[0063] In this process, the inner coating 25 in a formed area of ​​the electrochemically active area 18 of the first separator plate 2a forms a different surface texture than in a non-formed area 22 of the first separator plate 2a. S20 Providing a second separator plate 2b with an inner surface 24, an outer surface 26 and an inner coating 25 arranged on the inner surface 24. S22 Forming the second separator plate 2b to form fluid guiding structures 16, 16' in the electrochemically active area 18, wherein the inner coating 25 is arranged in the area of ​​the fluid guiding structures 16'.

[0064] The inner coating 25 forms a different surface texture in a formed area of ​​the electrochemically active area 18 of the second separator plate 2b than in a non-formed area 22 of the second separator plate 2b due to the forming process. S30 Connecting, preferably by material bonding, the first separator plate 2a with the second separator plate 2b.

[0065] When joining, the first separator plate 2a and the second separator plate 2b can be welded together, for example by means of at least a weld joint 21. The inner coating is often destroyed and removed in the area of ​​the weld joint 21. S40 Application of an outer coating 27 to the outside 26 of the first separator plate 2a in the area of ​​the fluid guidance structures 16 and an outer coating 27 to the outside 28 of the second separator plate 2b in the area of ​​the fluid guidance structures 16.

[0066] The outer coating 27 is applied, for example, by PVD, particularly after forming and / or joining the separator plates 2a, 2b. If the weld joint 21 is located outside the electrochemically active area 18, local damage to the outer coating 27 outside the electrochemically active area 18 increases the corrosion risk to an acceptable extent. In this case, steps S30 and S40 can be reversed in their order. According to an exemplary embodiment, the outer coating 27 is applied simultaneously to the outer surface 26 of the first separator plate 2a and the outer coating 27 to the outer surface 26 of the second separator plate 2b, whereby the same or different coating thicknesses of the same coating materials are possible on both outer surfaces.

[0067] They can also be applied sequentially if, for example, different outer coatings 27 with different compositions are desired on the anode and cathode sides. In some embodiments, the bipolar plate 2 can remain in the same tool, allowing the different outer layers 27 to be applied directly one after the other in the same process step.

[0068] In a subsequent process step, a further coating, in particular section by section, can be applied to the corrugated roof of the corrugated arrangement 12, cf. polymeric sealing 30 of the Fig. 3.

[0069] The process is particularly suitable for the production of the bipolar plate 2 described above. Features of the process and of the bipolar plate 2 can thus be combined. Reference symbol list: 1 electrochemical system 2 Separator plate or bipolar plate 2a first single plate 2b second single plate 3 End plate 4 End plate 5 Media connection 6 stacks 7 z-direction 8 x-direction 9 y-direction 10 Membrane electrode unit 11a-c Through openings 12 Sealing arrangement 12a-d sealing beads 13a-c procedures 14 Gas diffusion layer 15 Membrane reinforcement layer 16, 16' channels 17 fluid-carrying area, flow field 18 electrochemically active areas 19 Cavity 20 Distribution or collection area 21, 21a, 21b Welded joint 22 flat, unformed area 23 lines in the SEM image along which EDX measurements were taken 24 Inside 25 Interior coating 26 Outside 27 Exterior coating 30 polymer sealant

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