Method for producing a bipolar plate, bipolar plate for an electrochemical cell and electrochemical cell

Abstract

Method for producing a bipolar plate (14e, 16e, 14e', 16e'), in particular for producing a bipolar plate (14e, 16e, 14e', 16e') for an electrochemical cell (10, 10'), comprising: - providing a substrate (42e) having a substrate surface (68e) and - applying a coating (46e) to at least a portion of the substrate surface (68e), in which method, in a plasma spraying process, a rough and / or porous cover layer (54e) of the coating (46e) is applied to at least a portion of the substrate surface (68e) with a rough and / or porous cover layer surface (72e) facing away from the substrate (42e), wherein a protective layer (56e) of the coating (46e) is applied to the cover layer surface (72e), characterized in that the coating (46e) is applied with a roughness which has a value in a range of approximately 0.6 to approximately 0.85, and in that the protective layer (56e) is applied in such a way that the roughness of the cover layer (54e) is maintained or substantially maintained.

Description

[0001] The present invention relates to a method for producing a bipolar plate, in particular for producing a bipolar plate for an electrochemical cell, comprising: - Providing a substrate having a substrate surface and - applying a coating to at least part of the substrate surface, in which method a rough and / or porous cover layer of the coating is applied to at least part of the substrate surface with a rough and / or porous cover layer surface facing away from the substrate in a plasma spraying process, wherein a protective layer of the coating is applied to the cover layer surface.

[0002] Furthermore, the present invention relates to a bipolar plate for an electrochemical cell, comprising a substrate having a substrate surface and having a coating applied to at least part of the substrate surface, which coating comprises a rough and / or porous cover layer applied to the substrate surface in a plasma spraying process and having a rough and / or porous cover layer surface facing away from the substrate, wherein the coating comprises a protective layer and that the cover layer surface is provided or covered with the protective layer.

[0003] Furthermore, the invention relates to an electrochemical cell, in particular an electrolysis cell or a fuel cell, comprising at least two electrodes, wherein at least one of the at least two electrodes is designed in the form of a bipolar plate.

[0004] Methods and bipolar plates of the type described above are known, in particular, from DE 10 2013 213 015 A1. In these methods, a layer is applied to a substrate using a plasma spraying process, and the layer is sealed. The sealing process, in particular, smooths out or closes roughness and pores in the applied layer. Overall, a substantially smooth layer is created on the substrate.

[0005] However, a smooth surface of the applied layer has the disadvantage that it is very wetted by water. The contact angle of a water droplet on the layer is comparatively small, typically below 30°. The known sealed layer therefore exhibits essentially hydrophilic properties. However, this has the disadvantage that water generated, particularly in a fuel cell, adheres to the surface of the sealed layer, making it difficult to remove.

[0006] DE 11 2005 002 439 T5 discloses a bipolar plate with improved stability. US Pat. No. 6,967,065 B1 discloses a separator for a proton exchange fuel cell. Coating processes and corrosion-protective coatings for electrodes are described in DE 10 2006 031 791 A1.

[0007] It is therefore an object of the present invention to improve a method and a bipolar plate of the type described above.

[0008] This object is achieved according to the invention in a method of the type described at the outset in that the coating is applied with a roughness which has a value in a range from approximately 0.6 to approximately 0.85, and that the protective layer is applied in such a way that the roughness of the cover layer is retained or substantially retained.

[0009] The coating thus comprises at least two layers, namely the cover layer, which is applied directly to the substrate, namely its substrate surface, and a protective layer, which is applied to the rough and / or porous cover layer surface. In particular, it is advantageous if the protective layer is applied in such a way that the porosity of the cover layer is retained or substantially retained. In particular, the protective layer is not intended to close or compensate for pores and roughness in the cover layer, but rather to maintain them. This results in a microstructure of the coating as a whole that has significantly more hydrophobic properties compared to the prior art.The contact angle of a water droplet on the coating proposed according to the invention is large compared to the bipolar plates known from the prior art; in particular, it can be greater than 70°. This prevents large-area wetting of the coating by water generated, for example, in a fuel cell. The generated water can be removed much more easily than with bipolar plates produced by processes known from the prior art. The coating is preferably carbon-free. This has the particular advantage that the conductivity of the coating cannot be impaired by the presence of carbon. The substrate is particularly well protected against corrosion if the coating, i.e., in particular the cover layer, is a dense and non-porous layer.According to the invention, the coating is applied with a roughness ranging from approximately 0.6 to approximately 0.85. The specified roughness values are defined as the ratio between the geometric area of the coating and the actual surface of the coating, i.e., roughness=A. geometrisch / A tatsächlich .

[0010] It is advantageous to apply a cover layer to the substrate that is or contains titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), chromium nitride (CrN), or titanium suboxide (Ti4O7). These materials, in particular, enable the relatively cost-effective application of rough and / or porous cover layers by plasma spraying or thermal spraying. In particular, relatively thick cover layers can be applied cost-effectively, providing the substrate with significantly better protection against corrosion than if only a thin gold layer were applied directly to the substrate, which is easily damaged by mechanical stress.

[0011] Good corrosion protection for the substrate can be achieved, in particular, if the top layer is applied to the substrate surface with an average coating thickness in a range of approximately 5 µm to approximately 500 µm. It is particularly advantageous if the average coating thickness is in a range of approximately 10 µm to approximately 100 µm. The thicker the top layer, the better the corrosion protection for the substrate.

[0012] Preferably, the protective layer is applied to the cover layer surface with a constant or substantially constant protective layer thickness. This has the particular advantage that the porosity and / or roughness of the cover layer surface is essentially maintained despite the application of the protective layer. Maintaining the porosity or roughness, in particular, makes it possible to maintain a hydrophobic property of the cover layer, which enables the removal of generated water from the bipolar plate.

[0013] The protective layer serves the primary purpose of preventing passivation of the cover layer, which occurs particularly through the formation of titanium oxide (TiO2) when titanium applied to the substrate by plasma spraying comes into contact with air. However, this passivation is undesirable in bipolar plates because it reduces the conductivity of the bipolar plate. In particular, the protective layer can be formed from a highly conductive metal.

[0014] It is advantageous if the protective layer is applied to the cover layer with a thickness in a range from approximately 10 nm to approximately 5,000 nm. In particular, it is advantageous if the protective layer thickness is in a range from approximately 20 nm to approximately 500 nm. Particularly with a material for forming the protective layer that is expensive, relatively thin protective layers can preferably be applied. A thin protective layer also has the advantage that the roughness and / or porosity of the cover layer can be substantially retained.

[0015] A protective layer can be easily applied to the top layer by electrochemical deposition. Electrochemical deposition of the protective layer also has the advantage that it allows for a simple, essentially constant protective layer thickness.

[0016] A bipolar plate with the desired properties can be produced particularly simply and cost-effectively by applying only one cover layer to the substrate and only one protective layer to the cover layer. In this case, the coating comprises only two layers.

[0017] A gold layer is preferably applied as a protective layer. This is ideal for passivating a titanium top layer, for example, i.e., preventing titanium oxide from forming on the surface of the top layer. Furthermore, gold makes it possible to completely cover and protect the top layer despite a very thin layer thickness. Gold also possesses excellent conductivity and inertness, which can ensure, in particular, very good conductivity of the bipolar plate and good corrosion protection of the top layer.

[0018] According to a further preferred variant of the method according to the invention, it can be provided that the protective layer is applied with a protective layer thickness such that a mass of the protective layer amounts to approximately 0.1% to approximately 2% of a total mass of the cover layer and the protective layer. The mass of the protective layer in the total mass is preferably approximately 1%, and the mass of the cover layer is approximately 99%. Such a structure enables, in particular, optimal passivation of the substrate by the cover layer, which can be made relatively thick. The material for the cover layer can be selected in particular at low cost. Thus, only a particularly thin protective layer is required to passivate the cover layer, so that it can also be made from a relatively expensive material in order to optimize the properties of the bipolar plate.

[0019] It is advantageous if the top layer and the protective layer are applied in such a way that an interface contact resistance measured according to the standard method of the US Department of Energy is less than about 15 mΩ cm 2 Preferably, the interface contact resistance is less than about 10 mΩ cm 2 The lower the contact resistance, the higher the efficiency of an electrochemical cell equipped with such a bipolar plate.

[0020] Water management, i.e., in particular, the drainage of water formed, for example, in a fuel cell from a surface of the bipolar plate, can be improved in particular by applying the cover layer and the protective layer in such a way that a water droplet contact angle is greater than approximately 70°. It is particularly advantageous if the water droplet contact angle is greater than approximately 80°.

[0021] It is advantageous if the coating is applied with a roughness value of 0.62. The specified roughness value is defined as the ratio between the geometric area of the coating and the actual surface of the coating, i.e. roughness = A geometrisch / A tatsächlich .

[0022] To form a bipolar plate, it is advantageous to provide a plate-shaped substrate.

[0023] Bipolar plates can be formed particularly cost-effectively if a substrate is provided from steel, copper, and / or aluminum. In particular, the substrate can be stainless steel, which can combine good electrical conductivity and corrosion protection properties.

[0024] It is advantageous if the substrate is provided with multiple flow channels on one or both sides. This allows gases and / or liquids to be supplied and / or discharged easily and safely through the flow channels.

[0025] Advantageously, the substrate surface is roughened using a sandblasting and / or grinding process. This can improve the adhesion of the top layer. It is particularly advantageous to roughen the substrate surface before the top layer is applied. This can improve the adhesion of the top layer.

[0026] The object set out at the outset is further achieved according to the invention in a bipolar plate of the type described at the outset in that the roughness of the coating has a value in a range from approximately 0.6 to approximately 0.85 and that the protective layer is designed in such a way that the roughness of the cover layer is maintained.

[0027] The protective layer makes it possible, in particular, to use materials to form the cover layer that are themselves susceptible to corrosion or tend to passivation. In both cases, this can lead to reduced conductivity, which is undesirable for the bipolar plate. The protective layer makes it possible, in particular, to prevent passivation or corrosion of the cover layer. This is particularly advantageous for a layer consisting of or containing titanium, since this is passivated upon contact with oxygen by forming a thin titanium oxide layer (TiO2). Furthermore, the protective layer can be designed in such a way that the porous cover layer surface of the cover layer is retained. This makes the coating overall rough and / or porous, which can minimize wetting of the coating, in particular by water, from the bipolar plate.A bipolar plate constructed in this way is particularly well-suited for use in electrochemical cells operating at low temperatures, such as low-temperature fuel cells or electrolysis cells, since particularly large amounts of water can be produced or present during low-temperature operation, which must be removed. According to the invention, the roughness of the coating has a value in a range from approximately 0.6 to approximately 0.85. The specified roughness values are defined as the ratio between the geometric area of the coating and the actual surface of the coating, i.e., roughness=A. geometrisch / A tatsächlich .

[0028] A cost-effective bipolar plate can be formed, in particular, by using a cover layer made of or containing titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), chromium nitride (CrN), or titanium suboxide (Ti4O7). Thick cover layers can thus be applied relatively inexpensively to protect the substrate from corrosion.

[0029] Particularly well-functioning bipolar plates can be obtained when the cover layer has an average thickness in a range of approximately 5 µm to approximately 500 µm. In particular, a cover layer thickness in a range of approximately 10 µm to approximately 100 µm is preferred. This allows an optimal cover layer thickness to be selected to reliably protect the substrate from corrosion and to minimize the costs of manufacturing the bipolar plate.

[0030] According to a further preferred embodiment of the invention, it can be provided that the protective layer has a constant or substantially constant protective layer thickness. Such protective layers make it possible, in particular, to maintain or substantially maintain a roughness and / or porosity of the cover layer. A surface of the coating is then not microscopically smooth, but microscopically rough, with a roughness and / or porosity that is predetermined by the sprayed-on cover layer. Thus, the hydrophobic property of the cover layer can be maintained due to its roughness, in particular even after the application of the protective layer.

[0031] It is advantageous if the protective layer thickness is in a range of approximately 10 nm to approximately 5,000 nm. In particular, the protective layer thickness is in a range of approximately 20 nm to approximately 500 nm. Protective layer thicknesses in the specified ranges offer optimal corrosion protection of the top layer while simultaneously optimizing costs and minimizing production time.

[0032] Preferably, the protective layer is deposited electrochemically. A protective layer applied to the cover layer in this way makes it possible, in particular, to automatically apply protective layers with essentially constant thicknesses.

[0033] An optimal structure for good bipolar plate functionality while simultaneously minimizing manufacturing effort and costs can be achieved, in particular, by having the bipolar plate comprise only a single cover layer and / or a single protective layer. In particular, the substrate coating can comprise only exactly two layers: a cover layer and a protective layer.

[0034] Optimal protection of the cover layer can be achieved in particular by making the protective layer or containing gold. In particular, a thin gold layer can prevent automatic passivation of, for example, a titanium cover layer. Furthermore, a protective gold layer can be applied so thinly that roughness and / or porosity of the cover layer can be retained. However, it can also be advantageous if the protective layer contains iridium (Ir), platinum (Pt), boron-doped diamond (BDD), boron-doped silicon carbide (SiC:B), titanium metal oxide (T 1-x M x O2) with M=tungsten, molybdenum (Mo) or niobium (Nb) or contain these elements or mixtures individually or mixed.

[0035] It is particularly advantageous if the protective layer has a protective layer thickness such that a mass of the protective layer corresponds to approximately 0.1% to approximately 2% of the total mass of a cover layer and the protective layer. It is particularly advantageous if a mass of the protective layer corresponds to approximately 1% of the total mass. In particular, the cover layer can be optimized to protect the substrate against corrosion, the protective layer to protect the cover layer against corrosion or to passivate the latter. In particular, if the protective layer is made of gold, the overall costs of the coating can be minimized because, compared to state-of-the-art gold coatings, only a very thin gold layer needs to be applied to the cover layer to form a bipolar plate with the desired properties.

[0036] It is advantageous if the coating has an interfacial contact resistance measured according to the standard method of the US Department of Energy, which is less than about 15 mΩ cm 2 In particular, the interface contact resistance is preferably less than about 10 mΩ cm 2 Such interface contact resistances enable particularly high current densities.

[0037] Furthermore, it is advantageous if the coating has a water droplet contact angle greater than approximately 70°. Preferably, it is greater than approximately 80°. A coating with such a water droplet contact angle is essentially sufficiently hydrophobic that the coating is not wetted by water across its entire surface. This allows water generated in a fuel cell, for example, to be easily and safely drained away from the bipolar plate.

[0038] It is advantageous if the roughness of the coating has a value of 0.62. The specified value for roughness is defined as the ratio between the geometric area of the coating and the actual surface of the coating, i.e. roughness = A geometrisch / A tatsächlich .

[0039] To form a bipolar plate, it is advantageous if the substrate is plate-shaped.

[0040] Preferably, the substrate is made of steel, copper, and / or aluminum. Advantageously, the substrate is made of stainless steel. Stainless steel, in particular, already exhibits a certain degree of corrosion resistance.

[0041] However, the coating proposed according to the invention can provide even better protection against corrosion for the substrate.

[0042] Furthermore, it is advantageous if the substrate is provided with multiple flow channels on one or both sides. Flow channels, in particular, enable optimized fluid management, i.e., the supply and removal of fluids from the surface of the bipolar plate can be implemented particularly easily.

[0043] A cover layer adheres particularly well if the substrate has a substrate surface roughened by a sandblasting process and / or a grinding process, to which the cover layer in particular is applied.

[0044] According to a further preferred embodiment of the invention, the substrate can have at least two substrate surfaces facing away from one another, and at least one of the at least two substrate surfaces can be completely or substantially completely covered with the coating. In particular, both substrate surfaces can also be covered with the coating. This allows, for example, stacks of fuel cells and / or stacks of electrolysis cells to be easily produced.

[0045] The object set out at the outset is further achieved according to the invention in an electrochemical cell of the type described at the outset in that the bipolar plate is designed in the form of one of the bipolar plates described above.

[0046] The electrochemical cell proposed according to the invention then also has the advantages described above in connection with preferred embodiments of bipolar plates.

[0047] The electrochemical cell is preferably in the form of a polymer electrolyte fuel cell, polymer electrolyte water electrolyzer, or a redox flow battery. For example, the electrochemical cell can comprise dimensionally stable anodes, which are particularly important for the electrochemical chlorine industry. Furthermore, the fabrication of highly hydrophobic metallic coatings on substrates is also possible in the manner described above. In particular, these can be electrochemical cells operating at low temperatures, for example low-temperature fuel cells or electrolysis cells, during the operation of which a particularly large amount of water can be produced or present, which must be removed. In particular, the electrochemical cell can be an electrolysis cell with a proton exchange membrane, the cathode of which is designed in the form of one of the bipolar plates described above.

[0048] The following description of preferred embodiments of the invention serves to explain it in more detail in conjunction with the drawings. They show: Fig. 1: a schematic sectional view of a fuel cell with two bipolar plates; Fig. 2: an enlarged view of area A in Fig. 1 with a coating according to the state of the art; Fig. 3: a view similar Fig. 2 with a coating according to the present invention; Fig. 3A: an enlarged schematic view of area B from Fig. 3; Fig. 4: a schematic sectional view of the structure of an electrolysis cell; Fig. 5A: a scanning electron microscope image of a section of the bipolar plate at 100× magnification; Fig. 5B: a scanning electron microscope image of a section of the bipolar plate at 400× magnification; Fig. 5C: a scanning electron microscope image of a section of the bipolar plate at 700× magnification; Fig. 5D: a scanning electron microscope image of a section of the bipolar plate at 5,000× magnification; Fig. 6A: a photographic representation of a water droplet on a bipolar plate according to the prior art; Fig. 6B: a photographic representation of a water droplet on a bipolar plate according to the present invention; Fig. 7: Interface contact resistances as a function of compaction pressure of gold / stainless steel samples (state of the art) and gold / titanium / stainless steel samples measured according to the standard of the US Department of Energy (DOE); Fig. 8: Comparison of the time courses of the currents generated by a polymer electrolyte fuel cell when using gold-coated bipolar plates (prior art) and gold / titanium-coated bipolar plates according to the invention; and Fig. 9: Comparison of the temporal course of the cathode pressure of a polymer electrolyte fuel cell when using gold-coated bipolar plates (state of the art) and gold / titanium-coated bipolar plates according to the invention.

[0049] In Fig. 1 schematically shows a sectional view through an electrochemical cell, designated overall by the reference numeral 10, which is designed in the form of a fuel cell 12.

[0050] The cell 10 comprises two bipolar plates 14 and 16. The bipolar plate 14 forms an anode of the fuel cell 12, and the bipolar plate 16 forms a cathode 20 thereof. Both bipolar plates 14 and 16 are preferably made of stainless steel.

[0051] Flow channels 22 and 24 are formed in both the bipolar plate 14 and the bipolar plate 16, respectively. These have a rectangular or substantially rectangular cross-section. The two bipolar plates 14 and 16 are arranged parallel to one another in such a way that the flow channels 22 and 24 formed in the upper sides 25 and 27 of the bipolar plates 14 and 16 are each open toward the other bipolar plate 14 and 16, respectively.

[0052] The upper side 25 of the bipolar plate 14 carries an anodic gas diffusion layer 26 covering the flow channels 22, and the upper side 27 of the bipolar plate carries a cathodic gas diffusion layer 28 covering the flow channels 24 of the bipolar plate 16.

[0053] A microporous layer 30 and a catalyst layer 32 are applied to the anodic gas diffusion layer 26, and a microporous layer 34 and a catalyst layer 36 are applied to the cathodic gas diffusion layer. A proton exchange membrane 38 is arranged between the two microporous layers 30 and 34.

[0054] The anodic gas diffusion layer 26, the microporous layer 30, the catalyst layer 32, the proton exchange membrane 38, the catalyst layer 36, the microporous layer 34 and the cathodic gas diffusion layer 28 form a multilayer membrane electrode assembly 40 which is arranged between the upper sides 25 and 27 of the two bipolar plates 14 and 16.

[0055] During operation of the fuel cell 12, hydrogen flows through the flow channels 22 of the anode 18, and oxygen flows through the flow channels 24 of the cathode 20. Furthermore, water droplets formed as a reaction product during the electrochemical reaction of oxygen (O2) and hydrogen (H2) in the fuel cell 12 are also discharged through the flow channels 24 of the cathode 20.

[0056] The Fig. The structure shown in Figure 1 is known from the prior art.

[0057] In Fig. 2 shows a schematic representation of a section A of the bipolar plate 16, as is known from the prior art.

[0058] A substrate 42, preferably in the form of a steel plate, for example made of stainless steel, is first provided with the flow channels 24. For example, these are produced on the upper side 27 of the bipolar plate 16 by milling or etching.

[0059] The known from the state of the art, in Fig. The bipolar plate 16 shown in Figure 2 is provided with a coating 46 on its upper side 27 and on surfaces 50 of the flow channels 24. This coating 46 can be, for example, gold or a titanium layer applied by plasma spraying, the pores of which are sealed by a polymer. This coating 46 is highly hydrophilic. This is particularly evident in the fact that water droplets 48 wet a surface 50 of the flow channels 24 over a large area. A contact angle 52 between water droplets 48 and surface 50 is less than approximately 30°.

[0060] Fig. Figure 6A shows the image of a water droplet on a substrate 42 provided with the described coating 46 known from the prior art. It can be clearly seen that the contact angle θ Kontakt is less than 30°. The surface 50 is therefore wetted over a large area.

[0061] Fig. Figure 3 shows a schematic representation of a bipolar plate 16e formed according to the invention and capable of replacing the bipolar plate 16 in the cell 10. The substrate 42e is provided with a cover layer 54e applied by plasma spraying, in particular by a vacuum plasma spraying process. The cover layer 54e may, in particular, be or contain titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), chromium nitride (CrN), or titanium suboxide (Ti4O7).

[0062] A protective layer 56e is applied to the cover layer 54e. The protective layer 56e is preferably formed by electrochemical deposition. In particular, the protective layer 56e can be an electrochemically deposited layer of gold.

[0063] As in the Fig. 3 and Fig. As can be clearly seen in Figure 3A, the protective layer 56e has a constant or substantially constant protective layer thickness 58e. In this way, the roughness and / or porosity of the cover layer 54e are maintained. A surface 50e of the two-layer coating 46e, which comprises only the cover layer 54e and the corrosion layer 56e, is thus substantially maintained.

[0064] Due to the roughness of the cover layer 54e, which is not compensated for by the protective layer 56e and / or whose pores are not filled, as is the case in the prior art, a hydrophobic coating 46e is formed overall. Water droplets 48e wet the coating 46e only poorly. A contact angle 52e of the water droplets 48e is formed that is significantly greater than 70°. In particular, it can be greater than 80°. Fig. Figure 3 schematically shows a contact angle that is greater than 90°.

[0065] In Fig. 6B is a contact angle θ Kontakt shown for a water droplet 48e on a substrate with a coating 46e comprising the cover layer 54e and the protective layer 56e. It can be seen that the adhesion forces between the water molecules of the water droplet 48e and the bipolar plate 16e are significantly lower than for water droplets 48 on the bipolar plate 16. Large-volume water droplets 48e are formed, which can be easily removed from the flow channels 24e.

[0066] In Fig. 4 schematically shows, by way of example, a structure of an electrochemical cell, designated overall by the reference numeral 10', in the form of a polymer electrolyte water electrolyzer 60.

[0067] Similar to the Fig. In the fuel cell 12 shown in Figure 1, two bipolar plates 14e' and 16e' are arranged parallel to one another with flow channels 22e' and 24e' arranged facing one another. Between the bipolar plates 14e' and 16e', a membrane electrode assembly 40' is arranged, which has a proton exchange membrane 38', which is adjacent on both sides to catalyst layers 32' and 34', which in turn are adjacent on one side to an anodic gas diffusion layer 26' and on the other side to a cathodic gas diffusion layer 28'. These, in turn, are in surface contact with the upper sides 25' and 27' of the bipolar plates 14e' and 16e', respectively.

[0068] Water (H2O) is introduced into the cell 10' via an inlet 62, which is in fluid communication with the flow channels 24e', and a direct voltage is applied to the two bipolar plates 14e' and 16e'. The introduced water is split in the cell 10' into hydrogen (H2) and oxygen (O2). The produced hydrogen flows out of an outlet 64, which is in fluid communication with the flow channels 22e'. The oxygen produced by the electrolysis, along with any residual water that has not been split, is discharged from an outlet 66, which is in fluid communication with the flow channels 24e'.

[0069] The structure of the bipolar plates 14e' and 16e' corresponds to the structure of the bipolar plates 14e and 16e, i.e. all four bipolar plates have the same basic structure of the coating 46e.

[0070] The schematically shown in the Fig. 3 and Fig. The coating 46e shown in Figure 3A can be produced as follows. For example, a sandblasted 1.4301 stainless steel plate measuring 10 cm by 10 cm and 20 mm thick is used as the substrate 42e. A dense titanium cover layer 54e is applied to this substrate 42e by vacuum plasma spraying. The substrate is preheated to approximately 250 °C, and individual parameters, such as the type of plasma torch nozzle, the throughput rate of the titanium powder being sprayed, and the flow rates of argon, nitrogen, and hydrogen, are carefully monitored. The enthalpy for plasma generation is 21.27 MJkg -1The titanium powder preferably has a grain size of less than 45 µm and is sprayed into a vacuum chamber at a pressure of 50 mbar to prevent the formation of the oxide TiO2, which greatly reduces the conductivity of the coating 46e. The cover layer 54e can, in particular, comprise four successively sprayed layers and a total average coating thickness of 45 µm.

[0071] In the next step, the protective layer 56e is applied to the cover layer 54e by electrochemical deposition. The gold source used for this purpose is an acidic solution of potassium dicyanoaurate(I) (K[Au(CN2)]) with a pH of 1.5 and an aurate content of 2.95 g / l. This corresponds to 68.2%.

[0072] The metallization of the cover layer 54e was carried out at a current density of 70 mA / cm 2 for about 8 min. During this time, a total amount of 0.53 mg / cm 2Gold is deposited. This results in a coating 46e that has a gold content of approximately 1% or less and a titanium content greater than approximately 99%.

[0073] A comparison of the coating 46e and the substrate 42e without coating results in a corrosion potential of 0.49 V relative to a hydrogen reference electrode (RHE) and a corrosion current i corr less than 10 µAcm -2 for the coating 46e. Under the same conditions, i.e. oxygen-saturated sulfuric acid H s SO4 at 24°C, the substrate 42e without coating showed a corrosion potential of -0.28 V relative to a hydrogen reference electrode (RHE) and a 1000 times higher corrosion current i corr .

[0074] The roughness of the coating 46e preferably has a value in a range of about 0.6 to about 0.85; in particular, the roughness may have a value of 0.62. The specified values for the roughness are defined as the ratio between the geometric area of the coating and the actual surface of the coating, i.e., roughness = A geometrisch / A tatsächlich .

[0075] Plasma spraying of the titanium top layer 54e results in a very rough and porous titanium layer. This is Fig. 5A to 5D. Compared to the surface 50e of the coating 46e, the substrate 42e has a significantly less rough substrate surface 68.

[0076] As particularly in Fig. As can be clearly seen in Figure 5D, a protective layer thickness 58e is essentially constant. It preferably ranges from 10 nm to approximately 5,000 nm. The very rough and porous cover layer 54e has a cover layer thickness 70e, which on average preferably ranges from approximately 13 µm to approximately 62 µm.

[0077] The coating 46e is thus, despite the protective layer 56e, essentially as rough as the cover layer 54e, so that overall a highly hydrophobic surface 50e of the coating 46e is formed, which is only wetted with difficulty by water droplets 48e.

[0078] Fig. Figure 7 shows the interfacial contact resistance of the bipolar plates 16 (prior art) and 16e (invention) as a function of the compaction pressure and measured according to the standard of the US Department of Energy (DOE).

[0079] In Fig. 7 also shows a dashed rectangle area that corresponds to the DOE specification for 2015, namely a contact resistance of maximum 20 mΩ cm 2 at a compaction pressure of maximum 140 N / cm 2 . In Fig. 7 it can be seen that this requirement cannot be achieved by bipolar plates 16 according to the state of the art.

[0080] In contrast, the interface contact resistance of the bipolar plates 16e at the same pressure is below 10 mΩ cm 2 within the DOE's specifications. The significant increase in performance results in particular from the high roughness of the cover layer 54e due to the vacuum plasma spraying of the same onto the substrate 42e, which not only improves the adhesion of the protective layer 56e made of gold to the cover layer 54e made of titanium, but also reduces wetting by water droplets 48e.

[0081] Fig. Figure 8 shows the temporal evolution of the current generated by the fuel cell 12 with the bipolar plates 14e and 16e under conditions that promote water accumulation. The cell temperature T Zelle was 50 °C, a nozzle temperature T Düse 80 °C. Other parameters of the measurement are RH» 100 %, flow a / c 209 / 664 and cell voltage V const = 600 mV. These measurement conditions apply to the bipolar plates 14 and 16 according to the prior art with the coating 46 and to the bipolar plates 14e and 16e having a coating 46e according to the invention.

[0082] As in Fig. As can be clearly seen in Figure 8, a fuel cell with bipolar plates 14 and 16 according to the current state of the art exhibits periodically occurring current peaks caused by the accumulation of water droplets in the flow field, i.e., in the flow channels 24. In contrast, the number and severity of such current peaks can be minimized by using the bipolar plates 14e and 16e. Furthermore, the invention allows for a 17% increase in current and thus also water yield. A fuel cell 12 with bipolar plates 14e and 16e according to the invention is thus clearly superior to the current state of the art.

[0083] The described phenomenon is also evident in the temporal course of the cathode pressure, which in Fig. 9 for a fuel cell 12 equipped with bipolar plates 14 and 16 according to the prior art, and for a fuel cell 12 with bipolar plates 14e and 16e according to the invention. The pressure peaks that periodically occur in the conventional coating 46, which exclusively comprises a layer of gold, result from the formation of water droplets 48, the removal of which from the cell 10 is greatly hampered due to the heavy wetting of the coating 46. The operating conditions for measuring the cathode pressure as a function of time correspond to those described above in connection with Fig. 8 in connection with the measurement of current as a function of time.

[0084] In particular, through the Fig. 8 and Fig.The measurements of the temporal profiles of current and cathode pressure shown in Figure 9 confirm the improved water management in fuel cells 12 with bipolar plates 14e and 16e compared to the prior art. The release of produced water is particularly facilitated by the invention, whereby the coating 46e proposed according to the invention distinguishes itself from known coatings 46 from the prior art.

[0085] The use of bipolar plates 14e and 16e with the described coating 46e results in improved thermal and mechanical properties. The accumulation of produced water in fuel cells is reduced. Furthermore, in particular, a 30% higher current efficiency at low voltages and high water production rates are achieved compared to a coating 46 made of pure gold.

[0086] A significantly thicker cover layer 70e, preferably more than 40 µm, compared to a pure gold layer with a thickness of approximately 3 µm, also promotes the corrosion protection of the substrate 42e. The described high surface roughness of the coating 46e also reduces the contact resistance at the interface to approximately 8.6 mΩ cm. 2 , i.e. less than 10 mΩ cm 2 .

[0087] The described process technologies can be easily adapted to large-scale production. This makes it possible, in particular, to produce bipolar plates for polymer electrolyte fuel cells and bipolar plates for polymer electrolyte water electrolyzers on a large scale. Electrodes for redox flow batteries and dimensionally stable anodes for the electrochemical industry can also be easily produced on a large scale. The proposed coating 46e also enables the fabrication of highly hydrophobic, metallic, highly conductive coatings. List of reference symbols 10, 10' electrochemical cell 12 fuel cells 14, 14e, 14e' bipolar plate 16, 16e, 16e' bipolar plate 18 Anode 20 Cathode 22, 22e' flow channel 24, 24e' flow channel 25, 25' top 26, 26' anodic gas diffusion layer 27, 27' top 28, 28' cathodic gas diffusion layer 30 microporous layer 32 catalyst layer 34 microporous layer 36 catalyst layer 38, 38' proton exchange membrane 40, 40' membrane electrode array 42, 42e substrate 46, 46e coating 48, 48e water drops 50, 50e surface 52, 52e contact angle 54e top layer 56e protective layer 58e protective layer thickness 60 polymer electrolyte water electrolyzer 62 Entrance 64 Outlet 66 Outlet 68, 68e substrate surface 70e top layer thickness 72e top layer surface

Claims

[1] A method for producing a bipolar plate (14e, 16e, 14e', 16e'), in particular for producing a bipolar plate (14e, 16e, 14e', 16e') for an electrochemical cell (10, 10'), comprising: - providing a substrate (42e) having a substrate surface (68e) and - applying a coating (46e) to at least a portion of the substrate surface (68e), in which method, in a plasma spraying process, a rough and / or porous cover layer (54e) of the coating (46e) is applied to at least a portion of the substrate surface (68e) with a rough and / or porous cover layer surface (72e) facing away from the substrate (42e), wherein a protective layer (56e) of the coating (46e) is applied to the cover layer surface (72e), characterized bythat the coating (46e) is applied with a roughness having a value in a range of about 0.6 to about 0.85, and that the protective layer (56e) is applied such that the roughness of the cover layer (54e) is maintained or substantially maintained. [2] Method according to claim 1, characterized by that a cover layer (54e) is applied to the substrate (42e), which is or contains titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), chromium nitride (CrN), titanium suboxide (Ti4O7). [3] Method according to one of claims 1 or 2, characterized by that the cover layer (54e) is applied to the substrate surface (68e) with an average cover layer thickness (70e) in a range from about 5 µm to about 500 µm, in particular with an average cover layer thickness (70e) in a range from about 10 µm to about 100 µm. [4] Method according to one of the preceding claims, characterized bythat the protective layer (56e) is applied to the cover layer surface (72e) with a constant or substantially constant protective layer thickness (58e). [5] Method according to one of the preceding claims, characterized by that the protective layer (56e) is applied to the cover layer (54e) with a protective layer thickness (58e) in a range from about 10 nm to about 5000 nm, in particular with a protective layer thickness (58e) in a range from about 20 nm to about 500 nm. [6] Method according to one of the preceding claims, characterized by that the protective layer (56e) is applied to the cover layer (54e) by electrochemical deposition. [7] Method according to one of the preceding claims, characterized by that only one cover layer (54e) is applied to the substrate (42e) and only one protective layer (56e) is applied to the cover layer (54e). [8] Method according to one of the preceding claims, characterized bythat as a protective layer (56e) a gold layer or a layer consisting of iridium (Ir), platinum (Pt), boron-doped diamond (BDD), boron-doped silicon carbide (SiC:B), titanium metal oxide (T 1-x M x O2) with M=tungsten, molybdenum (Mo) or niobium (Nb), or containing these elements or mixtures individually or mixed. [9] Method according to one of the preceding claims, characterized by that the protective layer (56e) is applied with a protective layer thickness (58e) such that a mass of the protective layer (56e) is about 0.1% to about 2% of a total mass of the cover layer (54e) and the protective layer (56e). [10] Method according to one of the preceding claims, characterized by that the cover layer (54e) and the protective layer (56e) are applied in such a way that an interfacial contact resistance measured according to the standard method of the US Department of Energy is less than about 15·10 -7 Ω m2 is, in particular, less than about 10·10 -7 Ω m 2 . [11] Method according to one of the preceding claims, characterized by that the cover layer (54e) and the protective layer (56e) are applied such that a water drop contact angle (52e) is greater than about 70°, in particular greater than about 80°. [12] Method according to one of the preceding claims, characterized by that the coating (46e) is applied with a roughness having a value of 0.

62. [13] Method according to one of the preceding claims, characterized by that the substrate (42e) is provided in a plate-shaped manner. [14] Method according to one of the preceding claims, characterized by that the substrate (42e) is provided from a steel, copper and / or aluminum, in particular from a stainless steel. [15] Method according to one of the preceding claims, characterized bythat the substrate (42e) is provided on one or both sides with a plurality of flow channels (22e, 24e, 22e', 24e'), in particular before the cover layer (54e) is applied. [16] Method according to one of the preceding claims, characterized by that the substrate surface (68e) is roughened in a sandblasting process and / or in a grinding process, in particular before the cover layer (54e) is applied. [17] Bipolar plate (14e, 16e, 14e', 16e') for an electrochemical cell (10), comprising a substrate (42e) having a substrate surface (68e) and having a coating (46e) applied to at least part of the substrate surface (68e), which coating (46e) comprises a rough and / or porous cover layer (54e) applied to the substrate surface (68e) in a plasma spraying process and having a rough and / or porous cover layer surface (72e) facing away from the substrate (42e), wherein the coating (46e) comprises a protective layer (56e) and that the cover layer surface (72e) is provided or covered with the protective layer (56e), characterized by that the roughness of the coating (46e) has a value in a range of about 0.6 to about 0.85 and that the protective layer (56e) is formed such that the roughness of the cover layer (54e) is maintained. [18] Bipolar plate according to claim 17, characterized bythat the cover layer (54e) is or contains titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), chromium nitride (CrN), titanium suboxide (Ti4O7). [19] Bipolar plate according to one of claims 17 or 18, characterized by that the cover layer (54e) has an average cover layer thickness (70e) in a range from about 5 µm to about 500 µm, in particular in a range from about 10 µm to about 100 µm. [20] Bipolar plate according to one of claims 17 to 19, characterized by that the protective layer (56e) has a constant or substantially constant protective layer thickness (58e). [21] Bipolar plate according to claim 20, characterized by that the protective layer thickness (58e) has a value in a range from about 10 nm to about 5000 nm, in particular in a range from about 20 nm to about 500 nm. [22] Bipolar plate according to one of claims 17 to 21, characterized by an electrochemically deposited protective layer (56e). [23] Bipolar plate according to one of claims 17 to 22, characterized by only a single covering layer (54e) and / or only a single protective layer (56e). [24] Bipolar plate according to one of claims 17 to 23, characterized by that the protective layer (56e) is or contains gold or consists of iridium (Ir), platinum (Pt), boron-doped diamond (BDD), boron-doped silicon carbide (SiC:B), titanium metal oxide (T 1-x M x O2) with M=tungsten, molybdenum (Mo) or niobium (Nb), or contains these elements or mixtures individually or mixed. [25] Bipolar plate according to one of claims 17 to 24, characterized by that the protective layer (56e) has a protective layer thickness (58e) such that a mass of the protective layer (56e) corresponds to about 0.1% to about 2% of a total mass of the cover layer (54e) and the protective layer (56e). [26] Bipolar plate according to one of claims 17 to 25, characterized bythat the coating (46e) has an interfacial contact resistance measured according to the standard method of the US Department of Energy, which is less than about 15·10 -7 Ω m 2 is, in particular less than about 10.10 -7 Ω m 2 . [27] Bipolar plate according to one of claims 17 to 26, characterized by that the coating (46e) has a water drop contact angle (52e) which is greater than about 70°, in particular greater than about 80°. [28] Bipolar plate according to one of claims 17 to 27, characterized by that the roughness of the coating (46e) has a value of 0.

62. [29] Bipolar plate according to one of claims 17 to 28, characterized by that the substrate (42e) is plate-shaped. [30] Bipolar plate according to one of claims 17 to 29, characterized by that the substrate (42e) is made of a steel, copper and / or aluminum, in particular of a stainless steel. [31] Bipolar plate according to one of claims 17 to 30, characterized by that the substrate (42e) is provided with several flow channels (22, 24) on one or both sides. [32] Bipolar plate according to one of claims 17 to 31, characterized by that the substrate (42e) has a substrate surface (68e) roughened in a sandblasting process and / or in a grinding process, on which in particular the cover layer (54e) is applied. [33] Bipolar plate according to one of claims 17 to 32, characterized by that the substrate (42e) has at least two substrate surfaces (68e) facing away from one another and that at least one of the at least two substrate surfaces (68e) is completely or substantially completely covered with the coating (46e). [34] Electrochemical cell (10, 10'), in particular an electrolysis cell (60) or a fuel cell (12), comprising at least two electrodes, wherein at least one of the at least two electrodes is in the form of a bipolar plate (14e, 16e, 14e', 16e'), characterized by that the bipolar plate (14e, 16e, 14e', 16e') is in the form of a bipolar plate (14e, 16e, 14e', 16e') according to one of claims 17 to 33. [35] Electrochemical cell (10, 10') according to claim 34, characterized by that the electrochemical cell (10, 10') is in the form of a polymer electrolyte fuel cell (12), a polymer electrolyte water electrolyzer (60), a redox flow battery or an electrochemical cell with a dimensionally stable electrode for the electrochemical chlorine industry.

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