BIPOLAR PLATE FOR ELECTROCHEMICAL CELLS AND METHOD FOR PRODUCING THE SAME

Description

[0001] The invention relates to a coated bipolar plate for electrochemical energy converters, as is commonly used in fuel and / or electrolysis cells. The invention further relates to a method for producing the coated bipolar plate according to the invention. State of the art

[0002] An electrochemical energy converter (e.g. fuel cell or electrolysis cell) comprises two electrodes (anode and cathode) which are arranged in an inert fiber structure so as to be electrically insulated and mechanically separated from one another by a solid, semi-permeable electrolyte membrane or a liquid electrolyte. They form the so-called membrane electrode assembly (MEA). In fuel cells, the oxidation reaction takes place at the anode and the reduction reaction at the cathode; in electrolyzers, the reverse is true. Depending on the cell type, the electrolytes can be ion-conductive polymer membranes, dissolved alkalis or acids, alkali carbonate melts, or ceramics. A porous, electrically conductive gas diffusion layer made of carbon fleece / fabric or metal foam borders the electrode on both sides.An electrochemical energy converter has a bipolar plate (also called flow distributor or current collector plate) on both sides, which are usually made of electrically conductive carbon composite materials or metals.

[0003] The focus is on fuel cells or electrolyzers that operate with a solid polymer electrolyte membrane. A distinction is made between low-temperature polymer electrolyte fuel cells / electrolysis cells, which operate at approximately 85 °C, and high-temperature polymer electrolyte fuel cells / electrolysis cells, which operate in the temperature range between 120 and 180 °C. In both variants, a proton- or hydroxide-ion-conducting ionomer membrane acts as the electrolyte. It is gas-tight and non-electron-conducting. On each side, a catalyst layer (electrode), a porous gas diffusion layer, and a bipolar plate are connected. Three phases (catalyst as electron conductor, ionomer as proton conductor, and reactant) are always in contact on the electrode surface.The reactants (hydrogen and oxygen in fuel cells, and water in electrolysis cells) are introduced via the channel structure of the bipolar plates and distributed evenly over the catalyst surface with the help of the porous gas diffusion layer. The bipolar plate's function is to mechanically stabilize the electrochemical cell, supply and remove the reactants on both sides, and dissipate the generated electrical current.

[0004] Bipolar plates must therefore be mechanically stable, as they are a mechanically supporting element within fuel cells and electrolyzers and must withstand thermal expansion, high contact pressures, and, in mobile applications, vibrations and shocks. Bipolar plates must also have high electrical and thermal conductivity in order to efficiently conduct the generated electrical current and transfer thermal energy to the cooling medium. To do this, they must have a dense surface finish in order to neither absorb reactants nor electrolyte and to spatially separate them from the cooling medium. They also regulate the water balance (water supply in electrolyzers and water discharge in fuel cells). Furthermore, they require high stability against electrochemical corrosion at high temperatures and external potentials. Bipolar plates must, among other things,Concentrated phosphoric acid at temperatures up to 180 °C and electrochemical potentials up to approximately 2.2 V (vs. reversible hydrogen electrode). Electrochemical corrosion at the bipolar plate / electrolyte interface is the decisive criterion in the material selection of bipolar plates.

[0005] The current state of the art for both low- and high-temperature applications is the use of graphite-polymer composite materials. These are organic polymers such as polypropylene, polyphenylene sulfide, phenolic and vinyl ester resins with carbon black or graphite particles. These materials offer significant improvements in robustness and series production via injection molding or hot pressing compared to pure graphitic materials.

[0006] The polymer matrix of such graphite-polymer composite materials improves mechanical stability through elastic properties, but at the expense of electronic and thermal conductivity due to the insulating effect of the polymer components. Furthermore, graphite-based bipolar plates have a relatively high material thickness (> 2 mm).

[0007] This is where the enormous advantages of metallic bipolar plates lie. These can usually be produced using simple and inexpensive manufacturing processes, such as stamping or high-pressure forming. Furthermore, they exhibit good ductility and significantly higher mechanical stability against shocks and vibrations, which can often lead to cracks and subsequent gas leakage. Furthermore, they possess sufficiently high electronic and thermal conductivity for their application in a fuel cell or electrolyzer.

[0008] A significant challenge of metallic bipolar plates, which have so far been prevented from widespread introduction into fuel cell technology, is the susceptibility of metallic materials as bipolar plates to corrosion in acidic / alkaline and humid environments.

[0009] While low-temperature fuel cells / electrolyzers have moderate corrosion conditions (electrolyte concentration < 0.5 MH 2 SO 4 at ~ 85 °C), high-temperature fuel cells / electrolyzers have more drastic conditions due to the concentrated phosphoric acid used (up to 16 M) and the high temperature of up to 180 °C. This leads to the corrosion of metallic bipolar plates in contact with the electrolyte, and the released metal ions can adversely poison the catalyst and the polymer electrolyte membrane (Nafion ®< or polybenzimidazole membrane). In low-temperature applications with Nafion ®< membranes, the reduction in proton conductivity due to the incorporation of metal ions has already been investigated. Another corrosion phenomenon is the formation of non- orInsufficiently conductive passivation layers (metal oxides, hydroxides, phosphates) on the metal surface of a bipolar plate, which are associated with an increase in electrical contact resistance. This has also been extensively investigated for sulfuric acid conditions (Nafion ® membrane). In addition to the high operating temperature, the electrochemical potential is also a decisive factor influencing corrosion. Potentials of approximately 1 V can occur in fuel cells, and even up to 2.2 V in electrolyzers (vs. reversible hydrogen electrode).

[0010] The aforementioned disadvantages of corrosion can be reduced by applying durable and electronically conductive coatings to the metallic bipolar plate. Coatings such as ceramic nitrides and carbides based on titanium, chromium, aluminum, silicon, or zirconium, as well as graphitic or gold-based coatings produced by physical or chemical vapor deposition (PVD / CVD) are known to date. Electrochemical deposition processes of metal borides (e.g., NiCoB, Ni2B, or Ni3B), gold, or conductive organic polymers such as polyaniline or polypyrrole are also known from the literature. With all coating options, the difficulty always lies in producing defect-free layers that offer high long-term stability. Even the smallest defects such as cracks or holes (e.g.,pinholes) cause the electrolyte to spread under the coating and lead to corrosion damage.

[0011] KR 2013 0074342 A discloses a method for coating a metallic bipolar plate using a suspension which, in addition to graphene, contains further additives, such as conductive polymers.

[0012] Furthermore, WO 2015 / 074752 A1 describes a graphene coating on a bipolar plate, in the production of which a suspension of graphene oxide (graphite treated with hydrogen peroxide) is applied to stainless steel together with at least one adhesion promoter and is subsequently subjected to a thermal treatment.

[0013] Another possibility is to plate stainless steel as a bipolar plate material with a thin niobium layer, which forms stable and electronically conductive oxide layers and thus passivates the metal.

[0014] In principle, when selecting coatings, in addition to chemical and electrochemical stability, the electrical conductivity and the thermal expansion coefficient between the metallic bipolar plate material and the applied coating material must be taken into account in order to avoid surface cracks.

[0015] Bipolar plates in polymer electrolyte fuel cells are subject to electrochemical corrosion due to the acidic environment, as well as the influence of temperature and electrochemical potential. Only precious metals such as gold or very rare and expensive metals such as tantalum, which forms a stable and conductive oxide layer under the aforementioned conditions, can be used for such harsh conditions. Inexpensive stainless steels and nickel-based alloys, on the other hand, exhibit excessively high corrosion rates and / or the formation of non-conductive passive layers. This leads to a relatively rapid decrease in performance and, detrimentally, to the aging of the fuel cell.

[0016] However, in order to utilize the key advantages of metallic bipolar plates, a cost-effective, corrosion-resistant and electronically conductive coating as well as a coating process suitable for mass production are required. Task and solution

[0017] The object of the invention is to provide a further metallic bipolar plate for low- or high-temperature polymer electrolyte fuel cells / electrolyzers, which, due to a suitable coating, is inexpensive to manufacture, sufficiently stable and corrosion-resistant and also has the necessary electronic and thermal conductivities for use in an electrochemical energy converter.

[0018] It is further the object of the invention to provide a method for producing the aforementioned metallic bipolar plate for use in a low- or high-temperature polymer electrolyte fuel cell / electrolyzer.

[0019] The objects of the invention are achieved by a bipolar plate according to the main claim and by a method for its production according to the secondary claim. Advantageous embodiments of the bipolar plate or the production method are found in the dependent claims. Subject of the invention

[0020] The core idea of ​​the invention is to provide a metallic bipolar plate with a coating comprising at least partially reduced graphene oxide (GO) layers, which provides effective corrosion protection for metallic materials and, moreover, enables a simple and inexpensive coating process.

[0021] Suitable materials for such a bipolar plate include all metallic materials commonly used for bipolar plates, including iron-based steels, austenitic stainless steels and alloys with a high chromium, nickel and / or molybdenum content and additions of niobium, titanium and / or copper, manganese, tungsten, tantalum and vanadium, copper alloys and precious metals such as gold and platinum.

[0022] It has been shown that a coating with a graphene-like material consisting of one or more at least partially reduced graphene oxide layers is a promising option for reducing corrosion in metallic materials. Such a coating advantageously exhibits sufficient stability and the necessary electrical conductivity for use in an electrochemical cell.

[0023] Graphene is generally understood as a monolayer of carbon that exists in a two-dimensional, hexagonally interconnected plane. In contrast, graphite exists in a three-dimensional structure consisting of parallel, flat graphene layers. Both graphene and graphite are electrically conductive.

[0024] Graphite oxide is a non-stoichiometric compound of carbon, oxygen, and hydrogen. When graphite oxide is dissolved in a polar solvent and treated with ultrasound, a homogeneous colloidal suspension of flakes of non-conductive monomolecular layers (graphene oxide) is formed.

[0025] To produce the coating according to the invention comprising one or more at least partially reduced graphene oxide layers, a chemical synthesis is carried out. Starting with graphite powder, a graphite oxide powder is first produced, which is then converted into a stable graphene oxide (GO) suspension via ultrasonic dispersion. Such a suspension is considered stable if it still shows no sedimentation even after more than 30 days.

[0026] By depositing this suspension on a metallic carrier substrate, individual thin graphene oxide layers can then be applied and subsequently reduced to at least partially reduced graphene oxide (rGO), which is referred to below as graphene-like material.

[0027] The oxygen content in graphene oxide is in the range of 20–60 wt.%, in particular between 25 wt.% and 50 wt.% (remainder carbon), and can be significantly reduced after reduction in the graphene-like material depending on the reduction conditions. A reduction of more than 50%, in particular of approximately 75%, is advantageous. The at least partial reduction of the graphene oxide to graphene in the coating is referred to in the context of this invention as at least partially reduced graphene oxide (rGO), and the coating material is therefore referred to as graphene-like. The reduction in the oxygen content can be easily verified via XPS measurement, whereby the decrease in the intensity of the C-OH and C=O peaks can be regarded as a measure of the reduction of the graphene oxide to graphene.

[0028] The layer thicknesses of the individual deposited graphene oxide layers vary depending on the application, deposition method, and concentration of the GO suspension and are typically between 10 nm and 1 µm. Spray, dip, or spin coating methods are particularly suitable for deposition.

[0029] Advantageously, the coating and reduction steps are carried out alternately in several successive process steps until the desired layer thickness is achieved.

[0030] If only the coating steps were initially carried out to the desired layer thickness, the subsequent reduction would have to be carried out to greater layer depths, which is usually more difficult to achieve. Depending on the desired layer thickness, the GO concentration is typically 0.5–5 mg / ml for the spray process and approximately 5–10 mg / ml for dip and spin coating. The required layer thicknesses can vary depending on the conditions for use in low-temperature or high-temperature polymer electrolyte fuel cells / electrolyzers.

[0031] All known reduction methods (thermal, chemical, electrochemical, and laser-induced) are suitable for the at least partial reduction of the applied graphene oxide. Thermally reduced graphene oxide is also abbreviated to trGO below. This at least partial reduction renders the graphene oxide layers sufficiently electrically conductive, advantageous for use in a bipolar plate. Sufficient conductivity is defined as an electrical conductivity of at least 50 S / cm, preferably more than 100 S / cm.

[0032] In fuel cells / electrolyzers, the contact resistance between the bipolar plate and the adjacent gas diffusion layer is crucial. Metallic surfaces tend to form passive layers in contact with atmospheric oxygen and aqueous and oxygen-containing solutions, which significantly reduce electronic conductivity. This passivation effect leads to a significant performance reduction in fuel cells and electrolyzers when using uncoated metallic materials.

[0033] A crucial parameter for the inventive coating of a bipolar plate with a graphene-like material is therefore the contact resistance at the interface between the bipolar plate and the gas diffusion layer. Precious metals such as gold or platinum exhibit very low contact resistance because they are not subject to surface passivation. However, for cost reasons, precious metals are generally not considered as bipolar plate materials. For inexpensive stainless steels and nickel- and chromium-based alloys, the contact resistance has a tremendous impact on the performance of fuel cells and electrolyzers.

[0034] Furthermore, the metallic bipolar plate is spatially shielded from chemical electrolyte attack by the at least partially reduced graphene oxide coating. This requires that the reduced graphene oxide coating is free of defects such as cracks or pinholes. Low-temperature fuel cells and electrolyzers typically operate at temperatures of 80–90 °C and in a sulfuric acid aqueous environment (usually < 0.5 MH 2 SO 4 ). High-temperature fuel cells and electrolyzers operate at higher temperatures of 120–180 °C and acidities of ~16 MH 3 PO 4 . The inventive "graphene-like" bipolar plate coating has been found to be stable to the aforementioned temperatures and acidic ambient conditions.

[0035] In addition to chemical corrosion resistance to the corrosive electrolyte, the at least partially reduced graphene oxide coating also exhibits high electrochemical stability at external potentials up to 1 V for fuel cells and 2.2 V for electrolyzers. The stated potential values ​​are measured against the reversible hydrogen electrode as a reference standard. The reduction in anodic corrosion current densities at external potential by the reduced graphene oxide coating according to the invention represents a decisive improvement over previously conventional graphitic composite materials, uncoated metallic materials, and coating concepts known from the literature.

[0036] In addition, the at least partially reduced graphene oxide coating according to the invention exhibits higher expansion coefficients than graphitic and ceramic coating concepts known from the literature. At the above-mentioned operating temperatures of fuel cells and electrolyzers, the metallic bipolar plate experiences measurable material expansion, which can adversely lead to cracks and flaking in rigid and inelastic coatings. This inevitably leads to corrosion phenomena, an increase in contact resistance, and consequently to a decrease in cell performance. The high elastic modulus of the at least partially reduced graphene oxide layers according to the invention regularly prevents temperature-dependent degradation of the coating.Bending tests of coated metallic substrates have also shown that even at bending angles of up to 45°, no defects (cracks and flaking) of the at least partially reduced graphene oxide coating are observed.

[0037] All conventional metallic materials, including iron-based steels, austenitic stainless steels and alloys with a high chromium, nickel and / or molybdenum content and additions of niobium, titanium and / or copper, manganese, tungsten, tantalum and vanadium, copper alloys and precious metals such as gold and platinum, can be considered as materials for a suitable metallic bipolar plate.

[0038] The graphene-like coating according to the invention produced on the metallic bipolar plate advantageously exhibits very good adhesion to the metallic material of the bipolar plate. Furthermore, this coating has been found to be advantageously flexible. Furthermore, corrosion resistance in acidic or basic media, at high temperatures and electrical / electrochemical potentials can be significantly increased compared to an uncoated metallic bipolar plate.

[0039] Since the graphene-like coating according to the invention also exhibits sufficiently good electrical conductivity, it can be advantageously used as a promising coating for metallic bipolar plates for use in fuel cells and electrolyzers, i.e., in electrochemical cells in general. This opens up completely new design possibilities that cannot be transferred to graphitic materials. Since metallic bipolar plates typically have a material thickness of ~ 100 µm (cf. graphitic composite materials > 2 mm), significantly higher gravimetric and volumetric power densities of a fuel cell / electrolysis stack can be achieved with the graphene-like coating according to the invention.

[0040] Furthermore, the graphene-like coating according to the invention can be produced using the aforementioned coating process, which is significantly simpler and more cost-effective than known alternative coating technologies, e.g., the complex physical or chemical vapor deposition.

[0041] Advantageously, the graphene-like coating according to the invention is not limited to use in electrochemical cells. Further applications of the graphene-like coating according to the invention include the general coating of metallic components at risk of corrosion, such as pipelines, ship hulls, vehicle bodies, metallic electrochemical and chemical reactors, etc., or steel components in buildings, especially bridges and similar structures. Special description section

[0042] The subject matter of the invention is explained in more detail below by way of example with reference to some figures relating to bipolar plates, without thereby restricting it.

[0043] The requirements placed on a bipolar plate for use in an electrochemical cell are, on the one hand, high chemical and electrochemical stability in the cell environment, sufficient electronic and thermal conductivity, high ductility and the possibility of series production.

[0044] The Figure 1 a shows a metallic bipolar plate (material 1.4404) with embossed channel structure (flow field) without the reduced graphene oxide coating according to the invention.

[0045] The Figure 1 bIn contrast, shows a metallic bipolar plate (material 1.4404) with an embossed channel structure (flow field) with an at least partially reduced graphene oxide coating according to the invention. The coating was applied by spraying using an aqueous graphene oxide suspension with a concentration of 2 mg / ml. The reduction to the reduced graphene oxide layers was carried out thermally on a hot plate at temperatures up to 500 °C. After each spraying process, the currently applied layer was thermally reduced before the next coating curtain followed. The total layer thickness of this bipolar plate is 250 nm.

[0046] The Figure 2 shows a cross-section of the metallic bipolar plate (material 1.4404) with the graphene-like coating according to the invention. Application and reduction method (thermally reduced) analogous to Figure 1The total layer thickness is also approximately 250 nm. Cross sections were prepared using ion polishing technology.

[0047] The Figure 3 shows a cross-section of the metallic bipolar plate (material 1.4404) with a thermally reduced graphene oxide coating according to the invention. Application and reduction method comparable to that of the Figure 1 The layer thickness here is approximately 250 nm. The cross-section was prepared using a scalpel cut. The layer structure of individual reduced graphene oxide layers is clearly visible.

[0048] The Figure 4shows the contact resistance between uncoated and coated bipolar plates and the adjacent gas diffusion layer (carbon fleece) as a function of contact pressure. The thermally reduced graphene oxide coating on material 1.4404 (trGO / 1.4404) according to the invention with a layer thickness of 200 nm advantageously shows a reduction in contact resistance by more than an order of magnitude compared to a non-reduced graphene oxide coating on 1.4404 (GO / 1.4404). For comparison, the contact resistance of an uncoated bipolar plate made of material 1.4404 with a surface passivation layer naturally formed in atmospheric oxygen and with a mechanically abraded (polished) surface is shown. The material sample 1.4404 was measured directly after mechanical polishing. However, since surface passivation occurs within a few hours in atmospheric oxygen or in aqueous and oxygen-containing solutions, the material sample 1 shows4404 with passive layer the contact resistances expected during operation in fuel cells or electrolyzers.

[0049] The contact resistance of thermally reduced graphene oxide layers on 1.4404 (trGO / 1.4404) with a layer thickness of ~100 nm decreases after thermal reduction from 1700 mΩ cm 2< to 120 mΩ cm 2< at a contact pressure of 140 N cm -2< and from 775 mΩ cm 2< to 62 mΩ cm 2< at a contact pressure of 300 N cm -2< by more than an order of magnitude compared to non-reduced graphene oxide layers (GO / 1.4404). The contact resistance of trGO / 1.4404 is even lower than that of the uncoated 1.4404 material, which is passivated in atmospheric oxygen. It has been shown that contact resistances < 100 mΩ cm 2< are necessary for fuel cell operation. This requirement can be achieved for the thermally reduced graphene oxide layers produced according to the invention on a bipolar plate.

[0050] The Figure 5shows the course of the free corrosion potential with a temperature increase to 130 °C in a long-term test of 30 days. The test was carried out in a three-electrode measuring cell in 175 ml of 85 wt.% H 3 PO 4. In the uncoated material 1.4404, a rapid degradation of the passive layer can be seen, which is accompanied by a decrease in the free corrosion potential with increasing temperature. The temperature increase is represented in the diagram by diamonds with a temperature fluctuation of 5 °C. The thermally at least partially reduced graphene oxide coatings on material 1.4404 with a thickness of 10 nm and 100 nm show only a slight improvement under these drastic conditions. Thermally reduced graphene oxide layers with a thickness of approx. 250 nm, on the other hand, have a corrosion potential of 435 mV (vs. reversible hydrogen electrode) even after 30 days.This indicates that the metal surface of the bipolar plate is effectively protected from acid attack under the coating. Under real fuel cell / electrolysis conditions, significantly lower amounts of electrolyte (~ 1 mg / cm²) are typically in contact with the bipolar plate, so in these cases, even thinner coating thicknesses are sufficient to protect the metal substrate from corrosion.

[0051] As a further experiment, a potentiodynamic corrosion test was conducted in 175 ml of 1 MH 3 PO 4 at room temperature in an electrochemical three-electrode measuring cell. After 100 cycles in the potential range 0 - 1.3 V (vs. reversible hydrogen electrode) at a scan speed of 100 mV / s, an uncoated copper sample with a thickness of 100 µm (as a potential bipolar plate material) had completely dissolved, while a copper sample coated with at least partially thermally reduced graphene oxide (trGO) (as a configuration of a bipolar plate according to the invention) showed slight defects only at the edges. This can be explained, among other things, by the fact that the sealing ring had damaged the thermally reduced graphene oxide coating during disassembly of the measuring cell.

[0052] To produce the graphene oxide (GO) suspension, the chemical synthesis described below was performed. The functionalization of graphite with hydrophilic groups (including epoxy, hydroxyl, and carboxyl groups) followed by ultrasonic dispersion ensures a stable graphene oxide (GO) suspension, which is reduced after the coating process, as described above. The removal of the hydrophilic groups regenerates the aromatic system. Example synthesis (modified synthesis according to Hummer):

[0053] Place graphite in a round-bottomed flask, add 400 ml of H 2 SO 4 / H 3 PO 4 (360 / 40 ml) and stir. Then slowly add KMnO 4 in portions so that the temperature remains relatively constant. Cool in an ice bath, as the reaction is highly exothermic. Then bring to 50 °C and stir for 18 h. Then allow to cool to room temperature and pour onto 500 ml of ice. Then add 7 ml of 27% H 2 O 2. The resulting graphite oxide powder is then centrifuged, washed several times with ethanol and water and dried. This is followed by direct ultrasonic dispersion of the graphite oxide powder using a sonotrode in a protic polar solvent (60 min per 100 ml of suspension at 1 mg / ml concentration with an intensity of approx. 100 W / cm 2 < sonotrode area). Finally, centrifuge and dry again (product: graphene oxide particles). In the form of a suspension, direct coating of the substrate is possible.

[0054] In a simple embodiment of the process, the at least partial reduction of graphene oxide to graphene takes place in an oven or on a hot plate, preferably in a protective gas atmosphere (nitrogen, argon) or in atmospheric oxygen in a temperature range of 200–500°C. The thermal energy input reduces functional groups (escape of CO / CO 2 ) and regenerates the aromatic system. This was confirmed by thermogravimetric analysis (TGA). Furthermore, XPS spectroscopy can clearly distinguish whether the coating applied to a bipolar plate contains graphene, graphite oxide, or the graphene-like structure according to the invention.

[0055] Other methods for producing the graphene oxide (GO) suspension were also tested. Chemical synthesis is carried out using a strong reducing agent such as hydrazine in solution or in the gas phase. Graphene oxide layers are reduced using hydrazine to form chemically reduced graphene oxide layers (crGO). The electrochemical reduction of graphene oxide layers to form electrochemically reduced graphene oxide layers (erGO) was carried out in an electrolyte (e.g., potassium dihydrogen phosphate) in the cathodic polarization range (down to -1 V vs. a reversible hydrogen electrode). A disadvantage of chemical and electrochemical reduction is the contamination of the coating with foreign ions from the reducing agent. In fuel cells / electrolyzers, this can lead to contamination of the polymer electrolyte membrane or the catalyst. Laser-induced reduction is achieved by directly irradiating the graphene oxide coating with a laser beam.For this purpose, adjustments of intensity, energy, pulse duration, etc. are necessary to achieve effective reduction to laser-reduced graphene oxide coating (LrGO) on the one hand and to avoid damage to the coating on the other.

[0056] Chemical and electrochemical reduction therefore disadvantageously introduce impurities into the coating, and laser reduction frequently causes partial defects. Therefore, within the scope of this invention, thermal reduction is considered particularly simple and effective, and therefore particularly advantageous.

Claims

1. Method for producing a bipolar plate, comprising the steps: - graphite oxide is dissolved in water and treated with ultrasound, forming a homogeneous colloidal suspension comprising flocs of non-conductive graphene oxide - this is applied to a metallic bipolar plate, so that at least one electrically non-conductive graphene oxide layer is produced, - the applied graphene oxide layer or layers is or are subjected to a reduction step so that an at least partially reduced, electrically conductive graphene-like coating is produced.

2. Method according to claim 1, in which a reduction step is performed after each applied graphene oxide layer.

3. Method according to one of claims 1 to 2, in which the stable suspension comprising graphene oxide is applied by means of a spray, dip or spin coating process.

4. Method according to one of claims 1 to 3, in which the reduction of the deposited graphene oxide layer is carried out chemically, electrochemically, laser-induced or thermally.

5. Method according to one of claims 1 to 4, in which temperatures of up to max. 500 °C are used for the thermal reduction.

6. Method according to one of claims 1 to 5, in which a metallic bipolar plate is used which comprises an alloy with chromium, nickel and / or molybdenum contents, a copper alloy, a precious metal, an austenitic stainless steel with additions of tungsten, tantalum and / or lanthanum, or a ferritic chromium-containing steel with small contents of hafnium, tungsten, tantalum and / or vanadium.

7. Method according to one of claims 1 to 6, in which the oxygen content in the deposited graphene oxide layer is reduced by more than 50 %, in particular by more than 75 %.

8. Metallic bipolar plate for use in an electrochemical cell, produced according to one of claims 1 to 7, characterized in that the bipolar plate has an electrically conductive graphene-like coating which consists of one or more graphene oxide layers which are at least partially reduced.

9. Metallic bipolar plate according to claim 8, in which the graphene-like coating has a layer thickness of between 10 nm and 1 µm10. Metallic bipolar plate according to one of claims 8 to 9, comprising iron-based steels, austenitic stainless steels and alloys with high chromium, nickel and / or molybdenum contents and additions of niobium, titanium and / or copper, manganese, tungsten, tantalum and vanadium, copper alloys and precious metals such as gold and platinum.