BIPOLAR PLATE, FUEL CELL AND ELECTROLYSER

Description

[0001] The invention relates to a bipolar plate of an energy converter, in particular a fuel cell or an electrolyzer, according to the preamble of patent claim 1. The invention further relates to a fuel cell or an electrolyzer with such a bipolar plate.

[0002] Electrochemical systems such as fuel cells, in particular polymer electrolyte fuel cells, and conductive current collecting plates for such fuel cells and electrolyzers as well as current collectors in galvanic cells and electrolyzers are known.

[0003] An example of this is the bipolar or monopolar plates in fuel cells, particularly in oxygen half-cells. The bipolar or monopolar plates are made of carbon plates (e.g., graphoil plates), which contain carbon as a key component. These plates tend to be brittle and are comparatively thick, thus significantly reducing the fuel cell's power output. A further disadvantage is their lack of physical (e.g., thermomechanical) and / or chemical and / or electrical stability.

[0004] It is also common practice to manufacture the current-collecting plates of fuel cells from metallic (particularly austenitic) stainless steels. The advantage of these plates is that they can be achieved with a thickness of less than 0.5 mm. This thickness is desirable to minimize both the installation space and the weight of the fuel cell. A problem with these plates is that surface oxides form during fuel cell operation, resulting in an unacceptably high surface resistance and / or electrochemical degradation (such as corrosion).

[0005] From the published patent applications DE 10 2010 026 330 A1, DE 10 2013 209 918 A1,

[0006] DE 11 2005 001 704 T5 and DE 11 2008 003 275 T5 state that in order to meet the requirements, for example for the use of bipolar plates in fuel cells, austenitic stainless steels as a carrier are coated with a gold layer having a band thickness of up to 2 nm. This solution to the requirements has several disadvantages. For example, a gold layer, even one that is only 2 nm thick, is still too expensive for mass applications. A much greater disadvantage can be seen in a basic property of the chemical element gold. Gold is more noble than the carrier material than stainless austenitic steel (stainless steel) and therefore causes the carrier to dissolve under unfavorable operating conditions in the fuel cells (e.g. pitting corrosion), which reduces the service life. In environments containing chlorides in particular (e.g. aerosols), corrosion is unavoidable.

[0007] In particular, another disadvantage is that gold is not stable for high-load applications, e.g. at electrolysis conditions above 1500 mV standard hydrogen unit, in both acidic and basic environments.

[0008] Also known from the prior art are layers on the substrate in the form of so-called hard material coatings based on nitride or carbide. One example of this is titanium nitride, which, however, tends to form oxide metal complexes during operation of a fuel cell, even forming closed surface layers. As a result, the surface resistance increases to high values, as is the case with stainless steel. Processes for coating with chromium nitride or chromium carbonitride are described, for example, in patents DE 199 37 255 B4 and EP 1273060 B1, and in published patent application DE 100 17 200 A1.

[0009] Depending on their composition, the hard coatings exhibit very good operating properties (e.g., corrosion resistance, abrasion resistance, high contour fidelity), but they carry the risk of anodic dissolution if concentration chains form in the fuel cell under unfavorable operating conditions. This anodic dissolution occurs when internal electrochemical short circuits occur in the fuel cell, such as the formation of a water film between an active electrode of a membrane electrode assembly of the fuel cell and the bipolar plate, creating a so-called local element, or an unexpected and undesirable reaction element.

[0010] Also known are multiple coatings based on nitrides with very thin gold or platinum layers. Thus, satisfactory operating results for a fuel cell can be achieved with precious metal layer thicknesses of over 2 µm. The fundamental problem of dissolution remains at high anodic potentials. This layer thickness ensures a virtually pore-free top layer and thus reduces the risk of pitting corrosion.

[0011] Also known are so-called dimensionally stable anodes. These involve the formation of single-phase or multi-phase oxides with ruthenium oxide and / or iridium oxide using refractory metals. While this type of layer is very stable, it results in excessively high electrical resistance. A similar situation occurs when a surface of the substrate, generally made of a precious metal, is doped with iridium.

[0012] WO 2009 108 102 A1 describes an electrode for an electrochemical cell comprising an electrically conductive substrate with an electrically conductive, corrosion-resistant coating applied thereto, at least in partial areas. The coating comprises a multi-element material with a carbide or nitride composition.

[0013] Therefore, the following requirements must be met by the metallic supports or bipolar plates for a PEM fuel cell or an electrolyzer used in these exemplary electrochemical systems, in particular for energy conversion: High corrosion resistance to a surrounding medium, and / or high resilience to anodic or cathodic polarizing loads, low surface resistance of a surface of the carrier or its coating facing an electrolyte, and low production costs of the carrier, in particular, for example, of an electrically conductive conductor in the form of bipolar plates for the use of fuel cells for mobile applications.

[0014] Thus, the object of the present invention is to provide an improved bipolar plate generally for an energy converter, in particular for a fuel cell or an electrolyzer. Furthermore, the object of the invention is to provide a fuel cell equipped with such a bipolar plate with an improved layer system and an electrolyzer equipped therewith.

[0015] The object is achieved according to the invention by a bipolar plate of an energy converter, in particular a fuel cell or an electrolyzer, according to claim 1. The bipolar plate comprises a metallic substrate and a layer system applied at least in partial areas of the surface of the substrate, wherein the layer system comprises a cover layer and a base layer system. The cover layer consists of a homogeneous or heterogeneous solid metallic solution having a metal lattice which either contains, in a first alternative, a first chemical element from the group of noble metals in the form of iridium as a host metal and, dissolved in the metal lattice of the host metal, further contains at least one further non-metallic chemical element from the group comprising nitrogen, carbon, boron, fluorine, hydrogen, wherein the at least one non-metallic chemical element is present in a concentration in the range from 0.1 at.% to 65 at.-% is present in the cover layer, or in a second alternative contains a first chemical element from the group of noble metals in the form of iridium and a second chemical element from the group of noble metals in the form of ruthenium as host metals and contains, dissolved in the metal lattice of the host metal, at least one further non-metallic chemical element from the group comprising nitrogen, carbon, boron, fluorine, hydrogen, wherein the at least one non-metallic chemical element is present in a concentration in the range from 0.1 at.% to 65 at.% in the cover layer (3a).

[0016] According to the invention, the cover layer in the first alternative comprises a) more than 35 at.% iridium and also carbon; or b) more than 35 at.% iridium and also carbon and traces of hydrogen; or c) more than 35 at.% iridium and also carbon and fluorine, optionally also traces of hydrogen.

[0017] According to the second alternative, the cover layer comprises d) in total more than 35 at.% iridium and ruthenium and also carbon; or e) in total more than 35 at.% iridium and ruthenium and also carbon and traces of hydrogen; or f) in total more than 35 at.% iridium and ruthenium and also carbon and fluorine, optionally also traces of hydrogen.

[0018] The object is further achieved according to the invention by a fuel cell, in particular a polymer electrolyte fuel cell, comprising at least one bipolar plate according to the invention.

[0019] The object is further achieved according to the invention by an electrolyzer comprising at least one bipolar plate according to the invention.

[0020] Advantageous embodiments with expedient and non-trivial further developments of the invention are specified in the subclaims.

[0021] The top layer is electrically conductive and electrocatalytically active as well as corrosion-protective.

[0022] A homogeneous metallic solution is understood to mean that the non-metallic chemical elements mentioned are dissolved in the metal lattice in such a way that the lattice type of the host metal or host metal alloy does not change substantially.

[0023] Depending on the deposition conditions, the top layer can be metastable or stable in the thermodynamic sense.

[0024] It has been shown that with a carbon-containing top layer, thus through the use of the metalloid or non-metallic chemical element carbon, the conductivity of the top layer is higher than that of gold, and that at the same time its oxidation stability in an acidic solution is significantly higher than a voltage of 2000 mV of a standard hydrogen electrode. Depending on the design, measured specific electrical resistances are below 5 mΩ cm -2 (under standardized conditions).

[0025] In comparison, the specific electrical resistance of gold is approximately 10 mΩ cm -2< at room temperature.

[0026] Another important advantage is that the iridium does not oxidize and dissolve at voltages above the value E = 2.04 - 0.059 lg pH- -0.0295 lg (IrOa) 2-<. In the solid solution, the low-valency iridium is thus stabilized to such an extent that the otherwise usual oxidation at approximately 1800 mV in 1 mol / l (1N concentrated) sulfuric acid (H 2 SO 4 ) no longer occurs. The measure of stabilization is the gain in free partial mixing energy ΔG mixing of the solid solutions or compounds.

[0027] The cover layer preferably has a layer thickness of at least 1 nm to a maximum of 10 nm.

[0028] For example, with a layer thickness of approximately 10 nm, using (Ir,Nb)C 1-x results in only 4 µg Ir per cm² of the cover layer. For a 10 nm thick gold layer, more than 20 µg gold per cm² are required. The advantage of the cover layer compared to a gold layer is its high oxidation stability up to voltages well above 2000 mV compared to a standard hydrogen electrode in 1N concentrated sulfuric acid.

[0029] The top layer preferably also contains at least one metal from transition group IV and / or V of the periodic table of chemical elements. The advantage of using these metals – either elemental or in compound form – is that they form self-protecting, stable, and conductive oxides under corrosion conditions.

[0030] The at least one non-metallic chemical element is present in the cover layer at a concentration in the range of 0.1 at.% to 65 at.%, in particular from 10 to 30 at.%. In particular, the non-metallic chemical element carbon is present in the cover layer at a concentration range of 10 to 25 at.%.

[0031] The hydrogen or optionally present hydrogen according to the top layer compositions b), c), e) and f) is present only in traces.

[0032] Furthermore, the cover layer can contain at least one chemical element from the group of base metals. The at least one chemical element from the group of base metals is preferably aluminum, iron, nickel, cobalt, zinc, cerium, or tin and / or is contained in the cover layer in a concentration range of 0.01 to 5 at.%.

[0033] In a further advantageous embodiment of the cover layer, it comprises at least one chemical element from the group of refractory metals, in particular titanium and / or zirconium and / or hafnium and / or niobium and / or tantalum. It has been shown that the addition of the refractory metals also partially controls the H2O2 and ozone produced during electrolysis.

[0034] The covering layer is preferably formed as a multinary compound with the addition of refractory metals.

[0035] The cover layer, comprising at least one refractory metal, exhibits high conductivity and high corrosion resistance, particularly in a temperature range from 0 to approximately 200 °C. Thus, multinary solid iridium- and / or ruthenium-containing cover layers provide outstanding properties for long-term use in applications such as fuel cells.

[0036] A further advantage arises from the coating of electrical conductors, particularly metallic bipolar plates, regardless of whether the electrical conductor, such as a bipolar plate, is designed for low-temperature polymer electrolyte fuel cells or for high-temperature polymer electrolyte fuel cells. The particular advantage is that the coating layer, with a density between 10-13 gcm -3, has a density almost half that of a pure precious metal. This allows the use of expensive precious metals and / or their compounds to be reduced, particularly through the formation of multinary compounds with the other elements.

[0037] The at least one chemical element from the group of refractory metals is preferably contained in the cover layer in a concentration range of 0.01 to 5 at.%.

[0038] If the at least one chemical element from the group of base metals is present in the form of tin, this and the at least one chemical element from the group of refractory metals are contained together in the top layer in a concentration range of 0.01 to 5 at.%.

[0039] It has proven effective for the top layer to contain at least one additional chemical element from the group of precious metals in a concentration range of 0.01 to 10 at.%. The chemical elements from the group of precious metals are, in particular, platinum, gold, silver, rhodium, and palladium.

[0040] It has proven effective if all chemical elements from the group of precious metals, i.e. together with iridium and ruthenium, are contained in the top layer in the concentration range of more than 35 to 99 at.%.

[0041] Corrosion protection on metallic substrates, such as steels, especially stainless steels, or titanium, is further improved by applying the top layer to the sublayer system formed between the substrate and the top layer. This is particularly advantageous when corrosive ambient media are present, especially when the corrosion media contain chloride.

[0042] Underoxidation, i.e. oxidation of the surface of a substrate with a top layer applied to this surface, normally leads to delamination of the precious metal layers on top.

[0043] The layer system on the bipolar plate includes the top layer and the base layer system.

[0044] In particular, the underlayer system comprises at least one underlayer which comprises at least one chemical element from the group titanium, niobium, hafnium, zirconium, tantalum.

[0045] The base layer system has in particular a first base layer in the form of a metallic alloy layer comprising the chemical elements titanium and niobium, in particular 20 - 50 wt.% niobium and the remainder titanium.

[0046] The base layer system has in particular a second base layer comprising at least one chemical element from the group titanium, niobium, zirconium, hafnium, tantalum and furthermore at least one non-metallic element from the group nitrogen, carbon, boron, fluorine.

[0047] In a particularly preferred embodiment, the base layer system comprises a second base layer comprising the chemical elements a) titanium, niobium and furthermore carbon and fluorine, or b) titanium, niobium and furthermore nitrogen, is formed in particular from (Ti 67 Nb 33 )N 0.8-1.1.

[0048] The second backing layer is preferably arranged between the first backing layer and the cover layer.

[0049] The second underlayer may further contain up to 5 at.% oxygen.

[0050] A further advantage of choosing a multinary compound for the top layer is that, although it forms oxides under high anodic voltages of up to 3500 mV compared to a standard hydrogen electrode or in the presence of hydrogen peroxide or ozone, these oxides are electrically conductive and self-healing. They tend to form inert and conductive mixed oxide layers with the second base layer.

[0051] The bipolar plate according to the invention comprises the metallic substrate and the layer system applied to at least partial areas of the surface of the substrate. In particular, the layer system is applied over the entire surface of one or both sides of the plate-shaped substrate. The metallic substrate is formed, in particular, from steel or titanium, preferably from stainless steel. The thickness of the substrate is preferably less than 1 mm and is in particular equal to 0.5 mm.

[0052] A fuel cell according to the invention, in particular a polymer electrolyte fuel cell comprising at least one bipolar plate according to the invention, has proven particularly advantageous in terms of electrical values ​​and corrosion resistance. Such a fuel cell therefore has a long service life of more than 10 years or more than 5,000 vehicle operating hours.

[0053] With an electrolyzer according to the invention, which operates on the reverse principle of a fuel cell and uses electrical current to induce a chemical reaction, i.e., a material transformation, comparably long service lives can be achieved. In particular, the electrolyzer is suitable for hydrogen electrolysis.

[0054] Advantageously, a cover layer thickness of less than 10 nm is sufficient to protect against resistance-increasing oxidation of the second base layer. To ensure reliable corrosion protection, sublayers of the base layer system are formed from at least one refractory metal, which are applied to the steel, in particular stainless steel, in at least two layers: first as a metal or alloy layer (= first base layer) and then as a metalloid layer (= second base layer). The double layer formed under the cover layer with the aid of the two layers ensures, on the one hand, electrochemical adaptation to a carrier material, i.e., the material from which the carrier is made, and, on the other hand, pore formation due to oxidation and hydrolysis processes is excluded.

[0055] Electrochemical adaptation to the substrate material is necessary because both the metalloid layer (= second base layer) and the top layer are very noble. If pores were to form, high local element potentials would build up, resulting in unacceptable corrosion currents. The first metallic base layer is preferably made of titanium, niobium, zirconium, tantalum, or hafnium, or alloys of these metals. These are less noble than the steel, particularly stainless steel, substrate material. During corrosion processes, they initially react to form insoluble oxides or voluminous, sometimes gel-like hydroxo compounds of these refractory metals. As a result, the pores close and protect the base material from corrosion. This process represents a self-healing process of the coating system.

[0056] In particular, a second underlay layer in the form of a nitride layer serves as a hydrogen barrier and thus protects the substrate, in particular made of stainless steel, of the bipolar plate as well as the metallic first underlay layer from hydrogen embrittlement.

[0057] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and the figure. The features and combinations of features mentioned above in the description can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0058] The figure shows a bipolar plate 1 comprising a substrate 2 made of stainless steel and a layer system 3 applied over the entire surface of one side of the substrate 2. The layer system 2 comprises a cover layer 3a and a base layer system 4 comprising a first base layer 4a and a second base layer 4b.

[0059] In a first embodiment, a metallic substrate 2 in the form of a conductor, here for a bipolar plate 1 of a polymer electrolyte fuel cell for the conversion of (reformed) hydrogen, is made of a stainless steel, in particular of a so-called authentic steel with very high known requirements regarding corrosion resistance, e.g. with the DIN ISO material number 1.4404, manufactured.

[0060] By means of a coating process, for example a vacuum-based coating process (PVD), a layer system 3 is formed on the substrate 2 of the bipolar plate 1, wherein the substrate 2 is coated in one process pass, first with a first base layer 4a in the form of a 1.5 µm thick titanium layer, then with a second base layer 4b of approximately the same thickness in the form of a titanium nitride layer, and finally with a cover layer 3a in the form of a 25 nm thick layer. The cover layer 3a corresponds to a layer layer that is open on one side, since only one cover layer surface of a further layer, here the second base layer 4b, is designed to contact it. Thus, the free surface 30 of the cover layer 3a in a fuel cell is arranged directly adjacent to and exposed to an electrolyte, in particular a polymer electrolyte.

[0061] In a second embodiment, the metallic substrate 2 for the bipolar plate 1 is first coated with a first base layer 4a in the form of a metallic alloy layer with a thickness of several hundred nm, the metallic alloy layer having the composition Ti 0.9 Nb 0.1. Subsequently, a second base layer 4b with a thickness of another several hundred nm and the composition Ti 0.9 Nb 0.1 N 1-x is applied. A cover layer 3a with a thickness of several nm is applied thereon.

[0062] The advantage is an exceptionally high stability against oxidation of the bipolar plate 1 according to the invention. Even with a permanent load of +3000 mV compared to a normal hydrogen electrode, no increase in resistance is detected in sulfuric acid solution, which has a pH value of 3.

[0063] Externally, the free surface 30 of the cover layer 3a, i.e., the surface of the cover layer 3a facing away from the substrate 2, remains silvery shiny even after 50 hours of continuous exposure at +2000 mV compared to a standard hydrogen electrode. Even in a scanning electron microscope examination, no traces of corrosion extending through the thickness of the cover layer 3a toward or reaching the substrate 2 are visible.

[0064] The cover layer 3a of the second embodiment can be applied using both the sputtering technique and a cathodic ARC coating process, also known as vacuum arc evaporation. Despite a higher droplet count—in other words, an increased number of metal droplets compared to sputtering technology—the cover layer 3a produced using the cathodic ARC process also exhibits the advantageous properties of high corrosion resistance with time-stable surface conductivity of the cover layer 3a according to the invention produced using the sputtering technique.

[0065] In a third embodiment, the layer system 3 according to the invention is formed on a substrate 2 in the form of a structured perforated stainless steel sheet. The substrate 2 was electrolytically polished in an H 2 SO 4 / H 3 PO 4 bath prior to the application of a layer system 3. After the application of a single base layer in the form of a tantalum carbide layer several thousand nm thick, a cover layer 3a is applied in the form of a layer several hundred nm thick.

[0066] The advantage of the underlayer formed from tantalum carbide is not only its exceptional corrosion resistance but also that it does not absorb hydrogen and thus serves as a hydrogen barrier to the substrate 2. This is particularly advantageous when titanium is used as the substrate material.

[0067] The layer system 3 of the third embodiment is suitable for use in an electrolysis cell for generating hydrogen at current densities i that are greater than 500 mA cm -2<.

[0068] The advantage of the metalloid layer or the second base layer, which in the simplest case is made of titanium nitride, is its low electrical resistance of 10-12 mΩ cm -2< . Likewise, the top layer can also be formed without a second base layer or metalloid layer, possibly increasing the resistance.

[0069] In Table 1, rows 2 and 5 show exemplary layer systems with their characteristic values ​​for use in bipolar plates according to the invention. Rows 1, 4, and 6 show designs of a cover layer for bipolar plates not according to the invention. Table 1: Layers and selected characteristic values Layer system / layer thickness Surface resistance Corrosion current at 2000 mV standard hydrogen column in µA cm -2< in aqueous sulfuric acid solution (pH3) at T=800 °C Oxidation stability at 200 mV measured as change in surface resistance in mΩ cm -2< 1 Gold / 3 µm (for reference) 9 > 100 pitting current 9 -10 2 Ti / 0.5 µm 8 0,001 .12 TiN / 1 µm (Nb 0.1 Ir 0.9 )C 1-δ / 10nm 3 TiNb / 0.5 µm 10-11 0,001 10 -11 TiN / 1 µm (Nb 0.1 Ir 0.9 )Ni 1.05 / 10nm 4 TiNb / 0.1 µm 7-5 0,01 4-6 IrC / 10 nm 5 Ta / 0.05 µm 10 0,001 17-18 TaC / 0.5 µm (Ta,Ir)C / 5 nm 6 ZrB 2 / 0.3 µm (Zr 0.3 Ir 0.7 )B 2-δ / 10 nm 7 Pitting reaction after 4 hours of stress

[0070] Table 1 shows only a few exemplary coating systems. Advantageously, suitable coating systems exhibit no increase in resistance under an anodic stress of +2000 mV compared to a standard hydrogen column in sulfuric acid solution at a temperature of 80 °C for several weeks. The coating systems applied in high vacuum using a sputtering or ARC process, or in medium vacuum using PECVD (plasma-enhanced chemical vapor deposition) were partially darkened after this exposure period. However, no visible signs of corrosion or significant changes in surface resistance occurred. List of reference symbols

[0071] 1Bipolar plate 2Substrate 3Layer system 3aCover layer 4Underlayer system 4aFirst underlayer 4bSecond underlayer 30Free surface

Claims

1. Bipolar plate (1) of an energy converter, comprising a metallic substrate (2) and a layer system (3) applied at least in partial regions of the surface of the substrate (2), wherein the layer system (3) comprises a top layer (3a) and a backing layer system (4), the top layer (3a) consisting of a homogeneous or heterogeneous solid metallic solution having a metal lattice which either, in a first alternative, contains a first chemical element from the group of noble metals in the form of iridium as a host metal and, dissolved in the metal lattice of the host metal, further contains at least one further non-metallic chemical element from the group comprising nitrogen, carbon, boron, fluorine, hydrogen, wherein the at least one non-metallic chemical element is present in a concentration in the range from 0.1 at. % to 65 at. % in the top layer (3a), or in a second alternative contains a first chemical element from the group of noble metals in the form of iridium and a second chemical element from the group of noble metals in the form of ruthenium as host metals and containing dissolved in the metal lattice of the host metal at least one further non-metallic chemical element from the group comprising nitrogen, carbon, boron, fluorine, hydrogen, wherein the at least one non-metallic chemical element is present in a concentration in the range from 0.1 at. % to 65 at. % in the top layer (3a), wherein the top layer (3a) in the first alternative. a) comprises more than 35 at. % iridium and furthermore carbon; or b) comprising more than 35 at. % iridium and further comprising carbon and trace amounts of hydrogen; or c) comprises more than 35 at. % iridium and furthermore carbon and fluorine, optionally furthermore trace amounts of hydrogen; and wherein the top layer (3a) in the second alternative. d) comprises in total more than 35 at. % iridium and ruthenium and furthermore carbon; or e) comprising in total more than 35 at. % iridium and ruthenium and further comprising carbon and trace amounts of hydrogen; or f) comprises in total more than 35 at. % iridium and ruthenium and furthermore carbon and fluorine, optionally furthermore trace amounts of hydrogen.

2. Bipolar plate (1) according to claim 1, wherein the top layer (3a) in the first alternative and the second alternative further contains at least one metal from the IVth and / or the Vth subgroup of the periodic table of the chemical elements.

3. Bipolar plate (1) according to claim 1 or 2, wherein the top layer (3a) in the first alternative and the second alternative contains at least one chemical element from the group of base metals.

4. Bipolar plate (1) according to claim 3, wherein the at least one chemical element from the group of base metals is aluminium, iron, nickel, cobalt, zinc, cerium or tin.

5. Bipolar plate (1) according to one of claims 3 or 4, wherein the at least one further chemical element from the group of base metals is contained in the top layer (3a) in the concentration range from 0.01 to 5 at%.

6. Bipolar plate (1) according to one of the claims 1 to 5, wherein the top layer (3a) in the first alternative and the second alternative comprises at least one chemical element from the group of refractory metals.

7. Bipolar plate (1) according to claim 6, wherein the top layer (3a) comprises at least one chemical element from the group of refractory metals in the form of titanium and / or zirconium and / or hafnium and / or niobium and / or tantalum.

8. Bipolar plate (1) according to claim 6 or 7, wherein the at least one chemical element from the group of refractory metals is contained in the top layer (3a) in the concentration range from 0.01 to 5 at%.

9. Bipolar plate (1) according to one of claims 3 to 8, wherein the at least one chemical element from the group of base metals in the form of tin and the at least one chemical element from the group of refractory metals are together contained in the top layer (3a) in the concentration range from 0.01 to 5 at%.

10. Bipolar plate (1) according to one of the claims 1 to 9, wherein the top layer (3a) in the first alternative and the second alternative comprises at least one additional chemical element from the group of noble metals comprising platinum, gold, silver, rhodium, palladium, in a concentration range of 0.01 to 10 at%.

11. Bipolar plate (1) according to one of the previous claims, wherein all chemical elements from the group of noble metals, including iridium and ruthenium, are contained in the top layer (3a) in the concentration range from 35 to 99 at%.

12. Bipolar plate (1) according to one of the previous claims, wherein the non-metallic chemical element carbon contained in the top layer (3a) is present in the concentration range of 10 to 25 at%.

13. Bipolar plate (1) according to one of the previous claims, wherein the top layer (3a) has a layer thickness of at least 1 nm to a maximum of 50 nm.

14. Bipolar plate (1) according to one of the claims 1 to 13, wherein the backing layer system (4) comprises at least one backing layer (4a, 4b) comprising at least one chemical element from the group titanium, niobium, hafnium, zirconium, tantalum.

15. Bipolar plate (1) according to claim 14, wherein the backing layer system (4) comprises at least one first backing layer (4a) in the form of a metallic alloy layer comprising the chemical elements titanium and niobium.

16. Bipolar plate (1) according to claim 14 or 15, wherein the backing layer system (4) comprises a second backing layer (4b) comprising at least one chemical element from the group titanium, niobium, hafnium, zirconium, tantalum and further at least one non-metallic element from the group nitrogen, carbon, boron, fluorine.

17. Bipolar plate (1) according to claim 16, wherein the second backing layer (4b) is arranged between the first backing layer (4a) and the top layer (3a).

18. Bipolar plate (1) according to claim 16 or 17, wherein the second backing layer (4b) contains up to 5 at% oxygen.

19. Fuel cell, in particular a polymer electrolyte fuel cell, comprising at least one bipolar plate (1) according to one of claims 1 to 18.

20. Electrolyser comprising at least one bipolar plate (1) according to any one of claims 1 to 18.