EP4264717A1

EP4264717A1 - Layer and layer system and electrically conductive plate and electrochemical cell

Info

Publication number: EP4264717A1
Application number: EP21819340.7A
Authority: EP
Inventors: Romina BAECHSTAEDT; Moritz Wegener; Jan Martin STUMPF; Edgar Schulz; Ricardo Henrique Brugnara; Joachim Weber
Original assignee: Schaeffler Technologies AG and Co KG
Current assignee: Schaeffler Technologies AG and Co KG
Priority date: 2020-12-16
Filing date: 2021-11-26
Publication date: 2023-10-25
Also published as: JP2023543875A; WO2022127976A1; US20240047703A1; CN116348634A

Abstract

The invention relates to a layer (3a), in particular for forming an electrically conductive plate (1) for an electrochemical cell (50), wherein the layer (3a) contains a first chemical element from the group of precious metals in the form of ruthenium in a concentration in the range of 50 to 99 at.% and at least a second chemical element in the form of silicon in a concentration of < 10 at.%. The invention furthermore relates to a layer system, an electrically conductive plate and an electrochemical cell.

Description

Layer and layer system, as well as electrically conductive plate and electrochemical cell

The invention relates to a layer, in particular for forming an electrically conductive plate for an electrochemical cell. Furthermore, the invention relates to a layer system with such a layer and an electrically conductive plate with such a layer system. The invention also relates to an electrochemical cell, in particular a fuel cell, an electrolyzer or a redox flow cell, with at least one such electrically conductive plate.

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

An example of this are the bipolar or monopolar plates in fuel cells, especially in an oxygen half-cell. The bipolar or monopolar plates are in the form of carbon plates (e.g. graph oil plates) which contain carbon as an essential component. These plates tend to be brittle and are comparatively thick, so that they significantly reduce the power volume of the fuel cell. Another disadvantage is their lack of physical (e.g. thermomechanical) and/or chemical and/or electrical stability.

Also known is the production of the current collecting plates of the fuel cell from metallic (in particular austenitic) stainless steels. The advantage of these panels is that the panel thickness that can be achieved is less than 0.5 mm. This thickness is desirable so that both the space and the weight of the fuel cell can be kept as small as possible. The problem with these plates is that surface oxides are formed when the fuel cell is in operation, so that a surface resistance is impermissibly increased and/or electrochemical decomposition (such as corrosion, for example) occurs. Laid-Open Specifications DE 10 2010 026 330 A1, DE 10 2013 209 918 A1, DE 11 2005 001 704 T5 and DE 11 2008 003 275 T5 describe the coating of austenitic stainless steels as carriers to achieve the requirement, for example for the use of bipolar plates in fuel cells with a gold layer, which is in a band range of up to 2 nm. There are several disadvantages to this requirement solution. For example, a gold layer, even if 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 nobler than the carrier material than stainless austenitic steel (stainless steel) and under unfavorable operating conditions in the fuel cells causes the carrier to dissolve (e.g. pitting or pitting corrosion), which results in a reduction in service life. Corrosion cannot be prevented, particularly in environments containing chloride (eg aerosols).

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.

Layers on the carrier in the form of so-called hard material layers based on nitride or carbide are also known from the prior art. An example of this is titanium nitride, which, however, tends to form oxidic metal complexes through to closed surface layers during operation of a fuel cell. As a result, the surface resistance increases to high values, as with stainless steel. Processes for coating with chromium nitride or chromium carbonitride can be found, for example, in the patent specifications DE 199 37 255 B4 and EP 1273060 B1 and the published application DE 100 17 200 A1.

Depending on the composition, the hard material layers have very good operating properties (e.g. resistance to corrosion, abrasion resistance, high contour accuracy), but they harbor 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 in the fuel cell, such as the formation of a water film between Between an active electrode of a membrane-electrode unit of the fuel cell and the bipolar plate, a so-called local element or an unexpected and undesired reaction element arises.

Also known are multiple coatings based on nitrides with very thin layers of gold or platinum. Thus, satisfactory operating results for a fuel cell can be achieved with layer thicknesses of the noble metals of more than 2 μm. The fundamental problem of dissolution remains at high anodic potentials. The layer thickness ensures an almost pore-free top layer and thus reduces the risk of pitting corrosion.

So-called dimensionally stable anodes are also known. Here, single-phase or multi-phase oxides with ruthenium oxide and/or iridium oxide are formed with the aid of refractory metals. Although this type of layer is very stable, the electrical resistance is too high. The same is also true when a surface of the carrier, generally made of a noble metal, is doped with iridium.

DE 10 2009 010 279 A1 describes a bipolar fuel cell plate comprising a conductive metal plate which is anodized and on which a conductive layer is subsequently deposited by means of an atomic layer deposition process. In particular, the conductive layer comprises at least one of titanium oxynitride, gold, platinum, carbon, ruthenium or ruthenium oxide.

DE 10 2016 202 372 A1 discloses a layer consisting of a homogeneous or heterogeneous solid metallic solution or compound containing a first chemical element from the group of noble metals in the form of iridium and/or a second chemical element from the group of noble metals in the form of ruthenium and at least one other non-metallic chemical element from the group consisting of nitrogen, carbon, boron, fluorine and hydrogen.

Thus, in the electrochemical systems mentioned by way of example, in particular for energy conversion, embodied metallic carriers or a bipo lar plate for a PEM fuel cell or an electrolyser to make the following requirements:

- High corrosion resistance to a surrounding medium, and/or

- high resilience to anodic or cathodic polarizing loads,

- low surface resistance of an electrolyte facing surface of the carrier or its coating, and

- Low production costs of the carrier, in particular, for example, an electrically conductive conductor in the form of bipolar plates for the use of fuel cells for mobile use.

WO 2018/145 720 A1 discloses a plate-shaped electrode made of composite material for a redox flow cell, which is structured for optimal distribution of fluid.

It is therefore the object of the present invention to provide an improved layer or an improved layer system in general for an energy converter, in particular for a bipolar plate of a fuel cell or an electrolyzer or an electrode of a redox flow cell. Furthermore, it is the object of the invention to specify an electrically conductive plate with an improved layer system and an electrochemical cell equipped therewith.

The object is achieved according to the invention by a layer, in particular for forming an electrically conductive plate for an electrochemical cell, the layer containing a first chemical element from the group of noble metals in the form of ruthenium in a concentration in the range from 50 to 99 at. % and at least one second chemical element in the form of silicon in a concentration of <10 at.%.

Silicon forms stable compounds, especially oxides, which have a positive effect on the resistance of the layer to electrochemical attack.

The silicon is preferably contained in a concentration in the range from 3 to <10 at %.

The object is further achieved by a layer system, in particular for an electrically conductive plate of an electrochemical cell, comprising a top layer and a bottom layer system, the top layer being in the form of the layer according to the invention.

The object is also achieved according to the invention by an electrochemical cell, in particular in the form of a fuel cell, an electrolyzer or a redox flow cell, comprising at least one electrically conductive plate according to the invention.

Advantageous configurations with expedient and non-trivial developments of the invention are specified in the dependent claims.

The layer according to the invention is electrically conductive and electrocatalytically active and designed to protect against corrosion.

The layer according to the invention preferably contains at least one further second chemical element from the group consisting of nitrogen, carbon, boron, fluorine, hydrogen and oxygen.

The at least one second chemical element is preferably present in the layer in a concentration in the range from 1 at.% to 40 at.%.

The at least one second chemical element is preferably dissolved in the metal lattice of the ruthenium in such a way that the lattice type of the host metal or the host metal alloy essentially does not change.

The layer according to the invention preferably comprises a) ruthenium, silicon and carbon; or b) ruthenium, silicon, carbon and hydrogen; or c) ruthenium, silicon, carbon and fluorine, optionally also hydrogen; or d) ruthenium, silicon, carbon and oxygen; or e) ruthenium, silicon, carbon, oxygen and hydrogen. It has been shown that with a carbon-containing layer, i.e. through the use of the metalloid or non-metallic chemical element carbon, the conductivity of the layer is higher than with gold and that at the same time its oxidation stability in an acidic solution is significantly above a voltage of 2000 mV standard hydrogen electrode. Depending on the embodiment, measured specific electrical resistances are below 5 mΩ cm′ ² (under standardized conditions).

In comparison, the specific electrical resistance of gold is about 10 mΩ cm' ² at room temperature.

The layer preferably also has 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 and/or tungsten. The at least one chemical element from the group of refractory metals is contained in the layer in particular in a concentration range of 0.01 to 10 at. It has been shown that the addition of the refractory metals partially controls the H2O2 and ozone formed during the electrolysis.

The layer preferably also contains 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 formed by aluminum, iron, nickel, cobalt, zinc, cerium, tin. The at least one further chemical element from the group of base metals is contained in the layer, in particular in a concentration range of 0.01 to 10 at.

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 together are contained in the layer in particular in a concentration range of 0.01 to 10 at.

The layer also preferably has at least one additional chemical element from the group comprising indium, platinum, gold, silver, rhodium, palladium in a concentration range of 0.01 to 25 at %. It is preferred if the layer has a layer thickness in the range from 0.5 to 500 nm.

It has proven useful if all chemical elements from the group of noble metals, i.e. together with ruthenium, are contained in the layer in a concentration range of > 50 to 99 At.-%.

The corrosion protection on metallic carriers, such as those made of steel, in particular high-grade steel, or titanium, is further improved by applying the layer according to the invention to an underlayer system formed between the carrier and the layer. This is particularly advantageous when corrosive media are present, especially when the corrosive media contain chloride.

Under-oxidation, i.e. oxidation of the surface of a support with a layer applied to this surface, normally leads to the delamination of precious metal layers on top.

The object is also achieved by a layer system, in particular for an electrically conductive plate of an electrochemical cell, comprising a cover layer and an underlayer system, the cover layer being in the form of the layer according to the invention.

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

The underlayer system has in particular a first underlayer in the form of a metallic alloy layer comprising the chemical elements titanium and niobium, in particular 20-50% by weight niobium and the remainder titanium.

In particular, the underlayer system has a second underlayer comprising at least one chemical element from the group titanium, niobium, zirconium, hafnium, tantalum and also at least one non-metallic element from the group nitrogen, carbon, boron, fluorine.

In a particularly preferred embodiment, the underlayer system has a second underlayer comprising the chemical elements a) titanium, niobium and also carbon and fluorine, or b) titanium, niobium and also nitrogen, is in particular made of (Ti6?Nb33)No.8- i,i formed.

The second backing layer is preferably positioned between the first backing layer and the top layer.

The second sub-layer can further contain up to 5 at.% oxygen.

Advantageously, a thickness of the layer or cover layer according to the invention of less than 10 nm is sufficient to protect against resistance-increasing oxidation of the second base layer. In order to provide reliable protection against corrosion, partial layers of the underlayer system are made of at least one refractory metal, which are applied in at least two layers to the steel, in particular stainless steel, first as a metal or alloy layer (=first underlayer) and then as a metalloid layer (=second backing layer). The double layer formed with the help of the two-layer structure under the layer according to the invention ensures on the one hand an electrochemical adaptation to a substrate material, i.e. the material from which the substrate for receiving the layer system is formed, and on the other hand pore formation due to oxidation and hydrolysis processes is excluded.

Electrochemical adaptation to the substrate material is necessary because both the metalloid layer (=second underlying layer) and the layer according to the invention or the top layer are very precious. In the event of pore formation, high local element potentials would build up, resulting in impermissible corrosion currents. The metallic first substrate layer is formed from preferably titanium or niobium or zirconium or tantalum or hafnium or from alloys of these metals, the baser are as the substrate material in the form of steel, in particular stainless steel, and initially react during corrosion processes to form insoluble oxides or voluminous, partly gel-like hydroxo compounds of these refractory metals. As a result, the pores become blocked and protect the base material from corrosion. The process represents a self-healing of the layer system.

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

The object is also achieved by an electrically conductive plate, in particular a bipolar plate of a fuel cell or an electrolyzer or an electrode of a redox flow cell, having a metallic substrate and a layer system according to the invention applied at least in partial areas of the surface of the substrate.

In particular, the layer system is applied over the full area to one or both sides of the substrate. The metallic substrate is formed in particular from steel or titanium, preferably from high-grade steel. A thickness of the substrate is preferably less than 1 mm and is in particular equal to 0.5 mm.

Finally, the object is achieved for an electrochemical cell, in particular in the form of a fuel cell, an electrolyzer or a redox flow cell, comprising at least one electrically conductive plate according to the invention.

A fuel cell according to the invention, in particular a polymer electrolyte fuel cell, comprising at least one electrically conductive plate according to the invention in the form of a bipolar plate, has proven to be 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 5000 motor vehicle operating hours. Comparably long service lives can be achieved with an electrolyzer according to the invention, which works with the opposite principle of action with regard to a fuel cell and brings about a chemical reaction, ie a material conversion, with the aid of electric current. In particular, the electrolyzer is one that is suitable for hydrogen electrolysis.

With a redox flow cell according to the invention comprising at least one electrically conductive plate according to the invention in the form of an electrode, long service lives and power densities can be achieved.

Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments and the figures. The features and feature combinations mentioned above in the description can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the invention.

So shows

Figure 1 shows an electrically conductive plate in section,

FIG. 2 an electrode with a flow field,

FIG. 3 a redox flow cell or a redox flow battery with a redox flow cell,

Figure 4 shows an electrolyzer in section and

FIG. 5 shows a fuel cell stack in a three-dimensional view.

Figure 1 shows a sectional view of an electrically conductive plate 1, comprising a substrate 2 made of stainless steel and a layer system 3 applied over the entire surface on one side of the substrate 2. The layer system 3 comprises a cover layer 3a and an underlayer system 4 comprising a first underlayer 4a and a second underlayer 4b.

In a first embodiment, a metallic substrate 2 in the form of a con ductor, here for a bipolar plate of a polymer electrolyte fuel cell for the conversion of (reformed) hydrogen, made of stainless steel, in particular a so-called authentic steel with very high known requirements regarding corrosion resistance, eg with the DIN ISO material number 1.4404.

The layer system 3 is formed on the substrate 2 by means of a coating process, for example a vacuum-based coating process (PVD), with the substrate 2 in one process step initially having a first underlying layer 4a in the form of a 1.5 μm thick titanium layer, then having an approximately equal thick second base layer 4b in the form of a titanium nitride layer and finally coated with a top layer 3a in the composition RuSiC. 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 make contact with it. Thus, the free surface 30 of the cover layer 3a in a fuel cell is arranged facing an electrolyte, in particular a polymer electrolyte.

In a second exemplary embodiment, the metallic substrate 2 is initially coated with a first underlying layer 4a in the form of a metallic alloy layer with a thickness of several 100 nm, the metallic alloy layer having the composition Tio.9 Nbo.i. A further application of a second base layer 4b then takes place with a thickness of a further several 100 nm of the composition Tio.9 Nbo.i Ni- _X . A cover layer 3a with a thickness of several nm in the composition RuSiC is applied thereto.

The advantage is an extraordinarily high stability against oxidation of the plate 1 according to the invention. Even with a permanent load of +3000 mV compared to a standard hydrogen electrode, no increase in resistance is found in a sulfuric acid solution, which has a pH of 3.

The cover layer 3a according to the invention of the first and second exemplary embodiment can be applied both by means of the sputtering technique and by means of a cathodic ARC coating method, also known as vacuum arc evaporation. Despite a higher number of droplets, in other words, one compared to the sput tertechnology increased number of metal droplets, the cover layer 3a according to the invention produced in the cathodic ARC method also has 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.

In a third exemplary 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 has been electrolytically polished in a H2SO4/HsPO4 bath before a layer system 3 is applied. After the application of a single underlying layer in the form of a tantalum carbide layer several 1000 nm thick, a cover layer 3a in the form of RuSiCHO is applied.

The advantage of the underlayer formed from tantalum carbide consists not only in its extraordinary resistance to corrosion but also in the fact that it does not absorb any hydrogen and thus serves as a hydrogen barrier for the substrate 2 . This is particularly advantageous if titanium is used as the substrate material.

The layer system 3 according to the invention of the third exemplary embodiment is suitable for use in an electrolytic cell for generating hydrogen at current densities i that are greater than 500 mA cm ^-2 .

The advantage of the metalloid layer lying in between and/or closed on both sides in the layer system or of the second underlying layer, which in the simplest case is formed from titanium nitride, for example, is its low electrical resistance of 10-12 mΩ cm ^-2 . Likewise, the layer or cover layer according to the invention can also be formed without a second underlying layer or metalloid layer, with a possible increase in resistance.

Table 1 shows some coating systems with their characteristic values.

Table 1 : Layers and selected characteristic values

Table 1 shows only a few exemplary layer systems. Advantageously, the layer systems according to the invention show no increase in resistance over several weeks at an anodic load of +2000 mV compared to standard hydrogen electrode in sulfuric acid solution at a temperature with a value of 70-80°C. The layer systems applied in a high vacuum using a sputtering or ARC method or in a fine vacuum using a PECVD method (plasma-enhanced chemical vapor deposition method) were partially darkened after this exposure time. However, there were no visible signs of corrosion or significant changes in surface resistance.

FIG. 2 shows a three-dimensional view of a plate 1 in the form of an electrode comprising a substrate 2 in the form of a metal sheet made of stainless steel with a profile 40 that forms a flow field 7 . In the substrate 2 there is a profiling 40 on both sides for forming a flow field 7 in each case, resulting in a three-dimensional structuring of the surface of the electrode. The substrate 2 is on both sides with a layer system 3, which is to be flown by an electrolyte in a redox flow cell 8 (see FIG. 3).

FIG. 3 shows an electrochemical cell 50 in the form of a redox flow cell 8 or a redox flow battery with a redox flow cell 8. The redox flow cell 8 comprises two plates 1a, 1b in the form of electrodes , a first reaction space 10a and a second reaction space 10b, each reaction space 10a, 10b being in contact with one of the electrodes. The reaction spaces 10a, 10b are separated from one another by an ion exchange membrane 9a. A liquid anolyte 11a is pumped from a tank 13a via a pump 12a into the first reaction chamber 10a and passed between the plate 1a and the ion exchange membrane 9a. A liquid catholyte 11b is pumped from a tank 13b via a pump 12b into the second reaction chamber 10b and passed between the plate 1b and the ion exchange membrane 9a. Ion exchange takes place across the ion exchange membrane 9a, electrical energy being released at the electrodes due to the redox reaction.

FIG. 4 shows an electrochemical cell 50 in the form of an electrolysis cell 20 of an electrolyzer comprising a polymer electrolyte membrane 9 which separates an anode side A and a cathode side K from one another. A catalyst layer 21a, 21b, each comprising a catalyst material and a fluid diffusion layer 22a, 22b, is arranged adjacent to the catalyst layer 21a, 21b on both sides of the polymer electrolyte membrane 9. The fluid diffusion layers 22a, 22b are each disposed adjacent an electrically conductive plate 24a, 24b, with the fluid diffusion layers 22a and 22b being formed of expanded metal. The plates 24a, 24b each have flow channels 23a, 23b on their sides facing the fluid diffusion layers 22a, 22b in order to improve the supply of reaction medium (water) and the removal of reaction products (water, hydrogen, oxygen).

FIG. 5 schematically shows a fuel cell stack 100 comprising a plurality of electrochemical cells 50 in the form of fuel cells 90. Each fuel cell 90 comprises a polymer electrolyte membrane 9 which is adjacent to plates 1c, 1d in the form of bipolar plates on both sides. Each bipolar plate has a substrate 2 made of stainless steel, which is covered on both sides with a layer system 3 (see FIG. 1). The bipolar plate has an inflow area with openings 80a and an outlet area with further openings 80b, which are used to supply a fuel cell 90 with process gases and coolant and to remove reaction products from the fuel cell 90 and coolant. The bipolar plate also has a gas distributor structure 7 ′ on each side, which is arranged facing the polymer electrolyte membrane 9 .

Reference character list

1, 1a, 1b, 1c, 1d, 24a, 24b electrically conductive plate

2 substrate

3 layer system

3a layer, top layer

4 underlayer system

4a first backing layer

4b second backing layer

7 river field

7' gas distribution structure

8 redox flow cell

9 polymer electrolyte membrane

9a ion exchange membrane

10a first reaction space

10b second reaction space

11a anolyte

11 b catholyte

12a, 12b pump

13a, 13b Tank

20 electrolytic cell

21a, 21b catalyst layer

22a, 22b fluid diffusion layer

23a, 23b flow channel

30 free surface

40 profiling

50 electrochemical cell

80a, 80b opening

90 fuel cell

100 fuel cell stacks

K cathode side

A anode side

Claims

Claims Layer (3a), in particular for forming an electrically conductive plate (1) for an electrochemical cell (50), the layer (3a) containing a first chemical element from the group of noble metals in the form of ruthenium in a concentration in the range of 50 up to 99 At.-% and contains at least one second chemical element in the form of silicon in a concentration of <10 At.-%. Layer (3a) according to Claim 1, containing at least one further second chemical element from the group consisting of nitrogen, carbon, boron, fluorine, hydrogen and oxygen. Layer (3a) according to Claim 2, in which the at least one second chemical element is present in the layer (3a) in a concentration in the range from 1 at% to 40 at%. Layer (3a) according to any one of claims 1 to 3, wherein this a) comprises ruthenium, silicon and carbon; or b) comprises ruthenium, silicon, carbon and hydrogen; or c) ruthenium, silicon, carbon and fluorine, optionally further comprising hydrogen; or d) comprises ruthenium, silicon, carbon and oxygen; or e) ruthenium, silicon, carbon, oxygen and hydrogen. Layer (3a) according to one of the preceding claims, wherein the layer (3a) further contains 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 and/or tungsten, having. Layer (3a) according to claim 5, wherein the at least one chemical element from the group of refractory metals is contained in the layer (3a) in a concentration range of 0.01 to 10 at. Layer (3a) according to one of the preceding claims, wherein the layer (3a) further contains at least one chemical element from the group of base metals. Layer (3a) according to claim 7, wherein the at least one chemical element from the group of base metals is aluminium, iron, nickel, cobalt, zinc, cerium, tin. Layer (3a) according to claim 7 or 8, wherein the at least one further chemical element from the group of base metals is contained in the layer (3a) in a concentration range of 0.01 to 10 at. . Layer (3a) according to one of claims 5 or 6 in conjunction with one of claims 7 to 9, 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 together in Concentration range of 0.01 to 10 at .-% are included in the layer (3a). . Layer (3a) according to one of the preceding claims, wherein the layer (3a) further contains at least one additional chemical element from the group comprising indium, platinum, gold, silver, rhodium, palladium, in a concentration range of 0.01 to 25 At.- % having. . Layer (3a) according to one of the preceding claims, characterized in that the layer (3a) has a layer thickness in the range from 0.5 to 500 nm. . Layer system (3), in particular for an electrically conductive plate (1) of an electrochemical cell (50), comprising a cover layer (3a) and an underlayer system (4), the cover layer (3a) in the form of a layer (3a) according to claims 1 to 12 is formed. . Layer system (3) according to claim 13, wherein the base layer system (4) has at least one base layer (4a, 4b) comprising at least one chemical element from the group titanium, niobium, hafnium, zirconium, tantalum. - 19 - . Layer system (3) according to claim 14, wherein the base layer system (4) has at least one first base layer (4a) in the form of a metallic alloy layer comprising the chemical elements titanium and niobium. . Layer system (3) according to claim 14 or 15, wherein the base layer system (4) has a second base layer (4b) comprising at least one chemical element from the group titanium, niobium, hafnium, zirconium, tantalum and also at least one non-metallic element from the group nitrogen , carbon, boron, fluorine. . Layer system (3) according to Claim 16, characterized in that the second base layer (4b) is arranged between the first base layer (4a) and the cover layer (3a). . Layer system according to Claim 16 or 17, characterized in that the second underlying layer (4b) contains up to 5 at.% oxygen. . Electrically conductive plate (1, 1a, 1b, 1c, 1d, 24a, 24b), in particular a bipolar plate of a fuel cell (90) or an electrolyzer (20) or an electrode of a redox flow cell (8), having a metallic substrate (2) and a layer system (3) according to one of claims 13 to 18 applied at least in partial areas of the surface of the substrate (2). Electrochemical cell (50), in particular in the form of a fuel cell (90), an electrolyzer (20) or a redox flow cell (8), comprising at least one electrically conductive plate (1, 1a, 1b, 1c, 1d , 24a, 24b) according to claim 19. . Electrochemical cell (50) according to Claim 20, which is a fuel cell (90), in particular a polymer electrolyte fuel cell, comprising at least one plate (1c, 1d) according to Claim 19 in the form of a bipolar plate. - 20 -

22. Electrochemical cell (50) according to claim 20, which is an electrolyzer (20) comprising at least one plate (1a, 1b) according to claim 19 in the form of a bipolar plate. 23. Electrochemical cell (50) according to claim 20, wherein it is a redox

Flow cell (8) comprising at least one plate (24a, 24b) according to claim 19 in the form of an electrode.