FR3132168A1 - Component for fuel cell - Google Patents

Abstract

Component for fuel cell or alkaline electrolyte electrolyzer provided with an anti-corrosion coating, as well as such a fuel cell or electrolyser with alkaline electrolyte, the component comprising a substrate (31), electrically conductive, and an anti-corrosion coating (32) deposited on at least one surface of the substrate (31), the anticorrosion coating (32) comprising at least one main layer based on tantalum nitride doped with one or more doping elements chosen from the family of transition metals or lanthanides. Fig. 3.

Description

Fuel cell component

This presentation concerns a component for a fuel cell or electrolyzer with an alkaline electrolyte provided with an anti-corrosion coating, as well as such a fuel cell or electrolyzer with an alkaline electrolyte.

Such a component, having an electrical conduction function, can in particular equip an alkaline fuel cell (AFC: “Alkaline Fuel Cell”), a metal/air cell or even a direct urea fuel cell (DUFC: “Direct Urea Fuel”). Cell").

Conductive components, such as end plates, bipolar plates and interconnectors, used in fuel cells are exposed to both oxidizing conditions, due in particular to the presence of oxygen and/or water, and corrosive, due in particular to acid effluents from the electrolyte, extremely hard leading, in the absence of adequate protection, to rapid deterioration inducing a loss of conductivity of these components as well as pollution of the environment of the battery fuel, in particular its electrolyte and/or its catalysts, by the products resulting from this corrosion.

In particular, fuel cells with alkaline electrolyte are exposed to difficult alkaline corrosion conditions due in particular to alkaline effluents from the electrolyte.

In order to limit this corrosion, a first option is to use very specific alloys that naturally have high corrosion resistance. This may include Inconel 625. However, such special alloys are expensive. In addition, their natural resistance to corrosion may prove insufficient in certain applications.

A second option is to use more traditional materials for such conductive components but to protect them with an anti-corrosion coating. Numerous materials have thus been tested in the scientific literature, including coatings of graphite or conductive metal oxides.

However, in addition to very good corrosion protection properties, this anti-corrosion coating must benefit from good stability and sufficient electrical conductivity so as not to hinder the electrical operation of the fuel cell. In particular, the conductivity of the coating must remain greater than 100 S/cm. In addition, the coating must be sufficiently thin, for example less than 5 µm, so as not to modify the geometry of the component, particularly when the latter includes channels.

However, to date, both in the field of alkaline fuel cells (AFC: “Alkaline Fuel Cell”), and direct urea fuel cells (DUFC: “Direct Urea Fuel Cell”), the only known solution allowing to achieve an acceptable degradation rate (less than 40 µV/h), is the coating of pure gold, with a thickness of 200 to 500 nm.

However, it will be easy to understand that such a pure gold coating significantly increases the cost of the fuel cell. Thus, when all of the conductive components of the fuel cell are equipped with such a coating, the cost of this coating alone can represent up to half of the total cost of the fuel cell. In addition, even the pure gold coating ends up wearing out in part, thus polluting the environment of the fuel cell: in particular, gold particles can be deposited on certain components of the fuel cell and create electrical connections in undesired locations, which reduces the efficiency of the fuel cell.

There is therefore a real need for a component for a fuel cell or electrolyzer with an alkaline electrolyte, as well as for a fuel cell or an electrolyzer with an acid electrolyte comprising such a component, which are devoid, at least in part, of the disadvantages inherent in aforementioned known configurations.

The present disclosure relates to a component for a fuel cell or alkaline electrolyte electrolyzer, comprising
a substrate, electrical conductor, and
an anti-corrosion coating deposited on at least one surface of the substrate,
in which the anti-corrosion coating comprises at least one main layer based on tantalum nitride doped with one or more doping elements chosen from the family of transition metals or lanthanides.

As will be specified below in the detailed description, such a layer of tantalum nitride doped in this manner offers very good protection against corrosion while benefiting from good electrical conductivity. In particular, the inclusion of one or more dopants makes it possible to increase the chemical stability of tantalum nitride, usually metastable, by forming solid solutions.

The component thus obtained is therefore capable of ensuring its current conduction function efficiently and sustainably, despite the strongly oxidizing conditions prevailing in the fuel cell or the electrolyser.

In particular, thanks to the great stability of the doped tantalum nitride, the coating practically does not degrade over time: this reduces the risk, on the one hand, of seeing the conductivity of the component drop over time and, on the other hand, on the other hand, to pollute the environment of the fuel cell or the electrolyzer, in particular its electrolyte or its catalysts, and therefore to reduce its efficiency. Thus, such a coating makes it possible to achieve an extremely long lifespan for the component, of the order of 20,000 to 30,000 hours.

Furthermore, the cost of obtaining a doped tantalum nitride coating is significantly lower than that of a pure gold coating, which makes it possible to significantly reduce the overall cost of the fuel cell or the electrolyzer.

Tantalum nitride also has the advantage of remaining stable up to approximately 3000°C, which allows its use in a very broad spectrum of applications, including at very high temperatures.

In certain embodiments, the main layer is made essentially of tantalum nitride doped with one or more doping elements chosen from the family of transition metals or lanthanides. The main layer is thus essentially uniform.

In some embodiments, the main layer is two-phase or multi-phase. In particular, the main layer may have a composition gradient, for example in the direction perpendicular to the substrate.

In certain embodiments, the main layer is made essentially of tantalum nitride. On the other hand, the main layer may present a variation in composition affecting the crystal structure and/or the doping of the tantalum nitride. In particular, a crystal structure gradient has the advantage of improving the accommodation of stresses between the coating and its substrate, which improves the mechanical properties of the complete system, by limiting cracks and/or delamination of the coating.

In certain embodiments, the electrolyte of the fuel cell or the electrolyzer is liquid and/or solid.

In some embodiments, the electrolyte of the fuel cell or electrolyzer is a solution of potassium hydroxide KOH and/or sodium hydroxide NaOH.

In certain embodiments, the component has an electrical conduction function within the fuel cell or the electrolyzer. In particular, the component can be an end plate, a bipolar plate or even an interconnector for a fuel cell or electrolyzer.

In some embodiments, the substrate is metallic.

In some embodiments, the substrate is made of steel. It may in particular be stainless steel, for example 316L stainless steel. Indeed, thanks to the anti-corrosion protection offered by the anti-corrosion coating, it is possible to use a relatively inexpensive material for the substrate, which is particularly the case for stainless steel, even if it does not inherently have anti-corrosion properties. very high.

In certain embodiments, the substrate is made of titanium, aluminum, nickel, or an alloy based on at least one of these elements. These metals are also relatively inexpensive.

In some embodiments, the substrate is non-metallic. It may in particular be made of graphite or a composite material, for example with an organic or ceramic matrix.

In some embodiments, the crystal system of the main layer tantalum nitride is hexagonal. Indeed, in addition to excellent corrosion resistance, hexagonal tantalum nitride benefits from conductivity almost as good as that of gold.

In some embodiments, the crystal system of the main layer tantalum nitride is cubic. Indeed, although a little less conductive than hexagonal tantalum nitride, cubic tantalum nitride also benefits from excellent corrosion resistance, particularly when it is doped as is the case here.

In certain embodiments, the total dopant content within the main layer is between 1 ppm and 10% at, preferably between 10 ppm and 1% at, more preferably between 0.2 and 0.5% at .

In certain embodiments, the main doping element is chosen from zirconium, hafnium, titanium, vanadium, niobium, chromium or molybdenum.

In certain embodiments, the doping element used, preferably single, is titanium (Ti). Indeed, titanium has good resistance to alkaline environments. In addition, it is a fairly inexpensive item. In particular, the lifespan of an alkaline electrolyte fuel cell being relatively short, it is rare for the fuel cell to operate for a sufficiently long period to see a significant internal pollution phenomenon appear: under these conditions, it is superfluous to select more resistant but more expensive materials.

In certain embodiments, the doping element used, preferably single, is vanadium (V). Indeed, vanadium very easily forms a solid solution with tantalum nitride, which increases its stability. In addition, vanadium is difficult to oxidize: consequently, even if a small fraction of vanadium is released into the environment of the fuel cell or the electrolyzer, its impact on the electrolyte or the catalysts will be very low.

In certain embodiments, the content of the main dopant within the main layer is between 1 ppm and 10% at, preferably between 10 ppm and 1% at, more preferably between 0.2 and 0.5% at .

In certain embodiments, the thickness of the anti-corrosion coating is between 5 nm and 5 µm, preferably between 10 nm and 1 µm, more preferably between 100 nm and 300 nm.

In certain embodiments, the anti-corrosion coating comprises several superimposed layers, preferably between 2 and 10 layers, the material of the main layer constituting a main material, and the anti-corrosion coating comprising at least one secondary layer based on a secondary material different from the main material. Such a multilayer structure makes it possible to reinforce the mechanical resistance of the coating, each interface between two distinct layers helping to deflect any cracks appearing in the material. As a result, it is possible to reduce the risk of a crack propagating to the substrate, allowing the substrate to remain protected against corrosion.

In some embodiments, the anti-corrosion coating comprises at least three layers each comprising a different material.

In certain embodiments, each layer of the coating comprises a thickness of between 1 and 500 nm, preferably between 10 and 100 nm.

In some embodiments, the uppermost layer of the anti-corrosion coating is a main layer made of the main material. The first line of protection is thus provided by the main material, which is generally the one benefiting from the best anti-corrosion properties. However, in other embodiments, the uppermost layer of the anti-corrosion coating could be a secondary layer.

In some embodiments, the main material constitutes at least 30% by volume, preferably at least 50% by volume, of the anti-corrosion coating. Overall, this ensures particularly high anti-corrosion protection.

In certain embodiments, the anti-corrosion coating comprises alternating layers based alternately on the main material and the secondary material. Such alternation is particularly effective in stopping cracks before reaching the substrate.

In some embodiments, the secondary material is crystallographic tantalum nitride having a different crystal system and/or doping than the primary material. In particular, the secondary material may comprise one or more different doping elements, or be undoped. In this way, the secondary material has anti-corrosion properties which remain very high, which ensures satisfactory anti-corrosion protection even in the event of cracking of the main layer.

In certain embodiments, the secondary layer consists essentially of the secondary material.

In some embodiments, the secondary layer is two-phase or multi-phase. In particular, the secondary layer may have a composition gradient, for example in the direction perpendicular to the substrate.

In certain embodiments, the secondary layer is made essentially of tantalum nitride. On the other hand, the secondary layer may present a variation in composition affecting the crystal structure and/or the doping of the tantalum nitride. In particular, crystal structure gradient layers have the advantage of improving the accommodation of stresses between a given layer and the lower layer, which improves the mechanical properties of the complete system, by limiting cracks and/or delamination. from the coating to the interfaces. The doping gradient can similarly make it possible to best accommodate two successive layers, avoiding a sudden change in composition which could generate a more mechanically fragile interface.

In some embodiments, the primary layer is deposited using a co-spray process. This co-sputtering process can in particular combine high-power pulsed magnetron cathode sputtering (designated by the acronym “HiPIMS” in the English language literature for “High-Power Impulse Magnetron Sputtering”) using a tantalum target and magnetron cathode sputtering using a target comprising the doping element. Examples of a high-power pulsed magnetron cathode sputtering process are described in particular in document FR 3 097 237.

The present presentation also relates to a fuel cell or an electrolyzer, with alkaline electrolyte, comprising at least one component according to any of the preceding embodiments.

In certain embodiments, the fuel cell is of the alkaline fuel cell (AFC) type.

In certain embodiments, the fuel cell is of the metal/air cell type. This may in particular be a Zn/Air battery, an Al/Air battery, a Mg/Air battery or a Li/Air battery.

In certain embodiments, the fuel cell is of the direct urea fuel cell (DUFC) type.

In certain embodiments, the fuel cell is of the type supplied with gaseous fuel, preferably with dihydrogen H ₂ .

In certain embodiments, the fuel cell is configured to be supplied with urea CO(NH ₂ ) ₂ .

In this presentation, we consider that a part or part of a part is made from a given material when this material represents the majority material, by mass, in the composition of the part or part of a part.

In this presentation, it is considered that a part or part of a part is made essentially of a given material when it is formed at least 80%, preferably 90%, more preferably 99%, by this material.

The aforementioned characteristics and advantages, as well as others, will appear on reading the detailed description which follows, of examples of embodiment of the component and the fuel cell proposed. This detailed description refers to the accompanying drawings.

The accompanying drawings are schematic and aim above all to illustrate the principles of the presentation.

In these drawings, from one figure to another, identical elements (or parts of elements) are identified by the same reference signs. In addition, elements (or parts of elements) belonging to different embodiments but having a similar function are identified in the figures by numerical references incremented by 100, 200, etc.

There is a schematic view of a fuel cell according to the presentation.

There is a diagram of a fuel cell cell of the .

There is a schematic view of a first example component.

There schematically illustrates a device allowing the production of a coating according to the presentation.

There is a graph illustrating corrosion test results in the context of the first example.

There illustrates, in section and from the front, the microstructure of a cubic TaN coating deposited by a high-power pulsed magnetron cathode sputtering process.

There is a graph illustrating the results of corrosion tests in the context of a second example.

There is a schematic view of a third example component.

There is a graph illustrating corrosion test results in the context of the third example.

There illustrates, in section and from the front, the microstructure of a hexagonal TaN coating deposited by a high-power pulsed magnetron cathode sputtering process.

There illustrates in a comparative manner, in section and from the front, the microstructure of a hexagonal TaN coating deposited by a conventional magnetron sputtering process.

In order to make the presentation more concrete, examples of components and fuel cells are described in detail below, with reference to the accompanying drawings. It is recalled that the invention is not limited to these examples.

There schematically illustrates a fuel cell 1 according to the invention. Such a fuel cell comprises two end plates 11, 12 between which cells 20 are stacked in a stacking direction ), a first bipolar plate 21, a first diffusion layer 22, a first electrode 23, an electrolyte 24, a second electrode 25, a second diffusion layer 26 and a second bipolar plate 27.

The bipolar plates 21, 27 have the function of distributing the reagents and, where appropriate, the heat transfer fluid which cools the cell when said cell has reached its nominal operating speed: the bipolar plates 21, 27 are thus provided with a network of channels 21a, 27a on each of their faces. The bipolar plates 21, 27 also have the function of conducting an electric current between the successive cells 20. Thus, each bipolar plate 21, 27 is located at the interface between two successive cells 20, the second bipolar plate 27 of the N ^th cell 20 constituting the first bipolar plate 21 of the (N+1) ^th cell 20, thus connecting electrically in series the N ^th and (N+1) ^th cells 20.

The end plates 11, 12 play the same role as the bipolar plates 21, 27 except that they are provided at the ends of the stack, thus closing respectively the left side of the first cell 20 and the right side of the stack. the last cell 20: the end plates 11, 12 therefore only have a network of channels 11a, 12a on one of their faces. Also conductive, they constitute the terminals of the fuel cell as a whole: thus, the terminals of the electrical load 2 to be supplied, for example a motor, can be connected to each of the terminal plates 11, 12.

The function of the diffusion layers 22, 26 is to allow the diffusion of the reagents from the bipolar plate 21, 27 towards the electrode 23, 25 concerned, and of the reaction products, from this same electrode 23, 25 towards the bipolar plate 21 , 27. This diffusion can in particular be made possible by grooves or a network of porosities for example.

The electrodes 23, 25 are the seat of the electrochemical half-reactions ensuring the operation of the fuel cell 1: the first electrode 23 thus forms the anode while the second electrode 25 forms the cathode. The electrodes 23, 25 are porous, preferably microporous, in order to allow access to the reagents and the evacuation of the reaction products. The first electrode 23 and/or the second electrode 25 is provided with a catalyst making it possible to catalyze the electrochemical half-reaction in question.

The function of the electrolyte 24 is to authorize the migration of certain ions between the anode 23 and the cathode 25 while preventing the passage of electrons resulting from the oxidation half-reaction at the level of the anode 23. The electrons e ^- thus formed are then conducted towards the cathode 25 of the immediately preceding cell 20 where they are consumed by the reduction half-reaction. The electrons formed by the first cell 20 are for their part collected by the first end plate 11, supply the load 2, and join the cathode 25 of the last cell 20 via the second terminal plate 12.

There illustrates more precisely the operation of a cell 20 in the context of a first exemplary embodiment. In this first example, the fuel cell 1 is an alkaline fuel cell.

In such a fuel cell 1, the fuel supplied to the anode 23 is dihydrogen H ₂ while air is supplied to the cathode 25. The electrolyte 24 consists of an aqueous solution of potassium hydroxide KOH capable of allowing OH ^- hydroxide ions to pass while retaining e ^- electrons. The electrolyte also prevents the passage of any gas. Due to the alkaline environment, it is possible to choose a non-precious metal at the anode as catalyst: for example, it can be iron, cobalt or nickel. At the cathode, silver or iron can be used as a catalyst.

Anode 23 is thus the seat of the following oxidation half-reaction:
2H ₂ + 4OH ^- → 4H ₂ O + 2e ^-

Cathode 25 is for its part the seat of the following reduction half-reaction:
O ₂ + 2H ₂ O + 4e ^- → 4OH ^-

Thus, overall, the operating equation of fuel cell 1 is as follows:
2H ₂ + O ₂ → 2H ₂ O

In such an alkaline electrolyte cell, the electrolyte 24 is an aqueous solution such that H ₂ O water molecules are always present at the level of the cathode 25 in order to allow the reduction half-reaction. The oxidation half-reaction at the level of the anode 23 makes it possible to reconstitute the H ₂ O water molecules consumed at the cathode 25 while producing an excess of water which is evacuated through the diffusion layer 22.

Due to this highly corrosive environment, the end plates 11, 12 and the bipolar plates 21, 27 must be capable of resisting corrosion while continuing to ensure their electrical conduction function.

There then schematically represents such a component, designated under the generic reference 30, according to the first exemplary embodiment. Component 30 thus comprises a substrate 31, made for example of 316L stainless steel, and an anti-corrosion coating 32 deposited on the substrate 31, the anti-corrosion coating 32 having a thickness e ₁ of 380 nm.

In this first example, the anti-corrosion coating 32 comprises a single layer made of tantalum nitride TaN doped with zirconium Zr. This compound, of formula Ta _1-x Zr _x N (with 0 <

In the present example, the crystal system of this compound is cubic, more precisely presenting a face-centered cubic lattice. However, this compound is also capable of crystallizing in a hexagonal form, which is also quite suitable.

In the present example, zirconium is present at 5.8 +/- 0.6 at% within this compound.

Such an anti-corrosion coating 32 can be deposited on the substrate 31 using the device 50 shown schematically on the . This device 50 makes it possible to carry out co-sputtering by high-power pulsed magnetron cathode sputtering.

The device 50 comprises a chamber 51 intended to receive a plasma gas, for example consisting of a mixture of argon and nitrogen. The device further comprises a source of plasma gas (not shown) in communication with the chamber 51. The pressure in the chamber 51 is set between 0.1 and 7 Pa; the gas mixture comprises between 1 and 95 at% nitrogen. In this example, the pressure is set at 0.15 Pa and the gas mixture of argon and nitrogen comprises 10% nitrogen.

Chamber 51 comprises a tantalum target 52, constituting a first cathode, and a zirconium target 53, constituting a second cathode. The two targets 52 and 53 are arranged on the same side of the chamber 51, forming an angle of 90° relative to each other.

The substrate 31 to be covered, constituting the anode, is arranged within the chamber 51 opposite the targets 52, 53, perpendicularly and centered in relation to the bisector of the two targets 52, 53.

In the present example, the first target 52 comprises tantalum in an amount of more than 99% in atomic percentages, and preferably in an amount of more than 99.9% in atomic percentages. The second target 53 comprises zirconium in an amount of more than 99% in atomic percentages, and preferably in an amount of more than 99.9% in atomic percentages.

During coating, the tantalum target 52 is polarized by superimposing a continuous polarization and a pulsed polarization. The polarization of the first target 52 is imposed by a first electrical power supply device 54 comprising a voltage pulse generator and a DC voltage generator electrically connected to the target 52. The voltage pulse generator makes it possible to impose the polarization pulsed to the tantalum target 52. The DC voltage generator makes it possible to impose the DC polarization on the target 52. Such a configuration is described in particular in the document FR 3 097 237. The width of the pulses can be between 1 and 500 µs; their frequency between 100 and 5000 Hz; and their voltage between 300 and 2000 V. In the present example, the pulse width is equal to 30 µs, their frequency is equal to 1000 Hz and their voltage is equal to 1000V.

Analogously, the zirconium target 53 is polarized using a second electrical power supply device 55 configured to ensure radio-frequency polarization.

In the chamber 51, the application of a voltage between the target 52 and the substrate 31 in the presence of an atmosphere comprising nitrogen makes it possible to create a plasma. Electrons are generated by target 52 and can ionize the atoms constituting the plasma by collision. It is possible to introduce into the chamber 51 near the target 52 one or more permanent magnets, not shown, whose magnetic field confines the electrons generated near the target 51 and increases the probability that the collision between an electron and a plasma atom takes place there. When such a collision takes place, a type of high energy is generated, and the latter can bombard the target 52 and tear off, by elastic shock, particles from the target 52. The particles of the target 52 thus torn off can then be separated. deposit on the substrate 31 to form the coating 32.

Analogously and simultaneously, particles of the target 53 are also torn from the target 53 and sprayed onto the substrate 31. It is possible to adjust the quantity of doping particles, here zirconium, in the coating 32 by adjusting the electrical power supplied to the second target 53. For example, in the present example, a power of 5W is applied to the zirconium target 53.

The substrate 31 can be heated during coating by a heating member not shown. Alternatively, the substrate 31 may not be heated during coating. The temperature of the substrate can for example be greater than or equal to 20°C during coating, for example between 20°C and 600°C, or even between 30°C and 500°C. The temperature makes it possible to provide the substrate with thermal energy, and thus to allow a certain mobility of the atoms favoring the recombination of the atoms deposited on the surface of the substrate 31.

There now illustrates corrosion test results concerning this first embodiment. This cyclic voltammetry test was carried out in the laboratory, at room temperature, by immersing in a bath of 0.5 mol/L phosphoric acid. a working electrode made from the material to be tested as well as a platinum counter-electrode, by varying the potential applied to the working electrode in relation to the counter-electrode and recording its current response.

Curve 61 then corresponds to a working electrode in 316L stainless steel, that is to say the material of the substrate 31. Curve 62 corresponds to a working electrode in 316L stainless steel entirely covered with a coating of cubic tantalum nitride doped at a level of 5.8 +/- 0.6% at with zirconium Zr, that is to say a coating 32 according to the first embodiment. Curve 63 corresponds to a working electrode in 316L stainless steel entirely covered with a coating of cubic tantalum nitride doped to a level of 6.0 +/- 0.4% at with zirconium Zr, that is to say a coating 32 according to an alternative embodiment.

We can thus see on the that the curves 62 and 63, corresponding to the anti-corrosion coatings 32 according to the first embodiment and its variant, present stability levels significantly wider than the curve 61 corresponding to the material of the substrate 31, thus confirming the protection of the substrate 31 by the anti-corrosion coating 32.

This test also makes it possible to measure the corrosion potential and current for these different samples:

316L Ta _1-x Zr _x N – 5.8% Ta _1-x Zr _x N – 6.0% E_corr (V/Pt) 0.4 0.6 0.6 I_corr (µA/cm²) 27.2 18.4 17.5

It can thus be seen that the corrosion current I _corr of the anti-corrosion coatings 32 according to the present embodiment and its variant is more than 30% lower than that of the comparative sample of the substrate 31, thus confirming the excellent anti-corrosion properties of these coatings. .

Furthermore, the illustrates the microstructure of a cubic tantalum nitride coating, in section and from the front, deposited by a high-power pulsed magnetron cathode sputtering process according to the present example. The scale shown at the bottom of each view corresponds to 1µm. It can be seen from these two views that the microstructure of the coating is fine and homogeneous, which implies good mechanical resistance with, in particular, good resistance to the propagation of cracks within the coating. For comparison, such a coating deposited by a conventional magnetron sputtering process results in an easily fracturable columnar microstructure.

It is also important to note that the anti-corrosion coatings 32 according to the first embodiment and its variant benefit from a conductivity well above 100 S/cm, which ensures them sufficient conductivity to conduct electrons satisfactorily within of the fuel cell or electrolyzer. Indeed, at 160°C, the conductivity of cubic tantalum nitride deposited by high-power pulsed magnetron cathode sputtering is equal to 538 S/cm.

In this first example, the tantalum nitride TaN forming the anti-corrosion coating 32 is doped with zirconium. However, in other examples, other doping elements, from transition metals or lanthanides, could be used instead of or in addition to zirconium.

In particular, in a second embodiment, the anti-corrosion coating comprises a single layer made of tantalum nitride TaN doped with hafnium Hf. This compound, of formula Ta _1-x Hf _x N, has a stable crystal structure in which zirconium atoms substitute for tantalum Hf atoms, thus forming a solid solution.

In this second example, the crystal system of this compound is also cubic, presenting more precisely a face-centered cubic lattice. However, this compound is also capable of crystallizing in a hexagonal form, which is also quite suitable.

In this second example, hafnium is present at 5.7 +/- 0.5 at% within this compound.

A process entirely similar to that of the first example can be used to deposit this anti-corrosion coating on the substrate, by replacing the zirconium target 53 with a hafnium target.

Analogous to the , there illustrates corrosion test results concerning this second embodiment. This cyclic voltammetry test was carried out under the same conditions as the test carried out for the first embodiment.

Curve 161 then corresponds to a working electrode made of 316L stainless steel, that is to say the material of the substrate. Curve 162 corresponds to a working electrode in 316L stainless steel entirely covered with a coating of cubic tantalum nitride doped to a level of 5.7 +/- 0.7% at with hafnium Hf, that is to say a coating according to the second embodiment. Curve 163 corresponds to a working electrode in 316L stainless steel entirely covered with a coating of cubic tantalum nitride doped to a level of 5.3 +/- 0.5% at with hafnium Hf, that is to say a coating according to an alternative embodiment of the second example.

We can thus see on the that the curves 162 and 163, corresponding to the anti-corrosion coatings according to the second embodiment and its variant, present stability levels significantly wider than the curve 161 corresponding to the material of the substrate, thus confirming the protection of the substrate by the anti-corrosion coating.

This test also makes it possible to measure the corrosion potential and current for these different samples:

316L Ta _1-x Hf _x N – 5.3% Ta _1-x Hf _x N – 5.7% E_corr (V/Pt) 0.4 0.6 0.5 I_corr (µA/cm²) 27.2 22.1 23.7

It can thus be seen that the corrosion current I _corr of the anti-corrosion coatings according to the second embodiment and its variant is more than 10% lower than that of the comparative sample of the substrate, thus confirming the excellent anti-corrosion properties of these coatings.

There schematically represents a component 230 according to a third embodiment. Again, component 230 comprises a substrate 231, made for example of 316L stainless steel, and an anti-corrosion coating 232 deposited on the substrate 231.

In this third example, the anti-corrosion coating 232 comprises a plurality of superimposed layers 233, more precisely ten layers in the present example, each having a thickness e _n of 50 nm for a total coating thickness e _t of 500 nm.

In this third example, the anti-corrosion coating 232 comprises two types of layers 233 deposited alternately: main layers 233a, made of a main material, and secondary layers 233b, made of a secondary material. Here, the uppermost layer of the anti-corrosion coating 232, exposed to the environment, is a main layer 233a.

In this third example, the main material, that is to say the material of the main layers 233a, is cubic tantalum nitride TaN doped with zirconium Zr. In particular, this main material may correspond to the material of the anti-corrosion coating 32 of the first embodiment.

In this third example, the secondary material, that is to say the material of the secondary layers 233b, is undoped hexagonal tantalum nitride TaN.

This multilayer anti-corrosion coating 232 can be deposited on the substrate 231 using the same device 50 shown schematically on the and described in the context of the first embodiment. Indeed, the main layers 233a can be deposited by co-sputtering as described above; the secondary layers 233b can for their part be deposited by high-power pulsed magnetron cathode sputtering using the first target 52 alone, that is to say by not applying any power to the second target 53.

Thus, it is possible to produce all of the layers 233 of the coating 232 in a single step and using a single device 50, by controlling over time the electrical polarizations applied to each of the targets 52, 53.

In particular, in this third example, the pressure in chamber 51 is set at 5 mTorr, or approximately 0.7 Pa; the gas mixture is composed of argon and nitrogen with 25 at% nitrogen. The main layers are deposited with the same pulse parameters as those of the first embodiment. The secondary layers are deposited with the following pulse parameters: pulse width equal to 50 µs; pulse frequency equal to 1000 Hz; and pulse voltage equal to 700V.

The anti-corrosion performances of the main material have been described in the context of the first embodiment. There illustrates the results of corrosion tests concerning the secondary material. This cyclic voltammetry test was carried out under the same conditions as the test carried out for the first embodiment, except that the working electrode was tested in several baths of phosphoric acid at increasing concentrations: 0, 1 mol/L; 0.5 mol/L; and 1 mol/L.

The three curves 261, 262 and 263 thus correspond to the same working electrode in 316L stainless steel fully covered with an undoped hexagonal tantalum nitride coating. Curve 261 corresponds to a bath concentration of 0.1 mol/L; Curve 262 corresponds to a bath concentration of 0.5 mol/L; and Curve 263 corresponds to a bath concentration of 1 mol/L.

We can thus see on the that the curves 261, 262 and 263, corresponding to the secondary material also have stability levels significantly wider than that of the substrate 231 in 316L stainless steel.

This test also makes it possible to measure the corrosion potential and current for 316L stainless steel (Table 3) and undoped hexagonal tantalum nitride (Table 4) for these different bath concentrations.

0.1 mol/L 0.5mol/L 1 mol/L E_corr (V/Pt) 0.9 0.4 0.5 I_corr (µA/cm²) 14 27.2 49.7 Stability domain (V) 2.1 1.4 1.4

0.1 mol/L 0.5mol/L 1 mol/L E_corr (V/Pt) 0.4 0.4 0.4 I_corr (µA/cm²) 17.6 22.2 35.2 Stability domain (V) 2.2 2.1 2.1

We thus see that the corrosion current I _corr of undoped hexagonal tantalum nitride increases much more slowly than that of 316L stainless steel with the increase in the acidity of the environment, thus revealing better resistance to corrosion.

Furthermore, the illustrates the microstructure of a hexagonal tantalum nitride coating, in section and face, deposited by a high-power pulsed magnetron cathode sputtering process according to the present example. The scale shown at the bottom of each view corresponds to 1µm. We can see from these two views that, in the same way as for its cubic counterpart of the , the microstructure of the coating is fine and homogeneous, which implies good mechanical resistance with, in particular, good resistance to the propagation of cracks within the coating. For comparison, such a coating deposited by a conventional magnetron sputtering process results in an easily fracturable columnar microstructure, as is visible on the .

Additionally, the inventors determined that the cubic and hexagonal phases of tantalum nitride had significantly different Young's moduli. Indeed, the Young's modulus measured by nano-indentation of the cubic phase is 430 GPa while that of the hexagonal phase is 560 GPa.

As a result, cracks tend to be deflected at the interface between a main layer 233a and a secondary layer 233b. Therefore, the superposition of several alternating main layers 233a and secondary layers 233b makes it possible to significantly slow down the propagation of cracks within the coating 232.

Finally, it is important to note that hexagonal tantalum nitride benefits from even higher conductivity than cubic tantalum nitride, which further improves its electrical conduction function. Thus, at 160°C, the conductivity of cubic tantalum nitride deposited by high-power pulsed magnetron cathode sputtering is equal to 4,045 S/cm.

In this third example, the primary material is doped cubic tantalum nitride while the secondary material is undoped hexagonal tantalum nitride. However, other multilayer configurations are also possible.

For example, the main material may be hexagonal tantalum nitride, or two-phase tantalum nitride, predominantly cubic or hexagonal. The main material may also comprise a different doping element or even one or more additional doping elements.

Also for example, the secondary material can be doped. However, if the crystal system of the second material is the same as that of the main material, the doping will preferably be different from that of the main material.

Thus, in particular, in a fourth embodiment, the coating comprises alternating layers of doped hexagonal tantalum nitride and undoped hexagonal tantalum nitride.

In a fifth embodiment, the coating comprises alternating layers of doped hexagonal tantalum nitride and cubic tantalum nitride, doped or not.

Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the different illustrated/mentioned embodiments can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than a restrictive sense.

It is also obvious that all the characteristics described with reference to a process can be transposed, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device can be transposed, alone or in combination, to a process.

Claims (12)

Component for a fuel cell or alkaline electrolyte electrolyzer, comprising
a substrate (31), electrical conductor, and
an anti-corrosion coating (32) deposited on at least one surface of the substrate (31),
in which the anti-corrosion coating (32) comprises at least one main layer based on tantalum nitride doped with one or more doping elements chosen from the family of transition metals or lanthanides. Component according to claim 1, in which the substrate (31) is metallic, preferably steel. Component according to claim 1 or 2, wherein the crystal system of the tantalum nitride of the main layer (32) is hexagonal or cubic. Component according to any one of claims 1 to 3, in which the total dopant content within the main layer (32) is between 1 ppm and 10% at, preferably between 10 ppm and 1% at, preferably still between 0.2 and 0.5% at. Component according to any one of claims 1 to 4, in which the main doping element is chosen from zirconium, hafnium, titanium, vanadium, niobium, chromium or molybdenum. Component according to any one of Claims 1 to 5, in which the thickness of the anti-corrosion coating (32) is between 5 nm and 5 µm, preferably between 10 nm and 1 µm, more preferably between 100 nm and 1 µm. 300nm. Component according to any one of claims 1 to 6, in which the anti-corrosion coating (232) comprises several layers (233a, 233b) superimposed, preferably between 2 and 10 layers,
wherein the material of the main layer (233a) constitutes a main material, and
in which the anti-corrosion coating (232) comprises at least one secondary layer (233b) based on a secondary material different from the main material. Component according to claim 7, in which the main material constitutes at least 30% by volume, preferably at least 50% by volume, of the anti-corrosion coating. Component according to claim 7 or 8, in which the anti-corrosion coating comprises an alternation of layers based alternately on the main material and the secondary material. Component according to any one of claims 7 to 9, in which the secondary material is tantalum nitride having a crystal system and/or doping different from the main material. Fuel cell or electrolyzer, with alkaline electrolyte, comprising at least one component (30) according to any one of claims 1 to 10. Fuel cell or electrolyzer according to claim 11, of the alkaline fuel cell (AFC) type, of the metal/air cell type or of the direct urea fuel cell (DUFC) type.