Electrode material coated with a tantalum nitride coating of hexagonal crystallographic structure.

Abstract

Electrode material coated with a tantalum nitride coating of hexagonal crystallographic structure. The invention relates to an electrode material comprising a metallic substrate (11) coated with a protective coating (12) of tantalum nitride TaN of hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN, the protective coating being semiconducting and its band gap having a width of at most 50 meV. Figure for the abstract: Fig. 1.

Description

Title of the invention: Electrode material coated with a tantalum nitride coating of hexagonal crystallographic structure. technical field

[0001] The present presentation relates to the field of hydrogen production processes by photocatalysis of water and / or wireless photoelectrolysis of water (called in English "Wireless Solar Water Splitting") and more specifically to the electrodes enabling the realization of such a process. Previous technique

[0002] The production of gaseous dihydrogen (H2) is a major current energy challenge, dihydrogen being an energy source which does not release carbon dioxide when used.

[0003] In order for it to truly be a carbon-free energy source, however, the dihydrogen must not be produced from petroleum derivatives, unlike the vast majority of current dihydrogen.

[0004] Among the processes enabling the production of dihydrogen without requiring fossil resources are the photocatalysis of water or the wireless photoelectrolysis of water. The latter makes it possible to obtain dihydrogen by decomposition of water, the reaction however requiring an input of energy in the form of photons.

[0005] This reaction is not currently developed on an industrial scale, largely due to existing kinetic limitations at the electrodes enabling redox electrochemical reactions during photocatalysis.

[0006] Although copper(I) oxide electrodes (Cu2O) offer satisfactory performance and have even demonstrated in the laboratory that they effectively enable the photocatalysis of water, their stability precludes industrial-scale application. Indeed, copper(I) oxide transforms within minutes of contact with water into copper(II) oxide (CuO) and then into metallic copper (Cu), and the electrodes quickly become inoperative.

[0007] There therefore remains a need for electrode materials enabling the photocatalysis of water with performance comparable to that of copper(I) oxide (Cu2O) but with better long-term stability. Description of the invention

[0008] The invention is specifically aimed at meeting the need stated above.

[0009] To this end, it proposes an electrode material comprising a metallic substrate covered by a tantalum nitride (TaN) protective coating of structure hexagonal crystallographic or hexagonal doped tantalum nitride TaN, the protective coating being semiconducting and its band gap has a width of at most 50 meV.

[0010] Such an electrode material allows the photocatalysis of water to produce dihydrogen H2, while being stable over much longer periods than prior art electrodes.

[0011] In this application, water photocatalysis is understood as the decomposition reaction of water into dihydrogen H2 and dioxygen O2 catalyzed by the supply of photons of sufficient energy.

[0012] On the one hand, the coating of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN ensures excellent resistance to oxidation of the underlying substrate, but also excellent electrical conductivity to the whole electrode.

[0013] For the intended applications, it is the hexagonal crystallographic structure of the tantalum nitride coating (TaN) that advantageously provides these properties. Indeed, in this crystallographic structure, the conductivity of tantalum nitride is approximately 10 times greater than that of tantalum nitride with a cubic structure.

[0014] The band gap width is understood in the usual sense of this term in the field of semiconductor materials and is intended to characterize the energy gap between the valence band and the conduction band.

[0015] The band gap width of the electrode materials described above ensures that the substrate has the properties necessary to allow photocatalysis.

[0016] In one embodiment, the substrate comprises copper(I) oxide Cu2O, silicon Si, boron arsenide BAs, tungsten diselenide WSe2, zinc sulfide ZnS, indium phosphide InP, gallium-indium phosphide GaInP2 or a mixture of several of these compounds.

[0017] In one embodiment, the substrate is formed of one of the following compounds: copper(I) oxide Cu2O, silicon Si, boron arsenide BAs, tungsten diselenide WSe2, zinc sulfide ZnS, indium phosphide InP or gallium-indium phosphide GaInP2.

[0018] All the materials mentioned effectively enable the photo-catalysis of water and are compatible with a protective coating comprising tantalum nitride TaN with hexagonal crystallographic structure or hexagonal tantalum nitride TaN.

[0019] In one embodiment, the coating is made of undoped tantalum nitride TaN with a hexagonal crystallographic structure.

[0020] In one embodiment, the coating is made of doped hexagonal tantalum nitride.

[0021] Regardless of the dopant and the dopant content, it is important that the doped tantalum nitride remains of hexagonal crystallographic structure.

[0022] On the other hand, the invention is not limited either by the nature of the dopant, or by the quantity of the latter provided that the doped tantalum nitride TaN remains of hexagonal crystallographic structure.

[0023] In one embodiment, the coating is made of TabxAxN doped tantalum nitride of hexagonal crystallographic structure with A designating a dopant corresponding to one or more elements chosen from alkali metals, alkaline earth metals and / or transition metals and with x strictly positive and less than 0.05.

[0024] The specification that x is strictly positive in the preceding definition is not intended to exclude from the invention an undoped tantalum nitride TaN coating, but rather aims to avoid redundancy with this embodiment described separately.

[0025] In one embodiment, the dopant A may be one or more elements selected from alkali metals, alkaline earth metals and / or transition metals.

[0026] Such a dopant can be chosen to further functionalize the coating, for example by increasing its corrosion resistance and / or improving the stability of the crystalline phase, and / or its optical properties.

[0027] In one embodiment, dopant A is a single dopant selected from alkali metals, alkaline earth metals and / or transition metals.

[0028] In one embodiment, the dopant may be one or more elements selected from potassium K, sodium Na, calcium Ca, magnesium Mg, yttrium Y, vanadium V, copper Cu and titanium Ti.

[0029] In one embodiment, the dopant may be a single element chosen from potassium K, sodium Na, calcium Ca, magnesium Mg, yttrium Y, vanadium V, copper Cu and titanium Ti.

[0030] In one embodiment, x is less than or equal to 0.05.

[0031] For example, x can take a value between 0.001 and 0.05, or even between 0.01 and 0.05.

[0032] Indeed, this dopant content is a sufficient, but not necessary, condition to ensure that the structure of tantalum nitride remains hexagonal despite the presence of a dopant.

[0033] In one embodiment, the tantalum nitride coating can be written Tab XAXN with A being one or more dopants chosen from potassium K, sodium Na, calcium Ca, magnesium Mg, yttrium Y, vanadium V, copper Cu and titanium Ti, and x between 0 excluded and 0.05 inclusive.

[0034] According to another aspect of it, the invention also relates to a process for manufacturing dihydrogen H2 by photocatalysis of water, the process comprising a step exposure of an electrode comprising a material described above to a light source.

[0035] In one embodiment, the light source can be the sun.

[0036] On the one hand, the substrate chosen for the electrode material ensures excellent supply or capture of electrons and therefore makes it possible to supply the energy necessary for the water decomposition reaction.

[0037] On the other hand, the protective coating of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN ensures excellent preservation of these properties over time. Brief description of the drawings

[0038] [Fig-1] Fig. 1 schematically represents an electrolytic cell including a material as described.

[0039] [Fig.2] The [Fig.2] represents a diffractogram obtained by X-ray diffraction for an element coated with a tantalum nitride coating. Description of the implementation methods

[0040] The invention is now described by means of figures, which are present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0041] In one embodiment, the transmittance of the protective coating is greater than or equal to 50% or even greater than or equal to 70%.

[0042] Transmittance is understood here as the proportion of light radiation passing through the coating.

[0043] For example, the transmittance can be measured, for a given wavelength of the radiation, using a spectrophotometer, for example at a wavelength chosen between 400 nm and 700 nm.

[0044] Such a property ensures excellent efficiency for the entire process. Indeed, transmittance quantifies the proportion of incident light rays that pass through the coating. This must be as high as possible to allow excellent capture of the photons emitted by the light source by the underlying substrate.

[0045] In one embodiment, the light source can be the sun.

[0046] Sunlight is understood here as continuous light radiation between 400 and 700 nm, with the exception of absorption lines due to interaction with the Earth's or solar atmosphere.

[0047] Preferably, the thickness of the coating may be less than or equal to 1.0 pm, for example between 0.01 and 0.1 pm.

[0048] Such a thickness offers an excellent compromise between protection against oxidation of the substrate while ensuring low absorption of light by the protective coating, which ensures excellent performance of the electrode.

[0049] In one embodiment, the electrode material can be formed from the substrate directly coated by a coating of hexagonal TaN tantalum nitride or doped hexagonal TaN tantalum nitride.

[0050] It is thus understood that no layer is disposed between the substrate and the coating. In other words, the substrate is directly coated on at least one face, and preferably all its faces, by a protective coating of hexagonal tantalum nitride TaN or doped hexagonal tantalum nitride TaN.

[0051] In one embodiment, the electrode material does not comprise any material other than the substrate and the coating comprising hexagonal tantalum nitride TaN or doped hexagonal tantalum nitride TaN.

[0052] In one embodiment, the protective coating of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN may comprise more than 90% by weight of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN, or even more than 95% by weight, or even more than 99% by weight, or even be made up of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN.

[0053] The photocatalysis process can be described, in connection with [Fig. 1] which represents a photocatalysis cell 100 comprising an electrode material according to the invention.

[0054] Here, and without limiting the foregoing, the cell comprises a tank 21, comprising water 22, at the bottom of which is disposed an electrode comprising a material as described above, i.e. comprising a metallic substrate 11 covered by a protective coating 12 of tantalum nitride TaN of hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN, the protective coating being semiconducting and its band gap has a width of at most 50 meV.

[0055] The photons 32 from the light source, here the sun 31, are absorbed by the substrate 11 of the electrode which creates an electron-hole pair, denoted e and h+.

[0056] It will be noted that the photons 32 pass through the coating 12 comprising hexagonal tantalum nitride TaN or doped hexagonal tantalum nitride TaN.

[0057] The electron e moves from the valence band to the conduction band, because the energy of the absorbed photon is greater than the gap in the band gap of the electrode substrate.

[0058] Since an electron e moves from the valence band to the conduction band, a hole (denoted h+) is formed in the valence band.

[0059] The hole can then pass through the coating 12 due to the good conductivity of the latter, and then manages to react with a water molecule and oxide it according to the reaction: 2H2O + 4h+ = O2+ 4H+.

[0060] O2 is a gaseous species, which is evacuated in the form of a gas, while the proton H+ remains in the water 22.

[0061] The electrons e meanwhile pass through an electrical circuit 13 and reach the cathode 14 where the protons H+ are reduced according to the reaction: 2H+ + 2e = H2.

[0062] Thus, at the scale of the photocatalysis cell 100, we obtain the following balance: 2H2O = O2 + 2H2.

[0063] Of course, oxygen O2 and hydrogen H2 can be recovered separately from each other using known methods that are not described here. For example, it would suffice to isolate the cathode 14 where dihydrogen is generated from the rest of the tank 21.

[0064] In one embodiment, the coating 12 of tantalum nitride TaN with hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN can be obtained by a high-power pulsed magnetron sputtering method (designated by the acronym "HiPIMS" in the English-language literature for "High-Power Impulse Magnetron Sputtering").

[0065] In such a method, a metallic tantalum or doped metallic tantalum target is used. The hexagonal tantalum nitride coating is carried out under a nitrogen-containing atmosphere, the polarization of the target being controlled during the coating by imposing on it the superposition of a continuous polarization at a potential between -199 V and -100 V and a pulsed polarization whose peaks are at a potential between -900 V and -701 V.

[0066] Such parameters for high-power pulsed magnetron sputtering ensure that the resulting coating of doped or undoped tantalum nitride has a hexagonal crystallographic structure.

[0067] In order to characterize the hexagonal crystallographic structure, one can for example obtain a small-angle X-ray diffraction spectrum to verify that it includes the characteristic lines of the hexagonal crystallographic structure.

[0068] Alternatively, the target used may be a doped metallic tantalum target, comprising a dopant level ensuring the deposition of the desired amount of dopant.

[0069] Alternatively, a pure metallic tantalum target can be used, and one or more secondary target(s) made up of the dopant(s).

[0070] Details of the high-power pulsed magnetron sputtering method can be found in patent application PCT / FR2020 / 050801.

[0071] Unlike prior art electrodes made of copper(I) oxide, electrodes as described in this application exhibit excellent resistance to oxidation.

[0072] Thus, it was observed that even after 1 hour in a hot 80% phosphoric acid bath, the metallic substrate having been heated to 160 °C, a 316L steel substrate coated with tantalum nitride of hexagonal structure with a thickness of 0.5 pm, deposited by a high-power pulsed magnetron sputtering method, showed no signs of corrosion.

[0073] Although such a steel is not compatible with an application as an electrode material substrate, this example clearly demonstrates the corrosion resistance of the hexagonal TaN tantalum nitride coating in concentrated acidic media and temperature.

[0074] Figure 2 shows a small-angle X-ray diffraction spectrum obtained for the hexagonal structure tantalum nitride coating.

[0075] Fig. 2 shows the theoretical lines of the undoped tantalum nitride diffractogram of hexagonal structure 41a to 41h, as well as the actual diffractogram obtained 51 for the sample described above.

[0076] The characteristic line 5la of the hexagonal structure tantalum nitride is found on the latter, as well as the characteristic lines of a 316L steel 51b and 51c.

Claims

Demands

1. Electrode material comprising a substrate (11) covered by a protective coating (12) of tantalum nitride TaN having a hexagonal crystallographic structure or of doped hexagonal tantalum nitride TaN, the protective coating being semiconducting and its band gap having a width of at most 50 meV, in which the substrate (11) is formed of one of the following compounds: copper(I) oxide Cu2O, silicon Si, boron arsenide B As, tungsten diselenide WSe2, zinc sulfide ZnS, indium phosphide InP or gallium-indium phosphide GaInP2 and in which the substrate is directly coated by the hexagonal tantalum nitride TaN or doped hexagonal tantalum nitride TaN coating.

2. Electrode material according to claim 1, wherein the protective coating (12) is Tab XAXN doped tantalum nitride of hexagonal crystallographic structure with A denoting a dopant corresponding to one or more elements selected from alkali metals, alkaline earth metals and / or transition metals and with x strictly positive and less than 0.

05.

3. Electrode material according to claim 2, wherein the dopant A is one or more elements selected from potassium K, sodium Na, calcium Ca, magnesium Mg, yttrium Y, vanadium V, copper Cu and titanium Ti.

4. Electrode material according to claim 2 or 3, wherein x takes a value between 0.01 and 0.

05.

5. A process for manufacturing dihydrogen H2 by photocatalysis of water, the process comprising a step of exposing an electrode comprising a material according to any one of claims 1 to 4 to a light source.

6. A method for manufacturing dihydrogen H2 according to claim 5, wherein the light source is the sun (31).

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