EP2842142A1

EP2842142A1 - Electrode, device including same and manufacturing method thereof

Info

Publication number: EP2842142A1
Application number: EP13725820.8A
Authority: EP
Inventors: Pascal Gentile; Saïd SADKI; Fleur THISSANDIER; Emmanuel Hadji
Original assignee: Universite Joseph Fourier Grenoble 1; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Universite Joseph Fourier Grenoble 1; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2012-04-23
Filing date: 2013-04-22
Publication date: 2015-03-04
Also published as: FR2989838A1; WO2013160816A1; KR20150005647A; FR2989838B1; US20150332859A1

Abstract

The invention relates to an electrode, to the method for manufacturing same and to a device for storing and delivering electric power including same. The electrode of the invention includes a mounting, made of a conductor or semiconductor material, said mounting including, on at least one of the surfaces thereof, at least one nanostructure made of a semiconductor material, characterised by also including a protective layer made of a material selected among a dielectric material having an intrinsic capacity of 1.5 10^-6 to 10 10^-6 F/cm² and a metal, said layer having a thickness that is less than the height of said nanostructure, and covering the at least one nanostructure. The invention is useful in the field of electrochemical storage and, in particular, in the field of ultracapacitors.

Description

ELECTRODE, DEVICE COMPRISING SAME AND METHOD FOR MANUFACTURING SAME

The invention relates to an electrode, to a device for storing and restoring electrical energy comprising it, and to a method for manufacturing this electrode.

The electrodes are very widely used in many fields, and in particular, in the storage and restitution of electrical energy devices.

Among these devices for storing and restoring electrical energy, supercapacitors are being studied more and more.

In a supercapacitor, the key point is the developed surface of the electrodes.

There are several types of supercapacitors.

The first type is electrochemical double layer capacitor (EDLC) supercapacitors, which are electrical energy storage devices that store and release energy through the separation of ionic charges at the interface between a electrode and an electrolyte.

As the stored energy is inversely proportional to the thickness of the double layer, these supercapacitors have a very high energy density compared to conventional dielectric capacitors.

They are able to store a large amount of charge that can be delivered at much higher powers than rechargeable batteries.

A second type of supercapacitor is represented by pseudo-capacitive supercapacitors.

The materials used in these supercapacitors involve fast faradic reactions at the electrode / electrolyte interface.

There are two main categories of pseudocapacitive supercapacitors: supercapacitors based on metal oxides and those based on intrinsically conductive polymers.

The electrochemical capacity is due to reversible oxidation-reduction reactions occurring on the surface of the electrode material in the presence of electrolyte when a voltage is applied. These materials must therefore be able to undergo rapid oxidation-reduction reactions involving a maximum number of oxidation levels in a potentials window compatible with the electrochemical stability of the electrolyte used.

Thus, a third type of supercapacitor is represented by supercapacitors based on metal oxides.

In these supercapacitors, the electrochemical capacity is due to oxidation-reduction reactions at the surface and in the volume of the material of the electrode.

This quantity depends on the quantity of charges transferred, which itself depends on the applied voltage.

The oxides of the transition metals have a high number of oxidation states.

They can be prepared with a large surface area and some oxides are electronically conductive.

The fourth type of supercapacitor is represented by supercapacitors based on electronically conductive polymers.

Intrinsic electronic conductive polymers can receive electrons by electrochemical reduction (negative doping, n-doping) or electrons by oxidation (positive doping, p-doping).

The doping / dedoping phenomenon is electrochemically reversible, the intrinsic electronic conductive polymers can store charges and restore them in supercapacitor and battery applications.

The electronically conductive polymers have high electrochemical capacities because the doping / dedoping process involves all the mass and the volume of the polymer.

In the charged state, they possess large electronic conductivities. The doping / dedoping process is fast and the resistances obtained are weak. They have high power and energy densities, respectively of 4000 W.kg ^-1 and 10 Wh.kg-1 for poly (3-methylthiophene).

Very recently, in the field of renewable energies, silicon nanowires have been studied for use in supercapacitors. Silicon nanowires were first observed in 1964.

They made it possible to demonstrate that it was possible to achieve growth of silicon wires of less than 1 μπι in diameter,

They have been developed to catalyze the decomposition of a gaseous precursor, silicon for metallic impurities and locating the growth of wires at the location of metal seeds.

During the years 1964-1965, the bases of the catalyzed growth mechanism "whiskers", named "vapor-liquid-solid" (VLS) were laid.

About thirty years later, the growth of silicon strands from silane (SiH ₄ ) with diameters of the order of 10 nm has been demonstrated.

The term "nanowires" was then used to describe this new type of nanostructures.

The growth of VLS nanowires has received strong interest in the potential of this new type of material in nanoelectronics, NEMS, sensors, photovoltaic energy, biosensor.

From this VLS method, it was possible to realize more complex 3D structures: Cones and Trees.

These "branched" nanostructures consist of a trunk of silicon material (the nanowire itself) from which the substructures are elaborated (the branches).

Today, silicon nano-trees represent a unique opportunity to radically improve the performance of micro-super-capacitors thanks to their specific architectures and their extremely large surface area.

Thus, it is known to use silicon nanowires to increase the effective surfaces of the electrodes of supercapacitors, but also of all the electrochemical devices, and in particular of storage and restitution of electrical energy.

These electrodes have a large active surface but on the other hand the supercapacity is not stable over time: it degrades very quickly, especially at the beginning of life. Thus, the object of the invention is to obtain electrodes with a very large active surface whose supercapacity is stable over time.

For this purpose, the invention proposes to create a controlled interface between the silicon or any other conductive or semiconductor material constituting the electrode, and the electrolyte.

For this purpose, the invention proposes an electrode comprising a support, made of a conductive or semi-conductive material, this support comprising, on at least one of its surfaces, at least one nanostructure made of a semiconductor material, characterized in that it further comprises a protective layer of a material selected from a dielectric material having an intrinsic capacitance of between 1.5 10 ^-6 and 10 10 ^-6 F / cm ² and a metal, said layer having a thickness less than the height of said nanostructure, and covering the at least one nanostructure.

The intrinsic capacity of a material is understood to mean the capacity of this material alone (for example in the absence of an electrolyte for an electrode).

Preferably, the support is made of a conductive or semiconductive material selected from stainless steel, carbon, silicon, germanium, gallium arsenide (GaAs), alloys in all proportions of silicon and germanium (SiGe) , and indium phosphide (InP).

More preferably, the support is silicon.

Preferably, the nanostructure is a semiconductor material selected from silicon, germanium, gallium arsenide, alloys in all proportions of silicon and germanium and indium phosphide.

More preferably, the nanostructure is silicon.

When the protective layer is made of a dielectric material, this material is preferably chosen from SiO ₂ , a hafhiurn silicate, a zirconium silicate, hafnium dioxide, zirconium dioxide, silicon nitride, ruthenium (RuO ₂ ), manganese dioxide (MnO ₂ ), vanadium oxide (V ₂ O ₅ ), iron oxide (Fe ₃ O ₄ ).

Preferably, particularly when the nanostructure is silicon, the dielectric material is Si0 ₂ . When the protective layer is metal, this metal is preferably selected from gold (Au), platinum (Pt), silver (Ag), nickel (Ni), and titanium (Ti) and thickness of this layer is between 2 and 20 nm inclusive, preferably between 3 and 20 nm inclusive, more preferably between 3 and 8 nm

In a preferred embodiment, the electrode according to the invention further comprises a layer of an intrinsically conductive polymer material, on the protective layer of a dielectric material or metal, this layer of an intrinsically conductive polymer material and the protective layer. a dielectric material or metal having a total thickness less than the height of the nanostructure, preferably between 2 and 20 nm, more preferably between, even more preferably between 3 and 8 nm.

Preferably, the intrinsically conductive polymer material is chosen from poly (3,4-diethylenedioxythiophenes), poly (2,7 carbazoles), poly (3,6 carbazoles), poly (anilines) and poly (pyrroles). .

More preferably, the intrinsically conductive polymer material is poly (3,4-ethylene dioxythiophene).

The invention also proposes a device for storing and restoring electrical energy, characterized in that it comprises at least one electrode according to the invention.

In a first preferred embodiment, the device according to the invention comprises at least two identical electrodes according to the invention.

In a second preferred embodiment, the device according to the invention comprises at least two electrodes, one of which is an electrode according to the invention and the other is made of carbon, preferably with a large specific surface area (> 1500 m ² / boy Wut).

The invention also proposes a method for manufacturing an electrode comprising a support made of a conductive or semiconductor material, this support comprising, on at least one of its surfaces, at least one nanostructure made of a semiconductor material, characterized in it comprises a step of forming a protective layer of a material selected from a dielectric material having an intrinsic capacitance of between 1.5 10- and 10 10 -F / cm and a metal, on said nanostracture, this layer having a thickness less than the height of said nanostructure.

Preferably, the nanostructure is obtained by chemical vapor deposition (CVD) of the semiconductor material, on said surface of the support.

Also preferably, the protective layer is deposited by chemical or thermal oxidation of the material constituting the nanostructure.

This protective layer can also be obtained by nitriding the material constituting said nanostructure.

In a preferred embodiment, the method of the invention further comprises a step of depositing a layer of a conductive polymer on the protective layer of a dielectric material or metal, this layer made of a polymer material intrinsic conductor and the layer of a dielectric material or metal having a total thickness less than the height of the nanostructure.

The invention will be better understood and other characteristics and advantages thereof will appear more clearly on reading the explanatory description which follows and which is made with reference to the figures in which:

FIG. 1 shows the evolution of the voltage as a function of time in gai vano static charge / discharge cycles of a device composed of two electrodes according to the invention comprising nanostructures made of oxidized silicon. The working electrode is in oxidized N + silicon and the counter-electrode in oxidized P + + silicon, and the electrolyte is NEt ₄ BF ₄ , PC, 1M.

FIG. 2 shows the evolution of the surface capacitance of a device with two oxidized silicon nanostructure electrodes (curve denoted Np-100) and the electrolyte is of NEt ₄ BF ₄ , PC, 1M, as a function of the number of cycles. galvanostatic charge / discharge, the cycling being performed at plus or minus 5 μΑ.οπι ² between 0.01V and IV,

FIG. 3 shows, on the one hand, the cyclic voltammetric curve at 2 mV.s -1 of a nanostructured P ++ doped silicon sample (curve noted prior art) and, on the other hand, that of a sample. of oxidized nanostructured P ++ doped silicon (curve noted invention) obtained in a free bath using an electrolyte which is 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMI-TFSI) (ionic liquid),

FIG. 4 represents the evolution over time of cyclic voltammetry curves at 20 mV.s.sup.- ¹ of an electrode according to the invention comprising nanostructures made of oxidized N + + silicon, and the electrolyte is 1-ethyl-3- methylimidazolium bis (trifluoromethylsulfonyl) imide (EMI-TFSI) (ionic liquid),

FIG. 5 shows the cyclic voltammetric curve between -1 V and -0.3 V vs Ag / Ag ⁺ at 20 mV.s ^-1 obtained with a device comprising a nano-structured P ++ doped silicon-coated working electrode covered with a protective layer of gold, in a free bath, using, as the electrolyte, NE4BF4, PC, 1M, as reference electrode an Ag ⁺ / Ag electrode and a counter-electrode in Pt, and

FIG. 6 shows the static galvano charge / discharge curve obtained with the same device as in FIG. 5.

The aim of the invention is to provide electrodes, in particular for energy storage and retrieval devices, and more particularly for supercapacitive supercapacitors and / or supercapacitors, whose electrodes comprise a three-dimensional silicon structure, the nanostructures having a high surface density.

Indeed, the performances of an electrochemical capacity are directly proportional to its developed surface and it is not possible with the current electrodes to improve them beyond a certain limit.

The invention therefore proposes to use nanostructures, for example made of silicon, whose morphology and nano structuring makes it possible to increase the exchange surface at the electrode / electrolyte interface very strongly.

To obtain a device with a capacitive behavior and to increase its stability over time, the invention proposes to deposit, on the nanostructures, a layer of a dielectric material and / or electroactive that will have a function of protecting the nano structure.

Such a nanostructured electrode, when it is silicon, can be directly integrated into a microelectronic circuit. Thus, the invention proposes an electrode comprising a support made of a conductive or semi-conducting material comprising on at least one of its surfaces at least one nanostructure made of a semiconductor material.

The conducting or semiconductor material whose support is constituted is preferably chosen from stainless steel, carbon, silicon, germanium, gallium arsenide (AsGa), silicon and germanium alloys (SiGe ), and indium phosphide (InP).

Preferably, the support is made of silicon.

The nanostructures made of a semiconductor material will preferably be made of a semiconductor material chosen from silicon, germanium, gallium arsenide, an alloy of silicon and germanium and indium phosphide.

Preferably, the nanostructures are made of silicon.

These nanostructures can be obtained by catalyzed growth in chemical vapor deposition.

They can also be obtained by etching.

When the support of the electrodes is in silicon, the nanostructures will be obtained as well by epitaxial growth or vapor deposition (CVD) as by etching the structures directly in the silicon.

When the support of the electrodes is metal, the growth will be only by crystal growth or CVD.

The invention is based on the surprising discovery that when a protective layer of a material is dielectric having an intrinsic capacitance of between 1.5 10 ^-6 and 10 10 ^-6 F / cm ² is a metal and a thickness of some nanometers, is formed on the nanostructures, the electrode thus obtained has, in the presence of an electrolyte, surprising capacitive responses for applications, in particular supercapacitors, in the presence of organic electrolytes, ionic liquids or ionogels.

This protective layer should not have a thickness greater than the height of the nanostructures: it must not completely fill the interstructure space. In other words, it should not make the surface of the electrode flat because otherwise the interest of structuring is lost.

Advantageously, the layer is conformal, that is to say that it will follow the shapes of the nanostructure. This layer is also uniform over the entire surface of the nanostructures. In other words, it will have to completely cover the nanostructures, and have in all points of these nanostructures a sufficient thickness to protect them effectively.

This thickness depends on the density of the nanostructures.

In general, the thickness of the protective layer coating the nanostructures should not exceed 20 nm and should not be less than 2 nm. Preferably, the thickness of the protective layer will be greater than or equal to 3 nm.

This protective layer must also be sufficiently covering to protect the surface of the nanostructures, and in the case of a layer of a dielectric material, to maintain a high capacity of the electrode in the presence of an electrolyte.

Preferred dielectric materials for forming this protective layer are the oxides and nitrides of transition elements such as SiO ₂ , silicon nitride, high-k materials such as hafnium silicate, zirconium silicate, hafnium dioxide and zirconium dioxide.

In this case, the protective layer gives the nanostructure a passive protection.

But the dielectric material may also be a ruthenium oxide (RuO ₂ ), a manganese dioxide (MnO ₂ ), a vanadium oxide (V ₂ O 5), an iron oxide (Fe ₃ O ₄ ).

In this case, the protective layer actively protects the nanostructures: in addition to its role of protecting the nanostructures, the layer has a charge accumulation action that is added to the electrolyte charges.

When the dielectric material is SiO ₂ or silicon nitride or a high-k material, the desired m ² capacity of the electrode is as high as possible with a continuous protective layer. We are talking here about the intrinsic capacity of the material, taken alone. In particular, in the case of Si0 ₂ , a protective layer with a thickness of less than 2 nm is not sufficiently protective and the capacity per m ² of the layer formed is only 1.73 x 10 ^-6 F / cm. ² . For this reason, preferably, the protective layer, in the case of Si0 ₂ , should preferably have a thickness greater than or equal to 3 nm.

Therefore, in the case of Si0 ₂ , the protective layer should have a thickness greater than 2 nm and less than 8 nm, more preferably between 3 and 8 nm.

A layer with a thickness greater than or equal to 8 nm is too large with a capacity per cm ² - 9.6 x 10 ^-6 F / cm ² .

Thus, in the general case, the capacity must be between 1.73 x 10 ^-6 and 9.6 x 10 ^-6 F / cm ² or more generally 1, 5 10 ^'6-10 ^{10 -6} F / cm ² .

It is therefore deduced that in the case of silicon nitride, with a ε = 10, it will be necessary for the thickness of the layer to be between 5 and 20 nm, and in the case of "high-k" materials with ε- 1.6, it will be necessary that the thickness of the formed layer is between 3.2 nm and 12.8 nm.

Therefore, the protective layer should have, with these materials, a thickness between 2 and 20 nm, preferably between 5 and 20 nm.

This protective layer may be formed by chemical or thermal oxidation.

For example, in the case of the formation of a protective layer of SiO ₂ , on a silicon support, the chemical route consists in quenching the support-nanostructure assembly in an oxidizing acid solution such as sulfuric acid or nitric acid.

The oxidation reaction will be self-limited by the formation of the SiO ₂ layer of nanometric thickness.

Still in the case of the formation of a protective layer of SiO ₂ on the silicon nanostructures, the thermal pathway consists in carrying out thermal annealing in an oxidizing atmosphere.

This technique is totally mastered in the microelectronics industry. It makes it possible to obtain SiO ₂ layers with a thickness of the order of one nanometer.

Here, the formed layer conforms to the shape of the nanostructures (it conforms) and completely covers the surface of the nanostructures with a sufficient thickness. To obtain a layer of SiO ₂ having these properties, it must be formed by one of the means described above. Indeed, a simple oxidation by exposure to oxygen in the air, as in the case of the formation of native silicon oxide on silicon nanostructures, does not make it possible to obtain a continuous protective layer and a thickness in all points sufficient to play the role of protection of the protective layer according to the invention.

Finally, the formation of the protective layer can be carried out by chemical vapor deposition (CVD) of a dielectric material different from SiO ₂ , for example RuO ₂ or MnO ₂ .

Of course, the deposit must be a conformal deposit, that is to say that the thickness of the layer formed must follow very precisely the contours of the nanostructures, to maintain the interest of the nano structuration.

However, the capacitive performance of this protective layer can be further improved by covering this protective layer by means of an electroactive layer made of an intrinsically conductive polymer material.

Indeed, conductive polymers are conductive materials that improve the current densities and therefore the power and energy of the capacitors and add to the protective layer volume type capacity.

Suitable conductive polymers will be apparent to those skilled in the art but they are preferably selected from poly (3,4-ethylene dioxythiophenes), poly (2,7 carbazoles), poly (3,6 carbazoles), poly (anilines) and poly (pyrroles).

The deposition of this layer of intrinsic conductive polymers on the protective layer formed can be carried out electrochemically (for example by electropolymerization) or by sublimation or by evaporation or by conventional deposition (spin coating, resin deposit ...). The thickness of this protective layer can be controlled by measuring the synthesis charge, for example by coulometry.

This thickness will be adjusted to preserve the nanostructuration of the son constituting the nanostructures.

Thus, the total thickness of the protective layer and the conductive polymer layer must not make nanostructure disappear due to the nanostructures, and the total thickness of these two layers must not be equal to or exceed the height of the nanostructures.

The electrode of the invention can be used in a device for storing and restoring electrical energy.

Such a storage device generally comprises at least two electrodes.

Thus, only one of these electrodes can be an electrode according to the invention.

But at least two of these electrodes may both be electrodes according to the invention, advantageously identical.

When only one of these electrodes is an electrode according to the invention, the other carbon electrode of large specific surface area.

In a preferred embodiment, when the electrode is an electrode according to the invention, it is an electrode whose support is silicon, the nanostructure then being grown on this support and being itself in silicon.

The protective layer covering the nanostructure is then preferably a silica layer (Si0 ₂ ) obtained by oxidation of the silicon surface.

In an even more preferred embodiment, this electrode, whose support and nanostructures are made of silicon and are coated with a layer of silicon oxide, also comprises a layer of conductive polymer, which is poly (3,4 ethylene dioxythiophene).

The electrodes according to the invention are manufactured by a process which is also an object of the invention.

This method comprises a step of forming, on at least one nanostructure, a semiconductor material formed on a support made of a material semiconductor or conductor, a protective layer of a material selected from a dielectric material having an intrinsic capacitance of between 1.5 10 ^-6 and 10 10 -F / cm and a metal, which layer has a thickness less than height of the nanostructure.

The height of the nanostructure corresponds to the distance between the surface of the support and the surface of the nanostructure, when taken in the same plane.

The support of the electrodes is preferably made of a conductive or semiconductor material selected from stainless steel, carbon, silicon, germanium, gallium arsenide, alloys of silicon and germanium and indium phosphide .

More preferably, the support is silicon.

As for the nanostructure, it is preferably a semiconductor material selected from silicon, germanium, gallium arsenide, alloys of silicon and germanium, and indium phosphide.

More preferably, it is silicon.

By nanostructure, we mean both a wire, a nanowire, a nano-tree, etc.

Such a nanostructure must have a form factor greater than 5, advantageously greater than 20, and more preferably greater than 100. This form factor applies to wires, branches and tree trunks.

When the protective layer covering the structure is made of a dielectric material, this dielectric material is preferably chosen from silica, hafnium silicate, zirconium silicate, hafnium dioxide, zirconium dioxide and silicon nitride. , ruthenium dioxide (RuO ₂ ), manganese dioxide (MnO ₂ ), vanadium oxide (V ₂ O 5), iron oxide (Fe ₃ O ₄ ).

Hafnium silicate, zirconium silicate, hafnium dioxide and zirconium dioxide are so-called "high-k" materials.

Preferably, this layer is a silica layer, in particular when the nanostructure and / or the support are made of silicon.

Indeed, in this case, the layer of dielectric material can be formed by simple oxidation of the surface of the nanostructure and / or the support. But, the so-called protective layer and nanostructuration of the nanostructure may also be metal.

In this case, preferably, the metal is selected from gold (Au), platinum (Pt), silver (Ag), nickel (Ni) and titanium (Ti) and the thickness of this layer must be between 2 and 20 nm inclusive, preferably between 3 and 20 nm, more preferably between 3 and 8 nm inclusive, to form a sufficiently protective layer while maintaining a good capacity.

In a preferred embodiment, the electrode manufacturing method of the invention further comprises a step of depositing a layer of an intrinsically conductive polymer material on the protective layer of a dielectric material or metal.

Preferably, this layer of an intrinsically conductive polymer material is a layer of a material chosen from poly (3,4-ethylene dioxythiophenes), poly (2,7 carbazoles), poly (3,6 carbazoles), poly ( anilines) and poly (pyrroles).

More preferably, this layer is poly (3,4-ethylene dioxythiophene).

When two layers are formed on the nanostructures, these two layers together must not planarize the surface of the electrode.

For this reason, the thickness of these two layers must be less than the height of the nanostructure. It must also be, regardless of the fact that only one or two layers are deposited on the nanostructures, layer (s) deposited (s) conformably on the nanostructure, that is to say, marrying the forms of nanostructure.

In order to better understand the invention, will now be described by way of illustration and not limitation, examples of implementation.

Example 1

On a silicon support, a catalyst metal, here gold, is deposited. This metal is the seat of the growth of 3D silicon nanostructures.

The shaping of the catalyst is obtained by dewetting a thin film of the metal by heating, which leads to the formation of nanodroplets. However, the shaping of the catalyst can also be carried out by depositing colloidal metal catalyst.

The second step is then the formation of the nanostructures from the deposited metal.

In this example, the nanostructures are made of silicon and are obtained by chemical vapor deposition using silane.

But, one can also use disilane.

To dope these nanostructures during their growth, compounds containing boron, phosphorus or arsenic, depending on the desired doping, are added to the gaseous stream of silane.

Branched structures (nano-trees) are obtained using the catalysts on the main trunk.

These catalysts can come from:

i) the migration of the main catalyst during the growth of the trunk (in this case, the growth is carried out in one or two stages), or

ii) a new catalyst deposit after trunk growth. The silicon nanostructures will form one or two electrodes depending on the final component. Thus, if the final component must have a low price, only one electrode will be formed, but to have high performance two electrodes according to the invention will be used.

The third step is the step of creating the protective layer of the nanostructures.

In this example, the nanostructures being silicon, chemical oxidation is the simplest solution to implement to obtain a silica layer on the nanostructures.

However, when we want to obtain a protective layer in another metal oxide, it will be deposited in dynamic electrochemical mode, as cyclic voltammetry, chronopotentiometry on the surface of silicon nanostructures.

It is also possible to deposit this protective layer by sublimation or evaporation. In this example, the height of the nanostructures is 10 μπι, which corresponds to their length, and the protective layer has a thickness of 3 nm.

Characterization of the electrode of Example 1:

A device with two identical electrodes according to Example 1, with a surface area of 1 cm ² , was manufactured on a silicon support, one of which was based on a network of P + doped silicon nanowires, ie -This strongly doped P (high electron deficiency, thus excess of holes, considered positively charged, in this case by a doping of 4 10 ¹⁹ boron atoms / cm ³ ), and the other, a network of nanowires in N + doped silicon, that is to say strongly doped N (excess of electrons, considered negatively charged, in this case by a doping of 4 10 ¹⁹ phosphorus atoms / cm ³ ), each having a density of 5 10 ⁸ nanowires / cm ² .

The nanowires have a diameter of 100 nm and are coated with a protective layer of Si0 ₂ with a thickness of 3 nm.

The device further comprises:

an organic electrolyte which is a solution of tetraethylammonium tetrafluoroborate, in propylene carbonate at a concentration of 1 mol.L ^-1 (NEt ₄ BF ₄ , PC, 1M),

- a Celgard ^® separator,

- aluminum collectors,

the whole being held between two insulating Teflon materials.

This device has been tested in galvanostatic charge / discharge cycle.

The cycling is carried out at plus or minus 5 μ A / cm between 0.01V and IV.

FIG. 1 represents the variation of the potential in volts as a function of the cycling time of the device of example 1. As can be seen in FIG. 1, the shape of the cycle is almost ideal and characteristic of that of a supercapacitor.

The evolution of the surface capacitance of the device has also been determined.

The evolutions of this capacity in μΡ / cm as a function of the number of cycles, are represented in FIG.

As can be seen in FIG. 2, the surface capacity is stable during cycling. Example 2

In another embodiment, the electrode of Example 1 was coated in addition to a layer of an intrinsically conductive polymer material which is here poly (3,4-ethylene dioxythiophene).

Similarly, a layer of poly (2,7 carbazoles), poly (3,6 carbazoles) or poly (anilines) could have been deposited.

This layer is deposited in this example by voltammetry.

It can also be deposited by chronopotentiometry, or vaporization and evaporation when the conductive polymer is soluble in a solvent.

We then gained about a factor 100 on the value of the capacity without degrading the stability.

Example 3

With the same electrode of Example 1, a so-called free-bath device (that is to say without separator and in a large volume of electrolyte, here of the order of 5 ml) with an electrolyte was manufactured. which was 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMI-TFSI) which is an ionic liquid.

The electrode according to the invention has a surface of 0.2 cm ² , the counter electrode was platinum and the reference electrode Ag / Ag ⁺ .

Comparative example.

The same device as in Example 3 was manufactured, but the silicon nanowire array was not oxidized, as in the prior art.

Results.

The devices of Example 3 and Comparative Example were tested in cyclic voltammetry at 2mV.s- ¹ using an Ag ⁺ / Ag reference electrode.

The cyclic voltametry curves obtained are shown in FIG.

In FIG. 3, the curve denoted "invention" is the curve obtained with the device of example 3 and the curve denoted "prior art" is the cyclic voltametry curve obtained with the device of the comparative example. As seen in FIG. 3, the cyclic voltammetric curve of the device of example 3 is similar to that of a supercapacitor (horizontal curve) while the cyclic voltammetric curve of the device of the comparative example is not. not.

For the device of Example 3, the evolution over time of the cyclic voltametry curves at 20m Vs ^-1 was also examined.

The curves obtained are shown in FIG. 4 in which the curve noted (1) represents the cyclic voltametry curve at the initial moment and the curve noted (2) represents the cyclic voltametry curve obtained after 2 hours of cycling.

The stability of the electrodes of this device is maintained as shown by the curves which are horizontal and very close to each other; there is no loss of capacity.

Example 4

An electrode according to the invention was manufactured with a protective layer of gold.

For this purpose, silicon nanowires were grown on an N + doped silicon substrate by CVD (chemical vapor deposition), using silane.

The catalyst used was gold colloids 50 nm in diameter. Silicon nanowires of about 50 nm in diameter and about 5 μηι in length, N + doped, were obtained at a density of about 5 × 10 ⁸ nanowires / cm ² .

Silicon nanowires, forming the nanostructures, were coated with a protective layer of gold with a thickness of 8 nm, by evaporation of gold.

The electrode obtained was used in an electrochemical test setup comprising 3 electrodes, a working electrode, which is the one obtained above, a reference electrode Ag ⁺ / Ag and a counter-electrode in platinum (Pt). .

The electrode used was a solution of NEUBF ₄ , PC, 1M, as in Example 1.

This device was tested in cyclic voltammetry between -1 V and -0.3 V vs Ag ⁺ / Ag at 20 mV.s ^-1 . The curve obtained is shown in FIG.

As can be seen in FIG. 5, the curve obtained is very close to that of a supercapacitor.

This device has also been tested in charge cycles and static galvano discharge.

Cycling is performed at ± 8 μΑ.ατη ^-2 between -0.97 V and -0.4 V vs

Ag ⁺ / Ag.

The curve obtained is shown in FIG.

As can be seen in FIG. 6, this curve is close to that obtained with a supercapacitor.

The surface capacitance of this device was calculated from these two curves: the average surface capacitance is 45 μΈ.αχι, which is more important than the average surface capacitance of the same device but in which the protective layer is a layer of silicon oxide, this capacity being 26 μΡχηι ^-2 .

The device of this example can therefore store more energy than the device in which the protective layer is a silicon oxide layer because the energy stored by a device is proportional to its average surface capacity.

A device identical to that of Example 4 but having no protective layer does not have a capacitive behavior.

Claims

An electrode comprising a support made of a conductive or semiconductor material, this support comprising, on at least one of its surfaces, at least one nanostructure made of a semiconductor material, characterized in that it further comprises a layer protective protector made of a material selected from a dielectric material having an intrinsic capacitance of between 1.5 ^× 10 ^-6 and 10 10 -F / cm and a metal, said layer having a thickness less than the height of said nanostructure, and covering the less a nanostructure.

2. Electrode according to claim 1, characterized in that the support is made of a conductive or semiconductor material selected from stainless steel, carbon, silicon, germanium, gallium rarsenide (AsGa), alloys in all proportions of silicon and germanium (SiGe), and indium phosphide (InP), preferably the support is silicon.

3. Electrode according to claim 1 or 2, characterized in that the nanostructure is a semiconductor material selected from silicon, germanium, gallium arsenide, alloys in all proportions of silicon and germanium and phosphide indium, preferably the semiconductor material is silicon.

4. Electrode according to any one of the preceding claims, characterized in that the protective layer is a dielectric material selected from Si0 ₂ , a hafnium silicate, a zirconium silicate, hafnium dioxide, zirconium dioxide , silicon nitride, ruthenium dioxide (Ru0 ₂ ), manganese dioxide (Μη (な), vanadium oxide (V2O5), iron oxide (Fe ₃ C ⁾ 4), preferably dielectric material is Si0 ₂ .

5. Electrode according to any one of claims 1 to 3, characterized in that said protective layer is a metal selected from gold (Au), platinum (Pt), silver (Ag), nickel ( Ni) and titanium (Ti) and the thickness of this layer is between 2 and 20 nm inclusive, preferably between 3 and 20 nm inclusive, more preferably between 3 and 8 nm inclusive.

6. Electrode according to any one of claims 1 to 5, characterized in that it further comprises a layer of a polymer material intrinsically conductive, on the protective layer of a dielectric material or metal, this layer of an intrinsically conductive polymer material and the protective layer of a dielectric material or metal having a total thickness less than the height of the nano structure, preferably included between 2 and 20 nm, more preferably between 3 and 20 nm, still more preferably between 3 and 8 nm.

7. Electrode according to claim 6, characterized in that the intrinsic conductive polymer material is selected from poly (3,4 ethylene dioxythiophenes), poly (2,7 carbazoles), poly (3,6 carbazoles), poly (anilines) and poly (pyrroles).

8. Device for storing and restoring electrical energy, characterized in that it comprises at least one electrode according to any one of claims 1 to 7.

9. Device according to claim 8, characterized in that it comprises at least two identical electrodes according to any one of claims 1 to 7.

10. Device according to claim 8, characterized in that it comprises at least two electrodes, one of which is an electrode according to any one of claims 1 to 7 and the other is carbon, preferably with a specific surface area greater than 1500 m ² / g.

11. A method of manufacturing an electrode comprising a support made of a conductive or semiconductor material, this support comprising on at least one of its surfaces, at least one nanostructure of a semiconductor material, characterized in that it comprises a step of forming a protective layer of a material selected from a dielectric material having a capacitance between

6 6 2

1.5 10- and 10 10 -F / cm and a metal on said nanostructure, this layer having a thickness less than the height of said nanostructure.

12. The method of claim 11, characterized in that it further comprises a step of manufacturing the nanostructure by chemical vapor deposition (CVD).

13. The method of claim 11 or 12, characterized in that the protective layer is formed by chemical or thermal oxidation of the surface not in contact with the support of the nanostructure.

14. The method of claim 11 or 12, characterized in that the protective layer is formed by nitriding the surface not in contact with the support of the nanostructure.

15. Method according to any one of claims 1 1 to 14, characterized in that it further comprises a step of depositing a layer of a conductive polymer on the protective layer of a dielectric material or metal, this layer of an intrinsically conductive polymer material and the layer of a dielectric material or metal having a total thickness less than the height of the nanostructure.