EP4153345A1

EP4153345A1 - Ion-selective composite membrane

Info

Publication number: EP4153345A1
Application number: EP21734394.6A
Authority: EP
Inventors: Bruno Mottet; Benoit LABORIE; Mohammed KECHADI
Original assignee: Sweetch Energy
Current assignee: Sweetch Energy
Priority date: 2020-05-20
Filing date: 2021-05-19
Publication date: 2023-03-29
Also published as: JP2023526099A; BR112022023364A2; US20230226499A1; CA3184153A1; FR3110460B1; CN115697538A; WO2021234294A1; FR3110460A1

Abstract

The present invention relates to an ion-selective composite membrane having a thickness of between 4 µm and 100 µm, comprising at least one inner layer (2) disposed between two outer layers (1, 3), wherein: - the outer layers (1, 3) are each formed of a first material comprising a network of nanofibres and/or crosslinked microfibres and pores with a diameter of between 10 nm and 10 µm, - the inner layer (2) is formed of a second material comprising nanoparticles functionalised at the surface by charged groups and/or groups which become charged in the presence of water and having pores with a diameter of between 1 and 100 nm.

Description

ION SELECTIVE CONDUCTION COMPOSITE MEMBRANE

STATE OF THE ART

Ion selective conduction membranes play an essential role in many industrial processes.

A large number of these processes is in fact based on a selective conduction of ions according to the sign of their charge between two volumes separated by a membrane, under the effect of a stress on either side of this interface, for example. a pressure gradient, an electric potential gradient or a concentration gradient.

The most commonly used membranes with selective conduction of ions according to the sign of their charge are known as ion exchange membranes (MEIs). A distinction is made between cation exchange membranes (MECs), which allow the circulation of cations, and anion exchange membranes (MEAs) in which anions can circulate. These MEIs are prepared from grains of ion exchange resins dispersed in an inert polymeric binder (homogeneous MEIs) or else by introducing functional groups directly into the structure of a polymer constituting the membrane (heterogeneous MEIs).

MEIs are for example used in the fields of water treatment for the extraction of undesirable substances from a fluid to be treated, for example to desalinate brackish or sea water. In desalination processes, the extraction of Na ions ⁺ and Cl occurs by migration of ions through an alternation of membranes allowing the anions (MEAs) or cations (MECs) to pass selectively under the action of an electric field. At the end of the treatment, fresh water is recovered on the one hand and brine on the other.

Membranes with selective conduction of ions according to the sign of their charge are also used in processes for storing electrical energy in the form of electrolytic hydrogen or conversely using this hydrogen as a source of electrical energy (hydrogen fuel cells) . These processes involve an electrochemical reaction, the electrolysis of water. The electrolysis of water is carried out in an electrolyser, a device which comprises a set of electrolysis cells contiguous and connected to a source of electrical energy via electrodes. Each electrolysis cell is typically formed by bringing two metal plates called electrodes into contact with a solid or liquid electrolytic medium. In the case of a liquid electrolytic medium, the electrolytic cell comprises electrodes immersed in an aqueous solution containing both the water necessary for the reaction and electrolytes, soluble chemical compounds and conductors of current such as potash KOH ( alkaline electrolysis) or sulfuric acid H ₂ SO ₄ (acid electrolysis). The two electrodes are connected to an electricity generator which makes it possible to increase their difference in electric potential. When the latter passes a certain threshold, we observe the passage of a current in the circuit, and molecular oxygen (O2) is formed on the anode (electrode connected to the positive pole of the generator) and molecular hydrogen (H2) forms on the cathode (electrode connected to the negative pole of the generator). For example, in the case of acid hydrolysis, at the anode, the water molecules decompose according to the equation H2O -> 2 H ⁺ + 2e + ½ O2 and at the cathode, the protons are reduced according to the equation H ⁺ + le -> ½ H2, a flow of hydronium ions between the anode and the cathode. To avoid the spontaneous recombination of H2 and O2 into explosive gases, it is necessary to place between the electrodes a separating membrane allowing the protons to pass but not H2 and O2. More recently, proton exchange membrane electrolysis (PEM) uses cells in which the electrolytic medium is a solid polymer electrolyte in the form of a cation exchange membrane. In these cells, porous metal electrodes (Ep) are directly in contact with an ECM (M), the Ep-M-Ep assembly being on either side brought into contact with an aqueous solution. In these cells, the membrane material acts both as a separating membrane and as a solid electrolyte.

However, in general, MEIs conduct ionic currents weakly and constitute an important ohmic contribution to electrodialysis and reverse electrodialysis systems. In most cases, this limits the current density that can be applied to the electrodes to a few hundred mA.cm- ² , and therefore the operating range of technologies using MEIs. In addition, the preparation of these membranes turns out to be very expensive, which is why the major part of the maintenance investments of the membrane processes is devoted to the replacement of these membranes.

MEIs can also be used for the production of electricity from an electrolyte gradient, in particular from a salinity gradient.

Thus, the reverse electrodialysis pathway (RED) is based on the use of membranes whose basic property is the selective transport of ions according to the sign of their charge. A RED device typically consists of an alternation of MEAs and MECs separated by spacer membranes to form passages allowing fluids to flow. The circulation of an alternation of salt water and fresh water in these cells makes it possible to establish an ionic flow at each of the MEI of the device. At the ends of this stack of membranes, electrodes collect the electric current generated by the overall ionic flow.

One of the problems encountered by devices for generating electricity from a salinity gradient, such as current RED devices, is that these present a very low electricity production capacity, due to the fact that current MEIs develop electrical powers per unit of membrane area (ie membrane powers) of only a few W / m ² of membrane.

An approach to this problem is set out in the international application published on April 24, 2014 under the number WO 2014/060690. In this approach, nanoporous membranes have been proposed, the internal surface of the pores of which is covered with boron nitride or more generally with mixtures of the elements boron, carbon and nitrogen. These nanoporous membranes exploit diffusion-osmosis phenomena within the pores and develop membrane powers of the order of kW / m ² . More recently, it has also been proposed, in the international application published on March 9, 2017 under number WO 2017/037213, nanoporous membranes whose internal pore surface is covered with titanium oxide, making it possible to achieve membrane powers of order 5 kW / m ² . However, this approach involves the use of membranes based on boron nitride or titanium oxide, the preparation of which on a larger scale than that of the laboratory is complex and extremely expensive given the materials required. Furthermore, the materials used in these membranes are harmful to the environment, and present a risk if they are released into the environment.

To date, there is no membrane with selective conduction of ions according to the sign of their charge, developing high membrane powers under the effect of a salinity gradient, which is simple and economical to prepare, while at the same time presenting a limited risk to the environment.

DISCLOSURE OF THE INVENTION

Thus, an aim of the invention is to provide a membrane with selective conduction of ions according to the sign of their charge which is economical and easy to produce, while being capable of developing a high membrane power when it is integrated into cells. devices for producing electricity from an electrolyte gradient, in particular a salinity gradient, or in reverse devices for purification or desalination of water.

Another object of the invention is to provide a membrane with selective conduction of ions according to the sign of their charge, prepared from materials which present little or no risk to the environment.

These objects are achieved by the invention described below. Composite membrane

The first object of the invention is a composite membrane with selective conduction of ions having a thickness of between 4 μm and 100 μm comprising at least one internal layer (2), arranged between two external layers (1), (3) in which :

- the outer layers (1, 3) are each formed from a first material comprising a network of nanofibers and / or crosslinked microfibers and pores with a diameter of between 10 nm and 10 μm,

- The internal layer (2) is formed from a second material comprising nanoparticles functionalized at the surface by charged groups and / or which become charged in the presence of water and having pores with a diameter of between 1 and 100 nm.

The inventors have discovered that, unexpectedly, the composite membrane of the invention develops a very high membrane power, of the order of several hundred W / m ² of membrane, preferably at least 300 W / m ^2. , more preferably at least 500 W / m ² , under the effect of a salinity gradient.

Without wishing to be bound by a particular theory, the inventors believe that this very high membrane power is determined by the surface charge of the materials used in the layers of the membrane of the invention in association with the porosity of the outer layers (1,3 ) and the inner layer (2) and the composite membrane.

In particular, still according to the inventors, this association of porosity and load surface gives the composite membrane nanofluidic properties, and would influence the selective passage of ions through the membrane, according to a specific and unexpected mechanism, which would not be observed in the case where the materials constituting the membrane present greater porosities.

Structure of the composite membrane

The thickness of the composite membrane is advantageously between 4 μm and

75 pm.

The thickness of each of the outer layers (1,3) is advantageously between 2 μm and 45 μm, preferably between 2 μm and 30 μm, more preferably between 2 μm and 25 μm. The outer layers advantageously have the same thickness.

The thickness of the internal layer (2) is for its part preferably between 10 nm and 10 μm, and more advantageously between 10 nm and 2 μm, preferably between 10 nm and 1 μm, preferably between 10 nm and 800 nm, preferably between 10 nm and 400 nm, and more preferably between 200 nm and 500 nm. Preferably, the thickness of each of the outer layers (1,3) is advantageously between 2 μm and 45 μm, and the thickness of the internal layer (2) is between 10 nm and 10 μm.

According to the inventors, the very small thickness of the internal layer makes it possible to obtain excellent permeability while obtaining high selective conduction of ions.

In the invention, the thickness of the composite membrane and of the various layers is measured by scanning electron microscopy of sections of dry membrane.

The composite membrane preferably comprises less than 10% by weight of second material relative to the weight of first material, preferably between 2% and 8% by weight of second material relative to the weight of first material, more preferably between 3% and 5% by weight of second material based on the weight of first material.

The surface charge density of the internal wall of the pores of the composite membrane is advantageously between 0.001 and 3 C / m ² , preferably between 0.1 and 1 C / m ² .

The surface charge density of the composite membrane is measured by dosimetry.

Inner layer (2)

Second material

According to the invention, the term "nanoparticle" denotes a 3-dimensional object, in which at least one external dimension is on the nanometric scale (i.e. at least one dimension is in a range between 1 to 100 nm).

The second material advantageously comprises the nanoparticles in the form of individual nanoparticles, that is to say of nanoparticles which are not aggregated or in other words linked covalently to one another.

The second material advantageously comprises at least 50% by mass of nanoparticles, at least 95% by mass of nanoparticles, more preferably at least 99% of nanoparticles, relative to the mass of second material.

Advantageously, the nanoparticles are not in the form of nanotubes.

The nanoparticles are preferably lamellar nanoparticles.

According to the invention, the term “lamellar nanoparticle” denotes a nanoparticle comprising atoms in the form of monolayers of atoms linked together by covalent bonds. Lamellar nanoparticles can consist of a single monolayer of atoms (2D materials) or of a stack of 2 to 5 monolayers of atoms linked together by weak bonds, such as Van der Waals forces. In other words, a lamellar nanoparticle is a 3-dimensional object in which a first external dimension is located on the nanometric scale and the other two dimensions are significantly greater than the first dimension, and vary in particular between the nanometric scale and the scale. micrometric.

The lamellar nanoparticles preferably have a median size (also designated by the acronym “D50”) of between 5 and 50 μm, preferably between 10 and 20 μm, more preferably of 15 μm.

D50 means that 50% by weight of the particles have a smaller size.

According to the invention, the terms “monolayer”, “bilayer”, “oligo-layers”, relating to the lamellar nanoparticles, respectively denote a lamellar nanoparticle consisting of a monolayer of atoms, of two monolayers of atoms, and of 3. with 5 atom monolayers. Lamellar bilayer and oligolayer nanoparticles are typically stabilized by weak interactions between monolayers of atoms, such as Van der Waals interactions.

The lamellar nanoparticles are preferably lamellar nanoparticles of a metal oxide, in particular of SnO2 or of T1O2, lamellar nanoparticles of a dichalcogenide of a transition metal such as molybdenum disulphide M0S2 _, lamellar carbon nanoparticles, or a mixture of these.

The lamellar carbon nanoparticles are advantageously lamellar nanoparticles of monolayer graphene, bilayer graphene, trace graphene or a mixture thereof.

According to the invention, the term “single-layer graphene” denotes a crystalline two-dimensional material consisting of carbon in a particular allotropic form, which can be represented as a planar honeycomb. More particularly, monolayer graphene is a sheet formed by a single atomic plane of sp ² hybridized carbon. It can therefore be qualified as a monolayer.

According to the invention, the term “bilayer graphene” (or BLG; “Bi-Layer Graphene” in English) denotes a material consisting of a stack of 2 monolayers of graphene stabilized by interactions of van der Waals type between the 2 monolayers. of graphene. BLG can be obtained by exfoliation of graphite or by chemical vapor deposition (CVD).

According to the invention, the term “trace-layer graphene” (or FLG: “Few-Layer Graphene” in English) denotes a material consisting of a stack of 3 to 5 graphene sheets, stabilized by interactions of the van der type. Waals between the different graphene planes.

Single layer graphene lamellar nanoparticles are preferred. According to a preferred embodiment, the second material advantageously comprises at least 50% by mass of single-layer graphene, more preferably at least 95% by mass of single-layer graphene. The single-layered graphene lamellar nanoparticles preferably have a median size (also designated by the acronym “D50”) of between 5 and 50 μm, preferably between 10 and 20 μm, more preferably of 15 μm.

The lamellar molybdenum disulfide nanoparticles are advantageously lamellar nanoparticles of monolayer molybdenum disulfide, bilayer molybdenum disulfide, oligo-layered molybdenum disulfide or a mixture thereof.

Depending on the sign of their charge, the groups which are charged or which become charged in the presence of water confer a negative or positive surface charge on the internal layer (2) of the composite membrane when it is placed in the presence of water.

Any charged group or which becomes charged in the presence of water known to those skilled in the art and making it possible to increase the surface charge of graphene particles can be used within the framework of the present invention.

In one embodiment, the nanoparticles are surface functionalized by negatively charged groups and / or which become negatively charged in the presence of water.

The negatively charged groups and / or which become negatively charged in the presence of water are advantageously chosen from the epoxy group, the hydroxyl group, the carbonyl group, the carboxyl group, the sulfonate group -SO3, the carboxyalkylate group R-CO2 with R a C1-C4 and preferably C1-alkyl, the aminodiacetate group -N (CH ₂ C0 ₂ -) 2, the phosphonate group PO3 ² ; the amidoxine group -C (= NH ₂ ) (NOH), the aminophosphonate group -CH ₂ -NH-CH ₂ -PC> 3 ² , the thiol group -SH, and mixtures thereof.

Preferably, the nanoparticles functionalized at the surface with negatively charged groups or which become negatively charged in the presence of water are lamellar graphene oxide nanoparticles (or GO, in English “graphene oxide”).

The lamellar graphene oxide nanoparticles carry groups which are negatively charged or which become negatively charged in the presence of water, advantageously chosen from the epoxy group, the hydroxyl group, the carbonyl group, the carboxyl group, and mixtures thereof.

In one embodiment, the nanoparticles are surface functionalized by positively charged groups and / or which become positively charged in the presence of water.

Advantageously, the positively charged groups and / or which become positively charged in the presence of water are chosen from the quaternary ammonium group -N (R) 3 ⁺ with R a C1-C4 alkyl, the tertiary ammonium group -N (H) R) 2 ⁺ with R a C1-C4 alkyl, preferably a C1 alkyl, the dimethylhydroxyethylammonium group -N (C2H ₄ OH) CHB) 2 ⁺ , and their mixtures.

Outer layers (1.3)

First material

According to the invention, the expression “nanofiber” designates a 3-dimensional cellulose-based object in which 2 of the 3 external dimensions are located at the nanometric scale (ie 2 of the 3 dimensions range from 1 to 100 nm), the ^3rd external dimension being significantly greater than that of the other two dimensions, and are not necessarily at the nanoscale.

The nanofibers thus have a diameter ranging from 1 to 100 nm, preferably ranging from 1 to 70 nm, and more preferably ranging from 4 to 30 nm, in particular from 4 to 20 nm. In addition, their length is advantageously between 0.5 and 100 μm, in particular between 0.5 and 50 μm, for example between 0.5 and 10 μm, for example also between 0.5 and 2 μm.

According to the invention, the expression “microfiber” designates a 3-dimensional object in which 2 of the 3 external dimensions are located on the micrometric scale (ie 2 of the 3 dimensions go in a range going from 0.1 to 10 μm). , the 3 ^rd external dimension being significantly greater than that of the other two dimensions.

The microfibers thus have a diameter ranging from 0.1 μm to 10 μm, advantageously ranging from 0.1 μm to 5 μm, more advantageously ranging from 0.1 μm to 2 μm, in particular ranging from 0.1 μm to 1 μm, 0.1 pm to 7 pm, or 0.1 pm to 0.2 pm

In addition, their length is advantageously between 0.5 μm and 100 μm, in particular between 1 μm and 50 μm, for example between 1 μm and 10 μm, for example also between 1 μm and 5 μm.

The nanofibers and / or the microfibers advantageously have a form factor advantageously greater than 10, preferably greater than 100.

According to the invention, the expression “form factor”, relating to nanofibers and / or to microfibers, denotes the ratio of their length L to their diameter d (L / d).

The diameter of nanofibers and / or microfibers can be measured by TEM or SEM.

According to the invention, the term “crosslinked”, relating to nanofibers and / or microfibers, means that said fibers are connected to each other by covalent chemical bonds (sometimes called “bridges”) so as to form a three-dimensional network. In other words, they are not simply agglomerated by or self-assembled through weak bonds.

The first material plays a structuring role in the composite membrane, in particular in that it makes it possible to maintain the functionalized nanoparticles described above in the form of a second layer (2) arranged between the outer layers (1,3).

Furthermore, the first material of the outer layers (1, 3) ensures the integrity of the inner layer (2), in particular when, during its use, the latter is subjected to a stress such as a pressure gradient from side to side. on the other side of the membrane.

The nanofibers and / or the microfibers advantageously carry charged groups or which become charged in the presence of water.

In a first embodiment, the charged groups and / or which become charged in the presence of water from the outer layer (1) are of opposite sign to the charged groups and / or which become charged in the presence of water from the outer layer. (3). In this embodiment, the composite membrane is a bipolar composite membrane.

In a second embodiment, the charged groups and / or which become charged in the presence of water of the two outer layers (1, 3) have the same sign, advantageously the same sign as that of the charged groups or which become charged in the presence of water. water of the functionalized nanoparticles described above.

This has the advantage of increasing the surface charge of the whole of the composite membrane of the invention.

According to the inventors, the presence of these charged groups or which become charged in the presence of water of the same sign within the internal layer (2) and the external layers (1,3) of the composite membrane makes it possible to obtain an effect synergistic, i.e. an unexpected improvement in selective ion conduction across the composite membrane.

In this embodiment, the first material therefore plays a role in the structure of the composite membrane and in its capacity to ensure selective conduction of ions.

Furthermore, the covalent chemical bonds involved in the crosslinking of nanofibers and / or microfibers can also carry charged groups and / or which become charged in the presence of water, as is the case, for example, when the crosslinking agent used is citrate. In this case, the chemical crosslinking bonds play a role both in the structure and in the electrical surface charge of the nanoporous material.

In one embodiment, the nanofibers and / or the microfibers consist of an electrically conductive material, such as for example activated carbon as described below. In this embodiment, the outer layers (1,3) can conduct electrons, and therefore play the role of capacitive electrode when the composite membrane is used in a membrane process for electrolysis or reverse electrolysis, preferably. an electrodialysis or reverse electrodialysis process. In other words, the outer layers conduct the electric current necessary for the electrolytic reaction or for the implementation of the electrodialysis, or else collect the current generated by the reaction of electrolysis or reverse electrodialysis.

According to this embodiment, when the composite membrane is used in a reverse electrodialysis process, the fluid can circulate in the porosity of the outer layers (1,3), and the electrical energy produced by reverse electrodialysis is directly harvested. by nanofibers and / or microfibers of the outer layers (1,3).

Thus, composite membranes according to this embodiment make it possible to manufacture devices for reverse electrodialysis, in which it is not necessary to use spacers (in English "spacer") to form passages allowing fluids to flow. circulate between the membranes, as is the case in the RED type devices presented above.

This has the advantage of drastically reducing the resistance associated with the spacing between the membranes (in English “bulk”), commonly referred to by the term bulk resistance, and therefore of obtaining systems developing higher membrane powers.

Organic material

The first material of the outer layers (1,3) advantageously comprises nanofibers and / or microfibers of an organic material.

According to the invention, an organic material is a material essentially comprising carbon, oxygen and hydrogen.

The organic material consists essentially of carbon, oxygen and hydrogen, i.e. it consists of at least 90 mole% of carbon, oxygen and hydrogen, preferably at least 95 mole% of carbon, oxygen and hydrogen, more preferably at least 97 mole% of carbon, oxygen and hydrogen.

According to a preferred embodiment, the organic material comprises from 70 to 100% by mole of carbon, from 0 to 30% by mole of hydrogen and from 0 to 15% by mole of oxygen.

Also, the organic material is advantageously devoid of fluorine, an element that is commonly found in ion exchange membranes (MEIs).

The organic material is advantageously chosen from cellulose, activated carbon, or a mixture of these. Cellulosic matrix

In one embodiment, the first material is a cellulosic matrix comprising nanofibers and / or crosslinked cellulose microfibers.

According to the invention, the term “crosslinked”, relating to nanofibers and / or cellulose microfibers, means that said fibers are connected to each other by covalent chemical bonds (sometimes called “bridges”) so as to form a three-dimensional network under cellulose matrix form. In other words, they are not simply agglomerated by or self-assembled through weak bonds.

The network of cellulose nanofibers and / or microfibers advantageously has pores with a diameter of between 10 and 1000 nm.

The cellulose nanofibers advantageously have a diameter ranging from 1 to 100 nm, preferably ranging from 1 to 70 nm, and more preferably ranging from 4 to 30 nm, in particular from 4 to 20 nm. In addition, their length is advantageously between 0.5 and 100 μm, in particular between 0.5 and 50 μm, for example between 0.5 and 10 μm, for example also between 0.5 and 2 μm.

Cellulose microfibers advantageously have a diameter ranging from 100 nm to 1000 nm, preferably ranging from 100 nm to 700 nm, and more preferably ranging from 100 to 200 nm. In addition, their length is advantageously between 0.5 μm and 100 μm, in particular between 1 μm and 50 μm, for example between 1 μm and 10 μm, for example also between 1 μm and 5 μm.

The nanofibers and / or the cellulose microfibers advantageously have a form factor advantageously greater than 30, preferably greater than 100.

Advantageously, the cellulose matrix comprises at least 90% by mass of cellulose nanofibers and / or microfibers, at least 95% by mass of cellulose nanofibers and / or microfibers, more preferably at least 99% of nanofibers and / or cellulose microfibers, relative to the mass of cellulose matrix.

The nanofibers and / or the cellulose microfibers can be obtained by techniques known to those skilled in the art, in particular by mechanical, enzymatic or chemical treatment of a lignocellulosic material of natural origin such as wood.

In the case of wood, these treatments have the particular effect of separating the cellulose from the other constituents of the wood such as lignin and hemicellulose. For this, the natural cellulose fibers are pre-or post-treated chemically, in particular with enzymes, and / or mechanically to initiate the destructuring before mechanical treatment in a homogenizer. It is known that the size and in particular the diameter of the cellulose fibers of said material depending on the treatment that is subjected to the natural cellulose source.

Thus, the nanofibers and / or the cellulose microfibers can be obtained by mechanical treatment of wood fibers, the mechanical treatment being carried out so as to provide sufficient mechanical energy to burst the fibers of the natural cellulose by destroying at least in part hydrogen bonds that hold the microfibrils together. Mechanical treatment is often preceded by a chemical or enzymatic treatment step. For example, this treatment step can be an oxidation treatment, in particular using an oxidant such as TEMPO (2,2,6,6-tetramethylpiperidin-1-yl) oxy). The product thus obtained is often designated under the name “NanoFibrillated Cellulose” (abbreviated “NFC”) or “cellulose nanofibers” (abbreviated “CNF”) or “MicroFibrillated Cellulose” (abbreviated “MFC”) in the literature.

In general, MFC materials are prepared from a less thorough mechanical and / or chemical treatment than that used to obtain NFCs, so MFCs generally have fibers of larger diameters than those observed in NFCs. However, there is no unambiguous definition of MFC and NFC / CNF, so these terms are often used interchangeably in the literature.

The nanofibers and / or the cellulose microfibers are preferably nanofibers and / or the nanocellulose microfibers.

The nanofibers and / or the cellulose microfibers can include impurities originating from its preparation process. These impurities can in particular hemicellulose or lignin.

Thus, the cellulose matrix can in particular comprise at most 5% by mass of hemicellulose, more preferably at most 3% by mass of hemicellulose, or alternatively at most 1% by mass of hemicellulose.

The cellulose matrix can in particular comprise at most 5% by mass of lignin, more preferably at most 3% by mass of lignin, or alternatively at most 1% by mass of lignin.

The cellulose nanofibers and / or microfibers of the invention inherently carry a negative surface charge because the cellulose monomers naturally carry alcohol groups at their C2, C3 or C6 carbon atoms.

In one embodiment, the intrinsic negative surface charge of the cellulose nanofibers and / or microfibers of the invention can be increased by functionalizing them with groups which are negatively charged and / or which become negatively charged in the presence of water. This embodiment is particularly advantageous when the charged groups and / or which become charged in the presence of water of the functionalized nanoparticles of the second layer (2) have a negative sign. Indeed, this has the advantage of increasing the surface charge of the whole of the composite membrane of the invention.

The charged groups and / or which become charged in the presence of water carried by the microfibers and / or the nanofibers are advantageously chemically bonded covalently to the surface of said cellulose microfibers and / or nanofibers.

Any charged group and / or which becomes charged in the presence of water in the latter known to those skilled in the art and making it possible to increase the charge density of the microfibers and / or of the cellulose nanofibers of the invention can be used in the within the scope of the present invention.

Advantageously, the groups which are negatively charged and / or which become negatively charged in the presence of water carried by the nanofibers and / or the cellulose microfibers are chosen from the sulfonate group -SO3, the carboxylate group -CO2, the carboxyalkyl group R-CO2 with R a C1-C4 and preferably C1-alkyl, the aminodiacetate group -N (CH ₂ C0 ₂ -) 2, the phosphonate group PO3 ² ; the amidoxine group -C (= NH ₂ ) (NOH), the aminophosphonate group -CH ₂ -NH-CH ₂ -PC> 3 ² , the thiol group -SH, and mixtures thereof.

The carboxylate group -C0 ₂ and the carboxyalkyl group R-C0 ₂ with R a C 1 -C 4 and preferably C 1 alkyl are preferred.

Thus, cellulose nanofibers and / or microfibers carrying carboxylate -CO ₂ groups (/.e. Oxidized cellulose nanofibers and / or microfibers) can for example be obtained by oxidation, for example by TEMPO oxidation, of nanofibers and / cellulose microfibers. The oxidation preferably occurs on the primary alcohol group carried by the C6 carbon atom of the monomers of the nanofibers and / or of the cellulose microfibers.

Cellulose nanofibers and / or microfibers carrying R-C0 ₂ carboxyalkylate groups (ie carboxylalkylated cellulose nanofibers and / or microfibers) can for example be obtained by etherification of cellulose nanofibers and / or microfibers. Etherification preferably occurs on the alcohol groups carried by the C2, C3 or C6 carbon atoms of monomers of cellulose nanofibers and / or microfibers.

In another embodiment, the intrinsic negative surface charge of the nanofibers and / or cellulose microfibers of the invention can be reversed by functionalizing them with charged groups and / or which become charged in the presence of water exhibiting an electric charge. positive.

This embodiment is preferred when the charged groups and / or which become charged in the presence of water of the functionalized nanoparticles of the second layer (2) have a positive sign. Any charged group and / or which becomes charged in the presence of water known to those skilled in the art and making it possible to confer a positive surface charge on cellulose nanofibers and / or microfibers can be used in the context of the present invention.

Advantageously, the positively charged groups and / or which become positively charged in the presence of water of negative charge are chosen from the quaternary ammonium group -N (R) 3 ⁺ with R a C1-C4 alkyl, the tertiary ammonium group -N (H) R) 2 ⁺ with R a C1-C4 alkyl, preferably a C1 alkyl, the dimethylhydroxyethylammonium group -N (C2H ₄ OH) CH _B ) 2 ⁺ , and mixtures thereof.

Quaternary ammonium groups are preferred.

In a particular embodiment, the nanofibers and / or the microfibers of the outer layers (1, 3) advantageously carry charged groups or which become charged in the presence of water, and the charged groups and / or which become charged in the presence of The water of the outer layer (1) are of opposite sign to the charged groups and / or which become charged in the presence of water of the other outer layer (3). In this embodiment, the composite membrane is a bipolar composite membrane. Activated carbon based material.

In one embodiment, the first material is an activated carbon felt comprising nanofibers and / or crosslinked activated carbon microfibers.

According to the invention, the term “crosslinked”, relating to nanofibers and / or activated carbon microfibers, means that said fibers are connected to each other by covalent chemical bonds (sometimes called “bridges”) so as to form a three-dimensional network. in the form of activated carbon felt. In other words, they are not simply agglomerated by or self-assembled through weak bonds.

The activated carbon felt advantageously has a thickness of between 5 and 60 μm, preferably between 5 and 50 μm, more preferably between 5 and 45 μm.

The pores of the activated carbon felt advantageously have a diameter of between 1 and 10 μm.

The activated carbon microfibers advantageously have a diameter ranging from 0.1 to 10 μm, preferably ranging from 1 to 10 μm, and more preferably ranging from 2 to 10 μm. In addition, their length is advantageously between 10 and 500 μm, in particular between 20 and 400 μm, for example between 20 and 300 μm, for example also between 1 and 200 μm.

The activated carbon felt preferably comprises activated carbon microfibers.

The nanofibers and / or the activated carbon microfibers advantageously have a form factor advantageously greater than 10, preferably greater than 50.

Advantageously, the activated carbon felt comprises at least 90% by mass of nanofibers and / or activated carbon microfibers, at least 95% by mass of nanofibers and / or of activated carbon microfibers, more preferably at least 99% of activated carbon nanofibers and / or microfibers, relative to the mass of activated carbon felt.

The nanofibers and / or the activated carbon microfibers can be obtained by techniques known to those skilled in the art, in particular by partial combustion and thermal decomposition of a fibrous carbonaceous precursor.

They can typically be obtained by a process consisting in carbonizing fibers of a resin of an organic carbon precursor (wood, fruit stones, walnut shells) or mineral (peat, coal, lignite), then in activating them by lignite. using an activating agent. The carbon atoms are then in the form of planes of aromatic rings assembled randomly in a geometry comparable to that of crumpled paper.

Nanofibers and / or activated carbon microfibers consist essentially of carbon, i.e. they consist of at least 60 mole% of carbon, preferably at least 70 mole% of carbon. carbon, more preferably at least 80 mole% carbon, the remainder being elements such as oxygen and hydrogen.

According to a preferred embodiment, the nanofibers and / or the activated carbon microfibers comprise from 60 to 100% by mole of carbon, from 0 to 30% by mole of hydrogen and from 0 to 15% by mole of oxygen.

In addition, the nanofibers and / or microfibers of activated carbon intrinsically carry a negative surface charge, due to the fact that the ends of the polyaromatic units constituting the activated carbon carry oxygen and hydrogen atoms in the form of hydoxyl, carboxylic acid, lactone, phenol, chromene and pyrone.

Nanofibers and / or activated carbon microfibers conduct electricity.

Process

The second object of the invention is a process for manufacturing a composite membrane in accordance with the first object of the invention, characterized in that it comprises the steps consisting in: i) filtering a solution comprising nanofibers and / or microfibers on a filtration medium so as to form a first internal layer (1) comprising nanofibers and / or microfibers; ii) filtering a solution of particles of nanoparticles functionalized at the surface with charged groups and / or which become charged in the presence of water on the first layer (1) obtained at the end of stage i) so as to form a inner layer (2) on said first outer layer (1); iii) filtering a solution of nanofibers and / or microfibers so as to form a second outer layer (3) comprising nanofibers and / or microfibers on the inner layer (2) obtained at the end of step ii); iv) filtering a crosslinking solution capable of crosslinking the nanofibers and / or the microfibers of the outer layers (1,3); v) drying the product of step iv), preferably in an oven; vi) removing the filtration medium, so as to obtain a composite membrane.

The nanofibers and / or the microfibers and the functionalized nanoparticles are as defined in the first subject of the invention.

The process is simple, easy to implement, economical and makes it possible to control the thickness of each of the layers of the composite membrane.

The filtration of stages i), ii), iii) and iv) is advantageously carried out with a vacuum pump, preferably at 1 bar of vacuum.

The filtration of step i) can optionally be followed by a step ii) consisting in filtering a crosslinking solution on the outer layer (1) obtained at the end of step i).

The filtration of step ii) can optionally be followed by a step iii) consisting in filtering a crosslinking solution on the second layer obtained at the end of step ii).

The solution of nanofibers and / or microfibers used in steps i) and iii) comprises from 0.1% to 1% by weight of cellulose nanofibers and / or microfibers, preferably from 0.3% to 0, 6% by weight of cellulose nanofibers and / or microfibers.

The nanofibers and / or the microfibers of the solution of steps i) and iv) can be functionalized, as detailed in the first subject of the invention.

The solution of particles of functionalized nanoparticles used in step ii) comprises from 0.001% to 0.01% by weight of functionalized nanoparticles, preferably from 0.003% to 0.006% by weight of functionalized nanoparticles.

The crosslinking solution used in step v) advantageously comprises from 0.005 M to 0.02 M of one or more crosslinking agents, preferably from 0.008 M to 0.012 M of one or more crosslinking agents.

The drying of step v) is advantageously carried out at a temperature allowing the crosslinking reaction to take place and below a temperature damaging the fibers and / or the nanofibers. Preferably, the drying is carried out at a temperature between 80 ° C and 150 ° C, in particular between 80 ° C and 120 ° C, more preferably between 80 ° C and 100 ° C.

As detailed above, the crosslinking agent preferably carries charged groups and / or which become charged in the presence of water. Citrate is preferred.

At the end of step vi), the composite membrane is in the form of a dry material.

The method may further comprise a step vii) consisting in applying to the composite membrane obtained at the end of step vi) a pressure of between 3 bar and 4 bar at a temperature ranging from 60 ° C to 95 ° C. , preferably ranging from 80 ° C to 90 ° C, for a period of at least 5 minutes, so as to mechanically reinforce said ion-selective conduction membrane.

The pressure application of step vii) can be carried out using a press, in particular a heat press.

Any other technique known to those skilled in the art can be envisaged, whether it is discontinuously (ie by batch) or continuously, for example by the technique known as “roll-to-roll” (“roll-to-roll Processing” in English) in which the membrane is produced continuously and then stored in the form of a roll.

Use

A third subject of the invention is the use of the composite membrane according to the first subject of the invention or prepared according to the process defined in the second subject of the invention as a membrane with selective ion conduction.

This conduction is advantageously carried out under the effect of a stress exerted on either side of the composite membrane, preferably an electric potential gradient or a concentration gradient.

A fourth subject of the invention is also the use of the composite membrane according to the first subject of the invention or prepared according to the process defined in the second subject of the invention for the extraction of ionic or ionizable substances from water to treating, for the extraction of organic compounds from a water to be treated, for carrying out an electrolysis reaction or for carrying out a reverse electrodialysis reaction, in particular for the production of electricity , in particular for the production of electricity from a salinity gradient.

The composite membrane can be used for the extraction of ionic or ionizable substances from a water to be treated. The composite membrane can in particular be used in processes for extracting ionic or ionizable substances from a water to be treated, such as desalination and deionization. It may for example be the treatment of water polluted by elements chosen from manganese in ionized form and iron in ionized form, and / or by substances such as nitrate ions, ammonium ions, carbonate ions. , or organic compounds in ionic form. This treatment can in particular be carried out under the action of a concentration (filtration) or an electric potential (electrodialysis) gradient on either side of the composite membrane.

In other words, the composite membrane can be used in any type of ionic separation process in an aqueous medium under the action of an electric potential on either side of the composite membrane.

Electro-desalination (commonly referred to as “desalination”) is an electrodialysis technique aimed at extracting the ions contained in seawater, in particular sodium and chloride ions. Electrodialysis aims to remove all types of ions from relatively concentrated ion solutions, in particular from industrial effluents. Electrodeionization is an electrodialysis technique used to extract solutions of low ion concentration, typically solutions that have already been treated by reverse osmosis, and which is in particular useful for obtaining ultrapure water. Electrodeionization is particularly used in the pharmaceutical field.

When the composite membrane is bipolar, it can be used in a bipolar electrolysis process, advantageously bipolar electrodialysis. The composite membrane can also be used to extract one or more organic compound (s) from a water to be treated, preferably an alcohol or an alkane, advantageously C ₁ -C ₁₂ , for example methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, glycerol, methane, ethane, propane, butane and mixtures thereof.

The composite membrane can also be used for carrying out an electrolysis reaction. In this case, to the migration of ions through the composite membrane under the effect of an electric potential gradient, oxidation and reduction reactions are added at the electrodes. It may for example be a reaction of electrolysis of water for the production of hydrogen under the action of electric potential on either side of the composite membrane.

The composite membrane can also be used for carrying out a reverse electrolysis reaction, in particular for the production of electricity.

The composite membrane is preferably used for the manufacture of a device intended to generate an electric current by reverse electrodialysis, under the effect of an electrolyte concentration gradient, preferably a salinity gradient, exerted on both sides. other side of the composite membrane. DESCRIPTION OF FIGURES

Figure 1 is a schematic sectional view of a membrane according to the invention, in which the outer layers (1,3) are formed of a cellulosic matrix comprising nanofibers and / or crosslinked cellulose microfibers and the inner layer (2) is formed from a material comprising nanoparticles functionalized at the surface by charged groups and / or which become charged in the presence of water.

EXAMPLES

The present invention will be better understood on reading the following examples which illustrate the invention without limitation.

Example 1: Preparation of a composite membrane in accordance with the invention

Material and raw materials

The material used in this example is listed below: - A Buchner filter

- A vacuum pump at lbar

- 0.1 µm PVDF filter paper

- An oven The raw materials used in this example are listed below:

- Cellulose nanofibers negatively charged by carboxymethylation or TEMPO oxidation;

- Citric acid, 99% by volume;

- Graphene oxide sold by the company Sigma Aldrich under the reference No. 777676.

Preparation of the composite membrane

The preparation process used in this example is detailed below:

^■ 1.75 ml of nanocellulose solution are filtered on the Buchner filter with a PVD filter. The vacuum pump is set to 1 bar of vacuum;

^■ Once all the solution has been filtered, 5 ml of citric acid solution are filtered over it (which will act as a crosslinking agent between the nanofibers);

^■ Once the citric acid has been filtered, 7 ml of graphene oxide solution are filtered;

^■ Once the graphene oxide solution has been filtered, 1.75 ml of nanocellulose solution are refiltered;

^■ Once all the solution has been filtered, 5 ml of citric acid solution are filtered over it (which will act as a crosslinking agent between the nanofibers); ^■ Once all the citric acid solution has been filtered stops the pump, the Buchner device is opened and the filter paper with its filtrate is removed.

The filter / filtrate paper assembly is then placed in a study oven at 85 ° C. for 15 minutes (drying and crosslinking reaction).

The membrane is finally peeled off from its filtration medium, to make things easier, it may possibly be soaked beforehand in an isopropanol solution.

The membranes thus obtained are composed of 17.5 g / m ² of nanocellulose.

The nanocellulose contents were varied by mass contents of graphene oxide. Nanocellulose contents of less than 10 mg / m ² do not make it possible to obtain membranes having sufficient mechanical strength.

For reasons of mechanical strength and ionic resistance, these values of 17 g / m ² of cellulose and 4% by weight of graphene oxide seem optimal.

These membranes have an inner layer of graphene oxide having a thickness of about 100 nm, and outer layers of cellulose each having a thickness of about 10 µm.

Measurement of membrane power

The tests were carried out with a device consisting of two independent reservoirs each containing a solution of sodium chloride (NaCl) dissolved at 1 M for the concentrated solution, then 0.1 M, 0.01 M and 0.001 M in dilute solution making it possible to define the gradient of Rc of 10, 100 and 1000 between the two reservoirs.

The two reservoirs are separated by a composite membrane in accordance with the invention obtained as detailed in Example 1.

Ag / AgCl silver grid electrodes are immersed in each of the reservoirs on either side of the membrane to measure the electric current produced through the membranes. The results of these measurements are shown in Table 1.

Table 1

With :

U Osmo the potential linked to the membrane from which the Nernst potential of the electrodes is deduced (U Nernst)

I Osmo the current linked to the membrane, calculated by measuring the electrical resistance of the membrane according to Ohm's law I = U / R

P Osmo Max is calculated by the formula Pmax = (U x l) / 4

The membrane powers are expressed in W / m ² by multiplying by 10,000 the values obtained on 1 cm ² of composite membrane.

It has also been observed that by applying a pressure of 3 to 4 bars to the membrane between two metal plates during heating to 85 ° C., the mechanical stability of the membrane is improved by 10 to 20%. Comparative Example 2: Membrane not in accordance with the invention not comprising graphene oxide

Preparation of membranes not in accordance with the invention not comprising graphene oxide The materials used are those detailed in Example 1.

The preparation process used in this comparative example is as follows:

It is filtered on the Buchner filter with a PVDF filter 3.5 ml of nanocellulose solution. The vacuum pump is set to 1 bar of vacuum. Once all the solution has been filtered, 10 ml of citric acid solution (which acts as a crosslinking agent between the nanofibers) are filtered over it.

Once all the filtered citric acid solution stops the pump, the buchner device is opened and the filter paper is removed with its filtrate. The filtrate filter paper assembly is then placed in a study oven at 85 ° C. for 15 minutes (drying and crosslinking reaction).

The membranes thus obtained are composed of 17.5 g / m ² of nanocellulose and 0.34 g / m ² of graphene oxide (2% by mass).

Membrane power of membranes not in accordance with the invention

The device used is in all respects similar to that detailed in Example 1 except for the membrane which in this comparative example does not include graphene oxide. The results of these measurements are shown in Table 2.

Table 2

With :

U Osmo the potential linked to the membrane from which the Nernst potential of the electrodes is deduced (U Nernst) I Osmo the current linked to the membrane, calculated by measuring the electrical resistance of the membrane according to Ohm's law I = U / RP Osmo Max is calculated by the formula Pmax = (U x I) / 4

The membrane powers are expressed in W / m ² by multiplying by 10,000 the values obtained on 1 cm ² of membrane.

Claims

1. Ion-selective conduction composite membrane having a thickness between 4 µm and 100 µm comprising at least one inner layer (2) disposed between two outer layers (1,3), in which:

2. The membrane of claim 1, wherein the thickness of each of the outer layers (1,3) is advantageously between 2 µm and 45 µm, and the thickness of the inner layer (2) is between 10 nm and 10 pm.

3. Membrane according to any one of claims 1 or 2, in which the nanoparticles are lamellar nanoparticles, preferably lamellar nanoparticles of a metal oxide, of a dichalcogenide of a transition metal such as molybdenum disulphide. , carbon, or a mixture thereof, more preferably lamellar nanoparticles of graphene oxide.

4. The membrane according to any one of the preceding claims, in which the groups ionized, the groups which are charged and / or which become charged in the presence of water, have a negative electric charge, preferably chosen from the epoxy group, the hydroxyl group, the carbonyl group, the carboxyl group, the sulfonate group -SO ₃ , the carboxyalkylate group R-CO ₂ with R a C1-C4 and preferably C1 alkyl, the aminodiacetate group -N (CH ₂ CC> ₂ ) ₂ , the phosphonate group PO ₃ ² ; the amidoxine group -C (= NH ₂ ) (NOH), the aminophosphonate group -CH ₂ -NH-CH ₂ -PO ₃ ² , the thiol group -SH, and mixtures thereof.

5. Membrane according to any one of claims 1 to 3, in which the charged groups and / or which become charged in the presence of water have a positive electric charge, preferably chosen from the quaternary ammonium group -N (R) 3. ⁺ with R a C1-C4 alkyl, the tertiary ammonium group -N (H) R) 2 ⁺ with R a C1-C4 alkyl, from preferably a C1-alkyl, the dimethylhydroxyethylammonium group -N (C2H ₄ OH) CHB) 2 ⁺ , and mixtures thereof.

6. Membrane according to any one of the preceding claims, in which the nanofibers and / or the crosslinked microfibers are nanofibers and / or microfibers of an organic material, preferably of cellulose or of activated carbon.

7. Membrane according to any one of the preceding claims, in which the nanofibers and / or the crosslinked microfibers bear on their surface groups which are charged and / or which become charged in the presence of water, said groups advantageously exhibiting a charge of the same sign. than that of the charged groups and / or which become charged in the presence of water of the functionalized nanoparticles of the inner layer (2).

8. A method of manufacturing a composite membrane according to any one of claims 1 to 7 comprising the steps of: i) filtering a solution comprising nanofibers and / or microfibers on a filtration medium so as to form a first. inner layer (1) comprising nanofibers and / or microfibers; ii) filtering a solution of particles of nanoparticles functionalized at the surface with charged groups and / or which become charged in the presence of water on the first layer (1) obtained at the end of stage i) so as to form a inner layer (2) on said first outer layer (1); iii) filtering a solution of nanofibers and / or microfibers so as to form a second outer layer (3) comprising nanofibers and / or microfibers on the inner layer (2) obtained at the end of step ii); iv) filtering a crosslinking solution capable of crosslinking the nanofibers and / or the microfibers of the outer layers (1,3); v) drying the product of step iv), preferably in an oven; vi) removing the filtration medium, so as to obtain a composite membrane.

9. Use of the composite membrane as defined in any one of claims 1 to 7 or prepared according to the process as defined in claim 8 as an ion selective conduction membrane.

10. Use of the composite membrane as defined in any one of claims 1 to 7 or prepared according to the process as defined in claim 8 for the extraction of ionic or ionizable substances from a water to be treated, for l 'extraction of organic compounds from a water to be treated, for carrying out an electrolysis reaction, or for carrying out a reverse electrodialysis reaction, in particular for the production of electricity, in particular of electricity production from a salinity gradient.