SALINITY GRADIENT ENERGY PRODUCTION DEVICE THROUGH A CROSSLINKED CHITOSAN FIBERS MEMBRANE

Abstract

The present invention relates to a device for producing electrical energy comprising: a) a first reservoir A (20A) intended to receive an electrolytic solution (22A) of concentration CA in a solute and comprising an electrode (30A) in contact with the electrolytic solution of concentration CA; b) a second reservoir B (20B) intended to receive an electrolytic solution (22B) of concentration CB in the same solute, CB being lower than CA, and comprising an electrode (30B) in contact with the electrolytic solution of concentration CB; c) a membrane (10) separating the two reservoirs, said membrane comprising pores allowing the diffusion of electrolytes from reservoir A to reservoir B through said pore(s);and d) a device (32) for capturing and then supplying the electrical energy generated by the potential differential existing between the two electrodes, characterized in that the membrane comprises at least one layer formed from a chitosan-based material comprising a network of nanofibers and / or crosslinked chitosan microfibers.;

Description

Title of the invention: DEVICE FOR PRODUCING ENERGY BY SALINITY GRADIENT THROUGH A MEMBRANE BASED ON CROSSLINKED CHITOSAN FIBERS STATE OF THE ART

[0001] Energy production by salinity gradient is one of the renewable energy sources with the greatest potential on a global scale.

[0002] Among the various technologies currently under consideration, the reverse electrodialysis (RED) method 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 is typically made up of alternating ion exchange membranes between which salt water and fresh water are circulated alternately. The circulation of alternating salt water and fresh water between these ion exchange membranes (IEMs) makes it possible to establish an ionic flow at each of the IEMs of the device. At the ends of this stack of membranes, electrodes collect the electric current generated by the overall ionic flow.

[0003] One of the problems encountered by devices for producing electricity from a salinity gradient, such as current RED devices, is that these have a very low electricity production capacity, due to the fact that current membranes develop electrical powers per unit of membrane surface (i.e. membrane powers) of only a few W / m2 of membrane.

[0004] In particular, MEIs weakly conduct ionic currents and constitute a significant ohmic contribution to reverse electrodialysis systems. In addition, the preparation of these membranes is very expensive, which is why the majority of maintenance investments in membrane processes are devoted to the replacement of these membranes.

[0005] 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 whose internal surface of the pores 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 / m2. More recently, it has also been proposed, in the international application published on March 9, 2017 under the number WO 2017 / 037213, nanoporous membranes non-porous membranes whose internal surface of the pores is coated with titanium oxide, making it possible to achieve membrane powers of the order of 5 kW / m2. 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 potentially harmful, and present a risk if they are released into the environment.

[0006] There is therefore, in view of the above, a need for a device allowing the production of non-polluting, economical electrical energy and which makes it possible to obtain an energy production per square meter of membrane which is of the order of kW / m2. Presentation of the invention

[0007] The inventors have discovered that a device for producing electrical energy from a salinity gradient comprising a membrane comprising a layer formed from a network of chitosan nanofibers and / or microfibers makes it possible to obtain an energy production per square meter of membrane which is of the order of kW / m2.

[0008] The use of such membranes also makes it possible to facilitate the development on a larger scale of a salinity gradient energy production device and to reduce its cost.

[0009] Thus, one aim of the invention is to provide a salinity gradient energy production device capable of developing high membrane power, and using economical and easy-to-prepare membranes, which also presents a limited risk to the environment. DESCRIPTION OF FIGURES

[0010] [Fig.l] schematically represents an example of an electrical energy production device according to the present invention, comprising two reservoirs 20A and 20B, respectively reservoir A and reservoir B, separated by a membrane 10. Each of the two reservoirs contains an electrolytic solution 22A and 22B of respective concentration CA and CB in the same solute, in which an electrode 30A and 30B is immersed. The two electrodes 30A and 30B are connected to a device for capturing and then supplying the generated electrical energy. Each reservoir A and B can be any device or natural environment, open or closed, capable of containing a liquid. In order to generate a flow of ions through the membrane, the concentrations CA and CB in the same solute of the electrolytic solutions 22A and 22B are necessarily different.In the context of the present invention, it is arbitrarily considered that CB is less than CA, which causes a circulation of the solute ions from reservoir A to reservoir B. The membrane 10, separating the two reservoirs A and B, comprises pores allowing the diffusion of the electrolytes from reservoir A to reservoir B through the pores. The . diffusion will take place from reservoir A to reservoir B. The pores have an average section allowing both water molecules and solute ions to circulate. The electrodes 30A and 30B can be partially or entirely immersed in the solutions 22A and 22B. It is also possible to provide for the electrodes to be in the form of at least part of a wall of the reservoirs. The device (32) makes it possible to capture and then supply the electrical energy spontaneously generated by the potential differential existing between the two electrodes 30A and 30B. It can consist of simple cables connecting a battery, a light bulb or any other form of electrical consumer.

[0011] [Fig.2] schematically represents an example of a membrane (10) according to the invention comprising a single layer (101) formed from a chitosan-based material comprising crosslinked chitosan nanofibers and / or microfibers.

[0012] [Fig.3] schematically represents an example of a membrane (10) according to the invention, wherein the membrane is a composite membrane comprising two outer layers (101,103) each formed from a chitosan-based material comprising crosslinked chitosan nanofibers and / or microfibers between which is arranged an inner layer (102) formed from a material comprising nanoparticles functionalized on the surface by charged groups and / or which become charged in the presence of water. DETAILED DESCRIPTION OF THE INVENTION

[0013] The first object of the invention is a device for the production of electrical energy comprising:

[0014] - a first reservoir A (20A) intended to receive an electrolytic solution (22A) of CA concentration in a solute and comprising an electrode (30A) in contact with the electrolytic solution of CA concentration;

[0015] - a second reservoir B (20B) intended to receive an electrolytic solution (22B) of concentration CB in the same solute, CB being lower than CA, and comprising an electrode (30B) in contact with the electrolytic solution of concentration CB;

[0016] - a membrane (10) separating the two reservoirs, said membrane comprising pores allowing the diffusion of electrolytes from reservoir A to reservoir B through said pore(s); and

[0017] - a device (32) for capturing and then supplying the electrical energy generated by the potential differential existing between the two electrodes,

[0018] characterized in that the membrane comprises at least one layer formed from a chitosan-based material comprising a network of nanofibers and / or crosslinked microfibers of chitosan.

[0019] The electrical energy production device according to the present invention comprises two reservoirs, respectively reservoir A (20A) and reservoir B (20B), separated by a membrane 10. Each of the two reservoirs A and B are intended to receive electrolytic solutions (22A, 22B) of respective concentrations CA and CB in the same solute, in which an electrode 30A and 30B is immersed. The two electrodes (30A, 30B) are connected to a device for capturing and then supplying the electrical energy generated.

[0020] In order to generate a flow of ions across the membrane, the concentrations CA and CB in the same solute of the electrolytic solutions (22A, 22B) are necessarily different.

[0021] In the context of the present invention, it will be arbitrarily considered that CB is less than CA, which results in a circulation of solute ions from reservoir A to reservoir B.

[0022] The membrane (10), separating the two reservoirs A and B, comprises pores allowing the diffusion of electrolytes from reservoir A to reservoir B through said pore(s); Diffusion will take place from reservoir A to reservoir B. The pores have an average section allowing both water molecules and solute ions to circulate.

[0023] The thickness of the membrane is advantageously between 2 μm and 100 μm, preferably between 2 μm and 75 μm.

[0024] The membrane advantageously comprises from 5 g to 30 g of chitosan-based material per m2 of membrane, preferably from 10 g to 20 g of chitosan-based material per m2 of membrane, preferably from 15 g to 20 g of chitosan-based material per m2 of membrane.

[0025] The electrodes (30A, 30B) may be partially or entirely immersed in the electrolytic solutions (22A, 22B). It is also possible to provide that the electrodes are in the form of at least a part of a wall of the tanks.

[0026] The device (32) makes it possible to capture and then supply the electrical energy spontaneously generated by the potential differential existing between the two electrodes (30A) and (30B). It can consist of simple cables connecting a battery, a light bulb or any other form of electrical consumer.

[0027] In the device according to the invention, the electrical energy is generated thanks to the difference in the concentrations CA and CB in the same solute of the electrolytic solutions which causes the mobility of the electrolytes, more particularly of the ions originating from said electrolytes, from the most concentrated solution to the least concentrated solution, through the porosity of the material(s) of the membrane and under the influence of their surface properties, in particular their surface charge.

[0028] The inventors discovered that, completely unexpectedly, a membrane based on chitosan nanofibers and / or microfibers develops a mem power very high brane, of the order of several hundred W / m2 of membrane, under the effect of a salinity gradient.

[0029] Without wishing to be bound by a particular theory, the inventors believe that this unexpected membrane power is determined by the surface charge of the chitosan nanofibers and / or microfibers, as well as by the geometry of the network they form, which allow very good selective conduction of ions through the membrane.

[0030] In particular, still according to the inventors, the porosity and the surface charge within the network of chitosan nanofibers and / or microfibers unexpectedly influence the selective passage of ions through the membrane, thus allowing the membrane to develop an unexpected membrane power.

[0031] The surface charge density of the inner wall of the pores of the membrane is advantageously between 0.001 and 3 C / m2, preferably between 0.1 and 1 C / m2.

[0032] The surface charge density of the membrane can be measured by dosimetry. Chitosan (or chitosan)

[0033] Chitosan or chitosan is a polysaccharide composed of the random distribution of D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit) linked in B-(1-4). It is produced by chemical (in alkaline medium) or enzymatic deacetylation of chitin.

[0034] The degree of acetylation (DA) is the percentage of acetylated units relative to the number of total units. It can be determined by Fourier transform infrared spectroscopy (FT-IR) or by titration with a strong base.

[0035] The degree of deacetylation (DD) is related to the degree of acetylation (DA) by the formula: DD (%) = 100 - DA (%).

[0036] For the purposes of the present application, chitosan means a copolymer of D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit) linked in B-(1-4) having a degree of deacetylation (DD) greater than or equal to 50% and less than or equal to 90%, or in other words a degree of acetylation (DA) greater than or equal to 10% and less than or equal to 50%.

[0037] The chemical formula of chitosan according to the present application is represented in the figure below with a degree of acetylation (DA) greater than or equal to 10% and less than or equal to 50%.

[0038] [Chem.l]

[0039] Chitosan is insoluble in organic solvents, as well as in water at neutral and basic pH due to the numerous hydrogen bonds between the hydroxyl, amine, acetamide and ether functions. On the other hand, it becomes soluble in acidic medium thanks to the protonation of its free amine functions.

[0040] In the present application, the terms chitosan or chitosan will be used interchangeably. Chitosan nanofibers and / or microfibers

[0041] According to the invention, the term "crosslinked", relating to chitosan 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 the form of a chitosan-based matrix. In other words, they are not simply agglomerated by or self-assembled via weak bonds.

[0042] The covalent chemical bonds involved in the crosslinking of chitosan nanofibers and / or microfibers may also carry charged groups and / or groups that become charged in the presence of water, as is the case, for example, when the crosslinking agent used is citrate. In this case, the crosslinking chemical bonds play a role in both the structure and the electrical surface charge of the outer layers (101,103).

[0043] The network of chitosan nanofibers and / or microfibers advantageously has pores with a diameter of between 1 and 1000 nm, preferably between 10 and 1000 nm.

[0044] According to the invention, the expression “nanofiber” of chitosan designates an object to 3-dimensional and chitosan-based in which 2 of the 3 external dimensions are at the nanoscale (i.e. 2 of the 3 dimensions are in the range of 1 to 100 nm), the 3rd external dimension being significantly larger than the other two dimensions, and not necessarily at the nanoscale.

[0045] The chitosan 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 pm, in particular between 0.5 and 50 pm, for example between 0.5 and 10 pm, for example again between 0.5 and 2 pm.

[0046] According to the invention, the expression “microfiber” of chitosan designates a 3-dimensional object in which 2 of the 3 external dimensions are located on the micrometric scale (i.e. 2 of the 3 dimensions are in a range from 0.1 to 1 μm), the 3rd external dimension being significantly greater than that of the other two dimensions.

[0047] The chitosan microfibers advantageously have a diameter ranging from 100 to 1000 nm, preferably ranging from 100 to 700 nm, and more preferably ranging from 100 to 200 nm. In addition, their length is advantageously between 0.5 and 100 μm, in particular between 1 and 50 μm, for example between 1 and 10 μm, for example still between 1 and 5 μm.

[0048] The chitosan nanofibers and / or microfibers advantageously have a form factor advantageously greater than 30, preferably greater than 100.

[0049] Advantageously, the chitosan-based material comprises between 50% and 100%, preferably between 80% and 100% by mass of chitosan nanofibers and / or microfibers, relative to the mass of chitosan-based material.

[0050] The chitosan nanofibers and / or microfibers can be obtained by techniques known to those skilled in the art, from a chitosan solution, from commercial chitosan or even from chitin.

[0051] Chitin is the second most abundant biopolymer after cellulose. It is present in the exoskeletons of crabs, shrimps and insects and in the cell walls of fungi. Due to its linear structure, chitin has a high crystallinity and is arranged in nanofibers. The nanofibers are essentially incorporated into a protein matrix. Chitin nanofibers can thus be obtained by implementing a mechanical treatment allowing their deagglomeration after prior removal of the proteins and minerals present (Ifuku S. et al 2014). After purification, a process for obtaining chitin nanoparticles consists of pre-dispersing the chitin particles in a liquid medium and then carrying out a grinding / dispersion operation (Ifuku S. et al 2014). The implementation of a system called Star Burst makes it possible, for example, to obtain chitin nanofibers.In this process, a high-pressure water jet system equipped with a collision chamber comprising beads enables the nanofibrillation of chitin.

[0052] Chitosan is obtained by deacetylation of chitin. After deacetylation of chitin, a mechanical disintegration mechanism similar to that described above for chitin can make it possible to obtain chitosan nanofibers. A method for obtaining chitosan nanoparticles thus consists of dispersing the particles of chitosan and then perform a grinding / dispersion operation using the Star Burst high-pressure water jet system (Dutta AK et al 2013). It was observed that the higher the number of dispersion cycles, the smaller the diameter of chitosan nanofibers.

[0053] The process for obtaining chitosan nanofibers described above can be carried out directly from commercial chitosan (Dutta AK et al 2013).

[0054] Chitosan nanofibers can also be obtained from a chitosan solution using the electrospinning technique. Due to the high surface energy of chitosan solutions, another polymer is generally added to the chitosan solution to facilitate the production of nanofibers by electrospinning.

[0055] Obtaining chitosan microfibers from chitosan solutions is also known to those skilled in the art.

[0056] Chitosan microfibers can also be prepared from a less extensive mechanical treatment than that used to obtain nanofibers, and generally have fibers with diameters greater than those observed in chitosan nanofibers. However, there is no unambiguous definition of chitosan microfibers and chitosan nanofibers, so these terms are often used interchangeably in the literature. The product thus obtained is often referred to in French as "chitosan nanofibers", and in English as "nanofibrillated chitosan" (abbreviated here as "NFCh"), "chitosan nanofibers" (abbreviated as "ChNF") or "microfibrillated Chitosan" (abbreviated as "MFCh") for chitosan microfibers.

[0057] The chitosan nanofibers and / or microfibers are preferably nanofibers and / or microfibers comprising chitosan nanofibers.

[0058] The chitosan nanofibers and / or microfibers of the invention intrinsically carry a positive surface charge because the (deacetylated) glucosamine units naturally carry amine groups. In one embodiment, the intrinsic positive surface charge of the chitosan nanofibers and / or microfibers of the invention can be increased by functionalizing them with positively charged groups and / or groups that become positively charged in the presence of water. This embodiment is particularly advantageous when the charged groups and / or groups that become charged in the presence of water of the functionalized nanoparticles of the inner layer (102) are of positive sign. Indeed, this has the advantage of increasing the surface charge of the entire composite membrane of the invention.

[0059] The charged groups and / or groups which become charged in the presence of water carried by the microfibers and / or nanofibers are advantageously chemically covalently bonded to the surface of said chitosan microfibers and / or nanofibers.

[0060] The presence of hydroxyl groups and amine groups in the molecule chitosan allows the grafting of charged groups and / or which become charged in the presence of water.

[0061] 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 the chitosan nanofibers of the invention can be used in the context of the present invention.

[0062] 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(C2H4OH)CH3)2+, and mixtures thereof.

[0063] Quaternary ammonium groups are preferred.

[0064] As an example, Ruihua H. et al (2012) describes the grafting of quaternary ammonium groups onto chitosan during the synthesis of N-(2-hydroxypropyl)-3-trimethyl ammonium chloride chitosan by reaction between chitosan and glycidyl trimethyl ammonium chloride.

[0065] A positive surface charge can also be imparted to chitosan by reaction between the amine group of chitosan and methyl iodide, leading to the formation of -N(CH3)3+ groups (Hoven VP et al, 2007).

[0066] In another embodiment, the intrinsic positive surface charge of the chitosan nanofibers and / or microfibers of the invention can be reversed by functionalizing them with charged groups and / or which become charged in the presence of water having a negative electrical charge.

[0067] This embodiment is preferred when the charged groups and / or which become charged in the presence of water of the functionalized nanoparticles of the inner layer (102) are of negative sign.

[0068] 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 negative surface charge on chitosan nanofibers and / or microfibers can be used in the context of the present invention.

[0069] Advantageously, the charged groups and / or groups which become charged in the presence of negatively charged water carried by the chitosan nanofibers and / or microfibers are chosen from the sulfonate group -SO3, the carboxylate group -CO2, the carboxyalkyl group R-CO2 with R being a C1-C4 and preferably C1 alkyl, the aminodiacetate group -N(CH2CO2)2, the phosphonate group PO32; the amidoxine group -C(=NH2)(NOH), the aminophosphonate group -CH2-NH-CH2-PO32, the thiol group -SH, and mixtures thereof.

[0070] For example, the introduction of sulfonate groups on the surface of chitosan can be obtained by reaction between the amine group of chitosan and the acid 5-formyl-2-furan sulfonic acid (Hoven VP et al. 2007).

[0071] The carboxylate group -CO2 and the carboxyalkyl group R-CO2 with R a C1-C4 alkyl and preferably C1 are preferred.

[0072] Thus, chitosan nanofibers and / or microfibers carrying -CO2 carboxylate groups (i.e. oxidized chitosan nanofibers and / or microfibers) can for example be obtained by oxidation, for example by TEMPO oxidation, of chitosan nanofibers and / or microfibers. The oxidation occurs on the primary alcohol group carried by the C3 carbon atom of the monomers of the chitosan nanofibers and / or microfibers. Single-layer membrane

[0073] In a first embodiment, the invention relates to a device according to the invention whose membrane comprises a single layer (101) formed from a chitosan-based material as defined above.

[0074] The inventors have shown that, surprisingly, a membrane comprising a single layer (101) formed from a network of crosslinked chitosan nanofibers and / or microfibers makes it possible to develop surprisingly high membrane powers compared to membranes of the prior art, and compatible with industrial exploitation.

[0075] The thickness of the membrane comprising a single layer (101) is advantageously between 2 μm and 50 μm, preferably between 5 μm and 20 μm, more preferably between 10 μm and 20 μm.

[0076] In the invention, the thickness of the membrane and of the different layers is measured by scanning electron microscopy of sections of dry membrane. Process for preparing a single-layer membrane

[0077] A membrane comprising a single layer can easily be prepared by a process comprising the steps of:

[0078] i) filtering a solution comprising chitosan nanofibers and / or microfibers on a filtration support so as to form a layer comprising nanofibers and / or microfibers;

[0079] ii) filtering a crosslinking solution capable of crosslinking the chitosan nanofibers and / or microfibers of the layer obtained in step i);

[0080] iii) drying the product of step ii), preferably in an oven;

[0081] iv) removing the filtration support, so as to obtain a membrane comprising a layer.

[0082] The process is simple, easy to implement, economical and allows the thickness of the membrane to be controlled.

[0083] The chitosan nanofibers and / or microfibers used in the process are such as defined in the first subject of the invention.

[0084] The filtration of steps i) and ii) is advantageously carried out with a vacuum pump, preferably under 1 bar of depression.

[0085] The chitosan nanofiber and / or microfiber solution comprises from 0.1% to 1% by weight of chitosan nanofiber and / or microfiber, preferably from 0.3% to 0.6% by weight of chitosan nanofiber and / or microfiber.

[0086] The nanofibers and / or microfibers of the solution from step i) can be functionalized, as detailed in the first subject of the invention.

[0087] The crosslinking solution used in step ii) 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.

[0088] As detailed above, the crosslinking agent preferentially carries charged groups and / or which become charged in the presence of water.

[0089] The crosslinking agent may be citric acid (in its citrate form) or epichlorohydrin.

[0090] The membrane may also be prepared by casting a solution comprising chitosan nanofibers and / or microfibers according to a method comprising the steps of:

[0091] i) depositing by casting on a support a solution comprising chitosan nanofibers and / or microfibers and a crosslinking agent capable of crosslinking the chitosan nanofibers and / or microfibers so as to form a layer comprising nanofibers and / or microfibers;

[0092] ii) drying the product of step i), preferably in an oven;

[0093] iii) removing the support, so as to obtain a membrane comprising a layer.

[0094] The membrane can also be prepared by spraying or pulverizing a solution comprising chitosan nanofibers and / or microfibers according to a method comprising the steps of:

[0095] i) depositing by spraying onto a support a solution comprising chitosan nanofibers and / or microfibers and a crosslinking agent capable of crosslinking the chitosan nanofibers and / or microfibers so as to form a layer comprising nanofibers and / or microfibers;

[0096] ii) drying the product of step i), preferably in an oven;

[0097] iii) removing the support, so as to obtain a membrane comprising a layer.

[0098] According to a variant of the spraying method, the method comprises the steps of:

[0099] i) depositing by spraying onto a support a solution comprising chitosan nanofibers and / or microfibers so as to form a layer comprising nanofibers and / or microfibers;

[0100] ii) spray-depositing a crosslinking solution capable of crosslinking the chitosan nanofibers and / or microfibers of the layer obtained in step i);

[0101] iii) drying the product of step ii), preferably in an oven;

[0102] iv) removing the support, so as to obtain a membrane comprising a layer.

[0103] The casting and spraying techniques are well known to those skilled in the art and can be carried out using any suitable means. For example, the spraying can be carried out using a spray nozzle making it possible to spray the composition to be deposited in the form of droplets.

[0104] Any other technique known to those skilled in the art is conceivable, whether discontinuous (i.e. batchwise), or continuous, for example by the so-called “roll-to-roll” technique (“roll-to-roll processing” in English) in which the membrane is produced continuously then stored in the form of a roll. Composite membrane

[0105] In a second embodiment, the device comprises a composite membrane comprising two external layers (101, 103) each formed from a chitosan-based material as defined above, between which is arranged an internal layer (102) formed from a second material comprising nanoparticles functionalized by charged groups and / or which become charged in the presence of water.

[0106] The second material advantageously has pores with a diameter of between 10 and 100 nm.

[0107] The inventors have discovered that, unexpectedly, such a composite membrane develops a membrane power significantly higher than that of a membrane comprising a single layer (101) as defined above.

[0108] Without wishing to be bound by a particular theory, the inventors believe that this improvement in membrane power is due to a synergistic effect between, on the one hand, the properties of the network of chitosan nanofibers and / or microfibers, and on the other hand those of the layer of nanoparticles functionalized by charged groups. The thickness of the composite membrane is advantageously preferably between 4 μm and 100 μm, more preferably between 4 μm and 75 μm.

[0109] The thickness of each of the external layers (101, 103) is advantageously between 2 pm and 45 pm, preferably between 2 pm and 30 pm. Said external layers advantageously have the same thickness. The thickness of the internal layer (102) is advantageously between 10 nm and 2 pm, 10 nm and 1 pm, 10 nm and 800 nm, preferably between 10 nm and 400 nm, preferably between 200 nm and 500 nm.

[0110] Preferably, the thickness of each of the external layers (101, 103) is advantageously between 2 μm and 25 μm, and the thickness of the internal layer (102) is between 10 nm and 2 pm.

[0111] According to the inventors, the very low thickness of the internal layer (102) makes it possible to obtain excellent permeability while significantly increasing the selective conduction of ions.

[0112] Preferably, the membrane comprises less than 15% by weight of second material relative to the weight of chitosan-based material, preferably between 2% and 8% by weight of second material relative to the weight of chitosan-based material, more preferably between 3% and 5% by weight of second material relative to the weight of first material.

[0113] In this embodiment, the chitosan-based material of the outer layers (101, 103) ensures the integrity of the inner layer (102), in particular when, during its use, the latter is subjected to a stress such as a pressure gradient on either side of the membrane.

[0114] Preferably, the nanofibers and / or microfibers of the external layers (101, 103) carry charged groups or groups which become charged in the presence of water, said groups advantageously having a charge of the same sign as that of the charged groups or groups which become charged in the presence of water of the functionalized nanoparticles of the internal layer (102).

[0115] This has the advantage of increasing the surface charge of the entire membrane.

[0116] According to the inventors, the presence of these charged groups or those which become charged in the presence of water of the same sign within the internal layer (102) and the external layers (101, 103) of the membrane make it possible to obtain a synergistic effect, namely an unexpected improvement in the selective conduction of ions through the membrane.

[0117] In this embodiment, the chitosan-based material therefore plays a role in the structure of the membrane and in its capacity to ensure selective conduction of ions.

[0118] Furthermore, the covalent chemical bonds involved in the crosslinking of nanofibers and / or microfibers may also carry charged groups and / or groups that become charged in the presence of water, as is the case, for example, when the crosslinking agent used is citrate. In this case, the crosslinking chemical bonds play a role in both the structure and the electrical surface charge of the outer layers (101,103). Functionalized nanoparticles

[0119] According to the invention, the term “nanoparticle” designates a 3-dimensional object, in which at least one external dimension is located at the nanometric scale (i.e. at least one dimension is in a range between 1 and 100 nm).

[0120] The second material advantageously comprises the nanoparticles in the form of individual nanoparticles, i.e. nanoparticles which are not aggregated or in other words covalently linked to each other.

[0121] 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.

[0122] Advantageously, the nanoparticles are not in the form of nanotubes.

[0123] The nanoparticles are preferably lamellar nanoparticles.

[0124] According to the invention, the term "lamellar nanoparticle" designates a nanoparticle comprising atoms in the form of monolayers of atoms linked together by covalent bonds. The lamellar nanoparticles can consist of a single monolayer of atoms (2D materials) or a stack of 2 to 5 monolayers of atoms linked together by weak bonds, such as Van der Waals forces.

[0125] In other words, a lamellar nanoparticle is a 3-dimensional object in which a first external dimension is located at 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 micrometric scale.

[0126] The lamellar nanoparticles preferably have a median size (also designated by the acronym “D50”) of between 5 pm and 50 pm, preferably of between 10 pm and 20 pm, more preferably of 15 pm.

[0127] According to the invention, the terms “monolayer”, “bilayer”, “oligo-layers”, relating to lamellar nanoparticles, designate a lamellar nanoparticle consisting respectively of a monolayer of atoms, two monolayers of atoms, and 3 to 5 monolayers of atoms. The bilayer and oligolayer lamellar nanoparticles are typically stabilized by weak interactions between the monolayers of atoms, such as Van der Waals interactions.

[0128] The lamellar nanoparticles are preferably lamellar nanoparticles of a metal oxide, in particular SnO2 or TiO2, lamellar nanoparticles of a dichalcogenide of a transition metal such as molybdenum disulfide MoS 2, lamellar nanoparticles of carbon, or a mixture thereof.

[0129] The lamellar carbon nanoparticles are advantageously lamellar nanoparticles of monolayer graphene, bilayer graphene, oligolayer graphene or a mixture thereof.

[0130] Monolayer graphene particle nanoparticles are preferred.

[0131] According to the invention, single-layer graphene is a crystalline two-dimensional material consisting of carbon in a particular allotropic form, which can be represented as a planar honeycomb. More particularly, single-layer graphene is a sheet consisting of a single atomic plane of sp2 hybridized carbon. It can therefore be described as a single layer.

[0132] According to the invention, bilayer graphene (or BLG; “Bi-Layer Graphene” in English) is a material consisting of a stack of 2 graphene monolayers stabilized by van der Waals type interactions between the 2 graphene monolayers. BLG can be obtained by exfoliation of graphite or by chemical vapor deposition (CVD).

[0133] According to the invention, “few-layer graphene” (or FLG: “Few-Layer Graphene” in English) is a material consisting of a stack of 3 to 5 graphene sheets, stabilized by van der Waals type interactions between the different graphene planes.

[0134] The lamellar carbon nanoparticles are advantageously lamellar nanoparticles of monolayer molybdenum disulfide, bilayer molybdenum disulfide, oligolayer molybdenum disulfide or a mixture thereof.

[0135] Depending on the sign of their charge, the charged groups or those that become charged in the presence of water confer a negative or positive surface charge to the inner layer (102) of the composite membrane when it is placed in the presence of water.

[0136] Any charged group or group which becomes charged in the presence of water known to those skilled in the art and which makes it possible to increase the surface charge of graphene particles can be used in the context of the present invention.

[0137] In one embodiment, the nanoparticles are surface functionalized with negatively charged groups and / or which become negatively charged in the presence of water.

[0138] The negatively charged groups and / or which become negatively charged in the presence of water are advantageously chosen from the epoxide group, the hydroxyl group, the carbonyl group, the carboxyl group, the sulfonate group -SO3, the carboxyalkylate group R-CO2 with R a C1-C4 alkyl and preferably a C1, the aminodiacetate group -N(CH2CO2 )2, the phosphonate group PO32; the amidoxine group -C(=NH2 )(NOH), the aminophosphonate group -CH2-NH-CH2-PO32, the thiol group -SH, and mixtures thereof.

[0139] Preferably, the nanoparticles functionalized on the surface by negatively charged groups or which become negatively charged in the presence of water are lamellar nanoparticles of graphene oxide (or GO, in English “graphene oxide”).

[0140] The lamellar graphene oxide nanoparticles carry negatively charged groups or groups which become negatively charged in the presence of water, advantageously chosen from the epoxide group, the hydroxyl group, the carbonyl group, the carboxyl group, and mixtures thereof.

[0141] In one embodiment, the nanoparticles are surface functionalized by positively charged groups and / or which become positively charged in the presence of water.

[0142] 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(C2H4OH)CH3)2+, and mixtures thereof. Process for preparing a composite membrane

[0143] The composite membrane according to the second embodiment can be prepared by a method comprising the steps of:

[0144] i) filtering a solution comprising chitosan nanofibers and / or microfibers on a filtration support so as to form a first external layer (101) comprising chitosan nanofibers and / or microfibers;

[0145] ii) filtering a solution of functionalized nanoparticle particles on the external layer (101) obtained at the end of step i) so as to form an internal layer (102) on said first external layer (101);

[0146] iii) filtering a solution of chitosan nanofibers and / or microfibers so as to form a second external layer (103) comprising nanofibers and / or microfibers on the internal layer (102) obtained at the end of step ii);

[0147] iv) filtering a crosslinking solution capable of crosslinking the chitosan nanofibers and / or microfibers of the external layers (101,103);

[0148] v) drying the product of step iv) in the oven;

[0149] vi) removing the filtration support, so as to obtain a composite membrane.

[0150] The chitosan nanofibers and / or microfibers and the nanoparticles functionalized on the surface by charged groups and / or which become charged in the presence of water are as defined in the first subject of the invention.

[0151] The process is simple, easy to implement, economical and allows the thickness of each of the layers of the composite membrane to be controlled.

[0152] The filtration of steps i), ii), iii) and iv) is advantageously carried out with a vacuum pump, preferably under 1 bar of depression.

[0153] The filtration of step i) can optionally be followed by a step ij consisting of filtering a crosslinking solution on the external layer obtained at the end of step i).

[0154] The filtration of step ii) can optionally be followed by a step ii3) consisting of filtering a crosslinking solution on the internal layer (102) obtained at the end of step ii).

[0155] The solution of nanofibers and / or microfibers used in steps i) and iii) comprises from 0.1% to 1% by weight of chitosan nanofibers and / or microfibers, preferably from 0.3% to 0.6% by weight of chitosan nanofibers and / or microfibers.

[0156] The nanofibers and / or microfibers of the solution of steps i) and iii) can be functionalized, as detailed in the first subject of the invention.

[0157] The solution of functionalized nanoparticles used in step ii) comprises from 0.001% to 0.01% by weight of nanoparticles, preferably from 0.003% to 0.006% by weight of functionalized nanoparticles.

[0158] The crosslinking solution used in step iv) 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.

[0159] The drying of step v) is advantageously carried out at a temperature allowing the crosslinking reaction to occur and lower than a temperature damaging the fibers and / or 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.

[0160] As detailed above, the crosslinking agent preferentially carries charged groups and / or which become charged in the presence of water.

[0161] Any other technique known to those skilled in the art is conceivable, whether discontinuous (i.e. batchwise), or continuous, for example by the so-called “roll-to-roll” technique (“roll-to-roll processing” in English) in which the membrane is produced continuously then stored in the form of a roll. Other components of the device

[0162] The reservoirs A and B of the device according to the invention each contain an electrolytic solution (22A, 22B) of respective concentration CA and CB in the same solute, CB being lower than CA.

[0163] Each reservoir A and B can be any natural device or environment, open or closed, capable of containing a liquid.

[0164] By placing electrolytic solutions of different concentrations in the two reservoirs A and B, an osmotic flow is generated between the two reservoirs, preferably by diffusio-osmosis, i.e. without any osmotic pressure appearing. In another embodiment, the concentration gradient could also be obtained by temperature gradient between the two reservoirs by acting on the solubility of the salt as a function of the temperature.

[0165] In the context of the present invention, the concentration ratio Rc (Rc being equal to the ratio of the concentration of the most concentrated solution / the concentration of the least concentrated solution) may be between 1 and 109. Preferably, the concentration ratio CA / CB is greater than 1 and less than or equal to 109, advantageously greater than 10 and less than or equal to 105.

[0166] Electrolytic solutions are aqueous solutions comprising electrolytes. The electrolytes may be of any chemical nature as long as they dissolve in the solution in the form of charged ions. Preferably, these ions will come from dissolved salts such as LiCl, NaCl, KCl, CaCl2 and MgCl2. Electrolytic solutions can be:

[0167] - synthetic solutions;

[0168] - natural solutions, such as fresh water from lakes or rivers, groundwater, brackish water, seawater;

[0169] - industrial production water, oil production water or biological solutions.

[0170] Preferably, said electrolytic solutions are aqueous solutions comprising a solute chosen from alkali metal halides or alkaline earth metal halides, preferably chosen from LiCl, NaCl, KC1, CaCl2 and MgCl2, more preferably the solute is NaCl.

[0171] To improve the osmotic flow generated on either side of the membrane according to the invention, the pH of the solutions can be adjusted according to the isoelectric point of the material(s) constituting the membrane.

[0172] In the context of the present invention, pHiso means the pH of the isoelectric point of the material(s) constituting the membrane. pHiso is measured by methods known to those skilled in the art, in particular by the acid / base potentiometric titration method.

[0173] Even more favorably, to increase the asymmetry of the device and amplify the quantity of electrical energy produced by the device, a pH gradient may also be established between the two reservoirs, the pH difference between the two solutions will be greater than 1, preferably greater than 2.

[0174] Each of the reservoirs A and B of the device according to the invention also comprises an electrode (30A, 30B) arranged so as to come into contact with the electrolytic solution (22A, 22B).

[0175] Different types of electrodes can be used to recover the potential or electric current developed between the two reservoirs.

[0176] All types of electrodes capable of collecting the flow of Na+ or Cl ions can be used, and preferably electrodes composed of Silver and Silver Chloride (Ag / AgCl), Carbon and Platinum (C / Pt-), Carbon (C-), Graphite or even Iron complexes of the type | Fc(CN)6]4 / |FcfCNjôp .

[0177] The electrodes may be partially or completely immersed in the electrolytic solutions. It could also be provided that the electrodes are in the form of at least part of a wall of the tanks.

[0178] The electrodes may in particular be circulation electrodes (in English “redox-flow”). The principle of these electrodes is based on an oxidation reaction and a reduction reaction at each of the electrodes.

[0179] The electrodes are preferably capacitive or supercapacitive electrodes. The principle of these electrodes is based on an interaction of the electrodes and the electrolyte which leads to the spontaneous appearance of an accumulation of charges at the interfaces.

[0180] These electrodes are connected together to a device (32) making it possible to capture and then supply the electrical energy spontaneously generated by the potential differential existing between them. These electrodes can in particular be connected by simple cables connecting a battery, a light bulb or any other form of electrical consumer.

[0181] The device thus described makes it possible to harvest the electrical energy resulting from the charged ionic flow crossing the nano-fluidic membrane.

[0182] In a particular embodiment of the invention, the device may comprise N reservoirs (20) and N1 membranes (10), N being an integer, in particular between 3 and 100, in particular between 3 and 50.

[0183] In this device, the reservoirs and the membranes are as defined above. The assembly will therefore consist of an alternation of reservoirs alternately containing a concentrated electrolytic solution and a less concentrated electrolytic solution, separated from each other by membranes. Process for producing electrical energy

[0184] The second subject of the invention is a method for producing electrical energy using a device as described in the first subject of the invention comprising the following steps:

[0185] i) providing an electrolytic solution (22A) of solute concentration CA in the reservoir A (20A), so that the electrode (30A) with which it is equipped is in contact with said solution (22A),

[0186] ii) providing an electrolytic solution (22B) of concentration CB in the same solute, CB being lower than CA, in the reservoir B (20B), so that the electrode (30B) with which it is equipped is in contact with said solution (22B),

[0187] iii) allowing the electrolytes to diffuse from reservoir A to reservoir B through the membrane (10),

[0188] iv) capturing the electrical energy generated by the potential differential existing between the two electrodes, using the device (32).

[0189] Steps i) and ii) are preferably implemented by supplying the electrolytic solution of concentration CA and the electrolytic solution of concentration CB in the form of a continuous flow.

[0190] More generally, these different steps will be easily achievable by those skilled in the art, using their general knowledge. BIBLIOGRAPHICAL REFERENCES

[0191] Ifuku S. et al. Molecules 2014, 19, 18367-1838

[0192] Dutta AK et al, Carbohydrate Polymers 97 (2013) 363-367

[0193] Ruihua H. et al, J Mater Sci (2012) 47: 845-851

[0194] Hoven VP et al, Carbohydrate Polymers 68 (2007) 44-53

Claims

Claims

1. Device for producing electrical energy comprising: a) a first reservoir A (20A) intended to receive an electrolytic solution (22A) of concentration CA in a solute and comprising an electrode (30A) in contact with the electrolytic solution of concentration CA; b) a second reservoir B (20B) intended to receive an electrolytic solution (22B) of concentration CB in the same solute, CB being lower than CA, and comprising an electrode (30B) in contact with the electrolytic solution of concentration CB; c) a membrane (10) separating the two reservoirs, said membrane comprising pores allowing the diffusion of electrolytes from reservoir A to reservoir B through said pore(s);and d) a device (32) for capturing and then supplying the electrical energy generated by the potential differential existing between the two electrodes, characterized in that the membrane comprises at least one layer formed from a chitosan-based material comprising a network of nanofibers and / or crosslinked chitosan microfibers.;

2. Device according to claim 1, wherein the thickness of the membrane is between 2 pm and 100 pm, preferably between 2 pm and 75 pm.

3. A device according to any preceding claim, wherein the membrane comprises from 5 g to 30 g of chitosan-based material per m2 of membrane, preferably 10 to 20 g of chitosan-based material per m2 of membrane, preferably 15 to 20 g of chitosan-based material per m2 of membrane.

4. Device according to any one of claims 1 to 3, wherein the nanofibers and / or crosslinked microfibers of chitosan are functionalized with positively charged groups and / or which become positively charged in the presence of water, preferably groups selected 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(C2H4OH)CH3)2+, and mixtures thereof.

5. Device according to any one of claims 1 to 3, wherein the nanofibers and / or crosslinked microfibers of chitosan are functionalized by negatively charged groups and / or which become negatively charged in the presence of water, preferably groups selected from the sulfonate group -SO3, the carboxylate group -CO2, the carboxyalkyl group R-CO2 with R a C1-C4 and preferably Cl alkyl, the aminodiacetate group -N(CH2CO2 )2, the phosphonate group PO23; the amidoxine group -C(=NH2)(NOH), the aminophosphonate group -CH2-NH-CH2-PO32, the thiol group -SH, and mixtures thereof.

6. Device according to any one of the preceding claims in which the membrane comprises a single layer (101) formed of a chitosan-based material comprising a network of nanofibers and / or crosslinked microfibers of chitosan.

7. Device according to any one of claims 1 to 5 wherein the membrane is a composite membrane comprising two external layers (101, 103) each formed from a chitosan-based material comprising a network of nanofibers and / or crosslinked microfibers of chitosan, between which is arranged an internal layer (102) formed from a second material comprising nanoparticles functionalized by charged groups and / or which become charged in the presence of water.

8. Device according to the preceding claim, in which the thickness of each of the external layers (101, 103) is between 2 pm and 25 pm, and the thickness of the internal layer (102) is between 10 nm and 2 pm.

9. Device according to any one of claims 7 to 8, wherein the nanoparticles are lamellar nanoparticles, preferably lamellar nanoparticles of a metal oxide, of a dichalcogenide of a transition metal such as molybdenum disulfide, of carbon, or a mixture thereof, more preferably lamellar nanoparticles of graphene oxide functionalized on the surface by positively charged groups or which become positively charged in the presence of water.

10. A method of producing electrical energy using a device as described in any one of the preceding claims, comprising the following steps: i) providing an electrolytic solution (22A) of solute concentration CA in the reservoir A (20A), so that the electrode (30A) with which it is equipped is in contact with said solution (22A), ii) providing an electrolytic solution (22B) of concentration CB in the same solute, CB being lower than CA, in the reservoir B (20B), so that the electrode (30B) with which it is equipped is in contact with said solution (22B), iii) allowing the electrolytes to diffuse from reservoir A to reservoir B through the membrane (10), iv) capturing the electrical energy generated by the potential differential existing between the two electrodes, using the device (32).

11. Method according to the preceding claim, characterized in that said electrolytic solutions are aqueous solutions comprising a solute chosen from alkali metal halides or alkaline earth metal halides, preferably chosen from LiCl, NaCl, KC1, CaCl2 and MgCl2.

12. Method according to any one of claims 10 to 11, characterized in that the CA / CB concentration ratio is greater than 1 and less than or equal to 109, preferably greater than 10 and less than or equal to 105.

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