Abstract:
An electrolyte-separating membrane includes a carrier made of a porous and permeable synthetic thermoplastic material that is larger than 0.8 mm in thickness and an active layer made of a material able to induce insertion and deinsertion reactions for selective transfer of cations through the membrane. The active layer is deposited on the carrier and is hermetic. The material of the active layer may in particular be a molybdenum cluster chalcogenide. The invention aims to provide an electrolyte-separating membrane that is able to transfer cations selectively and that may be manufactured with large dimensions. The invention also relates to a cation transfer method employing this membrane and to a process for manufacturing said membrane, in particular by selective laser sintering of a powdered polymer.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an electrolyte-separating membrane for selective transfer of cations through the membrane, to a process for manufacturing said membrane and to a process for selective transfer of cations through said membrane. 
       PRIOR ART 
       [0002]    A membrane formed with chalcogenides with a molybdenum cluster, is already known, notably Mo 6 X 8  phases called Chevrel phases, described in International Patent Application WO 2009/007598, used for transferring cations through said membrane. 
         [0003]    This document states that cations may be transported through the membrane in a material of formula Mo 6 X 8  (with X=S, Se or Te) called Chevrel phases where reversible oxidation-reduction systems occur of the type: 
         [0000]      Mo 6 X 8   +x M n+   +xne   −           M x Mo 6 X 8    
         [0000]    wherein n is an integer and M n+  is a metal cation. These systems are diversified by the nature of the cation M n+ , of the chalcogen X and of the stoichiometry x of the ternary M n+ . 
         [0004]    In an experimental set-up applying the selective transfer process, the transfer membrane is placed between two compartments respectively including an electrode in platinum-plated titanium which operates as an anode and a metal electrode (in stainless steel, aluminium, copper or platinum-coated titanium) or in glassy carbon which operates as a cathode. The first compartment contains a first electrolyte which contains different cations of an effluent to be treated. The second compartment contains a second electrolyte intended to receive the selected cations. 
         [0005]    An electric direct current is established between the anode and the cathode. In the global electrochemical operation of the set of both compartments, insertion of the cation occurs at the M x Mo 6 S 8 /first electrolyte interface (effluent to be treated, mixture of the cations M′, M′ n+ , M″ n+ , for example), according to: 
         [0000]      Mo 6 X 8   +x M n+   +xne   −           M x Mo 6 X 8    
         [0000]    The de-insertion of this same cation M n+  at the M x Mo 6 S 8 /second electrolyte interface (solution for adding value to M n+  for example) is carried out reciprocally according to 
         [0000]      M x MO 6 X 8           MO 6 X 8 +XM n+   +xne   −   +x M n+   
         [0006]    The mobility of the metal cation in the Chevrel phase thus allows transfer of the desolvated cation M n+  from one medium to the other without transfer of any other chemical species of either one of the compartments. 
         [0007]    A transfer membrane as a disc is obtained by hot sintering of a mixture of composition powder adapted to the stoichiometry of the desired material. In this way, discs of active material with a thickness from 2 to 5 millimeters are thereby obtained. 
         [0008]    The tests with membranes consisting of selenated and sulfur-containing phases have shown that in particular the cations of the following metals may be transferred from one electrolyte to the other: iron, manganese, cobalt, nickel, copper, zinc, cadmium. The current density limits obtained are comprised between 10 and 20 A/m 2 , with Faradic yields of more than 90%, or even more than 98%, and with very good selectivity. 
         [0009]    The tests have also shown that the transfer rate increased with decreasing of the thickness of the membrane. However, the required mechanical strength of the membrane limits the reduction of its thickness. 
         [0010]    In document WO 2012/010766 A1, the making of a transfer membrane including two layers was proposed. A supporting layer is made for example in a porous ceramic material and a second so-called active layer is deposited on the support. The deposition of the second layer is achieved by coating with a composition comprising the active material as a powder, a binder and a solvent. The solvent is evaporated subsequently. The thereby produced active layer has a thickness of the order of 50 to 100 μm and is sealed. On the other hand, the support is pervious and allows an electrolyte contained on the side of the support to attain the active layer. The reactions discussed earlier may develop. The use of such a membrane retains the previous results in terms of Faradic yields, but gives the possibility of attaining current densities multiplied by five or more. 
         [0011]    This document also states that the host lattice of the active layer may also be selected from metal chalcogenides or compounds of lithium and of a metal as an oxide, phosphate or fluoride, the metal being selected from nickel, cobalt, iron, manganese, vanadium or titanium. Such host lattices are also capable of transferring metal cations, and in particular lithium. 
         [0012]    The manufacturing of the supports in ceramic is easy on small samples, but it is difficult to strongly increase the dimensions of these supports, which limits the possibilities of industrially utilizing the process. 
         [0013]    The invention is directed to providing an electrolyte-separating membrane with an active layer of a material capable of developing insertion and de-insertion reactions for selective transfer of cations through the membrane which may be manufactured with large dimensions. 
       DESCRIPTION OF THE INVENTION 
       [0014]    Considering these goals, the object of the invention is a membrane including a support consisting of a porous and pervious material and of an active layer of a material capable of developing insertion and de-insertion reactions for selective transfer of cations through the membrane, the active layer being deposited on the supporting and leak-proof layer, characterized in that the support is in a thermoplastic synthetic material. 
         [0015]    By using a synthetic material for the support, one gets rid of the large size manufacturing problems of the ceramic supports. It is ascertained that it is possible to manufacture such supports with large dimensions while retaining the transfer performances through the active layer. In particular, it is ascertained that the active layer may be applied on the synthetic support in the same way as on the ceramic support. The nature of the support is compatible with most of the electrolytes which may be used in applications with selective transfer of cations, such as solutions of acid or of salts. 
         [0016]    According to Particular Features:
       the porosity of the support is comprised between 20 and 60% of the volume fraction, the samples with a porosity in this range gave the possibility of demonstrating the operation of the selective transfer process;   the synthetic material is selected from a group including polyamide, amide copolymers, polyacetates, polyethylenes and polyether-ether-ketone; these thermoplastic materials are sufficiently resistant in mechanical terms and in chemical terms in connection with the nature of the electrolytes used; they are also suitable for a manufacturing method as discussed hereafter;   the material of the active layer is a metal chalcogenide;   more particularly, the metal chalcogenide is for example a chalcogenide with a molybdenum cluster (Mo n X n+2  or M x Mo n X n+2 ) wherein M is a metal and X is a chalcogen selected from the group of S, Se and Te;   alternatively, the material of the active layer is a compound of lithium and of a metal as an oxide, phosphate or fluoride or of a combination of these forms, the metal being selected from a group including nickel, cobalt, iron, manganese, vanadium, titanium and tungsten;   the active layer has a thickness of more than 50 μm; the sealing conditions of the active layer are attained from this thickness.       
 
         [0023]    The object of the invention is also a method for manufacturing a membrane as described earlier, according to which a solution is prepared, comprising an active material as a powder, a binder and a solvent, and the surface of a support in porous material is then coated with said solution and the solvent is evaporated in order to form a leak-proof active layer on the support, characterized in that the support is in a thermoplastic synthetic material. 
         [0024]    Particularly, the support is obtained by additive manufacturing of layers and selective laser sintering of a polymer as a powder. Successive layers of polymeric material as a powder are spread out and certain areas are selectively heated in order to partly melt the powder grains so that they agglomerate. The heating power is adjusted so that the melting of the grains is not complete so that spaces made between the grains remain. These spaces then form the pores of the support which is thereby built. Certain pores remain open and impart perviousness to the support. The surface of the thereby obtained support is not smooth and the active layer which is deposited thereon by coating adheres well to the support. 
         [0025]    According to other features, the powdered polymer has a grain size comprised between 1 and 120 μm, preferably between 10 and 80 μm, or even between 40 and 75 μm. The size of the grains also determines the size of the pores of the support which preferably has to be close to the size of the grains of the active material powder. Actually it is in this way that the sealing of the active layer is obtained the most easily, i.e. with the minimum thickness for the active layer. Obtaining a small thickness gives the possibility of preserving the current density which may be obtained. 
         [0026]    The object of the invention is also a selective extraction method for cations by electrochemical transfer according to which a first electrolyte containing cations and a second electrolyte are separated by a separation membrane, characterized in that a transfer membrane as defined earlier is used as a separation wall of the electrolytes, and transfer of the cations through said transfer membrane is ensured by generating a potential difference (ΔE) between either one anode in the first electrolyte and a cathode in the second electrolyte, or between an anode in the first electrolyte and said transfer membrane, so as to cause insertion of the cations into the active layer of the transfer membrane on the side of the first electrolyte, diffusion of the cations into the active layer, and then their de-insertion in the second electrolyte. 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0027]    The invention will be better understood and other particularities and advantages will appear upon reading the description which follows, the description referring to the appended drawings wherein: 
           [0028]      FIG. 1  is a sectional view of a transfer membrane according to the invention; 
           [0029]      FIG. 2  is a view of a facility for building a membrane according to the invention by selective sintering with a laser; 
           [0030]      FIGS. 3 and 4  are schematic views of a test set up for checking the porosity or the sealing of the membrane of  FIG. 1 ; 
           [0031]      FIG. 5  is a block diagram of a selective transfer device between two electrolytes. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    A transfer membrane  2  according to the invention is formed with a porous support  21  in a synthetic material onto which a thin active layer  22  has been deposited. The manufacturing of the leak-proof membrane  2  is carried out in a first phase for manufacturing the support  21 , and in a second phase for applying the active layer  22  on the support  21 . By leak-proof is meant the absence of any detectable transfer of an aqueous solution from one side to the other of the active layer exposed to the solution, the membrane acting as a container of the solution. 
         [0033]    Elaboration of the Porous Substrate 
         [0034]    The support  21  is manufactured, in this embodiment of the invention, by selective laser sintering of thermoplastic polymer powder. 
         [0035]    Powders are used having a grain size comprised between 1 and 120 μm, but for which the essential portion of the grain size is centered between 10 and 80 μm, or even between 40 and 75 μm. The main tests were conducted with polyamide 6, polyamide 11 or polyamide 12. 
         [0036]    The selective laser sintering device  5  comprises a powder supply tank  52  in which is placed the powdered polymer, a roller  53  for providing and distributing powder, as well as a laser  54 . The laser is for example an infrared radiation laser with variable power (10 to 60 W). The laser beam is directed via a mirror  55  to the powder area which one wishes to sinter, under a preferably neutral atmosphere, for example under a nitrogen atmosphere. 
         [0037]    The method uses a manufacturing platform  56  heated to a temperature close to the melting temperature of the selected polymer. In a particular embodiment, the mixture of powders is heated up to a temperature lower than the melting temperature of the polymer by a few degrees Celsius, for example up to a temperature of less than the melting temperature of the relevant polymer by 1 to 10° C. The energy required for the melting is then provided by the laser  54 . 
         [0038]    The laser  54  traces the shape of the membrane, layer by layer and locally provides, at each successive stratum of powders, sufficient heat energy for bringing the polymer to a temperature causing its at least partial melting at the surface of the grains. The non-sintered powders naturally ensure support of the following layers. The manufacturing platform  56  is movable and moves down by the thickness of a layer (100 μm), the displacement of the vertical part being ensured by a piston. A new powder layer is then spread out by the roller  53  and the cycle starts again in order to build the porous support, layer by layer, from bottom to top. Instead of the roller  53 , it is also possible to use another mechanical system such as for example a doctor blade. 
         [0039]    By partly carrying out the melting of the powders, the grains agglomerate while remaining in grain form and leaving pores between them before again solidifying. The apparent density of the thereby produced samples, measured by the apparent specific gravity over the theoretical specific gravity of the material of the powders, is comprised between 40 and 80%. The corresponding porosity level is therefore comprised between 20 and 60%, the majority of the samples having a porosity level comprised between 30 and 50%. An immersion of the support in water for 15 min shows that the support  21  absorbs water and therefore that a portion of the pores are open. 
         [0040]    The thickness of the support may be selected according to mechanical stresses against which the membrane should resist, but samples with a thickness of 1 mm were produced successfully. 
         [0041]    The validation of the permeability of the support was achieved by means of pH paper and of an OH −  tracer of a soda solution at 0.05 mol/L i.e. 2 g/L in 50% by volume of water and 50% by volume of ethanol in the following way illustrated by the diagram of  FIG. 3 : The support is deposited on pH paper; some tracer is poured on the support; the changing color of the pH paper after a few seconds gave the possibility of confirming good porosity and permeability of the support since the tracer crossed the support. 
         [0042]    Elaboration of the Active Layer 
         [0043]    The second phase for manufacturing the membrane consists in physically coating one face of the porous support  21 . In the example which is shown, coating is achieved with a suspension of a Chevrel phase, of formula Mo 6 X 8 , with X being a chalcogen, in a volatile solvent. The active layer is prepared from powdered compounds Mo 6 S 8  or Mo 6 Se 8  which form the active mass. An addition of poly(vinylidene fluoride), designated as PVDF in the international nomenclature and hereafter plays the role of a binder. 
         [0044]    The phases Mo 6 S 8  and Mo 6 Se 8  are derived from a ceramic synthesis as for example described in document WO 2012/010766, on pages 10 and 11 of the description. 
       Elaboration of the Active Layer by Coating 
       [0045]    Case of the Chevrel Phase Matrix 
         [0046]    A suspension consisting of 95% of Chevrel phases as a powder and 5% of PVDF is formed in 1-methyl-2-pyrrolidone, called NMP subsequently, in an amount of 0.95 g of the solid Mo 6 S 8  or Mo 6 Se 8  phase, 0.05 g of PVDF dispersed in 2 ml of NMP. The whole is subject to stirring for 2 hours. 
         [0047]    The treating of the support is carried out by applying the suspension of Mo 6 S 8  or Mo 6 Se 8  NMP-PVDF on the surface of the porous support with a brush in order to cover in a more uniform way the whole surface. Next, the whole is put into the oven for 1 h in order to remove the NMP solvent. Under these conditions, the resulting film of Mo 6 S 8  and Mo 6 Se 8  adheres to the surface of the support  21  with thicknesses of the order of 80 μm. Further, the sealing tests are conducted according to  FIG. 4 , by applying a border  6  to the sample  2  around the active layer  22  and by pouring a solution of soda at 2 g/L into water/ethanol in the thereby formed reservoir. The transfer membrane  2  is made on pH paper, the active layer  22  being partly above. The tests confirm proper occlusion of the pores of the porous support, because the pH paper does not change color, even after 6 hours. Electrical conductivity tests show good electric contact between the grains. 
         [0048]    The different tests have shown that the sealing is attained when the thickness of the active layer is of the order of 80 μm. 
         [0049]    Case of the Oxide Matrix of the LiN y O z  Type 
         [0050]    According to another example, the membrane is manufactured with as active material, a Li x CoO 2 , LiMn 2 O 4 , LiV 3 O 8 , LiNiO 2 , LiWO 3 , or LiMnO 2  matrix generically designated by the formula Li x M y O z . The elaboration principle remains coating of the porous support with a suspension of Li x M y O z . 
         [0051]    The coating solution is prepared from a powdered mixture Li x M y O z  which forms the active material at 80% by mass, from 10% PVDF which plays the role of a binder and from 10% carbon which ensures electric conductivity. The mixture is intimately homogenized in a mortar. 
         [0052]    A suspension is produced in 1-methyl-2-pyrrolidone with stirring for 2 hours in an amount of 1 g of powder mixture for 2 ml of NMP. 
         [0053]    The coating of the support is carried out by applying over the whole surface the Li x M y O z /PVDF/graphite suspension in NMP by means of a brush for covering in the most uniform way the whole surface. Next, the whole is put into the oven for 1 h in order to remove the NMP solvent. Under these conditions, the resulting oxide film adheres to the surface of the support with thicknesses of the order of 80 μm. Further, the sealing tests confirm proper occlusion of the pores of the porous support. Electric conductivity tests show good electric behavior of the film. 
         [0054]    Regardless of the type of matrix, in order to track the interface potentials, it is necessary to set into place an electric contact around the membrane by means of a graphite lacquer  23 . The contour of the membrane is painted and paint overflows onto the face of the active layer  22 . 
         [0055]    Selective Transfer Process 
         [0056]    The diagram of  FIG. 5  shows a device for applying a selective transfer process using transfer membranes according to the invention. The device includes a tank  1  including two compartments  11  and  12 , adapted for receiving an electrolyte and separated by a separation partition  13  in which is placed a transfer membrane  2 , sealably mounted in the partition  13 . 
         [0057]    The device also includes an anode A 1  placed in the first compartment  11  and a cathode C 2  placed in the second compartment  12 . A potential difference ΔE may be applied between the anode A 1  and the cathode C 2  by means known per se, in order to impose and control a current i between the electrolytes E 1  and E 2 . The anode A 1  is for example an electrode in platinum-coated titanium and the cathode C 2  is for example a metal electrode (in stainless steel, aluminium, copper or platinum-coated titanium) or in glassy carbon. 
         [0058]    The active layer  22  is placed on the side of the first compartment  11 , even if the system also operates when it is on the side of the second compartment  12 . A movable contact system with a spring  44  ensures electric connection with the contour of the membrane  2  covered with graphite lacquer, and allows the latter to be connected to a control device, notably adapted for measuring the interface potential Ei 1 , Ei 2  of the transfer membrane  2  relatively to reference electrodes  33 ,  34  respectively positioned in each compartment  11 ,  12  of the tank  1 , as illustrated in  FIG. 5 . 
         [0059]    The application of the device is typically carried out in the following way: 
         [0060]    The compartments  11  and  12  are filled with the desired electrolyte, for example and in a by no means limiting way, with a solution of mixtures of cations M (i) ) in a concentration at least equal to 10 −3  mol/L as a first electrolyte E 1  in the first compartment  11 , and with 100 ml of 0.5 M Na 2 SO 4  as a second electrolyte E 2  in the second compartment  12 , with M (i) ) being one or several metal cations which one wishes to separate and respectively designated by M n+ , M′ n+ , M″ n′+ . The anode A 1  is placed in the first compartment  11  and the cathode C 2  in the second compartment  12 , and the contact  44  of the membrane is connected with potentiometric control means, connected to the reference electrodes  33 ,  34  immersed in the electrolytes E 1  and E 2 . It is thus possible to control the interface potentials and accordingly adjust the intensity between the anode A 1  and the cathode C 2 , so as to obtain a current density referred to the operational surface area of the transfer membrane  2 , or of the whole of the transfer membranes positioned in parallel, for example comprised between 2 and 200 A/m 2 . 
         [0061]    Overall intensio-static conditions are established between the anode A 1  and the cathode C 2 . Let us designate by HL, for host lattice, the material of the active layer  22 . In the overall electrochemical operation of the whole of the two compartments, the electrolyte E 1  being an initial solution to be treated including a mixture of cations of different metals and of identical or different charges, M n+ , M′ n+ , M″ n′+  for example and the electrolyte E 2  being a solution for adding value to the metal M, there occurs:
       insertion of the cation M n+  at the interface of the active layer  22  with the electrolyte E 1 , according to:       
 
         [0000]      HL+ x M n+   +xne   − =&gt;M X HL       de-insertion of this same cation at the interface of the active layer  22  with the electrolyte E 2  (solution for adding value to M n+  for example), which is carried out reciprocally according to:         
         [0000]      M x HL=&gt;HL+ xne   −   +x M n+   
         [0064]    The mobility of the metal cation in the host lattice thus allows transfer of the de-solvated cation M n+  from one medium to the other without any transfer of any other chemical species from either one of the compartments. 
         [0065]    It will further be noted generally that the electrolytes placed in both compartments  11 ,  12  including the anode A 1  and the cathode C 2 , may be different, notably by the nature of the background salts, by the acidity level, by the presence of complexing agents, by the nature of the solvents, notably non-aqueous organic or mineral solvents (DMSO, DMF, ionic liquids, solid electrolytes, etc). Thus for example, it is possible to carry out ion transfer from a sulfate medium to a chloride medium without diffusion of said medium. 
       Example 1 
       [0066]    The support is made in polyamide 12, in several samples for which the porosity is comprised between 30 and 50% and the thickness is 0.9±0.1 mm. The active layer is based on LiMn 2 O 4  and has a thickness of the order of 80 μm. 
         [0067]    The first electrolyte E 1  is a 0.5 mol/L aqueous solution of lithium sulfate, i.e. 7 g/L of lithium and 0.5 mol/L of sodium sulfate, i.e. 11.5 g/L of sodium. 
         [0068]    The second electrolyte is a 0.05 mol/L sodium sulfate solution, i.e. 7.8 g/L of potassium. 
         [0069]    A stable current with an intensity of 110 A/m 2  is established. Thus 28 g·h −1 ·m −2  of lithium are thus selectively transferred from the first to the second electrolyte with a Faradic yield of 94%. 
       Example 2 
       [0070]    The support is made in polyamide 12, in several samples for which the porosity is comprised between 30 and 50%. The active layer is based on Mo 6 S 8  and has a thickness of the order of 80 μm. 
         [0071]    The first electrolyte E 1  is a 0.1 mol/L aqueous cobalt sulfate solution at, i.e. 5.9 g/L of cobalt and of 0.1 mol/L nickel sulfate, i.e. 5.8 g/L of nickel. 
         [0072]    The second electrolyte is a 0.05 mol/L aqueous sodium sulfate solution, i.e. 2.3 g/L of sodium. 
         [0073]    A stable current with an intensity of 70 A/m 2  is established. Thus 154 g·h −1 ·m −2  of cobalt are transferred from the first to the second electrolyte with a Faradic yield of 98%. The nickel is not transferred. 
       Example 3 
       [0074]    The support is made in polyamide 12, in several samples, for which the porosity is comprised between 30 and 50%. The active layer is based on Chevrel phases Mo 6 Se 8  and with a thickness of the order of 80 μm. 
         [0075]    The first electrolyte E 1  is an aqueous cadmium sulfate solution at 1 mol/L, i.e. 
         [0000]    112 g/L of cadmium and of nickel sulfate at 1 mol/L, i.e. 59 g/L of nickel. 
         [0076]    The second electrolyte is a 0.05 mol/L solution of sodium sulfate, i.e. 2.3 g/L of sodium. 
         [0077]    A stable current with an intensity of 70 A/m 2  is established. Thus 293 g·h −1 ·m −2  of cadmium are selectively transferred from the first to the second electrolyte with a Faradic yield of 99%. The nickel is not transferred.