Patent Application
20130126354
The invention relates to an electrolyte separation wall for the selective transfer of cations through the wall, a manufacturing process for the said wall and a selective cation transfer process through the said wall. The invention also relates to an electrolytic type process ensuring transfer of cations, through an adapted wall, from a first electrolyte solution containing one or more categories of ions of same charge or of different charges to a second electrolytic solution.
A process of this type is already known using as separation wall a wall formed of chalcogenides with molybdenum clusters, notably the Mo6X8 phases called Chevrel phases described in the international patent application WO 2009/007598. This process is also the subject of the following publications:
Chevrel compounds for cationic transfer—Principle and application on Co2+ ion ; S. Seghir, C. Boulanger, S. Diliberto, J. M. Lecuire, M. Potel, O. Merdrignac-Conanec; dans Electrochemistry Communications, 10, 2008, 1505-1508.
These documents describe that cations can be transported through the wall made of a material with formula Mo6X8 (where X=S, Se, Te) called Chevrel phases where reversible oxidation-reduction systems of following type occur:
Mo6X8+xMn++xne−MxMo6X8
x being a number varying typically between 0 and 4.
These systems are diversified by the nature of the cation Mn+, of the chalcogene X and of the stoichiometry x of the ternary.
In an experimental setup using the selective transfer process, the transfer wall is placed between two compartments including respectively a platinum-coated titanium electrode which operates as anode and a stainless steel electrode which operates as cathode. The first compartment contains a first electrolyte which contains various cations of an effluent to be treated. The second compartment contains a second electrolyte intended to receive the selected cations.
A direct electrical current is established between the anode and the cathode. In the global electrochemical operation of the set of two compartments, intercalation of the cation occurs at the MxMo6S8/first electrolyte interface (effluent to be treated, mix of cations Mn+, M′n+, M″n+ different from each other for example), according to:
Mo6X8+xMn++xne−MxMo6X8
The deintercalation of this same cation Mn+ at the MxMo6S8/second electrolyte interface (Mn+ valorisation solution for example) is done conversely according to:
MxMo6X8Mo6X8+xne−+xMn+
The mobility of the metallic cation in the Chevrel phase thus allows the transfer of the disolvated cation Mn+ from one medium to the other without the transfer of any other chemical species from one or the other of the compartments.
A transfer wall in disc form is obtained by hot sintering of a composition powder mix adapted to the stoichiometry of the material required. Thus, discs of active material with a thickness of 2 to 5 millimetres are obtained.
Tests with walls consisting of selenium and sulphur phases have shown that in particular the cations of the following metals can be transferred from one electrolyte to the other: iron, manganese, cobalt, nickel chrome, copper, zinc, cadmium. The current density limits obtained are between 10 and 20 A/m2, with Faradaic yields higher than 90%, or even higher than 98%, and a very good selectivity.
The tests also showed that the transfer rate limit increased with the reduction in the thickness of the wall. However, the required mechanical strength of the wall limits the reduction of its thickness.
Moreover, lithium transfer could not be obtained with such walls. Lithium is, however, increasingly demanded in industry, especially for electric car batteries.
The aim of the invention is therefore to provide a selective transfer wall allowing a good transfer rate and with an enlarged choice of transferable cations.
With these targets in mind, the subject of the invention is an electrolyte separation wall including a sealed active layer made of a material capable of developing intercalation and deintercalation reactions for the selective transfer of cations through the wall, characterised in that it includes a support layer consisting of a porous material acting as support for the active layer.
The inventors have succeeded in making a wall with a porous support which provides the mechanical strength and an active layer the thickness of which can be very low. They have observed that the porous support does not obstruct the electrochemical reactions which occur at the level of the active layer. By reducing the thickness of the active layer, the transfer rate achieved is well above the rate limit according to prior art which is one of the targets of the invention.
The porous material is chosen, for example, among mullite, silica, glass fibre, quartz or a ceramic. These materials offer the qualities required to fulfil the role of the wall, that is, the mechanical strength, resistance to the products contained in the electrolytes and porosity.
The porosity of the porous material is for example between 0.4 and 0.6. This value expresses the ratio of material in relation to the volume taken up. It comprises a good trade-off between the volume of the electrolyte present in the porous support and the mechanical strength of the said support.
Particularly, the material of the active layer is a binary or ternary material behaving as a host network with cation reversible accommodation properties according to an oxidation-reduction reaction. The inventors observed that the Chevrel phases are not the only materials which can develop intercalation/deintercalation reactions to form a selective transfer wall but, more generally, host networks which are stable and where oxidation-reduction reactions occur.
The material of the active layer is for example a metallic chalcogenide.
Particularly, the metallic chalcogenide is a chalcogenide with molybdenum clusters (MonXn+2 or MxMonXn+2), X being a chalcogene taken among S (Sulphur), Se (Selenium) and Te (Tellurium), and M being a metal. The number n is chosen for example among 1, 1.5, 2, 3, 4, 5, 6 and 9.
According to another composition, the material of the active layer is a compound of lithium and a metal in oxide form, phosphate or fluoride or a combination of these forms, the metal being chosen among nickel, cobalt, iron, manganese, vanadium, titanium and chrome. It can be seen that these materials are capable of developing intercalation and deintercalation reactions and of transferring cations selectively, especially lithium.
According to a transfer wall manufacturing process, a solution is prepared including an active material in powder form, a binder and a solvent, then the surface of a support layer made of a porous material is coated with the said solution and the solvent is evaporated to form a sealed active layer on the support layer.
It can be seen that the active layer obtained is sealed which guarantees that the electrolytes do not mix when the wall separates them. Also, in spite of the initial powder formulation, the active layer is electrically conductive which shows that the grains are in contact with each other and allow the oxidation-reduction reactions to develop over the complete surface area of the active layer. The layer obtained is very fine in compliance with the target fixed at the start.
The binder is for example poly(vinylidene fluoride).
The solvent is for example 1-methyl-2-pyrrolidone.
The result is already satisfactory when the material in powder form is present in a proportion of 80% in weight excluding the solvent.
The material in powder form has, for example, a grain size between 30 and 100 μm.
According to an improvement, the solution includes, among other things, graphite in powder form. This allows the electrical conductivity of the active layer to be completed.
According to an improvement, the active layer is polished until the support layer appears through the active layer. This thus reduces the thickness of the active layer.
In spite of this reduction, the sealing is preserved and the yield of the wall is not affected. An increase in the limit value of the electrical current density is observed.
The subject of the invention is also a selective cation extraction process by electrochemical transfer characterised in that it uses as electrolyte separation wall a transfer wall as described above and transfer of the cations is ensured through the said transfer wall by generating a potential difference between, on the one hand, the first electrolyte and, on the other hand, the second electrolyte or the said transfer wall to induce an intercalation of the cations in the transfer wall on the side of the first electrolyte, a diffusion of the cations in said wall then their deintercalation in the second electrolyte.
According to other characteristics:
at least one of the electrolytes is non-aqueous. The electrolytes can be different between the compartments, notably by differentiation of the nature of the background salts, by their levels of acidity, by the presence of complexants, by the nature of the solvents notably mineral or organic non-aqueous solvents such as, for example, DMSO, DMF, ionic liquids, solid electrolytes, etc.
the transfer wall is electrically connected to a device measuring the potential between the said wall and reference electrodes located respectively in each electrolyte and the potential applied between the said electrolytes is adjusted to suit.
the potential difference is generated between the first electrolyte and the transfer wall and the deintercalation of the cations on the side of the second electrolyte is a chemical deintercalation by a chemical oxidising agent in the second electrolyte.
a succession of cation transfers is ensured through transfer walls arranged successively between end electrolytes and with one or more intermediary electrolytes between the various transfer walls.
the transferred metal is electrodeposited on the cathode.
at least two transfer walls of different natures separate the first compartment from respective compartments in parallel for selective transfers of different cations with specific modulated intercalation electrolyses on each of the transfer walls engaged. The transfers to separate compartments allow the specific simultaneous recovery of each of the metals, for example, for a source solution containing Lithium and Cobalt ions with use of an active layer made of Mo6S8 for the transfer of the cobalt and a second layer made of LiMn2O4 selective transfer of lithium.
The invention will be better understood and other features and advantages of the invention will become apparent on reading the following description, the description making reference to the appended drawings where:
A transfer wall in disc form 2 in compliance with the invention is formed of a porous support 21 on which a fine active layer 22 is deposited. The manufacture of sealed discs is done, in a first phase, by the manufacture of the porous support 21 and, in a second phase, by the application of the active layer 22 to the support 21.
The porous support 21 is commercially available in mullite, quartz or ceramic. As an example, an embodiment is detailed below, which is taken from the protocol given by the article by Garcia-Gabaldon et coll. on the manufacture of ceramic membranes based on kaolin and alumina developing a modulable porosity for their applications as separation membranes in electrochemistry: Effect of porosity on the effective electrical conductivity of different ceramic membranes used as separators in electrochemical reactors, Journal of Membranes Sciences 280 (2006) 536-544.
The protocol is as follows: firstly, a mix intended for 5 g of material consists of:
2.52 g of Kaolin (hydrated aluminium silicate) Al2Si2O5(OH)4,
1 g of off-the-shelf potato startch.
The powder mix is homogenised in a porcelain mortar then wet with a minimum volume of acetone to prevent aggregates from forming. This mix is dried in free air for 14 h. The powder obtained is then reground manually in the mortar for 10 minutes then by fractions of around 1 g, it is formed into discs by pressing in a 25 mm diameter die under a pressure of 2 tonnes for 5 minutes. The compacted discs are 1 mm thick. The samples are submitted to two successive heat treatment operations.
A first heating operation to 300° C. allows the oxidation in air of the potato starch. This organic binder is eliminated in 1 hour and thus creates the porosity. An additional treatment at 1100° C. for 8 to 24 hours ensures a satisfactory mechanical strength. After this heat treatment, two discs are obtained with a diameter of 24 mm and a thickness of 1 mm. The surface area is 4.5 cm2.
An analysis by X-ray diffraction has been done on the porous disc (
The evaluation of the porosity of the disc was tested using pH paper and nitric acid HNO3 in the following manner shown on the diagram of
The second phase in the manufacture of the disc consists in the physical coating of one face of the porous support 21. In the example shown, the coating is done with a Chevrel phase suspension, formula Mo6X8, where X is a chalcogene, in a volatile solvent. The working electrode is prepared from pulverulent compounds Mo6S8 or Mo6Se8 which comprise the active mass. Added poly(vinylidene fluoride), also called PVDF, plays the role of a binder.
The Mo6Se8 phase is obtained from a ceramic synthesis from the Moo +2MoSe2 mix homogenised and compressed cold into cylinders at a pressure of 250 MPa, done in sealed molybdenum crucible in arc furnace, under partial argon pressure, then heated for 50 h at 1300° C. The same grinding 50 μm screening treatment is also applied to this compound.
The purity of the synthetic powders is checked by their X-ray diffraction diagrams obtained on a diffractometer.
The synthesis of the sulphur-based Chevrel phase is done by means of a ternary phase with an intermediate metal such as, for example, Cu3Mo6S8. The synthesis of this ternary compound is done in sealed silica tubes in a vacuum at 1000° C. for 50 h. The initial mix is comprised of micrometric powders of Cu, MoS2 and Mo homogenised in a ball grinder for 30 minutes and compressed cold under a pressure of 250 MPa.
The molybdenum powder is deoxidised under hydrogen current at 1000° C. for 3 h and the MoS2 powder is prepared in sealed silica tubes by gradually heating the stoichiometric mix of the elements up to 800° C.
The grain size of the pulverulent products engaged is within a range of 30 to 100 micrometres.
The Mo6Se8 phase is obtained from a ceramic synthesis from the Mo+2MoSe2 mix homogenised and compressed cold into cylinders at a pressure of 250 MPa, done in sealed molybdenum crucible in the arc furnace, under partial argon pressure, then heated for 50 h at 1300° C. The purity of the reaction products obtained is checked by their X-ray diffraction diagrams obtained on a diffractometer.
Application of the Active Layer to Porous Support by Coating
A suspension consisting of 95% Chevrel phases in powder form and 5% PVDF is formed in the 1-methyl-2-pyrrolidone, called NMP below, with 0.1 g of the solid Mo6X8 phase, 0.005 g of PVDF dispersed in 1 ml of NMP. The whole is stirred for 2 hours.
Using a Pasteur pipette, several drops of the Mo6X8 NMP-PVDF suspension is placed on the surface of the porous support disc to cover as uniformly as possible the complete surface area. Then, the whole is placed in the oven for 1 h to eliminate the NMP solvent. Under these conditions, the resulting film of Mo6S8 or Mo6Se8 adheres to the surface of the disc with thicknesses of around 80 μm. Also, the sealing tests in compliance with
Coating techniques using the spin-coating principle have also been used. They give coats of the same configuration as previously.
For the synthesis of the Mo6S8 binary phase, copper chemical deintercalation is done by electrochemical means after the coating has been applied.
According to another example, the wall is manufactured with as active material a matrix meeting the general LiXMYOZ formula where y and z are integers, for example, including but not limited to LixCoO2, LiMn2O4, LiV3O8, LiNiO2 or LiMnO2. The active material can also include a mix of metals M. The manufacturing principle remains a coating of the porous support by an LiXMYOZ suspension.
The coating solution is prepared from a pulverulent mix LiXMYOZ which comprises 80% in weight of the active material, 10% PVDF which plays the role of the binder and 10% carbon which ensures the electrical conductivity. The mix is thoroughly homogenised in a mortar.
A suspension is made in the 1-methyl-2-pyrrolidone with stirring for 2 hours with 0.2 grams of powder mix for 1 ml of NMP.
With the Pasteur pipette, several drops of the LixMyOz NMP-PVDF solution are placed on the surface of the porous support disc to cover as uniformly as possible the complete surface area. This operation can also be done using a spin-coating technique. Then, the whole is placed in the oven for 1 h to eliminate the NMP solvent. Under these conditions, the resulting oxide film adheres to the surface of the disc with thicknesses of around 80 μm. Also, the sealing tests confirm the correct occlusion of the pores of the porous disc. Electrical conductance tests demonstrate good electrical behaviour of the film.
Whatever the type of matrix, an electrical contact must be placed around the disc using graphite lacquer 23 to follow the interface potentials. The contour of the disc is daubed with the lacquer overlapping onto the face of the active layer 22.
The diagram on
The device also includes an anode A1 placed in the first compartment 11 and a cathode C2 placed in the second compartment 12. A potential difference AE can be applied between the anode Al and the cathode C2 by means known themselves to impose and check a current i between the electrolytes E1 and E2.
The active layer 22 is placed on the side of the first compartment 11, even if the system also operates when it is placed on the side of the second compartment 12. A spring-mounted mobile contact system 44 ensures an electrical connection with the contour of the disc 2 covered by graphite lacquer and allows the disc to be connected to a control device, adapted notably to measure the interface potential Ei1, Ei2 of the disc in relation to the reference electrodes 33, 34 placed respectively in each compartment 11, 12 of tank 1, as shown on
Typically, the device is used as follows:
The compartments 11 and 12 are filled with the required electrolyte, for example, and in an in no way limitative manner, 100 ml of 0.5 M Na2SO4+M(i)SO4 as first electrolyte E1 in the first compartment 11, and 100 ml of 0.5 M Na2SO4 as second electrolyte E2 in the second compartment 12, with M(i) being one or more metallic cations that are to be separated. The anode A1 is placed in the first compartment 11 and the cathode C2 in the second compartment 12, and the contact 44 of the disc is connected with the potentiometric control means, connected to the reference electrodes 33, 34 immersed in the electrolytes E1 and E2. The interface potentials can thus be checked and adjusted to suit the global potential ΔE applied between the anode A1 and the cathode C2, to obtain a current density related to the operational surface area of the transfer wall 2, or of all of the transfer walls arranged in parallel included, for example, between 2 and 200 A/m2.
A global intensiostatic state is established between the anode A1 and the cathode C2. Let us call RH, for host network, the material of the active layer 22. In the global electrochemical operation of the two compartments, the electrolyte E1 being an original solution to be treated including a mix of cations of different metals and of identical or different charges, Mn+, M′n+, M″n′+ for example, and the electrolyte E2 being a valorisation solution of metal M, the following occurs:
the intercalation of the cation Mn+ at the interface of the active layer 22 with the electrolyte E1, as follows:
RH+xMn++xne−MxRH
the deintercalation of this same cation at the interface of the active layer 22 with the electrolyte E2 (valorisation solution of Mn+ for example), which is done vice versa as follows:
MxRH+xne−+xMn+
The mobility of the metallic cation in the host network thus allows the transfer of the disolvated cation Mn+ from one medium to the other without transfer of any other chemical species from one or the other of the compartments.
Also note, in a general manner, that the electrolytes placed in the two compartments 11, 12, including the anode A1 and the cathode C2, can be different, notably by the nature of the background salts, by the level of acidity, by the presence of complexants, by the nature of the solvents, notably organic or mineral non-aqueous solvents (DMSO, DMF, ionic liquids, solid electrolytes, etc.). It is thus possible, for example, to do an ionic transfer from a sulphate medium to a chloride medium without scattering of the said medium.
In the variant on
In the case of the variant on
In another variant, shown on
Example 1
A porous disc 21 covered by a sulphur Chevrel phase based active layer 22 is used as transfer wall 2, as described previously, in a setup in compliance with
It can be seen that the faradaic yields are interesting as they are above 90% and that the current densities are well above those of prior art (around 16 A/m2).
For the sulphur phase, the quantitative transfer is established at rates between 2.10−2 mol/h/m2 at low current density (3.2 A/m 2) and 3 mol/h/m2 at high current density (70 A/m2). The latter value appears to be a limit rate for Mo6S8 junction thicknesses estimated at 80 μm. Concerning the disc with active material Mo6Se8, the transfer rates in these selenium phases are located at the same order of magnitude as for the sulphur phase, that is 5.10−2 mol/h/m2 for 3.2 A/m2 and 4 mol/h/m2 for 70 A/m2 only for cations Cd2+, Zn2+, Mn2+, Cu2+, and In3+. Tables 3 and 4 give the transfer rates for a current density of 70 A/m2 for each element.
In the cation selective transfer process, the first electrolyte E1 contains a mix of cations only one type of which is transferred through the wall. The selectivity is induced by the fact that during the electrolysis operation, the voltage applied between the two faces of the active layer 22 allows the intercalation and the deintercalation of only one type of cations. To transfer the other cations, a higher potential must be applied, which is not the aim of the process. The type of cation which is transferred has a minimum intercalation potential and a maximum deintercalation potential which are expressed in relation to the reference potential given by a saturated calomel electrode (SCE).
Transfer experimentations have been done for synthetic mixes of cations such as: Co/Ni, Cd/Zn, Cd/Ni, Zn/Mn, Cd/Co, Co/Fe, Ni/Fe and Cd/Co/Ni.
In the following examples, done from two equimolar cation mixtures (0.1 M), the transfer selectivity is expressed by a transfer selectivity rate of cation Mn+ represented by the ratio Mtn+/ΣMitn+ of the quantity of cations transferred Mtn+ for the species considered with the sum of cations transferred of all species Mitn+ in the compartment 2, for example Cot/(Cot+Nit) for the Co2+ +Ni2+ mix.
This ratio therefore approaches 100% as selectivity increases and takes a value of 50% if no selectivity develops.
Tables 5 and 6 give a summary of the selectivity rates obtained for the various mixes with different current densities. The values given for the different current densities correspond to the average of the selectivity rates obtained at each hour during the electrolysis of 1 to 7 hours.
It can be seen that the selectivity rate depends on the current density. The selectivity is high for high current densities thus inducing high transfer rates. The nature of the active layer 22 of the wall plays an important role in the selective transfer of cations. The transfer selectivity of Cd2+ or of Zn2+ in the presence of Ni2+ is truly improved up to 99% using a selenium matrix. The same observation can be made for the case of Cd2+ in the Cd/Co mix. The selectivity is not affected by the low thickness of the active layer 22.
In this example, the active layer 22 is made with material LiCoO2 with a thickness of around 80 μm. Such a material is used for example in the positive electrode of lithium ion batteries. The first electrolyte is a 1 M aqueous solution of Li+ in 1M Na2SO4 medium. The second electrolyte which acts as valorisation solution is a 1M aqueous solution of Na2SO4. The results are given in table 7.
This example is similar to example 2, except that the second electrolyte which acts as valorisation solution is a solution of a propylene carbonate solvent and perchlorate ammonium tetrabutyl. The anode is made of platinum-coated titanium and the cathode is made of stainless steel. The results are given in table 8.
From a film produced by depositing Mo6S8-PVDF on a porous disc according to the protocol described above, the excess material of the film is eliminated by manual polishing, by abrasion with SiC disc with a grain size of 2400 for several seconds until the colouring of the porous support appears. Thus, the part of the transfer materials is left only in the pores. This operation done and tested in the same manner as the previous experimentations offers clearly more advantageous transfer performances with an applicable current density of around 80 A/m2, higher than previously, without disturbing the faradaic yields.
In this example, the active layer is made with material LiMn2O4 with a thickness of around 80 μm. The first electrolyte is an aqueous solution containing cations Li+ (1M) and Co2+ (0.5M) in sulphate medium. The second electrolyte which acts as valorisation solution is an Na2SO4 solution with a concentration of 1M. The results are given in table 9.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1056066 | Jul 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2011/051602 | 7/6/2011 | WO | 00 | 1/22/2013 |
