Method for recycling aqueous electrolyte based on quinone compounds of a redox flow battery

Abstract

The present invention relates to a method for recycling an aqueous electrolyte of a redox flow battery to be recycled, the aqueous electrolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound being at least one oxidized or reduced form of a reducing / oxidizing couple, the oxidized form of which is a compound comprising a quinone unit, for example a benzoquinone unit, a naphthoquinone unit or an anthraquinone unit, preferably an anthraquinone unit, characterized in that it comprises a step of precipitating (300) the electroactive compound.

Description

[0001] Process for recycling aqueous electrolyte based on quinone compounds from redox flow battery

[0002] The present invention relates to a method for recycling aqueous electrolyte from a redox flow battery.

[0003] State of the art

[0004] A redox flow battery is a system that uses liquids (called electrolytes) to store energy. Redox flow batteries store electricity and generate it through a redox reaction. They typically have two compartments separated by an ion exchange membrane, in which current collectors (electrodes) are usually immersed.

[0005] One of the problems with current battery storage technologies in general is their use of minerals and metals, the extraction of which has a massive impact on the environment. In addition, their complex design and use of composite materials prevents simple, economical, and efficient recycling of critical materials. While they claim to solve the environmental impact of energy production through renewable energy storage (and therefore reduce CO2 emissions / kWh of electricity produced), these technologies have a very mixed life cycle analysis. They lead to resource depletion and significant pollution due to the waste generated by end-of-life batteries.

[0006] For existing technologies, recycling methods have been published in the literature, such as:

[0007] Li-ion battery (EP1269554B1): method for recycling and separating critical materials. Such a method is complex to implement and expensive;

[0008] Lead-acid battery (CA2986001A1): electrochemical process for recovering lead for closed-loop reuse;

[0009] Vanadium redox flow battery: recycling methods have already been proposed by mixing the two electrolytes, and must be operated continuously during cycling to counteract the crossover of vanadium ions through the membrane (Zhang, Y., Liu, L., Xi, J., Wu, Z., & Qiu, X. (2017). The benefits and limitations of electrolyte mixing in vanadium flow batteries. Applied Energy, 204, 373-381 ); Zn-Br redox flow battery: recovery process by neutralization of bromine (CN103236570B).

[0010] Current recycling solutions do not exist for redox flow batteries which use redox couples based on organic or organometallic compounds, particularly organic compounds, solubilized in an aqueous medium.

[0011] KEMIWATT uses electrolytes based on organic and organometallic compounds, dissolved in an aqueous medium, to limit the impact of this technology on the environment and the depletion of resources (use of critical metals / rare earths). However, to date, there is no recycling solution for such batteries.

[0012] Aims of the invention

[0013] The present invention aims to solve the technical problem of providing a recycling method for a redox flow battery using redox couples based on organic and / or organometallic compounds in aqueous solution.

[0014] The present invention aims in particular to solve the technical problem of providing a method for recycling aqueous electrolytes for a redox flow battery.

[0015] The present invention aims in particular to solve the technical problem of providing a simple method for treating aqueous electrolytes for end-of-life redox flow batteries in order to isolate the electroactive compound(s), purify them, and in particular use them as raw material for new electrolytes.

[0016] In particular, the present invention aims to solve the technical problems stated above by limiting the impact on the environment and the depletion of natural resources or by limiting the amount of new organic / organometallic compounds used in the electrolytes, in particular in the negolyte. Finally, the present invention aims to solve the technical problem of reducing the manufacturing costs of redox flow batteries.

[0017] Detailed description of the invention

[0018] The present invention makes it possible to solve one, and preferably all, of the technical problems posed by the present invention.

[0019] To strengthen the eco-compatibility and economic competitiveness of redox flow batteries using aqueous electrolytes comprising organic and / or organometallic compounds, the inventors have discovered and developed a process and a system for recycling the electroactive compounds of the electrolytes, in particular to reuse them in new redox flow batteries and thus create a circular economy around such a redox flow battery.

[0020] Advantageously, recycling according to the present invention comprises the isolation of the electroactive compound(s) included in the electrolyte at the end of the battery's life, in order to subsequently recover it / them, either directly by marketing it / them in another application, or preferably by reintroducing it / them into a new redox flow battery in the form of a new electrolyte. A redox flow battery can advantageously be recycled once it has lost at least 20% of its initial capacity.

[0021] Thus, the present invention makes it possible to limit the quantity of material newly introduced for the production of redox flow batteries and / or to limit the consumption of raw materials, natural or synthesized.

[0022] Thus, the present invention relates to a method for recycling an aqueous electrolyte from a redox flow battery to be recycled, the aqueous electrolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound being at least one oxidized or reduced form of an oxido / reduction couple whose oxidized form is a compound comprising a quinone unit, for example a benzoquinone unit, a naphthoquinone unit or an anthraquinone unit, preferably an anthraquinone unit, characterized in that it comprises a step of precipitation of the electroactive compound.

[0023] By electroactive compound is meant an organic or organometallic compound forming part of an oxidation-reduction couple, and designates indifferently either the oxidant (the oxidized form) of the oxidation-reduction couple, or the reducing agent (the reduced form) of the oxidation-reduction couple, or the mixture of the oxidant and the reducing agent of the oxidation-reduction couple.

[0024] Aqueous electrolyte means the aqueous solutions comprising the electroactive compound(s) and arranged in the positive and negative compartments of a redox flow battery.

[0025] The electrolyte in the positive compartment of the redox flow battery is called posolyte, and the electrolyte in the negative compartment of the redox flow battery is called negolyte.

[0026] Preferably, the aqueous electrolyte recycled by the process according to the invention is a negolyte.

[0027] Preferably, the method is characterized in that it successively comprises:

[0028] - a step of collecting an aqueous electrolyte from a redox flow battery comprising at least one electroactive compound,

[0029] - the step of precipitation of the electroactive compound, whereby a suspension is obtained, - a step of separation of the suspension, whereby a solid residue and an effluent are obtained, and

[0030] - optionally, a step of rinsing with water the solid residue obtained after the separation step, whereby a rinsed solid residue is obtained.

[0031] The water rinsing step may comprise trituration of the solid residue, and / or a second separation step to obtain a rinsed solid residue and a second effluent. The second separation step may be carried out at the same time as the rinsing and / or trituration of the solid residue.

[0032] The process may also optionally include a step of drying, possibly partial, the solid residue or the rinsed solid residue.

[0033] The steps of the method can be implemented by any technique known to those skilled in the art.

[0034] The collection step is preferably carried out by pumping the electrolyte(s) from the redox flow battery to be recycled to a container, preferably directly at the battery use site. According to one embodiment, the collection step further comprises a step of transferring the electrolyte(s) from the container to a reactor.

[0035] The collection step is preferably carried out after a complete discharge step of the redox flow battery. In other words, the electrolyte collected during the collection step is preferably a negolyte whose electroactive compound is in its oxidized form and / or a posolyte whose electroactive molecule is in its reduced form.

[0036] The aqueous electrolyte collected from the redox flow battery is a spent aqueous electrolyte since it has undergone at least one charge and / or discharge cycle. Preferably, the spent aqueous electrolyte is collected at the end of the battery life cycle.

[0037] The separation step is preferably carried out by filtration, for example using a centrifugal decanter.

[0038] The solid residue obtained at the end of the separation step includes the precipitated electroactive compound(s).

[0039] The water rinsing and water trituration step improves the purity of the solid residue, and in particular eliminates the precipitating agent used if it is weakly volatile. It is particularly suitable for the recycling of a negolyte.

[0040] The drying step is not mandatory, especially when the process includes a water rinsing step, because the solid residue can be formulated even if it includes residual rinsing water.

[0041] The process according to the invention is therefore preferably without a step of drying the solid residue, rinsed or not. The absence of a drying step, or the reduction of the drying time and / or temperature by implementing a partial drying step makes it possible to minimize the energy costs of the process.

[0042] The drying step, when present, can be carried out by heating and / or by placing the solid residue under reduced pressure.

[0043] Preferably, the method according to the invention is characterized in that the precipitation step comprises the addition of an anti-solvent of the electroactive compound and / or the addition of an acid or a base and / or the addition of a salt in the aqueous electrolyte.

[0044] Preferably, the step of adding an anti-solvent is carried out in a reactor vessel, with stirring.

[0045] Antisolvent means an organic solvent in which the electroactive compound is less soluble than in water.

[0046] Preferably, the anti-solvent is chosen for its ability to lower the solubility of the electroactive compound in the initial aqueous medium, preferably chosen from solvents in which the electroactive compound is 5 times less soluble than in water, more preferably 10 times less soluble, advantageously 100 times less soluble. In other words, the ratio between the solubility of the electroactive compound in water and the solubility of the electroactive compound in the anti-solvent is preferably greater than or equal to 5, more preferably greater than or equal to 10, advantageously greater than or equal to 100. The solubility of the electroactive compound in water or the anti-solvent is the maximum concentration, in g / mol at 25°C, at which the electroactive compound can dissolve in water or the anti-solvent, respectively, forming a homogeneous mixture, i.e. without the formation of a precipitate.

[0047] Preferably, the anti-solvent is an organic solvent, preferably chosen from the group of water-miscible aprotic and protic polar solvents, more preferably chosen from alcohols, preferably aliphatic alcohols, advantageously saturated aliphatic alcohols, such as methanol, ethanol, or 1-propanol and iso-propanol and organic solvents comprising a nitrile function, such as acetonitrile, or a ketone function, such as acetone, or any of their mixtures. The use of a mixture of at least two anti-solvents increases the amount of precipitated electroactive compound.

[0048] According to the invention, an acid is an acid in the Brônsted sense, that is to say a chemical species capable of giving up a proton H + . Preferably, the acid is characterized by a pKa strictly less than 7.

[0049] Preferably, the acid is a strong acid or a weak acid. A strong acid is an acid that reacts completely with water. A strong acid has a pKa of less than 0. The strong acid may be selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, hydroiodic acid, hydrobromic acid, perchloric acid, permanganic acid, manganic acid, chloric acid, phosphoric acid, or any mixture thereof. The weak acid may comprise at least one carboxylic acid function, such as formic acid, acetic acid, benzoic acid, citric acid, lactic acid, oxalic acid, or maleic acid. Preferably, the acid is a strong acid. The use of a strong acid increases the amount of precipitated electroactive compound. Preferably, the acid is sulfuric acid or acetic acid, advantageously sulfuric acid.

[0050] Preferably, the amount of acid added to the aqueous electrolyte corresponds to the amount of acid necessary to obtain a pH less than or equal to 10, preferably less than or equal to 8, preferably less than or equal to 7, more preferably less than or equal to 6. More preferably, the amount of acid added to the aqueous electrolyte corresponds to the amount of acid necessary to obtain a pH less than or equal to 10 and greater than or equal to 1, preferably less than or equal to 8 and greater than or equal to 2, more preferably less than or equal to 6 and greater than or equal to 3.

[0051] Preferably, the acid is added with stirring.

[0052] According to the invention, a base is a base in the Brônsted sense, that is to say a chemical species capable of capturing a proton H + . Preferably, the base is characterized by a pKa strictly greater than 7.

[0053] Preferably, the base is an inorganic base. The base may be selected from the group consisting of alkali hydroxides, such as NaOH or KOH, and alkali carbonates, such as NaCOs or K2CO3.

[0054] Preferably, the amount of base added to the aqueous electrolyte corresponds to the amount of base necessary to obtain a pH greater than or equal to 7, preferably greater than or equal to 8, preferably greater than or equal to 10. More preferably, the amount of base added to the aqueous electrolyte corresponds to the amount of base necessary to obtain a pH less than or equal to 14 and greater than or equal to 7, preferably less than or equal to 13 and greater than or equal to 10.

[0055] Preferably, the salt is an inorganic salt, preferably KCl or NaCl, or an organic salt, preferably sodium acetate or ammonium carbonate.

[0056] Preferably, the inorganic salt is chosen from inorganic salts whose cation corresponds to the cation or to one of the cations included in the aqueous electrolyte to be recycled.

[0057] The addition of an anti-solvent of the electroactive compound, the addition of an acid or a base and the addition of a salt in the aqueous electrolyte can be combined two by two or be implemented all together to optimize the precipitation of the electroactive compound, depending on its solubility.

[0058] Preferably, the precipitation step comprises the addition of an acid to the aqueous electrolyte, preferably until a pH of less than or equal to 10, preferably less than or equal to 8, preferably less than or equal to 7, more preferably less than or equal to 6, is obtained, or the addition of a base, preferably until a pH of greater than or equal to 7, preferably greater than or equal to 8, preferably greater than or equal to 10 is obtained.

[0059] In particular, precipitation was found to be instantaneous for quinones comprising one or more substituents of the -R type 1 and / or -AR 1 and / or -OA- R 1 , with R 1 = OH or COOH, preferably with R 1 COOH representative.

[0060] The choice of using an acid or a base therefore depends on the initial pH of the electrolyte to be recycled, with the aim of reaching the pH values ​​above at which the electroactive compound precipitates.

[0061] According to one embodiment, the precipitation step comprises the addition of a strong acid, preferably sulfuric acid, the volume of strong acid added representing between 0.1% and 40% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 15%, preferentially between 0.1% and 12%, depending on the initial pH of the solution and its composition.

[0062] According to another embodiment, the precipitation step comprises the addition of a weak acid, preferably acetic acid, the volume of weak acid added representing between 0.1% and 60% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 55%, preferentially between 0.1% and 50%.

[0063] According to yet another embodiment, the precipitation step comprises the addition of a base in the form of an aqueous solution of an alkali hydroxide whose alkali hydroxide concentration is between 1 and 25 moles per liter, preferably between 4 and 20 moles per liter, preferentially 4 and 8 moles per liter, the volume of base added representing between 0.1% and 40% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 30%, preferentially between 0.1% and 22%, depending on the initial pH of the solution and its composition.

[0064] The precipitation of electroactive compounds does not depend specifically on the concentration of electroactive compounds in the electrolyte. It is mainly the pH required to precipitate the electroactive compounds that guides the amount of acid to be added during the precipitation step.

[0065] Preferably, during the precipitation step, the aqueous electrolyte is at a temperature between 5°C and 40°C, preferably between 5°C and 20°C. Preferably, the electroactive compound of the aqueous electrolyte is at least one form of an oxido / reduction couple whose oxidized form is a compound comprising an anthraquinone unit. Preferably, the aqueous electrolyte is a negolyte.

[0066] A compound comprising an anthraquinone unit according to the invention is preferably a compound of formula (F):

[0067] [Chem 1] in which X 1 , X 2 , X 3 , X 4 , X 5 , X6 , X 7 and X 8 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms,

[0068] A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms, and / or one of its salts, in particular a sodium or potassium salt.

[0069] Preferably at least one of X 1 , X 2 , X 3 , X 4 , X 5 , X 6 , X 7 and X 8 comprises an OH, SO3H, COOH function, and / or any of their salts, in particular sodium or potassium salt.

[0070] Preferably, X 1 , X 2, X 3 , X 4 , X 5 , X 6 , X 7 and X 8 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group and an -OA-SO3H group.

[0071] Preferably five or six groupings among X 1 , X 2 , X 3 , X 4 , X 5 , X 6 , X 7 and X 8 are hydrogens. In particular, mention may be made of the compounds described in PCT application WO2021123334.

[0072] This application describes in particular compounds in oxidized form of formula (I):

[0073] [Chem 2] w and / or one of its salts, in particular a sodium or potassium salt.

[0074] In the structure of formula (I), X 1 , X 2 , X 4 , X 5 , X 6 , X7 and X 8 are independently selected from a hydrogen atom, a linear, cyclic or branched, saturated or unsaturated, optionally substituted C1-C10 hydrocarbon group, an OH group or an -O- AR group 1 ,

[0075] A represents a linear, cyclic or branched, saturated or unsaturated, optionally substituted C1-C10 hydrocarbon group;

[0076] R 1 represents COOH or SO3H; and in the structure of formula (I) one and only one of X 1 , X 2 , X 4 , X 5 , X 6 , X 7 and X 8 represents OH, and one and only one of X 1 , X 2 , X 4 , X 5 , X 6 , X 7 and X 8 represents -OAR 1 .

[0077] According to another embodiment, the electroactive compound of the aqueous electrolyte is at least one form of an oxido / reduction couple whose oxidized form is a compound comprising a naphthoquinone unit.

[0078] A molecule based on a naphthoquinone unit according to the invention is preferably a compound of formula (G):

[0079] [Chem 3] in which Z 1 , Z 2 , Z 3 , Z 4 , Z 5 and Z 6are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms, A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms, and / or one of its salts, in particular a sodium or potassium salt.

[0080] Preferably at least one of Z 1 , Z 2 , Z 3 , Z 4 , Z 5 and Z 6 comprises an OH, SO3H, COOH function, and / or any of their salts, in particular sodium or potassium salt.

[0081] Preferably, Z 1 , Z 2 , Z 3 , Z 4 , Z 5 and Z 6are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group and an -OA-SO3H group.

[0082] Preferably four or five groupings among Z 1 , Z 2 , Z 3 , Z 4 , Z 5 and Z 6 are hydrogens. Preferably, Z 2 = Z 3 = Z 4 = Z 5 = Z 6 = H.

[0083] According to another embodiment, the electroactive compound of the aqueous electrolyte is at least one form of an oxido / reduction couple whose oxidized form is a compound comprising a benzoquinone unit.

[0084] A molecule based on a benzoquinone unit according to the invention is preferably a compound of formula (H):

[0085] [Chem 4] or a compound of formula (K):

[0086] [Chem 5] in which Z 1 , Z 2 , Z 3 and Z 4 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms,

[0087] A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms, and / or one of its salts, in particular a sodium or potassium salt.

[0088] Preferably at least one of Z 1 , Z 2 , Z 3 and Z 4 comprises an OH, SO3H, COOH function, and / or any of their salts, in particular sodium or potassium salt, in particular an SO3H function.

[0089] Preferably, Z 1, Z 2 , Z 3 and Z 4 are independently selected from the group consisting of a hydrogen atom, an OH group, a methyl or ethyl group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group and an -OA-SO3H group.

[0090] Preferably, at least two of Z 1 , Z 2 , Z 3 and Z 4 are different from one hydrogen atom. Preferably, two or three of Z 1 , Z 2 , Z 3 and Z 4 are different from a hydrogen atom.

[0091] According to one embodiment, in the compound of formula (H), Z 4 = H. Preferably, Z 4 = H and Z 1 , Z 2 and Z 3 are independently selected from the group consisting of a methyl or ethyl group, a COOH group and an SO3H group. Advantageously, Z 4 = H, Z 1 = Z3 = Me and Z 2 = SO3H or COOH, preferably SO3H.

[0092] According to one embodiment, in the compound of formula (J), Z 1 = Z 3 = H. Preferably, Z 1 = Z 3 = H and Z 2 and Z 4 are independently selected from the group consisting of a methyl or ethyl group, a COOH group and an SO3H group. Advantageously, Z 1 = Z 3 = H, and Z 2 = Z 4 = SO3H or COOH, preferably SO3H.

[0093] The oxidized form of the redox / reduction couple included in the electrolyte can in particular be chosen from the following compounds:

[0094] AQDS(2,7): 9,10-anthraquinone-2,7-disulfonate disodium AQDS(1,5): 9,10-anthraquinone-1,5-disulfonate disodium

[0095] AQDS(1,8): 9,10-anthraquinone-1,8-dipotassium disulfonate

[0096] AQDS(2,6): 9,10-anthraquinone-2,6-disulfonic acid

[0097] AQS(2): 9,10-anthraquinone-2-sulfonate sodium

[0098] AQS(2)DH (Alizarin red s): 3,4-dihydroxy-9,10-anthraquinone-2-sulfonate sodium

[0099] AQS(2)NBr: 1 -amino-4-bromo-9,10-anthraquinone-2-sulfonate sodium

[0100] NQ(1 ,4)HB (Lapachol): 2-hydroxy-3-(3-methyl-2-butenyl)-1 ,4-naphthoquinone

[0101] NQ(1 ,4)H: 2-hydroxy-1 ,4-naphtoquinone

[0102] NQ(1 ,4)DHCI: 2,3-dichloro-5,8-dihydroxy-1 ,4-naphtoquinone

[0103] AQDH(2,6) (Anthraflavic acid): 2,6-dihydroxy-9,10-anthraquinone

[0104] AQDH(1 ,8) (Chrysazin): 1 ,8-dihydroxy-9,10-anthraquinone

[0105] AQDH(1 ,5) (Anthrarufin): 1 ,5-dihydroxy-9,10-anthraquinoneAQTH(1 ,2)

[0106] (Quinalizarin): 1 ,2,5,8-tetrahydroxy-9,10-anthraquinone

[0107] AQDH(1 ,2) (Alizarin): 1 ,2-dihydroxy-9,10-anthraquinone

[0108] AQDH(1 ,4) (Leucoquinizarin): 2,3-dihydro-9,10-dihydroxy-1 ,4-anthraquinone

[0109] AQDH(1 ,8)MH (Aloemodin): 1 ,8-dihydroxy-3-(hydroxymethyl)-9,10-anthraquinone

[0110] ARSNa: 1 ,2-dihydroxy-3-(sulfonate de sodium)-9,10-anthraquinone

[0111] ARSK: 1 ,2-dihydroxy-3-(sulfonate de potassium)-9,10-anthraquinone

[0112] AQTrHM (Emodin): 1 ,3, 8-trihydroxy-6-methylanthracene-9, 10-dione

[0113] AQTH(1 ,4): 1 ,4,5,8-tetrahydroxy-9,10-anthraquinone

[0114] AQDH(1 ,8)CA (Rhein): 4,5-dihydroxy-9,10-anthraquinone-2-carboxylic acid

[0115] HQ(1 ,4)S: 2,5-dihydroxybenzenesulfonate potassium

[0116] HQ(1 ,2): Ortho hydroquinone

[0117] HQ(1 ,4): Para hydroquinone

[0118] HQ(1 ,2)DS (Tiron): 4,5-dihydroxy-1 ,3-benzenedisulfonate disodium monohydrate

[0119] HQ(1 ,4)DH: 2,5-dihydroxy-1 ,4-benzoquinone

[0120] BQ(1,4)DHDCI (Chloranilic acid): 2, 5-dichloro-3, 6-1,4-benzoquinone

[0121] HQ(1,4)TCI: Tetrachloro hydroquinone

[0122] BQ(1,4)TH: Tetrahydroxy-1,4-benzoquinone

[0123] HQ(1,4)TF: 1,2,4,5-tetrafluoro-3,6-dihydroxybenzene.

[0124] (M3CH): compound of formula (F) with X 1 = OH, X 4 = -O-(CH2)3-COOH and X 2 = X 3 = X 5 = X 6 = X 7 = X 8 = H.

[0125] DBEAQ: compound of formula (F) with X 2 = X 6 = -O-(CH2)3-COOH and X 1 = X 3 = X 4 = X 5 = X 7 = X 8 = H.

[0126] BQDS: 1,2-dihydrobenzoquinone-3,5-disulfonic acid (compound of formula (K) with Z 1 = Z 3 = H, and Z 2 = Z 4 = SO3H). DHDMBS: 3,6-dihydroxy-2,4-dimethylbenzene sulfonic acid (compound of formula H with Z4 = H, Z 1 = Z 3 = Me and Z 2 = SO3H)

[0127] According to one embodiment, the method according to the invention is characterized in that it further comprises a step of formulating the solid residue comprising dissolving the solid residue in an aqueous medium to obtain a recycled electrolyte. The solid residue is optionally rinsed and / or dried.

[0128] This formulation step may further include the addition of other constituents to the recycled electrolyte, such as additives.

[0129] The choice of other constituents depends on the performance required for the recycled electrolyte.

[0130] The method according to the invention may further comprise a step of introducing the recycled electrolyte obtained in the formulation step (700) into the negative or positive compartment of a redox flow battery.

[0131] Preferably, the electrolyte is a negolyte, and the method according to the invention comprises a chemical oxidation step between the collection step and the precipitation step, comprising bringing the negolyte into contact with an oxidant capable of oxidizing the reduced form of the oxido / reducing couple.

[0132] This step is preferably implemented when the negolyte has been collected from a redox flow battery that has not been fully discharged prior to the collection step.

[0133] Thus, preferably, the electroactive compound to be precipitated is the oxidant of the oxido / reduction couple included in the negolyte.

[0134] By oxidant capable of oxidizing the reducing agent of the redox / reducing couple, we mean any compound belonging to an redox / reducing couple different from the redox / reducing couple included in the negolyte and whose standard redox potential is strictly greater than the standard redox potential of the redox / reducing couple included in the negolyte.

[0135] Preferably, the step of bringing the negolyte into contact with a reducing agent capable of reducing the oxidant of the redox / reducing agent pair is bringing the negolyte into contact with air, the oxygen of which spontaneously oxidizes the reducing agent of the redox / reducing agent pair of the negolyte. Alternatively, the oxidant is chosen from the group consisting of metal oxides, for example silver or chromium oxide, inorganic oxidants, for example ozone, diiodine, oxone (KHSO5), organic oxidants chosen from:

[0136] - peracids, for example metachloroperbenzoic acid (mCPBA) or magnesium monoperoxyphthalate (MMPP);

[0137] - peroxides, for example tert-butyl hydroperoxide (tBuOOH);

[0138] - N-oxides, e.g. N-methylmorpholine N-oxide (NMO) or (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO);

[0139] - pyridine-chromium couples, for example pyridinium dichromate (PDC) or pyridinium chlorochromate (PCC);

[0140] - 2-iodoxybenzoic acid (IBX) or the periodinane derivative of Dess Martin; and

[0141] - 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ).

[0142] Alternatively, any oxidized form of the electroactive compound of a posolyte can also be used as an oxidant for the negolyte reductant.

[0143] According to another embodiment, the electrolyte is a posolyte and the method according to the invention comprises a chemical reduction step between the collection step (200) and the precipitation step (300), comprising bringing the posolyte into contact with a reducing agent capable of reducing the oxidized form of the oxido / reducing couple.

[0144] This step is preferably implemented when the posolyte has been collected from a redox flow battery that has not been fully discharged prior to the collection step.

[0145] Thus, preferably, the electroactive molecule to be precipitated is the reducer of the oxido / reducing couple included in the posolyte.

[0146] By reducing agent capable of reducing the oxidized form of the redox / reducing couple, we mean any compound belonging to an redox / reducing couple different from the redox / reducing couple included in the posolyte and whose standard redox potential is lower than the standard redox potential of the redox / reducing couple included in the posolyte.

[0147] Preferably, the step of bringing the posolyte into contact with a reducing agent capable of reducing the oxidant of the oxido / reducing couple comprises the addition of a reducing agent to the aqueous posolyte. Preferably, the addition of a reducing agent to the aqueous posolyte is carried out by controlling the pH, which must preferably remain greater than or equal to 8.

[0148] Preferably, the reducing agent is selected from the group consisting of H2O2, Na2SOs, Na2S2C>4, Na2S2C>3, N2H4(hydrazine), l2(iodine) and organic reducing agents such as ascorbic acid, citric acid, and glucose derivatives. According to a variant, the method according to the invention further comprises a step of treating the effluent obtained at the end of the separation step to obtain a treated effluent. The treated effluent can be reused in the precipitation step.

[0149] The method according to the invention may further comprise a step of verifying the purity of the solid residue, for example by chemical and / or electrochemical analysis.

[0150] According to one embodiment of the method of the invention, the aqueous electrolyte to be recycled may comprise at least one additive. In this embodiment, depending on the solubility of the additive, the additive is either recycled with the electroactive compound, and is therefore included in the solid residue, or included in the effluent obtained at the end of the method.

[0151] An additive is any compound capable of increasing certain physicochemical properties of the electrolyte.

[0152] The invention also relates to a system for recycling an aqueous electrolyte of a redox flow battery comprising:

[0153] - a device (30) for collecting an aqueous electrolyte (10) from a redox flow battery (20), the aqueous electrolyte (10) comprising at least one electroactive compound and an aqueous solvent, and

[0154] - a device (40) for precipitating the electroactive compound by adding an anti-solvent of the electroactive compound and / or adding an acid or a base and / or adding a salt, providing a suspension.

[0155] The collection device preferably comprises a collection tank for the collected aqueous electrolyte and a device capable of transferring the electrolyte from the redox flow battery to the collection tank. The collection tank is fluidically connected to the negative or positive tank of the redox flow battery to be recycled. The aqueous electrolyte collected from the redox flow battery is a used aqueous electrolyte since it has undergone at least one charge and / or discharge cycle. Preferably, the used aqueous electrolyte is collected at the end of the battery life cycle.

[0156] The device according to the invention may further comprise a first storage tank comprising an anti-solvent of the electroactive compound of the collected aqueous electrolyte and / or an acid or a solution of a base and / or a solution of a salt as defined in the description of the method according to the invention, preferably a storage tank of an acid. The first storage tank is in fluid connection with the precipitation device. According to one embodiment, the recycling system according to the invention comprises a discharge device capable of oxidizing the reducing agent of the redox / reducing couple of the negolyte or capable of reducing the oxidant of the redox / reducing couple of the posolyte. The discharge device is preferably in fluid connection with a second storage tank comprising an oxidant capable of oxidizing the reducing agent of the redox / reducing couple or a reducing agent capable of reducing the oxidant of the redox / reducing couple as defined above.The discharge device is preferably fluidically connected to the collection tank and the precipitation device.

[0157] Preferably, the recycling system according to the invention further comprises a separation device for separating the suspension from the precipitation device into a solid residue and an effluent. For example, the separation device may be a centrifugal decanter.

[0158] The solid residue obtained in the separation device comprises the precipitated electroactive compound(s).

[0159] According to one embodiment, the recycling system according to the invention comprises a system for rinsing the solid residue, preferably rinsing with water, to obtain a rinsed solid residue.

[0160] The separation device is preferably in fluid connection with the precipitation device and the drying or formulation device.

[0161] According to one embodiment of the invention, the separation device is capable of partially or completely drying the solid residue, possibly rinsed. According to this embodiment, the drying device is included in the separation device.

[0162] Alternatively, the device according to the invention comprises a device for drying the solid residue by heating and / or by placing the solid residue under reduced pressure, possibly rinsed.

[0163] According to one embodiment, the device of the invention further comprises a device for treating the effluent from the separation device to obtain a treated effluent. The treatment device is in fluid connection with the storage tank and / or with the precipitation device.

[0164] Preferably, the recycling system according to the invention further comprises a formulation device formulating the solid residue in the form of a recycled electrolyte.

[0165] The device according to the invention may further comprise a formulation tank comprising an aqueous solution optionally comprising one or more additives, as defined above. The formulation tank is in fluid connection with the formulation device. At the outlet of the formulation device, the recycled electrolyte may be introduced into the positive or negative compartment of a new redox flow battery, preferably by a fluid connection.

[0166] Preferably, the recycling system according to the invention is for implementing the method according to the invention.

[0167] Advantage of the invention

[0168] It is particularly surprising that such electroactive compounds can be recycled by precipitation. The recycling process according to the invention is particularly easy to implement and therefore particularly innovative. This process makes it possible to obtain very good recycling yields of electroactive compounds.

[0169] Quite surprisingly, the electroactive compounds recycled by the process of the invention are reusable for further cycling in a new redox flow battery with very satisfactory performance, particularly in terms of capacity and / or ohmic resistance (< 2 Q.cm 2 ) and stability during repeated operating cycles of the redox battery, which is substantially stable over several tens or hundreds of cycles. Such performance was not expected by those skilled in the art.

[0170] Another advantage of the method of the invention is that a small amount (in proportion to the volume treated) of reagents is used. In addition, these reagents are readily available (and already used in many other applications) and inexpensive. For example, acetic acid or sulfuric acid do not pose a significant environmental threat.

[0171] Furthermore, precipitation is rapid, and the process according to the invention does not generate pollution of the recycled electroactive compounds: the purification of the solid residue only involves a rinsing step, and possibly evaporation. Advantageously, the efficiency of the process according to the invention and its low cost allow industrialization of the process and the system according to the invention.

[0172] Unless explicitly stated otherwise, the expressions “from X to Y” and “between X and Y” designate intervals whose limits X and Y are included.

[0173] Figures

[0174] [Fig 1] Figure 1 is a schematic block diagram of a method according to the invention. After cycling 100 of a redox flow battery, at least one aqueous electrolyte of the redox flow battery is collected during a collection step 200. The electroactive compound(s) included in the aqueous electrolyte is (are) optionally brought into contact with an oxidant or a reducing agent in order to be discharged during a chemical oxidation or reduction step (250). The electroactive compound(s) included in the aqueous electrolyte is (are) precipitated during a precipitation step 300, preferably by adding an anti-solvent of the electroactive molecule and / or the addition of an acid or a base and / or the addition of a salt in the aqueous electrolyte. The suspension obtained at the end of the precipitation step 300 is then separated into a solid residue and an effluent during a separation step 400.A solid residue comprising the electroactive compound(s) and an effluent are obtained. The solid residue may be rinsed with water and triturated during a water rinsing and water trituration step 500. The solid residue, rinsed or not, may optionally be dried during a drying step 600 in order to reduce the amount of water and / or solvent present in the solid residue. The solid residue may then be formulated during a formulation step 700 in order to obtain a recycled electrolyte. The recycled electrolyte may be used in a new redox flow battery, alone or in a mixture with an electrolyte comprising one or more native electroactive compound(s), i.e. one that has never been used in a charge and / or discharge cycle of a redox flow battery.In parallel, the effluent obtained at the end of the separation step can be treated during a treatment step 800, in order to obtain a treated effluent capable of being reused in the precipitation step 300 during a subsequent implementation of the method according to the invention.

[0175] [Fig 2] Figure 2 is a schematic representation of a recycling system 1 of an aqueous electrolyte of a redox flow battery 10 according to the invention. Figure 2 deals with the case of recycling a negolyte, but can be transposed in its entirety to a posolyte.

[0176] A negolyte 20 from a redox flow battery 10 is collected in a collection device 30, optionally conveyed to a discharge device 35, and then conveyed to a precipitation device 40. An acid or a base and / or an anti-solvent and / or a solution of a salt from a storage tank 45 is added to the negolyte 20 in the precipitation device 40 in order to precipitate the electroactive compound(s) of the negolyte 20. The resulting suspension is separated, preferably by filtration, in a separation device 50. A solid residue 52 comprising the electroactive compound(s) of the negolyte 20 and an effluent 54 are obtained. The effluent 54 is collected in an effluent collection device 80, and can be treated and then returned to the storage tank 45. Optionally, the solid residue 52 is rinsed with water and triturated with water, then separated a second time to remove the wash water.Optionally, the residue 52 (rinsed or not) is dried or partially dried, either directly in the separation device, or after having been transferred to a drying device 60. The drying device makes it possible to heat the residue 52 under controlled temperature and / or to put it under reduced pressure and thus to reduce the quantity of water and any solvents present in the solid residue 52. The solid residue 52 is then conveyed to a formulation device 70. An aqueous solution possibly comprising additives is also introduced into the formulation device 70 from a formulation tank 75 in order to prepare a recycled negolyte 78. The recycled negolyte 78 can then be introduced into the negative compartment of a new redox flow battery 90.

[0177] [Fig 3] Figure 3 is a graph representing the accessible capacity in battery cycling (TRL 4) (as a percentage of the theoretical capacity of the electrolytes) of a battery comprising electrolytes with native electroactive compound and a battery comprising electrolytes with recycled electroactive compound.

[0178] [Fig 4] Figure 4 is a graph representing the battery resistance (TRL 4) measured by polarization curve of a battery comprising electrolytes with native electroactive compound and a battery comprising electrolytes with recycled electroactive compound.

[0179] [Fig 5] Figure 5 is a graph representing the accessible capacity in battery cycling (TRL 4) (as a percentage of the theoretical capacity of the electrolytes) of a battery comprising electrolytes with only native electroactive compounds, and a battery comprising a posolyte with native electroactive compound and a negolyte with recycled electroactive compound.

[0180] [Fig 6] Figure 6 is a graph representing the battery resistance (TRL 4) measured by polarization curve of a battery comprising electrolytes with only native electroactive compounds, and a battery comprising a posolyte with native electroactive compound and a negolyte with recycled electroactive compound.

[0181] The present invention will now be described with the aid of non-limiting examples.

[0182] Examples:

[0183] Example 1: Recycling and battery testing of the ferrocvanide / ferricvanide redox couple The recycling process was implemented on electrolytes used in batteries (>350 cycles and 6 months of cycling). The results present both the characteristics of the recycling process and the performance of batteries including recycled electrolytes.

[0184] At the end of cycling, the electroactive compound(s) of the ferrocyanide / ferricyanide redox couple (depending on the final charge state of the electrolytic solution) of the posolyte is (are) first chemically reduced, for example by adding H2C>2, controlling the pH (which should preferably remain above 8) and with stirring, in order to obtain a posolyte comprising 100% ferrocyanide.

[0185] Precipitation is then carried out by adding 96% ethanol (in liquid form) to the posolyte under magnetic stirring; the quantity required to cause precipitation of the ferrocyanide depends on the temperature of the solution and its concentration. The ferrocyanide concentration in the posolyte is 0.34 M. Precipitation is instantaneous and visually detectable. The addition of ethanol must be controlled, because if it exceeds a certain volume, the effect is counterproductive and the ferrocyanide redissolves in the solvent mixture.

[0186] The solution is then filtered (for example in the laboratory on filter paper (5-10 pm)), then the solid residue obtained is dried by evaporation of the residual traces of solvent (water + ethanol).

[0187] At the negolyte level, the electroactive compound used is, in its oxidized form, the following molecule:

[0188] [Chem 6]

[0189] .(concentration: 0.2 M).

[0190] The fraction of the electroactive molecule (M3CH) of the negolyte being in reduced form is discharged (i.e. is oxidized) automatically in the air by the action of the oxygen in the air. Then, the precipitation of the electroactive molecule is carried out by acidification of the negolyte solution, up to a pH value less than or equal to 6. The process has been tested with several types of acid (strong acid e.g. sulfuric acid, weak acid e.g. acetic acid), leading to equivalent results. The quantity of acid to be added depends only on the volume of electrolyte to be reprocessed and its initial pH. It is added while stirring. As soon as the pH value is less than or equal to 6, precipitation is instantaneous. The filtration of the effluent can be carried out on a large pore size filter, because the cake obtained is very compact and forms a block.The precipitate must then be rinsed with water to remove traces of acid, then spread out to facilitate the drying step, and remove residual traces of solvent.

[0191] The nature and quantity of solvent used for each electrolyte, as well as the yields and purities obtained, are presented in Table 1. The required quantities of solvent are 10 and 30% by volume respectively for the negolyte and the posolyte. This addition tends to decrease for the posolyte when the concentration of electroactive compound increases. The yields are higher than 65%, with an improvement expected by the implementation of an optimized industrial process. The purity of the recycled electroactive compound obtained after simple drying is estimated by quantitative proton NMR (qNMR). 1H) with the presence of an internal standard. This purity is 92 and 93% respectively, which proves the ease of removal of the solvent used for precipitation. By comparison, the purity of these same native electroactive compounds is approximately 97% for anthraquinone and 96% for the ferrocyanide salt.

[0192] Quantitative NMR method: 1 H NMR spectra were recorded on a BRUKER AC 300 P spectrometer (300 MHz). Maleic acid (Acros Organics) was used as an internal standard to assess the purity of the compounds.

[0193] [Table 1] Figures 3 and 4 present the performances obtained with a battery comprising electrolytes comprising one or more native electroactive compounds and with a recycled battery, i.e. comprising a negolyte and a posolyte formulated from recycled electroactive compound(s). The initial pH of the electrolytes is 13.

[0194] The accessible capacity (Figure 3) is identical for both batteries (the visible difference between the two curves is included in the reproducibility error), which surprisingly proves that recycling by precipitation of the electroactive compounds has no impact on their electrochemical activity. The evolution of this capacity during cycling is stable.

[0195] The measured battery resistance (Figure 4) is also equivalent for both batteries, and remains constant over cycling. This result surprisingly confirms that the solvents used for precipitation have no impact on the system performance.

[0196] Comparison of the two battery tests highlights that the active materials of an aqueous organic flow redox battery can be recycled by precipitation and reused in a new storage system without performance degradation.

[0197] A negolyte comprising M3CH as an electroactive compound in combination with an additive was also recycled following the protocol below. The nature and quantity of solvent used, as well as the yields and purities obtained by quantitative NMR with the presence of an internal standard, are presented in Table 2.

[0198] [Table 2]

[0199] The results in the table above show that the presence of additives in the negolyte does not disturb the recycling process, as similar recycling performances are obtained with and without additives.

[0200] Figures 5 and 6 present the performances obtained with a battery comprising electrolytes with one or more native electroactive compound(s) and with a partially recycled battery, i.e. comprising a native posolyte but a negolyte formulated from recycled electroactive compound(s), the negolyte initially comprising at least one additive. The initial pH of the electrolytes is 12.

[0201] The results obtained are similar to those of the fully recycled battery without additives (Figures 3 and 4): the accessible capacity (Figure 5) and the resistance measured in the battery (Figure 6) are identical before and after recycling. This demonstrates that the method and the device according to the invention are applicable to negolytes, even in the case where they include additives.

[0202] Example 2: Recycling of other electroactive compounds

[0203] Electroactive compounds for negolyte other than anthraquinone (M3CH) were efficiently recycled using the method according to the invention.

[0204] Their structure, recycling conditions and performance are presented in the following table:

[0205] [Table 4]

[0206]

[0207] The molecules were either purchased or synthesized to perform the recycling tests. They were dissolved under conditions similar to those listed in the literature on organic flow batteries. Several tests were then carried out to evaluate the possibility of recycling the molecules without degrading their chemical structure. The integrity of the molecules after recycling was verified by NMR and the recycling efficiency was evaluated by weighing after drying, then calculated with the purity obtained by quantitative NMR. The results reveal good recycling possibilities for most of the molecules tested; an improvement in efficiency is observed when using a strong acid, which allows to go below the pKa of the functional groups and therefore to precipitate more of the active species.

Claims

CLAIMS 1. Method for recycling an aqueous electrolyte from a redox flow battery to be recycled, the aqueous electrolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound being at least one oxidized or reduced form of an oxido / reduction couple whose oxidized form is a compound comprising a quinone unit, for example a benzoquinone unit, a naphthoquinone unit or an anthraquinone unit, preferably an anthraquinone unit, characterized in that it comprises a step of precipitation (300) of the electroactive compound.

2. Method according to claim 1, characterized in that it successively comprises: a step of collecting (200) an aqueous electrolyte from a redox flow battery comprising at least one electroactive compound, - the precipitation step (300) of the electroactive compound, whereby a suspension is obtained, - a step of separation (400) of the suspension, whereby a solid residue and an effluent are obtained, and - optionally, a step of rinsing with water (500) the solid residue obtained after the separation step, whereby a rinsed solid residue is obtained.

3. Method according to claim 1 or 2, characterized in that the precipitation step (300) comprises the addition of an anti-solvent of the electroactive compound and / or the addition of an acid or a base and / or the addition of a salt in the aqueous electrolyte.

4. Method according to any one of claims 1 to 3, characterized in that the precipitation step (400) comprises the addition of an acid to the aqueous electrolyte, preferably until a pH of less than or equal to 10, preferably less than or equal to 8, preferably less than or equal to 7, more preferably less than or equal to 6, is obtained, or the addition of a base, preferably until a pH of greater than or equal to 7, preferably greater than or equal to 8, preferably greater than or equal to 10 is obtained.

5. Method according to any one of claims 1 to 4, characterized in that the precipitation step (400) comprises the addition of a strong acid, preferably sulfuric acid, the volume of strong acid added representing between 0.1% and 40% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 15%, preferentially between 0.1% and 12%, depending on the initial pH of the solution and its composition.

6. Method according to any one of claims 1 to 4, characterized in that the precipitation step (400) comprises the addition of a weak acid, preferably acetic acid, the volume of weak acid added representing between 0.1% and 60% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 55%, preferentially between 0.1% and 50%.

7. Method according to any one of claims 1 to 4, characterized in that the precipitation step (400) comprises the addition of a base in the form of an aqueous solution of an alkali hydroxide whose alkali hydroxide concentration is between 1 and 25 moles per liter, preferably between 4 and 20 moles per liter, the volume of base added representing between 0.1% and 40% of the volume of the aqueous electrolyte to be treated, preferably between 0.1% and 30%, preferentially between 0.1% and 22%.

8. Method according to any one of claims 2 to 7, characterized in that it further comprises a step of formulation (700) of the solid residue comprising the dissolution of the solid residue in an aqueous medium to obtain a recycled electrolyte.

9. Method according to any one of claims 1 to 8, characterized in that the electroactive compound is at least one form of an oxido / reduction couple whose oxidized form is a compound of formula (F) in which X 1 , X 2 , X 3 , X 4 , X 5 , X 6 , X 7 and X 8 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms, A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms. and / or one of its salts, in particular a sodium or potassium salt.

10. Method according to any one of claims 1 to 8, characterized in that the electroactive compound is at least one form of an oxido / reduction couple whose oxidized form is a compound of formula (G): in which Z 1 , Z 2 , Z 3 , Z 4 , Z 5 and Z 6 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms, A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms, and / or one of its salts, in particular a sodium or potassium salt.

11. Method according to any one of claims 1 to 8, characterized in that the electroactive compound is at least one form of an oxido / reduction couple whose oxidized form is a compound of formula (H): or a compound of formula (K): in which Z 1 , Z 2 , Z 3 and Z 4 are independently selected from the group consisting of a hydrogen atom, an OH group, a COOH group, an SO3H group, an -A-COOH group, an -OA-COOH group, an -A-SO3H group, an -OA-SO3H group and a linear, cyclic or branched, saturated or unsaturated hydrocarbon group comprising from 1 to 10 carbon atoms, A representing a linear, cyclic or branched, saturated or unsaturated hydrocarbon group, comprising from 1 to 10 carbon atoms, and / or one of its salts, in particular a sodium or potassium salt.

12. Method according to any one of claims 2 to 11, characterized in that the aqueous electrolyte is a negolyte, and the method further comprises a chemical oxidation step (250) between the collection step (200) and the precipitation step (300), comprising bringing the negolyte into contact with an oxidant capable of oxidizing the reduced form of the oxido / reducing couple.

13. Method according to any one of claims 2 to 11, characterized in that the aqueous electrolyte is a posolyte, and the method further comprises a chemical reduction step (250) between the collection step (200) and the precipitation step (300), comprising bringing the posolyte into contact with a reducing agent capable of reducing the oxidant of the oxido / reducing couple.

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