Method and apparatus for removing impurities from an electrolyte

Abstract

The present disclosure provides methods and apparatus for preparing electrolyte solutions containing undesired impurities at μg / L levels. The methods generally involve electrochemically reducing the impurities to a precipitated, plated, or volatilized form and removing the reduced form from the electrolyte solution. The present disclosure describes methods and apparatus for carrying out such methods, as well as electrochemical solutions obtained or obtainable from such methods.

Description

Technical Field

[0001] The present disclosure relates to a method and apparatus for preparing an electrolyte containing an electrolyte containing a redox active material, and a purified electrolyte prepared therefrom.

[0002] [Cross - reference to Related Applications] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 749,459, filed Oct. 23, 2018, the entire disclosure of which is incorporated herein by reference.

Background Art

[0003] Problems associated with certain impurities containing arsenic, antimony, tin, and other such metals in redox flow battery systems and general battery systems are well known. For example, U.S. Pat. No. 9,647,290, U.S. Pat. No. 9,985,311, and U.S. Patent Application Publication No. 2018 / 0102561 discuss problems associated with arsenic, antimony, and germanium precipitating from vanadium flow battery systems, and describe that it is necessary to maintain the levels of these substances as well as other metals and metalloids including Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo by the selection of raw materials.

[0004] Both U.S. Patent Application Publication No. 2010 / 0143781 and U.S. Patent Application Publication No. 2010 / 0261070 describe problems associated with impurities of Hg, Ni, Co, and Cu in iron - chromium - based redox flow battery electrolytes, and attempt to address or avoid problems associated with these substances using zinc amalgam or other inorganic reducing agents. These methods appear to be applicable only to solutions of iron ions and chromium containing acid.

Summary of the Invention

Problems to be Solved by the Invention

[0005] There are few, if any, available general methods that provide such solutions in either vanadium-based or other more diverse systems related to chemical action, and there are only a few examples at most. In particular, the chemical actions related to the metal-ligand coordination compounds described in this specification have not been addressed so far, and no methods for reducing these types of impurities in the systems of these compounds have been described at all.

[0006] This application relates to addressing these and other problems.

[0007] The present disclosure relates to methods and apparatuses for reducing impurity levels, electrolyte solutions obtained or obtainable from these methods and apparatuses, and batteries, electrochemical cells and / or electrochemical systems, fuel cells, electrochemical storage systems, and redox flow batteries and flow battery systems containing these electrolytes.

Means for Solving the Problems

[0008] A particular embodiment of the present disclosure is a method for preparing an electrolyte solution having a reduced impurity level, the method comprising: (1) electrochemically reducing at least the impurities present at an initial concentration in an initial electrolyte solution further comprising a redox-active electrolyte under conditions sufficient to produce an electrochemically treated electrolyte solution containing a reductant of the redox-active electrolyte and a reductant of the impurities; (2) optionally further separating the reductant of the impurities from the electrochemically reduced solution to obtain a final electrolyte solution having a final impurity concentration lower than the initial impurity concentration in the initial electrolyte solution.

[0009] In some embodiments, the concentration of the redox-active electrolyte in the redox-active electrolyte solution is in the range of 0.5 M to 5 M, or a partial range thereof.

[0010] In a particular embodiment, the concentration of the impurities in the final electrolyte solution is (i) less than about 10 mg ([mg / L]) per liter of redox active electrolyte solution of one or more predetermined impurities, less than 5 mg / L, less than 2.5 mg / L, less than 1 mg / L, less than 500 μg ([μg / L]) per liter of redox active electrolyte solution of one or more predetermined impurities, less than 250 μg / L, less than 100 μg / L, less than 50 μg / L, less than about 40 μg / L, less than about 30 μg / L, less than about 20 μg / L, less than about 10 μg / L, less than about 5 μg / L, or less than about 1 μg / L of one or more predetermined impurities, or, (ii) less than about 10 mg ([mg / mol]) per mole of redox active electrolyte in the redox active electrolyte solution of one or more predetermined impurities, less than 5 mg / mol, less than 2.5 mg / mol, less than 1 mg / mol, less than 500 μg ([μg / mol]) per mole of redox active electrolyte in the redox active electrolyte solution of one or more predetermined impurities, less than 250 μg / mol, less than 100 μg / mol, less than 50 μg / mol, less than about 40 μg / mol, less than about 30 μg / mol, less than about 20 μg / mol, less than about 10 μg / mol, less than about 5 μg / mol, or less than about 1 μg / mol of one or more predetermined impurities are contained in the redox active electrolyte solution. In some embodiments, this is less than 5 μg / L or 5 μg / mol for Sb and As, and less than 10 μg / L or 5 μg / mol for Ge and Sn. In some embodiments, the impurities are in the form of antimony, arsenic, germanium, tin, or combinations thereof. In other aspects, the impurities may also include Hg, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Mo.

[0011] Reductive impurities can be separated from the electrolyte solution by plating, precipitation, or volatilization depending on the nature of the impurities, the nature of the electrolyte solution (e.g., based on the pH and nature of the accompanying redox active species), and the reduction conditions used. In certain aspects of these methods, the electrochemical reduction is carried out at a redox potential more negative than the reduction potential of the impurities.

[0012] Embodiments in which reducible impurities result in volatile reduction impurities upon reduction, for example, where the reduction impurities include one or more of arsine (AsH3), germane (GeH4), stannane (SnH4), or stibine (SbH3), an additional method is to electrochemically treat the electrolyte solution, (a) heating the electrochemically treated electrolyte solution at a temperature in the range of, for example, 20 °C to about 105 °C, or, (b) purging or sparging the electrochemically treated electrolyte solution or the electrolyte solution heated in step (a) with an inert gas such as nitrogen or argon, further comprising conditioning by one or both of the above.

[0013] The heating step and the purging step can be provided individually, simultaneously, or sequentially. These operations can be carried out while the conditions for electrochemical reduction are present or absent, preferably such that reoxidation of the reduction impurities at elevated temperatures is minimized or avoided.

[0014] Any or all of these preceding steps can be carried out while the electrolyte is already placed within a battery, an electrochemical cell and / or an electrochemical system, a fuel cell, an electrochemical storage system, and a redox flow battery and a flow battery system. In a preferred embodiment, these methods are carried out prior to transporting such a system, for example, into a storage tank or other container, in order to simplify the operation of the battery, fuel cell, or flow battery.

[0015] These electrolyte solutions can be handled and operated as shown in the methods described heretofore. That is, in this case, the redox active electrolyte is charged with its reduced form, but most preferably after all reduction impurities are removed, an additional step is carried out such that the reduced redox active electrolyte can be at least partially discharged.

[0016] Accordingly, additional embodiments include methods that further comprise oxidizing the reduced form of the redox active electrolyte in the final electrolyte solution. Such oxidation can be carried out using (i) an oxidizing gas, for example, a gas mixture containing air or oxygen, (ii) an oxidizing chemical such as hydrogen peroxide, (iii) electrochemically, or (iv) a combination thereof. Whichever method is chosen, it should be understood that the choice of oxidant should not compromise the integrity of the purified electrolyte solution, for example, by introducing other impurities.

[0017] These methods are generally applicable not only to most chemical systems including vanadium or iron-chromium systems, but also to systems containing redox active metal ligand coordination compounds described elsewhere in this specification, organic redox active substances described elsewhere in this specification, or combinations thereof.

[0018] The present disclosure specifically represents the purified compositions obtained or obtainable from these methods. That is, and to avoid misunderstanding, the present disclosure encompasses those compositions regardless of whether they have actually been carried out by these methods. Such electrolyte solutions, as an independent embodiment, contain a suitable redox active electrolyte substance at a concentration of at least 0.7 M with a lower impurity level than that described elsewhere herein. Impurities as so defined herein may include one or more of As, Ge, Hg, and Sb, and / or Ag, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Ge, Ir, Mg, Mn, Mo, Ni, Pb, Pd, Pt, Re, Ru, Sb, Sc, Sn, Sr, V, and Zn. In certain embodiments, these solutions contain one or more of antimony, arsenic, germanium, and / or tin at levels below 5 μg / L or 5 μg / mol (redox active electrolyte in the redox active electrolyte solution).

[0019] Furthermore, flow batteries containing one or more of the disclosed electrolytes are also within the scope of the present disclosure.

[0020] Additional embodiments of the present disclosure include those apparatuses useful for practicing these methods and producing these purified electrolyte compositions. These apparatuses are apparatuses comprising at least one electrochemical cell comprising a first half-cell chamber and a second half-cell chamber separated by a cation exchange membrane, (i) the first half-cell chamber comprises a first electrode, preferably a carbon electrode, in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and (ii) the second half-cell chamber comprises a second electrode in contact with a second aqueous electrolyte, wherein the second aqueous electrolyte comprises one or more salts containing an aprotic cation at a concentration of at least 0.1 M, and the second electrode preferably comprises a catalyst for generating O2 containing nickel such as Ni foam, stainless steel mesh, stainless steel felt, Ni oxide, Ni hydroxide, Ni oxyhydroxide, or Ni-Fe oxide,

[0021] An additional embodiment is an electrochemical cell comprising a first half-cell chamber and a second half-cell chamber separated by a cation exchange membrane, (i) the first half-cell chamber comprises a first electrode, preferably a carbon electrode, such as carbon cloth, carbon felt, or carbon paper, in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and (ii) the second half-cell chamber comprises a second electrode in contact with a second aqueous electrolyte having a pH of at least 2 and containing one or more salts having an aprotic cation at a concentration of at least 0.1 M, (iii) wherein the second electrode comprises a catalyst for generating O2, preferably platinum, or at least one of oxides of cobalt, iridium, iron, manganese, nickel, ruthenium, tin, or combinations thereof, most preferably IrO2,

[0022] In yet another embodiment, the electrochemical cell comprises a first half-cell chamber and a second half-cell chamber separated by a membrane, where (i) The first half-cell chamber comprises a first electrode in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and (ii) The second half-cell chamber comprises a second electrode containing a catalyst for generating O2, and the second half-cell chamber does not contain (is free of) an aqueous electrolyte.

[0023] Yet other embodiments are methods of practicing the disclosed methods that include operating the disclosed electrochemical cells, including passing a sufficient current through the cell under conditions sufficient to reduce the concentration of reducible impurities in the first aqueous electrolyte to a predetermined level, e.g., the levels described elsewhere herein, of the electrochemical devices described herein.

[0024] This application will be better understood when read in conjunction with the accompanying drawings. The drawings, which are for illustrative purposes only, show exemplary embodiments of the subject matter, but the subject matter disclosed herein is not limited to the specific methods, apparatus, and systems disclosed. Further, the drawings are not necessarily drawn to scale. **Brief Description of the Drawings**

[0025]

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DETAILED DESCRIPTION OF THE INVENTION

[0026] The present disclosure relates to an electrolyte solution (posolyte or negolyte or both) and a method for preparing an electrolyte solution. In particular, the present disclosure relates to the preparation of an electrolyte solution containing impurities at a level below a predetermined threshold level.

[0027] The present disclosure can be more readily understood by reference to the following detailed description in conjunction with the accompanying figures and examples, all of which form a part of the present disclosure. The present disclosure is not limited to the specific products, methods, conditions, or parameters described and / or shown herein, and the terms used herein are for the purpose of describing specific embodiments only as examples and are not intended to limit any subject matter recited in the claims. Similarly, unless otherwise indicated, any description of a possible mechanism of action or mode of operation or reason for improvement is to be construed as illustrative only, and the present disclosure should not be constrained by the correctness of any such proposed mechanism of action or mode of operation or reason for improvement. Throughout this specification, it is recognized that the detailed description and embodiments refer to a method for preparing an electrolyte solution with reduced impurities, including a redox-active electrolyte solution, the electrolyte solution itself with reduced impurities, and an electrochemical device and a method of using these electrochemical devices useful for practicing the method. Embodiments or detailed descriptions used to describe or characterize any one of these categories should be construed as referring to all of these categories. That is, when the present disclosure describes and / or claims features or embodiments related to a system or device or a method of preparing or using a system or device, such detailed description and / or claims are intended to extend these features or embodiments to embodiments in each of these contexts (i.e., system, device, method, and composition).

[0028] It should also be understood that certain features of the invention described herein in the context of separate embodiments can be provided in combination or in a single embodiment for clarity. That is, unless clearly inconsistent or specifically excluded, each individual embodiment is considered combinable with any other embodiment(s), and such combinations are considered another embodiment. Conversely, the various disclosed features described in the context of a single embodiment can also be provided separately or in any partial combination for brevity. Finally, while embodiments can be described as part of a series of steps or as part of a more general structure, each of the above steps can also be considered an independent embodiment in itself and combinable with other steps.

[0029] When a list is presented, unless otherwise indicated, each individual element of that list and all combinations of that list are to be understood as separate embodiments. For example, a list of embodiments presented as "A, B, or C" should be interpreted to include the embodiments of "A", "B", "C", "A or B", "A or C", "B or C", and "A, B, or C".

[0030] The present disclosure relates, inter alia, to electrochemical methods and apparatuses that can reduce unwanted impurities from redox active electrolytes, in some cases reducing some impurities to low mg / L or mg / mol levels, or even to μg / L or μg / mol levels. The present disclosure also relates to electrolyte compositions having such reduced impurity levels, and redox batteries containing such electrolytes.

[0031] [Description of General Method] Accordingly, certain embodiments of the present disclosure provide a method of preparing an electrolyte solution having a reduced impurity level, the method comprising electrochemically reducing at least impurities present at an initial concentration in an initial electrolyte solution that also contains a redox-active electrolyte under conditions sufficient to produce an electrochemically treated electrolyte solution containing a reduced form of the redox-active electrolyte and a reduced form of the impurities. In related embodiments, the method may further comprise separating the reduced form of the impurities from the electrochemically treated solution to obtain a final electrolyte solution having a final impurity concentration lower than the initial impurity concentration.

[0032] These methods include electrochemically reducing the impurities to produce a reduced form of the impurities. In some embodiments, the electrochemical reduction is performed in an electrochemical cell using the skill of one of ordinary skill in the art. The impurities may be soluble in the electrolyte or may be present as a solid in the redox-active electrolyte. One of ordinary skill in the art will be able to perform the electrochemical reduction easily using the skill of one of ordinary skill in the art and the teachings of this specification. Generally, the electrochemical reduction is performed by passing a current through the electrolyte solution. If desired, the reduction is performed at an oxidation / reduction potential more negative than the reduction potential of the impurities. The required oxidation / reduction potential is selected based on the impurities to be removed from the electrolyte.

[0033] Discussion of the intended meaning of the terms "electrolyte," "redox-active electrolyte," "impurities," and "reduced form of the impurities" is provided elsewhere in this specification. Further, the above methods are applicable regardless of whether the initial electrolyte solution containing the impurities and the redox-active electrolyte has a pH that is alkaline, neutral, or acidic, which is a characteristic that represents the nature of the particular redox-active electrolyte being used.

[0034] Obviously, the nature and initial concentration of the impurities depend on the nature and concentration of the redox-active electrolyte and other substances used in the formulation of the initial electrolyte solution. For example, vanadium-containing electrolytes are known to contain metal elements such as Sb, As, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, or Sn. The same types of these impurities may also be present in other metal-containing redox-active electrolytes. In other embodiments, the impurities include one or more forms of antimony, arsenic, germanium, mercury, tin, or combinations thereof.

[0035] In some of these embodiments, the concentration of the redox-active electrolyte, particularly a redox-active electrolyte containing a metal or metalloid, is at least 0.5 M. Other specifically expressed concentrations are described elsewhere in this specification in the context of the final redox-active electrolyte solution.

[0036] By the methods described herein, the final concentration of impurities in the final electrolyte solution can be adjusted to a predetermined threshold level. For example, in some embodiments, the final concentration of impurities in the final electrolyte solution can be any of the impurity levels described herein, but in a preferred embodiment, these impurity levels are defined in the following respects: (i) less than about 50 μg of impurities per liter of the final electrolyte solution, preferably less than 10 μg / L, more preferably less than 5 μg / L, and even more preferably less than 1 μg / L, or, (ii) Less than about 50 μg ([μg / mol]) of impurities per mole of the redox active electrolyte in the final electrolyte solution, preferably less than 10 μg / mol, more preferably less than 5 μg / mol, even more preferably less than 1 μg / mol. Using the conditions described herein, these levels have been achieved for various impurities. In certain independent embodiments, for example, the method provides a level of germanium of less than 10 μg / L or less than 10 μg / mol, and / or a level of tin of less than 10 μg / L or less than 10 μg / mol, and / or a level of arsenic of less than 5 μg / L or less than 5 μg / mol, and / or a level of antimony of less than 5 μg / L or less than 5 μg / mol, and / or a level of mercury of 5 μg / L or less, and the final electrolyte solution contains these. Of course, as described elsewhere herein, the method can provide one or more of these impurities at higher levels, and the final electrolyte solution can contain these.

[0037] The method can also be characterized by their efficiency in removing impurities and reducing their initial levels to a final predetermined level. In some embodiments, the final impurity level represents a 50% reduction of the impurities relative to their initial levels. In other independent embodiments, the method provides a final redox active electrolyte solution in which the final impurity concentration is at least 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 99.9% lower than the concentration in the initial solution.

[0038] In some embodiments, when the reduced form of the impurities is separated by plating on or within the positive electrode of the electrochemical cell, the impurities are removed or their concentration is reduced. In other embodiments, the process yields a reduced form of the impurities that precipitate from the electrochemically treated electrolyte solution. Once precipitated, the reduced form of the impurities can be removed by filtration, decantation, or other known methods of removing solids from a solution. In some embodiments, the precipitated reduced form of the impurities is removed by flocculation sedimentation, filtration, or membrane separation. In a further embodiment, the precipitated reduced form of the impurities is removed by filtration. Filtration can be effected by passing the electrolyte containing the reduced form of the impurities through, for example, a filter of chelating resin or a column packed with beads of chelating resin.

[0039] In other embodiments, the nature of the impurities is such that the reduced form of the impurities is a volatile hydride. In particular, this class of impurities includes, for example, antimony, arsenic, germanium, or tin, where the volatile hydride is arsine (AsH3), germane (GeH4), stannane (SnH4), or stibine (SbH3).

[0040] For the electrochemical reduction to be effective, it is carried out at a redox potential more negative than the reduction potential of the impurities, in the presence of a redox-active electrolyte. Since different impurities have different reduction potentials, one of ordinary skill in the art should select an appropriate reduction potential for the target impurities. For this purpose, it is not important whether the impurities are reduced directly under the conditions to which they are applied or indirectly from the reduced form of the redox-active material (or vice versa), as long as the impurities are reduced. Although the presence of a predetermined redox-active electrolyte may affect the reduction potential of the impurities, to a first approximation, the reduction potential of the impurities can be determined or known independently of the presence of the redox-active electrolyte, and for this reason, determining the exact reduction potential will be within the ability of one of ordinary skill in the art. Also, the effect of pH on standard reduction potentials is known, and again one of ordinary skill in the art will be able to accurately predict the appropriate potential to apply to effect the desired conversion.

[0041] When such treatment results in the formation of volatile reducing impurities such as volatile hydrides, especially when produced in a large container, an additional step may be useful to remove such volatile substances. Such a "conditioning" step may include heating the electrochemically treated electrolyte solution at a temperature above ambient temperature up to the boiling point of the electrochemically treated electrolyte solution or the final electrolyte solution under inert atmosphere conditions. In the case of most commercially relevant systems, the boiling point of the final aqueous electrolyte solution is about 105°C to 110°C. At that time, in some embodiments, such heating may be applied to a temperature within the range of 20°C to 25°C, 25°C to 30°C, 30°C to 35°C, 35°C to 40°C, 40°C to 45°C, 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, 60°C to 65°C, 65°C to 70°C, 70°C to 75°C, 75°C to 80°C, 80°C to 85°C, 85°C to 90°C, 90°C to 95°C, 95°C to 100°C, or 100°C to 105°C, or 105°C to 110°C, such as 35°C to 95°C, more preferably 45°C to 85°C or one or more of them. In the case of an aqueous system, it is further understood that increasing the temperature of the electrolyte promotes the oxidation of the charged redox-active electrolyte or reducing impurities, and as a result, hydrogen may be generated. To avoid this, the temperature should be selected so that such oxidation is avoided (e.g., so that the electrolyte remains charged and / or prevents the escaping hydride from being oxidized to a soluble non-volatile state). Alternatively or additionally, the operator can maintain the electrolyte solution at an appropriate potential (i.e., a potential more negative than the reduction potential of the impurities and / or the redox-active electrolyte) during the heating process.

[0042] In addition to or instead of heating, such "conditioning" steps can include purging or sparging an electrochemically treated electrolyte solution or a heated electrolyte solution with an inert gas such as nitrogen or argon. Again, it is important to keep the reducing impurities in their volatile form. The purge can also be carried out using mixing at a rate sufficient to remove the reduced forms of the impurities. The inventors have found that purging with optional mixing facilitates the efficient transfer of the reduced forms of the dissolved impurities to the gas phase and their removal from the electrolyte.

[0043] When applied, heating and / or sparging can be carried out simultaneously or sequentially through one or more cycles.

[0044] When at least a portion of the reduced forms of the impurities are removed and the level of impurities in the final electrolyte solution reaches an appropriate level, the redox active material can perhaps be in its fully reduced / charged state. Regardless of safety reasons or any other reasons (e.g., to minimize unintended oxidation with hydrogen generation during transportation of the final electrolyte solution), it may be desirable to lower the charge state of the final electrolyte solution (i.e., at least partially discharge the redox active electrolyte). This can be achieved by any suitable oxidation method. For example, this can be done chemically, by purging with an oxidizing gas such as air or oxygen or by using a chemical oxidizing agent such as hydrogen peroxide, or electrochemically, by using a hydrogen generation catalyst (e.g., activated carbon, carbon cloth, carbon felt, carbon paper, Ti mesh, Ti felt, foamed Ti mesh, Pt plated Ti mesh, or combinations thereof), or by some combination of these methods. The reagents or methods for such oxidation are preferably selected so as not to introduce harmful substances.

[0045] Oxidation can be carried out at ambient temperature or at a reduced or elevated temperature. In certain embodiments, the final electrolyte solution is oxidized at one or more temperatures of at least 65 °C or higher, preferably at a temperature of about 85 °C or higher, more preferably at a temperature of about 105 °C or higher, and also up to the boiling point of the final electrolyte solution. In certain situations, the application of heat and / or sparging may be useful, and in fact, it can be intentionally used to further concentrate the redox-active electrolyte in the final redox-active solution.

[0046] [Electrochemical device (HERM device)] In this regard, the present disclosure has focused on methods for removing these reducible impurities and electrolytes with reduced impurities, but the present disclosure also encompasses devices / systems useful for practicing these methods.

[0047] To be operative, the chemical action requires an apparatus / system that "injects" electrons into the electrolyte solution, i.e., into the electrolyte solution containing the redox active material, at a potential sufficient to reduce reducible impurities. The effect or nature of the counterbalancing cation that must accompany the injected electrons has not been discussed heretofore. However, the specific choice of the apparatus / system used (hereinafter referred to as the "HERM apparatus" or Hybrid Electrochemical Removal Modular apparatus) depends on the electrolyte to be "purified", i.e., the solution containing the redox active material from which the impurities are removed. In the selection of the system, it is also necessary to consider that the amount of impurities (even if interpreted as 100 mg / L or 100 ppm) is very small relative to the redox active material (typically greater than 0.5 M) in any actual electrolyte solution to be purified, along with the charge state of the redox active material. For example, consider a hypothetical one-liter solution of an electrolyte containing a 50% charged state 1 M redox active material containing 100 ppm of reducible impurities. Almost all of the electrons injected into such a system will be used for the reduction of the redox active material, and only a small fraction will be used for the reduction of the reducible impurities. The number of electrons required to lower the charge state of the redox active material may be several orders of magnitude greater than the number of electrons required to reduce the reducible substance. And in such a system, these electrons will be accompanied by an equal amount of compensating cations.

[0048] Perhaps even more important is that by injecting hydrogen ions at these levels, regardless of whether the electrolyte being treated is acidic or alkaline, generally a concentration far higher than the hydrogen ion content of the electrolyte being treated results (even at pH = 2, [H +=0.01 M). Such injection will have a great impact on the change in the pH of the treated electrolyte. This can be solved by adjusting the pH of the electrolyte solution with an appropriate base after the completion of the reduction process. However, even the addition of such a base may introduce a significant amount of impurities into the purified electrolyte solution. A more sophisticated solution is to use a HERM device that simultaneously injects cations of alkali metals or alkaline earth metals (or other aprotic) along with electrons.

[0049] Devices previously described for pH equilibration of the system include the device described in International Publication No. 2015 / 048074 ('074 application). The content thereof is incorporated herein by reference in its entirety for all purposes. The '074 application teaches an apparatus and method for simultaneously equilibrating the electron and proton content of an operating electrolyte. As described therein, the apparatus described as a rebalancing cell or equilibration cell describes an apparatus and method for injecting both electrons and protons into a redox-active electrolyte solution. Although described in principle as being attached to a flow battery, this description also describes a rebalancing cell as a stand-alone device. Some of the embodiments described herein discussed in the '074 application include embodiments in which the equilibration cell comprises the following. (1) A first half-cell chamber and a second half-cell chamber, wherein the first half-cell chamber comprises a first electrode in contact with a first aqueous electrolyte of a redox flow battery, and the second half-cell chamber comprises a second electrode in contact with a second aqueous electrolyte, and the second electrode comprises a catalyst for generating O2. Here, in some of these embodiments, the pH of the second aqueous electrolyte is at least 2, preferably greater than about 7, and more preferably in the range of about 9 to about 14. In other embodiments, no second electrolyte is added. (2) A first half-cell chamber comprising a first electrode in contact with a first aqueous electrolyte of a redox flow battery, and a second half-cell chamber comprising a second electrode in contact with a second aqueous electrolyte, wherein the second electrode comprises a sacrificial carbon electrode material that generates O2 and / or CO2. The two half-cell chambers are separated by an ion exchange ionomer membrane. (3) A first half-cell chamber comprising a first electrode in contact with a first aqueous electrolyte of a redox flow battery, and a second half-cell chamber having a second electrode containing a catalyst that generates O2, wherein the electrode is not in contact with the second aqueous electrolyte. The two half-cell chambers are separated by an ion exchange ionomer membrane.

[0050] In these embodiments, it is described that the first aqueous electrolyte comprises the negative working electrolyte ("negolite") of the redox flow battery. In various embodiments, the electrochemistry associated with the second half-cell of the equilibrium cell at acidic or neutral pH values is given by Equation (1): 2H2O → 2O2 + 4H + + 4e - ·····(1) and at more basic pH values, the electrochemistry associated with the second half-cell of the equilibrium cell is given by Equation (2): 4OH - → 2H2O + O2 + 4e - ·····(2) as described by.

[0051] The corresponding electrochemical reactions associated with the first half-cell are Equations (3) to (5): M n + e - → M n-1 ·············(3) and IMP n+ + ne - → IMP 0 ·········(4) or IMP n+ + ne - + nH + → IMP.Hn ···(5) may be described in Here, in the formula, M n and M n-1 represent the oxidized form and the reduced form of the redox active species in the negolite, respectively, and in the present application, correspond to the redox active substances in the solution to be treated. Here, "IMP n+ " refers to the initial form of the impurity (for example, As 3+ ), and IMP 0 and IMP·H n refer to the reduced metal form or hydride form of the impurity, respectively. In this context, such two reaction schemes are represented by formula (6) and formula (7): As + 3H + + 3e - → AsH3 ········(6) As + 3H2O + 3e - → AsH3 + 3OH - ···(7) can be understood by

[0052] Since the '074 application is directed to the simultaneous equilibration of the electron and proton contents of the working electrolyte solution, in both situations, it is stated that charge balance is given to the negolite by transporting protons through a membrane from the second half-cell to the first half-cell of the pH correction cell. As one of the results of this design, the '074 application describes that when the second half-cell chamber comprises a second electrode containing a catalyst that generates O2 operating in an alkaline environment, the two half-cell chambers are separated by a bipolar membrane consisting of one cation exchange ionomer membrane and one anion exchange ionomer membrane sandwiching a metal oxide membrane that promotes the dissociation of water. It is described that the equilibrium cell is operated while deploying an alkaline electrolyte in the second half-cell chamber by using the bipolar membrane.

[0053] However, in this case, due to the above reasons, it is not very desirable to simultaneously inject electrons and protons (as described in the '074 application), and it is more desirable to simultaneously inject electrons and other non-protonic cations into the electrolyte solution. Therefore, by introducing an alkali metal cation or an alkaline earth metal cation (or further an ammonium cation) into the second electrolyte at a concentration sufficient to prevent the need for pH adjustment of the first aqueous electrolyte after electrochemical treatment, the apparatus described in the '074 application can be reconfigured to simultaneously inject electrons and non-protonic cations. In a preferred embodiment, the alkali metal cation or alkaline earth metal cation (or further ammonium cation) in the second electrolyte is present at a much higher concentration (e.g., more than 10 times, more than 100 times, more than 500 times, more than 1000 times, more than 5000 times, or more than 10000 times greater than the concentration of protons in the second electrolyte). In certain embodiments, these non-protonic cations can be an alkali metal cation or an alkaline earth metal cation, such as Li + 、Na + 、K + 、[NH n R 4-n + (R = alkyl), or a mixture thereof. These non-protonic cations can be present in the second aqueous electrolyte in a concentration range from at least 0.1 M to their saturation concentration, or in the ranges of 0.1 M - 0.2 M, 0.2 M - 0.3 M, 0.3 M - 0.4 M, 0.4 M - 0.5 M, 0.5 M - 0.6 M, 0.6 M - 0.7 M, 0.7 M - 0.8 M, 0.8 M - 0.9 M, 0.9 M - 1 M, 1 M - 1.25 M, 1.25 M - 1.5 M, 1.5 M - 2 M or higher, or any combination thereof. Exemplary counterions for these cations include hydroxide ions, phosphate ions, or sulfate ions.

[0054] ​In this case, a system similar to the system described in the '074 application (including its hardware aspect) can be used as a HERM device and attached to a flow battery as described in the '074 application, or attached to a storage tank or container and / or used separately from the working system. Since the HERM device is not used in combination with an operating flow battery system, the voltage and other efficiencies of the HERM device are not important for the economy of such a flow battery system.

[0055] In a particular embodiment of the present invention, the electrochemical cell comprises a first half-cell chamber and a second half-cell chamber separated by a membrane, wherein the first half-cell chamber is in contact with a first electrode, preferably a carbon electrode such as carbon cloth, carbon felt or carbon paper, in contact with a first aqueous electrolyte containing a redox active electrolyte (e.g., negolite) and reducible impurities, and the second half-cell chamber comprises a second electrode in contact with a second aqueous solution, preferably acidic but having a pH of at least 2, wherein the second electrode comprises a catalyst for generating O2, preferably platinum, or at least one of oxides of cobalt, iridium, iron, manganese, nickel, ruthenium, tin or combinations thereof, most preferably IrO2.

[0056] In other embodiments, the present disclosure contemplates an electrochemical cell comprising a first half-cell chamber and a second half-cell chamber separated by a membrane, wherein the first half-cell chamber comprises a first electrode, preferably a carbon electrode, in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and the second half-cell chamber comprises a second electrode in contact with a second aqueous solution, wherein the second electrode comprises a catalyst for generating O2, preferably nickel such as Ni foam, stainless steel mesh, stainless steel felt, Ni oxide, Ni hydroxide, Ni oxyhydroxide, or Ni-Fe oxide.

[0057] In some of these embodiments, the membrane is a cation exchange membrane (“CEM”). In these embodiments, the membrane does not include an incorporated anion exchange membrane and is not a bipolar membrane as described in the ’074 application. Materials useful for these cation exchange membranes include perfluorosulfonic acid membranes or polyfluorosulfonic acid membranes (NAFION® membranes, AQUIVION® membranes, or FLEMION® membranes) that optionally include perfluoropolyvinyl ether, copolymers of tetrafluoroethylene, sulfonated hydrocarbon membranes (sulfonated polyether ether ketone, sulfonated polyphenyl sulfone). Other exemplary perfluorinated membrane materials include copolymers of tetrafluoroethylene and one or more fluorinated acid functional comonomers. Other useful perfluorinated electrolytes include copolymers of tetrafluoroethylene (TFE) and FSO2-CF2CF2CF2CF2-O-CF=CF2.

[0058] In some embodiments, the second aqueous electrolyte does not contain a redox active material. In still other embodiments, the second aqueous electrolyte contains aprotic cations at levels discussed elsewhere herein for this purpose. Preferably, the type and proportion of aprotic cations in the second solution substantially or actually match those of the electrolyte solution being treated (in the first half-cell). The term “substantially match” refers to a distribution where the cation content in the redox-containing electrolyte does not require adjustment after treatment. The mobility of each cation through the membrane may require that the relative proportion of cations in the second electrolyte solution be slightly different from the corresponding proportion in the electrolyte being treated. The ability to determine these differences can be identified by one of ordinary skill in the art without undue experimentation, if necessary.

[0059] In yet other independent embodiments, the pH of the first aqueous electrolyte and the second aqueous electrolyte is less than 7 for both, about 7 for both, or greater than 7 for both. Such devices and methods can be used to treat highly acidic redox-active electrolytes such as those present in, for example, vanadium flow batteries or iron-chromium flow batteries, and alkaline or pH-neutral redox-active electrolytes based on, for example, metal-ligand coordination compounds such as titanium-based negolite materials (and including the broader range of negolites described herein).

[0060] Preferably, the pH of the first aqueous electrolyte and the second aqueous electrolyte differ by less than 5 pH units. In other independent embodiments, the electrolytes differ by less than 4 pH units, less than 3 pH units, less than 2 pH units, or less than 1 pH unit. In higher cases, it is also possible to use the strategy of using a pH buffer layer as described in International Application No. PCT / US2018 / 054798, filed October 8, 2018.

[0061] As noted above, the second electrode generally includes a catalyst that generates O2. In some of these embodiments, the second electrode includes a metal oxide catalyst suitable for electrochemically generating O2 from water. In addition to the ability to generate O2, these oxidation catalysts are preferably either corrosion-resistant under the pHs contemplated for this application, insufficient catalysts to reduce water to hydrogen, or both. Catalysts that corrode under the acidic oxidation conditions or basic oxidation conditions of the second working aqueous electrolyte can cross over to the first pH-correcting half-cell, interfering with the intended effect of the pH-correcting cell or, worse, interfering with the operation of the flow battery. If such crossover catalysts are highly efficient catalysts for hydrogen generation under the reducing conditions of the first half-cell, it can be envisioned that hydrogen generation at the negative electrode of the first half-cell or the working flow battery could raise safety concerns. Therefore, the present invention contemplates preferentially using oxides of cobalt, iridium, iron, manganese, nickel, ruthenium, tin, or combinations thereof for use in the second electrode. Iridium oxide is particularly preferred due to its good catalytic activity for O2 generation and high corrosion resistance.

[0062] When the second half-cell chamber contains an alkaline electrolyte, catalysts such as nickel oxide or nickel-iron oxide are particularly preferred due to their good catalytic activity for O2 generation and high corrosion resistance in base. Suitable materials in this case include Ni foam, stainless steel mesh, stainless steel felt, Ni oxide, Ni hydroxide, Ni oxyhydroxide, or Ni-Fe oxide.

[0063] In some embodiments, the second electrode of the HERM device contains carbon. Such electrodes are well known in the art and include graphite-like carbon, glassy carbon, amorphous carbon, boron- or nitrogen-doped carbon, diamond-like carbon, carbon onion, carbon nanotube, carbon cloth, carbon felt, carbon paper, and graphene. The carbon material can generate O2 despite a somewhat high overpotential, but it is inevitable that the carbon electrode itself oxidizes to CO2. Therefore, the carbon electrode exhibits semi-sacrificial properties. In other embodiments, the second electrode can also include Ti mesh, Ti felt, foamed Ti mesh, stainless steel mesh, and stainless steel felt.

[0064] In still other embodiments, the electrochemical cell comprises a first half-cell chamber and a second half-cell chamber separated by a membrane, where (i) the first half-cell chamber comprises a first electrode in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and (ii) the second half-cell chamber comprises a second electrode containing a catalyst for generating O2, and the second half-cell chamber does not contain (is free of) an aqueous electrolyte.

[0065] In this configuration, the water required for the O2 generation reaction is provided by water from the aqueous electrolyte in the first half-cell chamber that is transported across the membrane. To avoid situations that limit mass transport, the transport of water across the membrane needs to be faster than the consumption of water at the metal oxide catalyst. On the second half-cell chamber side of the membrane, a metal oxide O2 generation catalyst (e.g., IrO x ) is coated, and as a result, the water transported across the membrane from the first half-cell chamber is directly oxidized to molecular oxygen and protons. This configuration can significantly simplify the design of the electrochemical device. For example, by directly connecting the metal oxide catalyst on the membrane to the titanium end plate, the need for a titanium mesh that functions as the flow field for the second aqueous electrolyte can be eliminated. The only additional design feature is the pores for the molecular oxygen generated by the metal oxide catalyst. Additionally, water needs to be periodically added to the electrolyte tank for the negolite to replenish the water consumed in the O2 generation reaction. Optionally, this makeup water can be generated in situ by combining the generated O2 from the second half-cell chamber with the H2 generated in the second half-cell chamber of the electrochemical cell of the HERM device and the negolite section of the first cell. This water generation process can be catalyzed by a noble metal catalyst (e.g., Pt, Pd, etc.).

[0066] An additional embodiment is a method of operating any of these HERM devices described herein, each method including applying a potential across the first and second electrodes of the device to supply a current to the device in the presence of an aqueous electrolyte to be treated. Specific conditions for such operation are described elsewhere in this specification. When operating to remove impurities from the electrolyte solution containing these redox active materials, parasitic hydrogen generation does not interfere with its main function. This allows the use of less expensive materials such as stainless steel in the construction of the electrodes.

[0067] Further embodiments include incorporating a battery, an electrochemical cell, a fuel cell, and / or a flow battery as part of an energy storage system in a larger system that includes, for example, a cell stack, storage tanks and piping for containing and transporting electrolytes, control hardware and software (which may include a safety system), and at least one power conditioning unit. In such a system, the storage tank contains an electroactive material. The control software, hardware, and any safety system include any sensors, mitigation devices, and electronic / hardware control and safety devices that ensure the safe, autonomous, and efficient operation of the flow battery or other energy storage system.

[0068] Such a storage system can also convert input and output power to voltages and currents optimal for the energy storage system or its application by including a power conditioning unit at the front end of the energy storage system. In an example of an energy storage system connected to a power grid, during the charging cycle, the power conditioning unit will convert the input AC electricity to DC electricity at a voltage and current suitable for the electrochemical stack. During the discharge cycle, the stack generates DC power and the power conditioning unit converts it to AC power at a voltage and frequency suitable for power grid applications. Such an energy storage system is well-suited for continuous charge or discharge cycles of several hours. Accordingly, the system is suitable for providing a mechanism to smooth the energy supply / demand profile and stabilize intermittent power generation assets (e.g., from renewable energy sources). In so doing, it should be understood that various embodiments of the present invention include electrical energy storage applications where such long charge or discharge times are beneficial. For example, non-limiting examples of such applications include those where the system of the present invention is connected to a power grid, including renewable energy integration, peak load shifting, grid farming, base load power generation / consumption, energy arbitrage, siting of transmission and distribution assets, support of weak power grids, and / or frequency regulation. Further, the above-described device or system can be used to supply stable power for applications not connected to a power grid or microgrid, such as for remote camps, forward operating bases, off-grid telecommunication, or remote sensors.

[0069] [Redox-active electrolyte solution with reduced impurities] The present disclosure more specifically represents the final electrolyte solutions available for the disclosed methods. These embodiments include electrolyte solutions having the redox-active electrolytes and impurities and impurity levels disclosed herein, whether or not these electrolyte solutions were prepared (``obtained'') by the methods disclosed herein. As used herein, the term ``obtainable from'' indicates that a predetermined electrolyte solution could have been prepared by the method of the present invention (i.e., including the characteristics of the electrolyte solutions described herein), but was actually prepared by an alternative method.

[0070] These final ``impurity-reduced'' electrolyte solutions contain at least one redox-active electrolyte, one or more impurities at the levels described herein, and optionally, further additives (surfactants, viscosity modifiers, buffers, and electrolytes of non-redox-active molecules / species), depending on the desired performance of the substance.

[0071] The selection of at least one redox active material is flexible and includes, for example, salts of vanadium, iron, chromium, or various metal ligand coordination compounds. These latter types of materials are described elsewhere in this specification. In particular, the present disclosure emphasizes the usefulness of the methods of the present invention and the resulting redox active electrolyte solutions, where the redox active material includes a metal ligand coordination compound containing titanium, particularly as negolite.

[0072] In the inventors' experience with bulk commodity chemicals, significant impurities are found in the raw materials, and it is common for them to correspond to impurities in the final bulk formulated electrolyte. Corrosives are a common source of impurities. The ranges of some typical impurities from common precursor materials are shown in Tables 1A and 1B below. These impurities accumulate in the electrolyte of the final flow battery, depending on the method used, and may be found at high concentrations. See also Table 2 for equivalent data on titanium-catecholate derivatives prepared from commercial sources.

[0073] For example, when an economical grade of NaOH is used in several equivalents for the synthesis of an organic redox active material, the resulting material may be rich in impurities such as Ni. An electrolyte formulated from such a material may tend to precipitate or have an increased parasitic hydrogen generation in a flow battery.

[0074] JPEG0007697880000001.jpg73170

[0075] JPEG0007697880000002.jpg78170

[0076] JPEG0007697880000003.jpg54170

[0077] Impurities from vanadium sources can be even higher.

[0078] JPEG0007697880000004.jpg135170

[0079] In these redox-active electrolyte solutions containing a redox-active electrolyte solution with reduced impurities, the concentration of at least one redox-active substance is at least 0.7 M. In various embodiments, the concentration of at least one redox-active substance is defined by one or more ranges of 0.7 M to 0.8 M, 0.8 M to 0.9 M, 0.9 M to 1 M, 1 M to 1.2 M, 1.2 M to 1.4 M, 1.4 M to 1.6 M, 1.6 M to 1.8 M, 1.8 M to 2 M, 2 M to 2.2 M, 2.2 M to 2.4 M, 2.4 M to 2.6 M, 2.6 M to 2.8 M, 2.8 M to 3 M, 3 M to 3.5 M, or 3.5 M to 4 M. The level of impurities in the final redox-active solution is only relevant in the context of these actually useful concentrations (or higher), and thus should be interpreted in these contexts.

[0080] Impurities are generally described as including one or more of Ag, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Ge, Hg, Ir, Mg, Mn, Mo, Ni, Pb, Pd, Pt, Re, Ru, Sb, Sc, Se, Sn, Sr, Te, V, and Zn, but in certain embodiments, impurities are described as including one or more of As, Ge, Hg, and Sb.

[0081] In these contexts, in some embodiments, the levels of these impurities can be defined on a mass basis (independent of the amount of redox-active electrolyte in the solution), typically on a ppm or ppb basis, with respect to the total mass of the redox-active electrolyte solution. Alternatively, the level of impurities can be defined as a weight basis per unit volume of the redox-active electrolyte solution, for example, on a mg / L or μ / L basis. When the density of the redox-active electrolyte solution is 1 g / L, the corresponding ppm to mg / L and ppb to μg / L numerical values are equivalent.

[0082] Accordingly, in an independent embodiment, the redox active electrolyte solution contains less than about 10 mg ([mg / L]) of one or more predetermined impurities per liter of the redox active electrolyte solution, less than 5 mg / L of one or more predetermined impurities, less than 2.5 mg / L of one or more predetermined impurities, less than 1 mg / L of one or more predetermined impurities, less than 500 μg ([μg / L]) of one or more predetermined impurities per liter of the redox active electrolyte solution, less than 250 μg / L of one or more predetermined impurities, less than 100 μg / L of a predetermined impurity, less than 50 μg / L of one or more predetermined impurities, less than about 40 μg / L of one or more predetermined impurities, less than about 30 μg / L of one or more predetermined impurities, less than about 20 μg / L of one or more predetermined impurities, less than about 10 μg / L of one or more predetermined impurities, less than about 5 μg / L of one or more predetermined impurities, or less than about 1 μg / L of one or more predetermined impurities. Again, for clarity, these statements are intended to apply individually and independently of each other to each substance described as an impurity. For example, by these stated ranges, one specifically represented composition specifying As, Sb, and Ge may contain, for example, less than 5 μg / L of As, less than 10 μg / L of Sb, and less than 50 μg / L of Ge.

[0083] In some embodiments, the electrolyte solution contains less than about 50 μg / L of one or more of As, Ge, Hg, and Sb. In other preferred embodiments, the electrolyte solution contains less than about 5 μg / L of one or more of Ag, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Ge, Hg, Ir, Mg, Mn, Mo, Ni, Pb, Pd, Pt, Re, Ru, Sb, Sc, Sn, Sr, V, and Zn. In even more preferred embodiments, the electrolyte solution contains less than about 5 μg / L of one or more of Sb, As, Ge, Sn, or combinations thereof. In other preferred embodiments, the electrolyte solution contains less than about 5 μg / L of As, Hg, or combinations thereof. In still more preferred embodiments, the electrolyte solution contains less than about 5 μg / L of Sb, As, or combinations thereof. In yet other preferred embodiments, the electrolyte solution contains less than about 10 μg / L of Ge, Sn, or combinations thereof.

[0084] However, since the main sources of incidental impurities are typically the metals used as the redox-active metal (e.g., vanadium) or the redox-active metal ligand coordination compounds (e.g., titanium catecholate), these impurity levels can also (perhaps more appropriately) be defined in terms of the amount of the redox-active electrolyte in the redox-active electrolyte solution on a mass basis. In such cases, the relevant criterion is the parts by weight of the impurity per mole of the redox-active electrolyte in the redox-active electrolyte solution (i.e., on a mg / mol or μg / mol basis). Again, when the concentration of the redox-active electrolyte in the redox-active electrolyte solution is 1 M, the mg / mol or μg / mol values are numerically equivalent to ppm or ppb, respectively. However, in practice, additional embodiments include embodiments where the impurity levels are described in terms of mg / L or μg / L per molar concentration of the redox-active electrolyte in the redox-active electrolyte solution (e.g., a definition of 10 μg of impurity / L (solution) per 0.8 M of redox-active electrolyte solution would correspond to [10 μg / L] / [0.8 mol / L] or 12.5 μg of impurity / mol (of the redox-active electrolyte in the redox-active electrolyte solution)).

[0085] In this case, in an independent embodiment, using a 1M standard, the redox active electrolyte solution contains one or more predetermined impurities of less than about 10 mg per mole of the redox active electrolyte in the redox active electrolyte solution (hereinafter referred to as [mg / mol]), or less than 5 mg / mol, less than 2.5 mg / mol, less than 1 mg / mol, less than 500 μg / mol, less than 100 μg / mol, less than 50 μg / mol, less than 40 μg / mol, less than 30 μg / mol, less than 20 μg / mol, about less than 10 μg / mol, less than 5 μg / mol, or less than 1 μg / mol of one or more predetermined impurities per mole of the redox active electrolyte in the redox active electrolyte solution.

[0086] Similarly, in some embodiments, the electrolyte solution contains one or more of As, Ge, Hg, and Sb of less than about 50 μg / mol. In other preferred embodiments, the electrolyte solution contains one or more of Ag, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Ge, Hg, Ir, Mg, Mn, Mo, Ni, Pb, Pd, Pt, Re, Ru, Sb, Sc, Sn, Sr, V, and Zn of less than about 5 μg / mol. In a more preferred embodiment, the electrolyte solution contains one or more of Sb, As, Ge, Sn, or combinations thereof of less than about 5 μg / mol. In other preferred embodiments, the electrolyte solution contains As, Hg, or combinations thereof of less than about 5 μg / mol. In an even more preferred embodiment, the electrolyte solution contains Sb, As, or combinations thereof of less than about 5 μg / mol. In still other preferred embodiments, the electrolyte solution contains Ge, Sn, or combinations thereof of less than about 10 μg / mol.

[0087] [Use of an Electrochemical (Redox) System with an Electrolyte Solution Having Reduced Impurities] The redox active electrolyte solution of the present invention can also be used for flow batteries or fuel cells. Accordingly, the present disclosure includes flow batteries including at least one half-cell containing one of the electrolyte solutions with reduced impurities described herein, and systems comprising these flow batteries.

[0088] [Terms] Throughout this specification, terms should be given their ordinary meaning as understood by those skilled in the relevant art. However, to avoid misunderstanding, the meaning of certain terms is specifically defined or clarified.

[0089] In the present disclosure, unless the context clearly indicates otherwise, the singular forms ("a," "an," and "the") also include plural references, and references to a particular numerical value include at least that particular value. Thus, for example, a reference to "a material" is a reference to at least one of such materials and their equivalents known to those skilled in the art.

[0090] When a value is expressed as an approximation by use of the descriptor "about," it will be understood that that particular value forms another embodiment. In general, the use of the term "about" indicates an approximation that can vary depending on the desired characteristics to be obtained by the disclosed subject matter and should be interpreted based on its function in the specific context in which it is used. Those skilled in the art will be able to interpret this in the ordinary course. In some cases, the number of significant digits used for a particular value can be one non-limiting way of determining the scope of the term "about." In other cases, the stepwise changes used for a series of values can be used to determine the applicable range of the term "about" for each value. Where present, all ranges are inclusive and combinable. That is, a reference to a value shown in a range includes all values within that range.

[0091] Unless otherwise specified, the term "aqueous" refers to a solvent system that contains at least about 98 wt% water, based on the total weight of the solvent. In some applications, a co-solvent that is soluble, miscible, or partially miscible (emulsified with surfactants or other means) may be present, thereby, for example, extending the range of fluidity of water (e.g., alcohol / glycol). When specified, additional independent embodiments include embodiments where the "aqueous" solvent system contains at least about 55 wt%, at least about 60 wt%, at least about 70 wt%, at least about 75 wt%, at least about 80 wt%, at least about 85 wt%, at least about 90 wt%, at least about 95 wt%, or at least about 98 wt% water, based on the total solvent. In some situations, the aqueous solvent consists essentially of water and may contain substantially no or no co-solvent or other species. The solvent system may be at least about 90 wt%, at least about 95 wt%, or at least about 98 wt% water and, in some embodiments, may contain no co-solvent or other species.

[0092] Unless otherwise specified, the term "non-aqueous" refers to a solvent system that contains less than about 10 wt% water and generally contains at least one organic solvent. Additional independent embodiments include embodiments where the "non-aqueous" solvent system contains less than about 50 wt%, less than about 40 wt%, less than about 30 wt%, less than about 20 wt%, less than about 10 wt%, less than about 5 wt%, or less than about 2 wt% water, based on the total solvent.

[0093] As used herein, the terms "aqueous electrolyte", "electrolyte solution", and variations thereof (generally referred to as "electrolytes") are intended to mean a solvent system that contains at least one substance and has a higher conductivity than a solvent system that does not contain the substance.

[0094] The terms "electrochemically active electrolyte" or "redox active electrolyte", and variations thereof, convey their ordinary meaning to those skilled in the art of electrochemistry. These terms are typically intended to mean an electrolyte composition (compound or solution) capable of redox transfer, i.e., changing its oxidation state or valence state upon application of a potential by the capture or release of at least one electron. In the context of metal-ligand coordination compounds, the metal can be said to have multiple accessible valence states and be redox active, or the ligand can be capable of accepting / releasing electrons and be said to be redox active, or the redox active species can include one or both of a redox active metal and ligand.

[0095] Electrolytes can contain various redox active species. Examples of electrolytes include, among others, vanadium-based electrolytes containing vanadium ions as active materials for both electrodes, iron-chromium-based electrolytes containing iron ions as the positive electrode active material and chromium ions as the negative electrode active material, manganese-titanium-based electrolytes containing manganese ions as the positive electrode active material and titanium ions as the negative electrode active material, and manganese-titanium-based electrolytes containing manganese ions and titanium ions at both electrodes.

[0096] In some embodiments, the electrochemically active electrolyte comprises a metal ligand coordination compound. In other embodiments, the electrochemically active electrolyte comprises a metal ligand coordination compound comprising a redox-active metal ion and / or a redox-inactive metal ion. Preferably, the redox-active metal ion or redox-inactive metal ion is Al, Ca, Co, Cr, Sr, Cu, Fe, Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, V, Zn, or Zr, or a combination thereof. In other embodiments, the electrochemically active electrolyte comprises an organic active material. Preferably, the organic active material is carbon, an aromatic hydrocarbon, or a combination thereof. Examples of aromatic compounds include, but are not limited to, quinone, hydroquinone, viologen, pyridinium, pyridine, acridinium, or catechol. In some preferred embodiments, the electrolyte comprises a vanadium-based electrolyte.

[0097] As used herein, the term "redox couple" is a technical term generally recognized by skilled electrochemists and refers to the oxidized form (electron acceptor) and reduced form (electron donor) of the chemical species of a predetermined redox reaction. Fe(CN)6 3- / Fe(CN)6 4- The pair is merely one non-limiting example of a redox couple. Similarly, the term "redox-active metal ion" is intended to mean that the metal undergoes a change in oxidation state under the conditions of use. As used herein, the term "redox couple" can refer to a pair of organic or inorganic substances.

[0098] In addition to the redox-active material, the electrolyte can contain additional components such as a solvent, a buffer, a supporting electrolyte, a viscosity modifier, a wetting agent, etc. The electrolyte can also contain, as impurities, metal elements including heavy metals, as defined herein. Examples of solvents include aqueous solutions containing at least one of H2SO4, K2SO4, Na2SO4, H3PO4, H4P2O7, K2HPO4, Na3PO4, K3PO4, HNO3, KNO3, HCl, NaNO3, NaOH, or KOH. Alternatively, the solvent can be an organic acid solvent.

[0099] As used herein, the term "impurity" implies its recognized meaning of referring to unwanted chemical species, typically metals or metalloids, that are not intended to be involved in the function of an electrochemical cell, as in the context of a fuel cell or a flow battery, and are often detrimental to the safe and efficient use of the electrochemical cell. It is typically present incidentally and introduced as an impurity (in the sense of the conventional term) along with the intended substances. For example, as discussed elsewhere herein, in some systems, the presence of incidental arsenic, antimony, and other such substances forms precipitates during the operation of a flow battery. Although not considered to be of any use in the operation of a flow battery or a fuel cell, their presence can have an adverse effect on performance. The distinction of defining an impurity as being undesirable or not contemplated for a selected redox-active electrolyte is important. This is because a metal or metalloid that may be considered or even selected for use as a redox-active substance in some applications may be regarded as an impurity when present in other electrolytes. For example, in an electrolyte composition in which a metal ligand coordination compound containing titanium is the selected redox-active electrolyte, the presence of vanadium in the same electrolyte solution is likely to be regarded as an impurity despite the fact that vanadium is selected as the main redox-active electrolyte in other systems.

[0100] As used herein, the term "reduced form of an impurity" means a form of the impurity that has a lower oxidation state than the impurity in the initial electrolyte solution. For example, when most metals or metalloids are present in solution as cationic species or as impurities having a formal positive charge as used herein, the metallic form of such an impurity (i.e., having a formal zero valence state) or the hydride form, e.g., arsine, stibine, germane, etc., has a formal negative valence state and is thus considered a reduced form of the corresponding impurity. The reduced form of an impurity can include any reduced form of the impurity. In some embodiments, the reduced form of the impurity is volatile. In further embodiments, the reduced form of the impurity is a volatile hydride. Common reduced forms of impurities include, but are not limited to, arsine (AsH3), germane (GeH4), stannane (SnH4), stibine (SbH3), or combinations thereof. In some embodiments, the reduced form of the impurity is AsH3. In other embodiments, the reduced form of the impurity is GeH4. In further embodiments, the reduced form of the impurity is SnH4. In still other embodiments, the reduced form of the impurity is SbH3. In further embodiments, the reduced form of the impurity is elemental mercury.

[0101] As used herein, the term "inorganic substance" can include "metal-ligand coordination compounds" or simply "coordination compounds" known to those of ordinary skill in electrochemistry and inorganic chemistry. (Metal-ligand) coordination compounds can include metal ions bonded to atoms or molecules. The bonded atoms or molecules are called "ligands". In certain non-limiting embodiments, the ligands can include molecules containing C atoms, H atoms, N atoms, and / or O atoms. That is, the ligands can include organic molecules. In some embodiments, the coordination compound is water, a hydroxide, or a halide ion (F - , Cl - , Br - , I -Although it includes at least one ligand that is not [as described in the reference], the present invention is not limited to these embodiments. Additional embodiments include the metal-ligand coordination compounds described in U.S. Patent No. 9,768,463, which is hereby incorporated by reference herein, at least with respect to the teachings of the coordination compounds.

[0102] As used herein, the terms "negative electrode" and "positive electrode" are defined relative to each other such that, regardless of the actual potential at which they operate during both charge and discharge cycles, the negative electrode operates (or is designed or intended to operate) at a more negative potential than the positive electrode (or vice versa). The negative electrode may or may not actually operate at a negative potential relative to the reversible hydrogen electrode.

[0103] In the present disclosure, the negative electrode associated with the first aqueous electrolyte of the equilibrium cell may include the same material as or a different material from the negative electrode of the flow battery, but they share a common electrolyte. In contrast, the positive electrode associated with the second aqueous electrolyte of the equilibrium cell almost always includes a different material from the positive electrode of the flow battery, in which case the positive electrolyte of the flow battery is almost always compositionally different from and physically isolated from the second electrolyte of the equilibrium cell.

[0104] The terms "negolite" and "posolite" generally refer to the electrolytes associated with the negative electrode and positive electrode, respectively. However, as used herein, the terms "negolite" and "posolite" are limited to the respective electrolytes of the flow battery. As contemplated herein, the negative working electrolyte (negolite) of the flow battery includes a coordination compound or a metal-ligand coordination compound. In certain embodiments, the negolite includes a metal-ligand coordination complex having the following formula: M(L 1 ) x (L 2 ) y (L 3 ) z m Here, in the formula, M is Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Si, Sn, Ti, V, W, Zn, or Zr, and L 1 、L 2 、and L 3 are each independently ascorbate, catecholate, citrate, glycolate, or a polyol (including ligands derived from ethylene glycol, propylene glycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g., α-hydroxyacetate, or derived from glycolic acid), β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, phthalate, pyrogallate, sarcosinate, salicylate, or lactate, x, y, and z are independently 0, 1, 2, or 3, 1 ≦ x + y + z ≦ 3, and m is +1, 0, -1, -2, -3, -4, or -5.

[0105] Related independent embodiments provide that (a) x = 3, y = z = 0, (b) x = 2, y = 1, z = 0, (c) x = 1, y = 1, z = 1, (d) x = 2, y = 1, z = 0, (e) x = 2, y = z = 0, or (f) x = 1, y = z = 0. In individual preferred embodiments, M is Al, Cr, Fe, or Ti, and x + y + z = 3. In a more preferred embodiment, the negolite contains a metal ligand coordination compound of titanium. In other preferred embodiments, the negolite contains a metal ligand coordination compound of vanadium.

[0106] In other embodiments, the redox active material relates to a compound having the following formula: M n Ti(L1)(L2)(L3) as described. Here, in the formula, L1 is catecholate, and L2 and L3 are each independently selected from catecholate, ascorbate, citrate, glycolate, polyol, gluconate, glycinate, hydroxyalkanoate, acetate, formate, benzoate, malate, maleate, phthalate, sarcosinate, salicylate, oxalate, urea, polyamine, aminophenolate, acetylacetone, or lactate, each M is independently Na, Li, or K, and n is an integer of 0 or 1 to 6, provided that when both L1 and L2 are catecholate, L3 is not oxalate, urea, catecholate, or acetylacetone.

[0107] In some embodiments, the catecholate includes 1,2-dihydroxybenzene, 1,2,3-trihydroxybenzene, 1,2,4-trihydroxybenzene, or a mixture thereof. Preferred embodiments are the following formula: M n Ti(catecholate)2(hydroxycatecholate), or M n Ti(catecholate)3 and include compositions having the same.

[0108] In other embodiments, the redox-active composition is one or more compositions having the following formula: M n Ti(L1)(L2)(L3) or includes the composition. Here, in the formula, L1 is catecholate, and L2 and L3 are each independently selected from catecholate, ascorbate, citrate, glycolate, polyol, gluconate, glycinate, hydroxyalkanoate, acetate, formate, benzoate, malate, maleate, phthalate, sarcosinate, salicylate, oxalate, urea, polyamine, aminophenolate, acetylacetone, or lactate, each M is independently Na, Li, or K, and n is an integer of 0 or 1 to 6. L1, L2, or L3 may also be the following chemical formula I:

Chemical formula

[0109] Other suitable active substances may include "organic active substances". Organic active substances may include molecules or supramolecules that do not contain transition metal ions. Furthermore, it is understood that organic active substances are interpreted to include molecules or supramolecules that are dissolved in an aqueous solution. Suitable organic active substances can undergo a change in oxidation state during the operation of an electrochemical energy storage system. Thus, the molecule or supramolecule can accept or donate electrons during the operation of the system.

[0110] The terms "catecholate", "glycolate", "polyol", "hydroxyalkanoate", "benzoate", "phthalate", "urea", and "polyamine" mean that these ligands are H, C 1~6 alkoxy, C 1~6 alkyl, C 1~6 alkenyl, C 1~6 alkynyl, a 5- or 6-membered aryl or heteroaryl, boric acid or its salt, C0~6 It means that it can be optionally substituted with at least one group independently selected from alkylene-carboxylic acid or its salt, cyano, halogen, hydroxyl, nitro, sulfonate, sulfonic acid or its salt, phosphonate, phosphonic acid or its salt, or polyglycol (preferably polyethylene glycol).

[0111] Alcanoates include α-form, β-form, and γ-form. Polyamines include, but are not limited to, diamines and triamines such as ethylenediamine, ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). Catecholates include all compositions containing a 1,2-dihydroxybenzene moiety. Such moieties include not only hydroxycatecholates (including pyrogallates), but also the substituents listed herein. Substituents include, but are not limited to, alkyl, alkenyl, and alkynyl (each referring to a branched or linear structure and a structure optionally substituted with one or more carboxyl groups, halogen groups, hydroxyl groups, or other electron-withdrawing or electron-donating groups). Substituents also include 5-membered or 6-membered aryl or heteroaryl, including phenyl, pyridinyl, furyl, pyrrolyl, imidazolyl, triazole, or thiophenyl. By adding an electron-withdrawing substituent or an electron-donating substituent to the periphery of an aromatic ring, the redox potential of a redox-active ligand can be adjusted.

[0112] The terms "parts per million (ppm)" and "parts per billion (ppb)" are defined on a mass basis relative to the total mass of the electrolyte solution to which the term applies. As described elsewhere in this specification, the terms "mg / mol" and "μg / mol" refer to the concentration of impurities relative to the amount of redox-active electrolyte in the same redox-active electrolyte solution.

[0113] The term "stack" or "cell stack" or "electrochemical cell stack" refers to a collection of individual electrochemical cells that are electrically connected. The cells can be electrically connected in series or in parallel. The cells may or may not be fluidly connected.

[0114] [Embodiments] Aspect 1. A method for preparing an electrolyte solution having a reduced impurity level, comprising: a. electrochemically reducing impurities present at an initial concentration in an initial electrolyte solution further comprising a redox-active electrolyte at a concentration of at least 0.5 M under conditions sufficient to produce an electrochemically treated electrolyte solution containing a reductant of the redox-active electrolyte and a reductant of the impurities; b. separating the reductant of the impurities from the electrochemically treated electrolyte solution to obtain a final electrolyte solution containing impurities having a final concentration lower than the initial concentration of the impurities; The method comprising.

[0115] Aspect 2. The final concentration of the impurities in the final electrolyte solution is a predetermined threshold level, (i) one or more predetermined impurities of less than about 10 mg / L, less than 5 mg / L, less than 2.5 mg / L, less than 1 mg / L, less than 500 μg / L, less than 250 μg / L, less than 100 μg / L, less than 50 μg / L, less than about 40 μg / L, less than about 30 μg / L, less than about 20 μg / L, less than about 10 μg / L, less than about 5 μg / L, or less than about 1 μg / L, or (ii) impurities of less than about 10 mg ([mg / mol]) per mole of the redox-active electrolyte in the redox-active electrolyte solution, less than 5 mg / mol, less than 2.5 mg / mol, less than 1 mg / mol, less than 500 μg / mol, less than 250 μg / mol, less than 100 μg / mol, less than 50 μg / mol, less than about 40 μg / mol, less than about 30 μg / mol, less than about 20 μg / mol, less than about 10 μg / mol, less than about 5 μg / mol, or less than about 1 μg / mol of one or more predetermined impurities, The method of Aspect 1, which is.

[0116] Aspect 3. The method according to Aspect 1 or 2, wherein the impurity contains antimony, arsenic, germanium, tin, or a combination thereof in a form.

[0117] Aspect 4. The method according to any one of Aspects 1 to 3, wherein the electrochemical treatment is carried out in an electrochemical cell.

[0118] Aspect 5. The method according to Aspect 4, wherein the reduced form of the impurity is separated by plating in the positive electrode of the electrochemical cell.

[0119] Aspect 6. The method according to any one of Aspects 1 to 5, wherein the reduced form of the impurity precipitates from the electrochemically treated electrolyte solution.

[0120] Aspect 7. The method according to Aspect 6, wherein the precipitated reduced form of the impurity is removed by filtration.

[0121] Aspect 8. The method according to any one of Aspects 1 to 4, wherein the reduced form of the impurity is a volatile hydride.

[0122] Aspect 9. The method according to Aspect 8, wherein the volatile hydride is arsine (AsH3), germane (GeH4), stannane (SnH4), stibine (SbH3), or a combination thereof.

[0123] Aspect 10. The method according to any one of Aspects 1 to 9, wherein the electrochemical treatment is electrochemical reduction and is carried out at a redox potential more negative than the reduction potential of the impurity.

[0124] Aspect 11. The electrochemically treated electrolyte solution is (a) heating the electrochemically treated electrolyte solution at a temperature in the range of 20 °C to about 105 °C, (b) purging the electrochemically treated electrolyte solution or the electrolyte solution heated in step (a) using an inert gas, or (c) a combination of step (a) and step (b), The method according to any one of Aspects 1 to 10, further comprising conditioning by

[0125] Aspect 12. The method according to Aspect 11, wherein the temperature in the heating step (a) is in the range of about 35°C to about 95°C, or more preferably about 45°C to about 85°C.

[0126] Aspect 13. The method according to Aspect 11 or 12, wherein the inert gas is nitrogen or argon.

[0127] Aspect 14. The method according to any one of Aspects 11 to 13, wherein steps (a) and (b) are carried out simultaneously.

[0128] Aspect 15. The method according to any one of Aspects 11 to 13, wherein steps (a) and (b) are carried out sequentially.

[0129] Aspect 16. The method according to any one of Aspects 1 to 15, further comprising oxidizing the reductant of the redox-active electrolyte in the final electrolyte solution.

[0130] Aspect 17. The method according to Aspect 16, wherein the oxidation is carried out by purging the final electrolyte solution with an oxidizing agent such as oxygen, preferably air.

[0131] Aspect 18. The method according to any one of Aspects 1 to 15, wherein the oxidation is carried out using hydrogen peroxide.

[0132] Aspect 19. The method according to any one of Aspects 16 to 18, wherein the oxidation is carried out while heating the final electrolyte solution.

[0133] Aspect 20. The method according to Aspect 16 or 19, wherein the oxidation is carried out at a temperature of about 65°C or higher, preferably about 85°C or higher, more preferably about 105°C or higher.

[0134] Aspect 21. The method according to Aspect 16, wherein the oxidation is carried out using a hydrogen generation catalyst.

[0135] Aspect 22. The method according to Aspect 21, wherein the hydrogen generation catalyst is activated carbon, carbon cloth, carbon felt, carbon paper, Ti mesh, Ti felt, foamed Ti mesh, Pt-plated Ti mesh, or a combination thereof.

[0136] Aspect 23. The redox active electrolyte is (i) a metal ligand coordination compound containing Al, Ca, Co, Cr, Sr, Cu, Fe, Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, V, Zn, or Zr, (ii) an organic active substance, preferably carbon, aromatic hydrocarbon, such as quinone, hydroquinone, viologen, pyridinium, pyridine, acridinium, or catechol, or (iii) a combination of (i) or (ii), The method according to any one of Aspects 1 to 22.

[0137] Aspect 24. The method according to Aspect 23, wherein the redox active electrolyte contains a metal ligand coordination compound containing a combination of Co, Cr, Cu, Fe, Mn, Mo, Ru, Sn, Ti, V, or Zr.

[0138] Aspect 25. An electrolyte solution prepared by the method according to any one of Aspects 1 to 24.

[0139] Aspect 26. An electrolyte solution comprising (i) a redox active electrolyte having a concentration of at least 0.5 M, preferably containing a metal ligand coordination compound containing a metal or metalloid, more preferably titanium, and (ii) impurities present in an amount of less than about 500 μg per liter of the electrolyte solution or less than 500 μg per mole of the redox active electrolyte, An electrolyte solution.

[0140] Aspect 27. The electrolyte solution according to Aspect 25 or 26, containing one or more of As, Ge, Hg, and Sb in an amount of less than about 50 μg / L.

[0141] Aspect 28. An electrolyte solution according to any one of Aspects 25 to 27, comprising any one form of antimony, arsenic, germanium, tin, or a combination thereof, in an amount of less than about 20 μg / L, less than about 15 μg / L, less than about 10 μg / L, or less than about 5 μg / L.

[0142] Aspect 29. An electrolyte solution according to any one of Aspects 25 to 28, comprising one or more of antimony, arsenic, germanium, tin, or a combination thereof, in an amount of less than about 5 μg / L.

[0143] Aspect 30. An electrochemical cell comprising an electrolyte solution according to any one of Aspects 25 to 29.

[0144] Aspect 31. A redox flow battery comprising at least one electrochemical cell according to Aspect 30.

[0145] Aspect 32. An electrochemical cell comprising a first half-cell chamber and a second half-cell chamber separated by a cation exchange membrane, wherein the first half-cell chamber comprises a first electrode, preferably a carbon electrode, in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and the second half-cell chamber comprises a second electrode in contact with a second aqueous electrolyte, wherein the second aqueous electrolyte comprises one or more salts containing an aprotic cation at a concentration of at least 0.1 M, and the second electrode preferably comprises a catalyst for generating O2, including nickel such as Ni foam, stainless steel mesh, stainless steel felt, Ni oxide, Ni hydroxide, Ni oxyhydroxide, or Ni-Fe oxide.

[0146] Aspect 33. An electrochemical cell comprising a first half-cell chamber and a second half-cell chamber separated by a cation exchange membrane, wherein the first half-cell chamber comprises a first electrode, preferably a carbon electrode, such as carbon cloth, carbon felt, or carbon paper, in contact with a first aqueous electrolyte containing a redox active material and reducible impurities, and The second half-cell chamber is in contact with a second electrode in contact with a second aqueous electrolyte having at least 2 pH and containing one or more salts having an aprotic cation at a concentration of at least 0.1 M. The second electrode comprises an electrochemical cell comprising a catalyst that generates O2, preferably platinum, or at least one of oxides of cobalt, iridium, iron, manganese, nickel, ruthenium, tin, or combinations thereof, most preferably IrO2.

[0147] Aspect 34. The electrochemical cell of aspect 32 or 33, wherein the reducible impurity forms a volatile reductant, such as arsine (AsH3), germane (GeH4), stannane (SnH4), or stibine (SbH3), upon electrochemical reduction.

[0148] Aspect 35. The second aqueous electrolyte contains Na + ions and / or K + ions and has a pH of at least 7, for any one of the electrochemical cells of aspects 32-34.

[0149] Aspect 36. A method of operating an electrochemical cell of any one of aspects 32-35, the method comprising passing a sufficient current through the electrochemical cell under conditions sufficient to reduce the concentration of the reducible impurity in the first aqueous electrolyte to a predetermined level, preferably less than 10 μg / L, in the first aqueous electrolyte. [Examples]

[0150] The following examples are presented to illustrate some of the concepts described within the present disclosure. Each example is considered to illustrate a specific individual embodiment of the composition, preparation method, and method of use, but no example should be considered to limit the more general embodiments described herein.

[0151] [Example 1] As described above, in certain embodiments, the electrochemical cell comprises a first half-cell chamber and a second half-cell chamber separated by a membrane, where the first half-cell chamber is in contact with a first electrode, preferably a carbon electrode such as carbon cloth, carbon felt or carbon paper, in contact with a first aqueous electrolyte containing a redox-active electrolyte (e.g., negolite) and reducible impurities, and the second half-cell chamber comprises a second electrode in contact with a second aqueous solution that is preferably acidic but has a pH of at least 2, where the second electrode comprises a catalyst for generating O2, preferably platinum, or at least one of oxides of cobalt, iridium, iron, manganese, nickel, ruthenium, tin or combinations thereof, most preferably IrO2.

[0152] To demonstrate an exemplary embodiment in this context, an experiment was conducted using an iridium oxide catalyst coated on a cation exchange membrane with a carbon electrode as the negative electrode and a carbon cloth isolated from the membrane by a non-conductive layer as the positive electrode (see Fig. 3(A)). The non-conductive layer was made of a material that was electronically insulating but fluid permeable, such as a grid or a foam. It has been found that a melamine foam is useful for the operation of this non-conductive layer. The non-conductive layer separated the reduction of neolith from the flow of acid from the negative electrode that could cause the decomposition of neolith as described elsewhere in this specification. In this configuration, an electrochemical cell was used as a flow cell in which a redox active electrolyte (neolith) solution containing a titanium ligand coordination compound (e.g., a titanium-catecholate complex) flowed through the positive electrode. Water for the negative electrode was supplied by diffusion from the neolith solution. In practice, this design has been limited to charging the redox active electrolyte solution at a current density of about 30 mA or 40 mA per square centimeter. It was thought that the current density was limited by the transport of acid to the positive electrode that caused the decomposition of the titanium complex. Despite these initial limitations, such a configuration has been demonstrated to provide a proof of concept of the principles described herein, including a final redox active electrolyte solution containing a titanium-catecholate complex at a concentration greater than 0.7 M and containing arsenic at a level of less than 10 μg / L as an exemplary impurity.

[0153] [Example 2] As described above, in certain embodiments, the electrochemical cell comprises a first half-cell chamber and a second half-cell chamber separated by a membrane, where the first half-cell chamber comprises a first electrode, preferably a carbon electrode, in contact with a first aqueous electrolyte containing a redox active material and a reducible impurity, and the second half-cell chamber comprises a second electrode in contact with a second aqueous alkaline (anolyte) solution, where the second electrode preferably comprises a nickel, such as Ni foam, stainless steel mesh, stainless steel felt, Ni oxide, Ni hydroxide, Ni oxyhydroxide, or a catalyst for generating O2 containing Ni-Fe oxide.

[0154] In an exemplary embodiment in this context, the negative electrode solution was a concentrated solution of sodium hydroxide, potassium hydroxide, phosphate buffer, or a mixture thereof (see Figure 3(B)). The negative electrode consisted of a normal alkaline oxidation catalyst, in these cases nickel or nickel-coated steel. The positive electrode was a carbon electrode. The positive and negative electrodes were separated by a cation exchange membrane, for example, NAFION® perfluorosulfonic acid resin or sulfonated polyether ether ketone (s-PEEK). This cell was also designed as a flow cell in which both a redox active electrolyte (negolite) solution containing a titanium ligand coordination compound (e.g., a titanium-catecholate complex) and the negative electrode solution were pumped throughout the cell. Again, acceptable results as described in Example 1 were achieved.

[0155] [Example 3] 25 cm 2 The HERM device of was constructed using a Ni foam negative electrode (346 g / m 2 , 1.6 mm thickness, porosity: ≥95%, 80 ppi - 110 ppi, MTI Inc.), a Nafion N117 cation exchange membrane, and a carbon cloth positive electrode. 1 M KOH was used as the negative electrode solution. Using this device, a Ti-catecholate negolite with a concentration of 1.2 M containing 3.1 ppm As and 2.9 ppm Sn as measured by ICP-OES was purified. The charging of the negolite proceeded at 180 mA / cm 2 and reached a solution potential of -1.36 V versus Ag / AgCl. After sparging with nitrogen and magnetically stirring at 45 °C for 16 hours, the electrolyte was refluxed and further sparged with nitrogen for 20 hours to condition the electrolyte. At the end of these conditioning steps, less than 1.0 ppb As and 1.6 ppm Sn were seen by ICP-OES, indicating that the concentrations of these impurities had decreased by >99% and 45% respectively.

[0156] [Example 4] 25 cm 2The HERM device was constructed using a carbon cloth negative electrode facing the IrO₂-coated surface of a Nafion N117 membrane. A melamine foam non-conductive layer was placed between the membrane and the carbon cloth positive electrode. Using this device, Ti-catecholate negolite with a concentration of 1.2 M containing 3.1 ppm of As was purified. The charging of the negolite proceeded at 40 mA / cm 2 and the solution potential became -1.34 V versus Ag / AgCl. The electrolyte was conditioned by sparging with nitrogen and magnetically stirring at 65 °C for 2.5 hours. After this conditioning step, it was found that the electrolyte contained 6.7 ppb of As and the As concentration was reduced by more than 99%.

[0157] [Example 5] Ti-catecholate negolite with a concentration of 1.2 M contaminated with more than 20 mg / L of Zn was purified. As a result of charging this electrolyte to -1.42 V versus Ag / AgCl, extensive Zn plating occurred on the carbon positive electrode of the HERM device (Figure 4 shows the Zn plated on the HERM positive electrode).

[0158] As will be understood by those skilled in the art, in view of these teachings, many modifications and variations of the present invention are possible and all such are contemplated herein.

[0159] The disclosure of each patent, patent application, and publication cited or described herein is hereby incorporated by reference in its entirety for all purposes as part of this specification.

Claims

1. A method for preparing an electrolyte solution having a reduced impurity level, comprising: a. electrochemically treating an initial electrolyte solution containing a redox-active electrolyte at a concentration of at least 0.5 M to electrochemically reduce impurities present at an initial concentration in the initial electrolyte solution, wherein the redox-active electrolyte is a metal ligand coordination complex containing Al, Ca, Co, Cr, Sr, Cu, Fe, Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, V, Zn, or Zr; b. heating the electrochemically treated electrolyte solution at a temperature in the range of 20°C to 105°C; c. purging the electrochemically treated electrolyte solution or the electrolyte solution heated in step b using an inert gas; d. separating the reduced form of the impurities from the electrochemically treated electrolyte solution to obtain a final electrolyte solution containing impurities having a final concentration lower than 10 mg ([mg / mol]) per mole of the redox-active electrolyte in the redox-active electrolyte solution or lower than 10 mg / L. A method comprising the above steps.

2. The method according to claim 1, wherein the impurities include antimony, arsenic, germanium, tin, or a combination thereof.

3. The method according to claim 1, wherein the electrochemical treatment is carried out in an electrochemical cell.

4. The method according to claim 3, wherein the reduced form of the impurities is separated by plating in the positive electrode of the electrochemical cell.

5. The method according to claim 1, wherein the reduced form of the impurities precipitates from the electrochemically treated electrolyte solution.

6. The method according to claim 5, wherein the precipitated reduced form of the impurities is removed by filtration.

7. The method according to claim 1, wherein the reduced form of the impurities is a volatile hydride.

8. The volatile hydride is arsine (AsH3), germane (GeH ４ ), stannane (SnH ４ ), stibine (S ｂ H ３ ), or a combination thereof. The method according to claim 7.

9. The method according to claim 1, wherein the electrochemical treatment is electrochemical reduction and is carried out at a redox potential more negative than the reduction potential of the impurities.

10. The method according to claim 1, wherein the temperature in step b of the heating is in the range of 35°C to 95°C.

11. The method according to claim 1, wherein the inert gas is nitrogen or argon.

12. The method according to claim 1, wherein steps b and c are carried out simultaneously.

13. The method according to claim 1, wherein steps b and c are carried out sequentially.

14. The method according to claim 1, further comprising oxidizing a reductant of the redox active electrolyte in the final electrolyte solution.

15. The method according to claim 14, wherein the oxidation is carried out by purging the final electrolyte solution with an oxidizing agent.

16. The method according to claim 14, wherein the oxidation is carried out using hydrogen peroxide.

17. The method according to claim 14, wherein the oxidation is carried out while heating the final electrolyte solution.

18. The method according to claim 14, wherein the oxidation is carried out at a temperature of 65 °C or higher.

19. The method according to claim 14, wherein the oxidation is carried out using a hydrogen generation catalyst.

20. The method according to claim 19, wherein the hydrogen generation catalyst is activated carbon, carbon cloth, carbon felt, carbon paper, Ti mesh, Ti felt, foamed Ti mesh, Pt-plated Ti mesh, or a combination thereof.

21. The method according to claim 1, wherein the redox active electrolyte further comprises an organic active substance.

22. The method according to claim 1, wherein the metal ligand coordination complex comprises Co, Cr, Cu, Fe, Mn, Mo, Ru, Sn, Ti, V, or Zr.

23. The metal ligand coordination complex is represented by the formula: M(L １ ) ｘ (L ２ ) ｙ (L ３ ) ｚ ｍ and Here, in the formula, M is Al, Ca, Co, Cr, Fe, Mg, Mn, Mo, Sn, Ti, V, Zn, or Zr, and L １ , L ２ , and L ３ are each independently ascorbate, catecholate, citrate, glycolate, or polyol, gluconate, glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, phthalate, pyrogallate, sarcosinate, salicylate, or lactate, x, y, and z are independently 0, 1, 2, or 3, 1 ≦ x + y + z ≦ 3, and m is +1, 0, -1, -2, -3, -4, or -5. The method according to claim 1.

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