EP4713308A2 - Improved production of reaction products from an electrosynthetic reaction - Google Patents

Abstract

Conventional redox flow batteries can be limited by the solubility of raw materials that are used to form the negolyte. Disclosed herein are methods for improving the solubility of negolyte raw materials, by conversion to more soluble compounds that retain a desired reactivity with negolyte reagents under operating conditions.

Description

IMPROVED PRODUCTION OF REACTION PRODUCTS FROM AN ELECTROSYNTHETIC REACTION

REFERENCE TO RELATED APPLICATIONS

[0001] This application is being filed on May 14, 2024, as a PCT International Patent application and claims the benefit of and priority to U.S. Provisional Patent Application Serial No. 63/502,397, filed May 15, 2023, the disclosure of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] This invention relates generally to electrochemical synthesis of product compounds. More specifically, the invention relates to product compounds that are made through chemical or electrochemical reduction of a raw material to create a reactive intermediate, which is then reacted with a reagent to form the product compound. By use of a redox-active compound called a solubility shuttle, product compounds that have high water solubility can be made easily from raw materials that have poor or no water solubility to begin with. Some of the product compounds are useful as negolyte active materials for redox flow batteries, are additionally composed only of earth-abundant elements, and possess high chemical and electrochemical stability.

STATEMENT OF GOVERNMENT INTEREST

[0003] Certain aspects of the present invention were made with government support from the U.S. Department of Energy under award number DE-EE0009795. The government has certain rights to the invention.

BACKGROUND

[0004] In the production of specialty chemicals, including the production of chemical reactants for organic redox flow batteries (RFBs), the cost of manufacture is critically important for the commercial viability of systems or materials that make use of those specialty chemicals. The overall cost of manufacture is highly dependent on certain aspects such as yield, cost of raw materials, ease of workup and purification, waste disposal, production rate, and so on.

[0005] An innovative process for producing a RFB negolyte is described in the commonly-owned patent application US 63/215,079 (“the ‘079 application”), the contents of which are incorporated by reference herein in their entirety.

[0006] The ‘079 application is an electrochemical process that starts by converting a substituted anthraquinone raw material into a dihydroxy anthracene species (the reduced form of anthraquinone), by means of electrochemical reduction using an electrochemical system. The dihydroxyanthracene species behaves as a reactive intermediate, reacting with a negolyte reagent (frequently an aldehyde but also an interconvertible aldehyde such as a reducing sugar like glucose) to form a carbon-carbon bond at the 2-position of the anthraquinone/anthracene core which has the net effect of attaching a side chain to the core. Subsequent heating re-forms the anthraquinone core through a disproportionation reaction, or the anthraquinone core can also be re-formed through exposure to atmospheric oxygen or electrochemical re-oxidation in the electrochemical system. Depending on the choice of the negolyte reagent, the side chain imparts desirable properties to the product RFB negolyte that is formed. Such desirable properties include increased water solubility which increases the energy' density' of the RFB, and more negative reduction potential which increases the cell voltage and therefore also the energy density. Other desirable properties include improved chemical and electrochemical stability when the product RFB negolyte is cycled in a RFB.

[0007] One unaddressed limitation of the process in the ‘079 application is the solubility of the anthraquinone raw- material. The raw material is delivered to the electrochemical system in a liquid medium, whether as a dissolved species or as a solid slurry, where it is flowed through or past an electrode that delivers electrons to form the reduced form of the anthraquinone raw material, which is a reactive intermediate. Frequently, the raw material has considerably lower solubility' in the solvent (e.g., water) than the reactive intermediate or the product RFB negolyte. However, electrochemical systems frequently employ many components that are prone to clogging with even a small amount of suspended solid, such as porous electrodes that are designed for fluid to flow through them, or flow plates that have narrow channels.

[0008] A raw material with low solubility may have to be added very' slowly to the reaction vessel so as to stay below the solubility’ limit, a cosolvent may have to be used, or a non-electrochemical method such as hydrogenation or chemical reduction with a reducing agent may have to be employed to convert all or part of the raw material into the more soluble reactive intermediate. All the preceding methods come with some drawbacks, such as increased process complexity or the introduction of additional chemical species or solvents that may have to be cleaned up after the reaction is complete. [0009] This invention solves the problem of poor raw material solubility by introducing the use of a “solubility shuttle’; a chemical species that is present in the reaction mixture. The chemical species has high solubility in its reduced and oxidized states, and has a more negative reduction potential than the anthraquinone raw material. In other words, the reduced form of the solubility shuttle is able to react with and reduce the anthraquinone raw material to form the reactive intermediate that has increased solubility. The solubility shuttle can be continually circulated to the electrochemical system for regeneration by electrochemical reduction, and the reduced solubility⁷ shuttle returned to a mixing reservoir where the anthraquinone raw material is added and dissolved as a result of conversion into the reactive intermediate. The solubility shuttle can be any chemical species, even the product RFB negolyte itself.

SUMMARY

[0010] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify required or essential features of the claimed subject matter. Nor is this summary' intended to be used to limit the scope of the claimed subject matter.

[0011] This disclosure is directed to methods for improving the ease of handling reactants in electrochemical reactions, and liquid compositions comprising electrochemically active reagents and solubilizing agents. These agents improve the solubility⁷ of the raw material by the use of solubilizing compounds within the reaction mixture.

[0012] A variety of additional inventive aspects will be set forth in the description that follows. The inventive aspects can relate to individual features and to combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary' and explanatory⁷ only and are not restrictive of the broad inventive concepts upon which the embodiments disclosed herein are based. BRIEF DESCRIPTION OF THE DRAWING

[0013] The accompanying drawing, which are incorporated in and constitute a part of the description, illustrate several aspects of the present disclosure. A brief description of the drawing is as follows:

[0014] FIG. 1 is a schematic view of an electrochemical system compatible with solubility shuttle methods disclosed herein and comprising a divided electrochemical flow cell.

DEFINITIONS

[0015] To define more clearly the terms used herein, the following definitions are provided. Unless otherwise indicated, the following definitions are applicable to this disclosure. If a term is used in this disclosure but is not specifically defined herein, the definition from the IUPAC Compendium of Chemical Terminology, 2^nd Ed (1997), can be applied, as long as that definition does not conflict with any other disclosure or definition applied herein, or render indefinite or non-enabled any claim to which that definition is applied. To the extent that any definition or usage provided by any document incorporated herein by reference conflicts with the definition or usage provided herein, the definition or usage provided herein controls.

[0016] While compositions and methods are described herein in terms of “comprising” various components or steps, the compositions and methods also can “consist essentially of’ or “consist of’ the various components or steps, unless stated otherwise. For example, a solubility shuttle consistent with aspects of the present invention can comprise; alternatively, can consist essentially of; or alternatively, can consist of; an anthraquinone compound, a flavin compound, a phenazine compound, a viologen compound, vanadium (II/III) ions, chromium(II/III) ions, metal-ligand complexes, and other species commonly encountered as RFB negolytes.

[0017] Generally, groups of elements are indicated using the numbering scheme indicated in the version of the periodic table of elements published in Chemical and Engineering News, 63(5), 27, 1985. In some instances, a group of elements can be indicated using a common name assigned to the group; for example, alkali metals for Group 1 elements, alkaline earth metals for Group 2 elements, transition metals for Group 3-12 elements, and halogens or halides for Group 17 elements. [0018] For any particular formula or name that is presented, any general formula or name presented also encompasses all conformational isomers, regioisomers, and stereoisomers that can arise from a particular set of substituents.

[0019] The term “contacting” is used herein to describe compositions, processes, and methods in which the materials or components are combined together in any order, in any manner, and for any length of time, unless otherwise specified. For example, the materials or components can be blended, mixed, slurried, dissolved, reacted, treated, compounded, or otherwise contacted or combined in some other manner or by any suitable method or technique.

[0020] With respect to amounts of compositional components and other physical attributes described herein, the terms “substantially” or “about” may be used to refer to terms of degree. For example, a substantially soluble substance need not be purely soluble, and should be expected to have some insolubility. Dimensions, rates, and other measurable quantities should be understood to be approximate as determined by machining tolerances, manufacturing variability, measurement limitations, and the like. A “high-purity” material as described herein should be understood to refer to a purity greater than typical, conventional purity for that material, which could be greater than 80%, 90%, 95%, or 99%, or even higher depending upon the material being obtained.

[0021] Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the typical methods, devices, and materials are herein described.

[0022] All publications and patents mentioned herein are incorporated herein by reference in their entirety for the purpose of describing and disclosing, for example, the constructs and methodologies that are described in the publications and patents, which might be used in connection with the presently described invention.

[0023] Several types of ranges are disclosed in the present invention. When a range of any type is disclosed or claimed, the intent is to disclose or claim individually each possible number that such a range could reasonably encompass, including end points of the range as well as any sub-ranges and combinations of sub-ranges encompassed therein. For example, when a solubility within a certain range is disclosed or claimed, the intent is to disclose or claim individually every⁷ possible number that such a range could encompass, consistent with the disclosure herein. For example, the disclosure that a solubility is in a range from 0.01 mol/L to 10 mol/L, as used herein, refers to a solubility of 0. 1 mol/L, 1 mol/L, 2 mol/L, 3 mol/L, 4 mol/L, 5 mol/L, 6 mol/L. 7 mol/L, 8 mol/L, 9 mol/L, or 10 mol/L. as well as any range between these two numbers (for example, from 1 mol/L, to 2 mol/L,). The unit “M” is also used to mean “moles per liter” and is used interchangeably with “mol/L”.

[0024] In general, an amount, size, formulation, parameter, range, or other quantity or characteristic is “about” or “approximate” whether or not expressly stated to be such. Whether or not modified by the term “about” or “approximately.” the claims include equivalents to the quantities or characteristics.

DETAILED DESCRIPTION

[0025] The information that follows describes embodiments with reference to the accompanying figures, however, may be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein.

[0026] The present disclosure relates to electrochemical processes comprising providing in a first tank of an electrochemical system in fluid connection to a negative electrode, an aqueous solution comprising a first redox-active species that has the properties of and functions as a solubility shuttle; providing an oxidizable species to a second tank of the electrochemical system in fluid connection to a positive electrode; circulating the aqueous solution in the first tank to the negative electrode and the oxidizable species in the second tank to the positive electrode; applying a reducing voltage such that the solubility’ shuttle is electrochemically reduced and the oxidizable species is electrochemically oxidized; and adding a negolyte raw material and a negolyte reagent to the first tank. In certain aspects, the solubility shuttle can have a lower reduction potential than the negolyte raw material such that the negolyte raw material is reduced by reaction with the reduced form of the solubility- shuttle, thereby forming a reduced negolyte raw material as a reactive intermediate and the oxidized form of the solubility shuttle, the latter of which remains in solution.

[0027] The present disclosure also relates to aqueous negolyte solutions comprising a substituted anthraquinone with a hydroxyl group or an amine group at the 1 -position and no substitution at the 2-position, wherein the amine group can be an unsubstituted amine, a monoalkylated amine, or a dialk lated amine; a substituted dihydroxyanthracene corresponding to the reduced form of the substituted anthraquinone; a second quinone having a reduction potential lower than that of the substituted anthraquinone and an aqueous solubility greater than 0.1 mol/L at pH 14; a negolyte reagent; a product RFB negolyte: a base such as sodium hydroxide or potassium hydroxide, and water.

[0028] The present disclosure relates to electrochemical systems for conducting electrochemical reactions which generate a current, as an energy storage device. In certain aspects, systems disclosed herein can comprise a divided electrochemical cell, a first tank in fluid connection to a negative electrode of the divided electrochemical cell, a second tank in fluid connection to a positive electrode of the divided electrochemical cell, and a plurality of pumps to circulate the contents of each tank to the respective electrodes and back. In certain aspects, the first tank can comprise any aqueous negolyte solution disclosed herein. In other aspects, the second tank can comprise an oxidizable chemical species capable of supplying electrons to the negative electrode via the contact with the positive electrode.

[0029] Among various kinds of quinones that have been reported for flow batteries in the prior art, anthraquinones that are made by the Marschalk reaction (whether chemically or electrochemically as in the ‘079 application) have a known utility as negolytes in electrochemical processes within redox flow batteries. Generally, methods proceed according to Scheme 1 below, using 1,8-DHAQ as an example of a negolyte raw material, in the presence of base, to form 1,8,9, 10-tetrahydroxy anthracene, the reduced form of the negolyte raw material, which is a reactive intermediate. The 1,8.9.10- tetrahydroxyanthracene reacts with glyoxylic acid (as an example of a negolyte reagent) to form a second intermediate which then disporportionates. The resulting quinone is reduced again to form a third intermediate which disproporti onates a second time to form the product RFB negolyte, 2,7-bis(carboxymethyl)-l,8-dihydroxyanthraquinone (1,8- DCDHAQ). A similar process would exist for 1 -hydroxyanthraquinone, wherein the first disproportion process directly produces the desired product, 2-(carboxy methyl)- 1- hydroxy anthraquinone rather than requiring a second reduction and disproportionation.

Scheme 1

[0030] The process above requires the negolyte raw material, in its oxidized and reduced form, and any subsequent intermediates, to be dissolved in aqueous base so that it can be effectively transported to the electrochemical system for reduction, however, some negolyte raw materials such as 1,8-DHAQ have poor solubility in aqueous base, which limits the final concentration of the product RFB negolyte. Methods disclosed herein can achieve a more concentrated product RFB negolyte without the need for additional onerous process steps, allowing the electrochemical process to go further to completion, in some cases eliminating the need for downstream evaporation steps to concentrate the negolyte product. In certain aspects, depicted in Scheme 2, a compound used as a solubility shuttle may be included in the negolyte mixture or solution, which acts to improve the apparent solubility of the negolyte raw material. Generally, the solubility shuttle can have a reduction potential comparable to or more negative than the raw material, such that the solubility shuttle is able to be reduced within the electrochemical system, and subsequently reduce the negolyte raw material from the oxidized form to a reduced form (i.e.. reduced negolyte raw material) where it is more soluble. As shown below, a small amount of THAQ as the solubility shuttle may be fed to the electrochemical system and reduced to the hexahydroxy anthracene, which then reacts with 1,8-DHAQ located outside the cell to form 1,8,9, 10-tetrahydroxy anthracene. The 1,8,9,10-tetrahydroxyanthracene having higher solubility in the aqueous solution, compared to the oxidized form that is 1.8-DHAQ. may react with glyoxylic acid and undergo further reductions and disproportionations as in Scheme 1 above to form 1,8- DCDHAQ at a high concentration. During the course of the reaction, or after the reaction is complete, the solubility shuttle can be regenerated (in other words, reduced from its oxidized form back to its reduced form) through electrochemical reduction at the negative electrode of the same electrochemical system or a second electrochemical system. Alternatively, the solubility shuttle can be regenerated chemically by adding a reducing agent or by performing catalytic hydrogenation. low solubility high solubility

Scheme 2

[0031] In other aspects, as depicted in Scheme 3, the product RFB negolyte is itself the solubility shuttle compound. Here 1,8-DCDHAQ takes the place of THAQ and the rest of the reaction proceeds as in Scheme 2. Note that if the solubility shuttle is also the product RFB negolyte, the reaction mixture must first be “seeded⁷’ with an existing amount of product RFB negolyte. low solubility high solubility Scheme 3

[0032] The processes depicted in Schemes 1 - 3 are not limited to 1,8-DHAQ as the negolyte raw material, but can be extended to any anthraquinone that can undergo the Marschalk reaction, whether chemical or electrochemical. This applies to anthraquinones that are substituted with a hydroxyl group or an amine group at the 1-position and no substitution at the 2-position, wherein the amine group can be an unsubstituted amine, a monoalkylated amine, or a dialkylated amine. Examples of negolyte raw materials include 1,3-DHAQ, 1,4-DHAQ, 1,5-DHAQ, 1,6-DHAQ, 1,7-DHAQ, 1,8-DHAQ, 1- hydroxyanthraquinone (1-HAQ), 1,3 -diaminoanthraquinone (1,3-DAAQ), 1,4-DAAQ, 1,5-DAAQ, 1.6-DAAQ. 1,7-DAAQ, 1,8-DAAQ, 1 -aminoanthraquinone (1-AAQ), 1- hydroxy-4-aminoanthraquinone, 1 -hy droxy-5-aminoanthraquinone, 1 -hydroxy-8- aminoanthraquinone, and many such combinations wherein the amino group is optionally monosubstituted or disubstituted.

[0033] In other aspects of the invention, depicted in Scheme 4 and separately described in the commonly owned application on the reductive amination submitted on the same day herewith, entitled “System and Process for Electrochemical Functionalization of Substituted Anthraquinones'’), some amines may undergo an alternative pathway under certain reaction conditions. Instead of a Marschalk reaction, a diaminoanthraquinone can form an imine with the negolyte reagent which is then reduced to the amine through a disproportionation reaction. This process is not a Marschalk reaction, but rather a reductive amination reaction. Scheme 4

[0034] However, even if the product RFB negolyte is formed through one or more reductive amination reactions, or a combination of amination reactions and Marschalk reactions, the same invention of using a solubility shuttle is still applicable, as shown in Scheme 5. This applies to anthraquinones that are substituted with one or more amine groups, wherein the amine group can be an unsubstituted amine, or a monoalkylated amine. Examples of negolyte raw materials include 1-AAQ, 1,2-DAAQ, 1,3-DAAQ, 1,4- DAAQ, 1,5-DAAQ, 1,6-DAAQ, 1,7-DAAQ, 1,8-DAAQ, 1-AAQ, l-hydroxy-4- aminoanthraquinone, 1 -hydroxy-5 -aminoanthraquinone, 1 -hydroxy-8- aminoanthraquinone, and many such combinations wherein the amino group is optionally monosubstituted, such as 1 -(methylamino)anthraquinone, 1- (carboxymethylamino)anthraquinone, and so on.

Scheme 5

[0035] Methods disclosed herein can be conducted using any electrochemical system described herein. Generally electrochemical systems can comprise a divided electrochemical cell, or stack comprising multiple divided electrochemical cells, a first tank in fluid connection to a negative electrode of the cell or stack, and a second tank in fluid connection to a positive electrode of the cell or stack. The system can further comprise a power supply to power the various components and provide an electrical potential, and pumps to circulate the contents of each tank to the respective electrodes and back. In certain aspects, the cell or cells that comprise the stack is divided by an interface which could be a cation exchange membrane, an anion exchange membrane, a bipolar membrane, or a porous separator. The first tank is designed to hold a catholyte solution and the second tank is designed to hold an anolyte solution.

[0036] In other aspects, the second tank can comprise a chemical species capable of providing electrons (being oxidized in the process) to the positive electrode and from there to the negative electrode (e.g., water, potassium ferrocyanide, sodium ferrocyanide, hydrazine, etc.).

[0037] Systems can further comprise a bypass that allows fluid to be circulated within each tank using the existing pumps without having to enter the cell or stack. Systems may also comprise any amount of heating elements associated with the first or second tank, or electrochemical cell, to provide temperature control over the electrochemical reaction or reagents in storage.

[0038] Systems can further comprise a component or subsystem for further separating undissolved, suspended raw material from the first tank, located between the first tank and the cell or stack, such that all or substantially all of the undissolved, suspended raw material is diverted away from the cell or stack and back into the first tank and only liquid enters the cell or stack. Such components or subsystems include filters, centrifuges, and hydrocyclone separators. Examples of filters include screw presses, filter presses, inline filters, filter bags, and filter cartridges. Examples of centrifuges include decanter centrifuges, bowl centrifuges, or tubular centrifuges.

[0039] Methods disclosed herein can therefore comprise steps which incorporate the solubility shuttle into conventional electrochemical processes to achieve the surprising improvements to solubility, leading to increases in reagent and product concentrations and overall reaction efficiency.

[0040] In some aspects of the invention, the negolyte raw material has a solubility in the aqueous catholyte solution that is less than 1 .0 mol/L, 0.75 mol/L, 0.5 mol/L, 0.25 mol/L, 0.1 mol/L, 0.05 mol/L, 0.02 mol/L, 0.01 mol/L, 0.005 mol/L, 0.002 mol/L, or 0.001 mol/L.

[0041] In certain aspects of the invention, the reduced form of the negolyte raw material (i.e., the reduced negolyte raw material) has a solubility in the aqueous catholyte solution that is greater than 0.01 mol/L, 0.02 mol/L, 0.05 mol/L, 0. 1 mol/L, 0.25 mol/L, 0.5 mol/L, 0.75 mol/L, or 1.0 mol/L. [0042] In some aspects of the invention, the oxidized form of the solubility shuttle and the reduced form of the solubility shuttle both have a solubility in the aqueous catholyte solution that is greater than 0.01 mol/L, 0.02 mol/L, 0.05 mol/L, 0.1 mol/L, 0.25 mol/L, 0.5 mol/L, 0.75 mol/L, or 1.0 mol/L.

[0043] In certain aspects of the invention, all the reaction intermediates, in their oxidized or reduced forms, have a solubility in the aqueous catholyte solution that is greater than 0.01 mol/L, 0.02 mol/L, 0.05 mol/L, 0.1 mol/L, 0.25 mol/L, 0.5 mol/L, 0.75 mol/L, or 1 .0 mol/L.

[0044] In some aspects of the invention, the negolyte material can be added to the catholyte solution as a solid (powder, chunks, flakes, and so on), as a slurry with a fluid, as a melt, or the catholyte solution can be added to the negolyte material instead. An external mixing vessel that is separate from the first tank of the electrochemical system can also be employed. Mixing may be effected by various methods such as using screw mixers, paddle mixers, low and high shear mixers, planetary mixers, and so on. Dissolution or mixing may also be aided by other methods such as ultrasonication, elevated temperature, a high, low, or neutral pH, a base or alkali with mixed cations (such as sodium hydroxide and potassium hydroxide), an acid with mixed anions (such as hydrochloric acid and sulfuric acid), use of supporting salts or mixtures thereof, and so on.

[0045] In certain aspects of the invention, the anolyte in the second tank can additionally comprise a supporting salt or mixtures thereof. If the reaction to take place at the positive electrode is a proton-coupled reaction (i.e. it generates protons, consumes protons, generates hydroxide, or consumes hydroxide), the second tank can additionally comprise a base or alkali with single or mixed cations (such as sodium hydroxide and potassium hydroxide), or an acid with single or mixed anions (such as hydrochloric acid and sulfuric acid). Additionally, when the reaction that takes place at the positive electrode is a proton-coupled reaction, the pH of the anolyte can change as acid is produced or hydroxide is consumed. In further embodiments of the reaction, the concentration of acid or base in the anolyte is maintained by periodic or continuous addition of acid or base, as appropriate. The acid or base can be added as a solid, a neat liquid, a concentrated solution, or a dilute solution. The base may also be a reagent, such as sodium hydride or metallic sodium, that reacts with water to form hydroxide ions. The acid or base concentration in the anolyte can be different from the acid or base concentration in the catholyte. The acid or base counterion(s) in the anolyte can be different from the acid or base counterion(s) in the catholyte. In some aspects of the invention, the anolyte can comprise an acid while the catholyte comprises a base; in other aspects, the anolyte can comprise a base while the catholyte comprises an acid.

[0046] In some aspects of the invention, ions are conducted across the interface of the divided electrochemical cells. The movement of ions drags water with them, a phenomenon known as electroosmosis. This tends to transport water in the direction of ion motion. In addition, water can be produced or consumed by the electrochemical reaction at the positive electrode. Acid or base can be added at a concentration or a physical state (e.g. solid rather an a solution) chosen to ensure that the volume, or the proton or hydroxide concentration, or both the volume and the proton or hydroxide concentration of the anolyte remain approximately constant throughout the operation of the process.

[0047] In certain aspects, methods can comprise filling the first tank with an aqueous solution of the solubility shuttle at a low concentration, e.g. 0.001 - 0.1 mol/L, circulating the solution in the first tank to the negative electrode and the liquid or solution in the second tank to the positive electrode, and applying a voltage such that the solubility shuttle is electrochemically reduced and the contents of the second tank are electrochemically oxidized (e.g. water is converted to oxygen and protons, sodium ferrocyanide is converted to sodium ferricyanide, hydrazine is converted to nitrogen, etc.).

[0048] In other aspects of the invention, especially where the solubility shuttle is the same as the product RFB negolyte, the solubility shuttle can be employed at a higher starting concentration, e.g. 0.5 - 1.5 mol/L, 0.7 - 1.2 mol/L, or 0.9 - 1.1 mol/L, and the process designed to increase the overall volume of catholyte but not substantially change the concentration of product RFB negolyte (or solubility shuttle) in the catholyte.

[0049] In other aspects of the invention, the process can be operated as a batch, semibatch, or continuous flow process. For example, instead of being all present at the start of the process, additional negolyte raw material and negolyte reagent can be continually added to the catholyte and additional KOH/NaOH added to the anolyte while current is being passed through the electrochemical system and some portion of the catholyte stream exiting the electrochemical system is removed for downstream processing (e.g. further concentration in an evaporator, filtration, and so on), or utilization as a RFB active material.

[0050] After a certain amount of time has passed, or after a certain amount of charge has been passed, add the negolyte raw material and a negolyte reagent (e.g. glyoxylic acid) to the first tank at a rate approximately equivalent to the rate of passing charge (i.e. the current) through the cell or stack, taking into account the appropriate stoichiometry between the negolyte reagent and the number of electrons required to effect the reduction on the negolyte raw material. The rate of addition, the current to the cell or stack, and the flow rate of the solution in the first tank, should be controlled in order to minimize the amount of undissolved solid that enters the cell or stack.

[0051] After a threshold amount of time has passed, or a threshold amount of charge has been passed, or a threshold amount of negolyte raw material has been added, stop passing current. At this point, methods disclosed herein can continue circulating the solution in the first tank, without passing any current, for a predetermined amount of time. The temperature at this stage may also be adjusted. After the negolyte raw material has been fully converted to the product RFB negolyte, the product RFB negolyte may be concentrated, isolated or purified if desired, using methods typical to one skilled in the art, e.g. evaporation, distillation, drying, neutralization, precipitation, centrifugation, filtering, washing, recrystallization, and so on.

[0052] Alternatively, a bypass may be employed through which the fluid flows from the first tank to go through the bypass instead of through the cell or stack. In such aspects, a stoichiometric or sub-stoichiometric amount of the negolyte raw material and the negolyte reagent can be added. The contents of the first tank can then be circulated through the bypass until the undissolved solid content falls below a threshold level. Thereafter, the bypass can be shut off to redirect the fluid from in the first tank back into the cell or stack and resume the electric current. In some embodiments, the bypass takes the form of a hydrocyclone separator wherein the underflow that is enriched in suspended solids is returned to the first tank, and the overflow that is depleted of suspended solids can be directed into the cell or stack, or shut off from the cell or stack, as desired.

[0053] The steps above may be repeated until a threshold amount of charge has been passed, or a threshold amount of negolyte raw material has been added, or a threshold concentration of product RFB negolyte has been reached. This alternative method may be especially helpful if the dissolution of the negolyte raw material is slow. [0054] Without being bound by theory, it is contemplated that the methods and systems disclosed herein have the following advantages. Relative to previous processes, the methods disclosed herein are able to achieve a high product RFB negolyte concentration without using specialized electrochemical cells that tolerate suspended solids.

[0055] In certain aspects, solubility shuttles disclosed herein can have high solubility in water at the operating pH in both the reduced and the oxidized form, e.g. >0. 1 mol/L at pH 14. In other aspects, a solubility shuttle that has a less negative standard reduction potential than the negolyte raw material can nevertheless can have a more negative reduction potential than the negolyte raw material under the non-standard operating conditions (temperature, pH, concentration, solvent, etc.). Examples of potential solubility shuttles include quinones (e.g. substituted benzoquinones, substituted naphthoquinones, and substituted anthraquinones), substituted flavins, substituted phenazines, viologens, inorganic species such as chromium(II)/chromium(III), metalligand complexes such as chromium(II)/chromium(III) ethylenediaminetetraacetate and others readily apparent to those of skill in the art.

[0056] In addition, because the reaction between the reduced form of the solubility shuttle and the negolyte raw material to form the oxidized form of the solubility shuttle and the reduced negolyte raw material is an equilibrium, the reduction potential of the solubility shuttle does not necessarily have to be more negative than the reduction potential of the negolyte raw material. If the reduction potential of the solubility shuttle is equal or comparable to (e.g. up to 1 mV, 2 mV, 5 mV, 10 mV, 20 mV, 40 mV, 60 mV, 80 mV, 100 mV, or 120 mV less negative than) the reduction potential of the negolyte raw material, some portion of the negolyte raw material will still be reduced to form the reactive intermediate that is the reduced negolyte raw material, in accordance with the Nernst Equation. For instance, where the solubility shuttle has a reduction potential 14 mV less negative than that of the negolyte raw material, the reduced negolyte raw material will still be present at about 1:3 ratio with the negolyte raw material, in equilibrium. The reduced negolyte raw material then reacts with the negolyte reagent to form a different chemical species. The continued consumption of the reduced negolyte raw material ends up pushing the abovementioned equilibrium to the right until all the negolyte raw material has been fully reduced and subsequently converted to a different, soluble, chemical species. [0057] The solubility shuttles described above can be applicable to electrochemical processes comprising a variety of reagents and reactants. In certain aspects, negolyte reagents can include aldehydes and ketones if the quinone is substituted with an amino group, or amines if the quinone is substituted with a carbonyl group (as defined in the commonly owned application on the reductive amination submitted on the same day herewith, entitled “System and Process for Electrochemical Functionalization of Substituted Anthraquinones”), or interconvertible aldehydes or ketones such as glucose, fructose, and so on. Examples of negolyte raw materials include quinones (e.g. substituted benzoquinones, substituted naphthoquinones, and substituted anthraquinones).

EXAMPLES

[0058] The present disclosure is further illustrated by the following examples, which are not to be construed in any way as imposing limitations to the scope of this invention. Various other aspects, embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to one of ordinary skill in the art without departing from the spirit of the present invention or the scope of the appended claims.

Materials and General Procedures

[0059] A 200 cm² “MP Cell” (ElectroCell) was constructed generally as shown by the schematic diagram of FIG. 1. The MP Cell had a carbon cathode between two nickel anodes, Nafion 115 membranes separating the chambers of the MP Cell, polypropylene flow frames, and EPDM gaskets. During operation, the contents of the cathode reservoir (the “catholyte”, or reaction mixture) are reduced and oxygen gas is evolved at the anode. The cell was ran in a recirculation mode and the catholyte and anolyte solutions were continually pumped into the electrochemical cell and back again to their original reservoirs. The catholyte reservoir was approximately 2 L in volume and was kept under an inert atmosphere (N2 gas) to prevent reoxidation of the reaction mixture by atmospheric oxygen. The anolyte reservoir volume was also approximately 2 L and was open to the atmosphere. Both reservoirs were insulated and equipped with heating tape and a reflux condenser. An external mixing vessel (2 L, 3 -neck round-bottom flask, heating mantle, and overhead stirrer) was connected to a bidirectional peristaltic pump that pumps to and from the mixing vessel and catholyte reservoir. The speed and direction of mixing was controlled. A syringe pump and tubing were installed to supply glyoxylic acid to the catholyte reservoir. The syringe can add up to 60 mL and the speed of addition can be controlled. The glass external mixing vessel was equipped with a syringe line for sample collection.

[0060] The following materials were obtained from the specified vendors and used without further purification. 1,3,5,7-tetrahydroxyanthraquinone (THAQ) was obtained from Sinoconvoy New Material, Co., Ltd., China. 1,8-dihydroxyanthraquinone (1,8- DHAQ), 96%, was obtained from Atul Ltd., India. NaOH, 45% in water, KOH, 50% in water, and HPLC solvents were obtained from Sigma-Aldrich. Glyoxylic Acid, 50% in water, “Cosmetics Grade,” was obtained from Jinan Huashihang Chemical Co., Ltd., China. Sodium ferrocyanide decahydrate and potassium ferrocyanide trihydrate were obtained from Kodia Chem Ltd., China.

[0061] High-performance liquid chromatography (HPLC) measurements were performed using an Agilent- 1100 Series HPLC equipped with a UV detector and Agilent ZORBAX SB-C 18 column. Product purity was determined using peak integration area % at 254 nm. HPLC method information: 1.0 mL/min flow, 30 min. total run time, mobile phase A: H2O with 0.1% v/v trifluoroacetic acid, mobile phase B: acetonitrile with 0.1% v/v trifluoroacetic acid, solvent gradient 90: 10 A:B to 10:90 A:B and back to 90: 10 A:B, 40 °C column oven temperature, 3.0 pL injection volume. Samples for HPLC analysis were prepared in 50:50 A:B at 1 mM from a 20-50 mM solution in potassium hydroxide or sodium hydroxide.

Example 1

[0062] A RFB cell with a geometric electrode area of 50 cm² was constructed using cell hardware purchased from Fuel Cell Technologies, with AvCarb EP40 pre-activated carbon paper electrodes, EPDM gaskets, and a FuMATech E-620(K) cation exchange membrane. The anolyte reservoir was charged with 500 mL of aqueous solution comprising 0.3 mol/L of sodium ferrocyanide, 0.3 mol/L of potassium ferrocyanide, 0.5 mol/L of NaOH, and 0.5 mol/L of KOH. The catholyte reservoir (no mixing vessel) was charged with 100 mL of aqueous solution comprising 0.1 mol/L of THAQ, 1.9 mol/L of NaOH, and 1.9 mol/L of KOH and equipped with a magnetic stirrer. Both anolyte and catholyte reservoirs were kept under nitrogen. [0063] The pumps to the electrochemical cell were started (peristaltic pumps, ~50 mL/min flow rate) and the catholyte and anolyte were heated to 50°C. Electrical current was passed galvanostatically at a current density of 50 mA/cm² until 2 molar equivalents of electrons per mole of THAQ present was passed, as indicated by a sharp rise in the cell voltage above -1.6V.

[0064] While maintaining the temperature at 50°C, the electrical current and pumps were shut off and 1.92 grams of 1,8-DHAQ (8 mmol) was added to the catholyte reservoir under a stream of nitrogen gas. After stirring for 15 minutes, 2.21 mL of 50% glyoxylic acid (20 mmol) was added dropwise over ~1 minute and stirring was continued for another 15 minutes.

[0065] Next, the pumps were restarted and the electrical current was resumed at 50 mA/cm². After 6 molar equivalents of charge were passed with respect to the added 1,8- DHAQ, the current, and pumps were stopped, again indicated by a sharp rise in the cell voltage above -1.6V. Five subsequent additions of 1,8-DHAQ (each 1.92 grams) followed by 50% glyoxylic acid (each 1.10 mL) and passing current were performed in a similar manner.

[0066] At this point, the catholyte contained ~125 mL of THAQ at a concentration of 0.08 mol/L and 1,8-DCDHAQ at a concentration of 0.32 mol/L, and a total excess hydroxide concentration of ~1 mol/L. The catholyte solution was then drained from the catholyte chamber and mixing vessel. The drained catholyte solution was exposed to air. The 0.4 mol/L quinone solution (comprising THAQ and 1,8-DCDHAQ) can be used as is as a flow battery negolyte without further purification or processing.

Example 2

[0067] The MP Cell described above was used to conduct an electrochemical synthesis of DCDHAQ from 1,8-DHAQ using 1,8-DCDHAQ as the solubility shuttle, as summarized in Scheme 1 and Scheme 3 above. Generally, negolyte starting material 1,8- DHAQ was reacted with negolyte reagent glyoxylic acid to form 1,8-DCDHAQ.

[0068] The anolyte reservoir was charged with a mixture of 400 mL of DI water, 306 mL of 50% NaOH (5.82 mol), and 499 mL of 45% KOH (5.82 mol). The catholyte reservoir and mixing vessel were collectively charged with a mixture of -400 mL of 0.7 mol/L 1,8-DCDHAQ carried over from a previous experiment with an excess hydroxide concentration of 0.35 mol/L NaOH and 0.35 mol/L KOH (280 mmol), 313.8 mL of 50% NaOH (5964 mmol, 3.55 equivalents wrt 1,8-DHAQ), 511.7 mL of 45% KOH (5964 mmol, 3.55 equivalents), and sufficient water (-400 mL) to bring the solution volume to -1600 mL. Approximately half of the catholyte was in the catholyte reservoir and half was in the mixing vessel. The syringe pump was primed with a total of 390 mL of 50% glyoxylic acid (522.4 grams, 3528 mmol, 2.1 equivalents, added as multiple batches of 60 mL.

[0069] The cathode reservoir and mixing vessel headspaces were purged with nitrogen for -5 minutes and maintained under this inert atmosphere. The anode reservoir was allowed to be open to the air. The pumps to the electrochemical cell were started (peristaltic pumps, -100 mL/rnin flow rate) and the catholyte and anolyte were heated to 50°C. Electrical current was passed galvanostatically at a current density of 50 mA/cm² with respect to the carbon cathode until 2 molar equivalents of electrons per mole of 1,8- DCDHAQ present was passed, as indicated by a sharp rise in the cell voltage above -2.0V.

[0070] While maintaining the temperature at 50°C, the electrical current and transfer pump were shut off and 30.0 grams of 1,8-DHAQ (125 mmol) were added to the mixing chamber under a stream of nitrogen gas. After stirring for 10 minutes, the transfer pump was turned on at a rate of 5 mL/minute, the syringe pump began addition of 50% glyoxylic acid solution at a rate of -14.5 mL/hour (delivering a slight stoichiometric excess with respect to the electrical current), and the electrical current was resumed at 50 mA/cm² with respect to the carbon cathode. After 6 molar equivalents of charge were passed with respect to the added 1,8-DHAQ, the current, transfer pump, and syringe pump were stopped, again indicated by a sharp rise in the cell voltage above -2.0V. Three subsequent additions of 1,8-DHAQ (60.0 grams, 120.0 grams, and 193.6 grams) were performed in a similar manner. Since each addition of 1,8-DHAQ is greater than the last, the amount of glyoxylic acid added per cycle is also greater by a proportional amount.

[0071] At this point, the negolyte contained -2800 mL of DCDHAQ solution at a 0.7 mol/L concentration. The catholyte solution was then drained from the catholyte chamber and mixing vessel. The drained catholyte solution was exposed to air and concentrated on a vacuum or thermal evaporator to a 1 mol/L concentration of DCDHAQ (and therefore free hydroxide). The 1 mol/L solution can be used as is as a flow battery negolyte without further purification or processing. Example 3

[0072] The MP Cell described above was used to conduct an electrochemical synthesis of DCDHAQ from 1,8-DHAQ using 1,8-DCDHAQ as the solubility shuttle, as summarized in Scheme 1 and Scheme 3 above.

[0073] The anolyte reservoir was charged with a mixture of 400 mL of DI water, 306 mL of 50% NaOH (5.82 mol), and 499 mL of 45% KOH (5.82 mol). In the catholyte reservoir, 2.2 L of a solution containing 0.64 M 1,8-DCDHAQ (1.408 mol) and 3.5 M hydroxide with Na:K ratio of ~1 : 1 (corresponding to 0.95 M free hydroxide) was reduced at the cathode of an MP Cell at 10 A (50 mA/cm²) for ~ 3 hours at 50°C until 22.4 Ah of charge (0.836 mol of electrons) was passed. The solution was then transferred into a mixing vessel that contained 50.2 g of solid 1,8-DHAQ (96% pure by HPLC, 0.201 mol wrt. assay purity). Both catholyte reservoir and mixing vessel were purged with nitrogen. The solution/solid was mixed for about 30 minutes, by which time all the 1,8-DHAQ had dissolved. Then 60 mL of 50 wt. % glyoxylic acid was added at a flow rate of 1 mL/min. The solution was stirred for another hour before being returned to the catholyte tank. An additional 2 equivalents of charge (11.2 Ah, 0.418 mol of electrons) with respect to the solid 1,8-DHAQ was passed and the solution allowed to react at 50°C overnight. The solution was then fully discharged using the electrochemical cell. Approximately 2.25 L of a solution containing 0.71 M 1,8-DCDHAQ (1.598 mol) was produced. The solution contained 1 .87 M of sodium, 1 .92 M of potassium, and ~0.95 M of free hydroxide.

[0074] Note that additional sodium and potassium were transported from the anolyte across the cation exchange membrane during the course of passing the current. Additional hydroxide is produced through the disproportionation steps which exactly balances the amount of hydroxide consumed from the addition of 1,8-DHAQ and glyoxylic acid.

[0075] The process can be repeated to produce more DCDHAQ. The additional volume of solution was drained from the catholyte chamber and mixing vessel. The drained catholyte solution was exposed to air and concentrated on a vacuum or thermal evaporator to a 1 M concentration of 1,8-DCDHAQ. The 1 M solution can be used as is as a flow battery negolyte without further purification or processing. Alternatively, the 0.69 M 1,8-DCDHAQ solution can be used as is without any downstream evaporation of water to increase the concentration. [0076] The solubility shuttles tested were able to rapidly act on and “dissolve” the negolyte raw material in each case more quickly than expected, despite having only slightly more negative reduction potentials. “Dissolve” here means that the negolyte raw material is converted from the solid to some form in solution, and is meant to include the dissolved raw material in both its oxidized and reduced forms (i.e. the reactive intermediate or reduced negolyte raw material). Generally, the more negative the reduction potential of the solubility shuttle is, relative to the negolyte raw material, the larger is the thermodynamic driving force for the negolyte raw material to dissolve. In the case of 1,8-DCDHAQ and 1,8-DHAQ, the reaction product unexpectedly served as an effective solubility shuttle even though the reduction potentials of the two species are nearly identical. This probably arises the high concentration of the solubility shuttle and the low concentration of the dissolved negolyte raw material leads to an additional thermodynamic driving force for the redox-mediated dissolution process, or more familiarly as the reaction quotient Q in the Nernst Equation. Conversely, in the absence of any starting solubility shuttle, the negolyte raw material such as 1,8-DHAQ was very difficult to dissolve completely in aqueous base and frequently casued failure of the electrochemcial cell due to clogging.

[0077] Having described the preferred aspects and implementations of the present disclosure, modifications and equivalents of the disclosed concepts may readily occur to one skilled in the art. However, it is intended that such modifications and equivalents be included within the scope of the claims which are appended hereto.

Claims

1 . A process for forming a redox flow batery (RFB) negolyte, comprising: reacting a reduced form of a solubility shutle and a negolyte raw material to form a reduced negolyte raw material and an oxidized form of the solubility⁷ shutle; and reacting the reduced negolyte raw material with a negolyte reagent to form a product RFB negolyte or a negolyte intermediate which is subsequently converted to the product RFB negolyte; wherein the reduced negolyte raw material has an aqueous solubility that is greater than that of the negolyte raw material; and wherein the solubility shutle has a reduction potential that is comparable to or more negative than the negolyte raw material, such that the negolyte raw material is reduced by reaction with the solubility shutle to form the reduced negolyte raw material.

2. The process of claim 1, wherein reacting the solubility shutle and the negolyte raw material comprises mixing the negolyte raw material into an aqueous solution comprising the reduced form of the solubility shuttle.

3. The process of claim 1 , wherein reacting the reduced negolyte raw material and a negolyte reagent to form the negolyte intermediate comprises: forming a second aqueous solution comprising the reduced negolyte raw material and the negolyte reagent in an aqueous solution.

4. The process of claim 3, wherein the negolyte intermediate has a higher solubility than the reduced negolyte raw material in the aqueous solution.

5. The process of claim 1. further comprising regenerating the solubility shuttle by reducing the oxidized form of the solubility shutle to a reduced form of the solubility shutle.

6. The process of claim 1, wherein regenerating the solubility shuttle comprises: circulating the aqueous solution comprising the oxidized form of the solubility shuttle to a negative electrode of a divided electrochemical cell; circulating an aqueous solution comprising an oxidizable species to a positive electrode of the divided electrochemical cell; and applying a voltage between the negative electrode and the positive electrode, thereby converting the solubility shuttle from the oxidized form to the reduced form at the negative electrode, and oxidizing the oxidizable species at the positive electrode.

7. The process of claim 1, wherein reacting the solubility shuttle and the negolyte raw material comprises the mixing of the negolyte raw material with the aqueous solution comprising the reduced form of the solubility shuttle in a mixer.

8. The process of claim 1, wherein: the negolyte raw material in its oxidized form has a solubility of less than 0. 1 M, 0.2 M, 0.5 M, or 1.0 M in the aqueous solution; the reduced negolyte raw material has a solubility of greater than 0.01 M. 0. 1 M, 0.2 M, 0.5 M, or 1.0 M in the aqueous solution; or any combination thereof.

9. The process of claim 1, wherein the solubility shuttle is formed in situ.

10. The process of claim 9, wherein forming the solubility shuttle comprises: providing in a first tank of an electrochemical system in fluid connection to a negative electrode, an aqueous solution comprising a solubility⁷ shuttle precursor; providing an oxidizable species to a second tank of the electrochemical system in fluid connection to a positive electrode; circulating the aqueous solution in the first tank to the negative electrode and the oxidizable species in the second tank to the positive electrode; and applying a voltage between the negative electrode and the positive electrode such that the solubility shuttle precursor is electrochemically reduced to form the solubility shuttle in its oxidized or reduced form, and the oxidizable species is electrochemically oxidized.

11. The process of claim 6 or 10, wherein the oxidizable species comprises water, and applying the voltage converts the water to oxygen and creates protons, or equivalently consumes hydroxide.

12. The process of claim 11, wherein the oxidizable species comprises ferrocyanide.

13. The process of claim 11, wherein the oxidizable species comprises hydrazine.

14. The process of claim 11, wherein the oxidizable species comprises a quinone derivative in its reduced form, including a hydroquinone, dihydroxynaphthalene, or dihydroxy anthracene.

15. The process of claim 1, w herein the negolyte raw' material is a quinone.

16. The process of claim 1, wherein the negolyte reagent is glyoxylic acid.

17. The process of claim 1, wherein the solubility shuttle is 1, 3,5,7- tetrahydroxy anthraquinone.

18. The process of claim 1, wherein the solubility' shuttle is the same as the product RFB negolyte.

19. The process of claim 1 , wherein the subsequent conversion of the negolyte intermediate to the product RFB negolyte includes disproportionation.

20. An electrochemical system comprising: a divided electrochemical cell; a first tank in fluid connection to a negative electrode of the divided electrochemical celt; a second tank in fluid connection to a positive electrode of the divided electrochemical cell; a plurality' of pumps to circulate contents of each tank to the respective electrodes and back; and a separator located between the first tank and the divided electrochemical cell, the separator configured to accept fluid with suspended solids, and produce a first separated fluid stream that is depleted of suspended solids and is directed to the divided electrochemical cell, and a second separated fluid stream that is enriched in suspended solids.

21. The electrochemical system of claim 20, wherein the second separated fluid stream is directed back into the first tank.

22. The electrochemical system of claim 20. wherein the divided electrochemical cell is divided by an interface selected from a cation exchange membrane, an anion exchange membrane, a bipolar membrane, or a porous interface.

23. The electrochemical system of claim 20, further comprising heaters associated with the first tank, the second tank, or both.

24. The electrochemical system of claim 20, wherein the second tank includes a chemical species capable of supplying electrons to the positive electrode that is water, ferrocyanide, or hydrazine.

25. The electrochemical system of claim 20, further comprising a power supply.

26. The electrochemical system of claim 20, further comprising a bypass to allow fluid to be circulated within each tank without having to enter the divided electrochemical cell.

27. The electrochemical system of claim 20 or 21, wherein the separator comprises a filter, a centrifuge, a hydrocyclone separator, or a combination thereof.

28. The electrochemical system of claim 26. wherein the separator is present within the bypass.