A redox flow battery

Abstract

There is provided a redox flow battery (1) comprising: a negative compartment (4) comprising a negative electrode (6) in contact with a first aqueous-based electrolyte solution (2) including a negative electrolyte (2a) comprising an organic redox-active compound dissolved in a first aqueous-based solvent; a positive compartment (5) comprising a positive electrode (7) in contact with a second aqueous-based electrolyte solution (3) including a positive electrolyte (3a) in a second aqueous-based solvent; electrical conductive means (8) for establishing electrical conduction between said positive electrode and said negative electrode, and an external load (9) for directing electrical energy into or out of the redox flow battery; a separator component (10) separating the first aqueous-based electrolyte solution in the negative compartment from the second aqueous-based electrolyte solution in the positive compartment; wherein the organic redox-active compound is a naphthalene diimide, NDI, according to formula (I).

Description

A REDOX FLOW BATTERYTECHNICAL FIELD

[0001] The present disclosure relates to the field of redox flow batteries, especially redox flow batteries comprising an organic redox-active compound being NDI.BACKGROUND

[0002] Society faces a large challenge in transitioning to sustainable generation of electricity from renewable energy sources, such as wind power and solar power. One of the biggest obstacles to this transition is the intermittence of such sources, leading to a mismatch of supply and demand, a problem most commonly solved by installing large-scale energy storage.

[0003] A type of energy storage is the redox flow battery which is a type of rechargeable battery where energy is stored in two liquid electrolyte solutions contained in separate compartments. The electrolytes, typically containing different redox pairs, flow through an electrochemical cell that converts chemical energy to electrical energy and vice versa. Metals such as iron, chromium and vanadium as well as organic compounds such as anthraquinone and benzoquinone have been considered for use in the electrolytes of redox flow batteries.

[0004] Redox flow batteries are an emerging technology for large-scale energy storage, offering scalability, long cycle life, and the ability to decouple energy storage capacity from power generation. Organic flow batteries, which use redox-active organic molecules, are of particular interest due to their potential cost advantages and environmental benefits compared to traditional vanadium-based systems.However, the development of organic flow batteries has been hindered by limitations in the solubility, stability, and electrochemical performance of the organic molecules used in the electrolytes. These limitations reduce the energy density and cycling efficiency of the batteries, making them less competitive for commercial applications. There is a need for novel redox-active organic compounds that address these issues while maintaining compatibility with aqueous-based systems, improving energy density, stability, and overall batery performance.SUMMARY

[0005] The present inventors have realized that the need for improving the energy storage capacity and / or efficacy of the battery can be adressed by the invention of a redox flow battery comprising an organic redox-active compound being naphthalene diimide, NDI, according to formula (I) below.

[0006] According to a first aspect, a redox flow battery comprising: a negative compartment comprising a negative electrode in contact with a first aqueous-based electrolyte solution including a negative electrolyte comprising an organic redoxactive compound dissolved in a first aqueous-based solvent; a positive compartment comprising a positive electrode in contact with a second aqueous-based electrolyte solution including a positive electrolyte in a second aqueous-based solvent; electrical conductive means for establishing electrical conduction between said positive electrode and said negative electrode, and an external load for directing electrical energy into or out of the redox flow battery; a separator component separating the first aqueous-based electrolyte solution in the negative compartment from the second aqueous-based electrolyte solution in the positive compartment; wherein the organic redox-active compound is a naphthalene diimide, NDI, according to formula (I):(I)or a salt thereof.

[0007] The redox flow battery of the first aspect, in particular the NDI according to formula (I) comprised the negative electrolyte is improved with regards to thestability of the battery and / or efficacy of the battery. The NDI according to formula (I) exhibit an advantageous solubility in the first aqueous-based electrolyte solution, resulting in an increased energy density of the redox flow battery. Moreover, the NDI according to formula (I) may be cheaper to manufacture as their precursors are abundant and cheap. Traditional NDIs often suffer from low solubility in aqueousbased solutions, limiting their concentration in the electrolytes and, consequently, the energy storage capacity of the battery. By modifying the amines in the NDI molecule with carboxyalkyl side chains based on glutamic acid and / or aspartic acid, the solubility of the NDI compounds is distinctly increased. These modifications enable the NDI to achieve solubility levels of at least 600 mM in deionized water (DI) at room temperature. The first aqueous-based electrolyte solution including the organic redox-active compound may simply be referred to as a negative electrolyte or an aqueous-based negative electrolyte. Correspondingly, the second aqueous-based electrolyte solution including the positive electrolyte may simply be referred to as a positive electrolyte or an aqueous-based positive electrolyte.

[0008] The improved solubility of the NDI allows for higher concentrations of the redox-active compound in the electrolyte, thereby increasing the charge storage capacity and energy density of the battery. For example, due to the high concentration in the negative compartment of the flow battery, the energy density can reach values of at least 20 Wh / litre. The ability to maintain such high solubility without precipitation issues also supports better cycling stability. Hereby, consistent battery performance can be ensured over extended periods, and / or material loss can be minimized. As a result, the redox flow battery of the first aspect may be better suited for large-scale energy storage applications requiring both high capacity and efficiency.

[0009] The NDI is preferably deprotonated. Thus, in the first aqueous-based electrolyte solution, the previously mentioned side chains of the NDI have formed carboxylate groups (-COO⁻). The deprotonated carboxylate groups increase the hydrophilicity of the NDI molecule, significantly improving its solubility in water or aqueous-based solvents, such as the first aqueous-based electrolyte solution. The deprotonated NDI typically exist as a salt, with counterions such as sodium, potassium, or ammonium stabilizing the negatively charged carboxylate groups. The deprotonated NDI may furthermore exhibit favourable redox behaviour, seeminglyconcurrent two electron redox process instead of the two separated one electron redox processes normally seen for NDI. This ensures efficient charge and discharge cycles and may contribute to higher voltage efficiency in the battery.

[0010] According to one embodiment, the deprotonated NDI has a solubility in deionized water at room temperature of at least 600 mM, or at least 700 mM, or at least 800 mM, or at least 900 mM, or at least 1 M. Thus, when deprotonated, the NDI according to formula (I) may reach solubility levels at room temperature of at least 600 mM, or at least 700 mM, or at least 800 mM, or at least 900 mM, or at least 1 M. The solubility is here referring to the solubility in deionized water (DI). This high solubility represents a substantial improvement over prior art NDIs, which are often limited by low solubility that constrains their concentration in the electrolyte solution. The increased solubility allows for the preparation of more concentrated negative electrolyte solutions, directly contributing to higher charge storage capacity and improved energy density of the redox flow battery. Furthermore, the enhanced solubility typically results in better utilization of the redox-active compound, reducing the likelihood of precipitation and associated performance degradation over time. This advantage is particularly relevant for large-scale energy storage applications. Moreover, the ability to maintain high solubility in deprotonated form also supports stable cycling behaviour, as previously described, because the active material remains available for redox reactions throughout extended charge and discharge cycles.

[0011] According to one embodiment, the deprotonated NDI has a solubility in deionized water at room temperature of between 600 mM and 2.5 M, e.g. between 700 mM and 2.5 M or between 800 mM and 2.5 M, or between 900 mM and 2.5 M, or between 1 M and 2.5 M. Thus, when deprotonated, the NDI according to formula (I) may reach solubility levels in deionized water at room temperature of at least 600 mM, or at least 700 mM, or at least 800 mM, or at least 900 mM, or at least 1 M and / or up to a solubility level of 1.5 M, 2 M or 2.5 M. For example, when deprotonated, the NDI according to formula (I) may reach solubility levels in deionized water at room temperature of at least 600 mM, or at least 700 mM, or at least 800 mM, or at least 900 mM, or at least 1 M and / or up to a solubility level of 1.5 M, or 2 M, or 2.5 M.

[0012] According to one embodiment, the NDI is a salt, preferably a salt of sodium, potassium or ammonium. Thus, the deprotonated NDI may exist as a salt, with counterions being sodium, potassium, or ammonium. The use of sodium, potassium, or ammonium salts for the NDI is particularly advantageous due to their solubility and compatibility with aqueous-based electrolyte systems. The corresponding counterions of these salts are highly hydrophilic and stabilize the deprotonated carboxylate groups on the NDI molecule. Hereby, aggregation and / or precipitation may be reduced or even prevented.

[0013] According to one embodiment, the NDI is a salt based on alkyl amines, imidazolium, or other suitable cations. Thus, the NDI may be a salt with positive counterions such as sodium, potassium, ammonium, alkyl amines or imidazolium.

[0014] For embodiments in which the NDI is an ammonium salt, the salt may be a salt of primary, secondary, tertiary, or quaternary ammonium. Hereby, solubility may be further tuned and may further enhance the stability of the negative electrolyte, e.g. across a wide pH range.

[0015] According to one embodiment, the NDI is a compound according to formula (la):(Ia)

[0016] Thus, the NDI according to formula (I) may be that of the sub-formula (la). In other words, the side chains of the NDI may be based on aspartic acid. The NDI according to formula (la) may be referred to as aspartic acid NDI. The NDI according to formula (la) exhibits an advantageous solubility in the first aqueous¬ based electrolyte solution, resulting in an increased energy density of the redox flow battery.

[0017] According to one embodiment, the NDI is a compound according to formula (lb):(lb)

[0018] Thus, the NDI according to formula (I) may be that of the sub-formula (lb). In other words, the side chains of the NDI may be based on glutamic acid. The NDI according to formula (lb) may be referred to as glutamic acid NDI. The NDI according to formula (lb) exhibits an advantageous solubility in the first aqueous¬ based electrolyte solution, resulting in an increased energy density of the redox flow battery. Furthermore, NDI according to formula (lb) demonstrates a unique two- electron reduction at a single potential, potentially enhancing the nominal voltage and efficiency of the battery.

[0019] According to one embodiment, the NDI is a compound according to formula (la) and / or formula (lb). Thus, the side chains of the NDI maybe derived from aspartic acid or glutamic acid. By modifying the NDI accordingly, an NDI having high solubility in the electrolyte solution may be obtained while simultaneously keeping down the cost of the modification.

[0020] The redox flow battery may further comprise means capable of establishing flow of the first and second aqueous-based electrolyte solutions past said positive and negative electrodes, respectively. Such means may e.g. be pumps or the like. By the inclusion of means to establish flow of the electrolyte solutions, a uniform distribution of the active material across the electrodes may be provided. Hereby, concentration gradients can be reduced and the efficiency of redox reactions improved.

[0021] According to one embodiment, the first aqueous-based electrolyte solution in the negative compartment has pH of at least 4, such as e.g. at least 5 or at least 6. Hereby, the risk of precipitation of the NDI is reduced or even avoided. Thus, by using a pH of at least 4, or at least 5, or at least 6, the formation of the relatively insoluble acid form of the NDI may be reduced or even avoided.

[0022] According to one embodiment, the pH at the negative compartment is higher compared to the pH at the positive compartment. Thus, the pH of the first aqueous-based electrolyte solution may be higher than the pH of the second aqueousbased electrolyte solution.

[0023] According to one embodiment, the NDI is a first organic redox-active compound, and the positive electrolyte comprises a second organic redox-active compound. The use of a first organic redox-active compound, such as the NDI previously described, in combination with a second organic redox-active compound in the positive electrolyte allows for tailored redox potential ranges. Hereby, the overall voltage and energy density of the battery may be adapted.

[0024] The second organic redox-active compound may e.g. be at least one of the following: cyclic nitroxyl radical derivative or a salt thereof; ferrocyanide or a derivative thereof. Preferably, the cyclic nitroxyl radical derivative is selected from a tetramethylpiperidineoxyl (TEMPO) derivative, proxyl nitroxide derivative, phenyltetramethylnitronyl nitroxide derivative and nortropine N-oxyl derivative,more preferably the cyclic nitroxyl radical derivative is a TEMPO derivative. These cyclic nitroxyl radical derivatives have a particularly good combination of cost, solubility in the electrolyte solution and conductivity when used in a redox flow battery. The cyclic nitroxyl radical derivative is preferably present in its salt form.

[0025] According to one embodiment, the second organic redox-active compound has a solubility in deionized water at room temperature of between 200 mM and 5 M, e.g. between 300 mM and 5 Mor between 400 mM and 5 M, or between 500 mM and 5 M. The second organic redox-active compound may here e.g. be a cyclic nitroxyl radical derivative. Thus, when comprising a charge (being in the form of a salt), the second organic redox-active compound may typically reach solubility levels in deionized water at room temperature of at least 200 mM, or at least 300 mM, or at least 400 mM, or at least 500 mM, and / or up to a solubility level of 1.5 M, or 1 M, or 2.5 M, or 5 M. For example, when comprising a charge, the second organic redox-active compound may reach solubility levels at room temperature of at least 200 mM, or at least 300 mM, or at least 400 mM, or at least 500 mM, and / or up to a solubility level of 1.5 M, or 2.0 M, or 2.5 M, or 5 M. The solubility is here referring to the solubility in deionized water (DI).

[0026] According to one embodiment, the energy density of an electrolyte system formed by the positive electrolyte and the negative electrolyte is at least 20 Wh / litre, such as e.g. at least 25 Wh / litre, at least 30 Wh / litre or at least 35 Wh / litre. The high energy density of at least 20 Wh / liter enables the storage of more energy per unit volume. Hereby, the size (and costs) of the redox flow battery can be reduced and / or the efficiency can be increased. Thus, the operating voltage, and hence the power output, of the battery can be increased. For example, the energy density of the electrolyte system, or of the redox flow battery, is between 20 Wh / litre and 100 Wh / litre, such as e.g. between 25 Wh / litre and 100 Wh / litre, or between 30 Wh / litre or 35 Wh / litre and 100 Wh / litre.

[0027] According to one embodiment, the volumetric capacity of the first aqueous-based electrolyte solution is at least 15 Ah / litre, such as e.g. at least20 Ah / litre, at least 25 Ah / litre or at least 30 Ah / litre. For example, the volumetric capacity of the first aqueous-based electrolyte solution is between 15 Ah / litre and 80 Ah / litre, such as e.g. between 20 Ah / litre and 80 Wh / litre, or between 25 Ah / litre or 30 Ah / litre and 80 Ah / litre.

[0028] According to one embodiment, the nominal voltage between the positive and negative compartments is at least 1 Volt, such as at least 1.1 Volts or at least 1.2 Volts.

[0029] The separator component may be configured to prevent, or substantially prevent, the positive electrolyte in the positive compartment and the negative electrolyte in the negative compartment from intermingling with each other, while permitting the passage of non-redox-active species between the first and second aqueous-based electrolyte solutions. The separator component may be an anionic or cationic exchange membrane. Preferably, the separator component is an anionic exchange membrane.

[0030] The negative electrolyte and / or the positive electrolyte may further comprise supporting electrolytes. The supporting electrolytes may be selected from sulfuric acid, hydrobromic acid, chloric acid, perchloric acid, hydrochloric acid, citric acid, carbonic acid, phosphonic acid, phosphoric acid, formic acid, acetic acid; chloride salts of sodium, potassium, magnesium, calcium and ammonium; sodium, potassium, calcium and magnesium salts of carbonate, bicarbonate, phosphate, biphosphate, sulfate, bisulfate, nitrate, citrate, chlorate and perchlorate; and mixtures thereof. Preferably, the supporting electrolytes are selected from NH4C1, NaCl, KC1 and mixtures thereof. The supporting electrolytes may serve to make the electrolyte solutions ionically conductive and provide the system with mobile charge carriers.

[0031] With reference to the previously presented solubility of the NDI, and according to one embodiment, the deprotonated NDI may have a solubility in an aqueous-based solution of (DI) water and 1 M supporting electrolyte (as previously exemplified) at room temperature of at least 200 mM, or of at least 300 mM, or of at least 400 mM, or of at least 500 mM, or of at least 600 mM, or of at least 700 mM, or of at least 800 mM, and / or up to a solubility of 1.5 M, or 2.0 M, or 2.5 M. For example, the solubility is between 200 mM and 1.5 M, e.g. between 300 mM and 1.5 M or between 400 mM and 1.5 M, or between 500 mM and 1.5 M, or between 600 M and 1.5 M, or between 700 mM and 1.5 M, or between 800 mM and 1.5 M. For example, the deprotonated NDI may have a solubility in the first aqueous-based electrolyte solution at room temperature of between 200 mM and 1.5 M, e.g. between 300 mM and 1.5 M or between 400 mM and 1.5 M, or between 500 mM and 1.5 M, orbetween 600 M and 1.5 M, or between 700 mM and 1.5 M, or between 800 mM and 1.5 M.

[0032] The minimum concentration of the NDI in the redox flow battery of the present disclosure may be 0.3 M based on the molar concentration of the supporting electrolyte salt. The molar concentration of the second organic redox-active compound, such as e.g. tetramethylpiperidineoxyl or a derivative thereof, of the present disclosure may be 0.3 M based on the molar concentration of the supporting electrolyte salt.

[0033] The disclosed aspects, examples (including any preferred examples), and / or accompanying claims may be suitably combined with each other as would be apparent to anyone of ordinary skill in the art. Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Fig. 1 shows a schematic view of an aqueous organic redox flow battery.

[0035] Fig. 2 shows exemplified electrolyte pairs for use in the redox flow battery of Fig. 1.

[0036] Fig. 3 shows cyclic voltammetry tests for aspartic acid NDI and glutamic acid NDI.DETAILED DESCRIPTION

[0037] Aqueous organic redox flow batteries have gained interest due to their high performance, low cost and lower environmental impact compared to other batteries. However, there is still a need to reduce the environmental effect and the operation cost of these batteries.

[0038] The disclosure focuses on redox-active organic compounds based on naphthalene diimides (NDIs) for use in aqueous redox flow bateries. The present inventors have realized that by functionalizing the NDI core with side chains derived from glutamic acid and / or aspartic acid, current problems with low solubility and suboptimal electrochemical properties may be mitigated. The NDIs of the present invention exhibit significantly higher solubility in water and / or in the correspondingaqueous-based electrolyte solution, enabling improved energy densities in redox flow batteries. These compounds can be used in deprotonated (salt) form, with various counterions such as ammonium or potassium, to further optimize performance. The system-level implementation of these NDIs in redox flow batteries, including e.g. compatible positive electrolytes like ferrocyanide or nitroxyl radicals, results in a practical balance of energy density, cycling stability, and environmental sustainability. This combination of molecular design and electrochemical performance makes the invention a promising step forward for organic flow batteries, particularly for large-scale energy storage applications.

[0039] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. Like numbers refer to like elements throughout the description.

[0040] Fig. 1 is a schematic view of a redox flow battery 1 according to the present disclosure.

[0041] The redox flow battery comprises two aqueous-based electrolyte solutions, a first aqueous-based electrolyte solutions 2 including a negative electrolyte 2a and a second aqueous-based electrolyte solution 3 including a positive electrolyte 3a. In embodiments, the first aqueous-based electrolyte solution 2 may be the same as the negative electrolyte 2a, and the second aqueous-based electrolyte solution 3 may be the same as the positive electrolyte 3a.

[0042] The negative electrolyte 2a comprises a first organic redox-active compound being a NDI, or a salt thereof. The NDI is dissolved in a first aqueous, or aqueous-based solvent. The positive electrolyte 3a comprises a second organic redox-active compound such as cyclic nitroxyl radical derivatives. However, the second organic redox-active compound may alternatively be ferrocyanide or a derivative thereof. The cyclic nitroxyl radical derivative is dissolved in a second aqueous, or aqueous-based solvent. The first and the second aqueous solvent may be the same, or different, aqueous solvent. The volumes of the negative and positive electrolytes 2a, 3a are typically similar (or the same), and may e.g. differ by only 10 %, preferably 5 % (percentage based on volume). That is, the negative electrolyte 2a and the positive electrolyte 3a may differ relative each other by 10 vol.%, preferably 5 vol.%.

[0043] The negative electrolyte 2a and / or the positive electrolyte 3a may further comprise supporting electrolytes. According to one example, the first aqueous-based electrolyte solution 2 in the negative compartment 4 has pH of at least 4, such as e.g. at least 5 or at least 6. The pH of the first aqueous-based electrolyte solution 2 in the negative compartment 4 may e.g. be between 6 and 8, such as e.g. 7 or approximately 7 (e.g. a pH of between 6.5 and 7.5). According to one example, the second aqueousbased electrolyte solution 3 in the positive compartment 5 has pH of between 2 and 6, or a pH of 7, or approximately 7 (e.g. a pH of between 6.5 and 7.5). According to one embodiment, the pH of the first aqueous-based electrolyte solution 2 in the negative compartment 4 is between 6.5 and 7.5, and the pH of the second aqueous-based electrolyte solution 3 in the positive compartment 5 is between 2 and 6.

[0044] The negative electrolyte 2a is in Fig. 1 comprised in a first compartment, or negative compartment 4, and the positive electrolyte 3a is comprised in a second compartment, or positive compartment 5. Furthermore, the negative compartment 4 comprises a negative electrode 6 in contact with the negative electrolyte 2a and the positive compartment 5 comprises a positive electrode 7 in contact with the positive electrolyte 3a. The electrodes 6, 7 may be carbon felt. The battery 1 may further comprise electrically conductive means 8 for establishing electrical conduction between the negative electrode 6 and the positive electrode 7, and external means 9, such as a power supply, for directing electrical energy into or out of the redox flow battery 1. The external means 9 may be referred to as external power means, or external load. The nominal voltage between the positive and negative compartments 4, 5 may e.g. be at least 1 Volt, such as at least 1.1 Volts or at least 1.2 Volts.

[0045] The electrically conductive means 8 may comprise a first conductive plate 12 such as a current collector, in contact with the negative electrode 6 and a second conductive plate 13 such as a current collector, in contact with the positive electrode 7 wherein both the first plate 12 and the second plate 13 are connected to an outer circuit comprising the external means 9.

[0046] The redox flow battery 1 further comprises a separator component 10 separating the negative electrolyte 2a in the negative compartment 4 from the positive electrolyte 3a in the positive compartment 5. The separator component 10 is preferably a membrane which enables separation of the negative and positive electrolytes 2, 3, more preferably the separator component 10 substantially preventsthe negative and positive electrolytes 2,3 from mixing while permitting the passage of non-redox-active species, such as water and counterions, between the electrolytes 2, 3. The separator component 10 maybe an anionic or cationic exchange membrane. The respective negative and positive electrodes 6,7 may be stacked on each side of the separator component 10 in their respective compartment 4, 5.

[0047] The redox flow battery 1 may also comprise means 11 capable of establishing a flow of the negative and positive electrolytes 2, 3 past the respective negative and positive electrodes 6, 7. The means 11 may e.g. be one or more pumps.

[0048] In the following, the first aqueous-based electrolyte solution 2 including the negative electrolyte 2a and the second aqueous-based electrolyte solution 3 including the positive electrolyte 3a will be exemplified.

[0049] The NDI in the first aqueous-based electrolyte solution 2 and the negative electrolyte 2a is that of formula (I):(I)or a salt thereof.

[0050] Typically, the NDI is present in its deprotonated form in the negative electrolyte 2a. The deprotonated NDI in the negative electrolyte 2a typically exist as a salt. The salt may e.g. be a salt of sodium, potassium or ammonium. Thus, for such examples, the positive counterions to the negatively charged deprotonated NDI in thenegative electrolyte 2a is sodium, potassium, or ammonium. As a further alternative, the NDI is a salt of primary, secondary, tertiary or quaternary ammonium.

[0051] The deprotonated NDI in the negative electrolyte 2a, e.g. as previously exemplified, may have a solubility in deionized water at room temperature of between 600 mM and 2.5 M, e.g. between 700 mM and 2.5 M or between 800 mM and 2.5 M, or between 900 mM and 2.5 M, or between 1 M and 2.5 M. Typically, the deprotonated NDI in the negative electrolyte 2a has a solubility in deionized water at room temperature of at least 600 mM, or at least 700 mM, or at least 800 mM, or at least 900 mM, or at least 1.

[0052] For example, the NDI may be a compound according to formula (la):(la)

[0053] Thus, the NDI in the negative electrolyte 2a may be based on aspartic acid side chains to the nitrogen atoms in the NDI molecule.

[0054] According to another example, the NDI is a compound according to formula (lb):oo(lb)

[0055] Thus, the NDI in the negative electrolyte 2a may be based on glutamic acid side chains to the nitrogen atoms in the NDI molecule.

[0056] The second aqueous-based electrolyte solution 3 and the positive electrolyte 3a may comprise an organic redox-active compound, or a second organic redox-active compound. The second organic redox-active compound may e.g. be: a cyclic nitroxyl radical derivative; or ferrocyanide or a derivative thereof.

[0057] The cyclic nitroxyl radical derivative may be selected from a tetramethylpiperidineoxyl (TEMPO) derivative, proxy! nitroxide derivative, phenyltetramethylnitronyl nitroxide derivative and nortropine N-oxyl derivative, preferably the cyclic nitroxyl radical derivative is a TEMPO derivative.

[0058] The cyclic nitroxyl radical derivative of the present disclosure is preferably a salt. Having the cyclic nitroxyl radical derivate in its salt form increases the solubility of the cyclic nitroxyl radical derivatives and thus improves the efficacy of the redox flow battery. More preferably, the cyclic nitroxyl radical derivative is a salt of chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate, most preferably a salt of chloride or sulfate. These specific cyclicnitroxyl radical derivative salts have a particularly good combination of cost, solubility in the electrolyte solution and conductivity when used, in a redox flow battery.

[0059] In this disclosure TEMPO derivatives are derivatives of (2,2, 6,6-Tetramethylpiperidin-1-yl)oxyl, formula (II).AAATH3C— / V — CH3 / \H3C I CH3o'(II)

[0060] The TEMPO derivatives may comprise one or more of the group according to formula (II). The TEMPO derivatives may further comprise at least two groups according to formula (II) and thus forms a TEMPO oligomer. In a preferred embodiment, the TEMPO derivative comprises solubilizing groups such as primary, secondary or tertiary amine, carboxylic acid, sulfonic acid and / or phosphonic acid.

[0061] The TEMPO derivative may be selected fromderivatives further comprise counter ions and are thus present in the form of salts. The TEMPO derivatives comprising cationic charges may comprise counter ions such as chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate,citrate or carbonate. The TEMPO derivatives comprising anionic charges may comprise counter ions such as sodium, potassium or ammonium.

[0062] The second organic redox-active compound in the positive electrolyte 3a, e.g. as previously exemplified, may have a solubility in deionized water at room temperature of between 200 mM and 5 M, e.g. between 300 mM and 5 M or between 400 mM and 5 M, or between 500 mM and 5 M.

[0063] The relatively high solubility of the NDI in the negative electrolyte 2a and / or of the second organic redox-active compound (e.g. tetramethylpiperidineoxyl or a derivative thereof) in the positive electrolyte 3a results in an advantageous energy density of the redox flow battery. For example, high solubility of the NDI in the negative electrolyte 2a results in an energy density of an electrolyte system formed by the positive electrolyte 3a and the negative electrolyte 2a of at least 20 Wh / litre, such as e.g. at least 25 Wh / litre, at least 30 Wh / litre or at least 35 Wh / litre.

[0064] In a preferred embodiment, the redox flow battery 1 comprises NDI according to formula (I), (la) or (lb) and a tetramethylpiperidineoxyl derivative saltselected fromH3C I. CH3and °. The TEMPO derivatives further comprise counter ions such as chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate and are thus present in the form of salts.

[0065] The supporting electrolytes previously mentioned may be selected from sulfuric acid, hydrobromic acid, chloric acid, perchloric acid, hydrochloric acid, citric acid, carbonic acid, phosphonic acid, phosphoric acid, formic acid, acetic acid; chloride salts of sodium, potassium, magnesium, calcium and ammonium; sodium, potassium, calcium and magnesium salts of carbonate, bicarbonate, phosphate, biphosphate, sulfate, bisulfate, nitrate, citrate, chlorate and perchlorate; and mixtures thereof. Preferably, the supporting electrolytes are selected from NH4C1, NaCl, KC1 and mixtures thereof.

[0066] The minimum concentration of the NDI in the redox flow battery 1 of the present disclosure may be 0.3 M based on the molar concentration of the supporting electrolyte salt.

[0067] Example of electrolyte pairs for use in a redox flow battery, e.g. that of Fig. 1, are shown in Fig. 2. For each of the negative electrolyte and positive electrolyte, the various options of organic redox-active compounds are separated by solid lines. Any of the options of organic redox-active compound for the negative electrolyte may form an electrolyte pair with any of the options of organic redoxactive compound for the positive electrolyte.EXAMPLES

[0068] The solubility of deprotonated NDI according to formula (I) was tested for four samples: two different versions of NDI, a first version according to formula (la) (also referred to as aspartic acid NDI) and a second version according to formula (lb)(also referred to as glutamic acid NDI), and for two different types of salts, potassium and ammonium-based salts. High concentration solutions were prepared by dissolving appropriate amounts of NDI (approximate 1 g) up to a fixed volume of deionized water. All solutions were placed in an ultrasonic bath to ensure complete dissolution of all compounds. Additionally, the solutions were left undisturbed at room temperature for 24 h to ensure no precipitation would form. The results are shown in Table 1.Sample Negative electrolyte Weight Volume Concentration of (g) (mL) electrolyte (M = mol / L) Deprotonated form of1 NDI formula (la) 1.0411 2.0 0.8PotassiumDeprotonated form of2 NDI formula (la) 0.9064 2.0 0.8AmmoniumDeprotonated form of3 NDI formula (lb) 1.3575 2.0 1PotassiumDeprotonated form of4 NDI formula (lb) 1.1891 2.0 1Ammonium

[0069] Thus, the solubility for each sample 1-4 was at least 0.8 M or 800 mM (i.e. higher than 600 mM, even higher than 700 mM).

[0070] For cyclic voltammetry tests, each version of NDI, i.e. aspartic acid NDI and glutamic acid NDI, was dissolved to a concentration of 5 mM in a solution of 1 M KC1 and electrochemically analyzed with a 3-electrode setup, using polished glassy carbon as working electrode, platinum as counter electrode and Ag / AgCl in saturated KC1 as reference electrode.

[0071] Each solution was bubbled with nitrogen and kept under this inert atmosphere during data acquisition. The cyclic voltammetry was conducted using a BioLogic SP-50e potentiostat. Cycling potentials between 1 and -1 V vs Ag / AgCl wereapplied, at scan rates of 20; 50; 100; 250; and 500 mV / s. The data presented was acquired during the first cycle at each scan rate, see Fig. 3. In Fig.3, the cyclic voltammetry test for aspartic acid NDI is shown to the left and the cyclic voltammetry test for glutamic acid NDI is shown to the right.

[0072] In conclusion, it has been shown that the issues with low solubility when using NDI as an electrolyte in a redox flow battery can be mitigated when modifying the NDI molecule with carboxyalkyl side chains based on glutamic acid and / or aspartic acid. Moreover, the cyclic voltammetry tests for aspartic acid NDI and glutamic acid NDI provide proof that the use of NDI according to formula (I) is suitable for use in the negative electrolyte in a redox flow battery.

Claims

1. CLAIMS1. A redox flow battery (1) comprising:3.a negative compartment (4) comprising a negative electrode (6) in contact with a first aqueous-based electrolyte solution (2) including a negative electrolyte (2a) comprising an organic redox-active compound dissolved in a first aqueous-based solvent;4.a positive compartment (5) comprising a positive electrode (7) in contact with a second aqueous-based electrolyte solution (3) including a positive electrolyte (3a) in a second aqueous-based solvent;5.electrical conductive means (8) for establishing electrical conduction between said positive electrode and said negative electrode, and an external load (9) for directing electrical energy into or out of the redox flow battery;6.a separator component (10) separating the first aqueous-based electrolyte solution in the negative compartment from the second aqueous-based electrolyte solution in the positive compartment;7.wherein the organic redox-active compound is a naphthalene diimide, NDI, according to formula (I):

9.

10. (I)11.or a salt thereof.

2. The redox flow battery of claim 1, wherein the NDI is deprotonated.

3. The redox flow battery of claim 2, wherein the NDI has a solubility in deionized water at room temperature of at least 600 mM.

4. The redox flow battery of claim 2, wherein the NDI has a solubility in deionized water at room temperature of between 600 mM and 2.5 M, e.g. between 700 mM and 2.5 M or between 800 mM and 2.5 M, or between 900 mM and 2.5 M, or between 1 M and 2.5 M.

5. The redox flow battery of any of claims 1-4, wherein the NDI is a salt, preferably a salt of sodium, potassium or ammonium.

6. The redox flow battery of any of claims 1-4, wherein the NDI is a salt, preferably a salt of primary, secondary, tertiary or quaternary ammonium.

7. The redox flow battery of any of claims 1-6, wherein the NDI is a compound according to formula (la):

18.

19. (la)8. The redox flow battery of any of claims 1-6, wherein the NDI is a compound according to formula (lb):

21.

22. (Ib)9. The redox flow battery of any one of claims 1-8, further comprising means (11) capable of establishing flow of the first and second aqueous-based electrolyte solutions past said positive and negative electrodes, respectively.

10. The redox flow battery of any one of claims 1-9, wherein the first aqueous-based electrolyte solution in the negative compartment has pH of at least 4, such as e.g. at least 5 or at least 6.

11. The redox flow battery of any one of claims 1-10, wherein the NDI is a first organic redox-active compound, and wherein the positive electrolyte comprises a second organic redox-active compound.

12. The redox flow battery of claim 11, wherein the second organic redox-active compound is one of the following: cyclic nitroxyl radical derivative or salt thereof,; ferrocyanide or a derivative thereof.

13. The redox flow battery of any of claims 11-12, wherein the second organic redox-active compound has a solubility in deionized water at room temperature of between 200 mM and 5 M, e.g. between 300 mM and 5 M or between 400 mM and 5 M, or between 500 mM and 5 M.

14. The redox flow battery of any of claims 1-13, wherein the energy density of an electrolyte system formed by the positive electrolyte and the negative electrolyte is at least 20 Wh / litre, such as e.g. at least 25 Wh / litre, at least 30 Wh / litre or at least 35 Wh / litre.

15. The redox flow battery of any of claims 1-14, wherein the nominal voltage between the positive and negative compartments is at least 1 Volt, such as at least 1.1 Volts or at least 1.2 Volts.

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