Redox flow battery comprising NDI and cyclic nitroxyl radical derivatives

Abstract

There is provided a redox flow battery (1) comprising a negative electrolyte solution (2) and a positive electrolyte solution (3) wherein the negative electrolyte solution (2) comprises an optionally modified naphthalene diimide, NDI, or a salt thereof, and the positive electrolyte solution (3) comprises a cyclic nitroxyl radical derivative having at least two cyclic nitroxyl radical groups, or a salt thereof.

Description

REDOX FLOW BATTERY COMPRISING NDI AND CYCLIC NITROXYL RADICAL DERIVATIVESTECHNICAL FIELD

[0001] The present disclosure relates to the field of redox flow batteries and in particular to electrolytes used in redox flow batteries.BACKGROUND

[0002] Society faces a large challenge in transitioning from fossil energy sources to a 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 the sources, leading to a mismatch between 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 (one being oxidized and one being reduced during charging, and vice versa during discharging), 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. Naphtalene diimide (NDI) is a redox-active molecule that has gained attention for use in redox flow batteries due to its suitable electrochemical properties such as its ability to reduce with two electrons, high electron transfer kinetics and low reduction potentials. Thus, the development of a high energy density system using NDI is desirable.SUMMARY

[0004] NDIs, although possessing desirable properties, exhibit self-association due to their large aromatic structures. This self-association leads to limited solubility, and a decreased osmotic coefficient of the NDI-containing electrolyte, effectivelylimiting the maximum concentration of the positive electrolyte redox-active molecule. The present inventors have realized that by combining an NDI with a cyclic nitroxyl radical derivative comprising at least two cyclic nitroxyl radical groups, the above issues maybe overcome.

[0005] Accordingly, a redox flow battery is provided comprising a negative electrolyte solution and a positive electrolyte solution wherein the negative electrolyte solution comprises an optionally modified naphthalene diimide, NDI, or a salt thereof, and the positive electrolyte solution comprises a cyclic nitroxyl radical derivative having at least two cyclic nitroxyl radical groups, or a salt thereof.

[0006] By combining an optionally modified NDI with a cyclic nitroxyl radical derivative according to the present disclosure in a redox flow battery, the water transport (due to the osmotic effect) that is typically observed from the positive electrolyte to the negative NDI-containing electrolyte will be reduced or even reversed. Thus, a battery having balanced osmotic pressures at high energy densities is obtained, resulting in improved battery stability and efficacy.

[0007] The cyclic nitroxyl radical group may be selected from a tetramethylpiperidineoxyl, proxyl nitroxide, phenyltetramethylnitronyl nitroxide and nortropine N-oxyl, preferably tetramethylpiperidineoxyl.

[0008] The cyclic nitroxyl radical derivative maybe compound according to formula (I)Zl\ ^ZlLor a salt thereof;(I)Me - - Me / \Me I Me wherein Zi is a cyclic nitroxyl radical group selected fromoL is a linker combining the two cyclic nitroxyl radical groups, optionally L further comprises at least one additional cyclic nitroxyl radical group.

[0009] The cyclic nitroxyl radical group, Zi, is, preferably, a tetramethylpiperidineoxyl group and thus the cyclic nitroxyl radical derivative is a compound according to formula (II)L is a linker combining the two tetramethylpiperidineoxyl groups, optionally L further comprises at least one additional tetramethylpiperidineoxyl group.

[0010] Tetrmethylpiperidineoxyl in the present disclosure is to be understood to mean (2,2,6,6-tetramethylpiperidin-i-yl)oxyl. Thus, tetramethylpiperidineoxyl derivatives is understood to mean (2,2,6,6-tetramethylpiperidin-i-yl)oxyl derivatives and tetramethylpiperidineoxyl group is understood to mean (2, 2,6,6-tetramethylpiperidin-i-yl)oxyl group. Tetramethylpiperidineoxyl maybe abbreviated as TEMPO.

[0011] 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 cyclic nitroxyl radical derivative salts have a particularly good combination of cost,solubility in the electrolyte solution and conductivity when used in a redox flow battery.

[0012] L may be selected fromchloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate,citrate or carbonate. Preferably L is selected from. The cyclic nitroxyl radical derivatives having the linkers above have a high solubility, particularly in an aqueous solvent, and maybe produced cheaply.

[0013] More preferably, the cyclic nitroxyl radical derivative is selected fromand , most preferably it is selected from

[0014] The optionally modifed NDI may be a modified NDI and can then be a compound according to formula (III)(III)or a salt thereof;wherein Ri and R4 are independently selected from -H, -CN, -S(0)20H, -X or a hydrocarbyl group comprising 2-20 carbons and at least one substituent R7;R2, R3, R5 and R6 are independently selected from -H, -CN, -S(0)20H, -X, -N02, a primary, secondary, tertiary or quaternary amine, or a hydrocarbyl group comprising 2-20 carbons and at least one substituent R7;substituent R7 is selected from a primary, secondary, tertiary or quaternary amine, a phosphonate, -S(0)20H or -C(O)OH, -OH or -(CH2CH2O)I-5CH3; andX is a halogen.

[0015] A modified NDI according to formula (III) has particularly suitable properties for use as an electrolyte in a redox flow battery such as high solubility and improved redox potentials.

[0016] The modified NDI is preferably a salt. Having the modified NDI in its salt form increases the solubility of the modified NDI and thus improves the efficacy of the redox flow battery. More preferably, the modified NDI is a salt of sodium, potassium, ammonium, chloride, methylsulfonate, sulfate or hydrogen sulfate. If the modified NDI is anionic, i.e., an NDI comprising anionic charges, the modified NDI is preferably a salt of sodium, potassium or ammonium. If the modified NDI is cationic, i.e., an NDI comprising cationic charges, the modified NDI is preferably a salt of chloride, methylsulfonate, sulfate or hydrogen sulfate. These specific salts have a particularly good solubility in the electrolyte solution.

[0017] Preferably, R2, R3, R5 and R6 in the compound according to formula (III) are -H.

[0018] Preferably, Ri=Rq in the compound according to formula (III).

[0019] Ri and / or R4 maybe hydrocarbyl groups comprising 2-20 carbons and at least one substituent R7. Preferably, the substituent R7 is a primary, secondary, tertiary or quaternary amine, more preferably a tertiary or quaternary amine. Ri andR4 may be independently selected from2-6Rs; R8 may be a hydrocarbyl group comprising 1-5 carbons, such as methyl, ethyl and propyl and Y’ is a counterion such as chloride, methylsulfonate, sulfate or hydrogen sulfate.Preferably, R8 is a methyl. By modifying the NDI accordingly, an NDI having high solubility in the electrolyte solution maybe obtained while simultaneously keeping down the cost of the modification.

[0020] The NDI maybe a compound according to formula (IV)or a salt thereof.(IV)

[0021] The cyclic nitroxyl radical derivative and the modified NDI may comprise anionic or cationic charges, preferably both the cyclic nitroxyl radical derivative and the NDI comprise cationic charges. The charged cyclic nitroxyl radical derivative and the charged NDI are thus in their salt form.

[0022] The redox flow battery of the present disclosure may further comprise:- a first compartment comprising a negative electrode in contact with the negative electrolyte solution;- a second compartment comprising a positive electrode in contact with the positive electrolyte solution;- electrical conductive means for establishing electrical conduction between said positive electrode and said negative electrode;- an external means for directing electrical energy into or out of the redox flow battery; and- a separator component separating the negative electrolyte solution in the first compartment from the positive electrolyte solution in the second compartment.

[0023] The separator component maybe an anionic or cationic exchange membrane. Preferably, the separator component is an anionic exchange membrane. More preferably, the separator component is an anionic exchange membrane if the cyclic nitroxyl radical derivative comprises cationic charges and a cationic exchange membrane if the cyclic nitroxyl radical derivate comprises anionic charges. This will result in reduced crossover of the NDI and the cyclic nitroxyl radical derivative through the membrane due to ionic repulsion.

[0024] The redox flow battery may further comprise means capable of establishing flow of the positive and negative electrolyte solutions past said positive and negative electrodes, respectively. Such means may e.g. be pumps or the like.

[0025] The positive electrolyte solution may further comprise a first aqueous solvent and the negative electrolyte solution may further comprise a second aqueous solution. Using aqueous solvents reduces the environmental impact and flammability of the redox flow battery.

[0026] The negative electrolyte solution and / or the positive electrolyte solution may further comprise supporting electrolytes. The supporting electrolytes maybe 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 NH4CI, 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.

[0027] The minimum concentration of the optionally modified NDI in the redox flow battery of the present disclosure maybe 0.25 M based on the molar concentration of the cyclic nitroxyl radical groups. The molar concentration of the cyclic nitroxyl groups maybe o.1-1.0 M, such as 0.2-0.9 M such as 0.3-0.8 M.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Fig. 1 shows a schematic view of an aqueous organic redox flow battery.

[0029] Fig. 2 shows the electrolytes used in the examples section.

[0030] Fig. 3 shows battery capacity (on discharge) over multiple cycles for Battery 1 - DMAPA NDI x TMA-TEMPO.

[0031] Fig.4 shows battery capacity (on discharge) over multiple cycles for Battery 2 - DMAPA NDI x DMA-TEMPO dimer.

[0032] Fig.5 shows battery capacity (on discharge) over multiple cycles for Battery 3 - DMAPA NDI x TMPDA-TEMPO dimer.DETAILED DESCRIPTION

[0033] In this disclosure NDI’s intrinsic problem of low osmotic coefficient in redox flow batteries due to self-association has been overcome by combining an NDI as the negative electrolyte with a cyclic nitroxyl radical derivative having at least two cyclic nitroxyl radical groups as the positive electrolyte. The redox flow battery according to the present disclosure has an optimized osmotic pressure at high concentrations and thus has an improved battery efficacy.

[0034] The cyclic nitroxyl radical group may be selected from a tetramethylpiperidineoxyl, proxyl nitroxide, phenyltetramethylnitronyl nitroxide and nortr opine N-oxyl.

[0035] 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.

[0036] Fig. 1 shows a schematic view of an aqueous organic redox flow battery (1) according to the present disclosure.

[0037] The redox flow battery comprises two electrolyte solutions, a negative electrolyte solution 2 and a positive electrolyte solution 3. The negative electrolytesolution 2 comprises an optionally modified naphthalene diimide, NDI, or a salt thereof. Preferably, the optionally modified NDI is dissolved in a first aqueous solvent. The positive electrolyte solution 3 comprises a cyclic nitroxyl radical derivative having at least two cyclic nitroxyl radical groups or a salt thereof.Preferably, the cyclic nitroxyl radical derivative is dissolved in a second aqueous solvent. The first and the second aqueous solvent maybe the same aqueous solvent. The volume of the respective electrolyte solution 2,3 is similar, i.e., differs only with 10 %, preferably 5 %. That is, the negative electrolyte solution 2 and the positive electrolyte solution 3 may differ relative each other by 10 vol.%, preferably 5 vol.%.

[0038] The negative electrolyte solution 2 and / or the positive electrolyte solution 3 may further comprise supporting electrolytes.

[0039] The respective electrolyte solution 2,3 maybe present in a first compartment 4 and a second compartment 5. Furthermore, the first compartment 4 may comprise a negative electrode 6 in contact with the negative electrolyte solution 2 and the second compartment 5 may comprise a positive electrode 7 in contact with the positive electrolyte solution 3. The electrodes maybe carbon felt. The battery may further comprise electrical 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 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. The redox flow battery 1 may further comprise a separator component 10 separating the negative electrolyte solution 2 in the negative compartment 4 from the positive electrolyte solution 3 in the positive compartment 5. The separator component 10 is preferably a membrane which enables separation of the two electrolyte solutions 2,3 in each compartment 4,5, more preferably the separator component 10 substantially prevents the electrolyte solutions 2,3 from mixing while permitting the passage of non-redox-active species, such as water and counterions, between the electrolyte solutions 2,3. The separator component 10 maybe an anionic or cationic exchange membrane. An anionic exchange membrane is preferably used when the cyclic nitroxyl radical derivativecomprises cationic charges. A cationic exchange membrane is preferably used if the cyclic nitroxyl radical derivate comprises anionic charges. This will result in a reduced crossover of the cyclic nitroxyl radical derivative through the membrane due to ionic repulsion. The respective electrodes 6,7 are stacked on each side of the separator / membrane in their respective compartment 4, 5.

[0040] The redox flow battery 1 may also comprise means 11 capable of establishing a flow of the electrolyte solutions 2,3 past the respective electrodes 6,7 such as pumps.

[0041] Preferably, the cyclic nitroxyl radical derivative is a compound according to formula (I)Zl\ ^Zl(I)wherein Zi is a cyclic nitroxyl radical group selected fromL is a linker combining the two cyclic nitroxyl radical groups, optionally L further comprises at least one additional cyclic nitroxyl radical group.

[0042] The cyclic nitroxyl radical group, Zi, is, preferably, a tetramethylpiperidineoxyl group and thus the cyclic nitroxyl radical derivative is a compound according to formula (II)(II)L is a linker combining the two tetramethylpiperidineoxyl groups, optionally L further comprises at least one additional tetramethylpiperidineoxyl group. When thecyclic nitroxyl radical derivative is a salt, L may be selected from*Me Me Me Me MeAA N +>* - N N - * C Aand and A-is a counterion such as chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate. Preferably L is selected from Me Me Me MeA- Me^ / MeN** and

[0043] The cyclic nitroxyl radical derivative may be selected fromand , preferably it is selected from

[0044] Preferably, the NDI is a compound according to formula (III)(III) or a salt thereof;wherein Ri and R4 are independently selected from -H, -CN, -S(0)20H, -X or a hydrocarbyl group, such as alkanes, comprising 2-20 carbons and at least one substituent R7;R2, R3, R5 and R6 are independently selected from -H, -CN, -S(0)20H, -X, -N02, a primary, secondary, tertiary or quaternary amine, or a hydrocarbyl group, such as alkanes, comprising 2-20 carbons and at least one substituent R7;substituent R7 is selected from a primary, secondary, tertiary or quaternary amine, aphosphonate, -S(0)20H or -C(O)OH, -OH or -(CH2CH2O)I-5CH3; and X is a halogen.

[0045] Preferably, R2, R3, R5 and R6 in the compound according to formula (III) are -H.

[0046] Preferably, R1=R4 in the compound according to formula (III).

[0047] Ri and / or R4 may be hydrocarbyl groups, such as alkanes, comprising 2- 20 carbons and at least one substituent R7. Preferably, the substituent R7 is a primary, secondary, tertiary or quaternary amine, more preferably a tertiary orY- T I8^R8quaternary amine. Ri and R4 may be independently selected from2-68andrV .N.2-68; R8 may be a hydrocarbyl group comprising 1-5 carbons, such as methyl, ethyl and propyl and Y’ is a counterion such as chloride, methylsulfonate, sulfate or hydrogen sulfate. Preferably, R8 is a methyl.

[0048] The NDI maybe a compound according to formula (IV)MeMe or a salt thereof.(IV)

[0049] The NDI and the cyclic nitroxyl radical derivative preferably comprise anionic or cationic charges and are thus in their salt form. This is especially preferred when used together with an aqueous solvent, i.e., in an aqueous redox flow battery.Having the NDI and the cyclic nitroxyl radical derivative in their salt form increases their solubility and thus improves the efficacy of the battery. It is preferred that the NDI and the cyclic nitroxyl radical derivative both comprise the same type of charges, i.e., anionic or cationic charges. More preferably, both NDI and cyclic nitroxyl radical derivative comprise cationic charges.

[0050] The modified NDI is preferably a salt of sodium, potassium, ammonium, chloride, methylsulfonate, sulfate or hydrogen sulfate. If the modified NDI is anionic, i.e., an NDI comprising anionic charges, the modified NDI is preferably a salt of sodium, potassium or ammonium. If the modified NDI is cationic, i.e., an NDI comprising cationic charges, the modified NDI is preferably a salt of chloride, methylsulfonate, sulfate or hydrogen sulfate. These specific salts have a particularly good solubility in the electrolyte solution.

[0051] The cyclic nitroxyl radical derivative is preferably a salt of chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate, most preferably a salt of chloride or sulfate. These specific cyclic nitroxyl radical derivative salts have a particularly good solubility in the electrolyte solution.

[0052] In a preferred embodiment, the redox flow battery 1 comprises an NDI according to formula (IV) and a TEMPO derivative salt selected from

[0053] 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 ofsodium, 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.

[0054] The minimum concentration of the optionally modified NDI in the redox flow battery of the present disclosure maybe 0.25 M based on the molar concentration of the TEMPO groups.EXAMPLES

[0055] A redox flow battery cell with flat flow fields and 5 cm2carbon felt electrodes was used in this example. The carbon felt (SIGRACELL® GFD 2.5 EA thermal activated) was 2.5 mm thick, and was compressed to 2.1 mm. The carbon felt was used without pre-treatment. A Fumasep FAA-3-30-PE membrane (anionic membrane) was used as received from the manufacturer. Chorny SN100CQ peristaltic pumps set to a rotation speed of 60 RPM were used with Masterflex™ Tygon™ E-LFL Pump Tubing. Additionally, rigid PVDF / FEP tubing from Vici Jour (part no. JR-T-6130-M10) was used in parts of the battery setup. The entire redox flow battery setup (flow cell, pumps and solution reservoirs) was operated under a nitrogen atmosphere, by using an airtight custom-made enclosure. A BioLogic BCS-815 battery tester was used to run the batteries, by performing multiple charging and discharging cycles at a constant current of 300 mA between the cutoffs of 1.8 V and 0.6 V. The set-up of the battery in this example is similar to, or corresponding to, that of Fig. 1.

[0056] In this example a chloride salt of dimethylaminopropylamine-NDI (DMAPA-NDI) was used as the negative electrolyte, see Fig. 2. Three different TEMPO derivatives were used as the positive electrolyte: chloride salt of N,N,N',N'-tetramethyl-i,3-propanediamine- TEMPO dimer (TMPDA-TEMPO dimer), chloride salt of dimethylamine- TEMPO dimer (DMA-TEMPO dimer) and chloride salt of trimethylamine- TEMPO (TMA-TEMPO), see Fig. 2. TMPDA-TEMPO dimer and DMA-TEMPO dimer comprise two TEMPO groups and fall under the scope of the present disclosure. TMA-TEMPO comprises one TEMPO group and is thus used as a reference. The electrolyte solutions were prepared by dissolving the negative andpositive electrolytes in an aqueous solution of KC1 at the desired concentrations, see Table i.

[0057] Table 1 shows the electrolyte properties in the different batteries. The NDI derivative is the negative electrolyte and the TEMPO derivative is the positive electrolyte.Battery Electrolyte Concentration Concentration of KC1 (mol / L) of electrolyte(mol / L)1 DMAPANDI 0.3 1TMA-TEMPO 0.6 0.4 2 DMAPANDI 0.3 1DMA-TEMPO dimer 0.3 0.43 DMAPANDI 0.3 1TMPDA-TEMPO dimer 0.3 0.4

[0058] All three batteries were run for seven days. After the testing period was over, a discharge cycle was performed (constant current discharge at 2000 mA until the voltage of the cell reaches 0.3 V followed by a constant voltage) until the current was nearly o mA. Finally, the setup was disassembled, and the final solution volumes were measured, see Table 2.

[0059] Table 2 shows the electrolyte solution volumes before and after battery cycling.Battery Electrolyte Initial solution volume Final solution volume (mL) (mL)1 DMAPANDI 12.0 9.0TMA-TEMPO 12.0 13-02 DMAPANDI 12.0 10.6 DMA-TEMPO dimer 12.0 9.0 3 DMAPANDI 10.0 7-9TMPDA-TEMPO 10.0 7.2 dimer

[0060] During the operation of the battery and during the volume measuring procedure, some volume is lost. This is due to liquid being left inside the cell and the tubing. These losses are equivalent for both sides and can thus be neglected.

[0061] It was observed that significant osmotic water transport occurs from the negative side (NDI) to the positive side (TEMPO derivative) in battery 1, see Table 2. In batteries 2 and 3, wherein a TEMPO derivative comprising two TEMPO groups was used as the positive electrolyte, a reduction of the osmotic water transport towards the positive side was observed, see Table 2. For batteries 2 and 3, the water transport was reversed compared to battery 1, i.e., the water transport occurred from the positive side to the negative side. This is beneficial due to NDIs poor solubility and improves the stability of the battery as it is less likely that NDI will start precipitating and thus clog the battery.

[0062] The battery capacity of all three batteries was good and showed that all three batteries work properly, see Fig. 3-5. It was further observed that the graph for battery 1 was more uneven than the graph for batteries 2 and 3. This indicates that the issues with osmotic pressure are affecting the battery stability.

[0063] In conclusion, it has been shown that the issues with osmotic pressure that arise when using NDI as an electrolyte in a redox flow battery are significantly reduced, even reversed, when combining the NDI with a TEMPO derivative comprising at least two TEMPO groups. Thus, combining an NDI with a TEMPO derivative of the present disclosure gives rise to a redox flow battery having improved stability and efficacy at high concentrations.

Claims

CLAIMS1. A redox flow battery (1) comprising a negative electrolyte solution (2) and a positive electrolyte solution (3) wherein the negative electrolyte solution (2) comprises an optionally modified naphthalene diimide, NDI, or a salt thereof, and the positive electrolyte solution (3) comprises a cyclic nitroxyl radical derivative having at least two cyclic nitroxyl radical groups, or a salt thereof.

2. The redox flow battery (1) according to claim 1, wherein the cyclic nitroxyl radical derivative is a compound according to formula (I)Zl\ / ZlLor a salt thereof;(I)wherein Zi is a cyclic nitroxyl radical group selected fromL is a linker combining the two cyclic nitroxyl radical groups, optionally L further comprises at least one additional cyclic nitroxyl radical group.

3. The redox flow battery (1) according to claim 2, wherein the cyclic nitroxyl radical derivative is a compound according to formula (II)(ID4. The redox flow battery (1) according to any of the preceding claims, wherein the cyclic nitroxyl radical derivative is a salt, more preferably a salt of chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate, most preferably a salt of chloride or sulfate.

5. The redox flow battery (1) according to any one of claims 2-4, wherein L is Me Me Me MeMe Meandzi ; and A- is a counterion such as chloride, sulfate, hydrogen sulfate, phosphate, methylsulfonate, nitrate, acetate, citrate or carbonate.

6. The redox flow battery (1) according to any one of the preceding claims, wherein the optionally modified NDI is a modified NDI and a compound according to formula (III)(III)or a salt thereof;wherein Ri and R4 are independently selected from -H, -CN, -S(0)20H, -X or a hydrocarbyl group comprising 2-20 carbons and at least one substituent R7;R2, R3, R5 and R6 are independently selected from -H, -CN, -S(0)20H, -X, - NO2, a primary, secondary, tertiary or quaternary amine, or a hydrocarbyl group comprising 2-20 carbons and at least one substituent R7;substituent R7 is selected from a primary, secondary, tertiary or quaternary amine, a phosphonate, -S(0)20H or -C(O)OH, -OH or -(CH2CH2O)I-5CH3; and X is a halogen.

7. The redox flow battery (1) according to claim 6, wherein the optionally modified NDI is a salt, preferably a salt of sodium, potassium, ammonium, chloride, methylsulfonate, sulfate or hydrogen sulfate.

8. The redox flow battery (1) according to any one of claims 6-7, wherein R2, R3, R5 and R6 are -H.

9. The redox flow battery (1) according to any one of claims 6-8, wherein Ri=Rq.

10. The redox flow battery (1) according to any one of claims 6-9, wherein Ri and / or R4 are hydrocarbyl groups comprising 2-20 carbons and at least one substituent R7.

11. The redox flow battery (1) according to claim 10, wherein the substituent R7 is a primary, secondary, tertiary or quaternary amine, preferably a tertiary or quaternary amine.

12. The redox flow battery (1) according to claim 11, wherein Ri and R4 areindependently selected fromR8; R8 is independently selected from a hydrocarbyl group comprising 1-5 carbons such as methyl, ethyl and propyl; andY- is a counterion such as chloride, methylsulfonate, sulfate or hydrogen sulfate.

13. The redox flow battery (1) according to any one of the preceding claims, comprising:- a first compartment (4) comprising a negative electrode (6) in contact with the negative electrolyte solution (2);- a second compartment (5) comprising a positive electrode (7) in contact with the positive electrolyte solution (3);- electrical conductive means (8) for establishing electrical conduction between said positive electrode and said negative electrode;- an external means (9) for directing electrical energy into or out of the redox flow battery (1); and- a separator component (10) separating the negative electrolyte solution (3) in the first compartment (4) from the positive electrolyte solution (2) in the second compartment (5).

14. The redox flow battery (1) according to claim 13, wherein the separator component (10) is an anionic exchange membrane.

15. The redox flow battery (1) according to any one of the preceding claims, wherein the positive electrolyte solution (3) comprises a first aqueous solvent and the negative electrolyte solution (2) comprises a second aqueous solution.

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