Non-aqueous redox flow batteries

Abstract

Non-aqueous redox flow battery (RFB) comprising: - a positive compartment wherein a positive electrode is placed and wherein a non-aqueous liquid positive electrolyte is made to flow; - a negative compartment wherein a negative electrode is placed and wherein a non-aqueous liquid negative electrolyte is made to flow; - an ion exchange membrane placed between the positive compartment and the negative compartment; wherein : - said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having a general formula (I): wherein: - G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5-ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4; - R1, R2 and R3, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; in at least an organic solvent; - said non-aqueous liquid negative electrolyte comprises a solution of at least one compound having a general formula (II) or (III): wherein: - R6 and R7, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; - R8 represents a hydrogen atom; or a C1-C20 alkyl group, preferably C1- C6, linear or branched, saturated or unsaturated, preferably saturated, an optionally substituted cycloalkyl group, an optionally substituted aryl group; an optionally substituted heteroaryl group; - G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5-ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4; - X- represents a tetrafluoroborate anion (BF4-), a hexafluorophosphate anion (PF6'), a bistrifluoromethanesulfonylimidate anion [(CF3SO2)N-]; in at least an organic solvent. Said non-aqueous redox flow battery (RFB) may be advantageously used in devices that require medium to high power output (e.g., about 100 kW - 100 MW) for several hours (i.e. > 1 hour) such as, for example, devices for storing energy from industrial plants or alternative energy sources (such as, solar or wind power) for subsequent use (for example, for domestic use) or for sale.

Description

[0001] NON-AQUEOUS REDOX FLOW BATTERIES

[0002] The present invention relates to non-aqueous redox flow batteries (RFB).

[0003] More particularly, the present invention relates to a non-aqueous redox flow battery (RFB) comprising: a positive compartment wherein a positive electrode is positioned and wherein a non-aqueous liquid positive electrolyte is made to flow; a negative compartment wherein a negative electrode is positioned and wherein a non-aqueous liquid negative electrolyte is made to flow; an ion exchange membrane positioned between the positive compartment and the negative compartment; wherein: said non-aqueous liquid positive electrolyte comprises a solution of at least one compound comprising a triphenylalkyl and 2, 2,6,6- tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below in at least one organic solvent; said non-aqueous liquid negative electrolyte comprises a solution of at least one compound comprising at least one dipyridyl group having general formula (II) provided below or at least one compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below in at least one organic solvent.

[0004] Said non-aqueous redox flow battery (RFB) may be advantageously used in devices that require medium to high power output (e.g., about 100 kW - 100 MW) for several hours (i.e. > 1 hour) such as, for example, devices for storing energy from industrial plants or alternative energy sources (such as, solar or wind power) for subsequent use (for example, for domestic use) or for sale.

[0005] The present invention also relates to a compound comprising a triphenylalkyl and 2,2,6,6-tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below, and to a compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below.

[0006] Redox flow batteries (RFBs) are becoming an increasingly promising technology in energy storage due to their low environmental impact and safe operation.

[0007] Redox flow batteries (RFBs) are a type of rechargeable battery wherein electrolytes containing solutions of one or more electroactive species, are made to flow through an electrochemical cell that converts chemical energy directly into electrical energy. Said electrochemical cell normally consists of a negative compartment (or negative half-cell) and a positive compartment (or positive halfcell), separated by an ion-exchange membrane. By storing these electrolytes in external reservoirs, the power components (i.e. the power output that depends on the size and design of said electrochemical cell) and the energy components (i.e. the stored energy that depends on the size of said external reservoirs and the concentration of the electrolytes contained therein) are decoupled, with a clear gain in terms of flexibility in the applications thereof.

[0008] The characteristic feature of said solutions of one or more electroactive species is their high energy density, which depends on various factors such as, for example, the concentration in solution of the reacting electroactive species, the number of electrons transferred into the positive or negative compartment (or halfcell) and the reaction potential.

[0009] The first generation of aqueous redox flow batteries (RFBs) is represented by vanadium redox flow batteries (RFBs), the so-called “All Vanadium” (henceforth referred to as “VRFBs” for simplicity). In “VRFBs”, the electroactive species consist of acid solutions of the four different oxidation states of vanadium: i.e., vanadium in oxidation state (II) [V(II)] and vanadium in oxidation state (III) [ V(III)] in the negative compartment and vanadium in oxidation state (IV) [V(IV)] and vanadium in oxidation state (V) [V(V)] in the positive compartment. Generally, in said “VRFBs”, the open circuit potential difference (or standard potential) (E°) of the cell is comprised between about 1.2 V and 1.6 V, the typical concentration of the electroactive species in the electrolyte is 2 M [in 5 M aqueous solution of sulphuric acid (H2SO4)], with an energy density comprised between 20 Wh / 1 and 30 Wh / 1. One of the advantages of said “VRFBs” is precisely the use of single-element electrolytes in both compartments, thus making contamination through the membrane negligible. However, the maximum concentration of the different vanadium species in the electrolyte is limited due to their poor solubility and stability, especially in the case of vanadium in oxidisation state (V) [V(V)], which is subject to thermal precipitation above 40°C, and the open circuit potential difference (E°) is affected by the stability window of water (i.e. water electrolysis).

[0010] More details related to said “VRFBs” may be found, for example in: Sum E. et al., “ Journal of Power Sources” (1985), Vol. 15, Issues 2-3, pg. 179-190; Sum E. et al., “ Journal of Power Sources” (1985), Vol. 16, Issue 2, pg. 85-95; Aaron D. S. et al., “Journal of Power Sources” (2012), Vol. 206, pg. 450-453.

[0011] Over the years, other types of aqueous redox flow batteries (RFBs) have been studied.

[0012] For example, Huskinson B. et al., in “Nature” (2014), Vol. 505, pg. 195- 198, describe a metal-free aqueous flow battery with inexpensive carbon electrodes, using the redox pair quinone / hydroquinone in the negative compartment and the redox pair Bo / Br' in the positive compartment. The use of organic molecules instead of metals is said to represent a promising new route to cost-effective electricity storage.

[0013] In a later study, Lin K. et al., in “Science” (2015), Vol. 349, Issue 6255, pg. 1529-1532, the redox pair Bn / Br', is replaced by the redox pair ferrocyanide / ferri cyanide in the positive compartment. The resulting aqueous flow batteries have the advantage of including non-toxic, non-flammable compounds, and are safe for both operators and the environment.

[0014] Since, as mentioned above, in aqueous redox flow batteries (RFBs), the open circuit potential difference (E°) is affected by the stability window of water (i.e. water electrolysis), further studies have been made concerning the use of electrolytes comprising metal-organic electroactive species soluble in organic solvents.

[0015] For example, Chakrabarti M. H. et al., in “Electrochimica Acta” (2007), Vol. 52, pages 2189-2195, describe electrolytes comprising metal-organic species in acetonitrile: in particular, an electrolyte comprising the redox couple ruthenium acetyl acetonate [Ru(acac)2], in both the positive and negative compartments, is described, which shows high stability and solubility in acetonitrile. The non- aqueous redox flow bateries (RFBs) obtained are said to have high efficiency.

[0016] Kaur A. P. et al., in “Energy Technology' (2015), Vol. 3, pg. 476-480, describe a non-aqueous redox flow battery (RFB) wherein the electrolyte in the positive compartment (catholyte) comprises a phenothiazine derivative, specifically 3,7-bis(trifluoromethyl)-7V-ethylphenoxythiazine (BCF3EPT) and the electrolyte in the negative compartment (anolyte) comprises 2,3,6-trimethyl quinoxaline. Phenothiazine derivatives have high stability and solubility in carbonate-based solvents (for example, propylene carbonate): however, Kaur A. P. et al. believe that further studies will be necessary in order to improve the performance of non-aqueous redox flow batteries (RFBs) containing them.

[0017] Li Z. et al., in “Electrochemical and Solid-State Letters’" (2011), Vol. 14, Issue 12, A171-A173, describe non-aqueous redox flow batteries (RFBs) using 2,2,6,6-tetramethyl-l-piperinyloxy / NaClO4 / acetonitrile as electrolyte in the positive (catholyte) compartment and A-methyl-phthalimide / NaClOVacetonitrile as electrolyte in the negative (anolyte) compartment. The aforementioned nonaqueous redox flow battery (RFB), subjected to charge-discharge tests, are said to have stable charge-discharge curves and high Coulombic efficiency (90%) for the first 20 cycles.

[0018] Gong K. et al., in “Energy & Environmental Science"" (2015), Vol. 8, pg. 3515-3530, describe various types of non-aqueous redox flowbateries (RFBs): in particular, the use of different organic solvents, different supporting electrolytes and different redox couples. Among others, they describe an ultra-high voltage non-aqueous redox flow battery (RFB) [i.e. having an ultra-high open circuit potential difference (E°)], i.e. 4.5 V, when using an electrolyte comprising biphenyl and a 1 M lithium hexafluorophosphate (LiPFe) solution in dimethylformamide (DMF) in the negative compartment and an electrolyte comprising octafluoronaphthalene and a 1 M lithium hexafluorophosphate (LiPFe) solution in polycarbonate (PC) in the positive compartment.

[0019] US patent application US 2013 / 0224538 describes a non-aqueous redox flow battery (RFB) comprising a negative electrode immersed in a non-aqueous liquid negative electrolyte, a positive electrode immersed in a non-aqueous liquid positive electrolyte, and a cation-permeable separator (e.g., a porous membrane, film, foil or panel) placed between the non-aqueous liquid negative electrolyte and the non-aqueous liquid positive electrolyte. During charge-discharge, electrolytes circulate around their respective electrodes. Each of the electrolytes comprises an electrolyte salt (e.g., a sodium or lithium salt), a transition metal-free redox reagent, and optionally an electrochemically stable organic solvent. Each redox reagent is selected from an organic compound comprising an unsaturated conjugate part, a boron compound, and a combination thereof. The organic redox reagent contained in the non-aqueous liquid positive electrolyte is selected to have a higher redox potential than the redox reagent contained in the non-aqueous liquid negative electrolyte. The aforementioned non-aqueous redox flow battery (RFB) is said to be more efficient than known redox flow batteries (RFBs).

[0020] US patent application US 2021 / 0036355 describes a redox flow battery (RFB) comprising a negative electrode (also referred to as “anode”) immersed in a first liquid electrolyte (also referred to as “negative electrolyte” or “anolyte”) a positive electrode (also referred to as “cathode”) immersed in a second liquid electrolyte (also referred to as “positive electrolyte” or “catholyte”) and a cation- permeable separator (e.g., a membrane or other cation-permeable material) separating the negative electrode / anolyte from the positive electrode / catholyte. The redox reagent of the catholyte comprises a compound containing a 2, 2,6,6- tetramethylpiperidinyloxy group (TEMPO) and the redox reagent of the anolyte may comprise a quinoxaline (e.g., a quinoxaline bearing at least one electron donor substituent), a dipyridyl ketone, a viologen (e.g., a bis-benzyl viologen salt, a bisethyl viologen salt, a bis-methyl viologen salt, and the like), a benzophenone, an anthraquinone [e.g., a salt of anthraquinone-2,7-disulphonic acid (AQDS), a salt of anthraquinone-2-sulphonic acid (AQS), anthraflavic acid (2,6- dihydroxyanthraquinone)], a metal ion with redox activity (e.g., a vanadium ion), and / or other similar materials. The aforementioned redox flow battery (RFB) is said to overcome functional or performance and / or cost limitations that hinder the large-scale adoption of known redox flow batteries (RFBs). Neither the specific use of viologen as an anolyte nor the use of specific catholyte / anolyte pairs is mentioned in the above patent application.

[0021] Since, as mentioned above, redox flow batteries (RFBs) are becoming an increasingly promising technology in the field of energy storage due to their low environmental impact and safe operation, the study of new redox flow batteries (RFBs), in particular non-aqueous ones, is still of great interest.

[0022] The Applicant therefore set out to solve the problem of finding a nonaqueous redox flow battery (RFB) capable of good performance, i.e. having both a good open circuit potential difference (E°) and good energy density (e).

[0023] The Applicant has now found that the use of a non-aqueous liquid electrolyte comprising at least one compound comprising a triphenylalkyl and 2, 2,6,6- tetraalkylpiperidinyloxy (TEMPO) groups having the specific general formula (I) provided below in at least one organic solvent in the positive compartment and the use of a non-aqueous liquid electrolyte comprising at least one compound comprising a dipyridyl group having general formula (II) provided below or at least one compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below in at least one organic solvent in the negative compartment, provides a non-aqueous redox flow battery (RFB) capable of giving good performance, i.e. both a good open circuit potential difference (E°) and good energy density (e). Furthermore, both the compound comprising a triphenylalkyl and 2, 2,6,6- tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below, and the compound comprising a dipyridyl group having the general formula (II) provided below, as well as the compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below, exhibit good stability during charge-discharge cycles of said non-aqueous redox flow battery (RFB) and high solubility in the organic solvent used. Furthermore, both the compound comprising a triphenylalkyl and 2,2,6,6-tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below, and the compound comprising a dipyridyl group having the general formula (II) provided below, as well as the compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below, are non-toxic compounds and, therefore, not harmful either from the environmental point of view or to operators’ health. Lastly, both the compound comprising a triphenylalkyl and 2,2,6,6-tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below, and the compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below, may be synthesised by simple processes and using starting materials that are readily available commercially and, consequently, economically advantageous.

[0024] It is therefore an object of the present invention to provide a non-aqueous Redox Flow Battery (RFB) comprising: a positive compartment wherein a positive electrode is placed and wherein a non-aqueous liquid positive electrolyte is made to flow; a negative compartment wherein a negative electrode is placed and wherein a non-aqueous liquid negative electrolyte is made to flow; an ion exchange membrane placed between the positive compartment and the negative compartment; wherein: said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having a general formula (I): wherein:

[0025] G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2-

[0026] CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4;

[0027] Ri, R2 and R3, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; in at least an organic solvent; said non-aqueous liquid negative electrolyte comprises a solution of at least one compound having a general formula (II) or (III): wherein:

[0028] Re and R7, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated;

[0029] Rs represents a hydrogen atom; or a C1-C20 alkyl group, preferably Ci- Ce, linear or branched, saturated or unsaturated, preferably saturated, an optionally substituted cycloalkyl group, an optionally substituted aryl group; an optionally substituted heteroaryl group;

[0030] G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4;

[0031] X" represents a tetrafluoroborate anion (BF4‘), a hexafluorophosphate anion (PFe'), a bistrifluoromethanesulfonylimidate anion [(CF3SO2)N-]; in at least an organic solvent.

[0032] For the purpose of the present description and of the following claims, the definitions of the numerical intervals always comprise the extreme values unless otherwise specified.

[0033] For the purpose of the present description and of the following claims, the term "comprising" also includes the terms "which essentially consists of' or "which consists of.

[0034] In accordance with a preferred embodiment of the present invention, in said general formula (I):

[0035] G represents a C1-C20 alkylene group, preferably butylene;

[0036] Ri, R2 and R% equal to each other, represent a C1-C20 alkyl group, preferably a methyl.

[0037] In accordance with a preferred embodiment of the present invention, in said general formula (II):

[0038] Re represents a C1-C20 alkyl group, preferably an ethyl; X" represents a hexafluorophosphate anion (PFe").

[0039] In accordance with a preferred embodiment of the present invention, in said general formula (III):

[0040] Rs represents a C1-C20 alkyl group, preferably a methyl;

[0041] G represents a C1-C20 alkylene group, preferably butylene;

[0042] X" represents a hexafluorophosphate anion (PFe").

[0043] For the purpose of the present description and of the following claims, the term “C1-C20 alkylene group” means a divalent alkyl group -(CH2)n- wherein n is an integer comprised between 1 and 20, linear or branched, saturated or unsaturated. Specific examples of the C1-C20 alkylene group are: methylene, ethylene, propylene, butylene, pentyl ene, hexylene, / .w-propylene, / .w-butylidene.

[0044] For the purpose of the present description and of the following claims, the term “C1-C20 alkyl group” means a linear or branched, saturated or unsaturated alkyl group having from 1 to 20 carbon atoms. Specific examples of C1-C20 alkyl groups are: methyl, ethyl, / / -propyl, / .w-propyl, / / -butyl, zso-butyl, / c / V-butyl, n- pentyl, / / -hexyl, ethyl-hexyl, / / -heptyl, / / -octyl, nonyl, / / -decyl, / / -dodecyl.

[0045] For the purpose of the present description and of the following claims, the term “polyethyleneoxy group” means a group having oxyethylene units in the molecule. Specific examples of polyethyleneoxy group are: methyl enethylenoxyl, methyl enedi ethyl enoxyl , methyl enetri ethyl enoxyl , methyl enetetraethyl enoxyl .

[0046] For the purpose of the present description and of the following claims, the term “cycloalkyl groups” means cycloalkyl groups having from 3 to 30 carbon atoms. Said cycloalkyl group may optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12 thioalkoxy groups; C3-C24 trialkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12 mono- or di-alkylamine groups; nitro groups. Specific examples of cycloalkyl groups are: cyclopropyl, 2,2-difluorocyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, methoxy cyclohexyl, fluorocyclohexyl, phenyl cyclohexyl, decalin, abiethyl. For the purpose of the present description and of the following claims, the term “aryl group” means an aromatic carbocyclic group containing from 6 to 60 carbon atoms. Said aryl group may optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12 thioalkoxy groups; C3-C24 trialkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12 mono- or di-alkylamine groups; nitro groups. Specific examples of aryl groups are: phenyl, methylphenyl, trimethylphenyl, methoxyphenyl, hydroxyphenyl, phenyl oxyphenyl, fluorophenyl, pentafluorophenyl, chlorophenyl, bromophenyl, nitrophenyl, dimethylaminophenyl, naphthyl, phenylnaphthene, phenanthrene, anthracene.

[0047] For the purpose of the present description and of the following claims, the term “heteroaryl groups” means heterocyclic aromatic, penta- or hexa-atomic groups, also benzocondensed or heterobicyclic, containing from 4 to 60 carbon atoms and from 1 to 4 heteroatoms selected from nitrogen, oxygen, sulfur, silicon, selenium, phosphorus. Said heteroaryl groups may optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12 thioalkoxy groups; C3-C24 tri-alkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12 mono- or di-alkylamine groups; nitro groups. Specific examples of heteroaryl groups are: pyridine, methylpyridine, methoxypyridine, phenylpyridine, fluoropyridine, pyrimidine, pyridazine, pyrazine, triazine, tetrazine, quinoline, quinoxaline, quinazoline, furan, thiophene, hexyl thiophene, bromothiophene, dibromothiophene, pyrrole, oxazole, thiazole, isothiazole, oxadiazole, tiadiazole, pyrazole, imidazole, triazole, tetrazole, indole, benzofuran, benzothiophene, benzooxazole, benzothiazole, benzooxadiazole, benzothiadiazole, benzopyrazole, benzimidazole, benzotriazole, triazolepyridine, triazolepyrimidine, coumarin.

[0048] The aforementioned compounds having general formula (I) and (III), may be prepared by processes known in the art as described, for example, in: Zheng Z. et al., “ChemCatChem" (2014), Vol. 6, Issue 6, pg. 1626-1634; Kumar A. et et al., “ Journal of Energy Chemistry' (2023), Vol. 78, pg. 222-231; Shi X. et al., “ Journal of Molecular Catalysis A: Chemicar (2011), Vol. 341, pg. 57-62; Manusha P. et al., “ Journal of Molecular Liquid" (2020), Vol. 301, pg. 112412; Chai J. et al., “ACS Applied Materials & Interfaces'" (2020), Vol. 12, Issue 13, pg. 1562-15270; Zhang Y. et al., “CCS Chemistry" (2023), Vol. 5, pg. 1917-1930. Further details concerning the preparation of the above compounds having general formula (I) and (II) may be found in the following examples.

[0049] The aforementioned compounds having general formula (II) are commercially available.

[0050] The aforementioned liquid electrolytes may comprise at least one supporting electrolyte. The supporting electrolyte is able to maintain a balance of charge between the electrolyte in the negative compartment and the electrolyte in the positive compartment without, however, participating in the reaction. Generally, the supporting electrolyte must be chemically inert in the potential range considered, must have a high ionic conductivity to ensure low resistance to current flow, and must not hinder electronic exchange on the electrode surface.

[0051] In accordance with an embodiment of the present invention, the aforementioned liquid electrolytes comprise at least one supporting electrolyte selected, for example, from potassium hexafluorophosphate (KPF&) tetrabutylammonium hexafluorophosphate (TBAPFc,), tetraethylammonium tetrafluoroborate (TEABF4), tetrabutylammonium tetrafluoroborate (TBABF4), or mixtures thereof. Potassium hexafluorophosphate (KPFg) is preferred.

[0052] In accordance with a preferred embodiment of the present invention, said organic solvent may be selected, for example, from acetonitrile, 3- methoxyproprionitrile, diethyl carbonate, y-butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), V-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate, YA -di methyl acetamide, or mixtures thereof. Acetonitrile, 3-methoxyproprionitrile, are preferred.

[0053] It should be noted that for the purpose of the present invention, it is preferable to use the same solvent in both the positive and negative compartments, so as to prevent possible diffusion problems through the ion exchange membrane with consequent contamination problems between the two compartments.

[0054] It should also be noted that the aforementioned compounds having general formula (I), (II) and (III) have good solubility in the organic solvent used, i.e. solubility comprised between 0.03 M and 2 M, preferably comprised between 0.04 M and 1.5 VI

[0055] In accordance with a preferred embodiment of the present invention, said ion exchange membrane may be selected from polymeric membranes such as, for example: ion exchange membranes such as, for example, membranes based on a styrene-divinylbenzene copolymer or on a chloromethylstyrene- divinylbenzene copolymer containing amino groups, membranes based on poly(ether ether ketones), membranes based on a divinylbenzene- vinylpyridine copolymer containing a quaternary pyridine group; membranes based on an aromatic polysulfonic copolymer containing a chloromethyl group and amino groups, membranes based on polytetrafluoethyl ene (PTFE); cation exchange membranes such as, for example, membranes based on a fluoropolymer-copolymer based on tetrafluoroethylene sulfonate, membranes based on poly (ether ether ketones), membranes based on polysulfones, membranes based on polyethylene, membranes based on polypropylene, membranes based on ethyl ene-propyl ene copolymers, membranes based on polyimides, membranes based on polyvinyl fluorides. Anion exchange membranes which may be advantageously used for the purpose of the present invention and which are commercially available are NEOSEPTA® AMX, NEOSEPTA® AHA, NEOSEPTA® ACS made by Astom, lonac MA3475 of Lanxess, Teflon® of DuPont, Fumasep FAA-3 and Fumasep “ FAP-330-PE by Fumatech.

[0056] Cation exchange membranes which may be advantageously used for the purpose of the present invention and which are commercially available are NEOSEPTA® CMX, NEOSEPTA® CIMS, made by Astom, Nafion® made by DuPont.

[0057] Preferably, the negative electrode may comprise at least one metal such as, for example, platinum, copper, aluminium, nickel, stainless steel; or at least one carbon-containing material such as, for example, carbon black, activated carbon, amorphous carbon, graphite, graphene, a carbon nanostructured material, or mixtures thereof. Said negative electrode may be porous, grooved or smooth.

[0058] Preferably, the positive electrode may comprise, at least one metal such as, for example, platinum, copper, aluminium, nickel, stainless steel; or at least one carbon-containing material such as, for example, carbon black, activated carbon, amorphous carbon, graphite, graphene, a carbon nanostructured material; or mixtures thereof. Said positive electrode may be porous, grooved or smooth.

[0059] The present invention will now be illustrated in greater detail through an embodiment with reference to Figure 1 reported below.

[0060] In particular, Figure 1 schematically represents an embodiment of a nonaqueous redox flow battery (RFB) in accordance with the present invention. In this regard, the non-aqueous redox flow battery (RFB) (1) comprises a positive compartment (6a) wherein a positive electrode (6) is positioned wherein a nonaqueous liquid positive electrolyte (not shown in Figure 1) is made to flow, a negative compartment (8a) wherein a negative electrode (8) is placed wherein a non-aqueous liquid negative electrolyte (not shown in Figure 1) is made to flow, an ion exchange membrane (7) positioned between the positive compartment (6a) and the negative compartment (8a).

[0061] The positive compartment (6a) is connected to a reservoir (2) containing the non-aqueous liquid positive electrolyte comprising a solution of at least one compound having general formula (I) in at least one organic solvent, by means of an inlet pipe (3) and a pump (4a) (for example, a peristaltic pump) and an outlet pipe (5) so as to allow feeding and discharging of said non-aqueous liquid positive electrolyte during the operating cycle (i.e. during the charge-discharge phase).

[0062] The negative compartment (8a) is connected to a reservoir (12) containing the non-aqueous liquid negative electrolyte comprising a solution of at least one compound having general formula (II) or of at least one compound having general formula (III) in at least one organic solvent, by means of an inlet pipe (11) and a pump (4b) (for example, a peristaltic pump) and an outlet pipe (10) so as to allow feeding and discharging of said non-aqueous liquid negative electrolyte during the operating cycle (i.e. during the charge-discharge phase).

[0063] A voltmeter (9) is connected to the positive electrode (6) and to the negative electrode (8).

[0064] During the charging phase of the non-aqueous redox flow battery' (RFB) (1), a potential difference is applied between the positive and negative electrodes by means of the voltmeter (9) while simultaneously the non-aqueous liquid positive electrolyte is supplied, via the pump (4a) from the positive electrolyte reservoir (2) to the positive compartment (6a) and the non-aqueous liquid negative electrolyte is supplied, via the pump (4b) from the negative electrolyte reservoir (12) to the negative compartment (8a). Said non-aqueous liquid positive electrolyte present in the positive compartment (6a) undergoes an oxidation reaction at the positive electrode (6) and said non-aqueous liquid negative electrolyte present in the negative compartment (8a) undergoes a reduction reaction at the negative electrode (8): through the ion exchange membrane (7) there is a flow of the ions involved in the aforementioned oxidation and reduction reactions in opposite directions in order to balance the charges. During the discharge phase of the non-aqueous redox flow battery’ (RFB) (1), reverse reactions take place. The aforementioned charging and discharging phases may be summarised as follows: negative electrode:

[0065]

[0066] wherein:

[0067] G, Ri, R2, R3, Rx and X' have the same meanings described above; e’::::electrons.

[0068] During the operating cycle (i.e. during the charge-discharge phase) both the non-aqueous liquid positive electrolyte and the non-aqueous liquid negative electrolyte are continuously pumped into the positive and negative compartments, respectively, in order to continuously supply said positive and negative compartments.

[0069] The energy stored in the non-aqueous (1) redox flow battery (RFB) may be directly used for the operation of the apparatus wherein it is inserted, or it may be transferred to an electrical network during periods of peak use to supplement the power supply. An alternating current / direct current (AC / DC) converter (not shown in Figure I) may possibly be used to facilitate the transfer of energy to and from an alternating current (AC) supply network.

[0070] As mentioned above, the present invention also relates to a compound comprising a triphenyl alkyl and 2,2,6,6-tetraalkylpiperidinyloxy groups (TEMPO) having the specific general formula (I) provided below, and to a compound comprising at least one dipyridyl group and at least one imidazole group having the specific general formula (III) provided below. Accordingly, it is a further object of the present invention a compound having general formula (I): wherein G, Ri, R2 and R?, have the same meanings reported above. Further subject matter of the present invention is also a compound having general formula (III): wherein G, Rs and X", have the same meanings reported above.

[0071] The present invention will be further illustrated below by means of the following examples, which are provided for indicative purposes only and without limitation of this invention.

[0072] EXAMPLE 1

[0073] Synthesis of 4.4\4”-((((ethane- I J -triyltris benzene-4J -diyl ))tris oxyl ))tris-

[0074] (butane-4. l -diyl ))-tris(oxyl ))tris(2.2.6.6-tetramethylpiperidin- l -oxyl ) having formula (la)

[0075] (1) Synthesis of 4-(2.2 6-tetramethylpiperidin- l -oxyl )butylbromide having formula (A) In a 250 ml flask, equipped with magnetic stirring, in an inert atmosphere, 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl (4-OH-TEMPO) (Merck) (1.869 g; 10.85 mmol) was added to a mixture of 1,4-dibromobutane (Merck) (7.028 g; 32.55 mmol), tetrabutyl ammonium hydrogen sulphate (TBAHSO4) (Merck) (0.150 g; 0.44 mmol) and sodium hydroxide (NaOH) (Merck) (5.425 g; 135.63 mmol) in distilled water (50 ml): the reaction mixture obtained was stirred, at room temperature (25°C), for 24 hours. Subsequently, the reaction mixture was placed in a 500 ml separating funnel: an aqueous solution of sodium chloride (NaCl) (Merck) (3 x 100 ml) was added to said reaction mixture and the whole was extracted with ethyl acetate (Merck) (3 x 100 ml) obtaining an aqueous phase and an organic phase. The entire organic phase (obtained by combining the organic phases deriving from the three extractions) was separated and subsequently anhydrified over anhydrous sodium sulphate (Merck) and evaporated in a Rotovapor. The residue obtained was purified by elution on a silica gel chromatographic column [(eluent: heptane / ethyl acetate 8 / 2) (Merck)], obtaining 1.8 g of 4-(2,2,6,6-tetramethylpiperidin-l-oxyl)butylbromide having formula (A) as red oil (yield 54%).

[0076] (2) _ Synthesis _ of _ 4,4’ ,4” -((((ethane- 1,1,1 -tri yltri s(b enzene-4, 1 - diyl))tris(oxyl))tris-(butane-4,l-diyl))-tris(oxyl))tris(2,2,6,6.- tetramethylpiperidin-l-oxyl) having formula (la)

[0077] In a 250 ml flask, equipped with a magnetic stirrer, thermometer and coolant, in an inert atmosphere, 4-(2,2,6,6-tetramethylpiperidin-l- oxyl)butylbromide obtained as described above was added to a mixture of 1,1,1 - tris(4-hydroxyphenyl)ethane (Merck) (1.997 g; 6.52 mmol) and potassium carbonate (K2CO3) (Merck) (5.390 g; 39.00 mmol) in acetonitrile (ACN) (Merck) (50 ml): the reaction mixture obtained was heated to 90°C and stirred, at said temperature, for 24 hours. Subsequently, after cooling to room temperature (25°C), the reaction mixture was placed in a 500 ml separating funnel: a 1 M aqueous solution of sodium hydroxide (NaOH) (Merck) (3 x 100 ml) was added to said reaction mixture and the whole was extracted with ethyl acetate (Merck) (3 x 100 ml) obtaining an aqueous phase and an organic phase. The entire organic phase (obtained by combining the organic phases deriving from the three extractions) was separated and subsequently anhydrified on sodium sulphate (Aldrich) and evaporated in a Rotovapor. The residue obtained was purified by elution on a silica gel chromatographic column [(eluent: heptane / ethyl acetate 1 / 1) (Merck)], obtaining 5.139 g of 4,4’,4”-((((ethane-l,l,l-triyltris(benzene-4,l- diyl))tris(oxyl))tris(butane-4,l-diyl))tris-(oxyl))tris(2,2,6,6-tetramethylpiperidin- 1-oxyl) having formula (la) as a waxy red solid (yield 80%).

[0078] EXAMPLE 2

[0079] Synthesis of l,3,5-tris(l-[4-(l-methyl-lJ / -imidazol-3-ium-3-yl)butyl]-4-(pyridin-

[0080] 4-yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa)

[0081]

[0082] (1) Synthesis of 3-(4-bromobutyl)-l-methyl-17 -imidazol-3-ium bromide having formula (B) In a 250 ml flask, equipped with a magnetic stirrer, thermometer and coolant, in an inert atmosphere, 1 -methylimidazole (Merck) (0.891 g; 10.85 mmol) was added, to a solution of 1,4-dibromobutane (Merck) (7.028 g; 32.55 mmol), in acetonitrile (ACN) (Merck) (50 ml): the reaction mixture obtained was heated to 75°C and stirred, at said temperature, for 24 hours. Subsequently, after cooling to room temperature (25°C), the solvent was evaporated in a Rotavapor; the residue obtained was washed with ethylacetate (Merck) (10 ml) and subsequently dried in a vacuum oven at 50°C, for 18 hours, obtaining 2.59 g of 3- (4-bromobutyl)-l-methyl-l / 7-imidazol-3-ium bromide having formula (B) as a yellowish oil (yield 80%). (2) Synthesis of l-14-(l-methyl-lH-imidazol-3-ium-3-yl)butyl1-4-(Dyridin-4- yl)pyridin-l-ium dibromide having formula (C)

[0083] In a 250 ml flask, equipped with a magnetic stirrer, thermometer and coolant, in an inert atmosphere, the compound 3 -(4-bromobutyl)-l -methyl- 1H- imidazol-3-ium bromide having formula (B) (1.20 g; 3.30 mmoles) obtained as described above was added to a solution of 4,4’ -bipyridine (Merck) (2.65 g 13.22 mmoles) in acetonitrile (ACN) (Merck) (50 ml): the reaction mixture obtained was heated to 75°C and stirred, at said temperature, for 24 hours. Subsequently, after cooling to room temperature (25°C), the precipitate was collected, washed with ethyl acetate (Merck) (10 ml) and subsequently dried in a vacuum oven, at 24°C, for 24 hours, obtaining 1.37 g of l-[4-(l-methyl-17 / -imidazol-3-ium-3-yl)butyl]- 4-(pyridin-4-yl)pyridin-l-ium dibromide having formula (C) as a yellow solid (80% yield).

[0084] (3) Synthesis of k3,5-tris(l-14-(l-methyl-lJ -imidazol-3-ium-3-yl)butyl1-4- (pyridin-4-yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa)

[0085] In a 250 ml flask, equipped with a magnetic stirrer, thermometer and coolant, in an inert atmosphere, the compound l-[4-(l-methyl-lH-imidazol-3- ium-3-yl)butyl]-4-(pyridin-4-yl)pyridin-l-ium bromide having formula (C) obtained as described above (1.20 g; 2.31 mmoles) was added to a solution of l,3,5-tris(bromomethyl)benzene (Merck) (0.25g; 0.70 mmoles) in N,N- dimethylformamide (DMF) (Merck) (50 ml): the reaction mixture obtained was heated to reflux and stirred, at said temperature, for 4 hours. Subsequently, after cooling to room temperature (25°C), the precipitate was collected, washed with ethyl acetate (Merck) (10 ml) and subsequently dried in a vacuum oven, at 24°C, for 4 hours. The dried precipitate obtained was placed in a 250 ml flask equipped with a magnetic stirrer, in an inert atmosphere, and dissolved in 60 ml of distilled water: potassium hexafluorophosphate (KPFe) (Merck) (2.32 g; 12.60 mmol) was added to the mixture obtained, and the whole was heated to 60°C and stirred, at said temperature, for 18 hours. The precipitate obtained was collected, washed with distilled water (10 ml) and then dried in a vacuum oven, at 60°C, for 18 hours, obtaining 1.05 g of l,3,5-tris(l-[4-(l-methyl-17 / -imidazol-3-ium-3-yl)butyl]-4- (pyridin-4-yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa) as a white solid (65% yield).

[0086] EXAMPLE 3

[0087] Cyclic voltammetry measurements

[0088] Cyclic voltammetry measurements were performed in a hemi-cell with a three-electrode configuration, with glassy carbon working electrode, platinum counter electrode and silver / silver chloride (Ag / AgCl) reference electrode. The oxidation-reduction potentials E°'ox / Red were derived from the position of the forward peak (Epr) and the return peak (Epr): and the values were normalised with respect to the intersolvent ferrocene / ferrocenium (Fc / Fc+) pair.

[0089] Evaluations were performed on an Autolab PGSTAT 128N analytical instrument at scan rates of 10, 20, 50, 70, 100, and 200 mV / s. All evaluations were carried out in triplicate at room temperature (25°C). For the purpose, use was made of solutions containing: ethyl viologen PF6 having formula (Ila) (Merck)

[0090] (5 x 10'4M) and potassium hexafluorophosphate (KPFg) (Merck) (0.1 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte of the negative compartment) (EV);

[0091] 4,4’,4”-((((ethane-l,l,l-triyltris(benzene-4,l-diyl))tris(oxyl))tris(butane- 4,l-diyl))tris-(oxyl))tris(2,2,6,6.-tetramethylpiperidin-l-oxyl) having formula (la) (5 x 10'4M) obtained in Example 1 and potassium hexafluorophosphate (KPFeO (Merck) (0.1 M) in acetonitrile (Merck) (nonaqueous liquid positive electrolyte of the positive compartment) (la); 1, 3, 5-tris(l-[4-(l -methyl- lH-imidazol-3-ium-3-yl)butyl]-4-(pyridin-4- yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa) (5 x 10'4M) obtained in Example 2 and potassium hexafluorophosphate (KPFe) (Merck) (0.1 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte of the negative compartment) (Illa).

[0092] Figure 2 [the abscissa shows the potential (E) measured in volts (V) and the ordinate shows the current density (i) measured in amperes (A)] shows the cyclic voltagram obtained from the above solutions (la) e (Ila)) in acetonitrile, at a scan rate of 200 mV / s. It may be seen that a high open-circuit potential difference (E°) of 1.55 V is obtained calculated according to the following formula:

[0093] E° = (E°i) - (E°2) wherein:

[0094] (E°i) is the oxidation reduction potential for the (EV) relative to the second peak calculated as described above and is equal to -1.24 vs (Fc / Fc+);

[0095] (E°2) is the oxidation-reduction potential for (la) calculated as described above and is equal to 0.31 V vs (Fc / Fc+).

[0096] Figure 3 [the abscissa shows the potential (E) measured in volts (V) and the ordinate shows the current density (i) measured in amperes (A)] shows the cyclic voltagram obtained from the above solutions (la) e (Illa) in acetonitrile, at a scan rate of 200 mV / s. It may be seen that a high open-circuit potential difference (E°) of 1.55 V is obtained calculated according to the following formula:

[0097] E° = (E°2) - (E°i) wherein:

[0098] (E°i) is the oxidation-reduction potential for (Illa) relative to the second peak calculated as described above and is equal to -1.24 V vs (Fc / Fc+);

[0099] (E°2) is the oxidation-reduction potential for (la) calculated as described above and is equal to 0.31 V vs (Fc / Fc+).

[0100] EXAMPLE 4

[0101] Non-aqueous redox flow battery (RFB) charge / discharge tests [electrolytes: compound having formula (la) and compound having formula (Ila) in acetonitrilel The charge-discharge tests were earned out using a graphite electrochemical cell with a Fumasep® FAP-330-PE (Fumatech) membrane, having a surface area of approximately 4 cm , placed between two butyl rubber seals, between two electrodes consisting of carbon felts (SGL Carbon) having a surface area of approximately 4 cm2. The electrochemical cell was then assembled and closed with screws. It was subsequently placed inside a nitrogen inert container (“glovebox”).

[0102] The cell is connected to two glass tanks containing the liquid positive electrolyte and liquid negative electrolyte described below via technoprene inlet pipes and a two-head peristaltic pump (Watson Marlow SCI-Q 323E / D).

[0103] The following electrolytes were used for this purpose:

[0104] 4,4’,4”-((((ethane-l,l,l-triyltris(benzene-4,l-diyl))tris(oxyl))tris(butane- 4,l-diyl))tris-(oxyl))tris(2,2,6,6.-tetramethylpiperidin-l-oxyl) having formula (la) obtained in Example 1 (0.05 M) and potassium hexafluorophosphate (KPFe) (TBABF4) (Merck) (0.5 M) in acetonitrile (Merck) (non-aqueous liquid positive electrolyte of the positive compartment) (la); ethyl viologen PF6 having formula (Ila) (Merck) (0.05 M) and potassium hexafluorophosphate (KPFs) (Merck) (0.5 M) in acetonitrile (Merck) (nonaqueous liquid negative electrolyte of the negative compartment) (EV);

[0105] 25 ml of the above solutions were introduced into the respective compartments.

[0106] Charge and discharge tests were performed using a potentiostat manufactured by BioLogic, SP150e, interfaced with the dedicated EC -lab software, developed by Bio-Logic, which also allows real-time monitoring of the operating variables and data export. The tests were carried out under the following conditions: charging phase: the battery was charged in galvanostatic mode with a current of 30 mA up to the end-of-charge voltage of 1.6 V, followed by a potentiostatic charge at 2 V up to 5 mA; discharge phase: the battery was galvanostatically discharged with a current of 30 mA to the end-of-discharge voltage of 0.5 V. Figure 4 [the abscissa shows the time (Time) measured in seconds (s); the ordinate shows the cell potential (E) measured in volts (V)] shows the charge / discharge curve obtained. In particular, the graph of the first 30 charge / discharge cycles is depicted, from which a good cyclability of the molecules involved during the various cycles may be seen.

[0107] In Figure 5 [the abscissa shows the number of cycles; the ordinate shows the Coulombic efficiency (Efficiency) measured as the ratio of discharged capacity to charged capacity and the capacity retention expressed as the ratio of discharged capacity at ith cycle to initial capacity]. It may be seen that the Coulombic efficiency for all cycles is above 0.98 except for the first cycle where it is around 0.95. In addition, Figure 5 shows a retention of initial capacity greater than 85%, evidence of good stability of the active species.

[0108] EXAMPLE 5 (invention)

[0109] Non-aqueous redox flow battery (RFB) charge / discharge tests [electrolytes: compound (la) and compound (Illa) in acetonitrile]

[0110] The charge-discharge tests were performed using a graphite electrochemical cell with a Fumasep® FAP-330-PE membrane (Fumatech), having a surface area of approximately 4 cm2, placed between two butyl rubber seals, between two electrodes consisting of carbon felts (SGL Carbon) having a surface area of approximately 4 cm2. The electrochemical cell was then assembled and closed with screws. It was subsequently placed inside a nitrogen inert container (“glovebox”).

[0111] The cell is connected to two glass tanks containing the positive and negative electrolytes described below via technoprene inlet pipes and a two-head peristaltic pump (Watson Marlow SCI-Q 323E / D).

[0112] The following electrolytes were used for this purpose:

[0113] 4,4’,4”-((((ethane-l,l,l-triyltris(benzene-4,l-diyl))tris(oxyl))tris(butane- 4,l-diyl))tris-(oxyl))tris(2,2,6,6.-tetramethylpiperidin-l-oxyl) having formula (la) obtained in Example 1 (0.05 M) and potassium hexafluorophosphate (KPFe) (TBABF4) (Merck) (0.5 M) in acetonitrile (Merck) (non-aqueous liquid positive electrolyte of the positive compartment) (la);

[0114] 1, 3, 5-tris(l-[4-(l -methyl- lH-imidazol-3-ium-3-yl)butyl]-4-(pyridin-4- yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa) (0.05 M) and potassium hexafluorophosphate (KPFe) (Merck) (0.5 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte of the negative compartment) (Illa);

[0115] 25 ml of the above solutions were introduced into the respective compartments.

[0116] Charge and discharge tests were carried out using a potentiostat manufactured by BioLogic, SP150e, interfaced with the dedicated EC -lab software, developed by Bio-Logic, which allows real-time monitoring of operating variables such as current and voltage and data export. The tests were carried out under the following conditions: charging phase: the battery was charged in galvanostatic mode with a current of 30 mA up to the end-of-charge voltage of 2.3 V, followed by a potentiostatic charge at 2.3 V up to a current of 5 mA; discharge phase: the battery was galvanostatically discharged with a current of 30 mA to the end-of-discharge voltage of 0.5 V.

[0117] Figure 6 [the abscissa shows the time (Time) measured in seconds (s); the ordinate shows the cell potential (E) measured in volts (V)] shows the charge / discharge curve obtained. In particular, the graph of the first 30 charge / discharge cycles is depicted, from which a good cyclability of the molecules involved during the various cycles may be seen.

[0118] In Figure 7 [the abscissa shows the number of cycles; the ordinate shows the measured Coulombic efficiency (Efficiency) as the ratio of discharged capacity to charged capacity and the capacity retention expressed as the ratio of discharged capacity at ith cycle to initial capacity]. It shows an almost uniform Coulombic efficiency except for the first cycle where it is around 0.9. In addition, Figure 7 shows a retention of initial capacity of over 85%, evidence of increased stability of the active species.

[0119] EXAMPLE 6 Energy density calculation

[0120] The energy density (e) of a non-aqueous redox flow battery (RFB) is defined as the chemical energy contained in both compartments (i.e. positive and negative compartment) of the charged battery per unit volume.

[0121] For each individual compartment (i.e. positive and negative compartment) of the non-aqueous redox flow battery (RFB), the specific capacity ( / ?c) [expressed in ampere hours / litre (Ah / 1)] of the solution may be defined according to the following equation: pc= 26,8 • cone ■ n wherein cone is the concentration of the active species and n is the number of electrons involved in the reaction.

[0122] The energy density (e) [expressed in watt-hours / litre (Wh / 1)] of the nonaqueous redox flow battery (RFB) is defined according to the following equation: wherein: min is the minimum value between the two products in the numerator; pc+ is the specific capacity measured at the positive pole [expressed in ampere hours / litre (Ah / 1)];

[0123] V+ is the volume of the solution of the positive non-aqueous liquid electrolyte [expressed in litres (1)]; pc- is the specific capacity measured at the negative pole [expressed in ampere hours / litre (Ah / 1)];

[0124] V- is the volume of the solution of the non-aqueous liquid negative electrolyte [expressed in litres (1)];

[0125] Eo is the thermodynamic reaction potential in discharge [expressed in volts (V)].

[0126] In order to have a balanced non-aqueous redox flow battery (RFB) with equal charge at both the negative and positive poles, the following equation must be satisfied:

[0127] Pc+-V+= pc_ -V_ whereinc+, V+, pc. and V., have the same meanings described above.

[0128] In order to achieve high energy densities, it is therefore important to maximise the following parameters: the concentration in solution of the reacting species; the number of electrons transferred in the positive and negative compartments; the electrochemical cell potential.

[0129] In the case of a non-aqueous redox flow battery (RFB) containing the following solutions:

[0130] 4,4’,4”-((((ethane-l,l,l-triyltris(benzene-4,l-diyl))tris(oxyl))tris(butane- 4,l-diyl))tris-(oxyl))tris(2,2,6,6.-tetramethylpiperidin-l-oxyl) having formula (la) (5 x 10'2M) and potassium hexafluorophosphate (KPFe) (Merck) (0.5 M) in acetonitrile (Merck) (non-aqueous liquid positive electrolyte of the positive compartment) degassed with argon (Ar);

[0131] 1, 3, 5-tris(l-[4-(l -methyl- lH-imidazol-3-ium-3-yl)butyl]-4-(pyridin-4- yl)pyridin-l-ium nonahexafluorophosphate)benzene having formula (Illa) (5 x 10‘2M) and potassium hexafluorophosphate (KPF6) (Merck) (0.5 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte of the negative compartment) degassed with argon (Ar); the theoretical energy density (e) is 185 Wh / 1, said theoretical energy density (e) having been calculated considering that: the process is tri-electronic; the maximum concentration of the species is 1.0 M; the open-circuit potential difference value (E°) is 2.3 V.

Claims

CLAIMS1. Non-aqueous redox flow batery (RFB) comprising: a positive compartment wherein a positive electrode is placed and wherein a non-aqueous liquid positive electrolyte is made to flow; a negative compartment wherein a negative electrode is placed and wherein a non-aqueous liquid negative electrolyte is made to flow; an ion exchange membrane placed between the positive compartment and the negative compartment; wherein: said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having a general formula (I):wherein:G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4;Ri, R2 and R3, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated;in at least an organic solvent; said non-aqueous liquid negative electrolyte comprises a solution of at least one compound having a general formula (II) or (III):wherein:Re and R7, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated;Rs represents a hydrogen atom; or a C1-C20 alkyl group, preferably Ci- Ce, linear or branched, saturated or unsaturated, preferably saturated, an optionally substituted cycloalkyl group, an optionally substituted aryl group; an optionally substituted heteroaryl group;G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from eachother, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer comprised between 1 and 4;X" represents a tetrafluoroborate anion (BF4‘), a hexafluorophosphate anion (PFe'), a bistrifluoromethanesulfonylimidate anion [(CF3SO2)N']; in at least an organic solvent.

2. Non-aqueous redox flow battery (RFB) according to claim 1, wherein in said general formula (I):G represents a C1-C20 alkylene group, preferably butylene;Ri, R2 and R equal to each other, represent a C1-C20 alkyl group, preferably a methyl.

3. Non-aqueous redox flow battery (RFB) according to claim 1 or 2, wherein in said general formula (II):Re represents a C1-C20 alkyl group, preferably an ethyl;X" represents a hexafluorophosphate anion (PFe").

4. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, wherein in said general formula (III):Rs represents a C1-C20 alkyl group, preferably a methyl;G represents a C1-C20 alkylene group, preferably butylene;X" represents a hexafluorophosphate anion (PFe").

5. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, wherein the aforementioned liquid electrolytes comprise at least one support electrolyte selected from potassium hexafluorophosphate (KPFe), tetrabutylamm onium hexafluorophosphate (TB APFe), tetraethylamm onium tetrafluoroborate (TEABF4), tetrabutylammonium tetrafluoroborate (TBABF4), or mixtures thereof; preferably potassium hexafluorophosphate (KPFe).

6. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, wherein said organic solvent is selected from acetonitrile, 3- methoxyproprionitrile, diethyl carbonate, dimethyl carbonate, y-butyrolactone(GBL), propylene carbonate (PC), ethylene carbonate (EC), 7V-methyl-2- pyrrolidone (NMP), fluoroethylene carbonate, 7V,7V-dimethylacetamide, or mixtures thereof; preferably from acetonitrile, 3-methoxyproprionitrile.

7. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, wherein said ion exchange membrane is selected from polymeric membranes such as: anion exchange membranes such as membranes based on a styrene- divinylbenzene copolymer or on a chloromethylstyrene-divinylbenzene copolymer containing amino groups, membranes based on poly(ether ether ketones), membranes based on a divinylbenzene-vinylpyridine copolymer containing a quaternary pyridine group; membranes based on an aromatic polysulfonic copolymer containing a chloromethyl group and amine groups, membranes based on polytetrafluoethylene (PTFE); cation exchange membranes such as membranes based on a fluoropolymercopolymer based on tetrafluoroethylene sulfonate, membranes based on poly(ether ether ketones), membranes based on polysulfones, membranes based on polyethylene, membranes based on polypropylene, membranes based on ethyl ene-propylene copolymers, membranes based on polyimides, membranes based on polyvinyl fluorides.

8. Compound having general formula (I):wherein:G represents a C1-C20 alkylene group, preferably C1-C6, linear orbranched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2-CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer between 1 and 4;Ri, R2 and R3, equal to or different from each other, represent a C1-C20 alkyl group, preferably C1-C6 linear or branched, saturated or unsaturated, preferably saturated.

9. Compound having general formula (III):wherein:Rs represents a hydrogen atom; or a C1-C20 alkyl group, preferably Ci- Ce, linear or branched, saturated or unsaturated, preferably saturated, an optionally substituted cycloalkyl group, an optionally substituted aryl group; an optionally substituted heteroaryl group;G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or a -R4-O- R5- ether group wherein R4 and R5, equal to or different from eachother, represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a -[CH2- CH2-O]n-CH2- polyethyleneoxy group wherein n is an integer between 1 and 4; - X" represents a tetrafluoroborate anion (BF4‘), a hexafluorophosphate anion (PFe'), a bistrifluoromethanesulfonylimidate anion [(CF3SO2)N-].

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