WO2026028172A1 - Non-aqueous redox flow batteries - Google Patents

Abstract

Non-aqueous redox flow battery (RFB) comprising: - a positive compartment in which a positive electrode is placed and in which a positive non-aqueous liquid electrolyte is made to flow; - a negative compartment in which a negative electrode is placed and in which a non-aqueous liquid negative electrolyte is made to flow; - an ion exchange membrane placed between the positive compartment and the negative compartment; in which: - said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having general formula (I) or (II): (I) (II) in which: - R, equal or different from each other, preferably equal to each other, are selected from C₁-C₂₀ alkyl groups, preferably C₂-C₁₀, linear or branched, optionally substituted cycloalkyl groups, optionally 31 substituted aryl groups, optionally substituted heteroaryl groups, polyethyleneoxy groups R₁-O-[CH₂-CH₂-O]_m- in which R₁ represents a hydrogen atom, or is selected from C₁-C₂₀ alkyl groups, preferably C₂-C₁₀, linear or branched, and m is an integer comprised between 1 and 4; - n is an integer comprised between 1 and 10, preferably comprised between 2 and 4; - G represents a C₁-C₂₀ alkylene group, preferably C₁-C₆, linear or branched, saturated or unsaturated, preferably saturated, or an ether group -R₂-O-R₃- in which R₂ and R₃, equal or different from each other represent a C₁-C₂₀ alkyl group, preferably C₁-C₆, linear or branched, saturated or unsaturated, preferably saturated; or a polyethyleneoxy group -[CH₂-CH₂-O]_p-CH₂- in which p is an integer comprised between 1 and 4; in at least one organic solvent; - said non-aqueous liquid negative electrolyte comprises a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent. Said non-aqueous redox flow battery (RFB) can be advantageously used in devices that require medium to high power output (e.g., about 100 kW - 100 20 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

NON-AQUEOUS REDOX FLOW BATTERIES *** *** *** The present invention relates to non-aqueous redox flow batteries (RFB). More particularly, the present invention relates to a non-aqueous redox flow battery (RFB) comprising: a positive compartment in which a positive electrode is placed and in which a non-aqueous liquid positive electrolyte is made to flow; a negative compartment in which a negative electrode is placed and in which a non-aqueous liquid negative electrolyte is made to flow; an ion exchange membrane placed between the positive compartment and the negative compartment; in which: said non-aqueous liquid positive electrolyte comprises a solution of at least one compound comprising 2,5-di-tert-butylbenzene groups substituted with an alkoxyl group having specific general formula (I) or (II) provided below in at least one organic solvent; said non-aqueous liquid negative electrolyte comprises a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent. Said non-aqueous redox flow battery (RFB) can 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 from alternative energy sources (such as solar or wind power) for subsequent use (for example, for domestic use) or for sale. The present invention also relates to a compound comprising 2,5-di-tert- butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) provided below. Redox flow batteries (RFBs) are becoming an increasingly promising technology in energy storage due to their low environmental impact and safe operation. Redox flow batteries (RFBs) are a type of rechargeable batteries in which 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 half- cell), 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. 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 half-cell) and the reaction potential. The first generation of aqueous redox flow batteries (RFBs) are represented by vanadium redox flow batteries, 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/l and 30 Wh/l. 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). More details related to said “VRFBs” can 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. Over the years, other types of aqueous redox flow batteries (RFBs) have been studied. 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 Br2/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. In a later study, Lin K. et al., in “Science” (2015), Vol.349, Issue 6255, pg. 1529-1532, the redox pair Br2/Br-, is replaced by the redox pair ferrocyanate/ ferricyanate 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. 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. at water electrolysis), further studies have been made concerning the use of electrolytes comprising metal-organic electroactive species soluble in organic solvents. For example, Chakrabarti M. H. et al., in “Electrochimica Acta” (2007), Vol. 52, pg. 2189-2195, describe electrolytes comprising metal-organic species in acetonitrile: in particular, an electrolyte comprising the redox couple ruthenium acetylacetonate [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 batteries (RFBs) obtained are said to have high efficiency. Kaur A. P. et al., in “Energy Technology” (2015), Vol. 3, pg. 476-480, describe a non-aqueous redox flow battery (RFB) in which the electrolyte in the positive compartment (catholyte) comprises a phenothiazine derivative, specifically 3,7-bis(trifluoromethyl)-N-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. 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-1-piperinyloxy/NaClO4/acetonitrile as electrolyte in the positive (catholyte) compartment and N-methyl-phthalimide/NaClO4/acetonitrile as electrolyte in the negative (anolyte) compartment. The aforementioned non- aqueous 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. Gong K. et al., in “Energy & Environmental Science” (2015), Vol. 8, pg. 3515-3530, describe various types of non-aqueous redox flow batteries (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 (LiPF6) solution in dimethylformamide (DMF) in the negative compartment and an electrolyte comprising octafluoronaphthalene and a 1 M lithium hexafluorophosphate (LiPF₆) solution in polycarbonate (PC) in the positive compartment. 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). 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 (for example, 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 can comprise a quinoxaline (for example, a quinoxaline bearing at least one electron donor substituent), a dipyridyl ketone, a viologen (for example, a bis-benzyl viologen salt, a bis-ethyl viologen salt, a bis-methyl viologen salt, and the like), a benzophenone, an anthraquinone [for example, 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). 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. The Applicant therefore set out to solve the problem of finding a non- aqueous redox flow battery (RFB) capable of good performance, i.e. having both a good open circuit potential difference (E°) and good energy density (^_e). The Applicant has now found that the use of a non-aqueous liquid electrolyte comprising at least one compound comprising 2,5-di-tert- butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) 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 benzothiadiazole or derivative thereof 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. a good open circuit potential difference (E°). In addition, the compound comprising 2,5-di-tert- butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) provided below shows good stability during charge- discharge cycles of the non-aqueous redox flow battery (RFB) and high solubility in the organic solvent used. In addition, the compound comprising 2,5-di-tert- butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) provided below is non-toxic and, therefore, not harmful either from an environmental or operator health perspective. Finally, the compound comprising 2,5-di-tert-butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) provided below can be synthesised by means of simple procedures and using starting materials that are readily available commercially and thus cost-effective. It is therefore an object of the present invention to provide a non-aqueous redox flow battery (RFB) comprising: - a positive compartment in which a positive electrode is placed and in which a positive non-aqueous liquid electrolyte is made to flow; - a negative compartment in which a negative electrode is placed and in which a non-aqueous liquid negative electrolyte is made to flow; - an ion exchange membrane placed between the positive compartment and the negative compartment; in which: - said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having general formula (I) or (II): in which: - R, equal or different from each other, preferably equal to each other, are selected from C1-C20 alkyl groups, preferably C2-C10, linear or branched, optionally substituted cycloalkyl groups, optionally substituted aryl groups, optionally substituted heteroaryl groups, polyethyleneoxy groups R₁-O-[CH₂-CH₂-O]_m- in which R₁ represents a hydrogen atom, or is selected from C1-C20 alkyl groups, preferably C2-C10, linear or branched, and m is an integer comprised between 1 and 4; - n is an integer comprised between 1 and 10, preferably comprised between 2 and 4; - G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or an ether group -R2-O-R3- in which R2 and R3, equal or different from each other represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a polyethyleneoxy group -[CH₂-CH₂-O]_p-CH₂- in which p is an integer comprised between 1 and 4; in at least one organic solvent; - said non-aqueous liquid negative electrolyte comprises a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent. For the purpose of the present description and the following claims, the definitions of the numerical ranges always comprise the extreme values unless otherwise specified. For the purpose of the present description and of the following claims, the term “comprises” also includes the terms “essentially consists of” or “consists of”. In accordance with a preferred embodiment of the present invention, in said general formula (I): - R, equal to each other, represent a C1-C20 alkyl group, preferably a methyl; - n is 4. In accordance with a preferred embodiment of the present invention, in said general formula (II): - R, equal to each other, represent a C1-C20 alkyl group, preferably a methyl; - G represents a polyethyleneoxy group -[CH₂-CH₂-O]_m- in which m is 3. In accordance with a preferred embodiment of the present invention, said benzothiadiazole or a derivative thereof can be selected, for example, from benzothiadiazoles having general formula (III): in which R4, R5, R6 and R7, equal or different from each other, represent a hydrogen atom, or a halogen atom such as, for example, chlorine, fluorine, bromine, iodine, preferably fluorine; or represent a -CN group; or are selected from: C₁-C₁₀ alkyl groups, preferably C₁-C₄, linear or branched, saturated or unsaturated, C1-C10 alkoxyl groups, preferably C1-C4, linear or branched, saturated or unsaturated, carboxylic acid esters having general formula R8-COO- R₉ in which R₈ and R₉, equal or different from each other, are selected from C₁- C₁₀ alkyl groups, preferably C₁-C₄, linear or branched, saturated or unsaturated, sulphonic acid esters having general formula R8-OSO2-R9 in which R8 and R9 have the same meanings as described above, thioesters having general formula R₈-SO-R₉ in which R₈ and R₉ have the same meanings as described above, -(O- CH₂-CH₂)_q-OH groups in which q is an integer comprised between 1 and 4, -(O- CH(CH3)-CH2)z-OH in which z is an integer comprised between 1 and 4, optionally substituted aryl groups, optionally substituted heteroaryl groups. In accordance with a preferred embodiment of the present invention, in said general formula (III): - R4, R5, R6 and R7, equal to each other, represent a hydrogen atom. For the purpose of the present description and the following claims, the term “C₁-C₂₀ alkyl groups” and the term “C₁-C₁₀ alkyl groups” indicate linear or branched alkyl groups, saturated or unsaturated, having 1 to 20 carbon atoms and 1 to 10 carbon atoms, respectively. Specific examples of C1-C20 and C1-C10 alkyl groups are: methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n- pentyl, n-hexyl, ethyl-hexyl, n-heptyl, n-octyl, nonyl, n-decyl, n-dodecyl. For the purpose of the present description and the following claims, the term “C1-C10 alkoxyl groups” indicates groups comprising an oxygen atom to which a linear or branched, saturated or unsaturated C1-C10 alkyl group is bonded. Specific examples of C1-C10 alkoxyl groups are: methoxyl, ethoxyl, n- propoxyl, iso-propoxyl, n-butoxyl, iso-butoxyl, t-butoxyl, pentoxyl, hexyloxyl, 2- ethylhexyloxyl, heptiloxyl, octyloxyl, nonyloxyl, decyloxyl. For the purpose of the present description and the following claims, the term “cycloalkyl groups” indicates cycloalkyl groups having from 3 to 30 carbon atoms. Said cycloalkyl groups can optionally be substituted with one or more groups, equal 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 alkoxyl groups; C₁-C₁₂ thioalkoxyl groups; C₃-C₂₄ tri-alkylsilyl groups; polyethyleneoxy groups; cyan 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, methoxycyclohexyl, fluorocyclohexyl, phenylcyclohexyl, decalin, abietyl. For the purpose of the present description and following claims, the term “aryl groups” indicates aromatic carbocyclic groups containing from 6 to 60 carbon atoms. Said aryl groups can optionally be substituted with one or more groups, equal 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 alkoxyl groups; C₁-C₁₂ thioalkoxyl groups; C₃-C₂₄ tri-alkylsilyl groups; polyethyleneoxy groups; cyan groups; amino groups; C₁-C₁₂ mono- or di-alkylamine groups; nitro groups. Specific examples of aryl groups are: phenyl, methylphenyl, trimethylphenyl, methoxyphenyl, hydroxyphenyl, phenyloxyphenyl, fluorophenyl, pentafluorophenyl, chlorophenyl, bromophenyl, nitrophenyl, dimethylaminophenyl, naphthyl, phenylnaphthene, phenanthenene, anthracene. For the purpose of the present description and following claims, the term “heteroaryl groups” indicates aromatic heterocyclic penta- or hexa-atomic groups, also benzocondensates 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 can optionally be substituted with one or more groups, equal 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 alkoxyl groups; C1-C12 thioalkoxyl groups; C3-C24 tri-alkylsilyl groups; polyethyleneoxy groups; cyan 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, hexylthiophene, bromothiophene, dibromothiophene, pyrrole, oxazole, thiazole, iso isooxazole, isothiazole, oxadiazole, tiadiazole, pyrazole, imidazole, triazole, tetrazole, indole, benzofuran, benzothiophene, benzooxazole, benzothiazole, benzooxadiazole, benzothiadiazole, benzopyrazole, benzimidazole, benzotriazole, triazolopyridine, coumarin. For the purpose of the present description and the following claims, the term “C1-C20 alkylene groups” indicates divalent alkyl groups -(CH2)n- in which n is an integer comprised between 1 and 20, linear or branched, saturated or unsaturated. Specific examples of the C₁-C₂₀ alkylene groups are: methylene, ethylene, propylene, butylene, pentylene, hexylene, iso-propylene, iso-butylidene. For the purpose of the present description and the following claims, the term “polyethyleneoxy groups” indicates groups having oxyethylene units in the molecule. Specific examples of polyethyleneoxy groups are: methylene- ethylenethoxyl, methylenediethylenoxyl, methylene-tetraethylenoxyl, tetraoxyethylene, 3-oxo tetraoxyl, 3,6-dioxoheptyloxyethyl, 3,6,9- trioxadecyloxyl, 3,6,9,12-tetraoxadecyloxyl. The aforementioned compounds having general formula (I) and (II), can be prepared by processes known in the art as described, for example, in: Marcos R. et al, “Journal of the American Chemical Society” (2008), Vol. 130, pg. 16838- 16839; Chen Y. et al., “The Journal of Organic Chemistry” (1999), Vol.64, pg. 6870-6873. Further details concerning the preparation of the aforementioned compounds having general formula (I) and (II) can be found in the following examples. The aforementioned liquid electrolytes can 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. 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 (KPF6) tetrabutylammonium hexafluorophosphate (TBAPF6), tetraethylammonium tetrafluoroborate (TEABF₄), tetrabutylammonium tetrafluoroborate (TBABF₄), or mixtures thereof. Tetrabutylammonium tetrafluoroborate (TBABF₄) is preferred. In accordance with a preferred embodiment of the present invention, said organic solvent can be selected, for example, from acetonitrile, 3- methoxyproprionitrile, diethyl carbonate, ^-butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate, N,N-dimethylacetamide, or mixtures thereof. Acetonitrile, 3-methoxyproprionitrile, are preferred. 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. It should also be noted that the aforementioned compounds having general formula (I) and (II) 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 M. In accordance with a preferred embodiment of the present invention, said ion exchange membrane can be selected from polymeric membranes such as, for example: - ion exchange membranes such as, for example, membranes based on a styrene-divinylbenzene copolymer or 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 polytetrafluoethylene (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 ethylene-propylene copolymers, membranes based on polyimides, membranes based on polyvinyl fluorides. Anion exchange membranes which can 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, Ionac MA3475 of Lanxess, Teflon® of DuPont, Fumasep ^® FAA-3 and Fumasep^® FAP-330-PE by Fumatech. Cation exchange membranes which can 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. Preferably, the negative electrode can 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 can be porous, grooved or smooth. Preferably, the positive electrode can 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 can be porous, grooved or smooth. The present invention will now be illustrated in greater detail through an embodiment with reference to Figure 1 reported below. In particular, Figure 1 schematically represents an embodiment of a non- aqueous 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) in which a positive electrode (6) is placed in which a non- aqueous liquid positive electrolyte (not shown in Figure 1) is made to flow, a negative compartment (8a) in which a negative electrode (8) is placed in which a non-aqueous liquid negative electrolyte (not shown in Figure 1) is made to flow, an ion exchange membrane (7) placed between the positive compartment (6a) and the negative compartment (8a). 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) or (II) 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). The negative compartment (8a) is connected to a reservoir (12) containing an 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). A voltmeter (9) is connected to the positive electrode (6) and to the negative electrode (8). 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 above-mentioned charging and discharging steps can be summarised as follows: negative electrode: positive electrode: in which: - R has the same meanings as described above; - e- = electrons. 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. The energy stored in the non-aqueous (1) redox flow battery (RFB) can be directly used for the operation of the apparatus in which it is inserted, or it can 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 1) can possibly be used to facilitate the transfer of energy to and from an alternating current (AC) supply network. As mentioned above, the present invention also relates to a compound comprising 2,5-di-tert-butylbenzene groups substituted with an alkoxyl group having the specific general formula (I) or (II) provided below. Accordingly, it is a further object of the present invention to provide a compound having general formula (I) or (II): in which R, n and G, have the same meanings as above. 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. EXAMPLE 1 Synthesis of 5,5'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1- diyl))bis(oxy))-bis(1,4-di-tert-butyl-2-methoxybenzene) having formula (Ia) In a 100 ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, to a solution of 2,5-di-tert-butyl-4- methoxyphenol (Merck) (4.72 g; 20 mmoles) in anhydrous tetrahydrofuran (THF) (Merck) (100 ml), sodium hydride (NaH) (Merck) (0.60 g; 15 mmoles) was added, in small portions, in 10 minutes: the resulting reaction mixture was left, in an inert atmosphere, under stirring, at room temperature (25°C), for 1 hour. Subsequently, a mixture of tetraethylene glycol di(p-toluenesulphonate) (Merck) (5.02 g; 10 mmol) in anhydrous tetrahydrofuran (THF) (Merck) (5 ml) was added drop by drop at room temperature (25°C) in 15 minutes. The resulting reaction mixture was heated to 70°C and kept under stirring, at said temperature, for 16 hours. Subsequently, after cooling to room temperature (25°C), the reaction mixture was placed in a 500 ml separating funnel: a saturated 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 on sodium sulphate (Aldrich) and evaporated in a Rotovapor. The residue obtained was purified by elution on a silica gel chromatography column [(eluent: n-heptane/ethylacetate 1/1) (Aldrich)], obtaining 5.04 g of 5,5'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1- diyl))bis(oxy))bis(1,4-di-tert-butyl-2-methoxybenzene) having formula (Ia) as a straw-yellow oil (yield 80%). EXAMPLE 2 Synthesis of 1,27-bis(2,5-di-tert-butyl-4-methoxyphenoxy)-14,14-bis(13-(2,5-di- tert-butyl-4-methoxyphenoxy)-2,5,8,11-tetraoxydecyl)-3,6,9,12,16,19,22,25- octaoxa-heptacosane having formula (IIa) (1) Synthesis of 11-(triphenylmethyloxy)-3,6,9-trioxyundecan-1-ol having formula (A) The synthesis of 11-(triphenylmethyloxy)-3,6,9-trioxyundecan-1-ol was carried out as reported by Pilkington-Miksa M. A. et al., in “European Journal of Organic Chemistry” (2008), pag. 2900-2914, at p. 2906, left column, “Experimental Section”. (2) Synthesis of 13-(2,5-di-tert-butyl-4-methoxyphenoxy)-1,1,1-triphenyl- 2,5,8,11-tetra-oxatridecane having formula (B) In a 250 ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, to a solution of 2,5-di-tert-butyl-4- methoxyphenol (Merck) (3.54 g; 15 mmoles) in anhydrous tetrahydrofuran (THF) (Merck) 75 ml) sodium hydride (NaH) [60% dispersion in mineral oil (Merck)] (0.90 g; 22.5 mmoles) was added, in small portions, in 10 minutes: the resulting reaction mixture was left, under stirring, at room temperature (25°C), for 30 minutes. Subsequently, a mixture of 11-(triphenylmethyloxy)-3,6,9- trioxyundecan-1-ol (8.86 g; 15 mmol) obtained as described above in anhydrous tetrahydrofuran (THF) (Merck) (10 ml) was added drop by drop at room temperature (25°C) in 15 minutes. The resulting reaction mixture was heated to 70°C and kept under stirring, at said temperature, for 16 hours. Subsequently, after cooling to room temperature (25°C), the reaction mixture was placed in a 500 ml separating funnel: a 0.1 M 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 dried over sodium sulphate (Merck) and evaporated. The residue obtained was purified by eluting it on a silica gel chromatography column [(eluent: n-heptane/ethylacetate 1/1) (Merck)], obtaining 8.35 g of 13-(2,5-di- tert-butyl-4-methoxyphenoxy)-1,1,1-triphenyl-2,5,8,11-tetraoxytridecane as a straw-yellow oil (85% yield). (3) Synthesis of 2-(2-(2-(2-(2,5-di-tert-butyl-4- methoxyphenoxy)ethoxy)ethoxy)ethoxy)ethane-1-ol having formula (C) In a 250-ml flask, equipped with magnetic stirring, in an inert atmosphere, to a solution of 13-(2,5-di-tert-butyl-4-methoxyphenoxy)-1,1,1-triphenyl- 2,5,8,11-tetraoxytridecane obtained as described above (6.55 g; 10 mmoles) in methanol (Merck) (100 ml), p-toluenesulphonic acid (pTsOH) (Merck) (0.034 g; 0.2 mmoles) was added: the resulting reaction mixture was kept, under stirring, at room temperature (25°C), for 16 hours. Subsequently, the reaction solvent was removed by evaporation in Rotovapor, the residue obtained was dissolved in ethylacetate (Merck) (20 ml) and placed in a 500 ml separating funnel: subsequently, a saturated solution of sodium chloride (NaCl) (Merck) (3 x 100 ml) was added and the whole was extracted with ethylacetate (Merck) (3 x 100 ml), resulting in an aqueous 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 eluting it on a silica gel chromatography column [(eluent: n-heptane/ethylacetate 1/1) (Merck)], obtaining 2.94 g 2-(2-(2-(2-(2,5-di-tert-butyl-4-methoxyphenoxy)ethoxy)- ethoxy)ethoxy)ethane-1-ol as a straw-yellow oil (yield 70%). (4) Synthesis of 1,27-bis(2,5-di-tert-butyl-4-methoxyphenoxy)-14,14-bis(13-(2,5- di-tert-butyl-4-methoxyphenoxy)-2,5,8,11-tetraoxytridecyl)- 3,6,9,12,16,19,22,25-octaoxa-heptacosane having formula (IIa) In a 100-ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, to a solution of 2-(2-(2-(2,5-di-tert-butyl-4- methoxyphenoxy)ethane-1-ol mmoles obtained as described above (2.94 g; 10 mmoles), in dimethylformamide anhydrous (DMF) (Merck) (50 ml) sodium hydride (NaH) [60% dispersion in mineral oil (Merck)] (0.60 g; 15 mmoles) was added, in small portions, in 10 minutes: the resulting reaction mixture was kept, under stirring, at room temperature (25°C), for 1 hour. Subsequently, a mixture of pentaerythritol tetrabromide (0.97 g; 2.5 mmol) (Merck) in N,N- dimethylformamide anhydrous (DMF) (Merck) (5 ml) was added, drop by drop, at room temperature (25°C), in 15 minutes. The resulting reaction mixture was heated to 100°C and kept under stirring, at said temperature, for 16 reaction hours. Subsequently, after cooling to room temperature (25°C), the reaction mixture was placed in a 500 ml separating funnel: subsequently, a saturated solution of sodium chloride (NaCl) (Merck) (3 x 100 ml) was added 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 eluting on a silica gel chromatography column [(eluent: n-heptane/ethylacetate 1/1) (Merck)], obtaining 2.17 g of 1,27-bis(2,5-di-tert-butyl-4-methoxyphenoxy)-14,14-bis(13- (2,5-di-tert-butyl-4-methoxyphenoxy)-2,5,8,11-tetraoxytridecyl)- 3,6,9,12,16,19,22,25-octaoxaheptacosane having formula (IIa) as a straw yellow oil (yield 70%). EXAMPLE 3 Cyclic voltammetry measurements Cyclic voltammetry measurements were carried out 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 (E_pf) and the return peak (E_pr): and the values were normalised with respect to the intersolvent ferrocene/ferrocenium (Fc/Fc⁺) pair. 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: - 2,1,3-benzothiadiazole (Merck) (5 x 10^-2 M) (Merck) and tetrabutylammonium tetrafluoroborate (TBABF4) (Merck) (0.1 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte compartment) (BTD); - 5,5'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1-diyl))bis(oxy))- bis(1,4-di-tert-butyl-2-methoxybenzene) having formula (Ia) (5 x 10^-2 M) obtained in Example 1 and tetrabutylammonium tetrafluoroborate (TBABF4) (Merck) (0,1 M) in acetonitrile (Merck) (non-aqueous liquid positive electrolyte) (Ia); - 1,27-bis(2,5-di-tert-butyl-4-methoxyphenoxy)-14,14-bis(13-(2,5-di-tert- butyl-4-methoxyphenoxy)-2,5,8,11-tetraoxatridecyl)-3,6,9,12,16,19,22,25- octaoxaheptacosane having formula (IIa) (5 x 10^-2 M) obtained in Example 2 and tetrabutylammonium tetrafluoroborate (TBABF4) (Merck) (0.1 M) in acetonitrile (Merck). Figures 2 and 3 [the abscissa shows the potential (E) measured in volts (V) and the ordinate shows the current density (i) measured in amperes (A)] show the cyclic voltagram obtained from the above solutions (Ia) and (IIa) in acetonitrile, at a scan rate of 200 mV/s, respectively. Figure 4 [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 (Ia) and (BTD) in acetonitrile, at a scan rate of 200 mV/s. It can be seen that a high open-circuit potential difference (E°) of 2.16 V is obtained calculated according to the following formula: E° = (E°1) - (E°2) in which: - (E°₁) is the oxidation reduction potential for the (BTD) relative to the second peak calculated as described above and is equal to -1.85 vs (Fc/Fc⁺); - (E°₂) is the oxidation-reduction potential for (Ia) calculated as described above and is equal to 0.31 V vs (Fc/Fc⁺). EXAMPLE 4 Non-aqueous redox flow battery (RFB) charge/discharge tests [electrolytes: compound having formula (Ia) and compound having formula (IIa) in acetonitrile] The charge-discharge tests were carried out using a graphite electrochemical cell with a Fumasep^® FAP-330-PE (Fumatech) membrane, 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. Subsequently, it was placed inside a nitrogen inert container (“glovebox”). 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). The following electrolytes were used for this purpose: - 5,5'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1-diyl))bis(oxy))- bis(1,4-di-tert-butyl-2-methoxybenzene) having formula (Ia) (5 x 10^-2 M) obtained in Example 1 and tetrabutylammonium tetrafluoroborate (TBABF₄) (Merck) (0,1 M) in acetonitrile (Merck) (non-aqueous liquid positive electrolyte) (Ia); - 2,1,3-benzothiadiazole (Merck) (5 x 10^-2 M) (Merck) and tetrabutylammonium tetrafluoroborate (TBABF₄) (Merck) (0.1 M) in acetonitrile (Merck) (non-aqueous liquid negative electrolyte compartment) (BTD); 25 ml of the above solutions were introduced into the respective compartments. 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 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 40 mA up to the end-of-charge voltage of 2.8 V, followed by a potentiostatic charge at 2.8 V up to 5 mA; - discharge phase: the battery was galvanostatically discharged with a current of 15 mA to the end-of-discharge voltage of 0.5 V. Figure 5 [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 7 charge/discharge cycles is depicted, from which a good cyclability of the molecules involved during the various cycles can be seen. In Figure 6 [the abscissa shows the number of cycles; the ordinate shows the measured Coulombic efficiency (Efficiency) measured as a %, as the ratio of discharged capacity to charged capacity and capacity retention expressed as the ratio of discharged capacity to initial capacity]. Figure 6 shows that the Columbic efficiency, initially above 85% decreases slightly to 80% at the seventh cycle while the initial capacity retention remains above 65%.

Claims

CLAIMS 1. Non-acqueous redox flow battery (RFB) comprising: - a positive compartment in which a positive electrode is placed and in which a non-aqueous liquid positive electrolyte is made to flow; - a negative compartment in which a negative electrode is placed and in which a non-aqueous liquid negative electrolyte is made to flow; - an ion exchange membrane placed between the positive compartment and the negative compartment; in which: - said non-aqueous liquid positive electrolyte comprises a solution of at least one compound having general formula (I) or (II): in which: - R, equal or different from each other, preferably equal to each other, are selected from C₁-C₂₀ alkyl groups, preferably C₂-C₁₀, linear or branched, optionally substituted cycloalkyl groups, optionally substituted aryl groups, optionally substituted heteroaryl groups, polyethyleneoxy groups R₁-O-[CH₂-CH₂-O]_m- in which R₁ represents a hydrogen atom, or is selected from C1-C20 alkyl groups, preferably C2-C10, linear or branched, and m is an integer comprised between 1 and 4; - n is an integer comprised between 1 and 10, preferably comprised between 2 and 4; - G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or an ether group -R₂-O-R₃- in which R₂ and R₃, equal or different from each other represent a C1-C20 alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a polyethyleneoxy group -[CH₂-CH₂-O]_p-CH₂- in which p is an integer comprised between 1 and 4; in at least one organic solvent; - said non-aqueous liquid negative electrolyte comprises a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent. 2. Non-aqueous redox flow battery (RFB) according to claim 1, in which in said general formula (I): - R, equal to each other, represent a C₁-C₂₀ alkyl group, preferably a methyl; - n is 4. 3. Non-aqueous redox flow battery (RFB) according to claim 1, in which in said general formula (II): - R, equal to each other, represent a C₁-C₂₀ alkyl group, preferably a methyl; - G represents a polyethyleneoxy group -[CH₂-CH₂-O]_m- in which m is 3. 4. Non-aqueous redox flow battery (RFB) according to any of the preceding claims, in which said benzothiadiazole or a derivative thereof is selected from benzothiadiazoles having general formula (III): in which R4, R5, R6 and R7, equal or different from each other, represent a hydrogen atom, or a halogen atom such as chlorine, fluorine, bromine, iodine, preferably fluorine; or represent a -CN group; or are selected from: C₁-C₁₀ alkyl groups, preferably C₁-C₄, linear or branched, saturated or unsaturated, C₁-C₁₀ alkoxyl groups, preferably C1-C4, linear or branched, saturated or unsaturated, carboxylic acid esters having general formula R8-COO-R9 in which R8 and R9, equal or different from each other, are selected from C₁-C₁₀ alkyl groups, preferably C₁-C₄, linear or branched, saturated or unsaturated, sulphonic acid esters having general formula R8-OSO2-R9 in which R8 and R9 have the same meanings as described above, thioesters having general formula R8-SO-R9 in which R₈ and R₉ have the same meanings as described above, -(O-CH₂-CH₂)_q- OH groups in which q is an integer between 1 and 4, -(O-CH(CH₃)-CH₂)_z-OH in which z is an integer between 1 and 4, optionally substituted aryl groups, optionally substituted heteroaryl groups. 5. Non-aqueous redox flow battery (RFB) according to claim 4, in which in said general formula (III): - R4, R5, R6 and R7, equal to each other, represent a hydrogen atom. 6. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, in which said liquid electrolytes comprise at least one supporting electrolyte selected from potassium hexafluorophosphate (KPF₆), tetrabutylammonium hexafluorophosphate (TBAPF6), tetraethylammonium tetrafluoroborate (TEABF4), tetrabutylammonium tetrafluoroborate (TBABF4), or mixtures thereof; preferably it is tetrabutylammonium tetrafluoroborate (TBABF₄). 7. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, in which said organic solvent is selected from acetonitrile, 3- methoxyproprionitrile, diethyl carbonate, dimethyl carbonate, γ-butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2- pyrrolidone (NMP), fluoroethylene carbonate, N,N-dimethylacetamide, or mixtures thereof; preferably from acetonitrile, 3-methoxyproprionitrile. 8. Non-aqueous redox flow battery (RFB) according to any one of the preceding claims, in which said ion exchange membrane is selected from polymeric membranes such as: - anion exchange membranes such as membranes based on a styrene- divinylbenzene copolymer or 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 polytetrafluoroethylene (PTFE); - cation exchange membranes such as 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 ethylene-propylene copolymers, membranes based on polyimides, membranes based on polyvinyl fluorides. 9. Compound having general formula (I) or (II): in which: - R, equal or different from each other, preferably equal to each other, are selected from C1-C20 alkyl groups, preferably C2-C10, linear or branched, optionally substituted cycloalkyl groups, optionally substituted aryl groups, optionally substituted heteroaryl groups, polyethyleneoxy groups R₁-O- [CH2-CH2-O]m- in which R1 represents a hydrogen atom, or is selected from C1-C20 alkyl groups, preferably C2-C10, linear or branched, and m is an integer comprised between 1 and 4; - n is an integer comprised between 1 and 10, preferably comprised between 2 and 4; - G represents a C1-C20 alkylene group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated, or an ether group -R₂-O-R₃- in which R₂ and R₃, equal or different from each other, represent a C₁-C₂₀ alkyl group, preferably C1-C6, linear or branched, saturated or unsaturated, preferably saturated; or a polyethyleneoxy group -[CH2-CH2-O]p-CH2- in which p is an integer comprised between 1 and 4.