Electroactive compounds

Abstract

The present invention refers to an electrolyte comprising a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted. The present invention also relates to a redox flow battery comprising the electrolyte of the invention, to an energy storage and / or delivery system comprising said battery, to a method of storing and / or delivering electricity comprising said battery, to the use of said battery, to the use of an energy system comprising said battery and to the use of the use of the viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted as active species of an electrolyte.

Description

[0001] ELECTROACTIVE COMPOUNDS

[0002] DESCRIPTION

[0003] TECHNICAL FIELD

[0004] The present invention relates to the field of electrolytes. More specifically, the present invention relates to the field of electrolytes, in particular for redox flow batteries.

[0005] BACKGROUND

[0006] Redox-flow batteries (RFBs) are promising choices for grid-scale storage systems because of the unique advantages they present, such as their flexible, modular design and fast response (Wang et al. Adv. Funct Mater, 2013, 23(8):970-986). The name ‘redox’ refers to electrochemical reduction and oxidation reactions through which energy is stored in electrolytes. A common redox flow battery configuration comprises a first and a second electrode in contact with a first and a second electrolyte comprising redox active species (named anolyte and catholyte). In this redox flow battery configuration, the anolyte and catholyte are usually separated by a separator such as an ion-selective membrane.

[0007] Aqueous organic redox flow batteries (AORFBs) are bringing much attention, since they usually use as redox active species, organic and organometallic molecules based on Earth abundant elements. At neutral pH, alkyl viologen-derivatives such as 1,1'-bis[3- sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi), are the most commonly used redox active species in the anolyte of AORFBs. However, it has been observed that the presence of small amounts of oxygen in said anolyte triggers an accelerated degradation of those viologen-derivatives. It seems that the generation of hydroxide anions from the reduction of oxygen present in the anolyte accelerates the degradation of alkyl viologen-derivatives such as 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi), via dealkylation through nucleophilic attack, leading to a dramatic decrease in capacity retention of the battery. Complete absence of oxygen for long periods (>15 years) at large scale (>MWh) is very challenging, if not impossible, to be achieved. Moreover, the use of basic media in AORFBs is desired since it results in higher current density and lower power cost. Alkaline or basic media also enables the implementation of simple and effective rebalancing strategy to reverse the capacity loss due to Faradaic imbalance. Rubio-Presa et al. (Rubio-Presa et al. ACS Materials Lett. 2023, 5, 798-802) proposed an alkyl viologen derivative having a methyl group in the alkyl chain at the a-position with respect to the N atoms. In particular, the proposed alkyl viologen derivative in Rubio- Presa et al. was 3, 3'-([4,4'-bipyridine]-1 , 1 '-diium-1 , 1 '-diyl)bis(butane-1 -sulfonate) (BS3Bu-Vi) and showed a slower degradation at mild-basic conditions (pH 9-11) than BSPr-Vi. Rubio-Presa et al. attributed said stability to a greater difficulty for the nucleophilic attack due to steric hindrance. However, BS3Bu-Vi still degrades at high pH values such as those above 11 . The structural modification present in BS3Bu-Vi only slows down the degradation kinetics by steric effect. This allows the battery to operate under mild alkaline conditions during a certain amount of time. However, this steric hindrance is not sufficient to prevent damage at longer times or higher pH conditions.

[0008] Therefore, despite the above-mentioned systems, it is desirable to develop more stable viologen derivatives for redox flow batteries that lead to increased energy density, power density, energy efficiency and overall performance.

[0009] BRIEF DESCRIPTION OF THE INVENTION

[0010] The authors of the present invention have achieved a complete inhibition of the degradation pathway of viologens derivatives by changing the electronic nature of the carbon directly linked to the N-atoms of the bipyridinium core of the viologen. The authors of the present invention have surprisingly found that the introduction of an aryl group completely prevents the nucleophilic attack from hydroxide anions. Then, the modified viologens can be used in aqueous electrolytes in a redox flow battery which can operate at basic pH or highly basic pH for long time with a high current density and low power cost, enabling the implementation of simple and effective rebalancing strategy to reverse the capacity loss due to Faradaic imbalance. This type of battery can be used in an energy store and / or delivery system or in a method of storing electricity.

[0011] Thus, in a first aspect, the invention relates to an electrolyte comprising a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted; and wherein the electrolyte is an aqueous electrolyte having a pH equal or between about 9 and about 14.

[0012] In a second aspect, the invention is directed to a redox flow battery comprising: a) a positive electrode and a negative electrode; b) the electrolyte according to its aspect or any of its particular embodiments; and c) optionally, a separator.

[0013] In a third aspect, the present invention is directed to an energy storage and / or delivery system comprising at least one redox flow battery according to its aspect or any of its particular embodiments.

[0014] In a fourth aspect, the invention is directed to a method of storing electricity comprising the steps of: a) providing a redox flow battery according to according to its aspect or any of its particular embodiments; b) oxidizing the redox active species of the electrolyte at the positive electrode to the corresponding oxidized state, while the redox active species of the electrolyte are reduced to the corresponding reduced state at the negative electrode.

[0015] In a fifth aspect, the invention is directed to a method of delivering electricity comprising the steps of: a) providing a redox flow battery according to its aspect or any of its particular embodiments; b) reducing the redox active species of the electrolyte at the positive electrode to their reduced state while the redox active species of the electrolyte are oxidized to the corresponding oxidized state at the negative electrode.

[0016] In an additional aspect, the invention is directed to the use of the redox flow battery according to its aspect or any of its particular embodiments to store and / or deliver electricity.

[0017] In yet another aspect, the invention is directed to the use of energy storage and / or delivery system according to its aspect or any of its particular embodiments to store and / or deliver electricity.

[0018] An additional aspect is directed to the use of a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted, as redox active species of an electrolyte. Particularly the viologen compound of said use may have the features and / or characteristics defined when describing the viologen compound in the electrolyte aspect or in any of its particular embodiments.

[0019] All features of the products such as the electrolyte and redox flow battery and / or the systems described in this specification (including the claims, description, and drawings) may be combined, except combinations of mutually exclusive features. The same applies to all features and steps of the methods described in this specification. All features and / or steps of the methods described in this document may be combined with those of the products and / or systems (including the claims, description and drawings) described in this document, and vice versa, in any combination, except for combinations of mutually exclusive features.

[0020] FIGURES

[0021] Figure 1.1H NMR spectrum of 4-carboxyphenyl viologen in D2O (300 MHz).

[0022] Figure 2.1H NMR spectrum of 3-carboxyphenyl viologen in D2O (300 MHz).

[0023] Figure 3.1H NMR spectrum of 2,3-dicarboxyphenyl viologen in DMSO-cfe (300 MHz).

[0024] Figure 4.1H NMR spectrum of 3,4-dicarboxyphenyl viologen in DMSO-cfe (300 MHz).

[0025] Figure 5.1H NMR spectrum of 3,5-dicarboxyphenyl viologen in DMSO-cfe (300 MHz).

[0026] Figure 6.1H NMR spectrum of 2,5-dihydroxyphenyl viologen in DMSO-cfe (300 MHz).

[0027] Figure 7.1H NMR spectra of 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi), before (lower spectrum) and after (upper spectrum) being in a basic aqueous solution at pH 14 during 5 min.

[0028] Figure 8.1H NMR spectra of 1 ,T-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) before (lower spectrum) and after (upper spectrum) being in a basic aqueous solution at pH 14 during 5 min.

[0029] Figure 9.1H NMR spectra of 2,5-dihydroxyphenyl viologen dichloride (B-2,5-DHP-Vi) before (lower spectrum) and after (upper spectrum) being in a basic aqueous solution at pH 14 during 24 h.

[0030] Figure 10. Cyclic voltammetry results for (A) 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi), (B) 1 ,T-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi), (C) 2,5- dihydroxyphenyl viologen dichloride (B-2,5-DHP-Vi), and (D) 2,3-dicarboxyphenyl viologen dichloride. Conditions: 25 mM electroactive species, 1 M KCI and a scan rate 100 mV / s. Neutral pH (pH 7) for A and B, and basic pH (pH 14) for C and D.

[0031] Figure 11. A) Redox flow battery performance using 1 ,1 '-bis[3-sulfonatopropyl]-4,4'- bipyridinium (BSPr-Vi) as redox active species of the anolyte at pH 14; B) battery performance using 1 ,1 '-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) as redox active species of the anolyte at pH 14; and C) battery performance using 2,5- dihydroxyphenyl viologen dichloride as redox active species of the anolyte at pH 14.

[0032] Figure 12.1H NMR spectra of 1-((3,5-dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen in D2O.

[0033] Figure 13.1H NMR spectra of 1-((4-carboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen in D2O.

[0034] Figure 14.1H NMR spectra of 4-cyanophenyl viologen in D2O.

[0035] Figure 15.1H NMR spectra of 1 ,1'-bis(2,5-dihydroxyphenyl)-3-methyl viologen in D2O.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] With regard to the terms used in the present description, unless otherwise defined, scientific and technical terms used in connection with the present invention shall have the meanings that are commonly understood by those of ordinary skill in the art. The meaning and scope of the terms should be clear, however, in the event of any latent ambiguity, definitions provided herein take precedent over any dictionary or extrinsic definition.

[0038] As used herein, the singular forms “a” “an” and “the” include plural reference unless the context clearly dictates otherwise.

[0039] It will be further understood that the terms "comprises" and / or "comprising”, when used in this specification, specify the presence of stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, and / or components thereof. The term “comprises” encompasses the terms “consisting essentially of” and “consisting of”. The term “consisting essentially of” regarding a composition or composition of compounds is interpreted to mean that additional components may be present, provided that they do not significantly alter the fundamental characteristics of the composition. Preferably, the term “consisting essentially of” means unspecified compounds or components may be present up to 5%, 4%, 3%, 2%, 1 % or 0.5% by weight based on total weight of the composition.

[0040] As used herein, the terms "about" or “around” mean a slight variation of the value specified, preferably within 10 percent of the value specified. Further, to provide a more concise description, some of the quantitative expressions given herein are not qualified with any of the terms “about" or “around”. It is understood that whether any of the terms “about" or “around” is used explicitly or not, every quantity given herein is meant to refer to the actual given value and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximation due to the experimental and / or measurement conditions for such given value.

[0041] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. As an illustration, a numerical range of "about 1 % to about 5%" or “equal or between around 1 % and around 5%” should be interpreted to include not only the explicitly recited values of 1% to 5%, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3 and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting only one numerical value. It should also be understood that ranges formed by combination of any of the end points of different disclosed ranges and / or particular values therein are included in the present disclosure.

[0042] Electrolyte

[0043] In a first aspect, the invention relates to an electrolyte comprising: a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted; and wherein the electrolyte is an aqueous electrolyte having a pH equal or between about 9 and about 14.

[0044] Thus, the electrolyte of the first aspect comprises water and has a pH value ranging from about 9 to about 14, being 9 and 14 included.

[0045] In an embodiment, the electrolyte comprises a viologen compound or a salt thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted.

[0046] In a particular embodiment, the electrolyte comprises a viologen compound, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted.

[0047] In the context of the present invention, a “viologen compound” is understood as commonly known in the art, non-limiting examples of viologen compounds are 4,4’- bipyridinium compounds, substituted 4,4’-bipyridinium compounds, derivatives of 4,4’- bipyridinium compounds or salts or solvates thereof. In an embodiment, the viologen compound comprises at least two nitrogen atoms. Thus, the aforementioned “viologen compound” can be unsubstituted or can be substituted at one or more available positions with the suitable groups disclosed in each case. As used herein, the singular form “viologen compound” includes the plural reference unless the context clearly dictates otherwise.

[0048] The term “aryl” refers to an aromatic group having equal or between 6 and 18 carbon atoms (“Ce-Ci8 aryl”), preferably equal or between 6 and 12 carbon atoms (“C6-C12 aryl”), more preferably equal or between 6 and 10 and even more preferable having 6 carbon atoms, comprising 1 , 2 or 3 aromatic nuclei, including for example and in a non-limiting sense, phenyl, biphenyl, naphthyl, indenyl, phenanthryl or terphenyl. Preferably, aryl refers to phenyl (Ph) or biphenyl, more preferably refers to phenyl (Ph). .

[0049] The term “alkyl” refers to a lineal or branched hydrocarbon chain radical or group consisting of carbon and hydrogen atoms, containing no unsaturation, having equal or between 1 to 12 (“C1-C12 alkyl”), preferably from 1 to 6 (“Ci-Ce alkyl”), more preferably from 1 to 3 (“C1-C3 alkyl”), which is bound to the rest of the molecule through a single bond. Illustrative non-limiting examples of alkyl groups include methyl, ethyl, n-propyl, i- propyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, etc.

[0050] The term “alkenyl” refers to a lineal or branched hydrocarbon chain radical or group containing from 2 to 12 (“C2-C12 alkenyl”), preferably from 2 to 6 (“C2-C6 alkenyl”), more preferably from 2 to 3 (“C2-C3 alkenyl”), carbon atoms and which contains at least one double bond and is attached to rest of the molecule by a single bond. Illustrative nonlimiting examples of alkenyl include ethenyl, propenyl, allyl, butenyl, and 1-methyl-2- butenyl.

[0051] The term “alkynyl” refers to a lineal or branched hydrocarbon chain radical or group containing from 2 to 12 (“C2-C12 alkynyl”), preferably from 2 to 6 (“C2-C6 alkynyl”), more preferably from 2 to 3 (“C2-C3 alkynyl”), carbon atoms and which contains at least one triple bond and is attached to rest of the molecule by a single bond. Non-limiting examples of alkynyl groups include ethynyl, propynyl and butynyl.

[0052] The term “acyl” used alone or as a part of a larger moiety, refers to groups formed by removing a hydroxyl group from a carboxylic acid.

[0053] The term “alkylaryl” refers to an aryl group as defined above substituted with an alkyl group as defined above, such as (Ci-C6)alkyl(C6-Cis)aryl, (Ci-C6)alkyl(C6-Cio)aryl and (Ci-C3)alkyl(C6-C )aryl.

[0054] The term “arylalkyl” refers to an alkyl group as defined above substituted with an aryl group as defined above, such as (C6-Ci8)aryl(Ci-Ce)alkyl, (C6-C )aryl(Ci-C6)alkyl and (C6-C )aryl(Ci-C3)alkyl. Examples of such groups include benzyl, phenylethyl, phenylpropyl, naphthylmethyl, etc.

[0055] The term “alkylene” refers to a divalent group having a specified size. Illustrative examples of alkylene are methylene, ethylene, isoproylene and the like. Alkylene groups can contain straight-chain and branched-chain forms, as well as combinations of these. The divalency of alkylene groups does not include the optional substituent of the alkylene group.

[0056] The term “alkyleneimine” refers to a radical derived from a saturated aliphatic chain containing alkylenimine units which is bound to the rest of the molecules through a single bond. It may be linear or branched. The term "alkyleneimine" has the general formula - NR a -OR -R'NR'-, wherein each R or R' is independently alkyl as defined above.

[0057] The term “halogen” refers to fluorine (F), chlorine (Cl), bromine (Br), or iodine (I); preferably chlorine (Cl), bromine (Br), or iodine (I).

[0058] The aforementioned groups can be unsubstituted or can be substituted at one or more available positions with the suitable groups disclosed in each case. In a particular embodiment, it can be substituted with one, two or three, preferably with one or two, more preferably with one of said suitable groups.

[0059] The invention also provides “salts” of the compounds described in the present description. By way of illustration, said salts can be acid addition salts, base addition salts, or metal salts, and can be synthesized from the original compounds containing a basic or acidic residue by means of conventional chemical methods known in the art. Such salts are generally prepared, for example, by reacting the free acid or base forms of said compounds with a stoichiometric amount of the suitable base or acid in water or in an organic solvent or in a mixture of both. Illustrative examples of said acid addition salts include inorganic acid addition salts such as, for example, hydrochloride, hydrobromide, hydroiodide, sulfate, perchlorate, nitrate, phosphate, etc., organic acid addition salts such as, for example, acetate, formate, maleate, fumarate, citrate, oxalate, succinate, tartrate, malate, mandelate, methanesulfonate, p-toluenesulfonate, camphorsulfonate, etc. Illustrative examples of base addition salts include inorganic base salts such as, for example, ammonium salts, and organic base salts such as, for example, ethylenediamine, ethanolamine, / V, / V-dialkylenethanolamine, triethanolamine, glutamine, basic amino acid salts, etc. Illustrative examples of metal salts include, for example, sodium, potassium, calcium, magnesium, aluminum, and lithium salts. In a particular embodiment, the salt is an acid addition salt.

[0060] The compounds of the invention can be in the form of salts, solvates or stereoisomers. Similarly, when a particular compound, or group of compounds, is excluded from the claims, any salts, solvates, or stereoisomers thereof are also to be considered as excluded, unless stated otherwise.

[0061] Likewise, the compounds described in the present description can be obtained both as free compounds and as solvates (for example, hydrates, alcoholates, etc.), both forms being included within the scope of the present invention. Solvation methods are generally known in the state of the art. As used herein, the term "stereoisomer" is a general term for all isomers of individual molecules that differ only in the orientation of their atoms in space, and include enantiomers and diastereomers.

[0062] When the compounds of the invention have chiral centers, they can exist in different stereoisomeric forms, such as enantiomeric or diastereomeric forms. Thus, any given compound referred to herein is intended to represent any one of a racemate, one or more enantiomeric forms and one or more diastereomeric forms. All the stereoisomers including enantiomers and diastereoisomers of the compounds referred to herein, and mixtures thereof (including racemic mixtures, enantiomerically enriched mixtures and diastereomerically enriched mixtures), are considered within the scope of the present invention. Also, the invention further contemplates any E / Z possible isomers.

[0063] The electrolyte of the invention comprises the viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different between them (i.e. the aryl groups are the same or different from each other); and wherein said aryl groups are optionally substituted, as a redox active species. In addition, the 4,4’-bipyridinium of the viologen compound of the electrolyte of the invention can be unsubstituted or can be substituted at one or more available positions with the suitable groups disclosed in each case. In a particular embodiment, the 4,4’-bipyridinium of the viologen compound of the electrolyte of the invention can be substituted with one, two or three, preferably with one or two, more preferably with one of said suitable groups.

[0064] In a particular embodiment, any of the carbons of the 4,4’-bipyridinium of the viologen compound of the electrolyte, is substituted; preferably is substituted by a radical selected from: -Y, -OY, -COOY, -SY and -NY2; wherein Y represents: -H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine.

[0065] In a more particular embodiment, any of the carbons of the 4,4’-bipyridinium of the viologen compound of the electrolyte, is substituted by an alkyl group optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, - CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine; preferably is substituted by an alkyl group; more preferably by a Ci-Ce alkyl group; much more preferably by a C1-C3 alkyl group; even much more preferably is substituted by a methyl group.

[0066] In a more particular embodiment, one carbon of the 4,4’-bipyridinium of the viologen compound of the electrolyte, is substituted by an alkyl group optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine; preferably is substituted by an alkyl group; more preferably by a C1- Ce alkyl group; much more preferably by a C1-C3 alkyl group; even much more preferably by a methyl group.

[0067] The viologen compound of the electrolyte of the invention can be a symmetric compound or an asymmetric compound. In a preferred embodiment, the viologen compound is symmetric.

[0068] Each of the nitrogen atoms of the 4,4’-bipyridinium of the viologen compound of the electrolyte are substituted with one aryl group wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted.

[0069] The expression directed to the aryl groups substituting the nitrogen atoms of the 4,4’- bipyridinium of the viologen compound of the electrolyte being equal or different, means that these aryl groups may or may not be identical. In a particular embodiment, the aryl groups joined to the nitrogen atoms of the viologen compound of the electrolyte of the invention are the same (i.e. they are identical); preferably wherein the viologen compound is symmetric.

[0070] In another particular embodiment, the viologen compound of the electrolyte is symmetric.

[0071] The aryl groups of the viologen compound of the electrolyte of the invention are optionally substituted; preferably are substituted at one or two positions; more preferably one or two carbons of each of the aryl groups are substituted.

[0072] Said aryl groups can be substituted by at least a radical selected from the group consisting of: -Y, -OY, -COOY, -SY, -CN and -NY2; wherein Y represents: -H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine. In a particular embodiment, the aryl groups of the viologen compound of the electrolyte of the invention are substituted by at least a radical selected from the group consisting of: -Y -OY, - CN and -COOY; and wherein Y is selected from the group consisting of H and an alkyl organic radical optionally substituted with -OR’, wherein R’ is H, alkyl, alkylene oxide or alkyleneimine; preferably wherein the aryl groups are substituted by at least a radical selected from the group consisting of : -Y -OY, - CN and -COOY; and wherein Y is H; even more preferably the aryl groups are substituted by at least a radical selected from: -OY and -COOY; and wherein Y is H.

[0073] Alternatively, said aryl groups can be substituted by a radical selected from: -Y, -OY, - COOY, -SY and -NY2; wherein Y represents: -H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine.

[0074] In a particular embodiment, the aryl groups of the viologen compound of the electrolyte of the invention are substituted by a radical selected from: -Y -OY and -COOY; and wherein Y is selected from H and an alkyl organic radical optionally substituted with - OR’, wherein R’ is H, alkyl, alkylene oxide or alkyleneimin; preferably the aryl groups are substituted by a radical selected from: -Y -OY and -COOY; and wherein Y is H; even more preferably the aryl groups are substituted by a radical selected from: -OY and - COOY; and wherein Y is H.

[0075] In a particular embodiment, the aryl groups joined to the nitrogen atoms of the viologen compound of the electrolyte of the invention are substituted with the same groups; preferably are substituted with the same groups at one or two available positions.

[0076] In a particular embodiment, the viologen compound of the electrolyte of the invention is a compound of formula (I): wherein:

[0077] - any of R1, R2, R3, R4is independently selected from -Y, -OY, -COOY, -SY, -CN and -NY2, wherein Y represents:

[0078] H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine;

[0079] - any of ml , m2, is independently selected from an integer selected from 0 to 5;

[0080] - any of m3, m4 is independently selected from an integer selected from 0 to 4;

[0081] - n and p, are integer positive numbers; and

[0082] - N is an anion.

[0083] In a particular embodiment, the viologen compound is a compound of formula (I) wherein:

[0084] - any of R1and R2is independently selected from -Y, -OY, -COOY, -SY, -CN and — NY2, wherein Y represents:

[0085] H, or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine; and

[0086] - any of R3and R4is independently selected from H or an alkyl organic radical optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine.

[0087] In a particular embodiment, the viologen compound is a compound of formula (I) wherein any of R1, and R2is independently selected from -H, -OH, -COOH, or -CN; preferably wherein any of R1, and R2is independently selected from OH, -COOH, or -CN.

[0088] In another particular embodiment, the viologen compound is a compound of formula (I) wherein any of R3, and R4is independently selected from H, or an alkyl organic radical; preferably an akyl organic radical; more preferably a C1-C3 alkyl organic radical; even much more preferably a methyl organic radical.

[0089] In an embodiment, ml and m2 refer to the number of positions of the aryl ring which are substituted. In an embodiment, ml and / or m2 is an integer independently selected from 0 to 5. In a preferred embodiment, ml and / or m2 is an integer independently selected from 0, 1 and 2.

[0090] In an embodiment, m3 and m4 refer to the number of positions of the piridyl ring which are substituted. In an embodiment, m3 and / or m4 is an integer independently selected from 0 to 4. In a preferred embodiment, m3 and / or m4 is an integer independently selected from 0, 1 and 2.

[0091] In a particular embodiment, the viologen compound is a compound of formula (I) wherein: R1and R2are the same; and / or R3and R4are the same; and / or ml and m2 are the same; and / or m3 and m4 are the same.

[0092] In an alternative particular embodiment, the viologen compound of the electrolyte of the invention is a compound of formula (I'): wherein:

[0093] - R1is selected from -Y, -OY, -COOY, -SY and -NY2, wherein Y represents:

[0094] H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine;

[0095] - m is an integer independently selected from 0 to 5;

[0096] - n and p, are integer positive numbers; and

[0097] - N is an anion.

[0098] In an embodiment, R1of the compound of formula (I') is selected from -Y, -OY and - COOY, wherein Y is selected from H and an alkyl organic radical optionally substituted with -OR’. In a preferred embodiment R’ is H, alkyl, alkylene oxide or alkyleneimine. In a preferred embodiment, R’ is H. In another preferred embodiment, R’ is alkyl, selected from the group consisting of methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, pentyl and hexyl. In a more preferred embodiment, R’ is an alkyl selected from the group consisting of methyl and ethyl.

[0099] The aryl groups of the viologen compound of the electrolyte of the invention are optionally substituted; in particular they can be optionally substituted in one or various positions of the aryl ring. Therefore, the aryl ring of the viologen can be unsubstituted, monosubstituted, disubstituted, trisubstituted, tetrasubstituded or pentasubstituted.

[0100] As stated above, m refers the number of positions of the aryl ring which are substituted of the compound of formula (I'). In an embodiment, m is an integer independently selected from 0 to 5. In a preferred embodiment, m is an integer independently selected from 0, 1 and 2.

[0101] N is an anion. In a preferred embodiment, N is an anion having 1 , 2, 3, 4, or 6 negative charges; preferably an anion selected from the group consisting of Cl Br, I BF4; PF6AsF6; SbF6; AICI4; HSO4; CIO4; CH3SO3; CF3CO2; (CF3SO2)2N; (FSO2)2N; SO4; CF3SO3; (C2F5SO2)2N; and (C2F5SO2)(CF3SO2)N.

[0102] In a preferred embodiment, N is a halogen anion; preferably N is selected from F; Cl; Br, and I; preferably is selected from Cl; Br, and I’. In a most preferred embodiment, N is Ch.

[0103] In a particular embodiment of the invention, n is an integer positive number; preferably is 1.

[0104] In a particular embodiment of the invention, p is an integer positive number; preferably is 1 or 2.

[0105] In an embodiment, the compound of formula (I) or the compound of formula (I') has a net charge of 0 (i.e. is a neutral compound).

[0106] In an embodiment, one or two of the aryl groups of the viologen compound of the electrolyte are substituted with at least a substituent selected from the group consisting of -OH, -COOH, and -CN; preferably the two aryl groups of the viologen compound of the electrolyte are substituted with at least a substituent selected from the group consisting of -OH, -COOH, and -CN.

[0107] In a particular embodiment, the viologen compound comprises a cation selected from the group consisting of:

[0108] 4-carboxyphenyl viologen;

[0109] 3-carboxyphenyl viologen;

[0110] 2.3-dicarboxyphenyl viologen;

[0111] 3.4-dicarboxyphenyl viologen;

[0112] 3.5-dicarboxyphenyl viologen;

[0113] 2.5-dihydroxyphenyl viologen;

[0114] 1-((3,5-dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen;

[0115] 1-((4-carboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen;

[0116] 4-cyanophenyl viologen;

[0117] 1 ,1'-bis(2,5-dihydroxyphenyl)-3-methyl viologen; and a mixture thereof.

[0118] In a more particular embodiment, the viologen compound is selected from the group consisting of:

[0119] 4-carboxyphenyl viologen dichloride;

[0120] 3-carboxyphenyl viologen dichloride;

[0121] 2.3-dicarboxyphenyl viologen dichloride;

[0122] 3.4-dicarboxyphenyl viologen dichloride;

[0123] 3.5-dicarboxyphenyl viologen dichloride;

[0124] 2.5-dihydroxyphenyl viologen dichloride;

[0125] 1-((3,5-dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride;

[0126] 1-((4-carboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride;

[0127] 4-cyanophenyl viologen dichloride;

[0128] 1 ,1'-bis(2,5-dihydroxyphenyl)-3-methyl viologen dichloride; and a mixture thereof.

[0129] In a particular embodiment, the viologen compound of the electrolyte of the invention is a carboxyphenyl viologen or a hydroxyphenyl viologen.

[0130] In another particular embodiment, the viologen compound of the electrolyte of the invention is selected from the group consisting of 4-carboxyphenyl viologen dichloride; 3-carboxyphenyl viologen dichloride; 2,3-dicarboxyphenyl viologen dichloride; 3,4- dicarboxyphenyl viologen dichloride; 3,5-dicarboxyphenyl viologen dichloride; and 2,5- dihydroxyphenyl viologen dichloride.

[0131] As stated above, the term “electrolyte” can be referred to a catholyte and / or to an anolyte as commonly known in the art. In a preferred embodiment, the electrolyte is an anolyte.

[0132] In the context of the present invention, the term “anolyte” is understood as the portion of an electrolyte near an anode, in particular in a cell in which the cathode and anode are in separate compartments such as in redox flow batteries.

[0133] The viologen compound of the electrolyte of the present invention as described above in any of its particular embodiments, is a redox active species; preferably a redox active species of the anolyte.

[0134] In the context of the present invention, "redox active species" refers to redox species in which one active species (the reducing agent) undergoes oxidation (loses electrons) while another species (the oxidizing agent) undergoes reduction (gains electrons). For example, redox active species can be redox couples or redox pairs as known in the art.

[0135] The electrolyte is an aqueous electrolyte. The expression “aqueous electrolyte” is understood as an electrolyte comprising water or a mixture of water and at least an additional solvent wherein the at least additional solvent is in less amount than water; preferably comprising water. In a particular embodiment, the only solvent of the electrolyte is water.

[0136] In an embodiment, the electrolyte of the invention, further comprises:

[0137] - at least an additional solvent; and

[0138] - optionally a salt.

[0139] In a particular embodiment, the at least an additional solvent is selected from a polar solvent, a non-polar solvent and / or mixtures thereof.

[0140] In a particular embodiment, the at least an additional solvent is at least a polar solvent.

[0141] Having regard to the present disclosure, the selection of suitable polar solvents is well within the knowledge of the expert in the art. Non-limiting examples of suitable polar solvents are alcohols (for example methanol, ethanol, isopropanol, n-propanol, t-butanol or benzyl alcohol), carbonates (for example ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, or vinylene carbonate), ethers (for example tetrahydrofuran, methyltetrahydrofuran, dioxolane, dimethyl ether, di methoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, or polyethylene glycol) sulfones (for example ethyl methyl sulfone, dimethyl sulfone, or tetramethylene sulfone), sulfoxides (for example dimethylsulfoxide) amides and sulfamides (for example dimethylformamide), nitriles (for example acetonitrile), nitrobenzene and mixtures therefrom.

[0142] Having regard to the present disclosure, the selection of suitable non-polar solvents is well within the knowledge of the expert in the art. In an embodiment, the non-polar solvent is selected from the group consisting of hydrocarbon solvents (for example pentane, hexane, cyclohexane, or heptane), carbon tetrachloride, aromatic solvents (for example toluene, benzene, or xylene) and some ethers (for example diethyl ether).

[0143] In an embodiment, the invention relates to an electrolyte consisting essentially of or consisting of: a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted;

[0144] - water; optionally a pH regulator; and optionally a salt; wherein the electrolyte is an aqueous electrolyte having a pH equal or between about 9 and about 14.

[0145] In the particular embodiments wherein the electrolyte comprises a salt, the person skilled in the art would know that any type of salt may be suitable for the present invention. Non- limitative examples of salts suitable for the electrolytes are LiTFSi; TBAPFe; UCIO4; BU4BF4; LiPFe; BU4NCIO4, K3PO4, NaCI, KCI, Na2SC>4 and / or mixtures thereof; preferably NaCI, KCI, Na2SC>4 and / or mixtures thereof.

[0146] In a particular embodiment, the electrolyte of the invention comprises a salt; preferably a metal halide salt; more preferably a metal halide salt comprising Na or K; even much more preferably NaCI or KCI.

[0147] In an embodiment, the salt is in the electrolyte in an amount of between 0.1 and 2.5 M; preferably of between 0.5 and 2 M; more preferably between 0.8 M and 1.5 M.

[0148] In a particular embodiment, the electrolyte of the invention comprises a pH regulator; preferably a base; preferably an inorganic base such as a hydroxide; more preferably NaOH or KOH.

[0149] In an embodiment, the pH regulator is in the electrolyte in an amount of between 0.1 and 2.5 M; preferably of between 0.5 and 2 M; more preferably between 0.6 M and 1.2 M.

[0150] In a more preferred embodiment the electrolyte further comprises 1 M of KCI and 0.8 M of KOH.

[0151] In a particular embodiment, the electrolyte of the invention has a pH equal or higher than about 9, 9.5, 10, 10.5, 11 , 11.5, 12, 12.5, 13, 13.5 or 14.

[0152] In a particular embodiment, the electrolyte of the invention has a pH equal or between about 9 and about any of 9.5, 10, 10.5, 11 , 11.5, 12, 12.5, 13, 13.5 or 14.

[0153] In a particular embodiment, the electrolyte of the invention has a pH between about 10 and about 14, a pH between about 11 and about 14, a pH between about 12 and about 14, a pH between about 13 and about 14, or a pH equal to about 14.

[0154] In a preferred embodiment, the electrolyte of the invention has a pH between about 11 and about 14, preferably between about 12 and about 14, even more preferably between about 13 and about 14.

[0155] In another embodiment, the pH of the electrolyte of the invention is at least 10, at least 11 , at least 12 or at least 13. In a preferred embodiment, the pH of the invention is at least 10, more preferably at least 11 , even more preferably at least 12. In the context of the present invention the pH of a solution such as the electrolyte of the invention is measured by any technique known in the art such as electrochemical methods of pH measuring, for example using a pH meter at standard conditions (at about 25°C).

[0156] As indicated above, surprisingly, the viologen compounds of the invention as described in any of the particular embodiments mentioned above present the unexpected advantage with respect to other viologen compounds of the state of the art of their stability at basic pH values, even in extreme basic media. This is due to the presence of an aryl group directly linked to the nitrogen atoms of the 4,4’-bipyridinium rings, which avoids the nucleophile attack of the OH' group, and consequently avoids the degradation pathway that suffer the alkyl viologens.

[0157] Redox flow battery

[0158] The electrolyte of the invention can be advantageously used in a redox flow battery.

[0159] Therefore, another inventive aspect of the invention relates to a redox flow battery comprising: a) a positive electrode and a negative electrode; b) the electrolyte defined in its aspect or in any of its particular embodiments; and c) optionally, a separator.

[0160] In a particular embodiment the redox flow battery comprises: a) a positive electrode and a negative electrode; b) the electrolyte defined in its aspect or in any of its particular embodiments as anolyte, and a catholyte; c) a separator between the anolyte and the catholyte.

[0161] In a particular embodiment, the catholyte and the anolyte of the redox flow battery have the same or a different composition; preferably a different composition.

[0162] In the context of the present invention, the term “redox” refers to electrochemical reduction and oxidation reactions which help to store energy in a battery during charge and deliver energy during discharge.

[0163] The redox battery of the invention is configured to act as an energy storage and delivery system. In particular, the redox-battery can be a secondary or rechargeable battery, i.e. it can be configured to be reversibly charged and discharged.

[0164] In the context of the present invention, the term “flow battery” is intended to refer to a battery system in which reactants and products can be transported into and out of the battery.

[0165] In the context of the present invention the term “electrodes” refer to the positive and negative electrodes. The electrodes are where the oxidation or reduction of redox active species of the electrolyte take place. The electrodes of the invention comprise, at least, a positive electrode and a negative electrode. Both electrodes may have connection terminals that may be electrically connected to each other by a connection wire through which electrons are able to travel.

[0166] Non-limiting examples of suitable electrodes in the battery of the present invention are selected from a carbon matrix or a metallic matrix; particularly are selected from a porous carbon matrix or a porous metallic matrix; more preferably carbon paper, carbon felt, graphite felt, reticulated porous carbon, metallic mesh or metallic foam. In a preferred embodiment, the positive electrode and / or the negative electrode are made of a carbon based material, more preferably are carbon paper, carbon felt, graphite felt, or reticulated porous carbon; even much more preferably carbon felt.

[0167] In an embodiment, the separator is placed between the anolyte and the catholyte. In an embodiment, the anolyte and the catholyte are separated by the separator.

[0168] Any separator known in the art related to redox flow batteries may be used in the present invention. In a particular embodiment, the separator is made of an electrically insulating material, an ion exchange membrane or a cellulosic material. Non-limiting examples of electrically insulating materials suitable for the separator are insulating polymers such as polycarbonate or polypropylene, cellulose based materials such as paper, etc. In another particular embodiment, the separator is an ion exchange membrane; preferably a polymeric ion exchange membrane such as Nation™ perfluorinated membrane, or a hydrocarbon-based ion-exchange membrane. In a more particular embodiment, the separator comprises a cellulosic material; preferably a paper sheet.

[0169] In an embodiment, the redox flow battery of the invention is an aqueous organic redox flow battery (AORFBs). In an embodiment, the redox flow battery of the invention may work in a dynamic or static mode; preferably in a dynamic mode.

[0170] The redox-flow battery of the present invention may comprise means for or means adapted for connecting with a power / load source. The power / load source may be any external electrical device as known in the art such as an electrical grid, an electric vehicle, a domestic appliance or a sensor, that draws / transfers energy from / to the battery. In general, the power / load source has controllable voltages and / or current supplies or uptakes.

[0171] Energy storage and / or delivery system

[0172] In another aspect, the invention is directed to an energy storage and / or delivery system comprising at least one redox flow battery according to its aspect or in any of its particular embodiments described above. In particular, the redox flow battery may act as a secondary and / or rechargeable battery, i.e. the redox flow battery may be configured to be reversibly charged and discharged.

[0173] Methods of operation of the battery

[0174] As mentioned before, the redox-flow battery of the present invention in its aspect or in any of its particular embodiments is configured to act as an energy storage and delivery system, i.e. it is configured to be reversibly charged and discharged.

[0175] Method of storing electricity

[0176] Therefore, another aspect of the present invention is directed to a method of storing electricity comprising the steps of: a) providing a redox flow battery according to its aspect or any of its particular embodiments; b) oxidizing the redox active species of the electrolyte at the positive electrode to the corresponding oxidized state, while the redox active species of the electrolyte are reduced to the corresponding reduced state at the negative electrode.

[0177] In an embodiment, the viologen compound of the electrolyte of the present invention is a redox active species of the electrolyte of the redox flow battery. In a particular embodiment, when the redox flow battery of the invention comprises a catholyte and an anolyte, the viologen compound of the electrolyte of the present invention is a redox active species of the anolyte of the redox flow battery.

[0178] An illustrative non-limiting example of an embodiment of the battery of the invention works as storage system (charging mode) is described as follows: during the charging process of an embodiment of the battery of the present invention, the redox active species of the catholyte are oxidized. Then, the electrons released on the positive electrode of the battery of the present invention move through an external circuit, i.e. means for or adapted for connecting with the power / load source, to do useful work. At the same time, the redox active species of the anolyte (i.e. the viologen compound of the electrolyte as a redox active species) are reduced (capture electrons) to the corresponding reduced state at the negative electrode. Catholyte and anolyte may be separated by a separator. During the charging process, ions migrate from one electrolyte to the other crossing the intermediate liquid phase to keep electro-neutrality between them.

[0179] Method of delivering electricity

[0180] Another aspect of the present invention is directed to a method of delivering electricity comprising the steps of: a) providing a redox flow battery according to its aspect or any of its particular embodiments; and b) reducing the redox active species of the electrolyte at the positive electrode to their reduced state while the redox active species of the electrolyte are oxidized to the corresponding oxidized state at the negative electrode.

[0181] In an embodiment, the viologen compound of the electrolyte of the present invention is a redox active species of the electrolyte of the redox flow battery.

[0182] In a particular embodiment, when the redox flow battery of the invention comprises a catholyte and an anolyte, the viologen compound of the electrolyte of the present invention is a redox active species of the anolyte of the redox flow battery.

[0183] An illustrative non-limiting example on how the battery of the invention works as delivery system (discharging mode) is described as follows: During the discharging process of an embodiment of the battery of the present invention, the redox active species of the catholyte are reduced at one electrode. At the same time, the anolyte redox active species are oxidized to the corresponding oxidized state at the negative electrode (i.e. the viologen compound redox active species of the electrolyte). Catholyte and anolyte may be separated by a separator. During the discharging process, ions migrate from one electrolyte to the other crossing the intermediate liquid phase to keep electro-neutrality between them.

[0184] Energy storage and / or delivery system

[0185] Another additional aspect of the invention is directed to an energy storage and / or delivery system comprising at least one redox battery according to its aspect or any of its particular embodiments.

[0186] Applications

[0187] Additional aspects of the present invention are directed to the use of the redox-flow battery as defined above in its aspect or in any of its particular embodiments, to store and / or deliver energy. To this end, the redox flow battery of the present invention may be used individually, as modular redox-flow battery system, or in combination with other energy storage technologies (e.g., supercapacitors, etc.) and may be integrated into or with various systems and / or devices to improve efficiency, address energy demands, etc.

[0188] Furthermore, the redox battery of the invention may be used in a variety of applications having different energy delivery and / or storage needs, including, but not limited to, very large scale applications (e.g., utilities, functioning as a green energy source for a smart grid, energy storage for use in combination with renewable energy resources such as wind and solar power, etc.) and smaller applications (e.g. backup power, residential power, electromobility sector, etc.).

[0189] Another additional aspect of the invention is directed to the use of an energy storage and / or delivery system which comprises the redox flow battery according to the present invention to store and / or deliver electricity.

[0190] An additional aspect is directed to the use of a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted, as redox active species of an electrolyte; preferably of an aqueous electrolyte; more preferably as redox active species of an anolyte; even much more preferably as redox active species of an aqueous anolyte.

[0191] In a particular embodiment, the viologen compound is a compound of formula (I) or a compound of formula (I') according to any of the embodiments described above.

[0192] All the features or characteristics described above for the viologen compound of the electrolyte in any of its particular embodiments may be applied to the viologen compound of the use aspect defined above.

[0193] All the features or characteristics described above for the viologen compound, of for the the electrolyte in any of its particular embodiments may be applied to the redox flow battery, the energy storage and / or delivery system of the invention, the methods of the invention and to the viologen compound or salt or solvate thereof for use as redox active species in an electrolyte. Moreover, all the characteristics described above for the redox flow battery in any of its particular embodiments may be applied to the energy storage and / or delivery system of the invention, and to the methods of the invention.

[0194] The present invention will be described in further detail with reference to the following examples. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0195] EXAMPLES

[0196] Example 1: Synthesis of 4-ca rboxy phenyl viologen

[0197] 4-carboxyphenyl viologen dichloride was synthesized as follows:

[0198] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield).

[0199] Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and ethyl 4-aminobenzoate (496 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 3 days and the ethanol was removed by distillation. The residue was dissolved in water (25 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 4-(ethoxycarbonyl) phenyl viologen dichloride (441 mg; 84% yield).

[0200] Step 3: To a 25 mL round bottom flask was added 4-(ethoxycarbonyl)phenyl viologen dichloride (441 mg) and concentrated HCI concentrated (10 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (378 mg; 96% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of this compound in D2O is showed in Figure 1.

[0201] Example 2: Synthesis of 3-carboxyphenyl viologen

[0202] 3-carboxyphenyl viologen dichloride was synthesized as follows:

[0203] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield).

[0204] Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and methyl 3-aminobenzoate (458 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 3 days and the ethanol was removed by distillation. The residue was dissolved in water (25 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 3-(methoxycarbonyl)phenyl viologen dichloride (480 mg; 96% yield).

[0205] Step 3: To a 25 mL round bottom flask was added 3-(methoxycarbonyl)phenyl viologen dichloride (480 mg) and concentrated HCI concentrated (10 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (434 mg; 96% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of this compound in D2O is showed in Figure 2.

[0206] Example 3: Synthesis of 2,3-dicarboxyphenyl viologen

[0207] 2,3-dicarboxyphenyl viologen dichloride was synthesized as follows:

[0208] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield).

[0209] Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and dimethyl 3-aminophthalate (630 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 4 days and the ethanol was removed by distillation. The residue was dissolved in water (25 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 2,3-(dimethoxycarbonyl)phenyl viologen dichloride (503 mg; 82% yield).

[0210] Step 3: To a 25 mL round bottom flask was added 2,3-(dimethoxycarbonyl)phenyl viologen dichloride (503 mg) and concentrated HCI concentrated (10 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (429 mg; 94% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of this compound in DMSO-cfe is showed in Figure 3.

[0211] Example 4: Synthesis of 3,4-dicarboxyphenyl viologen

[0212] 3,4-dicarboxyphenyl viologen dichloride was synthesized as follows:

[0213] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield).

[0214] Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and dimethyl 4-aminophthalate (630 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 4 days and the ethanol was removed by distillation. The residue was dissolved in water (25 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 3,4-(dimethoxycarbonyl)phenyl viologen dichloride (552 mg; 90% yield).

[0215] Step 3: To a 25 mL round bottom flask was added 3,4-(dimethoxycarbonyl)phenyl viologen dichloride (552 mg) and concentrated HCI concentrated (10 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (477 mg; 95% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of this compound in DMSO-cfe is showed in Figure 4. Example 5: Synthesis of 3,5-dicarboxyphenyl viologen

[0216] 3,5-dicarboxyphenyl viologen dichloride was synthesized as follows:

[0217] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield).

[0218] Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and dimethyl 5-aminoisophthalate (630 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 4 days and the ethanol was removed by distillation. The residue was dissolved in water (25 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 3,5-(dimethoxycarbonyl)phenyl viologen dichloride (561 mg; 91% yield).

[0219] Step 3: To a 25 mL round bottom flask was added 3,5-(dimethoxycarbonyl)phenyl viologen dichloride (561 mg) and concentrated HCI concentrated (10 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (472 mg; 93% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of this compound in DMSO-cfe is showed in Figure 5.

[0220] Example 6: Synthesis of 2, 5-di hydroxyphenyl viologen

[0221] 2,5-dihydroxyphenyl viologen dichloride was synthesized as follows:

[0222] To a 250 mL round bottom flask was added p-benzoquinone (1.38 g), 4,4’-bipyridine (1 g) and glacial acetic acid (25 mL). The reaction mixture was heated under reflux for 3 hours and then excess cone, hydrochloric acid was added. After cooling to room temperature, diethyl ether (150 mL) was added to the mixture and the obtained deep purple solid was filtered off, washed with cold water and dried under vacuum. Purification by recrystallization from water / acetone gave rise to the solid product (2.1 g; 72% yield). The1H NMR spectrum of this compound in DMSO-cfe is showed in Figure 6.

[0223] Example 7: Chemical stability

[0224] The chemical stability in basic pH media of some of the compounds synthesized in examples 1-6 was explored by1H NMR. Figure 7 shows the1H NMR spectra of the state- of-the-art viologen, 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium, (BSPr-Vi), Figure 8 shows the1H NMR spectra of 1 ,T-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) and Figure 9 shows the1H NMR spectra of the 2,5-dihydroxyphenyl viologen dichloride (B-2,5-DHP-Vi) synthesized on example 6, before (lower spectrum) and after (upper spectrum) being in a basic aqueous solution comprising KOH, at pH 14, during 5 min for BSPr-Vi and BS3Bu-Vi; and 24 hours for B-2,5-DHP-Vi. It was observed that BSPr-Vi and BS3Bu-Vi degraded almost instantaneously at pH 14. The pH of the aqueous solution was neutralized to pH 7 before the spectra acquisition to avoid paramagnetic species.

[0225] The1H NMR spectrum shows that BSPr-Vi quickly degrades in basic media (even before 5 min of exposure) while the aryl viologen tested, B-2,5-DHP-Vi, remains chemically stable even after being during 24 h in an aqueous solution at pH 14. Thus, results showed that the change in the electronic nature of the carbon, from Csp3(alkyl chain) to Csp2(aryl group), directly bonded to the N-atoms of the bipyridinium core allows for complete inhibition of the degradation by the cleavage of the / V-substituents by nucleophilic attack of the hydroxide anions at extreme alkaline condition (pH 14), in contrast to the strategy described in the state of the art, based on steric hindrance of the alkyl / V-substituents, in which case the viologen derivative does not withstand extreme alkaline conditions. Example 8: Characterization by cyclic voltammetry.

[0226] Cyclic voltammetry was performed for a 1 M of KCI aqueous solution further comprising 25 mM of 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi) (Figure 10A), 1,1'-bis[1- methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) (Figure 10B), 2,5- dihydroxyphenyl viologen dichloride (B-2,5-DHP-Vi) (Figure 10C) or2,3-dicarboxyphenyl viologen dichloride (Figure 10D). Data was recorded at a scan rate of 100 mV / s. Cyclic voltammetry results were obtained at neutral pH (pH 7) on Fig. 10 A and B and at basic pH (pH 14) on Fig. 10 C and D.

[0227] Cyclic voltammetry results show that the electrochemical behaviour of the 2,5- dihydroxyphenyl viologen dichloride in basic media (C in Figure 10) is very similar to both 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi) and 1 ,1 '-bis[1-methyl-3- sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) in neutral pH (A and B in Figure 10, respectively) showing two reversible cathodic I anodic peaks.

[0228] The cyclic voltammetry results of the 2,3-dicarboxyphenyl viologen dichloride (D in Figure 10) show a potential shift of -200 mV with respect to the 2,5-dihydroxyphenyl viologen dichloride (B in Figure 10). In addition, they show two sets of reversible redox peaks located near -0.39 V and -0.86 V for 2,3-dicarboxyphenyl viologen dichloride, and -0.58 V and -0.95 V for 2,5-dihydroxyphenyl viologen dichloride.

[0229] Thus, the differences in the location of the reversible redox peaks of the cyclic voltammetry results of 2,5-dihydroxyphenyl viologen dichloride and 2,3-dicarboxyphenyl viologen dichloride, indicates that the redox potential of aryl viologen derivatives can be tuned by changing the functional groups attached to the aryl group.

[0230] Example 9: Performance as redox active species of the anolyte of a redox flow battery working at pH 14.

[0231] Figure 11 shows a comparison of the performance of 1 ,1 '-bis[3-sulfonatopropyl]-4,4'- bipyridinium (BSPr-Vi) and 2,5-dihydroxyphenyl viologen dichloride as redox active species of an aqueous anolyte of a redox flow battery working at pH 14.

[0232] The rechargeable redox flow batteries were made using a 0.2 M of 1 ,1'-bis[3- sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi) (Figure 11A), 1 ,1'-bis[1-methyl-3- sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) (Figure 11 B) or 2,5-dihydroxyphenyl viologen dichloride (Figure 11C) in 1 M KCI and 0.8 M KOH aqueous solution as electrolyte of the negative side of the cell (i.e. anolyte). The redox flow batteries were tested working at a current of 20 mA cm"2with 1 .2 V and 0.5 V as charge and discharge cut-off limits, respectively.

[0233] The batteries using an electrolyte comprising 1 ,1'-bis[3-sulfonatopropyl]-4,4'- bipyridinium (BSPr-Vi) or 1 ,T-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu- Vi) failed rapidly, losing all its capacity to store energy within 1 cycle. However, the redox flow battery using an electrolyte comprising 2,5-dihydroxyphenyl viologen dichloride was able to continue working during 3 weeks (Figure 11C) at pH 14. Indeed, the capacity fading rate for 2,5-dihydroxyphenyl viologen dichloride in basic media was only 0.03 %TT1. Surprisingly, this result is better than results reported for redox flow batteries using state-of-the-art viologen derivatives such as 1 ,T-bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi) (being 0.45 %TT1) or 1 ,T-bis[1-methyl-3-sulfonatopropyl]-4,4'-bipyridinium (BS3Bu-Vi) (being 0.15 %TT1) working at neutral pH.

[0234] Thus, results showed a significant improvement in the stability and a reduction in the capacity fading rate of redox flow batteries with electrolytes comprising aryl viologen derivatives in comparison with those comprising alkyl viologen derivatives such as 1,1'- bis[3-sulfonatopropyl]-4,4'-bipyridinium (BSPr-Vi) when working at basic pH.

[0235] Actually, Fig. 11C shows that viologen compounds wherein each of the nitrogen atoms are substituted with one aryl group are stable as electroactive species in the electrolyte of a redox flow battery for weeks wherein viologen compounds wherein each of the nitrogen atoms are substituted with alkyl groups degrades within minutes at pH 14, (see Figures 11A and 11 B). Thus, these results confirm that aryl viologen derivatives are surprisingly more stable under basic pH than alkyl viologen derivatives. Moreover, results showed that they do not suffer degradation through nucleophilic attack from hydroxide anions.

[0236] Moreover, surprisingly the performance results of the redox flow batteries were significantly better for redox flow batteries with electrolytes comprising aryl viologen derivatives working at basic pH than for those with electrolytes comprising state-of-the- art alkyl viologen derivatives when working at neutral pH. In particular, its performance over a longer period of time while maintaining a high current density and low energy costs.

[0237] Finally, the authors have observed an improved solubility of viologen compounds wherein each of the nitrogen atoms are substituted with one aryl group, when said aryl group is substituted with carboxyl and / or hydroxyl groups, in aqueous media under basic pH, in particular at pH values over 9 or 10, when compared with the solubility of other viologen compounds wherein each of the nitrogen atoms are substituted with one aryl group optionally substituted with other non-deprotonatable functional groups.

[0238] Example 10: Synthesis of 1 -((3, 5-dicarboxy)phenyl)-1 ’-(2, 5-dihydroxyphenyl) viologen

[0239] 1-((3,5-Dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride was synthesized as follows:

[0240] Step 1 : A solution of 4,4’-bipyridine (3.5 g) and 1-chloro-2,4-dinitrobenzene (3 g), in acetone (25 mL) was heated under reflux for 13 hours. The solid product was isolated by filtration, washed three times with dichloromethane and dried under vacuum to provide 1 -(2,4-dinitrophenyl)-[4,4’-bipyridin]-1 -ium chloride as a greyish white solid (3.2 g; 60% yield).

[0241] Step 2: To a 250 mL round bottom flask was added 1-(2,4-dinitrophenyl)-[4,4’-bipyridin]- 1-ium chloride (1.08 g), anhydrous ethanol (75 ml) and dimethyl 5-aminoisophthalate (942 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 3 days and the ethanol was removed by distillation. The residue was dissolved in water (40 mL) and washed three times with diethyl ether (30 mL). The aqueous phase was dried under vacuum to provide 1-(3,5-bis(methoxycarbonyl)phenyl)-[4,4’-bipyridin]-1-ium chloride (1.04 g; 90% yield).

[0242] Step 3: To a 50 mL round bottom flask was added p-benzoquinone (335 mg), 1-(3,5- bis(methoxycarbonyl)phenyl)-[4,4’-bipyridin]-1-ium chloride (595 mg) and glacial acetic acid (11 mL). The reaction mixture was heated under reflux for 16 hours and then excess cone, hydrochloric acid was added. After cooling to room temperature, diethyl ether (50 mL) and acetone (50 mL) were added to the mixture and the obtained deep garnet solid was filtered off, washed with acetone and dried under vacuum to give rise to the solid product (180 mg; 22% yield).

[0243] Step 4: To a 25 mL round bottom flask was added 1-(3,5-bis(methoxycarbonyl)phenyl)- 1’-(2,5-dihydroxyphenyl) viologen dichloride (180 mg) and concentrated HCI concentrated (5 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (155 mg; 91 % yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of 1-(3,5- dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen in D2O is showed in Figure 12.

[0244] Example 11: Synthesis of 1-((4-carboxy)phenyl)-1 ’-(2, 5-di hydroxyphenyl) viologen

[0245] 1-((4-Carboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride was synthesized as follows:

[0246] Step 1 : A solution of 4,4’-bipyridine (3.5 g) and 1-chloro-2,4-dinitrobenzene (3 g), in acetone (25 mL) was heated under reflux for 13 hours. The solid product was isolated by filtration, washed three times with dichloromethane and dried under vacuum to provide 1 -(2,4-dinitrophenyl)-[4,4’-bipyridin]-1 -ium chloride as a greyish white solid (3.2 g; 60% yield).

[0247] Step 2: To a 250 mL round bottom flask was added 1-(2,4-dinitrophenyl)-[4,4’-bipyridin]- 1-ium chloride (1.08 g), anhydrous ethanol (75 ml) and ethyl 4-aminobenzoate (750 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 4 days and the ethanol was removed by distillation. The residue was dissolved in water (40 mL) and washed three times with diethyl ether (30 mL). The aqueous phase was dried under vacuum to provide 1-(4-ethoxycarbonylphenyl)-[4,4’-bipyridin]-1-ium chloride (765 mg; 75% yield).

[0248] Step 3: To a 50 mL round bottom flask was added p-benzoquinone (324 mg), 1-(4- ethoxycarbonylphenyl)-[4,4’-bipyridin]-1-ium chloride (510 mg) and glacial acetic acid (12 mL). The reaction mixture was heated under reflux for 16 hours and then excess cone, hydrochloric acid was added. After cooling to room temperature, diethyl ether (50 mL) was added to the mixture and the obtained deep garnet solid was filtered off, washed with diethyl ether and acetone, and dried under vacuum to give rise to the solid product (407 mg; 56% yield).

[0249] Step 4: To a 25 mL round bottom flask was added 1-(4-ethoxycarbonylphenyl)-1’-(2,5- dihydroxyphenyl) viologen dichloride (407 mg) and concentrated HCI concentrated (5 mL). The solution was then heated to 90 °C for 24 hours before cooling to room temperature. The solid product (345 mg; 90% yield) was isolated as the chloride salt by filtration and washed with acetone. The1H NMR spectrum of 1-(4-Carboxy)phenyl)-1’- (2,5-dihydroxyphenyl) viologen in D2O is showed in Figure 13.

[0250] Example 12: Synthesis of 4-cyanophenyl viologen

[0251] 4-Cyanophenyl viologen dichloride was synthesized as follows:

[0252] Step 1 : A solution of 4,4’-bipyridine (3.7 g) and 1-chloro-2,4-dinitrobenzene (17 g), in acetonitrile (75 mL) was heated under reflux for 72 hours. The reaction mixture was diluted with acetonitrile (50 mL) and filtered. The filtered cake was refluxed with ethanol (350 mL). After cooling to room temperature, a greyish white solid was filtered and dried under vacuum to provide 2,4-dinitrophenyl viologen dichloride (6.4 g; 52% yield). Step 2: To a 100 mL round bottom flask was added 2,4-dinitrophenyl viologen dichloride (562 mg), anhydrous ethanol (25 ml) and 4-aminobenzonitrile (356 mg) under an inert N2 atmosphere. The reaction mixture was heated to 90 °C for 4 days and the ethanol was removed by distillation. The residue was dissolved in water (30 mL) and washed three times with diethyl ether (20 mL). The aqueous phase was dried under vacuum to provide 4-cyanophenyl viologen dichloride (306 mg; 71% yield). The1H NMR spectrum of 4-cyanophenyl viologen in D2O is showed in Figure 14.

[0253] Example 13: Synthesis of 1 , 1 '-bis(2,5-dihydroxyphenyl)-3-methyl viologen

[0254] 1 ,1'-Bis(2,5-dihydroxyphenyl)-3-methyl viologen dichloride was synthesized as follows:

[0255] To a 100 mL round bottom flask was added p-benzoquinone (540 mg), 3-methyl-4,4’- bipyridine (425 mg) and glacial acetic acid (10 mL). The reaction mixture was heated under reflux for 20 hours and then excess cone, hydrochloric acid was added. After cooling to room temperature, diethyl ether (75 mL) was added to the mixture and the obtained brown solid was filtered off, washed with acetone and dried under vacuum. Purification by recrystallization from water / acetone gave rise to the solid product (792 mg; 69% yield). The1H NMR spectrum of 1 ,1'-bis(2,5-dihydroxyphenyl)-3-methyl viologen in D2O is showed in Figure 15.

[0256] Example 14. Electrolytes

[0257] In the present example aqueous electrolytes of a redox flow battery comprising the viologen compounds synthesized as described on previous examples 1- 6, and 10-13 as electroactive species, and water are prepared. The pH of those electrolytes is adjusted to be basic, in particular with values between about 9 and about 14.

Claims

CLAIMS1. An electrolyte comprising: a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted; wherein the electrolyte is an aqueous electrolyte having a pH equal to or between about 9 and 14.

2. The electrolyte according to claim 1 , wherein the viologen compound is a compound of formula (I):wherein:- any of R1, R2, R3, R4is independently selected from -Y, -OY, -COOY, -SY, - CN and -NY2, wherein Y represents:H or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine;- any of ml , m2, is independently selected from an integer selected from 0 to 5;- any of m3, m4 is independently selected from an integer selected from 0 to 4;- n and p, are integer positive numbers; and- N is an anion.

3. The electrolyte according to claim 2, wherein the viologen compound is a compound of formula (I) wherein: any of R1and R2is independently selected from -Y, -OY, -COOY, -SY, -CNand -NY2, wherein Y represents:H, or an organic radical chosen from alkyl, alkenyl, alkynyl, acyl, aryl, alkylaryl, arylalkyl, alkylene oxide and alkyleneimine, optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine; and- any of R3and R4is independently selected from H or an alkyl organic radical optionally substituted with at least a substituent selected from the group consisting of F, Cl, Br, I, -CN, -OR’, -SR’ and, -NR’R”, wherein R’ and R” are independently selected from H, alkyl, alkylene oxide and alkyleneimine.

4. The electrolyte according to any of claims 2 to 3, wherein the viologen compound is a compound of formula (I) wherein:- any of R1, and R2is independently selected from -H, -OH, -COOH, or -CN; and / or- any of R3, and R4is independently selected from H, or an alkyl organic radical.

5. The electrolyte according any of claims 3 to 4, wherein n is 1 ; and / or wherein p is 1 or 2; and / or wherein N is a halogen anion.

6. The electrolyte according to any of claims 1 to 5, wherein one or two of the aryl groups of the viologen compound are substituted with at least a substituent selected from the group consisting of -OH, -COOH, and -CN.

7. The electrolyte according to any of claims 1 to 5, , wherein the viologen compound is selected from the group consisting of:4-carboxyphenyl viologen dichloride;3-carboxyphenyl viologen dichloride;2,3-dicarboxyphenyl viologen dichloride;3.4-dicarboxyphenyl viologen dichloride;3.5-dicarboxyphenyl viologen dichloride;2.5-dihydroxyphenyl viologen dichloride;1-((3,5-dicarboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride;1-((4-carboxy)phenyl)-1’-(2,5-dihydroxyphenyl) viologen dichloride;4-cyanophenyl viologen dichloride;1 ,1'-bis(2,5-dihydroxyphenyl)-3-methyl viologen dichloride; and a mixture thereof.

8. The electrolyte according to any of claims 1 to 7, wherein the viologen compound or a salt or solvate thereof is symmetric.

9. The electrolyte according to any of claims 1 to 8, wherein the electrolyte is an anolyte.

10. The electrolyte according to any of claims 1 to 9, wherein the electrolyte has a pH equal or between about 10 and about 14.11 . A redox flow battery comprising: a) a positive electrode and a negative electrode; b) the electrolyte defined in any of claims 1 to 10; and c) optionally a separator.

12. An energy storage and / or delivery system comprising at least one redox flow battery according to claim 11 .

13. A method of storing electricity comprising the steps of: a) providing a redox flow battery according to claim 11 ; b) oxidizing the redox active species of the electrolyte at the positive electrode to the corresponding oxidized state, while the redox active species of the electrolyte are reduced to the corresponding reduced state at the negative electrode.

14. A method of delivering electricity comprising the steps of: a) providing a redox flow battery according to claim 11 ; b) reducing the redox active species of the electrolyte at the positive electrode to their reduced state while the redox active species of the electrolyte are oxidized to the corresponding oxidized state at the negative electrode.

15. Use of the redox flow battery according to claim 11 or the energy storage and / or delivery system according to claim 12, to store and / or deliver electricity.

16. Use of a viologen compound or a salt or solvate thereof, wherein each of the nitrogen atoms of the viologen are substituted with one aryl group; wherein said aryl groups are equal or different; and wherein said aryl groups are optionally substituted as redox active species in an electrolyte.

17. The use of claim 16; wherein the electrolyte is aqueous.

18. The use of claim 17; wherein the electrolyte has a pH over about 9.

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