Redox flow battery

Abstract

The invention provides a redox flow battery comprising an organic redox species and hydrogen which provides an energy storage solution with high energy density and good long-term capacity retention. The use of this organic redox species together with a hydrogen anolyte creates a system that is also resistant to issues of electrolyte cross over and which avoids the use of metallic redox species.

Description

REDOX FLOW BATTERYFIELD OF INVENTION

[0001] The invention relates to redox flow batteries, electrolyte solutions for use in redox flow batteries, uses of electrolytes in redox flow batteries, and kits for redox flow batteries.BACKGROUND

[0002] For all types of energy infrastructure, the cost and security of energy supplies is a critical economic issue worldwide. As the means of producing grid electricity evolves, renewable energy represents a growing fraction. However, solar and wind energy are intermittent resources, and only a few energy storage systems are adaptable to storing gridscale quantities of energy in an economically viable way. The flow battery (FB) design has been considered a scalable design of electrochemical energy storage.

[0003] The traditional FB pumps liquids containing charge storing metallic or halide ions, complexes, or organic molecules through the porous electrodes in the cells. This provides active reactant for the electrochemical redox reactions on the surface of the electrodes inside the cells, for the interconversion of electrical and chemical energy. The positive and negative electrodes are adjacent to the liquid flow channels and are typically made of graphitic carbon felts or cloths, and the two sides are separated by an ion-exchange membrane (IEM) or porous separator in a zero-gap arrangement. This allows the energy storage capacity (C) to scale with the amount of external electrolyte, independent of the cell stack size. FBs rarely require catalytic electrodes due to the reversible nature of many FB electrolytes on thermally or chemically -treated carbon electrode materials. The all-vanadium FB (VFB) is a commonly referenced example, as it is arguably the most mature FB chemistry. The VFB stores and releases energy using the conversion between dissolved V(l l) / V(l 11) salts on the negative side, and V(IV) / V(V) salts on the positive side, both in aqueous sulfuric acid solutions.

[0004] A key challenge is to lower the cost of the electrolyte solutions used in FBs. There are many chemistries other than vanadium that have been demonstrated. Recently, the development of organic energy storage molecules has been seen as a way to reduce the electrolyte cost and provide better price stability, often benchmarked by the price and price volatility of vanadium.

[0005] One issue faced with many organic FBs is the long-term capacity retention. Active material degradation leads to capacity loss. Another cause of long-term degradation is the crossover of the active species to the opposite side of the cell. This can be improved with more selective membrane technology, or mitigated by using a gas-liquid reversible fuel cell(RFC) arrangement, whereby one of the liquid electrolytes is replaced by a gas. Hydrogen is an excellent energy storage medium, with fast and reversible kinetics to form protons when oxidised in acidic environment, using platinum group metals catalysts. However, the round trip efficiency of producing hydrogen in an electrolyser and consuming it in a fuel cell is quite low (<50%) as although the hydrogen evolution / reduction processes occur at low overpotentials, both the water oxidation (in the electrolyser) and oxygen reduction (in the fuel cell) reactions require large overpotentials to proceed at appreciable rates.

[0006] Early development of the H2-X FB showed how the H2 / H+redox couple can work as the redox reaction at the negative electrode in a gas-liquid hybrid FB. Provided the X electrode is at a more positive redox potential, the hydrogen evolution reaction (HER) occurs during charging and the hydrogen oxidation reaction (HOR) occurs during discharge. This arrangement with a V(IV) / V(V) positive redox couple halves the cost of the vanadium electrolyte compared to the VFB and does not self-discharge in the presence of atmospheric oxygen. However, the problem of a rather low limit on the operating temperature (approximately 40 °C) remains. Other H2-X systems include the H2-Br2reversible fuel cell (RFC), in which the positive electrolyte capacity can exceed 200 Ah L1, but toxicity concerns, crossover, and catalyst dissolution are amongst the reasons why the H2-Br2system is still in early stage of development.

[0007] As would be appreciated by the person skilled in the art, the charge density of a flow battery is proportionate to the number of redox species per unit volume present in the system (and the amount of charge capable of being stored by each redox species). In the case of liquid electrolytes, the solubility of the charge carrier in the electrolyte is also a limiting factor - as species not dissolved in the electrolyte cannot typically participate in the electrochemical reaction (unless they are directly attached to an electrode).

[0008] As such, despite the progress made in this area, there is a need for a FB capable of providing: high charge density; long-term capacity retention; and that make use of safe, readily available materials. The invention addresses, or at least ameliorates, at least some of these issues.SUMMARY OF INVENTION

[0009] The invention provides a redox flow battery comprising an anolyte and a catholyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formula (la) to (If)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl;A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl;Wherein ring B is optional; and whereinrepresents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent.

[0010] Also provided as part of the invention is a catholyte comprising the organic redox species of any of formulae (la) to formula (If) for use in a redox flow battery, wherein the redox flow battery comprises a gaseous anolyte.

[0011] Also provided as part of the invention is the use of an organic redox species of any of formulae (la) to formula (If) as a catholyte in a redox flow battery, wherein the redox flow battery comprises a gaseous anolyte.

[0012] Also provided as part of the invention is a redox flow battery comprising an anolyte and a catholyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formula (la) to (li)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl;A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl; wherein ring B is optional; and wherein ■ represents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent; each E is, independently, an electron-donating group; wherein each electron-donating group (E) is independently selected from: -NR3H and -N(R3)2, wherein R3is independently selected from: C1-12 alkyl; C1-12 haloalkyl; C1-12 alkenyl; C1-12 haloalkenyl; or combinations thereof; each m is independently an integer selected from 1 to 4; each Y is, independently, selected from carbonyl or an electron directing group.

[0013] Each of the organic redox species, the redox flow batteries, and the use are as described herein.Definitions

[0014] As will be appreciated by one skilled in the art, a ‘redox flow battery’ comprises an electrochemical cell for the conversion of chemical energy into electricity. A redox flow battery comprises an anode compartment comprising an anode and an anolyte fluid (i.e. a gas or liquid) and a cathode compartment comprising a cathode and a catholyte fluid (i.e. a gas or liquid). A membrane is provided between the two compartments and is configured to exchange ions between the two compartments. In the present disclosure, the anolyte fluid is a gas and the catholyte fluid is a liquid. The redox flow battery may include multiple electrochemical cells, which may be in fluid and / or electrical communication with one another.

[0015] The compartments of electrolyte (catholyte and anolyte) fluid may be charged separately with two different ‘redox active species’ that are each able to undergo reversible reduction-oxidation reactions. This allows the redox active species in one compartment to undergo, for example, an oxidation reaction while the redox active species in the other compartment undergoes a reduction reaction. The redox reactions cause a net flow of electrons between the compartments, thus generating an electrical current.

[0016] In accordance with standard terminology in the field of redox flow batteries, the terms ‘anode and ‘cathode’ are defined by the functions of the electrodes in the power delivery mode. To avoid confusion, the same terms are maintained herein to denote the same electrodes whether in a power deliver mode of operation or an energy storage mode of operation.

[0017] As used herein, the term ‘catholyte’ and ‘anolyte’ refer respectively to an electrolyte solution provided in contact with the ‘cathode’ and ‘anode’ of the redox flow battery.

[0018] The term ‘alkyl’ refers to a straight chain or branched, substituted or unsubstituted (e.g. unsubstituted) group containing from 1 to 40 carbon atoms (optionally from 1 to 20, such as from 1 to 10, such as from 1 to 5, optionally 2 carbon atoms). An alkyl group may optionally be substituted at any position.

[0019] The term ‘electron-donating group’ as used herein refers to a functional moiety (E) that that is capable of donating at least some of its electron density to adjacent species. An electron-donating group typically possess a lone pair of electrons capable of participating in tautomerisation and / or resonance. Often, the electron-donating group is capable of protonation so as to stabilise delocalised electrons. There is no particular restriction on the choice of electron-donating group (E). However, typically the electron donating group is a mesomeric group. The term ‘mesomeric’ is intended to describe a group capable of forming resonance structures (or typically one that participates in a mesomeric effect), often in concert with other groups and bonds (typically n-bonds) of the remainder of the molecule to which it is attached, thereby allowing electrons (often lone pairs of electrons) to be delocalised around the molecule. Typically, the mesomeric group provides a positive mesomeric effect. Often the electron donating group can be protonated to form a positively charged moiety. It may be that each electron-donating group (E) is independently selected from: amino, carboxyl, sulfonate, hydroxy, alkoxy, thioether, heterocyclic moiety, or combinations thereof. Often, each electron-donating group (E) is independently selected from: amino, carboxyl or sulfonate groups. It may be that each electron-donating group (E) is independently an amino group. Often, the amino group will be a secondary or tertiary amino group; usually a tertiary aminogroup. Tertiary amines are particular useful, as a quaternary amine can be formed with the addition of a proton, redistributing electrons across the organic redox species. Typically examples of suitable secondary and tertiary amino groups include, but are not limited to: - NR3H and N(R3)2, wherein R3is independently selected from: C1.12 alkyl; C1.12 haloalkyl; C1-12 alkenyl; C1.12 haloalkenyl; or combinations thereof. Often, R3is C1-12 alkyl and usually R3is Ci.6alkyl; and more typically R3is C1-4 alkyl. It may be that R3is independently selected from: methyl, ethyl or propyl. Usually, R3is independently methyl or ethyl; and it may be that at least one of R3is methyl (though often both R3are methyl).

[0020] The electron-donating properties of several hundred of the most common substituents, reflecting all common classes of substituents have been determined, quantified, and published. The most common quantification of electron-donating properties is in terms of Hammett a values. Hydrogen has a Hammett a value of zero, while other substituents have Hammett a values that increase positively or negatively in direct relation to their electronwithdrawing or electron-donating characteristics.

[0021] Substituents with negative Hammett a values are considered electron-donating. See Lange's Handbook of Chemistry, 12th ed., McGraw Hill, 1979, Table 3-12, pp. 3-134 to 3-138, which lists Hammett a values for a large number of commonly encountered substituents.

[0022] Similarly, the term 'electron-directing group’ as used herein refers to a functional moiety (e.g. D) that is capable of donating or accepting at least some electron density to or from adjacent species, e.g. by mesomeric functions as described above in relation to the ‘electron-donating group’. The definitions and literature reference above for the ‘electrondonating group’ applies to the electron directing group, mutatis mutandis. Typical electronaccepting groups are able to accept single (radical) electron or a lone pair and include imino groups (e.g. iminohydroxy groups).

[0023] The term ‘polymer’ or ‘poly’ when used to qualify a molecule refers to a molecule whose structure comprises multiple repeating units. The molecule may have 5, 6, 7, 8, 9, 10, or more repeat units. The molecule may have many repeat units, such as 100, 1 ,000, 10,000, or more. The term ‘copolymer’ or ‘co-poly’ when used to qualify a molecule refers to a molecule whose structure comprises at least two types of repeating units. The molecule may have 5, 6, 7, 8, 9, 10, or more repeat units of each type. The molecule may have many repeat units of each type, such as 100, 1 ,000, 10,000, or more.

[0024] The term ‘halogen atom’, ‘halo’ or ‘halogen’, refers to a group 7 element of the Periodic Table of the Elements, such as, fluorine, chlorine, bromine and iodine, optionally fluorine or chlorine, optionally fluorine.

[0025] The term ‘carbocyclic compound’ as used herein refer to a saturated or unsaturated cyclic aliphatic or aromatic monocyclic or polycyclic (including fused, bridging and spirofused) ring system which has from 3 to 20 carbon atoms. A carbocyclic compound may have from 3 to 15, such as from 3 to 12, such as from 3 to 10, such as from 3 to 8 carbon atoms, such as from 3 to 6 carbons atoms. Carbocyclic compounds groups may be substituted or unsubstituted, branched or unbranched.

[0026] A ‘heterocyclic compound’ is a carbocyclic compound as described above, which additionally contains one or more heteroatoms. The heterocyclic compound may contain from 1 to 5 heteroatoms, such as from 1 to 4 heteroatoms, such as from 1 to 3 heteroatoms, such as 1 or 2 heteroatoms. Heterocyclic compounds may contain from 4 to 21 atoms, such as from 4 to 16 atoms, such as from 4 to 13 atoms, such as from 4 to 11 atoms, such as from 4 to 9 atoms, such as from 4 to 7 atoms, wherein at least one atom is a carbon atom. Suitable heteroatoms are selected from O, S, N, P and Se. When heterocyclic compounds have two or more heteroatoms, the heteroatoms may be the same or different. Heterocyclic compounds groups may be substituted or unsubstituted, branched or unbranched.

[0027] As used herein, the term ‘optionally substituted’ means that one or more of the hydrogen atoms in the optionally substituted moiety is replaced by a suitable substituent. Unless otherwise indicated, an ‘optionally substituted’ group may have a suitable substituent at each substitutable position of the group, and when more than one position in any given structure may be substituted with more than one substituent selected from a specified group, the substituent may be either the same or different at every position. Combinations of substituents envisioned by this invention are preferably those that result in the formation of stable compounds.

[0028] The term 'stable’, as used herein, refers to compounds that are chemically feasible and can exist for long enough at room temperature (i.e. 16-25°C) to allow for their detection, isolation and / or use in chemical synthesis.

[0029] The term ‘ring’ and ‘ring system’ as used herein, eg with respect to groups A and B, refers to single and fused cyclic species respectively, typically wherein the ring size of each cyclic species is in the range of 4 to 8 carbons inclusive; more typically in the range of 4 to 7 carbons inclusive; and most typically 5 or 6 carbons. It is typically the case that both the ‘ring’and ‘ring system’ are substantially electrically conjugated so as to facilitate the redistribution of electron density across the organic redox species.

[0030] Any of the above groups (for example, those referred to herein as ‘optionally substituted’, including alkyl, aryl and heteroaryl groups) may optionally comprise one or more substituents, preferably selected from silyl, sulfo, sulfonyl, formyl, amino, imino, nitrilo, mercapto, cyano, carboxyl, nitro, halogen, -NCO, -NCS, -OCN, -SCN, -C(=O)NR°R00, -C(=O)X°, -C(=O)R°, -NR°R°°, C1-12 alkyl, C1.12 alkenyl, C1-12 alkynyl, C6-12 aryl, C3-12 cycloalkyl, heterocycloalkyl having 4 to 12 ring atoms, heteroaryl having 5 to 12 ring atoms, C1-12 alkoxy, hydroxy, C1-12 alkylcarbonyl, C1-12 alkoxy-carbonyl, C1.12 alkylcarbonyloxy or C1-12 alkoxycarbonyloxy wherein one or more H atoms are optionally replaced by F or Cl and / or combinations thereof; wherein X° is halogen and R° and R00are, independently, H or optionally substituted C1.12 alkyl. Often, each of R° and R00are independently selected from: H or optionally substituted Ci-6alkyl; more typically H or optionally substituted C1.4 alkyl; and in some cases, each of R° and R00are independently selected from optionally substituted methyl, optionally substituted ethyl, and optionally substituted propyl. Often, R° and R00are independently selected from H, methyl or ethyl. The optional substituents may comprise all chemically possible combinations in the same group and / or a plurality of the aforementioned groups (for example amino and sulfonyl if directly attached to each other represent a sulfamoyl radical).

[0031] The term ‘alkenyl’ or ‘vinyl’ refers to an unsubstituted or substituted alkyl group comprising from 2 to 40 carbon atoms (optionally from 2 to 20, such as from 2 to 10, such as from 1 to 5, such as from 2 to 5, optionally 2 carbon atoms) that comprises, in the straight or branched hydrocarbon chain, one or more carbon-carbon double bonds.

[0032] The term ‘alkynyl’ refers to an unsubstituted or substituted alkyl group [comprising from 2 to 40 carbon atoms (optionally from 2 to 20, such as from 2 to 10, such as from 2 to 5, optionally 2 carbon atoms)] that comprises a straight or branched hydrocarbon chain comprising one or more carbon-carbon triple bonds.

[0033] The term ‘carbonyl’ refers to an unsubstituted or substituted -C(O)RAgroup, wherein RAis hydrogen, or an alkyl, alkenyl or alkynyl group.

[0034] The term ‘ester’ refers to an unsubstituted or substituted -C(O)ORB(C-linked ester) or -OCORB(O-linked ester) group, wherein RBis an alkyl, alkenyl or alkynyl group.

[0035] The term ‘amide’ refers to an unsubstituted or substituted -C(O)NRC2(C-linked amide) or -NRcCORD(N-linked amide) group, wherein each Rcand / or RDare, independently, hydrogen, or an alkyl, alkenyl or alkynyl group.

[0036] The term ‘ether’ refers to an unsubstituted or substituted -OREgroup, wherein REis or an alkyl, alkenyl or alkynyl group.

[0037] The term ‘amine’ refers to an unsubstituted or substituted -NRF2group, wherein RFis or an alkyl, alkenyl or alkynyl group.

[0038] The term ‘alkyl’, ‘aryl’, ‘heteroaryl’, etc. also include multivalent species, for example alkylene, arylene, ‘heteroarylene’ etc. Examples of alkylene groups include ethylene (-CH2- CH2-) and propylene (-CH2-CH2-CH2-).

[0039] As used herein, references to moieties comprising substituents may be depicted structurally, for example as shown below with respect to substituent ‘(E)m’, in which the substituent may be attached any one of the available substituent positions on the moiety. The use of wavy bonds herein (see below by way of example) also signifies how said moiety may be attached, optionally through one or more additional moieties, to a larger structure:

[0040] Whilst the specification may make reference to the invention “comprising” one or more features, it is also contemplated that the invention may “consist” or “consist essentially of” said one or more features.

[0041] It is often the case that R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl. For example, R1may be hydrogen or optionally substituted Ci-i2alkyl; usually R1is hydrogen or optionally substituted Ci.ealkyl; and more typically R1is hydrogen or optionally substituted Ci-4alkyl. It may be that R1is: hydrogen, optionally substituted methyl, optionally substituted ethyl or optionally substituted propyl. Usually, R1is hydrogen, optionally substituted methyl or optionally substituted ethyl; and it may be that R1is hydrogen or optionally substituted methyl (though often R1is hydrogen).DETAILED DESCRIPTION

[0042] There is provided, in a first aspect of the invention, a redox flow battery comprising a catholyte and an anolyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formulae (la) to (If)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl; A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionallysubstituted heteroaryl; wherein ring B is optional; and wherein represents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent.

[0043] It has surprisingly been found by the inventors that: not only do the organic redox species of the invention provide excellent long-term capacity retention but the combination ofthese organic redox species with a gaseous anolyte makes possible a redox flow battery possessing surprisingly high charge density. In particular, the organic redox species of the invention can be provided in stable electrolyte solutions at concentrations much higher than many traditional organic redox species. Moreover, the redox potential of these species is highly compatible with gaseous anolytes (such as the versatile H2 / H+redox half cell) which ameliorates issues of crossover.

[0044] It is typically the case that the redox species has a formulae according to (la), (lb) or (Ic). Often, the redox species has a formulae according to (la) or (lb). In some situations, the redox species has a formulae according to (la). It may be that the redox species has a formulae according to (lb). It may be that the redox species has a formulae according to (Ic).

[0045] As one skilled in the art would appreciate, ‘crossover’ refers to the mixing of electrolytes (ie catholyte and anolyte) that penetrate through the membrane separating the anode and cathode compartments. This reduces the efficiency of redox flow batteries as redox species are rendered inactive. Electrolyte crossover can lead to a gradual and irreversible decrease of battery performance.

[0046] By employing a gaseous electrolyte as one of the anolyte or catholyte, this issue can be ameliorated. In the event that a gaseous anolyte crosses over to the neighbouring cathode compartment, it will be appreciated that separation of the gaseous anolyte from a liquid catholyte can be achieved easily (eg simply by tapping off the gaseous anolyte, eg from an upper part of the anode compartment). Similarly, in the event that liquid catholyte crosses over into the anode compartment, the liquid can simply be pumped out of the anode compartment (e.g. from a lower part of an cathode compartment). The redox flow battery may be suitably configured to enable such tapping and / or pumping.

[0047] It is often the case that the organic redox species is substantially electrically conjugated, as this facilitates delocalisation to electrons across the organic redox species more easily. However, it is not essential for the organic redox species to be completely electrically conjugated in order for an oxidised form of the organic redox species to be stabilised. In some cases, the organic redox species is electrically conjugated.

[0048] Typically, at least one of A and B is independently a ring or ring system comprising an optionally substituted aryl. Often, each of A and B is independently a ring or ring system comprising an optionally substituted aryl. Often, each of A and B is independently a ring or ring system comprising an aryl.

[0049] It is typically the case that each of A and B is independently a ring or ring system comprising an optionally substituted aryl or optionally substituted heteroaryl, wherein said optionally substituted aryl or optionally substituted heteroaryl is substituted with at least one electron-donating group (E). Typically, at least one of A and B is independently a ring or ring system comprising an optionally substituted aryl, wherein said optionally substituted aryl is substituted with at least one electron-donating group (E). Typically, each of A and B is independently a ring or ring system comprising an optionally substituted aryl, wherein said optionally substituted aryl is substituted with at least one electron-donating group (E). Typically, at least one of A and B is independently a ring or ring system comprising an aryl, wherein said aryl is substituted with at least one electron-donating group (E). Typically, each of A and B is independently a ring or ring system comprising an aryl, wherein said aryl is substituted with at least one electron-donating group (E).

[0050] Without being bound by theory, it is thought that the presence of one or more electrondonating groups (E) stabilises the distribution of electrons across the organic redox species. This better enables the organic redox species to exist in a stable state, in both reduced and oxidised forms, ensuring reliable charging and discharging behaviour of the redox flow battery. It is typically the case that the flow battery of the invention is configured such that the organic redox potential (ie of the catholyte) is positive compared to the gaseous anolyte.

[0051] As one skilled in the art would appreciate, in use, the organic redox species alternates between one or more reduced forms (eg when the redox flow battery is discharged) and one or more oxidised forms (eg when the redox flow battery is charged). It is typically the case that the redox flow battery is charged when the gaseous anolyte is in a reduced state; and wherein the redox flow battery is discharged when the gaseous anolyte is in an oxidised state.

[0052] Typically, the reduced forms of the organic redox species possess two additional electrons compared with the oxidised forms. It is also often the case that the reduced form comprises at least one additional proton compared with the oxidised form. For example, with reference to the substituted phenothiazine methylene blue (3,7- bis(dimethylamino)phenothiazine), several different configurations are available based on the redox state of the redox flow battery (and the conditions in the redox flow battery), some of which are depicted below, including a reduced form (R-MB) and an oxidised form (MB):Scheme 1 . showing a range of protonation states of methylene blue in both oxidised (MB), 1+ radical, and reduced forms (R-MB)

[0053] As such, reference herein to specific structures, such as those of formulae (la) to (If), also embraces the various reduced and oxidised forms thereof as would be understood by one skilled in the art. Likewise, the resonance forms and tautomers of both oxidised and reduced forms are also encompassed by the structures described herein.

[0054] Wherein one or both of A or B represents a ring system, ie a series of fused cyclic species, it may be that a combination of optionally substituted aryl and optionally substituted heteroaryl species are provided as part of the same fused cyclic species. Examples of typical ring systems include, but are not limited to: indolyl, naphthalenyl, anthracenyl, tetracenyl or phenanthracenyl (each of which may be optionally substituted). Often, A and B each independently represent a single cyclic species. Typically, A and B are each independently selected from: phenyl, cyclopentadienyl, pyrryl, furyl pyridyl, pyrenyl, pyrryl, thiophenyl, thiazolyl or combinations thereof (each of which may be optionally substituted). It may be that A and B are each independently selected from: phenyl, cyclopentadienyl, pyrryl, furyl, thiophenyl, thiazolyl or combinations thereof (each of which may be optionally substituted). Typically, A and B are each independently selected from: phenyl, cyclopentadienyl, pyrryl, furyl, thiophenyl or combinations thereof (each of which may be optionally substituted). It may be that A and B are each independently selected from: phenyl, cyclopentadienyl, pyrryl or combinations thereof (each of which may be optionally substituted). Often, A and B are each independently selected from: phenyl, cyclopentadienyl or combinations thereof (each of which may be optionally substituted).

[0055] It is often the case that at least one of A or B is a ring ie a single cyclic species. It is typically the case that each of A and B is independently a ring ie a single cyclic species.

[0056] It is typically the case that A and B are substantially electrically conjugated, as this facilitates delocalisation to electrons across the organic redox species more easily.

[0057] It may be that at least one of A and B comprises a group according to optionally substituted formula (Ila) or optionally substituted formula (lib)wherein each E is, independently, an electron-donating group; m is an integer selected from 1 to 4; R2is independently selected from: hydrogen, optionally substituted Ci-i2alkyl; optionally substituted C1-12 haloalkyl; optionally substituted Ci-i2alkenyl and optionally substituted Ci-i2haloalkenyl; and m’ is an integer selected from 1 to 2. It is often the case that m’ is 1. Often, R2is hydrogen or optionally substituted Ci-i2alkyl; usually R2is hydrogen or optionally substituted Ci.6alkyl; and more typically R2is hydrogen or optionally substituted C1-4 alkyl. It may be that R2is: hydrogen, optionally substituted methyl, optionally substituted ethyl or optionally substituted propyl. Usually, R2is hydrogen, optionally substituted methyl or optionally substituted ethyl; and it may be that R2is hydrogen or optionally substituted methyl (though often R2is hydrogen). Wavy bonds indicate locations of attachment to the remainder of the molecule. In the event that there are any unfilled valencies (e.g. in the context of Formula (If), where only one wavy bond is needed for attachment), these are selected from hydrogen or E.

[0058] Often at least one of A and B comprises a group according to formula (Ila) or formula (lib). Typically, at least one of A and B comprises a group according to optionally substituted formula (lib). It may be that at least one of A and B comprises a group according to formula (lib). In some cases A and B each independently comprise a group according to optionally substituted formula (lib); and it may be that A and B each independently comprise a group according to formula (lib). It may be that A and B are each independently selected from optionally substituted formulae (Illa), (lllb) and (lllc):(Illa) (lllb) (lllc)Typically, that A and B are each independently selected from formulae (Illa), (lllb) and (lllc).

[0059] Where B is not present, a double bond will typically be present in its place, for instance as shown below in formula (Ig) and formula (Ih):wherein each of formulae (If) and (Ig) may be optionally substituted. It may be that, where B is not present, the organic redox species is a compounds according to optionally substituted formulae (II) or (Ij):whereinA, X, X’ and E are as defined herein; and n is an integer of 1 or 2; typically wherein n is 1.

[0060] Whilst it may be the case that ring or ring system B is optional, it is typical that each of A and B are present. As such, where A and B are identical, the compounds of formula (la), (lb) and (Ic) possess a rotational symmetry about the axis defined, as is the case for formula (la), through both X and X’. Whilst A and B may be different, it is often the case that A and B are the same.

[0061] It may be that, with respect to formula (la), each of X and X’ are independently selected from: NR1, O or S. It may be that each of X and X’ are independently NR1or S. Often, X is NR1. It may be the case that X’ is S. Typically, X is NR1; and X’ is S.

[0062] Further, it may be that, with respect to formula (lb), X is NR1, O or S; more typically X is NR1or O; and often X is NR1.

[0063] It may be that, with respect to formula (Ic), X is NR1, O or S; more typically X is NR1or O; and often X is NR1.

[0064] Regarding the redox species according to formula (Id), one or more electron donating group (E) may be provided as a substituent. It may be that, with respect to formula (Id), X is NR1, O or S; more typically X is NR1or O; and often X is NR1.

[0065] Alternatively, it may be that the organic redox species is a compound according to optionally substituted formula (IV)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is as defined herein; wherein each E is, independently, an electron-donating group; and wherein each m is independently an integer selected from 1 to 4.

[0066] Typically, each m is independently an integer selected from 1 to 3; more typically 1 to 2; and often m is 1. It is typically the case that, where multiple electron-donating groups are provided together, the electron-donating groups are spaced in a meta- or para- configuration with respect to adjacent electron donating groups; usually a meta- configuration is adopted. As one skilled in the art would appreciated, the positioning of the electron-donating group may vary so as to enable a suitable mesomeric effect, such that stabilising resonance forms are created within the organic redox species.

[0067] Typically, in formula (IV), each of X and X’ are independently selected from: NR1, O or S. It may be that each of X and X’ are independently NR1or S. Often, X is NR1. It is may be the case that X’ is S.

[0068] It may be that the organic redox species is an optionally substituted phenothiazines according to formula (V)(phenothiazine) (V)comprising one or more electron-donating groups (E), wherein each of ‘E’ and ‘m’ are as described herein.

[0069] It has been found that optionally substituted phenothiazines comprising one or more electron-donating groups are particularly suited for use redox flow batteries. In particular, not only do they possess a useful electrochemical potential that can be conveniently paired with other half-cells, but these species are stable in both reduced and oxidised forms and are especially soluble in concentrated acid solutions, capable of being dissolved at concentrations higher than many existing systems (often without the need of any additional co-solvent).

[0070] The organic redox species may possess an electrochemical potential in the range 0.2 V to 1.2 V (with respect to the reversible hydrogen electrode, RHE); in some case 0.2 V to 1.0 V. It is often the case organic redox species will possess an electrochemical potential in the range 0.2 V to 0.8 V (with respect to the reversible hydrogen electrode, RHE); in some case 0.3 V to 0.7 V; and more typically 0.4 V to 0.6 V. Typically, the electrochemical potential of the organic redox species will be greater than or equal to 0.5 V (with respect to RHE).

[0071] Typically, the organic redox species is selected from: optionally substituted 3,7- bis(dimethylamino)phenothiazine; optionally substituted [7-(dimethylamino)-4- nitrophenothiazin-3-ylidene]-dimethylazine; optionally substituted N',N'-dimethylphenothiazin- 5-ium-3,7-diamine; optionally substituted 3,7-Bis(diethylamino)phenoxazine; optionally substituted thionine; or combinations thereof. Often, the organic redox species comprises an optionally substituted 3,7-bis(dimethylamino)phenothiazine. Usually, the organic redox species comprises 3,7-bis(dimethylamino)phenothiazine.3,7-bis(dimethylamino)phenothiazine

[0072] Whilst these compounds are described with respect to their free-base form, it will be appreciated by one skilled in the art that each of these compounds will often be provided as a salt thereof. As explained elsewhere in the description, there is no particular limitation on the choice of counterion used with the organic redox species of the invention. However, it may be that the organic redox species is selected from: methylene blue; methylene green, azureA, basic blue 3, or combinations thereof. Typically, it may be that the organic redox species is methylene blue (ie the salt of 3,7-bis(dimethylamino)phenothiazine, often sold commercially as the chloride salt) which comprises a 3,7-bis(dimethylamino)phenothiazine cation and an accompanying anion (Z).methylene blue

[0073] It has been found by the inventors that methylene blue is a particularly useful catholyte, especially when paired with a hydrogen anolyte. The electrochemical potential of methylene blue is 0.52V (with respect to RHE). Methylene blue can also be dissolved at high concentrations, especially in concentrated acid solutions, creating a highly energy dense flow battery.

[0074] As one skilled in the art would appreciate, where the organic redox species is provided as a salt, there is no particular limitation on the choice of counterion. Typically, counterions will be chosen that do not interfere with the redox behaviour of the redox flow battery, that do not react with the separator material and that improve the conductivity of the electrolyte. Typical examples of suitable counterions (ie Z) include, but are not limited to: halides (such as chloride and fluoride), perchlorate, sulfate, bisulfate, phosphate, acetate, or combinations thereof. Of these, It is often the case that Z is perchlorate sulfate, bisulfate, phosphate, acetate, or combinations thereof. It may be that the Z is perchlorate, sulfate, bisulfate, phosphate or combinations thereof; and often it will be the case that Z is perchlorate or sulfate.

[0075] There is provided, in another aspect of the invention, a redox flow battery comprising an anolyte and a catholyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formulae (la) to (li)(lb) (la)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl;A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl;wherein ring B is optional; and wherein - represents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent; each E is, independently, an electron-donating group; wherein each electron-donating group (E) is independently selected from: -NR3H and -N(R3)2, wherein R3is independently selected from: C1-12 alkyl; C1-12 haloalkyl; C1-12 alkenyl; C1-12 haloalkenyl; or combinations thereof; each m is independently an integer selected from 1 to 4; each Y is, independently, selected from carbonyl or an electron directing group.

[0076] Features discussed in relation to the aspect having an organic redox species which is a compound according to any of formula (la) to (If) apply equally to the aspect having an organic redox species being a compound according to any of formulae (la) to (li), mutatis mutandis. X and X’ may be NR1(e.g. for formula (li)).

[0077] The organic redox species may be a compound according to any of formulae (Iga) to (Ila)whereinEach R3is independently selected from: C1-12 alkyl; C1-12 haloalkyl; C1-12 alkenyl; C1-12 haloalkenyl; or combinations thereof;Z is an electron directing group.Z may be an optionally substituted imino (e.g. hydroxyimino). Each R3may independently be C1.12 alkyl, e.g. methyl, ethyl or propyl. Each R3may independently be methyl. R3may be as defined elsewhere herein.

[0078] The organic redox species may be selected from: optionally substituted 3,7- bis(dimethylamino)phenothiazine; optionally substituted [7-(dimethylamino)-4- nitrophenothiazin-3-ylidene]-dimethylazine; optionally substituted N',N'-dimethylphenothiazin- 5-ium-3,7-diamine; optionally substituted 3,7-Bis(diethylamino)phenoxazine; optionally substituted 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid; optionally substituted chlorpromazine (3-(2-chlorophenothiazin-10-yl)- / V, / V-dimethylpropan-1 -amine); optionally substituted thionine; optionally substituted N,N,N',N'-tetramethyl-p-phenylenediamine; optionally substituted N,N,N’,N’-tetramethylbenzidene; optionally substituted violuric acid (5- (hydroxyimino)pyrimidine-2,4,6(1 H,3H,5H)-trione); or combinations thereof.

[0079] It is usually the case that the gaseous anolyte is hydrogen. Not only does the use of hydrogen, as a gaseous anolyte, mitigate many of the problems of crossover; but the energy density of the hydrogen side of the flow battery is limited only by the pressure under which the hydrogen is stored. Accordingly, the combination of methylene blue and hydrogen represents a highly energy dense flow battery that also avoids the use of metallic redox species.

[0080] The redox flow battery typically comprises a separator. The separator creates a permeable barrier between the anode and cathode compartments and permits the movement of specific charge carriers across the membrane. There is no particular restriction upon the choice of membrane and one skilled in the art would be familiar with many typically separators suitable in such applications. Often, the separator is selectively permeable. Typically the separator is either an ion-exchange membrane or a porous membrane. Where an ionexchange membrane is employed, it will often be the case that this is a proton-exchange membrane. Where a porous membrane is used, these will typically be a microporous separator. Typical examples of suitable membranes include, but are not limited to: sulfonated ion-exchange membranes (eg Nation™ or a sulphonated polyether-ether ketone), polyphosphazine membranes, 4, 4 '-diamine-3, 3'-dimethyl-biphenyl Trager’s Base membranes (DMBP-TB), polybenzimidazole (PBI) materials; or combinations thereof. It may be that thatthe separator is an ion-exchange membrane, such as a sulfontated ion-exchange membrane. Alternatively, the separator may be a porous membrane, such as a PBI material. Porous separators are often used in the present invention, as any crossover occurring through the inadvertent transit of a redox species through a pore can be addressed by way of the heterophasic nature of the anolyte and the catholyte.

[0081] There is no particular limitation on the choice of electrodes employed in the redox flow battery of the invention. It may be that one or both of the electrodes includes an electrocatalyst. It may be that only one of the electrodes comprises an electrocatalyst. For example, it is often the case that the anode comprises an electrocatalyst to facilitate the hydrogen evolution and oxidation reactions (HER / HOR), eg an electrocatalyst such as platinum. Alternatively, neither electrode may include an electrocatalyst. The electrodes may be graphitic, amorphous, or have a glassy structure. Often, the electrodes will be graphitic (such as, a carbon felt, carbon paper or carbon cloth electrode).

[0082] Usually the catholyte comprises an acid. Typically, the acid is a strong acid; and often the acids at least water miscible. Usually the acids are water soluble. One skilled in the art would be familiar with the term ‘strong acid’. However, typically a strong acid possesses a pKa of less than or equal to 3; more typically, less than or equal to 2; even more typically less than or equal to 1 ; and often the acid has a pKa of less than 0. Strong acids are often used as they substantially dissociate protons in aqueous media. Acids suitable for use in the invention typically include, but are not limited to: acetic acid, oxalic, phosphoric acid, hydrochloric acid, sulfuric acid, perchloric acid, methylsulfonic, hydrofluoric, hydrobromic acid, hydrogen iodide, trifluoromethane sulfonic acid, nitric acid, benzene sulfonic acid or combinations thereof. Typical examples of acids suitable for use in the invention include, but are not limited to: oxalic acid, phosphoric acid, hydrochloric acid, sulfuric acid, perchloric acid, or combinations thereof. Of these, phosphoric acid, hydrochloric acid, sulfuric acid, perchloric acid, and combinations thereof are often employed. It may be that the acid is selected from hydrochloric acid, sulfuric acid, or combinations thereof. Often, the acid comprises sulfuric acid. The use of strong acids, such as those outlined herein, are useful as they produce solutions with a higher conductivity than many other acids.

[0083] The concentration of acid is typically greater than or equal to about 2M; or often greater than or equal to 3.5M or often greater than or equal to 4M. More typically, the concentration of acid is greater than or equal to about 5M; than greater than or equal to about 6M; and in some situations greater than or equal to about 7M. Typically, the acid is provided as an aqueous solution.

[0084] The catholyte may have a pH of at most about 6, optionally at most about 5, optionally at most about 4, optionally at most about 3, optionally at most about 2, optionally at most about 1, optionally at most about 0. The catholyte may have a pH of at least about 0, optionally at least about 0.5, optionally at 10 least about 1 , optionally at least about 1.5, optionally at least about 2, optionally at least about 2.5.

[0085] The catholyte may comprise one or more additional additives including, but not limited to: solubility enhancers (to improve the ability of the catholyte solution to dissolve the organic redox species); conductivity enhancers to increase the electrical conductivity of the catholyte solution; pH buffers to resist changes in the pH of the catholyte solution; or combinations thereof. Examples of typical conductivity enhancers often used include: chlorides, sulfates, bisulfates, phosphates, acetates or combinations thereof. One skilled in the art would be familiar with the types of buffers, such as appropriate conjugate bases, compatible with the electrolytes used in the redox flow battery of the invention.

[0086] The inventors have found that the organic redox species of the invention can be dissolved at high concentrations in concentrated acid solutions whilst retaining stability. As one skilled in the art will appreciate, the catholyte typically comprises the maximum amount of organic redox species capable of dissolving in the catholyte solution. As such, the precise concentration of the organic redox species present in the catholyte varies; and it may be that a slight excess of organic redox species is provided so as to ensure saturation of the catholyte with the organic redox species (across a range of operational conditions). However, it may be that the concentration of organic redox species is in the range of equal to or greater than about 0.5 mol dm-3; more typically equally to or greater than about 0.75 mol dm-3; even more typically equally to or greater than about 1.0 mol dm-3; and often equal to or greater than 1.2mol dm-3. Often, the concentration of organic redox species in the catholyte is in the range of 0.5 mol dm-3to 5.0 mol dm-3; more typically in the range of 0.5 mol dm-3to 3.0 mol dm-3.

[0087] In particular, the organic redox species dissolves well, and remains stable, even in strong acid at concentrations equal to or above about 3.5M. As such, very little (if any) additional solvent is required to fully solubilise large amounts of the organic redox species. This creates a redox flow battery with surprisingly high charge density.

[0088] Another benefit of the invention is that the redox flow battery can be operated at temperatures higher than many convention redox flow batteries. Typically, the temperature of the redox flow battery is in the range of 10°C to 100°C; more typically 20°C to 90°C; even more typically 30°C to 80°C; and often in the range of 40°C to 70°C. Often the temperature of the redox flow battery is equal to or greater than 50°C; and often equal to or greater than60°C. Being able to operate the redox flow battery at higher temperature can result in improved efficiency of the charging and discharging operations. Moreover, many conventional flow battery chemistries become unstable at higher temperatures as the charge carries can breakdown and / or precipitate out of solution.

[0089] The pressure of the hydrogen in the electrochemical system is not especially limited. It may be the case that the redox flow battery stores hydrogen at one pressure in a dedicated storage unit (such as a storage tank); and uses hydrogen at another pressure within the electrochemical cell. Typically, during operation with the electrochemical cell, the pressure of hydrogen is in the range about 1 bar to about 10 bar; more typically about 1 bar to about 6 bar; more typically still about 1 bar to about 3 bar. Often, the pressure of hydrogen within the electrochemical cell will be about 1 bar. Where hydrogen is stored in redox flow battery in compartment at a different pressure, the pressure is not especially limited. However, typically, the hydrogen storage pressure is in the range of about 1 bar to 100 bar; more typically in the range of about 1 bar to 30 bar; more typically about 3 bar to about 25 bar; even more typically about 6 bar to about 20 bar; and more typically still about 10 bar to about 15 bar. Often the storage pressure of the hydrogen will be calibrated based on the energy density of the catholyte.

[0090] The redox flow battery may be a reversible flow battery configured to operate in a power delivery mode in which it generates electrical power by the reaction of redox active species; and in an energy storage mode in which it consumes electrical power to generate said redox active species. In a power delivery mode, a redox active species is oxidised at the anode and a redox active species is reduced at the cathode to form reacted (or "spent") redox active species. In the energy storage mode, electrochemical system is reversed and the "spent" catholyte species is electrochemically oxidised at the cathode to regenerate the corresponding active species.

[0091] A reversible redox flow battery has conduits both for supplying redox active species to the electrodes for the power delivery phase, and also for conducting the spent redox active species to a store, such as one or more storage vessels (e.g. one for spent anolyte and another for spent catholyte) so that it can be regenerated. Often the redox active species will be in the form of electrolyte that is exhausted following a power delivery phase and, in this case, conduits may be arranged to conduct exhausted (or spent) electrolyte to a store and supply it back to its half-cell during an energy storage mode, e.g. by the use of appropriate pumps. In contrast, fuel cells are not set up to operate in energy storage mode to electrochemically replenish exhausted electrolyte. In the case of RFC having a half-cell containing a gaselectrode, a compressor is generally provided to compress gas generated during the energy storage mode to enable it to be collected in a compressed gas storage tank for future power delivery phases. In contrast a fuel cell will generally not have such a compressor.

[0092] The redox flow battery may include one or more vessels configured to contain the liquid catholyte and / or gaseous anolyte containing spent redox active species, which one or more vessels are connectable, in the power delivery mode, to one or more conduits for receiving the catholyte containing spent redox active species from the cathode and / or anode compartment.

[0093] The redox flow battery may include a pressurised gas source vessel (e.g. configured to contain hydrogen), which gas source is connectable, in the power delivery mode, to the anode. The pressurised gas source vessel may be connectable, in the energy storage mode, to the anode to receive gas (e.g. hydrogen) generated in the energy storage mode.

[0094] The redox flow battery may include at least one compressor configured to pressurise gas generated at the anode in the energy storage mode for storage in the pressurised gas source vessel, and optionally also a gas expander-generator to deliver electricity as a result of expansion of the compressed gas. The battery can operate without a compressor, provided the gas storage tank is sufficiently large to accommodate the generated gas. The redox flow battery may comprise a means for circulating the hydrogen gas through the conduits between the storage vessel and the anode compartment, e.g. a pump or a fan. The redox flow battery may also additionally include a dryer or humidifier which dries (or humidifies) the hydrogen gas before it is stored in the source vessel or supplied to the cell. The redox flow battery may also be equipped with a hydrogen expander-generator to deliver electricity or improve hydrogen circulation within the flow battery as a result of compressed gas expansion.

[0095] It will be appreciated that the redox reactions involving hydrogen will not produce any "spent" species at the gas anode in the power delivery mode as the redox active hydrogen species is transformed into protons that are dissolved in the electrolyte. Protons are passed by the membrane separating the anode and cathode compartments from the anode side of the membrane to the cathode side of the membrane. The electrons produced during the oxidation of the hydrogen gas at the anode during the power delivery mode are collected by a current collector. However, any unreacted hydrogen gas may be transferred away from the anode compartment by one or more conduits and returned to a gas source vessel (which may be pressurised or unpressurised). In the energy storage mode, protons are selectively passed by the membrane separating the anode and cathode compartments from the cathode side ofthe membrane to the anode side of the membrane and protons are reduced at the anode to regenerate the hydrogen gas, which forms the anode redox active species.

[0096] There is also provided, in a second aspect of the invention, a catholyte solution as defined in the first aspect of the invention. The catholyte solution comprises an organic redox species as defined in the first aspect of the invention. It is often the case that the catholyte solution comprises an acid as described in the first aspect of the invention.

[0097] The concentration of acid is typically greater than or equal to about 2M; or often greater than or equal to 3.5M; or often greater than or equal to 4M. More typically, the concentration of acid is greater than or equal to about 5M; than greater than or equal to about 6M; and in some situations greater than or equal to about 7M. Typically, the acid is provided as an aqueous solution.

[0098] The catholyte solution may comprise one or more additional additives including, but not limited to: solubility enhancers (to improve the ability of the catholyte solution to dissolve the organic redox species); conductivity enhancers to increase the electrical conductivity of the catholyte solution; pH buffers to resist changes in the pH of the catholyte solution; or combinations thereof. Examples of typical conductivity enhancers often used include: halides, perchlorates, sulfates, bisulfates, phosphates, acetates or combinations thereof. One skilled in the art would be familiar with the types of buffers, such as appropriate conjugate bases, compatible with the electrolytes used in the redox flow battery of the invention.

[0099] Typically, the catholyte solution comprises an organic redox species according to formula (V) of the first aspect of the invention; and an acid as defined in the first aspect of the invention.

[0100] Usually, the compound of formula (V) is an optionally substituted methylene blue (a salt of 3,7-bis(dimethylamino)phenothiazine); and often the organic redox species comprises methylene blue (a salt of 3,7-bis(dimethylamino)phenothiazine).

[0101] Typically, the acid is selected from hydrochloric acid, sulfuric acid, or combinations thereof; and often, the acid comprises sulfuric acid. It may also be the case that the concentration of acid is greater than or equal to about 3.5M; or often greater than or equal to 4M. More typically, the concentration of acid is greater than or equal to about 5M; than greater than or equal to about 6M; and in some situations greater than or equal to about 7M.

[0102] It was considered surprising by the inventors that the organic redox species of the invention can be dissolved in the catholyte at such high concentrations in concentrated acid.Moreover, that such compositions were stable and achieved a correspondingly high energy density.

[0103] There is also provided in a third the use of the organic redox species as defined in the first aspect of the invention in a catholyte in a redox flow battery according to the first aspect of the invention.DESCRIPTION OF FIGURES

[0104] The invention is also described with respect to the following figures:

[0105] Figure 1: (a) cyclic voltammetry (CV) at a 50 mV s-1scan rate for 1 mmol methylene blue (MB) in Ar-sat. H2SO4solutions, and (b) the absorbance in absorbance units (a.u.) of 10 pM MB in water and H2SO4solutions.

[0106] Figure 2: Koutechy-Levich analysis - (a) IR-free and background-corrected RDE voltammetry profiles with 10 mV s1scan rate of 1 mM MB / Argon-sat. 1 M H2SO4solutions, (b) Levich plot of the limiting current vs the square root of the rotation rate between 400-2500 rpm, (c) Koutechy-Levich plot for the inverse of the RDE current at different overpotentials between 2.5 40 mV, and (d) Tafel plot of the estimated linear region of the absolute kinetic current density vs overpotential.

[0107] Figure 3: 10 mM cycling tests; membrane appearance after 10 cycles; and polymer membrane structure for an ion-exchange membrane (IEM) of: (a) Nation™ 212; (b) 4,4'- diamine-3,3'-dimethyl-biphenyi Trager’s Base (DMBP-TB); and (c) polybenzimidazole (PBI).

[0108] Figure 4: (a) figures of merit for 0.1 M MB RFC galvanostatic cycling at 50 mA cm2, (b) polarisation curves of 0.1 M and 1.0 MB (PA = phosphoric acid).

[0109] Figure 5. CVs of 1 mM 2-dimethylaminothianthrene in acetonitrile at 100 mV / s as described in Example 6.

[0110] Figure 6. CVs of 1 mM 2,6-bis(dimethylamino)thianthrene in acetonitrile at 100 mV s1as described in Example 6.

[0111] Figure 7. CVs of 1 mM ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) in acetonitrile at a scan rate of 100 mV s-1as described in Example 6.

[0112] Figure 8. CVs of 1 mM N-hydroxyphthalimide (NHPI) in acetonitrile at a scan rate of 100 mV s '1as described in Example 6.

[0113] Figure 9. CVs of 1 mM thianthrene in acetonitrile at a scan rate of 100 mV s-1as described in Example 6.

[0114] Figure 10. CVs of 1 mM 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate (ABTS), [7- (dimethylamino)-4-nitrophenothiazin-3-ylidene]-dimethylazine (MG), N,N,N',N'-tetramethyl-p- phenylenediamine (WB), N,N,N',N'-tetramethylbenzidine (TMB) and violuric acid (VIO) at a scan rate of 50 mV s1as described in Example 7.

[0115] Figure 11. RFC test of 5 mM 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate (ABTS) in 100 mL electrolyte cycled galvanostatically at 5 mA cm2.

[0116] Figure 12. RFC test of 5 mM N,N,N',N'-tetramethylbenzidine (TMB) in 100 mL electrolyte cycled galvanostatically at 5 mA cm-2.

[0117] Figure 13. RFC test of 5 mM chlorpromazine (CPZ) in 100 mL electrolyte cycled galvanostatically at 5 mA cm2.

[0118] Figure 14. RFC test of 5 mM violuric acid (VIO) in 100 mL electrolyte cycled galvanostatically at 5 mA cm2.EXAMPLESExample 1 - Chemicals and Materials Characterisation

[0119] Methylene blue (MB) chloride (Thermo Fisher Scientific) was used as received. Electrolyte solutions were prepared from 95% H2SO4(VWR) and 18.2 MQ cm ultrapure water from a Sartorius purification system. MB solution samples and RFC electrolytes were prepared by dissolving the appropriate mass of MB in 6M H2SO4. Rotating disk electrode (RDE) measurements were performed a using a polished glassy carbon disk of 5 mm diameter as the working electrode and an RDE rotor (Pine Instruments), with a saturated calomel reference electrode (SCE) and a graphite rod counter electrode in a 3-compartment cell. Voltammograms were recorded on an Autolab PGSTAT302N at a scan rate of 10 mV s1using freshly prepared 1 mol dm3H2SO4solutions containing 1 mmol dm-3solutions of MB anolyte, which were purged with Argon (Air Products, BIP Plus N6.6) prior to each measurement. For the solubility determination, 10 mmol samples of MB were stirred for at least 24 h with H2SO4solutions of various strengths at room temperature, syringe-filtered through a 0.2 pm PTFE membrane (Puradisc 25 TF, Whatman) to remove undissolved solids from the electrolyte, and the filtrate diluted using ultrapure water to within the range of calibration where the Beer- Lambert law was applicable.Example 2 - Preparation of DMBP-TB Membrane

[0120] A mixture of 4,4’-diamino-3,3’-dimethylbiphenyl (5.000 g, 23.5 mmol) in dimethoxymethane (DMM, 10.5 mL, 118 mmol) was cooled to 0°C and trifluoroacetic acid wasadded dropwise (TFA, 40 mL). The resulting mixture was warmed to room temperature and left stirring for 96 h. The resulting viscous solution was then poured into stirred aqueous ammonium hydroxide solution. The precipitate was filtered and washed with excess water, methanol and acetone. Purification was achieved by repeated precipitation by the slow addition of a chloroform solution of polymer into hexane. The purified polymer (4.51g, 77%) was dried overnight under vacuum. Thick self-supported DMBP-TB membranes were fabricated by solution casting of a polymer solution in chloroform using a doctor blade followed by solvent evaporation at room temperature.Example 3 - Membrane Electrode Assembly (MEA)

[0121] Nation™ 212 was pre-treated by soaking the membrane in 5% H2O2for 1 h at 80°C. This was repeated with ultrapure pure water, then 1 M H2SO4. DMBP-TB membranes were pre-treated in 1 M H2SO4overnight before use. PBI membranes (Celtec™, BASF) were used as received. The positive-side electrode used a layer of 4.6 mm carbon felt (Sigracell™), which was oxygen plasma treated (Diener™) before assembling the MEA to remove surface impurities from the electrode felt. For the H2-side, a 190 pm, 0.4 mgPtcm2electrode (ELE0201 , Johnson Matthey™ Fuel Cells) was used as received. The electrodes, membranes, and gaskets (Tygaflor™) were compressed with 4.0 Nm applied torque in a 5 cm2flow cell fixture (Scribner Associates) to produce a -20% electrode compression relative to the original electrode thicknesses.Example 4 - RFC Testing

[0122] Charge-discharge experiments and polarisation curves were recorded using a Scribner 857 RFB test station. Galvanostatic charge-discharge cycles were performed using a 5 cm2cell at ±50 mA cm-2to the maximum possible depth-of-discharge within cell voltage limits of 0.9 V and 0.3 V. The MEA was prepared as described above, specifically using a water doped Celtec™ PBI membrane. During polarisation the discharge current was increased stepwise. The high frequency resistance was analysed using a 10 mV amplitude at a constant frequency of 5 kHz. All measurements were conducted at room temperature. Hydrogen gas was produced from an electrolyser (60H-FUEL Hydrogen Generator, Parker™) flowed through the cell at a rate of 100 mL min-1(1 bar), set using a H2mass-flow controller (El-Flow Select, Bronkhorst). The H2humidity of the RFC hydrogen inlet line was measured at room temperature using a dew point transmitter (Optidew Dew-point Transmitter, Michell Instruments). During polarization experiments the H2relative humidity was set at 98-100% by flowing through a humidification column (Perma Pure MH-110-12S-2) before the cell inlet. Thepositive electrolyte was flowed at a constant rate of 50 mL min'1and without any protection from air.Example 5 - Characterisation of Methylene Blue(MB) / H2in H2SO4

[0123] The preliminary electrochemical characterisation of MB in sulfuric acid showed that the reaction was sufficiently reversible to warrant its application in the RFC, and like many organic molecules, it exhibited a two-electron energy storage capacity per molecule. The two- electron transfer occurs in a single reversible wave in 1 M, 3 M and 5 M H2SO4with a 29 mV peak-to-peak separation as shown in Figure 1 (a). The half-wave potential at 0.525 V vs RHE in 1 M H2SO4 corresponds to the H2-MB cell voltage.

[0124] The peak at 664 nm in water (DI water) displayed in Figure 1 b is attributed to the unprotonated molecular absorbance. The peak appearing at 748 nm has been attributed to the monoprotonated form, which is dominant in concentrations above 1 M H2SO4. This implies that in strong acid, the positive electrolyte cycles between the MB and reduced (R-MB) structures shown in Scheme 2. Diprotonation of R-MB buffers the proton efflux and influx during cell charging and discharging respectively.Scheme 2: (a) Positive-side and (b) negative-side H2-MB cell reactions

[0125] The kinetic rate constant k3for electrochemical MB reduction on glassy carbon (Figure 2) was found to be 0.044 cm s1and the diffusion coefficient Dois 2.5x106cm2s1. The kinetic rate constant of the electron transfer reaction is therefore high, and the diffusion coefficient is of similar magnitude to other reported organic and metallic FB electrolytes.

[0126] There are many different cell chemistries already that can be classified as H2-X RFCs. These include, but are not limited to, vanadium, BQDS, iron, cerium and bromine. Of the intrinsic power values plotted here, the most comparable to MB are from other H2-X systems. The faster redox kinetics and highervolumetric energy density of the MB electrolyte presented here gives a higher Pmtrinsic compared to VO27VO2+, and the H2-MB system indeed outperforms early iterations of the H2-V RFC in terms of the peak power density achieved at 100% SOC. The MB positive electrolyte also has a higher intrinsic power than a 1 M Fe"71" electrolyte. IronRFCs typically use chloride and sulfate-based counterion / electrolyte formulations. Iron sulfate (1.4 M Fe) and iron chloride (0.9 M Fe) in a comparable H2-Fe RFC system have achieved slightly lower peak power densities (147 and 207 mW cm-2respectively) than the H2-MB system.

[0127] Organic redox couples often undergo a two-electron per molecule redox switch. Basic blue 3 (BB3) is another dye molecule with similar structural features to MB. The biphenol molecule BPTS can achieve a high peak power of approximately 290 mA cm'2in an FB with a cell potential of 0.905 V. Examples of quinones included here are BDQS, DHBS, and DHDMBS. Quinone derivatives are prevalent candidates for low pH organic FB electrolytes.

[0128] Maintaining good transport of H+above all other ions through the membrane is important in the H2-X RFC arrangement. The transport of H+from the positive electrolyte provides reactants for the HER, and the transference number of H+should be close to one. In order to determine the stability of polymer electrolytes in the redox medium, the performances of three different membrane materials (Nation™, DMBP-TB and PBI) were tested in a 10 mM H2-MB RFC cell which was cycled through ten charge discharge cycles as shown in Figure 3. Nation™ is the brand name of a synthetic polymer with the structure shown in Figure 3 (a), and Nation™ lEMs have been used in low-temperature proton-exchange membrane (PEM) fuel cells, as well as the conventional choice of membrane in the H2-X RFC systems. DMBP-TB is an intrinsically microporous polymer which has been applied in redox flow batteries, fuel cells, and nanofiltration applications. PBI is a thermoplastic polymer that forms an IEM that is resistant to high temperature and an oxidising environment, with applications including high- temperature PEM fuel cells, which typically operate in the range 120-180°C. Like DMBP-TB, the charge functional groups in acidic conditions are protonated amine groups that allow proton transport.

[0129] After cycling, the appearance of the membranes all showed staining by the electrolyte due to the very high molar absorption coefficient of the dye. In the tests with DMBP-TB and PBI there was no apparent capacity loss over ten charge / discharge cycles. In the case of the Nafion™-containing MEA test, up to 22% (44 pmol cm2) may have been lost to impregnation of the MB / R-MB into the membrane, as the final concentration of the electrolyte was determined to be only 7.8 mM. As can be seen in Figure 3 (a), the discharge profiles lead to a good coulombic efficiency, and the cell failure occurs during charge, characterised by a high overpotential indicating H+transport from the negative electrode is being impeded. MB molecules in the widest dimension are 1 .43 nm on average. This is on the same order as the hydrophilic channels in Nation™, and fouling from a ‘plugging’ effect is possible. DMBP-TBshows good resistance to the acidic organic electrolyte but could not maintain long-term physical integrity in the liquid-gas arrangement. The phosphoric acid (PA) doped Celtec™ membrane (RBI) not only shows comparable performance to DM BP TB, it also showed better structural physical integrity after prolonged use.

[0130] An MEA containing water-doped Celtec™ PBI membrane was assembled, and a 0.1 M H2-MB was cycled at 50 mA cm'2through 100% capacity usage of the electrolyte. As shown in Figure 4, the round-trip energy efficiency remained over 76% and there was no observed capacity loss in this time period.

[0131] Sulfuric acid is an inorganic acid that was used as the supporting electrolyte. The solubility of MB in in sulfuric acid solutions of various ionic strength was investigated. If an insufficient amount was added to a certain amount of MB powder, the solid and the sulfuric acid solution mixed to become a wetted paste-like solid, often with a gold-coloured lustre, from which a solution and residue phase were indistinguishable. The balance of solvent added to form a liquid phase without totally dissolving the methylene blue was found using a constant stoichiometric ratio of approximately 1.8-2.0 H2SO4:MB. The ionic conductivity of sulfuric acid solutions peaks at 0.82 S cm1at 25 °C, at approx. 4 M H2SO4. This is high compared to acetic acid solutions, which peaks at under at 0.019 S cm1. We found that the solubility of MB in sulfuric acid solutions can reach at least 1.22 mol dm3, with a molality of just over 0.9 mol kg-1. This is equivalent to a single-tank capacity of 65.40 Ah L-1. Considering both the MB-H2cell potential and the capacity, the theoretical positive electrolyte energy density of the positive electrolyte solution is 34.3 Wh L’1, which is greater than for 1 .0 M VIV / Vin a H2-V system. The slight decrease in the voltage (and energy) efficiency during long-term cycling is predominantly due to a gradual increase in the overpotential during charging. This was not observed for a symmetric 1.2 M MB-MB cell cycled at 200 mA cm2, in which the charging profile only became slightly higher over time. The two steps in capacity at cycle #15 and #25 is associated with transferring crossover electrolyte from the gas side back to the liquid side. In the H2-MB cell, the charging overpotential was greater when the RFC was tested at 1.0 M concentration.

[0132] Other organic molecules that have been investigated include the tricyclic molecule thianthrene and two amine-functionalized derivatives, showing that the redox activity of the substructure is retained. We also show ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) have a reversible electrochemical redox processes in 1 M sulfuric acid at a potentials of 0.79 V and 1.30 V vs SHE respectively, and it is therefore is shown that ABTS, NHPI, and their derivative structures are viable acidic flow battery electrolyte candidates.Example 6 - Characterisation of redox species for application in redox flow batteries

[0133] Cyclic voltammetry (CV) of 2-dimethylaminothianthrene was performed at 100 mV s1cyclic using a 1mM concentration in 0.1 M TABP (Tetra-n-butylammonium hexafluorophosphate) in acetonitrile, containing 3A molecular sieves (Figure 5). The potential was swept over increasing potential windows with the third and final scan used for each potential range, which increased between a -0.5 V vs AgNO3reference electrode lower limit to upper limits of 0.5, 0.7, 0.9 and 1.1 V vs AgNO3. The cyclic voltammograms (CVs) were then calibrated to the Fc / Fc+potential and background CVs of 0.1 M TABP in dry acetonitrile were subtracted. CVs were performed in a single-compartment cell using an AgNO3reference electrode containing 0.1 M AgNO3in acetonitrile filling solution. A platinum wire was used as the counter electrode and the working electrode was a 3 mm diameter glassy carbon electrode.

[0134] The CVs of 2,6-bis(dimethylamino)thianthrene were recorded analogously to 2- dimethylaminothianthrene, but without background correction (Figure 6).

[0135] The CVs of ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonicacid) were recorded analogously to 2-dimethylaminothianthrene (Figure 7).

[0136] The CVs of N-hydroxyphthalimide (NHPI) were recorded analogously to 2- dimethylaminothianthrene, but without background correction (Figure 8).

[0137] Cyclic voltammetry of thianthrene was performed at 100 mV s1cyclic using a 1mM concentration in 0.1 M TABP (Tetra-n-butylammonium hexafluorophosphate) in acetonitrile, containing 3A molecular sieves between -0.5 - 1 .8 V vs AgNO3(Figure 9). The CVs were then background-corrected and calibrated to the Fc / Fc+potential.Example 7 - Chemicals and Materials Characterisation

[0138] Electrochemical characterisation data was obtained using a 5 mm polished glassy carbon disk as the working electrode in a rotor (Pine instruments), a graphite rod counter electrode, and a saturated calomel electrode (SCE) reference or an in-house made reversible hydrogen electrode (RHE) in a 3-compartment cell. All results were recorded on a Autolab PGSTAT302N and corrected to the RHE scale. Rotating disk electrode (RDE) measurements were recorded at a scan rate of 10 mV s-1. All RDE data was IR and background corrected. Cyclic voltammograms were measured at a scan rate of 50 mV s’1and background corrected. Diammonium 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), N-hydroxyphthalimide and chlorpromazine hydrochloride (Sigma Aldrich), N,N,N',N'-tetramethylbenzidine (ThermoScientific Chemicals) and violuric acid (Alfa Aesar) were used as received. Aqueous electrolyte solutions of were prepared using 95% H2SO4(Normapur, VWR) or 60% HCIO4(VWR) diluted with 18.2 MQ ultrapure water from a Sartorius system. All 2- and 3-electrode measurements were performed at room temperature.

Claims

CLAIMS1 ) A redox flow battery comprising an anolyte and a catholyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formulae (la) to (If)N - 8(If) whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl;A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl; wherein ring B is optional; and wherein:represents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent.2) The redox flow battery according to claim 1 , wherein the organic redox species is a compound according to any of formula (la) or (Ic).3) The redox flow battery according to claim 1 or 2, wherein at least one of A and B is independently a ring or ring system comprising an optionally substituted aryl.4) The redox flow battery according to any of claims 1 to 3, wherein A and B are each independently a ring or ring system comprising an optionally substituted aryl.5) The redox flow battery according to claim 1 or claim 2, wherein A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl, wherein said optionally substituted aryl or an optionally substituted heteroaryl is substituted with at least one electron-donating group (E).6) The redox flow battery according to claim 5, wherein at least one of A and B is independently a ring or ring system comprising an optionally substituted aryl, wherein said optionally substituted aryl is substituted with at least one electron-donating group (E).7) The redox flow battery according to any preceding claim, wherein at least one of A and B comprises a group according to optionally substituted formula (Ila) or optionally substituted formula (lib)(Ila) (Hb) wherein each E is, independently, an electron-donating group; R2is independently selected from: hydrogen, Ci-i2alkyl; C^2haloalkyl; C1.12 alkenyl and C1-12 haloalkenyl; m is an integer selected from 1 to 4; and m’ is an integer of 1 to 2.8) The redox flow battery according to any preceding claim, wherein the organic redox species is a compound according to optionally substituted formula (IV)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se; wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl; wherein each E is, independently, an electron-donating group; and wherein each m is independently an integer selected from 1 to 4.9) The redox flow battery according to any of claims 5 to 8, wherein E is a mesomeric group.10) The redox flow battery according to any of claims 5 to 9, wherein E is a secondary or tertiary amine.11) The redox flow battery according to any preceding claim, wherein each of X and X’ are independently selected from: NR1, O or S.12) The redox flow battery according to any preceding claim, wherein X is NR1; and X’ is S.13) The redox flow battery according to any preceding claim, wherein the organic redox species is an optionally substituted phenothiazines according to formula (V)comprising one or more electron-donating groups (E), wherein each electron-donating group (E) is independently selected from: -NR3H and -N(R3)2, wherein R3is independently selected from: Ci-i2alkyl; CM2haloalkyl; Ci-i2alkenyl; Ci-i2haloalkenyl; or combinations thereof; and wherein each m is independently an integer selected from 1 to 4.14) The redox flow battery according to any preceding claim, wherein the organic redox species is selected from: optionally substituted 3,7-bis(dimethylamino)phenothiazine; optionally substituted [7-(dimethylamino)-4-nitrophenothiazin-3-ylidene]-dimethylazine; optionally substituted N',N'-dimethylphenothiazin-5-ium-3,7-diamine; optionally substituted3,7-Bis(diethylamino)phenoxazine; optionally substituted thionine; or combinations thereof.15) The redox flow battery according to any preceding claim, wherein the organic redox species comprises an optionally substituted 3,7-bis(dimethylamino)phenothiazine.16) The redox flow battery according to any preceding claim, wherein the organic redox species comprises methylene bluemethylene blue wherein Z is a counterion (such as halide, perchlorate, sulfate, bisulfate, phosphate, acetate, or combinations thereof).17) The redox flow battery according to claims 1-15, wherein the organic redox species comprises an optionally substituted 3,7-bis(dimethylamino)-4-nitrophenothiazine.18) The redox flow battery according to claim 17, wherein the organic redox species comprises methylene greenmethylene green wherein Z is a counterion (such as halide, perchlorate, sulfate, bisulfate, phosphate, acetate, or combinations thereof).19) A redox flow battery comprising an anolyte and a catholyte; wherein the anolyte is gaseous; and wherein the catholyte comprises an organic redox species or salt thereof, wherein the organic redox species is a compound according to any of formulae (la) to (li)whereinX and X’ are each independently selected from: C(R1)2, NR1, O, S or Se; with the proviso that at least one of X or X’ is selected from: NR1, O, S or Se;wherein R1is each independently selected from: hydrogen, halogen or optionally substituted alkyl;A and B are each independently a ring or ring system comprising an optionally substituted aryl or an optionally substituted heteroaryl; wherein ring B is optional; and wherein represents a double bond when ring B is absent for formulae (la) to (le) or is hydrogen for formula (If) when ring B is absent; each E is, independently, an electron-donating group; wherein each electron-donating group (E) is independently selected from: -NR3H and -N(R3)2, wherein R3is independently selected from: C1-12 alkyl; C1-12 haloalkyl; C1-12 alkenyl; C1-12 haloalkenyl; or combinations thereof; each m is independently an integer selected from 1 to 4; each Y is, independently, selected from carbonyl or an electron directing group.20) The redox flow battery of claim 19, wherein each substituent independently is as defined in any one of claims 2 to 18.21 ) The redox flow battery of claim 19 or 20, wherein each E is as defined in any one of claims 9 or 10.22) The redox flow battery of any one of claims 19 to 21 , wherein the organic redox species is a compound according to any of formulae (Iga) to (lia)(iga) (lha)whereinEach R3is independently selected from: C1-12 alkyl; C1-12 haloalkyl; C1-12 alkenyl; C1-12 haloalkenyl; or combinations thereof;Z is an electron directing group; optionally wherein Z is an optionally substituted imino (e.g. hydroxyimino) and / or each R3is independently C1-12 alkyl (e.g. methyl, ethyl or propyl), optionally wherein each R3is methyl.23) The redox flow battery of any one of claims 19 to 22, wherein Z is an optionally substituted imino, e.g. hydroxyimino.24) The redox flow battery of any one of claims 19 to 22, wherein each R3is independently a C1-12 alkyl, e.g. methyl, ethyl or propyl.25) The redox flow battery of any one of claims 19 to 22, wherein each R3is methyl, ethyl or propyl, optionally methyl.26) The redox flow battery according to claim 19, wherein the organic redox species is selected from: optionally substituted 3,7-bis(dimethylamino)phenothiazine; optionally substituted [7-(dimethylamino)-4-nitrophenothiazin-3-ylidene]-dimethylazine; optionally substituted N',N'-dimethylphenothiazin-5-ium-3,7-diamine; optionally substituted 3,7- Bis(diethylamino)phenoxazine; optionally substituted 2,2'-azino-bis(3- ethylbenzothiazoline-6-sulfonic acid; optionally substituted chlorpromazine (3-(2- chlorophenothiazin-10-yl)- / V, / \ / -dimethylpropan-1 -amine); optionally substituted thionine; optionally substituted N,N,N',N'-tetramethyl-p-phenylenediamine; optionally substituted N,N,N’,N’-tetramethylbenzidene; optionally substituted violuric acid (5- (hydroxyimino)pyrimidine-2,4,6(1 H,3H,5H)-trione); or combinations thereof.27) The redox flow battery according to any preceding claim, wherein the gaseous anolyte is hydrogen.28) The redox flow battery according to any preceding claim, wherein the catholyte comprises an acid (optionally a strong acid).29) The redox flow battery according to claim 28, wherein the acid is selected from: phosphoric acid, hydrochloric acid, sulfuric acid, perchloric acid, and combinations thereof (optionally wherein the acid comprises sulfuric acid).30) The redox flow battery according to any of claims 28 to 29, wherein the concentration of the acid is greater than or equal to about 4M.31) The redox flow battery according to any preceding claim further comprising a separator.32) A catholyte as defined in any preceding claim.33) Use of the organic redox species as defined in any of claims 1 to 31 in a catholyte in the redox flow battery according to any of claims 1 to 31.34) A kit comprising: a redox flow battery as defined in any of claims 1 to 31; a catholyte according to claim 32; and a gaseous anolyte (optionally hydrogen).35) A redox flow battery, kit of parts, catholyte or use of an organic redox species substantially as described herein with reference to the accompanying description and figures.

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