Methods of enhancing cycling stability of redox flow batteries

Abstract

The invention provides a flow battery, the battery comprising a catholyte section comprising a catholyte tank, having a cathode, a catholyte storage compartment and a catalytic column, an anolyte section comprising an anolyte tank, having an anode, and an anolyte storage compartment, an anion-exchange membrane and a current collector, wherein the flow battery suffers from irreversible side-reactions during charging or discharging in the anolyte section; and the catalytic column is configured to house a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator in an oxidised state. The invention also provides a method of improving a cycling stability of a flow battery, the method comprising the steps of providing the flow battery, subjecting the battery to a plurality of charge and discharge cycles and restoring / preventing a loss of capacity by subjecting the liquid catholyte to an oxygen evolution reaction (OER).

Description

[0001] METHODS OF ENHANCING CYCLING STABILITY OF REDOX FLOW BATTERIES

[0002] FIELD OF INVENTION

[0003] The present invention provides redox flow batteries, more particularly redox flow batteries with enhanced cycling stability. The present invention also provides methods of enhancing cycling stability of redox flow batteries.

[0004] BACKGROUND

[0005] The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0006] Energy utilization underpins human progress, yet the fossil fuel-dominated energy system poses serious sustainability challenges. Renewable resources such as wind and solar power have been widely deployed worldwide, but their intermittency and unpredictability cause fluctuations in electricity generation. These variations jeopardize grid stability and often lead to curtailment, underscoring the urgent need for large-scale energy storage solutions. Electrochemical energy storage (EES) technologies offer a viable approach, with aqueous redox flow batteries (ARFBs) standing out for their safety, scalability, and operational flexibility arising from the decoupling of energy and power.

[0007] The vanadium redox flow battery (VFB), reported in 1985, represents the most mature ARFB system and has been successfully demonstrated in practice. However, its commercialization remains constrained by high material costs, limited energy density, and moderate thermal stability. To overcome these limitations, zinc-based flow batteries (ZFBs) have attracted considerable interest. They offer higher cell voltages, low toxicity, and the use of earth- abundant, cost-effective materials.

[0008] Despite these advantages, ZFBs face intrinsic challenges at the zinc anode. Repeated plating and stripping often lead to non-uniform deposition and dendrite formation, particularly under high current densities and deep cycling. Detached dendrites accumulate as “dead zinc,” causing irreversible capacity loss and shortened cycle life. Although strategies such as electrolyte additives, electrode engineering, and advanced membranes have improved zinc utilization and cycling stability, the persistence of dead zinc remains a bottleneck. Consequently, practical ZFBs often operate at limited capacity and low depth of discharge, leaving substantial room for further optimization. Nicola Poli and co-workers reported a method to restore the battery energy and capacity of a Vanadium redox flow battery, by counteracting the charge imbalance caused by air-oxidation and hydrogen evolution in the negative electrolyte (Nicola Polia, Michael Schafferc, Andrea Trovda, Jens Noackc, Massimo Guarnieri, Peter Fischerc. Novel electrolyte rebalancing method for vanadium redox flow batteries. Chemical Engineering Journal 405 (2021 ) 126583).

[0009] Teresa P'aez and co-workers reported an approach consists of a simple electrochemical charge balancing protocol, which allows water splitting to take place in one compartment to compensate the charges consumed in the opposite compartment by the undesired reactions in the aqueous organic redox flow battery (Teresa P'aez, Alberto Martinez-Cuezva, Rebeca Marcilla, Jesus Palma, Edgar Ventosa. Mitigating capacity fading in aqueous organic redox flow batteries through a simple electrochemical charge balancing protocol. Journal of Power Sources 512 (2021) 230516).

[0010] Thus, there is a need for alternative and / or improved methods for enhancing the cycling stability of redox flow batteries for solving at least the above-mentioned problems.

[0011] SUMMARY

[0012] Aspects and embodiments of the current invention will now be described by reference to the following numbered clauses.

[0013] 1 . A flow battery, the battery comprising : a catholyte section comprising a catholyte tank, having a cathode, a catholyte storage compartment and a catalytic column, where the catholyte tank and the catholyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte tank to the catholyte storage compartment and back to the catholyte tank, and where the catholyte storage compartment and catalytic column are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte storage compartment to the catalytic column and back to the catholyte storage compartment; an anolyte section comprising an anolyte tank, having an anode, and an anolyte storage compartment, where the anolyte tank and the anolyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid anolyte from the anolyte tank to the analyte storage compartment and back to the anolyte tank; an anion-exchange membrane disposed between the catholyte and anolyte tanks that allows anions to move from the catholyte tank to the anolyte tank; and a current collector attached to the catholyte and anolyte tanks, wherein: the flow battery suffers from irreversible side-reactions during charging or discharging in the anolyte section; and the catalytic column is configured to house a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state.

[0014] 2. The battery according to Clause 1 , wherein:

[0015] (a) the catholyte section further comprises: a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; and a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state, where the catalyst is housed in the catalyst column; and

[0016] (b) the anolyte section further comprises: a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator.

[0017] 3. The battery according to Clause 2, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.

[0018] 4. The battery according to Clause 3, wherein the ions in the salts that provide ions are selected from: hydroxide ions and / or chloride ions; and one or more of the groups consisting of ammonium ions, lithium ions, sodium ions, potassium ions, magnesium ions, calcium ions, optionally wherein the ions in the salts that provide ions are sodium ions and hydroxide ions.

[0019] 5. The battery according to Clause 3 or Clause 4, wherein the solvent is water.

[0020] 6. The battery according to any one of Clauses 3 to 5, wherein one or more of the following apply:

[0021] (a) the pH of the electrolyte is from 11 to 15;

[0022] (b) the concentration of the one or more compounds or salts that provide ions in the solvent is from 0.05 to 10 M, such as from 1 to 5 M, such as about 3 M. 7. The battery according to any one of Clauses 2 to 6, wherein the cathodic redox mediator is selected from one or more of the group consisting of [Fe(CN)6]37[Fe(CN6)]4', [MnO4]27 [MnO4]’, Ce3+ / 4+, Mn2+ / 3+, halogens, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the cathodic redox mediator is [Fe(CN)6]3- / [Fe(CN6)]4.

[0023] 8. The battery according to any one of Clauses 2 to 7, wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

[0024] 9. The battery according to any one of Clauses 2 to 8, wherein the anodic redox mediator is selected from one or more of the group consisting of [Zn(OH)4]27Zn, DHPS / DHPS- 2H,Zn / Zn2+, V2+ / 3+, Cr2+ / 3+, polyoxometalate (POM), viologen based molecules, quinone based molecules, phenazine-based molecules, alloxazine-based molecules and metal complexes (e.g. iron (III) triethanolamine / iron (II) triethanolamine).

[0025] 10. The battery according to Clause 9, wherein when the anodic redox mediator includes a redox couple that provides a metal deposition, then it also includes a further redox mediator that does not deposit a metal to prevent the build-up of said metal (e.g. when the aniodic redox mediator includes [Zn(OH)4]27Zn or Zn / Zn2+, then the redox mediator also includes DHPS / DHPS-2H).

[0026] 11. The battery according to any one of Clauses 2 to 10, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

[0027] 12. The battery according to any one of Clauses 2 to 10, wherein the catalyst in the catalyst column is selected from one or more of the group consisting of NiFe(OH)2@Ni, lrC>2, RuC>2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.

[0028] 13. A method of improving a cycling stability of a flow battery, the method comprising the steps of: (a) providing a flow battery that suffers from irreversible side-reactions during charging or discharging in the anolyte section, which comprises: a catholyte section comprising a catholyte tank, having a cathode and a catholyte storage compartment, where the catholyte tank and the catholyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte tank to the catholyte storage compartment and back to the catholyte tank, and a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; an anolyte section comprising an anolyte tank, having an anode, and an analyte storage compartment, where the anolyte tank and the analyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid anolyte from the anolyte tank to the analyte storage compartment and back to the anolyte tank, and a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator; an anion-exchange membrane disposed between the catholyte and anolyte tanks that allows anions to move from the catholyte tank to the anolyte tank; and a current collector attached to the catholyte and anolyte tanks

[0029] (b) subjecting the battery to a plurality of charge and discharge cycles; and

[0030] (c) either:

[0031] (i) restoring a loss of capacity by subjecting the liquid catholyte to an oxygen evolution reaction (OER) on a periodic basis such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER; or

[0032] (ii) preventing a loss of capacity by continuously subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is reduced converted to a reduced state by the OER such that no reduction in capacity is observed; or

[0033] (iii) preventing a loss of capacity by intermittently subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER, where the intermittent period is sufficient to prevent a reduction in capacity.

[0034] 14. The method according to Clause 13, wherein restoring the loss of capacity on a periodic basis is achieved through the application of a voltage to the battery sufficient to initiate an OER.

[0035] 15. The method according to Clause 13, wherein the catholyte section further comprises a catalytic column, where the catholyte storage compartment and catalytic column are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte storage compartment to the catalytic column and back to the catholyte storage compartment, such that: step (c)(i) of Clause 13 is conducted by periodically passing the entire liquid catholyte through the catalyst column to conduct an oxygen evolution reaction to restore the lost capacity; or step (c)(ii) of Clause 13 is conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity; or step (c)(iii) of Clause 13 is conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity.

[0036] 16. The method according to any one of Clauses 13 to 15, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.

[0037] 17. The method according to Clause 18, wherein the ions in the salts that provide ions are selected from: hydroxide ions and / or chloride ions; and one or more of the groups consisting of ammonium ions, lithium ions, sodium ions, potassium ions, and magnesium ions, calcium ions, optionally wherein the ions in the salts that provide ions are sodium ions and hydroxide ions.

[0038] 18. The method according to Clause 16 or Clause 17, wherein the solvent is water.

[0039] 19. The method according to any one of Clauses 13 to 18, wherein one or more of the following apply:

[0040] (a) the pH of the electrolyte is from 0 to 15 (e.g. the pH of the electrolyte is from 0 to 5, from 6 to 8 (e.g. 7), or from 9 to 15, e.g. from 1 1 to 15); and

[0041] (b) the concentration of the one or more compounds or salts that provide ions in the solvent is from 0.05 to 10 M, such as from 1 to 5 M, such as about 4 M.

[0042] 20. The method according to any one of Clauses 16 to 20, wherein the cathodic redox mediator is selected from one or more of the group consisting of [Fe(CN)6]37[Fe(CN6)]4', [MnC>4]27 [MnC i]', Ce3+ / 4+, Mn2+ / 3+, halogens, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the anode redox mediator is [Fe(CN)6]37[Fe(CN6)]4‘.

[0043] 22. The method according to any one of Clauses 16 to 21 , wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

[0044] 23. The method according to any one of Clauses 16 to 22, wherein the anodic redox mediator is selected from one or more of the group consisting of [Zn(OH)4]27Zn, DHPS / DHPS- 2H,Zn / Zn2+, V2+ / 3+, Cr2+ / 3+, polyoxometalate (POM), viologen based molecules, quinone based molecules, phenazine-based molecules, alloxazine-based molecules and metal complexes (e.g. iron (III) triethanolamine / iron (II) triethanolamine), optionally wherein when the anodic redox mediator includes a redox couple that provides a metal deposition, then it also includes a further redox mediator that does not deposit a metal to prevent the build-up of said metal (e.g. when the anodic redox mediator includes [Zn(OH)4]27Zn or Zn / Zn2+, then the redox mediator also includes DHPS / DHPS-2H).

[0045] 24. The method according to any one of Clauses 16 to 23, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

[0046] 25. The method according to any one of Clauses 16 to 24, wherein the catalyst in the catalyst column is selected from one or more of the group consisting of NiFe(OH)2@Ni, lrO2, RUO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.

[0047] BRIEF DESCRIPTION OF DRAWINGS

[0048] FIG. 1 is a schematic illustration of the operation of a redox-flow cell with a separate OER catalyst column connected with the catholyte tank according to an embodiment of the invention.

[0049] FIG. 2 depicts CV curves of [Zn(OH)4]27Zn and [Fe(CN)6]374- in 3 M NaOH solution. The working electrode is glassy carbon, the counter electrode is Pt plate and reference electrode is Hg / HgO. The scan rate is 50 mV / s.

[0050] FIG. 3 depicts (a) galvanostatic cycling performance of a capacity balanced AZIFB at 80% SOC under 50 mA / cm2. (b) The corresponding voltage profiles of the 100th, 149thand 150thcycle.

[0051] FIG. 4 depicts (a) galvanostatic cycling performance of a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. The battery was cycled with a cutoff voltage of 2.2 V for the first 146 cycles and then increased to 2.5 V for the following cycles, (b) Galvanostatic cycling performance of a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. The battery was cycled with a cutoff voltage of 2.5 V for all the cycles.

[0052] FIG. 5 depicts (a) galvanostatic cycling performance of a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. The catholyte was flow through the catalytic column after capacity decay for few hours and then stop flow through the catalyst column and reused for the AZIFB. (b) Galvanostatic cycling performance of a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. The catholyte was flow through the catalytic column for each cycle. Cutoff voltage is 2.2 V for both working modes.

[0053] DESCRIPTION

[0054] The present inventors have developed a method to enhance the cycling stability of redox flow batteries. This is achieved through the oxygen evolution reaction (OER) occurring at the cathodic side, which can be initiated directly on the electrode or within the external cathodic tank through redox-mediated reactions. As demonstrated in the Examples, with the incorporation of the OER process at the cathodic side, this significantly enhances the cycling stability of capacity balanced Zn-Fe flow batteries, particularly at high states of charge (SOC). In the case of direct OER on the electrode, the OER process is initiated on the electrode after the conversion of all [Fe(CN)6]4' to [Fe(CN)6]3'. For the case of redox-mediated OER process, the OER process is triggered in the cathodic tank with the assistance of OER catalysts after the complete conversion of [Fe(CN)6]4' to [Fe(CN)6]3-. The accumulated [Fe(CN)6]3' is efficiently consumed during the cycling process. This approach is also applicable to other capacity balanced batteries experiencing water / oxygen-induced side reactions, encompassing both OER and hydrogen evolution reaction (HER).

[0055] Thus, in a first aspect of the invention, there is provided a flow battery (100), the battery comprising: a catholyte section (102) comprising a catholyte tank (104), having a cathode, a catholyte storage compartment (106) and a catalytic column (108), where the catholyte tank 104) and the catholyte storage compartment (106) are fluidly connected to one another by a fluid pathway (110), so as to facilitate the circulation of a liquid catholyte (1 12) from the catholyte tank (104) to the catholyte storage compartment (106) and back to the catholyte tank (104), and where the catholyte storage compartment (106) and catalytic column (108) are fluidly connected to one another by a fluid pathway (114), so as to facilitate the circulation of a liquid catholyte (1 12) from the catholyte storage compartment (106) to the catalytic column (108) and back to the catholyte storage compartment (106); an anolyte section (116) comprising an anolyte tank (1 18), having an anode, and an anolyte storage compartment (120), where the anolyte tank (1 18) and the anolyte storage compartment (120) are fluidly connected to one another by a fluid pathway (122), so as to facilitate the circulation of a liquid anolyte (124) from the anolyte tank (1 18) to the analyte storage compartment (120) and back to the anolyte tank (1 18); an anion-exchange membrane (126) disposed between the catholyte (104) and anolyte tanks (118) that allows anions to move from the catholyte tank (104) to the anolyte tank (118); and a current collector attached to the catholyte (104) and anolyte tanks (118), wherein: the flow battery (100) suffers from irreversible side-reactions during charging or discharging in the anolyte section (1 16); and the catalytic column (108) is configured to house a catalyst (128) capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state.

[0056] It is believed that the battery arrangement and methodology disclosed herein allow for the continued use of a flow battery cell, that would otherwise become unusable due to irreversible side reactions occurring in the anolyte side of the flow battery, leading to the consumption (full oxidation) of the cathodic redox mediator. The mechanism of this is described below for a flow battery where [Zn(OH)4]27Zn and [Fe(CN)6]3 / 4' are used as anodic and cathodic redox mediators, respectively. While this consumption of the cathodic redox mediator can be rectified by replacement of the liquid catholyte, this may not always be practical or desirable. A practical way to increase the lifetime of the flow battery would be to increase the volume of the liquid catholyte so that there is either no need to change it over the expected operational lifetime of the battery, or to drastically reduce the number of liquid catholyte exchanges required. However, this makes the battery significantly bulkier and also increases material costs. However, the flow battery disclosed herein obviates the need to replace the catholyte manually by the incorporation of a catalyst column that allows for the regeneration of the cathodic redox mediator and keep the balanced charge state of the catholyte and anolyte. This allows for stable operation of the flow battery over a significantly expanded number of cycles without the need to replace the liquid catholyte. This means that the amount of liquid catholyte that is required can be reduced, which results in increased energy density and reduces the material costs for the flow battery as a whole. Thus, the use of the catalyst column allows for a longer- term operation of the flow battery, which provides for the advantages mentioned above. In addition, the use of a separate catalyst column allows for a redox-mediated oxygen evolution reaction (OER) to be conducted using a lower charge voltage, thereby avoiding a high voltage electrochemical reaction and this helps to protect the electrode and increases energy efficiency. Moreover, the redox-mediated OER process gradually consumes the excess OH“ generated by the anodic parasitic hydrogen evolution reaction, thereby stabilizing the electrolyte pH throughout cycling without noticeable fluctuation. In addition, by balancing electrolyte capacity through the redox-mediated OER, the flow battery can operate under a capacity-balanced condition even at high states of charge. This enables operation with a reduced electrolyte volume, leading to higher material utilization, improved energy density, and lower raw material costs.

[0057] In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0058] The phrase, “consists essentially of’ and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

[0059] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a compound” includes mixtures of two or more such compounds, reference to “a composition” includes mixtures of two or more such compositions, and the like.

[0060] Both the cathode and anode tanks contain electrodes, i.e. , the cathode and the anode, which can be a carbon, a metal, or a combination thereof. Preferably, these two electrodes have high surface area, to facilitate the desired electrolysis process. They can be made of a carbon, a metal, or a combination thereof.

[0061] An anion-exchange membrane is disposed between the catholyte and anolyte tanks, which allows anions to move from the cathode tank to the anode tank. A current collector may also be attached to the catholyte and anolyte tank.

[0062] As will be appreciated, the first aspect of the invention may simply refer to the catholyte section (including the catalytic column), the anolyte section, the anion-exchange membrane, and the current collector, but it does not need to include the liquid catholyte, the liquid anolyte or the catalyst. However, in further embodiments of the invention, these may all be present.

[0063] Thus, in an embodiment of the invention, the flow battery may contain a liquid catholyte in the catholyte section, a suitable catalyst, and a liquid anolyte in the anolyte section. Both the liquid catholyte and the liquid anolyte require the presence of a supporting electrolyte and the desired redox mediator. Therefore, in certain embodiments:

[0064] (a) the catholyte section may further comprise a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; and a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state, where the catalyst is housed in the catalyst column; and

[0065] (b) the anolyte section may further comprise a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator.

[0066] The supporting electrolyte referred to above in connection to the liquid catholyte and liquid anolyte may comprise a solvent and one or more compounds or salts that provide ions. A suitable solvent for use in the supporting electrolyte is water.

[0067] As mentioned above, the supporting electrolyte also comprises one or more compounds or salts that provide ions. Any suitable material may be used in this capacity. Suitable ions that may be mentioned herein include, but are not limited to protons, ammonium ions, lithium ions, sodium ions, potassium ions, zinc ions, magnesium ions, calcium ions, aluminum ions, chloride ions, and hydroxide ions (e.g. ammonium ions, lithium ions, sodium ions, potassium ions, chloride ions, and hydroxide ions). For example, sodium hydroxide may be used as a source of sodium and hydroxide ions.

[0068] Any suitable concentration of the one or more compounds or salts that provide ions in the solvent may be used. In certain embodiments, the concentration of the one or more compounds or salts that provide ions in the solvent may be from 0.05 to 10 M, such as from 1 to 5 M. In certain exemplary embodiments, the concentration of the one or more compounds or salts that provide ions in the solvent may be about 3 M.

[0069] The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, within 1%, within 0.5%, within 0.1%, within 0.05%, within 0.01 %, within 0.005%, or within 0.001% of a stated value or of a stated limit of a range, and includes the exact stated value or range.

[0070] In certain embodiments, the cathodic redox mediator may be selected from one or more of the group consisting of [Fe(CN)6]37[Fe(CN6)]4', [MnC ]2 / [MnC ]’, Ce3+ / 4+, Mn2+ / 3+, halogens, ferrocene and derivatives thereof, and TEMPO and derivatives thereof. In certain exemplary embodiments, the cathodic redox mediator may be [Fe(CN)6]37[Fe(CN6)]4.

[0071] In certain embodiments, the anodic redox mediator may be selected from one or more of the group consisting of [Zn(OH)4]27Zn, DHPS / DHPS-2H,Zn / Zn2+, V2+ / 3+, Cr2+ / 3+, polyoxometalate (POM), viologen based molecules, quinone based molecules, phenazine-based molecules, alloxazine-based molecules and metal complexes (e.g. iron (III) triethanolamine / iron (II) triethanolamine). In certain particular embodiments, when the anodic redox mediator includes a redox couple that provides a metal deposition, then the anodic redox mediator may also include a further redox mediator that does not deposit a metal to prevent the build-up of said metal. For example, when the aniodic redox mediator includes [Zn(OH)4]27Zn or Zn / Zn2+, then the redox mediator also includes DHPS / DHPS-2H. Viologens are 1 ,1 ’-disubstituted 4,4’-bipyridinium ions (where the nitrogen atoms of the pyridine rings are substituted by an alkyl group (e.g. Ci to C12 alkyl)), with a suitable counterion (e.g. Ch, F’, Br and I ). An example of a viologen of this type is paraquat. When used herein viologens may include related compounds, such as diquat and bipolaron. Thus, the term “viologens and derivatives thereof’ should be interpreted accordingly.

[0072] Phenazine and derivatives thereof may refer to phenazine itself, as well as phenothiazine derivatives and phenoxazine derivatives, which may be used as redox mediators and may have the following structure:

[0073] Racan be H or C1.20 alkyl, X can be N, O or S, each of the aromatic moieties is optionally substituted with one or more of the following groups: F, Cl, Br, I, NO2, COOR, R, CF3, and COR, in which R can be H or C1-20 alkyl.

[0074] Derivatives of ferrocene that may be mentioned herein include ferrocene derivatives having the structure:

[0075] In the above formulae, X is selected from H, F, Cl, Br, I, NO2, COOR, C1-20 alkyl, CF3, and COR, in which R is H or C1-20 alkyl; n is from 0 to 20.

[0076] Specific derivatives of ferrocene that may be mentioned herein include but are not limited to bromoferrocene, ferrocenylmethyl dimethyl ethyl ammonium bis(trifluoromethanesulfonyl)imide (Fc1 N1 12-TFSI), / V-(pyridin-2-ylmethylene)-1 -(2- (diphenylphosphino) ferrocenyl) ethanamine (FeCp2PPh2RCN), 1 ,1 -dimethylferrocene (DMFc), tetraferrocene, di(ethylsulfonic sodium) ferrocene (Ci4Hi6FeS2O6Na2), and di(trimethanesulfonic sodium) ferrocene (Ci6H22FeS20gNa2). In particular embodiments of the invention that may be mentioned herein, the derivative of ferrocene may be di(trimethanesulfonic sodium) ferrocene (Ci6H22FeS2O6Na2) or di(ethylsulfonic sodium) ferrocene (Ci4Hi6FeS2O6Na2).

[0077] TEMPO is 2,2,6,6-tetramethylpiperidin-1 -yl)oxyl and derivatives thereof that may be mentioned herein include, but are not limited to 4-Hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino- TEMPO, 4-cyano-TEMPO and 4-carboxy-TEMPO.

[0078] Alloxazine derivatives that may be mentioned herein may be those disclosed in WO 2016 / 144909, which are herein incorporated by reference.

[0079] Anthraquinone derivatives that may be mentioned herein may be those disclosed in WO 2012 / 147398, which are herein incorporated by reference. Further quinones that may be mentioned herein include, but are not limited to, naphtoquinone derivatives such as 2-hydroxy- 3-carboxy-1 ,4-naphthoquinone (2,3-HCNQ), and benzoquinone derivatives such as 2,5- dihydroxy-1 ,4-benzoquinone (1 ,4-DHBQ).

[0080] Polyoxometalates that may be used herein include, but are not limited to LiPOM, NaPOM, KPOM and HPOM.

[0081] Examples of halogen couples include, but are not limited to, l / IOs’, I7ls', Br2 / Br

[0082] Examples of sulfur-based anolytes include, but are not limited to, S27S22-, S227S42'

[0083] The flow batteries disclosed herein are ones that suffer from unavoidable and irreversible side reactions such as one or more of a hydrogen evolution reaction, metal corrosion, and an oxygen reduction reaction on the anodic side.

[0084] The flow batteries used herein may be alkaline, acidic or neutral. In alkaline-based flow batteries, the catholyte may typically utilize a ferricyanide / ferrocyanide redox couple, while the anolyte may consist of quinone-based, phenazine-based, al I oxazine-based molecules, and metal complexes, among others. Acidic-based flow batteries commonly employ catholytes like VO2+ / VO2+, Ce3+ / 4+, Mn2+ / 3+, and anolytes such as V2+ / 3+, Cr2+ / 3+, quinone-based molecules, and polyoxometalates (POM). Neutral-based flow batteries utilize catholytes like Br2 / Br , 2, 2,6,6- tetramethyl-1 -piperidinyloxy (TEMPO) based molecules, and metal complexes, while anolytes may include viologen-based, sulfur-based, and quinone-based molecules, among others. Without wishing to be bound by theory, it is believed that the cathodic redox mediators used in the device and methods disclosed herein should be selected so that their standard potential is higher than that of the oxygen evolution reaction (OER) for said system it is used in, which may be determined by an environmental factor, such as the pH of the system. This is because it is believed that a positive standard potential with sufficiently driving force for the cathodic redox mediator may be necessary to initiate the OER. For example, in alkaline conditions (e.g . , pH 14), the potential of the cathodic redox mediators may be selected to be higher than 0.4 V (vs. SHE). In neutral conditions (e.g., pH 7), the potential of the cathodic redox mediators may be selected to exceed 0.81 V (vs. SHE), and in acidic conditions (e.g., pH 0), the potential of the cathodic redox mediators may be selected to be greater than 1 .23 V (vs. SHE).

[0085] As such, in certain embodiments, the pH of the electrolyte may be from 1 1 to 15.

[0086] In certain embodiments, the cathodic redox mediator may be provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M. In certain exemplary embodiments, the cathodic redox mediator may be provided in the liquid catholyte at a total concentration of the redox mediator(s) of about 0.6 M. In certain embodiments, the anodic redox mediator may be provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M. In certain exemplary embodiments, the anodic redox mediator may be provided in the liquid anolyte at a total concentration of the redox mediator(s) of about 0.6 M.

[0087] It is noted that the catalyst in the catalyst column may be selected from one or more of the group consisting of NiFe(OH)2@Ni, lrO2, RuO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni. However, any material that can catalyse an oxygen evolution reaction (OER) may be used herein. It is noted that the listed OER catalysts can be utilized across a wide pH range. As such, the strategy described herein is applicable not only to alkaline conditions but also across the pH range (e.g. to neutral and acidic flow battery systems too, provided that the flow battery system suffers from an irreversible (and unavoidable) side-reaction in the anolyte compartment).

[0088] In a second aspect of the invention, there is provided a method of improving a cycling stability of a flow battery, the method comprising the steps of:

[0089] (a) providing a flow battery that suffers from irreversible side-reactions during charging or discharging in the anolyte section, which comprises: a catholyte section comprising a catholyte tank, having a cathode and a catholyte storage compartment, where the catholyte tank and the catholyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte tank to the catholyte storage compartment and back to the catholyte tank, and a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; an anolyte section comprising an anolyte tank, having an anode, and an analyte storage compartment, where the anolyte tank and the analyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid anolyte from the anolyte tank to the analyte storage compartment and back to the anolyte tank, and a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator; an anion-exchange membrane disposed between the catholyte and anolyte tanks that allows anions to move from the catholyte tank to the anolyte tank; and a current collector attached to the catholyte and anolyte tanks

[0090] (b) subjecting the battery to a plurality of charge and discharge cycles; and

[0091] (c) either:

[0092] (i) restoring a loss of capacity by subjecting the liquid catholyte to an oxygen evolution reaction (OER) on a periodic basis such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER; or

[0093] (ii) preventing a loss of capacity by continuously subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is reduced converted to a reduced state by the OER such that no reduction in capacity is observed; or

[0094] (iii) preventing a loss of capacity by intermittently subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER, where the intermittent period is sufficient to prevent a reduction in capacity.

[0095] In certain embodiments, restoring the loss of capacity on a periodic basis may be achieved through the application of a voltage to the battery sufficient to initiate an OER. In other embodiments, the catholyte section may further comprises a catalytic column, where the catholyte storage compartment and catalytic column are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte storage compartment to the catalytic column and back to the catholyte storage compartment, such that: step (c)(i) may be conducted by periodically passing the entire liquid catholyte through the catalyst column to conduct an oxygen evolution reaction to restore the lost capacity; or step (c)(ii) may be conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity; or step (c)(iii) may be is conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity.

[0096] As details of the flow battery have already been described above, they are omitted here for brevity.

[0097] As will be appreciated, the flow batteries disclosed herein may be operated in three modes.

[0098] (i) A batch mode, where charge / discharge cycling is continued until the capacity or state of charge of the battery has decayed to a level considered sub-optimal, at which stage a charge sufficient to initiate an OER is applied to regenerate the catholyte. The applied charge may be applied for a sufficiently long time to return the battery to its original state of charge and then the battery may be used again in charge / discharge cycles until it is depleted once again.

[0099] (ii) A continuous mode, where the loss of capacity is prevented by continually cycling the catholyte to an OER to prevent the loss of capacity. As described herein, this may be achieved through the use of a catalyst column attached to the catholyte compartment, which contains a catalyst that enables the OER to take place.

[0100] (iii) An intermittent mode, which makes use of an OER in the same manner as (ii), but which can be operated after the state of charge falls towards a level similar to that of (i) or is initiated when the state of charge starts to fall.

[0101] The intermittent mode can be employed by triggering the OER process when capacity decay is observed during long-term cycling. In practical applications, regeneration is based on the state of charge (SOC) of the batteries. For instance, when the charge capacity falls below 80% of its theoretical capacity (80% SOC), the regeneration process is initiated with catholyte continuously flowing through the catalytic column without charge / discharge processes. After that, the batteries are cycled again the same as the traditional zinc alkaline batteries (this applies to other flow batteries too). This approach prevents further capacity loss and stops the decline once it becomes noticeable. After regeneration, no significant capacity loss will be observed, and any decline will only occur when the catholyte's capacity drops below the practical SOC threshold, such as 80%. The decline is similar as the traditional zinc alkaline batteries. It will be appreciated that passing part of the catholyte through the catalyst column may occur at the same time as the battery is being used for charge and discharge cycling too. In certain embodiments, the pH of the electrolyte may be from 0 to 15 (e.g. the pH of the electrolyte is from 0 to 5, from 6 to 8 (e.g. 7), or from 9 to 15, e.g. from 11 to 15).

[0102] Any suitable concentration of the one or more compounds or salts that provide ions in the solvent may be used. In certain embodiments, the concentration of the one or more compounds or salts that provide ions in the solvent may be from 0.05 to 10 M, such as from 1 to 5 M. In certain exemplary embodiments, the concentration of the one or more compounds or salts that provide ions in the solvent may be about 4 M.

[0103] Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

[0104] EXAMPLES

[0105] Materials

[0106] All chemicals were purchased from Sigma-Aldrich and used without further purification. DHPS was synthesized using the method reported in literature (JACS 2021 , 143(1 ): 223-231 ). Nation 115 and Nation 212 (Dupont) membrane were purchased from Chemours. The carbon felt was purchased from Liaoyang Jingu Carbide Co., Ltd or SGL Carbon and used as received.

[0107] Example 1 : Alkaline Zn / Fe flow battery (AZIFB)

[0108] With capacity balanced aqueous alkaline Zn / Fe flow battery (AZIFB) as an example, where [Zn(OH)4]z7Zn and [Fe(CN)6]3 / 4' are used as anolyte and catholyte, respectively, the battery setup is shown in FIG. 1 with a separate oxygen evolution reaction (OER) catalytic column. FIG. 2 shows the cyclic voltammogram (CV) curves of these two redox couples. The standard potential of [Zn(OH)4]27Zn redox couple is around -1.26 V vs. SHE and the standard potential of OH7H2is around -0.80 V vs. SHE. Consequently, the corrosion of Zn by OH- and the direct hydrogen evolution reaction (HER) process on electrode may be initiated. Thus, during the charging process, electrons from the anodic side are utilized for both the water-induced side reaction and the Zn deposition reaction. On the cathodic side, all electrons are employed for the oxidation of [Fe(CN)6]4. These water-induced side reactions result in a coulombic efficiency (CE) less than 100%. During the discharge process, only the deposited Zn on the anodic side can be oxidized, as the water-induced side reaction is irreversible. Consequently, only a portion of [Fe(CN)6]3' (in proportion to CE) can be reduced at the cathodic side, leading to the gradual accumulation of [Fe(CN)6]3' in the catholyte. The whole reaction processes are summarized with following equations.

[0109] Anodic reaction:

[0110] Irreversible side reactions at anodic side:

[0111] Cathodic reaction:

[0112] OER for CE compensation:

[0113] For the capacity balanced AZIFB, 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS) was used as redox mediator to prevent the “dead zinc” accumulation. Galvanostatic charge and discharge measurements were conducted for a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. As shown in FIG. 3a, the cell ran steadily for around 100 cycles with an average CE of 99.51 % and then revealed a rapid capacity decay with a fading rate of 0.67 mAh per cycle. FIG. 3b showed the voltage profiles of the AZIFB at 100th, 149th and 150th cycles. It is evident that the discharge plateau for DHPS (the second discharge plateau) was considerably longer than the charge plateau (the first charge plateau) throughout the entire cycling process. The additional capacity in the discharge process stemmed from the "dead zinc" and was recuperated through the DHPS-mediated oxidation process. Notably, DHPS demonstrated remarkable stability, as there was no observable capacity loss based on the first charge plateau. However, in the case of the second charge plateau, concentration polarization appeared early in the 149th cycle compared to the 100th cycle, signifying the gradually consumption of redox-active species during the continuous cycling process, which could be [Zn(OH)4]2-or [Fe(CN)6]4'. Taking into account that the reduced CE (<100%) was attributed to the parasitic HER at the anodic side, it is suggested that the complete consumption of [Fe(CN)e]4' led to a catholyte limited cell. Without surprise, after replacing catholyte, the charge capacity increased from 0.29 Ah to 0.32 Ah, corresponding to a 100% recovery, indicating that there was no capacity loss of the anodic side and the gradually accumulation of [Fe(CN)6]3' at cathodic results in the unbalanced capacity.

[0114] Example 2: Direct OER

[0115] FIG. 4a shows the cycling stability of a capacity balanced AZIFB at a current density of 50 mA / cm2and 80% SOC. The battery underwent approximately 97 cycles, maintaining an average CE of 99.56%. Subsequently, a rapid capacity decay ensued at a rate of 0.55 mAh per cycle. Starting from the 147th cycle, the cutoff voltage during the charge process was raised from 2.2 V to 2.5 V to initiate the OER on the electrode for CE compensation. Evidently, the capacity rebounded, enabling more than 100 additional cycles with almost no capacity decay. To further validate the positive impact of the OER process on CE compensation, another capacity balanced AZIFB was cycled at a current density of 50 mA / cm2and 80% SOC. Simultaneously, a high cutoff voltage of 2.5 V was applied to trigger OER whenever CE compensation was required. Unlike the battery without the OER process, the battery with OER demonstrated stable cycling for 200 cycles with virtually no observed capacity decay (FIG. 4b).

[0116] Example 3: Redox-mediated OER

[0117] An alternative approach to initiate the OER process involves utilizing a redox-mediated process within a catalyst column. The first operational mode is intermittent mode, wherein the catholyte flows through the catalytic column for a few hours to trigger the OER process, after which the battery undergoes cycles without the OER process. As shown in FIG. 5a, following a rapid capacity decay, the catholyte was directed through the catalyst column to induce a redox-mediated OER process. Evidently, the capacity was restored, enabling stable cycling for approximately 7 cycles, after which the battery experienced a capacity decay at a fading rate of 0.53 mAh per cycle. The second mode is continuous mode, involving the catholyte flowing through the catalyst column for every cycle, thereby sustaining a redox-mediated OER process. As shown in FIG. 5b, the battery featuring the redox-mediated OER process exhibited capacity recovery in the catholyte, allowing for more than 200 cycles with almost no capacity decay and an average coulombic efficiency (CE) of 99.46%.

Claims

1. CLAIMS1 . A flow battery, the battery comprising : a catholyte section comprising a catholyte tank, having a cathode, a catholyte storage compartment and a catalytic column, where the catholyte tank and the catholyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte tank to the catholyte storage compartment and back to the catholyte tank, and where the catholyte storage compartment and catalytic column are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte storage compartment to the catalytic column and back to the catholyte storage compartment; an anolyte section comprising an anolyte tank, having an anode, and an anolyte storage compartment, where the anolyte tank and the anolyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid anolyte from the anolyte tank to the analyte storage compartment and back to the anolyte tank; an anion-exchange membrane disposed between the catholyte and anolyte tanks that allows anions to move from the catholyte tank to the anolyte tank; and a current collector attached to the catholyte and anolyte tanks, wherein: the flow battery suffers from irreversible side-reactions during charging or discharging in the anolyte section; and the catalytic column is configured to house a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state.

2. The battery according to Claim 1 , wherein:(a) the catholyte section further comprises: a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; and a catalyst capable of catalysing oxygen production when brought into contact with a cathodic redox mediator when the cathodic redox mediator is in an oxidised state, where the catalyst is housed in the catalyst column; and(b) the anolyte section further comprises: a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator.

3. The battery according to Claim 2, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.

4. The battery according to Claim 3, wherein the ions in the salts that provide ions are selected from: hydroxide ions and / or chloride ions; and one or more of the groups consisting of ammonium ions, lithium ions, sodium ions, potassium ions, magnesium ions, calcium ions, optionally wherein the ions in the salts that provide ions are sodium ions and hydroxide ions.

5. The battery according to Claim 3 or Claim 4, wherein the solvent is water.

6. The battery according to any one of Claims 3 to 5, wherein one or more of the following apply:(a) the pH of the electrolyte is from 11 to 15;(b) the concentration of the one or more compounds or salts that provide ions in the solvent is from 0.05 to 10 M, such as from 1 to 5 M, such as about 3 M.

7. The battery according to any one of Claims 2 to 6, wherein the cathodic redox mediator is selected from one or more of the group consisting of [Fe(CN)6]37[Fe(CN6)]4', [MnO4]27 [MnO4] , Ce3+ / 4+, Mn2+ / 3+, halogens, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the cathodic redox mediator is [Fe(CN)6]3- / [Fe(CN6)]4-.

8. The battery according to any one of Claims 2 to 7, wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

9. The battery according to any one of Claims 2 to 8, wherein the anodic redox mediator is selected from one or more of the group consisting of [Zn(OH)4]27Zn, DHPS / DHPS- 2H,Zn / Zn2+, V2+ / 3+, Cr2+ / 3+, polyoxometalate (POM), viologen based molecules, quinone based molecules, phenazine-based molecules, alloxazine-based molecules and metal complexes (e.g. iron (III) triethanolamine / iron (II) triethanolamine).

10. The battery according to Claim 9, wherein when the anodic redox mediator includes a redox couple that provides a metal deposition, then it also includes a further redox mediator that does not deposit a metal to prevent the build-up of said metal (e.g. when the aniodic redox mediator includes [Zn(OH)4]27Zn or Zn / Zn2+, then the redox mediator also includes DHPS / DHPS-2H).11 . The battery according to any one of Claims 2 to 10, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

12. The battery according to any one of Claims 2 to 10, wherein the catalyst in the catalyst column is selected from one or more of the group consisting of NiFe(OH)2@Ni, IrOz, RuC>2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.

13. A method of improving a cycling stability of a flow battery, the method comprising the steps of:(a) providing a flow battery that suffers from irreversible side-reactions during charging or discharging in the anolyte section, which comprises: a catholyte section comprising a catholyte tank, having a cathode and a catholyte storage compartment, where the catholyte tank and the catholyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte tank to the catholyte storage compartment and back to the catholyte tank, and a liquid catholyte that comprises a supporting electrolyte and a cathodic redox mediator capable of producing oxygen; an anolyte section comprising an anolyte tank, having an anode, and an analyte storage compartment, where the anolyte tank and the analyte storage compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid anolyte from the anolyte tank to the analyte storage compartment and back to the anolyte tank, and a liquid anolyte that comprises a supporting electrolyte and an anodic redox mediator; an anion-exchange membrane disposed between the catholyte and anolyte tanks that allows anions to move from the catholyte tank to the anolyte tank; and a current collector attached to the catholyte and anolyte tanks(b) subjecting the battery to a plurality of charge and discharge cycles; and(c) either:(i) restoring a loss of capacity by subjecting the liquid catholyte to an oxygen evolution reaction (OER) on a periodic basis such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER; or(ii) preventing a loss of capacity by continuously subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is reduced converted to a reduced state by the OER such that no reduction in capacity is observed; or(iii) preventing a loss of capacity by intermittently subjecting the liquid catholyte to an OER such that at least part of a cathodic redox mediator in an oxidised state is converted to a reduced state by the OER, where the intermittent period is sufficient to prevent a reduction in capacity.

14. The method according to Claim 13, wherein restoring the loss of capacity on a periodic basis is achieved through the application of a voltage to the battery sufficient to initiate an OER.

15. The method according to Claim 13, wherein the catholyte section further comprises a catalytic column, where the catholyte storage compartment and catalytic column are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of a liquid catholyte from the catholyte storage compartment to the catalytic column and back to the catholyte storage compartment, such that: step (c)(i) of Claim 13 is conducted by periodically passing the entire liquid catholyte through the catalyst column to conduct an oxygen evolution reaction to restore the lost capacity; or step (c)(ii) of Claim 13 is conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity; or step (c)(iii) of Claim 13 is conducted by continually taking a portion of the liquid catholyte from the catholyte storage compartment and passing it through the catalyst column to conduct an oxygen evolution reaction to prevent a loss of capacity.

16. The method according to any one of Claims 13 to 15, wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions.

17. The method according to Claim 18, wherein the ions in the salts that provide ions are selected from: hydroxide ions and / or chloride ions; and one or more of the groups consisting of ammonium ions, lithium ions, sodium ions, potassium ions, and magnesium ions, calcium ions, optionally wherein the ions in the salts that provide ions are sodium ions and hydroxide ions.

18. The method according to Claim 16 or Claim 17, wherein the solvent is water.

19. The method according to any one of Claims 13 to 18, wherein one or more of the following apply:(a) the pH of the electrolyte is from 0 to 15 (e.g. the pH of the electrolyte is from 0 to 5, from 6 to 8 (e.g. 7), or from 9 to 15, e.g. from 1 1 to 15); and(b) the concentration of the one or more compounds or salts that provide ions in the solvent is from 0.05 to 10 M, such as from 1 to 5 M, such as about 4 M.

20. The method according to any one of Claims 16 to 20, wherein the cathodic redox mediator is selected from one or more of the group consisting of [Fe(CN)6]37[Fe(CN6)]4', [MnO4]27 [MnC ', Ce3+ / 4+, Mn2+ / 3+, halogens, ferrocene and derivatives thereof, and TEMPO and derivatives thereof, optionally wherein the anode redox mediator is [Fe(CN)6]37[Fe(CN6)]4'.

22. The method according to any one of Claims 16 to 21 , wherein the cathodic redox mediator is provided in the liquid catholyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

23. The method according to any one of Claims 16 to 22, wherein the anodic redox mediator is selected from one or more of the group consisting of [Zn(OH)4]27Zn, DHPS / DHPS- 2H,Zn / Zn2+, V2+ / 3+, Cr2+ / 3+, polyoxometalate (POM), viologen based molecules, quinone based molecules, phenazine-based molecules, alloxazine-based molecules and metal complexes (e.g. iron (III) triethanolamine / iron (II) triethanolamine), optionally wherein when the anodic redox mediator includes a redox couple that provides a metal deposition, then it also includes a further redox mediator that does not deposit a metal to prevent the build-up of said metal (e.g. when the anodic redox mediator includes [Zn(OH)4]27Zn or Zn / Zn2+, then the redox mediator also includes DHPS / DHPS-2H).

24. The method according to any one of Claims 16 to 23, wherein the anodic redox mediator is provided in the liquid anolyte at a total concentration of the redox mediator(s) from 0.05 to 3 M, such as from 0.2 to 2 M, such as from 0.5 to 1 M or from 0.4 to 0.8 M, such as about 0.6 M.

25. The method according to any one of Claims 16 to 24, wherein the catalyst in the catalyst column is selected from one or more of the group consisting of NiFe(OH)2@Ni, IrOa, RUO2, other transition metal oxides (TMOs), carbides (TMCs), nitrides (TMNs), phosphides (TMPs), dichalcogenides (TMDs), and borides (TMBs), optionally wherein the catalyst is NiFe(OH)2@Ni.

Images