WO2019054947A1

WO2019054947A1 - A condensed phase aqueous redox flow battery

Info

Publication number: WO2019054947A1
Application number: PCT/SG2018/050471
Authority: WO
Inventors: Qing Wang; Chuankun JIA; Songpeng HUANG
Original assignee: National University Of Singapore
Priority date: 2017-09-14
Filing date: 2018-09-14
Publication date: 2019-03-21
Also published as: WO2019054947A8

Abstract

Disclosed herein is a condensed phase redox flow battery (RFB) comprising a catholyte tank and an anolyte tank, wherein the catholyte tank and/or the anolyte tank further comprise a solid electroactive material which is selected from one or more of the group consisting of Prussian blue, Prussian blue analogues and hydrates thereof. Preferably, the liquid catholyte comprises a supporting electrolyte and vanadium species V02+/V02+ as a p-type redox mediator and the liquid anolyte comprises a supporting electrolyte and vanadium species V2+/V3+ as an n-type redox mediator.

Description

A CONDENSED PHASE AQUEOUS REDOX FLOW BATTERY

Field of Invention The invention relates to a redox flow battery system. Background

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.

Redox flow batteries (RFBs) are considered as one of the most promising large-scale energy storage technologies due to the decoupling of energy storage and power output, flexible design and long cycle life. RFB is a type of electrochemical cell where chemical energy is provided by electroactive elements dissolved in liquids contained within the system and separated by a membrane.

To date, vanadium redox-flow battery (VRB) is the most developed RFB system. The battery exploits the ability of vanadium to exist in solution in four different oxidation states. However, the VRB system, has yet been fully commercialized. This is due to barriers such as their low energy density, high cost, narrow operating temperature and rapid capacity decay. Thus, non-aqueous RFBs, with a wide electrochemical window and high energy density of aqueous RFBs, have attracted great attention worldwide. While promising, non-aqueous RFBs suffer from low power density, poor cycling life, and safety concerns, which limit their practical application.

Redox targeting of battery materials has recently been devised for advanced redox flow battery (RFB) applications. RFBs based on this concept exhibit considerably enhanced energy density. However, the research of redox targeting-based RFBs in the past has been focused on non-aqueous systems, which severely suffer from low power performance, which may be due to the poor ionic conductivities of the membrane and organic electrolytes, as compared to their aqueous counterpart. There is a need for better redox targeting-based flow battery system endowed with low cost, high energy/power density, excellent thermal and cycling stability, which address the problems mentioned above. Summary of Invention

Aspects and embodiments of the current invention that address the above problems are provided in the following numbered clauses. 1. A condensed phase redox flow battery, comprising:

a catholyte section comprising a liquid catholyte, a catholyte tank and a cathode compartment, where the catholyte tank and the cathode compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of the liquid catholyte from the catholyte tank to the cathode compartment and back to the catholyte tank;

an anolyte section comprising a liquid anolyte, an anolyte tank and an anode compartment, where the anolyte tank and the anolyte compartment are fluidly connected to one another by a fluid pathway, so as to facilitate the circulation of the liquid anolyte from the anolyte tank to the anode compartment and back to the anolyte tank;

an ion selective membrane disposed between the cathode and anode compartments that allows electro-active charge balancing ions to move therebetween; and

a current collector attached to the cathode and anode compartments, wherein:

the catholyte tank and/or the anolyte tank further comprises a solid electro- active material that stores electro-active ions;

the liquid catholyte comprises a supporting electrolyte and V0²⁺/V0₂ ⁺ as a p- type redox mediator;

the liquid anolyte comprises a supporting electrolyte and V²⁺/V³⁺ as a n-type redox mediator;

the liquid catholyte, liquid anolyte, solid electro-active material that stores electro-active ions and separator are compatible for use together in a battery; and the solid electro-active material present in the catholyte tank and/or the anolyte tank is selected from one or more of the group consisting of Prussian blue, Prussian blue analogues, and hydrates thereof.

2. The battery according to Clause 1 , wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions, where the ions are selected from one or more of the group consisting of lithium ions, sodium ions, potassium ions, magnesium ions, aluminium ions, copper ions, protons, chloride ions, bromide ions and hydroxide ions.

3. The battery according to Clause 2, wherein the ions are selected from one or more of the group consisting of lithium ions, sodium ions, potassium ions, and protons, optionally wherein the supporting electrolyte is protons.

4. The battery according to Clause 2 or Clause 3, wherein the solvent is a polar aprotic solvent, a polar protic solvent or a combination thereof, optionally wherein the solvent is selected from one or more of the group consisting of water, an alcohol, a carbonate, an ether, an ester, a ketone, and a nitrile.

5. The battery according to any one of Clauses 2 to 4, wherein the supporting electrolyte is H₂S0₄ in water, optionally wherein the concentration of the H₂S0₄ in water is from 0.5 to 5 M, such as from 1 to 3 M, such as 2 M.

6. The battery according to any one of the preceding clauses, wherein, when present in the catholyte, the Prussian blue analogues have the formula M_xFe(CN)₆, where M is selected from Ni, Co, Mn, Cu, Zn, VO and 1≤x≤2.

7. The battery according to any one of the preceding clauses, wherein when present, the solid cathodic electro-active material is provided as granules.

8. The battery according to any one of the preceding clauses, wherein the liquid catholyte further comprises an additive that is selected from one or more of the group consisting of Fe^{2+ 3+}, Br7Br₂, ferroin, iron(ll/lll) tris(2,2'-bipyridine), iron(ll/lll) tris(2,2'- methylbipyridine), iron(ll/lll) tris(2,2'-methoxybipyridine).

9. The battery according to Clause 8, wherein the additive is selected from one or more of the group consisting of Fe^{2+ 3+} and Br7Br₂.

10. The battery according to any one of the preceding clauses, wherein in the liquid catholyte, the total concentration of the p-type redox mediator present in the supporting electrolyte is from 0.05 M to 2 M, such as from 0.4 M to 1.5 M. 11. The battery according to any one of the preceding clauses, wherein the weight : weight ratio of the supporting electrolyte to the solid cathodic electro-active material is from 1 :20 to 20: 1 , such as from 3: 1 to 15: 1 , such as from 5: 1 to 10: 1 , such as 6.67: 1. 12. The battery according to any one of the preceding clauses, wherein the mole : mole ratio of the solid cathodic electro-active material to the redox mediator in the catholyte section is from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3: 1.

13. The battery according to any one of the preceding clauses, wherein, when present in the anolyte, the Prussian blue analogues have the formula M_xFe(CN)₆, where M is selected from Cr, Co, Mn, and V, and 1≤x≤2.

14. The battery according to any one of the preceding clauses, wherein when present, the solid anodic energy storage material is provided as granules.

15. The battery according to any one of the preceding clauses, wherein the liquid anolyte further comprises Fe²⁺/Fe³⁺ as an additive.

16. The battery according to any one of the preceding clauses, wherein the total concentration of the n-type redox mediator present in the supporting electrolyte is from 0.05

M to 3 M, such as from 0.4 M to 1.5 M.

17. The battery according to any one of the preceding clauses, wherein the weight : weight ratio of the supporting electrolyte to the solid anodic electro-active material is from 1 :20 to 20: 1 , such as from 3: 1 to 15: 1 , such as from 5: 1 to 10: 1 , such as 6.67: 1.

18. The battery according to any one of the preceding clauses, wherein the mole : mole ratio of the solid anodic electro-active material to the redox mediator is from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3:1.

19. The battery according to any one of the preceding clauses, wherein the catholyte section comprises the solid electro-active material Prussian blue and the liquid catholyte comprises aqueous H₂S0₄ as the supporting electrolyte and V0²⁺ at a concentration of 0.6 M in the supporting electrolyte.

Brief Description of the Drawings Figure 1 Working principle of the CARB cell, a, Schematic illustration of a CARB battery consisting of an electrochemical cell and two energy storage tanks. The anolyte and catholyte are separated by a proton-conducting membrane in the cell, which prevents the crossover of redox mediators. Besides catholyte, Prussian blue (PB) granules are loaded in the cathodic tank for energy storage, b, Galvanostatic voltage profiles of a VRB and CARB cell at a current density of 30 mA-cm^'2. Both cells share the same anolyte and catholyte, except that there are 0.47 M equivalent concentration of PB granules in the cathodic tank of CARB cell. The catholyte and anolyte are 15 ml_ 0.40 M V0²⁺/V0₂ ⁺ and 150 ml_ 0.40 M V²⁺/V³⁺ in 2 M H₂S0₄, respectively.

Figure 2 Electrochemical performance of a CARB cell, a, Cycling stability of a CARB single cell using 20 ml_ 0.60 M V0²⁺/V0₂ ⁺ loaded with 3 g pretreated PB granules in catholyte and 40 mL 1.5 M V²⁺/V³⁺ in anolyte. A VRB with 20 ml_ 1.50 M vanadium species in both compartments was tested for comparison. The current density was 60 mA-cm^'2 for the tests, b, Turnover time of V0²⁺/V0₂ ⁺ couple as a function of areal cumulative discharge capacity and cycle number, c, Rate performance of a CARB cell at room temperature and the efficiencies at elevated temperatures, d, Dependences of cell voltage and power density on current density of the above CARB and VRB cells at 100% SOC.

Figure 3 CARB stack cell and its comparison with VRB. a, Photograph of a CRAB stack cell consisting of 5 single cells, b, Voltage profile of the CARB stack cell using 110 mL 0.60 M V0²⁺/V0₂ ⁺ loaded with 16.8 g PB in catholyte and 220 mL 1.50 M V²⁺/V³⁺ in anolyte. The current density is 60 mA-cm^'2. c, Cycling stability of the CARB stack cell. d. Cathodic tank energy density and cost of CARB and its comparison with those of VRB. The theoretical value was calculated based on pure PB as the sole energy storage material in the cathodic compartment.

Figure 4 Cyclic voltammograms of V0₂/V0₂ ⁺ and PB. a. CV measurements of PB loaded on carbon felt and VOS0₄ with carbon felt as the working electrode; b. CV measurements of PB and VOSO₄ with a glassy carbon rotating disk electrode (RDE). The supporting electrolyte was 2 M H₂S0₄.

Figure 5 Charge-discharge curves of a CARB cell with catholyte consisting of 0.60 M V0₂/V0₂ ⁺ and 3 g PB granules in 2 M H₂S0₄. The current density was 60 mA-cm^'2. Figure 6 Crossover test of catholyte after long cycling for 900 cycles.

Figure 7 Capacity retention, Coulombic efficiency and energy efficiency of VRB operated at a, 60 °C and b, 80 °C. The concentration of catholyte and anolyte is 1.50 M. The capacity is normalized based on the capacity of the first charging cycle.

Figure 8 Cyclic voltammograms of V^{2+ 3+} and Mn-Cr PBA in 2 M H₂S0₄.

Figure 9 shows the charge-discharge curve of a conventional redox flow battery and a condensed phase aqueous redox flow battery.

Figure 10. Characterizations of redox-targeting reactions of PB in the tank, a, ATR-FTIR spectra of standard PB and BG, fully charged/discharged PB, and V0₂ ⁺- oxidized PB granules, b, Schematic setup for in-situ XANES measurement of CARB cell, c, in-situ XANES spectra of PB during the charging process. The catholyte consists of 0.60 M V0²⁺/V0₂ ⁺ in 2.0 M H⁺. d, Evolution of the weight percentage of BG upon charging the CARB cell. The data was obtained by fitting the XANES spectra.

Description

To address the issues discussed above, a low-cost, robust and scalable catholyte (and/or anolyte) system is disclosed for use in a battery. For example, the catholyte may use Prussian blue (PB) as a solid energy storage material in conjunction with low concentration V0²⁺/ V0₂ ⁺ as redox mediator, which leads to a new "condensed-phase aqueous redox-flow battery (CARB)". The energy of CARB catholyte is stored in PB granules kept in the cathodic tank via redox targeting reactions with V0²⁺/V0₂ ⁺, with which the effective concentration of the redox species reaches 6.30 M, 3-4 times the conventional vanadium redox-flow battery (VRB) concentration. In addition, such a CARB battery has superb operation stability at elevated temperatures of up to 80°C, and nearly 100 percent capacity retention after more than 900 cycles (over 1765 hours). As will be appreciated, the same effects may be achieved using a corresponding anolyte and the effects obtained by the modified catholytes and anolytes disclosed herein may also be combined to further improve the effectiveness of the resultant batteries. As noted below, while the solvent used in the electrolytes may be water (thus providing the CARB system(s) described above), in other systems contemplated herein, it is possible for water to be absent. In the condensed phase aqueous redox flow battery system (and related systems described herein), the resulting batteries have a high energy density, high power density, low capital and maintenance cost and good cycling stability, which is based on a(n aqueous) redox flow system. The reversible storage and release of the energy are achieved based on reversible redox targeting reactions, which significantly increases the energy density of the state-of-the- art aqueous systems, eliminates the limitation of low solubility of redox species in electrolytes, and reduces the cost of flow batteries. The invention will shed light on developing high energy density, low cost flow battery systems for large-scale stationary and automotive energy storage.

Thus, this disclosure provides a rechargeable electrochemical energy storage device, i.e., a condensed phase redox flow battery system that can be configured for different applications, such as powering portable electronic devices and electrical vehicles, storing energy generated from remote power systems such as wind turbine generators and photovoltaic arrays, and providing emergency power as an uninterruptible power source. As such, there is disclosed a condensed phase redox flow battery (e.g. a condensed phase aqueous redox flow battery), comprising:

a current collector attached to the cathode and anode compartments, wherein:

the liquid anolyte comprises a supporting electrolyte and V²⁺/V³⁺ as a n-type redox mediator; the liquid catholyte, liquid anolyte, solid electro-active material that stores electro-active ions and separator are compatible for use together in a battery; and the solid electro-active material present in the catholyte tank and/or the anolyte tank is selected from one or more of the group consisting of Prussian blue, Prussian blue analogues, and hydrates thereof. In the above system, additional energy may be stored in the solid electro-active materials, which can be stored in one or both of the catholyte and anolyte tanks. Power is released in the electrochemical cell when the redox mediators in the catholyte and anolyte are circulated through their respective storage tanks and can regenerated via reversible chemical reduction and oxidation of said materials. As a result, the energy density of this system is a few times higher than that of the state-of-the-art aqueous flow battery systems. In addition, the cost of this system is much lower than current aqueous flow batteries using cheap solid materials to replace expensive electrolytes.

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.

When used herein, the term "compatible for use together in a battery" is intended to mean that the respective components should be complementary to one another in the goal of both storing and discharging power in a safe and effective manner. This is well understood by a person skilled in this field.

The separator divides the cathode compartment from the anode compartment. It can be an electro-active charge balancing ion conducting membrane (e.g., a lithium or sodium ion conducting membrane). The separator prevents cross-diffusion of the redox mediator and allows for movement of electro-active charge balancing ions (e.g., lithium ions, sodium ions, magnesium ions, aluminium ions, copper ions, protons, or a combination thereof). For example, the separator may be a lithium phosphorus oxynitride glass, a lithium thiophosphate glass, sodium phosphorus oxynitride glass, a sodium thiophosphate glass, a NASICON-type lithium conducting glass ceramic, a NASICON-type sodium conducting glass ceramic, a Garnet-type lithium or sodium conducting glass ceramic, a ceramic nanofiltration membrane, a lithium or sodium ion-exchange membrane, or suitable combinations thereof.

Both the cathode and anode compartments can contain electrodes in the battery system, i.e., the cathode and the anode, which electrodes can be a carbon, a metal, or a combination thereof. Preferably, these two electrodes have high surface area, with or without one or more catalysts, to facilitate the charge collection process. They can be made of a carbon, a metal, or a combination thereof. Examples of an electrode can be found in Skyllas-Kazacos, et. al., Journal of The Electrochemical Society, 158, R55-79 (201 1) and Weber, et. al., Journal of Applied Electrochemistry, 41 , 1 137-64 (201 1).

A flow battery according to the current invention is depicted in Fig. 1a. As shown, there is a flow battery system that includes a solid cathodic energy storage material 30 stored within a catholyte tank 10. A fluid pathway 50 that runs from the catholyte tank 10 to a cathode compartment 70 and back to the catholyte tank 10, which is intended to enable the circulation of a liquid catholyte from the storage tank 10 to the cathode compartment 70 and back again. The cathode compartment 70 is separated from the anode compartment 80 by an ion selective membrane 90 disposed therebetween. A fluid pathway 60 runs from the anode compartment 80 to the anolyte tank 20, which may also hold a solid anodic energy storage material. The fluid pathway 60 is intended to circulate a liquid anolyte from the storage tank 20 to the anode 80 and back again. Surrounding the cathode 70, anode, 80 and membrane 90, is a current collector (not shown), which component parts function as the cathode and anode, respectively. As will be appreciated, the catholyte and anolyte may be circulated through the fluid pathways by any suitable means, such as by use of a suitable pumping system.

When used herein, "electro-active charge balancing ions" means ions that are capable of being embedded (e.g., intercalated) in an electro-active material and which move from one electrode to the counter electrode to balance charges through the electrolyte and the separator during discharge of a rechargeable battery, and conversely during charging. Examples of electro-active charge balancing ions include, but are not limited to, lithium ions, sodium ions, potassium ions, magnesium ions, aluminium ions, copper ions, protons, chloride ions, bromide ions, hydroxide ions, and combinations thereof. An electro-active material is a material that can store and release ions during charging and discharging in a battery. If the electro-active material has a high potential (e.g., losing electrons during charging), it is referred to as a "cathodic electro-active material" herein. If the material has a low potential (e.g., acquiring electrons during charging), it is referred to as an "anodic electro-active material" herein. As will be appreciated, a solid electro-active material must be present in either the cathode tank or the anode tank, and suitable materials may be present in both the cathode and anode tanks.

Suitable solid cathodic electro-active materials may include, but are not limited to, Prussian blue, Prussian blue analogues, hydrates thereof and combinations thereof. Prussian blue has the idealized form Fe₇(CN)₁₈ and its hydrate is (Fe₄[Fe(CN)₆]₃) - yH₂0. Analogues of Prussian blue that may be mentioned herein include, but are not limited to those with the formula M_xFe(CN)₆, where M is selected from Ni, Co, Mn, Cu, Zn, and VO and 1≤x≤2. As will be appreciated, these Prussian blue analogues may also be provided in hydrated forms.

Suitable solid anodic electro-active materials may include, but are not limited to, Prussian blue, Prussian blue analogues, hydrates thereof and combinations thereof. Prussian blue has the idealized form Fe₇(CN)₁₈ and its hydrate is (Fe₄[Fe(CN)₆]₃)- yH₂0. Analogues of Prussian blue that may be mentioned herein include, but are not limited to those with the formula M_xFe(CN)₆, where M is selected from Cr, Co, Mn, and V and 1≤x≤2. As will be appreciated, these Prussian blue analogues may also be provided in hydrated forms.

The solid electro-active materials used herein may be provided in any suitable form (e.g. in the form of solid plates or a mesh). In particular, embodiments that may be mentioned herein, the solid electro-active materials may be provided in the form of granules. When in the form of granules, the solid solid electro-active materials may take any suitable size and shape. Examples of suitable sizes may include, but is not limited to, granules having an average diameter of from 0.5 mm to 2.5 mm, such as from 0.75 mm to 2 mm, such as from 1 mm to 1.75 mm, such as around 1.5 mm. As will be appreciated, when the solid electro-active materials are in the form of granules, the outlet for the liquid catholyte/anolyte may be smaller than the granules or may include a filter or pores that prevent the solid material from exiting the respective catholyte/anolyte tank.

As will be appreciated, the catholyte and anolyte also require a solvent, which forms part of the supporting electrolyte mentioned above. A suitable solvent is water, which may be used alone or in combination with an organic solvent suitable for use in a battery. Suitable organic solvents that may be mentioned herein include, but are not limited to an alcohol, a carbonate, an ether, an ester, a ketone, and a nitrile. Suitable alcohols that may be mentioned herein include, but are not limited to methanol, ethanol, n-propanol, iso-propanol and the like. Suitable carbonates include cyclic carbonates (such as propylene carbonate, ethylene carbonate, diethyl carbonate butylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, vinylene carbonate, and/or the like), and a linear carbonate (such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and the like). Suitable ethers include glyme solvent and cyclic or linear ethers other than a glyme (such as tetrahydrofuran (and derivatives thereof), 1 ,3-dioxane, 1 ,4- dioxane, 1 ,2-dimethoxy ethane, 1 ,4-dibutoxyethane, and the like). Suitable esters include linear esters (such as methyl formate, methyl acetate, methyl butyrate, and the like). Suitable nitriles include, but are not limited to acetonitrile, benzonitrile, and the like. Further solvents that may be mentioned include dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof. Suitable glyme solvents may be selected from one or more of the group consisting of ethylene glycol dimetheyl ether (monoglyme), diglyme, triglyme, tetraglyme, methyl nonafluorobutyl ether (MFE) and analogues thereof. Analogues of tetraglyme (CH₃(0(CH2)2)₄0CI-l3) that may be mentioned include, but are not limited to, compounds where one or both of its CH₃ end members may be modified to either -C₂H₅ or to -CH₂CH₂CI, or other similar substitutions. In certain embodiments of the invention that may be mentioned herein, the glyme solvent is tetraglyme.

As will be appreciated, water may be used in any of the systems described herein. As such, when water is present, the organic solvents may be absent or may be used in any suitable weight ratio with respect to water. For example, the additional solvents may be selected from one or more of the group selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, sulfolane, acetonitrile and tetraglyme. In particular embodiments that may be mentioned herein the solvent used in the catholyte and anolyte may be water alone or water in combination with a glyme solvent (e.g. water and tetraglyme).

Given that the electro-active materials are solid and are essentially held permanently within the respective tank of the flow battery, the liquid catholyte or anolyte (i.e. the electrolyte in the cathodic/anodic portion of the flow battery) requires the presence of a redox mediator. A redox mediator refers to a compound present (e.g., dissolved) in the electrolyte (catholyte or anolyte) that acts as a molecular shuttle transporting charges between the respective electrodes and the energy storage materials upon charging/discharging. A p-type redox mediator transports charges between the cathodic electrode and the cathodic energy storage material. An n-type redox mediator transports charges between the anodic electrode and the anodic energy storage material. Not being bound by any theory, upon charging, the p- type redox mediator is reduced on the surface of the cathodic energy storage material and is oxidized on the surface of the cathodic electrode, and the n-type redox mediator is oxidized on the surface of the anodic energy storage material and is reduced on the surface of the anodic electrode. Upon discharging, the reverse processes take place.

As mentioned above, the p-type redox mediator that is used in the currently claimed invention is V0²⁺/V0₂ ⁺. As will be appreciated any suitable salts of V0²⁺/V0₂ ⁺ may be used in the batteries disclosed herein. Any suitable concentration of the p-type redox mediator in the supporting electrolyte may be used. Suitable total concentrations of the p-type redox mediator that may be mentioned herein include those from 0.05 M to 2 M, such as from 0.4 M to 1.5 M. As will be appreciated, the above refers to redox pairs of materials, while the battery may substantively only contain one of the materials when fully charged and/or discharged. For example, when fully charged, the catholyte may substantively contain V0₂ ⁺ and when fully discharged the catholyte may substantively contain V0²⁺. The redox mediators may be provided as the sulfate or chloride salts

Additives that may be used in the liquid catholyte of the batteries disclosed herein include, but are not limited to Fe^{2+ 3+}, Br7Br₂, ferroin, iron(ll/lll) tris(2,2'-bipyridine), iron(ll/lll) tris(2,2'- methylbipyridine), iron(ll/lll) tris(2,2'-methoxybipyridine), and combinations thereof. In particular examples, the additive may be Fe^{2+ 3+} and/or Br7Br_2. As will be appreciated any suitable salts of the respective materials listed may be used in the batteries disclosed herein. Any suitable concentration of the additives in the supporting electrolyte may be used. Suitable total concentrations of the additive that may be mentioned herein include those from 0.05 M to 2 M, such as from 0.4 M to 1.5 M. As will be appreciated, the above refers to redox pairs of materials, while the battery may substantively only contain one of the materials when fully charged and/or discharged, as for the p-type redox mediator above. The additives may be provided as the sulfate, nitrate or chloride salts for all of the above except for Br7Br₂, where the salt counterion may be selected from potassium, sodium, lithium, aluminum or magnesium salts or the compound that provides the Br7Br₂ may be HBr. Any suitable mole : mole ratio of the solid cathodic electro-active material to the p-type redox mediator in the catholyte section may be used. For example, said mole : mole ratio may be from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3: 1. Similar mole : mole ratios may be used for the additive in the liquid catholyte.

As mentioned above, the n-type redox mediator used in the batteries disclosed herein is V²⁺/V³⁺ sulfates, nitrates or chlorides. Any suitable concentration of the n-type redox mediator in the supporting electrolyte may be used. Suitable total concentrations of the n- type redox mediator that may be mentioned herein include those from 0.05 M to 3 M, such as from 0.4 M to 1.5 M. As will be appreciated, the above refers to redox pairs of materials, while the battery may substantively only contain one of the materials when fully charged and/or discharged. For example, when fully charged, the anolyte may substantively contain V²⁺ and when fully discharged the anolyte may substantively contain V³⁺.

Additives that may be used in the liquid anolyte of the batteries may be Fe²⁺/Fe³⁺. As will be appreciated any suitable salt of the additive may be used in the batteries disclosed herein. For example, the Fe²⁺/Fe³⁺ additive may be provided as its sulfate, nitrate or chloride salts. Any suitable concentration of the additive in the supporting electrolyte may be used. Suitable total concentrations of the additive that may be mentioned herein include those from 0.05 M to 3 M, such as from 0.4 M to 1.5 M. As will be appreciated, the above refers to redox pairs of materials, while the battery may substantively only contain one of the materials when fully charged and/or discharged. For example, when fully charged, the anolyte may substantively contain Fe²⁺ and when fully discharged the anolyte may substantively contain Fe³⁺.

Any suitable mole : mole ratio of the solid anodic electro-active material to the n-type redox mediator in the catholyte section may be used. For example, said mole : mole ratio may be from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3: 1. Similar mole : mole ratios may be used for the additive in the liquid anolyte.

For the avoidance of doubt, when the concentration of a p- or n-type redox mediator (or additive) is referred to above, the concentration is based on the volume of the solvent present in the supporting electrolyte. When used herein "supporting electrolyte" refers to a liquid that comprises a solvent and one or more compounds or salts that provide ions (including the charge balancing ions) that are dissolved in the solvent. Suitable ions that may be dissolved in the solvent include, but are not limited to lithium ions, sodium ions, potassium ions, magnesium ions, aluminium ions, copper ions, protons, chloride ions, bromide ions, hydroxide ions and suitable combinations thereof (whether in the catholyte or anolyte). In more particular examples that may be mentioned herein, the ions may be selected from one or more of the group consisting of lithium ions, sodium ions, potassium ions, and protons. More particular examples that may be mentioned herein include those in which the ions of supporting electrolyte are protons. As will be appreciated, the ions mentioned above refer to the required ion(s) needed to make the battery function (e.g. as charge-balancing ions). As such, it will be understood that these ions are provided to the battery system in the form of a suitable compound or salt, meaning that other components are also present as well in the supporting electrolyte. For example, the protons may be provided by water and/or a suitable acid (e.g. H₂S0₄), while ions such as lithium may be provided by a suitable salt form, such as LiCI.

In particular embodiments that may be mentioned herein, the ions of the supporting electrolyte may be protons. In such cases, the protons of the supporting electrolyte may be derived from aqueous H₂S0₄, which may have any suitable concentration, such as from 0.5 to 5 M, such as from 1 to 3 M, such as 2 M.

As will be appreciated, the weight : weight ratio of the supporting electrolyte to the solid electro-active material may have any suitable value. For example, the ratio of the supporting electrolyte to the solid cathodic and/or anodic electro-active material may independently be from 1 :20 to 20: 1 , such as from 3: 1 to 15: 1 , such as from 5: 1 to 10: 1 , such as 6.67: 1.

The catholyte and anolyte tanks, the cathode and anode, the ion selective membrane the current collector and the flow paths used herein may be any conventionally used in the field of flow batteries. No particular limitation is placed on these components.

In one example of a system, the flow battery disclosed herein may contain:

Prussian blue as the solid cathodic energy storage material;

no solid anodic energy storage material;

an aqueous catholyte comprising V0²⁺ at a concentration of 0.6 M and H₂S0₄ at a concentration of 2 M; and an aqueous anolyte comprising V at a concentration of 0.6 M and H₂S0₄ at a concentration of 2 M, where the solvent of the anolyte and catholyte may be water and teraglyme or only water. In this embodiment, there are two sets of reactions concomitantly taking place at the electrodes and tanks. The following equations show the electrochemical reactions on the cathode and anode respectively during charging:

Anodic Side:

V³⁺ + _e→ V²⁺ Cathodic side:

V0²⁺ + H₂0→ V0₂ ⁺ + 2H⁺ + e

3V0₂ ⁺ + Fe₄[Fe(CN)₆].(x+6)H₂0→ 3V0²⁺ + {Fe₄[Fe(CN)₆]₃}³⁺.XH₂0.60H^" + 3H₂0

During the charging process, V³⁺ is reduced to V²⁺ in the anodic compartment. Meanwhile, V0²⁺ is oxidized to V0₂ ⁺ in the cathodic compartment. The oxidized V0₂ ⁺ then oxidizes the PB in the cathodic tank. The above process continues until the PB is completely oxidized to Berlin

green (BG), from which the PB contributes additional energy to the VRB system. The redox reactions outlined above are reversed when the battery is discharged.

In one example of a system, the flow battery disclosed herein may contain:

Prussian blue as the solid anodic energy storage material;

no solid cathodic energy storage material;

an aqueous catholyte comprising V0²⁺ at a concentration of 0.6 M and H₂S0₄ at a concentration of 2 M; and

an aqueous anolyte comprising V³⁺ at a concentration of 0.6 M and H₂S0₄ at a concentration of 2 M, where the solvent of the anolyte and catholyte may be water and teraglyme or only water. In this embodiment, there are two sets of reactions concomitantly taking place at the electrodes and tanks. The anode side may further contain Fe^{2+ 3+} in the supporting electrolyte in some embodiments. The following equations show the electrochemical reactions on the cathode and anode respectively during charging:

Cathodic side:

V0²⁺ + H₂0→ V0₂ ⁺ + 2H⁺ + e

Anodic Side: Fe + e <→ Fe (when present)

V³⁺ + _e <→ V²⁺

4V²⁺ + Fe₄[Fe(CN)₆]₃→ 4V³⁺ + {Fe₄[Fe(CN)₆]₃}^4" During the charging process, the V0²⁺ is oxidized to V0₂ ⁺ on the cathodic side. Meanwhile, the oxidized redox molecule (Fe³⁺) in the anodic compartment will be reduced to Fe²⁺. After the full reduction of the Fe³⁺ ions, the V³⁺ will subsequently be reduced to V²⁺ at the electrode and then the reduced V²⁺ will then react with the Prussian Blue in the anodic tank to generate a reduced species.

As will be appreciated, the catholyte and anolyte systems above can be combined into one battery so as to provide Prussian blue on both sides, which may increase the energy capacity of the battery yet further. As will be appreciated, the energy density of the new catholyte (and anolyte) is significantly higher than conventional materials used in flow batteries. In addition, with the new catholyte and anolyte, the electrolyte imbalance and capacity fading are considerably alleviated, and the cycle life becomes much longer compared to a conventional VRB system. For example, in the catholyte system described above, this leads to the reduction of V0²⁺/V0₂ ⁺ concentration in the catholyte, which lowers the cycling cost and allows the operation of VRB at elevated temperature.

The battery system of this invention may have a higher energy density than those of traditional redox flow batteries. Compared to lithium ion batteries, this system does not require a bulky conducting additive and a voluminous binder, saving room for more electro- active materials and thus further increasing its energy density. In addition, the battery system can be rapidly refueled by replacing it's the catholyte and anolyte tanks with a charged one (in a similar way to refilling a fuel tank for an internal combustion engine). The catholyte and anolyte tanks can then be recharged externally. The catholyte and anolyte tanks contains the bulk of the electro-active materials of the battery system. During the operation, there is only a small amount of the redox mediator flows into the catholyte and anolyte compartments. Thus, the safety of the cell is greatly improved.

The problems the invention solves and advantages over existing devices include the following. 1) In comparison with current aqueous redox flow batteries, the present invention greatly increases the energy density.

2) In comparison with current aqueous redox flow batteries, the present invention dramatically lowers the cost of the system.

3) In comparison with current non-aqueous redox flow batteries, the present invention significantly increases the power density while keeping high energy density.

4) In comparison with current non-aqueous redox flow batteries, the present invention avoids using flammable organic solvents. All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims. Hereinafter, embodiments of the invention are illustrated in more detail with reference to the following examples. However, the present disclosure is not limited thereto. Furthermore, what is not described in this disclosure may be sufficiently understood by those who have knowledge in this field and will not be illustrated herein. Examples

Example 1

Working principle of the CARB cell Herein describes a redox targeting-based flow battery system— condensed-phase aqueous redox-flow battery (CARB), endowed with low cost, high energy/power density, excellent thermal and cycling stability, to address the challenges confronted by VRB.

The CARB cell is built upon VRB cell while with hydrated Prussian blue (PB) granules kept in the cathodic energy tank as the energy storage material (Fig. 1 a). As will be evident in the subsequent examples, PB granules or PB analogue materials may also be kept in the anodic energy tank. Catholyte and anolyte containing V0²⁺/V0₂ ⁺ and V²⁺/V³⁺ in sulfuric acid, respectively, are circulated through the cell and energy tanks for power generation. As revealed in Fig. 4, V0²⁺/V0₂ ⁺ has a redox potential at around 0.90 V (vs. Ag/AgCI), which is nearly the same as that of PB/Berlin green (BG). Therefore, the Nernstian potential difference induced by the activity changes of V0²⁺/V0₂ ⁺ upon charging/discharging would make the reactions energetically favorable for PB to be chemically oxidized by V0₂ ⁺ to BG, and reversely for BG to be chemically reduced by V0²⁺ to PB. This is evidenced by the considerably enhanced catalytic current of V0²⁺/V0₂ ⁺ on PB-modified electrode in the cyclic voltammetric measurement (Fig. 4). The corresponding electrochemical and chemical reactions are described as below:

VO²* + H₂0 VO$ + 2H+ + e^" (on the cathode)

f ) VOl -}- 3H+ + Fe{ⁿiFe^ll{CN)₆ ^■ xH₂0)_s - 3V0²* Ρβ ^! [Ρβ^!ίΙ (€Ν) (ΟΗ) · (x^■■■■ 1)H₂0}₃ - 3H_zO ( the tank)

0.)

Upon charging, V0²⁺ ions are firstly oxidized to V0₂ ⁺ on the cathode (eq.1) which are then pumped into the tank to chemically oxidize the hydrated PB to BG and the energy is stored in the material (eq.2). The conversion of PB to BG accompanies extraction of protons, which makes up the consumption of H⁺ by V0₂ ⁺ to regenerate V0²⁺. As a result, the total H⁺ concentration (or pH) of catholyte remains constant regardless of the state of charge (SOC), considering equimolar H⁺ migrates to the anolyte for charge balancing. Upon discharging, V0²⁺ is oxidized by BG to V0₂ ⁺ in the tank and subsequently the energy is released by the reduction of V0₂ ⁺ on the cathode. The discharge process conversely involves proton insertion in BG, and the pH of catholyte is independent of the state of discharge (SOD). This is different from VRB, in which the pH of catholyte keeps decreasing during charging process while increasing during discharging process. Hence, with the redox targeting reactions between PB/BG and V0²⁺/V0₂ ⁺, the energy is reversibly stored in the immobile solid material in the tank, and an enhancement in capacity is expected.

CARB cell measurement

A CARB cell with catholyte containing 0.40 M V0²⁺/V0₂ ⁺ and 0.47 M hydrated PB in the tank was investigated to assess the viability of the above redox targeting reactions for battery application. Materials and pre-preparation

Prussian Blue (PB, Sigma Aldrich), vanadium(IV) oxide sulfate hydrate (VOS0₄, Shanghai Huating Chemicals Factory Co., Ltd.), Nafion 115 membrane (Ion power), iron(lll) chloride hexahydrate (FeCI₃-6H₂0, Sigma Aldrich), polyvinylidene fluoride (PVDF, Alfa Aesar), ethyl cellulose (EC, Fluka Analytical) and sulfuric acid (H₂S0₄, 98%, Honeywell) were used as received.

Insoluble PB has a defective structure with a Fe-deficient void in the center of unit cell. The uptake of various hydrated cations in the void presents intriguing physical chemical properties, which have led to different applications. Here the PB granules were soaked in Fe³⁺ solution prior to use, to facilitate the hydration process and generate more redox sites.

Vanadium redox-flow cells with an active area of 13.3 cm² were fabricated to study the redox targeting reactions between PB and V0₂/V0₂ ⁺ and extensively evaluate the CARB cell performance. Nafion 115 membrane and carbon felt were used as the separator and electrode, respectively. Conductive plastic plates were used as current collector. To validate the redox targeting reactions, 15 ml_ 0.40 M VOS0₄ and 150 ml_ 0.40 M V²⁺/V³⁺ in 2 M H₂S0₄ was used as the catholyte and anolyte, respectively. 2 g PB powder was added into the cathodic tank as the energy storage material. To facilitate the hydration process, 3 g PB granules were soaked in 20 ml_ 0.30 M FeCI₃ solution for a few days prior to use.

Results A considerably extended voltage plateau of V0²⁺/V0₂ ⁺ is observed in the presence of PB (Fig. 1 b), indicating the solid material contributes to the catholyte capacity mediated by the vanadium species.

Example 2

Characterizations of redox-targeting reactions of PB in the tank

Fourier transform infrared (FTIR) spectroscopic measurement: FTIR spectra were collected with PerkinElmer Frontier MIR/FIR system by 16 scans with a nominal resolution of 1 cm^"1 through an attenuated total reflection (ATR) mode. Before the measurement, the samples were cleaned with Dl water and dried in a vacuum oven at 80 °C for 24 h. The sample was pressed in a 3 mm² sampling area and against the diamond crystal with the high pressure clamp to make the samples in intimate contact with the crystal. Spectra were collected from 4000 to 400 cm^"1.

FTIR Results

Fourier transform infrared (FTIR) spectroscopy was preformed to probe the redox targeting reactions between V0²⁺/V0₂ ⁺ and PB/BG (Fig. 10a). Compared with pristine PB, a peak appears at 2170 cm^"1 after charging, which is assigned to the vibration of Fe'"-C≡N as presented in the chemically synthesized BG, indicating PB is oxidized to BG by V0₂ ⁺. In addition, the vibrations at around 900-1200 and 1650 cm^"1 become noticeably stronger arising from the deprotonation of hydrated PB upon oxidation by V0₂ ⁺, which leads to the formation of hydroxide in the lattice. Such a change introduces vibrations of Fe'"-OH in the FTIR spectra, broadly consistent with the DFT calculations.

X-ray absorption spectroscopic measurement: The X-ray absorption near edge structure (XANES) measurement of PB in CARB was performed at XAFCA beamline of the Singapore Synchrotron Light Source (SSLS). A custom designed cell was employed to carry out the in- situ measurement under transmission mode. The set up consists of a CARB cell and an in- situ XANES cell. A CARB cell with an active area of 13.3 cm² was fabricated to real-time generate redox active mediators for the redox targeting reactions between PB and V0₂/ V0₂ ⁺. In the CARB cell, 30 ml_ 0.40 M VOS0₄ and 40 ml_ 1.50 M V²⁺/V³⁺ in 2 M H₂S0₄ was used as the catholyte and anolyte, respectively. 3 g PB granules was added into cathodic tank as the energy storage material. Nafion 115 membrane and carbon felt was used as the separator and electrode, respectively. Conductive plastic plates were used as current collector. The cell was charge-discharged at a constant current density of 30 mA crrf². In order to avoid the corrosion of carbon felt and conductive plastic plates, the cut-off voltage was set at 0.40-1.65 V. An in-situ XANES cell with an area of 4 cm² was connected to CARB cell for the in-situ measurement, in which 5 mg PB powder was loaded onto a carbon paper. The redox targeting reaction between PB and catholyte occurred when the catholyte was circulated through the cathodic tank of CARB and the in-situ cell. The X-ray beam penetrated through the in-situ cell and XANES signals were collected. To obtain the weight percentage of PB and BG during the charging/discharging, linear curve fitting (LCF) was carried out for the XANES spectra.

The in-situ and ex-situ XAS measurements were conducted with the setup described in Fig. 10b. Upon charging the CARB cell, the pump was stopped at 300 s, 600 s, 1000 s, 1600 s and 3200 s for in-situ XANES measurement. The signals were recorded for 6 minutes with 3 data sets collected at each stop point. The data for the fully charged and discharged products in the tank were also collected with the same settings in the ex-situ measurement. The above measurements were conducted at the XAFCA beamline (Y. Du, Y. Zhu, S. Xi, P. Yang, H. O. Moser, M. B. Breese, A. Borgna, J Synchrotron Radiat, 2015, 22, 839-843) of the Singapore Synchrotron Light Source. Fe foil was used for the energy correction. The XANES data processing and linear curve fitting were performed using standard methods with ATHENA software package (B. Ravel, M. Newville, J Synchrotron Radiat, 2005, 12, 537-541).

XANES Results X-ray absorption near-edge spectroscopy (XANES) is a useful technique to determine the valence state of active materials of flow batteries. To gain a better understanding of the redox targeting reaction in the cathodic tank, XANES were performed to scrutinize the valance changes of Fe element in PB at different stages of charging/discharging. A flow- through cell with a thin layer of PB particles loaded on carbon paper and connected with an active CARB battery was used for the in-situ XANES measurement (Fig. 10b. During the charging process, the K-edge of Fe shifted toward higher binding energy (Fig. 10c), indicative of an increase in the average valence of Fe in PB. The reaction appears to be swift in the first 300 s due to the high concentration V0₂ ⁺ initially generated in the tank of CARB cell flushing through the PB in the in-situ cell. The reaction between V0₂ ⁺ and PB then slacked off before sufficient V0₂ ⁺ was reproduced in the catholyte by charging the CARB cell for 1000 s (Fig. 10d). Eventually around 62% PB was oxidized to BG till the end of charging. This is corroborated by ex-situ measurement of PB collected from the tank of the CARB cell, in which about 51 % PB reacted after charging. The slightly higher material utilization in the flow-through cell is presumably a result of better accessibility of V0₂ ⁺ in PB powder than the granules in the tank. Upon discharging, the K-edge of Fe shifted backward revealing a decrease in the valance of Fe in BG, which is quantified by fitting the spectra that nearly 90% of the formed BG was reduced back to PB by V0²⁺. These results show good reversibility of the redox targeting reactions between V0²⁺/V0₂ ⁺ and PB/BG, crucial to the operation of CARB cell.

Example 3

Electrochemical performance of a CARB cell

A CARB cell was built using 1.50 M V²⁺/V³⁺, 0.60 M V0²⁺/V0₂ ⁺ in 2.0 M H₂S0₄ as anolyte and catholyte, respectively. 3 g PB granules were pretreated and added into the cathodic tank. In detail, 20 ml_ 0.60 M VOS0₄ in 2 M H₂S0₄ was used as catholyte with the pretreated PB granules (3 g) kept in the cathodic tank for energy storage. In the anodic side, 40 mL 1.50 M V²⁺/V³⁺ solution was used to balance the capacity of catholyte. Extensive galvanostatic measurement was carried out with an Arbin battery tester. For cycling stability test, a CARB cell was charged/discharged at a constant current density of 60 mA-crrf ² and a cutoff voltage of 0.40-1.70 V. For rate performance measurement, a separate CARB cell was charge/discharged at a varying current density from 40 to 160 mA-crrf². For thermal stability test, the catholyte was heated at 45, 60 and 80 °C with an oil bath during the galvanostatic measurement. For power density test, a CARB and a VRB cell were firstly charged to 100% SOC and then the discharge voltage was recorded at controlled current at a step of 1 mA-s^"1 to obtain the maximum output power.

Performance

The discharge voltage of the CARB cell is around 1.20 V at a current density of 60 mA-cm^'2 and the capacity increased considerably after adding PB in the tank (Fig. 5). In Fig. 5, the higher voltage plateau corresponds to the reaction of V0₂/V0₂ ⁺ on the cathode, which is considerably extended through the redox targeting reactions with PB/BG in the tank.

The discharge capacity initially increased with the hydration of PB and then broadly stayed constant before it gradually dropped after 700 cycles due to the imbalance of anolyte. Nearly 100% capacity retention was achieved for 900 cycles (Fig. 2a) without additional electrolyte balancing, revealing the excellent stability of CARB cell. Areal cumulative discharge capacity and turnover time of V0²⁺/V0₂ ⁺ were employed to assess the cycling stability of CARB cell stack and catholyte, respectively (Fig. 2b). For 900 cycles, the total turnover time for each vanadium species is 2142 (2.38 times per cycle on average), indicating each unit cell of the pretreated PB contributes >2.64 e^" (excl. crossover, Fig. 6) in the redox targeting reaction, which corresponds to a utilization of 87.9%.

After cycled for 900 cycles, 1 mL catholyte was taken out from the tank and diluted, and the concentration was measured by UV-vis (see Fig. 6) The concentration of catholyte is around 0.8 M, indicating around 0.2 M vanadium ions crossover from anolyte to catholyte.

The Coulombic efficiency, voltage efficiency and energy efficiency of the CARB cell is 95%, 80% and 87% at a current density of 40 mA-cm^'2 (Fig. 2c). The peak power density was determined to be 240 mW-cm^"2 at 100% SOC, around 20% higher than that of VRB with 1.50 M catholyte and anolyte (Fig. 2d). As the V0₂ ⁺ concentration in CARB is only 0.4 times that in VRB, the enhancement of power is deemed a result of facilitated interfacial charge transfer on the cathode.

Thermal stability

In addition, it is known that V0₂ ⁺ is prone to degrade at elevated temperatures by forming precipitates when the concentration is relatively high (see Table 1 in Example 7). This limits the operating temperature of VRB generally lower than 45°C and brings about additional complexity as well as cost for cooling. In contrast, CARB is immune from the thermal degradation of V0₂ ⁺ since it employs low concentration catholyte. As shown in Fig. 2c, even slightly improved performance was obtained with the 0.60 M catholyte cycled at temperatures up to 80°C for long time. This allows the application of CARB cell adapted to broader environmental conditions without auxiliary cooling. Colour of PB granules

The color of pristine PB granules is blue, which becomes green after charging in the CARB cell, indicating PB is oxidized to BG by V0₂ ⁺. In addition, the morphology of PB granules remained unchanged after prolonged cycling and no PB powder was observed in catholyte, revealing good mechanical stability of the PB granules.

Example 4

CARB stack cell A CARB stack consisting of 5 single cells (20 cm²) was assembled with external manifold design to further assess the viability of the redox targeting-based flow battery for practical applications (Fig. 3a).

CARB stack cell measurement: A CARB stack cell consisting of five 20 cm² single cells connected in series was assembled. 1 10 ml_ 0.60 M V0₂/V0₂ ⁺ loaded with 16.8 g pretreated PB granules was used as the catholyte and 220 ml_ 1.50 M V²⁺/V³⁺ as the anolyte. The stack cell was charge/discharged at a current density of 60 mA-cm^'2. The cut-off voltage was set from 2.0 V to 8.5 V.

The voltage profiles of the CARB stack cell are shown in Fig. 3b. It has a discharge voltage plateau of around 6.20 V at a current density of 60 mA-cm^'2, under which nearly no capacity and efficiency fading was observed after continuously cycling for more than 130 cycles (Fig. 3c), indicating the superior cycling stability of CARB system. The pretreated PB granules in the tank contribute around 42.4% to the total capacity (excl. anolyte crossover), further corroborating the viability of redox targeting reactions at stack cell level.

Example 5

Preliminary study of redox targeting reaction in the anolyte.

A similar redox targeting concept could be feasibly applied to the anolyte if a proton-hosting material with matched potential to V²⁺/V³⁺ is identified. One example is the PB analogue material— manganese(ll) hexacyanochromate(lll), which has identical potential to that of V²⁺/V³⁺ (Fig. 8). Thus, a considerably boosted energy density of CARB can be foreseen by implementing the redox targeting concept in both the cathodic and anodic tanks. Manganese (II) hexacyanochromate (III) (denoted as Mn-Cr PBA) was synthesized by dropwise addition of 40 ml_ of 0.05 M K₃Cr(CN)₆ (Sigma Aldrich) into 40 ml_ 0.0725 M Mn(CH₃COO)₂-4H₂0 (Sigma Aldrich) in dark under vigorous stirring for 12 h. The resulting precipitate was centrifuged and washed with distilled water and ethanol three times and dried under vacuum at room temperature for 24 h. To prepare the working electrode, 60 wt.% Mn-Cr PBA, 20 wt.% carbon black and 20 wt.% polyvinylidene fluoride were grounded in an agate mortar in 1- methyl-2-pyrrolidone to form a slurry. Slurry of Mn-Cr PBA were then coated on FTO glass (fluorine-doped Sn0₂, TEC15). The electrodes were dried under vacuum at room temperature for 24 h prior to use. The cyclic voltammetry (CV) measurements were conducted with a multichannel potentiostat (Metrohm Autolab, PGSTAT302N) under N₂ protection. A three-electrode setup consisted of a Ag/AgCI reference electrode (3.0 M KCI), and a Pt foil counter electrode was employed. The electrolyte was 2 M H₂S0₄. For the CV test of v^{2+ 3+}, carbon felt was used as the working electrode and 100 mM VCI₃ in 2 M H₂S0₄ was used as the electrolyte. The scan rate was 5 mV-s^"1. As shown in Fig. 8, both the v^{2+ 3+} and Mn-Cr PBA have the same potential at around -0.51 V (vs. Ag/AgCI), indicating good plausibility of redox targeting reaction between V^{2+ 3+} and Mn-Cr PBA.

The above studies have unequivocally demonstrated a redox targeting-based catholyte system, with which the CARB battery presents unprecedentedly enhanced cycling and thermal stability without compromising the energy density. More prominently, the CARB battery has considerably lower catholyte cost (lower vanadium and potentially lower H₂S0₄ concentrations), superior power performance, and wider operating temperature range, which translate into a further reduction of the total cost of the battery system in terms of materials, cell stack and maintenance, critically confronted by VRB. Moving forward, there is ample room for CARB to further boost its energy density by applying redox targeting concept to both the anolyte and catholyte, and by optimizing the packing and microstructures (i.e. dimension, porosity, tortuosity, etc.) of the energy storage material granules in the tank, which enable a multifold enhancement of energy density. The redox targeting concept breaks the boundary between solid phase and liquid phase energy storage, which as applied to various battery chemistries, provides an intriguing and effective approach to the development of cost-effective and high-energy and power density flow batteries systems for large-scale energy storage.

Example 6

An anolyte containing Prussian blue for use in a vanadium redox-flow battery (VRB)

Prussian blue (PB) is used as a proton hosting material in the anodic tank. The working principle of the device is described as follows: The component of a condensed phase aqueous redox flow battery consists of a cell stack (1), anodic and cathodic tanks (2), and pumps (3). In a cell stack, Nafion membrane is used as the separator to prevent the crossover of redox molecules and allow proton to pass through. Carbon felt is used as the electrode. The anolyte and catholyte is 0.75 M Fe³⁺ + 0.75 M V³⁺ + 2 M H2SO4 (20 ml_) and 1.5 M V0²⁺ + 2 M H2SO4 (40 ml_), respectively. Besides, solid powder of PB (2g) is added into the anodic tank for hosting protons. During the charging process, the V0²⁺ is oxidized to V02⁺ in cathodic side (reaction 1). Meanwhile, the oxidized redox molecule (Fe³⁺) in the anodic compartment will be reduced to Fe²⁺ (reaction 2). After the fully reduced of Fe³⁺ ions, the V³⁺ will subsequently be reduced to V²⁺ at the electrode (reaction 3). The reduced V²⁺ will then react with PB in the anodic tank (redox targeting reaction 4). The above process will continue until the PB is completely reduced. The corresponding electrochemical and chemical reactions are shown below:

Cathode side

V0²⁺ +H₂0→ V0₂ ⁺ + 2H⁺ + e (reaction 1) Anode side

Fe³⁺+e→Fe²⁺ (reaction 2)

V³⁺+e→V²⁺ (reaction 3)

4V²⁺ ₊Fe₄[Fe(CN)₆]₃→4V³⁺ ₊ {Fe₄[Fe(CN)₆]₃}⁴ _{(redox targetjng reactjon 4)}

During the discharging process, the V0₂ ⁺ is reduced to V0²⁺ in cathode side. Meanwhile, the reduced redox species V²⁺ will be oxidized to V³⁺ in the anodic side. After the V²⁺ is fully oxidized, the Fe²⁺ ions will subsequently be oxidized to Fe³⁺ at the electrode. The oxidized Fe³⁺ will then react with PB in the anodic tank (reaction 5). The charging process will go on until all PB is oxidized. For the charging process, the corresponding redox targeting reaction is shown below:

4Fe³⁺ ₊{Fe₄[Fe(CN)₆]₃}⁴ →4Fe²⁺ ₊ Fe₄[Fe(CN)₆]_{3 (redox targeting reaction 5)} Figure 9 shows the charge-discharge curve of a conventional redox flow battery and a condensed phase aqueous redox flow battery. Clearly, in the presence of PB in the anodic tank, the capacity is greatly extended without changing the concentration and volume of both anolyte and catholyte. This will then greatly enhance the energy density of the battery. Example 7

Thermal stability of the catholytes of VRB and CARB.

Table 1. Stability of different catholyte at different temperature

Note: NP means "no precipitation".

20 mL catholyte with different V0₂ ⁺ concentration was heated in a water bath at different temperatures. Yellowish-brown precipitate was observed after heating 2.0 M and 1.5 M catholyte, at 45 °C for 3 h and 48 h, respectively (see Table 1). In contrast, no precipitation was found at 45 °C for 0.6 M catholyte. When the heating temperature was increased to 60 °C, the precipitation was observed after 0.2 h, 0.6 h and >336 h for 2.0 M, 1.5 M and 0.6 M catholyte, respectively. When the heating temperature was increased to 80 °C, the precipitation was observed after 0.1 h, 0.2 h and >10 h for 2.0 M, 1.5 M and 0.6 M catholyte, respectively. These results reveal the relatively good thermal stability of 0.6 M catholyte at elevated temperatures. In addition, considering the redox targeting reaction instantaneously regenerates V0₂ ⁺ to V0₂, which lowers the effective concentration of V0₂ ⁺ and further enhances the thermal stability of catholyte.

VRB single cells were assembled to study the thermal stability at elevated temperatures. 20 mL 1.5 M VOS0₄ + 2 M H₂S0₄ and 20 mL 1.5 M V²⁺/V³⁺ were used as the catholyte and anolyte, respectively. Nafion 115 membrane and carbon felt were used as separator and electrodes respectively. Conductive plastic plate was used as current collector. The cell was charge-discharged at a constant current density of 60 mA crrf². To avoid corrosion of the carbon felt and the conductive plastic plates, the cut-off voltage was set at 0.7-1.7 V. The energy efficiency, voltage efficiency and charge capacity constantly decreased at 60 °C and 80 °C, due to vanadium precipitations on the surface of carbon felt in the cathodic side at high temperatures. Yellowish precipitate was observed at the bottom of cathodic tank. In contrast, the efficiencies of CARB cell increased with increasing temperature (Fig. 2c) and no precipitation was observed. The results reveal good thermal stability of CARB cell at elevated temperatures up to 80 °C.

Claims

1. A condensed phase redox flow battery, comprising:

a current collector attached to the cathode and anode compartments, wherein:

2. The battery according to Claim 1 , wherein the supporting electrolyte comprises a solvent and one or more compounds or salts that provide ions, where the ions are selected from one or more of the group consisting of lithium ions, sodium ions, potassium ions, magnesium ions, aluminium ions, copper ions, protons, chloride ions, bromide ions and hydroxide ions.

3. The battery according to Claim 2, wherein the ions are selected from one or more of the group consisting of lithium ions, sodium ions, potassium ions, and protons, optionally wherein the supporting electrolyte is protons.

4. The battery according to Claim 2 or Claim 3, wherein the solvent is a polar aprotic solvent, a polar protic solvent or a combination thereof, optionally wherein the solvent is selected from one or more of the group consisting of water, an alcohol, a carbonate, an ether, an ester, a ketone, and a nitrile.

5. The battery according to any one of Claims 2 to 4, wherein the supporting electrolyte is H₂S0₄ in water, optionally wherein the concentration of the H₂S0₄ in water is from 0.5 to 5 M, such as from 1 to 3 M, such as 2 M.

6. The battery according to any one of the preceding claims, wherein, when present in the catholyte, the Prussian blue analogues have the formula M_xFe(CN)₆, where M is selected from Ni, Co, Mn, Cu, Zn, VO and 1≤x≤2.

7. The battery according to any one of the preceding claims, wherein when present, the solid cathodic electro-active material is provided as granules.

8. The battery according to any one of the preceding claims, wherein the liquid catholyte further comprises an additive that is selected from one or more of the group consisting of Fe^{2+ 3+}, Br7Br₂, ferroin, iron(ll/lll) tris(2,2'-bipyridine), iron(ll/lll) tris(2,2'- methylbipyridine), iron(ll/lll) tris(2,2'-methoxybipyridine).

9. The battery according to Claim 8, wherein the additive is selected from one or more of the group consisting of Fe^{2+ 3+} and Br7Br₂.

10. The battery according to any one of the preceding claims, wherein in the liquid catholyte, the total concentration of the p-type redox mediator present in the supporting electrolyte is from 0.05 M to 2 M, such as from 0.4 M to 1.5 M.

11. The battery according to any one Claims 8 to 10, wherein the weight : weight ratio of the supporting electrolyte to the solid cathodic electro-active material is from 1 :20 to 20: 1 , such as from 3: 1 to 15: 1 , such as from 5: 1 to 10: 1 , such as 6.67: 1.

12. The battery according to any one of the preceding claims, wherein the mole : mole ratio of the solid cathodic electro-active material to the redox mediator in the catholyte section is from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3: 1.

13. The battery according to any one of the preceding claims, wherein, when present in the anolyte, the Prussian blue analogues have the formula M_xFe(CN)₆, where M is selected from Cr, Co, Mn, and V, and 1≤x≤2.

14. The battery according to any one of the preceding claims, wherein when present, the solid anodic energy storage material is provided as granules.

15. The battery according to any one of the preceding claims, wherein the liquid anolyte further comprises Fe²⁺/Fe³⁺ as an additive.

16. The battery according to any one of the preceding claims, wherein the total concentration of the n-type redox mediator present in the supporting electrolyte is from 0.05 M to 3 M, such as from 0.4 M to 1.5 M.

17. The battery according to any one of the preceding claims, wherein the weight : weight ratio of the supporting electrolyte to the solid anodic electro-active material is from 1 :20 to 20: 1 , such as from 3: 1 to 15: 1 , such as from 5: 1 to 10: 1 , such as 6.67: 1.

18. The battery according to any one of the preceding claims, wherein the mole : mole ratio of the solid anodic electro-active material to the redox mediator is from 0.1 : 1 to 20: 1 , such as from 0.5: 1 to 5: 1 , such as from 0.6: 1 to 3:1.