CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Chinese Application No. 201110203069.0, filed on July 20, 2011 and U.S. Provisional Application Serial No. 61/488,163, filed May 20, 2011. The contents of the parent applications are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to storage of electricity in a redox flow electrochemical cell. The redox pairs of the cell are partly or all from the different reduction/oxidation states of complexed metal ions selected from late 3d transition metals of low cost.

BACKGROUND

Because most renewable energy sources such as wind and solar energy has intermittent nature, having a large capacity energy storage is essential to large scale utilization of renewable energy. Reflow battery (RFB) is one such kind of high potential energy storage device.

In other areas such as civilian and naval ships, as well as submarine power source, may also require such large scale energy storage device.

The high cost of vanadium metal put limit on large scale adoption of vanadium reflow battery (VRB), therefore there is a urgent need to reduce the cost of the battery materials.

In a US patent related to VRB, US 6562514, the charging voltage for VRB is usually quite high, usually above 1.5 V. Also during charging in acidic solution the electrolysis water led to further decline in energy efficiency. Another drawback of water electrolysis is that it will generate extra acid and base, cause electrolyte composition to change overtime, thereby affecting the stability of the battery operation. Therefore there is a need for an improved battery design to minimize water electrolysis. DESCRIPTION OF DRAWINGS

Figure 1 demonstrates an exemplary structure of a flow battery according to the present invention (in which reference 1 represents a membrane or diaphragm; reference 2 represents a redox species in catholyte; reference 3 represents an electric current collector plate; reference 4 represents conducting porous materials; reference 5 represents a catholyte; and reference 6 represents an anolyte).

Figure 2 demonstrates an exemplary structure of a flow battery based on Mn ions (in which reference 1 represents ion exchange membrane; reference 2 represents a redox species in catholyte; reference 3 represents an anolyte; reference 4 represents a catholyte; reference A represents electrode structure A; and reference B represents electrode structure B.

Figure 3 is a graph showing a charging/discharging curve of an all Mn based reflow battery.

Figure 4 is a graph showing a charging/discharging curve of Fe(CN)-Br ₂ reflow battery.

Figure 5 is a graph showing a charging/discharging curve of Cu(NH ₃ )-Br ₂ reflow battery.

DETAILED DESCRIPTION

Definition

"Reflow battery" (or "flow battery") is also called redox reflow battery. It is a type of battery that uses separate flow of anolyte and catholyte, through battery stack to generate electricity or store electricity. Anolyte and catholyte are the electrolytes in the anode compartment and cathode compartment, respectively. They are also referred to as working solutions. In the present invention, the starting chemical composition of catholyte and anolyte can be the same or can be different.

"Redox" is an abbreviation for oxidation and reduction, which are

electrochemical reactions that cause oxidation state changes for atoms, ions, molecules or coordination compounds. "Complexes," also called coordination compounds or chelating complexes, are a type of chemical compounds that consist of atoms or ions in the center and ligand molecules or ions at its surroundings. The chemical bond between the center and ligand is partial or complete coordination bond. One typical example is copper(I) ion complex with ammonia molecule (NH ₃ ). Generally speaking, multi-nucleus multi-ligand complexes also belong to this category.

"Transition metals" are the elements located in d block of periodic table. The group number that is assigned to the metal corresponds to the filling of d orbital, from s2dl (group II) to s2d9 (group XI).

"Late 3d transition metals" refer to the group of elements consisting of Mn, Fe,

Co, Ni, and Cu.

Cathode and Anode: In electrochemical cell, the cathode is the electrode where reduction occurs when battery is discharging and the anode is the electrode where oxidation occurs when battery is discharging, cathode has high electrode potential and anode has lower electrode potential. For a battery, the cathode is also called positive electrode and anode negative electrode.

"Working solutions" is the general term for the catholyte and anolyte. The former has a higher redox potential and latter has a lower redox potential.

"Electric current collector" is also referred to as a biopolar plate. It is formed from the anode of one single battery cell fused with the cathode of another single battery cell. Its main function is to provide structure support and to conduct current.

"Percolation threshold" refers to the minimum concentration of particles above which a continuous pathway can be fromed from one electrode to another electrode. Usually the particles have random dispersion in a medium, but they can also be processed to be aligned or to induce low order to control local concentration, particle orientation, inter particle interaction etc. Filler can be a single substance or a mixture of many.

"Mean particle size D50" is a value describing the size of particles. It refers to the diameter of the particle that corresponds to accumulative numeric distribution of 50%. Its physical meaning is that 50% of particle are bigger than this value and 50% particles are smaller than this value (percentage can be number percentage or volume percentage). For non-spherical shape particles, the diameter refers to the diameter of a particle that has equal volume to the d50 particle.

"Redox materials," also refer to redox pairs, are a combination of high and low oxidation state of the same materials. For example Cu(II), which stands for plus two valent copper ion or Cu ²⁺ , and Cu(I), which stands for plus one valent copper ion or Cu ⁺ , can form a redox pair. In the present invention, the form of high or low oxidation state element can exist in the forms of neutral molecule or ions, such as complexes formed between the ions and water, hydroxide, oxygen containing ligand, nitrogen containing ligands. For example, in Mn ⁷⁺ /Mn ⁶⁺ , Mn ⁷⁺ exists in the form of permanganate MnO ⁴ -, and Mn ⁶⁺ exist in the form of manganate Mn0 ₄ ² . In Cu ²⁺ /Cu ⁺ , Cu ²⁺ and Cu ⁺ can exist in the form of their complexes with water, ammonia, and ligands, and mixture of ligands. In the present invention, redox pairs refer to both coordinated or non-coordinated metal ion redox pair, unless otherwise specified.

"Ligand" refers to a type of compounds that can form coordination complexes with any one of the late 3d transition elements. The ligand comprises one or more functional groups: organic amino (primary, secondary, or tertiary amino groups, aromatic or non-aromatic amino groups), phosphonate, thioalcohol, phosphate, hydroxyl, ammonia (NH ₃ ), or can be selected from a group consisting of bromide, chloride, fluoride, iodide, and cyanide.

Generally speaking, water and hydroxides can also be ligands, however, due to the ubiquitous nature of water and hydroxides, they are not considered as ligand in the present disclosure. Ligand can be mono-dentate or multi-dentate. Examples of multi- dentate ligand include 2, 2'- bispyridine, oxalate, citrate, EDTA. Multi-dentate ligand can also takes the form of a ring such as crown ether.

Ligand coordinated metal ions generally take the form of the following formula [M(X)(L) _s ] ^a+ , where M is a metal, X is the valence state denoted in roman numeric numbers I, II, III, etc, L is the ligand which can be anions or neutral molecules, s is an integer selected from 2-6, and a is the total charge of this coordination complex. This formula also describes the mixed ligand coordination complex wherein the L can be chosen from more than one type of ligand molecules. When more than one valence states are involved, such as between Fe(III) and Fe(II), they form ligand coordinated redox pair.

Within this application, the terms "carboxylate" or "phthalate", or "salicylate" include "carboxylic acid", "phthalic acid" or "salicylic acid", respectively, and denote both free acid and salt forms of the ligands.

Redox pairs formed by halogens of different oxidation states refer to redox pair among zero valence halogen, minus one valence halogen and halogen oxyacids. Here the zero valence halogen include chlorine, bromine, iodine, and interhalogen compounds having the formula of [X _n Y _m ] ^w , in which each of X and Y is CI, Br, or I, X can be equal to Y; n and m are integers from 1-6, w is the total charge, usually from -3 to 0. Typical examples of zero valence halogen compounds are Cl ₂ , Br ₂ , 1 ₂ , BrCl, I ₃ ^~ , ClBr ₂ ^" , BrCl ₂ ^" , Br ₃ ^~ , IC1 ₂ ^" , and IBr ₂ ^" . Halogen oxyacids have the general formula: [XO _p ] ^_1 , in which p is an integer from 1-4, and X is CI, Br or I.

Detailed Description

To overcome the drawbacks of vanadium reflow battery, an electrochemical flow battery is proposed here, which comprises an electrode structure A, an electrode structure B, a catholyte, an anolyte, and characterized in that:

1) said catholyte is in contact with electrode structure A;

2) said anolyte is in contact with electrode structure B, and the said anolyte is based on one or a mixture of the following ligand-coordinated redox pair:

[Mn(X)(L) _s ] ^a+ /[Mn(Y)(L) _t ] ^b+ ,[Cu(II)(L) _s ] ^a+ /[Cu(I)(L) _t ] ^b+ , [Fe(III)(L) _s ] ^a+ /[Fe(II)(L) _t ] ^b+ , where each of X and Y denotes a valence state of a Mn ion and is an integer from 2-6, and X is greater than Y, and L is a ligand that forms coordination compounds with the ions of late 3d elements (i.e., Mn, Cu, or Fe), and s and t are integers selected from 2-6;

3) said battery has a separating membrane or an ion exchange membrane, spatially positioned in between the electrode structure A and B; and

4) said electrode structure A and B have electric conductivity above 10 ^~3 S/m. The catholyte or anolyte according to present invention has hydroxide anions in the concentration range of 10 ^~10 -30 mol/L, preferably at 10 ^~2 -30 mol/L. The catholyte according to present invention contains at least one of the following redox pairs: coordinated or non-coordinated Cu ³⁺ /Cu ²⁺ , coordinated or non-coordinated Mn ⁷⁺ /Mn ⁶⁺ , those formed by halogens of different oxidation states, those formed by halogen oxy acids of different oxidation states.

As one embodiment, Mn, Fe, and Cu are chosen as anolyte redox species to lower the materials cost for the battery. Mn, Cu, and Fe are currently priced at 1/10, 1/4, and 1/30 of V, respectively. The three elements are widely abundant in earth's crust and have multiple valence state. All three elements have excellent redox properties and are beneficial for energy storage batteries. Although current invention focuses on these three metals, the concept of using ligands with metal ions can be applied to other metals as well.

In terms of anolyte redox species choices, the anolyte according to present invention is chosen from late 3d transition metals, that is, Mn, Cu, and Fe. Accordingly, the following anolyte design can be implemented. First, the anolyte can be based on Mn redox species including the following redox pair : Mn ³⁺ /HMn0 ₂ Mn ³⁺ /Mn ²⁺ ,

Mn ⁵⁺ /Mn ³⁺ , Mn ⁵⁺ /Mn ²⁺ , and Mn0 ₄ ² 7Mn ⁵⁺ . The anolyte can also contain at least one ligand and said ligand can form coordination compounds with the Mn species mentioned above. The anolyte can be in contact with electrode structure B. Similarly, the anolyte can be based on Cu ²⁺ /Cu ⁺ or Fe ³⁺ /Fe ²⁺ , and the anolyte can be in contact with electrode structure B.

Furthermore, a mixture of redox species can be used for anolyte, such as mixing two of the three following redox pairs in one anolyte solution: Mn /Mn , Cu /Cu , and Fe ³⁺ /Fe ²⁺ , in which x and y are integers from 1 to 6, and x is greater than y.

To increase the battery's energy density, it is generally necessary for the redox species to have high solubility in water. In one embodiment, the concentration of the metal ion redox species is above 0.2 mol/L, preferably above 1.0 mol/L, and more preferably above 2.0 mol/L. In addition, during the charging/discharging process, acid or base can be generated and large pH swing can occur. This requires the redox species to maintain high solubility even in the presence of high pH swings. This can be especially challenging when the pH is at high level due to poorer solubility of most metal ions at high H. For example, Fe has Ksp = 4x 10 ^" . When the Fe concentration is 1 mol/land the pH>2, Fe ³⁺ will form Fe(OH) ₃ precipitation. As another example, Cu ²⁺ has

20 2 2

Ksp = 2.2x 10 ^" . When the pH>4 and the Cu concentration is 1 mo 1/1, Cu will form Cu(OH) ₂ precipitation. Therefore, there is a need to improve solubility of metal ions in wide pH ranges.

In one embodiment according to present invention, ligands are added into the electrolyte to improve the solubility of metal ions in neutral or high pH. In acidic pH, the presence of ligands can also help to stabilize the metal species that has lower solubility even at a low pH value.

In another embodiment, the ligand that forms complexes with metal ions can include one or more functional groups, such as organic amine (primary, secondary, or tertiary, aromatic or non-aromatic), phosphonate, thioalcohol, phosphate, hydroxyl, or ammonia (NH ₃ ), or can be selected from a group consisting of bromide, chloride, fluoride, iodide, cyanide. Other organic functional groups including carboxylate, alkoxy, aldehyde, and ketone can also help in stabilizing the redox species in either catholyte or anolyte. Other effective ligands or chelating functional groups include hydroxylamines, amino acid, nitrogen containing heterocycles, iminodiacetic acids, nitrilotriacetic acids, nitriles. Ligand can be mono-dentate or multi-dentate. Examples of multi-dentate ligand include 2, 2'- bispyridine, oxalate, citrate, and EDTA. Multi-dentate ligand can also take the form of a ring such as crown ether. In both anolyte and catholyte, the ligand concentration lies in the range of 0.02-30 mol/L. Use of these ligands can greatly improve the redox species' solubility and/or dispersibility during charging and discharging, hence stabilized voltage is achieved. As one example, use of ammonia helps stabilized the Cu ²⁺ /Cu ⁺ redox pair in relative alkaline condition and avoids the copper species from precipitation out of electrolyte. Other multi-dentate ligands such as EDTA, polyethyleneimine, oligo-ethyleneimine are also effect ligand for complexing to late 3d metals. As an another example, in a battery based on Mn ions, the use of EDTA, bispyridine, pyridine, cyanide, thiocyanide, fluoride, chloride, bromide, flurocarboxylic acid, or carboxylic acid can form higher solubility coordination compounds with Mn ions, such as [Mn(edta)], K[Mn(cdta)], Mn ⁿ [Mn ⁱⁿ (edta)] ₂ , [Mn(py) ₆ ]Br, (pyH)Y[MnCl ₄ ], K4[Mn(CN) ₆ ], [Mn(acac) ₃ ], K ₃ [Mn(CN) ₆ ], K ₂ [MnF ₆ ], K ₂ [Mn(CN) ₆ ], M ₂ [MnF ₆ ] (M=N0 ₂ ⁺ ,NF ₄ ,or alkli metals), MMnF ₅ (M=0 ₂ ⁺ ,or alkali metals),

[MnCl ₄ bipy], MnF ₂ NH ₃ , Mn(NH ₃ ) ₆ (CN) ₂ , Mn(NH ₃ ) ₂ (CN) ₂ , Mn(SCN) ₂ ,

Mn(NH ₃ ) ₄ (SCN) ₂ , Mn(HCOO) ₂ , Mn(CH ₃ COO) ₂ , Mn(CF ₃ COO) ₂ and other mono- carboxylate or multi-carboxylate complexes with Mn ions. Ligands having both sulfonate and carboxylate groups, such as sulfosuccinic acid and sulfophthahc acid can also be used as ligands that can boost solubility of complexed metal ions for flow batteries using transition metals as redox species.

Another group of ligands can be chosen from multi-substituted aromatic-ring containing carboxylates in their acids or their salt forms, which include

benzenedicarboxylic acids, benzenetricarboxylic acids, benzenetetracarboxylic acids, benzenepentacarboxylic acid and benzenehexacarboxylic acid, napthalenedicarboxylic acids, napthalenetricarboxylic acids and naphthalenetetracarboxylic acid. It is to be understood that the carboxylic groups can be directly attached to the ring or indirectly via one methylene (CH ₂ ) group. It is also to be understood that the carboxylic group (or groups) can be randomly attached to the ring and not necessarily following a certain order. For example, the benezenedicarboxylic acids can consist of 1,2-, or 1,3-, and 1,4- benzenedicarboxylic acids. Yet another group of ligands for complexing late 3d transition metals are the mono-, di- or trisulfonates of the above mentioned aromatic ring- containing carboxylates. Representative compounds are 4-sulfophthalic acid, 5-sulfo- 1,2,4-benzenetricarboxylic acid.

In another embodiment, multi-substituted aromatic ring containing phenol, quinolinol, or naphthols can also be chosen to complex the late 3d transition metals. These compounds include salicylic acid, 5-sulfosalicylic acid, 5-bromosalicylic acid, 5- chlorosalicylic acid, 4,5-dihydroxybenzene-l,3-disulfonic acid, 8-hydroxyquinoline and its sulfonate-substituted derivatives, and pyrocatechol and its sulfonoated derivatives.

The aromatic rings on the above mentioned ligands can greatly improve the ligands' electrochemical stability due to resonance structures of the rings, and the multi- substitution by complexing and/or solubilizing groups such as carboxylate, phenolate, and sulfonate can further improve the solubility of the final complexed metals, thereby increasing the current density of the flow battery.

It is also applicable from cost standpoint to use a mixture of a multi-substituted aromatic ring-containing ligand and one or a few less expensive traditional ligands such as phosphonate ions, phosphate ions, oxalate ions, halogen ions, thiosulfate, cyanate, thiocyanate, citrate, tartarate, and other carboxylate group-containing ligands.

Furthermore, all organophosphonates and phosphonocarboxylates can also be used to complex late 3d transition metals for improving stability and/or battery output power. This group of compounds includes the acid or the salt form of the following compounds:

amino trimethylene phosphonic acid (ATMP),

1 - hydroxy ethylidene-l,l-diphosphonic acid (HEDP),

ethylene diamine terra (methylene phosphonic acid),

ethylene diamine terra (methylene phosphonic acid) (EDTMPA),

diethylene triamine penta (methylene phosphonic acid) (DTPMPA),

2- phosphonobutane -1,2,4-tricarboxylic acid (PBTCA),

2-hydroxy phosphonoacetic acid (HPAA),

hexamethylenediaminetetra (methylenephosphonic acid) HMDTMPA, polyamino polyether methylene phosphonate (PAPEMP), and

bis(hexamethylene triamine penta (Methylene Phosphonic Acid)).

When using Mn ions as anolyte redox species, the battery's catholyte can contain Mn /Mn . Mn can exist in the form of permanganate (Mn0 ₄ ), and Mn can exist in the form of manganite (Mn0 ₄ ² ). Alternatively, Br ₂ /Br , BrO Br ₂ , or Br0 ₃ Br ₂ can be used in the catholyte. When using Mn ³⁺ /Mn ²⁺ in the anolyte, the Mn ²⁺ can exist in several forms, one of which is HMn0 ₂ . Mn ³⁺ can be complexed Mn ³⁺ . Preferred ligand for complexed Mn ³⁺ is NH ₃ . Other ligands include CN ^{~ 1} , OH ^"1 , multidendate such as EDTA, ethylenediamine, or a mixture of ligands. Use of ligands can greatly reduce the precipitation of insoluble species such as Mn0 ₂ during battery charging and discharging. Other suitable Mn based anode redox pair is Mn ⁵⁺ /Mn ²⁺ . When using Fe ions as anolyte redox species, the anolyte preferably contains cyanide ions, and iron ions and cyanide ions form coordination compounds with cyanides. As one embodiment, the use of ligand expands the output voltages for the battery, for example, when using cyanide for Fe ions, the Fe(CN) ₆ ³ /Fe(CN) ₆ ⁴ has standard potential 0.358V, while Fe ³⁺ + e = Fe ²⁺ has standard voltage of 0.771V. This demonstrate the use of ligand to increase output voltage and hence battery energy and power density. The catholyte of the battery can contain Br ₂ /Br , BrO 7Br ₂ , or Br0 ₃ 7Br ₂ .

When using Cu ions as anolyte redox species, the solution (anolyte or catholyte) preferably contains ammonia or organic amine compounds, or carboxylic acids such as small molecule carboxylic acids (e.g., citric acid or tartaric acid) or polymers containing carboxylic acid groups (e.g., poly(acrylic acid) or poly(maleic acid)). The ammonia, carboxylic acids, or amines can form coordination compounds with copper ions. The catholyte of the battery can contain Br ₂ /Br , BrO 7Br ₂ , or Br0 ₃ Br ₂ .

In terms of catholyte redox choices, the halogen of different valence states or halogen oxy acid of different valence states can be used. For example, Br ₂ /Br ,

BrO Br ₂ , and Br0 ₃ 7Br ₂ are suitable choice of catholyte redox species in providing oxidizing power for the battery. Similarly, chlorine and iodine can also have different valence states and form different oxy acids that have higher standard potential than anode redox species. It is worth noting that in the presence of halide, the elemental halogen can be coordinated with halide ions to form complexed halogen such as Br ₃ which has higher solubility than Br ₂ . The catholyte typically needs to be in contact with electrode structure A which is the cathode.

Both electrode structure A and B are consisted of conducting electric current collector plate and conducting porous materials and there is electric contact between the two. Electrode structure A functions as the cathode and provide a place where cathode redox species can receive electrons. Electrode structure B functions as the anode and provides a place where anode redox species can give off electrons.

The electrode structure A and/or B preferably includes at least one porous or high surface area conducting material, which can be selected from porous metals (such as porous nickel, porous titanium), porous carbon materials (such as carbon powder, carbon non-woven fiber, carbon woven fiber, graphite powder, carbon nanotube), conducting polymers including those polymers with intrinsic high conductivity and those polymer composites that are compounded with intrinsic conductive materials. Examples of intrinsic conducting polymers include polyaniline and polypyrrole. Polymer composite usually contains polymer that is not intrinsically conducting but when compounded with an intrinsic conducting material in highly dispersed form at above percolation threshold, exhibit good conductivity.

The electrode structure A and/or B preferably contain at least one catalyst, which is selected from the elemental form or compound form of at least one of the following elements: Ti, V, Cr, Co, Ni, Zr, Nb, Mo, Ru, Rh, Pd, Au, and Pb.

In terms of construction, the battery generally includes a membrane or diaphragm that separates the two electrodes (i.e., the cathode and the anode). One purpose of the membrane or diaphragm is to minimize the crossover of the electrolytes which can lower battery efficiency. Preferred membranes include perfluorinated polymers such as NAFION (by Dupont), Flemion and Aciplex, and perfluorinated membrane produced by Dow Chemical. In terms of membrane category, the membrane can be selected from: ion exchange membranes, meso and microporous membranes, and nanofiltration membranes. It is required that the membrane material is stable under electrochemical

oxidation/reduction environment.

The conducting porous materials of electrodes preferably have conductivity of above 10 ^"3 S/m, more preferably above 1 S/m, and even more preferably above 100 S/m. The void volume preferably is above 10%, and more preferably above 25%. In addition, the conducting plate with roughness above 0.5 micron can also be used without use of conducting porous materials. In this situation, the electrode structure is made of the electric current collector plate which has roughness above 0.5 micron. The conducting porous materials can be selected from the following: porous nickel, carbon black, porous titanium, non-woven carbon fiber, woven carbon fiber, graphite powder, and carbon nanotube. In general, the electric current collector plate within electrode structures A and B is in electric contact with the conducting porous materials, so that electric current can be collected from conducting porous material to current collector.

Electrode structures A and B generally have certain mechanical rigidity because they need to provide space for electrolyte to flow within, and need to be chemically stable in the presence of an electrolyte. In addition, electrode structures A and B can provide the path of least electrical resistance when charging and discharging the battery.

To improve the electrode reaction efficiency, it is preferred to have a catalyst on the electrode structure A or B. Preferred catalysts can be selected from the elemental form or compound containing at least one the following elements: Ti, V, Cr, Co, Ni, Zr, Nb, Mo, Ru, Rh, Pd, Au, and Pb. The catalyst can be dispersed onto conducting porous materials, such as dispersed onto porous carbon based materials. These catalysts can improve the redox reaction rate on the porous materials. These catalysts can optionally have certain degree of solubility in electrolyte, however, dispersed onto the conducting porous materials is the preferred method of using the catalyst in the present battery.

As one embodiment of present invention, the current collector is coated with a conducting polymer composite coating to reduce the corrosion of the current collector plate in electrolyte.

The battery according to present invention has preferred charging voltage less than 1.5 V, and preferably less than 1.23 V, so that the water electrolysis is minimized. Charging and discharging voltages can be tuned by use of a catalyst, redox species, ligand and external charging/discharging control circuit. The choice of the ligand and redox species determines the thermodynamic voltage. In contrast, in a conventional vanadium reflow battery, the charging voltage is usually above 1.5 V and evolution of hydrogen or oxygen will occur regardless of electrode materials choices, which may put extra stress on battery stacks. As will be seen in examples below, the Mn-Br ₂ battery and Cu-Br ₂ battery have lower charging voltages than 1.23V, thereby minimizing the energy waste by water electrolysis and disruption of electrolyte composition due to oxygen or hydrogen evolution. The battery according to present invention can be used for energy storage application in general. Such energy storage device is particularly suitable for wind power station, solar power station, micro-electric grid, peak shaving, smart grid energy storage, underwater vehicles, and ship propulsion power source.

The contents of all publications cited herein (e.g., patents, patent application publications, and articles) are hereby incorporated by reference in their entirety.

The following examples are illustrative and not intended to be limiting.

Examples

Figure 1 shows the section view of the battery used for example 1-3. The membrane used was Nafion perfluoro membrane that separated the electrode structures A and B. Catholyte 5 and anolyte 6 were in contact with electrode structures A and B, respectively. When charging the battery, an external voltage was applied to the battery while the electrolytes were flowing using peristaltic pumps. Once charging was finished, the device was connected to an external load so that battery was discharged. The outer side of electrode structures A and B was made of current collector plate 3 and inner side was made of conducting porous materials 4.

Example 1 : Complexed all Mn reflow battery

Battery negative electrode (anode) reaction: Mn ²⁺ → Mn+ ³⁺ + e-

Battery positive electrode (cathode) reaction: Mn0 ₄ ^~ + e-→ Mn0 ₄ ^2~

In this example, the Mn species in catholyte and anolyte were both 1 mol/1, the voltage was determined to be IV. The electrode current collector was made of graphite and the conducting porous materials were made of carbon cloth. The apparent surface area of the electrode was 20 cm ² . To minimize the Mn0 ₂ from forming, the anolyte and catholyte both contained 8 mol/L KOH. Therefore, enough K ₂ (Mn0 ₄ ) was dissolved in 8 mol/1 KOH so that active Mn solubility was 1 mol/L. To further improve high valence Mn solubility, the temperature of the battery was controlled at 50°C. For anolyte, 1 mol/L MnCl ₂ was dissolved in 8 mol/L KOH as anolyte precursor solution. To make anolyte solution, the MnCl ₂ solution was electrochemically oxidized to Mn ³⁺ in the presence of 2M ammonia. Ammonia greatly increased the trivalent Mn solubility. The anolyte and catholyte prepared in this manner were charged and discharged according to Figure 3.

In the beginning of charging, 100 mL each of catholyte and anolyte was pumped into cathode A and anode B, respectively (Figure 2), and the battery was charged at constant current of 20 mA/cm ² for 1 hour and then discharged at the same current density. Figure 3 shows the charging/discharging characteristic of this battery. The charging voltage was less than 1.2V to avoid electrolysis of water, and discharging current was above 0.8V, otherwise insoluble Mn0 ₂ could form that would block the flow.

As a control experiment, a repeat experiment was run with identical conditions as above, except that there was no 2M NH ₃ added to electrolyte. The resulting battery lost 10% performance after 5 cycles due to excessive Mn0 ₂ formation.

Example 2: Complexed Fe-Br reflow battery

Anode reaction: [Fe(CN) ₆ ] ^3" ^ [Fe(CN) ₆ ] ^4" + e- Cathode reaction: 2Br-→ Br ² + 2e-

At the beginning of charging, 50 mL of anolyte (IM K ₃ [Fe(CN) ₆ ] in IM NaOH solution) was pumped into anode structure, and 50mL catholyte (IM NaBr in IM NaOH solution) was pumped into cathode structure. The battery was charged at constant current density of 20 mA/cm ² for 1 hour. Figure 4 gives the charging/discharging characteristic chart. This battery had a relative lower output voltage around 0.6-0.7 V.

As a control, an attempt to make Fe-Br battery in neutral or alkaline pH failed due to insolubility of Fe(OH) ₃ when cyanide was not used as a ligand.

Example 3 : Complexed Cu-Br reflow battery in alkaline pH

Anode reaction: [Cu(NH ₃ ) _m ] ²⁺ + e- ^ [Cu(NH ₃ ) _n ] ⁺ + (m-n) NH ₃

Cathode reaction: 2Br- <→Br ₂ +2e-

In the above reactions, m can be 4 and n can be 3, water and hydroxide anions can also participate in forming coordination compounds. At the beginning of charging, 1 M [Cu(NH ₃ ) ₄ (H ₂ 0)x] ²⁺ solution was injected into anode structure and 75 mL catholyte (1 M NaBr) was injected into catholyte. The battery was charged at constant current density of 20 mA/cm ² for 1 hour. Figure 5 shows the characteristic charging/discharging chart. As shown in Figure 5, this battery had an excellent voltage profile and its charge/discharge voltages liedbelow that of water hydrolysis but not much lower. The voltage profile is relatively flat in both charging and discharging process.

As a control, an attempt to make Cu-Br battery in neutral or alkaline pH failed due to insolubility of Cu(OH) ₂ when ammonia or amine compounds were not added as a ligand.

Example 4: Complexed Cu-Br reflow battery in acidic pH

Complexed Cu-Br reflow battery in acidic pH has the following battery charging reactions:

Anode reaction: Cu ²⁺ + 2Br ^_1 + e <→ [CuBr ₂ ] ^_1

Cathode reaction: 2Br ^~ <→Br ² +2e ^~

During battery discharging, the above reaction is reversed, and the anode reaction is oxidation of Cu(I) to Cu(II). Although bromide is the main complexing ligand, water and hydroxide anions can also participate in forming coordination compounds. Bromine, once generated, can also exist in the complexed form such as Br ₃ ^" .

At the beginning of charging, 1 M CuBr ₂ solution was injected into anode structure and 75mL catholyte (1 M HBr) was injected into the cathode structure. The battery was charged at constant current density of 20 mA/cm ² for 1 hour. This way bromine was generated at the cathode and Cu(I) species was generated at the anode. The battery then went through discharging at constant current of 80 mA/cm ² for 10 minutes; the average voltage at this discharging current density is 0.85V.

Alternatively, the soluble Cu(I) in acidic medium could be made directly from mixing HBr and CuBr powder at > 1 : 1 molar ratio, and good solubility of Cu(I) was obtained due to excellent complexing power of bromide ions. Once bromine was introduced at the cathode, the flow battery was ready to operate (discharge) without charging.

As a control, a CuBr powder without extra bromide source was used as a source for Cu(I) at the anode. However, a functional battery could not be made due to poor solubility of CuBr without extra bromide (well below 0.1 g/L level) as a ligand.

Examples 5, 6 and 7: Aromatic Ring-Containing Complexed Iron Redox Flow Battery

Examples 5-7 are examples of using complexed iron as anode electrolyte. All these examples used a 0.3 M Br ₂ /Br ^~ redox as the catholyte and 0.1 M Fe(III)/Fe(II) as the anolyte. The only variables were the ligand type and the ligand concentration. pH were adjusted using NaOH. In these examples, all cells were initially charged to 95% state of charge and discharged to 50% state of charge, at a constant current density of 20 mA/cm ² .

Table 1

From the above Table 1 , it can be seen that cell in Example 5 using EDTA as ligand suffered greater loss of potential after 100 cycles. In contrast, the multi- substituted aromatic ligands, i.e., sulfosalicylic acid and sulfophthalic acid, both exhibited enhanced potential retention after repeated cycles. Also, a comparison between Examples 5-7 and Example 8 demonstrates that using ligands to complex Fe greatly enhanced open circuit potential and increased power density. Various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the appended claims.