CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from US Provisional Patent Application Serial No. 62/287,523, filed on January 27, 2016, the entirety of which is expressly incorporated by reference herein for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to rechargeable flow batteries, and more specifically to the charge-carrying compounds utilized in these batteries.

BACKGROUND OF THE INVENTIO

Rechargeable flow batteries, including, for example, those disclosed in US Patent Application Publication No. US2012/03266:72 entitled Reversible Polarity Operation And Switching Method For ZnBr Flow Battery When Connected To Common DC Bus, which is expressly incorporated herein by reference in its entirety for all purposes, often involve electrolytes incorporating metal ion redox couples/compounds, including but not limited to vanadium, zinc -bromine, and other compounds, within the electrolyte. The electrolyte compositions in most commercial flow battery technologies utilize aqueous or water-based electrolyte(s), although organic compounds in non-aqueous media are gaining more attention.

For aqueous electrolyte-based flow batteries, light transition metals such as vanadium

(V), iron (Fe), and zinc (Zn) are the primary candidates for use as the active compounds in the electrolyte compositions. However, in order to avoid hydrogen or oxygen generation within the battery, only those redox couples having a standard electrode/redox potential within a narrow electrochemical stability window bounded by water electrolysis can be employed. More specifically, this window is defined by the electrode/redox potential for oxygen generation at positive electrode and the electrode/redox potential for hydrogen formation at negative electrode and as such necessarily excludes some electrode redox couples from being able to be utilized for flow battery applications in aqueous media based on their redox potentials being outside this window,

To attempt to expand the number of electrode redox couples that can be utilized in these flow battery applications, some transition metal ions complexed with differen Iigands can dramatically alter the redox potential for those ions, causing the redo potential to be increased or decreased and thus to fail within the window bounded by water electrolysis. This creates many new potential opportunities for certain ions to be complexed with appropriate ligands and utilized in aqueous media flow battery applications. In addition, redox couple reaction kinetics transport properties will be changed, depending on change of electrochemical potential, ion solubility and concentration, ion conductivity and viscosity, and etc. Hence flow battery performance may be improved with appropriate Uganda complexed with metal ions in electrolyte.

For example, the ferric/ferrous standard redox potential is 0.77V vs. with regard to a standard hydrogen electrode (SHE), which can be decreased to 0,36V vs. SHE with cyanide complexes [Fe(C )6] ² [Fe(C )6] ³ ~ added/complexed onto the iron ions. In this example. cyanide (CN ^" ) is a electron-donating Iigand. Thus, the negative ferricyanide/ferrocy nkle redox couple may be suitable for a flow battery negative electrode, coupled with organic quitione positive redox. (See, K. Lin, and et a!., Science, v349 (2015) p.1529), which is expressly incorporated herein by reference in its entiret for all purposes.

Still other types of ligands have an electron-accepting property, which generally increases metal redox potential. For example, standard redox potential of [Fe(phen)3] ³ V [Fe(phen)3] ²⁺ , where the iigand is phenanthroline (phen), is about 1.06V vs. SHE, as opposed to 0.77V vs. SHE for the ferric/ferrous standard redox couple. If utilized as a redox couple on the positive electrode, the complex formed with the ferric/ferrous and the phenanthroline Iigand can increase flow battery voltage and improve iron-based flow battery energy density,

For a non-aqueous media flow battery, redox couples on both positive and negative electrodes can also be modified with appropriate ligands. Similarly, the potential advantages include a higher cell voltage or higher battery energy and power, improved reaction rates on electrodes for better efficiencies, avoidance of undesirable side-reactions, and an improved battery durability during its life-time usage. Performance of today's state-of-the-art zinc-based flow batteries, such as zinc-bromine flow battery, needs to be improved for successful commercial applications. Compared to lithium-ion battery as a main competitive energy storage device, clearly current zinc-bromine flow battery has much lower energy efficiency, energy density, and power density.

For different metal ions, such as zinc ion, iron ions, or vanadium ions utilized in flow battery electrolyte, there are unique advantages and challenges due to their unique redox potential and kinetics, solubility and conductivity, operation pH range, and etc. The extent of metal ion binding interaction with various complex ligand(s) can be vastly different. Hence this results in different extent of electrochemical potential change when metai complexes are formed with ligands. Metai-ligand complexes can be drastically different, e.g., have drastically different behavior, from metal ions in an aqueous solution, depending on both metal ions and complexing ligands. For a specific metal ion, its suitable ligands and formed complexes containing electrolyte compositions need to be tailored for its flow battery application. In some cases, a suitable !igand for one metal ion may be a risk choice for another metal ion. For example, ferrieyanide/ferrocyanide negative redox can be paired with organic quinone positive redox for an alkaline flow battery. But the cyanide ligand is very poisonous in acidic electrolyte. Hence great care on electrolyte leakage and pH control must be required if cyanide is completed zinc ion in acidic electrolyte for zinc based flow battery.

SUMMARY OF THE INVENTION

Briefly described, according to an exemplary embodiment of the invention, Zn ² ~ ions are complexed with different ligand(s) to form a main active redox compound for us in an electrolyte flow battery. The Zn ²⁺ -ligand complex significantly enhances the operation of the flow battery by altering the redox potential of the Zn/Zi ' complex, thereby enhancing the charge efficiency, the voltage efficiency and/or the energy efficienc of the flow battery over multiple charge discharge cycles for the battery. To date, no zinc complex has been utilized in zinc based electrolyte flow batteries. Thus, the invention presents an improved zinc-based flow battery with zinc metal complexes in the electrolyte.

According to another exemplary embodiment of the invention, the Zn ²⁺ -ligand complex can be added to an existing Zn/Zn ²⁺ redox couple in an elecirofyte flow battery as an additive for performance improvement of the battery inc luding the Zn/Zn ²⁺ redox couple.

These and other aspects, advantages and exemplary embodiments of the invention will be made apparent from the following detailed description taken together with the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures illustrate the best mode currently contemplated of practicin the invention.

in the drawings:

FIG. 1 is a schematic diagram of a stack of alternatively disposed zinc-bromine battery components, cooperating with electrolyte reservoirs according to an exemplary embodiment of the invention. FIG. 2 is a perspective, exploded view of a stack of alternately disposed zinc-bromine battery components according to an exemplary embodiment of the invention.

FIG. 3 is a schematic diagram of a zinc -bromine battery cell showing electrolyte flow to and from the reservoirs and through the battery according to an exemplary embodiment of the invention.

FIG. 4 is a graph of the charge efficiency of an electrolyte flow battery constructed according to one exemplar)' embodiment of the invention over number of charge/discharge cycles,

FIG, 5 is a graph of the voltage efficiency of an electrolyte flow battery constructed according to one exemplary embodiment of the invention over number of charge/discharge cycles.

FIG. 6 is a graph of the energy efficiency of an electrolyte flow battery constructed according to one exemplary embodiment of the invention over number of charge/discharge cycles.

FIG. 7 is a graph of the discharge energy density (mW/cm ² ) of an electrolyte flow battery constructed according to one exemplary embodiment of the invention over number of charge/discharge cycles.

FIG. 8 is a graph ^' of the discharge power density (mW/cm ² ) of an electrolyte flow battery constructed according to one exemplary embodiment of the invention over number of charge/discharge cycles.

DETAILED DESCRIPTION OF THE INVENTION

Referring more particularly to the drawings, one particular exemplary embodiment of an electrolyte flow battery including zinc complexes, as are known in the art, such as US Patent Nos. 4,049,886; 5,002,841; 5, 188,9.55 and 5,650,239, US Patent Application Publication No. 2012/0326672, each of which is expressly incorporated by reference herein for all purposes in its entirety, and which each disclose a zinc-bromine battery, is shown in an exploded vie and is designated generally by the numeral 10 in FIG. I . The zinc-bromine battery 10 includes a series of electrodes U and separators 12, welded together to form a stack 1 3 of electrochemical cells. Each battery 10 includes a predetermined number of electrodes 1 1 and separators 12 and, thus, a predetermined number of electrochemical cells. As best seen in FIG. 2, respective endb!ocks 14 are disposed at each end of the batter 1.0. The endblocks 14 each have a pair of openings 15 in which a pair of terminal studs 1 are positioned. The terminal studs 16 are electrically coupled to the battery's terminal electrodes 17 which may be mounted directly adjacent to the. endblocks. The terminal studs provide a convenient means through which current may enter and leave the battery. Each terminal electrode is a current collector means capable of collecting current from, and distributing current to, the electrochemical cells of the battery. Although not shown, it should be understood that terminal electrodes are mounted on. or are adjacent to, each end block.

Referring back to FIG. 1, aqueous, or optionally non-aqueous, catholyte is stored in a catholyte reservoir 20, A catholyte pump 22 pumps aqueous catholyte through a common catholyte manifold 24 into each cathodic half cell as indicated by the arrows labeled A in FIG. 1 , and back to the catholyte reservoir 20 through a catholyte return manifold 26.

Similarly, aqueous, or optionally non-aqueous, anolyte is stored in an anolyte reservoir

30 and pumped through an anolyte inlet manifold 32 by an anolyte pump 34. The anolyte flows through each anodic half-cell, one of which is disposed between each cathodic half-cell, and back to the anolyte reservoir 30 through an anolyte return manifold 36, as indicated by the arrows labeled B in FIG. 1 . Thus, the electrochemical cells of the battery 10 are coupled in fluid flowing relation to the reservoirs 20 and 30 through the manifolds .24, 26, 32, and 36.

Each electrode and separator includes a thin sheet of electrode or separator material, respectively. These sheets are individually mounted in a nonconducttve flow frame 40. Preferably, the nonconductive flow frame is made from a polymeric material such as polyethylene. Long, winding electrolyte inlet and outlet channel patterns are incorporated into one or both sides of the separator frame, the electrode frame, or both. The geometry of the channels, contributes to the electrical resistance required to reduce shunt currents which result in cell power losses. A leak-free internal seal is maintained along the channels and about the common perimeter of adjacent separators and electrodes.

As can be more readily seen by reference to the schematic representation of FIG. 3, during charge electron flow through the battery 10 results in zinc being plated on an anode or zinc electrode 100 which is in an anodic half-ceil 1 10. During the same time bromine is evolved at a cathode or bromine electrode 120 which is in a cathodic half-cell 130. When the bromine is evolved it is immediately complexed with a quaternary salt and is removed from the battery to the catholyte reservoir 30. The complexed bromine or dense second phase is separated by gravity from bromine in the reservoir. Normally, on discharge, the complexed bromine or second phase is returned to the battery stack where bromine is reduced to bromide ion and zinc metal is oxidized to zinc ion. In exemplary embodiments of this invention, various iigands are utilized that form a complex with zinc ions used as an electrolyte in an electrolyte flow battery. The selected Iigands can act as either electron donor or electron acceptor when complexed with the zinc, such that the resulting zinc complex can have a resultant decrease or increase in the redox potential from the non-complexed zinc ions rendering the zinc complexes suitable for use in an electrolyte flow battery. This decrease or increase in the redox potential can be selected based on the ligand used in order to thermodynamicaUy favor zinc metal dissolution or zinc plating, as desired. For example, an electron-donor ligand can reduce Zn ² 72n standard redox potential of -0.76V vs SHE (standard hydrogen electrode). If .this formed zinc complex is utilized as a redox couple on a negative electrode of a zinc based flow battery, cell voltage will be increased together with energy density, and may also speed up zinc metal dissolution during batter discharge hence better battery efficiency. But this zinc complex redox potential shall not fall far below -1.0V vs SHE, since hydrogen formation on negative electrode may be significantly increased.

Many zinc complexes formed with the organic Iigands may have a limited solubility or be insoluble in water. However, they can be mixed with non-complexing zinc redox compounds or even utilized as an additive, which may still provide performance benefits in: the electrolyte flow batteries in which they are utilized. The concentration/amount of the ligand that can be added to form the complex with the zinc or as an additive, can be between 0.01M to 5.0M, or between 0.05M to 0,5 . One detailed example with only 0.2M disodium ethylenediaminetetraacetate (EDTA) as the zinc complex ligand in 2.5M ZnC anolyte will be demonstrated later.

According to some exemplary embodiments of the invention, certain electron-donor type Iigands that can be complexed with zinc ions include but are not limited to i\ Br ^" , CI ^" , F ^" , OH ^" , thiocyanate ion (SCN ^" ), oxalate ion, cyanide ion (CN ^" ), ethy nediamine (en), ethylenediaminetetraacetate (EDTA) ions, and citrate ions, among others including multiple mixed Iigands. in some additional exemplary embodiments of the invention, certain electron- acceptor type Iigands thai can be complexed with zinc ions include but are not limited to phenanthroline (phen) or its substituted derivatives, boron compounds, such as BCb, BBn, and others, and aluminum compounds, such as AiCb, A!Brs, and others including multiple mixed Iigands. Often the applied Iigands concentratio can be u to their maximum solubility in electrolyte. But the desirable ligand concentration may be lower due to other limitations such as water electrolysis side reactions when redox potential drastically shifted. According to■ one particular exemplary embodiment of the invention, disodium EDTA (with a limited solubility up to about 0.4M in water) was added to an aqueous solution of zinc ions in order to form a zinc-EDTA complex according to reaction 1 . This completing of the zinc ions drives reaction 2 towards zinc dissolution during discharge, of a zinc-bromine flow battery. When disodium EDTA (0.2M) was added to the anolyte including the zinc ions formed of 2.5M ZnCfc + 3M NaC!. + ^■ 0.3 ^' M 1. -Ethyl- 1 -methylpyrrolidjnium (MEP) in a batter}' having a .catholyte with 2.5M ZnBf2÷ 3M NaBr + ^■ 0.3M MEP, the resulting ZnEDTA compiex improved the first cycle charge efficiency (FIG. 4), voltage efficiency (FIG. 5), and energy efficiency (FIG. 6) over an electrolyte flow battery utilizing zinc ions in the anolyte without the addition of the disodium EDTA ligand, the reference in FIGS. 4-6. in the following charge-discharge cycles, battery efficiencies, discharge energy density (FIG. 7} and discharge power density (FIG. 8) all improved with EDTA utilized in zinc ion electrolyte.

Zn^(aq) + EDTA ^2" (aq) - [ZnEDTA](aq) (reaction 1 )

Zn(s)=Zn ²⁺ (aq) ÷ 2e ^" (reaction 2)

Other ligands, such as sodium citrate, wit -a much higher solubility in water may allow cell performance to be further improved, in addition, the zinc complexes using ligands of this type is not limited to a zinc-bromine electrolyte flow battery. The zinc complex redox couple formed by these ligands can be paired with any other redox couple, including, but not limited to, VQzVVO ² " ^" , ierric/ferrous, quinone/hydroquinone, among others, in an electrolyte flow battery with a practical cell voltage.

Various other embodiments of the invention are contemplated as being within the scope of the filed claims particularl pointing out and distinctly claiming the subject matter regarded as the invention.