Patent Description:
Redox flow batteries (RFB) are secondary (rechargeable) fuel cells and battery - fuel cell hybrids (<FIG>). Unlike traditional batteries, RFBs decouple system energy and power, and so like afuel cell have a total system energy that scales with the size of electroactive material storage tanks and a system power that scales with the size of the electrochemical reactor. This trait makes them ideal for storing large amounts of electricity at low cost, since additional electroactive material is usually far less expensive than a larger reactor. For this reason, RFBs are currently used and being further developed as massive, grid-level electricity storage devices. This is especially valuable for intermittent renewable energy sources, which, in order to stabilize their output to current electrical grids, require a source of backup electricity when the given resources (wind, solar) becomes temporarily unavailable.

Many RFB chemistries have been developed, yet they universally use oxidants that undergo <NUM> or <NUM> e- reductions (n = <NUM> or <NUM>) and/or have low to moderate solubility (Cmax), greatly limiting (<NUM>) their volumetric energy densities and (<NUM>) their power densities. This applies to V0<NUM>+, Ce<NUM>+, Fe<NUM>+ (<NUM> e-), Br<NUM> (<NUM> e- and O<NUM> (<NUM> e-)), the oxidants in all-vanadium, Zn/Ce<NUM>+, Cr<NUM>+/Fe<NUM>+, H<NUM>/Br<NUM>, and H<NUM>/O<NUM> RFBs, respectively. While O<NUM> and Br<NUM> have high n, their values for Cmax are much smaller than those for the other species (<NUM> and <NUM> vs. > <NUM>). The lower energy and power densities associated with RFBs result in very large battery sizes relative to competing technologies, such as lead-acid and Li-ion batteries. The sheer size of such batteries contributes to unwanted facilities, operational, and materials costs, as well as restrictions an battery siting.

With the aim to provide significantly higher energy and power density <CIT> developed a cathodic half-cell based on Cr<NUM>O<NUM><NUM>-. In order to overcome the problem that Cr<NUM>O<NUM><NUM>-'s reduction product, Cr<NUM>+, cannot be readily oxidized (i.e. recharged) heterogeneously at known electrode catalysts and combined it with an electron mediator capable of homogeneously oxidizing CrIII to CrVI using an ECcal mechanism in solution.

However, said half-cell's electrolyte contains Cr<NUM>O<NUM><NUM>- as well as highly reactive electron mediator species (such as Ce<NUM>+) at high concentration in an acidic environment, which causes deleterious interactions with many electrode materials not made from noble metal. While noble metals may be useful as an electrode material, their economic disadvantages are prohibitive to their use in large scale applications such as RFBs.

<CIT>, <CIT>, <CIT>, <CIT> disclose the use of glassy carbon for Ce<NUM>+ oxidation. However, glassy carbon is expensive, difficult to machine and prone to fracture, thus not very suitable for RFB applications. Furthermore, most flow batteries with acid electrolyte degrade carbon-based electrodes as disclosed in <CIT>. <NPL> that Pt-Ir and SGL carbon electrodes are best for Ce<NUM>+, they proceed with the far more expensive Pt-Ir anyway and later disclosed in <NPL> that carbon electrodes degrade rapidly, so that Pt-Ir electrodes are required for Zn/Ce RFBs operating with methane sulfonic acid. <NPL> that pristine carbon paper can be used for Ce<NUM>+ oxidation, but not only have such electrodes a low surface area, they are also rapidly degraded (as witnessed by the reduction of the Ce<NUM>+ peak with increasing with cycle number), which the authors mentioned as: "the deterioration of the CP surface was also confirmed by the decrease of the electrical conductivity.

Embodiments, wherein a carbon material has been used for the chemically much more aggressive combination of a CrIII/CrVI redox couple with a Ce<NUM>+/Ce<NUM>+ redox couple and a strong acid such as methane sulfonic acid, sulfuric acid and the like have not been disclosed yet.

Accordingly, it was the object of the present invention to develop a suitable electrode material essentially not consisting of noble metal for a positive (cathodic) half-cell employing the CrIII/CrVI redox couple in combination with a Ce<NUM>+/Ce<NUM>+ redox couple and a strong acid such as methane sulfonic acid, sulfuric acid and the like.

In a first aspect, the present invention provides a rechargeable redox flow battery (RFB) comprising an electrochemical cell comprising at least one positive electrode in a positive half-cell and a negative electrode in a negative half-cell, an ion-conducting membrane between the two half-cells, wherein the membrane is designed for dual acidic anolyte and catholyte or designed for an acidic catholyte and an alkaline anolyte; at least two storage tanks for catholyte and anolyte, optionally one or more pumps to circulate stored catholyte and anolyte through the cathodic and anodic half cells, respectively, and at least one anolyte and at least one catholyte;.

In another aspect, the present invention also relates to the use of the redox flow battery as described herein to store electrical energy for grid-level energy storage, homeowner energy storage, remote locations, firming or load leveling of intermittent renewable electricity generation site, preferably wind and solar farms, micro-hydropower, geothermal energy, tidal power, energy arbitrage, portable and/or personal electronics, electric vehicles such as ships, submarines, planes, unmanned underwater vehicles (UUVs) or unmanned aerial vehicles (UAVs), military electronics equipment, satellites and other manned or unmanned spacecraft, or other applications where rechargeable RFBs can be beneficially employed.

A further aspect of the present invention is a method for operating the positive half-cell of the RFB and for storing electrical energy in said RFB comprising:.

An "ECcat mechanism" (also denoted as EC' in<NPL>) is described in the following general manner: it consists of an electrochemical step (hereafter E) in which a given species A is converted to species B. This is followed by a subsequent chemical-catalytic step (hereafter Ccat) between B and species C, which regenerates species A from B and produces product D from C. The regeneration of A represents a catalytic process that gives an apparent increase in concentration of A near the electrode surface, generating higher than expected electrochemical current for the reduction of A to B: <MAT> <MAT>.

Since regenerated A must remain at the electrode surface to be detected, high currents are detected at (<NUM>) shorter time periods between consumption and regeneration of A and (<NUM>) decreased flow rates of solution across the electrode, which decrease the time for transport and the rate of transport for A leaving the electrode surface.

The observed "electrochemical potential," or simply "potential," E, of a given soluble redox
species (e.g., Ce<NUM>+) participating in a reversible reduction (e.g. Ce<NUM>+ + <NUM> e- ⇆ Ce<NUM>+) is defined by the Nernst equation: <MAT> where Cox and Crod are the respective concentrations of the oxidized and reduced forms of the given redox species (e.g. Ce<NUM>+ and Ce<NUM>+), n is the number of electrons involved in the conversion of the oxidized to the reduced form of the given redox species, R is the universal gas constant, T is the temperature, and F is Faraday's constant. The term E<NUM> is the standard potential of the redox species, which is the observed potential (E) when Cox = Crod. In the context of the invention, the Nernst equation implies that the potential of a given reversible redox species varies from its E<NUM> depending on the ratio of the concentrations of the oxidized to reduced form. Thus a ratio of Ce<NUM>+ to Ce<NUM>+ of <NUM> to <NUM> shifts the E of Ce<NUM>+ positive by <NUM> V, and a ratio of <NUM> to <NUM> shifts E positive by <NUM> V.

A "rotating disk electrode," RDE, is used in RDE voltammetry to achieve laminar flow across an electrode surface. When planar RDEs are used, as in the results presented herein, RDE voltammetry allows direct assessment of a multitude of fundamental parameters of a given fuel or oxidant for use in RFBs. Because RDE voltammetry assesses fundamental parameters, rather than a given fuel cell's or RFB's performance, RDE voltammetry results are universally comparable across all possible systems.

The "mass transport limited current", iL of an electrochemical reaction in RDE voltammetry is the maximum obtainable current at a given RDE rotation rate. Since the iL is a function of the steady-state transport that originates from laminar flow across the RDE, the IL is independent of potential, and appears as a horizontal line in an RDE voltammogram.

The half-wave potential, E<NUM>/<NUM>, of a given electrochemical reaction in RDE voltammetry is the potential at which ½ of the iL is achieved. A reasonably accurate measurement of E<NUM>/<NUM> must be performed at fairly low current density; at high current density in analytical glassware, solution resistance distorts the RDE voltammogram with a diagonal line that significantly shifts the E<NUM>/<NUM>. To achieve low current density for a high n oxidant like Cr<NUM>O<NUM><NUM>- , low concentration (<NUM> to <NUM>) and low to moderate rotation rate (<NUM> to <NUM> rpm) must be used. In the context of the Invention, the E<NUM>/<NUM> is useful as a first-order assessment of the operating potential of a given electrode, and thus the operating voltage of a RFB.

The terms "NHE" and "Ag/AgCl" refer to two standard types of reference electrodes used to measure the E of an electrode, where NHE is the Normal Hydrogen Electrode and Ag/AgCl is the silver / silver chloride electrode, and a given EAg/AgCl = EHHE - <NUM> V. Unless stated otherwise, all unreferenced potentials are shown as V vs. Ag/AgCl, not V vs. NHE.

The term storage tank refers to vessels wherein one or more liquids and/or gases can be stored. The liquids and/or gases may be separated from each other by a baffle or other suitable partition walls.

The terms anolyte and catholyte are familiar to the person skilled in the art of RFBs and are described e.g. in <NPL>.

Typically, the catholyte of the redox flow battery as described herein is a liquid comprising a CrIII/CrVI redox couple, a Ce<NUM>+/Ce<NUM>+ redox couple as an reversible electron mediator, a strong acid such as methane sulfonic acid, sulfuric acid and the like as an electrolyte for conductivity (and pH control) and a solvent.

In a preferred embodiment, the anolyte of the redox flow battery as described herein is a liquid comprising a solution of a fuel capable of reversible oxidation; an electrolyte for conductivity (and pH control) and a solvent.

Using the combination of a CrIII/CrVI redox couple with a Ce<NUM>+/Ce<NUM>+ redox couple together with a strong acid such as methane sulfonic acid, sulfuric acid and the like provided some difficulties in finding a suitable electrode material.

Surprisingly, the problem could be solved by a rechargeable redox flow battery (RFB) comprising an electrochemical cell comprising at least one positive electrode in a positive half-cell and a negative electrode in a negative half-cell, an ion-conducting membrane between the two half-cells, wherein the membrane is designed for dual acidic anolyte and catholyte or designed for an acidic catholyte and an alkaline anolyte; at least two storage tanks for catholyte and anolyte, optionally one or more pumps to circulate stored catholyte and anolyte through the cathodic and anodic half cells, respectively, and at least one anolyte and at least one catholyte;.

The strong acid used for the catholyte includes acids with a pKa of <NUM> or less, e.g. methane sulfonic acid, HNO<NUM>, H<NUM>SO<NUM>, HClO<NUM>, H<NUM>PO<NUM>, or mixtures thereof, preferably methane sulfonic acid. As the acid can be (reversibly) consumed during the operation of the RFB, the term strong acid encompasses in a preferred embodiment also the deprotonated form of the respective acid if these would undergo transformation to such acids during the typical operation of the RFB.

The electrolyte for the anolyte includes acids with a pKa of <NUM> or less e.g. methane sulfonic acid, HNO<NUM>, H<NUM>SO<NUM>, HClO<NUM>, H<NUM>PO<NUM>, or mixtures thereof, and also electrolytes for conductivity such as MClO<NUM>, MNO<NUM>, M<NUM>SO<NUM>, MF, MCl, MBr, or MI, where M = Li, Na, or K, tetra-n-butylammonium X, where X = F, Cl, Br, I, or hexafluorophosphate.

The reversible fuel for the anolyte is typically a liquid comprising a redox couple selected from the group consisting of Zn<NUM>+/Zn<NUM>; H+/H<NUM>; VIII/VII; CrII/Cr<NUM>: CrIII/CrII; AIII/Al<NUM>; ZrIV/Zr<NUM>; CoII/Co<NUM>; NlII/Nl<NUM>; CdII/Cd<NUM>; InIII/InII/InI/In; GaIII/GaI/Ga; SnII/Sn: SnIV/SnII; SbIII/Sb<NUM>; PbII/Pb<NUM>; LiI/Ll<NUM>; NaI/Na<NUM>; and/or the oxidized and reduced conjugates of anthraquinone <NUM>,<NUM>-disulfonate, and mixtures thereof.

Accordingly, the cathode can be made from any polymorph of conductive (sp<NUM>) carbon or doped diamond, including carbons that are partially nonconductive (sp<NUM>), partially oxidized (with various oxygen functionalities, such as -CO, -CO<NUM>H, and quinone or semiquinone moieties), partially fluorinated, or have been subjected to various heat treatments. Such polymorphs include, but are not limited to, graphite, carbon black, vulcanized rubber, glassy carbon, carbon papers, carbon felts, charcoal, and boron-doped diamond. Typical examples are carbon black, carbon felt, graphite, carbon and graphite felt, carbon flakes, carbon paper, carbon fiber, carbon nanotubes, carbon nanofibers, graphene and/or glassy carbon, preferably from carbon felt, carbon and graphite felt, and/or carbon black, most preferably from carbon and graphite felt. In particular embodiments the carbon of the positive electrode can comprise catalytically active metals selected from Pd, Ag, Pt, Ni, Ir, Ru, Rh, Au, can be doped with nitrogen or can be activated e.g. by thermal treatment, acid treatment, hydrogen peroxide treatment and/or plasma treatment.

In a preferred embodiment, the Ce<NUM>+/Ce<NUM>+ redox couple is used in the form of cerium(ill) methanesulfonate, which means partially, mainly or only Ce(CH<NUM>SO<NUM>)<NUM> is employed as the Ce<NUM>+ species. Ce(CH<NUM>SO<NUM>)<NUM> has a high solubility of ><NUM> In methane sulfonic acid, CH<NUM>SO<NUM>H. This compound can be prepared from the inexpensive precursors cerium carbonate, Ce<NUM>(CO<NUM>)<NUM>, and methane sulfonic acid. At the same time, CrVI/Cr<NUM>+ is preferably employed in the form of chromium(III) methanesulfonate, which means partially, mainly or only Cr(CH<NUM>SO<NUM>)<NUM> is employed as the Cr<NUM>+ species. This compound can be prepared by dissolving CrO<NUM> in water to form H<NUM>Cr<NUM>O<NUM>, subsequently reducing the CrVI to Cr<NUM>+ with hydrogen peroxide (H<NUM>O<NUM>) followed by addition of methane sulfonic acid. In this embodiment, methane sulfonic acid is preferably employed as a strong acid in the catholyte.

In a preferred embodiment, Zn is employed as the fuel for the anodic half-cell. Preferably Zn is employed in form the Zn<NUM>+ / Zn<NUM> redox couple wherein the Zn<NUM>+ species consists partially, mainly or only of Zn(CH<NUM>SO<NUM>)<NUM>, i.e. zinc(II). This compound can be prepared from ZnO and CH<NUM>SO<NUM>H. In this embodiment, methane sulfonic acid is preferably employed as a strong acid in the anolyte.

With respect to the above, "partially" refers to an amount of <NUM> to <NUM> mol% of the metal ions, "mainly" to an amount of <NUM> to <NUM> mol%, preferably <NUM> to <NUM> mol% of the metal ions, and "completely" to more than <NUM> mol% to <NUM> mol% of the metal ions.

In one embodiment of the invention, the initial battery formulation contains Zn in form of Zn<NUM>, e.g. a zinc plate and Cr in form of Cr<NUM>O<NUM><NUM>-.

In a preferred embodiment of the invention, the initial battery formulation contains Zn in form of a Zn<NUM>+ and Cr in form of Cr<NUM>+. This embodiment provides the following advantages:.

This configuration allows for all species to coexist in the solution and yet still produce enough acid to consume all of the CrVI generated. The preferable molar ratio of Cr(CH<NUM>SO<NUM>)<NUM> to Ce(CH<NUM>SO<NUM>)<NUM> in the anolyte is in the range from <NUM>:<NUM> to <NUM>:<NUM>, particularly preferably from <NUM>:<NUM> to <NUM>:<NUM> and most preferably <NUM>,<NUM>:<NUM>.

When carbon electrodes are subjected to potentials above +<NUM> V vs. Ag/AgCl in an acidic media, the carbon begins to oxidize. Oxidation of carbon increases its surface area. This in turn increases the size of carbon's native surface peaks, which are generated from the reversible oxidation and reduction of semiquinone-like species. Thus the extent of peak growth is used as an assessment of stability of the carbon electrode at the applied potential - the greater the size of the peaks, the greater the surface area, and hence the greater the amount of oxidative damage to the carbon surface from the applied potential.

Surprisingly, it has been found that at applied potentials of up to +<NUM> V vs. Ag/AgCl, the growth of the peaks is slowing down over the number of cycles and reaches a limiting value. At applied potentials of higher than <NUM>. 5V vs. Ag/AgCl the peak growth is continuous and does not reach a limiting value. This reflects a continuous degradation of the carbon at potentials ≥ +<NUM> V.

In order to achieve a high number of cycles the potential applied to the positive half-cell during charging of the battery should essentially not exceed <NUM>,<NUM> V, preferably <NUM>,<NUM> V and most preferably <NUM>,<NUM> V vs. Ag/AgCl. Essentially means that less than <NUM> % of the charge, preferably less than <NUM> % of the charge, more preferably less than <NUM>% of the charge, particularly preferably less than <NUM> % of the charge and most preferably less than <NUM>% of the charge should occur at a potential above the specified one.

Another embodiment of the invention is a method for operating a positive half-cell in a rechargeable redox flow battery. (RFB) comprising an electrochemical cell comprising at least a positive electrode in a positive half-cell and a negative electrode in a negative half-cell, an ion-conducting membrane between the two half-cells, and at least a anolyte and at least a catholyte; wherein the catholyte comprises the CrIII/CrVI redox couple, a strong acid, the Ce<NUM>+/Ce<NUM>+ redox couple and at least s solvent, wherein the positive electrode is made from carbon, wherein the potential applied to the positive half-cell during charging of the battery should essentially not exceed <NUM>,<NUM> V, preferably <NUM>,<NUM> V and most preferably <NUM>,<NUM> V vs. Ag/AgCl.

In a preferred embodiment, the anolyte contains In<NUM>+ and/or the anode In<NUM>. Thus, the spontaneous oxidation of Zn<NUM> under formation of H<NUM> in aqueous media can be suppressed. Sub-stoichiometric amounts of In with regard to Zn are sufficient, such as less than <NUM> mol%, preferably less than <NUM> mol% more preferably less <NUM> mol% and most preferably <NUM> mol% and less are sufficient. When Zn and In are co-deposited on the anode, the resulting ZnggIn<NUM> alloy has a surface with greatly reduced activity for H<NUM> evolution. The molar amount of In with regard to Zn In the anolyte and anode is less than <NUM> mol%, preferably less than <NUM> mol% more preferably less than <NUM> mol% and most preferably not more than <NUM> mol%.

While high current efficiencies for Zn deposition at carbon electrodes can be achieved by high concentrations of Zn(CH<NUM>SO<NUM>)<NUM> in CH<NUM>SO<NUM>H (methane sulfonic acid), Zn showed a current efficiency of <NUM>% for <NUM> Zn(CH<NUM>SO<NUM>)<NUM> in <NUM> CH<NUM>SO<NUM>H only when stripping at very low voltage efficiency in order for Zn stripping to outpace Zn decomposition to H<NUM>.

Indium shows <NUM> to <NUM>% current efficiency for <NUM> InCl<NUM> in <NUM> CH<NUM>SO<NUM>H, with significantly lower current efficiencies at longer deposition times. This suggests that In rapidly decomposes to H<NUM> and In<NUM>+ when left in solution.

However, Zn-In showed up to <NUM>% current efficiency analytically and <NUM> to <NUM>% current efficiency at the device level. Moreover, Zn and In significantly stabilize one another to suppress oxidation of Zn and/or In under H<NUM> formation. Economically advantageous In ingots can be easily dissolved in CH<NUM>SO<NUM>H as a substitute for expensive InCl<NUM>.

Zn-In shows <NUM>% current efficiency at <NUM>% estimated voltage efficiency using the following potential program:.

In step a), the deposited Zn-In can cover the carbon electrode and block carbon from creating H<NUM> as Zn-In has a higher overpotential for H<NUM> evolution than carbon and the sooner the carbon is covered, the less H<NUM> is generated by passing current through the electrode.

Step b) lasts for a longer period of time (e.g. <NUM>). At potential of -<NUM> V (+/- <NUM>. 1V) more Zn-In is deposited at lower overpotential. Said potential represents only <NUM> V (+/- <NUM>. 1V) overpotential for Zn deposition.

Step c) also lasts for a longer period of time (e.g. <NUM>). A somewhat high overpotential for stripping (<NUM> V) seems advantageous to strip off Zn-In fast enough to limit the competing decomposition to H<NUM>.

In step d), Zn-In is stripped for a short period of time to remove layers of In that protect Zn (and prevent it from being stripped at -<NUM> V). Though Zn and In codeposit at the same potential, In requires higher potentials for stripping than Zn.

A further improvement of charge efficiency can be effected by combining the embodiments according to the invention with a (catalytic) H<NUM> recombinator to ensure that the battery remains balanced during multiple cycles. H<NUM> recombinators are devices known to the person skilled in the art and used e.g. in commercially available Zn-Br<NUM> RFBs.

In the present invention, the solvent(s) for the catholyte and anolyte can be selected from the group consisting of water or a nonaqueous solvent, such as acetonitrile, dimethyl sulfoxide, dimethyl formamide, methanol, ethanol, <NUM>-propanol, isopropanol, ether, diglyme, tetrahydrofuran, glycerol, and mixtures thereof.

In another aspect, the present Invention relates to the use of the redox flow battery as described herein to store electrical energy for grid-level energy storage, homeowner energy storage, remote locations, firming or load leveling of intermittent renewable electricity generation site, preferably wind and solar farms, micro-hydropower, geothermal energy, tidal power, energy arbitrage, portable and/or personal electronics, electric vehicles such as ships, submarines, planes, unmanned underwater vehicles (UUVs) or unmanned aerial vehicles (UAVs), military electronics equipment, satellites and other manned or unmanned spacecraft, or other applications where rechargeable RFBs
can be beneficially employed.

Those of skill in the art will realize that many modifications and variations can be employed without departing from the spirit and scope of the invention. The present invention is now further illustrated by reference to the following, non-limiting examples.

A saturated solution of Ce<NUM>(CO<NUM>)<NUM> was prepared in water, with excess Ce<NUM>(CO<NUM>)<NUM> lying on the bottom of the glassware. Pure (<NUM>) CH<NUM>SO<NUM>H was then added to the solution while stirring until some of the Ce<NUM>(CO<NUM>)<NUM> dissolves. At this stage, excess solid Ce<NUM>(CO<NUM>)<NUM> was still present at the bottom of the glassware. The supernatant was then separated from the solid Ce<NUM>(CO<NUM>)<NUM>, placed in a watch glass or petri dish and heated in an oven to <NUM> to <NUM> to evaporate the water and excess CH<NUM>SO<NUM>H, leaving a precipitate of Ce(CH<NUM>SO<NUM>)<NUM>.

ZnO was dissolved in CH<NUM>SO<NUM>H, and the O<NUM>- counterion is reacted to H<NUM>O in acid, yielding a solution containing only Zn(CH<NUM>SO<NUM>)<NUM> and CH<NUM>SO<NUM>H and water, which could be used without the need to precipitate Zn(CH<NUM>SO<NUM>)<NUM>.

A saturated solution of H<NUM>Cr<NUM>O<NUM> was prepared by dissolving CrO<NUM> in water. Then, the saturated H<NUM>Cr<NUM>O<NUM> was added dropwise to a solution of <NUM>% pure (<NUM>) CH<NUM>SO<NUM>H and <NUM>% of <NUM>%-by-mass (<NUM>) H<NUM>O<NUM> under cooling in a cold-water bath to prevent the solution from boiling. The resulting solution was stirred and additional H<NUM>O<NUM> added dropwise until all of the CrVI has been reduced to Cr<NUM>+, which was evident by a change in coloration of the solution. Subsequently, a solution of <NUM> Cr<NUM>+ was prepared by adding H<NUM>Cr<NUM>O<NUM> to CH<NUM>SO<NUM>H in a <NUM>:<NUM> ratio, which corresponds to a <NUM>:<NUM> or <NUM>:<NUM> ratio of CrO<NUM> to CH<NUM>SO<NUM>H.

Ce(CH<NUM>SO<NUM>)<NUM> was added to the solution obtained from the synthesis of chromium(III) methane sulfonate to obtain an anolyte starting material.

Claim 1:
Rechargeable redox flow battery (RFB) comprising an electrochemical cell comprising at least a positive electrode in a positive half-cell and a negative electrode in a negative half-cell, an ion-conducting membrane between the two half-cells, wherein the membrane is designed for dual acidic anolyte and catholyte or designed for an acidic catholyte and an alkaline anolyte; at least two storage tanks for catholyte and anolyte, optionally one or more pumps to circulate stored catholyte and anolyte through the cathodic and anodic half cells, respectively, and at least one anolyte and at least one catholyte; wherein the anolyte comprises at least one fuel capable of reversible oxidation, and a Zn2+/Zn0 redox couple and In3+ at least one electrolyte for conductivity and at least one solvent; and the catholyte comprises a CrIII/CrVI redox couple, at least one strong acid, a Ce<NUM>+/Ce<NUM>+ redox couple and at least one solvent,
wherein the positive electrode is made from carbon,
wherein the molar amount of In with regard to Zn in the anolyte and anode is less than <NUM> mol%, preferably less than <NUM> mol% more preferably less than <NUM> mol% and most preferably not more than <NUM> mol%.