WO2020078895A1

WO2020078895A1 - Dichromate / zink redox flow batteries

Info

Publication number: WO2020078895A1
Application number: PCT/EP2019/077765
Authority: WO
Inventors: David Finkelstein; Pierre Bonin; Marian CHATENET
Original assignee: Lanxess Deutschland GmbH
Current assignee: Lanxess Deutschland GmbH
Priority date: 2018-10-16
Filing date: 2019-10-14
Publication date: 2020-04-23
Anticipated expiration: 2021-04-16
Also published as: EP3641040C0; EP3641040B1; EP3641040A1

Abstract

The present invention relates to a rechargeable redox flow battery (RFB) comprising an electrochemical cell with at least one positive electrode in a positive half-cell and a negative electrode (anode) in a negative half-cell, wherein the negative half-cell comprises the Zn 2+ /Zn 0 redox couple, at least methane sulfonic acid or its anion as an electrolyte for conductivity; and the positive half-cell comprises the Cr IIl /Cr VI redox couple, the Ce 4+ /Ce 3+ redox couple as a electrochemically reversible electron mediator at least methane sulfonic acid or its anion as an electrolyte, and at least one solvent.

Description

DICHROMATE / ZINK REDOX FLOW BATTERIES

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The invention describes novel redox flow battery systems, which are electrochemical cells

used for electricity storage.

Redox flow batteries (RFB) are secondary (rechargeable) fuel cells and battery-fuel cell hybrids (Fig.1 ). Unlike traditional batteries, RFBs decouple system energy and power, and so like a fuel 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) become temporarily unavailable.

Many RFB chemistries have been developed, yet they universally use oxidants that undergo 1 or 2 e reductions (n = 1 or 2) and/or have low to moderate solubility (C_max), greatly limiting (1 ) their volumetric energy densities and (2) their power densities. This applies to V0²⁺, Ce⁴⁺, Fe³⁺ (1 e ), Br₂ (2 e and 0₂ (4 e ), the oxidants in all-vanadium, Zn/Ce⁴⁺, Cr²⁺/Fe³⁺, H₂/Br₂, and H₂/0₂ RFBs, respectively. While 0₂ and Br₂ have high n, their values for C_max are much smaller than those for the other species (0.001 and 0.2 M vs. > 1 M). 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 WO 2016/202950 developed a cathodic half-cell based on Cr₂0₇ ² _. In order to overcome the problem that Cr₂0₇ ² 's reduction product, Cr³⁺, cannot be readily oxidized (i.e. recharged) heterogeneously at known electrode catalysts and combined it with an electron mediator capable of homogeneously oxidizing Cr^IN to Cr^vl using an EC_cat mechanism in solution.

However, said half-cell’s electrolyte contains Cr₂0₇ ² as well as highly reactive electron mediator species (such as Ce⁴⁺) at high concentration in an acidic environment, which causes deleterious interactions with typical counterions and/or electrolyte of the corresponding anodic half-cell comprising the fuel system.

Accordingly, it was the object of the present invention to develop a suitable complementary anodic half-cell to the cathodic half-cell based on Cr₂0₇ ² in order to provide a working redox flow battery.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a rechargeable redox flow battery (RFB) comprising an electrochemical cell comprising at least a positive electrode (cathode) in a positive half-cell and a negative electrode (anode) 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 the Zn²⁺/Zn° redox couple, at least methane sulfonic acid or its anion as an electrolyte for conductivity and at least one solvent; and

the catholyte comprises the Cr^NI/Cr^vl redox couple, at least methane sulfonic acid or its anion one electrolyte for conductivity and the Ce⁴⁺/Ce³⁺ redox couple as the electrochemically reversible electron mediator and at least one solvent.

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 storing electrical energy comprising:

a) charging the RFB according to any one of claims 1 to 9 by supplying a current power source to the positive and negative electrode,

b) discharging the RFB by connecting the positive and negative electrode to a load.

BRIEF DESCRIPTION OF THE FIGURES

FIG 1 : Schematic illustration of a redox flow battery comprising (1 ) anolyte storage tank; (2) catholyte storage tank; (3) anolyte chamber; (4) catholyte chamber; (5) anode; (6) cathode; (7) selectively permeable ion-conducting membrane; (8) pump for anolyte; (9) pump for catholyte.

FIG 2: Zn / Cr-Ce Flow Battery Testing

Conditions Oxidant side: 1 M Na₂Cr₂0₇, 0.5 M Ce(CH₃S0₃)₃, 2 M CH₃S0₃H (50 ml.) Conditions Fuel side: 1 M Zn²⁺, 2.5 M CH₃S0₃H (50 ml.)

CV Scan Rate 100 mV/s; Chronoamperometry at 0.5 V

FIG 3: Zn-ln Half Cell using optimized potential program

Conditions: 2 M Zn(CH₃S03)2, 0.020 M InCIs, 1 M CHsSOsH

FIG 4: Zn-ln vs Zn Half Cell

Conditions for Zn-ln: 2 M Zn(CH₃S03)2, 0.020 M InCIs, 1 M CHsSOsH

Conditions for Zn: 2 M Zn(CH3S03)2, 1 M CH3SO3H

FIG 5: Zn-ln Device Testing in a Zn-ln / Zn-ln Redox Pump

Conditions: 2 M Zn(CH₃S03)2, 0.020 M InCIs, 1 M CHsSOsH

FIG 6: Zn-ln / Cr-Ce Redox Flow Battery Testing

Fuel: 2 M Zn(CH₃S03)₂, 0.020 M InCIs, 1 M CHsSOsH

Oxidant: 1.33 M Cr(CH₃S03)₃, 0.5 M Ce(CH₃S03)₃

Charge 2.7 V, Discharge 0.5 V DETAILED DESCRIPTION

An EC_cat mechanism" (also denoted as EC' in Bard and Faulkner in "Electrochemical Methods: Fundamentals and Applications", 2^nd Ed. 2001 ) 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:

E: A + 1 e— >B

C_cat- B+ C-> A+ D

Since regenerated A must remain at the electrode surface to be detected, high currents are detected at (1 ) shorter time periods between consumption and regeneration of A and (2) 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., Ce⁴⁺) participating in a reversible reduction (e.g. Ce⁴⁺ + 1 e

Ce³⁺) is defined by the Nernst equation:

E = E° + RT/nF ln(C_0X/C_red) where C_ox and C_reci are the respective concentrations of the oxidized and reduced forms of the given redox species (e.g. Ce⁴⁺ and Ce³⁺), n is the number of electrons per molecule 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° is the standard potential of the redox species, which is the observed potential (E) when C_0x = C_red· In the context of the invention, the Nernst equation implies that the potential of a given reversible redox species varies from its E° depending on the ratio of the oxidized to reduced form present in solution. Thus a ratio of Ce⁴⁺ to Ce³⁺ of 99 to 1 shifts the E of Ce⁴⁺ positive by 0.120 V, and a ratio of 99.96 to 0.04 shifts E positive by 0.200 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", i_L of an electrochemical reaction in RDE voltammetry is the maximum obtainable current at a given RDE rotation rate. Since the i_L is a function of the steady-state transport that originates from laminar flow across the RDE, the i_L is independent of potential, and appears as a horizontal line in an RDE voltammogram.

The half-wave potential, E_1/2, of a given electrochemical reaction in RDE voltammetry is the potential at which V₂ of the i_L is achieved. A reasonably accurate measurement of E_1/2 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_1/2. To achieve low current density for a high n oxidant like Cr₂0₇ ² , low concentration (1 to 10 mM) and low to moderate rotation rate (50 to 3000 rpm) must be used. In the context of the invention, the E_1/2 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/AgCI" 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/AgCI is the silver / silver chloride electrode, and a given E_Ag/A_gci = E_NHE - 0.197 V. Unless stated otherwise, all unreferenced potentials are shown as V vs. Ag/AgCI, 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 Weber, A.Z., Mench, M.M., Meyers, J.P. et al. J. Appl. Electrochem., 201 1 , 41 , 1 137-1 164. In a preferred embodiment, the catholyte of the redox flow battery as described herein is a liquid comprising the Ce⁴7Ce³⁺ redox couple as an reversible electron mediator, 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.

When using a Zn²⁺/Zn° fuel system in an anodic half-cell in combination with the cathodic half-cell based on Cr₂0₇ ² it was critical to find a suitable electron mediator couple which is capable of oxidizing Cr^IN to Cr^vl and at the same provide a counter-ion which does neither deleterious affect the electron mediator couple nor Cr^IN/Cr^vl or Zn²⁺/Zn°.

Selecting the Ce⁴⁺/Ce³⁺ redox couple as the electron mediator couple provided a challenge to find a suitable counter-ion, as most common counter-ions cannot be used. For example N0₃ and CI0 are reduced at potentials less negative than Zn° and prevent Zn° deposition, S0₄ ² already precipitates Ce⁴⁺/Ce³⁺ species at low concentration, Cl , I , and Br are oxidized at potentials less positive than Ce³⁺ and thus consumed before Ce³⁺ oxidation.

The solution to the above problem was to use methane sulfuric acid as an electrolyte in the catholyte and anolyte. The effect will be improved by using the Zn²⁺/Zn° redox couple in form of zinc(ll) methane sulfonate, the Cr^NI/Cr^vl redox couple in form of chromium(lll) methane sulfonate and the Ce⁴⁺/Ce³⁺ redox couple in form of cerium(lll) methane sulfonate.

Using the Ce⁴⁺/Ce³⁺ redox couple in the form of cerium(lll) methane sulfonate, means employing partially, mainly or only Ce(CH₃S0₃)₃ as the Ce³⁺ species. Ce(CH₃S0₃)₃ has a high solubility of >2 M in methane sulfonic acid, CH₃S0₃H. This compound can be prepared from the inexpensive precursors cerium carbonate, Ce₂(C0₃)₂, and methane sulfonic acid.

Using the Zn²⁺ / Zn° redox in the form of zinc(ll) methane sulfonate means employing is partially, mainly or only Zn(CH₃S0₃)₂ as the Zn²⁺ species. This compound can be prepared from ZnO and CH₃S0₃H. Since ZnO dissolves very well in acid, only Zn(CH₃S0₃)₂ and CH₃S0₃H are left in the final solution. Hence, there is no need to precipitate Zn(CH₃S0₃)₂ for using it in the present invention.

Using the Cr^vl/Cr³⁺ in the form of chromium(lll) methane sulfonate means employing partially, mainly or only Cr(CH₃S0₃)₃ as the Cr³⁺ species. This compound can be prepared by dissolving Cr0₃ in water to form H₂Cr₂0₇, subsequently reducing the Cr^vl to Cr³⁺ with hydrogen peroxide (H₂0₂) followed by addition of methane sulfonic acid.

With respect to the above,“partially” refers to an amount of 20 to 50 mol% of the metal ions,“mainly” to an amount of 55 to 99 mol%, preferably 70 to 90 mol% of the metal ions, and“completely” to more than 99 mol% to 100 mol% of the metal ions.

The rechargeable redox flow battery (RFB) according to the invention typically comprises 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 the Zn²⁺/Zn° redox couple, methane sulfonic acid or its anion and at least one solvent; and

the catholyte comprises the Cr^NI/Cr^vl redox couple, methane sulfonic acid or its anion, the Ce⁴⁺/Ce³⁺ redox couple and at least one solvent.

In one embodiment of the invention, the initial battery formulation contains Zn in form of Zn°, e.g. a zinc plate and Cr in form of Cr₂0₇ ² .

In a preferred embodiment, the initial battery formulation contains Zn in form of a Zn²⁺ and Cr in form of Cr³⁺. This embodiment provides the following advantages:

• The Zn° (fuel) side of the battery can be constructed / shipped without pre-formed Zn° deposits. That is, the Zn° electrode can be built simply from carbon plates and does not need a film of Zn° pre-deposited. The fuel solution can contain Zn²⁺, which is stable, unlike Zn°, which readily decomposes in water to form H₂ gas.

• The entire battery can be constructed / shipped in a discharged state. Both the fuel (Zn°) and the oxidant (Cr^vl) are in the discharged states of Zn²⁺ and Cr³⁺, making the solution less reactive and less toxic for shipping and on-site construction of large, grid-level batteries.

• The Cr^vl (oxidant) side of the battery generates its own acid instead of becoming acid-starved during battery operation. If the battery is started from Zn° and Cr₂0₇ ^2-, then 1 M Cr₂0₇ ² solution would require 14 M methane sulfonic acid in order to be completely reduced to Cr³⁺. As such high concentration of methane sulfonic acid would limiting the solubility of Cr³⁺ species, and thus decrease the performance of the battery.

This configuration allows for all species to coexist in the solution and yet still produce enough acid to consume all of the Cr^vl generated. The preferable molar ratio of Cr(CH₃S0₃)₃ to Ce(CH₃S0₃)₃ in the anolyte is in the range from 1 :1 to 5:1 , particularly preferably from 2:1 to 3:1 and most preferably 2,7:1.

In a preferred embodiment, the anolyte contains ln³⁺ and/or the anode ln°. Thus, the spontaneous oxidation of Zn° under formation of H2 in aqueous media can be suppressed. Sub-stoichiometric amounts of In with regard to Zn are sufficient, such as less than 50 mol%, preferably less than 15 mol% more preferably less 5 mol% and most preferably 1 mol% and less are sufficient. When Zn and In are co-deposited on the anode, the resulting Znggln-i alloy has a surface with greatly reduced activity for H₂ evolution.

Thus, the molar amount of In with regard to Zn in the anolyte and anode is less than 50 mol%, preferably less than 15 mol% more preferably less than 5 mol% and most preferably not more than 1 mol%

While high current efficiencies for Zn deposition at carbon electrodes can be achieved by high concentrations of Zn(CH₃S0₃)₂ in CH₃S0₃H (methane sulfonic acid), Zn showed a current efficiency of 83% for 2 M Zn(CH₃S0₃)2 in 1 M CH₃S0₃H only when stripping at very low voltage efficiency in order for Zn stripping to outpace Zn decomposition to H₂.

In on the other hand showed 67 to 77% current efficiency for 0.1 M lnCI₃ in 3 M CH₃S0₃H, with significantly lower current efficiencies at longer deposition times. This suggests that In rapidly decomposes to H₂ and ln³⁺ when left in solution.

However, Zn-ln showed up to 100% current efficiency analytically and 70 to 90% current efficiency at the device level. Moreover, Zn and In significantly stabilize one another to suppress oxidation of Zn and/or In under H₂ formation. Economically advantageous In ingots can be easily dissolved in CH₃S0₃H as a substitute for expensive lnCI₃.

Zn-ln shows 87% current efficiency at 83% estimated voltage efficiency using the following potential program:

a) a potential of -1.5 (+/- 0.1 ) V vs. Ag/AgCI is applied for a short period of time (shorter than the following steps b) an c)) to rapidly deposit Zn-ln,

b) a potential of -1.1 (+/- 0.1 ) V is applied to deposit much more Zn-ln,

c) a potential of -0.7 (+/- 0.1 ) V is applied until the oxidation current greatly decreases, d) a potential of 0 to -0.3 V is applied for a shorter period than steps b) an c) to remove layers of In that protect Zn and prevent it from being stripped at -0.7 V.

In step a), the deposited Zn-ln can cover the carbon electrode and block carbon from creating H₂ as Zn-ln has a higher overpotential for H2 evolution than carbon and the sooner the carbon is covered, the less H₂ is generated by passing current through the electrode.

Step b) lasts for a longer period of time (e.g. 1 h). At potential of -1.1 V (+/- 0.1V) more Zn-ln is deposited at lower overpotential. Said potential represents only 0.1 V (+/- 0.1V) overpotential for Zn deposition.

Step c) also lasts for a longer period of time (e.g. 1 h). A somewhat high overpotential for stripping (0.3 V) seems advantageous to strip off Zn-ln fast enough to limit the competing decomposition to H₂.

In Step d), Zn-ln is stripped for a short period of time to remove layers of In that protect Zn (and prevent it from being stripped at -0.7 V). Though Zn and In co-deposit at the same potential, In requires higher potentials for stripping than Zn, as seen in the voltammograms according to Figure 4.

A further improvement of charge efficiency can be effected by combining the embodiments according to the invention with a H₂ recombinator to ensure that the battery remains balanced during multiple cycles. H₂ recombinators are devices known to the person skilled in the art and used e.g. in commercially available Zn-Br₂ 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, 1 -propanol, isopropanol, diethyl 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.

EXAMPLES

Synthesis of cerium(lll) methane sulfonate, Ce(CH₃S0₃)₃

A saturated solution of Ce₃(C0₃)₂ was prepared in water, with excess Ce₃(C0₃)₂ remaining unsolved on the bottom of the glassware. Pure (15.4 M) CH₃S0₃H was then added to the solution while stirring until some of the Ce₃(C0₃)₂ dissolves. At this stage, excess solid Ce₃(C0₃)₂ was still present at the bottom of the glassware. The supernatant was then separated from the solid Ce₃(C0₃)₂, placed in a watch glass or petri dish and heated in an oven to 120 to 150°C to evaporate the water and excess CH₃S0₃H, leaving a precipitate of Ce(CH₃S0₃)₃.

Synthesis of zinc(ll) methane sulfonate, Zn(CH₃S0₃)₂

ZnO was dissolved in CH₃S0₃H, and the O² counter-ion is reacted to H₂0 in acid, yielding a solution containing only Zn(CH₃S0₃)₂ and CH₃S0₃H, which could be used without the need to precipitate Zn(CH₃S0₃)₂

Synthesis of chromium(lll) methane sulfonate Cr(CH₃S0₃)₃

A saturated solution of H₂Cr₂0₇ was prepared by dissolving Cr0₃ in water, which was then added dropwise to a solution of 50% pure (15.4 M) CH₃S0₃H and 50% of 30%-by- mass (9.8 M) H₂0₂ under cooling in a cold-water bath to prevent the solution from boiling. The resulting solution was stirred and additional H₂0₂ added dropwise until all of the Cr^vl has been reduced to Cr³⁺, which was evident by a change in coloration of the solution. Subsequently, a solution of 2 M Cr³⁺ was prepared by adding H₂Cr₂0₇ to CH₃S0₃H in a 1 :6 ratio, which corresponds to a 2:6 or 1 :3 ratio of Cr0₃ to CH₃S0₃H.

Synthesis of an anolyte starting material

Ce(CH₃S0₃)₃ was added to the solution obtained from the synthesis of chromium(lll) methane sulfonate to obtain an anolyte starting material.

Battery Operation A

The battery was started from Zn° plate in 50 ml. of 2.5 M CH₃S0₃H and 50 mL of 1 M Cr₂0₇ ^2- in 2 M CH₃S0₃H, current dropped rapidly while the total acid in both solutions had been consumed. Given that the total system has 0.175 mol H⁺ and the reaction has a ratio of 14 H⁺ : 6 e , there is enough acid to pass 7,240 C of charge. Current dropped rapidly when this charge was passed and [H⁺] plummeted during the first 1 .5 h. After 7,240 C has passed by t = 1 .5 h, the current leveled off as the system became dependent on obtaining H⁺ from H₂0 instead of CH₃S0₃H.

Reaction:

Cr₂0₇ ² + 14 H⁺ + 6 e 2 Cr³⁺ + 7 H₂0

Reaction including all species in the oxidant-side solution:

H₂Cr₂0₇ + 12 CH₃S0₃H + 0.5 Ce³⁺ + 1 .5 CH₃S0₃ + 6 e

2 Cr³⁺ + 13.5 CH₃S0₃ + 0.5 M Ce³⁺ + 7 H₂0

Battery Operation B

The battery was started from Cr³⁺ and Zn²⁺, whereupon significant acid was generated in-situ from water on the Cr^vl (oxidant) side during Cr^vl production. The excess H⁺ generated will travel to the Zn²⁺ side to help conduct charge through solution and maintain charge balance during the reaction.

Oxidant-Side Reaction (calculated as ¾ x the reaction above):

1 ½ Cr³⁺ + 4 ¾ H₂0 ¾ Cr₂0₇ ² + 9 ½ H⁺ + 4 e

Oxidant-side Reaction including All Species in Solution:

1 ½ Cr³⁺ + 5.5 CH₃S0₃ + 0.5 M Ce³⁺ + 4 ¾ H₂0 ¾ H₂Cr₂0₇ + 5.5 CH₃S0₃H + 0.5 Ce³⁺ + 2.5 H⁺ + 4 e

Fuel-Side Reaction (calculated as 2 x the reaction above):

2 Zn²⁺ + 4 CH₃S0₃ + 1 CH₃S0₃H + 4 e 2 Zn° + 4 CH₃S0₃ + 1 CH₃S0₃H Zn-ln Half Cell:

Voltammetry of 2 M Zn(CH₃S0₃)₂, 0.020 M InCIs, 1 M CFbSOsH:

1 . First a potential of -1 .5 V vs. Ag/AgCI was applied for approx. 10 min to rapidly deposit Zn-ln, which covers the carbon electrode and blocks carbon from creating H₂. That is, because Zn-ln has a higher overpotential for H₂ evolution than carbon, the sooner the carbon is covered, the less H₂ is generated by passing current through the electrode.

2. Then a potential of -1 .1 V is applied for ca 1 h to deposit much more Zn-ln at lower overpotential. This represents only 0.1 V overpotential for Zn deposition.

3. The Zn-ln is then oxidized (stripped) at -0.7 V for a long period of time, until the oxidation current greatly decreases. A somewhat high overpotential for stripping (0.3 V) is necessary because the Zn-ln must be stripped off fast enough to decrease its available time to decompose to H₂.

4. Finally, Zn-ln is stripped at 0 to -0.3 V for a short period of time to remove layers of In that protect Zn and prevent it from being stripped at -0.7 V. Though Zn and In co- deposit at the same potential, In requires higher potentials for stripping than Zn, as seen in the voltammograms according to Figure 3.

The voltages used in Steps 2 and 3 correspond to a voltage efficiency of 83%. The cell has an open-circuit voltage of ~2.2 V, so Step 2 corresponds to charging at 2.3 V and Step 3 discharging at 1 .9 V.

Comparison Zn-ln vs Zn Half Cell

Voltammetrie of 2 M Zn(CH₃S03)₂, 0.020 M InCIs, 1 M CHsSOsH (Zn-ln) and 2 M Zn(CH₃S0₃)2, 1 M CH₃S0₃H (Zn), respectively. The results are shown in Figure 4.

Compared to Zn, Zn-ln improves current efficiency from 60 to 90% when tested at identical voltage efficiencies using the potential program described above. Moreover, compared to Zn, Zn-ln improves current efficiency from 83 to 93% and increases charging current density by 25% when tested at less efficient Edeposition -1 -5 V, E_stripping 0 V (i.e. Charge 2.7 V, Discharge 1 .2 V) Zn-ln Device Testing in a Zn-ln / Zn-ln Redox Pump:

Testing 2 M Zn(CH₃S0₃)₂, 0.020 M lnCI₃, 1 M CH₃S0₃H resulted in a 90% current efficiency in a device-level redox pump setup, in which Zn-ln is deposited at a carbon electrode on one side and Zn is stripped from Zn plate on the other side as shown in Figure 5

Zn-ln / Cr-Ce Flow Battery Testing

Testing a battery using 2 M Zn(CH₃S0₃)₂, 0.020 M lnCI₃, 1 M CH₃S0₃H as the fuel, 1.33 M Cr(CH₃S0₃)₃, 0.5 M Ce(CH₃S0₃)₃ as oxidant at voltages of 2,7 V for charging and 0.5 V for discharging respectively resulted in 86 % current efficiency as shown in Figure

6.

Claims

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;

2. RFB according to Claim 1 , wherein Zn²⁺/Zn° is used in form of zinc(ll) methane sulfonate, the Cr^NI/Cr^vl redox couple is used in form of chromium(lll) methane sulfonate and Ce³⁺ in the Ce⁴⁺/Ce³⁺ redox couple is used in form of cerium(lll) methane sulfonate.

3. RFB according to Claim 1 or 2 wherein the solvent, is selected from the group consisting of water or a non-aqueous solvent, such as acetonitrile, dimethylsulfoxide, dimethylformamide, methanol, ethanol, 1 -propanol, isopropanol, diethyl ether, diglyme, tetrahydrofuran, glycerol, and mixtures thereof.

4. RFB according to any of Claims 1 to 3 wherein the initial battery contains Zn in form of Zn° and Cr in form of Cr₂0₇ ² .

5. RFB according to any of Claims 1 to 3 wherein the initial battery contains Zn in form of Zn²⁺ and Cr in form of Cr³⁺, preferably in form of zinc(ll) methane sulfonate and chromium(lll) methane sulfonate.

6. RFB according to any of Claims 2 to 5 wherein the molar ratio of Cr(CH₃S0₃)₃ to Ce(CH₃S0₃)₃ in the anolyte is in the range from 1 :1 to 5:1 , particularly preferably from 2:1 to 3:1 and most preferably 2,7:1

7. RFB according to any of Claims 1 to 6 wherein the anolyte contains In3+ and/or the negative electrode contains In⁰.

8. RFB according to Claim 8 wherein the molar amount of In with regard to Zn in the anolyte and anode is less than 50 mol%, preferably less than 15 mol% more preferably less than 5 mol% and most preferably not more than 1 mol%.

9. RFB according to any of Claims 2 to 6 which further comprises a H₂ recombinator.

10. Method for operating a RFB according to any of Claims 7 to 9 wherein the following potential program is applied to the negative half cell:

a) a first potential of -1.5 (+/- 0.1 ) V vs. Ag/AgCI is applied for a shorter period than the following steps b) an c) to rapidly deposit Zn-ln,

b) a potential of -1.1 V (+/- 0.1V) is applied to deposit more Zn-ln,

c) a potential of -0.7 V (+/- 0.1V) is applied until the oxidation current greatly, d) a potential of 0 to -0.3 V is applied for a shorter period than steps b) and c) to remove layers of In that protect Zn.

1 1. Use of the RFB according to any one of claims 1 to 9 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.

12. A method for storing electrical energy comprising: