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 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 <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 CeIII to CrVI using an ECcat 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 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 Cr<NUM>O<NUM><NUM>- in order to provide a working redox flow battery.

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 Zn<NUM>+/Zno 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 CrIII/CrVI redox couple, at least methane sulfonic acid or its anion one electrolyte for conductivity and the Ce<NUM>+/Ce<NUM>+ 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:.

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 Coat) 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:.

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 Cred 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 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<NUM> is the standard potential of the redox species, which is the observed potential (E) when Cox = Cred. 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 oxidized to reduced form present In solution. 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 = ENHE - <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>.

In a preferred embodiment, the catholyte of the redox flow battery as described herein is a liquid comprising the Ce<NUM>+/Ce<NUM>+ 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 Zn<NUM>+/Zn<NUM> fuel system in an anodic half-cell in combination with the cathodic half-cell based on Cr<NUM>O<NUM><NUM> it was critical to find a suitable electron mediator couple which is capable of oxidizing CrIII to CrVI and at the same provide a counter-ion which does neither deleterious affect the electron mediator couple nor CrIII/CrVI or Zn<NUM>+/Zn<NUM>.

Selecting the Ce<NUM>+/Ce<NUM>+ 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 NO<NUM>- and ClO<NUM>- are reduced at potentials less negative than Zn<NUM> and prevent Zn<NUM> deposition, SO<NUM><NUM>- already precipitates Ce<NUM>+/Ce<NUM>+ species at low concentration, Cl-, I-, and Br- are oxidized at potentials less positive than Ce<NUM>+ and thus consumed before Ce<NUM>+ 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 Zn<NUM>+/Zn<NUM> redox couple in form of zinc(II) methane sulfonate, the CrIII/CrVI redox couple in form of chromium(III) methane sulfonate and the Ce<NUM>+/Ce<NUM>+ redox couple in form of cerium(III) methane sulfonate.

Using the Ce<NUM>+/Ce<NUM>+ redox couple in the form of cerium(III) methane sulfonate, means employing partially, mainly or only Ce(CH<NUM>SO<NUM>)<NUM> 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.

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

Using the CrVI/Cr<NUM>+ in the form of chromium(III) methane sulfonate means employing partially, mainly or only Cr(CH<NUM>SO<NUM>)<NUM> 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.

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.

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;.

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 Cr<NUM>O<NUM><NUM>-.

In a preferred embodiment, 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>.

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 Zn<NUM>In<NUM> alloy has a surface with greatly reduced activity for H<NUM> evolution.

Thus, 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>.

In on the other hand showed <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 H2 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 co-deposit at the same potential, In requires higher potentials for stripping than Zn, as seen in the voltammograms according to <FIG>.

A further improvement of charge efficiency can be effected by combining the embodiments according to the invention with a 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, 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.

A saturated solution of Ce<NUM>(CO<NUM>)<NUM> was prepared in water, with excess Ce<NUM>(CO<NUM>)<NUM> remaining unsolved 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>- counter-ion 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, 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, which was then 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.

The battery was started from Zn<NUM> plate in <NUM> of <NUM> CH<NUM>SO<NUM>H and <NUM> of <NUM> Cr<NUM>O<NUM><NUM>- in <NUM> CH<NUM>SO<NUM>H, current dropped rapidly while the total acid in both solutions had been consumed. Given that the total system has <NUM> mol H+ and the reaction has a ratio of <NUM>+ : <NUM> e-, there is enough acid to pass <NUM>,<NUM> C of charge. Current dropped rapidly when this charge was passed and [H+] plummeted during the first <NUM>. After <NUM>,<NUM> C has passed by t = <NUM>, the current leveled off as the system became dependent on obtaining H+ from H<NUM>O instead of CH<NUM>SO<NUM>H.

Reaction:     Cr<NUM>O<NUM><NUM>- + <NUM>+ + <NUM> e- → <NUM> Cr<NUM>+ + <NUM><NUM>O.

Reaction including all species in the oxidant-side solution:.

H<NUM>Cr<NUM>O<NUM> + <NUM>·CH<NUM>SO<NUM>H + <NUM> Ce<NUM>+ + <NUM> CH<NUM>SO<NUM>- + <NUM> e- → <NUM> Cr<NUM>+ + <NUM> CH<NUM>SO<NUM>- + <NUM> Ce<NUM>+ + <NUM><NUM>O.

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

Oxidant-Side Reaction (calculated as <MAT> x the reaction above): <MAT>.

Oxidant-side Reaction including All Species in Solution: <MAT> + <NUM>+ + <NUM> e-.

Fuel-Side Reaction (calculated as <NUM> x the reaction above):.

<NUM> Zn<NUM>+ + <NUM> CH<NUM>SO<NUM>- + <NUM> CH<NUM>SO<NUM>H + <NUM> e- → <NUM> Zn<NUM> + <NUM> CH<NUM>SO<NUM>- + <NUM> CH<NUM>SO<NUM>H.

Voltammetry of <NUM> Zn(CH<NUM>SO<NUM>)<NUM>, <NUM> InCl<NUM>, <NUM> CH<NUM>SO<NUM>H:.

The voltages used in Steps <NUM> and <NUM> correspond to a voltage efficiency of <NUM>%. The cell has an open-circuit voltage of ~<NUM> V, so Step <NUM> corresponds to charging at <NUM> V and Step <NUM> discharging at <NUM> V.

Voltammetrie of <NUM> Zn(CH<NUM>SO<NUM>)<NUM>, <NUM> InCl<NUM>, <NUM> CH<NUM>SO<NUM>H (Zn-In) and <NUM> Zn(CH<NUM>SO<NUM>)<NUM>, <NUM> CH<NUM>SO<NUM>H (Zn), respectively. The results are shown in <FIG>.

Compared to Zn, Zn-In improves current efficiency from <NUM> to <NUM>% when tested at identical voltage efficiencies using the potential program described above. Moreover, compared to Zn, Zn-In improves current efficiency from <NUM> to <NUM>% and increases charging current density by <NUM>% when tested at less efficient Edeposition -<NUM> V, Estripping <NUM> V (i.e. Charge <NUM> V, Discharge <NUM> V).

Testing <NUM> Zn(CH<NUM>SO<NUM>)<NUM>, <NUM> InCl<NUM>, <NUM> CH<NUM>SO<NUM>H resulted in a <NUM>% current efficiency in a device-level redox pump setup, in which Zn-In is deposited at a carbon electrode on one side and Zn is stripped from Zn plate 'on the other side as shown in <FIG>.

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 the Zn<NUM>+/Zn<NUM> redox couple, methane sulfonic acid or its anion and at least one solvent;
and
the catholyte comprises the CrIII/CrVI redox couple, methane sulfonic acid or its anion, the Ce<NUM>+/Ce<NUM>+ redox couple and at least one solvent,
wherein the anolyte contains In<NUM>+ and/or the negative electrode contains In<NUM>.