WO2022173785A1 - Aqueous redox flow batteries with redox-active solid additives - Google Patents

Abstract

Systems, devices, and methods for storing and discharging an aqueous redox flow battery (RFB) are provided. An aqueous redox flow battery includes an aqueous electrolyte including a redox- active additive and a redox mediator. The redox-active additive can be or include but is not limited to titanium (Ti), lead (Pb), iron (Fe), zinc (Zn), tin (Sn), copper (Cu), nickel (Ni), cobalt (Co), bismuth (Bi), sodium (Na), lithium (Li), magnesium (Mg), compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof. The redox mediator can be or include but is not limited to a transition metal coordination compound optionally comprising a ligand selected from triethanolamine, triisopropanolamine, bipyridine, porphyrins, bridging oxides, and any derivatives thereof.

Description

AQUEOUS REDOX FLOW BATTERIES WITH REDOX-ACTIVE SOLID

ADDITIVES

CROSS REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No. 63/148,508 filed February 11, 2021, the disclosure of which is hereby incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS This invention was made with government support under Grant Nos. CHE- 1904436 and DMR-1719797, awarded by the National Science Foundation. The government has certain rights in the invention.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In an aspect, an aqueous redox flow battery includes an aqueous electrolyte comprising a redox-active additive and a redox mediator. The redox-active additive can increase an energy storage capacity of the aqueous redox flow battery by at least 10%, compared to the energy storage capacity of an aqueous redox flow battery without the redox-active additive in the aqueous electrolyte. The redox-active additive can be or include a redox-active solid. The redox-active additive can be or include titanium (Ti), lead (Pb), iron (Fe), zinc (Zn), tin (Sn), copper (Cu), nickel (Ni), cobalt (Co), bismuth (Bi), sodium (Na), lithium (Li), magnesium (Mg), compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof. The redox-active additive can be or include Ti, Zn, Li, compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof. The redox-active additive can be or include a redox- active metal. The redox-active additive can be or include zinc metal. The zinc metal can be in the form of a powder, mossy zinc, zinc mesh, electrodepo sited zinc, zinc foam, zinc pellets or any combination thereof. The redox-active additive can be in the form of a powder. The redox-active additive can be insoluble in at least one of a reduced form or an oxidized form in the aqueous electrolyte. The water-soluble redox mediator can have a redox potential within about +/- 200 mV of the redox potential of the redox-active additive. The redox mediator can be water-soluble. The redox mediator can be or include a water- soluble aqueous transition metal-containing redox mediator. The redox mediator can be or include a transition metal coordination compound optionally comprising a ligand selected from triethanolamine, triisopropanolamine, bipyridine, porphyrins, bridging oxides, and any derivatives thereof. The redox mediator can be or include a water-soluble organometallic compound. The redox mediator can be or include an aqueous organoiron compound. The redox mediator can be selected from [Fe(TEOA)OH]^1_/2 , [EcfPPAjOHJ ^{l _/2}\ and any combination thereof. The redox mediator can be dissolved in the negative electrolyte electrolyte and/or the positive electrolyte. The electrolyte(s) can be or include the redox mediatonredox-active additive at a molar ratio of from 1:0.1 to 1:25. The aqueous redox flow battery can comprise a volumetric power density of from 5 Wh/L to 200 Wh/L. The aqueous redox flow battery can be substantially free of dendrites formed from the redox-active additive.

In an aspect, an aqueous redox flow battery includes a negative electrolyte tank. The negative electrolyte tank can include an aqueous electrolyte including a redox-active additive and a water-soluble redox mediator of the preceding aspect. The aqueous redox flow battery can include a negative electrode in fluid communication with the negative electrolyte tank. The aqueous redox flow battery can include a positive electrolyte tank comprising a positive electrolyte. The aqueous redox flow battery can include a positive electrode in fluid communication with the positive electrolyte tank. The aqueous redox flow battery can include an ion-permeable separator between the negative electrode and the positive electrode.

In an aspect, a method of operating a redox flow battery of the preceding aspect includes charging the aqueous redox flow battery by drawing a current density from 10 mA/cm² to 400 mA/cm² at a voltage from 0.5 V to 1.8 V from the aqueous redox flow battery.

DETAILED DESCRIPTION OF FIGURES

The foregoing aspects and many of the attendant advantages of the present disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, described below.

FIG. 1 is a schematic diagram illustrating a redox flow battery with electron transport in the circuit, ion transport in the electrolyte and across the membrane, active species crossover, and mass transport in the electrolyte, in accordance with embodiments of the present disclosure.

FIG. 2A is a schematic diagram illustrating an all-slurry flow redox battery, in accordance with embodiments of the present disclosure.

FIG. 2B is a schematic diagram illustrating a metal/slurry flow redox battery., in accordance with embodiments of the present disclosure.

FIG. 3 is a schematic diagram illustrating a redox-targeting flow battery., in accordance with embodiments of the present disclosure.

FIG. 4 is a block flow diagram of an example process 400 for operating an aqueous RFB configured as described above, in accordance with embodiments of the present disclosure.

FIG. 5 is a graph of example cyclic voltammograms of [Fe(TEOA)OH]² and [Fe(CN)₆]³ , in accordance with embodiments of the present disclosure. The [Fe(TEOA)OH]² solution contained 0.5 M FeCh^FbO and 1.0 M TEOA. The pH was adjusted to 13.96 with 10.2 equivalents of NaOH. The current was multiplied by 3 for clarity. The [Fe(CN)₆]³ solution contained 1.0 M K3[Fe(CN)6] dissolved in 0.5 M NaOH and had a pH of 13.97. The sweep rate was v = 50 mV s ¹ in both measurements. The electrochemical reactions are indicated above each voltammogram.

FIG. 6 is a graph of example room-temperature absorption spectra of charged 0.5 M [Fe(TEOA)OH]² and discharged [Fe(TEOA)OH] aqueous anolyte solutions (pH -13.9, sodium hydroxide aqueous mixture, pathlength = 2 mm), in accordance with embodiments of the present disclosure. In some embodiments, the discharged solution was prepared by electrochemical oxidation.

FIG. 7A is a graph of example absorption spectra of 0.1 M [Fe(TEOA)OH] after additions of Zn powder, in accordance with embodiments of the present disclosure.. The graph was prepared from solutions for which a total of 13.5 mg Zn powder (0.6 equivalents) was added in 3 approximately equal portions.

FIG. 7B is a graph of example absorption spectra of 0.1 M [Fe(TEOA)OH]² after additions of ZnO powder, in accordance with embodiments of the present disclosure. The graph was prepared from solutions for which a total of 34.6 mg ZnO powder (-0.8 equivalents) was added in 6 approximately equal portions.

FIG. 8 is a graph of example redox flow battery cell potential data for five charge/discharge cycles at 40 mA cm ², in accordance with embodiments of the present disclosure. Voltage limits were set to define the range 1.6 - 0.5 V. In the example redox flow battery cell, the catholyte was 1.0 M [Fe(CN)₆]³ in 0.5 M NaOH solution (pH 13.97). The anolyte was 0.5 M [Fe(TEOA)OH]² in NaOH solution (pH -13.9) with a 2: 1 TEOA:Fe stoichiometry.

FIG. 9A is a graph of example cell potential (V) data as a function of capacity (mAh) for two full redox flow battery charge/discharge cycles, in accordance with embodiments of the present disclosure. The voltage limits were set between 1.6 - 0.5 V with an 80 mA cm ² current density. The catholyte was 1.0 M [Fe(CN)₆]³ in 0.5 M NaOH solution (pH 13.97). The anolyte was 0.5 M [Fe(TEOA)OH]² in NaOH solution (pH -13.9) with a 2:1 TEOA:Fe stoichiometric excess.

FIG. 9B is a graph of example capacity (mAh, left ordinate) and relative change in capacity (%, right ordinate) plotted as a function of the equivalents of Zn powder added to the data in FIG. 9A, in accordance with embodiments of the present disclosure. In the graph, charge capacity is indicated by squares and discharge capacity is indicated by triangles.

FIG. 10A is a graph of example discharge capacity (mAh, left ordinate) and discharge efficiency (%, right ordinate) data as a function of cycle number for a full redox flow battery, in accordance with embodiments of the present disclosure. In the data presented, the catholyte was 1.0 M K3[Fe(CN)6] in 1.0 M NaOH. The anolyte was 0.4 M [Fe(TEOA)OH]² in 4.0 M NaOH with a 2.5:1 TEOA:Fe ratio and varying amounts of added Zn. Voltage limits were set between 1.6 - 0.5 V with a 40 mA cm ² current density.

FIG. 10B is a graph of example discharge capacity (mAh, left ordinate) and discharge efficiency (%, right ordinate) data as a function of cycle number for a full redox flow battery, in accordance with embodiments of the present disclosure. In the data presented, the catholyte was 1.0 M K3[Fe(CN)6] in 1.0 M NaOH. The anolyte was 0.4 M [Fe(TEOA)OH]² in 4.0 M NaOH with a 4: 1 TEOA:Fe ratio and varying amounts of added Zn. Voltage limits were set between 1.6 - 0.5 V with a 40 mA cm ² current density

FIG. 11 is a graph of cell potential data as a function of time for a redox flow battery for which addition of solid to anolyte increases discharge time, in accordance with embodiments of the present disclosure.

FIG. 12A is a greyscale image of a flow cell for a redox flow battery testing station, in accordance with embodiments of the present disclosure. FIG. 12B is a greyscale image of a redox flow battery testing station including a flow loop and controller, in accordance with embodiments of the present disclosure.

FIG. 13 is a graph of example cell potential data as a function of capacity for a redox flow battery illustrating the increase of capacity upon addition of additive, in accordance with embodiments of the present disclosure.

FIG. 14 is a graph of example cell potential data as a function of capacity for a redox flow battery illustrating the increase of capacity upon addition of additive, in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Definitions

In simplified terms, an "electrochemical cell" is a device capable of either deriving electrical energy from chemical reactions or facilitating chemical reactions through the introduction of electrical energy. An electrochemical cell has two half-cells. Each half cell includes an electrode and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode. A plurality of electrochemical cells electrically connected together in series within a common housing is generally referred to as an electrochemical "stack."

A "redox (reduction/oxidation) flow battery" is a special type of electrochemical system in which an electrolyte containing one or more dissolved electroactive species flows through a plurality of electrochemical cells. A common redox flow battery electrochemical cell configuration includes a positive electrode (also referred to interchangeably as a cathode) and a negative electrode (also referred to interchangeably as an anode) separated by an ion exchange membrane or a separator, and two circulating electrolyte solutions (positive and negative electrolyte flowstreams generally referred to as the "catholyte" and "anolyte," respectively). The energy conversion between electrical energy and chemical potential occurs instantly at the electrodes once the liquid electrolyte begins to flow through the cells. As used herein, the term "battery" is used interchangeably with "cell" or "electrochemical cell."

As used herein, the term "dendrites" refers to the needle-like dendritic crystals that form on the surface of an electrode during charging/discharging of a battery.

The compounds described herein can be asymmetric ( e.g ., having one or more stereocenters). All stereoisomers, such as enantiomers and diastereomers, are intended unless otherwise indicated.

Compounds of the present disclosure that contain asymmetrically substituted carbon atoms can be isolated in optically active or racemic forms. Methods on how to prepare optically active forms from optically active starting materials are known in the art, such as by resolution of racemic mixtures or by stereoselective synthesis. Many geometric isomers of olefins, C=N double bonds, and the like can also be present in the compounds described herein, and all such stable isomers are contemplated in the present disclosure. Cis and trans geometric isomers of the compounds of the present disclosure are described and can be isolated as a mixture of isomers or as separated isomeric forms.

Compounds of the disclosure also include tautomeric forms. Tautomeric forms result from the swapping of a single bond with an adjacent double bond together with the concomitant migration of a proton. Tautomeric forms include pro to tropic tautomers which are isomeric protonation states having the same empirical formula and total charge. Example prototropic tautomers include ketone - enol pairs, amide - imidic acid pairs, lactam - lactim pairs, amide - imidic acid pairs, enamine - imine pairs, and annular forms where a proton can occupy two or more positions of a heterocyclic system, for example, 1H- and 3H-imidazole, 1H-, 2H- and 4H- 1,2, 4-triazole, 1H- and 2H- isoindole, and 1H- and 2H-pyrazole. Tautomeric forms can be in equilibrium or sterically locked into one form by appropriate substitution.

Compounds of the disclosure can also include all isotopes of atoms occurring in the intermediates or final compounds. Isotopes include those atoms having the same atomic number but different mass numbers. For example, isotopes of hydrogen include tritium and deuterium.

In some embodiments, the compounds of the disclosure, and salts thereof, are substantially isolated. By "substantially isolated" is meant that the compound is at least partially or substantially separated from the environment in which it was formed or detected. Partial separation can include, for example, a composition enriched in the compound of the disclosure. Substantial separation can include compositions containing at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%, at least about 97%, or at least about 99% by weight of the compound of the disclosure, or salt thereof.

As used herein, with respect to measurements and/or quantities, "substantially free," "essentially free," "negligible," or similar terms can include a presence, composition, level, or quantity that is nonzero but has a de minimis effect on a target value, such as current density, power density, or the like. As an example, an aqueous redox flow battery that is substantially free of dendrites can include one or more dendrites at a quantity and/or density (e.g., dendrites per area) that does not significantly impair the overall performance of the redox flow battery.

As used herein, with respect to measurements and/or quantities, "about" means +/- 5%.

As used herein, a recited range includes the end points, such that from 0.5 mole percent to 99.5 mole percent includes both 0.5 mole percent and 99.5 mole percent. Discussion:

As the global demand for energy continues to increase, so does the need for reliable clean energy sources. Although solar and wind power have gained traction as viable sources of clean energy, their energy production is intermittent, thus necessitating reliable large-scale energy storage solutions. Lithium-ion batteries have become ubiquitous in small electronics and electric vehicles, but they do not meet the scalability and safety requirements to power large portions of the grid. To that end, redox flow batteries (RFBs) represent a viable long-term energy storage solution.

FIG. 1 is a schematic diagram illustrating an example redox flow battery 100 with electron transport in the circuit, ion transport in the electrolyte and across a membrane, active species crossover, and mass transport in the electrolyte, in accordance with embodiments of the present disclosure. Example RFB 100 includes a catholyte reservoir 105, an anolyte reservoir 110, holding a catholyte 115 and an anolyte 120, respectively. Electrolytes are fluidically coupled with a flow cell including a cathode 125 and an anode 130 and configured to flow past a conducting surface of the electrodes during charging and discharging. The direction of the arrows in FIG. 1 reflects the direction of electrolyte flow during discharging, but it is understood that reversing the direction of flow of the electrolytes corresponds to charging operation. Electrolytes flow through respective flow channels 135, separated by a separator 140. Electrodes 125-130 are electrically coupled via electronic and/or power circuitry 145.

A distinguishing feature of RFBs is that they separate the energy storage and power components. Employing a modular design allows energy storage and power cycling to be decoupled and addressed individually, improving safety relative to sealed lithium-ion batteries. RFBs store energy in liquid electrolytes 115 and 120 that can be held in tanks, which can be scaled up to meet power demands on a megawatt scale. During charge and discharge, electrolytes 115 and 120 can be pumped through one or more electrochemical cells 150 where redox-active molecules undergo electrochemical reactions at electrodes 125-130. The size and number of electrochemical cells 150 can be scaled up with increasing power demand on a grid-scale (e.g., in a stack). The negative electrolyte 120, referred to herein as an "anolyte," yields electrons during discharge cycles. The electrons are collected through external circuit 145 and returned to the electrochemical cell, where they reduce the positive electrolyte 115, referred to herein as a "catholyte." The process is reversed during charging cycles, where a voltage is applied to the electrochemical cell 150 to recharge the battery. Meanwhile, charge balance is achieved by flow of counter-ions from one electrolyte to the other through a semi-permeable membrane 140.

Advantageously, RFBs represent a significant improvement over current grid-scale charge storage technology. Unfortunately, commercialization efforts to date have been slowed by expensive and limited materials, costly engineering designs, and, to an extent, the difficult manufacturing of ligands. Conventional approaches include all-vanadium cells (e.g., electrodes and electrolyte include vanadium). While this approach avoids problems of degradation due to electrolyte crossover, vanadium is costly, rare, and not electrochemically optimized for use in RFB applications. Advantageously, systems described herein improve RFB systems by employing safe, inexpensive, and earth- abundant materials. In addition, the electrolyte formulations described herein operate at relatively higher voltages, higher currents, and higher energy densities. Although electrolyte tanks can theoretically be scaled up to very large volumes for higher energy needs, it becomes impractical and costly to employ large tanks.

One strategy to boost the efficiency of RFBs while keeping their total volume small is to increase electrolyte energy capacity. In some embodiments, energy capacity is increased by incorporating redox-active solids into the electrolyte system. Redox-active solids are typically characterized by significantly higher energy densities relative to electrolytes. Therefore, employing redox-active solids in an RFB architecture can increase the energy density of the system without substantially increasing volume of electrolyte to a point where tank size becomes impractical. To that end, three system configurations are contemplated: slurry -based, metal/slurry, and redox-targeting RFBs.

FIG. 2A is a schematic diagram illustrating an all-slurry flow redox battery 200, in accordance with embodiments of the present disclosure. Slurry -based RFBs contain suspensions of solid redox-active materials 205 and/or conductive solids 210 in supporting electrolyte. The solids 205 directly undergo electrochemical reactions at the electrodes as they are circulated through the cell stack. Slurry-based RFBs take advantage of the higher energy densities of solids, but their complex fluid dynamics and slow reaction kinetics can introduce sigmoidal performance with respect to process parameters such as flowrate, load voltage, operating current, etc. For example, suspended solids 205-210 can settle in tanks 105-110 at relatively low flow/mix rates and can cause erosion and reduced performance due to residence time issues at relatively high flowrates. To that end, the techniques described herein with respect to FIG. 3 and Example 1 can improve the performance of RFBs, with or without inclusion of suspended solids 205-210.

FIG. 2B is a schematic diagram illustrating a metal/slurry flow redox battery 250, in accordance with embodiments of the present disclosure. Metal/slurry RFBs (Figure 2b) employ a slurry on one side of the battery and a metal plate electrode 255 on the other side. Metal electrodes can dissolve and reform during charge-discharge cycling. In some cases, relatively large numbers of cycles can result in parasitic dendrite formation on the surface of metal plate electrode 255. As would be understood by a person having ordinary skill in the electrochemical arts, parasitic dendrite formation can result in short circuiting of electrochemical cells, surface fouling, or other performance impairments. To that end, the techniques described herein with respect to FIG. 3 and Example 1 can improve the performance of RFBs by replacing at least a portion of the metal/electrolyte flow redox configurations.

FIG. 3 is a schematic diagram illustrating a redox-targeting flow battery 300, in accordance with embodiments of the present disclosure. Redox-targeting flow batteries (or "flow-mediated" batteries) store redox-active solids 305 in the electrolyte tanks 105-110. In some embodiments, the electrolyte functions as a redox mediator that transports electrons between the electrochemical cell 150 and the redox-active solids 305 in the tank. In case of anolyte 120, redox-active molecules (A) first become electrochemically oxidized (A⁺) at anode 130. Catholyte and Anolyte molecules 310 are circulated back into tanks 105- 110, where they undergo chemical reactions with redox-active solid 305 to regenerate electrolytes 115-120. When charging, electrochemical reactions in equations (1) and (2) are reversed. Similarly, catholyte 115 is reduced at electrode 125 and oxidized by direct chemical reaction with redox active solid 305 in catholyte tank 105. This way, the overall charge capacity of the RFB is significantly improved based at least in part on regeneration of electrolytes 115-120. Advantageously, at least partially replacing suspended solids 205-210 with the redox active solid 305 configurations of example system 300 can reduce or entirely eliminate performance impairment associated with slurry circulation or parasitic dendrite formation on electrodes.

A ¹ A⁺ + e^~ (1)

A⁺ + S A + S⁺ (2)

Conventional attempts at redox-targeting RFBs have targeted catholyte- side configurations, where Prussian blue solids added to an aqueous catholyte of ferricyanide/ferrocyanide ([Fe(CN)₆]^{3 /4} ) were applied to increase in RFB capacitance. Similarly, LiFePCU solid has been used in conjunction with ferrocene and 1,1'- dibromoferrocene to increase RFB capacity. Problematically, the LiFePCU system employs organic solvents that are prohibitive for large-scale applications on the basis of cost, safety, and environmental impact. Redox-targeting systems applied to anolyte-side configurations exhibit similar challenges with respect to performance and complexity. For example, T1O2 can serve as a redox-active solid 305, with cobaltocene and decamethylcobaltocene as redox mediator molecules 310. Such organometallic compounds exhibit significant air and moisture sensitivity, limiting demonstrations of T1O2 systems to lab-scale demonstrations in a strictly inert environment, which is understandably inapplicable to pilot, commercial, and/or industrial scale application. Similarly, while a polyaniline/carbon-black composite added to an aqueous y^4+/3+ anolyte can potentially function as in a redox targeting RFB, (e.g providing a boost in capacity for the catholyte (Fe^2+/3+)), the g^4+/3+ redox couple is understood to be unsuitable as a redox mediator due to its relatively slow reaction kinetics. Similarly, Prussian blue can potentially function as a redox-active additive in Br2/Br catholytes but is limited to a relatively low operating voltage (0.67 V) that has negative implications for commercial application, as power and reaction rate depend at least in part on operating voltage. To that end, a robust, aqueous redox-targeting anolyte with high operating voltage, using low-cost materials and showing high cyclability, involves application of different redox active solids 305 and electrolytes 115-120.

In some embodiments, an aqueous redox flow battery in accordance with example systems 100, 200, 250, or 300 includes an aqueous electrolyte 115 and/or 120 including a redox-active additive and the redox mediator. The redox-active additive can be or include redox- active solid 305. In some embodiments, the redox active additive is different for cathode-side and anode-side half-cells. For example, in example system 300, catholyte tank 105 can include a first redox-active solid 305-1 that is different from a second redox active solid 305-2 that is disposed in anolyte tank 110.

In some embodiments, the redox-active additive includes titanium (Ti), lead (Pb), iron (Fe), zinc (Zn), tin (Sn), copper (Cu), nickel (Ni), cobalt (Co), bismuth (Bi), sodium (Na), lithium (Li), magnesium (Mg), compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof. In some embodiments, the redox-active additive includes Ti, Zn, Li, compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof. In some embodiments, the redox- active additive comprises a redox-active metal.

By incorporating redox-active additives described above, aqueous redox flow batteries of systems 100, 200, 250, and/or 300 can exhibit increased energy storage capacity. For example, energy storage capacity of aqueous redox flow batteries can be increased by at least 1%, by at least 5%, by at least 10%, by at least 15%, by at least 20%, by at least 25%, by at least 30%, by at least 40%, by at least 45%, by at least 50%, by at least 55%, by at least 60%, by at least 65%, by at least 70%, by at least 75%, by at least 80%, by at least 85%, by at least 90%, by at least 95%, by at least 100%, or more, including fractions and interpolations thereof, as compared to the energy storage capacity of an aqueous redox flow battery deployed the redox-active additive in the aqueous electrolytes 115 and/or 120.

In some embodiments, the redox-active additive comprises zinc metal. The zinc metal can be in the form of a powder, mossy zinc, zinc mesh, electrodeposited zinc, zinc foam, zinc pellets or any combination thereof. Similarly, the redox-active additive can be in the form of a powder. In some embodiments, the redox-active additive is insoluble in at least one of a reduced form or an oxidized form in the aqueous electrolyte 115 and/or 120. In some embodiments, the water-soluble redox mediator 310 comprises a redox potential within about +/- 10 mV, within about +/- 20 mV, within about +/- 30 mV, within about +/- 40 mV, within about +/- 50 mV, within about +/- 60 mV, within about +/- 70 mV, within about +/- 80 mV, within about +/- 90 mV, within about +/- 100 mV, within about +/- 110 mV, within about +/- 120 mV, within about +/- 130 mV, within about +/- 140 mV, within about +/- 150 mV, within about +/- 160 mV, within about +/- 170 mV, within about +/- 180 mV, within about +/- 190 mV, within about +/- 200 mV, within about +/- 210 mV, within about +/- 220 mV, within about +/- 230 mV, within about +/- 240 mV, within about +/- 250 mV, within about +/- 260 mV, within about +/- 270 mV of the redox potential, within about +/- 280 mV, within about +/- 290 mV, within about +/- 300 mV, within about +/- 310 mV, within about +/- 320 mV, within about +/- 330 mV, within about +/- 340 mV, or within about +/- 350 mV of the redox-active additive, including fractions and interpolations thereof. The suitability of a redox mediator 310 to be paired with redox active solid 305 corresponds with the similarity of redox potential of the two or more materials. For example, deviations larger than +/- 200 mV in redox potential between the redox-active additive and the redox mediator can reduce the voltage efficiency of the respective half cells that impair RFB performance.

In some embodiments, the redox mediator is water-soluble, such that electrolytes 115 and/or 120 can include water and a dissolved redox mediator. For example, the redox mediator can be or include a water-soluble aqueous transition metal salt, complex, coordination compound, or other molecule 310. In the context of example system 300, each half cell can include the same or different redox mediator. For example, cathode-side electrolyte 115 can include a first redox mediator and anode-side electrolyte 120 can include a second redox mediator, where first mediator and second mediator can be the same or different.

In some embodiments, the redox mediator can be or include a transition metal coordination compound optionally including a ligand selected from triethanolamine, triisopropanolamine, bipyridine, porphyrins, bridging oxides, and any derivatives thereof. To that end, the redox mediator can be or include a water-soluble organometallic compound. For example, the redox mediator can be or include an aqueous organoiron compound, including but not limited to [Fe(TEOA)OH] ^1_/2- and/or [Fe(TiPA)OH]

As such, the redox mediator can be dissolved in electrolytes 115 or 120, in addition to or in place of a particulate slurry. In some embodiments, example system 100, 200, 250, or 300 can include electrolytes 115 and/or 120 that incorporate the redox mediator and can include redox- active additive at a molar ratio of from about 1:0.1 to about 1:25, where the ratio notation describes the relative molar quantity of redox-active additive for a unit mole of redox mediator. For example, the redox-active additive can be included in tanks 105 and/or 110 at a molar ratio of about 0.01, about 0.1, about 0.5, about 1.0, about 1.5, about 2.0, about 2.5, about 3.0, about 3.5, about 4.0, about 4.5, about 5.0, about 5.5, about 6.0, about 6.5, about 7.0, about 7.5, about 8.0, about 8.5, about 9.0, about 9.5, about 10, about 11, about 12, about 13, about 14, about 15, about 16, about 17, about 18, about 19, about 20, about 21, about 22, about 23, about 24, about 25, about 30, about 35, about 40, about 45, about 50, for a mole of redox mediator, including fractions and interpolations thereof. With respect to the operational window of an RFB, higher molar ratios increase the volumetric capacity of the system. As solids tend to exhibit relatively higher energy density than electrolytes, a higher molar quantity of redox-active additive improves the ability of the RFB to transfer charge from electrolyte to additive while also controlling for dispersion and solubility limitations of solid additives in aqueous electrolytes. That being said, chemical or physical constraints can limit the molar ratio, such that beyond a given higher limit the performance of an RFB can plateau where redox active additive can settle or accumulate in non-functional regions of tanks 105 and/or 110.

To that end, electrolyte 115 and/or 120 can include the redox mediator at a concentration from about 0.1 M to about 5 M. For example, redox mediator concentration can be about 0.1 M, about 0.2 M, about 0.3 M, about 0.4 M, about 0.5 M, about 0.6 M, about 0.7 M, about 0.8 M, about 0.9 M, about 1.0 M, about 1.1 M, about 1.2 M, about 1.3 M, about 1.4 M, about 1.5 M, about 1.6 M, about 1.7 M, about 1.8 M, about 1.9 M, about 2.0 M, about 2.1 M, about 2.2 M, about 2.3 M, about 2.4 M, about 2.5 M, about 2.6 M, about 2.7 M, about 2.8 M, about 2.9 M, about 3.0 M, about 3.1 M, about 3.2 M, about 3.3 M, about 3.4 M, about 3.5 M, about 3.6 M, about 3.7 M, about 3.8 M, about 3.9 M, about 4.0 M, about 4.1 M, about 4.2 M, about 4.3 M, about 4.4 M, about 4.5 M, about 4.6 M, about 4.7 M, about 4.8 M, about 4.9 M, about 5.0 or greater, including fractions and interpolations thereof. With respect to redox mediator concentration, higher concentrations improve energy density of electrolytes, limited by saturation point and precipitation dynamics. The electrolyte concentration can be further improved by adding suspended or dispersed redox-active additive to electrolytes to be circulated with electrolytes and redox mediator.

Using parameters described above, example systems 100, 200, 250, and or 300 can be configured to operate as an aqueous redox flow battery. In some cases, the aqueous redox flow battery can achieve a volumetric power density from about 5 Wh/L to about 200 Wh/L. In this context, volumetric power density can be used as a figure of merit that carries a unit of Watt-hours per liter, which describes a cell-size normalized electrical performance of the aqueous redox flow battery. To that end, relative performance of embodiments of the present disclosure as compared to conventional technologies can be determined based at least in part on the value of volumetric power density. As described in more detail in reference to EXAMPLE 1, embodiments of the present disclosure can exhibit significant improvement with respect to the volumetric power density, which indicates significantly better suitability for application in grid- scale electricity storage relative to other flow cell batteries and/or lithium-based storage technologies. Volumetric power density is a common figure of merit for RFBs, much like mAh is a common figure of merit for conventional batteries. In general, higher volumetric power densities reflect improved power per unit of electrolyte volume. Improved volumetric power density in turn improves charge/discharge time for an RFB and improves peak power output. In the context of all vanadium RFBs, a typical power density is about 20 Wh/L.

In some embodiments, the volumetric power density of a system configured to operate as a redox flow battery, in accordance with the materials and parameters described above, can achieve a volumetric power density of about 5 Wh/L or less, from about 5 Wh/L to about 10 Wh/L, from about 5 Wh/L to about 15 Wh/L, from about 5 Wh/L to about 20 Wh/L, from about 5 Wh/L to about 25 Wh/L, from about 5 Wh/L to about 30 Wh/L, from about 5 Wh/L to about 35 Wh/L, from about 5 Wh/L to about 40 Wh/L, from about 5 Wh/L to about 45 Wh/L, from about 5 Wh/L to about 50 Wh/L, from about 5 Wh/L to about 55 Wh/L, from about 5 Wh/L to about 60 Wh/L, from about 5 Wh/L to about 65 Wh/L, from about 5 Wh/L to about 70 Wh/L, from about 5 Wh/L to about 75 Wh/L, from about 5 Wh/L to about 80 Wh/L, from about 5 Wh/L to about 85 Wh/L, from about 5 Wh/L to about 90 Wh/L, from about 5 Wh/L to about 95 Wh/L, from about 5 Wh/L to about 100 Wh/L, from about 5 Wh/L to about 105 Wh/L, from about 5 Wh/L to about 110 Wh/L, from about 5 Wh/L to about 120 Wh/L, from about 5 Wh/L to about 125 Wh/L, from about 5 Wh/L to about 130 Wh/L, from about 5 Wh/L to about 135 Wh/L, from about 5 Wh/L to about 140 Wh/L, from about 5 Wh/L to about 145 Wh/L, from about 5 Wh/L to about 150 Wh/L, from about 5 Wh/L to about 155 Wh/L, from about 5 Wh/L to about 160 Wh/L, from about 5 Wh/L to about 165 Wh/L, from about 5 Wh/L to about 170 Wh/L, from about 5 Wh/L to about 175 Wh/L, from about 5 Wh/L to about 180 Wh/L, from about 5 Wh/L to about 185 Wh/L, from about 5 Wh/L to about 190 Wh/L, from about 5 Wh/L to about 195 Wh/L, from about 5 Wh/L to about 200 Wh/L, of about 200 Wh/L or greater, including fractions and interpolations thereof.

In contrast to the system 250 described in reference to FIG. 2B, embodiments of the aqueous redox flow battery can be maintained substantially free of dendrites formed from the redox-active additive. As previously described, dendrites formed of metal and/or redox-active additives can significantly impair the performance and lifetime of and RFB, for example, by shorting one or more cells 150 of the RFB or by puncturing separator 140 and causing cross-contamination of electrolytes between half cells (e.g., when the RFB includes multiple cells in parallel and/or in series). At least in part due to the localization of electrochemical reactions (1) and (2) with the redox-active additive to tanks 105 and/or 110 plating of redox-active additives onto electrodes 125-130 is relatively disfavored, thereby reducing the formation of dendrites at or near electrodes 125 and/or 130.

To that end, embodiments of an aqueous redox flow battery include negative electrolyte tank 110 including a negative aqueous electrolyte 120 comprising a redox-active additive 305-2 and a water-soluble redox mediator as described above. The aqueous RFB can include negative electrode 130 fluidically coupled with the negative electrolyte tank 110 and a positive electrolyte tank 105 including a positive electrolyte 115. In some embodiments, positive electrolyte 115 includes a water-soluble redox mediator as described above. Positive electrode 125 can be fluidically coupled with positive electrolyte tank 105. In this way, cell 150 can be divided into two half cells by an ion-permeable separator 140 between 130 negative electrode and positive electrode 125.

FIG. 4 is a block flow diagram of an example process 400 for operating an aqueous RFB configured as described above, in accordance with embodiments of the present disclosure. In some embodiments, operating the aqueous RFB in accordance with example process 400 includes one or more operations that can be reordered, omitted, repeated, cycled, and/or performed at least partially in parallel. In this way, the sequence of operations illustrated in FIG. 4 is intended to illustrate an example rather than a limiting embodiment. For example, in a multi-cell RFB, embodiments include systems where at least one cell 150 can charge and at least one cell 150 can discharge concurrently, for example, through inclusion of electronic control circuitry that permits different cells 150 to be electronically coupled with different load/charge circuits. In this way, power delivery can be matched to load demand without sacrificing excess capacity. For example, through dynamic switching of power circuits as part of electronics 145, example process 400 can improve the performance, responsiveness, and efficiency of an aqueous RFB that includes multiple cells 150. Advantageously, electrochemical reactions occurring in tanks 105 and 110 (e.g., at redox-active additive) are agnostic as to the number of charging cells and discharging cells. Instead, the net reaction balance between charging and discharging determines at least in part the reactions that occur at redox-active additive. In this way, the flowrates through respective tanks 105 and 110 can be controlled by monitoring system operating parameters at tanks 105 and 110 and by net electrical parameters at circuitry 145, instead of measuring flowrate through individual cells.

At block 405, example process 400 includes charging the aqueous redox flow battery. Charging can include inducing a current through cell 150 at a current density of from about 10 mA/cm2 to about 400 mA/cm2. As current can be a function of an applied voltage, block 405 can include applying a voltage from about 0.5 V to about 1.8 V to the aqueous redox flow battery using an external power circuit (e.g., circuitry 145 of FIG. 3). For example, charging can include inducing the reverse of reactions (1) and (2) on the anode side to store charge as chemical potential energy in tank 110. In this way, charging can be applied to both tanks 105 and 110 or individual tanks 105 or 110. The voltage is determined by the difference between the reduction potentials of the anolyte and catholyte. Higher operating currents increase the power, which is a significant challenge preventing grid-scale application of RFB systems.

At Block 410, example process 400 includes storing charge in tanks 105 and 110 for a period of time. When operating as a distributed storage technology or in a grid-scale installation, tanks 105 and/or 110 can store charge for a period of time until demand triggers a switch from charging to discharge operation. In this way, example systems 100, 200, 250 and/or 300 can service peaking demand and/or can function as distributed storage for intermittent energy sources including but not limited to distributed solar, wind, tidal, or other environmentally sustainable sources of electrical generation capacity.

At block 415, example process 400 includes discharging the aqueous RFB, for example, by reversibly coupling cell 150 to an external load (e.g., as part of circuitry 145). In an illustrative example, the aqueous RFB can be physically connected to a load circuit via a mechanical relay that is actuated by control circuitry configured to monitor electricity supply and demand on a circuit to which the aqueous RFB is connected (e.g., a local or regional distribution grid) and to switch cell(s) 150 from charge to discharge operation in response to demand exceeding supply. Advantageously, in switching from charge to discharge operation, flow direction can be maintained without affecting performance of cell(s) 150. Flow directions can be maintained due, at least in part, because the redox-active additive can be reduced or oxidized by the water-soluble redox mediator in tanks 105 and/or 110, thereby relocating at least a part of the electrochemical reaction to tanks 105 and/or 110.

Advantageously, when configured as described above the aqueous redox flow battery can remain essentially free of dendrites formed from the redox-active additive over multiple cycles of example process 400. For example, where the redox-active additive and redox mediator are included as part of the negative half-cell, the anode can remain essentially free of dendrites formed from the redox-active additive when subjected to about 10 charge/discharge cycles, about 20 cycles, about 30 cycles, about 40 cycles, about 50 cycles, about 60 cycles, about 70 cycles, about 80 cycles, about 90 cycles, about 100 cycles, or more than 100 cycles, including fractions and interpolations thereof. Dendrite formation is generally understood to impair the functioning of aqueous RFBs. As such, lifetime of cell(s) 150 is directly related to the number of charge/discharge cycles (e.g., iterations of at least a portion of the operations of example process 400) over which dendrite formation is essentially negligible. Advantageously, configured as described above, aqueous RFBs can exhibit improved performance as power storage and delivery systems while also exhibiting improved lifetime, relative to the conventional approaches described in reference to FIGs. 1-2B.

Similar to the reduced formation of dendrites, the aqueous redox flow battery, configured as described above, can operate over a similar number of charge/discharge cycles with essentially negligible formation of deposits of redox-active additive on electrodes 125 or 130. For example, the anode can remain essentially fee of deposits when subjected to at least 100 charge/discharge cycles. In this context, deposits refers to plating during discharge or charge, as appropriate, of electrodes 125 or 130 with additive. Such deposits can introduce resistive barriers and can affect the electrochemical reactions (e.g., the galvanic properties of the electrochemical cell) and impair performance of cell(s) 150. For that reason, lifetime of the aqueous RFB is directly related to the number of charge/discharge cycles over which electrodes 125 and/or 130 remain essentially free of deposits.

In some embodiments, example process 400 optionally includes refreshing redox additive and/or redox mediator at block 420. Refreshing in this context refers to medium exchange and or replacing the contents of cell(s) 150 and/or tanks 105 and/or 110. Block 420 can improve the operational lifetime of example systems 100, 200, 250, or 300, for example, by replacing fouled additive or spent mediator with fresh materials. In some embodiments, block 420 can be repeated once for a number of cycles. For example, block 420 can be implemented every 100 charge/discharge cycles, every 1000 charge/discharge cycles, every 10,000 charge/discharge cycles, or more, including fractions and interpolations thereof. Understandably, maintenance procedures as in block 420 can include significant downtime for systems, such that exceedingly frequent refreshing operations can impair system performance. Conversely, however, exceedingly infrequent refreshing can impair performance through fouling or other undesirable competing reactions between electrolytes 115 and/or 120 and system components.

EXAMPLE 1: REDOX- ACTIVE SOLID ADDITIVES FOR AQUEOS REDOX FLOW

BATTERIES

In some embodiments, an inexpensive, earth-abundant aqueous redox-targeting anolyte includes an iron-based anolyte 120 as suitable redox mediator for reversible redox reactions with solid zinc additives 305 in water. As described in more detail in reference to Example 1, mediator/solid redox reactions were be monitored using UV-vis absorption spectroscopy and an iron-based anolyte 120 system was evaluated for flow battery applications (e.g., for use in example system 300) in conjunction with an iron-based catholytes 115. In this context, aqueous RFB s have been demonstrated that operate at 1.1 V and demonstrate improved cyclability relative to techniques described in reference to FIGs. 2A-2B and the preceding redox-targeting configurations. Addition of the redox-active solid 305 showed an 84% increase in capacity relative to the same RFB prior to solid addition.

Materials. Triethanolamine (TEOA, 99.0%) and zinc oxide powder (ZnO, > 99.0%) were purchased from Sigma Aldrich. Iron (II) chloride tetrahydrate (FcCh'dfLO, 99%) was purchased from Alfa Aesar. Iron (III) chloride hexahydrate (FcCl^b!LO, 99.7%) and sodium hydroxide pellets were purchased from Fisher Chemicals. Potassium ferricyanide (K3[Fe(CN)6, 99.9%) and zinc powder (purified) were purchased from J.T. Baker. All chemicals were used without further purification.

Electrolyte preparation. The 1.0 M [Fe(CN)₆]³ catholyte was prepared by dissolving K3[Fe(CN)6] in 0.5 M NaOH solution. The [Fe(TEOA)OH]² anolyte was prepared by first dissolving 3.96 g (20 mmol) FcCh'dfFO in 21 mL of deionized water with stirring and N2 sparging for 20 minutes to form a light green solution. Then, 2 equivalents of TEOA (5.4 mL, 40 mmol) were added to the Fe²⁺ flask, upon which a pastel- blue slurry was formed. Meanwhile, 9 - 10 equivalents of NaOH (7.22 g, 180 mmol) were dissolved in 10 mL of deionized water. The NaOH solution was cooled, then added to the Fe²⁺-TEOA flask, changing the pastel-blue slurry to a forest-green solution. These syntheses were also scaled up 12.5 times to yield 500 mL of each electrolyte. 0.1 M [Fe(TEOA)OH] was prepared for absorption measurements by dissolving 1.08 g (4 mmol) FcCh'bHiO in 10 mL of deionized water with stirring and N2 sparging, followed by the addition of 1 mL (7.5 mmol) TEOA. Meanwhile, 8.00 g NaOH were dissolved in 10 mL deionized water. The NaOH solution was cooled to room temperature and injected into the Fe³⁺ precursor flask. The final volume was adjusted to 40 mL.

Electrochemical characterization. Cyclic voltammograms were performed using an Autolab potentiostat with a 3-mm diameter glassy carbon working electrode, a 6-mm leakless Ag/AgCl reference electrode (eDAQ), and a platinum wire counter electrode. Solutions were sparged with N2 for at least five minutes prior to measurement.

Flow battery measurements. Fuel cell hardware was purchased from Fuel Cell Technologies (Albuquerque, NM). The active area was 5 cm² with a machined serpentine flow pattern on graphite blocks that were treated with a cured furan resin. 0.01" silicone gaskets were used on both plates. The electrodes were carbon cloth (Panex PW03, 0.4 mm uncompressed thickness, Zoltek Carbon Fiber). The ion exchange membrane (Nafion 115) was soaked in 1.0 M NaOH solution for at least an hour prior to assembling the cell. Electrolytes were stored in 50 mL centrifuge tubes with holes in their caps. A Masterflex L/S peristaltic pump and Chem-Durance Bio L/S 14 tubing were used to pump the electrolytes through the electrochemical cell (100 - 150 rpm). A Squidstat potentiostat (Admiral Instruments) was used for charge/discharge measurements. The potential limits were set from 1.6 - 0.5 V, and the current was set between 200 - 400 mA (40 - 80 mA cm²). All measurements were performed at room temperature with no active temperature control at the electrochemical cell or electrolyte tanks. Setup as described is illustrated in FIGs. 12A-B.

Electrolyte Characterization. An anolyte consisting of the [Fe(TEOA)OH]² /[Fe(TEOA)OH] redox pair was chosen due to its low cost components, water solubility, and attractive electrochemical properties. FIG. 5 shows typical cyclic voltammetry data for the Fe-based electrolytes measured at a scan rate of v = 50 mV s ¹. [Fe(TEOA)OH]² loses an electron upon discharge at a redox potential of -1.02 V vs Ag/AgCl (eq 3), and [Fe(CN)₆]³ gains an electron upon discharge at a redox potential of +0.27 V vs Ag/AgCl (eq 4). The RFB composed of these redox pairs is therefore expected to have a maximum cell potential of 1.29 V.

[Fe(TEOA)OH]^~ + e^~ ¹ [ Fe(TEOA)OH ^~ (3)

E₁/2 = —1.02 V vs Ag/AgCl

£^" _I/2 = +0.27 V vs Ag/AgCl

FIG. 6 shows UV-vis absorption spectra of the fully charged ([Fe(TEOA)OH]² , dashed line) and fully discharged ([Fe(TEOA)OH] , solid line) form of the anolyte. [Fe(TEOA)OH] was prepared by electrochemical oxidation. The fully charged anolyte is characterized by a strong absorption band in the NIR around 920 nm. In the fully discharged anolyte, this NIR absorption feature is mostly absent and the spectrum is characterized by peaks in the visible around 473 and 560 nm. The [Fe(TEOA)OH]² spectrum shows substantial scattering at high concentrations.

Redox reactions. Zinc metal was identified as a candidate for redox-targeting due to its redox potential and low cost. It was determined that the redox potential of Zn should be sufficient to reduce [Fe(TEOA)OH] to [Fe(TEOA)OH]² (eq 5). To that end, redox reactions were monitored between [Fe(TEOA)OH] and Zn powder by absorption spectroscopy. (FIG. 7A) shows the absorption spectrum of [Fe(TEOA)OH] (black solid curve), measured at an iron concentration of 0.1 M to facilitate transmission. As portions of Zn powder (-0.6 equivalents total) were added to this solution, the NIR absorption feature grew in intensity, indicating that [Fe(TEOA)OH] was being reduced to [Fe(TEOA)OH]² (black dashed curve). Conversely, it was determined that ZnO should be sufficiently oxidizing to discharge [Fe(TEOA)OH]² and form [Fe(TEOA)OH] . (FIG. 7B) shows the absorption spectrum of [Fe(TEOA)OH]² (black solid curve), also at 0.1 M concentration. As portions of ZnO powder were added (-0.8 equivalents total), the characteristic NIR peak diminished in intensity, indicating the oxidation of [Fe(TEOA)OH]² to form [Fe(TEOA)OH] (black dashed curve). These absorption data provide direct evidence of redox cycling between [Fe(TEOA)OH]² and [Fe(TEOA)OH] that are chemically enabled by Zn/ZnO. It is noted that the speciation of zinc after Zn oxidation (denoted "Zn²⁺" in eq 5) is not known and may not be ZnO; this chemistry is currently under investigation.

[Fe(TE0A)0H]^_ + 1/2 Zn° [Fe(TE0A)0H]^2- + 1/2 Zn²⁺ (5)

Flow battery performance :. Having established the feasibility of [Fe(TEOA)OH]^{2 /_} and Zn/ZnO as a redox-targeting anolyte pair, the system was evaluated by lab-scale flow battery measurements. RFB experiments were conducted using 40 mL of 0.5 M [Fe(TEOA)OH]² as the anolyte with a 2:1 TEOA:Fe ratio and pH -13.9 (see experimental). The well-studied and inexpensive [Fe(CN)₆]^{3 /4} redox couple was chosen as the catholyte. 40 mL of 1.0 M catholyte were used with the pH adjusted to -13.97 using sodium hydroxide (see experimental). Whereas this electrolyte pair has been reported, its use in conjunction with redox targeting has not been investigated previously. FIG. 8 shows charge/discharge voltage curves measured at a current density of 40 mA cm ². The voltage limits were set limiting the operation range to 1.6 - 0.5 V to avoid unwanted side reactions. At this current density, the average charge and discharge times were 1.98 and 1.91 hours, respectively.

Next, the effect of Zn addition to the anolyte tank was investigated. FIG. 9A plots the charge (top curves) and discharge (bottom curves) capacity (mAh) vs voltage for two full charge/discharge cycles, before and after the addition of Zn powder to the anolyte tank, measured at a current density of 80 mA cm ². The volumetric charge capacity (Ah L ¹) is shown on the top axis. As shown in FIG. 9A, the addition of increasing amounts of Zn powder systematically increases both the charge and discharge capacity. These data are summarized in FIG. 9B, which plots capacity (mAh, left axis) and increase in capacity (%, right axis) against the equivalents of Zn added to the anolyte tank. Charge data are indicated by squares and discharge data are indicated by triangles. These data indicate that each equivalent of Zn increases the discharge capacity by -131% on average, suggesting -1.3 e^~ per equivalent of Zn added the anolyte. A volumetric power density of 10 Wh L ¹ is estimated from the volumetric discharge capacity after adding 0.6 equivalents of Zn (9.1 Ah L ¹) and the average discharge voltage (-1.1 V). This power density already compares favorably to other published RFB systems and is expected to increase substantially with further optimization.

Electrochemical cells were disassembled after RFB measurements and inspected visually. No zinc deposits were observed by eye on the electrodes, membrane, or in the flow fields, regardless of whether the cell was disassembled after full charging or complete discharge. This observation suggests that soluble Zn²⁺ species are not themselves being reduced at the electrodes, and instead suggests reduction of Zn²⁺ by soluble [Fe(TEOA)OH]² . This chemistry is presently under investigation.

Interestingly, performance of the [Fe(TEOA)OH] ^/2 electrolyte with added Zn was found to depend on the ligand ratio. FIG. 10A and FIG. 10B show discharge capacity (mAh) and efficiency for six charge/discharge cycles with and without added Zn, respectively. The efficiency is the ratio of measured capacity to theoretical capacity (429 mAh). Here, the Fe-TEOA concentration was lowered to 0.4 M and the TEOA:Fe ratio was varied. By increasing the TEOA:Fe ratio from 2:1 to 2.5:1, the discharge capacity is increased to -355 mAh (FIG. 10A), despite a lower Fe concentration (0.4 M vs 0.5 M). Further increasing the TEOA:Fe ratio to 4:1 showed a less efficient initial performance, but upon addition of 0.4 equivalents of Zn the discharge capacity increases to -370 mAh (FIG. 10B). This discharge capacity corresponds to an overall efficiency of 86%, surpassing literature reports for RFBs based on Fe-TEOA electrolytes (Table 1).

Table 1. Comparison of [Fe(TEOA)OH]^_/2· -based RFB efficiencies.

Comparative Fe-TEOA Catholyte Vol. Theo. Eff. Example # cone. (M) Cap. Cap. [%]

[Ah/L] [Ah/

LI

*Example 4 used triisopropanolamine (TiPA) as the ligand.

Aqueous redox-targeting anolyte : Use of a redox-targeting anolyte system in RFBs is described that uses an inexpensive aqueous redox mediator paired with an inexpensive, earth- abundant redox-active solid additive. In some embodiments, implementing anolyte systems with catholyte systems provides further performance improvements, for example, following optimization of the anolyte 120 composition ( e.g ., amount of Zn added), and by variation of the ligands coordinating the Fe^2+/3+ redox mediator. For example, an iron- triisopropanolamine (Fe-TiPA) complex can be implemented as having electrochemical properties similar to Fe-TEOA. Thus, the redox-targeting strategy described herein is generalizable to a broad class of inorganic or organometallic compounds with redox potentials configured for pairing with Zn additive (e.g., about -1.02 V vs Ag/AgCl).

Redox-active solid additive: Measurements presented herein demonstrate increased RFB capacity with the addition of Zn powder to anolyte tank 110. RFB performance can be improved and/or optimized based at least in part on parametric-guided modification of the quantity and form of the added Zn redox active solids 305. Table 2: Influence of additive on capacity of RFB embodiments

Table 3: Influence of additive on capacity of RFB embodiments

Tables 2 and 3 illustrate the influence of additive inclusion and of stirring electrolyte 120 in anolyte tank 110 on RFB performance in the RFB system described in the context of Example 1. Data presented were determined from cell potential measurements made using the setup illustrated in FIGs. 12A-12B, as illustrated in FIG. 13 and FIG. 14 for Table 2 and Table 3, respectively.

Inclusion of redox-active additive and stirring or other mechanical agitation can improve one or more performance metrics for RFB, as well as performing at or above comparative examples described in Table 1 with respect to one or more properties shared in common with the examples. For example, with 0.4 molar additive, efficiency in Table 2 increases to 98.21 percent, which exceeds each comparative example described in Table 1. Similarly, discharge time increases, as illustrated in FIG. 11. Conclusion:

The above results demonstrate of an aqueous redox-targeting RFB cell 150 that employs a soluble iron redox mediator. Earth-abundant zinc metal was identified as a suitable solid electron reservoir 305. Redox reactions between anolyte 120 and solid reservoir 305 were confirmed by absorption spectroscopy. RFB experiments showed large and reversible increases in both charge and discharge capacity upon the addition of Zn to the Fe-TEOA anolyte tank 110, with no change in RFB discharge voltage (-1.10 V). An 84% increase in capacity was demonstrated following addition of 0.6 equivalents of Zn to the anolyte, and greater increases are anticipated with greater Zn additions. No deposition of zinc metal was observed on the electrodes 130 or in the flow fields after anolyte 120 charging, arguing against direct Zn²⁺ reduction at the electrode and instead supporting the role of Fe-TEOA as a redox mediator. The Fe-TEOA/Zn anolyte system demonstrated herein represents a new class of aqueous, earth- abundant redox-targeting electrolytes for grid-scale power storage.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

As used herein and unless otherwise indicated, the terms "a" and "an" are taken to mean "one", "at least one" or "one or more". Unless otherwise required by context, singular terms used herein shall include pluralities and plural terms shall include the singular.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." Words using the singular or plural number also include the plural and singular number, respectively. Additionally, the words "herein," "above," and "below" and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of the application.

Aspects of the disclosure can be modified, if necessary, to employ the systems, functions, and concepts of the above references and application to provide yet further embodiments of the disclosure. These and other changes can be made to the disclosure in light of the detailed description.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

Specific elements of any foregoing embodiments and examples can be combined or substituted for elements in other embodiments or examples. Moreover, the inclusion of specific elements in at least some of these embodiments and examples may be optional, wherein further embodiments may include one or more embodiments that specifically exclude one or more of these specific elements. Furthermore, while advantages associated with certain embodiments of the disclosure have been described in the context of these embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the disclosure.

It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein.

Example devices, methods, and systems are described herein. It should be understood the words "example," "exemplary," and "illustrative" are used herein to mean "serving as an example, instance, or illustration." Any embodiment or feature described herein as being an "example," being "exemplary," or being "illustrative" is not necessarily to be construed as preferred or advantageous over other embodiments or features.

Furthermore, the particular arrangements shown in the FIGURES should not be viewed as limiting. It should be understood that other embodiments may include more or less of each element shown in a given FIGURE. Further, some of the illustrated elements may be combined or omitted. Yet further, an example embodiment may include elements that are not illustrated in the FIGURES.

The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the disclosure. In this regard, no attempt is made to show structural details of the disclosure in more detail than is necessary for the fundamental understanding of the disclosure, the description taken with the drawings and/or examples making apparent to those skilled in the art how the several forms of the disclosure may be embodied in practice.

The description of embodiments of the disclosure is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. While the specific embodiments of, and examples for, the disclosure are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the disclosure, as those skilled in the relevant art will recognize.

Claims

CLAIMS The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An aqueous redox flow battery, comprising: an aqueous electrolyte comprising a redox-active additive and a redox mediator.

2. The aqueous redox flow battery of Claim 1, wherein the redox-active additive increases an energy storage capacity of the aqueous redox flow battery by at least 10%, compared to the energy storage capacity of an aqueous redox flow battery without the redox-active additive in the aqueous electrolyte.

3. The aqueous redox flow battery of Claim 1 or Claim 2, wherein the redox- active additive comprises a redox-active solid.

4. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive comprises titanium (Ti), lead (Pb), iron (Fe), zinc (Zn), tin (Sn), copper (Cu), nickel (Ni), cobalt (Co), bismuth (Bi), sodium (Na), lithium (Li), magnesium (Mg), compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof.

5. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive comprises Ti, Zn, Li, compounds thereof, oxides thereof, complexes thereof, salts thereof, or any combination thereof.

6. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive comprises a redox-active metal.

7. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive comprises zinc metal.

8. The aqueous redox flow battery of Claim 7, wherein the zinc metal is in the form of a powder, mossy zinc, zinc mesh, electrodeposited zinc, zinc foam, zinc pellets or any combination thereof.

9. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive is in the form of a powder.

10. The aqueous redox flow battery of any one of the preceding claims, wherein the redox-active additive is insoluble in at least one of a reduced form or an oxidized form in the aqueous electrolyte.

11. The aqueous redox flow battery of any one of the preceding claims, wherein the water-soluble redox mediator has a redox potential within about +/- 200 mV of the redox potential of the redox- active additive.

12. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator is water-soluble.

13. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator comprises a water-soluble aqueous transition metal-containing redox mediator.

14. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator comprises a transition metal coordination compound optionally comprising a ligand selected from triethanolamine, triisopropanolamine, bipyridine, porphyrins, bridging oxides, and any derivatives thereof.

15. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator comprises a water-soluble organometallic compound.

16. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator comprises an aqueous organoiron compound.

17. The aqueous redox flow battery of Claim 16, wherein the redox mediator comprises [Fe(TEOA)OH]¹-^/2 or [Fe(TⁱPA)OH] ^1_/2\

18. The aqueous redox flow battery of any one of the preceding claims, wherein the redox mediator is dissolved in the electrolyte.

19. The aqueous redox flow battery of any one of the preceding claims, wherein the negative electrolyte comprises the redox mediatonredox-active additive at a molar ratio of from 1:0.1 to 1:25.

20. The aqueous redox flow battery of any one of the preceding claims, wherein the negative electrolyte comprises the redox mediator at a concentration of from 0.1 M to 5 M.

21. The aqueous redox flow battery of any one of the preceding claims, wherein the aqueous redox flow battery comprises a volumetric power density of from 5 Wh/L to 200 Wh/L.

22. The aqueous redox flow battery of any one of the preceding claims, wherein the aqueous redox flow battery is substantially free of dendrites formed from the redox- active additive.

23. An aqueous redox flow battery, comprising a negative electrolyte tank comprising a negative aqueous electrolyte comprising a redox-active additive and a water-soluble redox mediator of any one of Claims 1 to 22; a negative electrode in fluidically coupled with the negative electrolyte tank; a positive electrolyte tank comprising a positive electrolyte; a positive electrode in fluid communication with the positive electrolyte tank; and an ion-permeable separator between the negative electrode and the positive electrode.

24. A method of operating a redox flow battery of Claim 23, comprising charging the aqueous redox flow battery by inducing a current via at a current density from about 10 mA/cm2 to about 400 mA/cm² at a voltage from about 0.5 V to about 1.8 V from the aqueous redox flow battery.

25. The method of Claim 24, further comprising discharging the aqueous redox flow battery by reversibly coupling a load with the aqueous redox flow battery.

26. The method of Claim 24 or Claim 25, wherein the aqueous redox flow battery does not form dendrites of the redox-active additive in the anode when subjected to at least 100 charge/discharge cycles.

27. The method of any one of Claims 24 to 26, wherein the aqueous redox flow battery does not form redox-active additive deposits on the anode when subjected to at least 100 charge/discharge cycles.

28. The method of any one of Claims 24 to 27, wherein the aqueous redox flow battery comprises a volumetric power density of 5 Wh/L or more.

29. The method of any one of Claims 24 to 28, wherein the aqueous redox flow battery does not comprise an organic solvent.

30. The method of any one of Claims 24 to 29, wherein the positive electrolyte comprises an iron-containing redox-active material.

31. The method of any one of Claims 24 to 30, wherein the positive electrolyte comprises [Fe(CN)g]^{4 /3} .

32. The method of any one of Claims 24 to 31, wherein the redox-active additive is reduced or oxidized by the water-soluble redox mediator in the negative electrolyte.