Systems and methods for using redox mediators or catalysts

Abstract

The present disclosure generally relates to redox mediators or catalysts for various uses, such as in redox flow batteries. The redox mediator may be a 2,2,6,6-tetramethylpiperidine-l-oxyl or TEMPO-based redox mediator in aspects. In certain cases, such redox mediators may be useful in electrochemical devices, for example, in a battery such as a flow battery. In some embodiments, the flow battery may be based on bromide or another halide, e.g., acting as a catholyte. Other aspects are generally directed to flow batteries containing such redox mediators, methods of making or using such redox mediators, kits involving such redox mediators, or the like.

Description

[0001] SYSTEMS AND METHODS FOR USING REDOX MEDIATORS OR CATALYSTS

[0002] RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 552,006, filed February 9, 2024, entitled “Systems and Methods for Using Redox Mediators or Catalysts,” by Sullivan, et al., incorporated herein by reference in its entirety.

[0004] FIELD

[0005] The present disclosure generally relates to redox mediators or catalysts for various uses, such as in redox flow batteries.

[0006] BACKGROUND

[0007] As dispatchable fossil fuel power generation is replaced with intermittent solar and wind energy, energy storage is needed to stabilize the grid and time-shift renewable generation to meet demand. This has economic, reliability, and emission implications. Lithium-ion batteries currently dominate new installations due to their manufacturing at scale and operational know-how, in part from the electric vehicle market. However, lithium-ion systems have fire safety concerns, supply chain constraints, and short duration configuration, limiting their value in the booming grid market. Thus, alternative solutions are needed to address electric grid sustainability at scale.

[0008] Aqueous flow batteries are a promising technology for stationary grid energy storage. Unlike conventional solid-state batteries (i.e., lithium ion), these systems decouple power (kW) and capacity (kWh) components. Redox active species are dissolved in water to serve as the anolyte (liquid anode) and catholyte (liquid cathode) and stored in electrolyte reservoirs. These electrolytes are pumped through a power cell stack where the species are reduced and oxidized at current collector surfaces (the anode and cathode) while separated by a membrane, charging and discharging electricity to and from the grid. Although numerous specific systems exist, flow battery archetypes are intrinsically more scalable and fire-safe for stationary applications.

[0009] Vanadium flow batteries are the most mature flow battery technology, having been heavily researched by NASA in the 1970s and deployed commercially since the early 2000s. However, their commercial progress has been stunted by their performance limitations and lithium-ion battery competition. Additionally, the raw vanadium electrolyte materials have a volatile supply chain with economics that are not able to compete with lithium-ion products. Thus, lower cost electrolyte materials are needed for widespread market impact. Halide catholytes are one such potential material. Approximately, iodide has a formal reduction potential of +0.536 V vs. SHE (standard hydrogen electrode) and bromide of +1.0 V vs. SHE, making them both fit for catholyte application in conjunction with various anolytes. For example, zinc-bromine is a common anolyte-catholyte pair for hybrid flow battery applications and has several commercial deployments. Despite being abundantly available through mineral ores and saltwater brines, bromide catholyte has limited performance that hinders product scalability, safety, and value. Specifically, these electrolytes require complex system controls and engineering due to phase separation during charging, formation of toxic bromine vapors during operation, and limitations to energy density, energy efficiency, and cycling stability. Thus, novel solutions are needed to address the kinetic, phase, and stability challenges of halide catholytes.

[0010] SUMMARY

[0011] The present disclosure generally relates to redox mediators or catalysts for various uses, such as in redox flow batteries. The subject matter of the present disclosure involves, in some cases, interrelated products, alternative solutions to a particular problem, and / or a plurality of different uses of one or more systems and / or articles.

[0012] For instance, some aspects are generally directed to ionic redox-active homogenous mediators for halide catholytes. In one set of embodiments, the mediators may include nitroxyl radical moieties, for example, (2,2,6,6-tetramethylpiperidit-l-yl)oxyl (known as “TEMPO”), which may bear negative functionalizations in some embodiments. In some embodiments, e.g., when paired with halide catholyte, for example bromide, such mediators may serve various purposes. This includes, for example, improving the electrode kinetics, reducing membrane crossover, or enhancing the phase stability of polyhalides, etc. In some cases, when deployed in catholytes (e.g., in flow batteries) such mediators may effectively increase coulombic efficiency, energy efficiency, energy density, or cycling stability performance. In addition, in certain embodiments, such mediators may potentially simplify certain practical system design or safety considerations.

[0013] One aspect is generally drawn to an electrochemical device. In one set of embodiments, the electrochemical device comprises an anode compartment containing an anolyte, a cathode compartment containing a halide catholyte and a mediator, and an ionexchange membrane between the anode compartment and the cathode compartment. In some cases, the mediator comprises at least one nitroxyl moiety and at least one anionic moiety.

[0014] The electrochemical device, in another set of embodiments, comprises an anode compartment containing an anolyte; a cathode compartment containing a halide catholyte and a mediator; and an ion-exchange membrane between the anode compartment and the cathode compartment. In some embodiments, the mediator has a structure: wherein Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; and A is an anionic moiety.

[0015] According to another set of embodiments, the electrochemical device comprises an anode compartment containing an anolyte, a cathode compartment containing a halide catholyte and a mediator, and an ion-exchange membrane between the anode compartment and the cathode compartment. In some embodiments, the mediator has a structure: where Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; A is an anionic moiety; n is between 2 and 1000; and * indicates a terminal group.

[0016] Another aspect is generally drawn to a composition. In accordance with one set of embodiments, the composition includes a halide concentration, e.g., of at least 0.1 M, and a compound having a structure: where Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; and A is an anionic moiety.

[0017] The composition, in another set of embodiments, comprises a halide concentration, e.g., of at least 0.1 M, and a compound having a structure: where Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; A is an anionic moiety; n is between 2 and 1000; and * indicates a terminal group.

[0018] Another aspect is generally drawn to a method. In one set of embodiments, the method comprises reacting a halide to produce a polyhalide in the presence of a mediator having a structure: where Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; and A is an anionic moiety.

[0019] The method, in another set of embodiments, comprises reacting a halide to produce a polyhalide in the presence of a mediator having a structure: where Y is an amine, an amide, an ester, an ether, or a covalent bond; L is an organic linker or a covalent bond; A is an anionic moiety; n is between 2 and 1000; and * indicates a terminal group.

[0020] In another aspect, the present disclosure encompasses methods of making one or more of the embodiments described herein, for example, a TEMPO-based redox mediator or catalyst. In still another aspect, the present disclosure encompasses methods of using one or more of the embodiments described herein, for example, a TEMPO-based redox mediator or catalyst. Other advantages and novel features of the present disclosure will become apparent from the following detailed description of various non-limiting embodiments of the disclosure when considered in conjunction with the accompanying figures.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Non-limiting embodiments of the present disclosure will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the disclosure shown where illustration is not necessary to allow those of ordinary skill in the art to understand the disclosure. In the figures:

[0023] Fig. 1 illustrates the reversible electrochemical oxidation of aqueous halide salts into polyhalides, which are in equilibrium with dissociation into dihalogen phases, in accordance with some embodiment;

[0024] Fig. 2 illustrates a mediator interacting with a halide catholyte, in another embodiment;

[0025] Fig. 3 illustrate TEMPO 4-position functionalization (Y) and a trend on cathodic potential to N-oxoammonium, according to yet another embodiment;

[0026] Fig. 4 is a non-limiting schematic of a reaction involving a mediator comprising a TEMPO moiety, in still another embodiment;

[0027] Figs. 5A-5B illustrate a non-limiting example of a battery charging and discharging, in accordance with yet another embodiment;

[0028] Fig. 6 is a schematic of a flow battery, in yet another embodiment;

[0029] Fig. 7 illustrates various nitroxyl moieties, in still other embodiments;

[0030] Fig. 8 illustrates anionic moieties, in other embodiments;

[0031] Fig. 9 illustrates mediators with various anionic groups, in accordance with certain embodiments;

[0032] Fig. 10 illustrates various anionic amine-functionalized mediators, in other embodiments;

[0033] Fig. 11 illustrates various zwitterionic amine-functionalized mediators, in yet other embodiments;

[0034] Fig. 12 illustrates various anionic ether or amide-functionalized mediators, in still other embodiments;

[0035] Fig. 13 illustrates various anionic mediators, in other embodiments; Fig. 14 illustrates various mediators with one nitroxyl radical TEMPO core, in still other embodiments;

[0036] Fig. 15 illustrates various dimeric or oligomeric nitroxyl moieties, in still other embodiments;

[0037] Fig. 16 illustrate various polymeric nitroxyl moieties, in yet other embodiments;

[0038] Fig. 17 illustrates redox behavior for PS-N+TEMPO, in one embodiment;

[0039] Fig. 18 illustrates redox behavior for PS-Eth-TEMPO, in another embodiment;

[0040] Fig. 19 illustrates that PS-Eth-TEMPO has a lower cathodic potential than PS- N+TEMPO, in yet another embodiment;

[0041] Fig. 20 illustrates redox behavior of KBr, in another embodiment;

[0042] Fig. 21 illustrates redox behavior for PS-N+TEMPO in the presence of a mediator, in yet another embodiment;

[0043] Fig. 22 illustrates redox behavior for PS-Eth-TEMPO in the presence of a mediator, in still another embodiment; and

[0044] Fig. 23 illustrates a representative charge-discharge curve from a PS-N+TEMPO and bromide catholyte flow battery, in another embodiment.

[0045] DETAILED DESCRIPTION

[0046] The present disclosure generally relates to redox mediators or catalysts for various uses, such as in redox flow batteries. The redox mediator may be a 2, 2, 6, 6- tetramethylpiperidine-l-oxyl or TEMPO-based redox mediator in aspects. In certain cases, such redox mediators may be useful in electrochemical devices, for example, in a battery such as a flow battery. In some embodiments, the flow battery may be based on bromide or another halide, e.g., acting as a catholyte. Other aspects are generally directed to flow batteries containing such redox mediators, methods of making or using such redox mediators, kits involving such redox mediators, or the like.

[0047] In certain aspects, mediators such as those described herein may be used in various redox-driven electrochemical devices, such as batteries, fuel cells, supercapacitors, or the like. In some cases, the electrochemical device may be a battery, such as a flow battery. Redox generally refers to a transfer of electrons, e.g., during operation of the electrochemical device, or when a voltage is applied. A flow battery is a battery having liquids present on separate sides of a membrane (e.g., separating an anode compartment from a cathode compartment), where ions can flow through the membrane from one liquid to the other, while electrons flow around the membrane and can be harnessed for power. In some cases, the membrane may be an ion- selective membrane, such as an anion- selective membrane or a cation-selective membrane. In some embodiments, one or both liquids may flow, e.g., from a reservoir, past the membrane, or through the device. In some cases, one or both liquids may be recirculated, e.g., to and from the reservoir. In some embodiments, one or both of the liquids may be aqueous, e.g., using water as a solvent, e.g., as in an aqueous flow.

[0048] In some embodiments, the battery may be a flow battery. A flow battery, or a redox flow battery, is a type of electrochemical device where chemical energy is provided by chemical components dissolved in liquids that are present on separate sides of a membrane, such as an ion selective membrane. As discussed, ions can flow through the membrane while electrons flow around the membrane, e.g., through an external circuit. In some cases, liquids may flow on one or both sides of the membrane, e.g., pumped to and / or from a reservoir. The energy capacity of the battery can thus be controlled by controlling the volume of liquid within the reservoir. The liquid on the anode side is called the anolyte, while the liquid on the cathode side is called the catholyte, both generally referred to as the electrolyte. However, despite such advantages, flow batteries have seen only limited commercial development, and thus, improvements are still needed.

[0049] Redox-active chemical species may be present within the electrolyte solutions in a battery (e.g., in a flow battery), where they can function as the anode and cathode electrolytes in some embodiments. The electrolyte in the anode compartment may be referred to as the anolyte, while the electrolyte in the cathode compartment may be referred to as the catholyte. During discharge of the electrochemical device, electrons flow from the anolyte to the catholyte (e.g., through a load). Oxidation (loss of electrons) occurs in the anolyte within the anode compartment, while reduction (gain of electrons) occurs in the catholyte within the cathode compartment. The electrons may flow from the anolyte, to an anode current collector, through an electrical circuit, to a cathode current collector, to the catholyte. In addition, to balance charge, cations (positively charged ions) may flow across the ionexchange membrane from the anode compartment to the cathode compartment, and / or anions (negatively charged ions) may flow across the ion-exchange membrane from the cathode compartment to the anode compartment. During charging, this process is reversed. Electrons move from the catholyte to the anolyte (typically requiring energy to cause the electrons to flow in that direction). Reduction (gain of electrons) occurs in the anolyte within the anode compartment, while oxidation (loss of electrons) occurs in the catholyte within the cathode compartment. The electrons may flow from the catholyte, to a cathode current collector, through an electrical circuit, to an anode current collector, to the anolyte. In addition, to balance charge, cations (positively charged ions) flow across the ion-exchange membrane from the cathode compartment to the anode compartment, and / or anions (negatively charged anions) flow across the ion-exchange membrane from the anode compartment to the cathode compartment. In some cases, the liquid within the cathode may flow to and / or from a cathodic reservoir, and / or the liquid within the anode may flow to and / or from an anodic reservoir. In some cases, one or more liquid pumps may be used to cause liquid flow to occur.

[0050] A non-limiting schematic diagram of a battery is shown in Fig. 5, as an illustrative non-limiting example. Fig. 5A shows a battery 10 in discharging mode. In anode compartment 20, an anolyte having a charge state n (i.e., A11) is oxidized to produce an electron (e‘) and a more positively charged state (An+1). The electrons may be collected by anode current collector 25, and can flow through load 40 to reach cathode current collector 35 in cathode compartment 30. In cathode compartment 30, the electrons from cathode current collector 35 may reduce the catholyte (having a charge state Cm+1) to produce a catholyte with a more negatively charged state m (i.e., Cm). In addition, counter-cations M+may flow from anode compartment 20 to cathode compartment 30 across ion exchange membrane 50, for example, which may be selectively permeable to the counter-cations. In these examples, n and m may independently be any suitable value, indicating a charge state, depending on the anolyte and the catholyte. For instance, n and m may independently be -3, -2, -1, 0, 1, 2, 3, etc.

[0051] Fig. 5B shows battery 10 in charging mode, where these processes are essentially reversed. Electrons are pumped from the catholyte to the analyte via energy source 45, which inputs energy into the system to be stored in the battery. In anode compartment 20, the electrons combine with anolyte (having a charge state An+1) to produce an anolyte having a more negatively charged state (A11), while the catholyte (having a charge state Cm) in cathode compartment 30 releases electrons to produce a more positively charged state (Cm+1). Counter-cations M+also can flow from cathode compartment 30 to anode compartment 20 across ion exchange membrane 50.

[0052] In some embodiments, for example, when battery 10 is a flow battery, one or both compartments may have fluid flowing therethrough. For example, as is shown in battery 10 in Fig. 6, pump 27 may recirculate a fluid containing the anolyte between anode compartment 20 and anode reservoir 22, and / or pump 37 may recirculate a fluid containing the catholyte between cathode compartment 30 and cathode reservoir 32.

[0053] A variety of redox species can be used as anolytes or catholytes in a battery, such as a flow battery, including inorganic species (e.g., sulfide / poly sulfide, Zn / ZnX2 (X=I, Br, Cl), Cr2+ / Cr3+, ferro / ferricyanide) or organic species (e.g., viologens, anthraquionones, phenazines, thiolate). Viologen derivatives, such as bis(3-trimethylammonio)propyl viologen tetrachloride, and other pyridyl derivatives, may be used in certain embodiments. Nonlimiting examples of suitable redox species include any of those described in US Pat. Apl. Pub. Nos. 2022 / 0363663 and 2023 / 0006250.

[0054] For example, some aspects are generally directed to electrolytes based on halides that may be present in a redox-driven electrochemical device. The halide electrolytes may involve one or more counter-cations (M) and one or more halides (X). See, e.g., Fig. 1, where M is a counter-cation or a mixture of counter-cations (e.g., Zn+2‘, Li+, Na+, K+, NH4+, etc.), and X is a halide or mixture of halides (e.g., one or more of I’, Br’, Cl’, F’, etc.).

[0055] A halide such as MX can be electrochemically oxidized, e.g., in a multi-step electrochemical redox reaction (such as is shown in Fig. 1) to form a polyhalide anion (X2n+f, where n may be any integer of at least 1, e.g., Xs’, Xs’, X?’, Xg’, Xu’, Xis’, etc.). Thus, the halide may be redox active, e.g., a halide catholyte may be specifically oxidized or reduced when a voltage is applied between the anode compartment and the cathode compartment. Non-limiting examples of halides include polybromides (e.g., Brf, Brs’, Br?’, BiV, etc.), polyiodides (e.g., Is’, Is', I?’, I9’, etc.), polychlorides (e.g., Cis’, Cis’, Cl?’, CI9’, etc.), or the like. In some cases, a polyhalide may comprise more than one type of halide, e.g., Br?!’, FBr’, BrsCI'. BrCh’, IsCf, ICh’, BrlQ’, etc. However, in some embodiments, such electrochemical reactions may result in sluggish redox kinetics. The oxidized polyhalide species may be in equilibria with dissociation into dihalogen (X2). Exacerbated at high concentrations and higher orders of polyhalides (e.g., X5, X7, etc.), the dissociation may facilitate aqueous-liquid and / or aqueous-solid phase separation, the formation of toxic halogen vapors, or other undesirable effects. Furthermore, the due to their small size and non-polar nature, polyhalides and dihalogens have high permeabilities through membrane separations, such as through cation-exchange membranes, which may result in self-discharge or the generation of heat during operation.

[0056] In some cases, an electrolyte may exhibit a halide concentration of at least 0.01 M, at least 0.02 M, at least 0.03 M, at least 0.05 M, at least 0.1 M, at least 0.2 M, at least 0.3 M, at least 0.5 M, at least 1 M, at least 2 M, at least 3 M, at least 5 M, at least 10 M, etc. In some cases, one or more halides may be present as a halide salt. For example, the halide salt may be an alkali metal salt (e.g., comprising lithium, sodium, potassium, etc.), an alkali metal salt (e.g., comprising beryllium, magnesium, calcium, etc.), a Row 4 transition metal salt (e.g., comprising scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, etc.), an aluminum salt, etc. In some cases, the halide salt may comprise an organic cation, for example, a quaternary ammonium, a protonated ammonium (e.g., a quaternary ammonium where any of the 4 groups bound to the nitrogen is a proton), an aromatic nitrogen cation moiety, phosphonium, etc.

[0057] Accordingly, the performance of the battery can be enhanced in certain aspects using a mediator. A mediator may facilitate such electrochemical reaction by facilitating the transport of electrons and / or ions, e.g., through an electrolyte. For example, a mediator in the cathode compartment may exchange electrons with a catholyte (for example, a halide catholyte), then diffuse to the cathode current collector and exchange electrons there, or vice versa, depending on whether the battery is being charged or discharged. Thus, a halide catholyte may be able to exchange an electron with the mediator when a voltage is applied between the anode compartment and the cathode compartment. Similarly, as another example, a mediator in the anode compartment may exchange electrons with an anolyte, then diffuse to the anode current collector and exchange electrons there, or vice versa, again depending on whether the battery is being charged or discharged. In some cases, there may be one or more mediators present in the cathode compartment, and / or one or more mediators present in the anode compartment.

[0058] A variety of mediators may be used in various embodiments. For example, a mediator may be a redox mediator, e.g., a mediator that is redox active by participating by mediating one or more redox reactions. In some cases, the mediator may be a redox-active homogenous mediator, e.g., contained within solution (e.g., within the catholyte and / or within the anolyte). The mediator may improve the redox kinetics, membrane crossover, the stability of the catholytes, or the like. In the catholyte, both the halide and the mediator may undergo electrochemical oxidation state changes. The halide and mediator redox reactions may occur in tandem and / or in sequence in bulk solution and / or at an electrode surface. In some cases, a single mediator may be used, although in some embodiments, 2, 3, 4, 5, or more mediators may be used, e.g., in a single compartment or more than one compartment in a battery (such as a flow battery) or other electrochemical device (including any of those described herein). The mediator may have multiple functionalities or multi-functional moieties in certain embodiments, e.g., as discussed herein. In addition, in some embodiments, a mediator may be water soluble.

[0059] A mediator may be present in an electrolyte at various concentrations. In some cases, an electrolyte may exhibit a mediator concentration of at least 0.01 M, at least 0.02 M, at least 0.03 M, at least 0.05 M, at least 0.1 M, at least 0.2 M, at least 0.3 M, at least 0.5 M, at least 1 M, at least 2 M, at least 3 M, at least 4 M, at least 5 M, etc. In some cases, the concentration may be less than 5 M, less than 4 M, less than 3 M, less than 2 M, less than 1 M, less than 0.5 M, less than 0.3 M, less than 0.2 M, less than 0.1 M, less than 0.05 M, less than 0.03 M, less than 0.02 M, less than 0.01 M, etc. In addition, in certain embodiments, combinations of any of these ranges are also possible, e.g., the concentration of a mediator may be between 0.01 M and 0.05 M, between 0.05 M and 3 M, between 0.1 M and 0.3 M, etc. If mediators are present in both an anolyte and a catholyte, the mediators may be the same or different, and the mediators may independently be present in the same or different concentrations.

[0060] In some cases, the mediator may be in the electrolyte at various molar (moles per liter) concentration ratios with the catholyte redox species, e.g., halides. In some cases, an electrolyte may exhibit a mediator to catholyte, e.g., halide, ratio of 1 to 1, less than 1 to 1, less than 0.9 to 1, less than 0.75 to 1, less than 0.5 to 1, less than 0.1 to 1, and / or more than 1 to 1, more than 1.1 to 1, more than 1.25 to 1, more than 1.5 to 1, more than 2 to 1, etc. Combinations of any of these ranges are also possible in other embodiments.

[0061] In one set of embodiments, the mediator may comprise at least one redox-active nitroxyl portion or radical. For example, the mediator may have 1 nitroxyl present. The mediator may also have 2 nitroxyls present in some embodiments. In certain embodiments, the mediator may have at least 2 nitroxyl moieties, at least 5 nitroxyl moieties, at least 10 nitroxyl moieties, at least 20 nitroxyl moieties, etc. In some cases, there may be between 2 and 10 nitroxyl moieties, between 10 and 20 nitroxyl moieties, etc. In some cases, there may be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more nitroxyls present in a mediator. In some cases, the nitroxyl portion of the mediator may also be referred to as a “core.”

[0062] As a non-limiting example, a redox-active nitroxyl radical may include a 2, 2,6,6- tetramethylpiperidine-l-oxyl redox moiety, also known as TEMPO. Other non-limiting examples of nitroxyl moieties are shown in Fig. 7. Such redox moieties may, in certain embodiments, possess formal reductional potentials approximately between +0.80 V and +1.0 V vs. SHE. In some embodiments, tis less than the formal reduction potential of bromide, estimated at +1.0 V vs. SHE. Upon charging, a nitroxyl radical within the mediator may be oxidized into a N-oxoammonium cation in certain cases, which is a relatively “soft” cation as an organic positive charge with resonance.

[0063] In some embodiments, a mediator may have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more nitroxyl radical “cores” present within the mediator structure. If more than one nitroxyl radical core is present, the nitroxyl radical core may independently be the same or different. As non-limiting examples, all of the nitroxyl radical cores may be TEMPO, or one or more of the nitroxyl radical cores may be TEMPO, while other nitroxyl radical cores may also be present, e.g., including those shown in Fig. 7. The nitroxyl radical cores may, according to certain embodiments, act in various roles. For instance, in certain embodiments, a nitroxyl radical core may be oxidized at potentials just below that of bromide or other halides. The resulting N-oxoammonium cations may electrostatically coordinate with the bromide or other halide anions, e.g., due to opposite charges.

[0064] In addition, in some embodiments, the oxidized N-oxoammonium may be able to orbitally interact with the bromide, other halides, or halide mixtures in certain embodiments. This may, in some cases, provide a low unoccupied molecular orbital (LUMO) that the HOMO of the bromide or other halide can mix with. As a result, the halide atoms may become partially oxidized, which may facilitate oxidation to polyhalides (X2n+f) such as polybromide (Br2n+f), e.g., at various surfaces, such as electrode surfaces, and / or in solution. Similarly, polyhalides may become partially reduced, which may facilitate reduction to halides such as bromide (Br‘), e.g., at various surfaces, such as electrode surfaces. In addition, in certain cases, upon formation of polyhalides such as polybromides, the “soft” organic anions may be able to coordinate with the N-oxoammonium cation. This electrostatic and orbital interaction may be able to stabilize the polyhalide, reducing its dissociation into halogens (e.g., Ch, Br2, etc.), phase separation, vaporization, or the like.

[0065] Generally, “soft” ionic groups and ions have a smaller charge to size ratio than “hard” ionic groups and ions. While the “soft” or “hard” nature of an ionic group does not readily lend itself to quantification, soft and hard ionic groups can be identified based on how these groups interact with ions in an aqueous solution. For the purposes of this disclosure, a hard ionic (anionic or cationic) group on a complexing agent is characterized in that said ionic group prefers to interact with (i.e., is more strongly attracted to) the counter ion (for example a charge-balancing cation or anion) of the ionic redox species, rather than with the soft ionic group of the complexing agent. The ionic redox species and the other ionic group of the complexing agent are, therefore, categorized as “soft” because they are left to coordinate with one another. These electrostatic interactions may be helpful in some embodiments between certain charge pairs, e.g., when coordination between the multiple ions may be in equilibria together with complex solution dynamics as well as potentially orbital effects. Due to their relatively large sizes and relatively low charge densities, the organic cationic groups of the complexing agents will be soft relative to any metal cation and NH Due to their relatively small size and relatively high charge densities, counter anions, such as Cl’, Br", COf, SO42', OH", NCh', CHaSCh', CFaSCh’, and the like, are examples of hard anions.

[0066] In some embodiments, a mediator may comprise at least one redox-inert anionic moiety, such as sulfonate (-S(O2)O‘), sulfate (-SO42'), carboxylate (-COO ), phosphate (-PO43' ), phosphonate (-PO(OR)2'X etc. Each R, where present, may independently be hydrogen, alkyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, alkyl-N(R”)x, alkyl-S(O)x, alkyl alcohol, polyether (e.g., polyethylene glycol), etc., where R” (when present) may be hydrogen, alkyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, an oxygen protecting group, and a nitrogen protecting group, and x is independently 2 or 3. The alkyl may be, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, etc. A cycloalkyl may include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, etc. In some cases, the carboxylate may be present as -COOH or -COOR, where R is as provided above. Anionic groups such as these, in certain embodiments, can be linked to nitroxyl radicals, such as TEMPO, using readily available reagents and scalable reactions known to those of ordinary skill in the art. For example, 1,3-propoanesultone or gamma-butyrolactone can be linked to a TEMPO through a simple SN2 reaction to form the corresponding alkyl sulfonate or alkyl carboxylate structure.

[0067] In certain embodiments, a mediator may have various 4-position moieties or functionalization, including but not limited to, ether, amines, esters, amides, alkyls, carboxylates, sulfates, phosphates, etc. These may be synthesized from TEMPO reagents, such as 4-oxo-2,2,6,6-tetramethyl-l-piperidinyloxy (4-oxo-TEMPO, CAS 2896-70-0), 4- hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-hydroxy-TEMPO, CAS 2226-96-2), 4- amino-2,2,6,6-tetramethylpiperidinyloxy (4-Amino-TEPMO, CAS 14681-88-4), etc., e.g.:

[0068] In some cases, functionalizing the 4 position may impact the formal reduction potential of the nitroxyl radical, ranging from approximately +0.8 V to +1.0 V vs. SHE in the general trend of amine > amide = ester > ether > alkane. In some cases, a mediator may comprise an amine-TEMPO having a cathodic potential that matches that of bromide, but in other embodiments, other functionalization or mixtures of functionalization can be utilized with various halides or mixtures of halides, etc. In one set of embodiments, a mediator may have a structure: where Y may be, for example, an amine, an amide, an ester, an ether, or the like; L may be an organic linker (e.g., as discussed herein); and A may be an anionic moiety (e.g., as discussed herein). For example, Y may be -O-, -OC(O)-, -NH-, -NR-, -N+RR’-, -NRC(O)R’, etc., where R and R’ may each independently be, for example, hydrogen, an alkyl, cycloalkyl, a heterocyclyl, an aryl, a heteroaryl, an alkyl amine, an aryl amine, a cycloalkyl amine, a heterocycloalkyl amine, a heteroaryl amine, an alkyl sulfate, an aryl sulfate, a cycloalkyl sulfate, a heterocycloalkyl sulfate, a heteroaryl sulfate, an alkyl alcohol, a polyether (e.g., polyethylene glycol), or other moieties, etc. In one set of embodiments, R may be another nitroxyl radical, such as TEMPO or the nitroxyl moieties shown in Fig. 7. In addition, in some cases, either or both of Y and / or L may be absent (e.g., Y and / or L may each independently be a covalent bond). Specific non-limiting examples of these include:

[0069] Additional examples include, but are not limited to:

[0070]

[0071] A non-limiting schematic of a reduction rection involving such structures is shown in Fig. 4. In this figure, electrons pass from a halide catholyte (represented as X in the structures shown herein) to a mediator, and from the mediator to a current collector. In this way, the mediator is able to facilitate the transport of electrons and / or ions, e.g., through an electrolyte to a current collector. Accordingly, with reference to Figs. 5 A and 5B, a mediator may assist in the movement of electrons from a current collector to a catholyte or an anolyte, or vice versa, e.g., such that the catholyte or anolyte can be reduced or oxidized.

[0072] Non-limiting examples of organic linkers (e.g., L in the structures shown herein) that can be used in such mediators include alkyls, substituted alkyl, alkenes, substituted alkenes, aryls, substituted aryls, heterocycles, substituted heterocycles, heteroaromatics, substituted heteroaromatics, alkyl alcohol, polyether (e.g., polyethylene glycol), etc. In some cases, a TEMPO moiety and / or organic linker may be neutrally charged or positive. For example, a quaternary ammonium, a protonated ammonium, an aromatic nitrogen cation moiety, etc. can be incorporated into the TEMPO moiety and organic linker. Specific non-limiting examples include alkyl ammoniums, nitrogen heterocycle ammoniums (e.g., morpholiniums, triethylenediammonium (DABCO), piperidiniums, pyrrolidinium, etc.,) aromatic nitrogen cations (e.g., imidazoliums, pyridiniums, etc.), or the like.

[0073] Aliphatic quaternary ammonium groups include alkyl quaternary ammonium groups, such as trimethyl quaternary ammonium groups and triethyl quaternary ammonium groups. Examples of cycloaliphatic quaternary ammonium groups and cationic nitrogen containing heteroaromatic groups include imidazolium groups, benzimidazolium groups, pyridinium groups, bipryridinium groups, l,4-diazabicyclo[2.2.2]octane-l,4-diium groups, aziridinium groups, azetidinium groups, pyrrolidinium groups, piperidinium groups, morpholinium groups, piperazinium group, and imidazolidinium groups. A linker may be bonded to or substituted with one, two, three, or more anionic moieties (e.g., A in the structures shown herein), in various embodiments. Examples of anionic moieties that the linkers may be bonded to or substituted with include, but are not limited to, sulfonates, sulfates, phosphonates, phosphates, carboxylates, etc. If two or more anionic moieties are present, they may independently be the same or different. Further, additional non-limiting examples of anionic moieties that may be used in various embodiments are shown in Fig. 8.

[0074] However, it should be understood that a linker is not necessarily required, e.g., E may represent a covalent bond. For example, in some embodiments, an anionic substituent can be directly functionalized to a mediator without a linker, e.g., to the 4-position of a TEMPO moiety. For example, the mediator may include a 4-carboxy-TEMPO moiety, or a 4-sulfate- TEMPO moiety. As other examples, sulfonate, phosphate, and phosphonate groups may be directly functionalized to the mediator, e.g., to the 4-position of a TEMPO moiety. As yet another example, anionic moieties can be directly bond to an amine (e.g., at the 4-position of a TEMPO) without a linker in some embodiments, e.g., producing amine- sulfate, aminesulfonate, amine-phosphate, amine-phosphonate, amine-carboxylate functionalities. Nonlimiting examples of such compositions include: where each A may independently be an anionic moiety (e.g., sulfonates, sulfates, phosphonates, phosphates, carboxylates, or other anionic moieties such as those described herein), and each R may each independently be, for example, hydrogen, an alkyl, cycloalkyl, a heterocyclyl, an aryl, a heteroaryl, an alkyl amine, an aryl amine, a cycloalkyl amine, a heterocycloalkyl amine, a heteroaryl amine, an alkyl sulfate, an aryl sulfate, a cycloalkyl sulfate, a heterocycloalkyl sulfate, a heteroaryl sulfate, or other moieties, or other R moieties such as any of those described herein. Additional examples include, but are not limited to: where each A may independently be an anionic moiety (e.g., sulfonates, sulfates, phosphonates, phosphates, carboxylates, or other anionic moieties such as those described herein), and each R may each independently be, for example, hydrogen, an alkyl, cycloalkyl, a heterocyclyl, an aryl, a heteroaryl, an alkyl amine, an aryl amine, a cycloalkyl amine, a heterocycloalkyl amine, a heteroaryl amine, an alkyl sulfate, an aryl sulfate, a cycloalkyl sulfate, a heterocycloalkyl sulfate, a heteroaryl sulfate, or other moieties, or other R moieties such as any of those described herein.

[0075] The mediator may be a negatively charged, positively charged, neutral, zwitterionic (e.g., having both anionic and cationic charges present, in the same or different numbers), etc., in various embodiments. These may be determined, for example, when the mediator is in a nitroxyl radical state. In some embodiments, the mediator has a positive charge, e.g., such that it can complex with a halide or a polyhalide. However, in some embodiments, the mediator may have negative functionalization, e.g., one or more anionic moieties. In some cases, the mediator may comprise a nitroxyl radical that can be oxidized to an N- oxoammonium cation during charging, for example, at an electrode surface or by complexing with polybromide or another polyhalide. In some cases, an additional redox-inert organic cation moiety can provide additional complexation ability, even in a reduced state. Thus, in the reduced state, e.g., with an uncharged nitroxyl radical, the mediator may be anionic or zwitterionic in some embodiments. In some cases, the mediator may be uncharged except for charges on the anionic moiety. Examples of mediators with one nitroxyl radical TEMPO core include, but are not limited to, those shown in Fig. 9. Examples of mediators with two or more nitroxyl radical TEMPO cores and an amine 4-position functionalization that are anionic include, but are not limited to, those shown in Fig. 10. Examples of mediators with two or more nitroxyl radical TEMPO cores and an amine 4-position functionalization that are zwitterionic include, but are not limited to, those shown in Fig. 11. Examples of mediators with two or more nitroxyl radical TEMPO cores and an ether or amide 4-position functionalization that are anionic include, but are not limited to, those shown in Fig. 12. Additional examples of mediators with two nitroxyl radical TEMPO cores that are anionic include, but are not limited to, those shown in Fig. 13. Additional examples of mediators with one nitroxyl radical TEMPO core are shown in Fig. 14.

[0076] In certain embodiments, the mediator may be present in a relatively acidic environment, which may affect the charge of the mediator. For example, in some embodiments, the mediator may be present in an anolyte or a catholyte that has a pH of less than 7, less than 6, less than 5, less than 4, less than 3, less than 2, etc. However, in some embodiments, the mediator may be present in a relatively basic environment. For example, the pH may be at least 7, at least 8, at least 9, at least 10, at least 11, or at least 12, etc. In addition, in some cases, the mediator may be water soluble, e.g., dissolved in an anolyte or in a catholyte. This may be achieved, for example, by covalently linking a hydrophilic anionic moiety or functionalization (e.g., including any of those described herein) to the mediator (although in other embodiment, the mediator may be water soluble without such moieties). In some cases, a hydrophilic anionic moiety or functionalization (e.g., any of those described herein) may be linked to a cation, e.g., an organic cation, such as a soft organic cation. In addition, the anionic moiety may reduce membrane crossover (e.g., flow of an ion across a membrane) in certain cases. In some cases, the movement of a polyhalide through an ion exchange membrane may be reduced, e.g., due to the size and / or negative charge of the mediator / polyhalide complex. Thus, size exclusion and / or coulombic repulsion may reduce the amount of movement across the membrane.

[0077] In some cases, the mediator can include a nitroxyl radical core, such as TEMPO monomer, dimers, oligomers, or polymers bearing at least one anionic charge. Without wishing to be bound by any theory, it is believed that these dimer and polymer structures may be able to coordinate multiple polyhalides, e.g., due to the presence of multiple N- oxoammoniums in the oxidized state. Accordingly, in some cases, the mediator may comprise 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more nitroxyl radical groups. For example, the mediator may have a structure such as: where Y may be, for example, an amine, an amide, an ester, an ether, or the like; L may be an organic linker (e.g., as discussed herein); A may be an anionic moiety (e.g., as discussed herein); and * indicates any suitable polymeric terminal group (for example, hydrogen, an alkyl, cycloalkyl, a heterocyclyl, an aryl, a heteroaryl, an alkyl amine, an aryl amine, a cycloalkyl amine, a heterocycloalkyl amine, a heteroaryl amine, an alkyl sulfate, an aryl sulfate, a cycloalkyl sulfate, a heterocycloalkyl sulfate, a heteroaryl sulfate, an alkyl alcohol, a polyether (e.g., polyethylene glycol), or other moieties, etc.). In some embodiments, the TEMPO-based mediator may be momomer, an oligomer, or a polymer. For example, n may be at least 2, at least 3, at least 5, at least 10, at least 15, at least 20, at least 30, at least 50, at least 100, at least 200, at least 300, at least 500, at least 1000, etc. In some cases, n may be no more than 1000, no more than 500, no more than 300, no more than 200, no more than 100, no more than 50, no more than 30, no more than 20, no more than 15, no more than 10, no more than 5, no more than 3, etc. Combinations of any of these are also possible in other embodiments, e.g., n may be between 10 and 20, between 30 and 100, between 500 and 1000, between 5 and 50, etc.

[0078] In some cases, the mediator may be associated in solution with a halogen such as bromide, iodide, chloride, or fluoride, or a polyhalide, for example, a polyhalide comprising bromide (for example, tribromide or Bra'), or other polyhalides such as those described herein. More than one type of halogen may be present, e.g., 2, 3, 4, or more halides may be present, e.g., in a mixed-halide solution. For example, the mediator may participate in a redox reaction involving halide oxidation / reduction. As opposed of using nitroxyl radicals as the sole or dominant redox-active component undergoing oxidation / reduction in the catholyte in the presence of spectator ions, such as halides, e.g., chloride or bromide, the use of nitroxyl radicals as mediators may utilize both the nitroxyl radicals and the halides as redox-active components in the catholyte in certain embodiments, e.g., undergoing reversible oxidation / reduction processes together in homogenous solution or at the electrode surface. In some cases, counter-cations, e.g., metal cations (for example, Zn2+, Ei+, Na+, K+) or organic cations (for example, NH4+), may act as spectator ions and, in some cases, when used with cation-exchange membranes, may act as charge-balancing ions that diffuse through the membrane to balance the transfer of electrons through the circuit. One non-limiting example is shown in Fig. 2.

[0079] Thus, the mediator may be present in solution, e.g., in an anode solution with the anolyte, or in a cathode solution with the catholyte. In some cases, more than one mediator may be present, e.g., there may be one, two, three, four, or more mediators in the anode solution, and / or one, two, three, four, or more mediators in the cathode solution.

[0080] For example, in various aspects, the mediator may be used in an electrochemical device, such as a battery, that uses various halide-based electrolytes, for example, bromide, iodide, mixed-halides, etc. For example, a mediator may be present in a catholyte, or in an anolyte. In one embodiment, for example, a catholyte may be a halide catholyte, such as a bromide catholyte, a chloride catholyte, etc. Other examples include mixed halide systems, including iodide, bromide, chloride, and / or fluoride. One, two, three, or more halides may be present. In some cases, bromide, or another halide, may be present in a mixed halide system at a mole fraction of at least 20 mol%, at least 30 mol%, at least 40 mol%, at least 50 mol%, at least 60 mol%, at least 70 mol%, at least 75 mol%, at least 80 mol%, at least 85 mol%, at least 90 mol%, at least 95 mol%, etc. In addition, in some embodiments, the halide may not comprise bromide at all. For example, in some embodiments, the halide may be an iodide and chloride mixture. In some embodiments, a mediator can be used in conjunction with any individual halide, with any mixture of two or more halides in any ratio, etc.

[0081] Referring now to Fig. 2, one mechanism of a halide catholyte charge / discharge with a mediator such as described herein is now described. In this specific nonlimiting example, the mediator contains a nitroxyl radical that is a TEMPO redox core with 4-position functionalization Y, an organic anionic group A, and an organic linker L connecting the anionic group A to the TEMPO redox core. X is a halide comprising bromide, and M is a counter-cation. Due to its formal reduction potential being below that of bromide, the nitroxyl radical can first be oxidized to an N-oxoammonium cation. This loss of an electron to the anolyte through the cell circuit may coincide with the diffusion of a counter-cation, M+, to the anolyte across the ion exchange membrane. As a result, the mediator can become zwitterionic in nature, and / or partially coordinate with the MX salt in equilibrium through electrostatic interactions. Additionally, the HOMO of X- may have orbital overlap with the LUMO of the N-oxoammonium. Thus, when charging continues and the halide is oxidized into polyhalide, the redox overpotential may be reduced and / or electrode kinetics enhanced in some embodiments. As the relatively non-polar X211+1 polyhalide is formed, the M+cation may have a stronger coulombic attraction force with the mediator anion, leaving the polyhalides to electrostatically couple with the organic cation.

[0082] The oxidation potential and the resulting HOMO-LUMO interaction of a halide with N-oxoammonium may depend on the 4-position functionalization. The formal reduction potential of TEMPOs can be tuned from approximately +0.8 V to +1.0 V vs. SHE depending on 4-position functionalization (see Fig. 3). For certain applications, a TEMPO may be functionalized with an amine, e.g., to produce a formal potential typically of about +0.90 V vs. SHE, which is just below that of bromide at +1.0 V vs. SHE. This results in less of a difference in potential when charging the cell with bromide and in increased HOMO-LUMO energy matching.

[0083] U.S. Provisional Patent Application Serial No. 63 / 552,006, filed February 9, 2024, entitled “Systems and Methods for Using Redox Mediators or Catalysts,” by Sullivan, et al., is incorporated herein by reference in its entirety.

[0084] The following examples are intended to illustrate certain embodiments of the present disclosure, but do not exemplify the full scope of the disclosure.

[0085] EXAMPLE 1

[0086] Two representative nitroxyl radicals bearing anionics charge were synthesized in this example as halide mediators or catalysts. These were propylsulfonate ether TEMPO (PS- Eth-TEMPO) and propyl sulfonate dimethylammonium TEMPO (PS-N+TEMPO) (Fig. 14). Both utilize a sulfonate anionic moiety covalently linked to the TEMPO 4-position with an alkyl chain. Although both have the same anionic moiety and linker, the difference in 4- position functionalization permits a comparison between mediators of different charge, one anionic and one zwitterionic, as well as formal reduction potential.

[0087] To synthesis PS-Eth-TEMPO, the following procedure was used. In a 500 mL two neck round bottom flask, sodium hydride (60%) was taken (2.48 g, 62.4 mmol) in toluene (50 mL). To this solution, 4-hydroxy TEMPO (5.00 g, 29.0 mmol) was added at room temperature under a N2 atmosphere. The resulting reaction mixture was stirred for 30 minutes at room temperature. Afterwards, a solution 1,3-propanesultone (4.25 g, 34.8 mmol) in toluene (30 mL) was added dropwise. The reaction mixture was heated at 85 °C for 24 hours. After this time, reaction mixture was cooled to room temperature and distilled out the solvent under reduced pressure by rotary evaporation to get a crude product as an orange residue. The crude product was purified by the silica gel column chromatography using methanol / dichloromethane to get the TEMPO as an orange solid (6.50 g, 73% yield). The synthesis of the PS-N+TEMPO used the following procedure. 4-dimethylamino TEMPO (1.99 g, 10 mmol, 1 equiv.) and 1,3-propanesultone (1.46 g, 12 mmol, 1.2 equiv.) were mixed, dissolved in acetone, and the mixture was sealed in a thick-walled pressure bottle under N2 protection. The system was stirred at 50 °C for 12 h. Ethyl ether (50 mL) was added and stirred for 20 min. Finally, the precipitate was obtained through filtration and washed with acetone (20 mL x 3). The product was dried at 40 °C under vacuum overnight and the yield was 92% (4.1 g).

[0088] The electrochemical properties of the resultant mediators were examined via cyclic voltammetry (CV) analysis. For these experiments, a 3-electrode setup was utilized with a glassy carbon electrode (GCE) as the working electrode, a platinum wire as the counter electrode, and a standard calomel electrode (SCE) as the reference electrode (+0.241 V vs. SHE). 1 M NaCl was used as the supporting salt with a scan rate of 100 mV / s at room temperature.

[0089] First, the redox behavior of the TEMPO mediators was tested by themselves at 0.1 M. PS-N+TEMPO (Fig. 17) and PS-Eth-TEMPO (Fig. 18) both exhibited reversible redox behavior at positive voltages. As shown in Fig. 19, PS-Eth-TEMPO possessed a lower cathodic potential of +0.8 IV vs. SHE, while that of PS-N+TEMPO was shifted positive at +0.84V vs. SHE. This difference is believed to be due to the electronic effects of the 4- position functionalization. Using the automated PSTrace software to correct for capacitive current, PS-Eth-TEMPO exhibited a peak cathodic current (zp,c) of 0.120 mA and a peak anodic current (zp,fl) of -0.166 mA, resulting in an anodic over cathodic peak current ratio of 0.966. PS-N+TEMPO exhibited a ip,cof 0.123 mA and a ip,aof -0.114 mA, resulting in a ratio of 0.927. The ratio effectively represents the reversibility of the redox processes, with a value of 1 representing an ideally reversible process. Both mediators demonstrated a nearly ideal redox process.

[0090] Second, the redox behavior of KBr was investigated at 0.1 M (Fig. 20). Unlike the TEMPO mediators, bromide did not exhibit ideally reversible redox behavior. The oxidation process did not show the presence of a diffusion limited region. This may be due to the formation of polybromides and bromine that may diffuse away from the electrode surface during oxidation, which was supported by observing bromine liquid formed and diffused during the experiment. Furthermore, a large reduction overpotential was observed as the anodic peak was shifted towards lower voltages with large peak separation from the cathodic peak. This may be due to the sluggish kinetics of the polybromide reduction process at the electrode surface. Finally, the redox behavior of KBr was investigated in the presence of the TEMPO mediators, both at 0.1 M concentrations. Interestingly, both PS-N+TEMPO (Fig. 21) and PS- Eth-TEMPO (Fig. 22) still exhibited reversible redox behavior with an increase in the anodic over cathodic peak current ratio compared to TEMPO themselves. For PS-Eth-TEMPO, the ratio increased from 0.966 to 1.08, while for PS-N+TEMPO it increased from 0.927 to 2.05. These peak ratios may increase due to the mediators facilitating the reduction of polybromide, which was formed during the oxidative scans. However, instead of seeing two discreet reduction peaks — one for the TEMPO reduction and one for the bromide reduction — only one reduction peak was observed. Without wishing to be bound by any theory, this may suggest that the polybromide and N-oxoammonium species orbitals may overlap and undergo a single electrode reduction process. Furthermore, the formation and diffusion of bromine liquid was no longer observed at the electrode surface during the oxidation process, supporting the hypothesis that the mediator electrostatically coordinates with the polybromides to inhibit dissociation into bromine and enabling a homogenous catholyte.

[0091] The PS-N+TEMPO mediator showed a more pronounced effect in CV with bromide than PS-Eth-TEMPO. With out wishing to be bound by any theory, this may be hypothesized to be due to the higher cathodic potential of the mediator with an ammonium 4-position functionalization, which is closer to that of bromide. It can be predicted that this may result in an increased orbital match between the LUMO of the N-oxoammonium of the oxidized mediator and the HOMO of the polybromide species. This hypothesis is suggested by at least two observations. First, the oxidation peak of the mediator with bromide was suppressed (i.e., lower current) compared to that of the mediator alone, suggesting an electrochemical interaction between the species. Second, the reduction peak of the polybromide with the PS- N+TEMPO mediator was shifted positive by nearly +200 mV compared to polybromide without the mediator. This may suggest that the reduction overpotential can be dramatically decreased and the electrode kinetics improved with the addition of the mediator. Thus, these results support that nitroxyl radicals bearing anionic moiety can complex with bromide and affect the electrochemical redox processes to varying degrees, e.g., depending on the properties of the mediator.

[0092] EXAMPLE 2

[0093] After characterizing the electrochemical performance of the two halide redox mediators, PS-N+TEMPO was selected for additional flow battery characterization in this example. A custom flow battery single-cell device was used with a 4 cm2active area and peristaltic pumps. 5 mL of 0.5 M l,r-bis[3-sulfonatopropyl]-4,4’-bipyridinium ((SPr^V) was used as the anolyte and 5 mL of 0.02 M PS-N+TEMPO and 0.02 KBr was used as the catholyte in a catholyte-limiting configuration. 1 M NaCl supporting salt was used in both electrolytes with a SELEMION™ CMVN cation-exchange membrane. Graphite felts with a thickness of 3 mm (GFD 3 EA, SIGRACELL®) were pre-treated at 400 °C in air for 6 h before being used as electrodes on both sides of the cell. The cell was galvanically cycled at 10 mA / cm2with a capacity cutoff.

[0094] Fig. 23 shows a representative charge-discharge curve from the PS-N+TEMPO and bromide catholyte flow battery experiment in this example. In this experiment, catholyte was charge to a capacity (mAh) in which, stoichiometrically, each of the nitroxyl radicals would be oxidized and 66% of the bromide would be oxidized (Brf). Charging halide catholyte to the 66% state-of-charge, stoichiometrically representing the trihalide sate, is standard practice in the operation of halide catholyte to mitigate phase separation of the hydrophobic polyhalides species. Based on the fact the individual redox species exhibited significantly different cathodic potentials in the CV — a formal reduction potential +0.84V vs. SHE for PS- N+TEMPO and an onset oxidation potential of over +1.2V vs. SHE for bromide — it was expected that two charge-discharge plateaus separated by approximated 0.36 V would be observed. However, surprisingly, no distinct regions were observed, and instead a single charge-discharge plateau occurs. This qualitatively may support that the nitroxyl radical is indeed orbitally interacting with the bromide / polybromide species, and improving the electrochemical reversibility of the halide catholyte system. Furthermore, no bromine liquid, aqueous-oil or aqueous-solid phase separation, or bromine vapors were observed during the cycling, confirming homogenous phase stability. This may suggest that the anionic substituent allows homogenous complexation of the oxidized polybromide species.

[0095] Overall, the impact of redox-active nitroxyl radicals with anionic functionalization on the electrochemical performance of halide catholyte was experimentally demonstrated in these examples. Specifically, both PS-Eth-TEMPO and PS-N+TEMPO, anionic and zwitterionic mediators, respectively, improved the reversibility of polybromide reduction in CV. Additionally, PS-N+TEMPO spurred noteworthy electrochemical perturbations to the bromide oxidation and reduction process in CV and flow battery testing, signifying the opportunity structure-property tuning of the mediators to match the properties of the halide catholyte. The mediators retained homogenous phase in aqueous solution in the presence of polybromide during CV and flow battery, supporting the importance of the hydrophilic anionic moiety in allowing a water-soluble soft cation (i.e., N-oxoammonium) and soft anion (i.e., polybromide) complex. Thus, mediators comprising nitroxyl radicals with anionic moieties have unique capabilities as redox-active homogeneous catalysts that can be exploited in tandem with halide catholytes in aqueous electrochemical applications.

[0096] While several embodiments of the present disclosure have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the functions and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within the scope of the present disclosure. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the teachings of the present disclosure is / are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the disclosure described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the disclosure may be practiced otherwise than as specifically described and claimed. The present disclosure is directed to each individual feature, system, article, material, kit, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and / or methods, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent, is included within the scope of the present disclosure.

[0097] In cases where the present specification and a document incorporated by reference include conflicting and / or inconsistent disclosure, the present specification shall control. If two or more documents incorporated by reference include conflicting and / or inconsistent disclosure with respect to each other, then the document having the later effective date shall control.

[0098] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0099] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

[0100] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and / or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and / or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0101] As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of’ or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.”

[0102] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. When the word “about” is used herein in reference to a number, it should be understood that still another embodiment of the disclosure includes that number not modified by the presence of the word “about.”

[0103] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0104] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of’ and “consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

CLAIMSWhat is claimed is:

1. An electrochemical device, comprising: an anode compartment containing an anolyte; a cathode compartment containing a halide catholyte and a mediator; and an ion-exchange membrane between the anode compartment and the cathode compartment, wherein the mediator comprises at least one nitroxyl moiety and at least one anionic moiety.

2. The electrochemical device of claim 1, wherein the electrochemical device is a battery.

3. The electrochemical device of claim 2, wherein the battery is a flow battery.

4. The electrochemical device of claim 3, wherein the flow battery is an aqueous redox flow battery.

5. The electrochemical device of any one of claims 1-4, wherein the ion-exchange membrane is a cation-exchange membrane.

6. The electrochemical device of any one of claims 1-5, wherein the anolyte is contained within aqueous solution.

7. The electrochemical device of any one of claims 1-6, wherein the halide catholyte is contained within aqueous solution.

8. The electrochemical device of claim 7, wherein the aqueous solution has a pH of less than 7.

9. The electrochemical device of claim 7, wherein the aqueous solution has a pH of at least 7.

10. The electrochemical device of any one of claims 1-9, wherein the halide catholyte and mediator are configured to exchange one or more electrons when a voltage is applied between the anode compartment and the cathode compartment.

11. The electrochemical device of any one of claims 1-10, wherein the halide catholyte comprises bromide.

12. The electrochemical device of any one of claims 1-11, wherein the halide catholyte is at least 50 mol% bromide.

13. The electrochemical device of any one of claims 1-12, wherein the halide catholyte is at least 75 mol% bromide.

14. The electrochemical device of any one of claims 1-13, wherein the halide catholyte is at least 95 mol% bromide.

15. The electrochemical device of any one of claims 1-14, wherein the halide catholyte comprises iodide.

16. The electrochemical device of any one of claims 1-15, wherein the halide catholyte comprises chloride.

17. The electrochemical device of any one of claims 1-16, wherein the halide catholyte comprises fluoride.

18. The electrochemical device of any one of claims 1-17, wherein the halide catholyte has only one halide.

19. The electrochemical device of any one of claims 1-18, wherein the halide catholyte comprises at least two halides.

20. The electrochemical device of any one of claims 1-19, wherein the halide catholyte comprises a halide salt.

21. The electrochemical device of claim 20, wherein the halide salt comprises an alkali metal salt.

22. The electrochemical device of claim any one of claims 20 or 21, wherein the halide salt comprises an alkaline earth metal salt.

23. The electrochemical device of claim any one of claims 20-22, wherein the halide salt comprises a Row 4 transition metal salt.

24. The electrochemical device of claim any one of claims 20-23, wherein the halide salt comprises an aluminum salt.

25. The electrochemical device of claim any one of claims 20-24, wherein the halide salt comprises an organic cation.

26. The electrochemical device of claim 25, wherein the organic cation comprises a quaternary ammonium.

27. The electrochemical device of any one of claims 25 or 26, wherein the organic cation comprises a protonated ammonium.

28. The electrochemical device of any one of claims 25-27, wherein the organic cation comprises an aromatic nitrogen.

29. The electrochemical device of any one of claims 25-28, wherein the organic cation comprises phosphonium.

30. The electrochemical device of any one of claims 1-29, wherein the mediator comprises a 2,2,6,6-tetramethylpiperidit-l-yl)oxyl (TEMPO) moiety.

31. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- po sition amine.

32. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- po sition ether.

33. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- po sition amide.

34. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- position ester.

35. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- position carboxylate.

36. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- position sulfate.

37. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- po sition phosphate.

38. The electrochemical device of claim 30, wherein the TEMPO moiety comprises a 4- po sition alkyl.

39. The electrochemical device of any one of claims 1-38, wherein the mediator is negatively charged in a nitroxyl radical state.

40. The electrochemical device of any one of claims 1-39, wherein the mediator is zwitterionic in a nitroxyl radical state.

41. The electrochemical device of any one of claims 1-40, wherein in a nitroxyl radical state, the mediator is uncharged except for charges on the anionic moiety.

42. The electrochemical device of any one of claims 1-41, wherein the at least one anionic moiety comprises a sulfonate.

43. The electrochemical device of any one of claims 1-42, wherein the at least one anionic moiety comprises a sulfate.

44. The electrochemical device of any one of claims 1-43, wherein the at least one anionic moiety comprises a phosphonate.

45. The electrochemical device of any one of claims 1-44, wherein the at least one anionic moiety comprises a phosphate.

46. The electrochemical device of any one of claims 1-45, wherein the at least one anionic moiety comprises a carboxylate.

47. The electrochemical device of any one of claims 1-46, wherein the mediator comprises a cationic moiety.

48. The electrochemical device of claim 47, wherein the cationic moiety comprises a quaternary ammonium.

49. The electrochemical device of any one of claims 47 or 48, wherein the cationic moiety comprises a protonated ammonium.

50. The electrochemical device of any one of claims 47-49, wherein the cationic moiety comprises an aromatic nitrogen.

51. The electrochemical device of any one of claims 1-50, wherein the mediator has only one nitroxyl moiety.

52. The electrochemical device of any one of claims 1-50, wherein the mediator has only two nitroxyl moieties.

53. The electrochemical device of any one of claims 1-50, wherein the mediator comprises at least 3 nitroxyl moieties.

54. The electrochemical device of any one of claims 1-50, wherein the mediator comprises at least 10 nitroxyl moieties.

55. The electrochemical device of any one of claims 1-54, wherein the mediator comprises a linker between a nitroxyl moiety and an anionic moiety.

56. The electrochemical device of claim 55, wherein the linker comprises an alkyl.

57. The electrochemical device of any one of claims 55 or 56, wherein the linker comprises a substituted alkyl.

58. The electrochemical device of any one of claims 55-57, wherein the linker comprises a heterocyclyl.

59. The electrochemical device of any one of claims 55-58, wherein the linker comprises an aryl.

60. The electrochemical device of any one of claims 55-59, wherein the linker comprises a substituted aryl.

61. The electrochemical device of any one of claims 55-60, wherein the linker comprises a hetero aromatic.

62. The electrochemical device of any one of claims 1-61, wherein a nitroxyl moiety is covalently bonded to an anionic moiety.

63. An electrochemical device, comprising: an anode compartment containing an anolyte; a cathode compartment containing a halide catholyte and a mediator; and an ion-exchange membrane between the anode compartment and the cathode compartment, wherein the mediator has a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond; andA is an anionic moiety.

64. The electrochemical device of claim 63 wherein Y is an amine.

65. The electrochemical device of claim 63 wherein Y is an ether.

66. The electrochemical device of claim 63 wherein Y is an amide.

67. The electrochemical device of claim 63 wherein Y is an ester.

68. The electrochemical device of claim 63 wherein Y is a carboxylate.

69. The electrochemical device of claim 63 wherein Y is a sulfate.

70. The electrochemical device of claim 63 wherein Y is a phosphate.

71. The electrochemical device of claim 63 wherein L is an alkyl.

72. The electrochemical device of claim 63 wherein L is a substituted alkyl.

73. The electrochemical device of claim 63 wherein L is a heterocyclyl.

74. The electrochemical device of claim 63 wherein L is a covalent bond.

75. The electrochemical device of claim 63 wherein A comprises a sulfonate.

76. The electrochemical device of claim 63 wherein A comprises a sulfate.

77. The electrochemical device of claim 63 wherein A comprises a phosphonate.

78. The electrochemical device of claim 63 wherein A comprises a phosphate.

79. The electrochemical device of claim 63 wherein A comprises a carboxylate.

80. The electrochemical device of claim 63 wherein Y is a covalent bond and L is a covalent bond, whereby the compound has a structure:

81. A composition, comprising: a halide concentration of at least 0.1 M; and a compound having a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond; andA is an anionic moiety.

82. The method of claim 81, wherein Y is a covalent bond and L is a covalent bond, whereby the compound has a structure:

83. A method, comprising: reacting a halide to produce a polyhalide in the presence of a mediator having a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond; andA is an anionic moiety.

84. The method of claim 83, further comprising removing electrons from the halide to produce the polyhalide.

85. The method of any one of claims 83 or 84, wherein the halide comprises bromide.

86. The method of any one of claims 83-85, wherein the polyhalide comprises poly bromide.

87. The method of any one of claims 83-86, wherein the polyhalide comprises a heteropolyhalide .

88. The method of any one of claims 83-87, wherein the polyhalide comprises an isopolyhalide.

89. The method of any one of claims 83-88, wherein Y is a covalent bond and L is a covalent bond, whereby the compound has a structure:

90. An electrochemical device, comprising: an anode compartment containing an anolyte; a cathode compartment containing a halide catholyte and a mediator; and an ion-exchange membrane between the anode compartment and the cathode compartment, wherein the mediator has a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond;A is an anionic moiety; n is between 2 and 1000; and* indicates a terminal group.

91. A composition, comprising: a halide concentration of at least 0.1 M; and a compound having a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond;A is an anionic moiety; n is between 2 and 1000; and* indicates a terminal group.

92. A method, comprising: reacting a halide to produce a polyhalide in the presence of a mediator having a structure:wherein:Y is an amine, an amide, an ester, an ether, or a covalent bond;L is an organic linker or a covalent bond;A is an anionic moiety; n is between 2 and 1000; and * indicates a terminal group.

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