Pyridinium redox materials and methods

Abstract

The present disclosure generally relates to pyridinium redox materials for various uses, such as in redox flow batteries. The pyridine may be a bipyridine, such as a 4,4'-bipyridine, in aspects. In certain cases, such anolytes 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 a N-monosubstituted 4,4'-bipyridinium, e.g., acting as an anolyte. Other aspects are generally directed to flow batteries containing such proton-coupled pyridiniums, methods of making or using such proton-coupled pyridiniums, kits involving such proton-coupled pyridiniums, or the like.

Description

[0001] PYRIDINIUM REDOX MATERIALS AND METHODS

[0002] RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 634,815, filed April 16, 2024, entitled “Pyridinium Redox Materials and Methods,” by Sullivan, et al., incorporated herein by reference in its entirety.

[0004] FIELD

[0005] The present disclosure generally relates to pyridinium redox materials 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 knowhow, 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.

[0010] Aqueous organic redox-active materials have been researched extensively for the past decade. As an alternative to inorganic species, like vanadium, these systems utilize water-soluble organic species that undergo reversible redox reactions. Most of these molecules comprise aromatic conjugation systems that are able to stabilize electrons and holes. Viologens are one such class of anolytes based on 4,4 ’-bipyridine cores with substitutions at both 4-positions, resulting in di-cationic pyridinium salts in the fully oxidized state. Viologen derivatives are considered non-proton-coupled in their redox reactions and are stable in near-neutral pH ranges. For application in a wide pH range, structural innovation is needed for modified electrochemical properties.

[0011] SUMMARY

[0012] The present disclosure generally relates to pyridinium redox materials 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.

[0013] For instance, some aspects are generally directed to A-monosubstituted bipyridines, for example, for use as an anolyte in redox flow battery, or for other applications. In one set of embodiments, the bipyridine may be a 4,4’ -bipyridine derivative. Some embodiments are directed toward tripyridines, for example, with at with one N-substitution and at least one N- unsubstituted pyridyl. Other multipyridines are contemplated as well. The pyridines, is some embodiments, may have anionic, cationic, and or charge-neutral substitutions, for example, N- substitutions and / or C-substitutions. In some embodiments, when paired in an acidic pH electrolyte (e.g., in flow battery), such anolytes may serve various purposes. This includes, for example, proton or cation ion-exchange membrane conduction. In some embodiments, when paired in alkaline pH electrolyte (e.g., in flow battery), such anolytes may serve other purposes. This includes, for example, a more negative redox potential and a two-electron process. In addition, in certain embodiments, such anolytes may potentially improve system energy density and power density. One aspect is generally directed to a composition, comprising a redox-active pyridinium having at least one / ' / -substituted pyridyl nitrogen and at least one / ' / -unsubstituted pyridyl nitrogen.

[0014] Another aspect is generally drawn to an electrochemical device. In one set of embodiments, the device comprises an anode compartment containing an anolyte, the anolyte comprising a redox-active pyridinium having at least one / / -substituted pyridyl nitrogen and at least one / / -unsubstituted pyridyl nitrogen; a cathode or cathode compartment containing a catholyte; and a membrane between the anode compartment and the cathode compartment.

[0015] Yet another aspect is generally directed to a composition comprising any of the structures shown in Figs. 6-12.

[0016] Some aspects are drawn to an electrochemical device comprising any of the structures shown in Figs. 6-12. In some embodiments, the electrochemical device may include an anode compartment containing an anolyte, a cathode or cathode compartment containing a catholyte, and a membrane between the anode compartment and the cathode compartment. In some cases, the anolyte may comprise any of the structures shown in Figs. 6-12.

[0017] In another aspect, the present disclosure encompasses methods of making one or more of the embodiments described herein, for example, a N-monosubstituted 4,4 ’-bipyridine, or a structure such as is shown in Figs. 6-12, etc. In still another aspect, the present disclosure encompasses methods of using one or more of the embodiments described herein, for example, a N-monosubstituted 4,4’ -bipyridine, or a structure such as is shown in Figs. 6-12, etc.

[0018] 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.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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: Fig. 1 illustrates a viologen reaction, in one embodiment;

[0021] Fig. 2 illustrates a bipyridine reaction, in another embodiment;

[0022] Fig. 3 illustrates a bipyridine redox reaction, in yet another embodiment;

[0023] Figs. 4A-4B illustrate a non-limiting example of a battery charging and discharging, in accordance with yet another embodiment;

[0024] Fig. 5 is a schematic of a flow battery, in yet another embodiment;

[0025] Fig. 6 illustrates various pyridiniums with anionic N-substituents, in certain embodiments;

[0026] Fig. 7 illustrates various pyridiniums with cationic N-substituents, in some embodiments;

[0027] Fig. 8 illustrates various pyridiniums with charge-neutral N-substituents, in certain embodiments;

[0028] Fig. 9 illustrates various pyridiniums with C-substituents, in some embodiments;

[0029] Fig. 10 illustrates various bipyridines, in certain embodiments;

[0030] Fig. 11 illustrates various tripyridines, in some embodiments;

[0031] Fig. 12 illustrates various extended pyridiniums, in certain embodiments;

[0032] Fig. 13 illustrates synthesis of a 4,4 ’-bipyridine bearing a single N-propylsulfonate, in one embodiment;

[0033] Fig. 14 illustrates an NMR spectrum of the compound in Fig. 13;

[0034] Fig. 15 illustrates synthesis of a 4,4 ’-bipyridine bearing N, N-propylsulfonate functionalization, in another embodiment;

[0035] Fig. 16 illustrates an NMR spectrum of the compound in Fig. 15;

[0036] Fig. 17 illustrates CV of various pyridinium redox materials at pH 14, in one embodiment;

[0037] Fig. 18 illustrates CV of various pyridinium redox materials at pH 10, in another embodiment;

[0038] Fig. 19 illustrates CV of various pyridinium redox materials at pH 7, in yet another embodiment;

[0039] Fig. 20 illustrates CV of various pyridinium redox materials at pH 4, in still another embodiment;

[0040] Fig. 21 illustrates CV of various pyridinium redox materials at pH 1 , in yet another embodiment; Fig. 22 illustrates charge-discharge profiles of various pyridinium redox materials at pH 10, in one embodiment;

[0041] Fig. 23 illustrates charge-discharge profiles of various pyridinium redox materials at pH 7, in another embodiment;

[0042] Fig. 24 illustrates charge-discharge profiles of various pyridinium redox materials at pH 4, in still another embodiment.

[0043] DETAILED DESCRIPTION

[0044] The present disclosure generally relates to pyridinium redox materials for various uses, such as in redox flow batteries. The pyridine may be a bipyridine, such as a 4,4’ -bipyridine, in aspects. In certain cases, such anolytes 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 a N- monosubstituted 4,4’ -bipyridinium, e.g., acting as an anolyte. Other aspects are generally directed to flow batteries containing such proton-coupled pyridiniums, methods of making or using such proton-coupled pyridiniums, kits involving such proton-coupled pyridiniums, or the like.

[0045] In certain aspects, pyridinium redox materials 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. 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.

[0046] 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.

[0047] 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 ion-exchange membrane from the anode compartment to the cathode compartment, 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, 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. A non-limiting schematic diagram of a battery is shown in Fig. 4, as an illustrative nonlimiting example. Fig. 4A shows a battery 10 in discharging mode. In anode compartment 20, an anolyte having a charge state n (i.e., An) 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.

[0048] Fig. 4B 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 (An), while the catholyte (having a charge state Cm) in cathode compartment 30 releases electrons to produce a more positively charged state (Cm+1). Countercations M+also can flow from cathode compartment 30 to anode compartment 20 across ion exchange membrane 50.

[0049] 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. 5, 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.

[0050] 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 / polysulfide, Zn / ZnX2 (X=I, Br, Cl), Cr2+ / Cr3+, V2+ / V3+, V4+ / V5+), organic species (e.g., viologens, anthraquionones, phenazines, thiolate, TEMPOs), metal-organic species (e.g., ferro / ferricyanide, metal-bipyridines, metal- polyaminocarboxylates), and halogen species. TEMPO derivatives, such as any of those described in US Pat. Apl. Pub. No. 63 / 552,006, filed February 9, 2024 (incorporated herein by reference in its entirety), and other nitroxyl radical derivatives, may be used in certain embodiments. Additional non-limiting examples of suitable redox species include any of those described in US Pat. Apl. Pub. Nos. 2022 / 0363663 and 2023 / 0006250.

[0051] In one aspect, certain embodiments relate to redox-active pyridiniums with at least one pyridyl nitrogen with an N-substitution and at least one pyridyl nitrogen that is N-unsubstituted (i.e., N: or NH). The substituent may be any organic group, including any of those described herein. In some embodiments, the pyridinium may be an N-monosubstituted bipyridine. As used herein, hydrogen is not considered a “N-substitution” functionalization on a pyridyl atom. The binding strength of hydrogen is weaker than that of a typical covalent bond (e.g., a N-C bond). In some cases, the nature of the N-H bond may depend on the activity of the H (proton) in solution and the basicity of the pyridinyl nitrogen, both of which may be dynamic during electrochemical charge / discharge application.

[0052] N-monosubstituted bipyridines, as discussed herein, do not include viologens. Viologens are N,N’ -disubstituted 4,4’-biypridines, in which both pyridyl nitrogen atoms are functionalized, e.g., to form dicationic pyridiniums in the oxidized state. Viologens may undergo a two-step reduction process in which electrons are sequentially stored in the bipyridyl conjugation system without a directly proton-coupled process as shown in Fig. 1. In contrast, N-monosubstituted 4,4’ -bipyridines, also known as monosubstituted viologens, have only one functionalized pyridyl nitrogen atom, as the other is unsubstituted (i.e., N: or NH). Without wishing to be bound by any theory, it is believed that because a nucleophilic lone pair is left free at the other pyridyl nitrogen site, the redox process becomes partially proton-coupled, and the electrochemical properties may be significantly altered. See, e.g., Fig. 2. As used herein, the term “proton-coupled” refers to a redox process in which the mechanism and behavior, such as the formal potential, number of steps, electron transfer rates, electrochemical reversibility, etc., are directly altered by the activity of proton in solution. In particular, as used herein, the term “proton-coupled” does not strictly refer to a process that is an exact 1 to 1 ratio of proton to electron and does not exactly follow the Nernst equation potential shift with pH.

[0053] In some embodiments, certain pyridiniums such as those described herein can be deployed in alkaline pH solutions with enhanced chemical stability. It is believed that such pyridiniums may exhibit increased thermodynamic stability in alkaline solution with only one N- functionalization, decreasing the electrostatic attraction and increasing the LUMO energy. In some cases, pyridiniums such as those disclosed herein may be used for alkaline anolytes. Examples of pyridiniums include, but are not limited to, N-monosubstituted bipyridines, including any of those described below.

[0054] Without wishing to be bound by any theory, it is believed that N-monosubstituted bipyridines or other redox-active pyridiniums have the capability to undergo a one-step, two- electron redox process in alkaline media, for example, as shown in Fig. 3. The N- monosubstituted bipyridines are believed to have a pyridyl lone-pair that is able to bind proton. When the pyridines are reduced at the electrode in alkaline condition, the additional electron may occupy the relatively high-energy LUMO (compared to viologen), which may possess orbital overlap and electrostatic attraction with proton. Thus, a congruent electron plus proton may be accompanied, which may result in an overall two-electron, one-proton, and one-step redox process. With the formation of a hydrogen covalent bond or strong proton coordination at the unsubstituted pyridyl site, the reduced pyridine may become more thermodynamically stable in alkaline condition, which may spur electrochemical robustness for application longevity. During electrochemical oxidation, this bond may be reversed in some cases, as electrons are removed from the highest occupied molecular orbital (HOMO), and the molecule regains a cationic charge centered on the N-substituted pyridyl nitrogen atom. Thus, in accordance with certain embodiments, the N-monosubstituted bipyridine redox process may be reversible in alkaline media.

[0055] According to certain embodiments, certain pyridiniums such as those described herein may possess stable electrochemical reversibility in acidic media with alternate properties. Without wishing to be bound by any theory, certain pyridines having an unsubstituted pyridyl lone pair may have a basic pKa, meaning that they may become protonated to form a pyridinium cation in acidic pH. This N-pyridyl hydrogen “bond” is not permanent and may be dissociated as the proton activity changes, e.g., during electrochemical cycling. In some cases, the substituted pyridine may be effectively “pre-charged” with proton before the proton-coupled electron process. In some cases, a pyridine such as those described herein may possess a lower- energy LUMO (a more positive reduction potential).

[0056] In acidic solution, in certain embodiments, certain pyridines may undergo a two-step, two-electron process at a relatively more positive potential with altered electron transfer kinetics and chemical stability. One non-limiting example is shown in Fig. 3 with an N-monosubstituted 4,4’ -bipyridine. Acidic conditions may offer certain system advantages in electrochemical devices, such as increased ion conduction through proton-exchange membranes.

[0057] Although the redox process of certain pyridiniums such as those described herein may change with pH, the acidic and basic mechanisms should not be thought of as discrete states. For example, proton-coupled pyridiniums, including but not limited to N-monosubstituted bipyridines, may be deployed in pH-neutral media where one or a mixture of the “proton- coupled” and “pre-charged proton-coupled” electrochemical mechanisms may occur. Without wishing to be bound by any theory, in some embodiments, the redox mechanism of the pyridiniums may depend on the strength of the orbital and electrostatic interactions of proton with the unsubstituted pyridyl lone pair. Although often depicted as a covalent bond, this interaction is not a permanent bond and is instead may be a complex equilibrium between the proton, the pyridyl nitrogen, the solvent, and other species in solutions. In other words, the observed redox mechanics of a pyridinium may be dependent on the activity of proton. Thus, perturbations in electrochemical behavior may vary based on solution pH, species concentration, N-functionalization identity, counter-ions, supporting salt, solution additives, electrolyte state-of- charge, operating temperature, electrode surface, etc. Thus, in some embodiments certain pyridiniums, including but not limited to N-monosubstituted bipyridines, can be deployed from alkaline pH to acidic pH with unique and tunable properties.

[0058] Accordingly, certain aspects are generally directed to bipyridines, tripyridines, or other higher-order multipyridines. Such pyridines may be formed of two or more pyridinyl groups covalently bound together, e.g., in a linear fashion. For instance, the bipyridine may be formed of two pyridinyl groups covalently bound together. A tripyridine may be formed of three pyridinyl groups covalently bound together. Higher-order multipyridines may have 4, 5, 6, etc. pyridinyl groups. Such pyridines may have at least one N-substituted pyridyl and at least one N- unsubstituted pyridyl. For instance, a tripyridine may have 1 N-unsubstituted pyridyl and 2 N- substituted pyridyls, or 2 N-unsubstituted pyridyls and 1 N-substituted pyridyl. Thus, certain embodiments are generally directed to N-substitutions of one or more pyridyl nitrogen atoms in a pyridine. The substituent may be not H in some cases, and / or may be any organic group.

[0059] In one set of embodiments, the organic N-substituent comprises one or more negatively charged groups. Examples include, but are not limited to, sulfates (ROSO3 ), sulfonates (RSO3 ), phosphates (ROPO32), phosphate esters (ROP(OR’)O2-), phosphonates (RPO32), phosphonates esters (RP(OR’)O2-), and carboxylates (RCOO ). Examples of negatively charged organic substituents include but are not limited to nitrates (RONO2 ), carbamates (RO2CNH2 ), acetates (RCOO ), thiocyanates (RSCN ), isothiocyanates (RNCS ), perchlorates (ROCIO3 ), cyanates (ROCN ), isocyanates (RNCO ), and borates (RB(OR)2-). The backbone structures to which these substituents may be attached may encompass a wide range of organic frameworks. These include, without limitation, alkyl groups (straight-chain, branched, and cyclic), substituted alkyls (where one or more hydrogen atoms have been replaced by other atoms or groups), alkenes (straight-chain, branched, and cyclic), substituted alkenes (where one or more hydrogen atoms have been replaced by other atoms or groups), aryl groups (derived from aromatic rings), and substituted aryls (aryl groups further modified with additional functional groups, either to the aromatic ring itself or to substituents on the ring). Other examples include, without limitation, heterocyclic compounds (containing one or more heteroatoms such as nitrogen, oxygen, or sulfur), fused ring systems (where heterocyclic rings are joined to other heterocyclic or aromatic rings), and alkyl halides (where one or more halogen atoms are substituted for hydrogen).

[0060] In some embodiments in which the pyridinium has an anionic N-substituent, various properties may be enhanced. The anionic charge of the anionic N-substituent may, in certain cases, enhance the water solubility for application in aqueous solution, such as in aqueous flow battery. The anionic substituent may, in some embodiments, reduce crossover through cationexchange membranes. The anionic substituent may be, in certain embodiments, an inner salt as the pyridinium cation coordinates with the substituent anion or may have a counter-ion or mixture of counter-ions. The term “inner salt” refers to a molecule which has equal number of cation and anion that electrostatically pair together and result in a product without any additional counter-ions. Counter cations may include, without limitation, proton (H+), potassium (K+), sodium (Na+), lithium (Li+), ammonium (NHC), quaternary ammoniums (NRC), zinc (Zn2+), and iron (Fe2 / 3+), as well as other organic cations, metal cations, or metal-organic cations. Couter anions may include, without limitation, hydroxide (OH ), chloride (Cl ), bromide (Br ), iodide (T ), fluoride (F ), sulfate (SO4 ), phosphate (PO4 ), chlorate (CIO4 ), nitrate (NO2 ), borate (BO3 ), sulfide / polysulfide (e.g., S2 ), and acetate (CH3COO ), as well as other organic anions, metal anions, or metal-organic anions. Specific non-limiting examples of these are shown in Fig. 6.

[0061] In one set of embodiments, the organic N-substituent comprises one or more positively charged groups. Examples include, but are not limited to, quaternary ammoniums (RNR3+), imidazoliums, and pyridiniums. In some embodiments, the quaternary ammonium may be an alkyl or substituted alkyl ammonium. In other embodiments, the quaternary may be cyclic, including but not limited to a l,4-diazabicyclo[2.2.2]octane-l,4-diium group, a aziridinium group, an azetidinium group, a pyrrolidinium group, a piperidinium group, a morpholinium group, or a piperazinium group. The backbone structures to which these substituents may be attached may encompass a wide range of organic frameworks. These include, without limitation, alkyl groups (straight-chain, branched, and cyclic), substituted alkyls (where one or more hydrogen atoms have been replaced by other atoms or groups), alkenes (straight-chain, branched, and cyclic), substituted alkenes (where one or more hydrogen atoms have been replaced by other atoms or groups), aryl groups (derived from aromatic rings), and substituted aryls (aryl groups further modified with additional functional groups, either to the aromatic ring itself or to substituents on the ring). Other examples include, without limitation, heterocyclic compounds (containing one or more heteroatoms such as nitrogen, oxygen, or sulfur), fused ring systems (where heterocyclic rings are joined to other heterocyclic or aromatic rings), and alkyl halides (where one or more halogen atoms are substituted for hydrogen).

[0062] In some embodiments in which the pyridinium has a cationic N-substituent, various properties may be enhanced. The cationic charge may enhance the water solubility for application in aqueous solution, such as in aqueous flow battery. In some cases, the cationic substituent may reduce crossover through anion-exchange membranes. The pyridinium may have a counter-anion or mixture of counter- anions in certain embodiments. Couter anions may include, without limitation, hydroxide (OH ), chloride (Cl ), bromide (Br ), iodide (I ), fluoride (F ), sulfate (SO4 ), phosphate (PO4 ), chlorate (CIO4 ), nitrate (NO2 ), borate (BO3 ), sulfide / polysulfide (S2), and acetate (CH3COO ), as well as other organic anions, metal anions, or metal-organic anions. Specific non-limiting examples of these are shown in Fig. 7.

[0063] In one set of embodiments, the organic N-substituent comprises one or more chargeneutral groups. Examples include without limitation, alkyl groups (straight-chain, branched, and cyclic), substituted alkyls (where one or more hydrogen atoms have been replaced by other atoms or groups), alkenes (straight-chain, branched, and cyclic), substituted alkenes (where one or more hydrogen atoms have been replaced by other atoms or groups), aryl groups (derived from aromatic rings), and substituted aryls (aryl groups further modified with additional functional groups, either to the aromatic ring itself or to substituents on the ring). Other examples include, without limitation, heterocyclic compounds (containing one or more heteroatoms such as nitrogen, oxygen, or sulfur), fused ring systems (where heterocyclic rings are joined to other heterocyclic or aromatic rings), and alkyl halides (where one or more halogen atoms are substituted for hydrogen). In some embodiments, the substituent may comprise charge-neutral hydrophilic moieties, including but not limited to alcohols, ethers, polyethylene glycols, amines, thiols, cyanos, and acid esters (e.g., sulfates, phosphates, sulfonates, phosphates, carboxylates).

[0064] In some embodiments in which the pyridinium has a charge-neutral N-substituent, various properties may be enhanced. For example, the neutral charge may reduce the solution viscosity for flow applications, such as in aqueous flow battery. The charge-neutral substituent may reduce crossover through size-exclusion membranes in some embodiments. The N- monosubstituted bipyridines with a charge-neutral substituent may have a counter-anion or mixture of counter-anions in some cases. Couter anions may include, without limitation, hydroxide (OH ), chloride (Cl ), bromide (Br ), iodide (I ), fluoride (F ), sulfate (SO4 ), phosphate (PO4 ), chlorate (CIO4 ), nitrate (NO2 ), borate (BO3 ), sulfide / polysulfide (S2), and acetate (CH3COO ), as well as other organic anions, metal anions, or metal-organic anions. Specific non-limiting examples of these are shown in Fig. 8.

[0065] In some embodiments, the pyridinium may possess a C-substitution or combination of one or more C-substitutions, e.g., in addition to having one or more N-substituents. Examples of C-substitutions include, without limitation, alkyls (e.g., methyl), substituted alkyls (e.g., alkyl alcohols), aldehydes, amines, carboxylic / carboxylates, sulfates, sulfonates, phosphates, phosphonates, amides, halogens, and cyanos. In some embodiments, C-substitution may perturbate the N-monosubstituted bipyridine properties, including redox kinetics, formal potential, water solubility, solution viscosity, membrane crossover, chemical stability, and synthetic cost. Specific non-limiting examples of these are shown in Fig. 9.

[0066] In certain aspects, the pyridiniums may be bipyridines in which one of the pyridyl nitrogen atoms has one / ' / -substituted pyridyl nitrogen and one / ' / -unsubstituted pyridyl nitrogen. The bipyridine may be a N-monosubstituted 4,4 ’-bipyridine. Other redox-active N- monosubstituted bipyridines are contemplated in other embodiments, such as, without limitation, 2,4’ -bipyridines, 3, 4’ -bipyridines, 2,2’-bipyridines, 2,3 ’-bipyridines, and 3, 3 ’-bipyridines. For non-symmetric bipyridines, i.e., 3, 4’ -bipyridines, 2,4’-bipyridines, 2,3 ’-bipyridines, either pyridyl nitrogen atom may bear the N-substitution. See, e.g., Fig. 10. In certain embodiments, a mixture of bipyridines and / or other pyridiniums such as any of those described herein may be present. For example, in some cases, various bipyridine cores with different N-substitutions may be present. As a non-limiting example, N-monosubstituted 4,4’-bipyrindes with differing N-substitution may be used in tandem and a N-monosubstituted 4,4’-bipyrine may be used in tandem with a N-monosubstituted 3, 4 ’-bipyridine with identical or differing N-substitution.

[0067] As mentioned, in some cases, tripyridine or higher-order pyridiniums are possible in certain embodiments. For example, in some embodiments, the pyridinium may be a tripyridine in which a single molecular structure comprises three pyridine cores. The pyridyl cores may be covalently linked by a linker that retains the conjugation system between pyridyls. Examples include without limitation phenyls, diazines, triazines, and heteroaromatics, which may be substituted or unsubstituted. In some embodiments, the tripyridine may have one or more N- substituted pyridyl nitrogens and one or more A- un substituted pyridyl nitrogens. The tripyridine may have various anionic, cationic, and uncharged N-substituents or combinations thereof, e.g., as described herein. The pyridyl cores may also comprise various C-substituents in certain embodiments, e.g., as described herein. In some embodiments, tripyridines may exhibit three- electron storage, enhanced energy density, deterred membrane crossover, altered electrochemical and physical properties, or the like. See, e.g., Fig. 11.

[0068] In some embodiments, the pyridinium may be an extended bipyridine or other extended multipyridine comprising one A-substituted pyridyl nitrogen and one A-unsubstituted pyridyl nitrogen. The term “extended,” as used herein, refers to a structure in which a conjugated organic linker connects the two pyridyl cores, e.g., such that there are one or more atoms between the pyridine groups. Non-limiting examples of extended pyridine conjugated linkers include, without limitation, C=C, C=C, N=N, SC4H2 heteroaromatic rings, CeFE aromatic rings, S2N2C4 heteroaromatic rings, and C2N2O aromatic rings, etc. In certain cases, additional substitutions (e.g., C-substitutions and / or N-substitutions such as any of those described herein) may be included on these aromatic linkers. Without wishing to be bound by any theory, it is believed that with extended conjugated systems between the pyridyl cores, the properties of the pyridinium may be altered, such as the redox mechanism, formal potential, electron kinetics, chemical stability, water solubility, and membrane crossover, etc. Those of ordinary skill in the art will be familiar with techniques for synthesizing extended viologens and other similar structures, e.g., such as those described in Carrington, et al., Nature, 623:949-955 (2023); Luo, Angew. Chem. Int. Ed., 57(l):231-235 (2018); Zhang, et al., ACS Appl. Mater. Interfaces, 14(43):48727-48733 (2022); Moriguchi, et al., Crystal Structure Theory and Applications , 10(2):27, 2021; or Tang, et al., JACS Au, 2(5): 1214-1222 (2022).

[0069] U.S. Provisional Patent Application Serial No. 63 / 634,815, filed April 16, 2024, entitled “Pyridinium Redox Materials and Methods,” by Sullivan, et al., is incorporated herein by reference in its entirety.

[0070] The following examples are intended to illustrate certain embodiments of the present disclosure, but do not exemplify the full scope of the disclosure.

[0071] EXAMPLE 1

[0072] To showcase the altered electrochemical properties of the proton-coupled pyridiniums, a 4,4’ -bipyridine bearing only one N-substitution and a viologen bearing the same substitution were synthesized for characterization in this non-limiting example.

[0073] A 4,4’ -bipyridine bearing a single N-propylsulfonate functionalization was synthesized according to Fig. 13, as follows. In a 500 mL Schlenk flask, 4,4’ -bipyridine (15.6 g, 10 mmol) was dissolved in 100 mL of acetonitrile under a nitrogen atmosphere, and the mixture was heated to 70 °C. A solution of 1,3-propylsultone (12.2 g, 10 mmol) in 50 mL of acetonitrile was then added dropwise to the mixture. After stirring overnight, the reaction was cooled to room temperature. The resulting white precipitate was filtered, washed three times with acetonitrile and ethanol, and dried under vacuum overnight. 4’-pyridine-l-[3-sulfonatopropyl]-4-pyridinium was obtained with a yield of 98% (27.2 g). Fig. 14 shows 'H NMR (D2O, 400 MHz), 8 = 8.89 (d, 2H), 8.63 (d, 2H), 8.30 (d, 2H), 7.78 (d, 2H), 4.70 (t, 2 H), 2.89 (t, 2H), 2.36-2.41 (m, 2H).

[0074] A 4,4’ -bipyridine bearing a N, N-propylsulfonate functionalization was synthesized according to Fig. 15, as follows. In a 500 mL Schlenk flask, 1,3-propylsultone (26.8 g, 220 mmol) was dissolved in 150 mL of toluene under N2, and the resulting solution was heated to 110 °C. A solution of 4,4’ -bipyridine (15.6 g, 100 mmol) in 100 mL of toluene was added dropwise to the heated solution. After stirring overnight, the reaction was cooled to room temperature. The resulting white precipitate was collected by filtration, washed three times with acetonitrile, and dried under vacuum overnight. The yield was 95% (38.0 g). Fig. 16 shows ’H NMR (D2O, 400 MHz), 8 = 9.05 (d, 4H), 8.46 (d, 4H), 4.78 (t, 4H), 2.91 (t, 4H), 2.39-2.44 (m, 4H). EXAMPLE 2

[0075] In this example, cyclic vltammetry (CV) was used to explore the redox behavior of the pyridinium materials synethsized in Example 1. A 3-electrode setup was used with glassy carbon working electrode (GCE), platinum wire counter electrode (Pt), and Standard Colomel reference electrode (SCE, +0.241 vs. standard hydrogen electrode (SHE)). For each test, 0.050 M of redox active analyte was used in 1 M of salt. Tests were performed at various pH: pH 1 in 1 M HC1, pH 4 in HC1 diluted with 1 M NaCl, pH 7 in 1 M NaCl, pH 10 in NaOH diluted with IM NaCl, and pH 14 in 1 M NaOH. A scan rate of 50 mV / s was used for each CV with at least three total scans.

[0076] Fig. 17 shows the CV (V vs. SCE) of the pyridinium redox materials at pH 14. As expected, the N,N-substituted materials shows two separate electron peaks, one at a more positive potential around -0.55 V vs. SCE and a second near -E0 V vs. SCE. The N-substituted materials shows a sharper current peak at more negative potential of - El V vs. SCE. The sharper peak shape signifies a 2-electron, one-step process. Interestingly, the N, N-substituted pyrdinium shows a third redox peak near that of the N-substituted peak around -1.1 V vs. SCE. It is hypohtesized that this peak is indeed the N-substituted species that forms from the decay of the N, N-substituted species in strong base. Thus, this experiment supports that the N, N-substituted species is not electroehcmially stable in strong base, while the N-substituted species shows a 2- electron process at more negative potential.

[0077] Fig. 18 shows the CV (V vs. SCE) of the pyridinium redox materials at pH 10. As expected, the N, N-substituted materials shows two separate electron peaks, one at a more positive potential around -0.60V vs. SCE and a second near -1.1 V vs. SCE. The N-substituted materials shows a sharper current peak at more negative potential of -1.0 V vs. SCE. Interestingly, the N-substituted pyrdinium shows two oxidation peaks (or a shoulder oxidation peak), suggesting a 2-electron process. The peaks occuring at two disinguishable potentials suggests either a mixture of redox mechanisms for the same species in solution (e.g., “proton- coupled” vs. “pre-charged proton-coupled”) or that the redox process is partially proton-coupled. Furthermore, the N-substituted peaks are shifted towards more positive voltage, which is expected of a proton-coupled process - as the pH is lowered, the potential will become more positive accoring to the Nernest equation and Le Chatelier’s principle. Thus, this experiment supports that the N-substituted species shows a 2-electron process at more negative potential and that the redox process is partially proton-coupled.

[0078] Figs. 19 and 20 show the CVs (V vs. SCE) of the pyridinium redox materials at pH 7 and pH 4, respectively. Interestingly, the peak shapes and potentials remain fairly unchanged between pH 10 and 4 for both pyridinium species. However, it should be noted that different behavior may be observed for the N-substitutued pyridinium with varying conditions, such as N- functionalization, supporting salt, concentration, and temperature. These factors will alter the activity of proton in solution or the pKb of the unsubstituted N-position, which will alter the proton-coupled mechanism and equilibrium. The results here suggest, that in the given elecolyte conditions, the redox mechanism remains unchanged in near-neutral pH for these materials.

[0079] Fig. 21 shows the CV (V vs. SCE) of the pyridinium redox materials at pH 1. The N,N- substituted materials shows two separate electron peaks, one at a more positive potential around - 0.75 V vs. SCE and a second near -0.95 V vs. SCE. The N-substituted species now also shows two dinsguishable electron peaks, one near -0.75 V vs. SCE and one near -0.90 V vs. SCE. Once again, this shift towards more positive potential suggests a redox process that is partially proton- coupled accoring to the Nernst equation and Le Chatelier’s principle. In fact, the N-substittued first electron peak potential nearly matches that of the N, N-substitutued first-electron peak, supporting the “pre-charged” proton redox mechanism theory. This may be due to the fact that the expected pKa of the N-substituted species is above pH 1, meaning that it is likely that the unsubstitutued N position is protonated in this condition. Thus, this experiment supports that proton-coupled nature of the N-substituted material that exhibits signfiicantly altered redox behavior depending on pH and unique compared to that of viologen materials.

[0080] EXAMPLE 3

[0081] After characterizing the electrochemical behavior of the representative pyridinium redox materials in Examples 1 and 2, flow battery characterization was performed in this example. A custom flow battery single-cell device was used with a 4 cm2active area and peristaltic pumps inside a nitrogen glovebox (below 1 ppm O2). CMVN cation-exchange membrane was used as the separator. Graphite felts with a thickness of 3 mm (GFD 3 EA, SIGRACELL®) were pretreated 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 100 mA / cm2with a voltage cutoff. 5 mL of 0.1 M 4-propylsulfonate TEMPO was used as the excess catholyte for each test in anolyte-limiting configuration. 5 mL of 0.050 M propylsulfonate pyridinium materials was used as the anolyte. Tests were performed for both materials in three different electrolyte conditions: pH 10 with NaOH base and 1 M NaCl, pH 7 with 1 M NaCl, and pH 4 with HC1 and 1 M NaCl.

[0082] Fig. 22 shows a representative charge-discharge profile for the N-substitited and N,N- substituted species at pH 10. As expected from the CV results, the N-substituted species shows roughly a 300 mV voltage increase compared to that of the N, N-substituted species. Both the N- substituted and N, N-substituted materials demonstrated reversible and stable charge-discharge processes over 10 cycles as shown in Tables 1 and 2, respectively. Table 1. Flow battery cyling results of the N-substituted pyridinium at pH 10.

[0083] Table 2. Flow battery cyling results of the N, N-substituted pyridinium at pH 10.

[0084] Fig. 23 shows a representative charge-discharge profile for the N-substitited and N,N- substituted species at pH 7. As expected from the CV results, the N-substituted species shows roughly a 300 mV voltage increase compared to that of the N, N-substituted species. These reulsts are similar to the pH 10 cycling performance. Both the N-substituted and N, N-substituted materials demonstrated reversible and stable charge-discharge processes over 10 cycles as shown in Tables 3 and 4, respectively.

[0085] Table 3. Flow battery cyling results of the N-substituted pyridinium at pH 7.

[0086] Table 4. Flow battery cyling results of the N, N-substituted pyridinium at pH 7.

[0087]

[0088] Fig. 24 shows a representative charge-discharge profile for the N-substitited and N,N- substituted species at pH 4. As expected from the CV results, the N-substituted species shows roughly a 300 mV voltage increase compared to that of the N, N-substituted species. These reulsts are similar to the pH 10 and pH 7 cycling performance for the N-substitutued species. Interestingly, however, the N, N-substituted species demonstrats drastically detterred redox performance at this more acidic pH compared to the N-substituted species, demonstrating the poentially advantageous propoerties of the proton-coupled pyridinium redox process. Both the N-substituted and N, N-substituted materials demonstrated reversible charge-discharge processes over several cycles as shown in Tables 5 and 6, respectively.

[0089] Table 5. Flow battery cyling results of the N-substituted pyridinium at pH 4.

[0090] Table 6. Flow battery cyling results of the N,N-substituted pyridinium at pH 4.

[0091] Overall, these results showcase the reversible and stable flow battery cycling capabilities of the N-substituted pyridinium materials across a basic-to-acidic pH range with unique electrochemical performance compared to that of viologen. These materials may be used as pyridinium redox materials in varying electrolyte pH conditions.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.”

[0096] 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.

[0097] 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.”

[0098] 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.

[0099] 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.”

[0100] 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.

[0101] 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, the anolyte comprising a redoxactive pyridinium having at least one / ' / -substituted pyridyl nitrogen and at least one N- unsubstituted pyridyl nitrogen; a cathode or cathode compartment containing a catholyte; and a membrane between the anode compartment and the cathode compartment.

2. The electrochemical device of claim 1 , wherein the redox-active pyridinium is not a viologen.

3. The electrochemical device of any one of claims 1 or 2, wherein the redox-active pyridinium is a bipyridine.

4. The electrochemical device of claim 3, wherein the redox-active bipyridine is a 4,4’- bipyridine.

5. The electrochemical device of any one of claims 1-4, wherein one or more of the at least one / / -substituted pyridyl nitrogen is substituted with an anionic substituent.

6. The electrochemical device of claim 5, wherein the anionic substituent comprises sulfonate or sulfonic acid.

7. The electrochemical device of any one of claims 5 or 6, wherein the anionic substituent comprises sulfate or sulfuric acid.

8. The electrochemical device of any one of claims 5-7, wherein the anionic substituent comprises phosphonate or phosphonic acid.

9. The electrochemical device of any one of claims 5-8, wherein the anionic substituent comprises phosphate or phosphoric acid.

10. The electrochemical device of any one of claims 5-9, wherein the anionic substituent comprises carboxylate or carboxylic acid.

11. The electrochemical device of any one of claims 1-10, wherein one or more of the at least one / ' / -substituted pyridyl nitrogen is substituted with a cationic substituent.

12. The electrochemical device of claim 11, wherein the cationic substituent comprises a protonated ammonium.

13. The electrochemical device of any one of claims 11 or 12, wherein the cationic substituent comprises a quaternary ammonium.

14. The electrochemical device of any one of claims 11-13, wherein the cationic substituent comprises a cyclic or heteroatomic cyclic ammonium.

15. The electrochemical device of any one of claims 11-14, wherein the cationic substituent comprises a nitrogen-containing aromatic.

16. The electrochemical device of any one of claims 1-15, wherein one or more of the at least one ^-substituted pyridyl nitrogen is substituted with a charge-neutral substituent.

17. The electrochemical device of claim 16, wherein the charge-neutral substituent is an alkyl.

18. The electrochemical device of any one of claims 16 or 17, wherein the charge-neutral substituent is a branched alkyl.

19. The electrochemical device of any one of claims 16-18, wherein the charge-neutral substituent is a substituted alkyl.

20. The electrochemical device of any one of claims 16-19, wherein the charge-neutral substituent is a phenyl or a substituted phenyl.

21. The electrochemical device of any one of claims 16-20, wherein the charge- neutral substituent is a heteroaromatic.

22. The electrochemical device of any one of claims 16-21, wherein the charge- neutral substituent is an amide.

23. The electrochemical device of any one of claims 16-22, wherein the charge- neutral substituent is a nitrite.

24. The electrochemical device of any one of claims 16-23, wherein the charge- neutral substituent is sulfonic ester.

25. The electrochemical device of any one of claims 16-24, wherein the charge- neutral substituent is a sulfonic phenyl.

26. The electrochemical device of any one of claims 16-25, wherein the charge- neutral substituent is a phosphonic ester.

27. The electrochemical device of any one of claims 16-26, wherein the charge- neutral substituent is a phosphonic phenyl.

28. The electrochemical device of any one of claims 16-27, wherein the charge- neutral substituent is an alcohol.

29. The electrochemical device of any one of claims 16-28, wherein the charge- neutral substituent is a polyol.

30. The electrochemical device of any one of claims 16-29, wherein the charge- neutral substituent is an ether.

31. The electrochemical device of any one of claims 16-30, wherein the charge- neutral substituent is a polyether.

32. The electrochemical device of any one of claims 1-31, wherein each pyridinium is not C- substituted.

33. The electrochemical device of any one of claims 1-32, wherein the pyridinium comprises at least 2 substantially identical pyridines.

34. The electrochemical device of any one of claims 1-33, wherein at least one pyridine of the pyridinium is a C-substituted pyridine.

35. The electrochemical device of claim 34, wherein the C-substitution comprises an alkyl.

36. The electrochemical device of any one of claims 34 or 35, wherein the C-substitution comprises an alcohol.

37. The electrochemical device of any one of claims 34-36, wherein the C-substitution comprises an ether.

38. The electrochemical device of any one of claims 34-37, wherein the C-substitution comprises a carboxylate or carboxylic acid.

39. The electrochemical device of any one of claims 34-38, wherein the C-substitution comprises a sulfonate or sulfonic acid.

40. The electrochemical device of any one of claims 34-39, wherein the C-substitution comprises a phosphonate or phosphoric acid.

41. The electrochemical device of any one of claims 34-40, wherein the C-substitution comprises an amine.

42. The electrochemical device of any one of claims 34-41, wherein the C-substitution comprises an ester.

43. The electrochemical device of any one of claims 34-42, wherein the C-substitution comprises an amide.

44. The electrochemical device of any one of claims 34-43, wherein the C-substitution comprises an aldehyde.

45. The electrochemical device of any one of claims 34-44, wherein the C-substitution comprises a cyano.

46. The electrochemical device of any one of claims 1-45, wherein each pyridine of the pyridinium is independently a C-substituted pyridine.

47. The electrochemical device of any one of claims 1-46, wherein the pyridinium comprises pyridines connected by a conjugated organic linker.

48. The electrochemical device of claim 47, wherein the linker comprises a N=N.

49. The electrochemical device of any one of claims 47 or 48, wherein the linker comprises a C=C.

50. The electrochemical device of any one of claims 47-49, wherein the linker comprises a phenyl or substituted phenyl.

51. The electrochemical device of any one of claims 47-50, wherein the linker comprises a thiazolo.

52. The electrochemical device of any one of claims 47-51, wherein the linker comprises a chalcogenophene.

53. The electrochemical device of any one of claims 47-52, wherein the linker comprises an oxadiazole.

54. The electrochemical device of any one of claims 47-53, wherein the pyridinium is a tripyridinium.

55. The electrochemical device of claim 54, wherein the linker comprises a phenyl or substituted phenyl.

56. The electrochemical device of any one of claims 54 or 55, wherein the linker comprises a pyridine, diazine, triazine, pyrimidine, or pyrazine.

57. The electrochemical device of any one of claims 1-56, wherein the electrochemical device is a battery.

58. The electrochemical device of claim 57, wherein the battery is a flow battery.

59. The electrochemical device of any one of claims 57 or 58, wherein the battery is a hybrid flow battery.

60. The electrochemical device of claim 59, wherein the flow battery is an aqueous redox flow battery.

61. The electrochemical device of any one of claims 59 or 60, wherein the flow battery is a hybrid aqueous redox flow battery.

62. The electrochemical device of any one of claims 1-61, wherein the anolyte has a pH above 9 in a discharged state.

63. The electrochemical device of any one of claims 1-62, wherein the anolyte has a pH below 5 in a discharged state.

64. The electrochemical device of any one of claims 1-63, wherein the membrane is an ionexchange membrane.

65. The electrochemical device of any one of claims 1-64, wherein the membrane is a cationexchange membrane.

66. The electrochemical device of any one of claims 1-65, wherein the membrane is a protonexchange membrane.

67. The electrochemical device of any one of claims 1-66, wherein the membrane is an anion-exchange membrane.

68. The electrochemical device of any one of claims 1-67, wherein the membrane is a sizeexclusion membrane.

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