Aqueous redox flow batteries comprising matched ionomer membranes

Abstract

This invention is directed to aqueous redox flow batteries comprising ionically charged redox active materials and ionomer membranes, wherein the charge of the redox active materials is of the same sign as that of the ionomer, so as to confer specific improvements.

Claims

What is claimed: 1. A flow battery comprising: a first aqueous electrolyte comprising a first redox active material, the first redox active material being a first metal ligand coordination compound bearing a first ionic charge; a second aqueous electrolyte comprising a second redox active material, the second redox active material being a second metal ligand coordination compound bearing a second ionic charge; a first electrode in contact with said first aqueous electrolyte; a second electrode in contact with said second aqueous electrolyte; and a separator comprising an ionomer membrane disposed between said first and second aqueous electrolytes, the ionomer membrane comprising a polymer with ionized repeating units and electrically neutral repeating units; wherein the first redox active material and the second redox active material differ from one another, the first ionic charge has a sign that is the same in an oxidized form and a reduced form of the first redox active material, the second ionic charge has a sign that is the same in an oxidized form and a reduced form of the second redox active material, and the signs of the first ionic charge and the second ionic charge are individually the same as a charge sign of the ionized repeating units in the ionomer membrane; and wherein the flow battery operates or is capable of operating: (a) where the first or second redox active materials comprise 3% or less of the molar flux of ions passing through the ionomer membrane; or (b) with a round trip current efficiency that is at least about 95%; or (c) at a current density of at least 100 mA/cm 2 with a round trip voltage efficiency of at least about 90%; or (d) with the electrolytes having an energy density of at least about 30 Wh/L; or (e) with a combination of any two or more of (a), (b), (c), and (d). 2. The flow battery of claim 1 , wherein the ionomer membrane has a thickness of less than 100 μm. 3. The flow battery of claim 1 , wherein the ionomer membrane comprises a fluoropolymer. 4. The flow battery of claim 1 , wherein the ionized repeating units comprise covalently attached or embedded sulfonate, carboxylate, quaternary ammonium, sulfonium, phosphazenium, or guanidinium residues, or salts thereof. 5. The flow battery of claim 1 , wherein at least one of the first and second redox active materials exhibits substantially reversible electrochemical kinetics. 6. The flow battery of claim 1 , wherein at least one of the first electrode and the second electrode presents a surface of an allotrope of carbon to the first aqueous electrolyte or the second aqueous electrolyte. 7. The flow battery of claim 1 , wherein first electrode presents a surface of an allotrope of carbon to the first aqueous electrolyte and the second electrode presents a surface of an allotrope of carbon to the second aqueous electrolyte. 8. The flow battery of claim 1 , wherein the first and second aqueous electrolytes each exhibit a pH in a range of about 7 to about 13, about 8 to about 13, about 9 to about 13, about 10 to about 13, about 10 to about 12, or about 11. 9. The flow battery of claim 1 , wherein both the first and second ionic charges are negative in the oxidized and reduced forms of the first redox active material and the second redox active material. 10. The flow battery of claim 9 , wherein the first and second ionic charges range from −2 to −5 in the first redox active material and the second redox active material. 11. The flow battery of claim 1 , where both the first and second ionic charges are positive in the oxidized and reduced forms of the first redox active material and the second redox active material. 12. The flow battery of claim 11 , wherein the first and second ionic charges range from +2 to +5 in the first redox active material and the second redox active material. 13. The flow battery of claim 1 , wherein the flow battery exhibits a round trip current efficiency of at least 98% over a state-of-charge in a range from about 35% to about 65%. 14. The flow battery of claim 1 , further comprising: an external electrical circuit in electrical communication with the first and second electrodes, said external electrical circuit being capable of charging or discharging the flow battery. 15. The flow battery of claim 1 , wherein the first and second aqueous electrolytes have an energy density of at least 10 Wh/L, at least 20 Wh/L, or at least 30 Wh/L. 16. A method of operating the flow battery of claim 1 , said method comprising: charging said flow battery by an input of electrical energy or discharging said flow battery by a removal of electrical energy. 17. A method of operating the flow battery of claim 1 , said method comprising: applying a potential difference across the first and second electrodes, with an associated flow of electrons, so as to: (a) reduce the first redox active material while oxidizing the second redox active material; or (b) oxidize the first redox active material while reducing the second redox active material. 18. A system comprising: the flow battery of claim 1 and the system further comprising: (a) a first chamber containing the first aqueous electrolyte and a second chamber containing the second aqueous electrolyte; (b) at least one electrolyte circulation loop in fluidic communication with each chamber, said at least one electrolyte circulation loop comprising a storage tank and piping for containing and transporting the first and second aqueous electrolytes; (c) control hardware and software; and (d) an optional power conditioning unit. 19. The system of claim 18 , the system being adapted for connection to an electrical grid configured so as to provide renewables integration, peak load shifting, grid firming, baseload power generation/consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, frequency regulation, or a combination thereof. 20. The system of claim 18 , the system being configured to provide stable power for remote camps, forward operating bases, off-grid telecommunications, or remote sensors.
CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 61/739,140 filed Dec. 19, 2012, and is a continuation-in-part of U.S. patent application Ser. No. 13/948,497, filed Jul. 23, 2013, which is a continuation-in-part from U.S. patent application Ser. No. 13/795,878, filed Mar. 12, 2013, which itself claims priority to U.S. Application No. 61/739,145, filed Dec. 19, 2012, U.S. Application No. 61/738,546, filed Dec. 18, 2012, U.S. Application No. 61/683,260, filed Aug. 15, 2012, and U.S. Application No. 61/676,473, filed Jul. 27, 2012, each of which applications is incorporated herein by reference in its entirety for any and all purposes. TECHNICAL FIELD This disclosure relates to the field of energy storage systems, including electrochemical energy storage systems, batteries, and flow battery systems and methods of operating the same. BACKGROUND There exists a long-felt need for safe, inexpensive, easy-to-use, and reliable technologies for energy storage. Large scale energy storage enables diversification of energy supply and optimization of the energy grid. Existing renewable-energy systems (e.g., solar- and wind-based systems) enjoy increasing prominence as energy producers explore non-fossil fuel energy sources, however storage is required to ensure a high quality energy supply when sunlight is not available and when wind does not blow. Flow battery energy storage systems have been proposed for large-scale energy storage. But existing storage systems suffer from a variety of performance and cost limitations, including, for example, system scalability, round trip energy efficiencies (RT Eff ), cycle life, and other areas. Despite significant development effort, no flow battery technology has yet achieved widespread commercial adoption, owing to the materials and engineering hurdles that make system economics unfavorable. Accordingly, there is a need in the art for improved flow batteries. SUMMARY The present invention addresses these challenges. In certain embodiments, the present disclosure provides, in certain aspects, low-cost energy storage using low-cost battery active materials that are comprised of charged metal ligand coordination compounds and separated by an ionomer membrane within the electrochemical cell. The invention comprises the matching of the sign of the charge of the active materials with the sign of the ionomer membrane to enable high electrochemical cell performance. Typically, flow battery membranes are made conductive by the incorporation of a charged polymer or ionomer. For example, negatively charged ionomers are selected to transport positively charged ions between the electrodes of the cell (e.g., protons, sodium, and/or potassium ions). The prior art teaches design principles for ionomer membranes to yield high conductivity (e.g., through the fabrication of thin membranes), however such design principles often lead to high active material crossover with typical flow battery active materials. That is, existing conductive membranes do not exhibit optimal performance characteristics (e.g., high conductivity for ion transport with low active material crossover in compact designs) in such systems. The present disclosure describe cell design principles and operating cell embodiments that overcome these deficiencies. In particular, in the disclosure describes the sign of the charge of the metal-ligand coordination compound is chosen to match the sign of the ionomer membrane, so as to induce ionic repulsion between the membrane and active material and prevent active material crossover. Such configurations are shown here to yield highly selective ion transport between the negative and positive electrodes of the flow battery. Certain embodiments of the present invention provide flow batteries, each flow battery comprising: a first aqueous electrolyte comprising a first redox active material; a second aqueous electrolyte comprising a second redox active material; a first electrode in contact with said first aqueous electrolyte; a second electrode in contact with said second aqueous electrolyte and a separator comprising an ionomer membrane disposed between said first and second aqueous electrolytes; wherein the sign of the net ionic charge of the first, second, or both redox active materials matches that of the ionomer membrane; and wherein the flow battery operates or is capable of operating: (a) where the first or second redox active materials comprise 3% or less of the molar flux of ions passing through the ionomer membrane; or (b) with a round trip current efficiency that is at least about 95%; or (c) at a current density of at least about 100 mA/cm 2 with a round trip voltage efficiency of at least about 90%; or (d) with the electrolytes having an energy density of at least about 30 Wh/L; or (e) with a combination of any two or more of (a), (b), (c), and (d). Various additional features provide for the use of metal ligand coordination compound; the use of various membrane compositions and thicknesses; the use of electrodes presenting a surface of an allotrope of carbon to the respective electrolyte; the use of various pH ranges; matched membrane-electrolyte systems; flow batteries exhibiting specific round trip current efficiencies and energy densities of at least 98% over a state-of-charge in a range of from about 35 to about 65%. Additional embodiments provide for methods of operating the inventive flow batteries, each method comprising charging said battery by the input of electrical energy or discharging said battery by the removal of electrical energy. The specification also discloses methods of operating the inventive flow batteries, each method comprising applying a potential difference across the first and second electrode, with an associated flow of electrons, so as to: (a) reduce the first redox active material while oxidizing the second redox active material; or (b) oxidize the first redox active material while reducing the second redox active material. The teachings of the present disclosure also includes systems, each system comprising a flow battery of any one of the inventive flow batteries, and further comprising: (a) a first chamber containing the first aqueous electrolyte and a second chamber containing the second aqueous electrolyte; (b) at least one electrolyte circulation loop in fluidic communication each electrolyte chamber, said at least one electrolyte circulation loop comprising storage tanks and piping for containing and transporting the electrolytes; (c) control hardware and software; and (d) an optional power conditioning unit. Such systems may be connected to an electrical grid configured to provide renewables integration, peak load shifting, grid firming, baseload power generation/consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, frequency regulation, or a combination thereof. Such systems may also be configured to provide stable power for remote camps, forward operating bases, off-grid telecommunications, or remote sensors. BRIEF DESCRIPTION OF THE DRAWINGS The present application is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the subject matter, there are shown in the drawings exemplary embodiments of the subject matter; however, the presently disclosed subject matter is not limited to the specific methods, devices, and systems disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings: FIG. 1 depicts a schematic of an exemplary flow battery. FIG. 2 provides stability performance data obtained during 250 charge/discharge cycles for a 5 cm 2 system based on Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− , as described in Example 2. FIG. 3 provides a charge/discharge trace for a flow battery of the present invention as described in Example 2. This example contains Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− as first and second electrolytes, respectively. The battery was charged from 0% SOC to 60% SOC and then discharged to 40% SOC at a current density of 200 mA/cm 2 and a RT Voltage efficiency of ˜76%. FIG. 4 provides current efficiency data obtained for a system based on Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− , as described in Example 3. FIG. 5 provides voltage efficiency data, as a function of current density, for a system based on Ti 4+/3+ (cat) 2 (pyrogallate) 2−/3− and Fe 3+/2+ (CN) 6 3−/4− , as described in Example 4. FIG. 6 provides voltage efficiency data, as a function of current density, for a system based on Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− , as described in Example 4. FIG. 7 provides a charge/discharge trace for a flow battery of the present invention. This example contains Fe 3+/2+ (cat) 3 3−/4− and Fe 3+/2+ (CN) 6 3−/4− as first and second electrolytes, respectively. The battery was charged from 0% SOC to 60% SOC and then discharged to 40% SOC at a current density of 100 mA/cm 2 and a RT voltage efficiency of ca. 82%. FIG. 8 provides data for cell voltage during charge-discharge cycling for 1 M Fe(CN) 6 as positive couple and 1 M Ti(lactate) 2 (salicylate) as negative couple, both at pH 11, in a 5 cm 2 active area flow battery at a current density of 150 mA/cm 2 except for the area noted as 100 mA/cm 2 . FIG. 9 provides cell voltage in volts plotted versus test time in hours during charge-discharge cycling and iV traces between each cycle for 1 M Fe(CN) 6 as positive couple and 1 M Ti(lactate) 2 (α-hydroxyacetate) as negative couple, both at pH 11, in a 5 cm 2 active area flow battery at a current density of 150 mA/cm 2 . DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The present disclosure may be understood more readily by reference to the following description taken in connection with the accompanying Figures and Examples, all of which form a part of this disclosure. It is to be understood that this disclosure is not limited to the specific products, methods, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of any claimed disclosure. Similarly, unless specifically otherwise stated, any description as to a possible mechanism or mode of action or reason for improvement is meant to be illustrative only, and the invention herein is not to be constrained by the correctness or incorrectness of any such suggested mechanism or mode of action or reason for improvement. Throughout this text, it is recognized that the descriptions refer both to methods of operating a device and systems and to the devices and systems providing said methods. That is, where the disclosure describes and/or claims a method or methods for operating a flow battery, it is appreciated that these descriptions and/or claims also describe and/or claim the devices, equipment, or systems for accomplishing these methods. In the present disclosure the singular forms “a,” “an,” and “the” include the plural reference, and reference to a particular numerical value includes at least that particular value, unless the context clearly indicates otherwise. Thus, for example, a reference to “a material” is a reference to at least one of such materials and equivalents thereof known to those skilled in the art, and so forth. When a value is expressed as an approximation by use of the descriptor “about,” it will be understood that the particular value forms another embodiment. In general, use of the term “about” indicates approximations that can vary depending on the desired properties sought to be obtained by the disclosed subject matter and is to be interpreted in the specific context in which it is used, based on its function. The person skilled in the art will be able to interpret this as a matter of routine. In some cases, the number of significant figures used for a particular value may be one non-limiting method of determining the extent of the word “about.” In other cases, the gradations used in a series of values may be used to determine the intended range available to the term “about” for each value. Where present, all ranges are inclusive and combinable. That is, references to values stated in ranges include every value within that range. It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. That is, unless obviously incompatible or specifically excluded, each individual embodiment is deemed to be combinable with any other embodiment(s) and such a combination is considered to be another embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any sub-combination. Finally, while an embodiment may be described as part of a series of steps or part of a more general structure, each said step may also be considered an independent embodiment in itself. When a list is presented, unless stated otherwise, it is to be understood that each individual element of that list and every combination of that list is to be interpreted as a separate embodiment. For example, a list of embodiments presented as “A, B, or C” is to be interpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A or C,” “B or C,” or “A, B, or C.” Electrochemical energy storage systems typically operate through the interconversion of electrical and chemical energy. Various embodiments of electrochemical energy storage systems include batteries, capacitors, reversible fuel cells and the like, and the present invention may comprise any one or combination of these systems. Unlike typical battery technologies (e.g., Li-ion, Ni-metal hydride, lead-acid, etc.), where energy storage materials and membrane/current collector energy conversion elements are unitized in a single assembly, flow batteries transport (e.g., via pumping) redox active energy storage materials from storage tanks through an electrochemical stack, as in exemplary FIG. 1 , which is described elsewhere herein in further detail. This design feature decouples the electrical energy storage system power (kW) from the energy storage capacity (kWh), allowing for considerable design flexibility and cost optimization. In some embodiments, flow batteries according to the present disclosure may also be described in terms of a first chamber comprising a first or negative electrode contacting a first aqueous electrolyte; a second chamber comprising a second or positive electrode contacting a second aqueous electrolyte; and a separator disposed between the first and second electrolytes. The electrolyte chambers provide separate reservoirs within the cell, through which the first and/or second electrolyte flow so as to contact the respective electrodes and the separator. Each chamber and its associated electrode and electrolyte defines its corresponding half-cell. The separator provides several functions which include, e.g., (1) serving as a barrier to mixing of first and second electrolytes; (2) electronically insulating to reduce or prevent short circuits between the positive and negative electrodes; and (3) to provide for ion transport between the positive and negative electrolyte chambers, thereby balancing electron transport during charge and discharge cycles. The negative and positive electrodes provide a surface for electrochemical reactions during charge and discharge. During a charge or discharge cycle, electrolytes may be transported from separate storage tanks through the corresponding electrolyte chambers. In a charging cycle, electrical power is applied to the system wherein the active material contained in the second electrolyte undergoes a one-or-more electron oxidation and the active material in the first electrolyte undergoes a one-or-more electron reduction. Similarly, in a discharge cycle the second electrolyte is reduced and the first electrolyte is oxidized producing electrical power. Certain specific embodiments of the present invention include flow batteries, each flow battery comprising: a first aqueous electrolyte comprising a first ionically charged redox active material; a second aqueous electrolyte comprising a second ionically charged redox active material; a first electrode in contact with said first aqueous electrolyte; a second electrode in contact with said second aqueous electrolyte and a separator comprising an ionomer membrane disposed between said first and second aqueous electrolytes; wherein the sign of the net ionic charge of the first, second, or both redox active materials matches that of the ionomer membrane. In some of these embodiments, the flow battery operates or is capable of operating: (a) where the first or second redox active materials comprise 3% or less of the molar flux of ions passing through the ionomer membrane; or (b) with a round trip current efficiency that is at least about 95%; or (c) at a current density of at least about 100 mA/cm 2 with a round trip voltage efficiency of at least about 90%; or (d) with the electrolytes having an energy density of at least about 30 Wh/L; or (e) in some combination any two or more of (a), (b), (c), and (d). The flow batteries may further comprise an external electrical circuit in electrical communication with the first and second electrodes, said circuit capable of charging or discharging the flow battery. Reference to the sign of the net ionic charge of the first, second, or both redox active materials relates to the sign of the net ionic charge in both oxidized and reduced forms of the redox active materials under the conditions of the operating flow battery. Further exemplary embodiments provide that (a) the first ionically charged redox active material has an associated net positive or negative charge and is capable of providing an oxidized or reduced form over an electric potential in a range the negative operating potential of the system, such that the resulting oxidized or reduced form of the first redox active material has the same charge sign (positive or negative) as the first redox active material, the ionomer membrane also having a net ionic charge of the same sign; and (b) the second ionically charged redox active material has an associated net positive or negative charge and is capable of providing an oxidized or reduced form over an electric potential in a range of the positive operating potential of the system, such that the resulting oxidized or reduced form of the second redox active material has the same charge sign (positive or negative sign) as the second redox active material, the ionomer membrane also having a net ionic charge of the same sign; or both (a) and (b). These matching charges of the first and/or second electrolytes and the stationary phase of the membrane, provides a selectivity such that, in individual embodiments, about 3% or less, about 2% or less, about 1% or less, about 0.5% or less, about 0.2% or less, about 0.1% or less, about 0.01% or less, about 0.001% or less, or about 0.0001% or less of the molar flux of ions passing through the membrane is attributable to the first or second ionically charged redox active material (i.e., independent embodiments where the selectivities are up to about 1,000,000, with exemplary ranges having an independent lower value of about 50, 100, 250, 500, 1000, or about 10,000 and an upper value of about 1,000,000, or about 100,000, or about 10,000, or about 1000, or about 100). The term “molar flux of ions” refers to the amount of ions passing through the separator membrane, balancing the charge associated with the flow of external electricity/electrons. That is, the flow battery is capable of operating or operates with the substantial exclusion of the ionically charged redox active materials by the ionomer membrane. Independent embodiments of those flow batteries wherein the sign of the net ionic charge of the first, second, or both redox active materials matches that of the ionomer membrane include those where one or more of the following features are individually or collectively present: (i) where, during the operation of the flow battery, the first or second redox active materials comprise about 3% or less of the molar flux of ions passing through the ionomer membrane; (ii) where, the round trip current efficiency is at least about 70%, at least about 80%, at least about 90%, or at least about 95%; (iii) where the round trip current efficiency is at least about 95%; (iv) where the sign of the net ionic charge of the first, second, or both redox active materials is the same in both oxidized and reduced forms of the redox active materials and matches that of the ionomer membrane; (v) where the ionomer membrane has a thickness of about 100 microns or less, about 75 microns or less, about 50 microns or less, or about 25 microns or less. (vi) where the flow battery is capable of operating at a current density of at least about 100 mA/cm 2 with a round trip voltage efficiency of at least about 60%; of at least about 70%; of at least about 80%; or of at least about 90%; (vii) where the energy density of the electrolytes is at least about 10 Wh/L, at least about 20 Wh/L, or at least about 30 Wh/L. In certain further embodiments of these flow batteries, at least one of the first or second redox active material or both first and second redox active materials comprise a metal ligand coordination compound. The term “metal ligand coordination compound” is defined below. Where the first and second redox active materials comprise first and second metal ligand coordination compounds, respectively, the first metal ligand coordination compound may be the same or different than the second metal ligand coordination compound. Also as described below, one or both of the first and second redox materials may exhibit substantially reversible electrochemical kinetics. Facile electrochemical kinetics, and especially reversible electrochemical kinetics are important for decreasing energy wasting electrode overpotentials in both battery charge and discharge modes. In certain embodiments, these substantially reversible electrochemical kinetics are achievable or achieved using electrodes presenting a surface of an allotrope of carbon to the respective electrolyte. One or both of the electrodes may present a surface of an allotrope of carbon to the respective electrolyte. Unless otherwise specified, the term “substantially reversible electrochemical kinetics” refers to the condition wherein the voltage difference between the anodic and cathodic peaks is less than about 0.3 V, as measured by cyclic voltammetry, using an ex-situ apparatus using a flat glassy carbon disc electrode and recording at 100 mV/s. However, additional embodiments provide that the voltage difference between the anodic and cathodic peaks is less than about 0.2 V, less than about 0.1 V, less than about 0.075 V, or less than about 0.059 V, under these same testing conditions These aqueous electrolytes of these flow batteries may independently have a pH in a range of about 1 to about 13 pH units. In other independent embodiments, the pH of each of the first or second aqueous electrolytes or the pH of both the first and second aqueous electrolytes each exhibits a pH in a range of about 7 to about 13, about 8 to about 13, about 9 to about 13, about 10 to about 13, about 10 to about 12, or about 11. In other independent embodiments, the pH of the first aqueous electrolyte is within about 2 pH units, about 1 pH unit, or about 0.5 pH units of the pH of the second aqueous electrolyte. Additional embodied ranges for pH are provided below. In specific embodiments, both the first and second ionically charged redox active materials and their respective oxidized or reduced forms are negatively charged, and the ion selective membrane having a stationary phase that also has a net negative charge, so as to be selectively permeable to cations to the substantial exclusion of the negatively charged redox active materials. The first and second redox active materials and their respective oxidized or reduced forms may independently exhibit charges in a range of −2 to −5. The term “substantial exclusion” refers to the ability of the membrane to limit the molar flux of ions passing through the membrane attributable to the first or second ionically charged redox active material to about 3% or less of the total ion flux during the operation of the flow battery. In related independent embodiments, the flux of ions attributable to the first or second ionically charged redox active material is about 2% or less, about 1% or less, about 0.5% or less, about 0.2% or less, about 0.1% or less of the total ion flux during the operation of the flow battery. In other embodiments, both the first and second ionically charged redox active materials and their respective oxidized or reduced forms are positively charged, the ion selective membrane having a stationary phase that also has a net positive charge, so as to be selectively permeable to anions to the substantial exclusion of the positively charged redox active materials. The first and second redox active materials and their respective oxidized or reduced forms may independently exhibit charges in a range of +2 to +5 over the respective potential ranges. The term “substantial exclusion” is as described above. These flow batteries of the present invention include those capable of or actually providing excellent round trip current efficiencies. In certain embodiments, the flow batteries described above, when operating exhibit a round trip current efficiency of at least 98% over a state-of-charge in a range of from about 35 to about 65%. In other independent embodiments, the flow batteries exhibit round trip current efficiency of at least about 98.5, 99, 99.5, or 99.8% over a state-of-charge in a range of from about 35 to about 65%. In still other embodiments, these efficiencies are achieved over a state-of-charge in a range of from about 40 to about 60% or about 50%. The flow batteries of the present invention also provide superior open circuit potentials and energy densities. In certain independent embodiments, the flow batteries of the present invention exhibit open circuit potential of at least about 1.4 V, at least about 1.6 V, or at least about 2 V. In other independent embodiments, the flow batteries of the present invention are able to provide an energy density of at least 10 Wh/L, at least about 20 Wh/L, or at least about 30 Wh/L. To this point, the various embodiments have been described mainly in terms of individual flow batteries. It should be appreciated that, where possible, the descriptions should be read as including flow batteries that are operating or capable of operating with the specified characteristics. Similarly, the descriptions should be read as including systems of flow batteries, wherein the system comprises at least two of the flow batteries described herein. An exemplary flow battery is shown in FIG. 1 . As shown in that figure, a flow battery system may include an electrochemical cell that features a separator 20 (e.g., a membrane) that separates the two electrodes of the electrochemical cell. Electrode 10 is suitably a conductive material, such as a metal, carbon, graphite, and the like. Tank 50 may contain first redox material 30 , which material is capable of being cycled between an oxidized and reduced state. A pump 60 may affect transport of the first active material 30 from the tank 50 to the electrochemical cell. The flow battery also suitably includes a second tank (not labeled) that contains the second active material 40 . The second active material 40 may or may not be the same as active material 30 . A second pump (not labeled) may affect transport of second redox material 40 to the electrochemical cell. Pumps may also be used to affect transport of the active materials from the electrochemical cell to the tanks of the system. Other methods of effecting fluid transport—e.g., siphons—may be used to transport redox material into and out of the electrochemical cell. Also shown is a power source or load 70 , which completes the circuit of the electrochemical cell and allows the user to collect or store electricity during operation of the cell. It should be understood that FIG. 1 depicts a specific, non-limiting embodiment of a flow battery. Accordingly, devices according to the present disclosure may or may not include all of the aspects of the system depicted in FIG. 1 . As one example, a system according to the present disclosure may include active materials that are solid, liquid, or gas and/or solids, liquids, or gases dissolved in solution, or slurries. Active materials may be stored in a tank, in a vessel open to the atmosphere, or simply vented to the atmosphere. In some cases, a user may desire to provide higher charge or discharge voltages than available from a single battery. In such cases, and in certain embodiments, then, several batteries are connected in series such that the voltage of each cell is additive. An electrically conductive, but non-porous material (e.g., a bipolar plate) may be employed to connect adjacent battery cells in a bipolar stack, which allows for electron transport but prevents fluid or gas transport between adjacent cells. The positive electrode compartments and negative electrode compartments of individual cells are suitably fluidically connected via common positive and negative fluid manifolds in the stack. In this way, individual electrochemical cells can be stacked in series to yield a desired operational voltage. In additional embodiments, the cells, cell stacks, or batteries are incorporated into larger energy storage systems, suitably including piping and controls useful for operation of these large units. Piping, control, and other equipment suitable for such systems are known in the art, and include, for example, piping and pumps in fluid communication with the respective electrochemical reaction chambers for moving electrolytes into and out of the respective chambers and storage tanks for holding charged and discharged electrolytes. The energy storage and generation systems described by the present disclosure may also include electrolyte circulation loops, which loops may comprise one or more valves, one or more pumps, and optionally a pressure equalizing line. The energy storage and generation systems of this disclosure can also include an operation management system. The operation management system may be any suitable controller device, such as a computer or microprocessor, and may contain logic circuitry that sets operation of any of the various valves, pumps, circulation loops, and the like. In some embodiments, a flow battery system may comprise a flow battery (including a cell or cell stack); storage tanks and piping for containing and transporting the electrolytes; control hardware and software (which may include safety systems); and an optional power conditioning unit. The flow battery cell stack accomplishes the conversion of charging and discharging cycles and determines the peak power of energy storage system, which power may in some embodiments be in the kW range. The storage tanks contain the positive and negative active materials; the tank volume determines the quantity of energy stored in the system, which may be measured in kWh. The control software, hardware, and optional safety systems suitably include sensors, mitigation equipment and other electronic/hardware controls and safeguards to ensure safe, autonomous, and efficient operation of the flow battery energy storage system. Such systems are known to those of ordinary skill in the art. A power conditioning unit may be used at the front end of the energy storage system to convert incoming and outgoing power to a voltage and current that is optimal for the energy storage system or the application. For an example of an energy storage system connected to an electrical grid, in a charging cycle the power conditioning unit would convert incoming AC electricity into DC electricity at an appropriate voltage and current for the electrochemical stack. In a discharging cycle, the stack produces DC electrical power and the power conditioning unit converts to AC electrical power at the appropriate voltage and frequency for grid applications. The energy storage systems of the present disclosure are, in some embodiments, suited to sustained charge or discharge cycles of several hour durations. As such, the systems of the present disclosure may be used to smooth energy supply/demand profiles and provide a mechanism for stabilizing intermittent power generation assets (e.g., from renewable energy sources). It should be appreciated, then, that various embodiments of the present disclosure include those electrical energy storage applications where such long charge or discharge durations are valuable. For example, non-limiting examples of such applications include those where systems of the present disclosure are connected to an electrical grid include, so as to allow renewables integration, peak load shifting, grid firming, baseload power generation consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, and/or frequency regulation. Cells, stacks, or systems according to the present disclosure may be used to provide stable power for applications that are not connected to a grid, or a micro-grid, for example as power sources for remote camps, forward operating bases, off-grid telecommunications, or remote sensors. Flow battery energy storage efficacy is determined by both the round trip DC-DC energy efficiency (RT EFF ) and the energy density of the active materials (measured in Wh/L). The RT EFF is a composite of voltage and current efficiencies for both the battery charge and discharge cycles. In electrochemical devices, voltage and current efficiencies are functions of the current density, and while voltage and current efficiency typically decrease as current density (mA/cm 2 ) increases, high current densities are often desirable to reduce electrochemical stack size/cost required to achieve a given power rating. Active material energy density is directly proportional to the cell OCV (OCV=open circuit voltage), the concentration of active species, and the number of electrons transferred per mole of active species. High energy densities are desirable to reduce the volume of active materials required for a given quantity of stored energy. It should be appreciated that, while the various embodiments described herein are described in terms of flow battery systems, the same strategies and design/chemical embodiments may also be employed with stationary (non-flow) electrochemical cells, batteries, or systems, including those where one or both half cells employ stationary electrolytes. Each of these embodiments is considered within the scope of the present invention. TERMS Throughout this specification, words are to be afforded their normal meaning, as would be understood by those skilled in the relevant art. However, so as to avoid misunderstanding, the meanings of certain terms will be specifically defined or clarified. The term “active material” is well known to those skilled in the art of electrochemistry and electrochemical energy storage and is meant to refer to materials which undergo a change in oxidation state during operation of the system. Active materials may comprise a solid, liquid, or gas and/or solids, liquids, or gasses dissolved in solution. In certain embodiments, active materials comprise molecules and/or supramolecules dissolved in solution. Active materials with a composition of matter described by this invention may be used in energy storage systems in such a way that they are paired with other active materials to form a positive couple and a negative couple wherein said other active materials are described by the present invention or are previously known in the art or a combination thereof, inclusive of soluble, semi-solid, intercalation, capacitive or pseudo-capacitive, and plating-type active materials. The concentration of the molecules may be at least 2 M (for example to about 3 M or 4 M), between 1 and 2 M, about 1.5 M, between 0.5 M and 1M, or less than about 0.5 M. In certain embodiments, the active material may comprise a “metal ligand coordination compound,” which are known to those skilled in the art of electrochemistry and inorganic chemistry. A metal ligand coordination compound may comprise a metal ion bonded to an atom or molecule. The bonded atom or molecule is referred to as a “ligand”. In certain non-limiting embodiments, the ligand may comprise a molecule comprising C, H, N, and/or O atoms. In other words, the ligand may comprise an organic molecule. The metal ligand coordination compounds of the present disclosure are understood to comprise at least one ligand that is not water, hydroxide, or a halide (F − , Cl − , Br − , I − ). Metal ligand coordination compounds may comprise a “redox active metal ion” and/or a “redox inert metal ion”. The term “redox active metal ion” is intended to connote that the metal undergoes a change in oxidation state under the conditions of use. As used herein, the term “redox inert” metal ion is intended to connote that the metal does not undergo a change in oxidation state under the conditions of use. Metal ions may comprise non-zero valence salts of, e.g., Al, Ca, Co, Cr, Sr, Cu, Fe, Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, Zn, Zr, V, or a combination thereof. The skilled artisan would be able to recognize the circumstances where a given non-zero valence metal would be redox active or inactive under the prescribed electrolyte environments. In specific embodiments, the first, second, or both first and second redox active material comprise a metal ligand coordination complex having a formula comprising M(L1) x (L2) y (L3) z m , M is Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Sn, Ti, W, Zn, or Zr; L1, L2, and L3 are each independently ascorbate, a catecholate, citrate, a glycolate or polyol (including ligands derived from ethylene glycol, propylene glycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g., α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, a phthalate, a pyrogallate, sarcosinate, salicylate, or lactate; x, y, and z are independently 0, 1, 2, or 3, and 1≦x+y+z≦3; and m is +1, 0, −1, −2, −3, −4, or −5. Related and independent embodiments provide that (a) x=3, y=z=0; (b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0; or (f) x=1, y=z=0. In individual preferred embodiments, M is Al, Cr, Fe, or Ti and x+y+z=3. In other specific embodiments, the first, second, or both first and second redox active material comprise a hexacyanide metal ligand coordination complex, for example comprising chromium, iron, manganese, molybdenum, or ruthenium, preferably a chromium, iron, or manganese hexacyanide, such as ferricyanide or ferrocyanide. In other embodiments, the active material may comprise an “organic active material”. An organic active material may comprise a molecule or supramolecule that does not contain a transition metal ion. It is further understood that organic active materials are meant to comprise molecules or supramolecules that are dissolved in aqueous solution. And organic active material is capable of undergoing a change in oxidation state during operation of the electrochemical energy storage system. In this case, the molecule or supramolecule may accept or donate an electron during operation of the system. Unless otherwise specified, the term “aqueous” refers to a solvent system comprising at least about 98% by weight of water, relative to total weight of the solvent. In some applications, soluble, miscible, or partially miscible (emulsified with surfactants or otherwise) co-solvents may also be usefully present which, for example, extend the range of water's liquidity (e.g., alcohols/glycols). When specified, additional independent embodiments include those where the “aqueous” solvent system comprises at least about 55%, at least about 60 wt %, at least about 70 wt %, at least about 75 wt %, at least about 80%, at least about 85 wt %, at least about 90 wt %, at least about 95 wt %, or at least about 98 wt % water, relative to the total solvent. It some situations, the aqueous solvent may consist essentially of water, and be substantially free or entirely free of co-solvents or other species. The solvent system may be at least about 90 wt %, at least about 95 wt %, or at least about 98 wt % water, and, in some embodiments, be free of co-solvents or other species. In addition to the redox active materials described below, the aqueous electrolytes may contain additional buffering agents, supporting electrolytes, viscosity modifiers, wetting agents, and the like. The term “bipolar plate” refers to an electrically conductive, substantially nonporous material that may serve to separate electrochemical cells in a cell stack such that the cells are connected in series and the cell voltage is additive across the cell stack. The bipolar plate has two surfaces such that one surface of the bipolar plate serves as a substrate for the positive electrode in one cell and the negative electrode in an adjacent cell. The bipolar plate typically comprises carbon and carbon containing composite materials. The term “cell potential” is readily understood by those skilled in the art of electrochemistry and is defined to be the voltage of the electrochemical cell during operation. The cell potential may be further defined by Equation 1: Cell Potential=OCV−η pos −η neg −iR   (1) where OCV is the “open circuit potential”, η pos and η neg are the overpotentials for the positive and negative electrodes at a given current density, respectively, and iR is the voltage loss associated with all cell resistances combined. The “open circuit potential” or OCV may be readily understood according to Equation 2: OCV= E + −E −   (2) where E + and E − are the “half-cell potentials” for the redox reactions taking place at the positive and negative electrodes, respectively. The half-cell potentials may be further described by the well-known Nernst Equation 3: E=E°−RT/nF ln( X red /X ox )  (3) wherein E° is the standard reduction potential for redox couple of interest (e.g., either the positive or negative electrode), the R is the universal gas constant, T is temperature, n is the number of electrons transferred in the redox couple of interest, F is Faraday's constant, and X red /X ox is the ratio of reduced to oxidized species at the electrode. The OCV of a battery system may be measured by using standard techniques when the current flow between the first and second electrode is equal to zero. In this condition the voltage difference between the first and second electrodes corresponds to the OCV. The OCV of a battery system depends on the state of charge (SOC) of said system. Without being bound to the correctness of any theory, the OCV of an ideal battery will change with state of charge according to the Nernst equation (equation 4 above). For simplicity in this application all OCVs will be referenced to their values at 50% SOC. Those of ordinary skill in the art will recognize that at higher SOCs the OCV of a battery will increase, and at lower SOCs the OCV will decrease from the value at 50% SOC. The term “charge” refers to the “net charge” or total charge associated with an active material or ionomer moiety. The term “current density” refers to the total current passed in an electrochemical cell divided by the geometric area of the electrodes of the cell and is commonly reported in units of mA/cm 2 . In certain embodiments of the present invention, current densities are in a range of from about 50 mA/cm 2 , from about 100 mA/cm 2 or from about 200 mA/cm 2 , to about 200 mA/cm 2 , to about 300 mA/cm 2 , to about 400 mA/cm 2 , or to about 500 mA/cm 2 . The term “current efficiency” (I EFF ) may be described as the ratio of the total charge produced upon discharge of the system to the total charge passed upon charge. In some embodiments, the charge produced on discharge or passed on charge can be measured using standard electrochemical coulomb counting techniques well known to those of ordinary skill in the art. Without being bound by the limits of any theory, the current efficiency may be a function of the state of charge of the flow battery. In some non-limiting embodiments the current efficiency can be evaluated over an SOC range of about 35% to about 60%. The term “energy density” refers to the amount of energy that may be stored, per unit volume, in the active materials. Energy density, as used herein, refers to the theoretical energy density of energy storage and may be calculated by Equation 4: Energy density=(26.8 A-h/mol)×OCV×[ e − ]  (4) where OCV is the open circuit potential at 50% state of charge, as defined above, (26.8 A-h/mol) is Faraday's constant, and [e − ] is the concentration of electrons stored in the active material at 99% state of charge. In the case that the active materials largely comprise an atomic or molecular species for both the positive and negative electrolyte, [e − ] may be calculated as: [ e − ]=[active materials]× n/ 2  (5) where [active materials] is the concentration (mol/L or M) of the active material in either the negative or positive electrolyte, whichever is lower, and n is the number of electrons transferred per molecule of active material. The related term “charge density” refers to the total amount of charge that each electrolyte may contain. For a given electrolyte: Charge density=(26.8 A-h/mol)×[active material]× n   (6) where [active material] and n are as defined above. The term “energy efficiency” may be described as the ratio of the total energy produced upon discharge of the system to the total energy consumed upon charge. The energy efficiency (RT EFF ) may be computed by Equation 7: RT EFF =V EFF,RT ×I EFF   (7) As used herein, the term “evolution current” describes the portion of the electrical current applied in an energized flow battery configuration which is associated with the evolution (generation) of a particular chemical species. In the current context, then, when a sufficient overpotential vide infra) is applied in a flow battery such that either or both oxygen evolves at the positive electrode or hydrogen evolves at the negative electrode, that portion of the current associated with the evolution of oxygen or hydrogen is the oxygen evolution current or hydrogen evolution current, respectively. In certain preferred embodiments, there is no current associated with hydrogen evolution, oxygen evolution, or both hydrogen and oxygen evolution. This may occur when the positive half-cell is operating at a potential less than the thermodynamic threshold potential or the threshold overpotential of the positive electrode (i.e., no oxygen produced; see explanation of terms below) or the negative half-cell cell is operating at a potential more positive than the thermodynamic threshold potential or the threshold overpotential of the negative electrode (i.e., no hydrogen produced), or both. In separate embodiments, the batteries operates within 0.3 V, within 0.25 V, within 0.2 V, within 0.15 V, or within 0.1 V of either the thermodynamic threshold potential or the threshold overpotential of the respective positive or negative electrodes. In embodiments wherein gas is evolved, the portion of current associated with gas evolution (either hydrogen or oxygen or both) is suitably about 20% or less, about 15% or less, about 10% or less, about 5% or less, about 2% or less, or about 1% or less of the total applied current. Lower gas evolution currents are considered particularly suitable for battery (cell or cell stack) efficiencies. The term “excluding” refers to the ability of a separator to not allow certain ions or molecules to flow through the separator and typically is measured as a percent. The term “mobile ion” is understood by those skilled in the art of electrochemistry and is meant to comprise the ion which is transferred between the negative and positive electrode during operation of the electrochemical energy storage system. The term “mobile ion” may also refer to as an ion that carries at least about 80% of the ionic current during charger/discharge. As used herein, the terms “negative electrode” and “positive electrode” are electrodes defined with respect to one another, such that the negative electrode operates or is designed or intended to operate at a potential more negative than the positive electrode (and vice versa), independent of the actual potentials at which they operate, in both charging and discharging cycles. The negative electrode may or may not actually operate or be designed or intended to operate at a negative potential relative to the reversible hydrogen electrode. The negative electrode is associated with the first aqueous electrolyte and the positive electrode is associated with the second electrolyte, as described herein. The term “overpotential” is well understood by those skilled in the art of electrochemistry and is defined by the difference in voltage between an electrode during operation of an electrochemical cell and the normal half-cell potential of that electrode, as defined by the Nernst equation. Without being bound by theory, the term overpotential is meant to describe the energy, in excess of that required by thermodynamics, to carry out a reaction at a given rate or current density. The term “overpotential” also describes a potential more positive than the thermodynamic onset voltage for oxygen evolution from water at the positive electrode and more negative than the thermodynamic onset voltage for hydrogen evolution from water at the negative electrode. Similarly, as used herein, the term “threshold overpotential” refers to the overpotential at which either hydrogen or oxygen gas begins to evolve at the respective electrode. Note that an electrochemical system comprising “imperfect” (i.e., less than ideal catalytically) electrodes can be operated in three regions: (a) at a potential “below” the thermodynamic onset potential (i.e., more positive than the thermodynamic onset potential of the negative electrode and more negative than the thermodynamic onset potential of the positive electrode; no gas evolving so no gas evolution current); (b) at a potential between the thermodynamic threshold potential and threshold overpotential (no gas evolving and still no evolution current); and (c) beyond the threshold overpotential (gas evolving and exhibiting a gas evolution current). Such threshold overpotentials can be identified by those skilled in the art for a given system, for example, by measuring gas evolution as a function of applied half-cell potential (using e.g., a mass spectrometer), in the presence or absence of an electroactive material. See also below. The gas evolution threshold potentials are also affected by the nature of the electrolytes. Certain chemicals are known to inhibit the evolution of hydrogen and oxygen in electrolytic cells, either because of some activity in the bulk electrolyte or because of their ability to coat or otherwise deactivate their respective electrodes; for example, macromolecules or oligomers or salts, such as chloride or phosphate, on Pt surfaces. Accordingly, in certain embodiments, then, either the first or second or both first and second electrolytes comprise at least one compound increases the hydrogen or oxygen threshold overpotential of the system, respectively. As used herein, the terms “regenerative fuel cell” or “reversible fuel cell” or “flow battery” or “flow energy device” connote the same or similar type of device, which utilizes the same battery configuration (including cell or cell stack) for both energy storage and energy generation. The term “reversible hydrogen electrode,” or RHE, is used in its conventional meaning. That is, a reversible hydrogen electrode (RHE) is a reference electrode. The potential of the RHE, E(RHE) corresponds to the potential for Equation 8: 2 + +2 e − H 2   (8) When the reaction of Equation 8 is carried out at equilibrium at a given pH and 1 atm H 2 . This potential can be reference to a normal hydrogen electrode, E(NHE), by the following relation: E (RHE)= E (NHE)−0.059×pH=0.0 V−0.059×pH  (9) where E(NHE) is the potential for the normal hydrogen electrode (NHE=0.0 V), defined as the potential for the reaction of Equation 8 at standard state (1M H + , 1 atm H 2 ). Thus a potential of 0 V vs. RHE corresponds to a voltage of 0 V vs. NHE at pH 0 and −0.413 V vs. NHE at pH 7. The term “selectivity” is well known to those of ordinary skill in the art of electrochemistry and refers to the ability of a membrane to allow a ratio of the movement of mobile ions to active materials through a membrane. For example, a membrane that allows a 50:1 ratio of mobile ions to active materials to pass through would have a selectivity of 50. The terms “separator” and “membrane” refer to an ionically conductive, electrically insulating material disposed between the positive and negative electrode of an electrochemical cell. The polymer electrolytes useful in the present disclosure may be anion or cation conducting electrolytes. Where described as an “ionomer,” the term refers to a polymer comprising both electrically neutral and a fraction of ionized repeating units, wherein the ionized units are pendant and covalently bonded to the polymer backbone. The fraction of ionized units may range from about 1 mole percent to about 90 mole percent, but may be further categorized according to their ionized unit content. For example, in certain cases, the content of ionized units are less than about 15 mole percent; in other cases, the ionic content is higher, typically greater than about 80 mole percent. In still other cases, the ionic content is defined by an intermediate range, for example in a range of about 15 to about 80 mole percent. Ionized ionomer units may comprise anionic functional groups comprising carboxylates, sulfonates, phosphonates, salts of a carboxy acid, sulfonic acid, phosphonic acid, and the like. These functional groups can be charge balanced by, mono-, di-, or higher-valent cations, such as alkali or alkaline earth metals. Ionomers may also include polymer compositions containing attached or embedded quaternary ammonium, sulfonium, phosphazenium, and guanidinium residues or salts. The polymers useful in the present disclosure may comprise highly fluorinated or perfluorinated polymer backbones. Certain polymer electrolytes useful in the present disclosure include copolymers of tetrafluoroethylene and one or more fluorinated, acid-functional co-monomers, which are commercially available as NAFION™ perfluorinated polymer electrolytes from E. I. du Pont de Nemours and Company, Wilmington Del. Other useful perfluorinated electrolytes comprise copolymers of tetrafluoroethylene (TFE) and FSO 2 —CF 2 CF 2 CF 2 CF 2 —O—CF═CF 2 . The term “stack” or “cell stack” or “electrochemical cell stack” refers to a collection of individual electrochemical cells that are in electrically connected. The cells may be electrically connected in series or in parallel. The cells may or may not be fluidly connected. The term “state of charge” (SOC) is well understood by those skilled in the art of electrochemistry, energy storage, and batteries. The SOC is determined from the concentration ratio of reduced to oxidized species at an electrode (X red /X ox ). For example, in the case of an individual half-cell, when X red =X ox such that X red /X ox =1, the half-cell is at 50% SOC, and the half-cell potential equals the standard Nernstian value, E°. When the concentration ratio at the electrode surface corresponds to X red /X ox =0.25 or X red /X ox =0.75, the half-cell is at 25% and 75% SOC respectively. The SOC for a full cell depends on the SOCs of the individual half-cells and in certain embodiments the SOC is the same for both positive and negative electrodes. Measurement of the cell potential for a battery at OCV, and using Equations 2 and 3 the ratio of X red /X ox at each electrode can be determined, and therefore the SOC for the battery system. The term “supporting electrolyte” is well-known in the arts of electrochemistry and energy storage, and is intended to refer to any species which is redox inactive in the window of electric potential of interest and aids in supporting charge and ionic conductivity. In the present case, a supporting electrolyte does not substantially compromise the solubility of the coordination compound or complex. Non-limiting examples include salts comprising an alkali metal, ammonium ion including an ammonium ion partially or wholly substituted by alkyl or aryl groups, halide (e.g., Cl − , Br − , I − ), chalcogenide, phosphate, hydrogen phosphate, phosphonate, nitrate, sulfate, nitrite, sulfite, perchlorate, tetrafluoroborate, hexafluorophosphate, or a mixture thereof, and others known in the art. The term “voltage efficiency” may be described as the ratio of the observed electrode potential, at a given current density, to the half-cell potential for that electrode (×100%), wherein the half-cell potential is calculated as described above. Voltage efficiencies can be described for a battery charging step, a discharging step, or a “round trip voltage efficiency”. The round trip voltage efficiency (V EFF,RT ) at a given current density can be calculated from the cell voltage at discharge (V Discharge ) and the voltage at charge (V charge ) using Equation 10: V EFF,RT =V Discharge /V Charge ×100%  (10) Exemplary Operating Characteristics The present disclosure provides a variety of technical features of the disclosed systems and methods. It should be understood that any one of these features may be combined with any one or more other features. For example, a user might operate a system featuring an electrolyte that includes an organic active material (e.g., a quinone), wherein that electrode has a pH of about 3. Such a system might also feature a membrane separator having a thickness of about 35 microns. It should be further understood that the present disclosure is not limited to any particular combination or combinations of the following features. Certain embodiments of the present invention provides method of operating a flow battery, each method comprising charging said battery by the input of electrical energy or discharging said battery by the removal of electrical energy. Further embodiments provide applying a potential difference across the first and second electrode, with an associated flow of electrons, so as to: (a) reduce the first redox active material while oxidizing the second redox active material; or (b) oxidize the first redox active material while reducing the second redox active material. Complementary methods provide those where each method comprises applying a potential difference across the first and second electrode, with an associated flow of electrons, so as to: (a) oxidize the first redox active metal-ligand coordination compound; or (b) reduce the second redox active metal-ligand coordination compound; or (c) both (a) and (b). In traditional flow battery operation, mobile ions comprise proton, hydronium, or hydroxide. In various embodiments of the present disclosure, one may transport ions other than proton, hydronium, or hydroxide (e.g., when these ions are present in comparatively low concentration, such as below 1M). Separate embodiments of these methods of operating a flow battery include those wherein the mobile ion does not consist essentially of protons, hydronium, or hydroxide. In these embodiments, less than 50% of the mobile ions comprise protons, hydronium, or hydroxide. In other embodiments, less than about 40%, less than about 30%, less than about 20%, less than about 10%, less than about 5%, or less than about 2% of the mobile ions comprise protons, hydronium, or hydroxide. Exemplary mobile ions in these embodiments include alkali metal or alkaline earth metal cations (especially Li + , Na + , K + , Mg 2+ , Ca 2+ or Sr 2+ ). In some embodiments of the present disclosure, it is advantageous to operate between pH 1 and 13 (e.g. to enable active material solubility and/or low system cost). In this case one or both electrolytes is characterized as having a pH of between about 1 and about 13, or between about 2 and about 12, or between about 4 and about 10, or even between about 6 and about 8. In some embodiments, the pH of the electrolyte may be maintained by a buffer. Typical buffers include salts of phosphate, borate, carbonate, silicate, trisaminomethane (Tris), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), and combinations thereof. A user may add an acid (e.g., HCl, HNO 3 , H 2 SO 4 and the like), a base (NaOH, KOH, and the like), or both to adjust the pH of a given electrolyte as desired. In some embodiments, the pH of the first and second electrolytes are equal or substantially similar; in other embodiments, the pH of the two electrolytes differ by a value in the range of about 0.1 to about 2 pH units, about 1 to about 10 pH units, about 5 to about 12 pH units, about 1 to about 5 pH units, about 0.1 to about 1.5 pH units, about 0.1 to about 1 pH units, or about 0.1 to about 0.5 pH. In this context, the term “substantially similar,” without further qualification, is intended to connote that the difference in pH between the two electrolytes is about 1 pH unit or less. Additional optional embodiments provide that the pH difference is about 0.4 or less, about 0.3 or less, about 0.2 or less, or about 0.1 or less pH units. The disclosed systems and methods may also comprise active materials and membrane ionomers that are charged. As described above, the term “charge” refers to the “net charge” or total charge associated with an active material or ionomer moiety. The charged species may be anionic or cationic. In certain desired embodiments of the present disclosure it is advantageous for the active materials and membrane ionomers to comprise charges of the same sign (e.g. to prevent transfer of the active material across the membrane). Systems and methods according to the present disclosure also feature active materials comprising metal-ligand coordination compounds. Metal-ligand coordination compounds may be present at, e.g., a concentration of at least about 0.25 M, at least about 0.35 M, at least about 0.5 M, at least about 0.75 M, at least about 1 M, at least about 1.25 M, at least about 1.5 M, at least about 2 M, or 2 M (for example to about 3 M, to about 4 M, or about 5 M). The metal-ligand coordination compound may be further characterized with respect to the nature of the oxidizable or reducible species. For example, in some cases, the redox potential of the metal-ligand coordination compound may be defined by transitions entirely within the metal center—i.e., the redox potential is defined by the accessibility of energies associated with transitions between various valence states within the metal. In other cases, the oxidation/reduction may be localized within the ligand system. In still other cases, the oxidation/reduction may be distributed throughout the entire redox active complex, such that both the metal and the ligand system sharing in the distribution of charge. In particular embodiments of the present disclosure, the metal-ligand coordination compound may comprise ligands which are mono-, bi-, tri-, or multidentate. Monodentate ligands bind to metals through one atom, whereas bi-, tri-, or multidentate ligands bind to metals through 2, 3, or more atoms, respectively. Examples of monodentate ligands include halogens (F − , Cl − , Br − , I − ), cyanide (CN − ), carbonyl or carbon monoxide (CO), nitride (N 3− ), oxo (O 2− ), hydroxo (OH − ), sulfide (S 2− ), pyridine, pyrazine, and the like. Other types of ligand bonding moieties include amino groups (NR 3 ), amido groups (NR 2 ), imido groups (NR), alkoxy groups (R—CO − ), siloxy (R—SiO − ), thiolate (R—S − ), and the like, which may comprise mono-, bi-, tri-, or multidentate ligands. Examples of bidentate ligands include catechol, bipyridine, bipyrazine, ethylenediamine, diols (including ethylene glycol), and the like. Examples of tridentate ligands include terpyridine, diethylenetriamine, triazacyclononane, trisaminomethane, and the like. The disclosed systems and methods may feature electrochemical cell separators and/or ionomer membranes that have certain characteristics. In this disclosure, the terms membrane and separator are used interchangeably. The membranes of the present disclosure may, in some embodiments, feature a membrane separator having a thickness of about 500 microns or less, about 300 microns or less, about 250 microns or less, about 200 microns or less, about 100 microns or less, about 75 microns or less, about 50 microns or less, about 30 microns or less, about 25 microns or less, about 20 microns or less, or about 15 microns of less, or about 10 microns or less, for example to about 5 microns. Separators are generally categorized as either solid or porous. Solid membranes typically comprise an ion-exchange membrane, wherein an ionomer facilitates mobile ion transport through the body of the polymer. The facility with which ions conduct through the membrane can be characterized by a resistance, typically an area resistance in units of ohm-cm 2 . The area resistance is a function of inherent membrane conductivity and the membrane thickness. Thin membranes are desirable to reduce inefficiencies incurred by ion conduction and therefore can serve to increase voltage efficiency of the energy storage device. Active material crossover rates are also a function of membrane thickness, and typically decrease with increasing membrane thickness. Crossover represents a current efficiency loss that must be balanced with the voltage efficiency gains by utilizing a thin membrane. Porous membranes are non-conductive membranes which allow charge transfer between two electrodes via open channels filled with conductive electrolyte. Porous membranes are permeable to liquid or gaseous chemicals. This permeability increases the probability of chemicals passing through porous membrane from one electrode to another causing cross-contamination and/or reduction in cell energy efficiency. The degree of this cross-contamination depends on, among other features, the size (the effective diameter and channel length), and character (hydrophobicity/hydrophilicity) of the pores, the nature of the electrolyte, and the degree of wetting between the pores and the electrolyte. Such ion-exchange separators may also comprise membranes, which are sometimes referred to as polymer electrolyte membranes (PEMs) or ion conductive membranes (ICMs). The membranes according to the present disclosure may comprise any suitable polymer, typically an ion exchange resin, for example comprising a polymeric anion or cation exchange membrane, or combination thereof. The mobile phase of such a membrane may comprise, and/or is responsible for the primary or preferential transport (during operation of the battery) of at least one mono-, di-, tri-, or higher valent cation and/or mono-, di-, tri-, or higher valent anion, other than protons or hydroxide ions. Additionally, substantially non-fluorinated membranes that are modified with sulfonic acid groups (or cation exchanged sulfonate groups) may also be used. Such membranes include those with substantially aromatic backbones, e.g., poly-styrene, polyphenylene, bi-phenyl sulfone (BPSH), or thermoplastics such as polyetherketones or polyethersulfones. Examples of ion-exchange membranes comprise NAFION™ perfluorinated polymer electrolytes. Battery-separator style porous membranes, may also be used. Because they contain no inherent ionic conduction capability, such membranes are typically impregnated with additives in order to function. These membranes are typically comprised of a mixture of a polymer, and inorganic filler, and open porosity. Suitable polymers include those chemically compatible with the electrolytes of the presently described systems, including high density polyethylene, polypropylene, polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE). Suitable inorganic fillers include silicon carbide matrix material, titanium dioxide, silicon dioxide, zinc phosphide, and ceria and the structures may be supported internally with a substantially non-ionomeric structure, including mesh structures such as are known for this purpose in the art. The open circuit potential (OCV) of an electrochemical cell is a relevant operating characteristic of electrochemical energy storage systems. In certain embodiments, the OCV may be comparatively large (e.g. at least 1 V, and upwards to about 2 V, about 3 V, or about 4 V). Such comparatively large open circuit potentials are known to enable high cell voltage efficiencies, high AC-AC conversion efficiencies, high energy storage densities, and low system costs. Traditional flow batteries with aqueous electrolytes and soluble active materials may operate with an OCV less than about 1.2 V. An electrochemical cell according to the present disclosure is suitably characterized by an open circuit potential of at least about 1.4 V. In some embodiments, the open circuit voltage (OCV) of the flow battery is at least about 1.2 volts, at least about 1.3 V, at least about 1.4 V, at least about 1.5 V, at least about 1.6 V, at least about 1.7 V, at least about 1.8 V, at least about 1.9 V, or at least about 2 V. As described above, higher open circuit voltages are associated with higher power densities. Systems and methods according to the present disclosure may exhibit a particular current density at a given round trip voltage efficiency. Methods for determining current density at a given round trip voltage efficiency are known to those skilled in the art of electrochemistry and electrochemical energy storage. To serve as a metric for electrochemical cell performance, a specified current density must be linked to a measured voltage efficiency. Higher current densities for a given round trip voltage efficiency enable lower cost electrochemical cells and cell stacks. In certain embodiments, it is desired to operate a flow battery with a current density at least about 50 mA/cm 2 at V EFF,RT at least about 50%. In other embodiments, the current density will be at least about 50 mA/cm 2 at V EFF,RT at least about 60%, at least about 75%, at least about 85%, at least about 90%. In other embodiments, the current density will be at least 100 mA/cm 2 at V EFF,RT at least about 50%, at least about 60%, at least about 75%, at least about 85%, at least about 90% and the like. In other embodiments, the current density will be at least 200 mA/cm 2 at V EFF,RT at least about 50%, at least about 60%, at least about 75%, at least about 85%, at least about 90%, and above. In certain embodiments, these efficiencies may be achieved when the current density is in a range of having a lower limit of about 50 mA/cm 2 , from about 100 mA/cm 2 or from about 200 mA/cm 2 and having an upper limit of about 200 mA/cm 2 , to about 300 mA/cm 2 , to about 400 mA/cm 2 , or to about 500 mA/cm 2 . Electrolytes that include an organic active material, either in the absence or presence of metal coordination, are considered suitable for one or both half-cells of the disclosed systems and methods. Suitable organic active materials include carbon, aromatic hydrocarbons, including quinones, hydroquinones, viologens, pyridinium, pyridine, acridinium, catechol, other polycyclic aromatic hydrocarbons, and the like. Suitable organic active materials may also include sulfur, including thiol, sulfide, and disulfide moieties. Suitable organic active materials may be soluble in water in concentrations at least about 0.1 M, at least about 0.5 M, at least about 1 M, at least about 1.5 M, at least about 2 M, and above, for example to about 3 M, about 4 M, or about 5 M. The disclosed systems and methods may also be characterized in terms of their half-cell potentials. Both the negative and positive electrode may exhibit a half-cell potential. An electrochemical cell according to the present disclosure may, in some embodiments, have a half-cell potential for the negative electrode less than about 0.5 V vs. RHE, less than about 0.2 V vs. RHE, less than about 0.1 V vs. RHE, less than about 0.0 V vs. RHE, less than about −0.1 V vs. RHE, less than about −0.2 V vs. RHE, less than about −0.3 V vs. RHE, less than about −0.5 V vs. RHE, for example, to about −2 V vs. RHE. An electrochemical cell according to the present disclosure may, in some embodiments, have a half-cell potential for the positive electrode at least about 0.5 V vs. RHE, at least about 0.7 V vs. RHE, at least about 0.85 V vs. RHE, at least about 1.0 V vs. RHE, at least about 1.1 V vs. RHE, at least about 1.2 V vs. RHE, at least about 1.3 V vs. RHE, at least about 1.4 V vs. RHE and the like, for example, to 2 V vs. RHE. The disclosed systems and methods may also be characterized in terms of their energy density, as defined above. Flow batteries of the present disclosure may operate with an energy density of about 5 Wh/L, between about 5 Wh/L and about 15 Wh/L, between about 10 Wh/L and about 20 Wh/L, between about 20 Wh/L and about 30 Wh/L, between about 30 and about 40 Wh/L, between about 25 Wh/L and about 45 Wh/L, and above 45 Wh/L. Separate embodiments provide upper energy densities of about 70 Wh/L, about 60 Wh/L, or about 50 Wh/L. Among the many specific embodiments considered within the scope of the present invention are these: Embodiment 1 A flow battery comprising: a first aqueous electrolyte comprising a first redox active material; a second aqueous electrolyte comprising a second redox active material; a first electrode in contact with said first aqueous electrolyte; a second electrode in contact with said second aqueous electrolyte and a separator comprising an ionomer membrane disposed between said first and second aqueous electrolytes; wherein the sign of the net ionic charge of the first, second, or both redox active materials matches that of the ionomer membrane; and wherein the flow battery operates or is capable of operating: (a) where the first or second redox active materials comprise 3% or less of the molar flux of ions passing through the ionomer membrane; or (b) with a round trip current efficiency that is at least 95%; or (c) at a current density of at least about 100 mA/cm 2 with a round trip voltage efficiency of at least about 90%; or (d) with the electrolytes having an energy density of at least about 30 Wh/L; or (e) with a combination of any two or more of (a), (b), (c), and (d). Embodiment 2 The flow battery of Embodiment 1, wherein the sign of the net ionic charge of the first, second, or both redox active materials is the same in both oxidized and reduced forms of the redox active materials and matches that of the ionomer membrane. Embodiment 3 The flow battery of Embodiment 1 or 2, wherein the ionomer membrane has a thickness of 100 micron or less. Embodiment 4 The flow battery of any of the preceding Embodiments, where at least one of the first or second redox active material or both first and second redox active materials comprise a metal ligand coordination compound. Embodiment 5 The flow battery of any one of the preceding Embodiments, wherein the first and second redox active materials comprise first and second metal ligand coordination compounds, respectively, the first metal ligand coordination compound being different than the second metal ligand coordination compound. Embodiment 6 The flow battery of any one of the preceding Embodiments, the ion selective membrane comprising a fluoropolymer. Embodiment 7 The flow battery of any one of the preceding Embodiments, the ion selective membrane comprising an ionomer having covalently attached or embedded sulfonate, carboxylate, quaternary ammonium, sulfonium, phosphazenium, and guanidinium residues or salts thereof. Embodiment 8 The flow battery of any one of the preceding Embodiments, where either one or both of the first and second redox materials exhibit substantially reversible electrochemical kinetics. Embodiment 9 The flow battery of any one of the preceding Embodiments, wherein at least one of the electrodes presents a surface of an allotrope of carbon to the respective electrolyte. Embodiment 10 The flow battery of any one of the preceding Embodiments, wherein both of the electrodes presents a surface of an allotrope of carbon to the respective electrolyte. Embodiment 11 The flow battery of any one of the preceding Embodiments, where the pH of each of the first or second aqueous electrolytes or the pH of both the first and second aqueous electrolytes each exhibits a pH in a range of about 7 to about 13, about 8 to about 13, about 9 to about 13, about 10 to about 13, about 10 to about 12, or about 11. Embodiment 12 The flow battery of any one of the preceding Embodiments, where both the first and second ionically charged redox active materials and their respective oxidized or reduced forms are negatively charged, and the ion selective membrane having a stationary phase that also has a net negative charge, so as to be selectively permeable to cations to the substantial exclusion of the negatively charged redox active materials. Embodiment 13 The flow battery of Embodiment 12, wherein the first and second redox active materials and their respective oxidized or reduced forms independently exhibit charges in a range of −2 to −5. Embodiment 14 The flow battery of any one of Embodiments 1 to 12, where both the first and second ionically charged redox active materials and their respective oxidized or reduced forms are positively charged, the ion selective membrane having a stationary phase that also has a net positive charge, so as to be selectively permeable to anions to the substantial exclusion of the positively charged redox active materials. Embodiment 15 The flow battery of Embodiment 14, wherein the first and second redox active materials and their respective oxidized or reduced forms independently exhibit charges in a range of +2 to +5 over the respective potential ranges. Embodiment 16 The flow battery of any one of the preceding Embodiments, which when operating exhibits a round trip current efficiency of at least 98% over a state-of-charge in a range of from about 35 to about 65%. Embodiment 17 The flow battery of any one of the preceding Embodiments, further comprising an external electrical circuit in electrical communication with the first and second electrodes, said circuit capable of charging or discharging the flow battery. Embodiment 18 The flow battery of any one of the preceding Embodiments, the electrochemical cell capable of providing an energy density of at 10 Wh/L, at least 20 Wh/L, or at least 30 Wh/L. Embodiment 19 A method of operating a flow battery of any one of the preceding Embodiments, said method comprising charging said battery by the input of electrical energy or discharging said battery by the removal of electrical energy. Embodiment 20 A method of operating a flow battery of any one of Embodiments 1 to 18, said method comprising applying a potential difference across the first and second electrode, with an associated flow of electrons, so as to: (a) reduce the first redox active material while oxidizing the second redox active material; or (b) oxidize the first redox active material while reducing the second redox active material. Embodiment 21 A system comprising a flow battery of any one of Embodiments 1 to 18, and further comprising: (a) a first chamber containing the first aqueous electrolyte and a second chamber containing the second aqueous electrolyte; (b) at least one electrolyte circulation loop in fluidic communication each electrolyte chamber, said at least one electrolyte circulation loop comprising storage tanks and piping for containing and transporting the electrolytes; (c) control hardware and software; and (d) an optional power conditioning unit. Embodiment 22 The system of claim 21 , the system connected to an electrical grid configured to provide renewables integration, peak load shifting, grid firming, baseload power generation/consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, frequency regulation, or a combination thereof. Embodiment 23 The system of claim 21 , the system configured to provide stable power for remote camps, forward operating bases, off-grid telecommunications, or remote sensors. As those skilled in the art will appreciate, numerous modifications and variations of the present invention are possible in light of these teachings, and all such are contemplated hereby. For example, in addition to the embodiments described herein, the present invention contemplates and claims those inventions resulting from the combination of features of the invention cited herein and those of the cited prior art references which complement the features of the present invention. Similarly, it will be appreciated that any described material, feature, or article may be used in combination with any other material, feature, or article, and such combinations are considered within the scope of this invention. The disclosures of each patent, patent application, and publication cited or described in this document are hereby incorporated herein by reference, each in its entirety. EXAMPLES The following Examples are provided to illustrate some of the concepts described within this disclosure. While each Example is considered to provide specific individual embodiments of composition, methods of preparation and use, none of the Examples should be considered to limit the more general embodiments described herein. Example 1 Example 1.1—Materials Sodium hexacyanoferrate(II) decahydrate 99%, Na 4 Fe(CN) 6 .10H 2 O; potassium hexacyanoferrate(II) trihydrate 98+%, K 4 Fe(CN) 6 .3H 2 O; potassium hexacyanoferrate(III) ACS 99.0% min; K 3 Fe(CN) 6 ; ethylene glycol, propylene glycol, glycerol, lactic acid (80-85% aqueous solution); glycine, glycolic acid (67% aqueous solution); maleic acid; malic acid; phthalic acid; salicylic acid; gluconic acid; citric acid; sarcosine; iron (III) sulfate; iron (III) chloride; titanium oxysulfate; manganese (II) sulfate; and chromium (III) sulfate were purchased from Alfa Aesar (Ward Hill, Mass.) as ACS grade or better unless specified above and were used without additional purification. Ammonium bislactatobishydroxytitanium (IV) was purchased from Sigma Aldrich (St. Louis, Mo.) as a 50% aq. solution and was used without further purification. Potassium hexacyanochromate(III), K 3 [Cr(CN) 6 ] and potassium hexacyanomanganate(III), K 3 [Mn(CN) 6 ] were purchased from Sigma-Aldrich (St. Louis, Mo.) and used without additional purification. Complexes could be synthesized by several methods. Homoleptic tris-ligated complexes were most easily synthesized by stirring a 3:1 aqueous mixture of ligand and metal salt while slowly adding an alkali metal hydroxide solution until the pH was between 8 and 13, the typical window of stability for the complexes of interest. Certain mixed ligand species, for example Ti(lactate) 2 (salicylate), could also be synthesized by this method. Mono and bis α-hydroxy acid complexes of iron and titanium were synthesized by the portion-wise addition of 2 equivalents of sodium bicarbonate to stirred solutions of the metal sulfates (2-3 M) and the appropriate proportion of the appropriate ligand. For example, 6 mmol of TiOSO 4 and 6 mmol of glycolic acid were stirred, and 12 mmol of NaHCO 3 was added slowly, allowing gas evolution to subside between additions. The pH of the resulting solutions was about 3.5 for the solutions of ML 1 and about 2 for the solutions of ML 2 . The solubility of these complexes relative to aquated metals is evidenced by the stability with respect to precipitation of metal oxides of TiL 1 and TiL 2 solutions at such high pHs. In a control experiment where no ligand was added, wholesale and irreversible precipitation of TiO 2 was observed when more than 1 equivalent of NaHCO 3 was added, corresponding to a pH of about 1. Complexes with additional ligands could be synthesized by adding an appropriate amount of ML 1 or ML 2 solution synthesized as described in the previous paragraph to a solution of the desired additional ligand mixed with a suitable base, such as potassium carbonate or potassium hydroxide. Mixed ligand analogs of the Mn, Cr, Ti, and Fe compounds may be prepared by similar reaction schemes. Titanium bis-lactate L′ complexes could also be synthesized using (NH 4 ) 2 Ti(lactate) 2 (OH) 2 (available from Sigma Aldrich as a 50% solution) as a synthon. In this case, L′ (e.g., salicylic acid) was added, and after about an hour of stirring, an aqueous solution of 2 eq. alkali metal hydroxide was added to deprotonate ammonium, drive off ammonia over the course of about 24 hours of stirring uncapped in a fume hood, and provide the desired metal complex as a sodium/potassium salt, e.g., NaKTi(lactate) 2 (salicylate). Disodium titanium(IV) triscatecholate, Na 2 Ti(catecholate) 3 was synthesized by a modification of a procedure described by Davies, see Davies, J. A.; Dutramez, S. J. Am. Ceram. Soc. 1990, 73. 2570-2572, from titanium(IV) oxysulfate and pyrocatechol. Sodium hydroxide was used in place of ammonium hydroxide to obtain the sodium salt. Sodium potassium titanium(IV) trispyrogallate, NaKTi(pyrogallate) 3 was made analogously, first as the ammonium salt, (NH 4 )Ti(pyrogallate) 3 , and subsequently converted to the sodium potassium salt by heating in a mixture of aqueous sodium hydroxide and aqueous potassium hydroxide. The mixed ligand titanium complexes sodium potassium titanium(IV) biscatecholate monopyrogallate, sodium potassium titanium(IV) biscatecholate-monolactate, sodium potassium titanium (IV) biscatecholate monogluconate, sodium potassium titanium(IV) biscatecholate monoascorbate, and sodium potassium titanium(IV) bis catecholate monocitrate were made from a titanium catecholate dimer, Na 2 K 2 [TiO(catecholate)] 2 . For the synthesis of the tetrapotassium salt see Borgias, B. A.; Cooper, S. R.; Koh, Y. B.; Raymond, K. N. Inorg. Chem. 1984, 23, 1009-1016. A one-to-one mixture of titanium dimer with the desired chelate (pyrogallol, lactic acid, gluconic acid, ascorbic acid, or citric acid) gave the mixed ligand species. Sodium potassium titanium(IV) monocatecholate monopyrogallate monolactate was made in a similar fashion by addition of both pyrogallol and lactic acid to the catecholate containing dimer. Mixed ligand analogs of the Al, Cr, Fe, and Mn compounds may be prepared by similar reaction schemes. Mixed ligand analogs of the Al, Cr, Fe, and Mn compounds may be prepared by similar reaction schemes. Sodium potassium iron(III) triscatecholate, Na 1.5 K 1.5 Fe(catecholate) 3 was prepared according to the procedure outline by Raymond et. al., see Raymond, K. N.; Isied, S. S., Brown, L. D.; Fronczek, F. R.; Nibert, J. H. J. Am. Chem. Soc. 1976, 98, 1767-1774. The only modification was the use of a mixture of sodium hydroxide and potassium hydroxide as the excess base in place of potassium hydroxide. Sodium titanium(IV) triscitrate, Na 4 Ti(citrate) 3 , was synthesized by analogy to the method used for sodium titanium(IV) triscatecholate described above except using citric acid in place of catechol. These starting materials were obtained from Alfa Aesar (Ward Hill, Mass.), were of reagent grade or better, and were used as received. Sodium aluminum(III) biscitrate monocatecholate, Al(citrate) 2 (catecholate), was synthesized in analogy to the method used for sodium titanium(IV) triscatecholate described above except using two equivalents of citric acid and one equivalent of catechol to a solution of aluminum(III) sulfate. These starting materials were obtained from Alfa Aesar (Ward Hill, Mass.), were of reagent grade or better, and were used as received. Example 1.2—Cyclic Voltammetry Cyclic voltammetry data was recorded using a 760c potentiostat (CH Instruments, Austin, Tex.) with iR correction. Tests were conducted using glassy carbon working electrodes (Bioanalytical Systems, Inc., West Lafayette, Ind.), Ag/AgCl reference electrodes (Bioanalytical Systems, Inc. West Lafayette, Ind.) and platinum wire counter electrodes (Alfa Aesar, Ward Hill, Mass.). Working electrodes were polished according to the supplier's instructions before each experiment. Reference electrodes were calibrated against a “master” Ag/AgCl electrode known to have a potential of +0.210 V vs. NHE as known by those skilled in the art of electrochemistry. Solutions were sparged with argon for at least 5 minutes before each experiment. All experiments were performed at ambient temperatures (17-22° C.). No supporting electrolytes were added unless otherwise specified. All data were collected at a scan rate of 100 mV/s unless otherwise specified. Under these conditions, hydrogen evolution became significant at potentials more negative than −0.80 V vs. RHE and oxygen evolution became significant at potentials more positive than +2.20 V vs. RHE. Representative electrochemical data are provided in the following Tables. TABLE 2A Exemplary electrochemical couples described herein; half-cell potentials generated by cyclic votammetry, using glassy carbon electrodes Charge E 1/2 , V vs. Solubility Density Couple RHE pH (Molar), 25° C. (Ah/L) Al(citrate) 2 (catecholate) 2−/3− 1.25 11.5 0.5 13.4 Fe(catecholate) 3 2−/3− −0.50 11 1.5 40.2 Ti(catecholate) 3 2−/3− −0.45 11 1.0 26.8 Ti(pyrogallate) 3 2−/3− −0.55 9.8 1.6 42.9 Ti(catecholate) 2 (pyrogallate) 2−/3− −0.50 11 1.5 40.2 Ti(catecholate) 2 (ascorbate) 2−/3− −0.55 10 1.5 40.2 Ti(catecholate) 2 (gluconate) 2−/3− −0.60 9 1.5 40.2 Ti(catecholate) 2 (lactate) 2−/3− −0.49 9 1.5 40.2 Ti(catecholate)(pyrogallate)(lactate) 2−/3− −0.70 8.5 1.5 40.2 Ti(citrate) 3 −0.04 5 2.0 53.6 Fe(CN) 6 3−/4− 1.18 11 1.5 40.2 Cr(CN) 6 3−/4− −0.60 9 1.5 40.2 Mn(CN) 6 3−/4− −0.60 9 1.5 40.2 TABLE 2B Exemplary electrochemical couples described herein E 1/2 , Solubility Charge V vs. (Molar), Density Couple RHE pH 25° C. (Ah/L) Ti IV/III (lactate) 1 −0.34 3.6 1.75 46.9 Ti IV/III (lactate) 1 −0.40 5.6 1.75 46.9 Ti IV/III (lactate) 1 −0.54 9 1.75 46.9 Ti IV/III (lactate) 2 −0.03 2 1.75 46.9 Ti IV/III (lactate) 2 −0.40 3.6 1.75 46.9 Ti IV/III (lactate) 2 −0.40 9 1.75 46.9 Ti IV/III (lactate) 1 (malate) 2 −0.40 9.9 1.5 40.2 Ti IV/III (malate) 2 (salicylate) −0.48 10 1.5 40.2 Ti IV/III (lactate) 2 (glycinate) −0.50 9.9 1.5 40.2 Ti IV/III (lactate) 2 (salicylate) −0.48 10 1.5 40.2 Ti IV/III (salicylate) 2 (lactate) −0.50 9.8 1.5 40.2 Ti IV/III (α-hydroxyacetate) 2 (salicylate) −0.48 10 1.5 40.2 Ti IV/III (malate) 2 (salicylate) −0.50 10 1.5 40.2 Ti IV/III (α-hydroxyacetate) 2 (lactate) −0.50 10 1.5 40.2 Ti IV/III (lactate) 2 (α-hydroxyacetate) −0.50 10 1.5 40.2 Ti IV/III (lactate) 3 −0.45 10 1.75 46.9 Ti IV/III (salicylate) 3 −0.25 8.6 0.5 13.4 Fe III/II (salicylate) 3 −0.10 9.3 0.5 13.4 Fe III/II (malate) 3 −0.30 9.2 1.0 26.8 Fe III/II (α-hydroxyacetate) 3 −0.50 8.1 1.0 26.8 Fe III/II (lactate) 2 (salicylate) 1 −0.39 8.7 1.0 26.8 Fe III/II (lactate) 2 (glycinate) 1 +0.30 6.7 1.0 26.8 Fe III/II (lactate) 2 +0.45 2.6 1.5 40.2 Fe III/II (lactate) 1 +0.11 3.1 1.5 40.2 Fe(CN) 6 3−/4− +1.18 11 1.5 40.2 Al(citrate) 2 (catecholate) 2−/3− +1.25 11.5 0.5 13.4 Fe III/II (H 2 O) 6 +0.77 0 2 53.6 Ce IV/III (H 2 O) x +1.75 0 0.5 13.4 TABLE 3A Calculated OCVs and theoretical energy density (Wh/L) for various other electrolyte couple pairs calculated from data in Table 2. Fe(CN) 6 3−/4− Al(cit) 2 (cat) 2−/3− Energy Energy OCV Density OCV Density Couple (V) (Wh/L) (V) (Wh/L) Mn(CN) 6 3−/4− 1.78 35.8 1.85 12.4 Fe(catecholate) 3 2−/3− 1.68 33.8 1.75 11.7 Ti(catecholate) 3 2−/3− 1.63 21.8 1.70 11.4 Ti(pyrogallate) 3 2−/3− 1.73 34.8 1.80 12.1 Ti(catecholate) 2 (pyrogallate) 2−/3− 1.68 33.8 1.75 11.7 Ti(catecholate) 2 (ascorbate) 2−/3− 1.73 34.8 1.80 12.1 Ti(catecholate) 2 (gluconate) 2−/3− 1.78 35.8 1.85 12.4 Ti(catecholate) 2 (lactate) 2−/3− 1.67 33.6 1.74 11.7 Ti(catecholate)(pyrogallate)- 1.73 34.8 1.80 12.1 (lactate) 2−/3− Ti(citrate) 3 1.22 24.5 1.29 8.6 TABLE 3B Calculated OCVs and theoretical energy density (Wh/L) for various electrolyte couple pairs calculated from data in Table 2. Fe(CN) 6 3−/4− Al(cit) 2 (cat) 2−/3− Energy Energy OCV Density OCV Density Couple (V) (Wh/L) (V) (Wh/L) Ti IV/III (lactate) 1 1.60 34.9 1.67 25.2 Ti IV/III (lactate) 2 1.46 31.8 1.53 23.1 Ti IV/III (lactate) 3 1.57 34.2 1.64 24.7 Ti IV/III (salicylate) 3 1.29 17.3 1.36 9.1 Ti IV/III (lactate) 1 (malate) 2 1.51 30.4 1.58 21.2 Ti IV/III (malate) 2 (salicylate) 1.60 32.2 1.67 22.4 Ti IV/III (lactate) 2 (glycinate) 1.61 32.4 1.68 22.5 Ti IV/III (lactate) 2 (salicylate) 1.60 32.2 1.67 22.4 Ti IV/III (salicylate) 2 (lactate) 1.61 32.3 1.68 22.5 Ti IV/III (α-hydroxyacetate) 2 (salicylate) 1.60 32.2 1.67 22.4 Ti IV/III (malate) 2 (sal) 1.62 32.6 1.69 22.6 Ti IV/III (α-hydroxyacetate) 2 (lactate) 1.62 32.6 1.69 22.6 Ti IV/III (lactate) 2 (α-hydroxyacetate) 1.62 32.6 1.69 22.6 Fe III/II (salicylate) 3 1.18 15.8 1.25 8.4 Fe III/II (malate) 3 1.37 23.0 1.44 14.5 Fe III/II (α-hydroxyacetate) 3 1.51 25.3 1.58 15.9 TABLE 4 Calculated OCVs and theoretical energy density (Wh/L) for various electrolyte couple pairs calculated from data in Table 2 in mildly acidic solutions. 2M Fe III/II , pH 2 0.5M Ce IV/III , pH 2 Energy Energy OCV Density OCV Density Couple (V) (Wh/L) (V) (Wh/L) Ti IV/III (lactate) 1 1.32 33.2 2.30 34.7 Ti IV/III (lactate) 2 0.92 23.1 1.90 28.6 Example 1.3—Experimental Procedure for a 5 cm 2 Active Area Flow Battery Cell hardware designed for 5 cm 2 active area and modified for acid flow was obtained from Fuel Cell Technologies (Albuquerque, N. Mex.). Carbon felt, nominally 3 mm thick, was obtained from Alfa Aesar (Ward Hill, Mass.) and MGL 370 carbon paper was obtained from Fuel Cell Earth (Stoneham, Mass.). Felts were dip-coated with a suspension of Vulcan XC-72 carbon (Cabot Corp., Boston, Mass.) and NAFION™ (Ion-Power, New Castle, Del.) and air-dried before use and carbon papers were used as received. NAFION™ HP, XL, or NR-212 cation exchange membranes were obtained from Ion-Power in the H + form and were used as received. VITON™ gaskets were obtained from McMaster Carr (Robinsville, N.J.) and were cut to allow for a 5 cm 2 active area with ˜1 cm 2 areas left above and below the felts for electrolyte ingress and egress from the positive and negative compartments of the cell. The cell was assembled using gaskets that provided a compression of ˜25% of the measured thickness of the felts or papers. The membranes and electrodes were not pretreated before assembly. The electrolyte reservoirs were fashioned from Schedule 80 PVC piping with PVDF tubing and compression fittings. Masterflex™ L/S peristaltic pumps (Cole Parmer, Vernon Hills, Ill.) were used with Tygon™ tubing. Electrolytes were sparged with UHP argon through an oil-filled bubbler outlet before electrochemical testing and a head pressure of argon was maintained during the testing. An Arbin Instruments BT2000 (College Station, Tex.) was used to test the electrochemical performance, and a Hioki 3561 Battery HiTESTER (Cranbury, N.J.) was used to measure the AC resistance across the cell. In a typical experiment, 50 mL each of electrolyte containing active material for the positive and negative electrode were loaded into separate reservoirs and sparged with argon for 20 minutes while circulating the electrolytes through the cell. The electrolytes were charged to 40% SOC (calculated from the concentrations of the active materials and the volumes of the electrolyte), the iV response of the cell was obtained, and then the electrolytes were cycled between 40 and 60% SOC. An analog output from the Hioki battery tester was recorded to monitor changes in the membrane and contact resistances. Example 2 A redox flow battery cell was assembled according to the methods described in Example 1 using titanium tris-catecholate (Ti 4+/3+ (cat) 3 2−/3− ) and ferri/ferro-cyanide (Fe 3+/2+ (CN) 6 3−/4− ) metal ligand coordination compounds as active materials for the negative and positive electrolytes, respectively. The active materials were prepared at concentrations of 0.5 M in 0.5 M pH 11 Na 2 SO 4 supporting electrolyte (negative electrolyte, or negolyte) or no supporting electrolyte (positive electrolyte, or posolyte) and were flowed at 100 mL/min through the flow battery cell assembled using 5 cm 2 carbon felt electrodes and a NAFION™ cation selective membrane (50 μm thick) in Na form. The cell was initially charged from 0 to 50% state of charge before several charge/discharge cycles was collected by charging and discharging the battery at a current density of ˜150 mA/cm 2 and monitoring the resulting cell potential, FIG. 2 . At open circuit, a cell potential of 1.63 V was observed as expected for equilibrium cell potential at 50% SOC based on the externally measured E 1/2 values for Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− . Charge/discharge cycling revealed well behaved, reproducible voltage/current vs. time traces, demonstrating promising durability, FIG. 2 . An RT voltage efficiency of 69% was measured for this system at 150 mA/cm 2 . Typical resistances measured by the Hioki Battery Tester for the membrane and contact resistance component of cells built with NR212, XL, and HP membranes were 0.77, 0.60, and 0.5 ohm-cm 2 , respectively. FIG. 3 displays the charge/discharge characteristics for a flow battery of the present invention wherein the negative and positive active materials comprise Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− , respectively. The cell potential increases as the battery is charged and decreases as the battery is discharged. Example 3 A redox flow battery cell was assembled according to the methods described in Example 1.3 using titanium tris-catecholate (Ti 4+/3+ (cat) 3 2−/3− ) and ferri/ferro-cyanide (Fe 3+/2+ (CN) 6 3−/4− ) metal ligand coordination compounds as active materials for the negative and positive electrolytes, respectively. In a typical cell, stable voltages were observed upon repeatedly charging to 60% SOC and discharging to 40% SOC (see FIG. 4 ) when the discharge energy for each cycle was 99.8% of the charge energy, indicative of 99.8% roundtrip current efficiency. This was achieved by using a constant current density (e.g., 150 mA/cm 2 ) for both charge and discharge but with a discharge time that was slightly shorter than (i.e., 99.8% of) the charge time. Under these conditions, the open circuit voltages at 40 and 60% SOC were stable for extended periods of time. Crossover flux data were obtained by measuring the concentrations of Fe and Ti in each electrolyte at the beginning and end of a suitably lengthy battery test, typically one to two weeks in duration for a membrane area of 7 cm 2 . The concentrations were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) experiments performed by Evans Analytical Group, Syracuse, N.Y. The moles of Fe in the Ti-containing electrolyte before the test were subtracted from the number of moles in the same electrolyte at the end of the test. This was converted to a flux by dividing the moles by the membrane area and the test duration. In the present example, the active materials were prepared at concentrations of 0.5 M in 0.5 M pH 11 Na 2 SO 4 electrolyte and were flowed at 100 mL/min through the flow battery cell assembled using 5 cm 2 carbon felt electrodes and a NAFION™ cation selective membrane (50 μm thick) in Na + form. The cell was initially charged from 0 to 50% state of charge before several charge/discharge cycles at a current density of 100 mA/cm 2 . The cell was cycled between 40% and 60% SOC for 283 cycles over the course of a 163 hour experiment. The test was then terminated and a sample of the positive electrolyte was analyzed for Ti content. From the Ti concentration in the positive electrolyte, the total exposed membrane area (7 cm 2 ), and the time of exposure (163 hrs) a flux of 5×10 mol Ti cm −2 day −1 could be calculated, see Table 5. The selectivity for pumping Na + or K + ions across the membrane over the Ti complex over the course of this example can be calculated by computing the quantity of ions passed in each discharge cycle (in this case 60% to 40% SOC), and comparing this quantity to twice the quantity of Ti in the positive electrolyte at the end of the experiment (accounting for the charge of −2 for the Ti(cat) 3 complex and +1 for Na + or K + ions). In this case, 5×10 −3 mol of Na + /K + were passed in each cycle, and over the 283 cycles of the experiment approximately 1.42 mol of Na + /K + were passed. Since the quantity of Ti in the positive electrolyte was measured as 2.3×10 −6 mol, a selectivity of ˜3×10 5 can be determined (1.42 mol Na + /K + divided by 2×2.3×10 −6 mol Ti). TABLE 5 Thickness Initial Volume Membrane Titanium Flux Estimated time Membrane (μm) Concentration (M) (L) Area (m 2 ) (mol/cm 2 day) to 5% xover NR212- 50 0.5 0.05 0.0007 5 × 10 −8 196 years subscale data Typical fluxes for metal ligand coordination complexes in cells operated at 100 mA/cm 2 with boiled DuPont NAFION™ NR212 membranes (50 micron thick) were 5.0×10 −8 mol cm −2 day −1 for ferri/ferrocyanide and 6.5×10 −8 mol cm −2 day −1 for titanium triscatecholate. Thus the iron and titanium complexes comprise 5.6×10 −5 % and 7.2×10 −5 %, respectively, of the total molar flux of ions passing through the unboiled membrane. For unboiled DuPont NAFION™ HP (20 micron thick), the measured fluxes were 1.1×10 −5 and 3.3×10 −6 mol cm −2 day −1 for the above iron and titanium complexes, respectively. Thus the iron and titanium complexes comprise 0.012% and 0.0037%, respectively, of the total molar flux of ions passing through the unboiled membrane. These data indicate that the average round trip current efficiencies over the tests for both the boiled and unboiled membranes are greater than 99.9%. These results are believed to be representative and typical for the compounds described herein. Example 4 A redox flow battery cell was assembled according to the general methods described in Example 1.3, again using titanium bis-catecholate mono-pyrogallate (Ti 4+/3+ (cat) 2 (gal) 2−/3− ) and ferri/ferro-cyanide (Fe 3+/2+ (CN) 6 3−/4− ) metal ligand coordination compounds as active materials for the negative and positive electrolytes, respectively. In this example the carbon felt electrodes were replaced with TORAY™ carbon paper electrodes that were catalyzed with Vulcan carbon and NAFION™ in a manner similar to that of Example 2. Additionally, flow fields of the “interdigitated” type were employed. The active material solution concentrations were increased to 1.5 M and the cell performance was evaluated by monitoring the cell potential on both charge and discharge cycles as a function of current density. As can be seen in FIG. 5 , the cell maintains round trip voltage efficiencies of 84%, 79%, and 73% at current densities of 150, 200, and 250 mA/cm 2 , respectively. In this configuration the flow battery active materials exhibited an energy density of 32.79 Wh/L. The results of analogous experiments using Ti 4+/3+ (cat) 3 2−/3− and Fe 3+/2+ (CN) 6 3−/4− are shown in FIG. 6 and FIG. 7 . Example 5 A redox flow battery cell was assembled according to the methods described in Example 1.3 using titanium bis-lactate mono-salicylate ([Ti 4+/3+ (lactate) 2 (salicylate)] 2−/3− ) and ferri/ferro-cyanide ([Fe 3+/2+ (CN) 6 ] 3−/4− ) metal ligand coordination compounds as active materials for the negative and positive electrolytes, respectively. The active material solutions were prepared at concentrations of 1 M with no additional supporting electrolyte and were flowed at 100 mL/min through the flow battery cell assembled using 5 cm 2 carbon paper electrodes and a NAFION™ cation selective membrane (25 μm thick) in the Na + form. The cell was initially charged from 0 to 25% state of charge before charge/discharge cycles were collected by charging and discharging the cell at 150 or 100 mA/cm 2 and monitoring the resulting cell potential, FIG. 8 (where visually wider cycles were taken at 100 instead of 150 mA/cm 2 ). At open circuit, a cell potential of 1.60 V was observed as expected for equilibrium cell potential at 50% SOC based on the externally measured E 1/2 values for [Ti 4+/3+ (lactate) 2 (salicylate)] 2−/3− and [Fe 3+/2+ (CN) 6 ] 3−/4− . Charge/discharge cycling revealed well behaved, reproducible voltage/current vs. time traces, demonstrating promising durability, FIG. 8 . An RT voltage efficiency of 67% was measured for this system at 150 mA/cm 2 . Typical resistances measured by the Hioki Battery Tester for the membrane and contact resistance component of cells built with NR212, XL, and HP membranes were 0.77, 0.60, and 0.5 ohm-cm 2 , respectively. Example 6 A redox flow battery cell was assembled according to the methods described in Example 1.3 using titanium bis-lactate mono-glycolic acid ([Ti 4+/3+ (lactate) 2 (α-hydroxyacetate)] 2−/3− ) and ferri/ferro-cyanide ([Fe 3+/2+ (CN) 6 ] 3−/4− ) metal ligand coordination compounds as active materials for the negative and positive electrolytes, respectively. In a typical cell, stable voltages were observed upon repeatedly charging to 75% SOC and discharging to 25% SOC (see FIG. 9 ) when the discharge energy for each cycle was 99.8% of the charge energy, indicative of 99.8% roundtrip current efficiency. This was achieved by using a constant current density (e.g., 150 mA/cm 2 ) for both charge and discharge but with a discharge time that was slightly shorter than (i.e., 99.8% of) the charge time. Under these conditions, the open circuit voltages at 25 and 75% SOC were stable for extended periods of time.

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