Abstract: Flow batteries can include a first half-cell containing a first aqueous electrolyte solution, a second half-cell containing a second aqueous electrolyte solution, and a separator disposed between the first half-cell and the second half-cell. The first aqueous electrolyte solution contains a first redox-active material, and the second aqueous electrolyte solution contains a second redox-active material. At least one of the first redox-active material and the second redox-active material is a nitroxide compound or a salt thereof. Particular nitroxide compounds can include a doubly bonded oxygen contained in a ring bearing the nitroxide group, a doubly bonded oxygen appended to a ring bearing the nitroxide group, sulfate or phosphate groups appended to a ring bearing the nitroxide group, various heterocyclic rings bearing the nitroxide group, or acyclic nitroxide compounds.

Abstract: Electrolyte solutions for flow batteries and other electrochemical systems can contain an active material that is capable of transferring one or more electrons per molecule during an oxidation-reduction cycle. Doubly bridged aromatic groups or their coordination compounds can be particularly suitable active materials. Flow batteries can include a first half-cell containing a first electrolyte solution, and a second half-cell containing a second electrolyte solution, in which at least one of the first electrolyte solution and the second electrolyte solution contains an active material having at least two aromatic groups doubly bridged by a carbonyl moiety and a bridging moiety containing a bridging atom selected from carbon, nitrogen, oxygen, sulfur, selenium and tellurium. Such bridged compounds can directly function as the active material, or coordination compounds containing the bridged compounds as at least one ligand can serve as the active material.

Abstract: The circulation rates of the electrolyte solutions in a flow battery can impact operating performance. Adjusting the circulation rates can allow improved performance to be realized. Flow battery systems having adjustable circulation rates can include a first half-cell containing a first electrolyte solution, a second half-cell containing a second electrolyte solution, at least one pump configured to circulate the first electrolyte solution and the second electrolyte solution at adjustable circulation rates through at least one half-cell in response to a value of Pexit/I or I/Penter, and at least one sensor configured to measure net electrical power entering or exiting the flow battery system, and an amount of electrical current passing through the whole cell. I is the electrical power passing through the whole cell. Pexit is net electrical power exiting the system in a discharging mode, and Penter is net electrical power entering the system in a charging mode.

Abstract: The present invention relates to methods and apparatuses for determining the ratio of oxidized and reduced forms of a redox couple in solution, each method comprising: (a) contacting a first stationary working electrode and a first counter electrode to the solution; (b) applying a first potential at the first working electrode and measuring a first constant current; (c) applying a second potential at the first working electrode and measuring a second constant current; wherein the sign of the first and second currents are not the same; and wherein the ratio of the absolute values of the first and second currents reflects the ratio of the oxidized and reduced forms of the redox couple in solution.

Abstract: Parasitic reactions, such as production of hydrogen and oxidation by oxygen, can occur under the operating conditions of flow batteries and other electrochemical systems. Such parasitic reactions can undesirably impact operating performance by altering the pH and/or state of charge of one or both electrolyte solutions in a flow battery. Electrochemical balancing cells can allow rebalancing of electrolyte solutions to take place. Electrochemical balancing cells suitable for placement in fluid communication with both electrolyte solutions of a flow battery can include: a first chamber containing a first electrode, a second chamber containing a second electrode, a third chamber disposed between the first chamber and the second chamber, an ion-selective membrane forming a first interface between the first chamber and the third chamber, and a bipolar membrane forming a second interface between the second chamber and the third chamber.

Abstract: Titanium catecholate complexes can be desirable active materials for flow batteries and other electrochemical energy storage systems, particularly when incorporated in aqueous electrolyte solutions. It can be desirable to avoid introducing certain organic solvents and/or extraneous salts into aqueous electrolyte solutions. Methods for synthesizing titanium catecholate complexes that can help avoid the unwanted introduction of organic solvents and/or extraneous salts into aqueous electrolyte solutions can include: providing an aqueous solution containing a catechol compound, reacting a titanium reagent with the catechol compound in the aqueous solution to form an intermediate titanium catecholate complex, isolating the intermediate titanium catecholate complex as a solid or slurry, and reacting a ligatable compound with the intermediate titanium catecholate complex in the presence of a base to produce a salt form titanium catecholate complex containing at least one additional ligand.

Abstract: Active materials for flow batteries can include various coordination compounds formed from transition metals. Some compositions containing coordination compounds can include a substituted catecholate ligand having a structure of in a neutral form or a salt form, in which Z is a heteroatom functional group bound to the substituted catecholate ligand at an open aromatic ring position and n is an integer ranging between 1 and 4. When more than one Z is present, each Z can be the same or different. Electrolyte solutions can include such coordination compounds, and such electrolyte solutions can be incorporated within a flow battery.

Abstract: This invention is directed to aqueous redox flow batteries comprising redox-active metal ligand coordination compounds. The compounds and configurations described herein enable flow batteries with performance and cost parameters that represent a significant improvement over that previous known in the art.

Abstract: This invention is directed to aqueous redox flow batteries comprising ionically charged redox active materials and separators, wherein the separator is about 100 microns or less and the flow battery is capable of (a) operating with a current efficiency of at least 85% with a current density of at least about 100 mA/cm2; (b) operating with a round trip voltage efficiency of at least 60% with a current density of at least about 100 mA/cm2; and/or (c) giving rise to diffusion rates through the separator for the first active material, the second active material, or both, of about 1×10?7 mol/cm2-sec or less.

Abstract: Flow batteries and other electrochemical systems can contain an active material that is a coordination complex having at least one monosulfonated catecholate ligand or a salt thereof bound to a metal center. The monosulfonated catecholate ligand has a structure of More particularly, the coordination complex can be a titanium coordination complex with a formula of DgTi(L1)(L2)(L3), in which D is a counterion selected from H, NH4+, Li+, Na+, K+, or any combination thereof g ranges between 3 and 6; and L1, L2 and L3 are ligands, where at least one of L1, L2 and L3 is a monosulfonated catecholate ligand. Methods for synthesizing such monosulfonated catecholate ligands can include providing a neat mixture of catechol and up to about 1.3 stoichiometric equivalents of sulfuric acid, and heating the neat mixture at a temperature of about 80° C. or above to form 3,4-dihydroxybenzenesulfonic acid or a salt thereof.

Abstract: Electrolyte solution circulation rates in a flow battery can impact operating performance. Although adjusting the circulation rates can allow improved performance to be realized, it can be difficult to levelize circulation rates over multiple electrochemical cells of an electrochemical cell stack due to a non-uniform pressure drop that occurs at an outlet of each electrochemical cell. Accordingly, flow batteries capable of realizing improved operating performance can include: an electrochemical cell stack containing a plurality of electrochemical cells in electrical communication with one another; an inlet manifold containing an inflow channel fluidically connected to an inflow side of each of the electrochemical cells; an outlet manifold containing an outflow channel fluidically connected to an outflow side of each of the electrochemical cells; and an insert disposed in the outflow channel. The insert has a variable width along a length of the outflow channel.

Abstract: Parasitic reactions, such as production of hydrogen and oxidation by oxygen, can occur under the operating conditions of flow batteries and other electrochemical systems. Such parasitic reactions can undesirably impact operating performance by altering the pH and/or state of charge of one or both electrolyte solutions in a flow battery. Electrochemical balancing cells configured for addressing the effects of parasitic reactions can include: a first chamber containing a first electrode, a second chamber containing a second electrode, a third chamber disposed between the first chamber and the second chamber, an ion-selective membrane forming a first interface between the first chamber and the third chamber, and a bipolar membrane forming a second interface between the second chamber and the third chamber. Such electrochemical balancing cells can be placed in fluid communication with at least one half-cell of a flow battery.

Abstract: Electrochemical cells, such as those present within flow batteries, can include at least one electrode with one face being more hydrophilic than is the other. Such electrodes can lessen the incidence of parasitic reactions by directing convective electrolyte circulation toward a separator in the electrochemical cell. Flow batteries containing the electrochemical cells can include: a first half-cell containing a first electrode with a first face and a second face that are directionally opposite one another, a second half-cell containing a second electrode with a first face and a second face that are directionally opposite one another, and a separator disposed between the first half-cell and the second half-cell. The first face of both the first and second electrodes is disposed adjacent to the separator. The first face of at least one of the first electrode and the second electrode is more hydrophilic than is the second face.

Abstract: This invention is directed to aqueous redox flow batteries comprising ionically charged redox active materials at least one of which is a sulfonated catechol.

Abstract: Coordinatively saturated titanium (IV) coordination compounds containing catecholate ligands can be desirable active materials for flow batteries and other electrochemical energy storage systems. Such coordination compounds can be formed advantageously via an intermediate composition containing a coordinatively unsaturated titanium (IV) coordination compound. More specifically, such compositions can include a coordinatively unsaturated titanium (IV) coordination compound having a coordination number of 5 or less and containing two catecholate ligands, wherein the composition is substantially free of non-ligated catechol compound.

Abstract: The invention concerns flow batteries comprising: a first half-cell comprising: (i) a first aqueous electrolyte comprising a first redox active material; and a first carbon electrode in contact with the first aqueous electrolyte; (ii) a second half-cell comprising: a second aqueous electrolyte comprising a second redox active material; and a second carbon electrode in contact with the second aqueous electrolyte; and (iii) a separator disposed between the first half-cell and the second half-cell; the first half-cell having a half-cell potential equal to or more negative than about ?0.3 V with respect to a reversible hydrogen electrode; and the first aqueous electrolyte having a pH in a range of from about 8 to about 13, wherein the flow battery is capable of operating or is operating at a current density at least about 25 mA/cm2.

Abstract: During operation of flow battery systems, the volume of one or more electrolyte solutions can change due to solvent loss processes. An electrochemical balancing cell can be used to combat volume variability. Methods for altering the volume of one or more electrolyte solutions can include: providing a first electrochemical balancing cell containing a membrane disposed between two half-cells, establishing fluid communication between a first aqueous electrolyte solution of a flow battery system and a first half-cell of the first electrochemical balancing cell, and applying a current to the first electrochemical balancing cell to change a concentration of one or more components in the first aqueous electrolyte solution. Applying the current causes water to migrate across the membrane, either to or from the first aqueous electrolyte solution, and a rate of water migration is a function of current.

Abstract: Productive electrochemical reactions can often occur most effectively in proximity to a separator dividing an electrochemical cell into two half-cells. Parasitic reactions can often occur at locations more removed from the separator. Parasitic reactions are generally undesirable in flow batteries and other electrochemical systems, since they can impact operating performance. Flow batteries having a decreased incidence of parasitic reactions can include, a first half-cell containing a first electrode, a second half-cell containing a second electrode, a separator disposed between the first half-cell and the second half-cell and contacting the first and second electrodes, a first bipolar plate contacting the first electrode, and a second bipolar plate contacting the second electrode, where a portion of the first electrode or the first bipolar plate contains a dielectric material.

Abstract: Parasitic reactions, such as production of hydrogen and oxidation by oxygen, can occur under the operating conditions of flow batteries and other electrochemical systems. Such parasitic reactions can undesirably impact operating performance by altering the pH and/or state of charge of one or both electrolyte solutions in a flow battery. Electrochemical balancing cells can allow rebalancing of electrolyte solutions to take place. Electrochemical balancing cells suitable for placement in fluid communication with both electrolyte solutions of a flow battery can include: a first chamber containing a first electrode, a second chamber containing a second electrode, a third chamber disposed between the first chamber and the second chamber, an ion-selective membrane forming a first interface between the first chamber and the third chamber, and a bipolar membrane forming a second interface between the second chamber and the third chamber.

Abstract: An energy storage unit (ESU) is provided herein. The ESU includes a housing forming an interior volume configured to store a plurality of batteries that can collectively provide a maximum power level. The ESU includes a segmented branch interconnect system including a power transfer assembly that includes a first power interconnect system and a second power interconnect system. The second power interconnect system is configured to be coupled to a first adjacent power interconnect system of a first adjacent ESU. The power transfer assembly includes one or more conductors coupled to the first power interconnect system and the second power interconnect system, and is configured to transfer electrical power therebetween. The power transfer assembly is rated to transfer a power level that is at least three times the maximum power level provided by the plurality of batteries stored in the housing.