ALL-VANADIUM SULFATE ACID REDOX FLOW BATTERY SYSTEM

FIELD

This invention concerns embodiments of an all-vanadium sulfate acid electrolyte comprising chloride ions and phosphate ions for use in a redox-flow battery system.

BACKGROUND

A redox flow battery (RFB) stores electrical energy in reduced and oxidized species dissolved in two separate electrolyte solutions. The anolyte and the catholyte circulate through a cell electrode separated by a membrane or separator. Redox flow batteries are advantageous for energy storage because they are capable of tolerating fluctuating power supplies, repetitive charge/discharge cycles at maximum rates, overcharging, overdischarging, and because cycling can be initiated at any state of charge.

However, among the many redox couples upon which redox flow batteries are based, a number of disadvantages exist. For example, many systems utilize redox species that are unstable, are highly oxidative, are difficult to reduce or oxidize, precipitate out of solution, and/or generate volatile gases. One of the main challenges confronting RFB systems is the intrinsically low energy density compared with other reversible energy storage systems such as lithium-ion batteries. With the voltage limitation of the aqueous systems, this issue is typically tackled by increasing the active species concentration in the electrolyte. However, the active species concentration is limited by the solubility and the stability of the active redox ions in the electrolyte solutions. Therefore, a need exists for RFB systems having a greater energy density.

SUMMARY

Embodiments of an all-vanadium sulfate acid flow battery system are disclosed. The system includes an anolyte comprising V²⁺ and V³⁺ in an aqueous supporting solution, and a catholyte comprising V⁴⁺ and V⁵⁺ in an aqueous supporting solution. The aqueous supporting solution for each of the anolyte and the catholyte includes sulfate ions, protons, chloride ions, and phosphate ions. The chloride ions may be provided by an inorganic chloride salt. The phosphate ions may be provided by an inorganic phosphate salt. In any or all of the above embodiments, the aqueous supporting solution may further include magnesium ions, ammonium ions, or magnesium ions and ammonium ions. In any or all of the above embodiments, [V] may be ≧1.0 M in the anolyte and [V] may be ≧1.0 M in the catholyte.

In any or all of the above embodiments, the aqueous supporting solution may comprise 0.05-0.5 M chloride ions. In any or all of the above embodiments, the chloride ions may be provided by MgCl₂. In any or all of the above embodiments, the aqueous supporting solution may comprise 0.05-0.5 M phosphate ions. In any or all of the above embodiments, the phosphate ions may be provided by ammonium phosphate. In some embodiments, the phosphate ions are provided by (NH₄)₂HPO₄.

In any or all of the above embodiments, the battery system may have a Mg:V molar ratio within a range of 1:100 to 1:1, a Cl:V molar ratio within a range of 1:50 to 1:2, a phosphate:V molar ratio within a range of 1:50 to 1:2, a NH₄+:V molar ratio within a range of 1:50 to 1:3, a Cl:phosphate molar ratio within a range of 10:1 to 1:10, a NH₄+:Mg molar ratio within a range of 60:1 to 1:5, or any combination thereof. In any or all of the above embodiments, the sulfate concentration may be 4.5-6 M.

In any or all of the above embodiments, the aqueous supporting solution may include H₂O, H₂SO₄, MgCl₂, and ammonium phosphate. In some embodiments, the aqueous supporting solution comprises 0.025-0.25 M MgCl₂and 0.05-0.5 M ammonium phosphate.

In any or all of the above embodiments, the anolyte may comprise 1.0-2.5 M vanadium as V²⁺ and V³⁺, 0.025-0.5 M magnesium ions, 0.05-0.5 M chloride ions, 0.05-1.5 M ammonium ions, and 0.05-0.5 M phosphate ions, and the catholyte may comprise 1.0-2.5 M vanadium as V⁴⁺ and V⁵⁺, 0.025-0.25 M magnesium ions, 0.0.5-0.5 M chloride ions, 0.05-0.1.5 M ammonium ions, and 0.05-0.5 M phosphate ions. In some embodiments, the anolyte and catholyte independently comprise 0.5-0.1 M MgCl₂and 0.1-0.2 M (NH₄)₂HPO₄.

In any or all of the above embodiments, the anolyte and catholyte may comprise, consist essentially of, or consist of a solution prepared by combining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide 1.0-2.5 M VOSO₄-3.5 M H₂SO₄, 0.025-0.25 M MgCl₂, and 0.05-0.5 M ammonium phosphate. In some embodiments, the ammonium phosphate is (NH₄)₂HPO₄.

In any or all of the above embodiments, the system may have an operating cell temperature within a range of −5° C. to 50° C. In any or all of the above embodiments, the system may further include an anode, a cathode, and a separator or membrane separating the anolyte and the catholyte. In some embodiments, the anode and cathode are graphite or carbon-based electrodes.

Embodiments of an anolyte and catholyte system for use in an all vanadium sulfate acid redox flow battery system include an aqueous anolyte comprising V²⁺, V³⁺, sulfate, magnesium ions, ammonium ions, chloride ions, and phosphate ions; and an aqueous catholyte comprising V⁴⁺, V⁵⁺, sulfate, magnesium ions, ammonium ions, chloride ions, and phosphate ions. In some embodiments, the anolyte and the catholyte independently comprise 0.025-0.25 M MgCl₂. In any or all of the above embodiments, the anolyte and the catholyte may independently comprise 0.05-0.5 M ammonium phosphate. In any or all of the above embodiments, the anolyte and catholyte system may have a Cl:V molar ratio within a range of 1:50 to 1:2 and a phosphate:V molar ratio within a range of 1:50 to 1:2.

The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cyclic voltammogram on a carbon electrode of electrolyte solutions containing 2 M VOSO₄-3.5 M H₂SO₄with and without added components. The scan was carried out at room temperature at a scan rate of 50 mV/s.

FIG. 2 is a graph showing the charge capacity, discharge capacity, charge energy, and discharge energy of an embodiment of the all vanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂as a function of the number of cycles at −5° C.

FIG. 3 is a graph showing the coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) of an embodiment of the all vanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂as a function of the number of cycles at −5° C.

FIG. 4 is a graph showing the charge capacity, discharge capacity, charge energy, and discharge energy of an embodiment of the all vanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂as a function of the number of cycles at 50° C.

FIG. 5 is a graph showing the coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) of an embodiment of the all vanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂as a function of the number of cycles at 50° C.

FIGS. 6A and 6B are molecular models showing optimized DFT (density functional theory) configurations for [V₂O₃(H₂O)₇]⁴⁺ (FIG. 6A) and [VO₂Mg(H₂O)₉]³⁺ (FIG. 6B) solvates; H₂O and H₃O⁺ solvation molecules are shown in stick images.

FIG. 7 shows ⁵¹V NMR spectra of vanadate-based electrolytes with and without added components; the spectra were measured at 250K under 11 Tesla.

DETAILED DESCRIPTION

Embodiments of an all-vanadium sulfate acid flow battery system are disclosed. The system includes a liquid-phase anolyte comprising V²⁺ and V³⁺ and a liquid-phase catholyte comprising V⁴⁺ and V⁵⁺. The anolyte and catholyte comprise sulfuric acid and a dual-component system to increase solubility and/or stability of the vanadium species, wherein the components comprise chloride ions and phosphate ions. The term “phosphate ions” includes PO₄³⁻, HPO₄²⁻, and H₂PO₄₋ ions, and all combinations thereof.

Embodiments of the system include a higher concentration of vanadium and/or have an increased operating temperature range compared to vanadium sulfate systems in the absence of the chloride ions and phosphate ions.

I. Definitions and Abbreviations

The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.

Unless otherwise indicated, all numbers expressing quantities of components, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those persons of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.

In order to facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:

Capacity: The capacity of a battery is the amount of electrical charge a battery can deliver. The capacity is typically expressed in units of mAh, or Ah, and indicates the maximum charge a battery can produce over a period of one hour. The specific capacity is measured with respect to either volume or weight of the active materials, which leads to the concept of specific volumetric capacity and specific gravimetric capacity, respectively.

Cell: As used herein, a cell refers to an electrochemical device used for generating a voltage or current from a chemical reaction, or the reverse in which a chemical reaction is induced by a current. Examples include voltaic cells, electrolytic cells, redox flow cells, and fuel cells, among others. Multiple single cells can form a cell assembly, often termed a stack. A battery includes one or more cells, or even one or more stacks. The terms “cell” and “battery” are used interchangeably when referring to a battery containing only one cell.

Coulombic efficiency (CE): The efficiency with which charges are transferred in a system facilitating an electrochemical reaction. CE may be defined as the amount of charge exiting the battery during the discharge cycle divided by the amount of charge entering the battery during the charging cycle.

Electrochemically active element: An element that is capable of forming redox pairs between different oxidation and reduction states, i.e., ionic species with differing oxidation states. In a flow battery, an electrochemically active element refers to the chemical species that significantly participate in the redox reaction during the charge and discharge processes contributing to the energy conversions that ultimately enable the battery to deliver/store energy. As used herein, the term “electrochemically active element” refers to an element that constitutes at least 5% of the redox active materials participating in redox reactions during battery cycling after initial charging.

Electrolyte: A substance containing free ions that behaves as an ionically conductive medium. In a redox flow battery, some of the free ions are electrochemically active elements. An electrolyte in contact with the anode, or negative half-cell, may be referred to as an anolyte, and an electrolyte in contact with the cathode, or positive half-cell, may be referred to as a catholyte. With respect to vanadium sulfate acid redox flow battery systems, the electrolyte conventionally refers to vanadium species in an aqueous sulfuric acid solution. As used herein, the terms “anolyte” and “catholyte” refer to vanadium species in an aqueous “supporting solution.” The supporting solution, or supporting electrolyte, is an aqueous solution comprising sulfate ions, chloride ions, phosphate ions, protons, and other counterions introduced through added components that are not redox active. The anolyte and catholyte are often referred to as the negative electrolyte and positive electrolyte, respectively, and these terms can be used interchangeably.

Energy efficiency (EE): The product of coulombic efficiency and voltage efficiency. EE=CE×VE.

Half-cell: An electrochemical cell includes two half-cells. Each half-cell comprises an electrode and an electrolyte. A redox flow battery has a positive half-cell in which ions are oxidized, and a negative half-cell in which ions are reduced during charge. Opposite reactions happen during discharge. In an all vanadium redox flow battery, VO²⁺ ions in the positive half-cell are oxidized to VO₂+ ions (V⁴⁺ oxidized to V⁵⁺), and V³⁺ ions in the negative half-cell are reduced to V²⁺ ions during charge.

Proton: Hydrogen ions (H⁺) and water-solvated hydrogen ions, e.g., H₃O⁺, [H₅O₂]⁺, [H₉O₄]⁺.

Voltage efficiency: The voltage produced by the battery while discharging divided by the charging voltage

II. All-Vanadium Sulfate Acid Redox Flow Battery System

Redox flow batteries (RFBs) can provide electrical energy converted from chemical energy continuously, and are promising systems for energy storage to integrate renewable energies (e.g., solar and/or wind energy) into electrical supply grids. A RFB system comprises a positive half-cell and a negative half-cell. The half-cells are separated by a membrane or separator, such as an ion-conductive membrane or separator. The positive half-cell contains a catholyte and the negative half-cell contains an anolyte. The anolyte and catholyte are solutions comprising electrochemically active elements in different oxidation states. The electrochemically active elements in the catholyte and anolyte couple as redox pairs. During use, the catholyte and anolyte are continuously circulating through the positive and negative electrodes, respectively, where the redox reactions proceed providing the conversion between chemical energy and electrical energy or vice-versa. To complete the circuit during use, positive and negative electrodes of a RFB are electrically connected through current collectors with an external load.

Among various RFBs, the all-vanadium redox flow battery (VRFB) is currently considered one of the most promising candidates for grid scale energy storage. As those of ordinary skill in the art understand, the term “all-vanadium” means that the major redox active materials (i.e., at least 95%, at least 97%, or at least 99% of the redox active materials) participating in redox reactions during battery cycling (charging/discharging) after initial charging are vanadium ion redox pairs, i.e., V²⁺, V³⁺, V⁴⁺⁻ (VO²⁺), V⁵⁺ (VO₂₊). Other redox pairs may participate during initial charging of the redox flow battery.

The VRFB has several advantages such as high energy efficiency, quick response time, long lifespan, low self-discharge, no crossover issues, and low maintenance cost. However, the disadvantages of low energy density and poor stability and solubility of flow battery electrolyte solutions limit its applications.

Embodiments of the disclosed all-vanadium sulfate acid redox flow battery system have an anolyte comprising V²⁺ and V³⁺ in an aqueous supporting solution and a catholyte comprising V⁴⁺ and V⁵⁺ in an aqueous supporting solution, wherein the aqueous supporting solution comprises sulfate ions, protons, and a dual-component system comprising chloride ions and phosphate ions to stabilize and increase the solubility of the vanadium species. Without wishing to be bound by a particular theory, it appears that the combination of sulfate ions, protons, chloride ions, and phosphate ions may act synergistically to stabilize and increase solubility of the vanadium species. Suitable electrodes include graphite and/or carbon based electrodes, e.g., graphite felt, graphene, carbon felt, carbon foam electrodes, and so on. In some embodiments, the cathode and the anode are graphite electrodes. The system further includes a separator or membrane, such as an ion-conducting separator or membrane, separating the anolyte and the catholyte. The vanadium may be provided, for example, by dissolving vanadium (IV) oxide sulfate hydrate (VOSO₄.xH₂O) in sulfuric acid. The system is operable, without an external heat management device, over an operating temperature range of −5° C. to 50° C. with vanadium concentrations up to 2.5 M, such as from 1.0-2 M. The flow battery system may be operated as a closed system, which advantageously prevents rapid oxidation of V²⁺ and V³⁺ by air and minimizes electrolyte loss.

Traditional vanadium sulfate RFB systems previously have been limited to a maximum concentration of 1.5 M vanadium in the absence of chloride, e.g., 1.5 M VOSO₄-3.5 M H₂SO₄, over an operating temperature range of 10° C. to 35° C. At greater concentrations, V⁵⁺ species precipitate at elevated temperatures (e.g., at 50° C.) and V⁴⁺ species precipitate at lower temperatures (e.g., less than 0° C.). For example, at concentrations >1.7 M, V⁵⁺ has poor thermal stability at temperatures ≧40° C. V²⁺ and V³⁺ ions also may precipitate at lower temperatures.

The disclosed supporting solutions include a dual-component system to maintain electrolyte stability, thereby enabling increased vanadium concentration and thus the energy density of the system. In particular, the supporting solutions comprise a dual-component system that increases the solubility and stability of vanadium (IV) and vanadium (V) species. Effective components do not adversely react with the redox species, e.g., do not change the potential of redox reaction between V⁵⁺ and V⁴⁺, and do not react with sulfate. Effective components also are electrochemically stable over the flow battery operation voltage window. Desirably, the added components are effective at low concentrations relative to the vanadium species and sulfuric acid; high added component concentrations may exacerbate vanadium species precipitation, particularly V³⁺, and/or form sulfate precipitates.

Embodiments of the disclosed supporting solutions comprise sulfate ions (e.g., provided by sulfuric acid), protons, and a dual-component system comprising chloride ions and phosphate ions. In one embodiment, the protons are hydrogen ions (H⁺). In another embodiment, the protons are solvated hydrogen ions, e.g., H₃O⁺, [H₅O₂]⁺, [H₉O₄]⁺, and combinations thereof. In an independent embodiment, the protons are a combination of H⁺, H₃O⁺, [H₄O₂]⁺, and/or [H₉O₄]⁺. The protons may be provided by sulfuric acid. The chloride ions may be provided by an inorganic chloride salt. As used herein, the term “inorganic chloride salt” refers to a salt composed of metal or ammonium cations and chloride anions. The phosphate ions may be provided by an inorganic phosphate salt. The term “inorganic phosphate salt” refers to a salt composed of metal or ammonium cations and phosphate ions (H₂PO₄⁻, HPO₄²⁻, PO₄³⁻, and combinations thereof). The chloride ions and phosphate ions increase the stability and/or solubility of the vanadium species in the anolyte and catholyte.

For a number of years following the development of all-vanadium redox battery systems, those of ordinary skill in the art determined and stated, that is, the conventional wisdom and understanding was, that HCl could not be present in supporting solutions for all-vanadium redox battery systems and particularly in supporting solutions for the catholyte. For example, those of ordinary skill in the art believed that V⁵⁺ is unstable in HCl solutions. Further, HCl is corrosive and can produce dangerous HCl vapors and Cl₂vapors if overcharging, especially at higher temperatures. The inventors however, unexpectedly discovered that, in some cases, very low concentrations of chloride ion, e.g., 0.1-0.2 M, can be advantageous in stabilizing the electrolyte. The inventors also unexpectedly discovered that very low concentrations of chloride ion stabilize the anolyte and catholyte at higher temperatures, e.g., above 35° C. In a preferred embodiment of the invention, chloride ions in the disclosed supporting solutions are provided by an inorganic chloride salt. The use of an inorganic chloride salt makes the supporting solutions less corrosive than supporting solutions comprising HCl in addition to H₂SO₄, and also advantageously reduces system corrosion and HCl vapors within the system. Inorganic chloride salts are preferable to organic chloride salts, which can reduce V⁵⁺ to V⁴⁺.

Magnesium chloride, MgCl₂, was surprisingly found to be a particularly effective inorganic chloride salt. Without wishing to be bound by a particular theory, Mg²⁺ ions may further augment stability of the vanadium species. Although calcium chloride may be a more effective anti-freezing agent with respect to vanadium, calcium can react with sulfate to form precipitated calcium sulfate. In some embodiments, the supporting solution includes 0.05-0.1 M MgCl₂, thereby providing 0.1-0.2 M chloride ions.

While inorganic chloride salts are effective at higher temperatures within the redox flow battery's operating temperature range, inorganic chloride salts may not sufficiently stabilize the vanadium species at lower temperatures, e.g., less than 10° C. Thus, it is beneficial to include a second added component that stabilizes the anolyte and catholyte at low temperatures. Phosphate ions were found to stabilize the electrolytes at low temperatures and increase the solubility of vanadium (IV) species. In some embodiments, the supporting solution comprises 0.1-0.2 M phosphate ions. Phosphate ions may be provided by an inorganic phosphate salt. In some embodiments, phosphate ions are provided by ammonium phosphate. As used herein, the term “ammonium phosphate” refers to (NH₄)H₂PO₄, (NH₄)₂HPO₄, (NH₄₃PO₄, and combinations thereof. In certain examples, phosphate ions were provided by ammonium hydrogen phosphate (ammonium phosphate dibasic), (NH₄)₂HPO₄.

Cyclic voltammetry showed that MgCl₂and (NH₄)₂HPO₄do not react with the vanadium species (see, e.g., FIG. 1). MgCl₂did not noticeably affect the scans. However, (NH₄)₂HPO₄was found to have a catalytic effect, providing higher peak currents of electrolyte with positive scanning from V⁴⁺ to V⁵⁺ and V⁵⁺ to V⁴⁺. Furthermore, the peak voltage for V⁴⁺ to V⁵⁺ was shifted 75 mV, indicating that (NH₄)₂HPO₄facilitates the oxidation. On the negative side, presence of (NH₄)₂HPO₄shifted the peak voltage 100 mV and produced peak currents that were wider and higher, indicating that (NH₄)₂HPO₄increases redox activity in the anolyte.

When the anolyte and catholyte include the dual-component system comprising chloride ions and phosphate ions, the anolyte and catholyte may independently have a total vanadium concentration (i.e., V²⁺/V³⁺, V⁴⁺/V⁵⁺) greater than 1.0 M, such as within the range of 1.0-2.5 M, 1.5-2.5 M, or 1.5-2.0 M. Increasing the vanadium concentration correspondingly increases the energy capacity of the system. In some examples, the total vanadium concentration in each of the anolyte and the catholyte is 2 M, an increase of 33% compared to vanadium-sulfate systems without the disclosed dual-component system comprising chloride ions and phosphate ions, thereby providing an increased energy capacity of 33%. A vanadium-sulfate redox flow battery system without the disclosed dual-component system and having a total vanadium concentration of 1.5M has an energy density of 20.1 Wh/L, based on theoretical voltage and 80% of state of charge (SOC). In contrast, a vanadium-sulfate redox flow battery system with the disclosed dual-component system and having a total vanadium concentration of 2M has an energy density of 26.8 Wh/L, based on theoretical voltage and 80% SOC.

The cell reactions during battery charge/discharge cycling, after initial charging, are as shown below for a standard vanadium sulfate redox flow battery and a redox flow battery as disclosed herein:

At the negative electrode of a vanadium sulfate RFB:

V³⁺+e⁻ custom-character V²⁺ E°=−0.26 V

At the positive electrode of a vanadium sulfate RFB:

VO²⁺+2H⁺+e⁻ custom-character VO₂₊+H₂O E°=1.00 V

Embodiments of the disclosed all vanadium sulfate redox flow battery systems include an anolyte aqueous supporting solution and a catholyte aqueous supporting solution that comprise sulfate, chloride ions, and phosphate ions. The anolyte aqueous supporting solution and catholyte aqueous supporting solution may further comprise magnesium ions, ammonium ions, or magnesium and ammonium ions. The anolyte further comprises V²⁺ and V³⁺ under battery charge/discharge conditions. The catholyte further comprises V⁴⁺ and V⁵⁺ under battery discharge/discharge conditions.

In some embodiments, the anolyte aqueous supporting solution and catholyte aqueous supporting solution independently comprise 0.05-0.5 M, such as 0.1-0.2 M, chloride ions provided by an inorganic chloride salt. The inorganic chloride salt may be MgCl₂. In some embodiments, the anolyte aqueous supporting solution and catholyte aqueous supporting solution independently comprise 0.05-0.5 M, such as 0.1-0.2 M, phosphate ions. Phosphate ions may be provided by an inorganic phosphate salt. In some embodiments, the phosphate salt is ammonium phosphate, e.g., ammonium phosphate dibasic.

In some embodiments, the anolyte and catholyte supporting solutions independently comprise, or consist essentially of, H₂O, H₂SO₄, magnesium ions, chloride ions, ammonium ions, and phosphate ions. In an independent embodiment, the anolyte and catholyte independently comprise, or consist essentially of, H₂O, VOSO₄, H₂SO₄, MgCl₂, and ammonium phosphate. As used herein, “consists essentially of” means that the electrolyte includes no other components that materially affect battery performance. The electrolyte may include components that do not materially affect battery performance during charging/discharging, for example, non-electrochemically active species such as alkali metal cations. The anolyte and catholyte may independently comprise, or consist essentially of, H₂O, VOSO₄, H₂SO₄, 0.025-0.25 M MgCl₂, and 0.05-0.5 M ammonium phosphate. In some embodiments, the anolyte and catholyte may independently comprise, or consist essentially of, H₂O, VOSO₄, H₂SO₄, 0.05-0.1 M MgCl₂, and 0.1-0.2 M ammonium phosphate. In an independent embodiment, the anolyte and catholyte consist of H₂O, VOSO₄, H₂SO₄, MgCl₂, and ammonium phosphate. The anolyte and catholyte may independently consist of H₂O, VOSO₄, H₂SO₄, 0.025-0.25 M MgCl₂, and 0.05-0.5 M ammonium phosphate. In some embodiments the anolyte and catholyte independently consist of H₂O, VOSO₄, H₂SO₄, 0.05-0.1 M MgCl₂, and 0.1-0.2 M ammonium phosphate. The ammonium phosphate may be (NH₄)₂HPO₄.

In some embodiments, during battery charge/discharge, the anolyte has a total [V]≧1.0 M as V²⁺ and V³⁺, and the catholyte has a total [V]≧1.0 M as V⁴⁺ (e.g., VO²⁺) and V⁵⁺ (e.g., VO₂⁺). The anolyte and catholyte may independently have a total concentration of 1.0-2.5 M vanadium or 1.5-2.5 M vanadium, such as a concentration of 2 M vanadium.

In one embodiment, the anolyte and catholyte independently have a Cl:V molar ratio within a range of 1:50 to 1:2, such as from 1:25 to 1:4, or from 1:20 to 1:10. In some examples, the Cl:V molar ratio was 1:20.

In an independent embodiment, the anolyte and catholyte independently have a phosphate:V molar ratio within a range of 1:50 to 1:2, such as from 1:25 to 1:4, or from 1:20 to 1:10. In some examples, the phosphate:V molar ratio was 1:13.

In an independent embodiment, the anolyte and catholyte independently have a Mg:V molar ratio within a range of 1:100 to 1:1, such as 1:50 to 1:8, or from 1:40 to 1:20. In some examples, the Mg:V molar ratio was 1:40.

In an independent embodiment, the anolyte and catholyte independently have a NH₄⁺:V molar ratio within a range of 1:50 to 1:3, such as from 1:25 to 1:1.3, or 1:10 to 1:5. In some examples, the NH₄+:V molar ratio was 1:6.7.

In an independent embodiment, the anolyte and catholyte independently have a Cl:SO₄molar ratio within a range of 1:120 to 1:9, such as from 1:60 to 1:25, or 1:55 to 1:35. In some examples, the Cl:SO₄molar ratio was 1:55.

In an independent embodiment, the anolyte and catholyte independently have a phosphate:SO₄molar ratio within a range of 1:120 to 1:9, such as from 1:60 to 1:25, or 1:50 to 1:25. In some examples, the phosphate:SO₄molar ratio was 1:37 or 1:27.

In an independent embodiment, the anolyte and catholyte independently have a Cl:phosphate molar ratio within a range of 10:1 to 1:10, such as from 5:1 to 1:5, from 2:1 to 1:2, or from 1:1 to 1:2. In some examples, the Cl:phosphate molar ratio was 1:1.5.

In an independent embodiment, the anolyte and catholyte independently have a NH₄⁺:Mg molar ratio within a range of 60:1 to 1:5, such as from 12:1 to 1:1, or from 8:1 to 4:1. In some examples, the NH₄⁺:Mg molar ratio was 6:1 or 8:1.

In an independent embodiment, the anolyte and catholyte independently have a Mg:Cl molar ratio of 1:2.

In an independent embodiment, the anolyte and catholyte independently have a NH₄+:phosphate molar ratio within a range of 3:1 to 1:1, such as from 2.5:1 to 1.5:1. In some examples, the NH₄+:phosphate molar ratio was 2:1.

In an independent embodiment, the anolyte and catholyte independently comprise, consist essentially of, or consist of a solution prepared by combining H₂O, VOSO₄—H₂SO₄, a magnesium ion source, a chloride ion source, an ammonium ion source, and a phosphate ion source to provide 1.0-2.5 M vanadium, 4.5-6 M SO₄²⁻, 0.025-0.25 M Mg²⁺, 0.05-0.5 M Cl⁻, 0.05-1.5 M NH₄⁺, and 0.05-0.5 M phosphate. The anolyte and catholyte may independently comprise, consist essentially of, or consist of a solution prepared by combining H₂O, VOSO₄—H₂SO₄, a magnesium ion source, a chloride ion source, an ammonium ion source, and a phosphate ion source to provide 1.0-2.5 M vanadium, 4.5-6 M SO₄²⁻, 0.05-0.1 M Mg²⁺, 0.1-0.2 M Cl⁻, 0.1-0.6 M NH₄⁺, and 0.1-0.2 M phosphate. Suitable ion sources may include, but are not limited to, MgCl₂, (NH₄)₂HPO₄, (NH₄)H₂PO₄, (NH₄)₃PO₄, Mg(OH)₂, HCl, NH₄OH, H₃PO₄, NH₄Cl, MgHPO₄, Mg(H₂PO₄)₂, Mg₃(PO₄)₂, and NH₄MgPO₄.

In an independent embodiment, the anolyte and catholyte independently comprise, consist essentially of, or consist of a solution prepared by combining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide 1.0-2.5 M vanadium, 4.5-6 M SO₄²⁻, 0.025-0.25 M MgCl₂, and 0.05-0.5 M ammonium phosphate. The anolyte and catholyte may independently comprise, consist essentially of, or consist of a solution prepared by combining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide 1.0-2.5 M vanadium, 4.5-6 M SO₄²⁻, 0.05-0.1 M MgCl₂, and 0.1-0.2 M ammonium phosphate or 0.15-2 M ammonium phosphate. The ammonium phosphate may be (NH₄)₂HPO₄. In some examples, the anolyte and catholyte comprise 2 M vanadium, 5.5 M SO₄²⁻, 0.05 M MgCl₂, and 0.15 M (NH₄)₂HPO₄.

Embodiments of the disclosed all vanadium sulfate acid redox flow battery systems are operable over a temperature range from -5° C. to 50° C. and a current density range from 1 mA/cm²to 1000 mA/cm². In some embodiments, the disclosed systems are operable over a temperature range from −5° C. to 50° C. with a coulombic efficiency >90%, such as >95% or >97% at a current density from 10 mA/cm²to 320 mA/cm². The disclosed systems have an energy efficiency and a voltage efficiency >75%, >80%, or >85% over the operating temperature range at a current density from 10 mA/cm²to 320 mA/cm². Coulombic, energy, and/or voltage efficiency may depend at least in part on other parameters, such as flow field, pump rate, etc.

At −5° C., embodiments of the disclosed all vanadium sulfate redox flow battery systems including 2 M VOSO₄-3.5 M H₂SO₄, 0.1-0.2 M Cl⁻ provided by an inorganic chloride salt, and 0.1-0.2 M phosphate ions have a charge capacity and discharge capacity greater than 1.2 Ah, a charge energy greater than 2 Wh, and a discharge energy greater than 1.6 Wh over at least 75 cycles. In such embodiments, the voltage efficiency and energy efficiency is greater than 80%, with a coulombic efficiency greater than 95%. At 50° C., the same systems have a charge capacity and discharge capacity from 1.8-2.2 Ah, a charge energy from 2.9-3.2 Wh, and a discharge energy from 2.7-3 Wh over at least 25 cycles. The voltage and energy efficiencies are greater than 85%, with a coulombic efficiency greater than 95%. Over the operating temperature range of −5° C. to 50° C. and a current density range from 1 mA/cm²to 1000 mA/cm², the coulombic, energy, and voltage efficiencies of the disclosed systems remain substantially constant (i.e., varying by less than 5%) over at least 25 cycles, at least 50 cycles, at least 70 cycles, or at least 80 cycles (see, e.g., FIGS. 3 and 5). Charge and discharge capacities also remain substantially constant (see, e.g., FIGS. 2 and 4).

In some examples where the electrolytes included 2 M vanadium, 5.5 M sulfate, 0.05 M MgCl₂, and 0.15 M (NH₄)₂HPO₄, the coulombic efficiency was 97-99% and the energy efficiency was 80-89% over the temperature range from -5° C. to 50° C. at a current density of 50 mA/cm². The charge and discharge capacities were 1.3-2.2 Ah, the charge energy was 2-3.2 Wh, and the discharge energy was 1.7-2.7 Wh.

EXAMPLES
Materials and Methods

Vanadium sulfate was purchased from Noah Technology. All other chemicals were purchased from Sigma-Aldrich.

Solution Preparation and Stability Testing

The V⁴⁺ electrolyte solutions were prepared by dissolving VOSO₄.xH₂O in sulfuric acid aqueous solutions. The electrolyte solutions containing V²⁺, V³⁺ and V⁵⁺ cations were prepared electrochemically by charging the V⁴⁺ solutions in a flow cell. The stability evaluations were carried out at a temperature range of −5 to 50° C. in a temperature-controlled bath. For the added component effect study, a known amount of each added component (i.e., chloride, phosphate) was added into the electrolyte solutions before starting the stability evaluations. All the stability tests were carried out statically without any stirring or shaking. During the evaluation, each sample was scanned once a day for precipitation and solution color change. The concentrations of electrolyte solution were analyzed by inductively coupled plasma/atomic emission spectrometry (ICP/AES, Optima 7300DV, Perkin Elmer) techniques after appropriate dilution. Three emission lines were chosen for each element as a crosscheck for spectral interference. The calibration standards were matrix-matched in water.

Flow Cell Test of 2 M Vanadium Sulfate Acid Electrolyte Solution with Mixture of (NH₄)₂HPO₄and MgCl₂as Added Components

The flow cell tests were conducted at 50, 25 and −5° C. respectively in a single flow cell with graphite plates as positive and negative current collectors. Graphite felt (“GFD5”, SGL Carbon group, Germany) was employed as both positive and negative electrodes, with geometrical area of 10 cm²and heat treatment at 400° C. in air for 6h. A Nafion® 115 membrane (perfluorosulfonic acid/PTFE copolymer in the acid form) soaked in deionized water was used as a separator in the flow cell. The electrolyte solutions containing 2M V⁴⁺ ion with 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂in 5.5 M H₂SO₄were employed and the flow rate was fixed at 20 mL/min. Charging was conducted at 50 mA/cm²to 1.6 voltage (80% SOC) and discharged to 0.8 voltage with the same current density. To keep the charge balance at positive and negative sides, the V⁵⁺ solution (ca. 100% SOC) at positive side after the first charge process was replaced by the original V⁴⁺ solution with the same volume. Cycling test was performed to investigate capacity degradation and durability of the electrolyte 2 M VOSO₄-3.5 M H₂SO₄-0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂. Both the flow cell and electrolyte reservoirs are inside an environment chamber (Thermal Product Solution, White Deer, Pa.) maintained at −5° C. during the test.

Example 1
Stability of All-Vanadium Sulfate Acid Electrolytes Without Added Components

The highest concentration of all-vanadium sulfate acid electrolyte solution was 1.5 M over the operating temperature range of −5 to 50° C. as shown in Table 1. As shown in Table 1, species of V⁴⁺ and V⁵⁺ easily precipitated out from electrolyte solutions.

TABLE 1

Stability of Vⁿ⁺ sulfate acid solutions

(Italicized data shows the conditions and times

at which the indicated species precipitated.)

Time to

Vⁿ⁺ Species
Vⁿ⁺, M
Total Sulfate, M
T, ° C.
precipitation

V³⁺
2.0
5.5
−5
Stable >10 days

2.0
5.5
25
Stable >10 days

2.0
5.5
40
Stable >10 days

2.0
5.5
50
Stable >10 days

V⁴⁺

2.0

5.5

−5

Stable <18 hours

(VO²⁺)

2.0

5.5

25

Stable <95 hours

2.0
5.5
40
Stable >10 days

2.0
5.5
50
Stable >10 days

V⁵⁺
2.0
5.5
−5
Stable >10 days

(VO₂⁺)
2.0
5.5
25
Stable >10 days

2.0

5.5

40

Stable <95 hours

2.0

5.5

50

Stable <18 hours

1.5
5.0
50
Stable >10 days

Example 2
Stability of All-Vanadium Sulfate Acid Electrolytes with Added Components

Inorganic salt anti-freezing components were chosen because organic salts could react with V⁵⁺ species and reduce V⁵⁺ species to V⁴⁺ which is unsuitable for the all-vanadium flow battery system. To determine the best added components for the all-vanadium sulfate acid flow battery system, especially to improve the stability of V⁴⁺ and V⁵⁺ species, several inorganic salts were evaluated as shown in Table 2.

TABLE 2

Salts

Chemical Name
Formula
Lowest Practical Temp. ° C.

Calcium Chloride
CaCl₂
−29

Magnesium Chloride
MgCl₂
−15

Potassium Acetate
KC₂H₃O₂
−9

Lithium Chloride
LiCl

Ammonium Acetate
NH₄C₂H₃O₂

Sodium Chloride
NaCl
−9

Ammonium Sulfate
(NH₄)₂SO₄
−7

Urea
NH₂CONH₂
−7

Ammonium Phosphate
(NH₄)₂HPO₄

Dibasic

Ammonium Phosphate
NH₄H₂PO₄

Monobasic

Ammonium Chloride
NH₄Cl

Sodium Sulfate
Na₂SO₄

The added salts were evaluated at temperatures ranging from −5 to 50° C. The best salts were mixtures of ammonium phosphate dibasic and magnesium chloride, which could stabilize all vanadium species of V³⁺, V⁴⁺ and V⁵⁺ in a sulfate acid system. Although calcium chloride is a more efficient anti-freezing agent, calcium ions could react with sulfate acid to form undissolved calcium sulfate. Other anti-freezing agents or salts were either not strong enough to stabilize the V³⁺, V⁴⁺ and/or V⁵⁺ at temperature range of −5 to 50° C. or could react with sulfate acid to form undissolved salts. Ammonium phosphate dibasic and magnesium chloride individually were not strong enough to stabilize all the four vanadium species.

As shown in Table 1, V³⁺ is stable at temperatures ranging from −5 to 50° C. However, any added salt could cause precipitation in highly concentrated electrolyte solutions. Therefore it is preferable to include minimal concentrations of added salts to avoid precipitation of V³⁺. For chloride based salts, it has been shown that chloride ion can stabilize V⁵⁺ species in electrolyte solution (Li et al., Advanced Energy Materials 2011, 1:394-400). However, the stability studies showed that chloride ion did not improve the solubility of V⁴⁺ species. It also was demonstrated that ammonium phosphate dibasic did not improve the solubility and stability of V⁵⁺ electrolyte, but was a good salt for stabilizing V⁴⁺ species. The test results for all vanadium sulfate acid electrolytes with different salts are shown in Table 3.

TABLE 3

Stability of Vⁿ⁺ sulfate acid solutions with salts

(Italicized data shows the temperatures and times at which the indicated species precipitated.)

Time to

V (M)
SO₄²⁻(M)
Composition
Temperature ° C.
Salt
precipitate

2.0
5.5
V⁵⁺

50, 40, 0, −5, −10, −15
Blank

<4 days

V⁴⁺
50, 0, −5, −10, −15

<4 days

V³⁺
50, 0, −5, −10, −15

≧48 days

2.0
5.5
V⁵⁺
50, 40, 0, −5, −10, −15
0.05M MgCl₂

V⁴⁺
50, 0, −5, −10, −15

<1 day

V³⁺

50, 0, −5, −10, −15

<5 days

2.0
5.5
V⁵⁺
50, 40, 0, −5, −10
0.15M

<2 days

V⁴⁺
50, 0, −5, −10
(NH₄)₂HPO₄

<8 days

V³⁺
50, 0, −5, −10, −15

2.0
5.5
V⁵⁺
50, 40, 25, 0, −5
0.1M
≧14 (50, 40° C.) days

V⁴⁺
50, 0, −5
(NH₄)₂HPO₄
≧12 (−5° C.) days

V³⁺
50, 0, −5, −10
0.1M MgCl₂
>60 days

2.0
5.5
V⁵⁺
50, 40, 25, 0, −5
0.15M
≧21 (50, 40° C.) days

V⁴⁺
50, 0, −5
(NH₄)₂HPO₄
≧20 (−5° C.) days

V³⁺
50, 0, −5, −10, −15
0.05M MgCl₂
≧41 days (50° C.)

2.0
5.5
V⁵⁺
50, 40, 25, 0, −5 C.
0.2M
≧21 days (50, 40° C.)

V⁴⁺
50, 0, −5, −10, −15 C.
(NH₄)₂HPO₄
≧22 days (0° C.)

V³⁺
50, 0, −5, −10 C.
0.05M MgCl₂
≧21 (50° C.) days

The results demonstrated that a mixture of (NH₄)₂HPO₄and MgCl₂could stabilize 2 M all-vanadium sulfate electrolytes. Mixtures of 0.15 or 0.2 M (NH₄)₂HPO₄and 0.05 M MgCl₂were able to stabilize 2 M all-vanadium sulfate electrolytes for more than 20 days at a temperature range from −5 to 50° C.

Example 3
Properties and Performance of All-Vanadium Sulfate Acid Electrolyte Solutions with (NH₄)₂HPO₄and MgCl₂

Electrochemical properties of the all vanadium mixed acid electrolyte with and without added salts were investigated using the cyclic voltammogram (CV) method. The CV scan of 2.0 M VOSO₄and 3.5 M H₂SO₄electrolyte with and without 0.15 M (NH₄)₂HPO₄and 0.05 M MgCl₂are shown in FIG. 1.

As shown in FIG. 1, both ammonium phosphate dibasic and magnesium chloride were unreactive with the vanadium species. The CV scans of 2 M VOSO₄-3.5 M H₂SO₄electrolyte with and without 0.05 M MgCl₂were almost identical, indicating that magnesium chloride does not react with vanadium species at all. As shown, however, ammonium phosphate dibasic had a catalytic effect on both positive and negative electrolytes. When positive scanning from V⁴⁺ to V⁵⁺ and from V⁵⁺ to V⁴⁺, both the peak currents of electrolyte with ammonium phosphate dibasic were higher than that of electrolyte without ammonium phosphate dibasic. Also the peak voltage was shifted 75 mV from 1.15V to 1.075V when scanning from V⁴⁺ to V⁵⁺, indicating ammonium phosphate dibasic can help V⁴⁺ to V⁵⁺ transfer. For negative side electrolytes, the peak voltage was shifted about 100 mV from −0.835V to −0.735V and the peak currents were much wider and higher than that without ammonium phosphate dibasic electrolytes. The results indicated that ammonium phosphate dibasic not only can improve the stability and solubility of electrolytes, but also can increase the activity of redox reaction of electrolytes.

The cell performance of the all vanadium sulfate acid electrolyte solution was evaluated in a single RFB cell. As shown in FIGS. 2 and 4, charge capacity, discharge capacity, charge energy, and discharge energy remained substantially constant for at least 80 cycles at −5° C. and for at least 25 cycles at 50° C. As shown in FIGS. 3 and 5, respectively the columbic efficiency was 99% at −5° C. and 97% at 50° C. Energy efficiency was 80% at −5° C. and 89% at 50° C.; and voltage efficiency was about 80%. Voltage efficiency was similar to energy efficiency.

Example 4
Vanadate Solvate Structures

Understanding vanadate solvation structures facilitates rational design of redox flow battery electrolytes. Vanadium solvate structures were analyzed using density functional theory (DFT)-based calculations with the NWchem 6.1 package (developed and maintained by the Environmental Molecular Science Laboratory, at Pacific Northwest National Laboratory, Richland, Wash/; Valiev et al., Comput Phys. Commun. 2010, 181:1477). All of the calculations were done at the B3LYP theory level with dispersion correction (D3) using the 6-31G** (all-electron valence double zeta with polarization function) Gaussian-type basis set without any geometrical constraints. To capture the solvent ensemble effect, COSMO (an implicit solvent model) was employed with dielectric constant (ε) of 29.8 to represent the 5M sulfuric acid solution. Initially, the V⁵⁺ based solvates with water molecules alone were analyzed with COSMO model. For the global energy minimum structures, explicit solvent molecules (both H₂O and H30+) were evenly spread over the V⁵⁺ based solvates and subsequently treated with COSMO based solvation model (FIG. 6A). This cluster model approach with layers of explicit and implicit solvent molecules represents the highly acidic environment within the electrolyte system.

The vanadate solvate structure (FIG. 6A) revealed that the water molecule bonded with metal cation, i.e., [V₂O₃(H₂O)₇]⁴⁺, is prone to losing a proton due to its electrostatic interaction with the metal center even under highly acidic conditions (represented by H₃O⁺ ions). The relatively higher electronegativity of the vanadate cation leads to transfer of electron density from the molecular orbital of the water molecules to the empty orbitals of the metal cation. This charge transfer weakens the O—H bond in the coordinated water molecules and makes it more acidic, leading to proton loss and subsequently hydroxo-species which are prone to hydrolysis-based polymerization leading to V₂O₅precipitation. Although the proton loss can be controlled by temperature and pH of the solvent system, the higher charge cations (Z≧4+) could still be susceptible to proton loss. Hence it is advantageous to counter the initial proton loss through addition of suitable added components, which can disrupt the hydrolysis-based polymerization by coordinating directly with the vanadate solvate structure.

A Mg²⁺-based added component was selected because its lower electronegativity can inhibit the proton loss from the coordinated water molecules and because it could also directly coordinate with the vanadate molecule through a bridging oxygen configuration. DFT analysis revealed a lower formation energy (˜0.30 eV) for the Mg²⁺-coordinated vanadate solvate structure (FIG. 6B) corroborating the advantages of the Mg²⁺-based added component. Guided by this computational analysis, electrolytes were prepared with a high concentration (≧2M) of vanadate and a low concentration (0.1 M) of MgCl₂as an added component system. The ⁵¹V NMR analysis of pristine and added component vanadate-based electrolytes is shown in FIG. 7. The added component-based electrolyte reveals the formation of new species indicated by a low-field resonance (peak B ˜−592 ppm) relative to the pristine vanadate electrolyte (peak A ˜−577 ppm), which corroborated the DFT-based prediction.

To further validate the identity of this new species under added component conditions, the ⁵¹V NMR chemical shift was calculated for [V₂O₃(H₂O)₇]⁴⁺ and [VO₂Mg(H₂O)₉]³⁺ solvates as −562 and -584 ppm, respectively. The DFT derived chemical shifts are in good agreement with the observed chemical shifts of peak A and peak B, and confirm the formation of a Mg²⁺-coordinated vanadate solvate structure. Such Mg²⁺ coordination can effectively disrupt the hydrolysis-based network formation between successive vanadate molecules and thereby suppress the V₂O₅precipitation.

Additional subject matter is disclosed in U.S. Pat. No. 8,628,880, which is incorporated by reference in its entirety herein.

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.

ALL-VANADIUM SULFATE ACID REDOX FLOW BATTERY SYSTEM

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