METHODS FOR THE PREPARATION OF ELECTROLYTES FOR CHROMIUM-IRON

REDOX FLOW BATTERIES

Majid KESHAVARZ and Aravamuthan VARADARAJAN

RELATED APPLICATIONS [001] The present application claims priority to U.S. Nonpro visional Application Serial No. 12/721,411, filed on March 10, 2010, which is herein incorporated by reference in its entirety.

BACKGROUND

Technical Field

[002] The present disclosure relates to methods for preparing electrolytes and electrolyte solutions useful in reduction-oxidation (redox) flow batteries.

Discussion of Related Art

[003] There is an increasing demand for novel and innovative electric power storage systems. Reduction-oxidation (redox) batteries have become an attractive means for such energy storage. In certain applications, a redox flow battery or a redox flow cell may include positive and negative electrodes disposed in separate half-cell compartments. The two half-cells may be separated by a porous or ion-selective membrane, through which ions are transferred during a redox reaction. Electrolytes (anolyte and catholyte) are flowed through the half-cells as the redox reaction occurs, often with an external pumping system. In this manner, the membrane in a flow cell battery operates in an aqueous electrolyte environment. In some applications, an iron-ion containing aqueous hydrochloric acid solution may be used as the catholyte, while a chiOmium-ion containing aqueous hydrochloric acid solution may be used as the anolyte. In some applications, a mixture of chromium and iron containing solutions may be used on both sides of the redox cell. The use of mixed reactants eliminates the requirement for a highly- selective membrane since the composition of both half cells is identical in the discharged state.

[004] In some redox flow batteries, certain metal impurities contained in the electrolyte solution can cause side reactions at the negative electrode, which can result in the evolution of hydrogen gas that adversely affects the coulombic efficiency of the battery. While the use of high-purity raw materials such as high-grade iron chloride and high-grade chromium chloride can suppress such hydrogen gas-forming reactions, such materials are typically too expensive for use in redox batteries on a commercial scale.

[005] Therefore, there exists a need to develop methods for preparing and purifying electrolyte solutions from inexpensive raw materials.

SUMMARY

[006] Consistent with some of the embodiments described herein, a method for preparing a redox flow battery electrolyte is provided. In some embodiments, the method includes dissolving a raw material that includes at least one source of chromium ions in a higher oxidation state in an acidic aqueous liquid to form a solution of chromium ions; subjecting the solution to a reductant to form Chromium (III) ions; and adding a source for Iron (II) ions.

[007] Further embodiments and aspects of the invention are discussed with respect to the following figures, which are incorporated in and constitute a part of this specification. BRIEF DESCRIPTION OF THE DRAWINGS

[008] Figure 1 illustrates a redox flow cell filled with electrolyte solution consistent with the embodiments described herein.

[009] Figure 2 illustrates a method of producing an electrolyte according to some embodiments of the present invention.

[010] Relative sizes and distances depicted in the figure are for convenience of illustration and have no further meaning.

DETAILED DESCRIPTION

[01 1] Some embodiments of the present invention provide for the processing of raw materials that are sources of chromium (VI) ions and/or iron. In some embodiments, the raw materials contain impurities, for example the raw materials are also a source of certain transition- metal ions. However, in many of the raw materials, the impurities are present in concentrations insufficient to significantly affect performance of a battery that utilizes the resulting electrolyte.

[012] Consistent with some embodiments, the methods described herein include dissolving a raw material containing chromium in a higher oxidation state such as chromium (VI) in an acidic aqueous solution. In some embodiments, the acidic aqueous solution is then treated with a reductant. The reductant may be an organic reagent, an inorganic reagent, an electrochemical process, or any other way of reducing the high oxidation state chromium to a Cr(III) state.

[013] Following the reduction treatment, in some embodiments iron metal is added, which can also serve to complete the reduction of the higher oxidation state chromium and provide Fe(II) (Fe ²⁺ ) to form an electrolyte formulation. Further, the acid concentration of the solution may be adjusted. [014] Consistent with some embodiments, the methods describe herein allow for the production of electrolyte solutions with low or minimal amounts of ionic impurities from sources containing high oxidation state chromium, for example Cr(VI). In some embodiments, the reduction process can be electrochemical. In some embodiments, the reduction process can be by addition of an organic reductant such formic acid, methanol, glucose or sucrose. In some embodiments, the reduction process can be by addition of an inorganic reductant such as sodium bisulfite or sodium dithionite. In some embodiments, the excess reductant and volatile impurities can be distilled off once the reduction process is complete, resulting in a redox electrolyte solution containing chromium ions. Iron is subsequently added in the same pot as part of the reducing mixture and or electrolyte constituent to adjust concentration of ions in the resulting electrolytes.

[015] Low-cost raw materials containing sources of chromium ions and/or iron ions are desirable in that they can be processed for use as reduction-oxidation (redox) battery electrolytes. Some of the raw materials contain chromium in the +6 oxidation state (Chromium (VI)), including chromium trioxide (Cr0 ₃ ), sodium dichromate (Na ₂ Cr ₂ 0 ₇ ), potassium dichromate (K ₂ Cr ₂ 0 ), sodium chromate (Na ₂ Cr0 ₄ ), potassium chromate (K ₂ Cr0 ₄ ), and chromyl chloride (Cr0 ₂ Cl ₂ ). Those raw materials may also contain impurities, such as sources of certain metal species. However, these impurities may be present in the raw materials in small amounts that do not interfere with the battery operation or are reduced by the reductant to non-interfering species. Accordingly, some methods described herein provide an aqueous redox electrolyte substantially free of certain metal impurities. [016] As used in the present specification, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicates otherwise.

[017] As described herein, the term "reductant" refers to a reducing agent having strong reducing properties with a high overpotential for hydrogen reduction in aqueous acidic solutions. An example of high hydrogen overpotential would include reducing agents that exhibit little to no evolution of hydrogen gas in an aqueous acidic solution. eductants utilized in some embodiments exhibit little or no interference (in their oxidized form) with the electrochemical process of the resulting electrolyte.

[018] As described herein, the term "impurities" refers to those materials that deposit under reductive conditions and have low hydrogen overpotential. Examples of low hydrogen overpotential include materials that evolve hydrogen gas in an aqueous acidic environment.

[019] Figure 2 illustrates a process 200 of producing an electrolyte according to some embodiments of the present invention. As shown in Figure 2, in step 202 a raw material is dissolved or extracted in an acidic aqueous solution to form a solution of chromium ions. The solution may also include iron ions. In certain embodiments, the acidic aqueous solution may include hydrochloric acid (HC1). In some embodiments, HC1 may be partially or completely replaced with other Br0nsted acids, for example, sulfuric acid, sulfonic acid, phosphoric acid, phosphonic acid, perchloric acid, and mixtures thereof. In certain embodiments, a mixture of hydrochloric acid (HQ) and sulfuric acid may be used.

[020] The raw material dissolved in the acidic aqueous solution in step 202 may further contain sources of certain impurities. Such impurities may include, but are not limited to, metal ions such as antimony, bismuth, cadmium, cobalt, copper, lead, manganese, molybdenum, niobium, nickel, titanium, tungsten, and zinc. In some embodiments, the raw material may contain sources of nickel ions, cobalt ions and/or sources of copper ions. In some embodiments, the raw material can be chromium trioxide.

[021] In some embodiments, the raw material can be a sodium or potassium dichromate, can be a sodium or potassium chromate, can be chromyl chloride, can be iron metal, or may be a combination of these materials. In some embodiments, raw materials are chosen such that the typical impurity levels are low enough to be within an acceptable range of concentrations. For

9-1- example, in some embodiments, the level of Co ions is below 10 ppm and may be between 1 and 10 ppm. In some embodiments, the level of Ni ²⁺ ions is below 10 ppm and may be between 1 and 10 ppm. In some embodiments, the level of Cu ²⁺ ions is below 10 ppm and may be between 1 and 10 ppm. In some embodiments, the level of Cu ²⁺ ions is below 1.0 ppm and may be between 0.1 and 1.0 ppm.

[022] From step 202, process 200 proceeds to reduction process 204 where a reducing step is performed to reduce the chromium ions in the solution to the chiOmium (III) (Cr ³⁺ ) oxidation state. Reduction can be accomplished either chemically by addition of a reductant or electrochemically.

[023] Electrolysis can be accomplished by submerging a single electrode or pairs of spaced electrodes in the solution. A few examples of the reduction process are provided below utilizing various electrode systems. One skilled in the art will recognize other processes that may also be utilized in reduction process 204. These processes are also considered to be within the scope of this disclosure.

[024] In some embodiments, reduction of Cr(VI) to Cr(III) can be effected

electrochemically. In certain circumstances, the reduction of Cr(VI) to Cr(III), can be accompanied by forming Fe ions using iron electrodes. The iron electrode should have impurity levels within the acceptable range of concentrations and therefore be substantially free from metals such as Cu, Ni, Pd, Pt, Au, and Ru, which when present in the electrolyte contribute to performance loss in a flow cell that utilizes the resulting electrolyte. However, Oxidation of Fe ²⁺ to Fe ³⁺ and precipitation of Fe ₂ 0 ₃ and chromite (FeCr ₂ 04) can interfere with an

electrochemical process.

[025] In certain embodiments, electrolysis can be carried out with Cr0 ₃ in sulfuric acid. A variety of electrodes, such as carbon felt, glassy carbon, polypropylene graphite composite, titanium, mixed metal oxide (MMO), or others, can be used. Chromium(III) sulfate and oxygen are formed. The chromium sulfate can be converted to the hydroxide using calcium hydroxide and then to the chloride with HC1. Iron can be directly dissolved in the chromium chloride solution, or separately dissolved in HC1 and mixed with CrCl ₃ . Lower concentrations of Cr(VI) and a low current density can be used. The current density usually drops as reduction process 204 proceeds.

[026] In certain embodiments, electrolysis can be carried out with Cr0 ₃ in hydrochloric acid. A variety of electrodes, such as carbon felt, glassy carbon, polypropylene graphite, or others, can be used. CliiOmium(III) chloride and chlorine are formed. Iron can be directly dissolved in the chromium chloride solution, or separately dissolved in HC1 and mixed with CrCl ₃ . However, in some embodiments lower concentration of Cr(VI) and low current density should be used. For example, in some embodiments the Cr(VI) concentration is kept below 1.0 M. In some embodiments, the Cr(VI) concentration is kept below 1.0 M. In some embodiments, the Cr(VI) concentration can be below 0.5 M or even below 0.1 M. Further, in some

embodiments the acid or base concentration can be between 1 and 4 M. [027] The current density usually drops as the reduction proceeds. In some embodiments, the voltage between the electrodes can be kept between 1 and 5 voltages and the electrode spacing between 1 and 5 centimeters. However, other concentrations, electrode voltages, and cell spacings can be utilized.

[028] In certain embodiments, Cr(VI) can be reduced using inorganic reducing agents. The reducing agent can be sodium dithionite (Na ₂ S ₂ 0 ₄ ) or sodium bisulfite (NaHS0 ₃ ), for example, resulting in a reduction product that contains Na salts along with Cr(III). The solution can be used for electrolyte preparation as sodium ion is not expected to be of serious

consequence. However, if necessary, the sulfate can be converted to the hydroxide, filtered and then redissolved in HCl to obtain the chloride, which may also eliminate most of the sodium ions in the final electrolyte. Iron and HCl can be added to obtain the final electrolyte formulation.

[029] In certain embodiments, reduction of Cr(VI) can be effected by organic reducing agents. Examples of organic reducing agents include monohydric alcohols such as methanol, ethanol and polyhydric alcohols, such as ethylene glycol and glucose. Formic acid for conversion of Cr0 ₃ to chromium formates is also an example.

[030] In certain embodiments, formic acid can be used for the reduction of Cr(VI) as defined in Equation 1 :

Cr0 ₃ + 1.5 HCOOH + Fe + 5 HCl - CrCl ₃ + FeCl ₂ + 1.5 C0 ₂ + 3 ¾0 + H ₂ (1) Stoichiometric amount (1.5 equivalents) or less than stoichiometric amount (1.0 equivalent) of formic acid can be used and iron powder is added to complete the reduction and provide the source for Fe2+ ions in the electrolyte solution.

[031] In certain embodiments, oxalic acid can be used for the reduction as shown in Equation 2. However, the oxalate salt of chromium(III) may form and in such case can be converted to the chloride through the hydroxide.

Cr0 ₃ + 1.5 (COOH) ₂ + Fc + 5 IIC1 -> CrCl ₃ + FeCl ₂ + 3 C0 ₂ + 3 H ₂ 0 + ¾ (2)

[032] In certain embodiments methanol is the reductant as depicted in Equation 3 : C1O3 + 0.5 CH ₃ OH + Fe + 5 HC1 - CrCl ₃ + FeCl ₂ + 0.5 C0 ₂ + 2.5 H ₂ 0 + H ₂ (3)

[033] Reducing agents with higher carbon content can also be employed for the reduction. As shown in Equation 4, sugars such as glucose can be used in certain embodiments:

C1 3 + 0.125 C ₆ H ₁₂ 0 ₆ + Fe + 5 HC1 -» CrCl ₃ + FeCl ₂ + 0.75 C0 ₂ + 2.25 H ₂ 0 + ¾ (4)

[034] In certain embodiments, higher carbohydrates such as cane sugar, raw sugar or sucrose can also be employed. However, it may be desirable to use smaller organic molecules in order to keep the level of organic carbon (from partially oxidized organic moieties) in the final electrolyte low, as these organic impurities may interfere with the redox processes in the battery.

[035] In certain embodiments, where Cr(VI) species and HC1 is evolved, considerable amount of chlorine gas may evolve. In those embodiments, provision can be made for venting the gas, for example, into the hood. The generation of chlorine can be minimized by using less concentrated HC1 and/or carrying out the initial addition of the reducing agent at sub-ambient temperature, for example, using an ice-bath. In some embodiments, the Cr0 ₃ concentration can be between 1 and 3 M and the HCL concentration between 1 and 12 M.

[036] Since the starting Cr(VI) materials contain considerably lower levels of deleterious impurities such as Ni, CO and Cu ions, the reduction method yields electrolytes with low levels of these impurities. In some embodiments, the amount of nickel ions may be about 10 ppm or less. In some embodiments, the amount of nickel ions may be about 1 ppm or less. In some embodiments, the amount of nickel ions may be about 10 ppb or less. In some embodiments, the amount of cobalt and copper ions may be about 10 ppm or less. In some embodiments, the amount of cobalt and copper ions may be about 10 ppb or less.

[037] Upon completion of the reduction reaction, process 200 proceeds to process step 206, where iron powder can be added to produce the electrolyte of desired composition. In certain embodiments, iron chloride can be prepared separately and added to the Cr(III) solution after reduction.

[038] From process step 206, process 200 may proceed to filtering step 208. To remove insoluble materials, the solution may be filtered. Suitable filtration methods may include the use of a solid filter, such as filter paper or fritted discs, and a filter aid such as Celite ^® , Celpure ^® or Celatom ^® .

[039] Once filtered, in step 210 concentrated HC1 or HC1 gas may be added to solution to achieve the desired acid concentration. After the desired ionic species are formed, the desired electrolyte can be formulated for use as a redox battery electrolyte by adding the appropriate amount of catalysts and additives.

[040] Figure 1 illustrates a redox flow cell 100 filled with electrolyte solution consistent with embodiments described. Redox flow cell 100 includes two half-cells 108 and 110 separated by permeable membrane 106. Half-cells 108 and 110 include electrodes 102 and 104 respectively, in contact with electrolytes 124 and 126, respectively, such that an anodic reaction occurs at the surface of one of electrodes 102 or 104 and a cathodic reaction occurs at the surface of the other one of electrodes 102 or 104. In some embodiments, multiple redox flow cells 100 can be electrically coupled (e.g., stacked) either in series to achieve higher voltage and/or in parallel in order to achieve higher current. When filled with electrolyte, one half-cell (e.g., 108 or 110) of redox flow cell 100 contains an anolyte and the other half-cell contains a catholyte, the anolyte and catholyte being collectively referred to as electrolytes. Reactant electrolytes may be stored in separate tanks and dispensed into the cells 108 and 110 via pipes coupled to cell input/output (I/O) ports 112, 114 and 116, 118, respectively, often using an external pumping system.

[041] At least one electrode 102 and 104 in each half-cell 108 and 110 provides a surface on which the redox reaction takes place and from which charge is transferred. Redox flow cell 100 operates by changing the oxidation state of its constituents during charging or discharging. The two half-cells 108 and 110 are connected in series by the conductive electrolytes, one for anodic reaction and the other for cathodic reaction. In operation (i.e., charge or discharge), electrolyte (i.e., anolyte or catholyte) is flowed through half-cells 108 and 1 10 through I/O ports 112, 114 and 116, 118, respectively, as the redox reaction takes place. Positive ions or negative ions pass through permeable membrane 106 separating the two half-cells 108 and 110 as the redox flow cell 100 charges or discharges. Reactant electrolytes 124 and 126 are flowed through half-cells 108 and 1 10, as necessary and in a controlled manner to supply electrical power or be charged by load/source 120.

[042] In the examples below, the following abbreviations have the following meanings. If an abbreviation is not defined, it has its generally accepted meaning. milliliters

liters

degrees Celsius

molarity

normality

parts per million

parts per billion

nitrogen gas cm = centimeters

HN0 ₃ = nitric acid

PbCl ₂ = lead chloride

Co = cobalt

Cu copper

Ni = nickel

C1O3 = chromium trioxide

K ₂ Cr ₂ 0 ₇ = potassium dichromate

Na ₂ S ₂ 0 ₄ = sodium dithionite

DI = deionized

g = gram

hr = hour

min minute

rt room temperature

Cr(VI) REDUCTION PROCEDURES

A. Electrochemical reduction Example 1

[043] Into a solution (50 mL) of Na ₂ Cr ₂ 0 ₇ (0.025 M) in H ₂ S0 ₄ (4 M) two carbon felt electrodes with active area 3X2 cm ² and separated by 2 cm were immersed in the solution. Using a power supply, a voltage of 3.4 V to 5.5 V was applied over a period of 4 hr, during which time the current dropped from 512 mA to 109 mA. Aliquots were taken at different time intervals and the concentrations of Cr ³⁺ and Cr ⁶⁺ were measured using UV-VIS spectrophotometry by measuring the absorbance at 586 nm and 349 nm, respectively. The electrodes were removed, iron powder (0.1 g, 1.79 mmol) was added, and the mixture stirred to dissolve the iron. After filtration, the concentrations of Cr ³⁺ and Fe ²⁺ were measured by UV-VIS spectrophotometry to be 0.062 M and 0.063 M, respectively, and the acid concentration by pH titration to be approximately 4.2 M. The concentrations of Co, Ni and Cu were determined by ICPMS to be 0.046 ppm, 0.0015 ppm and 0.0007 ppm, respectively.

Example 2

[044] Into a solution (50 mL) of Na ₂ Cr ₂ 0 ₇ (0.025 M) in HC1 (4 M) two carbon felt electrodes with active area 4X2.5 cm ² and separated by 2 cm were immersed in the solution. Using a power supply, a voltage of 2.5 V to 4.2 V was applied over a period of 4 hr, during which time the current dropped from 515 mA to 22 mA. Aliquots were taken at different time intervals and the concentrations of Cr ³⁺ and Cr ⁶⁺ were measured using UV-VIS

spectrophotometry by measuring the absorbance at 587 nm and 346 nm, respectively. The electrodes were removed, iron powder (0.047 g, 0.84 mmol) was added, and the mixture stirred to dissolve the iron. After filtration, the concentrations of Cr ³⁺ and Fe ²⁺ were measured by UV- VIS spectrophotometry to be 0.049 M and 0.046 M, respectively, and the acid concentration by pH titration to be approximately 3.8 M. The concentrations of Co, Ni and Cu were determined by ICPMS to be 0.017 ppm, 1.3 ppm and 0.047 ppm, respectively.

Example 3

[045] Into a solution (50 mL) of Cr0 ₃ (0.05 M) in HC1 (4 M) two carbon felt electrodes were immersed (active area 3X3 cm ² separated by 2 cm. Using a power supply, a voltage of 3.9 V to 4.3 V was applied over a period of 2.45 hr, during which time the current dropped from 519 mA to 34 mA. Aliquots were taken at different time intervals and the concentrations of Cr ³⁺ and Cr ⁶⁺ were measured using UV-VIS spectrophotometry by measuring the absorbance at 580 nm and 340 nm, respectively. The electrodes were removed, iron powder (0.093 g, 1.68 mmol) was added, and the mixture stirred to dissolve the iron. After filtration, the concentrations of Cr and Fe ²⁺ were measured by UV-VIS spectrophotometry to be 0.06 M and 0.09 M, respectively, and the acid concentration by pH titration to be approximately 0.27 M. The concentrations of Co, Ni and Cu were determined by ICPMS to be 0.021 ppm, 0.00076 ppm and 0.00032 ppm, respectively.

B. Chemical reduction with inorganic reducing agents

Example 4

[046] To a solution of sodium dichromate (1 M, 50 mL) in 4 M HC1, NaHS0 ₃ (9 g; 85 mmol) was added in small portions over 30 min, during which time the temperature rose from ambient to 77 °C. The mixture was allowed to cool and Fe powder (3.63 g, 65 mmol) was added in small amounts over 1 hr. The solution was stirred for 12 hr, diluted with water (25 mL), and filtered through Celatom FW-80. The filtrate was analyzed by UV-VIS spectrophotometry for Cr ³⁺ and Fe ²⁺ concentrations which were 1.82 M and 0.7 M, respectively.

Example 5

[047] To a solution of Cr0 ₃ (7.5 g; 0.075 mole) in 4 M HC1 (50 mL), sodium dithionite, Na ₂ S ₂ 0 ₄ , (6.53 g; 0.0375 mol) was added in small amounts over 2 hi'. The reaction temperature rose from 22 °C to 70 °C. The mixture was cooled to ambient temperature and iron powder (2.8 g; 0.05 mole) was added in small amounts over 3.5 hr. The mixture was stirred for 12 hr and filtered as above. The concentrations of Cr ³⁺ and Fe ²⁺ were determined spectrophotometrically to be 1.36 M and 0.85 M respectively, and the acid concentration by pH titration to be 0.09 M. The concentrations of Co, Ni and Cu were 0.58 ppm, 0.11 ppm and 0.23 ppm respectively, as measured by ICPMS.

Example 6

[048] To a solution of Cr0 ₃ (3.75 g; 0.0375 mole) in 4 M HC1 (25 mL), sodium dithionite, Na ₂ S ₂ 0 ₄ , (1.63 g; 0.0375 mole) was added in small amounts over 3 hr. The reaction temperature rose from 22 °C to 65 °C. The mixture was cooled to ambient temperature and iron powder (1.4 g; 0.025 mole) was added in small amounts over 4 hr. An additional quantity of dithionite (0.81g; 0.019 mole) was added slowly. The mixture was treated with 12 M HC1 (2 mL). The mixture was stirred for 12 hr and filtered as above. The concentrations of Cr ³⁺ and Fe were determined spectrophotometrically to be 1.77 M and 0.93 M, respectively, and the acid concentration by pH titration to be 0.33 M. The concentrations of Co, Ni and Cu were 0.44 ppm, 0.57ppm and 0.47 ppm, respectively, as measured by ICPMS.

C. Chemical reduction with organic reducing agents

Example 7

[049] A solution of Cr0 ₃ (150 g; 1.5 mole) in 9 M HC1 (1000 mL) was treated with HCOOH (103.57 g; 2.25 mole) in small amounts over 18 hr during which the temperature varied from 33 °C to 47 °C. After the addition was complete, the mixture was heated to 57 °C and stirred for 8 hr to complete the reduction. The mixture was cooled to ambient temperature and Fe powder (325 mesh; 55 g; 1 mole) was added in small amounts. After completion of addition, the mixture was stirred at ambient temperature for 16 hr, and filtered through a bed of Celatom FW-80. The acid concentration was raised by passing HC1 gas into the solution. The

concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.4 M and 0.92 M, respectively. The acid concentration was measured by pH titration to be 2.25 M. Concentrations of other elements measured by ICPMS were: C 690 ppm; Co 0.01 ppm; Ni 1.2 ppm; and Cu below detection limit.

Example 8

[050] A solution of Cr0 ₃ (15 g; 0.15 mole) in 1 M HC1 (100 mL) was treated with HCOOH (10.32 g; 0.225 mole) in small amounts over 0.25 hr during which the temperature varied from 24 °C to 54 °C. HCl (12M; 30 mL) was added in three installments. The mixture was then heated to 68 °C over 19 hr and more HCl (12 M; 17 mL) was added. The solution was cooled to ambient temperature and Fe powder (5.52 g; 0.1 mole) was added gradually. After the addition was complete, the mixture was stirred for 8 In- and filtered through a bed of Celatom FW-80. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 0.9 M and 0.66 M, respectively. The acid concentration was measured by pH titration to be 0.16 M. Concentrations of other elements measured by ICPMS were: C 690 ppm; Co 0.0057 ppm; Ni 0.95 ppm; and Cu 0.001 lppm.

Example 9

[051] A solution of Cr0 ₃ (75 g; 0.75 mole) in 4.5 M HCl (500 mL) was treated with HCOOH (51.78g; 1.125 mole) in small amounts over 4 hr during which the temperature varied from 26 °C to 74 °C. When the solution cooled to 30 °C, it was then heated to 69 °C over 19 hr. The solution was cooled to ambient temperature and Fe powder (28 g; 0.5 mole) was added gradually. During the addition of Fe, HCl (12M; 70 mL) was added in four installments. After the addition was complete, the mixture was stirred for 19 hr and filtered through a bed of Celatom FW-80. The acid concentration was raised by passing HCl gas into the solution. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.36 M and 0.96 M respectively. The acid concentration was measured by pH titration to be 2.37 M. Concentrations of other elements measured by ICPMS were: C 610 ppm; Co 0.048 ppm; Ni 2 ppm; and Cu 0.32ppm.

Example 10

[052] A solution of Cr0 ₃ (75 g; 0.75 mole) in 6 M HCl (500 mL) was treated with HCOOH (51.78g; 1.125 mole) in small amounts over 8 hr during which the temperature varied from 31 °C to 78 °C. When the solution cooled to 27 °C, it was then heated to 69 °C over 19 hi ¹ . The solution was cooled to ambient temperature and Fe powder (28 g; 0.5 mole) was added gradually. During the addition of Fe, HC1 (12M; 40 mL) was added in four installments. After the addition was complete, the mixture was stirred for 22 hr and filtered through a bed of Celatom FW-80. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.35 M and 0.9 M, respectively. The acid concentration was measured by pH titration to be 0.99 M. Concentrations of other elements measured by ICPMS were: C 570 ppm; Co 0.048 ppm; Ni 0.88 ppm; and Cu O.Olppm. The amount of total of organic carbon (TOC) was analyzed to be less than 0.4%.

Example 11

[053] A solution of Cr0 ₃ (75 g; 0.75 mole) in 6 M HC1 (500 mL) was treated with HCOOH (34.52 g; 0.75 mole) in small amounts over 1 hr during which the temperature varied from 23 °C to 60 °C. When the solution cooled to 30 °C, it was then heated to 63 °C over 11 hr. The solution was cooled to ambient temperature and Fe powder (325 mesh, 28 g; 0.5 mole) was added gradually. During the addition of Fe, HC1 (12M; 84 mL) was added in four installments. After the addition was complete, the mixture was stirred for 24 hr and filtered through a bed of Celatom FW-80. The solution was concentrated by distilling the solvent under vacuum. The residual material (260 mL) was treated with HC1 (6 M; 150 mL) and H ₂ 0 (50 mL). The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.32 M and 0.99 M, respectively. The acid concentration was measured by pH titration to be 2.25 M. Concentrations of other elements measured by ICPMS were: C 100 ppm; Co 0.014 ppm; Ni 1.5 ppm; and Cu 0.18 ppm. Example 12

[054] A solution of 6 M HC1 (100 mL) was cooled in an ice-bath to 4 °C. Cr0 ₃ (15 g; 0.15 mole) was added in small amounts over 1 hr. HCOOH (5.75g; 0.15 mole) was added in small amounts over 5 min. The mixture was allowed to warm to ambient temperature and then heated to 66 °C over 11 hr. The solution was cooled to ambient temperature and Fe powder (5.6 g; 0.1 mole) was added gradually. After the addition of Fe, HC1 (12M; 20 mL) was added in four installments. The mixture was stirred for 20 hr and filtered through a bed of Celatom FW- 80. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.26 M and 0.93 M, respectively. The acid concentration was measured by pH titration to be 1.31 M.

Concentrations of other elements measured by ICPMS were: C 670 ppm; Co 0.24 ppm; Ni 1.6 ppm; and Cu 0.078 ppm.

Example 13

[055] A solution of Cr0 ₃ (3.75 g; 0.0375 mole) in 4 M HC1 (50 mL) was treated with oxalic acid, (COOH) ₂ (7.09 g; 0.056 mole) in small amounts over 3 hr during which the temperature varied from 23 °C to 84 °C. After the addition, when the solution cooled to ambient temperature, Fe powder (325 mesh, 1.4 g; 0.025 mole) was added gradually. After the addition was complete, the mixture was stirred for 4 hr and filtered through a bed of Celatom FW-80. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.35 M and 1.12 M, respectively. The acid concentration was measured by pH titration to be 0.03 M.

Concentrations of other elements measured by ICPMS were: C 5800 ppm; Co 0.77 ppm; Ni 0.0011 ppm; and Cu 0.001 ppm. Example 14

[056] A solution of Cr0 ₃ (3.75 g; 0.0375 mole) in 4 M HC1 (25 mL) was treated with methanol (0.6 g; 0.0185 mole) drop wise during which the temperature varied from 23 °C to 81 °C. After 3 hr, when the solution cooled to ambient temperature, Fe powder (325 mesh, 1.4 g; 0.025 mole) was added gradually. After the addition was complete, HC1 (12 M; 2 mL) was added and the mixture was stirred for 18 hr. The mixture was then filtered through a bed of Ceiatom FW-80. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.84 M and 0.95 M, respectively. The acid concentration was measured by pH titration to be 0.19 M. Concentrations of other elements measured by ICPMS were: C 1200 ppm; Co 0.44 ppm; Ni 0.57 ppm; and Cu 0.27 ppm.

Example 15

[057] A solution of 9.5 M HC1 (500 mL) was cooled in an ice-bath to 8 °C. Cr0 ₃ (75 g; 0.75 mole) was added in small amounts over 2 hr. Methanol (12.02 g; 0375 mole) was added in small amounts over 6 hr. The mixture was allowed to warm to ambient temperature and stirred for 14 hr. The solution was cooled to ambient temperature and Fe powder (28 g; 0.5 mole) was added over 1 hr. The mixture was stirred for 22 hr and filtered through a bed of Ceiatom FW-80. The filter cake was washed with HC1 (6 M; 3X10 mL) and the washings were combined with the filtrate. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.56 M and 1.09 M, respectively. The acid concentration was measured by pH titration to be 2.25 M. Concentrations of other elements measured by ICPMS were: C 1600 ppm; Co 0.004 ppm; Ni 1.4 ppm; and Cu 0.0049 ppm. The amount of total of organic carbon (TOC) was analyzed to be less than 0.4%. Example 16

[058] To a solution of Cr0 ₃ (150 g; 1.5 mole) in 8 M HCl (1000 mL), glucose (50 g; 0.277 mole) was added in small amounts over 4.5 hr during which time the temperature varied from 33 to 97 °C. After 12 hr, when the solution cooled to ambient temperature, Fe powder (325 mesh, 55.85 g; 1.0 mole) was added over 6.5 hr. After the addition was complete, the mixture was stirred for 16 hr. The mixture was then filtered through a bed of Celatom FW-80. The acid concentration was raised by passing HCl gas into the solution. The concentrations of Cr and Fe were measured by UV-VIS spectroscopy to be 1.5 M and 1.3 M, respectively. The acid concentration was measured by pH titration to be 2.58 M. Concentrations of other elements measured by ICPMS were: C 3500 ppm; Co 0.0051 ppm; Ni 0.99 ppm; and Cu 0.076 ppm. Example 17

[059] To a solution of Cr0 ₃ (1 M; 8 mL) in 4 M HCl (1000 mL), raw sugar (sucrose; 0.345 g; 1 mmol) was added at which time the temperature was within a range from 22 °C to 42 °C. After 4 hr, during which time the solution cooled to ambient temperature, the mixture was filtered through a bed of Celatom FW-80. The concentration of Cr was measured by UV-VIS spectroscopy to be 1.26 M. The acid concentration was measured by pH titration to be 1.73 M. Concentrations of other elements measured by ICPMS were: C 6600 ppm; Co 0.093 ppm; Ni 0.0012 ppm; and Cu 0.04 ppm. The solution can be formulated to the desired electrolyte composition by the addition of appropriate amounts of iron and HCl.

Example 18

[060] Figure 1 depicts a flow cell. In the flow cell, a total volume of electrolyte as produced in Example 7 above in each compartment 124 and 126 was roughly 150 mL. The charging current density was 40 mA/cm ² . Charge/discharge cycles were carried out within the state of charge limits of 20% and 80 %. Polarization measurement gave area resistivity values of 1.79 Ω cm ² for charge and 1.75 Ω cm ² for discharge for the first cycle, and 1.93 Ω cm ² for charge and 1.88 Ω cm ² for the discharge.

[061] Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.