Preparation of iron / vanadium mixed electrolyte for flow batteries

Abstract

An electrolyte, a method for producing the electrolyte, and a flow cell battery are provided. The electrolyte contains about 1.0 M to about 1.5 M iron ions and about 1.0 M to about 1.5 M vanadium ions.

Description

[Technical field]

[0001] (Priority Claim) This application claims priority to U.S. Patent Application No. 17 / 671,948, filed February 15, 2022, which is incorporated by reference in its entirety.

[0002] The present disclosure relates to improved reactant utilization and energy density in iron / vanadium (Fe / V) mixed chloride and sulfonate flow batteries. [Background technology]

[0003] Energy storage is increasingly in demand to implement renewable energy resources and improve the reliability and efficiency of the power grid. Energy storage is seen as a key enabler of the transition from the current fossil fuel economy to a zero-carbon economy.

[0004] Almost all countries in the Middle East and North Africa (MENA) region are blessed with abundant renewable energy sources due to their unique geographical location. For example, the direct solar radiation (insolation) in the MENA region is 6 kWh / m 2 The MENA region is expected to achieve over 100,000MW of renewable energy in 2020. However, renewable energy is intermittent, which requires large-scale energy storage systems to ensure continuous and reliable power output. Overall development plans should adopt and focus on energy storage technologies suited to the needs and climate of the MENA region. Long-lasting batteries, such as flow batteries, with capacities up to multi-megawatt levels, are considered a suitable option to time-shift this large amount of power when required.

[0005] Flow batteries are energy storage technologies that store power as chemical energy in solutions that flow from separate storage tanks called catholyte and anolyte. Flow batteries are one of the most promising storage options and may be more flexible than their competitors, for example because they can be scaled up at low cost. In a flow battery, electrolytes are circulated through an electrochemical cell and separated by an ion exchange membrane (IEM). Electricity is converted to chemical energy in the electrochemical cell, stored, and released during discharge. A defining feature of flow batteries is their ability to vary their energy and power capacities independently. The energy capacity is defined by the volume of electrolyte stored in the tanks and the concentration of redox pair species, while the power rating is defined by the size of the electrodes and the number of cells in the stack. Summary of the Invention

[0006] One embodiment exemplified herein provides an electrolyte for a flow cell battery, the electrolyte comprising about 1.0M (molar: molar concentration) to about 1.5M iron ions and about 1.0M to about 1.5M vanadium ions.

[0007] Another embodiment illustrated herein provides a method of preparing an electrolyte for a flow cell battery. The method includes heating an acid solution, adding a first vanadium salt to the acid solution, and adding a second vanadium salt to the acid solution. The acid solution is stirred until both vanadium salts are dissolved. An iron salt is dissolved in the acid solution.

[0008] Another embodiment illustrated herein provides a flow cell battery. The flow cell battery includes an electrochemical cell. The electrochemical cell includes an ion exchange membrane, an anode current collector, and a cathode current collector, the space between the ion exchange membrane and the anode current collector forming a first channel, the space between the ion exchange membrane and the cathode current collector forming a second channel, and the ion exchange membrane is configured to pass ions between the first channel and the second channel. The flow cell battery includes a first tank configured to flow the anolyte at a first rate through the first channel, and a second tank configured to flow the catholyte at a second rate through the second channel. The catholyte, or both the catholyte and the anolyte, include about 1.0M to about 1.5M iron ions and about 1.0M to about 1.5M vanadium ions. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram of a flow battery that uses two electrolytes. [Diagram 2] FIG. 2 shows a method of operating the Fe / V flow cell to obtain higher energy density. [Diagram 3] FIG. 3 is a schematic diagram of the procedure for making a mixed ionic electrolyte solution. [Figure 4] FIG. 4 is a process flow diagram of a method for making a mixed ionic electrolyte solution. [Diagram 5] FIG. 5 is a schematic diagram illustrating the charge and discharge behavior of an electrochemical cell containing a mixture of iron and vanadium ions in both the catholyte and anolyte in equal volumetric ratios. [Figure 6] FIG. 6 is a plot showing the charging performance of the flow cell battery of FIG. [Figure 7] FIG. 7 is a conceptual diagram illustrating the operation of an electrochemical cell containing a mixture of iron and vanadium ions in both the catholyte and anolyte, showing the change in operation due to the high charging voltage of 1.6 V. [Figure 8] FIG. 8 is a plot of the charging of the electrochemical cell of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Electrolysis is commonly used to prepare separate catholyte and anolyte solutions, but this method is time consuming and inefficient. The examples described herein provide a chemical method for producing a mixed electrolyte solution of iron and vanadium ions that can be used as both the anolyte and catholyte in a flow cell battery. The method involves direct chemical dissolution of each component in an acid mixture to form a mixed ion solution.

[0011] In some embodiments described herein, a portion of the vanadium ions in the catholyte is replaced with V 5+ The charging voltage is increased to 1.6 volts (V) to convert V in the catholyte. 4+ Some of the ions are oxidized to V 5+ During charging, at voltages up to 1.4 V, the Fe ions in the catholyte 2+ The ion is Fe 2+ At higher voltages, V 4+ The ion is oxidized to V 5+ Form ions.

[0012] However, as described herein, at higher temperatures, V 5+ These compounds can cause deposition of V 5+ A 100% ion ratio starts at about 40°C. This causes damage to the flow cell battery. As explained in Table 1, V 4+ V for ions 5+ Limiting the concentration ratio of the ions can prevent this damage.

[0013] The data in Table 1 show the V 4+ V for ions 5+ The table shows the stability test of the battery at ion concentration ratios of about 50%, about 60%, about 70%, and about 80%. In the test at about 60°C, V5+ Ion and V 4+ It is shown that the flow cell battery exhibits stable behavior when the ion concentration ratio decreases to about 60%.

[0014] [Table 1]

[0015] Thus, in the embodiments described herein, V formed in the catholyte 5+ The amount of ions is controlled, V 4+ Approximately 50% to 60% or more of the ions are V 5+ Prevents conversion to ions. V 5+ By limiting the proportion of vanadium ions to less than approximately 60% of the total amount of vanadium ions, the V 5+ This reduces the chance of ion precipitation and expands the temperature range to meet operating requirements in high temperature environments such as those in the Middle East and North Africa (MENA) countries. Additionally, iron ions are more stable at high temperatures than vanadium ions, so the presence of iron ions does not affect the stability of the electrolyte.

[0016] To take advantage of the increased energy density, changing the ratio of anolyte to catholyte from 1:1 to 3:2 improves the reactant utilization of ions in the catholyte. During discharge, V in the catholyte is first 5+ Ion is V 4+ ions, which are then reduced to Fe 3+ The ion is Fe 2+ In the anolyte, the iron ions are reduced to V 3+ / V 2+ It does not participate in reactions that use half cells.

[0017] Therefore, it is possible to produce batteries with higher energy density that are stable at high temperatures. Because iron is less expensive than vanadium, the capital costs of iron / vanadium mixed acid flow batteries are also reduced.

[0018] 1 is a diagram of a flow battery 100 that uses two electrolytes. In the flow battery 100, energy is stored in electrolytes 102 and 104. Herein, electrolytes 102 and 104 are referred to as anolyte 102 and catholyte 104. Electrolytes 102 and 104 are stored in tanks 106 and 108 and are separately pumped from tanks 106 and 108 to an electrochemical cell 110 by a dedicated pump 112.

[0019] In some embodiments, an ion exchange membrane 114 is used in the electrochemical cell 110. The ion exchange membrane 114 separates the electrolytes 102 and 104 to prevent energy loss due to short circuits, while allowing protons or other ions to pass between the two sides and maintain electrical neutrality during charge and discharge cycles. In some embodiments, the ion exchange membrane 114 is sulfonated tetrafluoroethylene, available commercially, for example as NAFION® from DuPont Chemical Company, Wilmington, Virginia. The ion exchange membrane 114 generally controls the efficiency of the flow battery 100 and is a significant contributor to the cost of the flow battery 100. Thus, in some embodiments, the ion exchange membrane 114 is omitted and the electrolytes 102 and 104 are generally prevented from mixing by laminar flow or are not required by the battery design, such as when a single electrolyte solution is used.

[0020] As the electrolytes 102 and 104 are pumped through the electrochemical cell 110, they pass through channels 116 and 118. The channels 116 and 118 may comprise a porous electrode material such as felt or Raney nickel, among others, to allow ions and electrons to flow between the electrolytes 102 and 104. In some embodiments, for example when the ion exchange membrane 114 is omitted, the channels 116 and 118 may be narrow to enhance laminar flow.

[0021] During the generation of power, the anolyte 102 is oxidized and releases electrons to the anode current collector 120. The electrons are transferred to a load 124 by a line 122. After powering the load 124, the electrons are transferred back to the electrochemical cell 110 by another line 126. The electrons re-enter the electrochemical cell 110 from the cathode current collector 128 and reduce the catholyte 104.

[0022] The anolyte 102 and catholyte 104 are regenerated during a charge cycle. During a charge cycle, a power source 130 removes electrons from the cathode current collector 128 through line 132, oxidizing the catholyte 104 to its initial state in the reverse reaction of Equation 2. Electrons are supplied from the power source 130 through another line 134 to the anode current collector 120, reducing the anolyte 102 to its initial state in the reverse reaction of Equation 1.

[0023] One of the most established technologies for flow batteries is based on vanadium redox chemistry and is called the vanadium redox flow battery (VFB). In a VFB, vanadium ions are dissolved in an aqueous acid supporting electrolyte. 2+ / V 3+ and V 4+ / V 5+ These are often based on the redox pair:

[0024] For a vanadium ion flow cell battery, during discharge, the reaction of the anolyte 102 at the anode current collector 120 is shown in Equation 1:

[0025]

number

[0026] During discharge, the reaction of the catholyte 104 at the cathode current collector 128 is shown in Equation 2.

[0027]

number

[0028] In a VFB, vanadium ions are dissolved in an aqueous acid supporting electrolyte. Since the first demonstration of the VFB, several studies have been carried out to investigate the effects of various electrochemical, chemical and physical variables and to optimize the associated system operating parameters while minimizing capacity fade.

[0029] VFB is currently in commercial use, but 5+ There are issues associated with performance at temperatures above about 40°C where ions may precipitate. These issues may limit their use in high temperature environments such as those in the MENA countries.

[0030] Therefore, V 5+ Flow batteries based on Fe / V redox chemistry have been explored as a potential option to lower the cost of large-scale energy storage, as the use of Fe ions instead of vanadium ions could significantly improve the operating temperature range of all-vanadium flow batteries. Furthermore, iron is less expensive than vanadium, thus lowering the material cost of flow batteries.

[0031] In an Fe / V flow battery, during a discharge cycle of the flow battery, the catholyte 104 contains Fe(III) that is reduced to Fe(II) at the cathode current collector 128(+) according to the reaction shown below, while the anolyte 102 contains V(II) that is oxidized to V(III) at the anode current collector 120(-) according to the reaction shown below:

[0032]

number

[0033] However, practical implementation of Fe / V flow batteries can be challenging: for example, cross-contamination of the two half-cell electrolytes by iron and vanadium ions passing through the membrane can cause discharge decay and shorten the electrolyte lifetime.

[0034] Additionally, Fe / V redox flow batteries have lower energy density compared to VFB systems due to lower cell operating voltages across the cathode and anode, and less efficient utilization of reactants in the catholyte 104. This is typically due to lower reactant concentrations in the electrolyte than in VFB. Iron ions have low solubility in hydrochloric and sulfuric acids, so the total concentration of iron and vanadium ions in an Fe / V / HCl cell is approximately 1.5 mol L−1. -1 is limited to.

[0035] 2 is a method 200 of operating an Fe / V flow cell to obtain higher energy density. The method begins at block 202 with preparation of a solution for iron and vanadium ions. This can be done, for example, by dissolving a vanadium salt in a strong acid, such as concentrated hydrochloric acid, to form a solution with a concentration of about 1.5 M based on solubility. Next, an iron salt, for example, is dissolved in the acid to form a mixed ion solution containing about 1.5 M vanadium ions in addition to iron ions.

[0036] At block 204, the anolyte tank of the flow cell battery is filled with the mixed ionic solution. At block 206, the catholyte tank is filled with the mixed ionic solution.

[0037] At block 208, the anolyte and catholyte flow through the electrochemical cell to generate electricity. In the embodiment described herein, the power output is controlled by the total charging voltage and the volume ratio of catholyte to anolyte, for example, the catholyte to anolyte ratio is 2:3. This can be adjusted by controlling the flow rate through the electrochemical cell. At block 210, a load is powered from the power available from the flow cell battery.

[0038] At the positive electrode side of the Fe / V (chloride / sulfide) mixed flow cell battery, Fe 2+ / Fe 3+ Plus V 4+ / V 5+By using, for example, the energy density of the Fe / V flow cell battery can be improved by about 30% by optimizing the volume ratio with flow rate. The improved energy density and reagent utilization could potentially reduce the capital cost of the Fe / V mixed acid flow battery by 25%-30% or more. Furthermore, this mixed electrolyte battery can be used with V 4+ From V 5+ The 60% limit on conversion to CO2 allows flow cell batteries to withstand higher temperatures than VFBs, for example above about 40°C. This allows flow cell batteries to meet operational requirements in the MENA region.

[0039] Figure 3 is a schematic diagram of a procedure 302 for making a mixed ionic electrolyte solution. This procedure corresponds to block 202 in Figure 2. The solutions are mixed in a reactor 304, which can be a batch reactor, a continuous stirred tank reactor (CSTR), or a loop reactor (including a static mixer). The choice of reactor is based on mixing time and heating. In some embodiments, the reactor is a batch reactor that allows for mixing times of several hours.

[0040] An acid 306 is added to the reactor 304. The acid 306 comprises hydrochloric acid, such as concentrated hydrochloric acid. In some embodiments, the acid can be a mixture of hydrochloric acid and sulfuric acid.

[0041] The acid 306 in the reactor 304 is heated, for example, to about 50° C. to about 70° C., or about 60° C., to dissolve the vanadium salts. Equal amounts of vanadium(III) oxide 308 and vanadium(V) oxide 310 are added to the heated acid 306 in the reactor 304, forming a solution having a vanadium ion concentration of about 1.0 M to about 1.5 M with an average charge of about 3.5+ after dissolution. In some embodiments, the vanadium ion concentration is about 1.25 M. The solution is stirred to dissolve the vanadium salts in the hot acid.

[0042] Next, an iron salt 312, such as iron(II) chloride, is added to the acid solution of vanadium ions, and after dissolution, Fe 2+A solution is formed with an ion concentration of about 1.0 M to about 1.5 M. In some embodiments, the iron atom concentration is about 1.25 M. Other iron salts 312 can be used, but the use of these salts is subject to the V 5+ It may affect the solubility limits of ions.

[0043] The solution is agitated to completely dissolve and then passed through filter 314 to form a mixed ionic electrolyte solution. Filter 314 can be a ceramic or other material selected to withstand the hot acid without corroding.

[0044] In some embodiments, if a different type of reactor 304 is selected, such as a loop reactor, a higher heat flux can be used to heat the acid 306 during dissolution. However, the temperature that the acid 306 is allowed to reach during heating is limited by the solubility of vanadium (V) ions in the acid 306.

[0045] Figure 4 is a process flow diagram of the method of block 202 for producing a mixed ionic electrolyte solution. Like numbered items are as described with respect to Figure 2. The method 202 begins at block 402 with the addition of an acid solution to a reactor. If the reactor is a batch reactor, the entire amount of acid is added at once. If the reactor is a continuous reactor, circulation is started through the reactor. At block 402, the acid solution is heated to promote dissolution.

[0046] At block 404, a first vanadium salt is added to the reactor. At block 406, a second vanadium salt is added to the reactor. The vanadium salts may be mixed and added together prior to addition. At block 408, the reactor is agitated to dissolve the vanadium salt. In one embodiment, the reactor is agitated at a temperature of about 60° C. for about 12 hours to dissolve the vanadium salt. The chemical reaction for dissolving the vanadium salt is shown in Equation 1:

[0047]

number

[0048] After the vanadium salt has dissolved, heating can be discontinued and the reactor allowed to cool.

[0049] At block 410, an iron salt is added. At block 412, the reactor is stirred to dissolve the iron salt. In some embodiments, the reactor is stirred at a temperature of about 20° C. for about 20 hours. It is not necessary to cool the reactor to this temperature before adding the iron salt. The chemical reaction for dissolving the iron salt is shown in Equation 2:

[0050]

number

[0051] At block 414, the solution is filtered to remove undissolved solids. The filtered solution is a mixed ionic electrolyte that can be used as a catholyte and anolyte in a flow cell battery, as described in the Examples below.

[0052] In some embodiments, the order of the blocks may differ from that described above. For example, an iron salt may first be dissolved in an acid solution, as described with respect to blocks 410 and 412. Then, a vanadium salt may be dissolved to prepare the final electrolyte solution, as described with respect to blocks 404 through 408. EXAMPLES

[0053] experiment

[0054] (Preparation of electrolyte) A mixed ion solution containing Fe and V ions was prepared as described herein. The mixed reactant solution contained 1.25-1.5M Fe and an equimolar amount of vanadium, e.g., 1.25-1.5M V, and 6.6-8.6M Cl. -The preparation was carried out by dissolving 1.25-1.5 M iron(II) chloride (FeCl2) powder in 6.6-8.6 M hydrochloric acid (HCl) solution at room temperature (20-25 °C) and stirring for 24 h.

[0055] A 50 / 50 molar mixture of V2O3 and V2O5 was then added to the FeCl2 solution to achieve equimolar amounts of iron and vanadium. The solution was heated to 60°C and stirred for 12 hours. The resulting electrolyte solution was 3.5+ / Fe 2+ It is a mixed solution of 50% V 4+ And 50% V 3+ and Fe 2+ and a solution.

[0056] (Electrochemical Test Cell) The mixed reaction solutions were tested as both catholyte and anolyte in an electrochemical test cell battery. The terms catholyte and anolyte have been retained for consistency, even though these solutions are not flowing. With reference to FIG. 1, the catholyte 104 is on the positive side during discharge and receives electrons from the load. The anolyte 102 is on the negative side during discharge and provides electrons to the load. The volume ratio was determined by the amount added to each side of the test cell. The test cells were separated by a sulfonated polytetrafluoroethylene membrane available under the NAFION® brand from Chemours, Inc., Wilmington, Delaware, USA.

[0057] (Example 1: Fe / V mixed electrolyte with 0% vanadium utilization rate in the cathode fluid) In the first tests, a charging voltage of 1.4 V and a catholyte to anolyte volume ratio of 1:1 were used. Using this charging voltage, only the iron ions in the catholyte react and only the vanadium ions in the anolyte react. This is further explained with reference to Figure 5.

[0058] 5 is a schematic diagram of the charging and discharging operation of an electrochemical cell 502 containing a mixture of iron and vanadium ions in equal volumetric ratios in both the catholyte 104 and the anolyte 102. In this example, the cell is charged at a voltage of 1.4 V. During charging, the Fe 2+ is Fe 3+ In the anolyte 102, charging causes the electrolyte to be oxidized to V 3+ V 2+ When charged to 1.4 V, the vanadium ions in the catholyte 104 do not participate in the reaction. The iron ions in the anolyte 102 do not participate in the reaction.

[0059] During discharge, Fe in the catholyte 104 3+ The ion is Fe 2+ In the anolyte 102, V is reduced to 2+ Ion is V 3+ As during charging, the iron ions in the anolyte 102 do not participate in the reaction.

[0060] Figure 6 is a plot of the charging performance of the flow cell battery of Figure 5. When charged at 1.4 V, approximately 3.3 ampere-hours (Ah) of charge capacity is obtained.

[0061] (Example 2: Fe / V mixed electrolyte with 50% vanadium utilization rate in the cathode fluid) FIG. 7 is a schematic diagram of the operation of an electrochemical cell 502 containing a mixture of iron and vanadium ions in both the catholyte 104 and anolyte 102, illustrating the change in operation caused by a high charging voltage of 1.6 V. The high charging voltage increases the energy density of the flow cell battery. The increased energy density can be achieved by increasing the utilization of the reactants in the electrolyte. Since the vanadium component is the more expensive component of the Fe / V flow battery system, increasing the utilization of vanadium not only improves the energy density, but also reduces the cost of the electrolyte.

[0062] To take advantage of the increased energy density of the catholyte 104, the catholyte / anolyte volume ratio was set to 2:3. Specifically, the catholyte 104 contained 2 molar volumes of iron ions and 2 molar volumes of vanadium ions for a total of 4 molar volumes. The anolyte 102 contained 3 molar volumes of iron ions and 3 molar volumes of vanadium ions for a total of 6 molar volumes. This allows a 3 / 8 saving in the total amount of vanadium used compared to a battery without a mixed electrolyte.

[0063] In this example, during charging, Fe 2+ Fe 3+ is oxidized to Fe 2+ After the ion is completely oxidized, V 4+ is oxidized to V 4+ and V 5+ A mixture of ions is formed. The charge is V 4+ Only 50-60% of the ions are V 5+ This maintains the stability of the solution at high temperatures. For example, at an operating temperature of about 40°C, V 5+ The probability of ion deposition decreases. In the anolyte 102, V 3+ V 2+ As in Example 1, the iron ions in the anolyte 102 do not participate in the reaction.

[0064] During discharge, first, V 5+ Ion is V 4+ It is reduced to ions. V 5+ When the ions are reduced, the Fe in the catholyte 104 3+ The ion is Fe 2+ During discharge, the V in the anolyte 102 is reduced to 3+ Ion is V 2+ As during charging, the iron ions in the anolyte 102 do not participate in the reaction.

[0065] As a result of the high charging voltage and the change in volume ratio, more than 50% of the vanadium in the positive electrolyte (catholyte) is available, thus improving the energy density of the Fe / V cell by 30% compared to the Fe / V cell operated at the lower charging voltage, as described with respect to Figures 5 and 6.

[0066] FIG. 8 is a plot of the charging of the electrochemical cell of FIG. 7. The test was performed on a 48 cm 2 The electrolyte used for both the anolyte and catholyte was 1.25 M (molar) V, 1.25 M Fe, 6.6 M Cl. - , and 0.8M SO4 2- During charging, the catholyte Fe 2+ is Fe 3+ is oxidized to Fe 2+ When all of the V in the catholyte is converted, the high charging voltage 4+ V 5+ As described herein, charging is performed by 4+ Only about 60% of V 5+ The anolyte is controlled so that it is not converted to V during charging, enhancing thermal stability at temperatures of about 40°C. 3+ V 2+ is reduced to.

[0067] (Embodiment) The embodiments described in the Examples herein provide an electrolyte for a flow cell battery, the electrolyte including about 1.0M (molar) to about 1.5M iron ions and about 1.0M to about 1.5M vanadium ions.

[0068] In one embodiment, the electrolyte comprises about 1.25 M iron ions. In one embodiment, the electrolyte comprises about 1.25 M vanadium ions. In one embodiment, the electrolyte comprises equimolar concentrations of iron ions and vanadium ions.

[0069] In one embodiment, the electrolyte contains about 5M to about 8M chloride ions (Cl - ).

[0070] In one embodiment, the electrolyte contains about 6.6 M chloride ions. In one embodiment, the electrolyte contains about 0.5 M to about 1.0 M sulfate ions (SO4 2- In one embodiment, the electrolyte comprises about 0.8M sulfate ions.

[0071] In one aspect, the electrolyte comprises a base solution comprising hydrochloric acid. In one aspect, the electrolyte comprises a catholyte for a flow cell battery. In one aspect, the electrolyte comprises an anolyte for a flow cell battery.

[0072] Another embodiment described in the Examples herein provides a method of preparing an electrolyte for a flow cell battery. The method includes heating an acid solution, adding a first vanadium salt to the acid solution, and adding a second vanadium salt to the acid solution. The acid solution is stirred until both vanadium salts are dissolved. An iron salt is dissolved in the acid solution.

[0073] In one embodiment, iron(II) chloride is added to the acid solution. In one embodiment, the method comprises the chemical reaction: FeCl → Fe 2+ + 2Cl - In one embodiment, the method includes dissolving an iron salt to form a solution of about 1.0 M to about 1.5 M iron ions. In one embodiment, the method includes dissolving an iron salt to form a solution of about 1.25 M iron ions.

[0074] In one embodiment, the method includes dissolving a mixture of vanadium(III) oxide (V2O3) and vanadium(V) oxide (V2O5) in an acid solution.

[0075] In one aspect, the method comprises: Chemical Reaction: JPEG2025505790000007.jpg13152 Includes.

[0076] In one embodiment, the method includes dissolving a first vanadium salt and a second vanadium salt to form a solution of about 1.0 M to about 1.5 M vanadium ions. In one embodiment, the method includes dissolving a first vanadium salt and a second vanadium salt to form a solution of about 1.25 M vanadium ions.

[0077] Another embodiment illustrated herein provides a flow cell battery. The flow cell battery includes an electrochemical cell. The electrochemical cell includes an ion exchange membrane, an anode current collector, and a cathode current collector, where the space between the ion exchange membrane and the anode current collector forms a first channel, and the space between the ion exchange membrane and the cathode current collector forms a second channel, and the ion exchange membrane is configured to allow ions to pass between the first channel and the second channel. The flow cell battery includes a first tank configured to flow the anolyte at a first rate through the first channel, and a second tank configured to flow the catholyte at a second rate through the second channel. The catholyte, or both the catholyte and the anolyte, include about 1.0M to about 1.5M iron ions and about 1.0M to about 1.5M vanadium ions.

[0078] In one embodiment, the catholyte, or both the catholyte and the anolyte, comprises an equimolar solution of about 1.25M to about 1.5M iron ions and about 1.25M to about 1.5M vanadium ions.

[0079] In one embodiment, the catholyte, or both the catholyte and the anolyte, contain about 5M to about 8M chloride ions (Cl - In one embodiment, the catholyte, or both the catholyte and the anolyte, comprises about 6.6M chloride ions.

[0080] In one embodiment, the catholyte, or both the catholyte and the anolyte, contain about 0.5M to about 1.0M sulfate ion (SO 2- In one embodiment, the catholyte, or both the catholyte and the anolyte, comprises about 0.8M sulfate ions.

[0081] In one embodiment, the first rate is greater than the second rate. In one embodiment, the ratio of the first rate to the second rate is 3:2. In one embodiment, both the catholyte and the anolyte contain about 1.25M vanadium (V), about 1.25M iron (Fe), about 6.6M chloride ions (Cl - ), and about 0.8M sulfate ion (SO4 2- ).

[0082] Other embodiments are within the scope of the following claims.

Claims

1. 1. An electrolyte for a flow cell battery, comprising: An electrolyte solution comprising about 1.0M (molar) to about 1.5M iron ions and about 1.0M to about 1.5M vanadium ions.

2. 10. The electrolyte of claim 1 comprising about 1.25M iron ions.

3. 10. The electrolyte of claim 1 comprising about 1.25M vanadium ions.

4. 10. The electrolyte of claim 1 comprising equimolar concentrations of iron ions and vanadium ions.

5. About 5M to about 8M chloride ion (Cl － 2. The electrolyte of claim 1 , comprising:

6. 6. The electrolyte of claim 5 comprising about 6.6M chloride ions.

7. About 0.5M to about 1.0M sulfate ion (SO ４ ２－ 2. The electrolyte of claim 1 , comprising:

8. 8. The electrolyte of claim 7 comprising about 0.8M sulfate ions.

9. 10. The electrolyte of claim 1 comprising a base solution comprising hydrochloric acid.

10. 13. The electrolyte of claim 1 comprising a catholyte for a flow cell battery.

11. 13. The electrolyte of claim 1 comprising an anolyte for a flow cell battery.

12. 1. A method for preparing an electrolyte for a flow cell battery, comprising: Heating the acid solution; adding a first vanadium salt to the acid solution; adding a second vanadium salt to the acid solution; stirring until both vanadium salts are dissolved; dissolving an iron salt in the acid solution; A method comprising:

13. 13. The method of claim 12, comprising adding iron(II) chloride to the acid solution.

14. Chemical Reaction: The method of claim 12, comprising:

15. 13. The method of claim 12, comprising dissolving an iron salt to form a solution of about 1.0 M to about 1.5 M iron ions.

16. 13. The method of claim 12, comprising dissolving an iron salt to form a solution of about 1.25 M iron ions.

17. Vanadium(III) oxide (V ２ O ３ ) and vanadium(V) oxide (V ２ O ５ 13. The method of claim 12, comprising dissolving a mixture of:

18. Chemical Reaction: The method of claim 12, comprising:

19. 13. The method of claim 12, comprising dissolving the first vanadium salt and the second vanadium salt to form a solution of about 1.0 M to about 1.5 M vanadium ions.

20. 13. The method of claim 12, comprising dissolving the first vanadium salt and the second vanadium salt to form a solution of about 1.25 M vanadium ions.

21. 1. A flow cell battery, comprising: An electrochemical cell, the electrochemical cell comprising: An ion exchange membrane; an anode current collector; a cathode current collector; a space between the ion exchange membrane and the anode current collector forms a first channel, a space between the ion exchange membrane and the cathode current collector forms a second channel, and the ion exchange membrane is configured to allow ions to pass between the first channel and the second channel; a first tank configured to flow anolyte at a first rate through the first channel; a second tank configured to flow catholyte through the second channel at a second rate; the catholyte, or both the catholyte and the anolyte, comprises from about 1.0 M to about 1.5 M iron ions and from about 1.0 M to about 1.5 M vanadium ions; Flow cell battery.

22. 22. The flow cell battery of claim 21 , wherein the catholyte, or both the catholyte and the anolyte, comprise an equimolar solution of about 1.25M to about 1.5M iron ions and about 1.25M to about 1.5M vanadium ions.

23. The catholyte, or both the catholyte and the anolyte, contain about 5M to about 8M chloride ions (Cl － 22. The flow cell battery of claim 21 , comprising:

24. 22. The flow cell battery of claim 21, wherein the catholyte, or both the catholyte and the anolyte, contain about 6.6 M chloride ions.

25. The catholyte, or both the catholyte and the anolyte, contain about 0.5 M to about 1.0 M sulfate ions (SO ４ ２－ 22. The flow cell battery of claim 21 , comprising:

26. 22. The flow cell battery of claim 21, wherein the catholyte, or both the catholyte and the anolyte, comprise about 0.8 M sulfate ions.

27. 22. The flow cell battery of claim 21, wherein the first rate is greater than the second rate.

28. 22. The flow cell battery of claim 21, wherein the ratio of said first rate to said second rate is 3:

2.

29. Both the catholyte and the anolyte contained approximately 1.25M vanadium (V), approximately 1.25M iron (Fe), approximately 6.6M chloride ions (Cl － ), and about 0.8M sulfate ions (SO ４ ２－ 22. The flow cell battery of claim 21 , comprising:

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