Increasing reactant utilization in FE / V flow batteries

Abstract

Methods and systems are provided for using a flow cell battery with a mixed Fe / V electrolyte. An exemplary method includes flowing an anolyte through a first channel in an electrochemical cell, the first channel being formed in a space between an anode current collector and an ion exchange membrane. Flowing a catholyte through a second channel in the electrochemical cell, the second channel being formed in a space between a cathode current collector and an ion exchange membrane, the first and second channels being separated by the ion exchange membrane, the catholyte including a mixed electrolyte including both iron and vanadium ions. Flowing ions through the ion exchange membrane to oxidize the anolyte and reduce the catholyte. An electric current is generated between the anode current collector and the cathode current collector.

Description

[Technical field]

[0001] This application claims priority to U.S. Patent Application No. 17 / 651,116, filed February 15, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure is directed to increasing reactant utilization and energy density in Fe / V mixed chloride and sulfonate flow batteries. [Background technology]

[0003] Energy storage is increasingly in demand with the introduction of renewable energy resources and improving the reliability and efficiency of the power grid, which is seen as a key enabling factor for the transition from the current fossil 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, direct solar radiation is 6 kWh / m 2 The total energy demand in the MENA region is over 1000MW. However, renewable energy is intermittent and requires large-scale energy storage systems for continuous and reliable power output. Overall development plans should adopt and focus on appropriate energy storage technologies suited to the needs and climate of the MENA region. Long-duration batteries, such as flow batteries, with capacities at the multi-megawatt level, are considered a suitable option to time-shift this large amount of power when required.

[0005] Flow batteries are an energy storage technology that stores power as chemical energy in solutions that flow from separate storage tanks called catholytes (catholyte) and anolytes (anolyte). Flow batteries are one of the most promising storage options, potentially cheaper and more flexible than their competitors, for example, due to their low cost and scalability. In a flow battery, electrolytes are circulated through an electrochemical cell, which is separated by an ion exchange membrane (IEM). Electricity is converted to chemical energy in the electrochemical cell for storage and then released during discharge. Unique to 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 tank and the concentration of redox couple species, while the rated power is defined by the size of the electrodes and the number of cells in the stack. Summary of the Invention

[0006] An embodiment described in the Examples herein provides a flow cell battery. The flow cell battery includes an electrochemical cell, the electrochemical cell including an ion exchange membrane, an anode current collector, and a cathode current collector. A space between the ion exchange membrane and the anode current collector forms a first channel, and a space between the ion exchange membrane and the cathode current collector forms a second channel. 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 anolyte through the first channel at a first rate, and a second tank configured to flow catholyte through the second channel at a second rate. The catholyte, or both the catholyte and the anolyte, include a mixture of iron ions and vanadium ions.

[0007] Another embodiment described in the Examples herein provides a method of generating an electric current. The method includes flowing anolyte through a first channel in an electrochemical cell, the first channel being formed in a space between an anode current collector and an ion exchange membrane. Flowing catholyte through a second channel in the electrochemical cell, the second channel being formed in a space between a cathode current collector and an ion exchange membrane, the first channel and the second channel being separated by the ion exchange membrane, the catholyte comprising a mixed electrolyte containing both iron and vanadium ions. Flowing ions through the ion exchange membrane to oxidize the anolyte and reduce the catholyte. An electric current is generated between the anode current collector and the cathode current collector. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 shows a flow battery using two electrolytes.

[0009] [Diagram 2] A method for manipulating the Fe / V flow cell to obtain higher energy density is shown.

[0010] [Diagram 3] FIG. 1 is a schematic diagram showing the charging and discharging behavior of an electrochemical cell containing a mixture of iron and vanadium ions in equal volume ratios of both catholyte and anolyte up to a charging voltage of 1.4 V.

[0011] [Figure 4] This plots the performance of the flow cell battery in Figure 3.

[0012] [Diagram 5] FIG. 1 is a schematic diagram showing the operation of a flow cell battery containing a mixture of iron and vanadium ions in both the catholyte and anolyte, illustrating the change in operation due to higher charging voltages and different volume ratios.

[0013] [Figure 6]6 is a plot of the performance of the electrochemical cell of FIG. 5.

[0014] [Figure 7] FIG. 13 is a plot of the energy density of electrolytic cells comparing the two protocols over multiple charge / discharge cycles.

[0015] [Figure 8] 1 is a plot of the cycling efficiency of an Fe / V electrolytic cell over 200 cycles.

[0016] [Figure 9] This plots the capacity of an Fe / V battery over 200 cycles. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The embodiments described herein provide a flow cell battery that utilizes an electrolyte mixture of iron and vanadium ions as both the anolyte and catholyte. To increase the energy density, the charging voltage can be increased from 1.4V to 1.6V.

[0018] Specifically, the higher charging voltage is due to the V 4+ Oxidize some of the ions to V 5+ ions are formed. During charging, Fe in the catholyte 2+ The Fe ion is activated at voltages up to 1.4 V. 2+ At higher voltages, V 4+ The ion is oxidized to V 5+ Form ions.

[0019] However, as described herein, at higher temperatures, V 5+ This can result in the precipitation of compounds such as V 5+ A 100% ion fraction begins at about 40°C. This causes damage to the flow cell battery. The damage occurs at V 4+V for ions 5+ This can be prevented by limiting the concentration of the ions ratio.

[0020] Table 1 shows the V 5+ Ion Pair V 4+ The stability tests of the battery at temperatures above about 40° C., such as about 55° C. and about 60° C., are shown for the cases where the ratio of ions is about 50%, about 60%, about 70%, and about 80%. The test at about 60° C. is V 4+ V for ions 5+ The concentration ratio of the ions decreased to about 60%, indicating that the flow cell battery exhibited stable behavior.

[0021] [Table 1]

[0022] Thus, in the embodiments described herein, V formed in the catholyte 5+ The amount of ions is V 4+ Approximately 50% to 60% of the ions are V 5+ The V 5+ Limiting the proportion of vanadium ions to less than about 60% of the total vanadium ions present ensures that the V 5+ It reduces the possibility of ion precipitation and increases the temperature range to meet the operating requirements in high temperature environments such as those found in the Middle East and North Africa. Furthermore, the presence of iron ions does not affect the stability of the electrolyte since iron ions are more stable at high temperatures than vanadium ions.

[0023] To take advantage of the increased energy density, the ratio of anolyte to catholyte is changed from 1:1 to 3:2, improving the reactant utilization of ions in the catholyte. During discharge, the 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 anolyte, the iron ion is reduced to V 3+ / V 2+Used in half-cells and does not participate in reactions.

[0024] Therefore, it is possible to produce batteries with higher energy density that are stable at higher temperatures. Iron is less expensive than vanadium, which also reduces the capital cost of Fe / V mixed acid flow batteries.

[0025] 1 illustrates a flow battery 100 that uses two electrolytes. In the flow battery 100, energy is stored in electrolytes 102 and 104, referred to herein as anolyte 102 and catholyte 104. The electrolytes 102 and 104 are stored in tanks 106 and 108 and are separately pumped from the tanks 106 and 108 to an electrochemical cell 110 by a dedicated pump 112.

[0026] 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 sides during charge and discharge cycles to maintain electrical neutrality. In some embodiments, the ion exchange membrane 114 is sulfonated tetrafluoroethylene, available commercially, for example under the name NAFION® from DuPont Chemical of 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 kept from mixing by laminar flow or are made unnecessary by the battery design, such as when a single electrolyte solution is used.

[0027] 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 include porous electrode materials such as felt or Rainy 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.

[0028] During the generation of power, the ions in the anolyte 102 are oxidized and lose 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 ions in the catholyte 104.

[0029] 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. VFBs are often referred to as V 2+ / V 3+ and V 4+ / V 5+ Based on redox couples.

[0030] In the case of a vanadium ion flow cell battery, during discharge, the reaction of the anolyte 102 on the anode current collector 120 is shown in Equation 1.

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number

[0031] 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 via line 132, oxidizing the ions in the catholyte 104, e.g., to the initial state, e.g., the reverse of Equation 2. Electrons are supplied from the power source 130 via another line 134 to the anode current collector 120, reducing the ions in the anolyte 102, e.g., to the initial state, e.g., the reverse of Equation 1.

[0032] In a VFB, vanadium ions are dissolved in an aqueous acid supporting electrolyte. Since the first demonstration of a VFB, several studies have been conducted to investigate the effect of various electrochemical, chemical, and physical variables to optimize the relevant system operating parameters with negligible capacity fade.

[0033] VFB is currently in commercial use, but at high temperatures above about 40°C, V 5+ There are performance-related challenges, such as the tendency for ions to precipitate and damage the flow battery, which could limit its use in high-temperature environments such as those in MENA countries.

[0034] Therefore, flow batteries based on Fe / V redox chemistry have the following characteristics: 5+ The use of Fe ions in the cathode instead of vanadium ions potentially offers a significant improvement in the operating temperature window over all vanadium flow batteries and is therefore offered herein as an option to reduce the cost for large scale energy storage. Additionally, iron is less expensive than vanadium, resulting in lower material costs for flow batteries.

[0035] In an Fe / V flow battery, during the discharge cycle of the flow battery, the catholyte 104 converts Fe to Fe+ at the cathode current collector 128(+) according to the reaction shown below: 2+ Fe is reduced to 3+ while the anolyte 102 contains V at the anode current collector 120(-). 3+ V is oxidized to 2+ Including:

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number

number

[0036] However, Fe / V flow batteries may be difficult to commercialize: cross-contamination of the two half-cell electrolytes by passage of iron and vanadium ions across the membrane can lead to discharge collapse and reduce the lifetime of the electrolyte.

[0037] Additionally, Fe / V redox flow batteries have a lower energy density than VFB systems due to lower cell operating voltages across the cathode and anode, and inefficient utilization of reactants in the catholyte 104. This is generally due to lower reactant concentrations in the electrolyte than in VFB. Fe ions have low solubility in hydrochloric and sulfuric acids, so the total concentration of Fe and V ions in an Fe / V / HCl cell is approximately 1.5 mol L -1 is limited to.

[0038] 2 is a method 200 for operating an Fe / V flow cell to obtain higher energy density. The method begins in block 202 with the preparation of a solution of iron and vanadium ions. This can be done, for example, by dissolving iron metal in a strong acid, such as concentrated HCl, to form a solution with a maximum concentration of 1.5M based on solubility. Vanadium metal is then dissolved in the acid to form a mixed ion solution having, for example, about 1.5M vanadium ions in addition to iron ions.

[0039] In block 204, an anolyte tank in the flow cell battery is filled with the mixed ionic solution. In block 206, a catholyte tank is filled with the mixed ionic solution.

[0040] At block 208, the anolyte and catholyte flow through an electrochemical cell to generate electricity. In the embodiment described herein, the output is controlled by the total charge voltage and the volume ratio of catholyte to anolyte, for example, the ratio of catholyte to anolyte 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 electricity generated from the flow cell battery.

[0041] Fe on the positive side, i.e., catholyte side, of a mixed V / Fe chloride / sulfide flow battery 2+ / Fe 3+ In addition to the half-cell reaction, V 4+ / V 5+ The use of half-cell reactions, for example, allows for an increase in the energy density of Fe / V flow batteries by about 30% by optimizing the volumetric ratio by flow rate. As explained with respect to Figures 5 and 6, a flow rate or volume ratio of catholyte to anolyte of 3:2 results in a large amount of Fe in the catholyte. 2+ Ion and V 4+ The improved energy density and reagent utilization could reduce the capital cost of Fe / V mixed acid flow batteries by 25% to 30% or more. In addition, the resistance to high temperatures, such as above 40°C, is a significant improvement over the V 4+ V 5+ This is higher than VFB due to the imposed 60% limit on conversion to CO2, which allows the flow cell battery to meet the operating requirements in the MENA region.

[0042] Working Example

[0043] experiment

[0044] Preparation of electrolyte

[0045] A mixed ion solution containing Fe and V ions was prepared as described herein. The mixed reaction solution was 1.25-1.5M Fe / 1.25-1.5M V and 6.6-8.6M Cl. -The solution was prepared by dissolving 1.25-1.5M iron chloride (FeCl2) powder in 6.6-8.6M hydrochloric acid (HCl) at room temperature (20-25°C) and stirring for 24 hours.

[0046] A 50 / 50 molar ratio mixture of V2O3 / V2O5 was then added to the FeCl2 solution. The solution was heated to 60°C and stirred for 12 hours. The resulting electrolyte solution contained 50% V 4+ and 50% V 3+ Fe 2+ The solution was added, V 3.5+ / Fe 2+ It is a mixed solution.

[0047] Electrochemical Test Cell

[0048] The mixed reactant solutions were used in an electrochemical test cell battery as both catholyte and anolyte. Although these solutions were not flowing, the terms catholyte and anolyte are retained for consistency. With reference to FIG. 1, catholyte 104 is on the + side during discharge and accepts electrons from the load, and anolyte 102 is on the - 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.

[0049] Example 1: Mixed Fe / V electrolyte with 0% vanadium utilization in the catholyte.

[0050] Initial testing used a charging voltage of 1.4 V and a 1:1 volume ratio of catholyte and anolyte. With 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 FIG. 3.

[0051] 3 is a schematic diagram illustrating the charging and discharging behavior of an electrochemical cell 302 containing a mixture of iron and vanadium ions in both the catholyte 104 and the anolyte 102 in the same volume ratio. In this example, the cell is charged at a voltage of 1.4 V. During charging, Fe 2+ is the Fe in catholyte 104 3+ In anolyte 102, the charge causes the V 3+ V 2+ When charged to 1.4 V, the vanadium ions in catholyte 104 do not participate in the reaction, and neither do the iron ions in anolyte 102.

[0052] Fe in catholyte 104 during discharge 3+ The ion is Fe 2+ while V in anolyte 102 is reduced to 2+ Ion is V 3+ As during charging, the iron ions in the anolyte 102 do not participate in the reaction.

[0053] Figure 4 is a plot of the charging performance of the flow cell battery of Figure 3. Charging at 1.4 V provides approximately 3.3 ampere-hours (Ah) of charge capacity.

[0054] Example 2: Mixed Fe / V electrolyte with 50% vanadium utilization in the catholyte.

[0055] FIG. 5 is a schematic diagram illustrating the operation of an electrochemical cell containing a mixture of iron and vanadium ions in both catholyte 104 and anolyte 102, showing the change in operation due to a higher charging voltage of 1.6 V. This increases the energy density of the flow cell battery. The increase in 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 vanadium utilization not only improves the energy density, but also reduces the electrolyte cost.

[0056] To take advantage of the increased energy density in Catholyte 104, the volume ratio of Catholyte / Anolyte was set to 2:3. Specifically, Catholyte 104 contained 2 molar volumes of iron ions and 2 molar volumes of vanadium ions for a total of 4 molar volumes. Anolyte 102 contained 3 molar volumes of iron ions and 3 molar volumes of vanadium ions for a total of 6 molar volumes. This saves 3 / 8 of the total amount of vanadium used compared to a battery without mixed electrolytes.

[0057] In this embodiment, during charging, Fe 2+ First, Fe in catholyte 104 3+ is oxidized to Fe 2+ After the ion is completely oxidized, V 4+ is oxidized to V 4+ and V 5+ Forms ionic hybrids. V 4+ Only 50-60% of the ions are V 5+ The loading is controlled to convert the ions into ions. This maintains the stability of the solution at higher temperatures, e.g., V at an operating temperature of about 40°C. 5+ This reduces the possibility of ions being precipitated. 3+ V 2+ For example, as in Example 1, the iron ions in the anolyte 102 do not participate in the reaction.

[0058] V in catholyte 104 during discharge 5+ The ion is first V 4+ It is reduced to ions. V 5+ When the ions are reduced, the Fe in catholyte 104 3+ The ion is Fe 2+ During discharge, the V in Anolyte 102 is reduced to 3+ is V 2+ As during charging, the iron ions in the anolyte 102 do not participate in the reaction.

[0059] As a result of the higher charging voltage and the change in volume ratio, more than 50% of the vanadium in the positive electrolyte (catholyte) can be utilized, thus achieving a 30% improvement in the energy density of the Fe / V cell compared to the Fe / V cell operated at the lower charging voltage, as described with respect to Figures 3 and 4.

[0060] FIG. 6 is a plot of the charging of the electrochemical cell of FIG. 5. The test was performed at 48 cm 2 The electrolyte used for both the anolyte and catholyte was 1.25 Molar (M) V, 1.25 M Fe, 6.6 M Cl. - , and 0.8M SO4 2- During charging, the Fe in the catholyte 2+ Fe 3+ All Fe is oxidized to 2+ When is converted, the high charging voltage causes the V in the catholyte to 4+ V 5+ As described herein, charging is performed at V 4+ Approximately 60% of the V 5+ This increases thermal stability by preventing the conversion of V to V during charging. 3+ V 2+ is reduced to.

[0061] 7 is a plot of the energy density of an electrolytic cell comparing the two protocols over multiple charge / discharge cycles. In these tests, the catholyte, anolyte, and membrane were as described with respect to FIG.

[0062] As described herein, the first, or common protocol (FIGS. 3 and 4) uses a 1:1 volume ratio of catholyte to anolyte and produces a voltage of up to 1.4 V. As shown in FIG. 7, the energy density of the common protocol is approximately 12 watt-hours per liter (Wh / l).

[0063] In the new protocol (Figures 5 and 6), the volume ratio of anolyte to catholyte is increased to 2:3, generating a voltage up to 1.6 V. As shown in Figure 7, the energy density of the new protocol is up to 16 Wh / l.

[0064] FIG. 8 is a plot showing the cycling efficiency of an Fe / V electrolytic cell over 200 cycles. - / SO4 2- The long-term cycling performance of an Fe / V redox flow battery using electrolytes, catholytes, anolytes, and membranes as described with respect to FIG. 6 was tested, and the coulombic, voltage, and energy efficiencies remained constant over 200 cycles.

[0065] FIG. 9 is a plot showing the capacity of the Fe / V battery over 200 cycles. The Fe / V cell also exhibited excellent capacity retention as shown in FIG. 9, with no obvious capacity loss throughout 200 cycles. The discharge energy density also remained constant over 200 cycles, indicating that the Fe / V flow battery can operate reliably for an extended period of time. The excellent stable performance of the Fe / V flow battery over extended cycles allows the system to operate with minimal system maintenance.

[0066] Embodiment

[0067] An embodiment described in the Examples herein provides a flow cell battery. The flow cell battery includes an electrochemical cell, the electrochemical cell including an ion exchange membrane, an anode current collector, and a cathode current collector. A space between the ion exchange membrane and the anode current collector forms a first channel, and a space between the ion exchange membrane and the cathode current collector forms a second channel. 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 anolyte through the first channel at a first rate, and a second tank configured to flow catholyte through the second channel at a second rate. The catholyte, or both the catholyte and the anolyte, include a mixture of iron and vanadium ions.

[0068] In one embodiment, the mixture is an equimolar mixture of iron and vanadium ions.

[0069] In one embodiment, the first rate is greater than the second rate. In one embodiment, the ratio between the first rate and the second rate is 3:2. In one embodiment, the flow cell battery includes a pump configured to flow anolyte through the first channel at the first rate and a pump to flow catholyte through the second channel at the second rate.

[0070] In one embodiment, the catholyte contains both vanadium and iron ions. In one embodiment, the anolyte contains both vanadium and iron ions. In one embodiment, the catholyte contains 1.25 molar (M) V, 1.25M Fe, 6.6M Cl. - , and 0.8M SO4 2- In one embodiment, the catholyte is the same as the anolyte.

[0071] In one embodiment, the anode current collector is a carbon-based electrode.In one embodiment, the cathode current collector is a carbon-based electrode.

[0072] In one embodiment, the first channel comprises a porous electrode material, hi one embodiment, the porous electrode material is felt or Rainey nickel.

[0073] In one embodiment, the second channel comprises a porous electrode material, hi one embodiment, the porous electrode material is felt or Rainy nickel.

[0074] In one embodiment, the flow cell battery includes a load connected between the anode current collector and the cathode current collector. In one embodiment, the flow cell battery includes a power source connected to the anode current collector and the cathode current collector.

[0075] In one embodiment, the catholyte is 50% to 60% V 5+ Contains vanadium ions in an oxidized state.

[0076] Another embodiment described in the Examples provides a method of generating an electric current. The method includes flowing an anolyte through a first channel in an electrochemical cell, the first channel being formed in a space between an anode current collector and an ion exchange membrane. The method includes flowing a catholyte through a second channel in the electrochemical cell, the second channel being formed in a space between a cathode current collector and an ion exchange membrane, the first channel and the second channel being separated by the ion exchange membrane, the catholyte comprising a mixed electrolyte containing both iron and vanadium ions. The ions flow through the ion exchange membrane to oxidize the anolyte and reduce the catholyte. An electric current is generated between the anode current collector and the cathode current collector.

[0077] In one embodiment, the method comprises flowing catholyte through the second channel at a rate that is 3 / 2 the rate of anolyte through the first channel.

[0078] In one embodiment, the method includes preparing a mixed electrolyte to include an equimolar solution of about 1.25 molar (M) to 1.5 M vanadium ions and about 1.25 M to 1.5 M iron ions, and using the mixed electrolyte as a catholyte. In one embodiment, the method includes using the mixed electrolyte as an anolyte.

[0079] In one embodiment, the method comprises administering about 6.6M Cl - ions and approximately 0.8M SO4 2- forming a mixed electrolyte to include the ions.

[0080] Other embodiments are within the scope of the following claims.

Claims

1. An ion exchange membrane; an anode current collector; a cathode current collector, wherein 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; and a first tank configured to flow anolyte through the first channel at a first rate; a second tank configured to flow catholyte through the second channel at a second rate, the catholyte or both the catholyte and the anolyte being comprised of a mixture of iron and vanadium ions; Flow cell battery.

2. The mixture is an equimolar mixture of iron ions and vanadium ions.

2. The flow cell battery of claim 1.

3. the first rate is greater than the second rate; 2. The flow cell battery of claim 1.

4. the ratio of the first rate to the second rate is 3:2; 2. The flow cell battery of claim 1.

5. a pump configured to flow the anolyte through the first channel at the first rate, and a pump for flowing the catholyte through the second channel at the second rate.

2. The flow cell battery of claim 1.

6. The catholyte contains both vanadium and iron ions.

2. The flow cell battery of claim 1.

7. The anolyte contains both vanadium and iron ions.

2. The flow cell battery of claim 1.

8. The catholyte was 1.25 molar (M) V, 1.25M Fe, 6.6M Cl. － , and 0.8M SO ４ ２－ Including, The flow cell battery of claim 6.

9. The catholyte is the same as the anolyte; The flow cell battery of claim 8.

10. The anode current collector is a carbon-based electrode.

2. The flow cell battery of claim 1.

11. The cathode current collector is a carbon-based electrode.

2. The flow cell battery of claim 1.

12. the first channel comprises a porous electrode material; 2. The flow cell battery of claim 1.

13. The porous electrode material is felt or Rainy nickel; 13. The flow cell battery of claim 12.

14. the second channel comprises a porous electrode material; 2. The flow cell battery of claim 1.

15. The porous electrode material is felt or Rainy nickel; 15. The flow cell battery of claim 14.

16. further comprising a load connected between the anode current collector and the cathode current collector.

2. The flow cell battery of claim 1.

17. further comprising a power source connected to the anode current collector and the cathode current collector.

2. The flow cell battery of claim 1.

18. The catholyte is V ５＋ Contains 50% to 60% of said vanadium ions in an oxidized state; 2. The flow cell battery of claim 1.

19. flowing an anolyte through a first channel in an electrochemical cell, the first channel being formed in a space between an anode current collector and an ion exchange membrane; flowing a catholyte through a second channel in the electrochemical cell, the second channel being formed in a space between a cathode current collector and the ion exchange membrane, the first channel and the second channel being separated by an ion exchange membrane, the catholyte including a mixed electrolyte comprising both iron and vanadium ions; flowing ions through the ion exchange membrane to oxidize the anolyte and reduce the catholyte; generating an electric current between the anode current collector and the cathode current collector. A method for generating electrical current.

20. flowing the catholyte through the second channel at a rate that is 3 / 2 the rate of the anolyte through the first channel; 20. The method of claim 19.

21. forming the mixed electrolyte from an equimolar solution of about 1.25 molar (M) to 1.5M vanadium ions and about 1.25M to 1.5M iron ions; and using the mixed electrolyte as the catholyte.

20. The method of claim 19.

22. using the mixed electrolyte as the anolyte; 22. The method of claim 21.

23. About 6.6M Cl － ions and approximately 0.8M SO ４ ２－ forming the mixed electrolyte containing ions; 22. The method of claim 21.

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