Iron-chromium redox flow battery systems and methods of using the same

Abstract

A redox flow battery system comprising: an anolyte having chromium ions in solution, wherein at least a portion of the chromium ions form a chromium complex with at least one of: NH3, NH4 + , CO(NH2)2, SCN ‑ , or CS(NH2)2; a catholyte having iron ions in solution; a first half-cell comprising a first electrode in contact with the anolyte; a second half-cell comprising a second electrode in contact with the catholyte; and a first separator separating the first half-cell from the second half-cell. Other redox flow battery systems and methods of using redox flow battery systems are also disclosed.

Description

[0001] Related Patent Applications

[0002] This patent application claims the benefit of U.S. provisional patent application serial numbers 63 / 120,204, 63 / 126,408, 63 / 127,048, 63 / 131,738, 63 / 147,182, 63 / 154,547, 63 / 174,352, and 63 / 114,160; and is a continuation-in-part of each of U.S. patent application serial numbers 17 / 362,610, 17 / 362,543, 17 / 362,521, 17 / 362,499, and 17 / 362,468, all of which are incorporated herein by reference in their entirety. TECHNICAL FIELD

[0003] The present invention relates to the field of redox flow battery systems and methods of making and using redox flow battery systems. BACKGROUND

[0004] Over the past decade, the cost of renewable energy generation has rapidly decreased, and will continue to decrease as more renewable energy generation elements, such as solar panels, are used. However, renewable energy, such as solar, hydroelectric, and wind resources, is typically intermittent, and the pattern of user loads is typically not aligned with the intermittency of these resources. A cost-effective and reliable energy storage system is needed to store power generated by renewable energy when it is available, and to provide power to users when power generated by renewable energy is not sufficient. SUMMARY

[0005] One embodiment is a redox flow battery system comprising: an anolyte having chromium ions in solution, wherein at least a portion of the chromium ions form a chromium complex with at least one of: NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2; a catholyte having iron ions in solution; a first half-cell comprising a first electrode in contact with the anolyte; a second half-cell comprising a second electrode in contact with the catholyte; and a first membrane separating the first half-cell from the second half-cell.

[0006] In at least some embodiments, the chromium complex comprises a compound or ion having the formula: [Cr 3+ (J) x (M) y (H2O) z wherein x, y, and z are non-negative integers, x + y + z = 6, and x is at least 1, J is selected from the group consisting of: NH3, NH4 + , CO(NH2)2, SCN -or CS(NH2)2, and each M is different from J, and is independently selected from the group consisting of Cl - , F - , Br - , I - , NH4 + , NH3, ethylenediaminetetraacetic acid (EDTA), CN - , SCN - , S 2- , O-NO2 - , OH - , NO2 - , CH3CN, C5H5N, NC5H4-C5H4N, C 12 H8N2, CO(NH2)2, CS(NH2)2, P(C6H5)3, -CO, CH3-CO-CH2-CO-CH3, NH2-CH2-CH2-NH2, NH2CH2COO - , O-SO2 2- , or P(o-tolyl)3.

[0007] In at least some embodiments, J is NH3or NH4 + , and at least one M is CO(NH2)2. In at least some embodiments, the chromium complex further includes at least one counterion selected from the group consisting of ammonium, chloride, bromide, iodide, fluoride, sulfate, or nitrate. In at least some embodiments, J is NH3or NH4 + . In at least some embodiments, J is CO(NH2)2or CS(NH2)2. In at least some embodiments, the chromium complex is formed in situ.

[0008] In at least some embodiments, the redox flow battery system further includes a balancing device including a balancing electrolyte including vanadium ions in solution, a third half-cell including a third electrode in contact with the anolyte or the catholyte, a fourth half-cell including a fourth electrode in contact with the balancing electrolyte, and a reducing agent in the balancing electrolyte or that can be introduced into the balancing electrolyte for reducing vanadyl ions. In at least some embodiments, the reducing agent is NH3, NH4 + , CO(NH2)2, or CS(NH2)2. In at least some embodiments, the reducing agent is an organic compound.

[0009] Another embodiment is a redox flow battery system including an anolyte having chromium ions in solution and at least one nitrogen-containing compound selected from NH3, NH4 + , CO(NH2)2, SCN -or CS(NH2)2; a catholyte having iron ions in solution; a first half-cell comprising a first electrode in contact with the anolyte; a second half-cell comprising a second electrode in contact with the catholyte; and a first separator separating the first half-cell from the second half-cell.

[0010] In at least some embodiments, the nitrogen-containing compound is NH3or includes NH4 + In at least some embodiments, the nitrogen-containing compound is CO(NH2)2or CS(NH2)2. In at least some embodiments, at least a portion of the chromium ions form chromium complexes with at least one of NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2.

[0011] In at least some embodiments, the chromium complex includes a compound or ion having the formula [Cr 3+ (J) x (M) y (H2O) z where x, y, and z are non-negative integers, x + y + z = 6, and x is at least 1, J is selected from the group consisting of NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2, and each M is different from J and is independently selected from the group consisting of Cl - , F - , Br - , I - , NH4 + , NH3, ethylenediaminetetraacetic acid (EDTA), CN - , SCN - , S 2- , O-NO2 - , OH - , NO2 - , CH3CN, C5H5N, NC5H4-C5H4N, C 12 H8N2, CO(NH2)2, CS(NH2)2, P(C6H5)3, -CO, CH3-CO-CH2-CO-CH3, NH2-CH2-CH2-NH2, NH2CH2COO - , O-SO2 2- , or P(o-tolyl)3.

[0012] In at least some embodiments, J is NH3or NH4 + , and at least one M is CO(NH2)2. In at least some embodiments, J is NH3or NH4 + .

[0013] In at least some embodiments, the redox flow battery system further comprises: a balancing device comprising a balancing electrolyte, the balancing electrolyte comprising vanadium ions in solution; a third half-cell comprising a third electrode in contact with the anolyte or the catholyte; a fourth half-cell comprising a fourth electrode in contact with the balancing electrolyte; and a reducing agent in or introducible into the balancing electrolyte for reducing vanadyl ions. In at least some embodiments, the reducing agent is NH3, NH4 + , CO(NH2)2, or CS(NH2)2. In at least some embodiments, the reducing agent is an organic compound.

[0014] Another embodiment is a method for hydrogen production. The method comprises providing a redox flow battery, the battery comprising: an anolyte comprising chromium ions in solution, a catholyte comprising iron ions in solution, a first half-cell comprising a first electrode in contact with the anolyte, a second half-cell comprising a second electrode in contact with the catholyte, and a first separator separating the first half-cell from the second half-cell; connecting the electrolytic cell to the redox flow battery, the electrolytic cell comprising a third electrode and a fourth electrode and water; discharging the redox flow battery into the electrolytic cell to produce hydrogen gas by electrolysis of water in the electrolytic cell; removing the hydrogen gas from the electrolytic cell; and storing the hydrogen gas in a container.

[0015] One embodiment is a method of making an electrolyte for a redox flow battery. The method comprises reducing a chromium ore using a carbon source to convert the chromium ore into an iron / chromium alloy having carbon particles; dissolving the iron / chromium alloy having carbon particles in sulfuric acid to form a first solution; adding calcium chloride or barium chloride to the first solution to produce a second solution comprising FeCh and CrCh; and adding an acid to the second solution to form the electrolyte.

[0016] In at least some embodiments, the chromium ore comprises at least one metal impurity, and the method further comprises reducing at least one metal impurity in the electrolyte using an electrode of the redox flow battery to form particles of the at least one metal impurity; collecting the particles of the at least one metal impurity; and removing the collected particles using a cleaning solution. In at least some embodiments, collecting the particles comprises collecting at least a portion of the particles on an electrode of the redox flow battery system.

[0017] In at least some embodiments, the electrode includes interdigitated openings or notches for collecting the particles. In at least some embodiments, the at least one metal impurity includes at least one of nickel, antimony, bismuth, zinc, platinum, palladium, gold, or copper. In at least some embodiments, the cleaning solution includes ferric ions. In at least some embodiments, the cleaning solution is at least a portion of a catholyte of the redox flow battery system. In at least some embodiments, the cleaning solution includes hydrogen peroxide or ferric chloride.

[0018] In at least some embodiments, the carbon source includes graphite, activated carbon, coal, charcoal, or carbon monoxide gas. In at least some embodiments, the method further includes adding FeCl3or CrCl3to obtain a preselected ratio of FeCl3 / CrCl3in the electrolyte.

[0019] Another embodiment is a method of preparing an electrolyte for a redox flow battery. The method includes dissolving chromium scrap in HC1 or H2SO4to produce an electrolyte including CrCl3or Cr2(SO4)3, respectively, and at least one metal impurity; reducing the at least one metal impurity in the electrolyte using an electrode of the redox flow battery to form particles of the at least one metal impurity; collecting the particles of the at least one metal impurity; and removing the collected particles using a cleaning solution.

[0020] In at least some embodiments, collecting the particles includes collecting at least a portion of the particles on an electrode of the redox flow battery system. In at least some embodiments, the electrode includes interdigitated openings or notches for collecting the particles. In at least some embodiments, the at least one metal impurity includes at least one of nickel, antimony, bismuth, zinc, platinum, palladium, gold, or copper.

[0021] In at least some embodiments, the cleaning solution includes ferric ions. In at least some embodiments, the cleaning solution is at least a portion of a catholyte of the redox flow battery system. In at least some embodiments, the cleaning solution includes hydrogen peroxide or ferric chloride.

[0022] In at least some embodiments, the method further includes adding FeCl2, Fe+FeCl3, Fe(OH)2, or Fe+Fe(OH)3to obtain a preselected ratio of FeCl3 / CrCl3in the electrolyte. In at least some embodiments, the chromium scrap includes Cr(OH)3.

[0023] Another embodiment is a method for hydrogen production. The method includes providing a redox flow battery, the battery including: an anolyte including chromium ions in solution, a catholyte including iron ions in solution, a first half-cell including a first electrode in contact with the anolyte, a second half-cell including a second electrode in contact with the catholyte, and a first separator separating the first half-cell from the second half-cell; connecting the electrolytic cell to the redox flow battery, the electrolytic cell including a third electrode and a fourth electrode and water; discharging the redox flow battery into the electrolytic cell to produce hydrogen gas by electrolysis of the water in the electrolytic cell; removing the hydrogen gas from the electrolytic cell; and storing the hydrogen gas in a container.

[0024] One embodiment is a redox flow battery system, including: an anolyte; a catholyte; a first half-cell including a first electrode in contact with the anolyte; a second half-cell including a second electrode in contact with the catholyte; a separator separating the anolyte in the first half-cell from the catholyte in the second half-cell; at least one state measuring device configured to intermittently, periodically, or continuously measure a value indicative of a state of charge of the anolyte or the catholyte, respectively, before entering or after exiting the first half-cell or the second half-cell; and a controller connected to the at least one state measuring device for generating a temporal energy profile of the anolyte or the catholyte, respectively, using the measurements.

[0025] In at least some embodiments, the at least one state measuring device includes an anolyte state measuring device configured to intermittently, periodically, or continuously measure a value indicative of a state of charge of the anolyte before entering or after exiting the first half-cell, and a catholyte state measuring device configured to intermittently, periodically, or continuously measure a value indicative of a state of charge of the catholyte before entering or after exiting the second half-cell.

[0026] In at least some embodiments, the redox flow battery system further includes an anolyte pump configured to pump the anolyte into and out of the first half-cell, and a catholyte pump configured to pump the catholyte into or out of the second half-cell. In at least some embodiments, the controller is configured to use the temporal energy profile to vary a pumping speed of the anolyte pump and the catholyte pump.

[0027] In at least some embodiments, the at least one state measurement device is configured to measure a voltage of the anolyte or the catholyte. In at least some embodiments, the controller is configured to determine a state of charge of the anolyte or the catholyte from the measurements made by the at least one state measurement device and use the state of charge in the time-energy curve. In at least some embodiments, the controller is configured to determine an average oxidation state of the anolyte or the catholyte from the measurements made by the at least one state measurement device and use the average oxidation state in the time-energy curve. In at least some embodiments, the controller is configured to generate the time-energy curve using the measurements and an estimate of the diffusion of charged species within the anolyte or the catholyte.

[0028] Another embodiment is a method of operating any of the redox flow battery systems described above. The method includes intermittently, periodically, or continuously measuring a value indicative of a state of charge of the anolyte or the catholyte before entering or after exiting the first half-cell or the second half-cell, respectively; and generating a time-energy curve for the anolyte or the catholyte, respectively, using the measurements.

[0029] In at least some embodiments, the redox flow battery system further includes an anolyte pump configured to pump the anolyte into and out of the first half-cell and a catholyte pump configured to pump the catholyte into or out of the second half-cell; the method further includes using the time-energy curve to vary a pumping rate of the anolyte pump and the catholyte pump. In at least some embodiments, the measuring includes measuring a voltage of the anolyte or the catholyte.

[0030] In at least some embodiments, generating the time-energy curve includes determining a state of charge of the anolyte or the catholyte from the measurements made by the at least one state measurement device and using the state of charge in the time-energy curve. In at least some embodiments, generating the time-energy curve includes determining an average oxidation state of the anolyte or the catholyte from the measurements made by the at least one state measurement device and using the average oxidation state in the time-energy curve. In at least some embodiments, generating the time-energy curve includes generating the time-energy curve using the measurements and an estimate of the diffusion of charged species within the anolyte or the catholyte.

[0031] Another embodiment is a non-transitory computer-readable medium having processor-executable instructions for operating any of the above redox flow battery systems. When installed onto a device, the processor-executable instructions cause the device to perform actions. The actions include intermittently, periodically, or continuously measuring values indicative of a state of charge of the anolyte or the catholyte, respectively, before entering or after exiting the first half-cell or the second half-cell; and generating a time-energy curve for the anolyte or the catholyte, respectively, using the measurements.

[0032] In at least some embodiments, the actions further include using the time-energy curve to vary a pumping speed of an anolyte pump and a catholyte pump of the redox flow battery system. In at least some embodiments, the measurements include measuring a voltage of the anolyte or the catholyte.

[0033] In at least some embodiments, generating the time-energy curve includes determining a state of charge or an average oxidation state of the anolyte or the catholyte from the measurements by the at least one state measuring device and using the state of charge or the average oxidation state in the time-energy curve. In at least some embodiments, generating the time-energy curve includes generating the time-energy curve using an estimate of a diffusion of a charge-carrying species within the anolyte or the catholyte.

[0034] One embodiment is a redox flow battery system, comprising: an anolyte; a catholyte; a first half-cell comprising a first electrode in contact with the anolyte; a second half-cell comprising a second electrode in contact with the catholyte; a separator separating the anolyte in the first half-cell from the catholyte in the second half-cell; at least one charge measuring device configured for intermittently, periodically, or continuously measuring power provided by a charging source; and a controller connected with the at least one charge measuring device for generating a time-energy curve for the anolyte or the catholyte, respectively, using the measurements.

[0035] One embodiment is a redox flow battery system comprising an anolyte; a catholyte; an anolyte tank configured to hold at least a portion of the anolyte; a catholyte tank configured to hold at least a portion of the catholyte; and a primary redox flow battery device comprising: a first half-cell comprising a first electrode in contact with the anolyte, a second half-cell comprising a second electrode in contact with the catholyte, a first separator separating the first half-cell from the second half-cell, a first anolyte pump configured to move the anolyte between the first half-cell and the anolyte tank, and a first catholyte pump configured to move the catholyte between the second half-cell and the catholyte tank. The redox flow battery system further comprises a secondary redox flow battery device comprising: a third half-cell comprising a third electrode in contact with the anolyte, a fourth half-cell comprising a fourth electrode in contact with the catholyte, a first separator separating the third half-cell from the fourth half-cell, a second anolyte pump configured to move the anolyte between the third half-cell and the anolyte tank, and a second catholyte pump configured to move the catholyte between the fourth half-cell and the catholyte tank; wherein the peak power delivery capacity of the secondary redox flow battery device is less than the peak power delivery capacity of the primary redox flow battery device.

[0036] In at least some embodiments, the redox flow battery system further comprises a controller connected to the primary redox flow battery device and the secondary redox flow battery device. In at least some embodiments, the controller is configured to switch from operation of the primary redox flow battery device to operation of the secondary redox flow battery device when the discharge power falls below a first predetermined level. In at least some embodiments, the controller is configured to switch from operation of the secondary redox flow battery device to operation of the primary redox flow battery device when the discharge power exceeds a second predetermined level. In at least some embodiments, the first predetermined level and the second predetermined level are the same. In at least some embodiments, the second predetermined level is greater than the first predetermined level.

[0037] In at least some embodiments, the controller is configured to increase operation of the secondary redox flow battery device into operation of the primary redox flow battery device when the discharge power exceeds a third predetermined level. In at least some embodiments, the ratio of the peak power delivery capacity of the secondary redox flow battery device to the peak power delivery capacity of the primary redox flow battery device is in the range of 1 :5 to 1 :200.

[0038] In at least some embodiments, the redox flow battery system further includes a first electrode structure including a first non-conductive substrate having a first surface and a second surface opposite the first surface, and including a first separator and first conductive elements extending through the first non-conductive substrate, wherein the first non-conductive substrate prevents the anolyte and the catholyte from flowing through the first non-conductive substrate, and each first conductive element includes a first end exposed at the first surface of the first non-conductive substrate and a second end exposed at the second surface of the first non-conductive substrate, wherein the first electrode is disposed on the first surface of the first non-conductive substrate, the second electrode is disposed on the second surface of the first non-conductive substrate, and wherein the first electrode includes the first ends of the first conductive elements and the second electrode includes the second ends of the first conductive elements.

[0039] In at least some embodiments, the redox flow battery system further includes a second electrode structure including a second non-conductive substrate having a first surface and a second surface opposite the first surface, and including a second separator and second conductive elements extending through the second non-conductive substrate, wherein the second non-conductive substrate prevents the anolyte and the catholyte from flowing through the second non-conductive substrate, and each second conductive element includes a first end exposed at the first surface of the second non-conductive substrate and a second end exposed at the second surface of the second non-conductive substrate, wherein the third electrode is disposed on the first surface of the second non-conductive substrate, the fourth electrode is disposed on the second surface of the second non-conductive substrate, and wherein the third electrode includes the first ends of the second conductive elements and the fourth electrode includes the second ends of the second conductive elements.

[0040] In at least some embodiments, the anolyte includes chromium ions and at least a portion of the chromium ions form a chromium complex with at least one of: NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2.

[0041] In at least some embodiments, the redox flow battery system further includes a balancing device containing a balancing electrolyte including vanadium ions in solution, a fifth half-cell including a fifth electrode in contact with the anolyte or the catholyte, a sixth half-cell including a sixth electrode in contact with the balancing electrolyte, and a reducing agent in or introducible into the balancing electrolyte for reducing vanadyl ions. In at least some embodiments, the second anolyte pump and the second catholyte pump have a smaller pumping capacity than the first anolyte pump and the second anolyte pump.

[0042] Another embodiment is a method of operating any of the redox flow battery systems described above. The method includes charging the redox flow battery system by connecting a power source to at least one of the primary redox flow battery device or the secondary redox flow battery device; and discharging the redox flow battery system by connecting a load to at least one of the primary redox flow battery device or the secondary redox flow battery device.

[0043] In at least some embodiments, discharging includes switching from the primary redox flow battery device being connected to the load to the secondary redox flow battery device being connected to the load when the discharge power falls below a first predetermined level. In at least some embodiments, discharging includes switching from the secondary redox flow battery device being connected to the load to the primary redox flow battery being connected to the load when the discharge power exceeds a second predetermined level. In at least some embodiments, the first predetermined level and the second predetermined level are the same. In at least some embodiments, the second predetermined level is greater than the first predetermined level.

[0044] In at least some embodiments, discharging includes connecting both the secondary redox flow battery device and the primary redox flow battery to the load when the discharge power exceeds a third predetermined level. In at least some embodiments, a ratio of a peak power delivery capacity of the secondary redox flow battery device to a peak power delivery capacity of the primary redox flow battery device is in a range of 1 :5 to 1 :200.

[0045] One embodiment is a redox flow battery system, comprising: an anolyte; a catholyte; a first electrode structure comprising a substrate having a first surface and a second surface opposite the first surface, a first electrode disposed on the first surface of the substrate, a second electrode disposed on the second surface of the substrate, and a conductive element extending through the substrate, wherein the substrate prevents the anolyte and the catholyte from flowing through the substrate, and each conductive element comprises a first end exposed at the first surface of the substrate and a second end exposed at the second surface of the substrate, wherein the first electrode comprises the first end of the conductive element and the second electrode comprises the second end of the conductive element; a first half-cell, wherein the first electrode is in contact with the anolyte; and a second half-cell, wherein the second electrode is in contact with the catholyte.

[0046] In at least some embodiments, the conductive element comprises at least one of a metal wire, a carbon fiber, a graphite fiber, or silicon carbide. In at least some embodiments, the substrate comprises at least one of a plastic, a resin, a carbon or graphite plate, or a metal or alloy plate. In at least some embodiments, the substrate comprises a polypropylene or polyethylene perfluoroalkoxy alkane, a polyvinylidene fluoride, a polytetrafluoroethylene, a polyvinyl chloride, or a chlorinated polyvinyl chloride.

[0047] In at least some embodiments, at least one of the first electrode or the second electrode includes an additional conductive material. In at least some embodiments, the additional conductive material includes a metal, carbon fibers, carbon felt, or silicon carbide.

[0048] In at least some embodiments, the anolyte includes chromium ions in solution and the catholyte includes iron ions in solution. In at least some embodiments, the anolyte further includes iron ions in solution and the anolyte includes chromium ions in solution.

[0049] In at least some embodiments, the redox flow battery system further includes: a balancing device including a balancing electrolyte; a second electrode structure including a substrate having a first surface and a second surface opposite the first surface, a third electrode disposed on the first surface of the substrate, a fourth electrode disposed on the second surface of the substrate, and conductive elements extending through the substrate, wherein the substrate prevents the anolyte and the catholyte from flowing through the substrate, and each conductive element includes a first end exposed at the first surface of the substrate and a second end exposed at the second surface of the substrate, wherein the third electrode includes the first end of the conductive elements and the fourth electrode includes the second end of the conductive elements; a third half-cell including the third electrode in contact with the anolyte; and a fourth half-cell including the fourth electrode in contact with the catholyte.

[0050] In at least some embodiments, the balancing electrolyte includes vanadium ions in solution and the balancing device further includes a reducing agent in the balancing electrolyte or introducible into the balancing electrolyte for reducing divanadium ions. In at least some embodiments, at least a portion of the chromium ions form chromium complexes with at least one of: NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2.

[0051] Another embodiment is a method of making an electrode structure. The method includes: providing a substrate configured to prevent electrolyte from flowing through the substrate; and extending a plurality of conductive elements through the substrate such that each conductive element includes a first end exposed at a first surface of the substrate and a second end exposed at a second surface of the substrate, wherein a first electrode includes the first ends of the conductive elements and a second electrode includes the second ends of the conductive elements.

[0052] In at least some embodiments, the extending includes molding the substrate around portions of the conductive elements. In at least some embodiments, the extending includes pushing the conductive elements through the substrate. In at least some embodiments, the method further includes heating the substrate to flow the substrate around the conductive elements.

[0053] In at least some embodiments, the method further includes disposing of an additional first conductive material on a first surface of the substrate, wherein the first electrode further comprises the additional first conductive material. In at least some embodiments, the method further includes disposing of an additional second conductive material on a second surface of the substrate, wherein the second electrode further comprises the additional second conductive material.

[0054] Another embodiment is a method of operating any of the above-described redox flow battery systems. This method includes charging the redox flow battery system by connecting a power source to a first electrode and a second electrode; and discharging the redox flow battery system by connecting a load to the first electrode and the second electrode.

[0055] In at least some embodiments, the anolyte comprises chromium ions in solution, and the catholyte comprises iron ions in solution. In at least some embodiments, the anolyte further comprises iron ions in solution, and the anolyte comprises chromium ions in solution. Attached Figure Description

[0056] Non-limiting and non-complete embodiments of the invention are described with reference to the following accompanying drawings. In the drawings, unless otherwise stated, the same reference numerals refer to the same parts throughout the various figures.

[0057] To better understand the present invention, reference will be made to the following specific embodiments in conjunction with the accompanying drawings, wherein:

[0058] Figure 1 This is a schematic diagram of one embodiment of the redox flow battery system according to the present invention;

[0059] Figure 2 This is a schematic diagram of one embodiment of the electrode of the redox flow battery system according to the present invention;

[0060] Figure 3 This is a flowchart of an embodiment of removing or reducing impurities in a redox flow battery system according to the present invention;

[0061] Figure 4 This is a schematic diagram of another embodiment of the redox flow battery system according to the present invention, wherein the cathode electrolyte is transferred to a second half-cell for maintenance;

[0062] Figure 5A This is a schematic diagram of one embodiment of the system according to the present invention, which includes a redox flow battery system and a balancing device;

[0063] Figure 5B It is based on the present invention Figure 5A A schematic diagram of one implementation scheme of the system's balancing device;

[0064] Figure 5C is a schematic diagram of another embodiment of a system according to the present invention comprising a redox flow battery system and a balancing device;

[0065] Figure 5D is a schematic diagram of a balancing device of a system according to the present invention; Figure 5C

[0066] Figure 5E is a schematic diagram of another embodiment of a balancing device according to the present invention;

[0067] Figure 6A is a schematic diagram of an electrolyte tank of a redox flow battery system according to the present invention with a pressure release valve;

[0068] Figure 6B is a schematic diagram of an electrolyte tank of a redox flow battery system according to the present invention with a liquid containing U-tube device for pressure release;

[0069] Figure 6C is a schematic diagram of an electrolyte tank of a redox flow battery system according to the present invention with a device for gas migration between tanks;

[0070] Figure 7 is a schematic diagram of another embodiment of a redox flow battery system according to the present invention with a second container;

[0071] Figure 8 is a schematic diagram of another embodiment of a redox flow battery system according to the present invention with temperature zones;

[0072] Figure 9A is a schematic cross-sectional view of an embodiment of an electrode structure according to the present invention;

[0073] Figure 9B is a schematic cross-sectional view of an electrode structure according to the present invention in a redox flow battery device; Figure 9A

[0074] Figure 10 is a schematic diagram of an embodiment of a redox flow battery system according to the present invention with a primary redox flow battery device and a secondary redox flow battery device;

[0075] Figure 11 is a schematic diagram of an embodiment of a redox flow battery system according to the present invention connected to an electrolysis cell for production of hydrogen;

[0076] Figure 12 ​​is a schematic diagram of one embodiment of a redox flow battery system with a particulate filter according to the present invention;

[0077] Figure 13 is a plot of charge capacity versus charge voltage for a balanced redox flow battery system and two unbalanced redox flow battery systems according to the present invention;

[0078] Figure 14 is a flowchart of one embodiment of a method of determining average oxidation state (AOS) of a redox flow battery system according to the present invention;

[0079] Figure 15 is a flowchart of another embodiment of a method of determining average oxidation state (AOS) of a redox flow battery system according to the present invention;

[0080] Figure 16 is a plot of self-discharge time versus discharge voltage for an unbalanced redox flow battery system according to the present invention;

[0081] Figure 17 is a flowchart of a third embodiment of a method of determining average oxidation state (AOS) of a redox flow battery system according to the present invention;

[0082] Figure 18 is a schematic diagram of another embodiment of a redox flow battery system according to the present invention, including a cell for measuring open circuit voltage (OCV);

[0083] Figure 19 is a flowchart of one method of determining storage or charge capacity by measuring end OCV according to the present invention;

[0084] Figure 20 is a flowchart of one method of determining AOS using measured values of OCV according to the present invention;

[0085] Figure 21 is a schematic perspective view of a plurality of electrolyte tanks disposed in a storage area; and

[0086] Figure 22 is a schematic diagram of one embodiment of a redox flow battery system with a state of charge measuring device according to the present invention; and

[0087] Figure 23 is a flowchart of one embodiment of a method of operating a redox flow battery system using a time-energy curve. DETAILED DESCRIPTION

[0088] The present invention relates to the field of redox flow battery systems and methods of making and using redox flow battery systems. The present invention also relates to iron-chromium (Fe-Cr) redox flow battery systems utilizing chromium complexes with nitrogen-containing ligands in electrolytes and methods of making and using Fe-Cr redox flow battery systems. The present invention also relates to iron-chromium (Fe-Cr) redox flow battery systems and methods of making chromium-containing electrolytes for use in such systems. The present invention also relates to redox flow battery systems and methods of operating redox flow battery systems utilizing temporal energy profiles. The present invention also relates to redox flow battery systems utilizing primary and secondary redox flow battery devices and methods of making and using redox flow battery systems. The present invention also relates to redox flow battery systems utilizing bipolar electrode structures and methods of making and using redox flow battery systems.

[0089] Redox flow battery systems are a promising technology for storing energy generated from renewable energy sources (e.g., solar, wind, and hydroelectric energy) as well as non-renewable and other energy sources. As described herein, in at least some embodiments, a redox flow battery system can have one or more of the following properties: long lifetime; reusable energy storage; or tunable power and storage capacity.

[0090] Figure 1 One embodiment of a redox flow battery system 100 is shown. It will be recognized that other redox flow battery systems 100 can include more or fewer elements and that the elements can be arranged in different ways than shown in the illustrated embodiment. It will also be recognized that the following description of components, methods, systems, etc. can apply to other redox flow battery systems that differ from the illustrated embodiment.

[0091] Figure 1 The redox flow battery system 100 of FIG. 1 includes two electrodes 102, 104 and associated half-cells 106, 108 separated by a separator 110. The electrodes 102, 104 can be in contact with or separated from the separator. Electrolyte solutions flow through the half-cells 106, 108 and are referred to as an anolyte 112 and a catholyte 114. The redox flow battery system 100 further includes an anolyte tank 116, a catholyte tank 118, an anolyte pump 120, a catholyte pump 122, an anolyte distribution device 124, and a catholyte distribution device 126. The anolyte 112 is stored in the anolyte tank 116 and flows through the anolyte distribution device 124 to the half-cell 106 at least in part by action of the anolyte pump 120. The catholyte 114 is stored in the catholyte tank 118 and flows through the catholyte distribution device 126 to the half-cell 108 at least in part by action of the catholyte pump 122. It will be recognized that although the anolyte 112 and the catholyte 114 are shown as being stored in separate tanks 116, 118, the anolyte 112 and the catholyte 114 can be stored in a common tank.Figure 1 The illustrated embodiments include a single component of each, but other embodiments may include any one or more of the illustrated components. For example, other embodiments may include a plurality of electrodes 102, a plurality of electrodes 104, a plurality of anolyte tanks 116, a plurality of cathode electrolyte tanks 118, a plurality of half-cells 112 or a plurality of half-cells 114, or any combination thereof.

[0092] The anolyte and the cathode electrolyte are electrolytes, and they can be the same electrolyte or different electrolytes. During the flow of energy into or out of the redox flow battery system 100, the electrolyte in one of the half-cells 106 and 108 is oxidized and loses electrons, while the electrolyte in the other half-cell is reduced and gains electrons.

[0093] like Figure 1 As shown, the redox flow battery system 100 can be connected to a load / power source 130 / 132. In charging mode, the redox flow battery system 100 can be charged or recharged by connecting the flow battery to the power source 132. The power source 132 can be any power source, including but not limited to fossil fuel power, nuclear power, other batteries or cells, and renewable power sources such as wind, solar, or hydropower. In discharging mode, the redox flow battery system 100 can provide energy to the load 130. In charging mode, the redox flow battery system 100 converts electrical energy from the power source 132 into chemical potential energy. In discharging mode, the redox flow battery system 100 converts the chemical potential energy back into electrical energy to supply the load 130.

[0094] The redox flow battery system 100 can also be connected to a controller 128 that can control the operation of the redox flow battery system. For example, the controller 128 can connect or disconnect the redox flow battery system 100 from a load 130 or a power supply 132. The controller 128 can control the operation of the anolyte pump 120 and the cathode electrolyte pump 122. The controller 128 can control the operation of valves associated with the anolyte tank 116, the cathode electrolyte tank 118, the anolyte distribution system 124, the cathode electrolyte distribution system 126, or the half-cells 106, 108. The controller 128 can be used to control the general operation of the redox flow battery system 100, including switching between charging mode, discharging mode, and optional maintenance mode (or any other suitable system operating mode). In at least some embodiments, the controller or the redox flow battery system can control the temperature within the half-cell or elsewhere in the system. In at least some implementations, during operation, the temperature of the half-cell (or the overall system or a portion of the system) is controlled to be no greater than 65, 60, 55 or 50 degrees Celsius.

[0095] In at least some embodiments, the anolyte pump 120 or the catholyte pump 122 (or both) can be operated to increase or maintain the temperature of the anolyte / catholyte or half-cell. Operation of the pump generates heat that can be at least partially transferred to the anolyte or catholyte. In at least some embodiments, if the temperature of the anolyte or catholyte (or the corresponding half-cell 106, 108) falls below a predetermined value, the anolyte pump 120 or the catholyte pump 122 is activated or increased in operation, respectively, to generate heat that is at least partially transferred to the anolyte or catholyte, respectively.

[0096] Any suitable controller 128 can be used, including but not limited to one or more computers, laptops, servers, any other computing device, etc., or any combination thereof, and can include, for example, one or more processors, one or more memories, one or more input devices, one or more display devices, etc. The controller 128 can be connected to the redox flow battery system by any wired or wireless connection, or any combination thereof. The controller 128 (or at least a portion of the controller) can be located locally to the redox flow battery system 100, or partially or entirely located non-locally to the redox flow battery system.

[0097] The electrodes 102, 104 can be made of any suitable material, including but not limited to graphite or other carbon materials (including solid, felt, paper, or cloth electrodes made of graphite or carbon), gold, titanium, lead, etc. The two electrodes 102, 104 can be made of the same or different materials. In at least some embodiments, the redox flow battery system 100 does not include any homogeneous or metallic catalysts for the redox reactions in the anolyte or catholyte or both. This can limit the types of materials that can be used for the electrodes.

[0098] The separator 110 separates the two half-cells 106, 108. In at least some embodiments, the separator 110 allows the transport of selected ions (e.g., H + - or iron or chromium ions, or any combination thereof) during charging or discharging of the redox flow battery system 100. In some embodiments, the separator 110 is a microporous membrane. Any suitable separator 110 can be used, examples of suitable separators including but not limited to ion transfer membranes, anion transfer membranes, cation transfer membranes, microporous separators, etc., or any combination thereof.

[0099] As Figure 9A ​As shown, the alternatives to the electrodes 102, 104 and the separator 110 are an electrode structure (which can be referred to as a "bipolar electrode") 905 that functions as both an anode, a cathode, and a separator. The electrode structure 905 includes a substrate 910 having electrically conductive elements 911 that extend through the substrate from a first surface of the substrate to a second surface of the substrate. As shown, these electrically conductive elements 911 are exposed in both half-cells 906, 908. The substrate 910 can function as the separator 110. Figure 9B

[0100] The substrate 910 is made of a material that is impermeable or substantially blocks or impedes the flow of both anolyte and catholyte through the substrate. Examples of such materials include plastics (e.g., polypropylene, polyethylene, perfluoroalkoxy alkane (PFA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), etc.), resins, carbon or graphite plates, metal plates, metal alloy plates, etc. In at least some embodiments, the substrate is non-conductive. In at least some embodiments, the substrate is conductive, e.g., a graphite plate. In at least some embodiments, the electrical conductivity of the substrate 910 between the first and second surfaces is less than the electrical conductivity of the electrically conductive elements 911.

[0101] The electrically conductive elements 911 extend through the substrate 910 so that both anolyte and catholyte can contact them. Examples of suitable electrically conductive elements include, but are not limited to, metal wires (e.g., titanium iron, copper, zinc, silver, gold, or platinum wire), carbon fibers, graphite fibers arranged for conducting electrons through the substrate, silicon carbide, etc., or any combination thereof. The electrically conductive elements 911 form part (or all) of the electrodes 902, 904 disposed on opposite first and second surfaces of the substrate 910. Optionally, the electrodes 902, 904 can include additional electrically conductive material, e.g., metal, carbon fibers, carbon felt, silicon carbide, etc., that does not extend through the substrate 910, but is disposed on the surface of the substrate or extends into the substrate but not through the substrate.

[0102] In at least some embodiments, the substrate 910 can be molded with the electrically conductive elements 911 arranged so that the electrically conductive elements will extend through the substrate. In at least some embodiments, the electrically conductive elements 911 can be inserted or pushed through the substrate 910. In at least some embodiments, after the electrically conductive elements 911 are passed through the substrate, the substrate 910 can be heated to flow the substrate material and embed the middle portions of the electrically conductive elements within the substrate.

[0103] ​While the electrode structure 905 is disclosed in the context of a Fe-Cr redox flow battery system, it will be recognized that the electrode structure 905 can be used in other redox flow battery systems, including but not limited to vanadium redox flow battery systems, vanadium-bromine redox flow battery systems, vanadium-iron redox flow battery systems, zinc-bromine redox flow battery systems, all-iron redox flow battery systems, organic aqueous redox flow battery systems, and the like. The electrode structure 905 can also be used in other electrochemical systems and methods.

[0104] Redox flow battery systems can be safe, reliable, and provide a reusable energy storage medium. However, it is challenging to determine a redox flow battery system that has a long life expectancy of storing energy (e.g., a flow battery system that retains its storage capacity over many charge / discharge cycles) and is made from materials that have abundant availability (e.g., materials that are abundant on Earth, commercially mined, and available in relatively large quantities). Current lithium and vanadium batteries use materials that have limited availability. The storage capacity of many conventional battery systems also decreases when subjected to 10, 50, or 100 or more charge / discharge cycles. Another challenge for aqueous redox flow battery systems is controlling or avoiding the evolution of hydrogen gas or oxygen gas from water.

[0105] As described herein, a suitable and useful redox flow battery system is a Fe-Cr redox flow battery system that utilizes Fe-Cr redox chemistry. Iron and chromium are generally readily available for purchase, and at least in some embodiments, the storage capacity of the Fe-Cr redox flow battery system does not decrease by more than 10% or 20% after at least 100, 200, 250, or 500 charge / discharge cycles, or can be configured using a maintenance procedure to retain at least 70%, 80%, or 90% of the storage capacity after at least 100, 200, 250, or 500 charge / discharge cycles. 3+ / Fe 2+ and Cr 3+ / Cr 2+ As described herein, a suitable and useful redox flow battery system is a Fe-Cr redox flow battery system that utilizes Fe-Cr redox chemistry. Iron and chromium are generally readily available for purchase, and at least in some embodiments, the storage capacity of the Fe-Cr redox flow battery system does not decrease by more than 10% or 20% after at least 100, 200, 250, or 500 charge / discharge cycles, or can be configured using a maintenance procedure to retain at least 70%, 80%, or 90% of the storage capacity after at least 100, 200, 250, or 500 charge / discharge cycles.

[0106] In at least some embodiments, the electrolyte of the Fe-Cr redox flow battery system (i.e., the catholyte or the anolyte) includes an iron-containing compound or a chromium-containing compound (or both) dissolved in a solvent. In some embodiments, both the anolyte and the catholyte contain both an iron-containing compound and a chromium-containing compound. The concentrations of the two compounds in the anolyte and the catholyte can be the same or different. In other embodiments, the catholyte includes only an iron-containing compound and the anolyte includes only a chromium-containing compound.

[0107] In at least some embodiments, the chromium-containing compound can be, for example, chromium chloride, chromium sulfate, chromium bromide, or the like, or any combination thereof. In at least some cases, it has been found that chloride-complexed chromium ions (e.g., Cr(H20)5Cl 3+ / 2+ ) have faster reaction kinetics and lower H2 production compared to at least some other chromium ion complexes (e.g., Cr(H20)6 2+ / + ). Thus, it can be beneficial to include chloride in the anolyte (e.g., from the chromium-containing compound, the solvent, or both).

[0108] It has been found that chromium can form complexes with nitrogen-containing ligands that are at least in some cases more stable than chloride complexes of chromium. In at least some cases, in a Fe-Cr redox flow battery, chromium complexes with nitrogen-containing ligands can have higher redox activity, or can result in fewer side reactions (e.g., hydrogen generation). In at least some embodiments, the chromium-containing compound can be a chromium complex including at least one of the following nitrogen-containing ligands: ammonia (NH3), ammonium (NH4 + ), urea (CO(NH2)2), thiocyanate (SCN - ), or thiourea (CS(NH2)2), or any combination thereof. Examples of complexes with different nitrogen-containing ligand combinations include chromium complexes with ammonia and urea.

[0109] In at least some embodiments, the chromium complex has the formula [Cr 3+ (J) x (M) y (H20) z ], where x is a positive integer, y and z are non-negative integers, x + y + z = 6, J is selected from the group consisting of NH3, NH4 + , CO(NH2)2, SCN - , or CS(NH2)2, and each M is different from J and is independently selected from the group consisting of Cl - , F - , Br - , I - , NH4 + , NH3, ethylenediaminetetraacetic acid (EDTA), CN - , SCN - , S 2- , O-NO2 - , OH - , NO2 - , CH3CN, C5H5N, NC5H4-C5H4N, C 12H8N2, CO(NH2)2, CS(NH2)2, P(C6H5)3, -CO, CH3-CO-CH2-CO-CH3, NH2-CH2-CH2-NH2, NH2CH2COO - , O-SO2 2- , or P(o-tolyl)3. Such chromium-containing compounds can also include any suitable counterion, including but not limited to ammonium, chloride, bromide, iodide, fluoride, sulfate, nitrate, and the like or any combination thereof. One example of a chromium complex is Cr(NH3) x Cl y (H2O) z where x and z are in the range of 1 to 6, and y is in the range of 1 to 3. In at least some embodiments, the ratio of Cr to NH3 (or other nitrogen-containing ligand) can be less than 1 for the anolyte or catholyte electrolyte, so long as a portion of the chromium ions are complexed with ammonia (or other nitrogen-containing ligand).

[0110] In at least some embodiments, the chromium complex can be generated in situ in the electrolyte (anolyte or catholyte electrolyte or both) by exposing a chromium salt (e.g., chromium chloride, chromium sulfate, chromium bromide, and the like or any combination thereof) or other chromium compound to a ligand-containing compound (e.g., ammonia, ammonium chloride, urea, potassium thiocyanate, sodium thiocyanate, or thiourea).

[0111] In at least some embodiments, the molar ratio of nitrogen-containing ligand to chromium in the electrolyte (anolyte or catholyte electrolyte or both) is in the range of 1 : 10 to 10: 1. In the case of chromium complexes with ammonia and urea, the molar ratio of ammonia to urea is in the range of 1 : 10 to 10: 1. It should be understood that at least some nitrogen-containing ligands can not complex with chromium. For example, at least some nitrogen-containing ligands can complex with iron, or can not complex in the electrolyte.

[0112] The iron-containing compound can be, for example, iron chloride; iron sulfate; iron bromide; an iron complex including at least one of ammonia (NH3), ammonium (NH4 + ), urea (CO(NH2)2), thiocyanate (SCN - ), or thiourea (CS(NH2)2) as a ligand; or any combination thereof. In at least some embodiments, the iron complex can include the same ligands or a subset of these ligands as the chromium complex in the same electrolyte (anolyte or catholyte electrolyte or both).

[0113] The solvent can be water; an aqueous acid, such as hydrochloric acid, hydrobromic acid, sulfuric acid, etc.; or an aqueous solution including a soluble salt of a weak acid or weak base (e.g., ammonium chloride). In at least some embodiments, the water content of the anolyte or catholyte (or both) is at least 40, 45, or 50 wt.%. In at least some embodiments, both the catholyte and the anolyte of the Fe-Cr redox flow battery system include ferric chloride and chromium chloride dissolved in hydrochloric acid. In at least some embodiments, the catholyte of the Fe-Cr redox flow battery system includes ferric chloride dissolved in hydrochloric acid and the anolyte includes chromium chloride dissolved in hydrochloric acid.

[0114] In at least some embodiments, a nitrogen-containing compound can also provide benefits relative to the solvent, even if the nitrogen-containing compound is not a ligand for chromium or iron. For example, urea or thiourea in the electrolyte (anolyte or catholyte or both) can neutralize the HC1 in the electrolyte, which can reduce HC1 vapor during battery operation. As another example, a solvent with ammonium or ammonium ions (e.g., replacing all or part of the hydrochloric acid with ammonium chloride) can result in an effective electrolyte (anolyte or catholyte or both) with lower acidity.

[0115] In at least some embodiments, both the catholyte and the anolyte of the Fe-Cr redox flow battery system include ferric chloride and chromium chloride dissolved in hydrochloric acid. In at least some embodiments, the catholyte of the Fe-Cr redox flow battery system includes ferric chloride dissolved in hydrochloric acid and the anolyte includes chromium chloride dissolved in hydrochloric acid.

[0116] In at least some embodiments, the molar concentration of iron in the catholyte or anolyte or both is in the range of 0.5 to 2, or at least 1 M. In at least some embodiments, the molar concentration of chromium in the anolyte or catholyte or both is in the range of 0.1 to 2, or at least 0.2, 0.5, or 1 M. In at least some embodiments, the molar concentration of hydrochloric acid or other aqueous acid or base is in the range of 0.5 to 2. In at least some embodiments, the molar concentration of ammonia or ammonium ions is in the range of 0.5 to 4.

[0117] As one example of a method of forming an anolyte or catholyte with a chromium complex, a mixture of 100 grams of FeCl2-4H2O (35 wt.%), CrCl3-6H2O (45 wt.%), and NH4Cl (20%) was added to a 250 mL beaker and dissolved in distilled ionized water to form a 150 mL solution. The solution was stirred at 50°C to 60°C to accelerate dissolution. 2 mL of 37 wt.% HC1 was added to the solution to adjust the pH.

[0118] In another example, a mixture of 100 grams of FeCl2*4H2O (32 wt%), CrCl3*6H2O (43 wt%), and CO(NH2)2 (25%) was added to a 250 mL beaker and dissolved in distilled, ionized water to form a 150 mL solution. 1.5 mL of 37 wt% HC1 was added to the solution to adjust the pH.

[0119] As a third example, a mixture of 100 grams of FeCl2*4H2O (35 wt%), CrCl3*6H2O (45 wt%), and NH4C1 (10%) and CO(NH2)2 (10%) was added to a 250 mL beaker and dissolved in distilled, ionized water to form a 150 mL solution. 2 mL of 37 wt% HC1 was added to the solution to adjust the pH.

[0120] One challenge with previous Fe-Cr redox flow batteries was the production or evolution of hydrogen gas (H2) on the negative electrode as a result of the redox reactions. In at least some cases, increasing the utilization of chromium in the redox flow battery can increase the production of hydrogen gas.

[0121] It is generally desirable to limit or reduce the production of hydrogen gas in a redox flow battery. It has been found that limiting the use of chromium results in lower hydrogen generation while maintaining sufficient energy density in the redox flow battery system. In at least some embodiments, the utilization of chromium in the anolyte of the redox flow battery system is limited to no more than 80%, 70%, or 60% or less. In at least some embodiments, the utilization of chromium in the anolyte is limited by the amount of iron in the catholyte, or by 100% utilization of iron in the catholyte.

[0122] By managing the relative amounts of chromium and iron in the redox flow battery system, the utilization of chromium can be at least partially managed. The term "molar ratio" as used herein refers to the ratio of the molar amount of one component relative to the molar amount of a second component. In at least some embodiments, the molar ratio of chromium in the anolyte to iron in the catholyte (Cr(anolyte) / Fe(catholyte)) is not 1, but rather the molar ratio of Cr(anolyte) / Fe(catholyte) is at least 1.25 or greater (e.g., at least 1.43, 1.67 or greater). In at least some embodiments, the molar amount of iron in the catholyte is no more than 80%, 70%, or 60% or less of the molar amount of chromium in the anolyte. In at least some embodiments, the lesser amount of available iron limits the utilization of available chromium to no more than 80%, 70%, or 60%. In at least some embodiments, both the anolyte and the catholyte are mixed iron / chromium solutions.

[0123] In at least some embodiments, the concentration of iron in the catholyte differs from the concentration of chromium in the anolyte to produce a desired molar ratio. In at least some embodiments, the molar amount of iron in the catholyte is no more than 80%, 70%, or 60% or less of the concentration of chromium in the anolyte.

[0124] In at least some embodiments, the concentration of iron in the catholyte and the concentration of chromium in the anolyte are the same. In such embodiments, the molar ratio of chromium to iron in the anolyte and catholyte can be selected by selecting the volumes of the anolyte and catholyte, respectively. In at least some embodiments, the volume ratio of the anolyte to the catholyte is at least 1.25: 1 or greater (e.g., at least 1.43: 1 or 1.67: 1 or greater), and when the concentration of chromium in the anolyte and the concentration of iron in the catholyte are the same, the molar ratio is equal to the volume ratio. In at least some embodiments, the volume of the catholyte is no more than 80%, 70%, or 60% of the volume of the anolyte.

[0125] In some embodiments, the volumes of the anolyte and catholyte can be based on the volumes of the respective half-cells 106, 108. In some embodiments, the volumes of the anolyte and catholyte can be based on the volumes of the respective catholyte and anolyte portions of the redox flow battery system 100. For example, the catholyte portion can include the half-cell 108, the catholyte tank 118, and the catholyte distribution device 126. The anolyte portion can include the half-cell 106, the anolyte tank 116, and the anolyte distribution device 124.

[0126] It will be recognized that different combinations of iron and chromium concentrations and different catholyte and anolyte volumes can be used to achieve a desired molar ratio of chromium in the anolyte to iron in the catholyte. In at least some of these embodiments, the volume of the catholyte is no more than 95%, 90%, 80%, 70%, or 60% of the volume of the anolyte.

[0127] In at least some cases, it was found that a higher H + concentration in the anolyte promotes hydrogen generation. To reduce hydrogen generation by the anolyte, the H + concentration in the initial anolyte can be lower than the H + concentration in the initial catholyte. In at least some embodiments, the H + concentration in the initial anolyte is at least 10%, 20%, 25%, or 50% lower than the H + concentration in the initial catholyte.

[0128] Table 1 shows the anolyte to catholyte volume ratio of 1 : 1 at different states of charge (SOC), where the state of charge represents the percentage of the initial active ion species in the anolyte and catholyte that is converted to reduced / oxidized ion species. It should be recognized that the concentration of H + changes to maintain charge balance between the anolyte and catholyte. In Table 1, the initial anolyte is 1.25 M Fe 2+ , 1.25 M Cr 3+ , and 1.25 M H + , and the initial catholyte is 1.25 M Fe 2 + , 1.25 M Cr 3+ , and 2.5 M H + . These particular concentrations are chosen so that the H + concentration is equal at 50% SOC.

[0129] Table 1

[0130]

[0131] Table 2 shows the anolyte to catholyte volume ratio of 2: 1 at different states of charge (SOC). In Table 2, the initial anolyte is 1.25 M Fe 2+ , 1.25 M Cr 3+ , and 1.5625 M H + , and the initial catholyte is 1.25 M Fe 2+ , 1.25 M Cr 3+ , and 2.5 M H + . These particular concentrations are chosen so that the H + concentration is equal when the anolyte is at 25% SOC and the catholyte is at 50% SOC. The difference in SOC between the anolyte and catholyte is due to the anolyte having twice the volume of the catholyte.

[0132] Table 2

[0133]

[0134] Another challenge for Fe-Cr redox flow battery systems, as well as other redox flow battery systems, is the presence of metal impurities, such as nickel, antimony, zinc, bismuth, platinum, gold, or copper. In at least some cases, these metal impurities increase the production of hydrogen gas on the negative electrode surface. Such metal impurities can be present as natural impurities or as part of the refining or manufacturing of the iron and chromium compounds or other parts of the redox flow battery system or through any other mechanism.

[0135] In at least some embodiments, the redox flow battery system 100 can be configured to remove these impurities or reduce their levels. As shown in FIG. 3, in at least some embodiments, to remove these impurities or reduce their levels, the redox flow battery system 100 is configured to electrochemically reduce at least some of the impurities to a metallic form (step 350), collect the resulting metallic particles using a particulate filter or other device (e.g., the interdigitated electrode described below) (step 352), and remove these impurities using a cleaning solution containing an oxidizing species (step 354). Figure 3

[0136] In at least some embodiments, the impurities are reduced in the anolyte as part of the redox reaction. When reduced during charging, the impurities form metallic particles or particulates. The redox flow battery system 100 can include a particulate filter at the half-cell 106 or elsewhere to capture the metallic particles or particulates. In some embodiments, the negative electrode 102 can assist in filtering the metallic particles or particulates. To facilitate removal of the impurities, the negative electrode 102 can have an interdigitated structure, as shown in FIG. 4. The interdigitated structure includes empty or jagged channels 240 for collecting the metallic impurity particles during operation of the redox flow battery system 100. As described below, these particles can be removed from the electrode during a maintenance cycle. Figure 2 Figure 12 FIG. 5 illustrates examples of alternative locations for the particulate filter 121, including but not limited to upstream or downstream of the anolyte / catholyte tanks 116, 118; downstream or upstream of the half-cells 106, 108, etc., or any combination thereof. In at least some embodiments, the pore size of the filter 121 is in the range of 1-10 microns. In at least some embodiments, the positive and negative assignments of the electrodes 102, 104 can be reversed, such that impurities in both electrolyte tanks can be removed.

[0137] In at least some embodiments, the Fe-Cr redox flow battery systems described herein are configured to use a solution with an oxidizing species (e.g., Fe 3+ ) to remove these impurities. As part of the maintenance of the redox flow battery system 100, during a maintenance cycle, a solution of Fe 3+ (or other oxidizing) can be flowed through the anolyte portion of the system to remove impurities from the electrode 102 or elsewhere in the system. In at least some embodiments, the Fe 3+ ​​The solution can be or be part of the catholyte. Alternative oxidizing solutions include, but are not limited to, hydrogen peroxide solutions, ferric chloride solutions, nitric acid, and the like. In at least some embodiments, the redox flow battery system 100 can include a cleaning solution tank (not shown) that can be connected to the anolyte / catholyte distribution devices 124, 126 (or to the filter 121) to periodically clean the filter.

[0138] In at least some embodiments, the removal or reduction of metal impurities is performed during manufacture of the redox flow battery system, prior to the redox flow battery system beginning operation, or during operation of the redox flow battery system, or any combination thereof. It should be appreciated that these methods and systems for removing metal impurities are not limited to Fe-Cr redox flow battery systems, but can also be used for other redox flow battery systems, such as vanadium, vanadium-bromine, vanadium-iron, zinc-bromine, and organic redox flow battery systems.

[0139] It has also been discovered that, in at least some embodiments, occasional exposure of the electrodes 102 to the catholyte 114 can promote passivation of the electrode 102 surface and reduce hydrogen gas generation. For example, in one Fe-Cr redox flow battery system, after 17 charge / discharge cycles, treatment of the electrodes 102 with the catholyte 114 for 1 hour reduced the hydrogen gas generation rate from 38.9 ml / min to 10.2 ml / min. In at least some embodiments, operation of the redox flow battery system can periodically (or when initiated or requested by an operator) include a maintenance period during which the half-cell 106 or electrodes 102 are exposed to the catholyte (or an electrolyte having, for example, the components specified above for the catholyte) for a period of time (e.g., 5, 10, 15, 30, 45, 60 minutes or more). During the maintenance, the catholyte can be introduced into the half-cell 106 or electrodes 102 once, periodically, intermittently, or continuously. In at least some of these embodiments, the catholyte 114 can be returned to the catholyte tank 118 after the maintenance period. In at least some embodiments, the maintenance period can be performed when the state of charge of the anolyte is at least 50%, 75%, or 90%.

[0140] Figure 4 One embodiment of a redox flow battery system is illustrated that includes a switch 434 for disconnecting the anolyte distribution system 124 from the half-cell 106 and connecting the catholyte distribution system 126 to the half-cell 106 to flow the catholyte 114 into the half-cell 106. This arrangement can be used to reduce or remove metal impurities or to passivate the electrodes 102 or any combination thereof. The pump 122 can be used to flow the catholyte 114 into the half-cell 106 or to remove the catholyte 114 from the half-cell 106 when the maintenance is complete.

[0141] The storage capacity of a Fe-Cr redox flow battery system can decrease over time, at least in part due to Cr 2+ / Cr 3+ The low standard potential of the pair results in the production of at least some level of hydrogen gas on the anolyte side of the system. As a result, the average oxidation state (AOS) of the active species in the system increases, the system becomes unbalanced, and the storage capacity decreases. Accordingly, it would be useful to have a method or apparatus to at least partially restore the storage capacity by restoring the AOS.

[0142] In at least some embodiments, the AOS of a Fe-Cr redox flow battery system can be described as: AOS = ((moles of Fe 3+ in the catholyte and anolyte) * 3 + (moles of Fe 2+ in the catholyte and anolyte) * 2 + (moles of Cr 3+ in the anolyte and catholyte) * 3 + (moles of Cr 2+ in the anolyte and catholyte) * 2) / (moles of Fe in the catholyte and anolyte + moles of Cr in the anolyte and catholyte).

[0143] In at least some embodiments, the presence of ammonia or urea in the electrolyte (e.g., as a ligand for the chromium complex) can facilitate rebalancing of the system and restoration of the storage capacity. In at least some embodiments, the following electrolytic reactions occur at the electrodes:

[0144] 7H2O + 2Cr 3+ - 6e - → Cr2O7 2- + 14H + E0 = +1.33 V

[0145] Fe 3+ + e - → Fe 2+ E0 = +0.77 V

[0146] Chromate ions can react with urea or ammonia to regenerate Cr 3+ , thereby rebalancing the system:

[0147] Cr2O7 2- + 8H + + CO(NH2)2 → 2Cr 3+ + CO2 + N2 + 6H2O

[0148] Cr2O7 2- + 8H + + 2NH3 → 2Cr 3+ + N2 + 7H2O

[0149] In at least some implementations, the generated nitrogen or carbon dioxide can be released to prevent pressurization of the redox flow battery system.

[0150] Alternatively, in at least some embodiments, to rebalance the redox flow battery system, the redox flow battery system includes a balancing device combined with an anolyte or a catholyte to rebalance the system and restore its storage capacity. In at least some embodiments, the balancing device utilizes a vanadium source to produce vanadium oxide (VO₂O₃). 2+ ) and vanadium dioxane (VO) 2+ The system uses ionic substances and reducing agents (such as oxidizable hydrocarbon compounds) to rebalance the system and restore its storage capacity. The following embodiments illustrate the addition of a balancing device to an Fe-Cr redox flow battery system. It should be understood that such a balancing device can be used in other redox flow battery systems, or other chemical and / or electrochemical systems.

[0151] Figure 5A The illustration shows a portion of a redox flow battery system 100 and an embodiment of a balancing device 500. Figure 5B An embodiment of the balancing device 500 is illustrated. In this embodiment, the cathode electrolyte 114 and the balancing electrolyte 562 (e.g., containing VO) 2+ / VO2 + The electrolyte and reducing agent 563 are used in combination to rebalance the redox flow cell system 100. The balancing device 500 includes: a cathode electrolyte tank 118; balancing electrodes 552, 554; balancing half-cells 556, 558; a balancing diaphragm 560; a cathode electrolyte balancing pump 572; a cathode electrolyte balancing distribution system 576; a balancing tank 566; an optional reducing agent tank 567; a balancing electrolyte pump 570; a balancing electrolyte distribution device 574; and a potential source 561. In at least some embodiments, the reducing agent may be urea or ammonia, which may be present as a ligand for chromium or iron complexes, or may be provided as a reducing agent.

[0152] The following reaction equation illustrates an example of using an iron-based cathode electrolyte 114, a vanadium oxide-containing equilibrium electrolyte 562, and a reducing agent 563 containing urea or ammonia to rebalance the system.

[0153] VO 2+ +H2O+Fe 3+ →VO2 + +Fe 2+ +2H +

[0154] 6VO2 + +6H ++ CO(NH2)2→ 6VO 2+ + CO2+ N2+ 5H2O

[0155] 6VO2 + + 6H + + 2NH3→ 6VO 2+ + N2+ 6H2O

[0156] In at least some embodiments, the generated nitrogen gas or carbon dioxide can be released to prevent pressurization of the redox flow battery system.

[0157] The following reaction equations illustrate another example of rebalancing the system using an iron-based cathode electrolyte 114, a balancing electrolyte 562 containing vanadyl ions, and a reducing agent 563 containing fructose, and an external potential of at least 0.23 V applied by the potential source 561:

[0158] VO 2+ + H2O + Fe 3+ → VO2 + + Fe 2+ + 2H +

[0159] 24VO2 + + 24H + + C6H 12 O6→ 24VO 2+ + 6CO2+ 18H2O

[0160] Through the reactions shown in the above two examples, the AOS of the redox flow battery system 100 can be reduced, and the H + ions lost in hydrogen generation can be recovered. In at least some embodiments, this rebalancing (or recovery of AOS or recovery of storage capacity) does not use any metal catalysts, as such catalysts typically increase the production of hydrogen. In at least some embodiments, the VO 2+ of the balancing electrolyte 562 can be considered a homogeneous catalyst, as the VO 2+ ions are regenerated using the reducing agent 563. In at least some embodiments, the reduction of VO2 + ions occurs in the balancing half-cell 566.

[0161] In at least some embodiments, the oxidation of the reducing agent 563 can occur in the balancing tank 566 instead of in the half-cell 556, and can not require the application of an external potential, as long as the VO2 +Ions are available. Suitable reducing agents include sugars (e.g., fructose, glucose, sucrose, etc., or any combination thereof), carboxylic acids (e.g., formic acid, acetic acid, propionic acid, oxalic acid, etc., or any combination thereof), aldehydes (e.g., formaldehyde, acetaldehyde, etc., or any combination thereof), alcohols (e.g., methanol, ethanol, propanol, etc., or any combination thereof), ammonia, urea, thiourea, ammonium ions, other hydrocarbons, or hydrogen. In at least some embodiments, the reducing agent is soluble or at least partially soluble in water.

[0162] In at least some embodiments, the reducing agent 563 is added to the balancing electrolyte 562 periodically, intermittently, or continuously from the reducing agent tank 567. In at least some embodiments, the rebalancing process (to restore storage capacity or to restore AOC) occurs continuously, intermittently, or periodically. For example, the catholyte balancing pump 572 and the balancing electrolyte pump 570 can be run continuously, intermittently, or periodically. In at least some embodiments, the catholyte pump 122 can also be used as the catholyte balancing pump 572. In addition, the catholyte balancing dispensing device 576 can include a valve to connect to or disconnect from the catholyte tank 118.

[0163] Figure 5C and 5D Another embodiment of a redox flow battery system 100 with a balancing device 500' is illustrated that operates using an anolyte 112 (and corresponding anolyte pump 572' and anolyte balancing dispensing device 576') instead of a catholyte. In at least some embodiments, the anolyte pump 120 can also be used as the anolyte balancing pump 572'.

[0164] The following reaction equations illustrate one example of rebalancing the system using a chromium-based anolyte 112, a balancing electrolyte 562 containing vanadyl ions, and a reducing agent 563 containing fructose, and an external potential of at least 1.40 V applied by the potential source 561:

[0165] VO 2+ + H2O + Cr 3+ → VO2 + + Cr 2+ + 2H +

[0166] 24 VO2 + + 24H + + C6H 12 O6→ 24 VO 2+ + 6CO2+ 18H2O

[0167] Other reducing agents, including those listed above, can be used in place of fructose.

[0168] Figure 5EAnother embodiment of a balancing device 500" is illustrated, which can be adapted to operate with catholyte or anolyte and corresponding catholyte / anolyte tanks 118 / 116 connected to the rest of the redox flow battery system 100. This embodiment incorporates an intermediate tank 584 and two intermediate half-cells 586, 588 between the catholyte / anolyte tanks 118 / 116 and the balancing tank 562 and corresponding half-cells 556 / 558. (As with the balancing tank, there can be intermediate pumps and intermediate distribution devices, as well as an intermediate separator between the two half-cells 586, 588 and a power source potential to apply a potential between the electrodes of the two half-cells 586, 588.) In one embodiment, the intermediate electrolyte in the intermediate tank 584 comprises V 2+ 3+ ions.

[0169] The following reaction equations illustrate one example of rebalancing of the system using the balancing device 500" and the redox flow battery system 100 Figure 1 ) with catholyte 114.

[0170] VO 2+ + H2O - e - → VO2 + + 2H + (half-cell 556)

[0171] V 3+ + e - → V 2+ (half-cell 558)

[0172] V 2+ - e - → V 3+ (half-cell 586)

[0173] Fe 3+ + e - → Fe 2+ (half-cell 588)

[0174] 24 VO2 + + 24H + + C6H 12 O6→ 24 VO 2+ + 6 CO2 + 18 H2O (balancing tank 562 or half-cell 556 or both)

[0175] Other reducing agents, including those listed above, can be used in place of fructose.

[0176] Another embodiment uses an anolyte (Cr 2+ / Cr 3+ ​This replaces the cathode electrolyte in combination with the intermediate and equilibrium electrolytes. Another embodiment uses an anolyte and Fe... 2+ / Fe 3+ intermediate electrolyte replaces V 2+ / V 3+ Intermediate electrolyte.

[0177] It will be appreciated that the balance arrangement described herein can be used in other redox flow battery systems, especially those capable of producing hydrogen. Examples of such redox flow battery systems include, but are not limited to, Zn-Br or Zn-Cl redox flow battery systems, vanadium-based (e.g., all-vanadium, V-Br, V-Cl, or V-polyhalide) redox flow batteries; Fe-V or other iron-based redox flow battery systems (e.g., all-iron redox flow battery systems); or organic redox flow battery systems.

[0178] In some implementations, in Fe 2+ During an overcharge state, chlorine gas (Cl2) can be generated on the cathode electrolyte side of the redox flow battery system 100. The chlorine gas can be confined in the cathode electrolyte top space, such as cathode electrolyte tank 118 or half-cell 108, or any combination thereof. The continuous generation of chlorine gas increases the pressure in the enclosed cathode electrolyte top space. In at least some embodiments, this can cause chlorine gas to travel via connector 638c (…). Figure 6C The chlorine gas migrates to the top space of the anolyte, and the connection 638c optionally includes one or more valves or switches 639 to control the flow. In at least some embodiments, at least a portion of the chlorine gas can be absorbed by the anolyte. In at least some embodiments, the following reaction can occur between the chlorine gas and the anolyte to chemically discharge the overcharged system:

[0179] 2Cr 2+ +Cl2→2Cr 3+ +2Cl -

[0180] 2Fe 2+ +Cl2→2Fe 3+ +2Cl -

[0181] In at least some embodiments, the redox flow battery system 100 may include a pressure relief system to control the pressure in the top space of the cathode electrolyte or anolyte. For example, a pressure reducing valve 638a ( Figure 6A ) or U-shaped tube device containing liquid 638b Figure 6BThe pressure vessel can be connected to the top space of the cathode electrolyte to control pressure. Similarly, a pressure reducing valve or a liquid-containing U-tube device can be connected to the top space of the anolyte. In at least some embodiments, the gas in the top space of the anolyte or cathode electrolyte can be exchanged with the ambient atmosphere via a bidirectional gas pressure control system (e.g., the U-tube device). In at least some embodiments, the U-tube device can also be used as a gas leak detector. In at least some embodiments, the liquid in the U-tube device can contain an acid level indicator that can be used to estimate the amount of acidic gas released into the environment by the redox flow battery system.

[0182] In at least some cases, leaked acidic solutions and chemical vapors from the electrolyte and chemical products of redox reactions can damage electronic components (e.g., controller 128, switches, valves, pumps, sensors, etc.) in the redox flow battery system 100. Furthermore, leaks can lead to environmental damage or pollution.

[0183] In at least some embodiments, all or a portion of the redox flow battery system 100, comprising an anolyte or a catholyte or both, may be located in a second container 790. Figure 7 In the second container 790, the acid-absorbing material is contained, such as sodium carbonate, sodium bicarbonate, calcium carbonate, or calcium oxide. In at least some embodiments, the second container may contain sufficient acid-absorbing material to neutralize at least 10, 25, 40, 50, 60, 70, 75, 90% or more of the anolyte or catholyte or both.

[0184] In some embodiments, components of the redox flow battery system 100 containing anolyte and catholyte, such as anolyte or catholyte tanks 116, 118, half-cells 106, 108, at least some portions of anolyte or catholyte distribution systems 124, 126, electrodes 102, 104, etc., are maintained at a temperature of at least 50, 60, 70, or 80 degrees Celsius or higher during charging or discharging in temperature zone 892. Figure 8 As shown. One or more heating devices 894 can be used to maintain the temperature of these components. Furthermore, one or more electronic components of the redox flow battery system, such as one or more controllers 128, pumps 120, 122, one or more sensors, one or more valves, etc., are maintained at a temperature not exceeding 40, 35, 30, 25, or 20 degrees Celsius or lower. One or more cooling devices 896 can be used to maintain the temperature of these components.

[0185] In at least some embodiments, chromite can be used as a starting material to obtain a chromium-containing compound for use in the anolyte or catholyte described above. Chromite (a mixture of iron and chromium oxides represented by the chemical formula FeO-Cr2O3) is treated with a carbon source under high temperature (at least 1000 °C) and reducing conditions to convert the ore into a porous Fe-Cr alloy with a predetermined content of unburnt carbon particles.

[0186] FeCr2O4 + C → Fe-Cr-C + CO + CO2

[0187] Examples of suitable carbon sources include, but are not limited to, graphite, coal, activated carbon, charcoal, carbon monoxide gas, and carbon-containing materials that contain carbon in an oxidation state less than 4, which can remove oxygen from the chromite in the form of carbon monoxide or carbon dioxide.

[0188] The Fe-Cr-C particles are pulverized to a predetermined size and then dissolved in hot sulfuric acid in air to produce FeSO4 and Cr2(SO4)3. Optionally, the solution is filtered to remove insoluble components.

[0189] Fe + Cr + H2SO4 → Fe2(SO4)3 + Cr2(SO4)3 + H2

[0190] Calcium chloride or barium chloride is added to the solution to remove most of the sulfate anions. The solution is filtered to remove calcium sulfate or barium sulfate and other insoluble components.

[0191] CaCl2 + Fe2(SO4)3 + Cr2(SO4)3 → CaSO4 + FeCl3 + CrCl3

[0192] BaCl2 + Fe2(SO4)3 + Cr2(SO4)3 → BaSO4 + FeCl3 + CrCl3

[0193] Optionally, by adding FeCl3 or CrCl3 to the solution, the Cr:Fe ratio can be adjusted to obtain the final desired ratio (e.g., 3:2 Cr:Fe). The solution is cooled to crystallize the FeCl3 / CrCl3 mixture and remove impurities. Optionally, one or more recrystallizations can be used to remove impurities.

[0194] The FeCl3 / CrCl3 mixture can be used as an electrolyte by adding water and reducing iron powder to produce a FeCl2 / CrCl3 solution. If needed, the heat from the reaction of the reducing iron and FeCl3 can be used to heat the solution.

[0195] Fe + 2FeCl3 → 2FeCl2 + heat (about 168.3 kilojoules per mole of iron)

[0196] In at least some embodiments, H2SO4 or HCl is added to the solution to produce the final electrolyte composition.

[0197] Other chromium materials can also be used. Such chromium materials can include chromium waste, such as electroplating waste, leather tanning waste, etc. (including those containing chromium...). 6+ The chromium-containing compound can first be reduced by an agent such as iron powder or Fe. 2+ or Cr 3+ (Compound reduction). These chromium materials can be dissolved in acids such as hydrochloric acid or sulfuric acid to form chromium salts. The pH of the dissolved chromium can be increased to pH > 3, 5, 7 or higher to produce Cr(OH)3. In at least some embodiments, the choice of acid and pH can provide other chromium compounds, such as:

[0198] Cr(OH)3 + 3HCl → CrCl3 + 3H2O

[0199] 2Cr(OH)3+3H2SO4→Cr2(SO4)3+6H2O

[0200] Alternatively, adding FeCl2, Fe+FeCl3, Fe(OH)2, or Fe+Fe(OH)3 combined with HCl can produce an electrolyte composition.

[0201] Chromite and chromium scrap may contain impurities such as silica, alumina, iron, and other metals (Ni, Mn, Cu, Sb, Bi, etc.). Some of these metals can act as hydrogen generation catalysts, promoting hydrogen production during the operation of Fe-Cr redox flow batteries. As mentioned above, metal impurities can be reduced to metal particles in the anolyte and can be removed using filters, for example... Figure 12 Filter 121 is used to remove these particles. In at least some embodiments, the pore size of filter 121 is in the range of 1-10 micrometers. An oxidizing solution, such as one containing Fe, is used. 3+ The filter is periodically cleaned with a solution or cathode electrolyte solution from a flow battery to dissolve the small metal particles trapped by the filter, and the used cleaning solution is removed from the system. The trapped and removed metal cations can be recycled for further applications.

[0202] In at least some implementation schemes, such as Figure 10 As shown, the redox flow battery system 100 may include a secondary redox flow battery device 201 utilizing the same anolyte 112 and catholyte 114. The secondary redox flow battery device 201 includes: two electrodes 202, 204; associated half-cells 206, 208; a separator 210; an anolyte pump 220; a catholyte pump 222; an anolyte distribution device 224; and a catholyte distribution device 226.Figure 10 As shown, the secondary redox flow battery device 201 also includes the anolyte tank 116 and the catholyte 118 used by the primary redox flow battery device 101 and is operated by the same controller 128, connected or connectable to the same load / power source 130 / 132. (Although not shown, the secondary redox flow battery device 201 can also include a second anolyte tank 116 and a second catholyte tank 118, as described below.) Figure 10 It can appear that the anolyte and catholyte tanks 116, 118 are shown as being split between the primary and secondary redox flow battery devices 101, 201, but it should be understood that the entirety of the anolyte and catholyte tanks 116, 118 are part of each of the primary and secondary redox flow battery devices 101, 201.

[0203] In at least some embodiments, the secondary redox flow battery device 201 has smaller half-cells 206, 208 or anolyte / catholyte pumps 220, 222 than the primary redox flow battery device 101, has a more limited pumping capacity (or any combination of these features). In at least some embodiments, the ratio of the peak power delivery capacity of the secondary redox flow battery device 201 to the peak power delivery capacity of the primary redox flow battery device 101 is in the range of 1:5 to 1:200 or in the range of 1:1.1 to 1:10 or in the ratio of 1:1.5 to 10. In at least some embodiments, the peak power delivery capacity of the secondary redox flow battery device is less than the peak power delivery capacity of the primary redox flow battery device.

[0204] In at least some embodiments, by placing the secondary redox flow battery device 201 in series or parallel configuration relative to the load 130, the secondary redox flow battery device 201 can be used to supplement the primary redox flow battery device 101 during high power delivery.

[0205] In at least some embodiments, the secondary redox flow battery device 201 can be used to replace the primary redox flow battery device 101 during low power delivery to provide time periods in which the primary redox flow battery device 101 is not operating or is in a resting state. The secondary redox flow battery device 201 continues to use the same anolyte / catholyte 112, 114 during periods in which the primary redox flow battery device 101 is not operating to discharge these electrolytes and reduce or prevent self-discharge or overheating. In at least some embodiments, the secondary redox flow battery device 201 can be used to restart the primary redox flow battery device 101.

[0206] In at least some embodiments, the controller 128 is configured to switch from the primary redox flow battery device 101 to the secondary redox flow battery device 201 when the discharge power falls to no higher than a first predetermined level. In at least some embodiments, the controller 128 is configured to switch from the secondary redox flow battery device 201 to the primary redox flow battery device 101 when the discharge power is at least a second predetermined level. In at least some embodiments, the first and second predetermined levels are the same. In other embodiments, the second predetermined level is greater than the first predetermined level. In at least some embodiments, the controller 128 is configured to add the secondary redox flow battery device 201 to the primary redox flow battery device 101 when the discharge power is at least a third predetermined level.

[0207] In at least some embodiments, the secondary redox flow battery device 201 can be used as a backup power source. In at least some embodiments, when the primary redox flow battery device 101 is placed in a resting or non-operational state, the electrolytes 112, 114 in the half-cells 106, 108 of the primary redox flow battery device can be discharged by connecting the secondary redox flow battery device 201 as a load 130 to the primary redox flow battery device (e.g., to avoid damage to the primary redox flow battery device). In at least some embodiments, the anode and cathode electrolyte pumps 120, 122 are stopped or relatively slowly pumped during this process, as the purpose is simply to discharge the electrolytes 112, 114 in the half-cells 106, 108. Conversely, when the secondary redox flow battery device 201 is placed in a resting or non-operational state, the electrolytes 112, 114 in the half-cells 206, 208 of the secondary redox flow battery device can be discharged by connecting the primary redox flow battery device 201 as a load 130 to the secondary redox flow battery device.

[0208] It will be appreciated that a redox flow battery system can include one or more primary redox flow battery devices 202 and one or more secondary redox flow battery devices 201. It will be appreciated that one or more secondary redox flow battery devices 201 can be incorporated into any other embodiments described herein, and any modifications described herein for a redox flow battery system can also be applied to a secondary redox flow battery device 201.

[0209] As Figure 11As shown, the redox flow battery can be used to produce hydrogen gas by electrolysis of water 1112 in an electrolytic cell 1130 connected to the redox flow battery system 100. The electrolytic cell 1130 includes electrodes 1102, 1108, and a hydrogen removal device 1125 transports hydrogen gas 1113 produced by electrolysis of water in the electrolytic cell 1130 to a hydrogen gas storage tank 1117. Separation of hydrogen gas 1113 from oxygen is known and can be performed using any suitable method.

[0210] The redox flow battery system 100 provides a convenient and useful mechanism for producing hydrogen gas. The redox flow battery system 100 can be charged and then used to electrolyze water in the electrolytic cell 1130. In contrast to many conventional devices for hydrogen gas production that require AC electricity to be converted to DC electricity to hydrolyze water, the redox flow battery system 100 inherently produces DC electricity sufficient to hydrolyze water to produce hydrogen gas.

[0211] Alternatively or additionally, in at least some embodiments, the hydrogen gas 1113 is a byproduct of the operation of the redox flow battery system 100. A hydrogen gas removal device (similar to the hydrogen gas removal device 1125) transports hydrogen gas 1113 produced in the first half-cell 106 or the second half-cell 108 to a hydrogen gas storage tank 1117. Byproducts of the charging of the redox flow battery system 100 can be sold as additional products.

[0212] In a redox flow battery system, the average oxidation state (AOS) of the active species in the catholyte or anolyte (or both) changes, especially under conditions where hydrogen gas is produced in the system or oxygen intrusion. As a result of the change in AOS, the system can become unbalanced, the system storage capacity can decrease, or side reactions (such as hydrogen gas generation) can accelerate, or any combination of these effects.

[0213] It is useful to know the AOS of a redox flow battery system. Conventional AOS determination methods include sampling and performing off-line potentiometric titration analysis, obtaining in-situ UV-Visible measurements; or in-situ potential difference measurements relative to a reference electrode. These techniques can be slow or can be relatively inaccurate.

[0214] In contrast to these conventional techniques, a relatively fast and accurate method is presented herein. The method includes measuring the ion capacity in at least one electrolyte during a low-potential charging process, and using the charging capacity and a known electrolyte volume to determine the AOS.

[0215] In at least some embodiments, the anolyte / catholyte tanks 116, 118 can each be a set of tanks. For example, in at least some embodiments, the anolyte / catholyte tanks 116, 118 each include, as Figure 21A main electrolyte tank 2116 and one or more supplemental electrolyte tanks 2118 are shown. In at least some embodiments, as Figure 21 shown, the main electrolyte tank 2116 is cylindrical, rather than cuboid or cuboid. In at least some embodiments, cuboids or cuboids are prone to leaking, particularly for relatively large storage tanks, while cylindrical tanks are less likely to leak.

[0216] In at least some embodiments, the tank storage area 125 of a redox flow battery system can be cuboid, as Figure 21 shown, or another non-cylindrical shape. As Figure 21 shown, the cylindrical main electrolyte tank 2116 can not fill the tank storage area 125. To provide additional electrolyte storage, one or more (e.g., one, two, three, four, five, six, or more) supplemental electrolyte tanks 2118 can be included in the tank storage area 125. The supplemental electrolyte tanks 2118 can be in series or in parallel with the main electrolyte tank 2116. The supplemental electrolyte tanks 2118 can be cylinders, cubes, cuboids, or any other suitable shape. If the supplemental electrolyte tanks 2118 have a much smaller volume (e.g., no more than 5%, 10%, or 25%) than the main electrolyte tank 2116, then the concern for (or likelihood of) leaks is reduced. The supplemental electrolyte tanks 2118 can all have the same shape or volume, or can have different shapes or volumes.

[0217] Figure 13 Three different charge curves for a redox flow battery system are shown. The first charge curve 990 is for a balanced system. The second charge curve 992 and the third charge curve 994 are for two different unbalanced systems. In the unbalanced systems, there are low-potential charge regions 992a, 994a and high-potential charge regions 992b, 994b. By performing one or more measurements in the low-potential charge regions 992a, 994a, the AOS can be determined.

[0218] This AOS determination method will be illustrated using the Fe-Cr redox flow battery system described above. However, it should be understood that these methods can be used for any other suitable redox flow battery system. In this example, both the catholyte and the anolyte include iron and chromium ions. For a balanced system, when the redox flow battery system is fully discharged, there are only Fe 2+ and Cr 3+ in both the anolyte and the catholyte. Upon charging, the system undergoes the following redox reactions:

[0219] Fe 2+- → Fe 3+ + e (E o = +0.77 V, positive electrode)

[0220] Cr 3+ +e - →Cr 2+ (E o = -0.40V, negative terminal)

[0221] like Figure 13 As shown in the first charging curve 990, in at least some embodiments, the potential difference between the positive and negative electrolytes can immediately increase to greater than 0.9V.

[0222] However, if the system is not in equilibrium, AOS increases due to side reactions. For example, Fe can be found in the discharge electrolyte of an unbalanced Fe-Cr redox flow battery system. 2+ / Fe 3+ / Cr 3+ A mixture of ions. For such an ionic mixture, when a redox flow battery system is charged, the system first undergoes the following redox reaction:

[0223] Fe 2+- →Fe 3+ +e (positive electrode)

[0224] Fe 3+ +e - →Fe 2+ (negative electrode)

[0225] These reactions occur at low potentials until all Fe... 3+ Ions are consumed in the negative electrode electrolyte. This corresponds to Figure 13 The low-potential charging regions 992a and 994a are only reached after all Fe has been consumed. 3+ After the ions are reduced, chromium ions will be reduced at a higher charging potential (as shown in the high potential charging regions 992b and 994b).

[0226] The charging capacity of this low-potential charging process can be used to determine the Fe content in the anolyte. 3+ The amount of ions. For example, Figure 13 Dividing the charging capacity at the inflection point of 993 by 26.8 Ah / mol (the charge on one mole of electrons) yields the Fe content in the anolyte. 3+ The molar amount. This, combined with the volumes of the positive and negative electrode electrolytes, can be used to determine the AOS of a redox flow battery system.

[0227] This information can also be used to rebalance redox flow battery systems, for example, using the balancing device described above. Figure 5A and 5BThe balancing device 500, the balancing electrolyte 562 containing vanadium ions, the reducing agent 563 containing fructose, and the external potential of at least 0.23V applied by the potential source 561 produce the following reaction:

[0228] VO 2+ +H2O+Fe 3+ →VO2 + +Fe 2+ +2H +

[0229] 24VO2 + +24H + +C6H 12 O6→24VO 2+ +6CO2 + 18H2O

[0230] AOS can be rebalanced by providing a reducing agent 563 (fructose) to the equilibrium electrolyte 562. Ideally, an amount equal to Fe... 3+ Multiplying the molar amount of fructose by (1 / 24) of the molar amount of fructose can rebalance AOS. It should be recognized that, due to non-ideal factors in the system, more fructose may be needed to fully rebalance AOS.

[0231] Figure 14 This is a flowchart of one implementation of the method for determining AOS. In step 1060, the charging capacity during low-potential charging is measured. For example, the charging capacity can be measured when the charging curve changes from a low-potential charging period to a high-potential charging period (e.g., during...). Figure 13 The charging capacity is determined at inflection points 993 and 995 in the circuit. In step 1062, the AOS can then be determined using the measured values ​​(or a set of measured values) and the known volumes of the anolyte and catholyte, as well as the known iron and chromium concentrations of the catholyte and anolyte. In at least some embodiments, prior to step 1060, the anolyte 112 and catholyte 114 in the redox flow battery system 100 can be fully discharged by applying an external potential or by completely mixing the anolyte and catholyte. In at least some embodiments, the catholyte and anolyte can be completely mixed before determining the charging capacity.

[0232] Another implementation does not require a fully discharged system, but any state of charge condition can be determined using coulometric titration. Figure 15 This is a flowchart of one implementation of the method. In step 1166, an ionic species (e.g., Fe) in the electrolyte is measured by stopping the flow of an electrolyte (cathode electrolyte or anolyte) and performing an in-situ titration of a first ionic species in the non-flowing electrolyte. 3+ or Cr 3+The amount of ion species can be titrated using either the balancing device 500 or, preferably, another electrolyte continuously flowing through the redox flow cell system. For example, this can be achieved by stopping the flow of the cathode electrolyte and using vanadium ions from a) the balancing device 500 or Cr ions from b) the anolyte. 2+ (It can continue to flow through the redox battery system to ensure sufficient Cr) 2+ Titration of all Fe ions 3+ To titrate Fe 3+ Determine the Fe in the cathode electrolyte 3+ The amount.

[0233] In step 1168, the second ionic species (e.g., Cr) in the electrolyte is measured by stopping the flow of one electrolyte (anolyte or catholyte) and performing an in-situ titration of the second ionic species in the non-flowing electrolyte. 2+ or Fe 2+ The amount of the second ionic species is determined. Preferably, the second ionic species is measured in an electrolyte different from the electrolyte in which the first ionic species is measured, such that one ionic species is measured in the cathode electrolyte and another ionic species is measured in the anolyte. In at least some embodiments, the measurement of the first ionic species will change the amount or concentration of the second ionic species, such that the measurement of the second ionic species will be adjusted to account for this change. The balancing device 500 or preferably other electrolytes continuously flowing through the redox flow cell system can be used to titrate the ionic species. For example, the flow of the anolyte can be stopped and vanadium ions in a) the balancing device 500 or Fe ions in b) the cathode electrolyte can be used. 3+ (It can continue to flow through the redox battery system to ensure sufficient Fe) 3+ Titration of all Cr ions 2+ To titrate Cr 2+ Determine the Cr in the cathode electrolyte 2+ The amount.

[0234] In step 1170, the AOS can then be determined using these two measurements, the initial concentrations or amounts of iron and chromium, the volume of the half-cell, and the volumes of the anolyte and catholyte.

[0235] In another implementation, the discharge process can be observed instead of the charging process. For charged or partially charged systems, the discharge or self-discharge process can be used based on two different electrochemical pairs (Fe in this article). 2+ / Fe 3+ For Cr 2+ / Cr 3+ ) and an electrochemical pair with different concentrations (Fe 2+ / Fe 3+ The difference in discharge rate between the two can be used to estimate the additional Fe in the anolyte.3+ The change in discharge rate of two electrochemical couples in an electrochemical device is much faster than the change in discharge rate of one electrochemical couple with different concentrations. As a result, as shown in FIG. 12, a turning point 1297 can be observed in the discharge curve 1296. In at least some embodiments, this turning point 1297 can be used to estimate the amount of extra Fe Figure 16 3+ Figure 17 is an embodiment of a method of determining AOS using a discharge curve. In step 1374, the discharge rate is measured during an initial discharge. In step 1376, the turning point of the voltage discharge is determined, and the open circuit voltage is measured. In step 1378, the open circuit voltage after the turning point can be used to estimate the extra concentration of the active electrochemical couple (here, Fe 3+ / Fe 2+ ) using the Nernst equation. In at least some embodiments, the end of discharge is chosen to be the point where no more than 1%, 5%, or 10% of the Cr 2+ ions in the anolyte are present. In step 1380, the AOS can be determined from the estimated concentrations of the electrolytes and the known volumes. This is the reverse of the method shown in FIG. 13. Figure 14

[0236] In at least some embodiments, the methods of determining AOS described above and shown in FIGS. 12-14 can be performed in situ using the half-cells 106, 108, the anolyte 112, the catholyte 114, other elements of the redox flow battery system 100, or elements of the balancing system 500. In other embodiments, the methods of determining AOS can include flowing a portion of the anolyte 112 or the catholyte 114, or both, into one or more other half-cells for measurement. In other embodiments, the methods of determining AOS can include removing a portion of the anolyte 112 or the catholyte 114, or both, and performing the measurements outside of the redox flow battery system 100. Figures 13 to 18

[0237] In at least some embodiments, the determined AOS can be used to estimate the amount of hydrogen gas produced or the yield of other byproducts. In at least some embodiments, after determining the AOS as described above in FIGS. 12-14, an operation can be performed to balance the device to rebalance the redox flow battery system and restore the AOS. Such an operation can include, for example, determining the amount of reducing agent 563 to add to the balancing electrolyte 562. Figure 14 15

[0238] ​​​​​​For a given redox flow battery system, there is a fixed ratio of electrolyte volume within the cell stack and throughout the cell system. The volume of electrolyte within the cell stack is always much smaller than the volume outside the electrolyte tank. Therefore, charging and discharging the electrolyte within the cell stack is much faster than charging and discharging the electrolyte throughout the system. A solution for quickly measuring the available capacity of a redox flow battery is to charge or discharge the electrolyte in the cell stack only without electrolyte flow under given operating conditions, and then convert the result to the entire system based on system design parameters.

[0239] It is often desirable to know the available energy or storage capacity of a redox flow battery system, or changes in that energy or storage capacity. Conventional methods for such determinations include, for example, sampling from a redox flow battery system for off-line titration or other analysis, in situ ultraviolet-visible (UV-Vis) measurements, or in situ potential difference measurements relative to a reference electrode. These methods can be slow or inaccurate.

[0240] It has been discovered that during the operation of a redox flow battery system, the end OCV (open circuit voltage) of a redox flow battery system changes for a given set of discharge conditions as the active materials become unbalanced. The difference in end OCV is directly related to its available storage or charge capacity. In addition, the end OCV can be directly related to the AOS. In particular, for a given redox flow battery system, there is a reliable relationship between the end OCV and the system storage or charge capacity or AOS at the same discharge rate. Under certain conditions, for example, the production of H2or O2intrusion into the system on the anolyte side of the system, the active species in the system become unbalanced. As a result, the system storage or charge capacity decreases and the side reactions are further accelerated. The relationship between the OCV and the system storage or charge capacity or AOS also changes.

[0241] For example, for one embodiment of a Fe-Cr redox flow battery system, the end OCV under the same discharge conditions can be used as an indicator of the system available storage or charge capacity. Accordingly, the end OCV can also be used as an indicator of the AOS. 2 The discharge energy and the associated end OCV after discharging to 0.60 V are given in the table below.

[0242] End OCV (V) 0.874 0.872 0.865 0.860 0.854 0.850 0.848 Discharge Energy (Wh) 890 900 926 949 968 986 1001

[0243] As shown in the table, the end OCV under the same discharge conditions can be used as an indicator of the system available storage or charge capacity. Accordingly, the end OCV can also be used as an indicator of the AOS.

[0244] Accordingly, in at least some embodiments, the available storage capacity or charge capacity or AOS can be determined by discharging the system under a given set of conditions (e.g., at a preselected discharge rate or power), and then measuring the end OCV. This end OCV can be compared to a pre-determined OCV curve, a pre-determined lookup table or other calibration table, chart, etc.; or applied to a pre-determined mathematical relationship to determine the storage capacity or charge capacity or AOS of the redox flow battery system.

[0245] In at least some embodiments, the discharge and end OCV measurements can be performed using the half-cells 106, 108 and the electrodes 102, 104. In at least some embodiments, the entire redox flow battery system is discharged.

[0246] For a redox flow battery system, there is typically a fixed ratio of electrolyte volume within the cell stack (e.g., the half-cells 106, 108) and the entire system. The electrolyte volume within the cell stack is typically much smaller than the volume in the electrolyte tanks 116, 118. Accordingly, charging and discharging the electrolyte within the half-cells 106, 108 can be much faster than charging and discharging the electrolyte in the entire system. Accordingly, in at least some embodiments, the flow of electrolyte (anolyte and catholyte) can be stopped during this storage capacity determination, such that only the electrolyte in the half-cells 106, 108 is discharged. Typically, the end OCV measurement of the electrolyte in the half-cells 106, 108 is indicative of the entire redox flow battery system as a whole.

[0247] Further, it can be advantageous to perform the discharge and end OCV measurements using a cell that is even smaller than the half-cells (e.g., the half-cells 106, 108) of the cell stack. In at least some embodiments, as shown in FIG. 14, the redox flow battery system 100 can include an OVC cell 1401 having half-cells 1406, 1408 and electrodes 1402, 1404. In at least some embodiments, the volume of the OVC cell 1401 can be at least 25%, 30%, 40%, 50%, 60%, or 75% smaller than the half-cells 106, 108 and the electrodes 102, 104. As shown in FIG. 14, the OVC cell 1401 can include a smaller volume of electrolyte than the half-cells 106, 108 and the electrodes 102, 104. Figure 18 Figure 18 ​As shown, the OCV cell 1401 can be in series with the flow of electrolyte (anolyte and catholyte) through the half-cells 106, 108. Alternatively, the OCV cell 1401 can be external to the flow of electrolyte via the anolyte distribution device 124 and the catholyte distribution device 126, and the OCV cell 1401 can be filled with electrolyte using valves, switches, or the like. Having a relatively small OCV cell 1401 can be advantageous because the discharge process in the OCV cell can be faster than the discharge process in the half-cells 106, 108 (electrolyte flow stops), resulting in a faster end OCV measurement and determination of the storage capacity. Thus, in at least some embodiments, the flow of electrolyte (anolyte and catholyte) can be stopped during this storage capacity determination, such that only the electrolyte in the OCV cell 1401 is discharged.

[0248] Figure 19 is a flow chart of one embodiment of a method of determining the storage capacity or charge capacity or AOS of a redox flow battery system. In step 1574, the redox flow battery system is discharged at a preselected discharge rate. In at least some embodiments, the actual discharge rate differs from the preselected discharge rate by no more than 1%, 5%, or 10%. In at least some embodiments, the discharge is a self-discharge of the redox flow battery system. In at least some embodiments, the end of discharge point is when the amount of Cr 2+ ions in the anolyte is no more than 1%, 5%, or 10% of the total chromium. Other end of discharge points can be used.

[0249] In step 1576, after the discharge, the end OCV is measured. In step 1578, the measured end OCV is compared to a pre-determined OCV curve, a pre-determined lookup table or other calibration table, chart, or the like; or applied to a pre-determined mathematical relationship that relates the end OVC to the concentration of one or more active species, the storage capacity or charge capacity or AOS of the redox flow battery system. In at least some embodiments, the pre-determined end OVC curve; the pre-determined lookup table or other calibration table, chart, or the like; or the pre-determined mathematical relationship is obtained by experimentally measuring the end OVC of a redox flow battery system having different values of the concentration of one or more active species, the storage capacity or charge capacity or AOS using the preselected discharge rate.

[0250] In optional step 1580, when the concentration of one or more active species or the storage capacity or charge capacity is determined in step 1578, the concentration of one or more active species or the storage capacity or charge capacity can be used to determine the AOS using the electrolyte volume.

[0251] In at least some embodiments, when the state of charge or AOS is determined and indicates that the system is unbalanced, any of the techniques described above can be employed to rebalance the redox flow battery system.

[0252] The AOS can also be determined using other measurements of the OCV. Figure 20 is a flowchart of one embodiment of a method of determining the AOS of a redox flow battery system. In step 1680, the OCV of the redox flow battery system is measured. In step 1682, one or both of the following procedures is performed: a) measuring the amount of one iron ion species (e.g., Fe 3+ or Fe 2+ ) in the catholyte; or b) measuring the amount of one chromium ion species (e.g., Cr 3+ or Cr 2+ ) in the anolyte. It will be recognized that measurements of other ion species in the anolyte or catholyte (or other types of redox flow battery systems) can be used in this step. In at least some embodiments, the two half-cells used to measure the amount of one iron ion species or one chromium ion species can be part of a cell stack of the redox flow battery system, such as half-cells 106, 108. In other embodiments, the two half-cells are not part of a cell stack, but can be, for example, OCV cells 1401 of Figure 18 In at least some embodiments, the measurements are made with no flow of electrolyte in the redox flow battery system. In at least some embodiments, the flow of the electrolyte for which measurements are being made is stopped while the flow of the other electrolyte is maintained to facilitate the complete titration of the ion species being measured. In at least some embodiments, the measurement of one iron ion species or one chromium ion species can utilize the balancing device 500 of Figure 5A and the measurement step can include reducing one iron ion species or one chromium ion species and oxidizing vanadium ions, followed by regenerating the vanadium ions by reducing the dioxovanadium ions using a reducing agent as described above. In at least some embodiments, the measurement of one iron ion species or one chromium ion species can be performed offline or can be performed in situ.

[0253] In step 1684, the AOS is determined using i) a measured amount of an iron ion species, a measured amount of a chromium ion species, or a measured amount of both an iron ion species and a chromium ion species, ii) the measured OCV, and iii) a relationship between the amount or concentration of an iron ion species or a chromium ion species in the catholyte or anolyte, respectively, and the OCV. In at least some embodiments, the AOS can be determined from a predetermined OCV curve, a lookup table, a calibration table, or a mathematical relationship that relates OCV to one of the concentration or amount of an iron ion species, the concentration or amount of a chromium ion species, or the concentration or amount of both an iron ion species and a chromium ion species.

[0254] In at least some embodiments, the predetermined OCV curve, lookup table, calibration table, or mathematical relationship can be a relationship between the OCV of the balanced system and a measured or calculated ion species. The measured OCV can provide an expected concentration or amount of that ion species. A difference between 1) the expected concentration or amount of the ion species and 2) the measured concentration or amount of the ion species can be used to determine the AOS.

[0255] In at least some embodiments, the predetermined OCV curve; the predetermined lookup table or other calibration table, chart, etc.; or the mathematical relationship that is applied is obtained by experimentally measuring the OCV of the balanced system for different amounts or concentrations of the selected ion species.

[0256] In at least some embodiments, when the AOS is determined and indicates that the system is unbalanced, any of the techniques described above can be employed to rebalance the redox flow battery system.

[0257] The charging and discharging of the anolyte / catholyte can depend on a variety of factors, including the pumping rate, the power delivered by the charging source, or the power required by a load. In at least some cases, the charging / discharging of the anolyte / catholyte in the half-cell is not constant, but varies over time. This can result in a temporal variation in the AOS (average oxidation state) or SOC (state of charge) of the anolyte / catholyte entering or leaving the half-cell, particularly when the anolyte / catholyte mixing is limited or absent. (In at least some cases, it can be challenging to mix the anolyte / catholyte to achieve a relatively uniform AOS or SOC throughout the anolyte / catholyte.)

[0258] In at least some embodiments, the AOS or SOC (or other values indicative of AOS or SOC) of the anolyte 112 or catholyte 114 (or both) can be measured or estimated intermittently, periodically, or continuously as the anolyte or catholyte enters or exits the half-cell 106, 108 (or elsewhere along the anolyte or catholyte distribution device 124, 126) to record a time-energy curve of the anolyte 112 or catholyte 114. The time-energy curve can represent the AOS or SOC of different portions of the anolyte / catholyte and can correspond to a curve of the AOS or SOC of the entire volume of the anolyte / catholyte.

[0259] In at least some embodiments, the time-energy curve can correspond to measured, calculated, or estimated values of the AOS or SOC or quantities indicative of the AOS or SOC (e.g., measured voltages of the anolyte or catholyte). In at least some embodiments, the AOS or SOC is determined or estimated for the time-energy curve. In at least some embodiments, the time-energy curve includes (or is based on) intermittent, periodic, or continuous measurements or other calculated values indicative of the AOS or SOC (e.g., scaled or relative in a linear or non-linear manner).

[0260] In at least some embodiments, the open circuit voltage (or other measured values indicative of the AOS or SOC) of the anolyte 112 or catholyte 114 (or both) and the charge / discharge current are measured or estimated intermittently, periodically, or continuously as the anolyte or catholyte passes through the half-cell. In at least some embodiments, the open circuit voltage and the charge / discharge current can be used to determine the AOS or SOC and this can be used to generate the time-energy curve. In at least some embodiments, a small open circuit voltage measurement cell can be used to measure the open circuit voltage (OCV) of the anolyte and catholyte before or after passing through the half-cell using, for example, a small portion of the anolyte and catholyte diverted to the open circuit voltage measurement cell.

[0261] In at least some embodiments, the redox flow battery system 100 is maintained to reduce mixing or diffusion of the anolyte / catholyte such that the temporal energy profile of the anolyte 112 or catholyte 114 along the anolyte / catholyte distribution devices 124, 126 and within the volume of the anolyte / catholyte is reliable. In at least some embodiments, the temporal energy profile can be corrected to account for diffusion within the anolyte / catholyte using an estimated diffusion coefficient and one or more factors, such as a) the time for the anolyte / catholyte to travel through the anolyte / catholyte distribution devices after exiting a half-cell until reentering a half-cell, b) the AOS or SOC of temporally adjacent portions of the anolyte / catholyte, c) the temperature of the anolyte / catholyte, electrolyte flow rate, etc., or any combination thereof.

[0262] In at least some embodiments, the temporal energy profile can be used to vary the speed of the anolyte / catholyte pumps 120, 122 to provide sufficient power to the load 130. For example, when the temporal energy profile indicates that the anolyte / catholyte 112, 114 entering a half-cell 106, 108 has a lower charge as indicated by a measured or estimated AOS or SOC, the pumping speed of the anolyte / catholyte pumps 120, 122 can be increased. The pumping speed can also vary based on the amount of power drawn by the load 130.

[0263] Figure 22 A redox flow battery device is illustrated that includes one or more state measurement devices 123 to make measurements to facilitate the measurement, determination, or estimation of the AOS or SOC (or an amount indicative of the AOS or SOC) of the anolyte 112 or catholyte 114 to produce a temporal energy profile of the anolyte or catholyte. For example, the state measurement devices 123 can measure the voltage of the anolyte or catholyte, measure the concentration of active species (e.g., Cr 3+ , Cr 2+ , Fe 3+ , or Fe 2+ ) of the anolyte 112 or catholyte 114 by a spectrometer, etc. Figure 22 Examples of locations for the state measurement devices 123 are illustrated, but it should be understood that such state measurement devices can be located anywhere along the flow path of the anolyte 112 or catholyte 114, including but not limited to before or after entering a half-cell 106, 108.

[0264] In at least some embodiments, the state measurement devices 123 are positioned to only measure the anolyte 112 or catholyte 114. This arrangement can work best if the anolyte / catholyte is kept in balance.

[0265] In at least some implementations, it is possible to use, by means of Figure 22 The charging measurement device 127 shown attributes at least a partial determination or estimation of the time-energy curve by intermittently, periodically, or continuously measuring the power supplied by the charging source 132 to obtain a time-charging curve. In at least some embodiments, the charging curve may be combined with the synchronous pumping rates of the anolyte / cathode electrolyte pumps 120, 122 to obtain the time-energy curves of the anolyte / cathode electrolytes 112, 114. In at least some embodiments, the charging or discharging current is continuously recorded during redox flow battery operation. This can be combined with, for example, the actual pumping rate and a measured OCV (e.g., the OCV of the anolyte / cathode electrolyte entering or leaving the half-cell) to generate the time-charging curve.

[0266] In at least some embodiments, controller 128 receives measurements from state measuring device 123 or charge measuring device 127. In at least some embodiments, controller 128 determines, calculates, records, or stores time-energy curves or time-charge curves, or any combination thereof. In at least some embodiments, controller 128 adjusts the pumping rate of anolyte pump 120 or catholyte pump 122 based on the time-energy curve. Pumping rates can also be recorded intermittently, periodically, or continuously. In at least some embodiments, pumping rates can be determined using pump power consumption or actual flow rate measurements.

[0267] Controller 128 may include at least one processor 129 and at least one memory 131. Controller 128 may utilize any suitable processor 129 including one or more hardware processors, which may be local to the user or non-local to the user or other components of the computer. As described below, processor 129 is configured to execute instructions provided to the processor.

[0268] Any suitable memory 131 can be used in controller 128. Memory 131 illustrates a computer-readable medium, namely a computer-readable storage medium. Computer-readable storage media can include, but is not limited to, non-volatile, non-transitory, removable, and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules, or other data). Examples of computer-readable storage media include RAM, ROM, EEPROM, flash memory or other storage technologies, CD-ROM, digital versatile disc (“DVD”) or other optical storage, cassette tape, magnetic tape, disk storage or other magnetic storage devices, cloud storage, or any other medium that can be used to store desired information and is accessible by a processor.

[0269] Figure 23The illustration shows one implementation of a redox flow battery system, for example... Figure 22 The redox flow battery system 100. In step 2302, the state measuring device 123 is used to intermittently, periodically, or continuously measure values ​​indicating the state of charge of the anolyte 112 or the catholyte 114 before entering or after entering the half-cell 106 or the second half-cell 108. For example, the state measuring device 123 measures the open-circuit voltage of the anolyte or the catholyte.

[0270] Alternatively, the charging measurement device 127 is used to intermittently, periodically, or continuously measure the power supplied by the charging source 132.

[0271] In step 2304, the measured values ​​are used to generate a time-energy profile for the anolyte or catholyte. In at least some embodiments, the AOS or SOC of the anolyte or catholyte is determined by measurements performed by at least one state measurement device and used to generate the time-energy profile. In at least some embodiments, an estimate of the diffusion of charged species within the anolyte or catholyte is also used to generate the time-energy profile.

[0272] In step 2306, during the discharge, the system controller 128 adjusts the pumping speed of the anolyte / cathode electrolyte pumps 120 and 122 based on the time-energy curve to provide the power required by the load.

[0273] It will be recognized that, Figure 22 Redox flow battery system and Figure 23 The operation method of the redox flow battery system is not limited to Fe-Cr redox flow battery, but can be used in other redox flow battery systems, including but not limited to vanadium redox flow battery systems, vanadium-bromine redox flow battery systems, vanadium-iron redox flow battery systems, zinc-bromine redox flow battery systems, all-iron redox flow battery systems, organic aqueous redox flow battery systems, etc.

[0274] The methods, systems, and apparatus described herein can be implemented in many different forms and should not be construed as limited to the embodiments described herein. Therefore, the methods, systems, and apparatus described herein can take the form of a completely hardware implementation, a completely software implementation, or an implementation combining software and hardware aspects. Consequently, the following detailed description should not be considered limiting. The methods described herein can be performed using any type of processor and any suitable type of apparatus including a processor.

[0275] It should be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, and the methods disclosed herein, can be implemented by computer program instructions. These program instructions can be provided to a processor to produce a machine, such that the instructions, which execute via the processor, create means for implementing the actions specified in the flowchart block or blocks. The computer program instructions can also be executed by a processor to cause the processor to perform a series of operational steps to produce a computer implemented process. The computer program instructions can also cause at least some of the operational steps to be performed in parallel. Moreover, some steps can also be executed across more than one processor, such as might be present in a multi-processor computer system. Further, one or more processes can also be performed concurrently with other processes, or even on the same processor, and with different sequences of one or multiple processes other than described. Also, steps can be added or omitted in some embodiments. Furthermore, one or more process can also be performed concurrently with other processes, or even on the same processor, and with different sequences of one or multiple processes other than described.

[0276] The computer program instructions can be stored on any suitable computer- readable media that include, but are not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tapes, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by a processor (either locally or non-locally for the computer).

[0277] The above specification provides a description of the manufacture and use of the application. Since the application can be practiced in many embodiments, the application is to be construed as not being limited to only the embodiments described herein, featuring specifically the particular sequence of steps described. Many changes and modifications can be made to the application, by one having ordinary skill in the art, without departing from the scope of the application in its broadest form.

Claims

1. A redox flow battery system, comprising: The anolyte comprises chromium ions, iron ions, and hydrochloric acid with a molar concentration in the range of 0.5 to 2, wherein at least a portion of the chromium ions forms a chromium complex with at least one of the following: NH3, CO(NH2)2, SCN. - Or CS(NH2)2, wherein the chromium complex comprises having the formula [Cr 3+ (J) x (M) y (H2O) z Compounds or ions of ] Where x, y, and z are non-negative integers, x + y + z = 6, and x is at least 1, y is at least 1, and z is at least 1. J can be selected from NH3, CO(NH2)2, and SCN. - or the group consisting of CS(NH2)2, and Each M is different from J and is independently chosen by Cl. - F - ,Br - I - NH4 + NH3, EDTA, CN - SCN - S 2- O-NO2 - OH - NO2 - , CH3CN, C5H5N, NC5H4-C5H4N, C 12 H8N2, CO(NH2)2, CS(NH2)2, P(C6H5)3, -CO, CH3-CO-CH2-CO-CH3, NH2-CH2-CH2-NH2, NH2CH2COO - O-SO2 2- Or the group consisting of P(o-tolyl)3; The cathode electrolyte contains iron ions, chromium ions, and hydrochloric acid with a molar concentration in the range of 0.5 to 2. The first half-cell includes a first electrode in contact with the anolyte; The second half-cell includes a second electrode in contact with the cathode electrolyte; and The first diaphragm separates the first half-cell from the second half-cell.

2. The redox flow battery system according to claim 1, wherein J is NH3 and at least one M is CO(NH2)2.

3. The redox flow battery system according to claim 1, wherein the chromium complex further comprises at least one counterion selected from the group consisting of ammonium, chloride, bromide, iodide, fluoride, sulfate or nitrate.

4. The redox flow battery system according to claim 1, wherein J is NH3.

5. The redox flow battery system according to claim 1, wherein J is CO(NH2)2 or CS(NH2)2.

6. The redox flow battery system according to claim 1, wherein the chromium complex is formed in situ.

7. A redox flow battery system, comprising: The anolyte comprises chromium ions, iron ions, and hydrochloric acid in a molar concentration ranging from 0.5 to 2, and at least one nitrogen-containing compound selected from: NH3, CO(NH2)2, SCN. - Or CS(NH2)2, wherein the chromium ion comprises at least one having the formula [Cr 3+ (J) x (M) y (H2O) z Chromium ions, Where x, y, and z are non-negative integers, x + y + z = 6, and x is at least 1, y is at least 1, and z is at least 1. J can be selected from NH3, CO(NH2)2, and SCN. - or the group consisting of CS(NH2)2, and Each M is different from J and is independently chosen by Cl. - F - ,Br - I - NH4 + NH3, EDTA, CN - SCN - S 2- O-NO2 - OH - NO2 - , CH3CN, C5H5N, NC5H4-C5H4N, C 12 H8N2, CO(NH2)2, CS(NH2)2, P(C6H5)3, -CO, CH3-CO-CH2-CO-CH3, NH2-CH2-CH2-NH2, NH2CH2COO - O-SO2 2- Or the group consisting of P(o-tolyl)3; The cathode electrolyte contains iron ions, chromium ions, and hydrochloric acid with a molar concentration in the range of 0.5 to 2. A first half-cell includes a first electrode in contact with the anolyte; The second half-cell includes a second electrode in contact with the cathode electrolyte; and A first separator separates the first half-cell from the second half-cell.

8. The redox flow battery system according to claim 7, wherein the nitrogen-containing compound is NH3.

9. The redox flow battery system according to claim 7, wherein the nitrogen-containing compound is CO(NH2)2 or CS(NH2)2.

10. The redox flow battery system according to claim 7, wherein J is NH3 and at least one M is CO(NH2)2.

11. The redox flow battery system according to claim 7, wherein J is NH3.

12. A non-transitory computer-readable medium having processor-executable instructions for operating a redox flow battery system according to claim 1, wherein when the processor-executable instructions are mounted on a device, the device is enabled to perform actions, the actions including: The values ​​indicating the state of charge of the anolyte or catholyte are measured intermittently, periodically, or continuously before entering or after entering the first or second half-cell; and Time-energy curves for the anolyte or catholyte were generated using the measured values.

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