Method and system for measuring ferrous iron concentration in electrolyte of Fe / Cr flow battery

Abstract

The present invention discloses a method and system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery. The measurement method includes: mixing the positive electrolyte and the negative electrolyte in equal volume to obtain a mixed electrolyte; mixing the mixed electrolyte with an acid and an excessive amount of soluble dichromate so that the divalent iron in the mixed electrolyte is fully oxidized to trivalent iron to obtain an acidic test solution; introducing a pair of electrolysis electrodes and a pair of measurement electrodes into the acidic test solution, performing direct current constant current electrolysis on the acidic test solution through the electrolysis electrodes to reduce the trivalent iron in the acidic test solution, and simultaneously detecting the potential between the measurement electrodes by a potentiometer and recording the electrolysis duration required when the potential suddenly drops; obtaining the concentration of divalent iron in the electrolyte of the Fe / Cr flow battery according to the added amount of the soluble dichromate, the current magnitude of the constant current electrolysis, and the electrolysis duration. This measurement method has the advantages of accurate measurement results, high reliability, etc.

Description

Technical Field

[0001] The present invention belongs to the field of flow batteries, and more particularly, to a method and system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery. Background Art

[0002] A flow battery system is a new type of large-scale electrochemical energy storage device. Compared with traditional energy storage batteries, the flow battery system has the advantages of independent power and capacity, low self-discharge, good safety, and no pollutant emissions. Existing flow battery electrochemical redox systems include all-vanadium flow batteries, zinc-bromine flow batteries, iron-chromium flow batteries, etc. Among them, the iron-chromium flow battery, as one of the flow batteries, also has the following excellent characteristics: high efficiency, low cost, long life, fast response speed, wide operating temperature range, etc. The positive and negative electrode electrolytes of the Fe / Cr flow battery contain Fe 3+ / Fe 2+ 、Cr 3+ / Cr 2+ mixed ions, which not only play a role in conducting electricity, but also are active substances that store chemical energy converted from electrical energy in the electrolyte. The concentration of ions in different valence states directly affects the performance of the battery. Therefore, it is very important to accurately measure the concentration of iron ions in different valence states in the electrolyte.

[0003] Existing methods for measuring the concentration of divalent iron ions mainly include spectrophotometry, redox titration, complexometric titration, etc. The corresponding national standards are GB 6730.8-86, GB / T 6730.65-2009, GB / T223.73-2008, JB / T6237.3-2008, etc. The principle of the above measurement methods is to reduce trivalent iron to divalent iron, and directly analyze the concentration of trivalent iron through a conventional spectrophotometer, or judge the titration end point through color change. However, factors such as the mutual interference of the colors of various ions in the electrolyte used in the iron-chromium flow battery will affect the determination of the end point and make it impossible to accurately measure the concentration of trivalent iron ions. Therefore, the method for measuring the concentration of divalent iron ions in the electrolyte of the iron-chromium flow battery still needs to be further improved. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems in the related art to some extent. For this reason, an object of the present invention is to provide a method and system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery. The measurement accuracy of this measurement method is independent of the color change of the electrolyte, and the dichromate introduced by the measurement and the chromium ions in the electrolyte do not affect the measurement result. The measurement accuracy can reach 10 -6 ~0.01 mol / L, and has the advantages of accurate measurement results, high reliability, etc.

[0005] The present invention is mainly proposed based on the following problems:

[0006] Among the existing methods for measuring the concentration of divalent iron ions in electrolytes, there is a method of titrating in an acidic solution medium with potassium thiocyanate as an indicator, reducing trivalent iron with ascorbic acid, determining the titration end point according to the color change, and calculating the corresponding iron ion concentration from the corresponding electrolyte volume. The electrolyte of the iron-chromium flow battery turns dark green after dilution. After adding potassium thiocyanate, it forms a blood-red complex with Fe 3+ in the electrolyte. After adding ascorbic acid, trivalent iron is reduced to colorless. At the end point of the reaction, the solution is bright green. However, this method judges the reaction end point by the naked eye, and the electrolyte needs to be diluted many times to see the color change, which will introduce errors and cannot accurately measure the iron ion concentration. Moreover, it is the concentration of ferrous ions in the electrolyte that affects the performance of the iron-chromium flow battery, while the reaction measures the concentration of trivalent iron ions. The concentration of ferrous ions needs to be obtained by measuring the total iron concentration and then subtracting the concentration of trivalent iron ions, which will also introduce errors. Further, the titration process requires slow titration with a standard ascorbic acid solution, and ferrous ions in the electrolyte may be oxidized during sample transfer, sample titration, etc., increasing the trivalent iron ions and causing the measurement result to deviate from the actual value. There is also a method of reducing trivalent iron to divalent with ascorbic acid. At pH 5.7, divalent iron forms a wine-red complex with o-phenanthroline, and its absorbance is measured at a wavelength of 510 nm with a spectrophotometer to obtain the concentration of trivalent iron. However, this method is only applicable to solutions containing divalent iron at a concentration of 5-200 μg / L, and the electrolyte is acidic. Therefore, the electrolyte needs to be diluted about 10,000 times, resulting in large errors. In addition, o-phenanthroline and some metal colored substances will interfere with the measurement. Moreover, a standard solution curve needs to be drawn and the mass of iron is obtained from the working curve, and the process is cumbersome and the iron ion concentration cannot be directly obtained.

[0007] For this reason, according to the first aspect of the present invention, the present invention proposes a method for measuring the concentration of divalent iron in the electrolyte of an Fe / Cr flow battery. According to an embodiment of the present invention, the method includes:

[0008] (1) Mix the positive electrolyte and the negative electrolyte in equal volumes to obtain a mixed electrolyte;

[0009] (2) Mix the mixed electrolyte with an acid and an excessive amount of soluble dichromate to fully oxidize the divalent iron in the mixed electrolyte to trivalent iron, obtaining an acidic test solution;

[0010] (3) Introduce a pair of electrolysis electrodes and a pair of measurement electrodes into the acidic test solution, perform direct current constant current electrolysis on the acidic test solution through the electrolysis electrodes, reduce the trivalent iron in the acidic test solution, and at the same time use a potentiometer to detect the potential between the measurement electrodes and record the electrolysis duration required when the potential suddenly drops;

[0011] (4) Obtain the concentration of ferrous iron in the Fe / Cr flow battery electrolyte according to the addition amount of the soluble dichromate, the magnitude of the constant current electrolysis current, and the electrolysis duration.

[0012] According to the method for measuring the concentration of ferrous iron in the Fe / Cr flow battery electrolyte in the above embodiments of the present invention, the inventors found that before charging the Fe / Cr flow battery, the concentrations of ferrous iron and ferric iron in the positive electrode electrolyte and the negative electrode electrolyte are the same; as the charging process proceeds, ferrous iron in the positive electrode electrolyte is converted into ferric iron, and trivalent chromium in the negative electrode electrolyte is converted into divalent chromium, and the electrode potential determines that the reaction of ferric iron being converted into ferrous iron does not occur in the negative electrode electrolyte. That is to say, only the ferrous iron in the positive electrode electrolyte undergoes an oxidation reaction, and the amount of trivalent chromium undergoing a reduction reaction in the negative electrode electrolyte is the same as the amount of ferrous iron undergoing an oxidation reaction in the positive electrode electrolyte. In the present invention, by mixing the positive electrode electrolyte and the negative electrode electrolyte in equal volume, the ferric iron obtained by oxidation in the positive electrode electrolyte can be reduced again by the divalent chromium in the negative electrode electrolyte, thereby avoiding the influence of divalent chromium ions on subsequent electrolytic measurements and ensuring the accuracy of the measurement results; further, in an acidic condition, an excess of dichromate is used to oxidize the ferrous iron in the mixed electrolyte to obtain an acidic test solution containing only ferric iron and trivalent chromium; then, the acidic test solution is further subjected to constant current electrolysis, so that the ferric iron in the test solution can be reduced to ferrous iron again, and the excess dichromate further oxidizes the ferrous iron under acidic conditions, and the two reach a dynamic equilibrium. When the potential of the measurement electrode suddenly drops, it indicates that the dichromate in the acidic test solution has been completely consumed. Thus, according to the magnitude of the constant current electrolysis current, the electrolysis duration, and the total addition amount of the dichromate, it is possible to know the amount of dichromate consumed for oxidizing ferrous iron before electrolysis, and then obtain the concentration of ferrous iron in the Fe / Cr flow battery electrolyte. In summary, the measurement accuracy of this measurement method is independent of the color change of the electrolyte, and the dichromate introduced by the measurement and the chromium ions in the electrolyte do not affect the measurement results. The measurement accuracy can reach 10 -6 ~0.01 mol / L, having the advantages of accurate measurement results, high reliability, etc., and solving the problem in the prior art that the concentration of ferrous iron ions in the electrolyte of the iron-chromium flow battery cannot be directly and accurately measured.

[0013] In addition, the method for measuring the concentration of ferrous iron in the Fe / Cr flow battery electrolyte according to the above embodiments of the present invention may further have the following additional technical features:

[0014] In some embodiments of the present invention, steps (1) to (4) are carried out in an inert atmosphere.

[0015] In some embodiments of the present invention, step (1) further includes: stirring the mixed electrolyte.

[0016] In some embodiments of the present invention, step (2) further includes: (2-1) mixing the mixed electrolyte with the acid to obtain an acidic electrolyte; (2-2) mixing the acidic electrolyte with the excessive soluble dichromate to obtain the acidic test solution.

[0017] In some embodiments of the present invention, in step (2), the molar ratio of the total iron in the mixed electrolyte to the dichromate radical in the soluble dichromate is 6:(1-1.1), and the total iron includes divalent iron and trivalent iron.

[0018] In some embodiments of the present invention, in step (2), the concentration of hydrogen ions in the acidic test solution is 2-6 mol / L.

[0019] In some embodiments of the present invention, in step (2), the soluble dichromate is at least one selected from potassium dichromate, sodium dichromate, ammonium dichromate and silver dichromate.

[0020] In some embodiments of the present invention, in step (2), the acid is selected from sulfuric acid and / or hydrochloric acid.

[0021] In some embodiments of the present invention, in step (3), a coulometric titration device is used to perform the constant current electrolysis and the detection on the test solution.

[0022] In some embodiments of the present invention, the positive electrode and the negative electrode of the electrolysis / measurement electrode are separated by a diaphragm sleeve or are respectively placed in different containers and connected by a salt bridge.

[0023] In some embodiments of the present invention, the positive electrode of the electrolysis electrode uses a single platinum wire, and the single platinum wire is placed in a glass tube containing an acid. The negative electrode of the electrolysis electrode uses a double platinum plate; the positive electrode of the measurement electrode uses a single platinum plate, and the negative electrode of the measurement electrode uses a black tungsten wire, and the black tungsten wire is placed in a glass tube containing a salt solution.

[0024] In some embodiments of the present invention, the acid in the glass tube is the same as the acid used in step (2), and the difference in the concentration of hydrogen ions in the glass tube and the concentration of hydrogen ions in the acidic test solution is not more than 10%.

[0025] In some embodiments of the present invention, in step (4), the concentration of divalent iron in the Fe / Cr flow battery electrolyte is: C = 3×(M - It / 6F) / V, where C is the concentration of divalent iron in the Fe / Cr flow battery electrolyte, in mol / L; M is the amount of dichromate added to the mixed electrolyte, in mol; I is the current of the constant current electrolysis, in A; t is the electrolysis duration required for the potential to drop suddenly, in s; F is the Faraday constant, in C / mol -1 ; V is the volume of the positive electrolyte or the negative electrolyte, in L.

[0026] In some embodiments of the present invention, the value of F is 96485 - 96500 C / mol.

[0027] According to the second aspect of the present invention, the present invention provides a system for implementing the method for measuring the concentration of divalent iron in the Fe / Cr flow battery electrolyte described above. According to the embodiments of the present invention, the system includes:

[0028] A mixing device, which includes a mixing tank, a positive electrolyte inlet, and a negative electrolyte inlet. The positive electrolyte inlet is connected to the positive electrolytic cell, and the negative electrolyte inlet is connected to the negative electrolytic cell;

[0029] A reaction device, which includes a reaction tank, a mixed electrolyte inlet, an acid inlet, and a soluble dichromate inlet. The mixed electrolyte inlet is connected to the mixing tank;

[0030] A titration device, which includes a liquid storage tank, an electrolytic generator, and an indicator. The liquid storage tank is connected to the reaction tank. The electrolytic generator has a pair of electrolytic electrodes and a DC constant current power supply connected to the electrolytic electrodes. The indicator has a pair of measuring electrodes and a potentiometer connected to the measuring electrodes. The electrolytic electrodes and the measuring electrodes are adapted to be immersed in the liquid to be measured in the liquid storage tank, and the positive and negative electrodes of the electrolytic electrodes are adapted to be separated by a diaphragm sleeve or connected by a salt bridge. The electrolytic generator is adapted to perform DC constant current electrolysis on the liquid to be measured in the liquid storage tank, and the indicator is adapted to detect the potential between the measuring electrodes using the potentiometer.

[0031] According to the system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to the above embodiments of the present invention, the positive electrolyte and the negative electrolyte can be mixed in equal volume by a mixing device, and the mixed electrolyte can be fully oxidized by a reaction device to obtain an acidic test solution. Then, a titration device is used to perform oxidation-reduction on the acidic test solution to obtain the charge required to consume the remaining dichromate in the acidic test solution. Thus, by comprehensively considering the magnitude of the current in constant-current electrolysis, the electrolysis duration, and the total amount of dichromate added, it is possible to know the amount of dichromate consumed in oxidizing divalent iron before electrolysis, and further obtain the concentration of divalent iron in the electrolyte of the Fe / Cr flow battery. Therefore, when using this system to measure the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery, the measurement accuracy is independent of the color change of the electrolyte, and the dichromate introduced during the measurement and the chromium ions in the electrolyte do not affect the measurement result, and the measurement accuracy can reach 10 -6 ~0.01 mol / L, having the advantages of accurate measurement results, high reliability, etc., and solving the problem in the prior art that the concentration of divalent iron ions in the electrolyte of an iron-chromium flow battery cannot be directly and accurately measured.

[0032] In some embodiments of the present invention, the system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery further includes: a protection device, the protection device includes a box body and an operation space located inside the box body, the box body has an inert gas inlet, an inert gas outlet, and operation gloves, and the protection device is adapted to seal and protect the mixing device, the reaction device, and the titration device in an inert atmosphere.

[0033] In some embodiments of the present invention, the positive electrolyte inlet is hermetically connected to the positive electrolytic cell, and the negative electrolyte inlet is hermetically connected to the negative electrolytic cell.

[0034] In some embodiments of the present invention, a first metering feeder is provided at the positive electrolyte inlet, and a second metering feeder is provided at the negative electrolyte inlet.

[0035] In some embodiments of the present invention, the mixing device further includes a stirring device.

[0036] In some embodiments of the present invention, the stirring device is a stirring rod, a stirring paddle, an ultrasonic generator, or glass balls.

[0037] In some embodiments of the present invention, the electrolysis generator further includes a timer and / or a current measuring instrument.

[0038] In some embodiments of the present invention, the titration device is a coulometric titration device.

[0039] In some embodiments of the present invention, the mixing tank, the reaction tank, and the liquid storage tank are the same tank body.

[0040] In some embodiments of the present invention, an oxygen detector is further provided in the protection device.

[0041] In some embodiments of the present invention, the protection device is connected to a vacuum pumping device.

[0042] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. Description of the Drawings

[0043] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0044] Figure 1 is a flowchart of a method for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to an embodiment of the present invention.

[0045] Figure 2 is a schematic structural diagram of a system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to an embodiment of the present invention.

[0046] Figure 3 is a schematic structural diagram of a system for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to another embodiment of the present invention. Detailed Description of the Embodiments

[0047] Embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the drawings are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0048] According to a first aspect of the present invention, the present invention provides a method for measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery. According to an embodiment of the present invention, as Figure 1As shown, the method includes: (1) mixing the positive electrode electrolyte and the negative electrode electrolyte in equal volumes to obtain a mixed electrolyte; (2) mixing the mixed electrolyte with an acid and an excessive amount of soluble dichromate so that ferrous iron in the mixed electrolyte is fully oxidized to ferric iron to obtain an acidic test solution; (3) introducing a pair of electrolysis electrodes and a pair of measurement electrodes into the acidic test solution, performing direct current constant current electrolysis on the acidic test solution through the electrolysis electrodes, reducing ferric iron in the acidic test solution, and simultaneously using a potentiometer to detect the potential between the measurement electrodes and recording the electrolysis duration required when the potential suddenly drops; (4) obtaining the concentration of ferrous iron in the Fe / Cr flow battery electrolyte according to the addition amount of the soluble dichromate, the magnitude of the constant current electrolysis current, and the electrolysis duration. The measurement accuracy of this measurement method is independent of the color change of the electrolyte, and the added dichromate and chromium ions in the electrolyte during measurement do not affect the measurement result, and the measurement accuracy can reach 10 -6 ~0.01 mol / L, having the advantages of accurate measurement results, high reliability, etc., and solving the problem in the prior art that the concentration of ferrous iron ions in the Fe / Cr flow battery electrolyte cannot be directly and accurately measured.

[0049] It should be noted that the concentration of ferrous iron in the Fe / Cr flow battery electrolyte obtained by measurement in the present invention refers to the concentration of ferrous iron in the Fe / Cr flow battery electrolyte before charging; in addition, the "excessive" in the excessive soluble dichromate is based on the iron content in the electrolyte, and the acid is excessive relative to the dichromate.

[0050] Next, refer to Figure 1 to describe in detail the method for measuring the concentration of ferrous iron in the Fe / Cr flow battery electrolyte in the above embodiments of the present invention.

[0051] S100: Mix the positive electrode electrolyte and the negative electrode electrolyte in equal volumes to obtain a mixed electrolyte

[0052] According to the embodiments of the present invention, the inventor found that before charging the Fe / Cr flow battery, the concentrations of ferrous iron and ferric iron in the positive electrode electrolyte and the negative electrode electrolyte are the same; as the charging process progresses, ferrous iron in the positive electrode electrolyte is converted into ferric iron, and trivalent chromium in the negative electrode electrolyte is converted into divalent chromium, and the electrode potential determines that the reaction of ferric iron being converted into ferrous iron will not occur in the negative electrode electrolyte. That is to say, only the ferrous iron in the positive electrode electrolyte undergoes an oxidation reaction, and the amount of trivalent chromium in the negative electrode electrolyte undergoing a reduction reaction is the same as the amount of ferrous iron in the positive electrode electrolyte undergoing an oxidation reaction. In the present invention, by mixing the positive electrode electrolyte and the negative electrode electrolyte in equal volumes, the ferric iron obtained by oxidation in the positive electrode electrolyte can be reduced again by the divalent chromium in the negative electrode electrolyte, thereby avoiding the influence of divalent chromium ions on subsequent electrolysis measurement and ensuring the accuracy of the measurement result.

[0053] According to a specific embodiment of the present invention, the mixed electrolyte can be further subjected to a stirring treatment, whereby the positive electrode electrolyte and the negative electrode electrolyte can be fully mixed, which is more conducive to eliminating the influence of divalent chromium ions on subsequent electrolytic measurements and ensuring the accuracy and reliability of the measurement results. Further, a stirring rod, a stirring paddle, an ultrasonic generator or glass balls can be used to stir the mixed electrolyte. For example, a number of glass balls can be added to the mixed electrolyte to further improve the uniformity of the mixed electrolyte.

[0054] S200: Mix the mixed electrolyte with an acid and an excessive amount of soluble dichromate to fully oxidize the divalent iron in the mixed electrolyte to trivalent iron, obtaining an acidic test solution.

[0055] According to an embodiment of the present invention, in an acidic condition, the divalent iron in the mixed electrolyte can be oxidized by an excessive amount of dichromate ions to obtain an acidic test solution containing only trivalent iron and trivalent chromium. Specifically, the mixed electrolyte can be pre-mixed with an acid to obtain an acidic electrolyte; then the acidic electrolyte is mixed with an excessive amount of soluble dichromate to obtain an acidic test solution. In the present invention, by pre-mixing the mixed electrolyte with an acid, it can further ensure that the divalent chromium in the mixed electrolyte is fully oxidized by the trivalent iron.

[0056] According to a specific embodiment of the present invention, the molar ratio of the total iron in the mixed electrolyte to the dichromate ions in the soluble dichromate can be 6:(1-1.1), and the total iron includes divalent iron and trivalent iron. The inventor found that if the molar ratio of the total iron in the mixed electrolyte to the dichromate ions in the soluble dichromate is too large, it cannot ensure that the divalent iron in the mixed electrolyte is fully oxidized by the dichromate ions, and if the molar ratio of the total iron in the mixed electrolyte to the dichromate ions in the soluble dichromate is too small, more dichromate ions are consumed in the subsequent electrolytic measurement, which will affect the measurement efficiency. In the present invention, by controlling the soluble dichromate to the above addition amount, the measurement efficiency can be further improved on the basis of ensuring the accuracy of the detection result.

[0057] According to still another specific embodiment of the present invention, the concentration of hydrogen ions in the acidic test solution can be 2-6 mol / L. The inventor found that by controlling the acidic test solution to the above acidic condition in the present invention, the oxidizing property of the dichromate ions can be further improved, ensuring that the dichromate ions can quickly oxidize the divalent iron obtained by electrolytic reduction again during the subsequent electrolytic measurement.

[0058] According to another specific embodiment of the present invention, the types of soluble dichromate and acid in the present invention are not particularly limited, and those skilled in the art can select according to actual needs. For example, the soluble dichromate can be at least one selected from potassium dichromate, sodium dichromate, ammonium dichromate, and silver dichromate; the acid can be selected from sulfuric acid and / or hydrochloric acid, etc., thereby further improving the accuracy and reliability of the detection results.

[0059] S300: Introduce a pair of electrolysis electrodes and a pair of measurement electrodes into the acidic test solution, perform direct current constant current electrolysis on the acidic test solution through the electrolysis electrodes, reduce the trivalent iron in the acidic test solution, and at the same time use a potentiometer to detect the potential between the measurement electrodes and record the electrolysis duration required when the potential suddenly drops.

[0060] According to the embodiment of the present invention, performing constant current electrolysis on the acidic test solution can reduce the trivalent iron in the test solution to divalent iron again, and the excessive dichromate further oxidizes the divalent iron under acidic conditions, and the two reach a dynamic equilibrium. When the potential of the measurement electrode suddenly drops, it indicates that the dichromate in the acidic test solution has been completely consumed. According to the magnitude of the constant current electrolysis current and the electrolysis duration, the amount of dichromate consumed during the electrolysis measurement process can be obtained. Further combining with the total addition amount of dichromate, the amount of dichromate consumed for oxidizing divalent iron before electrolysis can be known, and then the concentration of divalent iron in the Fe / Cr flow battery electrolyte can be obtained.

[0061] According to a specific embodiment of the present invention, a coulometric titration device can be used to perform constant current electrolysis and detection on the test solution, thereby further improving the accuracy and reliability of the detection results.

[0062] According to still another specific embodiment of the present invention, the positive electrode and the negative electrode of the electrolysis / measurement electrodes can be separated by a diaphragm sleeve or placed in different containers and connected by a salt bridge, thereby further reducing errors and improving the detection accuracy. For example, the positive electrode of the electrolysis electrode can be immersed in an acid solution, and the hydrogen ions in the acid solution obtain electrons to generate hydrogen gas. The acid solution can be placed in a glass tube with a diaphragm, and the negative electrode of the measurement electrode can be placed in a glass tube containing a salt solution.

[0063] According to yet another specific embodiment of the present invention, the positive electrode of the electrolysis electrode can be a single platinum wire, and the single platinum wire is placed in a glass tube containing an acid. The bottom of the glass tube can have a diaphragm. The negative electrode of the electrolysis electrode can be a double platinum sheet; the positive electrode of the measurement electrode can be a single platinum sheet, and the negative electrode of the measurement electrode can be a black tungsten wire. The black tungsten wire is placed in a glass tube containing a saturated potassium sulfate solution. The bottom of the glass tube can have a diaphragm, which can further help reduce errors and improve the detection accuracy. Further, the type of acid in the glass tube is not particularly limited, and those skilled in the art can select according to actual needs. For example, the acid in the glass tube can be the same as the acid used in step S200. The concentration of hydrogen ions in the glass tube can be 2 to 6 mol / L, preferably the same as or close to the concentration of hydrogen ions in the sample to be measured, which can further facilitate the generation of hydrogen, thereby improving the efficiency of constant current electrolysis. Further, the difference between the concentration of hydrogen ions in the glass tube and the concentration of hydrogen ions in the acidic sample to be measured can be no more than 10%, which can further reduce errors and improve the detection accuracy.

[0064] According to yet another specific embodiment of the present invention, the magnitude of the current used in constant current electrolysis can be 1 to 100 mA, for example, it can be 10 to 100 mA, etc., which can further improve the detection accuracy.

[0065] S400: Obtain the concentration of divalent iron in the Fe / Cr flow battery electrolyte according to the added amount of soluble dichromate, the magnitude of the current in constant current electrolysis, and the electrolysis duration

[0066] According to the embodiment of the present invention, the concentration of divalent iron in the Fe / Cr flow battery electrolyte is:

[0067] C = 3×(M - It / 6F) / V,

[0068] where C is the concentration of divalent iron in the Fe / Cr flow battery electrolyte, in mol / L; M is the amount of dichromate added to the mixed electrolyte, in mol; I is the current in constant current electrolysis, in A; t is the electrolysis duration required for the potential to suddenly drop, in s; F is the Faraday constant, in C / mol; V is the volume of the positive electrode electrolyte or the negative electrode electrolyte, in L.

[0069] According to a specific embodiment of the present invention, the value range of the Faraday constant F can be 96485 to 96500 C / mol. Thus, the appropriate magnitude of the Faraday constant can be selected according to different detection accuracy requirements.

[0070] According to a specific embodiment of the present invention, steps S100 to S400 are preferably carried out under an inert atmosphere. The inventors have found that when measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery, the divalent iron ions in the electrolyte may be oxidized during processes such as sample transfer and sample titration, resulting in an increase in trivalent iron ions and causing the measurement result to deviate from the actual value. However, this problem has not been recognized in existing detection methods. In the present invention, by carrying out the entire measurement process within a protective atmosphere, the problem of divalent iron ions being oxidized during sample transfer, titration, etc. can be effectively avoided, thereby further improving the accuracy and stability of the measurement result.

[0071] In summary, for the method of measuring the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to the above embodiments of the present invention, the inventors have found that before charging the Fe / Cr flow battery, the concentrations of divalent iron and trivalent iron in the positive electrolyte and the negative electrolyte are the same; as the charging process proceeds, divalent iron in the positive electrolyte is converted into trivalent iron, and trivalent chromium in the negative electrolyte is converted into divalent chromium. The electrode potential determines that the reaction of trivalent iron being converted into divalent iron does not occur in the negative electrolyte. That is to say, only the divalent iron in the positive electrolyte undergoes an oxidation reaction, and the amount of trivalent chromium undergoing a reduction reaction in the negative electrolyte is the same as the amount of divalent iron undergoing an oxidation reaction in the positive electrolyte. In the present invention, by mixing the positive electrolyte and the negative electrolyte in equal volumes, the trivalent iron obtained by oxidation in the positive electrolyte can be reduced again by the divalent chromium in the negative electrolyte, thereby avoiding the influence of divalent chromium ions on subsequent electrolytic measurements and ensuring the accuracy of the measurement result; further, under acidic conditions, the divalent iron in the mixed electrolyte is oxidized by an excessive amount of dichromate to obtain an acidic test solution containing only trivalent iron and trivalent chromium; then, the acidic test solution is further subjected to constant current electrolysis, so that the trivalent iron in the test solution is reduced to divalent iron again, and the excessive dichromate further oxidizes the divalent iron under acidic conditions, and the two reach a dynamic balance. When the potential of the measuring electrode suddenly drops, it indicates that the dichromate in the acidic test solution has been completely consumed. Thus, according to the magnitude of the constant current electrolysis current, the electrolysis duration, and the total amount of dichromate added, the amount of dichromate consumed for oxidizing divalent iron before electrolysis can be known, and then the concentration of divalent iron in the electrolyte of the Fe / Cr flow battery can be obtained. In summary, the measurement accuracy of this measurement method is independent of the color change of the electrolyte, and the dichromate introduced by the measurement and the chromium ions in the electrolyte do not affect the measurement result. The measurement accuracy can reach 10 -6 ~0.01 mol / L, having the advantages of accurate measurement results, high reliability, etc., and solving the problem in the prior art that the concentration of divalent iron ions in the electrolyte of an iron-chromium flow battery cannot be directly and accurately measured.

[0072] According to a second aspect of the present invention, the present invention provides a system for implementing the method for measuring the concentration of divalent iron in the electrolyte of an Fe / Cr flow battery. According to an embodiment of the present invention, as Figure 2 shown, the system includes: a mixing device 100, a reaction device 200, and a titration device 300. When using this system to measure the concentration of divalent iron in the electrolyte of an Fe / Cr flow battery, the measurement accuracy is independent of the color change of the electrolyte, and the dichromate introduced during the measurement and the chromium ions in the electrolyte do not affect the measurement result. The measurement accuracy can reach 10 -6 ~0.01 mol / L, with the advantages of accurate measurement results and high reliability, solving the problem in the prior art that the concentration of divalent iron ions in the electrolyte of an iron-chromium flow battery cannot be directly and accurately measured. The following refers to Figures 2 - 3 to describe in detail the system for measuring the concentration of divalent iron in the electrolyte of an Fe / Cr flow battery according to the above embodiments of the present invention.

[0073] Mixing device 100

[0074] According to an embodiment of the present invention, the mixing device includes a mixing tank 110, a positive electrolyte inlet 120, and a negative electrolyte inlet 130. The positive electrolyte inlet 120 is connected to the positive electrolytic cell, and the negative electrolyte inlet 130 is connected to the negative electrolytic cell. This mixing device is suitable for mixing the positive electrolyte and the negative electrolyte in equal volumes to obtain a mixed electrolyte, thereby not only avoiding the influence of divalent chromium ions on subsequent electrolytic measurements but also ensuring the accuracy of the measurement results.

[0075] According to a specific embodiment of the present invention, the positive electrolyte inlet 120 can be hermetically connected to the positive electrolytic cell, and the negative electrolyte inlet 130 can be hermetically connected to the negative electrolytic cell, thereby effectively avoiding the problem that divalent iron ions in the electrolyte may be oxidized during the transfer of the positive electrolyte and the negative electrolyte, resulting in an increase in trivalent iron ions and causing the measurement result to deviate from the actual value.

[0076] According to still another specific embodiment of the present invention, a first metering feeder (not shown) can be provided at the positive electrolyte inlet 120, and a second metering feeder (not shown) can be provided at the negative electrolyte inlet 130, which can further facilitate the equal-volume mixing of the positive electrolyte and the negative electrolyte and avoid the problem of inaccurate measurement results caused by uneven mixing of the positive electrolyte and the negative electrolyte.

[0077] According to yet another specific embodiment of the present invention, the mixing device 100 may further include a stirring device (not shown), which is adapted to stir the mixed electrolyte in the mixing tank, thereby being more conducive to eliminating the influence of divalent chromium ions on subsequent electrolytic measurements and ensuring the accuracy and reliability of the measurement results. Further, the stirring device may be a stirring rod, a stirring paddle, an ultrasonic generator, or glass beads, etc. For example, a number of glass beads may be added to the mixed electrolyte to further improve the uniformity of the mixed electrolyte, or an ultrasonic device may be used to mix the positive electrolyte and the negative electrolyte in equal volumes.

[0078] Reaction device 200

[0079] According to an embodiment of the present invention, the reaction device 200 includes a reaction tank 210, a mixed electrolyte inlet 220, an acid inlet 230, and a soluble dichromate inlet 240. The mixed electrolyte inlet 220 is connected to the mixing tank 110. The reaction device is adapted to mix the mixed electrolyte with an acid and an excess of soluble dichromate so that divalent iron in the mixed electrolyte is fully oxidized to trivalent iron to obtain an acidic test solution. For example, the mixed electrolyte and the acid may be pre-mixed to obtain an acidic electrolyte; then the acidic electrolyte is mixed with an excess of soluble dichromate to obtain an acidic test solution.

[0080] According to a specific embodiment of the present invention, the reaction device 200 may also further include a stirring device (not shown), which is adapted to stir the reaction solution in the reaction tank, thereby being more conducive to the full oxidation of divalent iron in the reaction solution by dichromate ions.

[0081] Titration device 300

[0082] According to an embodiment of the present invention, the titration device 300 includes a liquid storage tank 310, an electrolytic generator 320, and an indicator 330. The liquid storage tank 310 is connected to the reaction tank 320. The electrolytic generator 320 has a pair of electrolytic electrodes 321 and a DC constant current power supply 322 connected to the electrolytic electrodes 321. The indicator 330 has a pair of measuring electrodes 331 and a potentiometer 332 connected to the measuring electrodes 331. The electrolytic electrodes 321 and the measuring electrodes 331 are adapted to be immersed in the liquid to be measured in the liquid storage tank 310, and the positive and negative electrodes of the electrolytic electrodes 321 are adapted to be separated by a diaphragm sleeve or connected by a salt bridge. The electrolytic generator 320 is adapted to perform DC constant current electrolysis on the liquid to be measured in the liquid storage tank 310, and the indicator 320 is adapted to detect the potential between the measuring electrodes 331 by using the potentiometer 332. The titration device is adapted to perform DC constant current electrolysis on the acidic liquid to be measured through the electrolytic electrodes, so that ferric iron in the acidic liquid to be measured is reduced. At the same time, the potential between the measuring electrodes is detected by using the potentiometer, and the electrolysis duration required when the potential suddenly drops is recorded. Thus, according to the magnitude of the current in the constant current electrolysis and the electrolysis duration, the amount of dichromate consumed during the electrolytic measurement can be obtained. Further, in combination with the total amount of dichromate added, the amount of dichromate consumed for oxidizing ferrous iron before electrolysis can be known, and then the concentration of ferrous iron in the Fe / Cr flow battery electrolyte can be obtained.

[0083] According to a specific embodiment of the present invention, the electrolytic generator 320 may further include a timer 323 and / or a current measuring instrument 324, whereby the electrolysis duration required when the potentiometer suddenly drops can be recorded by using the timer, and whether the magnitude of the current changes during the constant current electrolysis can be checked by using the current measuring instrument.

[0084] According to still another specific embodiment of the present invention, the titration device 300 may be a Coulometric titration device, which can further facilitate controlling the conditions of the constant current electrolysis process, thereby further improving the accuracy and reliability of the detection results.

[0085] According to yet another specific embodiment of the present invention, the mixing tank 110, the reaction tank 210, and the liquid storage tank 310 may be the same tank body, which can further avoid possible detection errors during the solution transfer process, thereby further improving the measurement accuracy.

[0086] According to yet another specific embodiment of the present invention, the positive electrode of the electrolytic electrode may be a single platinum wire, and the single platinum wire may be placed in a glass tube containing acid. The bottom of the glass tube may have a diaphragm. The negative electrode of the electrolytic electrode may be a double platinum plate; the positive electrode of the measuring electrode may be a single platinum plate, and the negative electrode of the measuring electrode may be a black tungsten wire. The black tungsten wire may be placed in a glass tube containing a saturated potassium sulfate solution, and the bottom of the glass tube may have a diaphragm, which can further facilitate reducing errors and improving the detection accuracy.

[0087] According to a specific embodiment of the present invention, as Figure 3 shown, the system for measuring the ferrous iron concentration in the electrolyte of the Fe / Cr flow battery may further include a protection device 400. The protection device 400 may include a box body 410 and an operation space 420 located inside the box body 410. The box body 410 has an inert gas inlet 411, an inert gas outlet 412, and an operation glove 413. The protection device 400 is adapted to seal and protect the mixing device 100, the reaction device 200, and the titration device 300 with an inert atmosphere, so that the entire measurement process can be carried out in a protective atmosphere, effectively avoiding the problem that ferrous ions are oxidized during the transfer and titration of the electrolyte, thereby further improving the accuracy and stability of the measurement results.

[0088] According to another specific embodiment of the present invention, the protection device 400 may further be provided with an oxygen detector (not shown), whereby the oxygen content inside the protection device can be monitored in real time to ensure that the entire measurement process is carried out under an inert atmosphere.

[0089] According to still another specific embodiment of the present invention, the protection device 400 may further be connected to a vacuum pumping device (not shown), whereby the protection device can be pre-evacuated and then filled with an inert gas, which is more conducive to exhausting the air inside the protection device, thereby further avoiding the problem that ferrous ions are oxidized during the transfer and titration of the electrolyte, and further improving the accuracy and stability of the measurement results.

[0090] In summary, for the system for measuring the ferrous iron concentration in the electrolyte of the Fe / Cr flow battery according to the above embodiments of the present invention, the mixing device can be used to mix the positive electrolyte and the negative electrolyte in equal volume, and the reaction device can be used to fully oxidize the mixed electrolyte to obtain an acidic test solution. Then, the titration device can be used to perform oxidation-reduction on the acidic test solution to obtain the charge required to consume the remaining dichromate in the acidic test solution. Thus, by comprehensively considering the magnitude of the current in the constant-current electrolysis, the electrolysis duration, and the total amount of dichromate added, the amount of dichromate consumed for oxidizing ferrous iron before electrolysis can be known, and further the concentration of ferrous iron in the electrolyte of the Fe / Cr flow battery can be obtained. Therefore, when using this system to measure the ferrous iron concentration in the electrolyte of the Fe / Cr flow battery, the measurement accuracy is independent of the color change of the electrolyte, and the dichromate introduced during the measurement and the chromium ions in the electrolyte do not affect the measurement results. The measurement accuracy can reach 10 -6 ~0.01 mol / L, having the advantages of accurate measurement results and high reliability, and solving the problem in the prior art that the concentration of ferrous ions in the electrolyte of the iron-chromium flow battery cannot be directly and accurately measured.

[0091] The scheme of the present invention will be explained below in conjunction with the embodiments. It will be appreciated by those skilled in the art that the following embodiments are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used are not indicated by the manufacturer and are all conventional products that can be obtained commercially.

[0092] Determination of the concentration of ferrous iron in the electrolyte of the same Fe / Cr flow battery at different charge states

[0093] Example 1

[0094] Determination of the concentration of divalent iron in the electrolyte of Fe / Cr flow battery before charging

[0095] (1) Before charging the Fe / Cr flow battery, 2.5 mL of the positive and negative electrolytes were transferred into a conical flask, and several glass balls were added and mixed to obtain a mixed electrolyte;

[0096] (2) adding 50 mL of 3 mol / L sulfuric acid to the mixed electrolyte obtained in step (1), and then adding 10 mL of 0.10136 mol / L potassium dichromate solution (excess), and mixing thoroughly to obtain an acidic test solution;

[0097] (3) Use a coulometric titrator to perform a DC constant current titration on the acidic test solution. The coulometric titrator uses a single platinum wire as the positive electrode, a 3 mol / L sulfuric acid solution in the glass tube, and a double platinum sheet as the negative electrode. The measuring electrode uses a single platinum sheet as the positive electrode, a black tungsten wire as the negative electrode, and a saturated potassium sulfate solution in the glass tube. The coulometric titration electrolysis current is 50 mA, the moment when the potential suddenly drops is the titration endpoint, and the electrolysis time is 604 s; wherein, the electrolysis time is performed in 6 groups of parallel tests according to steps (1) to (3), and the average value of the electrolysis time required for each group to reach the titration endpoint is read;

[0098] (4) The concentration of divalent iron in the Fe / Cr flow battery electrolyte is calculated to be: 1.15 mol / L.

[0099] Example 2

[0100] Determination of the concentration of ferrous iron in the electrolyte of a Fe / Cr flow battery during charging

[0101] (1) After the Fe / Cr flow battery is charged for 0.5 hours, 2.5 mL of the positive and negative electrolytes are transferred into a conical flask, and several glass balls are added and mixed to obtain a mixed electrolyte;

[0102] (2) adding 50 mL of 3 mol / L sulfuric acid to the mixed electrolyte obtained in step (1), and then adding 10 mL of 0.10136 mol / L potassium dichromate solution (excess), and mixing thoroughly to obtain an acidic test solution;

[0103] (3) Use a coulometric titrator to perform a DC constant current titration on the acidic test solution. The coulometric titrator uses a single platinum wire as the positive electrode, a 3 mol / L sulfuric acid solution in the glass tube, and a double platinum sheet as the negative electrode. The measuring electrode uses a single platinum sheet as the positive electrode, a black tungsten wire as the negative electrode, and a saturated potassium sulfate solution in the glass tube. The coulometric titration electrolysis current is 50 mA, the moment when the potential suddenly drops is the titration endpoint, and the electrolysis time is 750 s; wherein, the electrolysis time is performed in 6 groups of parallel tests according to steps (1) to (3), and the average value of the electrolysis time required for each group to reach the titration endpoint is read;

[0104] (4) The concentration of divalent iron in the Fe / Cr flow battery electrolyte is calculated to be: 1.14 mol / L.

[0105] Results and Conclusions:

[0106] By comparing Examples 1 to 2, it can be seen that the method for determining the concentration of divalent iron in the electrolyte of a Fe / Cr flow battery according to the above embodiment of the present invention is applicable to Fe / Cr flow batteries under different charging states, and the measurement results are accurate, with small errors and high precision. In addition, the concentration of divalent iron measured in Example 2 is slightly lower than the measured value in Example 1, indicating that side reactions may have occurred during the charging process, resulting in the consumption of divalent iron.

[0107] In the present invention, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly defined. In addition, unless otherwise clearly stipulated and defined, the term "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral whole; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0108] In the description of this specification, the descriptions referring to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0109] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for determining the concentration of divalent iron in the electrolyte of an Fe / Cr flow battery, characterized in that, Comprising: (1) Mixing the positive electrolyte and the negative electrolyte in equal volumes to obtain a mixed electrolyte; (2) Mixing the mixed electrolyte with an acid and an excessive amount of soluble dichromate so that divalent iron in the mixed electrolyte is fully oxidized to trivalent iron to obtain an acidic test solution; (3) Introducing a pair of electrolysis electrodes and a pair of measurement electrodes into the acidic test solution, performing direct current constant current electrolysis on the acidic test solution through the electrolysis electrodes, reducing trivalent iron in the acidic test solution, and simultaneously detecting the potential between the measurement electrodes using a potentiometer and recording the electrolysis duration required for the potential to drop suddenly; (4) Obtaining the concentration of divalent iron in the Fe / Cr flow battery electrolyte according to the addition amount of the soluble dichromate, the current magnitude of the constant current electrolysis, and the electrolysis duration.

2. The method according to claim 1, wherein Steps (1) to (4) are carried out under an inert atmosphere.

3. The method according to claim 1 or 2, characterized in that, Step (1) further includes: stirring the mixed electrolyte.

4. The method according to claim 1 or 2, characterized in that, Step (2) further includes: (2-1) Mixing the mixed electrolyte with the acid to obtain an acidic electrolyte; (2-2) Mixing the acidic electrolyte with the excessive amount of soluble dichromate to obtain the acidic test solution.

5. The method according to claim 1 or 2, characterized in that, In step (2), the molar ratio of the total iron in the mixed electrolyte to the dichromate radical in the soluble dichromate is 6:(1-1.1), and the total iron includes divalent iron and trivalent iron.

6. The method according to claim 1 or 2, characterized in that, In step (2), the concentration of hydrogen ions in the acidic test solution is 2-6 mol / L.

7. The method according to claim 1 or 2, characterized in that, In step (2), the soluble dichromate is at least one selected from potassium dichromate, sodium dichromate, ammonium dichromate, and silver dichromate.

8. The method according to claim 1 or 2, characterized in that, In step (2), the acid is selected from sulfuric acid and / or hydrochloric acid.

9. The method according to claim 1, characterized in that, In step (3), a coulometric titration device is used to perform the constant current electrolysis and the detection on the test solution.

10. The method according to claim 9, wherein The positive electrode and the negative electrode of the electrolysis / measurement electrode are separated by a diaphragm sleeve or are respectively placed in the same container and are in contact with the test solution through a salt bridge.

11. The method according to claim 10, wherein The positive electrode of the electrolysis electrode uses a single platinum wire, and the single platinum wire is placed in a glass tube containing an acid. The negative electrode of the electrolysis electrode uses a double platinum plate; the positive electrode of the measurement electrode uses a single platinum plate, and the negative electrode of the measurement electrode uses a black tungsten wire, and the black tungsten wire is placed in a glass tube containing a salt solution; The acid in the glass tube is the same as the acid used in step (2), and the difference in the concentration of hydrogen ions in the glass tube and the concentration of hydrogen ions in the acidic test solution is not more than 10%.

12. The method according to claim 1, wherein In step (4), the concentration of divalent iron in the Fe / Cr flow battery electrolyte is: C = 3×(M - It / 6F) / V, where C is the concentration of divalent iron in the Fe / Cr flow battery electrolyte, with the unit of mol / L; M is the amount of dichromate radical added to the mixed electrolyte, with the unit of mol; I is the current of the constant current electrolysis, with the unit of A; t is the electrolysis duration required for the potential to drop suddenly, with the unit of s; F is the Faraday constant, with the unit of C / mol; V is the volume of the positive electrolyte or the negative electrolyte, with the unit of L. The value of F is 96485 - 96500 C / mol.

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