A method and system for in-situ reduction of over-oxidized vanadium electrolyte

Abstract

This invention provides a method and system for in-situ reduction of over-oxidized vanadium electrolyte, relating to the field of vanadium redox flow battery technology. The method includes the following steps: (1) mixing the vanadium electrolyte in the positive electrode storage tank and the negative electrode storage tank to obtain a mixed solution; (2) determining the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixed solution; (3) calculating the transfer charge Q required to convert vanadium ions in the vanadium electrolyte in the negative electrode storage tank into trivalent vanadium ions, and the electrolysis time t, followed by charging and electrolysis; (4) adding a reducing agent to the vanadium electrolyte in the positive electrode storage tank to convert vanadium ions in the vanadium electrolyte in the positive electrode storage tank into tetravalent vanadium ions. The method of this invention achieves capacity recovery of the vanadium electrolyte, and the entire recovery process is simple and efficient, requiring no replacement of the electrolyte and electrodes or disassembly of the battery.

Description

Technical Field

[0001] This invention relates to the field of vanadium redox flow battery technology, and in particular to a method and system for in-situ reduction of excessively oxidized vanadium electrolyte. Background Technology

[0002] In recent years, renewable energy storage and reuse has become a crucial link in achieving carbon peaking and carbon neutrality, and in striving to build a clean, low-carbon, safe, and efficient energy system. Under this premise, low-cost, long-life energy storage of renewable energy is a prerequisite for its integration into traditional power grid systems. Compared to lithium-ion batteries, which have higher energy storage costs and limited power-energy decoupling capabilities, redox flow battery technology is one of the most promising alternatives to existing grid-scale energy storage. It possesses high scalability and energy-power decoupling control characteristics, making it better suited for the storage and reuse of highly volatile and discontinuous renewable energy sources such as wind and solar power.

[0003] Vanadium redox flow batteries (VRBs) have become one of the most promising large-scale energy storage technologies due to their power and energy decoupling characteristics, high safety, and excellent scalability. However, the inevitable performance degradation (including energy efficiency and capacity) of VRBs during long-term cycling significantly reduces the lifespan of VRB systems, thus hindering their commercial development. Generally, the capacity loss during VRB operation is mainly caused by the imbalance and cross-contamination of different vanadium species. During the reaction process, due to factors such as electric field, concentration gradient, pressure difference, and side reactions, the four valence states of vanadium ions (V4, V6, V7) are affected. 2+ V 3+ V 4+ (VO 2+ V 5+ (VO2 + The different transport rates of vanadium ions on the membrane lead to an imbalance in vanadium ion concentration and valence state. In addition, water molecules are also transferred along with protons and vanadium ions in a dragging manner, resulting in a volume imbalance between the negatively charged electrolyte and the positively charged electrolyte.

[0004] To mitigate capacity decay during VRFB cycling tests, several strategies have been developed, including novel low-vanadium permeability ion exchange membranes, hydraulic pressure adjustment, osmotic pressure balancing, and alteration of charge / discharge current. However, capacity loss continues to accumulate with increasing cycle count, making the development of effective capacity recovery methods to extend cycle life a significant concern. In all-vanadium redox flow batteries, V 3.5+ Equivalent to V 3+ and V 4+ (VO 2+ An equimolar mixture of ions can be considered as an electrolyte in equilibrium.

[0005] Currently, the most direct method for restoring capacity after cycling is to remix the electrolytes at the negative and positive electrodes. While this method successfully balances the concentration and volume on both sides, problems such as increased electrolyte valence due to side reactions and air oxidation caused by incomplete battery sealing still exist (C(V)). 3+ ) <C(V 4+ )).

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method and system for in-situ reduction of over-oxidized vanadium electrolyte. The method of this invention solves the problems in the prior art, such as the increase in electrolyte valence state caused by side reactions after remixing the negative and positive electrode electrolytes, and air oxidation caused by incomplete battery sealing. The method of this invention, through appropriate pipeline design, can fully mix the unbalanced positive and negative electrode electrolytes. For electrolytes with an average vanadium valence state greater than 3.5 after mixing, the capacity of the vanadium electrolyte can be restored. Moreover, the entire recovery process is simple and efficient, without the need to replace the electrolyte and electrodes or disassemble the battery.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0009] In a first aspect, the present invention provides a method for in-situ reduction of excessively oxidized vanadium electrolyte, the method comprising the following steps:

[0010] (1) Mix the vanadium electrolyte in the positive electrode storage tank and the negative electrode storage tank to obtain a mixed solution;

[0011] (2) Determine the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture;

[0012] (3) Calculate the amount of charge Q required to convert vanadium ions in the vanadium electrolyte in the negative electrode storage tank into trivalent vanadium ions, and the electrolysis time t after which charging is performed for electrolysis.

[0013] (4) Add a reducing agent to the vanadium electrolyte in the positive electrode storage tank to convert vanadium ions in the vanadium electrolyte in the positive electrode storage tank into tetravalent vanadium ions.

[0014] Preferably, in step (1), a pressure difference of less than 50mV between the vanadium electrolyte in the positive electrode storage tank and the vanadium electrolyte in the negative electrode storage tank indicates uniform mixing.

[0015] Preferably, in step (2), the method for determining the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture is as follows:

[0016] The absorbance of the mixed solution was detected by ultraviolet-visible absorption spectroscopy, and the concentrations of trivalent vanadium ions (C1) and tetravalent vanadium ions (C2) in the mixed solution were determined by combining the standard curves of trivalent vanadium ion electrolyte and tetravalent vanadium ion electrolyte.

[0017] Wherein, the concentration of tetravalent vanadium ions C2 is greater than the concentration of trivalent vanadium ions C1.

[0018] Preferably, in step (3), the transferred charge Q is calculated using the following formula:

[0019] Q = C² × V × F

[0020] Where Q represents the amount of electricity transferred, C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and F represents the Faraday constant.

[0021] Preferably, in step (3), the electrolysis time t is calculated using the following formula:

[0022]

[0023] Where C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank, F represents the Faraday constant, i represents the current density, S represents the effective area, and N represents the number of battery stacks.

[0024] Preferably, in step (3), the current density i = 120 mA / cm² -2 .

[0025] Preferably, in step (3), after the electrolysis is completed, the vanadium electrolyte in the positive electrode storage tank is composed of tetravalent vanadium ions and pentavalent vanadium ions, wherein the concentration of tetravalent vanadium ions is 2C1 and the concentration of pentavalent vanadium ions is C2-C1.

[0026] Preferably, in step (3), after the electrolysis is completed, the vanadium electrolyte in the negative electrode storage tank contains only trivalent vanadium ions, wherein the concentration of trivalent vanadium ions is C2+C1.

[0027] Preferably, in step (4), the amount m of the reducing agent added is calculated by the following formula:

[0028]

[0029] Where m represents the amount of reducing agent added, C1 represents the concentration of trivalent vanadium ions determined in step (2), C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and M represents the molar mass of the reducing agent.

[0030] Preferably, in step (4), the reducing agent includes any one or a combination of at least two of sodium borohydride, oxalic acid, hydrogen, or formic acid, preferably oxalic acid.

[0031] Preferably, in step (4), heating and stirring are required after adding the reducing agent.

[0032] Preferably, the heating and stirring temperature is 45-60°C, and the heating and stirring time is 2-5 hours.

[0033] Preferably, step (4) is followed by step (5):

[0034] Charge and discharge the battery to increase and restore its capacity; or mix the vanadium electrolyte in the positive electrode reservoir and the negative electrode reservoir again to obtain a vanadium electrolyte with an average valence state of 3.5.

[0035] The charging and discharging procedure is as follows:

[0036] First at 120mA·cm -2 Charge the battery at a given current density, with the cutoff voltage set to 1.5V×N or 1.55V×N; then charge at a current density of 120mA·cm⁻¹. -2 The battery is discharged, and the cutoff voltage is set to 0.9V×N; where N is the number of single cell groups contained in the battery stack.

[0037] In a second aspect, the present invention provides a system for in-situ reduction of excessively oxidized vanadium electrolyte, the system comprising: an electrode stack, a positive electrode storage tank, and a negative electrode storage tank;

[0038] The positive electrode storage tank and the negative electrode storage tank are connected by a pipeline, which includes a positive electrode electrolyte outlet section pipeline, a mixing section pipeline and a negative electrode electrolyte outlet section pipeline;

[0039] The positive electrode electrolyte outlet section pipeline is connected to the positive electrode storage tank and is equipped with a first valve; the negative electrode electrolyte outlet section pipeline is connected to the negative electrode storage tank and is equipped with a fourth valve; and a fifth valve is installed in the middle of the mixing section pipeline.

[0040] The mixing section pipeline is respectively provided with a first branch and a second branch connected to the fuel cell stack;

[0041] The first branch is located between the first valve and the fifth valve, and the inlet of the first branch is connected to the mixing section pipeline, and the outlet of the first branch is connected to the fuel cell stack; and a positive electrode pump and a second valve are provided on the first branch.

[0042] The second branch is located between the fourth valve and the fifth valve, and the inlet of the second branch is connected to the mixing section pipeline, and the outlet of the second branch is connected to the fuel cell stack; and a negative electrode pump and a third valve are provided on the second branch.

[0043] The positive electrode storage tank and the fuel cell stack are directly connected via a first return liquid pipeline, and the negative electrode storage tank and the fuel cell stack are directly connected via a second return liquid pipeline.

[0044] Preferably, the fuel cell stack includes at least one set of fuel cell stacks.

[0045] Preferably, the battery stack includes end plates, electrodes, and ion exchange membranes.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] (1) The present invention, through appropriate pipeline design, fully mixes the unbalanced positive and negative electrode electrolytes, targeting electrolytes with an average vanadium valence greater than 3.5 after mixing;

[0048] (2) The present invention determines the concentration distribution of trivalent vanadium and tetravalent vanadium in the mixed vanadium electrolyte by ultraviolet absorption spectroscopy, thereby calculating the average valence state of the vanadium electrolyte.

[0049] (3) To restore capacity, this invention first divides the mixed electrolyte into two equal parts to balance concentration and volume. Then, based on the average valence state of vanadium calculated from the ultraviolet absorption spectrum, the amount of oxalic acid required as a reducing agent for the positive electrode electrolyte is calculated. Considering that the reaction rate of oxalic acid with tetravalent vanadium is slow, while the reaction rate with pentavalent vanadium is fast, this method first charges the battery and then adds oxalic acid as a reducing agent to the positive electrode to carry out the reduction reaction, thereby restoring the capacity of the vanadium electrolyte.

[0050] (4) The entire recovery process of the in-situ reduction of over-oxidized vanadium electrolyte described in this invention is simple and efficient, and does not require replacement of electrolyte and electrodes or disassembly of battery. Attached Figure Description

[0051] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0052] Figure 1 This is a flowchart of the in-situ reduction method for excessively oxidized vanadium electrolyte according to the present invention.

[0053] Figure 2 This is a schematic diagram of the structure of the in-situ reduction of excessive oxidation of vanadium electrolyte system described in this invention.

[0054] Among them, 10 is the fuel cell stack, 11 is the end plate, 12 is the electrode, 13 is the ion exchange membrane, 20 is the positive electrode storage tank, 30 is the negative electrode storage tank, 101 is the positive electrode electrolyte outlet section pipeline, 102 is the mixing section pipeline, 103 is the negative electrode electrolyte outlet section pipeline, 104 is the first branch, 105 is the second branch, 106 is the first return pipeline, 107 is the second return pipeline, 1 is the first valve, 2 is the second valve, 3 is the third valve, 4 is the fourth valve, 5 is the fifth valve, 6 is the positive electrode pump, and 7 is the negative electrode pump.

[0055] Figure 3 This is a standard curve showing the ultraviolet absorption versus concentration of trivalent and tetravalent vanadium ions.

[0056] Figure 4 The charge-discharge curves before and after recycling are shown in Example 1. Detailed Implementation

[0057] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0058] It should be noted that specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many ways other than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0059] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] Firstly, such as Figure 1 As shown, the present invention provides a method for in-situ reduction of excessively oxidized vanadium electrolyte, the method comprising the following steps:

[0061] (1) Mix the vanadium electrolyte in the positive electrode storage tank and the negative electrode storage tank to obtain a mixed solution;

[0062] (2) Determine the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture;

[0063] (3) Calculate the amount of charge Q required to convert vanadium ions in the vanadium electrolyte in the negative electrode storage tank into trivalent vanadium ions, and the electrolysis time t after which charging is performed for electrolysis.

[0064] (4) Add a reducing agent to the vanadium electrolyte in the positive electrode storage tank to convert vanadium ions in the vanadium electrolyte in the positive electrode storage tank into tetravalent vanadium ions.

[0065] In this invention, the method for in-situ reduction of over-oxidized vanadium electrolyte can thoroughly mix imbalanced positive and negative electrode electrolytes, specifically for electrolytes where the average vanadium valence state after mixing is greater than 3.5. This method uses ultraviolet absorption spectroscopy to determine the concentration distribution of trivalent and tetravalent vanadium in the mixed vanadium electrolyte, thereby calculating the average valence state of the vanadium electrolyte. To restore capacity, the mixed electrolyte is first divided into two equal parts to balance concentration and volume. Then, based on the average vanadium valence state calculated from the ultraviolet absorption spectrum, the amount of oxalic acid required as a reducing agent for the positive electrode electrolyte is calculated. Considering that the reaction rate of oxalic acid with tetravalent vanadium is slow, while its reaction rate with pentavalent vanadium is fast, this method first charges the battery, then adds oxalic acid as a reducing agent to the positive electrode, and reacts at 45°C for a sufficiently long time to achieve capacity recovery of the vanadium electrolyte. The entire recovery process is simple and efficient, requiring no replacement of the electrolyte and electrodes or disassembly of the battery.

[0066] As an optional embodiment of the present invention, in step (1), the pressure difference between the vanadium electrolyte in the positive electrode storage tank and the vanadium electrolyte in the negative electrode storage tank is less than 50mV to represent uniform mixing. For example, it can be 50mV, 49mV, 48mV, 47mV, 46mV, 45mV, 40mV, 35mV, 30mV, 20mV, 10mV, etc.

[0067] As an optional embodiment of the present invention, in step (1), by means of appropriate piping design (such as...) Figure 2 (As shown) to ensure thorough mixing of the unbalanced positive and negative electrolytes, more specifically, the mixing includes the following steps:

[0068] (a) With all valves closed, open valve 1, valve 3 and valve 5, close valve 2 and valve 4, and simultaneously turn on the negative electrode pump 7 to pump the vanadium electrolyte in the positive electrode storage tank 20 into the negative electrode storage tank 30. After the electrolyte has been completely pumped in, first close the negative electrode pump 7, valve 1 and valve 5, keep valve 3 open, and then open valve 4 before turning on the negative electrode pump 7 again to allow the electrolyte in the negative electrode storage tank 30 to undergo its first circulation in the fuel cell stack 10.

[0069] (b) Close the negative electrode pump 7 and all valves, open the second valve 2, the fourth valve 4 and the fifth valve 5, and simultaneously turn on the positive electrode pump 6 to pump the vanadium electrolyte in the negative electrode storage tank 30 into the positive electrode storage tank 20; after it has been completely pumped in, first close the positive electrode pump 6 and the fifth valve 5, keep the first valve 1 and the second valve 2 in the open state, and then turn on the positive electrode pump 6 to make the mixed electrolyte pumped into the positive electrode storage tank 20 undergo a second circulation in the stack 10.

[0070] (c) Repeat steps (a) and (b);

[0071] (d) Keep the positive electrode pump 6 and the negative electrode pump 7 in the closed state, and open the first valve 1, the second valve 2, the third valve 3, the fourth valve 4 and the fifth valve 5 to keep the volume of vanadium electrolyte in the positive electrode storage tank 20 consistent with the volume of vanadium electrolyte in the negative electrode storage tank 30. The volume of electrolyte on both sides is recorded as V.

[0072] As an optional embodiment of the present invention, in step (a), the time of the first cycle is 10 to 30 minutes, for example, it can be 10 minutes, 12 minutes, 14 minutes, 16 minutes, 18 minutes, 20 minutes, 22 minutes, 24 minutes, 26 minutes, 28 minutes, 30 minutes, etc.

[0073] As an optional embodiment of the present invention, in step (b), the time of the second cycle is 10 to 30 minutes, for example, it can be 10 minutes, 12 minutes, 14 minutes, 16 minutes, 18 minutes, 20 minutes, 22 minutes, 24 minutes, 26 minutes, 28 minutes, 30 minutes, etc.

[0074] As an optional embodiment of the present invention, in step (c), the number of repetitions is 1 to 10 times, for example, 1 time, 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, 10 times, etc., preferably 3 to 50 times.

[0075] As an optional embodiment of the present invention, step (d) further includes the following detection step to ensure that the electrolyte is mixed uniformly: turn on the positive electrode pump 6 and the negative electrode pump 7, measure the battery voltage, and when the battery voltage is below 50mV, it indicates that the mixture is uniformly mixed.

[0076] As an optional embodiment of the present invention, in step (2), the method for determining the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture is as follows:

[0077] The absorbance of the mixed solution was detected by ultraviolet-visible absorption spectroscopy, and the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixed solution were determined by combining the standard curves of trivalent vanadium ion electrolyte and tetravalent vanadium ion electrolyte.

[0078] As an optional embodiment of the present invention, in step (2), the concentration of tetravalent vanadium ions C2 is greater than the concentration of trivalent vanadium ions C1.

[0079] As an optional embodiment of the present invention, in step (3), the transferred charge Q is calculated by the following formula:

[0080] Q = C² × V × F

[0081] Where Q represents the amount of electricity transferred, C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and F represents the Faraday constant.

[0082] As an optional embodiment of the present invention, in step (3), the electrolysis time t is calculated by the following formula:

[0083]

[0084] Where C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank, F represents the Faraday constant, i represents the current density, S represents the effective area, and N represents the number of battery stacks.

[0085] As an optional embodiment of the present invention, in step (3), the current density i = 120 mA / cm² -2 .

[0086] As an optional embodiment of the present invention, in step (3), after the electrolysis is completed, the vanadium electrolyte in the positive electrode storage tank is composed of tetravalent vanadium ions and pentavalent vanadium ions; wherein, the concentration of tetravalent vanadium ions is 2C1 and the concentration of pentavalent vanadium ions is C2-C1.

[0087] As an optional embodiment of the present invention, in step (3), after the electrolysis is completed, the vanadium electrolyte in the negative electrode storage tank contains only trivalent vanadium ions; wherein the concentration of trivalent vanadium ions is C2+C1.

[0088] As an optional embodiment of the present invention, in step (4), the amount m of the reducing agent added is calculated by the following formula:

[0089]

[0090] Where m represents the amount of reducing agent added, C1 represents the concentration of trivalent vanadium ions determined in step (2), C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and M represents the molar mass of the reducing agent.

[0091] As an optional embodiment of the present invention, in step (4), the reducing agent includes any one or a combination of at least two of sodium borohydride, oxalic acid, hydrogen or formic acid, preferably oxalic acid.

[0092] As an optional embodiment of the present invention, in step (4), heating and stirring are required after adding the reducing agent.

[0093] As an optional embodiment of the present invention, in step (4), the heating and stirring temperature is 45 to 60, for example, it can be 45, 46, 48, 50, 52, 54, 56, 58, 60, etc.

[0094] As an optional embodiment of the present invention, in step (4), the heating and stirring time is 2 to 5 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, etc.

[0095] As an optional embodiment of the present invention, step (5) is further included after step (4):

[0096] Charge and discharge the battery to increase and restore its capacity.

[0097] Alternatively, the vanadium electrolyte in the positive electrode storage tank and the negative electrode storage tank can be mixed again to obtain a vanadium electrolyte with an average valence state of 3.5.

[0098] As an optional embodiment of the present invention, the charging and discharging procedure is as follows:

[0099] First at 120mA·cm -2 Charge the battery at a given current density, with the cutoff voltage set to 1.5V×N or 1.55V×N; then charge at a current density of 120mA·cm⁻¹. -2 The battery is discharged, and the cutoff voltage is set to 0.9V×N; where N is the number of single cell groups contained in the battery stack.

[0100] Secondly, the present invention provides a system for in-situ reduction of excessively oxidized vanadium electrolyte, such as... Figure 2 As shown, the system includes: a fuel cell stack 10, a positive electrode storage tank 20, and a negative electrode storage tank 30.

[0101] The positive electrode storage tank 20 and the negative electrode storage tank 30 are connected by a pipeline, which includes a positive electrode electrolyte outlet section pipeline 101, a mixing section pipeline 102, and a negative electrode electrolyte outlet section pipeline 103.

[0102] The positive electrode electrolyte outlet section pipeline 101 is connected to the positive electrode storage tank 20 and is equipped with a first valve 1; the negative electrode electrolyte outlet section pipeline 103 is connected to the negative electrode storage tank 30 and is equipped with a fourth valve 4; and a fifth valve 5 is provided in the middle of the mixing section pipeline 102.

[0103] The mixing section pipeline 102 is provided with a first branch 104 and a second branch 105 that are connected to the fuel cell stack 10.

[0104] The first branch 104 is located between the first valve 1 and the fifth valve 5, and the inlet end of the first branch 104 is connected to the mixing section pipeline 102, and the outlet end of the first branch 104 is connected to the fuel cell stack 10; and a positive electrode pump 6 and a second valve 2 are provided on the first branch 104.

[0105] The second branch 105 is located between the fourth valve 4 and the fifth valve 5, and the inlet end of the second branch 105 is connected to the mixing section pipeline 102, and the outlet end of the second branch 105 is connected to the fuel cell stack 10; and the second branch 105 is equipped with a negative electrode pump 7 and a third valve 3.

[0106] The positive electrode storage tank 20 and the fuel cell stack 10 are directly connected through the first return liquid pipeline 106.

[0107] The negative electrode storage tank 30 and the fuel cell stack 10 are directly connected via a second return pipeline 107.

[0108] As an optional embodiment of the present invention, the fuel cell stack includes at least one set of fuel cell stacks.

[0109] As an optional embodiment of the present invention, the battery stack includes end plates, electrodes, and ion exchange membranes.

[0110] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0111] Example 1

[0112] This embodiment provides a method for in-situ reduction of excessively oxidized vanadium electrolyte, the method comprising the following steps:

[0113] S1, Mixing of positive and negative electrolytes:

[0114] Open valve 1, valve 3, and valve 5, and close the other valves. At the same time, turn on the negative electrode pump to pump the vanadium electrolyte in the positive electrode storage tank into the negative electrode storage tank. After the electrolyte has been completely pumped in, turn off the negative electrode pump and valves 1 and 5, keep valves 3 and 4 open, and turn on the negative electrode pump to circulate the negative electrode electrolyte in the stack for 20 minutes.

[0115] Turn off the negative electrode pump, keep the second valve 2, the fourth valve 4, and the fifth valve 5 open, and the first valve 1 closed. Turn on the positive electrode pump to allow the electrolyte to completely enter the positive electrode storage tank. Then turn off the positive electrode pump and the fifth valve 5, keep the first valve 1 and the second valve 2 open, and turn on the positive electrode pump to circulate the mixed electrolyte in the stack for 20 minutes.

[0116] Repeat the above operation 4 times, turn off the pump, open all valves, let stand for 5 minutes, the positive and negative electrode solutions are the same volume, close the three-way valve, turn on the positive and negative electrode pumps, the battery voltage display is 30mV, indicating that the electrolyte has been mixed evenly, at this time the electrolyte volume on both sides is 19.35L.

[0117] S2. Determination of vanadium ion concentration:

[0118] The absorbance of the mixed solution was detected by ultraviolet-visible absorption spectroscopy, and the results were analyzed using standard curves of trivalent vanadium ion electrolyte and tetravalent vanadium ion electrolyte (e.g., ...). Figure 3 As shown), the mixed electrolyte was diluted 100 times to determine the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixed solution: where, V 3+ The concentration was 0.3103 mol / L, V 4+ (VO 2+ The concentration was 1.3520 mol / L.

[0119] S3, Charging Electrolysis:

[0120] The selected batteries have an effective area of ​​15cm × 15cm, with 10 groups, and a current density of 120mA·cm. -2 Based on the above formula for calculating electrolysis time t, the required electrolysis time is calculated as t = 1.3520 mol / L × 19.35 L × 96485 C·s -1 / (120mA·cm -2 ×225cm 2 (×10)=9348.8s.

[0121] S4, Positive Electrolyte Reduction:

[0122] After charging and electrolysis are completed, oxalic acid (m = 1.3417 mol / L / 2 × 19.35 L × 90 g / mol) = 1168.3 g is added to the positive electrode storage tank. The tank is then stirred and heated at 45°C for 4 hours to adjust the concentration of V in the positive electrode storage tank. 5+ (VO2 + Completely transformed into V 4+ (VO 2+ ).

[0123] S5, Vanadium-based electrolyte:

[0124] After mixing the positive and negative electrode electrolytes evenly according to step S1, a full vanadium electrolyte with an average valence state of 3.5 can be obtained.

[0125] Subsequent charge-discharge tests were performed on the all-vanadium electrolyte (the charge-discharge procedure is as follows: first at 120 mA·cm⁻¹). -2 The battery was charged at a current density with a cutoff voltage set to 15.5V; then charged at a current density of 120mA·cm⁻¹. -2 Discharge the battery (with the cutoff voltage set to 9V) at 120 mA cm⁻¹ -2 At current density, the capacity of the recovered electrolyte is significantly increased compared to before recovery (e.g., Figure 4 As shown in the figure, this proves the feasibility of the method.

[0126] This invention utilizes appropriate pipeline design to thoroughly mix imbalanced positive and negative electrode electrolytes. For electrolytes with an average vanadium valence greater than 3.5 after mixing, this method uses ultraviolet absorption spectroscopy to determine the concentration distribution of trivalent and tetravalent vanadium in the mixed vanadium electrolyte, thereby calculating the average valence state of the vanadium electrolyte. To restore capacity, the mixed electrolyte is first divided into two equal parts to balance concentration and volume. Then, based on the average vanadium valence state calculated from the ultraviolet absorption spectrum, the amount of oxalic acid required as a reducing agent for the positive electrode electrolyte is calculated. Considering that the reaction rate of oxalic acid with tetravalent vanadium is slow, while its reaction rate with pentavalent vanadium is fast, this method first charges the battery, then adds oxalic acid as a reducing agent to the positive electrode, and reacts for a sufficient time at 40–60°C to achieve capacity recovery of the vanadium electrolyte. Furthermore, the entire recovery process is simple and efficient, requiring no replacement of electrolytes and electrodes or disassembly of the battery.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for in-situ reduction of excessively oxidized vanadium electrolyte, characterized in that, The method includes the following steps: (1) Mix the vanadium electrolyte in the positive electrode storage tank and the negative electrode storage tank to obtain a mixed solution; (2) Determine the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture; (3) Calculate the amount of charge Q required to convert vanadium ions in the vanadium electrolyte in the negative electrode storage tank into trivalent vanadium ions, and the electrolysis time t after which charging is performed for electrolysis; In step (3), after the electrolysis is completed, the vanadium electrolyte in the positive electrode storage tank is composed of tetravalent vanadium ions and pentavalent vanadium ions, wherein the concentration of tetravalent vanadium ions is 2C1 and the concentration of pentavalent vanadium ions is C2-C1; the vanadium electrolyte in the negative electrode storage tank contains only trivalent vanadium ions, wherein the concentration of trivalent vanadium ions is C2+C1. (4) Add a reducing agent to the vanadium electrolyte in the positive electrode storage tank to convert vanadium ions in the vanadium electrolyte in the positive electrode storage tank into tetravalent vanadium ions; In step (4), the amount m of the reducing agent added is calculated by the following formula: in, C1 represents the amount of reducing agent added, C2 represents the concentration of trivalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and M represents the molar mass of the reducing agent.

2. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (1), a pressure difference of less than 50 mV between the vanadium electrolyte in the positive electrode storage tank and the vanadium electrolyte in the negative electrode storage tank indicates that they are mixed evenly.

3. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (2), the method for determining the concentrations of trivalent vanadium ions C1 and tetravalent vanadium ions C2 in the mixture is as follows: The absorbance of the mixed solution was detected by ultraviolet-visible absorption spectroscopy, and the concentrations of trivalent vanadium ions (C1) and tetravalent vanadium ions (C2) in the mixed solution were determined by combining the standard curves of trivalent vanadium ion electrolyte and tetravalent vanadium ion electrolyte. Wherein, the concentration of tetravalent vanadium ions C2 is greater than the concentration of trivalent vanadium ions C1.

4. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (3), the transferred charge Q is calculated using the following formula: Where Q represents the amount of electricity transferred, C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank in step (2), and F represents the Faraday constant.

5. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (3), the electrolysis time t is calculated using the following formula: Where C2 represents the concentration of tetravalent vanadium ions determined in step (2), V represents the volume of vanadium electrolyte in the positive or negative electrode storage tank, F represents the Faraday constant, i represents the current density, S represents the effective area, and N represents the number of battery stacks. And / or, the current density i = 120 mA cm⁻¹ -2 .

6. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (4), the reducing agent includes any one or a combination of at least two of sodium borohydride, oxalic acid, hydrogen, or formic acid.

7. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 6, characterized in that, In step (4), the reducing agent is oxalic acid.

8. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, In step (4), after adding the reducing agent, heating and stirring are required.

9. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 8, characterized in that, The heating and stirring temperature is 45~60℃, and the heating and stirring time is 2~5 h.

10. The method for in-situ reduction of excessively oxidized vanadium electrolyte according to claim 1, characterized in that, Step (4) is followed by step (5): Charge and discharge the battery to increase and restore its capacity; or mix the vanadium electrolyte in the positive and negative electrode storage tanks again to obtain a vanadium electrolyte with an average valence of 3.

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11. A system for implementing the method of in-situ reduction of over-oxidized vanadium electrolyte according to any one of claims 1 to 10, characterized in that, The system includes: a fuel cell stack, a positive electrode storage tank, and a negative electrode storage tank; The positive electrode storage tank and the negative electrode storage tank are connected by a pipeline, which includes a positive electrode electrolyte outlet section pipeline, a mixing section pipeline and a negative electrode electrolyte outlet section pipeline; The positive electrode electrolyte outlet section pipeline is connected to the positive electrode storage tank and is equipped with a first valve; the negative electrode electrolyte outlet section pipeline is connected to the negative electrode storage tank and is equipped with a fourth valve; and a fifth valve is installed in the middle of the mixing section pipeline. The mixing section pipeline is respectively provided with a first branch and a second branch connected to the fuel cell stack; The first branch is located between the first valve and the fifth valve, and the inlet of the first branch is connected to the mixing section pipeline, and the outlet of the first branch is connected to the fuel cell stack; and a positive electrode pump and a second valve are provided on the first branch. The second branch is located between the fourth valve and the fifth valve, and the inlet of the second branch is connected to the mixing section pipeline, and the outlet of the second branch is connected to the fuel cell stack; and a negative electrode pump and a third valve are provided on the second branch. The positive electrode storage tank and the fuel cell stack are directly connected via a first return liquid pipeline, and the negative electrode storage tank and the fuel cell stack are directly connected via a second return liquid pipeline.

12. The system according to claim 11, characterized in that, The fuel cell stack includes at least one set of fuel cell stacks; And / or, the battery stack includes end plates, electrodes, and ion exchange membranes.

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