A method of recovery of electrolyte for a zinc-bromine flow battery

Abstract

This invention discloses a method for restoring the electrolyte in a zinc-bromine flow battery, belonging to the field of flow batteries. After debromination of the zinc-bromine flow battery electrolyte, a carbonate additive is added. This additive is stable in the zinc-bromine battery electrolyte, does not participate in the charge / discharge reaction during battery charging and discharging, regulates the electrolyte pH, and does not introduce new impurities into the electrolyte. After long-term battery operation, to ensure stable battery operation, the electrolyte needs to be debrominated periodically. When the battery is in standby mode, the large amount of elemental zinc deposited at the negative electrode reacts with hydrogen ions in the electrolyte, thereby reducing capacity and affecting the battery's coulombic efficiency. When the electrolyte pH drops significantly, a small amount of additive is added to the solution. The carbonate hydrolysis consumes the hydrogen ions in the electrolyte, thereby increasing the electrolyte pH value, reducing the consumption of elemental zinc at the negative electrode during standby, and improving the battery's standby performance.

Description

Technical Field

[0001] This invention relates to a method for restoring electrolyte in a zinc-bromine flow battery, belonging to the field of flow batteries. Background Technology

[0002] Zinc-bromine flow batteries are a new type of low-cost, high-efficiency, and environmentally friendly flow battery. They have advantages such as high energy density and current efficiency, simple and easy-to-operate devices, long service life, and low cost. They are mainly used in grid peak shaving, renewable energy power generation such as wind and solar power, and electric vehicles.

[0003] For zinc-based flow batteries, the poor stability of the zinc anode has always been a major factor restricting the development of this type of battery. To ensure stable battery operation, the electrolyte needs to be restored periodically to remove excess bromine. This process causes the electrolyte pH to drop. During battery standby, the zinc generated at the anode will react with protons in the electrolyte, resulting in capacity loss. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for restoring the electrolyte in a zinc-bromine flow battery. Utilizing the principle that carbonate and bicarbonate hydrolysis produces alkalinity, after bromine removal from the zinc-bromine flow battery electrolyte, one or more aqueous solutions of carbonate and bicarbonate are added to the electrolyte. The pH of the electrolyte is adjusted and increased by utilizing the alkalinity principle of carbonate and bicarbonate hydrolysis. During battery standby, this reduces corrosion of the zinc element at the negative electrode, improves the battery's coulombic efficiency, and enhances battery performance.

[0005] This invention combines the characteristics of zinc-bromine flow batteries. After debromination by inter-mixed electrolysis, one or more aqueous solutions of carbonates and bicarbonates are added to the electrolyte to adjust the electrolyte pH to near neutral, thereby reducing the corrosion of the negative electrode during standby power generation.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for restoring the electrolyte of a zinc-bromine flow battery involves removing bromine from the electrolyte when the coulombic efficiency of the zinc-bromine flow battery decreases. After bromine removal, one or more aqueous solutions of carbonate additives are added to the electrolyte to raise the pH value of the electrolyte to 6-7, thus completing the restoration of the zinc-bromine flow battery electrolyte.

[0008] Furthermore, the positive electrolyte chamber and the negative electrolyte chamber of the zinc-bromine flow battery are connected by a pipeline, and a valve is provided on the pipeline. The valve is closed when the battery is running. The method includes the following steps:

[0009] 1) After the battery has been running for a period of time, discharge it completely;

[0010] 2) Open the valve to mix the positive and negative electrolytes of the battery. Add a reducing agent to the positive and / or negative electrolyte storage tank to remove the excess bromine in the electrolyte.

[0011] 3) When the electrolyte becomes colorless, the pH of the electrolyte will drop sharply. Add a certain amount of carbonate additives to the positive and / or negative electrode electrolyte storage tanks until the pH of the electrolyte is 6-7.

[0012] 4) Close the valve and start the battery charging and discharging operation.

[0013] Furthermore, the positive and negative electrolytes of the zinc-bromine single-flow battery are both neutral aqueous solutions containing zinc ions. The zinc and bromine raw materials are zinc bromide. The zinc ion concentrations in the positive and negative electrolytes are the same, and the supporting electrolyte KCl concentrations are the same. The zinc ion concentration in the electrolyte is 2-4 mol / L, preferably 2 mol / L, and the KCl concentration is 2-5 mol / L, preferably 3 mol / L.

[0014] Furthermore, in the zinc-bromine flow battery, the electrolyte flows with the circulation pump, and an ion exchange membrane is provided between the positive and negative electrodes. The negative electrode chamber includes an electrolyte inlet and outlet, and the negative electrode electrolyte inlet and / or outlet are connected to the electrolyte storage tank via the circulation pump and pipeline. The positive electrode chamber includes an electrolyte inlet and outlet, and the positive electrode electrolyte inlet and / or outlet are connected to the electrolyte storage tank via the circulation pump and pipeline.

[0015] Furthermore, the time period mentioned in step 1) is when the battery coulombic efficiency drops by more than 5%, at which point it can be determined that the battery needs to be restored to its performance.

[0016] Furthermore, the reducing agent includes one or more of formic acid and hydrazine hydrochloride, with a concentration of 0.5 mol / L to 2 mol / L, preferably 0.8-1.2 mol / L, and the amount added is 0.5-3% of the total volume of the electrolyte.

[0017] Furthermore, the carbonate additive includes one or a mixture of two of potassium carbonate, potassium bicarbonate, sodium bicarbonate, or sodium carbonate, with a molar concentration of 0.005 mol / L to 0.01 mol / L for the single solution or the mixture, preferably 0.007 mol / L.

[0018] Further, the carbonate additive has a molar concentration of 0.005 mol / L to 0.008 mol / L of potassium carbonate or a molar concentration of 0.008 mol / L to 0.01 mol / L of potassium bicarbonate solution.

[0019] The carbonate additives described herein are stable in zinc-bromine battery electrolytes, do not participate in charge-discharge reactions during battery charging and discharging, regulate electrolyte pH, and do not introduce new impurities into the electrolyte. After long-term battery operation, to ensure stable operation, the electrolyte needs periodic debromination (bromine accumulates significantly after a period of operation). A common method for restoration is to add a reducing agent (formic acid, hydrazine hydrochloride, etc.) to the electrolyte. This generates a large number of hydrogen ions, causing the solution pH to decrease. When the battery is in standby mode, the large amount of zinc deposited at the negative electrode reacts with hydrogen ions in the electrolyte, reducing capacity and affecting the battery's coulombic efficiency. When the electrolyte pH drops significantly, adding a small amount of carbonate or bicarbonate to the solution utilizes carbonate hydrolysis to consume hydrogen ions in the electrolyte, thereby increasing the electrolyte pH and reducing the consumption of zinc at the negative electrode during standby, thus improving the battery's standby performance.

[0020] The beneficial effects of this invention are:

[0021] This invention addresses the zinc corrosion problem at the negative electrode in zinc-bromine flow batteries during standby power generation. After debromination of the electrolyte, one or more aqueous solutions of carbonates and bicarbonates are added. The pH of the electrolyte is adjusted by utilizing the principle of alkalinity resulting from the hydrolysis of carbonate and bicarbonate ions, thus increasing the electrolyte pH value. This reduces zinc corrosion at the negative electrode during standby power generation, improves coulombic efficiency, and enhances battery performance.

[0022] During the battery's standby period after charging, the electrolyte circulation pump stops operating, leaving a small amount of electrolyte residue on the negative electrode. During each debromination and recovery process, the added reducing agent reacts with the bromine in a redox reaction, generating hydrogen ions and lowering the electrolyte pH. During standby, the acidic electrolyte corrodes the zinc on the negative electrode, reducing the stack capacity and affecting its performance. Because carbonates and bicarbonates are weakly alkaline in aqueous solution, adding a certain amount of carbonate or bicarbonate ions to the electrolyte after debromination utilizes their weakly alkaline hydrolysis properties to adjust the electrolyte pH to near neutral, reducing corrosion of the zinc on the negative electrode. This improves the stack capacity retention, coulombic efficiency, and overall standby performance. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the zinc-bromine redox flow battery structure described in this invention.

[0024] In the diagram, 1. Battery; 2. Positive electrolyte storage tank; 3. Negative electrolyte storage tank; 4. Positive circulation pump; 5. Negative circulation pump; 6. Positive electrolyte storage tank outlet; 7. Positive electrolyte storage tank inlet; 8. Negative electrolyte storage tank outlet; 9. Negative electrolyte storage tank inlet; 10. Positive electrolyte inlet; 11. Positive electrolyte outlet; 12. Negative electrolyte inlet; 13. Negative electrolyte outlet; 14. Valve A; 15. Valve B. Detailed Implementation

[0025] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0026] Example 1

[0027] like Figure 1 A zinc-bromine flow battery includes a battery 1, a positive electrolyte storage tank 2, and a negative electrolyte storage tank 3. The battery 1 includes a negative electrode chamber containing the negative electrode and a positive electrode chamber containing the positive electrode. The negative electrode chamber containing the negative electrode includes a negative electrolyte inlet 12 and a negative electrolyte outlet 13. The negative electrolyte inlet 12 is connected to the negative electrolyte storage tank outlet 8 on the negative electrolyte storage tank 3 through a negative electrode inlet pipe. The negative electrolyte outlet 13 is connected to the negative electrolyte storage tank inlet 9 on the negative electrolyte storage tank 3 through a negative electrode outlet pipe. A negative electrode circulation pump 5 is provided on the negative electrode inlet pipe or the negative electrode outlet pipe. The electrolyte in the negative electrolyte storage tank circulates between the negative electrode chamber containing the negative electrode and the negative electrolyte storage tank through the negative electrode circulation pump 5.

[0028] The positive electrode chamber includes a positive electrode electrolyte inlet 10 and a positive electrode electrolyte outlet 11. The positive electrode electrolyte inlet 10 is connected to the positive electrode electrolyte storage tank outlet 6 on the positive electrode electrolyte storage tank 2 through a positive electrode inlet pipe. The positive electrode electrolyte outlet 11 is connected to the positive electrode electrolyte storage tank inlet 7 on the positive electrode electrolyte storage tank 2 through a positive electrode outlet pipe. A positive electrode circulation pump 4 is provided on the positive electrode inlet pipe or the positive electrode outlet pipe. The electrolyte in the positive electrode electrolyte storage tank circulates between the positive electrode chamber and the positive electrode electrolyte storage tank through the positive electrode circulation pump 4.

[0029] A pipeline is provided between the positive electrolyte outlet 11 and the negative electrolyte outlet 13, and a valve B15 is provided on the pipeline; a pipeline is provided between the positive electrolyte inlet 10 and the negative electrolyte inlet 12, and a valve A14 is provided on the pipeline.

[0030] Example 2

[0031] The electrolyte for both positive and negative electrodes is 2 mol / L ZnBr2 + 3 mol / L KCl + 0.8 MME P, 40 ml. A single cell consists of a positive electrode plate and a 6x6 cm positive electrode. 2Graphite plate, positive electrode frame, carbon felt, diaphragm, carbon felt, negative electrode frame, negative electrode 6x6cm 2 Graphite plate, negative electrode plate. After 100 battery cycles, the positive and negative electrolytes are mixed, and a 1 mol / L formic acid solution is added as a reducing agent to remove excess bromine. Once the electrolyte is free of bromine, a 0.007 mol / L potassium carbonate solution is added to adjust the electrolyte pH to 6.5. The battery is then operated normally, fully charged, and left to stand for 24 hours before discharging. The charge / discharge current density is 40 mA / cm². 2 The maximum charging capacity is 100mAh / cm². 2 Battery performance is shown in Table 1.

[0032] Table 1

[0033] <![CDATA[Battery charging surface capacity mAh / cm 2 > Shelving time (h) CE / % VE / % EE / % 100 24 80 86 69

[0034] Example 3

[0035] The electrolyte for the positive and negative electrodes is 2 mol / L ZnBr2 + 3 mol / L KCl + 0.8 MMEP, 40 ml. The single cell consists of a positive electrode plate and a 6x6 cm positive electrode. 2 Graphite plate, positive electrode frame, carbon felt, diaphragm, carbon felt, negative electrode frame, negative electrode 6x6cm 2 Graphite plate, negative electrode plate. After 100 battery cycles, the positive and negative electrolytes are mixed, and a 1 mol / L formic acid solution is added as a reducing agent to remove excess bromine. Once the electrolyte is free of bromine, a 0.01 mol / L potassium bicarbonate solution is added to adjust the electrolyte pH to 6.5. The battery is then operated normally, fully charged, and left to stand for 24 hours before discharging. The charge / discharge current density is 40 mA / cm². 2 The maximum charging capacity is 100mAh / cm². 2 Battery performance is shown in Table 2.

[0036] Table 2

[0037] <![CDATA[Battery charging surface capacity mAh / cm 2 > Shelving time (h) CE / % VE / % EE / % 100 24 83 85 71

[0038] Data from Tables 1 and 2 show that adjusting the electrolyte pH to near neutral significantly slows down the corrosion rate of zinc in the battery's negative electrode. During a 24-hour standby period, the discharge coulombic efficiency still exceeds 80%. This method effectively suppresses negative electrode capacity loss and improves the battery's standby capability.

[0039] Comparative Example 1

[0040] The electrolyte for the positive and negative electrodes is 2 mol / L ZnBr2 + 3 mol / L KCl + 0.8 MMEP, 40 ml. The single cell consists of a positive electrode plate and a 6x6 cm positive electrode. 2 Graphite plate, positive electrode frame, carbon felt, diaphragm, carbon felt, negative electrode frame, negative electrode 6x6cm 2 Graphite plate, negative electrode plate. After 100 battery cycles, the positive and negative electrolytes are mixed, and a 1 mol / L formic acid solution is added as a reducing agent to remove excess bromine. Once the electrolyte is free of bromine, no further action is taken. The battery is then operated normally, fully charged, and left to stand for 24 hours before discharging. The charge / discharge current density is 40 mA / cm². 2 The maximum charging capacity is 100mAh / cm². 2 Battery performance is shown in Table 3.

[0041] Table 3

[0042] <![CDATA[Battery charging surface capacity mAh / cm 2 > Shelving time (h) CE / % VE / % EE / % 100 24 60 80 48

[0043] As shown in Table 3, the battery experiences significant capacity loss during 24-hour standby, due to the acidic nature of the electrolyte and the reaction of the zinc element generated at the negative electrode with the acid.

[0044] Comparative Example 2

[0045] The electrolyte for both positive and negative electrodes is 2 mol / L ZnBr2 + 3 mol / L KCl + 0.8 MME P, 40 ml. A single cell consists of a positive electrode plate and a 6x6 cm positive electrode. 2 Graphite plate, positive electrode frame, carbon felt, diaphragm, carbon felt, negative electrode frame, negative electrode 6x6cm 2 Graphite plate, negative electrode plate. After 100 battery cycles, the positive and negative electrolytes are mixed, and a 1 mol / L formic acid solution is added as a reducing agent to remove excess bromine. Once the electrolyte is free of bromine, a 0.01 mol / L potassium acetate solution is added to adjust the electrolyte pH to 6.5. The battery is then operated normally, fully charged, and left to stand for 24 hours before discharging. The charge / discharge current density is 40 mA / cm². 2 The maximum charging capacity is 100mAh / cm². 2 Battery performance is shown in Table 1.

[0046] Table 4

[0047] <![CDATA[Battery charging surface capacity mAh / cm 2 > Shelving time (h) CE / % VE / % EE / % 100 24 81 65 53

[0048] As shown in Table 4 regarding battery performance, although the coulombic efficiency loss is relatively small during 24-hour standby, the battery voltage efficiency is very low. Unlike carbonate and bicarbonate ions, acetate ions cannot decompose themselves and can only remain in the electrolyte as acetate ions (carbonate and bicarbonate ions can decompose into carbon dioxide and water). Introducing acetate ions into the electrolyte will affect battery performance (voltage efficiency) and will not achieve the technical effect intended by this invention.

Claims

1. A method for restoring electrolyte in a zinc-bromine flow battery, characterized in that: When the coulombic efficiency of the zinc-bromine flow battery decreases, the electrolyte is debrominated. After debromination, one or more aqueous solutions of carbonate additives are added to the electrolyte to raise the pH value of the electrolyte to 6-7, thus completing the recovery of the zinc-bromine flow battery electrolyte. The positive electrolyte chamber and the negative electrolyte chamber of the zinc-bromine flow battery are connected by a pipeline, and a valve is installed on the pipeline. The valve is closed when the battery is running. The method includes the following steps: 1) After the battery has been running for a period of time, discharge it completely; 2) Open the valve to mix the positive and negative electrolytes of the battery. Add a reducing agent to the positive and / or negative electrolyte storage tank to remove the excess bromine in the electrolyte. 3) After the electrolyte becomes colorless, add a certain amount of one or more aqueous solutions of carbonate additives to the positive and / or negative electrode electrolyte storage tank until the pH value of the electrolyte is 6-7. 4) Close the valve and begin battery charging and discharging. The reducing agent includes one or more of formic acid and hydrazine hydrochloride.

2. The method according to claim 1, characterized in that: The positive and negative electrolytes of the zinc-bromine single-flow battery are both neutral aqueous solutions containing zinc ions. The zinc and bromine raw materials are zinc bromide. The zinc ion concentrations in the positive and negative electrolytes are the same, and the supporting electrolyte KCl concentration is the same. The zinc ion concentration in the electrolyte is 2-4 mol / L, and the KCl concentration is 2-5 mol / L.

3. The method according to claim 1, characterized in that, In a zinc-bromine flow battery, the electrolyte flows with a circulating pump. An ion exchange membrane is installed between the positive and negative electrodes. The negative electrode chamber contains the electrolyte inlet and outlet. The negative electrode electrolyte inlet and / or outlet are connected to the electrolyte storage tank via a pipeline through the circulating pump. The positive electrode chamber contains the electrolyte inlet and outlet. The positive electrode electrolyte inlet and / or outlet are connected to the electrolyte storage tank via a pipeline through the circulating pump.

4. The method according to claim 1, characterized in that: Step 1) refers to a period of time when the battery coulombic efficiency drops by more than 5%, at which point it can be determined that the battery needs performance recovery.

5. The method according to claim 1, characterized in that: The reducing agent concentration is 0.5 mol / L to 2 mol / L, and the amount added is 0.5% to 3% of the total volume of the electrolyte.

6. The method according to claim 1, characterized in that: The carbonate additives include one or a mixture of two of potassium carbonate, potassium bicarbonate, and sodium bicarbonate, with a single solution or mixture having a molar concentration of 0.005 mol / L to 0.01 mol / L.

7. The method according to claim 1, characterized in that: The carbonate additive is potassium carbonate with a molar concentration of 0.005 mol / L to 0.008 mol / L or potassium bicarbonate solution with a molar concentration of 0.008 mol / L to 0.01 mol / L.

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