A method for recovering electrolyte of alkaline zinc-iron flow battery

Abstract

The present invention relates to the field of flow battery technology, particularly to the field of alkaline zinc-iron flow batteries. A method for online electrolyte recovery in alkaline zinc-iron flow batteries is described, which involves introducing an electrolyte recovery stack into a zinc-iron flow battery stack testing system. When the positive electrode electrolyte needs to be restored, the electric valve at the zinc-iron flow battery stack inlet is closed, and the electric valve for electrolyte recovery stack is opened. This solves the problem of increased stack polarization and capacity attenuation caused by accumulation of positive electrode active material during stack operation, thereby improving the stack's cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of liquid flow batteries, in particular to the field of alkaline zinc-iron liquid flow batteries. Background Art

[0002] With the increasing depletion of fossil energy, the development and utilization of renewable energy sources such as wind and solar energy has become a focus of global attention. Because wind and solar energy are intermittent and unstable due to factors such as weather, this can impact the grid during the integration of renewable energy generation, affecting power supply quality and grid stability. Energy storage technology can address this issue, ensuring the efficient and stable operation of renewable energy generation. Energy storage technologies are primarily categorized as physical and chemical. Chemical energy storage, represented by flow batteries, offers the greatest advantages for large-scale energy storage due to its many advantages, including independent power and capacity, rapid response, simple structure, ease of design, long cycle life, and environmental friendliness.

[0003] Zinc-iron flow batteries have attracted widespread attention in the industry due to their rich active material resources, high energy density, and low system cost. During the long-term cycle of alkaline zinc-iron flow battery stacks, there is a problem of accumulation of positive electrode active materials due to the corrosion of the negative electrode zinc. As the cycle progresses, due to the problem of zinc corrosion at the negative electrode, the corresponding positive electrode active material accumulation gradually increases, resulting in a gradual increase in battery polarization, a decrease in battery performance, and a gradual decay in capacity. In existing recovery methods, reducing substances (hydrogen peroxide, hydrazine hydrate, etc.) are usually used to regularly reduce and restore the positive electrode active materials. However, since the reducing substances will consume the hydroxide in the electrolyte during the addition process, the battery polarization increases, and the battery performance decreases after recovery. As the number of recovery times increases, the battery polarization gradually increases, resulting in an increase in the recovery frequency. In addition, if the consumed alkali is supplemented after the addition of the reducing substance recovery agent, the ionic strength of the positive electrode electrolyte will gradually increase, affecting the stability of the electrolyte until the active material precipitates. Summary of the Invention

[0004] Technical Problem to be Solved by the Present Invention (Objective of the Invention)

[0005] During long-term cycling, alkaline zinc-iron flow battery stacks experience accumulation of active material in the positive electrode due to zinc corrosion at the negative electrode. As cycling progresses, this corrosion causes a corresponding increase in active material in the positive electrode, leading to increased battery polarization, decreased battery performance, and gradual capacity decay. This invention constructs an alkaline zinc-iron flow battery system that utilizes a recovery stack to achieve online recovery of the positive electrode electrolyte, addressing the battery capacity decay issue and improving the battery's cycling performance.

[0006] The complete technical solution provided by the present invention is described in detail with reference to the accompanying drawings.

[0007] The online electrolyte recovery method for an alkaline zinc-iron flow battery comprises introducing an electrolyte recovery stack into a zinc-iron flow battery stack test system. When the positive electrode electrolyte needs to be recovered, the electric valve at the zinc-iron flow battery stack inlet is closed, and the electric valve for the electrolyte recovery stack is opened.

[0008] The recovery condition for the positive electrode electrolyte of the alkaline zinc-iron flow battery is recovery when the charging capacity of the battery stack drops below 80% of the initial capacity.

[0009] The working process of the working battery is its normal battery charging and discharging process; the working current density is 20-60mA / cm 2 The charging cutoff conditions are both time and voltage cutoffs. If either the time or voltage condition is reached, the stack switches to the discharge state. The charge cutoff voltage for a single cell is 2.1V, and the discharge cutoff voltage for a single cell is 0.8V. After the working battery is discharged, the positive electrode electrolyte recovery process begins.

[0010] The working process of the recovery battery is its discharge process, and the working current density of the recovery battery stack is 10-40mA / cm 2 , preferably 10-20mA / cm 2 , the discharge cut-off voltage is 0V.

[0011] The electrolyte recovery stack positive electrode circulation pump is connected to the zinc-iron liquid flow battery stack positive electrode storage tank.

[0012] A zinc sheet is added inside the negative electrode of the recovery stack, and its area is the same as that of the negative electrode.

[0013] The positive and negative electrodes of the recovery stack are carbon felt or graphite felt, and the zinc sheet is added on the negative electrode carbon felt surface close to the diaphragm.

[0014] The negative electrode storage tank of the recovery stack is an aqueous solution of alkali, and the alkali is one or more of NaOH, KOH, and LiOH. - The concentration is 4-8 M, preferably 6-8 M.

[0015] Beneficial effects brought by the technical solution of the present invention

[0016] 1. Solve the problem of increased polarization and capacity attenuation of the battery stack caused by the accumulation of positive electrode active materials during the operation of the battery stack, and improve the cycle stability of the battery stack.

[0017] 2. Compared with adding reducing substances into the electrolyte, there is no need to introduce other reducing substances into the electrolyte, which ensures the long-term cycle stability of the electrolyte and reduces the frequency of electrolyte recovery.

[0018] 3. This method is simple to operate and easy to implement, and can achieve online recovery of the electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 An alkaline zinc-iron flow battery with an electrolyte recovery system;

[0020] Among them, 1-electrolyte recovery stack negative electrode storage tank, 2-first circulation pump, 3-electrolyte recovery stack, 4-second circulation pump, 5-zinc-iron liquid flow battery positive electrode electrolyte storage tank, 6-third circulation pump, 7-zinc-iron liquid flow battery stack, 8-fourth circulation pump, 9-zinc-iron liquid flow battery negative electrode electrolyte storage tank, 10-first electric valve, 11-second electric valve DETAILED DESCRIPTION

[0021] The alkaline zinc-iron dual-flow battery includes a battery stack consisting of one or more single cells connected in series, a positive electrolyte storage tank 5, and a negative electrolyte storage tank 9. The positive electrolyte in the positive electrolyte storage tank circulates in the chamber where the positive electrode of the zinc-iron flow battery is located, and the negative electrolyte in the negative electrolyte storage tank circulates in the chamber where the negative electrode of the zinc-iron flow battery is located.

[0022] A single cell consists of a stacked positive current collector, a positive electrode frame, a positive electrode, a separator, a negative electrode, a negative electrode frame, and a negative current collector. Positive and negative terminal plates are located at each end of the stack, or at each end of a single cell in the case of a single cell. The positive electrode is housed in a through-hole in the center of the positive electrode frame, and the positive electrolyte in the cell is sealed in a closed cavity formed by the positive electrode frame, the positive current collector, and the separator, serving as the positive electrode cavity. The negative electrode is housed in a through-hole in the center of the negative electrode frame, and the negative electrolyte in the cell is sealed in a closed cavity formed by the negative electrode frame, the negative current collector, and the separator, serving as the negative electrode cavity. The electrolyte in the electrolyte storage tank of a zinc-iron flow battery is pumped into the positive and negative electrode cavities.

[0023] The structures of the alkaline zinc-iron flow battery stack (as a working battery) and the electrolyte recovery stack (as a recovery battery) are both the alkaline zinc-iron dual-flow battery described above; and the alkaline zinc-iron flow battery stack and the electrolyte recovery stack share a zinc-iron flow battery positive electrode electrolyte storage tank 5;

[0024] 1- electrolyte recovery stack negative electrode storage tank, 2- first circulation pump, 3- electrolyte recovery stack, 4- second circulation pump, 5- zinc-iron flow battery positive electrode electrolyte storage tank, constitute the electrolyte recovery stack; zinc sheet is added inside the negative electrode of the recovery stack;

[0025] 5-zinc-iron liquid flow battery positive electrode electrolyte storage tank, 6-third circulation pump, 7-zinc-iron liquid flow battery stack, 8-fourth circulation pump, 9-zinc-iron liquid flow battery negative electrode electrolyte storage tank, constitute an alkaline zinc-iron liquid flow battery stack;

[0026] The positive electrode electrolyte storage tank 5 of the zinc-iron flow battery is connected to the positive electrode electrolyte inlet of the electrolyte recovery stack via the first electric valve 10 and the second circulation pump 4; the positive electrode electrolyte storage tank 5 of the zinc-iron flow battery is connected to the positive electrode electrolyte inlet of the alkaline zinc-iron flow battery stack via the second electric valve 11 and the third circulation pump 6;

[0027] Example 1

[0028] The positive and negative electrode materials of the battery stack are both carbon felt, with an area of ​​1000cm 2 The alkaline zinc-iron flow battery stack was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane, and the stack had 10 sections. The cathode electrolyte composition was 0.4 mol L -1 Na4Fe(CN)6+0.4mol L -1 K4Fe(CN)6+0.4mol L -1 NaOH + 0.4 mol L -1 KOH; negative electrode electrolyte is 0.6 mol L -1 EDTA + 2.4 mol L -1 NaOH + 0.6 mol L -1 of ZnBr2, and using 7.5 mol L -1 Adjust the pH of the electrolyte to 12 with NaOH. The volume of the positive electrode electrolyte is 60 L; the volume of the negative electrode electrolyte is 60 L. At this time, the second electric valve 11 is opened, the first electric valve 10 is closed, the fourth circulation pump 8 and the third circulation pump 6 are working, and the current is 40 mA cm -2 The stack is charged for 2 h at a current density of 1000 Ω / min and the upper limit of the charging voltage is 21 V, 40 mA cm -2 The battery was discharged to 8V under the condition of current density.

[0029] The positive and negative electrode materials of the electrolyte recovery stack are both carbon felt, with an area of ​​1000cm 2 , zinc sheet is placed on the negative electrode carbon felt close to the diaphragm side surface, with an area of ​​1000cm 2 The electrolyte recovery stack of the alkaline zinc-iron flow battery was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane. The negative electrode electrolyte storage tank of the recovery stack is 8 mol L -1 NaOH aqueous solution, volume 60L.

[0030] When the charging capacity of the alkaline zinc-iron flow battery stack drops to 75% of the initial capacity, after the alkaline zinc-iron flow battery stack is discharged, the second electric valve 11 is closed and the first electric valve 10 is opened to start the positive electrode electrolyte recovery. The electrolyte recovery stack is at a discharge current density of 20mA / cm 2 Discharge until the stack voltage is 0V.

[0031] Using this recovery method, after the electrolyte is restored and the stack is discharged once, the stack charging capacity is restored to 80mAh / cm 2 The battery stack only needs to be restored after running 400 cycles.

[0032] Example 2

[0033] The positive and negative electrode materials of the battery stack are both carbon felt, with an area of ​​1000cm 2 The alkaline zinc-iron flow battery stack was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane, and the stack had 10 sections. The cathode electrolyte composition was 0.4 mol L -1 Na4Fe(CN)6+0.4mol L -1 K4Fe(CN)6+0.4mol L -1 NaOH + 0.4 mol L -1 KOH; negative electrode electrolyte is 0.6 mol L -1 EDTA + 2.4 mol L -1 NaOH + 0.6 mol L -1 of ZnBr2, and using 7.5 mol L -1 Adjust the pH of the electrolyte to 12 with NaOH. The volume of the positive electrode electrolyte is 60 L; the volume of the negative electrode electrolyte is 60 L. At this time, the second electric valve 11 is opened, the first electric valve 10 is closed, the fourth circulation pump 8 and the third circulation pump 6 are working, and the current is 40 mA cm -2 The stack is charged for 2 h at a current density of 1000 Ω / min and the upper limit of the charging voltage is 21 V, 40 mA cm -2 The battery was discharged to 8V under the condition of current density.

[0034] The positive and negative electrode materials of the electrolyte recovery stack are both carbon felt, with an area of ​​1000cm 2 , Place zinc sheet on the negative electrode carbon felt close to the diaphragm side surface, with an area of ​​1000cm 2 , restore the negative electrode electrolyte tank of the battery stack to 8mol L -1 NaOH aqueous solution, volume 60L.

[0035] When the stack charge capacity drops to 75% of the initial capacity, after the stack discharge is completed, close the electric valve 11 and open the electric valve 10 to start the positive electrode electrolyte recovery. (The above process is carried out 4 times in total) The stack discharge current density is restored to 10mA / cm 2 , 20mA / cm 2 , 30mA / cm 2 , 40mA / cm 2 , discharge until the stack voltage is 0V.

[0036] The recovery stack uses 10mA / cm 2, 20mA / cm 2 At the discharge current density, the stack can be restored after 400 cycles, and the stack charging capacity is restored to 80mAh / cm 2 When the stack is restored, the current is 30 mA / cm 2 , 40mA / cm 2 At the discharge current density, the stack was run 350 times and needed to be restored once every 300 times. The stack charging capacity was restored to 80mAh / cm 2 This is mainly due to the increase in the discharge current density of the recovery stack, which leads to increased battery polarization, decreased discharge capacity, and a decrease in the accumulated trivalent iron reduction number in the positive electrode.

[0037] Example 3

[0038] The positive and negative electrode materials of the battery stack are both carbon felt, with an area of ​​1000cm 2 The alkaline zinc-iron flow battery stack was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane, and the stack had 10 sections. The cathode electrolyte composition was 0.4 mol L -1 Na4Fe(CN)6+0.4mol L -1 K4Fe(CN)6+0.4mol L -1 NaOH + 0.4 mol L -1 KOH; negative electrode electrolyte is 0.6 mol L -1 EDTA + 2.4 mol L -1 NaOH + 0.6 mol L -1 of ZnBr2, and using 7.5 mol L -1 Adjust the pH of the electrolyte to 12 with NaOH. The volume of the positive electrode electrolyte is 60 L; the volume of the negative electrode electrolyte is 60 L. At this time, the second electric valve 11 is opened, the first electric valve 10 is closed, the fourth circulation pump 8 and the third circulation pump 6 are working, and the current is 40 mA cm -2 The stack is charged for 2 h at a current density of 1000 Ω / min and the upper limit of the charging voltage is 21 V, 40 mA cm -2 The battery was discharged to 8V under the condition of current density.

[0039] The positive and negative electrode materials of the electrolyte recovery stack are both carbon felt, with an area of ​​1000cm 2 , Place zinc sheet on the negative electrode carbon felt close to the diaphragm side surface, with an area of ​​1000cm 2 , the negative electrode electrolyte storage tanks of the recovery stack are 3 mol L -1 , 4 mol L -1 , 5 mol L -1 , 6 mol L -1 , 7 mol L -1 , 8 mol L -1NaOH aqueous solution, volume 60L.

[0040] When the stack charge capacity drops to 75% of the initial capacity, after the stack discharge is completed, close the electric valve 11, open the electric valve 10, and start the positive electrode electrolyte recovery. 2 Discharge until the stack voltage is 0V.

[0041] <![CDATA[Alkali concentration / mol L -1 > Restored stack discharge capacity / Ah Number of cycles after recovery 4 10.1 200 5 12.3 250 6 16.4 300 7 17.9 350 8 19.7 400

[0042] When the alkali concentration in the negative electrode electrolyte of the recovery stack is 3 mol L -1 When the alkali concentration is too low, zinc oxide is generated during the discharge process of the stack, causing the recovery stack to become blocked and fail. -1 , 5 mol L -1 , 6 mol L -1 , 7 mol L -1 , 8 mol L -1 When the stack is restored, the charging capacity of the stack can be restored to 80mAh / cm 2 When the alkali concentration is 4 mol L -1 , 5 mol L -1 , 6 mol L -1 , 7 mol L -1 , 8 mol L -1 When the battery stack is running for 200 cycles, 250 cycles, 300 cycles, 350 cycles, and 400 cycles, respectively, it needs to be restored. This is mainly because as the alkali concentration increases, the electrolyte conductivity increases, the battery polarization decreases, and the discharge capacity of the restored battery stack increases.

[0043] Comparative Example 1

[0044] The positive and negative electrode materials of the battery stack are both carbon felt, with an area of ​​1000cm 2 The alkaline zinc-iron flow battery stack was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane, and the stack had 10 sections. The cathode electrolyte composition was 0.4 mol L -1 Na4Fe(CN)6+0.4mol L -1 K4Fe(CN)6+0.4mol L -1 NaOH + 0.4 mol L -1 KOH; negative electrode electrolyte is 0.6 mol L -1 EDTA + 2.4 mol L -1 NaOH + 0.6 mol L -1 of ZnBr2, and using 7.5 mol L -1Adjust the electrolyte pH to 12 with NaOH. The volume of positive electrode electrolyte is 60L; the volume of negative electrode electrolyte is 60L; at 40mA cm -2 The stack is charged for 2 h at a current density of 1000 Ω / min and the upper limit of the charging voltage is 21 V, 40 mA cm -2 The battery was discharged to 8V under the condition of current density.

[0045] When the stack charging capacity drops to 75% of the initial capacity, after the stack discharge is completed, a 30% by mass H2O2 aqueous solution is added to the positive electrode electrolyte to start the positive electrode electrolyte recovery until the trivalent iron in the positive electrode electrolyte is completely reduced.

[0046] Using this recovery method, the stack charging capacity is restored to the initial charging capacity of 80mAh / cm 2 Since the introduction of H2O2 consumes hydroxide in the electrolyte and causes increased battery polarization, the stack needs to be restored after 300 cycles. In addition, the restoring agent reacts with the alkali metal salt to produce alkaline hydrogen peroxide, and the reaction is as follows:

[0047] H2O2+OH - →OOH - +H2O

[0048] When H2O2 is used as the recovery agent, each recovery will consume the alkali in the electrolyte, and the battery polarization will gradually increase. As the number of recovery times increases, the battery polarization gradually increases, resulting in an increase in the recovery frequency.

[0049] Comparative Example 2

[0050] The positive and negative electrode materials of the battery stack are both carbon felt, with an area of ​​1000cm 2 The alkaline zinc-iron flow battery stack was assembled with sulfonated polyetheretherketone (SPEEK) ion conductive membrane, and the stack had 10 sections. The cathode electrolyte composition was 0.4 mol L -1 Na4Fe(CN)6+0.4mol L -1 K4Fe(CN)6+0.4mol L -1 NaOH + 0.4 mol L -1 KOH; negative electrode electrolyte is 0.6 mol L -1 EDTA + 2.4 mol L -1 NaOH + 0.6 mol L -1 of ZnBr2, and using 7.5 mol L -1 Adjust the electrolyte pH to 12 with NaOH. The volume of positive electrode electrolyte is 60L; the volume of negative electrode electrolyte is 60L; at 40mA cm- 2 The charging voltage limit of the stack is 21V, 40mA cm- 2The battery was discharged to 8V under the condition of current density.

[0051] Without any recovery method, as the number of cycles increases, the battery polarization gradually increases and the battery capacity decays. After 400 cycles, the battery capacity decays to 30% of the initial charge capacity.

Claims

1. A method for recovering an electrolyte of an alkaline zinc-iron flow battery, characterized in that: It includes two alkaline zinc-iron dual-flow batteries, one as a working battery and the other as a recovery battery; The two alkaline zinc-iron dual-flow batteries each include a battery stack consisting of one single cell or two or more single cells connected in series, a positive electrolyte storage tank, and a negative electrolyte storage tank. The positive electrolyte in the positive electrolyte storage tank circulates in the chamber where the positive electrode of the zinc-iron flow battery is located through a liquid pump, and the negative electrolyte in the negative electrolyte storage tank circulates in the chamber where the negative electrode of the zinc-iron flow battery is located through a liquid pump. The working battery and the recovery battery share a positive electrode electrolyte storage tank, and the negative electrode storage tank of the recovery battery is an alkaline aqueous solution. When the positive electrode electrolyte of the working battery needs to be restored, the working battery liquid pump is stopped and the recovery battery liquid pump is started. Zinc sheets are added to the negative electrode of the recovery battery stack, and the working battery operates as a normal battery charge and discharge process. After the working battery is discharged, the positive electrode electrolyte of the working battery is restored. The working process of the recovery battery is its discharge process, and the working current density of the recovery battery stack is 10-40 mA / cm 2 , the discharge cut-off voltage is 0V.

2. The recovery method according to claim 1, wherein: The base is one or more of NaOH, KOH, and LiOHH; the aqueous solution of the base contains OH - The concentration is 4-8M.

3. The recovery method according to claim 2, wherein: OH in the aqueous solution of the base - The concentration is 6-8M.

4. The recovery method according to claim 1, wherein: The zinc sheet is placed on the negative electrode surface close to the diaphragm, and its area is the same as that of the negative electrode.

5. The recovery method according to claim 1, wherein: The positive and negative electrodes of the recovery battery stack are respectively carbon felt or graphite felt, and a zinc sheet is added on the negative electrode carbon felt surface close to the diaphragm side.

6. The recovery method according to claim 1, wherein: The recovery condition for the positive electrode electrolyte of the alkaline zinc-iron flow battery used as the working battery is when the charging capacity of the battery stack drops below 80% of the initial capacity.

7. The recovery method according to claim 1, characterized in that: The recovery battery stack cathode liquid pump is connected to the working battery stack cathode liquid storage tank.

8. The recovery method according to claim 1, wherein: A single cell includes a positive electrode current collector, a positive electrode frame, a positive electrode, a separator, a negative electrode, a negative electrode frame, and a negative electrode current collector stacked in sequence; a positive terminal plate and a negative terminal plate are provided at each end of the battery stack, or at each end of a single cell in the case of a single cell; the positive electrode is accommodated in a through-hole in the middle of the positive electrode frame, and the positive electrolyte in the single cell is sealed in a closed cavity surrounded by the positive electrode frame, the positive electrode current collector, and the battery separator, serving as a positive electrode cavity; the negative electrode is accommodated in a through-hole in the middle of the negative electrode frame, and the negative electrolyte in the single cell is sealed in a closed cavity surrounded by the negative electrode frame, the negative electrode current collector, and the battery separator, serving as a negative electrode cavity; The electrolyte in the electrolyte storage tank of the zinc-iron flow battery flows into the positive and negative electrode cavities respectively through pumps.

9. The recovery method according to claim 1, characterized in that: The operating current density of the battery stack is restored to 10-20 mA / cm 2 .

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