Alkaline zinc-iron flow battery electrolyte recovery method

Abstract

The invention relates to the technical field of flow batteries, in particular to the field of alkaline zinc-iron flow batteries, and in the alkaline zinc-iron flow battery electrolyte online recovery method, an electrolyte is introduced into a zinc-iron flow battery stack test system to recover a stack. And when the positive electrolyte needs to be recovered, closing the electric valve at the liquid inlet of the zinc-iron flow battery stack, and opening the electrolyte to recover the electric valve of the stack, thereby solving the problems of stack polarization increase and capacity attenuation caused by accumulation of positive active materials in the running process of the stack, and improving the cycling stability of the stack.

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 such as wind energy and solar energy has become the focus of attention of all countries. Since wind energy and solar energy are discontinuous and unstable due to factors such as weather, this will have an impact on the power grid during the process of renewable energy power generation and grid connection, affecting the power supply quality and grid stability. Energy storage technology can solve this problem and ensure the efficient and stable operation of renewable energy power generation and grid connection. Energy storage technology is mainly divided into two categories: physical energy storage and chemical energy storage. Among them, chemical energy storage represented by flow batteries has the most advantages in large-scale energy storage due to its many advantages such as independent power and capacity, rapid response, simple structure, easy 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. In the long-term cycle process of alkaline zinc-iron flow battery stacks, there is a problem of accumulation of positive active materials due to the corrosion of the negative zinc. As the cycle progresses, due to the problem of zinc corrosion at the negative electrode, the corresponding accumulation of positive active materials 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 restore the positive active materials. However, since the reducing substances will consume the hydroxide in the electrolyte at the same time during the addition of the electrolyte, 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 adding the reducing substance recovery agent, the ionic strength of the positive electrolyte will gradually increase, affecting the stability of the electrolyte until the active substances are precipitated. Summary of the invention

[0004] Technical problem to be solved by the present invention (purpose of the invention)

[0005] During the long-term cycle of the alkaline zinc-iron flow battery stack, there is a problem of accumulation of positive active substances due to the corrosion of the negative zinc. As the cycle progresses, due to the problem of zinc corrosion at the negative electrode, the corresponding accumulation of positive active substances gradually increases, resulting in a gradual increase in battery polarization, a decrease in battery performance, and a gradual decay in capacity. The present invention constructs an alkaline zinc-iron flow battery system, which achieves online recovery of the positive electrode electrolyte by using a recovery stack, solves the problem of battery capacity decay, and improves the battery cycle performance.

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

[0007] The method for online recovery of electrolyte in an alkaline zinc-iron flow battery is to introduce 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 recovering the electrolyte stack is opened.

[0008] The positive electrode electrolyte recovery condition of the alkaline zinc-iron flow battery is to recover 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 cut-off condition is a dual cut-off of time and voltage. When either the time or voltage reaches the cut-off condition, the battery stack switches to the discharge state. The charge cut-off voltage of a single battery is 2.1V, and the discharge cut-off voltage of a single battery is 0.8V. After the working battery is discharged, the positive electrode electrolyte recovery of the working battery 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 side.

[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 4-8M, preferably 6-8M.

[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 due to 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, thus ensuring the long-term cycle stability of the electrolyte and reducing the frequency of electrolyte recovery.

[0018] 3. This method is simple to operate, 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 comprises a battery stack formed by one single battery or two or more single batteries 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] The 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; the two ends of the battery stack, or the two ends of a single cell in the case of a single cell, are respectively provided with a positive terminal plate and a negative terminal plate; the positive electrode is accommodated in a through hole in the middle of the positive electrode frame, and the positive electrode 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, 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 electrode 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, as a negative electrode cavity. The electrolyte in the electrolyte storage tank of the zinc-iron liquid flow battery flows in the positive and negative electrode cavities respectively through a pump.

[0023] The structures of the alkaline zinc-iron liquid flow battery stack (as a working battery) and the electrolyte recovery stack (as a recovery battery) are both the above-mentioned alkaline zinc-iron dual-flow battery; and the alkaline zinc-iron liquid flow battery stack and the electrolyte recovery stack share a zinc-iron liquid 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 liquid 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 7.5 mol L -1 NaOH adjusts the electrolyte pH to 12. The volume of the positive electrolyte is 60L; the volume of the negative electrolyte is 60L; 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, at 40mA cm -2 The stack is charged for 2 h at a current density of 21 V and 40 mA cm -2 Discharge to 8V under current density conditions.

[0029] Electrolyte recovery stack positive and negative electrode materials are carbon felt, area 1000cm 2 A zinc sheet is placed on the negative electrode carbon felt near the diaphragm side, with an area of ​​1000 cm 2 The electrolyte recovery stack of the alkaline zinc-iron flow battery was assembled with sulfonated polyetheretherketone (SPEEK) ion-conducting 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 discharge of the alkaline zinc-iron flow battery stack is completed, the second electric valve 11 is closed, the first electric valve 10 is opened, and the positive electrode electrolyte recovery is started. The electrolyte recovery stack is discharged at a 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 7.5 mol L -1 NaOH adjusts the electrolyte pH to 12. The volume of the positive electrolyte is 60L; the volume of the negative electrolyte is 60L; 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, at 40mA cm -2 The stack is charged for 2 h at a current density of 21 V, 40 mA cm -2 Discharge to 8V under current density conditions.

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

[0035] When the stack charging capacity drops to 75% of the initial capacity, after the stack discharge is completed, the electric valve 11 is closed and the electric valve 10 is opened to start the positive electrode electrolyte recovery. (The above process is carried out 4 times in total) The discharge current density of the recovery stack is 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 When the discharge current density is 400, the stack can be restored once, and the stack charging capacity is restored to 80 mAh / cm 2 When the stack is restored, 30 mA / cm 2 , 40mA / cm 2 At the discharge current density, the stack was run 350 times and needed to be restored after 300 times. The stack charging capacity was restored to 80 mAh / cm 2 This is mainly due to the increase in the discharge working current density of the recovery stack, which leads to an increase in battery polarization, a decrease in discharge capacity, and a decrease in the number of ferric iron reduction accumulated 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 7.5 mol L -1 NaOH adjusts the electrolyte pH to 12. The volume of the positive electrolyte is 60L; the volume of the negative electrolyte is 60L; 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, at 40mA cm -2 The stack is charged for 2 h at a current density of 21 V and 40 mA cm -2 Discharge to 8V under current density conditions.

[0039] Electrolyte recovery stack positive and negative electrode materials are carbon felt, area 1000cm 2 A zinc sheet is placed on the negative electrode carbon felt near 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 charging capacity drops to 75% of the initial capacity, after the stack discharge is completed, the electric valve 11 is closed and the electric valve 10 is opened to 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 battery stack, causing the recovery battery stack to become blocked and fail. -1 , 5 mol L -1 , 6 mol L -1 , 7 mol L -1 , 8 mol L -1 After the stack is restored, the stack charging capacity can be restored to 80mAh / cm 2 When the base 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 runs for 200 cycles, 250 cycles, 300 cycles, 350 cycles, and 400 cycles, 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 7.5 mol L -1Adjust the electrolyte pH to 12 with NaOH. The positive electrolyte volume is 60 L; the negative electrolyte volume is 60 L; at 40 mA cm -2 The stack is charged for 2 h at a current density of 21 V, 40 mA cm -2 Discharge to 8V under current density conditions.

[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 will consume the hydroxide in the electrolyte and cause the battery polarization to increase, the battery stack needs to be restored once every 300 cycles. In addition, since the restoring agent reacts with the alkali metal salt to generate alkaline hydrogen peroxide, 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 will gradually increase, 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 7.5 mol L -1 Adjust the electrolyte pH to 12 with NaOH. The positive electrolyte volume is 60 L; the negative electrolyte volume is 60 L; at 40 mA cm- 2 The stack is charged for 2 hours at a current density of 21 V, 40 mA cm- 2Discharge to 8V under current density conditions.

[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 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 respectively include a battery stack consisting of one single battery or two or more single batteries connected in series, a positive electrode electrolyte storage tank, and a negative electrode electrolyte storage tank. The positive electrode electrolyte in the positive electrode 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 electrode electrolyte in the negative electrode 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 operation of the working battery liquid pump is stopped, and the operation of the recovery battery liquid pump is started.

2. The recovery method according to claim 1, characterized in that: The negative electrode storage tank of the recovery stack is an aqueous solution of alkali, wherein the alkali is one or more of NaOH, KOH, and LiOH; OH in the alkali solution - The concentration is 4-8M; preferably 6-8M.

3. The recovery method according to claim 1, characterized in that: A zinc sheet is added inside the negative electrode of the recovery stack, and the zinc sheet is placed on the side surface of the negative electrode close to the diaphragm, and its area is the same as that of the negative electrode.

4. The recovery method according to claim 1, characterized in that: The positive and negative electrodes of the recovery 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.

5. The recovery method according to claim 1, characterized in that: The working process of the working battery is its normal battery charging and discharging 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 10-40mA / cm 2 , preferably 10-20mA / cm 2 , the discharge cut-off voltage is 0V.

6. The recovery method according to claim 1, characterized in that: The positive electrode electrolyte recovery condition of the alkaline zinc-iron flow battery (working battery) is to recover 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 electrolyte recovery stack cathode liquid pump is connected to the zinc-iron liquid flow battery stack cathode liquid storage tank.

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

Images