Alkaline zinc-iron flow battery negative electrode electrolyte

Abstract

The invention discloses a negative electrode electrolyte of an alkaline zinc-iron flow battery. The negative electrode electrolyte is a mixed aqueous solution formed by a zinc source, a strong base and cyclodextrin and / or a derivative thereof. According to the invention, cyclodextrin and / or a derivative thereof are / is taken as a negative electrode electrolyte additive, and abundant hydroxyl groups on cyclodextrin and / or the derivative thereof and free water in a negative electrode form a strong hydrogen bond network to hinder migration of water; meanwhile, through the ether bond adsorption effect of the zinc deposition layer and cyclodextrin, an electrochemical window and a double-electrode-layer structure of the negative electrode electrolyte are changed, the inhibition effect on zinc dendritic crystal growth and by-product formation is achieved, and the situation that the zinc dendritic crystal punctures a battery diaphragm to cause battery failure and the situation that the cycle life is shortened due to by-products are avoided; therefore, the performance and cycle life of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of alkaline zinc-iron flow batteries, and particularly relates to a negative electrolyte for an alkaline zinc-iron flow battery. Background Art

[0002] With the continuous increase in global energy demand and the environmental pollution and climate change problems brought about by the dependence on fossil energy, the development of renewable new energy has become extremely urgent. Among them, renewable energy sources such as wind energy and solar energy, although having the characteristics of cleanness and sustainability, their intermittency and instability are challenges for their stable operation when integrated into the power grid. Currently, large-scale electrochemical energy storage technologies are expected to be able to efficiently store excess electrical energy and release it in a timely manner, thereby ensuring the stable power supply of renewable energy. Among many electrochemical energy storage technologies, flow batteries have great advantages in large-scale energy storage because they separate the energy storage active substances from the electrode reactions, achieving independent energy and power decoupling, and having the advantages of high safety, low environmental pollution, and long service life.

[0003] Currently, the widely applied flow battery technologies mainly include: all-vanadium flow batteries, zinc-bromine flow batteries, and iron-chromium flow batteries. Among them, the main reason restricting the further commercial application of all-vanadium flow batteries is the large price fluctuation of vanadium resources and high cost. Although zinc-bromine batteries can provide high voltage, the overflow of bromine gas at the positive electrode has certain safety hazards. Iron-chromium flow batteries have the problem of power density caused by slow chemical reaction kinetics.

[0004] Alkaline zinc-iron flow batteries have become a research hotspot because of their environmental friendliness, rich reserves of zinc and iron, high energy density, and extremely high safety. The existing positive electrolyte of alkaline zinc-iron flow batteries uses an alkaline solution of ferrocyanide, and the negative electrolyte uses a solution obtained by dissolving zinc salt or zinc oxide in strong alkali. In the operation process of the battery with this electrolyte system, due to the inconsistent osmotic pressure of the positive and negative electrolytes, the irreversible water migration caused by ion migration is serious, resulting in serious electrolyte migration, thereby causing the decline of the battery voltage efficiency and further leading to the attenuation of the battery energy efficiency, which greatly increases the maintenance cost of the battery. At the same time, during the charge and discharge process of the alkaline zinc-iron flow battery, zinc deposition and dissolution occur on the electrode on the negative electrode side. When operating at a high current density, serious zinc dendrites or dendrite-like deposits will be generated, and the continuously growing dendrites will pierce the diaphragm, causing battery short circuit. Summary of the Invention

[0005] In order to solve the water migration problem caused by ion concentration difference in the existing alkaline zinc-iron flow battery, the purpose of the present invention is to provide a negative electrolyte for an alkaline zinc-iron flow battery containing cyclodextrin and / or its derivatives, which can solve the water migration problem, prevent the growth of zinc dendrites, inhibit the hydrogen evolution side reaction, and improve the performance and cycle life of the battery.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solutions:

[0007] A negative electrolyte for an alkaline zinc-iron flow battery is a mixed aqueous solution formed by a zinc source, a strong base, and cyclodextrin and / or its derivatives.

[0008] Further, the cyclodextrin and / or its derivatives are α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin.

[0009] Further, the zinc source is zinc oxide, zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc nitrate, or zinc carbonate.

[0010] Further, the strong base is sodium hydroxide or potassium hydroxide.

[0011] Further, after the zinc source is dissolved in the strong base, Zn(OH)4 is formed 2- , and its concentration is 0.01 - 2 mol / L -1 ; the concentration of the strong base is 0.01 - 6 mol / L -1 ; the concentration of cyclodextrin and / or its derivatives is 0.01 - 0.4 mol / L -1 .

[0012] Even further, after the zinc source is dissolved in the strong base, Zn(OH)4 is formed 2- , and its concentration is 0.1 - 1 mol / L -1 ; the concentration of the strong base is 2 - 5 mol / L -1 ; the concentration of cyclodextrin and / or its derivatives is 0.02 - 0.1 mol / L -1 .

[0013] An electrolyte for an alkaline zinc-iron flow battery includes a positive electrolyte and the above negative electrolyte, and the positive electrolyte is a mixed aqueous solution formed by ferrocyanide and a strong base.

[0014] Further, the ferrocyanide is potassium ferrocyanide or sodium ferrocyanide, and the concentration is 0.02 - 1 mol / L -1 ; the strong base is sodium hydroxide or potassium hydroxide, and the concentration is 0.01 - 6 mol / L -1 .

[0015] Even further, the concentration of the ferrocyanide is 0.2 - 0.8 mol / L -1 ; the concentration of the strong base is 2 - 5 mol / L -1 .

[0016] We know that in an alkaline zinc-iron flow battery, the active materials [Zn(OH)4 2– , Fe(CN)6 4– and Fe(CN)63–] It rarely occurs through the membrane. Therefore, the electrolyte migration behavior of the alkaline zinc-iron flow battery during cycling mainly involves the transmembrane migration of water. Due to the large difference in ionic strength between the positive and negative electrodes, the ionic strength is calculated using this equation:

[0017]

[0018] where I (mol L –1 ) is the ionic strength of the solution, C i (mol L –1 ) is the concentration of the ion, and Z i is the charge number of the ion.

[0019] For example, in an alkaline zinc-iron flow battery, the initial negative electrolyte consists of 0.2 mol L –1 Na2Zn(OH)4 and 3 mol L –1 NaOH, while the initial positive electrolyte consists of 0.4 mol L –1 K4Fe(CN)6 and 3 mol L –1 NaOH; the total concentration of ions in the negative electrolyte is 3.6 mol L –1 , much lower than that of the positive electrolyte (7 mol L –1 ). The osmotic pressure, which is closely related to the ion concentration, will surely cause water to migrate from the negative electrode to the positive electrode, thus seriously affecting the performance of the battery. In addition, the zinc anode has poor reversibility due to dendritic growth, and the side reaction of hydrogen evolution and the accompanying by-products will reduce the cycle stability and further shorten its life due to the continuous consumption of the electrolyte and the zinc anode.

[0020] Based on this, the present invention proposes to use cyclodextrin and / or its derivatives as additives for the negative electrolyte. By utilizing the abundant hydroxyl groups on cyclodextrin and / or its derivatives to form a strong hydrogen bond network with the free water in the negative electrode, the migration of water is hindered; at the same time, the adsorption effect of the zinc deposition layer and the ether bond of cyclodextrin changes the electrochemical window and the double-layer structure of the negative electrolyte, realizing the inhibition of zinc dendritic growth and by-product formation, avoiding the battery failure caused by zinc dendrites piercing the battery diaphragm and the reduction of the cycle life caused by by-products, thereby improving the performance and cycle life of the battery. Description of the Drawings

[0021] Figure 1 It is the cycle performance graph of the alkaline symmetric zinc flow battery of Comparative Example 1.

[0022] Figure 2 It is the cycle performance graph of the alkaline zinc-iron flow battery of Comparative Example 2.

[0023] Figure 3 It is the cycle performance graph of the alkaline zinc-iron flow battery of Comparative Example 3.

[0024] Figure 4 It is the cycle performance diagram of the alkaline symmetric zinc flow battery of Example 1.

[0025] Figure 5 It is the cycle performance diagram of the alkaline zinc-iron flow battery of Example 2.

[0026] Figure 6 It is the physical diagram of the remaining volume after the negative electrolyte circulation in Examples 2-4. Specific Embodiments

[0027] The following is a further description in conjunction with specific embodiments, so that the advantages and features of the present invention can be more easily understood by those skilled in the art. The raw materials used in the following examples are all commercially available conventional products.

[0028] In the embodiments of the present invention, the positive and negative electrodes are heat-treated hydrophilic porous carbon felt electrodes, and a graphite plate with a serpentine flow field is used as the bipolar plate; the ion conductive membrane is a cation exchange membrane.

[0029] Preparation method of the negative electrolyte containing HP-β-CD: Dissolve 0.2 mol L -1 ZnO in 3.4 mol L -1 NaOH, add HP-β-CD after forming a homogeneous solution, and stir well at 25 °C for 0.1 to 4 hours to obtain.

[0030] Comparative Example 1

[0031] Alkaline zinc symmetric flow battery: The positive electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH; the negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH.

[0032] Single cell assembly method: The single cell is assembled in the following order: positive end plate, copper plate, graphite plate, 1.5×1.5×cm 2 zinc sheet with a thickness of 0.06 cm, positive electrode 2×2 cm 2 carbon felt, cation exchange membrane, negative electrode 2×2 cm 2 carbon felt, graphite plate, copper plate, negative end plate.

[0033] Test conditions: Electrode effective area: 2×2 cm 2 ; The battery adopts a constant current charge and discharge mode, at 60 mAcm -2Charge for 6 min under the current density condition of 60 mA cm, and then use the voltage cut-off as the condition. -2 Discharge to -0.5 V under the current density condition.

[0034] As Figure 1 shown, under the current density of 60 mA cm -2 and the deposition capacity of 6 mAh cm -2 conditions, when HP-β-CD (2-hydroxypropyl-β-cyclodextrin) is not added to the alkaline zinc symmetric battery, the battery cycle stability is extremely poor and starts to fail after 7 h, unable to ensure long-term cycling.

[0035] Comparative Example 2

[0036] Alkaline zinc symmetric flow battery: The positive electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.4 mol L -1 glucose; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.4 mol L -1 glucose.

[0037] Single cell assembly method: The single cell is assembled in the following order: positive end plate, copper plate, graphite plate, zinc sheet with a thickness of 0.06 cm and a size of 1.5×1.5×cm 2 positive electrode carbon felt with a size of 2×2 cm 2 cation exchange membrane, negative electrode carbon felt with a size of 2×2 cm 2 graphite plate, copper plate, negative end plate.

[0038] Test conditions: Electrode effective area: 2×2 cm 2 ; The battery adopts a constant current charge and discharge mode. Charge for 6 min under the current density condition of 60 mA cm -2 , and then use the voltage cut-off as the condition. Discharge to -0.5 V under the current density condition of 60 mA cm -2 .

[0039] As Figure 2 shown, under the current density of 60 mA cm -2 and the deposition capacity of 6 mAh cm -2 conditions, when glucose is added to the alkaline zinc symmetric battery, the battery cycle stability is extremely poor and starts to fail after 4 h, unable to ensure long-term cycling.

[0040] Comparative Example 3

[0041] Alkaline zinc-iron flow battery: The positive electrolyte composition is 0.4 mol L -1 Fe(CN)6 4- + 3 mol L -1 NaOH; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH; The volumes of the positive and negative electrolytes are each 12 mL.

[0042] The single cell is assembled in the following order: positive end plate, copper plate, graphite plate, positive electrode 2×2 cm 2 carbon felt, cation exchange membrane, negative electrode 2×2 cm 2 carbon felt, graphite plate, copper plate, negative end plate.

[0043] Test conditions: Electrode effective area: 2×2 cm 2 ; The battery adopts a constant current charge-discharge mode, charges for 15 min under the current density condition of 80 mA cm -2 , and then discharges until 0.1 V under the current density condition of 80 mA cm -2 .

[0044] As Figure 3 shown, under the current density of 80 mA cm -2 and the deposition capacity condition of 200 mAh cm -2 , when HP-β-CD is not added in the alkaline zinc-iron battery, the cycle stability of the battery is extremely poor, and it starts to fail after 21 cycles, unable to ensure long-term cycling.

[0045] Example 1

[0046] Alkaline zinc symmetric flow battery: The positive electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.05 mol L -1 HP-β-CD; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.05 mol L - 1 HP-β-CD.

[0047] The test conditions and the single cell assembly method are the same as those in Comparative Example 1.

[0048] As Figure 4 shown, under the current density of 60 mA cm -2 and the deposition capacity of 6 mAh cm -2After adding HP-β-CD to the alkaline zinc symmetric flow battery under the deposition capacity condition, the charging end voltage of the flow battery slowly rises after 120 h of cycling, and both the cycling stability and reversibility of the battery are significantly improved.

[0049] Example 2

[0050] Alkaline zinc-iron flow battery: The positive electrolyte composition is 0.4 mol L -1 Fe(CN)6 4- + 3 mol L -1 NaOH; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.02 mol L -1 HP-β-CD; The volume of the positive and negative electrolytes is 12 mL each.

[0051] The test conditions and the single-cell assembly method are the same as those in Comparative Example 2.

[0052] As Figure 5 shown, compared with the performance of the blank battery, adding 0.02 mol L -1 HP-β-CD to the negative electrolyte significantly improves the long-cycle performance of the battery. The battery can operate for 330 cycles, and the volume of the negative electrolyte decreases very little after operation, indicating that the effect of inhibiting hydrogen evolution by HP-β-CD is obvious. The Coulombic efficiency is 99.3%, the voltage efficiency is 74.32%, and the energy efficiency is 73.85%.

[0053] Example 3

[0054] Alkaline zinc-iron flow battery: The positive electrolyte composition is 0.4 mol L -1 Fe(CN)6 4- + 3 mol L -1 NaOH; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.05 mol L -1 HP-β-CD; The volume of the positive and negative electrolytes is 12 mL each.

[0055] The test conditions and the single-cell assembly method are the same as those in Comparative Example 2.

[0056] When the concentration of HP-β-CD is 0.05 mol L -1 , the cycling performance of the battery is improved, and it can operate for more than 350 cycles. The Coulombic efficiency is 98.95%, the voltage efficiency is 67.75%, and the energy efficiency is 67.04%.

[0057] Example 4

[0058] Alkaline zinc-iron flow battery: The positive electrolyte composition is 0.4 mol L -1 Fe(CN)6 4- + 3 mol L -1 NaOH; The negative electrolyte composition is 0.2 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.1 mol L -1 HP-β-CD; The volume of the positive and negative electrolytes is 12 mL each.

[0059] The test conditions and the single-cell assembly method are the same as those in Comparative Example 2.

[0060] When the concentration of HP-β-CD is 0.1 mol L -1 the battery can operate for more than 400 cycles, and the negative electrolyte hardly decreases, indicating that the addition of HP-β-CD significantly inhibits the hydrogen evolution side reaction on the negative side of the flow battery; however, the battery polarization increases, the energy efficiency decreases, the Coulomb efficiency is 97.22%, the voltage efficiency is 66.59%, and the energy efficiency is 64.74%.

Claims

1. An alkaline zinc-iron flow battery negative electrode electrolyte, characterized in that: The negative electrode electrolyte is a mixed aqueous solution formed by a zinc source, a strong base, and cyclodextrin and / or its derivatives.

2. The negative electrode electrolyte according to claim 1, characterized in that The cyclodextrin and / or its derivative is α-cyclodextrin, β-cyclodextrin or γ-cyclodextrin.

3. The negative electrode electrolyte according to claim 1, characterized in that The zinc source is zinc oxide, zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc nitrate or zinc carbonate.

4. The negative electrode electrolyte according to claim 1, characterized in that The strong base is sodium hydroxide or potassium hydroxide.

5. The negative electrode electrolyte according to claim 1, characterized in that The zinc source is dissolved in a strong base to generate Zn(OH)4 2- , its concentration is 0.01~2mol L -1 ; The concentration of strong base is 0.01~6mol L -1 ; The concentration of cyclodextrin and / or its derivatives is 0.01 to 0.4 mol L -1 .

6. The negative electrode electrolyte according to claim 5, characterized in that The zinc source is dissolved in a strong base to generate Zn(OH)4 2- , the concentration is 0.1~1 mol L -1 ; The concentration of strong base is 2 to 5 mol L -1 ; The concentration of cyclodextrin and / or its derivatives is 0.02 to 0.1 mol L -1 .

7. An alkaline zinc-iron flow battery electrolyte, characterized in that: It comprises a positive electrode electrolyte and the negative electrode electrolyte according to any one of claims 1 to 6, wherein the positive electrode electrolyte is a mixed aqueous solution formed by ferrocyanide and a strong base.

8. The electrolyte according to claim 7, characterized in that The ferrocyanide is potassium ferrocyanide or sodium ferrocyanide, and the concentration is 0.02-1 mol L -1 ; Strong base is sodium hydroxide or potassium hydroxide, with a concentration of 0.01 to 6 mol L -1 .

9. The electrolyte according to claim 8, characterized in that The concentration of ferrocyanide is 0.2-0.8 mol / L -1 ; The concentration of strong base is 2 to 5 mol L -1 .

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