Positive electrode electrolyte for zinc-manganese flow battery

Abstract

The invention discloses a positive electrode electrolyte for a zinc-manganese flow battery, the positive electrode electrolyte comprises a manganese salt, an additive and a supporting electrolyte, and the additive is one or a combination of more than two of chromium chloride, magnesium chloride, nickel sulfate, cobalt sulfate, ferrous sulfate, copper sulfate and aluminum sulfate. By introducing metal ions into the electrolyte, the electrolytic process of Mn < 2 + > / MnO2 can be catalyzed and enhanced, and by utilizing the doping effect of the metal ions, oxygen vacancies in MnO2 can be remarkably increased, a more active electronic state can be generated, charge transfer kinetics can be promoted, and the deposition and stripping performance of MnO2 can be further optimized; therefore, the high-rate performance and the cycling stability of the zinc-manganese flow battery are improved. The zinc-manganese flow battery provided by the invention has the advantages of being green, pollution-free, low in cost, safe, environment-friendly and the like, so that the zinc-manganese flow battery has a wide application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of zinc-manganese flow batteries, and particularly relates to a positive electrolyte for zinc-manganese flow batteries. Background Art

[0002] In recent years, zinc-manganese flow batteries have received extensive attention due to their advantages such as low cost, rich resources, high energy density, and high electrochemical window. However, in practical applications, the growth of low-conductivity MnO2 at the positive electrode during the charging process of zinc-manganese flow batteries will lead to an increase in ohmic polarization. At the same time, a relatively thick deposition layer is inevitably formed, resulting in the generation of dead MnO2, that is, MnO2 that is peeled off or incompletely dissolved, leading to poor electronic conductivity and cycle stability. At the same time, under high areal capacity conditions, the electrolysis kinetics of MnO2 / Mn 2+ is sluggish, resulting in a large overpotential of the battery; furthermore, during long-term cycling, dead MnO2 will accumulate and fall off with the hindrance of ion transport, resulting in severe capacity loss, thereby leading to poor rate performance and cycle stability.

[0003] In view of the fact that the poor conductivity of MnO2 and the formation of inactive MnO2 in the prior art limit the deposition and dissolution of its high areal capacity, a new strategy needs to be developed to solve the problem of slow electrolysis kinetics of MnO2 / Mn 2+ and further improve the rate performance and cycle stability of zinc-manganese flow batteries. Summary of the Invention

[0004] Aiming at the problems existing in the background art, the purpose of the present invention is to provide a positive electrolyte for zinc-manganese flow batteries to solve the problem of slow electrolysis kinetics of MnO2 / Mn 2+ during the charging process of existing traditional zinc-manganese flow batteries and improve its rate performance and cycle stability.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A positive electrolyte for zinc-manganese flow batteries includes a manganese salt, an additive, and a supporting electrolyte. Among them, the additive is one or a combination of two or more of chromium chloride, magnesium chloride, nickel sulfate, cobalt sulfate, ferrous sulfate, copper sulfate, and aluminum sulfate.

[0007] Preferably, the concentration of the additive is 0.1 mol / L - 2 mol / L.

[0008] Preferably, the concentration of the manganese salt is 0.2 mol / L - 4 mol / L.

[0009] Preferably, the manganese salt is one or a combination of two or more of manganese sulfate, manganese chloride, and manganese nitrate.

[0010] Preferably, the supporting electrolyte includes one or a combination of two or more of potassium ions, sodium ions, and sulfuric acid. The potassium ions or sodium ions in the supporting electrolyte are used to increase the conductivity of the electrolyte solution and maintain the osmotic pressure balance of the positive and negative electrode electrolytes, and the sulfuric acid in the supporting electrolyte is used to adjust the pH value of the electrolyte solution.

[0011] Preferably, the concentration of sodium ions in the supporting electrolyte is 1 mol / L to 4 mol / L.

[0012] Preferably, the source of sodium ions in the supporting electrolyte includes one or a combination of two or more of sodium sulfate, sodium chloride, and sodium nitrate.

[0013] Preferably, the positive electrode electrolyte further includes deionized water.

[0014] The second aspect of the present invention provides a zinc-manganese flow battery, which includes the positive electrode electrolyte for the zinc-manganese flow battery described above.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) The present invention uses metal salts as additives, and by introducing metal ions (such as Cr 3+ , Co 2+ , Ni 2+ , Fe 2+ , Mg 2+ , Cu 2+ or Al 3+ in the positive electrode electrolyte, one or more of them are used to catalyze and enhance the Mn 2+ / MnO2 electrolysis process. Based on the valence state difference between the doped metal ions and Mn being balanced by oxygen vacancy defects, it can significantly increase the oxygen vacancies in MnO2, generate more active electron states, promote charge transfer kinetics, and further optimize the deposition and stripping performance of MnO2, thereby improving the high-rate performance and cycle stability of the zinc-manganese flow battery.

[0017] (2) The zinc-manganese flow battery provided by the present invention has the advantages of being green and pollution-free, low cost, safe and environmentally friendly, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1 It is a schematic diagram of the basic structure of a zinc-manganese flow battery;

[0020] Figure 2 SEM images of the microscopic morphology of electrode deposition: (a) Zinc-manganese flow battery without adding chromium chloride after 20 cycles; (b) Zinc-manganese flow battery with chromium chloride added to the positive electrolyte after 20 cycles;

[0021] Figure 3 At a current density of 10 - 40 mA·cm -2 and a fixed areal capacity of 5 mAh cm -2 , the figure shows the results of the rate performance test of zinc-manganese flow batteries with and without adding chromium chloride;

[0022] Figure 4 At a current density of 20 mA·cm -2 and a charging time of 1 h, the figure shows the results of the cycle performance of zinc-manganese flow batteries with and without adding chromium chloride;

[0023] Figure 5 At a fixed areal capacity of 20 mAh·cm 2 and a current density of 20 mA·cm 2 with a charging time of 1 h, the figure shows the results of the rate performance test of zinc-manganese flow batteries with and without adding chromium chloride;

[0024] Markings in the figure: 1. Negative electrode; 2. Positive electrode; 3. Negative electrolyte storage tank; 4. Positive electrolyte storage tank; 5. Diaphragm; 6. Negative peristaltic pump; 7. Positive peristaltic pump. Detailed implementation manners

[0025] In the following description, specific details such as specific system structures and technologies are presented for the purpose of illustration rather than limitation, so as to thoroughly understand the embodiments of the present invention. However, those skilled in the art should clearly understand that the present invention can also be implemented in other embodiments without these specific details.

[0026] Example 1

[0027] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate (active material), chromium chloride (additive), sulfuric acid and sodium sulfate (supporting electrolyte), and deionized water (solvent). Among them, the concentration of manganese sulfate in the positive electrolyte is 1 mol / L, the concentration of chromium chloride is 0.1 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L (used to ensure the same osmotic pressure between the positive and negative electrodes). The preparation steps are as follows: Weigh 3.4145 g of manganese sulfate monohydrate, 0.5438 g of chromium chloride and 2.8988 g of sodium sulfate in a beaker and dissolve them in deionized water. Then, use a pipette to add 0.5330 ml of concentrated sulfuric acid dropwise into the beaker, and pour it into a 20 ml volumetric flask and mix well to obtain the positive electrolyte.

[0028] Example 2

[0029] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate, nickel sulfate, sulfuric acid, sodium sulfate, and deionized water. Among them, the concentration of manganese sulfate in the positive electrolyte is 1 mol / L, the concentration of nickel sulfate is 0.1 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L;

[0030] The specific preparation steps are as follows: Weigh 3.4145 g of manganese sulfate monohydrate, 0.5364 g of nickel sulfate hexahydrate, and 2.8988 g of sodium sulfate in a beaker and dissolve them with deionized water. Then, use a pipette to transfer 0.5330 ml of concentrated sulfuric acid and drop it into the beaker, and pour it into a 20 ml volumetric flask and mix well to obtain the positive electrolyte. After testing, for the flow battery assembled based on this positive electrolyte, under the conditions of a current density of 20 mA·cm -2 The long-term cycling test results under the condition of a current density and charging for 0.5 h show that, compared with the battery without additives, the cycling life of the flow battery with nickel sulfate additive is increased by 2 times.

[0031] Example 3

[0032] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate, magnesium chloride, sulfuric acid, sodium sulfate, and deionized water. Among them, the concentration of manganese sulfate in the positive electrolyte is 1 mol / L, the concentration of magnesium chloride is 0.3 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L. The specific preparation steps are the same as those in Example 1. After testing, for the flow battery assembled based on this positive electrolyte, under the conditions of a current density of 20 mA·cm -2 The long-term cycling test results under the condition of a current density and charging for 0.5 h show that, compared with the battery without additives, the cycling life of the flow battery with magnesium chloride additive is increased by 2.2 times.

[0033] Example 4

[0034] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate, aluminum sulfate, sulfuric acid, sodium sulfate, and deionized water. Among them, the concentration of manganese sulfate in the positive electrolyte is 1 mol / L, the concentration of aluminum sulfate is 0.5 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L. The specific preparation steps are the same as those in Example 1. After testing, for the flow battery assembled based on this positive electrolyte, under the conditions of a current density of 20 mA·cm -2 The long-term cycling test results under the condition of a current density and charging for 0.5 h show that, compared with the battery without additives, the cycling life of the flow battery with aluminum sulfate additive is increased by 2.3 times.

[0035] Example 5

[0036] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate, copper sulfate, sulfuric acid, sodium sulfate, and deionized water. Among them, the concentration of manganese sulfate in the positive electrolyte is 1.5 mol / L, the concentration of copper sulfate is 0.3 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L. The specific preparation steps are the same as those in Example 1. After testing, for the flow battery assembled based on this positive electrolyte, under the conditions of a current density of 20 mA·cm -2 The long-term cycling test results under the condition of a current density and charging for 0.5 h show that, compared with the battery without additives, the cycling life of the flow battery with the addition of copper sulfate additive has increased by 2.2 times.

[0037] Example 6

[0038] The positive electrolyte of a zinc-manganese flow battery includes manganese sulfate, copper sulfate, cobalt sulfate, sulfuric acid, sodium sulfate, and deionized water. Among them, the concentration of manganese sulfate in the positive electrolyte is 1.5 mol / L, the concentration of copper sulfate is 0.3 mol / L, the concentration of cobalt sulfate is 0.2 mol / L, the concentration of sulfuric acid is 0.5 mol / L, and the concentration of sodium sulfate is 1 mol / L. The specific preparation steps are the same as those in Example 1. After testing, for the flow battery assembled based on this positive electrolyte, under the conditions of a current density of 20 mA·cm -2 The long-term cycling test results under the condition of a current density and charging for 0.5 h show that, compared with the battery without additives, the cycling life of the flow battery with the addition of copper sulfate and cobalt sulfate has increased by 2.3 times.

[0039] Comparative Example 1

[0040] It is basically the same as the steps in Example 1, except that chromium chloride is not added to the zinc-manganese flow battery.

[0041] 1. Assemble the battery

[0042] Refer to Figure 1 , and use the positive electrolyte prepared by the present invention to assemble a zinc-manganese flow battery. Its basic structure mainly includes: a negative electrode 1, a positive electrode 2, a negative electrolyte storage tank 3, a positive electrolyte storage tank 4, a separator 5, a negative peristaltic pump 6, and a positive peristaltic pump 7; the positive electrolyte storage tank 4 is filled with the prepared positive electrolyte, which includes a manganese salt, an additive, a supporting electrolyte potassium ion or sodium ion, and sulfuric acid. Among them, sulfuric acid is used as a supporting electrolyte to adjust the solution pH, and potassium ion or sodium ion is used as a supporting electrolyte to increase the electrolyte conductivity and maintain the osmotic pressure balance between the positive and negative electrolytes.

[0043] The negative electrode electrolyte storage tank 3 is filled with negative electrode electrolyte, which includes zinc sulfate with a concentration of 1 mol / L, and acetic acid and sodium acetate with concentrations of 2 mol / L as supporting electrolytes (used to maintain the pH of the negative electrode). The specific configuration steps of the negative electrode electrolyte are as follows: Weigh 5.7801 g of zinc sulfate and 3.3143 g of sodium acetate into a beaker, and then use a pipette to transfer 2.287 mL of acetic acid into a 20 ml volumetric flask and mix well.

[0044] Both the negative electrode 1 and the positive electrode 2 of the zinc-manganese flow battery use carbon felt as the electrode, and the area of both is 2×2.5 cm 2 , and the battery separator selects a commercial Nafion117 proton exchange membrane, and the area of the proton exchange membrane is 3×4 cm 2 .

[0045] The negative electrode electrolyte in the negative electrode electrolyte storage tank 3 is pumped into the negative electrode 1 through the negative electrode peristaltic pump 6. The negative electrode active substance in the negative electrode electrolyte will undergo an electrochemical reaction on the surface of the negative electrode 1, and then the negative electrode electrolyte flows out of the negative electrode 1 and re-enters the negative electrode electrolyte storage tank 1; the positive electrode electrolyte in the positive electrode electrolyte storage tank 4 enters the positive electrode 2 through the positive electrode peristaltic pump 7. The positive electrode active substance in the positive electrode electrolyte will undergo an electrochemical reaction on the surface of the positive electrode 2, and then the positive electrode electrolyte flows out of the positive electrode 2 and re-enters the positive electrode electrolyte storage tank 4; due to the existence of the separator 5, the negative electrode electrolyte on the surface of the negative electrode 1 and the positive electrode electrolyte on the surface of the positive electrode are independent of each other, and only protons and sodium ions can pass through the separator.

[0046] During charging and discharging, the positive and negative electrode reaction principles are shown in the following formulas (1) and (2):

[0047] Negative electrode:

[0048] Positive electrode:

[0049] The present invention proposes to use metal salts as additives in the positive electrode electrolyte, and introduce Cr 3+ , Co 2+ , Ni 2+ , Fe 2+ , Mg 2+ , Cu 2+ , Al 3+ One or more of the doping ions such as can pass through with Mn during the entire charging process 2+Co-deposit and replace some manganese atoms to regulate the electronic structure of MnO2, and reversibly dissolve back into the electrolyte during discharge, thus establishing a dynamic doping mechanism. Due to the electronegativity difference and electron redistribution, the doped metal atoms replace manganese atoms to optimize the electron migration path, enhance the electronic structure (oxygen vacancies) of MnO2, and increase the active O 2p electronic states, promote charge transfer, further optimize the deposition and stripping performance of MnO2, and ultimately improve the cycle performance and rate performance of the battery.

[0050] (1) Characterize the electrode deposition microtopography of the zinc-manganese flow battery without chromium chloride and the zinc-manganese flow battery with chromium chloride after 20 cycles respectively. The results are shown in Figure 2 .

[0051] From Figure 2 (a) The results show that after 20 cycles, in the electrode deposition microtopography of the zinc-manganese flow battery with chromium chloride, it can be found that a large amount of (undissolved) manganese dioxide agglomerates unevenly on the carbon fiber. And the undissolved manganese dioxide during these cycles will lead to a low Coulombic efficiency of the battery and is easy to fall off and block the positive electrode pipeline.

[0052] Figure 2 (b) The results show that in the electrode deposition microtopography of the zinc-manganese flow battery with chromium chloride added to the positive electrolyte, no obvious manganese dioxide accumulation is observed after 20 cycles. This indicates that the reversibility of manganese dioxide has been improved after adding chromium chloride.

[0053] (2) At a current density of 10 - 40 mA·cm -2 and a fixed areal capacity of 5 mAh·cm -2 , perform rate performance tests on the flow battery (CE: Coulombic efficiency; EE: energy efficiency). The results are shown in Figure 3 .

[0054] From Figure 3 the results, it can be seen that without adding chromium chloride, the Coulombic efficiency and energy efficiency of the zinc-manganese flow battery are very low, both the Coulombic efficiency and energy efficiency are less than 80%. While after adding a small amount of chromium chloride, both the Coulombic efficiency and energy efficiency of the zinc-manganese flow battery have been significantly improved. The Coulombic efficiency is maintained above 95%, and the energy efficiency is up to 80% at a current density of 20 mA cm -2 and can be maintained at a relatively high level. This result shows that the reversibility and rate performance of the zinc-manganese flow battery have been significantly improved after adding chromium chloride.

[0055] (3) Under the conditions of a current density of 20 mA·cm -2 and charging for 0.5 h, perform long cycle tests on the zinc-manganese flow battery without adding chromium chloride and the zinc-manganese flow battery with chromium chloride added. The results are shown inFigure 4 .

[0056] As Figure 4 can be seen from the results, the cycle stability of the flow battery without adding chromium chloride is extremely poor, the Coulomb efficiency is very low (50%-70%), and the cycle life is relatively short (~100 cycles). After adding a small amount of chromium chloride, the obtained zinc-manganese flow battery can cycle 220 times and the Coulomb efficiency is maintained at a high level, indicating that the cycle stability and reversibility of the flow battery after adding chromium chloride have been greatly improved.

[0057] (4) To further improve the utilization rate of the electrolyte, the fixed surface capacity during charging was increased to 20 mAh·cm 2 , and the rate performance of the battery was tested at a current density of 20 mAh·cm 2 and under the condition of charging for 1 h. The results are shown in Figure 5 .

[0058] As Figure 5 can be seen from the results, the manganese sulfate-based flow battery after adding chromium chloride can stably cycle 120 times and maintain a high level of Coulomb efficiency, that is, the zinc-manganese flow battery can still operate stably at a higher surface capacity.

[0059] The above experimental results show that the positive electrolyte for zinc-manganese flow batteries provided by the present invention uses manganese sulfate as the positive active material, and one or more of Cr 3+ , Co 2+ , Ni 2+ , Fe 2+ , Mg 2+ , Cu 2+ , Al 3+ are used as additives, which can catalyze and enhance the Mn 2+ / MnO2 electrolysis process. Through the doping effect of the additives, the oxygen vacancies in MnO2 are significantly increased, generating more active electron states, promoting the charge transfer kinetics, and further optimizing the deposition and stripping performance of MnO2, thereby improving the high-rate performance and cycle stability of the zinc-manganese flow battery.

[0060] The present invention is not limited to the above specific embodiments. Those of ordinary skill in the art starting from the above concepts and making various transformations without creative labor fall within the protection scope of the present invention.

Claims

1. A positive electrolyte for a zinc-manganese flow battery, comprising a manganese salt, an additive, and a supporting electrolyte, characterized in that, The additive is one or a combination of two or more of chromium chloride, magnesium chloride, nickel sulfate, cobalt sulfate, ferrous sulfate, copper sulfate, and aluminum sulfate.

2. The positive electrode electrolyte for a zinc-manganese flow battery according to claim 1, wherein The concentration of the additive is 0.1 mol / L - 2 mol / L, and the concentration of the manganese salt is 0.2 mol / L - 4 mol / L.

3. The positive electrode electrolyte for a zinc-manganese flow battery according to claim 1, wherein, The manganese salt is one or a combination of two or more of manganese sulfate, manganese chloride, and manganese nitrate.

4. The positive electrode electrolyte for zinc-manganese flow battery according to claim 1, characterized in that, The supporting electrolyte includes one or a combination of two or more of potassium ions, sodium ions, and sulfuric acid.

5. The positive electrolyte for zinc-manganese flow battery according to claim 4, characterized in that, The concentration of sodium ions in the supporting electrolyte is 1 mol / L to 4 mol / L.

6. The positive electrolyte for zinc-manganese flow battery according to claim 5, characterized in that, The source of sodium ions in the supporting electrolyte includes one or a combination of two or more of sodium sulfate, sodium chloride, and sodium nitrate.

7. A zinc-manganese flow battery, characterized in that, It includes the positive electrode electrolyte for a zinc-manganese flow battery according to any one of claims 1-6.

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