Strong-alkalinity total-iron flow battery electrolyte and preparation method thereof

Abstract

The invention relates to a strong alkaline total iron flow battery electrolyte and a preparation method thereof, the strong alkaline total iron flow battery electrolyte comprises a positive electrode electrolyte and a negative electrode electrolyte, the positive electrode electrolyte is a strong alkaline aqueous solution of ferrocyanate, the negative electrode electrolyte is a strong alkaline aqueous solution of iron salt and an organic ligand, the pH value of the positive electrode electrolyte is consistent with that of the negative electrode electrolyte, and the pH value of the positive electrode electrolyte is consistent with that of the negative electrode electrolyte. The pH is greater than or equal to 14.0. In the negative electrode electrolyte, an organic ligand is used as a complexing agent and forms a stable six-coordinate structure with iron ions, so that the stability of the negative electrode electrolyte is enhanced; the complexing constant of the organic ligand and iron is relatively large, so that the generation of ferric hydroxide precipitates in positive and negative electrolyte can be effectively inhibited even under an alkaline condition, and the stability of iron ions is improved; besides, the ligand molecules are large in size, the steric effect of the formed chelate is large, the transmembrane migration degree of iron ions / ferrous ions is reduced, the cycle life of the total-iron flow battery is greatly prolonged, and the total-iron flow battery can be widely applied to various aqueous flow batteries.

Description

Technical Field

[0001] The present invention relates to the field of liquid flow batteries, and in particular to a strong alkaline all-iron liquid flow battery electrolyte and a preparation method thereof Background Art

[0002] In recent years, alkaline all-iron flow batteries have developed rapidly. By the end of 2023, there have been commercial energy storage application demonstration projects, which are showing a trend of surpassing the development of acidic all-iron flow batteries. Gong et al. used Fe-TEOA and [Fe(CN)6] 3- / 4- The first fully soluble all-iron flow battery using a redox couple has been developed, significantly improving the battery's energy efficiency compared to traditional acidic all-iron batteries. The alkaline iron-based negative electrode electrolytes disclosed in patents such as CN113764714B and CN114709459B all use TEA or TEA derivatives as primary ligands, with small organic molecules as auxiliary ligands, forming stable hexacoordinate complexes with iron ions. However, these small organic molecules are easily detached and penetrate the diaphragm during long-life cycling, causing the Fe-TEOA / TEA derivatives to convert to pentacoordinate complexes, resulting in rapid capacity decay over long cycle life.

[0003] Although acidic all-iron flow batteries have commercial demonstration devices, these systems still face challenges such as hydrogen evolution, iron dendrites, and ferric hydroxide precipitation. Therefore, it is urgent to develop a strong alkaline all-iron flow battery electrolyte to address these fundamental issues that affect the long-term operation of existing electrolytes, such as hydrogen and oxygen evolution, iron dendrites, and ferric hydroxide precipitation. Summary of the Invention

[0004] The present invention addresses the defects of the electrolyte of alkaline all-iron liquid flow batteries in the prior art and provides an electrolyte for a strong alkaline all-iron liquid flow battery and a preparation method thereof. The strong alkaline environment protects the electrode material, reduces electrode corrosion and dissolution of active substances, thereby ensuring the long-term stable operation of the battery.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is: A strong alkaline all-iron flow battery electrolyte, characterized by comprising a positive electrode electrolyte and a negative electrode electrolyte, wherein the positive electrode electrolyte is an alkaline aqueous solution composed of ferrocyanide and a supporting electrolyte, with a total iron concentration of 0.1 to 0.5 mol / L; the negative electrode electrolyte is an alkaline aqueous solution composed of a ferric salt, an organic ligand, a stabilizer, and a supporting electrolyte, with a total iron concentration of 0.1 to 0.5 mol / L, a molar ratio of organic ligand to iron of 1.0 to 2.0, and a molar ratio of stabilizer to Fe of 0.25 to 1.0; the supporting electrolyte concentration is 0 to 2.0 mol / L, and the pH is adjusted to ≥ 14.0 by an inorganic base.

[0006] The ferrocyanide is one or more of sodium ferrocyanide, potassium ferrocyanide, lithium ferrohydride, and ammonium ferrocyanide.

[0007] The iron salt is one or more of ferric chloride, ferric sulfate, ferric nitrate, ferric hydroxide, and ferric carbonate.

[0008] The organic ligand is 2,6-bis[ N, N -bis(hydroxyethyl)aminomethyl]pyridine, 2,6-bis(aminomethyl)pyridine tetraacetate, N2,N2,N6,N6-tetrakis(2-hydroxyethyl)-2,6-pyridinedicarboxamide, 2,2′-[2,6-pyridinediylbis[methylene(methylimine)]]bis-1,3-propylene glycol.

[0009] The stabilizer is one or more of sorbitol, mannitol, gluconic acid, and gluconic acid.

[0010] The supporting electrolyte is one or more of potassium chloride, sodium chloride, sodium sulfate, potassium sulfate, potassium nitrate, sodium nitrate, and ammonium chloride.

[0011] The alkali solution is one or more of ammonia water, sodium hydroxide, potassium hydroxide and lithium hydroxide.

[0012] The pH values ​​of the positive electrode electrolyte and the negative electrode electrolyte are kept consistent and adjusted with alkaline solution, with the pH being ≥ 14.0.

[0013] The positive electrode electrolyte preparation method comprises the following steps: weighing a certain mass of ferrocyanide, a stabilizer, and a supporting electrolyte, dissolving them in an appropriate amount of distilled water, then adjusting the pH of the solution to ≥ 14.0 with an alkaline solution, stirring at 40-60°C for 24 hours, and cooling to room temperature to obtain the positive electrode electrolyte.

[0014] The negative electrode electrolyte preparation method comprises the following steps: weighing a certain mass of iron salt, organic ligand, stabilizer and supporting electrolyte and dissolving them in an appropriate amount of distilled water, then adjusting the pH of the solution to ≥ 14.0 with alkaline solution, stirring at 40-60°C for 24 hours, and cooling to room temperature to obtain the negative electrode electrolyte.

[0015] Compared with the prior art, the present invention has the following advantages: The present invention uses an organic ligand as the active substance of the negative electrode electrolyte, which contains 4 hydrophilic groups (-COOH), 2 nitrogen atoms and a pyridine group. It chelates with iron ions to form a stable six-membered ring network structure. Even under high temperature alkaline conditions, it is difficult to dissociate metallic iron ions, greatly improving the cycle stability of the negative electrode electrolyte.

[0016] The molecular size of the active substances in the positive and negative electrode electrolytes is relatively large, and the permeability through the ion exchange membrane is greatly reduced, which can effectively block the migration of active substances. The assembled battery has high coulombic efficiency, good stability and low capacity attenuation rate.

[0017] The pH of the all-iron liquid flow battery electrolyte of the present invention is ≥14.0, creating a strongly alkaline environment. Together with the alkaline positive electrode electrolyte (such as ferrocyanide), the strongly alkaline all-iron liquid flow battery basically does not experience problems such as hydrogen and oxygen evolution, iron hydroxide, and iron dendrites during battery operation, thereby greatly extending the cycle life of the all-iron liquid flow battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The structural formula of iron-2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine complex Figure 2 Cyclic voltammetry curves (CV curves) of 200 negative electrode electrolytes in Example 1 Figure 3 Coulombic efficiency, voltage efficiency, and energy efficiency of the strong alkaline all-iron flow battery composed of Example 1 Figure 4 Capacity retention of all-iron flow batteries composed of Example 1 and Comparative Examples 1 to 3 DETAILED DESCRIPTION

[0019] The present invention is further described in detail below through comparative examples and examples.

[0020] 1 Preparation process of all-iron flow battery electrolyte 1.1 Preparation process of positive electrode electrolyte: Weigh a certain mass of ferrocyanide, stabilizer, and supporting electrolyte and dissolve them in an appropriate amount of distilled water. Then adjust the pH of the solution to ≥ 14.0 with alkaline solution, stir at 40 ~ 60 ° C for 24 hours, and cool to room temperature to obtain the positive electrode electrolyte.

[0021] 1.2 Negative electrolyte preparation process: Weigh a certain amount of iron salt, organic ligand, stabilizer and supporting electrolyte and dissolve them in an appropriate amount of distilled water. Then adjust the pH of the solution to ≥14.0 with alkaline solution, stir at 40 ~ 60℃ for 24 hours, and cool to room temperature to obtain the negative electrode electrolyte.

[0022] Example 1 Positive electrolyte: 0.1 mol potassium ferrocyanide, 1 mol potassium chloride, adjust the pH to ≥14.0 with potassium hydroxide, and dilute to 1 L to prepare a positive electrolyte containing 0.1 M ferrocyanide.

[0023] Anode electrolyte: 0.1 mol ferric chloride, 0.1 mol 2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine, 0.05 mol sorbitol, 1 mol potassium chloride, adjust the pH to ≥ 14.0 with potassium hydroxide, and adjust the volume to 1 L to prepare a negative electrode electrolyte containing 0.1M iron-2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine.

[0024] Example 2 Positive electrolyte: 0.2 mol sodium ferrocyanide, 1 mol sodium chloride, adjust the pH ≥ 14.0 with sodium hydroxide, and dilute to 1 L to prepare a positive electrolyte containing 0.2 M ferrocyanide.

[0025] Anode electrolyte: 0.2 M iron sulfate, 0.2 mol 2,6-bis(aminomethyl)pyridine tetraacetate, 0.05 mol mannitol, 1 mol sodium sulfate. Adjust the pH to ≥ 14.0 with sodium hydroxide and dilute to 1 L to prepare a negative electrode electrolyte containing 0.2 M iron-2,6-bis(aminomethyl)pyridine tetraacetate.

[0026] Example 3 Positive electrolyte: 0.3 mol lithium ferrocyanide, 0.5 mol potassium sulfate, 0.5 mol potassium sulfate, adjust the pH ≥ 14.0 with lithium hydroxide, and make up to 1 L to prepare a positive electrolyte containing 0.3 M ferrocyanide.

[0027] Negative electrolyte: 0.3 mol ferric nitrate, 0.3 mol N2,N2,N6,N6-tetrakis(2-hydroxyethyl)-2,6-pyridinedicarboxamide, 0.15 mol gluconic acid, 0.5 mol potassium sulfate, 0.5 mol potassium sulfate, adjust the pH ≥14.0 with lithium hydroxide, and make up to 1 L to prepare a negative electrolyte containing 0.3 M iron-N2,N2,N6,N6-tetrakis(2-hydroxyethyl)-2,6-pyridinedicarboxamide.

[0028] Example 4 Positive electrolyte: 0.4 mol ammonium ferrocyanide, 0.5 mol potassium nitrate, 0.5 mol sodium nitrate, adjust the pH to ≥14.0 with potassium hydroxide, and make up to 1 L to prepare a positive electrolyte containing 0.4 M ferrocyanide.

[0029] Anode electrolyte: 0.4 mol iron carbonate, 0.4 mol 2,2′-[2,6-pyridinediylbis[methylene(methylimine)]]bis-1,3-propylene glycol, 0.3 mol glucosanic acid, 0.5 mol potassium nitrate, 0.5 mol sodium nitrate, pH adjusted to ≥ 14.0 with potassium hydroxide, and the volume was adjusted to 1 L to prepare a negative electrode electrolyte containing 0.4 M iron-2,2′-[2,6-pyridinediylbis[methylene(methylimine)]]bis-1,3-propylene glycol.

[0030] Example 5 Positive electrolyte: 0.5 mol ammonium ferrocyanide, 1 mol ammonium chloride, adjust the pH to ≥ 14.0 with ammonia water, and dilute to 1 L to prepare a positive electrolyte containing 0.5 M ferrocyanide.

[0031] Anode electrolyte: 0.5 mol ferric hydroxide, 0.5 mol 2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine, 0.5 mol sorbitol, 1 mol ammonium chloride, adjusted to pH ≥ 14.0 with aqueous ammonia, and the volume was adjusted to 1 L to prepare a negative electrode electrolyte containing 0.5 M iron-2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine.

[0032] Comparative Example 1 Positive electrolyte: 0.5 mol potassium ferrocyanide, 2 mol potassium chloride, adjust the pH to neutral with sodium hydroxide, and dilute to 1 L to prepare a positive electrolyte containing 0.5 M ferrocyanide.

[0033] Anode electrolyte: 0.5 mol ferric chloride, 0.5 mol 2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine, 2 mol potassium chloride, adjust the pH to neutral with sodium hydroxide, and dilute to 1 L to prepare a negative electrode electrolyte containing 0.5 M iron-2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine.

[0034] Comparative Example 2 Positive electrolyte: 0.5 mol sodium ferrocyanide, 1.0 mol potassium chloride, and 1 mol potassium hydroxide were dissolved in distilled water and the volume was adjusted to 1 L to prepare the positive electrolyte.

[0035] Anode electrolyte: 0.5 M Fe-TEA was prepared by adding 0.5 mol ferric chloride, 1.0 mol triethanolamine (TEA), 4 mol KOH, 1 mol potassium chloride, and a certain amount of deoxygenated distilled water to a volume of 1 L to prepare a cathode electrolyte containing 0.5 M Fe-TEA.

[0036] Comparative Example 3 The positive and negative electrolytes were ferrous chloride (total iron concentration: 1.0 M) and hydrochloric acid concentration: 2.0 M.

[0037] 2 Electrochemical performance test of all-iron flow battery 2.1 Cyclic voltammetry curve (CV curve) Experimental conditions: Cyclic voltammetry (CV) curve: three-electrode mode (electrochemical workstation: Shanghai Chenhua Instrument Co., Ltd. CHI660E), working electrode: glassy carbon electrode (diameter 6 mm), counter electrode: platinum sheet electrode (15*15*0.1 mm), reference electrode: Ag / AgCl, scanning voltage range: -0.6 ~ -1.1 V; scanning voltage: 50 mv / s, number of cycles: 200 groups, the electrolyte changes after 200 groups of CV curves of the negative electrode electrolyte in Examples 1 to 5 and Comparative Examples 1 to 3 are shown in Table 1 below, and the 200 groups of CV curves of the negative electrode electrolyte in Example 1 are as follows Figure 2 .

[0038] Table 1 Electrochemical experimental phenomena of negative electrode electrolyte Note: The total iron concentration in Examples 1 to 5 and Comparative Examples 1 to 3 was diluted to 0.1 mol / L with deoxygenated distilled water.

[0039] It can be clearly seen from Table 1 above that after 200 sets of cyclic voltammetry curves (CV curves) of the negative electrode electrolytes in Examples 1 to 5, no iron was generated on the working electrode, no bubbles were generated, and no red flocculent precipitate was present in the electrolyte; however, the negative electrode electrolytes in Comparative Examples 1 to 2 produced iron on the working electrode, accompanied by bubble generation, and the electrolyte became turbid; the negative electrode electrolyte in Comparative Example 3 produced a large amount of iron on the working electrolyte. Figure 2 It can be seen that the CV curves of the 200 groups of negative electrode electrolytes in Example 1 have very good coincidence, which indicates that the negative electrode electrolyte in Example 1 has very good stability.

[0040] In summary, this shows that the negative electrode electrolytes of Examples 1 to 5 have good stability, while the negative electrode electrolytes of Comparative Examples 1 to 3 have poor cycle stability.

[0041] 2.2 All-iron flow battery performance test (1) Assembly of all-iron flow battery The single cell was assembled in the following order: positive electrode aluminum end plate, gold-plated copper plate, graphite current collector, positive electrode 4 cm * 4 cm * 4.35 mm graphite felt (Liaoning Jingu Carbon Materials Co., Ltd.), ion exchange membrane Nafion 212 (soaked in the negative electrode electrolyte for 1 day and then repeatedly washed with distilled water before use), negative electrode 4 cm * 4 cm * 4.35 mm graphite felt (Liaoning Jingu Carbon Materials Co., Ltd.), graphite current collector, gold-plated copper plate and negative electrode aluminum end plate.

[0042] Liquid flow battery: It is composed of the above-mentioned positive electrode electrolyte and negative electrode electrolyte, positive electrode electrolyte tank, negative electrode electrolyte tank, circulation pump, circulation pipeline and single battery circuit connected in series.

[0043] (2) All-iron flow battery test conditions Charge and discharge tester: CT-4008T from Dongguan Xinwei Testing Technology Co., Ltd., charge and discharge mode: constant current charging mode, the volume of the positive and negative electrolytes are both 35 ml, the flow rate of the positive and negative electrolytes is 120 ml / min, nitrogen is introduced before charging and discharging to remove oxygen, current density: 80 mA / cm 2 The temperature was 35°C. The charge and discharge cut-off voltages of Examples 1 to 5 were 1.65 V and 0.5 V, respectively. The charge and discharge cut-off voltages of Comparative Examples 1 to 3 were 0.5 to 1.7 V, respectively. The number of charge and discharge cycles was 200. Table 2 shows the coulombic efficiency, voltage efficiency, and energy efficiency of Examples 1 to 5 and Comparative Examples 1 to 3. Figure 3 The coulombic efficiency of the alkaline flow battery composed of Example 1 is shown. Voltage efficiency, energy efficiency, Figure 4 The figure shows the capacity retention rate of the all-iron flow batteries composed of Example 1 and Comparative Examples 1-3.

[0044] Table 2 All-iron flow battery performance It can be clearly seen from Table 2 above that the coulombic efficiency, voltage efficiency and energy efficiency of Examples 1-5 are significantly higher than those of Comparative Examples 1-3.

[0045] from Figure 4 It can be seen that the capacity retention rate of the all-iron liquid flow battery composed of Example 1 is still higher than 90% after 200 charge and discharge cycles, and the capacity retention rate is relatively high; however, the capacity retention rate of the all-iron liquid flow batteries composed of Comparative Examples 1 to 3 is relatively low after 200 charge and discharge cycles, and the capacity decays rapidly.

[0046] After the experiment was completed, the all-iron flow battery stacks composed of Examples 1-5 and Comparative Examples 1-3 were disassembled. We found that the ion exchange membranes and graphite felts in the flow batteries composed of Examples 1-5 were clean, with no iron hydroxide precipitation or elemental iron formation, and the electrolytes remained clear and transparent. However, the flow batteries composed of Comparative Examples 1-3 had obvious red flocculent precipitation of elemental iron or iron hydroxide on the graphite felts, as well as on the ion exchange membranes, and both the positive and negative electrolytes were turbid. This indicates that the charge and discharge performance of the alkaline all-iron flow batteries composed of Examples 1-5 is significantly better than that of Comparative Examples 1-3, and that the positive and negative electrolytes of the present invention have high charge and discharge activity, few side reactions, low ion permeability, and good cycle stability.

[0047] In summary, the present invention provides an alkaline all-iron flow battery electrolyte, which uses an organic ligand as the negative electrode electrolyte active material. The ligand contains multiple hydrophilic groups (-COOH), two nitrogen atoms, and a pyridine group, which chelates with iron ions to form a stable six-membered ring network structure. Even under high-temperature alkaline conditions, it is difficult to dissociate metallic iron ions, greatly improving the cycle stability of the negative electrode electrolyte. The all-iron flow battery electrolyte of the present invention has a pH ≥ 14.0, a strongly alkaline environment, and together with the alkaline positive electrode electrolyte (such as ferrocyanide), the strongly alkaline all-iron flow battery is essentially free of hydrogen and oxygen evolution, iron hydroxide, and iron dendrites during battery operation, significantly extending the cycle life of the all-iron flow battery and finding wide application in various aqueous flow batteries.

Claims

1. A strong alkaline all-iron flow battery electrolyte, characterized by: The invention comprises a positive electrode electrolyte and a negative electrode electrolyte, wherein the positive electrode electrolyte is a strongly alkaline aqueous solution composed of ferrocyanide and a supporting electrolyte, and the total iron concentration is 0.1 to 0.5 mol / L; the negative electrode electrolyte is a strongly alkaline aqueous solution composed of an iron salt, an organic ligand, a stabilizer, and a supporting electrolyte, and the total iron concentration is 0.1 to 0.5 mol / L, the molar ratio of the organic ligand to iron is 1.0 to 2.0, and the molar ratio of the stabilizer to Fe is 0.25 to 1.0; the concentration of the supporting electrolyte is 0 to 2.0 mol / L, and the pH value is adjusted to pH ≥ 14.0 by an inorganic base.

2. A strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that The ferrocyanide in the positive electrode electrolyte is one or more of sodium ferrocyanide, potassium ferrocyanide, lithium ferrohydride, and ammonium ferrocyanide.

3. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that The iron salt in the negative electrode electrolyte is one or more of ferric chloride, ferric sulfate, ferric nitrate, ferric hydroxide, and ferric carbonate.

4. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that: The organic ligand in the negative electrode electrolyte is one or more of 2,6-bis[N,N-bis(hydroxyethyl)aminomethyl]pyridine, 2,6-bis(aminomethyl)pyridine tetraacetate, N2,N2,N6,N6-tetrakis(2-hydroxyethyl)-2,6-pyridinedicarboxamide, and 2,2′-[2,6-pyridinediylbis[methylene(methylimine)]]bis-1,3-propylene glycol.

5. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that: The stabilizer in the negative electrode electrolyte is one or more of sorbitol, mannitol, gluconic acid, and gluconic acid.

6. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that: The supporting electrolyte is one or more of potassium chloride, sodium chloride, ammonium chloride, sodium sulfate, potassium sulfate, ammonium sulfate, potassium nitrate, and sodium nitrate.

7. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that: The inorganic base is one or more of ammonia water, sodium hydroxide, potassium hydroxide and lithium hydroxide.

8. The strong alkaline all-iron flow battery electrolyte according to claim 1, characterized in that: The pH values ​​of the positive electrode electrolyte and the negative electrode electrolyte are kept consistent and adjusted with an inorganic base, and the pH values ​​are both pH ≥ 14.

0.

9. The electrolyte of a strong alkaline all-iron flow battery according to claims 1 to 8, characterized in that: The positive electrode electrolyte preparation method comprises the following steps: weighing a certain mass of ferrocyanide and a supporting electrolyte, dissolving them in an appropriate amount of distilled water, then adjusting the pH of the solution to ≥ 14.0 with an inorganic base, stirring at 40-60°C for 24 hours, and cooling to room temperature to obtain the positive electrode electrolyte.

10. The electrolyte of a strong alkaline all-iron flow battery according to claims 1 to 8, characterized in that: The negative electrode electrolyte preparation method comprises the following steps: weighing a certain mass of iron salt, organic ligand, stabilizer and supporting electrolyte and dissolving them in an appropriate amount of distilled water, then adjusting the pH of the solution to ≥ 14.0 with alkaline solution, stirring at 40-60°C for 24 hours, and cooling to room temperature to obtain the negative electrode electrolyte.

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