Aqueous iron-based complex electrolyte, preparation method thereof and total iron flow battery

Abstract

The invention discloses an aqueous iron-based complex electrolyte, a preparation method thereof and an all-iron flow battery. The electrolyte is prepared from the following raw materials: iron salt, an organic ligand, a supporting electrolyte and / or an auxiliary electrolyte and water, the organic ligand contains three tertiary amine groups and five hydroxyl functional groups, the organic ligand and iron ions can form a stable saturated coordination structure, the excessive non-coordinated groups are beneficial to maintaining the structural stability of the iron-based complex, generation of elemental iron in the battery circulation process is avoided, and the coulombic efficiency and the circulation stability of the all-iron battery are remarkably improved; and free ligand molecules are more difficult to penetrate through the diaphragm due to larger steric hindrance, so that the problem of mutual channeling is avoided while the stability of the iron-based complex is maintained, and the problem of capacity fading of the total iron flow battery is effectively solved.

Description

Technical Field

[0001] The invention relates to an aqueous iron-based complex electrolyte and a preparation method thereof and an all-iron liquid flow battery, belonging to the technical field of electrochemical energy storage and liquid flow batteries. Background Art

[0002] Among many electrochemical energy storage technologies, flow batteries have two significant advantages over other battery systems: higher safety and environmental friendliness, because their active substances are dissolved in aqueous electrolytes and the energy storage and conversion process generally does not involve solid-phase reactions. They are considered one of the next generation of high-safety, large-scale, long-term energy storage technologies. All-vanadium flow batteries are the most mature flow battery technology currently developed, and the key bottleneck hindering their industrial development is the cost issue. Alkaline all-iron flow batteries have received increasing attention in recent years. First, iron resources are abundant and raw material costs are low. Second, compared with traditional acidic all-iron flow batteries, they avoid iron dendrites and hydrogen evolution side reactions and have greater application prospects.

[0003] Alkaline all-iron flow batteries use aqueous iron-based complexes as the active materials of the positive and negative electrolytes. Since iron ions or ferrous ions have a six-coordinated regular octahedral configuration, the researchers commonly use ferrocyanide (Fe(CN)6 4- ), the research on this complex is relatively mature, so the research focus of alkaline all-iron flow batteries is the negative electrode iron-based complex.

[0004] The researchers first used a water-soluble complex formed by the coordination of tridentate ligand triethanolamine (TEOA) and iron ions (CN103700872A) as the negative electrode iron-based complex. Since the TEOA molecule has fewer coordination groups, it is difficult to form a strong coordination effect with iron ions. During the operation of the battery, the iron-based complex will gradually undergo structural dissociation, resulting in low battery charge and discharge efficiency and rapid capacity decay during the cycle process. Patent CN113764714A discloses an aqueous all-iron liquid flow battery. The negative electrode electrolyte uses bis(2-hydroxyethyl)amino(trihydroxymethyl)methane (BIS-TRIS) or 3-[NN-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) with more coordination groups as iron-based ligands. Although the coordination ability with iron ions is improved to a certain extent, an excess of 50% to 100% of ligand molecules still needs to be added to the electrolyte to maintain the structural stability of the iron-based complex, and the capacity decay problem of the all-iron battery is not fundamentally solved. Literature reports (Chemical Engineering Journal, 2024, 487: 150491) further studies have shown that the current capacity decay of all-iron batteries is mainly due to the excessive free ligands (such as DIPSO and BIS-TRIS) in the negative electrode electrolyte penetrating the membrane to the positive electrode, and reacting chemically with the positive electrode ferrocyanide, resulting in a mismatch between the positive and negative electrode capacities, and thus the overall battery capacity gradually decreases. In summary, the number of coordination groups and the structural size of the iron-based ligand molecule may affect the capacity stability of the all-iron battery. Summary of the invention

[0005] The purpose of the present invention is to provide an aqueous iron-based complex electrolyte, a preparation method thereof and an all-iron liquid flow battery. The organic ligand in the electrolyte adopts N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA) and its derivatives with rich coordination groups and large molecular size, which can not only form a stable saturated hexacoordinate structure with iron ions, but also the excess uncoordinated groups help to maintain the structural stability of the iron-based complex, avoid the formation of iron element during the battery cycle, and significantly improve the coulombic efficiency and cycle stability of the all-iron battery; and the free ligand molecules are difficult to penetrate the diaphragm due to their large steric hindrance, while maintaining the stability of the iron-based complex, avoiding the problem of mutual crosstalk, and effectively solving the capacity attenuation problem of the all-iron liquid flow battery.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: An aqueous iron-based complex electrolyte, wherein the electrolyte comprises iron salt, organic ligand, supporting electrolyte and / or auxiliary electrolyte, and water; The organic ligand contains three tertiary amine groups and five hydroxyl functional groups, and includes one or more of the compounds shown in Formula I or their salts: , The R1-R5 are respectively one of the following groups: hydrogen, methyl, methoxy, ethyl or ethoxy.

[0007] Preferably, the iron salt is one or more of ferric sulfate, ferric chloride, ferric acetate and ferric nitrate.

[0008] Preferably, the organic ligand is N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine.

[0009] Preferably, the supporting electrolyte is one or more of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate and lithium carbonate; The auxiliary electrolyte is one or more of potassium chloride, sodium chloride, ammonium chloride, potassium nitrate, sodium nitrate, potassium sulfate, sodium sulfate and ammonium sulfate.

[0010] Preferably, the concentration of iron salt in the electrolyte is 0.005-2.5 mol / L, preferably in the range of 0.25-2 mol / L; The molar ratio of the organic ligand to the iron in the iron salt is (0.2-4):1, preferably in the range of (0.5-2):1; The concentration of the supporting electrolyte is 1-10 mol / L, preferably in the range of 3-5 mol / L; The concentration of the auxiliary electrolyte is 0-2 mol / L and is not 0, and preferably ranges from 0.2-1 mol / L.

[0011] The method for preparing any of the above-mentioned aqueous iron-based complex electrolytes is to first add an organic ligand to an aqueous solution containing an iron salt, then add a supporting electrolyte and / or an auxiliary electrolyte and control the solution temperature within the range of 0-60°C, stir the reaction, and filter after completion to obtain the electrolyte.

[0012] Preferably, the solution temperature is controlled within the range of 5-15°C after adding the supporting electrolyte; The stirring reaction is carried out at a speed of 300-1000 rpm for 2-48 hours.

[0013] An all-iron liquid flow battery comprises a positive electrode electrolyte, a negative electrode electrolyte, a diaphragm and a current collector; wherein the negative electrode electrolyte is any of the above-mentioned aqueous iron-based complex electrolytes.

[0014] Preferably, the cathode electrolyte is ferrocyanide or ferrocyanide, with a concentration of 0.005-1.5 mol / L; The diaphragm is any one of a sulfonated polyetheretherketone membrane (SPEEK), a perfluorosulfonic acid membrane (Nafion211, Nafion212 or Nafion117), and a porous ion conducting membrane, preferably a SPEEK membrane; The current collector is graphite or carbon felt.

[0015] A method for reducing the transmembrane permeation rate of a ligand in an all-iron liquid flow battery is to use any of the above-mentioned aqueous iron-based complex electrolytes as the negative electrode electrolyte of the all-iron liquid flow battery; The membrane used in the all-iron flow battery is any one of a sulfonated polyetheretherketone membrane (SPEEK), a perfluorosulfonic acid membrane (Nafion211, Nafion212 or Nafion117) and a porous ion conducting membrane; The concentration of the iron salt is 0.25-2 mol / L, the molar ratio of the organic ligand to the iron in the iron salt is (0.5-2):1, the concentration of the supporting electrolyte is 3-5 mol / L, and the concentration of the auxiliary electrolyte is 0.2-1 mol / L.

[0016] The beneficial effects of the present invention are: The organic ligand molecular structure used contains abundant coordination groups (3 tertiary amine groups and 5 hydroxyl functional groups), which can efficiently coordinate with iron ions to form a saturated six-coordinate structure. The excess uncoordinated groups help maintain the structural stability of the iron-based complex, avoid the formation of iron in the battery cycle process, and significantly improve the coulombic efficiency and cycle stability of the all-iron battery. The organic ligand molecules used have excess coordination groups. Under the condition of a certain excess ratio of coordination groups to iron ions, the free ligand content in the electrolyte of the present invention is greatly reduced. At the same time, the ligand molecules are larger in size, and the greater steric hindrance further reduces the ligand permeation rate through the membrane, and the capacity attenuation problem of the all-iron battery related to ligand shuttling is significantly improved. The raw material cost is low, the preparation process is simple, and the feasibility of industrial scale-up is high, making it suitable for large-scale application of all-iron liquid flow batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Comparison of the ligand permeation rate of the electrolyte in Examples 1-4 and Comparative Examples 1-4; Figure 2 The capacity decay curves of the batteries with the electrolytes in Example 1 and Comparative Examples 1-2 are shown. DETAILED DESCRIPTION

[0018] Example 1: A 0.5M Fe(PDETA) complex electrolyte, the preparation method comprising the following steps: (1) Add 0.005 mol of ferric sulfate to deionized water, stir until completely dissolved, then add 0.015 mol of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA), and stir for a while until the solution is mixed evenly; (2) Slowly adding 0.08 mol of potassium hydroxide to the above solution, controlling the temperature of the mixed solution to be below 10° C. by ice bath, and then stirring the mixture to react for 24 hours; (3) The filtrate obtained by filtering the above solution was added with deionized water to make the volume to 20 mL to prepare the electrolyte.

[0019] Example 2: A 0.5M Fe(PDETA) complex electrolyte, prepared by the same method as in Example 1 except that 0.005 mol of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA) is added in step (1).

[0020] Example 3: A 0.5M Fe(PDETA) complex electrolyte, prepared by the same method as in Example 1 except that 0.01 mol of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA) is added in step (1).

[0021] Example 4: A 0.5M Fe(PDETA) complex electrolyte, prepared by the same method as in Example 1 except that 0.02 mol of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA) is added in step (1).

[0022] Example 5: A 0.5M Fe(PDETA) complex electrolyte, prepared in the same manner as Example 1 except that 0.04 mol of potassium hydroxide is added in step (2).

[0023] Example 6: A 0.5M Fe(PDETA) complex electrolyte solution is prepared in the same manner as in Example 1 except that an ice bath is not used to control the solution temperature in step (2).

[0024] Example 7: A 1.0M Fe(PDETA) complex electrolyte is prepared in the same manner as in Example 1, except that 0.01 mol of ferric sulfate is added in step (1), stirred until completely dissolved, and then 0.03 mol of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA) is added.

[0025] Comparative Example 1: A 0.5M Fe(TEA) complex electrolyte, prepared by the same method as in Example 1 except that 0.015 mol of triethanolamine (TEA) was added in step (1) instead of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA).

[0026] Comparative Example 2: A 0.5M Fe(TEA) complex electrolyte, prepared by the same method as in Example 1 except that 0.02 mol of triethanolamine (TEA) was added in step (1) instead of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA).

[0027] Comparative Example 3: A 0.5M Fe(TEA) complex electrolyte, prepared by the same method as in Example 1 except that 0.03 mol of triethanolamine (TEA) was added in step (1) instead of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA).

[0028] Comparative Example 4: A 0.5 M Fe(BISTRIS) complex electrolyte, prepared by the same method as in Example 1 except that 0.015 mol of bis(2-hydroxyethyl)amino(trihydroxymethyl)methane (BISTRIS) was added in step (1) instead of N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine (PDETA).

[0029] The permeation rate of the ligand in the electrolyte through the membrane was tested using an H-type diffusion cell. The left side of the diffusion cell contained 30 mL of 0.5M iron-based complex electrolyte, the right side contained 30 mL of 5M KOH solution, and the middle was blocked by a SPEEK membrane. Samples were taken from the right side of the cell at intervals for UV absorption peak testing, and the sample concentration was determined by drawing a standard curve for the corresponding ligand.

[0030] Figure 1 The permeation concentration change curves of the ligands in the electrolytes of Examples 1-4 and Comparative Examples 1-4 are shown, where the slope k value characterizes the size of the permeation rate, which is summarized in Table 1. Comparison of Examples 1-4 shows that for Fe (PDETA) electrolyte, the permeation rate value of the PDETA ligand through the membrane is small, and as the ligand concentration increases, the change trend of its permeation rate value through the membrane is not obvious; while comparing Comparative Examples 1-3 shows that for Fe (TEA) electrolyte, the permeation rate value of the TEA ligand through the membrane is nearly 30 times higher than that of the PDETA ligand, and its permeation rate value through the membrane shows an obvious linear increase trend with the concentration, which is consistent with the relationship in the traditional permeation model that the permeation rate is proportional to the concentration difference, and the permeation process of the PDETA ligand molecule is significantly hindered due to the strong coordination effect of multiple groups and the spatial size effect, which does not conform to the traditional permeation model; comprehensive comparison shows that the permeation rate of the PDETA ligand through the membrane is significantly reduced compared with both TEA and BISTRIS ligands.

[0031] Table 1 Permeation concentration variation curve of ligand in electrolyte of Examples 1-4 and Comparative Examples 1-4 Electrolyte Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Permeation rate k value 0.06 0.05 0.05 0.07 1.47 2.03 2.98 0.51 The electrolytes (10 mL) of Examples 1-7 and Comparative Examples 1-4 were used as the negative electrode, 0.5 mol / L Na4Fe(CN)6 (20 mL) electrolyte was used as the positive electrode, and sulfonated polyetheretherketone (SPEEK) membrane was used as the diaphragm to assemble the all-iron liquid flow battery. 2 The charge and discharge tests were carried out at different current densities, with cut-off voltages of 1.75V and 0.5V respectively.

[0032] Table 2 summarizes the battery performance of Examples 1-7 and Comparative Examples 1-4 electrolytes within 100 charge and discharge cycles. Comparative Examples 1-4 show that as the PDETA ligand concentration increases, the battery's first discharge capacity gradually increases, indicating that the utilization rate of iron ions is gradually improved, and when the ligand to iron ion concentration ratio is 1.5 (Example 1), the battery's comprehensive performance is optimal; when the ligand concentration ratio is 0.5 (Example 2), the battery's capacity retention rate decreases slightly, indicating that the ligand concentration in the electrolyte is insufficient to completely complex the iron ions, and some free iron ions penetrate the membrane to cause the negative electrode capacity to decay; when the ligand concentration ratio is higher than 2 (Example 4), the battery's energy efficiency is significantly reduced, which is due to the increase in electrolyte viscosity, which leads to an increase in battery polarization, but the battery capacity retention rate does not change significantly compared to Example 1, indicating that free ligand molecules hardly penetrate the membrane. Therefore, the appropriate proportion of ligands in the electrolyte plays a key role in the structural stability and battery efficiency of the iron-based complex.

[0033] However, even if the concentration of TEA ligand is increased (Comparative Examples 1-3), the capacity retention rate of the battery does not show a significant improvement. This is mainly because the low concentration of TEA ligand is not enough to complex iron ions, and the unstable structure of the iron-based complex leads to the attenuation of the negative electrode capacity, while the high concentration of TEA ligand has a too fast penetration rate through the membrane, and reacts chemically with the positive electrode active material to cause the attenuation of the positive electrode capacity. It can be seen that the optimal concentration of the ligand depends on two aspects: one is the complexing ability of the ligand to iron ions, and the appropriate excess ligand ratio can prevent the dissociation of the iron-based complex and inhibit the penetration of iron ions through the membrane, and the second is the penetration rate of the ligand molecule itself. Compared with Comparative Examples 1 and 4, when the ligand concentration is the same, the first discharge capacity of Example 1 is higher, indicating that the coordination efficiency of PDETA with iron ions is higher than that of TEA and BISTRIS, so the utilization rate of active materials in Fe (PDETA) electrolyte is higher, and the battery has higher coulomb efficiency and capacity retention rate, which indicates that the Fe (PDETA) complex is more stable in structure during the battery cycle and the penetration rate of PDETA ligand molecules through the membrane is lower.

[0034] In addition, the comparison results of Examples 1 and 5 show that a low supporting electrolyte concentration will lead to a decrease in battery energy efficiency and will also affect the cycle stability of the complex structure; the comparison results of Examples 1 and 6 show that the solution temperature rises sharply due to the addition of a strong base during the electrolyte preparation process, which makes it easier for iron ions to precipitate in the form of ferric hydroxide, and the actual active complex concentration in the electrolyte decreases, thereby significantly reducing the battery discharge capacity, and also affecting the cycle stability of the electrolyte, so it is necessary to control the solution temperature during the electrolyte preparation process; the results of Examples 1 and 7 show that an overall increase in the concentration of the Fe(PDETA) complex can achieve an exponential increase in the battery discharge capacity, and at the same time the battery efficiency will not show a significant decrease, which indicates that the reaction activity of the active substance in the high-concentration complex electrolyte remains good.

[0035] Table 2 Battery performance of electrolytes of Examples 1-7 and Comparative Examples 1-4 within 100 charge-discharge cycles Electrolyte First discharge capacity (Ah / L) Coulomb efficiency (%) Energy efficiency (%) Capacity retention rate (%) Example 1 11.46 99.55 85.10 99.98 Example 2 8.32 98.10 86.12 80.35 Example 3 11.21 99.22 85.77 92.08 Example 4 11.68 99.39 81.23 99.46 Example 5 12.53 99.04 82.15 95.23 Example 6 9.36 99.33 84.03 99.52 Example 7 24.08 99.80 84.40 99.55 Comparative Example 1 8.78 96.83 87.30 14.10 Comparative Example 2 9.11 98.20 86.75 20.23 Comparative Example 3 9.12 98.59 82.10 31.80 Comparative Example 4 9.05 99.15 85.26 93.49 Figure 2 The battery capacity decay curves of the electrolytes in Example 1 and Comparative Examples 1 and 4 are shown. The capacity decay of Comparative Example 1 exceeds 50% after 50 cycles, and Comparative Example 4 begins to show rapid capacity decay after 100 cycles. In contrast, Example 1 shows excellent cycle stability, and the capacity retention rate does not show obvious decay after 1400 cycles of stable operation (fluctuations caused by changes in ambient temperature). It can be seen that the Fe(TEA) electrolyte exhibits rapid capacity decay at the beginning of the battery cycle due to the unstable complex structure, while the Fe(BISTRIS) electrolyte has a similar coordination structure, and the capacity decay is slowed down, but the same decay trend will still appear after a certain period of circulation, and the high stability saturated coordination structure formed by Fe(PDETA) in the electrolyte of Example 1 enables the electrolyte to always maintain good capacity stability during long-term operation.

[0036] The above is only a preferred implementation of the patent of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the patent of the present invention. These improvements and modifications should also be regarded as the scope of protection of the patent of the present invention.

Claims

1. An aqueous iron-based complex electrolyte, characterized in that: The electrolyte comprises iron salt, organic ligand, supporting electrolyte and / or auxiliary electrolyte, and water; The organic ligand contains three tertiary amine groups and five hydroxyl functional groups, and includes one or more of the compounds shown in Formula I or their salts: ， The R1-R5 are respectively one of the following groups: hydrogen, methyl, methoxy, ethyl or ethoxy.

2. The aqueous iron-based complex electrolyte according to claim 1, characterized in that: The iron salt is one or more of ferric sulfate, ferric chloride, ferric acetate and ferric nitrate.

3. The aqueous iron-based complex electrolyte according to claim 1, characterized in that: The organic ligand is N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine.

4. The aqueous iron-based complex electrolyte according to claim 1, characterized in that: The supporting electrolyte is one or more of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate and lithium carbonate; The auxiliary electrolyte is one or more of potassium chloride, sodium chloride, ammonium chloride, potassium nitrate, sodium nitrate, potassium sulfate, sodium sulfate and ammonium sulfate.

5. The aqueous iron-based complex electrolyte according to claim 1, characterized in that: In the electrolyte, the concentration of iron salt is 0.005-2.5 mol / L; The molar ratio of the organic ligand to the iron in the iron salt is (0.2-4):1; The concentration of supporting electrolyte is 1-10 mol / L; The concentration of auxiliary electrolyte is 0-2 mol / L.

6. The method for preparing the aqueous iron-based complex electrolyte according to any one of claims 1 to 5, characterized in that: The method comprises first adding the organic ligand into an aqueous solution containing dissolved iron salt, then adding a supporting electrolyte and / or an auxiliary electrolyte and controlling the solution temperature within the range of 0-60°C, stirring the reaction, and filtering after the reaction is completed to obtain the electrolyte.

7. The method for preparing an aqueous iron-based complex electrolyte according to claim 6, characterized in that: After adding the supporting electrolyte, the solution temperature was controlled within the range of 5-15°C; The stirring reaction is carried out at a speed of 300-1000 rpm for 2-48 hours.

8. An all-iron liquid flow battery, characterized in that: It comprises a positive electrode electrolyte, a negative electrode electrolyte, a separator and a current collector; wherein the negative electrode electrolyte is the aqueous iron-based complex electrolyte described in any one of claims 1-5.

9. The all-iron liquid flow battery according to claim 8, characterized in that: The positive electrode electrolyte is ferrocyanide or ferrocyanide, and the concentration is 0.005-1.5 mol / L; The diaphragm is any one of a sulfonated polyetheretherketone membrane, a perfluorosulfonic acid membrane and a porous ion conducting membrane; The current collector is graphite or carbon felt.

10. A method for reducing the transmembrane permeation rate of a ligand in an all-iron liquid flow battery, characterized in that: The aqueous iron-based complex electrolyte described in any one of claims 1 to 5 is used as the negative electrode electrolyte of the all-iron liquid flow battery; Wherein, the membrane for the all-iron liquid flow battery is any one of a sulfonated polyetheretherketone membrane, a perfluorosulfonic acid membrane and a porous ion conducting membrane; The concentration of the iron salt is 0.25-2 mol / L, the molar ratio of the organic ligand to the iron in the iron salt is (0.5-2):1, and the concentration of the supporting electrolyte is 3-5 mol / L.

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