Bipyridine iron complex and its preparation method, electrolyte

Abstract

This invention specifically discloses a bipyridine iron complex, its preparation method, and an electrolyte. The general structural formula of the bipyridine iron complex is shown in Formula I: (Formula I); in Formula I, R1~R6 may be the same or different, and each is independently selected from electron-withdrawing groups, electron-donating groups, or sterically hindered groups. The bipyridine iron complex disclosed in this invention promotes the redistribution of charge between the Fe center and the ligand by introducing electron-withdrawing or electron-donating groups, or by introducing sterically hindered groups to inhibit intermolecular aggregation through steric hindrance effects, thereby optimizing the electronic state distribution, enhancing the contribution of local excited states, suppressing the excessive occupation of charge-transfer states, and thus improving molecular stability. Furthermore, when this bipyridine iron complex is used as an active material in a flow battery, it enables the flow battery to maintain a high capacity retention rate during long-term cycling.

Description

Technical Field

[0001] This invention belongs to the field of flow battery technology, specifically relating to a bipyridine iron complex, its preparation method, and an electrolyte. Background Technology

[0002] With the rapid development of renewable energy, large-scale energy storage technology has become crucial for the stable operation of power grids. Flow batteries, due to their advantages such as independently designable power and capacity, high safety, and long lifespan, are considered one of the ideal large-scale energy storage solutions. Among flow battery systems, organic flow batteries use organic molecules as active materials, offering advantages such as designable molecular structures, abundant resources, and environmental friendliness, making them a research hotspot.

[0003] Bipyridine iron complexes are an important class of organometallic compounds, widely studied for their potential as active materials in positive electrode electrolytes of flow batteries due to their high solubility, suitable redox potential, and good initial electrochemical reversibility. However, traditional bipyridine iron complexes are prone to molecular structural instability during long-term cycling, leading to capacity decay, which limits their feasibility in practical applications. Summary of the Invention

[0004] This invention provides a bipyridine iron complex, its preparation method, and an electrolyte. The bipyridine iron complex optimizes the electronic state distribution by introducing electron-withdrawing, electron-donating, or sterically hindered groups, enhances the contribution of local excited states, and suppresses the excessive occupation of charge-transfer states, thereby improving molecular stability.

[0005] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows: The first aspect of this invention provides a bipyridine iron complex, the general structural formula of which is shown in Formula I: (Formula I); In Formula I, R1 to R6 may be the same or different, and each is independently selected from electron-withdrawing groups, electron-donating groups or steric hindrance groups.

[0006] As can be seen from the above technical solutions, the bipyridine iron complex provided by the first aspect of the present invention promotes the redistribution of charge between the Fe center and the ligand by introducing electron-withdrawing or electron-donating groups, or by introducing steric hindrance groups to inhibit intermolecular aggregation by utilizing steric hindrance effects, thereby promoting the rational distribution of charge between the Fe center and the ligand, thus optimizing the electronic state distribution, enhancing the contribution of local excited states, inhibiting the excessive occupation of charge transfer states, and thus improving molecular stability.

[0007] A second aspect of the present invention provides a method for preparing a bipyridine iron complex, comprising the following steps: Substituent-containing bipyridine is added to methanol, and after stirring, a methanol solution of substituent-containing bipyridine is obtained. An aqueous solution of FeCl2·4H2O was added dropwise to a methanol solution of the substituent-containing bipyridine, and the mixture was stirred, crystallized, filtered, and dried to obtain a bipyridine iron complex.

[0008] As can be seen from the above technical solutions, the preparation method of the bipyridine iron complex provided by the second aspect of the present invention can prepare a bipyridine iron complex with high purity and stable structure by causing a substituent-containing bipyridine to undergo a coordination reaction with FeCl2·4H2O. In addition, the preparation method is simple to operate, has mild reaction conditions, and controllable cost, which is conducive to realizing large-scale production.

[0009] A third aspect of the present invention provides an electrolyte comprising the bipyridine iron complex of the above embodiments or the bipyridine iron complex prepared by the preparation method of the above embodiments.

[0010] As can be seen from the above technical solutions, the electrolyte provided in the third aspect of the present invention uses the bipyridine iron complex of the above embodiments or the bipyridine iron complex prepared by the preparation method of the above embodiments as the active material. When the electrolyte is applied to a flow battery, the flow battery can maintain a high capacity retention rate during long-term cycling. Attached Figure Description

[0011] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1 This is a schematic flowchart of a method for preparing bipyridine iron complexes provided in some embodiments of the present invention; Figure 2 This is the proton nuclear magnetic resonance spectrum provided in Embodiment 1 of the present invention; Figure 3 This is the hydrogen nuclear magnetic resonance spectrum provided in Embodiment 2 of the present invention; Figure 4 This is the charge transfer diagram provided in Embodiment 2 of the present invention; Figure 5 This is the ultraviolet-visible absorption spectrum provided in Embodiment 2 of the present invention; Figure 6 This is a cyclic voltammogram of the flow battery provided in Embodiment 2 of the present invention; Figure 7 This is a cycle stability curve provided by the flow battery of Embodiment 2 of the present invention; Figure 8 This is a cycle stability curve provided by the flow battery composed of Comparative Example 1. Detailed Implementation

[0013] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] The flowchart shown in the attached diagram is for illustrative purposes only and does not necessarily include all content and operations / steps, nor does it necessarily have to be performed in the order described. For example, some operations / steps can be broken down, combined, or partially merged, so the actual execution order may change depending on the actual situation.

[0015] It should be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0016] It should also be understood that the term “and / or” as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0017] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0018] This invention provides a bipyridine iron complex, the general structural formula of which is shown in Formula I: (Formula I); In Formula I, R1 to R6 may be the same or different, and each is independently selected from electron-withdrawing groups, electron-donating groups or steric hindrance groups.

[0019] In this invention, electron-withdrawing groups, by pulling electrons from the Fe center, promote the redistribution of charge between the Fe center and the ligands, thereby optimizing the electronic state distribution, enhancing the contribution of local excited states, and suppressing the over-occupation of charge-transfer states, thus improving the molecular stability of the bipyridine iron complex. Similarly, electron-donating groups, by pushing electrons towards the Fe center, promote the redistribution of charge between the Fe center and the ligands, optimizing the electronic state distribution, enhancing the contribution of local excited states, and suppressing the over-occupation of charge-transfer states, thus improving the molecular stability of the bipyridine iron complex. Steric hindrance groups, through steric hindrance effects, reduce intermolecular aggregation, avoid electronic state disorder caused by aggregation, promote the rational distribution of charge between the Fe center and the ligands, thereby enhancing the contribution of local excited states, suppressing the over-occupation of charge-transfer states, and thus improving the molecular stability of the bipyridine iron complex.

[0020] In some embodiments, the electron-withdrawing group is selected from any one of sulfonic acid group, quaternary ammonium group, methylene quaternary ammonium group, nitro group, cyano group, and trifluoromethyl group.

[0021] All of the electron-withdrawing groups mentioned above can pull electrons from the Fe center, causing the charge to redistribute between the Fe center and the ligand, thereby optimizing the electronic state distribution, enhancing the contribution of local excited states, suppressing the excessive occupation of charge-transfer states, and thus improving the molecular stability of bipyridine iron complexes.

[0022] In some embodiments, the electron-donating group is selected from any one of methyl, methoxy, ethyl, benzyl, and amino. These electron-donating groups can all push electrons toward the Fe center, thereby optimizing the electronic state distribution, enhancing the contribution of local excited states, suppressing the over-occupation of charge-transfer states, and thus improving the molecular stability of the bipyridine iron complex.

[0023] In some embodiments, the steric hindrance group is selected from any one of tert-butyl, isopropyl, phenyl, adamantyl, and triphenylphosphine. These steric hindrance groups can reduce intermolecular aggregation through steric hindrance effects, avoid electronic state disorder caused by aggregation, promote the rational distribution of charge between the Fe center and the ligand, thereby enhancing the contribution of local excited states, inhibiting the excessive occupation of charge-transfer states, and thus improving the molecular stability of the bipyridine iron complex.

[0024] In some embodiments, the bipyridine iron complex is selected from any of the compounds shown in the following structural formulas: , , ; R1 to R6 are identical to each other and are selected from electron-withdrawing groups.

[0025] The aforementioned electron-withdrawing groups can be located at the meta or para position of each pyridine ring in the bipyridine iron complex. Each pyridine ring has an electron-withdrawing group attached to the same position, and the types of electron-withdrawing groups are the same. In other words, the electron-withdrawing groups can be uniformly located at the meta position or the para position of each pyridine ring. This makes the electronic state distribution of the entire molecule more uniform, further promoting the rational distribution of charge between the Fe center and the ligand, and improving the molecular stability.

[0026] In some embodiments, the bipyridine iron complex is selected from any of the compounds shown in the following structural formulas: , .

[0027] In this embodiment, the first compound shown in the structural formula is a tris(4,4'-bis(quaternary ammonium)-2,2'-bipyridine) iron complex, and the second compound shown in the structural formula is a tris(4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine) iron complex. Both compounds utilize an electron-withdrawing group attached to the para position of each pyridine ring. This group draws electrons from the Fe center, promoting a redistribution of charge between the Fe center and the ligand, thereby optimizing the electronic state distribution, enhancing the contribution of locally excited states, suppressing excessive occupation of charge-transfer states, and ultimately improving molecular stability.

[0028] As can be seen from the above technical solutions, the bipyridine iron complex provided by the present invention promotes the redistribution of charge between the Fe center and the ligand by introducing electron-withdrawing or electron-donating groups, or by introducing steric hindrance groups to inhibit intermolecular aggregation by utilizing steric hindrance effects, thereby promoting the rational distribution of charge between the Fe center and the ligand, thus optimizing the electronic state distribution, enhancing the contribution of local excited states, inhibiting the excessive occupation of charge-transfer states, and thus improving molecular stability.

[0029] Please see Figure 1 The present invention provides a method for preparing a bipyridine iron complex, specifically including steps S101 to S102.

[0030] Step S101: Add the substituent-containing bipyridine to methanol and stir to obtain a methanol solution of the substituent-containing bipyridine.

[0031] This step utilizes the good solubility of methanol in substituted bipyridines. By stirring, the substituted bipyridines are uniformly dispersed in the solvent, providing a uniform reaction environment for the subsequent coordination reaction with Fe and avoiding side reactions caused by excessively high local concentrations of ligands.

[0032] In some embodiments, the substituent-containing bipyridine is selected from any one of bipyridine containing a quaternary ammonium group, bipyridine containing a methylene quaternary ammonium group, bipyridine containing a methyl group, bipyridine containing an amino group, and bipyridine containing a tert-butyl group.

[0033] Among them, quaternary ammonium and methylene quaternary ammonium groups are electron-withdrawing groups, methyl and amino groups are electron-donating groups, and tert-butyl is a sterically hindered group. By selecting bipyridines substituted with electron-withdrawing or electron-donating groups, the redistribution of charge between the Fe center and the ligand can be promoted, thereby optimizing the electronic state distribution, enhancing the contribution of local excited states, inhibiting the excessive occupation of charge transfer states, and thus improving the molecular stability of bipyridine iron complexes. Alternatively, by selecting bipyridines substituted with sterically hindered groups, the steric hindrance effect can be used to reduce intermolecular aggregation, avoid electronic state disorder caused by aggregation, promote the rational distribution of charge between the Fe center and the ligand, thereby enhancing the contribution of local excited states, inhibiting the excessive occupation of charge transfer states, and thus improving the molecular stability of bipyridine iron complexes.

[0034] In some embodiments, the substituent-containing bipyridine is selected from any one of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine, 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 4,4'-diamino-2,2'-bipyridine, and 4,4'-di-tert-butyl-2,2'-bipyridine. In these substituent-containing bipyridines, the substituents are all located at the para position of each pyridine ring, resulting in a more uniform electronic state distribution throughout the molecule, promoting a rational distribution of charge between the Fe center and the ligand, and enhancing the molecular stability of the bipyridine iron complex.

[0035] In some embodiments, the synthetic steps of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine include: reacting 4,4'-dibromo-2,2'-bipyridine with an aqueous solution of Me3N in dichloromethane, and lyophilizing the resulting solution to obtain 4,4'-bis(quaternary ammonium)-2,2'-bipyridine. By replacing the bromine atom at the 4,4' position in the 4,4'-dibromo-2,2'-bipyridine molecule with a quaternary ammonium group provided by Me3N, a quaternary ammonium group can be introduced at the 4,4' position. This quaternary ammonium group is an electron-withdrawing group, which improves the molecular stability of the bipyridine iron complex by promoting the redistribution of charge between the Fe center and the ligand, optimizing the electronic state distribution, enhancing the contribution of local excited states, and suppressing the over-occupation of charge-transfer states.

[0036] In some embodiments, the synthetic steps of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine include: reacting 4,4'-dibromomethyl-2,2'-bipyridine with an aqueous solution of Me3N in dichloromethane, and lyophilizing the resulting solution to obtain 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine. By replacing the bromine atom on the bromomethyl group at the 4,4' position of the 4,4'-dibromomethyl-2,2'-bipyridine molecule with a quaternary ammonium group provided by Me3N, a methylene quaternary ammonium group can be introduced at the 4,4' position. This methylene quaternary ammonium group is an electron-withdrawing group, which improves the molecular stability of the bipyridine iron complex by promoting the redistribution of charge between the Fe center and the ligand, optimizing the electronic state distribution, enhancing the contribution of local excited states, and suppressing the over-occupation of charge-transfer states.

[0037] In some embodiments, the mass ratio of the substituent-containing bipyridine to methanol is 1:(8~12), which ensures complete dissolution of the substituent-containing bipyridine while maintaining a suitable concentration of the ligand in the reaction system. If the mass ratio is greater than 1:8, the coordination reaction between the ligand and Fe will be uneven, leading to the formation of byproducts; if the mass ratio is less than 1:12, the content of the substituent-containing bipyridine will be too low, affecting the yield of the bipyridine-iron complex. For example, the mass ratio of the substituent-containing bipyridine to methanol can be any typical but non-limiting value or a range between any two values, such as 1:12, 1:11.5, 1:11, 1:10.3, 1:10, 1:9.7, 1:9, 1:8.8, 1:8.2, 1:8.

[0038] Step S102: Add an aqueous solution of FeCl2·4H2O dropwise to a methanol solution of bipyridine containing a substituent, stir, crystallize, filter, and dry to obtain the bipyridine iron complex.

[0039] First, 0.4 mmol to 0.6 mmol of FeCl₂·4H₂O was added to 4 mL to 6 mL of deionized water to prepare an aqueous solution of FeCl₂·4H₂O. Then, the aqueous solution of FeCl₂·4H₂O was slowly added dropwise to a methanol solution of bipyridine containing a substituent, and the mixture was stirred with nitrogen for 1 to 3 hours. After the reaction was complete, a red solution was obtained. Next, the red solution was dried under vacuum to remove some of the methanol solvent, and 4 mL to 6 mL of deionized water was added to promote crystallization. After filtration, a crude red product was obtained. Finally, the crude red product was dried under vacuum to obtain a red powder, which is the bipyridine iron complex.

[0040] In some embodiments, the molar ratio of the substituent-containing bipyridine to FeCl2·4H2O is (2~5):1, which ensures the interaction between the ligand and Fe²⁺. +Sufficient coordination leads to the formation of a bipyridine-iron complex. If the molar ratio is greater than 5:1, a large number of uncoordinated ligands will remain in the reaction system, increasing the purity of the subsequent product; if the molar ratio is less than 2:1, it will result in Fe²⁺… + The inability to achieve complete coordination reduces the yield of the bipyridine-iron complex. For example, the molar ratio of the substituent-containing bipyridine to FeCl2·4H2O can be any typical but non-limiting value, such as 2:1, 2.2:1, 2.7:1, 3.0:1, 3.5:1, 3.8:1, 4:1, 4.6:1, 5:1, or any range between any two values.

[0041] As can be seen from the above technical solution, the preparation method of the bipyridine iron complex provided by the present invention can prepare a high-purity and structurally stable bipyridine iron complex by causing a coordination reaction between a substituent-containing bipyridine and FeCl2·4H2O. In addition, the preparation method is simple to operate, has mild reaction conditions, and controllable cost, which is conducive to realizing large-scale production.

[0042] This invention provides an electrolyte comprising the bipyridine iron complex of the above embodiments or the bipyridine iron complex prepared by the preparation method of the above embodiments.

[0043] For example, the flow battery includes a battery stack, a positive electrode reservoir, and a negative electrode reservoir; wherein the positive electrode reservoir is the electrolyte provided in the embodiments of the present invention, and the electrolyte includes the bipyridine iron complex of the above embodiments or the bipyridine iron complex prepared by the preparation method of the above embodiments.

[0044] In some embodiments, the molar concentration of the bipyridine iron complex in the electrolyte is 0.05 mol / L to 1.0 mol / L, which provides sufficient active material and ensures the electrochemical performance of the battery. If the molar concentration of the bipyridine iron complex is greater than 1.0 mol / L, it will increase the migration resistance of ions in the electrolyte, exacerbate polarization during battery charging and discharging, and reduce the battery's voltage efficiency and energy efficiency. If the molar concentration of the bipyridine iron complex is less than 0.05 mol / L, the content of active material in the electrolyte is insufficient, leading to a decrease in battery energy density. For example, the molar concentration of the bipyridine iron complex can be any typical but non-limiting value or an interval between any two values, such as 0.05 mol / L, 0.07 mol / L, 0.08 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.35 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 0.96 mol / L, 1.0 mol / L.

[0045] As can be seen from the above technical solutions, the electrolyte provided by the present invention uses the bipyridine iron complex of the above embodiments or the bipyridine iron complex prepared by the above embodiment as the active material. When the electrolyte is applied to a flow battery, the flow battery can maintain a high capacity retention rate during long-cycle operation.

[0046] The following detailed description of the bipyridine iron complex, its preparation method, and electrolyte provided by the present invention is based on specific embodiments and experimental data.

[0047] Example 1 This embodiment provides a bipyridine iron complex, the structural formula of which is: It is a tri(4,4'-bis(quaternary ammonium)-2,2'-bipyridine) iron complex.

[0048] The preparation method of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine includes the following steps: 0.471 g (1.5 mmol) of 4,4'-dibromo-2,2'-bispyridine is added to 20 mL of a 50% (w / w) aqueous solution of Me3N and reacted in dichloromethane for 24 h. The resulting solution is then freeze-dried to obtain 4,4'-bis(quaternary ammonium)-2,2'-bipyridine with a mass of 0.32 g, a molar amount of 1.185 mmol, and a yield of 79%.

[0049] Please see Figure 1 The preparation method of the bipyridine iron complex includes the following steps: In a 25 mL round-bottom flask, 0.408 g (1.5 mmol) of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine was added to 5 mL of methanol. After stirring, a methanol solution of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine was obtained. 0.099 g (0.5 mmol) of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine was then added to 5 mL of methanol. FeCl₂·4H₂O (mmol) was added to 5 mL of deionized water to prepare an aqueous solution of FeCl₂·4H₂O. The aqueous solution of FeCl₂·4H₂O was slowly added dropwise to a methanol solution of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine, and stirred with nitrogen for 2 h. After the reaction was complete, a red solution was obtained. The red solution was then dried under vacuum to remove some of the methanol solvent, and 5 mL of deionized water was added to promote crystallization. The solution was filtered to obtain a crude red product. Finally, the crude red product was dried under vacuum to obtain a red powder, which is the tri(4,4'-bis(quaternary ammonium)-2,2'-bipyridine) iron complex with a mass of 0.27 g, a molar amount of 0.31 mmol, and a yield of 66.1%.

[0050] The 1H NMR spectrum of the tri(4,4'-bis(quaternary ammonium)-2,2'-bipyridine) iron complex is shown below. Figure 2As shown, the spectral analysis is as follows: the proton characteristic peak of the quaternary ammonium group appears at a chemical shift of δ3.56 ppm, and the proton characteristic peak of the aromatic hydrogen on the bipyridine skeleton appears at chemical shifts of δ7.84 ppm, δ8.51 ppm, and δ8.63 ppm. This proves that the tris(4,4-bis(quaternary ammonium)-2,2'-bipyridine) iron complex was successfully synthesized, and its molecular structure is correct.

[0051] Example 2 This embodiment provides a bipyridine iron complex, the structural formula of which is: It is a tri(4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine) iron complex.

[0052] The preparation method of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine includes the following steps: 0.513 g (1.5 mmol) of 4,4'-dibromo-2,2'-bispyridine is added to 20 mL of a 50% (w / w) aqueous solution of Me3N and reacted in dichloromethane for 24 h. The resulting solution is then freeze-dried to obtain 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine with a mass of 0.46 g, a molar amount of 0.425 mmol, and a yield of 89.7%.

[0053] Please see Figure 1 The preparation method of the bipyridine iron complex includes the following steps: In a 25 mL round-bottom flask, 0.45 g (1.5 mmol) of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine was added to 5 mL of methanol. After stirring, a methanol solution of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine was obtained. 0.099 g (0.5 mmol) of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine was then added to 5 mL of methanol. FeCl₂·4H₂O (mmol) was added to 5 mL of deionized water to prepare an aqueous solution of FeCl₂·4H₂O. The aqueous solution of FeCl₂·4H₂O was slowly added dropwise to a methanol solution of 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine, and stirred with nitrogen for 2 h. After the reaction was complete, a red solution was obtained. The red solution was then dried under vacuum to remove some of the methanol solvent, and 5 mL of deionized water was added to promote crystallization. The solution was filtered to obtain a crude red product. Finally, the crude red product was dried under vacuum to obtain a red powder, which is the tris(4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine) iron complex with a mass of 0.323 g, a molar amount of 0.338 mmol, and a yield of 71.8%.

[0054] The 1H NMR spectrum of the tri(4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine) iron complex is shown below. Figure 3As shown, the spectral analysis is as follows: the proton characteristic peak of the quaternary ammonium group appears at a chemical shift of δ3.11 ppm, the proton characteristic peak of the methylene group appears at a chemical shift of δ4.68 ppm, and the proton characteristic peak of the aromatic hydrogen on the bipyridine skeleton appears between chemical shifts of δ7.65 ppm and 8.95 ppm. This proves that the tris(4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine) iron complex was successfully synthesized, and its molecular structure is correct.

[0055] Comparative Example 1 This comparative example provides a bis(2,2'-bipyridine)ferric iron, the structural formula of which is: .

[0056] Test data results like Figure 4 As shown, by analyzing the electronic state distribution of the Fe center (horizontal axis 1) and the three ligands (horizontal axis 2-4) in Example 2, it can be seen that the local excited (LE) state signal (corresponding to blue) is significantly enhanced in the hole, electron and hole-electron overlap regions, while the charge transfer (CT) state signal (corresponding to red) is suppressed.

[0057] like Figure 5 As shown, this figure is the UV-Vis absorption spectrum of Example 2, where the horizontal axis represents wavelength (unit: nm) and the vertical axis represents absorbance; from Figure 5 It can be seen that the position and intensity of the characteristic absorption peaks of Example 2 did not change significantly before electrochemical cycling (black curve) and after cycling (red curve), which indicates that the molecular structure of Example 2 has good stability during electrochemical cycling.

[0058] like Figure 6 As shown, this figure is a cyclic voltammogram of the flow battery constructed in Example 2, where the horizontal axis represents potential (unit: V) and the vertical axis represents current (unit: mA); from Figure 6 It can be seen that the oxidation peak (red) and reduction peak (blue) are symmetrical and the potential difference is 1.71V, which indicates that the flow battery has rapid electron transfer kinetics and good redox reversibility.

[0059] like Figure 7 As shown, this figure is a cycle stability curve of the flow battery composed in Example 2, where the horizontal axis represents the number of cycles (unit: times) and the vertical axis represents the capacity (unit: mAh); from Figure 7It can be seen that the flow battery maintains a coulombic efficiency of around 100% over 1400 cycles, while the charge capacity and discharge capacity retention rates remain stable at around 90%, indicating that the flow battery has good cycle stability.

[0060] like Figure 8 As shown in the figure, this is a cycle stability curve of the flow battery composed of Comparative Example 1, where the horizontal axis represents the number of cycles (unit: times) and the vertical axis represents the capacity (unit: mAh); from Figure 8 It can be seen that after 200 cycles, the charging capacity and discharging capacity of the flow battery both show a significant downward trend. This indicates that the flow battery of Comparative Example 1 has poor cycle stability and a fast capacity decay rate, making it difficult to meet the requirements of long-term cycle use.

[0061] In summary, the bipyridine iron complex provided in this embodiment of the invention can promote the redistribution of charge between the Fe center and the ligand, optimize the electronic state distribution, enhance the contribution of local excited states, and suppress the excessive occupation of charge transfer states, thereby improving molecular stability. In addition, when this bipyridine iron complex is used as an electrolyte active material in a flow battery, it can enable the flow battery to maintain a high capacity retention rate during long-term cycling.

Claims

1. A bipyridine iron complex, characterized in that, The general structural formula of the bipyridine iron complex is shown in Formula I: (Formula I); In Formula I, R1 to R6 may be the same or different, and each is independently selected from electron-withdrawing groups, electron-donating groups or steric hindrance groups.

2. The bipyridine iron complex according to claim 1, characterized in that, The electron-withdrawing group is selected from any one of sulfonic acid group, quaternary ammonium group, methylene quaternary ammonium group, nitro group, cyano group, and trifluoromethyl group; And / or, the electron-donating group is selected from any one of methyl, methoxy, ethyl, benzyl, and amino; And / or, the steric hindrance group is selected from any one of tert-butyl, isopropyl, phenyl, adamantyl, and triphenylphosphine.

3. The bipyridine iron complex according to claim 1, characterized in that, The bipyridine iron complex is selected from any one of the compounds shown in the following structural formulas: 、 、 ； R1 to R6 are identical to each other and are selected from electron-withdrawing groups.

4. The bipyridine iron complex according to claim 1, characterized in that, The bipyridine iron complex is selected from any one of the compounds shown in the following structural formulas: 、 。 5. A method for preparing a bipyridine iron complex as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Substituent-containing bipyridine is added to methanol, and after stirring, a methanol solution of substituent-containing bipyridine is obtained. An aqueous solution of FeCl2·4H2O was added dropwise to a methanol solution of the substituent-containing bipyridine, and the mixture was stirred, crystallized, filtered, and dried to obtain a bipyridine iron complex.

6. The method for preparing the bipyridine iron complex according to claim 5, characterized in that, The substituent-containing bipyridine is selected from any one of bipyridine containing a quaternary ammonium group, bipyridine containing a methylene quaternary ammonium group, bipyridine containing a methyl group, bipyridine containing an amino group, and bipyridine containing a tert-butyl group.

7. The method for preparing the bipyridine iron complex according to claim 5, characterized in that, The mass ratio of the substituent-containing bipyridine to methanol is 1:(8~12). And / or, the molar ratio of the substituent-containing bipyridine to FeCl2·4H2O is (2~5):

1.

8. The method for preparing the bipyridine iron complex according to claim 5, characterized in that, The substituent-containing bipyridine is selected from any one of 4,4'-bis(quaternary ammonium)-2,2'-bipyridine, 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 4,4'-diamino-2,2'-bipyridine, and 4,4'-di-tert-butyl-2,2'-bipyridine.

9. The method for preparing the bipyridine iron complex according to claim 8, characterized in that, The synthesis steps of the 4,4'-bis(quaternary ammonium)-2,2'-bipyridine include: reacting 4,4'-dibromo-2,2'-bipyridine with an aqueous solution of Me3N in dichloromethane, and freeze-drying the resulting solution to obtain the 4,4'-bis(quaternary ammonium)-2,2'-bipyridine; And / or, the synthesis steps of the 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine include: reacting 4,4'-dibromomethyl-2,2'-bispyridine with an aqueous solution of Me3N in dichloromethane, and lyophilizing the resulting solution to obtain the 4,4'-bis(dimethylquaternary ammonium)-2,2'-bipyridine.

10. An electrolyte, characterized in that, The electrolyte comprises the bipyridine iron complex as described in any one of claims 1 to 4 or the bipyridine iron complex prepared by any one of claims 5 to 9.

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