A sterically protected single-electron bipyridine derivative and its application in liquid flow batteries

Abstract

The present invention relates to the field of negative electrode materials for liquid flow batteries, and in particular to a sterically protected single-electron bipyridine derivative and its application in liquid flow batteries. 2,2'-alkoxy-4,4'-bipyridine (Bpy-OR), an organic negative electrode material for liquid flow batteries with low redox potential and electrochemical stability, can improve the voltage and cycle life of organic liquid flow batteries. Bpy-OR adopts a strategy of steric protection, by introducing an alkoxy functional group at the 2nd position of the two pyridine rings of 4,4'-bipyridine, and utilizing the steric hindrance of the functional group to avoid side reactions (mainly dimerization and nucleophilic reactions to nitrogen atoms) of active materials during the redox process. In addition, the introduced alkoxy functional group is an electron-withdrawing group, which can further reduce the reduction potential and thus increase the battery voltage.

Description

Technical Field

[0001] The present invention relates to the field of negative electrode materials for liquid flow batteries, and in particular to a sterically protected single-electron bipyridine derivative and an application thereof in liquid flow batteries. Background Art

[0002] Redox flow batteries are a type of energy storage system with high cycle efficiency and controllable properties and have broad prospects. Compared with inorganic flow battery systems, organic flow batteries using organic molecules as redox active materials have the advantages of lower cost, higher voltage, good reversibility, and adjustable structure.

[0003] As typical negative electrode active materials for organic flow batteries, 4,4'-bipyridine and its derivatives have attracted research interest due to their diverse structures and simple chemical modification. The most common 4,4'-bipyridine derivative is methyl viologen, which can be synthesized by reacting bipyridine with methyl iodide, and the reaction yield is high and can be mass-produced. When methyl viologen is used in combination with 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl free radical, the battery capacity is 13.40Ah L -1 , the capacity loss rate of 24h is 3.6%. This high capacity loss rate is caused by the dimerization of the intermediate viologen cation. In order to prevent dimerization, some researchers used Coulomb repulsion to introduce quaternary ammonium salt groups into viologen, and the capacity loss rate of the battery was reduced to 0.03%. In addition, sulfonated functionalized viologen also has a similar effect.

[0004] Although 4,4'-bipyridine and its derivatives have made great progress in the development and research of aqueous organic flow batteries with long cycle life, there are few reports on non-aqueous flow batteries. Although bipyridine series materials have a relatively negative redox potential (-1.8V vs. Ag / AgCl) in organic solvents, their application in battery electrodes has not been deeply studied. In addition, although some negative potential non-aqueous anode materials (quinones and azos) have been reported, they all have disadvantages such as high redox potential and short cycle life. Summary of the invention

[0005] The purpose of the present invention is to provide an organic negative electrode material for a liquid flow battery that is simple to synthesize, has a low redox potential, and is electrochemically stable, so as to improve the voltage and cycle life of the organic liquid flow battery.

[0006] The present invention provides a bipyridine-based organic molecular flow battery negative electrode material 2,2'-alkoxy-4,4'-bipyridine (Bpy-OR) designed based on a steric hindrance protection strategy, and its general structural formula is as follows:

[0007]

[0008] Among them, R=Me, Et, iPr, tBu.

[0009] When R=Me in the structure, the structure of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) is as follows:

[0010]

[0011] The present invention also provides a method for preparing Bpy-OMe, which uses zinc powder and nickel triphenylphosphine dibromide as catalysts for catalytic coupling reaction to prepare Bpy-OMe. The specific synthesis steps are as follows:

[0012] Weigh zinc powder, nickel triphenylphosphine dibromide and tetraethylammonium iodide, add them to a 250 mL two-necked round-bottom flask in sequence, ventilate 3 times to remove oxygen in the flask, inject tetrahydrofuran under the protection of nitrogen, stir for 30 minutes, inject a tetrahydrofuran solution containing 2-alkoxy-4-bromopyridine under the protection of nitrogen, heat and stir at 50° C. for 2 hours, after the reaction is completed, cool the mixture to room temperature and pour it into ammonia water, then extract with chloroform, filter the mixed solution to remove solid impurities, and retain the filtrate; after the filtrate is layered, take the lower organic liquid, and extract the upper aqueous solution with chloroform twice; after the organic liquids are combined, wash with saturated brine 3 times, dry with anhydrous sodium sulfate, and then remove the solvent by vacuum rotary evaporation to obtain a crude product; finally, extract by column chromatography to obtain a white solid powder product, namely 2,2'-alkoxy-4,4'-bipyridine.

[0013] The present invention also provides a method for applying Bpy-OR as a negative electrode active material in an organic flow battery, the specific contents of which are as follows:

[0014] Preparation of negative electrode electrolyte based on Bpy-OR active material: weigh a certain amount of negative electrode active material Bpy-OR, add it into a conventional electrolyte to prepare a solution with a concentration of 0.05M to 1.0M, which is the negative electrode electrolyte.

[0015] Conventional electrolytes include an organic solvent and an electrolyte salt, wherein the organic solvent is preferably, but not limited to, acetonitrile solvent, the electrolyte salt is preferably, but not limited to, tetrabutylammonium hexafluorophosphate, and the concentration of the electrolyte salt is preferably, but not limited to, 1.0M.

[0016] Prepare the positive electrode electrolyte: Weigh a certain amount of positive electrode active material and add it to a conventional electrolyte to prepare a solution with the same concentration as the negative electrode electrolyte, which is the positive electrode electrolyte.

[0017] The positive electrode active material includes, but is not limited to, organic molecules such as ferrocene that have a relatively high redox potential and a relatively high solubility in an organic solvent.

[0018] Assemble and test the flow battery: The battery system includes positive and negative terminal plates, positive and negative current collectors, porous diaphragms, and positive and negative electrode reservoirs, which are filled with electrolytes (positive and negative electrode electrolytes). In a nitrogen-filled glove box, use a pipette to fill the positive and negative electrode reservoirs with positive / negative electrolytes, seal the battery, and perform electrochemical performance tests on the battery on the battery test system.

[0019] The diaphragm is a film with high ion conductivity, including Daramic AA-800, PP film, etc.

[0020] The current collector includes but is not limited to graphite carbon felt, conductive carbon layer and the like.

[0021] Beneficial Effects

[0022] By adopting the strategy of steric protection, an alkoxy functional group is introduced at the 2nd position of the two pyridine rings of 4,4'-bipyridine, and the steric hindrance of the functional group is used to avoid the side reactions of the active material during the redox process (mainly dimerization reaction and nucleophilic reaction to nitrogen atoms). In addition, the introduced alkoxy functional group is an electron-withdrawing group, which can further reduce the reduction potential and thus increase the battery voltage. Furthermore, through molecular structure design, the present invention comprehensively considers the steric protection effect, the π-π interaction of organic molecules, the energy levels of the lowest unoccupied orbital (LUMO) and the highest occupied orbital (HOMO) of organic molecules, and designs an organic negative electrode material with overall excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described in the form of exemplary embodiments, which will be described in detail by way of the accompanying drawings. These embodiments are not restrictive, and in these embodiments, the same number represents the same structure, wherein:

[0024] Figure 1 is the electrostatic potential (ESP) of 4,4'-bipyridine and 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe).

[0025] Figure 2 It is the HOMO-LUMO energy level diagram of 4,4'-bipyridine and 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe).

[0026] Figure 3 It is the interaction energy and distance between different valence states of 4,4'-bipyridine (Bp) and 2,2'-methoxy-4,4'-bipyridine (MeOBP).

[0027] Figure 4 Synthesis route of Bpy-OMe.

[0028] Figure 5This is the hydrogen NMR spectrum of Bpy-OMe.

[0029] Figure 6 The cyclic voltammograms of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) and 4,4'-bipyridine (Bpy).

[0030] Figure 7 This is the cyclic voltammogram of Bpy-OMe after 100 cycles.

[0031] Figure 8 This is a graph of the cyclic charge-discharge capacity and efficiency of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe)|ferrocene flow battery.

[0032] Fig. 9 This is the capacity-voltage diagram of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe)|ferrocene flow battery.

[0033] Fig.10 This is a graph of the cyclic charge-discharge capacity and efficiency of 4,4'-bipyridine (Bpy)|ferrocene flow battery.

[0034] Fig.11 This is the capacity-voltage diagram of 4,4'-bipyridine (Bpy)|ferrocene flow battery. DETAILED DESCRIPTION

[0035] The following description is intended to enable those of ordinary skill in the art to implement and use the present invention, and the description is provided in the context of a specific application and its requirements. It will be apparent to those of ordinary skill in the art that various changes may be made to the disclosed embodiments. In addition, the general principles defined in the present invention may be applied to other embodiments and application scenarios without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited to the disclosed embodiments, but should be given the broadest scope consistent with the scope of the invention patent.

[0036] Example 1, Synthesis of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) and its application in flow batteries

[0037] The synthetic route of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) is as follows Figure 4As shown, specifically: weigh 0.49g (7.50mmol) of zinc powder, 1.12g (1.50mmol) of triphenylphosphine nickel dibromide and 1.29g (1.5mmol) of tetraethylammonium iodide, add them into a 250mL two-necked round-bottom flask in sequence, ventilate the flask three times to remove oxygen in the flask, inject 15mL of tetrahydrofuran (THF) under the protection of nitrogen, stir for 30 minutes, inject 0.93g (5mmol) of 2-methoxy-4-bromopyridine in tetrahydrofuran solution under the protection of nitrogen, and heat and stir at 50°C for 2 hours. After the reaction is completed, the mixture is cooled to room temperature and poured into 30 mL of 2M ammonia water, and then 100 mL of chloroform is added for extraction. The mixed solution is filtered to remove solid impurities and the filtrate is retained; after the filtrate is layered, the lower organic liquid is taken, and the upper aqueous solution is extracted twice with 50 mL of chloroform; after the organic liquids are combined, they are washed three times with saturated brine, dried over anhydrous sodium sulfate, and then vacuum evaporated to remove the solvent to obtain a crude product; finally, it is purified by column chromatography to obtain a white solid powder product, namely Bpy-OMe.

[0038] Figure 5 This is the hydrogen NMR spectrum of Bpy-OMe, which can prove that Bpy-OMe was successfully synthesized.

[0039] Compared with 4,4'-bipyridine, after the introduction of methoxy as a functional group, the negative electrostatic potential energy surface area around the active site N atom on the pyridine ring of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) is reduced and surrounded by the positive red electrostatic potential energy surface of the methoxy group, indicating that due to the steric protection effect of the methoxy group, the attack of the material molecules in the redox process can be effectively reduced, thereby improving the stability of the redox intermediates and improving its cyclic stability. Figure 1 shown.

[0040] The LUMO energy level of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) is 0.23 V higher than that of 4,4'-bipyridine. The energy level diagram of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of 4,4'-bipyridine and 2,2'-methoxy-4,4'-bipyridine is shown in Figure 2 shown.

[0041] The π-π interaction of organic molecules has a great effect on the electrochemical application of molecules. On the one hand, it will reduce the distance between molecules and thus reduce the solubility of the material, thus affecting the cycle stability of the battery; on the other hand, a certain π-π interaction can average the charge distribution on the reduced N atom of the bipyridine material, thereby improving the stability of the reduced free radical molecules. There is a certain π-π interaction in the two molecular structures of the three different valence states of 2,2'-methoxy 4,4'-bipyridine (Bpy-OMe), and the interaction energy is also greater than the value between the different valence states of 4,4'-bipyridine and 4,4'-biquinoline molecules. Figure 3 is the interaction energy and distance between different valence states of 4,4'-bipyridine (Bp) and 2,2'-methoxy 4,4'-bipyridine (MeOBP). In addition, the π-π interaction isosurface area increases first and then decreases with the increase of reduced state molecules, among which the neutral state and reduced 2,2'-methoxy 4,4'-bipyridine (MeOBP–MeOBP - ) have the largest π-π interaction. Figure 3 shown.

[0042] 2,2'-Methoxy-4,4'-bipyridine (Bpy-OMe) flow battery applications:

[0043] Prepare a negative electrode electrolyte based on Bpy-OMe active material: weigh the negative electrode active material Bpy-OMe, add it into a conventional electrolyte to prepare a negative electrode electrolyte with a concentration of 0.20M.

[0044] Prepare positive electrode electrolyte: weigh the positive electrode active material and add it to the conventional electrolyte to prepare a positive electrode electrolyte with a concentration of 0.20M.

[0045] Assemble and test the flow battery: The battery system includes positive and negative terminal plates, positive and negative current collectors, porous diaphragms, and positive and negative electrode reservoirs, which are filled with the above-mentioned positive and negative electrode electrolytes. In a nitrogen-filled glove box, use a pipette to fill the positive / negative electrode reservoirs with positive / negative electrolytes, seal the battery, and perform electrochemical performance tests on the battery test system.

[0046] The electrochemical performance of Bpy-OMe|ferrocene flow battery was tested by cyclic voltammetry (CV). 2,2'-methoxy-4,4'-bipyridine has good electrochemical reversibility, and its reduction potential in an electrolyte with acetonitrile as solvent is -1.89V (vs.Ag / AgCl). Ferrocene (potential: 0.51V vs.Ag / AgCl) was selected as the positive electrode material, and the battery voltage can reach 2.4V, and it can remain stable for 200 cycles in the battery cycle test. Figure 7 and Figure 8 shown.

[0047] Figure 6 The cyclic voltammograms of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) and 4,4'-bipyridine (Bpy). The introduction of methoxy groups into 4,4'-bipyridine (Bpy) can reduce the redox potential of the material, making it more negative, so that it can be used as a negative electrode material to match the positive electrode material to obtain a higher charge and discharge voltage.

[0048] Figure 8 This is a graph of the cyclic charge-discharge capacity and efficiency of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe)|ferrocene flow battery.

[0049] Fig. 9 This is the capacity-voltage diagram of the 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) | ferrocene flow battery. According to the data, the first cycle discharge capacity of the 2,2'-methoxy-4,4'-bipyridine | ferrocene flow battery is 2.51Ah / L, which is 46.8% of the theoretical capacity. After 200 cycles of charge and discharge, the battery discharge voltage is 2.20V, the battery coulomb efficiency remains at 95%, and the energy efficiency remains at 85%. The discharge capacity of the 200th cycle is 2.06Ah / L, the capacity retention rate is 82%, and the capacity decay rate per cycle is 0.09%.

[0050] Example 2, Synthesis of 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) and its application in flow batteries 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe) was synthesized according to the method and steps of Example 1.

[0051] 2,2'-Methoxy-4,4'-bipyridine (Bpy-OMe) flow battery applications:

[0052] Prepare a negative electrode electrolyte based on Bpy-OMe active material: weigh the negative electrode active material Bpy-OMe, add it into a conventional electrolyte to prepare a negative electrode electrolyte with a concentration of 0.30M.

[0053] Prepare the positive electrode electrolyte: weigh the positive electrode active material and add it to the conventional electrolyte to prepare a positive electrode electrolyte with a concentration of 0.30M.

[0054] Assemble and test the flow battery: The battery system includes positive and negative terminal plates, positive and negative current collectors, porous diaphragms, and positive and negative electrode reservoirs, which are filled with electrolytes (positive and negative electrode electrolytes). In a nitrogen-filled glove box, use a pipette to fill the positive and negative electrode reservoirs with positive / negative electrolytes, seal the battery, and perform electrochemical performance tests on the battery on the battery test system.

[0055] Example 3, Synthesis of 2,2'-methoxy-4,4'-bipyridine (Bpy-OEt) and its application in flow batteries

[0056] The specific steps for the synthesis of 2,2'-ethoxy-4,4'-bipyridine (Bpy-OEt) are:

[0057] Weigh 0.49 g (7.50 mmol) of zinc powder, 1.12 g (1.50 mmol) of triphenylphosphine nickel dibromide and 1.29 g (1.5 mmol) of tetraethylammonium iodide, add them into a 250 mL two-necked round-bottom flask in sequence, evacuate the flask three times to remove oxygen, inject 15 mL of tetrahydrofuran (THF) under nitrogen protection, stir for 30 minutes, and then inject 1.01 g (5 mmol) of 4-bromo-2-ethoxypyridine in tetrahydrofuran solution under nitrogen protection, and heat and stir at 50°C for 2 hours. After the reaction is completed, the mixture is cooled to room temperature and poured into 30 mL of 2M ammonia water, and then 100 mL of chloroform is added for extraction. The mixed solution is filtered to remove solid impurities and the filtrate is retained; after the filtrate is layered, the lower organic liquid is taken, and the upper aqueous solution is extracted twice with 50 mL of chloroform; after the organic liquids are combined, they are washed three times with saturated brine, dried over anhydrous sodium sulfate, and then vacuum evaporated to remove the solvent to obtain a crude product; finally, it is purified by column chromatography to obtain a white solid powder product, namely Bpy-OEt.

[0058] Comparative Example 1, Application of 4,4'-bipyridine (Bpy) in flow batteries

[0059] Preparation of negative electrode electrolyte based on Bpy active material: Weigh the negative electrode active material Bpy and add it into a conventional electrolyte to prepare a negative electrode electrolyte with a concentration of 0.20M.

[0060] Prepare positive electrode electrolyte: weigh the positive electrode active material and add it to the conventional electrolyte to prepare a positive electrode electrolyte with a concentration of 0.20M.

[0061] Assemble and test the flow battery: The battery system includes positive and negative terminal plates, positive and negative current collectors, porous diaphragms, and positive and negative electrode reservoirs, which are filled with electrolytes (positive and negative electrode electrolytes). In a nitrogen-filled glove box, use a pipette to fill the positive and negative electrode reservoirs with positive / negative electrolytes, seal the battery, and perform electrochemical performance tests on the battery on the battery test system.

[0062] Fig.10 This is a graph of the cyclic charge-discharge capacity and efficiency of 4,4'-bipyridine (Bpy)|ferrocene flow battery.

[0063] Fig.11 This is the capacity-voltage diagram of 4,4'-bipyridine (Bpy)|ferrocene flow battery.

[0064] The first cycle discharge capacity of 4,4'-bipyridine|ferrocene static flow battery is 4.50Ah L -1 , which is 84% ​​of the theoretical capacity. After 200 charge and discharge cycles, the battery discharge voltage is 1.75V, the battery coulomb efficiency remains at 95%, the energy efficiency remains at 50%, the 200th cycle discharge capacity is 0.23Ah L-1, the capacity retention rate is only 5.11%, and the capacity decay rate per cycle is 0.47%, which indicates that in actual battery tests, the capacity decay of 4,4'-bipyridine is faster and the stability is lower.

[0065] Compared with Example 1, compared with 4,4'-bipyridine (Bpy) before the introduction of the functional group, after the introduction of the methoxy group into 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe), the battery voltage becomes higher, and due to the steric protection of the methoxy group, the stability of the active material is improved, thereby significantly improving the battery cycle stability.

[0066] In Example 1, Example 2 and Comparative Example 1, the conventional electrolyte is a 1M acetonitrile solution of tetrabutylammonium hexafluorophosphate; the positive electrode active material is ferrocene; the separator is Daramic AA-800; and the current collector is graphite carbon felt.

Claims

1. An organic liquid flow battery, characterized in that: The liquid flow battery comprises positive and negative terminal plates, positive and negative current collectors, a porous diaphragm, and positive and negative electrode storage tanks, wherein the positive and negative electrode storage tanks are filled with electrolyte; The electrolyte filled in the negative electrode storage tank of the flow battery is: a solution prepared by adding a bipyridine derivative into a conventional electrolyte to form a concentration of 0.05 M to 1.0 M, which is the negative electrode electrolyte; The bipyridine derivative is 2,2'-alkoxy-4,4'-bipyridine (Bpy-OR), and its general structural formula is: , where R=Me, Et, iPr, tBu.

2. The organic flow battery according to claim 1, characterized in that: The electrolyte filled in the positive electrode storage tank of the liquid flow battery is: the positive electrode active material ferrocene is added into a conventional electrolyte to prepare a solution with a concentration of 0.05 M to 1.0 M, which is the positive electrode electrolyte.

3. The organic flow battery according to claim 1, characterized in that: The assembly method of the liquid flow battery is as follows: using a pipette gun in a glove box filled with nitrogen to fill the positive / negative electrode electrolytes in the positive and negative electrode storage tanks respectively, and sealing the liquid flow battery system.

4. The organic flow battery according to claim 1, characterized in that: The preparation method of the bipyridine derivative is as follows: zinc powder, nickel triphenylphosphine dibromide and tetraethylammonium iodide are weighed, and added into a two-necked round-bottom flask in sequence, and the oxygen in the flask is removed by evacuation for 3 times, and tetrahydrofuran is injected under the protection of nitrogen, and after stirring for 30 minutes, a tetrahydrofuran solution containing 2-alkoxy-4-bromopyridine is injected under the protection of nitrogen, and heated and stirred at 50° C. for 2 hours. After the reaction is completed, the mixture is cooled to room temperature and poured into ammonia water, and then extracted with chloroform, and the mixed solution is filtered to remove solid impurities, and the filtrate is retained; after the filtrate is separated, the lower organic liquid is taken, and the upper aqueous solution is extracted twice with chloroform; after the organic liquid is combined, it is washed with saturated brine for 3 times, dried with anhydrous sodium sulfate, and then vacuum rotary evaporation is performed to remove the solvent to obtain a crude product; finally, it is extracted by column chromatography to obtain a white solid powder product, namely 2,2'-alkoxy-4,4'-bipyridine.

5. The organic flow battery according to claim 1, characterized in that: The alkoxy group is methoxy, ethoxy, or isopropoxy.

6. The organic flow battery according to claim 1, characterized in that: The bipyridine derivative is 2,2'-methoxy-4,4'-bipyridine (Bpy-OMe), and its structure is: 。

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