Pyrrolidine nitroxide free radical connected with hydrophilic group through carbon chain, preparation method of pyrrolidine nitroxide free radical and application of pyrrolidine nitroxide free radical in flow battery

Abstract

The invention discloses a pyrrolidine nitroxide free radical connected with a hydrophilic group through a carbon chain, a preparation method of the pyrrolidine nitroxide free radical and application of the pyrrolidine nitroxide free radical in a flow battery. The positive electrode of the aqueous organic flow battery comprises a positive electrode electroactive material and an aqueous solution supporting electrolyte; the positive electrode electrolyte active substance is pyrrolidine nitroxide free radical connected with a hydrophilic group through a carbon chain; and the supporting electrolyte is a single-component or multi-component chloride ion inorganic salt. The hydrophilic group and the pyrrolidine nitroxide free radical are connected through the carbon chain, so that the problems of low water solubility and low stability of the existing pyrrolidine nitroxide free radical serving as a positive electrode electroactive material of the aqueous organic flow battery can be solved at the same time.

Description

Technical Field

[0001] This invention relates to a pyrrolidine nitroxide radical, specifically to a pyrrolidine nitroxide radical with hydrophilic groups linked by a carbon chain, its preparation method, and its application in flow batteries. Background Technology

[0002] To mitigate global climate change and reduce carbon emissions, the use of clean energy sources such as solar and wind power is gradually increasing. However, the power generation from these sources is non-steady-state and requires large-scale energy storage technologies. Flow batteries are one of the most promising large-scale energy storage technologies. The working principle of a flow battery is that a peristaltic pump drives electrolyte from an external storage tank into the battery body for charging and discharging. Therefore, capacity and power can be designed separately, offering the advantage of low power cost.

[0003] Compared to organic flow batteries using organic solvents, aqueous flow batteries have become the mainstream due to their superior conductivity and safety. Aqueous flow batteries initially used inorganic materials as electroactive materials, such as vanadium and zinc bromine, but their development was limited by high mining costs and low solubility. Organic electroactive materials, on the other hand, are not limited by resources and their performance is easily tunable, making them a focus of attention. The mainstream organic electroactive materials currently fall into four categories: quinones, viologens, aromatic heterocycles, and TEMPO. Among them, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) is the only cathode material. TEMPO can undergo highly reversible redox reactions and has a high redox potential. It can be used in neutral environments and has low corrosivity, making it a highly promising electroactive material for flow batteries.

[0004] In 2016, Schubert's group reported the first highly water-soluble 4-trimethylammonium TEMPO (TEMPTMA) for aqueous flow batteries, with a solubility of 3 M in pure water and a redox potential of 0.79 V (vs AgCl / Ag). Subsequently, high-performance TEMPO derivatives have been reported as positive electrode active materials, but none have demonstrated superior overall performance compared to TEMPTMA. While modifying the 4-position of TEMPO has improved its water solubility, its stability remains a significant issue. Currently, the substituents at the 4-position of TEMPO derivatives are electron-withdrawing groups, leading to decomposition of the ammonium cations after charging due to excessive electronegativity, resulting in battery capacity decay. Furthermore, during charge and discharge, the TEMPO solution becomes acidified, driving disproportionation and generating irreversible hydroxylamine molecules, further contributing to capacity reduction. Therefore, to achieve highly stable aqueous organic redox flow batteries, it is essential to find new positive electrode active materials that can fundamentally improve stability.

[0005] 2,2,5,5-Tetramethylpyrrolidine nitroxide radicals exhibit significantly better anti-disproportionation properties than TEMPO, and their redox potential is even higher when the functional groups are the same. However, pyrrolidine nitroxide radicals have not yet attracted much attention, with only a few reported cases. In 2021, Song's research group reported the first 3-carboxamido-2,2,5,5-tetramethylpyrrolidine nitroxide radical for use in aqueous flow batteries (see Chinese invention patent CN111628185B). However, 3-carboxamido-2,2,5,5-tetramethylpyrrolidine nitroxide radicals lack hydrophilic groups, and their water solubility is far from meeting the requirements of aqueous flow batteries. Chinese invention patent CN114031539B discloses an amidated cyclic nitroxide free radical compound, its preparation method and application. However, the flow battery using 2,2,5,5-tetramethylpyrrolidine nitroxide free radical as the active material will also be acidified during the charging and discharging process. The amide bond will be broken by hydrolysis in an acidic environment, and the structure has chemical instability.

[0006] In 2025, Boutamine's group reported the first highly water-soluble 3-trimethylammonium-2,2,5,5-tetramethylpyrrolidine nitroxide radical for use in aqueous flow batteries (K. Boutamine et al. 3-TMA PROXYL: A High-Potential, Highly Soluble Nitroxide for Enhanced Stability and Performance in Aqueous Organic Redox Flow Batteries. Energy Storage Materials, 2025, Vol. 80, pp. 104379-104386). However, the trimethylammonium functional group, which strongly pulls electrons, is directly connected to the pyrrolidine nitroxide radical. The strong inductive effect of the trimethylammonium functional group causes the pyrrolidine nitroxide radical to rearrange, resulting in the compound losing its electroactivity. Therefore, the battery test results given by this technology are not as stable as TEMPTMA. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a pyrrolidine nitroxide radical with both high water solubility and high stability, which is linked to hydrophilic groups via a carbon chain, and its preparation method, thereby solving the problem of insufficient stability of pyrrolidine nitroxide radicals in existing technologies.

[0008] Another objective of this invention is to provide the application of the pyrrolidine nitroxide radical, which is linked to a hydrophilic group via a carbon chain, in the preparation of a positive electrode electroactive material for an aqueous organic flow battery.

[0009] To achieve the objectives of this invention, the following technical solution is adopted:

[0010] The pyrrolidine nitroxide radical, which links a hydrophilic group through a carbon chain, has the following structural formula:

[0011]

[0012] n is the number of carbon atoms in the carbon chain, n≥1; R is -NH2, -N(CH3)2, -N + (CH3)3、-[N(CH3)2] + (CH2) n N + (CH3)3(n≥1), -NHCOCH2N + (CH3)3, -OH, -O(CH2) n N + (CH3)3(n≥1), -OCOCH2N + (CH3)3, -OCH3, -OPO3 2- and -OSO3 - One of them.

[0013] To further achieve the purpose of this invention, preferably, the pyrrolidine nitroxide radical connected by a carbon chain to a hydrophilic group is a 3-trimethylammoniummethyl-2,2,5,5-pyrrolidine nitroxide radical, a 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical, or a 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical.

[0014] The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains includes the following steps:

[0015] 1) 4-O-TEMPO reacts with iodine and ammonia in a Favorskii rearrangement reaction to generate 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical;

[0016] 2) The 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical is hydrolyzed under alkaline or acidic conditions to generate 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical;

[0017] 3) React 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical with lithium aluminum hydride to generate 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical;

[0018] 4) 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical is condensed with a halide or esterified with a carboxylic acid, phosphoric acid, or sulfuric acid to obtain a pyrrolidine nitroxide radical with a hydrophilic group linked by a carbon chain.

[0019] Preferably, the molar ratio of 4-oxy-TEMPO, iodine and ammonia is 1:(1-5):(1-10), and the temperature of the rearrangement reaction is 0-50℃.

[0020] Preferably, the molar ratio of the 3-amide-2,2,5,5-pyrrolidine nitroxide radical to the base or acid is 1:(1-10), and the hydrolysis reaction temperature is 0-100℃.

[0021] Preferably, the alkali is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide; and the acid is hydrochloric acid, sulfuric acid, and nitric acid.

[0022] Preferably, the molar ratio of the 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical or the 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to lithium aluminum hydride is 1:(1-10); the reaction temperature with lithium aluminum hydride is 0-100℃.

[0023] The molar ratio of the 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to the halide is 1:(1-10), and the condensation reaction temperature is 0-100℃.

[0024] The application of pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains in the preparation of positive electrode active materials for aqueous organic flow batteries: The positive electrode of the aqueous organic flow battery includes a positive electrode active material and an aqueous solution of supporting electrolyte; the active material of the positive electrode electrolyte is pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains; the supporting electrolyte is a single-component or multi-component chloride ion inorganic salt.

[0025] Preferably, the concentration of the pyrrolidine nitroxide radicals with hydrophilic groups linked to the carbon chain is 0.01 mol / L to 5 mol / L.

[0026] Preferably, the chloride ion inorganic salt is LiCl, NaCl, KCl, NH4Cl, MgCl2 or CaCl2; the concentration of the chloride ion inorganic salt is 0.01 to 5 mol / L.

[0027] Compared with the prior art, the present invention has the following advantages and superior effects:

[0028] 1) The pyrrolidine nitroxide radical of the present invention, which connects hydrophilic groups through a carbon chain, has better anti-disproportionation properties than the piperidine nitroxide radical derivative (TEMPO) currently being studied, and can effectively suppress the decay of battery capacity.

[0029] 2) The present invention has excellent chemical stability by connecting hydrophilic groups and pyrrolidine nitroxide radicals through carbon chains, and the carbon chains are not easy to break; through the blockage of carbon chains, especially the strong inductive effect of hydrophilic groups, the pyrrolidine nitroxide radicals will not be affected, thereby causing them to rearrange and deactivate; it can simultaneously solve the problems of low water solubility or low stability of pyrrolidine nitroxide radical derivatives in the prior art. Attached Figure Description

[0030] Figure 1 The image shows the 1H NMR spectrum of the 3-amide-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 1 of this invention.

[0031] Figure 2 The image shows the 1H NMR spectrum of the 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 2 of this invention.

[0032] Figure 3 The image shows the 1H NMR spectrum of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitrogen oxide radical obtained in Example 3 of this invention.

[0033] Figure 4 The image shows the 1H NMR spectrum of the 3-carboxy-2,2,5,5-tetramethylpyrrolidine nitroxide radical obtained in Example 4 of this invention.

[0034] Figure 5 The image shows the 1H NMR spectrum of the 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitrogen oxide radical obtained in Example 5 of this invention.

[0035] Figure 6 This is a comparison chart of the cyclic voltammetric test results of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical and TEMPTMA in Example 6.

[0036] Figure 7 The image shows the UV absorption curve of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical in Example 7.

[0037] Figure 8 The graph shows the linear relationship of UV absorbance of aqueous solutions of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals at various concentrations in Example 7.

[0038] Figure 9This is a comparison chart of the long-cycle capacity of the symmetric battery assembled with 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical and TEMPTMA in Example 8.

[0039] Specific implementation methods

[0040] To better understand the present invention, it will be further described below with reference to the accompanying drawings and specific embodiments. However, the implementation of the present invention is not limited thereto. The described embodiments are some, but 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.

[0041] Currently available pyrrolidine nitroxide radical derivatives cannot simultaneously solve the problems of water solubility and stability, and their performance in flow batteries has failed to surpass that of TEMPTMA. This invention discovers that pyrrolidine nitroxide radicals linked to hydrophilic groups via carbon chains exhibit water solubility and redox potential comparable to TEMPO, and possess better resistance to reduction and are less prone to disproportionation reactions. Furthermore, the alkyl chain, being a non-electron-withdrawing group, significantly enhances stability by linking the hydrophilic group and the pyrrolidine nitroxide radical through the alkyl chain.

[0042] The pyrrolidine nitroxide radical of the present invention, which links hydrophilic groups via a carbon chain, has the following structural formula:

[0043]

[0044] n is the number of carbon atoms in the carbon chain, n≥1; R is -NH2, -N(CH3)2, -N + (CH3)3、-[N(CH3)2] + (CH2) n N + (CH3)3(n≥1), -NHCOCH2N + (CH3)3, -OH, -O(CH2) n N + (CH3)3(n≥1), -OCOCH2N + (CH3)3, -OCH3, -OPO3 2- and -OSO3 - One of them.

[0045] For example, the pyrrolidine nitroxide radical of the present invention, which connects hydrophilic groups through a carbon chain, is a 3-trimethylammoniummethyl-2,2,5,5-pyrrolidine nitroxide radical, a 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical, or a 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical.

[0046] The reaction formula for preparing the pyrrolidine nitroxide radical by linking hydrophilic groups via a carbon chain in this invention is as follows:

[0047]

[0048] Specifically, the present invention provides a method for preparing pyrrolidine nitroxide radicals by linking hydrophilic groups via carbon chains, comprising the following steps:

[0049] 1) 4-O-TEMPO reacts with iodine and ammonia in a Favorskii rearrangement to generate 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical; Reaction formula: The preferred molar ratio of 4-oxy-TEMPO, iodine and ammonia is 1:(1-5):(1-10), and the preferred temperature for the rearrangement reaction is 0-50℃.

[0050] 2) The 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical undergoes a hydrolysis reaction under alkaline or acidic conditions to generate a 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical; Reaction formula: The preferred molar ratio of 3-amide-2,2,5,5-pyrrolidine nitroxide radical to base or acid is 1:(1-10); the preferred hydrolysis reaction temperature is 0-100℃. The preferred base is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide; the preferred acid is hydrochloric acid, sulfuric acid, and nitric acid.

[0051] 3) Reacting 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radicals or 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radicals with lithium aluminum hydride generates 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radicals or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radicals, respectively; Reaction formula: as well as Preferably, the molar ratio of 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to lithium aluminum hydride is 1:(1-10); the reaction temperature with lithium aluminum hydride is 0-100℃.

[0052] 4) The 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical or the 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical is reacted with halides via condensation reactions (e.g., the reaction of 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical with chloromethane yields 3-trimethylammoniummethyl-2,2,5,5-pyrrolidine nitroxide radical); or with carboxylic acids, phosphoric acid, or sulfuric acid via esterification reactions to obtain pyrrolidine nitroxide radical derivatives with hydrophilic groups linked by carbon chains. Reaction formula: The molar ratio of 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to halide is 1:(1-10), and the preferred temperature for the condensation reaction is 0-100℃.

[0053] The present invention utilizes pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains as the positive electrode active material in an aqueous organic flow battery. The positive electrode of the aqueous organic flow battery comprises the positive electrode active material and an aqueous solution supporting the electrolyte. The active material of the positive electrode electrolyte is a pyrrolidine nitroxide radical with hydrophilic groups linked by carbon chains, at a concentration of 0.01 mol / L to 5 mol / L. The supporting electrolyte is a single-component or multi-component chloride ion inorganic salt. The chloride ion inorganic salt is LiCl, NaCl, KCl, NH4Cl, MgCl2, or CaCl2; the concentration of the chloride ion inorganic salt is 0.01 to 5 mol / L.

[0054] Aqueous neutral flow batteries typically use aluminum plates as end plates; the electrolyte is a neutral aqueous salt solution; two graphite or titanium plates serve as flow field plates for the positive and negative electrode solutions; graphite felt or carbon paper are used as reaction electrodes, with an anion exchange membrane separating the positive and negative electrodes. This invention uses pyrrolidine nitroxide radicals linked to hydrophilic groups via carbon chains as the positive electrode organic active material; viologen and its derivatives or zinc can be used as the negative electrode active material, with a concentration of 0.01 mol / L to 5 mol / L; the volume ratio of the positive to negative electrolyte is 1:(1-5).

[0055] Example 1

[0056] Preparation of 3-amide-2,2,5,5-pyrrolidine nitroxide radical: Weigh 4.02 g of iodine and transfer it to an Erlenmeyer flask. Add 80 mL of toluene to dissolve it and set aside. Weigh 2723.7 mg of 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxo radical (4-oxo-TEMPO) and 2.74 g of potassium hydroxide, respectively, and transfer them to a 250 mL single-necked flask. Add 80 mL of ammonia water to the flask, add a magnetic stir bar, and place a constant-pressure dropping funnel on the flask. Transfer the prepared iodine-toluene solution to the constant-pressure dropping funnel, cover with a ground glass stopper, and then place the entire reaction apparatus on a magnetic stirrer. Adjust the stopcock of the constant-pressure dropping funnel to add the iodine-toluene solution dropwise to the flask. After the iodine-toluene solution has been added, stir the reaction mixture for another 24 hours to stop the reaction. Add 20 g of sodium chloride to the flask, transfer the mixture to a separatory funnel for extraction and separation, discard the toluene layer, extract the aqueous layer three times with dichloromethane, combine the dichloromethane solutions, dry, filter, and concentrate under reduced pressure to obtain 1143.9 mg of yellow 3-amide-2,2,5,5-pyrrolidine nitroxide free radical solid, with a yield of 42%, which does not require further purification.

[0057] Figure 1The 1H NMR spectrum of 3-amide-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 1 of this invention. 1 ¹H NMR (500 MHz, DMSO-d⁶) δ 2.00 (t, J = 11.8 Hz, ¹H), 1.54 (dd, J = 12.5, 7.8 Hz, ¹H), 1.25 (s, ¹H), 1.17 (s, ³H), 1.09 (s, ³H), 1.04 (s, ³H), 0.91 (s, ³H). Characteristic peak A corresponds to the hydrogen atoms of the four methyl groups at positions 2 and 5, characteristic peak B corresponds to the hydrogen atoms of the carbon atom at position 3 which is connected to the functional group, and characteristic peak C corresponds to the hydrogen atoms of the methylene group at position 4. Therefore, this example yielded 3-amide-2,2,5,5-pyrrolidine nitroxide radicals.

[0058] Example 2

[0059] Preparation of 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical: Weigh 555 mg of 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical and transfer it to a 100 mL single-necked flask. Dissolve it in 20 mL of ultra-dry tetrahydrofuran and seal with a rubber stopper. Place the flask in an ice bath and add 3 mL of lithium aluminum hydride (2.5 mol in THF) dropwise using a syringe. After the lithium aluminum hydride is added, return the reaction to room temperature and stir for 12 hours. After the reaction is complete, place the flask in an ice bath and quench the lithium aluminum hydride by adding 5 mL of water dropwise. Then transfer the mixture to a separatory funnel and extract three times with ethyl acetate. Combine the ethyl acetate solutions, dry, filter, and concentrate under reduced pressure to obtain 256 mg of yellow 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical solid, with a yield of 50%, requiring no further purification.

[0060] Figure 2 The image shows the 1H NMR spectrum of the 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 2 of this invention. 1¹H NMR (400MHz, Deuterium Oxide) δ 3.13–3.07 (m, 1H), 2.86 (dd, J = 12.8, 10.7 Hz, 1H), 2.46 (dddd, J = 12.8, 10.4, 6.5, 3.6 Hz, 1H), 2.20 (dd, J = 13.2, 6.5 Hz, 1H), 1.97–1.86 (m, 1H), 1.43 (d, J = 4.0 Hz, 6H), 1.37 (s, 3H), 1.23 (s, 3H). Characteristic peak A corresponds to the hydrogens of the four methyl groups at positions 2 and 5, characteristic peak B corresponds to the hydrogen of the methylene group at position 4, characteristic peak C corresponds to the hydrogen of the carbon atom attached to the aminomethyl group at position 3, and characteristic peak D corresponds to the hydrogen of the methylene group attached to the amino group. It can be seen that 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radicals were prepared in this embodiment.

[0061] Example 3

[0062] Preparation of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical: Weigh 513 mg of 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical and transfer it to a 100 mL single-necked flask. Add 20 mL of ultra-dry tetrahydrofuran to dissolve it, then add 638.7 mg of iodomethane and seal with a rubber stopper. Place the flask on a magnetic stirrer and stir for 24 hours. After the reaction is complete, filter the mixture and collect the residue. Dissolve the residue in water and add chloride ion exchange resin to replace iodide ions with chloride ions. Concentrate the aqueous solution of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical under reduced pressure to obtain a yellow solid of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical. The crude product was recrystallized from methanol / acetone to obtain 597 mg of pure 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solid, with a yield of 80%.

[0063] Figure 3 The image shows the 1H NMR spectrum of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitrogen oxide radical obtained in the embodiments of the present invention. 1¹H NMR (400MHz, Deuterium Oxide) δ 3.40 (d, J = 13.5Hz, 1H), 3.23 (dd, J = 13.9, 8.4Hz, 1H), 3.04 (s, 9H), 2.18 (dt, J = 15.8, 9.1Hz, 2H), 1.83 (q, J = 11.9, 8.3Hz, 1H), 1.23 (d, J = 6.4Hz, 9H), 0.98 (s, 3H). Characteristic peak A corresponds to the hydrogen atoms of the four methyl groups at positions 2 and 5; characteristic peak B corresponds to the hydrogen atom of the carbon atom attached to the trimethylammonium methyl group at position 3; characteristic peak C corresponds to the hydrogen atom of the methylene group at position 4; characteristic peak D corresponds to the hydrogen atom of the methyl group on the trimethylammonium group; and characteristic peak E corresponds to the hydrogen atom of the methylene group on the trimethylammonium methyl group. Therefore, this example yielded a 3-trimethylammoniummethyl-2,2,5,5-pyrrolidine nitroxide radical.

[0064] Example 4

[0065] Preparation of 3-carboxy-2,2,5,5-tetramethylpyrrolidine nitroxide radical: Weigh 555 mg of 3-amide-2,2,5,5-pyrrolidine nitroxide radical and transfer it to a 100 mL single-necked flask. Add water and sodium hydroxide, and stir overnight. After the reaction is complete, neutralize with sulfuric acid solution, then transfer the mixture to a separatory funnel and extract three times with ethyl acetate. Combine the ethyl acetate solutions, dry, filter, and concentrate under reduced pressure to obtain 446 mg of yellow 3-carboxy-2,2,5,5-pyrrolidine nitroxide radical solid, with a yield of 80%. No further purification is required.

[0066] Figure 4 The image shows the 1H NMR spectrum of the 3-carboxy-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 4 of this invention. 1 ¹H NMR (500 MHz, DMSO-d6) δ 2.70–2.62 (m, 1H), 1.98 (t, J = 12.2 Hz, 1H), 1.70 (dd, J = 12.8, 8.0 Hz, 1H), 1.24 (s, 3H), 1.13 (s, 3H), 1.08 (s, 3H), 0.97 (s, 3H). Characteristic peak A corresponds to the hydrogens of the four methyl groups at positions 2 and 5, characteristic peak B corresponds to the hydrogen of the methylene group at position 4, characteristic peak C corresponds to the hydrogen of the carbon atom at position 3 which is connected to the carboxyl group, and characteristic peak D is the solvent peak of the deuterated reagent DMSO-d6. It can be seen that 3-carboxy-2,2,5,5-pyrrolidine nitroxide radical was prepared in this example.

[0067] Example 5

[0068] Preparation of 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical: Weigh 558 mg of 3-carboxy-2,2,5,5-pyrrolidine nitroxide radical and transfer it to a 100 mL single-necked flask. Dissolve it in 20 mL of ultra-dry tetrahydrofuran and seal with a rubber stopper. Place the flask in an ice bath and add 3 mL of lithium aluminum hydride (2.5 mol in THF) dropwise using a syringe. After the lithium aluminum hydride is added, return the reaction to room temperature and stir for 12 hours. After the reaction is complete, place the flask in an ice bath and quench the lithium aluminum hydride by adding 5 mL of water dropwise. Then transfer the mixture to a separatory funnel and extract three times with ethyl acetate. Combine the ethyl acetate solutions, dry, filter, and concentrate under reduced pressure to obtain 258 mg of yellow 3-hydroxymethyl-2,2,5,5-pyrrolidine nitroxide radical solid, with a yield of 50%, requiring no further purification.

[0069] Figure 5 The image shows the 1H NMR spectrum of the 3-hydroxymethyl-2,2,5,5-pyrrolidine nitroxide radical obtained in Example 5 of this invention. 1 ¹H NMR (500MHz, DMSO-d6) δ 3.50 (dd, J = 10.5, 6.4Hz, 1H), 3.29 (dd, J = 10.5, 7.6Hz, 1H), 1.87 (dq, J = 11.1, 7.3Hz, 1H), 1.71 (dd, J = 12.5, 7.7Hz, 1H), 1.33 (t, J = 11.7Hz, 1H), 1.09 (s, 3H), 1.07 (s, 3H), 1.06 (s, 3H), 0.89 (s, 3H). Characteristic peak A corresponds to the hydrogens of the four methyl groups at positions 2 and 5; characteristic peak B corresponds to the hydrogen of the carbon atom attached to the hydroxymethyl group at position 3; characteristic peak C corresponds to the hydrogen of the methylene group at position 4; characteristic peak D corresponds to the hydrogen of the methylene group attached to the hydroxyl group; and characteristic peak E is the solvent peak of the deuterated reagent DMSO-d6. It can be seen that 3-hydroxymethyl-2,2,5,5-pyrrolidine nitroxide radicals were prepared in this embodiment.

[0070] Example 6

[0071] Cyclic voltammetry tests and comparisons were performed on 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals and TEMPTMA.

[0072] Cyclic voltammetry can measure the redox potential of a compound. The battery voltage is calculated as the difference between the redox potential of the positive electrode and the redox potential of the negative electrode. Therefore, when the negative electrode material is the same, the higher the redox potential of the positive electrode material, the higher the voltage of the battery.

[0073] Weigh out 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals and dissolve them in 30 mL of 1 mol / L potassium chloride solution. Stir and shake to prepare a 0.001 mol / L solution of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals. Perform cyclic voltammetry tests on the prepared 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solutions using a three-electrode system. Use a glassy carbon electrode as the working electrode, Ag / AgCl as the reference electrode, and a platinum electrode as the counter electrode. Scan rate is 100 mV / s for one cycle. The cyclic voltammetry test procedure for TEMPTMA is the same.

[0074] Figure 6 This is a comparison of the cyclic voltammetric test results of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical and TEMPTMA in this embodiment. The test results show that the redox potential of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical is 0.72 V (vs Ag / AgCl), while the redox potential of TEMPTMA is 0.79 V (vs Ag / AgCl), indicating that their redox potentials are essentially equivalent.

[0075] Example 7

[0076] The solubility of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical in pure water was tested.

[0077] A 100 mL stock solution of 0.01 M 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical was prepared by weighing 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical. The solution from this stock solution was then used to prepare aqueous solutions of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical at concentrations of 0.05 M, 0.06 M, 0.07 M, 0.08 M, 0.09 M, 0.1 M, 0.15 M, 0.2 M, 0.25 M, 0.3 M, and 0.35 M. First, the 0.05 M solution was quantitatively analyzed using a UV spectrophotometer, and the UV absorption curves of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical aqueous solution at this concentration were obtained. The obtained curves are shown below. Figure 7 From this curve, the relationship between absorption wavelength and absorbance can be derived, and it can be seen from... Figure 7 The maximum absorption wavelength was observed to be 230 nm. Qualitative analysis was then performed on the remaining concentration solutions, measuring their absorbance at 230 nm. A graph was plotted showing the absorbance of each concentration solution at 230 nm, and the resulting curve is shown below. Figure 8The linear regression equation was derived, which is the linear relationship between the concentration of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide free radical aqueous solution and the absorbance. The concentration can be determined by substituting the absorbance measured for an unknown concentration of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide free radical aqueous solution into the equation.

[0078] Example 8

[0079] Symmetric cell tests and comparisons were conducted on 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals and TEMPTMA.

[0080] Symmetrical battery testing uses positive electrode active materials in both the positive and negative electrode electrolytes, eliminating the influence of negative electrode active materials and providing a reliable method for evaluating the stability of active materials. The charge-discharge reactions of the positive electrode active materials in the positive and negative half-cells of a symmetric battery are shown in the following equations. During charging, the reaction at the positive electrode involves the oxidation of free radical pyrrolidine nitrogen oxides to generate cationic pyrrolidine ammonium oxide cations, while the reaction at the negative electrode involves the reduction of cationic pyrrolidine ammonium oxide cations to generate free radical pyrrolidine nitrogen oxides. During discharging, the reaction at the positive electrode involves the reduction of cationic pyrrolidine ammonium oxide cations to generate free radical pyrrolidine nitrogen oxides, while the reaction at the negative electrode involves the oxidation of free radical pyrrolidine nitrogen oxide cations to generate cationic pyrrolidine ammonium oxide cations.

[0081] Positive half-cell reaction

[0082] Negative electrode half-cell reaction

[0083] Weigh out 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals and dissolve them in 10 mL of 2 mol / L potassium chloride solution, then shake and stir to prepare a 0.1 mol / L 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solution. Weigh out methyl viologen and dissolve it in 6 mL of 2 mol / L potassium chloride solution, then shake and stir to prepare a 0.1 mol / L methyl viologen solution. Use 5 mL of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solution as the positive electrode solution and 6 mL of the methyl viologen solution as the negative electrode solution. The battery is tested at a cutoff voltage of 1.7 V and a current density of 40 mA cm⁻¹. -2Constant current and constant voltage charging was performed under a cutoff current of 12mA. After charging was completed, all the charged 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solution was removed and mixed with the uncharged 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical solution. 6 ml of this mixture was used as the negative electrode solution and 4 ml as the positive electrode solution. Constant voltage charge-discharge cycles were performed with a charging voltage of 0.35V, a discharging voltage of -0.35V, and a charge-discharge cutoff current of 12mA for 1000 cycles, performing a symmetrical battery charge-discharge test. The symmetrical battery test procedure for TEMPTMA was the same as above.

[0084] Figure 9 This is a comparison chart of the long-cycle capacity of the symmetric battery assembled with 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical and TEMPTMA in this example. Figure 9 It is evident that after 1000 charge-discharge cycles, the capacity of 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radicals decreased only slightly, while the capacity of TEMPTMA decreased by almost half.

[0085] As can be seen from the test results above, in this invention, the redox potential of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical is slightly lower than that of TEMPTMA, while its water solubility and symmetric cell stability are both higher than those of TEMPTMA. Overall, the comprehensive performance of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical surpasses that of TEMPTMA, exhibiting excellent water solubility and stability. The test results of the 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical and the 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical are basically comparable to those of the 3-trimethylammonium methyl-2,2,5,5-pyrrolidine nitroxide radical.

[0086] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A pyrrolidine nitroxide radical linked by a carbon chain to a hydrophilic group, characterized in that... It has the following structural formula: n is the number of carbon atoms in the carbon chain, n≥1; R is -NH2, -N(CH3)2, -N + (CH3)3、-[N(CH3)2] + (CH2) n N + (CH3)3(n≥1), -NHCOCH2N + (CH3)3, -OH, -O(CH2) n N + (CH3)3(n≥1), -OCOCH2N + (CH3)3, -OCH3, -OPO3 2- and -OSO3 - One of them.

2. The pyrrolidine nitroxide radical with hydrophilic groups linked by a carbon chain according to claim 1, characterized in that, The pyrrolidine nitroxide radical connected by a carbon chain to a hydrophilic group is 3-trimethylammoniummethyl-2,2,5,5-pyrrolidine nitroxide radical, 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical, or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical.

3. The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains as described in claim 1, characterized in that... Includes the following steps: 1) 4-O-TEMPO reacts with iodine and ammonia in a Favorskii rearrangement reaction to generate 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical; 2) The 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical is hydrolyzed under alkaline or acidic conditions to generate 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical; 3) React 3-amide-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-carboxyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical with lithium aluminum hydride to generate 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical; 4) 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical or 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical is condensed with a halide or esterified with a carboxylic acid, phosphoric acid, or sulfuric acid to obtain a pyrrolidine nitroxide radical with a hydrophilic group linked by a carbon chain.

4. The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains according to claim 3, characterized in that, The molar ratio of 4-oxy-TEMPO, iodine and ammonia is 1:(1-5):(1-10), and the temperature of the rearrangement reaction is 0-50℃.

5. The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by carbon chains according to claim 3, characterized in that, The molar ratio of the 3-amide-2,2,5,5-pyrrolidine nitroxide radical to the base or acid is 1:(1-10), and the hydrolysis reaction temperature is 0-100℃.

6. The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by a carbon chain according to claim 5, characterized in that, The alkali is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide; the acid is hydrochloric acid, sulfuric acid, and nitric acid.

7. The method for preparing pyrrolidine nitroxide radicals with hydrophilic groups linked by a carbon chain according to claim 3, characterized in that, The molar ratio of the 3-aminomethyl-2,2,5,5-pyrrolidine nitroxide radical or the 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to lithium aluminum hydride is 1:(1-10); the reaction temperature with lithium aluminum hydride is 0-100℃. The molar ratio of the 3-aminomethyl-2,2,5,5-tetramethylpyrrolidine nitroxide radical to the halide is 1:(1-10), and the condensation reaction temperature is 0-100℃.

8. The application of the pyrrolidine nitroxide radical linked by a carbon chain to a hydrophilic group as described in claim 1 in the preparation of a positive electrode electroactive material for an aqueous organic flow battery, characterized in that, The positive electrode of an aqueous organic flow battery includes a positive electrode active material and an aqueous solution of a supporting electrolyte; the active material of the positive electrode electrolyte is a pyrrolidine nitroxide radical linked by a carbon chain to a hydrophilic group; the supporting electrolyte is a single-component or multi-component chloride ion inorganic salt.

9. The application of the pyrrolidine nitroxide radical linked by a carbon chain to a hydrophilic group as described in claim 8 in the preparation of a positive electrode electroactive material for an aqueous organic flow battery, characterized in that, The concentration of the pyrrolidine nitroxide radicals with hydrophilic groups linked to the carbon chain is 0.01 mol / L to 5 mol / L.

10. The application of the pyrrolidine nitroxide radical linked by a carbon chain to a hydrophilic group as described in claim 8 in the preparation of a positive electrode electroactive material for an aqueous organic flow battery, characterized in that, The chloride ion inorganic salt is LiCl, NaCl, KCl, NH4Cl, MgCl2 or CaCl2; the concentration of the chloride ion inorganic salt is 0.01 to 5 mol / L.

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