Preparation method of branched polybenzimidazole membrane

Abstract

The invention discloses a preparation method of a branched polybenzimidazole membrane, which comprises the following steps: synthesizing 4, 4 ', 4' '-(benzene-1, 3, 5-trioxy) tribenzoic acid through substitution and hydrolysis reaction; the preparation method comprises the following steps: under the protection of nitrogen, carrying out high-temperature polycondensation on 4, 4 ', 4'-(benzene-1, 3, 5-trioxyl) tribenzoic acid, 4, 4 '-dicarboxyl diphenyl ether and 3, 3'-diaminobenzidine serving as raw materials to obtain a branched polybenzimidazole polymer; and dissolving the branched polybenzimidazole polymer in dimethyl sulfoxide, casting to form a membrane, acidizing and washing to obtain the branched polybenzimidazole membrane material. According to the invention, the branching degree is controlled by regulating and controlling the dosage ratio of 4, 4 ', 4' '-(benzene-1, 3, 5-trioxyl) tribenzoic acid to 4, 4'-dicarboxyl diphenyl ether, so that the ionic conductivity of the membrane can be remarkably improved, and the membrane has excellent application potential of the all-vanadium redox flow battery.

Description

Technical Field

[0001] The invention belongs to the technical field of battery separator preparation, and more specifically, the invention relates to a method for preparing a branched polybenzimidazole membrane. Background Art

[0002] The rapid development of human economy has led to a sharp increase in energy demand, while the massive consumption of fossil energy has caused serious environmental and social problems. Therefore, wind and solar energy, which are clean and renewable, have been greatly developed as alternative energy sources. Due to the urgent need to overcome the natural randomness and intermittent characteristics of renewable energy, large-scale energy storage systems with large capacity have attracted widespread attention. Energy storage devices can provide amplitude modulation and frequency modulation, smooth power output and regulate power generation, thereby achieving the continuity, stability and controllability of renewable energy generation. Because of this, large-scale energy storage technology with large capacity is the key to the development of renewable energy. Batteries can store the electrical energy generated by renewable energy in the form of chemical energy and convert chemical energy into electrical energy as needed. Therefore, battery technology can accelerate the utilization of renewable energy. Based on this, the all-vanadium liquid flow battery (VFB) energy storage technology with the advantages of high safety, long cycle life, recyclable electrolyte, and high cost-effectiveness of life cycle has shown broad application prospects in large-scale energy storage systems.

[0003] At present, the membrane commonly used in VFB is a perfluorosulfonated ion exchange membrane (such as Nafion membrane). This membrane has good chemical stability and high ion conductivity. However, the obvious hydrophilic / hydrophobic microphase separation structure forms a large-sized ion transmission channel, which allows hydrated vanadium ions to pass through the membrane, resulting in poor ion selectivity and reducing the coulombic efficiency of VFB. In addition, the price of perfluorosulfonated ion exchange membranes is expensive. For example, the market price of Nafion membranes produced by DuPont is currently $700m. -2 This greatly limits the commercialization and industrialization of VFB. Therefore, the development of non-perfluorinated ion exchange membranes with high ion conductivity and selectivity, excellent chemical stability, and low cost and environmental protection is crucial to accelerate the commercial development of VFB.

[0004] Polybenzimidazole (PBI), as an aromatic heterocyclic polymer, has the advantages of excellent thermal stability, outstanding chemical stability and good mechanical properties. Pure PBI is a non-conductive polymer, but under acidic conditions, PBI is protonated and absorbs acid and water, thereby achieving ion conduction. Based on this feature, acid-doped PBI membranes have become a type of membrane material for VFB. However, the ion conductivity of dense PBI membranes cast from organic solutions without additional treatment is relatively low. Therefore, in order to prepare PBI membranes with high ion conductivity, many researchers have studied this and adopted different strategies, including: preparing porous membranes or composite membranes, grafting bulky or hydrophilic side chains, and synthesizing PBI with different structures. SH Moon et al. used polyethylene glycol as a pore former and prepared nanoporous PBI membranes by solution casting. Under optimal conditions, the ion conductivity of the PBI membrane (PBI-10%) was as high as 1.61mS cm -1 , which is about three times that of dense PBI membrane (0.55mS cm -1 ). XMYan et al. successfully prepared long-chain sulfonated polybenzimidazole (PBI-PS) membrane. As the sulfonation degree increased from 108% to 200%, the area resistance of the PBI-PS membrane increased from 1.35Ωcm 2 Reduced to 0.51Ωcm 2 , ionic conductivity from 4.00mS cm -1 Increased to 10.78 mS cm -1 DJ Chen et al. designed a PBI membrane with dual channels (B-PBI) by introducing pyridine groups into the PBI main chain. The area resistance of the B-PBI membrane (<0.2Ωcm 2 ) is less than one third of PBI membrane (>0.6Ωcm 2 ), thereby significantly enhancing the ion conductivity of the B-PBI membrane. Therefore, the present invention aims to overcome the shortcomings of the ion conductivity of the PBI membrane, prepare a branched PBI membrane with good chemical stability and ion conductivity, and correspondingly provide its preparation method and main application fields. Summary of the invention

[0005] An object of the present invention is to solve at least the above problems and / or disadvantages and to provide at least the advantages which will be described hereinafter.

[0006] In order to achieve these purposes and other advantages according to the present invention, a branched polybenzimidazole membrane is provided, the chemical formula of which is:

[0007]

[0008] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0009] Step 1: under nitrogen protection, add phosphorus pentoxide and methane sulfonic acid into a reactor, stir at a certain temperature until dissolved; add 3,3'-diaminobenzidine, heat and stir until dissolved; add 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat and stir until dissolved; then add 4,4'-dicarboxy diphenyl ether, heat and stir to react for a certain time, after the reaction liquid becomes viscous, pour it into a pre-cooled detergent to obtain a noodle-like solid, neutralize it with an alkaline substance to a weak alkalinity, filter it, wash it with a detergent for multiple times, and dry it to obtain a branched polybenzimidazole polymer;

[0010] Step 2: Add the branched polybenzimidazole polymer and dimethyl sulfoxide obtained in step 1 into a reactor, stir until dissolved to obtain a casting solution, pour the casting solution onto a dry and clean glass plate, cast into a film, take out the film after drying, immerse it in an acidic solution for acid doping, and then wash it with a detergent to obtain a branched polybenzimidazole film.

[0011] Preferably, in the step 1, phosphorus pentoxide and methane sulfonic acid are added to the reactor and stirred at 30-60°C for 2-4 hours; 3,3'-diaminobenzidine is added and stirred at 60-100°C for 2-4 hours; 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid is added and stirred at 90-110°C for 2-4 hours; 4,4'-dicarboxy diphenyl ether is added and stirred at 100-150°C for 2-8 hours; the product is dried at 40-80°C for 12-36 hours.

[0012] Preferably, in the step 1, the mass ratio of phosphorus pentoxide to methanesulfonic acid is 1:7-13; the molar ratio of 3,3'-diaminobenzidine, 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid and 4,4'-dicarboxy diphenyl ether is 1.0:0.0-0.2:0.7-1.0; and for every 4-6 mmol of 3,3'-diaminobenzidine added, the mass of phosphorus pentoxide added is 4.0-6.0 g.

[0013] Preferably, in step 1, the alkaline substance is one or more of sodium bicarbonate, sodium carbonate and sodium hydroxide; and the detergent is one or more of deionized water, distilled water and ultrapure water.

[0014] Preferably, in the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0015] Step I: Place p-fluorobenzonitrile and phloroglucinol in N,N-dimethylformamide, add potassium carbonate, and react at 120-150° C. for 20-36 hours; after the reaction is complete, cool to room temperature, pour into deionized water for precipitation, refrigerate at 0-5° C. for 20-36 hours, filter, and dry at 40-80° C. for 12-36 hours to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0016] Step II: Place 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of ethanol and water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, and react at 90-110° C. for 20-36 hours; after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 40-80° C. for 12-36 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

[0017] Preferably, in step I, the molar ratio of phloroglucinol, fluorobenzonitrile and potassium carbonate is 1.0:2.8-3.2:2.8-3.2, and the molar volume ratio of phloroglucinol and N,N-dimethylformamide is 1-2 mmol:4.0-8.0 mL.

[0018] Preferably, in step II, the molar volumes of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile, ethanol and water are 1-2 mmol: 16.0-32.0 mL: 4.0-8.0 mL.

[0019] Preferably, in the step 2, the mass volume ratio of the branched polybenzimidazole polymer and dimethyl sulfoxide is 0.5-1.0 g:15.0-30.0 mL; the drying condition is 12-36 h at 60-100 ° C; the acidic solution is 1.0-4.0 mol / L H2SO4 solution, and the soaking time is 5-9 days; the detergent is one or more of deionized water, distilled water and ultrapure water;

[0020] In the step 2, the thickness of the obtained branched polybenzimidazole membrane is 25 to 50 μm.

[0021] The invention discloses an application of a branched polybenzimidazole membrane, wherein the branched polybenzimidazole membrane is applied to a diaphragm of an all-vanadium liquid flow battery.

[0022] The present invention has at least the following beneficial effects:

[0023] (1) The present invention uses 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, 4,4'-dicarboxy diphenyl ether and 3,3'-diaminobenzidine as raw materials to prepare a branched polybenzimidazole membrane, which can significantly improve the interaction between molecular chains and the spatial free volume, so that the ion conductivity of the branched polybenzimidazole membrane is optimized, overcoming the disadvantage of low ion conductivity of the polybenzimidazole membrane in VFB use, thereby further enhancing the VFB efficiency.

[0024] (2) The efficiency of VFB assembled using the branched polybenzimidazole membrane prepared by the present invention can be higher than that of the battery using Nafion212 membrane, indicating that the branched polybenzimidazole membrane provided by the present invention can better meet the application requirements of VFB.

[0025] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The synthetic route of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid of the present invention;

[0027] Figure 2 The invention provides a preparation route for the branched polybenzimidazole membrane;

[0028] Figure 3 is the hydrogen nuclear magnetic resonance spectrum of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid of the present invention;

[0029] Figure 4 The infrared spectra of the branched polybenzimidazole film prepared in Example 3 and the polybenzimidazole film prepared in Comparative Example 1;

[0030] Figure 5 The figure is a comparison of the coulombic efficiency of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the commercial Nafion 212 membrane;

[0031] Figure 6 This is a comparison chart of the energy efficiency of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the commercial Nafion 212 membrane;

[0032] Figure 7 The voltage efficiency comparison diagram of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the commercial Nafion 212 membrane;

[0033] Figure 8 This is a comparison chart of the coulombic efficiency of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the polybenzimidazole membrane prepared in Comparative Example 1;

[0034] Fig. 9 This is a comparison diagram of the energy efficiency of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the polybenzimidazole membrane prepared in Comparative Example 1;

[0035] Fig.10 This is a comparison diagram of the voltage efficiency of VFB using the branched polybenzimidazole membrane prepared in Example 3 and the polybenzimidazole membrane prepared in Comparative Example 1. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement the invention with reference to the description.

[0037] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0038] Embodiment 1:

[0039] like Figure 2 As shown, a method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0040] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; add 0.15mmol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat to 100°C and stir for 3h; then add 4.775mmol of 4,4'-dicarboxyl diphenyl ether, heat to 120°C and stir for 6h. After the reaction liquid becomes viscous, pour it into pre-cooled deionized water to obtain noodle-like solids, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for several times, and dry it at 60°C for 24h to obtain branched polybenzimidazole polymer;

[0041] Step 2: Add 1.0 g of the branched polybenzimidazole polymer obtained in step 1 and 25.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a branched polybenzimidazole membrane (PBI-B-3) with a thickness of 37 μm.

[0042] like Figure 1 As shown, in the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0043] Step I: 0.12 mol of p-fluorobenzonitrile and 0.04 mol of phloroglucinol were placed in 160.0 mL of N,N-dimethylformamide, and then 0.12 mol of potassium carbonate was added, and the mixture was reacted at 140° C. for 28 h. After the reaction was complete, the mixture was cooled to room temperature, poured into deionized water for precipitation, refrigerated at 0-5° C. for 24 h, filtered, and dried at 60° C. for 24 h to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0044] Step II: 0.01 mol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile was placed in a mixed solution of 160.0 mL of ethanol and 40.0 mL of water, potassium hydroxide was added to adjust the pH value of the reaction solution to 13-14, and the reaction was carried out at 100°C for 24 hours. After the reaction was complete, the reaction solution was cooled to room temperature, and hydrochloric acid was slowly added dropwise to adjust the pH value of the reaction solution to 5-6. The reaction solution was allowed to stand and filtered, and dried at 60°C for 24 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid. The chemical structure of the branched polybenzimidazole membrane prepared in this embodiment is as follows:

[0045]

[0046] Embodiment 2:

[0047] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0048] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; add 0.30mmol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat to 100°C and stir for 3h; then add 4.55mmol of 4,4'-dicarboxyl diphenyl ether, heat to 120°C and stir for 4h. After the reaction solution becomes viscous, pour it into pre-cooled deionized water to obtain noodle-like solids, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for several times, and dry it at 60°C for 24h to obtain branched polybenzimidazole polymer;

[0049] Step 2: Add 1.0 g of the branched polybenzimidazole polymer obtained in step 1 and 25.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a branched polybenzimidazole membrane (PBI-B-6) with a thickness of 34 μm.

[0050] In the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0051] Step I: 0.12 mol of p-fluorobenzonitrile and 0.04 mol of phloroglucinol were placed in 160.0 mL of N,N-dimethylformamide, and then 0.12 mol of potassium carbonate was added, and the mixture was reacted at 140° C. for 28 h. After the reaction was complete, the mixture was cooled to room temperature, poured into deionized water for precipitation, refrigerated at 0-5° C. for 24 h, filtered, and dried at 60° C. for 24 h to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0052] Step II: Place 0.01 mol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of 160.0 mL of ethanol and 40.0 mL of water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, react at 100°C for 24 hours, and after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 60°C for 24 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

[0053] Embodiment 3:

[0054] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0055] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; add 0.45mmol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat to 100°C and stir for 3h; then add 4.325mmol of 4,4'-dicarboxyl diphenyl ether, heat to 120°C and stir for 3h. After the reaction liquid becomes viscous, pour it into pre-cooled deionized water to obtain noodle-like solids, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for several times, and dry it at 60°C for 24h to obtain branched polybenzimidazole polymer;

[0056] Step 2: Add 1.0 g of the branched polybenzimidazole polymer obtained in step 1 and 28.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a branched polybenzimidazole membrane (PBI-B-9) with a thickness of 36 μm.

[0057] In the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0058] Step I: 0.12 mol of p-fluorobenzonitrile and 0.04 mol of phloroglucinol were placed in 160.0 mL of N,N-dimethylformamide, and then 0.12 mol of potassium carbonate was added, and the mixture was reacted at 140° C. for 28 h. After the reaction was complete, the mixture was cooled to room temperature, poured into deionized water for precipitation, refrigerated at 0-5° C. for 24 h, filtered, and dried at 60° C. for 24 h to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0059] Step II: Place 0.01 mol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of 160.0 mL of ethanol and 40.0 mL of water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, react at 100°C for 24 hours, and after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 60°C for 24 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

[0060] The hydrogen nuclear magnetic resonance spectrum of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid prepared in this example is as follows Figure 3 The peak with chemical shift at 6.7 ppm originates from the benzene ring hydrogen of phloroglucinol; the peaks at 7.2 ppm and 7.9 ppm originate from the benzene ring hydrogen of p-fluorobenzonitrile, and the peak at 12.8 ppm corresponds to the carboxyl hydrogen. The integrated area ratio of the four chemical shift peaks is about 1:2:2:1, which is consistent with the theory, indicating that 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid has been successfully prepared by substitution and hydrolysis reactions.

[0061] The infrared spectrum of the branched polybenzimidazole membrane prepared in this example is as follows: Figure 4 As shown. At 1400~1500cm -1 The absorption peak at 1230cm -1 and 802cm -1 The absorption vibration peak at 1598cm -1 The stretching vibration peak of C=N is at 1286cm -1 The stretching vibration peak of CN also appeared at 1166cm -1 The absorption peak at comes from the ether bond between benzene rings. Based on the above results, it is preliminarily proved that the branched polybenzimidazole membrane has been successfully prepared.

[0062] The branched polybenzimidazole membrane prepared in this example and the commercial Nafion 212 membrane were used to assemble the VFB to verify the battery performance of the membrane. 1.7 mol / L VO 3.5+ +4.7mol / LH2SO4 solution 30.0mL, the electrolyte was transported into the battery by a peristaltic pump, and the battery was tested for constant current charge and discharge (current density 300~80mA / cm 2 ), the voltage range is 0.8~1.65V. The coulombic efficiency, energy efficiency and voltage efficiency can be calculated using the following formula:

[0063] Coulomb efficiency CE = discharge capacity / charge capacity × 100%

[0064] Energy efficiency EE = discharge energy / charge energy × 100%

[0065] Voltage efficiency VE = energy efficiency / coulomb efficiency × 100%

[0066] The coulombic efficiency results are as follows Figure 5 The energy efficiency results are shown in Figure 6 As shown, the voltage efficiency results are as follows Figure 7 As shown, it can be seen that compared with the commercial Nafion 212 membrane, the branched polybenzimidazole membrane prepared in this example has a higher efficiency of the all-vanadium redox flow battery, indicating that the branched polybenzimidazole membrane of the present invention has excellent application potential in all-vanadium redox flow batteries.

[0067] Embodiment 4:

[0068] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0069] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; add 0.60mmol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat to 100°C and stir for 3h; then add 4.1mmol of 4,4'-dicarboxyl diphenyl ether, heat to 110°C and stir for 3h. After the reaction solution becomes viscous, pour it into pre-cooled deionized water to obtain noodle-like solids, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for several times, and dry it at 60°C for 24h to obtain branched polybenzimidazole polymer;

[0070] Step 2: Add 1.0 g of the branched polybenzimidazole polymer obtained in step 1 and 30.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a branched polybenzimidazole membrane (PBI-B-12) with a thickness of 34 μm.

[0071] In the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0072] Step I: 0.12 mol of p-fluorobenzonitrile and 0.04 mol of phloroglucinol were placed in 160.0 mL of N,N-dimethylformamide, and then 0.12 mol of potassium carbonate was added, and the mixture was reacted at 140° C. for 28 h. After the reaction was complete, the mixture was cooled to room temperature, poured into deionized water for precipitation, refrigerated at 0-5° C. for 24 h, filtered, and dried at 60° C. for 24 h to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0073] Step II: Place 0.01 mol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of 160.0 mL of ethanol and 40.0 mL of water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, react at 100°C for 24 hours, and after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 60°C for 24 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

[0074] Embodiment 5:

[0075] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0076] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; add 0.75mmol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat to 100°C and stir for 3h; then add 3.875mmol of 4,4'-dicarboxyl diphenyl ether, heat to 100°C and stir for 3h. After the reaction solution becomes viscous, pour it into pre-cooled deionized water to obtain a noodle-like solid, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for several times, and dry it at 60°C for 24h to obtain a branched polybenzimidazole polymer;

[0077] Step 2: Add 1.0 g of the branched polybenzimidazole polymer obtained in step 1 and 30.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a branched polybenzimidazole membrane (PBI-B-15) with a thickness of 33 μm.

[0078] In the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises:

[0079] Step I: 0.12 mol of p-fluorobenzonitrile and 0.04 mol of phloroglucinol were placed in 160.0 mL of N,N-dimethylformamide, and then 0.12 mol of potassium carbonate was added, and the mixture was reacted at 140° C. for 28 h. After the reaction was complete, the mixture was cooled to room temperature, poured into deionized water for precipitation, refrigerated at 0-5° C. for 24 h, filtered, and dried at 60° C. for 24 h to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile;

[0080] Step II: Place 0.01 mol of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of 160.0 mL of ethanol and 40.0 mL of water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, react at 100°C for 24 hours, and after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 60°C for 24 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

[0081] Comparative Example 1:

[0082] A method for preparing a branched polybenzimidazole membrane comprises the following steps:

[0083] Step 1: Under nitrogen protection, add 5.0g of phosphorus pentoxide and 50.0g of methane sulfonic acid into the reactor, stir at 50°C for 3h; add 5mmol of 3,3'-diaminobenzidine, heat to 80°C and stir for 3h; then add 5mmol of 4,4'-dicarboxyl diphenyl ether, heat to 130°C and stir for 6h. After the reaction liquid becomes viscous, pour it into pre-cooled deionized water to obtain noodle-like solids, neutralize it with sodium bicarbonate to weak alkalinity, filter it, wash it with deionized water for multiple times, and dry it at 60°C for 24h to obtain polybenzimidazole polymer;

[0084] Step 2: Add 1.0 g of the polybenzimidazole polymer obtained in step 1 and 20.0 mL of dimethyl sulfoxide into the reactor and stir until dissolved to obtain a casting solution. Pour the casting solution onto a dry and clean glass plate and cast it into a film. After drying at 80°C for 24 hours, take out the film and immerse it in a 3.0 mol / L H2SO4 solution for 7 days for acid doping, and then wash it with deionized water to obtain a polybenzimidazole film (PBI-B-0) with a thickness of 35 μm.

[0085] The infrared spectrum of the polybenzimidazole film prepared in this comparative example is as follows Figure 4 As shown. At 1400~1500cm -1 The absorption peak at 1230cm -1 and 802cm -1 The absorption vibration peak at 1598cm -1 The stretching vibration peak of C=N is at 1286cm -1 The stretching vibration peak of CN also appeared at 1166cm -1 The absorption peak at comes from the ether bond between the benzene rings, and the absorption peak here is significantly weaker than that in Example 3. Based on the above results, it is preliminarily proved that the polybenzimidazole membrane has been successfully prepared.

[0086] The polybenzimidazole membrane prepared in this comparative example and the branched polybenzimidazole membrane prepared in Example 3 were used to assemble VFB to verify the battery performance of the membrane. 1.7 mol / L VO 3.5+ +4.7mol / L H2SO4 solution 30.0mL, the electrolyte was transported into the battery by a peristaltic pump, and the battery was tested for constant current charge and discharge (current density 300~80mA / cm 2 ), the voltage range is 0.8~1.65V. The coulombic efficiency, energy efficiency and voltage efficiency can be calculated using the following formula:

[0087] Coulomb efficiency CE = discharge capacity / charge capacity × 100%

[0088] Energy efficiency EE = discharge energy / charge energy × 100%

[0089] Voltage efficiency VE = energy efficiency / coulomb efficiency × 100%

[0090] The coulombic efficiency results are as follows Figure 8 The energy efficiency results are shown in Fig. 9 As shown, the voltage efficiency results are as follows Fig.10As shown, it can be seen that compared with the branched polybenzimidazole membrane prepared in Example 3, the polybenzimidazole membrane prepared in this comparative example has a low VFB efficiency, indicating that the branched polybenzimidazole membrane prepared in Example 3 has excellent application potential in all-vanadium liquid flow batteries.

[0091] The number of devices and processing scales described here are used to simplify the description of the present invention. Applications, modifications and variations of the present invention will be obvious to those skilled in the art.

[0092] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes, and they can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and the illustrations shown and described herein.

Claims

1. A branched polybenzimidazole membrane, characterized in that: Its chemical structure is:

2. A method for preparing a branched polybenzimidazole membrane as claimed in claim 1, characterized in that: The following steps are involved: Step 1: under nitrogen protection, add phosphorus pentoxide and methane sulfonic acid into a reactor, stir at a certain temperature until dissolved; add 3,3'-diaminobenzidine, heat and stir until dissolved; add 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid, heat and stir until dissolved; then add 4,4'-dicarboxy diphenyl ether, heat and stir to react for a certain time, after the reaction liquid becomes viscous, pour it into a pre-cooled detergent to obtain a noodle-like solid, neutralize it with an alkaline substance to a weak alkalinity, filter it, wash it with a detergent for multiple times, and dry it to obtain a branched polybenzimidazole polymer; Step 2: Add the branched polybenzimidazole polymer and dimethyl sulfoxide obtained in step 1 into a reactor, stir until dissolved to obtain a casting solution, pour the casting solution onto a dry and clean glass plate, cast into a film, take out the film after drying, immerse it in an acidic solution for acid doping, and then wash it with a detergent to obtain a branched polybenzimidazole film.

3. The method for preparing a branched polybenzimidazole membrane according to claim 2, wherein: In the step 1, phosphorus pentoxide and methane sulfonic acid are added to a reactor and stirred at 30-60° C. for 2-4 hours; 3,3'-diaminobenzidine is added and stirred at 60-100° C. for 2-4 hours; 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid is added and stirred at 90-110° C. for 2-4 hours; 4,4'-dicarboxyl diphenyl ether is added and stirred at 100-150° C. for 2-8 hours; the product is dried at 40-80° C. for 12-36 hours.

4. The method for preparing a branched polybenzimidazole membrane according to claim 2, wherein: In the step 1, the mass ratio of phosphorus pentoxide to methanesulfonic acid is 1:7-13; the molar ratio of 3,3'-diaminobenzidine, 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid and 4,4'-dicarboxy diphenyl ether is 1.0:0.0-0.2:0.7-1.0; for every 4-6 mmol of 3,3'-diaminobenzidine added, the mass of phosphorus pentoxide added is 4.0-6.0 g.

5. The method for preparing a branched polybenzimidazole membrane according to claim 2, wherein: In the step 1, the alkaline substance is one or more of sodium bicarbonate, sodium carbonate and sodium hydroxide; the detergent is one or more of deionized water, distilled water and ultrapure water.

6. The method for preparing a branched polybenzimidazole membrane according to claim 2, wherein: In the step 1, the preparation method of 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid comprises: Step I: Place p-fluorobenzonitrile and phloroglucinol in N,N-dimethylformamide, add potassium carbonate, and react at 120-150° C. for 20-36 hours; after the reaction is complete, cool to room temperature, pour into deionized water for precipitation, refrigerate at 0-5° C. for 20-36 hours, filter, and dry at 40-80° C. for 12-36 hours to obtain the intermediate 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile; Step II: Place 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile in a mixed solution of ethanol and water, add potassium hydroxide to adjust the pH value of the reaction solution to 13-14, and react at 90-110° C. for 20-36 hours; after the reaction is complete, cool to room temperature, slowly add hydrochloric acid to adjust the pH value of the reaction solution to 5-6, let it stand and filter, and dry at 40-80° C. for 12-36 hours to obtain the final product 4,4',4"-(benzene-1,3,5-trioxy)tribenzoic acid.

7. The method for preparing a branched polybenzimidazole membrane according to claim 6, characterized in that: In the step I, the molar ratio of phloroglucinol, fluorobenzonitrile and potassium carbonate is 1.0:2.8-3.2:2.8-3.2, and the molar volume ratio of phloroglucinol and N,N-dimethylformamide is 1-2 mmol:4.0-8.0 mL.

8. The method for preparing a branched polybenzimidazole membrane according to claim 6, characterized in that: In the step II, the molar volumes of 4,4',4"-(benzene-1,3,5-trioxy)tribenzonitrile, ethanol and water are 1-2 mmol: 16.0-32.0 mL: 4.0-8.0 mL.

9. The method for preparing a branched polybenzimidazole membrane according to claim 2, wherein: In the step 2, the mass volume ratio of the branched polybenzimidazole polymer and dimethyl sulfoxide is 0.5-1.0 g:15.0-30.0 mL; the drying condition is 12-36 h at 60-100 ° C; the acidic solution is 1.0-4.0 mol / L H2SO4 solution, and the soaking time is 5-9 days; the detergent is one or more of deionized water, distilled water and ultrapure water; In the step 2, the thickness of the obtained branched polybenzimidazole membrane is 25 to 50 μm.

10. Use of a branched polybenzimidazole membrane prepared by the method for preparing a branched polybenzimidazole membrane according to any one of claims 2 to 9, characterized in that: The branched polybenzimidazole membrane is applied to the diaphragm of all-vanadium redox flow battery.

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