A porous polybenzimidazole membrane and a preparation method thereof, and a flow battery

Abstract

The application belongs to the technical field of liquid flow batteries, and discloses a porous polybenzimidazole membrane, a preparation method thereof and a liquid flow battery, wherein the preparation method comprises the following steps: S1, a nucleophilic substitution reaction is performed on polybenzimidazole and a functional compound to obtain polybenzimidazole grafted with a functional side chain; S2, a casting solution containing polybenzimidazole grafted with a functional side chain is coated into a wet membrane; and S3, the wet membrane is subjected to preliminary setting through pre-drying, and then is subjected to heat treatment to form pores, so that the porous polybenzimidazole membrane is obtained. The porous polybenzimidazole membrane prepared by the preparation method has excellent conductivity, high selectivity and good mechanical properties, and can fully meet the requirements of ion-conducting membranes for liquid flow batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of liquid flow batteries, and particularly relates to a porous polybenzimidazole membrane, a preparation method thereof and a liquid flow battery. BACKGROUND

[0002] Under the background of sustainable development, various renewable energies such as wind energy and solar energy are widely used, but the above-mentioned renewable energies have the characteristics of intermittency and volatility, therefore, in order to realize the smooth output of new energy power generation, energy conversion and storage technology has been widely studied. Among many energy storage technologies, liquid flow batteries are considered an ideal long-time energy storage technology because of their low cost, long cycle life, high efficiency, and independently adjustable power and capacity.

[0003] Among the key components of the liquid flow battery stack, the ion-conducting membrane is responsible for separating the positive and negative active materials and allowing the passage of balancing ions, and plays a crucial role in the operation of the liquid flow battery; an ideal ion-conducting membrane should have excellent conductivity, high selectivity and good mechanical strength. The most widely used membrane in liquid flow batteries is a perfluorosulfonic acid membrane, which has the advantages of good chemical stability and high conductivity; however, its relatively low ion selectivity and high cost limit further application. Therefore, it is crucial to develop low-cost, high-performance ion-conducting membranes.

[0004] Currently, non-fluorinated polymers have become a membrane material that has attracted much attention, with the advantages of lower cost and environmental friendliness, and have broad application prospects in the construction of separators for liquid flow batteries. Among them, polybenzimidazole (PBI) has become the most feasible polymer due to its excellent chemical stability. However, the molecular chains of polybenzimidazole membranes prepared by direct solvent evaporation are closely arranged, the ion mass transfer channels are limited, and the conductivity is low, which limits the performance of liquid flow batteries, so it is necessary to precisely regulate the structure of the polybenzimidazole membrane and introduce rich porous mass transfer channels therein.

[0005] Phase inversion method is widely used to construct porous structures in polymer membranes, but this method requires accurate control of conditions such as humidity and temperature, and it is difficult to precisely regulate the pore structure. Soft / hard template method is also used to prepare porous polymer membranes, but the removal of the template agent is usually required, which is a complicated process and is difficult to use for industrialized continuous production.

[0006] Therefore, it is of great significance to develop a preparation method of a polybenzimidazole membrane that can realize precise regulation of sub-nanometer pores and simultaneously introduce functional groups, for improving the performance of liquid flow batteries. SUMMARY

[0007] In view of the deficiencies of the prior art, the present application aims to provide a porous polybenzimidazole membrane, a preparation method thereof and a flow battery, the porous polybenzimidazole membrane provided by the present application has excellent conductivity, high selectivity and good mechanical properties, and can fully meet the requirements of ion-conducting membranes for flow batteries.

[0008] To achieve the object of the present application, the following technical solutions are adopted in the present application:

[0009] In a first aspect, the present application provides a preparation method of a porous polybenzimidazole membrane, the preparation method comprising:

[0010] S1. carrying out a nucleophilic substitution reaction on polybenzimidazole (PBI) and a functional compound to obtain polybenzimidazole grafted with functional side chains;

[0011] S2. coating a casting solution containing polybenzimidazole grafted with functional side chains into a wet film;

[0012] S3. carrying out preliminary setting of the wet film through pre-drying, and then carrying out heat treatment to form pores, to obtain the porous polybenzimidazole membrane.

[0013] In the preparation method provided by the present application, the functional compound is grafted on the PBI main chain through a substitution reaction, then coated into a film, and finally decomposed by using programmed heat treatment, so that N2 gas is generated in situ to form pores, and the functional groups (sulfonic acid groups) are left on the inner wall of the pores, thus the preparation method provided by the present application can simultaneously construct porous channels and functionalize on the dense PBI film, and the obtained PBI film has high proton conductivity and low vanadium ion permeability, and when applied to ion-conducting membranes of flow batteries, the battery exhibits high coulombic efficiency, high energy efficiency and excellent long-cycle stability.

[0014] Preferably, the functional compound contains a sulfonyl hydrazine group (-SO2NHNH2) and an electrophilic reaction group at the same time, and preferably the electrophilic reaction group includes a carboxyl group (-COOH) and / or a halogen (-X, such as -Cl, -Br, etc.).

[0015] Preferably, the functional compound includes 4-bromobenzenesulfonyl hydrazide and / or 4-sulfonyl hydrazide benzoic acid.

[0016] Preferably, the ratio of the molar mass of the functional compound to the molar mass of the structural unit of the polybenzimidazole is (0.5-1.5):1, such as 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc.

[0017] Preferably, the nucleophilic substitution reaction is carried out under catalysis of a catalyst.

[0018] Preferably, the catalyst comprises any one or a combination of at least two of potassium carbonate, sodium carbonate or potassium hydroxide.

[0019] Preferably, the mole ratio of the added mole amount of the catalyst to the repeating unit of the polybenzimidazole is (1-3):1, such as 1:1, 1.5:1, 2:1, 2.5:1, 3:1, etc.

[0020] Preferably, the nucleophilic substitution reaction is carried out at a temperature of 60-100℃, such as 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, etc., for a time of 10-40 h, such as 10 h, 15 h, 20 h, 25 h, 30 h, 35 h, 40 h, etc.

[0021] The present application can precisely control the grafting amount to be 50-150% by controlling the feeding ratio, the temperature and the time of the nucleophilic substitution reaction. If the grafting amount is too small, the resulting polybenzimidazole membrane has less pore structure, affecting its performance. If the grafting amount is too high, it affects the chemical stability of the membrane.

[0022] Preferably, the nucleophilic substitution reaction is carried out in an organic solvent, preferably the organic solvent comprises any one or a combination of at least two of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP) or tetrahydrofuran (THF).

[0023] Preferably, the nucleophilic substitution reaction is carried out under a protective atmosphere.

[0024] Preferably, step (1) further comprises, after the reaction is completed, carrying out precipitation, filtration, washing and drying.

[0025] In the present application, in order to fully decompose the sulfonamide group, the present application adopts a programmed temperature heat treatment method, the temperature is raised to 100℃, then raised to 150℃ in a gradient heating and holding manner, wherein 100-150℃ is the decomposition temperature of the sulfonamide group, in this process, the sulfonamide group (-SO2NHNH2) at the end of the grafting branch is decomposed by heat to produce N2 and -SO3H groups, the generated N2 gas forms sub-nanometer scale pores in the polymer matrix membrane, and the -SO3H group remains in the inner wall of the pores, giving the polybenzimidazole membrane excellent hydrophilicity and proton conduction ability.

[0026] Preferably, the method for heat treatment pore formation comprises using a gradient heating-holding manner for multiple times to raise the temperature from 90℃ to 150℃.

[0027] Preferably, the method for heat treatment pore-forming comprises: heating to 90-110°C and holding for 10-30 min, for example, 10 min, 15 min, 20 min, 25 min, 30 min, etc., and then heating to 105-125°C and holding for 10-30 min, for example, 10 min, 15 min, 20 min, 25 min, 30 min, etc., and finally heating to 150°C and holding for 10-30 min, for example, 10 min, 15 min, 20 min, 25 min, 30 min, etc.

[0028] It should be noted that in the present application, the temperature of the second holding needs to be greater than the temperature of the first holding.

[0029] Preferably, the polybenzimidazole comprises any one or a combination of at least two of poly 2,2'-(m-phenyl)-5,5'-benzimidazole (mPBI), poly 2,2'-(p-phenyl)-5,5'-benzimidazole (pPBI), poly 2,2'-(p-diphenyl ether)-5,5'-benzimidazole (OPBI), or poly(2,5-benzimidazole) (ABPBI).

[0030] Preferably, the organic solvent used by the casting solution comprises any one or a combination of at least two of N,N-dimethylformamide, N,N-dimethylacetamide, or dimethyl sulfoxide.

[0031] Preferably, the mass percentage content of the polybenzimidazole grafted with the functional side chain is 5-15%, for example, 5%, 6%, 8%, 10%, 12%, 14%, 15%, etc., based on the total mass of the casting solution being 100%.

[0032] Preferably, the method for coating comprises blade coating, further comprising flow coating, and in the coating process, the thickness of the finally obtained wet film can be controlled by controlling the height of the doctor blade, and preferably the rate of the flow coating can be 0.05-0.2 m / min, for example, 0.05 m / min, 0.08 m / min, 0.1 m / min, 0.12 m / min, 0.15 m / min, 0.18 m / min, 0.2 m / min, etc.

[0033] Preferably, the thickness of the wet film is 200-500 μm, for example, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, etc.

[0034] Preferably, the pre-drying temperature is 60-80℃, such as 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, etc., and the time is 1-3 h, such as 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc.

[0035] Preferably, the preparation method comprises:

[0036] (1) polybenzimidazole (PBI) is dissolved in an organic solvent, 4-bromobenzenesulfonyl hydrazide and / or 4-sulfonyl hydrazide benzoic acid and a catalyst are added, and the reaction is carried out at 60-100℃ for 10-40 h. After the reaction, the product is precipitated, filtered, washed and dried to obtain polybenzimidazole grafted with functional side chains;

[0037] (2) the polybenzimidazole grafted with functional side chains is dissolved in an organic solvent to prepare a casting solution with a concentration of 5-15%. The casting solution is used to prepare a wet film with a thickness of 200-500 μm by a flow coating method;

[0038] (3) the wet film is pre-dried at 60-80℃ for 1-3 h;

[0039] (4) the pre-dried wet film is programmed to heat to 100℃ and keep for 10-30 min, then heated to 120℃ and kept for 10-30 min, and then heated to 150℃ and kept for 10-30 min to obtain the porous polybenzimidazole film.

[0040] The preparation method provided by the application can anchor the thermal decomposition functional side chains on the PBI main chain, and realize precise pore formation and simultaneous introduction of sulfonic acid groups through in-situ thermal decomposition reaction, so as to prepare a functional film with adjustable ion transmission channels, so as to realize high proton selectivity and high ion conductivity at the same time.

[0041] In the second aspect, the application provides a porous polybenzimidazole film prepared by the preparation method of the first aspect.

[0042] The preparation method provided by the application has the following advantages:

[0043] i. the film has porous channels with adjustable sizes formed by thermal decomposition gas inside the film;

[0044] ii. the inner wall of the channel is anchored with sulfonic acid groups (-SO3H), which can form a hydrophilic proton transmission channel;

[0045] iii. the ion exchange capacity IEC of the porous polybenzimidazole film is in the range of 0.5-2.0 mmol / g, preferably in the range of 0.8-1.5 mmol / g;

[0046] iv. The proton conductivity of the porous polybenzimidazole membrane (25℃) is not less than 15 mS / cm, and the vanadium ion permeation rate is not higher than 1.0*10 -7 cm 2

[0047] In a third aspect, the present application provides an application of the porous polybenzimidazole membrane of the second aspect in ion conduction.

[0048] In a fourth aspect, the present application provides a flow battery comprising an ion conduction membrane, wherein the ion conduction membrane is the porous polybenzimidazole membrane of the second aspect.

[0049] Compared with the prior art, the present application has the following beneficial effects:

[0050] (1) Precise control of pore channels: using the size-determined N2 molecules generated by thermal decomposition of functional side chains as pore-forming agents, the pore channel size can be precisely controlled on the sub-nanometer scale, effectively realizing ion sieving;

[0051] (2) Synchronous completion of functionalization and pore formation: while thermal decomposition forms pores, the -SO3H groups are permanently anchored in the pore walls in situ, avoiding subsequent sulfonation treatment steps, simplifying the process, and achieving uniform functional group distribution;

[0052] (3) Adjustable structure and performance: by simply adjusting the grafting degree of the functional side chain and the thermal treatment process (temperature, time), the pore structure, sulfonic acid group content, and hydrophilic / hydrophobic properties of the membrane can be precisely controlled, thereby optimizing its ion conductivity and selectivity;

[0053] (4) Excellent performance: the prepared porous PBI membrane has high proton conductivity (derived from continuous hydrophilic pores and sulfonic acid groups) and extremely low vanadium ion permeation rate (derived from sub-nanometer sieving pores), and when used in a vanadium redox flow battery, it exhibits high coulombic efficiency and high energy efficiency, as well as excellent long-cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 NMR spectrum of OPBI prepared by the present application;

[0055] Figure 2 Infrared spectrum of OPBI prepared by the present application;

[0056] Figure 3 Surface and cross-sectional SEM images of the porous polybenzimidazole membrane obtained in Example 1;

[0057] Figure 4 Cross-sectional SEM image of the polybenzimidazole membrane provided in Comparative Example 1;

[0058] Figure 5 ​This is a comparison chart of the water absorption rates of the porous polybenzimidazole membrane obtained in Example 1 and the polybenzimidazole membrane provided in Comparative Example 1.

[0059] Figure 6 This is a comparison chart of the swelling rates of the porous polybenzimidazole membrane obtained in Example 1 and the polybenzimidazole membrane provided in Comparative Example 1.

[0060] Figure 7 The all-vanadium redox flow single cells prepared from the porous polybenzimidazole membranes provided in Examples 1-3 operate at 80-200 mA·cm⁻¹. -2 Comparison of Coulomb efficiency under varying current;

[0061] Figure 8 The all-vanadium redox flow single cells prepared from the porous polybenzimidazole membranes provided in Examples 1-3 operate at 80-200 mA·cm⁻¹. -2 Comparison of voltage efficiency under varying current;

[0062] Figure 9 The all-vanadium redox flow single cells prepared from the porous polybenzimidazole membranes provided in Examples 1-3 operate at 80-200 mA·cm⁻¹. -2 Comparison of energy efficiency under varying current;

[0063] Figure 10 The porous polybenzimidazole membrane provided in Example 1 was tested at 120 mA·cm⁻¹. -2 Long-cycle performance of a single cell at current density;

[0064] Figure 11 The porous polybenzimidazole membrane provided in Example 1 was tested at 120 mA·cm⁻¹. -2 Capacity decay graph of a single cell at current density;

[0065] Figure 12 The alkaline zinc-iron flow single cell prepared using the porous polybenzimidazole membrane provided in Example 1 operates at 40-160 mA·cm⁻¹. -2 Coulomb efficiency diagram under varying current;

[0066] Figure 13 The alkaline zinc-iron flow single cell prepared using the porous polybenzimidazole membrane provided in Example 1 operates at 40-160 mA·cm⁻¹. -2 Voltage efficiency diagram under varying current;

[0067] Figure 14 The alkaline zinc-iron flow single cell prepared using the porous polybenzimidazole membrane provided in Example 1 operates at 40-160 mA·cm⁻¹. -2 Energy efficiency diagram under varying current;

[0068] Figure 15The alkaline zinc-iron flow single cell prepared from the porous polybenzimidazole membrane provided in Example 1 operates at 80 mA·cm⁻¹. -2 Long-cycle performance diagram under varying current. Detailed Implementation

[0069] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0070] Unless otherwise specified, the raw materials involved in the following specific embodiments of the present invention are all conventional materials in the art, and can be purchased from commercially available products. Some raw material information is as follows:

[0071] Poly(p-diphenyl ether)-5,5'-bibenzimidazole: OPBI, prepared in-house, preparation method as follows:

[0072] 10 g of P2O5 and 90 g of methanesulfonic acid (98%) were added to a 250 mL three-necked flask and magnetically stirred at 80 °C for 2 h under a nitrogen atmosphere. Then, 3,3'-diaminobenzidine (3.21 g) and 4,4'-diphenyl ether dicarboxylic acid (3.87 g) were added and magnetically stirred for 4 h to dissolve them and form a homogeneous solution. The reaction solution was then heated to 140 °C and stirred continuously for 6 h until a certain solution viscosity was reached. The high-viscosity reaction solution was slowly poured into a saturated sodium bicarbonate solution to form OPBI precipitate. The precipitate was washed repeatedly with deionized water and ethanol several times and finally dried under vacuum at 130 °C for 24 h.

[0073] Characterization of the basic chemical structure of OPBI polymers, such as Figure 1 The 1H NMR spectrum shows characteristic signals at 7.3, 7.6, 7.8, 8, 8.3, and 13 ppm. Further infrared characterization was performed. Figure 2 The corresponding characteristic peak appears at 3400 cm⁻¹. -1 3180 cm -1 1640 cm -1 and 1243 cm -1 The characteristic peaks at 1608 cm⁻¹ belong to non-hydrogen-bonded free NH groups, self-associative hydrogen-bonded NH groups, C=N stretching peaks, and stretching peaks of aromatic ether bonds, respectively; the benzimidazole functional group is confirmed by the following stretching characteristic peaks: 1608 cm⁻¹ -1 The position represents the conjugated ring vibration between the benzene and imidazole rings, 1447 cm. -1 The position represents the in-plane vibration of the 2,6-disubstituted benzimidazole ring, 1280 cm⁻¹. -1 The position represents the respiration of the imidazole ring and 806 cm. -1The above characterizations prove the formation of OPBI chemical structure.

[0074] Poly 2,2'-(m-phenyl)-5,5'-biphenylbenzimidazole: mPBI, self-made, the preparation method is as follows:

[0075] 3,3'-diaminobenzidine (2.14 g) and isophthalic acid (1.66 g) were dissolved in polyphosphoric acid at 140°C, and stirred at 170°C and 200°C for 24 h respectively, the high-viscosity reaction solution was slowly poured into saturated sodium bicarbonate solution to form mPBI precipitate, which was repeatedly washed with deionized water and ethanol for several times, and finally vacuum dried at 130°C for 24 h.

[0076] Example 1

[0077] This example provides a porous polybenzimidazole membrane and a preparation method thereof, as follows:

[0078] S1. 1 g OPBI powder was dissolved in 40 mL DMSO, 4-bromobenzenesulfonyl hydrazide and catalyst potassium carbonate with a molar ratio of 1:1 to OPBI structural unit were weighed and added into the solution, and the reaction was carried out at 80°C for 24 h under nitrogen protection; after the reaction was completed, the reaction solution was poured into a large amount of acetone for precipitation, and the solid was collected by filtration, washed with acetone for three times, and vacuum dried at 60°C for 24 h to obtain polybenzimidazole grafted with functional side chains;

[0079] S2. 0.5 g of polybenzimidazole grafted with functional side chains was dissolved in 4.5 g of DMSO to prepare a casting solution, and a film was cast on a glass plate by using a doctor blade with a height of 200 μm;

[0080] S3. The wet film was first pre-dried in an oven at 70°C for 1 h, and then subjected to programmed heat treatment: the temperature was increased to 100°C at a rate of 1°C / min, and maintained for 20 min; then the temperature was increased to 120°C, and maintained for 20 min; finally, the temperature was increased to 150°C, and maintained for 30 min; after natural cooling, the film was peeled off from the glass plate to obtain a porous polybenzimidazole membrane.

[0081] Example 2

[0082] This example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0083] The difference from Example 1 is that in this example, the molar ratio of 4-bromobenzenesulfonyl hydrazide to OPBI structural unit is 1.5:1, and the grafting degree is 150%.

[0084] Example 3

[0085] This example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0086] The difference from Example 1 is that in this example, the molar ratio of 4-bromobenzenesulfonyl hydrazide to OPBI structural units is 0.5:1, and the grafting degree is 50%.

[0087] Example 4

[0088] This example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0089] The difference from Example 1 is that in this example, the thickness of the wet film is 500 μm by controlling the height of the doctor blade.

[0090] Example 5

[0091] This example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0092] S1. 1 g of mPBI powder was dissolved in 40 mL of DMSO, and 4-sulfonamidobenzoic acid and a catalyst potassium carbonate with a molar ratio of 1:1 to the ABPBI structural units were weighed and added to the solution. Under nitrogen protection, the reaction was carried out at 100°C for 12 h. After the reaction was completed, the reaction solution was poured into a large amount of acetone for precipitation, and the solid was collected by filtration and washed with acetone three times. Vacuum drying was carried out at 60°C for 24 h to obtain polybenzimidazole grafted with functional side chains.

[0093] S2. 1.5 g of polybenzimidazole grafted with functional side chains was dissolved in 8.5 g of DMSO to prepare a casting solution, and a doctor blade with a height of 300 μm was used to cast a film on a glass plate.

[0094] S3. The wet film was first pre-dried in an oven at 60°C for 3 h, and then subjected to programmed heat treatment: heating to 110°C at 1°C / min, holding for 10 min, then heating to 115°C, holding for 10 min, and finally heating to 150°C, holding for 20 min. After natural cooling, the film was peeled off from the glass plate to obtain a porous polybenzimidazole membrane.

[0095] Example 6

[0096] This example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0097] S1. 1 g of mPBI powder was dissolved in 40 mL of DMSO, and 4-sulfonamidobenzoic acid and a catalyst potassium carbonate with a molar ratio of 1:1 to the ABPBI structural units were weighed and added to the solution. Under nitrogen protection, the reaction was carried out at 100°C for 12 h. After the reaction was completed, the reaction solution was poured into a large amount of acetone for precipitation, and the solid was collected by filtration and washed with acetone three times. Vacuum drying was carried out at 60°C for 24 h to obtain polybenzimidazole grafted with functional side chains.

[0098] S2. 0.5 g of polybenzimidazole powder grafted with functional side chains was dissolved in 9.5 g of DMSO to prepare a casting solution, and a film was cast on a glass plate using a doctor blade with a height of 400 μm;

[0099] S3. The wet film was first pre-dried in an oven at 80°C for 1 h, and then subjected to programmed heat treatment: the temperature was raised to 90°C at a rate of 1°C / min, maintained for 30 min, then raised to 105°C, maintained for 20 min, and finally raised to 150°C, maintained for 30 min. After natural cooling, the film was peeled off from the glass plate to obtain a porous polybenzimidazole film.

[0100] Comparative Example 1

[0101] This comparative example provides a polybenzimidazole film and a preparation method thereof, as follows:

[0102] 0.5 g of OPBI was dissolved in 4.5 g of DMSO to prepare a casting solution, and a film was cast on a glass plate using a doctor blade with a height of 200 μm. The subsequent treatment was performed according to Example 1.

[0103] Comparative Example 2

[0104] This comparative example provides a porous polybenzimidazole film and a preparation method thereof, as follows:

[0105] (1) A three-necked flask was charged with 2.0 g (5 mmol) of OPBI, 0.857 g (3.25 mmol) of 18-crown-6 ether, and 0.857 g (2.38 mmol) of dibenzo-18-crown-6 ether (mass ratio 2.3:1:1), followed by the addition of a solution of N,N-dimethylacetamide 50 mL. Under nitrogen protection, mechanical stirring was performed to uniformly mix the mixture, which was first allowed to react at room temperature for 2 h, and then at a temperature of 60°C for 12 h. After stopping heating, the solution was poured into a centrifuge tube after cooling to room temperature, and centrifugation was performed for the next step;

[0106] (2) The centrifuged mixture was cast as a casting solution on a clean glass plate with a thickness of 200 μm, and dried in an oven at 60°C for 24 h to obtain a polybenzimidazole film polymer film;

[0107] (3) The obtained polybenzimidazole film was immersed in a 3 mol / L sulfuric acid solution at room temperature for 12 h, and then washed with water for 5 h to remove residual sulfuric acid. Then it was immersed in a methanol solution at room temperature for 24 h, and washed with water for 5 h to remove residual methanol, to obtain a porous polybenzimidazole film.

[0108] Comparative Example 3

[0109] This comparative example provides a porous polybenzimidazole film and a preparation method thereof.

[0110] The difference from Example 1 is that in this comparative example, the molar ratio of 4-bromobenzenesulfonyl hydrazide and OPBI structural units is 1.9:1, and the grafting degree is 190%.

[0111] Comparative Example 4

[0112] The comparative example provides a porous polybenzimidazole membrane and a preparation method thereof.

[0113] The difference from Example 1 is that in this comparative example, the molar ratio of 4-bromobenzenesulfonyl hydrazide and OPBI structural units is 0.3:1, and the grafting degree is 30%.

[0114] Performance test

[0115] The performance of the polybenzimidazole membrane provided by the example and the comparative example is tested by the following method:

[0116] (1) Microstructure: The microstructure of the polybenzimidazole membrane obtained by SEM observation is as follows:

[0117] Figure 3 The surface and cross-section SEM images of the porous polybenzimidazole membrane obtained in Example 1 are shown. As can be seen from the microstructure of the surface (a), the surface of the obtained polybenzimidazole membrane presents a porous structure on one side, and a dense structure on the other side, without obvious defects and porous structure (b). The dense side surface can effectively block the active substance. At the same time, as can be seen from the micro-morphology of the cross-section (c), the thickness is about 20 μm, and the bulk phase presents a porous structure, which is conducive to the rapid conduction of the balanced ions.

[0118] Figure 4 The cross-section SEM image of the polybenzimidazole membrane provided by Comparative Example 1 is shown. As can be seen from the figure, the PBI membrane obtained in Comparative Example 1 generally presents a dense structure.

[0119] (2) Mechanical property: Since a strong compression force is needed to reduce the contact resistance between the carbon felt electrode and the membrane, a higher requirement is needed for the mechanical strength of the membrane. The tensile strength of the membrane is tested by a universal testing machine, and the loading rate is 5 mm / min;

[0120] (3) Water uptake (WU) and swelling ratio (SR): The system used for testing the flow battery is a water environment, and the ion transmission mechanism usually follows the Grutthuss mechanism, that is, the ion is conducted by the breaking and forming of hydrogen bond. Therefore, the content of water molecules in the diaphragm will directly affect the conductivity of the diaphragm, so the water uptake of the membrane is tested to evaluate the richness of the water channels in the diaphragm, and the stability of the channel size of the membrane after water absorption will have a significant impact on the barrier property of the diaphragm, so the swelling ratio of the membrane is further tested;

[0121] The water uptake and swelling ratio are calculated by the following method:

[0122]

[0123] wherein: W wet and W dry are the mass of the membrane in wet and dry state, respectively, d wet and d dry are the diameter of the membrane in wet and dry state, respectively;

[0124] The test results are as follows:

[0125] Figures 5-6 are the comparison chart of water absorption and swelling rate of the porous polybenzimidazole membrane obtained in Example 1 and the polybenzimidazole membrane provided in Comparative Example 1, and the specific data are shown in Table 1:

[0126] Table 1

[0127]

[0128] It can be seen from the examples and performance tests that the porous polybenzimidazole membrane provided by the application has excellent mechanical properties and can meet the stable application in the battery. It can be seen from the water absorption test results that the porous polybenzimidazole membrane provided by the application effectively introduces rich mass transfer water channels in the porous structure, and the swelling rate is less than 7%, which indicates that the membrane channel after water absorption still has good dimensional stability.

[0129] The above tests show that the porous membrane construction strategy can effectively introduce rich porous mass transfer channels in the dense polymer membrane, so the prepared membrane is further tested for the performance of the redox flow battery to evaluate the performance of the membrane and establish the relationship between the structure and performance of the membrane. In this system, the separator needs to quickly conduct hydrogen ions while effectively blocking active vanadium ions in the electrolyte on both sides, which has a decisive influence on the performance of the battery.

[0130] Single cell performance evaluation:

[0131] The performance of the membrane is evaluated by using the self-built redox flow single cell evaluation system. The single cell tested is composed of the following parts: two end plates, two carbon felt electrodes (the compression rate is 20% during assembly), two current collectors and the membrane to be tested (the effective test area of the membrane is 10.5 cm 2 ), the carbon felt electrode needs to be pretreated as follows: soaked in a mixed solution of ethanol and water (the volume ratio of ethanol and water is 1:5) for 24 h to remove the dirt attached to the carbon felt, dried, and then heated at 400℃ for 12 h for activation.

[0132] Vanadium system:

[0133] The positive electrolyte is 1.5 M VO 2+ / VO2+ and 3 M H2SO4, the negative electrode is 1.5 M V 2+ / V 3+ and 3 M H2SO4, the test system needs to be protected by inert gas, the variable current performance of the membrane is obtained by charging and discharging test under variable current density of 80, 100, 120, 140, 160, 180, 200 mA·cm -2 and cut-off voltage of 0.8 V and 1.7 V, and the long cycle stability of the membrane is obtained by charging and discharging cycle under current density of 120 mA·cm -2 .

[0134] Figures 7-11 respectively, and the specific data is shown in Table 2:

[0135] Table 2

[0136]

[0137] It can be known from the embodiments and performance tests that the porous polybenzimidazole membrane provided by the application has excellent barrier performance to vanadium ions when applied in a flow battery, and has excellent mass transfer performance and energy efficiency, in particular:

[0138] It can be known from Examples 1-3 that the coulombic efficiency increases with the increase of current density, and the coulombic efficiency reflects the barrier performance of the membrane. The charging and discharging time of the battery is shortened at a higher current density, thereby reducing the mutual diffusion of active substances. It can be known from Example 1 that the coulombic efficiency under different test current densities is higher than 99%, indicating that the prepared porous polybenzimidazole membrane has excellent barrier performance to vanadium ions. Example 2 indicates that a higher grafting degree can make the obtained porous polybenzimidazole membrane have more abundant porous channels, and Example 3 indicates that the porous polybenzimidazole membrane is more dense.

[0139] At the same time, it can be known from the voltage efficiency that the mass transfer performance of the porous polybenzimidazole membrane can be significantly improved due to the rich mass transfer channels and the introduction of sulfonic acid groups. The grafting degree of Example 3 is 50%, and the porous channels are less, so the voltage efficiency is significantly reduced, but it can still meet the application requirements.

[0140] It can be known from the energy efficiency that the preparation method provided by the application can effectively improve the comprehensive performance of the porous polybenzimidazole membrane. Under variable current density of 80-200 mA·cm -2 , the polymer membrane in Example 1 exhibits excellent performance of 88.8-73.9%.

[0141] It can be known from the long cycle performance test that the porous polybenzimidazole membrane provided by the application has excellent stability, wherein the porous polybenzimidazole membrane provided by example 1 can be stably cycled more than 500 times, and the efficiency does not obviously attenuate, which indicates that the porous membrane construction strategy does not affect the chemical stability of the membrane; it can be known from the discharge capacity attenuation performance test in the cycle test that the capacity retention rate of the porous polybenzimidazole membrane provided by example 1 is still 97.6% after 200 cycles, and the low attenuation rate further proves the effective blocking performance of the diaphragm.

[0142] It can be known from the comparison of the examples and the comparative examples that the porous polybenzimidazole membrane obtained by the preparation method provided by the application has high proton conductivity and extremely low vanadium ion permeability, and when used in a full vanadium redox flow battery, the porous polybenzimidazole membrane shows high coulomb efficiency and high energy efficiency, and excellent long cycle stability.

[0143] In order to verify the application universality of the constructed porous membrane in the redox flow battery, the example membrane 1 is used for testing in an alkaline zinc-iron redox flow battery system.

[0144] The electrolyte of the alkaline zinc-iron system is 0.2 M Na4Fe(CN)6, 0.2 M K4Fe(CN)6, 0.2 M ZnO and 2 M NaOH at the positive and negative electrodes, the variable current performance is obtained by charging and discharging test at variable current densities of 40, 60, 80, 100, 120, 140 and 160 mA·cm -2 , and the long cycle stability is obtained by charging and discharging cycle at a current density of 80 mA·cm -2 .

[0145] The coulomb efficiency (CE), energy efficiency (EE) and voltage efficiency (VE) of the single cell can be calculated by the following formula, so as to evaluate the performance of the membrane:

[0146]

[0147]

[0148]

[0149] Wherein, I d and I c respectively represent the discharge current and the charging current in the charging and discharging process, V d and V c respectively represent the discharge voltage and the charging voltage in the charging and discharging process.

[0150] The test results are as follows:

[0151] Figures 12-15The figures show the coulombic efficiency, voltage efficiency, and energy efficiency of the alkaline zinc-iron flow cell prepared with the porous polybenzimidazole membrane provided in Example 1. As can be seen from the figures, the alkaline zinc-iron flow cell prepared with the porous polybenzimidazole membrane provided in Example 1 achieves energy efficiency in the range of 40-160 mA·cm⁻¹. -2 The energy efficiency at current densities is 89.8-69.3%; further, at 80 mA·cm⁻¹, the energy efficiency is [missing information]. -2 Cyclic tests were conducted under a constant charge and discharge current. Figure 15 The figure shows the long-cycle performance of the alkaline zinc-iron flow cell prepared by the porous polybenzimidazole membrane provided in Example 1. As can be seen from the figure, the porous polybenzimidazole membrane in Example 1 can be stably cycled for more than 500 times without significant efficiency decay. This proves that the porous polybenzimidazole membrane prepared by the method provided in this invention can also maintain excellent stability in alkaline systems, indicating that the prepared porous polybenzimidazole membrane can be applied to both acidic and alkaline systems and has a certain degree of universality.

[0152] The present invention has been illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a porous polybenzimidazole membrane, characterized in that, The preparation method includes: S1. Polybenzimidazole and a functional compound undergo a nucleophilic substitution reaction to obtain polybenzimidazole grafted with functional side chains; S2. Coat a casting solution containing polybenzimidazole grafted with functional side chains into a wet film; S3. The wet film is pre-dried for initial shaping, and then heat-treated to create pores to obtain the porous polybenzimidazole membrane; The functional compound contains both a sulfonylhydrazine group and an electrophilic group, wherein the electrophilic group includes a carboxyl group and / or a halogen group; The ratio of the molar amount of the functional compound to the molar amount of the structural unit of the polybenzimidazole is (0.5-1.5):

1.

2. The method for preparing the porous polybenzimidazole membrane according to claim 1, characterized in that, The nucleophilic substitution reaction was carried out under the catalysis of a catalyst; The nucleophilic substitution reaction is carried out at a temperature of 60-100℃ for a time of 10-40 h.

3. The method for preparing the porous polybenzimidazole membrane according to claim 2, characterized in that, The functional compounds include 4-bromobenzenesulfonyl hydrazide and / or 4-sulfonyl hydrazide benzoic acid.

4. The method for preparing the porous polybenzimidazole membrane according to claim 1, characterized in that, The heat treatment method for creating pores includes: raising the temperature to above 90°C and holding it for 10-30 minutes, and then using a heating-holding method until the temperature is raised to 150°C and held for 10-30 minutes. In the heating-holding method, the holding time for each heating is independently selected from 10-30 minutes.

5. The method for preparing the porous polybenzimidazole membrane according to any one of claims 1-4, characterized in that, The polybenzimidazole includes any one or a combination of at least two of poly2,2'-(m-phenyl)-5,5'-bibenzimidazole, poly2,2'-(p-phenyl)-5,5'-bibenzimidazole, poly2,2'-(p-diphenyl ether)-5,5'-bibenzimidazole, or poly(2,5-benzimidazole).

6. The method for preparing the porous polybenzimidazole membrane according to any one of claims 1-4, characterized in that, The organic solvent used in the casting solution includes any one or a combination of at least two of N,N-dimethylformamide, N,N-dimethylacetamide or dimethyl sulfoxide. Based on the total mass of the casting solution as 100%, the mass percentage of the polybenzimidazole grafted with functional side chains is 5-15%. The coating method includes scraping; The thickness of the wet film is 200-500 μm.

7. The method for preparing the porous polybenzimidazole membrane according to any one of claims 1-4, characterized in that, The pre-drying temperature is 60-80℃, and the time is 1-3 hours.

8. A porous polybenzimidazole membrane prepared by the method of any one of claims 1-7.

9. The application of the porous polybenzimidazole membrane as described in claim 8 in ion conduction.

10. A flow battery, characterized in that, It includes an ion-conducting membrane, wherein the ion-conducting membrane is the porous polybenzimidazole membrane as described in claim 8.

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