A crosslinked sulfonated polybenzimidazole membrane, its preparation method and application

Abstract

The present invention discloses a preparation method of a cross-linked sulfonated polybenzimidazole membrane, comprising the following steps: under a nitrogen atmosphere, adding polybenzimidazole into a first solvent, adding anhydrous potassium carbonate powder after reaction, then slowly adding a sulfonating agent, reacting, and obtaining sulfonated polybenzimidazole through post-treatment; dissolving the sulfonated polybenzimidazole in a second solvent, adding a cross-linking agent, heating up for reaction, and obtaining cross-linked sulfonated polybenzimidazole through post-treatment; dissolving the cross-linked sulfonated polybenzimidazole in a third solvent, drying to obtain a cross-linked sulfonated polybenzimidazole membrane, and activating to obtain an activated cross-linked sulfonated polybenzimidazole membrane. The cross-linked sulfonated polybenzimidazole membrane provided by the present invention has both excellent proton conductivity and ion selectivity, has excellent Coulomb efficiency and voltage efficiency in a vanadium redox flow battery, and has optimal battery performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of diaphragms for flow batteries, and specifically relates to a crosslinked sulfonated polybenzimidazole membrane, a preparation method thereof, and an application thereof. Background Art

[0002] In energy storage technologies, electrochemical energy storage technologies have received extensive attention and have developed rapidly. In particular, redox flow batteries (RFBs) are considered to be an ideal choice for large-scale long-term energy storage due to their overall safety, flexible design, high conversion efficiency, and long cycle life. Currently, all-vanadium flow batteries (VRFBs) have been commercially applied and have good development prospects.

[0003] The diaphragm is a core component in VRFBs and must meet the requirements of high proton conductivity, low vanadium penetration, good mechanical properties, and excellent chemical stability. In all-vanadium flow batteries, the Nafion series membranes of DuPont Company are currently the most widely used, with good mechanical stability and proton conductivity. However, the high water absorption rate of perfluorosulfonic acid membranes (PFSA) exacerbates the permeability of vanadium ions, resulting in low ion selectivity; moreover, the cost of Nafion membranes accounts for more than 40% of the total cost of a 1-kilowatt battery stack, and such a high material cost also hinders the widespread commercialization of VRFBs. Therefore, there is an urgent need to develop new flow battery diaphragms with low cost and high performance.

[0004] As one of the most promising non-fluorinated ion-conducting membranes, polybenzimidazole (PBI) membranes have excellent vanadium-blocking performance and chemical stability and have been widely used in VRFBs. However, PBI membranes lack ion-exchange groups, resulting in very low conductivity of their own. Researchers have explored the effects of various ionic groups on the conductivity of PBI in the past few years. Among them, the proton dissociation ability of sulfonic acid groups is much higher than that of other proton-conducting groups. Introducing sulfonic acid groups is an effective way to improve the ionic conductivity of PBI membranes. However, it may potentially reduce the mechanical strength, chemical stability, and ion selectivity of the membranes. To solve these problems, other modification strategies can be used in combination to achieve optimal performance. Among them, covalent crosslinking is the main means to improve the mechanical stability and chemical stability of the membranes, which can effectively reduce the excessive swelling of PBI membranes and cross-contamination of electrolytes to balance the mass transfer ability and stability of sulfonated PBI membranes. And using a suitable crosslinking agent can form a wide crosslinking network inside the membrane, broaden the ion transport channels, and optimize the pore structure to ensure high ion selectivity and chemical stability while improving the ionic conductivity of PBI membranes. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method of a crosslinked sulfonated polybenzimidazole membrane.

[0006] Another object of the present invention is to provide an application of the crosslinked sulfonated polybenzimidazole membrane prepared by the method in the preparation of a flow battery.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] In a first aspect of the present invention, a method for preparing a crosslinked sulfonated polybenzimidazole membrane is provided, comprising the following steps:

[0009] In the first step, under a nitrogen atmosphere, polybenzimidazole is added to a first solvent, and stirred for 1 to 3 h (preferably 2 h) at a temperature of 60 to 100 °C (preferably 80 °C), anhydrous potassium carbonate powder is added, and the molar ratio of polybenzimidazole to anhydrous potassium carbonate powder is 1:1 to 2 (preferably 1:1.5), and then a sulfonating agent is slowly added, and the molar ratio of polybenzimidazole to the sulfonating agent is 8 to 20:1 (preferably 10:1), and the reaction is carried out for 1 to 48 h (preferably 24 h) at a temperature of 30 to 50 °C (preferably 40 °C), and sulfonated polybenzimidazole is obtained after post-treatment;

[0010] The sulfonating agent is selected from at least one of 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, vinyl sulfonic acid lactone, benzenesulfonic acid lactone, ethylenesulfonic acid lactone, and functionalized sulfonic acid lactone derivatives;

[0011] In the second step, the sulfonated polybenzimidazole is dissolved in a second solvent to obtain a solution with a mass fraction of 1 to 10% (preferably 2%), a crosslinking agent is added, and the molar ratio of sulfonated polybenzimidazole to the crosslinking agent is 1 to 20:1 (preferably 10:1, 5:1), and the temperature is raised to 60 to 100 °C (preferably 80 °C) and the reaction is carried out for 1 to 48 h (preferably 24 h), and crosslinked sulfonated polybenzimidazole is obtained after post-treatment;

[0012] The crosslinking agent is selected from at least one of glycerol triglycidyl ether (GTE), ethylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, and pentaerythritol glycidyl ether;

[0013] In the third step, the crosslinked sulfonated polybenzimidazole is dissolved in a third solvent, dried to obtain a crosslinked sulfonated polybenzimidazole membrane, and activated to obtain an activated crosslinked sulfonated polybenzimidazole membrane.

[0014] The first solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid.

[0015] The post-treatment steps in the first step: Pour the mixture into acetone to precipitate the polymer, and wash it several times with water and ethanol until the solution becomes colorless, then dry it.

[0016] The drying in the first step is to vacuum-dry the precipitate at a temperature of 50-70 °C (preferably 60 °C) for 1-48 h (preferably 24 h).

[0017] The preparation method of the polybenzimidazole includes the following steps:

[0018] Under a nitrogen atmosphere, phosphorus pentoxide and methanesulfonic acid with a mass ratio of 1:1-20 (preferably 1:10) are mixed, and 3,3'-diaminobenzidine (DAB) and 4,4'-dicarboxydiphenyl ether (OBA) with a molar ratio of 1:1-2 (preferably 1:1) are added. The mass ratio of phosphorus pentoxide to 3,3'-diaminobenzidine is 1-5:1 (preferably 3.1:1). Under stirring conditions, the reaction system is heated to 90-110 °C (preferably 100 °C) and stirred and mixed for 1-3 h (preferably 2 h), then the temperature is raised to 130-150 °C (preferably 140 °C) and reacted for 1-5 h (preferably 4 h) to obtain the polybenzimidazole.

[0019] The number-average molecular weight (Mn) of the polybenzimidazole is 34175 g·mol -1 , and the weight-average molecular weight (Mw) is 57958 g·mol -1 , and the polydispersity index (PDI) is 1.69.

[0020] The second solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid.

[0021] The post-treatment steps in the second step: Pour the mixture into acetone to precipitate the polymer, and wash it several times with water and ethanol until the solution becomes colorless, then dry it.

[0022] The drying in the second step is to vacuum-dry the precipitate at a temperature of 50-70 °C (preferably 60 °C) for 1-48 h (preferably 24 h).

[0023] Dissolve the crosslinked sulfonated polybenzimidazole in a third solvent to obtain a solution with a mass fraction of 1-10% (preferably 2%).

[0024] The drying step in the third step: Dry the above solution at a temperature of 60-100 °C (preferably 80 °C) for 1-48 h (preferably 24 h).

[0025] The third solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid.

[0026] The thickness of the crosslinked sulfonated polybenzimidazole membrane is 20-30 μm (preferably 25 μm).

[0027] In the activation step of the third step: The prepared membrane is placed in a phosphoric acid solution with a mass concentration of 85 wt% and left standing for 1-48 h (preferably 24 h) for activation pretreatment, and then washed repeatedly with deionized water.

[0028] In the second aspect of the present invention, a crosslinked sulfonated polybenzimidazole membrane is provided, and the crosslinked sulfonated polybenzimidazole membrane is prepared by the above preparation method.

[0029] In the third aspect of the present invention, an application of the crosslinked sulfonated polybenzimidazole membrane in preparing a flow battery is provided.

[0030] The flow battery is selected from at least one of an all-vanadium flow battery, a lithium-ion flow battery, a zinc-bromine flow battery, a zinc-cerium flow battery, a zinc-nickel flow battery, a lead flow battery, an iron-chromium flow battery, a polysulfide / sodium bromide flow battery, or an all-iron flow battery.

[0031] Due to the adoption of the above technical solutions, the present invention has the following advantages and beneficial effects:

[0032] The crosslinked sulfonated polybenzimidazole membrane provided by the present invention modifies the molecular structure of PBI through grafting and crosslinking reactions. The grafted hydrophilic sulfonic acid groups provide more proton transport sites for the membrane, and a dynamic hydrogen bond network conducive to proton transport is constructed through crosslinking reactions, providing a good ion transport channel for protons, significantly improving the proton conductivity of the membrane. At the same time, the continuous crosslinked network formed by covalent bonds effectively restricts the diffusion of active substances, enabling it to maintain excellent ion selectivity. The crosslinked sulfonated polybenzimidazole membrane provided by the present invention can exhibit excellent performance in VRFB.

[0033] The preparation method of the crosslinked sulfonated polybenzimidazole membrane provided by the present invention grafts sulfonic acid groups with relatively high conductivity on the PBI main chain as ion exchange groups, and at the same time uses glycerol triglycidyl ether (GTE) as a covalent crosslinking agent to prepare a crosslinked sulfonated polybenzimidazole membrane. On the one hand, the synergistic effect of sulfonic acid groups and protonated imidazole groups provides more ion exchange sites for protons. On the other hand, the dynamic hydrogen bond network formed by ether bonds and hydroxyl groups between chain segments provides a good ion transport channel for protons, significantly improving the proton conductivity of the membrane. At the same time, the tight crosslinked network also enables the membrane to maintain excellent ion selectivity and chemical stability. The all-vanadium redox flow battery assembled with it exhibits excellent electrochemical performance and stability. The present invention provides an effective strategy for the design of high-performance PBI membranes, thereby enhancing the competitiveness of PBI membranes in the field of all-vanadium redox flow batteries.

[0034] The crosslinked sulfonated polybenzimidazole membrane provided by the present invention has both excellent proton conductivity and ion selectivity, and has excellent Coulomb efficiency and voltage efficiency in all-vanadium redox flow batteries, showing the best battery performance. Brief Description of the Drawings

[0035] Figure 1 It is a schematic diagram of vanadium ion penetration test.

[0036] Figure 2 It is the XRD pattern of OPBI-SO3H-GTE membrane and pure OPBI membrane.

[0037] Figure 3 It is the SEM image and EDS elemental mapping schematic diagram of OPBI-SO3H-GTE membrane.

[0038] Figure 4 It is the schematic diagram of the water absorption rate and swelling rate test results of OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0039] Figure 5 It is the IEC schematic diagram of Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0040] Figure 6 It is the water contact angle schematic diagram of OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0041] Figure 7 It is the schematic diagram of proton conductivity and surface resistance of Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0042] Figure 8Schematic diagram of vanadium ion permeability of Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0043] Figure 9 Schematic diagram of variable current performance of Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane.

[0044] Figure 10 Is the long-term cycling performance graph of OPBI-SO3H-GTE membrane at 200 mA·cm -2 below.

[0045] Figure 11 Schematic diagram of self-discharge curves of Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. Detailed implementation manners

[0046] To illustrate the present invention more clearly, the present invention will be further described below in conjunction with preferred embodiments. Those skilled in the art should understand that the content specifically described below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention.

[0047] By reasonably selecting modification strategies and carefully regulating the structure and properties of the polymer side chains, the present invention can improve the proton conductivity of PBI membranes while maintaining their high chemical stability. The present invention uses inexpensive and highly reactive 1,3-propane sultone as the sulfonation reagent for PBI, and the epoxy ring is opened at a temperature of 40 °C to graft sulfonic acid groups onto the PBI side chain. The process is simple and stable. In addition, glycerol triglycidyl ether (GTE) is used as a covalent crosslinking agent, which has abundant epoxy active sites to ensure the crosslinking effect. Moreover, numerous ether bonds and hydroxyl groups between the segments after crosslinking not only broaden the free volume of the membrane, but also form a dynamic hydrogen bond network to provide more ion transport channels for protons, further improving the mass transfer ability of the membrane.

[0048] Example 1

[0049] Preparation of aromatic ether type polybenzimidazole (OPBI)

[0050] Add 10 g of phosphorus pentoxide and 100 g of methanesulfonic acid into a three-necked flask. The mass ratio of phosphorus pentoxide to methanesulfonic acid is 1:10. Under a nitrogen atmosphere, add 3,3'-diaminobenzidine (DAB) (15 mmol, 3.21 g) and 4,4'-dicarboxydiphenyl ether (OBA) (15 mmol, 3.87 g). The molar ratio of 3,3'-diaminobenzidine (DAB) to 4,4'-dicarboxydiphenyl ether (OBA) is 1:1. The mass ratio of phosphorus pentoxide to 3,3'-diaminobenzidine is 3.1:1. Under stirring conditions, heat the reaction system to 100 °C and stir and mix for 2 h, then raise the temperature to 140 °C and react for 4 h. After the reaction is completed, slowly pour the solution into saturated NaHCO3 under stirring conditions for precipitation to neutralize the residual methanesulfonic acid in the product. Cut the precipitated product into pieces and filter and wash it with water and ethanol for several times until the solution is neutral. Finally, vacuum dry the OPBI polymer at 120 °C for 24 h to obtain 4.97 g of OPBI. The number-average molecular weight (Mn) is 34175 g·mol -1 , and the weight-average molecular weight (Mw) is 57958 g·mol -1 . The calculated polydispersity index (PDI) is 1.69.

[0051]

[0052] Preparation of sulfonated polybenzimidazole (OPBI-SO3H) membrane

[0053] Under a nitrogen atmosphere, add OPBI (2.5 mmol, 1 g) into 50 g of dimethyl sulfoxide and stir for 2 h at a temperature of 80 °C. After complete dissolution, add anhydrous K2CO3 powder (3.75 mmol, 0.52 g) as a reaction catalyst. The molar ratio of OPBI to anhydrous K2CO3 powder is 1:1.5. Then slowly add 1,3-propanesultone (0.25 mmol, 0.03 g). The molar ratio of OPBI to 1,3-propanesultone is 10:1. React at a temperature of 40 °C for 24 h. After the reaction is completed, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at a temperature of 60 °C for 24 h to obtain 0.84 g of OPBI-SO3H.

[0054] Dissolve 0.5 g of OPBI-SO3H solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super petri dish (Japanese Flat-glass flat glass culture dish, FS-30, diameter 32 mm, height 15 mm), and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H film with a thickness of 25 μm and a uniform surface. Place the prepared film in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it several times with deionized water to obtain the activated OPBI-SO3H film.

[0055] Preparation of crosslinked sulfonated polybenzimidazole (OPBI-SO3H-GTE) membrane

[0056] Dissolve OPBI-SO3H (0.95 mmol, 0.5 g) solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add glycerol triglycidyl ether (GTE) (0.095 mmol, 0.025 g). The molar ratio of OPBI-SO3H to GTE is 10:1. Heat up to 80 °C and react for 24 h. After the reaction is completed, pour the mixture into acetone to precipitate the polymer, and wash it several times with water and ethanol until the solution becomes colorless. Finally, dry the precipitate under vacuum at 60 °C for 24 h to obtain 0.42 g of OPBI-SO3H-GTE product.

[0057] Dissolve 0.5 g of OPBI-SO3H-GTE solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super petri dish, and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H-GTE film with a thickness of 25 μm and a uniform surface. Place the prepared film in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it several times with deionized water to obtain the activated OPBI-SO3H-GTE film.

[0058]

[0059] Comparative Example 1

[0060] Preparation of pure OPBI membrane

[0061] Dissolve 0.5 g of OPBI solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a superflat dish and place it in an oven at 80 °C for drying for 24 h to obtain a PBI film with a thickness of 25 μm and a uniform surface. Place the prepared film in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it with deionized water multiple times to obtain the activated OPBI film.

[0062] Comparative Example 2

[0063] Commercially available Nafion 212 membrane.

[0064] Testing methods for the samples prepared in Example 1 and Comparative Examples 1 - 2

[0065] Swelling ratio and water absorption rate

[0066] All the membranes used in preparing the membrane samples below are activated membranes.

[0067] Put the cut membrane sample into a vacuum drying oven at 60 °C for drying for 24 h. After taking out the sample from the oven, quickly weigh the mass of the dry membrane, denoted as M dry , and measure its diameter, denoted as D dry . Immerse it in deionized water for 24 h. After the immersion, use filter paper to remove the water on the surface of the membrane, and then immediately weigh the mass M wet and diameter D wet . To ensure the accuracy of the data and reduce errors, each membrane sample is measured three times and the average value is taken. The SR (swelling ratio) and WU (water absorption rate) of the membrane are obtained from the following two equations respectively:

[0068]

[0069]

[0070] where, M dry is the dry weight of the membrane, and M wet is the wet weight of the membrane after water absorption. D dry is the diameter of the membrane in the dry state, and D wet is the diameter of the membrane in the wet state.

[0071] Ion exchange capacity

[0072] Put the prepared membrane sample into a vacuum drying oven at 60 °C and dry it for 24 h. After taking out the sample from the oven, accurately weigh the mass m of the dry membrane, put it into 10 mL of sulfuric acid with a concentration of 1 mol / L and soak it for 24 h. After the soaking is completed, rinse the free acid on the membrane surface with deionized water and dry the moisture on the membrane surface with filter paper. Using phenolphthalein as an indicator, titrate with a 0.01 mol / L NaOH standard solution and record the volume of the standard solution consumed. The calculation formula for the ion exchange capacity (IEC) is as follows:

[0073]

[0074] where m is the dry weight of the membrane, C NaOH is the molar concentration of NaOH, and V’ and V are the volume readings after and before titration, respectively.

[0075] Proton conductivity

[0076] Use an electrochemical workstation to measure the proton conductivity (σ) and surface resistance (AR) of various membranes by the alternating current impedance method. The frequency range is between 10 -2 -10 6 Hz, and the alternating current amplitude is 5 mV. The calculation formulas for proton conductivity and surface resistance are as follows:

[0077]

[0078] AR = A × R

[0079] where σ is the proton conductivity of the membrane (mS·cm -1 ), L is the thickness of the membrane between the positive and negative electrode plates (cm), A is the cross-sectional area of the membrane sample (cm 2 ), R is the actual measured value of the membrane impedance (Ω), and AR is the surface resistance of the membrane (Ω·cm 2 ).

[0080] Vanadium ion permeation test

[0081] The permeation of vanadium ions is tested by the method of counter-diffusion. First, dissolve vanadyl sulfate (VOSO4) in 3.0 M H2SO4 to prepare standard vanadyl sulfate solutions with concentrations of 0.1 mol·L -1 , 0.075 mol·L -1 , 0.03 mol·L -1 , 0.015 mol·L -1 , 0.005 mol·L -1 and 0.0025 mol·L -1 to calibrate the concentration-absorbance standard curve of V 4+ in VOSO4. As Figure 1As shown Figure 1 is a schematic diagram of vanadium ion permeation test. The prepared membrane was clamped in a diffusion cell. On the left and right sides were 50 mL of [1.5 M VOSO4 + 3.0 M H2SO4 solution] and 50 mL of [1.5 M MgSO4 + 3.0 M H2SO4] respectively. To avoid concentration polarization, magnetic stirring was maintained on both sides and the same rotation speed was kept. Every 24 h, 4 mL of sample was taken from the side containing MgSO4 solution, and then 4 mL of [1.5 M MgSO4 + 3.0 M H2SO4] solution was replenished to maintain ionic balance. The absorbance corresponding to the solution obtained by testing with a UV-visible spectrophotometer was used, and the corresponding V 4+ concentration was obtained according to the calibrated standard curve. Finally, the vanadium ion permeability was calculated by the following formula.

[0082]

[0083] where V B is the volume of the solution (mL), t is the time (min), A is the tested membrane area (cm 2 ), L is the thickness of the membrane (cm), P is the vanadium ion permeability of the membrane (cm 2 ·min -1 ), C A is the V 4+ concentration in the VOSO4 solution (mol·L -1 ), C B (t) is the V 4+ concentration in the MgSO4 solution (mol·L -1 ).

[0084] Basic characterization of the membrane

[0085] Figure 2 are the XRD patterns of OPBI-SO3H-GTE membrane and pure OPBI membrane. It can be seen from the figure that the pure OPBI membrane with a dense structure shows a single strong diffraction peak only at 2θ = 23.6° due to π-π stacking, and its chain segment spacing corresponds to 4.21 Å. Compared with the OPBI membrane, for the OPBI-SO3H-GTE membrane, in addition to the diffraction peak at 2θ = 22.8°, a new diffraction peak at 2θ = 16.9° (5.45 Å) was also observed. This indicates that in the local space where grafting and cross-linking reactions occur, due to the presence of long side chains, the intermolecular distance is enlarged, resulting in an increase in the free volume of the polymer chain.

[0086] Figure 3It is a schematic diagram of the SEM image and EDS elemental spectrum of the OPBI-SO3H-GTE membrane. Among them, (a) is the SEM surface image; (b) is the SEM cross-sectional image; (c) is the schematic diagram of the EDS elemental spectrum. It can be seen from the figure that the membrane is smooth, transparent and dense, with a thickness of about 25 μm, and the S element is evenly distributed in the membrane.

[0087] Figure 4 It is a schematic diagram of the water absorption rate and swelling rate test results of the Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. It can be seen from the figure that the water absorption rate of the OPBI-SO3H-GTE membrane is 25.35%, which is 2.4 times that of the pure OPBI membrane (10.52%), and its anti-swelling performance is enhanced compared with the OPBI-SO3H membrane. This is because the grafted sulfonic acid groups have good hydrophilicity, and the grafting and cross-linking of the long side chains expand the free volume of the chain, allowing more water molecules to be accommodated inside the membrane, which will significantly enhance the proton transport process with water as the carrier. After the cross-linking reaction, a continuous cross-linked network is formed inside the membrane, and the anti-swelling performance of the membrane is enhanced.

[0088] Figure 5 It is a schematic diagram of the IEC of the Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. It can be seen from the figure that the OPBI-SO3H-GTE membrane has the highest IEC value (1.57 mmol / g), significantly higher than that of the Nafion212 membrane (0.84 mmol / g) and the OPBI membrane (0.45 mmol / g). The negatively charged SO3 - anion of sulfonic acid group is H + which provides abundant ion exchange sites, resulting in a high ion exchange capacity of the membrane. After cross-linking, the long side chains expand the ion conduction channels of the membrane, allowing it to absorb more water molecules and promoting ion transport.

[0089] Figure 6 It is a schematic diagram of the water contact angle of the OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. After grafting and cross-linking, the water contact angle of the membrane decreases from 80.69° to 28.6°, which further illustrates that the hydrophilic sulfonic acid groups, ether bonds and hydroxyl groups inside the membrane can enable it to adsorb more water molecules and significantly improve its ion transport ability.

[0090] Figure 7 It is a schematic diagram of the proton conductivity and surface resistance of the Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. It can be seen from the figure that the proton conductivity of the pure OPBI membrane is only 30.27 mS·cm -1 which is lower than that of the Nafion212 membrane (47.3 mS·cm-1 ), after the grafting and crosslinking reactions, the conductivity of the OPBI-SO3H-GTE membrane reached 74.86 mS·cm -1 , and its surface resistance was as low as 13.40 Ω·cm 2 , much lower than that of the Nafion212 membrane (21.18 Ω·cm 2 ). This is because the introduction of sulfonic acid groups increases the hydrophilicity of the membrane and provides more proton transport sites, thus promoting proton transport; after crosslinking, the content of hydroxyl groups and ether bonds in the membrane is increased, and these hydroxyl groups and ether bonds form a dynamic hydrogen bond network conducive to proton transport, providing a good ion transport channel for protons and greatly reducing the surface resistance of the membrane.

[0091] Figure 8 is a schematic diagram of the vanadium ion permeability of the Nafion212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. It can be seen from the figure that the vanadium ion permeabilities of the OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane are all much lower than that of the Nafion212 membrane. The crosslinked OPBI-SO3H-GTE membrane shows a relatively low vanadium ion permeability (12.63×10 -8 cm 2 ·min -1 ), which indicates that the crosslinked network formed by covalent bonds can effectively limit membrane swelling, reduce molecular gaps, and thus hinder the transmembrane transfer of vanadium ions.

[0092] Flow battery performance

[0093] The performance of the membrane was tested at room temperature using a flow battery device. The flow battery device mainly consists of the following components: a membrane with an effective area of 4 cm 2 , two activated carbon felt electrodes, two graphite current collectors, several gaskets, several bolts and nuts, and two PTFE housings. The structure of the vanadium battery from left to right is as follows: PTFE housing, gasket, graphite current collector, gasket, electrode frame, carbon felt electrode, separator. The structure on the right is the same as that on the left and is symmetrically distributed. Each of the above components is fixed by bolts and nuts passing through both ends. The membrane sample is firmly clamped between the two electrodes in the battery to isolate the positive and negative electrolyte solutions. The positive and negative electrolyte solutions are both vanadium ion solutions dissolved in sulfuric acid.

[0094] After assembling the flow battery, a variable current charge-discharge performance test was carried out. The variable current density range was 80 - 220 mA·cm -2 , with an interval of 20 mA·cm -2 , and the cut-off voltage was 0.8 - 1.7 V. At 200 mA·cm -2Long-term cyclic tests were carried out, and the cut-off voltage was the same as that in the variable current test. Self-discharge tests were carried out at 50% state of charge (SOC), and the change of voltage with the shelf time during the test was recorded. The coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) of the flow battery were calculated using the following formulas.

[0095]

[0096]

[0097]

[0098] where I c is the charging current of the battery (mA), I d is the discharging current of the battery (mA), V c is the charging voltage of the battery (V), V d is the discharging voltage of the battery (V), and t is the time (s).

[0099] Figure 9Schematic diagrams of the variable current performance of Nafion 212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. Among them, (a) is the schematic diagram of the Coulomb efficiency (CE) of Nafion 212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane, (b) is the schematic diagram of the voltage efficiency (VE) of Nafion 212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane, and (c) is the schematic diagram of the energy efficiency (EE) of Nafion 212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. It can be seen from the figure that the Coulomb efficiency of OPBI-SO3H-GTE membrane (98.49%-99.68%) is the highest, which indicates that the tight cross-linked network formed in the membrane effectively enhances the ion selectivity of the membrane. The test results of the voltage efficiency are consistent with the surface resistance results of various membranes. The voltage efficiency of OPBI-SO3H-GTE membrane (91.98%-78-95%) is significantly better than other membranes, which is due to the existence of efficient proton transport channels in it. On the one hand, the presence of sulfonic acid groups and protonated imidazoles provides more transport sites for protons. On the other hand, the presence of flexible side chains not only broadens the free volume of the membrane, but also the dynamic hydrogen bond network formed by ether bonds and hydroxyl groups between chain segments provides more ion transport channels for protons, thus improving its voltage efficiency. Based on the excellent Coulomb efficiency and voltage efficiency, OPBI-SO3H-GTE membrane exhibits the best comprehensive performance, and its energy efficiency is 90.60%-78.34%. The comprehensive performance of OPBI-SO3H membrane grafted only with sulfonic acid groups (87.48%-74.74%) has certain disadvantages compared with OPBI-SO3H-GTE membrane, but it is still higher than that of pure OPBI membrane (86.36%-72.45%) and commercial Nafion 212 (90.60%-78.34%) membrane, which also shows that the cross-linked sulfonated polybenzimidazole membrane has good application scenarios in VRFB.

[0100] Figure 10 is the long-term cycling performance graph of OPBI-SO3H-GTE membrane at 200 mA·cm -2 The all-vanadium redox flow battery assembled with OPBI-SO3H-GTE membrane was cycled about 1200 times in total, and the cycling time was 720 h. Its good ion selectivity gives it excellent Coulomb efficiency (>99%), and its energy efficiency is almost constant throughout the test process, remaining at about 80%. This indicates that the cross-linked sulfonated polybenzimidazole membrane with grafted and cross-linked structures can not only efficiently conduct protons during the dynamic charge and discharge process of VRFB, but also has excellent long-term chemical stability under the strong acidic and strong oxidizing conditions of the electrolyte.

[0101] Figure 11 It is a schematic diagram of the self-discharge curves of Nafion 212 membrane, OPBI membrane, OPBI-SO3H membrane and OPBI-SO3H-GTE membrane. The self-discharge time of the OPBI-SO3H-GTE membrane can reach 54 h, which is much longer than that of the OPBI membrane (34 h) and the Nafion 212 membrane (24 h), which is consistent with the 2+ penetration results of VO ions and the Coulomb efficiency of the battery, proving that the cross-linked sulfonated polybenzimidazole membrane prepared by the present invention has excellent ion selectivity.

[0102] Example 2

[0103] Preparation of sulfonated polybenzimidazole (OPBI-SO3H) membrane

[0104] Under a nitrogen atmosphere, OPBI (2.5 mmol, 1 g) was added to 50 g of dimethyl sulfoxide and stirred at 80 °C for 2 h. After complete dissolution, anhydrous K2CO3 powder (3.75 mmol, 0.52 g) was added as a reaction catalyst. The molar ratio of OPBI to anhydrous K2CO3 powder was 1:1.5. Then, 1,4-butanesultone (0.25 mmol, 0.034 g) was slowly added. The molar ratio of OPBI to 1,4-butanesultone was 10:1. The reaction was carried out at 40 °C for 24 h. After the reaction, the mixture was poured into acetone to precipitate the polymer, and washed several times with water and ethanol until the solution became colorless. Finally, the precipitate was vacuum dried at 60 °C for 24 h to obtain 0.86 g of OPBI-SO3H.

[0105] Preparation of cross-linked sulfonated polybenzimidazole (OPBI-SO3H-GTE) membrane

[0106] The solid of OPBI-SO3H (0.93 mmol, 0.5 g) was dissolved in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Glycerol triglycidyl ether (GTE) (0.093 mmol, 0.024 g) was added. The molar ratio of OPBI-SO3H to GTE was 10:1. The temperature was raised to 80 °C and the reaction was carried out for 24 h. After the reaction, the mixture was poured into acetone to precipitate the polymer, and washed several times with water and ethanol until the solution became colorless. Finally, the precipitate was vacuum dried at 60 °C for 24 h to obtain 0.43 g of OPBI-SO3H-GTE product.

[0107] Dissolve 0.5 g of OPBI-SO3H-GTE solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super flat dish and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H-GTE film with a thickness of 25 μm and a uniform surface. Place the prepared film in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it with deionized water multiple times to obtain the activated OPBI-SO3H-GTE film.

[0108] Example 3

[0109] Preparation of Sulfonated Polybenzimidazole (OPBI-SO3H) Membrane

[0110] Under a nitrogen atmosphere, add OPBI (2.5 mmol, 1 g) to 50 g of dimethyl sulfoxide and stir at 80 °C for 2 h. After complete dissolution, add anhydrous K2CO3 powder (3.75 mmol, 0.52 g) as a reaction catalyst. The molar ratio of OPBI to anhydrous K2CO3 powder is 1:1.5. Then slowly add 1,3-propanesultone (0.25 mmol, 0.03 g). The molar ratio of OPBI to 1,3-propanesultone is 10:1. React at 40 °C for 24 h. After the reaction is completed, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.84 g of OPBI-SO3H.

[0111] Preparation of Crosslinked Sulfonated Polybenzimidazole (OPBI-SO3H-GDE) Membrane

[0112] Dissolve 0.95 mmol (0.5 g) of OPBI-SO3H solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add ethylene glycol diglycidyl ether (GDE) (0.095 mmol, 0.016 g). The molar ratio of OPBI-SO3H to GDE is 10:1. Heat up to 80 °C and react for 24 h. After the reaction is completed, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.41 g of OPBI-SO3H-GDE product.

[0113] Dissolve 0.5 g of OPBI-SO3H-GDE solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super flat dish and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H-GDE membrane with a thickness of 25 μm and a uniform surface. Place the prepared membrane in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it with deionized water multiple times to obtain the activated OPBI-SO3H-GDE membrane.

[0114] Example 4

[0115] Preparation of Sulfonated Polybenzimidazole (OPBI-SO3H) Membrane

[0116] Under a nitrogen atmosphere, add OPBI (2.5 mmol, 1 g) to 50 g of dimethyl sulfoxide and stir at 80 °C for 2 h. After complete dissolution, add anhydrous K2CO3 powder (3.75 mmol, 0.52 g) as a reaction catalyst. The molar ratio of OPBI to anhydrous K2CO3 powder is 1:1.5. Then slowly add 1,3-propanesultone (0.5 mmol, 0.061 g). The molar ratio of OPBI to 1,3-propanesultone is 5:1. React at 40 °C for 24 h. After the reaction, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.92 g of OPBI-SO3H.

[0117] Preparation of Crosslinked Sulfonated Polybenzimidazole (OPBI-SO3H-GTE) Membrane

[0118] Dissolve 0.95 mmol (0.5 g) of OPBI-SO3H solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add glycerol triglycidyl ether (GTE) (0.095 mmol, 0.025 g). The molar ratio of OPBI-SO3H to GTE is 10:1. Raise the temperature to 80 °C and react for 24 h. After the reaction, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.44 g of OPBI-SO3H-GTE product.

[0119] Dissolve 0.5 g of OPBI-SO3H-GTE solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super flat dish and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H-GTE membrane with a thickness of 25 μm and a uniform surface. Place the prepared membrane in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it with deionized water multiple times to obtain the activated OPBI-SO3H-GTE membrane.

[0120] Example 5

[0121] Preparation of Sulfonated Polybenzimidazole (OPBI-SO3H) Membrane

[0122] Under a nitrogen atmosphere, add OPBI (2.5 mmol, 1 g) to 50 g of dimethyl sulfoxide and stir at 80 °C for 2 h. After complete dissolution, add anhydrous K2CO3 powder (3.75 mmol, 0.52 g) as a reaction catalyst. The molar ratio of OPBI to anhydrous K2CO3 powder is 1:1.5. Then slowly add 1,3-propane sultone (0.25 mmol, 0.03 g). The molar ratio of OPBI to 1,3-propane sultone is 10:1. React at 40 °C for 24 h. After the reaction, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.84 g of OPBI-SO3H.

[0123] Preparation of Crosslinked Sulfonated Polybenzimidazole (OPBI-SO3H-GTE) Membrane

[0124] Dissolve 0.95 mmol (0.5 g) of OPBI-SO3H solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add glycerol triglycidyl ether (GTE) (0.19 mmol, 0.049 g). The molar ratio of OPBI-SO3H to GTE is 5:1. Raise the temperature to 80 °C and react for 24 h. After the reaction, pour the mixture into acetone to precipitate the polymer, and wash it with water and ethanol several times until the solution becomes colorless. Finally, vacuum dry the precipitate at 60 °C for 24 h to obtain 0.44 g of OPBI-SO3H-GTE product.

[0125] Dissolve 0.5 g of OPBI-SO3H-GTE solid in 25 g of dimethyl sulfoxide to obtain a solution with a mass fraction of 2%. Add 2.0 mL of the solution to a super flat dish and place it in an oven at 80 °C for drying for 24 h to obtain an OPBI-SO3H-GTE membrane with a thickness of 25 μm and a uniform surface. Place the prepared membrane in 10 mL of a phosphoric acid solution with a mass concentration of 85 wt% and let it stand for 24 h for activation pretreatment. After taking it out, wash it with deionized water multiple times to obtain the activated OPBI-SO3H-GTE membrane.

[0126] Table 1 shows the performance results of the membranes prepared in the examples and comparative examples of the present invention.

[0127] Table 1

[0128]

[0129] From the data in Table 1, it can be seen that Example 1 has the optimal comprehensive performance. Due to the efficient proton transport channels and cross-linked network in the OPBI-SO3H-GTE membrane, its Coulomb efficiency and voltage efficiency are 99.40% and 80.73% respectively at 200 mA·cm -2 which are better than those of the pure OPBI membrane in Comparative Example 1 (98.73%, 75.36%) and the Nafion212 membrane in Comparative Example 2 (97.54%, 75.60%). Based on the excellent Coulomb efficiency and voltage efficiency, its energy efficiency reaches over 90% and 80% respectively at 80 mA·cm -2 and 200 mA·cm -2 which is the highest among all the examples. While the energy efficiencies of the pure OPBI membrane in Comparative Example 1 and the Nafion212 membrane in Comparative Example 2 are only 74.41% and 73.74% at a high current density of 200 mA·cm -2 This shows that the cross-linked sulfonated polybenzimidazole membrane prepared by the present invention has higher ionic conductivity while maintaining its own excellent selectivity and stability, showing great application potential.

[0130] The above description is only a preferred embodiment of the present invention and does not impose any form of limitation on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art of this patent, without departing from the scope of the technical solution of the present invention, can make some changes or modifications to the above-mentioned disclosed technical content to be equivalent change equivalent embodiments. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the present invention.

Claims

1. A preparation method of a crosslinked sulfonated polybenzimidazole membrane, characterized in that, It includes the following steps: In the first step, under a nitrogen atmosphere, polybenzimidazole is added to the first solvent, stirred for 1 - 3 h at a temperature of 60 - 100 °C, anhydrous potassium carbonate powder is added, the molar ratio of polybenzimidazole to anhydrous potassium carbonate powder is 1:1 - 2, and then a sulfonating agent is slowly added. The molar ratio of polybenzimidazole to the sulfonating agent is 8 - 20:1, and the reaction is carried out for 1 - 48 h at a temperature of 30 - 50 °C. After post-treatment, sulfonated polybenzimidazole is obtained; The polybenzimidazole is aromatic ether type polybenzimidazole OPBI; The sulfonating agent is selected from at least one of 1,3 - propane sultone and 1,4 - butane sultone; In the second step, the sulfonated polybenzimidazole is dissolved in the second solvent to obtain a solution with a mass fraction of 1 - 10%, a cross-linking agent is added, the molar ratio of sulfonated polybenzimidazole to the cross-linking agent is 1 - 20:1, the temperature is raised to 60 - 100 °C and the reaction is carried out for 1 - 48 h. After post-treatment, cross-linked sulfonated polybenzimidazole is obtained; The cross-linking agent is selected from glycerol triglycidyl ether; In the third step, the cross-linked sulfonated polybenzimidazole is dissolved in the third solvent, dried to obtain a cross-linked sulfonated polybenzimidazole membrane, and activated to obtain an activated cross-linked sulfonated polybenzimidazole membrane; The activation step in the third step: The prepared membrane is placed in an 85 wt% phosphoric acid solution and left standing for 1 - 48 h for activation pretreatment, and then washed repeatedly with deionized water.

2. The preparation method of the crosslinked sulfonated polybenzimidazole membrane according to claim 1, wherein The first solvent is selected from at least one of N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, N - methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid; The post-treatment step in the first step: The mixture is poured into acetone to precipitate the polymer, and washed several times with water and ethanol until the solution becomes colorless, and then dried.

3. The preparation method of the crosslinked sulfonated polybenzimidazole membrane according to claim 1, wherein The preparation method of the polybenzimidazole includes the following steps: Under a nitrogen atmosphere, phosphorus pentoxide and methanesulfonic acid with a mass ratio of 1:1 - 20 are mixed, 3,3’ - diaminobenzidine and 4,4’ - dicarboxydiphenyl ether with a molar ratio of 1:1 - 2 are added, the mass ratio of phosphorus pentoxide to 3,3’ - diaminobenzidine is 1 - 5:

1. Under stirring conditions, the reaction system is heated to 90 - 110 °C and stirred and mixed for 1 - 3 h, and then the temperature is raised to 130 - 150 °C and the reaction is carried out for 1 - 5 h to obtain the polybenzimidazole.

4. The preparation method of the cross-linked sulfonated polybenzimidazole membrane according to claim 1, characterized in that, The second solvent is selected from at least one of N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, N - methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid; The post-treatment step in the second step: The mixture is poured into acetone to precipitate the polymer, and washed several times with water and ethanol until the solution becomes colorless, and then dried.

5. The preparation method of the crosslinked sulfonated polybenzimidazole membrane according to claim 1, characterized in that, The cross-linked sulfonated polybenzimidazole is dissolved in the third solvent to obtain a solution with a mass fraction of 1 - 10%.

6. The preparation method of the cross-linked sulfonated polybenzimidazole membrane according to claim 1, characterized in that, The third solvent is selected from at least one of N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, N - methylpyrrolidone, tetrahydrofuran, and methanesulfonic acid.

7. The preparation method of the crosslinked sulfonated polybenzimidazole membrane according to claim 1, characterized in that, The thickness of the cross-linked sulfonated polybenzimidazole membrane is 20 - 30 μm.

8. A crosslinked sulfonated polybenzimidazole membrane, characterized in that, The crosslinked sulfonated polybenzimidazole membrane is prepared by the preparation method described in any one of claims 1 to 7.

9. Use of the crosslinked sulfonated polybenzimidazole membrane according to claim 8 in the preparation of a flow battery.

10. The application according to claim 9, wherein The flow battery is selected from at least one of an all-vanadium flow battery, a lithium-ion flow battery, a zinc-bromine flow battery, a zinc-cerium flow battery, a zinc-nickel flow battery, a lead flow battery, an iron-chromium flow battery, a polysulfide sodium / bromine flow battery, or an all-iron flow battery.

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