Branched polybenzimidazole composite membrane for all-vanadium redox flow battery and preparation method of branched polybenzimidazole composite membrane

Abstract

The invention relates to the technical field of diaphragms for flow batteries, in particular to a branched polybenzimidazole composite membrane for an all-vanadium flow battery and a preparation method of the branched polybenzimidazole composite membrane. The preparation method comprises the following steps: step 1, synthesizing branched PBI; step 2, preparing a branched PBI base membrane; and 3, performing interfacial polymerization modification to form a functional separation layer. The invention aims to provide an efficient and reliable solution for solving the technical problem through material innovation and process optimization on the basis of a synergistic strategy of a branched PBI base membrane and interfacial polymerization modification.

Description

A branched polybenzimidazole composite membrane for vanadium redox flow batteries and its preparation method Technical Field

[0001] This invention relates to the field of membrane technology for flow batteries, and in particular to a branched polybenzimidazole composite membrane for vanadium redox flow batteries and its preparation method. Background Technology

[0002] As the global energy structure shifts towards renewable energy, the scale of grid connection for intermittent power sources such as wind power and photovoltaics is expanding, making the demand for long-duration energy storage technology increasingly urgent. Long-duration energy storage can not only smooth out the output fluctuations of renewable energy and ensure the stable operation of the power grid, but also serve as a key support for building a new power system based on new energy sources (Science, 334(6058), 928–935). Among the many energy storage technologies, vanadium redox flow batteries are widely recognized as one of the most promising large-scale long-duration energy storage technologies due to their inherent safety, long cycle life, power and capacity decoupling, and flexible site selection (Journal of The Electrochemical Society, 158(8), R55–R79).

[0003] In the core components of VRFB (Vibration-Return Fiber), the ion-conducting membrane plays a crucial role. It not only needs to block the cross-linking of vanadium ions in the positive and negative electrode electrolytes to prevent self-discharge, but also needs to efficiently conduct protons (H+). + To form a complete current loop. Therefore, an ideal membrane must have high ion selectivity (i.e., high proton conductivity and low vanadium ion permeability), excellent chemical stability, moderate swelling capacity and low cost (Chemical Society Reviews, 46(12), 3516–3530).

[0004] Currently, perfluorosulfonic acid membranes are widely used in commercial VRFB (Vibration Recycling Fusion Battery), with the Nafion series membranes produced by DuPont (now Chemours) being the most typical example. These membrane materials have excellent chemical stability and extremely high proton conductivity, thanks to their unique microphase separation structure of perfluorocarbon backbone and hydrophilic sulfonic acid groups (Chemical Reviews, 104(10), 4535–4586). However, Nafion membranes have two inherent shortcomings in VRFB applications: First, the size of their hydrophilic channels (approximately 2–4 nm) is much larger than the kinetic diameter of hydrated vanadium ions (e.g., V(IV) ≈ 0.6 nm), resulting in poor vanadium blocking performance, low battery coulombic efficiency, and rapid capacity decay; Second, the perfluoropolymer synthesis process is complex and the raw material cost is high, which seriously restricts their large-scale application in large-scale energy storage power stations (ChemSusChem, 4(10), 1388–1406).

[0005] To overcome the technical and economic bottlenecks of Nafion membranes, academia and industry have been committed to developing new non-fluorinated or partially fluorinated membrane materials, mainly including sulfonated polyarylethers, sulfonated polysulfones, and polybenzimidazole (PBI) materials. Among them, PBI has attracted much attention due to its outstanding thermal stability, mechanical strength, and inherent vanadium blocking potential. PBI can form proton transport channels by doping its basic imidazole groups with phosphoric acid or sulfuric acid, but its dense segmental structure often leads to low intrinsic proton conductivity, usually requiring operation under high acid doping levels, which may cause material swelling and a decrease in mechanical properties (Journal of Materials Chemistry A, 9(35), 19179–19207).

[0006] In recent years, polymer molecular structure design has become a key approach to improving membrane performance. Studies have shown that branched polymers, due to their three-dimensional structure and covalent branching, can maintain good solubility and processability while forming a large free volume similar to a cross-linked network, providing more pathways for ion transport (Chemical Society Reviews, 49(23), 8609–8630). Introducing branched structures into PBI is expected to reduce chain entanglement and improve proton transport efficiency while maintaining its inherent dimensional stability and barrier properties.

[0007] In summary, developing a novel composite membrane that combines high proton conductivity, excellent vanadium barrier properties, good mechanical and chemical stability, and low cost is an urgent need for the development of vanadium redox flow battery technology. This invention, based on a synergistic strategy of "branched PBI base film" and "interfacial polymerization modification," aims to provide an efficient and reliable solution to the aforementioned technical challenges through material innovation and process optimization. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a branched PBI composite film for all vanadium redox flow batteries that has high proton conductivity, high vanadium resistance and excellent stability, as well as its preparation method.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for preparing a branched polybenzimidazole composite membrane for vanadium redox flow batteries includes the following steps:

[0011] Step 1: Synthesis of Branched PBI

[0012] Using 3,3'-diaminobenzidine, specific tricarboxylic acid monomers (such as 1,3,5-tris(4-carboxyphenyl)benzene) and dicarboxylic acid monomers (such as 2,2'-bis(4-carboxyphenyl)hexafluoropropane) as reactants, a branched PBI polymer with a three-dimensional structure was synthesized through a temperature-programmed polycondensation reaction in a polyphosphoric acid solvent. The structural formula is shown in Formula (I). This branched structure is key to forming a large free volume and reducing proton transport resistance.

[0013]

[0014] Step 2: Preparation of branched PBI base film

[0015] The aforementioned branched PBI polymer was dissolved in an organic solvent (such as DMAc) to form a uniform casting solution. A gel-like base film with a microporous cross-linked network structure was then prepared by a casting and vacuum drying process. This base film provides support for the intrinsic vanadium barrier properties and mechanical strength.

[0016] Step 3: Interface aggregation and modification to form a functional separation layer

[0017] On the surface of the branched PBI base membrane, a dense functional separation layer is constructed using interfacial polymerization technology. Specifically, the base membrane is first immersed in an aqueous solution containing a polyamine (such as 2,2'-dipyridine) as shown in formula (II), and then it is brought into contact with an organic phase solution containing a crosslinking agent (such as tribenzyl bromide), so that the monomers of the two phases undergo an in-situ polymerization reaction at the membrane interface to form an ultrathin, dense separation skin layer rich in charged groups, the structural formula of which is shown in formula (III).

[0018]

[0019]

[0020] Preferably, the reaction temperature in step one is 180–220°C and the reaction time is 24–48 h. More preferably, the polyphosphoric acid is heated to 120°C and stirred to melt, the three monomers are added and stirred to dissolve, the temperature is raised to 140°C for prepolymerization for 6 h, and the temperature is raised to 220°C for polymerization for 4 h.

[0021] Preferably, the molar ratio of the dicarboxylic acid monomer, the tricarboxylic acid monomer, and the 3,3'-diaminobenzidine monomer in step one is 4.55:0.3:5.

[0022] Preferably, the concentration of the polyamine monomer in step three is 0.1 wt.% to 10 wt.%.

[0023] Preferably, the crosslinking agent in step three is tribenzyl bromide.

[0024] Preferably, the concentration of the tribenzyl bromide organic solution is 0.1 wt.% to 2 wt.%, and the soaking time is 2 to 12 h, more preferably 1.5 wt.% for 8 h.

[0025] Preferably, the drying temperature in step three is 50–80°C and the time is 15–120 min, and more preferably it is 60°C and 25 min.

[0026] A branched polybenzimidazole composite membrane for use in vanadium redox flow batteries is prepared using the above-described preparation method.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) Branched three-dimensional base film constructs "high-speed proton channel" to solve the industry problem of balancing conductivity and stability.

[0029] Traditional dense PBI membranes exhibit high proton conduction resistance, while porous membranes tend to sacrifice mechanical strength. This invention employs branched PBI as the base membrane material. Its three-dimensional branched structure helps reduce polymer chain entanglement and forms covalently bonded 'free volume cavities,' thereby improving proton conduction efficiency. This not only creates a low-resistance, continuous transport channel for protons, resulting in a conductivity (>240 mS / cm) far exceeding that of Nafion 212, but also, thanks to its cross-linked, stable structure, achieves a synergistic effect of high conductivity and low swelling rate (<10.5%), overcoming the common technical bottleneck of "high conductivity inevitably leading to high swelling" from the material source.

[0030] (2) The interface is aggregated to construct the "intelligent ion sieving" functional layer, realizing the precise sieving of protons and vanadium ions.

[0031] To address the core defect of Nafion membranes—poor vanadium blocking performance due to excessively large hydrophilic channels—this invention utilizes interfacial polymerization technology to generate a dense functional layer in situ on the base membrane surface. This layer, through the synergistic effect of Donan repulsion (charge sieving) and sub-nanometer pore size (size sieving), achieves highly efficient vanadium ion blocking while maintaining rapid proton conduction. This results in a proton / vanadium ion selectivity (H / V) of over 44,500 for the composite membrane, nearly three orders of magnitude higher than Nafion 212 (~48.3), thus solving the battery self-discharge problem and significantly improving coulombic efficiency.

[0032] (3) The “interface interlocking” structure realizes the integration of the functional layer and the base film, overcoming the bottleneck of interface stability under long-term operation of the composite membrane.

[0033] Conventional composite membranes are prone to swelling and delamination at their interlayer interfaces during long-term operation. This invention utilizes an "interface interlocking" technology to achieve deep interpenetration and entanglement between the polymer chains of the skin layer generated by interfacial polymerization and the branched PBI base membrane segments, forming covalent cross-links. This robust interfacial structure ensures excellent compatibility and dimensional synergy between the functional and support layers in the electrolyte environment, effectively suppressing interlayer delamination caused by swelling differences, and endowing the composite membrane with an ultra-long cycle life and durability surpassing conventional multilayer structures. Attached Figure Description

[0034] Figure 1 is a photograph of the branched PBI base film;

[0035] Figure 2 is a photograph of the prepared PBI composite membrane.

[0036] Figures 3a and 3b are surface and cross-sectional views of the prepared PBI composite membrane;

[0037] Figure 4 shows the performance degradation of the prepared PBI composite membrane during long-cycle operation in a single flow battery cell. Detailed Implementation

[0038] To further illustrate the technical solution of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the claims of the present invention.

[0039] Example 1

[0040] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1052 g of 1,3,5-tris(4-carboxyphenyl)benzene, and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to the flask, and the mixture was stirred until a homogeneous suspension formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, dry under vacuum at 110°C for at least 24 hours.

[0041] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0042] The PBI substrate membrane was immersed in a 2wt.% 2,2'-dipyridine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0043] Example 2

[0044] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1052 g of 1,3,5-tris(4-carboxyphenyl)benzene, and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to the flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, dry under vacuum at 110°C for at least 24 hours.

[0045] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0046] The PBI substrate membrane was immersed in a 2 wt.% aqueous solution (10 mL) of 4,4'-trimethylenebis(1-methylpiperidine) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% solution of tribenzyl bromide in 10 mL of trimethylbenzyl bromide and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with trimethylbenzyl bromide and ethanol to obtain the PBI composite membrane.

[0047] Example 3

[0048] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1052 g of 1,3,5-tris(4-carboxyphenyl)benzene, and 1.3767 g of 2,2'-ditrifluoromethyl-4,4'-biphenyldicarboxylic acid were carefully added to the flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the sample was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, it was dried under vacuum at 110°C for at least 24 hours.

[0049] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0050] The PBI substrate membrane was immersed in a 2 wt.% pentamethyldiethylenetriamine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed to remove excess aqueous solution and allowed to air dry at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0051] Example 4

[0052] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1052 g of 1,3,5-tris(4-carboxyphenyl)benzene, and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to the flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, dry under vacuum at 110°C for at least 24 hours.

[0053] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0054] The PBI substrate membrane was immersed in a 2 wt.% pentamethyldiethylenetriamine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed to remove excess aqueous solution and allowed to air dry at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0055] Example 5

[0056] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 h.

[0057] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0058] The PBI substrate membrane was immersed in a 2 wt.% pentamethyldiethylenetriamine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed to remove excess aqueous solution and allowed to air dry at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0059] Example 6

[0060] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 h.

[0061] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0062] The PBI substrate membrane was immersed in a 2 wt.% aqueous solution (10 mL) of 4,4'-trimethylenebis(1-methylpiperidine) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% solution of tribenzyl bromide in 10 mL of trimethylbenzyl bromide and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with trimethylbenzyl bromide and ethanol to obtain the PBI composite membrane.

[0063] Example 7

[0064] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 h.

[0065] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0066] The PBI substrate membrane was immersed in a 2wt.% 2,2'-dipyridine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0067] Example 8

[0068] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 1.3767 g of 2,2'-ditrifluoromethyl-4,4'-biphenyl dicarboxylic acid were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 hours. The reaction temperature was then increased to 170 °C and maintained for 1 hour to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 hours to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 hours.

[0069] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0070] The PBI substrate membrane was immersed in a 2wt.% 2,2'-dipyridine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0071] Example 9

[0072] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 0.866 g of tetrafluoroterephthalic acid were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 hours. The reaction temperature was then increased to 170 °C and maintained for 1 hour to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 hours to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 hours.

[0073] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0074] The PBI substrate membrane was immersed in a 2wt.% 2,2'-dipyridine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0075] Example 10

[0076] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was then heated to 140 °C and held for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1160 g of 4,4',4”-(1,3,5-triazine-2,4,6-triyl)tris(benzoylhydrazine), and 0.866 g of tetrafluoroterephthalic acid were carefully added to a flask, and the mixture was stirred until a homogeneous suspension was formed after approximately 6 hours. The reaction temperature was then increased to 170 °C and maintained for 1 hour to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 hours to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 hours.

[0077] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane.

[0078] The PBI substrate membrane was immersed in a 2 wt.% aqueous solution (10 mL) of 4,4'-trimethylenebis(1-methylpiperidine) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% solution of tribenzyl bromide in 10 mL of trimethylbenzyl bromide and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with trimethylbenzyl bromide and ethanol to obtain the PBI composite membrane.

[0079] Comparative Example 1

[0080] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine and 1.5690 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to the flask, and the mixture was stirred until a homogeneous suspension formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, the precipitate was dried under vacuum at 110 °C for at least 24 h.

[0081] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a linear PBI membrane.

[0082] The PBI substrate membrane was immersed in a 2wt.% 2,2'-dipyridine aqueous solution (10 mL) containing 0.1 wt.% sodium dodecyl sulfate for 2 h. Then, the substrate membrane was removed, excess aqueous solution was drained, and the membrane was air-dried at ambient temperature. Next, the membrane was immersed in a 1.5 wt.% tribenzyl bromide solution (10 mL) in mesitylene and kept at 25 °C for 8 h. The resulting PBI composite membrane was dried at 60 °C for 20 min, and then rinsed with mesitylene and ethanol to obtain the PBI composite membrane.

[0083] Comparative Example 2

[0084] In a 100 mL three-necked round-bottom flask equipped with a condenser, magnetic stirrer, and dry nitrogen protection system, 45 g of polyphosphoric acid was introduced, and the solution was heated to 140 °C and maintained for at least 1 h until the PPA melted. Under vigorous stirring, 0.8571 g of 3,3'-diaminobenzidine, 0.1052 g of 1,3,5-tris(4-carboxyphenyl)benzene, and 1.4278 g of 2,2'-bis(4-carboxyphenyl)hexafluoropropane were carefully added to the flask, and the mixture was stirred until a homogeneous suspension formed after approximately 6 h. The reaction temperature was then increased to 170 °C and maintained for 1 h to promote prepolymerization. Finally, the temperature was increased to 220 °C and maintained for at least 4 h to complete the polymerization reaction. After cooling to room temperature, the mixture was precipitated in a saturated sodium bicarbonate solution, and the precipitate was then soaked in the solution overnight to remove residual PPA. After filtration, the precipitate was purified by washing repeatedly with deionized water and anhydrous ethanol. Finally, dry under vacuum at 110°C for at least 24 hours.

[0085] Weigh 0.5g of polymer and dissolve it in 20mL of DMAc. Let it stand to remove bubbles to obtain a casting solution. Coat the casting solution onto a clean glass plate and vacuum dry it at 80℃ for 24h to obtain a branched PBI gel membrane without a functional separation layer.

[0086] The performance of the flow battery separators prepared in each embodiment and comparative example is shown in Table 1:

[0087]

[0088] Table 1.

Claims

1. A method for preparing a branched polybenzimidazole composite membrane for a vanadium redox flow battery, characterized in that... The process includes the following steps: Step 1: Synthesis of branched PBI. Using 3,3'-diaminobenzidine, tricarboxylic acid monomers, and dicarboxylic acid monomers as reactants, a branched PBI polymer with a three-dimensional structure is synthesized in a polyphosphoric acid solvent through a programmed temperature-controlled polycondensation reaction. Step 2: Preparation of the branched PBI base film. The branched PBI polymer prepared in Step 1 is dissolved in an organic solvent to form a uniform casting solution. A gel-state base film with a microporous cross-linked network structure is prepared by casting and vacuum drying. Step 3: Interfacial polymerization modification to form a functional separation layer. On the surface of the branched PBI base film, a functional separation layer is constructed through interfacial polymerization. Specifically, the branched PBI base film is first immersed in an aqueous solution containing polyamines, and then the immersed branched PBI base film is contacted with an organic solution containing a cross-linking agent, so that the monomers of the two phases undergo in-situ polymerization at the membrane interface. After drying, an ultrathin, dense separation skin rich in charged groups is formed.

2. The method for preparing the branched polybenzimidazole composite membrane for vanadium redox flow batteries as described in claim 1, characterized in that: In step one, the reaction temperature is 180~220 ℃ and the reaction time is 24~48 h. More preferably, the polyphosphoric acid is heated to 120 ℃ and stirred to melt, the three monomers are added and stirred to dissolve, the temperature is raised to 140 ℃ for prepolymerization for 6 h, and the temperature is raised to 220 ℃ for polymerization for 4 h.

3. The method for preparing the branched polybenzimidazole composite film for vanadium redox flow batteries as described in claim 1, characterized in that: The molar ratio of the dicarboxylic acid monomer, the tricarboxylic acid monomer, and the 3,3'-diaminobenzidine monomer mentioned in step one is 4.55:0.3:

5.

4. The method for preparing the branched polybenzimidazole composite film for vanadium redox flow batteries as described in claim 1, characterized in that: In step three, the concentration of polyamine monomers is 0.1 wt.% to 10 wt.%.

5. The method for preparing the branched polybenzimidazole composite film for vanadium redox flow batteries as described in claim 1, characterized in that: The crosslinking agent mentioned in step three is tribenzyl bromide.

6. The method for preparing the branched polybenzimidazole composite film for vanadium redox flow batteries as described in claim 5, characterized in that: The concentration of the tribenzyl bromide organic solution is 0.1 wt.%~2 wt.%, and the soaking time is 2~12 h, more preferably 1.5 wt.% for 8 h.

7. The method for preparing the branched polybenzimidazole composite film for vanadium redox flow batteries as described in claim 1, characterized in that: In step three, the drying temperature is 50~80 ℃ and the time is 15~120 min, more preferably 60 ℃ and 25 min.

8. A branched polybenzimidazole composite membrane for use in vanadium redox flow batteries, characterized in that: Prepared using the preparation method described in any one of claims 1-7.

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