Application of side chain functionalized polybenzimidazole porous membranes in bromine-based flow batteries

Abstract

The application discloses a kind of side chain functionalized polybenzimidazole porous ion conducting membrane applied in liquid flow battery, especially relates to the application of the membrane in bromine-based liquid flow battery.The membrane is prepared by grafting bromide quaternary ammonium group-containing side chain on the main chain of polybenzimidazole through nucleophilic substitution reaction with bromide ammonium bromide compound, and then using the side chain functionalized polybenzimidazole as raw material to prepare polybenzimidazole porous ion conducting membrane.The bromide quaternary ammonium group on the side chain can be complexed with bromine, which can fix bromine diffusing to the negative electrode on the membrane surface or membrane pores, thereby preventing bromine from migrating from the positive electrode side to the negative electrode side through the membrane and reacting with the active material on the negative electrode side to cause self-discharge of the battery.Therefore, the side chain functionalized polybenzimidazole porous membrane has high selectivity, and can be applied to bromine-based liquid flow battery system without bromine complexing agent in the electrolyte system, thereby reducing the cost of the battery system.

Description

Technical Field

[0001] This invention provides the preparation and application of a side-chain functionalized polybenzimidazole porous ion-conducting membrane, particularly its application in the field of bromine-based flow batteries, and removes the application of bromine complexing agents in the electrolyte of bromine-based flow batteries. Background Technology

[0002] In recent years, there has been a growing call for the use of renewable and clean energy. However, renewable energy power generation, such as wind and solar power, is significantly affected by seasonal, meteorological, and geographical conditions, resulting in discontinuous and unstable power output with large fluctuations and poor adjustability. This can potentially have a significant impact on the power grid. Therefore, with the rapid rise of renewable energy sources such as wind and solar power and the smart grid industry, energy storage technology has become a focus of attention.

[0003] Energy storage technologies are broadly categorized into physical energy storage and chemical energy storage. Physical energy storage includes pumped hydro storage, compressed air storage, and flywheel energy storage. Chemical energy storage primarily includes lead-acid batteries, sodium-sulfur batteries, flow batteries, and lithium-ion batteries. However, each energy storage technology has its suitable application areas. Chemical energy storage technologies suitable for large-scale energy storage mainly include flow batteries, sodium-sulfur batteries, lead-acid batteries, and lithium-ion batteries. Considering the advantages and disadvantages of various energy storage technologies, flow battery energy storage technology has received more widespread attention. Among them, Br2 / Br - Br2 / Br2 redox couples are characterized by high potential, high solubility, and low cost. In the field of flow batteries, they are particularly popular. - Bromine-based flow batteries with bromine as the positive electrode active couple have been extensively studied. However, the development of bromine-based flow batteries still faces many challenges. In particular, bromine has strong volatility and strong diffusivity. During battery operation, bromine generated at the positive electrode diffuses to the negative electrode, which not only reduces the content of active material at the positive electrode but also affects the active material at the negative electrode, causing battery self-discharge, reducing battery efficiency, and seriously affecting battery capacity and cycle life.

[0004] Ion-conducting membranes are one of the key materials in bromine-based flow batteries. They function to prevent cross-mixing of active materials in the positive and negative electrode electrolytes, while simultaneously transferring charge carriers to form a complete battery circuit, acting as a crucial barrier against bromine diffusion. However, currently used membrane materials in bromine-based flow batteries suffer from insufficient selectivity, failing to effectively suppress bromine diffusion and reduce battery self-discharge and capacity decay. Therefore, structural design and performance optimization of ion-conducting membrane materials are essential for the further development of bromine-based flow batteries, aiming to develop membrane materials that combine high ion selectivity, high ion conductivity, high stability, and low cost. Summary of the Invention

[0005] The purpose of this invention is to prepare a side-chain functionalized polybenzimidazole porous membrane for bromine-based flow batteries. The quaternary ammonium bromide groups on the side chains complex with diffused bromine, fixing it to the membrane surface and pores. This inhibits bromine from diffusing from the positive electrode to the negative electrode and reacting with the active material, thus preventing self-discharge. Therefore, the electrolyte of the bromine-based flow battery does not require a complexing agent, thereby reducing the influence of the bromine complexing agent on the redox activity of bromine and lowering battery costs. Furthermore, the quaternary ammonium bromide groups have extremely high hydrophilicity, which facilitates carrier migration, giving the membrane high ionic conductivity.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The side-chain functionalized polybenzimidazole porous membrane is characterized in that it is used in a bromine-based flow battery.

[0008] The membrane has a porous structure with a porosity of 20%-90% and a pore size distribution range of 0.001-500 nm.

[0009] The raw material for the side-chain functionalized polybenzimidazole porous membrane is all or part of the side-chain functionalized polybenzimidazole.

[0010] The side-chain functionalized polybenzimidazole has a side chain containing a quaternary ammonium bromide group and is prepared by a nucleophilic substitution reaction of polybenzimidazole and a brominated ammonium bromide compound.

[0011] The brominated ammonium bromide compounds are (5-bromopentyl)-trimethylammonium bromide, (3-bromopropyl)-trimethylammonium bromide, (4-bromobutyl)-trimethylammonium bromide, (2-bromoethyl)-trimethylammonium bromide, (4-bromobutyl)-tributylammonium bromide, (3-bromopropyl)-tripropylammonium bromide, (2-bromoethyl)-triethylammonium bromide, (12-bromododecyl)-trimethylammonium bromide, (10-bromodecyl)-trimethylammonium bromide, (16-bromohexadecyl)-trimethylammonium bromide, (14-bromotetradecyl)-trimethylammonium bromide, and (4-bromobutyl)-methyldimethylammonium bromide. One or more of the following: n-butylammonium bromide, (10-bromodecyl)-decyldimethylammonium bromide, (8-bromooctyl)-octyldimethylammonium bromide, (12-bromododecyl)-dodecyldimethylammonium bromide, (14-bromotetradecyl)-tetradecyldimethylammonium bromide, (12-bromododecyl)-dimethylbenzylammonium bromide, (18-bromooctadecyl)-trimethylammonium bromide, (2-bromoethyl)-benzyldiethylammonium bromide, (4-bromobutyl)-benzyldibutylammonium bromide, (6-bromoheptyl)-triheptylammonium bromide, and (8-bromooctyl)-trioctylammonium bromide.

[0012] The polybenzimidazole is one or more of the following three types, and its structure is as follows:

[0013] Where n is between 20 and 10000.

[0014] The application preparation process includes the following steps:

[0015] (1) Dissolve polybenzimidazole in a solvent to obtain a homogeneous polybenzimidazole solution with a mass concentration between 1% and 50%;

[0016] (2) The ammonium bromide compound is dissolved in the above-mentioned polybenzimidazole solution, wherein the molar ratio of the ammonium bromide compound to the imidazole group in the polybenzimidazole is between 0.1 and 2, preferably between 0.1 and 1.2;

[0017] (3) Stir the solution from step (2) at a reaction temperature of 20-200℃ for 10 min-240 h to obtain a side-chain functionalized polybenzimidazole solution;

[0018] (4) Prepare a porous membrane from the solution prepared in step (3).

[0019] The solvent is at least one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0020] The molar ratio (R) of the ammonium bromide compound to the imidazole group in the polybenzimidazole is calculated as follows: ;

[0021] Where m1 and m2 are the masses (in g) of the ammonium bromide compound and polybenzimidazole in the solution, respectively, and M1 and M2 are the molecular weights (in mol / g) of the ammonium bromide compound and the benzimidazole monomer, respectively.

[0022] The molar ratio of the ammonium bromide compound to the imidazole group in polybenzimidazole determines the degree of functionalization of polybenzimidazole; the higher the molar ratio, the higher the degree of functionalization.

[0023] The process of preparing side-chain functionalized benzimidazole porous membrane by immersion phase transformation method is as follows: the solution prepared in step (3) is uniformly coated on a glass plate, and then immersed in a non-solvent at room temperature to solidify into a membrane with a thickness between 50-1000 μm; the non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol and acetonitrile.

[0024] In addition to using the above-mentioned immersion phase transformation method to prepare side-chain functionalized polybenzimidazole porous membranes, this type of membrane can also be prepared by one or more of the following methods: vapor phase transformation, thermally induced phase separation, solvent evaporation induced phase separation, etc.

[0025] The process of preparing side-chain functionalized polybenzimidazole porous membranes by vapor phase transition method is as follows: the solution prepared in step (3) is uniformly coated on a glass plate, and then placed in a non-solvent vapor at 40-80 ℃ and 50%-100% humidity for 10 min-2 h for phase transition curing to form a membrane with a thickness between 50-1000 μm; the non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol, and acetonitrile.

[0026] The process of preparing side-chain functionalized polybenzimidazole porous membranes using the solvent template method is as follows: In step (2), silica particles are added simultaneously to obtain a uniform solution. The prepared solution is uniformly coated onto a glass plate, and then immersed in a non-solvent at room temperature to solidify into a membrane. The membrane is then immersed in a 1-6 M sodium hydroxide solution to etch the silica, thus obtaining porous polybenzimidazole membranes. The membrane thickness is between 50-1000 μm; the non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol, and acetonitrile.

[0027] The side-chain functionalized polybenzimidazole porous membrane is used as a separator in bromine-based flow batteries, including but not limited to zinc / bromine flow batteries, hydrogen / bromine flow batteries, lithium / bromine flow batteries, quinone / bromine flow batteries, magnesium / bromine flow batteries, sodium polysulfide / bromine flow batteries, or vanadium / bromine flow batteries, etc.

[0028] The quaternary ammonium bromide groups on the side chains of the polybenzimidazole porous membrane with functionalized side chains can complex with diffused bromine, thus fixing the bromine on the membrane surface or in the membrane pores. This prevents bromine migration from the positive electrode to the negative electrode, effectively mitigating the battery self-discharge problem caused by bromine diffusion during battery operation. This allows bromine-based flow batteries to use electrolyte systems without bromine complexing agents. Consequently, the impact of adding complexing agents to the electrolyte on the battery electrode reaction kinetics is weakened, and battery costs are reduced.

[0029] The bromine-free electrolyte system contains only active substances and supporting electrolytes.

[0030] Beneficial results of the present invention

[0031] 1. Side-chain functionalized polybenzimidazole porous membranes are commonly used in vanadium redox flow batteries, utilizing the Donnan repulsion effect of positively charged nitrogen-containing groups on the side chains on positively charged vanadium ions to suppress vanadium ion cross-transport. In contrast, the side-chain functionalized polybenzimidazole porous membrane for zinc-bromine flow batteries prepared in this invention exhibits high selectivity. Its quaternary ammonium bromide groups on the side chains can complex with diffused bromine, thus fixing the bromine on the membrane surface or within the membrane pores, preventing bromine migration from the positive to the negative electrode and effectively mitigating the battery self-discharge problem caused by bromine diffusion during battery operation. Furthermore, when using this type of membrane, stable performance, low capacity retention, and long cycle life can be achieved in the electrolyte without the addition of bromine complexing agents. This reduces the adverse effects of bromine complexation on the kinetics of bromine redox reactions and lowers battery costs.

[0032] 2. The side-chain functionalized polybenzimidazole porous membrane for zinc-bromine flow batteries prepared by this invention has a porous structure and highly hydrophilic quaternary ammonium groups, which can greatly promote ion transport and give the membrane high conductivity.

[0033] 3. The side-chain functionalized polybenzimidazole porous membrane for zinc-bromine flow batteries prepared by this invention has adjustable side chain type, degree of functionalization, and type of quaternary ammonium bromide group, which can achieve controllable regulation of the performance of zinc-bromine flow batteries.

[0034] 4. The side-chain functionalized polybenzimidazole porous membrane for zinc-bromine flow batteries prepared by this invention broadens the types and application range of membrane materials for zinc-bromine flow batteries. Attached Figure Description

[0035] Figure 1 Chemical structure of PBI.

[0036] Figure 2 The preparation process of FPBI.

[0037] Table 1. Comparison of properties of unfunctionalized and functionalized PBI porous membranes.

[0038] Figure 3 Schematic diagram of the complexation of FPBI with bromine.

[0039] Table 2 compares the performance of zinc-bromine flow batteries assembled with unfunctionalized and functionalized PBI porous membranes.

[0040] Figure 4 Cyclic performance of zinc-bromine flow batteries assembled with PBI-1.

[0041] Figure 5 Cyclic performance of the zinc-bromine flow battery assembled with FPBI-1.

[0042] Figure 6 The structure of PBI used in Example 5. Detailed Implementation

[0043] Taking a zinc-bromine flow battery as an example, the cycle performance test conditions for a zinc-bromine flow battery are as follows: the end plates are made of stainless steel, the bipolar plates are made of graphite plates, and both the positive and negative electrodes use carbon felt as electrodes, with an effective electrode area of ​​36 cm². 2 Both the positive and negative electrode electrolytes are 2 mol / L zinc bromide + 3 mol / L potassium chloride (without complexing agent). The volume of each electrolyte is 60 mL. The battery uses a constant current charge-discharge mode at 40 mA cm⁻¹. -2 Charged for 60 min under a current density condition, then under voltage cutoff condition, 40 mA cm -2 Discharged to 0.8 V under the current density conditions.

[0044] The following embodiments are further illustrations of the present invention, but are not intended to limit the scope of the invention.

[0045] Example 1

[0046] The preparation method of side-chain functionalized polybenzimidazole (FPBI) porous membrane is as follows:

[0047] (1) 1 g of PBI was dissolved in 19 g of N,N-dimethylacetamide (DMAc) to obtain a homogeneous solution with a mass concentration of 5%. The structure of the PBI used (monomer molecular weight of 308 g / mol) is as follows. Figure 1 As shown.

[0048] (2) Dissolve 1.79 g of (4-bromobutyl)-trimethylammonium bromide (4BTAB, molecular weight 275 g / mol) in the above solution. The molar ratio of 4BTAB to imidazole groups is 1.

[0049] (3) React the above solution at 50 °C for 24 h; the reaction equation is as follows. Figure 2 As shown.

[0050] (4) Pour the solution obtained after the reaction onto a clean and flat glass plate, and then immerse the whole plate in a non-solvent (water) at 25°C for more than 0.5 h to obtain an FPBI porous membrane (defined as FPBI-1 porous membrane, with a membrane thickness of 100 μm, a porosity of 80%, and a pore size distribution range of 0.01-50 nm).

[0051] The preparation method of unfunctionalized PBI porous membrane is as follows:

[0052] (1) Dissolve 1 g of PBI in 19 g of N,N-dimethylacetamide (DMAc) to obtain a homogeneous solution with a mass concentration of 5%;

[0053] (2) Pour the above solution onto a clean and flat glass plate, and then immerse the whole plate in a non-solvent (water) at 25 °C for more than 0.5 h to obtain a PBI porous membrane (defined as PBI-1 porous membrane) with a thickness of 100 μm.

[0054] The performance of the FPBI-1 porous membrane was tested and compared with that of the PBI-1 porous membrane, as shown in Table 1. First, the side chain containing the quaternary ammonium bromide group has extremely high hydrophilicity, which is beneficial for promoting ion migration; therefore, the FPBI-1 porous membrane exhibits high ion conductivity. Second, the side chain of FPBI-1 contains the quaternary ammonium bromide group, which can complex with bromine (…). Figure 3 The FPBI-1 porous membrane is immobilized on the membrane surface and within the membrane by the complexation and immobilization of bromine diffusing towards the negative electrode, thereby inhibiting bromine diffusion and giving the FPBI-1 porous membrane high selectivity. However, the PBI-1 porous membrane cannot effectively prevent bromine diffusion. Therefore, the sheet resistance of the FPBI-1 porous membrane is lower than that of the PBI-1 porous membrane, proving that the FPBI-1 porous membrane has higher ionic conductivity (Table 1). The bromine permeability of the FPBI-1 porous membrane is shown in Table 1. Its bromine-immobilization effect makes its bromine permeability lower than that of the PBI-1 porous membrane, indicating that FPBI-1 also has excellent ion selectivity. Therefore, side-chain functionalization of PBI can make the membrane possess both high conductivity and high selectivity.

[0055] FPBI-1 and PBI-1 porous membranes were used in zinc-bromine flow batteries. When assembling a zinc-bromine flow battery using the PBI-1 porous membrane, the electrolyte lacked a bromine complexing agent (the role of the bromine complexing agent is to use organic compounds to complex with bromine to form larger bromine complexes, thus preventing bromine diffusion through the membrane based on the pore size sieving effect. Furthermore, when bromine complexes with the complexing agent, it can inhibit its corrosion of battery components such as electrodes, membranes, and current collectors), and the membrane lacked the ability to complex bromine. Consequently, the current collector was severely corroded by bromine, leading to abnormal and unstable battery operation and very poor performance. Figure 4 Conversely, when assembling zinc-bromine flow batteries using FPBI-1 porous membranes, the high ionic conductivity and bromine-complexing and immobilizing ability of the FPBI-1 porous membranes enable operation at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery assembled with the FPBI-1 porous membrane exhibited stable performance, with a coulombic efficiency of 99.09% and a voltage efficiency of 86.63%, demonstrating high battery performance (Table 2). After self-discharge testing, the zinc-bromine flow battery assembled with the FPBI-1 porous membrane maintained a capacity retention of up to 85% (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled with the FPBI-1 porous membrane achieved a capacity retention of up to 85% at 40 mA cm⁻¹. -2 Under operating current density conditions, it can operate continuously and stably for more than 150 cycles, and the battery performance remains stable, demonstrating excellent stability. Figure 5 Therefore, using brominated ammonium bromide-based compounds to functionalize the side chains of PBI can effectively improve the selectivity and conductivity of the membrane. It can suppress the self-discharge and capacity decay of the battery and improve the cycle performance of the battery without the presence of bromine complexing agents in the electrolyte. Furthermore, eliminating the complexing agent from the electrolyte can greatly reduce costs.

[0056] Example 2

[0057] FPBI porous membranes (defined as FPBI-2 porous) were prepared according to the method described in Example 1, except that the membrane thickness was 700 μm.

[0058] The performance of the composite membrane FPBI-1 porous membrane was tested. Because increasing the thickness of the porous membrane hinders ion transport and reduces the ionic conductivity, the sheet resistance of the FPBI-2 porous membrane was higher than that of the FPBI-1 porous membrane, demonstrating that the FPBI-2 porous membrane has relatively higher ionic conductivity (Table 1). Furthermore, the increased porous membrane thickness implies an increased content of quaternary ammonium bromide groups, which can complex with more bromine, better preventing bromine diffusion to the negative electrode, resulting in lower bromine permeability, i.e., better ion selectivity. The bromine permeability of the FPBI-2 porous membrane is shown in Table 1. Its better complexation and fixation of bromine makes its bromine permeability lower than that of the FPBI-1 porous membrane, proving that the FPBI-2 porous membrane also has excellent ion selectivity.

[0059] The prepared FPBI-2 porous membrane was used in a zinc-bromine flow battery. The increased content of quaternary ammonium bromide groups enhanced the membrane's ability to complex and immobilize bromine, better preventing bromine diffusion to the negative electrode. This resulted in higher coulombic efficiency, capacity retention, and cycle performance in the assembled zinc-bromine flow battery. However, the membrane's ion transport capacity decreased, which was detrimental to improving the battery's voltage efficiency. Therefore, at 40 mA cm⁻¹... -2 Under the operating current density conditions, the battery exhibited stable performance, with a coulombic efficiency of 99.25%, higher than that of the FPBI-1 porous membrane-assembled zinc-bromine flow battery under the same conditions; however, its voltage efficiency was 86.04%, lower than that of the FPBI-1 assembled zinc-bromine flow battery under the same conditions. After self-discharge testing, the battery's capacity retention rate was as high as 87%, higher than that of the FPBI-1 porous membrane-assembled zinc-bromine flow battery under the same conditions. Furthermore, the FPBI-2 porous membrane-assembled zinc-bromine-zinc-iron flow battery could operate stably for more than 200 cycles continuously, maintaining stable performance and demonstrating superior cycle performance compared to the FPBI-1 porous membrane-assembled zinc-bromine flow battery under the same conditions.

[0060] Example 3

[0061] FPBI porous membranes (defined as FPBI-3 porous) were prepared according to the method described in Example 1, except that the molar ratio of 4BTAB and imidazole groups was 1.3.

[0062] The performance of the composite membrane FPBI-3 porous membrane was tested. The increased molar ratio of 4BTAB to imidazole groups allows the imidazole groups of PBI to undergo more nucleophilic substitution reactions with 4BTAB, increasing the functionalization of the FPBI side chains. This means the FPBI-3 porous membrane has more highly hydrophilic side chains containing brominated quaternary ammonium groups, which better promotes ion transport and improves the membrane's ion conductivity. Furthermore, it can complex with more bromine, better preventing bromine diffusion to the negative electrode, resulting in lower bromine permeation and thus better ion selectivity. Therefore, the sheet resistance of the FPBI-3 porous membrane is lower than that of the FPBI-1 porous membrane, proving that FPBI-3 has higher ion conductivity (Table 1). The bromine permeation of the FPBI-3 porous membrane is lower than that of the FPBI-1 porous membrane, indicating that the FPBI-3 porous membrane also possesses excellent ion selectivity.

[0063] The prepared FPBI-3 porous membrane was used in a zinc-bromine flow battery. Its improved ion transport and bromine complexation capabilities contributed to enhanced coulombic efficiency, voltage efficiency, capacity retention, and cycle performance. (At 40 mA cm⁻¹) -2 Under the operating current density conditions, the zinc-bromine flow battery assembled with the FPBI-3 porous membrane exhibited stable performance, with a coulombic efficiency of 99.32% and a voltage efficiency of 88.14%, which are higher than those of the zinc-bromine flow battery assembled with the FPBI-porous membrane 1 under the same conditions (Table 2). After the self-discharge test, the battery's capacity retention rate was as high as 88%, which is higher than that of the zinc-bromine flow battery assembled with the FPBI-1 porous membrane under the same conditions (Table 2). However, the mechanical properties of the FPBI-3 porous membrane decreased, affecting the performance of the assembled zinc-bromine zinc-iron flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, it can operate continuously and stably for more than 100 cycles, and the battery performance remains stable, but it is inferior to the cycle performance of the zinc-bromine flow battery assembled with FPBI-1 porous membrane under the same conditions.

[0064] Example 4

[0065] FPBI porous membranes (defined as FPBI-4 porous membranes) were prepared according to the method described in Example 1, except that the brominated ammonium bromide compound used was (2-bromoethyl)-benzyldiethylammonium bromide. Similar to 4BTAB, (2-bromoethyl)-benzyldiethylammonium bromide also reacts with PBI to generate functionalized PBI.

[0066] The performance of the FPBI-4 porous membrane was tested and compared with that of the PBI-1 porous membrane. The FPBI-4 porous membrane contains hydrophilic quaternary ammonium bromide groups, which promote ion migration, resulting in high ion conductivity and lower sheet resistance than the PBI-1 porous membrane (Table 1). Furthermore, the side chains of the FPBI-4 porous membrane contain quaternary ammonium bromide groups, which can complex with bromine, thereby inhibiting bromine diffusion and giving the FPBI-4 porous membrane high selectivity. Therefore, the bromine permeability of the FPBI-4 porous membrane is lower than that of the PBI-1 porous membrane. This demonstrates that different brominated ammonium bromide compounds can also be used to functionalize the side chains of PBI, improving the conductivity and selectivity of the prepared FPBI porous membrane.

[0067] When FPBI-4 porous membranes are used in zinc-bromine flow batteries, their high ion conductivity and high bromine complexation and fixation capacity enable them to achieve high conductivity at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery assembled with the FPBI-4 porous membrane exhibited stable performance, with a coulombic efficiency of 99.06% and a voltage efficiency of 86.60%, demonstrating high battery performance (Table 2). After self-discharge testing, the zinc-bromine flow battery assembled with the FPBI-1 porous membrane maintained a capacity retention of up to 84% (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled with FPBI-1 achieved a capacity retention of up to 84% at 40 mA cm⁻¹. -2 Under the operating current density conditions, it can operate continuously and stably for more than 150 cycles, and the battery performance remains stable, demonstrating excellent stability.

[0068] Example 5

[0069] FPBI (defined as FPBI-5 porous) and PBI (defined as PBI-5 porous) porous membranes were prepared according to the method described in Example 1, the difference being that the structure of the PBI used was as follows: Figure 6 As shown.

[0070] The performance of the FPBI-5 porous membrane was tested and compared with that of the FPBI-1 and PBI-5 porous membranes. FPBI-5 contains hydrophilic quaternary ammonium bromide groups, which promote ion migration, resulting in high ion conductivity and lower sheet resistance compared to the PBI-5 porous membrane. Furthermore, the side chains of FPBI-5 contain quaternary ammonium bromide groups, which can complex with bromine, thereby inhibiting bromine diffusion and giving the FPBI-5 porous membrane high selectivity. Therefore, the bromine permeability of the FPBI-5 porous membrane is lower than that of the PBI-1 porous membrane. This demonstrates that different brominated ammonium bromide compounds can be used to functionalize the side chains of PBI membranes with different structures, improving the conductivity and selectivity of the prepared FPBI porous membranes. Moreover, the sheet resistance and bromine permeability of the FPBI-5 porous membrane are similar to those of the FPBI-1 porous membrane, indicating that changes in the PBI structure do not significantly affect the properties of the prepared side-chain functionalized membranes (Table 1).

[0071] The FPBI-5 porous membrane was used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery assembled with the FPBI-5 porous membrane exhibited stable performance, with a coulombic efficiency of 99.08% and a voltage efficiency of 86.63%, demonstrating high battery performance similar to that of the FPBI-1 porous membrane (Table 2). After self-discharge testing, the zinc-bromine flow battery assembled with the FPBI-5 porous membrane maintained a capacity retention of up to 85%, comparable to the capacity retention of the zinc-bromine flow battery assembled with the FPBI-1 porous membrane under the same conditions (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled with FPBI-5 showed good performance at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery can operate stably for more than 150 cycles continuously, maintaining stable performance and exhibiting excellent stability. Its cycle performance is similar to that of a zinc-bromine flow battery assembled with the FPBI-1 porous membrane under the same conditions. These results indicate that changes in the PBI structure do not significantly affect the battery performance of the prepared side-chain functionalized membrane.

[0072] Example 6

[0073] (1) Dissolve 1 g of PBI in 19 g of DMAc to obtain a homogeneous solution with a mass concentration of 5%. The structure of PBI is as follows: Figure 1 As shown;

[0074] (2) Dissolve 1.79 g of 4BTAB in the above solution. The molar ratio of 4BTAB to imidazole groups is 1.

[0075] (3) React the above solution at 50 °C for 24 h; the reaction equation is as follows. Figure 2 As shown.

[0076] (4) Pour the solution obtained after the reaction onto a clean and flat glass plate, and then place the whole plate in a non-solvent (water) vapor at a temperature of 50 ℃ and a humidity of 100% for more than 10 min to carry out phase transformation. Finally, place the glass plate in water for more than 0.5 h to obtain an FPBI porous membrane (defined as FPBI-6 porous membrane) with a thickness of 100 μm.

[0077] The preparation method of unfunctionalized PBI porous membrane is as follows:

[0078] (1) Dissolve 1 g of PBI in 19 g of DMAc to obtain a homogeneous solution with a mass concentration of 5%. The structure of PBI is as follows: Figure 1 As shown;

[0079] (2) Pour the above solution onto a clean and flat glass plate, and then place the whole plate in a non-solvent (water) vapor at a temperature of 50 °C and a humidity of 100% for more than 10 min to carry out phase transformation. Finally, place the glass plate in water for more than 0.5 h to obtain a PBI porous membrane (defined as PBI-6 porous membrane) with a thickness of 100 μm.

[0080] The performance of the FPBI-6 porous membrane was tested and compared with that of the PBI-6 porous membrane. The side chains of the quaternary ammonium bromide groups in the FPBI-6 porous membrane possess extremely high hydrophilicity and bromine complexing ability, giving it both high conductivity and high selectivity. Therefore, the sheet resistivity of the FPBI-6 porous membrane is lower than that of the PBI-6 porous membrane, and its bromine permeability is also lower. Furthermore, due to the sponge-like pore structure of the porous membrane prepared by vapor phase conversion, the sheet resistivity of the FPBI-6 porous membrane is slightly higher and its bromine permeability is slightly lower than that of the FPBI-1 porous membrane (Table 1).

[0081] The FPBI-6 porous membrane was used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, the FPBI-1 assembled battery exhibited stable performance, with a coulombic efficiency of 99.29% and a voltage efficiency of 86.21%, demonstrating high battery performance (Table 2). After self-discharge testing, the zinc-bromine flow battery assembled with the FPBI-6 porous membrane maintained a capacity retention of up to 86% (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled with the FPBI-6 porous membrane achieved a capacity retention of up to 86% at 40 mA cm⁻¹. -2 Under the operating current density conditions, it can operate stably for more than 150 cycles continuously, maintaining stable battery performance and demonstrating excellent stability. However, when the PBI-6 porous membrane is used in a zinc-bromine flow battery, the battery cannot operate normally and cannot achieve stable performance because it lacks the ability to effectively suppress bromine diffusion and the electrolyte does not contain a complexing agent.

[0082] Comparative Example 1

[0083] FPBI porous membranes were prepared according to the method described in Example 1, except that the thickness of the FPBI porous membrane was 20 μm (defined as FPBI-D1 porous membrane).

[0084] The properties of the FPBI-D1 porous membrane are shown in Table 1. A decrease in the content of quaternary ammonium bromide groups in the membrane reduces its ability to complex and fix bromine, but a decrease in membrane thickness increases its ion transport capacity. When the FPBI-D1 porous membrane is used in a zinc-bromine flow battery, at 40 mA cm⁻¹... -2 Under the operating current density conditions, the battery short-circuited after two cycles. This is because the membrane was too thin and its mechanical properties were too poor to prevent bromine diffusion, and zinc dendrites easily pierced the membrane, causing the battery to short-circuit.

[0085] Comparative Example 2

[0086] FPBI porous membranes were prepared according to the method described in Example 1, except that the thickness of the FPBI porous membrane was 1200 μm (defined as FPBI-D2 porous).

[0087] The properties of the FPBI-D1 porous membrane are shown in Table 1. Its excessive thickness results in extremely high sheet resistivity. When the FPBI-D2 porous membrane was used in a zinc-bromine flow battery, it achieved a resistivity of 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate because the membrane impedance is too high.

[0088] Comparative Example 3

[0089] FPBI porous membranes were prepared according to the method described in Example 1, except that the molar ratio of 4BTAB to imidazole groups was 3. The prepared FPBI porous membranes were very brittle and fragile, and their mechanical properties were unsuitable for use in zinc-bromine flow batteries.

[0090] Comparative Example 4

[0091] FPBI porous membranes were prepared according to the method described in Example 1, except that the molar ratio of 4BTAB to imidazole groups was 0.0001 (defined as FPBI-D4 porous membrane).

[0092] The performance of the FPBI-D4 porous membrane was tested, as shown in Table 1. Because the amount of 4BTAB is too small, only a very small number of imidazole groups of PBI can undergo nucleophilic substitution reactions with it. The functionalization degree of FPBI-D4 is very low, and the ability of functionalization to promote ion transport and inhibit bromine diffusion is limited. Therefore, the sheet resistance and bromine permeability of the FPBI-D4 porous membrane are similar to those of the PBI-1 porous membrane.

[0093] The FPBI-D4 porous membrane was used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under operating current density conditions, because the FPBI-D4 porous membrane cannot effectively suppress bromine diffusion, the battery cannot operate stably in the absence of a complexing agent in the electrolyte. Therefore, the extremely low degree of side chain functionalization cannot significantly alter the properties of the PBI porous membrane.

[0094] Comparative Example 5

[0095] This comparative example prepared porous membranes using the following method:

[0096] (1) Dissolve 1.79 g of 4BTAB in 19 g of DMAc and stir at room temperature to obtain a homogeneous 4BTAB solution.

[0097] (2) The PBI-1 porous membrane prepared in Example 1 was immersed in 4BTAB solution and reacted at 50 °C for 24 h.

[0098] (3) Take out the membrane in step (2), rinse off the remaining 4BTAB with water, and place it in water for later use (defined as FPBI-D5 porous membrane).

[0099] The performance of the FPBI-D5 porous membrane was tested, as shown in Table 1. Because PBI is in a cured state, its imidazole groups cannot react with 4BTAB, resulting in the performance of the FPBI-D5 porous membrane being extremely similar to that of the PBI-1 porous membrane. Furthermore, the zinc-bromine flow battery assembled with the FPBI-D5 porous membrane also failed to operate stably under conditions where no complexing agent was present in the electrolyte.

[0100] Comparative Example 6

[0101] This comparative example prepared porous membranes using the following method:

[0102] (1) Dissolve 1.79 g of 4BTAB and 1 g of PFSA (as a binder) in 19 g of DMAc and stir at room temperature to obtain a uniform 4BTAB / PFSA solution.

[0103] (2) The prepared 4BTAB / PFSA solution was uniformly coated on one side surface of the PBI-1 porous membrane substrate prepared in Example 1 by scraping. The substrate was then transferred to a hot plate at 50°C and heated for 4 h. After cooling at room temperature, a 4BTAB / PFSA / PBI-1 composite membrane with a dense 4BTAB / PFSA coating (defined as FPBI-D6 porous membrane) was obtained. The thickness of the dense 4BTAB / PFSA coating was 10 μm.

[0104] The performance of the FPBI-D6 porous membrane was tested, and the results are shown in Table 1. Because the 4BTAB / PFSA coating is dense, the FPBI-D6 porous membrane exhibits extremely high sheet resistance. When the FPBI-D6 porous membrane was used in a zinc-bromine flow battery, it achieved a resistance of 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate because the membrane impedance is too high.

[0105] Comparative Example 7

[0106] FPBI porous membranes were prepared according to the method described in Example 1, except that the electrolyte composition was 2 mol / L zinc bromide + 3 mol / L potassium chloride + 0.8 mol / L N,N-methylethylpyrrolidine bromide (bromine complexing agent).

[0107] The FPBI-1 porous membrane was tested under conditions where the electrolyte contained a bromine complexing agent. The presence of the bromine complexing agent reduced the activity of the bromine-containing electrolyte in the redox reaction, thereby increasing battery polarization. Under He Rong charging conditions, the coulombic efficiency, voltage efficiency, capacity retention, and cycle performance of the battery all decreased. Therefore, at 40 mA cm⁻¹... -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled in this comparative example is 94.63%, and the voltage efficiency is 66.08%, which is lower than the coulombic efficiency and voltage efficiency of the zinc-bromine flow battery assembled in Example 1 under the same conditions (Table 2). After the self-discharge test, the capacity retention rate of the zinc-bromine flow battery assembled in this comparative example is only 50%, which is lower than the capacity retention rate of the zinc-bromine flow battery assembled in Example 1 under the same conditions (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled in this comparative example exhibits a lower capacity retention rate at 40 mA cm⁻¹. -2 Under the operating current density conditions, it can run continuously and stably for more than 50 cycles, but its cycle performance is inferior to that of the zinc-bromine flow battery assembled in Example 1 under the same conditions.

[0108] Table 1. Comparison of properties of unfunctionalized and functionalized PBI porous membranes

[0109] membrane 4BTAB to imidazole group molar ratio Film thickness (μm) <![CDATA[Sheet resistance (Ω cm 2 )]]> <![CDATA[Bromine transmittance (mol L -1 h -1 )]]> Remark PBI-1 --- 100 2.85 0.000842 Example 1 FPBI-1 1 100 1.32 0.000080 Example 1 FPBI-2 1 700 1.52 0.000053 Example 2 FPBI-3 1.3 100 1.21 0.000046 Example 3 FPBI-4 1 100 1.34 0.000081 Example 4 FPBI-5 1 100 1.33 0.000082 Example 5 FPBI-6 1 100 1.41 0.000076 Example 6 FPBI-D1 1 20 1.05 0.000132 Comparative Example 1 FPBI-D2 1 1200 9.87 0.0000025 Comparative Example 2 FPBI-D4 0.0001 100 2.86 0.000847 Comparative Example 4 FPBI-D5 0 100 2.84 0.000899 Comparative Example 5 FPBI-D6 --- --- 10.63 0.0000012 Comparative Example 6

[0110] Table 2. Performance comparison of zinc-bromine flow batteries assembled with unfunctionalized and functionalized PBI porous membranes.

[0111]

Claims

1. The application of side-chain functionalized polybenzimidazole porous membranes in bromine-based flow batteries, characterized by: The membrane has a porous structure with a porosity of 20%-90% and a pore size distribution range of 0.001-500 nm; the raw material for the side-chain functionalized polybenzimidazole porous membrane is all or part of the side-chain functionalized polybenzimidazole. Side-chain functionalized polybenzimidazole has a side chain containing a quaternary ammonium bromide group and is prepared by a nucleophilic substitution reaction of polybenzimidazole and a brominated ammonium bromide compound. The brominated ammonium bromide compounds are (5-bromopentyl)-trimethylammonium bromide, (3-bromopropyl)-trimethylammonium bromide, (4-bromobutyl)-trimethylammonium bromide, (2-bromoethyl)-trimethylammonium bromide, (4-bromobutyl)-tributylammonium bromide, (3-bromopropyl)-tripropylammonium bromide, (2-bromoethyl)-triethylammonium bromide, (12-bromododecyl)-trimethylammonium bromide, (10-bromodecyl)-trimethylammonium bromide, (16-bromohexadecyl)-trimethylammonium bromide, (14-bromotetradecyl)-trimethylammonium bromide, and (4-bromobutyl)-methylammonium bromide. One or more of the following: di-n-butylammonium bromide, (10-bromodecyl)-decyldimethylammonium bromide, (8-bromooctyl)-octyldimethylammonium bromide, (12-bromododecyl)-dodecyldimethylammonium bromide, (14-bromotetradecyl)-tetradecyldimethylammonium bromide, (12-bromododecyl)-dimethylbenzylammonium bromide, (18-bromooctadecyl)-trimethylammonium bromide, (2-bromoethyl)-benzyldiethylammonium bromide, (4-bromobutyl)-benzyldibutylammonium bromide, (6-bromoheptyl)-triheptylammonium bromide, and (8-bromooctyl)-trioctylammonium bromide; The preparation process includes the following steps: (1) Dissolve polybenzimidazole in a solvent to obtain a homogeneous polybenzimidazole solution with a mass concentration between 1% and 50%; (2) Dissolve the ammonium bromide compound in the above-mentioned polybenzimidazole solution, wherein the molar ratio of the ammonium bromide compound to the imidazole group in the polybenzimidazole is between 0.1 and 2; (3) Stir the solution from step (2) at a reaction temperature of 20-200℃ for 10 min-240 h to obtain a side-chain functionalized polybenzimidazole solution; (4) Prepare a porous membrane from the solution prepared in step (3); The thickness of the porous polybenzimidazole membrane is between 50 and 1000 μm; the bromine-based flow battery uses an electrolyte system without bromine complexing agents.

2. The application according to claim 1, characterized in that: Polybenzimidazole is one or more of the following three types, and its structure is as follows: Where n is between 20 and 10000.

3. The application according to claim 1, characterized in that: The solvent is at least one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide. The molar ratio (R) of the ammonium bromide compound to the imidazole group in the polybenzimidazole is calculated as follows: Where m1 and m2 are the masses of the ammonium bromide compound and polybenzimidazole in the solution, respectively, in g; and M1 and M2 are the molecular weights of the ammonium bromide compound and the benzimidazole monomer, respectively, in g / mol. The molar ratio of the ammonium bromide compound to the imidazole group in polybenzimidazole determines the degree of functionalization of polybenzimidazole; the higher the molar ratio, the higher the degree of functionalization.

4. The application according to claim 1, characterized in that: The process of preparing side-chain functionalized polybenzimidazole porous membrane by immersion phase transformation method is as follows: the solution prepared in step (3) is uniformly coated on a glass plate, and then immersed in a non-solvent at room temperature to solidify into a membrane with a thickness between 50-1000 μm; the non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol and acetonitrile. Alternatively, the process of preparing side-chain functionalized polybenzimidazole porous membranes by vapor phase transition method is as follows: the solution prepared in step (3) is uniformly coated on a glass plate, and then placed in a non-solvent vapor at 40-80 ℃ and 50%-100% humidity for 10 min-2 h for phase transition curing to form a membrane with a thickness between 50-1000 μm; the non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol, and acetonitrile. Alternatively, the process of preparing a side-chain functionalized polybenzimidazole porous membrane using the solvent template method is as follows: In step (2), silica particles are added simultaneously to obtain a uniform solution. The prepared solution is uniformly coated on a glass plate and then immersed in a non-solvent at room temperature to solidify into a membrane. Then, the membrane is immersed in a 1-6 M sodium hydroxide solution to etch the silica, and the thickness of the porous polybenzimidazole membrane is between 50-1000 μm. The non-solvent is at least one or more of water, methanol, ethanol, propanol, butanol, and acetonitrile.

5. The application as described in claim 1, characterized in that, The side-chain functionalized polybenzimidazole porous membrane is used as a separator in bromine-based flow batteries, including but not limited to zinc / bromine flow batteries, hydrogen / bromine flow batteries, lithium / bromine flow batteries, quinone / bromine flow batteries, magnesium / bromine flow batteries, sodium polysulfide / bromine flow batteries, or vanadium / bromine flow batteries.

6. The application as described in claim 1, characterized in that, The quaternary ammonium bromide groups of the side chains of the polybenzimidazole porous membrane with side chains complex with diffused bromine, thereby fixing the bromine on the membrane surface or in the membrane pores. This prevents the migration of bromine from the positive electrode to the negative electrode, effectively mitigating the problem of battery self-discharge caused by bromine diffusion during battery operation. This allows the bromine-based flow battery to use an electrolyte system without bromine complexing agents. Consequently, the impact of adding complexing agents in the electrolyte on the battery electrode reaction kinetics is weakened, and the battery cost is reduced.

7. The application as described in claim 1 or 6, characterized in that, The bromine-free electrolyte system contains only active substances and supporting electrolytes.

Images