A polybenzimidazole microporous ion-conducting membrane and a one-step electro-spraying method for preparing the same

Abstract

This invention proposes a polybenzimidazole microporous ion-conducting membrane and its one-step electrospray preparation method. Polybenzimidazole electrospray solutions are prepared using organic solvents with different boiling points. Electrospray parameters are adjusted under a high-voltage electrostatic field, and the polybenzimidazole electrospray solution is dispersed into nanodroplets using this field. A receiving roller rotates to spread and layer the droplets, followed by rapid drying and solidification, directly preparing the polybenzimidazole microporous membrane. This invention can directly prepare microporous membranes using non-ionic polymer materials, and the membrane preparation process is simple and easily scaled up. It overcomes the problem of high mass transfer resistance of non-ionic polymers, enabling highly selective proton conduction. Simultaneously, the high stability of non-ionic polymers improves the oxidative stability and mechanical strength of the ion-conducting membrane, showing broad application prospects in the field of new energy storage flow batteries.

Description

Technical Field

[0001] This invention belongs to the field of new energy storage flow batteries, specifically relating to a polybenzimidazole microporous ion-conducting membrane and its one-step electrospray preparation method. This method is simple to operate, has adjustable porosity, and is easy to scale up. Background Technology

[0002] Currently, human society faces the dual challenges of energy crisis and environmental pollution, making the development of conversion and storage technologies for renewable energy sources such as solar and wind power an urgent priority. Flow batteries, with their high design flexibility, represent an advanced large-scale energy storage technology. As a key material in flow batteries, the ion-conducting membrane needs to rapidly conduct protons and separate active ions in the anode and cathode electrolytes, significantly impacting the efficiency and cycle life of flow batteries, thus becoming a key research focus.

[0003] The structure-activity relationship between the microstructure of ion-conducting membranes and their ion-conducting mechanisms is significant. Dense membranes, exemplified by commercially available Nafion membranes, utilize ion-conducting groups, such as sulfonic acid groups, to self-assemble into hydrophilic microphases with a diameter of approximately 3-5 nm. This allows protons to be transported via both carrier mechanisms (diffusion as hydrated hydrogen ions) and hopping mechanisms (rapid transport along hydrogen bond networks). However, these hydrophilic channels also facilitate the permeation of hydrated vanadium ions (approximately 0.6-0.8 nm in diameter), and functional groups like sulfonic acid groups can easily induce degradation of the ion-conducting membrane under the strong oxidizing environment of the battery, leading to decreased ion selectivity and battery capacity decay. Unlike dense membranes, microporous membranes can conduct protons independently of ion-conducting groups, conducting protons through their internal pores. When the pore size is between the hydrated diameter of a proton (approximately 0.24 nm) and the hydrated diameter of a vanadium ion, they also exhibit a highly efficient size sieving effect. For example, the nonionic polymer material polybenzimidazole (PBI) has attracted considerable attention due to its excellent chemical inertness, mechanical strength, and thermal stability.

[0004] Precise control of pore morphology is key to improving the efficiency of microporous membranes. The pore-forming processes for proton exchange membranes in energy storage flow batteries mainly include template methods and phase inversion methods. The template method is the most classic strategy. Its principle is to mix removable small molecules, inorganic salts, or polymers into the casting solution as pore-forming templates. After film formation, the template is removed, leaving interconnected pores in situ. (See literature...) J. Membr. Sci. 2020, 611Patent 118359 prepared porous polybenzimidazole (PBI) membranes for use in vanadium redox flow batteries using monodisperse SiO2 solid spheres and 3 M NaOH as templates and etching solutions, respectively. Patent ZL202111419128.8 proposed a method for preparing PBI-based ion exchange membranes containing hydrophilic ion-sieving micropores. Using sulfate-based ion clusters in a dense membrane precursor as templates, the sulfate groups are hydrolyzed to convert them into small-sized hydroxyl groups, generating a large number of 4-8 Å sieve pores in situ, achieving high selectivity (100 mA / cm²). 2 It achieves a balance between high energy efficiency (85.8%), low capacity decay (only 0.22% / cycle), and high stability. The phase inversion method utilizes mass transfer exchange in a non-solvent environment to initiate liquid-liquid phase separation, thereby solidifying into a porous structure, as shown in the literature. ChemSusChem 2025, 18 e202401576 prepared porous polybenzimidazole membranes using the phase inversion method, with current densities ranging from 100 to 400 mA / cm². 2 It exhibits superior battery efficiency compared to Nafion 212 within the range. However, common methods for preparing porous PBI membranes suffer from complex multi-step processes, difficulty in controlling pore structure, or reduced oxidation stability due to the distribution of hydrophilic groups such as sulfonic acid conductive groups, hydroxyl groups, and amine groups within the pore structure, which limits the large-scale application of PBI in the flow battery field. Summary of the Invention

[0005] This invention proposes a one-step electrospray method for preparing polybenzimidazole microporous ion-conducting membranes. A high-voltage electrostatic field is used to disperse a PBI electrospray solution into nanodroplets. A receiving roller rotates, causing the droplets to spread, overlap, and rapidly dry and solidify, directly forming a polybenzimidazole ion-conducting membrane with a microporous structure. This invention can directly prepare microporous membranes using nonionic polymer materials, simplifying the membrane fabrication process and overcoming the high mass transfer resistance problem of nonionic polymers. It also leverages the high stability of nonionic polymers, enabling high energy efficiency and cycle stability when applied to vanadium redox flow batteries.

[0006] The technical solution of the present invention is as follows: This invention provides a one-step electrospray method for preparing polybenzimidazole microporous ion-conducting membranes. A polybenzimidazole (PBI) electrospray solution is prepared using organic solvents with different boiling points. Electrospray parameters are adjusted under a high-voltage electrostatic field, and the PBI electrospray solution is sprayed onto a receiving roller. The rotating receiving roller causes the droplets to spread, overlap, and rapidly dry and solidify before peeling off, directly preparing a polybenzimidazole ion-conducting membrane with a microporous structure. The pore size of the micropores in the polybenzimidazole ion-conducting membrane prepared with organic solvents of different boiling points varies; the lower the boiling point of the organic solvent, the larger the pore size.

[0007] Furthermore, the PBI electro-spraying solution refers to PBI dissolved in organic solvents with different boiling points, wherein the mass fraction of PBI is 1%-10%, and the organic solvent is N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), or N,N-dimethylacetamide (DMAC).

[0008] Furthermore, the aforementioned electro-injection parameters refer to a voltage range of 4-10 kV and a solution propulsion rate of 0.6-1.4 mL / h. -1 The spinneret's flat sweeping range is 50-150 mm, the distance from the spinneret to the receiving roller is 2-10 cm, the receiving roller's rotation speed is 100-800 rpm, the ambient temperature is 15-45 ℃, and the ambient relative humidity is 20-50%.

[0009] This invention also provides a polybenzimidazole microporous ion-conducting membrane prepared using the above method. The polybenzimidazole microporous ion-conducting membrane refers to a polybenzimidazole microporous ion-conducting membrane with a microporous structure, a pore size ranging from micrometers to nanometers, and a porosity of 19-22%. The thickness of the polybenzimidazole microporous ion-conducting membrane prepared by this method is 1-40 μm, and a high flow battery efficiency can be obtained when the thickness of the polybenzimidazole microporous ion-conducting membrane is 30-40 μm.

[0010] The beneficial effects of this invention are as follows: The one-step electrospinning method for preparing polybenzimidazole microporous ion-conducting membranes provided by this invention can directly utilize non-ionic polymer materials to prepare microporous membranes, resulting in a simple membrane fabrication process. Compared with commonly used electrospinning methods that can only prepare porous fiber network membranes, this invention can prepare microporous membranes in one step. By selecting organic solvents with different boiling points and matching the droplet drying speed and droplet superposition frequency on the receiving roller, the pore size and porosity of the microporous membrane can be effectively controlled. This invention overcomes the problem of high mass transfer resistance of non-ionic polymers, enabling highly selective proton conduction. Simultaneously, by utilizing the high stability of non-ionic polymers, the oxidative stability and mechanical strength of the ion-conducting membrane are improved, showing broad application prospects in the field of new energy storage flow batteries. Attached Figure Description

[0011] Figure 1The images show the morphology of PBI films prepared under different process conditions. (a1)-(a2) represent PBI-Cast; (b1)-(b2) represent PBI-nmp; (c1)-(c2) represent PBI-dmso; and (d1)-(d2) represent PBI-dmac. In the optical images, the cast film (PBI-Cast) is a dense, uniform, and transparent film. Among the one-step electrosprayed microporous films prepared with different solvents, the PBI-nmp and PBI-dmso films prepared with high-boiling-point solvents are also relatively transparent, indicating smaller pore sizes. In contrast, the PBI-dmac film prepared with the relatively low-boiling-point solvent DMAC has lower transparency, indicating larger pore sizes. In the scanning electron microscope images, the stacked pore structure of the PBI-dmac film is clearly visible, with pore sizes reaching the micrometer scale. The pore sizes of the PBI-nmp and PBI-dmso films are smaller, ranging from hundreds of nanometers to nanometers.

[0012] Figure 2 The figures represent the porosity of different membranes. It can be seen that the porosity of the microporous membrane prepared by this invention is much higher than that of the dense cast membrane.

[0013] Figure 3 The performance of vanadium redox flow batteries with PBI films prepared under different process conditions is shown, where (a) is coulombic efficiency; (b) is voltage efficiency; (c) is energy efficiency; and (d) is the long-cycle energy efficiency of PBI-NMP. It can be seen that the performance is good in the range of 60-200 mA / cm². -2 The coulombic efficiencies of the microporous membranes PBI-nmp and PBI-dmso are >99.5%, while the coulombic efficiency of the microporous membrane PBI-dmac is relatively low (18.5%-79.2%), indicating that changing the electrospray solvent can effectively adjust the pore size and achieve highly selective conduction of hydrogen and vanadium ions. The microporous membranes all exhibit high voltage efficiencies, indicating that their high porosity allows for the formation of interconnected proton conduction channels through water and acid absorption. In contrast, the cast membrane, due to its dense structure and lack of sulfonic acid functional groups, has extremely low voltage efficiencies and cannot be used for charge-discharge operations. The microporous membrane PBI-nmp demonstrates the highest battery energy efficiency and cycle stability at 200 mA cm⁻¹. -2 The energy efficiency is 78.1%, and the battery can operate stably for nearly 2000 cycles without replacing the electrolyte, with an energy efficiency decay rate of only 0.0029% / cycle. Detailed Implementation

[0014] The experimental scheme of the present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0015] Example 1 Weigh 0.5 g of PBI powder and dissolve it in 49.5 g of NMP to prepare a PBI electro-injection solution. The electro-injection parameters were set as follows: voltage 4 kV, solution injection rate 0.6 mL / h.-1 The spinneret sweep range was 150 mm, the distance from the spinneret to the receiving roller was 10 cm, the receiving roller speed was 800 rpm, the ambient temperature was 45 ℃, and the relative humidity was 20%. After spraying, the high-voltage power supply was turned off, and the film was allowed to dry and cure fully before being peeled off to obtain the PBI-nmp film. The film thickness was approximately 1 μm when the electrospraying time was 1.5 h, and approximately 30 μm when the electrospraying time was 45 h, with a porosity of 19.6%. The PBI-nmp film with a thickness of 30 μm had a much higher water absorption rate at room temperature (20.1%) than the PBI cast dense film (2.5%), while the swelling rate (6.5% vs 5.2%) was comparable. The porous membrane structure can fully absorb water, significantly reducing the sheet resistivity to 0.17 ohm cm⁻¹. 2 It was assembled into an all-vanadium redox flow battery, operating at 60-200 mAcm. -2 At 200 mA / cm², the coulombic efficiency is greater than 99.5%, indicating that the PBI-nmp membrane can effectively block vanadium ion permeation; the voltage efficiency is 92.7%-77.7%, indicating that the micropores in the membrane provide relatively interconnected proton conduction channels; and the energy efficiency is 92.1%-76.9%. -2 Under these conditions, the battery operated stably for nearly 2000 cycles.

[0016] Example 2 Weigh 1g of PBI powder and dissolve it in 19g of DMSO to prepare a PBI electro-spraying solution. The electro-spraying parameters were set as follows: voltage 7 kV, solution injection rate 1.0 mL / h. -1 The spinneret sweep range was 100 mm, the distance from the spinneret to the receiving roller was 6 cm, the receiving roller speed was 450 rpm, the ambient temperature was 30 ℃, and the relative humidity was 35%. After spraying, the high-voltage power supply was turned off, and the film was allowed to dry and cure fully before being peeled off to obtain the PBI-dmso film. The porosity of the above one-step electrosprayed film (PBI-dmso) was 20.1%, and the film thickness was 40 μm. The water absorption rate at room temperature (20.8%) was much higher than that of the PBI cast dense film (2.5%). The porous membrane structure can fully absorb water, significantly reducing the sheet resistivity to 0.23 ohm cm⁻¹. 2 It was assembled into an all-vanadium redox flow battery, operating at 60-200 mA / cm². -2 The coulombic efficiency is greater than 99.5%, indicating that the PBI-dmso membrane can effectively block vanadium ion permeation; the voltage efficiency is 86.1%-56.1%, and the energy efficiency is 85.2%-56.1%.

[0017] Example 3 Weigh 1g of PBI powder and dissolve it in 9g of DMAC to prepare a PBI electro-injection solution. The electro-injection parameters were set as follows: voltage 10 kV, solution injection rate 1.4 mL / h. -1The spinneret sweep range was 50 mm, the distance from the spinneret to the receiving roller was 2 cm, the receiving roller speed was 100 rpm, the ambient temperature was 15 ℃, and the relative humidity was 50%. After spraying, the high-voltage power supply was turned off, and the film was allowed to dry and cure fully before being peeled off to obtain the PBI-dmac film. The porosity of the above one-step electrosprayed film (PBI-dmac) was 21.5%, and the film thickness was 40 μm. The water absorption rate at room temperature (22.6%) was much higher than that of the PBI cast dense film (2.5%). The porous membrane structure can fully absorb water, significantly reducing the sheet resistivity to 0.19 ohm cm⁻¹. 2 It was assembled into an all-vanadium redox flow battery, operating at 60-200 mA / cm². -2 At this level, the coulombic efficiency is 18.5%-79.2%, the voltage efficiency is 89.2%-55.4%, and the energy efficiency is 48.8%-16.5%. Compared with cast films, which cannot be charged and discharged due to their dense structure and lack of sulfonic acid functional groups, microporous PBI-dmac films can effectively improve battery efficiency.

Claims

1. A method for one-step electrospray preparation of polybenzimidazole microporous ion-conducting membranes, characterized in that: Polybenzimidazole (PBI) electrospray solutions were prepared using organic solvents with different boiling points. The electrospray parameters were adjusted under a high-voltage electrostatic field, and the PBI electrospray solution was sprayed onto a receiving roller. The rotation of the receiving roller caused the droplets to spread, overlap, and rapidly dry and solidify before being peeled off, thus directly preparing a polybenzimidazole ion-conducting membrane with a microporous structure. The pore sizes of the micropores in the polybenzimidazole ion-conducting membranes prepared with organic solvents of different boiling points were different. The lower the boiling point of the organic solvent, the larger the pore size.

2. The method for one-step electrospray preparation of polybenzimidazole microporous ion-conducting membranes as described in claim 1, characterized in that: The PBI electro-spraying solution refers to PBI dissolved in organic solvents with different boiling points, wherein the mass fraction of PBI is 1%-10%, and the organic solvent is N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, or N,N-dimethylacetamide.

3. The method for one-step electrospray preparation of polybenzimidazole microporous ion-conducting membranes as described in claim 1, characterized in that: The aforementioned electro-injection parameters refer to a voltage range of 4-10 kV and a solution injection rate of 0.6-1.4 mL / h. -1 The spinneret's flat sweeping range is 50-150 mm, the distance from the spinneret to the receiving roller is 2-10 cm, the receiving roller's rotation speed is 100-800 rpm, the ambient temperature is 15-45 ℃, and the ambient relative humidity is 20-50%.

4. The polybenzimidazole microporous ion-conducting membrane prepared by the method according to claims 1-3.

5. The polybenzimidazole microporous ion-conducting membrane as described in claim 4, characterized in that: The aforementioned polybenzimidazole microporous ion-conducting membrane refers to a polybenzimidazole microporous ion-conducting membrane with a microporous structure, a pore size ranging from micrometers to nanometers, and a porosity of 19-22%.

6. The polybenzimidazole microporous ion-conducting membrane as described in claim 5, characterized in that: The thickness of the polybenzimidazole microporous ion-conducting membrane is 1-40 μm, and a high flow battery efficiency can be obtained when the thickness of the polybenzimidazole microporous ion-conducting membrane is 30-40 μm.

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