A graphene-based composite ion barrier film and a method of preparing the same

Abstract

This invention discloses a graphene-based composite ion-barrier membrane and its preparation method. The preparation method includes: mixing an aqueous dispersion of graphene oxide with an aqueous dispersion of amino / sulfonic acid-based bifunctional graphene quantum dots, adjusting the pH to 4.0-5.5 at 40-60°C, and sonicating to obtain a composite colloid; uniformly coating the composite colloid onto the surface of a pretreated porous polymer-based membrane, drying, hot-pressing, cross-linking in an aldehyde solution, and finally protonating, washing, and drying. This invention uses a porous polymer as the base membrane and constructs a nanocomposite functional layer on its surface, composed of graphene oxide sheets and amino / sulfonic acid-based bifunctional graphene quantum dots self-assembled. This barrier exhibits extremely low vanadium ion permeability, excellent proton conductivity, and long cycle life, significantly improving the coulombic efficiency, energy efficiency, and cycle stability of vanadium redox flow batteries.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage materials, specifically to a graphene-based composite ion-barrier membrane and its preparation method. Background Technology

[0002] With the global energy structure transitioning towards cleaner and lower-carbon energy, the installed capacity of renewable energy sources such as wind power and photovoltaics is rapidly increasing. However, their intermittency and volatility pose a severe challenge to the stable operation of the power grid, necessitating the development of efficient, safe, and economical large-scale energy storage technologies. Flow batteries, especially vanadium redox flow batteries (VRFB), are considered one of the most promising large-scale energy storage technologies due to their outstanding advantages such as power-capacity decoupling, long cycle life, high safety, and environmental friendliness. In VRFB systems, the ion-conducting membrane is one of the core components determining battery performance, lifespan, and cost. Its core function is to isolate the positive and negative electrolytes and prevent cross-contamination of active materials while allowing protons (H+) that support charge balance to pass through. + High efficiency is crucial. An ideal ion-conducting membrane should simultaneously possess high proton conductivity, low vanadium ion permeability, excellent chemical and mechanical stability, and reasonable cost. Currently, the ion-conducting membrane widely used in commercial VRFBs is the perfluorosulfonic acid membrane, such as the Nafion series membranes produced by DuPont. While these membranes exhibit excellent proton conductivity and good chemical stability, they also have the following significant drawbacks: the production process of perfluorosulfonic acid membranes is complex, and the raw materials are expensive, resulting in membrane costs accounting for 30% to 40% of the total cost of the fuel cell stack, severely restricting the large-scale application of VRFBs; the size of the hydrophilic channels within the membrane is typically 2 to 4 nm, much larger than hydrated vanadium ions (such as VO₂). 2+ The membrane's size (approximately 0.8 nm) makes it unable to effectively block vanadium ion migration, leading to high battery self-discharge rate, decreased coulombic efficiency, and capacity decay. Furthermore, long-term operation in high-temperature environments (40°C) will accelerate the degradation of the membrane's mechanical and barrier properties.

[0003] To overcome the shortcomings of perfluorosulfonic acid membranes, academia and industry have successively developed various alternatives, including non-fluorinated polymer membranes (such as sulfonated polyether ether ketone SPEEK and sulfonated polysulfone SPSF), porous membranes (such as Daramic), and organic-inorganic composite membranes. Among these, composite membranes based on two-dimensional nanomaterials (such as graphene, molybdenum disulfide, and MXene) have attracted widespread attention due to their tunable interlayer structure and unique mass transfer properties. Graphene oxide (GO), as an important derivative of graphene, is rich in oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface, giving it good hydrophilicity and dispersibility. GO sheets can self-assemble into membranes in water through π-π stacking and hydrogen bonding. The nanochannels (0.3-0.8 nm) formed between the sheets can theoretically achieve ion sieving: allowing small hydrated protons (0.2 nm) to pass through rapidly while blocking larger hydrated vanadium ions. However, pure GO membranes face the following key bottlenecks in practical applications: GO membranes swell significantly in aqueous or acidic environments, increasing interlayer spacing and leading to a sharp decline in ion sieving capacity; this is especially true in strongly acidic (H2SO4) and strongly oxidizing (V2SO4) environments like VRFB. 5+ In certain environments, the oxygen-containing functional groups of GO are easily reduced or degraded, leading to membrane structure damage and performance degradation. Furthermore, the negatively charged oxygen-containing groups on GO sheets exert electrostatic repulsion on proton transport, and the ordered water channels between layers are prone to collapse under low humidity. In recent years, graphene quantum dots (GQDs), as an emerging zero-dimensional carbon nanomaterial, have been attempted to be introduced into ion-conducting membranes. GQDs are small (typically <10 nm), have a large specific surface area, are easily functionalized, and possess good dispersibility. Studies show that GQDs can be dispersed as fillers in polymer matrices or inserted into the GO interlayer as intercalating agents to adjust the membrane's free volume and hydrophilicity. However, current technologies mostly use GQDs as simple physical additives, and their surface chemical properties and interactions with GO sheets are not finely controlled, resulting in limited improvement in the ion selectivity of the composite membrane. Under long-term operation, GQDs are prone to aggregation or dissolution, leading to significant stability issues. In addition, most existing studies focus on the ion conduction and barrier properties of membranes in the short term. There is a lack of systematic understanding of the structural evolution, failure mechanism and lifetime prediction of membranes in real VRFB long-term cycling, variable operating conditions and impurity presence, which restricts their progress from laboratory to engineering application.

[0004] Therefore, developing a novel composite ion-conducting membrane that can synergistically regulate ion sieving size and intramembrane electrostatic environment, while possessing excellent structural stability and long lifespan, is a key breakthrough for reducing costs and increasing efficiency in VRFB technology and realizing large-scale commercial applications. Summary of the Invention

[0005] To address the shortcomings of existing vanadium redox flow battery ion conduction membranes in terms of ion selectivity, proton conductivity, and long-term stability, the present invention aims to provide a graphene-based composite ion barrier membrane and its preparation method. This barrier membrane is based on a composite of graphene oxide and amino / sulfonic acid-based bifunctional graphene quantum dots. By constructing a microstructure with gradient sieve pore size and a controllable electrostatic environment, it achieves efficient vanadium ion blocking and rapid proton conduction, while also possessing excellent chemical and mechanical stability, significantly improving the overall performance and cycle life of vanadium redox flow batteries.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a graphene-based composite ion-barrier film includes the following steps: The aqueous dispersion of graphene oxide and the aqueous dispersion of amino / sulfonic acid bifunctional graphene quantum dots were mixed at a mass ratio of (5:1) to (15:1), and the pH value was adjusted to 4.0 to 5.5 at 40 to 60°C. Then, the mixture was sonicated to obtain a composite colloid. The composite colloid is uniformly coated on the surface of the pretreated porous polymer base film, dried, and a functional layer is formed. The composite colloid and the functional layer form a composite film. After hot pressing, the composite membrane is cross-linked in an aldehyde solution, and finally protonated, washed, and dried to obtain a graphene-based composite ion-barrier membrane.

[0007] Furthermore, the graphene oxide is in the form of sheets with a lateral dimension of 10~50μm, a thickness of 1~3nm, and a carbon-oxygen atomic ratio of 1.5~2.5.

[0008] Furthermore, the amino / sulfonic acid bifunctional graphene quantum dots have a particle size of 3-8 nm, an amino content of 5-15 at.%, and a sulfonic acid content of 3-10 at.%. The ultrasound was performed at 40-50℃, with an ultrasound power of 100-300W and an ultrasound duration of 2-4 hours.

[0009] Furthermore, the preparation process of the amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion is as follows: citric acid and L-cysteine ​​are added to water, and then a hydrothermal reaction is carried out at 180~220℃ for 8~12h to obtain the amino / sulfonic acid bifunctional graphene quantum dot crude dispersion. The crude dispersion of amino / sulfonic acid bifunctional graphene quantum dots was filtered, dialyzed, concentrated under reduced pressure or freeze-dried, and then added to water to obtain amino / sulfonic acid bifunctional graphene quantum dots.

[0010] Furthermore, the mass ratio of citric acid to L-cysteine ​​is 3:1 to 5:1.

[0011] Furthermore, the concentration of the graphene oxide aqueous dispersion is 2~8 mg / mL, the concentration of the amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion is 1~4 mg / mL, and the graphene oxide aqueous dispersion and the amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion are in a mass ratio of (5:1)~(15:1).

[0012] Furthermore, the porous polymer base membrane is polyethersulfone or sulfonated polyetheretherketone, with an average pore size of 20~100nm and a porosity of >50%; the pretreatment process of the porous polymer base membrane is: treatment with 100~200W oxygen plasma for 3~5min.

[0013] Furthermore, the thickness of the functional layer is 1.0~3.0μm. The hot pressing temperature is 90~110℃, the pressure is 5~10MPa, and the time is 15~20min.

[0014] Furthermore, the aldehyde is one or more of glutaraldehyde, adipaldehyde, terephthalaldehyde, or glyoxal; The aldehyde solution had a mass concentration of 25%, and the crosslinking treatment was carried out at a temperature of 40-50℃ for 4-6 hours. The protonation conditions are: soaking in 0.5~2.0 mol / L H2SO4 solution for 24~48 h.

[0015] A graphene-based composite ion-barrier membrane.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: In the graphene-based composite ion-barrier membrane of this invention, the graphene oxide (GO) sheets and amino / sulfonic acid-based bifunctional graphene quantum dots (N,S-GQDs) are not simply physically mixed, but rather form a highly ordered "sandwich" or "core" structure through electrostatic interactions, hydrogen bonds, and possible covalent bonding. N,S-GQDs are partially embedded between the GO sheets and partially anchored to the surface or edges of the GO sheets. This structure creates gradient-varying nanochannels; the channels are narrower (0.3-0.5 nm) near the core of the GO sheets, mainly relying on size sieving; near the N,S-GQDs-modified regions, the channel size is slightly larger, but the surface charge density is high, mainly relying on electrostatic interactions. The preparation cost of the main raw materials GO and N,S-GQDs is much lower than that of perfluorosulfonic acid resin, and the preparation process is relatively simple, which is expected to significantly reduce the membrane material cost of VRFB. This preparation method can be combined with existing membrane processing technologies (such as casting and coating), and has the potential for large-scale production.

[0017] Furthermore, this invention, through precise control of the GO sheet spacing and intercalation of amino / sulfonic acid-based bifunctional graphene quantum dots (N,S-GQDs), forms a gradient pore size distribution from sub-nanometer to several nanometers, enabling efficient physical barrier of vanadium ions with varying degrees of hydration, thus achieving gradient size sieving. The positively charged amino groups (-NH3) on the surface... + In acidic environments, it effectively blocks VOCs through electrostatic repulsion. 2+ Isocations; simultaneously, negatively charged sulfonic acid groups (-SO3) - ) is a proton (H3O) + The GO membrane provides abundant hopping sites, forming continuous proton conduction pathways. The ordered spatial distribution of positive and negative charge sites constitutes a dynamic "electrostatic gating" effect. N,S-GQDs act as nano-"rivets," crosslinking GO sheets through multi-point interactions (such as the formation of amide bonds between carboxyl groups on GO and amino groups on GQDs), significantly suppressing the swelling of the GO membrane in the electrolyte and enhancing the structural integrity and long-term operational stability of the entire functional layer.

[0018] This invention utilizes a dual mechanism of "gradient size sieving" and "electrostatic synergistic gating," resulting in a composite membrane with a vanadium ion permeability reduction of over 60% compared to Nafion 212 membranes, while maintaining a long-term stable coulombic efficiency of over 98.5%. Abundant sulfonic acid groups provide low-barrier proton hopping sites, and ordered nanochannels reduce proton transport resistance, achieving a proton conductivity of 0.08~0.10 S / cm, comparable to or even superior to Nafion membranes. The covalent and non-covalent cross-linked network effectively suppresses membrane swelling and structural relaxation. After 1000 hours of accelerated aging at 60°C in a 1.6 mol / L VOSO4 / 3 mol / L H2SO4 electrolyte, the membrane's performance degradation rate is <5%. Detailed Implementation

[0019] To make the technical problems solved by the present invention, the technical solutions, and the beneficial effects clearer, the following specific embodiments provide a further detailed description of the present invention. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of the invention.

[0020] The graphene-based composite ion-barrier membrane of the present invention has a multilayer composite structure, including a carrier layer and a functional layer. The carrier layer is a porous polymer-based membrane with channels, used to provide mechanical support; the functional layer is a nanocomposite barrier layer formed by the self-assembly of graphene oxide sheets and amino / sulfonic acid-based bifunctional graphene quantum dots, and is coated or composited onto the surface of the carrier layer or penetrates the channels of the carrier layer.

[0021] Furthermore, the porous polymer-based membrane is made of polyethersulfone (PES) or sulfonated polyether ether ketone (SPEEK), with an average pore size of 20~100nm and a porosity of >50%. Its function is to provide stable porous support for the functional layer and allow electrolyte to pass through.

[0022] Furthermore, the graphene oxide (GO) sheets preferably have a lateral dimension of 10~50μm, a thickness of 1~3nm, and a carbon-oxygen atom ratio of 1.5~2.5, and these sheets serve as the main framework for constructing nanochannels.

[0023] Furthermore, the amino / sulfonic acid bifunctional graphene quantum dots (N,S-GQDs) have a particle size of 3-8 nm, and amino (-NH2) and sulfonic acid (-SO3H) groups are simultaneously introduced onto the surface of graphene oxide via a chemical method. The amino content is 5-15 at.%, and the sulfonic acid content is 3-10 at.%, with these quantum dots serving as functional intercalating agents and charge modifiers.

[0024] The present invention discloses a method for preparing a graphene-based composite ion-barrier film, comprising the following steps: S1, Raw material preparation and dispersion; Graphene oxide (GO) was prepared using a modified Hummers method, and a GO aqueous dispersion with a controllable C / O ratio (concentration 2–8 mg / mL) was obtained by centrifugation and dialysis. The GO had a lateral dimension of 10–50 μm, a thickness of 1–3 nm, and a C / O ratio of 1.5–2.5. Specifically, GO was prepared by the following process: 5 g of graphite was slowly added to 120 mL of concentrated H₂SO₄ (98% by mass) under ice-water bath conditions, and stirred for 10 min. Then, 2.5 g of NaNO₃ and 15 g of KMnO₄ were added sequentially, and the reaction temperature was controlled at 0–10 °C. After reacting for 1–2 h, the mixture was transferred to a 35 °C water bath and stirred continuously for 2 h to obtain a dark green pre-oxidized slurry.

[0025] The above-mentioned dark green pre-oxidized slurry was slowly poured into 250 mL of deionized water, heated to 98±2℃, and stirred for 1 h. Then, 400 mL of deionized water was added for dilution, and a 30% H2O2 solution was slowly added dropwise until the system turned bright yellow to terminate the reaction and remove residual oxidant.

[0026] The reaction product was repeatedly centrifuged and washed with deionized water (8000 rpm, 15 min each time) until the pH of the supernatant was close to neutral (pH 7). The precipitate was then dispersed in deionized water and ultrasonically exfoliated at 300 W for 2 h to obtain a brown GO dispersion.

[0027] The above-mentioned brownish-red GO dispersion was placed in a dialysis bag with a molecular weight cutoff of 8000-14000 Da and dialyzed in flowing deionized water for 72 hours to completely remove residual metal ions and small molecule acids. After dialysis, the dispersion was moderately concentrated by rotary evaporation to obtain a homogeneous GO aqueous dispersion with a carbon-to-oxygen atomic ratio (C / O) of 1.5-2.5 and a concentration of 2-8 mg / mL.

[0028] Preparation of N,S-GQDs: N,S-GQDs were synthesized via a hydrothermal method using citric acid and L-cysteine ​​as precursors. After dialysis purification, an aqueous dispersion of N,S-GQDs was prepared. Specifically, the preparation process was as follows: Citric acid and L-cysteine ​​were weighed at a mass ratio of 3:1 to 5:1 and dissolved together in an appropriate amount of deionized water. The mixture was magnetically stirred at room temperature for at least 30 minutes to form a clear and transparent solution. The solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE), sealed, and placed in an oven. The reaction system was heated to 100°C at a rate of 3°C / min, starting from room temperature (25°C) and holding for 0.5 hours. From 100°C, the temperature was increased to 150°C at a rate of 2°C / min and held for 1 hour. Finally, the temperature was increased to 180-220°C at a rate of 1°C / min and maintained within this temperature range for 8-12 hours to complete carbonization and surface functionalization. After the reaction is complete, allow the reactor to cool naturally to room temperature. The resulting reaction solution is a brownish-yellow to brownish-red transparent liquid, i.e., a crude dispersion of N,S-GQDs.

[0029] The crude dispersion of N,S-GQDs was first filtered through a 0.22 μm microporous membrane to remove any trace carbonaceous particles. The filtrate was then placed in a dialysis bag with a molecular weight cutoff of 1000 Da for purification, and dialyzed in flowing deionized water for 48–72 h to completely remove unreacted small molecule precursors and salts.

[0030] The purified dialysis bag fluid was collected, concentrated under reduced pressure or freeze-dried at 40-60℃, and then redispersed to finally prepare an N,S-GQDs aqueous dispersion with a concentration of 1-4 mg / mL. The N,S-GQDs had a particle size of 3-8 nm, an amino content of 5-15 at.%, and a sulfonic acid content of 3-10 at.%.

[0031] S2, controllable self-assembly of composite colloids; Aqueous dispersions of GO and N,S-GQDs were mixed at a mass ratio of (5:1) to (15:1). The pH of the mixture was adjusted to 4.0–5.5 by adding dilute hydrochloric acid (0.1 mol / L) dropwise at 40–60 °C. Under these conditions, the carboxyl groups at the edges of the GO sheets were negatively charged due to ionization, while the protonated amino groups on the surface of N,S-GQDs were positively charged, resulting in a strong electrostatic attraction between the two. Subsequently, the mixture was ultrasonically treated at 40–50 °C and 100–300 W for 2–4 hours, promoting molecular-level self-assembly of GO and N,S-GQDs through electrostatic interactions, hydrogen bonding, and π-π stacking, forming a uniform and stable composite colloid.

[0032] S3, Base film pretreatment; The selected porous polymer substrate membrane was ultrasonically cleaned sequentially with ethanol and deionized water to remove surface contaminants. Then, it was treated with oxygen plasma at a power of 100-200W for 3-5 minutes to obtain a pretreated porous polymer substrate membrane. Oxygen plasma treatment introduces polar groups such as hydroxyl and carboxyl groups onto the substrate membrane surface, significantly improving its surface energy and water resistance, ensuring the uniformity and adhesion of subsequent functional layer coatings.

[0033] S4, the construction and solidification of functional layers; The composite colloid obtained in step S2 is uniformly coated onto the surface of the porous polymer-based membrane pretreated in step S3 using a spin coating method. By adjusting the coating process parameters (spin rotation speed and colloid concentration), the thickness of the single-layer wet film can be controlled. After multiple spin coatings, the wet film is placed at 60~80℃ for vacuum drying for 4~8 hours to form a functional layer with a thickness of 1.0~3.0μm, thereby forming a composite film between the composite colloid and the functional layer.

[0034] S5, crosslinking and post-treatment; To enhance the bonding strength within the functional layer and between the functional layer and the base membrane, and to prevent peeling during use, cross-linking treatment is required. First, the composite membrane is placed in a hot press and maintained at 90–110°C and 5–10 MPa for 15–20 minutes. Second, the hot-pressed membrane is placed in a 25% (w / w) aldehyde solution (glutaraldehyde, adipaldehyde, terephthalaldehyde, or glyoxal) and cross-linked at 40–50°C for 4–6 hours. This allows the amino groups on the GO and GQDs to undergo a Schiff base reaction with the aldehyde groups, forming a covalent cross-linked network, significantly improving the membrane's dimensional stability and solvent resistance. Third, the cross-linked membrane is immersed in a 0.5–2.0 mol / L H₂SO₄ solution for 24–48 hours to fully protonate the sulfonic acid groups within the membrane (-SO₃H), ensuring the membrane reaches swelling equilibrium under VRFB operating conditions. Finally, the membrane surface was washed with a large amount of deionized water until the pH was neutral (pH 7), and then air-dried at 20~30℃ to obtain a graphene-based composite ion-barrier membrane, which was then stored in a nitrogen atmosphere.

[0035] The following are specific examples.

[0036] Example 1 This embodiment provides a method for preparing a graphene-based composite ion-barrier film, including the following steps: Graphite oxide was prepared using a modified Hummers method, and GO with C / O = 2.0 was obtained by centrifugation and dialysis, and then prepared into an aqueous dispersion of GO at 5 mg / mL. Citric acid and L-cysteine ​​were weighed at a mass ratio of 4:1 and dissolved together in an appropriate amount of deionized water. The mixture was magnetically stirred at room temperature for at least 30 minutes to form a clear and transparent solution. The solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE), sealed, and placed in an oven. The reaction system was heated to 100°C at a rate of 3°C / min, starting from room temperature (25°C) and holding for 0.5 hours. From 100°C, the temperature was increased to 150°C at a rate of 2°C / min and held for 1 hour. Finally, the temperature was increased to 200°C at a rate of 1°C / min and maintained within this temperature range for 10 hours to complete carbonization and surface functionalization.

[0037] After the reaction is complete, allow the reactor to cool naturally to room temperature. The resulting reaction solution is a brownish-yellow to brownish-red transparent liquid, i.e., a crude dispersion of N,S-GQDs.

[0038] The coarse dispersion was first filtered through a 0.22 μm microporous membrane to remove any trace carbonaceous particles. The filtrate was then placed in a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed in flowing deionized water for 60 hours to thoroughly remove unreacted small molecule precursors and salts.

[0039] The purified dialysis bag fluid was collected, concentrated under reduced pressure or freeze-dried at 40-60℃, and then redispersed to finally prepare an N,S-GQDs aqueous dispersion with a concentration of 2.5 mg / mL. The N,S-GQDs had a particle size of 3-8 nm, an amino content of 10 at.%, and a sulfonic acid content of 6 at.%.

[0040] The GO aqueous dispersion and the N,S-GQDs aqueous dispersion were mixed at a mass ratio of 10:1, the pH was adjusted to 4.8 with 0.1 mol / L hydrochloric acid, and the mixture was treated with ultrasound at 40℃ and 200W for 2 hours to obtain a uniform brown composite colloid.

[0041] A PES base film with an average pore size of 50 nm was selected, and it was ultrasonically cleaned with ethanol and deionized water in sequence, and then treated with oxygen plasma (150 W, 3 min) to obtain the treated PES base film.

[0042] 15 mL of composite colloid was coated onto the treated PES substrate membrane (5 cm in diameter) using a spin coating method, resulting in a wet film thickness of approximately 200 μm. The membrane was then dried in a 60 °C oven for 12 h to form a 3 μm thick functional layer, thus obtaining the composite membrane.

[0043] The composite membrane was then hot-pressed at 100℃ and 8MPa for 15 min. It was then crosslinked in glutaraldehyde solution (50℃) for 4 h. Finally, it was immersed in 1.0 mol / L H2SO4 for 36 h and washed with deionized water to obtain a graphene-based composite ion-barrier membrane, denoted as composite membrane M1.

[0044] Example 2 The mass ratio of GO to N,S-GQDs was adjusted to 8:1, the chemical crosslinking step was cancelled, and only hot-press crosslinking was performed. The remaining steps were the same as in Example 1, and the composite membrane M2 was obtained.

[0045] Example 3 SPEEK (65% sulfonation) was used as the porous base membrane, and the remaining steps were the same as in Example 1 to obtain composite membrane M3.

[0046] Example 4 In the composite colloid preparation step, the pH value was adjusted to 5.5, and the remaining steps were the same as in Example 1, to obtain composite membrane M4.

[0047] Example 5 The composite colloid was spin-coated onto the pretreated PES base film (first layer 1000 rpm, 30 s; second layer 2000 rpm, 30 s), repeated three times. The drying, crosslinking, and protonation steps were the same as in Example 1, resulting in composite film M5 with a total functional layer thickness of approximately 3 μm.

[0048] Example 6 This embodiment provides a method for preparing a graphene-based composite ion-barrier film, including the following steps: Graphite oxide was prepared using a modified Hummers method, and GO with C / O = 1.5 was obtained by centrifugation and dialysis, and then prepared into an 8 mg / mL GO aqueous dispersion. Citric acid and L-cysteine ​​were weighed at a mass ratio of 4:1 and dissolved together in an appropriate amount of deionized water. The mixture was magnetically stirred at room temperature for at least 30 minutes to form a clear and transparent solution. The solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE), sealed, and placed in an oven. The reaction system was heated to 100°C at a rate of 3°C / min, starting from room temperature (25°C) and holding for 0.5 hours. From 100°C, the temperature was increased to 150°C at a rate of 2°C / min and held for 1 hour. Finally, the temperature was increased to 200°C at a rate of 1°C / min and maintained within this temperature range for 10 hours to complete carbonization and surface functionalization.

[0049] After the reaction is complete, allow the reactor to cool naturally to room temperature. The resulting reaction solution is a brownish-yellow to brownish-red transparent liquid, i.e., a crude dispersion of N,S-GQDs.

[0050] The coarse dispersion was first filtered through a 0.22 μm microporous membrane to remove any trace carbonaceous particles. The filtrate was then placed in a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed in flowing deionized water for 60 hours to thoroughly remove unreacted small molecule precursors and salts.

[0051] The purified dialysis bag fluid was collected, concentrated under reduced pressure or freeze-dried at 40-60℃, and then redispersed to finally prepare an N,S-GQDs aqueous dispersion with a concentration of 1 mg / mL. The N,S-GQDs had a particle size of 3-8 nm, an amino content of 5 at.%, and a sulfonic acid content of 7 at.%.

[0052] The GO aqueous dispersion and the N,S-GQDs aqueous dispersion were mixed at a mass ratio of 5:1, the pH was adjusted to 4.5 with 0.1 mol / L hydrochloric acid, and the mixture was treated with ultrasound at 40℃ and 150W for 3 h to obtain a uniform brown composite colloid.

[0053] A SPEEK base film with an average pore size of 50 nm was selected, and it was ultrasonically cleaned with ethanol and deionized water in sequence, and then treated with oxygen plasma (100 W, 5 min) to obtain the treated SPEEK base film.

[0054] 15 mL of composite colloid was coated onto the treated SPEEK base film (5 cm in diameter) using a spin coating method, and then dried in an oven at 60 °C for 12 h to form a functional layer with a thickness of 3 μm. Thus, the composite colloid and the functional layer form a composite film.

[0055] The composite membrane was then hot-pressed at 90℃ and 7MPa for 20 min. It was then placed in a hexamethylene aldehyde solution and crosslinked at 50℃ for 4 h. Finally, it was immersed in 1.0 mol / L H2SO4 for 30 h, washed with deionized water, and the graphene-based composite ion-barrier membrane was obtained.

[0056] Example 7 This embodiment provides a method for preparing a graphene-based composite ion-barrier film, including the following steps: Graphite oxide was prepared using a modified Hummers method, and GO with C / O = 2.5 was obtained by centrifugation and dialysis, and then prepared into an aqueous dispersion of GO at 2 mg / mL. Citric acid and L-cysteine ​​were weighed at a mass ratio of 5:1 and dissolved together in an appropriate amount of deionized water. The mixture was magnetically stirred at room temperature for at least 30 minutes to form a clear and transparent solution. The solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE), sealed, and placed in an oven. The reaction system was heated to 100°C at a rate of 3°C / min, starting from room temperature (25°C) and holding for 0.5 hours. From 100°C, the temperature was increased to 150°C at a rate of 2°C / min and held for 1 hour. Finally, the temperature was increased to 180°C at a rate of 1°C / min and maintained within this temperature range for 12 hours to complete carbonization and surface functionalization.

[0057] After the reaction is complete, allow the reactor to cool naturally to room temperature. The resulting reaction solution is a brownish-yellow to brownish-red transparent liquid, i.e., a crude dispersion of N,S-GQDs.

[0058] The coarse dispersion was first filtered through a 0.22 μm microporous membrane to remove any trace carbonaceous particles. The filtrate was then placed in a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed in flowing deionized water for 48–72 h to thoroughly remove unreacted small molecule precursors and salts.

[0059] The purified dialysis bag fluid was collected, concentrated under reduced pressure or freeze-dried at 40-60℃, and then redispersed to finally prepare an N,S-GQDs aqueous dispersion with a concentration of 4 mg / mL. The N,S-GQDs had a particle size of 3-8 nm, an amino content of 10 at.%, and a sulfonic acid content of 3 at.%.

[0060] Take N,S-GQDs with an amino content of 10 at.% and a sulfonic acid content of 6 at.% and prepare a 2.5 mg / mL N,S-GQDs aqueous dispersion.

[0061] The GO aqueous dispersion and the N,S-GQDs aqueous dispersion were mixed at a mass ratio of 12:1, the pH was adjusted to 4 with 0.1 mol / L hydrochloric acid, and the mixture was treated with ultrasound at 45℃ and 300W for 2 hours to obtain a uniform brown composite colloid.

[0062] A PES base film with an average pore size of 50 nm was selected, and it was ultrasonically cleaned with ethanol and deionized water in sequence, and then treated with oxygen plasma (200 W, 3 min) to obtain the treated PES base film.

[0063] 15 mL of composite colloid was coated onto the treated PES base film (5 cm in diameter) using a spin coating method, and then dried in an oven at 60 °C for 12 h to form a functional layer with a thickness of 2 μm. Thus, the composite colloid and the functional layer form a composite film.

[0064] The composite membrane was then hot-pressed at 110℃ and 5MPa for 15 min. It was then placed in a terephthalaldehyde solution and crosslinked at 40℃ for 6 h. Finally, it was immersed in 1.0 mol / L H2SO4 for 48 h and washed with deionized water to obtain a graphene-based composite ion-barrier membrane.

[0065] Example 8 This embodiment provides a method for preparing a graphene-based composite ion-barrier film, including the following steps: Graphite oxide was prepared using a modified Hummers method, and GO with C / O = 2.3 was obtained by centrifugation and dialysis, and then prepared into an aqueous dispersion of GO at 4 mg / mL. Citric acid and L-cysteine ​​were weighed at a mass ratio of 3:1 and dissolved together in an appropriate amount of deionized water. The mixture was magnetically stirred at room temperature for at least 30 minutes to form a clear and transparent solution. The solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE), sealed, and placed in an oven. The reaction system was heated to 100°C at a rate of 3°C / min, starting from room temperature (25°C) and holding for 0.5 hours. From 100°C, the temperature was increased to 150°C at a rate of 2°C / min and held for 1 hour. Finally, the temperature was increased to 220°C at a rate of 1°C / min and maintained within this temperature range for 8 hours to complete carbonization and surface functionalization.

[0066] After the reaction is complete, allow the reactor to cool naturally to room temperature. The resulting reaction solution is a brownish-yellow to brownish-red transparent liquid, i.e., a crude dispersion of N,S-GQDs.

[0067] The coarse dispersion was first filtered through a 0.22 μm microporous membrane to remove any trace carbonaceous particles. The filtrate was then placed in a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed in flowing deionized water for 72 hours to thoroughly remove unreacted small molecule precursors and salts.

[0068] The purified dialysis bag fluid was collected, concentrated under reduced pressure or freeze-dried at 40-60℃, and then redispersed to finally prepare an N,S-GQDs aqueous dispersion with a concentration of 4 mg / mL. The N,S-GQDs had a particle size of 3-8 nm, an amino content of 15 at.%, and a sulfonic acid content of 10 at.%.

[0069] The GO aqueous dispersion and the N,S-GQDs aqueous dispersion were mixed at a mass ratio of 15:1, the pH was adjusted to 5.5 with 0.1 mol / L hydrochloric acid, and the mixture was treated with ultrasound at 50℃ and 100W for 4 h to obtain a uniform brown composite colloid.

[0070] A PES base film with an average pore size of 50 nm was selected, and it was ultrasonically cleaned with ethanol and deionized water in sequence, and then treated with oxygen plasma (160 W, 4 min) to obtain the treated PES base film.

[0071] 15 mL of composite colloid was coated onto the treated PES base film (5 cm in diameter) using a spin coating method, and then dried in an oven at 60 °C for 12 h to form a functional layer with a thickness of 1 μm. Thus, the composite colloid and the functional layer form a composite film.

[0072] The composite membrane was then hot-pressed at 100℃ and 10MPa for 17 min. It was then placed in a 1:1 mass ratio solution of glyoxal and glutaraldehyde and crosslinked at 45℃ for 5 h. Finally, it was immersed in 1.0 mol / L H2SO4 for 24 h and washed with deionized water to obtain a graphene-based composite ion-barrier membrane.

[0073] Comparative Example 1 Without adding N,S-GQDs, a pure GO composite membrane D1 was obtained by forming a film on a PES-based membrane using only GO dispersion and hot pressing.

[0074] Comparative Example 2 Unfunctionalized ordinary graphene quantum dots (without N or S) were used instead of N,S-GQDs and compounded with GO, with the rest being the same as in Example 1, to obtain composite film D2.

[0075] Comparative Example 3 Composite membrane D3 was obtained by combining graphene quantum dots (without sulfonic acid groups) with GO using a single amino functionalized graphene quantum dot, with the rest being the same as in Example 1.

[0076] Comparative Example 4 The GO and N,S-GQDs dispersions were mixed by simple mechanical stirring (without pH adjustment and ultrasonic self-assembly), and then a composite membrane D4 was obtained.

[0077] Comparative Example 5 We used the commercially available Nafion 212 membrane directly as a comparison.

[0078] Key performance testing: All membranes from Examples 1-5 and Comparative Examples 1-5 were cut into circular samples with an effective area of ​​5 cm², assembled into VRFB single cells, and tested. The test method is as follows: the electrolyte is 1.6 MV. + 3.5 / 4.5 in 3 mol / L H₂SO₄, tested current density 80 mA / cm⁻¹ -2 The temperature is controlled at 25±1℃.

[0079] 1) Surface resistance: Tested using the four-probe method, surface resistance <0.5 Ω·cm 2 It is acceptable.

[0080] 2) Vanadium permeability: vanadium ion flux was calculated using the two-chamber diffusion cell method (unit: mol·cm⁻¹). -2 ·h -1 ).

[0081] 3) Cycle capacity retention: Constant current charge-discharge cycle test of vanadium redox flow battery (current density 80 mA / cm²) 2 ).

[0082] 4) Coulombic efficiency: Tested using constant current charge and discharge mode of vanadium redox flow battery.

[0083] 5) Energy efficiency: Tested using constant current charge and discharge mode of vanadium redox flow battery.

[0084] 6) Proton conductivity: tested using the four-probe method.

[0085] The performance test results are shown in Table 1.

[0086] Table 1 Key performance test results for each membrane sample

[0087] As can be seen from the data in Table 1, the composite membrane prepared in the embodiments of the present invention has the best overall performance, and the sheet resistivity of the composite membrane is all below 0.5 Ω. cm 2 The composite membranes prepared in this invention meet the requirements for VRFB (Vibration-Return Battery) applications. Their proton conductivity is comparable to that of commercial Nafion 212 membranes, while their vanadium permeability is only 1 / 5 to 1 / 7 that of Nafion 212 membranes, significantly reducing the battery's self-discharge rate and improving its long-term cycle stability. Compared to the comparative examples, the composite membranes (M1-M5) prepared in this invention exhibit the best overall performance, significantly outperforming both the comparative examples and commercial Nafion 212 membranes in key indicators such as vanadium permeability, coulombic efficiency, energy efficiency, and cycle capacity retention. Example 3 (SPEEK-based membrane) demonstrates the best performance, achieving a coulombic efficiency of 99.4%, an energy efficiency of 88.8%, and a capacity retention of 98.7% after 500 cycles. This is attributed to the synergistic effect of the SPEEK-based membrane's proton conductivity and the functional layers. Comparative Example M1 and Comparative Examples D1-D4 show that the bifunctional modification of N,S-GQDs and the controllable self-assembly process are key to improving membrane performance: the lack of N,S-GQDs (D1), the use of unfunctionalized or single-functionalized GQDs (D2, D3), and the absence of self-assembly (D4) all lead to increased vanadium permeability, decreased proton conductivity, and worsened cycle stability of the membrane, demonstrating the importance of the dual mechanism of "gradient size sieving" and "electrostatic synergistic gating".

[0088] The barrier membrane of this invention uses a porous polymer (PES or SPEEK) as the base membrane, on which a nanocomposite functional layer is constructed by the self-assembly of graphene oxide (GO) sheets and amino / sulfonic acid bifunctional graphene quantum dots (N,S-GQDs). Size sieving (i.e., controllable self-assembly of the composite colloid) is achieved through precisely controlled gradient nanochannels, ion-selective gating is achieved using the synergistic electrostatic interaction of amino and sulfonic acid groups, and the structural stability of the membrane is enhanced by covalent and non-covalent cross-linked networks. This composite membrane exhibits extremely low vanadium ion permeability (more than 60% lower than Nafion 212 membranes), excellent proton conductivity (0.08~0.10 S / cm), and long cycle life (capacity retention >96% after 500 cycles), significantly improving the coulombic efficiency, energy efficiency, and cycle stability of vanadium redox flow batteries. The preparation process of this invention is controllable and low-cost, compatible with existing membrane processing technologies, and has the potential for large-scale production, which is of great significance for promoting the commercial application of vanadium redox flow batteries.

[0089] The above description is only of the preferred embodiment of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of the present invention are also within the scope of protection of the present invention.

[0090] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

Claims

1. A method for preparing a graphene-based composite ion-barrier film, characterized in that, Includes the following steps: The aqueous dispersion of graphene oxide and the aqueous dispersion of amino / sulfonic acid bifunctional graphene quantum dots were mixed at a mass ratio of (5:1) to (15:1), and the pH value was adjusted to 4.0 to 5.5 at 40 to 60°C. Then, the mixture was sonicated to obtain a composite colloid. The composite colloid is uniformly coated on the surface of the pretreated porous polymer base film, dried, and a functional layer is formed. The composite colloid and the functional layer form a composite film. After hot pressing, the composite membrane is cross-linked in an aldehyde solution, and finally protonated, washed, and dried to obtain a graphene-based composite ion-barrier membrane.

2. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, Graphene oxide is in the form of sheets with a lateral dimension of 10~50μm, a thickness of 1~3nm, and a carbon-to-oxygen atomic ratio of 1.5~2.

5.

3. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The amino / sulfonic acid bifunctional graphene quantum dots have a particle size of 3-8 nm, an amino content of 5-15 at.%, and a sulfonic acid content of 3-10 at.%. The ultrasound was performed at 40-50℃, with an ultrasound power of 100-300W and an ultrasound duration of 2-4 hours.

4. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The preparation process of amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion is as follows: citric acid and L-cysteine ​​are added to water, and then a hydrothermal reaction is carried out at 180~220℃ for 8~12h to obtain amino / sulfonic acid bifunctional graphene quantum dot crude dispersion. The crude dispersion of amino / sulfonic acid bifunctional graphene quantum dots was filtered, dialyzed, concentrated under reduced pressure or freeze-dried, and then added to water to obtain amino / sulfonic acid bifunctional graphene quantum dots.

5. The method for preparing a graphene-based composite ion-barrier membrane according to claim 4, characterized in that, The mass ratio of citric acid to L-cysteine ​​is 3:1 to 5:

1.

6. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The concentration of the graphene oxide aqueous dispersion is 2~8 mg / mL, the concentration of the amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion is 1~4 mg / mL, and the graphene oxide aqueous dispersion and the amino / sulfonic acid bifunctional graphene quantum dot aqueous dispersion are in a mass ratio of (5:1)~(15:1).

7. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The porous polymer base membrane is made of polyethersulfone or sulfonated polyetheretherketone, with an average pore size of 20~100nm and a porosity of >50%. The pretreatment process of the porous polymer base membrane is as follows: it is treated with oxygen plasma of 100~200W for 3~5min.

8. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The thickness of the functional layer is 1.0~3.0μm. The hot pressing temperature is 90~110℃, the pressure is 5~10MPa, and the time is 15~20min.

9. The method for preparing the graphene-based composite ion-barrier membrane according to claim 1, characterized in that, The aldehyde is one or more of glutaraldehyde, adipaldehyde, terephthalaldehyde, or glyoxal; The aldehyde solution had a mass concentration of 25%, and the crosslinking treatment was carried out at a temperature of 40-50℃ for 4-6 hours. The protonation conditions are: soaking in 0.5~2.0 mol / L H2SO4 solution for 24~48 h.

10. A graphene-based composite ion-barrier membrane prepared by the method according to any one of claims 1-9.

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