An ultrathin b-oriented zsm-5 zeolite molecular sieve membrane, a preparation method and application in vanadium redox flow battery

Abstract

The present application relates to a kind of ultra-thin b orientation ZSM-5 zeolite molecular sieve membrane, preparation method and application in vanadium oxidation liquid flow battery, belong to liquid flow battery technical field. Low defect density, thin and [0k0] plane out orientation ZSM-5 zeolite separation membrane made of zeolite nanosheet realizes enhanced ion selectivity and ion conductivity, so as to improve battery performance. Therefore, VFB equipped with ZSM-5 zeolite membrane shows excellent vanadium resistance, self-discharge time reaches 116.2 hours, far more than Nafion 212 (45.9 hours). In addition, under 80mA cm ‑2 Current density, the performance of VFB remains stable in 1000 cycles.

Description

Technical Field

[0001] This invention relates to an ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane, its preparation method, and its application in vanadium oxide flow batteries, belonging to the field of flow battery technology. Background Technology

[0002] Stationary electrochemical energy storage (EES) has attracted considerable attention due to its ability to balance the unstable power provided by intermittent renewable energy sources. Vanadium oxide flow batteries (VFBs) are among the most promising large-scale energy storage technologies due to their adjustable capacity, environmental friendliness, high safety, and excellent efficiency. The membrane is one of the most critical components in VFBs, responsible for separating the cathode and anode while allowing protons to transport within the membrane, forming a complete circuit. Traditional polymer membranes are widely used as proton conduction membranes in VFBs due to their excellent stability and high proton conductivity. However, their poor ion selectivity caused by severe swelling significantly limits their further application in VFBs.

[0003] In recent years, porous membranes with molecular sieving capabilities have shown great application potential in VFBs (volatile organic compounds). In particular, molecular sieve materials, such as zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), possess narrower pore size distributions and more ordered interconnected channels compared to polymer porous membranes, promising high ion conductivity and precise separation. However, their instability under harsh conditions such as strong acids and strong oxidation limits their use. MFI-type zeolites, a type of aluminosilicate or silicate material with a three-dimensional porous structure, have been shown to exhibit exceptional acid and oxidation resistance. They possess sinusoidal channels of 0.51 × 0.55 nm along the a-axis and straight channels of 0.53 × 0.56 nm along the b-axis, perfectly blocking vanadium ions (>0.6 nm) in the electrolyte. Previous studies have found that ZSM-5 zeolite (partial Si in the MFI zeolite framework)… 4+ By Al 3+ The alternative has better hydrophilicity and proton transport capability and has shown good battery performance in VFB[1]. However, traditional three-dimensional (3D) zeolite membranes have thicker membrane layers and random pore orientation, which limits the rate of proton permeation[2].

[0004] Non-patent literature 1: Z. Xu, I. Michos, ZSCao, WHJing, XHGu, K. Hinkle, S. Murad, JHDong, Proton-Selective Ion Transport in ZSM-5Zeolite Membrane, J Phys ChemC, 120(2016)26386-26392.

[0005] Non-patent literature 2: P.Kumar, DWKim, N.Rangnekar, H.Xu, EOFetisov, S.Ghosh, H.Zhang, Q.Xiao, M.Shete, JISiepmann, T.Dumitrica, B.McCool, M.Tsapatsis, KAMkhoyan, One-dimensional intergrowths in two-dimensional zeolitenanosheets and their effect on ultra-selective transport, Nat Mater,19(2020)443. Summary of the Invention

[0006] The technical problem this invention aims to solve is the low proton permeability and poor battery performance of membranes with molecular sieving properties used in vanadium oxide flow batteries. This invention employs an ultrathin b-oriented ZSM-5 zeolite separation membrane with low defect density, made of zeolite nanosheets, to achieve enhanced ion selectivity and ion conductivity, thereby improving battery performance.

[0007] An ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane includes a support layer and a selective separation layer. The selective separation layer is composed of ZSM-5 nanosheets, which are all [0k0] out-of-plane oriented. The pore size of the ZSM-5 nanosheets is 0.51-0.56 nm.

[0008] The separation layer is selected to have a thickness of 100-500 nm.

[0009] The support layer is a porous ceramic material.

[0010] The preparation method of the above-mentioned ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane includes the following steps:

[0011] Step 1: Obtain a suspension dispersion of ZSM-5 nanosheets;

[0012] Step 2: The support layer is brought into contact with the suspension dispersion to create ZSM-5 nanosheets on the surface as seed crystals.

[0013] Step 3: The support layer is placed in the first synthesis solution for hydrothermal synthesis, washed, and then calcined to obtain a molecular sieve membrane.

[0014] In step 1, the ZSM-5 nanosheets are prepared by the following method:

[0015] Step a: Tetraethoxysilane, bis-1,5-(tripropylammonium)pentamethyldiiodide, KOH, and water are mixed in a weight ratio of 80:3-5:10-30:5000-15000 to form a second synthesis solution, which is then hydrolyzed to form a precursor sol.

[0016] Step b: After adding seed crystals to the precursor sol, hydrothermal synthesis is carried out;

[0017] In step c, NaAlO2 solution is added, and the Si / Al molar ratio is adjusted to 20-30. Hydrothermal synthesis continues, and the product is washed and centrifuged to obtain ZSM-5 nanosheets.

[0018] In step b, the hydrothermal synthesis conditions are 370-450K for 2-5 days; in step c, the hydrothermal synthesis conditions are 370-450K for 0.5-1 days.

[0019] In step 1, the concentration of the suspension dispersion is 0.005-0.5 wt%.

[0020] In step 2, the contact time is 10-100 seconds, and drying is performed after contact.

[0021] The preparation method of the first synthetic solution is as follows: first, a hydrolysis reaction and pre-crystallization are carried out according to the weight ratio of silicon source: template agent: water of 20:1-5:10000-15000 to generate a sol; the hydrolysis time is 5-20h; the pre-crystallization condition is 0.5-5h at 350-500K.

[0022] In step 3, the hydrothermal treatment temperature is 350-420K, and the calcination conditions are 650-850K for 5-10 hours.

[0023] The above-mentioned ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane is used in vanadium oxide flow batteries.

[0024] Beneficial effects

[0025] Vanadium redox flow batteries (VFBs) are widely considered one of the most reliable stationary electrochemical energy storage (EES) technologies due to their high modularity, long cycle life, and environmental friendliness. Membranes with high ion selectivity and conductivity are crucial for developing high-performance VFBs.

[0026] Two-dimensional zeolite nanosheets are used to prepare ultrathin zeolite films with a thickness of only a few nanometers, allowing only horizontal stacking. In this work, an ultrathin b-oriented ZSM-5 zeolite film was successfully designed, and the [0k0]-oriented straight-pore channels effectively shorten the proton transport distance. The zeolite layer composed of high aspect ratio nanosheets has a low defect density. The ultrathin zeolite film constructed with zeolite nanosheets achieves high permeability and high ion selectivity. Attached Figure Description

[0027] Figure 1 Design principle of ultrathin b-oriented ZSM-5 zeolite membrane.

[0028] Figure 2 a) SEM image, b) HRTEM image of ZSM-5 zeolite nanosheets, c) pore size distribution of ZSM-5 zeolite nanosheets, d) AFM image and AFM height distribution, e) N2 adsorption isotherms of raw and processed ZSM-5 molecular sieve nanosheets.

[0029] Figure 3 SEM images of the surface and cross-section of α-alumina sheets (a) and (b) sections; SEM images of the surface and cross-section of α-alumina coated with nanosheets (c) and (d) sections; SEM images of the surface and cross-section of zeolite films after hydrothermal treatment for 12 hours (e) and (f) sections; SEM images of the surface and cross-section of zeolite films after hydrothermal treatment for 24 hours (g) and (h) sections; SEM images of the surface and cross-section of zeolite films after hydrothermal treatment for 36 hours (i) and (j) sections; SEM images of the surface and cross-section of zeolite films after hydrothermal treatment for 48 hours (k) and (l) sections; m) XRD patterns of different films.

[0030] Figure 4 a) Permeability of different gases within 2D ZSM-5 zeolite membranes; b) Vanadium permeability of different membranes; c) Variation of area resistivity with sulfuric acid concentration; d) Proton conductivity of ZSM-5 molecular sieve membranes and Nafion 212.

[0031] Figure 5 a) Schematic diagram of the operating mechanism of ZSM-5 zeolite membrane in VFB, ZSM-5 zeolite membrane at 20 mA cm⁻¹ -2 Up to 120mA cm -2 The relationship between b) battery performance and c) charge / discharge curves.

[0032] Figure 6 a) 3D zeolite membrane and 3D zeolite membrane at 80 mA cm -2 A comparison of battery performance, b) 2D and 3D ZSM-5 zeolite films at b) 80mA cm -2 and c)100mA cm -2 The charge-discharge curves are shown in Figure d), and the schematic diagram of the proton transport path in the [0k0] and [h0h] direction channels of the ZSM-5 molecular sieve is shown in Figure d).

[0033] Figure 7 Self-discharge tests of different membranes.

[0034] Figure 8 a) 80mA cm -2 The discharge capacity retention and charge-discharge cycle performance of the ZSM-5 zeolite membrane. Detailed Implementation

[0035] Preparation of ZSM-5 nanosheets

[0036] ZSM-5 nanosheets were obtained by a bottom-up synthesis method. The synthesis method of bis-1,5-(tripropylammonium)pentamethyldiiodide (dC5) and diatomite nanoseed crystals used in the preparation process was based on the technique reported by Jeon et al. [3]. The precursor sol composition of the synthesized MFI nanosheets was 80 tetraethoxysilane (TEOS):3.75dC5:20KOH:9500H2O. The precursor sol was hydrolyzed at room temperature for 16 hours and filtered through a 0.45-micron syringe filter. The filtered precursor sol was then mixed with prepared silicate-1 seeds, with a silica content to gel molar ratio of 1:800 in the seed suspension. The mixture was transferred to a Teflon-lined stainless steel pressure cooker and hydrothermally treated at 413 K for 3.5 days. Then, 1M NaAlO2 solution (Si / Al molar ratio of 25) was added to the reaction mixture, and hydrothermal treatment was continued for 1 day [4]. The synthesized products typically contain two-dimensional ZSM-5 zeolite nanosheets. The prepared ZSM-5 nanosheets are usually treated with an alkaline salt solution (0.1M KOH + 2M KCl), followed by repeated centrifugation and washing with deionized water. Finally, the ZSM-5 nanosheets are dispersed in H₂O at a concentration of 0.02 wt%.

[0037] Non-patent document 3: MYJeon, D.Kim, P.Kumar, PSLee, N.Rangnekar, P.Bai, M.Shete, B.Elyassi, HSLee, K.Narasimharao, SNBasahel, S.Al- Thabaiti,WQXu,HJCho,EOFetisov,R.Thyagarajan,RFDeJaco,W.Fan,KAMkhoyan,JISiepmann,M.Tsapatsis,Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets,Nature,543(2017)690.

[0038] Non-patent literature 4: ZSCao, SXZeng, Z.Xu, A.Arvanitis, SWYang, XHGu, JHDong, Ultrathin ZSM-5zeolite nanosheet laminated membrane for high-fluxdesalination of concentrated brines, Sci Adv, 4 (2018).

[0039] The 3D ZSM-5 membranes used in the following experiments were prepared according to existing techniques (Z.Xu,I.Michos,ZSCao,WHJing,XHGu,K.Hinkle,S.Murad,JHDong,Proton-Selective Ion Transport in ZSM-5Zeolite Membrane,J Phys Chem C,120(2016)26386-26392.), and their performance was compared with that of b-oriented ZSM-5 membranes under the same conditions.

[0040] Preparation of beta-oriented ultrathin zeolite films

[0041] ZSM-5 nanosheets were coated onto the surface of α-alumina sheets using a dip-coating method. Specifically, the ZSM-5 suspension was treated at least three times alternately with an ultrasonic machine and a vortex mixer, each treatment lasting one minute to ensure uniform dispersion. The sides and back of the α-alumina sheets were wrapped with PTFE threaded sealing tape, and then the surface of the α-alumina sheets was horizontally immersed in the suspension for 40 seconds and dried at 333K for 6 hours.

[0042] ZSM-5 nanosheet coatings on α-alumina sheets were hydrothermally grown to close the nanoscale gaps between the nanosheets. Specifically, a sol with a molar ratio of 20 TEOS:3 tetrapropylammonium hydroxide (TPAOH):13500 H2O was hydrolyzed for 12 hours, and the hydrolyzed sol was incubated at 423 K for 2 hours for pre-crystallization. After cooling, α-alumina sheets coated with ZSM-5 nanosheets were immersed in the sol and hydrothermally treated at 383 K for 12-48 hours, with the best quality and higher orientation observed after 36 hours. During the hydrothermal treatment, the α-alumina sheets were horizontally fixed in the middle of the reactor using a PTFE support, with the nanosheet layer facing downwards, to ensure that the crystals formed during synthesis did not leave traces on the zeolite membrane surface. The surface of the initial membrane was repeatedly clarified with a brush and deionized water and dried overnight in a blast furnace at 353 K. To evaluate various properties, the zeolite membrane was calcined at 723 K for 6 hours at a heating rate of 30 K / h. -1 .

[0043] Test methods

[0044] Single gas permeation test

[0045] Single gas permeation tests were conducted using CO2, N2, CH4, and SF6.

[0046] Vanadium ion permeability

[0047] Vanadium ion permeability was tested using an H-type diffusion cell. The membrane was sealed in a PMMA ring with epoxy resin adhesive, and both sides were sealed with silicone gaskets to prevent leakage. The feed side was filled with 50 mL of 1.5 M VOSO4 dissolved in 3.0 M H2SO4. To eliminate the effects of ionic strength and osmotic pressure, the permeate side was filled with 50 mL of 1.5 M MgSO4 dissolved in 3.0 M H2SO4. The diffusion cell was continuously stirred on both sides using a PTFE rotor to eliminate concentration polarization near the membrane. At regular intervals, 3 mL of solution was taken from the permeate side and 3 mL of the original permeate side solution was added. VA ion permeability was measured using a UV-Vis spectrophotometer (Perkin Elmer Lambda 950). 4+ The concentration. Permeability is calculated using the following formula.

[0048]

[0049] Where V B (L) is the solution volume on the osmotic side of the diffusion cell. P (mol m -2 h -1 ) is V 4+ penetration rate, C A (mol L -1 ) is the concentration on the feed side, C B (t)(mol L -1 A(m) represents the concentration on the osmotic side at different times; 2 ) is the effective area of ​​the membrane, and L(m) is the thickness of the membrane.

[0050] Area resistivity test

[0051] The area resistivity of the membranes was tested using a vanadium ion permeability testing apparatus. 10 × 10 mm platinum electrodes were inserted into each side of the diffusion cell. Both sides of the diffusion cell were filled with 50 mL of sulfuric acid solution. Each membrane was immersed in the corresponding concentration of sulfuric acid solution for 24 hours before testing. Area resistivity (AR) was determined using electrochemical impedance spectroscopy (EIS, Solartron 1470E+1260A) at a frequency range of 1 kHz to 1 MHz and calculated using the following formula.

[0052] AR = (R1 - R2) × S

[0053]

[0054] Where S(cm) 2 ) represents the effective area of ​​the membrane. Where σ(Scm) -1 R is the electrical conductivity of the membrane. R1 and R2 are the electronic resistances of the conductive cells with and without the membrane, respectively. L (cm) is the thickness of the membrane.

[0055] Battery performance test

[0056] The electrochemical performance of the zeolite membrane was tested using a homemade single-cell apparatus. The end plates and electrode frame were made of PMMA, and the current collector was a graphite plate without electrolyte channels. Commercially available graphite felt was used for both the positive and negative electrodes. A Neware CT-4008 was used to test charge-discharge performance. The charging cutoff voltage was 1.7V, and the discharging cutoff voltage was 0.8V. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the single cell were defined by the following formulas.

[0057]

[0058]

[0059]

[0060] Characterization of zeolite nanosheets

[0061] This patent describes the preparation of an ultrathin b-type ZSM-5 zeolite film. The method first successfully fabricates ZSM-5 zeolite nanosheets using a bottom-up approach, such as... Figure 2 As shown in Figure a, the nanosheets exhibit a typical rhomboid shape. Figure 2 Image b is a TEM image of the zeolite nanosheets, showing the ordered regularity and rapid ion-selective channels of ZSM-5. The pore size of the prepared zeolite nanosheets is approximately 0.56 nm. Figure 2 (c) It can separate hydrated protons (approximately 0.27 nm) and hydrated vanadium ions (>0.6 nm). During the assembly of the nanosheets, the thickness is 7–9 nm. Figure 2 The zeolite nanosheets (d) are horizontally distributed only on the surface of the support, which is beneficial for preparing β-axis zeolite films. Furthermore, the ultrathin ZSM-5 nanosheets have a high aspect ratio, which is conducive to forming continuous nanosheet layers. The chemical stability of the film material is crucial to the lifetime of the VFB, and is particularly important at 1.5 MV. 2-5+ The stability of ZSM-5 zeolite nanosheets was tested by immersion in +3.0M H2SO4 or 1.5MV H2SO4. 5+ Immersion in +3.0M H2SO4 improved the adsorption-desorption properties of nitrogen, which may be due to the additional sample activation effect. Figure 2(e). The activated pores further confirm the stability of ZSM-5 zeolite nanosheets in harsh environments of strong acid and strong oxidation.

[0062] Characterization of ZSM-5 nanosheets and b-axis ZSM-5 nanosheet films

[0063] A prerequisite for preparing high-quality zeolite membranes is obtaining a continuous and complete seed layer. An α-alumina ceramic sheet with a pore size of approximately 100 nm and a thickness of approximately 1 mm was selected as the support. Figure 3 (a and b). After two consecutive dip-coatings, a continuous zeolite nanosheet layer was obtained, and no exposed alumina was found. Figure 3 c). The sheet-like stacking of ZSM-5 nanosheets can be observed in the SEM images of the cross-section. Figure 3 The α-alumina support with nanosheet coating exhibits a distinct diffraction peak at 2θ≈8.8°, corresponding to the

[020] out-of-plane orientation of ZSM-5. Protons can rapidly migrate through the nearly cylindrical straight-pore channels inside the [0k0] oriented ZSM-5 zeolite film. However, there are many nanoscale gaps between the continuous nanosheet stacks, through which vanadium ions can easily penetrate. The nanosheet layers were post-treated by hydrothermal treatment to close the nanoscale gaps between the zeolite nanosheets. Since the in-plane growth rate of zeolite is much faster than the out-of-plane growth rate, hydrothermal treatment can form highly intergrowthed ultrathin zeolite layers. We investigated zeolite films after hydrothermal treatment for 12 hours, 24 hours, 36 hours, and 48 hours, respectively. After 12 hours of hydrothermal treatment, ZSM-5 nanosheets were deposited on the surface of α-alumina without significant growth ( Figure 3 (e and f). Microscopic morphology confirms that after 24 hours of hydrothermal treatment, the gaps between the nanosheets are closed, and a good symbiotic relationship between the nanosheets is observed. However, flux can be detected in the N2 permeation experiment, which proves that the zeolite membrane is not completely dense. Figure 3 The presence of unclosed gaps in the zeolite layer (g and h) leads to low ion selectivity. Figure 3 i and Figure 3 The image shows representative surface and cross-sectional SEM images of the membrane sample synthesized after 36 hours of hydrothermal treatment. The membrane thickness is ~350 nm, and no N2 permeation was detected. XRD patterns also confirm that the zeolite layer maintains good [0k0] out-of-plane orientation after 36 hours of hydrothermal treatment, consistent with the microstructure. Figure 3 (m). However, after 48 hours of hydrothermal treatment, the nanosheets overgrowth (m). Figure 3 The [0k0] out-of-plane orientation of the zeolite film was not maintained due to the k and l values. This is further evidenced by the enhanced diffraction peaks of

[101] in the XRD pattern. Figure 3 (m). Therefore, 36 hours is the final optimized hydrothermal treatment time.

[0064] Precision sieving of zeolite membranes

[0065] The ideal selectivity of N2 / SF6 has always been an important indicator for evaluating the quality of zeolite membranes. The single-gas permeability of H2, CO2, N2, CH4, and SF6 was tested at 298 K. Except for CO2, the permeability is related to the molecular dynamic diameter, because MFI-type zeolites have a strong adsorption effect on CO2. Figure 4 (a) The single-gas permeability of N2 reaches 7.06 × 10⁻⁶. -7 mol m -2 s -1 Pa -1 The ideal separation coefficient of N2 / SF6 reaches 10¹³.

[0066] The ion selectivity of the ZSM-5 zeolite membrane was evaluated using a vanadium ion permeation experiment. For example... Figure 4 As shown in b, the α-alumina support with an effective pore size of approximately 100 nm cannot block vanadium ions, leading to V on the permeation side. 4+ The concentration increased sharply. After preparing an ultrathin ZSM-5 zeolite layer on the α-alumina surface, the vanadium ion permeability decreased significantly, from the initial 0.75 mol / L. -1 h -1 Decreased to 0.07 mmol / L -1 h -1 This is far lower than the permeability of Nafion 212 (0.13 mmol / L). -1 h -1 This result indicates that the vanadium retention is mainly contributed by the ultrathin zeolite layer, because the ordered 0.56 nm pores within the zeolite layer effectively inhibit the permeation of vanadium ions (>0.6 nm) through a pore size repulsion effect. The ionic conductivity of the ZSM-5 zeolite membrane was measured by electrochemical impedance spectroscopy (EIS). Figure 4 Figure c shows the sheet resistivity of Nafion 212, α-alumina substrate, and ZSM-5 zeolite layer at different sulfuric acid concentrations. It can be seen that the AR of Nafion 212 fluctuates little under different acid concentrations, only ranging from 1.04 × 10⁻⁶. -1 Ωcm -2 It decreased to 5.84 × 10 -2 Ωcm -2 This is because Nafion primarily transfers protons through sulfonic acid groups within the membrane. Conversely, with increasing sulfuric acid concentration, the sheet resistivity of both the α-alumina substrate and the ZSM-5 zeolite layer significantly decreased. When the sulfuric acid concentration increased from 0.5 M to 3.0 M, the resistivity (AR) of α-alumina increased from 9.03 × 10⁻⁶. -1 Ωcm -2 Decreased to 3.5×10 -1 Ωcm -2The AR of the ZSM-5 zeolite layer is 2.79 × 10⁻⁶. -1 Ωcm -2 It decreased to 3.39 × 10 -3 Ωcm -2 This is because the transfer of protons in porous membranes depends on the electrolyte adsorbed in the pores, and the H+ in the electrolyte... + Increased concentration accelerates proton transfer. Furthermore, the ultra-low surface resistivity of the ZSM-5 zeolite layer is also attributed to the self-inhibited out-of-plane growth of the zeolite nanosheets; the ultrathin zeolite layer and near-cylindrical straight-pore channels ensure rapid proton transport. Figure 4 As shown in Figure d, the proton conductivity of the ZSM-5 zeolite membrane is significantly higher than that of Nafion 212. In 3.0 M sulfuric acid solution, the proton conductivity of Nafion 212 is approximately 0.05 S cm⁻¹. -1 The ZSM-5 zeolite membrane has a proton conductivity as high as 0.16 S cm⁻¹. -1 .

[0067] Battery performance test

[0068] Figure 5 Figure 'a' is a schematic diagram of a typical VFB. The battery performance of different membranes was evaluated using a self-made VFB module. In the VFB using an α-alumina sheet, severe interpenetration of the electrolytes on both sides prevented normal charge-discharge cycling. After preparing a ZSM-5 zeolite layer on α-alumina, the VFB equipped with the ZSM-5 zeolite membrane achieved a current density of 40 mA cm⁻¹. -2 It exhibits excellent battery performance (CE = 93.9%, VE = 87.6%, EE = 82.3%). Figure 5 (b) CE gradually increases with increasing current density, reaching a maximum at a current density of 120 mA cm⁻¹. -2 At that time, CE reached 97.6%. As the polarization effect inside the battery becomes more severe with the increase of current density, VE gradually tends to decrease. Figure 5 c shows the ZSM-5 zeolite membrane at different current densities (20-120 mA cm⁻¹). -2 The charge / discharge curves are shown below. As the current density increases, the initial charging voltage gradually increases, while the initial discharging voltage gradually decreases, leading to a decrease in VE. Furthermore, the increase in polarization causes the charge / discharge capacity to decrease with increasing current density.

[0069] Figure 6 A comparison of 3D and 2D ZSM-5 zeolite films at 80 mA / cm² was made. -2The battery performance was compared. High aspect ratio nanosheets tend to form zeolite films with lower defect density; therefore, the CE of submicron-thick zeolite layers is comparable to that of three-dimensional zeolite films a few micrometers thick. However, the VE of two-dimensional zeolite films increased by 23.3%, and ultimately the EE increased by 22.5%. Figure 6 b shows two ZSM-5 zeolite membranes at 80 mA cm⁻¹ -2 The charge-discharge curves show that the initial charging voltage of the two-dimensional zeolite film is 1.30V and the initial discharging voltage is 1.36V, while those of the three-dimensional zeolite film are 1.47V and 1.14V, respectively. The higher discharge plateau and lower charging plateau contribute to the better emission potential (VE) of the two-dimensional zeolite film. This improvement in VE is not solely due to the sharp decrease in film thickness. Figure 6 As shown in d, compared to the sinusoidal channels within the three-dimensional zeolite membrane, the b-oriented zeolite membrane possesses a nearly cylindrical straight-through channel with lower transmission resistance. When the current density increases to 100 mA cm⁻¹... -2 At that time, the severe polarization effect inside the battery equipped with the three-dimensional zeolite film caused a sharp drop in charge and discharge capacity, while the two-dimensional zeolite film with excellent proton conductivity still maintained good performance. This verifies the superiority of the ultrathin b-oriented zeolite film. Figure 6 (c).

[0070] The self-discharge rate is closely related to the ion selectivity of the membrane. Figure 7 The self-discharge rates of α-alumina, Nafion 212, and ZSM-5 zeolite films were shown. In the hydrated state, the ion clusters formed by the sulfonic acid groups in Nafion further expanded, leading to cross-mixing of vanadium ions, and the open-circuit voltage (OCV) of the battery dropped to 0.8V after 45.9 hours. Since the effective pore size of α-alumina is approximately 100 nm, it cannot prevent electrolyte cross-mixing, and the OCV plummeted to 0 after 1.8 hours. However, after fabricating an ultrathin ZSM-5 zeolite layer on α-alumina, the 0.56 nm pore size of the zeolite effectively suppressed electrolyte cross-mixing and vanadium ion permeation, achieving a self-discharge time of 116.2 h, exceeding that of Nafion 212, consistent with the results of vanadium ion permeation tests. The ultra-long self-discharge time verifies the excellent ion selectivity of the ultrathin b-oriented ZSM-5 zeolite film. The lifetime of the ion-conducting membrane is crucial for flow batteries. Long-term cycling tests were conducted at 80 mA cm⁻¹. -2 It was performed at a current density. For example... Figure 8 As shown in Figure a, after 1000 cycles, the battery performance remained good (initial: CE = 97.5%, VE = 77.2%, EE = 75.2%, final: CE = 97.8%, VE = 76.9%, EE = 75.2%), indicating that the ZSM-5 zeolite membrane has good stability. Due to the high ion selectivity of the ZSM-5 zeolite membrane, its discharge capacity decay rate is 0.36% per cycle. Figure 8 (b).

Claims

1. The application of an ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane in a vanadium oxide flow battery, characterized in that, The ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane includes a support layer and a selective separation layer. The selective separation layer is composed of ZSM-5 nanosheets, all of which have an out-of-plane [0k0] orientation. The pore size of the ZSM-5 nanosheets is 0.51-0.56 nm. The separation layer is selected to have a thickness of 100-500 nm; the support layer is a porous ceramic material. The preparation method of ultrathin b-oriented ZSM-5 zeolite molecular sieve membrane includes the following steps: Step 1: Obtain a suspension dispersion of ZSM-5 nanosheets; Step 2: The support layer is brought into contact with the suspension dispersion to create ZSM-5 nanosheets on the surface as seed crystals. Step 3: The support layer is placed in the first synthesis solution for hydrothermal synthesis, washed, and then calcined to obtain a molecular sieve membrane; The preparation method of the first synthetic solution is as follows: first, hydrolysis and pre-crystallization are carried out according to the weight ratio of silicon source: template agent: water of 20:1-5:10000-15000 to generate sol; the hydrolysis time is 5-20 hours. The pre-crystallization conditions are 350-500K for 0.5-5 hours; In step 3, the hydrothermal treatment temperature is 350-420K, and the calcination conditions are 650-850K for 5-10 hours.

2. The application according to claim 1, characterized in that, In step 1, the ZSM-5 nanosheets are prepared by the following method: Step a: Tetraethoxysilane, bis-1,5-(tripropylammonium)pentamethyldiiodide, KOH, and water are mixed in a weight ratio of 80:3-5:10-30:5000-15000 to form a second synthesis solution, which is then hydrolyzed to form a precursor sol. Step b: After adding seed crystals to the precursor sol, hydrothermal synthesis is carried out; In step c, NaAlO2 solution is added, and the Si / Al molar ratio is adjusted to 20-30. Hydrothermal synthesis continues, and the product is washed and centrifuged to obtain ZSM-5 nanosheets.

3. The application according to claim 2, characterized in that, In step b, the hydrothermal synthesis conditions are 370-450 K for 2-5 days; in step c, the hydrothermal synthesis conditions are 370-450 K for 0.5-1 days.

4. The application according to claim 1, characterized in that, In step 1, the concentration of the suspension dispersion is 0.005-0.5 wt%.

5. The application according to claim 1, characterized in that, In step 2, the contact time is 10-100 seconds, and drying is performed after contact.

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