CN115411292B - Molecular cross-linked molecular sieve nanosheet hybrid membrane, preparation method and application thereof in flow battery - Google Patents

Abstract

The invention belongs to the technical field of new materials, and relates to a preparation method and application of a molecular cross-linked molecular sieve nanosheet hybrid membrane. The method adopts surface functional molecular crosslinking to introduce a catalyst with-SO on the surface of the two-dimensional molecular sieve ₃ H、‑NH ₂ And (3) dispersing molecular sieve nano-sheets crosslinked by functional molecules in a polymer solution to obtain uniform casting solution, and then preparing the hybrid membrane by solution casting. The molecular cross-linked organic side chain effectively improves the organic-inorganic interface compatibility and the stress transmission of the inorganic filler and the polymer phase, can be combined with the ion exchange groups on the polymer chain and induces the rearrangement thereof to form a continuous ion transmission channel, and enhances the mechanical strength, the stability, the ion selectivity and the proton conductivity of the membrane.

Description

A molecular cross-linked molecular sieve nanosheet hybrid membrane, preparation method and application in a flow battery application in

technical field

The invention belongs to the technical field of new materials, and relates to a preparation method and application of a molecular cross-linked molecular sieve nanosheet hybrid membrane.

Background technique

With the transformation of the world's energy structure and the response to global climate change, how to efficiently use renewable energy has become a major challenge we face. As a large-scale energy storage technology, flow batteries have the characteristics of environmental friendliness, high efficiency, adjustable power and capacity, long cycle life, and low maintenance cost. The role of cutting peaks and filling valleys. As an important part of the flow battery, the proton-conducting membrane plays the main function of separating the positive and negative electrolytes and transferring protons, and its performance will greatly affect the performance of the battery system. Therefore, the development of low-cost and high-performance proton-conducting membrane materials is of great significance for the development of large-scale energy storage flow battery systems.

In recent years, sulfonated non-fluorine polymers have been widely studied as a low-cost ion-exchange membrane material. It can adjust the proton conductivity of the membrane by adjusting the degree of sulfonation. However, too high a degree of sulfonation often leads to Lead to more serious swelling phenomenon and poor ion selectivity and mechanical properties. Co-hybridization of functional inorganic fillers with polymer matrices is a general strategy to improve the performance of separators, which can well integrate the processing characteristics of organic polymers and the diverse functional properties of nanofillers. Porous materials such as zeolites, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are new types of functional fillers for the preparation of high-performance proton-conducting membranes. Their inherently regular pores can effectively intercept larger active substances. At the same time, carriers such as protons can be allowed to pass through quickly. It is worth noting that zeolite molecular sieves have three-dimensional interconnected rigid and regular channels, which can recognize ions or molecules in the Angstrom size range. Two-dimensional zeolite nanosheet (ZN)-based membranes exhibit excellent performance in ion and molecular separation due to the large lateral dimensions and the nanoscale thickness in the preferred straight channel direction. High-aspect-ratio ZNs, which can maximize ion discrimination and minimize proton transport pathways, have great potential as functional building blocks for the development of novel proton-conducting membranes (Non-Patent Documents 1–54). Although such inorganic porous materials have shown great promise as the basic building blocks of proton-conducting membranes, they still face challenges in practical applications, such as the poor compatibility of the organic-inorganic interface, and foreign fillers are easy to change the polymerization. The distribution of ion exchange groups in the phase makes it difficult to form continuous proton transfer channels. Therefore, it is of great practical significance to develop suitable strategies to fully utilize the application potential of this kind of inorganic nanofillers.

Non-Patent Document 1: Kim D, Jeon MY, Stottrup BL, Tsapatsis M. para-Xylene ultra-selective zeolite MFI membranes fabricated from nanosheet monolayers at the air–water interface. Angew Chem IntEd. 2018; 57:480-485.

Non-Patent Document 2: Min B, Yang S, Korde A, Kwon YH, Jones CW, Nair S. Continuous zeolite MFI membranes fabricated from 2D MFI nanosheets on ceramic hollowfibers. Angew Chem Int Ed. 2019; 58:8201-8205.

Non-Patent Document 3: Dai L, Xu F, Huang K, et al. Ultrafast water transport intwo-dimensional channels enabled by spherical polyelectrolyte brushes with controllable flexibility. Angew Chem IntEd. 2021; 60: 19933-19941.

Non-Patent Document 4: Liu Y, Qiang W, Ji T, et al. Uniform hierarchical MFInanosheets prepared via anisotropic etching for solution-based sub-100-nm-thick oriented MFI layer fabrication. Sci Adv.2020; 6:eaay5993.

Contents of the invention

The purpose of the present invention is to solve the problems of poor compatibility and low battery performance of ordinary two-dimensional zeolite molecular sieve nanosheets used in flow battery diaphragm materials. This patented method uses surface functional molecular cross-linking to introduce organic side chains with functional groups such as -SO ₃ H and -NH ₂ on the surface of two-dimensional molecular sieves, and disperses the functional molecular cross-linked molecular sieve nanosheets in the polymer solution to obtain uniform casting. Membrane solution, and then the hybrid membrane was prepared by solution casting method. This kind of molecularly cross-linked organic side chain effectively improves the organic-inorganic interface compatibility and the stress transfer between the inorganic filler and the polymer phase, and at the same time can combine with the ion-exchange groups on the polymer chain and induce its rearrangement to form a continuous Ion transport channels that enhance the mechanical strength, stability, ion selectivity, and proton conductivity of the membrane.

A hybrid membrane of molecularly cross-linked molecular sieve nanosheets, including polymers and molecular sieve nanosheets, the surface of the molecular sieve nanosheets is grafted with a silane coupling agent, and the silane coupling agent has amino and sulfonic acid groups , mercapto, hydroxymethyl, ureido, or one or more of ethoxy, methoxy or isocyanate functional groups.

The polymer is sulfonated polyetheretherketone, perfluorosulfonic acid resin, polyether block polyamide, polydimethylsiloxane, polyethersulfone, sulfonated polyethersulfone, polybenzimidazole or poly imide.

The molecular sieve nanosheets are selected from ZSM-5 molecular sieve nanosheets, ZSM-35 molecular sieve nanosheets or ZSM-22 nanosheets.

The silane coupling agent is 3-mercaptopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, hydroxymethyl Triethoxysilane, ethyltriethoxysilane, ureapropyltriethoxysilane, trimethoxysilane or isocyanatopropyltriethoxysilane.

The weight ratio of the molecular sieve nano sheet to the polymer is 0.1:100-30:100.

The thickness of the hybrid film is 20-150 μm.

The preparation method of the above-mentioned molecular cross-linked molecular sieve nanosheet hybrid membrane comprises the following steps:

Step 1, carry out the crosslinking reaction of the molecular sieve nanosheet and the silane coupling agent in the first solvent, and dry the product after washing;

Step 2, dispersing the molecularly crosslinked molecular sieve nanosheets and polymers obtained in step 1 in a second solvent to obtain a casting solution;

In step 3, the casting solution is subjected to film-forming treatment to obtain a hybrid film.

In the step 1, the weight ratio of the molecular sieve nanosheets to the silane coupling agent is 1:0.1˜1:1.

The condition of the cross-linking reaction is reflux at 80-120° C. for 24-48 hours.

The first solvent is selected from toluene, benzene or xylene.

In the step 2, the second solvent is selected from N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, propanol, butanol, propylene glycol, dimethyl sulfoxide, One or more of isopropyl acetate and tetrahydrofuran.

The mass fraction of the polymer in the casting liquid is 1%-30%.

Beneficial effect

The invention successfully carries out a series of surface functional modification on the two-dimensional molecular sieve nano-sheet through molecular cross-linking, and uses it as a filler to prepare a molecular sieve nano-sheet hybrid film with functional molecular cross-linking. Molecularly cross-linked molecular sieve nanosheets effectively enhance the mechanical strength, ion selectivity and proton conductivity of the separator. The method of the present invention fully exerts the selective screening effect of the molecular sieve nanosheets, improves the compatibility between the molecular sieve nanosheets and the polymer phase through molecular cross-linking, and fully exploits the advantages of the molecular sieve nanosheets in the selective transmission of protons and in batteries. The application in the diaphragm provides a new idea and strategy.

Description of drawings

Figure 1 is a schematic diagram of the preparation and structure of functional molecular cross-linked zeolite nanosheets (ZN) reinforced membranes. (a) Schematic diagram of the hybrid approach for functional design of amino-functionalized ZN (ZN-NH ₂ ) and sulfonic acid-functionalized ZN (ZN-SO ₃ H). (b) Schematic structure of proton channels in (i) SPEEK membrane and hybrid membrane embedded with (ii) ZN- _NH2 and (iii) ZN- _SO3H .

Figure 2 is the characterization of ZN. Transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy (EDS) results (inset) of (a) ZN, (b) ZN-SO ₃ H and (c) ZN-NH ₂ . (d) X-ray photoelectron spectroscopy (XPS) results, (e) thermogravimetric analysis (TG) curve, (f) X-ray diffraction (XRD) pattern, (g) adsorption-desorption isotherm of N2 at 77K, ( h) Pore size distribution curves, and (i) ion exchange capacity (IEC) results of ZNs.

Figure 3 is the morphology of the hybrid film doped with ZN. Surface, cross-sectional, and local magnification scanning electron microscope (SEM) images of (a) SPEEK film and hybrid films doped with (b) ZN, (c) ZN-NH2, and (d) ZN-SO3H. From left to right: surface image, cross-sectional image, and partially enlarged image. The ZN loading was 4 wt%.

Figure 4 is the film properties. (a) Water absorption and swelling rate, (b) Tensile strength, (c) TG curve, (d) Time-dependent diffusion concentration of VO2+ on the permeation side, (e) Proton conductivity and surface resistance, and (f) Ion selectivity of different membranes.

Figure 5 is the battery performance. (a) Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of VFB cells equipped with different membranes. (b) Comparison of the EE of the S/ZN-SO3H-4% membrane with the reported SPEEK-based membrane applied to VFB at different current densities. Organic and inorganic represent SPEEK-based membranes modified by organic polymer blending and inorganic nanofiller mixing, respectively. (c) CE, VE, and EE of VFB cells equipped with different membranes, subjected to long-cycle tests at a current density of 120 mA cm-2. (d) Discharge capacity retention of different films in a long-cycle test at a current density of 120 mA cm-2.

Detailed ways

The preparation steps of the hybrid membrane of the present invention are described in detail as follows:

Step (1), preparation of molecularly cross-linked molecular sieve nanosheets: add molecular sieve nanosheets and silane coupling agent to solvent A in sequence, stir at 100-500 rpm, heat to 80-120°C under an inert atmosphere, condense and reflux for 24-48 hours , after the reaction mixture is cooled to room temperature, the reaction mixture is centrifuged at 5000-10000 rpm, the supernatant is poured off, the collected reaction product is washed with absolute ethanol for 3-5 times, and dried in an oven at 80°C to obtain Molecularly crosslinked molecular sieve nanosheets.

Step (2), preparation of molecular crosslinked molecular sieve nanosheet casting solution: add molecular crosslinked molecular sieve nanosheet prepared in step (1) to solvent B, and ultrasonically disperse molecular crosslinked molecular sieve nanosheet evenly for 12 hours to obtain a dispersion. Then add the polymer into the dispersion liquid, ultrasonicate for 6 hours and then stir at 50-350 rpm for 24 hours to fully dissolve the polymer, and obtain the molecular cross-linked molecular sieve nanosheet casting solution.

Step (3), preparation of molecularly crosslinked molecular sieve nanosheet hybrid membrane: Spread the molecularly crosslinked molecular sieve nanosheet casting solution prepared in step (2) on a glass plate, dry at 60°C for 6h to evaporate the solvent, and then place the glass The plate is transferred to a vacuum drying oven for drying at 100-140° C. for 12 hours to obtain a molecularly cross-linked molecular sieve nanosheet hybrid membrane.

In step (1), the solvent A is toluene, benzene or xylene.

In step (1), the molecular sieve nanosheets are ZSM-5 molecular sieve nanosheets, ZSM-35 molecular sieve nanosheets or ZSM-22 nanosheets.

In step (1), the described

In step (1), the inert atmosphere is nitrogen, helium or argon atmosphere.

In step (2), the solvent B is N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, propanol, butanol, propylene glycol, dimethylsulfoxide, One or more of isopropyl acetate and tetrahydrofuran.

In step (2), the polymer is sulfonated polyetheretherketone, perfluorosulfonic acid resin, polyether block polyamide, polydimethylsiloxane, polyethersulfone, sulfonated polyethersulfone, Polybenzimidazole or polyimide.

In step (1), the weight ratio of the molecular sieve nanosheets to the silane coupling agent is 1:0.1˜1:1.

In step (2), the weight ratio of the molecularly cross-linked molecular sieve nanosheets to the polymer is 0.1:100-30:100.

In step (2), the mass fraction of the polymer in the casting solution is 1%-30%.

In step (3), the thickness of the hybrid film is 20-150 μm.

The synthesis of embodiment 1ZSM-5 zeolite nanosheet (ZN)

ZN was synthesized by a two-step hydrothermal reaction. The precursor sol was configured according to the molar ratio of 10SiO ₂ :2.4TPAOH:0.87NaOH:114H ₂ O. The mixture was stirred overnight at room temperature, then hydrothermally reacted at 50° C. under normal pressure for 6 days, and then heated to 100° C. for three days. After the reaction, the zeolite nano-crystal seeds were washed with deionized water and collected by centrifugation. The template bis-1,5-tripropylpentamethylene diiodide (dC5) is synthesized by the thorough alkylation reaction of 1,5-diaminopentane and 1-iodopropane, and then using ethanol, 2- Butanone and ethyl acetate were purified. In the second step, the precursor sol is prepared according to the molar ratio of 80TEOS:3.75dC5:20KOH:9500H ₂ O, the precursor sol is hydrolyzed at room temperature overnight and then mixed with the zeolite nano-crystal seeds synthesized in the first step, the zeolite nano-crystal seeds The ratio of silica to precursor sol is 1:800. After the mixture was hydrothermally reacted at 140° C. for 3.5 days, 1M NaAlO ₂ (Si/Al was 25) was added thereto and the hydrothermal reaction was continued for 0.5 days. The reaction product was treated with alkaline salt solution to remove amorphous silica, then washed with deionized water, and collected by centrifugation to obtain ZN. The as-synthesized ZN was calcined at 500°C in air for 6 hours to remove the templating agent and activate it, with a heating rate of 1°C min ^-1 .

The synthesis of embodiment 2ZN-SO ₃ H

Add 0.5g activated ZN and 15mL toluene into the three-necked flask and stir well. Under a nitrogen atmosphere, add 0.25g of 3-mercaptopropyltriethoxysilane to the mixture, heat slowly to 110°C for 48h under reflux, wash with toluene and ethanol several times, centrifuge and dry at 60°C for 6h to obtain the reaction product. Under a nitrogen atmosphere, the product was stirred and oxidized in H ₂ O ₂ aqueous solution at room temperature for 24 h, and then the product was soaked in 10 mL of 0.1M H ₂ SO ₄ for 6 h to protonate the sulfonic acid group. Finally, the product was centrifuged, washed, and dried overnight at 80° C. to obtain ZN-SO ₃ H powder.

Embodiment 3ZN-NH _Synthesis of:

Add 0.5g activated ZN and 15mL toluene into the three-necked flask and stir well. Under a nitrogen atmosphere, add 0.25g of 3-aminopropyltriethoxysilane to the mixture, slowly heat to 110°C and reflux for 48h, wash with toluene and ethanol several times, centrifuge and dry overnight at 80°C to obtain ZN _-NH2 powder.

Preparation of embodiment 4SPEEK/molecular sieve hybrid membrane

SPEEK was prepared by sulfonating PEEK in sulfuric acid (98wt.%) at 50°C for 5h. Then SPEEK/MOF hybrid membranes were prepared by the following method.

Dissolve ZN, ZN-SO ₃ H, ZN-NH ₂ and SPEEK in Examples 1-3 respectively in N,N-dimethylformamide, so that the concentration of SPEEK is , and make ZN, ZN-SO ₃ H or ZN-NH ₂ accounts for 1-5% of the mass of SPEEK, and the concentration of SPEEK in N,N-dimethylformamide is about 14wt%, and it is used as a casting solution after mixing evenly. The casting solution was flatly spread on a glass plate, dried at 60°C for 6h to evaporate the solvent, and then the glass plate was transferred to a vacuum drying oven at 100-140°C for 12h to obtain a molecularly cross-linked molecular sieve nanosheet hybrid membrane.

Test Methods

1. Swelling rate and water absorption rate

The SR (swelling rate) and WU (water absorption rate) of the membrane are obtained by the following two equations, respectively:

where d and W are the diameter and mass of the membrane in wet and dry state, respectively.

2. Mechanical properties

The mechanical properties of the films were obtained on a universal testing machine with a tensile speed of 10 mm min ⁻¹ . Cut the sample into strips of 20mm×5mm, and wipe off the water on the surface of the wet film before testing. In order to reduce the test error, three samples were tested for each film, and the obtained data were averaged. The formula for calculating the tensile strength of the film sample is as follows:

In the formula, F _Max is the maximum tension; W and D are the width and thickness of the sample film, respectively.

3. Proton transfer rate

The proton transfer rate of the membrane was measured on an electrochemical workstation (Solartron analytical 1470E+1260A) by AC impedance spectroscopy. The sample is sandwiched between two circular titanium sheets held in place by a button cell clip. The test frequency is between 10 ³ and 10 ⁶ Hz with an AC amplitude of 5 mV. The proton transfer rate of the membrane is measured by the latest method of Professor Li's team, and the calculation formula is as follows:

where σ is the proton transfer rate of the membrane. L is the thickness of the sample. R is the resistance of the membrane. A is the effective area of the membrane, that is, the area of the titanium sheet.

The membrane surface resistance can be calculated by the following formula:

R _A = R × A

R _A is the film area resistance.

4. Vanadium ion permeation rate and ion selectivity

Tested in an H-type pair diffusion cell with an effective area of ^1.77cm2 . One side of the diffusion cell is filled with 50mL 1.5MVOSO ₄ /3.0MH ₂ SO ₄ solution, and the other side of the diffusion cell is filled with the same volume of 1.5M MgSO ₄ /3.0MH ₂ SO ₄ solution to balance the ionic strength and osmotic pressure. Magnetic stirring was used to reduce concentration polarization during the test. The product was measured in MgSO ₄ /H ₂ SO ₄ solution every 24 hours, and the absorbance was measured with a UV-Vis spectrophotometer at 762nm. VO2 ⁺ concentrations were determined using standard absorbance/concentration curves. The penetration rate of vanadium can be calculated by the following formula:

Among them, V _B is the volume of MgSO ₄ /H ₂ SO ₄ solution, which is 50ml in this experiment; C _B (t) is the concentration of VO ²⁺ ions in MgSO ₄ /H ₂ SO ₄ solution at time t; C _A is VOSO ₄ /H The concentration of VO ²⁺ in ₂ SO ₄ solution can be regarded as a constant when the test time is not too long to simplify the calculation; A and L are the effective membrane area and membrane thickness respectively; P is the permeation rate of vanadium ions.

Ion selectivity is defined as the ratio of proton transfer rate to VO2 ⁺ permeation rate, calculated as follows:

5. Ion exchange capacity

The ion exchange capacity is tested by the traditional titration method. The sample is immersed in a saturated NaCl solution for 24 hours, and the H+-containing solution obtained by the replacement is titrated with a 0.03M NaOH solution, and the ion exchange capacity can be calculated according to the following formula:

c _NaOH is the concentration of NaOH solution; V _NaOH is the volume of NaOH solution; W is the quality of the sample

6. Single battery performance

The performance of the VRFB was tested using a self-made flow battery system consisting of a membrane (with an effective area of 4 cm ² ), two carbon felt electrodes, two graphite plate current collectors, and a pair of shells. The membrane separates the negative electrolyte (10ml 1.5MV ²⁺ /V ³⁺ 3MH ₂ SO ₄ ) and the positive electrolyte (10ml 1.5M VO ²⁺ /VO2+3M H ₂ SO ₄ ), and is sandwiched between the two electrode. For the charge and discharge test, the cut-off voltages of charge and discharge are 1.7V and 0.8V respectively, and the current density of the test is 40-200mA cm ^-2 . For the cycle test, a constant current density of 120mA cm ^-2 was tested at the same cut-off voltage. During the test, nitrogen protection was used. The Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the battery are calculated by the following formulas:

Among them, C _d and C _c are discharge capacity and charge capacity, respectively; E _d and E _c are discharge energy and charge energy, respectively.

Characterization of Molecular Sieve Nanosheets (ZN)

As shown in the ac regions of Fig. 2, pristine ZN, ZN- _NH2, and ZN- _SO3H all exhibit the typical characteristics of single-layer or multilayer 2D nanosheets. Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) detected nitrogen and sulfur elements in ZN-NH ₂ and ZN-SO ₃ H, respectively. ad regions) no other elements were detected, which demonstrates the successful grafting of _-NH2 and _-SO3H functional groups. Thermogravimetric analysis (TGA) showed that ZN has excellent thermal stability, and the weight loss behavior of ZN- _NH2 and ZN- _SO3H can be attributed to the decomposition of organic side chains grafted on the surface of ZN (region e of Fig. ). X-ray diffraction (XRD) patterns, _N2 adsorption–desorption isotherms, and pore size distribution curves indicated that the crystal structure and intrinsic pores of ZN were well preserved after surface functional modification (fh region of Fig. 2). In addition, both ZN-NH ₂ (0.45mmolg ^-1 ) and ZN-SO ₃ H (0.57mmolg-1 ) exhibited higher ion exchange capacity (IEC) than ZN (0.27mmolg ^-1 ) (region i of Fig. 2 ), which are expected to provide more exchange sites for proton transport.

Membrane Morphology

The above-mentioned ZN, ZN-NH ₂ and ZN-SO ₃ H were respectively incorporated into the SPEEK matrix to prepare a proton conduction membrane, and the molecular sieve loading was controlled at 1wt%-5wt%. The corresponding film is expressed as S/ZX%, where S is SPEEK, Z represents ZN, ZN- _NH2 or ZN- _SO3H , and X% is the weight ratio of nanosheets to SPEEK. The thickness of the different films is well controlled at 49 ± 2 μm, and the thickness of the hybrid films increases with increasing ZN content. Due to the uniform dispersion of ZN in the polymer matrix, all the hybrid films are almost identical to the pure SPEEK films in terms of apparent physical properties such as color, opacity, and flexibility. The cross-sectional morphology was characterized by scanning electron microscopy (SEM). The SPEEK film and ZN-containing hybrid film are uniform, dense, and defect-free, and the incorporated ZN is uniformly suspended in the polymer phase (Fig. 3). Unlike the smooth cross-section of the pure SPEEK film (region a of Fig. 3), the ZN-incorporated film exhibits a rougher cross-section with a specific wrinkled network structure. Due to the uniform distribution of ZN and abundant interfacial interactions, the network structure It becomes denser with increasing ZN loading (region bd of Fig. 3). Importantly, no apparent gap between the ZN and polymer phases caused by interfacial incompatibility was observed in the ZN- _NH2 and ZN- _SO3H -incorporated films (Fig. 3, c, d regions). (region b of Fig. 3), which indicated that the surface affinity between ZN and the polymer was effectively enhanced by grafting -NH ₂ and -SO ₃ H functional groups.

membrane properties

Water absorption and swelling are two key parameters of proton-conducting membranes for flow batteries, where the former is related to the transport of protons and the latter reflects the dimensional stability of the membrane. Due to the inherent microporous structure of molecular sieves (region a of Figure 4), the water absorption capacity of the ZN-embedded membrane is higher than that of the pure SPEEK membrane (region a of Figure 4), and the water absorption of the hybrid membrane increases with the increase of ZN loading. In addition, due to the introduction of hydrophilic _-SO3H functional groups, this enhanced water absorption capacity is more pronounced in ZN- _SO3H -incorporated membranes compared with ZN and ZN- _NH2 . At the same time, due to the steric confinement effect of the rigid zeolite framework, the swelling rate of the polymer with water adsorption is well restricted (region a of Figure 4). The swelling ratios of all hybrid membranes decreased with increasing ZN loading. In particular, due to the formation of acid-base pairs between the _-SO3H groups in the SPEEK matrix and the _-NH2 functional groups in ZN- _NH2 , the dimensional stability of the ZN- _NH2 -incorporated membranes was further enhanced, so the incorporation The membranes incorporating ZN- _NH2 exhibited higher swelling resistance than ZN and ZN- _SO3H .

Membrane stability, including mechanical, chemical, and thermal stability, is an indispensable parameter for determining long-term stability during battery assembly and operation. Compared with the pure SPEEK film, the addition of ZN improves the tensile strength of the film, which originates from the dispersion strengthening effect of ZN filler addition. In addition, under the same ZN loading, the ZN- _NH2 embedded membrane exhibited better tensile strength than ZN and ZN- _SO3H membranes, because the synergistic effect of the acid-base pair contributes to better extraction from the polymer Stress transfer from the matrix to the nanofillers (region b of Figure 4). Chemical stability is another critical parameter of membranes for VFB applications due to the corrosion of electrolytes containing VO ²⁺ . The degradation of the membrane is usually caused by the strong oxidation of ^VO ions during charge and discharge. Absorption spectra revealed no detectable changes in the vanadium species in the soaking solution after immersing the membrane in a solution containing ^VO ions for 15 days, which indicated the excellent chemical stability of the membrane. Furthermore, the thermal stability was assessed by thermogravimetric analysis (TGA) (region c of Fig. 4). There was almost no obvious difference in the weight loss behavior of the films, which indicated that the incorporation of ZN did not change the thermodynamic properties of the films.

The ion selectivity and conductivity of the membrane are the two most important parameters that directly determine the performance of the battery. A separator with higher ion selectivity can effectively prevent the cross-contamination of active substances in the positive and negative electrolytes and slow down the self-discharge of the battery. The vanadium permeability of the membrane was first studied by vanadium ion permeation experiments. As shown in region d of Fig. 4, the beneficial effect of intercalated ZN on blocking the permeation of vanadium ions is evident. Once ZN was incorporated into the SPEEK polymer matrix, the vanadium permeation rate decreased significantly for all membranes, and the vanadium permeation rate decreased with increasing ZN loading. The enhanced barrier effect of vanadium ions can be attributed to the complete blocking of vanadium ions with a diameter larger than 0.6 nm by the rigid channels of the zeolites and the significantly prolonged vanadium ion diffusion path. It is worth noting that at 4wt% ZN loading, the vanadium permeability of the membrane doped with ZN- _NH2 is slightly lower than that of the membrane doped with ZN and ZN- _SO3H , because the interface formed by ZN- _NH2 and SPEEK interacts with each other. The effect further prevents the formation of large ion channels (region d of Figure 4).

All the films embedded with ZN, ZN- _NH2 , or ZN- _SO3H exhibited lower sheet resistance and higher proton conductivity compared with pure SPEEK film and commercial Nafion 212 (region e of Fig. 4). The proton conductivity of the membranes doped with different kinds of ZN showed a similar trend, and the proton conductivity increased with the increase of ZNs content, reached a maximum at 4 wt%, and then decreased. The increased conductivity of the hybrid membrane is attributed to the hydrophilic ionized surface and b-axis straight channel of the incorporated ZN, which provides another internal channel for the fast capture and transport of protons. However, too much ZN will cause severe aggregation and hinder the transport of protons. Furthermore, the interfacial interactions between the polymer matrix and the surface functional groups construct a continuous long-range and low energy barrier proton transfer channel along the interface between SPEEK and ZN (region b of Figure 1). Therefore, at the same 4wt% ZN loading, the use of ZN-NH ₂ and ZN-SO ₃ H as fillers resulted in an increase in proton conductivity higher than that of ZN, following the S/ZN-SO ₃ H-4% film>S/ZN The order of -NH ₂ -4% film>S/ZN-4% film. The proton conductivity of the S/ZN-SO ₃ H-4% membrane reaches 92.4mS cm ^-1 , which is about 1.6 times that of the pure SPEEK membrane (56.7mS cm ^-1 ) and 2.4 times that of the commercial Nafion212 membrane (39.3mS cm -1 ^{). 1} ). Ion selectivity, defined as the ratio of proton conductivity to vanadium permeability, usually reflects the overall performance of the membrane. All the hybrid membranes exhibited higher ion selectivity due to the significant enhancement of vanadium ion barrier capacity and proton conductivity, which corresponded well with the trend of proton conductivity (region f of Fig. 4). The S/ZN-SO ₃ H-4% membrane exhibited the highest ion selectivity (6.0×10 ⁵ S min cm ^-3 ) among all the membranes in this patent, much higher than the pure SPEEK membrane (1.6×10 ⁵ S min cm ^-3 ) and commercial Nafion 212 membrane (0.8×10 ⁵ S min cm ^-3 ).

battery performance

The membrane incorporating 4wt% ZN exhibited the most attractive comprehensive properties, such as enhanced water absorption, better swelling resistance, higher ion selectivity and conductivity, good stability in harsh environments, and is expected to have Excellent battery performance. .In order to further verify the availability and superiority of hybrid membranes, VFB was carried out on S/ZN-4% membrane, S/ZN-NH ₂ -4% membrane, S/ZN-SO ₃ H-4% membrane, SPEEK membrane Single cell tests and commercial Nafion 212 membranes. All the films doped with ZN, ZN- ^NH2 or ZN- ^SO3H exhibited better CE, VE and EE than pure SPEEK films at current densities ranging from 40 to 200 mA cm ^-2 (region a of Fig. 5 ). It is worth noting that the CE of the ZN-embedded membrane is significantly higher than that of the pure SPEEK membrane and Nafion 212 over the entire tested range, especially at low current densities, which is consistent with the vanadium permeation results (region d of Figure 4). Due to the high overpotential and ohmic polarization at high current density, the VE of the film gradually decreases with increasing current density (region a of Figure 5). Nevertheless, VFBs equipped with ZN-embedded membranes still exhibit significantly higher VEs than those equipped with pure SPEEK membranes because the high proton conductivity effectively reduces the ohmic polarization. At the same current density, the VE of the batteries mixed with different kinds of ZN films still followed the order of S/ZN-SO ³ H-4% film>S/ZN-NH ² -4% film>S/ZN-4% film . This agrees well with the proton conductivity results (region e of Figure 4). Overall, the excellent CE and VE indicated that the incorporation of ZN simultaneously enhanced the vanadium ion barrier ability and proton conductivity, and broke the trade-off between ion selectivity and proton conductivity. Therefore, all ZN-incorporated films exhibited enhanced EE (region a of Fig. 5), with the highest EE for the S/ZN- _SO3H -4% film. The CE and EE of the cell with the S/ZN- _SO3H -4% film reached 99.0% and 85.0% at a current density of 120mAc ^m-2 , in sharp contrast to 97.8% and 78.4% of the pure SPEEK film and Nafion 212 film, respectively. 96.3% and 80.1%.

Panel b of Fig. 5 summarizes the cutting-edge reports on SPEEK membranes modified by mixing organic blends and inorganic nanofillers for VFB applications. It is clear that the EE of the cell with the S/ZN-SO3H-4% membrane is superior to most of the reported membranes in the large current density range, which is benefited from the simultaneous enhancement of ion selectivity and conductance by the incorporation of functional ZN. Rate.

In order to further confirm the stability and reliability of the hybrid membrane, VFB cells equipped with S/ZN-SO ₃ H-4% membrane, S/ZN-NH ₂ -4% membrane and SPEEK membrane at a current density of 120mA cm ^-2 The down run is used for long loop tests (region c of Figure 5). The cell efficiencies of the S/ZN-SO ₃ H-4% and S/ZN-NH ₂ -4% membranes were relatively stable with no significant drop during the entire test, while the efficiency of the SPEEK membrane dropped sharply after 80 cycles Due to rupture of the membrane during operation. Furthermore, ZN-embedded membranes, especially ZN- _NH2 membranes, exhibited significantly higher capacity retention compared with SPEEK membranes, since membranes with high ion selectivity can effectively resist electrolyte cross-contamination ( Region d of Figure 5). The capacity retention rate of the SPEEK membrane was 74.3% after 80 cycles, and the capacity fading was 0.32% per cycle and 6.36% per day. In contrast, the capacity retentions of S/ZN-SO ₃ H-4% and S/ZN-NH ₂ -4% films after 250 cycles were 57.0% and 64.0%, respectively, which indicated that the capacity fade was higher than that of SPEEK Films were significantly lower (0.17% per cycle and 3.35% per day for S/ZN- _SO3H -4%, 0.14% per cycle and 2.54% per day for S/ZN- _NH2-4 %).

Claims (1)

1. The flow battery is characterized in that a molecular cross-linked molecular sieve nano-sheet hybrid membrane is adopted as a diaphragm, the molecular cross-linked molecular sieve nano-sheet hybrid membrane is a proton conduction membrane, the molecular cross-linked molecular sieve nano-sheet hybrid membrane comprises a polymer and a molecular sieve nano-sheet, and a silane coupling agent is grafted on the surface of the molecular sieve nano-sheet; the silane coupling agent is 3-mercaptopropyl triethoxysilane; the polymer is sulfonated polyether-ether-ketone; the molecular sieve nanosheets are selected from ZSM-5 molecular sieve nanosheets; the molecular sieve nano-sheet accounts for 1-5% of the mass of the polymer; the thickness of the hybridized film of the molecular cross-linked molecular sieve nano-sheet is 20-150 mu m; the preparation of the molecular cross-linked molecular sieve nanosheet hybrid membrane comprises the following steps:

(1) Synthesis of ZSM-5 zeolite nanosheets:

first, siO is added according to the mole ratio ₂ :TPAOH:NaOH:H ₂ O=10:2.4:0.87:114 configuring the first precursor sol; stirring the precursor sol at room temperature overnight, then carrying out hydrothermal reaction at 50 ℃ for 6 days under normal pressure, and then raising the temperature to 100 ℃ for 3 days; washing the obtained zeolite nano seed crystal with deionized water after the reaction is finished, and centrifugally collecting; the second step is that TEOS is used as template agent of bis-1, 5 tripropyl pentamethylene diimmonium iodide, KOH is used as H ₂ O=80:3.75:20:9500Preparing a second precursor sol, hydrolyzing the second precursor sol at room temperature overnight, mixing with the zeolite nano-seed crystal synthesized in the first step, performing hydrothermal reaction at 140 ℃ for 3.5 days, and adding 1M NaAlO ₂ Then the hydrothermal reaction is continued for 0.5 days; treating the reaction product with alkali salt solution to remove amorphous silica, washing with deionized water, centrifuging, collecting to obtain ZSM-5 zeolite nanosheets, calcining the synthesized ZSM-5 zeolite nanosheets in air at 500 ℃ for 6h to remove template agent and activate, and heating at a rate of 1 ℃ for min ^-1 ；

Wherein, the template agent of bis-1, 5-tripropyl pentamethylene ammonium diiodo is synthesized by thorough alkylation reaction of 1, 5-diaminopentane and 1-iodopropane, and then purified by ethanol, 2-butanone and ethyl acetate;

(2) Synthesis of sulfonic acid functionalized ZSM-5 zeolite nanoplatelets:

adding 0.5g of the activated ZSM-5 zeolite nano-plate and 15mL of toluene into a three-necked flask, and fully stirring; under nitrogen atmosphere, 0.25g of 3-mercaptopropyl triethoxysilane is added into the mixture, the mixture is slowly heated to 110 ℃ for reflux reaction for 48 hours, and then the reaction product is obtained by repeated washing with toluene and ethanol, centrifugation and drying at 60 ℃ for 6 hours; under nitrogen atmosphere, the product was purified under H ₂ O ₂ The mixture was oxidized in aqueous solution at room temperature for 24H with stirring, and then the product was oxidized in 10mL of 0.1M H ₂ SO ₄ Soaking for 6h to protonate the sulfonic acid group; finally, centrifuging and washing the product, and drying overnight at 80 ℃ to obtain sulfonic acid functionalized ZSM-5 zeolite nano-sheets;

(3) Preparation of molecular cross-linked molecular sieve nanosheet hybrid membrane:

dissolving sulfonic acid functionalized ZSM-5 zeolite nano sheets and SPEEK in N, N-dimethylformamide, enabling the sulfonic acid functionalized ZSM-5 zeolite nano sheets to account for 1-5% of the SPEEK in weight, enabling the concentration of the SPEEK in the N, N-dimethylformamide to be 14% by weight, uniformly mixing, then spreading the obtained mixture as casting solution on a glass plate, drying the casting solution at 60 ℃ for 6 hours to volatilize a solvent, transferring the glass plate to a vacuum drying oven for drying at 100-140 ℃ for 12 hours, and obtaining the molecular crosslinked molecular sieve nano sheet hybrid membrane; the molecular cross-linked molecular sieveProton conductivity of the nano-sheet hybrid membrane reaches 92.4 mS.cm ^-1 。