Graphene oxide-based monovalent cation-selective membranes and their applications

Abstract

This invention discloses a graphene oxide-based monovalent cation-selective membrane and its applications, belonging to the field of separation technology. The monovalent cation-selective membrane comprises graphene oxide, with tannic acid anchored between the graphene oxide layers via hydrogen bonding and π-π stacking to form a first cross-linked network. Dioctadecyl dimethyl ammonium bromide is anchored between the graphene oxide layers via electrostatic adsorption and forms a second cross-linked network with tannic acid through electrostatic attraction. Dioctadecyl dimethyl ammonium bromide is added to an alkaline aqueous solution of graphene oxide, followed by the addition of tannic acid and thorough mixing. The mixture is then filtered to form a membrane, which is subsequently dried and annealed to obtain the monovalent cation-selective membrane. This invention can simultaneously improve the monovalent / divalent cation sieving performance and ion flux of the membrane, while also enhancing its permeation energy generation capacity. It breaks the trade-off between selectivity (sieving characteristics) and flux in ion-selective membranes, providing a new approach and technical reference for the development of multifunctional ion exchange membranes for complex water bodies.

Description

Technical Field

[0001] This invention relates to the field of separation technology, specifically to graphene oxide-based monovalent cation-selective membranes and their applications. Background Technology

[0002] Oceans cover over 70% of the Earth's surface, containing abundant resources and energy with enormous development potential. Membrane technology encompasses various separation processes such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis, enabling efficient separation and conversion of substances. With its advantages of high efficiency, energy saving, and stability, membrane technology plays a crucial role in seawater development and is widely used in water purification, resource recovery, and energy extraction. Among these, reverse electrodialysis and the ion-selective membrane technology relied upon in the chlor-alkali industry are key representatives of membrane technology in the energy and chemical fields. Reverse electrodialysis (RED) is a process that converts chemical potential energy into electrical energy through the directional transmembrane migration of anions and cations driven by concentration gradients. The chlor-alkali industry uses ion-selective membranes as the core membrane to produce Cl2, NaOH, and H2 through the electrolysis of brine. The core component of both systems is an ion-selective membrane capable of achieving efficient and selective ion transport. However, in practical applications, various divalent ions (such as Mg2+) are commonly present. 2+ Ca 2 + and SO4 2- Traditional ion-selective membranes mostly lack the ability to distinguish between ions of different valence states. The presence of multivalent ions inevitably affects system performance, leading to problems such as reduced output power density, decreased product purity, and membrane fouling. Therefore, the development of ion-selective membranes that combine anion / cation selectivity with mono / divalent ion sieving capabilities is particularly urgent.

[0003] The anion / cation selectivity of ion-selective membranes typically stems from the repulsion of like ions and preferential transport of counterions by charged groups on the nanochannel surface. The sieving characteristics of monovalent / divalent ions mainly rely on the synergistic effect of size sieving and electrostatic interactions. The size sieving effect depends on angstrom / nanometer-scale confined channels, effectively blocking divalent ions with larger hydration radii through steric hindrance while allowing smaller monovalent ions to pass through. The valence-dependent electrostatic interaction manifests as a stronger electrostatic force between divalent ions and the membrane surface charge, thereby achieving differentiated migration and effective separation of monovalent / divalent ions. For example, patent application CN 117427500 A discloses an amphiphilic molecularly modified graphene-based ion-selective membrane, its preparation method, and its application. This membrane, prepared using bis(octadecyldimethylammonium bromide), sodium polyacrylate, and graphene oxide, selectively allows cations to pass through while stably generating electricity. However, this membrane allows both divalent and monovalent ions to pass through, failing to achieve precise sieving of monovalent / divalent ions. To achieve high anion / cation selectivity and precise sieving of monovalent / divalent ions, membranes are often required to possess high charge density and / or extremely narrow mass transfer channels. This inevitably exacerbates the resistance and energy barrier to ion transport, thus creating an inherent trade-off between selectivity (sieving characteristics) and flux. Further shrinking the confined channel can achieve precise sieving of monovalent / divalent ions, meaning the membrane's confined channel can only allow monovalent ions to pass through, while divalent ions cannot due to their larger hydration radius. However, this also reduces membrane flux, thereby reducing power generation capacity. Therefore, it is necessary to develop novel membrane materials that can overcome functional barriers and synergistically combine multiple separation mechanisms to achieve differentiated migration and effective separation of monovalent / divalent ions, reduce transmembrane resistance, and significantly increase ion flux. Summary of the Invention

[0004] The present invention addresses the aforementioned limitations of existing technologies by providing a graphene oxide-based monovalent cation-selective membrane and its applications. This invention employs tannic acid (TA) and dioctadecyl dimethyl ammonium bromide (DODAB) to synergistically modify graphene oxide (GO), simultaneously enhancing the membrane's cation selectivity, monovalent / divalent cation sieving performance, and ion flux, thereby improving the membrane's power generation capacity. This breaks the inverse relationship between selectivity (sieving characteristics) and flux in ion-selective membranes, providing a new design approach and technical reference for the development of multifunctional ion exchange membranes for complex water bodies.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a monovalent cation-selective membrane based on graphene oxide, the monovalent cation-selective membrane comprising graphene oxide, wherein tannic acid is anchored between the graphene oxide layers by hydrogen bonding and π-π stacking to form a first crosslinked network, and bis(octadecyldimethylammonium bromide) is anchored between the graphene oxide layers by electrostatic adsorption and forms a second crosslinked network with tannic acid by electrostatic attraction.

[0006] Preferably, the monovalent cation-selective membrane is prepared by the following method: (1) Disperse graphene oxide in pure water and adjust the pH to 11 to obtain GO aqueous solution. First, dissolve dioctadecyl dimethyl ammonium bromide in anhydrous ethanol to obtain DODAB alcohol solution. Disperse the DODAB alcohol solution in pure water to obtain DODAB solution. Dissolve tannic acid in pure water to obtain TA aqueous solution. (2) Add DODAB solution dropwise to GO aqueous solution, stir evenly, then add TA aqueous solution, stir evenly, then filter the membrane under vacuum, and after filtration, dry and anneal to obtain monovalent cation selective membrane.

[0007] Preferably, in step (1), the concentration of the GO aqueous solution is 0.01 mg / mL.

[0008] Preferably, in step (1), the concentration of the DODAB alcohol solution is 1 mg / mL and the concentration of the DODAB solution is 0.004 mg / mL.

[0009] Preferably, in step (1), the concentration of the TA aqueous solution is 0.02 mg / mL.

[0010] Preferably, in step (2), the volume ratio of the GO aqueous solution, DODAB solution and TA aqueous solution is 2:1:1.

[0011] Preferably, in step (2), the filter membrane is a mixed cellulose microporous filter membrane with a diameter of 50 mm and an effective pore size of 200 nm.

[0012] Preferably, in step (2), the drying temperature is 60°C and the time is 1 h; the annealing temperature is 120°C and the time is 4 h.

[0013] A second aspect of the present invention provides the application of a monovalent cation-selective membrane in at least one of the following 1) to 3): 1) Improve the screening capacity for mono / divalent cations; 2) Increase ion flux; 3) Electrolysis of brine to produce alkali.

[0014] Preferably, the monovalent / divalent cation sieve excludes anions and divalent cations, allowing only monovalent cations to be transported across the membrane.

[0015] The beneficial effects of this invention are: (1) This invention uses tannic acid (TA) and dioctadecyl dimethyl ammonium bromide (DODAB) to synergistically modify graphene oxide (GO). Through the cross-linking of TA between graphene oxide layers and the cross-linking network formed by DODAB and TA through electrostatic attraction, angstrom / nanometer-scale confined channels can be formed between graphene oxide layers. These channels effectively block divalent ions with larger hydration radii through steric hindrance, while allowing smaller monovalent ions to pass through. The abundant phenolic hydroxyl groups in the TA molecule provide sufficient reactive sites for ion transport, thereby increasing ion flux. Therefore, the monovalent / divalent cation sieving performance and ion flux of the membrane are simultaneously improved, while also enhancing the membrane's permeation energy generation capacity. This breaks the trade-off between selectivity (sieving characteristics) and flux in ion-selective membranes, providing a new design concept and technical reference for the development of multifunctional ion exchange membranes for complex water bodies.

[0016] (2) The ion-selective membrane prepared in this invention is effective against K + / Mg 2+ Na + / Mg 2+ and Li + / Mg 2+ The sieve ratios reached 87, 74, and 67, respectively, and 30.2 W / m³ was achieved under a 50-fold salinity gradient. -2 The membrane also demonstrates potential for application in the electrolysis of brine to produce high-purity sodium hydroxide, which is expected to simplify the pretreatment process and reduce production costs. Attached Figure Description

[0017] Figure 1 (a) Nanochannel structure diagram of GDT membrane nanochannels. After GO is modified with TA and DODAB, the resulting nanochannels only allow monovalent cations to pass through, while blocking divalent ions and anions; (b) Zeta potential of GDT solutions at different pH values; Figure 2(a) SEM image of the cross-section of the GDT membrane; (b) AFM image of the GO nanosheets; (c) AFM image of the GT nanosheets; (d) AFM image of the GDT nanosheets; (e) Comparison of water contact angles of GO, GD, GT and GDT membranes; (f) FT-IR spectra of GO, GD, GT and GDT membranes; (g) Zeta potential of GO, GD, GT and GDT membranes; (h) XRD patterns of the GO membrane under dry and wet conditions; (i) XRD patterns of the GT membrane under dry and wet conditions; (j) XRD patterns of the GD membrane under dry and wet conditions; (k) XRD patterns of the GDT membrane under dry and wet conditions, with the wet condition being immersion in 0.1 M NaCl for 5 h; (l) Water flux of the GT membrane and GDT membrane; Figure 3 (a) Potassium-magnesium sieve ratio of GO, GT, GD, GDT, GDP and GDT2 membranes; (b) Sodium-magnesium sieve ratio of GO, GT, GD, GDT, GDP and GDT2 membranes; (c) Lithium-magnesium sieve ratio of GO, GT, GD, GDT, GDP and GDT2 membranes; (d) IV curves of GO, GT, GD, GDT, GDP and GDT2 membranes under a 0.5 / 0.01 M NaCl gradient; Figure 4 (a) Schematic diagram of the device for collecting permeation energy; (b) Relationship between current density and power density of GDT membrane and load resistance under a 0.5 / 0.01 M NaCl gradient; (c) Output power density of GO, GT, GD, GDT, GDP and GDT2 membranes; Figure 5 (a) Schematic diagram of an apparatus for producing alkali by electrolysis of brine; (b) GDT membrane electrolysis of Mg-containing... 2+ (c) Mechanism diagram for preparing high-purity NaOH from crude brine; (d) Mg added to the cathode chamber solution after GDT membrane electrolysis. 2+ Cl - and Na + (d) The molar concentration and permeation rate of GO, GT, GD, GDT, GDP and GDT2 after membrane electrolysis, the newly added Mg in the cathode chamber solution. 2+ and Na + The molar concentration. Detailed Implementation

[0018] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, 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 application pertains.

[0019] As described in the background section, achieving high anion / cation selectivity and precise sieving of mono / divalent ions often requires membranes with high charge density and / or extremely narrow mass transfer channels. This inevitably exacerbates the resistance and energy barrier to ion transport, thus creating an inverse relationship between selectivity (sieving characteristics) and flux.

[0020] Based on this, the purpose of this invention is to provide a monovalent cation-selective membrane based on graphene oxide and its applications. This invention proposes to synergistically modify graphene oxide (GO) with tannic acid (TA) and dioctadecyl dimethyl ammonium bromide (DODAB) to simultaneously improve the membrane's cation selectivity and monovalent / divalent cation sieving performance. The electrostatic interaction between DODAB and GO, and the crosslinking effect between TA and GO, synergistically regulate and reduce the ion transport channel size, placing it between the typical sizes of monovalent and divalent cations, thereby endowing the membrane with significant monovalent / divalent cation sieving capabilities. Simultaneously, the positively charged DODAB can attract more negatively charged TA molecules into the GO interlayer: on the one hand, the introduction of TA enhances the negative charge on the membrane surface, improving cation selectivity; on the other hand, the abundant phenolic hydroxyl groups in the TA molecules provide sufficient reactive sites for ion transport, thereby increasing ion flux. Based on this design, the GO-DODAB-TA (GDT) composite membrane prepared by this invention simultaneously possesses excellent cation selectivity, monovalent / divalent cation sieving capabilities, and increased ion flux.

[0021] Graphene oxide (GO) films, due to their unique stacked structure containing both pristine graphite and oxide regions, enable ultrafast water transport and highly selective sieving of molecules and ions. However, the hydrophilicity of graphene oxide causes GO films to swell easily in aqueous solutions, weakening the nanoscale confinement of the interlayer space. Dioctadecyl dimethyl ammonium bromide (DODAB) consists of a positively charged hydrophilic "head group" and two hydrophobic long-chain "tails," and its positively charged quaternary ammonium group (N... + ) and the negatively charged groups on GO (such as -COOH, -OH) through N + -O - The interaction combines cation-π interaction with ionic interaction. This strong interaction significantly improves the stability of the GO membrane in aqueous solution and alleviates the swelling problem. On the other hand, it reduces the size of the ion transport channel, making it intermediate between monovalent and divalent ions, thereby achieving a sieving effect on monovalent / divalent ions.

[0022] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0023] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0024] Example 1: Preparation of GDT membrane (1) Take 1 mg of GO (prepared by Hummers method) and disperse it in 100 mL of pure water. Disperse it by ultrasonication for 5 min, and then adjust the pH to 11 with 1 mol / L NaOH solution to obtain GO aqueous solution.

[0025] Dissolve 50 mg of DODAB powder in 50 mL of anhydrous ethanol, sonicate in an ice-water bath for 40 min, and then heat in a 60 °C water bath for 2.5 h under sealed conditions to finally obtain a 1 mg / mL DODAB alcohol solution. Take 200 μL of the DODAB alcohol solution and disperse it in 50 mL of pure water to obtain a DODAB solution with a concentration of 0.004 mg / mL.

[0026] A TA solution with a concentration of 0.02 mg / mL was prepared by dissolving TA in pure water.

[0027] (2) First, 50 mL of 0.004 mg / mL DODAB solution was added dropwise to 100 mL of pH 11 GO aqueous solution (concentration 0.01 mg / mL). After stirring at room temperature (25℃) for 10 min, 50 mL of 0.02 mg / mL TA aqueous solution was added, and the mixture was stirred at room temperature for 10 min. Then, the mixture was vacuum filtered through a mixed cellulose microporous membrane (50 mm in diameter, 200 nm in effective pore size). After filtration, the membrane was dried in a 60℃ oven for 1 h to remove residual moisture, and then annealed in a 120℃ oven for 4 h to obtain the GDT membrane.

[0028] Comparative Example 1: Preparation of GO Membrane 1 mg of GO was dispersed in 100 mL of pure water and ultrasonically dispersed for 5 min to obtain a 0.01 mg / mL GO aqueous solution. The pH of the GO aqueous solution was then adjusted to 11 with 1 mol / L NaOH solution, and the solution was vacuum filtered through a mixed cellulose microporous membrane (50 mm in diameter, 200 nm in effective pore size). After filtration, the membrane was dried in a 60 °C oven for 1 h to remove residual moisture, and then annealed in a 120 °C oven for 4 h to obtain the GO membrane.

[0029] Comparative Example 2: Preparation of GD Membrane The difference from Example 1 is that no TA solution is added.

[0030] Comparative Example 3: Preparation of GT Membrane The difference from Example 1 is that no DODAB solution was added.

[0031] Comparative Example 4: Preparation of amphiphilic molecule-modified graphene-based ion-selective membranes (1) Dissolve 0.5 mg of dioctadecyl dimethyl ammonium bromide (DODAB, 99%) in 50 mL of deionized water and mix well to obtain an aqueous solution of dioctadecyl dimethyl ammonium bromide. Dissolve 0.2 mg of sodium polyacrylate in 50 mL of deionized water and mix well to obtain an aqueous solution of sodium polyacrylate.

[0032] (2) The bis(octadecyldimethylammonium bromide) solution obtained in step (1) was poured into an aqueous solution of graphene oxide with a pH of 10 and stirred for 1 minute. Then, the sodium polyacrylate solution was poured into the mixed solution and stirred for 1 minute to obtain a mixed solution with small and uniform flocculation. The mixed solution was poured onto cellulose ester membrane filter paper (pore size 200 nm) for vacuum filtration to form a membrane. The filtered membrane, along with the filter paper, was placed in a glass petri dish and dried in an oven at 40°C for 1 hour to ensure that the water in the membrane slowly escaped. Then, it was annealed in an oven at 120°C for 3 hours to ensure the stability of the membrane, thus obtaining an amphiphilic molecule-modified graphene-based ion-selective membrane (GDP membrane).

[0033] Comparative Example 5: Preparation of GDT2 membrane The difference from Example 1 is that DODAB was dispersed in pure water to obtain a DODAB solution with a concentration of 0.004 mg / mL. The resulting membrane is designated as the GDT2 membrane.

[0034] Example 2: The effect of pH on membrane performance The pH of the GO aqueous solution was adjusted to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 using 1 mol / L NaOH or 1 mol / L HCl solution. GDT mixed solutions were prepared using the above GO solutions. The Zeta potential of each membrane solution was then measured using a Zetasizer Nano ZS particle size analyzer. Each solution was tested five times, and the average value was taken. The test results are as follows: Figure 1 As shown in (b), the absolute value of the Zeta potential reaches its maximum at pH=11, at which point the mixed solution exhibits the highest negative charge density. This is attributed to the deprotonation of the abundant phenolic hydroxyl groups on the TA, generating negatively charged phenoxy ions (-O). - The deprotonation of TA increases the negative charge density, thus improving cation selectivity; simultaneously, it forms a stronger hydrogen bond acceptor, and the hydrogen bond network between TA and GO becomes denser and more stable. Through the dual regulation of GO by DODAB and TA, the GDT membrane exhibits both good cation selectivity and mono / divalent cation sieving ability.

[0035] DODAB carries a positively charged quaternary ammonium group (N). +) and the negatively charged groups on GO (such as -COOH, -OH) through N + -O - The interaction between ions and cation-π interactions, combined, effectively alleviates GO membrane swelling while reducing the size of ion transport channels, thus improving the sieving efficiency of monovalent / divalent ions. The presence of hydrophobic long chains reduces the hydrophilicity of the membrane, enhancing its performance stability. Tannic acid molecules consist of a glucose nucleus and ten galloyl groups. Under alkaline conditions, the phenolic hydroxyl group of tannic acid undergoes deprotonation, generating a negatively charged phenoxy ion (-O). - The presence of polyanions significantly increases the surface charge density of the GO membrane, and the positively charged DODAB further adsorbs more TA through electrostatic attraction, thus improving the cation selectivity of the GDT membrane. Furthermore, the abundant phenolic hydroxyl groups in the TA molecule provide ample reactive sites for ion transport, which is beneficial for increasing ion flux. The tight cross-linking between TA and GO through a hydrogen bond network makes the GO membrane structure more compact, further enhancing the membrane's size sieving effect. Therefore, the GDT membrane possesses excellent cation selectivity, monovalent ion selectivity, and high ion flux. Figure 1 (a)

[0036] Example 3: Characterization The surface morphology of the GDT film was tested using scanning electron microscopy (SEM), such as... Figure 2 As shown in (a), the cross-section of the GDT membrane is similar to that of the pure GO membrane, both exhibiting a tightly packed layered structure, which constitutes a two-dimensional nanochannel for ion transmembrane transport. Figure 2 (b) Figure 2 As shown in (d), observing the atomic mechanical microscopy (AFM) images of GO, GT, and GDT reveals that the thickness of GO nanosheets is approximately 2 nm, the thickness of GT nanosheets is approximately 5 nm, and the thickness of GDT nanosheets reaches 8 nm.

[0037] The increased thickness of the GDT nanosheets demonstrates the tight bonding between the GO nanosheets and DODAB and TA through multiple interactions. Furthermore, DODAB is an amphiphilic molecule containing two hydrophobic chains; therefore, its addition enhances the membrane's hydrophobicity. The hydrophobicity of DODAB was verified by water contact angle testing: the water contact angle of the GO membrane was 62.04°, the GT membrane was 50.44°, while the GDT membrane increased to 84.64°. This hydrophobic property also facilitates low-friction transmembrane ion transport. Figure 2 Middle (e).

[0038] Subsequently, Fourier transform infrared spectroscopy (FT-IR) was performed on the four types of films: GO, GD, GT, and GDT. Figure 2As shown in (f), the GDT membrane at 3423 cm⁻¹ -1 There is a distinct absorption peak at 1629 cm⁻¹, which is due to OH stretching. -1 The absorption peak at 3438 cm⁻¹ is a characteristic C=O absorption peak of carboxylate. It is similar to the GO peak at 3438 cm⁻¹. -1 Compared to the characteristic OH absorption peak in the vicinity, the GDT membrane exhibited a red shift, which may be due to the formation of hydrogen bonds between GO and DODAB and TA. Comparison of the zeta potentials of the four membranes revealed that the membrane potential increased with the addition of TA, with the GDT membrane possessing the highest zeta potential. This may be attributed to the adsorption of more TA molecules by the positively charged groups of DODAB. (See below) Figure 2 (g). Furthermore, the interlayer spacing of the membrane was determined by X-ray diffraction (XRD), with the wet interlayer spacing obtained after immersing the membrane in 0.1M NaCl solution for 5 h. For example... Figure 2 As shown in (h), although the GO film was annealed, it still swelled easily in solution, and the interlayer spacing increased from 0.73 nm to 1.18 nm. Figure 2 As shown in (i), the cross-linking effect between TA and GO significantly improves the swelling of the GT film; as Figure 2 As shown in (j), the strong electrostatic interaction between DODAB and GO increases the wet interlayer spacing of the GD film by only 0.04 nm, while the wet interlayer spacing of both the GT and GD films is smaller than that of the GO film. Figure 2 As shown in (k), through the dual regulation of DODAB and TA, the interlayer spacing of GDT reached 0.85 nm, and the wet interlayer spacing did not increase significantly, indicating that DODAB and TA improved the structural stability of GO. Although the interlayer spacing of the GDT membrane is larger than that of other membranes, it exhibits the best ion sieving effect. This may be because the DODAB and TA molecules themselves, while widening the interlayer spacing, also enhance the steric hindrance effect between the layers, making the actual size of the ion channel smaller than the interlayer spacing. Figure 2 As shown in Figure (l), by comparing the water flux of the GT membrane and the GDT membrane, it can be seen that the structure of the GDT membrane is more compact than that of the GT membrane.

[0039] Experimental Example 1: Ion Sieving and Selectivity The membranes prepared in Example 1 and Comparative Examples 1-5 were subjected to ion sieving and selectivity tests.

[0040] The membrane to be tested is wrapped with two perforated PI membranes on either side, forming a PI-membrane-PI structure. The thicknesses of the GO, GD, GT, GDT, GDP, and GDT2 membranes are approximately 500 nm. This structure is sandwiched between two chambers of a test electrolytic cell (a CH2010H type ground glass electrolytic cell with replaceable membranes). 0.1 M KCl, 0.1 M NaCl, 0.1 M LiCl, or 0.1 M MgCl2 solution are added to each chamber, respectively. The test area is 2 cm². 2 Using an electrochemical workstation (CHI760EB18569), cyclic voltammetry was employed to record IV curves. The scanning voltage range was -0.3 V to +0.3 V, with a step voltage of 0.001 V. Based on the conductivity of the obtained IV curves, the sieving ratio of the membrane for different ions was calculated. The sieving ratio is a key performance parameter describing sieving performance; for example, the sodium-magnesium sieving ratio S... Na / Mg It can be obtained from the following formula: Where G is electrical conductivity and z is the ionic valence state.

[0041] Using the same apparatus, 0.5 M NaCl solution and 0.01 M NaCl solution were used to simulate seawater and river water, respectively, and these solutions were filled into the left and right chambers. The test area was 0.0314 cm². 2 Using an electrochemical workpiece (CHI760E B18569), cyclic voltammetry was employed to record IV curves. The scanning voltage range was -0.3 V to +0.3 V, with a step voltage of 0.001 V. Based on the open-circuit voltage values ​​of the obtained IV curves, the cation selectivity of the membrane was compared.

[0042] according to Figure 3 (a) ~ Figure 3 In Example (c), the GDT membrane prepared in Example 1 showed better potassium-magnesium sieve ratio, sodium-magnesium sieve ratio, and lithium-magnesium sieve ratio than Comparative Examples 1, 2, 3, and 4. Regarding K... + / Mg 2+ Na + / Mg 2+ and Li + / Mg 2+ The sieving ratios reached 87, 74, and 67, respectively. Comparison shows that adding DODAB and TA separately improves the ion sieving ratio, and adding both simultaneously yields even better results. Figure 3As can be seen from Figure (d), the GDT membrane prepared in Example 1 maintains a good cation selectivity ratio while achieving a high cation selectivity comparable to the comparative example. The addition of TA is beneficial for improving cation selectivity and increasing ion flux. Although the short-circuit current of the GDT membrane prepared in Example 1 is slightly lower than that of the GT membrane in Comparative Example 3, it is significantly better than Comparative Example 3 in terms of balancing cation selectivity and cation-to-divalent ion sieving ratio. The sieving ratio of Comparative Example 4 is small because the amphiphilic graphene-based ion-selective membrane of Comparative Example 4 does not form a cross-linked network between the GO layers, but relies on electrostatic attraction and other effects for ion sieving. Therefore, this membrane can pass both monovalent and divalent cations, and cannot achieve cation / divalent ion sieving. The sieving effect of the GDT2 membrane in Comparative Example 5 is also significantly lower than that of the GDT membrane in Example 1. This is because in the preparation of the GDT membrane in Example 1, DODAB was first dissolved in anhydrous ethanol and then dispersed in water to prepare a DODAB solution. Dispersing DODAB in anhydrous ethanol first helps to form a nanovesicle solution with uniform particle size and stable dispersion. This allows for more uniform and orderly intercalation of DODAB between GO layers during film formation, resulting in a more uniform cross-linked network and improving the consistency and controllability of the membrane structure.

[0043] Experimental Example 2: Osmotic Energy Collection The output power density of the membranes prepared in Example 1 and Comparative Examples 1-5 was tested. A higher output power density indicates higher cation selectivity and ion flux of the membrane. Using... Figure 4 The apparatus shown in (a) collects osmotic energy. Two perforated PI membranes are used to wrap around the membrane to be tested, forming a PI-membrane-PI structure. This structure is sandwiched between two chambers of a test electrolytic cell (a CH2010H type ground glass electrolytic cell with replaceable membrane). 0.5 M and 0.01 M NaCl solutions are injected into the left and right chambers, respectively. The test area is 0.0314 mm². 2 The two chambers are connected to the load resistor via electrodes and wires. The Vt curves of the membrane with different load resistors are recorded using an electrochemical workstation (CHI760E B18569) to obtain the current density and output power density of the membrane under different resistances.

[0044] Example 1: Current density and output power density of the GDT film are as follows Figure 4 As shown in (b), the GDT film achieved 30.2 W / m² when the load resistance was approximately 7 kΩ. -2 The output power density is at a leading level compared to Comparative Examples 1-5, see [reference]. Figure 4 (c) demonstrates its potential for application in permeable energy harvesting.

[0045] Experimental Example 3: Electrolysis of brine to produce alkali The membranes prepared in Example 1 and Comparative Examples 1-5 were used to electrolyze brine to produce alkali. The apparatus for electrolyzing the mixed brine is as follows: Figure 5 As shown in (a), the membrane to be tested is wrapped with two perforated PI membranes on the left and right sides to form a PI-membrane-PI structure. This structure is sandwiched between two chambers of a test electrolytic cell (a CH2010H type ground glass electrolytic cell with replaceable membrane). A mixed solution of 0.5 M NaCl and MgCl2 (molar ratio 9:1) and a 0.5 M NaOH solution are injected into the left and right chambers respectively. A commercial ruthenium-iridium titanium electrode is used as the anode, and a commercial pure titanium electrode is used as the cathode. The test area is 2 cm². 2 A constant voltage of 2.7V was applied using an electrochemical workstation (CHI760E B18569), and electrolysis was performed for 1 hour. After electrolysis, the solution in the cathode chamber was collected, and the Na content in the cathode chamber solution was measured using inductively coupled plasma atomic emission spectrometry (ICP). + and Mg 2+ Concentration, determined by ion chromatography (IC). - concentration.

[0046] The mechanism of mixed brine electrolysis using GDT membrane in Example 1 is as follows: Figure 5 As shown in (b). After electrolysis begins, the Cl in the anode chamber... - An oxidation reaction occurs, producing Cl2. Water in the cathode chamber undergoes a reduction reaction, producing H2, while Na in the anode chamber... + Driven by voltage, it migrates through the GDT membrane into the cathode chamber, where it reacts with excess OH radicals. - The reaction produces NaOH; Mg in the anode chamber 2+ Under voltage-driven conditions, there is also a tendency for the Mg to migrate towards the cathode chamber, but the GDT film intercepts the Mg. 2+ This ensures the purity of the NaOH product in the cathode chamber.

[0047] After electrolysis, the increase in Na in the cathode chamber solution was tested. + Mg 2+ and Cl - The concentration, such as Figure 5 As shown in (c). The results showed that Na in the cathode chamber... + Concentration far exceeding Mg 2+ and Cl - This indicates that the GDT film is effective against Mg. 2+ and Cl - Its penetration rate is extremely low, while Na + The flux remained unaffected. Compared to comparative examples 1-5, the GDT membrane effectively intercepted Mg. 2+ High Na + Flux, see Figure 5(d) This demonstrates that the GDT membrane possesses excellent cation selectivity, monovalent / divalent cation sieving, and high ion flux, showcasing its application potential in the electrolysis of mixed brine to produce alkali. This simplifies the feed liquid treatment process, reduces the complexity of the preparation process, saves production costs and time, and is more conducive to the progress and development of the chlor-alkali industry.

[0048] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a graphene oxide-based monovalent cation-selective membrane, characterized in that, Includes the following steps: (1) Graphene oxide was dispersed in pure water and the pH was adjusted to 11 to obtain a GO aqueous solution. Dioctadecyl dimethyl ammonium bromide was first dissolved in anhydrous ethanol to obtain a DODAB alcohol solution. The DODAB alcohol solution was then dispersed in pure water to obtain a DODAB solution. Tannic acid was dissolved in pure water to obtain a TA aqueous solution. The concentration of the GO aqueous solution was 0.01 mg / mL. The concentration of the DODAB alcohol solution was 1 mg / mL. The concentration of the DODAB solution was 0.004 mg / mL. The concentration of the TA aqueous solution was 0.02 mg / mL. (2) DODAB solution was added dropwise to GO aqueous solution and stirred evenly. Then TA aqueous solution was added and stirred evenly. The filter membrane was then vacuum filtered. After filtration, it was dried and annealed to obtain a monovalent cation selective membrane. The volume ratio of GO aqueous solution, DODAB solution and TA aqueous solution was 2:1:

1. The filter membrane was a mixed cellulose microporous filter membrane with a diameter of 50 mm and an effective pore size of 200 nm. The drying temperature was 60 °C and the time was 1 h. The annealing temperature was 120 °C and the time was 4 h.

2. The monovalent cation-selective membrane obtained by the preparation method according to claim 1, characterized in that, The monovalent cation-selective membrane includes graphene oxide, tannic acid is anchored to the interlayer of graphene oxide through hydrogen bonding and π-π stacking to form a first cross-linked network, and dioctadecyldimethylammonium bromide is anchored to the interlayer of graphene oxide through electrostatic adsorption and forms a second cross-linked network with tannic acid through electrostatic attraction.

3. The use of the monovalent cation-selective membrane according to claim 2 in at least one of the following 1) to 3): 1) Improve the screening capacity for mono / divalent cations; 2) Increase ion flux; 3) Electrolysis of brine to produce alkali.

4. The application according to claim 3, characterized in that, The monovalent / divalent cation sieve excludes anions and divalent cations, allowing only monovalent cations to be transported across the membrane.

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