MXene-cmc composite membrane for salinity gradient energy harvesting and method of making the same

Abstract

The application discloses a preparation method of an MXene-CMC composite film for salinity gradient energy collection, and specifically comprises the following steps: step 1, preparing an MXene solution; step 2, preparing a CMC solution; step 3, placing the MXene solution prepared in step 1 into the CMC solution prepared in step 2, and dissolving the same through electromagnetic stirring to obtain a mixed dispersion liquid; step 4, taking a polycarbonate film, placing the polycarbonate film as a substrate into a suction filter bottle, taking the mixed dispersion liquid into the suction filter bottle to perform vacuum-assisted suction filtration, and obtaining a polycarbonate film covered with the MXene-CMC composite film; and step 5, stripping the polycarbonate film covered with the MXene-CMC composite film, filtering water, and obtaining the MXene-CMC composite film for salinity gradient energy collection. The application further discloses the MXene-CMC composite film for salinity gradient energy collection. The composite film prepared by the application has the characteristics of high power density, good stability and high economic value.

Description

Technical Field

[0001] This invention belongs to the field of marine salinity gradient energy development technology, and relates to an MXene-CMC composite membrane for salinity gradient energy harvesting. This invention also relates to a method for preparing the MXene-CMC composite membrane for salinity gradient energy harvesting. Background Technology

[0002] In recent years, with the rapid growth of global energy demand, the focus on salinity gradient energy has increased significantly. Calculations show that the free energy change caused by the mixing of seawater and river water can reach 2.72 kJ / L, meaning that every cubic meter of river water flowing into the ocean can generate approximately 0.8 kWh of energy, equivalent to the energy released when water falls from a 280-meter-high dam. The global potential of this energy source should not be underestimated. Furthermore, the energy often overlooked in the treatment of industrial wastewater is actually a considerable amount of blue energy. It is reported that the total global blue energy is as high as 30 terawatts (TW), of which theoretically usable resources are approximately 2.4 to 2.6 TW, equivalent to the generating capacity of more than 100 Three Gorges Dams, making it the second largest ocean energy source after wave energy. Therefore, the effective collection and utilization of ocean salinity gradient energy is expected to significantly alleviate the current energy shortage problem.

[0003] The core component of a salt gradient energy harvesting device based on RED technology is a selective ion exchange membrane. This device utilizes the salt concentration gradient as a driving force, selectively permeating ions with specific electrical charges through the ion exchange membrane to create a potential difference, directly converting salt gradient energy into electrical energy. In recent years, two-dimensional materials such as graphene, graphene oxide (GO), boron nitride (BN), carbon nanotubes, and transition metal dichalcogenides (TMDs) have attracted widespread attention in salt gradient power generation research. These materials, due to their excellent electrical properties, can achieve power densities ranging from several kW / m² to an astonishing 1000 kW / m² within a microscopic measurement area. However, when the effective test area is expanded to a macroscopic scale, concentration polarization causes the ion flux of the porous membrane to be significantly lower than the theoretical value of the sum of the individual pores, resulting in a sharp drop in power density. Currently, materials such as molybdenum disulfide, GO, and MXene exhibit relatively outstanding salt gradient power harvesting performance, but it is still below the minimum standard for commercial profitability (5 W / m²), and their mechanical strength is relatively low.

[0004] Sodium carboxymethyl cellulose (CMC) is an important water-soluble polysaccharide widely used in the food, pharmaceutical, and industrial sectors. Due to its unique properties, the development and application of CMC can help improve the performance and quality of various products. However, research on the application of CMC in permeation energy capture is relatively limited. In fact, the carboxyl groups (-COOH) introduced into CMC during the modification of natural cellulose give it a negative charge in solution, while the hydroxyl groups (-OH) in the original cellulose are retained. Therefore, CMC molecular chains can be linked to some nanosheets (such as MXene containing functional groups such as -F, -O-, and -OH) via hydrogen bonds, thereby increasing the surface charge and mechanical strength of the membrane, making it potentially valuable for high-performance permeation energy extraction. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting. The composite membrane prepared by this method has the characteristics of high power density, good stability, and high economic value.

[0006] Another object of the present invention is to provide an MXene-CMC composite membrane for salinity gradient energy harvesting.

[0007] The first technical solution adopted in this invention is a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, which specifically includes the following steps:

[0008] Step 1: Prepare MXene solution;

[0009] Step 2: Prepare CMC solution;

[0010] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0011] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0012] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0013] The first technical solution of this invention is further characterized by:

[0014] The specific process of step 1 is as follows: Take a quantitative amount of MXene dispersion into the reactor and perform ultrasonic treatment to obtain an MXene solution.

[0015] The concentration of MXene solution is 1-5 mg / mL.

[0016] The ultrasonic treatment time is 4-6 minutes.

[0017] The specific process of step 2 is as follows:

[0018] Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder;

[0019] Step 2.2: Dissolve CMC solid powder in deionized water and stir it with electromagnetic stirring to obtain a CMC solution.

[0020] The concentration of CMC solution is 2-10 mg / mL.

[0021] The electromagnetic stirring time is 24-36 hours.

[0022] In step 3, an electromagnetic stirrer is used for electromagnetic stirring. The speed of electromagnetic stirring is 1000-1500 rpm, and the electromagnetic stirring time is 12-24 hours.

[0023] The vacuum-assisted filtration time in step 4 is 3-7 days.

[0024] The second technical solution adopted in this invention is an MXene-CMC composite membrane for salinity gradient energy harvesting, which is prepared using the same method as that for salinity gradient energy harvesting.

[0025] The beneficial effects of this invention are as follows:

[0026] This invention provides an MXene-CMC composite membrane for salinity gradient energy harvesting and its preparation method. Sodium carboxymethyl cellulose (CMC) is selected as the binder between MXene nanosheets and enhances the surface charge of each nanosheet. Compared with pure MXene membranes, the composite membrane exhibits higher surface charge density and mechanical strength. Under simulated river water and real seawater mixing conditions, the composite membrane achieves an output power density of 17.10 W / m², while under simulated river water and salt lake conditions, this value increases to 53.78 W / m². Experimental and simulation results show that the increased radial size and surface charge of the nanocomposite membrane channels are key factors in achieving high output power. Furthermore, the MXene-CMC composite membrane can also be used to extract permeate energy from pH gradients; the high diffusion rate of H⁺ and the neutralization reaction result in an output power density as high as 200 W / m² under acidic and alkaline conditions. It is worth noting that the economic viability of the membrane is a major challenge for the industrial application of permeate energy harvesting. In the MXene-CMC composite membrane prepared in this application, the relatively expensive MXene (Ti3C2Tx) serves only as a framework, while the environmentally friendly and inexpensive sodium carboxymethyl cellulose is the dominant material. This composite membrane is significantly less expensive than the pure MXene membrane, while its energy harvesting performance far surpasses that of the pure MXene membrane, thus well meeting economic requirements. This provides a new approach for designing high-performance and inexpensive nanomembranes for extracting permeate energy and treating acidic and alkaline wastewater. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of the preparation method of the MXene-CMC composite membrane for salinity gradient energy harvesting according to the present invention;

[0028] Figure 2 This is a structural diagram of the CMC molecule in Example 6 of the present invention;

[0029] Figure 3 This is a schematic diagram of the MXene nanosheets in Embodiment 6 of the present invention;

[0030] Figure 4 This is a physical image of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0031] Figure 5 This is a cross-sectional schematic diagram of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0032] Figure 6 This is a surface SEM image of the pure MXene film in Example 6 of the present invention;

[0033] Figure 7 This is a surface SEM image of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0034] Figure 8This is a cross-sectional SEM image of the pure MXene membrane in Example 6 of the present invention;

[0035] Figure 9 This is a cross-sectional SEM image of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0036] Figure 10 This is the stress-strain curve of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0037] Figure 11 This is a schematic diagram of the contact angle of the MXene-CMC composite film in Embodiment 6 of the present invention;

[0038] Figure 12 These are the XRD spectra of the pure MXene membrane and the composite membrane in Example 6 of this invention;

[0039] Figure 13 These are zeta potential diagrams of dispersions with different blending ratios in Example 6 of the present invention;

[0040] Figure 14 This is the XPS spectrum of the MXene-CMC composite membrane in Example 6 of the present invention;

[0041] Figure 15 This is a schematic diagram of ion transmembrane transport in Embodiment 6 of the present invention;

[0042] Figure 16 This is an IV curve graph of the MXene-CMC composite membrane in Example 6 of the present invention measured in neutral potassium chloride solutions of different concentrations;

[0043] Figure 17 This is a graph showing the changes in conductivity and Debye length with KCl electrolyte concentration in Example 6 of the present invention.

[0044] Figure 18 This is the IT curve measured under the cyclic bias voltage in Embodiment 6 of the present invention;

[0045] Figure 19 This is the IV curve measured under electrolyte conditions in Example 6 of the present invention;

[0046] Figure 20 This is a schematic diagram of the apparatus for testing the permeation energy of the MXene-CMC composite membrane in Embodiment 6 of the present invention;

[0047] Figure 21 This is a schematic diagram showing the short-circuit current and open-circuit voltage generated by MXene-CMC composite films of different thicknesses in Embodiment 6 of the present invention;

[0048] Figure 22This is a power density diagram of MXene-CMC composite films of different thicknesses in Embodiment 6 of the present invention;

[0049] Figure 23 This is a graph showing the power density as a function of the external load resistance in Embodiment 6 of the present invention;

[0050] Figure 24 This is a current density diagram of the MXene-CMC composite membrane with optimal thickness and blending ratio under different concentration gradients in Example 6 of the present invention;

[0051] Figure 25 This is a power density diagram of different measurement areas in Embodiment 6 of the present invention;

[0052] Figure 26 This is a radial anion concentration distribution diagram of the nanochannel model under different channel radii in Embodiment 6 of the present invention;

[0053] Figure 27 This is a graph showing the power density and current density of the MXene-CMC composite membrane extracted permeation energy as a function of load resistance in Embodiment 6 of the present invention;

[0054] Figure 28 This is a schematic diagram comparing the salinity gradient power generation performance of Embodiment 6 of the present invention with other methods;

[0055] Figure 29 This is a distribution curve of ion concentration along the axial position of the simulated channel under different surface charge densities in Embodiment 6 of the present invention;

[0056] Figure 30 This is a schematic diagram illustrating the effect of surface charge density on diffusion current and diffusion voltage in Embodiment 6 of the present invention;

[0057] Figure 31 This is a schematic diagram illustrating the promoting effect of surface charge density on output power value in Embodiment 6 of the present invention;

[0058] Figure 32 This is a schematic diagram illustrating the effect of nanochannel diameter on current and selectivity in Embodiment 6 of the present invention;

[0059] Figure 33 This is a schematic diagram showing the relationship between the nanochannel diameter and output power in Embodiment 6 of the present invention;

[0060] Figure 34 This is a diagram showing the open-circuit voltage and short-circuit current at different pH gradients in Embodiment 6 of the present invention;

[0061] Figure 35 This is a schematic diagram showing the functional relationship between current density and load resistance in KCl and HCl electrolytes under the same concentration gradient in Example 6 of the present invention;

[0062] Figure 36 This is a schematic diagram illustrating the functional relationship between power density and load resistance in KCl and HCl electrolytes under the same concentration gradient in Example 6 of the present invention.

[0063] Figure 37 This is a schematic diagram showing the relationship between power density and efficiency as a function of pH gradient in Embodiment 6 of the present invention;

[0064] Figure 38 This is a schematic diagram showing the relationship between current density and load resistance at different pH gradients in Embodiment 6 of the present invention;

[0065] Figure 39 This is a graph showing the maximum output power density at different pH gradients in Example 6 of the present invention. Detailed Implementation

[0066] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0067] Example 1

[0068] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0069] Step 1: Prepare MXene solution;

[0070] Step 2: Prepare CMC solution;

[0071] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0072] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0073] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0074] Example 2

[0075] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0076] Step 1: Prepare MXene solution;

[0077] The specific process is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; the concentration of the MXene solution is 1-5 mg / mL; the ultrasonic treatment time is 4-6 min;

[0078] Step 2: Prepare CMC solution;

[0079] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0080] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0081] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0082] Example 3

[0083] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0084] Step 1: Prepare MXene solution;

[0085] The specific process is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; the concentration of the MXene solution is 1-5 mg / mL; the ultrasonic treatment time is 4-6 min;

[0086] Step 2: Prepare CMC solution;

[0087] The specific process is as follows:

[0088] Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder;

[0089] Step 2.2: Dissolve CMC solid powder in deionized water and stir it electromagnetically to ensure complete dissolution, thereby obtaining a CMC solution;

[0090] The CMC solution concentration is 2-10 mg / mL; the electromagnetic stirring time is 24-36 h;

[0091] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0092] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0093] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0094] Example 4

[0095] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0096] Step 1: Prepare MXene solution;

[0097] The specific process is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; the concentration of the MXene solution is 1-5 mg / mL; the ultrasonic treatment time is 4-6 min;

[0098] Step 2: Prepare CMC solution;

[0099] The specific process is as follows:

[0100] Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder;

[0101] Step 2.2: Dissolve CMC solid powder in deionized water and stir it electromagnetically to ensure complete dissolution, thereby obtaining a CMC solution;

[0102] The CMC solution concentration is 2-10 mg / mL; the electromagnetic stirring time is 24-36 h;

[0103] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0104] The electromagnetic stirring uses an electromagnetic stirrer with a stirring speed of 1000-1500 rpm and a stirring time of 12-24 hours.

[0105] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0106] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0107] Example 5

[0108] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0109] Step 1: Prepare MXene solution;

[0110] The specific process is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; the concentration of the MXene solution is 1-5 mg / mL; the ultrasonic treatment time is 4-6 min;

[0111] Step 2: Prepare CMC solution;

[0112] The specific process is as follows:

[0113] Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder;

[0114] Step 2.2: Dissolve CMC solid powder in deionized water and stir it electromagnetically to ensure complete dissolution, thereby obtaining a CMC solution;

[0115] The CMC solution concentration is 2-10 mg / mL; the electromagnetic stirring time is 24-36 h;

[0116] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0117] The electromagnetic stirring uses an electromagnetic stirrer with a stirring speed of 1000-1500 rpm and a stirring time of 12-24 hours.

[0118] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0119] Vacuum-assisted filtration takes 3-7 days;

[0120] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection.

[0121] Example 6

[0122] The present invention relates to a method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, as follows: Figure 1 As shown, the specific steps include the following:

[0123] Step 1: Prepare MXene solution;

[0124] The specific process is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; the concentration of the MXene solution is 1-5 mg / mL; the ultrasonic treatment time is 4-6 min;

[0125] Step 2: Prepare CMC solution;

[0126] The specific process is as follows:

[0127] Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder;

[0128] Step 2.2: Dissolve CMC solid powder in deionized water and stir it electromagnetically to ensure complete dissolution, thereby obtaining a CMC solution;

[0129] The CMC solution concentration is 2-10 mg / mL; the electromagnetic stirring time is 24-36 h;

[0130] Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion.

[0131] The electromagnetic stirring uses an electromagnetic stirrer with a stirring speed of 1000-1500 rpm and a stirring time of 12-24 hours.

[0132] like Figure 2 As shown, the CMC molecular chain contains abundant -OH and -COOH groups, such as Figure 3 As shown, by stirring, the CMC molecular chains and MXene nanosheets are fully cross-linked, and the resulting mixed dispersion exhibits a significant Tyndall effect, indicating that MXene and CMC are uniformly dispersed in the aqueous phase.

[0133] Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane.

[0134] Vacuum-assisted filtration takes 3-7 days;

[0135] Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covering the MXene-CMC composite membrane, filter out moisture, and so on. Figure 4 , 5 As shown, an MXene-CMC composite membrane for salinity gradient energy harvesting was obtained; the composite membrane was named MXene-CMC-x, where x represents the mass ratio of CMC to MXene;

[0136] Next, the dispersion and the MXene-CMC composite membrane were characterized. Figure 6 As shown, the scanning electron microscope (SEM) image of the pure MXene film reveals certain defects, such as... Figure 7 As shown, the MXene-CMC composite membrane exhibits a smoother and more even surface. Figure 8 , 9 As shown, the SEM cross-sectional images clearly reveal that the composite membrane is thicker and more robust than the pure MXene membrane. The nanosheets act like thin "bricks," becoming more stable through the "concrete" of CMC. The EDS spectra of the cross-section indicate that the major elements of MXene (C, O, F, Ti) are uniformly distributed within the nanocomposite membrane. Transmission electron microscopy (TEM) images characterize the size of the MXene nanosheets as approximately a few micrometers. The SEM images of CMC show a highly regular structure. Tensile tests demonstrate that the composite membrane possesses superior toughness and strength; a mere 2mm wide MXene-CMC-20 membrane can withstand a tensile force equivalent to the weight of a 1.6kg object. Figure 10 As shown, the stress strength reaches 135 MPa. Figure 11 As shown, the MXene-CMC composite membrane also exhibits good hydrophilicity, which helps promote ion transport within the composite membrane. Due to the abundant hydrophilic groups in CMC, the contact angle gradually decreases with increasing CMC content in the composite membrane, indicating enhanced hydrophilicity. X-ray diffraction (XRD) spectroscopy shows that, as... Figure 12 As shown, the angle corresponding to the characteristic peak (002) of the composite membrane decreases. According to Bragg's law, the d-spacing of the MXene and MXene-CMC composite membranes is calculated to be 1.45 nm and 1.50 nm, respectively, indicating that the composite membrane can achieve higher ion flux during ion transport. Furthermore, as... Figure 13 As shown, CMC exhibits excellent charge properties, with a zeta potential as high as -79.3 mV, significantly higher than MXene's -27.67 mV, which contributes to the improved ion selectivity of the composite film. The surface elemental state of the composite film was analyzed by X-ray photoelectron spectroscopy (XPS). The full spectrum of the MXene-CMC-20 composite film is shown below. Figure 14As shown, the binding energies of several characteristic elements are revealed. Further analysis using C1s single-spectrum analysis reveals that the composite membrane contains abundant oxygen-containing functional groups, thus exhibiting excellent charge properties.

[0137] The MXene-CMC composite membrane exhibits excellent ion transport properties. These properties were evaluated experimentally, with the tested membrane containing approximately 95% CMC (i.e., a CMC to MXene mass ratio of 20), a ratio obtained through power density optimization. Figure 15 As shown, electrolyte solutions of the same concentration were added to both sides of the composite membrane in the electrolytic cells. Potassium chloride was chosen as the electrolyte solution because potassium ions and chloride ions have the same diffusion coefficient. The ion transport characteristics of the composite membrane were evaluated by ion current-voltage (IV) measurements. Figure 16 As shown, the IV curves are displayed for KCl electrolyte concentrations of 0.01, 0.1, and 1 M. All curves exhibit linear ohmic characteristics, with negligible rectification effects, indicating a symmetrical structure of the composite membrane system. Conductivity measurements reveal that ion transport within the composite membrane channels is controlled by surface charge. Figure 17 As shown, the transmembrane conductivity of ions exhibits two distinct patterns with varying electrolyte concentrations. At high concentrations, the conductivity shows a linear relationship with concentration, consistent with the bulk value. However, when the concentration drops below 0.01 M, the ionic conductivity begins to deviate from the bulk value and gradually tends towards a fixed value. This is because in the high-concentration region, the double-layer thickness (i.e., ...) is significantly reduced. Figure 17 The Debye length (λD) differs significantly from the channel size, therefore the conductivity is concentration-dependent. In low-concentration regions, the double layer thickness is large and overlaps with the channel region, leading to surface charge dominating ion transport. The Debye length (double layer thickness) is calculated using the following formula:

[0138] (1);

[0139] Wherein, ε, ε0, R, T, nbulk, z, and F represent the dielectric constant of water, vacuum dielectric constant, universal gas constant, absolute temperature, solution concentration, ionic valence, and Faraday constant (107, 95, 79), respectively. It can be seen from formula (1) that the Debye length is negatively correlated with the ion concentration in the solution. Compared with the MXene membrane, the MXene-CMC-20 composite membrane exhibits higher conductivity, especially at lower concentrations, where the conductivity value is even an order of magnitude higher. This is mainly attributed to the incorporation of CMC, which widens the interlayer spacing of the two-dimensional membrane, thereby achieving more efficient ion transport. To evaluate the stability of the composite membrane during ion transport, a time-of-current (IT) test was performed by alternately applying an external voltage of ±1 V. Each measurement cycle lasted 10 minutes, for a total of 15 cycles. The test was conducted under acidic (pH=3), neutral (pH=7), and alkaline (pH=11) conditions, and the results showed no significant current decay within 9000 seconds, such as... Figure 18 As shown, this demonstrates that the composite membrane maintains good stability under three different environments, laying the foundation for subsequent measurements of energy conversion performance in acidic and alkaline environments. Furthermore, to further understand the confined ion transport, such as... Figure 19 As shown, the chemical potential-driven ion transport of several different chloride electrolytes was measured under a 50-fold concentration gradient. At zero bias voltage, all currents were observed to be positive, which is determined by the preferential cation transport characteristic of the MXene-CMC-20 composite membrane.

[0140] Next, the salt gradient energy conversion performance of the MXene-CMC composite membrane was investigated. For example... Figure 20As shown, the basic principle of salinity gradient energy conversion is illustrated: driven by chemical potential (i.e., concentration gradient), ions selectively permeate through the composite membrane, forming a potential difference, which then converts chemical energy into electrical energy through redox reactions on the electrodes. To investigate the effect of CMC content on the salinity gradient energy conversion performance of the composite membrane, composite membranes with different CMC contents were prepared. With the increase of CMC content, both the open-circuit voltage (VOC) and short-circuit current (ISC) initially showed an upward trend, followed by a decrease. This phenomenon can be attributed to the fact that the increase of CMC increases the charge density and widens the nanomembrane channels, thereby enhancing selectivity and ion flux. However, with further increases in CMC content, the cross-linked CMC molecular chains reach saturation, and excess CMC may block the membrane channels, leading to a decrease in ion flux. When the mass ratio of MXene to CMC is 1:20, the optimal trade-off between VOC and ISC is achieved, while the maximum output power density is reached. After determining that MXene-CMC-20 is the composite membrane with the best performance, its permeation energy conversion performance at different thicknesses was further investigated. Measurements under simulated seawater / river water conditions (i.e., NaCl solution with a 50-fold concentration gradient) showed that both the open-circuit voltage and short-circuit current reached their maximum values ​​when the membrane thickness was 60 μm. Figure 21 As shown, a maximum output power density of 12.3 W / m² was achieved. When the thickness is less than 60 μm, the membrane structure is easily damaged, affecting ion selectivity; while when the thickness is greater than 60 μm, the ion transport path increases, reducing transport efficiency. Furthermore, the MXene-CMC composite membrane also exhibited excellent permeate energy conversion performance under different concentration gradients. From... Figure 22 As can be seen, both the short-circuit current and open-circuit voltage increase with the increase of the concentration gradient, reaching 40 μA and 162 mV respectively at a concentration gradient of 500 times (0.01 M / 5 M NaCl solution, i.e., simulated river water / salt lake). Figure 23 , 24 As shown, the output current density and power density are positively correlated with the concentration gradient. When the concentration gradient increases from 5 to 500, the power density increases from 1.81 W / m² to 53.78 W / m², mainly due to the increase in Gibbs free energy. Furthermore, the high ionic conductivity of this membrane helps alleviate concentration polarization problems, making it advantageous for larger-scale applications. Figure 25 As shown, under a 50-fold concentration gradient, when the cross-sectional area of ​​ion transport is increased to 1.00 mm² and 2.25 mm², respectively, the maximum output power density can still reach 0.56 W / m² and 0.26 W / m².

[0141] Due to the cation selectivity of this composite membrane, several major cations in seawater were selected for salinity gradient energy conversion performance testing, with Cl⁻ being the primary anion. Results showed that the power density reached 13.97 W / m² and 10.02 W / m² in K⁺ and Li⁺ solutions, respectively, while it was 7.04 W / m² and 4.28 W / m² in divalent ions Mg²⁺ and Ca²⁺, respectively. The permeation performance of different ions was mainly affected by their differences in diffusion coefficients. Although the diffusion coefficient of calcium ions was slightly greater than that of magnesium ions, the radius of calcium ions was approximately 50% larger than that of magnesium ions. Therefore, calcium ions were more easily affected by the forces of negatively charged functional groups when passing through nanochannels, leading to their retention in the membrane channels. Studies have shown that the cross-linking effect of calcium ions can enhance the mechanical strength of the membrane, while the cross-linking effect of magnesium ions is weaker, thus the diffusion of calcium ions within the membrane is more restricted. Although the power density measured in Ca²⁺ solution was the lowest, its content in real seawater was also the lowest, only about 20% of that in Mg²⁺, and far lower than that in Na⁺. To further verify the energy conversion performance of this membrane system in a natural environment, we used natural seawater (seawater samples taken from the South China Sea) instead of the high-concentration electrolyte solution for measurements. Figure 26 As shown, in the natural seawater / simulated river water system, the output power density reached 17.10 W / m², higher than the value measured under a 50-fold NaCl gradient. This is likely due to the total cation concentration in natural seawater being greater than 0.5 M, and the composite membrane exhibiting good selective transport of several major cations in seawater. Figure 27 As shown, by comparing with some previous works, the output power density of this membrane system shows a significant advantage under similar measurement systems.

[0142] To further elucidate the mechanism of the high performance of the MXene-CMC composite membrane, a continuum model based on the Poisson-Nernst-Planck equations was used for simulation calculations. To make the calculations feasible, as... Figure 28 As shown, the ion transport path is simplified to a negatively charged single nanochannel. (As illustrated...) Figure 29 As shown, the simulation results intuitively reveal the concentration polarization phenomenon: at the interface between the low-concentration side and the nanochannel, an ion-enriched region appears, where the solution concentration is significantly higher than the concentration in the pool, resulting in an actual concentration difference that is smaller than the ideal concentration difference. Conversely, at the interface between the high-concentration side and the nanochannel, an ion dissipation region exists. Because the high concentration leads to a reduction in the thickness of the electric double layer, its effect on ions is weaker; therefore, the difference between the ion concentration in the dissipation region and the bulk ion concentration is not significant.

[0143] Subsequently, the influence of surface charge density on the ion-permeation power generation performance of nanochannels was studied. For example... Figure 30 As shown, with increasing surface charge density, the nanochannels attract more ions with opposite charges, thereby increasing the diffusion current and diffusion voltage, such as... Figure 31 As shown, this ultimately leads to a significant increase in the permeation power of the nanochannels. Another key factor in the ion transport performance of nanochannels is the channel diameter. For example... Figure 32 As shown, with the gradual increase in the diameter of the nanochannels, the double-layer effect gradually weakens. Although the ion selectivity decreases slightly, the ion flux increases significantly, thereby enhancing the output current. Figure 33 As shown, this improved the permeation power of the nanochannels. These results explain the superior performance of the MXene-CMC composite membrane in permeation power generation. In this application, compared with the pure MXene membrane, the MXene-CMC composite membrane exhibited stronger charge and larger interlayer spacing, successfully achieving two important conditions for improving the output power of the nanochannels. Therefore, its permeation energy conversion performance is significantly better than that of the pure MXene membrane.

[0144] Besides the abundant osmotic energy generated by the confluence of seawater and river water, acidic and alkaline wastewater produced in industrial activities also contains considerable osmotic energy. This resource has great potential and deserves in-depth exploration, extraction, and efficient utilization. Compared with other ions, such as the main ions in seawater, hydrogen ions and hydroxide ions in acids and alkalis have unique characteristics. They undergo a neutralization reaction: "H+" + +OH - →H2O”, which merges into the solvent, causes hydrogen ions on one side of the membrane to selectively permeate to the other side by the composite membrane, such as Figure 34 As shown, it is rapidly consumed by hydroxide ions, thereby causing H... + It can continuously migrate over. This process almost eliminates the polarization effect of hydrogen ions on the opposite side, significantly increasing the net concentration gradient. This differs from other ions, whose actual concentration gradient is smaller than the set concentration gradient due to the enrichment effect of ions on the low-concentration side around the membrane. Furthermore, the diffusion coefficient of hydrogen ions is much greater than that of other ions; therefore, under the same concentration gradient conditions (1 M - 0.001 M), the osmotic current density and power density generated by HCl are significantly higher than those of KCl. Previous studies have determined that, under the same concentration gradient, KCl can generate the largest salinity gradient power generation due to the high diffusion coefficient of K⁺. Figure 35 , 36 As shown, the power of HCl is nearly four times that of HCl. To better understand the difference in energy conversion performance under the two ionic conditions, we calculated the ion mobilities for both. The ion mobilities can be calculated using Einstein relation 136:

[0145] (2);

[0146] in μ It is ion mobility. Di It is the ion diffusion coefficient. q It is the charge number (C) of the ion.k B It is Boltzmann's constant. T It is temperature (K). H + and K + The diffusion coefficients are 9.311 × 10⁻⁶. -5 cm 2 s -1 and 1.957×10 -5 cm 2 s -1 Therefore, H can be calculated according to equation (2). + and K + The mobility rates are respectively μ 25℃ (H + =3.623 × 10 -7 m 2 V −1 s −1 and μ 25℃ (K + = 7.614 × 10⁻⁸ m 2 V −1 s −1 , consistent with the existing value. H + The mobility ratio is higher than K + It's almost an order of magnitude higher, which means H + The flux is much greater than K. + This results in higher power density.

[0147] Subsequently, the energy conversion performance of the composite membrane under different pH gradients was studied. In the experiments, since AgCl electrodes were used, 0.01 M NaCl was added as a supporting solution to the acid and alkaline solutions at each pH to prevent electrode damage due to reaction under alkaline conditions. In several different pH gradients studied, such as... Figure 37 As shown, the open-circuit voltage (VOC) is as high as 256 mV, and the short-circuit current (ISC) reaches 126 μA. Figure 38 As shown, with increasing pH gradient, the load resistance corresponding to the maximum output power density gradually decreases. This is mainly because the increased concentration gradient and neutralization reaction promote ion transport, thereby increasing the ionic conductivity of the membrane. Figure 39 As shown, with a pH gradient of 0-3, the output power density reaches 62.47 W / m², almost three times that of the ANF membrane under the same conditions, and ten times that of the pure MXene membrane. With a pH gradient of 0-14, the output power density is even higher, reaching approximately 200 W / m², far exceeding the value under a pure HCl concentration gradient. This is precisely due to the neutralization reaction's effect on H₂O. +The facilitating effect of transport. In summary, achieving high-performance permeation energy conversion under acidic and alkaline conditions depends not only on the superior performance of the composite membrane itself, but more importantly on the high mobility of hydrogen ions. Furthermore, H... + With OH - The neutralization reaction between ions effectively eliminates ion enrichment due to its unique properties, which is another crucial factor driving high-performance osmotic energy conversion. Neutralization is also a necessary process in acid and alkali wastewater treatment; therefore, extracting osmotic energy from acids and alkalis here can be considered a win-win situation, simultaneously achieving the utilization and treatment of wastewater.

[0148] We have developed a simple, economical, and high-performance two-dimensional composite membrane using a vacuum filtration method. In a natural seawater / simulated river system, this membrane exhibits excellent performance, achieving a power density of 17.10 W / m², far exceeding commercial benchmarks. Under a 500-fold concentration gradient (simulated salt lake / river water) condition, it achieves an even higher power density of 53.75 W / m² for salinity gradient energy harvesting. When applied to salinity gradient energy harvesting of simulated acidic and alkaline wastewater, the power density reaches approximately 200 W / m², thus broadening the membrane's application range. Another major advantage of the prepared composite membrane is its low cost. Although MXene is expensive, its content in the composite membrane is extremely small; while CMC, the main component of the composite membrane, is not only inexpensive and easy to extract and prepare, but its abundant natural reserves make it an excellent environmentally friendly material. The composite membrane synthesized using this environmentally friendly material achieves efficient integrated "utilization-treatment" of acidic and alkaline wastewater, realizing not only high-power-density energy harvesting but also effective wastewater treatment. This application represents a significant step forward in the extraction of osmotic energy from ion gradients and in the treatment and utilization of acidic and alkaline wastewater.

Claims

1. A method for preparing an MXene-CMC composite membrane for salinity gradient energy harvesting, characterized in that, Specifically, the steps include the following: Step 1: Prepare MXene solution; Step 2: Prepare CMC solution; Step 3: Add the MXene solution prepared in Step 1 to the CMC solution prepared in Step 2, and use electromagnetic stirring to fully dissolve it to obtain a mixed dispersion. Step 4: Take a polycarbonate membrane, place it in a vacuum filtration flask, take the mixed dispersion in the vacuum filtration flask and perform vacuum-assisted filtration to obtain a polycarbonate membrane covered with MXene-CMC composite membrane. Step 5: Peel the MXene-CMC composite membrane from the polycarbonate membrane covered with the MXene-CMC composite membrane, filter out the water, and obtain the MXene-CMC composite membrane for salinity gradient energy collection. The specific process of step 1 is as follows: a quantitative amount of MXene dispersion is taken into a reactor and subjected to ultrasonic treatment to obtain an MXene solution; The mass ratio of MXene to CMC is 1:20; The concentration of the MXene solution is 1-5 mg / mL; The ultrasonic treatment time is 4-6 minutes; The specific process of step 2 is as follows: Step 2.1: Take deionized water into the reactor and take a quantitative amount of CMC solid powder; Step 2.2: Dissolve CMC solid powder in deionized water and stir it electromagnetically to ensure complete dissolution, thereby obtaining a CMC solution; The concentration of the CMC solution is 2-10 mg / mL; The electromagnetic stirring time is 24-36 hours; The electromagnetic stirring in step 3 uses an electromagnetic stirrer with a stirring speed of 1000-1500 rpm and a stirring time of 12-24 hours. The vacuum-assisted filtration time in step 4 is 3-7 days.

2. An MXene-CMC composite membrane for salinity gradient energy harvesting, characterized in that, The MXene-CMC composite membrane for salinity gradient energy harvesting was prepared using the method described in claim 1.

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