Nanofiber membrane for CO2 reduction as well as preparation method and application of nanofiber membrane

Abstract

The invention provides a nanofiber membrane for CO2 reduction and a preparation method and application thereof, and belongs to the field of CO2 electrocatalytic reduction. The nanofiber membrane is of a porous structure, the average pore size of the nanofiber membrane is 1.8-2.5 microns, and the porosity of the nanofiber membrane is 42%-50%. And the thickness of the nanofiber membrane is 20-60 [mu] m. The nanofiber membrane is of a three-dimensional network structure which is formed by building a plurality of superfine fibers and is filled with nanoscale pores, and in the CO2 electroreduction process, the ethylene Faraday efficiency and the product selectivity can be remarkably improved. The invention also provides a preparation method of the nanofiber membrane, the porous nanofiber membrane is obtained by using natural cellulose through a mechanical dissociation method and optimizing the membrane thickness and the interface microenvironment, the porous nanofiber membrane has good ionic conductivity and gas barrier ability, and the ethylene Faraday efficiency can reach more than 70% under 700 mA cm <-2 >, which is improved by more than 10% compared with a traditional anion exchange membrane system.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic reduction of CO2, specifically relating to a nanofiber membrane for CO2 reduction, its preparation method, and its application. Background Technology

[0002] Traditional CO2RR technology typically requires the use of a high concentration of alkali metal cations in the catholyte to suppress hydrogen evolution side reactions. However, this leads to the dissolution of liquid products from CO2RR, such as formic acid, acetic acid, and ethanol, in the catholyte, mixing with the high concentration of alkali metal cations. This necessitates additional, energy-intensive operating units to separate the liquid products from the metal cations to obtain a product with high purity. Electrochemical CO2 reduction devices equipped with solid electrolytes can solve the problem of mixing liquid products with high concentrations of alkali metal cations. This results in the use of anion exchange membranes such as QAPPT and X37-50 in CO2RR. These anion exchange membranes are based on petroleum-based materials, are expensive, and have insufficient ion conductivity. Electrochemical CO2 reduction devices equipped with porous membranes can solve the problem of low ion conductivity.

[0003] Typical alkaline electrolyte electrochemical carbon dioxide reduction devices require an anion exchange membrane (AEM) to be coated on the cathode. Previous research has largely focused on using polymer ion exchange membranes. For example, invention patent CN107266629A discloses a method for preparing a polymer anion exchange membrane, including: thoroughly mixing a dicationic imidazole salt with a base membrane monomer, an initiator, and a crosslinking agent, stirring the mixture under heating until it becomes a viscous liquid, and then polymerizing it under ultraviolet light to form a membrane. This method overcomes the problem of dicationic imidazole salt precipitation and macroscopic phase separation during membrane formation, ensuring a high content of dicationic imidazole salt and uniform membrane formation.

[0004] For example, invention patent CN115785506A discloses the preparation and application of a cross-linked quaternized polybenzimidazole anion exchange membrane. The method involves dissolving a grafted modified quaternized polybenzimidazole precursor in dimethyl sulfoxide, adding dichloroalkane to react, and obtaining a viscous casting solution. This solution is poured onto a dry and clean glass plate, cast into a film, and dried. Finally, the obtained cross-linked quaternized polybenzimidazole anion exchange membrane is immersed in anhydrous ethanol, washed, and dried. The dried cross-linked quaternized polybenzimidazole anion exchange membrane is then immersed in KOH solution, washed with deionized water, and dried before being used as a membrane electrode diaphragm in the electrocatalytic reduction of carbon dioxide. This method can improve the ionic conductivity of the anion exchange membrane and the Faradaic efficiency of carbon monoxide.

[0005] However, traditional polymer ion exchange membranes are derived from petroleum-based raw materials, which does not meet the goals of sustainable development; moreover, they have low ion conduction efficiency, poor energy utilization efficiency, and are expensive, which is not conducive to large-scale application.

[0006] Therefore, developing a green and biodegradable ion exchange membrane to improve gas cross-diffusion and increase CO2 utilization is an urgent problem to be solved. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides a nanofiber membrane for CO2 reduction, its preparation method, and its application. Using natural cellulose, a porous nanofiber membrane is obtained through mechanical dissociation and optimization of membrane thickness and interfacial microenvironment. This membrane is used in the CO2 electrocatalytic reduction to ethylene system to improve the ethylene Faradaic efficiency and product selectivity, while also being environmentally friendly.

[0008] The technical solution adopted by this application to solve the above problems is as follows: This invention provides a nanofiber membrane for CO2 reduction, wherein the nanofiber membrane has a porous structure, an average pore size of 1.8-2.5 μm, and a porosity of 42%-50%. Furthermore, the thickness of the nanofiber membrane is 20-60 μm.

[0009] Nanofiber membranes are composed of multiple extremely fine fibers and have a three-dimensional network structure filled with nanoscale pores. In the CO2 electroreduction process, they can provide a huge specific surface area and an ideal channel structure, so as to efficiently carry out the CO2 electroreduction reaction.

[0010] During the CO2 electroreduction process, hydroxyl ions are generated at the cathode-catalyst interface and diffuse freely to the anode interface through the membrane. The porous structure of the nanofiber membrane facilitates the accumulation of hydroxyl ions at the reaction interface during free diffusion. In an alkaline microenvironment, the nanofiber membrane of the aforementioned thickness range promotes the formation of C-C coupling and prevents hydroxyl ions from occupying metal active sites, which could lead to hydrogen evolution reaction under high current and reduce the Faradaic efficiency of ethylene, thus significantly improving the Faradaic efficiency of ethylene.

[0011] On the other hand, the present invention also provides a method for preparing a nanofiber membrane for CO2 reduction, comprising the following steps: (1) Disperse natural cellulose in a solvent and then stir to break the cell wall to obtain a precursor solution; (2) The precursor solution was milled to obtain a cellulose dispersion; (3) The cellulose dispersion was ultrasonically filtered under vacuum to obtain a thin film, and the thin film was freeze-dried to obtain a nanofiber membrane.

[0012] This invention uses natural cellulose as raw material and employs a mechanical dissociation method involving impact, shearing, and friction during cell wall disruption and stirring to initially break up and shorten entangled natural cellulose fiber bundles. These bundles are then fully swollen and dispersed in a solvent. Further mechanical dissociation involves sand milling to apply intense shearing, compression, and friction to the cellulose, thereby stripping the micron-sized fiber bundles into nano-sized fibers. This process requires no chemical modification and creates multi-level channels from nanometer to micrometer scales, facilitating rapid diffusion and transport of CO2 gas, timely desorption of reaction products, and preventing blockage of active sites.

[0013] Preferably, the natural cellulose in step (1) includes one or more of wood flour and bamboo flour.

[0014] Preferably, the solvent in step (1) is deionized water.

[0015] Preferably, the mass ratio of natural cellulose to solvent in step (1) is 1:49-149.

[0016] Preferably, the stirring speed in step (1) is 20,000-30,000 r / min, and the stirring time is 5-10 min.

[0017] Within the above parameter range, the cell wall breaking stirring provides sufficient impact and shear force, which can effectively break the hydrogen bonds and physical entanglement between fibers, and avoid excessive heat generation that could lead to cellulose degradation or excessive molecular chain breakage.

[0018] Preferably, the grinding speed in step (2) is 2500-3500 r / min and the grinding time is 30-60 min.

[0019] High-intensity and long-term sand milling within the above parameter range generates shear force sufficient to overcome the strong hydrogen bonds between cellulose molecules, thereby splitting the fibers longitudinally and producing cellulose nanofibers with high aspect ratio and uniform size to the maximum extent, achieving deep nano-sizing.

[0020] Preferably, the mass concentration of the cellulose dispersion in step (2) is 0.5-2 wt%.

[0021] Preferably, the volume of the cellulose dispersion during ultrasonication in step (3) is 40-100 mL.

[0022] For the CO2 reduction reaction, the reactant (CO2) needs to diffuse into the active sites inside the membrane, and the product also needs to diffuse out from the inside. Nanofiber membranes obtained using cellulose dispersions within the aforementioned range exhibit suitable internal pore channels, balanced mass transfer resistance and number of active sites, significantly improving internal catalyst utilization and overall reaction efficiency. Furthermore, adjusting the cellulose dispersion content within the aforementioned range can further regulate the pore structure inside the nanofiber membrane, resulting in a larger specific surface area and more pore space, enabling the loading of a large amount of catalyst and providing abundant channels for gas and mass transport, leading to rapid mass transfer.

[0023] Preferably, the ultrasonic power in step (3) is 500-600 W and the ultrasonic time is 20-40 min.

[0024] Preferably, the freeze-drying temperature in step (3) is -10~-20℃, and the freeze-drying time exceeds 24-96h.

[0025] On the other hand, the present invention also provides the application of the nanofiber membrane for CO2 reduction described above in the electrocatalytic reduction of CO2 to olefins, wherein the nanofiber membrane serves as the membrane electrode diaphragm in the electrocatalytic reduction of CO2.

[0026] Preferably, the membrane electrode is prepared by: adding copper oxide to deionized water, ethanol and Nafion solution to obtain ink, uniformly drop-coating the ink onto carbon paper as a cathode; using an IrO2 / Ti electrode as an anode; and assembling the cathode, the nanofiber membrane and the anode to obtain the membrane electrode.

[0027] More preferably, the volume ratio of the deionized water, ethanol, and Nafion solution is 30:20:1; and the mass ratio of the deionized water to copper oxide is 3:5.

[0028] More preferably, the copper oxide loading is 2.0 mg / cm³. 2 .

[0029] Preferably, 1 M KOH is used as the electrolyte. The electrolyte is delivered to the anode by a peristaltic pump and flows. Dry CO2 is humidified by a gas humidification device and delivered to the cathode. Then, a voltage is applied to perform electrolysis, which promotes the electrocatalytic reduction of CO2. The flow rate of CO2 is 60 sccm.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. The nanofiber membrane provided in this application has a porous structure, composed of multiple extremely fine fibers, and is filled with nanoscale pores. During the CO2 electroreduction process, it can provide a huge specific surface area and an ideal channel structure. Based on this thickness, the porous nanofiber membrane can efficiently support and carry out the CO2 electroreduction reaction, and has good ion conductivity and gas barrier properties at 700 mA cm⁻¹. -2 Under these conditions, the ethylene faradaic efficiency can reach over 70%, which is more than 10% higher than that of traditional anion exchange membrane systems.

[0031] 2. This application uses natural cellulose as raw material, which is biodegradable. Based on the mechanical dissociation method, it first performs preliminary dissociation by breaking the cell wall and stirring, and then achieves deep nano-sizing by sand milling, forming multi-level channels from nano to micro scale. This is conducive to the rapid diffusion and transport of CO2 gas, timely desorption of reaction products, and prevention of blockage of active sites. The preparation method can be widely applied to electrolysis systems of different scales in membrane electrode reactors.

[0032] 3. The nanofiber membrane prepared in this application is inexpensive, with raw material costs reduced by more than 99%. Attached Figure Description

[0033] Figure 1 This is a scanning electron microscope (SEM) image of the nanofiber membrane surface in Example 1 of the present invention.

[0034] Figure 2 This is a scanning electron microscope (SEM) image of the cross-section of the nanofiber membrane in Example 1 of the present invention.

[0035] Figure 3 This is the electrochemical CO2 reduction data from Example 1 of the present invention.

[0036] Figure 4 This is the electrochemical CO2 reduction data for Example 2 of the present invention.

[0037] Figure 5 This is the electrochemical CO2 reduction data for Example 3 of the present invention.

[0038] Figure 6 This is the electrochemical CO2 reduction data for Example 4 of the present invention.

[0039] Figure 7 This is the electrochemical CO2 reduction data for Comparative Example 1 of this invention.

[0040] Figure 8 This is the electrochemical CO2 reduction data for Comparative Example 2 of this invention.

[0041] Figure 9 This is the electrochemical CO2 reduction data for Comparative Example 3 of this invention.

[0042] Figure 10 This is the electrochemical CO2 reduction data for Comparative Example 4 of this invention. Detailed Implementation

[0043] The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products. The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples.

[0044] Example 1 The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products. The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples.

[0045] Example 1 Preparation of nanofiber membranes: First, hardwood pulp board was mixed with deionized water at a mass ratio of 1:99 (5 g of hardwood pulp was used to prepare 500 g of aqueous suspension). Then, the mixture was pulped using a high-speed blender at 30,000 r / min for 10 minutes to obtain pulp. The obtained pulp was then milled in a sand mill at 2800 r / min for 40 minutes. Subsequently, 80 mL of the 1 wt% dispersion was ultrasonicated at 600 W for 30 minutes (in an ice bath, divided into three 10-minute intervals), and vacuum filtered to form a membrane. The obtained membrane was frozen at -20°C overnight and then dried in a freeze dryer for 24 hours to obtain the nanofiber membrane.

[0046] Microstructure characterization of nanofiber membranes: such as Figure 1 As shown, SEM images of the nanofiber membrane surface reveal a multi-scale network structure formed by interwoven nanofibers. The loosely arranged fibers provide ion conduction channels and gas barrier capabilities for the membrane material. The visible light and dark areas in the image correspond to the fiber bundles and the pores between the fibers, respectively. Figure 2 As shown, SEM images of the nanofiber membrane cross-section reveal a porous structure formed by the stacking of nanofibers along the thickness direction of the membrane. Numerous micropores and mesopores are formed between the fibers, which helps to create a localized alkaline microenvironment and stabilize the CO2 reduction reaction interface. Figure 2 The upper and lower boundaries correspond to the upper and lower surfaces of the membrane, respectively; and are composed of Figure 2 It can be seen that the thickness of the nanofiber membrane prepared in Example 1 is 50 μm.

[0047] CO2 electroreduction test: Inks were prepared by dispersing 10 mg of commercial copper oxide catalyst powder in a mixture of 600 μL deionized water, 400 μL ethanol, and 20 μL 5wt% Nafion solution. These inks were prepared at 2.0 mg / cm³. 2 The loading was uniformly sprayed onto 28BC carbon paper as the cathode electrode. Electrochemical reduction was carried out in the membrane electrode assembly. The cathode, the nanofiber membrane, and the anode were assembled using an IrO2 / Ti electrode as the anode to form the membrane electrode. The cathode, anode, and diaphragm were assembled using an end plate with a serpentine flow path and secured with screws under a clamping pressure of 0.5 MPa. The CO2 feed gas was humidified using a humidifier filled with deionized water. The gas flow rate was set to 60 sccm. 1M KOH anolyte was circulated using a peristaltic pump. Electrochemical CO2 reduction tests were conducted at different current densities using the constant current mode of a CHI660 workstation. The gaseous products were quantitatively determined by gas chromatography, and the Faraday efficiency was calculated using the following formula: FE = znF / Q × 100%, where z is the number of electrons transferred in each product molecule (e.g., 2 for CO, 12 for C2H4), n is the molar amount of the product (mol), F is the Faraday constant (96485 C / mol), and Q is the total charge transferred during CO2 reduction (C). The test results are as follows: Figure 3 As shown in Table 1.

[0048] Example 2 Example 2 is the same as Example 1, except that 40 mL of a 1 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 22 μm. The CO2 electroreduction test results are as follows... Figure 4 As shown in Table 1.

[0049] Example 3 Example 3 is the same as Example 1, except that 64 mL of a 1 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 38 μm. The CO2 electroreduction test results are as follows... Figure 5 As shown in Table 1.

[0050] Example 4 Example 4 is the same as Example 1, except that 96 mL of a 1 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 60 μm. The CO2 electroreduction test results are as follows... Figure 6 As shown in Table 1.

[0051] Comparative Example 1 Comparative Example 1 is the same as Example 1, except that a commercially available QAPPT anion exchange membrane is used instead of the nanofiber membrane. The CO2 electroreduction test results are as follows: Figure 7 As shown in Table 1.

[0052] Comparative Example 2 Comparative Example 2 is the same as Example 1, except that 160 mL of a 1 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 109 μm. The CO2 electroreduction test results are as follows... Figure 8 As shown in Table 1.

[0053] Comparative Example 3 Comparative Example 3 is the same as Example 1, except that 80 mL of a 3 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 52 μm. The CO2 electroreduction test results are as follows... Figure 9 As shown in Table 1.

[0054] Comparative Example 4 Comparative Example 4 is the same as Example 1, except that 80 mL of 0.3 wt% dispersion was used in the preparation of the nanofiber membrane to obtain a nanofiber membrane with a thickness of 50 μm. The CO2 electroreduction test results are as follows... Figure 10 As shown in Table 1.

[0055] Table 1 Catalytic performance of the membrane electrodes provided in the examples and comparative examples

[0056] from Figures 3-10 As shown in Table 1, the differences in ethylene selectivity of different membrane materials under the same test conditions indicate the performance advantage of nanofiber membranes in promoting CC coupling reaction when the thickness is appropriate. In addition, as the membrane thickness continues to increase, the mass transfer resistance at the electrolysis interface will also increase, and the product selectivity will also change.

[0057] Detection Example 1 Mercury intrusion porosimetry (MIP) was used to determine the pore structure information. The prepared nanofiber membrane was cut to a size matching the sample chamber and dried at 60 °C for 12 h to remove adsorbed water. The dried sample was placed in the sample chamber of the mercury intrusion porosimeter and connected to the pressure control system. Before testing, the sample chamber was evacuated to a vacuum level below 50 μm Hg to ensure smooth mercury entry into the sample pores during the test. During the test, the instrument applied external pressure to the nanofiber membrane in stages, gradually forcing mercury into different pores. Initially, macropore structure data was obtained in the low-pressure region (0-30 psia), followed by recording the mercury intrusion volume changes in micropores and mesopores in the high-pressure region (up to 60,000 psia). The instrument automatically collected the mercury intrusion volume at different pressures and plotted pressure-volume curves. After the test, the instrument software calculated parameters such as pore size distribution, median volumetric pore size, median area pore size, total mercury intrusion volume, and total specific surface area using the Washburn equation. Simultaneously, the bulk density was calculated by recording the volume change of the sample at 0.52 psia; the apparent (skeleton) density of the material was obtained by combining the maximum mercury expulsion volume during mercury intrusion porosimetry; and the porosity was estimated from the ratio of bulk density to skeleton density. Typical pore structure information of the nanofiber membrane can be obtained through the above methods, as shown in Table 2, including average pore size and porosity, providing a basis for evaluating its porous structure and interfacial behavior in the electrocatalytic process.

[0058] Table 2

[0059] Table 2 lists the typical pore structure and physical properties of the nanofiber membrane. The average pore size calculated using the 4V / A method can be used to comprehensively reflect the pore size of the material. Porosity represents the proportion of voids within the material. As shown in Table 2, the nanofiber membrane provided by this invention possesses typical high porosity, a porous network, and a suitable pore size distribution, which is beneficial for forming a localized alkaline microenvironment and improving the performance of CO2 electroreduction reactions. Combined with Table 1, it can be seen that under the conditions of an average pore size of 1.8-2.5 μm and a porosity of 42%-50%, when the membrane thickness is controlled at 50 μm, the nanofiber membrane exhibits higher ethylene selectivity under higher current density conditions, demonstrating the best overall performance.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A nanofiber membrane for CO2 reduction, characterized by, The nanofiber membrane has a porous structure with an average pore size of 1.8-2.5 μm and a porosity of 42%-50%. Furthermore, the thickness of the nanofiber membrane is 20-60 μm.

2. The method of claim 1, wherein the nanofiber membrane is prepared by electrospinning. Includes the following steps: (1) Disperse natural cellulose in a solvent and then stir to break the cell wall to obtain a precursor solution; (2) The precursor solution was milled to obtain a cellulose dispersion; (3) The cellulose dispersion was ultrasonically filtered under vacuum to obtain a thin film, and the thin film was freeze-dried to obtain a nanofiber membrane.

3. The production method according to claim 2, characterized by, In step (1), the mass ratio of natural cellulose to solvent is 1:49-149.

4. The preparation method according to claim 2, characterized in that, In step (1), the stirring speed is 20,000-30,000 r / min, and the stirring time is 5-10 min.

5. The preparation method according to claim 2, characterized in that, In step (2), the grinding speed is 2500-3500 r / min and the grinding time is 30-60 min.

6. The preparation method according to claim 2, characterized in that, The mass concentration of the cellulose dispersion in step (2) is 0.5-2 wt%.

7. The preparation method according to claim 2, characterized in that, In step (3), the volume of the cellulose dispersion during ultrasonication is 40-100 mL.

8. The preparation method according to claim 2, characterized in that, The ultrasonic power in step (3) is 500-600 W, and the ultrasonic time is 20-40 min; The freeze-drying temperature is -10~-20℃, and the freeze-drying time is 24-96 h.

9. The application of the nanofiber membrane according to claim 1 in the electrocatalytic reduction of CO2 to olefins, characterized in that, The nanofiber membrane serves as the diaphragm for the membrane electrode in the electrocatalytic reduction of CO2.

10. The application according to claim 9, characterized in that, The membrane electrode is prepared by adding copper oxide to deionized water, ethanol and Nafion solution to obtain ink, and uniformly dripping the ink onto carbon paper as a cathode; using an IrO2 / Ti electrode as an anode; and assembling the cathode, the nanofiber membrane and the anode to obtain the membrane electrode.

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