Preparation Method of Ag-Supported Self-Supported Carbon Film and Its Application as an Electrocatalytic CO2 Reduction Gas Diffusion Electrode

Abstract

The present invention provides a method for preparing an Ag-loaded self-supporting carbon film and its application as a gas diffusion electrode for electrocatalytic CO2 reduction, belonging to the technical field of electrocatalytic CO2 reduction; the Ag-loaded self-supporting carbon film prepared by the preparation method of the present invention has strong mechanical stability and a large number of catalytic reaction active sites, which can effectively improve the electrocatalytic CO2 reduction efficiency; the Ag-loaded self-supporting carbon film can be directly used as a gas diffusion electrode for electrocatalytic CO2 reduction after hydrophobic treatment, solving the problem of limited application of powdery silver-based catalysts in the prior art; through performance testing in a flow cell system, when used as a gas diffusion electrode, this catalyst can achieve a higher current density at a higher Faraday efficiency, and has the ability to operate stably for a long time. At the same time, after a long-time reaction of electrocatalytic CO2 reduction, the electrode material is more convenient to recycle.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalytic CO2 reduction, and specifically relates to a preparation method of an Ag-loaded self-supporting carbon film and its application as a gas diffusion electrode for electrocatalytic CO2 reduction. Background Art

[0002] It has been found that noble metal silver can be used for electrocatalytic reduction of CO2 to CO. Among the developed electrocatalyst materials, silver-based catalysts can still maintain high selectivity at high current densities. However, at present, most of the silver-based catalysts are mainly prepared by impregnating powdery carbon materials with good electrical conductivity such as carbon black or graphene into a silver salt solution, and then obtaining the powdery silver-based catalyst through a reduction step.

[0003] In the actual application process of the powdery silver-based catalyst material, it is necessary to first ultrasonically disperse it in a mixed solution of ethanol and a binder to make a slurry, and then coat it on a hydrophobic carbon paper to make a gas diffusion electrode. The preparation process is complex, and the addition of the binder will hinder electron transfer to a certain extent and affect the electrical conductivity of the electrode. In addition, after the gas diffusion electrode prepared from the powdery catalyst runs in a flow cell for a long time, the gas diffusion layer will be damaged during the high-current catalysis process, causing the catalyst to fall off from the hydrophobic carbon paper, resulting in the loss of active sites; the above problems limit the industrial application and popularization of silver-based catalyst materials in electrocatalytic CO2 reduction with high current and long time. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a preparation method of an Ag-loaded self-supporting carbon film. The Ag-loaded self-supporting carbon film catalyst prepared by this method has excellent electrochemical performance and stable mechanical properties, and can be directly used as a gas diffusion electrode for electrocatalytic CO2 reduction after hydrophobic treatment, solving the problem of limited application of powdery silver-based catalysts in the prior art.

[0005] In order to achieve the above purpose, the present invention is realized by the following technical solutions:

[0006] A preparation method of an Ag-loaded self-supporting carbon film, comprising the following steps:

[0007] (1) Cut the native linden wood into thin slices of a predetermined size along the direction perpendicular to the growth direction, dry the cut thin wood slices and immerse them in an Ag salt solution, shake them in a constant temperature shaker, and vacuum dry overnight to obtain Ag + @NW;

[0008] (2) Place Ag + @NW in a muffle furnace, first perform low-temperature pre-carbonization in an air atmosphere, and then perform high-temperature carbonization in an Ar atmosphere. The carbonized product is subjected to a post-treatment step to obtain Ag@CW;

[0009] (3) Place Ag@CW in a corundum boat, and place the corundum boat in the center of an atmospheric tube furnace. Use high-purity Ar as the protective gas, and switch the gas to CO2 gas during the heat preservation process to obtain Ag@CW-E;

[0010] (4) Place Ag@CW-E in a 0.1 M KHCO3 electrolyte solution, use Ag / AgCl as the reference electrode and a Pt mesh as the counter electrode for potentiostatic polarization to obtain an Ag-loaded self-supporting carbon film Ag@CW-EOR.

[0011] Preferably, in step (1), the pre-carbonization temperature is 250 - 270 °C, the time is 5.5 - 6.5 h, the carbonization temperature is 950 - 1050 °C, the time is 5.5 - 6.5 h, the gas flow rate is 40 mL / min, and the heating rate for both carbonizations is 4 - 5 °C / min.

[0012] Preferably, in step (1), the Ag salt solution is a 0.1 M AgNO3 solution.

[0013] Preferably, in step (1), the shaker oscillation conditions are set as follows: oscillate at 110 - 120 rpm at 25 °C for 11 - 12 h.

[0014] Preferably, in step (2), the post-treatment steps include successively polishing the carbonized product with sandpaper, ultrasonically cleaning it with water and ethanol until clean, and then drying it overnight in a vacuum oven.

[0015] Preferably, in step (3), set the temperature of the tube furnace to 700 - 800 °C, keep it warm for 50 - 70 min, the gas flow rate is 40 mL / min, and the heating rate is 4 - 5 °C / min.

[0016] Preferably, in step (4), the polarization process is as follows: first perform potentiostatic polarization at 2.0 V vs. Ag / AgCl for 110 - 120 s, and then polarize at -0.5 V vs. Ag / AgCl for 590 - 610 s.

[0017] An application of an Ag-loaded self-supporting carbon film prepared by the above method as an electrocatalytic CO2 reduction gas diffusion electrode.

[0018] Preferably, during application, place the Ag-loaded self-supporting carbon film Ag@CW-EOR on a heating plate at 65 - 75 °C, spray a 14 - 16 mg / mL PTFE emulsion on one side of Ag@CW-EOR, and form a hydrophobic layer after drying to obtain an electrocatalytic CO2 reduction gas diffusion electrode.

[0019] As can be seen from the above technical solution, the present invention provides a method for preparing an Ag-loaded self-supporting carbon film and its application as an electrocatalytic CO2 reduction gas diffusion electrode. The beneficial effects are as follows: First, the thin wood chips are dried and then impregnated in an Ag salt solution so that the thin wood chips can fully absorb Ag + , and a shaker is used for oscillation to make Ag + evenly adsorbed in the fibrous pores of the thin wood chips; by subjecting the thin wood chips adsorbed with Ag + to low-temperature pre-carbonization and high-temperature carbonization, Ag + is firmly loaded in the hierarchical pores of the carbon film in the form of Ag elemental substance, and the carbon material after two carbonizations has strong mechanical stability and electrical conductivity; when calcining with high-purity Ar as the protective gas, during the heat preservation process, high-purity Ar is switched to CO2 gas to utilize the etching effect of CO2 gas on the carbon material at high temperature, so that some of the Ag nanoparticles covered by carbon during the carbonization process can be fully exposed, thereby increasing more accessible catalytic active sites; then through constant potential polarization, Ag elemental substance is oxidized to Ag + , and then through low-potential polarization, Ag + is reduced to Ag elemental substance. The catalyst is surface-reconstructed through the polarization process to disperse large-sized Ag nanoparticles into smaller-sized particles, further increasing the number of catalytic reaction active sites. The Ag-loaded self-supporting carbon film prepared by this method has strong mechanical stability and a large number of catalytic reaction active sites, and can effectively improve the electrocatalytic CO2 reduction efficiency.

[0020] When the Ag-loaded self-supporting carbon film is used as an electrocatalytic CO2 reduction gas diffusion electrode, only hydrophobic treatment needs to be carried out on it to make it have hydrophobicity on the gas side and hydrophilicity on the electrolyte side, and then it can be directly used as a gas diffusion electrode without adding a binder and a conductive additive, nor making a slurry for coating, which simplifies the process of using a traditional powdered silver-based catalyst as a gas diffusion electrode, reduces the production cost, and is conducive to large-scale production and application.

[0021] Performance tests were carried out through a flow cell system. The test results show that when the Ag-loaded self-supporting carbon film is used as a gas diffusion electrode, a higher current density can be achieved at a higher Faraday efficiency, and after a long-term stability test, it still has a high current density, proving its ability to operate for a long time. In addition, the Ag-loaded self-supporting carbon film itself is a catalyst, and it is more convenient to recycle the electrode material after a long-term electrocatalytic carbon dioxide reduction reaction. Description of the Drawings

[0022] Figure 1 is an SEM picture of the Ag-loaded self-supporting carbon film.

[0023] Figure 2It is the XRD picture of the Ag-loaded self-supporting carbon film.

[0024] Figure 3 It is the XPS picture of the Ag-loaded self-supporting carbon film.

[0025] Figure 4 It is the test result of the contact angle after the hydrophobic treatment of the Ag-loaded self-supporting carbon film.

[0026] Figure 5 It is the test result of the catalytic performance of the Ag-loaded self-supporting carbon film used as a gas diffusion electrode for electrocatalytic CO2 reduction.

[0027] Figure 6 It is the test result of the catalytic stability of the Ag-loaded self-supporting carbon film used as a gas diffusion electrode for electrocatalytic CO2 reduction. Detailed implementation manners

[0028] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be emphasized that the specific embodiments described herein are only for better explaining the present invention, which are partial embodiments of the present invention rather than all embodiments, so they are not used to limit the present invention.

[0029] The present invention provides a preparation method of an Ag-loaded self-supporting carbon film, comprising the following steps:

[0030] (1) Cut the native linden wood along the direction perpendicular to the growth direction into thin slices of a predetermined size, dry the cut thin wood slices and then immerse them in an Ag salt solution, shake them in a constant temperature shaker and dry them overnight in a vacuum to obtain Ag + @NW;

[0031] In a specific implementation manner, cut the native linden wood along the direction perpendicular to the growth direction into thin slices of 3.5 cm × 2.2 cm × 2.0 mm, dry them in a blast drying oven at 80 °C for 12 h. Immerse the dried thin wood slices in a 0.1 M AgNO3 solution, shake them in a constant temperature shaker at 25 °C and 120 rpm for 12 h, and dry them overnight in a vacuum drying oven at 60 °C to obtain thin wood slices uniformly adsorbed with silver ions, that is, Ag + @NW.

[0032] (2) First perform low-temperature pre-carbonization on Ag + @NW in an air atmosphere, and then perform high-temperature carbonization in an N2 atmosphere. The carbonized product is subjected to a post-treatment step to obtain Ag@CW;

[0033] In the specific implementation process, Ag +@NW was placed in a muffle furnace, set at 260°C, and pre-carbonized for 6 hours in an air atmosphere, then carbonized for 6 hours in a tubular furnace with an Ar atmosphere set at 1000°C, with a N2 flow rate set at 40 mL / min, and a heating rate of 5°C / min for both steps. The post-treatment steps included sanding the carbonized product with sandpaper, ultrasonically cleaning with water and ethanol, and then drying it in a vacuum oven overnight, with the oven temperature set at 60°C. The wood chips were carbonized into hierarchical porous carbon materials through two carbonization processes, which had a high degree of crystallinity and good electrical conductivity. At the same time, Ag + During the carbonization process, Ag is firmly loaded in the multi-level pores of the carbon film in the form of a single substance.

[0034] (3) Ag@CW is placed in a corundum boat, which is placed in the center of an atmosphere tube furnace. High-purity Ar is used as the protective gas. During the insulation process, the gas is switched to CO2 gas to obtain Ag@CW-E.

[0035] Since some Ag nanosilver particles are covered by carbon in the Ag@CW material, the catalytic activity of Ag@CW is low. In a specific embodiment, the Ag@CW flakes are placed in a corundum boat, which is placed in the center of an atmosphere tube furnace. High-purity Ar is used as a protective gas, the gas flow rate is set to 40 mL / min, the tube furnace temperature is set to 750°C, and the temperature is kept for 1 hour. During the insulation process, the gas is switched to CO2 gas, and the heating rate is 5°C / min. By switching high-purity Ar to CO2 gas during the insulation process, CO2 reacts with carbon at high temperature, so that some Ag nanoparticles covered by carbon during the carbonization process can be fully exposed, thereby increasing more accessible catalytic active sites.

[0036] (4) Ag@CW-E was placed in a 0.1M KHCO3 electrolyte solution, with Ag / AgCl as the reference electrode and Pt mesh as the counter electrode, and polarization was performed to obtain Ag-loaded self-supporting carbon film Ag@CW-EOR with high catalytic activity.

[0037] In order to further increase the number of active sites in the catalytic reaction, Ag@CW-E was placed in 0.1M KHCO3 solution, with Ag / AgCl as the reference electrode and Pt mesh as the counter electrode, and polarized at 2.0 V vs. Ag / AgCl for 120 s to oxidize Ag into Ag. + , and then polarized at -0.5Vvs.Ag / AgCl for 600s to make Ag + After being reduced to single substance Ag, the surface of Ag@CW-E was reconstructed through a polarization process, so that large-sized Ag nanoparticles were dispersed into smaller particles, which effectively increased the number of active sites for the catalytic reaction and obtained a Ag-loaded self-supporting carbon film with high catalytic activity, namely, Ag@CW-EOR.

[0038] See Figure 1 , the morphology of Ag@CW-EOR was analyzed by SEM. Figure 1 In Figure 1 a and Figure 1 b, it can be seen that Ag@CW-EOR has a three-dimensional porous structure with many open and interconnected micropores. The pore diameter is in the range of 2 - 50 μm. This hierarchical porous structure is beneficial for rapid mass transfer and provides a large specific surface area, thus being able to provide more accessible catalytic active sites. Figure 1 c and d show that there are many nanoparticles evenly distributed in the pore channels of the carbon film.

[0039] See Figure 2 , the crystal structure of the sample was analyzed by X-ray diffraction (XRD). The results show that the diffraction peaks of Ag@CW-EOR are completely consistent with the standard card of elemental Ag, indicating that the silver nanoparticles are successfully loaded on the carbon substrate.

[0040] See Figure 3 , X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemical composition and valence state of Ag@CW-EOR. It can be seen from the Ag3d fine spectrum that the binding energies of the 3d 3 / 2 and 3d 5 / 2 of Ag species are located at 374.15 eV and 368.14 eV, which perfectly match the binding energy position of elemental silver.

[0041] Since the Ag@CW-EOR electrode itself is a hydrophilic material, in order to construct a normal three-phase interface and avoid the occurrence of the "flooding" phenomenon, the flow cell system requires the gas diffusion electrode to be hydrophobic on the gas flow side. Therefore, the prepared Ag-loaded self-supporting carbon film Ag@CW-EOR was placed on a heating plate at 65 - 75 °C, and a PTFE emulsion with a concentration of 14 - 16 mg / mL was sprayed on one side of the whole electrode. After drying, a hydrophobic layer was formed to obtain the gas diffusion electrode.

[0042] In the specific implementation, the Ag@CW-EOR with a specification of 2.5 cm × 1.0 cm × 1.0 mm was placed on a heating plate at 70 °C, and a 15 mg / mL PTFE emulsion was sprayed on one side of the whole electrode. Each spraying was 50 μL. After drying, the next spraying was carried out. A total of 0.5 mL was sprayed and dried to form a hydrophobic layer. Then, a hydrophobic carbon paper was attached to the gas side as the gas diffusion layer, and then directly assembled into a flow cell for testing. During the flow cell test, 0.5 M KHCO3 was used as the electrolyte, and Nifoam and Ag / AgCl electrodes were used as the counter electrode and reference electrode respectively. The anode and cathode were separated by a proton exchange membrane.

[0043] See Figure 4, the contact angle test was carried out on the hydrophobically treated Ag@CW-EOR. The Ag@CW-EOR sample was placed on a horizontal glass plate, and a drop of 0.5M KHCO3 was vertically dropped from a syringe. After standing for 30 s, the contact angle (the angle at the solid-liquid-gas three-phase interface) was calculated after the droplet state was completely stable. As Figure 4 shown in a, the droplet angle was 134°, showing strong hydrophobicity, indicating that Ag@CW-EOR can meet the hydrophobic requirements on the gas side of the gas diffusion electrode; Figure 4 b shows the side without hydrophobization treatment, and the angle was 27°, indicating good hydrophilicity and good mass transfer with the electrolyte. The Ag@CW-EOR gas diffusion electrode fully meets the conditions of the gas diffusion electrode.

[0044] See Figure 5 , the Ag@CW-EOR gas diffusion electrode was successfully assembled into a flow cell system for testing. In the figure, FE co is the CO Faraday efficiency, and j co is the CO current density. It can be seen that for Ag@CW-EOR at -0.54 V vs. RHE, the CO Faraday efficiency is as high as 85.46%. As the potential increases, the CO current density increases accordingly. For Ag@CW-EOR at -1.31 V vs. RHE, the CO current density is as high as 186.7 mA·cm -2 , and at this time the CO Faraday efficiency still reaches 70.35%. This indicates that Ag@CW-EOR has high electrocatalytic carbon dioxide reduction performance.

[0045] See Figure 6 , a stability test of the current density of the Ag@CW-EOR gas diffusion electrode at -1.27 V vs. RHE for 8 h was carried out. j Total is the total current density, FE co is the CO Faraday efficiency, and FE H2 is the H2 Faraday efficiency. As the polarization time prolongs, the total current density increases from 255.7 mA·cm -2 to 336.4 mA·cm -2 , and the retention rate of the CO Faraday efficiency is still 69.8%, which can prove its ability to operate for a long time and good stability.

[0046] The reason for the high activity of the Ag@CW-EOR is that the carbon film prepared by two-step carbonization has a well-defined hierarchical porous structure. This hierarchical porous structure is conducive to rapid mass transfer and provides a large surface area, enabling more silver nanoparticles, which serve as accessible catalytic active sites, to be loaded within the pores of the carbon film. By switching high-purity Ar to CO2 gas during the heat preservation process and utilizing the etching effect of CO2 gas on carbon materials at high temperatures, some of the Ag nanoparticles covered by carbon during the carbonization process can be fully exposed, thereby increasing more accessible catalytic active sites. In addition, surface reconstruction of the catalyst is carried out through potentiostatic polarization treatment to disperse large-sized Ag nanoparticles into smaller-sized particles, further increasing the number of catalytic reaction active sites. Through the combination of the above treatments, abundant Ag active sites are provided for the electrocatalytic reduction of CO2, showing a high Faraday efficiency and current density for CO products.

[0047] The foregoing disclosure is only for the preferred embodiments of the present invention. Of course, it cannot be used to limit the scope of the rights of the present invention. Those of ordinary skill in the art can understand the entire or part of the processes of implementing the above embodiments, and the equivalent changes made according to the claims of the present invention still fall within the scope covered by the invention.

Claims

1. A method for preparing an Ag-loaded self-supporting carbon film, characterized in that: It includes the following steps: (1) Cut the native basswood into thin slices of a predetermined size along the direction perpendicular to the growth direction. After drying the cut thin wood slices, immerse them in an Ag salt solution, oscillate them with a constant temperature shaker, and vacuum dry them overnight to obtain Ag + @NW; (2) Place Ag + @NW in a muffle furnace. First, perform low-temperature pre-carbonization in an air atmosphere, and then perform high-temperature carbonization in an Ar atmosphere. The carbonized product is subjected to a post-treatment step to obtain Ag@CW; (3) Place Ag@CW in a corundum boat, and place the corundum boat in the center of an atmospheric tube furnace. Use high-purity Ar as the protective gas, and switch the gas to CO2 gas during the heat preservation process to obtain Ag@CW-E; (4) Place Ag@CW-E in a 0.1M KHCO3 electrolyte solution, use Ag / AgCl as the reference electrode, and a Pt mesh as the counter electrode to perform potentiostatic polarization to obtain an Ag-loaded self-supporting carbon film Ag@CW-EOR; Among them, in the step (1), the pre-carbonization temperature is 250 - 270 °C, the time is 5.5 - 6.5 h, the high-temperature carbonization temperature is 950 - 1050 °C, the time is 5.5 - 6.5 h, the gas flow rate is 40 mL / min, and the heating rate for both carbonizations is 4 - 5 °C / min; in the step (4), the polarization process is: first perform potentiostatic polarization at 2.0 V vs. Ag / AgCl for 110 - 120 s, and then polarize at -0.5 V vs. Ag / AgCl for 590 - 610 s.

2. The preparation method of the Ag-loaded self-supporting carbon film according to claim 1, characterized in that: In the step (1), the Ag salt solution is a 0.1M AgNO3 solution.

3. The preparation method of the Ag-loaded self-supporting carbon film according to claim 1, characterized in that: In the step (1), the setting conditions of the shaker oscillation are: oscillate at 110 - 120 rpm at 25 °C for 11 - 12 h.

4. The preparation method of the Ag-loaded self-supporting carbon film according to claim 1, characterized in that: In the step (2), the post-treatment steps include successively polishing the carbonized product with sandpaper, ultrasonically cleaning it with water and ethanol, and then drying it overnight in a vacuum oven.

5. The preparation method of the Ag-loaded self-supporting carbon film according to claim 1, characterized in that: In the step (3), set the temperature of the tube furnace to 700 - 800 °C, keep it warm for 50 - 70 min, the gas flow rate is 40 mL / min, and the heating rate is 4 - 5 °C / min.

6. Application of an Ag-loaded self-supporting carbon film prepared by the method according to claim 1 as an electrocatalytic CO2 reduction gas diffusion electrode.

7. Use of the Ag-loaded self-supporting carbon film as an electrocatalytic CO2 reduction gas diffusion electrode according to claim 6, characterized in that: During application, place the Ag-loaded self-supporting carbon film Ag@CW-EOR on a heating plate at 65 - 75 °C, spray a 14 - 16 mg / mL PTFE emulsion on one side of Ag@CW-EOR, and form a hydrophobic layer after drying to obtain an electrocatalytic CO2 gas diffusion electrode.

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