Self-enhanced concentration gradient effect catalyst for electrocatalytic reduction of low-concentration CO2 and preparation method of self-enhanced concentration gradient effect catalyst

Abstract

The invention belongs to the field of preparation of nano carbon material-based electrocatalysts, and relates to a self-enhanced concentration gradient effect catalyst for electrocatalytic reduction of low-concentration CO2 and a preparation method of the self-enhanced concentration gradient effect catalyst for electrocatalytic reduction of low-concentration CO2. The carbon-based monatomic catalyst is prepared in situ by using a chemical vapor deposition method. According to the present invention, the catalyst has the ordered morphology that the metal monatomic carbon nano-tube is vertically grown on the porous carbon nano-sheet, the high CO selectivity under the low-concentration CO2RR condition can be achieved, and the length of the carbon nano-tube can be accurately controlled by regulating the growth time so as to optimize the self-enhancement CO2 concentration gradient effect and improve the electro-catalysis low-concentration CO2 reduction performance;

Description

Technical Field

[0001] The present invention belongs to the field of preparation of nano-carbon material-based electrocatalysts, and specifically relates to a self-enhanced concentration gradient effect catalyst for electrocatalytic reduction of low-concentration CO2 and a preparation method thereof. Background Art

[0002] Electrocatalytic carbon dioxide reduction technology (CO2RR) is a technology that uses electricity generated by renewable energy to convert CO2 into high-value-added chemicals or fuels under mild conditions. However, linear CO2 molecules are highly chemically inert and there is a serious competitive hydrogen evolution reaction, which leads to challenges such as sluggish reaction kinetics and high overpotential for the electrocatalytic CO2 reduction reaction. The development of various high-efficiency catalysts has effectively improved the selectivity and activity of CO2 catalytic conversion. However, electrocatalytic CO2 reduction is currently mainly carried out in a high-purity CO2 (>99%) atmosphere. The utilization of this high-purity CO2 requires a complex capture-regeneration-transport process. The amines and alkaline reagents used in this process are prone to corrosion of the equipment and require a large amount of heat energy. There are problems such as high cost, high energy consumption, and easy to cause secondary pollution. Therefore, directly using the CO2 contained in the flue gas emitted by the factory for electrocatalytic conversion can achieve carbon emission reduction goals while obtaining higher economic benefits.

[0003] However, the CO2 concentration in industrial flue gas is only about 3% to 15%, and it contains a large amount of nitrogen impurities, which makes the electrocatalytic reduction of low-concentration CO2 a serious challenge. That is, the introduction of N2 hinders the mass transfer and diffusion of CO2 between the gas / solid / liquid three-phase interface in the local environment during the CO2RR process. This is mainly manifested in two aspects: (1) N2 with high viscosity will cause the Reynolds number of the feed gas to decrease, increase the thickness of the mass transfer boundary layer, and make the CO2 concentration reaching the catalyst surface much lower than that of the pure concentration CO2 system, resulting in a serious hydrogen evolution side reaction (HER); (2) According to Fick's law, the reduction of CO2 feed concentration will weaken the concentration gradient effect of the catalyst layer, reduce the CO2 transmission flux, and limit the mass transfer and diffusion of CO2 at the catalyst layer, resulting in slow reaction kinetics and high overpotential. Therefore, the preparation of catalysts that can enhance CO2 mass transfer behavior is the key to achieving efficient catalytic conversion of low-concentration CO2RR. At present, the catalyst preparation strategies suitable for low concentration mainly include surface modification of functional groups, introduction of CO2 capture agents, etc., but these catalysts all have problems such as unclear reaction sites and unclear catalytic mechanisms. Therefore, designing and preparing catalysts with independent yet closely connected CO2 capture sites and catalytic conversion sites, shortening the thickness of the mass transfer boundary layer, enhancing the concentration gradient of CO2 at the catalyst interface layer, and promoting the CO2 mass transfer and diffusion behavior are the keys to achieving direct CO2RR of industrial flue gas. Summary of the Invention

[0004] In response to the deficiencies of the prior art, the present invention provides a self-enhanced concentration gradient effect catalyst for electrocatalytic low-concentration CO2 reduction and a preparation method thereof. The prepared catalyst can achieve specific capture of CO2 and high selectivity for CO under low-concentration CO2RR conditions.

[0005] The technical solution of the present invention to solve the technical problem is:

[0006] The first aspect of the present invention provides a method for preparing a self-enhanced concentration gradient effect catalyst for electrocatalytic low-concentration CO2 reduction, comprising the following steps:

[0007] (1) dissolving a soluble metal salt aqueous solution in a sodium carboxymethyl cellulose aqueous solution, and rapidly and vigorously stirring until a uniform transparent sol is formed; freezing the sol in liquid nitrogen to obtain a solid, immersing the solid in an ethanol solution containing sodium hydroxide or hydrochloric acid at -20 to -35°C, and after the ice crystals are completely dissolved, washing with ultrapure water at 24 to 26°C, and swelling in ultrapure water to obtain a gel, and refreezing the gel in liquid nitrogen, and freeze-drying to obtain a metal-sodium carboxymethyl cellulose aerogel; the mass ratio of the soluble metal salt to the sodium carboxymethyl cellulose in the transparent sol is 0.1 to 1:1;

[0008] (2) placing the metal-sodium carboxymethyl cellulose aerogel in a closed heating environment and heating it to 600-900°C at a heating rate of 1-20°C / min in an inert gas atmosphere; then continuously introducing an inert gas carrying a nitrogen source into the closed heating environment at a flow rate of 200-1000sccm, carrying the nitrogen source to the aerogel by the inert gas, and performing in situ growth of metal single-atom carbon nanotubes for 0.3-0.6h. After the growth is completed, stopping the introduction of the nitrogen source; then naturally cooling to room temperature in the inert gas, removing the metal particles formed during the reaction, and filtering and drying to obtain a porous carbon / metal single-atom carbon nanotube catalyst.

[0009] Furthermore, in step (1), the soluble metal salt is one or more of a soluble iron salt, a soluble cobalt salt, and a soluble nickel salt, and is further preferably one or more of cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate, ferric nitrate, ferric chloride, nickel chloride, nickel nitrate, or nickel sulfate.

[0010] Furthermore, in step (1), the concentration of the sodium carboxymethyl cellulose aqueous solution is 0.01 to 0.05 g / mL.

[0011] Furthermore, in step (1), the ethanol solution contains 1-2M sodium hydroxide or hydrochloric acid.

[0012] Furthermore, in step (1), the swelling time is 1 to 48 hours, and the ultrapure water is replaced every 2 to 24 hours.

[0013] Furthermore, in step (1), the freezing method adopts a traditional direct freezing method or a directional ice template method.

[0014] Furthermore, in step (2), the nitrogen source is a carbon-containing organic small molecule compound, and the carbon-containing organic small molecule compound is one or more of acetonitrile, ethylenediamine, and pyridine.

[0015] The second aspect of the present invention provides a catalyst prepared by the preparation method, wherein the morphology of the catalyst is a multi-level structure of metal-loaded single-atom carbon nanotubes vertically grown on porous carbon nanosheets.

[0016] The third aspect of the present invention provides the use of the catalyst in the electrocatalytic reduction of low-concentration CO2 to produce CO.

[0017] Furthermore, the application method is:

[0018] (1) Catalyst and polytetrafluoroethylene (PTFE) powder with a mass ratio of 95-105:1 were uniformly dispersed in isopropanol, water and 4-6 wt% perfluorosulfonic acid resin (Nafion) solution were added, ultrasonically dispersed, and drop-coated onto an SGL 39BB diffusion electrode with a loading of 0.8-1.2 mg / cm 2 , used as cathode working electrode;

[0019] (2) IrO2 was added to isopropyl alcohol and 4-6 wt% perfluorosulfonic acid resin solution, ultrasonically dispersed, and drop-coated onto platinum-coated titanium felt with a loading of 0.8-1.2 mg / cm 2 , used as the anode working electrode;

[0020] (3) Sustainion X37-50 Grade FA was used as an anion exchange membrane to assemble a membrane electrode. Simulated flue gas with different concentrations of CO2 was introduced into the cathode side of the membrane electrode through a humidifier filled with ultrapure water at a flow rate of 60 sccm. A 1 M potassium hydroxide solution was introduced into the anode side of the membrane electrode using a peristaltic pump at a flow rate of 10 sccm. The current density range was 50 to 250 mA / cm 2 The tank pressure is measured internally, and the outlet gas is tested for product using an online gas chromatograph.

[0021] The present invention mainly adopts the following innovations in the process of preparing a self-enhanced concentration gradient effect catalyst for electrocatalytic low-concentration CO2 reduction:

[0022] (1) The present invention prepares a metal-sodium carboxymethyl cellulose hydrogel with uniformly dispersed metal ions based on a coordination / hydrogen bond cross-linking strategy. The metal ions are anchored by the abundant hydroxyl and carboxyl functional groups of sodium carboxymethyl cellulose, and the strong cross-linking effect of hydrogen bonds is further utilized to ensure that the metal ions are uniformly anchored on the sodium carboxymethyl cellulose substrate, thereby providing a growth template for the subsequent growth of metal single-atom carbon nanotubes with adjustable length perpendicular to the porous carbon nanosheets. While the sodium carboxymethyl cellulose forms ultramicroporous carbon nanosheets during pyrolysis, the metal particles, under the action of a nitrogen source, in situ grow perpendicular to the porous carbon nanosheets to catalyze the growth of nitrogen-doped metal single-atom carbon nanotubes.

[0023] (2) Based on the swelling strategy, the present invention innovatively utilizes the water absorption properties of sodium carboxymethyl cellulose to introduce water molecules into the hydrogel cross-linked network, causing the hydrogel macrostructure to swell while increasing the distance between metal ions, thereby avoiding the problem of metal agglomeration during the subsequent in situ growth process.

[0024] (3) The present invention constructs ordered three-dimensional channels through the directional ice template method. Compared with the traditional method of directly freezing with liquid nitrogen, the directional ice template method using copper blocks as the cooling medium can prepare a uniform channel array of ~50 μm, which provides a channel for the flow of nitrogen source during the subsequent in situ growth process and avoids the occurrence of uneven growth, agglomeration and other phenomena.

[0025] (4) The present invention adopts an in-situ growth strategy and uses metal-sodium carboxymethyl cellulose aerogel as a growth substrate. Thanks to the top growth mechanism of carbon nanotubes, no metal single atoms will remain when the sodium carboxymethyl cellulose substrate is pyrolyzed into porous carbon nanosheets rich in ultra-micropores. Instead, metal particles grow in situ perpendicular to the porous carbon nanosheets under the action of a nitrogen source to catalyze the growth of nitrogen-doped metal single-atom carbon nanotubes, thus realizing the controllable preparation of multi-level morphologies of vertically grown metal-loaded single-atom carbon nanotubes on porous carbon nanosheets.

[0026] (5) The porous carbon nanosheets obtained by the present invention have a rich ultra-microporous structure (pore size <1nm), which can achieve the specific capture of CO2. In the low-concentration CO2RR process, the ultra-microporous structure is used to selectively capture CO2 to construct a local CO2-rich microenvironment, shorten the thickness of the mass transfer boundary layer, increase the CO2 concentration in the local environment, accelerate the mass transfer and diffusion rate of CO2 at the boundary layer, and inhibit the HER side reaction. The metal single-atom carbon nanotubes grown perpendicular to the porous carbon nanosheets can serve as catalytic active sites. The captured CO2 constructs a self-enhanced CO2 concentration gradient effect along the carbon tubes in the form of chemical adsorption and is directionally converted into CO. The CO2 enriched in the porous carbon nanosheets can utilize the "metal-N" sites on the carbon nanotubes to form a self-enhanced CO2 concentration gradient effect through chemical adsorption. Since the CO2 concentration gradient effect is positively correlated with the thickness of the catalyst layer, the present invention can accurately control the length of the carbon nanotubes by regulating the in-situ growth time, thereby optimizing the CO2 concentration gradient effect and improving the low-concentration CO2RR performance.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The carbon-based single-atom catalyst with self-enhanced CO2 concentration gradient obtained by the present invention can achieve high selectivity for CO under low-concentration CO2RR conditions. Specifically, the adsorption capacity of CO2 can reach 0.29mmol / g under 0.15bar and 298K conditions; in a membrane electrode system, under the inlet conditions of CO2:N2=0.15:0.85, at 250mA / cm 2 The selectivity of CO can reach 95.4% at industrial-grade current density, and even at extremely low CO2 concentration (CO2:N2=0.03:0.97, v / v) the selectivity of CO can reach 87.7%. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the SEM image of the Ni-sodium carboxymethyl cellulose aerogel prepared in Example 1;

[0030] Figure 2 FTIR image of Ni-sodium carboxymethyl cellulose aerogel prepared in Example 1;

[0031] Figure 3 XRD pattern of Ni-N-CNTs / PC NSs prepared in Example 1;

[0032] Figure 4 This is the SEM image of Ni-N-CNTs / PC NSs prepared in Example 1;

[0033] Figure 5This is the spherical aberration corrected HAADF-STEM image of Ni-N-CNTs / PC NSs prepared in Example 1;

[0034] Figure 6 This is the pore size distribution diagram of PC NSs prepared in Example 1;

[0035] Figure 7 CO2RR performance diagram of Ni-N-CNTs / PC NSs prepared in Example 1 at different CO2 feed concentrations;

[0036] Figure 8 This is a graph showing the cell pressure changes of Ni-N-CNTs / PC NSs prepared in Example 1 at different CO2 feed concentrations;

[0037] Figure 9 This is the low-concentration CO2RR performance diagram of Ni-N-CNTs / PC NSs prepared in Example 1 at different flow rates;

[0038] Figure 10 CO selectivity diagram of Ni-N-CNTs / PC NSs prepared in Example 1 at different CO2 concentrations;

[0039] Figure 11 This is a stability test diagram of the electrocatalytic reduction of low-concentration CO2 of Ni-N-CNTs / PC NSs prepared in Example 1 in 0.1M potassium bicarbonate solution;

[0040] Figure 12 TEM image of the catalyst obtained in Comparative Example 1;

[0041] Figure 13 TEM image of the catalyst obtained in Comparative Example 2;

[0042] Figure 14 This is the SEM image of the catalyst obtained in Comparative Example 3;

[0043] Figure 15 This is the SEM image of the catalyst obtained in Comparative Example 4;

[0044] Figure 16 This is a low-concentration CO2RR performance diagram of the catalyst obtained in Comparative Example 4;

[0045] Figure 17 The relationship between the growth time and carbon nanotube length of the catalysts prepared in Examples 5 to 8 is shown;

[0046] Figure 18 Figure 1 is a graph of Ni single atom content of the catalysts prepared in Example 1 and Comparative Examples 5 to 8;

[0047] Figure 19The catalyst prepared in Comparative Example 5 has a 2 Faradaic efficiency diagram of CO at current density;

[0048] Figure 20 The catalyst prepared in Comparative Example 6 has a 2 Faradaic efficiency diagram of CO at current density;

[0049] Figure 21 The catalyst prepared in Comparative Example 7 has a 2 Faradaic efficiency diagram of CO at current density;

[0050] Figure 22 The catalyst prepared in Comparative Example 8 has a 2 Faradaic efficiency diagram of CO at current density;

[0051] Figure 23 The low concentration TOF graphs of the catalysts obtained in Example 1 and Comparative Examples 5 to 8;

[0052] Figure 24 This is the CO2 concentration distribution diagram of the catalyst obtained by COMSOL simulation in Example 1 under membrane electrode conditions.

[0053] Figure 25 The low-concentration CO2RR relaxation time distribution diagram of the catalysts prepared in Example 1, Comparative Example 6 and Comparative Example 8 in the membrane electrode. DETAILED DESCRIPTION

[0054] The specific embodiments of the present invention are given below. The specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of the present invention.

[0055] The present invention provides a method for preparing a self-enhanced concentration gradient effect catalyst for electrocatalytic low-concentration CO2 reduction (hereinafter referred to as the method), comprising the following steps:

[0056] (1) dissolving sodium carboxymethyl cellulose in water and stirring thoroughly until there are no bubbles; dissolving a soluble metal salt in water to form a transparent solution and then pouring it into the sodium carboxymethyl cellulose solution, stirring rapidly and vigorously until a transparent sol is formed; freezing the sol in liquid nitrogen, and immersing the frozen solid in an ethanol solution containing sodium hydroxide or hydrochloric acid at -20 to -35°C; when the ice crystals are completely dissolved, washing with ultrapure water at 24 to 26°C, and swelling in ultrapure water to obtain a gel, and refreezing the gel in liquid nitrogen, and then freeze-drying to obtain a metal-sodium carboxymethyl cellulose aerogel; the soluble metal salt is one or more of a soluble iron salt, a soluble cobalt salt, and a soluble nickel salt; the mass ratio of the soluble metal salt to the sodium carboxymethyl cellulose in the transparent sol is 0.1 to 1:1;

[0057] Preferably, in step (1), the concentration of the sodium carboxymethyl cellulose solution is 0.01 to 0.05 g / mL.

[0058] Preferably, in step (1), the soluble metal salt is one or more of cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate, ferric nitrate, ferric chloride, nickel chloride, nickel nitrate, and nickel sulfate.

[0059] Preferably, in step (1), sodium hydroxide or hydrochloric acid is used to adjust the pH value. The ethanol solution contains 1-2M sodium hydroxide or hydrochloric acid.

[0060] Preferably, in step (1), the swelling time in ultrapure water is 1 to 48 hours, and the ultrapure water is replaced every 2 to 24 hours.

[0061] Preferably, in step (1), the freezing method adopts the traditional direct freezing method or the directional ice template method.

[0062] (2) The metal-sodium carboxymethyl cellulose aerogel is placed in a sealed heating environment and heated to 600-900°C at a heating rate of 1-20°C / min in an inert gas atmosphere. Then, an inert gas carrying a nitrogen source is continuously introduced into the sealed heating environment at a flow rate of 200-1000 sccm. The nitrogen source is carried to the aerogel by the inert gas, and the in-situ growth of metal single-atom carbon nanotubes is carried out for 0.3-0.6 hours. After the growth is completed, the introduction of the nitrogen source is stopped. After the aerogel is naturally cooled to room temperature in the inert gas, the introduction of the inert gas is stopped. The metal particles formed during the reaction are then removed, and the porous carbon / metal single-atom carbon nanotube catalyst is obtained after filtration and drying. The morphology of the catalyst is a multi-level structure of vertically grown metal single-atom carbon nanotubes on porous carbon nanosheets.

[0063] Preferably, in step (2), the nitrogen source is a nitrogen-containing organic small molecule compound, specifically acetonitrile, ethylenediamine or pyridine.

[0064] Preferably, in step (2), the flow rate of the inert gas carrying the nitrogen source is 200 to 1000 sccm.

[0065] This is illustrated by the following examples.

[0066] Example 1

[0067] (1) Weigh 1.0 g of sodium carboxymethyl cellulose and dissolve it in 40 mL of ultrapure water, stirring thoroughly until the solid is completely dissolved and there are no bubbles; at the same time, weigh 0.2 g of NiCl2 and dissolve it in 10 mL of ultrapure water, stirring thoroughly until it is completely dissolved to obtain a dark green transparent solution, which is poured into the sodium carboxymethyl cellulose solution at one time and stirred rapidly until a light green sol is formed; the sol is poured into a PTFE mold and frozen in liquid nitrogen using a directional ice template method; the frozen solid is immersed in a 1.5 M NaOH ethanol solution and cross-linked at -30 ° C until the ice crystals are completely dissolved; then, the residual NaOH and ethanol on the surface are washed with ultrapure water, and the solid is swelled in ultrapure water for 48 h, and the ultrapure water is replaced every 2 h, and then the solid is frozen again in liquid nitrogen using a directional ice template method, and the frozen solid is freeze-dried to obtain Ni-sodium carboxymethyl cellulose aerogel;

[0068] (2) Ni-sodium carboxymethyl cellulose aerogel was placed in a quartz boat and placed in a closed heating tube furnace; the temperature was raised to 750°C at a heating rate of 5°C / min in an argon atmosphere with an argon flow rate of 300sccm; when the temperature reached 750°C, the acetonitrile passage was opened, and the argon flowed through the acetonitrile and carried the acetonitrile into the tube furnace, where it came into contact with the aerogel and, under the catalytic action of Ni metal, the acetonitrile molecules were cracked to grow carbon nanotubes; after growing for 0.5h, the acetonitrile passage was closed, and the argon flow was retained; the argon was then naturally cooled to room temperature and the argon was turned off; the collected product was ground in a mortar and soaked in 1M dilute hydrochloric acid for 24h to remove the metal particles in the powder, and dried at 60°C to obtain a CO2 concentration gradient self-enhanced carbon-based Ni single-atom catalyst (Ni-N-CNTs / PCNSs).

[0069] The Ni-sodium carboxymethyl cellulose aerogel obtained in step 1 of Example 1 was subjected to electron microscopy scanning. Figure 1 As shown, it can be seen that the obtained Ni-sodium carboxymethyl cellulose aerogel has a uniform pore array of ∼50 μm.

[0070] Figure 2 This is the FTIR image of Ni-sodium carboxymethyl cellulose aerogel. It can be seen that the Ni in Ni-sodium carboxymethyl cellulose aerogel 2+ It is fixed in the aerogel network by coordinating with the –OH and –COO– functional groups in sodium carboxymethyl cellulose.

[0071] Figure 3 This is the XRD pattern of Ni-N-CNTs / PC NSs. It can be seen that the main diffraction peaks of Ni-N-CNTs / PC NSs obtained in step 2 correspond well to the PDF#41-1478 card of graphite carbon. At the same time, there is no diffraction peak of Ni, indicating that Ni does not exist in the form of particles.

[0072] Figure 4 is the SEM image of Ni-N-CNTs / PC NSs. It can be seen that Ni-N-CNTs grow vertically and uniformly on the surface of PCNSs, where the average length of Ni-N-CNTs is ~600 nm and the thickness of PC NSs is ~200 nm.

[0073] Figure 5 This is the spherical aberration corrected HAADF-STEM image of Ni-N-CNTs / PC NSs. It can be seen that the bright spots evenly distributed on the Ni-N-CNTs are the loaded metal Ni single atoms.

[0074] Figure 6 This is the pore size distribution diagram of PC NSs. It can be seen that PC NSs has a rich ultramicroporous structure, and the pore size is mainly 0.5 nm.

[0075] The Ni-N-CNTs / PC NSs prepared in Example 1 were used to electrocatalytically reduce low-concentration CO2 to produce CO. The specific steps are as follows:

[0076] (1) 20 mg of Ni-N-CNTs / PC NSs and 0.2 mg of 50 nm PTFE powder were added to 1000 μL of isopropanol and ultrasonically dispersed for 30 min. 900 μL of ultrapure water and 100 μL of Nafion (5 wt%) solution were added and ultrasonically dispersed for another 30 min. The mixture was then drop-coated onto an SGL 39BB diffusion electrode with a loading of 1.0 mg / cm 2 , used as the cathode working electrode; 20 mg of IrO2 was added to 1900 μL of isopropanol and 100 μL of Nafion (5 wt%) solution, ultrasonically dispersed for 30 min, and drop-coated on platinum-coated titanium felt with a loading of 1.0 mg / cm 2 , used as the anode working electrode; Sustainion X37-50 Grade FA was used as the anion exchange membrane, soaked in 1M potassium hydroxide solution for at least 24 hours before use, and rinsed with ultrapure water before assembling the membrane electrode;

[0077] (2) During the zero-gap membrane electrode electrochemical test, simulated flue gas with different CO2 contents (CO2:N2=15:85, 50:50, 100:0) was introduced into the cathode side of the membrane electrode through a humidifier filled with ultrapure water at a flow rate of 60 sccm, and a 1M potassium hydroxide solution was introduced into the anode side of the membrane electrode using a peristaltic pump at a flow rate of 10 sccm. The current density range was 50-250 mA / cm 2 The cell pressure was measured internally, and the outlet gas was tested for product using online gas chromatography, from which the selectivity of the catalyst for CO in a simulated flue gas environment was calculated.

[0078] (3) Replace the electrolyte with 0.1M potassium bicarbonate solution and re-drip the diffusion electrode. 2 The low-concentration CO2RR stability of the catalyst was tested under current density conditions. The electrolyte was replaced by potassium bicarbonate solution from potassium hydroxide solution to avoid serious carbonate / bicarbonate precipitation problems during the stability test.

[0079] The performance of the catalyst for electrocatalytic reduction of low concentration CO2 is as follows Figures 7-10 .

[0080] Figure 7 The CO2RR performance of Ni-N-CNTs / PC NSs prepared in Example 1 at different CO2 feed concentrations shows that during the membrane electrode operation, Ni-N-CNTs / PC NSs is basically unaffected by the CO2 feed concentration (15%, 50%, 100%). Specifically, at different CO2 feed concentrations, the CO2RR performance of Ni-N-CNTs / PC NSs is 50-250 mA / cm 2 The Faradaic efficiency (FE) of CO at a current density of CO ) is always above 90%. At 15% CO2 concentration and current density of 250 mA / cm 2 Time FE CO is 95.4%, while at 100% CO2 concentration, 250mA / cm 2 Corresponding FE CO The results show that the carbon nanosheets in Ni-N-CNTs / PC NSs enrich CO2 through their ultra-microporous structure, while the Ni-N-CNTs with the optimal length contribute to the synergistic catalytic conversion of CO2. Together, they establish a self-enhanced CO2 concentration gradient effect, promote CO2 mass transfer and diffusion, accelerate the CO2RR reaction kinetics, and achieve efficient CO2RR at low concentrations.

[0081] Figure 8 The cell pressure variation diagram of Ni-N-CNTs / PC NSs at different CO2 feed concentrations shows that during the membrane electrode operation, the cell pressure of Ni-N-CNTs / PC NSs remains basically stable at different CO2 feed concentrations (15%, 50%, 100%). At a CO2 feed concentration of 15%, the current density is 250mA / cm 2 The corresponding cell voltage is 3.8V, and at 100% CO2 concentration, 250mA / cm 2 The corresponding cell voltage is 3.7 V. This indicates that the self-enhanced CO2 concentration gradient effect induced by Ni-N-CNTs / PC NSs can effectively reduce the reaction overpotential.

[0082] Figure 9The CO2RR performance diagram of Ni-N-CNTs / PC NSs at different flow rates shows that during the membrane electrode operation, Ni-N-CNTs / PC NSs has a FE of 15% CO2 feed concentration. CO Not affected by the inlet gas flow rate, when the flow rate is 120sccm, FE CO is 98.1%. When the flow rate is reduced to 10 sccm, the FE CO It can still reach 98.5%. This shows that the carbon nanosheets in Ni-N-CNTs / PC NSs can shorten the mass transfer boundary layer, thereby maintaining efficient CO2RR performance at low flow rate and low CO2 concentration.

[0083] Figure 10 The CO selectivity diagram of Ni-N-CNTs / PC NSs at different CO2 concentrations is shown in Figure 2. Figure 10 It can be seen that the self-enhanced CO2 concentration gradient effect induced by Ni-N-CNTs / PC NSs with a suitable carbon nanotube length exhibits excellent ultra-low CO2 concentration tolerance. Notably, under the extreme condition of a CO2 feed gas concentration of only 3%, Ni-N-CNTs / PC NSs can still achieve a CO selectivity of up to 87.7%.

[0084] The stability test of Ni-N-CNTs / PC NSs in electrocatalytic reduction of low concentration CO2 in 0.1M potassium bicarbonate solution is shown in Figure 2. Figure 11 As shown. Figure 11 It can be seen that at 100mA / cm 2 Under the current density of 1000 nm and the inlet gas condition of CO2:N2=15:85, Ni-N-CNTs / PC NSs can operate stably for 12 h, and FE CO It always remains above 90% and the cell pressure remains basically stable, indicating that the prepared CO2 self-enhanced concentration gradient effect catalyst has good electrocatalytic stability.

[0085] Examples 2-3

[0086] The only difference from Example 1 is that the concentration of sodium carboxymethyl cellulose in step (1) was changed to 0.01 g / mL and 0.03 g / mL, respectively. The products obtained in Examples 2 and 3 all maintained the pore array structure, and the selectivity of CO after electrocatalytic reduction of low-concentration CO2 was >90%, indicating that the prepared catalysts have good selectivity for CO under low-concentration CO2 conditions.

[0087] Examples 4-5

[0088] The only difference from Example 1 is that the mass ratio of NiCl2 and sodium carboxymethyl cellulose in step (1) is changed to 0.1:1 and 0.3:1 respectively.

[0089] The difference between the products obtained in Examples 4 and 5 is that the coverage of carbon nanotubes on the surface of the porous carbon nanosheets is different. The selectivity of CO after electrocatalytic low-concentration CO2 reduction is >90%, indicating that the prepared catalyst has good selectivity for CO under low-concentration CO2 conditions.

[0090] Examples 6 to 9

[0091] The only difference from Example 1 is that the growth temperature in step (2) is changed to 600, 700, 800, and 900° C. respectively.

[0092] The products obtained in Examples 6 to 9 have similar morphologies to that of Example 1, and the selectivity of CO after electrocatalytic reduction of low-concentration CO2 is >90%, indicating that the prepared catalysts have good selectivity for CO under low-concentration CO2 conditions.

[0093] Examples 10 to 13

[0094] The only difference from Example 1 is that the gas flow rate in step (2) is changed to 200, 400, 800, and 1000 sccm respectively.

[0095] The products obtained in Examples 10 to 13 have similar morphologies to that of Example 1, and the selectivity of CO after low-concentration CO2RR is >90%, indicating that the prepared catalysts have good selectivity for CO under low-concentration CO2 conditions.

[0096] Examples 14-15

[0097] The only difference from Example 1 is that the growth time in step (2) is changed to 0.3 h and 0.6 h respectively.

[0098] The products obtained in Examples 14 and 15 had similar morphologies to those in Example 1. The carbon nanotubes were approximately 400 nm long when grown for 0.3 h, and approximately 800 nm long when grown for 0.6 h. The CO selectivity after low-concentration CO₂RR was >90%, demonstrating that the catalysts exhibited good CO selectivity at low CO₂ concentrations.

[0099] Comparative Examples 1-2

[0100] The only difference from Example 1 is that the swelling time in step (1) is changed to 1 h and 12 h respectively.

[0101] Figure 12This is a TEM image of the catalyst obtained in Comparative Example 1. It can be seen that the catalyst obtained in Comparative Example 1 does not grow carbon nanotubes, but forms an agglomerated carbon layer.

[0102] Figure 13 This is a TEM image of the catalyst obtained in Comparative Example 1. It can be seen that the catalyst obtained in Comparative Example 2 did not form the multi-level structure shown in Example 1, but showed a random distribution of porous carbon nanosheets and carbon nanotubes, and the carbon nanotubes tightly coated the nickel nanoparticles, making it difficult to remove the nickel particles by hydrochloric acid etching.

[0103] Comparative Example 3

[0104] The only difference from Example 1 is that the process of step (1) is changed to weighing 1.0 g of sodium carboxymethyl cellulose and 0.2 g of NiCl2, and uniformly grinding the sodium carboxymethyl cellulose and NiCl2 in an agate mortar for at least 30 minutes to obtain a light yellow powder. Step (2) is adjusted as follows: the mixed powder of sodium carboxymethyl cellulose and NiCl2 is placed in a quartz boat, and placed in a closed heating tube furnace; the temperature is raised to 750°C at a heating rate of 5°C / min under an argon atmosphere, with an argon flow rate of 300 sccm; when the temperature reaches 750°C, the hydrogen and acetonitrile passages are opened, and a hydrogen / argon mixture (30:270, v:v) is allowed to flow through the acetonitrile and carry the acetonitrile into the tube furnace. After 30 minutes of growth, the hydrogen and acetonitrile passages are closed, and the argon flow is retained.

[0105] Figure 14 This is an SEM image of the catalyst prepared in Comparative Example 3. It can be seen that the catalyst obtained in Comparative Example 3 exhibits a disordered structure rather than the ordered, hierarchical structure presented in Example 1. The Ni nanoparticles are completely encapsulated in the thick carbon layer, and no carbon nanotube formation is detected within the thick carbon layer. This is because the metallic Ni is easily covered by the thick carbon layer derived from sodium carboxymethylcellulose during heating, making it unable to act as a catalyst to promote carbon nanotube growth during the subsequent in situ growth process.

[0106] Comparative Examples 4 to 8

[0107] The only difference from Example 1 is that the growth time in step (2) is changed to 0, 0.2, 1.0, 1.5, and 3.0 h, respectively.

[0108] The growth time directly affects the length of Ni single-atom-loaded carbon nanotubes, thereby producing self-enhanced CO2 concentration gradient effects of different intensities.

[0109] Figure 15This is the SEM image of the catalyst prepared in Comparative Example 4. It can be seen that when the growth time is 0, the SEM observes the morphology of porous carbon nanosheets with a thickness of ~250 nm and uniform pores of ~5.5 nm on the surface. These pores are attributed to the Ni particles removed by acid etching. This result also shows that swelling for 48 h can make the Ni particles uniformly dispersed on the surface of the carbon nanosheets in nanometer size, effectively preventing Ni from agglomerating during the pyrolysis process.

[0110] Figure 16 The low concentration CO2RR performance of the catalyst prepared in Comparative Example 4 is shown in FIG. 1 . It can be seen that the simple porous carbon nanosheets prepared in Comparative Example 4 after 0 h of growth do not have low concentration CO2RR performance. 2 Only hydrogen can be detected within the range of 1.5 GHz. This indicates that Ni-N-CNTs are the catalytic active sites for the electrocatalytic reduction of CO2, while the porous carbon nanosheets mainly play the role of specifically capturing CO2 and promoting CO2 mass transfer and diffusion.

[0111] Figure 17 The relationship diagram of catalyst growth time-carbon nanotube length for the catalysts prepared in Examples 5 to 8 shows that as the growth time increases, the thickness of the porous carbon nanosheets remains at ~250nm. However, the length of Ni-N-CNTs increases with the increase of growth time. When the growth time is 0.2 (Comparative Example 5), 1.0 (Comparative Example 6), 1.5 (Comparative Example 7) and 3.0h (Comparative Example 8), the corresponding Ni-N-CNTs lengths are ~100nm, ~500nm, ~1100nm and ~1800nm, respectively. Based on the characteristic that the length of carbon nanotubes can be dynamically adjusted with the growth time, the intensity of the CO2 concentration gradient effect can be accurately controlled by precisely controlling its in-situ growth time, thereby effectively modulating the low-concentration CO2RR performance.

[0112] Figure 18 The Ni single atom content graph of the catalysts prepared in Example 1 and Comparative Examples 5 to 8 shows that the Ni single atom content is related to the growth time. As the growth time increases, the Ni single atom content will first rise and then remain stable. When the growth time is 0.2 (Comparative Example 5), 0.5 (Example 1), 1.0 (Comparative Example 6), 1.5 (Comparative Example 7) and 3.0 h (Comparative Example 8), the Ni single atom content is 0.75, 2.00, 2.14, 2.34 and 2.75 wt %, respectively. Among them, the catalyst with a growth time of 0.2 h has the lowest Ni single atom content, which is the main reason for its poor subsequent low-concentration CO2RR performance.

[0113] Figure 19 The catalyst prepared in Comparative Example 5 has a 2From the Faraday efficiency diagram of CO at a current density of 250 mA / cm, it can be seen that the Ni-N-CNTs / PC NSs prepared in Comparative Example 5 and grown for 0.2 h 2 The Faradaic efficiency of CO at a current density of 44.3% is achieved.

[0114] Figure 20 The catalyst prepared in Comparative Example 6 has a 2 From the Faraday efficiency diagram of CO at a current density of 250 mA / cm, it can be seen that the Ni-N-CNTs / PC NSs grown for 1 h prepared in Comparative Example 6 2 The Faradaic efficiency of CO at a current density of 1.5 wt% is 73.8%.

[0115] Figure 21 The catalyst prepared in Comparative Example 7 has a 2 From the Faraday efficiency diagram of CO at a current density of 250 mA / cm, it can be seen that the Ni-N-CNTs / PC NSs prepared in Comparative Example 7 and grown for 1.5 h 2 The Faradaic efficiency of CO at a current density of 3.5 wt% is 36.1%.

[0116] Figure 22 The catalyst prepared in Comparative Example 8 has a 2 From the Faraday efficiency diagram of CO at a current density of 100%, it can be seen that the Ni-N-CNTs / PC NSs grown for 3 h prepared in Comparative Example 8 has a high CO2 content in the range of 50-250 mA / cm 2 The current density range always maintains FE CO >90%, which indicates that Ni-N-CNTs can achieve efficient CO2-CO conversion as active sites. However, when nitrogen dilution was introduced into the system (N2:CO2=50:50, v / v), FE CO At 250mA / cm 2 When the CO2 concentration is further reduced to 15%, the FE CO At 250mA / cm 2 Combined with Example 1, it can be seen that excessively long Ni-N-CNTs will weaken the CO2 concentration gradient effect, reduce the CO2 transmission flux at the catalyst layer, and lead to slow CO2RR reaction kinetics.

[0117] In order to exclude the influence of Ni content in Ni-N-CNTs of different lengths on CO2RR performance caused by different growth time, the turnover frequency (TOF) was used to normalize the Ni single atom content and determine the intrinsic activity of Ni-N-CNTs / PC. Figure 23The TOF graphs of the catalysts prepared in Example 1 and Comparative Examples 5 to 8 show that although Comparative Example 5 (growth time of 0.2 h) has the most significant CO2 concentration gradient effect and also exhibits the highest TOF at different current densities, the FE CO At 250mA / cm 2 The CO2 concentration gradient of Example 1 (growth time of 0.5 h) has a suitable self-enhanced CO2 concentration gradient and catalytic active sites, so the CO2 concentration of Example 1 is 0.5 h, which is 0.5 h. 2 It reached 1.31×10 5 h –1 The high TOF of Example 8 (1.99×10 4 h -1 ) is about 8 times higher than the original data.

[0118] The performance of Comparative Examples 5 to 8 is inferior to that of Example 1. From the results of the low-concentration CO2RR performance in the membrane electrode, it can be seen that the introduction of porous carbon nanosheets and the length of Ni-N-CNTs play a key role in regulating the performance of electrocatalytic low-concentration CO2 reduction. Specifically, the porous carbon nanosheets can selectively enrich CO2 in a local environment through the ultra-microporous structure, thereby accelerating the CO2 mass transfer rate. Suitable Ni-N-CNTs can form a self-enhanced CO2 concentration gradient effect along the carbon nanotubes through chemical adsorption of the captured CO2, thereby increasing the transmission flux of CO2 at the catalyst layer and accelerating the CO2RR reaction kinetics. In order to further verify the above inference, the action mechanism of the self-enhanced concentration gradient effect catalyst was further studied.

[0119] Finite element simulation can intuitively analyze the concentration distribution of CO2 in porous carbon nanosheets and Ni-N-CNTs during membrane electrode operation (cell voltage is 3.0V). Figure 24 This is a COMSOL simulation of the CO2 concentration distribution of the catalyst obtained in Example 1 under membrane electrode conditions. It can be seen that the porous carbon nanosheets, with their rich ultramicroporous structure, selectively capture CO2 near the catalyst layer, subsequently forming a significant concentration gradient effect along the Ni-N-CNTs with a length of ~800 nm. When the Ni-N-CNTs exceed this length, the CO2 concentration along the Ni-N-CNTs gradually stabilizes, and the concentration gradient effect weakens.

[0120] In order to further evaluate the enhancement effect of the self-enhanced concentration gradient effect on CO2 mass transfer and diffusion, the relaxation time distribution during the low-concentration CO2RR process when the membrane electrode cell voltage ranged from 2.8V to 3.6V was obtained by deconvolution of the electrochemical impedance spectroscopy under working conditions. Figure 25The relaxation time distribution diagram of low concentration CO2RR in the membrane electrode of the catalyst of Example 1, Comparative Example 6 and Comparative Example 8 is shown. It can be seen that in the relaxation time range of 10 -5 ~10 -3 s, 10 -3 ~10 -1 s and 10 -1 There are three obvious peaks at 10s, corresponding to the interface contact resistance, charge transfer impedance and mass transfer diffusion impedance. As the length of Ni-N-CNTs increases (from Example 1 to Comparative Example 6 and Comparative Example 8), 10 -1 The mass transfer impedance peak at ~10s shifts to low frequency and the resistance value increases significantly, which indicates that excessively long Ni-N-CNTs will weaken the CO2 concentration gradient effect and reduce the CO2 diffusion flux, further confirming that the concentration gradient effect induced by the length of Ni-N-CNTs is directly related to the low-concentration CO2RR performance.

[0121] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a self-enhanced concentration gradient effect catalyst for electrocatalytic low-concentration CO2 reduction, characterized in that: The following steps are involved: (1) placing an aqueous solution of a soluble metal salt in an aqueous solution of sodium carboxymethyl cellulose, stirring rapidly and vigorously until a transparent sol is formed, freezing the sol in liquid nitrogen to obtain a solid, immersing the solid in an ethanol solution containing sodium hydroxide or hydrochloric acid at -20 to -35°C, washing it with ultrapure water at 24 to 26°C after the ice crystals are completely dissolved, and swelling it in ultrapure water to obtain a gel, freezing the gel again in liquid nitrogen, and freeze-drying it to obtain a metal-sodium carboxymethyl cellulose aerogel; the soluble metal salt is one or more of a soluble iron salt, a soluble cobalt salt, and a soluble nickel salt; the mass ratio of the soluble metal salt to the sodium carboxymethyl cellulose in the transparent sol is 0.1 to 1:1; (2) The metal-sodium carboxymethyl cellulose aerogel is placed in a closed heating environment and heated to 600-900°C at a heating rate of 1-20°C / min in an inert gas atmosphere. Then, an inert gas carrying a nitrogen source is continuously introduced into the closed heating environment at a flow rate of 200-1000 sccm to perform in-situ growth of metal single-atom carbon nanotubes for 0.3-0.6 hours. After the growth is completed, the introduction of the nitrogen source is stopped. After the inert gas is naturally cooled to room temperature, the metal particles formed during the reaction are removed, and the catalyst is obtained after filtration and drying.

2. The preparation method according to claim 1, characterized in that In step (1), the soluble metal salt is one or more of cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate, ferric nitrate, ferric chloride, nickel chloride, nickel nitrate, and nickel sulfate.

3. The preparation method according to claim 1, characterized in that In step (1), the concentration of the sodium carboxymethyl cellulose aqueous solution is 0.01 to 0.05 g / mL.

4. The preparation method according to claim 1, characterized in that In step (1), the ethanol solution contains 1-2M sodium hydroxide or hydrochloric acid.

5. The preparation method according to claim 1, characterized in that In step (1), the swelling time is 1 to 48 hours, and the ultrapure water is replaced every 2 to 24 hours.

6. The preparation method according to claim 1, characterized in that In step (1), the freezing method adopts the traditional direct freezing method or the directional ice template method.

7. The preparation method according to claim 1, characterized in that In step (2), the nitrogen source is a carbon-containing organic small molecule compound, and the carbon-containing organic small molecule compound is one or more of acetonitrile, ethylenediamine, and pyridine.

8. A catalyst prepared by the preparation method according to claims 1 to 7, characterized in that: The morphology of the catalyst is a multi-level structure of metal-loaded single-atom carbon nanotubes vertically grown on porous carbon nanosheets.

9. Use of the catalyst according to claim 8 in the electrocatalytic reduction of low-concentration CO2 to produce CO.

10. The use according to claim 9, characterized in that The application method is: (1) The catalyst and polytetrafluoroethylene powder with a mass ratio of 95-105:1 were uniformly dispersed in isopropanol, water and 4-6 wt% perfluorosulfonic acid resin solution were added, ultrasonically dispersed, and drop-coated onto the SGL 39BB diffusion electrode with a loading of 0.8-1.2 mg / cm 2 , used as cathode working electrode; (2) IrO2 was added to isopropyl alcohol and 4-6 wt% perfluorosulfonic acid resin solution, ultrasonically dispersed, and drop-coated onto platinum-coated titanium felt with a loading of 0.8-1.2 mg / cm 2 , used as the anode working electrode; (3) Sustainion X37-50 Grade FA was used as an anion exchange membrane to assemble a membrane electrode. CO2 simulated flue gas of different concentrations was passed through a humidifier filled with ultrapure water into the cathode side of the membrane electrode. A peristaltic pump was used to pass a 0.8-1.2 M potassium hydroxide solution into the anode side of the membrane electrode. The current density range was 50-250 mA / cm 2 The tank pressure is measured internally, and the outlet gas is tested for product using an online gas chromatograph.

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