Preparation method of integrated self-supporting electrode

Abstract

The invention discloses a preparation method of an integrated self-supporting electrode, which comprises the following steps: dissolving zinc salt, ligand and butylamine, mixing and fully reacting to form a colloidal ZIF-8 precursor; the preparation method comprises the following steps: preparing a colloidal ZIF-8 precursor, controlling the mass fraction of N, N-dimethylformamide in the colloidal ZIF-8 precursor to be 85-91%, mixing and stirring polyacrylonitrile, the colloidal ZIF-8 precursor and nickel salt, spinning into a uniform fiber membrane through an electrostatic spinning technology, and carrying out heating oxidation and calcination to obtain the integrated self-supporting electrode. And the prepared electrode is stable in overall structure and high in efficiency, and has a wide application prospect in the fields of electrocatalytic carbon dioxide reduction process and the like.

Description

Technical Field

[0001] The invention relates to the field of catalysis or colloid chemistry, and in particular to a method for preparing an integrated self-supporting electrode with a nickel single-atom catalyst as a main body. Background Art

[0002] The capture and resource utilization of carbon dioxide (CO2) is of great significance for coping with the increasingly serious energy and environmental crisis. The electrocatalytic reduction (eCO2RR) of CO2 using electricity generated by renewable energy such as solar energy and wind energy to produce high-value-added fuels and chemicals is an ideal solution to reduce CO2 emissions. There are many CO2 reduction products, including C1 products such as carbon monoxide (CO), formic acid (salt), and methane, as well as multi-carbon (C2+) products such as ethylene, ethanol, and propanol. Among the many reduction products, the reduction of CO2 to CO only involves the transfer of two electrons and two protons, which has certain kinetic advantages compared with other products. At the same time, the products of CO2 reduction to CO are usually only CO and the by-product H2. By adjusting the structure and composition of the active sites of the catalyst, the ratio of CO / H2 in the product can be effectively controlled, and the generated synthesis gas (CO / H2) can be directly used as a raw material for subsequent thermal catalytic reactions (such as Fischer-Tropsch reaction) to prepare high-value-added products. At the same time, the reduction of CO2 to CO can also be integrated into a series reactor as the first step of the eCO2RR to prepare C2+ products. In summary, under industrial current conditions (>200 mA·cm -2 ) Achieving high conversion rate and high selectivity of CO2 electroreduction to produce CO is a key step in realizing the resource utilization of CO2 to produce high value-added products.

[0003] At present, the reduction of CO2 to CO usually requires precious metals such as Au, Ag, Pd or their alloys as catalysts. However, precious metal catalysts have disadvantages such as high cost and low reserves. Moreover, with the large-scale industrialization of eCO2RR, the price of precious metal catalysts will inevitably soar further. Therefore, it is urgent to find cheap and efficient non-precious metal catalysts to promote the large-scale industrialization of eCO2RR. Among them, single metal atom MNC catalysts (M = Fe, Co, Ni and other transition metals) have the advantages of high atomic utilization, low cost and good selectivity. They are potential catalysts that can replace precious metal catalysts to achieve efficient CO2 conversion to CO. In particular, Ni single atom catalysts have attracted much attention due to their high selectivity and high stability in a wide potential range.

[0004] Rational design of catalysts and reactors to reduce the cell pressure of the electrolyzer, improve product selectivity, current density and stability is the key to promoting the large-scale industrialization of eCO2RR. In addition to catalysts, the optimization of reactor structure has an important impact on achieving industrial current density and improving CO2 conversion rate. The current density in traditional H-type reactors is limited by the solubility and diffusion rate of CO2 in the electrolyte, so it is not suitable for industrialization. The use of gas diffusion electrodes can break the limitation of CO2 solubility, so that the current density of eCO2RR electrolyzers can meet industrial needs. In particular, membrane electrode (MEA) electrolyzers use polymer anion / cation exchange membranes to replace traditional liquid electrolytes, minimize the distance between the cathode and the anode, and effectively reduce ohmic polarization. The catalytic layer in traditional membrane electrodes is prepared by mixing active substances, carriers (electronic conductors) and electrolyte solutions (ionic conductors) in a certain proportion, and the transmission channels of substances such as electrons, ions, gases and water are all in a disordered state. There are strong concentration polarization and ohmic polarization, which seriously restrict the current density and energy efficiency of membrane electrode electrolyzers. Secondly, due to the weak contact between the gas diffusion layer and the electrocatalyst, the electroactive substances are easily separated and fall off, thereby reducing the long-term performance of CO2 electroreduction. At the same time, the enriched catalyst layer will cause a local high alkaline environment under the reaction, which is easy to cause salt precipitation and reduce the overall stability.

[0005] Integrated self-supporting electrodes can integrate the gas diffusion layer with the catalyst layer, and have many advantages in application. For example, the monolithic structure can directly avoid the use of film-forming binders in the preparation of electrode sheets, reducing inactive components and their adverse effects on conductivity; in addition, electrospinning technology converts the catalyst layer into an ordered fiber structure, constructing an ordered transmission channel for substances such as electrons, ions, gases and water. Integrated self-supporting electrodes can effectively and evenly disperse catalytic active centers, making it possible to stably obtain industrial-grade current density. Carbon-based gas diffusion electrodes have the characteristics of good conductivity and adjustable surface wettability. However, at present, there is still a research gap in the development and application of carbon-based integrated self-supporting electrodes. On the one hand, the preparation of carbon-based integrated self-supporting electrodes puts forward higher requirements for material synthesis. Not only must there be a rich porous structure for enhancing mass transfer, but also sufficient support strength; at the same time, regulating the three-phase interface environment of the carbon-based integrated self-supporting electrode and ensuring enough accessible active sites is also a difficulty in the application of carbon-based integrated self-supporting electrodes. Therefore, the development of efficient carbon-based integrated self-supporting structure electrodes for CO2RR still faces huge challenges. Summary of the invention

[0006] 1. Technical issues to be resolved The technical problem to be solved by the present invention is to provide a method for preparing an integrated self-supporting electrode with stable structure and high efficiency.

[0007] (II) Technical solution In order to solve the above problems, the present invention provides a method for preparing an integrated self-supporting electrode, comprising the following steps: S1: dissolving zinc salt to form solution 1, dissolving ligand and butylamine to form solution 2, mixing the two solutions to react fully, and then separating the synthesized colloidal ZIF-8 (zeolite imidazolate framework) precursor by centrifugation; S2: The residual methanol in the colloidal ZIF-8 precursor was exchanged with N,N-dimethylformamide by solvent exchange method; S3: Control the mass fraction of N,N-dimethylformamide in the colloidal ZIF-8 precursor in the range of 85-91% by centrifugation; S4: mixing and stirring polyacrylonitrile, colloidal ZIF-8 precursor, and nickel salt to form a spinning solution; S5: spinning the spinning solution into a uniform fiber membrane through electrospinning technology; S6: heating and oxidizing the fiber membrane in air; S7: calcining the fiber membrane under a nitrogen atmosphere to obtain an integrated self-supporting electrode.

[0008] Preferably, step S4 further comprises: mixing polyacrylonitrile, colloidal ZIF-8 precursor, and nickel salt, and stirring for at least 24 hours, wherein the mass ratio of polyacrylonitrile to colloidal ZIF-8 precursor solid content is 1.0-1.2, the total solid content is 13.5%-15%, and the mass fraction of nickel is 1%-2%.

[0009] Preferably, when spinning a uniform fiber membrane in step S5, the electrospinning voltage is set to 14 kv-18 kv and the flow rate is set to 1.2 ml / min-2 ml / min.

[0010] Preferably, the step S6 further comprises: oxidizing the fiber membrane in air at 250° C. for 1 hour, with a heating rate of 2° C. / min.

[0011] Preferably, the step S7 further comprises: calcining the fiber membrane under a nitrogen atmosphere at a temperature of 900° C.-1050° C. for 2-4 hours at a heating rate of 5° C. / min to obtain an integrated self-supporting electrode.

[0012] (III) Beneficial effects The method for preparing the above-mentioned integrated self-supporting electrode of the present invention can form an integrated self-supporting electrode that can integrate the gas diffusion layer with the catalyst layer. The catalyst layer has both catalytic and gas transmission functions, which reduces the use of film-forming binders in the preparation of electrode sheets and improves the overall stability of the catalyst layer. At the same time, the electrostatic spinning technology constructs the catalyst layer into a three-dimensional ordered fiber structure, constructs an ordered transmission channel for substances such as electrons, ions, gases and water, and has a stable overall structure and high efficiency, which has broad application prospects in the fields of electrocatalytic carbon dioxide reduction processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the process of preparing an integrated self-supporting electrode according to an embodiment of the present invention.

[0014] Figure 2 This is a photograph of a nickel single-atom integrated self-supporting electrode made by the preparation method of the present invention in Example 1 of the present invention.

[0015] Figure 3 This is the X-ray diffraction pattern of Example 1 of the present invention.

[0016] Figure 4 This is a graph of the partial current density of the integrated self-supporting electrode prepared in Example 1 of the present invention.

[0017] Figure 5 This is a Faraday efficiency diagram of the integrated self-supporting electrode prepared in Example 1 of the present invention.

[0018] Figure 6 Schematic diagram of the membrane electrode of the integrated self-supporting electrode prepared in Example 1 of the present invention.

[0019] Figure 7 This is a Faraday efficiency diagram of the integrated self-supporting electrode prepared in Example 1 of the present invention used for a membrane electrode. DETAILED DESCRIPTION

[0020] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0021] Figure 1 1 is a schematic flow chart of a method for preparing an integrated self-supporting electrode according to an embodiment of the present invention, wherein the method comprises the following steps: S1: Dissolve the zinc salt to form solution 1, dissolve the ligand and butylamine to form solution 2, mix the two solutions to react fully, and then separate the synthesized colloidal ZIF-8 (zeolite imidazole ester framework) precursor by centrifugation. The details are as follows: In one embodiment of the present invention, step S1 includes the following steps: dissolving 1 mmol of zinc nitrate hexahydrate in 200 ml of methanol as solution 1, dissolving 2.5 mmol of 2-methylimidazole and 25 mmol of butylamine in 200 ml of methanol as solution 2. Pour solution 2 into solution 1, let stand for 1-1.5 hours to wait for sufficient reaction, and then separate the synthesized colloidal ZIF-8 precursor by centrifugation.

[0022] Among them, adding butylamine can adjust the acidity and alkalinity of the solution, thereby changing the dispersion form of the ZIF-8 precursor in the solution.

[0023] In order to obtain a better ZIF-8 precursor, the ZIF-8 precursor can be centrifuged and washed twice with methanol.

[0024] S2: The residual methanol in the colloidal ZIF-8 precursor was exchanged with N,N-dimethylformamide (DMF) by solvent exchange method.

[0025] S3: The mass fraction of N,N-dimethylformamide (DMF) in the colloidal ZIF-8 precursor was controlled in the range of 85-91% by centrifugation; In one embodiment of the present invention, step S3 comprises: adding N,N-dimethylformamide (DMF) to the colloidal ZIF-8 precursor, mixing evenly, and standing for 2-6 hours. Separating the colloidal ZIF-8 precursor by centrifugation. Then centrifuging and washing with N,N-dimethylformamide (DMF) to control the mass fraction of N,N-dimethylformamide (DMF) in the colloidal ZIF-8 precursor to be in the range of 85-91%.

[0026] S4: polyacrylonitrile (PAN), colloidal ZIF-8 precursor, and nickel salt are mixed and stirred to form a spinning solution.

[0027] In one embodiment of the present invention, step S4 includes: taking 2 g of PAN and adding it to a solution prepared by DMF, uniformly mixing the PAN solution with the colloidal ZIF-8 precursor, and adding nickel chloride hexahydrate at the same time, stirring for at least 24 hours, wherein PAN:ZIF-8=1.1 (±0.1) (solid content mass ratio), the total solid content is 13.5%-15%, and the mass fraction of nickel is 1%-2%.

[0028] S5: Spinning the spinning solution into a uniform fiber membrane through electrospinning technology. In one embodiment of the present invention, the electrospinning voltage is set to 14 kV-18 kV, and the flow rate is 1.2 ml / min-2 ml / min.

[0029] Electrospinning technology is a technology that uses high-voltage electrostatic fields to prepare ultrafine fiber membranes. Its core principle is to form a strong electric field between the spinneret and the grounded collection device through a high-voltage power supply to charge the spinning solution. When the injection pump pushes the spinning solution to the spinneret, the droplets form a Taylor cone under the action of the electric field force. When the electric field strength increases to a critical value, the electric field force will overcome the surface tension of the liquid and spray out from the Taylor cone. The jet stream is stretched violently, and at the same time, the jet stream oscillates and becomes unstable under the action of the high electric field, producing an irregular spiral motion with a very high frequency. In the high-speed oscillation, the jet stream is rapidly stretched, and the solvent also evaporates rapidly, eventually forming fibers with diameters ranging from nanometers to micrometers, which are randomly scattered on the collection device to form a fiber membrane.

[0030] S6: heating and oxidizing the fiber membrane in air. In one embodiment of the present invention, step S6 heats and oxidizes the fiber membrane in air at 250° C. for 1 hour, with a heating rate of 2° C. / min.

[0031] S7: calcining the fiber membrane in a nitrogen atmosphere to obtain an integrated self-supporting electrode. In one embodiment of the present invention, step S7 calcines the fiber membrane in a nitrogen atmosphere at 900°C-1050°C for 2-4 hours with a heating rate of 5°C / min. Finally, a nickel single atom integrated self-supporting electrode is obtained.

[0032] The complete preparation process of the integrated self-supporting electrode of Example 1 of the present invention is as follows: 1mmol zinc nitrate hexahydrate is dissolved in 200ml methanol as solution one, and 2.5mmol 2-methylimidazole and 25mmol butylamine are dissolved in 200ml methanol as solution two. Pour solution two into solution one, let stand for 1h to wait for full reaction, and then separate the synthesized colloidal ZIF-8 precursor by centrifugation. In order to obtain a better ZIF-8 precursor, the ZIF-8 precursor is centrifuged and washed twice with methanol. N,N-dimethylformamide (DMF) is added to the colloidal ZIF-8 precursor, mixed evenly, and allowed to stand for 6h. The colloidal ZIF-8 precursor is separated by centrifugation. Then centrifuge and wash with N,N-dimethylformamide (DMF) to control the mass fraction of N,N-dimethylformamide (DMF) in the colloidal ZIF-8 precursor to 90%. Take 2g of PAN and add it to the solution prepared by DMF, mix the PAN solution with the colloidal ZIF-8 precursor evenly, add nickel chloride hexahydrate at the same time, and stir for 24h, where PAN:ZIF-8=1.1 (solid content mass ratio), the total solid content is 13.5%, and the mass fraction of nickel is 1%. The above spinning solution is spun into a uniform fiber membrane by electrospinning technology. The electrospinning voltage is set to 16kv and the flow rate is 1.2 ml / min. The fiber membrane obtained by spinning is heated and oxidized at 250℃ in air for 1h, with a heating rate of 2℃ / min. The fiber membrane is then calcined at 900℃-1050℃ for 2h in a nitrogen atmosphere, with a heating rate of 5℃ / min. Finally, a nickel single-atom integrated self-supporting electrode is obtained.

[0033] Figure 2 This is a photograph of a nickel single-atom integrated self-supporting electrode made by the preparation method of Example 1 of the present invention. It can be seen that the integrated self-supporting electrode forms a uniform, stable sheet structure with certain mechanical strength.

[0034] Figure 3 The X-ray diffraction pattern (XRD) of the nickel single-atom integrated self-supporting electrode prepared in Example 1 of the present invention is as follows: Figure 3 It can be seen that the nickel single-atom integrated self-supporting electrode has only a broad peak of amorphous carbon in the range of 20°-30°, and no relevant diffraction peaks of nickel or zinc metal clusters are seen.

[0035] The electrocatalytic carbon dioxide reduction performance test was carried out on the nickel single-atom integrated self-supporting electrode obtained in the above Example 1, wherein the electrocatalytic carbon dioxide reduction test was carried out in the following three-electrode system: the reaction cell was an H cell in which the cathode chamber and the anode chamber were separated by an anion exchange membrane, the reference electrode was a saturated Ag / AgCl electrode, a platinum sheet was used as a counter electrode, and the working electrode preparation method was as follows: the nickel single-atom integrated self-supporting electrode was ground into a catalyst, 12 mg of the catalyst, 180 μl of 5 wt% Nafion, 600 μl of ethanol, and 200 μl of water were mixed, and ultrasonic dispersion was performed for 1 hour to finally obtain the prepared catalyst slurry; the prepared slurry was then evenly drop-coated on SGL 36BB carbon paper and used as a working electrode after drying; the electrolyte was a CO2-saturated 0.5 M KHCO3 aqueous solution, and CO2 gas was continuously introduced during the carbon dioxide reduction process.

[0036] In Example 1 of the present invention, the parameters of the electrocatalytic carbon dioxide reduction reaction include: temperature of 25°C; pressure of one standard atmosphere, i.e., 1 atm; and reduction potential range of preferably -1.3 to -2.0 V relative to a saturated Ag / AgCl electrode. Within the reduction potential range of -1.3 to -2.0 V, reduction potentials are taken at intervals of 0.1 V, for a total of 8 reduction potentials, to perform constant potential electrocatalytic carbon dioxide reduction. Figure 4 and Figure 5 All potentials are relative to the reversible hydrogen electrode (RHE), and the relevant potential calculations are based on the Nernst equation (Formula 1): E(vs.RHE)=E(vs.Ag / AgCl)+0.204V+0.0591×pH (Formula 1).

[0037] When testing the Faraday efficiency, the working electrode was kept at a constant potential for 15 minutes, the total current was recorded using an electrochemical workstation, and the gas products produced were detected using a Shimadzu GC-2014 gas chromatograph (Shimadzu, Japan). The applied voltage range during the test was -1.3 to -2.0 V, and only CO and H2 were detected in the gas phase products. The partial current density was calculated according to (Formula 2, Formula 3): Partial current = total current × Faraday efficiency (Formula 2), Partial current density = partial current / electrode area (Formula 3).

[0038] Figure 4 This is a graph of the current density of the integrated self-supporting electrode prepared in Example 1 of the present invention, see Figure 4 It can be seen that at -0.85 V, the CO current density of the working electrode prepared in Example 1 can reach 30 mA / cm 2 .

[0039] Figure 5 This is a Faraday efficiency diagram of the integrated self-supporting electrode prepared in Example 1 of the present invention, see Figure 5 It can be seen that the catalyst prepared in Example 1 of the present invention has a Faraday efficiency of 95% for reducing carbon dioxide to carbon monoxide at -0.65 V.

[0040] In order to verify that the nickel single-atom integrated self-supporting electrode prepared by the present invention can replace the preparation of traditional electrodes, performance tests were carried out using membrane electrodes. Figure 6 A schematic diagram of a membrane electrode of an integrated self-supporting electrode prepared according to an embodiment of the present invention, see Figure 6 It can be seen that the cathode and anode are separated by the anion exchange membrane Grade RT in the membrane electrode. In this embodiment, the cathode is a nickel single atom integrated self-supporting electrode prepared by the present invention, and the anode is a titanium fiber paper loaded with iridium oxide. 20sccm of CO2 (99.999%) is passed into the cathode flow channel, and 0.5M KHCO3 solution is passed into the anode flow channel. After applying a constant current (50mA, 100mA, 150mA, 200mA) to the membrane electrode, CO2 is reduced at the cathode, and the product is detected by gas chromatography to calculate the Faraday efficiency. The effective area of ​​the membrane electrode is 1 square centimeter, and the current density is calculated according to (Formula 3).

[0041] Figure 7 This is a Faraday efficiency diagram of the integrated self-supporting electrode prepared in Example 1 of the present invention for use as a membrane electrode. Figure 7 It can be seen that the nickel single-atom integrated self-supporting electrode prepared by the present invention has a high conductivity at 100 mA / cm 2 At a current density of 1.34 W, the Faradaic efficiency of carbon monoxide reaches 80%.

[0042] The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention. The protection scope of the present invention should be defined by the claims.

Claims

1. A method for preparing an integrated self-supporting electrode, characterized in that: The steps include: S1: dissolving zinc salt to form solution 1, dissolving ligand and butylamine to form solution 2, mixing the two solutions to react fully, and then separating the synthesized colloidal ZIF-8 precursor by centrifugation; S2: The residual methanol in the colloidal ZIF-8 precursor was exchanged with N,N-dimethylformamide by solvent exchange method; S3: Control the mass fraction of N,N-dimethylformamide in the colloidal ZIF-8 precursor in the range of 85-91% by centrifugation; S4: mixing and stirring polyacrylonitrile, colloidal ZIF-8 precursor and nickel salt to form a spinning solution; S5: spinning the spinning solution into a uniform fiber membrane through electrospinning technology; S6: heating and oxidizing the fiber membrane in air; S7: calcining the fiber membrane under a nitrogen atmosphere to obtain an integrated self-supporting electrode.

2. The method for preparing an integrated self-supporting electrode according to claim 1, characterized in that: The step S4 further comprises: mixing polyacrylonitrile, colloidal ZIF-8 precursor, and nickel salt, and stirring for at least 24 hours, wherein the mass ratio of polyacrylonitrile to colloidal ZIF-8 precursor solid content is 1.0-1.2, the total solid content is 13.5%-15%, and the mass fraction of nickel is 1%-2%.

3. The method for preparing an integrated self-supporting electrode according to claim 1, characterized in that: When spinning a uniform fiber membrane in step S5, the electrospinning voltage is set to 14 kv-18 kv and the flow rate is set to 1.2 ml / min-2 ml / min.

4. The method for preparing an integrated self-supporting electrode according to claim 1, characterized in that: The step S6 further comprises: oxidizing the fiber membrane in air at 250° C. for 1 hour at a heating rate of 2° C. / min.

5. The method for preparing an integrated self-supporting electrode according to claim 1, characterized in that: The step S7 further comprises: calcining the fiber membrane in a nitrogen atmosphere at a temperature of 900° C.-1050° C. for 2-4 hours at a heating rate of 5° C. / min to obtain an integrated self-supporting electrode.

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