Preparation Method of Integrated Self-Supporting Electrode

Abstract

The present invention discloses a preparation method of an integrated self-supporting electrode. By dissolving and mixing a zinc salt, a ligand and butylamine and reacting fully, a colloidal ZIF-8 precursor is formed; then the mass fraction of N,N-dimethylformamide in the colloidal ZIF-8 precursor is controlled within the range of 85-91%, and then polyacrylonitrile, the colloidal ZIF-8 precursor and a nickel salt are mixed and stirred, and through electrospinning technology, a uniform fiber membrane is spun, and then heat oxidation and calcination are carried out to obtain the integrated self-supporting electrode. The prepared electrode has a stable overall structure and high efficiency, and has broad application prospects in the fields of electrocatalytic carbon dioxide reduction process and the like.

Description

Technical Field

[0001] The present invention relates to the field of catalysis or colloid chemistry, and particularly to a preparation method of an integrated self-supporting electrode mainly composed of a nickel single-atom catalyst. Background Art

[0002] Capturing and resource-utilizing carbon dioxide (CO2) is of great significance for addressing the increasingly serious energy and environmental crises. Electrochemical catalytic reduction (eCO2RR) of CO2 using electrical energy generated from renewable energy sources such as solar energy and wind energy to prepare high-value-added fuels and chemical products is an ideal solution for reducing CO2 emissions. There are numerous CO2 reduction products, including C1 products such as carbon monoxide (CO), formic acid (salt), methane, and multi-carbon (C2+) products such as ethylene, ethanol, and propanol. Among these reduction products, the reduction of CO2 to CO only involves the transfer of two electrons and two protons, having certain kinetic advantages compared to other products. Meanwhile, the product of CO2 reduction to CO is usually only CO and by-product H2. By adjusting the structure and composition of the catalyst active sites, the ratio of CO / H2 in the product can be effectively regulated. The syngas (CO / H2) generated can be directly used as the raw material for subsequent thermocatalytic reactions (such as the 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 tandem reactor as the first-step reaction for preparing C2+ products by eCO2RR. In summary, achieving high-conversion and high-selectivity electroreduction of CO2 to CO under industrial current conditions (>200 mA·cm -2 ) is a key step in realizing the resource utilization of CO2 to prepare high-value-added products.

[0003] Currently, the reduction of CO2 to CO usually requires noble metals such as Au, Ag, Pd or their alloys as catalysts. However, noble metal catalysts have disadvantages such as high cost and low reserves. Moreover, with the large-scale industrialization of eCO2RR, the price of noble metal catalysts is bound to further skyrocket. Therefore, there is an urgent need to find cheap and efficient non-noble metal catalysts to promote the large-scale industrialization of eCO2RR. Among them, single-metal atom M-N-C catalysts (M = transition metals such as Fe, Co, Ni, etc.) have the advantages of high atomic utilization rate, low cost, and good selectivity, and are potential catalysts to replace noble metal catalysts to achieve efficient conversion of CO2 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] Rationally designing catalysts and reactors to reduce the electrolytic cell voltage, improve product selectivity, current density, and stability is the key to promoting the large-scale industrialization of eCO2RR. In addition to catalysts, optimizing the reactor structure has an important impact on achieving industrial current density and improving CO2 conversion rate. In traditional H-type reactors, the current density is limited by the solubility and diffusion rate of CO2 in the electrolyte, so it is not suitable for industrialization. Using gas diffusion electrodes can break through the limitation of CO2 solubility and enable the current density of the eCO2RR electrolytic cell to meet industrial requirements. In particular, membrane electrode assembly (MEA) electrolytic cells use polymer anion / cation exchange membranes to replace traditional liquid electrolytes, minimizing the distance between the cathode and the anode and effectively reducing ohmic polarization. In traditional membrane electrodes, the catalyst layer is prepared by mixing active substances, carriers (electronic conductors), and electrolyte solutions (ionic conductors) in a certain proportion. The transport channels for substances such as electrons, ions, gases, and water are all in a disordered state. There is strong concentration polarization and ohmic polarization, which severely restricts the current density and energy efficiency of membrane electrode electrolytic cells. Secondly, due to the weak contact between the gas diffusion layer and the electrocatalyst, electroactive substances are easily separated and shed, reducing the long-term performance of CO2 electroreduction. At the same time, the enriched catalyst layer will create a locally highly alkaline environment during the reaction, easily causing salting-out phenomena and reducing the overall stability.

[0005] Integrated self-supporting electrodes can integrate the gas diffusion layer and the catalyst layer, and have many advantages in applications. 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 ordered transport channels 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 poses higher requirements for material synthesis. It not only needs to have a rich porous structure to enhance mass transfer but also sufficient support strength; at the same time, regulating the three-phase interface environment of carbon-based integrated self-supporting electrodes to ensure a sufficient number of accessible active sites is also a difficulty in the application of carbon-based integrated self-supporting electrodes. Therefore, developing highly efficient carbon-based integrated self-supporting structure electrodes for CO2RR still faces huge challenges. Summary of the Invention

[0006] (I) Technical Problems to be Solved

[0007] The technical problem to be solved by the present invention is to provide a preparation method for an integrated self-supporting electrode with stable structure and high efficiency.

[0008] (II) Technical Solution

[0009] To solve the above problems, on the one hand, the present invention proposes a preparation method of an integrated self-supporting electrode, including the following steps:

[0010] S1: Dissolve zinc salt to form Solution 1, dissolve ligand and butylamine to form Solution 2, then mix the two solutions and react fully, and then centrifugally separate the synthesized colloidal ZIF-8 (zeolitic imidazolate framework) precursor;

[0011] S2: Through the solvent exchange method, exchange the residual methanol in the colloidal ZIF-8 precursor with N,N-dimethylformamide;

[0012] S3: By centrifugation, control the mass fraction of N,N-dimethylformamide in the colloidal ZIF-8 precursor in the range of 85-91%;

[0013] S4: Mix polyacrylonitrile, colloidal ZIF-8 precursor, and nickel salt and stir to form a spinning solution;

[0014] S5: Spin the above spinning solution into a uniform fiber membrane by electrospinning technology;

[0015] S6: Heat and oxidize the fiber membrane in air;

[0016] S7: Calcinate the fiber membrane in a nitrogen atmosphere to obtain an integrated self-supporting electrode.

[0017] Preferably, step S4 further includes: mixing polyacrylonitrile, colloidal ZIF-8 precursor, and nickel salt, and stirring for at least 24 h, wherein the mass ratio of polyacrylonitrile to the solid content of the colloidal ZIF-8 precursor is 1.0-1.2, the total solid content is 13.5%-15%, and the mass fraction of nickel is 1%-2%.

[0018] Preferably, when spinning the uniform fiber membrane in step S5, the electrospinning voltage is set to 14 kv - 18 kv, and the flow rate is 1.2 ml / min - 2 ml / min.

[0019] Preferably, step S6 further includes: first oxidizing the fiber membrane in air at a temperature of 250 °C for 1 hour, and the heating rate is 2 °C / min.

[0020] Preferably, step S7 further includes: calcining the fiber membrane in a nitrogen atmosphere at a temperature of 900 °C - 1050 °C for 2-4 hours, and the heating rate is 5 °C / min to obtain an integrated self-supporting electrode.

[0021] (III) Beneficial Effects

[0022] 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

[0023] Figure 1 Schematic diagram of the process of preparing an integrated self-supporting electrode according to an embodiment of the present invention.

[0024] 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.

[0025] Figure 3 This is the X-ray diffraction pattern of Example 1 of the present invention.

[0026] 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.

[0027] Figure 5 This is a Faraday efficiency diagram of the integrated self-supporting electrode prepared in Example 1 of the present invention.

[0028] Figure 6 Schematic diagram of the membrane electrode of the integrated self-supporting electrode prepared in Example 1 of the present invention.

[0029] 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

[0030] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0031] 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:

[0032] 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:

[0033] In an embodiment of the present invention, step S1 includes the following steps: Dissolve 1 mmol of zinc nitrate hexahydrate in 200 ml of methanol to form solution one, and dissolve 2.5 mmol of 2-methylimidazole and 25 mmol of butylamine in 200 ml of methanol to form solution two. Pour solution two into solution one, and let it stand for 1 - 1.5 h to wait for sufficient reaction, and then centrifuge to separate the synthesized colloidal ZIF-8 precursor.

[0034] 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.

[0035] In order to obtain a better ZIF-8 precursor, the ZIF-8 precursor can be centrifugally washed with methanol twice.

[0036] S2: Through the solvent exchange method, exchange the residual methanol in the colloidal ZIF-8 precursor for N,N-dimethylformamide (DMF).

[0037] S3: By centrifugation, control the mass fraction of N,N-dimethylformamide (DMF) in the colloidal ZIF-8 precursor within the range of 85 - 91%;

[0038] In an embodiment of the present invention, step S3 includes: Add N,N-dimethylformamide (DMF) to the colloidal ZIF-8 precursor, mix evenly, and let it stand for 2 - 6 h. Separate the colloidal ZIF-8 precursor by centrifugation. Then, centrifugally wash with N,N-dimethylformamide (DMF) to control the mass fraction of N,N-dimethylformamide (DMF) in the colloidal ZIF-8 precursor within the range of 85 - 91%.

[0039] S4: Mix and stir polyacrylonitrile (PAN), the colloidal ZIF-8 precursor, and nickel salt to form a spinning solution.

[0040] In an embodiment of the present invention, step S4 includes: Take 2 g of PAN and add it to a solution prepared with DMF, uniformly mix the PAN solution with the colloidal ZIF-8 precursor, and simultaneously add nickel chloride hexahydrate and stir for at least 24 h, where PAN:ZIF-8 = 1.1 (±0.1) (mass ratio of solids content), the total solid content is 13.5% - 15%, and the mass fraction of nickel is 1% - 2%.

[0041] S5: Spin the above spinning solution into a uniform fiber membrane through electrospinning technology. In an 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.

[0042] Electrospinning technology is a technique for preparing ultrafine fiber membranes using a high-voltage electrostatic field. Its core principle is to form a strong electric field between the spinneret and the grounded collection device through a high-voltage power supply, making the spinning solution charged. When the syringe pump pushes the spinning solution to the spinneret, the liquid 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 overcomes the surface tension of the liquid and sprays out from the Taylor cone. The jet stream is violently stretched, and at the same time, the jet stream oscillates unstably under the action of the high electric field, generating an extremely high-frequency irregular spiral motion. During the high-speed oscillation, the jet stream is rapidly thinned, and the solvent also rapidly volatilizes, finally forming fibers with diameters ranging from nanometers to micrometers and scattered randomly on the collection device to form a fiber membrane.

[0043] S6: Heat-oxidize the fiber membrane in air. In an embodiment of the present invention, in step S6, the fiber membrane is heat-oxidized in air at 250 °C for 1 h with a heating rate of 2 °C / min.

[0044] S7: Calcinate the fiber membrane in a nitrogen atmosphere to obtain an integrated self-supporting electrode. In an embodiment of the present invention, in step S7, the fiber membrane is calcined in a nitrogen atmosphere at 900 °C - 1050 °C for 2 - 4 h with a heating rate of 5 °C / min. Finally, a nickel single-atom integrated self-supporting electrode is obtained.

[0045] The complete preparation process of the integrated self-supporting electrode in Example 1 of the present invention is as follows: Dissolve 1 mmol of zinc nitrate hexahydrate in 200 ml of methanol as Solution 1, and dissolve 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 it stand for 1 h for sufficient reaction, and then centrifuge to separate the synthesized colloidal ZIF-8 precursor. To obtain a better ZIF-8 precursor, the ZIF-8 precursor is centrifuged and washed with methanol twice. Add N,N-dimethylformamide (DMF) to the colloidal ZIF-8 precursor, mix evenly, and let it stand for 6 h. Separate the colloidal ZIF-8 precursor 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 2 g of PAN and dissolve it in the solution prepared with DMF. Mix the PAN solution evenly with the colloidal ZIF-8 precursor, and at the same time add nickel chloride hexahydrate and stir for 24 h, where PAN:ZIF-8 = 1.1 (mass ratio of solids content), the total solid content is 13.5%, and the mass fraction of nickel is 1%. Spin the above spinning solution into a uniform fiber membrane by electrospinning technology. The electrospinning voltage is set to 16 kv and the flow rate is 1.2 ml / min. Heat and oxidize the fiber membrane obtained by electrospinning in air at 250 °C for 1 h with a heating rate of 2 °C / min. Then, calcine the fiber membrane in a nitrogen atmosphere at 900 °C - 1050 °C for 2 h with a heating rate of 5 °C / min. Finally, a nickel single-atom integrated self-supporting electrode is obtained.

[0046] Figure 2 Figure of the nickel single-atom integrated self-supporting electrode prepared 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 and sheet-like structure with certain mechanical strength.

[0047] Figure 3 X-ray diffraction pattern (XRD) of the nickel single-atom integrated self-supporting electrode prepared in Example 1 of the present invention. Figure 3 It can be seen that the nickel single-atom integrated self-supporting electrode only has a broad peak of amorphous carbon in the range of 20° - 30°, and no diffraction peaks related to nickel or zinc metal clusters are observed.

[0048] The nickel single-atom integrated self-supported electrode obtained in the above Example 1 was subjected to electrocatalytic carbon dioxide reduction performance testing. Among them, the electrocatalytic carbon dioxide reduction test was carried out in the following three-electrode system: The reaction cell was an H cell with an anion exchange membrane separating the cathode chamber and the anode chamber. The reference electrode was a saturated Ag / AgCl electrode, and a platinum sheet was used as the counter electrode. The working electrode was prepared as follows: The nickel single-atom integrated self-supported electrode was ground into a catalyst, and 12 mg of the catalyst, 180 μL of 5 wt% Nafion, 600 μL of ethanol, and 200 μL of water were mixed and ultrasonically dispersed for 1 hour to finally obtain the prepared catalyst slurry. Then, the prepared slurry was evenly drop-coated on Sigri 36BB carbon paper and dried to be used as the working electrode. The electrolyte was a 0.5 M KHCO3 aqueous solution saturated with CO2, and CO2 gas was continuously introduced during the carbon dioxide reduction process.

[0049] In Example 1 of the present invention, the parameters of the electrocatalytic carbon dioxide reduction reaction include: the temperature was 25 °C; the pressure was one standard atmosphere, that is, 1 atm; the reduction potential range was preferably -1.3 to -2.0 V, relative to the saturated Ag / AgCl electrode. Within the reduction potential range of -1.3 to -2.0 V, the reduction potential was sampled at 0.1 V intervals, with a total of 8 reduction potentials, and constant potential electrocatalytic carbon dioxide reduction was carried out. Figure 4 and Figure 5 All the potentials in [reference] are relative to the reversible hydrogen electrode (RHE), and the relevant potential calculations are carried out according to the Nernst equation (Formula 1):

[0050] E(vs.RHE) = E(vs.Ag / AgCl) + 0.204 V + 0.0591 × pH (Formula 1).

[0051] When testing the Faraday efficiency, the working electrode was held at a constant potential for 15 minutes, the total current was recorded using an electrochemical workstation, and the gas products generated were detected using a Shimadzu GC-2014 gas chromatograph (Shimadzu Corporation, Japan). The range of the applied voltage 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):

[0052] Partial current = Total current × Faraday efficiency (Formula 2),

[0053] Partial current density = Partial current / Electrode area (Formula 3).

[0054] Figure 4 This is the partial current density diagram of the integrated self-supported electrode prepared in Example 1 of the present invention. See Figure 4 It can be seen that at -0.85 V, for the working electrode prepared in Example 1, the partial current density of CO can reach 30 mA / cm2 。

[0055] Figure 5 This is the Faraday efficiency graph of the integrated self-supporting electrode prepared in Example 1 of the present invention. Refer to Figure 5 It can be seen that when the catalyst prepared in Example 1 of the present invention is at -0.65V, the Faraday efficiency of carbon dioxide reduction to carbon monoxide reaches 95%.

[0056] 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, a membrane electrode was used for performance testing. Figure 6 This is the schematic diagram of the membrane electrode of the integrated self-supporting electrode prepared in an embodiment of the present invention. Refer to Figure 6 It can be seen that in the membrane electrode, the anode and cathode are separated by an anion exchange membrane Grade RT. In this embodiment, the cathode is the nickel single-atom integrated self-supporting electrode prepared by the present invention, and the anode is a titanium fiber paper loaded with iridium oxide. 20 sccm of CO2 (99.999%) is introduced into the cathode flow channel, and a 0.5 M KHCO3 solution is introduced into the anode flow channel. After applying a constant current (50 mA, 100 mA, 150 mA, 200 mA) to the membrane electrode, CO2 is reduced at the cathode, and at the same time, the products are 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).

[0057] Figure 7 This is the Faraday efficiency graph of the integrated self-supporting electrode prepared in Example 1 of the present invention for the membrane electrode. Refer to Figure 7 It can be seen that in the membrane electrode test of the nickel single-atom integrated self-supporting electrode prepared by the present invention, at a current density of 100 mA / cm 2 the Faraday efficiency of carbon monoxide reaches 80%.

[0058] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also 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, and the protection scope of the present invention shall be defined by the claims.

Claims

1. Preparation method of an integrated self-supporting electrode, characterized in that The steps include: S1: dissolving zinc salt in methanol to form solution 1, dissolving ligand and butylamine in methanol 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 preparation method of the 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 preparation method of the 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 preparation method of the integrated self-supporting electrode according to claim 1, wherein 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 preparation method of the 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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