Iridium-zinc ultrathin nanosheet material, preparation method and use thereof
By preparing iridium-zinc ultrathin nanosheet materials and combining the heterogeneous interface between crystalline and amorphous phases, the problems of high cost and poor performance of Ir-based catalysts in the oxygen evolution reaction in acidic environments have been solved, achieving high activity and stable oxygen evolution performance, which is suitable for industrial applications in proton exchange membrane water electrolyzers.
Patent Information
- Application Number
- CN202411442610.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-16
- Publication Date
- 2025-10-24
- Estimated Expiration
- 2044-10-16
AI Technical Summary
Existing Ir-based noble metal catalysts suffer from high cost and poor performance when used in the oxygen evolution reaction of water splitting to produce hydrogen in acidic environments. In particular, the activity and stability of Ir-based catalysts are insufficient in proton exchange membrane water electrolysis, making it difficult to meet the needs of industrial applications.
By using iridium-zinc ultrathin nanosheets, iridium-zinc ultrathin nanosheets with atomic heterostructure are prepared. By combining the heterogeneous interface between the crystalline phase and the amorphous phase, strong electronic interactions are formed, which optimizes the electronic structure of the Ir active sites and reduces the reaction energy barrier, thereby improving the oxygen evolution reaction kinetics.
The iridium-zinc ultrathin nanosheet material achieved high activity and excellent stability in acidic environments, with an oxygen evolution overpotential of less than 271mV at a current density of 10mA cm-2. The slope of the Tafel curve and the charge transfer resistance were also significantly reduced. The stability test showed no significant decay after 115 hours, which is significantly better than traditional materials.
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Abstract
Description
TECHNICAL FIELD
[0001] The present application belongs to the technical field of hydrogen production materials by electrocatalytic water splitting in acidic environment, and particularly relates to an iridium-zinc ultrathin nanosheet material, a preparation method thereof and an application of the material to oxygen evolution electrode materials. BACKGROUND
[0002] With the continuous development of economy, the global demand for energy is expanding, and energy crisis and environmental problems are becoming increasingly prominent. Under this background, it is urgent to develop clean and sustainable energy. Hydrogen energy has the characteristics of high energy density (~120 MJ / kg), environmental friendliness and sustainability, and is an ideal energy to solve global environmental problems and energy demand. Traditional fossil fuel hydrogen production has the problems of low hydrogen purity and environmental pollution, while clean, safe and efficient water splitting for hydrogen production is a promising hydrogen production technology. However, the slow four-electron transfer in the oxygen evolution reaction is a great challenge to water splitting, and the reaction kinetics is relatively slow, which is the rate-limiting step in the water splitting reaction process. Proton exchange membrane water electrolysis (PEMWE) is considered to be one of the most efficient, stable and reliable hydrogen production technologies, which has the advantages of high hydrogen purity, high maximum allowable current density, high conversion rate and large hydrogen production capacity; however, stable electrolysis in acidic environment and low electrolytic cell voltage usually require Ir-based noble metal catalysts, which increases the cost of hydrogen production. Therefore, it is essential to develop Ir-based anode oxygen evolution electrocatalysts with high quality activity and stability in acidic medium to promote the industrial application of PEMWE.
[0003] Transition metal modification is a method that can simultaneously reduce the loading of noble metals and improve the IrO xA simple and effective strategy for acidic oxygen evolution performance. Due to the strong electronic interaction of the atomic heterostructure, it helps to regulate the electronic structure of the Ir active site and reduce the electron density, thereby optimizing the adsorption / desorption energy of the reaction intermediates on the surface of the active site, also reducing the reaction energy barrier and improving the oxygen evolution reaction (OER) kinetics. In addition, there are abundant water or hydroxide molecular adsorption sites in the amorphous phase, and the crystalline phase has good conductivity and stability, which can drive the transfer of electrons to the adsorbed molecules, promote the charge transfer between the adsorbed species and the surface of the catalyst, and the synergistic effect between the crystalline phase and the amorphous phase at the heterojunction can also accelerate the originally slow OER kinetics. Therefore, by modifying non-noble metals to construct atomic heterostructures and crystalline / amorphous phase interfaces, it is expected to further improve the OER performance. Metal Zn has the characteristics of non-toxic, cheap and easy to obtain, but its acidic OER activity and stability are poor; in the literature (Tuning the hybridization state of Ir-O to improve the OER activity and stability of iridium pyrochlore via Zn doping, Applied Surface Science, 2022.) Lu 1.8 Zn 0.2 The oxygen evolution overpotential of the Ir2O7 nanoparticles is 331 mV at a current density of 10 mA cm-2 -2 , and the mass activity is only 135.32 A g-1 at a voltage of 1.58 V vs. RHE -1 Ir , and its oxygen evolution stability is only 11 hours at a current density of 10 mA cm-2 -2 , although the activity is significantly improved after doping Zn (the mass activity of Lu2Ir2O7 is 108.75 A g-1 -1 Ir ), but its oxygen evolution activity and stability are still poor, and the stable running time is short.
[0004] Therefore, the present application is proposed. SUMMARY
[0005] To solve the problems of expensive and poor performance of existing water decomposition anode oxygen evolution electrode materials, the present application provides an iridium-zinc ultrathin nanosheet material with high activity and excellent stability. The iridium-zinc ultrathin nanosheet material prepared by the present application is used for oxygen evolution electrode, and the oxygen evolution performance is significantly improved.
[0006] The technical scheme of the present application is as follows:
[0007] The first aspect of the present application discloses an iridium-zinc ultra-thin nanosheet material with high activity and excellent stability, wherein the thickness of the nanosheet is 5-20 nm.
[0008] Preferably, the atomic ratio of Zn / Ir in the iridium-zinc ultra-thin nanosheet material is (11.5-4.9):1.
[0009] Preferably, the iridium-zinc ultra-thin nanosheet material has an atomic heterostructure, and a hetero-interface between a crystalline phase and an amorphous phase exists.
[0010] The second aspect of the present application discloses a preparation method of the iridium-zinc ultra-thin nanosheet material, comprising the following steps:
[0011] (1) Dissolve a zinc precursor and 2-methylimidazole in methanol respectively to obtain respective methanol solutions;
[0012] (2) Mix the two methanol solutions of step (1), separate, wash, and dry to obtain a white solid;
[0013] (3) Disperse the white solid in ultrapure water to obtain a dispersion liquid, dropwise add an iridium precursor solution to the dispersion liquid, mix, separate, wash, and dry to obtain a light yellow solid;
[0014] (4) Calcine the light yellow solid powder to obtain a gray-blue powder, which is the iridium-zinc ultra-thin nanosheet material.
[0015] Preferably, the zinc precursor of step (1) is Zn(NO3)2·6H2O, and the iridium precursor solution of step (3) is chloroiridic acid solution.
[0016] Preferably, the mixing temperature of step (2) is 25-35℃, and the mixing time is 3-4 hours; the washing method is to use methanol to wash at least three times; and the drying temperature is 50-80℃.
[0017] Preferably, the mixing temperature of step (3) is 35-45℃, and the mixing time is 10-12 hours; the washing method is to use water and ethanol to wash at least three times respectively in sequence; and the drying temperature is 50-80℃.
[0018] Preferably, the calcination temperature of step (4) is 380-450℃, and the calcination time is 4-5 hours.
[0019] The third aspect of the present application discloses the use of the iridium-zinc ultra-thin nanosheet material for an oxygen evolution electrode.
[0020] The present application has the following beneficial effects:
[0021] 1. The iridium-zinc metal oxide alloy material of the present application has an ultra-thin nanosheet structure, and the nanosheet thickness is 5-20 nm; when used for an oxygen evolution electrode, the ultra-thin nanosheet layer structure can strengthen the interface contact between the catalyst and the electrolyte, is conducive to fully exposing the active sites and increasing the electrochemical active area.
[0022] 2. The Ir and Zn atom heterostructure formed in the iridium-zinc ultra-thin nanosheet material of the present application has strong electronic interaction, which is helpful for regulating the electronic structure of the Ir active site and reducing the electron density; when used for an oxygen evolution electrode, the adsorption and desorption energy of the reaction intermediates on the active site surface is optimized, the reaction energy barrier is also reduced, and the oxygen evolution reaction kinetics is improved, so that excellent electron transfer ability and fast oxygen evolution reaction kinetics can be generated.
[0023] 3. The iridium-zinc ultra-thin nanosheet material of the present application has a crystalline phase and an amorphous phase heterostructure. There are abundant crystalline phase and amorphous phase hetero-interfaces, and appropriate proportions of metal oxygen and hydroxyl oxygen contents are generated; there are abundant water or hydroxide molecular adsorption sites in the amorphous phase, and the surface hydroxyl is easy to be converted into an electrophilic O I- species, which is conducive to the nucleophilic attack by water molecules or hydroxyl ligands, promotes O-O coupling to generate O2; the crystalline phase has good conductivity and stability, and can drive electron transfer to the adsorbed molecules, promoting the charge transfer between the adsorbed species and the catalyst surface; when used for an oxygen evolution electrode, the synergistic effect of the two phases can accelerate the originally slow oxygen evolution reaction kinetics, which is conducive to obtaining appropriate intermediate binding energy, thereby reducing the reaction energy barrier and improving the catalytic activity.
[0024] 4. The iridium-zinc ultra-thin nanosheet material of the present application has excellent oxygen evolution performance, and the oxygen evolution overpotential is only 271 mV when the current density reaches 10 mA cm -2 , which is lower than the 338 mV of iridium oxide and the oxygen evolution overpotential of zinc oxide (without activity); at the same time, the Tafel curve slope and charge transfer resistance are also lower than those of iridium oxide, and the iridium-zinc ultra-thin nanosheet material has excellent charge transfer ability and fast oxygen evolution reaction kinetics.
[0025] 5. The iridium-zinc ultra-thin nanosheet material of the present application has a stable iridium-zinc ultra-thin nanosheet structure, and in the oxygen evolution stability test under a current density of 10 mA cm -2 , the activity does not decrease significantly after running for 115 hours, which is significantly better than the performance of the Lu 1.8 Zn 0.2 Ir2O7 nanoparticles prepared by Zn doping reported in the literature.
[0026] 6、The preparation method of the iridium-zinc ultrathin nanosheet material is a simple impregnation-calcination method, and the high-activity iridium-zinc metal oxide alloy material with a crystalline phase / amorphous phase and an ultrathin nanosheet layer structure is synthesized for the first time. The preparation method has a wide raw material source and a simple synthesis path, and the high activity and strong stability of the iridium-zinc ultrathin nanosheet material are suitable for industrialized proton exchange membrane water electrolysis cells. The iridium-zinc ultrathin nanosheet material and the preparation method thereof are reported for the first time. BRIEF DESCRIPTION OF DRAWINGS
[0027] Figure 1 It is a scanning electron microscope graph of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application.
[0028] Figure 2 It is a transmission electron microscope graph of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application.
[0029] Figure 3 It is an X-ray diffraction graph of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application and the zinc oxide material prepared in Comparative Example 1.
[0030] Figure 4 It is an X-ray photoelectron spectroscopy of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application, the zinc oxide material prepared in Comparative Example 1 and the iridium oxide material of Comparative Example 2.
[0031] Figure 5 It is the oxygen evolution performance of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application, the zinc oxide material and the iridium oxide material in the comparative example: (a) polarization curve, (b) electrochemical impedance spectrogram and (c) Tafel curve.
[0032] Figure 6 It is the oxygen evolution stability test curve of the iridium-zinc ultrathin nanosheet material prepared in Example 1 of the present application. DETAILED DESCRIPTION
[0033] In order to make the purpose, preparation method and advantages of the present application more clear, the present application is further described below in combination with specific examples and drawings. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.
[0034] Example 1: Preparation of iridium-zinc ultrathin nanosheet material.
[0035] (1) 4.58 g of Zn(NO3)2·6H2O and 5.50 g of 2-methylimidazole were respectively dissolved in 200 mL of methanol to obtain respective methanol solutions;
[0036] (2) The 2-methylimidazole methanol solution was added to the stirring Zn(N03)2methanol solution, continuously stirred at a constant temperature of 30°C for 3 hours. The white solid was centrifugally separated and washed with methanol three times, and dried at 60°C under vacuum to obtain a ZIF-8 white solid powder;
[0037] (3) 100 mg of the ZIF-8 white powder was weighed out, dispersed in 60 mL of ultrapure water and ultrasonically treated for 1 hour; 0.5 mL of a 20 g / L chloroiridic acid solution was pipetted into a beaker and diluted with 10 mL of water; then the chloroiridic acid solution was added dropwise to the stirring ZIF-8 dispersion, the beaker was washed with 10 mL of water and added dropwise to the ZIF-8 dispersion, continuously stirred at a constant temperature of 40°C for 12 hours; then centrifugally separated, washed with water and ethanol three times in turn, and dried at a constant temperature of 60°C under vacuum to obtain a light yellow solid powder;
[0038] (4) The light yellow solid powder was placed in a muffle furnace, heated to 400°C at a rate of 5°C min -1 and calcined for 4 hours, naturally annealed to room temperature to obtain a grayish blue powder.
[0039] Comparative Example 1: Preparation of a zinc oxide material.
[0040] (1) 4.58 g of Zn(N03)2·6H20 and 5.50 g of 2-methylimidazole were separately dissolved in 200 mL of methanol;
[0041] (2) The 2-methylimidazole methanol solution was added to the stirring Zn(N03)2methanol solution, continuously stirred at a constant temperature of 30°C for 3 hours; the white solid was centrifugally separated and washed with methanol three times, and dried at 60°C under vacuum to obtain a ZIF-8 white solid powder;
[0042] (3) The ZIF-8 white solid powder was placed in a muffle furnace, heated to 450°C at a rate of 5°C min -1 and calcined for 4 hours, naturally annealed to room temperature to obtain zinc oxide. And the zinc oxide was used instead of the iridium-zinc ultrathin nanosheet to prepare an anode oxygen evolution working electrode.
[0043] Comparative Example 2: Commercial iridium oxide material.
[0044] The commercial iridium oxide was used instead of the iridium-zinc ultrathin nanosheet to prepare an anode oxygen evolution working electrode.
[0045] Preparation of the anodic oxygen evolution reaction working electrode: The materials obtained in the examples and comparative examples were made into ink and drop-coated on a glassy carbon electrode to prepare the working electrode. The steps are as follows: first, 4 mg of the solid powder obtained in the examples and comparative examples and 1 mg of carbon nanotubes were dissolved in a mixed solution consisting of 625.0 μL of ultrapure water, 312.5 μL of isopropanol, and 62.5 μL of Nafion, and the mixture was ultrasonically treated for 1 hour to obtain a catalyst ink; secondly, 3.5 μL of the catalyst ink was drop-coated on the glassy carbon electrode and dried at room temperature to obtain 0.2 mg cm -2 of catalyst loading.
[0046] Oxygen evolution reaction performance test: Using a saturated calomel electrode and a platinum mesh as the reference electrode and counter electrode respectively, the oxygen evolution performance was evaluated in 40 mL of 0.1 M HClO4 electrolyte solution at a constant temperature of 30°C. The oxygen evolution reaction activity was evaluated in the potential range of 1.2-1.8 V vs RHE at a rate of 2 mV s -1 The linear voltammetric scan was performed at a scan rate of 100 kHz. The voltage was set to 1.50 V vs RHE and the scan frequency range was 100 kHz-0.05 Hz for the oxygen evolution electrochemical impedance spectroscopy test. The current density was selected to be 10 mA cm -2 A long-term constant current stability test was performed to obtain the voltage-time (Et) curve.
[0047] Figure 1 This is a scanning electron microscope image of the iridium zinc ultrathin nanosheet material prepared in Example 1. Figure 1 It can be seen that the iridium-zinc metal oxide alloy prepared in Example 1 has an ultra-thin nanosheet structure, and the thickness of the nanosheet is about 10 nm.
[0048] Figure 2 This is a transmission electron microscope image of the iridium zinc ultrathin nanosheet material prepared in Example 1. Figure 2 (a) It can be seen that the nanosheet structure of the iridium zinc metal oxide alloy prepared in Example 1; Figure 2 (b) It can be found that the transmission electron microscopy image under high magnification clearly shows lattice fringes with lattice spacings of 0.249nm, 0.198nm and 0.166nm, which are slightly larger than the lattice spacings of the (101), (102) and (110) crystal planes of ZnO (0.247nm, 0.191nm, 0.162nm), confirming that Ir atoms are successfully embedded in the ZnO crystal to form an iridium-zinc metal oxide alloy; in addition, an amorphous structure is observed in the position close to the crystal, resulting in a rich crystalline and amorphous phase heterogeneous interface (white dotted line in the figure).
[0049] Figure 3 The X-ray diffraction patterns of the iridium zinc ultrathin nanosheet material prepared in Example 1 and the zinc oxide material prepared in Comparative Example 1 are shown;Figure 3 (a) It can be observed that both the iridium-zinc ultrathin nanosheets obtained in Example 1 and the zinc oxide obtained in Comparative Example 1 exhibit eight significant crystalline peaks of ZnO, which are respectively attributed to the (110), (002), (101), (102), (110), (103), (103) and (440) crystal planes of ZnO, and no significant Ir02crystalline peak is observed; from which Figure 3 (b) It can be found that the corresponding crystalline peaks in the iridium-zinc ultrathin nanosheets obtained in Example 1 are shifted to lower angles relative to the ZnO crystalline peaks, which is caused by the intercalation of Ir atoms into the ZnO lattice, indicating that the Ir atoms have been successfully intercalated into the ZnO lattice and formed iridium-zinc metal oxide alloy.
[0050] Figure 4 X-ray photoelectron spectroscopy of the iridium-zinc metal oxide alloy material prepared in Example 1, the zinc oxide prepared in Comparative Example 1 and the iridium oxide material of Comparative Example 2; from which Figure 4 (a) It can be seen that the Ir 4f peak position of the iridium-zinc ultrathin nanosheets of the present application is shifted to higher binding energy relative to the commercial iridium oxide material, while Figure 4 (b) It can be seen that the Zn 2p peak position of the iridium-zinc ultrathin nanosheets of the present application is negatively moved to lower binding energy relative to the zinc oxide material; which indicates that the electron transfer from Ir to Zn atoms occurs in the iridium-zinc ultrathin nanosheet material, and causes the Zn site to have higher electron density and the Ir site to have lower electron density; confirming that strong electronic interaction occurs between Ir and Zn atoms, and regulates the electronic structure of the Ir active site.
[0051] Figure 5 Oxygen evolution performance curves of the iridium-zinc metal oxide alloy material prepared in Example 1, the zinc oxide material prepared in Comparative Example 1 and the iridium oxide material of Comparative Example 2. From which Figure 5 (a) It can be seen that the iridium-zinc ultrathin nanosheet material prepared in Example 1 requires only 1.501 V voltage when reaching a current density of 10 mA cm -2 , and the overpotential is only 271 mV (the theoretical voltage for oxygen evolution is 1.23 V), which is lower than that of iridium oxide (1.568 V, overpotential of 338 mV) and zinc oxide (no activity), showing excellent oxygen evolution activity; at the same time Figure 5 (b) and Figure 5 (c) show that the iridium-zinc ultrathin nanosheet material prepared in the present application also has the smallest Tafel slope and charge transfer resistance value, which are as low as 35.4 mV dec -1 and 5.06 Ω cm 2 , which are much smaller than those of iridium oxide (73.2 mV dec -1 , 52.95 Ω cm 2), demonstrating its superior charge transfer ability, rapid oxygen evolution reaction kinetics, and higher catalytic activity. Therefore, the oxygen evolution performance of the iridium-zinc ultrathin nanosheet material prepared in Example 1 is far superior to that of the iridium oxide and zinc oxide materials in the comparative example.
[0052] Figure 6 This is the stability test curve of the iridium zinc ultrathin nanosheet material prepared in Example 1. Figure 6 It can be seen that its activity did not significantly decrease after 115 hours of oxygen evolution electrolysis, indicating its excellent stability during the water splitting process. Therefore, the iridium zinc ultrathin nanosheet material prepared in Example 1 of the present invention achieves a balance between high activity and excellent stability, and is expected to become a candidate material for the acidic oxygen evolution reaction (AOER) inexpensive and efficient.
[0053] Example 2
[0054] (1) The preparation of ZIF-8 white solid powder was the same as steps (1) and (2) in Example 1;
[0055] (2) Weigh 100 mg of ZIF-8 white powder and disperse it in 60 mL of ultrapure water by ultrasonication for 1 hour. Pipette 0.25 mL of 20 g / L chloroiridic acid solution into a beaker and dilute it with 10 mL of water. Then, add the chloroiridic acid solution dropwise to the stirred ZIF-8 dispersion. Wash the beaker with 10 mL of water and add it dropwise to the mixture. Stir continuously at a constant temperature of 40°C for 12 hours. Then, centrifuge to separate the light yellow solid, wash it three times with water and ethanol in sequence, and vacuum dry it at a constant temperature of 60°C to obtain a light yellow solid powder.
[0056] (3) The method for preparing gray-blue iridium-zinc metal oxide alloy powder by calcining light yellow solid powder is the same as step (4) of Example 1.
[0057] The acidic oxygen evolution activity of the iridium zinc ultrathin nanosheet material obtained in Example 2 is as follows: -2 The overpotential at the current density is 296 mV, and the charge transfer resistance is 11.31 Ω cm 2 , Tafel slope is 55.8mV dec -1 .
[0058] Example 3
[0059] (1) The preparation of ZIF-8 white solid powder was the same as steps (1) and (2) in Example 1.
[0060] (2) Take 100 mg of ZIF-8 white powder and disperse it in 60 mL of ultrapure water by ultrasonic for 1 hour, take 1.00 mL of 20 g / L chloroiridic acid solution into a beaker and dilute with 10 mL of water; then add the chloroiridic acid solution dropwise into the stirred ZIF-8 dispersion, wash the beaker with 10 mL of water and add it dropwise into the mixture, continuously stir at constant temperature of 40℃ for 12 hours; then centrifugal separate the light yellow solid, wash it with water and ethanol in turn for three times, and vacuum dry at constant temperature of 60℃ to obtain light yellow solid powder.
[0061] (3) The method for preparing gray-blue iridium-zinc metal oxide alloy powder by calcining the light yellow solid powder is the same as step (4) of Example 1.
[0062] The acidic oxygen evolution activity of the iridium-zinc ultrathin nanosheet material obtained in Example 3 is as follows: the overpotential is 279 mV at current density of 10 mA cm-2, the charge transfer resistance is 6.22 Ω cm-2, and the Tafel slope is 47.2 mV dec-1. -2 2 -1
[0063] The above examples are not a limitation of the present application, and the present application is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the essential scope of the present application also belong to the protection scope of the present application.
Claims
1. A method for preparing an iridium-zinc ultrathin nanosheet material, characterized in that, The iridium-zinc ultrathin nanosheet has a thickness of 5-20 nm; the iridium-zinc ultrathin nanosheet material has an atomic number ratio of Zn / Ir of (11.5-4.9) : 1; the iridium-zinc ultrathin nanosheet material has an atomic heterostructure and a crystalline phase and amorphous phase hetero-interface; The preparation method of the iridium-zinc ultrathin nanosheet material comprises the following steps: (1) dissolving a zinc precursor and 2-methylimidazole in methanol respectively to obtain respective methanol solutions; (2) mixing the two methanol solutions of step (1), separating, washing, and drying to obtain a white solid; (3) dispersing the white solid in water to obtain a dispersion liquid, adding an iridium precursor solution dropwise into the dispersion liquid, mixing, separating, washing, and drying to obtain a light yellow solid; (4) calcining the light yellow solid powder to obtain a gray-blue powder, which is the iridium-zinc ultrathin nanosheet material.
2. The production method according to claim 1, characterized by, The zinc precursor of step (1) is Zn(NO3)2·6H2O, and the iridium precursor solution of step (3) is chloroiridic acid solution.
3. The preparation method according to claim 1, characterized in that The mixing temperature of step (2) is 25-35℃, and the mixing time is 3-4 hours; the washing method is to use methanol to wash at least three times; and the drying temperature is 50-80℃.
4. The method of claim 1, wherein, The mixing temperature of step (3) is 35-45℃, and the time is 10-12 hours; the washing method is to use water and ethanol to wash at least three times respectively in sequence; and the drying temperature is 50-80℃.
5. The preparation method according to claim 1, characterized in that The calcination temperature of step (4) is 380-450℃, and the time is 4-5 hours.